A nanoporous membrane is used for the pervaporation process in which potable water is maintained, at atmospheric pressure, on the feed side of the membrane. The water enters the non- pervaporation (NPV) membrane device where it is separated into two streams — retentate water and permeated water. The permeated pure water is removed by applying low vapor pressure on the permeate side to create water vapor before condensation. This permeated water vapor is subsequently condensed by coming in contact with the cool surface of a heat exchanger with heat being recovered through transfer to the feed water stream.
A thermoelectric heat exchanger is used here to pump heat from the condensing surface to the feed water stream. Because the temperature differential across this heat exchanger is relatively small, the thermoelectric process is highly energy efficient. This new heat exchanger provides high surface area for vapor condensation with controllable temperature on the hot and cold sides to meet the operating temperature requirements for the evaporation of solvent-containing, heat-sensitive substances, such as pharmacological substances, vitamins, etc.
The heat exchanger is a sandwich, with the copper interface plates on both sides of the heat pumps interfacing with the radiators. The outside of the radiators is insulated with two cover plates bolted together to hold the entire heat exchange unit together. Four heat-pump units are connected in series and are controlled by one circuit. The control electronics are built around a standard commercial voltage regulator. This was chosen in part because it has a 6.2-volt reference output that is needed for the bridge circuit. The radiator on the incoming potable-water side is heating, and the vapor side is cooling.
The NPV process can be operated at close to room temperature, and is driven by the space vacuum (provided by a secondary loop controlled by a secondary vacuum valve with built-in redundancy for safety) applied on the permeate side with minimal energy consumption. A primary valve controls the inlet space vacuum. The nanoporous membrane serves as a barrier, not only between liquid and water vapor phases, but also between pure water and dissolved solids to be removed. The nanopore selectively adsorbs liquid water and excludes undesirable constituents such as particles, microbes (e.g., bacteria), viruses, and volatile organic compounds.
The system only requires a low-pressure gradient across the membrane [150 psi (≈1.03 MPa)], to achieve high water-flow rate. As a result, the novel membrane will not be prone to the fouling issues that are commonly seen in the RO system. The cross-flow design can also allow the concentration stream to sweep away retained molecules and prevent the membrane surface from clogging or fouling, making the system able to deliver medical-grade water to point of use. The overall process has no moving parts and has low maintenance requirements.
This work was done by Chung-Yi Tsai and Jerry Alexander of T3 Scientific Limited Liability for Johnson Space Center. For further information, contact the Johnson Commercial Technology Office at (281) 483-3809. MSC-24264-1/6-1