Separation of propylene/propane mixtures using Ag+–RTIL solutions. Evaluation and comparison of the performance of gas–liquid contactors

Over the past few years reactive absorption systems have been explored as alternative to traditional distillation columns in the separation of propylene/propane gas mixtures. This work evaluates the performance of two different contactors, a membrane contactor, MC, and a semi-batch stirred tank reactor, STR, in the separation of the gas mixtures based on the absorption of propylene in Ag+–BmimBF4 reactive media. The experimental study analysed the influence of operation variables such as gas feed composition (30–70%, v/v of C3H6), solvent type (aqueous solutions or ionic liquid) and silver salt concentration on the absorption flux of propylene at room temperature. After development of the mass balances to both contactors the characteristic parameters have been determined from the fitting of the experimental data of the absorption flux to the mathematical models. In MC employing low silver concentrations, 0.1 M, the propylene fluxes reached the highest values when the ionic liquid was used as a solvent whereas for higher silver ion concentrations, 0.25 M, the aqueous media attained lower overall resistance to mass transport than the corresponding ionic liquid and consequently higher propylene fluxes. Working with the STR high propylene fluxes were obtained, that were directly proportional to the inlet C3H6 partial pressure.Under similar operating conditions and 298 K of temperature the overall mass transfer parameters took the values of Koveralla = 5.9 × 10−4 s−1 for MC and Koveralla = 236 × 10−4 s−1 for STR respectively; although the use of the membrane contactor resulted in a lower value of the mass transport coefficient a considerable decrease of the overall resistance to mass transport (1/klHadiEA) can be induced by acting on the design and on the operating conditions due to the favourable influence of the higher absorption capacity of the olefin in the ionic liquid media (high values of Hadi) with the enhancement in the absorption rate due to the instantaneous reaction with silver ions, thus further optimization of the process could improve the performance of the MC.