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Improvement of reverse osmosis process stability in internally staged design under seasonal variation of feed water
... For the research done by Lu et al. [35], Lee et al. [36] and Jiang et al. [37], optimum membrane cleaning and replacing strategies were developed. Lim et al. [38] improved the RO plant stability with ISD under time variation of conditions. An ISD with an inter-stage energy recovery device could improve the fouling resistance. ...
... The current model has been validated by the IMSDesign using SWC6 MAX and ESPA2-LD MAX membranes at 20 • C and 30 • C in the supplementary materials. (38) 7 and 9 κ 7, t = κ 9, t = (Ex 6, t + Ex 25, t )/(Ex 7, t + Ex 9, t ) (39) 8 (48) where {H + } t (kg/m 3 ), C boric,t (kg/m 3 ), and C borate,t (kg/m 3 ) are the concentrations of hydrogen ion, boric acid, and borate ion at tth hour, respectively, K TB, t ′ (kg/m 3 ) is the apparent first acid dissociation constant for boric acid, C ch,mw,t is the concentration at membrane wall at tth hour. σ is the reflection coefficient. ...
This paper introduces a superstructure-based simultaneous optimum design and operation of seawater reverse osmosis (RO) system with boron removal under time-variant constraints. Three operational strategies are utilized to improve operability of the system under boron restrictions, including changing the activated number of pressure vessels, permeates split ratio, and pH values. Moreover, exergo-environmental and exergo-econoenvironmental analysis models are integrated. The economic efficiency, energy utilization, and environmental impact could be optimized in both design and operation phases. The effect of water tank area, membrane type, and boron concentration on water production are investigated with constant feed conditions. Choosing proper water tank volume and type of membrane could not only reduce cost but also better operation conditions, such as improving the efficiency of pumps and reducing feed pH of RO pass 2. When both the eco-costs/value ratio and emergy rate of product are added as a penalty, the annual operation cost (a reduction of 5.50 %), emergy rate (a reduction of 7.02 %) and eco-costs/value ratio (a reduction of 7.48 %) of the product could be reduced with only a small amount of water cost increment (about 2.66 %). Improving the efficiency of pumps would be an effective method to improve overall performance of the RO plant.
... A two-level design related to the RO network and operation optimization was proposed by Saif et al., 38 and the time-variant operation constraints were considered. Lim et al. 39 developed a numerical simulation model to improve the RO process stability with an internally staged design, and the seasonal variation of feed seawater quality and temperature was considered. Optimal control models were utilized to move the key manipulated variables to their target points. ...
... Simulation and experimental results demonstrated that energy consumption could be effectively reduced. 40−42 Recently, optimum operation methods based on the actual RO plants 39,43,44 were reported, which could provide useful information on both design and operating of the RO plants. ...
Simultaneous optimization of size and operation for the seawater reverse osmosis (RO) system is proposed through a superstructurebased mixed-integer differential-algebraic programming approach. The influences of operating conditions on the performance of pumps, RO membrane, and the properties of seawater are considered. The permeate split (PS) design, interstage permeate split (ISPS) design, and on−off model could give more operability and cost saving. For constant feed conditions, nearly constant profiles of control variables are obtained with a larger water tank. The maximum level in a water tank is an indicator to measure its buffer capacity. For the variation of temperature and time-of-use electricity price, although energy consumption can be effectively saved by one time shut off at a large water demand time period with high electricity price, not much water cost is reduced due to the increase of the plant size. Low load operation is an alternative solution. The ISPS design could balance the flux distributions inside the pressure vessel and extract more permeates with high quality; thus, the water cost and energy consumption could be reduced, compared with the interstage design (ISD) and the PS design. The uncertainties of the maximum concentration of product water and the membrane fouling factor are considered in a twostage stochastic program. It is useful for identifying a robust optimal design and operation solutions for membrane-based seawater desalination
Membrane-based separations for water purification and desalination have been increasingly applied to address the global challenges of water scarcity and the pollution of aquatic environments. However, progress in water purification membranes has been constrained by the inherent limitations of conventional membrane materials. Recent advances in methods for controlling the structure and chemical functionality in polymer films can potentially lead to new classes of membranes for water purification. In this Review, we first discuss the state of the art of existing membrane technologies for water purification and desalination, highlight their inherent limitations and establish the urgent requirements for next-generation membranes. We then describe molecular-level design approaches towards fabricating highly selective membranes, focusing on novel materials such as aquaporin, synthetic nanochannels, graphene and self-assembled block copolymers and small molecules. Finally, we highlight promising membrane surface modification approaches that minimize interfacial interactions and enhance fouling resistance.
Desalination membranes are essential for the treatment of unconventional water sources, such as seawater and wastewater, to alleviate water scarcity. Promising research efforts on novel membrane materials may yield significant performance gains over state-of-the-art thin-film composite (TFC) membranes, which are constrained by the permeability–selectivity tradeoff. However, little guidance currently exists on the practical impact of such performance gains, namely enhanced water permeability or enhanced water–solute selectivity. In this critical review, we first discuss the water permeabilities and water–solute selectivities of current TFC membranes and the effect that these active-layer properties have on the performance of desalination processes. We then highlight and provide context for recent module-scale modeling studies that have found limited impact of increased water permeability on the efficiency of desalination processes. Next we cover several important examples of water-treatment processes where inadequate membrane selectivity hinders process efficacy due to permeation of undesired feed solutes and process efficiency due to the need for additional process steps. We conclude with a brief discussion of how the need for enhanced selectivity may influence the design strategies of future membranes.
Recovery and salt rejection rate, which are directly related to the membrane properties and operating conditions (i.e., feed flow rate, pressure, temperature, and TDS concentration), are two main indicators for evaluating the performance of a reverse osmosis (RO) membrane process. A simple and rapid test with minimum data is inevitably requested to diagnose membrane performance at a certain operating conditions, because experiments at all operating conditions are inefficient and consumable work in the view of cost as well as man-hour. In this study, permeate flow rate and TDS concentration of three kinds of commercial RO membranes were carefully examined under various operating conditions [i.e., feed pressure (45–65 kgf/cm) and temperature (5–30°C)]. Based on the experimental data, membrane resistance models including temperature and pressure correction factors were developed for the rapid diagnosis of SWRO membrane performances. As a result, the models developed in this study have good agreements between observed and simulated data. Based on the procedure in this study, the performance of any type of RO membranes can be rapidly examined by simple model parameter inputs. Furthermore, the developed diagnosis tool for performance test of SWRO membranes can be practically applied to build database of membrane performance for designing the SWRO process with minimum data as well as to reduce the cost and effort for data acquisition.
The development of concentration polarization in a spiral wound reverse osmosis membrane channel and the depolarization effect of spacers are important concerns for understanding the performance of membrane processes. However, an accurate quantification of these effects derived from fundamental principles is impractical due to the complexity of the processes. In this study, a macroscopic method was developed to estimate the effect of concentration polarization on the performance of the spiral wound membrane modules. Concentration polarization in a channel filled with spacers was described as a combination of two extreme cases, namely the undisturbed concentration polarization and complete depolarization (uniform distribution across the channel height). With the introduction of a polarization factor for the degree of concentration polarization, a mathematical model was developed for the permeate flux in the spiral wound modules. The proposed model was solved numerically to simulate the performance of a long membrane channel under various operation conditions. The simulation results demonstrated that the model developed in this study was a feasible way to estimate concentration polarization in spiral wound modules. Excellent fitness was found between the numerical simulations and experimental observations of the average permeate fluxes in along membrane channel of spiral wound membrane modules.
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.
Water scarcity is a grand challenge that has always stimulated research interests in finding effective means for pure water production. In this context, reverse osmosis (RO) is considered the leading and the most optimized membrane-based desalination process that is currently dominating the desalination market. In this review, various aspects of RO desalination are reviewed. Theories and models related to concentration polarization and membrane transport, as well as merits and drawbacks of these models in predicting polarization effects, are discussed. An updated review of studies related to membrane modules (plate and frame, tubular, spiral wound, and hollow fiber) and membrane characterization are provided. The review also discusses membrane cleaning and different pre-treatment technologies in place for RO desalination, such as feed-water pre-treatment and biocides. RO pre-treatment technologies, which include conventional (e.g., coagulation-flocculation, media filtration, disinfection, scale inhibition) and non-conventional (e.g., MF, UF, and NF) are reviewed and their relative attributes are compared. As per the available literature, UF, MF and coagulation-flocculation are considered the most widely used pre-treatment technologies. In addition, this review discusses membrane fouling, which represents a serious challenge in RO processes due to its significant contribution to energy requirements and process economy (e.g., flux decline, permeate quality, membrane lifespan, increased feed pressure, increased pre-treatment and membrane maintenance cost). Different membrane fouling types, such as colloidal, organic, inorganic, and biological fouling, are addressed in this review. Principles of RO process design and the embedded economic and energy considerations are discussed. In general, cost of water desalination has dropped to values that made it a viable option, comparable even to conventional water treatment methods. Finally, an overview of hybrid RO desalination processes and the current challenges faced by RO desalination processes are presented and discussed.
Internally staged design (ISD) was introduced as a method to lower front reverse osmosis (RO) element fluxes, yielding operational benefits. Systematic selection of the best ISD combinations has not been conducted as it is dependent on operating conditions (i.e., RO recovery and average flux). This study aimed to optimize seawater reverse osmosis (SWRO) operations using ISD for better product water quality and energy efficiency. A total of 36 ISD combinations were configured with three commercial SWRO membranes and systematically examined under varying operating conditions which were simulations of typical SWRO operation. A Pareto-optimal front, a non-dominated feasible solution set, was created to understand the tradeoff relationship between permeate quality and specific energy consumption while considering all parameters. The Pareto-optimal front was classified into five phases, and an analysis of phase changes provided optimal parameter settings for SWRO operation. However, the occurrence of colloidal fouling altered optimal ISD combinations on the Pareto-optimal front. Particularly, when SWRO membranes were fouled at high-recovery and -flux conditions, ISD combinations employing three different membrane types exhibited superior performance in terms of energy efficiency. With these results, the best SWRO operation was suggested depending on its performance requirements, and the advantages of ISD were further delineated.
In this study, Internally Staged Design (ISD) was numerically optimized in order to demonstrate the long-term performance of full-scale seawater reverse osmosis (SWRO) membrane systems in the presence of colloidal foulants. To this end, a numerical model based on a finite difference method was developed and optimized with various feed water qualities, such as fouling potential and total dissolved solids (TDS). As a result, the optimized ISD exhibited greater water flux and higher energy efficiency in long-term operation (90 days) compared to conventional designs, where single-type membrane elements are employed over a pressure vessel while achieving potable TDS concentration in permeate (<400 mg/L). This enhanced performance was attributed to the reduction of colloidal fouling on the lead elements and the alleviation of concentration polarization (CP) at the tail elements. Among fouling mechanisms, cake-enhanced concentration polarization (CECP) played a dominant role in flux decline at the lead elements, whereas CP was the chief flux decline mechanism at the tail elements. In addition, it was found that the employment of a low-permeability membrane at the lead element is important to maintain acceptable TDS in the permeate at either high TDS or high fouling potential of feed water.
In this work, the optimal schedule of membrane cleaning and replacement of spiral-wound SWRO (seawater reverse osmosis) system was studied and analyzed under different feed conditions. Firstly, the spiral-wound RO process model described by differential and algebraic equations was established with the consideration of membrane fouling. Then the optimization problem was formulated to minimize the daily cost of the SWRO system, and a new method by which the optimization problem was transformed as an NLP problem was proposed to solve the problem effectively and quickly. The method was used for SWRO case study and was compared with the traditional method in the form of MINLP. Computing results demonstrate that the proposed method has the advantages of efficiency and stability. Moreover, it can get schedule of membrane cleaning and replacement with lower operational cost as well as give more detailed information of membrane channel. Lastly, influence of feed temperature and feed concentration of seawater was analyzed with the new method. Computing results show the stability of the new method and lead to new profiles of optimal operational variables and performance when feed temperature and salt concentration change, which are quite different from those in designed conditions.
This study proposes an optimal study of reverse osmosis (RO) seawater desalination system based on the split partial second pass (SPSP) design in order to fully utilize better quality and higher flux for the permeate at the front of the pressure vessel (PV). Differential equations in the membrane transport model are solved by finite difference method, which considers the longitudinal variation of the superficial velocity, the pressure and the salinity inside the PV. Superstructure optimization is adopted to determine optimal flow structure, appropriate membrane type, and operation conditions. Firstly, single product RO system has been optimized, cost breakdown and sensitive analysis reveal that membrane with large active area and high salt rejection is favored for SPSP. System recovery, pressure, and split ratio for the permeate of pass 1 should be adjusted with time in order to decrease total annualized cost. Secondly, multiple product RO system has been studied. Three-pass RO configuration with permeate re-processed and brine recycled is favored for the system with two permeate products. The optimal RO configuration with three permeate products is influenced by flow rates and required permeate concentrations. In general, SPSP could provide lower cost, lower energy consumption, and smaller system size than normal design.
A fouling model for the seawater reverse osmosis (SWRO) process is required to propose an appropriate strategy for membrane maintenance (i.e., membrane cleaning and replacement). In this study, a fouling model that considered the concentration polarization was properly developed to simulate the long-term performance of the SWRO process based on models from the literature. The model was practically applied to one-year operation data obtained from the Fujairah SWRO desalination plant with parameter estimation, and good agreement between the measured data and simulated results was obtained for both the rejection and recovery rates. Compared to an integrated model consisting of two models from the literature, the fouling model proposed in this study showed reliable performance for membrane fouling such as the increase in the permeate TDS concentration according to operating time. In addition, simulations for membrane maintenance were conducted based on the variation in membrane resistance, which reflects the fouling state of the membrane. These revealed that the rearrangement of membranes is not significantly effective without cleaning the fouled membrane. Instead, a partial replacement of membranes can be an effective maintenance scheme to increase the recovery rate. Accordingly, the results of the study presented here can be used to save operation and maintenance (O&M) costs in SWRO plants through the optimized management of fouled membranes.
The design of reverse osmosis (RO) network for desalination for a wide range of salinity and seawater temperature is considered here. We start with a flexible superstructure that contains all possible alternatives of a potential RO network and refine the network during the synthesis using model based optimization technique. The synthesis problem is posed to find out an optimal RO network which will minimize the total annualized cost consisting of capital costs (costs of sea water pre-treatment, pump, turbine and membrane modules) and operating costs (pumping energy, chemicals, maintenance and membrane replacement) while fulfilling a given fresh water demand and quality. The problem results in a mixed-integer non-linear programming (MINLP) problem due to the presence of both integer and non-integer decision variables. Outer approximation algorithm within gPROMS software is used to solve the optimization problem. The results revealed that the feed salinity and temperature have significant impact on the RO network design and operation.
Polymer membrane-based desalination (e.g., reverse osmosis (RO) and nanofiltration (NF)) has been extensively developed since the 1960s and is a well-established process. The separation performance of desalination membranes is usually described in terms of water flux (or permeance) and salt rejection. Based on a survey of available data, water permeance and NaCl rejection are often inversely correlated, and there may be an upper bound, similar to that observed in gas separation membranes, beyond which there are very few data points. However, water permeance and salt rejection are not intrinsic material properties since they are influenced by sample size (i.e., membrane thickness in the case of permeance) and measurement variables (e.g., pressure and salt concentration in the case of salt rejection). Use of water permeability, rather than water flux or permeance, and water/salt permeability selectivity, rather than rejection, in a tradeoff analysis provides a clearer comparison of properties that depend only on the fundamental transport characteristics of the materials under study. When water and salt transport data are presented on a log-log plot of water permeability versus water/NaCl permeability selectivity, a tradeoff relation and upper bound are observed. Both water/NaCl solubility and diffusivity selectivity contribute to high water/NaCl permeability selectivity, but diffusivity selectivity is the dominant factor. Both solubility selectivity and diffusivity selectivity exhibit tradeoff and upper bound features when plotted as a function of water solubility and water diffusivity, respectively; these correlations combine mathematically, in accord with the solution diffusion model, to yield the observed tradeoff relation and upper bound correlation between water permeability and water/salt selectivity.
Desalination market growth has triggered significant development in SWRO membrane and process development and the new extra high rejection and ultra low energy membranes from Dow, FILMTEC™ SW30XHR-400i and SW30ULE-400i, as well as the internally staged design concept, have been validated in extensive field testing and various commercial plants over the recent years and are now commercially available. These solutions from Dow can be used to increase membrane flux and system recovery and / or to reduce feed pressure. This yields capital cost and /or energy savings. These savings have been assessed in 4 different situations / geographies, using a thorough and validated cost model. These geographies are South Pacific (Australia), Persian Gulf (Saudi Arabia), with very different feed water qualities (in terms of feed salinity and temperature range) and product quality requirements (in terms of bromide, boron and salinity). Depending on the cost savings route chosen, there are strong differences in the consequences with regards to size of the RO stage (17–26% smaller), size of the pretreatment (9–12% smaller), and/or the feed pressure (2–6 bar lower). These cost savings are in the range of US cent 0.4–4.1/m³ water produced. This is equivalent to 0.7–6.5% water cost saving. Considering that these considerable cost savings are readily available since 2008 from Dow Water Solutions, the industry should start to significantly benefit from these in the coming years.
The seawater reverse osmosis (RO) desalination is an attractive and viable method for the production of fresh water in many areas. This paper addresses the optimal design of RO desalination system considering membrane cleaning and replacing during the 5-year maintenance period, and only a single stage configuration with pressure exchanger is analyzed. A mathematical model for the prediction of the performance of RO process is presented in detail. Simultaneously, this paper also addresses the new fouling model and the criterion of cleaning and replacing. Then the relevant economic models to the RO desalination process are developed, which relate the cost of investment and operation with the design variables, the structural variable, as well as the binary variable that determine the membrane regeneration. The optimum design problem can be formulated as a mixed-integer non-linear programming (MINLP) problem, which minimizes the total annualized cost. The mathematical programming problem is solved with GAMS software. As a result, the optimal operational parameters and the optimal cleaning and replacing scheduling are given. According to this model, one example is solved to illustrate the advantage and effectiveness of the suggested method.
This paper describes the determination of critical flux and the cake enhanced osmotic pressure (CEOP) effect of colloidal silica fouling in reverse osmosis (RO) using a sodium chloride tracer response technique. The tracer technique gave the transient values of concentration polarization, CP, and cake resistance, Rf. From these values, the rate of accumulation of deposit, dmf/dt, could be readily determined. The critical flux, Jcrit, was defined as the flux at which the rate of deposition of colloidal silica on the membrane, became zero, i.e. dmf/dt=0. It was found that the critical flux concept was applicable to colloidal silica fouling in the RO process. The critical flux was strongly influenced by the crossflow velocity, such that Jcrit∼v0.4. In addition, the critical Peclet number, (Jv/km)crit, which is a measure of the convective to diffusive forces, to trigger silica fouling was approximately 0.52. Furthermore, it was observed that the amount of particles convected to and finally deposited on the membrane surface, known as the fractional deposition constant, Φ, was strongly influenced by the crossflow velocity. At low crossflow (
After a brief introduction to membrane processes in general, and the reverse osmosis process in particular, the structure and properties of membranes and membrane transport theory are described. The mechanism of salt rejection and transport properties of membranes are discussed in detail. Solubility, diffusivity, and permeability of membranes to solutes and solvents are reviewed critically and compared with each other. Special attention is given to two particular types of membranes, cellulose acetate (CA) and aromatic polyamide (AP) membranes, which are often used for water desalination. The major portion of this study is devoted to the review and discussion of membrane transport theory with application to the reverse osmosis and ultrafiltration processes. It is shown that the solvent flux can be represented reasonably well by linear models such as the solution-diffusion model. This work is pertinent to desalination, gas separation, and waste treatment.
A Hybrid membrane Inter-stage Design (HID) combines membranes of different nominal flux and salt rejection – i.e. different models –, in the same pressure vessel of seawater reverse osmosis racks. The HID experiences reported in the literature show there are no quantitative analysis published and very few seawater reverse osmosis plants have used it to date. The main objective of this paper is to present the results of a thorough computer assessment of this innovative design for conventional seawater reverse osmosis installations. Two types of analysis have been conducted for all representative models of membranes. Firstly, membrane replacement with a view to reduce the energy consumption due to lower working pressures while the plant capacity remains unchanged. Secondly, to increase plant capacity by reducing the specific energy consumption while maintaining the power consumption of the plant. A general HID design criterion has been defined for the three main membrane manufacturers – i.e. Filmtec, Hydranautics and Toray. Moreover, the quantitative results are useful for retrofitting plants thanks to membrane replacement.Highlights► Hybrid membrane Inter-stage Design (HID) uses membranes of different nominal flux. ► This paper presents the result of a thorough assessment for conventional seawater reverse osmosis installations. ► A general design criterion of HIDs for the three main membrane manufacturers has been defined. ► We quantify the operation and maintenance cost savings, as well as capital cost savings. ► The specific energy consumption of HIDs is lower than the standard PV design but similar for different HIDs/brands analyzed.
A predictive model was developed in this study for simulating the development of membrane fouling with time in full-scale RO processes. The increase in resistance was used as an indicator of membrane fouling and it enabled one to better describe membrane fouling taking place in a long membrane channel. In this model, fouling potential of a feed water was defined as the increment in membrane resistance due to a unit volume of permeate passing through the membrane, which was directly measurable with a simple filtration experiment. By employing the concept of fouling potential, fouling property of a feed water could be directly related to fouling rate on a RO membrane. The local variations in flow properties and parameters were explicitly taken into consideration in the model in order to provide a more realistic description of a full-scale RO process. The results of the simulation studies demonstrated that a full-scale RO system could maintain a constant average permeate flux for a period of time even though fouling development had occurred right from the start of operation. The effects of water fouling potential, channel length, and membrane resistance on fouling development in full-scale RO processes were investigated.
As a means of optimizing desalination processes, site-specificity in the determination of seawater quality conditions is a crucial point for improving the overall energy efficiency of a seawater reverse osmosis (SWRO) process. To this end, field studies were carried out at 16 sampling sites along the shoreline in South Korea to investigate the site-specific features of seawater quality. Also, two mathematical models were developed for the simulation of SWRO processes dependent on seawater quality in macroscopic and microscopic contexts, respectively. As a result, the microscopic dynamic model revealed that concentration polarization in the vicinity of the membrane surface and permeate concentration are affected by the feed seawater concentration and pressure. Then, the application of Fujairah SWRO plant operation data to the macroscopic simulation of a non-isobaric SWRO process model resulted in significant energy savings in terms of operational pressure savings, reducing 0.3 bar from the annual average value in the first pass operational pressure. These findings suggest that a cost-effective SWRO operation can be feasible using non-isobaric pressure controls by considering site-specific feed seawater concentrations. Results of the study presented here can be applied to improving the energy efficiency in SWRO plants through the optimization of pressurized systems.
A new viscosity–temperature equation and corresponding chart have been developed to extend the range of the current ASTM viscosity–temperature charts. This new chart and equation extends the temperature and viscosity range for hydrocarbons and, for the first time, has the ability to extend to the low viscosity regime of halocarbons and low temperature fluids. The new equation and chart can linearize liquid viscosity data from 0.04cSt and covers the temperature range from −210 to 500C for halocarbons and hydrocarbons. With a modification to the temperature scaling, the new equation also has the ability to fit liquid metal viscosity data. The new chart and equation cannot accurately linearize the viscosity with respect to temperature of fluids exhibiting strong molecular bonding (water, ammonia), fluids whose molecular structure consists of long coils (some long chained silicones), or fluid mixtures in which one fluid precipitates out of solution (wax precipitation).
The operation of seawater reverse osmosis (SWRO) plants is systematically optimized with the objective to reduce the electricity charges, which constitute the largest portion of the operation costs. Variable operating conditions and a time-of-use electricity tariff rate schedule are considered. A well-established model is extended to model a reverse osmosis (RO) system equipped with highly efficient pressure exchangers (PXs) and variable frequency drives (VFDs). All major performance factors are considered, including membrane's permeability, concentration polarization, temperature effects, membrane fouling, and efficiency dependency of the VFD on the loading conditions. The proposed formulation minimizes the electricity cost per day, given a desired daily production rate. The problem is formulated as a mixed-integer nonlinear program (MINLP), allowing for periods without operation. The time domain (24 h) is discretized into 0.5 h increments, and a pseudo steady-state assumption is used. The optimal operation plan resulting in maximum electricity cost-savings is established with standard global optimization tools. The results show significant electricity and production cost-saving potentials. More savings in electricity can be achieved by oversizing the plant, thus allowing shutting-off at periods of high electricity cost. The cost-saving potentials are particularly promising in electricity markets in which the electricity rate schedule is widely distributed.
The design of various multistage RO systems under different feed concentration and product specification is presented in this work. An optimization method using the process synthesis approach to design an RO system has been developed. First, a simplified superstructure that contains all the feasible design in present desalination process has been presented. It offers extensive flexibility towards optimizing various types of RO system and thus may be used for the selection of the optimal structural and operating schemes. A pressure vessel model that takes into account the pressure drop and concentration changes in the membrane channel has also been given to simulate multi-element performance in the pressure vessel. Then the cost equation relating the capital and operating cost to the design variables, as well as the structural variables of the designed system have been introduced in the objective function. Finally the optimum design problem can be formulated as a mixed-integer nonlinear programming (MINLP) problem, which minimizes the total annualized cost. The solution to the problem includes optimal arrangement of the RO modules, pumps, energy recovery devices, the optimal operating conditions, and the optimal selection of types and number of membrane elements. The effectiveness of this design methodology has been demonstrated by solving several seawater desalination cases. Some of the trends of the optimum RO system design have been presented.
The performance of a spiral wound element, i.e. permeate flow and quality can be calculated by numerical integration of balance equations. The same is valid for the calculation of optimal element geometry. For this a computer program is required which needs reliable input data regarding geometrical parameters and transport characteristics. This paper presents some experimental data for pressure drop and mass-transfer characteristics in spiral wound elements and in spacer-filled flow channels. The experimental results and the computer program were used to calculate the performance of a spiral wound element and its possible design variations.
A novel global optimization algorithm to solve nonconvex problem is used to find the global optimal design of reverse osmosis networks for seawater desalination. The objective is to determine the optimal process design and operating conditions for a given water production. The networks were designed by using hollow fiber reverse osmosis modules. The Kimura–Sourirajan model was used for describing transport phenomena of solute and water transport through the membrane. The concentration polarization phenomenon has been taken into account. It was mathematically described using the film theory. The objective function to be minimized is the cost, which includes capital investment (membrane cost, pumping and energy recovery system, intake and pre-treatment systems, etc.) and operation and maintenance costs (membrane replacement, chemical treatment, spares, required and recovered energy, etc.). The proposed algorithm is deterministic and attains finite convergence to the global optimum. It is iterative and a main problem is solved each iteration. The main problem has convex constraints and a nonconvex objective function. The main problem solution indicates either a better solution for the original problem, or a region which can be discarded. Therefore, the feasible region to improve the objective function is reduced each iteration. The algorithm finishes when the whole region has been analysed and discarded. A bound reduction technique is performed in order to accelerate the convergence speed. The algorithm shows a good performance and efficient execution time. Different cases are solved in order to show the methodology and computational performance.
Reverse osmosis membrane technology has developed over the past 40 years to a 44% share in world desalting production capacity, and an 80% share in the total number of desalination plants installed worldwide. The use of membrane desalination has increased as materials have improved and costs have decreased. Today, reverse osmosis membranes are the leading technology for new desalination installations, and they are applied to a variety of salt water resources using tailored pretreatment and membrane system design. Two distinct branches of reverse osmosis desalination have emerged: seawater reverse osmosis and brackish water reverse osmosis. Differences between the two water sources, including foulants, salinity, waste brine (concentrate) disposal options, and plant location, have created significant differences in process development, implementation, and key technical problems. Pretreatment options are similar for both types of reverse osmosis and depend on the specific components of the water source. Both brackish water and seawater reverse osmosis (RO) will continue to be used worldwide; new technology in energy recovery and renewable energy, as well as innovative plant design, will allow greater use of desalination for inland and rural communities, while providing more affordable water for large coastal cities. A wide variety of research and general information on RO desalination is available; however, a direct comparison of seawater and brackish water RO systems is necessary to highlight similarities and differences in process development. This article brings to light key parameters of an RO process and process modifications due to feed water characteristics.
Results from well-controlled colloidal fouling experiments with reverse osmosis (RO) and nanofiltration (NF) membranes suggest the existence of a new source of flux decline for salt-rejecting membranes-cake-enhanced osmotic pressure. The physical mechanisms leading to this enhanced osmotic pressure are a combination of hindered back-diffusion of salt ions and altered cross-flow hydrodynamics within colloidal deposit layers, which lead to an enhanced salt concentration polarization layer. A model that accounts for both hindered diffusion of salt ions and altered hydrodynamics within colloidal deposit ("cake") layers is presented. The model successfully links permeate flux and salt rejection to cake-enhanced concentration polarization and provides new insight into the mechanisms through which salt-rejecting membranes foul. Experimental data support the model calculations and highlight the role of enhanced concentration polarization phenomena in the performance (i.e., water flux and salt rejection) of polymeric thin-film composite RO/NF membranes in environmental applications.
Optimal operation of RO system with daily variation of freshwater demand and seawater temperature
- K M Sassi
- I M Mujtaba
K.M. Sassi, I.M. Mujtaba, Optimal operation of RO system
with daily variation of freshwater demand and seawater
temperature, Comp. Chem. Eng., 59 (2013) 101-110.