Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 519 of 707

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 elim- inated 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 opera- tion; 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 with- in the scrubber unit and supporting equipment must be checked and maintained after installation and start-up. Even with prop- er operation and a good checklist, poor design can lead to less-than-desirable oper- ating conditions and downtime. The fol- lowing 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 main- tain the concentration gradient between the liquid and gas phase. The concen- tration 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 sig- nificantly. In some cases, if the gra- dient is lost, contaminants can be stripped from solution. In these cas- es, the inlet loading of a particular contaminant can be lower than the tested outlet concentration. As mentioned earlier, two tech- niques for sump replenishment are overflow and blowdown (the overflow 518

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