Design of Cost-Effective VOC-Recovery Systems

(Heilshorn, 1991a)

Wastewater flows into the top or bottom of an adsorption column, filled with porous activated carbon, and is distributed throughout the carbon bed.
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 wastewater; 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. 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 steam 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" wastewater exits the adsorber after the contact with the activated carbon.

A widely used technology with well established performance levels.
Can be used for low concentration inlet streams.
Can achieve high removal efficiencies.
Can efficiently handle fluctuations in wastewater flow rates and VOC cencentrations.

Higher complexity of operation than other technologies.
Larger space requirements than other technologies.
Water adosrption lowers the VOC adsorption capacity.
Exceeding the LFL in the exit regeneration stream (when using air regeneration) may lead to fires and/or explosions in the adsorber or the subsequent air handling equipment/condenser.

(Belhateche, 1995; Cusack et al, 1991)

Liquid-liquid extraction involves the separation of VOC's by contact with another liquid (solvent) in which the VOC's are more soluble.
Extraction solvent selection is based on:
- selectivity (ability of the solvent to extract much of the VOC but very little of the water)
- ease of regeneration (ability to separate the VOC from the extraction solvent, typically using distillation)
- low miscibility with the feed solution (the extraction solvent should not transfer to the exiting wastewater stream)
- significant density difference between the extraction solvent and the wastewater feed (aqueous and organic phases are generally separated by settling)
- moderate interfacial tension (impacts mixing capabilities)
- low viscosity (<10 cps minimizes resistance to mass transfer)
- low flammability and toxicity
- low cost and ready availability
Separation of the solvent-VOC waste can be handled via air stripping, steam stripping, distillation, or additional liquid-liquid extraction.
Separation of the exiting wastewater stream can occur via air stripping, steam stripping, activated carbon adsorption or biological treatment.
Process efficiency can be increased by increasing the flowrate of solvent to wastewater or by increasing the number of extraction stages.

Generally, liquid-liquid extraction is easy to operate.
Capital costs are relatively low; however, if additional separation (distillation) of exiting streams is required capital cost generally increases by a factor of 8-10 and operating costs increase by a factor of 20.
Can be used for heat sensitive materials.
Can be used to separate close-boiling mixtures, such as isomers.

VOC gaseous emissions may occur from the extraction unit.
Energy costs are high.
Additional treatment (distillation) of streams leaving the extraction unit is generally required.