The Basics of Demineralisation by Ion Exchange

Raw Water Supply Water comes into sites from many sources and can be potable (suitable for drinking), an industrial supply provided by the local water plc or the clients own supply extracted on site from a river / borehole. The drinking water use is sterile and includes all many of the natural elements we need to sustain a healthy life. All these ions however cannot be left in the water fed to boilers and to other processes. They would cause corrosion and deposits affecting performance and causing premature plant failure.

Raw Water Supply Water in the UK can come from many sources: Four principle types of supply are widely encountered Ground Waters – Pumped from boreholes or wells, these supplies have a high 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 high proportion 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)

Raw Water Supply Ions present in all natural waters: Cations Anions Sodium (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.

Typical IEx Plant Designs To achieve high water quality the majority of plants employed in the UK fit into the following categories: Cation – Anion (Main subject for today’s presentation) Cation – Anion – Polishing Cation Cation – Anion – Mixed Bed Reverse Osmosis – Ion Exchange Plant The cation and anion columns can employ either co-flow or counter-flow regeneration and in some cases they can also they employ a Degassing Tower after the cation unit.

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 at optimum rates without undue pressure drop.

Cation Exchange Resins Two principle types of cation resin: Weak Acid Cation – with carboxylic group (Resin – COOH) – Dealkalisation Process Strong Acid Cation – with sulphonic acid group (Resin - S03H) Regeneration is with an excess amount of dilute acid (sulphuric or hydrochloric acid).

Cation Unit Representation in service and after co-flow regeneration In Service OperationResin Resin - SO3H + Na  Resin - SO3Na + H 2Resin - SO3H + Ca  2Resin – SO3Ca + 2H Order of Selectivity: Fe > Ca > Mg > K > Na In Regeneration – (Typically with 5% HCl conc.) Resin – SO3Na + HCl  Resin – SO3H + NaCl + Excess Acid Resin – SO3Ca + H2SO4  Resin – SO3H + CaSO4 + Excess Acid Treated water contains high concentration of H+ ions so water exit cation has a low pH. Raw Water Calcium Magnesium Sodium H+ (unused) In Service

Anion Resins Strong 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.

Anion Unit Representation in service and after co-flow regeneration In Service OperationResin Resin – Amine OH + Cl  Resin – Amine Cl + OH 2Resin – Amine OH + SO4  2Resin – Amine SO4 + 2OH Order of selectivity: SO4 > NO3 > Cl > Bicarbonate / CO2 > Silica In Regeneration (Typically with 4% NaOH conc.) Resin – Amine Cl + NaOH  Resin – Amine OH + NaCl + Excess NaOH Resin – Amine SO4 + NaOH  Resin – Amine + Na2SO4 + Excess NaOH Treated water now contains OH- ions which combine with H+ ions to form pure water H2O. Raw Water Sulphate Nitrate Chloride Bicarbonate / CO2 Silica OH- (unused) In Service

Ion Exchange Resin Standard 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.

Ion Exchange Resin Grades Narrow Uniform Grade Resins Most 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 capacity Better Rinse Lower pressure drop Higher breaking weight Are more suitable to some specialist engineering designs (e.g. Packed beds)

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)

Degassing Towers Between the cation and anion stage on many large demin plants there is a degassing tower. (Normally if the bicarbonate content of the raw water supply is above 50 mg/l). These are a very efficient way of removing the bicarbonate present in the water mechanically and cheaply. When the Ca / Mg associated with the bicarbonate passes through a cation resin this happens. Ca(HCO3)2 + 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)

Counter flow (Example showing upflow regen.) Co-flow Co-flow vs Counter Flow Regeneration (Cation Representation) Counter flow (Example showing upflow regen.) Co-flow Service flow Co-flow Regeneration Counter Flow Regeneration After regen: After regen: Ca Mg Mg Na Na Na Na Na Ca Ca Ca Mg Mg Na Na Na With 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!

Co – Flow Regeneration The regeneration of the resin involves the following main steps with co-flow regeneration Backwash Bed Settle Establish motive water Regenerant Injection Slow / Displacement Rinse Fast Rinse

Co-Flow Regeneration 7 1 Valve Identifiers Inlet 5 Oulet Drain Feed Water 1 Valve Identifiers Inlet Oulet Drain Regen / Slow Rinse Inlet WWI WWO Vent (manual) 5 4 6 Regen 3 Effluent 2 Treated Water

Plant Operation / Treated Water Quality SAC / Degasser / SBA / Mixed Bed Treated water Quality Cation TWQ: Anion TWQ: MB TWQ: AT ALL TIMES!!!!! pH 2 – 3 Conductivity Increase (R water x 1.5 to 2) Trace Na / No hardness Co-flow Regen (Typ.) 0.5-2.0 mg/l Na Counter flow Regen (Typ.) 0.02-0.5 mg/l Na pH > 7 Typically 7.3 - 9 Conductivity low (Depending Sodium leakage exit cation) Reactive Silica low Co-flow Regen (Typ.) 0.05 – 0.3 mg/l SiO2 Counter flow Regen (Typ.) 0.025 – 0.1 mg/l SiO2 pH 7+ Conductivity 0.056 - 0.1 us/cm Na < 0.01 mg/l Silica < 10 - 20 ug/l 5 mg/l CO2 SAC Degasser SBA Mixed Bed

Minimum Level of Instrumentation for Cation – Anion – Polishing M Bed (Cation – Anion with co-flow regeneration) Pump Raw Water Flow Pressure Cation Anion Pressure Pressure Conductivity Silica (Optional depending on clients Treated Water specification) Treated Water

Minimum Level of Instrumentation for Cation – Anion – Polishing M Bed (Cation – Anion with co-flow regeneration) Flow Flow Pressure Pressure Pump Cation Anion LS Pressure Pressure LS Degasser Tower Conductivity Silica (Optional depending on clients Treated Water specification) Raw Water Tank Pump Treated Water Tank