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In order to achieve reliable operation for as long as possible, the design of these columns has to be carefully considered
In recent years, steam strippers have become integral parts of industrial plant operations. The goal when designing these types of columns is to have reliable operation for as long as possible. This article details the methodology used when designing steam strippers for wastewater service. An example of a steam-stripper design is also presented.
THE BASICS
Steam stripping for wastewater purification is a distillation process where light volatile organic compounds (VOCs) are removed from water (the heavier component). The VOCs present in the feedstream to these types of columns are usually in low concentrations in comparison to the water phase. The typical organics present are as follows:
* benzene
* toluene
* xylenes (o, m and p)
* ethyl benzene
* styrene
* chlorinated hydrocarbons
(This list may change depending on the service.)
Steam stripping takes place at high temperatures that are usually very close to the boiling point of water. The removal of volatile organics is a strong function of temperature. Steam stripping allows for the removal of heavy soluble organics that other stripping techniques (air stripping, for instance) will not remove.
The typical arrangement of a steamstripping tower is a column where the liquid feed is introduced at the top of the column while steam is introduced at the bottom of the column. The wastewater feedstream is heated and put in contact with steam in a packed or trayed tower (Figure 1).
The combined effects of the steam and heat cause organic material to transfer from the liquid phase to the vapor phase. The steam-stripping tower utilizes trays or packing internals to facilitate contact between the contaminated water stream and the steam stream. The volatile organic material is then carried out with the vapor. As contacting proceeds down the column, the wastewater becomes leaner with organic material while the vapor phase becomes more enriched with organic material as it travels up the column.
The overhead organic vapor that comes off the top of the column is condensed and recovered. The recovered hydrocarbons can either be treated with an incinerator or recycled back into the process. The purified water that comes off the...