Crystal-Distillation Hybrid To Separate MDI Isomeric

Disocyanatodiphenylmethane ( MDI ) is an important raw material for manufacture of polyurethane.there are mainly three isomers of MDI,namely,2,4'-MDI, 4,4'-MDI and 2,2'-MDI. Their freezing points and boiling points are nearly.Based on the isomer ratio of 2,4'-MDI in the isomer mixture, there are two products of MDI in the market, MDI-100,MDI-50.they all are very important.Distillation are mostly applied to separate the mixture of MDI in industry.there also are some crystallization methods being used to separate the mixture of MDI in patent.The process has more superiority and economic feasibility in coupling distillation and crystallization than using only one of them.Crystallization of the mixture of 2,4'-MDI and 4,4'-MDI were studied in different concentration .The effects of the operation parameters crystallization time and final temperature were investigated on crystallization experimentation.The experiment result indicated that with increasse of the isomer ratio of 2,4'-MDI in the isomer mixture,the freezing points will decresse.when crystallization time prolong,crystallization freezing ratio will increase and concentration of mother liquid will decrease.with increasing of the final temperature,the crysatllization rate will decrease and the concentration of the production will increase.It was also found that the process of MDI crystalllization will be on conditon that crystal is added on right temperature,and pure product will be obtained by four-stage crystallization when the concentration of 4,4'-MDI is 92.1%.The process of crystallization and distillation were simulated and calculated .and The effects of the crystallization and distillation were investigated. based on the virtue of them, A set of technical process of crystallization- distillation coupling to separation MDI isomeric is proposed.the analyse result indicated that the concentration of crystal production of 4,4-MDI can be improved to 99% in the new process, and the energy consumed of distillation in the new process just is 1/2-1/3 of that of traditonal distillation process.Three set of technical process of crystallization-distillation coupling to separation MDI isomeric is proposed. The effects of the circulare flux ,reflux ratio and energy expened were simulated and analysis ,the result indicated the circulare flux and the energy expened of the first crystallization-distillation coupling process is the hightest; the circulare flux of the second crystallization-distillation coupling process is the least; the energy expened of the three crystallization-distillation coupling process is the least and saving two crysllization equipment. So the three crystallization-distillation coupling process is the best choice.