Tech Fact
Removal of CO2 in the Production of Demineralized Water by Reverse Osmosis with Ion Exchange Mixed Bed Polishing
Application Carbon dioxide (CO2) in water is present as an equilibrium mixture of the dissolved − 2− Description gas CO2, the weak acid H2CO3, and the HCO3 and CO3 anions associated with bicarbonate and carbonate alkalinity. The exact equilibrium ratio of these four species depends on the pH and temperature of the water.
A common process to produce demineralized water is to use a double-pass reverse osmosis (RO) system followed by an ion exchange mixed bed polisher.
Typically, the pH of RO permeate is acidic with a pH range of 4 – 5. This pH range will
convert carbonate alkalinity and most bicarbonate alkalinity into dissolved CO2 gas.
If the CO2 is not removed prior to entering the mixed bed polisher, it will be converted by the ion exchange system to bicarbonate and carbonate anions, which will exchange onto the anion resin in the mixed bed polisher.
Depending on the CO2 levels: l Accounting for this load in new designs can result in large mixed bed polisher vessels, which increases capital expenses.
l For existing systems that see an increase in CO2, reduction in run length and increased chemical consumption can be observed, resulting in increased operating costs and water usage.
Solution DuPont™ Ligasep™ Degasification Modules use a proprietary polymethylpentene (PMP) hollow fiber membrane that provides an efficient transfer of gases between a liquid and a gas.
The membrane does not allow water to pass through the membrane but freely allows gas to pass through. Equilibrium between the liquid and gas phase is offset when a vacuum and a strip gas is applied to one side of the membrane. This creates a driving force to move dissolved gases from the water to the gas side of the membrane.
Ligasep™ Degasification Modules offer an efficient and stable way to decarbonate to
concentrations of 5 ppm or less of dissolved CO2 before feeding the water to the mixed bed resins.
Page 1 of 2 Form No. 45-D00503-en, Rev. 1 March 2020 Key Benefits l Improves process reliability l Efficient and stable way to decarbonate to concentrations of 5 ppm or less of
dissolved CO2, even for variable feedwater quality l Increases anion exchange run length in existing systems
l Reduces anion resin volume and vessel size for new systems
l Potentially reduces chemical consumption and operating costs (note: this will depend on system design and configuration)
l Eliminates potential cross-contamination from strip gas compared to decarbonators
Schematic Diagram
for illustration purposes only
DuPont Products l Ligasep™ LDM-040-HS or LDM-120-HS Degasification Modules l FilmTec™ Reverse Osmosis Elements
l AmberLite™ Ion Exchange Resins
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Page 2 of 2 Form No. 45-D00503-en, Rev. 1 March 2020