The effects of operational conditions on the mixing behavior of a rotary energy recovery device have been systematically investigated through the combined methods of computational fluid dynamics and validating experiments in this paper. The sliding mesh technique and species transport equations were applied in the simulation process. An innovative parameter of inflow length was defined to express the moving distance of a mixing section in rotor ducts. A theoretical formula between the inflow length and operational conditions was first established on the basis of the mass balance and computational fluid dynamics analysis. Simulation results revealed that the mixing has a polynomial relation with dimensionless inflow length, which was in good agreement with the experimental results. The obtained formulas between mixing and dimensionless flow length provide a simple way to calculate and predict the mixing behavior of the device, which will be beneficial to design and operate the rotary energy recovery device in a lower mixing level.

•The RS-ERD is innovatively applied in an actual SWRO desalination system.•The adaptability of the RS-ERD is investigated under different brine flow rates.•The RS-ERD maintains a high efficiency of 97% in different operating capacities.The adoption of energy recovery device (ERD) in seawater reverse osmosis (SWRO) system has become a practical way for reducing the power consumption and permeating cost of the desalting system. The performance requirement of the ERD basically depends upon the configurations and operating parameters of the SWRO system. The temperature and salinity of the seawater feed are two typical parameters that should be considered, whose fluctuations may induce significant changes in the membrane separation performance and thus the flow balance of the SWRO system, which requires the ERD having a good adaptability to meet the practical demands. In this paper, a SWRO desalination platform is built to evaluate the capacity flexibility of a reciprocating-switcher energy recovery device (RS-ERD). The experimental results show that the RS-ERD can run in 66.7%–150.0% of the design capacity (30 m3/h) and maintain a relative constant efficiency above 96.50% and leakage ratio about 2.62%–3.95%, which proves the wide-range adaptability of the RS-ERD to the varying conditions of the SWRO desalination system.

•The potential of zero charge (PZC) of ion-doped PPy/CNT electrodes were achieved.•Effects of the PZC on PPy/CNT electrode performance were studied.•Configuration of CDI cells was optimized based on PZC and potential distribution.•A new strategy for enhancing performances of CDI electrode and cells was proposed.In order to enhance the ion adsorption capacity and ion selective ability of electrode materials, carbon nanotube/polypyrrole (CNT/PPy) composites doped with chloride (Cl−) and dodecyl benzene sulfonate (DBS-) were used for capacitive deionization (CDI) process, respectively. Seven types of symmetric and asymmetric CDI cells based on the obtained composite electrodes and CNT electrode were assembled and tested. The experimental results indicated that the desalination performance for CDI cells typically depended on the potential of zero charge (PZC) of the electrodes and the potential distribution of the cells. The PZC of the CNT, CNT/PPy-Cl and CNT/PPy-DBS electrodes were measured to be 0 V, −0.1 V and 0.4 V vs. saturated calomel electrode, respectively. In the applied potential of 1.2 V, the largest effective working potential of 1.2 V and the highest adsorption capacity of 35.46 mg/g were found based on the CNT/PPy-Cl (anode)‖CNT/PPy-DBS (cathode) cell, in which the PZCs were all outside of the polarization window of the corresponding electrodes. While a diminished effective working potential and desalination performance can be observed when one of the PZCs at least was within the polarization window of the respective electrode. This work provides an effective approach for optimizing electrode configuration based on PZC analysis.

•The theoretical analysis was introduced into the optimization of energy recovery efficiency.•The optimal processing capacity was determined for the designed RERD and established SWRO system.•The maximal leakage was specified to satisfy the requirement of energy recovery efficiency.A rotary energy recovery device (RERD), as the energy saving equipment of seawater desalination system, has a great significance on reducing the energy consumption and permeate cost of the system. The high or low energy recovery efficiency directly determines whether the engineering application of the RERD can be widely used. In this paper, firstly, based on the calculation formula of energy recovery efficiency, the main impact factors of efficiency were analyzed theoretically, including processing capacity, the high and low pressure differentials, and the leakage of the device. Secondly, in order to carry out some required auxiliary tests smoothly, a new RERD was designed and manufactured, and a complete SWRO desalination system was also established. The results depicted that the energy recovery efficiency reached the maximum and the corresponding optimal processing capacity was about 12.15 m3/h when the pressure of the low pressure seawater was 1.0 bar, that of the high pressure brine was 60.0 bar, and leakage of the device was 0.6 m3/h. Furthermore, when processing capacity was kept 13 m3/h constant and the efficiency was required to exceed 94%, under the existing operating conditions, the maximal leakage of the device was needed no more than 0.43 m3/h.

•PPy/CNT doped with DBS− and Cl− were prepared and used in CDI electrodes.•Low charging resistance of PPy-DBS/CNT and PPy-Cl/CNT electrodes was obtained.•High electrosorption capacity of CDI cell was achieved by PPy/CNT electrodesThe composites of carbon nanotube (CNT) and the conducting polymer polypyrrole (PPy) doped with dodecyl benzene sulfonate (DBS−) and Cl− were prepared respectively by chemical oxidation method and made into two types of composite electrode (PPy-DBS/CNT and PPy-Cl/CNT) for capacitive deionization (CDI) cell. Enhanced electrochemical performance of the composite electrodes was confirmed by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) test. The results of CV showed that the specific capacitance of PPy/CNT doped with either DBS− or Cl− was about 2.3–2.5 times that of the CNT electrode. And EIS test demonstrated that the charging resistance for both the PPy-DBS/CNT and PPy-Cl/CNT electrodes was dramatically reduced and only about 10% that of the CNT electrode. By pairing the two composite electrodes with CNT electrodes, four cells were assembled to study the effects of electrode assembling patterns on the desalination performance of the CDI process. The results indicated that the CDI cells involving either the PPy-DBS/CNT electrode or the PPy-Cl/CNT electrode, or both of them exhibited much higher electrosorption capacity (from 40.8 to 72.36 mg/g) than that of the CDI cell assembled with two CNT electrodes (about 12.6 mg/g). And the composite electrodes kept good stability and fast regeneration rate after several desalting cycles. Thus, it is believed that the enhanced performance and optimized electrode assembly of the CDI cell should be with the composite electrodes that have the good ability of ion selective adsorption for cations or anions.

•Preparation parameters of coated PPy/CNT electrode were typically optimized.•Charging resistance of coated electrode was obviously lower than that of compressed electrode.•Coated electrode was notably superior to compressed electrode in specific capacitance and specific adsorption capacity.In order to obtain the high electrode performance in capacitive deionization (CDI), the coating methodology was introduced and incorporated in the fabrication of the novel polypyrrole/carbon nanotube (PPy/CNT) composite electrode. A typical optimization on the major preparation parameters of the coating-type PPy/CNT electrode was conducted, including the mass ratio of electrode material mixture, the coating thickness and the drying temperature. Based on the PPy/CNT composite, the performance of the optimized coated electrode and the compressed electrode was typically compared in terms of the electrochemical characteristic and desalination performance. The experimental results indicated that the performance of the coated electrode got to the optimum under the mass ratio of 8:1:1 for the PPy/CNT, polyvinylidene fluoride and graphite, the coating thickness of 0.3 mm and the drying temperature of 40 °C. The coated electrode was markedly superior to the traditional compressed electrode in terms of the specific mass capacitance and specific adsorption capacity. The specific mass capacitance and specific adsorption capacity of the coated electrode reached 180.61 F/g and 93.68 mg/g respectively, which were 1.57 and 1.37 times that of the compressed electrode. These conclusions would lay a good foundation for the practical application of the coating methodology and also the novel electrode composite materials in the CDI field.

Significant improvement in reverse osmosis (RO) membrane permeability makes the emergence of thermodynamic restriction in desalting process. Due to the filtration flux, both the accumulation of rejected solute and the subsequent concentration polarization layer contribute to the transmembrane osmotic pressure difference that needs to be overcome by the applied pressure. A theoretically derived formula predicting the permeate flux accounting for pressure drop, effect of applied pressure on solute rejection and concentration polarization is presented, and the obtained average rejection is used to calculate the operating energy consumption of RO process more accurately. On the basis of theoretical considerations, at the limit imposed by thermodynamic restriction, an energy consuming analysis model of RO process was developed to study the effects of RO operating parameters (e.g., recovery rate), energy recovery devices (ERDs) performance variables (efficiency and leakage ratio), and pump efficiencies on the specific energy consumption (SEC).

This work is focused on the theoretical investigation of internal leakage of a newly developed pilot-scale fluid switcher-energy recovery device (FS-ERD) for reverse osmosis (RO) system. For the purpose of increasing FS-ERD efficiency and reducing the operating cost of RO, it is required to control the internal leakage in a low level. In this work, the internal leakage rates at different leakage gaps and retentate brine pressures are investigated by computational fluid dynamics (CFD) method and validating experiments. It is found that the internal leakage has a linear relationship with the retentate brine pressure and a polynomial relationship with the scale of leakage gap. The results of the present work imply that low internal leakage and high retentate brine pressure bring benefits to achieve high FS-ERD efficiency.

•Surface texturing technology is innovatively applied to reduce the friction torque of the RERD•Operational performances of the textured RERD and the un-textured RERD are typically compared and analyzed.•A dramatic reduction of the friction torque and a significant increment of the efficiency are obtained for the textured RERD.A rotary energy recovery device (RERD) is widely used in a reverse osmosis desalination system. In order to maintain the device running in an available torque and obtaining a long service life, a portion of high pressure seawater is usually sacrificed to work as lubrication to reduce the friction torque of the device. However, when using such method, a certain amount of energy recovery efficiency will be lost due to the lubrication or leakage. This paper focuses on improving operating performance of a rotary energy recovery device by making textured grooves on the surfaces of two end covers of the RERD. The experimental results show that compared to the un-textured RERD, the textured RERD reduces the device's torque by half, and successfully improved the efficiency by 40% under the mating clearance of 0.035 mm, operating pressure of 6.0 MPa and rotary speed of 500 rpm. In addition, thanks to the groove textured surface which dramatically reduces the RERD's torque and permits the mating clearance to be minimized. Under the small clearance of 0.025 mm, the energy recovery efficiency of the textured RERD can be increased up to 96.3% at an operating pressure of 6.0 MPa which can well satisfy the requirement of the industry application.

•An innovative control strategy was adopted to realize the cyclic operation of the system.•The “co-ion” effect in CDI was revealed and monitored through outlet stream conductivity variations.•The MCDI and CDI processes were assessed in terms of current efficiency and specific energy consumption.•The MCDI was significantly superior to CDI in current efficiency and specific energy consumption.In this paper, graphite cloths were used as electrode materials to fabricate the capacitive deionization (CDI) cell and the ion-exchange membranes were introduced in the CDI cell to assemble the membrane capacitive deionization (MCDI) module. The desalination performances of the MCDI and the CDI cells were investigated under different working voltages and feed concentrations and assessed by adsorption capacity and specific energy consumption. The experimental results indicated that the introduction of the ion exchange membranes could effectively prevent the “co-ion” effect in the CDI and the current efficiency of MCDI was up to four times of the CDI. Under the experimental conditions, the adsorption capacity of MCDI is much higher than that of the CDI and also its specific energy consumption was dramatically reduced indicating the comprehensive economical advantage of MCDI technology.

•A mathematical model considering different concentration polarizations and the RSF was developed.•The predicted water flux and power density through the model were in good accordance with the experimental results;.•The concentration polarizations also been quantitatively evaluated under different conditions.•Osmotic pressure drop caused by CECP was proved to be notable for higher feed solution concentration.Pressure retarded osmosis (PRO) is a potential technology for harvesting renewable energy from the salinity gradients. However, the PRO performance can be dramatically influenced by concentration polarizations and reverse salt flux (RSF) of the forward osmosis membrane. A mathematical PRO model considering dilutive external concentration polarization (DECP), concentrative internal concentration polarization (CICP), concentrative external concentration polarization (CECP) and the RSF was developed to predict the water flux of the PRO, from which we predicted the power density achieved by the membrane. Experimental verification of the model was carried out by using a bench-scale PRO system. The experimental results demonstrated a good agreement with the actual model scenarios of the water flux and the power density. This study also quantitatively analyzed the proportions of the osmotic pressure drop caused by different factors. It was showed that the proportions of CICP rose from 7.7% to 50.4%, as the feed solution concentration increased from 0 M to 1.5 M under a constant osmotic pressure difference (0.5 M), which also led to a reduction in water flux from 11.55 L m− 2 h− 1 to 1.40 L m− 2 h− 1. Results also indicated that, at higher solution concentrations, the osmotic pressure drop caused by CECP is as notable as that of DECP.

•An innovative single-cylinder energy recovery device is introduced and developed.•Two new plates greatly improve power consumption and flow continuity of the device.•Functionality and reliability of the SC-ERD are well verified through experiments.Energy recovery devices (ERDs) are widely used in seawater reverse osmosis (SWRO) desalination systems, and have become a vital facility of the system due to their great contribution for significant reductions of the power consumption. The innovative single-cylinder ERD (SC-ERD) technology is an improved pressure exchanger concept which follows the principle of positive displacement, and provides more advantages than the current devices, including the simple installation, flexible operation and stable performance. This paper focuses on verifying the functionality of the SC-ERD and evaluating its operating performance based on the industrial conditions. Experiments are carried out by using two sets of SC-ERD working in parallel and the performance of the devices are evaluated by employing an inhouse emulational SWRO system with the flow rate of 30 m3/h and the operating pressure of 6.5 MPa. The experimental results show that two SC-ERDs in parallel have reached the basic function of the double-cylinders ERD (DC-ERD). The pressure fluctuation of the SC-ERD has been reduced about 80% compared with the DC-ERD under the same operating conditions and the energy recovery efficiency could still be remained as high as 98%.

•The flexibility of FS-ERD is proved under different pressures and capacities.•An optimized PLC control strategy with fault tolerance module is developed.•The optimized PLC controller can deal with the emergency condition of the FS-ERD.•Reliability of the FS-ERD is demonstrated under simulated emergency condition.Piston type energy recovery device (ERD) commonly depends upon the concerted actions of magnetic sensors and PLC controller to realize the energy recovery function. For ensuring operational stability and flexibility of the device, signal control mode is basically adopted in the standard PLC controller. However, as a typical emergency condition of the signal control mode, failure of the magnetic sensor may bring on function loss of the mode, which imperils the operational safety of the ERD and even the RO system. So in this paper, an optimized PLC control strategy with fault tolerance module is developed to resolve the possible control problem of piston-type ERD. The experimental results indicate that the optimized PLC control strategy can maintain reliable under simulated emergency condition of magnetic sensor failure.

•Pre-pressurizing/depressurizing grooves were typically introduced into the RERD.•The flow fluctuation of the slotted RERD was largely reduced about 75.3–77.2%.•The pressure variation of the slotted RERD was greatly reduced about 90.7–92.5%.•Correlative error between simulative and experimental results was less than 7.55%.In order to diminish the flow fluctuations of the rotary energy recovery device (RERD), the pre-pressurization and pre-depressurization grooves were introduced and typically slotted on the surface of the RERD's endcovers. Effects of the grooves on performance fluctuations of the RERD were researched by means of computational fluid dynamics (CFD) simulations and validating experiments. Simulative results indicated that when compared with the un-slotted RERD under flow rate of 12m3/h, rotary speed of 400 rpm, and operating pressure of 6.0 MPa, the flow fluctuation amplitudes of the slotted RERD were reduced about 75.3% for high pressure outlet and 77.2% for low pressure outlet respectively. Similarly, the pressure fluctuation amplitudes of the slotted RERD were dramatically reduced about 92.5% for high pressure outlet and 90.7% for low pressure outlet. In addition, the validating experiments were conducted under the same operating conditions. The results showed that flow fluctuation amplitudes of experiments were in good agreement with that of the simulations, and the difference between them were about 2.76–7.55%, verifying the simulation model was reasonable. Thus, it provides a feasible and effective way to use the pre-pressurization and pre-depressurization grooves to diminish the flow fluctuations of the RERD.

•Polypyrrole/carbon nanotube composites were prepared and used as cathode in CDI.•Specific capacitance of the composites was about 4.6 times that of carbon nanotubes.•Enhanced desalination performance for CDI cell was achieved.Polypyrrole/carbon nanotube (PPy/CNT) composites were prepared via chemical oxidation method. By choosing sodium dodecyl benzene sulfonate (SDBS) as the dopant in the preparation process, the obtained composites have the ability of selective adsorption for cations and are suitable for application as the cathode material of capacitive deionization cell. The scanning electron microscope and transmission electron microscopy analysis showed that the PPy/CNT composites were in nanotube morphology with the CNTs wrapped uniformly by the PPy layer. Electro-chemical characteristics and desalting performance of PPy/CNT composites were tested and analyzed. The results indicated that specific capacitance of PPy/CNT composites increased more than 3 times compared with the CNTs. The saturated adsorbing capacity of PPy/CNT–CNT cell (PPy/CNTs used as cathode) was evaluated as 43.99 mg/g, which is much higher than that of CNT–CNT cell (about 11.00 mg/g). So the PPy/CNT composites doped with DBS− have a great potential as the high-performance cathode material for capacitive deionization technology.

•The hydrostatic bearing was firstly introduced into the RERD•The hydrostatic bearing favors for the establishment of fluid film lubrication in RERD•The circular clearance regarded as fluid restrictor was optimized•Experimental results are in accordance with CFD simulationsBased on the structural characteristics of the rotary energy recovery device, the hydrostatic bearing was established on both sides of the rotor. Effects of the hydrostatic bearing on the resultant force and fluid film thickness were investigated by the methods of computational fluid dynamics simulation and validating experiments. Simulation results indicate that resultant force rises linearly with the increase of both operating pressure and thickness difference between the upper and lower fluid films, indicating that there exists a cooperative function between the upper and lower fluid films which favors for the self-adjustment of film thickness and resuming the rotor to a stable lubrication state when the pressure changes. The circular clearance regarded as fluid restrictor was optimized in order to adjust the fluid film thickness rapidly and the best circular clearance is about 0.03 mm. The experimental results indicate that the practical resistance of the rotor can well be reflected by the changing trend of the experimental rotor speed at operating pressure from 0.1 MPa to 6.0 MPa, and is in good accordance with the theoretical resistance calculated by using the simulation results, verifying that the simulation model was reliable. This study provides an applied structure for improving the frictional state of the rotor and prolonging the working life of the device.

•Power-saving water hydraulic actuator is innovatively used in piston type ERD.•Driving power of RS-ERD can be reduced up to 90.3% by avoiding previous idle work.•Performance of RS-ERD with water hydraulic actuator is experimentally investigated.•Reliability of water hydraulic actuator is verified in real case of NF-ERD system.A piston type energy recovery device (ERD) can reduce the power consumption of seawater desalination system effectively, but the necessity of the oil hydraulic actuator for the ERD weakens the economics of the device and brings peripheral issues such as increased footprint. In this paper, a water hydraulic actuator driven by the high pressure brine of the desalting system is proposed to resolve the above problem and applied in the piston type reciprocating-switcher ERD (RS-ERD) to testify its effectiveness. The superiority evaluation proves that a reduction of up to 90.3% of the driving power consumption can be obtained by using the water hydraulic actuator, whose capital and maintenance costs are significantly lower than that of the oil hydraulic actuator. The experiments in the emulate test platform show that the RS-ERD with the water hydraulic actuator is operationally feasible and achieves high energy recovery efficiency of above 97%, except the existence of pressure pulsation (23.1%) in high pressure brine stream. Additionally, long term demonstration in the nanofiltration desalination system proves that the RS-ERD with water hydraulic actuator has a good operational reliability whose pressure pulsation has been dramatically minimized to 3.3% while maintaining the efficiency as high as 96%.