Strippers

Stripping columns are used to remove unwanted materials from process streams. The air stripping column shown below is used to remove ammonia from water and waste water feedstreams.

(Copyright Ducon Technologies, Inc.,

Farmingdale, NY)

Air Stripping Columns

Air stripping columns are most widely used for the removal of volatile organic compounds (VOCs) from water. Air stripping columns are designed specifically for a given application.

(Copyright Monroe Environmental

Corporation, Monroe, MI)

General Information

The design of air stripping columns, such as the ones shown below, is governed by differences in volatility between the volatile organic compounds (VOCs) in the contaminated water.

(Copyright Monroe Environmental

Corporation, Monroe, MI)

The more volatile components transfer from the water to the air stream. The air exits the top of the column and contains the most volatile components, while the least volatile components exit with the water out the bottom of the column.

Equipment Design

After the contaminated water enters the column, it flows down the column and through the packing countercurrent to the air stream. The packing serves to increase the amount of contact between the two streams, and enhances the stripping as a result. After leaving the column, the air is either distributed into the atmosphere, or is collected for purification.

Sieve Tray Air Stripper

Packed Tower Air Stripper

(Copyright Monroe Environmental Corporation, Monroe, MI)

Air stripping columns contain either trays or packing, depending on the application. These trays and packing are the same as those used in distillation and absorption columns.

Tray types include sieve , valve , and bubble cap . The geometry of the trays affects the extent and type of contact between the vapor and liquid streams. Downcomers channel the liquid flowing from one tray down to the tray below.

Packing types include dumped packing and stacked packing . The packing channels the liquid flowing from the top to the bottom of the column.

Usage Examples

In the system below on the left, air is used to remove benzene derivatives, chloroethylenes, and ammonia from contaminated groundwater. On the right is an air stripping column used to remove VOCs at a potable water treatment plant.

(Copyright Monroe Environmental Corporation, Monroe, MI)

The air stripping columns pictured below are used to remove VOCs and dissolved gases from an incoming water feedstream.

(Copyright Layne Christensen Company, Mission Woods, KS)

The air strippers pictured below are used in a groundwater treatment plant. These units purify the incoming water through the use of trays rather than packing.

(Copyright QED Environmental Systems, Dexter, MI)

Advantages

Disadvantages

  • VOC removals in excess of 99.99% are possible with air stripping.
  • Air strippers are more economical when low volatility VOCs are to be removed.
  • Capable of achieving very high removal rates at very low pressure drops.
  • Packed air strippers are the most economical alternative for flows above 50 gpm.
  • Treatment of the exiting VOC laden air needed to prevent air pollution.
  • Fouling.

Steam Stripping Columns

General Information

Steam strippers are similar to air strippers with one exception: instead of air, they use steam to strip volatile components from a liquid stream.

Steam strippers allow for the removal of heavy soluble organic compounds that air strippers cannot remove. The temperature of these columns is usually very close to the boiling point of water.

In a typical steam stripping column, the liquid feed is introduced at the top of the column and the steam input is located at the bottom. When the liquid enters the column, it flows down the column and through the packing countercurrent to the steam. The packing enhances the stripping by increasing the amount of contact between the two streams. After leaving the column, the vapor is either distributed into the atmosphere or is collected for purification.

Equipment Design

Steam stripping columns contain either trays or packing, depending on the application. These trays and packing are the same as those used in distillation and absorption columns.

Tray types include sieve , valve , and bubble cap . The geometry of the trays affects the extent and type of contact between the vapor and liquid streams. Downcomers channel the liquid flowing from one tray down to the tray below.

Packing types include dumped packing and stacked packing . The packing channels the liquid flowing from the top to the bottom of the column.

The operating pressure of a steam stripper can influence the efficiency and reliability of the column. At lower operating pressures, greater volatility and lower operating temperatures are achieved. Stream strippers operating at vacuum can be expensive but highly efficient; they allow the use of plastic internals to fight the effects of corrosive systems. Operating the column at or near atmospheric pressure results in higher volatilities and lower operating temperatures without the extra expense of a vacuum system. At higher operating pressures, the solubility of the solute increases and the separation becomes more difficult to perform. Also, at higher pressures, more steam input to the column is required for the same separation to occur.

Preheating the feed before it enters the column is important because it reduces the amount of heat added to the column to achieve the required separation. The feed is preheated by either a heat recovery exchanger that cross exchanges the feed without the effluent coming off of the bottoms, a steam heat exchanger that cross exchanges the feed with a steam stream, or a combination of both.

Usage Examples

Steam stripping columns are most widely used in the recovery of solvents and solutes, and also in the removal of Volatile Organic Compounds (VOCs) from water.

Steam stripping for wastewater purification is a distillation process where over 99% of VOCs are removed from water. Some typical VOCs are benzene, toluene, xylenes, ethyl benzene, styrene, and chlorinated hydrocarbons.

The wastewater feedstream is pre-heated and put in contact with steam in a packed or tray column. Trays or packing units facilitate contact between the contaminated water and the steam stream. The combined effects of the steam and heat cause the organic materials to transfer from the liquid into the vapor phase. As contacting proceeds down the column, the vapor becomes rich in organic materials while the wastewater gets cleaner. The organic vapor leaves the column at the top and is condensed and recovered. These hydrocarbons can be recycled back into the stripper or treated with an incinerator. The purified water that leaves the bottom of the column has very low organic contaminate concentrations and it can be recycled back into the plant's water system.

Advantages

Disadvantages

  • Capable of achieving very high removal rates and low effluent concentrations.
  • Most economical removal technique at feed concentrations above 0.1% weight organics.
  • Can be made very energy efficient with heat recovery.
  • Minimizes air emissions and reduces loads to incineration.
  • Easy solute recovery.
  • Not useful to strip phenol, glycols, ethylene, propylene, glycerine, ionic compounds such as sulfonated organics, inorganic compounds (except in free gaseous dissolved form), and low volatility, highly water soluble compounds such as acetic acid.
  • Fouling.

Acknowledgements

Chemical Engineering, Access Intelligence, LLC

Ducon Technologies, Inc. , Farmingdale, NY

Monroe Environmental Corporation , Monroe, MI

Layne Christensen Company , Mission Woods, KS

QED Environmental Systems , Dexter, MI

Sigma-Aldrich Co. LLC , St. Louis, MO

Sulzer Chemtech Ltd. , Switzerland

Vendome Copper and Brass Works, Inc. , Louisville, KY

References

Ellerbe, R.W. "Steam Distillation/Stripping." Handbook of Separation Techniques for Chemical Engineers. 2nd ed. Ed. Philip A. Schweitzer. New York: McGraw-Hill, Inc., 1988. Print.

Kohl, Arthur L. "Absorption and Stripping." Handbook of Separation Process Technology. Ed.      Ronald W. Rousseau. New York: John Wiley & Sons 1987: 340-344, 399-400. Print.

McCabe, Warren L., Julian C. Smith, and Peter Harriott. Unit Operations of Chemical Engineering. 5th ed. McGraw-Hill, New York, 1993. Print.

Perry, Robert H., and Don W. Green. Perry's Chemical Engineers' Handbook. 7th ed. New      York: McGraw Hill 1997: 14-4 - 14-10, 14-23 - 14-26, 14-38 - 14-40. Print.

Zygula, Timothy M. "Designing Steam Stripping Columns for Wastewater." Chemical Engineering May 2008: 52-56. Print.

Developers

Brent Ladd

Matthew Robertson

Abigail Nalbandian

Kelsey Kaplan

Steve Cotton