Formulation of Multi-objective-Optimization Problem

There are two major costs that play an important role in the feasibility of retrofitting a distillation column to HMD. One of them is the capital cost of the membrane module along with compressor, exchangers and associated installation, piping and labor costs (that is, cost of retrofitting). The other is the utility cost, which includes condenser and reboiler 

Case Membrane feed flow (kmol/h) Ethylene permeance (kmol/h-m - bar) Membrane area (m ) Pressure difference across membrane (bar) Utility cost savings ( /yr) Annualized capital cost of retrofitting ( /yr) 

In order to minimize utility costs, design variables of the membrane unit can be chosen to give better separation. For example, a larger membrane area will lead to more separation for a given feed flowrate and permeate pressure. This may lead to a reduction in condenser and reboiler duties of the associated column. While there is a decrease in utility cost of the column, there is a corresponding increase in the capital cost of the membrane. This results 

The MOO is performed for maximizing utility cost savings and minimizing capital cost (that is, cost of retrofitting) simultaneously for retrofitting EF (Case 1 assuming reboiler duty as credit), EF (Case 2 assuming reboiler duty as cost), and PF (Case 3), with a 

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