ENCYCLOPEDIA OF ENVIRONMENTAL SCIENCE AND ENGINEERING - DESALINATION ppsx

194 D DESALINATION INTRODUCTION Our planet, the earth or gaia, is a water place. Water occu- pies 70% of the total earth’s area, about 360 ϫ 10 12 m 2 , and the total water volume that covers the earth’s surface is about 1.40 ϫ 10 18 m 3 . This makes the planet earth a very large water reservoir, nevertheless practically, these huge amounts of water are not directly usable, as 97% of this water is the seawater of the open seas and oceans and only 3% consist of fresh and brackish water, i.e., 0.042 ϫ 10 18 m 3 . From these 3.0% fresh water reserves only a por- tion (0.014%) is in liquid form in rivers, lakes and wells directly available to us for immediate use, the rest can be found as glacier, icebergs and very deep water of geological reservoirs. Fresh water, for all of historical times has been an uncon- trollable happening of nature wherever and however found. In the Bible “good land” is described as one “of brooks of water, of fountains, and depths that spring out of valleys and hills,” for water is a precious source of development and civ- ilization, because water and civilization are two inseparable conceptions. From antiquity up to our times, rivers, seas, oases, and oceans have attracted man to their shores. As a rule, towns and countries have grown along rivers. Egypt for example was considered as “the gift of the river.” Egypt is a typical historical example of the influence of water to the birth and development of a civilization. As a largely developed agricultural country, Egypt was able to master the river and had most of the time such an abundant harvest, that it became the main wheat-exporting country in the whole Mediterranean. The Egyptians learned to determine the seasons of the year by the behaviour of the river. Inundation, Emergence of the fields and Drought were their seasons. The first calendar was created in this way and out of this was derived the modern calendar. Unfortunately, fresh water, and even seawater has not had our proper attention, respect and treatment. Due to the increase of population, especially in certain regions, and the increase of living standards, water demand increased exponentially, and wells or other fresh water sources run dry. The great population increase multiplies the total withdrawal. In some areas twice or more as much water is being drawn out of the ground as sank into it, thus the water table drops every year a few meters and water shortage increases dra- matically, especially in dry years. In modern large civic centers, opening of the tap pro- vides us with as much fresh water as we are able to waste. The notion of lack of water is usually a matter we very seldom think of. However, at the same time there are places where water is so scarce that there are serious problems of existence, as is the case these days in many places in Africa. Generally it is not realized that fresh water represents only 3% of the total reserves in the world and that 75% of it is immobilized as ice. Modern agriculture also requires con- siderable amounts of good water to meet increasing food requirements. There is a tremendous requirement of water to run industrial plants, to produce all kinds of goods but also all kinds of polluting effluent. The quantity of water used varies from location to loca- tion throughout the day and throughout the year, as many factors influence this variation. The more important factors include the economics of a community, its geographic loca- tion, and the nearby availability of the water source or the transportation distance from the source. Climate is the most common cause of water lack or of water insufficiency. Sparse rainfall to feed streams, wells and the soil for agricultural production of crops exhausts water reserves. The most arid areas are the deserts, where no rain exists and some underground waters most of the time are salty or brackish. About 19% of the total land surface of the earth on all continents but Europe, is covered by des- erts, which are surrounded by semi-arid lands where existing water is insufficient. Coastal deserts, where the lack of water is as high as in the interior deserts, cover about 33,000 km 2 around the world, the greatest part of which is found in the Middle East, along the Persian Gulf, adjoining parts of the Arabian Sea and the Indian Ocean. The coastal deserts are divided into four main categories, according to their climatic conditions. © 2006 by Taylor & Francis Group, LLC DESALINATION 195 The tropical regions, where the temperature is about 30ЊC in summer and 22ЊC during winter. The subtropical regions, where in warm periods the temperature is about 30ЊC, and it ranges between 10 and 22ЊC in the cold months. The regions, as in the Mediterranean coasts, where during warm periods the temperatures are 22 to 33ЊC, and in cold months 10 to 22ЊC. The cool coastal deserts have in summer temperatures under 22ЊC and in winter time 10 to 22ЊC. The last desert regions are the cold places where summer is under 22ЊC and winter under 10ЊC. The largest single coastal desert of the third type, with the moderate climatic conditions, is that of the Mediterranean Sea and covers about 2650 km 2 . 1 Coastal deserts have an advantage over the interior deserts. They are climatically more pleasant, because they are cooler in summer and warmer in winter. Further, they have advantages over the interior deserts from the desalination point of view. Coastal deserts are surrounded by abundant sea water supply which is in the same level as the desalt- ing installation, and thus the intake of water can be pumped with less power consumption than the deep well salty or brackish waters of the inland deserts. The brine disposal is also easy, without problems, as it is discharged directly into the sea, whereas the disposal of brine in inland deserts may create serious problems. Also, coastal deserts are in favour over the inland ones concurring the transportation of the equipment and all other necessary supplies for a desalina- Saudi Arabia. Some of the most attractive areas and beaches of the world are almost devoid of water. Not only is this living space, and space for resort hotels lost, but, in some cases, profitable resources cannot be exploited. Thus, known min- erals on Egypt’s Red Sea coast cannot be mined, and fish- ing industries on South America’s Pacific coast, and other places around the world, cannot be expanded for lack of water. These present major losses in the world supplies of minerals and foods. WATER DEMAND AND USAGE The water cycle leaves about 9000 km 3 of water worldwide per year. This amount is enough to provide, with good qual- ity water, about 20 billion people, but this water is far from evenly divided, with major shortages in some regions and abundant quantities in other places. In a modern urban agglomeration, supply of water may satisfy domestic, municipal and industrial demand, as well as agricultural needs. There are no standards of general acceptance for the quality of water required by each group of users. Domestic demand includes all water consumed in housekeeping and gardening. A limit of 500 mg/L (ppm) for total dissolved solids with a maximum of 250 mg/L for chlo- ride and sulphate ions, respectively, is recommended by the World Health Organization (WHO). 2 Nevertheless, there is a large number of communities, which are still consuming water containing up to 1000 mg/L total dissolved solids and sometimes more. Physiological changes may result from the intake of large amounts of the main ions, as well as of some trace elements. Municipal requirements, beside the supply of water for domestic use, include all water needed by offices, public and commercial establishments, fire-fighting and irrigation of municipal parks. Although the standards for the latter uses are not strictly the same, as for drinking water, in practice all municipal water requirements are identical to drinking water since it is nearly always supplied by the same piping system. A large variety of quality standards is involved in the use of industrial water, depending on its specific use. They may vary from high-quality drinking water for food process- ing to completely demineralized water for specific uses. Limitations of salt content may be imposed in some cases for process water. Boiler feed water needs special treatment to minimize salt content and eliminate dissolved gases. Cooling water also needs some treatment to meet the process require- ments. River water and sea water can be used for cooling purposes and this is the usual practice in plants located on a river or near the seashore. About 70% of water withdrawn from the earth goes for agriculture purposes and the balance, 30%, for various uses, as household and industrial process water. Overirrigation the last years, brought salinization of the nearby water resources, affecting the soil and crop quality, as salts are accumulating in the soil. Irrigation water quality, which includes also drinking water for animals, depends to a large extent on the nature of the soil, the crops and the climate. The yield and quantity of some crops can be affected, not only by the total amount of dissolved solids, but also by the presence of certain specific salts. Thus if desalinated water is to be used in certain places the make-up of the product water will be necessary. The water withdrawn per year and per capita, concern- ing industry and agriculture is increasing by 8.5%, the main increase in the developed countries. The USA consumes 2500 m 3 per year per capita, Switzerland 500 and Ghana, a very poor African country, only 40 m 3 . 3 Meanwhile, the majority of fresh water streams are severely polluted, decreasing the quality water reserves. Self decontamination is not feasible in many cases and, thus, treatment methods have to be applied to degrade at least some of the pollutants in the water. On the other hand, sea water exist in huge amounts, given free. Although also pol- luted to some extent, it is a future source of fresh water as desalination is the future process to produce this valuable good quality water. SEA WATER The seas and oceans are great sources of material available to mankind, though their destiny is very low to be exploited, but high enough to make the water salty, unsuitable for drink- ing or processing purposes. Not all the seas around the world have the same amount of total dissolved solids, the amount of which range from 20,000 to 50,000 ppm. © 2006 by Taylor & Francis Group, LLC tion plant. Figure 1 shows a modern desalination plant in 196 DESALINATION and closed seas. Over seventy elements have been detected in seawater, some in very small to trace amounts. Their pro- portion in all oceans, independent of their concentration, is almost stable. 4 The four main metals—sodium, magnesium, calcium and potassium—and their combining ions, chlorides, includ- ing the other halogens and bicarbonates, are presented in major and the minor elements in seawater. FIGURE 1 Panoramic view of the Al-Jubail Saudi Arabia, phase II, MSF desalination plant. It is up to now the World largest desalination installation totaling a capacity of 947,000 m 3 /d (250 Mgd) fresh water production. Each unit has a capacity of 23,500 m 3 /d (6.2 Mgd). The plant was built for the Saline Water Conversion Corporation, of Saudi Arabia by the Japanese Companies of Sasakura and Mitsubishi. (Courtesy Sasakura Engineering Co., Japan) © 2006 by Taylor & Francis Group, LLC amounts beyond comprehension. In Table 2 are given the T able 1 gives the total dissolved salt of various oceans DESALINATION 197 Desalination eliminates the main elements from sea- water, producing fresh water and concentrated brine, almost saturated in the main salts, which are withdrawn to the sea. There are two main reasons that these salts are not exploited. The brine volumes are huge and cannot be han- dled easily. The present extraction technology is expensive for the relatively cheap materials. Nevertheless, there is some industry exploiting, in part, the concentrated brine. Today throughout the world more than 1,900,000 m 3 /d (500 MUSGPO) of fresh water is produced by the various desalination processes. 5 Usually twice to 2½ as much sea- water is processed, so that the solids concentration of the brine is doubled. It is estimated that the recovery from the withdrawn brine can be: Magnesium 2,306.000 t/y Bromine 116.100 t/y Calcium 728.000 t/y Copper 5.385 t/y Potassium 659.000 t/y Uranium 5.385 t/y Sulfate 4,855.000 t/y Gold 7.2 kg/y Calcium and magnesium are the main elements that cause scale formation. Scales are formed and precipitate inside desalination equipment simultaneously with other suspended solids content in the feed water sea or brackish. These materi- als precipitate in areas favored for deposition. In distillation plants these are the heat exchangers and, in reverse osmosis, the semipermeable membranes, cause the problems. These deposits are categorised in two main types, the sludge which is soft and can be easily washed out, and the scale which is hard, adheres to heat transfer surfaces and can be removed only by plant shutdown. Brackish waters are classified as waters with total dis- solved solids content ranging from 3,000 ppm to 20,000 ppm. The elements vary widely, depending on the rocks and soil coming in contact with the water. In some brackish waters large amounts of calcium sulfate are present up to satura- tion conditions, making the water bitter and unsuitable for any use. DESALINATION PROCESSES When all other possibilities to use existing natural water resources are exhausted or to augment fresh water supply by conventional methods fail, then desalting of seawater, or brackish water and/or of polluted water reserves might give the answer to local water problems. The cost of desalting has been drastically reduced over the past several years. This is TABLE 1 Total Dissolved Solids in Various Seas Ocean/Sea g/kg ppm Ocean/Sea g/kg ppm Baltic Sea 7.0 7,000 Pacific Ocean 33.6 33,600 Caspian Sea 13.5 13,500 Atlantic Ocean 36.0 33,600 Black Sea 20.0 20,000 Mediterranean Sea 39.0 39,000 White Sea 28.0 28,000 Red Sea 43.0 43,000 Northern Adriatic 29.0 29,000 Kara Bogar (Caspian) 164.0 164,000 Dead Sea 270.0 270,000 TABLE 2 Ionic Composition of Main Elements in Seawater 6 Ions g/kg Ions g/kg Chlorides Cl − 18.980 Copper Cu 0.003 × 10 −3 Sodium Na + 10.560 Uranium U 0.003 × 10 −3 Sulfates SO 2− 4 2.560 Magnesium Mg 2+ 1.270 Total TDS — 34.482 Calcium Ca 2+ 0.400 Water H 2 O 965.518 Potassium K + 0.380 Hydrogen Seawater characteristics Carbonates HCO 3 0.143 Salinity g/kg 34,330 Bromides Br − 0.065 Chlorinity g/kg 19,000 Boric acid H 3 BO 3 0.026 Chlorocity g/kg 19,950 Strondium Sr 2+ 0.014 Specific weight N/m 3 10,243 Fluorides F 0.001 © 2006 by Taylor & Francis Group, LLC 198 DESALINATION the result of the combined effort of scientists and engineers. However, it should not be forgotten that desalted water is an industrial product and its cost can never compete with the cost of natural fresh water supplies. The largest desalination plant is Nature. The hydrological cycle on earth begins by desalination of surface waters. As the sun’s energy evaporates the water from the oceans and the land surface waters, the vapors condense again on the earth’s surface, as desalted water, stored as snow, ice or through the soil returns to the rivers and seas. This water is the vital liquid for all creatures on the earth. The importance of water, as a matter of life, is quoted as far back as there are records in history. We read in the Old Testament: “Moses brought the sons of Israel from the Red Sea and they went into the desert of Shur. They marched three days in the wilderness and could not find water to drink. And when they arrived to Merra they could not drink the waters of Merra, for they were bitter. Therefore, he named this place ‘bitterness.’ And the people murmured against Moses, saying: What shall we drink? And Moses cried onto the Lord. And the Lord sweded a wood, which when he had cast into the waters, the waters were made sweet.” 7 Nobody has guessed what kind of wood this could be, but is the first known in his- tory, technical desalination. The effort of a desalination process is to separate one of the most common, and most useful and yet most unusual material—the water from the one of the next common mate- rial, the salt. Hundreds of processes have been proposed, based on the various properties of water and its saline solu- tions. Nevertheless only a few of these methods have reached such an advanced state of technology to be considered as safe processes for the commercial conversion of saline waters into fresh. Distillation processes, reverse osmosis and elec- trodialysis or in some cases combination of two processes. The expectations, connected with freezing processes, could not be met with current freezing technology in large scale industrial application. The required separation may be of water from salt, or of salt from water. Thus the desalination processes can be classi- fied, according to the operation reference parameter as follow: 1: Methods that separate water from salts 1.1: All distillation methods 1.2: Reverse osmosis 1.3: Crystallization (freezing and hydrates) 2: Methods that separate the salts from the water 2.1: Electrodialysis 2.2: Ion-exchange 2.3: Piezodialysis 2.4: Osmionic methods 3: Methods with phase change 3.1: All distillation methods 3.2: Crystallization 4: Methods without phase change 4.1: Reverse osmosis 4.2: Electrodialysis From the energy point of view, the methods are classi- fied as follow: 5: Methods using heat (thermal methods) 5.1: All distillation methods except mechanical vapor compression 6: Methods using mechanical energy 6.1: Mechanical vapor compression 6.2: Reverse-osmosis 7: Methods using electrical energy 7.1: Electrodialysis 8: Methods depending on chemical energy 8.1: Ion-exchange The methods which found practical applications in large scale industrial plants are: Distillation methods: which comprise the following modifications: 1: Multiple-Effect Evaporation ME 2: Multi-Stage-Flash Evaporation MSF 3: Vapor-Compression methods VC 4: Solar distillation method SD Distillation is the most developed process of removing water from a saline solution. It is applied up to very large capacities with various types of evaporators and accounts for about 59.4% of the total world plant capacity. 5 The latent heat of changing phase is an important factor in the overall process economics, but the degree of salinity of the raw water is of no importance. Multistage flash distillation and multi-effect evaporation are reduc- ing considerably the economic effect of the latent heat of vaporization. Reverse osmosis uses mechanical energy, as pres- sure, to drive the water out of the solution through semi- permeable membranes. The applied pressure must be higher than osmotic pressure, its value depending from the salt content of the brackish or seawater solution. The necessary counterpressure in reverse osmosis depends greatly upon the salt content of the raw water and imposes constraints on membrane life and performance, but also varying energy consumption according to the salinity of the raw water. Membrane life is an important cost factor. Today reverse osmosis plants account, worldwide, for 32.6% for plants having a capacity 100 to 4000 m 3 /d and 19.5% for capacities over 4000 m 3 /d. Electrodialysis is the most developed process for eliminating salts from aqueous solutions. The economics depend closely on the salt content of the raw water, as the consumption of electric energy is related to the total dis- solved solids removed from the solution. Electrodialysis may, therefore, preferably be applied for the purification of brackish waters. Reversal electrodialysis is a modification, by which poles are reversing every 20Ј and which assures the production of high-quality water and minimizes the rejection of brine. © 2006 by Taylor & Francis Group, LLC DESALINATION 199 Today electrodialysis accounts for 5.7% of plants with capacities 100 to 4000 m 3 /d and 2.5% for capacities over 4000 m 3 /d. Freezing processes found no commercial application though the simplicity of the method. They failed because the size of produced ice was very small and half of the fresh water was used to wash out the salt from ice surface, render- ing the method uneconomical. Independently from the method or procedure for sea or brackish water desalination the operation of a desalination plant includes some general steps to be followed. Figure 2 gives the procedures before and after the main desalina- tion step. ENERGY SOURCES Running a desalination plant many expenses arise, the high- est of which is energy cost. In a normal chemical plant energy cost is low, only 1 to 5% and, in some extreme occasion, 10% of the total operation cost. On the contrary, desalination is a high energy consuming procedure and the cost of necessary minimum energy to run the plant is 40% of the total cost. The main energy sources, depending on the method, are low pressure stream and electricity, two energy sources easily available in any industrialized region. Few other energy sources are given at lower cost or free of charge. These alter- native energies are suitable for small capacity plants and/or for remote and arid regions, where fuel and electricity are not available or the cost of fuel transportation renders its use uneconomic. Alternative energy sources include geothermal energy when and where is available, all kinds of waste heat and waste heat from nuclear plants. Renewable energy sources include wind energy, tidal energy, Ocean Thermal Energy Conversion (OTEC) and, above all, the abundant solar energy. Waste heat is available from chemical industry, power plants and nuclear power plants in large amounts, but in low heat content. Wind and tidal energy are available in certain specified regions, transforming the corresponding energy into electricity. OTEC takes advantage of the temperature difference between the ocean surface and about the 500 m depth of the tropical regions. Solar energy is for the time being the most promising renewable energy. In the earth’s sunny regions solar radiation is very intensive though also very spread out, thus the capture of solar energy depends on large areas. Although solar, wind and tidal energy are natural forces given free, the corresponding equipment for transformation of these energies into a usuable form are yet very expensive and the yield very low. DISTILLATION PROCESSES Aristotle, the ancient Greek philosopher, wrote: “Salt water, when it turns into vapor, becomes sweet, and the vapor does not form salt water again when it condenses.” Sailors have used simple evaporation apparatus to make drinking water for almost 400 years, and ocean going ships have tradition- ally used evaporators, often multiple-effect, as an accessory to steam boilers. The simplest way to evaporate water is the natural one, using solar heat. Sun is a free inexhaustible source of energy. However, this energy has not been captured and stored at its most concentrated form as yet. The way to use solar energy for desalination purposes depends on the desalination pro- cess. The simplest and most common method is the direct use of the solar energy in specific equipment called “solar stills” which act simultaneously as converters of solar energy to heat and as distillers. 8 Indirect use of solar energy, called “solar assisted” or “solar driven” desalination, captures the solar radiation using one of the modern procedures which transform the energy into either heat or electrical power. Horizontal tube, multiple- effect (HTME), multi-stage-flash (MSF) and thermal vapor compression (TVC) distillation methods are coupled to the REMOVAL OF COARSE MATERIAL FEED WATER PRETREATMENT SEAWATER INTAKE MATERIAL STORAGE DESALINATION INSTALLATION VAPOR, POWER CONDENSATE BRINE REJECTION FRESH WATER FRESH WATER DISTRIBUTION FRESH WATER STORAGE POST TREATMENT POWER STATION FIGURE 2 Flow diagram of the main procedures to be followed in the operation of a desalination plant. © 2006 by Taylor & Francis Group, LLC 200 DESALINATION heat source, though reverse osmosis (RO), electrodialysis (ED) and mechanical vapor compression (MVC) to the elec- trical power produced from the sun’s radiation. 9 As the incidence of solar radiation varies over the day, the time of the year, the degree of cloudy weather and the geographic location, conventional solar evaporation can never be a steady state operation. Moreover, convectional solar distillation is a single effect process and is character- ized by the thermal disadvantages of single stage operation. The intensity of solar radiation reaching the earth varies from zero to about 1047 W/m 2 . Part of this radiation may come directly from the sun, but sometimes as much as 10% of it comes as scattered light, even when the atmosphere is unobstructed by clouds. In cloudy weather the total radiation is greatly reduced and most of the light that passes through may be scattered light. The solar radiation striking a horizontal surface is great- est at noon, as the sun’s rays pass through the atmosphere with a minimum length of passage through the air. In the morning and the afternoon the rays are subject to increased absorption and scattering. Considering the latitude, maxi- mum radiation is at the equator. Hence the radiation inten- sity depends on the hour of the day, the day of the year, and the clarity of the atmosphere for a given location, as well as of the latitude of the earth at the point of observation. These limitations of the solar radiation render solar distilla- tion method and solar driven desalination a nonsteady state operation except if solar energy storage is provided, which in general increases installation costs. The daily production of conventional solar distillation is low, due to low performance of the stills. Depending on the intensity of solar radiation, the day of the month and the month of the year the fresh water production ranges from 1.5 to 5.51/m 2 d (0.036 to 0.130 gal/ft 2 d). 10 Increasing feedwater temperature the daily productivity increases as well. This can be done by connecting a solar still with a solar collector or by using the condensate from low pressure steam. Many other methods have been proposed, to augment the efficiency of solar stills, nevertheless without any success due to increase of the corresponding costs. To calculate the efficiency or the daily productivity of the solar stills have been proposed many mathematical models. Here two general equations are given: One concerns the operation of a conventional solar still and the second the productivity of a solar still connected to a solar collector. The daily output of a stagnant solar still is given by the equation: 11 M out ϭ F 1 H d ϩ F 2 (T ad Ϫ T wd ) ϩ F 3 (1) and the daily output of a solar still connected to a solar col- lector is: 12 MFHF F out p d swd ad 2 ϭϩ⌻Ϫ⌻ϩ( ) . (2) Both equations depend on construction and operational parameters. Much material is required to construct a solar still: glass or plastic for the cover, black basin surface to absorb the solar radiation, material for the basin, usually concrete or plastic, pumps and piping—metal or preferably plastic, for the feed water and the fresh water distribution. 13,14,15 Total cost of installation and operation of solar distillation plants is not very high if land is given free. They need large condensing areas and are vulnerable to storms. However, energy is free, except pumping, operation is simple, and maintenance cost very low. Although the advantage of cost-free energy is partly offset by increased amortization cost and the large installa- tion area, distillation with solar energy remains a favorable process for small-capacity water desalting at remote loca- tions where there is considerable solar radiation. Most solar distillation plants are being (or will be) erected in less devel- oped countries or in areas where there are limited mainte- nance facilities. Solar energy for evaporation was first used on a major scale about 1872 in Chile, where a glass-roofed unit had 4,400 m 2 to make 22.4 m 3 /d (ϳ6000 gpd) in a mining camp. 16 Today many units, glass covered or plastic ones, are installed in small capacities world wide, mainly in arid and remote glass covered, yet in operation, in Porto Santo (Madeira) Portugal, with an installation area of 1200 m 2 . 17 It seems to be very simple as a method, and really it is, because theoretically solar energy can replace any other energy source. From a technical point of view this is not yet totally feasible because either the corresponding technology is not fully developed or the market is still very expensive. Both procedures, solar distillation and solar driven desal- ination, depend on local insolation rates which vary from site to site for the same region, from the time of the day, the time of the year and the cloudy weather making desalination an unsteady state operation. Heat storage, if possible, improves productivity by extending operation during the nighttime or during cloudy days but also affects directly the economics of the method. However, for certain locations as remote, arid or semi-arid regions, where the small communities are poor and where the techniques and tools of water production and distribution developed in industrialized areas are not always appropriate to be used, solar desalination is admitted as the most suitable process. The other way of using solar energy for desalination purposes is the collection of solar energy by solar collectors or concentrators, with subsequent conversion of the solar energy to heat or electricity. This solar assisted desalination is expanding rapidly and many installations have been erected in commercial but as yet small capacity sizes. The simplest thermal conversion type of collector is a solar pond. A solar pond is a shallow body of water in which a stabilizing salinity gradient prevents thermal convection, thereby allowing the pond to act as a solar trap. The merit of solar ponds lies in their ability to collect solar energy in large scale and provide long-term heat storage. This long- term storage provides also increased flexibility of heat use. They can operate at all latitudes and are estimated to be less © 2006 by Taylor & Francis Group, LLC areas. Figure 3 is the photograph of a solar distillation plant, DESALINATION 201 expensive than flat plate collectors per unit area installed and per unit of thermal energy delivered. Solar ponds, being low- grade heat source, can be competitive with convectional heat sources in many applications. Flat-plate collectors, evacuated tube collectors and focus- sing collectors are used to produce hot water or steam as the heat medium for the distillation units. For reverse osmosis or electrodialysis units, photovoltaic devices are used or ther- mal conversion systems, e.g., central receivers, to drive the turbine generator. A very important aspect of the solar assisted desalination process is the cost of energy and water produced. However, experience has shown that cost estimates are different every- where. Labour, material cost, etc. depend on local circum- stances, so the cost of water is not the same at all places. Solar assisted desalination capacity is only a very small percentage, about 0.80%, of the total world capacity of convectional-fossil fuel fired desalination plants. A part, 0.60% is coupled to collectors or photovoltaic devices and 0.13% are wind-driven plants. The total capacity of worldwide solar-driven desalination plants is only about 15,250 m 3 /d, and wind driven as low as 2,530 m 3 /d. 5 irst known sketch for solar distil- lation equipment. 18 Distillation process, operated with conventional energy sources, i.e., low pressure steam, are applied up to very large capacities by using various types of evaporators and are clas- sified accordingly as follow: Multiple-effect evaporator (ME) Vertical tube evaporators VTE, falling or climbing type Horizontal tube evaporators HTE Multi-stage-flash evaporator (MSF) Vapor compression evaporator (VC) Thermal vapor compression TVC Vacuum vapor compression VVC Mechanical vapor compression MVC The term “evaporation” in the desalination refers espe- cially to the vaporization of water from an aqueous saline solution, as brackish or seawater, where the solid constitu- ents are practically nonvolatile, in the range of working FIGURE 3 Photograph of the Solar distillation plant in Porto Santo, Madeira, Portugal. It is the only solar plant in operation in Europe. Has a total evaporating area of 1,200 m 2 , and consists from two different kinds of solar stills, of the assymetrical type. The Greek design developed at the T.U. of Athens and the design developed by the university of Berlin. © 2006 by Taylor & Francis Group, LLC Figure 4 presents the f 202 DESALINATION temperatures and pressures. Thus, water alone is vaporized, which is the main product, and the dissolved solids remain in the residual liquid, the brine. In the chemical industry, when an evaporation process is applied, the water vapors are usually discarded and the emphasis is given to the recuperation of the dissolved solids. In desalination the term “distillation” predominates over the correct term “evaporation.” The process is performed in evaporators, where heat is supplied to the solution, to change phase. The productivity is expressed either as the net evapo- ration or “gain output ratio” (GOR), i.e., the kilos of pro- duced distilled water per kilo of boiler steam used, (kg/kg) or as performance ratio R. It is usually prefered, instead of the GOR, to use the term performance ratio which defines the mass of distillate produced per 2326 kJ (gal/1000 BTU) of heat input to the brine heater in case of MSF distilla- tion or to the first effect in case of multiple-effect evapo- rators. The latter definition is thermodynamically more accurate, as it refers to the enthalpy of the steam instead to the mass. Thermodynamic considerations lead to a common char- acteristic of all distillation process, that the percentage of evaporated water with respect to the circulating seawater is as much larger as is the difference between the maximum and minimum temperature of the saline solution. As the minimum temperature is defined by the temperature of the incoming seawater, enlarging of the temperature difference can only be obtained by increasing the initial maximum temperature of the salt water feed. Limitations due to the appearance of phenomena like scale formation and corro- sion, which are becoming more important at higher tempera- tures, define an allowable maximum temperature for each distillation process. An appropriate pretreatment of the salt water is necessary to make an increase of the feed water tem- perature possible. The economics of the distillation process might be affected by the following parameters: • The chemical additives for feed water pre- treatment • Scale formation which decreases performance • Increase of maintenance costs due to corrosion Corrosion may increase fixed changes, when more expensive materials of construction must be used. Multiple-Effect Distillation (ME) Theoretically, in single-effect distillation 1 kg of distillate will be produced for every kg of steam consumed and the gain output ratio of the plant will be 1. In fact, despite pre- heating of the feed, a large part of the enthalpy of the vapors, FIGURE 4 The first historically known solar distillation equipment, according to Giovanni Batista De La Porta. The sun evaporates the water inside the glass vessels and distilled water is collected beneath the vessels. “De distillations,” Libri IX, Rome 1608. © 2006 by Taylor & Francis Group, LLC DESALINATION 203 evolved in the single-effect evaporator, is lost in the con- denser. A better heat recuperation would be obtained if the heat, released by the condensing vapor, is not rejected in a condenser, but is used to heat the brine of a second evapora- tor and so on. This leads to the concept of multiple-effect distillation, where the vapors from one effect are used as the heat source of the next effect, as long as the difference in temperature between the condensing vapor and the solution is high enough to act as the driving force in the evaporation pro- cess, each effect being at progressively lower temperature and pressure. Vapor condensing because of lower boiling temperature, in each effect, produces fresh water as distil- late, whereas the vapor from the final effect is condensed by a circulating seawater cooling stream. Theoretically, an additional kg of distillate would be obtained in each consecutive effect for the same kg of steam initially introduced into the first effect and the plant gain output ratio would be equal to the number of effects in operation. However, this is not true in practice. Part of the condensation heat to be recovered is lost to the atmosphere, in design features and in the differences of temperature used as the plant’s driving force. Multiple-effect distillation process uses evaporators which are modified successors of evaporators that have been used in sugar and other process industries for more than 100 years and have been in use for seawater distillation about 90 years. The latter were originally built for shipboard use, the main requirements being for compactness, simplicity in operation and reliability. In land based industrial evapora- tor plants the requirements are mainly directed to the cost of product water with emphasis on cheaper materials of construction, high boiling temperatures, efficient descaling methods and the use of the cheapest type of evaporator. Previously multiple-effect distillation was second in importance of the distillation process, as medium capacity plants but day hardly is applied. Worldwide capacity of ME plants for units producing more than 100 m 3 /d of fresh water, is only 765,143 m 3 /day or 4.1% of total world capacity. 5 Long Tube Vertical Evaporator, LTVE Long tube evapo- rators consist of a series of long tubes arranged vertically inside the evaporator shell. Seawater feed may be from the top or from the bottom, called respectively falling or rising film LTV evaporators. In the falling film evaporator seawater is introduced at the top and the incoming seawater flows across an upper tube plate and is equally distributed to the tubes, and flows downward by gravity as a thin film. The principal advantage of the VTE process is that high heat-transfer can be achieved, which considerably reduces the required heat-transfer sur- face area. This forward feed is the usual method of feeding a multiple-effect-evaporator. The VTE rising film is similar to falling film evaporator except that seawater is introduced at the bottom of the first effect, thus reducing the overall pumping requirements. Heat transfer in the VTE evaporators is increased by using fluted tubes, which enhance heat transfer. Steam condenses outside the tubes, forming also a thin film of distillate. Surface tension forces are created, which are inversely proportional to the flute radius. This causes the condensate film to drain from the crests to the grooves, so that a very thin condensate layer is remaining on the crests, which promotes heat transfer. The flow sheet of a typical multiple-effect vertical tube a feed heater C which uses the product vapors as heating medium in the form of distilled water or vapor condensate. Vapors produced in the first effect condense outside the tubes of effect 2 and the brine is pumped from each effect to the top of the next. The average efficiency K of each effect is usu- ally between 0.85 and 0.95. Concerning N effects in a LTE system, as in Figure 5, the GOR is given by the equation: GOR ϭ K(1 Ϫ K) N /(1 Ϫ K) (3) Thus when K ϭ 0.95 and the number of effects N is 15, GOR ϭ 10 kg/kg. Doubling the effects to 30, the GOR is only 14.9, and it will attain a maximum of 19.0 for an infi- nite number of effects, when K ϭ 0.95. There are some economic limitations increasing the number of effects. The investment costs and consequently the fixed charges are increasing almost linearly with the number of effects. The costs of steam and water fall off rapidly at first, but the savings diminish progressively. The total cost of operating an evaporator leads to an optimum number of effects, at the point where the sum of fixed costs and the cost of utilities shows a minimum. The most probable number of effects will be between 10 and 20. The Multiple-Effect Horizontal-Tube Evaporator The (MEHT) type of evaporator operates on the same principle as the VTE evaporator, but the steam condenses on the inside of the horizontal tubes imparting its latent heat of conden- sation to the brine, which cascades and evaporates over the outside of the tubes. The brine falls to the next effect by gravity and the vapors formed in one effect are used in the next effect. The horizontal-tube evaporator eliminates the pumps required for each effect of the VTE brine circulation, by an arrangement in which the effects are stacked vertically on the top of each other. This compact arrangement of the ME evaporators, called also multiple-effect stack (MES), is constructed in low capacity units and though there are many advantages, it accounts for only 1% of the world capacity. 19 Multiple- effect-horizontal-tube evaporators are suitable to operate with solar energy plants. VAPOR COMPRESSION (VC) Vapor compression (VC) distillation takes advantage of the latent heat of the vapors produced in the process. Vapor produced by evaporation from a salt solution is superheated because of the boiling point elevation of the solution and © 2006 by Taylor & Francis Group, LLC distillation plant is shown in Figure 5. In each effect is adapted In Figure 6 a typical HTE evaporator is presented. [...]... the minimization of the amounts of thermal or mechanical energy and of the amount of equipment and, hence, the amounts of materials used Quite often, in such optimization studies, as energy goes down equipment goes up, and vice versa Thus the most economical balance must be struck Often this is at a point where the energy cost and the capital cost of the equipment are about equal Of the billion dollars... other unit-operation separation methods as these are shown in Figure 17 These four methods can be applied for the purification of water of solutions related or not to desalination methods and reverse osmosis and nanofiltration for desalination of seawater brackish or natural waters, as well Applied pressures are higher for the reverse osmosis method and low for microfiltration and, on the other hand, porosity... Delyannis and V Belessiotis, Solar desalination: Part I and II, Desalination and Water Reuse, 4, No 4, 9/14, and 5, 28/34, 1995 9 A.A Delyannis and E Delyannis, Solar Desalination, Desalination 50, 71/81, 1984 10 N Robinson, Solar Radiation, Elsevier Publ Co., 345 pp., 1966 11 V Belessiotis, K Voropoulos and E Delyannis, Experimental method for the determination of the daily output of a solar still, Desalination. .. diagram of a vacuum vapor compression unit MSF is the most widely applied distillation process, especially for large units, and despite the thermodynamic advantages of ME evaporation, all major plants installed are of the MSF principle because of the simplicity and reliability of the process It accounts for 51.5% of the world desalination capacity and 86.9% of total distillation processes The capacity of. .. Electrodialysis and ion-exchange are the separation processes for desalination Methods which separate water from a salt solution Reverse osmosis, nanofiltration, ultrafiltration and microfiltration are the main processes of this type of separation which are applied to desalination or to water purification © 2006 by Taylor & Francis Group, LLC 211 Ionic Processes Common salt, and other salts as well as acids and. .. ions) and the negative (for example, chlorine ions) Whereas in distillation processes the amount and kind of salts dissolved in the raw feed water are of no importance to the process and do not affect the economics, in all ionic processes the amount of dissolved salts is of primary importance In electrodialysis, the amount of salts to be removed affects the consumption of electrical energy and in ion-exchange... independent of each other and is mainly used for smaller desalination plants (Courtesy of Sasakura Engineering Co., Ltd., Japan) factors, thereby increasing the water recovery ratio for saline solutions and favourably affecting the economics of the desalting process Treatment of the seawater with H2SO4, on the other hand, reduces the corrosiveness of the water by eliminating dissolved oxygen and carbon... economic analysis of dual-purpose plants, as the two products are not necessarily consumed by a single market To obtain this requirement the size and characteristics of each component of the dual-purpose plant must be selected in such a way, taking the power and water demand curves in consideration, as to arrive at the optimum cost It should be noted that power and water demand may present daily and seasonal... have been proposed and many combinations have been tried, mainly for small capacity or pilot size plants Commercial application used the vertical tubemulti-stage-flash (VTE/MSF) and the vertical tube-vapor © 2006 by Taylor & Francis Group, LLC Scale Formation and Its Prevention Formation of scale deposits on and fouling of heat transfer surfaces is one of the most serious problems of distillation equipment... Voropoulos, E Delyannis and V Belessiotis, Thermohydraulic simulation of a solar distillation system under pseudo-steady-state conditions, 1997 13 E Delyannis and V Belessiotis, Solar application in desalination, Desalination 100, 1995 14 A Delyannis and E Howe, Report of W.P on recommended procedure for costing of solar stills C.S.I.R.O., Int Rept No 77, Melbourne, 1971 15 T Lawand, Systems for Solar . for all of historical times has been an uncon- trollable happening of nature wherever and however found. In the Bible “good land” is described as one of brooks of water, of fountains, and depths. optimization of a distillation process has as a first object the minimization of the amounts of thermal or mechan- ical energy and of the amount of equipment and, hence, the amounts of materials. degree of salinity of the raw water is of no importance. Multistage flash distillation and multi-effect evaporation are reduc- ing considerably the economic effect of the latent heat of vaporization.