Feature

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 with the overflow method being more common and simple to operate with no instrumentation other than a flow meter. Fresh water is added through an adjustable flow meter at a continuous rate, while the sump liquid overflows into the scrubber drain at a predetermined location. In the blowdown method, liquid is forced to drain by the recirculation pump. If blowdown is inadequate, the rate of scaling and algae growth will increase, as will sedimentation. Sump level controls and solenoid valves or flow control valves have to be provided in the recirculation piping to allow fluid to be discharged at a measured rate. In either method, the make-up water rate must be high enough to compensate for evaporation losses, which can range drastically depending system size and atmospheric conditions. This is the key point for keeping the concentration gradient in check.

Improper pump size. To determine pump size and selection for a given unit, it is necessary to perform hydraulic calculations for the recirculation system. Three parameters affect the required design head of a pump: friction losses through piping and fittings, pumping height, and pressure loss of nozzles. If add-in items, such as basket strainers, are not accounted for in the design of a system, the pump flow rate will suffer, thereby affecting efficiency.

Improper addition of scrubbing liquid. Inadequate addition of scrubbing liquid can significantly reduce performance of scrubbers. If ammonia is being scrubbed and sulfuric acid is the neutralizing agent, outlet readings can be higher than inlet readings if pH is not maintained.

Location of the pH probe. A common error with pH control systems is location of the pH probe versus the location of the chemical supply injection. Locating a pH probe within 12 inches from the chemical injection pipe will not give true indication of the pH of the scrubber liquid. The
pH controller and on/off switch for chemical injection will continually chase each other.

Excessive velocity profile. Unfortunately, scrubbers have velocity constraints that play a key role with performance. Once a scrubber is in operation the cross-sectional area has forever been established. If a unit is designed for 10,000 CFM and the fan is exhausting 14,000 CFM, the performance and efficiency decreases while the pressure loss increases. Exceeding the design velocity profile of a unit affects mist eliminator performance, absorption, and evaporation losses.

Channeling caused by plugged spray nozzles. Spray nozzles can be an operator’s nightmare and the cause of frequent and expensive unplanned shutdowns. Plugging should be expected when using scrubbers that incorporate spray nozzles. When a nozzle plugs, the area of packing directly below is not receiving liquid. This will create an area where no absorption is taking place and, therefore, decreases the efficiency of the scrubber.

Channeling Caused by Poor Air Distribution and Rectangular Housings. In vertical scrubbers, inlets are located 90 degrees from the air direction through the packed tower. The incoming air stream must make an abrupt 90-degree turn into the packing. Very few scrubbers are designed to account for this abrupt turn. (Air follows the path of least resistance.) Air will continue straight through the inlet to the back wall of the vessel where it is disturbed and will spiral and vortex up through the packed bed section. This channeling creates dead spots within the packed bed. The now channeled air streams will pass through the packed bed at higher velocities below the designed retention time. Air will also follow the same general undisrupted path through rectangular scrubber housings. Dead spaces are common in rectangular vertical and horizontal scrubber housings. Design for these units must also account for air distribution inefficiencies. Theoretical analyses suggest decreases in performance for units without proper design.

Biological growth. Build-ups of biological growth in packed bed sections and mist eliminators will adversely affect performance of scrubbers. In acid scrubbers, where pH is typically maintained in the 8–9 range, biological growth is a commonality. Without treatment, the growth can create areas of channeling and increase the pressure drop through the scrubber.

DESIGN CONSIDERATIONS FOR EASE OF MAINTENANCE AND OPTIMUM EFFICIENCY

Pumps. Scrubbers should include redundant pumps and ensure the control system is capable of automated switchover in case of loss of pump or low flow. Utilize pressure gauges and flow meters on discharge piping. Oversize pumps by 125% to ensure adequate capacity and operation.

Controlling pH. It is best to monitor pH away from the chemical injection area. To measure pH as it exits
the packed bed section, utilize a catch cup just below the packing to capture liquids falling from above. The catch shall be plumbed to the exterior portion of the unit where liquid will gravity flow through the pH probe and down back into the sump area. Chemical injection should be as close to the pump suction as possible. Utilize a pipe with small perforations to act as a distribution device as chemical is brought into the unit. Chemical should exit the pipe near the pump suction area. The holes in the pipe will allow sump water to mix with the neutralizing chemical prior to entering the recirculation piping. The pump impellers will provide an excellent means of turbulence and mixing to prevent the channeling of liquid through the piping and packed bed.

Instrumentation. Monitor and Alarm the following:

  • pH
  • Fresh water make-up
  • Pump flow rate
  • Pump pressure
  • Pressure drop (scrubber and mist eliminator)
  • Sump Levels
  • Blowdown
  • Sump temperature
  • Air flow should also be monitored in the duct system at a suitable location before the scrubber.

Access considerations. Design mist eliminators for ease of removal for inspection, cleaning and replacement. Mist eliminators should be encapsulated to prevent potential bypass. Access doors should be provided for an operator to inspect the packed bed section, sump area, pump area, and liquid distribution section. The access for the sump area should be above water level to prevent leak points. View ports should be provided for easy inspection of internals. (Borosilicate glass works best as a window; it resists fading, unlike clear PVC or Plexiglas, and withstands the heat of the high-intensity lights.) Locate widows between the water line and packing bottom, at the packed bed section, and at the liquid distribution section. Utilize slide shades to keep light from entering the scrubber where possible.

TECHNIQUES FOR REDUCTION OF BIOLOGICAL GROWTH

Following are some guidelines to reduce bacterial growth, which could impede scrubber function:

  • Acid wash the unit periodically or shock it with sodium hypochlorite (5% solution) to destroy algae and other biological organisms.
  • Use a chlorinating or brominating system to destroy algae and other biological organisms.
  • Use UV light devices for disinfecting supply and recirculation liquid.
  • Segregate VOC exhaust from scrubbed exhaust. Field experience indicates less evidence of growth with non-VOC exhaust.
  • Segregate all sources of phosphoric acid or other phosphates that feed algae and scrub them with a strong caustic solution at a pH of 10 to 11.
  • Field experiences suggest reduced growth in polypropylene constructed units versus FRP construction. Porosity and pinholes tend to be breeding areas, which are common in FRP units.
  • Utilize sliding shades over all clear view doors to prevent light from entering the unit.

CONCLUSION
This article touches on just a few common causes of reduced efficiencies in scrubber systems. Proper design of a high efficiency scrubber system requires much more than just a pump, vessel and spray header. Routine preventative maintenance schedules are important to avoid compounding problems and costly downtime. Reputable scrubber manufacturers can provide periodic preventative maintenance inspections and follow up reports which allow for trending of system parameters and early recognition of arising problems. For more information on wet packed bed fume scrubbers, please visit www.kchservices.com

BIO
Kyle Hankinson is Vice President of KCH Engineered Systems, a North Carolina- based manufacturer of pollution control exhaust systems and process equipment for the metal finishing industry. He is an NASF-Certified Electroplater-Finisher. Hankinson is also a Chief Warrant Officer and part-time helicopter pilot for the National Guard. He may be reached at (828) 245-9836 or via e-mail: khankinson@kchservices.com.
 

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