🍳Separation Processes Unit 5 – Absorption and Stripping

Key Concepts

• Absorption involves the transfer of a solute from a gas phase to a liquid phase
• Stripping is the reverse process, transferring a solute from a liquid phase to a gas phase
• Mass transfer is driven by the concentration gradient between the phases
• Equilibrium is reached when the concentration of the solute in both phases is equal
• Henry's law describes the relationship between the partial pressure of a gas and its solubility in a liquid
• The mass transfer coefficient quantifies the rate of mass transfer between phases
• The interfacial area is the surface area available for mass transfer to occur
• The concept of theoretical stages is used to analyze and design absorption and stripping columns

Fundamentals of Mass Transfer

• Mass transfer occurs due to the concentration difference between two phases
• Fick's law describes the diffusion of a solute through a medium driven by the concentration gradient
• The rate of mass transfer is proportional to the concentration gradient and the interfacial area
• The mass transfer coefficient is influenced by factors such as temperature, pressure, and fluid properties
• The two-film theory assumes that mass transfer resistance occurs in thin films on either side of the interface
• The penetration theory considers the unsteady-state diffusion of solute into the liquid phase
• The surface renewal theory assumes that the liquid surface is continuously replaced by fresh liquid elements
• The choice of mass transfer model depends on the specific system and operating conditions

Absorption Theory

• Absorption occurs when a solute is transferred from the gas phase to the liquid phase
• The solubility of the solute in the liquid phase is a key factor in absorption efficiency
• Henry's law relates the partial pressure of a gas to its concentration in the liquid phase at equilibrium
• The absorption rate is influenced by the gas-liquid interfacial area and the mass transfer coefficient
• The liquid phase is usually chosen to have a high solubility for the solute and low volatility
• Countercurrent flow of gas and liquid enhances the concentration gradient and improves absorption efficiency
  • The gas enters at the bottom of the column and flows upward
  • The liquid enters at the top of the column and flows downward
• The absorption factor (A) is a dimensionless parameter that relates the liquid and gas flow rates and the equilibrium curve

Stripping Theory

• Stripping involves the transfer of a solute from the liquid phase to the gas phase
• The driving force for stripping is the concentration difference between the liquid and gas phases
• Stripping is often used to remove volatile components from a liquid mixture
• The stripping factor (S) is a dimensionless parameter that relates the gas and liquid flow rates and the equilibrium curve
• The relative volatility of the solute compared to the liquid determines the ease of stripping
• Increasing the temperature or decreasing the pressure can enhance stripping by increasing the vapor pressure of the solute
• Steam stripping is a common technique where steam is used as the stripping gas
  • The steam provides heat for vaporization and acts as a carrier gas for the stripped solute

Equipment and Design

• Absorption and stripping are typically carried out in columns or towers
• Packed columns are filled with random or structured packing materials to increase the interfacial area
  • Examples of random packing include Raschig rings, Pall rings, and Berl saddles
  • Structured packing, such as Mellapak and Sulzer packing, provides more uniform flow distribution
• Tray columns use a series of perforated trays to create stages for mass transfer
  • Sieve trays have perforations that allow gas to pass through the liquid on the tray
  • Bubble-cap trays use caps over the perforations to create a bubbling action
• The choice between packed and tray columns depends on factors such as capacity, pressure drop, and fouling tendency
• The height equivalent to a theoretical plate (HETP) is used to compare the efficiency of different packing materials
• The column diameter is determined based on the desired gas and liquid flow rates and the allowable pressure drop

Operating Conditions and Parameters

• The operating temperature and pressure affect the solubility, diffusivity, and mass transfer rates
• Higher temperatures generally increase the diffusivity and mass transfer coefficients but may reduce solubility
• Higher pressures increase the solubility of gases in liquids according to Henry's law
• The gas-to-liquid ratio (G/L) is an important parameter that influences the absorption or stripping efficiency
  • A higher G/L ratio favors stripping, while a lower G/L ratio favors absorption
• The liquid and gas flow rates determine the residence time and the extent of mass transfer
• The choice of solvent for absorption should consider solubility, selectivity, stability, and regeneration ease
• The presence of impurities or contaminants can affect the mass transfer performance and require pretreatment

Calculations and Problem-Solving

• Material balances are used to determine the flow rates and compositions of the streams entering and leaving the column
• Equilibrium data, such as Henry's law constants or vapor-liquid equilibrium curves, are required for design calculations
• The number of theoretical stages can be determined using graphical methods like the McCabe-Thiele method or analytical methods like the Kremser equation
• The height of a transfer unit (HTU) and the number of transfer units (NTU) are used to calculate the column height
  • HTU represents the height of packing required for one transfer unit
  • NTU is the number of transfer units required to achieve the desired separation
• The overall mass transfer coefficient (KGa or KLa) is used to calculate the rate of mass transfer
• The pressure drop across the column is estimated using empirical correlations or packing manufacturer data
• Energy balances are necessary to account for heat effects, such as heat of absorption or heat of vaporization

Industrial Applications

• Absorption is widely used in the chemical, petrochemical, and environmental industries for gas purification and separation
• Examples of absorption processes include:
  • Removal of carbon dioxide from natural gas using amine solutions
  • Removal of hydrogen sulfide from refinery gas streams using alkaline solutions
  • Absorption of volatile organic compounds (VOCs) using organic solvents
• Stripping is used for the recovery of valuable components or the removal of undesired components from liquid streams
• Examples of stripping processes include:
  • Removal of oxygen from boiler feedwater to prevent corrosion
  • Recovery of ammonia from wastewater using steam stripping
  • Stripping of volatile organic compounds (VOCs) from contaminated groundwater
• The choice of absorption or stripping depends on the specific application, the nature of the components, and the desired purity
• Process simulators, such as Aspen Plus or HYSYS, are commonly used for the design and optimization of absorption and stripping processes in industry