1. Introduction
The energy demand of the world is continuously increasing due to new industrial developments and population growth [
1]. Internal combustion engines with different operation principles will remain the main form of propulsion for terrestrial, air, and maritime transportation in the next 20–30 years. Accordingly, hydrocarbons as engine fuels for internal combustion engines will continue to play an important role as demonstrated in
Figure 1 and
Figure 2a,b [
2]. The proportion of gasoline will decrease slightly, but will still account for a one-third share in 2040. This decrease is due to the better fuel economy and the spread of electric vehicles [
3] (
Figure 3). However, contrary to forecasts, electric vehicles cannot spread as rapidly as expected, due to reasons such as availability of raw materials, recycling, sustainability, etc.
Sustainable production of alternative engine fuels is a big challenge. Accordingly, different biofuels presently play a significant role for many countries in the world. The foreseen decrease of fossil reserves, global protection of the environment, and sustainability of mobility are the main driving forces for the research into renewable alternative energy sources [
4]. The latest directive of the European Union requires the production of fuels from non-edible, renewable, or waste feedstocks [
5]. Currently, the main bio-component of gasoline is bioethanol [
6], which is also a molecular constituent in ethyl-tert-butyl-eter (bio-ETBE), an important gasoline blending component [
7]. Despite the numerous advantages of ethanol, it has also many disadvantages, such as high solubility in water, increase of vapor pressure in gasoline, lower energy content, corrosion effects, high ozone producing potential, and high atmospheric reactivity [
8]. Moreover, it is mainly produced from food-based plants, e.g., corn. As a result of persistent research and development in the last few years, the construction of plants using lignocellulose as feedstock has already begun [
9]. However, their technological efficiency and reliability need to be proven [
10].
The conversion processes of alternative feedstocks, such as biomass and waste, to different fuels result in the formation of a significant amount of light hydrocarbons (e.g., C5-C7 fractions), mainly as by-products in the boiling range of gasoline. The following processes can result in light hydrocarbons from alternative sources:
Based on the results of the presented publications it was concluded that by-products containing mainly C
5-C
7 paraffins can be obtained from different alternative sources. These by-products have very low octane numbers (<55–65) and contain undesirable components, such as olefins, aromatics, oxygenates, and other corrosive compounds; therefore, they are not suitable as gasoline blending components (“drop in fuel”). Isomerization and dehydrocyclization (aromatization) are the main chemical processes for the quality improvement of n-paraffins to enhance their octane number. Via isomerization the products are iso-paraffins, and via the dehydrocyclization process the obtained products are aromatics. Aromatics, including benzene, have very high octane numbers, e.g., the research octane number (RON) of benzene is 101. However, they are carcinogenic, therefore their concentration in gasoline must be reduced. Gasoline standards strictly limit the concentration of benzene, e.g., in California, USA, the limit is 0.62 v/v%, and in the EU this value is 1.0 v/v% [
4].
No experimental results were provided in the publications about quality improvement, e.g., increasing the octane number of light gasoline fraction obtained from alternative sources. Based on the aforesaid reasons, the main research target of this paper was to investigate the quality improvement of benzene containing light hydrocarbons (rich in n-paraffins), from alternative sources via hydroisomerization. The main goal was also to convert the n-paraffins to branched paraffins with a higher octane number via hydrogenation, i.e., saturation of benzene to cyclo-hexane and its isomerization to methyl-cyclo-pentane.
3. Results and Discussion
Catalysts and the optimal process parameter combinations for the quality improvement of biogasoline fractions were determined based on preliminary experimental results to obtain the most favorable experimental results.
The yield of gas and liquid product mixtures obtained from the two different catalysts differed in the function of process parameters. The yield of the gas products obtained in Reactor I from feedstock “A” over the Pt/Al2O3/Cl catalyst was higher than 99.3% in every case. This was due to the low chlorine content of the catalyst, the high liquid hourly space velocity (LHSV), and the low experimental temperature applied in Reactor I. The low chlorine content of the catalyst results in lower acidity, and thus lower cracking activity. The yields of products obtained from Reactor II decreased up to 2.1%, which was higher than in the case of Reactor I. This was due to the higher chlorine content, and thus higher acidity and cracking activity of the catalyst.
According to the above, the yields of the liquid products varied from 97.2% to 99.4% in the function of process parameters, as shown in
Table 4. These yield values are high even from an industrial perspective. The lowest acceptable yield value for a light naphtha isomerization unit is 92%. Data in
Table 4 illustrate well that the gas production was low (0.6–2.8 %), due to the reasons mentioned above.
The yield of gas phase products (C
1–C
4) obtained from feedstock “B” on the Pt/H-Mordenite/Al
2O
3 catalyst was below 5% up to 250 °C at every LHSV. At higher temperatures and lower LHSV the yield of gas products sharply increased because the cracking reactions took place to a greater extent (
Table 5). About 45% of the gas phase product was i-butane, which can be utilized for alkylate production or as an LPG (propane-butane gas) blending component.
According to the yield data of gas products, the yield value of the liquid products changed in opposite tendency due to the above-mentioned cracking reactions (
Figure 5). The yield of liquid products changed between 85.4% and 99.3%. The lowest values were obtained under the strictest process parameters, at high temperature (T: 270 °C) and high residence time (LHSV: 1.0 h
−1) in the catalytic system. Curves in
Figure 5 illustrate well that iso-paraffins can be obtained with high yield from high n-paraffin containing biogasoline fractions for a wide range of process parameters.
Based on the gas and liquid product yields obtained from the different catalysts, it was concluded that the liquid yield obtained from Pt/Al2O3/Cl was significantly higher than in the case of Pt/H-Mordenite/Al2O3. The reason for this was because the cracking reactions took place to a lesser extent due to the lower isomerization temperature.
In order to evaluate the isomerization reaction results, the thermodynamic equilibrium concentration (ATEC) was determined for the individual components in the C
5 and C
6 fractions as a function of process parameters. The isomerization activity of the catalyst was monitored by the concentration of 2-methyl-butane (2-MB in C
5 fraction) and 2,2-dimethyl-butane (2,2-DMB in C
6 fraction). Only 2-MB can be formed from n-pentane during isomerization; 2,2-dimethyl-propane (2,2-DMP) cannot be formed due to the steric and reaction mechanism reasons. The 2,2-DMB component has the lowest reaction rate among the hexane isomers; its formation is the rate-determining step of the isomerization of n-hexane, and its equilibrium concentration depends mainly on the reaction temperature [
27].
Figure 6a,b demonstrate the ATEC values of 2-MB and 2,2-DMB in the liquid products obtained from feedstock “A” on the Pt/Al
2O
3/Cl catalyst as a function of process parameters.
As a comparison, ATEC values of 2-MB and 2,2-DMB in the liquid products obtained from feedstock “B” on the Pt/H-MordeniteAl
2O
3 catalyst as a function of process parameters are shown in
Figure 7a,b.
Curves in
Figure 6a,b and
Figure 7a,b demonstrate well that the concentrations of the two emphasized isomers increasingly approach the thermodynamic equilibrium concentrations by increasing the reaction temperatures and decreasing the LHSV. However, the extent of the increase in ATEC values lessened by increasing the temperature, especially at 125–145 °C and 250–270 °C, respectively, at lower LHSV. The reason for this is that the process parameters have less impact on the reaction rate near to the equilibrium concentrations. This is also supported by the fact that in case of the Pt/Al
2O
3/Cl catalyst, the ATEC values of 2-MB at the temperature of 125–145 °C and LHSV of 1.0–1.66 h
−1 were 87.8–93.0%, with an absolute difference of 5.2%, while, in the case of 2,2-DMB, these values changed between 67.5% and 85.6%, with an absolute difference of 18.1%. However, in the case of the Pt/H-Mordenite/Al
2O
3 catalyst, the ATEC values of 2-MB at a temperature of 250–270 °C and LHSV of 1.0–1.5 h
−1 were 87.0–92.8%, with an absolute difference of 5.8%, while, in the case of 2,2-DMB, these values changed between 32.1% and 86.8%, with an absolute difference of 54.7%.
Based on the results of a previous publication [
28] it was shown that among the C
6 i-paraffins obtained, the ATEC value of 2,3-dimethylbutane (2,3-DMB), 2-methylpropane (2-MP), and 3-methylpropane (3-MP) were significantly higher than in the case of 2,2-DMB. To prove this phenomenon in the present study, ATEC values of 2-MP obtained from both feedstocks on the applied catalysts are illustrated in
Figure 8a,b.
The highest ATEC values of the individual i-paraffins presented in the figures were obtained at 145 °C (Pt/Al
2O
3/Cl) and at 270 °C (Pt/H-Mordenite/Al
2O
3) at LHSV of 1.0 h
−1. However, the liquid yields were the lowest due to the high hydrocracking activity of catalysts at these process parameters. It is also important to emphasize that the highest possible equilibrium concentrations of 2-MB and 2,2 DMB at the highest experimental temperature are the lowest, because these values are decreasing approximately exponentially with increasing temperature due to the exothermic skeletal isomerization reactions [
4].
Based on the results obtained from the two different catalysts, the most favorable process parameter combinations for the isomerization of benzene containing bio-originated C
5-C
6 fractions are the following: temperature, 125–135 °C; LHSV, 1.0–1.33 h
−1 (Pt/Al
2O
3/Cl); and: temperature, 260 °C; LHSV, 1.0–1.5 h
−1 (Pt/H-Mordenite/Al
2O
3). The benzene content of the target products was <0.05 mg/kg, and oxygenate content was not detected, thus the hydrogenation was complete. The main properties of the liquid products obtained over both catalysts at favorable process parameter combinations are summarized in
Table 6. From the results, it was concluded that in the case of the Pt/Al
2O
3/Cl catalyst the yield of the liquid products and their research octane number was higher with ca. 5% and 4–5 units, respectively, compared to the results obtained on Pt/H-Mordenite/Al
2O
3. When the n-paraffins and the mono-methyl pentanes having low octane number were recirculated with 95%, the RON of the products could reach 92. This is a good result taking into account that the feedstock was benzene free. It is noted that benzene has a high research octane number (101), and its hydrogenation to cyclohexane results in a lower octane number (84). This decrease in octane should be compensated for by the production of isomers having a very high RON. These i-paraffins are free of sulfur and aromatics; consequently, during their application in internal combustion engines compared to current engine fuels, the pollutant emission is lower and contains less harmful pollutants.