Performance analysis of cascaded membrane distillation arrangements for desalination of brackish water

摘要

A previously validated model for direct contact membrane distillation (DCMD) was used to analyze the performance of a process for desalinating geothermal saline water. The water recovery rate for a single DCMD unit is extremely low. Using several MD units arranged in series and array patterns increases pure water production. Another advantage of the cascade structure is that most of the geothermal energy associated with brackish water is utilized. The number of units that can be used in a series pattern is determined by the temperature difference between the exit brine and inlet permeate streams of an MD unit. The number of stages that can be used in an array pattern is determined by the temperature difference between the exit permeate and brine streams through out the stage. Simulations indicated that 51% water recovery can be achieved when 40 MD units are used in an array pattern. The analysis revealed a16% increase in water recovery when the feed salinity is reduced from 3.7% to 0.2%. In addition, the gained output ratio can reach a value of 9 when the entire exit permeate stream is recycled to the MD unit as a feed stream at the expense of additional waste of heat.