Second law analysis of multieffect and multistage flash desalination plants

Abstract

Although the first law of thermodynamics indicates the overall performance of the desalting plants, it does not show the actual losses from each component (i.e. process) of these plants. These can be revealed by an analysis where the second law of thermodynamics is applied to the multieffect, once through multistage flash, and recirculation multistage flash desalting plants.The exergy losses from the different processes in each plant are determined for each stage or effect. The proceses of major exergy are the vapor condensation (condenser exergy loss), vapor flashing (flashing exergy loss) and feed heating in the brine heater (heating exergy loss). The other losses such as heat transfer through walls, pressure drop through demister and friction flow in pipes, etc. are of minor magnitudes. For each plant, four parameters are interrelated (i.e. number of stages or effects N, gain ratio D/S, top brine temperature TBT, and heat transfer area At). Now, as two parameters are kept constant, the exergy losses are plotted against the third or the fourth parameter.Finallly a comparison between the exergy losses is conducted for the three types of plants and for the same gain ratio. The major specific exergy losses are compared for the three plants. It is found that the multieffect plant is the least exergy wasting plant.