Abstract
Vacuum membrane distillation (VMD) is an attractive variant of the novel membrane distillation process, which is promising for various separations, including water desalination and bioethanol recovery through fermentation of agro-industrial by-products. This publication is part of an effort to develop a capillary membrane module for various applications, as well as a model that would facilitate VMD process design. Experiments were conducted in a laboratory pilot VMD unit, comprising polypropylene capillary-membrane modules. Performance data, collected at modest temperatures (37 °C to 65 °C) with deionized and brackish water, confirmed the improved system productivity with increasing feed-water temperature; excellent salt rejection was obtained. The recovery of ethanol from ethanol-water mixtures and from fermented winery by-products was also studied, in continuous, semi-continuous, and batch operating modes. At low-feed-solution temperature (27–47 °C), ethanol-solution was concentrated 4 to 6.5 times in continuous operation and 2 to 3 times in the semi-continuous mode. Taking advantage of the small property variation in the module axial-flow direction, a simple VMD process model was developed, satisfactorily describing the experimental data. This VMD model appears to be promising for practical applications, and warrants further R&D work.
Highlights
In membrane distillation (MD) the temperature difference between the warm feed and the cooler permeate side (Figure 1) drives the separation process
These experiments were conducted with the membrane module M2
A simplified model for the case of simple feed and continuous operation experiments was developed. Through this model it was found that the membrane can be characterized on the basis of pure water Vacuum membrane distillation (VMD) experiments through a permeability-parameter K value
Summary
In membrane distillation (MD) the temperature difference between the warm feed and the cooler permeate side (Figure 1) drives the separation process. Vacuum membrane distillation (VMD) is a significant variant of the MD process, where the permeation of volatile compound through the membrane is enhanced by applying vacuum at the permeate side. Due to the relatively low-suction pressure applied at the permeate side, and the membrane hydrophobicity, molecules of the volatile compound evaporate at the warmer feed-side of the membrane, move as vapor though the membrane pores, and are condensed in an external condenser [1,2,3]. The VMD mode of operation is considered [4,5,6] to have two advantages, in comparison to other MD variants: (a) relatively low-conductive heat loss and (b) reduced resistance to mass transfer.
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