Stripping (chemistry)
Stripping is a physical separation process where one or more components are removed from a liquid stream by a vapor stream.[1] In industrial applications the liquid and vapor streams can have co-current or countercurrent flows. Stripping is usually carried out in either a packed or trayed column.[2]
Theory
Stripping works on the basis of mass transfer. The idea is to make the conditions favorable for the component, A, in the liquid phase to transfer to the vapor phase. This involves a gas–liquid interface that A must cross. The total amount of A that has moved across this boundary can be defined as the flux of A, NA.
Equipment
Stripping is mainly conducted in trayed towers (
Trayed towers consist of a vertical column with liquid flowing in the top and out the bottom. The vapor phase enters in the bottom of the column and exits out of the top. Inside of the column are trays or plates. These trays force the liquid to flow back and forth horizontally while the vapor bubbles up through holes in the trays. The purpose of these trays is to increase the amount of contact area between the liquid and vapor phases.
Packed columns are similar to trayed columns in that the liquid and vapor flows enter and exit in the same manner. The difference is that in packed towers there are no trays. Instead, packing is used to increase the contact area between the liquid and vapor phases. There are many different types of packing used and each one has advantages and disadvantages.
Variables
The variables and design considerations for strippers are many. Among them are the entering conditions, the degree of recovery of the solute needed, the choice of the stripping agent and its flow, the operating conditions, the number of stages, the heat effects, and the type and size of the equipment.[2]
The degree of recovery is often determined by environmental regulations, such as for volatile organic compounds like chloroform.
Frequently,
The size of the equipment, and particularly the height and diameter, is important in determining the possibility of flow channeling that would reduce the contact area between the liquid and vapor streams. If flow channeling is suspected to be occurring, a redistribution plate is often necessary to, as the name indicates, redistribute the liquid flow evenly to reestablish a higher contact area.
As mentioned previously, strippers can be trayed or packed. Packed columns, and particularly when random packing is used, are usually favored for smaller columns with a diameter less than 2 feet and a packed height of not more than 20 feet. Packed columns can also be advantageous for
Trayed strippers can have sieve, valve, or bubble cap trays while packed strippers can have either
During operation, monitoring the pressure drop across the column can help to determine the performance of the stripper. A changed pressure drop over a significant range of time can be an indication that the packing may need to be replaced or cleaned.
Typical applications
Stripping is commonly used in industrial applications to remove harmful contaminants from waste streams. One example would be the removal of TBT and PAH contaminants from harbor soils.[4] The soils are dredged from the bottom of contaminated harbors, mixed with water to make a slurry and then stripped with steam. The cleaned soil and contaminant rich steam mixture are then separated. This process is able to decontaminate soils almost completely.
See also
- Steam stripping (Similar concept, but more specialized to Refinery Operations )
- Continuous distillation
- Distillation
- Distillation Design
- Fractionating column
- Packed bed
- Steam distillation
- Theoretical plate
References
- ISBN 0-07-112738-0.
- ^ ISBN 0-471-46480-5.
- ^ http://tech.money.pl/przemysl/patenty/pl-101135-424118.html Białecki ring. Polish Patent 101135 (1976)
- ^ The Steam Stripping Process: A Remediation Technique for TBT- and PAH-Contaminated Dredged Sediments and Soils, by A. Eschenbach et al.
- ISSN 0360-7275.