تاثیر اختلاط جریان در به حداقل رساندن هزینه انرژی اسمز معکوس

Various mixing operations between the feed, retentate and permeate streams are studied in this work to determine their effectiveness in decreasing the specific energy consumption (SEC) of single-stage (single-pass), two-pass and two-stage reverse osmosis (RO) processes operated at the limit of the thermodynamic restriction. The results show that in a single-stage RO process, partial retentate recycling to the feed stream does not change the SEC, while partial permeate recycling to the feed stream increases the SEC if targeting the same overall water recovery. Energy optimization of two-pass membrane desalination, with second-pass retentate recycling to the first-pass feed stream and operated at the limit imposed by the thermodynamic restriction, revealed the existence of a critical water recovery. When desalting is accomplished at recoveries above the critical water recovery, two-pass desalination with recycling is always less efficient than single-pass desalination. When desalting is accomplished at recoveries below the critical water recovery, an operational sub-domain exists in which the SEC for a two-pass process with recycling can be lower than for a single-pass counterpart, when the latter is not operated at its globally optimal state. For the two-stage RO process, diverting part of the raw feed to the second stage, in order to dilute the feed to the second-stage RO, does not decrease the minimal achievable SEC of a two-stage RO process. The various mixing approaches, while may provide certain operational or system design advantages (e.g., with respect to achieving target salt rejection for certain solutes or flux balancing), do not provide an advantage from an energy usage perspective.

Energy cost remains one of the most important factors contributing to the cost of water desalination via reverse osmosis (RO) processes [1], [2] and [3]. The introduction of highly permeable RO membranes has led to a significant reduction in the energy consumption in RO desalination [4] and [5]. As a result, the feasible operating pressure for the new generation of high permeability membranes is approaching the limit imposed by the thermodynamic restriction [6] and [7]. This constraint specifies that the feed-side pressure cannot be lower than the sum of the osmotic pressure of the exit brine stream and pressure losses (in the membrane channel) in order to ensure that permeate product water is produced along the entire membrane surface area [9]. As argued in a previous study [8], significant reduction in the cost of RO water desalination is less likely to arise from the development of significantly more permeable membranes, but it is more likely to arise from: (a) optimization of process configuration [9] and [11], (b) implementation of advanced control schemes (e.g., to account for feed salinity fluctuation [10] and even temporal fluctuation of electrical energy purchasing price), (c) utilization of low cost renewable energy sources, and (d) more effective and lower cost feed pretreatment and brine management strategies [12]. Recent studies have demonstrated that when a membrane desalting process can be operated up to the limit imposed by the thermodynamic restriction, there is an optimal product water recovery at which the specific energy consumption (i.e., energy consumption per volume of permeate produced) is minimized [9]. For example, the optimization model was successfully demonstrated in a recent study showing significant energy savings (up to 22%) under fluctuating feed salinity (up to 43%) [10]. It has been shown, via a formal optimization procedure, that the optimal operating condition shifts to higher recovery with increased membrane and brine management costs [9]. It has also been suggested that the energy consumption for membrane desalting would decrease with increased number of desalting stages where inter-stage pumps are utilized. More recently, a two-pass membrane desalination process was evaluated and compared to a single-pass process when both processes operate at the limit of the thermodynamic restriction [11]. Considerations of energy recovery, pump efficiency and the limitations imposed by membrane rejection level have led to the conclusion that a single-pass process is more energy efficient relative to a two-pass process. However, in our previous works [8], [9] and [11], the impact of various stream mixing and recycling configurations on the SEC of an RO plant was not systematically studied. Extending previous studies on RO optimization for operation at the thermodynamic limit, this work evaluates the effect of possible mixing/blending of various streams (feed, retentate, permeate) on the specific energy consumption (SEC) of RO desalination. To address this problem, the analysis begins with the simplest configuration: single-stage RO desalination, in which two possible recycling (partial retentate recycling and partial permeate recycling) operations are examined. Based on the results from the single-stage RO configuration, two-pass and two-stage desalting with recycling are then studied to determine the effect of various mixing/blending operations on the resulting SEC.

Various mixing operations between the feed, retentate and permeate streams were explored to assess their potential effectiveness for decreasing the specific energy consumption of single-stage, two-pass and two-stage RO desalination processes operated at the limit of the thermodynamic restriction. The analysis clarifies that in a single-stage RO process, partial retentate recycling to the feed stream does not change the SEC, while partial permeate recycling to the feed stream increases the SEC when targeting the same overall water recovery. For a two-stage RO process, diverting part of the raw water feed from the first-stage to the second-stage RO does not decrease the minimum achievable SEC in the two-stage RO process. For a two-pass membrane desalination, second-pass retentate recycling to the first-pass feed stream reduces the energy consumption relative to the case of no recycling. However, the optimal two-pass process always reduces to a single-pass (single-stage) process. In closure, the various mixing approaches considered in the present study, while may be useful for various operational reasons, do not provide an advantage from the viewpoint of energy use reduction.