Reaction engineering aspects of nitrile hydrogenation

Different reactor concepts were compared for the gas/solid/liquid three-phase hydrogenation of nitriles in the presence of liquid ammonia. In the laboratory a semi-batch stirred tank reactor containing nitrile dissolved in ammonia and with suspended catalyst with hydrogen gas entrainment from the bottom was used for an initial parameter study. The concentration of ammonia in the liquid phase was the most critical parameter for selectivity to primary amines. The selectivity dropped rapidly when the ammonia concentration in the liquid phase fell below a critical value. Much higher ammonia concentrations were needed in a micro-flow trickle bed reactor in which a solution of nitrile in ammonia and a hydrogen stream entered from the top. With increasing hydrogen flow the amount of ammonia discharged from the reactor along with the gas phase increased, inducing reduced selectivity to primary amines. Thus, the effective ammonia concentration in the liquid phase was much lower as the phase equilibrium re-established itself. With respect to process development, the two different reactor concepts for the hydrogenation of a high boiling compound (nitrile) in the presence of a volatile reactant (ammonia) are only comparable when working above the critical ammonia concentration. Under these conditions, basic parameters for scale-up can be generated in the laboratory. However, the actual concentration of highly volatile compounds in the liquid phase depends strongly on the fluid dynamics in the reactor which is intrinsically hard to predict for trickle-bed reactors. Thus, a pilot plant will be needed for final performance studies. Flow diagrams.