ADSORPTION

1. ADSORPTION Adsorption is the process in which matter is extracted from one phase and concentrated at the surface of a second phase. (Interface accumulation). This is a surface phenomenon as opposed to absorption where matter changes solution phase, e.g. gas transfer. This is demonstrated in the following schematic. If we have to remove soluble material from the solution phase, but the material is neither volatile nor biodegradable, we often employ adsorption processes. adsorbate: material being adsorbed adsorbent: material doing the adsorbing. (examples are activated carbon or ion exchange resin). In a typical adsorption process, a gaseous, liquid or solid molecule (theadsorptive) attached to a solid or liquid surface, the adsorbent, andforms the adsorbate, a complex between adsorptive and adsorbent.

2. d • Adsorption is the adhesion of atoms, ions, or molecules from a gas,liquid, or dissolved solid to a surface. • This process creates a film of the adsorbate on the surface ofthe adsorbent. • This process differs from absorption, in which a solute isdissolved by a liquid or solid (the absorbent). • Adsorption is a surface-based process while absorption involves thewhole volume of the material.

5. Surface Energy • Similar to surface tension, adsorption happen due to residual surface energy. • Inside matter, all the bonding requirements of atoms of the materialare filled by other atoms . • However, atoms on the surface are not completely surrounded by other atoms and therefore they can attract adsorbates. These surface molecules have additional energy to balance the forces. It takes energy to put molecules on the surface, since at least one of the interior bonds must be broken to get the molecule to the surface. This excess energy is called surface tension. Since it takes energy to create interfacial surfaces, the system will try to minimize the total interfacial surface area. Hence we see spherical droplet, meniscus etc.

6. Adsorption Mechanism • 2) Chemical adsorption • Results from a chemical interaction between the adsorbate and adsorbent. Therefore formed bond is much stronger than that for physical adsorption • Heat liberated during chemisorption is in the range of 20-400 kj/g mole

7. Adsorption Types • Exchange adsorption (ion exchange)– electrostatic due to charged sites on the surface. Adsorption goes up as ionic charge goes up and as hydrated radius goes down. • Physical adsorption: Van der Waals attraction between adsorbate and adsorbent. The attraction is not fixed to a specific site and the adsorbate is relatively free to move on the surface. This is relatively weak, reversible, adsorption capable of multilayer adsorption. • Chemical adsorption: Some degree of chemical bonding between adsorbate and adsorbent characterized by strong attractiveness. Adsorbed molecules are not free to move on the surface. There is a high degree of specificity and typically a monolayer is formed. The process is seldom reversible.     

8. Adsorption Process • Classified as Physical and Chemical Ads. • 1) Physical adsorption • The gas molecules adhere to the surface of the solid adsorbent as a result of intermolecular attractive forces (van der Waals forces) between them • The process is exothermic: the heat liberated is in the order of the the enthalpy of condensation of vapor (2-20 kJ/gmole) • The process is reversible (recovery of adsorbent material or adsorbed gas is possible) by increasing the temperature or lowering the adsorbate conc. • Physical adsorption usually directly proportional to the amount of solid surface area • Adsorbate can be adsorbed on a monolayer or a number of layers • The adsorption rate is generally quite rapid

9. The effect of molecular size and functional groups on adsorbability on activated carbon

10. 2) Chemical adsorption • Results from a chemical interaction between the adsorbate and adsorbent. Therefore formed bond is much stronger than that for physical adsorption • Heat liberated during chemisorption is in the range of 20-400 kj/g mole • It is frequently irreversible. On desorption the chemical nature of the original adsorbate will have undergone a change. • Only a monomolecular layer of adsorbate appears on the adsorbing medium

11. Virtually every solid surface has the capacity to adsorb solutes.

12. Activated Carbon Activated Alumina Silica Gel Molecular Sieves (zeolite) Sorbent Materials Polar and Non-polar adsorbents 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)?

13. Q: What if it is heated over 250 oC? H2O H2O H2O O OH OH OH OH heating hydrophobic hydrophilic

15. MANUFACTURE OF ACTIVATED CARBON: DIFFERENT RAW MATERIALS AC prepared from many sources: • Wood • Lignite • Coal • Nutshells • Bone

16. These raw materials are pyrolyzed at high temperature under low oxygen conditions (so we don’t get complete combustion). This forms a “char”. The char is then activated by heating to 300 – 1000 oC in the presence of steam, oxygen or C02. Result: “Activated carbon” which is highly porous, micro-crystalline material which resembles graphite plates with some specific functional groups (e.g. COOH, OH)

17. MANUFACTURE OF ACTIVATED CARBON FROM COAL

18. Multiple Hearth ↑ ACTIVATION FURNACE TYPES Rotary kiln

19. DIFFERENT PHYSICAL FORMS OF ACTIVATED CARBON

20. Surface area of the AC is huge. Most of the surface area is interior in micro- and macropores. Typical surface area is in the range of 300-1500 m2/gram. Quality and hardness of the AC are a function of the starting material and the activation process.

21. Pore size distribution: micropores: <2nm dia mesopores: 2nm to 20 nm macropores: > 20 nm Increasing magnification

23. Area of a few grams of activated carbon

24. Application Methodology

25. Granular Activated Carbon Columns

26. Gas Purification

27. Thermodynamics of surface adsorption • In solutions certain particles tend to concentrate at the surface. These particles are those that have low affinity for the water (solvent). These are hydrophobic molecules. Because they have low affinity for the solvent the can get to the surface easily since they have low bond energy in the bulk phase. The water system prefers to have these molecules at the surface because the placement at the surface requires less energy than a water molecule -- hydrophobic molecules decrease surface energy (surface tension) relative to a pure watersystem. • On the other hand if a particle has a high affinity for the solvent phase (hydrophilic) it will tend to remain in the bulk solution because of its strong bond with water. In fact, this bonding makes the water bonding stronger and, therefore, there is a larger energy required to get water molecules to the surface-- therefore, hydrophilic molecules increase surface tension, e.g. salts such as NaCl. • As particles concentrate at surface there becomes a "surface excess". Surface excess is defined as G. • isthe amount of solute adsorbed at the surface per unit surface area in excess of bulk concentration • units = moles/cm2

28. Surface excess can be defined as: Where "Volume" is the volume of the solution from which the adsorption is occurring onto the surface with total surface area = "surface area". A more fundamental definition is given by the Gibbs relationship. where: mi = the molar free energy of solute i. Ci is the bulk concentration of this solute.

29. The Gibb’s expression simply uses G as a proportionality constant to relate the change in solute molar free energy to surface tension (g) during adsorption. The underlying principle here is that for the adsorption process changes in the sum of all solute free energy must be accounted for in changes in the surface tension during the adsorption process. For a single solute:

30. Therefore: results in increases in G (surface concentration) results in decrease in G

31. ADSORPTION EQUILIBRIA If the adsorbent and adsorbate are contacted long enough an equilibrium will be established between the amount of adsorbate adsorbed and the amount of adsorbate in solution. The equilibrium relationship is described by isotherms. Define the following: qe = mass of material adsorbed (at equilibrium) per mass of adsorbent. Ce= equilibrium concentration in solution when amount adsorbed equals qe. qe/Ce relationships depend on the type of adsorption that occurs, multi-layer, chemical, physical adsorption, etc.

32. Some general isotherms are shown in the figure below.

33. Isotherm models: The figures below show that there are four common models for isotherms.

34. Langmuir Adsorption Isotherm In the Langmuir model the adsorbent surface is considered to possess a number of active interaction sites for adsorption. Langmuir derived a relation between adsorbed material and its equilibrium concentration. His assumption are: There are fixed adsorption sites on the surface of the adsorbent. At a given T&P some fraction of these sites are occupied by adsorbate molecules. Let this fraction be  Each site on the surface of the adsorbent can hold one adsorbate molecule. The heat of adsorption is the same for each site and is independent of . There is no interaction between molecules on different sites.

35. Surface is tilted w.r.t. to dense crystal planes • Atomically rough • TSK model - terrace-step-kink Terrace Step Kink

36. Adsorbed phase consists of species localised in a fraction  of the available sites. • No interaction between species on adjacent sites (i.e. random occupancy)

37. 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

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

40. Freundlich Isotherm Q: Calculate the equilibrium adsorptivity of 1000 ppm toluene in air on 4X10 mesh activated carbon at 298 K and 1 atm.

41. 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?

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

43. Aerosol & Particulate Research Lab

44. Four GAC columns in series

46. REACTIVATION OF ACTIVATED CARBON

47. 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

48. 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

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

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