Kinetics of front-end acetylene hydrogenation in ethylene production

The kinetics of acetylene hydrogenation in the presence of a large excess of ethylene was studied in a laboratory flow reactor. Experiments were carried out using a Pd/alpha-Al2O3 commercial catalyst-and a simulated cracker gas mixture (H-2/C2H2 = 50; 60% C2H4; 30% H-2, and traces of CO), at varying temperature (293-393 K) and pressure (2-35 atm). Competing mechanisms for acetylene and ethylene hydrogenation were formulated and the corresponding kinetic equations derived by rate-determining step methods. A criterion based upon statistical analysis was used to discriminate between rival kinetic models. The selected equations are consistent with the adsorption of C2H2 and C2H4 in the same active sites followed by reaction with adsorbed hydrogen atoms to form C2H4 and C2H6 in a one-step process. Good agreement between computed and experimental results was obtained using a nonisothermal reactor model that takes into account the existence of external temperature and concentration gradients. The derived kinetic equations together with a pseudohomogeneous model of an integral adiabatic now reactor were employed to simulate the conversion and the temperature profiles for a commercial hydrogenation unit.