Research On New Operation Mode Of Cyclic Total Reflux Batch Distillation

Cyclic total reflux operation, which has no reflux ratio, is a new batch distillation operation mode. It is characterized by repeating total reflux (TR) operation and total withdrawal (TW) operation, the two periods repeating until the end of the separation. The total reflux operation is generally used in establishing the concentration profile during the starting period. However, the total withdrawal operation is a special operation with rare researches. In this paper, the total withdrawal operation is studied.First of all, the total withdrawal operation after sufficient total reflux operation, which means the column reaches equilibrium, was studied according to simulation and experiments. The overhead concentration and the column's holdup concentration changes were primarily explored. According to simulation results and experimental data, the overhead concentration formed"overhead concentration platform"(OCP) phenomenon in the total withdrawal operation.Secondly, several influencing factors, such as feeding mixture, the column's theoretical tray number, tray holdup and feeding concentration, were studied theoretically and experimentally. Single factor experiments show that the inspected factors is positive correlation to the OCP time, while the feed concentration and theoretical tray number shows the greatest impact on that.Thirdly, the total withdrawal operation after insufficient total reflux operation, which the top concentration is bigger than the desired purity, was studied theoretically. The insufficient total reflux operation is classified according to the equilibrium degree. For a distillation column with N theoretical trays, N* is a constant relating to the minimum theoretical tray number. If the (N-N*)/2 tray holdup's concentration reaches the product's purity and then the infinite reflux operation is terminate, this will give the highest separation efficiency. During the total withdrawal operation, the temperature and concentration in the middle of the column changes most dramatic. Lastly, based on the OCP researches, a tri-temperature control method was proposed. The temperature at the tower's top, middle and upper area were measured. A temperature sensor was settled in the (N-N*)/2 tray to control the end of the infinite reflux operation; a theoretical method was proposed to determine the variation range of the middle temperature, and the column'middle temperature was used to end the total withdrawal operation; the top of the column was measured as the assistant temperature control point. The theoretical and experimental studies were made on the control method.The simulation results showed for the particular feeding concentrations, the cyclic total reflux operation without reflux drum shortened the operation time by 26.4%-30.1% than that under regular constant reflux operation in batch distillation. The tri-temperature control method was also compared with the top-middle temperature control method and the top temperature control method according to experiments. The experimental data showed that the tri-temperature is of higher separation efficiency, lower operation time, higher product purity and higher yield.