MULTISTAGE HYDROGENATION OF COAL - A NEW CONCEPT

Abstract

This chapter discusses the validity of the concept that certain retrogressive reactions can be effectively suppressed. Retrogressive reactions during hydroliquefaction of coal are well known. As a sequence of these reactions, distillate yield seems to reach a limiting value. Two opposing sets of reactions, one of progressive size degradation and the other of regressive build up, balance the effect of each other. The use of efficient catalyst and near ideal mass transfer condition shifts the equilibrium partly towards the desired direction but fails to eliminate the regressive reactions to any appreciable extent. The hydroliquefaction of coal is known to be predominantly a free radical process. Inter radical coupling and chain transfer reactions between a radical and any other molecular species are not size selective and, therefore, leads to size degradation and regressive molecular growth with equal propensity. Among the vast array of reactions, the most undesirable are the transitions of the smaller molecules and smaller radicals into bigger molecules and bigger radicals. A highly effective way of reducing or eliminating undesirable molecular growth can be attained by selectively transferring the smaller reaction products and intermediates to another phase where encounter with big radicals can be avoided through restricted solubility of the bigger species in that phase.