Optimising industrial water softeners with a brine elution study
Paul Sharpe , industry consultant - boiler and cooling at Kurita Americas , looks at a key way to soften boiler water in industry
Most industrial boiler systems require soft feedwater . Without this softening pre-treatment , boilers are at risk of developing hard insulating deposits that reduce heat transfer efficiency and increase energy use . Severe deposit formation can permanently damage boiler tubes or cause dangerous failures from overheating . Water softening is an ion exchange technique used for removing calcium and magnesium ions that form hard water scale , such as calcium and magnesium carbonate . The process involves exchanging source water calcium and magnesium cations for sodium cations that have been adsorbed onto resin beads filled with exchange sites . As water flows through the softener system , sodium ions are released ( exchanged ) from the resin , and the hard water ions are collected on the exchange sites . When a softener with new ( or regenerated ) resin is placed in service , sodium ions adsorbed on the exchange sites within the resin are immediately exchanged with calcium and magnesium cations . This produces soft water with very little residual hardness in the effluent water . The resin bed will continue to exchange its sodium ions with calcium and magnesium to a point where the exchange sites are reduced and hard water can be detected in the softener output . This is referred to as the ‘ saturation point ’ at which regeneration is needed . Cation softener regeneration requires a concentrated solution of sodium chloride to be rinsed through the resin bed ; this is a physico-chemical process that uses osmotic pressure and physical flow to remove the hard water ions from the resin . The regeneration process uses at least three distinct sequences :
• Backwash : Resin is counterflow backwashed , using a specified flow rate based on water temperature and manufacturer ’ s specifications noted in the service manual . Using the correct flow rate is important in preventing loss of resin ; if too high , the resin could be washed over to drain . Backwash serves to remove broken resin beads , dirt and debris , and reclassifies the bed for proper water flow during service
• Brine draw / Slow rinse : Brine rinse is the forward flow regeneration of the resin bed using a saturated sodium chloride ( NaCl ) salt solution diluted to 30-38 % saturation , using an eductor system and fresh water . The diluted brine is eluted through the resin bed using a distributor inside the softener for even flow . The concentration of the salt through the resin bed should be above 30 % saturation for a minimum of 30 minutes
• Rinse ( fast ): The rinse sequence is forward flow and uses municipal water or well water to flush out the remaining salt water . This rinsing also compacts and conditions the resin and helps to prevent channelling Incorrect regeneration timing and improper flow rates are often the cause of problems in ion exchange water quality . An elution study is used to collect process data during the regeneration and requires graphing the concentration of salt during brine draw versus time . The information is then used to troubleshoot and evaluate the ion exchange process .
Elution study
The equipment for an elution study comprises : a 250 ml graduated cylinder for collecting brine rinse samples ; a 1-100 % salometer for measuring percent saturation of the brine rinse samples ; and , a stopwatch or timer app for timing samples and the regeneration cycle Prior to the start of the study , check the concentrated brine tank with the salometer . The NaCl concentration should be 90 % or higher . If the
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