How to make DM water from raw water through various process in TPP.

  WATER CHEMISTRY

WATER TREATMENT 

What is water
    Water is the combination of two molecule of hydrogen and one molecule of oxygen.

            2H2+O → H2O

      In a thermal power plant main sources  is water so it can must be required  to plenty amount for produced electricity through steam flowing to steam turbine.
      So that the raw water is carried out from river ,sea , kennel  etc. This raw water contain some  impurities  to remove those impurity water treatment plant must be required.(the impurity are  salt and mineral, Organic material, Biological ,Oil and grease, Floating solid, Colloids, Colour , Order, Suspended solids etc).

Process of making DM water from raw water in WTP

                                              In water treatment plant raw water is carried out from river,lakes,sea etc.These raw water contains impurity like large material  which is removed through mesh,The process of removed large particle material is called Screening.
             After screening the raw water is storage a large sourrounded area where remove some bacteria by mixing  of chlorine in water in ths process is called Areation.
           After aeration process complete we get a clarify water through clarification by filter is called Pre-treatment of WTP. It involves Coagulation, Floculation after that clarifier used to remove sludge in water through pressure filter.
            After clarification active carbon  filter is used as a filter bed through which water is passed .it main function is to activate carbon by oxidation of water by free of chlorine .in active carbon filter a membrane        filter is used to removed non-reactive silica.
            After removing silica through active carbon filter we make DM water by post-treatment through Anion exchange resin, Cation exchange resin, Mix bed unit ,after  cat ion exchange the influent acid and all the bicarbonate present in water which convert CO2 .This CO2 is removed through degasser through degasser pimp otherwise co2 is removed in anion exchange method .and De-Mineralised water is prepare to formation of Steam.

Why DM water is used formation of steam not in raw water

                   Because DM water does not contain any impurities which is suitable for produced steam. If we use raw water it contain impurities like silica when the raw water is feed to boiler the silica is deposited on boiler water wall tube which reduced heat transfer and silica deposited inner surface of tube which occurred Scaling.

Method of  treatment of water in  WTP 

Two type of method used in water treatment plant
1> Pre-Treatment
      This method is used to avoid corrosion in boiler tube     
      silica, colloids, suspended solids, organic mater, bacteria,     
     physically mixed, floating solid etc removed in pre treatment plant

2>Post-treatment
     Post treatment is used to remove dissolve gasses and solid to make DM water

 1-PRE-TREATMENTPLANT 

          Pre treatment is used to remove suspended solids, colloids, organic matter etc. Process of removing those things steps are below given.

Ø Screening
 Screening is provide to avoid large size floating material comes from with water from river.the screening are 25mm dia hole and are spaces about 100mm apart and placed slightly inclination from vertically which is easy to remove large particle,here velocity of water should not exceed 0.5m/s. After screening a fine screen fitted through orifice in size of 6mm dia to removeparticle in size of 6mm dia to remove particle are also called travelling water screens.

Ø Raw water storage

After screen the raw material is storage large surrounded area  at 7 to 15 days for improvement of water quality and average water demand and also reduced bacteria, suspended solids and river algae etc.

 

Ø Prechlorination     
This process chlorine is injected into raw water for destroys the high bacteria ,oxidizes, iron and manganese are kill algae ,reduces colour etc.

Ø Aeration

The process is used to controlling taste, order, and Corrosion but not all case complete control gases are absorbed or liberated from water between natural gas contain in atmosphere and gas contain in water , the volatile matter and decomposing organic matter can be manganese in solution are also oxidized and precipitated out.

Ø      Coagulation
This process small particle suspension together to from large agglomerate. Fine particles and colloids are present in water which carry negative charges on their surface which are true coagulation into a large particle which depend upon frequency of collision. So it is very important process to accelerate sedimentation of suspended particles  coagulation aids is different type which are all long chain molecules carrying electrically active group like long chain poly acrylic amides which carries positive charges.

Ø    Flocculation
When ferric and aluminium salts added into water is formed insoluble hydroxide which comes out as flocks whose surface carry positive flocks attracts itself with the negative charged particles in water and also attracts and holds humic acid. When alums added in water aluminium hydroxide floc and sulphuric acid is reduced PH value of water normally raw water PH is alkaline, after addition of alum in water PH value may be down to coagulation pit. In rainy season when turbidity is high , alum dosing is high in that case PH may faill below coagulation PH. So Lime ca(OH2)  is used to increase PH to required level & helps to remove some hardness.

Ø Clarifiers

This system which water is cleared form most of the suspended solids with or without coagulation then there is different types of clarifiers.                 

1>Conventional type

2>Hopper to remove sludge  - it is used to remove sludge

Ø Filteration

Filtration means removing suspended solid out of fluids and colloids from water. in case of filtration no sands and mud are not filtration  here water which has been screened  and settled which contained suspended matter and colloids .colloids are those particles which does not show in microscope which size is 0.005 to 0.2 microns in diameter. The colloids is water absorb and scatter light passing through it this process remove suspended solids in water.

1>Deep bed filter

      It is used to removed sand always back washed  upward and san bed expand and solids are released

It is 2 type  -Up flow filter

                     -Down flow filter

            2>Slow and rapid sand filter

                 Its flow is 1-2kg /sqft/hr and 100litr/sq-m/hr

           3>Activated carbon filter

      It is used to remove bacteria growth it main function is oxidisation of water by free chlorine.

4>Membrane filter

   A Ultra filtration membranes used to remove non-reactive silica with a pressure of 5 bar water have total dissolve salts or more than 500 ppm.

 

 

2 POST TREATMENT

         Post treatment is used in DM plant maded ehich is 3 type of process.
a) Ion exchange method

b)chemical softonic method

c)Reverse osmosis method

 

Ø ION EXCHANGE METHOD

       In this method includes anion exchange resin and cation exchange reign for removing dissolve solid

Cat ion exchange method

 

     Cat ion is positive charge  but in acid which represent  H+ and its resin is R+ it is two type 

1. Strong acid cation exchange
            When the function group attached to resin matrix is strong acid      group is called SAC resin. It split all salts and influence by  PH of water its operational exchange capacity and regeneration efficiency of SAC is less than WAC.

2.Weak acid cation exchange
   When the functional group is of weak acid is called WAC resin it can spited weak electrolyte carbonate and bicarbonate only.

 Anion exchange method

      It can  denoted  OH-  and negative charge it is two type
1.Strong base anion resin
    Strong Based anion the functional group is strong base is called SBA.it performance is not influenced by water PH  and it exchange with both strong and weak acids.

2.Weak base Anion Exchange
   When the functional group attracted to weak base is called week base anion. It performance better at low PH and increase PH decreased it performance when PH more than 11
  
                                    efficiency SAB<WBA

Mixed bed unit

    This method both anion and cation exchanger passing through water mixed bed unit is formed and after mix bed unit DM water is made.
         

DEGASSIFIER

After the cation exchanger the influent is acid and all the bicarbonate present in water is converted to co2 , this co2  can remove in degasser ,the capital cost of degasser is very less and running cost also less. other wise co2 is remove  in anion exchanger. The theory of degasser based on following gas law.

DALTONS LAW
The total pressure exerted by a mixture of several gases to the sum of the partial pressure of individual gas. Again according to charls law the partial pressure of each gas is determined by the amount of that gas in the mixture.

HENRYS LAW
The solubility of the gas in water is directly proportional to the partial pressure or that gas is contact with water.

X = P/H

Where  X= Amount of gas dissolved in water
               P=Partial pressure of the gas in contact with water
               H= Constant at that temperature