Mixed Bed Internal Regeneration

Mixed bed polishing is a well understood ion-exchange based polishing technology and comprises a resin bed of cation and anion resins, intimately mixed, housed within a cylindrical vessel. The intimate mixture of resins provides a perfect environment for removal of dissolved ionic contaminants. Mixed beds are also somewhat tolerant to suspended solids (crud) within the feed water and can accommodate peaks of high suspended solids which can then be removed during regeneration.

Mixed bed ion exchange is a common method for treating low-TDS feedwaters to produce demineralized water of very high purity, generally < 0.1 µS/cm and < 10 ppb total dissolved solids.  Mixed bed polishing with internal regeneration is generally most appropriate when many or all of the following parameters are met :

  • Minimal wastewater / high recovery is important
  • Acid / Caustic chemicals already present on site for regeneration
  • Effluent neutralization facility can be used
  • Final treated quality within typical range as above

For the same applications, this technology normally competes with electrodeionisation (EDI / CEDI), normally depending upon the aforementioned factors.

  • High treated water quality
  • Hydraulic efficiency
  • Widely understood technology
  • Fail-safe technology; water quality maintained in the event of power failure

Mixed bed polishing is a well understood ion-exchange based polishing technology and comprises a resin bed of cation and anion resins, intimately mixed, housed within a cylindrical vessel.  The intimate mixture of resins provides a perfect environment for removal of dissolved ionic contaminants.  Mixed beds are also somewhat tolerant to suspended solids (crud) within the feed water and can accommodate peaks of high suspended solids which can then be removed during regeneration.   Generally, but not exclusively, provided as n+1 configuration (2 x 100%, 3 x 50%, etc.) such that when one vessel is in regeneration the standby can be placed online.  Regeneration duration typically ~3 hours.

As the water passes through the intimate mixture of strong cation and strong anion resin the presence of both hydrogen and hydroxide exchange sites results in high purity eluent.  When the appropriate number of exchange sites has been used the resin beds will be exhausted and require regeneration.  This may be detected by an increase in the conductivity of the treated water.  Under normal circumstances the mixed beds will be regenerated after a set throughput of condensate has been treated.

Mixed bed ion exchangers can be designed to a wide range of flows from < 5 m3/h to 750 m3/h each.  The technology lends itself to supply as multiple units to treat very high total flows.  The below water quality would be typical :

  • Conductivity : 0.1 µS/cm
  • TDS : < 10 ppb total
    • Na : < 10 ppb
    • Cl : < 5 ppb
    • SO4 : < 5 ppb
  • Silica : < 10 ppb (except high feed temperature)
  • TOC : < 100 ppb (limited capacity, feed quality typically < 200 ppb)
  • Fe + Cu : < 10 ppb
  • pH : Neutral
  • Efficiency : > 99.8% depending on feedwater quality

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