Acetylene, The Principles Of Its Generation And Use
by
F. H. Leeds and W. J. Atkinson Butterfield
Part 4 out of 9
observed.
_Class A.--Stationary Automatic Apparatus._
1. FOUNDATIONS.--(_a_) Must, where practicable, be of brick, stone,
concrete or iron. If necessarily of wood they shall be extra heavy,
located in a dry place and open to the circulation of air.
The ordinary board platform is not satisfactory. Wooden foundations shall
be of heavy planking, joists or timbers, arranged so that the air will
circulate around them so as to form a firm base.
(_b_) Must be so arranged that the machine will be level and unequal
strain will not be placed on the generator or connexions.
2. LOCATION.--(_a_) Generators, especially in closely built up
districts should preferably be placed outside of insured buildings in
generator houses constructed and located in compliance with Rule 9.
(_b_) Generators must be so placed that the operating mechanism will
have room for free and full play and can be adjusted without artificial
light. They must not be subject to interference by children or careless
persons, and if for this purpose further enclosure is necessary, it must
be furnished by means of slatted partitions permitting the free
circulation of air.
(_c_) Generators which from their construction are rendered
inoperative during the process of recharging must be so located that they
can be recharged without the aid of artificial light.
(_d_) Generators must be placed where water will not freeze.
3. ESCAPES OR RELIEF-PIPES.--Each generator must be provided with an
escape or relief-pipe of ample size; no such pipe to be less than 3/4-
inch internal diameter. This pipe shall be substantially installed,
without traps, and so that any condensation will drain back to the
generator. It must be carried to a suitable point outside the building,
and terminate in an approved hood located at least 12 feet above ground
and remote from windows.
The hood must be constructed in such a manner that it cannot be
obstructed by rain, snow, ice, insects or birds.
4. CAPACITY.--(_a_) Must be sufficient to furnish gas continuously
for the maximum lighting period to all lights installed. A lighting
period of at least 5 hours shall be provided for in every case.
(_b_) Generators for conditions of service requiring lighting period
of more than 5 hours must be of sufficient capacity to avoid recharging
at night. The following ratings will usually be found advisable.
(i) For dwellings, and where machines are always used intermittently, the
generator must have a rated capacity equal to the total number of burners
installed.
(ii) For stores, opera houses, theatres, day-run factories, and similar
service, the generator must have a rated capacity of from 30 to 50 per
cent, in excess of the total number of burners installed.
(iii) For saloons and all night or continued service, the generator must
have a rated capacity of from 100 to 200 per cent. in excess of the total
number of burners installed.
(_c_) A small generator must never be installed to supply a large
number of lights, even though it seems probable that only a few lights
will be used at a time. _An overworked generator adds to the cost of
producing acetylene gas_.
5. CARBIDE CHARGES.--Must be sufficient to furnish gas continuously for
the maximum lighting period to all burners installed. In determining
charges lump carbide must be estimated as capable of producing 4-1/2
cubic foot of gas to the pound, commercial 1/4-inch carbide 4 cubic feet
of gas to the pound, and burners must be considered as requiring at least
25 per cent. more than their rated consumption of gas.
6. BURNERS.--Burners consuming one-half of a cubic foot of gas per hour
are considered standard in rating generators. Those having a greater or
less capacity will decrease or increase the number of burners allowable
in proportion.
Burners usually consume from 25 to 100 per cent. more than their rated
consumption of gas, depending largely on the working pressure. The so-
called 1/2-foot burner when operated at pressures of from 20- to 25-
tenths inches water column (2 to 2-1/2 inches) is usually used with best
economy.
7. PIPING.--(_a_) Connexions from generators to service-pipes must
be made with right and left thread nipples or long thread nipples with
lock nuts. All forms of unions are prohibited.
(_b_) Piping must, as far as possible, be arranged so that any
moisture will drain back to the generator. If low points occur of
necessity in any piping, they must be drained through tees into drip cups
permanently closed with screw caps or plugs. No pet-cocks shall be used.
(_c_) A valve and by-pass connexion must be provided from the
service-pipe to the blow-off for removing the gas from the holder in case
it should be necessary to do so.
(_d_) The schedule of pipe sizes for piping from generators to
burners should conform to that commonly used for ordinary gas, but in no
case must the feeders be smaller than three-eighths inch.
The following schedule is advocated:
3/8 inch pipe, 26 feet, three burners.
1/2 inch pipe, 30 feet, six burners.
3/4 inch pipe, 50 feet, twenty burners.
1 inch pipe, 70 feet, thirty-five burners.
1-1/4 inch pipe, 100 feet, sixty burners.
1-1/2 inch pipe, 150 feet, one hundred burners.
2 inch pipe, 200 feet, two hundred burners.
2-1/2 inch pipe, 300 feet, three hundred burners.
3 inch pipe, 450 feet, four hundred and fifty burners,
3-1/2 inch pipe, 500 feet, six hundred burners.
4 inch pipe, 600 feet, seven hundred and fifty burners.
(_e_) Machines of the carbide-feed type must not be fitted with
continuous drain connexions leading to sewers, but must discharge into
suitable open receptacles which may have such connections.
(_f_) Piping must be thoroughly tested both before and after the
burners have been installed. It must not show loss in excess of 2 inches
within twelve hours when subjected to a pressure equal to that of 15
inches of mercury.
(_g_) Piping and connexions must be installed by persons experienced
in the installation of acetylene apparatus.
8. CARE AND ATTENDANCE.--In the care of generators designed for a
lighting period of more than five hours always clean and recharge the
generating chambers at regular stated intervals, regardless of the number
of burners actually used.
Where generators are not used throughout the entire year always remove
all water and gas and clean thoroughly at the end of the season during
which they are in service.
It is usually necessary to take the bell portion out and invert it so as
to allow all gas to escape. This should never be done in the presence of
artificial light or fire of any kind.
Always observe a regular time, during daylight hours only, for attending
to and charging the apparatus.
In charging the generating chambers of water-feed machines clean all
residuum carefully from the containers and remove it at once from the
building. Separate from the mass any unslacked carbide remaining and
return it to the containers, adding now carbide as required. Be careful
never to fill the containers over the specified mark, as it is important
to allow for the swelling of the carbide when it comes in contact with
water. The proper action and economy of the machine are dependent on the
arrangement and amount of carbide placed in the generator. Carefully
guard against the escape of gas.
Whenever recharging with carbide always replenish the water-supply.
Never deposit residuum or exhausted material from water-feed machines in
sewer-pipes or near inflammable material.
Always keep water-tanks and water-seals filled with clean water.
Never test the generator or piping for leaks with a flame, and never
apply flame to an outlet from which the burner has been removed.
Never use a lighted match, lamp, candle, lantern or any open light near
the machine.
Failure to observe the above cautions is as liable to endanger life as
property.
9. OUTSIDE GENERATOR HOUSES.--(_a_) Outside generator houses should
not be located within 5 feet of any opening into, nor shall they open
toward any adjacent building, and must be kept under lock and key.
(_b_) The dimensions must be no greater than the apparatus requires
to allow convenient room for recharging and inspection of parts. The
floor must be at least 12 inches above grade and the entire structure
thoroughly weather-proof.
(_c_) Generator houses must be thoroughly ventilated, and any
artificial heating necessary to prevent freezing shall be done by steam
or hot-water systems.
(_d_) Generator houses must not be used for the storage of calcium
carbide except in accordance with the rules relating to that subject
(_vide_ Chapter II.).
_Class B.--Stationary Non-Automatic Apparatus_.
10. FOUNDATIONS.--(_a_) Must be of brick, stone or concrete.
(_b_) Must be so arranged that the machine will be level and so that
strain will not be brought upon the connexions.
11. GAS-HOUSES.--(_a_) Must be constructed entirely of non-
combustible material and must not be lighted by any system of
illumination involving open flames.
(_b_) Must be heated, where artificial heating is necessary to
prevent freezing, by steam or hot-water systems, the heater to be located
in a separate building, and no open flames to be permitted within
generator enclosures.
(_c_) Must be kept closed and locked excepting during daylight
hours.
(_d_) Must be provided with a permanent and effective system of
ventilation which will be operative at all times, regardless of the
periods of operation of the plant.
12. ESCAPE-PIPES.--Each generator must be provided with a vent-pipe of
ample size, substantially installed, without traps. It must be carried to
a suitable point outside the building and terminate in an approved hood
located at least 12 feet above ground and remote from windows.
The hood must be constructed in such a manner that it cannot be
obstructed by rain, snow, ice, insects or birds.
13. CARE AND MAINTENANCE.--All charging and cleaning of apparatus,
generation of gas and execution of repairs must be done during daylight
hours only, and generators must not be manipulated or in any way tampered
with in the presence of artificial light.
This will require gasholders of a capacity sufficient to supply all
lights installed for the maximum lighting period, without the necessity
of generation of gas at night or by artificial light.
In the operation of generators of the carbide-feed type it is important
that only a limited amount of carbide be fed into a given body of water.
An allowance of at least one gallon of generating water per pound of
carbide must be made in every case, and when this limit has been reached
the generator should be drained and flushed, and clean water introduced.
These precautions are necessary to avoid over-heating during generation
and accumulation of hard deposits of residuum in the generating chamber.
(Rule 14, referring to the storage of carbide, has been quoted in Chapter
II. (page 19)).
RULES FOR THE CONSTRUCTION OF GENERATORS.
The following Rules are intended to provide only against the more
hazardous defects usually noted in apparatus of this kind. The Rules do
not cover all details of construction nor the proper proportioning of
parts, and devices which comply with these requirements alone are not
necessarily suitable for listing as permissible for use. These points are
often only developed in the examination required before permission is
given for installation.
_Class A.--Stationary Apparatus for Isolated Installations._
15. GENERAL RULES. GENERATORS.--(_a_) Must be made of iron or steel,
and in a manner and of material to insure stability and durability.
(_b_) Must be automatically regulated and uniform in their action,
producing gas only as immediate consumption demands, and so designed that
gas is generated without producing sufficient heat to cause yellow
discoloration of residuum (which will occur at about 500 deg. F.) or
abnormal pressure at any stage of the process when using carbide of any
degree of fineness.
The presence of excessive heat tends to change the chemical character of
the gas and may even cause its ignition, while in machines of the
carbide-feed type, finely divided carbide will produce excessive pressure
unless provision is made to guard against it.
(_c_) Must be so arranged that during recharging, back flow of gas
from the gasholder will be automatically prevented, or so arranged that
it will be impossible to charge the apparatus without first closing the
supply-pipe to the gasholder, and to the other generating chambers if
several are used.
This is intended to prevent the dangerous escape of gas.
(_d_) The water or carbide supply to the generating chamber must be
so arranged that gas will be generated long enough in advance of the
exhaustion of the supply already in the gasholder to allow the using of
all lights without exhausting such supply.
This provides for the continuous working of the apparatus under all
conditions of water-feed and carbide charge, and it obviates the
extinction of lights through intermittent action of the machine.
(_e_) No valves or pet-cocks opening into the room from the gas-
holding part or parts, the draining of which will allow an escape of gas,
are permitted, and condensation from all parts of the apparatus must be
automatically removed without the use of valves or mechanical working
parts.
Such valves and pet-cocks are not essential; their presence increases the
possibility of leakage. The automatic removal of condensation from the
apparatus is essential to the safe working of the machine.
U-traps opening into the room from the gas-holding parts must not be used
for removal of condensation. All sealed drip connexions must be so
arranged as to discharge gas to the blow-off when blown out, and the
seals must be self-restoring upon relief of abnormal pressure.
(_f_) The apparatus must be capable of withstanding fire from
outside causes.
Sheet-metal joints must be double-seamed or riveted and thoroughly
sweated with solder. Pipes must be attached to sheet-metal with lock-nuts
or riveted flanges.
This prohibits the use of wood or of joints relying entirely upon solder.
(_g_) Gauge glasses, the breakage of which would allow the escape of
gas, must not be used.
(_h_) The use of mercury seals is prohibited.
Mercury has been found unreliable as a seal in acetylene apparatus.(_i_)
Combustible oils must not be used in connexion with the
apparatus.
(_j_) The construction must be such that liquid seals shall not
become thickened by the deposit of lime or other foreign matter.
(_k_) The apparatus must be constructed so that accidental siphoning
of water will be impossible.
(_l_) Flexible tubing, swing joints, unions, springs, mechanical
check-valves, chains, pulleys, stuffing-boxes and lead or fusible piping
must not be used on acetylene apparatus except where failure of such
parts will not vitally affect the working or safety of the machine.
Floats must not be used excepting in cases where failure will result only
in rendering the machine inoperative.
(_m_) Every machine must be plainly marked with the maximum number
of lights it is designed to supply, the amount of carbide necessary for a
single charge, the manufacturer's name and the name of the machine.
16. GENERATING CHAMBERS.--(_a_) Must be constructed of galvanised
iron or steel not less than No. 24 U.S. Standard gauge in thickness for
capacities up to and including 20 gallons, not less than No. 22 U.S.
Standard gauge for capacities between 20 and 75 gallons, and not less
than No. 20 U.S. Standard gauge for capacities in excess of 75 gallons.
(_b_) Must each be connected with the gasholder in such a manner
that they will, at all times, give open connexion either to the gasholder
or to the blow-off pipe to the outer air.
This prevents dangerous pressure within or the escape of gas from the
generating chamber.
(_c_) Must be so constructed that not more than 5 pounds of carbide
can be acted upon at once, in machines which apply water in small
quantities to the carbide.
This tends to reduce the danger of overheating and excessive after-
generation by providing for division of the carbide charges in machines
of this type.
(_d_) Must be provided with covers having secure fastenings to hold
them properly in place and those relying on a water-seal must be
submerged in at least 12 inches of water. Water-seal chambers for covers
depending on a water-seal must be 1-1/2 inches wide and 15 inches deep,
excepting those depending upon the filling of the seal chambers for the
generation of gas, where 9 inches will be sufficient.
(_e_) Must be so designed that the residuum will not clog or affect
the working of the machine and can conveniently be handled and removed.
(_f_) Must be provided with suitable vent connexions to the blow-off
pipe so that residuum may be removed and the generating water replaced
without causing siphoning or introducing air to the gasholder upon
recharging.
This applies to machines of the carbide-feed type.
(_g_) Feed mechanism for machines of the carbide-feed type must be
so designed that the direct fall of carbide from the carbide holder into
the water of the generator is prevented at all positions of the feed
mechanisms; or, when actuated by the rise and fall of a gas-bell, must be
so arranged that the feed-valve will not remain open after the landing of
the bell, and so that the feed valve remains inoperative as long as the
filling opening on the carbide hopper remains open. Feed mechanisms must
always be far enough above the water-level to prevent clogging from the
accumulation of damp lime. For this purpose the distance should be not
less than 10 inches.
17. CARBIDE CHAMBERS.--(_a_) Must be constructed of galvanised iron
or steel not less than No. 24 U.S. Standard gauge in thickness for
capacities up to and including 50 pounds and not less than No. 22 U.S.
Standard gauge for capacities in excess of 50 pounds.
(_b_) Must have sufficient carbide capacity to supply the full
number of burners continuously and automatically during the maximum
lighting period.
This rule removes the necessity of recharging or attending to the machine
at improper hours. Burners almost invariably require more than their
rated consumption of gas, and carbide is not of staple purity, and there
should therefore be an assurance of sufficient quantity to last as long
as light is needed. Another important consideration is that in some
establishments burners are called upon for a much longer period of
lighting than in others, requiring a generator of greater gas-producing
capacity. Machines having several generating chambers must automatically
begin generation in each upon exhaustion of the preceding chamber.
(_c_) Must be arranged so that the carbide holders or charges may be
easily and entirely removed in case of necessity.
18. GASHOLDERS.--(_a_) Must be constructed of galvanised iron or
steel not less than No. 24 U.S. Standard gauge in thickness for
capacities up to and including 20 gallons, not less than No. 22 U.S.
Standard gauge for capacities between 20 and 75 gallons, and not less
than No. 20 U.S. Standard gauge for capacities in excess of 75 gallons.
Gas-bells, if used, may be two gauges lighter than holders.
Condensation chambers, if placed under holders, to be of same gauge as
holders.
(_b_) Must be of sufficient capacity to contain all gas generated
after all lights have been extinguished.
If the holder is too small and blows off frequently after the lights are
extinguished there is a waste of gas. This may suggest improper working
of the apparatus and encourage tampering.
(_c_) Must, when constructed on the gasometer principle, be so
arranged that when the gas-bell is filled to its maximum with gas at
normal pressure its lip or lower edge will extend at least 9 inches below
the inner water-level.
(_d_) Must, when constructed on the gasometer principle, have the
dimensions of the tank portion so related to those of the bell that a
pressure of at least 11 inches will be necessary before gas can be forced
from the holder.
(_e_) The bell portion of a gasholder constructed on the gasometer
principle must be provided with a substantial guide to its upward
movement, preferably in the centre of the holder, carrying a stop acting
to chock the bell 1 inch above the normal blow-off point.
This tends to insure the proper action of the bell and decreases the
liability of escaping gas.
(_f_) A space of at least three-quarters of an inch must be allowed
between the sides of the tank and the bell.
(_g_) All water-seals must be so arranged that the water-level may
be readily seen and maintained.
19. WATER-SUPPLY.--(_a_) The supply of water to the generator for
generating purposes must not be taken from the water-seal of any
gasholder constructed on the gasometer principle, unless the feed
mechanism is so arranged that the water-seals provided for in Rules 18,
(_c_), (_d_), and (_e_) may be retained under all
conditions. This provides for the proper level of water in the gasholder.
(_b_) In cases where machines of the carbide-feed type are supplied
with water from city water-mains or house-pipes, the pipe connexion must
discharge into the regularly provided filling trap on the generator and
not through a separate continuous connexion leading into the generating
chamber.
This is to prevent the expulsion of explosive mixtures through the
filling trap in refilling.
20. RELIEFS OR SAFETY BLOW-OFFS.--(_a_) Must in all cases be
provided, and must afford free vent to the outer air for any over-
production of gas, and also afford relief in case of abnormal pressure in
the machine.
Both the above-mentioned vents may be connected, with the same escape-
pipe.
(_b_) Must be of at least 3/4-inch internal diameter and be provided
with suitable means for connecting to the pipe loading outside of the
building.
(_c_) Must be constructed without valves or other mechanical working
parts.
(_d_) Apparatus requiring pressure regulators must be provided with
an additional approved safety blow-off attachment located between the
pressure regulator and the service-pipes and discharging to the outer
air.
This is intended to prevent the possibility of undue pressure in the
service-pipes due to failure of the pressure regulator.
21. PRESSURES.--(_a_) The working pressure at the generator must not
vary more than ten-tenths (1) inch water column under all conditions of
carbide charge and feed, and between the limits of no load and 50 per
cent. overload.
(_b_) Apparatus not requiring pressure regulators must be so
arranged that the gas pressure cannot exceed sixty-tenths (6) inches
water column.
This requires the use of the pressure relief provided for in Rule No. 20
(_a_).
(_c_) Apparatus requiring pressure regulators must be so arranged
that the gas pressure cannot exceed three pounds to the square inch.
The pressure limit of 3 pounds is taken since that is the pressure
corresponding to a water column about 6 feet high, which is about, the
limit in point of convenience for water-sealed reliefs.
22. AIR MIXTURES.--Generators must be so arranged as to contain the
minimum amount of air when first started or recharged, and no device or
attachment facilitating or permitting mixture of air with the gas prior
to consumption, except at the burners, shall be allowed.
Owing to the explosive properties of acetylene mixed with air, machines
must be so designed that such mixtures are impossible.
23. PURIFIERS.--(_a_) Must be constructed of galvanised iron or
steel not less than No. 24 U.S. Standard gauge in thickness.
(_b_) Where installed, purifiers must conform to the general rules
for the construction of other acetylene apparatus and allow the free
passage of gas.
(_c_) Purifiers must contain no carbide for drying purposes.
(_d_) Purifiers must be located inside of gasholders, or, where
necessarily outside, must have no hand-holes which can be opened without
first shutting off the gas-supply.
24. PRESSURE REGULATORS.--(_a_) Must conform to the rules for the
construction of other acetylene apparatus so far as they apply and must
not be subject to sticking or clogging.
(_b_) Must be capable of maintaining a uniform pressure, not varying
more than four-tenths inch water column, at any load within their rating.
(_c_) Must be installed between valves in such a manner as to
facilitate inspection and repairs.
_Class B.--Stationary Apparatus for Central Station Service._
Generators of over 300 lights capacity for central station service are
not required to be automatic in operation. Generators of less than 300
lights capacity must be automatic in operation and must comply in every
respect with the requirements of Class A.
25. GENERAL RULES. GENERATORS.--(_a_) Must be substantially
constructed of iron or steel and be protected against depreciation by an
effective and durable preventive of corrosion.
Galvanising is strongly recommended as a protection against oxidation,
and it may to advantage be reinforced by a thorough coating of asphaltum
or similar material.
(_b_) Must contain no copper or alloy of copper in contact with
acetylene, excepting in valves.
(_c_) Must be so arranged that generation will take place without
overheating; temperatures in excess of 500 deg. F. to be considered
excessive.
(_d_) Must be provided with means for automatic removal of
condensation from gas passages.
(_e_) Must be provided with suitable protection against freezing of
any water contained in the apparatus.
No salt or other corrosive chemical is permissible as a protection
against freezing.
(_f_) Must in general comply with the requirements governing the
construction of apparatus for isolated installations so far as they are
applicable.
(_g_) Must be so arranged as to insure correct procedure in
recharging and cleaning.
(_h_) Generators of the carbide-feed type must be provided with some
form of approved measuring device to enable the attendant to determine
when the maximum allowable quantity of carbide has been fed into the
generating chamber.
In the operation of generators of this type an allowance of at least 1
gallon of clean generating water per pound of carbide should be made, and
the generator should be cleaned after slaking of every full charge. Where
lump carbide is used the lumps may become embedded in the residuum, if
the latter is allowed to accumulate at the bottom of the generating
chamber, causing overheating from slow and restricted generation, and
rendering the mass more liable to form a hard deposit and bring severe
stresses upon the walls of the generator by slow expansion.
26. GENERATING CHAMBERS.--(_a_) Must each be connected with the
gasholder in such a manner that they will, at all times, give open
connexion either to the gasholder or to the blow-off pipe into the outer
air.
(_b_) Must be so arranged as to guard against appreciable escape of
gas to the room at any time during the introduction of the charges.
(_c_) Must be so designed that the residuum will not clog or affect
the operation of the machine and can conveniently be handled and removed.
(_d_) Must be so arranged that during the process of cleaning and
recharging the back-flow of gas from the gasholder or other generating
chambers will be automatically prevented.
27. GASHOLDERS.--(_a_) Must be of sufficient capacity to contain at
least 4 cubic feet of gas per 1/2-foot burner of the rating.
This is to provide for the requisite lighting period without the
necessity of making gas at night, allowance being made for the
enlargement of burners caused by the use of cleaners.
(_b_) Must be provided with suitable guides to direct the movement
of the bell throughout its entire travel.
28. PRESSURE RELIEFS.--Must in all cases be provided, and must be so
arranged as to prevent pressure in excess of 100-tenths (10) inches water
column in the mains.
29. PRESSURES.--Gasholders must be adjusted to maintain a pressure of
approximately 25-tenths (2.5) inches water column in the mains.
CHAPTER V
THE TREATMENT OF ACETYLENE AFTER GENERATION
IMPURITIES IN CALCIUM CARBIDE.--The calcium carbide manufactured at the
present time, even when of the best quality commercially obtainable, is
by no means a chemically pure substance; it contains a large number of
foreign bodies, some of which evolve gas on treatment with water. To a
considerable extent this statement will probably always remain true in
the future; for in order to make absolutely pure carbide it would be
necessary for the manufacturer to obtain and employ perfectly pure lime,
carbon, and electrodes in an electric furnace which did not suffer attack
during the passage of a powerful current, or he would have to devise some
process for simultaneously or subsequently removing from his carbide
those impurities which were derived from his impure raw materials or from
the walls of his furnace--and either of these processes would increase
the cost of the finished article to a degree that could hardly be borne.
Beside the impurities thus inevitably arising from the calcium carbide
decomposed, however, other impurities may be added to acetylene by the
action of a badly designed generator or one working on a wrong system of
construction; and therefore it may be said at once that the crude gas
coming from the generating plant is seldom fit for immediate consumption,
while if it be required for the illumination of occupied rooms, it must
invariably be submitted to a rigorous method of chemical purification.
IMPURITIES OF ACETYLENE.--Combining together what may be termed the
carbide impurities and the generator impurities in crude acetylene, the
foreign bodies are partly gaseous, partly liquid, and partly solid. They
may render the gas dangerous from the point of view of possible
explosions; they, or the products derived from them on combustion, may be
harmful to health if inspired, injurious to the fittings and decorations
of rooms, objectionable at the burner orifices by determining, or
assisting in, the formation of solid growths which distort the flame and
so reduce its illuminating power; they may give trouble in the pipes by
condensing from the state of vapour in bends and dips, or by depositing,
if they are already solid, in angles, &c., and so causing stoppages; or
they may be merely harmful economically by acting as diluents to the
acetylene and, by having little or no illuminating value of themselves,
causing the gas to emit less light than it should per unit of volume
consumed, more particularly, of course, when the acetylene is not burnt
under the mantle. Also, not being acetylene, or isomeric therewith, they
require, even if they are combustible, a different proportion of oxygen
for their perfect combustion; and a good acetylene jet is only calculated
to attract precisely that quantity of air to the flame which a gas having
the constitution C_2H_2 demands. It will be apparent without argument
that a proper system of purification is one that is competent to remove
the carbide impurities from acetylene, so far as that removal is
desirable or necessary; it should not be called upon to extract the
generator impurities, because the proper way of dealing with them is, to
the utmost possible extent, to prevent their formation. The sole
exception to this rule is that of water-vapour, which invariably
accompanies the best acetylene, and must be partially removed as soon as
convenient. Vapour of water almost always accompanies acetylene from the
generator, even when the apparatus does not belong to those systems of
working where liquid water is in excess, this being due to the fact that
in a generator where the carbide is in excess the temperature tends to
rise until part of the water is vapourised and carried out of the
decomposing chamber before it has an opportunity of reacting with the
excess of carbide. The issuing gas is therefore more or less hot, and it
usually comes from the generating chamber saturated with vapour, the
quantity needed so to saturate it rising as the temperature of the gas
increases. Practically speaking, there is little objection to the
presence of water-vapour in acetylene beyond the fear of deposition of
liquid in the pipes, which may accumulate till they are partially or
completely choked, and may even freeze and burst them in very severe
weather. Where the chemical purifiers, too, contain a solid material
which accidentally or intentionally acts as a drier by removing moisture
from the acetylene, it is a waste of such comparatively expensive
material to allow gas to enter the purifier wetter than need be.
EXTRACTION OF MOISTURE.--In all large plants the extraction of the
moisture may take place in two stages. Immediately after the generator,
and before the washer if the generator requires such an apparatus to
follow it, a condenser is placed. Here the gas is made to travel somewhat
slowly through one or more pipes surrounded with cold air or water, or is
made to travel through a space containing pipes in which cold water is
circulating, the precise method of constructing the condenser being
perfectly immaterial so long as the escaping gas has a temperature not
appreciably exceeding that of the atmosphere. So cooled, however, the gas
still contains much water-vapour, for it remains saturated therewith at
the temperature to which it is reduced, and by the inevitable law of
physics a further fall in temperature will be followed by a further
deposition of liquid water from the acetylene. Manifestly, if the
installation is so arranged that the gas can at no part of the service
and on no occasion fall to a lower temperature than that at which it
issues from the condenser, the removal of moisture as effected by such a
condenser will be sufficient for all practical purposes; but at least in
all large plants where a considerable length of main is exposed to the
air, a more complete moisture extractor must be added to the plant, or
water will be deposited in the pipes every cold night in the winter. It
is, however, useless to put a chemical drier, or one more searching in
its action than a water-cooled condenser, at so early a position in the
acetylene plant, because the gas will be subsequently stored in a water-
sealed holder, where it will most probably once again be saturated with
moisture from the seal. When such generators are adopted as require to
have a specific washer placed after them in order to remove the water-
soluble impurities, _e.g._, those in which the gas does not actually
bubble through a considerable quantity of liquid in the generating
chamber itself, it is doubtful whether a separate condenser is altogether
necessary, because, as the water in the washer can easily be kept at the
atmospheric temperature (by means of water circulating in pipes or
otherwise), the gas will be brought to the atmospheric temperature in the
washer, and at that temperature it cannot carry with it more than a
certain fixed proportion of moisture. The notion of partially drying a
gas by causing it to pass through water may appear paradoxical, but a
comprehension of physical laws will show that it is possible, and will
prove efficient in practice, when due attention is given to the facts
that the gas entering the washer is hot, and that it is subsequently to
be stored over water in a holder.
GENERATOR IMPURITIES.--The generator impurities present in the crudest
acetylene consist of oxygen and nitrogen, _i.e._, the main
constituents of air, the various gaseous, liquid, and semi-solid bodies
described in Chapter II., which are produced by the polymerising and
decomposing action of heat upon the carbide, water, and acetylene in the
apparatus, and, whenever the carbide is in excess in the generator, some
lime in the form of a very fine dust. In all types of water-to-carbide
plant, and in some automatic carbide-feed apparatus, the carbide chamber
must be disconnected and opened each time a fresh charge has to be
inserted; and since only about one-third of the space in the container
can be filled with carbide, the remaining two-thirds are left full of
air. It is easy to imagine that the carbide container of a small
generator might be so large, or loaded with so small a quantity of
carbide, or that the apparatus might in other respects be so badly
designed, that the gas evolved might contain a sufficient proportion of
air to render it liable to explode in presence of a naked light, or of a
temperature superior to its inflaming-point. Were a cock, however, which
should have been shut, to be carelessly left open, an escape of gas from,
rather than an introduction of air into, the apparatus would follow,
because the pressure in the generator is above that of the atmosphere. As
is well known, roughly four-fifths by volume of the air consist of
nitrogen, which is non-inflammable and accordingly devoid of danger-
conferring properties; but in all flames the presence of nitrogen is
harmful by absorbing much of the heat liberated, thus lowering the
temperature of that flame, and reducing its illuminating power far more
seriously. On the other hand, a certain quantity of air in acetylene
helps to prevent burner troubles by acting as a mere diluent (albeit an
inferior one to methane or marsh-gas), and therefore it has been proposed
intentionally to add air to the gas before consumption, such a process
being in regular use on the large scale in some places abroad. As Eitner
has shown (Chapter VI.) that in a 3/4-inch pipe acetylene ceases to be
explosive when mixed with less than 47.7 per cent. of air, an amount of,
say, 40 per cent. or less may in theory be safely added to acetylene; but
in practice the amount of air added, if any, would have to be much
smaller, because the upper limit of explosibility of acetylene-air
mixtures is not rigidly fixed, varying from about 50 per cent. of air
when the mixture is in a small vessel, and fired electrically to about 25
per cent. of air in a large vessel approached with a flame. Moreover,
safely to prepare such mixtures, after the proportion of air had been
decided upon, would require the employment of some additional perfectly
trustworthy automatic mechanism to the plant to draw into the apparatus a
quantity of air strictly in accordance with the volume of acetylene made
--a pair of meters geared together, one for the gas, the other for the
air--and this would introduce extra complexity and extra expense. On the
whole the idea cannot be recommended, and the action of the British Home
Office in prohibiting the use of all such mixtures except those
unavoidably produced in otherwise good generators, or in burners of the
ordinary injector type, is perfectly justifiable. The derivation and
effect of the other gaseous and liquid generator impurities in acetylene
were described in Chapter II. Besides these, very hot gas has been found
to contain notable amounts of hydrogen and carbon monoxide, both of which
burn with non-luminous flames. The most plausible explanation of their
origin has been given by Lewes, who suggests that they may be formed by
the action of water-vapour upon very hot carbide or upon carbon separated
therefrom as the result of previous dissociation among the gases present;
the steam and the carbon reacting together at a temperature of 500 deg. C.
or thereabouts in a manner resembling that of the production of water-gas.
The last generator impurity is lime dust, which is calcium oxide or
hydroxide carried forward by the stream of gas in a state of extremely
fine subdivision, and is liable to be produced whenever water acts
rapidly upon an excess of calcium carbide. This lime occasionally appears
in the alternative form of a froth in the pipes leading directly from the
generating chamber; for some types of carbide-to-water apparatus,
decomposing certain kinds of carbide, foam persistently when the liquid
in them becomes saturated with lime, and this foam or froth is remarkably
difficult to break up.
FILTERS.--It has just been stated that the purifying system added to an
acetylene installation should not be called upon to remove these
generator impurities; because their appearance in quantity indicates a
faulty generator, which should be replaced by one of better action. On
the contrary, with the exception of the gases which are permanent at
atmospheric temperature--hydrogen, carbon monoxide, nitrogen, and oxygen--
and which, once produced, must remain in the acetylene (lowering its
illuminating value, but giving no further trouble), extraction of these
generator impurities is quite simple. The dust or froth of lime will be
removed in the washer where the acetylene bubbles through water--the dust
itself can be extracted by merely filtering the gas through cotton-wool,
felt, or the like. The least volatile liquid impurities will be removed
partly in the condenser, partly in the washer, and partly by the
mechanical dry-scrubbing action of the solid purifying material in the
chemical purifier. To some extent the more volatile liquid bodies will be
removed similarly; but a complete extraction of them demands the
employment of some special washing apparatus in which the crude acetylene
is compelled to bubble (in finely divided streams) through a layer of
some non-volatile oil, heavy mineral lubricating oil, &c.; for though
soluble in such oil, the liquid impurities are not soluble in, nor do
they mix with, water; and since they are held in the acetylene as
vapours, a simple passage through water, or through water-cooled pipes,
does not suffice for their recovery. It will be seen that a sufficient
removal of these generator impurities need throw no appreciable extra
labour upon the consumer of acetylene, for he can readily select a type
of generator in which their production is reduced to a minimum; while a
cotton-wool or coke filter for the gas, a water washer, which is always
useful in the plant if only employed as a non-return valve between the
generator and the holder, and the indispensable chemical purifiers, will
take out of the acetylene all the remaining generator impurities which
need, and can, be extracted.
CARBIDE IMPURITIES.--Neglecting very minute amounts of carbon monoxide
and hydrogen (which may perhaps come from cavities in the calcium carbide
itself), as being utterly insignificant from the practical point of view,
the carbide impurities of the gas fall into four main categories: those
containing phosphorus, those containing sulphur, those containing
silicon, and those containing gaseous ammonia. The phosphorus in the gas
comes from calcium phosphide in the calcium carbide, which is attacked by
water, and yields phosphoretted hydrogen (or phosphine, as it will be
termed hereafter). The calcium phosphide, in its turn, is produced in the
electric furnace by the action of the coke upon the phosphorus in
phosphatic lime--all commercially procurable lime and some varieties of
coke (or charcoal) containing phosphates to a larger or smaller extent.
The sulphur in the gas comes from aluminium sulphide in the carbide,
which is produced in the electric furnace by the interaction of
impurities containing aluminium and sulphur (clay-like bodies, &c.)
present in the lime and coke; this aluminium sulphide is attacked by
water and yields sulphuretted hydrogen. Even in the absence of aluminium
compounds, sulphuretted hydrogen may be found in the gases of an
acetylene generator; here it probably arises from calcium sulphide, for
although the latter is not decomposed by water, it gradually changes in
water into calcium sulphydrate, which appears to suffer decomposition.
When it exists in the gas the silicon is derived from certain silicides
in the carbide; but this impurity will be dealt with by itself in a later
paragraph. The ammonia arises from the action of the water upon
magnesium, aluminium, or possibly calcium nitride in the calcium carbide,
which are bodies also produced in the electric furnace or as the carbide
is cooling. In the gas itself the ammonia exists as such; the phosphorus
exists mainly as phosphine, partly as certain organic compounds
containing phosphorus, the exact chemical nature of which has not yet
been fully ascertained; the sulphur exists partly as sulphuretted
hydrogen and partly as organic compounds analogous, in all probability,
to those of phosphorus, among which Caro has found oil of mustard, and
certain bodies that he regards as mercaptans. [Footnote: It will be
convenient to borrow the phrase used in the coal-gas industry, calling
the compounds of phosphorus other than phosphine "phosphorus compounds,"
and the compounds of sulphur other than sulphuretted hydrogen "sulphur
compounds." The "sulphur compounds" of coal-gas, however, consist mainly
of carbon bisulphide, which is certainly not the chief "sulphur compound"
in acetylene, even if present to any appreciable extent.] The precise way
in which these organic bodies are formed from the phosphides and
sulphides of calcium carbide is not thoroughly understood; but the system
of generation employed, and the temperature obtaining in the apparatus,
have much to do with their production; for the proportion of the total
phosphorus and sulphur found in the crude gas which exists as "compounds"
tends to be greater as the generating plant yields a higher temperature.
It should be noted that ammonia and sulphuretted hydrogen have one
property in common which sharply distinguishes them from the sulphur
"compounds," and from all the phosphorus compounds, including phosphine.
Ammonia and sulphuretted hydrogen are both very soluble in water, the
latter more particularly in the lime-water of an active acetylene
generator; while all the other bodies referred to are completely
insoluble. It follows, therefore, that a proper washing of the crude gas
in water should suffice to remove all the ammonia and sulphuretted
hydrogen from the acetylene; and as a matter of fact those generators in
which the gas is evolved in presence of a large excess of water, and in
which it has to bubble through such water, yield an acetylene practically
free from ammonia, and containing nearly all the sulphur which it does
contain in the state of "compounds." It must also be remembered that
chemical processes which are perfectly suited to the extraction of
sulphuretted hydrogen and phosphine are not necessarily adapted for the
removal of the other phosphorus and sulphur compounds.
WASHERS.--In designing a washer for the extraction of ammonia and
sulphuretted hydrogen it is necessary to see that the gas is brought into
most intimate contact with the liquid, while yet no more pressure than
can possibly be avoided is lost. Subdivision of the gas stream may be
effected by fitting the mouth of the inlet-pipe with a rose having a
large number of very small holes some appreciable distance apart, or by
bending the pipe to a horizontal position and drilling it on its upper
surface with numbers of small holes. Another method is to force the gas
to travel under a series of partitions extending just below the water-
level, forming the lower edges of those partitions either perfectly
horizontal or with small notches like the teeth of a saw. One volume of
pure water only absorbs about three volumes of sulphuretted hydrogen at
atmospheric temperatures, but takes up some 600 volumes of gaseous
ammonia; and as ammonia always accompanies the sulphuretted hydrogen, the
latter may be said to be absorbed in the washer by a solution of ammonia,
a liquid in which sulphuretted hydrogen is much more soluble. Therefore,
since water only dissolves about an equal volume of acetylene, the liquid
in the washer will continue to extract ammonia and sulphuretted hydrogen
long after it is saturated with the hydrocarbon. For this reason,
_i.e._, to avoid waste of acetylene by dissolution in the clean
water of the washer, the plan is sometimes adopted of introducing water
to the generator through the washer, so that practically the carbide is
always attacked by a liquid saturated with acetylene. Provided the liquid
in the generator does not become seriously heated, there is no objection
to this arrangement; but if the water is heated strongly in the generator
it loses much or all of its solvent properties, and the impurities may be
driven back again into the washer. Clearly if the waste lime of the
generator occurs as a dry or damp powder, the plan mentioned is not to be
recommended; but when the waste lime is a thin cream--water being in
large excess--it may be adopted. If the generator produces lime dust
among the gas, and if the acetylene enters the washer through minute
holes, a mechanical filter to remove the dust must be inserted between
the generator and the washer, or the orifices of the leading pipe will be
choked. Whenever a water-cooled condenser is employed after the
generator, in which the gas does not come in contact with the water, that
liquid may always be used to charge the generator. For compactness and
simplicity of parts the water of the holder seal is occasionally used as
the washing liquid, but unless the liquid of the seal is constantly
renewed it will thus become offensive, especially if the holder is under
cover, and it will also act corrosively upon the metal of the tank and
bell. The water-soluble impurities in acetylene will not be removed
completely by merely standing over the holder seal for a short time, and
it is not good practice to pass unnecessarily impure gas into a holder.
[Footnote: This is not a contradiction of what has been said in Chapter
III. about the relative position of holder and chemical purifiers,
because reference is now being made to ammonia and sulphuretted hydrogen
only.]
HARMFULNESS OF IMPURITIES.--The reasons why the carbide impurities must
be removed from acetylene before it is burned have now to be explained.
From the strictly chemical point of view there are three compounds of
phosphorus, all termed phosphoretted hydrogen or phosphine: a gas, PH_3;
a liquid, P_2H_4; and a solid, P_4H_2. The liquid is spontaneously
inflammable in presence of air; that is to say, it catches fire of itself
without the assistance of spark or flame immediately it comes in contact
with atmospheric oxygen; being very volatile, it is easily carried as
vapour by any permanent gas. The gaseous phosphine is not actually
spontaneously inflammable at temperatures below 100 deg. C.; but it oxidises
so rapidly in air, even when somewhat diluted, that the temperature may
quickly rise to the point of inflammation. In the earliest days of the
acetylene industry, directly it was recognised that phosphine always
accompanies crude acetylene from the generator, it was believed that
unless the proportion were strictly limited by decomposing only a carbide
practically free from phosphides, the crude acetylene might exhibit
spontaneously inflammable properties. Lewes, indeed, has found that a
sample of carbide containing 1 per cent of calcium phosphide gave
(probably by local decomposition--the bulk of the phosphide suffering
attack first) a spontaneously inflammable gas; but when examining
specimens of commercial carbide the highest amount of phosphine he
discovered in the acetylene was 2.3 per cent, and this gas was not
capable of self-inflammation. According to Bullier, however, acetylene
must contain 80 per cent of phosphine to render it spontaneously
inflammable. Berdenich has reported a case of a parcel of carbide which
yielded on the average 5.1 cubic foot of acetylene per lb., producing gas
which contained only 0.398 gramme of phosphorus in the form of phosphine
per cubic metre (or 0.028 per cent. of phosphine) and was spontaneously
inflammable. But on examination the carbide in question was found to be
very irregular in composition, and some lumps produced acetylene
containing a very high proportion of phosphorus and silicon compounds. No
doubt the spontaneous inflammability was due to the exceptional richness
of these lumps in phosphorus. As manufactured at the present day, calcium
carbide ordinarily never contains an amount of phosphide sufficient to
render the gas dangerous on the score of spontaneous inflammability; but
should inferior material ever be put on the markets, this danger might
have to be guarded against by submitting the gas evolved from it to
chemical analysis. Another risk has been suggested as attending the use
of acetylene contaminated with phosphine (and to a minor degree with
sulphuretted hydrogen), viz., that being highly toxic, as they
undoubtedly are, the gas containing them might be extremely dangerous to
breathe if it escaped from the service, or from a portable lamp,
unconsumed. Anticipating what will be said in a later paragraph, the
worst kind of calcium carbide now manufactured will not yield a gas
containing more than 0.1 per cent. by volume of sulphuretted hydrogen and
0.05 per cent. of phosphine. According to Haldane, air containing 0.07
per cent. of sulphuretted hydrogen produces fatal results on man if it is
breathed for some hours, while an amount of 0.2 per cent. is fatal in 1-
1/2 minutes. Similar figures for phosphine cannot be given, because
poisoning therewith is very rare or quite unknown: the cases of "phossy-
jaw" in match factories being caused either by actual contact with yellow
phosphorus or by inhalation of its vapour in the elemental state.
However, assuming phosphine to be twice as toxic as sulphuretted
hydrogen, its effect in crude acetylene of the above-mentioned
composition will be equal to that of the sulphuretted hydrogen, so that
in the present connexion the gas may be said to be equally toxic with a
sample of air containing 0.2 per cent. of sulphuretted hydrogen, which
kills in less than two minutes. But this refers only to crude acetylene
undiluted with air; and being a hydrocarbon--being in fact neither oxygen
nor common air--acetylene is irrespirable of itself though largely devoid
of specific toxic action. Numerous investigations have been made of the
amount of acetylene (apart from its impurities) which can be breathed in
safety; but although these point to a probable recovery after a fairly
long-continued respiration of an atmosphere charged with 30 per cent. of
acetylene, the figure is not trustworthy, because toxicological
experiments upon animals seldom agree with similar tests upon man. If
crude acetylene were diluted with a sufficient proportion of air to
remove its suffocating qualities, the percentage of specifically toxic
ingredients would be reduced to a point where their action might be
neglected; and short of such dilution the acetylene itself would in all
probability determine pathological effects long before its impurities
could set up symptoms of sulphur and phosphorus poisoning.
Ammonia is objectionable in acetylene because it corrodes brass fittings
and pipes, and because it is partially converted (to what extent is
uncertain) into nitrous and nitric acids as it passes through the flame.
Sulphur is objectionable in acetylene because it is converted into
sulphurous and sulphuric anhydrides, or their respective acids, as it
passes through the flame. Phosphorus is objectionable because in similar
circumstances it produces phosphoric anhydride and phosphoric acid. Each
of these acids is harmful in an occupied room because they injure the
decorations, helping to rot book-bindings, [Footnote: It is only fair to
state that the destruction of leather bindings is commonly due to traces
of sulphuric acid remaining in the leather from the production employed
in preparing it, and is but seldom caused directly by the products of
combustion coming from gas or oil.] tarnishing "gold-leaf" ornaments, and
spoiling the colours of dyed fabrics. Each is harmful to the human
system, sulphuric and phosphoric anhydrides (SO_3, and P_4O_10) acting as
specific irritants to the lungs of persons predisposed to affections of
the bronchial organs. Phosphorus, however, has a further harmful action:
sulphuric anhydride is an invisible gas, but phosphoric anhydride is a
solid body, and is produced as an extremely fine, light, white voluminous
dust which causes a haze, more or less opaque, in the apartment.
[Footnote: Lewes suggests that ammonia in the gas burnt may assist in the
production of this haze, owing to the formation of solid ammonium salts
in the state of line dust.] Immediately it comes in contact with
atmospheric moisture phosphoric anhydride is converted into phosphoric
acid, but this also occurs at first as a solid substance. The solidity
and visibility of the phosphoric anhydride and acid are beneficial in
preventing highly impure acetylene being unwittingly burnt in a room;
but, on the other hand, being merely solids in suspension in the air, the
combustion products of phosphorus are not so easily carried away from the
room by the means provided for ventilation as are the products of the
combustion of sulphur. Phosphoric anhydride is also partly deposited in
the solid state at the burner orifices, perhaps actually corroding the
steatite jets, and always assisting in the deposition of carbon from any
polymerised hydrocarbons in the acetylene; thus helping the carbon to
block up or distort those orifices. Whenever the acetylene is to be burnt
on the incandescent system under a mantle of the Welsbach or other type,
phosphorus, and possibly sulphur, become additionally objectionable, and
rigorous extraction is necessary. As is well known, the mantle is
composed of the oxides of certain "rare earths" which owe their practical
value to the fact that they are non-volatile at the temperature of the
gas-flame. When a gas containing phosphorus is burnt beneath such a
mantle, the phosphoric anhydride attacks those oxides, partially
converting them into the respective phosphates, and these bodies are less
refractory. A mantle exposed to the combustion products of crude
acetylene soon becomes brittle and begins to fall to pieces, occasionally
showing a yellowish colour when cold. The actual advantage of burning
acetylene on the incandescent system is not yet thoroughly established--
in this country at all events; but it is clear that the process will not
exhibit any economy (rather the reverse) unless the plant is provided
with most capable chemical purifiers. Phosphorus, sulphur, and ammonia
are not objectionable in crude acetylene because they confer upon the gas
a nauseous odour. From a well-constructed installation no acetylene
escapes unconsumed: the gas remains wholly within the pipes until it is
burnt, and whatever odour it may have fails to reach the human nostrils.
A house properly piped for acetylene will be no more conspicuous by its
odour than a house properly piped for coal-gas. On the contrary, the fact
that the carbide impurities of acetylene, which, in the absolutely pure
state, is a gas of somewhat faint, hardly disagreeable, odour, do confer
upon that gas a persistent and unpleasant smell, is distinctly
advantageous; for, owing to that odour, a leak in the pipes, an unclosed
tap, or a fault in the generating plant is instantly brought to the
consumer's attention. A gas wholly devoid of odour would be extremely
dangerous in a house, and would have to be scented, as is done in the
case of non-carburetted water-gas when it is required for domestic
purposes.
AMOUNTS OF IMPURITIES AND SCOPE OF PURIFICATION.--Partly for the reason
which has just been given, and partly on the ground of expense, a
complete removal of the impurities from crude acetylene is not desirable.
All that need be done is to extract sufficient to deprive the gas of its
injurious effects upon lungs, decorations, and burners. As it stands,
however, such a statement is not sufficiently precise to be useful either
to consumers of acetylene or to manufacturers of plant, and some more or
less arbitrary standard must be set up in order to define the composition
of "commercially pure" acetylene, as well as to gauge the efficiency of
any process of purification. In all probability such limit may be
reasonably taken at 0.1 milligramme of either sulphur or phosphorus
(calculated as elementary bodies) per 1 litre of acetylene, _i.e._,
0.0-1.1 grain per cubic foot; a quantity which happens to correspond
almost exactly with a percentage by weight of 0.01. Owing to the atomic
weights of these substances, and the very small quantities being
considered, the same limit hardly differs from that of 0.01 per cent. by
weight of sulphuretted hydrogen or of phosphine--it being always
recollected that the sulphur and phosphorus do not necessarily exist in
the gas as simple hydrides. Keppeler, however, has suggested the higher
figure of 0.15 milligramme of either sulphur or phosphorus per litre of
acetylene (=0.066 grain per cubic foot) for the maximum amount of these
impurities permissible in purified acetylene. He adopts this standard on
the basis of the results of observations of the amounts of sulphur and
phosphorus present in the gas issuing from a purifier charged with
heratol at the moment when the last layer of the heratol is beginning to
change colour. No limit has been given for the removal of the ammonia,
partly because that impurity can more easily, and without concomitant
disadvantage, be extracted entirely; and partly because it is usually
removed in the washer and not in the true chemical purifier.
According to Lewes, the maximum amount of ammonia found in the acetylene
coming from a dripping generator is 0.95 gramme per litre, while in
carbide-to-water gas it is 0.16 gramme: 417 and 70.2 grains per cubic
foot respectively. Rossel and Landriset have found 4 milligrammes (1.756
grains [Footnote: Milligrammes per litre; grains per cubic foot. It is
convenient to remember that since 1 cubic foot of water weighs 62.321 x
16 - 997.14 avoirdupois ounces, grammes per litre are approximately equal
to oz. per cubic foot; and grammes per cubic metre to oz. per 1000 cubic
feet.]) to be the maximum in water-to-carbide gas, and none to occur in
carbide-to-water acetylene. Rossel and Landriset return the minimum
proportion of sulphur, calculated as H_2S, found in the gaseous state in
acetylene when the carbide has not been completely flooded with water at
1.18 milligrammes per litre, or 0.52 grain per cubic foot; and the
corresponding maxima at 1.9 milligrammes, or 0.84 grain. In carbide-to-
water gas, the similar maxima are 0.23 milligramme or 0.1 grain. As
already stated, the highest proportion of phosphine yet found in
acetylene is 2.3 per cent. (Lewes), which is equal to 32.2 milligrammes
of PH_3 per litre or 14.13 grains per cubic foot (Polis); but this sample
dated from 1897. Eitner and Keppeler record the minimum proportion of
phosphorus, calculated as PH_3, found in crude acetylene, as 0.45
milligramme per litre, and the maximum as 0.89 milligramme per litre; in
English terms these figures are 0.2 and 0.4 grain per cubic foot. On an
average, however, British and Continental carbide of the present day may
be said to give a gas containing 0.61 milligramme of phosphorus
calculated as PH_3 per litre and 0.75 milligramme of sulphur calculated
as H_2S. In other units these figures are equal to 0.27 grain of PH_3 and
0.33 grain of H_2S per 1 cubic foot, or to 0.041 per cent. by volume of
PH_3 and 0.052 per cent. of H_2S. Yields of phosphorus and sulphur much
higher than these will be found in the journals and books, but such
analytical data were usually obtained in the years 1896-99, before the
manufacture of calcium carbide had reached its present degree of
systematic control. A commercial specimen of carbide was seen by one of
the authors as late as 1900 which gave an acetylene containing 1.12
milligramme of elementary sulphur per litre, i.e., 0.096 per cent, by
volume, or 0.102 per cent, by volume of H_2S; but the phosphorus showed
the low figure of 0.36 milligramme per litre (0.031 per cent, of P or
0.034 per cent, of PH_3 by volume).
The British Acetylene Association's regulations relating to carbide of
calcium (_vide_ Chap. XIV.) contain a clause to the effect that
"carbide which, when properly decomposed, yields acetylene containing
from all phosphorus compounds therein more than 0.05 per cent, by volume
of phosphoretted hydrogen, may be refused by the buyer." This limit is
equivalent to 0.74 milligramme of phosphorus calculated as PH_3 per
litre. A latitude of 0.01 per cent, is, however, allowed for the
analysis, so that the ultimate limit on which carbide could be rejected
is: 0.06 volume per cent. of PH_3, or 0.89 milligramme of phosphorus per
litre.
The existence in appreciable quantity of combined silicon as a normal
impurity in acetylene seems still open to doubt. Calcium carbide
frequently contains notable quantities of iron and other silicides; but
although these bodies are decomposed by acids, yielding hydrogen
silicide, or siliciuretted hydrogen, they are not attacked by plain
water. Nevertheless Wolff and Gerard have found hydrogen silicide in
crude acetylene, and Lewes looks upon it as a common impurity in small
amounts. When it occurs, it is probably derived, as Vigouroux has
suggested, from "alloys" of silicon with calcium, magnesium, and
aluminium in the carbide. The metallic constituents of these substances
would naturally be attacked by water, evolving hydrogen; and the
hydrogen, in its nascent state, would probably unite with the liberated
silicon to form hydrogen silicide. Many authorities, including Keppeler,
have virtually denied that silicon compounds exist in crude acetylene,
while the proportion 0.01 per cent. has been given by other writers as
the maximum. Caro, however, has stated that the crude gas almost
invariably contains silicon, sometimes in very small quantities, but
often up to the limit of 0.8 per cent.; the failure of previous
investigators to discover it being due to faulty analytical methods. Caro
has seen one specimen of (bad) carbide which gave a spontaneously
inflammable gas although it contained only traces of phosphine; its
inflammability being caused by 2.1 per cent. of hydrogen silicide.
Practically speaking, all the foregoing remarks made about phosphine
apply equally to hydrogen silicide: it burns to solid silicon oxide
(silica) at the burners, is insoluble in water, and is spontaneously
inflammable when alone or only slightly diluted, but never occurs in good
carbide in sufficient proportion to render the acetylene itself
inflammable. According to Caro the silicon may be present both as
hydrogen silicide and as silicon "compounds." A high temperature in the
generator will favour the production of the latter; an apparatus in which
the gas is washed well in lime-water will remove the bulk of the former.
Fraenkel has found that magnesium silicide is not decomposed by water or
an alkaline solution, but that dilute hydrochloric acid acts upon it and
spontaneously inflammable hydrogen silicide results. If it may be assumed
that the other silicides in commercial calcium carbide also behave in
this manner it is plain that hydrogen silicide cannot occur in crude
acetylene unless the gas is supposed to be hurried out of the generator
before the alkaline water therein has had time to decompose any traces of
the hydrogen silicide which is produced in the favouring conditions of
high temperature sometimes prevailing. Mauricheau-Beaupre has failed to
find silica in the products of combustion of acetylene from carbide of
varying degrees of purity. He found, however, that a mixture of strong
nitric and hydrochloric acids (_aqua regia_), if contaminated with
traces of phosphoric acid, dissolved silica from the glass of laboratory
vessels. Consequently, since phosphoric acid results from the phosphine
in crude acetylene when the gas is passed through aqua regia, silica may
be found on subsequently evaporating the latter. But this, silica, he
found, was derived from the glass and not through the oxidation of
silicon compounds in the acetylene. It is possible that some of the
earlier observers of the occurrence of silicon compounds in crude
acetylene may have been misled by the solution of silica from the glass
vessels used in their investigations. The improbability of recognisable
quantities of silicon compounds occurring in acetylene in any ordinary
conditions of generation is demonstrated by a recent study by Fraenkel of
the composition of the deposit produced on reflectors exposed to the
products of combustion of a sample of acetylene which afforded a haze
when burnt. The deposit contained 51.07 per cent. of phosphoric acid, but
no silica. The gas itself contained from 0.0672 to 0.0837 per cent. by
volume of phosphine.
PURIFYING MATERIALS.--When acetylene first began to be used as a domestic
illuminant, most generator builders denied that there was any need for
the removal of these carbide impurities from the gas, some going so far
as to assert that their apparatus yielded so much purer an acetylene than
other plant, where purification might be desirable, that an addition of a
special purifier was wholly unnecessary. Later on the more responsible
members of the trade took another view, but they attacked the problem of
purification in a perfectly empirical way, either employing some purely
mechanical scrubber filled with some moist or dry porous medium, or
perhaps with coke or the like wetted with dilute acid, or they simply
borrowed the processes adopted in the purification of coal-gas. At first
sight it might appear that the more simple methods of treating coal-gas
should be suitable for acetylene; since the former contains two of the
impurities--sulphuretted hydrogen and ammonia--characteristic of crude
acetylene. After removing the ammonia by washing with water, therefore,
it was proposed to extract the sulphur by passing the acetylene through
that variety of ferric hydroxide (hydrated oxide of iron) which is so
serviceable in the case of coal-gas. The idea, however, was quite
unsound: first, because it altogether ignores the phosphorus, which is
the most objectionable impurity in acetylene, but is not present in coal-
gas; secondly, because ferric hydroxide is used on gasworks to extract in
a marketable form the sulphur which occurs as sulphuretted hydrogen, and
true sulphuretted hydrogen need not exist in well-generated and well-
washed acetylene to any appreciable extent; thirdly, because ferric
hydroxide is not employed by gasmakers to remove sulphur compounds (this
is done with lime), being quite incapable of extracting them, or the
analogous sulphur compounds of crude acetylene.
About the same time three other processes based on somewhat better
chemical knowledge were put forward. Pictet proposed leading the gas
through a strong solution of calcium chloride and then through strong
sulphuric acid, both maintained at a temperature of -20 deg. to -40 deg. C.,
finally washing the gas in a solution of some lead salt. Proof that such
treatment would remove phosphorus to a sufficient degree is not
altogether satisfactory; but apart from this the necessity of maintaining
such low temperatures, far below that of the coldest winter's night,
renders the idea wholly inadmissible for all domestic installations.
Willgerodt suggested removing sulphuretted hydrogen by means of potassium
hydroxide (caustic potash), then absorbing the phosphine in bromine
water. For many reasons this process is only practicable in the
laboratory. Berge and Reychler proposed extracting both sulphuretted
hydrogen and phosphine in an acid solution of mercuric chloride
(corrosive sublimate). The poisonousness of this latter salt, apart from
all other objections, rules such a method out.
BLEACHING POWDER.--The next idea, first patented by Smith of Aberdeen,
but fully elaborated by Lunge and Cedercreutz, was to employ bleaching-
powder [Footnote: Bleaching-powder is very usually called chloride of
lime; but owing to the confusion which is constantly arising in the minds
of persons imperfectly acquainted with chemistry between chloride of lime
and chloride of calcium--two perfectly distinct bodies--the less
ambiguous expression "bleaching-powder" will be adopted here.] either in
the solid state or as a liquid extract. The essential constituent of
bleaching-powder from the present aspect is calcium hypochlorite, which
readily oxidises sulphuretted hydrogen, and more particularly phosphine,
converting them into sulphuric and phosphoric acids, while the acetylene
is practically unattacked. In simple purifying action the material proved
satisfactory; but since high-grade commercial bleaching-powder contains
some free chlorine, or some is set free from it in the purifier under the
influence of the passing gas, the issuing acetylene was found to contain
chlorine, free or combined; and this, burning eventually to hydrochloric
acid, is hardly less harmful than the original sulphur compounds.
Moreover, a mixture of acetylene, chlorine, and air is liable to catch
fire of itself when exposed to bright sunlight; and therefore the use of
a bleaching-powder purifier, or rather the recharging thereof, was not
unattended by danger in the early days. To overcome these defects, the
very natural process was adopted of diluting the bleaching-powder, such
diluent also serving to increase the porosity of the material. A very
unsuitable substance, however, was selected for the purpose, viz.,
sawdust, which is hygroscopic organic, and combustible. Owing to the
exothermic chemical action between the impurities of the acetylene and
the bleaching-powder, the purifying mass became heated; and thus not only
were the phenomena found in a bad generator repeated in the purifying
vessel, but in presence of air and light (as in emptying the purifier),
the reaction proceeded so rapidly that the heat caused inflammation of
the sawdust and the gas, at least on one occasion an actual fire taking
place which created much alarm and did some little damage. For a time,
naturally, bleaching-powder was regarded as too dangerous a material to
be used for the purification of crude acetylene; but it was soon
discovered that danger could be avoided by employing the substance in a
proper way.
HERATOL, FRANKOLINE, ACAGINE AND PURATYLENE.--Setting aside as unworthy
of attention certain compositions offered as acetylene purifying
materials whose constitution has not been divulged or whose action has
not been certified by respectable authority, there are now three
principal chemical reagents in regular use. Those are chromic acid,
cuprous chloride (sub- or proto-chloride of copper), and bleaching-
powder. Chromic acid is employed in the form of a solution acidified with
acetic or hydrochloric acid, which, in order to obtain the advantages
(_see_ below) attendant upon the use of a solid purifying material,
is absorbed in that highly porous and inert description of silica known
as infusorial earth or "kieselguhr." This substance was first recommended
by Ullmann, and is termed commercially "heratol" As sold it contains
somewhere about 136 grammes of chromic acid per kilo. Cuprous chloride is
used as a solution in strong hydrochloric acid mixed with ferric
chloride, and similarly absorbed in kieselguhr. From the name of its
proposer, this composition is called "frankoline." It will be shown in
Chapter VI. that the use of metallic copper in the construction of
acetylene apparatus is not permissible or judicious, because the gas is
liable to form therewith an explosive compound known as copper acetylide;
it might seem, therefore, that the employment of a copper salt for
purification courts accident. The objection is not sound, because the
acetylide is not likely to be produced except in the presence of ammonia;
and since frankoline is a highly acid product, the ammonia is converted
into its chloride before any copper acetylide can be produced. As a
special acetylene purifier, bleaching-powder exists in at least two chief
modifications. In one, known as "acagine," it is mixed with 15 per cent.
of lead chromate, and sometimes with about the same quantity of barium
sulphate; the function of the latter being simply that of a diluent,
while to the lead chromate is ascribed by its inventor (Wolff) the power
of retaining any chlorine that may be set free from the bleaching-powder
by the reduction of the chromic acid. The utility of the lead chromate in
this direction has always appeared doubtful; and recently Keppeler has
argued that it can have no effect upon the chlorine, inasmuch as in the
spent purifying material the lead chromate may be found in its original
condition unchanged. The second modification of bleaching-powder is
designated "puratylene," and contains calcium chloride and quick or
slaked lime. It is prepared by evaporating to dryness under diminished
pressure solutions of its three ingredients, whereby the finished
material is given a particularly porous nature.
It will be observed that both heratol and frankoline are powerfully acid,
whence it follows they are capable of extracting any ammonia that may
enter the purifier; but for the same reason they are liable to act
corrosively upon any metallic vessel in which they are placed, and they
therefore require to be held in earthenware or enamelled receivers. But
since they are not liquid, the casing of the purifier can be safely
constructed of steel or cast iron. Puratylene also removes ammonia by
virtue of the calcium chloride in it. Acagine would probably pass the
ammonia; but this is no real objection, as the latter can be extracted by
a preliminary washing in water. Heratol changes, somewhat obscurely, in
colour as it becomes spent, its original orange tint, due to the chromic
acid, altering to a dirty green, characteristic of the reduced salts of
chromium oxide. Frankoline has been asserted to be capable of
regeneration or revivification, _i.e._, that when spent it may be
rendered fit for further service by being exposed to the air for a time,
as is done with gas oxide; this, however, may be true to some extent with
the essential constituents of frankoline, but the process is not
available with the commercial solid product. Of all these materials,
heratol is the most complete purifier of acetylene, removing phosphorus
and sulphur most rapidly and thoroughly, and not appreciably diminishing
in speed or efficiency until its chromic acid is practically quite used
up. On the other hand, heratol does act upon pure acetylene to some
extent; so that purifiers containing it should be small in size and
frequently recharged. In one of his experiments Keppeler found that 13
per cent. of the chromic acid in heratol was wasted by reacting with
acetylene. As this waste of chromic acid involves also a corresponding
loss of gas, small purifiers are preferable, because at any moment they
only contain a small quantity of material capable of attacking the
acetylene itself. Frankoline is very efficacious as regards the
phosphorus, but it does not wholly extract the sulphur, leaving,
according to Keppeler, from 0.13 to 0.20 gramme of the latter in every
cubic metre of the gas. It does not attack acetylene itself; and if,
owing to its free hydrochloric acid, it adds any acid vapours to the
purified gas, these vapours may be easily removed by a subsequent passage
through a vessel containing lime or a carbide drier. Both being
essentially bleaching-powder, acagine and puratylene are alike in
removing phosphorus to a satisfactory degree; but they leave some sulphur
behind. Acagine evidently attacks acetylene to a slight extent, as
Keppeler has found 0.2 gramme of chlorine per cubic metre in the issuing
gas.
Although some of these materials attack acetylene slightly, and some
leave sulphur in the purified gas, they may be all considered reasonably
efficient from the practical point of view; for the loss of true
acetylene is too small to be noticeable, and the quantity of sulphur not
extracted too trifling to be harmful or inconvenient. They may be valued,
accordingly, mainly by their price, proper allowance being made for the
quantity of gas purified per unit weight of substance taken. This
quantity of gas must naturally vary with the proportion of phosphorus and
sulphur in the crude acetylene; but on an average the composition of
unpurified gas is what has already been given above, and so the figures
obtained by Keppeler in his investigation of the subject may be accepted.
In the annexed table these are given in two forms: (1) the number of
litres of gas purified by 1 kilogramme of the substance, (2) the number
of cubic feet purified per lb. It should be noted that the volumes of gas
refer to a laboratory degree of purification; in practice they may all be
increased by 10 or possibly 20 per cent.
_________________________________________________
| | | |
| | Litres | Cubic Feet |
| | per Kilogramme. | per Lb. |
|______________|___________________|______________|
| | | |
| Heratol | 5,000 | 80 |
| Frankoline | 9,000 | 144 |
| Puratylene | 10,000 | 160 |
| Acagine | 13,000 | 208 |
|______________|___________________|______________|
Another method of using dry bleaching-powder has been proposed by
Pfeiffer. He suggests incorporating it with a solution of some lead salt,
so that the latter may increase the capacity of the calcium hypochlorite
to remove sulphur. Analytical details as to the efficiency of this
process have not been given. During 1901 and 1902 Bullier and Maquenne
patented a substance made by mixing bleaching-powder with sodium
sulphate, whereby a double decomposition occurs, sodium hypochlorite,
which is equally efficient with calcium hypochlorite as a purifying
material, being produced together with calcium sulphate, which, being
identical with plaster of Paris, sets into a solid mass with the excess
of water present, and is claimed to render the whole more porous. This
process seemed open to objection, because Blagden had shown that a
solution of sodium hypochlorite was not a suitable purifying reagent in
practice, since it was much more liable to add chlorine to the gas than
calcium hypochlorite. The question how a solidified modification of
sodium hypochlorite would behave in this respect has been investigated by
Keppeler, who found that the Bullier and Maquenne material imparted more
chlorine to the gas which had traversed it than other hypochlorite
purifying agents, and that the partly foul material was liable to cause
violent explosions. About the same time Rossel and Landriset pointed out
that purification might be easily effected in all generators of the
carbide-to-water pattern by adding to the water of the generator itself a
quantity of bleaching-powder equivalent to 5 to 20 grammes for every 1
kilogramme of carbide decomposed, claiming that owing to the large amount
of liquid present, which is usually some 4 litres per kilogramme of
carbide (0.4 gallon per lb.), no nitrogen chloride could be produced, and
that owing to the dissolved lime in the generator, chlorine could not be
added to the gas. The process is characterised by extreme simplicity, no
separate purifier being needed, but it has been found that an
introduction of bleaching-powder in the solid condition is liable to
cause an explosive combination of acetylene and chlorine, while the use
of a solution is attended by certain disadvantages. Granjon has proposed
impregnating a suitable variety of wood charcoal with chlorine, with or
without an addition of bleaching-powder; then grinding the product to
powder, and converting it into a solid porous mass by the aid of cement.
The material is claimed to last longer than ordinary hypochlorite
mixtures, and not to add chlorine to the acetylene.
SUBSIDIARY PURIFYING MATERIALS.--Among minor reagents suggested as
purifying substances for acetylene may be mentioned potassium
permanganate, barium peroxide, potassium bichromate, sodium plumbate and
arsenious oxide. According to Benz the first two do not remove the
sulphuretted hydrogen completely, and oxidise the acetylene to some
extent; while potassium bichromate leaves some sulphur and phosphorus
behind in the gas. Sodium plumbate has been suggested by Morel, but it is
a question whether its action on the impurities would not be too violent
and whether it would be free from action on the acetylene itself. The use
of arsenious oxide dissolved in a strong acid, and the solution absorbed
in pumice or kieselguhr has been protected by G. F. Jaubert. The
phosphine is said to combine with the arsenic to form an insoluble
brownish compound. In 1902 Javal patented a mixture of 1 part of
potassium permanganate, 5 of "sulphuric acid," and 1 of water absorbed in
4 parts of infusorial earth. The acid constantly neutralised by the
ammonia of the crude gas is as constantly replaced by fresh acid formed
by the oxidation of the sulphuretted hydrogen; and this free acid, acting
upon the permanganate, liberates manganese peroxide, which is claimed to
destroy the phosphorus and sulphur compounds present in the crude
acetylene.
EPURENE.--A purifying material to which the name of epurene has been
given has been described, by Mauricheau-Beaupre, as consisting of a
mixture of ferric chloride and ferric oxide in the proportion of 2
molecules, or 650 parts, of the former with one molecule, or 160 parts,
of the latter, together with a suitable quantity of infusorial earth. In
the course of preparation, however, 0.1 to 0.2 per cent. of mercuric
chloride is introduced into the material. This mercuric chloride is said
to form an additive compound with the phosphine of the crude acetylene,
which compound is decomposed by the ferric chloride, and the mercuric
chloride recovered. The latter therefore is supposed to act only as a
carrier of the phosphine to the ferric chloride and oxide, by which it is
oxidised according to the equation:
8Fe_2Cl_6 + 4Fe_2O_3 + 3PH_3 = 12Fe_2Cl_4 + 3H_3PO_4.
Thus the ultimate products are phosphoric acid and ferrous chloride,
which on exposure to air is oxidised to ferric chloride and oxide. It is
said that this revivification of the fouled or spent epurene takes place
in from 20 to 48 hours when it is spread in the open in thin layers, or
it may be partially or wholly revivified _in situ_ by adding a small
proportion of air to the crude acetylene as it enters the purifier. The
addition of 1 to 2 per cent. of air, according to Mauricheau-Beaupre,
suffices to double the purifying capacity of one charge of the material,
while a larger proportion would achieve its continuous revivification.
Epurene is said to purify 10,000 to 11,000 litres of crude acetylene per
kilogramme, or, say, 160 to 176 cubic feet per pound, when the acetylene
contains on the average 0.05 per cent, by volume of phosphine.
For employment in all acetylene installations smaller than those which
serve complete villages, a solid purifying material is preferable to a
liquid one. This is partly due to the extreme difficulty of subdividing a
stream of gas so that it shall pass through a single mass of liquid in
small enough bubbles for the impurities to be removed by the time the gas
arrives at the surface. This time cannot be prolonged without increasing
the depth of liquid in the vessel, and the greater the depth of liquid,
the more pressure is consumed in forcing the gas through it. Perfect
purification by means of fluid reagents unattended by too great a
consumption of pressure is only to be effected by a mechanical scrubber
such as is used on coal-gas works, wherein, by the agency of external
power, the gas comes in contact with large numbers of solid surfaces kept
constantly wetted; or by the adoption of a tall tower filled with porous
matter or hollow balls over which a continuous or intermittent stream of
the liquid purifying reagent is made to trickle, and neither of these
devices is exactly suited to the requirements of a domestic acetylene
installation. When a solid material having a proper degree of porosity or
aggregation is selected, the stream of gas passing through it is broken
up most thoroughly, and by employing several separate layers of such
material, every portion of the gas is exposed equally to the action of
the chemical reagent by the time the gas emerges from the vessel. The
amount of pressure so consumed is less than that in a liquid purifier
where much fluid is present; but, on the other hand, the loss of pressure
is absolutely constant at all times in a liquid purifier, provided the
head of liquid is maintained at the same point. A badly chosen solid
purifying agent may exhibit excessive pressure absorption as it becomes
partly spent. A solid purifier, moreover, has the advantage that it may
simultaneously act as a drier for the gas; a liquid purifier, in which
the fluid is mainly water, obviously cannot behave in a similar fashion
For thorough purification it is necessary that the gas shall actually
stream through the solid material; a mere passage over its surface is
neither efficient nor economical of material.
DISPOSITION OF PURIFYING MATERIAL.--Although much has been written, and
some exaggerated claims made, about the maximum, volume of acetylene a
certain variety of purifying material will treat, little has been said
about the method in which such a material should be employed to obtain
the best results. If 1 lb. of a certain substance will purify 200 cubic
feet of normal crude acetylene, that weight is sufficient to treat the
gas evolved from 40 lb. of carbide; but it will only do so provided it is
so disposed in the purifier that the gas does not pass through it at too
high a speed, and that it is capable of complete exhaustion. In the coal-
gas industry it is usually assumed that four layers of purifying
material, each having a superficial area of 1 square foot, are the
minimum necessary for the treatment of 100 cubic feet of gas per hour,
irrespective of the nature of the purifying material and of the impurity
it is intended to extract. If there is any sound basis for this
generalization, it should apply equally to the purification of acetylene,
because there is no particular reason to imagine that the removal of
phosphine by a proper substance should occur at an appreciably different
speed from the removal of carbon dioxide, sulphuretted hydrogen, and
carbon bisulphide by lime, ferric oxide, and sulphided lime respectively,
Using the coal gas figures, then, for every 10 cubic feet of acetylene
generated per hour, a superficial area of (4 x 144 / 10) 57.6 square
inches of purifying material is required. In the course of Keppeler's
research upon different purifying materials it is shown that 400 grammes
of heratol, 360 grammes of frankoline, 250 grammes of acagine, and 230
grammes of puratylene each occupy a space of 500 cubic centimetres when
loosely loaded into a purifying vessel, and from these data, the
following table has been calculated:
__________________________________________________________
| | | | |
| | Weight | Weight | Cubic Inches |
| | per Gallon | per Cubic Foot | Occupied |
| | in Lbs. | in Lbs. | per Lb. |
|_____________|____________|________________|______________|
| | | | |
| Water | 10.0 | 62.321 | 27.73 |
| Heratol | 8.0 | 49.86 | 31.63 |
| Frankoline | 7.2 | 41.87 | 38.21 |
| Acagine | 6.0 | 31.16 | 55.16 |
| Puratylene | 4.6 | 28.67 | 60.28 |
|_____________|____________|________________|______________|
As regards the minimum weight of material required, data have been given
by Pfleger for use with puratylene. He states that 1 Kilogramme of that
substance should be present for every 100 litres of crude acetylene
evolved per hour, 4 kilogrammes being the smallest quantity put into the
purifier. In English units these figures are 1 lb. per 1.5 cubic feet per
hour, with 9 lb. as a minimum, which is competent to treat 1.1 cubic feet
of gas per hour. Thus it appears that for the purification of the gas
coming from any generator evolving up to 14 cubic feet of acetylene per
hour a weight of 9 lb of puratylene must be charged into the purifier,
which will occupy (60.28 / 9) 542 cubic inches of space; and it must be
so spread out as to present a total superficial area of (4 x 144 x 14 /
100) 80.6 square inches to the passing gas. It follows, therefore, that
the material should be piled to a depth of (542 / 80.6) 6.7 inches on a
support having an area of 80.6 square inches; but inasmuch as such a
depth is somewhat large for a small vessel, and as several layers are
better than one, it would be preferable to spread out these 540 cubic
inches of substance on several supports in such a fashion that a total
surface of 80.6 square inches or upwards should be exhibited. These
figures may obviously be manipulated in a variety of ways for the design
of a purifying vessel; but, to give an example, if the ordinary
cylindrical shape be adopted with four circular grids, each having a
clear diameter of 8 inches (_i.e._, an area of 50.3 square inches),
and if the material is loaded to a depth of 3 inches on each, there would
be a total volume of (50.3 x 3 x 4) = 604 cubic inches of puratylene in
the vessel, and it would present a total area of (50.3 x 4) = 201 square
inches to the acetylene. At Keppeler's estimation such an amount of
puratylene should weigh roughly 10 lb., and should suffice for the
purification of the gas obtained from 320 lb. of ordinary carbide; while,
applying the coal-gas rule, the total area of 201 square inches should
render such a vessel equal to the purification of acetylene passing
through it at a speed not exceeding (201 / 5.76) = 35 cubic feet per
hour. Remembering that it is minimum area in square inches of purifying
material that must govern the speed at which acetylene may be passed
through a purifier, irrespective probably of the composition of the
material; while it is the weight of material which governs the ultimate
capacity of the vessel in terms of cubic feet of acetylene or pounds of
carbide capable of purification, these data, coupled with Keppeler's
efficiency table, afford means for calculating the dimensions of the
purifying vessel to be affixed to an installation of any desired number
of burners. There is but little to say about the design of the vessel
from the mechanical aspect. A circular horizontal section is more likely
to make for thorough exhaustion of the material. The grids should be
capable of being lifted out for cleaning. The lid may be made tight
either by a clamp and rubber or leather washer, or by a liquid seal. If
the purifying material is not hygroscopic, water, calcium chloride
solution, or dilute glycerin may be used for sealing purposes; but if the
material, or any part of it, does absorb water, the liquid in the seal
should be some non-aqueous fluid like lubricating oil. Clamped lids are
more suitable for small purifiers, sealed lids for large vessels. Care
must be taken that condensation products cannot collect in the purifying
vessel. If a separate drying material is employed in the same purifier
the space it takes must be considered separately from that needed by the
active chemical reagent. When emptying a foul purifier it should be
recollected that the material may be corrosive, and being saturated with
acetylene is likely to catch fire in presence of a light.
Purifiers charged with heratol are stated, however, to admit of a more
rapid flow of the gas through them than that stated above for puratylene.
The ordinary allowance is 1 lb. of heratol for every cubic foot per hour
of acetylene passing, with a minimum charge of 7 lb. of the material. As
the quantity of material in the purifier is increased, however, the flow
of gas per hour may be proportionately increased, _e.g._, a purifier
charged with 132 lb. of heratol should purify 144 cubic feet of acetylene
per hour.
In the systematic purification of acetylene, the practical question
arises as to how the attendant is to tell when his purifiers approach
exhaustion and need recharging; for if it is undesirable to pass crude
gas into the service, it is equally undesirable to waste so comparatively
expensive a material as a purifying reagent. In Chapter XIV. it will be
shown that there are chemical methods of testing for the presence, or
determining the proportion, of phosphorus and sulphur in acetylene; but
these are not suitable for employment by the ordinary gas-maker. Heil has
stated that the purity of the gas may be judged by an inspection of its
atmospheric flame as given by a Bunsen burner. Pure acetylene gives a
perfectly transparent moderately dark blue flame, which has an inner cone
of a pale yellowish green colour; while the impure gas yields a longer
flame of an opaque orange-red tint with a bluish red inner zone. It
should be noted, however, that particles of lime dust in the gas may
cause the atmospheric flame to be reddish or yellowish (by presence of
calcium or sodium) quite apart from ordinary impurities; and for various
other reasons this appearance of the non-luminous flame is scarcely to be
relied upon. The simplest means of ascertaining definitely whether a
purifier is sufficiently active consists in the use of the test-papers
prepared by E. Merck of Darmstadt according to G. Keppeler's
prescription. These papers, cut to a convenient size, are put up in small
books from which they may be torn one at a time. In order to test whether
gas is sufficiently purified, one of the papers is moistened with
hydrochloric acid of 10 per cent. strength, and the gas issuing from a
pet-cock or burner orifice is allowed to impinge on the moistened part.
The original black or dark grey colour of the paper is changed to white
if the gas contains a notable amount of impurity, but remains unchanged
if the gas is adequately purified. The paper consists of a specially
prepared black porous paper which has been dipped in a solution of
mercuric chloride (corrosive sublimate) and dried. Moistening the paper
with hydrochloric acid provides in a convenient form for application
Berge's solution for the detection of phosphine (_vide_ Chapter
XIV.). The Keppeler test-papers turn white when the gas contains either
ammonia, phosphine, siliciuretted hydrogen, sulphuretted hydrogen or
organic sulphur compounds, but with carbon disulphide the change is slow.
Thus the paper serves as a test for all the impurities likely to occur in
acetylene. The sensitiveness of the test is such that gas containing
about 0.15 milligramme of sulphur, and the same amount of phosphorus, per
litre (= 0.0655 grain per cubic foot) imparts in five minutes a distinct
white mark to the moistened part of the paper, while gas containing 0.05
milligramme of sulphur per litre (= 0.022 grain per cubic foot) gives in
two minutes a dull white mark visible only by careful inspection. If,
therefore, a distinct white mark appears on moistened Keppeler paper when
it is exposed for five minutes to a jet of acetylene, the latter is
inadequately purified. If the gas has passed through a purifier, this
test indicates that the material is not efficient, and that the purifier
needs recharging. The moistening of the Keppeler paper with hydrochloric
acid before use is essential, because if not acidified the paper is
marked by acetylene itself. The books of Keppeler papers are put up in a
case which also contains a bottle of acid for moistening them as required
and are obtainable wholesale of E. Merek, 16 Jewry Street, London, E.C.,
and retail of the usual dealers in chemicals. If Keppeler's test-papers
are not available, the purifier should be recharged as a matter of
routine as soon as a given quantity of carbide--proportioned to the
purifying capacity of the charge of purifying material--has been used
since the last recharging. Thus the purifier may conveniently contain
enough material to purify the gas evolved from two drums of carbide, in
which case it would need recharging when every second drum of carbide is
opened.
REGULATIONS AS TO PURIFICATION.--The British Acetylene
Association has issued the following set of regulations as to purifying
material and purifiers for acetylene:
Efficient purifying material and purifiers shall comply with the
following requirements:
(1) The purifying material shall remove phosphorus and sulphur compounds
to a commercially satisfactory degree; _i.e._, not to a greater
degree than will allow easy detection of escaping gas through its odour.
(2) The purifying material shall not yield any products capable of
corroding the gas-mains or fittings.
(3) The purifying material shall, if possible, be efficient as a drying
agent, but the Association does not consider this an absolute necessity.
(4) The purifying material shall not, under working conditions, be
capable of forming explosive compounds or mixtures. It is understood,
naturally, that this condition does not apply to the unavoidable mixture
of acetylene and air formed when recharging the purifier.
(5) The apparatus containing the purifying material shall be simple in
construction, and capable of being recharged by an inexperienced person
without trouble. It shall be so designed as to bring the gas into proper
contact with the material.
(6) The containers in purifiers shall be made of such materials as are
not dangerously affected by the respective purifying materials used.
(7) No purifier shall be sold without a card of instructions suitable or
hanging up in some convenient place. Such instructions shall be of the
most detailed nature, and shall not presuppose any expert knowledge
whatever on the part of the operator.
Reference also to the abstracts of the official regulations as to
acetylene installations in foreign countries given in Chapter IV. will
show that they contain brief rules as to purifiers.
DRYING.--It has been stated in Chapter III. that the proper position for
the chemical purifiers of an acetylene plant is after the holder; and
they therefore form the last items in the installation unless a "station"
governor and meter are fitted. It is therefore possible to use them also
to remove the moisture in the gas, if a material hygroscopic in nature is
employed to charge them. This should be true more particularly with
puratylene, which contains a notable proportion of the very hygroscopic
body calcium chloride. If a separate drier is desirable, there are two
methods of charging it. It may be filled either with some hygroscopic
substance such as porous calcium chloride or quicklime in very coarse
powder, which retains the water by combining with it; or the gas may be
led through a vessel loaded with calcium carbide, which will manifestly
hold all the moisture, replacing it by an equivalent quantity of
(unpurified) acetylene. The objection is sometimes urged against this
latter method, that it restores to the gas the nauseous odour and the
otherwise harmful impurities it had more or less completely lost in the
purifiers; but as regards the first point, a nauseous odour is not, as
has previously been shown, objectionable in itself, and as regards the
second, the amount of impurities added by a carbide drier, being strictly
limited by the proportion of moisture in the damp gas, is too small to be
noticeable at the burners or elsewhere. As is the case with purification,
absolute removal of moisture is not called for; all that is needed is to
extract so much that the gas shall never reach its saturation-point in
the inaccessible parts of the service during the coldest winter's night.
Any accessible length of main specially exposed to cold may be
safeguarded by itself; being given a steady fall to a certain point
(preferably in a frost-free situation), and there provided with a
collecting-box from which the deposited liquid can be removed
periodically with a pump or otherwise.
FILTRATION.--The gas issuing from the purifier or drier is very liable to
hold in suspension fine dust derived from the purifying or drying
material used. It is essential that thin dust should be abstracted before
the gas reaches the burners, otherwise it will choke the orifices and
prevent them functioning properly. Consequently the gas should pass
through a sufficient layer of filtering material after it has traversed
the purifying material (and drier if one is used). This filtering
material may be put either as a final layer in the purifier (or drier),
or in a separate vessel known as a filter. Among filtering materials in
common use may be named cotton-wool, fine canvas or gauze, felt and
asbestos-wool. The gas must be fairly well dried before it enters the
filter, otherwise the latter will become choked with deposited moisture,
and obstruct the passage of the gas.
Having now described the various items which go to form a well-designed
acetylene installation, it may be useful to recapitulate briefly, with
the object of showing the order in which they should be placed. From the
generator the gas passes into a condenser to cool it and to remove any
tarry products and large quantities of water. Next it enters a washing
apparatus filled with water to extract water-soluble impurities. If the
generator is of the carbide-to-water pattern, the condenser may be
omitted, and the washer is only required to retain any lime froth and to
act as a water-seal or non-return valve. If the generator does not wash
the gas, the washer must be large enough to act efficiently as such, and
between it and the condenser should be put a mechanical filter to extract
any dust. From the washer the acetylene travels to the holder. From the
holder it passes through one or two purifiers, and from there travels to
the drier and filter. If the holder does not throw a constant pressure,
or if the purifier and drier are liable to cause irregularities, a
governor or pressure regulator must be added after the drier. The
acetylene is then ready to enter the service; but a station meter (the
last item in the plant) is useful as giving a means of detecting any leak
in the delivery-pipes and in checking the make of gas from the amount of
carbide consumed. If the gas is required for the supply of a district, a
station meter becomes quite necessary, because the public lamps will be
fed with gas at a contract rate, and without the meter there would be no
control over the volume of acetylene they consume. Where the gas finally
leaves the generating-house, or where it enters the residence, a full-way
stopcock should be put on the main.
GENERATOR RESIDUES.--According to the type of generator employed the
waste product removed therefrom may vary from a dry or moist powder to a
thin cream or milk of lime. Any waste product which is quite liquid in
its consistency must be completely decomposed and free from particles of
calcium carbide of sensible magnitude; in the case of more solid
residues, the less fluid they are the greater is the improbability (or
the less is the evidence) that the carbide has been wholly spent within
the apparatus. Imperfect decomposition of the carbide inside the
generator not only means an obvious loss of economy, but its presence
among the residues makes a careful handling of them essential to avoid
accident owing to a subsequent liberation of acetylene in some
unsuitable, and perhaps closed, situation. A residue which is not
conspicuously saturated with water must be taken out of the generator-
house into the open air and there flooded with water, being left in some
uncovered receptacle for a sufficient time to ensure all the acetylene
being given off. A residue which is liquid enough to flow should be run
directly from the draw-off cock of the generator through a closed pipe to
the outside; where, if it does not discharge into an open conduit, the
waste-pipe must be trapped, and a ventilating shaft provided so that no
gas can blow back into the generator-house.
DISPOSAL OF RESIDUES.--These residues have now to be disposed of. In some
circumstances they can be put to a useful purpose, as will be explained
in Chapter XII.; otherwise, and always perhaps on the small scale--
certainly always if the generator overheats the gas and yields tar among
the spent lime--they must be thrown into a convenient place. It should be
remembered that although methods of precipitating sewage by adding lime,
or lime water, to it have frequently been used, they have not proved
satisfactory, partly because the sludge so obtained is peculiarly
objectionable in odour, and partly because an excess of lime yields an
effluent containing dissolved lime, which among other disadvantages is
harmful to fish. The plan of running the liquid residues of acetylene
manufacture into any local sewerage system which may be found in the
neighbourhood of the consumer's premises, therefore, is very convenient
to the consumer; but is liable to produce complaints if the sewage is
afterwards treated chemically, or if its effluent is passed untreated
into a highly preserved river; and the same remark applies in a lesser
degree if the residues are run into a private cesspool the liquid
contents of which automatically flow away into a stream. If, however, the
cesspool empties itself of liquid matter by filtration or percolation
through earth, there can be no objection to using it to hold the lime
sludge, except in so far as it will require more frequent emptying. On
the whole, perhaps the best method of disposing of these residues is to
run them into some open pit, allowing the liquid to disappear by
evaporation and percolation, finally burying the solid in some spot where
it will be out of the way. When a large carbide-to-water generator is
worked systematically so as to avoid more loss of acetylene by solution
in the excess of liquid than is absolutely necessary, the liquid residues
coming from it will be collected in some ventilated closed tank where
they can settle quietly. The clear lime-water will then be pumped back
into the generator for further use, and the almost solid sludge will be
ready to be carried to the pit where it is to be buried. Special care
must be taken in disposing of the residues from a generator in which oil
is used to control evolution of gas. Such oil floats on the aqueous
liquid; and a very few drops spread for an incredible distance as an
exceedingly thin film, causing those brilliant rainbow-like colours which
are sometimes imagined to be a sign of decomposing organic matter. The
liquid portions of these residues must be led through a pit fitted with a
depending partition projecting below the level at which the water is
constantly maintained; all the oil then collects on the first side of the
partition, only water passing underneath, and the oil may be withdrawn
and thrown away at intervals.
CHAPTER VI
THE CHEMICAL AND PHYSICAL PROPERTIES OF ACETYLENE
It will only be necessary for the purpose of this book to indicate the
more important chemical and physical properties of acetylene, and, in
particular, those which have any bearing on the application of acetylene
for lighting purposes. Moreover, it has been found convenient to discuss
fully in other chapters certain properties of acetylene, and in regard to
such properties the reader is referred to the chapters mentioned.
PHYSICAL PROPERTIES.--Acetylene is a gas at ordinary temperatures,
colourless, and, when pure, having a not unpleasant, so-called "ethereal"
odour. Its density, or specific gravity, referred to air as unity, has
been found experimentally by Leduc to be 0.9056. It is customary to adopt
the value 0.91 for calculations into which the density of the gas enters
(_vide_ Chapter VII.). The density of a gas is important not only
for the determination of the size of mains needed to convey it at a given
rate of flow under a given pressure, as explained in Chapter VII., but
also because the volume of gas which will pass through small orifices in
a given time depends on its density. According to Graham's well-known law
of the effusion of gases, the velocity with which a gas effuses varies
directly as the square root of the difference of pressure on the two
sides of the opening, and inversely as the square root of the density of
the gas. Hence it follows that the volume of gas which escapes through a
porous pipe, an imperfect joint, or a burner orifice is, provided the
pressure in the gas-pipe is the same, a function of the square root of
the density of the gas. Hence this density has to be taken into
consideration in the construction of burners, i.e., a burner required to
pass a gas of high density must have a larger orifice than one for a gas
of low density, if the rate of flow of gas is to be the same under the
same pressure. This, however, is a question for the burner manufacturers,
who already make special burners for gases of different densities, and it
need not trouble the consumer of acetylene, who should always use burners
devised for the consumption of that gas. But the Law of effusion
indicates that the volume of acetylene which can escape from a leaky
supply-pipe will be less than the volume of a gas of lower density,
_e.g._, coal-gas, if the pressure in the pipe is the same for both.
This implies that on an extensive distributing system, in which for
practical reasons leakage is not wholly avoidable, the loss of gas
through leakage will be less for acetylene than for coal-gas, given the
same distributing pressure. If _v_ = the loss of acetylene from a
distributing system and _v'_ = the loss of coal-gas from a similar
system worked at the same pressure, both losses being expressed in
volumes (cubic feet) per hour, and the coal-gas being assumed to have a
density of 0.04, then
(1) (_v_/_v'_) = (0.40 / 0.91)^(1/2) = 0.663
or, _v_ = 0.663_v'_,
which signifies that the loss of acetylene by leakage under the same
conditions of pressure, &c., will be only 0.663 times that of the loss of
coal-gas. In practice, however, the pressures at which the gases are
usually sent through mains are not identical, being greater in the case
of acetylene than in that of coal-gas. Formula (1) therefore requires
correction whenever the pressures are different, and calling the pressure
at which the acetylene exists in the main _p_, and the corresponding
pressure of the coal-gas _p'_, the relative losses by leakage are--
(2) (_v_/_v'_) = (0.40 / 0.91)^(1/2) x (_p_/_p'_)^(1/2)
_v_ = 0.663_v'_ x (_p_/_p'_)^(1/2)
It will be evident that whenever the value of the fraction
(_p_/_p'_)^(1/2), is less than 1.5, _i.e._, whenever the pressure of
the acetylene does not exceed double that of the coal-gas present in
pipes of given porosity or unsoundness, the loss of acetylene will be
less than that of coal-gas. This is important, especially in the case of
large village acetylene installations, where after a time it would be
impossible to avoid some imperfect joints, fractured pipes, &c.,
throughout the extensive distributing mains. The same loss of gas by
leakage would represent a far higher pecuniary value with acetylene than
with coal-gas, because the former must always be more costly per unit of
volume than the latter. Hence it is important to recognise that the rate
of leakage, _coeteris paribus_, is less with acetylene, and it is
also important to observe the economical advantage, at least in terms of
gas or calcium carbide, of sending the acetylene into the mains at as low
a pressure as is compatible with the length of those mains and the
character of the consumers' burners. As follows from what will be said in
Chapter VII., a high initial pressure makes for economy in the prime cost
of, and in the expense of laying, the mains, by enabling the diameter of
those mains to be diminished; but the purchase and erection of the
distributing system are capital expenses, while a constant expenditure
upon carbide to meet loss by leakage falls upon revenue.
The critical temperature of acetylene, _i.e._, the temperature below
which an abrupt change from the gaseous to the liquid state takes place
if the pressure is sufficiently high, is 37 deg. C., and the critical
pressure, _i.e._, the pressure under which that change takes place
at that temperature, is nearly 68 atmospheres. Below the critical
temperature, a lower pressure than this effects liquefaction of the gas,
_i.e._, at 13.5 deg. C. a pressure of 32.77 atmospheres, at 0 deg. C.,
21.53 atmospheres (Ansdell, _cf._ Chapter XI.). These data are of
comparatively little practical importance, owing to the fact that, as
explained in Chapter XI., liquefied acetylene cannot be safely utilised.
The mean coefficient of expansion of gaseous acetylene between 0 deg. C.
and 100 deg. C., is, under constant pressure, 0.003738; under constant
volume, 0.003724. This means that, if the pressure is constant, 0.003738
represents the increase in volume of a given mass of gaseous acetylene
when its temperature is raised one degree (C.), divided by the volume of
the same mass at 0 deg. C. The coefficients of expansion of air are: under
constant pressure, 0.003671; under constant volume, 0.003665; and those
of the simple gases (nitrogen, hydrogen, oxygen) are very nearly the
same. Strictly speaking the table given in Chapter XIV., for facilitating
the correction of the volume of gas measured over water, is not quite
correct for acetylene, owing to the difference in the coefficients of
expansion of acetylene and the simple gases for which the table was drawn
up, but practically no appreciable error can ensue from its use. It is,
however, for the correction of volumes of gases measured at different
temperatures to one (normal) temperature, and, broadly, for determining
the change of volume which a given mass of the gas will undergo with
change of temperature, that the coefficient of expansion of a gas becomes
an important factor industrially.
Ansdell has found the density of liquid acetylene to range from 0.460 at
-7 deg. C. to 0.364 at +35.8 deg. C., being 0.451 at 0 deg. C. Taking the
volume of the liquid at -7 deg. as unity, it becomes 1.264 at 35.8 deg.,
and thence Ansdell infers that the mean coefficient of expansion per degree
is 0.00489 deg. for the total range of pressure." Assuming that the liquid
was under the same pressure at the two temperatures, the coefficient of
expansion per degree Centigrade would be 0.00605, which agrees more nearly
with the figure 0.007 which is quoted, by Fouche As mentioned before, data
referring to liquid (_i.e._, liquefied) acetylene are of no practical
importance, because the substance is too dangerous to use. They are,
however, interesting in so far as they indicate the differences in
properties between acetylene converted into the liquid state by great
pressure, and acetylene dissolved in acetone under less pressure; which
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