The Use of Hybrid Membrane/Distillation System for the Ethane/Ethylene Separation in Olefin Plants

Separation of ethylene from ethane in commercial processes is expensive both in capital and operating costs. This study has evaluated the feasibility of using facilitated transport membrane technology for improving the separation process. Simulation and optimization of various distillation/membrane hybrid configurations as well as stand-alone membrane cascade systems have been examined under different operating conditions of temperature and pressure. A detailed analysis has determined the optimum sequence that minimizes the energy consumption of the C2 splitter while maximizing the profitability of the ethylene plant. This study has shown that the reduction in refrigeration requirement is the key to a successful hybrid system and that the series hybrid configuration is the optimum design, providing the maximum processing savings for a grass-roots ethylene plant. A retrofit design has also been investigated for an existing ethylene plant with the new membrane process. The parallel hybrid scheme, with a permeate pressure of about 50 psia, shows the ultimate design. For this parallel hybrid system, a net annual savings in processing cost of more than 0.86 million US dollars can be achieved to provide a payout period of about 19 months. INTRODUCTION The low-temperature distillation column for the separation of ethylene from ethane has been the preferred technology for several decades. However, this binary separation process consumes about 36 percent of the refrigeration energy required in the ethylene plant [1]. The C2 splitter is commonly operated at high-pressure, utilizing closed-cycle propylene refrigeration, which is incorporated with the refrigeration systems serving other parts of the plant. Facilitated transport membrane technology (FTMT) is a less established separation technique. It has been demonstrated in laboratories and pilot plants for the selective separation of olefins from alkanes. This technology has shown significantly high selectivity and flux rate of ethylene over ethane [2,3]. The desired objectives for any ethylene separation process are to obtain a high-purity ethylene product combined with a high percentage recovery of the ethylene. The conventional distillation technology can accomplish both of these objectives. However, the accompanying high-energy consumption of the refrigeration system makes the present distillation process costly. On the other hand, the FTMT is capable of producing a high-purity ethylene product, but with a lower percentage of recovery. The lower percentage of recovery dictates the use of a multi-stage system incorporating additional compressors. The combination of membrane and distillation technologies to form a hybrid system is another design alternative for replacing the current distillation technology. The hybrid system incorporates the high-purity product aspect of both technologies as well as the high percentage recovery of the distillation process. In the study reported here, various configurations of the membrane/distillation hybrid systems have been investigated for the ethane/ethylene separation. Among the many configurations studied were those wherein the membrane was located at the top, at the bottom, in parallel, and in series with the distillation column. Stand-alone