5.1 Fundamentals of gas absorption and stripping

Gas absorption and stripping are crucial separation processes in chemical engineering. These techniques involve the transfer of solutes between gas and liquid phases, driven by concentration gradients and governed by principles like Henry's law and vapor-liquid equilibrium.

Equipment choices, operating conditions, and solvent selection greatly impact absorption and stripping performance. Factors like temperature, pressure, and interfacial area play key roles in optimizing these processes for applications ranging from natural gas purification to air pollution control.

Principles and Mechanisms of Gas Absorption and Stripping

Principles of gas absorption processes

Top images from around the web for Principles of gas absorption processes

• 

  physical chemistry - Understanding dissolution of CO₂ in Selexol and Rectisol - Chemistry Stack ... View original

  Is this image relevant?

• 

  21.8B: Henry’s Law - Medicine LibreTexts View original

  Is this image relevant?

• 

  Major Features of a Phase Diagram | Introduction to Chemistry View original

  Is this image relevant?

• 

  physical chemistry - Understanding dissolution of CO₂ in Selexol and Rectisol - Chemistry Stack ... View original

  Is this image relevant?

• 

  21.8B: Henry’s Law - Medicine LibreTexts View original

  Is this image relevant?

1 of 3

Top images from around the web for Principles of gas absorption processes

• 

  physical chemistry - Understanding dissolution of CO₂ in Selexol and Rectisol - Chemistry Stack ... View original

  Is this image relevant?

• 

  21.8B: Henry’s Law - Medicine LibreTexts View original

  Is this image relevant?

• 

  Major Features of a Phase Diagram | Introduction to Chemistry View original

  Is this image relevant?

• 

  physical chemistry - Understanding dissolution of CO₂ in Selexol and Rectisol - Chemistry Stack ... View original

  Is this image relevant?

• 

  21.8B: Henry’s Law - Medicine LibreTexts View original

  Is this image relevant?

1 of 3

• Mass transfer principles drive solute movement between phases
  • Concentration gradient creates driving force for diffusion
  • Solute molecules diffuse through stagnant films at interface
  • Interfacial mass transfer occurs at gas-liquid boundary
• Gas absorption process dissolves solute gas into liquid solvent
  • Solute transfers from bulk gas phase through interface into liquid
  • Rate depends on solubility, diffusivity, and interfacial area
• Stripping process removes dissolved gas from liquid phase
  • Reverses absorption, transferring solute from liquid to gas phase
  • Used to purify liquids or recover dissolved gases
• Henry's law relates partial pressure to concentration in solution
  • $p_A = H_A x_A$ where $p_A$ is partial pressure, $H_A$ is Henry's constant
  • Assumes dilute solution and moderate pressures (methane in water)
• Equilibrium considerations determine maximum absorption/stripping
  • Vapor-liquid equilibrium (VLE) data shows composition relationship
  • Operating line represents actual concentrations in column
  • Equilibrium curve shows theoretical maximum transfer possible

Factors in absorption performance

• Temperature affects solubility and mass transfer rates
  • Higher temps generally decrease gas solubility in liquids
  • But can increase diffusion and reaction rates (CO2 in amine solutions)
• Pressure influences driving force for mass transfer
  • Higher pressure typically promotes absorption of gases
  • Lower pressure enhances stripping and solute removal
• Gas-liquid contact area impacts overall transfer rate
  • Increased interfacial area improves absorption efficiency
  • Achieved through packing, trays, or spray nozzles
• Solvent selection crucial for process effectiveness
  • High solubility of target gas improves absorption
  • Selectivity allows separation of specific components (MEA for CO2)
• Flow rates and ratios affect column performance
  • Optimizing gas-to-liquid ratio maximizes efficiency
  • Too little solvent limits absorption, excess wastes energy
• Mass transfer coefficients quantify resistance to transfer
  • Gas-side resistance often dominates for sparingly soluble gases
  • Liquid-side resistance important for highly soluble species
  • Overall coefficient combines both resistances

Equipment for gas absorption

• Packed columns provide large contact area
  • Random packing (Raschig rings) or structured packing used
  • Liquid distributors ensure even flow over packing surface
• Tray columns use horizontal plates for gas-liquid contact
  • Sieve trays have simple perforations for gas flow
  • Bubble cap trays provide better liquid-gas mixing
  • Valve trays offer adjustable gas flow paths
• Spray towers atomize liquid for simple gas treatment
  • High throughput but limited mass transfer efficiency
  • Used for particle scrubbing or preliminary absorption
• Venturi scrubbers accelerate gas for particle removal
  • High relative velocity between gas and liquid droplets
  • Limited application in absorption due to short contact time
• Column internals support packing and distribute flow
  • Packing support plates prevent settling and channeling
  • Liquid collectors and redistributors maintain even wetting
• Flow configurations optimize contact between phases
  • Countercurrent maximizes driving force along column height
  • Cross-flow used in some specialized applications

Physical vs chemical absorption

• Physical absorption relies on solubility differences
  • Reversible process allows easier solvent regeneration
  • CO2 removal using cold methanol (Rectisol process)
• Chemical absorption involves reaction with solvent
  • Often achieves higher selectivity and absorption capacity
  • Amine scrubbing removes acid gases (CO2, H2S) from natural gas
• Absorption mechanisms differ between types
  • Physical: Van der Waals forces, hydrogen bonding dominate
  • Chemical: Covalent bonds form, acid-base reactions occur
• Regeneration approaches vary based on mechanism
  • Physical: Often use pressure or temperature swing
  • Chemical: May require more energy to reverse reactions
• Solvent selection criteria depend on absorption type
  • Physical: Focus on solubility parameters, volatility
  • Chemical: Reactivity with target components, stability
• Process design implications vary between types
  • Physical: Simpler equipment, often lower temperatures
  • Chemical: May need corrosion-resistant materials, heat management