10.1 Absorption and Stripping
4 min read•august 13, 2024
Absorption and stripping are key mass transfer processes used in chemical engineering. They involve the exchange of components between gas and liquid phases, with absorption removing gases from streams and stripping removing volatiles from liquids.
These processes are crucial for pollution control, product purification, and resource recovery. Understanding the principles and design considerations for absorption and stripping columns is essential for effective mass transfer operations in industrial settings.
Principles and Applications of Absorption and Stripping
Absorption Process
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- Absorption removes a soluble component from a gas stream by dissolving it in a liquid solvent
- The gas-liquid contact occurs in an absorption column or tower
- The for mass transfer in absorption is the concentration gradient between the gas and liquid phases
- The solubility of the component in the liquid phase and its partial pressure in the gas phase determine the equilibrium distribution
Stripping Process
- Stripping removes a volatile component from a liquid stream by contacting it with a gas (typically steam or air)
- The liquid-gas contact takes place in a stripping column
- Stripping is the reverse process of absorption
- The driving force for mass transfer in stripping is the concentration gradient between the liquid and gas phases
Applications of Absorption
- Removing pollutants from gas streams (CO2 from natural gas, SO2 from flue gases)
- Recovering valuable components from gas streams
- Purifying gases
- Absorbing ammonia in water to produce aqueous ammonia
Applications of Stripping
- Removing volatile impurities from liquid streams (oxygen from boiler feed water)
- Recovering solvents from liquid streams
- Desorbing gases from liquids
- Recovering volatile organic compounds (VOCs) from wastewater
- Regenerating rich solvents in gas sweetening processes
Factors Affecting Column Performance
Operating Conditions
- Temperature affects gas solubility in liquids and removal of volatile components
- Higher temperatures reduce gas solubility in absorption, decreasing efficiency
- Higher temperatures favor the removal of volatile components in stripping
- Pressure influences partial pressure of solute gas and solubility
- Higher pressures increase solute gas partial pressure and solubility in absorption, improving efficiency
- Lower pressures promote the removal of volatile components in stripping
- Gas and liquid flow rates impact contact time and interfacial area for mass transfer
- Higher gas flow rates in absorption increase gas-liquid contact but may cause flooding
- Higher liquid flow rates in stripping enhance volatile component removal but may lead to entrainment
Column Design Parameters
- Column dimensions (height and diameter) affect residence time and gas-liquid contact
- Taller columns provide more contact time and improve separation efficiency
- Larger diameters accommodate higher flow rates
- Packing characteristics (type, size, surface area) influence interfacial area and mass transfer rates
- Structured packings offer high surface area and low pressure drop
- Random packings are cheaper and easier to install
- Tray designs (sieve trays, valve trays) impact gas-liquid contact and mass transfer efficiency
Design and Sizing of Absorption and Stripping Columns
Design Procedure
- Define the separation task (feed composition, desired product purity, flow rates)
- Obtain physical properties of components (solubility, diffusivity, equilibrium data)
- Use equilibrium-based methods (McCabe-Thiele, Kremser equation) for preliminary sizing
- These methods assume ideal stages and neglect mass transfer limitations
- Employ rate-based methods (NTU-HTU, ) for more accurate sizing
- These methods consider mass transfer resistances, interfacial area, and driving forces
- Determine column height based on the number of theoretical stages or transfer units required
- Calculate column diameter based on gas and liquid flow rates, preventing flooding or entrainment
- Perform pressure drop calculations to ensure acceptable operating limits
- Select packing or trays based on capacity, efficiency, pressure drop, and cost
Simulation and Optimization
- Use simulation software (Aspen Plus, ProMax) for rigorous modeling and optimization
- Consider complex thermodynamics, mass transfer, and hydraulic behavior in simulations
- Optimize operating conditions, column dimensions, and internals for improved performance and cost-effectiveness
Efficiency and Effectiveness of Absorption and Stripping Operations
Performance Indicators
- Removal efficiency quantifies the percentage of the target component removed from the feed stream
- Calculated as the ratio of the amount of component removed to the amount in the feed
- Recovery percentage measures the fraction of the target component recovered in the desired product stream
- Indicates the effectiveness of the separation process in terms of product yield
- Separation factor compares the relative concentrations of the target component in the product and waste streams
- Higher separation factor indicates better separation performance
Energy Consumption and Solvent Selection
- Energy consumption is mainly associated with pumping fluids and regenerating solvents
- Minimize energy usage through process optimization and heat integration
- Solvent selection plays a crucial role in absorption effectiveness
- Ideal solvents have high selectivity, high capacity, low volatility, low viscosity, and easy regeneration
- Solvents should be environmentally friendly and cost-effective
Environmental Impact and Economic Analysis
- Environmental impact assessment considers emissions, waste generation, and resource consumption
- Minimize environmental footprint through proper solvent management, emission control, and waste treatment
- Techno-economic analysis evaluates the feasibility and profitability of absorption and stripping operations
- Consider capital costs, operating expenses, and revenue generation
- Helps in making informed decisions regarding process implementation and optimization
Key Terms to Review (18)
Aeration: Aeration is the process of introducing air or oxygen into a liquid or solid, which is essential for various biological and chemical processes. This technique is widely used in environmental engineering, particularly in the treatment of wastewater and in processes like absorption and stripping, where the transfer of gases is crucial for enhancing reaction rates and improving system efficiency.
Amine Solvents: Amine solvents are organic compounds that contain one or more amino groups (-NH2, -NHR, or -NR2) and are commonly used in processes such as absorption and stripping to selectively interact with specific solutes. They are particularly effective for capturing carbon dioxide and other acidic gases due to their ability to form chemical bonds, making them essential in various industrial applications including gas treatment and separation processes.
Bubble Point: The bubble point is the temperature at which a liquid mixture begins to form vapor bubbles at a given pressure. This point is crucial in understanding phase equilibrium and helps determine how mixtures behave during processes like absorption, stripping, distillation, and extraction. The bubble point plays a significant role in the efficiency of separation processes, indicating the conditions under which a mixture can start vaporizing.
Driving Force: In the context of mass transfer processes, the driving force refers to the gradient that causes a substance to move from one area to another. This can be a difference in concentration, temperature, or pressure that propels the movement of species during processes like absorption and stripping, ultimately facilitating the transfer of materials between phases.
Gas Absorption: Gas absorption is the process where a gas interacts with a liquid, leading to the transfer of one or more components from the gas phase into the liquid phase. This process is crucial in various industrial applications, such as pollution control and the extraction of valuable compounds from gases. By utilizing different solvents and operational conditions, the efficiency of gas absorption can be enhanced to meet specific requirements in chemical processes.
Henry's Law: Henry's Law states that the amount of gas dissolved in a liquid at a constant temperature is directly proportional to the partial pressure of that gas above the liquid. This law is crucial in understanding processes where gases are absorbed into liquids and can be applied to analyze how gases behave in various industrial operations.
Liquid stripping: Liquid stripping is a process used to remove solutes from a liquid phase, typically involving a counter-current flow where a stripping agent (often a gas) is introduced to facilitate the transfer of the solute from the liquid into the gas phase. This method is crucial in separating desired components from mixtures, enhancing the purity and efficiency of various industrial processes.
Mass transfer coefficient: The mass transfer coefficient is a measure of the mass transfer rate per unit area per unit concentration difference, which quantifies how effectively mass is transferred between phases or within a single phase. It plays a vital role in understanding processes such as convection, diffusion, and phase change, providing insights into how efficiently substances move from one location to another under different conditions.
Natural gas processing: Natural gas processing refers to the series of operations and technologies used to separate impurities and other hydrocarbons from raw natural gas to make it suitable for commercial use. This process not only removes contaminants like water, carbon dioxide, and hydrogen sulfide but also separates valuable byproducts such as ethane, propane, and butane, which can be further utilized in various industrial applications.
Overall mass transfer efficiency: Overall mass transfer efficiency refers to the effectiveness with which a component is transferred from one phase to another during processes like absorption and stripping. It is an important metric that combines both the physical properties of the components involved and the design of the equipment used in these processes, influencing how effectively the mass transfer occurs.
Packed Column: A packed column is a type of equipment used in various separation processes, particularly for gas-liquid contact applications such as absorption and stripping. It consists of a vertical cylindrical vessel filled with packing material that provides a large surface area for mass transfer between the gas and liquid phases, promoting efficient interaction and transfer of components. This design allows for enhanced contact time and improved separation efficiency in processes where the transfer of mass is crucial.
Packing Material: Packing material refers to the solid or porous substances placed within absorption or stripping columns to facilitate the mass transfer of components between phases. This material enhances contact area and promotes efficient interaction between gas and liquid phases, which is crucial for effective separation processes in chemical engineering.
Pollutant removal: Pollutant removal refers to the process of extracting or reducing harmful substances from air, water, or soil to improve environmental quality and protect public health. This process is essential in various applications, particularly in industrial settings, where it helps minimize the impact of emissions and discharges on the environment. Effective pollutant removal techniques ensure compliance with environmental regulations and contribute to sustainable practices.
Raoult's Law: Raoult's Law states that the partial vapor pressure of each component in an ideal solution is equal to the vapor pressure of that pure component multiplied by its mole fraction in the solution. This law helps in understanding how components interact during phase changes and is crucial for processes involving mass transfer, such as absorption, stripping, distillation, and extraction.
Stage efficiency: Stage efficiency refers to the effectiveness of a single stage in a mass transfer operation, such as absorption or stripping, in achieving the desired separation or transfer of components. It is expressed as the ratio of the actual mass transfer performance of the stage to the maximum possible performance if it were ideal. This term connects deeply to how well a system performs under given conditions, influencing design and operational decisions.
Tray column: A tray column is a vertical vessel used in processes like absorption and stripping, featuring horizontal trays that allow for the contact between gas and liquid phases. The trays facilitate mass transfer by enabling the upward flow of gas and the downward flow of liquid, promoting interactions that are essential for separating components based on their volatility or solubility.
Vapor-liquid equilibrium: Vapor-liquid equilibrium (VLE) is the condition where a vapor and its corresponding liquid phase coexist at a specific temperature and pressure. At this point, the rate of evaporation of the liquid equals the rate of condensation of the vapor, resulting in stable concentrations of both phases. This balance is essential in processes involving absorption and stripping, as it governs how substances transfer between phases and influences overall system efficiency.
Water as an absorbent: Water as an absorbent refers to its ability to dissolve and hold various substances, particularly gases, in a process called absorption. This property makes water a vital component in various chemical processes, particularly in the removal of contaminants from gases through mass transfer operations. Its unique characteristics enable efficient absorption mechanisms, leading to applications in industries such as environmental engineering and chemical manufacturing.