Abstract
Membrane distillation (MD) is a technology that can treat feed solutions with higher osmotic pressure, as well as produce high-purity water. However, the water production cost of the MD process is expensive. In this study, to decrease the water production cost, we attempted to evaluate the effect of membrane characteristics on the long-term stability of a vacuum MD (VMD) system. We fabricated four different types of polyvinylidene difluoride hollow fiber membranes, and operated a VMD system with 3.5 wt% NaCl aqueous solution at 65 °C as a feed under 11 kPa of air gap pressure. Consequently, in the proposed VMD system, it is found that the liquid entry pressure (LEP) is the most important factor. When LEP was higher than 0.37 MPa, the pilot-scale module was very stable for long-term operations, and the vapor flux was approximately 19.3 kg/m2·h with a total salt retention factor of over 99.9% during the 300-h operation.
Highlights
In the 21st century, water shortages are a global issue owing to population growth, as well as the associated developments of resource mining, agriculture, and industry [1].The membrane process is one of the key technologies used to address water shortage.Presently, reverse osmosis (RO) is a mainstream process for water production, and largescale seawater desalination plants are operated with this process [2]
To measure the liquid entry pressure (LEP) of each membrane, both the bore and measure the liquid installed entry pressure (LEP) ofmodule each membrane, both water, the bore shellTo sides of the membrane in the lab-scale were filled with andand shell sides of the membrane installed in the lab-scale module were filled with water, pressure was applied to the bore side (Figure 4)
M-1, M-2, M-3, and M-4 lab-scale modules were evaluated to confirm the relationship between vacuum membrane distillation (MD) (VMD) performance, membrane morphology, and their physical properties such as porosity, pore size distribution, and LEP
Summary
In the 21st century, water shortages are a global issue owing to population growth, as well as the associated developments of resource mining, agriculture, and industry [1]. The RO process is a pressure-driven membrane process in which product water is obtained by applying a higher pressure than the osmotic pressure difference between the feed and permeate to the feed side. Heat efficiency is not high because heat conduction through the membrane is most likely to occur [10], and the temperature polarization decreases the flux [11]. It is possible to achieve high vapor flux and low heat conduction simultaneously by SGMD and VMD. In VMD, the highest vapor flux is expected since the high vapor pressure difference can be obtained by decompressing the permeate side. VMD systems can prevent feed water from contaminating the permeated water because the membrane and condenser can be placed farther apart than DCMD and AGMD.
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