(686d) Novel Hybrid Process Solution for a Low Capital and Energy Efficient Continuous Operation MDI Splitting Process.

MDI is produced via the commercially proven phosgene route; this reaction produces a crude isocyanate product containing the three diisocyanate isomers (4,4’, 2,4’ and 2,2’), oligomeric polyisocyanates, various byproducts and impurities from the reaction step. The present market requires separate products or blends thereof to make commercially relevant polyurethanes with the properties required by the market. The 2,4 and 2,2 diisocyanate isomers are difficult to separate and since the 2,4 isomer is the main isomer after 4,4 we will generally refer to the mixture as containing only 4,4 and 2,4 where the 2,4 fraction is assumed to contain the 2,2 isomer fraction also. The MDI splitting plant is responsible for producing the commercially viable fractions from this crude feed. Existing splitting plants are relatively expensive and energy intensive; GEA has launched a novel hybrid process that will simplify the splitting step, lower capital costs, reduce energy requirements and improve the final product quality. The typical MDI fractions of commercial importance can be generally classified as:

1. Polymeric MDI (mixtures of oligomeric and 2,4 and 4,4 MDI),
2. Pure MDI (mostly 4,4 with some 2,4 MDI) and
3. Mixed MDI Isomers (a range of 2,4 and 4,4 MDI excluding oligomeric)

Traditionally, MDI splitting plants are set up as multi-step distillation plants to split out oligomeric polyisocyanates and separate the isomers to generate the required MDI fractions. The GEA hybrid process solution does not separate the three isomers by distillation and therefore offers low energy consumption, improved product quality and lower capital cost through the novel combination of (1) evaporation/distillation processes and (2) continuous suspension-based melt crystallization to produce the specific MDI fractions.

A general principle in MDI process design is to avoid water or steam as direct utility to avoid the risk of water ingress in the MDI process and to operate at reduced temperatures to minimize product degradation. The evaporation/distillation unit must operate under rather extreme vacuum (2-10 mBar) to minimize operating temperature and preserve product quality. Appropriate heat transfer fluids are used as intermediates that provide necessary heating and the GEA vacuum utility is designed with ejectors driven by monochlorobenzene (MCB) vapors, thus complying with the need to avoid any potential moisture inside the MDI process.

The first step in the splitting process is always the separation of the higher oligomeric fraction from a part of the three diisocyanate isomers. The only possible route is by evaporation/distillation. Commercial crude MDI feedstocks can vary in their specific composition, the amount of vapor discharged in the evaporation/distillation process provides basic control of the diisocyanates concentration in the Polymeric MDI bottoms product, this is needed to give the desired viscosity. The mixed diisocyanate isomer vapors are condensed via the top of the column. This condenser allows the rapid cooling of the diisocyanate vapor stream and minimizes liquid holdup, thus strongly reducing the formation of unwanted dimers. The concentration of the toxic phenyl isocyanates and other lights is reduced to below necessary limits by a stripping element or, when required, a low energy distillation step can be added.

The condensed diisocyanate mixed isomers stream (distillate) free of oligomers and light impurities is subsequently fed via a small buffer tank continuously to the crystallization step for final separation and purification of the MDI isomers by suspension-based melt crystallization.

The two-stage melt crystallization unit includes wash columns for the final solid/liquid separation and produces both the Pure 4,4 MDI and the Mixed isomer fractions (b & c mentioned above). The distillate is fed to the crystallization unit where the final product purity is simply controlled by the equilibrium temperature of the slurry produced in the crystallizer. The wash column efficiently separates the Pure MDI crystal from the 2,4 enriched liquid. The liquid filtrate can be discharged directly (with 35wt% 2,4 composition) or crystallized again at a lower temperature in a second cold stage crystallizer generating the so called 50/50 MDI consisting of about half 2,4 (including the 2,2 isomer) MDI and half 4,4 MDI. Since this is near the eutectic point of the mixture higher concentrations of 2,4 MDI are not possible with crystallization and if higher compositions are needed, this second crystallization step can be totally or partially replaced with distillation. Due to the high separation efficiency in the wash columns, there is no need for an intermediate distillation step to reach the desired composition of the Pure 4,4 MDI.

Flexible operation is key to the design; higher 2,4 MDI compositions can be realized depending on the crude feed composition. The crystallization capacity is based on a pure 4,4 MDI output and increased 2,4 MDI can easily be handled on the same equipment (filtrate flow is not limiting). The ratio of 2,4 MDI in the feed to 2,4 MDI in the Pure 4,4 MDI product is thus maximized resulting in a high overall rejection in the Pure 4,4’ MDI fraction of the close boilers that are not removed in the distillation process.

This novel hybrid configuration provides significant advantages over the conventional process; more than 50% energy reduction can be realized through the combination of optimized distillation with an efficient crystallization system and reduced vacuum load The product quality is improved because the lower operating temperatures result in less thermal product degradation, less generation of toxic light impurities and less dimer formation. The total process can be designed with fewer and smaller components and therefore reduces the required CAPEX.