Crystal size effect in vacuum pressure-swing adsorption for propane/propylene separation

The aim of this work is to evaluate the performance parameters of a vacuum pressure swing adsorption (VSA-PSA) unit for propane/propylene separation using zeolite 4A with different crystal sizes. For this purpose, we have synthesized samples of zeolite 4A with different average crystal diameters: 0.3, 1.9, 10.0, and 22.0 mum. We measured the adsorption equilibrium and kinetics of pure propane and propylene on these samples. The adsorption equilibrium has been measured on a manometric equipment from 0 to 100 kPa pressure and at temperatures of 373, 423, and 473 K. The pure gas adsorption kinetics at the same temperatures has been measured by isothermal breakthrough curves using only 1.1% of pure hydrocarbon diluted with helium at atmospheric pressure. Both crystal diffusivity and equilibrium data show only slight differences with previously reported data on commercial extrudates. Experimental data with pellets with 3.8 mum average crystal diameter were measured in a PSA laboratory unit using a five-step cycle (pressurization, feed, rinse, depressurization to intermediate pressure, and low-pressure countercurrent blowdown). With the aid of simulations, we assessed the effect of different crystal sizes on the equipment performance. When a fixed purity is established as the target, the higher recovery and productivity were obtained with the smaller crystals.