MODELING AND SIMULATION OF SEVERAL HYBRID DISTILLATION SYSTEMS FOR THE ETHANE/ ETHYLENE SEPARATION

The separation of ethylene from ethane in a commercial process is quite expensive both in capital and operating costs. This study evaluates the feasibility of using facilitated transport membrane technology for improving the separation process. Simulation and optimization of various hybrid distillation/membrane configurations have been examined under different operating conditions of temperature and pressure. This analysis determines the best sequence that minimizes the energy consumption of the C2 splitter while maximizing profitability of the ethylene plant. A number of hybrid system configurations provide significant cost savings. Several of these hybrid configurations are illustrated in this study. The low-temperature distillation process for the separation of ethylene from ethane has been the preferred technology for several decades. This process consumes about 40 percent of the refrigeration energy required in the ethylene plant. The ethylene/ethane 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. Therefore, any reduction in the refrigeration load will affect not only the economics in the propylene closed-cycle but also the entire ethylene plant. Facilitated transport (FT) membrane technology is a less established separation technique. It has been demonstrated in the laboratory for the selective separation of ethylene from ethane. However, the economic effect of this technology when applied to a commercial system has not been analyzed and reported. The only attempts to date have been experimental studies. This study evaluates the economics of using this technology for the ethane/ ethylene separation. The desired objectives for this 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 objectives. However, the accompanying high-energy consumption of the refrigerant makes this technology costly. Facilitated transport membrane technology also produces a high purity product, but with a lower percentage of recovery. This lower percentage of recovery dictates the use of a multi-stage system incorporating additional compressors with a high power requirement. The hybrid technology, on the other hand, incorporates the high purity product aspect of both technologies, maximizing the distillation column’s high percentage recovery while minimizing the overall energy consumption of the system, and thus provides the potential for significant economic advantages. The reduction in the energy consumption of the C2 splitter is economically more important to this part of the process than the cost of the distillation column itself, since reducing the cost of the distillation column will only have a marginal effect on the overall process. However, reducing the energy consumption at this stage directly affects many of the other unit operations in the process which will further reduce the overall fuel consumption of the plant. In this study, various configurations of the membrane/distillation hybrid systems have been investigated. Among those configurations were those wherein the membrane was located at the top, at the bottom, in parallel, and in series with the distillation column. For each hybrid system, the optimum design is based on an economic comparison of the overall savings achieved in the total processing costs.