1. Air Stripping (Section 9 – 1)
Physical Treatment
Air Stripping
(Section 9 – 1)

2. Volatility Tendency to move from solution to gas phase Function of:
Vapor pressure (VP)
Molecular weight (MW)
Henry’s constant (H)
Solubility (S)
etc.

3. Henry’s Law Constant (H)

4. AWWA Equation Factors

5. Henry’s Law Constants

6. Equipment Spray systems Aeration in contact tanks Tray towers
Packed towers

7. Aeration in Tanks

8. Tray Towers

9. Packed Towers

10. Liquid Distribution Systems

11. Design of Air Stripping Column
Parameters
Chemical properties
Range of influent flow rates, temperatures, and concentrations
Range of air flow rates and temperatures
Operation as continuous or batch
Packing material

12. Packing

13. Fouling

14. Cleaning Packing

15. Comparison: Equipment

16. Design, in General Tower diameter function of design flow rate
Tower height function of required contaminant removal

17. Diameter of Column

18. Depth of Packing Design Equations
Assumptions:
Plug flow
Henry’s Law applies
Influent air contaminant free
Liquid and air volumes constant

19. Depth of Packing L = liquid loading rate (m3/m2/s)
KLa = overall mass transfer rate constant (s-1)
R = stripping factor
C = concentration

20. Stripping Factor (R) Process: mass balance on contaminant
Initial assumptions:
Previous
Plus
dilute solution
no accumulation
no reactions
100% efficient

21. Example: Removal Efficiency
Calculate the removal efficiency for an air stripper with the following characteristics.
Z = 12.2 m
QW = 0.28 m3/s
H’ =
QA = 5.66 m3/s
KLa = s-1
D = 4.3 m

22. Activity – Team
Ethylbenzene needs to be removed from a wastewater. The maximum level in the wastewater is 1 mg/L. The effluent limit is 35 g/L. Determine the height of an air stripping column. The following data is available:
KLa = s-1
QW = 7.13 L/s
T = 25 oC
D = 0.61 m
QA/QW = 20

23. More on Stripping Factor

24. KLa: Two-Film Theory CL CI PI PG Bulk Liquid Liquid Film Air Film
Bulk Air

25. KLa: Transfer Rate KLa (s-1) Determination:
KL = liquid mass transfer coefficient (m/s)
a = area-to-volume ratio of the packing (m2/m3)
Determination:
experimentally
Sherwood-Holloway equation
Onda correlations

26. KLa: Column Test System Test Small diameter column Packing material
Blower
Pump
Contaminated water
Test
Range of liquid loading rates
Range of air-to-water ratios

27. Column Test Continued Determining KLa
Plot sample (packing) depth vs. NTU (which varies based on Ce/Ci)
Slope = 1/HTU
KLa = L/HTU

28. Sampling Port Depth (m)
Example: Column Test
Sampling Port Depth (m)
TCE (µg/L)
230 2
143 4 82 6 48 8 28

29. Example continued

30. Sherwood-Holloway Equation
L = liquid mass loading rate (kg/m2/s)
 = liquid viscosity (1.002 x 10-3 Pa-s at 20 oC
 = water density (998.2 kg/m3 at 20 oC)
, n = constants (next slide)
DL = liquid diffusion coefficient (m2/s)
Wilke-Chang method
B T/

31. Sherwood-Holloway Constants
Packing
Size (mm)  n
Raschig rings 12
920
0.35 25
330
0.22 38
295 50
260
Berl saddles
490
0.28
560
525
Tile 75
360

32. DL: Wilke-Change Method
DL = liquid diffusion coefficient (cm2/s)
T = temperature (K)
 = water viscosity (0.89 cP at 25 oC)
V = contaminant molal volume (cm3/mol)

33. DL: Conversion Constant B

34. Onda Correlations Accounts for gas-phase and liquid-phase resistance
Better for slightly soluble gases
No empirical constants

35. Gas Pressure Drop
Physical parameter: describes resistance blower must overcome in the tower
Function of:
gas flow rate
water flow rate
size and type of packing
air-to-water ratio
Found from gas pressure drop curve

36. Example: Pressure Drop Figure
Determine the air and liquid loading rates for a column test to remove TCE. The stripping factor is 5 when 51-mm Intalox saddles are used at a pressure drop of 100 N/m2/m. The influent concentration is 230 g/L and the effluent concentration is 5 g/L. The temperature is 20oC.

37. Preliminary Design Determine height of packing
Z = (HTU) (NTU)
Zdesign = Z (SF)
Determine pressure drop and impact on effluent quality by varying air-to-water ratio (QA/QW) and the packing height (Z)

38. Activity – Team
Determine the dimensions of a full-scale air stripping tower to remove toluene from a waste stream if the flow rate is 3000 m3/d, the initial toluene concentration is 230 g/L, and the design effluent concentration is 1 g/L. Assume that the temperature of the system is 20 0C. A pilot study using a 30-cm diameter column, 25-mm Raschig rings, a stripping factor of 4, and a pressure drop of 200 N/m2/m generated the following data.
 Depth (m) [Toluene] (g/L)
2 52
4 21
6 6

39. Design Procedure
Select packing material. Higher KLa and lower pressure drop produce most efficient design.
Select air-to-water ratio and calculate stripping factor or select stripping factor and calculate operating air-to-water ratio.
Calculate air flow rate based on selected gas pressure drop and pressure drop curve.

40. Design Procedure Continued
Determine liquid loading rate from air-to-water ratio.
Conduct pilot studies using gas and liquid loading rates. Develop NTU data from Ce/Ci, and calculate KLa.
Determine tower height and diameter.
Repeat using matrix of stripping factors.

41. Comparison: QA/Qw & Z

42. Discharged Air
Recover and reuse chemical
Direct discharge
Treatment

43. Common Design Deficiencies
Poor efficiency due to low volatility
Poor effluent quality due to insufficient packing height/no. of trays
Poor design due to inadequate equilibrium data and/or characterization data
Inadequate controls for monitoring
Heavy entrainment due to no mist eliminator
Not sheltered so difficult to maintain in inclement weather
Lines freeze during winter shutdowns due to no drains or insulation

44. More Design Deficiencies
Tray Towers
Inadequate tray seals
Heavy foaming
Trays corroded
Packed Towers
Inadequate packing wetness due to poor loading and/or inadequate redistribution
No means to recycle effluent to adjust influent flow
Plugging due to heavy solids or tar in feed
Inadequate blower capacity

45. Steam Stripping (Section 9 – 3)
Physical Treatment
Steam Stripping
(Section 9 – 3)

46. Steam Stripping

47. Steam Stripping Design
Strippability of organics
Separation of organic phase from steam in decanter
Fouling

48. Rules of Thumb Strippability Separate phase formation
Any priority pollutant analyzed by direct injection on a gas chromatograph
Any compound with boiling point < 150 oC and H > atm-m3/mol
Separate phase formation
At least one compound with low solubility
Operating parameters
SS < 2%
Operating pressures as low as possible

49. Example – Feasibility Analysis
Mixture A
37 mg/L methanol
194 mg/L ethanol
114 mg/L n-butanol
Mixture B
37 mg/L methanol
194 mg/L ethanol
114 mg/L n-butanol
110 mg/L toluene
14 mg/L xylene

50. Common Design Deficiencies
High packing breakage due to thermal stresses
Heavy fouling due to influent characteristics & elevated temperature
Inadequate steam capacity
No control for steam flow
Dilute overhead product due to inadequate enriching section
Inadequate decanter to separate immiscible phase