Study On The Process And Design Method Of Multicomponent Batch Distillation

Although batch distillation has been dealt with more than 100 years, the design methods developed to date have some disadvantages, most of which are derived from FUG method for continuous distillation or from experiences under the assumption of no holdup and no slop cuts. The reason is that the laws of batch distillation are not understood clearly. In this paper batch distillation was studied thoroughly. For total reflux operation, the concept of degree of approaching equilibrium was used. On the basis of study on appropriate range of degree of approaching equilibrium the method of determining the number of theoretical plates was put forward. In product withdrawal period, the effects of parameters on reflux ratio were correlated by the degree of difficulty of separation (q) for binary and ternary systems. The results showed that the appropriate range of q is 0.4~0.6 for a separation task. In slop cut withdrawal period, the method of using residue composition as stopping criterion was developed, which was compared with the distillate stopping criterion by simulation. It can be concluded that the fractional recovery of heavy key component is improved and the distillation time is greatly shortened by using residue stopping criterion. Thus, the new operation mode of draining column holdup at the end of slop cut withdrawal was developed. And the effects of parameters, on reflux ratio for slop cut withdrawal period were simulated by constant holdup model under the objective function FJ for binary and ternary systems. The role of column holdup on the performance of batch distillation was studied systematically. And it was found that that R increases with plate holdup (UI) for q>0.7, but may drops when UI increases among a certain range for q0.7. It was also found that R increases with accumulator and condenser holdup (). Further studies on the dynamic role of column holdup showed that the ratio of column holdup to reboiler initial charge () acts as a factor. So an optimum reboiler initial charge exists for a column when q0.7,which was checked and proved by experiments with toluene-ethyl benzene system. The study on the effects of parameters on optimum was done for binary systems using maximum distillate as objective function. And the method of determining optimum was built. Design of batch distillation was the focus of the paper. The method of determining minimum reflux ratio using the pinch zone theory was built, which got rid of the disadvantages of Underwood equation. Two efficient design methods, the F-U-G method modified by constant holdup model and the short-cut design method incorporated with holdup, were developed. The methods took slop cut withdrawal and column holdup into account, and not only can complete the design tasks perfectly, but were used easily.The emerging batch distillation columns were also studied. Constant holdup model was built for stripping batch distillation columns. The relationship between the stripping batch distillation columns and the regular one was discovered. And F-U-G design method modified by constant holdup model was set forth, which provided a perfect and easy way to complete the design tasks. Two models, short cut model on the basis of degree of approaching equilibrium and constant holdup model, were built for multi-effect batch distillation columns (MEBAD), whose design method was developed on the basis of studies on total reflux.