A problem for optimal plant retrofitting, aiming to minimize the acrylonitrile released in the atmosphere from a Gas Treatment Department at Acrylic Fibre Plant, is considered. The applied retrofit policy involves (i) substitution of a used solvent with an appropriate selected mixture; (ii) flowsheet superstructure definition; and (iii) process parameters and mixture composition determination, so as the environmental regulations at minimum total retrofit cost to be reached. The pointed steps are described and the results obtained are discussed.