Modern Gasworks Practice

THE PRELIMINARY PURIFICATION OF COAL GAS 537 liquor is admitted, thus extracting the bulk of the sulphuretted hydrogen and carbon dioxide. 4. Second ammonia washer.—In which. water is used for removing any remain-ing ammonia, H2S and C02. The H2S is reduced to less th.an 0-02 per cent, by volume. 5. Final washer.—In the lower portion, of this washer, a ferrous hydrate sludge is used to extract the last traces of H2S and a further quantity of ammonia, as follows Iron sulphate is treated with ammoniacal liquor to give ammonium sulphate : FeS04 + 2 NH40H = (NH^SOa + Fe(OH)2. The H2S then combin.es with the iron hydrate formed: Fe(OH)a + H2S = FeS + 2H2O. The iron sulphide resulting from this reaction is afterwards made use of for absorbing cyanogen, as explained under heading (2). In the upper portion of the final washer, water or sulphuric acid is used for extracting the last traces of ammonia. The Petit Process This process is deserving of mention if only for the faet that the apparatus re-quired consists merely of two orelinary washer-serubbers. The first of the washers serves the purpose of absorbing the sulphuretted hydrogen from the gas (this being effeeted by the use of a special washing liquid), while in the second vessel revivi-fication of the saturated liquor is brouglit about. The washing medium made use of is preferably potassium carbonate (K2CO3), which has the advantage that ao precipitate is formed, so that the liquor always remains as clear as water. The gas after having the ammonia removed, but still containing carbon dioxide, is passed through the solution, when the carbon dioxide unites with the potassium carbonate in the presence of water to form a hydro-carbonate (KHC’O3). In addition, sulphuretted hydrogen is absorbed and yields a solution of potassium hydrosulphide, whilst the hydrocyanic acid is almost wholly eliminated and gives potassium cyanide. After a time the original washing liquor becomes saturated with impurities, and is subjected to a simple process of regeneration, after which. it is capable of further use. To effeet revivification a stream of air from a blower is passed through the fouled solution, when the sulphuretted hydrogen and hydrocyanic acid are immedi-ately liberated. It is interesting to note that an air biast would not liberate the impurities from a neutral solution, and in this case (owing to the original reaction with carbon, dioxide) a bicarbonate salt is formed. In the Petit process the sulphur of the original coal may still be recovered as a by-product, and to this end it is custo-mary to pass the air from the regenerator to a Claus kiln. for the production of free sulphur. In general, the air after leaving the regenerator will contain about 2 per cent, of sulphur. It is found that a small amount of the carbon dioxide is also liberated by the biast; con.sequen.tly the revivifiecL solution is capable of taking up from the gas a further quantity of carbon, dioxide in the next operation. The speed of the regeneration process may be appreciably inereased if a small proportion of carbon dioxide is admixed with. the air before its passage through the liquor. For this purpose a supply of waste gases may be taken from any con-