Acetylene, the Principles of Its Generation and Use Part 23

"METHODS FOR THE DETERMINATION OF TILE YIELD OF GAS FROM CALCIUM CARBIDE.

"The greatest precision is attained when the whole of the sample submitted to the a.n.a.lyst is gasified in a carbide-to-water apparatus, and the gas evolved is measured in an accurately graduated gasholder.

"The apparatus used for this a.n.a.lysis must not only admit of all the precautionary rules of gas-a.n.a.lytical work being observed, but must also fulfil certain other experimental conditions incidental to the nature of the a.n.a.lysis.

"(_a_) The apparatus must be provided with an accurate thermometer to show the temperature of the confining water, and with a pressure gauge, which is in communication with the gasholder.

"(_b_) The generator must either be provided with a gasholder which is capable of receiving the quant.i.ty of gas evolved from the whole amount of carbide, or the apparatus must be so constructed that it becomes possible with a gasholder which in not too large (up to 200 litres = say 7 cubic feet capacity) to gasify a larger amount of carbide.

"(_c_) The generator must be constructed so that escape of the evolved gas from it to the outer air is completely avoided.

"(_d_) The gasholder must be graduated in parts up to 1/4 per cent.

of its capacity, must travel easily, and be kept, as far as may be in suspension by counterweighting.

"(_e_) The water used for decomposing the carbide and the confining water must be saturated, before use, with acetylene, and, further, the generator must, before the a.n.a.lysis proper, be put under the pressure of the confining (or sealing) liquid."

The following is a description of a typical form of apparatus corresponding with the foregoing requirements:

"The apparatus, shown in the annexed figure, consists of the generator A, the washer B, and the gasholder C.

[Ill.u.s.tration: FIG. 22.--LARGE-SCALE APPARATUS FOR DETERMINING YIELD OF GAS FROM CARBIDE.]

"The generator A consists of a cylindrical vessel with sloping bottom, provided with a sludge outlet _a_, a gas exit-pipe _b_, and a lid _b'_ fastened by screws. In the upper part ten boxes _c_ are installed for the purpose of receiving the carbide. The bottoms of those boxes are flaps which rest through their wire projections on a revolvable disc _d_, which is mounted on a shaft _l_. This shaft pa.s.ses through a stuffing-box to the outside of the generator and can be rotated by moans of the chains _f_, the pulleys _g_ and _h_, and the winch _i_. Its rotation causes rotation of the disc _d_. The disc _d_, on which the bottoms of the carbide- holders are supported, is provided with a slot _e_. On rotating the disc, on which the supporting wires of the bottoms of the carbide-holders rest, the slot is brought beneath these wires in succession; and the bottoms, being thus deprived of their support, drop down. It is possible in this way to effect the discharge of the several carbide-holders by gradual turning of the winch _i_.

"The washer B is provided with a thermometer _m_ pa.s.sing through a sound stuffing-box and extending into the water.

"The gasholder C is provided with a scale and pointer, which indicate how much gas there is in it. It is connected with the pressure-gauge _n_, and is further provided with a control thermometer _o_.

The gas exit-pipe _q_ can be shut off by a c.o.c.k. There is a c.o.c.k between the gasholder and the washer for isolating one from the other.

"The dimensions of the apparatus are such that each carbide-holder can contain readily about half a kilogramme (say l lb.) of carbide. The gasholder is of about 200 litres (say 7 cubic feet) capacity; and if the bell is 850 mm. (= 33-1/2 inches) high, and 550 mm. (= 21-1/2 inches) in diameter it will admit of the position being read off to within half a litre (say 0.02 cubic foot)."

The directions of the German a.s.sociation for sampling a consignment of carbide packed in drums each containing 100 kilogrammes (say 2 cwt.) have already been given in the rules of that body. They differ somewhat from those issued by the British a.s.sociation (_vide ante_), and have evidently been compiled with a view to the systematic and rapid sampling of larger consignments than are commonly dealt with in this country.

Drawing a portion of the whole sample from every tenth drum is substantially the same as the British a.s.sociation's regulations for cases of dispute, viz., to have one unopened drum (_i.e._, one or two cwt.) per ton of carbide placed at the a.n.a.lyst's disposal for sampling.

Actually the mode of drawing a portion of the whole sample from every tenth vessel, or lot, where a large number is concerned, is one which would naturally be adopted by a.n.a.lysts accustomed to sampling any other products so packed or stored, and there in no reason why it should be departed from in the case of large consignments of carbide. For lots of less than ten drums, unless there is reason to suspect want of uniformity, it should usually suffice to draw the sample from one drum selected at random by the sampler. The a.n.a.lyst, or person who undertakes the sampling, must, however, exercise discretion as to the scheme of sampling to be followed, especially if want of uniformity of the several lots const.i.tuting the consignment in suspected. The size of the lumps const.i.tuting a sample will be referred to later.

The British a.s.sociation's regulations lead to a sample weighing about 3 lb. being obtained from each drum. If only one drum is sampled, the quant.i.ty taken from each position may be increased with advantage so as to give a sample weighing about 10 lb., while if a large number of drums is sampled, the several samples should be well mixed, and the ordinary method of quartering and re-mixing followed until a representative portion weighing about 10 lb. remains.

A sample representative of the bulk of the consignment having been obtained, and hermetically sealed, the procedure of testing by means of the apparatus already described may be given from the German a.s.sociation's directions:

"The first carbide receptacle is filled with 300 to 400 grammes (say 3/4 lb.) of any readily decomposable carbide, and is hung up in the apparatus in such a position with regard to the slot _e_ on the disc _d_ that it will be the first receptacle to be discharged when the winch _i_ is turned. The tin or bottle containing the sample for a.n.a.lysis is then opened and weighed on a balance capable of weighing exactly to 1/2 gramme (say 10 grains). The carbide in it is then distributed quickly, and as far as may be equally, into the nine remaining carbide receptacles, which are then shut and hung up quickly in the generator.

The lid _b'_ is then screwed on the generator to close it, and the empty tin or bottle, from which the sample of carbide has been removed, is weighed.

"The contents of the first carbide receptacle are then discharged by turning the winch _i_. Their decomposition ensures on the one hand that the sealing water and the generating water are saturated with acetylene, and on the other hand that the dead s.p.a.ce in the generator is brought under the pressure of the seal, so that troublesome corrections which would otherwise be entailed are avoided. After the carbide is completely decomposed, but not before two hours at least have elapsed, the c.o.c.k _p_ is shut, and the gasholder is run down to the zero mark by opening the c.o.c.k _q_. The c.o.c.k _q_ is then shut, _p_ is opened, and the a.n.a.lytical examination proper is begun by discharging the several carbide receptacles by turning the winch _i_. After the first receptacle has been discharged, five or ten minutes are allowed to elapse for the main evolution of gas to occur, and the c.o.c.k _p_ is then shut. Weights are added to the gasholder until the manometer _n_ gives the zero reading; the position of the gasholder C is then read off, and readings of the barometer and of the thermometer _o_ are made. The gasholder is then emptied down to the zero mark by closing the c.o.c.k _p_ and opening _q_. When this is done _q_ is closed and _p_ is opened, and the winch _i_ is turned until the contents of the next carbide receptacle are discharged. This procedure is followed until the carbide from the last receptacle has been gasified; then, after waiting until all the carbide has been decomposed, but in any case not less than two hours, the position of the gasholder is read, and readings of the barometer and thermometer are again taken. The total of the values obtained represents the yield of gas from the sample examined."

The following example is quoted:

Weight of the tin received, with its contained | carbide . . . . . ._| = 6325 grammes.

Weight of the empty tin . . . . = 1485 "

_______ Carbide used . . . = 4840 " = 10670 lb.

The carbide in question was distributed among the nine receptacles and gasified. The readings were:

________________________________________________ | | | | | | No. | Litres. | Degrees C. | Millimetres. | |______|__________|______________|_______________| | | | | | | 1 | 152.5 | 13 | 762 | | 2 | 136.6 | " | " | | 3 | 138.5 | " | " | | 4 | 161.0 | " | " | | 5 | 131.0 | " | " | | 6 | 182.5 | 13.5 | " | | 7 | 146.0 | " | " | | 8 | 163.0 | 14.0 | " | | 9 | 178.5 | " | " | |______|__________|______________|_______________|

After two hours, the total of the readings was 1395.0 litres at 13.5 C.

and 762 mm., which is equivalent to 1403.7 litres (= 49.57 cubic feet) at 15 C. and 760 mm. (or 60 F. and 30 inches; there is no appreciable change of volume of a gas when the conditions under which it is measured are altered from 15 C. and 760 mm. to 60 F. and 30 inches, or _vice versa_).

The yield of gas from this sample is therefore 1403.7/4.840 = 290 litres at 15 C. and 760 mm. per kilogramme, or 49.57/10.67 = 4.65 cubic feet at 60 F. and 30 inches per pound of carbide. The apparatus described can, of course, be used when smaller samples of carbide only are available for gasification, but the results will be less trustworthy if much smaller quant.i.ties than those named are taken for the test.

Other forms of carbide-to-water apparatus may of course be devised, which will equally well fulfil the requisite conditions for the test, viz., complete decomposition of the whole of the carbide without excessive rise of temperature, and no loss of gas by solution or otherwise.

An experimental wet gas-motor, of which the water-line has been accurately set (by means of the Gas Referees' 1/12 cubic foot measure, or a similar meter-proving apparatus), may be used in place of the graduated gasholder for measuring the volume of the gas evolved, provided the rate of flow of the gas does not exceed 1/6 cubic foot, or say 5 litres per minute. If the generation of gas is irregular, as when an apparatus of the type described above is used, it is advisable to insert a small gasholder or large bell-governor between the washer and the meter. The meter must be provided with a thermometer, according to the indications of which the observed volumes must be corrected to the corresponding volume at normal temperature.

If apparatus such as that described above is not available, fairly trustworthy results for practical purposes may be obtained by the decomposition of smaller samples in the manner described below, provided these samples are representative of the average composition of the larger sample or bulk, and a number of tests are made in succession and the results of individual tests do not differ by more than 10 litres of gas per kilogramme (or 0.16 cubic foot per pound) of carbide.

It is necessary at the outset to reduce large lumps of carbide in the sample to small pieces, and this must be done with as little exposure as possible to the (moist) air. Failing a good pulverising machine of the coffee-mill or similar type, which does its work quickly, the lumps must be broken as rapidly as possible in a dry iron mortar, which may with advantage be fitted with a leather or india-rubber cover, through a hole in which the pestle pa.s.ses. As little actual dust as possible should be made during pulverisation. The decomposition of the carbide is best effected by dropping it into water and measuring the volume of gas evolved with the precautions usually practised in gas a.n.a.lysis. An example of one of the methods of procedure described by the German a.s.sociation will show how this test can be satisfactorily carried out:

"A Woulff's bottle, _a_ in the annexed figure, of blown gla.s.s and holding about 1/4 litre is used as the generating vessel. One neck, about 15 mm. in internal diameter, is connected by flexible tubing with a globular vessel _b_, having two tubulures, and this vessel is further connected with a conical flask _c_, holding about 100 c.c.

The other neck is provided with tubing _d_, serving to convey the gas to the inlet-tube, with tap _e_, of the 20-litre measuring vessel _f_, which is filled with water saturated with acetylene, and communicates through its lower tubulure with a similar large vessel _g_. The generating vessel _a_ is charged with about 150 c.c.

of water saturated with acetylene. The vessel _f_ is filled up to the zero mark by raising the vessel _g_; the tap _e_ is then shut, and connexion is made with the tube _d_. Fifty grammes (or say 2 oz.) of the pulverised carbide are then weighed into the flask _c_ and this is connected by the flexible tubing with the vessel _b_.

The carbide is then decomposed by bringing it in small portions at a time into the bulb _b_ by raising the flask _c_, and letting it drop from _b_ into the generating vessel _a_, after having opened the c.o.c.k _e_ and slightly raised the vessel _f_. After the last of the carbide has been introduced two hours are allowed to elapse, and the volume of gas in _f_ is then read while the water stands at the same level in _f_ and _g_, the temperature and pressure being noted simultaneously."

A second, but less commendable method of decomposing the carbide is by putting it in a dry two-necked bottle, one neck of which is connected with _e_, and dropping water very slowly from a tap-funnel, which enters the other neck, on to the carbide. The generating bottle should be stood in water, in order to keep it cool, and the water should be dropped in at the rate of about 50 c.c. in one hour. It will take about three hours completely to gasify the 50 grammes of carbide under these conditions. The gas is measured as before.

[Ill.u.s.tration: FIG. 23.--SMALL-SCALE APPARATUS FOR DETERMINING YIELD OF GAS FROM CARBIDE.]

Cedercreutz has carried out trials to show the difference between the yields found from large and small carbide taken from the same drum. One sample consisted of the dust and smalls up to about 3/5 inch in size, while the other contained large carbide as well as the small. The latter sample was broken to the same size as the former for the a.n.a.lysis. Tests were made both with a large testing apparatus, such as that shown in Fig.

22, and with a small laboratory apparatus, such as that shown in Fig. 23.

The dust was screened off for the tests made in the large apparatus. Two sets of testings were made on different lots of carbide, distinguished below as "A" and "B," and about 80 grammes wore taken for each determination in the laboratory apparatus, and 500 grammes in the large apparatus. The results are stated in litres (at normal temperature and pressure) per kilogramme of carbide.

___________________________________________________________________ | | | | | | "A" | "B" | |_____________________________________________________|______|______| | | | | | Lot |Litres|Litres| | Small carbide, unscreened, in laboratory (1) | 276 | 267 | | apparatus . . . . . / (2) | 273 | 270 | | Average sample of carbide, unscreened, in (1) | 318 | 321 | | laboratory apparatus . . . / (2) | 320 | 321 | | Small carbide, dust freed, in large apparatus (1) | 288 | 274 | | Average sample of carbide, dust freed, in (2) | 320 | 322 | | large apparatus . . . . / | | | |_____________________________________________________|______|______|

As the result of the foregoing researches Cedercreutz has recommended that in order to sample the contents of a drum, they should be tipped out, and about a kilogramme (say 2 to 3 lb.) taken at once from them with a shovel, put on an iron base and broken with a hammer to pieces of about 2/5 inch, mixed, and the 500 grammes required for the a.n.a.lysis in the form of testing plant which he employs taken from this sample. Obviously a larger sample can be taken in the same manner. On the other hand the British and German a.s.sociations' directions for sampling the contents of a drum, which have already been quoted, differ somewhat from the above, and must generally be followed in cases of dispute.

Cedercreutz's figures, given in the above table, show that it would be very unfair to determine the gas-making capacity of a given parcel of carbide in which the lumps happened to vary considerably in size by a.n.a.lysing only the smalls, results so obtained being possibly 15 per cent. too low. This is due to two causes: first, however carefully it be stored, carbide deteriorates somewhat by the attack of atmospheric moisture; and since the superficies of a lump (where the attack occurs) is larger in proportion to the weight of the lump as the lump itself is smaller, small lumps deteriorate more on keeping than large ones. The second reason, however, is more important. Not being a pure chemical substance, the commercial material calcium carbide varies in hardness; and when it is merely crushed (not reduced altogether to powder) the softer portions tend to fall into smaller fragments than the hard portions. As the hard portions are different in composition from the soft portions, if a parcel is sampled by taking only the smalls, practically that sample contains an excess of the softer part of the original material, and as such is not representative. Originally the German Acetylene a.s.sociation did not lay down any rules as to the crushing of samples by the a.n.a.lyst, but subsequently they specified that the material should be tested in the size (or sizes) in which it was received. The British a.s.sociation, on the contrary, requires the sample to be broken in small pieces. If the original sample is taken in such fashion as to include large and small lumps as accurately as possible in the same proportion as that in which they occur in the main parcel, no error will be introduced if that sample is crushed to a uniform size, and then subdivided again; but a small deficiency in gas yield will be produced, which will be in the consumer's favour. It is not altogether easy to see the advantage of the British idea of crushing the sample over the German plan of leaving it alone; because the a.n.a.lytical generator will easily take, or its parts could be modified to take, the largest lumps met with.

If the sample is in very large ma.s.ses, and is decomposed too quickly, polymerisation of gas may be set up; but on the other hand, the crushing and re-sampling will cause wastage, especially in damp weather, or when the sampling has to be done in inconvenient places. The British a.s.sociation requires the test to be made on carbide parcels ranging between 1 and 2-1/2 inches or larger, because that is the "standard" size for this country, and because no guarantee is to be had or expected from the makers as to the gas-producing capacity of smaller material.

Manifestly, if a consumer employs such a form of generator that he is obliged to use carbide below "standard" size, a.n.a.lyses may be made on his behalf in the ordinary way; but he will have no redress if the yield of acetylene is less than the normal. This may appear a defect or grievance; but since in many ways the use of small carbide (except in portable lamps) is not advantageous--either technically or pecuniarily--the rule simply amounts to an additional judicious incentive to the adoption of apparatus capable of decomposing standard-sized lumps. The German and Austrian a.s.sociations' regulations, however, provide a standard for the quality of granulated carbide.

It has been pointed out that the German a.s.sociation's direction that the water used in the testing should be saturated with acetylene by a preliminary decomposition of 1/2 kilogramme of carbide is not wholly adequate, and it has been suggested that the preliminary decomposition should be carried out twice with charges of carbide, each weighing not less than 1 per cent. of the weight of water used. A further possible source of error lies in the fact that the generating water is saturated at the prevailing temperature of the room, and liberates some of its dissolved acetylene when the temperature rises during the subsequent generation of gas. This error, of course, makes the yield from the sample appear higher than it actually is. Its effects may be compensated by allowing time for the water in the generator or gasholder to cool to its original temperature before the final reading is made.

With regard to the measurement of the temperature of the evolved gas in the bell gasholder, it is usual to a.s.sume that the reading of a thermometer which pa.s.ses through the crown of the gasholder suffices. If the thermometer has a very long stem, so that the bulb is at about the mid-height of the filled bell, this plan is satisfactory, but if an ordinary thermometer is used, it is better to take, as the average temperature of the gas in the holder, the mean of the readings of the thermometer in the crown, and of one dipping into the water of the holder seal.

The following table gives factors for correcting volumes of gas observed at any temperature and pressure falling within its range to the normal temperature (60 F.) and normal barometric height (30 inches). The normal volume thus found is, as already stated, not appreciably different from the volume at 15 C. and 760 mm. (the normal conditions adopted by Continental gas chemists). To use the table, find the observed temperature and the observed reading of the barometer in the border of the table, and in the s.p.a.ce where these vertical and horizontal columns meet will be found a number by which the observed volume of gas is to be multiplied in order to find the corresponding volume under normal conditions. For intermediate temperatures, &c., the factors may be readily inferred from the table by inspection. This table must only be applied when the gas is saturated with aqueous vapour, as is ordinarily the case, and therefore a drier must not be applied to the gas before measurement.

Hammerschmidt has calculated a similar table for the correction of volumes of gas measured at temperatures ranging from 0 to 30 C., and under pressures from 660 to 780 mm., to 15 C. and 760 mm. It is based on the coefficient of expansion of acetylene given in Chapter VI., but, as was there pointed out, this coefficient differs by so little from that of the permanent gases for which the annexed table was compiled, that no appreciable error results from the use of the latter for acetylene also.

A table similar to the annexed but of more extended range is given in the "Notification of the Gas Referees," and in the text-book on "Gas Manufacture" by one of the authors.

The determination of the amounts of other gases in crude or purified acetylene is for the most part carried out by the methods in vogue for the a.n.a.lysis of coal-gas and other illuminating gases, or by slight modifications of them. For an account of these methods the textbook on "Gas Manufacture" by one of the authors may be consulted. For instance, two of the three princ.i.p.al impurities in acetylene, viz., ammonia and sulphuretted hydrogen, may be detected and estimated in that gas in the same manner as in coal gas. The detection and estimation of phosphine are, however, a.n.a.lytical operations peculiar to acetylene among common illuminating gases, and they must therefore be referred to.