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)

TR

HH

JTR

HH

eoreH

S

VP

C

C

C

PH

C

C

BT

A

T

BA

L

G

L

G

'

log

AWWA Equation Factors

Compound H x 10-3 JOxygen 1.45 7.11

Methane 1.54 7.22

Hydrogen sulfide 1.85 5.88

Carbon dioxide 2.07 6.73

Carbon tetrachloride 4.05 10.06

Trichloroethylene 3.41 8.59

Bezene 3.68 8.68

Chloroform 4.00 9.10

Henry’s Law Constants

Equipment

• Spray systems

• Aeration in contact tanks

• Tray towers

• Packed towers

Aeration in Tanks

Tray Towers

Packed Towers

Liquid Distribution Systems

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

Packing

Fouling

Cleaning Packing

Comparison: Equipment

Design, in General

• Tower diameter function of design flow rate

• Tower height function of required contaminant removal

Diameter of Column

504

.

L

QD

Depth of Packing Design Equations

Assumptions:

– Plug flow– Henry’s Law applies– Influent air contaminant

free– Liquid and air volumes

constant

Depth of Packing

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

))((

11

ln1

NTUHTUR

RCC

R

R

aK

LZ out

in

L

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

More on Stripping Factor

imum

operating

LG

LGR

ratiowatertoairimumltheoretica

ratiowatertoairoperatingactualR

min/

/

min

'

1)/( min HC

CCLG

in

outinimum

imumoperating LGRLG min)/()/(

KLa: Two-Film Theory

Bulk Liquid Bulk AirLiquid Film Air Film

CL

PG

CI

PI

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

Example: Column Test

Sampling Port Depth (m) TCE (µg/L)

0 230

2 143

4 82

6 48

8 28

Example continued

0

0.5

1

1.5

2

2.5

0 2 4 6 8 10

Z (m)

NT

U

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/

5.01

305.0

L

n

LL D

LDaK

Sherwood-Holloway Constants

Packing Size (mm) n

Raschig rings 12 920 0.35

25 330 0.22

38 295 0.22

50 260 0.22

Berl saddles 12 490 0.28

25 560 0.28

38 525 0.28

Tile 75 360 0.28

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)

6.0

71006.5

V

TxDL

DL: Conversion Constant B

Compound B x 1015

Carbon tetrachloride 2.76Trichloroethylene 2.86Benzene 3.04Chloroform 3.12Vinyl chloride 3.85Chloromethane 4.49Methane 6.18

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 2302 524 216 68 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

Physical Treatment

Steam Stripping

(Section 9 – 3)

Steam Stripping

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

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

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