450
MODERN GASWORKS PRACTICE
diiefly in conflict as to whether gas and tar should be separated at the earliest possible moment, or whether prolonged contact between the two should be permitted. As already stated, the heavy tars and high-boiling constituents are mostly separated out in the hydraulic main, altliough some portion of these is carried still fartlier forward by mechanical action. There can be little dispute, however, that such tars are detrimental from the point of view of quality, and should be isolated from the gas with all possible speed. For the same reason a tar seal in the hydraulic main is to be avoided. The solvent action of the heavy tar vesicles becomes more pronouuced as the temperature
drops. Thus removal of the heavy fog at an. early stage is probably desirable, leaving the lighter qualities to exert their action as a naphthalene solvent.
Colman states that heavy tar fog when allowed to go forward to the condensers affects the removal of the naphthalene solvents in two ways—one favour-ably and the other adversely. On the one hånd, it removes from the gas the vapours of the light oils, and thus tends to prevent them from reaching the cold end of the condenser, where their action 011 naphthalene would be most pronounced. On the other hand, such fog as gets forward to the cold end mixes with the light oils which separate there, thus increasing their content of naph-thalene and impairing their ability to take up more of this substance. The complete removal of naphthalene can only be
re not already saturated with the product.
«ffected by the action of solvents which
It would seem, however, that one of the most effective methods of dealing with the crude gas is to eliminate the heavy tars and heavy fog at an early stage, and to ensure the passage of the light oil fog as far forward as the outlet of the condenser. Some
form of mechanical action has the most effective results in breaking up and throwing down the heavier vesicles, and Colman has successfully applied the principle of centrifugal force in his “ Cyclone ” apparatus. This machine, shown in Fig. 278, is usually interposed in the foul main at a point about half-way between the hydraulic main and the condensers. It consists of a wrought or cast-iron cylinder, to the