Mathematical Modeling of Oil Shale Pyrolysis
Department of Chemical Engineering University of Utah, Salt Lake City, Utah
Pankaj Tiwari Jacob Bauman
Milind Deo
October, 19th , 2011
1 http://from50000feet.wordpress.com
Oil shale thermal treatment-Pyrolysis
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Background
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Research phase More than 80 years worldwide More than 40 years at LLNL (USA)
Key points Experimental studies
•Source material dependent
•System dependent
•Different results – Mechanism, kinetic and product distribution
•Formulation of heat and mass transfer effects
•Multiscale modeling
•Coupled physical and chemical phenomena
Modeling studies
Oil shale pyrolysis
Several Interrelated Physical and Chemical Phenomena
Heat transfer
Chemical reaction kinetics
Multiphase flow
Phase changes
Mineral alteration and interaction
Physical properties changes
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Heating the surface
Oil shale pyrolysis process Experimental approach
Sweep gas
Simplified modeling approach
Variation in r direction only
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Modeling of pyrolysis process
Shrinking core model Grain model
Single particle decomposition
Oil shale pyrolysis
Grain Model Particle-mesh size – TGA experiments
BC’s: Isothermal Nonisothermal
Modeling and simulation approach
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Heat Transfer Model (Shape and size)
Kinetic Model (Distributed reactivity)
Mass Transfer Model (Secondary reactions)
Thermodynamic Model (Distribution/lumping)
Properties of products
Heat capacity Equilibrium constant Density, etc.
Temperature distribution
Product distribution
Concentration profile
Product distribution
Quality and Yield
Operating conditions
Temperature Heating rate Pressure properties
Parameters
Raw material properties
Residence time distribution
Time-temperature history Pressure Porosity and permeability
Convection heat
Sweep/reactive gas
Model for oil shale thermal treatment
Changes in the physical properties
COMSOL Multiphysics
COMSOL Multiphysics
• COMSOL Multiphysics - finite element analysis and solver
software package for physics and engineering applications
• The main advantage of COMSOL is its ability to solve
coupled phenomena
• Many built-in modules including Chemical Reaction, Earth
Science, Acoustics, Heat transfer, etc.
• COMSOL also has a model library
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COMSOL Multiphysics Heat transfer module
Kinetic models
Three different kinetic models
Secondary reaction, coking and cracking
Darcy’s law - single phase flow
Transport of species module - mass based
Coupled governing equations Solved simultaneously
Appropriate changes in the physical properties
Mathematical model
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Governing equations
–ρ = overall density –Cp = heat capacity –k = thermal conductivity
–Q = Heat source/sink (heat absorbed by reactions)
•ci = Mass/concentration of i •DAB = diffusion coefficient =10-50
•ri = reaction rate •u = velocity vector
•Species transfer equation –Diffusion, convection and reaction term
•Heat transfer equation – Conduction and convection
•Rate equations
Kerogen decomposition rate,[kg or mol/(m3.s)]
Heat of reaction = - 370kJ/kg (Camp W.D., LLNL)
0
TuCpQTktTCp
iiiABi curcDtc
0
11 [Campbell et al., In -Situ (1978)
Physical properties- raw material
- rho_org = density of organic = 1050 [kg/m^3] - rho_shale = density of rock = 2700 [kg/m^3] - org = organic content = 0.18 wt% [unit less]
• Heat capacity of the raw material- function of oil yield and temperature = [ J/(kg*K)]
• Thermal conductivity of the raw material –function of oil yield and temperature = [W/(m*K)]
• Density of the raw material- function of organic contain (org) = [kg/m^3]
Heat equation • Grade -30gal/ton
• 18% organic matter
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Kerogen decomposition kinetic
• Oil shale pyrolysis- TGA
Kinetic Parameters of Kerogen Decomposition
Activation energy,- E
Pre-exponential factor -A
Seven heating rates – 0.5oC/min to 50C/min [100 interval]
0.E+00
1.E+14
2.E+14
3.E+14
4.E+14
5.E+14
6.E+14
7.E+14
8.E+14
9.E+14
1.E+15
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
A.f(α
), 1/s
Extent of conversion
Distribution of A.f(α)
0
50
100
150
200
250
300
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Activ
ation
energ
y, kJ/m
ol
Extent of conversion
Distribution of activation energy
Tiwari and Deo, AIChE Journal (2011)
Weight loss Conversion Kinetic model
Reaction mechanism
Single step mechanism
Kerogen a* Oil + b * Gas + c * Coke a : 63 b: 24 c: 13
Aa , Ea
Two step mechanism
Kerogen a* Oil + b * Gas + c * Coke a : 63 b: 24 c: 13 e: 80 f: 20
Aa , Ea
Oil d* Gas + e* Coke A , E
Multistep mechanism • Kerogen
decomposition • Oil phase reaction • Gas phase reaction • Char decomposition
Oil Shale
Kerogen
Liquid
Gas
Solid
Oil
Non-condesable Methane Char and Coke
Heavy oil Light oil
Products
[Campbell-1978]
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Reactions- Pyrolysis Reaction Networks
1. Kerogen ----> a1*HO + a2*LO + a3*Gas + a4*Char +a5*CH4
2. HO ----> b1*LO + b2*Gas + b3*Char + b4*CH4
3. LO ----> c3*Gas + c4*Char + c5*CH4
4. Gas ----> d4*Char + d5*CH4
5. Char ----> e3* Gas + e5*CH4 + e6*Coke
Stoichiometric coefficients- Mole or mass
Component KEROGEN HO LO GAS CHAR METHANE COKE
C 1479.000 31.751 11.189 3.354 1.004 1.000 1.185
H 2220.000 42.818 17.510 11.634 0.546 4.000 0.316
Ratio 1.501 1.349 1.565 3.468 0.544 4.000 0.267
MW 20000.550 424.492 152.034 52.011 12.604 16.042 14.552
Reaction scheme adopted from various sources –[Burnham and Braun] Bauman and Deo Energy & Fuels (2011)
[Aa , Ea]
Results TGA Scheme- Single particle
Isothermal-400C Noniosthermal-10C/min Single Step Mechanism
K O + G+ C
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Results TGA Scheme- Single particle
Isothermal-400C Noniosthermal-10C/min
Two Step Mechanism
K O + G+ C OG +C
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Results
Isothermal-400C Noniosthermal-10C/min
Multi Step Mechanism
TGA Scheme- Single particle
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Results
Isothermal-400C Noniosthermal-10C/min
Multi Step Mechanism
TGA Scheme- Single particle
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Heat application- Two cases
Surface heating Lab scale experiments
Center heating Reservoir thermal treatment
Surface heating- Products travel from cold to hot zone- fast secondary reactions Center heating- Products hit low temperature/pressure – condensation
1cm radius
Kinetic conversion- Combined isothermal and non-isothermal history 19
Results- No flow
Core sample -10[cm] radius
Isothermal-400C Noniosthermal-10C/min Multistep Mechanism
Surface heating
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Core sample -10[cm] radius
Isothermal-400C Noniosthermal-10C/min Multistep Mechanism
Results- No flow and no convection
Surface heating
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Core sample -10[cm] radius
Isothermal-400C Noniosthermal-10C/min
Multistep Mechanism
Results- No flow and no convection
Surface heating
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Core sample -10[cm] radius
Isothermal-400C Noniosthermal-10C/min
Multistep Mechanism
Results- No flow and no convection
Surface heating
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Products flow
ε = 0.003+(0.0146+0.0129 ∙(Grade_OS∙xK)-0.000046 ∙(Grade_OS ∙xK)2)
Porosity of oil shale
K = Dp2 ∙ ε 3/(150 ∙(1- ε)2)
Permeability of oil shale [Kozney –Carman]
Average pore diameter
Dp = 50e-6 [m]
Velocity field is determined by the pressure gradient, the fluid viscosity, and the structure of the porous medium
Continuity equation
Darcy flow
Baughman Gary L. [1978]
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Results- Darcy’s law Surface heating Core sample -10[cm] radius Multistep Mechanism With Convection
Velocity profile Pressure profile
Isothermal-400C
Nonisothermal-10C/min
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Results- Effect of convection Surface heating Core sample -10[cm] radius Multistep Mechanism Surface point
Reaction rates of product
No convection With convection
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Core sample -10[cm] radius Flux from Boundary- Average Isothermal-400C
Results- Comparison of the two different heating options
Center heating- isothermal-400C Surface heating- isothermal-400C
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Summary
• Local thermodynamics of the phase changes may alter the product distribution.
• Mineral reactions can be important to generate the gas pressure, may also
participate in the reaction network.
• The development of the comprehensive model will depend on Literature.
• Heterogeneity of raw material is crucial.
• Other physical process -Expansion and fractures.
• Reliable mechanism of product formation is required.
• Kinetics play an important role in product distribution/formation.
• Secondary reactions regulate the final products.
• Study of time-temperature is important to optimize the desired products.
• Many assumptions.
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Department of Energy [DOE] – Financial support
Member of Institute for Clean and Secure Energy [ICSE]
Member of Petroleum Research Center [PERC]
COMSOL Multiphysics- Academic License
Acknowledgement
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Literature • Mathematical modeling of In-situ oil shale retorting(George and
Harris 1977) • Pyrolysis kinetics for oil Shale particles(Granoff and Nuttall 1977) • PMOD: A flexible model of oil and gas generation, cracking and
expulsion(Braun and Burnham 1991) • Mathematical model of oil generation, degradation, and
expulsion(Braun and Burnham 1990) • Efficient formulation of heat and mass transfer in oil shale retort
models(Parker and Zhang 2006). Heat Conduction Modeling Tools for Screening In Situ Oil Shale Conversion Processes(Symington and SPiecker 2008)
• Practical kinetic modeling of petroleum generation and expulsion(Stainforth 2009)
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0.01K/min – profiles- Surface heating
10cm
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