adsorption modeling update
TRANSCRIPT
11/15/2006 1
Oklahoma State UniversitySchool of Chemical Engineering
Adsorption Modeling UpdateK. A. M. Gasem
R. L. Robinson, Jr.(Principal Investigators)
J. E. FitzgeraldS. A. Mohammad
J. S. Chen
Oklahoma State University
Sponsored by theCoal-Seq II Consortium
November 2006
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Oklahoma State UniversitySchool of Chemical Engineering
Improved Adsorption Models for Coalbed Methane Production
and CO2 Sequestration
Our goal is to develop reliable coal-structure-based generalized equilibrium models that are suitable for
generalized coalbed methane (CBM) adsorption predictions and reservoir simulations.
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Oklahoma State UniversitySchool of Chemical Engineering
Measure pure CO2 adsorption isotherms on wetBeulah-Zap, Wyodak, Illinois #6, Upper Freeport, and Pocahontas #3 coals.
Measure the adsorption of pure methane, nitrogen, and CO2 on activated carbon at two levels of moisture.
Measure the adsorption of pure methane, and nitrogen on three wet Premium Argonne coals.
All measurements will be at 328.2 K and pressures to 13.8 MPa.
Experimental Program Modified Objectives
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Oklahoma State UniversitySchool of Chemical Engineering
Model Development Objectives1. Develop an equation of state (EOS) that is more
accurate at high densities (hard-sphere limit).
2. Develop robust algorithms to account rigorously for the presence of moisture.
3. Assemble a database for the adsorption of CBM gases on carbon matrices with special emphasis on coals.
4. Generalize the model parameters of the most successful of 2-D EOS, OK or SLD models in terms of accessible coal characterizations.
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Oklahoma State UniversitySchool of Chemical Engineering
First Year AccomplishmentsAcquired new adsorption data on five dry Argonne Premium coals. These data constitute a valuable addition to the CBM adsorption database.
Refined the Hard-Sphere EOS to include accurate predictions of water properties.
Demonstrated the ability of the SLD-HS framework to represent our data within their experimental uncertainties.
Developed SLD-HS model generalizations for dry coals.
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Oklahoma State UniversitySchool of Chemical Engineering
A Status ReportEffect of moisture on CO2 adsorption
CO2 adsorption on wet Pocahontas coal
CO2 adsorption on wet activated carbon at three levels of moisture
Water adsorption algorithm
Excess volume model for pure gas adsorption
EOS mixture adsorption modeling
Adsorption Database
Future work
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Oklahoma State UniversitySchool of Chemical Engineering
Experimental studies are underway to investigatehow moisture content may affect significantly the:
Adsorption capacityMixture adsorption behaviorData interpretation and reconciliationAdsorbed-phase density
Confirmation run completed for CO2 adsorption on wet Pocahontas Coal.
Measurements completed for CO2adsorption on wet activated carbon at 131 °F.
Moisture Effects
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Oklahoma State UniversitySchool of Chemical Engineering
Effect of Moisture Content on Gas Adsorption: Literature Observations
Presence of moisture reduces the gas adsorption capacity.
Moisture content above the equilibrium moisture level does not affect significantly the adsorption on wet coals.
The key question here is: Quantitatively, how does moisture level below equilibrium moisture content affect adsorption behavior?
Thus our focus is on isotherm adsorption measurements at more than one level of moisture.
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Oklahoma State UniversitySchool of Chemical Engineering
Effect of Moisture Content on Gas Adsorption: CO2 Adsorption on Pocahontas Coal at 131°F
0.0
0.4
0.8
1.2
1.6
2.0
0 400 800 1200 1600 2000 2400
Pressure (psia)
Exc
ess
Ads
orpt
ion
(mm
ol/g
)
Pocahontas Coal (Dry)
Pocahontas Coal (0.65% Moisture)
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Oklahoma State UniversitySchool of Chemical Engineering
Effect of Moisture Content on Gas Adsorption: Pure-Gas Adsorption on Wet Tiffany Coal at 130°F
0.0
100.0
200.0
300.0
400.0
500.0
600.0
0 200 400 600 800 1000 1200 1400 1600 1800 2000
Pressure (psia)
Gib
bs A
dsor
ptio
n (S
CF/
ton)
N2 (11.7% Moisture)
CH4 (5.6% Moisture)
CH4 (16.5% Moisture)
CH4 (11.7% Moisture)
CO2 (11.7% Moisture)
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Oklahoma State UniversitySchool of Chemical Engineering
Determining Moisture Content of Activated CarbonAdsorption measurements were conducted on two levels of moisture (27% and 34% moisture).
A modified ASTM D1412 coal procedure was used to determine the moisture content of F-400 AC.
According to ASTM D1412 the equilibrium moisture for F-400 AC is about 27%.
Calgon Corporation claims that F-400 AC can hold up to 50% adsorbed water when soaked in water.
CO2 adsorption at 34% moisture content was lower than adsorption at 27% moisture.
Therefore, we suspect 27% moisture is below the saturation level of F-400 AC
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Oklahoma State UniversitySchool of Chemical Engineering
Effect of Moisture Content on Gas Adsorption: CO2 Adsorption on Activated Carbon at 131°F
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
0 200 400 600 800 1000 1200 1400 1600 1800 2000Pressure (psia)
Exc
ess
Ads
orpt
ion
(mm
ol/g
)
Dry
34% Moisture
27% Moisture
15 % Moisture
Changes in Concavity
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Oklahoma State UniversitySchool of Chemical Engineering
Effect of Density Correction: CO2 Adsorptionon Wet Activated Carbon at 131°F
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
0 200 400 600 800 1000 1200 1400 1600 1800 2000Pressure (psia)
Exc
ess
Ads
orpt
ion
(mm
ol/g
)
27% Moisture
27% Moisture (Density Correction)
5% Gas Density Correction
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Oklahoma State UniversitySchool of Chemical Engineering
Kinetics of CO2 Adsorption on Wet Activated Carbon
0
50
100
150
200
250
300
350
400
450
100 200 400 600 800 1000 1200 1400 1600 1800 2000Pressure Step (psia)
Equ
libra
tionT
ime
(hrs
)
27% Moisture
34% Moisture
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Oklahoma State UniversitySchool of Chemical Engineering
Kinetics of CO2 Adsorption on Wet Activated Carbon
Long equilibration times were observedEquilibration time for each pressure step lasted from one to three weeksLonger times were required for pressures below 1000 psia
Changes in concavity occurred below 1000 psia This may be attributed to (a) gas-phase CO2 stripping of moisture from the adsorbent, and/or (b) errors in the CO2 gas densities
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Oklahoma State UniversitySchool of Chemical Engineering
Effect of Moisture Content on Gas Adsorption: CO2 Adsorption on Illinois # 6 Coal at 131oF
0.0
0.4
0.8
1.2
1.6
2.0
0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200Pressure (psia)
Gib
bs A
dsor
ptio
n (m
mol
/ g
Coa
l)
Dry coal
4% Moisture
8% Moisture
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Oklahoma State UniversitySchool of Chemical Engineering
Effect of Moisture Content on Gas Adsorption:Literature Data - 1
Joubert’s study on effect ofmoisture on CH4 adsorption on Pittsburgh I Coal at 30°C
Negligible effect of moistureabove the equilibrium level on CH4 adsorption
Gregory et al.,1986, found similar results for CH4
adsorption on a set of coalsfrom Black Warrior Basin
Dry
4% Moisture
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Oklahoma State UniversitySchool of Chemical Engineering
Effect of Moisture Content on Gas Adsorption:Literature Data - 2
Adsorption of CH4 on wet activated carbon at 25 C reported by Zhou et al., 2001
They found that complete drying of a wet activated carbon is not necessary
Moisture content of less than 2% actually enhances CH4adsorption over the adsorption for dry activated carbon
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Oklahoma State UniversitySchool of Chemical Engineering
Desired Attributes of a Generalized Predictive Adsorption Model
The ability to predict mixed-gas adsorption from pure-gas isotherms.
The ability to generalize the pure-gas isotherms from the characteristics of the coal.
Robust algorithms to account rigorously for thepresence of moisture in the coal.
Toward this end, we use our newly developed HS-EOS within the SLD adsorption theory.
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Oklahoma State UniversitySchool of Chemical Engineering
EOS Modeling ApproachExtend HS-SLD adsorption model to mixture predictions for dry and wet coals
Thus far we have generalizations for pure-gas adsorption on dry coals
Generalize PR-SLD model parameters for adsorption predictions involving pure gases and mixtures, including both dry and wet coals.
Thus far we have an adsorption model for pure gases and mixtures
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Oklahoma State UniversitySchool of Chemical Engineering
Modeling of Pure-Water AdsorptionModeling supercritical-gas adsorption allows forone possible adsorbed phase-density in a pore at a given pressure and temperature.
However, models of water adsorption (liquid or vapor)show multiple roots within the pore.
Gibbs Energy Minimization and Stability Analysisprocedures are used to determine the correct phaseof water in the pore.
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Oklahoma State UniversitySchool of Chemical Engineering
Adsorption of Water using HS-SLD Model
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0 0.5 1 1.5 2 2.5 3 3.5 4Pressure (psia)
mas
s fra
ctio
n of
wat
er a
dsor
bed(
abso
lute
)
Dipole=1.00, L=1.2Dipole=0.80, L=1.2Dipole=0.60, L=1.2Dipole=1.00, L=5.0
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Oklahoma State UniversitySchool of Chemical Engineering
Accounting for the Moisture Effect: SLD-HS Model + Excess Volume Model
We use an excess volume term to represent themixture adsorbed-phase density in terms of the purecomponent adsorbed-phase densities:
( ) ( )Ex
20Hads
Abs20H
2COads
Abs2CO
adsvxx1 +
ρ+
ρ=
ρ
AbsO2H
Abs2CO
Ex xRTCxv =
The adsorbed-phase density, ρads, for CO2 is obtainedfrom the generalized SLD model. For water, the adsorbed-phase density is assumed to be 1.0 g/cc.
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Oklahoma State UniversitySchool of Chemical Engineering
Solution of Excess Volume Model
( )( )( )( )2CO,ads20H,ads
Ex2CO2COgas20H,ads
Ex20H,ads
Ex2CO2COgas20H,ads
Ex2COAbs
2CO /1y
v1yx
ρρ−ξ−ρ+ρ
ρ+ξ−ρ+ρξ=
For the CO2-water system, the absolute mole fractionof the adsorbed CO2 is obtained from a massbalance:
Ex,Totali
ExiEx
i nn≡ξwhere Abs,Total
i
AbsiAbs
in
nx =
( )igasAbsiadsads
Exi yxVn ρ−ρ=
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Oklahoma State UniversitySchool of Chemical Engineering
Solution of Excess Volume Model (Continued)
Given a trial solution of , the absolute mole fractioncan be explicitly written as:
Ex2COξ
( ) 4/1dxd2dd21x 2CO
idealAbs2CO2CO
22CO2CO
Abs2CO +−+±−=
( )( )RTCy2
11y1
d Ex2CO2COgas
2CO,ads20H,ads
Ex2CO2COgas
2COξ−ρ
⎟⎟⎠
⎞⎜⎜⎝
⎛
ρ−
ρξ−ρ+
=where
Adjust the trial solution until the following relationshipis satisfied: ( )
gasads
gasAbs
2COadsEx2CO
xρ−ρ
ρ−ρ=ξ
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Oklahoma State UniversitySchool of Chemical Engineering
Parameterization of Excess Volume Model
The excess volume constant C is a function of density:
AbsO2H
Abs2CO
Ex xRTCxv = gas29.0gas
1 CCC ρ+ρ
=
Constants C1 and C2 are functions of thefixed carbon (FC) content:
43
1 CFC100
CC +−
= 65
2 CFC100
CC +
−=
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Oklahoma State UniversitySchool of Chemical Engineering
Generalized Model of CO2 Adsorption on Beulah Zap Coal at 131 F
0.0
0.5
1.0
1.5
2.0
0 400 800 1200 1600 2000Pressure (psia)
Exce
ss A
dsor
ptio
n (m
mol
/g)
Beulah Zap (Dry)Beulah Zap (32% Moisture)
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Oklahoma State UniversitySchool of Chemical Engineering
Generalized Model of CO2 Adsorption on Pocahontas Coal at 131 F
0.0
0.4
0.8
1.2
0 400 800 1200 1600 2000Pressure (psia)
Exce
ss A
dsor
pion
(mm
ol/g
)
Pocahontas (Dry)Pocahontas (0.65% M oisture)
11/15/2006 30
Oklahoma State UniversitySchool of Chemical Engineering
Generalized Model of CO2 Adsorption on Upper Freeport Coal at 131 F
0.0
0.4
0.8
1.2
0 400 800 1200 1600 2000
Pressure (psia)
Exce
ss A
dsor
ptio
n (m
mol
/g)
Upper Freeport (Dry)Upper Freeport (1.1% Moisture)
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Oklahoma State UniversitySchool of Chemical Engineering
Generalized Model of CO2 Adsorption on Wyodak Coal at 131 F
0.0
0.5
1.0
1.5
2.0
0 400 800 1200 1600 2000Pressure (psia)
Exce
ss A
dsor
ptio
n (m
mol
/g)
Wyodak (Dry)Wyodak (28% Moisture)
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Oklahoma State UniversitySchool of Chemical Engineering
Generalized Model of CO2 Adsorption on Illinois Coal #6 at 131 F
0.0
0.4
0.8
1.2
1.6
2.0
0 400 800 1200 1600 2000Pressure (psia)
Exce
ss A
dsor
ptio
n (m
mol
/g)
Illinois #6 (Dry)Illinois #6 (9.2% Moisture)
11/15/2006 33
Oklahoma State UniversitySchool of Chemical Engineering
WAAE
Quality of HS-SLD Generalized Predictions for Adsorption on Coals
Overall WAAE is 0.81
i
N
i
calc nnabs
NWAAE ∑
=⎟⎟
⎠
⎞
⎜⎜
⎝
⎛ −=
1 exp
exp1σ
Objective Function:
Methane Nitrogen CO2 Wet CO2
Beulah Zap 0.37 0.36 0.46 0.70Wyodak 0.34 1.07 1.32 0.36Ilinois #6 0.32 0.20 0.96 1.50
Upper Freeport 0.43 0.71 1.37 0.80Pochahontas 0.62 1.79 0.98 0.78
11/15/2006 35
Oklahoma State UniversitySchool of Chemical Engineering
Adsorption Database An expanded adsorption database for CBM gases (CO2, CH4, N2) is being assembled.
Fifty-one new systems have been identified involving both pure
gases and mixtures.
Each “system” has at least one isotherm.
Database includes adsorption isotherms on coals and
activated carbon under wet and dry conditions.
Individual authors have been contacted to request their
numerical data.
11/15/2006 36
Oklahoma State UniversitySchool of Chemical Engineering
Future Work in Balance of Year 2Continue pure-gas adsorption measurements; specifically measure:
Methane and nitrogen on wet activated carbon
Methane and nitrogen on wet Argonne Premium coals
Evaluate mixing rules for the new HS-EOS.
Continue development of VLE / Adsorption algorithms.
Expand the model generalizations.
11/15/2006 37
Oklahoma State UniversitySchool of Chemical Engineering
2: Pure on Wet Activated Carbon
4: Model Generalization
3: Database Assembly
2: Algorithm Development
1: EOS Development
Model Development
3: Pure on Selected System
1: Pure on Wet Coals(Completed)
Experimental Work
Jan-May2007
Oct-Dec2006
Jul -Sep 2006
Apr -Jun 2006
Jan–Mar 2006
Oct-Dec 2005
Apr-Sep 2005
Task
Project Schedule
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Oklahoma State UniversitySchool of Chemical Engineering
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
0.0 0.1 0.2 0.3 0.4 0.5
Normalized Slit Width
Loca
l Den
sity
, g/c
cStrategy: Use rigorous methodologies rooted in fundamentals to develop reliable models…
Bulk Gas
Adsorbate
Mean Field Approximation
11/15/2006 39
Oklahoma State UniversitySchool of Chemical Engineering
Molecular Interactions
Gas Molecule
z L - z
Coal Surface
( ) ( ) ( )zLzz 2fs1fsfs −µ+µ=µ
ff
fs1 fs2
ρwall = 1 / b
Local Density
Area / 2
11/15/2006 40
Oklahoma State UniversitySchool of Chemical Engineering
Molecular Interactions
Gas Molecule
z L - z
Coal Surface
( ) ( ) ( )zLzz 2fs1fsfs −µ+µ=µ
ff
fs1 fs2
ρwall = 1 / b
Local Density
Area / 2
11/15/2006 41
Oklahoma State UniversitySchool of Chemical Engineering
The Simplified Local Density (SLD) Model
z L-z -
AdsorbentSurface
Adsorbed Phase
Bulk Phase[ ]bayPTf i
bulki ,,,,ˆ r
[ ]bzazxzPTf iadsi ),(),(),(,,ˆ rρ
[ ]zfsiΨ
( )( )
( ) ( )0
ˆ)(),(ˆ
ln =−Ψ+Ψ
+⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
kTzLz
yf
zzxf fsi
fsi
bulki
adsi
r
r ρEquilibrium Relationship: