Assessing the Impact of Operating Conditions on the Energy and Exergy Efficiency for Multi-Effect Vacuum Membrane Distillation Systems

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.

Performance Comparison of Cross- and Forward-Flow Configurations for Multiple-Effect Vacuum Membrane Distillation

“…Two different configurations of the V-MEMD unit were used in this study; the main difference between them is the way the feed water flows through the cascade effects. Forward-flow configuration (FFC) in experimental works was investigated [ 10 , 19 ] and theoretically studied by Najib et al [ 24 ]. With regard to this configuration, the feed water flows through cascade effects by moving from one effect to the next effect, where circulating water loses a portion of its mass and heat and ends up in brine water ( B 1), as shown in Figure 3 .…”

“…These control volumes are described in Figure 5 . Indeed, the conservation equations for each control volume were solved simultaneously by successive iterative procedures until overall convergence occurred, as explained by Najib et al [ 24 ].…”

“…This paper aims to enhance the performance of the multiple effects of vacuum membrane distillation (V-MEMD) by applying the CFC. This will be investigated theoretically by using a mathematical model developed by Najib et al [ 24 ], which in turn evaluates several key performance indicators (KPI). The results are expressed in terms of variation of productivity, recovery ratio, gain output ratio, specific thermal energy consumption, specific electrical energy consumption, specific thermal exergy consumption, exergetic efficiency, and exergy destruction in the V-MEMD system.…”

Advances in Membrane Distillation Module Configurations

“…Recently developed innovative MD configurations consider latent heat recovery and mitigation of TP phenomenon to enhance energy efficiency. Najib et al studied the energy and exergy efficiency of the V-MEMD system [124]. They calculated the specific thermal energy consumption (STEC) and specific electrical energy consumption (SEEC) of the V-MEMD system as 166 kWh/m 3 and 4.5 kWh/m 3 , respectively.…”

Experimental Study of a Heat Pump for Simultaneous Cooling and Desalination by Membrane Distillation

“…The specific electrical energy consumption (SEEC), which reports the amount of electrical energy required to produce one cubic meter of distillate water, is often neglected in studies examining MD systems [51]. For the coupling of a heat pump and a membrane distillation unit, it is important to study the SEEC.…”