2 Raw Water SupplyWater comes into sites from many sources and can be potable(suitable for drinking), an industrial supply provided by the localwater plc or the clients own supply extracted on site from a river /borehole.The drinking water use is sterile and includes all many of thenatural elements we need to sustain a healthy life.All these ions however cannot be left in the water fed to boilers and toother processes. They would cause corrosion and deposits affectingperformance and causing premature plant failure.
3 Raw Water Supply Water in the UK can come from many sources: Four principle types of supply are widely encounteredGround Waters – Pumped from boreholes or wells, these supplies have ahigh salts content. From deep boreholes the water quality remains very constant and it is normally high in hardness (calcium & magnesium) and high in alkalinity (bicarbonate). Normally dissolved organics are not present.Surface Waters – Upland sources low in dissolved solids but with a highproportion of dissolved organics.Surface Waters – Lower levels with moderate dissolved solids and with moderate to high organics.Mix of surface and ground waters of variable quality – (river supplements)
4 Raw Water SupplyIons present in all natural waters:Cations AnionsSodium (Na) Bicarbonate (HCO3) / Carbon Dioxide (CO2)Calcium (Ca) Sulphate (SO4)Magnesium (Mg) Chloride (Cl)Potassium (K) Nitrate (NO3)Iron (Fe) Silica (SiO2)In addition dissolved organics can be present which can be important on some sites with regard to resin selection.
5 Typical IEx Plant Designs To achieve high water quality the majority of plantsemployed in the UK fit into the following categories:Cation – Anion (Main subject for today’s presentation)Cation – Anion – Polishing CationCation – Anion – Mixed BedReverse Osmosis – Ion Exchange PlantThe cation and anion columns can employ either co-flow orcounter-flow regeneration and in some cases they can alsothey employ a Degassing Tower after the cation unit.
6 Ion Exchange Resin - Properties Synthetic Ion Exchangers require certain properties to perform demineralisation. The three main properties required are:a. Insoluble in, but permeable by water.b. An ability to exchange ions, with the different types of ions commonly encountered in water supplies. Active groups throughout the beads perform the ion exchange.c. To allow the passage of water through the resin bed atoptimum rates without undue pressure drop.
7 Cation Exchange Resins Two principle types of cation resin:Weak Acid Cation – with carboxylic group (Resin – COOH) – Dealkalisation ProcessStrong Acid Cation – with sulphonic acid group (Resin - S03H)Regeneration is with an excess amount of dilute acid (sulphuric or hydrochloric acid).
8 Cation Unit Representation in service and after co-flow regeneration In Service OperationResinResin - SO3H Na Resin - SO3Na + H2Resin - SO3H + Ca 2Resin – SO3Ca + 2HOrder of Selectivity: Fe > Ca > Mg > K > NaIn Regeneration – (Typically with 5% HCl conc.)Resin – SO3Na + HCl Resin – SO3H + NaCl + Excess AcidResin – SO3Ca + H2SO4 Resin – SO3H + CaSO4 + Excess AcidTreated water contains high concentration of H+ ions so water exit cation has a low pH.Raw WaterCalciumMagnesiumSodiumH+ (unused)In Service
9 Anion ResinsStrong base anion resins are employed on all demineralisation plants for producing high quality water.Either in separate anion units and or as the strong base anion component in mixed beds.Strong base anion resins will remove all anions present but require an excess of Sodium Hydroxide (Caustic Soda) to regenerate them.
10 Anion Unit Representation in service and after co-flow regeneration In Service OperationResinResin – Amine OH + Cl Resin – Amine Cl + OH2Resin – Amine OH + SO4 2Resin – Amine SO4 + 2OHOrder of selectivity: SO4 > NO3 > Cl > Bicarbonate / CO2 > SilicaIn Regeneration (Typically with 4% NaOH conc.)Resin – Amine Cl + NaOH Resin – Amine OH + NaCl + Excess NaOHResin – Amine SO4 + NaOH Resin – Amine + Na2SO4 + Excess NaOHTreated water now contains OH- ions which combine with H+ ionsto form pure water H2O.Raw WaterSulphateNitrateChlorideBicarbonate / CO2SilicaOH- (unused)In Service
11 Ion Exchange ResinStandard grade resins from all manufacturers are typically made 300 to 1200 microns with less than 1% less than 300 microns. Hence internal systems / nozzles are selected to have a maximum slot / aperture of 200 microns.In addition resin suppliers also make more uniform and specialist grades.
12 Ion Exchange Resin Grades Narrow Uniform Grade ResinsMost Narrow grade resins typically in the range of 400 – 800 microns (some of these resins have a very narrow distribution and a low uniformity coefficient )Standard grade resins 300 – 1200 microns.Narrow grade resins can offer:Higher capacityBetter RinseLower pressure dropHigher breaking weightAre more suitable to some specialist engineering designs (e.g. Packed beds)
13 Ion Exchange Resin Selection The Six Most Important Factors Affecting Resin Selection:Raw water quality (TDS and other contamiants)Treated water quality. (conductivity / silica specification)Engineering techniques employed. (co-flow or counter flow regen)Operating flow rate. (good kinetics)Process temperatures. (anion resins have low maximum temp limits)Presence of organic foulants. (anion resin resistant to fouling)
14 Degassing TowersBetween the cation and anion stage on many large demin plantsthere is a degassing tower. (Normally if the bicarbonate contentof the raw water supply is above 50 mg/l).These are a very efficient way of removing the bicarbonatepresent in the water mechanically and cheaply.When the Ca / Mg associated with the bicarbonate passesthrough a cation resin this happens.Ca(HCO3) Resin-2H+ Resin-Ca + H2CO3 (Carbonic acid)When the resin releases the H+ ions the water becomes acidic(pH 2-3 exit SAC). At low pH Carbonic acid is unstable.H2CO3 at low pH H2O + CO2.(forming pure water and carbon dioxide)
15 Counter flow (Example showing upflow regen.) Co-flow Co-flow vs Counter Flow Regeneration (Cation Representation)Counter flow (Example showing upflow regen.)Co-flowService flowCo-flow RegenerationCounter Flow RegenerationAfter regen:After regen:CaMg MgNa NaNa Na NaCa Ca CaMg MgNa NaNaWith counter flow regeneration the most highly regenerated portion of the ion exchange bed is at the unit outlet so leakage is significantly better in service operation!
16 Co – Flow RegenerationThe regeneration of the resin involves the followingmain steps with co-flow regenerationBackwashBed SettleEstablish motive waterRegenerant InjectionSlow / Displacement RinseFast Rinse
18 Plant Operation / Treated Water Quality SAC / Degasser / SBA / Mixed Bed Treated water QualityCation TWQ:Anion TWQ:MB TWQ:AT ALL TIMES!!!!!pH 2 – 3Conductivity Increase (R water x 1.5 to 2)Trace Na / No hardnessCo-flow Regen (Typ.)mg/l NaCounter flow Regen (Typ.)mg/l NapH > 7 TypicallyConductivity low(Depending Sodium leakage exit cation)Reactive Silica lowCo-flow Regen (Typ.)0.05 – 0.3 mg/l SiO2Counter flow Regen (Typ.)0.025 – 0.1 mg/l SiO2pH 7+Conductivity us/cmNa < 0.01 mg/lSilica < ug/l5 mg/l CO2SACDegasserSBAMixed Bed
19 Minimum Level of Instrumentation for Cation – Anion – Polishing M Bed (Cation – Anion with co-flow regeneration)PumpRaw WaterFlowPressureCationAnionPressurePressureConductivitySilica (Optional depending on clients Treated Water specification)Treated Water
20 Minimum Level of Instrumentation for Cation – Anion – Polishing M Bed (Cation – Anion with co-flow regeneration)FlowFlowPressurePressurePumpCationAnionLSPressurePressureLSDegasser TowerConductivitySilica (Optional depending on clients Treated Water specification)Raw WaterTankPumpTreated WaterTank