Absorption

(Ruddy and Carroll, 1993; US EPA, 1991)

Description

• VOC gaseous emissions flow into the bottom of a packed or tray column and is distributed throughout the absorption column.
• A heavy oil/hydrocarbon flows into the top of the column and the VOC is transferred (an amount based on solubility levels) from the gas to the oil via direct contact, thus, the airstream is "scrubbed."
• The VOC/oil mixture exits the bottom of the column and is subsequently distilled to allow separation of the VOC and the oil.
• "VOC-free" gas exits the top of the column.

Advantages

• Can achieve high recovery efficiencies (95-98%).
• Can be used for a wide range of gas flow rates (2,000-100,000 cfm).
• Can handle a wide range of inlet VOC concentrations (500-5,000 ppm).
• Good for high humidity (>50% r.h.) air streams.

Disadvantages

• May result in the generation of a wastewater stream.
• May result in column packing plugging or fouling if particulates are present in the gaseous waste stream.
• Some of the liquid absorbent may be transferred to the exit gas stream, thus creating a new pollution concern.

Activated Carbon Adsorption

(Ruddy and Carroll, 1993; US EPA, 1991, Stenzel, 1993)

Description

• VOC gaseous emissions flow into the top or bottom of an adsorption column, filled with porous activated carbon, and is distributed throughout the carbon bed.
• Two adsorption processes exist, temperature-swing adsorption (TSA) and pressure-swing adsorption (PSA). Temperature-swing adsorption is the approach commonly used for VOC recovery and the process description, advantages, and disadvantages listed in this section correspond to the temperature-swing adsorption process.
• Carbon adsorption beds can be fixed or moving, with respect to the carbon. For moving beds, the flow of activated carbon is countercurrent to the flow of the gas; however, fixed beds are more common in industry.
• The VOC is adsorbed onto the surface of the activated carbon and onto the surface of the pores. At some point the carbon becomes saturated with VOC and loses its capacity for additional adsorption. This results in the concept of "breakthrough" where significant quantities of VOC become apparent in the gas stream exiting the adsorption process. When this occurs the carbon must be regenerated for re-use or replaced with virgin carbon.
• Multiple fixed beds are generally employed so that as one or more beds are adsorbing at least one bed can be regenerating. Regenerating a bed of activated carbon typically involves the direct injection of steam, hot nitrogen or hot air to the bed which causes the VOC to release from the carbon and exit the bed via a vapor or condensate stream. The regenerated stream, containing a higher concentration of the VOC than the original wastewater stream, is subsequently condensed. If the VOC is immiscible in water, the condensate will form an aqueous layer and a solvent layer that can be separated using a decanter. If the VOC is miscible in water, additional distillation can be used to further separate the VOC and water.
• "VOC-free" gas exits the adsorber after contacting the activated carbon.

Advantages

• A widely used technology with well established performance levels.
• Can achieve high recovery efficiencies (90-98%).
• Can be used for a wide range of gas flow rates (100-60,000 cfm).
• Can handle a wide range of inlet VOC concentrations (20-5,000 ppm).
• Can efficiently handle fluctuations in gas flow rates and VOC concentrations.

Disadvantages

• VOC's having high heats of adsorption (typically ketones) can cause carbon bed fires.
• Carbon attrition properties (permanent bonding of small quantities of VOC through each adsorption cycle) requires the periodic replacement of carbon with virgin or reactivated carbon. Spent carbon may need to be disposed of as a hazardous waste depending on the VOC(s) adsorbed.
• Carbon efficiency decreases for high humidity (>50% r.h.) air streams.

Condensation

(Ruddy and Carroll, 1993; US EPA, 1991, Dunn and El-Halwagi, 1994a, Dunn and El-Halwagi, 1994b)

Description

• VOC gaseous emissions are cooled below the stream dew point to condense the stream VOC.
• Cooling occurs in indirect-contact heat transfer equipment (i.e. shell and tube heat exchangers, finned heat exchangers, etc.)
• Cooling mediums are usually cooling water, chilled water, and refrigerants.
• The cooling medium is recycled, re-cooled, and reused for additional VOC condensation.
• "VOC-free" gas exits the condenser.

Advantages

• Can achieve moderate recovery efficiencies (50-90%).
• Simple process that does not require contacting the VOC gas stream with other streams (i.e. oils, activated carbon), thus, minimizing contamination concerns.
• Efficiency improves as VOC concentration in inlet gas increases.
• Good for low volatility (high boiling point) VOC's.

Disadvantages

• May result in the generation of a wastewater stream.
• May require inert gas blanketing if inlet gas exceed the UEL to eliminate explosion hazards.
• The liquid produced via condensation may require treatment for water removal or may require additional separation (typically distillation) if multiple VOC are recovered.
• Cryogenic temperatures may be necessary and special equipment designs for these temperatures will be required.
• Condensation is typically used for low to moderate inlet gas flow rates (<20,000 cfm).
• Extensive cooling is required for low concentration VOC gaseous streams.
• Ice formation in heat transfer equipment may occur.