Dynamic Simulation And Control Of Distillation-vapor Permeation Hybrid Process On Ethylene-ethane System

For the relative volatility of ethylene/ethane is samll, it will need low temperature (25℃), high pressure (2.3MPa) and large reflux ratio to separate them using conventional distillation.. Moreover, the plate number and tower height are usually more than200and100m respectively. Therefore, the required equipment and operation costs are very high. Gas separation membrane technology has been developed rapidly in recent years as a new separation technology. It has a series of advantages, such as high efficiency, low energy consumption, adaptableness, easy operation and so on. For the ethylene/ethane separation, there is one potential pinch point at the feed plate of the distillation column. Close to the pinch point, the change in composition from tray to tray is small. This indicates that distillation is not very efficient in this composition region. But in the distillation-membrane hybrid process, a stream is led out at the pinch point from the distillation column, separated by membrane module, then the permeate gas and residue gas back to the rectifying and stripping sections separately. Membrane module undertakes a part of separation work, then the number of balance plate is reduced efficiently. For distillation column has been put into production, reflux ratio could be reduced. In addition, disturbances have different influences on product purity and recovery in the actual production. In order to ensure the product quality and system stability, process characteristics need to be investigated further to develop applicable control schemes combining actual conditions. Three different control schemes of distillation-vapor permeation hybrid process on ethylene/ethane system were put forward in this article. Then dynamic models were built on the base of the control schemes. At last the optimal scheme was confirmed with the comparison of parameters. The specific research content as flows.(1) Three different control schemes of distillation-vapor permeation hybrid process on ethylene/ethane system were put forward. Distillation column control requirements and main disturbance factors were analyzed at first. Based on the temperature control theory of rectifying section, overhead temperature control scheme was proposed on distillation-vapor permeation hybrid process. This control scheme could maintain the overhead product in high quality effectively. Based on the pressure control theory of distillation column and temperature control theory of stripping section, overhead pressure-bottom temperature control scheme was proposed on distillation-vapor permeation hybrid process. This control scheme could overcome the disturbance in stripping section and maintain the bottom product in high quality effectively. Base on the ratio control scheme, reflux ratio-reboil ratio control scheme were put forward. And corresponding integrated control scheme figures were given in the article.(2) Dynamic models of the distillation-vapor permeation hybrid process on ethylene/ethane system were built. The distillation-vapor permeation hybrid process steady model was built in UniSim Design with the literature data. Then UniSim Design simulation results and literature data were compared, and the error analysis was carried out. Finally it was confirmed that the distillation-vapor permeation hybrid process could be used for further research. Dynamic models were built in UniSim Design corresponding three control schemes. PID parameters of the models were present.(3) The comparison of distillation-vapor permeation hybrid process on ethylene/ethane system dynamic models. Disturbances were added to overhead temperature control dynamic model, overhead pressure-bottom temperature control dynamic model and reflux ratio-reboil ratio control dynamic model respectively. When the feed flow rate and heat value changed, parameters fluctuations of feed flow rate, overhead product composition, bottom product composition, overhead product flow rate, bottom product flow rate, reflux ratio, reboil ratio, condenser level, reboiler level, overhead temperature, overhead pressure and bottom temperature along with time were studied. Parameters fluctuation trend and stability of three dynamic models were compared when disturbances were added. Comparing these three dynamic models, the reflux ratio-reboil ratio control is proved to exhibit the best disturbance adaptability. Parameters stability is the best and stability time needed is the shortest for reflux ratio-reboil ratio control. However, the overhead temperature control scheme is easy to be popularized in practical application due to its simple construction and easy operation.