The Purifier

—The apparatus represented in the preceding figure is composed of a cylindrical iron ve el, with an air-tight cover screwed upon it, through which the cylinder n is also fixed air-tight. The bottom of this cylinder spreads out like the brim of a hat, forming a horizontal circular partition, which is pierced with holes. Through a stuffing box, in the cover of this interior cylinder, the vertical axis of the agitator pa es, which is turned by wheel and pinion work, in order to stir up the lime from the bottom of the water in the purifier. The ve el o serves for introducing fresh milk of lime, as also for letting it off by a stopcock when it has become too foul for further use. The quantity of lime should be proportioned to the quantity of sulphuretted hydrogen and carbonic acid contained in the gas. Supposing that in good coal gas there is 5 per cent. of these gases, about one pound and a half of lime will be requisite for every hundred cubic feet of coal gas generated, which amounts to nearly one-sixteenth of the weight of coal subjected to decomposition. This quantity of lime mixed with the proper quantity of water will form about a cubic foot of milk of lime. Consequently, the capacity of the purifier, that is, of the interior space filled with liquid, may be taken at four-sevenths of a cubic foot for every hundred cubic feet of gas pa ing through it in one operation; or for 175 cubic feet of gas, one cubic foot of liquor. After every operation, that is, after every five or six hours, the purifier must be filled afresh. Suppose that in the course of one operation 20,000 cubic feet of gas pa through the machine, this should be able to contain 20,000 175 = 114 cubic feet of milk of lime; whence its diameter should be seven feet, and the height of the liquid three feet. If the capacity of the ve el be le , the lime milk must be more frequently changed. In some of the large gas works of London the purifier has the following construction, whereby an uninterrupted influx and efflux of milk of lime takes place. Three single purifiers are so connected together, that the second ve el stands higher than the first, and the third than the second; so that the discharge tube of the superior ve el, placed somewhat below its cover, enters into the upper part of the next lower ve el; consequently, should the milk of lime in the third and uppermost ve el rise above its ordinary level, it will flow over into the second, and thence in the same way into the first; from which it is let off by the eduction pipe. A tube introduces the gas from the condenser into the first ve el, another tube does the same thing for the second ve el, ., and the tube of the third ve el conducts the gas into the gasometer. Into the third ve el, milk of lime is constantly made to flow from a cistern upon a higher level. By this arrangement, the gas pa ing through the several ve els in proportion as it is purified, comes progre ively into contact with purer milk of lime, whereby its purification becomes more complete. The agitator c , provided with two stirring paddles, is kept in continual rotation. The pre ure which the gas has here to overcome is naturally three times as great as with a single purifier of like depth. IMG:4147767755307473660_illo0548a.png:Purifier Fig. 485. is a simple form of purifier, which has been found to answer well in practice. Through the cover of the ve el A B , the wide cylinder e d is inserted, having its lower end pierced with numerous holes. Concentric with that cylinder is the narrower one s z , bound above with the flange a b , but open at top and bottom. The under edge g h of this cylinder descends a few inches below the end c d of the outer one. About the middle of the ve el the perforated shelf m n is placed. The shaft of the agitator l , pa es through a stuffing box upon the top of the ve el. The gas-pipe g , proceeding from the condenser, enters through the flange a b in the outer cylinder, while the gas-pipe h goes from the cover to the gasometer. A stopcock upon the side, whose orifice of discharge is somewhat higher than the under edge of the outer cylinder, serves to draw off the milk of lime. As the gas enters through the pipe g into the space between the two cylinders, it displaces the liquor till it arrives at the holes in the under edge of the outer cylinder, through which, as well as under the edge, it flows, and then pa es up through the apertures of the shelf m n into the milk of lime chamber; the level of which is shown by the dotted line. The stirrer, l , should be turned by wheel work, though it is here shown as put in motion by a winch handle. In order to judge of the degree of purity of the gas after its transmi ion through the lime machine, a slender syphon tube provided with a stopcock may have the one end inserted in its cover, and the other dipped into a ve el containing a solution of acetate of lead. Whenever the solution has been rendered turbid by the precipitation of sulphuret of lead, it should be renewed. The saturated and fetid milk of lime is evaporated in oblong cast-iron troughs placed in the ash-pit of the furnaces, and the dried lime is partly employed for luting the apparatus, and partly disposed of for a mortar or manure. By this purifier, and others of similar construction, the gas in the preceding parts of the apparatus, as in the retorts and the condenser, suffers a pre ure equal to a column of water about two feet high; and in the last described purifier even a greater pre ure. This pre ure is not disadvantageous, but is of use in two respects; 1. it shows by a brisk jet of gas when the apparatus is not air-tight, and it prevents common air from entering into the retorts; 2. this compre ion of the gas favours the condensation of the tar and ammoniacal liquor. The effect of such a degree of pre ure in expanding the metal of the ignited retorts is quite inconsiderable, and may be neglected. Two contrivances have, however, been proposed for taking off this pre ure in the purifier. IMG:4147767755307473660_illo0548b.png:Purefier In fig. 486. , m m are two similar ve els of a round or rectangular form, furnished at their upper border with a groove filled with water, into which the under edge of the cover fits, so as to make the ve el air-tight. The cover is suspended by a cord or chain, which goes over a pulley, and may be raised or lowered at pleasure. The ve els themselves have perforated bottoms, r r′ , covered with wetted mo or hay sprinkled over with slaked and sifted quicklime. The gas pa es through the loosely compacted matter of the first ve el, by entering between its two bottoms, rises into the upper space t , thence it proceeds to the second ve el, and, lastly, through the pipe u into the gasometer. This method, however, requires twice as much lime as the former, without increasing the purity of the gas. IMG:4147767755307473660_illo0551.png:Compre or The second method consists in compre ing the gas by the action of an Arch i me des screw, to such a degree, before it is admitted into the purifier, as that it may overcome the pre ure of the column of water in that ve el. Fig. 487. exhibits this apparatus in section. D D is the Arch i me des worm, the axis of which revolves at bottom upon the gudgeon e ; it po e es a three-fold spiral, and is turned in the opposite direction to that in which it scoops the water. The cistern which contains it has an air-tight cover. The gas to be purified pa es through the pipe C into the space D , over the water level d ; the upper cells of the worm, scoop in the gas at this point, and carry it downwards, where it enters at g into the cavity E of a second cistern. In order that the gas, after it escapes from the bottom of the worm, may not partially return through g into the cavity D , an annular plate g h is attached to its under edge, so as to turn over it. The compre ed gas is conducted from the cavity E through the pipe G into the purifying machine; a is a manometer, to indicate the elastic tension of the gas in D . On the top of the worm a mechanism is fitted for keeping it in constant rotation. A perfect purification of light-gas from sulphuretted hydrogen, either by milk of lime or a solution of the green sulphate of iron, is attended with some difficulty, when carried so far as to cause no precipitation of sulphuret in acetate of lead, because such a degree of washing is required as is apt to diminish its illuminating power, by abstracting the vapour of the rich oily hydro carburet which it contains. Moreover, the coal gas obtained towards the end of the distillation contains some sulphuret of carbon, which affords sulphurous acid on being burned, and can be removed by no easy method hitherto known. The lime in the purifier disengages from the carbonate and hydro sulphuret of ammonia carried over with the gas, especially when it has been imperfectly cooled in the condenser, a portion of ammoniacal gas, which, however, is not injurious to its illuminating power. The best agent for purifying gas would be the pyro lignite of lead, were it not rather expensive, because it would save the trouble of stirring, and require a smaller and simpler apparatus.