John Walton, Sergio Solis, Huanmin Lu, Charles Turner, Herbert Hein
University of Texas at El Paso
El Paso, Texas 79968
The major problem with desalination at inland locations is the fate of the concentrate solutions produced. In the absence of an ocean, there is no convenient location to deposit the concentrate. One potential solution is to sequentially extract additional water from the concentrates until a saturated NaCl solution remains. The saturated solution can then serve as a resource for the development of salinity gradient solar ponds (solar pond). The solar pond can serve as a final repository for salts initially present in the source water and provide low-grade heat energy to drive thermal desalination technologies. The total system represents a cost effective desalination operation with zero discharge.
A solar pond uses a salinity gradient of salt concentration to capture and store heat from the sun. In brief, a gradient of salt content is used to create a stagnant water layer that insulates the high specific gravity heat storage zone at the very bottom of the pond. (http://rorykate.ce.utep.edu). The major raw material required to construct a salinity gradient solar pond is a source of concentrated brine solution - i.e., the major waste product from desalination. The solar pond has a secondary benefit of providing a low cost source of low-grade heat energy that can assist with the desalination operation. Researchers at the University of Texas at El Paso (UTEP) have been investigating methods that couple solar pond technology with desalination to create a zero discharge system.
An example of heat provided by a solar pond is given in Figure 1. Temperature development in the upper convective zone (UCZ) and lower convective zone (LCZ) of the UTEP solar pond are given over time. The gradient was established on May 1, 1999. After establishment of the gradient the bottom layer of the pond (the LCZ) gains heat at about a degree per day until June 1. After June 1 the temperature of the LCZ is held constant by heat extraction. The heat extracted during the summer and fall has been used for electricity generation and desalination.
Figure 1. Temperature development in the UTEP solar pond.
An example of a desalination operation with zero discharge is given in Figure 2. Ground water is being desalinated to provide high quality water for boiler feed. The first treatment technology is nanofiltration with an approximate recovery of 75%. The concentrate from the nanofiltration system is fed to two thermal desalination technologies, flash distillation, and membrane distillation for further concentration and water recovery. The thermal technologies are driven by low-grade heat energy provided by the solar pond. The thermal technologies extract additional high quality water from the nanofiltration concentrate. The concentrate from the thermal technologies serves as feed to a brine concentration and recovery system (BCRS). The BCRS concentrates the solution to saturation with sodium chloride while producing additional fresh water. The remaining saturated solution serves as a brine solution source for the solar pond. In a large-scale operation additional acreage of solar pond is developed over time as more saturated salt solutions are produced.
Figure 2. Zero discharge desalination schematic.
One of the technologies currently being investigated as part of the zero discharge concept is membrane distillation (MD). This work is funded by the United States Bureau of Reclamation. MD differs from other membrane filtration technologies in that the driving force for desalination is the differential in vapor pressure of water rather than total pressure. The membranes for membrane distillation are hydrophobic allowing water vapor to pass but not liquid water.
A schematic of a distillation membrane is shown to the left. The hot (brackish) and cold water streams are passed through the membrane leading to net movement of pure water across the air gap to the condensation surface. Outside of the membrane unit (not shown) the cooling water is re-cooled and the brackish water stream is re-warmed. The heat exchanger arrangement for the warming and cooling (not shown) becomes complex in order to maximize energy efficiency. Pure product water and concentrated reject are removed from the recirculating streams as appropriate and makeup brine feed is introduced.
The minimum temperature difference (i.e., brine temperature – permeate temperature) required to drive membrane distillation is a function of brine temperature and salinity. For most brackish waters the minimum required temperature difference is less than one degree C. Practical production requires higher temperature differences but not dramatically higher.
The advantages of membrane distillation are a) it produces high quality distillate, b) water can be distilled at relatively low temperatures (30 to 100 ºC), c) low grade heat (solar, industrial waste heat, or desalination waste heat) may be used, and d) the water does not require extensive pretreatment to prevent membrane fouling as in pressure based membrane treatment. Potential disadvantages are uncertain economic cost and relatively low fluxes (~3–6 liter/m2/h) (Lawson, 1995, Lawson and Lloyd, 1997). Potential applications for membrane distillation are: a) solar desalination of brackish water, b) desalination at electrical generation power plants using waste heat, and c) desalination at geothermal sites.
A membrane distillation system from SCARAB was obtained for testing in conjunction with the solar pond. The system was set up to take advantage of heat from the UTEP solar pond. The following data were collected at the UTEP solar pond for use in an experimental matrix that includes hot side temperature, temperature drop, and salinity of input water. The experimental matrix is designed to allow comparison of results from a production MD module with a theoretical model developed in the current research project.
Figure 3. Dependence of flux on temperature drop across the membrane.
Data have been collected for water salinities of 0, 0.6 (~ seawater), 2, 3, and 4 molar concentrations. In each test the distillate and concentrate are recycled into the same tank, keeping constant salinity in the feed waters during the test. Note that the purpose of the test is to improve understanding of membrane distillation. In this context distilling already distilled water makes sense as it supplies flux and thermal efficiency data at one end point of concentration. The theoretical model indicates that higher hot side temperatures, greater temperature drop, and lower concentration of source water all increase the flux. It also indicates that flux begins at higher temperature drops for greater salinity water.
The flux results are shown in Figure 3. Flux is greater at greater temperature drops. At higher source water salinity, the flux begins at greater temperature differentials and is lower. The economic ratio varies between about 0.3 and 1 pounds of water /1,000 BTU of heat input. The MD module being tested makes no provision for recovering the latent heat of vaporization leading to a theoretical maximum economic ratio of 1.0 (note that is not a limitation of MD technology, only of the module being tested).
Desalination in conjunction with a salinity gradient solar pond is being investigated at the University of Texas at El Paso. The technology promises a cost effective system for desalination with zero discharge of concentrate. An emerging thermal desalination technology with great promise is membrane distillation.
This work was funded by the US Bureau of Reclamation under agreement number 98 FC 81 0048.
Lawson, K.W. and D.R. Lloyd, Membrane Distillation, Journal of Membrane Science 124 pp. 1-25, 1997.
Lawson, K.W., 1995, Membrane Distillation, Ph.D. Dissertation, University of Texas at Austin, UMI Microform 9603893.