The Alliance for Water Efficiency (AWE) and ...
Nebraska generating station saves 50% in capital expense by using membrane contactors to remove carbon dioxide to extend mixed-bed resin life
Lincoln Electric Systems (LES) of Lincoln, Neb., recently commissioned a membrane decarbonation system using Liqui-Cel 14-by-28 membrane contactors to remove CO2 prior to its mixed-bed deionizers. The system was installed at the Rokeby Generating Station (RGS) in Lincoln.
The RGS is the primary peaking power station of LES, totaling 255 mw and consisting of three dual-fuel combustion turbines. Its DI water system consisted of two single-pass, two-stage RO skids followed by a 31 cu ft (0.87 cu m) mixed-bed deionizer and two 250,000-gal (943-cu-m) storage tanks.
LES determined that the mixed-bed unit was producing only 30% of its expected capacity (90,000 gal actual instead of 300,000 gal expected, or 341 cu m actual instead of 1136 cu m expected). It was determined that the cause of the decreased capacity was dissolved CO2 in the water, which was overloading the anion resin. As the power capacity demand increased, LES had to act quickly to update its winter contingency plans.
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.
In 2005, LES began engineering a membrane decarbonation system using Liqui-Cel 14-by-28 membrane degasifiers. The system was designed to treat the combined water flow from both RO skids—approximately 150 gpm (0.6 cu m). 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.
Since the LES staff was able to design, fabricate and install the Liqui-Cel degassing system, the total capital cost was approximately 50% lower than the cost of a forced-draft degasifier. The compact design also allowed LES to build the system inside of an existing building with minimal modification. The low system-pressure drop through a Liqui-Cel membrane contactor system also eliminated the need for a repressurization pump, further lowering operating costs for LES.
The degas skid was installed downstream of the RO skids and before the mixed bed. In order to maximize efficiency, a cation bottle was installed between the RO skids and the degas skid to reduce the pH and convert the HCO3 to free CO2 gas.
LES temporarily installed three 3.3-cu-ft (0.09-cu-m) mixed-bed bottles downstream of the degas skid. The bottles were installed in parallel for a total capacity of 9.9 cu ft (0.28 cu m). During the test, only one of the RO units was operated, resulting in water flow of 40 gpm.
LES expected to achieve 138,000 to 168,000 gallons (522 cu m to 636 cu m) throughput with this design; it actually achieved 191,000 gallons (725 cu m). Specific conductivity was 0.5 μS/cm, and silica was 7.5 ppb. LES estimates the full-scale capacity will be approximately 617,000 gallons. This represents an increase in the total capacity of the system by a factor of 5.9.
Liqui-Cel membrane contactors offer a cost-effective, efficient option for removal of carbon dioxide from process water. Removal of carbon dioxide prior to the mixed-bed resins significantly improves regeneration times, thereby reducing operating costs and improving overall efficiency by minimizing downtime.