MISS. RAHIMAH BINTI OTHMAN (Email: rahimah@unimap.edu.my)

1. Chapter 6: ADSORPTION • MISS. RAHIMAH BINTI OTHMAN • (Email: rahimah@unimap.edu.my)

2. COURSE OUTCOMES

3. OUTLINES • Introduction to adsorption. • Adsorption equipments. • Adsorption Isotherms Analysis. • Principles of Adsorption. • Basic Equation for Adsorption. • AdsorberDesign Calculation. √ √ √ √

4. INTRODUCTION TO ADSORPTION 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 .

5. INTRODUCTION TO ADSORPTION 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 (H2S). • water softening and deionisation. • hydrocarbon fractionation. • pharmaceutical purification.

6. INTRODUCTION TO ADSORPTION *NATURE OF ADSORBENT • Porous material - Large surface area per unit mass - internal surface area greater than the external surface area - often 500 to 1000 m2/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.

7. Silica structure Zeolite structure

8. INTRODUCTION TO ADSORPTION Types of Adsorption 1. Ion exchange • Electrostatic attachment of ionic species to site of the opposite charge at the surface of an adsorbent 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

9. Adsorption Equipment • Fixed-bed adsorbers • Gas-drying equipment • Pressure-swing adsorption

10. Fixed-bed adsorbers Gas-drying Equipment ADSORPTION EQUIPMENT Adsorption From Liquids Pressure-swing Adsorption

11. FIXED-BED ADSORBERS • 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.

12. FIXED-BED ADSORBERS 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.

13. Fixed-bed adsorbers Gas-drying Equipment ADSORPTION EQUIPMENT Adsorption From Liquids Pressure-swing Adsorption

14. GAS-DRYING EQUIPMENT • The equipment for drying is similar to the shown in Fig. 25.1, but hot gas is used for regeneration. • The moist gas from the bed being generated may be vented, or much of the water may be removed in a condenser and the gas recirculatedthrough a heater to the bed. • For small dryers, electric heaters are sometimes installed inside the bed to provide the energy for regeneration. Fig. 25.1 Vapor-phase Adsorption System

15. Fixed-bed adsorbers Gas-drying Equipment ADSORPTION EQUIPMENT Adsorption From Liquids Pressure-swing Adsorption

16. PRESSURE-SWING ADSORPTION • Most often, adsorption is used as a purification process to remove small amounts of material, but, there is a number of applications involve separations of gas mixtures with moderate to high concentration of adsorbates. • These are called bulk separations, and they often use different operating procedures than for gas purification. • Pressure-swing adsorption (PSA) is a bulk separation process that is used for small-scale air separation plants and for concentration of hydrogen in process streams.

17. Fixed-bed adsorbers Gas-drying Equipment ADSORPTION EQUIPMENT Adsorption From Liquids Pressure-swing Adsorption

18. ADSORPTION FROM LIQUIDS • 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.

19. PACKED EXTRACTION TOWERS Tower packings; (a) Raschig rings, (b) metal Pall ring, (c) plastic Pall ring, (d) Berl saddle, (e) ceramic Intalox saddle, (f) plastic Super Intalox saddle, (g) metal Intalox saddle

20. Adsorption IsothermS ANALYSIS • 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).

21. TYPES OF ISOTHERMS Amount of adsorbed is independent of concentration down to very low values. Amount of adsorbed is proportional to the concentration in the fluid. Concave upward; low solid loadings are obtained and because it leads to quite long mass-transfer zones in the bed. (this shape are rare)

22. TYPES OF ISOTHERMS Nearly linear isotherm up to 50 percent humidity, and the ultimate capacity is about twice that for the other solids. Water is held most strongly by molecular sieves, and the adsorption is almost irreversible, but the pore volume not as great as for silica gel Fig. 25.3 Adsorption isotherms for water in air at 20 to 50 oC

23. Adsorption data for vapors on activated carbon Sometimes fitted to Freundlich isotherms, but data for wide range of pressures show isotherm slopes gradually decrease as the pressure is increased.

24. Amount of adsorbed depends on (T/V) log (fs/f), where: T: adsorption temperature (Kelvin). V: molar volume of the liquid at the boiling point fs:fugasity of the saturated liquid at adsorption temperature f: fugasity of the vapor For adsorption at atmospheric pressure; * fugasity = partial pressure = vapor pressure Volume adsorbed is converted to mass by assuming the adsorbed liquid has the same density as liquid at the boiling point.

25. QUESTION 1 EXAMPLE 25.1. Adsorption on BPL carbon is used to treat an airstream containing 0.2 percent n-hexane at 20 oC. (a) Estimate the equilibrium capacity for a bed operating to 20 oC. (b) How much would the capacity decrease if the heat of adsorption raised the bed temperature to 40 oC.

26. ANSWER • Estimate the equilibrium capacity for a bed operating to 20 oC. The MWn-hexane (C6H14)= 86.17, At 20o C (from Perry’s Handbook, 7thed.) Pʹ=120mm Hg ≈ fs. At the normal boiling point (68.7 oC), ρL=0.615 g/cm3. The adsorption pressure P is 760 mm Hg. (b) At 40 oC, Pˊ= 276 mm Hg

27. 4 TYPES OF ADSORPTION ISOTHERMS • 1. Linear Isotherms • - Adsorption amount is proportional to the concentration in the fluid • Irreversible – independent of concentration • 3. Langmuir Isotherm – favorable type • 4. FreundlichIsotherm – strongly favorable type

28. 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. • Wmax and K are constants determined experimentally by plotting 1/W against 1/c

29. 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 = log b + m log c • Constant determined from experimental data by plotting log W versus log c • Slope = m, intercept = b

30. 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, where 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. • The region where most of the change in concentration occurs is called the mass-transfer zone (MTZ), and the limits are often taken as c/c0 values of 0.95 to 0.05.

31. Mass Transfer Zone and Breakthrough

32. Concentration Profile In Fixed Beds

33. t1: no part of the bed is saturated. • From t1 to t2: the wave had moved down the bed. • t2: the bed is almost saturated for a distance LS, but is still clean at LF. • Little adsorption occurs beyond LF at time t2, and the adsorbent is still unused. • The MTZ where adsorption takes place is the region between LS and LF. • The concentration of the adsorbate on the adsorbent is related to the adsorbate concentration in the feed by the thermodynamic equilibrium. • Because it is difficult to determine where MTZ begins and ends, LF can be taken where C/CF = 0.05, with LS at C/CF = 0.95. • tB: the wave has moved through the bed, with the leading point of the MTZ just reaches the end of the bed. This is known as the breakthrough point. • Rather than using C/CF = 0.05, the breakthrough concentration can be taken as the minimum detectable or maximum allowable solute concentration in the effluent fluid, e.g. as dictated by downstream processing unit. Concentration Profile In Fixed Beds

34. Concentration profile in fixed beds Figure 25.6(a) Is the ratio of solute concentration to inlet solute concentration in the fluid.

35. Breakthrough Curves t1, t2, t3: the exit concentration is practically zero. Time for fluid living the bed.

36. Breakthrough Curves

37. Breakthrough Curves • tb– time when the concentration reaches break point • The feed is switched to a fresh adsorbent bed • Break point – relative concentration c/co 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)

38. Breakthrough Curves • t* is the ideal adsorption time for a vertical breakthrough curve • t* is also the time when c/co reaches 0.50 • Amount of adsorbed is proportional to the rectangular area to the left of the dashed line at t*

39. Breakthrough Curves • Solute feed rate (FA) = superficial velocity (uo) X concentration (co) Where: Wo = initial adsorbate loading Wsat= adsorbate at equilibrium with the fluid (saturation) L = length of the bed ρb = bulk density of the bed

40. LENGTH OF UNUSED BED (LUB) • For systems with favorable isotherm, the concentration profile in the mass-transfer zone acquires a characteristic shape and width that do not change as the zone moves down the bed. • Test with different bed lengths have breakthrough curve of the same shape, but with longer beds, the MTZis a smaller fraction of the bed length, and greater fraction of the bed is utilized. • The scale-up principles “The amount of unused solid or length of unused bed does not change with the total bed length.”

41. LENGTH OF UNUSED BED (LUB) • To calculate LUB, determine the total solute adsorbed up to the break point by integration; • The break point time, tb is calculated from the ideal time and the fraction of bed utilized:

42. QUESTION 2 EXAMPLE 25.2 EXAMPLE 25.2. The adsorption of n-butanol from air was studied in a small (10.16 cm diameter) with 300 and 600 g carbon, corresponding to bed lengths of 8 and 16 cm. (a) From the following data for effluent concentration, estimate the saturation capacity of the carbon and the fraction of the bed used at c/c0 = 0.05. (b) Predict the break-point time for a bed length of 32 cm. Data for n-butanol on Columbia JXC 4/6 carbon are as follows:

43. ANSWER The concentration profiles are plotted in Fig 25.8, and extended to c/c0=1.0 assuming the curves are symmetric about c/c0=0.5. Per square centimeter of bed cross section, the solute feed rate is The total solute adsorbed is the area above the graph multiplied by FA. For the 8 cm bed, the area is;

44. ANSWER This area corresponds to the ideal time that would be required to adsorb the same amount if the breakthrough curve were a vertical line. The mass of carbon per unit cross-sectional area of bed is; 8 x 0.461 = 3.69 g/cm2 Thus, • Trapezoidal rule: At the break point, where c/c0 = 0.05, and t = 2.4 h The amount adsorbed up to the break point is then Thus 50 percent of the bed capacity is unused, which can be represented by a length of 4 cm.

45. ANSWER (-cont’) For the 16-cm bed the breakthrough curve has the same initial slope as the curve for the 8-cm bed, and although data were not taken beyond c/c0 = 0.25, the curves are assume to be parallel. For the entire bed, At c/c0 = 0.05, t = 7.1 h, and At the break point, 74 percent of the bed capacity is used, which corresponds to an unused section of length 0.26 x 16 = 4.2 cm. Within experimental error, the lengths of unused bed agree, and 4.1 cm is the expected value for a still longer bed.

46. ANSWER (-cont’) (b) For L = 32 cm, the expected length of the fully used bed is; 32 - 4.1 = 27.9 cm. The fraction of the bed used is: The break-point time is

47. QUESTION 3 A waste stream of n-butanol vapor in air from a process was adsorbed by activated carbon particles in a packed bed having a diameter of 4 cm and length of 14 cm containing 79.2 g of carbon. The density of the activated carbon is 0.461 g/cm3. The inlet gas stream having a concentration, C0of 600 ppm and a density of 0.00115 g/cm3 entered the bed at the solute feed rate, FA of 0.063 g/cm2.s. Data in Table 3.1 give the concentrations of the fluid in the bed, C. The break point concentration is set at C/Co = 0.05. • QUESTION; • Plot a breakthrough curve. • Determine the break-point time. • Calculate the saturation capacity of the carbon, Wsat. • Calculate the length of unused bed (LUB).

48. ANSWER • Given; • Packed bed, D = 4 cm • L = 14 cm • adsorbent = 79.2 g • ρ carbon = 0.461 g/cm3 • Inlet gas stream, C0 = 600 ppm • ρ = 0.00115 g/cm3 • FA = 0.063 g/cm2.s • C/C0 = 0.05 • Plot a breakthrough curve.

49. ANSWER- cont’ • Determine the break-point time. From the breakthrough curve, breakthrough time at C/C0 = 0.05 is tb = 4.1 h

50. ANSWER- cont’ • Calculate the saturation capacity of the carbon, Wsat. • The total solute adsorbed is the area above the graph multiplied by FA Simpson’s Rule of integration. (pp. 872) • From the graph plotted, the following data • is obtained;