Mathematical Modeling of Oil Shale Pyrolysis

Transcription

1 October, 19 th, 2011 Mathematical Modeling of Oil Shale Pyrolysis Pankaj Tiwari Jacob Bauman Milind Deo Department of Chemical Engineering University of Utah, Salt Lake City, Utah 1

2 Oil shale thermal treatment-pyrolysis 2

3 Research phase Key points Experimental studies Source material dependent System dependent Background More than 80 years worldwide More than 40 years at LLNL (USA) Different results Mechanism, kinetic and product distribution Modeling studies Formulation of heat and mass transfer effects Multiscale modeling Coupled physical and chemical phenomena 3

4 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 4

5 Experimental approach Oil shale pyrolysis process Sweep gas Heating the surface Simplified modeling approach Variation in r direction only 5

6 Modeling of pyrolysis process Single particle decomposition Shrinking core model Grain model Oil shale pyrolysis Grain Model Particle-mesh size TGA experiments BC s: Isothermal Nonisothermal 6

7 Modeling and simulation approach Model for oil shale thermal treatment Changes in the physical properties Temperature distribution Product distribution Concentration profile Heat Transfer Model (Shape and size) Kinetic Model (Distributed reactivity) Parameters Raw material properties Operating conditions Temperature Heating rate Pressure properties Convection heat Sweep/reactive gas Properties of products Heat capacity Equilibrium constant Density, etc. Thermodynamic Model (Distribution/lumping) Mass Transfer Model (Secondary reactions) Residence time distribution Time-temperature history Pressure Porosity and permeability Product distribution Quality and Yield COMSOL Multiphysics 7

8 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 8

9 COMSOL Multiphysics Heat transfer module Kinetic models Mathematical model 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 9

10 Governing equations Heat transfer equation Conduction and convection Cp T t Heat of reaction = - 370kJ/kg k T Q Cp u T (Camp W.D., LLNL) 0 ρ = overall density Cp = heat capacity k = thermal conductivity Q = Heat source/sink (heat absorbed by reactions) Species transfer equation Diffusion, convection and reaction term c t Rate equations i D AB c 0 i r i u c i c i = Mass/concentration of i D AB = diffusion coefficient =10-50 r i = reaction rate u = velocity vector Kerogen decomposition rate,[kg or mol/(m3.s)] 10

11 Physical properties- raw material Heat equation Grade -30gal/ton 18% organic matter Density of the raw material- function of organic contain (org) = [kg/m^3] - 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)] [Campbell et al., In -Situ (1978) 11

12 Oil shale pyrolysis- TGA Kerogen decomposition kinetic Weight loss Conversion Kinetic model Kinetic Parameters of Kerogen Decomposition Seven heating rates 0.5 o C/min to 50C/min [100 interval] Distribution of activation energy Activation energy,- E Pre-exponential factor -A Activation energy, kj/mol A.f(α), 1/s Extent of conversion 1.E+15 Distribution of A.f(α) 9.E+14 8.E+14 7.E+14 6.E+14 5.E+14 4.E+14 3.E+14 2.E+14 1.E+14 0.E Extent of conversion Tiwari and Deo, AIChE Journal (2011) 12

13 Reaction mechanism Single step mechanism Kerogen A a, E a a* Oil + b * Gas + c * Coke a : 63 b: 24 c: 13 Two step mechanism [Campbell-1978] A a, E a Kerogen a* Oil + b * Gas + c * Coke a : 63 b: 24 c: 13 A, E Oil d* Gas + e* Coke e: 80 f: 20 Multistep mechanism Kerogen decomposition Oil phase reaction Gas phase reaction Char decomposition Oil Shale Kerogen Products Liquid Gas Solid Heavy oil Oil Light oil Non-condesable Methane Char and Coke 13

14 Reactions- Pyrolysis Reaction Networks 1. Kerogen ----> a 1* HO + a 2* LO + a 3* Gas + a 4* Char +a 5* CH 4 [A a, E a ] 2. HO ----> b 1* LO + b 2* Gas + b 3* Char + b 4* CH 4 3. LO ----> c 3* Gas + c 4* Char + c 5* CH 4 4. Gas ----> d 4* Char + d 5* CH 4 5. Char ----> e 3* Gas + e 5* CH 4 + e 6* Coke Stoichiometric coefficients- Mole or mass Component KEROGEN HO LO GAS CHAR METHANE COKE C H Ratio MW Reaction scheme adopted from various sources [Burnham and Braun] Bauman and Deo Energy & Fuels (2011) 14

15 Isothermal-400C Results TGA Scheme- Single particle Single Step Mechanism K O + G+ C Noniosthermal-10C/min 15

16 Results TGA Scheme- Single particle Isothermal-400C Two Step Mechanism K O + G+ C O G +C Noniosthermal-10C/min 16

17 Results TGA Scheme- Single particle Isothermal-400C Multi Step Mechanism Noniosthermal-10C/min 17

18 Results TGA Scheme- Single particle Isothermal-400C Multi Step Mechanism Noniosthermal-10C/min 18

19 Heat application- Two cases Surface heating Lab scale experiments 1cm radius 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 Kinetic conversion- Combined isothermal and non-isothermal history 19

20 Surface heating Isothermal-400C Results- No flow Core sample -10[cm] radius Multistep Mechanism Noniosthermal-10C/min 20

21 Results- No flow and no convection Surface heating Core sample -10[cm] radius Isothermal-400C Multistep Mechanism Noniosthermal-10C/min 21

22 Results- No flow and no convection Surface heating Isothermal-400C Core sample -10[cm] radius Multistep Mechanism Noniosthermal-10C/min 22

23 Results- No flow and no convection Surface heating Core sample -10[cm] radius Multistep Mechanism Isothermal-400C Noniosthermal-10C/min 23

24 Products flow Continuity equation Darcy flow Velocity field is determined by the pressure gradient, the fluid viscosity, and the structure of the porous medium Porosity of oil shale ε = ( (Grade_OS xk) (Grade_OS xk) 2 ) Average pore diameter D p = 50e-6 [m] Permeability of oil shale [Kozney Carman] K = D 2 p ε 3 /(150 (1- ε) 2 ) Baughman Gary L. [1978] 24

25 Surface heating Core sample -10[cm] radius Multistep Mechanism With Convection Results- Darcy s law Pressure profile Nonisothermal-10C/min Velocity profile Isothermal-400C 25

26 Surface heating Core sample -10[cm] radius Multistep Mechanism Surface point Results- Effect of convection No convection Reaction rates of product With convection 26

27 Results- Comparison of the two different heating options Core sample -10[cm] radius Flux from Boundary- Average Isothermal-400C Surface heating- isothermal-400c Center heating- isothermal-400c 27

28 Summary 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. 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. 28

29 Acknowledgement 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 29

30 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) 30

31 0.01K/min profiles- Surface heating 10cm 31