Presentation is loading. Please wait.

Presentation is loading. Please wait.

Physical Treatment Air Stripping (Section 9 – 1).

Similar presentations


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

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 (m 3 /m 2 /s) –K L a = 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 –Q W = 0.28 m 3 /s –H’ = 0.2315 –Q A = 5.66 m 3 /s –K L a = 0.0125 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: –K L a = 0.016 s -1 –Q W = 7.13 L/s –T = 25 o C –D = 0.61 m –Q A /Q W = 20 –T = 25 o C

23 More on Stripping Factor

24 K L a: Two-Film Theory Bulk LiquidBulk AirLiquid FilmAir Film CLCL PGPG CICI PIPI

25 K L a: Transfer Rate K L a (s -1 ) –K L = liquid mass transfer coefficient (m/s) –a = area-to-volume ratio of the packing (m 2 /m 3 ) Determination: –experimentally –Sherwood-Holloway equation –Onda correlations

26 K L a: Column Test System –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 K L a –Plot sample (packing) depth vs. NTU (which varies based on C e /C i ) –Slope = 1/HTU –K L a = L/HTU

28 Example: Column Test Sampling Port Depth (m)TCE (µg/L) 0230 2143 482 648 828

29 Example continued

30 Sherwood-Holloway Equation –L = liquid mass loading rate (kg/m 2 /s) –  = liquid viscosity (1.002 x 10 -3 Pa-s at 20 o C –  = water density (998.2 kg/m 3 at 20 o C) – , n = constants (next slide) –D L = liquid diffusion coefficient (m 2 /s) Wilke-Chang method B T/ 

31 Sherwood-Holloway Constants PackingSize (mm)  n Raschig rings129200.35 253300.22 382950.22 502600.22 Berl saddles124900.28 255600.28 385250.28 Tile753600.28

32 D L : Wilke-Change Method D L = liquid diffusion coefficient (cm 2 /s) T = temperature (K)  = water viscosity (0.89 cP at 25 o C) V = contaminant molal volume (cm 3 /mol)

33 D L : 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/m 2 /m. The influent concentration is 230  g/L and the effluent concentration is 5  g/L. The temperature is 20 o C.

37 Preliminary Design Determine height of packing –Z = (HTU) (NTU) –Z design = Z (SF) Determine pressure drop and impact on effluent quality by varying air-to-water ratio (Q A /Q W ) 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 m 3 /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 0 C. 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/m 2 /m generated the following data. Depth (m)[Toluene] (  g/L) 0230 252 4216 81.5

39 Design Procedure Select packing material. Higher K L a 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 C e /C i, and calculate K L a. Determine tower height and diameter. Repeat using matrix of stripping factors.

41 Comparison: Q A /Q w & 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 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 –Any priority pollutant analyzed by direct injection on a gas chromatograph –Any compound with boiling point 0.0001 atm-m 3 /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


Download ppt "Physical Treatment Air Stripping (Section 9 – 1)."

Similar presentations


Ads by Google