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8/7/2019 Gasem-CO2-presentation (CBM)
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March 6, 2002 1
Oklahoma State University
School of Chemical Engineering
Modeling of Gas Adsorptionon Coalbeds
K. A. M. GasemZ. Pan
J. E. FitzgeraldM. Sudibandriyo
R. L. Robinson, Jr.
Oklahoma State University
Sponsored by theU.S. Department of Energy
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March 6, 2002 2
Oklahoma State University
School of Chemical Engineering
RATIONALE
Modeling of the adsorption behavior of coalbed gases(methane, CO 2, nitrogen) is essential in CBMproduction and in CO 2 sequestration.
Further, knowledge of the competitive adsorption ofCBM gases is required to elucidate mechanisms forenhanced recovery of CBM and CO 2 sequestrationprocesses.
Reliable adsorption predictions cannot be generatedusing simple, empirical models. Accurate modelsrequire sound theory, judicious approximations, andaccurate experimental information.
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March 6, 2002 3
Oklahoma State University
School of Chemical Engineering
Cleat system wherefree gas resides
Micropores where
Adsorbed gas resides
Zoom-in viewof model micropore
Coalbed Adsorption Phenomenon
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March 6, 2002 4
Oklahoma State University
School of Chemical Engineering
Desorption
Diffusion toCleats
Free Gas DiffusionThrough Cleats
Reduce cleat pressure by producing water
Methane desorbs from matrix to cleats Methane and water flow to well bore
To Well-Bore
CoalMatrix
Water
Methane
Primary Recovery
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March 6, 2002 5
Oklahoma State University
School of Chemical Engineering
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
0 500 1000 1500 2000
Pressure, psia
Adsorption, m
mol/g
Absolute Adsorption on Fruitland Coal at 115F
CO 2
Nitrogen
Methane
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March 6, 2002 6
Oklahoma State University
School of Chemical Engineering
CoalMatrix
To WellBore
MethaneDesorption
CO 2Adsorption
Diffusion
CO 2Injection
WaterMethane
CO 2
CO 2 displaces methane after injection No initial breakthrough of CO
2 Higher cleat pressure results in faster flow
CO 2 Enhanced Recovery
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Oklahoma State University
School of Chemical Engineering
Theory / Practice
Theory: Improve our understanding of high-pressureadsorption through rigorous methodologies.
Practice: Provide reliable equilibrium adsorptionmodels for optimum CBM production andCO 2 sequestration.
Strategy: Use rigorous methodologies to developreliable adsorption models for industrial
practice. Goal: Develop reliable coal-structure-based
generalized predictions using simple,accessible characterizations.
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March 6, 2002 8
Oklahoma State University
School of Chemical Engineering
Current Issues
Adsorption modeling Coal characterization
Coal structure-based model generalizations
Estimates for adsorbed-phase density
Effect of moisture on modeling adsorption capacity
Matrix swelling
Binary and ternary pvT data
Balancing computational efficiency and reliability
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March 6, 2002 9
Oklahoma State University
School of Chemical Engineering
Current Issues: Adsorbed-Phase Density
Why do we need adsorbed-phase density? Current estimation methods:
! Traditional
! Experimental approximation! Model-based:
2D equations of state, SLD theory, Ono Kondo theory
What is the impact?
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Oklahoma State University
School 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
Local Density, g/cc
Molecular Interactions: Mean Field Approximation
Bulk Ga s
Adsorbate
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March 6, 2002 11
Oklahoma State University
School of Chemical Engineering
The excess adsorption is defined as follows
( )
( )bulkadsGibbs
VbulkadsGibbs
Vn
or
dVn
=
=
Excess and Absolute Adsorption
The absolute adsorption is defined as
ads
bulk
Gibbsadsadsabs
1
nVn
==
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March 6, 2002 12
Oklahoma State University
School of Chemical Engineering
Estimation of Phase Volume and Density
y = -0.360x + 0.365R2 = 0.994
0.00
0.05
0.10
0.15
0.20
0.250.30
0.35
0 0.2 0.4 0.6 0.8 1 1.2Density, g/cc
Adsorption, g CO2/g-carbon
Phase Density1.02 g/cc
0 400 800 1200 1600 2000
Pressure, psia9200
CO2 on activated carbon at 113 oF
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Oklahoma State University
School of Chemical Engineering
CO 2 and Ethane Adsorption on Activated Carbon (OSU)
0
2
4
6
8
10
0 2 4 6 8 10 12 14
Pressure (MPa)
Adsorption (mmol/g
Carbon Dioxide, Gibbs
Carbon Dioxide, AbsoluteEthane, Gibbs
Ethane, Absolute
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March 6, 2002 14
Oklahoma State University
School of Chemical Engineering
Impact of Adsorbed-Phase Density: CO 2 on Activated Carbon at 113 o F
0.0
2.0
4.0
6.0
8.0
10.0
0 200 400 600 800 1000 1200 1400 1600 1800 2000
Pressure (psia)
Adsorption (mmol/g AC)
ZGR -- 0.98 g/ccOK -- 1.00 g/ccExperimental -- 1.02 g/ccOKv -- variableSLDv -- variableZGRv -- variable
Traditional -- 1.18 g/ccGibbs Adsorption
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March 6, 2002 15
Oklahoma State University
School of Chemical Engineering
Current Issues: Adsorption Modeling
We seek simple, reliable adsorption equilibriummodels that are suitable for generalized predictionsand reservoir simulations.
Such models should be capable of! Precisely representing pure and mixture isotherms
! Facilitating a priori predictions
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March 6, 2002 16
Oklahoma State University
School of Chemical Engineering
Equilibrium Modeling: Three Methods
1. Enhanced forms of the Langmuir isotherms-- provide simple data correlation
2. Two-dimensional equations of state (2-D EOS)
(a) Cubic EOS (b) Segment-Segment EOS
-- facilitate generalized simulations
3. Simplified-Local-Density (SLD) models
-- account for surface structure and near-critical behavior
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Oklahoma State University
School of Chemical Engineering
The Langmuir & Loading Ratio Correlation (LRC)
P = pressure yi = gas phase mole fraction of component i
i = LRC exponent for component i i = amount adsorbed of component i Li, B i = Langmuir/LRC model coefficients
( )( )
+=
jii
ii
i
ii
i
PyB1PyB
L
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Oklahoma State University
School of Chemical Engineering
LRC Current Capability
The loading-ratio correlations:
Represent absolute pure-component and mixed-gastotal adsorption precisely
Yield reasonable predictions for these systems Represent individual-component adsorption in
mixtures less precisely, especially the less-
adsorbed ones Require adsorbed-phase density estimates
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Oklahoma State University
School of Chemical Engineering
LRC Representations: Illinois-6 Coal
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0 200 400 600 800 1000 1200 1400 1600 1800
Pressure (psia)
Absolute Adsorption (mmol/g coal) Pure CO2
Mixture TotalPure CH4
CO2 in Mixture
CH4 in Mixture
LRC
Mixture is 60/40 CH4/CO2
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March 6, 2002 20
Oklahoma State University
School of Chemical Engineering
2-D Equations of State (OSU, 1992)
[ ]A U W RT m
+
+ + =2 21
1( )
( )
=ji
x xi j ij =ji
x xi j ij
ij i j= +( ) / 2 ij i j=
EOS m U WVDW 1 0 0SRK 1 1 0PR 1 2 -1Eyring 1/2 0 0ZGR 1/3 0 0
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Oklahoma State University
School of Chemical Engineering
2D EOS Current Capability
2-D EOS models:
Describe CBM pure-component and mixed-gas totaladsorption data with sufficient precision
Yield reasonable predictions for these systems Represent individual-component adsorption less
precisely, especially the less-adsorbed ones
Employ inadequate repulsive terms Do not account for variations in coal structure
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March 6, 2002 22
Oklahoma State University
School of Chemical Engineering
2D EOS Representations: Illinois-6 Coal
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0 200 400 600 800 1000 1200 1400 1600 1800
Pressure (psia)
Absolute Adsorption (mmol/g coal)
Pure CO2Mixture TotalPure CH4CO2 in MixtureCH4 in MixtureZGR
Mixture is 60/40 CH4/CO2
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March 6, 2002 23
Oklahoma State University
School of Chemical Engineering
The EOS-SLD Adsorption Model
The fluid-solid interaction potential equals the sum of thepotentials between the gas molecule and the two sides of the slit.
Gas Molecule
z L - z
Coal Surface
( ) ( ) ( )zLzz 2fs1fsfs +=
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Oklahoma State University
School of Chemical Engineering
EOS-SLD Current Capability
The EOS-SLD models:
Account for variations in coal structure
Provide a viable framework for generalized predictions
Have produced promising preliminary results formixture adsorption modeling
Employ inadequate repulsive terms
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Oklahoma State University
School of Chemical Engineering
CO 2 Adsorption Using Modified SLD-PR
0
1
2
3
4
5
6
7
8
0 2 4 6 8 10 12 14
Pressure (MPa)
Gibbs Excess
(mmol/g)
Experimental Data OSU
Original SLD
Modified SLD
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Oklahoma State University
School of Chemical Engineering
Sample Pure-Gas Adsorption Model Results
%AAD
Nitrogen Methane CO 2 Ethane
Dry Activated Carbon
LRC 0.3 0.6 6.1 5.8
ZGR 0.4 0.7 5.2 5.6ZGR Gibbs 0.4 0.5 1.3 3.3Original SLD 0.5 0.8 14.9 29.7Modified SLD 0.4 0.6 2.2 6.1
Wet Fruitland Coal
LRC 1.1 0.7 3.3ZGR 1.9 0.7 3.1
Original SLD 1.5 0.6 3.9Modified SLD 1.1 0.6 3.6
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Oklahoma State University
School of Chemical Engineering
Conclusions 2-D EOS and the EOS-SLD models are better
equipped than Langmuir-type correlations for modelingCBM adsorption isotherms.
The EOS-SLD models appear both accurate and
amenable to structure-based generalization.
Improved mixing rules and additional mixture data arerequired to improve predictions for individual-
component adsorption. More efforts should be dedicated to structure-based
model generalizations.
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March 6, 2002 28
Oklahoma State University
School of Chemical Engineering
Modeling Work in Progress at OSU
Extend the EOS-SLD and Ono Kondo models tomixture predictions.
Implement other potential models for fluid-solid
Interactions. Incorporate other geometries within the EOS-SLD
framework.
Develop theoretically-based equations of state thatfeature more accurate fluid-fluid repulsive terms.