Floating Pressure Control of Vapor Recompression Distillation in Propane-propylene Separation

Abstract

The separation of propylene from propane is an energy-intensive distillation process. Vapor recompression is commonly used for the separation of propylene and propane. Most studies of vapor recompression were carried out at a given pressure. It is well known that the lower the pressure, the higher the volatility difference and less energy is required to perform the separation. In a traditional column, energy of the distillation can be minimized by operating at the lowest pressure possible. The limit is usually determined by the maximum cooling capacity in the condenser, which is in turn determined by the temperature of the cooling water. Such a practice is known as floating pressure control. In a vapor-recompression column, the condenser and reboiler were replaced by a heat exchanger. Auxiliary condenser and reboiler may or may not be present. The operating constraint is determined by the anti-surge control of the compressor. Furthermore, it is necessary to ensure that the compressed vapor which acts as steam in the reboiler is not substantially subcooled to avoid the vibration of the heat exchanger. In this study, the implementation of floating pressure control for a vapor-recompression propane-propylene column with an auxiliary condenser is studied using ASPEN Plus dynamics. A control scheme that includes basic inventory control and quality control was proposed. To keep the operation within safe region of the compressor surge curve, the split of the compressed vapor going to the bottom as heating medium and passing through the auxiliary condenser is adjusted. It is shown that the column can be operated under product purity requirements of top and bottom when column pressure is reduced by 1 kg/cm2.