Abstract Salinity gradient processes, such as Forward Osmosis and Pressure Retarded Osmosis, have been proven to be promising technologies for reducing the energy consumption in water treatment processes, for energy production, and for energy recovery. Based on the thermodynamic concepts, specifically Gibbs' Free Energy of Mixing, the concentration of the draw solution plays an important role in determining whether the selected salinity gradient process is economically feasible or not. An increase in the salinity of a draw solution does not only increase the osmotic pressure difference between the draw and feed solutions, but also allows a higher hydraulic pressure to be applied on the draw solution, which together greatly increases the volumetric flux of the draw solution per single pass when PRO is used. Even though higher power densities can be achieved by applying higher hydraulic pressures on the draw solution, this requires greater mechanical stability of the membrane to be able to withstand these higher hydraulic pressures. In order to increase the mechanical stability of the membranes, generally, thicker support layers can be applied, which have a direct negative impact on membrane permeability. Therefore, there is a limitation to the salinity of the draw solution which can be used in the PRO processes. This being dependent on the concentration of the hypersaline solution and hence overall hydraulic pressure, necessitating the use of an ultra-thick support layer for maximum energy production and/or recovery. In this theoretical and simulative optimization of the PRO process, we achieved the optimum energy recovery from a hypersaline solution (TDS ~ 300,000 mg/l) by using a multistage PRO (MPRO) system which included implementing variable applied feed pressures to each stage. The results showed that the volumetric flow rate of the hypersaline draw solution increased by up to a factor of 10 during the MPRO process in single pass, and the concentration of the hypersaline draw solution diluted up to 10× accordingly. Energy conversion efficiency of osmotic pressure to hydraulic pressure was found to be around 10% without variable feed pressure and approximately 20% with variable feed pressure.

中文翻译：

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多级压力延迟渗透 (MPRO) 的热力学优化与高盐溶液的可变进料压力

摘要 盐度梯度过程，如正向渗透和压力延迟渗透，已被证明是减少水处理过程中能源消耗、能源生产和能源回收的有前途的技术。基于热力学概念，特别是吉布斯混合自由能，驱动溶液的浓度在确定所选盐度梯度过程在经济上是否可行方面起着重要作用。驱动溶液盐度的增加不仅增加了驱动溶液和进料溶液之间的渗透压差，而且允许在驱动溶液上施加更高的液压，这一起大大增加了驱动溶液的体积通量使用 PRO 时每次通过。尽管可以通过对驱动溶液施加更高的液压来实现更高的功率密度，但这需要膜具有更高的机械稳定性以能够承受这些更高的液压。为了增加膜的机械稳定性，通常可以应用更厚的支撑层，这对膜渗透性有直接的负面影响。因此，可用于 PRO 工艺的汲取溶液的盐度存在限制。这取决于高盐溶液的浓度以及因此的总水压，需要使用超厚支撑层以实现最大的能量产生和/或回收。在这个 PRO 过程的理论和模拟优化中，我们通过使用多级 PRO (MPRO) 系统从高盐溶液 (TDS ~ 300,000 mg/l) 中实现了最佳能量回收，其中包括对每个阶段实施可变的进料压力。结果表明，在单程MPRO过程中，高盐度提取液的体积流量增加了10倍，高盐度提取液的浓度相应地稀释了10倍。发现渗透压到液压的能量转换效率在不可变进料压力的情况下约为 10%，而在可变进料压力下约为 20%。结果表明，在单程MPRO过程中，高盐度提取液的体积流量增加了10倍，高盐度提取液的浓度相应地稀释了10倍。发现渗透压到液压的能量转换效率在不可变进料压力的情况下约为 10%，而在可变进料压力下约为 20%。结果表明，在单程MPRO过程中，高盐度提取液的体积流量增加了10倍，高盐度提取液的浓度相应地稀释了10倍。发现渗透压到液压的能量转换效率在不可变进料压力的情况下约为 10%，而在可变进料压力下约为 20%。