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Considerations of energy consumption in desalination by reverse osmosis
Abstract
The primary factors affecting the energy consumption of a reverse osmosis plant are considered. These are the osmotic pressure of the feedwater, the feedwater temperature, the water recovery, and the relationship between the water flux and salt flux characteristics of the membrane. In addition, the required permeate quality may have several indirect effects on the energy consumption. Permeate quality standards may impose minimum operating pressures, limit the recovery, and/or require treatment with a full or partial second stage. As a general rule, the energy required increases with increasing feed salinity and increasing permeate quality.For any given recovery, a single stage system will require less energy than a partial two stage system. However, for a specified permeate quality, a partial two stage system can operate at a higher overall recovery and a lower energy consumption than a single stage systemEnergy recovery systems can recover between 50 and 90 percent of the available energy in a reverse osmosis unit, thus significantly lowering the energy consumption. Studies have shown that with an energy recovery system, the minimum energy consumption occurs at a first stage recovery of 30 to 35 percent. Currently, very few energy recovery systems are in use due to their high capital cost, but as energy recovery systems become more available and reliable, they will greatly increase the energy efficiency of reverse osmosis plants.
... RO especially has become more popular in recent decades due to decreasing costs and advancing technology [5]. In desalination, the mechanism for RO causes the outflow of water through a semipermeable membrane by the application of pressure to the saline feed solution that is greater than its osmotic pressure, or the minimum pressure that prevents the inflow of pure water via osmosis [6]. Since RO membranes are designed such that the convective water flow is typically higher than the diffusive flow of salts, there is an accumulation of salts rejected by the membrane at the membrane surface [7]. ...
... Since RO membranes are designed such that the convective water flow is typically higher than the diffusive flow of salts, there is an accumulation of salts rejected by the membrane at the membrane surface [7]. Because osmotic pressure increases with feed salinity, a major disadvantage of RO is its inability to desalinate highly concentrated salt solutions due to the increased osmotic pressure at the membrane surface which also causes an increased passage of salt through the membrane [6,7]. ...
Membrane distillation (MD) is an emerging technology for the desalination of brines. In some cases, liquid penetrates into the pores of the membrane, causing pore wetting. MD’s commercialization is hindered largely due to the occurrence of pore wetting phenomena since it results in the reduction of flux and/or permeate quality. Hence, it is of crucial importance for MD to prevent pore wetting from occurring. In this paper, the methods of detecting pore wetting and the membrane parameters related to this phenomenon are reviewed and possible sources of MD pore wetting occurrence are identified. Moreover, the methods to prepare membranes specifically designed for the mitigation of membrane wetting, such as the design of membrane materials, membrane surface modification, preparation of nanocomposite membranes by the addition of nanoparticles, dual-layered membranes, and membranes with a re-entrant structure are discussed. Finally, process-based approach for wetting mitigation, mainly by the pretreatment of the feed solution, is elucidated, and models for wetting phenomena are also outlined. Thus, attempts are made in this review to discuss all aspects of the pore wetting of MD membranes.
... Augmenting 2% in the pump efficiency leads to a considerable decrease in SEC, particularly for feedwater with high salinity (e.g., SW) [56]. Operational indicators, such as feed salinity, permeate quality, recovery rate, and feed temperature, influence the needed pressure and energy consumption [57]. Electric and mechanical pumps may furnish the required pressure [32]. ...
In this review, the new solar water treatment technologies, including solar water desalination in two direct and indirect methods, are comprehensively presented. Recent advances and applications of five major solar desalination
technologies include solar-powered humidification–dehumidification, multi-
stage flash desalination, multi-effect desalination, RO, and solar stills. Each technology’s productivity, energy consumption, and water production costs
are presented. Also, common methods of solar water disinfection have been reviewed as one of the common and low-cost methods of water treatment, especially in areas with no access to drinking water. However, although desalination
technologies have many social, economic, and public health benefits,
they are energy-intensive and negatively affect the environment. In addition,
the disposal of brine from the desalination processes is one of the most challenging and costly issues. In this regard, the environmental effects of desalination
technologies are presented and discussed. Among direct solar water
desalination technologies, solar still technology is a low-cost, low-tech, and
low-investment method suitable for remote areas, especially in developing countries with low financial support and access to skilled workers. Indirect solar-driven water desalination technologies, including thermal and membrane
technologies, are more reliable and technically more mature. Recently,
RO technology has received particular attention thanks to its lower energy
demand, lower cost, and available solutions to increase membrane durability.
The brine disposal can account for much of the water cost and potentially negatively
affect the environment. Therefore, in addition to efforts to improve
the efficiency and reduce the cost of solar technologies and water treatment
processes, future research studies should consider developing new solutions
to this issue.
... When pressure is applied to the saltwater feed solution during the desalination process that is larger than just its osmotic pressure or the minimal pressure that prohibits the intake of pure water by osmosis, water is ejected through a semipermeable membrane [8]. RO is not advised for desalinating highly concentrated salt solutions because of the increasing osmotic pressure which also causes an accelerated flow of salt across the membrane [9]. ...
Water is a critical component for humans to survive, especially in arid lands or areas where fresh water is scarce. Hence, desalination is an excellent way to effectuate the increasing water demand. Membrane distillation (MD) technology entails a membrane-based non-isothermal prominent process used in various applications, for instance, water treatment and desalination. It is operable at low temperature and pressure, from which the heat demand for the process can be sustainably sourced from renewable solar energy and waste heat. In MD, the water vapors are gone through the membrane's pores and condense at permeate side, rejecting dissolved salts and non-volatile substances. However, the efficacy of water and biofouling are the main challenges for MD due to the lack of appropriate and versatile membrane. Numerous researchers have explored different membrane composites to overcome the above-said issue, and attempt to develop efficient, elegant, and biofouling-resistant novel membranes for MD. This review article addresses the 21st-century water crises, desalination technologies, principles of MD, the different properties of membrane composites alongside compositions and modules of membranes. The desired membrane characteristics, MD configurations, role of electrospinning in MD, characteristics and modifications of membranes used for MD are also highlighted in this review.
... About 2% increase in pump efficiency results in a significant reduction in SEC, especially for a feedwater with high salinity [170]. Feed salinity, permeate quality, recovery rate, and feed temperature are operational parameters, which have effects on the required pressure values and thus energy consumption [172]. The required pressure can be supplied both by electricity and by mechanical pumps [52]. ...
This chapter discusses about the conventional, advanced, and new novel water treatment methods. The conventional method and the advanced method used in the present scenario seem to be more energy-intensive and the treatment cost is also very high. Owing to these drawbacks, recent research is focused on the new cost-effective novel technology. The recent technologies and the modifications made are vigorously studied to meet the domestic and industrial needs. Also, the need for solar-driven water treatment technology as a sustainable approach to solve the water crisis is briefly presented.
... About 2% increase in pump efficiency results in a significant reduction in SEC, especially for a feedwater with high salinity [170]. Feed salinity, permeate quality, recovery rate, and feed temperature are operational parameters, which have effects on the required pressure values and thus energy consumption [172]. The required pressure can be supplied both by electricity and by mechanical pumps [52]. ...
Due to water and energy crises around the world, the use of solar-powered processes to supply drinking water in urban, rural, and remote areas has attracted much attention. Among these categories, desalination of seawater and brackish water seems a sustainable option in meeting drinking water needs. Desalination is the process of removing salt from water to produce drinking water. Most desalination techniques, including reverse osmosis and electrodialysis techniques, have high costs of construction, repair, and maintenance, and also they require consuming electrical power. In addition, their high capacities make these techniques unsuitable for areas with a dispersed population. Solar distillation is a promising alternative for these techniques. In this chapter the second generation of solar water treatment technologies, including solar water desalination in two direct and indirect methods, is comprehensively presented. Recent advances and applications of five major solar desalination technologies such as solar-powered humidification–dehumidification (HDH), multistage flash desalination (MSF), multieffect desalination (MED), reverse osmosis (RO), and solar stills have been comprehensively reviewed. In particular, solar stills have been described in detail as one of the oldest yet simplest methods of direct solar water desalination. Productivity, energy consumption, and water production costs of each of the different technologies are presented. Also, common methods of solar water disinfection have been reviewed as one of the common and low-cost methods of water treatment, especially in areas with no access to drinking water. Although desalination technologies have many benefits in social, economic, and public health aspects, they are energy-intensive and therefore have a detrimental effect on the environment. In addition, the disposal of waste from desalination processes is one of the most challenging and costly issues. In this regard, the environmental effects of desalination technologies are presented and discussed.
... About 2% increase in pump efficiency results in a significant reduction in SEC, especially for a feedwater with high salinity [170]. Feed salinity, permeate quality, recovery rate, and feed temperature are operational parameters, which have effects on the required pressure values and thus energy consumption [172]. The required pressure can be supplied both by electricity and by mechanical pumps [52]. ...
In general, human civilization has placed itself in areas with locally sustainable water sources, in the form of runoff, and/or rivers and streams [1].
Water is an essential life-sustaining element; human life on the earth has
become sustainable due to the presence of water, and agricultural productivity on the earth is also possible only in the presence of water. Water
has been discussed in terms of quantity and quality, wherein the quantity
measures the total available form of water that can be used for human and
agricultural practice and the quality indicates the suitability of the water
for drinking, domestic, and irrigation purposes
... Doubling of the second law efficiencies will result in reducing the current energy usage for desalination by half. Raising the efficiencies to 20% to 40% is a very realistic goal since many engineering systems in operation have second law efficiencies well over 50% [1,2]. ...
... Doubling of the second law efficiencies will result in reducing the current energy usage for desalination by half. Raising the efficiencies to 20% to 40% is a very realistic goal since many engineering systems in operation have second law efficiencies well over 50% [1,2]. ...
... where the concentration polarisation module C represents the concentration of salts near the surface of the membrane divided by that in the bulk solution, J is the flux of water through the membrane and S is its permeability. The minimum flux that must be used depends on the purity required in the permeate, since smaller flux tends to lead to a more saline permeate [13]. Thus the flux needed to obtain a required level of purity can be calculated from the salt transport coefficient B of the membrane as: ...
Desalination of groundwater is essential in arid regions that are remote from both seawater and freshwater resources. Desirable features of a groundwater desalination system include a high recovery ratio, operation from a sustainable energy source such as solar, and high water output per unit of energy and land. Here we propose a new system that uses a solar-Rankine cycle to drive reverse osmosis (RO). The working fluid such as steam is expanded against a power piston that actuates a pump piston which in turn pressurises the saline water thus passing it through RO membranes. A reciprocating crank mechanism is used to equalise the forces between the two pistons. The choice of batch mode in preference to continuous flow permits maximum energy recovery and minimal concentration polarisation in the vicinity of the RO membrane. This study analyses the sizing and efficiency of the crank mechanism, quantifies energy losses in the RO separation and predicts the overall performance. For example, a system using a field of linear Fresnel collectors occupying 1000m2 of land and raising steam at 200°C and 15.5bar could desalinate 350m3/day from saline water containing 5000ppm of sodium chloride with a recovery ratio of 0.7.
... Doubling of the second law efficiencies will result in reducing the current energy usage for desalination by half. Raising the efficiencies to 20% to 40% is a very realistic goal since many engineering systems in operation have secondlaw efficiencies well over 50% [5,6]. It appears that the thermodynamic analysis of desalination processes as well as actual plant operational data are necessary to put various techniques in perspective, and the best analytical tool to do it is the second law analysis. ...
The exergy analysis of a 7250 m3/d reverse osmosis (RO) desalination plant in California was conducted by using actual plant operation data, and an alternative design was investigated to improve its performance. The RO plant is described in detail, and the exergies across the major components of the plant are calculated and illustrated using exergy flow diagrams in an attempt to assess the exergy destruction distribution. The primary locations of exergy destruction were the membrane modules in which the saline water is separated into the brine and the permeate, and the throttling valves where the pressure of liquid is reduced, pressure drops through various process components, and the mixing chamber where the permeate and blend are mixed. The largest exergy destruction occurred in the membrane modules, and this amounted to 74.07% of the total exergy input. The smallest exergy destruction occurred in the mixing chamber. The mixing accounted for 0.67% of the total exergy input and presents a relatively small fraction. The second law of efficiency of the plant was calculated to be 4.3%, which seems to be low. The analysis of the alternative design was based on the exergy analysis. It is shown that the second law of efficiency can be increased to 4.9% by introducing a pressure exchanger with two throttling valves on the brine stream, and this saved 19.8 kW electricity by reducing the pumping power of the incoming saline water.
Seawater desalination using a cost-effective reverse osmosis system is crucial for hot climate countries suffering from water scarcity. The most favorable seawater membrane characteristics were identified under typical Egyptian operating conditions. Twelve different commercially available membrane elements were investigated. A reverse osmosis system was designed and simulated using available software (e.g., ROSA and IMSDesign). The characteristics of the most promising membranes were identified for operation at Matruh (Mediterranean Sea) and Sharm El-Sheikh (Red Sea). The present work shows that the lowest cost of seawater desalination is obtained with membranes having high salt rejection, high permeate flow, high membrane active area, and permeate flux greater than 0.914 m³/(d·m²). Moreover, the cost of seawater desalination in summer is lower than in winter by 5% for Matruh and 2.7% for Sharm El-Sheikh. However, the impact of water salinity on the cost and specific energy consumption is higher than that of the seawater temperature. The cost of Mediterranean seawater desalination is lower than that of the Red Sea by 10.6%. Cost analysis at five different locations in Egypt shows that the highest cost takes place at Suez (Gulf of Suez), and the lowest cost occurs at Matruh (Mediterranean Sea).
Practitioner Points
• Twelve different membranes were investigated for use under typical hot climate conditions.
• The cost of seawater reverse osmosis (RO) desalination is lower in summer by 2.7%–5% compared with winter.
• RO desalination costs up to 10.6% less for the Mediterranean Sea compared with the Red Sea.
• The optimum membrane element performance characteristics were identified for use under hot climate conditions.
The sequestration of CO2 within stable mineral carbonates (e.g., CaCO3) represents an attractive emissions-reduction strategy because it offers an energy efficient, environmentally benign, and leakage-free alternative to geological storage. However, the pH levels of aqueous streams equilibrated with CO2-containing gas streams (pH ∼4) are lower than the pH required for carbonate precipitation (pH > 8). Thus, the use of regenerable ion exchange materials is proposed to induce alkalinity in CO2-containing aqueous streams to achieve the pH required for mineralization without the addition of expensive stoichiometric reagents such as caustic soda (e.g., NaOH). Herein, geochemical and process-modeling software was used to identify the optimum thermodynamic conditions and to quantify the energy intensity and CO2 reduction potential of a process that sequesters CO2 (dissolved in wastewater) as solid calcium carbonate (CaCO3). CaCO3 yields were maximized when the initial calcium to CO2 ratio in the aqueous phase was 1:1. The energy intensity of the process (0.22-2.10 MW·h/t of CO2 removed) was dependent on the concentration of CO2 in the gas phase (i.e., 5-50 vol %) and the produced water composition, with the nanofiltration and reverse osmosis steps used to recover magnesium and sodium ions requiring the most energy (0.07-0.80 MW·h/t of CO2 removed). Energy consumption was minimized under conditions where CaCO3 yields were maximized for all produced water compositions and CO2 concentrations. The ratio of net CO2 to gross CO2 removal for the process ranged from 0.05 to 0.90, indicating a net CO2 reduction across all conditions studied. The results from these studies indicate that ion exchange processes can be used as alternatives to the addition of stoichiometric bases to provide alkalinity for the precipitation of CaCO3 at the CO2 concentrations studied, thereby opening a pathway toward sustainable and economic mineralization processes.
Reverse osmosis, distillation, and freeze desalination processes were analyzed using the first and second-laws of thermodynamics with attention to the minimum separation work requirement and the flow exergy. The minimum work for complete separation was investigated by first considering reversible processes for which entropy generation and exergy destruction are zero. Minimum work relations for complete separation of mixtures were obtained and presented in various convenient forms. These relations were later employed to develop the minimum separation work for incomplete separation of saline water solution encountered in desalination plants. The minimum work input was determined for various salinities of incoming saline water and outgoing brine and product water, and the results were tabulated and plotted. The minimum work values show that a lower and an upper limit for the minimum work exist at corresponding recovery ratios of 0 percent and 100 percent. The plots of the minimum work versus recovery ratio at various salinities of the incoming saline water also show that there is an optimum value of the recovery ratio which decreases with increasing salinity. Using reverse osmosis, distillation, and freeze desalination processes, it is shown that the minimum separation work is independent of any hardware or process and thus the same for all processes.
Next, the exergy analysis of typical ideal and actual desalination processes was conducted together with the discussion of the minimum separation work requirement. The exergy changes of major components were calculated and illustrated using exergy flow diagrams for four desalination systems using actual plant operation data. Three systems were part of a brackish water desalination plant in California that incorporates RO (reverse osmosis), NF (nanofiltration), and EDR (electrodialysis reversal) units. Each unit produces about one million gallons of fresh water per day. The fourth plant is located near the city of Al-Jubail at the Arabian Gulf coast. This MSF (multi-stage flash) plant consists of 40 distillation units, and each unit consists of 22 flashing stages. The plant is capable of producing distilled water at a rate of 230 million gallons per day. Exergy flow rates were evaluated throughout the plant, and the exergy flow diagrams were prepared. The rates of exergy destruction and their percentage were indicated on the diagram so that the locations of highest exergy destruction could easily be identified. The analysis showed that most exergy destruction occurs in the pump/motor and the separation units. The fraction of exergy destruction in the pump/motor units is 39.7 percent for the RO unit, 23.6 percent for the NF unit, 54.1 percent for the EDR unit and 78 percent for the MSF unit. Therefore, using high-efficiency pumps and motors equipped with VFD drives can reduce the cost of desalination significantly. The second-law efficiencies for these systems were: 8.0 percent for the RO unit, 9.7 percent for the NF unit, 6.3 percent for the EDR unit, and 4 percent for the MSF unit. These very low efficiencies indicate that there are major opportunities in the plant to improve thermodynamic performance by reducing exergy destruction and thus the amount of energy supplied, making the operation of the plant more cost effective.
The use of membrane processes for controlling trihalomethanes (THMs) was investigated for Florida groundwater sources and one surface water source. All of the sources were used for public supply and had excessive THMs (>300 μg/L) in the finished water. The performance and projected cost of the membrane system using groundwater sources are reported in this article. Se investigó el uso de procesos de membranas para el control de trihalometanos (THMs) en fuentes de agua subterráneas y una fuente de agua superficial en Florida. Todas las fuentes eran usadas para el suministro público y tenían THMs excesivos (300 μg/L) en la muestra final. En este artículo se documenta el empleo y costo estimado del sistema de membrana que usa fuentes de agua subterráneas.
This paper summarizes the results of an analytical study carried out to determine the technical and economic feasibility of wind driven reverse osmosis desalination systems. Four different types of reverse osmosis are considered including a diesel engine driven system (used for a baseline comparison), two Autonomous Wind Diesel Systems (AWDS), and a mechanical type wind powered system. A major objective of this work was to model systems comprised of commercially available components (including horizontal axis wind turbines, high pressure positive displacement pumps for use in reverse osmosis applications, and reverse osmosis membranes), in order to determine their present economic performance. The general conclusions from this study show that the AWDS type of system can be economically competitive when higher wind velocities and fuel costs prevail, however, for most average wind velocities and fuel prices, the diesel powered systems are still the most economic. Also, extrapolation of the mechanical type wind driven system showed that using a larger wind turbine than the one initially modeled can result in the production of potable water which is cost competitive in the higher speed wind regimes and fuel cost scenarios.
Energy consumptions and costs of desalting systems are among the main parameters affecting the choice of certain desalting system and desalted water final cost. The cost of the consumed energy depends, beside the performance of the plant, on the quality of energy, i.e., either mechanical or thermal, and if thermal at what temperature. The cost depends also on the way this energy is applied (e.g. thermal energy can be directly applied from a boiler or from cogeneration steam turbine, or from heat recovery steam generator of a gas turbine). This paper uses the concept of availability to bring different kinds of energy to a common basis of comparison. The way energy applied was considered in comparing the energy consumptions of different desalting systems, namely, multi stage flash MSF, multi effect boiling MEB, thermovapor compression TVC, mechanical vapor compression MVC and reverse osmosis RO desalting systems. Rational basis for costing thermal and electrical energy were used in comparing the energy cost of different types of exsisting desalting systems.
Recent advances in gas turbine cycles and combined gas/steam combined cycles are presented. First and second law analysis of gas turbine cycles is presented and shows that substantial available energy (up to 30% of fuel availability) is carried out by gas turbine exhaust gases. This available energy can be utilized for further heat to work conversion or for direct process heating. Heat recovered from gas turbine exhaust gases in heat recovery steam generator (HRSG) can generate relatively high pressure steam to be supplied to Rankine steam power cycle to form gas/steam power cycle of high efficiency (40–45%). Steam can be generated at low pressure in the HRSG for direct supply to the brine heater of a multistage flash MSF desalting system. Possible combination of simple gas turbine cycle or combined gas/steam cycle with reverse osmosis or MSF sea water desalting systems are presented and evaluated. The evaluation indicates that most of these combination are more superior than combination with steam power plants from energy view points.
A typical ideal distillation process is proposed and analyzed using the first and second-laws of thermodynamics with particular attention to the minimum work requirement for individual processes. The distillation process consists of an evaporator, a condenser, a heat exchanger, and a number of heaters and coolers. Several Carnot engines are also employed to perform heat interactions of the distillation process with the surroundings and determine the minimum work requirement for processes. The Carnot engines give the maximum possible work output or the minimum work input associated with the processes, and therefore the net result of these inputs and outputs leads to the minimum work requirement for the entire distillation process. It is shown that the minimum work relation for the distillation process is the same as the minimum work input relation for an incomplete separation of incoming saline water, and depends only on the properties of the incoming saline water and the outgoing pure water and brine. Also, certain aspects of the minimum work relation found are discussed briefly.
Potential seawater desalination applications require an evaluation of both single-stage and two-stage reverse osmosis system configurations when selecting the most cost effective process design for a specific application. The preferred reverse osmosis staging should be derived from cost factors, membrane performance parameters, feedwater characteristics and other application site-specific considerations.This paper presents a comparative analysis of single-stage and two-stage seawater reverse osmosis systems, illustrating how the optimum design is influenced by the choice of operating parameters and related cost factors. Computer aided design techniques used by Fluid Systems in evaluating reverse osmosis array options are discussed.Performance characteristics of Fluid Systems' single-stage and high-flux seawater elements are used in the analyses to arrive at the optimum system designs for a range of feedwater salinities. The approach of using pilot plant test results for long-term performance predictions is described.
Economic comparison is made for three different plant sizes. Small plants with a capacity below 100 m3/d; medium-scale plants with a capacity between 100 and 200 m3/d, and large plants with a capacity above 200 m3/d.Compared are solar stills, solar-assisted desalination, reverse osmosis for small plants. Published comparisons of medium and large commercially available plants are reviewed.The economics of two new and one as yet undeveloped processes are compared. Dehumidification of air and a direct contact freezing process are new. Development of the butane freezing process for large plants can reduce water-cost by over 30%.
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