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 processequipment. Many wet scrubbers in operation are achieving less-than-expectedemission results and require frequent shutdown due to problems that can beeliminated or reduced with proper design and operation. The goal of this paperis to familiarize the owner/engineer/operator of common design and processerrors that lead to undesirable conditions, frequent maintenance, and safetyhazards. Design, process and operation suggestions will be provided in order tomaximize 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 scrubberis functioning properly if the pump is on and fan is drawing air. Various itemswithin the scrubber unit and supporting equipmentmust be checked and maintained after installationand start-up. Even with proper operationand a good checklist, poor design can leadto less-than-desirable operating conditionsand downtime. The following itemsare common causes of reduced efficiency:

Inadequate sump fluid replacement. Forscrubbers using overflow or blowdownto maintain fresh solution, the freshwater make-up rate must be adequateto maintain the concentration gradientbetween the liquid and gas phase. Theconcentration gradient for a given unitis dependent upon a number of variables—and, if not maintained, the efficiencyof a system can drop quicklyand significantly. In some cases, ifthe gradient is lost, contaminantscan be stripped from solution. Inthese cases, the inlet loading of aparticular contaminant can be lowerthan the tested outlet concentration.As mentioned earlier, two techniquesfor sump replenishment are overflow and blowdown (the overflow method being more common and simple to operate with no instrumentationother than a flow meter). Fresh water is added through an adjustable flow meterat a continuous rate, while the sump liquid overflows into the scrubber drain ata predetermined location. In the blowdown method, liquid is forced to drain bythe recirculation pump. If blowdown is inadequate, the rate of scaling and algaegrowth will increase, as will sedimentation. Sump level controls and solenoidvalves or flow control valves have to be provided in the recirculation piping toallow fluid to be discharged at a measured rate.In either method, the make-up water rate must be high enough to compensatefor evaporation losses, which can range drastically depending system size andatmospheric conditions. This is the key point for keeping the concentrationgradient in check.

Improper pump size. To determine pump size and selection for a given unit, it isnecessary to perform hydraulic calculations for the recirculation system. Threeparameters affect the required design head of a pump: friction losses throughpiping and fittings, pumping height, and pressure loss of nozzles. If add-initems, 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 liquidcan significantly reduce performance of scrubbers. If ammonia is being scrubbedand sulfuric acid is the neutralizing agent, outlet readings can be higher thaninlet readings if pH is not maintained.

Location of the pH probe. A common error with pH control systems is locationof the pH probe versus the location of the chemical supply injection. Locatinga pH probe within 12 inches from the chemical injection pipe will not give trueindication of the pH of the scrubber liquid. The pH controller and on/off switchfor chemical injection will continually chase each other.

Excessive velocity profile considerations. Unfortunately, scrubbers have velocityconstraints 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 designedfor 10,000 CFM, and the fan is exhausting 14,000 CFM, the performance andefficiency decreases while the pressure loss increases. Exceeding the designvelocity profile of a unit affects mist eliminator performance, absorption, andevaporation losses.

Channeling caused by plugged spray nozzles. Spray nozzles can be an operator’snightmare 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 verticalscrubbers, inlets are located 90 degrees from the air direction through thepacked tower. The incoming air stream must make an abrupt 90-degree turn intothe packing. Very few scrubbers are designed to account for this abrupt turn. (Airfollows the path of least resistance.) Air will continue straight through the inletto the back wall of the vessel where it is disturbed and will spiral and vortex upthrough the packed bed section. This channeling creates dead spots within thepacked bed. The now channeled air streams will pass through the packed bedat higher velocities below the designed retention time.Air will also follow the same general undisrupted path through rectangularscrubber housings. Dead spaces are common in rectangular vertical and horizontalscrubber housings. Design for these units must also account for air distributioninefficiencies. Theoretical analyses suggest decreases in performancefor units without proper design.

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

DESIGN CONSIDERATIONS FOR EASE OF MAINTENANCE AND OPTIMUMEFFICIENCY

Pumps. Scrubbers should include redundant pumps and ensure the controlsystem is capable of automated switchover in case of loss of pump or low flow.Utilize pressure gauges and flow meters on discharge piping. Oversize pumpsby 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 belowthe packing to capture liquids falling from above. The catch shall be plumbedto the exterior portion of the unit where liquid will gravity flow through thepH probe and down back into the sump area. Chemical injection should be asclose to the pump suction as possible. Utilize a pipe with small perforationsto act as a distribution device as chemical is brought into the unit. Chemicalshould exit the pipe near the pump suction area. The holes in the pipe willallow sump water to mix with the neutralizing chemical prior to entering therecirculation piping. The pump impellers will provide an excellent means ofturbulence and mixing to prevent the channeling of liquid through the pipingand 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 suitablelocation before the scrubber.

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

TECHNIQUES FOR REDUCTION OF BIOLOGICAL GROWTH

Following are some guidelines to reduce bacterial growth, which could impedescrubber 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 otherbiological organisms.

• Use UV light devices for disinfecting supply and recirculation liquid.

• Segregate VOC exhaust from scrubbed exhaust. Field experienceindicates less evidence of growth with non-VOC exhaust.

• Segregate all sources of phosphoric acid or other phosphates that feedalgae and scrub them with a strong caustic solution at a pHof 10 to 11.

• Field experiences suggest reduced growth in polypropyleneconstructed 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 fromentering the unit.

CONCLUSION

This article touches on just a few common causes of reduced efficiencies inscrubber systems. Proper design of a high-efficiency scrubber system requiresmuch more than just a pump, vessel and spray header. Routine preventativemaintenance schedules are important to avoid compounding problems andcostly downtime. Reputable scrubber manufacturers can provide periodic preventativemaintenance inspections and follow-up reports that allow for trendingof system parameters and early recognition of arising problems. For more informationon wet packed bed fume scrubbers, please visit www.kchservices.com.