Utility cost savings

Cost of retrofit (US ) Retrofitting level Utility cost (US /year) Utility cost savings (US /year) Fhyback period (year)... 

Case Design/Decision variables Utility cost savings ( /yr) Annualized capital cost of retrofitting ( /yr)... 

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

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... 

Maximize Utility Cost Savings = OPEX yb OPEX,... 

Figure 10.9 Non-dominated solutions for maximization of utility cost savings and minimization of capital cost for retrofitting EF to an HMD system, considering reboiler duty as utility credit (plot a) optimal values of decision variables corresponding to the Pareto-optimal front are in plots b-e.

The optimal value of membrane feed flowrate (Figure 10.10b) is initially near the lower bound before increasing non-linearly for increasing utility cost savings. The optimal values of membrane area and membrane permeance (Figure 10.10c and lO.lOd) follow the trend observed in Case 1. The optimal values of pressure difference across the membrane... 

The extreme point of the Pareto-optimal point before the spike ( 4.55 Million/yr utility cost savings at 0,462 Million/yr capital cost) is the best choice for rettofitting the PF to an HMD system. This optimal solution corresponds to a membrane feed flowrate of 300 kmol/h, membrane area of 1497 m, membrane permeance of 0.0149 kgmol/m -h-bar, and pressure difference across the membrane of 1560 kPa. It gives a net savings of 4.16 Million/yr (that is, 67% savings on the current utility cost), which is very attfactive with a payback period of just about one year. 

The optimal values of pressure difference across the membrane are scattered near the lower bound with a spike at the end (Figure 10.1 le). The spike is due to the rest of the decision variables reaching their respective upper limit. Hence, to further increase the utility cost savings, the pressure difference across the membrane has to be increased to allow more components to be separated by the membrane. However, it can be observed that varying the pressure difference across the membrane has a weak effect on utility cost savings compared with other decision variables such as membrane area. Increasing the pressure difference... 

---