Water. Desalination + reuse
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38 Far Site December 2016 Water. desalination + reuse one-stage, two-stage, ideal batch, batch, and semi-batch. One-stage and semi-batch are commercially run RO processes; semi-batch being the closed circuit desalina- tion system offered by Desalitech. Ideal batch refers to a hypotheti- cal fully pressurised batch RO process, and batch uses energy recover devices and an upressur- ised recycle tank. "We plotted the amount of energy in kWh per m3 of desali- nated water produced, against the recover ratio for each process. As expected, the best performing process was ideal batch, which had the lowest energy consump- tion. Conventional one-stage RO had the highest energy consump- tion for a recovery ratio of 50 per cent. We compared the processes and showed that you can save energy by using batch and semi- batch, or processes with in- creased staging, but the flip-side is that in some cases it increases capital costs on equipment or pumps. Batch and semi-batch are more complicated than the processes run commercially at the moment, but could offer advan- tages in terms of process robust- ness," says Deshmukh. The study considered the effect of pressure loss within mem- brane elements and piping, and found the batch or semi-batch processes, those approaching the theoretical minimum of 1.1 KwH/ m3, are "particularly hammered by any pressure losses in the pipes and the valves. It makes quite a dramatic effect due to the need to recycle so much volume", says Werber. For example, at a circulatory pressure loss of 0.1 bar, the two-stage process uses more energy that the ideal batch process; but at a pressure loss of 0.5 bar, two-stage RO is more ef- ficient than ideal batch RO. However, the authors conceded that in the field, cost considerations tend to over- shadow the benefits of an energy efficient system. "We were trying to show, if you were to build these plants, what the energy consump- tion would be," Werber says. The paper discusses cost and other considerations following the energy analysis. "Can batch or semi-batch processes save energy in reverse osmosis desalination?" by Jay Werber, Akshay Deshmuk, and Menachem Elimelech, Yale University, will be published in Desalination in January 2017. Yale team models energy use in batch RO processes From le , Akshay Deshmuk, professor Menachem Elimelech, and Jay Werber of Yale University, have studied the energy-saving effects of batch processes in reverse osmosis. A team at Yale University has modelled five RO desalination processes to understand what energy savings can be attained by changing process design. The paper, "Can batch or semi- batch processes save energy in reverse osmosis desalination?", follows on from a paper by Yale professor Menachem Elimelech published by Elsevier Science in 2011, which suggested that energy savings from improving mem- brane technology had plateaued for conventional, one-stage seawater RO. "Energy consumption in seawater RO over time has reduced from about 16kWh/m3 in the 1970s to 2kWh/m3 today. It's now within a factor of two of the theoretical minimum, which is about 1.1kWh/m3 for a certain set of assumptions. A lot of plants today are 2.5kWh/m3 to 3.5kWh/ m3. We wanted to build on profes- sor Elimelech's paper by taking a rigorous look at process design. The processes we modelled might not be the cheapest solutions, but if you did it, what would the en- ergy needs be?" says Jay Werber, co-first author. An MIT paper based on a similar idea, "Energy efficiency of batch and semi-batch reverse osmosis desalination", was pub- lished in Water Research journal in September. Werber and Akshay Deshmuk, co-first author of the Yale paper, modelled five RO processes: