Modern Gasworks Practice

THE CONDENSATION OF COAL GAS o of the liquid and solid particles and the more complex gases, whereas more permanent gases are unafiected by it, and are cooled only by contact witli cool surfaces. In considering the principles of condensation, the question of the removal of aqueous vapour may be dismissed, as it is inevitable that the whole of this must be eliminated, with the exception. of that portion with. which the gas is saturated at normal temperatures. So far as the hydrocarbon condensate is concerned, it is of advantage to sub-divide this in the folio wing manner :— (a) Heavy tars. (6) Medium tars. (c) Light tars and oil fog. Of the total quantity of tar obtained, about 60 to 65 per cent, will be deposited in the hydraulic main, and this portion is largely of a heavy character. The medium tars, of a somewhat lower specific gravity, are the next constituents to be thrown. down. The greater portion of these will be accounted for during the passage of the products through the main connecting the hydraulic with the condenser, whilst the lighter tars and extremely fine vesicles known as the “ oil fog ” may travel con-siderably further. In cases where condensation capacity is insufficient or the appar-atus is improperly controlled, considerable quantities of the heavier tar fog will be found to travel forward to the serubbers, so that eventually stoppage occurs. From the point of view of the retention in the gas of the maximum quantity of the hydro-carbons, the slow travel and gradual temperature drop associated with. the horizontal atmospheric condenser cannot be improved upon. In general, the temperature of the gas in the hydraulic main varies between 60° and 70° C. (140° and 158° Fahr.). Although this temperature is below the boiling point of the suspended tarry vapours the latter are prevented from depositing owing to their being carried forward mechani-cally by the moving bulk of gas. The constituents most liable to deposition are benzene, toluene, certain naphthas, and, to some extent, xylene, which have an important eflect on the ultimate illuminating power of the gas. A well-establish.ed rule of condensation states that the minimum temperature to which gas is reduced should not be lower than 10° C. (50° Fahr.). Dr. Carpenter has stated, however, that rapid condensation with a comparatively big temperature drop has been shown in practice to have very little effeet on quality. The experiment which this authority conducted were carried out with gas which had been subjected to an extremely sharp drop of from 60° C. to 5° C. The whole problem of condensation turns on the point as to the affinity pos-sessed by tar, both in the liquid and “ fog ” conditions, for absorbing certain low boiling hydrocarbons present in the gas. Gas as it leaves the hydraulic main may be looked upon as being partly saturated with naphthalene, although with. modern systems of carbonization and heavy charges the quantity present has been consider-ably reduced. There is little doubt that the heavier tars and tar “ fog ” with which. the gas is in contact in the initial lengths of the foul main possess a marked affinity for naphthalene. At the same time prolonged contact must inevitably result in the absorption of low-boiling constituents. Authorities on condensation are, therefore, G G SB