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Adsorption, ion exchange, and chromatography Adsorption, ion exchange, and chromatography

Adsorption, ion exchange, and chromatography - PowerPoint Presentation

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Adsorption, ion exchange, and chromatography - PPT Presentation

Adsorption ion exchange and chromatography 1 Introduction Sorption operations certain components of a fluid phase called solutes are selectively transferred to insoluble rigid particles ID: 767301

adsorbent adsorption adsorbed isotherms adsorption adsorbent isotherms adsorbed pressure heat gas adsorbate temperature surface amount type bed isotherm chemical

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Adsorption, ion exchange, and chromatography 1

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. 2

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

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 4

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 AMeso pore 20-500 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 5

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 vacuumrange 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 6

Specific surface area of an adsorbent 7

Representative Properties of Commercial Porous Adsorbents 8

Representative cumulative pore-size distributions of adsorbents. 9

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 2013/12/3 Aerosol & Particulate Research Lab10 10

Adsorption Process Classified as : Physical adsorption Chemical adsorption 11

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 exothermicThe 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 layersThe adsorption rate is generally quite rapid The amount of heat released during this process is equal to the heat of condensation 12

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 . 13

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

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. 16

1. Pure Gas Adsorption 17

Brunauer's five types of adsorption isotherms 18

Type I The simplest isotherm is Type I Corresponds to unimolecular adsorptionCharacterized by a maximum limit in the amount adsorbedThis type applies often to gases at temperatures above their critical temperatureDesirable isotherms with strong adsorption 19

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 LayersEach layer is assumed to have a heat of adsorption equal to the heat of condensation (vaporization).Desirable isotherms Strong adsorption20

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.21

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 IIIHysteresis can also occur throughout any isotherm when strongly adsorbed impurities are present 22

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 constantp is the partial pressure of the component in the gas.23

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

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. 25

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

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

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.28

Freundlich Isotherm correlate isotherms of Type I k and n are temperature-dependent constants. n = 1 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 29

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 30

Other Adsorption Isotherms Valenzuela and Myers Isotherm m, b, and t = constants (a given adsorbate -adsorbent system and temperatureHonig and Reyerson Isotherm31

Adsorption isotherms for multicomponent mixtures 32 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.

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2. Liquid Adsorption 35

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. 36

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

adsorption isotherms 38 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

SORPTION SYSTEMS 40

Common Commercial Methods for Adsorption Separations 41

1. Stirred-tank slurry operation 42 the spent adsorbent is removed from the slurry by sedimentation or filtration and discarded can operated continuously the rate of adsorption is rapidWith 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.

2. Cyclic fixed-bed, batch operation 43 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

3. Continuous countercurrent operation 44 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 feed entry desorbent entry extract (adsorbed) removalraffinate (non-adsorbed)EC, extract column;RC, raffinate column.UOP Sorbex process

Regeneration methods 46 Thermal (temperature)-swing-adsorption (TSA) method Inert-purge-swing method Pressure-swing-adsorption (PSA) cycleVacuum-swing-adsorptionDisplacement-purge (displacement desorption) cycle

1. thermal (temperature)-swing-adsorption (TSA) method 47 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

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

2. inert-purge-swing method 49 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 valueThe purge gas must be inexpensive so that it does not have to be purified before recycle

3. Pressure-swing-adsorption (PSA) cycle 50 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 minutesBecause of these short times, the beds need not be largeSmall plants often use PSA because that cycle is simpler. If adsorption takes place at near-ambient pressure and desorption undervacuum, the cycle is referred to as vacuum-swing-adsorption (VSA)

4. Displacement-purge cycle 51 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.