Isothermal and Nonisothermal Kinetic Analyses of Mahogany Oil Shale with TGA

Transcription

1 Isothermal and Nonisothermal Kinetic Analyses of Mahogany Oil Shale with TGA Pankaj Tiwari, Kyeongseok Oh and Milind Deo Chemical Engineering Dept, University of Utah 1

2 Table of contents Introduction Background Experiments (TGA) Pyrolysis (N 2 environment) Combustion (Air environment) Kinetic study (TGA) Isothermal Nonisothermal Results & Discussion Conclusions 2

3 Oil shale: Source of unconventional energy Organic matter Bitumen (soluble in organic solvent) Kerogen (significant portion of TOC, total organic carbon) Mineral constituents Carbonates: calcite, dolomite, etc. Kerogen, A chemically immature hydrocarbon - essentially, oil's geological ancestor Thermal/Chemical decomposition- Released petroleum-like substance Retorting (Pyrolysis)» In-Situ retorting» Surface retorting Products include Synthetic crude oil liquid Gases Residual solid Oil Shale Shale Oil jdwaggoner.wordpress.com/2008/03/ 3

4 Kerogen classification H/C (atomic ratio) Van Krevelen diagram I Green River Formation II III O/C (atomic ratio) Hutton et al., Energy Fuels

5 Previous studies: Mechanism Approach 1. Kerogen Heavy Oil + Light Oil + gas + CH 4 + char Heavy Oil Light Oil + gas + CH 4 +char Light Oil gas + CH 4 + char Gas CH 4 + char Char CH 4 + gas + coke All reactions are assumed to be first order Approach 2. Approach 3. Kerogen Bitumen Products Two steps Organic matter Intermediate Kerogen Products Single step And many others First order reaction with respect to decomposition of the reactants 5 Approach n.

6 Kinetic study Rate constant (K) Reaction order (n) Activation energy (Ea) The rate at which a kinetic process proceeds depends on timetemperature history The knowledge of kinetic parameters is important to optimize the process well locations, heat input rates, etc. 6

7 Thermo Gravimetric Analysis (TGA) Weight loss/derivative versus Time/Temperature Calibration Mass Temperature Operating condition Purge gas 60ml/min Weight --20~30 mg Particle size ~100 mesh size Environment N 2 and Air Temperature Isothermal and Non-isothermal Reproducibility TGA:Q

8 TGA: Reproducibility of data Organic Mineral 8

9 Conversion α = Kinetic study with TGA Conversion and rate equation ( W ( W 0 0 W W ) ) W 0 = Initial weight of the sample, mg W t = Weight of the sample at time t, mg W = Weight of the sample at the end of analysis, mg X = Normalization Factor t or α = ( W ( W W t ) X ) Arrhenius dependency : K = A. exp(-ea/rt) First order assumption dα = dt Ea A exp( RT 0 0 ) (1 α ) Constant Temperature dα dt = A exp( β Ea RT ) (1 α ) Constant Heating Rate dt β = dt

10 Kinetic study with TGA Conversion and rate equation ( W0 Wt ) α = ( W0 W ) dα A Ea = exp( ) (1 α) dt β RT Isothermal TGA curve Analysis criteria 100 o C/min 1000 o C (0.5 o C/min) Integral Method Differential Method Mathematical method Activation energy ( Ea) Pre-exponential factor (A) Start End Isothermal C

11 Kinetic study with TGA Conversion and rate equation ( W0 Wt ) α = ( W0 W ) dα A Ea = exp( ) (1 α) dt β RT Non-isothermal TGA curve Analysis criteria 100 o C/min 1000 o C (0.5 o C/min) Mathematical method Activation energy ( Ea) Pre-exponential factor (A) Direct Arrhenius Plot Coats & Redfern Friedman Method Maximum rate method Anthony & Howard Chen & Nuttall R max T start T end

12 Analysis criteria 100 o C/min 1000 o C (0.5 o C/min) Mathematical method Activation energy ( Ea) Pre-exponential factor (A) Kinetic study with TGA Conversion and rate equation ( W0 Wt ) α = ( W0 W ) dα A Ea = exp( ) (1 α) dt β RT Experimental conditions Isothermal Nonisothermal T ISOTHERMAL N 2 _ Normalized data Total time Isothermal _N 2 Initial weight o C min mg Time Start Isothermal condition Weight Loss Normalization factor Isothermal analyses cannot be performed at these temperatures, because most of the organic matter decomposes before this temperature is attained X

13 Kinetic study with TGA Conversion and rate equation ( W0 Wt ) α = ( W0 W ) dα A Ea = exp( ) (1 α) dt β RT Analysis criteria 100 o C/min 1000 o C (0.5 o C/min) Mathematical method Activation energy ( Ea) Pre-exponential factor (A) Experiment conditions Isothermal Nonisothermal NONISOTHERMAL Heating rate Initial weight Non-isothermal _N 2 T Start End Max wt % T Wt % Tmax Beta mg o C o C o C N 2 _environment Wt % 13 %/min

14 Kinetic study with TGA Isothermal: Integral method (first order) ln( 1 α ) = k.( t t o ) Integral method _N 2 Arrhenius plot _N 2 t o is the time at the start of the constant- temperature period (isothermal condition). Thermal induction part is eliminated from the kinetic analysis and correspondingly the W is corrected. α and x, are same and representing conversion N 2 Ea, KJ/mol A E+09

15 Non-isothermal Kinetic study with TGA Conversion 15

16 Kinetic study with TGA Non-isothermal: Differential method (Direct Arrhenius plot) dα = dt A exp( β Ea R T Simplified form for first order ) (1 α ) 1 dα A Ea ln = ln( ) ( ) (1 ) dt α β R T N2_Nonisothermal_Differential N 2 _Nonisothemal_Differential β R 2 slope Intercept Ea A E E E E+10 [ln ( 1/(1-x) dx/dt)] N2_0.5C N2_1C N2_2C N2_5C N2_10C N2_20C N2_50C 1/T, k 16

17 Kinetic study with TGA Non-isothermal: Integral method (Coats-Redfern) α d α = exp( (1 α ) β R 0 A β ln( 1 α) 2 R T A Ea ln ln( 1 ) = ln( ) ( ) 2 R T Ea Ea R T Chen-Nuttall ln( 1 α ) ln 2 T T T 0 = Ea T ) dt A R Ea ln( ) ( Ea β R T Coats-Redfern N 2 _Nonisothemal_Integral method β R 2 slope Intercept Ea A ) [ln(-ln(1-x)/t2] N 2 _Nonisothemal_Integral method N2_0.5C -12 N2_1C N2_2C -13 N2_5C N2_10C N2 20C N2_50C E E E E /T 17

18 Kinetic study with TGA Non-isothermal: Friedman method d α ln( ) dt [ A (1 )] ( ) = ln α Ea R T N 2 _Nonisothemal_Friedman α Slope Intercept R 2 Ea kj/mol A LnA E E E E E E E E E E E

19 Kinetic study with TGA Non-isothermal: Maximum rate method (First order) β ln( ) = ln( 2 T A R Ea ) ( Ea R m T m ) N 2 _Nonisothemal_Maximum rate Ea, KJ/mol A 1.03E+06 19

20 Results and Discussion Non-isothermal: Integral vs Differential method Activation Energies Activation energy increases as heating rate increase Higher activation energy using the integral method

21 Results and Discussion Non-isothermal: Integral vs Differential method Trade-off between A and E [lna = a + be] Good fit using both the methods

22 Results and Discussion Non-isothermal: Integral vs Differential method ln K vs 1/T relationship Both the methods suggest/support, pyrolysis is single stage process

23 Results and Discussion Isothermal_N 2 Weight loss increases as temperature increase 10-12% weight loss (TOC) at around ºC lnk vs 1/T plot suggests its single stage process Integral method gives 134 KJ/mol, activation energy 23

24 Results and Discussion Non-isothermal_N % weight loss (TOC) Maximum rate of weight loss is higher at higher temperature Pyrolysis temperature shifts to higher temperature as heating rate increase Four methods were used and they give distribution of activation energy Differential (74-147, KJ/mol) Integral (89-189, KJ/mol) Friedman ( , KJ/mol) Maximum rate method (78, KJ/mol) Activation energy increases as heating rate increase Differential and integral methods fit the data best Integral method gives higher activation energy than the differential method 24

25 Kinetic study with TGA Non-isothermal _Air 25

26 Isothermal _Air Results and Discussion Integral method_air Ea, KJ/mol A Non-isothermal _Air E+07 26

27 Conclusions TGA gives intrinsic kinetics with effective kinetic parameters Pyrolysis of Mahogany oil shale is single stage and first order process Integral method gives higher activation energy than differential method Friedman approach gives highest activation energy values Kinetic parameters are in the range of values reported in the literature Pyrolysis, mechanism and kinetic parameters depend on origin and geological condition More weight loss in air environment than N 2 environment Activation energy is less in air environment than N 2 for isothermal 27

28 Acknowledgement Department of Energy Utah Geological Survey- Samples Utah Heavy Oil Program members Petroleum Research Center members 28

29 Thank you

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