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# Physical Treatment - PowerPoint PPT Presentation

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

Air Stripping

(Section 9 – 1)

• Tendency to move from solution to gas phase

• Function of:

• Vapor pressure (VP)

• Molecular weight (MW)

• Henry’s constant (H)

• Solubility (S)

• etc.

• Spray systems

• Aeration in contact tanks

• Tray towers

• Packed towers

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

• 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 Equations

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

• R = stripping factor

• C = concentration

• Process: mass balance on contaminant

• Initial assumptions:

• Previous

• Plus

• dilute solution

• no accumulation

• no reactions

• 100% efficient

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

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: Two-Film Theory

CL

CI

PI

PG

Bulk Liquid

Liquid Film

Air Film

Bulk Air

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 air-to-water ratios

• Determining KLa

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

• Slope = 1/HTU

• KLa = L/HTU

•  = 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

• Accounts for gas-phase and liquid-phase resistance

• Better for slightly soluble gases

• No empirical constants

• 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

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.

• 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)

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

• 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.

• 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

• Recover and reuse chemical

• Direct discharge

• Treatment

• 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

• 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

• Tray Towers

• Heavy foaming

• Trays corroded

• Packed Towers

• No means to recycle effluent to adjust influent flow

• Plugging due to heavy solids or tar in feed

### Physical Treatment

Steam Stripping

(Section 9 – 3)

• Strippability of organics

• Separation of organic phase from steam in decanter

• Fouling

• 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 Analysis

• High packing breakage due to thermal stresses

• Heavy fouling due to influent characteristics & elevated temperature