7.4 Absorption and stripping

Principles and Applications of Absorption and Stripping

Fundamentals of Absorption and Stripping Processes

Absorption is a mass transfer operation where a soluble component in a gas mixture dissolves into a liquid solvent. Stripping is the reverse: a dissolved component is pulled out of a liquid by contact with a gas. These two processes are essentially mirror images of each other, and they're often paired in industrial systems (absorb a gas into a solvent, then strip it out to regenerate the solvent).

Both processes are governed by the solubility of gases in liquids, which depends on temperature, pressure, and the chemical nature of the gas-liquid pair. The driving force for mass transfer is the concentration gradient between phases:

• In absorption, the solute concentration is higher in the gas phase than the liquid phase, so the solute transfers gas → liquid until equilibrium is reached.
• In stripping, the solute concentration is higher in the liquid phase, so the solute transfers liquid → gas.

Applications of Absorption and Stripping Processes

Absorption applications:

• Removing acidic gases from natural gas ($CO_2$, $H_2S$)
• Recovering valuable components from process gas streams (ammonia, hydrocarbons)
• Purifying air or other gases by removing contaminants or moisture

Stripping applications:

• Removing volatile organic compounds (VOCs) like benzene and toluene from wastewater
• Regenerating solvents by removing dissolved gases or impurities
• Deaerating liquids (removing dissolved $O_2$ or $CO_2$ from boiler feedwater, for example)

Design of Absorption and Stripping Columns

Types of Absorption and Stripping Columns

The two main column types are packed towers and tray columns. The choice depends on gas and liquid flow rates, required separation efficiency, and allowable pressure drop.

Packed towers use a bed of packing material to create a large interfacial area for gas-liquid contact.

• Common packing materials include Raschig rings, Pall rings, and structured packing.
• Liquid flows downward through the packing while gas flows upward in a countercurrent arrangement. This countercurrent flow maximizes the concentration gradient along the entire column height.

Tray columns use a series of horizontal trays (or plates) to create discrete stages of gas-liquid contact.

• Liquid flows across each tray, then down to the next tray through downcomers.
• Gas rises through openings in the trays and bubbles through the liquid sitting on each tray.

Packed towers tend to have lower pressure drop and work well for corrosive systems, while tray columns handle high liquid rates well and are easier to clean.

Design Considerations and Analysis Methods

Designing an absorption or stripping column means determining the column diameter, height, and internals (packing type or tray configuration). The key inputs you need are:

• Gas and liquid flow rates
• Inlet and outlet solute concentrations
• The equilibrium curve (relates solute concentration in the gas phase to its concentration in the liquid phase at equilibrium)
• Mass transfer coefficients

Column diameter is set by the gas and liquid flow rates and the allowable pressure drop. Too small a diameter causes flooding; too large wastes capital.

Column height is determined by two quantities:

• NTU (Number of Transfer Units): a dimensionless measure of how difficult the separation is. More NTU = harder separation = taller column.
• HTU (Height of a Transfer Unit): the physical height of column needed to achieve one transfer unit. It depends on the mass transfer coefficient and flow rates.

The total packing height is then:

$Z = NTU \times HTU$

Two graphical tools are central to column analysis:

• The equilibrium curve shows where the gas and liquid phases would be in balance. It's determined by thermodynamics (Henry's law for dilute systems, for instance).
• The operating line shows the actual concentrations at each point in the column, determined by a mass balance on the gas and liquid streams. The vertical distance between the operating line and the equilibrium curve represents the driving force for mass transfer at any point.

Optimal Operating Conditions for Absorption and Stripping

Factors Affecting the Optimal Operating Conditions

Temperature and pressure directly affect gas solubility:

• Higher pressure increases gas solubility (favors absorption).
• Higher temperature decreases gas solubility (favors stripping).

So absorption columns typically operate at higher pressures and lower temperatures, while stripping columns do the opposite.

The liquid-to-gas ratio (L/G) is a critical design parameter. A higher L/G ratio improves separation efficiency because more solvent is available to absorb the solute. However, it also increases pumping costs and requires a larger column diameter. There's a minimum L/G ratio below which the desired separation is thermodynamically impossible (the operating line would cross the equilibrium curve). Practical designs typically use 1.2 to 1.5 times the minimum L/G ratio.

Solvent selection matters enormously for absorption. The ideal solvent has:

• High solubility for the target gas
• Good selectivity (absorbs what you want, not everything else)
• Easy regeneration (so you can strip and recycle it)

Common solvents include water, alkanolamines (MEA, DEA) for acid gas removal, and physical solvents like Selexol and Rectisol for high-pressure applications.

Optimization Methods and Tools

Finding the best operating conditions involves balancing separation efficiency against energy consumption and equipment costs. The main approaches are:

1. Process simulation using software like Aspen Plus, HYSYS, or ProMax to model performance across a range of conditions.
2. Pilot-scale experiments to validate simulation predictions and provide real data for scale-up.
3. Optimization studies using mathematical models and algorithms to find conditions that meet a specific objective (minimize energy use, maximize purity, or minimize total cost) subject to constraints like maximum pressure drop or minimum separation efficiency.

Performance Evaluation of Absorption vs. Stripping Processes

Key Performance Parameters

Several parameters quantify how well an absorption or stripping column performs:

• NTU (Number of Transfer Units): Represents the difficulty of the separation. Higher NTU means a harder separation requiring a taller column.
• HTU (Height of a Transfer Unit): The column height needed per transfer unit. Depends on mass transfer coefficients and flow conditions.
• Separation factor ($\alpha$): The ratio of equilibrium constants (K-values) for the components being separated. A higher $\alpha$ means the components are easier to separate.
• Absorption factor ($A$): A dimensionless ratio that relates the liquid flow rate, gas flow rate, and equilibrium. It's defined as:

$A = \frac{L}{mG}$

where $L$ is the liquid molar flow rate, $G$ is the gas molar flow rate, and $m$ is the slope of the equilibrium line. This parameter determines the minimum L/G ratio needed for a given separation.

• Stripping factor ($S$): The analogous parameter for stripping, essentially the inverse of the absorption factor ($S = mG/L$).

Performance Evaluation and Troubleshooting

Beyond these parameters, you'll also track removal efficiency, product purity, pressure drop, and energy consumption. These indicators help you compare designs, tune operating conditions, and diagnose problems.

Common issues and what they suggest:

Troubleshooting follows a straightforward process:

1. Collect and analyze operating data (temperatures, pressures, flow rates, compositions).
2. Compare actual performance against design specifications.
3. Identify the root cause by checking which parameters deviate from design values.
4. Adjust operating conditions, clean or replace internals, or modify the process design as needed.
5. Validate proposed fixes with simulations or pilot tests before full-scale implementation.