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Air Stripping (Section 9 – 1)

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Presentation on theme: "Air Stripping (Section 9 – 1)"— Presentation transcript:

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

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