OPTIMIZATION OF MASS EXCHANGE NETWORK BY MINLP

Abstract

Mass Exchange Network (MEN) is a systematic generation of mass exchangers to transfer certain components from rich stream(s) to lean stream(s). It can be counter-current direct current mass transfer operation which utilizes a number of MSAs. The use of MEN has been growing steadily for diverse applications like ,feed separation, product separation , product finishing and recovery of valuable materials. Recently MEN has been widely used in waste minimisation. Some very popular examples where MENs are extensively deployed includes removal of copper from Ammoniacal etching solution and a rinse water stream, removal of hydrogen disulphide from two rich streams: coke oven gas and tail gas from Claus unit, removal/recovery of phenols from waste streams of coal conversion plant, carbon dioxide removal in a formaldehyde plant. Broadly MENs can be used in two ways .first type include design of a new plant which is in design stage and second using it retrofit an existing plant for the purpose of improving its efficiency. These problems are intensive and require a rigorous and specialised approach for its solution. The synthesis of MEN networks can be broadly classified into two categories .In the first category design targets are achieved first and then a feasible MEN structure is drawn meeting all the criteria defined .The most prominent among this approach is pinch method where prior knowledge of structure is not required. The second method first draws all possible structure and then applies various mathematical tools to arrive at most optimum mass exchange networks. Pinch is a thermodynamic and heuristic based method which does not require prior knowledge of networks. The concept of pinch based design was given El.- Halwagy et al in 1989. The design starts at the pinch division which is also the most constrained part and moves away from it