Metal Finishing Guide Book

2013

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environmental controls CRITICAL FACTORS AFFECTING WET SCRUBBER PERFORMANCE BY KYLE HANKINSON, VICE PRESIDENT, KCH ENGINEERED SYSTEMS, FOREST CITY, N.C. Wet scrubbers are used for the abatement of chemical emissions from process equipment. Many wet scrubbers in operation are achieving less-than-expected emission results and require frequent shutdown due to problems that can be eliminated or reduced with proper design and operation. The goal of this paper is to familiarize the owner/engineer/operator of common design and process errors that lead to undesirable conditions, frequent maintenance, and safety hazards. Design, process and operation suggestions will be provided in order to maximize wet scrubber performance. The following three topics will be addressed: Causes of poor scrubber operation; design considerations for ease of maintenance and optimum efficiency; and techniques for reduction or elimination of biological growth. Causes of poor scrubber operation. It is implausible to assume that a scrubber is functioning properly if the pump is on and fan is drawing air. Various items within the scrubber unit and supporting equipment must be checked and maintained after installation and start-up. Even with proper operation and a good checklist, poor design can lead to less-than-desirable operating conditions and downtime. The following items are common causes of reduced efficiency: Inadequate sump fluid replacement. For scrubbers using overflow or blowdown to maintain fresh solution, the fresh water make-up rate must be adequate to maintain the concentration gradient between the liquid and gas phase. The concentration gradient for a given unit is dependent upon a number of variables—and, if not maintained, the efficiency of a system can drop quickly and significantly. In some cases, if the gradient is lost, contaminants can be stripped from solution. In these cases, the inlet loading of a particular contaminant can be lower than the tested outlet concentration. As mentioned earlier, two techniques for sump replenishment are overflow and blowdown (the overflow 578

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