A Study On The Melt Crystallization Process Of TDI

Tolylene Diisocyanate (TDI) is an important raw material for manufacture of polyurethane. There are mainly two kind isomers of TDI, namely, 2,4-TDI and 2,6-TDI. Based on the isomer ratio of 2,4-TDI in the isomer mixture, there are three products of TDI in the market, TDI-65, TDI-80, TDI-100. The price of TDI-100 is the highest. In this paper, the melt crystallization process of TDI-100 was systemically studied.Firstly, the differential scanning calorimetry (DSC) was used to measure the solid-liquid phase diagram of TDI system, which is appeared as the eutectic one according to the experimental results. The UNIFAC equation was applied to simulate the solid-liquid equilibria of TDI system. Some new groups were defined and the UNIFAC group-interaction parameters were evaluated. Compared with other activity-coefficient equations (the Margules, Wilson and NRTL equations), the UNIFAC equation is the best model for simulating the solid-liquid equilibria of TDI system according to the comparison of the calculated and experimental temperatures. The mathematical model of heat conduction with a moving boundary was applied to simulate the melt crystallization process which carried out outside a cylindrical crystallizer. The model used by Guardani was improved combined with the effects of the temperature of the cooling fluid on that of solid-liquid interface. The analytic iteration method was applied to solve the model. Under the condition of interest, the improved model can describe the melt crystallization process of TDI based on the comparison between the calculated and experimental results.Based on the above fundamental researches, the melt crystallization process for manufacturing TDI-100 was systemically investigated. The effects of the initial condition of crystallization, the cooling rate, the final crystallization temperature, the sweating temperature, the sweating rate and the time of the melt crystallization process on the yield and isomer ratio of TDI-100 were studied. Finally, an optimal melt crystallization technology for manufacturing TDI-100 was developed. By using of the new developed technology, the TDI-100 product with isomer ratio of more than 98.5% can be obtained from the TDI-80 raw material without adding any solvent. And the mother liquor produced by the melt crystallization process can be sold as TDI-65. Summarily, this new melt crystallization technology for producing TDI-100 has a goodforeground of industrial application.