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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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Physical treatment l.jpg

Physical Treatment

Air Stripping

(Section 9 – 1)


Volatility l.jpg
Volatility

  • Tendency to move from solution to gas phase

  • Function of:

    • Vapor pressure (VP)

    • Molecular weight (MW)

    • Henry’s constant (H)

    • Solubility (S)

    • etc.





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Equipment

  • Spray systems

  • Aeration in contact tanks

  • Tray towers

  • Packed towers






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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






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Design, in General

  • Tower diameterfunction of design flow rate

  • Tower height function of required contaminant removal



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Assumptions:

Plug flow

Henry’s Law applies

Influent air contaminant free

Liquid and air volumes constant

Depth of Packing Design Equations


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Depth of Packing

  • L = liquid loading rate (m3/m2/s)

  • KLa = overall mass transfer rate constant (s-1)

  • R = stripping factor

  • C = concentration


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Stripping Factor (R)

  • Process: mass balance on contaminant

  • Initial assumptions:

    • Previous

    • Plus

      • dilute solution

      • no accumulation

      • no reactions

      • 100% efficient


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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


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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



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KLa: Two-Film Theory

CL

CI

PI

PG

Bulk Liquid

Liquid Film

Air Film

Bulk Air


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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


K l a column test l.jpg
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


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Column Test Continued

  • Determining KLa

    • Plot sample (packing) depth vs. NTU (which varies based on Ce/Ci)

    • Slope = 1/HTU

    • KLa = L/HTU




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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/



D l wilke change method l.jpg
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)


D l conversion constant b l.jpg
DL: Conversion Constant B


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Onda Correlations

  • Accounts for gas-phase and liquid-phase resistance

  • Better for slightly soluble gases

  • No empirical constants


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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 l.jpg
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.


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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)


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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


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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 l.jpg
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 q a q w z l.jpg
Comparison: QA/Qw & Z


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Discharged Air

  • Recover and reuse chemical

  • Direct discharge

  • Treatment


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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


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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


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Physical Treatment

Steam Stripping

(Section 9 – 3)



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Steam Stripping Design

  • Strippability of organics

  • Separation of organic phase from steam in decanter

  • Fouling


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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


Example feasibility analysis l.jpg

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

Example – Feasibility Analysis


Common design deficiencies50 l.jpg
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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