(17) The energy consumption is only 30% of the vacuum distillation process. (9) Huntsman company used one-time suspension crystallization to increase the purity of 4,4′-MDI from 82 to >99%. (16) Therefore, compared with the static process, the dynamic process has a higher separation efficiency when the crystal growth rate is similar or even faster. (15) However, in the process of dynamic falling film crystallization and suspension crystallization, the crystal grows from the well-mixed melt caused by stirring or pumping, leading to high-quality transportation of the crystal surface and the impurities into the melt.
(9−14) In static melt crystallization, because of no mixing device, the melt growth is only caused by natural convection. Therefore, it is necessary to find a low-temperature, energy-saving, and environmentally friendly method for isocyanate separation to make up for the defects of rectification separation.Īccordingly, the melt crystallization method, which includes static melt crystallization, dynamic falling film melt crystallization, and suspension crystallization, has the advantages of high product purity, energy saving, and low requirements in equipment, (8) was especially suitable for the separation of the heat-sensitive mixtures with similar boiling points. What is more, 4,4′-MDI and 2,4′-MDI are heat-sensitive materials that self-polymerization occurs easily at high temperature, causing isocyanate loss. (6) Although rectification separation had achieved industrialization, it has the disadvantages of high energy consumption, (7) high cost, and high requirements in equipment.
also separated 95% 4,4′-MDI from the mixture of 4,4′-MDI, 2,2′-MDI, and 2,4′-MDI using the distillation separation technology under the conditions of 0.1–50 Mbar and 210–225 ☌, and results showed that the impurity content in 4,4′-MDI was below 50 ppm. Under the pressure range of 2–50 Mbar and temperature range of 150–250 ☌, 4,4′-MDI with a purity of 98.7% was obtained. used the distillation separation technology to separate 85.1% 4,4′-MDI from a mixture of 4,4′-MDI, 2,2′-MDI, and 2,4′-MDI. At present, rectification separation is mostly used in industry. It was also observed that compared with 2,4′-MDI, 4,4′-MDI molecules on the (110) crystal surface were easier to form crystal structures.Īpparently, the separation of 4,4′-MDI and 2,4′-MDI is not easy due to their high similarities in chemical structure, such as boiling point, polarities, and so on. Finally, the attachment energy (AE) model was used to observe and compare different crystal surfaces it was proposed that the aggregation of 4,4′-MDI was attributed to the polar attraction between isocyanate groups according to the results of the orientation correlation function. Furthermore, the separation phenomenon and the formation of the crystal structure were observed according to the radial distribution function (RDF) and orientation correlation function obtained from MD simulation. Results showed that compared with 2,4′-MDI, 4,4′-MDI molecules could form a more stable and symmetrical crystal structure due to their stronger charge density symmetry and electrostatic potential energy. The separation mechanism was simulated by density functional theory (DFT) and molecular dynamics (MD) simulation. In this work, the isomer mixture of 4,4′-diphenylmethane diisocyanate (MDI) and 2,4′-MDI was separated and purified by dynamic falling film melt crystallization, and 99.3% purity and 50.8% yield of 4,4′-MDI could be obtained under optimized conditions.
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