The Investigation On Purification Process Of TDI By Zone Melting

oluene diisocyanate, also named TDI, is one of the important raw materials of polyurethane industry. And it’s widely used in the production of foam, plastics, coatings, elastomers and other chemical products. Because it’s easy to react with water, amines, alcohols, alkalis, a compound containing active hydrogen atoms, so TDI is also used as the most important intermediate in organic synthesis. At present, the industrial production of TDI products, there are three main specifications: TDI-65, TDI-80 and TDI-100. And our country has realized industrialization production of TDI-80 and TDI-65, but so far there is still no production process of TDI-100. Therefore, to explore a kind of energy saving, environmental protection of the TDI-100 production method is imperative.Under this background, the preparation of TDI-100 by using zone melting and using a purity of 80% toluene diisocyanate(i.e. TDI-80) as the raw material is proposed in this paper. The outstanding advantage of zone melting is that the purified material does not have any contact with other solvents or chemicals, and it can obtain a high purity product that is difficult to produce by traditional methods.From experimental investigation, the conditions for the analysis of gas chromatography were determined. The chromatography column was an AT. XE- 60 with the size 30 m ×0.25 mm ×0.25 m. And the chromatography column temperature was 150 ℃; the temperature of vaporizing chamber was 250 ℃. The hydrogen flame ionization detector was selected, and its temperature was 250 ℃. The pattern of dector was FID. The carrier gas was high purity nitrogen and its flow rate was 1.0 m L/min, and injection volume was 0.15 L.The raw material of TDI-80 was pre purificated to 90%~91% by suspension crystallization. The results indicated that the purity of the product without washing filter cake after three times suspension crystallization was 90.1%, and the overall yield was only 12.61%. After one time suspension crystallization, the product washing filter cake by TDI-100 was 90.25%, the yield of the product was 57.32%, and the effect of pre purification was better than three times suspension crystallization. The effective distribution coefficient of impurity 2, 6-TDI under different conditions was measured by experimental method. The greater the zone melting temperature difference, the smaller the zone melting rate, the effective distribution coefficient is more far away from 1. The effect of zone melting temperature, zone melting rate, molten zone size, molen zone moving direction and zone melting passing times on the product purity of 2, 4-TDI were investigated by zone melting. The results show that the optimum technological conditions are zone meltin rate of 10 mm/h, temperature difference of 50 ℃, molten zone of 15 mm, molten zone moving straight up, zone melting passing 6 times, the yield of the product with a purity of more than 98.5% is 60%.In order to better simulate impurity 2, 6-TDI distribution after multiple zone melting, the numercial model of purification process of TDI by zone melting was established. And it proved that the model can satisfy the experimental data well through comparison of experimental and simulation value. In theory, the optimization of zone melting process is carried out. The results show that zone melting passing times has a significant influence on the distribution of impurities. With the increasing of the zone melting passing times, the more and more impurities were enriched at the end of the sample bar, and the impurity concentration in the front end of the sample bar is gradually reduced. The impurity concentration distribution curves approached the limit impurity concentration curves gradually as zone melting passing times increased. When the molten zone size is 0.15 L, the zone melting rate is 10mm/h, the purity of 80% product can meet the requirements of TDI-100(98.5%) after zone melting passing 8 times. Molten zone size had also important influence on impurity profiles. During the zone melting process, when zong melting passing times were less, longer molten zone size was helpful to zone melting and when zong melting passing times were more, shorter molten zone size was beneficial to zone melting.