After specific treatment of the SWS Bottoms for these contaminants, this water is not only considered a suitable quality for steam and cooling systems but actually becomes a superior quality similar to steam condensate. Further, this captured SWS-treated water produces substantial fuel value in the form of heat. Similar to steam condensate, this water can bypass normal boiler feedwater pretreatment systems such as ion exchange or reverse osmosis (RO) and can proceed directly to the boiler deaerator.
Occasionally, SWS units are not operated or maintained correctly. A system with a significant presence of the three aforementioned inorganic cations would be overlooked as a candidate for water reuse. Most commonly, these cations enter the SWS system either by cooling water intrusion from piping and condenser leaks or by using an unsuitable water injection source in the distillation column overheads for the forcing of the dewpoint to initiate condensation.
These two conditions ultimately cause major problems in the SWS units themselves, such as the deterioration of the SWS trays, which will cause serious SWS performance problems and will require repairs. These problems almost always are short term with respect to the presence of these inorganic cations in the Bottoms, as the SWS cannot operate very long under these conditions.
In addition to boiler feedwater supply, the same considerations regarding scale formation exist as the criteria for the justification of SWS water reuse as cooling tower supply water; this also has an attractive return on investment. The validation for this cooling tower make-up can be found in the increased cycles of concentration, which would be tolerable in the cooling towers with the treated SWS water. This translates to substantial reductions in the volume of supply water used, wastewater generated and cost of chemical treatment in cooling tower operations.
The economic basis for the justification of water reuse investment at the SWS is substantially more attractive for boiler feedwater than it is for cooling tower make-up water. The return on investment for SWS Bottoms reuse as boiler feedwater is based on the reduction in the cost of treating wastewater; the decrease in the cost of supply water pretreatment; and the capture of SWS heat, which reduces deaerator heating fuel costs.
Interestingly, for those plants required to meet selenium NPDES permit limits, the routing of the SWS Bottoms to the boilers inherently extracts more than 80 to 90 percent of the total selenium load to the wastewater treatment plant, thus eliminating any selenium removal needs in the facility in almost all cases.
About the Author: David Kujawski is vice president of Refinery Water Engineering and Associates in Nederland, Texas. He has 32 years experience in water and waste treatment in over 200 industrial plants and 46 oil refineries. Kujawski holds degrees in Environmental Engineering, Chemistry, and Marketing, and over his career has held positions at Nalco, Betz-Dearborn, Baker-Petrolite, US Filter-Siemens, Chevron El Segundo Refinery, Sybron Biochemical, and Ashland Oil.
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