Abstract
The merit of DCMD capability to utilize low-grade waste and/or alternative energy sources, and it is easier to set up and operate. DCMD received much attention in many countries, and we also used this system in our research. However, from a commercial point of view, DCMD is not implemented yet in industry due to its inefficient permeate rate. The composite (hydrophobic/hydrophilic) membranes instead of hydrophobic membranes are adopted to improve the permeate rate. The objects of this work are two, first is to observe the effect of flow rate and temperature to the permeate rate for hydrophobic membranes, the second part; we stack commercial hydrophobic and hydrophilic membranes together to control their thickness. The permeate rate increased with the increasing flow rate for all membrane modules, and the influence in feed side is stronger than in the permeate side. With the flow rate increased, the flux reaches a maximum values asymptotically for high flow rate, the effect of the flow rate is most obviously when the feed side temperature is at 60℃, as the temperature raise to 70℃, the permeate rate is affected strongly by vapor pressure. For the same total thickness of membrane, the permeate rates of hydrophobic/hydrophilic membranes are invariably greater than those of hydrophobic membranes. How to adjust the thicknesses of hydrophobic and hydrophilic layers respectively becomes a main issue in the research of composite membranes. For a fixed thickness of thin hydrophobic layer, the same phenomena are observed as the thickness of hydrophilic layer increases. This discrepancy between the effect of mass transfer resistance and that of thermal resistance is enhanced when the difference of temperature between feed and permeate sides increases. It is interesting that the opposite results are obtained for a constant thickness of thick hydrophobic layer, i.e., the permeate rate increasing with the thickness of hydrophilic layer. However, the lack of agreement is obtained for some results of hydrophobic/hydrophilic membranes. The possible reasons are that the gap between membranes due to the stack of the membranes results in the additional resistance and the variation of effective porosity on the experimental side, and also the porosity difference between hydrophobic and hydrophilic membranes.