The selective hydrogenation of acetylene in a short contact time reactor.

摘要

Acetylene is an egregious contaminant in polyolefin feedstocks. A novel approach is to selectively hydrogenate it through a short contact time reactor. A thin Pd/γ-Al₂O₃ catalytic membrane (about 5–8 μm thick) was prepared and observed to be void of minor defects. Forcing a dilute C₂H₂/H₂/Ar mixture through the layer produced high conversions coupled with high selectivity at high temperatures. The selectivity was observed to increase with temperature. Langmuir-Hinshelwood type rate expressions were derived for both acetylene consumption and ethane formation. Interestingly, parallel reaction pathways involving carbonaceous deposits exist for both. The rate expressions were applied in a gas dispersion model to explain the membrane reactor results. A solution to the corresponding coupled non-linear differential equations resulted in a Peclet number of 64.5 and a contact time of 9.5 * 10−3 sec through the reactor at 100°C. A critical thickness is predicted to exist for a maximum in ethylene concentration. A value of 2.5 μm at 200°C is representative. Lastly, permeability experiments agree with modeling results to show that Knudsen diffusion is absent despite a narrow pore size.