Abstract
The coupling of heat and mass transfer in membrane distillation (MD) process makes enhancing water vapor flux and determining MD membrane mass transfer coefficient (MTC) fairly challenging due to the development of temperature gradient near the membrane surface, referred to as temperature polarization (TP). As a result, the change in feed temperature at the membrane surface will be difficult to measure accurately. In this paper, the effect of TP was decoupled from the membrane MTC by preventing the liquid feed stream from contacting the membrane surface through the use of a novel custom-made vacuum MD (VMD) module design. Results showed that a temperature difference of 10 °C between the feed bulk and feed temperatures at the membrane surface/interface is estimated to take place in the typical VMD configuration, while the proposed flashed-feed VMD configuration eliminates TP effect and gives a flux 3.5-fold higher (200 kg/m2 h) under similar operating conditions. Therefore, it can be concluded that heat transfer coefficient is considered to be the main factor controlling resistance of water vapor flux in the typical VMD configuration. The measured MTC of the tested commercial membrane was found to be more accurate and the highest among all reported MTCs in the MD literature (2.44 × 10−6 kg/m2 s Pa). Additionally, a transmembrane temperature difference of 5 °C and 10 °C in the novel configuration can produce water vapor fluxes of about 9 kg/m2 h and 40 kg/m2 h, respectively, at a feed temperature of 70 °C, which is very attractive for scaling-up the process.
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•Flashed-feed VMD is proposed as a novel MD configuration.•It consist of preventing the liquid feed stream from contacting the membrane surface.•The effect of TP is eliminated by decoupling the heat and mass transfer.•A high flux can be obtained at small transmembrane temperature difference.•TP can reduce feed temperature by as much as 10 °C.