A STUDY OF FORCED FEED COMPOSITION CYCLING IN THE CATALYTIC SYNTHESIS OF AMMONIA

In this study, the influence of forced feed composition cycling on the time-average rate of ammonia synthesis reaction and the temperature distribution in the fixed bed catalytic reactor was investigated using a triply promoted iron catalyst. Experiments were carried out in two reactors: a gradientless recycle reactor and a near-adiabatic fixed bed reactor.; In the gradientless reactor, time-average reaction rates under periodic operation were 30-45% higher than the steady-state rates under identical operating conditions. The percentage improvement was influenced by the choice of cycling-period and cycle-split. Maximum percentage improvement was observed when the mean feed composition was 90% H(,2) + 10% N(,2). The presence of intra-particle concentration gradients reduced the percentage improvement due to cycling. This occurs because of the damping in the square-wave of feed composition by intraparticle diffusion.; The results of fixed bed reactor were very similar to those in the gradientless recycle reactor. The percentage improvement was in the range of 30-50%. Maximum percentage improvement occurred when the feed mixture was nitrogen deficient.; The temperature distribution in the fixed bed reactor was influenced significantly by the feed composition cycling. Average bed temperature increased due to the increase in the time-average rate. There was no significant effect of cycling on the magnitude of the hot-spot. The temperature distribution was more uniform in cycling than in steady-state. The relative standard deviation in temperature distribution increased with the cycle-period.; In an attempt to elucidate the reaction mechanism, both steady-state and transient response were used. Steady-state rate data fail to statistically discriminate among four models derived from different reaction mechanism. Also, the transient response was neither uniquely monotonic nor complex, but depended on the magnitude of the step-change. As a result, transient response technique fails to provide any useful insight into the reaction mechanism.; Models proposed in the literature by many workers to predict rate improvements due to cycling were examined. These and other models fail to predict any rate improvement in periodic operation.