Reducing Energy Consumption In Reverse Osmosis Desalination: Cyclic or Multi-Stage Operation?

Current commercially available reverse osmosis membranes are of sufficiently high permeability to enable RO desalination process operation near the thermodynamic limit while maintaining a practical level of permeate flux. As a result, the minimum specific energy consumption (Energy consumed per unit product water) is dictated by the water recovery and not membrane permeability. However, plant footprint can be reduced since higher flux can be attained with a lower membrane surface area. As concluded in our previous work [1], significantly reduction in the cost of RO desalination is more likely to arise from optimization of process configuration (e.g. [2], [3], [7]) and control schemes [4] (e.g., to account for feed salinity fluctuation [5] and even temporal fluctuation of electrical energy purchasing price), utilization of low cost renewable energy sources, as well as more effective and lower cost feed pretreatment and brine management [6]. For example, recent developments have suggested that cyclic (semi-batch) RO operation can result in lower energy consumption relative to continuous RO operation. Also, it has been proposed that the use of multi-stage RO operation (with booster pumps between RO stages) can lead to significant energy savings by allowing RO operation that enables closer approach toward thermodynamic process reversibility. Accordingly, in the present work we examine the above two approaches for reducing RO energy consumption. During cyclic RO, the full brine stream is mixed with the fresh feed to the process and the resulting stream enters the RO module. As a result, the energy of the brine stream is utilized while the salinity of the feed in the RO module increases with time, necessitating the increase of the feed side pressure to maintain a constant water production rate. Furthermore, the RO module is taken offline periodically to remove the salt accumulated in the RO module. In order to evaluate the energy consumption for such a process, a novel computational framework was developed for computing the SEC of the RO processes with full cyclic operation accounting for the impact of downtime. In contrast, multi-stage which allow continuous permeate production enable lower energy consumption relative to a single-stage (compared at the same permeate recovery), albeit at the expense of higher membrane surface area requirement and possibly higher capital cost. The results of the present analysis demonstrate that under certain operating conditions cycling or continuous operation (with and without energy recovery devices) can be favorable from an energy viewpoint and can lead to a reduction in the cost of water production by RO desalination.

References:
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