Abstract
A novel synthesis approach is developed to evaluate and optimise various hybrid membrane-distillation schemes. The problem is formulated as an optimisation problem, where the objective is to minimise the total operating cost of the separation scheme. Interactions between the separation and refrigeration systems are considered and opportunities for heat integration are exploited. A case study, ethylene/ethane separation, is presented to demonstrate the synthesis approach. Schemes that apply a membrane in parallel to and in series with a distillation column are explored.
Shortcut models that account for multiple feeds and products are used to represent the distillation column. The distillation model predictions are shown to be in good agreement with results of more rigorous simulations carried out using HYSYS. Established membrane models (Shindo et al., 1985) are applied and shown to be valid for the system of interest. A systematic approach is developed to account for heat recovery between: i) column feeds and products; ii) the membrane feed and products and iii) the associated refrigeration system.
The optimisation results reveal that a facilitated transport membrane reported in the literature (Pinnau and Toy, 2001) used in parallel with a distillation column can reduce the condenser duty by about 33 %, compared to a conventional distillation column operating at the same pressure (20 bar). However, recompression and sub-ambient cooling are required for the permeate stream, incurring operating costs. The total operating cost of the heat-integrated parallel hybrid scheme may be reduced by 11 %, compared to a conventional distillation column.
