1. Adsorption, ion exchange, and chromatography

2. Introduction
Sorption operations: certain components of a fluid phase, called solutes, are selectively transferred to insoluble, rigid particles suspended in a vessel or packed in a column.
Sorption includes selective transfer to the surface and/or into the bulk of a solid or liquid.
The substance that is transferred to the surafce is the adsorbate.
The material on which the adsorbate deposits is the adsorbent.

3. Industrial Example
Pressure-swing adsorption for the dehydration of air

4. SORBENTS (1) high selectivity to enable sharp separations
(2) high capacity to minimize the amount of sorbent needed
(3) favorable kinetic and transport properties for rapid sorption
(4) chemical and thermal stability
(5) hardness and mechanical strength
(6) a free-flowing tendency for ease of filling or emptying vessels
(7) high resistance to fouling for long life
(8) no tendency to promote undesirable chemical reactions
(9) the capability of being regenerated
(10) relatively low cost

5. SORBENTS
Most solids are able to adsorb species from gases and liquids. However, only a few have a sufficient selectivity and capacity to make them serious candidates for commercial adsorbents.
Micro pore <20 A
Meso pore A
Macro pore >500 A (50 nm)
The surface area-to-volume ratio
The specific surface area, Sg is area per unit mass of adsorbent is

6. Specific surface area of an adsorbent
Sg is measured by adsorbing gaseous nitrogen.
Typically, the BET apparatus operates at the normal boiling point of N2 (-195.8°C) by measuring the equilibrium volume of pure N2 physically adsorbed on several grams of the adsorbent at a number of different values of the total pressure in the vacuum
range of 5 to at least 250 mmHg.
Brunauer, Emmett, and assumed that the heat of adsorption during monolayer formation is constant and that the heat effect associated with subsequent layers is equal to the heat of condensation. The BET equation is

7. Specific surface area of an adsorbent

8. Representative Properties of Commercial Porous Adsorbents

9. Representative cumulative pore-size distributions of adsorbents.

10. 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 kj/g mole
2018/9/21
Aerosol & Particulate Research Lab 10

11. Adsorption Process Classified as: Physical adsorption
Chemical adsorption

12. Adsorption Process 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 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
The amount of heat released during this process is equal to the heat of condensation

13. Adsorption Process 2) Chemical adsorption
Occurs when there is sharing of electrons between adsorbent and adsorbate
The amount of heat released during this process is equal to the heat of reaction.
Heat liberated during chemisorption is much larger than the heat of vaporization.
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.
Chemisorption from a gas generally takes place only at temperatures greater than 200°C may be slow and irreversible.
Commercial adsorbents rely on physical adsorption; catalysis relies on chemisorption.

15. Factors affecting adsorption
Physical and chemical properties of the adsorbate
Properties of the adsorbent
Adsorption isotherm
Solubility pH
pH often affects the surface charge on the adsorbent as well as the charge on the solute.
Temperature
Adsorption reactions are typically exothermic as T increases extent of adsorption decreases.
Presence of other solutes

16. Adsorption Equilibrium
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.
This equilibrium is usually expressed in terms of
(1) concentration (if the fluid is a liquid) or partial pressure (if the fluid is a gas) of the adsorbate in the fluid
(2) solute loading on the adsorbent, expressed as mass, moles, or volume of adsorbate per unit mass or per unit BET surface area of the adsorbent.
This equilibrium isotherm places a limit on the extent to which a solute is adsorbed from a given fluid mixture on an adsorbent of given chemical composition and geometry for a given set of conditions.

17. 1. Pure Gas Adsorption

18. Brunauer's five types of adsorption isotherms

19. Type I The simplest isotherm is Type I
Corresponds to unimolecular adsorption
Characterized by a maximum limit in the amount adsorbed
This type applies often to gases at temperatures above their critical temperature
Desirable isotherms with strong adsorption

20. Types II Multimolecular adsorption
Observed for gases at temperatures below their critical but approaching, the saturation pressure (vapor pressure)
The heat of adsorption for the first adsorbed layer is greater than that for the succeeding Layers
Each layer is assumed to have a heat of adsorption equal to the heat of condensation (vaporization).
Desirable isotherms
Strong adsorption

21. Types III
Undesirable because the extent of adsorption is low except at high pressures
Corresponds to multimolecular adsorption where the heat of adsorption of the first layer is less than that of succeeding layers.
This type of isotherm is rarely observed
an example being the adsorption of iodine vapor on silica gel. In the limit, as the heat of adsorption of the first layer approaches zero, adsorption is delayed until the saturation pressure is approached.

22. Type IV and Type V
the number of layers is restricted by pore size and capillary
the maximum extent of adsorption occurs before the saturation pressure is reached
Type IV is the capillary-condensation version of Type II
Type V is the capillary-condensation version of Type III
Hysteresis can also occur throughout any isotherm when strongly adsorbed impurities are present

23. Adsorption isotherms for NH3 on charcoal
For ammonia boiling point is -33.3"C and the critical temperature is 132.4"C
isotherms are of Type I.
When the amount adsorbed is low (<25 cm3/g), the isotherms are almost linear and the following form of Henry's law, called the linear isotherm:
where q is amount adsorbed/unit mass of adsorbent
k is an empirical, temperature-dependent constant
p is the partial pressure of the component in the gas.

24. Adsorption isobars for NH3 on charcoal
As the temperature increases, the amount adsorbed decreases because of Le Chatelier's principle for an exothermic process.

25. Adsorption isotherms for pure propane vapor at 298-303 K
adsorption isotherms for a given pure gas at a fixed temperature vary considerably with the adsorbent.

26. Adsorption isotherms for water in air at 20 to 50'C.

27. Generalized adsorption correlation for Pittsburgh Chemical Co
Generalized adsorption correlation for Pittsburgh Chemical Co. BPL carbon (1040 m2/g).

28. Comparison of Equilibrium Adsorption of Pure Gases
20-40 mesh Columbia L Activated Carbon Particles (S, = 1,152 m2Ig) at 38°C and -1 atm
for a given adsorbent, the loading depends strongly on the gas.

29. Freundlich Isotherm correlate isotherms of Type I
k and n are temperature-dependent constants.
n = Henry's law equation
K decreases with increasing temperature
n increases with increasing temperature and approaches a value of 1 at high temperatures
assumptions:
a heterogeneous surface
nonuniform distribution of the heat of adsorption over the surface
1<n<5

30. Langmuir Isotherm correlate isotherms of Type I Assumtion:
chemisorption (the Langmuir adsorption isotherm is restricted to a monomolecular layer)
the surface of the pores of the adsorbent is homogeneous
the forces of interaction between adsorbed molecules are negligible.
Although originally devised by Langmuir for chemisorption, this eq. has been widely applied to physical adsorption data
K should change rapidly with temperature, but q, should not because it is related through v, by to the specific surface area of the adsorbent, Sg

31. Other Adsorption Isotherms
Valenzuela and Myers Isotherm
m, b, and t = constants (a given adsorbate-adsorbent system and temperature
Honig and Reyerson Isotherm

32. Adsorption isotherms for multicomponent mixtures
Adsorption isotherms are generally presented for a single component, but many applications involve multicomponent mixtures.
The Langmuir isotherm is easily modified for multiple adsorbates by adding terms to the denominator:
This equation is not very satisfactory for strongly adsorbed materials.

35. 2. Liquid Adsorption

36. When porous adsorbent particles are immersed in a pure gas, the amount of adsorbed gas is determined by the decrease in total pressure.
With a liquid, the pressure does not change, and no simple experimental procedure has been devised for determining the extent of adsorption of a pure liquid.
If the liquid is a homogeneous binary mixture, it is customary to designate one component the solute (1) and the other the solvent (2).
adsorption of the solvent is tacitly assumed not to occur.

37. adsorption isotherms Assuming: no adsorption of solvent
a negligible change in the total moles of liquid mixture

38. adsorption isotherms
When the solvent is not a Composite curve without negative Concentration
Composite isotherms or isothems of concentration change
>

39. When data for the binary mixture are only available in the dilute region, the amount of adsorption of the solvent may be constant and all changes in the total amount adsorbed are due to just the solute.
In that case, the adsorption mass of adsorbent isotherms are of the form of Figure a, which resembles the form obtained with pure gases.
It is then common to fit with adsorbent the data with concentration forms of the Freundlich equation

40. SORPTION SYSTEMS

41. Common Commercial Methods for Adsorption Separations

42. 1. Stirred-tank slurry operation
the rate of adsorption is rapid
With good agitation and small particles, the external resistance to mass transfer from the bulk liquid to the external surface of the adsorbent particles is small.
The main application of this mode of operation is the removal of very small amounts of dissolved, and relatively large molecules, such as coloring agents, from water.
the spent adsorbent is removed from the slurry by sedimentation or filtration and discarded
can operated continuously

43. 2. Cyclic fixed-bed, batch operation
Bed pressure drop decreases with increasing particle size
The solute transport rate increases with decreasing particle size
For purification or bulk separation of gases, the adsorbent is almost always regenerated in place

44. 3. Continuous countercurrent operation
It is difficult to circulate the solid adsorbent, as a moving bed, to achieve a steady-state operation
Only a few units were installed

45. UOP Sorbex process feed entry desorbent entry
extract (adsorbed) removal
raffinate (non-adsorbed)
EC, extract column;
RC, raffinate column.

46. Regeneration methods
Thermal (temperature)-swing-adsorption (TSA) method
Inert-purge-swing method
Pressure-swing-adsorption (PSA) cycle
Vacuum-swing-adsorption
Displacement-purge (displacement desorption) cycle

47. 1. thermal (temperature)-swing-adsorption (TSA) method
the adsorbent is regenerated by desorption at a temperature higher than that used during the adsorption step of the cycle.
The temperature of the bed is increased by:
(1) heating coils located in the bed followed by pulling a moderate vacuum
(2) heat transfer from an inert, non-adsorbing, hot purge gas, such as steam.
heating and cooling of the bed requires hours and the quantity of adsorbent in the bed is reasonable thus TSA is practical only for purification involving small rates of adsorption

48. Fluidized bed TSA
Purasiv process with a fluidized bed for adsorption and moving bed for desorption.
Application:
remove small amounts of solvents from air
removal of moisture, C02, and pollutants from gas streams.

49. 2. inert-purge-swing method
Desorption is at the same temperature and pressure
the gas used for purging is non-adsorbing (inert) or only weakly adsorbing
This method is used only when the solute is weakly adsorbed, easily desorbed, and of little or no value
The purge gas must be inexpensive so that it does not have to be purified before recycle

50. 3. Pressure-swing-adsorption (PSA) cycle
Adsorption takes place at an elevated pressure, whereas desorption occurs at near-ambient pressure
because the bed can be depressurized and repressurized rapidly, making it possible to operate at cycle times of seconds to minutes
Because of these short times, the beds need not be large
Small plants often use PSA because that cycle is simpler.
If adsorption takes place at near-ambient pressure and desorption under
vacuum, the cycle is referred to as vacuum-swing-adsorption (VSA)

51. 4. Displacement-purge cycle
a strongly adsorbed purge gas is used in the desorption step to displace the adsorbed species.
Another step is required to recover the purge gas.
This method is considered only where TSA, PSA, and VSA cannot be used because of pressure or temperature limitations.