Abstract
A comprehensive study was conducted to elucidate the effect of operating conditions on the performance of a multi-effect vacuum membrane distillation pilot plant. A theoretical assessment of the energy and exergy efficiency of the process was achieved using a mathematical model based on heat and mass transfer, which was calibrated using experimental data obtained from the pilot plant. The pilot plant was a solar vacuum multi-effect membrane distillation (V-MEMD) module comprising five stages. It was found that a maximal permeate mass flux of 17.2 kg/m2·h, a recovery ratio of 47.6%, and a performance ratio of 5.38% may be achieved. The resulting gain output ratio (GOR) under these conditions was 5.05, which is comparable to previously reported values. Furthermore, the present work systematically evaluated not only the specific thermal energy consumption (STEC), but also the specific electrical energy consumption (SEEC), which has been generally neglected in previous studies. We show that STEC and SEEC may reach 166 kWh/m3 and 4.5 kWh/m3, respectively. We also observed that increasing the feed flow rate has a positive impact on the process performance, particularly when the feed temperature is higher than 65 °C. Under ideal operational conditions, the exergetic efficiency reached 21.1%, and the maximum fraction of exergy destruction was localized in the condenser compartment. Variation of the inlet hot and cold temperatures at a constant differential showed an interesting and variable impact on the performance indicators of the V-MEMD unit. The difference with the lowest inlet temperatures exhibited the most negative impact on the system performance.
Highlights
The demand for freshwater is becoming a central issue for several countries worldwide
The results of this study will permit a more straightforward evaluation of the behavior of vacuum multi-effect membrane distillation (V-MEMD) systems through key indicators defined in terms of variation of the permeate mass flux, recovery ratio, gained output ratio, performance ratio, specific thermal energy consumption, specific electrical energy consumption, specific thermal exergy consumption, and exergetic efficiency
Cold water temperatures higher than 38 ◦ C result in a significant deterioration in unit performance associated with an increase in the specific thermal/electrical energy consumption and a decrease of the gain output ratio (GOR), PR and permeate mass flux
Summary
The demand for freshwater is becoming a central issue for several countries worldwide. The optimum values of the system performance indicators such as the productivity (Vd ), permeate mass flux (J), recovery ratio (R), gain output ratio (GOR), specific thermal energy consumption (STEC), and specific electrical energy consumption for the V-MEMD unit reached 31.8 L/h, 12.2 kg/m2 ·h, 36.8%, 4.25, 150.9 kWh/m3 , and 6.3 kWh/m3 , respectively. It gathers useful information and data on V-MEMD unit performance parameters including water production, permeate mass flux, recovery ratio, gain output ratio, specific electrical energy consumption, and specific thermal energy consumption. The results of this study will permit a more straightforward evaluation of the behavior of V-MEMD systems through key indicators defined in terms of variation of the permeate mass flux, recovery ratio, gained output ratio, performance ratio, specific thermal energy consumption, specific electrical energy consumption, specific thermal exergy consumption, and exergetic efficiency. This work emphasizes the effects of various input parameters on the performance indicators, illuminating the favorable operation domain for the V-MEMD system
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