Adsorption ≠ Absorption ! Absorption – a fluid phase is transferred from one medium to another Adsorption – certain components of a fluid (liquid or gas) phase are transferred to and held at the surface of a solid (e.g. small particles binding to a carbon bed to improve water quality) Adsorbent – the adsorbing phase (carbon, silica gel, zeolite) Adsorbate – the material adsorbed at the surface of adsorbent
Application of Adsorption Used in many industrial processes: –Adsorbing the desired product from fermentation broths –Isolation of proteins –Dehumidification –odour/colour/taste removal –gas pollutant removal (H 2 S) –water softening and deionisation –hydrocarbon fractionation –pharmaceutical purification
Nature of Adsorbent Porous material - Large surface area per unit mass - internal surface area greater than the external surface area - often 500 to 1000 m 2 /g. Granular (50μm - 12 mm diameter), small pellets or beads Suitable for packed bed use Activated carbon, silica gel, alumina, zeolites, clay minerals, ion exchange resins Separation occurs because differences in molecular weight, shape or polarity of components Rate of mass transfer is dependent on the void fraction within the pores
Types of Adsorption 1.Ion exchange –Electrostatic attachment of ionic species to site of the opposite charge at the surface of an adsorbent
Types of Adsorption 2.Physical Adsorption –result of intermolecular forces causing preferential binding of certain substances to certain adsorbents –Van der Waal forces, London dispersion force –reversible by addition of heat (via steam, hot inert gas, oven) –Attachment to the outer layer of adsorbent material
3.Chemisorption –result of chemical interaction –Irreversible, mainly found in catalysis –change in the chemical form of adsorbate
Fixed-bed Adsorber Adsorbent particles: 0.3 – 1.2 m deep supported on a perforated plate Feed gas passes down through the bed Downflow is preffered because upflow at high rates may fluidize the particles, causing attrition and loss of fines. The feed gas is switched to the other bed when the conc. Of solute in exit gas reaches a certain value. The bed is regenerate by steam / hot inert gas.
Regeneration To remove unwanted particles from the adsorbent surface after the adsorption process using steam/hot inert gas Steam condenses in the bed, raising the temp. of the solid, provide energy for desorption The solvent is condensed, separated from water. Then the bed is cooled and dried with inert gas
Adsorption from liquid Use of activated carbon to remove pollutants from aqueous wastes Use carbon beds up to 10 m tall, several ft in diameter, several bed operating in parallel. Tall beds are needed to ensure adequate treatment
Adsorption Isotherm Adsorption isotherm – equilibrium relationship between the concentration in the fluid phase and the concentration in the adsorbent particles. For gas – concentration in mole % or partial pressure For liquid – concentration in mg/L (ppm) or μg/L (ppb) Concentration of adsorbate on the solid = mass adsorbed (g) per unit mass of original adsorbent (g).
4 types of Adsorption Isotherms 1.Linear Isotherms - Adsorption amount is proportional to the concentration in the fluid 2.Irreversible – independent of concentration 3.Langmuir Isotherm 4.Freundlich Isotherm
LANGMUIR ISOTHERM Often been used to correlate equilibrium adsorption data for protein. Isotherms that convex upward are called favorable. Where: W = adsorbate loading (g absorbed/g solid) c = the concentration in the fluid (mg/L) K = the adsorption constant K >> 1 : the isotherm is strongly favorable. W max and K are constants determined experimentally by plotting 1/W against 1/c
FREUNDLICH ISOTHERM – strongly favourable –Describe the adsorption of variety of antibiotics, steroids and hormones. – high adsorption at low fluid concentration where b and m are constant -Linearize the equation: Log W = b + m log c -Constant determined from experimental data by plotting log W versus log c -Slope = m, intercept = b
FIGURE 25.3Adsorption isotherms for water in air at 20 to 50 0 C.
Principles of Adsorption In fixed bed adsorption, the concentrations in the fluid phase and the solid phase change with: a) time b) as well as the position in the bed. At first, most of the mass transfer takes place near the inlet of the bed. The fluid contacts the adsorbent. After a few minutes, the solid near the inlet is nearly saturated. Most of the mass transfer takes place farther from the inlet. The concentration gradient become S-shaped.
Concentration profile in fixed beds Figure 25.6(a)
t b – time when the concentration reaches break point The feed is switched to a fresh adsorbent bed Break point – relative concentration c/c o of 0.05 or 0.10 Adsorption beyond the break point would rise rapidly to about 0.50 Then, slowly approach 1.0 (concentration liq in = liq out)
t* is the ideal adsorption time for a vertical breakthrough curve t* is also the time when c/c o reaches 0.50 Amount of adsorbed is proportional to the rectangular area to the left of the dashed line at t*
Solute feed rate (F A ) = superficial velocity (u o ) X concentration (c o ) Where: W o = initial adsorbate loading W sat = adsorbate at equilibrium with the fluid (saturation) L = length of the bed ρ b = bulk density of the bed
Length of Unused Bed (LUB) To calculate LUB, determine the total solute adsorbed up to the break point by integration The break point time, t b is calculated from the ideal time and the fraction of bed utilized: