INTRODUCTION TO DESALINATION
Introduction
Desalination is the natural continuous process, which is essential for the water recycle. Precipitation, such as rain or snowfall on ground, which finally flows either in to the sea or goes back to the atmosphere through evaporation or percolates in the sub-soil is a process of natural desalination. Living beings uses this water directly from rain or from river, lakes or springs. During the travel of the surface water towards sea it dissolves minerals and other materials and becomes salty. Once it arrives to the oceans natural evaporation removes part of the water in to the atmosphere as cloud while remaining water available in the ocean becomes very salty. The evaporated water from the ocean is given back to the earth in the form of rain or snow, which again travels back to ocean and the cycle continues.
Availability of fresh water has been the main centre of growth of civilisation. However, there are lots of inequality existing on earth, which needs to be artificially corrected through incorporation of technologies such as thermal or membrane desalination. With the growth of world population the need of fresh water has also increased substantially which has resulted in growth of desalination installation as well. Logically the desalination activities are concentrated on those parts of the earth where availability of water is scares. This is precisely the reason why more than 80% of desalination plants are located in the water scares Middle East region. Unequal water distribution also exists within our country and fresh water desalination technology is getting concentrated more on water scares areas such as Gujarat, Tamil Nadu and Rajasthan. Besides producing desalted water for human consumption and Industrial requirement these technologies are also found to be advantageous in the recovery of water from waste streams. There is no reliable statistics available on number of plants, their capacities, technologies adopted and status on these plants in India. However, rough indications are that there are more than 1000 membrane based desalination plants of various capacities ranging from 20 m3/day to 10,000 m3/day. There are few thermal based desalination plants also.
In order to compute a reliable data base the reader is requested to provide as much information as he / she can provide on the desalination plants, their installations date, capacity, name of the supplier, present status etc. This will help us in producing a reliable document, which can be used by planners as well as users.
Commercially Available Desalting Processes
A desalting device essentially separates saline water into two streams: one with a low concentration of dissolved salts (the fresh water stream) and the other containing the remaining dissolved salts (the concentrate or brine stream). The device requires energy to operate and can use a number of different technologies for the separation. The various desalting processes are listed below :
Major Processes
Thermal
Multi Stage Flash Distillation (MSF)
Multiple Effect Distillation (MED)
Vapour Compression Distillation (VC)Membrane
Reverse Osmosis (RO)
Electrodialysis (ED)
Minor Processes
Freezing
Membrane Distillation
Solar Humidification
Thermal Process
Over 60 percent of the world's desalted water is produced with heat to distil fresh water from sea water. The distillation process mimics the natural water cycle in that saline water is heated, producing water vapour that is in turn condensed to form fresh water. In the laboratory or industrial plant, water is heated to the boiling point to produce the maximum amount of water vapour.
For this to be done economically in a desalination plant, the boiling point is controlled by adjusting the atmospheric pressure of the water being boiled. (The temperature required to boil water decreases as one moves from sea level to a higher elevation because of the reduced atmospheric pressure on the water. Thus, water can be boiled on top of Mt. McKinley in Alaska [elevation 6200 meters] at a temperature about 16°C less than boiling it at sea level). The reduction of the boiling point is important in the desalination process for two major reasons: multiple boiling and scale control. To boil water needs two import conditions: the proper temperature relative to its ambient pressure and enough energy for vaporisation. When water is heated to its boiling point and then the heat is turned off, the water will continue to boil only for a short time because the water needs additional energy (the heat of vaporisation) to permit boiling. Once the water stops boiling, boiling can be renewed by either adding more heat or by reducing the ambient pressure above the water. If the ambient pressure is reduced, then the water would then be at a temperature above its boiling point (because of the reduced pressure) and will boil with the extra heat from the higher temperature to supply the heat of vaporisation needed. As the heat of vaporisation is supplied, the temperature of the water will fall to the new boiling point.
To significantly reduce the amount of energy needed for vaporisation, the distillation desalting process usually uses multiple boiling in successive vessels, each operating at a lower temperature and pressure. This process of reducing the ambient pressure to promote boiling can continue downward and, if carried to the extreme with the pressure reduced enough, the point at which water would be boiling and freezing at the same time would be reached.
Aside from multiple boiling, the other important factor is scale control. Although most substances dissolve more readily in warmer water, some dissolve more readily in cooler water. Unfortunately, some of these substances like carbonates and sulfates are found in sea water. One of the most important is gypsum (CaSC4), which begins to leave solution when water approaches about 95°C. This material forms a hard scale that coats any tubes or containers present. Scale creates thermal and mechanical problems and, once formed, is difficult to remove. One way to avoid the formation of this scale is to keep the temperature below boiling point of the water.
These two concepts have made various forms of distillation successful in locations around the world. The process which accounts for the most desalting capacity is multi-stage flash distillation, commonly referred to as the MSF process.
Multi Stage Flash Distillation
In the MSF process, sea water is heated in a vessel called the brine heater. This is generally done by condensing steam on a bank of tubes that passes through the vessel which in turn heats the sea water. This heated sea water then flows into another vessel, called a stage, where the ambient pressure is such that the water will immediately boil. The sudden introduction of the heated water into the chamber causes it to boil rapidly, almost exploding or flashing into steam. Generally, only a small percentage of this water is converted to steam (water vapour), depending on the pressure maintained in this stage since boiling will continue only until the water cools (furnishing the heat of vaporisation) to the boiling point.
The concept of distilling water with a vessel operating at a reduced pressure is not new and has been used for well over century. In the 1950s, a unit that used a series of stages set at increasingly lower atmospheric pressures was developed. In this unit, the feed water could pass from one stage to another and be boiled repeatedly without adding more heat Typically, an MSF plant can contain from 4 to about 40 stages.
The steam generated by flashing is converted to fresh water by being condensed on tubes of heat exchangers that run through each stage. The tubes are cooled by the incoming feed water going to the brine heater. This, in turn, warms up the feed water so that the amount of thermal energy needed in the brine heater to raise the temperature of the sea water is reduced.
Multi-stage flash plants have been built commercially since the 1950s. They are generally built in units of about 4,000 to 30,000 cum/d (1 to 8 mgd). The MSF plants usually operate at the top feed temperatures (after the brine heater) of 90 -120°C. One of the factors that effects the thermal efficiency of the plant is the difference in temperature from the brine heater to the condenser on the cold end of the plant. Operating a plant at the higher temperature limits of 120°C tends to increase the efficiency, but it also increases the potential for detrimental scale formation and accelerated corrosion of metal surfaces.
Multiple Effect Distillation
The multi effect distillation (MED) process has been used for industrial distillation for a long time. One popular use for this process is the evaporation of juice from sugar cane in the production of sugar or the production of salt with the evaporative process. Some of the early water distillation plants used the MED process, but this process was displaced by the MSF units because of cost factors and their apparent higher efficiency. However, in 1980s, interest in the MED process has renewed, and a number of new designs have been built. Most of these new MED units have been built around the concept of operating on lower temperatures.
MED, like the MSF process, takes place in a series of vessels (effects) and uses the principal of reducing the ambient pressure in the various effects. This permits the sea water feed to undergo multiple boiling without supplying additional heat after the first effect. In an MED plant, the sea water enters the first effect and is raised to the boiling point after being pre-heated in tubes. The sea water is either sprayed or otherwise distributed onto the surface of evaporator tubes in a thin film to promote rapid boiling and evaporation. The tubes are heated by steam from a boiler, or other source, which is condensed on the opposite side of the tubes. The condensate from the boiler steam is recycled to the boiler for reuse.
Only a portion of the sea water applied to the tubes in the first effect is evaporated. The remaining feed water is fed to the second effect, where it is again applied to a tube bundle. These tubes are in turn being heated by the vapours created in the first effect. This vapour is condensed to fresh water product, while giving up heat to evaporate a portion of the remaining sea water feed in the next effect. This continues for several effects, with 8 or 16 effects being found in a typical large plant.
MED plants are. typically built in units of 2,000 to 10,000 cum/d (0.5 to 2.5 mgd). Some of the more recent plants have been built to operate with a top temperature (in the first effect) of about 70°C, which reduces the potential for scaling of sea water within the plant but in turn increases the need for additional heat transfer area in the form of tubes. Most of the more recent applications for the MED plants have been in some of the Carribbean areas. Although the number of MED plants is still relatively small compared to MSF plants, their numbers have been increasing.
Vapour Compression Distillation
The vapour compression (VC) distillation process is generally used for small and medium scale sea water desalting units. The heat for evaporating the water comes from the compression of vapour rather than the direct exchange of heat from steam produced in a boiler.
The plants which use this process are generally designed to take advantage of the principle of reducing the boiling point temperature by reducing the pressure. Two primary methods are used to condense vapour so as to produce enough heat to evaporate incoming sea water: a mechanical compressor or a steam jet. The mechanical compressor is usually electrically driven, allowing the sole use of electrical power to produce water by distillation.
With the steam jet-type of VC unit, also called a thermocompressor, a vemturi orifice at the steam jet creates and extracts water vapour from the main vessel, creating a lower ambient pressure in the main vessel. The extracted water vapour is compressed by the steam jet. This mixture is condensed on the tube walls to provide the thermal energy (heat of condensation) to evaporate the sea water being applied on the other side of the tube walls in the vessel.
VC Units are usually built in the 20 to 2,000 cum/d (0.005 to 0.5 mgd) range. They are often used for resorts, industries and drilling sites where fresh water is not readily available.
Membrane Processes
In nature, membranes play an important role in the separation of salts. This includes both the processes of dialysis and Osmosis that occur in the body. Membranes are used in two commercially important desalting processes: electrodialysis and RO. Each process uses the ability of membranes to differentiate and selectively separate salts and water. However, membranes are used differently in each of these processes.