The used Kaolim catalyst was subjected to X-ray analysis to verify its composition as shown in Fig. 3.
The diffractogram peaks representing kaolinite demonstrate that this oxide can be used as a catalyst. In petrochemical cracking processes the kaolin content directly influences the separation efficiency of petroleum derivatives fractions. This is because the active sites of kaolin are mainly formed by acidic sites of tetrahedral aluminum, which can favor cracking with a reduction in the temperature of the pyrolysis reactor (Czajczyńska et al. 2017). The significant influence of the catalyst can be verified in both responses analyzed from the ANOVA table constructed from the response surface model as shown in Table 2 and Table 3.
Table 2
Source | DF | Adj SS | Adj MS | F-Value | P-Value |
Model | 5 | 731.363 | 146.273 | 52.44 | 0.000 |
Linear | 2 | 244.108 | 122.054 | 43.76 | 0.000 |
Kaolim Mass | 1 | 218.325 | 218.325 | 78.28 | 0.000 |
heating ramp | 1 | 25.783 | 25.783 | 9.24 | 0.019 |
Square | 2 | 175.908 | 87.954 | 31.53 | 0.000 |
Kaolim Mass * Kaolim Mass | 1 | 123.890 | 123.890 | 44.42 | 0.000 |
heating ramp * heating ramp | 1 | 74.004 | 74.004 | 26.53 | 0.001 |
2-Way Interaction | 1 | 311.346 | 311.346 | 111.63 | 0.000 |
Kaolim Mass * heating ramp | 1 | 311.346 | 311.346 | 111.63 | 0.000 |
Error | 7 | 19.524 | 2.789 | | |
Lack-of-Fit | 3 | 11.052 | 3.684 | 1.74 | 0.297 |
Pure Error | 4 | 8.472 | 2.118 | | |
Total | 12 | 750.887 | | | |
Table 3
ANOVA for CG areas comparison response
Source | DF | Adj SS | Adj MS | F-Value | P-Value |
Source | 5 | 25258035568 | 5051607114 | 75771.88 | 0.000 |
Model | 2 | 7135491735 | 3567745867 | 53514.62 | 0.000 |
Linear | 1 | 4374579287 | 4374579287 | 65616.76 | 0.000 |
Kaolim Mass | 1 | 2760912448 | 2760912448 | 41412.47 | 0.000 |
heating ramp | 2 | 18122342232 | 9061171116 | 135913.57 | 0.000 |
Square | 1 | 25847473 | 25847473 | 387.70 | 0.000 |
Kaolim Mass * Kaolim Mass | 1 | 17965944736 | 17965944736 | 269481.25 | 0.000 |
heating ramp * heating ramp | 1 | 201601 | 201601 | 3.02 | 0.126 |
2-Way Interaction | 1 | 201601 | 201601 | 3.02 | 0.126 |
Kaolim Mass * heating ramp | 7 | 466680 | 66669 | | |
Error | 3 | 172068 | 57356 | 0.78 | 0.564 |
Lack-of-Fit | 4 | 294612 | 73653 | | |
Pure Error | 12 | 25258502248 | | | |
In both tables the p-value was less than 0.05 related to the level of significance. The use of the catalyst provided an increase in the conductivity of the PP Kaolim mixture inside the reactor, demonstrating the possibility of solving a process gap that is still much discussed by researchers (Lechleitner et al. 2021). The increase in the conductivity of the reaction medium can still be propitious to keep the molten material inside in the liquid phase, since the high viscosity can lead back to solidification if the medium temperature is not kept constant, above the melting point and the losses of heat to the external environment is reduced through coatings. The homogeneity of the liquid phase in the reactor favors greater dispersion on the catalyst surface, enabling better effectiveness in the development of the reaction mechanism.
Therefore, in order to reduce the activation energy of the pyrolysis system so that high yields with satisfactory selectivity are possible, an increase in the entropy of the thermodynamically closed system can be favored to ensure a more homogeneous and homogeneous fusion of the polymeric material in the reactor. with this, better productivity of condensable gases, mainly in the aromatic class. Thermodynamically entropy can be stated as follows in Eq. 2.
Where dS represents the entropy differential, dQ the heat flux differential associated with the reactor and coming from the electrical resistance and T the temperature of the reaction medium. Considering that the pyrolysis reactor is insulated with refractory material in order to reduce thermal exchanges with the external environment and considering the constant temperature in the steady state of 512°C, an alternative to increase entropy was the modification in the heat flow of the system with kaolin insert. Thus, as the heat flux variation is related to the specific heat of the PP and Kaolin mixture, considering that the specific heat of PP in relation to kaolin is about 100 times lower. The increase in kaolin mass can help in increasing the thermal conductivity of the rational system and with it in the increase of entropy, favoring better polymer decomposition into compounds with higher molecular weights. The kaolin mass factor coefficient in the regression function estimated by Ordinary Least Square (OLS) determined from the CCD model in terms of yield response was + 2.126, indicating that the increase in mass favors yield.
Although the yield in a reaction is a response of interest in pyrolysis processes, there is a restriction that must be considered in terms of reaction selectivity, which needs a better targeting of the rational mechanism to condition the production of a given compound of commercial interest in majority percentages. In this context, the GC-MS analysis showed that pyrolysis for all experiments produced chromatograms described in an interval referring to organic compounds with carbon numbers between C6-18, as well as the distribution of liquid, gas and solids as shown in Fig. 4.
Among the condensed compounds of crude oil extracted from Pyrolysis after distillation, it was possible to estimate from the GC-MS analysis, for each experiment, the percentage of aromatic and aliphatic compounds present. The increase of aromatic compounds in the oil can considerably improve fuel quality by improving the octane property. Figure 5 presents the relationship for each experiment of the levels of different organic groups.
It is possible to verify that the higher percentages of aromatic compounds from Fig. 5 are due to the greater masses of kaolin used in the experiment, referring to experiments 2 and 6. Even when we comparing this range of organic compounds in the pyrolyzed oil with the kerosene standard of aviation it is possible to verify that all the peaks contained in the oil are part of the kerosene standard, but some with lower concentrations as shown in Fig. 6.
Aromatic compounds that have higher numbers of carbons interconnected by ring unsaturation have higher vaporization points than aliphatic compounds. Thus, it is possible to see in Fig. 6 when comparing the pyrolysis oil chromatograms with the kerosene standard that there is a need to reduce the concentration of the organic compound referring to the peak between C9 and C10 and at the same time to increase some peaks referring to the intervals C11, C12, C13, C14, C15 and C16.
From Fig. 6, it is possible to predict that compounds with higher molecular weights that are separated by CG-MS in longer times have even lower concentrations compared to the kerosene standard, which favors an increase in the ratio between the total areas of each of the chromatograms, increasing the selectivity for the generation of aviation kerosene. Similarly, the yield response according to the high calorific capacity of kaolin, there is an increase in the heat flux inside the reactor, which favors the formation in pyrolysis of compounds with smaller carbon chains, as shown in Fig. 7.
The interaction between the kaolin mass and the heating ramp for the comparison response of the CG-MS peak areas demonstrates that smaller kaolin masses and smaller ramps favor a better overlap between the pyrolysis oil and aviation kerosene chromatograms.
Thus, it is possible to state a tradeoff problem where the increase in yield does not favor the selectivity of the generation of aviation kerosene, which will be solved concomitantly using the NBI algorithm.
3.1. Reaction Mechanism
According to the reaction mechanism proposed by Kruse et al., (2003) the first phase of chain fission can happen in a reversible reaction due to the action of heat between the polypropylene and some radicals formed in the products, which can occur the phenomenon of recombination and, consequently, a decrease in the pyrolysis yield. In this context, from Fig. 7 it is also possible to predict that the increase in the heating ramp up to a value of 33°C/min does not favor the selectivity of the reaction in the production of kerosene determined from GC-MS. However, above this heating ramp value, the selectivity can be favored due to the direction of the reaction to the side of the formation of products in the first stage of the mechanism that has a lower number of moles through the increase of steam in the system and consequently the pressure increase. The next steps in the mechanism of thermal decomposition of polypropylene are characterized by the hydrogenation of carbon chains, which can be established by the kaolinite structure present in kaolin. In this context, kaolin samples with larger surface areas can lead to a better selectivity process for kerosene production.
3.2. Optimization of Responses
The optimization of both responses simultaneously was performed according to the NBI algorithm from the construction of the Pareto Frontier (Pinto et al. 2019). This frontier has numerous process setups that will be used later for process validation. Table 4 presents the optimized responses as well as the setups for each factor.
Table 4
NBI-optimized values for responses Difference between the areas of the CG-MG and Yield (%) and setups for each pair of responses
Yield (%) | Difference between the areas of the CG-MG (%) | Kaolim Mass (g) | Heating Ramp (°C/min) |
47.09 | 42.61 | 22.73 | 24.03 |
46.95 | 41.26 | 22.14 | 23.00 |
46.56 | 40.08 | 21.54 | 22.15 |
46.00 | 39.00 | 20.94 | 21.43 |
45.31 | 38.02 | 20.33 | 20.81 |
44.50 | 37.11 | 19.72 | 20.28 |
43.61 | 36.26 | 19.10 | 19.82 |
42.63 | 35.46 | 18.48 | 19.43 |
41.59 | 34.71 | 17.85 | 19.09 |
40.49 | 33.99 | 17.21 | 18.80 |
39.34 | 33.31 | 16.55 | 18.57 |
38.14 | 32.66 | 15.89 | 18.38 |
36.90 | 32.04 | 15.20 | 18.24 |
35.61 | 31.45 | 14.50 | 18.16 |
34.28 | 30.89 | 13.77 | 18.13 |
32.92 | 30.35 | 13.00 | 18.16 |
31.52 | 29.83 | 12.18 | 18.26 |
30.07 | 29.34 | 11.30 | 18.46 |
28.58 | 28.88 | 10.31 | 18.80 |
27.03 | 28.47 | 9.12 | 19.37 |
The Pareto frontier for the tradeoff solution of the bio-objective problem related to Table 4 is shown in Fig. 8.
To validate the optimization, three points on the Frontier were chosen as shown in the circles in Fig. 8. These points were performed in the setups shown in Table 5.
Table 5
Setups chosen for validation of optimization
Yield (%) | Difference between the areas of the CG-MG (%) | Kaolim Mass (g) | Heating Ramp (°C/min) |
47.09 | 42.61 | 22.73 | 24.03 |
40.49 | 33.99 | 17.21 | 18.80 |
27.03 | 28.47 | 9.12 | 19.37 |
The number of three repetitions necessary for validation was determined from the power sample size test, attributing a value of 0.8 to the test power. The oil extracted in the three experiments is shown in Fig. 9.
The values of the two responses for comparison of the areas of the chromatograms and yield are shown in Table 6.
Table 6
Responses to compare the areas of the chromatograms and Yield
Average Yield (%) | SD* | Difference between the areas of the CG-MG (%) | SD* | Kaolim Mass (g) | Heating Ramp (°C/min) |
45.03 | 0.32 | 41.87 | 0.02 | 22.73 | 24.03 |
38.07 | 0.65 | 32.68 | 0.03 | 17.21 | 18.80 |
26.98 | 0.28 | 25.46 | 0.03 | 9.12 | 19.37 |
*SD – Standard Deviation |
In the optimized and validated experimental condition of temperature ramp and catalyst mass 19.37 °/min and 9.12 g, a TG-DTA analysis demonstrated that the first exothermic decomposition of PP occurs at 161°C as in the same experimental condition without the use of kaolin the temperature was 171.6°C. This temperature reduction in the pyrolysis process can influence the cost reduction of the process, especially when it comes to attribution to process scale up.
3.2.1. Thermal Analysis Evaluation
Thermal analysis was performed using a heating ramp equal to the optimal point identified by the experimental design equivalent to 18.80°C / min. The thermogram of polypropylene (PP) shown in Fig. 10. shows an initial degradation temperature of 378°C. The differential temperature analysis (DTA) shows the PP phase changes, with the endothermic peak at 171°C with an activation energy of -98.5 J/g representing the polymer melting temperature, while above 400°C endothermic events represent mass loss related to the release of volatile products and decomposition.
Kaolinite, according to, has an endothermic peak at 550°C referring to the dehydroxylation of kaolinite and formation of methaculin in exothermic pixo near 1000°C (SANTOS et al. 2019). The maximum temperature of the bio-oil production reaction medium is limited to 512°C, allowing the thermogram of the PP kaolin mixture to be evaluated based on only polymer changes.
The thermogram of the mixture Fig. 11. shows that there was a reduction in the initial degradation temperature to 261°C and formation of thermal decomposition steps for the polymer. The melting temperature of the mixture occurred in an endothermic event at 165°C (Delta H = -22.4 J/g) requiring less energy when compared to pyrolysis in the absence of catalyst. The next intense exothermic event occurs at 272.54°C (1.13 kJ/mol) indicating the occurrence of radical recombination and oxidation between the temperatures of 250°C and 350°C. The last exothermic event at 400°C with an activation energy of 35.73 J/g refers to the complete oxidative degradation of the polymer. The temograms indicate that the presence of kaolin slows down the polymer degradation process, allowing the occurrence of oxidation and recombination reactions that allow better selectivity of the pyrolysis process.
The evaluation of thermal analysis together with the mechanisms demonstrates that the process from the optimization can be glimpsed in a scenario of transposition from a laboratory scale to a pilot scale in order to verify the variability in conditions with higher production volumes and efficiency of heat exchangers applied in oil condensation and distillation.