Modern Gasworks Practice

404 MODERN GASWORKS PRACTICE Lewes states thai for the methane to fall below 30 per cent, in a coal gas made from a good coal indicates degradation of the products by overheating, and, although decomposition to a certain extent is essential, it may be said that with straight coal gas the economical limit is probably reached when the methane has been reduced to about 28 per cent. The tendency of hydrocarbons to dissociate is well illustrated by consideration of their structural formulæ and the arrangement of the carbon affinities. Thus we get— H H H—C —C —H I I H H Ethane (C2H6). H—C — C —EL Ethylene (C2H4). Acetylene (C2H2). In eacli case it will be seen that the carbon atoms are not wholly saturated with hydrogen, and in order that each affinity may be satisfied the carbon atoms nnit,p with one another. A bond between. the carbon atoms as indicated invariably gives rise to a state of instability, so that the molecule at this point tends to sever, and to recombine into two molecules with the addition of hydrogen. In the case of ethylene and acetylene this tendency becomes still more marked, as double and treble bonds exist between the carbon atoms. The force of attraction between the atoms virtually acts along a straight line, but in the case of tliese two compound« it may be looked upon as being bent from its proper path and curved round so as to unite with the other carbon atom. Hence a condition of strain is set up in the bond, which on but slight provocation straightens out and takes up another hydrogen atom. Where double or treble bonds exist they may be taken as an indication of instability; and the greater the number of bonds uniting the carbon atoms the greater will be the tendency to decompose. Bone supports this theory by showing that acetylene tends to break down across the triple bond, yielding free carbon and hydrogen, whilst if other hydrogen is taken up methane will be formed. Of the hydrocarbons in coal gas, benzene and its homologu.es (which are present to the extent of about 0-75 per cent.) are the most important. Their presence has been accounted for in various ways, but there is little doubt that they are high temperature degradation products. This is simply shown by th.e faet that whereas modern gas and tar emanating from heavy coal charges are of a distinctly paraffinoid nature, the hydrocarbons from the lighter charges, which afford ampie opportunity for “ over cracking,” contain a greater proportion of benzene derivatives. Many investigators have sought to prove that benzene is the direct result of the poly-merization of acetylene. Thus three molecules of acetylene (united in open. chains) undergo rearrangement to yield the closed benzene formation:—