Industrial VOC Air Purifier: Applications, Treatment Methods, Selection, Carbon Bed Sizing, Operation

An industrial VOC air purifier treats exhaust from hoods, booths and process equipment before it reaches the stack. Its design starts with where the vapors are captured and how many pounds of solvent per hour the stream carries.

Absorption in a wet scrubber works for water-soluble compounds: methyl and ethyl alcohol, acetone, phenols, formaldehyde and other aldehydes. They pass into the liquid on random or structured packing. Our TORNADO RP wet scrubbers treat 600–30,000 cfm per module and are built of polypropylene, PVDF or steel. Saturation of the solution limits efficiency. In a semiconductor project with 100 scfm and 15 ppmv of IPA, removal reached about 99.9% with fresh water and dropped below 90% once the recirculated liquid held 5% alcohol. A labyrinth droplet eliminator comes standard on this model, and heavier carryover calls for a separate unit from our mist and droplet eliminators.

Oil mist, smoke and paint overspray coat sorbent granules and block the pores. In machining exhaust, oil mist collectors remove the aerosol ahead of the adsorber.

Biofiltration relies on microorganisms in a moist bed that oxidize biodegradable compounds at low concentrations. It fits steady, large flows and tolerates long shutdowns poorly. Torch-Air manufactures biofilters for such duties.

Thermal and catalytic oxidation suit high mass loads, where sorbent replacement costs more than burning the vapors.

Equipment for industrial air purification for VOCs is chosen from measured stream data. Flow is needed twice: in scfm for the mass load and in acfm at actual temperature and pressure for sizing. Add the concentration of each compound, relative humidity, aerosol and dust content, operating schedule and the required outlet value.

The mass load is calculated from flow at stated standard conditions, while bed area and contact time use the actual flow through the vessel. Equilibrium capacity comes from an isotherm; the example uses the toluene isotherm in the EPA Air Pollution Control Cost Manual. Where no working-capacity data exist, the same manual allows a preliminary estimate of 50% of equilibrium capacity. Superficial velocity comes from the media supplier; vendor values cited there run from 60 to 85 ft/min.

The run time is a mass-balance estimate. Hours until the outlet reaches 5 ppmv depend on the mass-transfer zone and are read from the supplier's breakthrough curve at actual humidity and temperature. Above 50% RH water competes for adsorption sites, most noticeably at low concentrations, and capacity falls. At roughly 170 lb of media per shift, a regenerable system deserves a cost comparison.

The change-out point equals the outlet limit. In a lead-lag arrangement, gas is sampled between the two vessels; when the lead bed reaches the limit, it receives fresh media and moves to the lag position.

The analyzer must detect the target compounds. A photoionization detector responds only to substances whose ionization energy is below the lamp energy, so a 10.6 eV lamp misses formaldehyde (10.88 eV) and methanol (10.84 eV). Formaldehyde requires a dedicated method.

Ketones, aldehydes, organic acids and peroxides can react exothermically on carbon and form hot spots. Keeping the stream below 25% of the LEL does not rule out this thermal risk. Heating is limited by diluting the inlet air or spreading a high inlet load over time, and the vapors should be tested on the chosen carbon before start-up. Bed temperature and CO are monitored in service. Shutdown follows the unit design and an approved procedure, because air drawn over hot carbon can supply oxygen for a fire.

Spent media is characterized as waste before it leaves the site. Depending on what it holds, it may fall under RCRA hazardous waste rules or return for reactivation. On scrubbers, operators track the L/G ratio, pH or ORP, blowdown rate and pressure drop across the droplet eliminator.