An absorption tower is used to transfer a gas phase component into a liquid phase, while an adsorption tower is designed to remove or capture specific components from a gas or liquid stream using an adsorbent material.

In absorption towers, the principle is based on the solubility of gases in liquids. The gas is brought into contact with a liquid, and the target component dissolves into the liquid phase due to differences in concentration. This is commonly used in processes like removing CO2 from flue gases using amine solutions.

In adsorption towers, the principle involves attaching or adsorbing specific components onto the surface of a solid adsorbent material. This is achieved by bringing the gas or liquid stream into contact with the adsorbent, which has a high surface area. Examples include activated carbon adsorbing contaminants from water or molecular sieves capturing moisture from compressed air.

Terminology varies but generally involves terms like "adsorbent," which is the material that captures the target component, and "desorbent" for a substance that releases an adsorbed component. Types of adsorption towers include fixed bed, fluidized bed, and moving bed towers, each with different configurations for achieving efficient adsorption.

Design equations for absorption and adsorption towers depend on factors like mass transfer, flow rates, equilibrium data, and adsorption isotherms. These equations help engineers determine tower dimensions, operating conditions, and efficiency. Specific equations vary based on the type of tower and the nature of the adsorption or absorption process.

The principle of absorption and adsorption towers revolves around the interaction between different phases (gas and liquid, or gas and solid) and the properties of the substances involved.

Absorption Tower Principle:

In absorption, a gas phase component is transferred into a liquid phase. This process occurs due to differences in concentration between the gas phase and the liquid phase. The key driving forces are partial pressure differences and solubility. When the gas comes into contact with the liquid, the component with a higher partial pressure in the gas phase tends to dissolve into the liquid phase until equilibrium is reached.

Adsorption Tower Principle:

Adsorption involves attaching molecules of a gas or liquid onto the surface of a solid adsorbent material. This occurs due to attractive forces, such as Van der Waals forces or chemical interactions, between the adsorbate (the substance being adsorbed) and the adsorbent surface. The adsorbent material typically has a high surface area, providing ample sites for the adsorbate to adhere to. The driving force here is the potential energy reduction when molecules attach to the surface.

Theory:

In absorption, the theory is based on Henry's Law, which states that the concentration of a gas in a liquid is directly proportional to its partial pressure in the gas phase, assuming temperature remains constant.

For adsorption, theories include the Langmuir adsorption isotherm, which describes how adsorbate molecules form a monolayer on the surface and reach a saturation point, and the BET theory (Brunauer, Emmett, and Teller), which extends the Langmuir model to multilayer adsorption.

Both principles involve reaching equilibrium where the rates of transfer (absorption or adsorption) and desorption (release of absorbed or adsorbed molecules) become equal. Understanding the equilibrium relationships and factors affecting the rates of transfer is crucial for designing efficient absorption and adsorption towers.

Overall, these principles and theories guide the design and operation of absorption and adsorption towers for various industrial applications.

Terminology : 

Absorption Tower:

  • Absorbent: The liquid phase that captures and dissolves the target gas component.
  • Desorber: A unit or process that removes the absorbed component from the liquid phase.
  • Absorption Efficiency: The ratio of the amount of gas absorbed by the liquid to the amount that could have been absorbed under ideal conditions.
  • Absorption Factor:The ratio of the equilibrium concentration of the gas in the liquid phase to its partial pressure in the gas phase.
  • Liquid-Gas Ratio (L/G): The ratio of the flow rate of liquid to the flow rate of gas in an absorption tower.

Adsorption Tower:

  • Adsorbent: A solid material with a high surface area that captures and holds the target component from a gas or liquid stream.
  • Adsorbate: The component that is captured or adsorbed onto the surface of the adsorbent.
  • Desorption: The process of releasing or removing the adsorbed component from the adsorbent.
  • Adsorption Isotherm: A graph that shows the relationship between the amount of adsorbate adsorbed onto the adsorbent and the equilibrium concentration of the adsorbate in the gas or liquid phase.
  • Breakthrough: The point at which the adsorbent becomes saturated and the adsorbate starts to breakthrough into the effluent stream.
  • Regeneration: The process of restoring the adsorbent's capacity by removing the adsorbed component, often through techniques like heating or purging.

General:

  • Equilibrium: The state where the rates of adsorption (or absorption) and desorption are equal, resulting in no net change in concentrations.
  • Partial Pressure: The pressure exerted by a specific gas component in a mixture of gases.
  • Surface Area: The total area of the solid adsorbent's surface available for adsorption, influencing its capacity to adsorb molecules.

These terms provide a foundation for understanding the functioning and design of absorption and adsorption towers, their components, and the interactions that occur during the processes.


Types of Adsorption Towers:

1. Fixed Bed Adsorption Tower: In this type, the adsorbent material is packed in a fixed bed, and the gas or liquid stream flows through the bed. The adsorbent captures the target component as the stream passes through. Fixed bed towers are commonly used in various industries for purification and separation processes.

2. Fluidized Bed Adsorption Tower: In this configuration, the adsorbent particles are suspended in the gas or liquid stream, creating a fluidized bed. This enhances contact between the adsorbent and the stream, improving mass transfer rates. Fluidized bed towers are used when high throughput and rapid adsorption are required.

3. Moving Bed Adsorption Tower: This type involves the continuous movement of the adsorbent material through the tower. Fresh adsorbent is introduced at the top, while spent adsorbent exits at the bottom. This allows for efficient utilization of the adsorbent and is common in processes where adsorbent regeneration is necessary.

Design Equations:

Designing adsorption towers involves considering mass transfer, equilibrium data, and adsorption isotherms. While specific equations can vary based on the type of tower and the adsorption process, here are some general concepts:

1. Mass Transfer Rate Equation: The rate of adsorption is proportional to the concentration difference between the bulk fluid and the surface of the adsorbent. The general equation is:

Rate of Adsorption = K * (C_b - C_s)

   Where:

  •    K is the mass transfer coefficient.
  •    C_b is the concentration of the adsorbate in the bulk fluid.
  •    C_s is the concentration of the adsorbate at the surface of the adsorbent.

2. Adsorption Isotherms: These equations describe the relationship between the amount of adsorbate adsorbed onto the adsorbent and the equilibrium concentration in the gas or liquid phase. The Langmuir and BET isotherms are common models.

3. Bed Depth and Contact Time: Depending on the tower type, you might use equations to determine the appropriate bed depth and contact time to achieve desired levels of adsorption.

4. Breakthrough Analysis: For fixed bed and moving bed towers, you would analyze breakthrough curves to determine the point at which the adsorbent becomes saturated and needs replacement.

5. Regeneration: If regeneration of the adsorbent is required, equations for temperature, pressure, and time are used to optimize the regeneration process.

Remember, actual design equations can be quite complex and vary based on factors like specific adsorbent characteristics, flow rates, temperature, and more. Engineers use these equations to guide the sizing and operational parameters of adsorption towers to meet desired separation and purification goals.