Mass Transfer Analysis of Air-Cooled Membrane Distillation Configuration for Desalination

Abstract

It has been proposed that the air-cooled configuration for air gap membrane distillation is an effective way to simplify the system design and energy source requirement. This offers potential for the practical applications of membrane distillation on an industrial scale. In this work, membrane distillation tests were performed using a typical water-cooled membrane distillation (WCMD) configuration and an air-cooled membrane distillation (ACMD) configuration with various condensing plates and operating conditions. To increase the permeate flux of an ACMD system, the condensing plate in the permeate side should transfer heat to the atmosphere more effectively, such as using a more thermally conductive plate, adding fins, or introducing forced convection air flow. Importantly, a practical mass transfer model was proposed to describe the ACMD performance in terms of permeate flux. This model can be simplified by introducing specific correction values to the mass transfer coefficient of a WCMD process under the same conditions. The two factors relate to the capacity (B) and the efficiency (σ), which can be considered as the characteristic factors of a membrane distillation (MD) system. The experimental results are consistent with the theoretical estimations based on this model, which can be used to describe the performance of an MD process.

Finned Tubular Air Gap Membrane Distillation

“…As shown in Figure 4 a, under the conditions of air cooling, the transmembrane flux of membrane distillation can reach a maximum of 2.5 kg/m 2 /h. Compared with the flux of water cooling, under the condition of air temperature at 24 ± 1 °C, the transmembrane flux can reach only 1/10 of the transmembrane flux under water cooling (cooling water temperature at 24 ± 1 °C), and the corresponding σ value is about 0.10, which is lower than the σ value of 0.45 in the forced convection air-cooled membrane distillation device constructed by Cong et.al [ 18 ]. This is because the air flow inside the condensation tube is almost zero for radiation heat transfer between the condensation wall surface and the large space, resulting in the σ value of the tubular membrane distillation being lower than that of the plate membrane distillation under the same conditions.…”

“…Performance conditions of the membrane distillation device, such as temperature and concentration of the hot feed solution, membrane, and air gap parameters, affect the mass transfer capability of the membrane distillation process. The cooling conditions on the cold side can be characterized by the efficiency coefficient ( σ ) of the membrane distillation process, which can be described by Equation (2) to show the performance of the membrane distillation process [ 18 , 29 ]: where B value is the total mass transfer coefficient including the mass transfer of water vapor in the feed side, the membrane pores, and the air gap. It is related to the configuration of membrane distillation module and the parameters of the membrane, with units of kg/m 2 /Pa/s.…”

“…When air cooling is used in membrane distillation, the temperature difference between air temperature and condensing plate temperature is large. The σ value is used to correct the temperature of the condensing plate so that the transmembrane flux of the membrane distillation module can be predicted directly by calculating the saturated vapor pressure from the air temperature [ 18 ]. P f and P c are the saturated vapor pressures on the feed side and the cooling side, respectively, with units of Pa.…”

“…Performance conditions of the membrane distillation device, such as temperature and concentration of the hot feed solution, membrane, and air gap parameters, affect the mass transfer capability of the membrane distillation process. The cooling conditions on the cold side can be characterized by the efficiency coefficient (σ) of the membrane distillation process, which can be described by Equation (2) to show the performance of the membrane distillation process [18,29]:…”

“…Compared with the conventional air gap membrane distillation configuration (water cooling) is an effective way to simplify the system design and offers a potential way for the practical applications of membrane distillation on an industrial scale. Using air cooling in plate and frame air gap membrane distillation, the effect of water cooling can be achieved by forcing convection and increasing the heat transfer area [ 17 , 18 ].…”