Absorption: A Unit Operation for Gas Separation
Absorption is a crucial unit operation in chemical engineering that involves the separation of a gas mixture into its components. This process occurs when a gas mixture, comprising a solute gas and an insoluble gas, comes into contact with a liquid solvent. The soluble gas is then dissolved in the absorbent, allowing for the separation of the gas mixture.
Types of Absorption
Absorption can be a purely physical phenomenon, as seen in the removal of sulfur dioxide (SO2) from flue gases using an alkaline solution. This process is essential for reducing air pollution and mitigating the harmful effects of acid rain.
Environmental Applications
With the growing concern of global warming, researchers are exploring the absorption of carbon dioxide (CO2) from the atmosphere. This technology has the potential to play a critical role in reducing greenhouse gas emissions and mitigating climate change.
Gas Separating Membranes
Gas separating membranes operate at high pressures and pressure differentials, allowing for the efficient separation of gas mixtures. These membranes can be designed to target specific gas components, making them an attractive option for various industrial applications.
Factors Influencing Absorption
The absorption rate of a sparingly soluble gas in a liquid can be increased by enhancing the liquid-side mass transfer coefficient. Additionally, the equilibrium relation for solute distribution between a gas and liquid phase is described by the equation y = mx, where y is the mole fraction of the solute in the gas phase, x is the mole fraction of the solute in the liquid phase, and m is the equilibrium constant.
Industrial Applications
Absorption is a vital process in various industries, including:
- Removal of NH3 from dilute NH3-air mixtures in water, where gas-side resistance is more significant than liquid-side resistance.
- Absorption of benzene from coal gas in wash oil, where the major resistance to mass transfer comes from the gas side.
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Absorption Factor Calculator:
Mole Fraction and Absorption Factor
The mole fraction, y, is related to the mole ratio, Y, by the equation y = Y / (1 + Y). Additionally, the absorption factor, L/mG, plays a crucial role in determining the number of plates required. When L/mG is greater than one, the number of plates increases, indicating a more challenging separation process.
Henry's Law Constant Calculator
Mass Transfer Coefficient Calculator
Packed Column Diameter Calculator
Tray Efficiency Calculator
Gas Absorption and Scrubber Operations
The overall gas phase mass transfer coefficient (Ky) can be determined using the individual gas and liquid phase mass transfer coefficients (ky and kx) through the relation: 1/Ky = 1/ky + m/kx. Similarly, the overall liquid phase mass transfer coefficient (KL) is given by: 1/KL = 1/kL + 1/Hkg, where H is the Henry's Law constant, kg is the gas phase mass transfer coefficient, and kL is the liquid phase mass transfer coefficient. For a specific system, the equilibrium relation between the gas and liquid phases is described by the equation p = HC, where p is the partial pressure of the solute and C is its concentration in the liquid phase. Additionally, for a particular system, the equilibrium relation can also be expressed as y = 2x, where x and y are the mole fractions of the solute in the liquid and gas phases, respectively.
In gas absorption, when individual gas and liquid phase mass transfer resistances are equally important, the overall gas phase mass transfer coefficient (Ky) is typically 50% of the individual gas phase coefficient (ky). A stage is a device where mass transfer occurs between two immiscible phases, bringing them into intimate contact and resulting in equilibrium at the exit. Multistage contacting is preferred in chemical processing industries (CPI) due to lower solvent requirements and smaller equipment sizes. The Kremser equation is a useful tool for determining the number of stages required in a staged column, particularly when operating and equilibrium lines are straight. An ideal solvent for gas absorption should possess low vapor pressure and viscosity. Common solvents employed in various applications include ethylene glycol for natural gas dehydration, and mono- and diethanolamines, as well as aqueous NaOH solutions, for sweetening sour gases.
Design Considerations for Plate Columns
When designing a plate column for gas absorption, several factors come into play. For instance, if the gas and liquid flow rates are doubled while maintaining the same gas-phase concentration change, the number of plates required remains unchanged. This highlights the importance of optimizing flow rates to achieve efficient separation.
Height of Transfer Unit (HTU)
The height of transfer unit (HTU) is a critical parameter in gas absorption, providing insight into the ease of separation. HTU is equal to the height equivalent to a theoretical plate (HETP) when L/mG equals one or when the liquid phase resistance is negligible. The expression for HTU in the gas phase (HtOG) is given by (G / KGaPtyBM).
Case Study: Gas Absorption Process
Consider a dilute gas absorption process with HtG = 0.3 m, HtL = 0.24 m, and L/mG = 1.2. In this scenario, the HTU is calculated to be 0.5 m. This example illustrates the importance of HTU in designing efficient gas absorption systems.
Number of Transfer Units
The number of transfer units (NTU) is another key parameter in gas absorption, indicating the difficulty of separation. The NTU for the gas phase is defined as the overall change in gas phase concentration divided by the average driving force.
Non-Isothermal Absorption
In cases where absorption is accompanied by heat evolution, the number of plates required increases compared to isothermal absorption, even for the same degree of separation. This highlights the importance of considering thermal effects in gas absorption design.
Lewis Number
Finally, the Lewis number (Le) is defined as the ratio of the Schmidt number (Sc) to the Prandtl number (Pr). This dimensionless quantity plays a crucial role in characterizing the transport properties of fluids in gas absorption systems.