Desalination Process

DESALINATION PROCESS

• Only 0.5% of the earth's water is potable. The remaining 97% is ocean water and 2.5% is brakish water.

• Natural water can be classified according to their TDS values.

Due to the scarcity of fresh water, it has become necessary to convert salt water into potable fresh water, and the process is called Desalination. It also reduces the Total Dissolved Solids (TDS) in water.

Desalination is a costly process. The water obtained by desalination proves much costlier than the naturally available - treated water. Research is going on to reduce the cost of desalination.

Methods of Desalination:

The various methods which are generally adopted for the conversion of salt water into fresh water are enumerated below:

 (1) Desalination by evaporation and distillation.

(2) Electrodialysis method

(3) Reverse Osmosis method

(4) Freezing process

(5) Solar distillation method and

(6) Other methods.

 

1. Desalination by evaporation and distillation:

This is the most commonly used method of desalination. In this method, Sea or saline water is boiled in giant vessels called evaporators, to produce water vapours which are caught and condensed into fresh water.

• An evaporator consists of a metal box in which the salt water (brine) is heated by a nest of pipes carrying very hot steam.

• Heat passes from the steam through the pipe walls and boils the brine.

• The boiling brine evaporates and the vapour is led into a second box, where another nest of pipes, filled with cold brine, condenses it to fresh and pure water. The heat coming out from the vapour, during cooling operation, warms the cold brine, which is then sent to the evaporator.

• These evaporators are efficient but are prone to 'scaling'. The scale consists of lime stone, chalk and plaster of paris, which gets deposited on the metal box evaporator. This problem is overcome by pretreatment of the raw brine (salt water) through pH control and decarbonation or by lining the evaporator tubes with teflon.

In order to reduce the cost of distillation, the following new methods have been developed to conserve the heat energy and fuel used in the process:

(i) Vertical Tube Evaporators (VTE)

(ii) Multistage Flash Evaporators (MSF)

(iii) Multieffect Multistage flash evaporators (MEMS)

(iv) Combined process (VTE / MSF)

 

Vertical Tube Evaporators (VTE)

• A single effect evaporator is shown in Figure.2.24 oil to aboston

• The The steam from boiler heats and vaporises the salt water.

A Multiple Effect Evaporator is shown in Figure 2.25. In this, the water evaporates at the highest pressure in the first effect. The vapour is condensed in the second effect to evaporate an equal amount of vapour. Similarly, the third evaporator acts as condenser for the second and so on. The temperature and pressures in the successive evaporators will go on reducing so that salt water may boil at low temperature and pressure.

 

Multistage Evaporation

Water can be boiled at low temperatures, by reducing the pressure. The hot brine can be vaporised to a certain extent at a certain pressure; the colder brine left can also be vaporised at still lower pressures and so on. This principle is used in 'multi-stage' evaporators i.e. water is evaporated - condensed again and again in various stages. This technique efficiently utilises the heat energy.

A multi-stage evaporator is shown in figure 2.26. In this evaporator, the operation is carried out at low temperatures and pressures. The process is divided into different stages. The temperature and pressure reduces at each stage. The heated brine is led into the first compartment, which causes evaporation of some water, which is condensed and collected in another compartment.

The colder brine goes to another compartment, where pressure is still less, thus causing the evaporation of some of it, which is also condensed and collected. The process is repeated until the vacuum is as high as is practically feasible, and all possible evaporation has taken place. Finally, when the brine becomes so strong and temperature and pressure so low that it cannot be evaporated and the remaining heat cannot be extracted, then it is thrown into the oceans. Only this little heat is wasted in this method.

 

Multistage Flash Evaporation (MSF)

Flash Evaporation means the effect produced when hot salt water is suddenly cooled under less pressure. Water is vaporised suddenly like a flash and the actual boiling point is never reached.

The salt water is heated to the highest temperature using a heater. The hot brine under pressure is sprayed through nozzle into the first chamber which is at a lower pressure and cooler. The sudden change produces the flash action and the water will evaporate immediately. The salt water passes through a number of compartments (flash chambers) wherein flashing of the brine occurs at successively low pressures. The vapour released in flashing each stage condenses to heat the incoming sea water and gives fresh water

 

Multieffect Multistage Flash Evaporation (MEMS)

In this method, the overall temperature flashing range is broken into a number of temperature intervals. Within each interval, an individual single-effect multistage unit with its own heat input, heat recovery, heat rejection and recycling stream operates. The first effect receives heat from the external source while for the subsequent effects, the heat from the previous effect is rejected to the brine of the next effect. This utilises the heat input efficiently.

 

2. Desalination by Electrodialysis method:

In salt water, H,O molecules are bonded together with sodium and chlorine ions. These hydrogen-bonds between the H2O molecules and Na+ and Cl- ions must be broken up, in order to separate the salt from water. These bonds where broken by heat in the "method of distillation"; while in the "electrodialysis method", these bonds are broken with the help of electricity.

When an electric-current is passed through the salt-solution, the sodium and chlorine ions get freed from water molecules, and they start moving towards their oppositely charged electric poles. In other words, the +vely charged sodium ions will move towards the - ve pole.i.e., cathode, and the - vely charged chlorine ions will move towards the +ve anode. If these cations (+ ve ions) and anions (- ve ions) are allowed to segregate in different compartments, what is left, is fresh-water.

The segregation is achieved by means of thin plastic like sheets called 'membranes'.

A cell consists of a pair of cation and anion membranes installed in parallel between a pair of electrodes.

A stack consists of a number of membrane pairs installed parallel between a pair of electrodes (with spacers between each membrane).

For a big plant, stacks are hydraulically connected together, usually in parallel (sometimes in series, also), as shown in figure 2.31. In such a plant, therefore, a large number of small separations are hooked up together in parallel; and alternate kinds of membrane are stacked with thin spacers in between each pair of opposites. Pores give excess at each level, so that salt water could be forced in between the membranes, while less salty water could be pumped out at the other side, which can be recirculated through other stacks of membranes. Since one stack of membranes usually removes 50% salinity, very fine water could be produced in this manner, using three to four stacks.

Advantages of Electrodialysis method are:

 (i) Best method for salt water with moderate TDS (500 to 3000 mg/l)

 (ii) It is a compact machine.

(iii) The cost of buying and erecting the plant is small.

(iv) It is easy to operate. It can be started and stopped in a few minutes.

(v) This is ideal for purifying water in a small town or at a remote place.

(vi) They can be assembled for different outputs just by changing the number of units added together.

 

3. Desalination by Reverse Osmosis process :

In this method of desalination, the water molecules and the salt ions are separated by forcing the salt solution against a 'semi-permeable membrane' barrier, which permits the flow of water through itself but stops the salt. In Osmosis, when salt solution is separated from pure water by a semipermeable membrane, the pure water flows across the membrane until the pressure on the pure water side become equal to the osmotic pressure of the salt solution.

But in reverse osmosis, the natural osmotic pressure is opposed by applying an external high pressure on the side containing the salt solution. In other words, the natural osmosis process is reversed, the external pressure forces pure water from salt solution to move across the membrane towards the side containing pure water.

The external pressure applied should be atleast twice the osmotic pressure to achieve

a feasible flow. RO is usually operated at about 1000 Kg/cm2

The semi-permeable membranes are thin but dense and strong enough to withstand the high external pressure.

The classification of membranes based on pore size and particle size removed, is given below:

(i) Micro filtration (MF)

(ii) Ultra filtration (UF)

(iii) Nano filtration (NF); and

(iv) Reverse Osmosis (RO)

• Microfiltration (MF) and Ultra Filtration (UF) are microporous membranes which remove suspended small size particles by physical separation. They are low pressure processes.

• Nano filtration (NF) and Reverse Osmosis (RO) remove dissolved salts ions by osmosis. NF is known as low-pressure reverse osmosis. NF and RO are high pressure processes and the separation is by diffusion. RO rejects particles of size 0.0001 μm, whereas NF rejects particles of size greater than 0.001 μm (nanometer)

Reverse Osmosis can be used for desalination with low pressure membranes (as Nano filtration) of moderately salty waters (TDS of 1000 to 10000 mg/l); and with high pressure membranes for severely saline waters containing TDS above 10000 mg/l. Sufficiently good quality water containing TDS within 500 mg/l can be obtained by this method.

4. Desalination by Freezing process: This method is based upon the principle that when salt water freezes, the ice formed in the beginning is almost free from salt. This ice, when melted, can give us good water. The quality of water obtained is 20 satisfactory, but the cost of production is high and prohibitive. A freezing process involves cooling of the incoming sea water, freezing it to ice, separating the ice and brine liquor, melting the ice to give fresh water and using the concentrated brine to chill the incoming sea water. Liquefied hydrocarbons (n-butane) can be used as refrigerants. In direct contact refrigerating system, the refrigerated butane is mixed with the salt water. Alternatively, pressure freezing; a high-boiling hydrocarbon is frozen and mixed with prechilled sea water.

5. Desalination by Solar Evaporation Method :

The other desalination methods use thermal energy or mechanical power. But solar evaporation uses solar radiation as the source of heat energy. The equipment required is simple. The figure 2.37 shows a green-house type still, which may yield about 5 kg of fresh water/day/m2 of basin surface, under good sunshine conditions. Salt water is contained in a blacked bottom shallow through which absorbs the solar energy. The vapours rise and condence on the glass or plastic surface, so inclined as to cause the collected water to flow to a common reservoir. The glass surface must be frequently cleaned off dust etc. to keep them transparent.

6. Other methods of desalination: In this method, propane gas is allowed to combine with salt water under controlled conditions of temperature and pressure. A chemical reaction takes place between salt water and propane gas at temperatures higher than the freezing point of water, forming ice like crystals. These crystals reject the salt and accept only pure water in their composition. These crystals are separated from the brine, washed and decomposed to form water. The propane gas released

 

 

Selection of a particular method of Desalination :

2200 Selection of the correct process, especially in the 500 - 5000 mg/l range (TDS), requires careful evaluation of process efficiency, plant capital and running cost. The application of the various desalination processes has been in the following categories of TDS waters.

Distillation and reverse osmosis have been widely used. Almost 75% of all desalination plants are based on distillation.