Absorption vs. Stripping

Attribute	Absorption	Stripping
Process	Transfer of one or more solutes from a gas phase to a liquid phase	Transfer of one or more solutes from a liquid phase to a gas phase
Objective	To remove solutes from a gas stream	To remove solutes from a liquid stream
Driving Force	Concentration difference between the gas and liquid phases	Concentration difference between the liquid and gas phases
Mass Transfer	Mass transfer occurs from gas to liquid phase	Mass transfer occurs from liquid to gas phase
Equipment	Absorber tower or column	Stripper tower or column
Operating Temperature	Usually lower temperature	Usually higher temperature
Operating Pressure	Usually higher pressure	Usually lower pressure
Gas-Liquid Contact	Intimate contact between gas and liquid phases	Intimate contact between liquid and gas phases
Applications	CO2 capture, gas purification, solvent recovery	Volatile organic compound (VOC) removal, ammonia stripping

Introduction

In the field of chemical engineering, absorption and stripping are two important processes used for the separation of components from a mixture. Both processes involve the transfer of one or more components from a gas or liquid phase to a liquid phase. While absorption focuses on the transfer of components from a gas phase to a liquid phase, stripping involves the transfer of components from a liquid phase to a gas phase. In this article, we will explore the attributes of absorption and stripping, highlighting their similarities and differences.

Process Overview

Absorption: Absorption is a process where one or more components from a gas mixture are dissolved into a liquid phase. It is commonly used to remove pollutants or recover valuable components from gas streams. The gas mixture is brought into contact with a liquid solvent, and the desired components are selectively absorbed into the liquid phase. The liquid solvent, now containing the absorbed components, is then separated from the gas phase.

Stripping: Stripping, on the other hand, involves the transfer of one or more components from a liquid phase to a gas phase. It is often used to remove volatile components from a liquid stream or to concentrate a specific component. In stripping, the liquid stream is brought into contact with a gas stream, and the volatile components are transferred from the liquid phase to the gas phase. The gas stream, now containing the stripped components, is then separated from the liquid phase.

Mass Transfer Mechanisms

Absorption: The mass transfer mechanism in absorption is primarily governed by the difference in concentration between the gas phase and the liquid phase. The driving force for mass transfer is the concentration gradient, with the gas phase having a higher concentration of the desired component compared to the liquid phase. The component diffuses from the gas phase to the liquid phase until equilibrium is reached.

Stripping: In stripping, the mass transfer mechanism is also driven by the concentration gradient, but in the opposite direction. The liquid phase has a higher concentration of the volatile component compared to the gas phase, creating a driving force for mass transfer. The volatile component diffuses from the liquid phase to the gas phase until equilibrium is established.

Equipment and Operation

Absorption: Absorption processes typically involve packed columns or tray towers. Packed columns consist of a column filled with a packing material, which provides a large surface area for contact between the gas and liquid phases. Tray towers, on the other hand, contain a series of trays or plates that facilitate the contact between the two phases. The gas stream is introduced at the bottom of the column, while the liquid solvent is introduced at the top. As the gas rises through the column, it comes into contact with the descending liquid solvent, allowing for mass transfer to occur.

Stripping: Stripping processes also utilize packed columns or tray towers, similar to absorption. The main difference lies in the direction of flow. In stripping, the liquid stream is introduced at the top of the column, while the gas stream is introduced at the bottom. As the liquid flows downward, it comes into contact with the rising gas stream, facilitating the transfer of volatile components from the liquid to the gas phase.

Applications

Absorption: Absorption processes find wide applications in various industries. One common application is in the removal of pollutants from industrial gas streams. For example, in the flue gas desulfurization process, absorption is used to remove sulfur dioxide (SO2) from the flue gas emitted by power plants. Absorption is also employed in the separation and recovery of valuable components, such as the extraction of natural gas liquids (NGLs) from natural gas streams.

Stripping: Stripping processes are commonly used in the chemical and petrochemical industries. One notable application is in the removal of volatile organic compounds (VOCs) from wastewater. By stripping the VOCs from the liquid phase, the concentration of pollutants can be reduced to meet environmental regulations. Stripping is also utilized in the concentration of certain components, such as the concentration of ethanol from fermentation broth in the production of biofuels.

Energy Requirements

Absorption: Absorption processes generally require energy input in the form of heat to drive the mass transfer. This is particularly true when the desired component needs to be separated from a gas stream at a low concentration. The energy input is necessary to regenerate the liquid solvent and recover the absorbed component. The energy requirements can vary depending on the specific system and the desired separation efficiency.

Stripping: Stripping processes, on the other hand, often require energy input in the form of heat to vaporize the liquid stream and facilitate the transfer of components to the gas phase. The energy input is necessary to achieve the desired separation efficiency and concentration of the stripped component in the gas stream. The energy requirements can vary depending on the specific system and the desired concentration of the stripped component.

Conclusion

In conclusion, absorption and stripping are two important processes used for the separation of components from a mixture. While absorption focuses on the transfer of components from a gas phase to a liquid phase, stripping involves the transfer of components from a liquid phase to a gas phase. Both processes rely on concentration gradients to drive mass transfer, but in opposite directions. They also utilize similar equipment and operation principles, such as packed columns or tray towers. However, the energy requirements differ, with absorption requiring energy input for regeneration of the liquid solvent, while stripping requires energy input for vaporization of the liquid stream. Understanding the attributes of absorption and stripping is crucial for designing efficient separation processes in various industries.