Modern Gasworks Practice

466 MODERN GASWORKS PRACTICE away in solution to the seal-pots of the plant. The efficacy of the treatment obviously depends upon the quantity of naphthalene in the crude gas and upon the proportion of the naphtha vapours present. It is in this direction that gas produced in horizontal and inclined retorts diffeis from that derived from contimious vertical retorts; for, in the former case the quantity of naphthalene is comparatively high, while the lower-boiling naphthas are usually present in small proportion, only. On the other band, with contimious vertical retorts the proportion of naphthalene is small; and, owing to its paraffinoid nature, the gas usually contains a high proportion of the lower-boiling constituents. There should therefore be little difficulty -—providing sufficient temperature drop is effected—in reducing the naphthalene content of gas from continuous verticals to a safe figure by passage through the condensers. In viewing the problem from the standpoint of the horizontal retort it is necessary to remember that a moderately high, content of naphthalene will still be present after the process of condensation as it is ordinarily carried out; but the extent of the trouble which this quantity is likely to give is, again, very greatly dependent upon the proportion of low-boiling naphthas which also remain in the gas after the condensers. For instance, provided that the low-boiling vapours are present in such quantities as to introduce what may be described as a state of equilibrium with the naphthalene, tlien no trouble need be anticipated when the gas is subjected to lower temperatures in the district mains. In such, cases the two constituents will separate out together, the naphthalene will be dissolved in the naphtha con-densate, and both will be carried to and collected from the syphons. Unfortunately, however, gas from horizontal retorts, even when füll charges are employed, possesses decidedly benzenoid characteristics. In other words, in comparison with vertical retort gas, the proportion of low-boiling vapours is small. Consequently, after condensation, the proportion of low-boiling constituents to naphthalene is invariably such that the condition of equilibrium préviously referred to is seldom found in practice. Perhaps no better instance of this is required than the conditions which are so frequently found in condensers of ampie size and capacity which effect a temperature drop above the normal. It frequently happens that large deposita of solid naphthalene are found adj acent to the outlet pipes of such condensers, sliowing that the proportion of low-boiling condensible vapours in the gas is insufficient to dissolve the whole of the naphthalene produced by drastic cooling of the kind. So far as the removal of the condensible constituents is concerned it is found in practice that a portion of the lower-boiling substances and lighter tars are immune from the effect of condensation per se; and owing to their physical condition they are carried forward in a state of mechanical suspension. Accordingly, if the gas is not subjected to thorough mechanical treatment, such as wire-drawing or primary washing, the “ oil fog ” may travel some considerable distance. This, in faet, from the point of view of naphthalene removal, is desirable, for it is when the gas is at the lower temperatures that the solvent action of the light oil fog is most effeetive. On the other hånd, the heavy tar vesicles and heavy fog should be re-moved by employing some such device as the Cyclone extractor (Fig. 278) at an