10.1 Absorption and Stripping

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 driving force 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, mass transfer coefficient) 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