ERT 313 BIOSEPARATION ENGINEERING ABSORPTION

1. ERT 313BIOSEPARATION ENGINEERINGABSORPTION Prepared by: Pn. Hairul Nazirah Abdul Halim

2. Topic Outline • Introduction • Basic Principles • Applications • Gas – Liquid Equilibrium • Unit operation for Absorption: a) Packed tower b) Plate Column • Mass Transfer between Phases

3. Introduction • Absorption – between gas and liquid. • Solutes are absorbed from the gas phase into the liquid phase. • Absorption does not destroy the gases. • It simply transfers the contaminated gas to the liquid state. • Stripping or desorption - reverse of absorption

4. Basic Principles • The type of contacting liquid chosen depends on the: 1. Solubility of solute (contaminant gases) in the chosen contacting liquid. - pure water : NH3, acetic acid 2. Chemical reactivity between gas and liquid. - caustic solution: acid gases, HCl & SO2 - produce a salt

5. Applications 1. Absorbing SO2 from the flue gases by absorption in alkaline solutions 2. Hydrogenation of edible oils in food industry - hydrogen gas is bubbled into oil and absorbed. 3. Removal of CO2 from synthesis gases by absorbing it with hot potassium carbonate solution. (in ammonia production) 4. Absorbing dimethyl sulfide from the food processing industry

6. Gas-Liquid Equilibrium • Consider the SO2-air-water system. • An amount of gaseous SO2, air and water are put in a closed container and shaken repeatedly at a given temperature until equilibrium is reached. • Samples of the gas and liquid are analyzed to determine the partial pressure pA of SO2 in the gas and mol fraction xA in the liquid.

7. Gas-Liquid Equilibrium (con’t) • The equilibrium plot is shown in Figure 10.2-1. • The equilibrium relation between pA in the gas phase and xA can be expressed by a straight line Henry’s Law equation at low concentration: pA = H xA Where H = Henry’s law constant (mol frac gas/ mol frac liquid)

9. The data for some common gases with water are given in Appendix A.3 (Geankoplis, Transport Process and Separation Process Principles, 4th ed., Prentice Hall)

12. Absorption System • The most common design of absorption systems: 1. Packed Bed Column / Packed Tower 2. Plate Column

13. Unit Operation 1: PACKED TOWER • A common apparatus used in gas absorption is the packed tower as shown in Figure 18.1 • The device consist of: a) cylindrical column or tower b) gas inlet and distributing space at the bottom c) liquid inlet and distributor at the top d) gas & liquid outlets at the top & bottom, respectively e) tower packing – supported mass of inert solid shapes

15. PACKED TOWER

17. The liquid inlet - pure solvent or weak liquor - is distributed over the top of packing by the distributor - uniformly wets the surfaces of the packings • The distributor - is a set of perforated pipes (Fig. 18.1) - a spray nozzles in a large towers • The gas inlet - enter the distributing space below the packing - flow upward in the packing countercurrent to the flow of the liquid

18. PACKINGS • The packing - provides a large area of contact between the liquid and gas - encourage intimates contact between the phases • Common dumped packings is shown in Figure 18.2.

20. PACKINGS • Hollow or irregular packing units – high void spaces • Intalox saddles – the shape prevents pieces from nesting closely together - Increases the bed porosity • Porosity or void fraction: 60 – 90% • 3 principal types: i) dumped packings, (0.25 – 3 inch) ii) stacked packings, (2 – 8 inch) iii) structured/ordered packings. • Made from: plastic, metal or ceramic

21. Structured Packing Ceramic Intalox Saddle Packing

22. Contact between liquid & gas • Good contact between liquid & gas is the hardest to meet esp. in large tower • Channeling – occur at low liquid rates - some of the packing surface dry - chief reason for the poor performance - severe in tower filled with stacked packings - less severe in dumped packings - can be minimized by having the ratio of tower diameter to packing diameter, 8:1

23. FLOODING • Occur in countercurrent flow towers Inlet gas flow rate is so high It interferes with the downward flow of the solvent liquid. Cause an upward flow of the liquid through the tower

24. Most absorbers are designed to operate at no more than 70% of maximum gas velocity that can cause flooding. • Factors that may lead to flooding: 1. high inlet gas flow rates 2. low liquid circulation rates 3. small diameter towers

25. Pressure Drop & Limiting Flow rates • Figure 18.4 shows typical data for the pressure drop in a packed tower. • Pressure drop is due to fluid friction • Pressure drop - common way of determining if flooding is occuring / something else goes wrong inside the absorber. • The graph is plotted on logarithmic coordinates for ΔP (inches H20/ft packing) versus the gas flow rate, Gy (lb/ft2.h)

26. Loading & Flooding Point • Point K is the loading point • Point L is the flooding point for the given liquid flow. • Loadingpoint is a point where liquid hold up starts to increase and caused a change in the slope of the pressuredrop • Flooding point is a point where the gas velocity will result in the pressure drop start to become almost vertical. Liquid rapidly accumulates, the entire column filled with liquid.

28. Unit operation 2: PLATE COLUMN • Plate Column absorbers distribute a contacting liquid over plates situated one above the other. • The contacting liquid flows downward through the column from one plate to the other in a stepwise fashion. • The inlet gas rises through each plate through openings in the plate and comes into contact with the liquid. • Usually, a layer of foam and froth is formed above each plate resulting from the mixing of liquid and gas.

30. The gas not absorbed rises through the foam layer to the next plate for another stage of absorption. • Plate column absorbers result in a high removal efficiency since there are multiple stages of contact between liquid and gas. • More expensive than packed bed towers. • The advantages of plate columns are usually not justified in small operations where a packed bed tower will suffice.

31. Plate columns have certain advantages over packed bed towers: a) plate columns can handle high gas flow rates accompanied by a low liquid flowrate with little chance of flooding. b) little chance for channeling inside of a plate column compared to a packed bed tower. c) sediment build-up often can be easily removed in plate column absorbers (packed bed towers are harder to clean).

32. Principles of Absorption • Mass Transfer between phases • Rate of absorption • Calculation of tower height • Number of transfer unit • Material Balances: a) Packed Column b) Plate Column • Graphical Method: Theoretical Stages

33. Mass Transfer Between Phases Two-Film Theory • In absorption, solute from gas phase must diffuse into liquid phase. • The rate of diffusion in both phases affect the overall rate of mass transfer. • Assumption in Two-Film Theory: a) equilibrium is assumed at the interface b) the resistance to mass transfer in the two phases are added to get an overall resistance.

34. Mass Transfer Between Phases • Nomenclature: ky = mass-transfer coefficient in gas phase kx = mass-transfer coefficient in liquid phase Ky = Overall mass-transfer coefficient in gas phase Kx = Overall mass-transfer coefficient in liquid phase a = interfacial area per unit volume

35. The rate of absorption, r per unit volume of packed column is given by any of the following equations: where y and x refer to the mole fraction of the component being absorbed.

36. The overall coefficient: • Where m = the local slope of the equilibrium curve. • In Eq. (18.12), = the resistance of mass transfer in the gas film. = the resistance of mass transfer in the liquid film

37. In Eq. (18.12): The liquid film resistance control the rate of absorption • when kya = kxa and m > 1.0. • This means that any change in kxa has a nearly proportional effect on both Kya and Kxa on the rate of absorption, • whereas a change in kya has little effect.

38. In Eq. (18.12): The gas film resistance control the rate of absorption • when kya = kxa and m << 1.0 (very small) • Solubility of the gas is very high • Such as absorption of HCl in water and absorption of NH3 in water

39. Calculation of Tower Height Fig. 18.12 Diagram of packed absorption tower

40. Consider the packed column shown in Figure 18.12. • The cross section is S, the differential volume in height is S dZ. • The amount absorbed in section dZ is –V dy, which equals the absorption rate times the differential volume: • Rearrange for integration: • The equation for column height (ZT) can be written as follows:

41. Number of Transfer Units (NTU) • The integral part in Eq. (18.16) is called the number of transfer units NTU (NOy) = • The other part of Eq. (18.16) has the units of length and is called the height of a transfer unit (HTU) HOy: • Hence,

42. The number of transfer units is somewhat like the number of ideal stages (theoretical plates). • The NTU = ideal stage if the operating line and equilibrium line are straight and parallel as in Fig. 18.13 a.

43. For straight operating and equilibrium lines: • Where: • The corresponding equation based on the liquid phase:

44. 4 basic types of mass transfer coefficient: Gas Film: Liquid Film: Overall Gas: Overall Liquid:

45. Material Balances for Packed Column L = molal flow rate of the liquid phase V = molal flow rate of the gas phase x = liquid phase concentration y = gas phase concentration

46. Material balances for the portion of the column above an arbitrary section (dashed line) • Total material balance: • Material balance on component A Overall material equations • Total material balance: • Material balance on component A:

47. Rearrange Eq. (18.3) gives operating-line equation: • The operating line can be plotted on an arithmetic graph along with the equilibrium curve as shown in Fig. 18.10. • The operating line must lie above the equilibrium line in order for absorption to take place.

49. Absorption in Plate Column • Besides packed tower, gas absorption can be carried out in a column equipped with sieve trays or other types of plates. • Plate column is used instead of packed column because: a) to avoid the problem of liquid distribution in a large diameter tower b) to decrease the uncertainty in scaleup Plate Column

50. Material Balances for Plate Column • A general stage in the system is the nth stage, which is number n counting from the entrance of the L phase. yn+1 = mole fraction of component A in the V phase leaving stage n + 1. Ln = molal flow rate of the L phase leaving the nth stage.