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**Air Stripping (Section 9 – 1)**

Physical Treatment Air Stripping (Section 9 – 1)

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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.

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**Henry’s Law Constant (H)**

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AWWA Equation Factors

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Henry’s Law Constants

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**Equipment Spray systems Aeration in contact tanks Tray towers**

Packed towers

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Aeration in Tanks

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Tray Towers

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Packed Towers

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**Liquid Distribution Systems**

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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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Packing

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Fouling

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Cleaning Packing

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**Comparison: Equipment**

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**Design, in General Tower diameter function of design flow rate**

Tower height function of required contaminant removal

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Diameter of Column

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**Depth of Packing Design Equations**

Assumptions: Plug flow Henry’s Law applies Influent air contaminant free Liquid and air volumes constant

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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’ = QA = 5.66 m3/s KLa = 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 = s-1 QW = 7.13 L/s T = 25 oC D = 0.61 m QA/QW = 20

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**More on Stripping Factor**

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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) 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

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**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

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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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**Sampling Port Depth (m)**

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

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Example continued

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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/

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**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

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**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)

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**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

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**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) 2 52 4 21 6 6

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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.

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**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.

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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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**Steam Stripping (Section 9 – 3)**

Physical Treatment Steam Stripping (Section 9 – 3)

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Steam Stripping

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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 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

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**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

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**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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