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23291582 dm-plant

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  • 5. INDEX1) RAW WATER & IMPURITIES…………………………………………2) METHOD OF EXPRESSING DISSOLVED IMPURITIES……………3) IMPURITIES IN WATER, DIFFICULTIES CAUSED & MEANS OF TREATMENT…………………………………………………………...4) PRETREATMENT OF RAW WATER………………………………….5) DEMINERALISATION SYSTEM……………………………………….. 5.1 Demineralization process …………………………………………. 5.2 Ion exchange materials……………………………………………. 5.3 Different demineralization systems…………………………………6) OPERATION OF DM PLANT…………………………………………….. 6.1 WAC Exchanger………………………………………………….. 6.2 SAC Exchanger……………………………………………………. 6.3 Degasser system……………………………………………………. 6.4 WBA Exchanger…………………………………………………… 6.5 SBA Exchanger……………………………………………………. 6.6 Mixed Bed………………………………………………………...7) MONITORING OF DM PLANT…………………………………………8) OPTIMISATION OF DM PLANT OPERATION…………………………. 8.1 selection of regeneration system ……………………………………….. 8.2 selection of layout & resin types…………………………………………. 8.3 atmospheric degasifier………………………………………………. 8.4 outputs based on water quality............................................................... 6
  • 6. 1) RAW WATER & IMPURITIES:Water is one of the basic requirements in raising steam. In nature water is available inabundance. Its physical and chemical characteristics vary depending upon the source andstrata on which it flows. It picks up mineral salts from the soil, which go in to solution.Water, therefore contains mineral salts in dissolved condition, in varying proportions,composition and degree. It gets polluted further with multifarious organic and inorganicimpurities due to disposal of industrial and domestic wastes. Decayed vegetation and micro-organism also contribute to contamination. Besides dissolved salts water contains coarsesubstance in suspended form, constituting of silt and clay matters, generally termed asturbidity. Silicate matters are present in dissolved as well as in colloidal forms, proportion ofwhich varies depending mainly on the following conditions:- Temperature- Seasonal Conditions- Chemical characteristics of the particulate- Velocity of the flowTable 1 lists the major impurities of raw water, classed in three main groups: first, ionic anddissolved, second non-ionic and undissolved and third gaseous. The ionic impurities in thefirst group are sub divided in to cations and anions. Organic matter and colour appear in boththe first two groups, because there are many types: some dissolved and ionic, such ashamates, and others colloidal and non-ionic, such as tannins. Also there may be types oforganic matter that are dissolved and non-ionic. 7
  • 7. MAJOR IMPURITIES OF WATER:CATIONIC ANIONIC NON-IONIC AND GASEOUS UN-DISSOLVEDCalcium Bicarbonate Turbidity, silt, mud, Carbon dioxide dirt and other suspended matter.Magnesium Carbonate Colour Hydrogen SulphideSodium Hydroxide Organic matter AmmoniaPotassium Sulphate Colloidal silica MethaneAmmonium Chloride Micro-organisms OxygenIron Phosphate Bacteria ChlorineManganese Silica Oil Organic matter Corrosion products colour (condensate)HARDNESSPermanent Hardness: is due to presence of SO4, NO3, Cl of Ca++ & Mg++Temporary Hardness: is due to presence of HCO3 & CO3 of Ca++ & Mg++ in waterEquivalent Mineral Acidity: (E.M.A.):-This is the sum of all ions of SO4 + Cl + NO3 in raw water.Alkalinity is of 3 types.1. Bicarbonate Alkalinity2. Carbonate Alkalinity3. hydroxide or caustic AlkalinityTotal Alkalinity = HCO3 + CO3 + OHPhenolphthalein Alkalinity (p Alk) is determined by titration with an acid and the colour change (pink tocolorless) takes place at pH of about 8.3.Methyl Orange Alkalinity (M. Alkalinity):- During titration with an acid, the colour change takes placeat about pH of 4.3 (Orange to Pink) 8
  • 8. 2) METHOD OF EXPRESSING DISSOLVED IMPURITIES: Dissolved impurities may be expressed in terms of ions themselves or in terms of their equivalent. The preferred method of expression in water treatment field is in terms of equivalent of Calcium Carbonate abbreviated ‘as CaCo3.’ CaCo3 is a good common denominator because it has a molecular weight of 100, which facilitates calculations. Moreover in this form of analysis the sum of cations always equal to sum of anions. This method also aids in predicting the comparative analysis after various forms of treatment and analysis at consecutive steps in multi step demineralization. If analysis expressed in terms of ions, it can be converted to the form of expression in terms of equivalent CaCo3 (or as CaCo3 ) by dividing figures by equivalent weight of ions then multiplying by equivalent weight of CaCo3 (i.e. 50). For example if amount of Calcium in water is 40 ppm as Ca then during expressing it in terms of CaCo3 it becomes 40 ----- X 50 = 100 ppm as Caco3. (Here 20 is equivalent wt. of Ca) 20 (Equivalent weight of an ion is its molecular weight divided by its valances)Other units of analysis of water are:a) one grain/u.s. gallon = 17.1 ppmb) one grain/imperial gallon = 14.3 ppmc) one milligram/liter = 1 ppm Or one gram/m3 = 1 ppm[Note 1 U.S. gallon = 8.33 pounds 1 pound = 7000 grams 1 imperial gallon = 10 pounds 1 liter = 1,000,000 mg] 9
  • 9. 3) IMPURITIES IN WATER, DIFFICULTIES CAUSED AND MEANS OF TREATMENT: CONSTIT CHEMICAL DIFFICULTIES CAUSED MEANS OF TREATMENT UENT FORMULA 1. Turbidity None Imparts unsightly Coagulation, setting and filtration. expressed in appearance to analytical water. Deposits units in water lines and process equipment. 2. Colour None May cause foaming in Boilers. Coagulation and filtration, expressed in Hinders precipation methods chlorination, Adsorption by analytical such as iron removal and activated carbon. units softening. 3. Hardness Calcium and Chief source of scale in heat Softening, Demineralization, Magnesium exchange equipment, boilers, internal boiler water treatment. salts pipelines, etc. forms curds with expressed as soap and interferes with dyeing. CaCO3 4. Alkalinity HCO3, CO3 Foaming and carryover of solids Lime & Lime Soda Softening. and OH with steam. Embitterment of Acid treatment. Hydrogen expressed as boiler steel. Bicarbonate and Zeolite softening. CaCO3 carbonate produce CO2 in steam a source of corrosion in condensate lines.5.Free H2SO4, HCL, Corrosion Demineralizing. Dealkalization Mineral etc. by H ion exchange. Acid expressed Neutralization with alkalies. 6.Carbon CO2 Corrosion in water lines Aeration, De-aeration, Dioxide particularly steam and Neutralization with alkalis. condensate lines. 7. pH Hydrogen- PH varies according to acidic or pH can be increased by alkalis ion alkaline solids in water. Neutral and decreased by acids. concentration water have a pH of 6.0 – 8.0 defined as PH = log 1/H 8. Sulphate (SO4) Adds to the solids content of Demineralization water, but in itself, is not usually significant. Combines with Ca++ to form CaSO4 scale. 9. Chloride Cl Adds to the solids content Demineralization 10
  • 10. 4) PRE TREATMENT OF RAW WATER:The purpose of pre treatment is to render raw water fit as influent to a de-mineralizing unit.Pre treatment is done to make water free from suspended, colloidal and organic impurities.Since presence of such impurities adversely affect the de-ionisation effect and the finalquality of de-mineralized water, pre-treatment plays a vital role in water treatment. Thedifferent process involved in pre-treatment are:- a) Settling and Coagulation b) Filtrationa) SETTLING & COAGULATION:Used for removal of turbidity and suspended matter. The coarse, heavy particles ofsuspended matter gets easily removed by settling the water in a tank, but some suspendedimpurities, such as turbidity, micro-organisms and colour are very finely divided or even incolloidal form, so that they do not settle readily. Settling basins would have to beexcessively large, to remove these fine particles.Co-agulation is a process of breaking up of a colloidal solution resulting in the recipitation ofthe particle of the dispersed phase. It may be spontaneous and brought about by the additionsof an electrolyte which is termed as “coagulant”. Co-agulation, induced by adding chemicals(Coagulants) to the water, agglomerates the finely divided, suspended solids in to masses thatsettle more readily. This occurs in two ways:- • The particles of turbidity and colour have like electric charges on their surfaces, which keep them apart, because like electric charges repel one another and the co- agulant ions selected possess charges opposite to those on the suspended particles, so that they neutralize each other. • The coagulant reacts with the alkalinity of the water to form a gelatinous precipitate, called “floc” which enmeshes and entraps the finer of the suspended particles. Alum or Aluminum sulphate is the most commonly used coagulant, because it is the lowest in cost and least corrosive to handle. The reactions are shown below:- Al2 (SO4)3 + 3 Ca (HCO3)2 = 2Al (OH) 3 + 3CaSO4 + 6CO2 11
  • 11. The other coagulates are FeSO4 (Ferrous sulphate) and ferric sulphate {Fe2 (SO4)3} Factors influencing coagulation: • Organic matter, if present in appreciable amounts, inhibits coagulation and narrows the optimal pH range. For the oxidation of organic matter pre chlorination and narrows the optimal pH ranges. For the oxidation of organic matter pre chlorination is desirable, because it broadens the optimal pH range and there by makes the coagulation easier to control. • If the alkalinity in the water is insufficient to react with the dose of co-agulant, the pH is below the optimal range, and then the alkalinity must be increased by adding an alkali. Lime is the cheapest alkali. For Al2 (SO4)3 optimum pH is from 5.5 to 7.5.b) Filtration: Filtration is defined as passage of fluid through a porous medium to remove matters held in suspension. In water purification the matter to be removed includes:- 1. Suspended Silt 2. Clay 3. colloids 4. Micro Organisms including algae, bacteria and virus. The particles to be removed have approximate size as follows:- Material Particle Size, Milli-micron Silt 50,000 Bacteria 5,000 Viruses 50 Colloids 1 – 1,000 12
  • 12. 5) DEMINERALISATION SYSTEM:5.1 DEMINERALIZATION PROCESS & SYSTEMS: The process of demineralization water by ion exchange comprises of:- • Conversion of salts to their corresponding acids by hydrogen cat-ion exchanger. • Removal of acids by anion exchangers. • The two exchangers are normally in series. Normally cat-ion precedes the anion exchanger 5.2 ION EXCHANGE MATERIALS:Major ion exchange materials are synthetic resins made by the polymerization of variousorganic compounds.Most frequently used compounds are:- 1. Styrene 2. Die vinyl – Benzene • The long chained co-polymer formed from these compounds contains a major proportion of styrene (80-92%) and a minor portion of divinely Benzene (8- 20%). Divinyl Benzene acts as a cross link to hold the polymer chains together. • To make strong acid cation exchanger polymer is treated with concentrated sulphuric acid, which attaches – SO3H to the hydro carbon network, to make most anion exchanger resin the matrix is chloromethylated and animated. • The resin when dry shrinks so that chains come very close together and the bead cannot be readily penetrated by the ions, but when placed in water, it takes on water and swells, so that the chains spread apart and permits the diffusion of the ions. • The degree of swelling depends on the degree of cross linking, i.e. the number of cross links. The greater the no. of cross links, less the moisture holding capacity and the swelling. • From the kinetic point of view, for a steady exchange reaction it will be desirable to have as low as a degree of cross linkage as possible, but this 13
  • 13. would result in a high degree of swelling and an accompanying gelatinous structure having poor hydraulics properties. • The design of commercial ion exchange resin is therefore involves a choice of cross linking that represents a compromise between kinetic and hydraulic performance. Cation exchanger of the hydrogen type are either strongly acidic or weekly acidic. Strongly acidic resins contain the sulfonic acidic functional group SO3H where as weekly acidic resins contain carboxylic acidic group COOH. Similarly there are weekly basic anion exchanger resins and strongly basic anion exchanger resins. Strongly basic anion exchangers are again of two types, type I and type II. Type I resins have less exchange capacity, but more stability than type II. Type I have quaternary ammonium functional group. Type II has modified quaternary ammonium functional group where one of the methyl groups is replaced with an ethanol group. The week base anion exchangers have polyamine functional groups containing primary amine – NH2, secondary amine NHR and tertiary amines NR2. 5.3 DIFFERENT DEMINERALISATION SYSTEM: Various system combinations are available. Selection of particular system depends on quality of raw water available and the requirement of end product. The various demineralization systems have been shown below. Different Demineralization SystemDemineralization Application Typical Effluent RemarksSystem1. SA-WB SiO2 + CO2 are no Conductivity 10-30 Low equipment and limitation micro mhos/cm. SiO2 regenerate cost. un-changed2. SA-WB-D No limitation of SiO2 Conductivity 10-20 Low regenerate cost. but CO2 removal is micro mhos/cm required 14
  • 14. 3. SA-SB Low alkalinity raw Conductivity 5-15 Low equipment cost water. SiO2 removal micro mhos/cm. SiO2 high chemical cost. required. = 0.02-0.14. SA-D-SB High alkalinity of raw Conductivity 5-15 Low chemical cost. water. SiO2 removal micro mhos/cm. SiO2 required. = 0.02 – 0.15. SA-WB-D-SB Higher alkalinity, SO4 Conductivity 5-15 Low chemical with and Cl in raw water. micro mhos/cm. SiO2 high equipment cost. SiO2 removal required. = 0.02-0.16. WA-SA-D-WB-SB High hardness, Conductivity 5-15 Lowest chemical cost alkalinity, sulphate micro mhos/cm. SiO2 high equipment cost. and chloride. SiO2 = 0.02-0.1 removal required.7. SA-D-SB-SA-SB High alkalinity, high Conductivity 1-5 Low chemical cost, Na in raw water, high micro mhos/cm. SiO2 high equipment cost. purity treated water = 0.01-0.05 required.8. MB Low solids raw water Conductivity 1.0 Low equipment cost, high purity of treated micro mhos/cm SiO2 high chemical cost water required. = 0.01 – 0.059. SA-D-SB-MB-MB High alkalinity and Conductivity 1.0 Lower chemical cost, dissolved solid raw micro mhos/cm SiO2 higher equipment cost. water. High purity = 0.01 – 0.05 treated water.10. SA-D-SB-MB-MB High alkalinity & Conductivity 0.5 Lower chemical cost, dissolved solid raw micro mhos/cm SiO2 higher equipment cost. water. Ultra pure = 0.01 – 0.02 water required.KEYSA - Strong acid Cation ExchangerSB - Strong Base Anion ExchangerWB - Weak Base Anion ExchangerWA - Weak acid Cation ExchangerD - DegasserMB - Mixed Bed 15
  • 15. 6) OPERATION OF DM PLANT: A typical DM plant consists of cation, degasser and anion exchangers:Introducing weak acid cation exchanger and weak base anion along with typical de-mineralization chain and regenerating strong and weak exchangers by same acid/causticincrease the efficiency of plant. Also having mixed bed for polishing ex-anion waterimproves the quality of D.M. water to great extent. OVERVIEW OF DM PLANT Degasser tower Weak Strong Raw water Acid Acid inlet cation cation Pressure filter Degasser tank DM Water storage tank Weak Strong Ba se Mixed Ba se Anion Bed anion DM water supply to unitDegasser usually provided in between cation exchangers and anion exchangers to removecarbon dioxide that decipates during ion exchange in cation exchangers.The operating principles of weak acid cation exchanger (WAC), strong acid cation exchanger(SAC), Degasser, Weak Base Anion Exchanger (NBA), Strong Base Anion Exchanger(SBA) and mixed bed (MB) are discussed in following paragraphs.6.1 WEAK ACID CATION EXCHANGERS: 16
  • 16. Weak acid cation exchanger mainly removes Calcium & Magnesium alkalinity fromraw water. For simplicity cation resin is represented by H2R and equation for the servicecycle of WAC can be written as Ca Ca HCO3 + H2R -R + H2CO3 Mg Mg H2CO3 H2O + CO26.2 STRONG ACID CATION EXCHANGERS:Strong acid cation exchanger removes sulfates, chloride, nitrates and sodium salts. Theequation for service cycle of strong acid cation can be written as Ca So4 Ca H2So4 Cl2 + H2R -R + HCl Mg NO3 Mg HNO3 HCO3 CO3 H2C03 Na SO4 + H 2R NaR + H2So4 Cl2 HCl NO3 HNO3 H2CO3 H2O + CO2.Process of exchanging salts in cation exchange continues till resin looses its capacity toconvert salts into corresponding acids. After this the resin to be regenerated by usinghydrochloric acid or sulfuric acid.During regeneration resin will regain its capacity to exchange salts after which it can againremove salts from water.The equation for regeneration cycle can be written as follows: Ca Ca Mg R + HCl Mg Cl + RH Na Na 17
  • 17. Ca Ca Mg R + H2So4 Mg So4 + RH Na NaNote: Strong Acid Cation Resin can also remove alkaline salts of Calcium & Magnesium.However as WAC is precedes SAC, there won’t be any load of alkaline salts on SAC.When Weak Acid Cation and Strong acid cation are in series regeneration is done in throughfare system. Acid after regenerating strong acid cation exchanger passes through weak acidcation exchanger.Ion leakage and end points of exhaustion phase: During re-generation with down flow of acid the top of bed is more completelyconverted to the hydrogen form than the bottom, but unless uneconomical amounts of excessacid are employed, the bottom usually contains a band of sodium at the end of re-generation.As the next service run starts, the cations in the influent are exchanged for the hydrogen ionsin the top of the bed, releasing the sodium as cation leakage into the effluent. As the runprogresses, this sodium cation leakage decreases, because the sodium band at the bottom isgradually consumed.The cations in the water are converted to their corresponding acids. But the conversion is notcomplete. The difference between the total mineral acidity (corresponding to the sulphatesand chlorides in the influent) and the free mineral acidity in the effluent is equal to the cationleakage.At the end of the exhaustion run, at the break through, the FMA drops, and when theresulting increased cation leakage reaches the allowable limit, the unit is regenerated.Normally conductivity is compared during the run. The conductivity ratio is normallyconstant during the run. At the exhaustion the conductivity ratio changes indicating theexhaustion.Cation leakage is important because it affects the purity of the demineralizer effluent. Astrong base anion exchanger can remove only the acidity, not the sodium. It coverts the 18
  • 18. sodium salts to sodium hydroxide, which creates a high conductivity and pH value in theeffluent. Therefore for a low conductivity of dematerialized water cation leakage (Na) mustbe reduced. The several methods adopted area:- 1. Air Mixing of resin Regeneration. 2. Counter flow re-generation.6.3 DEGASSER SYSTEMIn de-mineralization process carbon dioxide generated by dissociation of carbonic acid atcation outlet water. H2CO3 H2O + CO2The CO2 generated if not removed increases load on SBA resin. So degassers orDecarborators are placed in cation & anion.Degassers usually made of acid proof materials (wood or rubber lined steel) as it have tohandle acidic water of cation exchangers. (Redwood or cypresses are usually preferredwoods).Typical degasser as shown; air blown at the bottom and rises counter current to thedownward trickling water. The spray pipes or trays divide water into droplets or thin filmsexposing new surfaces to gas phase. Tray also serves to agitate the water by splashing thusallow dissolved gases to leave water readily. Agitation overcome tendency of water to retaingas bubbles through surface tension and viscosity.Height of the tray stack or Rasching ring proportional to amount of influent CO2.Decarborators are designed with flow rates that range from 20 to 30 gal/min/sq.foot areas (1to 1.5 m3/min/m2). The height of Rasching –ring varies from 5 to 15 ft. 19
  • 19. Sectional view of degasser tower 20
  • 20. 6.4 WEAK BASE ANION EXCHANGER:Weak base anion exchangers can remove only the highly dissociated acids (like H2So4,HNO3, HCl) from the effluent of cation exchanger. They can not remove weakly dissociatedcarbonic acid from alkalinity or silica acid from the silica content in the water.Exhaustion reaction represented by equation. H2SO4 SO4 2 HCl + 2ROH (NO3)2 2R + 2H2O 2HNO3 NaThe regenerates of weak base anion resins may be NaOH, Na2CO3 or NH3Some typical application of WBA exchanger are mirror silvering, processing of ceramics,deproofing or cutting of alcohol in distilleries, plating, glass manufacture and automobilepointing.6.5 STRONG BASE ANION EXCHANGERThis removes weakly dissociated and the strongly dissociated acids.The reaction of strong base anion exchangers given in following equation: H2SO4 SO4 2HCl + 2ROH Cl2 2R + 2H2O 2H2CO3 (Co3)2Regeneration of Anion exchanger usually did by caustic soda. If weak base anion exchanger& strong base anion exchanger are in series regeneration done in thoroughfare system.Caustic soda after regenerating strong base anion exchanger passes through weak base anionexchanger. Following equation represents regeneration equation.SO4 Na2SO4Cl2 2NaCl(NO3)2 -2R + 2 NaOH 2 ROH + 2 Na (NO3)2SiO3 Na2SiO3CO3 Na2CO3 21
  • 21. 6.6 MIXED BED: OPERATION PRINCIPLE OF MIXED BED DEMINERALISER: 22
  • 22. In mixed bed both strong cation and strong anion exchangers are in same shell, rather inseparate shells. They are mixed together by compressed air after regeneration. Cation andanion particles being next to each other constitute a series of two bed pairs of beads.Prior to regeneration the two resins are separated by backwashing. Due to density differencebetween two types of beads, the two types of resins separate completely and settle one abovethe other (cation in the bottom and anion at the top). A screened interface pipe system,located between two resins, collects regenerate effluent. Acid usually follows upward andcaustic soda downward.Cation Regeneration usually proceeds anion, but two may be simultaneously also. Theadvantage of sequential regeneration is that the Calcium cations dissipated from the cationresins before carbonate ions from Anion resins, formation of Calcium Carbonate precipitatesis avoided thus avoiding fouling of interface screening.A downward blocking flow of water proceeds from top while acid flows upward. Bothblocking water as well as effluent acid escape through interface collector.The blocking flow avoids expansion of bed and also prevents acid from entering anion bedabove interface.An upward blocking flow of water or acid proceeds from bottom while caustic soda flowsdownward, so that later does not enter cation resin.Usually mixed bed used for polishing and follows a two bed pair. 23
  • 23. 7) MONITORING OF DM PLANT: S. DEFECTS CAUSES REMEDIESNO. a. Increase in ionic load Check by analysis b. Flow integrator/indicator defective Check c. Less amount of regenerant chemical Check used for regeneration Decrease in d. Resin fouled Give treatment for de-fouling capacity 1 e. Plant being used intermittently Regular running between two Check and ensure uniform regenerations f. Channeling in resin bed distribution of bed g. Resin dirty Give prolonged backwash h. Resin deteriorated Check /replace the resin i. Resin quantity become less Check and makeup the level a. Cation exhausted Check and regenerate b. Anion exhausted Check and regenerate c. Mixed bed exhausted Check and regenerate d. Resin in mixed bed not in mixed Again remix the resin by air and state rinse. e. Some valves particularly back wash Check and rectify. inlet valves passing Check feed water analysis. Note f. Sodium slip from cation high changes in Na / TC and Silica / Treated TA ratios. Use more chemicals water quality g. Silica slip from anion high 2 accordingly. not as specified h. Unit idle Check i. Unit is not sufficiently rinsed Rinse it to satisfactory quantity. Adjust the unit flow between j. Excessive low flow rate minimum and maximum flow rate. Check and ensure uniform k. Channeling of resin bed distribution in bed. l. Resin fouled Same as 1 (d ). m. Resin deteriorated Check the resin and replace. a. Resin not separated during back Give extended backwash after wash exhaustion of bed. Mixed bed b. Air mixing not proper Give extended air mixing. 3 quality not c. Final rinse not proper Give extended final rinse. good d. Some valves may be leaking and Check and rectify. contaminating the treated water 24
  • 24. a. Due to choked suction filter of Check and clean filter. degassed air blower High residual CO2 from Check blower discharge valve / 4 degasser b. Improper air flow to degasser damper / speed of blower and its discharge pressure. Check and take blower in line. c. Degasser blower not in operation. Reduce air flow rate by adjusting a. Very high air flow rate Flooding in V/V 5 Degasser b. Packed tower choked due to dust Open, check, clean or replace or broken packing material packing. a. Flow rate too low Check and increase flow rate. b. Backwash inlet valves not holding Check and rectify. Unit rinse c. Anion resin organically fouled Give alkaline brine treatment. 6 takes long d. Mixed Bed air mix not satisfactory Carryout air mixing again. time Faulty design, Check/rectify. e. Acid or alkali pockets formed in Temporarily give longer backwash unit and rinse again. a. Choked valves or suction strainer Check and rectify of pump Check water level in respective Flow rate b. Cavitations in pump tanks. too low c. Low inlet pressure Check water pressure. 7 d. Distribution or collector system Check and clean choked e. Resin trap at outlet unit choked Check and clean f. Control valves shut due to low off Increase off take. take. a. Defective valves Check and rectify / replace. Pressure Give extended backwash with open b. Packing of resin bed due to fines of drop across manhole, scrap fines from top of 8 resin bed bed increasing c. Collecting system choked Check and repeat backwash. d. Pressure gauge defective Check and rectify / replace. a. Due to excessive backwash Check inlet pressure and flow rate pressure or flow. and reduce it if necessary. Resin bed 9 Examine the system for any being lost b. Faulty collection system breakage. c. Inlet strainer damaged Check and replace. 10 Ejector not a. Low power water pressure Check and adjust. working b. Air lock in the unit Backwash and release air entrapped in unit. c. Choked or defective valves Examine and rectify 25
  • 25. d. Ejector nozzle may be choked Check and clean. Check for choking of regenerant e. Too much back Pressure from unit distribution / collecting system. Passing of inlet and outlet valves. f. Bulge in rubber linking of pipeline Check and rectify. Incorrect a. Choked orifice or impulse line Check and clean11 reading from b. Dirty glass or float Check and clean rotameters. a. Choked impulse line or orifice Check and clean Improper b. DP transmitter requires re reading Re calibrate calibration from flow c. Leakage in signal tube between recorder Check and repair transmitter and panel12 integrator Check instrument air pressure and take remedial measure d. Low air pressure for D. P. transmitter or recorder Level a. Improper contact between Check the contact and rectify. electrodes electrodes and control cabling. system for b. Short circuiting of electrodes due Clean and dry contacts of measuring to moisture , dirt etc moisture and dirt.13 and dilution tank not c. Improper working of level Check and repair. functioning controllers properly Leakage from acid injection or a. Improper adjustment of14 Check and adjust. unloading mechanical seal transfer pumps Check silica gel breather in acid Corrosion in a. Low concentration of H2SO4 storage tank and replace silica concentrate15 gel charge if necessary. d acid tanks b. Lining of HCl tank / pipe line and lines Rectify. damaged. Improper a. Defective solenoid valves. Check and rectify. opening b. Leaking in air line to solenoid Check and rectify. closing of valve to respective control valve.16 pneumatical c. Improper contact of micro ly operated switches giving false indication on Check and rectify. valves panel. 26
  • 26. 8) OPTIMISATION OF DM PLANT OPERATION: DM plant operation can be optimized by • Proper selection of Regeneration system. • Selection of layout & resin type. • Using atmospheric degasser. • Output based on water quality.8.1 SELECTION OF REGENERATION SYSTEM:Regeneration system of cation / anion exchanger is normally two types based on regenerateflow. When the flow of acid / caustic are in the same direction on the service flow theRegeneration system is known as cocurrent regeneration. And when the flow of acid /caustic are in opposite direction of service flow it is known as counter current regeneration.Counter current regeneration have following advantages. • Reduced chemical consumption • Improved water quality and • Less waste volumesCation / Anion exchangers which are regenerated by counter-current regeneration systemgive more output when compared to the exchangers that are regenerated by Co-current.This is illustrated in tables below 27
  • 27. STRONG ACID CATION RESIN Operating capacity verses Regeneration level: (Na = 40 %, Alkalinity = 50 %) Regeneration level Exchange Capacity (Kg of HCl / M3 of resin Kg CaCO3 / M3 of Resin Co – Current Counter Current 50 46 55.2 60 51 59.5 70 55 63.36 80 58.5 66.72 90 61.5 69.12 100 64 71.52 110 66.5 73.44 120 68.5 75.36 STRONG BASE ANION RESIN Operating capacity verses Regeneration level : (SO4 = 25 %, CO2 = 20 %, Silica = 25 %) Regeneration level Exchange Capacity(Kg of NaOH / M3 of resin Kg CaCO3 / M3 of Resin Co – Current Counter Current 40 26.2 30.2 50 27.6 32.2 60 29.4 34.0 70 31.3 35.4 8 33.6 36.8 100 36.3 38.2 120 38.2 39.1Counter-current regeneration systems provide a water quality of better than 2 µ S/cm andresidual silica of 0.020 to 0.050 mg/l as SiO2. Depending upon water composition andregeneration conditions, the specific conductivity could be as low as 0.2 µ S/cm. Thenormal counter-current endpoint is 4 µ S/cm conductivity. 28
  • 28. A maximum endpoint value of 0.3 mg/1 SiO2 above the average leakage should not beexceeded in order to avoid a high contamination of the polishing resin layer and unacceptablyhigh silica leakage during subsequent cycles. Silica leakage can be minimized by operatingthe plant at silica break rather than conductivity end point. This secures the lowest silicaleakage, but at the expense of a 5 – 10 % throughput reduction.8.2 SELECTION OF LAYOUT & RESIN TYPES:The plant configuration will depend on the feed water composition, the water qualityrequired and the economics of operation. The following general guidelines are given to helpin configuration and resin selection. (A) [SAC] – [WBA]: This combination of strong acid cation [SAC] and weak base anion [WBA] resins is used to obtain partially deionized water without removal of CO2 and SiO2. (B) [SAC] – [SBA]: The combination of strong acid cation and strong base anion [SBA] is preferred for treating low mineralized water or for small size plants. (C) [SAC] – [WBA] – [SBA] : This combination of strong acid cation with weak base and strong base anions is proven to be an excellent choice for larger plants as it provides an optimum balance between investment and running cost. It is well suited to treat waters with low alkalinity, when the FMA (Cl + NO3 + SO4) is typically > 60% of the total anions. The normal end-point for a WBA resin corresponds to the chloride breakthrough, which means that the downstream SBA resin is only removing the carbon dioxide and silica ions. This situation generally leads to a big discrepancy between WBA (large) and SBA (low) volumes. (D) [WAC] – [SAC] – [SBA]: The use of a weak acid cation [WAC] in front of a strong anion is preferred with feed waters containing a high proportion of temporary hardness (>60%) and low FMA. The normal end-point for a WAC resin is 10% alkalinity leak. In this condition, the down-stream SAC resin should remove the permanent hardness and the monovalent cations. This situation generally leads to a big discrepancy between WAC (large). This is the ideal combination for high hardness, high alkalinity and high FMA water, as well as large size plants. Again the cation and anion combinations can be in single or separate vessels. 29
  • 29. (E) [WAC] – [SAC] – [WBA] – [SBA]: This is the ideal combination for high hardness, high alkalinity & high FMA water, as well as large size plants. Again the Cation & Anion combination can be single or separate vessel.8.3 ATMOSPHERIC DEGASIFIER:The decision to install an atmospheric degasifier is based principally on economicalconsiderations. Removing carbon dioxide before it reaches the anion resins will reduceNaOH chemical consumption stoichiometrically and this should be balanced against the costof the degasifier. Generally the economical balance is not in favor of a degasifier for smallplants (up to about 10 m3/h or 50 gpm). For larger plants, if the total CO2 is greater than 80-100 mg/1 (ppm), the pay-back time for a degasifier should be short. For very large plants,the limit can be reduced to 50 mg/l CO2.8.4 OUTPUT BASED ON WATER QUALITY:Output of DM plant is depending on water quality, if water quality vary output will alsochanged accordingly. So regular monitoring of raw water quality is required and accordinglyoutput may be calculated.Some time conductivity of anion remains high since initial stage of service run. This may dueto either CaSO4 precipitation on SAC, Organic fouling & silica deposit on anion resin. 30