Features of The Most Common Desalination Processes - Coursework Example

TABLE OF CONTENTS INTRODUCTION 2 2. OVERVIEW OF MOST COMMON DESALINATION TECHNOLOGIES 4 2 THERMAL BASED DESALINATION PROCESSES (WITH PHASE CHANGE) 6 2.1.1 DISTILLATION 6 2.1.2 MULTI-STAGE FLASH DISTILLATION 7 2.1.3 MULTIPLE EFFECT DISTILLATION 8 2.1.4 VAPOR COMPRESSION DISTILLATION 9 2.1.5 EVAPORATION 11 2.1.6 FREEZING 12 2.2 MEMBRANE BASED DESALINTION PROCESSES (WITHOUT PHASE CHANGE) 14 2.2.1 REVERSE OSMOSIS 14 2.2.2 ELECTRODIALYSIS 17 2.2.3 NANOFILTRATION 18 2.3 COMBINATION OF THERMAL AND MEMBRANE BASED DESALINATION 18 2.3.1 MEMBRANE DISTILLATION 19 3. FUTURE PROSPECTS 20 REFERENCES 21 1. Introduction Water seems to be all around us; statistically, around 3/4th of earth is water and yet water scarcity is a major global issue. 97% of global water resource comprises of sea water which is rendered unfit for use because of its salinity. Of the remaining 3%, 67.8% is locked in glaciers and ice caps, leaving barely 1% to be used by all life forms on earth (fig. 1).(8) This added to the unquestionable fact that water is essential for any life form, its not difficult to comprehend the predicament of the balancing act nations have to go through in managing water resources. Besides the limited supply, the cause of water scarcity lies in the increasing demand due to ever increasing population, higher standards of living, escalating agriculture and industrial usage. As a result the list of countries undergoing water shortage is increasing: Australia, Israel, Iraq, Greece, Spain, Syria, Sri Lanka, U.S.A. and U.K. Further these Problems are expected to aggravate in coming years. Figure 1 (23) Besides the water conservation and recycling techniques, alternatives to use unavailaible water resources, specifically sea waters are being explored.(10) Desalination or reduction of salt content of brackish water, is one of the earliest methods of getting salt free water, known to man. In nature, it forms the source of hydrological cycle. Natural water contains a variety of salts viz. Sodium chloride, Calcium carboanate, magnesium sulphate etc in varied concentrations, which give a distinctive taste and odour to otherwise tasteless, odourless water. In absence of these, water would taste “flat” and would be unpalatable. Sea water has a very high concentration of these (table 1), imparting it an unpleasant taste and also unfit for consumption. Desalination refers to the process of reducing salt concentration of saline water so as to make it palatable and fit for consumption. In addition to salt removal, some of the techniques of desalination also remove suspended material, organic matter, bacteria and viruses. (22) Type of Water TDS value (mg/l) Sweet waters 0-1000 Brackish waters 1000-5000 Moderately saline waters 5000-10,000 Severely saline waters 10,000-30,000 Seawater >30,000 Table 1: Classification of natural waters based on TDS values. (20) 2 Overview of Most Common Desalination Techniques The basic technique of desalination involves feeding saline water to process chamber where it is subjected to either heat, water pressure or electrical energy. The outlets are two, one for desalinated and the other for concentrated salt water, which is discarded (fig.2). Figure 2 There are two basic techniques of desalination: Thermal desalination involving evapoartion of saline water followed by condensation of pure water vapour; or freezing of saline water followed by melting of pure ice. Membrane desalination involving passing saline water through selectively permeable membrane. On the basis of the TDS value of the saline water an appropriate method can be selected (table 2). The relative distribution of different desalination plants worldwide is given in figure 3. TABLE 2: DESALINATION METHODS BASED ON TDS VALUE OF FEEDWATER (20) Figure 3 (10) 2.1 Thermal based desalination processes (with phase change): The thermal based deslaination processes are based on either evaporation or freezing. Post evaporation, steam is collected, condensed to obtain pure water. Evaporation can be a normal single stage distillation process used at small scale or a multi stage process, it can take place over a heat transfer area i.e. boiling, or in the bulk liquid, i.e. flashing. 2.1.1 Distillation: It is the simplest process of collecting pure water by the combined process of distillation and condensation. Either fuel or solar energy can be used to eavporate salt water. This is a basic survival technique and can be used only at small scales. An assembly for small scale distillation is shown in fig. 4. Figure 4 (23) 2.1.2 Multi-stage flash distillation: Large scale distillation units involve the principle of flash distillation which is based on the theory that lowering the pressure in a sealed container would lower the boiling point of a liquid in the container. Thus in this method, a series of distillation chambers are used, each of which is at a lower temperature and pressure than the previous so that some water is flashed off or evaporated from each chamber (figure 5). Figure 5 The MSF power plant consists of a heat input, heat recovery and heat rejection sections. (15) Low pressure steam is used as a heat source. The incoming feed water is heated in a brine heater before entering the first stage, a small proportion of which immediately produces vapours or flashes off. At every consequtive stage, maintained at a gradually lower temperature and vapour pressure, an additional amount of brine flashes to steam. The collected steam is condensed to form distilled water on the outside of tube conveying incoming feedwater. Scale formation is a major problem of MSF plants and either a high temperature additive is used for scale control or acids are used (13), in which case a decarbonator is provided to remove CO2 converted from bicarbonate in sea water by the acid (12). A vaccum deaerator is also used to remove dissolved gases from saline water. The water quality is determined by the pressure in each stage, while the production rate is a function of number of stages used. However, more stages means more cost. An alternative to reduce cost is to mix 50% to 75% of waste concentrate from previous satge with incoming feed, thus providing heat as well as reducing pretreatment need. But this also means more corrosion and scaling of the plant due to higher salt content of the incoming feed. MSF is the most reliable and most used distillation technique of desalination, being used for three decades now, in over 55 countries accounting for over two third of commercial desalination market. (25) 2.1.3 Multiple-effect distillation (MED): MED or long tube vertical distillation (LTV) involves the use of steam to heat up the water in first stage, and the resulting vapour is used to evaporate the water in the subsequent stages, so that the temperature progressively falls at each stage (fig 6). The assembly consists of a series of vertically placed evaporators or effects, which consists of tubes that are preheated by steam from an outside source. Seawater is sprayed on the tubes to facillitate rapid evaporation. A portion of the water is evaporated and collected as it condenses at the opposite end of the tube. The remaining water moves on to second effect, where it is again sprayed on a bundle of tubes preheated using the heat of condensation of vapour from the first effect. Thus, repeated boiling of sea water is achieved without further heating. The process continues for 4 to 21 effects, the performance ratio for which varies from 10 to 18. (16) The steam economy of the MED is dependent on the number of effects, which in turn is determined by the temperature range available and the lowest temperature difference allowed from one effect to another. Figure 6: Schematic diagram of a horizontal tube multiple effect desalination plant (4) MED plants can be horizontal, vertical or submerged, of which the horizontal are the oldest of the desalination plants; first to be developed for large scale applications. (2) Thermodynamically also, the MED plants are the most effcient, their power consumption being much lower than MSF plants, and performance ratio much higher. An average MED plant recovers 405 to 60% of the volume of sea water as fresh water. But still the MED plants are not much in use, only recently the number of MED plants has started increasing. They account for 5% of world’s distillation capacity. 2.1.4 Vapor compression distillation (VCD): VCD, like MFD is also based on the principle that boiling point of a liquid decreases when vapour pressure is reduced in a sealed container, however unlike MFD, here heat for evaporating sea water comes exclusively from compression of vapour. Assembly units of a VCD use two modes to utilise heat of condensation of vapour for evaporating incoming feedwater: Mechanical Compressor (figure 7), which is electrically driven. Steam jet or thermocompressor (figure 8), in which a low pressure is created as water vapour emanates from a venturi orifice. This water vapour is mixed, compressed and at the walls of tube. This in turn provides heat enrgy to evaporate further incoming sea water sprayed on the other end of tube. Figure 7 Conceptual diagram of vapour compression desalination process (10) The VCD plants are simple, reliable and efficient. The enrgy requirement is only at start up (25), the rest being provided by the compressor. VCD operates at low temperature with the help of a high capacity compressor. This reduces scaling and corrosion. About 2% of world desalination plants are VCD plants. They are usually diesel powered and are used on ships, off shore oil rigs or resorts in regions with water scarcity. (5) 2.1.5 Evaporation: This technique is based on hydrological cycle i.e. involves the use of solar energy to evaporate sea water, and condense it to obtain pure water. (6) A number of cofigurations making use of this technique have been functional, but the most popularly used is greenhouse still (figure 8). In this assembly an air tight glass enclosure, with a blck bottom pool is used. The pool is filled with saline water, which evaporates using the solar energy absorbed by the pool bottom. The vapours rising from saline water condense on the cooler insides of the glass surface. The condensed droplets collect in troughs placed along lower edges of glass panel and are thereafetr chaneeled into storage tanks. Figure 8: Solar distillation syatem (10) There are three major limitations of the process, weather dependence, large solar collection area requirement and high capital cost. It has a high capital cost to operating cost ratio (4:1), for others it is 2:3. Overall cost of water produced is very high, $50 to $80 per 1000 gallons. (4) 2.1.6 Freezing: Freezing of salty water is characterized by exclusion of dissolved salts during formation of ice crystal formation. This forms the basis of freezing desalination. The procedure involves cooling of feedwater, partial freezing under controlled conditions to obtain ice crystals, separation of ice from sea water, melting, referigeration and heat rejection. (15) A number of methods have been described to carry out the above steps efficiently viz; triple point, secondary referigerant, indirect, eutectic and hydrate processes. (14) Figure 9: Vacuum Freeze Desalination (10) Freezing as a technique of desalination has several advantages: The referigerant used is n butane, which has multiple advantages of being inexpensive, abundant and environmentally safe. The process is highly energy efficient since the two main heat transfer processes of freezing and melting are regenerative. Infact, the energy requirement of the process is comparable to reverse osmosis. (21) Corrosion is minimum due to the process being carried out at low temperature. Still the process fails to be a success commercially because of several disadvantages which include sizing and design of components, operating and controlling the freezing and ice washing process, separation of ice crystals from unfrozen salt water and melting ice crystals. (17) The biggest limitations, however are the ones associated with the referigerant compressor, viz; Contamination of ice with lubricant used in the compressor. Risk of damage to the compressor by water vapour or droplets carried along with referigerant vapours. Precautions to manage this drawback involve use of demisters and desiccators which further complicate the procedure and also increase the input cost. Compressor efficiency is lowered when operated with low pressure referigerants such as n-butane at atmospheric pressure and freezing temperature of sea water. A hydraulic referigerant compressor used instead, overcomes some of these problems since it does not need a lubricant, and is not damaged by water vapour and droplets coming in contact with it. (18) The use of this compressor in freeze desalination has the potential to make it a commercially more viable and therefore, needs intensive research. 2.2 Membrane based desalination processes (without phase change): Membrane desalination process use permeable membranes to selectively remove dissolved salts when subjected to gradients of pressure or electric potential. The membrane used are synthetic polymeric membranes. The chief membrane based processes are reverse osmosis, electrodialysis and nanofiltration. 2.2.1 Reverse Osmosis (RO): Chemical potential of a solution is a function of its temperature, pressure and concentration. When two solutions of varying concentrations are placed together, separated by a semipermeable membrane, under conditions of constant temperature and pressure, there will be flow of water from higher to lower chemical potential, till equillibrium is achieved. The driving force being osmotic pressure (OP). This process can be reversed by subjecting the solution at higher concentration to a pressure in excess of OP, and the process is then known as reverse osmosis (RO). An RO desalination plant consists of components to facilitate the 4 major processes (1): Feed water treatment: Pretreatment is essential to overcome the undesirable characteristics of water which may cause membrane fouling. (9) Depending on the characteristics of feed water and membrane used pretreatment methods are chosen. Suspended solids are removed by filtration through trash racks and travelling screens. Further multimedia gravity filters containg anthracite, sillica and/or granite are used to rmoved dissolved solids. Chlorination, coagulation, acid addition, micron cartridge filtration and dechlorination are other pretreatments procedures. Microbial growth is kept under control by addition of sodium hypochlorite and sulfuric acid is used to control hydrolysis and scale formation. High pressure pumping: Raising the pressure of feed water to enable it to pass through membrane and enable rejection of salts is done by stainless steel pumps. Centrifugal pumps with pressure range of 50-80 bars are generally used. Membrane Seapartion: It is imporatnt to have a high surface area of membrane to facilitate larger quantity of water travelling through membrane simultaneously. Two of the most used configurations enabling this are (3) (figure 10): spiral wound with several alternating layers of membrane sheets and spacer fabric. hollow fine fibre with cellulose acetate and polyamide membranes are packed in a U shaped fibre bundle. Figure 10 (20) Permeate post treatment: Post reatment involves: pH adjustments removal of dissolved gases if any. disinfection Figure 11 (20) RO plants need no heating or phase change, the only energy requirement being pressure application to feedwater. Corrosion too is not a major problem as the procedure is performed under ambient conditions. Moreover, development of more durable and efficient membranes and energy recovery devices, during recent years have helped reduce the operating cost of RO desalination plants. (7) 2.2.2 Electodialysis: Electrodialysis utilizes electrical energy for the process of desalination and uses the principle of movement of salt ions towards their respective electrodes in presence of electric field. Cation exchange and anion exchange membranes, which selectively allow cations and anions respectively, further facilitate the desalination. The process involves pumping of saline water at low pressure between ion exchange membrane stacks with alternate stacking of anion and cation exchange membranes. Electrodes placed at the ends of stack establish a direct electric current which pulls salt ions towards their respective electrodes, eventually stacking them between alternate pair of membranes (figure 12). Figure 12 (4) Scaling or fouling of membranes used in an electrodialysis unit is kept under check by reversing the direction of flow of electric current every 15 minutes. (19) 2.2.3 Nanofiltration: Nanofiltration is a pressure driven filtration process using a selective membrane made of organic semipermeable material. The driving force for the separation being pressure difference on the two sides of the membrane. The membrane selection is a function of molecular weight and charge characteristics of the material passing through it. It allows only molecules with molecular weight less than 200D, while retaining larger molecules. It also retains polyvalent molecules, allowing only monvalent molecules. A naonofiltration membrane has two parts: Thin barrier layer or membrane to act as separating layer. Microporous sublayer or base fibre composd of hollow fibre reinforced with fibre glass to give compression and collapse pressure (26). Research to utilize nanofiltration technology for desalination is still in its nascent stage and evidently, it is expected to play a prominent role in providing usable water in days to come. 2.3 Combination of thermal and membrane based desalination processes: Hybrid desalination plants using a combination of two or more technologies are being tested in an attempt to reduce the capital cost of the desalination plant. Also the bottleneck of any desalination technology being energy input, the hybrid technology is being researched to optimize it. The most successful of these have been processes based on thermal distillation and membrane filtration. 2.3.1 Membrane Distillation: Membrane distillation is a hybrid technology, a thermally driven membrane process (review of memb proc). A hydrophobic microporous membrane is used in this process, which separates hot and cold water, allowing water vapour to pass through; while restricting the movement of liquid water, due to its hydrophobic nature. Thus, filtered distillate condenses on the colder surface, which is doubly purified. Figure 13 (24) For desalination of sea water, an elevated temperature of 80.C is maintained on one side of membrane, and the other side is maintained at 75.C. Thus a vapour pressure gradient is established, facilitating the movement of water vapour across the membrane. Types of MD based on differences in distillate collection, mechanism of transfer through membrane etc are (11): Direct Contact Air Gap Sweeping Gas Vacuum 3.Future Prospects: Desalination is expected to be the method of choice for overcoming water scarcity in coming days. Continuous endeavours to overcome bottlenecks of energy consumption and brine generation with ongoing researches will make the technology more lucrateive for industrial, irrigational and drinking purposes. Intensive research for selection of technology based on local weather, energy resource availability and brine characteristics is needed to find the method most suitable for an area. Use of renewable energy and safe disposal or possible use of concentrated brine should be the pivotal points of research. Environmental impacts of the process should be carefully looked into before making long term economic investements. References: 1. Baig MB, Al Kutbi, A. 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Desal. water reuse. 1999; 9(2): 21-29. 8. Choi, CQ... Nano World: Water, water everywhere nano. March 18, 2005; United Press International. http://www.wpherald.com (3/27/2005). 9. Durham B, Walton, A. Membrane pretreatment of reverse osmosis: long term experience on difficult waters. Desalination. 1999; 122(2):157-170. 10. Foundation for water research. Desalination for water supply. U.K. February 2006. Available at www.fwr.org/desal.pdf. 11. Gryta M. Osmotic MD and other membrane distillation variants. J. Membr. Sci. 2005; 246: 145–156. 12. Harris A. Sea water chemistry and Scale control, Desalination Technology Development and Practice in A. Porteous (Ed.). Applied Science Publishers, London, UK. 1983. p. 31-56. 13. Jambi FI, Wie JM. The Royal Commission Gas Turbine/HRSG/Desalination Cogeneration Plant, 1989 ASME COGEN-TURBO, 3rd International Symposium on Turbomachinery, Combined-Cycle and Cogeneration, American Society of Mechanical Engineers, New York. 1989. p. 275–280. 14. Johnson WE. The story of freeze desalting. Desal. water reuse. 1993; 3(4): 10-27. 15. Khawaji AD, Kutubkhanah, IK, Wie, J. Advances in seawater desalination technologies. Desal. 2008; 221: 47-69. 16. Michels T. Recent achievments of low temperature multiple effect desalination in the western area of Abu Dhabi, UAE. Desalination. 1999; 93: p. 1111-1118. 17. Rice W, Chau, DSC. Freeze desalination using hydraulic referigerant compressors. Desal. 1997; 109: 157-164. 18. Rice W, Whitfield, KL, Chau, DSC, Wood, BD. CFC and halon alternatives Conf. Washington DC. Proc., Internat. 1994. 19. Schmauss LR. A review of electrodialysis. Proceedings of the first biennial conference. “Is current technology the answer?” National water supply improvement association. 1986; p.28. 20. Smith M, Shaw, R. Desalination. available at http://www.lboro.ac.uk/well/resources/technical-briefs/40-desalination.pdf 21. Toups PRC. Evaluation of desalination technology for wastewater reuse. Office of water research and technology, Washington DC. August 1982. p. 46. 22. U.S. Congress, Office of technological assesment, Using desalination technologies for water treatment, OTA-BP-O-46. Washington DC. March 1988. 23. USGS. Available at http://ga.water.usgs.gov/edu/waterdistribution.html 24. Water treatment plant. Available at http://www.thewatertreatmentplant.com/membrane-distillation.html 25. Wesner GM. Desalting process and pretreatment. The role of desalting technology in water supply, waste water reuse and industrial applications: A compilation of papers presented as a series of technology transfer workshops. Office of water research and technology, Washington DC. 1981. p. 9-38. 26. ZENON Environmental, Inc. June 1995. Development of an advanced transverse flow nanofiltration membrane process for high performance desalination. Water Treatment Technology Report No.9. Read More