TB 57: Rokeby generating station saves 50% in capital expense by using liqui-cel® 14x28 membrane contactors to remove CO2 to extend mixed bed resin life

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Lincoln Electric Systems (LES) recently commissioned a membrane decarbonation system using Liqui-Cel® 14x28 Membrane Contactors to remove CO2 prior to their mixed bed deionizers. The system was installed at the Rokeby Generating Station (RGS) in Lincoln, Nebraska.

Background
The Rokeby Generating Station is LES’ primary peaking power station, totaling 255 MW and consisting of 3 duel fuel combustion turbines. The existing DI water system consisted of two single-pass, two-stage RO skids followed by a
31 ft3 (0.87 m3) mixed bed deionizer and two 250,000-gallon (943 m3) storage tanks.

LES determined that the mixed bed unit was producing only 30% of its expected capacity (90,000 gallons actual vs. 300,000 gallons expected or 341 m3 actual vs. 1136 m3 expected). It was determined that the cause of the decreased capacity was due to dissolved CO2 in the water, which was overloading the anion resin. As the power capacity demand increased, LES had to act quickly to update their winter contingency plans.

Treatment Options
Multiple treatment options including chemical treatment and the installation of a forced draft degasifier were considered. Ultimately, chemical treatment was considered too risky because of its negative impact on increasing scale on the RO membranes. A forced draft deaerator was also considered impractical due to the large capital expense and size constraints at LES.

System Design
In 2005, LES began engineering a membrane decarbonation system using Liqui-Cel 14x28 Membrane Degasifiers. The system was designed to treat the combined water flow from both RO skids, approximately 150 GPM (0.6 m3). The goal was to achieve approximately 90% reduction of dissolved CO2. Additionally, the system was designed to operate with vacuum assisted air sweep, using a liquid ring vacuum pump to draw atmospheric air through the Liqui-Cel Contactors.

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