Propane/propylene separation by pressure swing adsorption: sorbent comparison and multiplicity of cyclic steady states

The separation of propane/propylene mixtures is an important yet difficult separation. Two types of adsorbents, namely a π-complexation sorbent (AgNO3/SiO2), and a steric (based on size exclusion) sorbent (AlPO4-14), were previously proposed to be highly effective materials for C3H6/C3H8 separation using a vacuum pressure swing adsorption (PSA) cycle. In this work, high pressure C3H6 and C3H8 adsorption isotherms have been measured for the AgNO3/SiO2. The performance of the AgNO3/SiO2 sorbent has been compared with that of AlPO4-14 by simulating a four-step PSA cycle with two different grades of feed. The adsorption step was carried out at 7atm while desorption was conducted at either 1 or 0.2atm. It was found that although both sorbents provided more than 99% product purity at reasonably high recovery, the performance of AgNO3/SiO2 was better than the AlPO4-14 sorbent. The advantage of performing adsorption at a superatmospheric pressure was that the olefin product could be obtained at a higher pressure than that possible with a vacuum-swing cycle. For C3H6/C3H8 separation using AlPO4-14 sorbent, multiplicity of cyclic steady states was observed under certain operating conditions. One unstable and two stable steady states were observed within these regions for identical cycle conditions depending upon the initial temperature or sorbate concentration in the PSA bed at startup. The exact initial bed temperature and concentration at which there was a switch in the stable steady states was identified.