Liquid Stripping
Table of Contents
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].
Figure 1: Basic Flow Diagram of a Stripper |
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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.
Figure 2: Stripping/Absorption System |
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Design of a Stripping Column
Many things must be taken into consideration when designing a stripping column such as temperature, pressure, and flow rate of the gas and liquid entering, the temperature and pressure of the entire unit, the percentage recovery of the solute, the number of equilibrium stages and their efficiency, and the size and type of absorber. All of these must be analyzed in detail as they will all affect the cost, size, and manufacturing process of the column [2].
Equilibrium Stages
The number of equilibrium stages is a very important factor that has to be taken into consideration when designing a stripping column. Normally the desired amount of solute will not transfer into the gas stream after being contacted once, meaning more than stage where the components are contacted are required. The components of each stage leave in equilibrium with each other which results in the name equilibrium stages. The number of equilibrium stages can be determined either graphically using the McCabe-Thiele graphing method (Figure 3) or algebraically using the Kremser equation (Figure 4) [2].
Figure 3: McCabe-Thiele Graphical Method |
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Figure 4: Kremser Equation |
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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].
Figure 5: Absorption/Stripping Factor Relation |
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Equipment
The different types of equipment that are used for liquid stripping can also be used for the very similar separation processes gas absorption and distillation. A detailed breakdown of the equipment that can be used for these 3 processes can be found here.
Simulation Software
When designing a stripper, a lot of mathematical calculations are involved as previously mentioned which can be very time consuming. Before simulation software and programs, chemical engineers would have to do all of these calculations by hand although today there is a lot of software widely available that can do these calculations much faster. Two very similar software packages, Aspen HYSYS and Aspen PLUS which are both made by AspenTech, are two of the most widely used software packages among chemical engineers. The main difference between the two is the industries they are used in as HYSYS is mainly used in petroleum engineering while PLUS is fine chemistry as both of them do a better job modelling the processes used in the industries mentioned [4]. Another useful software packages in general for engineering is MATLAB which can be used to model many separation processes including gas absorption. Another very useful software suite that is specifically designed for process simulations would be Chemstations CHEMCAD which can model many separation processes including gas absorption [5].
Contributors:
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Faculty Advisor: Sarah Meunier