Life cycle assessment of N-methyl-2-pyrrolidone reduction strategies in the manufacture of resin precursors

Abstract

A case study has been conducted on the reduction of N-methyl-2-pyrrolidone (NMP) solvent waste in the manufacture of polyimide and polybenzoxazole precursors. The evaluation includes the environmental and economic life cycle assessment of solvent recovery and solvent substitution strategies. A two-step distillation process proved effective in recovering 95 % of the NMP at a purity of 99.97 % from an aqueous waste stream comprised 17 % NMP, 0.5 % hydroxyethyl methacrylate, 0.5 % trifluoroacetic acid, and 0.5 % hydrochloric acid. Yearly operating costs were reduced by 83 %, with the greatest impact on the reduction in virgin NMP purchase and hazardous waste disposal cost. Even if a capital acquisition was needed, the recovery option would still result in a net present value at 10 years of 3.12 MM $US. The environmental life cycle assessment (LCA) showed that a 44 % reduction of total emissions is possible with the solvent recovery process, impacting the virgin NMP and hazardous waste disposal life cycle emissions the most. The efficiency in the reduction in life cycle emissions is limited by the thermodynamics of the system, in particular the large composition of water in the waste stream which requires significant energy to distill, thus generating significant life cycle emissions. Solvent substitutes dimethyl sulfoxide and sulfolane reduce life cycle emissions by 44 and 47 %, respectively, when they replace NMP in the process, even without a recovery operation, due to their greener manufacturing profile. Although, when the recovery systems for the solvent substitutes are incorporated into the design, no further reductions in the environmental impact are seen. This demonstrates the need for a complete analysis of all the aspects of a greener design (including the recovery step), since the thermodynamic characteristics of the solvents are important when performing an LCA. Water reuse was also considered for the overall process, but not recommended due to the cost of recovering it from the waste stream to ultrapure water standards.