Steam boiler and its various types
Boiler feed water treatment
Mixed bed deionizer
In this lesson, we will discuss about mixed bed deionizers.
In mixed-bed deionizers the cation-exchange and anion-exchange resins are intimately mixed and contained in a single pressure vessel. The thorough mixture of Cation-exchangers and anion-exchangers in a single column makes a mixed-bed deionizer equivalent to a lengthy series of two-bed plants. As a result, the water quality obtained from a mixed-bed deionizer is appreciably higher than that produced by a two-bed plant. Although more efficient in purifying the incoming feed water, mixed-bed plants are more sensitive to impurities in the water Supply and involve a more complicated regeneration process. Mixed-bed deionizers are normally used to ‘polish’ the Water to higher levels of purity after it has been initially treated by either a two-bed deionizer or a reverse osmosis unit.
The procedure is described below:
Sodium hypochlorite (NaOCl) is injected to raw water to kill the algae or bacteria present in the raw water. Otherwise they may cause harm to the multi grade-filter (MGF).
The multi-grade filter is the primitive type of filter where sand, stone-chips, stones are used in stacks to remove the large size suspended particles from the raw water.
The net filter again removes medium-size suspended particles, where the raw water passes through the net minute vents.
Then by ultra-filtration very small suspended particles are removed. After long usage of ultra-filtration unit, it requires back-wash, and then it is back-washed with water and three chemicals, viz. HCL, NaOH and NaOCl (Sodium Hypochlorite). HCl Removes iron by dissolving it. It also removes the basic salts those are rejected on UFU.
NaOH ← It helps to remove acidic salt.
NaOCl ← To kill algae and bacteria inside the UFU.
After ultra-filtration the water is stored into RO feed tank and then pumped with RO feed pump of Reverse Osmosis Plant. In the channel the water is mixed with HCL (for pH controlling, as the water coming from RO plant or RO permeate water should have pH around 6.0) and SMBS (sodium meta bi-sulphate)[Na2S2O5]. Due to the presence of sodium hypochlorite the water is chlorinated. To remove excess chlorine SMBS is used. If excess chlorine is not removed then the semi-permeable membrane may get damaged. It is also mixed with anti-scaling reagent (AS), which reacts with those chemicals which form scale inside the channel.
Then the water is passed through micro-cartridge filter (MCF) which removes the other suspended particles and the precipitate formed by the reaction of anti-scaling reagent with the scaling chemicals.
In the this stage of boiler feed water treatment the water is fed to RO unit by H/P pump, where after successive filtration by 1st and 2nd stage RO it is fed to degasser unit.
After degasification the water is passed through D/M plant MB (mixed bed) resin and stored into D/M water storage tank.
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