Abstract:
Desalination processes are being used widely these days to produce fresh water to meet the growing demand for water. 50% of the desalination plants use MSF technology [10]. But for small amount of water production this process is uneconomical. Humidification-dehumidification process (HD) is a valuable option for small scale production in remote areas as it does not need sophisticated set up and has low maintenance. HD is mainly used with solar collectors to provide the necessary heat source. But HD process can even run at temperature as low as 500C. Hence it can make use of low grade heat from industries.
Introduction:
Water is an important commodity in our day to day life. With the rapid growth of cities, demand for fresh water is ever increasing. Water shortage affects 88% of the developing nations which are home to nearly half of the world population and the demand for water is growing at twice the rate of population growth [3]. The minimum per-capita water requirement has been estimated at 1200 litres per day. The actual amount varies depending on the standard of living [4]. Due to the effect of environmental pollution and ununiformed distribution, the supply of fresh water from rivers and rain is inadequate. To meet the ever increasing demand for fresh portable water alternate sources are being looked out for. About 97% of the earth's water is not fit for drinking as it is saline and only 3% is fresh water out of which only 0.5% is available as the rest of it is in the form of polar ice caps, glaciers and in atmosphere [1]. The biggest sources of water are the oceans and sea, but they contain salt in then. As per world health organisation (WHO) guidelines the permissible amount of salt contain is 500ppm and in certain cases 1000ppm. But sea water generally contains 35000-45000ppm of dissolved salts [2]. Fresh water can be obtained for sea water by desalination. Though the process of desalination is not new it is at present energy intensive. Different methods are being considered to desalinate sea water to produce fresh water economically. And the option of using waste heat from industries and renewable energy is also being considered.
History of desalination:
Desalination process has been there for a long time and it even finds reference in the bible during the time of Moses [5]. As early as in the fourth century BC, Aristotle described a method to evaporate impure water and then condense it to obtain potable water. Historically probably the first application of seawater desalination process can be dated back to AD 200 when Alexander of Aphrodisias in one of his drawings depicted sailors at sea boiling seawater with a sponge placed on the mouth of the vessel to absorb what was evaporated [2]. Solar distillation has been in practice for a long time. Until medieval times no important applications of desalination by solar energy existed. The stills or alembics were discovered in Alexandria, Egypt, during the Hellenistic period. Cleopatra the Wise, a Greek alchemist, developed many distillers of this type. The Arabs who overtook science and especially alchemy about the 7th century, named the distillers Al-Ambiq, from which came the name alembic [5].
Later on during the Renaissance, Giovani Batista Della Porta (1535-1615), one of the most important scientists of his time wrote many books which were translated into French, Italian and German. In the first edition of his book Magiae Naturalis which appeared in 1558 he mentions three desalination systems. In 1589 he issued the second edition comprising 20 volumes. In the volume on distillation he mentions seven methods of desalination, but the most important reference is in the 19th volume where he describes a solar distillation apparatus that converted brackish water into fresh water. He also describes, in the second chapter of volume 20, a method to obtain fresh water from the air, nowadays called the humidification-dehumidification method [5]. In 1870 the first American patent on solar distillation was granted to the experimental work of Wheeler and Evans. Almost everything we know about the basic operation of the solar stills and the corresponding corrosion problems is described in that patent. The inventors described the greenhouse effect, analysed in detail the cover condensation and re-evaporation, discussed the dark surface absorption and the possibility of corrosion problems. High operating temperatures were claimed as well as means of rotating the still in order to follow the solar incident radiation. The renewal of interest on solar distillation occurred after the First World War at which time several new devices had been developed such as: roof type, tilted wick, inclined tray and inflated stills [2]. And during the Second World War the U.S navy made use of modified solar stills to desalination sea water. By the end of 1991, a worldwide total of 8886 desalting units with a total capacity of 15.582 - 106 m3/d had been installed or contracted. The major proportion of the capacity is installed in Saudi Arabia (24.4%) followed by the US (15.2%), the United Arab Emirates 0.6%), and Kuwait (9.1%) [7]. In recent times governments have realised the importance of water and new desalination plants have been set up on a small scale or experimental basis.
Desalination processes:
Desalination process can be classified into phase change and single phase processes. This can be further classified as shown below.
Desalination processes:
Figure 1: desalination processes
For phases change desalination process multistage flash distillation (MSD), multi-effect desalination (MED) and vapour compression desalination (VC) are the once most commonly used technologies. And of the membrane process reverse osmosis is preferred although it is relatively new when compared to other desalination processes.
Multistage flash distillation is the most widely used desalination process. In this process saline water is heated to high temperature and passed into vessels of decreasing pressure leading to flash evaporation. The vapours are condensed by passing over feed water in a heat exchanger pre heating the feed water. The condensed vapour is collected as fresh water [7].
Multi-effect distillation is similar to multistage flash distillation. Only major difference being that the evaporation occurs at the heat exchanger surface [4].
Vapour-compression distillation process uses a compressor to increase the pressure of the vapours of saline water thus increasing the condensation temperature and it is used to provide energy needed to evaporate salt water [4].
Reverse osmosis is a process in which external pressure is applied to the salt solution side of the membrane. The external pressure should be greater than the osmotic pressure. This causes the water to flow across the membrane leaving salt behind [8].
Humidification dehumidification process makes use of heated saline water which is sprayed to humidify in coming air in the humidification chamber. The humidified air is cooled by passing it over the cool feed water in the dehumidification chamber. The moisture is condensed and collected [6].
Table 1: comparison of desalination processes [3].
Desalination
production cost
$/m3
MSF
0.8-1.87
MEE
0.46-1.49
VC
0.87-1.21
RO seawater
0.55-1.51
RO brackish
0.20-0.70
ED brackish
0.58
Multistage flash distillation is used mainly when higher production rates are required. Reverse osmosis process too has a higher production rate and lower production cost but it requires replacement of membrane every seven years. In humidification dehumidification process quantity of fresh water produced is low compared to process like MSF or RO. Desalination using solar stills where water is circulated in a solar collector has high set up cost and requires land to set up collector but has very low maintenance cost as it has very few moving parts.
Process
MSF
MED
TVC
MVC
RO
ED
HD
Operating temperature (oC)
<120
<70
<70
<70
<45
<45
50-90
Table 2: operating temperatures for different desalination processes [4, 6].
From table 2 it can be seen that reverse osmosis, electro dialysis, humidification-dehumidification process work at low temperature. Some of these processes like RO and HD can make use of low grade heat from industries to raise the temperature of feed water to an optimum level and get higher efficiency.
Process selection:
Mentioned below are the points that have to be kept in mind while selecting desalination process [2].
Suitability of the process for renewable energy application.
The effectiveness of the process with respect to energy consumption.
The amount of fresh water required in a particular application in combination with the range of applicability of the various desalination processes.
The seawater treatment requirements.
The capital cost of the equipment.
The land area required or could be made available for the installation of the equipment.
Use of renewable energy sources in desalination process:
Renewable energy such as solar energy, wind energy, geothermal energy, tidal energy can be combined with different desalination processes and improve the efficiency and production rate. Figure 1 shows the combinations that can be tried out in desalination process. Thermal processes in which pure water is obtained by distillation can make use of solar or geothermal energy to provide thermal energy for evaporation and processes like reverse osmosis, electro dialysis which require mechanical or electrical energy can be coupled with solar photovoltaic, wind, tidal/wave energy. Most of the cases water shortage occurs in sunny regions or windy coasts [9].
Figure 2: possible use of renewable energy for desalination. [9]
Humidification-Dehumidification (HD) process:
Common methods of desalination such as distillation, reverse osmosis and electrolysis are energy intensive techniques so these techniques will be cost effective only for large scale production of fresh water. However for a decentralised supply of water, where less amount of fresh water is required, HD process suits better as it is very flexible in production capacity and it also has moderate installation and operation costs.
Figure 3: saturation humidity of air as a function of temperature [3]
HD process is based on the principle that air can be mixed with important quantities of vapour. The amount of vapour that can be carried by air increases with temperature. Figure 2 shows the trend graphically. When it comes in contact with salt water, air absorbs certain quantity of vapour at the expense of sensitive heat of salt water causing cooling [10]. By cooling this humid air distilled water can be obtained. This can be carried out in a heat exchanger where feed water can be preheated by latent heat recovery.
Figure 4: Schematic diagram of HD process with solar air heater as heat source [12].
In HD process humidification can be carried out in single pass (once through) or multi pass. When comparing sensitive heat loss to fresh water produced, there is high sensitive heat loss and low fresh water production. In multi pass system we get a slightly higher ratio [10]. The basic HD cycle consists of a heat source, humidification chamber and dehumidification chamber as shown in figure 4. Heated salt water is passed in to the humidification chamber where it comes in contact with air, the air absorbs vapours. This humidified air is cooled in the dehumidification chamber where usually feed water is used as coolant to increase efficiency. An air blower is required to provide air supply. Large amount of air is needed to be recirculated because the quantity of water contained in saturated air is minimal. Thus air pumping alone may represent a prohibitive energy cost. Pinch technology can be used to find the optimum flow rate of air required to get maximum efficiency [11].
Factors affecting the efficiency of HD process:
The amount of energy supplied is a direct function of amount of fresh water produced. The temperature of feed water at the inlet of the humidification chamber is direct function of amount of fresh water produced. Minimum temperature required for optimum operation of this system is 40-50o C [6]. Ambient temperature has little effect on the system productivity. System productivity is strongly affected by air flow rate, saline water flow rate and cooling water flow rate [12]. Best results were obtained at the feed water temperature range of 80-900C [6]. In solar HD process the solar heat collector has the lowest efficiency [14].
Conclusion:
Desalination by HD is an efficient desalination process. It is best suited for dry arid regions which have enough solar energy to run the process. It is ideal for small housing colonies or small industries where small amount of water is required. Processes like MSF, MEB, OR are not efficient for small quantity production of fresh water. HD process has less maintenance when compared to the processes and hence can be used in remote areas and island. It also has less environmental effect when compared OR process [13].
The cost of solar HD process is relatively high because of higher investment required to set up solar collectors. One to reduce this is by coupling this system to utilise waste heat from industries. Low grade heat from industries can be made used of. Waste heat from industries as low as 40-500C can be utilised to preheat brine water before heating it in solar collectors thus reducing the size of solar collector. Small industries can set up this system to provide water required for human activities within the industries and any access water can be circulated to nearby housing areas or staff quarters. Different materials can be tried in the humidification tower to reduce fouling of surface due to salt water. Plastic is being looked as an alternate material [6].
