Propane/propylene separation by simulated moving bed I. Adsorption of propane, propylene and isobutane in pellets of 13X zeolite

The separation of propane-propylene mixture is the most energy consuming operation in the petrochemical industry. Various studies have been investigated to relieve the cryogenic distillation ordinarily used for this separation, and the adsorption technology appeared to be a promising option. Considering the encouraging results obtained by cyclic adsorption processes and notably by pressure swing adsorption, the simulated moving bed (SMB) has been suggested as a new and competitive alternative. The keystone of a SMB for a gas mixture separation is the choice of an adequate and pertinent adsorbent-desorbent couple. In this work, isobutane has been tested as a potential desorbent over 13X zeolite. A gravimetric method has been used to measure the adsorption equilibrium isotherms of propylene, propane, and isobutane on 13X zeolite pellets over a temperature range from 333K to 393K and pressure up to 160kPa. Experimental adsorption equilibrium isotherms were correlated with the Toth model. The 13X zeolite shows an intermediate loading capacity for isobutane at low pressures. Equilibrium capacities for propylene, propane, and isobutane at 373K and 110kPa were 2.12, 1.61, and 1.53mol/kg, respectively. The heats of adsorption at zero coverage for propylene, propane, and isobutane were found to be 42.4, 36.9, 41.6kJ/mol, respectively. Breakthrough curves of pure components were measured at 373K and 150kPa with different initial conditions (adsorbent bed saturated with nitrogen or isobutane). Experimental breakthrough curves were well-predicted by an exhaustive mathematical model taking into account the energy balance in the three phases (gas, solid, and wall column). Multi-component fixed bed adsorption experiments allowed us to observe that isobutane could displace an adsorbed propane/propylene mixture from the 13X zeolite and itself was fairly easily displaced from the adsorbent by this same mixture. These results confirmed the assumption that isobutane is a good desorbent for the adsorptive separation of C3H6/C3H8 mixture by a simulated moving bed.