Guidelines for Selecting Vacuum Systems in the MDI Separation Process

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Guide to Vacuum System Selection for the MDI Separation Process In the production of MDI (diphenylmethane diisocyanate), the task of the separation process is to refine the crude MDI obtained after the phosgenation reaction into pure MDI (MDI-100) and polymeric MDI (PM-200). Due to MDI’s high boiling point and extreme heat sensitivity (it tends to undergo self-polymerization, coking, or degradation at high temperatures), the separation process must be carried out under high vacuum and low temperature. Therefore, the selection of the vacuum system is key to ensuring product quality and the long-term operation of the equipment. 1. Why is a high vacuum required for MDI separation? Lowering the boiling point: Under normal pressure, the boiling point of MDI is extremely high, far exceeding its thermal decomposition temperature. By reducing the system pressure to extremely low levels (with the back pressure usually being 0.1–1.0 kPa or even lower), the separation process can be carried out at temperatures between 150°C and 200°C, thereby preventing the product from deteriorating. Preventing color darkening: MDI is extremely sensitive to oxygen and high temperatures; a high-vacuum environment effectively isolates it from air, ensuring that the product remains transparent in color. 2. Equipment selection scheme for vacuum systems: In large-scale MDI plants, a multi-stage pump combination system is typically employed to balance the pumping capacity and the ultimate vacuum level: (1) Main pumping pumps: Dry Screw Vacuum Pumps. Reasons for selection: They are the preferred choice for modern MDI plants. Compared to traditional water-ring pumps, dry pumps require no operating fluid in their chambers; thus, they produce no waste liquid pollution and do not cause moisture contamination to MDI products. Advantages: It can handle gases containing trace amounts of solvent (chlorobenzene) or residual HCl, and it has strong corrosion resistance. (2) Booster pumps: Roots pumps. Function: Serve as intermediate or foreline boosters. Rotary vane pumps have a very high pumping speed at low pressures, enabling them to rapidly increase the vacuum level of a system. Configuration: Typically, a “two-stage Roots pump + one-stage dry pump” or a “three-stage Roots pump combination” is used to achieve deep vacuum. 3. Key technical aspects in selection and design ① Material corrosion resistance (Material Selection): The exhaust gas obtained after separation may contain trace amounts of hydrogen chloride (HCl) and chlorobenzene solvents. Design requirements: The flow-through components of vacuum pumps (rotor, pump chamber) usually require special treatment, such as the use of nickel-based alloys, stainless steel lined with Teflon, or high-grade stainless steel, to prevent acidic corrosion. ② Sealing reliability: Once a vacuum system leaks and air enters, it can cause MDI to oxidize and local polymer formation, thereby blocking the column. Design requirements: A double-end mechanical seal shall be used, along with nitrogen injection for protection (nitrogen sealing), to ensure that the system remains in a state of absolute negative pressure enclosure. ③ For exhaust gas condensation and trapping, cold traps must be installed before the gases enter the vacuum pump system. Purpose: To condense the vast majority of MDI vapor and solvents in the exhaust gases, preventing them from entering the pump and causing scaling or damage to the rotor. ④ Safety Interlocks: The vacuum system must be interlocked with the heating system of the distillation tower. Logic: If the vacuum level experiences significant fluctuations or fails (due to power outages, pump failures, etc.), heating of the reboiler at the bottom of the tower must be stopped immediately to prevent MDI from undergoing intense self-polymerization reactions under high temperature and pressure, which could even lead to overpressure explosions. 4. Summary: Configuration trends of MDI vacuum systems; Component recommendations and key functions. Primary/secondary booster Roots pumps enhance pumping speed to achieve deep vacuum levels. The final stage uses dry screw pumps to remove non-condensable gases, ensuring an environmentally friendly and pollution-free operation. Pre-treatment heat exchangers and collection tanks are used to recover solvents, thereby protecting the pump system. The control system features variable frequency control and pressure interlocks to maintain stable operating pressures and ensure intrinsic safety