Figure 2.3

A Schematic Representation of a Liquid-Liquid Extraction Process for VOC Aqueous Wastes


Pervaporation

(Losin, et al., 1993)


Description

  • Pervaporation (a technology that combines permeation and evaporation) is a membrane-based process that operates on the principle of selective permeation of a VOC through the membrane.
  • For VOC removal from wastewater, a hydrophobic (organophilic) membrane (typically a rubbery polymer) is required.
  • Feed streams to pervaporation membranes are typically preheated. The warm wastewater feed enters the membrane housing at a high pressure and the VOC (and some water) preferentially permeates to the low pressure side of the membrane. As the VOC (referred to as the permeate) goes from high pressure to low pressure it flashes to the vapor state and a condensation process (typically using chilled water) is used to condense the VOC.
  • "VOC-free" wastewater (referred to as the retentate) exits the high pressure side of the membrane.

Advantages

  • Organics can be concentrated 10 to 50 times higher than in the original wastewater stream.
  • Systems are modular and compact.
  • Efficient at low VOC inlet concentrations (100-5000ppm).
  • Less energy intensive than reverse osmosis.
  • Can be used to separate close-boiling or azeotropic compounds.

Disadvantages

  • Restricted to smaller flowrates (<15 gpm).
  • Membrane fouling may occur.
  • Although a proven technology, no large scale industrial uses currently exist.

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Figure 2.4

A Schematic Representation of a Pervaporation Process for VOC Aqueous Wastes


Reverse Osmosis

(El-Halwagi, 1992)


Description

  • Reverse Osmosis is a membrane-based process that operates on the principle of selective permeation of a VOC through the membrane. Unlike pervaporation, no phase change occurs across the membrane and the water permeates from the feed side of the membrane to the permeate side of the membrane.
  • Wastewater streams fed to reverse osmosis membranes enter the membrane housing at a high pressure and the water (and some VOC) preferentially permeates to the low pressure side of the membrane.
  • "VOC-free" wastewater (referred to as the permeate) exits the low pressure side of the membrane.
  • The "reject" stream exits the high pressure side of the module and contains a higher concentration of VOC than the initial wastewater stream. This stream can be disposed of or distilled for further VOC recovery.

Advantages

  • Generally compact in size; therefore, a reverse osmosis system can be easily added to existing plant operations.
  • Flexibility of adding modules or bypassing modules can accommodate surges in wastewater loads and/or concentrations.

Disadvantages

  • Fouling of membranes may occur.
  • Can not be used on streams with a high osmotic pressure.
  • May require additional treatment of the reject stream to purify the VOC to levels acceptable for reuse in the plant.

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Figure 2.5

A Schematic Representation of a Reverse Osmosis Process for VOC Aqueous Wastes

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References

Belhateche, Dannelle H., Choosing Appropriate Wastewater Treatment Technologies, Chemical Engineering Progress, 91(8), 32-51, 1995.

Cusack, Roger W., Pierre Fremeaux and Don Glatz, A Fresh Look at Liquid-Liquid Extraction, Chemical Engineering, 98(2), 66-76, 1991.

DeRenzo D. J., ed., Pollution Control Technology for Industrial Wastewater, Noyes Data Corporation, Park Ridge, NJ, 403-416, 437-485, and 526-530, 1981.

El-Halwagi, M. M., Synthesis of Optimal Reverse Osmosis Networks for Waste Reduction, AIChE J., 38(8), 1185-1198, 1992.

Geankoplis, C. J., Transport Processes and Unit Operations, Second Edition, Allyn and Bacon, Inc., Boston, 1983.

Heilshorn, E.D., Removing VOC's from Contaminated Water-Part One, Chemical Engineering, 98(2), 120-124, 1991a.

Heilshorn, E.D., Removing VOC's from Contaminated Water-Part Two, Chemical Engineering, 98(3), 152-158, 1991b.

Henley, E. J. and J. D. Seader, Equilibrium-Stage Separation Operations in Chemical Engineering, John Wiley and Sons, New York, 1981.

King, C. J., Separation Processes, Second Edition McGraw-Hill, New York, 1980.

Losin, Martha S., Rao D. Mikkilineni, and Brian D. Miller. Pervaporation: An Emerging Technology for Wastewater Processing, 1993 AIChE Annual Meeting, St. Louis, Paper 4e. 1-28, 1993.

McCabe, W. L., J. C. Smith and P. Harriot, Unit Operations of Chemical Engineering, Fifth Edition, McGraw-Hill, New York, 1993.

Okoniewski, Bradley A., Remove VOCs from Wastewater by Air Stripping, Chemical Engineering Progress, 88(2), 89-93, 1992.

Perry, R. H. and D. Green, Perry's Chemical Engineers' Handbook, Sixth Edition, McGraw-Hill, New York, 1984.

Treybal, R. E., Mass Transfer Operations, Second Edition, McGraw-Hill, New York, 1980.

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