Shown in Table 1 are the operating conditions for the reactor model used to generate and find the kinetic parameters. The reactor is an ideal plug flow reactor with a constant operating temperature and pressure. Reactor temperature varied from 600°C to 800°C in 50 degrees increments. The reactor pressure was kept constant at 23 MPa, similar to the study where the reaction mechanism was obtained [10]. Inlet velocity was kept constant as well as the inlet concentration of PP at 5%, weight basis. The reactor length varied from 0.24m to 0.25m in 0.05m increments in a descending order, opposite to the reactor temperature. The logic behind varying the reactor length while maintaining the same flow velocity, is to obtain similar concentration profiles versus time from all temperatures. That help generate good quality data (concentrations versus residence time) and useful objective function values while changing the reactor temperature.
Shown in Table 2 is the reaction mechanism and kinetic parameter used to generate concentration versus residence time data from an ideal PFR. Time dependant concentrations are chosen since that’s the common output that can be obtain experimentally. Time dependency data can be obtained, experimentally, by sampling along the length of the PFR or changing its feeding rate (i.e. altering the residence time of reactants).
Shown in Table 3 are the parameters used to find the reaction rate constant values at 800°C. As mentioned in the approach section, these procedures make no assumptions about the range of reaction rate constants or the order of magnitude of the kinetic parameters. Any attempt to obtain the kinetic parameters (A and E) without having a preconceived idea about the possible range for each reaction resulted in negative reaction rates and numerical instabilities in the modeling tool, Cantera. Hence the approach start with looking for rate constants at the upper temperature limit (800°C in this study). As seen in Table 3, the search limit, for the rate constants, is four order of magnitudes and is the same for all reactions. Hence the population size for the GA model is relatively large; four order of magnitudes per variable. Given the large population size and the fact that the rate constants obtained from this step are not the final values that will be used to obtain the kinetic parameters, a relatively low number of iterations is acceptable.
Table 3
First iteration GA parameters at highest temperature
Rate constant (\({{s}{e}{c}}^{-1}\))
|
Search limit (all reactions)
|
Population size
|
Number of iterations
|
\({{k}}_{{r},{800}^{^\circ }{C}}\)
|
1E-5–1E-1
|
13 X 1E4
|
500
|
The objective function minimized by the GA model in this simulation (and all subsequent GA simulations for rate constants) is the singe value obtain from Eq. 1. The objective function is the sum of absolute differences between concentrations values obtain by the GA model and original data for all modeled species (9 species), at all time steps/intervals (150 time steps).
Equation 1. \(Objective\:function \left(OF\right)=\sum _{all\:time\:steps}\sum _{all\:species}\left|{\left[C\right]}_{GA}-{\left[C\right]}_{orig.}\right| (mol/L)\)
Shown in Table 4 are the GA results for the rate constant values at 800°C. Progression of the OF values is shown in the Appendix (Figure A 2). The final OF value (absolute deviation) was 0.38 mol/L. For convenience, a relative deviation from original data is calculated using Eq. 2. Relative deviation from original data, at 800°C, is ~ 1%. Relative deviations at all temperatures are shown in Table 10.
Table 4
Initial GA results at highest temperature (800°C)
Rate constants
(\({{s}{e}{c}}^{-1}\))
|
GA search results at 800°C
|
\({{k}}_{1,{800}^{^\circ }{C}}\)
|
2.03E-02
|
\({{k}}_{2,{800}^{^\circ }{C}}\)
|
4.84E-03
|
\({{k}}_{3,{800}^{^\circ }{C}}\)
|
1.95E-03
|
\({{k}}_{4,{800}^{^\circ }{C}}\)
|
2.96E-04
|
\({{k}}_{5,{800}^{^\circ }{C}}\)
|
8.29E-02
|
\({{k}}_{6,{800}^{^\circ }{C}}\)
|
4.43E-03
|
\({{k}}_{7,{800}^{^\circ }{C}}\)
|
5.40E-04
|
\({{k}}_{8,{800}^{^\circ }{C}}\)
|
1.81E-02
|
\({{k}}_{9,{800}^{^\circ }{C}}\)
|
1.28E-02
|
\({{k}}_{10,{800}^{^\circ }{C}}\)
|
1.83E-03
|
\({{k}}_{11,{800}^{^\circ }{C}}\)
|
7.11E-03
|
\({{k}}_{12, {800}^{^\circ }{C}}\)
|
8.87E-02
|
\({{k}}_{13,{800}^{^\circ }{C}}\)
|
1.33E-02
|
Given a successful / low relative deviation from original data for the rate constants obtained at the highest temperature, they are used to set the limits for subsequent GA search for rate constants at lower temperatures (see Table 5). At this point, a narrower and specific range is assigned for each reaction; for the subsequent temperature, 750°C, the rate constants upper search limits is the rate constant at 800°C and the lower limit is 2 order of magnitudes lower than the upper limit for each reaction. Same limits are applied for subsequent reactions. Because of the narrower and specific range for each rate constant, population size is smaller (2000 X number of variables) and number of iterations is low as well (500 iterations). Following this approached, the GA reached even lower OF/NOF in shorter time duration at subsequent temperatures. Results of the GA search for the rate constants at 750°C (as well as lower temperatures) are listed in Table 6. OF values versus iterations at each temperature are respectively shown in the appendix (Figure A 3, Figure A 4, Figure A 5 and Figure A 6).
Table 5
GA parameters for subsequent iterations at lower temperatures
Rate constants
(\({{s}{e}{c}}^{-1}\))
|
GA search limits
|
\({{k}}_{{r},{750}^{^\circ }{C}}\)
|
\([{k}_{r,{800}^{^\circ }C}/100-{k}_{r,{800}^{^\circ }C}], r= 1-13\)
|
\({{k}}_{{r},{700}^{^\circ }{C}}\)
|
\([{k}_{r,{750}^{^\circ }C}/100-{k}_{r,{750}^{^\circ }C}], r= 1-13\)
|
\({{k}}_{{r},{650}^{^\circ }{C}}\)
|
\([{k}_{r,{700}^{^\circ }C}/100-{k}_{r,{700}^{^\circ }C}], r= 1-13\)
|
\({{k}}_{{r},{600}^{^\circ }{C}}\)
|
\([{k}_{r,{650}^{^\circ }C}/100-{k}_{r,{650}^{^\circ }C}], r= 1-13\)
|
* Population size: 13 X 2000. Iterations: 500 |
Table 6
GA results for reaction rate constants at subsequent temperatures
Rate constant
(\({{s}{e}{c}}^{-1}\))
|
Temperature
|
750°C
|
700°C
|
650°C
|
600°C
|
\({{k}}_{1}\)
|
1.68E-02
|
1.54E-02
|
1.43E-02
|
1.30E-02
|
\({{k}}_{2}\)
|
1.19E-03
|
2.18E-04
|
3.78E-05
|
1.31E-05
|
\({{k}}_{3}\)
|
1.90E-03
|
1.50E-03
|
1.12E-03
|
8.12E-04
|
\({{k}}_{4}\)
|
1.31E-05
|
5.83E-07
|
1.56E-08
|
4.08E-10
|
\({{k}}_{5}\)
|
8.18E-02
|
7.85E-02
|
6.92E-02
|
5.94E-02
|
\({{k}}_{6}\)
|
1.70E-03
|
7.72E-04
|
3.45E-04
|
1.34E-04
|
\({{k}}_{7}\)
|
1.54E-05
|
3.42E-06
|
1.21E-06
|
7.07E-07
|
\({{k}}_{8}\)
|
1.63E-02
|
1.21E-02
|
9.93E-03
|
7.27E-03
|
\({{k}}_{9}\)
|
1.28E-02
|
1.27E-02
|
1.26E-02
|
1.11E-02
|
\({{k}}_{10}\)
|
7.69E-04
|
3.90E-04
|
1.70E-04
|
7.28E-05
|
\({{k}}_{11}\)
|
1.59E-03
|
2.94E-04
|
4.33E-05
|
7.53E-06
|
\({{k}}_{12}\)
|
1.75E-02
|
3.88E-03
|
7.72E-04
|
8.00E-05
|
\({{k}}_{13}\)
|
1.60E-03
|
9.74E-04
|
4.31E-04
|
1.07E-04
|
Equation 2. \(Normalized\:objective\:function \left(NOF\right)=\frac{OF}{\sum _{all time steps}\sum _{all species}{⌊C⌋}_{orig.}}\times 100 \left(\%\right)\)
Given the rate constants values at each temperature, linear regression is used to obtain the kinetic parameters for each reaction, by plotting ln(k) versus 1/T (see Fig. 1). Results of the linear regression are shown in Table 7.
Table 7
Kinetic parameters results using linear regression
Reaction ID
(r)
|
Pre-exponential factor
(\({{A}}_{{r}, {L}{R}} {{s}{e}{c}}^{-1}\))
|
Activation energy
(\({{E}}_{{r}, {L}{R}} {J}/{k}{m}{o}{l}\))
|
1
|
1.19E-01
|
1.63E + 07
|
2
|
1.39E + 09
|
2.37E + 08
|
3
|
1.19E-01
|
3.59E + 07
|
4
|
9.95E + 21
|
5.26E + 08
|
5
|
3.85E-01
|
1.33E + 07
|
6
|
1.24E + 04
|
1.34E + 08
|
7
|
7.49E + 07
|
2.41E + 08
|
8
|
1.10E + 00
|
3.63E + 07
|
9
|
2.25E-02
|
4.80E + 06
|
10
|
1.76E + 03
|
1.24E + 08
|
11
|
9.08E + 10
|
2.70E + 08
|
12
|
9.10E + 11
|
2.68E + 08
|
13
|
1.40E + 06
|
1.70E + 08
|
Kinetic parameters obtained by linear regression (Table 7) resulted in normalized objective function (relative deviation from original data) in the range of ~ 0.5% − 6.5%, with an average / normalized relative deviation of ~ 2.7%. Although acceptable, a better fit is obtained by re-running the GA search for all kinetic parameters for all reactions, using the values obtained by linear regression to help set the search limits for the GA. Narrower and specific search limits for each parameter are set as shown in Table 8. The pre-exponential factor (A) spans a range of 2 order of magnitude. However, given the exponential effect of the activation energy (E), a range of ± 20% was used as limits for the GA search. Population size was 400 X number of variables (26). Results of GA search for the kinetic parameters are shown in Table 9. The objective function minimized for finding the kinetic parameters is similar to that used to find the rate constants, except concentration values at all temperatures are added to the summation (see Eq. 3 and Eq. 4). Final values of the objective functions (absolute difference in concentrations of all species at all time intervals for all temperatures) are shown in Table 10) for convenience, relative difference (as defined by Eq. 4) is also shown in Table 10. NOF (relative difference between original concentration values and concentrations obtained by the kinetic parameters found via the GA) ranged from ~ 0.82–2.1% with an average deviation of ~ 1.2%.
Table 8
GA search parameters using kinetic parameters from linear regression
Reaction ID
(r)
|
Pre-exponential factor search range
\(\left({{s}{e}{c}}^{-1}\right)\)
|
Activation energy search range
\(({J}/{k}{m}{o}{l})\)
|
\({r}= 1-13\)
|
\({A}_{r,LR}/10\)
|
\({A}_{r, LR}\times 10\)
|
\({E}_{r, LR}-20\%\)
|
\({E}_{r,LR}+20\%]\)
|
* Population size: 26 X 400. Iterations: 1000 |
Table 9
GA search results for the kinetic parameters
Reaction
|
Pre-exponential factor
(\({{A}}_{{r},{G}{A}} {{s}{e}{c}}^{-1}\))
|
Activation energy
(\({{E}}_{{r},{G}{A}} {J}/{k}{m}{o}{l}\))
|
r1
|
1.24E-01
|
1.66E + 07
|
r2
|
9.29E + 09
|
2.53E + 08
|
r3
|
2.38E-01
|
4.12E + 07
|
r4
|
6.26E + 22
|
5.56E + 08
|
r5
|
2.77E-01
|
1.08E + 07
|
r6
|
4.59E + 04
|
1.44E + 08
|
r7
|
2.83E + 07
|
2.87E + 08
|
r8
|
2.63E + 00
|
4.15E + 07
|
r9
|
2.98E-02
|
5.54E + 06
|
r10
|
1.34E + 04
|
1.42E + 08
|
r11
|
8.20E + 11
|
2.89E + 08
|
r12
|
3.83E + 12
|
2.79E + 08
|
r13
|
6.70E + 06
|
1.78E + 08
|
Equation 3. \(Objective\:function \left({OF}_{A, E}\right)=\sum _{all\:temp.}\sum _{all\:time\:steps}\sum _{all species}\left|{\left[C\right]}_{GA}-{\left[C\right]}_{orig.}\right| (mol/L)\)
Equation 4. \(Normalized\:objective\:function \left({NOF}_{A,E}\right)=\frac{{OF}_{A,E}}{\sum _{all temp.}\sum _{all time steps}\sum _{all species}{⌊C⌋}_{orig.}}\times 100 \left(\%\right)\)
For the purpose of visual comparison between calculated concentrations using kinetic parameters obtained by the suggested approach and original data, Fig. 2, Fig. 3, Fig. 4 and Fig. 5 are utilized. These figures show concentrations of major species versus time at all temperatures. Figure 2 and Fig. 3 represent the behaviour of the main reactant (PP) and the main product (H2) respectively. Figure 4 and Fig. 5 represent the behaviour of two intermediates; C2H2 and CH4 respectively. Solid lines represent the original data. Square markers represent concentrations obtained from the procedural approach followed above.
Figure 6, Fig. 7, Fig. 8 and Fig. 9 demonstrate the relative importance of the suggested reaction mechanism in the literature as well as the kinetic parameters. Shown in Fig. 6 are three sets of rate constants at 600°C, k600,r; original data rate constants, rate constants calculated using kinetic parameters obtained by linear regression as well as the rate constants calculated using kinetic parameters obtained from the GA search. Shown in Fig. 7 are the rate constants at 800°C, k800,r. Shown in Fig. 8 and Fig. 9 are the respective sets of kinetic parameters; original Aorig, Eorig, ALR and ELR obtained by linear regression and AGA and EGA obtained by the GA search. The purpose of these figures is show the discrepancy in rate constants and kinetic parameters between the original data and the kinetic parameters obtained either by linear regression or GA, however, the excellent agreement in the final results of concentrations versus time shown in Fig. 2 to Fig. 5. Noting the logarithmic scale in Fig. 6 and Fig. 7, it’s obvious the rate constants for reactions 2, 4, 7, 12 and 13 are much different from the original data. This indicates that the process can be expressed via other mechanisms or some of these reaction routes are parallel and, effectively combined, can yield the same concentration profiles.
Also, given the log scale in Fig. 8 and Fig. 9, a more pronounced discrepancy in the values of calculated kinetic parameters (pre-exponential factor, Fig. 8) and (activation energy, Fig. 9) is noticed. That confirms the conclusion that the process can be modeled using different mechanisms. Also, it is possible that the compensation effect plays a role and could result in obtaining the similar values of rate constants using various combinations of kinetic parameters. The important conclusion in this study is, given the end goal represented in the objective function (reactor output versus time and temperature), there is no one answer (mechanism or set of kinetic parameters) can describe the process. Hence, a procedural / systematic approach should be followed while obtaining the kinetic parameters using optimization techniques. This procedural approach is presented in this investigation. The relative/normalized deviation in concentrations (9 species) versus time for at all temperatures (5 temperatures), at all time steps (150 increments) was 1.2%.
Table 10
Absolute differences and relative difference from original data for all methods
Method
|
600°C
|
650°C
|
700°C
|
750°C
|
800°C
|
All Ts
|
OF
(mol/L)
|
NOF
(%)
|
OF
(mol/L)
|
NOF
(%)
|
OF
(mol/L)
|
NOF
(%)
|
OF
(mol/L)
|
NOF
(%)
|
OF
(mol/L)
|
NOF
(%)
|
OF
(mol/L)
|
NOF
(%)
|
\({{k}}_{{G}{A}}\)
|
0.02
|
0.06
|
0.01
|
0.03
|
0.04
|
0.13
|
0.07
|
0.22
|
0.38
|
1.07
|
-
|
-
|
\({{A}}_{{r},{L}{R}} , {{E}}_{{r},{L}{R}}\)
|
0.72
|
2.3
|
0.15
|
0.49
|
0.54
|
1.74
|
0.59
|
1.86
|
2.31
|
6.51
|
4.32
|
2.69
|
\({{A}}_{{r},{G}{A}} , {{E}}_{{r},{G}{A}}\)
|
0.36
|
1.15
|
0.35
|
1.13
|
0.26
|
0.84
|
0.67
|
2.11
|
0.29
|
0.82
|
1.93
|
1.2
|