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Adsorption

Adsorption. Physical vs chemical adsorption Sorbent materials Mechanism Isotherm Effects of humidity Fixed-bed systems Regeneration Rotary bed and fluidized bed systems Pressure drop. Reading: Chap. 12. Definition.

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Adsorption

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  1. Adsorption • Physical vs chemical adsorption • Sorbent materials • Mechanism • Isotherm • Effects of humidity • Fixed-bed systems • Regeneration • Rotary bed and fluidized bed systems • Pressure drop Reading: Chap. 12 Aerosol & Particulate Research Lab

  2. Definition The concentration of gases, liquids or dissolved substances (adsorbate) on the surface of solids (adsorbent) Q: Examples in your daily life? Physical vs Chemical Physical Adsorption (van der Waals adsorption): weak bonding of gas molecules to the solid; exothermic (~ 0.1 Kcal/mole); reversible Chemisorption: chemical bonding by reaction; exothermic (10 Kcal/mole); irreversible Aerosol & Particulate Research Lab

  3. Activated Carbon Activated Alumina Silica Gel Molecular Sieves (zeolite) Sorbent Materials Polar and Non-polar adsorbents http://www.activatedcarbonindia.com/activated_carbon.htm Q: If an activated carbon granule is a solid particle with 1 mm diameter, what is the specific surface area (r = 0.7 g/cm3)? Air Pollution Engineering Manual., 1992 Aerosol & Particulate Research Lab

  4. Q: What if it is heated over 250 oC? H2O H2O H2O O OH OH OH OH heating hydrophobic hydrophilic Aerosol & Particulate Research Lab

  5. Air Pollution Engineering Manual., 1992 • Crystalline zeolite • Uniform pores to selectively separate compounds by size & shape Aerosol & Particulate Research Lab

  6. Adsorption Mechanism Aerosol & Particulate Research Lab

  7. Langmuir Isotherm Adsorption Isotherm: the mass of adsorbate per unit mass of adsorbent at equilibrium & at a given temperature Rate of adsorption (f: fraction of surface area covered) Rate of desorption f At equilibrium 1-f ( m: mass of adsorbate adsorbed per unit mass of adsorbent) Mono-layer coverage Aerosol & Particulate Research Lab

  8. Langmuir Isotherm ( p: partial pressure of the adsorbate) m Q: Low P? High P? p Aerosol & Particulate Research Lab

  9. Freundlich Isotherm Q: Calculate the equilibrium adsorptivity of 1000 ppm toluene in air on 4X10 mesh activated carbon at 298 K and 1 atm. Aerosol & Particulate Research Lab

  10. Effects of Humidity Amount of trichloroethylene adsorbed as a function of relative humidity Isotherm for toluene & trichloroethylene and water vapor (individual) Q: How can we adjust the system to reduce the impact of humidity? Aerosol & Particulate Research Lab

  11. Fixed-Bed Adsorption System Q: How will the OUTLET concentration as a function of TIME look like? Isotherm!!! Aerosol & Particulate Research Lab

  12. Aerosol & Particulate Research Lab

  13. Regeneration Q: In addition to steam, what else can we use? Q: Typically only 30 ~ 40% of the equilibrium isotherm is used. Why is that? Theodore & Buonicore, 1988 Aerosol & Particulate Research Lab

  14. Q: How will you select the regeneration time? • A well-designed system has steam consumption in the range of 1 to 4 lb of steam/lb of recovered solvent or 0.2 to 0.4 lb of steam/lb of carbon • In a continuous operation, a minimum of 2 adsorption units is required. Q: Three-units? Any advantage? Aerosol & Particulate Research Lab

  15. Rotary Bed System Mycock et al., 1995 Aerosol & Particulate Research Lab

  16. Fluidized-Bed System Q: Benefits? Aerosol & Particulate Research Lab

  17. Pressure Drop P: pressure drop (lb/ft2) D: bed depth (ft) e : void fraction G’: gas mass flux (lb/ft2-hr) mg: gas viscosity (lb/ft-hr) dp: carbon particle diameter (ft) Typical operating range: < 20 in H2O; 20 < V < 100 ft/min ==> determine the Maximum Adsorbent Bed Depth Q: Why? Aerosol & Particulate Research Lab

  18. Union Carbide Empirical Equation P: bed pressure drop, in H2O V: gas velocity, ~60-140 ft/min D: bed depth, ~5-50 inches dp: 4X6 mesh sized carbon Minimum Adsorbent Bed Depth Need to be at least longer than the MTZ CB: breakthrough capacity % CS: saturation capacity % XS: degree of saturation in the MTZ (usually 50%) D: bed depth Aerosol & Particulate Research Lab

  19. Other Systems: Nongenerable Canister adsorber Thin-bed adsorber Q: What need to be known to start the design of an adsorption bed system? Mycock et al., 1995 Aerosol & Particulate Research Lab

  20. Exercise I • An exhaust stream contains 1880 ppm of n-pentane at 95 oF. The flow rate to be treated is 5500 acfm. Carbon capacity is 3.5 lb n-pentane/100 lb AC. Carbon density is 30 lb/ft3. 2-bed system: 1 hr for adsorption and the other hr for regeneration. • Q: Mass flow rate of n-pentane? Volume of carbon bed? Flow velocity? Steam requirement? Pressure drop? Aerosol & Particulate Research Lab

  21. Exercise II • Conditions: 10,000 acfm of air @ 77 oF at 1 atm containing 2000 ppm toluene (MW = 92) to be treated. 95% removal efficiency by 4X10 mesh carbon expected (density of carbon = 30 lb/ft3) • Q: how many lb/hr of toluene to be removed? • Q: If regeneration at 212 oF, what’s the working capacity? • Q: Design an adsorption system with max DP of 8 inH2O, 4 hr cycle, two beds. Aerosol & Particulate Research Lab

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  24. Quick Reflection Aerosol & Particulate Research Lab

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