Transcript Physical Treatment: Air Stripping

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)
PG
CG VP
H 


CL
CL
S
H e
B

 A 
T 

or e
A


B


T

 H
log H 
J
RC T
H
H 
RC T
'
AWWA Equation Factors
Compound
-3
H x 10
J
Oxygen
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
 4Q 
D

 L 
0.5
Depth of Packing Design Equations
Assumptions:
– Plug flow
– Henry’s Law applies
– Influent air contaminant
free
– Liquid and air volumes
constant
Depth of Packing
 Cin

R  1  1

 L  R  Cout

  ( HTU )(NTU )
Z  
 ln 
R

 K L a  R  1  


– 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
More on Stripping Factor
actual operatingair  to  water ratio
R
theoretical min im umair  to  water ratio
G / L operating
R
G / L minimum
(G / L) minimum
Cin  Cout 1

Cin
H'
(G / L)operating  R(G / L)minimum
KLa: Two-Film Theory
CL
CI
PI
Bulk Liquid
Liquid Film
Air Film
PG
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 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
2.5
NTU
2
1.5
1
0.5
0
0
2
4
6
Z (m)
8
10
Sherwood-Holloway Equation
1 n

L
K L a   DL  0.305 


–
–
–
–
–
  


 DL 
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/
0.5
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
12
490
0.28
25
560
0.28
38
525
0.28
75
360
0.28
Berl saddles
Tile
DL: Wilke-Change Method
7
5.06x10 T
DL 
0.6
V
•
•
•
•
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
Compound
Carbon tetrachloride
Trichloroethylene
Benzene
Chloroform
Vinyl chloride
Chloromethane
Methane
B x 10
2.76
2.86
3.04
3.12
3.85
4.49
6.18
15
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-towater 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