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Physical Treatment. Air Stripping (Section 9 – 1). Volatility. Tendency to move from solution to gas phase Function of: Vapor pressure (VP) Molecular weight (MW) Henry’s constant (H) Solubility (S) etc. Henry’s Law Constant (H). AWWA Equation Factors. Henry’s Law Constants. Equipment.

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Volatility

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

Equipment

- Spray systems
- Aeration in contact tanks
- Tray towers
- Packed towers

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

Design, in General

- Tower diameterfunction of design flow rate
- Tower height function of required contaminant removal

Plug flow

Henry’s Law applies

Influent air contaminant free

Liquid and air volumes constant

Depth of Packing Design EquationsDepth of Packing

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

Stripping Factor (R)

- Process: mass balance on contaminant
- Initial assumptions:
- Previous
- Plus
- dilute solution
- no accumulation
- no reactions
- 100% efficient

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’ = 0.2315
- QA = 5.66 m3/s
- KLa = 0.0125 s-1
- D = 4.3 m

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 = 0.016 s-1
- QW = 7.13 L/s
- T = 25 oC
- D = 0.61 m
- QA/QW = 20
- T = 25 oC

KLa: Transfer Rate

- KLa (s-1)
- KL = liquid mass transfer coefficient (m/s)
- a = area-to-volume ratio of the packing (m2/m3)

- Determination:
- experimentally
- Sherwood-Holloway equation
- Onda correlations

KLa: Column Test

- System
- Small diameter column
- Packing material
- Blower
- Pump
- Contaminated water

- Test
- Range of liquid loading rates
- Range of air-to-water ratios

Column Test Continued

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

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/

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)

DL: Conversion Constant B

Onda Correlations

- Accounts for gas-phase and liquid-phase resistance
- Better for slightly soluble gases
- No empirical constants

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

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.

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)

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)

0 230

2 52

4 21

6 6

8 1.5

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.

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.

Comparison: QA/Qw & Z

Discharged Air

- Recover and reuse chemical
- Direct discharge
- Treatment

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

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

Steam Stripping Design

- Strippability of organics
- Separation of organic phase from steam in decanter
- Fouling

Rules of Thumb

- Strippability
- Any priority pollutant analyzed by direct injection on a gas chromatograph
- Any compound with boiling point < 150 oC and H > 0.0001 atm-m3/mol

- Separate phase formation
- At least one compound with low solubility

- Operating parameters
- SS < 2%
- Operating pressures as low as possible

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

Example – Feasibility AnalysisCommon 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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