Adsorption 2018/7/1

Definition Physical vs Chemical The concentration of gases, liquids or dissolved substances (adsorbate) on the surface of solids (adsorbent) 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 2018/7/1

Properties of Activated Carbon Sorbent Materials Activated Carbon Activated Alumina Silica Gel Molecular Sieves (zeolite) Polar and Non-polar adsorbents Properties of Activated Carbon Bulk Density 22-34 lb/ft3 Heat Capacity 0.27-0.36 BTU/lboF Pore Volume 0.56-1.20 cm3/g Surface Area 600-1600 m2/g Average Pore Diameter 15-25 Å Regeneration Temperature (Steaming) 100-140 oC Maximum Allowable Temperature 150 oC 2018/7/1

Properties of Activated Alumina Properties of Molecular Sieves Bulk Density Granules 38-42 lb/ft3 Pellets 54-58 lb/ft3 Specific Heat 0.21-0.25 BTU/lboF Pore Volume 0.29-0.37 cm3/g Surface Area 210-360 m2/g Average Pore Diameter 18-48 Å Regeneration Temperature (Steaming) 200-250 oC Maximum Allowable Temperature 500 oC Properties of Molecular Sieves Anhydrous Sodium Aluminosilicate Anhydrous Calcium Aluminosilicate Anhydrous Aluminosilicate Type 4A 5A 13X Density in bulk (lb/ft3) 44 38 Specific Heat (BTU/lboF) 0.19 - Effective diameter of pores (Å) 4 5 13 Regeneration Temperature (oC) 200-300 Maximum Allowable Temperature (oC) 600 Crystalline zeolite Uniform pores to selectively separate compounds by size & shape 2018/7/1

Properties of Silica Gel Bulk Density 44-56 lb/ft3 Heat Capacity 0.22-0.26 BTU/lboF Pore Volume 0.37 cm3/g Surface Area 750 m2/g Average Pore Diameter 22 Å Regeneration Temperature 120-250 oC Maximum Allowable Temperature 400 oC Q: What if it is heated over 250 oC? H2O H2O H2O OH OH OH O OH heating hydrophobic hydrophilic 2018/7/1

Adsorption Mechanism 2018/7/1

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 Mono-layer coverage ( m: mass of adsorbate adsorbed per unit mass of adsorbent) 2018/7/1

Langmuir Isotherm ( p: partial pressure of the adsorbate) m p 2018/7/1

Aerosol & Particulate Research Lab For liquid-solid a = mass of adsorbed solute required to saturate completely a unit mass of adsorbent K = experimental constant Ce = equilibrium concentration 2018/7/1 Aerosol & Particulate Research Lab

Freundlich Isotherm For liquid-solid 2018/7/1

Effects of Humidity Amount of trichloroethylene adsorbed as a function of relative humidity Isotherm for toluene & trichloroethylene and water vapor (individual) 2018/7/1

Fixed-Bed Adsorption System Isotherm!!! 2018/7/1 Aerosol & Particulate Research Lab

Aerosol & Particulate Research Lab 2018/7/1 Aerosol & Particulate Research Lab

Aerosol & Particulate Research Lab 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? 2018/7/1 Theodore & Buonicore, 1988 Aerosol & Particulate Research Lab

Aerosol & Particulate Research Lab 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? 2018/7/1 Aerosol & Particulate Research Lab

Aerosol & Particulate Research Lab Rotary Bed System Mycock et al., 1995 2018/7/1 Aerosol & Particulate Research Lab

Aerosol & Particulate Research Lab Fluidized-Bed System Q: Benefits? 2018/7/1 Aerosol & Particulate Research Lab

Aerosol & Particulate Research Lab 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? 2018/7/1 Aerosol & Particulate Research Lab

Aerosol & Particulate Research Lab 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 2018/7/1 Aerosol & Particulate Research Lab

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 2018/7/1 Aerosol & Particulate Research Lab

Aerosol & Particulate Research Lab 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? 2018/7/1 Aerosol & Particulate Research Lab

Aerosol & Particulate Research Lab 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. 2018/7/1 Aerosol & Particulate Research Lab

Aerosol & Particulate Research Lab 2018/7/1 Aerosol & Particulate Research Lab

Aerosol & Particulate Research Lab 2018/7/1 Aerosol & Particulate Research Lab

Aerosol & Particulate Research Lab Quick Reflection 2018/7/1 Aerosol & Particulate Research Lab