stripping (desorption)Reading time:
The aim is to remove gases dissolved in water so that they can be transferred into a gas phase in order to obtain water that contains very little dissolved gas. These gases are removed from the liquid phase and carried off in a major gas backflow designated scavenging gas. When the transfer laws are applied, we see that, in order to obtain very low levels of dissolved gas, we need to :
- lower the molar fraction of the gas concerned in the scavenging gas phase: stripping CO2 using air, and O2 using natural gas…;
- lower the total pressure of the gas phase: vacuum degassing of oxygen, CO2, CH4…;
- increase the degassing Henry constant: thermal at high temperature (O2, CO2).
Most of the dissolved gases mentioned in the foregoing are very low solubility in water types and desorption will be governed by the transfer to the liquid phase. For very soluble gases, (NH3), desorption is regulated by the gas phase.
The most frequently used industrial units are packing columns; their design is similar to that for distillation columns.
The height H of packing required for stripping purposes can be calculated as follows :
- either by the product H = HUT × NUT (in the case of low solubilitygases), where :
- HUT: HUT = Height of Unit of Transfer and this primarily depends on the characteristics of the packing
- NUT: Number of Units of Transfer and this depends exclusively on initial, final and interface concentration levels at the different stages:
- or by the product H = HETP x NTS (in the case of highly soluble gases), where :
- NTS : number of theoretical stages established by an analytical or graphic calculation;
- HETP: height equivalent to a theoretical plateau, primarily dependent on the packing.
For oxygen removal, at 15°C, the number of transfer units (or stages) will vary for saturated water from 8 to 12 for final concentrations of 50 to 10 μg · L–1.
CO2 desorption is not usually as meticulous. In a demineralisation unit, the target CO2 reduction from 70 to 10 mg · L–1 requires less than two theoretical stages.