Turning to the effect of condensation upon the substances in gas which are considered as impurities, reference may be made to a paper recently read at the Institute of Civil Engineers, by Harry Jones, C.E., Engineer to the Commercial Gas Co., and in which are given figures showing the amounts of the impurities present in the gas at different stages, from analyses made by the company's chemist. Gas made from ordinary Newcastle gas Coal was found, on entering the condensers, to contain as a mean of several experiments the following amounts of impurities :— From these figures it will be seen that a considerable reduction takes place in the amount of all impurities, except sulphur in other forms than as sulphuretted hydrogen, upon which no action is exerted. The partially purified gas then passes to the scrubbers and washers, by the action of which the impurities become still further reduced.
Effect of Scrubbing and Washing.—The portiou of the purifying plant which comprises the various forms of scrubbers and washers has for its primary object the elimination of ammonia from the gas, in such a way that a solution of ammonia is formed, of sufficient strength to be of commercial value. The effect of this portion of the plant is, however, not limited to the simple removal of ammonia, but also comprises a considerable reduction in the amounts of sulphuretted hydrogen and carbonic acid. The apparatus used simply consists of various mechanical contrivances, by which the gas is brought at a reduced velocity into intimate contact with water, or with wetted surfaces. The effect of this treatment will be best comprehended by giving the amounts of the various impurities in the gas at its entrance to the apparatus and at its exit :— It will be seen that the ammonia has entirely disappeared, while the proportions of sulphuretted hydrogen and carbonic acid have been considerably reduced, the greatest reduction having taken place in the amount of the last-named compound. The reduction of the two impurities and during the passage of the gas through the scrubbers and washers is due to the same cause that effects a diminution in the amounts of these impurities in the condenser, viz. combination with ammonia. The solutions of ammonia produced in the condenser, the washers, and the scrubbers, constitute, when mixed, the " ammoniacal liquor" of commerce ; its strength, that is, the quantity of ammonia which it contains, varies with the character of the coal, and the amount of water used in the scrubbers. From ordinary Newcastle gas-coal, the yield of a ton may be taken as about 28 gal. of "7-oz. liquor," i. e. liquor containing a sufficient amount of ammonia for 1 gal. to neutralize 7 oz. of sulphuric acid (sp. gr. 1.85).
The following Table shows the composition of ammooiacal liquors of different strengths :— The " fixed ammonia " shown in the last Table is that part of the total ammonia which does not exist in the form of volatile compounds, Each as the eulphydrate and carbonate, and which is consequently not shown by an ordinary estimation by standard acid. The "fixed ammonia"
consists principally of hyposulphate and sulphooyanate, with a little sulphate. The sulphocyanide ie chiefly derived from the mutual reaction of cyanogen, ammonia, and sulphuretted hydrogen. The hyposulphate and sulphate are most probably formed from the oxidation of ammonium aulphydrate by the oxygen dissolved in the water supplied to the scrubbers, and by the small quantity of air as an accidental impurity present in the gas. An abnormal amount of hyposulphate and sulphate is sometimes produced on first starting a scrubber containing fresh coke, probably owing to the oxygen condensed in the pores of the material. A similar production would occur if any crack or orifice existed in the retorts, through which, by the action of the exhauster, furnace gases charged with sulphurous acid were drawn in.
The gas, as it leaves the scrubbers, should be practically free from ammonia. Sometimes, however, difficulty is experienced in attaining the desired result, coincident with the employment of such a limited amount of water as to produce ammoniacal liquor of saleable strength. In such cases, supplementary means for removing the small amount of residual ammonia are sometimes adopted, chief among which may be mentioned the use of the material known as " carbon " or " acid carbon," the employment of sawdust moistened with weak sulphuric acid, and the use of crude sulphate of iron (copperas). The material known as " carbon " is sawdust which has been mixed with strong sulphuric acid, and subsequently subjected to heat, by which more or less complete carbonization has been effected. The action of this material as a purifying agent for the removal of ammonia is the same as that which takes place when sawdust moistened with weak sulphuric acid is used, and consists in the simple union of the sulphuric acid and ammonia to form ammonium sulphate, + 2N El, = In the case of copperas, the action is as follows :— FeSO, + + = + FeO.
Sulphate Ammonium Ferrous of Iron. Ammonia. Water. sulphate, oxide.
The sulphate of iron and the ammonia thus form sulphate of ammonia and protoxide of iron (ferrous oxide). The ferrous oxide then reacts on the sulphuretted hydrogen, sulphide of iron (ferrous sulphide) and water being produced, thus—Fe0 + = FeS + It remains to describe the changes which occur during the subsequent purification of the gas from the residual sulphuretted hydrogen and carbonic acid, as well as from carbon disulphide.
The gas, as it leaves the scrubbers, supposing those vessels to be thoroughly efficient, may be taken as containing the following average amounts of impurities Sulphur In