Reagent purity in relation to MDI/PMDI manufacture

摘要

The stepwise conversion of benzene to MDI/PMDI, via nitrobenzene, aniline and MDA/PMDA intermediates is well-known and operated commercially at very large scale. Extensive patent and other literature in the public domain teaches many aspects of the relevant technology. Still further improvements in the industrial plants can be achieved by, amongst other things, recognizing and mitigating against the important effects of relatively minor amounts of impurities in the reagents, catalysts and other chemicals used at each of the production stages. It is to be understood of course that the scientific and technical aspects (causes and effects) may be well known but that the level or impact of such effects may change with time such that new problems can occur or be defined where the solutions are merely extensions of existing knowledge. This is also true for new impurities generated within the production processes themselves. Thus, for example, trace metal impurities in the nitric or sulfuric acids used in nitrobenzene production can have deleterious effects on the operation of that process, as can trace metals in the hydrochloric acid or formalin used in the MDA process. So-called "N-methyl" groups (＞N-CH_3 functional groups), formed in MDA/PMDA by locally high concentrations of formaldehyde-type groups, lead to chlorinated impurities in MDI/PMDI and subsequently derived products and can be reduced in amount by production control (reaction temperature, formalin addition rate, reagent mixing energy).