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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) 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 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 Tennis court in a bean! Aerosol & Particulate Research Lab

  4. 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 Low P? High P? p Aerosol & Particulate Research Lab

  9. Freundlich Isotherm Calculate the equilibrium adsorptivity of 1000 ppm toluene in air on 4×10 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) How can we adjust the system to reduce the impact of humidity? Aerosol & Particulate Research Lab

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

  12. Aerosol & Particulate Research Lab

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

  14. 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. Three-units? Any advantage? Aerosol & Particulate Research Lab

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

  16. Fluidized-Bed System 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. 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 Breakthrough time Vs: saturation velocity s: carbon bed density Ce: pollutant concentration Ac: carbon bed cross-section X: adsorption capacity (isotherm, m) area Aerosol & Particulate Research Lab

  19. Other Systems: Nongenerable Canister adsorber Thin-bed adsorber 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 A gas stream flowing at 1 m3/s with a VOC contaminant at a concentration of 100 mg/m3 is to be treated in an activated carbon adsorption column that is 0.5 m in diameter and 2 m long. Determine the breakthrough time (tB) if the sorbent capacity is 120 g VOC per kg carbon. You may assume the packed density of the carbon is 300 kg/m3 and the volumes treated at breakthrough and at capacity are 85% and 95 % of the maximum sorbent capacity, respectively.

  22. Quick Reflection Aerosol & Particulate Research Lab

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