Energy integration study of a multi-effect evaporator
In the pulp and paper industry, multi-effect evaporators are used to evaporate water from black liquor solutions to allow its recycle as chemicals and fuel for the process. The thermodynamic principle of the multi-effect evaporator consists in a serie of reboilers operating at different pressures; the water evaporated at one stage is condensed and used as the heat source for another stage. Due to its strong integration with the process, it is worth to analyse the integration of the multi-effect evaporator with the rest of the process. To do so a thermo-economic analysis model of the evaporation system has been developped. The example is based on the evaporation system of a calcium bisulfite pulp manufacturing mill located in Switzerland. This system involves 3 multi-effect evaporators fed at different concentrations of black liquor. A systematic analysis of the system Grand composite curve has been developed to identify pertinent process modifications. From this analysis, several modifications like decreasing the deltaTmin of a stream, increasing or decreasing pressures of evaporation effects have been evaluated. For each of these measures, we have analysed the thermo-economic aspects, adapting the pinch analysis rules to account for the thermo-econmic benefit of integration. By these measures, the minimum energy requirement of the multi-effect evaporation system can be reduced by up to 20%. Resulting from the integration of the utility system, the related utility cost can be diminished of up to 23% from the base case model. The integration of heat pumping system and the utilities has then been analysed in order to reduce the exergy degradation in the energy conversion system. A thermoeconomic analysis including operating and investment cost estimation, evaluation of the Net Present Value and Payback Time of different process configurations corresponding to different energy savings scenarios has been performed. The integration of a heat pump system shows a reduction of the Net Present Value and an acceptable Payback Time compared to the base case model. A sensitivity analysis on electricity and natural gas prices has then been performed in order to better understand the economic condition of the integration.