ADSORPTION The removal of dissolved substances from solution using adsorbents such as activated carbon

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.

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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

Adsorption Process 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

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

Properties of Activated Carbon Adsorbent Material Silica gel Activated alumina Activated carbon Synthetic zeolite Molecular sieve Dehyrdating purposes 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

Properties of Silica Gel Properties of Activated Alumina Adsorbent Material 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 Properties of Activated Alumina 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 Adsorbent Materials 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

Adsorption Isotherm The amount of gas adsorbed per unit of adsorbent at equilibrium is measured against the partial pressure of the adsorbate in the gas phase gives equilibrium adsorption isotherm

Adsorption Isotherm In general, an adsorption isotherm relates the volume or mass adsorbed to the partial pressure or concentration of the adsorbate in the main gas stream at a given temperature The equilibrium concentration adsorbed is very sensitive to T There are many equations proposed to fit analyticaly the various experimental istoherms

Adsorption equilibrium is a dynamic concept achieved when the rate at which molecules adsorb on to a surface is equal to the rate at which they desorb. The capacity of an adsorbent for a particular adsorbate involves the interaction of three properties: the concentration C of the adsorbate in the fluid phase, the concentration Cs of the adsorbate in the solid phase and the temperatureT of the system.

If one of these properties is kept constant, the other two may be graphed to represent the equilibrium. The commonest practice is to keep the temperature constant and to plot C against Cs to give an adsorption isotherm. When Cs is kept constant, the plot of C against T is known as an adsorption isostere. In gas–solid systems, it is often convenient to express C as a pressure of adsorbate. Keeping the pressure constant and plotting Cs against T gives adsorption isobars. The three plots are shown for the ammonia-charcoal system in Figure 17.4 which is taken from the work of BRUNAUER.

At very low concentrations the molecules adsorbed are widely spaced over the adsorbent surface so that one molecule has no influence on another. For these limiting conditions it is reasonable to assume that the concentration in one phase is proportional to the concentration in the other, that is: Where ∆𝐻 the enthalpy change per mole of adsorbate as it transfers from gaseous to adsorbed phase.

Adsorption Isotherms, Langmuir In Langmuir isotherm assuming a unimolecular layer can be obtained by a kinetic approach: at equilibium knowing that the rate of adsorption is equal to the rate of desorption: ra=CaP(1-f) rd=Cdf ra: rate of adsorption Ca, Cd: constant P: partial pressure of the adsorbate f: is the occupied fraction of the total solid surface 16

Adsorption Isotherms, Langmuir Since we assume a monolayer coverage, the mass of adsorbate per unit mass of adsorbent (a) is also proportional to f: Langmuir Isotherm 17

BET (Brunauer, Emmett and Teller) isotherm: This is a more general, multi-layer model. It assumes that a Langmuir isotherm applies to each layer and that no transmigration occurs between layers. It also assumes that there is equal energy of adsorption for each layer except for the first layer.

CS =saturation (solubility limit) concentration of the solute CS =saturation (solubility limit) concentration of the solute. (mg/liter) qe = mass of material adsorbed (at equilibrium) KB = a parameter related to the binding intensity for all layers. Note: when Ce << CS and KB >> 1 and K = KB/Cs BET isotherm approaches Langmuir isotherm.