How Does HEEPM™ Work? (In-depth)
The patented High Efficiency Electro-Pressure Membrane (HEEPM™) process takes advantage of the respective differences and advantages of electrodialysis and nanofiltration or reverse osmosis by combining both processes in a manner optimizing the separation characteristics of each. Use of the integrated HEEPM™ approach overcomes inherent limitations of both membrane-based technologies, allowing these to operate optimally and economically, thereby achieving improved efficiencies, product recovery, and finished product quality. While electrodialysis systems typically also allow higher feed recoveries compared to pressure-based membrane systems such as reverse osmosis or nanofiltration, the unique HEEPM™ system can allow even additional feed recovery at reduced overall desalting cost compared to electrodialysis-only systems, nanofiltration-only and/or reverse osmosis-only systems. Recoveries as high as 99+% are possible using HEEPM™ since the feed concentration is maintained at a relatively constant level due to the combined separation actions of each membrane type. Since feed concentration is relatively constant, the osmotic pressure, and therefore productivity, of the nanofiltration/reverse osmosis membrane remains constant allowing almost complete reclamation of the feed. The resulting enhanced recovery can greatly improve the economic feasibility and cost effectiveness of a variety of desalination operations. Cost components of interest affected by improved recovery include pretreatment costs, value of recovered product, cost of disposal of concentrate, capital cost, and energy cost required to perform the additional recovery. Since the feed concentration is relatively constant over the whole range of recoveries, the rejection of the salts and productivity of the nanofiltration/reverse osmosis remains constant, resulting in an improved permeate product compared to nanofiltration/reverse osmosis only systems in which the permeate product quality would decrease as a function of recovery. By ensuring that each sub-system operates in the feed concentration range where it is most energy efficient and removal effectiveness for each membrane type is optimal, the integrated HEEPM™ process results in lower overall energy operating costs at high feed recoveries.
Figure 1 shows the typical behavior of a batch operated or series electrodialysis process. In a batch process, the feed salt concentration is lowered over time, while in a series process, the feed salt concentration becomes lower as the feed proceeds down the line of electrodialysis units. As shown in the figure, desalination slows as the feed salt concentration decreases due to:
- Lower solution conductivity as a result of the lower salt feed concentrations.
- Increased concentration polarization near the electrodialysis membrane surface.
- Increased driving potential between the concentrate and diluate (feed) stream as the feed stream becomes lower in concentration.
Figure 2 shows the typical feed and permeate concentration behavior of a batch or series operated nanofiltration/reverse osmosis process. In a process or series process, the feed stream becomes concentrated as permeate is recovered. Even if rejection remains approximately constant (in many cases, it can decline with recovery), the permeate quality becomes worse as the feed concentration increases.
In addition, as the feed concentration increases, the osmotic pressure of the feed solution also increases, lowering the membrane flux, as shown in Figure 3. As a result, for even moderately concentrated brine feeds (>10,000 ppm), recoveries >80% are seldom feasible with low pressure systems.
Figures 4 and 5 show the feed and permeate concentrations and membrane flux, respectively, for a HEEPM™ integrated system. The electrodialysis system maintains the feed concentration near the initial level over the whole recovery range. As a result, the permeate concentration and membrane flux also remain constant over the range of recoveries.
In addition, back-diffusion is minimized, allowing recovery of highly concentrated brine solutions in the electrodialysis concentrate stream, with concentrations >20 g/L possible. Such behavior holds even for sparingly conductive organic-water solutions, as shown in Figures 6 and 7.
Finally, the integrated HEEPM™ system can be operated such that even low pressure nanofiltration or reverse osmosis membranes can be utilized in recovery of high TDS streams. In such an arrangement, electrodialysis is operated alone for the initial desalination until the feed concentration is reduced to a level low enough to feed to the low pressure nanofiltration or reverse osmosis units, at which point normal HEEPM™ is resumed; sample results of this method are shown in Figure 8. This operational method can result in substantial energy savings or capital cost savings in some applications.
What Benefits Does HEEPM™ Offer?
How Does HEEPM™ Work? (Overview)
HEEPM™ Key Features
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