Liquid Stripping

What is Liquid Stripping?

Liquid Stripping is a process used to remove impurities and contaminants from a stream of liquid by contacting it with a pure stream of gas. By setting up favorable conditions for a mass transfer, the unwanted components of the liquid stream will be absorbed into the gas stream [1]. Stripping can either be physical or chemical where physical stripping is when there are no irreversible changes between the components and chemical stripping is when there is a reaction and irreversible changes do occur between the components. Chemical liquid stripping is often the much more efficient process. The opposite of liquid stripping is gas absorption and these processes are often coupled together. Stripping is best operated at a high temperature and at a low pressure for maximum efficiency [2].

Industry Usage

Liquid stripping, and unit operations in general, are often used in the processing of chemicals and pollution control. Although other substances such as steam can be used, pure air is often the gas used to be contacted with the liquid that is rich in solute. A common example of stripping is within the wastewater treatment industry where benzene is removed from wastewater [3]. Stripping columns are an essential to any gas treatment plant and the solvent is often recycled using liquid stripping and gas absorption together. As mentioned, gas absorption and liquid stripping are often used together as the exit streams of an absorber can often be recycled back into a stripping column as its input streams, and vice versa. A flow diagram of this process can be found in Figure 2.

Temperature and Pressure

Ideally stripping columns are operated at a low pressure and a high temperature to reduce the number of equilibrium stages. This is due to the effect the stripping factor and the K-value of a component have in determining the amount of equilibriums stages in a stripper. The k-value varies exponentially with temperature and is inversely proportional to pressure. Equations and calculations are different for each column as the flow rates of the gas and liquid, as well as the K-value of the components have an affect on the stripping factor. Due to all of these variables, for the benefit of efficiency and cost stripping columns are usually operated at a high temperature and a low pressure. As mentioned in the gas absorption wiki page, the absorption factor affects the number of equilibrium stages and this absorption factor is inversely proportional to the stripping factor as seen in Figure 5 [2].