mathematical modeling of oil shale pyrolysis · 2012-05-18 · mathematical modeling of oil shale...

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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