Control Of Crystal Size Distribution In The Crystallization Process Of γ-Aminobutyric Acid

With the rapid development of food,chemical and pharmaceutical industries,crystallization technology is not only a separation and purification process,but also a key part affecting downstream unit operation and quality of products.Crystal size distribution has received extensive attention because of its influence on the crystal performance.Crystal size distribution is closely related to the crystallization mode.What’s more,it is susceptible to the operation conditions of crystallization process,such as temperature and the flow rate of anti-solvent.Therefore,it is the key to realize the control of crystal size distribution in crystallization process through choosing the right crystallization mode,controlling the temperature and the flow rate of anti-solvent.In view of the selection of crystallization mode,the crystal size distribution model of γ-aminobutyric acid crystallization process was established,while in order to acquire the optimal control curves in crystallization process,the control method was studied based on the model.First,a crystal size distribution model of y-aminobutyric acid was established to simulate the crystallization process.The model consists of solubility equation,nucleation rate equation,growth rate equation,crystal balance equation and mass balance equation.For y-aminobutyric acid solubility equation,based on the study of the thermodynamic properties,taking into account the effects of temperature and ethanol content,the empirical equation of temperature and ethanol content was established.For the numerical simulation of crystallization process,the crystal size distribution and yield of crystal were studied under the way of cooling,anti-solvent and cooling combined with anti-solvent.The experimental results showed that the model and simulation method can reflect real crystallization process well.Furthermore,mean crystal size and yield of crystal could be improved by cooling combined with anti-solvent crystallization.Secondly,in view of acquiring optimal control curves of temperature and flow rate of ethanol to realize online control of crystal size distribution,an optimal control model for crystal size distribution of γ-aminobutyric acid crystallization process was established.In this model,the mean size of crystal was used as an index to evaluate crystal size distribution,while the process dynamic model and operating conditions constraints were also considered comprehensively.Based on the optimal control model,the problem was transformed into a nonlinear programming problem through control vector parameterization.And then optimal control curves of temperature and flow rate of ethanol were obtained by using sequential quadratic programming.Based on these two control curves,the on-line control strategy of crystal size distribution was discussed,the results show that mean size and yield of crystal were improved compared with the off-line control strategy.Finally,according to the systems engineering method,a crystallization process control system was designed,including a real-time control system and a host computer monitoring system.The real-time control system is used to control temperature and ethanol flow rate in crystallization process,while the host computer monitoring system is used to calculate and display the process parameters.