Gas Absorption and Stripping

Gas absorption and stripping are mass transfer operations widely used in chemical and environmental engineering for separating components from gas mixtures.

Gas Absorption

Gas absorption involves the transfer of solute components from a gas mixture to a liquid solvent. This process is driven by the concentration difference between the two phases.

Key aspects of gas absorption:

1. Rich gas (containing solute) contacts a weak liquor (absorbent)
2. Solute transfers from the gas to the liquid phase
3. Process continues until equilibrium is reached

Highlight: The absorption factor in gas absorption is a key parameter that influences the efficiency of the process. It is defined as the ratio of the slope of the operating line to the slope of the equilibrium curve.

Formula: Absorption Factor = L / $m * G$, where L is the liquid flow rate, G is the gas flow rate, and m is the slope of the equilibrium curve.

Stripping (Desorption)

Stripping, also known as desorption, is essentially the reverse process of gas absorption.

Key features of stripping:

1. Rich liquor (containing solute) contacts an inert gas
2. Solute transfers from the liquid to the gas phase
3. Results in a dilute gas (exit gas) with higher solute content

Example: Stripping is commonly used in wastewater treatment to remove volatile organic compounds (VOCs) from contaminated water using air as the stripping medium.

Both gas absorption and stripping operations often utilize column-type equipment, such as packed or tray columns, to facilitate efficient mass transfer between phases.

Vocabulary: The absorption column working principle involves countercurrent flow of gas and liquid phases, maximizing the driving force for mass transfer throughout the column height.

Liquid-Liquid Extraction

Liquid-liquid extraction is a separation method used to transfer a solute from one liquid phase to another immiscible or partially miscible liquid phase.

Extraction Equipment

Two common types of liquid-liquid extraction equipment are:

1. Single-stage mixer-settler
2. Multi-stage mixer-settler

Definition: Mixer-settlers are devices where the two liquid phases are thoroughly mixed by an impeller and then allowed to separate under the influence of gravity in a settling tank.

Key features of mixer-settlers:

• Thorough mixing promotes mass transfer
• Gravity separation based on density differences
• Suitable for both single-stage and multi-stage operations

Example: In a multi-stage mixer-settler arrangement, the underflow (more dense) phase from one stage becomes the feed for the next stage, allowing for increased extraction efficiency.

Extraction Efficiency

The effectiveness of liquid-liquid extraction is often quantified by extraction efficiency:

Formula: Extraction Efficiency = (Amount of solute extracted) / (Initial amount of solute in feed) * 100%

Factors affecting extraction efficiency:

1. Solvent selection
2. Number of stages
3. Phase ratio
4. Mixing intensity
5. Temperature

Highlight: Stage efficiency in liquid-liquid extraction is crucial for determining the number of stages required to achieve a desired separation. It compares the actual performance of each stage to that of an ideal equilibrium stage.

Understanding liquid-liquid extraction principles and calculations is essential for designing and optimizing extraction processes in various industries, including pharmaceuticals, food processing, and hydrometallurgy.

Vocabulary: The raffinate is the residual liquid from which the solute has been extracted, while the extract is the solvent phase containing the extracted solute.

Equilibrium Stage Concept

The equilibrium stage concept is a fundamental principle in separation processes, particularly in gas absorption, stripping, and extraction operations. This concept describes the ideal scenario where two phases come into intimate contact and reach thermodynamic equilibrium.

Definition: An equilibrium stage is a unit or equipment where two dissimilar phases are brought into intimate contact, allowing for the redistribution of components between the phases until equilibrium is established.

Key characteristics of equilibrium stages:

1. Intimate contact between phases
2. Sufficient mixing time for equilibrium establishment
3. Mechanical separation of resultant phases
4. No further net exchange of components at equilibrium

Highlight: The equilibrium stage represents the maximum possible composition change for given operating conditions, making it an ideal benchmark for actual process performance.

Actual Stages vs. Equilibrium Stages

In industrial applications, achieving true equilibrium is often impractical due to time and equipment constraints. This leads to the concept of actual stages:

Definition: An actual stage is a practical unit operation that approaches, but may not fully reach, equilibrium conditions.

The relationship between actual and equilibrium stages is quantified by stage efficiency:

Formula: Stage Efficiency = (Composition change in actual stage) / (Composition change in equilibrium stage)

Example: If an actual absorption column achieves 80% of the composition change that would occur in an ideal equilibrium stage, its stage efficiency would be 0.8 or 80%.

Understanding the difference between equilibrium and actual stages is crucial for:

1. Designing separation equipment
2. Optimizing process conditions
3. Predicting separation performance
4. Calculating the number of stages required for a given separation task

Vocabulary: Overall stage efficiency is often expressed as the ratio of the number of ideal (equilibrium) trays to the number of actual trays required for a specific separation.