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Research Project to Remove Siloxanes from Digester Gas
County Sanitation Districts of Los Angeles County
Industrial Silicone Polymer Lubricant Antiperspirant Skin
Care
DrainLandfills
DEFINITION OF SILOXANES AND FATE OF SILOXANE BEARING PRODUCTS
WWTP
Why Treat Digester Gas For Siloxane Removal?
When combusted, siloxanes form silicon dioxide, which can coat equipment, cause damage to equipment and cause a loss of heat transfer efficiency.
Heat Recovery Steam GeneratorsW/ Silicon Dioxide Deposits 20% Loss of Heating Value
Loss of Heat Value and Equipment Damage
$400,000/year
Clean
Treatment Costs $200,000/year
Digester Gas
Gas Turbine
Downstream Heat Recovery Equipment Becomes Coated with Silicon Dioxide Deposits
Existing Digester Gas Pretreatment
Digester Gas
Octamethylcyclotetrasiloxane (D4)
O
O O
O
Me2Si
Me2Si
Si Me2
SiMe2
Physical Properties of D4 Siloxane
Water Solubility
Low 74 micrograms/
L Vapor
Pressure High 1.0 mm Hg
at 25oC Henry’s Law
Constant High 3 to > 17
(Benzene is 2.2)
Existing JWPCP Gas Pretreatment System
• Venturi- Removes Dust• Mist Separator• Cooling Coil- 50oF• Compressor- 350 PSIG• Condenser- 40 oF. Installed a few years
after the original system to remove VOCs.
Siloxanes Concentrations at Various DigesterGas Pretreatment Locations
3.0
4.0
2.2
3.0
1.71.5
6.0
1.6 1.7
0.4
0.0
2.0
4.0
6.0
7.0
Meso. Dig.Thermo. Dig. Venturi Compressor Condenser
PPM
, (V/
V)
D4D5
Combined D4 AND D5 AT JWPCP AND OTHER TREATMENT PLANTS
2.1
1.20
0.20
1.05
0.670.83
1.03
0.530.75 0.63
0
0.5
1
1.5
2
2.5
JWPCP Condens
erVale
ncia Digest
erPalm
dale D
igeste
Hyperion Compre
ssoO.C. Plan
t 1 Com
pO.C. Plan
t 1 Dig.9
&10O.C. Plan
t 2 Com
pIEUA Plan
t 1 Comp
IEUA Plant 2
CompLas
Virgene
s
Siloxane Removal Technologies
• Liquid Absorption• Condensation• Solid Phase Adsorption
Liquid Absorption
Digester Gas In
Absorbent Liquid
Gas Flow(with Contaminants or Particulates)
Absorbent Liquid Flow
Absorbed Contaminant in Droplet
Nozzle
Digester Gas Out
Scrubber Testing
• Water Not Effective• Bench Scale Impingers Efficient
When Digester Gas is Bubbled into Solution (Polypropylene Glycol, Methanol, Ethoxy-based Detergent)
• Pilot-Plant Testing at 50 scfm showed that much more scrubbing agent needed in both Venturi and Packed Tower
Venturi-Condenser Pilot-Plant
Venturi /40oF Condenser Pilot-Plant Results
00.20.40.60.8
11.21.41.61.8
VenturiInfluent
VenturiEffluent
CondenserEffluent
D4D5
Silo
xane
Con
cent
ratio
n
Test Condenser
Percentage Siloxane Removal as Functionof Condenser Temperature
20
40
60
80
100
0-30 -20 -10 0 10 20 30 40 50
Perc
enta
ge S
iloxa
ne R
emov
al
With Coalescer
Additional Condenser Without Coalescer
Existing Plant Condenser (Alone)
Temperature (Degrees F)
Gas Adsorption with Solid MediaDirty Gas
Solid
Media
Gas Adsorbed onto Active Site of Solid Media
Solid Filter Gas Flow
Clean Gas
High Pressure ASME Rated Test Filter Vessels
Typical Siloxane Filter Breakthrough Curve
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
2 4 6 8 10 12Days of Operation
D4
Con
cent
ratio
n, p
pm (V
/V)
AAverage Influent Concentration During this Period was 1.35 ppm (V/V)
0 4,800 9600 14,400SCF/lb Graphite
Rapid Rise in Effluent Concentration at 7 days
Types of Solid Adsorbents• Polymeric Synthetic Proprietary Resin• Zeolite (Crystalline Aluminosilicates) • Silica Gel• Activated Carbon Graphite (with
Various Adsorption Capacity Ratings)• Activated Coconut Shell Based Carbon
Proprietary Resin
•Copolymer of Styrene/ Divinylbenzene
•Hydrophobic
•Low Affinity for Methane
•Micropores 100-200 Angstroms
Crystalline Aluminosilicates Clinoptilolite Zeolite
•Inexpensive
•High Sorptive Capacity
•Pore Size 11 Angstroms
•Adsorbs Large Molecules
•Typical Use of Zeolites is Adsorption of water in presence of non-polar solvents
Graphite Based Activated Carbon
•Carbon composed of polymorphous graphite
•Pore size 10-100 Angstroms
•Different Adsorbant Grades
Coconut Shell Based Carbon
•Inexpensive
•Good Experience with Coconut Shell Activated Carbon for Air Pollution Control
•Average Pore Size 20 Angstroms
Polymeric Resin Results
•Polymeric resin had high adsorptive capacity (20,000 scf gas/lb resin) on initial run but could not be regenerated with high efficiency.
•Along with siloxanes, other high molecular weight organics were adsorbed onto active sites in the resin.
•Overall rejection of the polymeric resin for this application. Resin good for low molecular weight contaminant such as methyl ethyl ketone (MEK)
Clinoptiolite Zeolite
0
1000
2000
3000
4000
5000
6000
7000
8000Sc
f Dig
este
r Gas
/lbZe
olite
Silica Gel Had Same Performance of Zeolite
Graphite Carbon Results
0
5000
10000
15000
20000
50/60/70 90CTC Number
SCF
Dig
este
r G
asFl
owPe
r L
b G
raph
ite
VirginMicrowaveKiln
80
Activated Carbon Results
1150012000
02000400060008000
100001200014000
Virgin
Regenerated
s
cf D
iges
ter
Gas
/lb.A
ctiv
ated
Car
n
02000
6000
10000
14000
18000
50/6
0/70
Coc
onut
Shel
l G.A
.C.
80 90
VirginMicrowaveKiln
Coconut Shell Solid Media PerformanceSC
F D
iges
ter G
as F
low
/Lb
Car
bon
Sil.G
el /Z
eol
Siloxanes at JWPCP 2000 and 2005
0.86
3.0
4.3
3.0
1.31.7
0.5
1.5
0.18
1.7
3.2
0.4
0.58
0
1
2
3
4
5
6
2000 2005 2000 2005 2000 2005
Mesophilic Digester Condenser Influent Condenser Effluent
Silo
xane
s C
once
ntra
tion
ppm
(v/v
)
D5D4D3
4.5
5.3
4.7 4.5
2.1
1.1
Potential Full Scale Design
•3- Filters; 2 on-line & 1 Stand By
•Each 30,000 lbs Graphite or Coconut Shell Carbon
•Each 10 ft. Diam. X 14 ft. Ht.
Estimated Capital & Operating Costs of 9,000 scfm Adsorption Filter System
System Graphite-Based
Coconut Shell NewAdditional
Condenser*CapitalCosts
$360,000 $360,000 $750,000
AnnualOperations/Mainten-
ance Costs
$160,000 $160,000 $85,000
*JWPCP already chills digester gas from 80oF to 40oF at capital cost of $500,000 and annual O/M of $64,000
Final Conclusions and Future Studies• Removal of siloxanes can be done
with adsorption and/or condensation
•Condensation/Adsorption costs $160,000 per year over 10 year period. Not treating costs $400,000/year
2006 Survey of Treatment Used
• LACSD: Carbon at Lancaster WRF Fuel Cell;Carbon and Silica Gel at Calabasas Landfill; Chiller at Palmdale Microturbine
• City of Los Angeles: Activated Carbon at Scattergood Electrical Station (Hyperion WWTP)
• Activated Carbon, Graphite and Chillers Used Elsewhere—”Second Generation” Low Temperature Chillers