Practical aspects of batch RO design for energy-efficient seawater desalination

Abstract

Batch operation of reverse osmosis (RO) has been proposed as a method to reduce seawater RO (SWRO) energy consumption and fouling propensity. In this paper, we use a transient numerical model of the RO process to investigate the impact of several practical loss mechanisms on the overall energetic performance of batch SWRO compared to a conventional continuous system. A critical variable that controls the energetic advantage of batch RO is the reset time between cycles. A large reset time necessitates higher operating flux and therefore results in increased energy consumption. On the other hand, ensuring a low cycle reset time requires higher energy for the refilling process. A batch SWRO design with an atmospheric pressure feed tank and pressure exchangers for energy recovery does not show promise for energy savings. Batch SWRO must be designed with a large number of short pressure vessels (with fewer membranes each) and lower energy recovery losses (e.g., by using pressurized feed storage) in order to reduce energy consumption by up to 8%. These modifications are more complex and hence capital expenses would determine overall feasibility of such designs to improve seawater desalination.

Impact of salt retention on true batch reverse osmosis energy consumption: Experiments and model validation

“…Both models account for concentration polarization and assume perfect salt rejection. Results from prior work [12] and our model indicate that fewer elements per pressure vessel are desirable in true batch RO systems. Today's single-stage continuous RO plants typically have 6-8 elements per pressure vessel [3].…”

“…However, theoretical studies have shown that batch RO with ERDs consumes more energy than true batch RO [10,11]. Significant energy is lost as the feed repeatedly cycles through the ERDs, which are not perfectly efficient [12].…”

The relative insignificance of advanced materials in enhancing the energy efficiency of desalination technologies

“…14,113,115 Staged, batch, and semi-batch processes lower the module-scale water recovery (wr) required to achieve a given system-scale recovery (WR), thereby lowering the average overpressure and thus reducing the DP required. 14,119,120 The continued development of high efficiency energy recovery devices (ERDs), such as turbines or isobaric pressure exchangers, is also essential to minimize the energy lost during brine depressurization. 46 Novel pressure exchanger designs have been able to achieve energy recoveries of around 95%, though these are typically limited to prespecified isobaric operating conditions.…”

Energy Savings in Desalination Technologies: Reducing Entropy Generation by Transport Processes

“…Figure 6 shows a simplified conceptual arrangement. Practical considerations may limit the energy savings for seawater operation in certain configurations, while brackish water operation has shown the potential for significant percentage gains [38,39]. This technology is precommercial.…”