Adsorption (part 1)

1. Adsorption(part 1) Instructor: Prof. Moo Been Chang Date: 2008/10/08 Graduate Institute of Environmental EngineeringNational Central University

2. Outline • What is the adsorption？ • Adsorption Isotherms • Adsorbent Material • The Application of Activated Carbon --VOCs control -- Dioxins (PCDD/Fs) control

3. What is the adsorption？

4. Adsorption Definition: the gases, liquids or dissolved substances (adsorbate) on the surface of solids (adsorbent) • Adsorption means the attachment of molecules to the surface of a solid. In contrast absorption means the dissolution of molecules within a collecting medium, which may be liquid or solid. • Generally, adsorbed materials are attached onto the surface of a material, like dust on a wall. Absorption mostly occurs into liquids, adsorption mostly onto solids.

5. Adsorption Requirements • adsorbent must have large surface areas, • adsorbent must have internal micropores and macropores (eg., activated carbon & zeolite), • selective adsorption (moisture must be avoided), • need good contact time for successful separation, • pre-treatment to lower gas composition required, • no mal-distribution of flow in the bed, • easy regeneration of bed should be possible, • continuous operation requires multiple beds in tandem.

6. Category of Adsorption Physisorption (Physical or van der Waals adsorptions): weak bonding of gas molecules to the solid exothermic (~ 0.1 kcal/mole) no physical or chemical changes reversible multilayer adsorption does not accompany catalysis Chemisorption chemical bonding by reaction exothermic (> 10 kcal/mole) adsorbent changes characteristics irreversible monolayer in most case definitely catalyzed

7. Adsorption Mechanism

8. Temperature Effect on Adsorption Q: Why adsorption capacity decreases when gas temperature is increased?

9. Thermodynamic Analysis According to 2nd law of thermodynamic △G = △H - T△S △G :Gibbs free energy； △H: enthaplpy ；△S : entropy Since the adsorption is a spontaneous reaction, so we can say that the entropy and Gibbs free energy are below 0.(△S＜0 ) (△G ＜0 ) . So the enthaplpy (△H) must be negative. (△H<0)

10. Thermodynamic Analysis The adsorption is an exothermic reaction. The higher temperature the lower efficiency ofadsorption

11. Adsorption Isotherms

12. Adsorption Isotherms Data relating adsorbed concentration (g/g of bed weight) to equilibrium gas phase concentration (g/ml of stream) is given in terms of adsorption isotherms. Wads = f (P,T) • Three common types of isotherms: • Langmuir • Freundlich • BET

13. Favorable and Unfavorable Adsorption

14. Langmuir Isotherm The earliest model of gas adsorption suggested by Langmuir (1916). The classical Langmuir model is limited to monolayer adsorption. It is assumed that gas molecules striking the surface have a given probability of adsorption. Molecules already adsorbed similarly have a given probability of desorption. At equilibrium, equal numbers of molecules desorb and adsorb at any time. The probabilities are related to the strength of the interaction between the adsorbent surface and the adsorbate gas.

15.  1- air adsorbate Langmuir Isotherm (cont’d) Rate of adsorption, Rate of desorption, At equilibrium, where, Wads = the mass of gas adsorbed at pressure P; Wmax = the mass of gas which covers the entire adsorbing surface with a monolayer; P= the partial pressure of interest in the gas phase;  = coverage; C = a constant for the gas/solid combination = ka/kd; ka = the adsorption rate coefficient; kd = the desorption rate coefficient.

16. 1 S = 1/Wmax P/Wads  C J = 1/CWmax 0 P P Langmuir Isotherm (cont’d) Some physisorption and most chemisoption processes follow this isotherm. It is the one with the best theoretical basis, which assumes that adsorption is limited to one monolayer on the surface. One can obtain the two constants by linearization of the isotherm:

17. Langmuir Isotherm (cont’d) It is particularly suited to represent binary and ternary systems.

18. Freundlich Isotherm The Fruendlich isotherm model is valid for heterogeneous surfaces, monolayer coverage. Common for most adsorption work since it fits almost all data. It is empirical in nature, although some theoretical foundations do exit.

19. n > 1 n = 1 Wads n < 1 P Freundlich Isotherm The expression： Wads = KFP1/n (KF and n are experimentally determined parameters) • When n = 1, the reaction is linear and called “partitioning”. • When n > 1, the reaction is said to be “favorable” as the incremental change in amount sorbed decreases with increasing concentrations. • While n < 1 is called “unfavorable” because the reverse is true. • Most natural adsorbents exhibit either linear or favorable adsorption. • The Langmuir and Fruendlich models for n < 1 are concave downwards, so both models can be calibrated to similar data..

20. n > 1 n = 1 log 1/n ln Wads Wads n < 1 ln KF P ln P Freundlich Isotherm (cont’d) lnWads = lnKF + 1/n lnP Wads = KFP1/n

21. Freundlich Isotherm Parameters Available for a wide variety of organic vapors on various activated carbon types Wads = KFP1/n

22. Brunauer-Emmett-Teller (BET) Isotherm • Brunauer, Emmett and Teller (BET) developed several models for gas adsorption on solids which have become the effective standard for surface area measurements. • BET isotherm is valid for multiple layers on homogeneous surfaces.

23. Brunauer-Emmett-Teller (BET) Isotherm The assumptions underlying the simplest BET isotherm are: Gas adsorbs on a flat, uniform surface of the solid with a uniform heat of adsorption due to van der Waals forces between the gas and the solid. There is no lateral interaction between the adsorbed molecules. After the surface has become partially covered by adsorbed gas molecules, additional gas can adsorb either on the remaining free surface or on top of the already adsorbed layer. The adsorption of the second and subsequent layers occurs with a heat of adsorption equal to the heat of liquefaction of the gas. multi-layers adsorption

24. Wads P BET Isotherm (cont’d) Work for almost any type of data on the adsorption of gases on solids. It describes every type of isotherm including the linear, and Langmuir isotherms. The theoretical basis is sound. For single component the equation is, for n  for finite n Note that n is the number of adsorbed monolayers, and x= P/P0. Where, P is the actual partial pressure of gas in the stream and P0 is the vapor pressure of the pure gas. Note: The BET simplifies to the Langmuir when relative pressure x< 0.01 and C >100 (Valsaraj et al., 1992).

25. S = (C-1)/CWmax J = 1/CWmax P/Po BET Isotherm (cont’d) To obtain the parameters in the BET equation, one needs to linearize the equation:

26. Empirical Equations for Adsorption (1). Correlation using a logarithmic series expansion such as: Note that a, b and c are constants specific to a typical compound.

27. The most common isotherm models [Dastgheib and Rockstraw, 2002]

28. Adsorbent Material

29. Adsorbent Material • Silica Gel • Molecular Sieves (zeolite) • Activated Carbon • Activated Alumina Polar and Non-polar adsorbents

30. Source: Air Pollution Engineering Manual., 1992 Source: Air Pollution Engineering Manual., 1992

31. Source: Air Pollution Engineering Manual., 1992 Source: Air Pollution Engineering Manual., 1992

32. H2O H2O H2O O OH OH OH OH heating hydrophobic hydrophilic Physical property might be changed as overheated.

33. Physical Properties of Adsorbents Source: Cooper and Alley (2002)

34. Activated carbon from various sources Source: Cooper and Alley (2002)

35. Adsorbent Material Activated Carbon

36. Adsorbent Material • Activated Carbon The most common adsorbent which apply to various works and utilizes to deodorization, decolor ,remove various toxic substances and so on. As the research show that about 280,000 tons activated carbons are consumed each year in the world.

37. Adsorbent Material • Activated carbons have unique porous structures, large specific surface area and porosity, and various surface functional groups. • These physical and chemical properties make activated carbons the most commonly employed adsorbents for removal of VOCs from gaseous and liquid phases.

38. Adsorbent Material The adsorption capacities and kinetics of activated carbons depends on their surface microstructure, including (a)specific surface area, (b)pore volume, pore size distribution and (c)various surface function groups (a) Specific Surface Area Generally speaking, the higher surface area can have higher adsorption capacity.

39. Adsorbent Material (b) Porosity The activated carbon have higher porous structure. Some researches indicate the surfaces of the pores of 1 g AC is equal 8 tennis courts. According to the IUPAC(International Union of Pure and Applied Chemistry, 1972) define the diameter of the pores. (1)macropore : diameter <2nm (2)mesoropore : diameter 2~50 nm (3)micropore : diameter >50 nm

40. Adsorbent Material • According to the research indicates that the diameter of air pollutants are on the range of 0.4~0.85 nm in general, so the proportion of the micropores are more important for those . Stenzel (1993) • The dioxin compounds are larger than those contaminants. • The diameter of dioxin is about 0.35~1.37nm.

41. Adsorbent Material (3) Surface Functional Groups. • Generally speaking, activated carbons are non-polar adsorbents which have higher affinity to non-polar organic matters. • The surface functional groups can affect the characteristics of adsorption, especially oxygen groups. The oxygen groups polar • Most oxygen groups can react with H2O molecular and reduce the adsorption capacities. (When H2O molecules exist. )

42. Adsorbent Material The figure of oxygen groups The research indicated that the oxygen groups could hinder the adsorption of the non-polar organic maters (i.e CCl4 ) . Ishizaki (1988)

43. Internal porosity Macropores (accessible to solvent and solute) Pore Structure of Activated Carbon

44. Pore Structure (cont’d)

45. Indicator (1) Molasses Number Decolorizing Index (2)Methylene Blue Number The adsorption indicator of aryl organic matters (3)Phenol Number Because of phenol molecules have higher solubility and often exist in the environmental pollution. Thus phenol number is an important indicator for adsorption ability. (4)Alky Benzene Sulphonate，ABS The adsorption indicator of large molecular (5) Iodine Number Iodine number B.E.T surface area

46. Adsorbent Material According to the difference forms, we can separate five styles: (1) Powder Activated Carbon( PAC) PACs have large external surface area and short diffusion path. The velocity of adsorption are most fast. (2) Granular Activated Carbon( GAC) The surface area of GACs are smaller than PACs, but the GACs have many advantages such as to fill easily, .to regenerate easily, have lower pressure drop and so on.

47. Adsorbent Material • (3)Spherical or Cylindrical Activated Carbon) • Spherical or Cylindrical Activated Carbon usually have higher mechanistic intensity. (4) Activated Carbon Fiber, (ACF) • ACFs have higher surface area than PACs and have lower pressure drop than GACs .But ACFs also have higher price than others.

48. Adsorbent Material (5) Impregnated Activated Carbon (IAC) • To put activated carbon into specific chemical solution and make these chemical substances to fix on the surface of activated carbon. • Activated carbon also can coating specific metals as a catalyst.

49. The influence of activated carbon adsorption The characteristic of adsorbent • Specific surface area • Surface Functional Groups • Porosity The characteristic of adsorbate • The molecular size of adsorbate • The polar of adsorbate • The concentration of adsorbate The factor of environment • Temperature • Moisture

50. The influence of Temperature The adsorption of naphthol in different temperature The adsorption is an exothermic reaction.