Gas Absorption
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Table of Contents
What is Gas Absorption?
Gas absorption, also known as gas scrubbing or gas washing, is a process used to remove impurities and contaminants from a stream of gas by contacting it with a liquid stream. Through setting up favorable conditions for a mass transfer, the unwanted components of the gas stream will dissolve into the liquid stream [1]. There are 2 types of gas absorption known as physical absorption and chemical or reactive absorption. Physical absorption occurs when there are no irreversible chemical reactions between the liquid and the gas. A common example of physical absorption would be CO2 being absorbed into water to make carbonated water [2]. Chemical absorption is when a chemical reaction does occur between the liquid and the gas. Chemical absorption is often much more quick and more efficient than physical absorption. The opposite of gas absorption is liquid stripping and these processes are often coupled together. Gas absorption is best operated at a low temperature and a high pressure for maximum efficiency [3].
Figure 1: Basic Flow Diagram of an Absorber |
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Industry Usage
Gas absorption, and unit operations in general, are often used in the processing of chemicals and pollution control. Although other substances can be used, pure water is often the liquid used to be contacted with the gas that is rich in solute. Absorption columns are an essential to any gas treatment plant and the solvent is often recycled using liquid stripping and gas absorption together. A common example of the usage of gas absorption would be removing CO2 from flue gasses and natural gases. By using a liquid to remove CO2 from a gas, the CO2 is not added back to the atmosphere and is instead trapped within the water where it can be disposed of in a much more environmentally friendly way. 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 [1]. A flow diagram of this process can be found in Figure 2.
Figure 2: Stripping/Absorption System |
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Design of an Absorber
Many things must be taken into consideration when designing an absorber 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 absorber [3].
Equilibrium Stages
The number of equilibrium stages is a very important factor that has to be taken into consideration when designing an absorber. Normally the desired amount of solute will not dissolve into the liquid 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 (Figure 3) or algebraically using the Kremser equation (Figure 4) [3].
Figure 3: Graphical Method |
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Figure 4: Kremser Equation |
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Temperature and Pressure
Ideally absorbers are operated at a high pressure and a low temperature to reduce the number of equilibrium stages. This is due to the effect the absorption factor and the K-value of a component have in determining the amount of equilibriums stages in an absorber. The k-value varies exponentially with temperature and is inversely proportional to pressure. Equations and calculations are different for each absorber as the flow rates of the gas and liquid, as well as the K-value of the components have an affect on the absorption factor. Due to all of these variables, for the benefit of efficiency and cost absorbers are usually operated at a low temperature and a high pressure. As mentioned in the liquid stripping wiki page, the stripping factor affects the number of equilibrium stages and this stripping factor is inversely proportional to the absorption factor as seen in Figure 5 [3].
Figure 5: Absorption/Stripping Factor Relation |
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Equipment
The different types of equipment that are used for gas absorption can also be used for the very similar separation processes stripping and distillation. A detailed breakdown of the equipment that can be used for these 3 processes can be found here.
Simulation Software
When designing an absorber, 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].
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