Design and Control of a Modified Vinyl Acetate Monomer Process

Abstract

The vinyl acetate monomer (VAM) process presents several challenging design problems because of the many design optimization variables and several important constraints. The process features a cooled tubular reactor, both gas and liquid recycle streams, two absorbers, and two distillation columns, one involving heterogeneous azeotropic distillation. The major design variables are reactor temperature, reactor size, reactor pressure, ethylene gas recycle and acetic acid liquid recycle. The major constraints are a maximum oxygen flammability limit, a maximum internal reactor temperature limit, and the need to stay above dewpoint temperatures in the reactor. This paper develops an economic optimum flowsheet based on the conditions and parameters provided by M. L. Luyben and B. D. Tyreus over a decade ago as a challenge problem for the academic community for design and control studies (Comput. Chem. Eng. 1998, 22, 867–877). The conceptual design developed produces 18% more VAM product for the same oxygen fresh feed flow rate used in the original design. Compared to the original nonoptimized design, the modified design has a reactor that is twice as large with reactor inlet conditions showing lower oxygen concentrations and higher acetic acid concentrations. Operating pressure is lower, liquid acetic acid recycle is larger, and gas ethylene recycle is smaller. A plantwide control scheme that provides effective disturbance rejection and is significantly different than the structure used in the original design is developed.