【3D Process Flow】Summary Post--MDI Separation Process-High Vacuum Distillation: The Core Technology for MDI Refining

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【3D Process Flow】Summary Post – Useful for understanding the process flow: https://bbs.hcbbs.com/thread-5711616-1-1.html ---------------------------------------------------------- Stage 1: Degassing and solvent recovery. The first key step is degassing and solvent recovery, with the aim of removing toxic and corrosive gases and reusing the solvents. Deaeration: The mixture first enters the Phosgene & HCl Stripper Column (a phosgene and hydrogen chloride stripping column that operates under vacuum and at high temperatures). Here, the vast majority of volatile HCl and phosgene is vaporized and sent to the Recovery Unit. Solvent recovery: The degassed bottom product (still containing a large amount of solvent) enters the subsequent MCB Stripper Column. This tower evaporates the MCB solvent with a lower boiling point under vacuum; after condensation, it is reused as Recovered MCB Solvent and circulated back to the phosgene gasification section. The product obtained at the bottom of the tower is Solvent-free Crude MDI, which contains almost no solvents; it includes monomeric MDI (4,4’-, 2,4’-, 2,2’-MDI) as well as polymeric MDI with higher molecular weights. Phase 2: Crude MDI separation (core separation). This is a fractional distillation column specifically designed for high-vacuum distillation. Due to the high boiling points (over 300°C) and strong thermal sensitivity of MDI isomers, conventional high-temperature distillation leads to severe decomposition and self-polymerization of the product. Therefore, it is crucial to lower the boiling point in an extremely high vacuum (usually only a few millibars). The separation tower performs rough separation based on differences in molecular weight and boiling point: The product at the bottom of the tower (Polymeric MDI - pMDI) consists of tricyclic and polycyclic MDI polymers with higher molecular weights (which have the highest boiling points), forming the residue at the bottom of the tower. This is the pMDI (polymerized MDI) stream, which is sent to the product storage tanks and is primarily used in the production of rigid polyurethane foams, such as insulation materials for refrigerators and building panels. Top product (Monomeric MDI Mixture): The dibicyclic MDI isomers with lower boiling points (4,4’-MDI and 2,4’-MDI) are evaporated, condensed, and discharged from the top of the tower as Monomeric MDI. This stream needs further, much more precise refinement. Stage 3: Separation of MDI monomer isomers (precision distillation) usually involves a series of distillation columns with a high number of theoretical plates (or packing height), and it also requires operation under extremely high vacuum and high reflux ratios. This is because the boiling points of the main isomers, 4,4’-MDI and 2,4’-MDI, are extremely close. Initial refining: The first Refining Column in this process serves to remove trace amounts of the isomers with the lowest boiling point (mainly 2,2’-MDI) as overhead distillates. Isomer fractionation: The key to separation is to isolate large amounts of 2,4’-MDI from 4,4’-MDI. o2,4’-MDI Rich Fraction: A fraction rich in the 2,4’-isomer is removed from the top or side stream of the tower. MDI of this grade is mainly used in specific coating and adhesive applications. oPure 4,4’-MDI: The final product is Pure 4,4’-MDI; a purity level of over 99.5% is generally required. This high-purity grade is discharged from the bottom of the tower and is essential for producing spandex fibers, high-performance cast elastomers, sealants, and thermoplastic polyurethanes (TPU). In summary, the separation and purification of MDI is not simply the removal of impurities ； It is a carefully designed series of processes aimed at recovering expensive reagents and maximizing the production of valuable products. By combining degassing, solvent recovery, rough separation and precise distillation, along with the integration of recycle streams, manufacturers can produce MDI in various grades, ranging from polymeric MDI used for insulation to ultra-pure monomer 4,4’-MDI used for advanced materials.