Energy-efficient Reverse Osmosis Research Articles

Ozark Graphene Nanopore for Efficient Water Desalination.

A nanoporous graphene membrane is crucial to energy-efficient reverse osmosis water desalination given its high permeation rate and ion selectivity. However, the ion selectivity of the common circular graphene nanopore is dependent on the pore size and scales inversely with the water permeation rate. Larger, circular graphene nanopores give rise to the high water permeation rate but compromise the ability to reject ions. Therefore, the pursuit of a higher permeation rate while maintaining high ion selectivity can be challenging. In this work, we discover that the geometry of graphene nanopore can play a significant role in its water desalination performance. We demonstrate that the ozark graphene nanopore, which has an irregular slim shape, can reject over 12% more ions compared with a circular nanopore with the same water permeation rate. To reveal the physical reason behind the outstanding performance of the ozark nanopore, we compared it with circular, triangular, and rhombic pores from perspectives including interfacial water density, energy barrier, water/ion distribution in pores, the ion-water RDF in pores, and the hydraulic diameter. The ozark graphene nanopore further explores the potential of graphene for efficient water desalination.

Numerical model-based analysis of energy-efficient reverse osmosis (EERO) process: Performance simulation and optimization

We conducted a feasibility study of the energy-efficient reverse osmosis (EERO) process, which is a multi-stage membrane system that integrates single-stage reverse osmosis (SSRO) and a countercurrent membrane cascade with recycle (CMCR). To this end, we developed a numerical model for the 1-2 EERO process (one SSRO stage with two stages in CMCR: one nanofiltration (NF) stage followed by one terminal RO stage), then validated the model using performance data obtained from commercial RO projection software. Retentate recycle ratio was one of the key parameters to determine energy efficiency of EERO. In addition, the implementation of NF membranes in the first stage of CMCR yielded additional improvement in EERO performance and played an important role in determining optimum salt rejection. An optimal design of the NF stage was successfully achieved by hybridization of different NF membranes in a vessel (internally staged design, ISD). Under the conditions optimized, EERO exhibited not only greater energy efficiency (3–25%), but lower concentration polarization (CP) and potentials of membrane fouling than conventional SSRO for ≥55% overall recoveries because of reduced water flux in the lead elements (averagely 34%). These findings can thus provide insight into optimal design and operation of the EERO process.

Preparation of polyamide thin film nanocomposite membranes containing silica nanoparticles via an in-situ polymerization of SiCl4 in organic solution

Thin film nanocomposite (TFN) membranes with improved permeability is important in the energy-efficient reverse osmosis membrane processes. In this paper, we demonstrate a facile method for the formation of polyamide/silica TFN membranes by an in-situ interfacial polymerization of aqueous amine and acid chloride/silicon tetrachloride (SiCl4) solutions. Water permeability is greatly enhanced with minimal loss in NaCl rejection. Polyamide/silica TFN membranes with 0.04% w/v SiCl4 exhibit a pure water permeability up to 9.7 L m−2 h−1 bar−1, which is 2.02 times more than that of the pristine thin film composite (TFC) membranes, while high rejection to NaCl (96.5%) is maintained. TEM tomograms confirm the existence of silica nanoparticles (NPs) in the thin PA film. More permeable voids are created in the cross-section of TFN membranes. Outer voids are buried underneath the PA protuberances and overhanging crumples. Inner voids and silica NPs are both encapsulated by polyamide. Top surfaces become more hydrophilic and negatively charged. With the optimal amount of SiCl4 loading, the formation of non-selective voids around the particles is minimized. Covalent and hydrogen bonds are formed between SiCl4 and PA polymers. These polyamide/silica nanocomposite membranes outperform many other TFN membranes in water permeability and salt rejection.

Process economics and operating strategy for the energy-efficient reverse osmosis (EERO) process

The energy-efficient reverse osmosis (EERO) desalination process was recently developed for cost-effective high total water recovery. It feeds the retentate from one or more single-stage reverse osmosis (SSRO) stages in series to a countercurrent membrane cascade with recycle (CMCR) consisting of a terminal reverse osmosis (RO) stage and one or more low salt-rejection stages. The CMCR permits retentate refluxing in the low salt-rejection stages and multi-pass processing of permeate. The novel 2-2 EERO process is advanced that involves two SSRO stages in series with a 2-stage CMCR. To address problems encountered in a pilot-scale test of the EERO process, it develops an operating strategy involving boosting the pressure to the low salt-rejection stage of the CMCR to compensate for using membranes with a higher salt rejection than required. A process embodiment for mitigating concentration polarization is also advanced. The first estimate of the total cost of water production for three EERO process configurations is made. The EERO process can reduce the osmotic pressure differential by 50% relative to conventional SSRO for the same total water recovery and can achieve 75% total water recovery at a lower total cost of water production than conventional SSRO operated at just 50% water recovery.

Energy-efficient reverse osmosis desalination: Effect of retentate recycle and pump and energy recovery device efficiencies

The development of commercial RO membranes offering high salt rejection and flux has focused more attention on improving desalination process technology. The energy-efficient reverse osmosis (EERO) process has recently been advanced that combines single-stage reverse osmosis (SSRO) with a countercurrent membrane cascade with recycle (CMCR). The SSRO retentate is the feed to the CMCR that employs countercurrent retentate and permeate flow, permeate recycle, and retentate self-recycling via the use of NF membranes in one or more stages. This reduces the foulant load on the RO stage in the CMCR and allows it to run at a higher recovery than conventional SSRO. This option as well as the effects of pump and energy-recovery device (ERD) efficiencies are considered here. For a typical 35g/L seawater feed and 0.35g/L water product, the 4-stage EERO process reduces the osmotic pressure differential (OPD) relative to conventional SSRO by 50% at all overall water recoveries. For pump and ERD efficiencies of 85% and 90%, respectively, the 3-stage EERO process has a 50% overall water recovery at an OPD of 42.7bar and a net specific energy consumption (SECnet) of 2.323kWh/m3, thereby reducing the OPD by 23.1% at the cost of increasing the SECnet by only 3.6% relative to conventional SSRO. For the same efficiencies the 4-stage EERO process can achieve a 75% overall water recovery at an OPD of 55.5bar and SECnet of 3.773kWh/m3, thereby reducing the OPD by 50% and the SECnet by 3.6% relative to conventional SSRO at that recovery.

Energy-efficient reverse osmosis desalination process

A novel energy-efficient reverse osmosis (EERO) process is proposed for which the retentate from single-stage reverse osmosis (SSRO) serves as the feed to a countercurrent membrane cascade with recycle (CMCR). The 3-stage EERO process employs two, whereas the 4-stage EERO process employs three stages in the CMCR. The EERO process is advantageous because of four features: (i) coupling SSRO with a CMCR; (ii) countercurrent retentate and permeate flow; (iii) permeate recycling; and (iv) retentate self-recycling owing to the use of one or more nanofiltration stages. The EERO process was compared to conventional SSRO for both processes operating at the thermodynamic limit and employing an energy-recovery device. For the same overall recovery the osmotic pressure differential is reduced by 33% and 50% relative to SSRO for the 3- and 4-stage ERRO processes, respectively. There is a critical recovery above which the EERO process also can reduce the specific energy consumption (SEC) relative to SSRO for the same recovery. For a typical seawater feed of 35g/L the 3-stage EERO process can achieve a 75% recovery at a net SEC of 2.746kWh/m3, an 11.0% reduction in the SEC relative to SSRO for the same recovery. The 4-stage EERO process can achieve a 75% recovery at the same net SEC as SSRO (3.086kWh/m3). Accounting for the additional membrane area required for the EERO process increases its cost relative to that for SSRO by at most 8%. An additional benefit of the EERO process relative to SSRO is the highly concentrated retentate that reduces the brine disposal volume or can be used to greatly increase the draw potential to harvest its osmotic potential energy via the pressure-retarded osmosis process.

Application of energy efficient reverse osmosis system for seawater desalination

A performance of an energy-efficient reverse osmosis (RO) system for seawater desalination was evaluated. The evaluation focused on energy saving performance of the RO system. The strategy adopted was to induce high recovery (50%) through the pressure increase. For this purpose, a pilot-scale RO plant (5 m 3/h) was built and installed in a southern port of Korea. The study showed that energy saving was possible by application of high pressure. The 50% recovery was achieved at 8.3 MPa, but the operation led to water quality degradation. The water quality problem was solved by the operation of two-stage mode. Under this condition, the target recovery was achieved at lower pressure (6.6∼7.5 MPa), which resulted in the saving of power consumption. No water quality degradation was observed. Further saving was possible by adding energy recovery turbine (ERT) and inverter system. When the RO system was operated for 50% recovery at two-stage mode while equipped with energy saving devices of ERT and inverter system, 44% of saving in power consumption was possible.