Energy Fuels 2010, 24, 475–482 Published on Web 10/19/2009
: DOI:10.1021/ef900857n
Effect of Operation Conditions on the Composition and Octane Number of Gasoline in the Process of Reducing the Content of Olefins in Fluid Catalytic Cracking (FCC) Gasoline Fusheng Ouyang,* Xu Pei, Xuhong Zhao, and Huixin Weng Research Institute of Petroleum Processing, East China University of Science & Technology, 130 Meilong Road, Shanghai 200237, People’s Republic of China Received August 5, 2009. Revised Manuscript Received September 27, 2009
Effects of operation conditions were studied on the compositions and research octane number (RON) of gasoline from fluid catalytic cracking (FCC) with the GOR-Q catalyst and a normal MLC-500 catalyst for reducing olefin content. The results show that the effect of the GOR-Q catalyst on decreasing the olefins content in FCC gasoline is obvious and most of olefins in gasoline have low carbon numbers. The influence of the operation conditions on olefin, aromatics, and isoparaffin content of FCC gasoline is apparent. The operation conditions at a reaction temperature of 110 μm microactivity metal analysis Al2O3 (%) Na2O (%) Re2O3 (%) Fe (%) V (μg/g) Ni (μg/g) Sb (/μg/g)
MLC-500
fresh catalyst
equilibrium catalyst
fresh catalyst
equilibrium catalyst
0.37
0.23 2.78 0.79 0.80 0.85
0.43
0.31
0.69
18.4 82.6 77
0.67 0.83
3.85 72.70 20.0 3.50 66
18.3
0.34 2.4 0.38 1400 6900 3300
81.7
12 61 27
77
60
49.7 0.14 1.9 0.24
43.6 0.62 0.36 759 8452 4465
Figure 1. Effect of reaction temperature and catalyst/oil (cat/oil) ratio on gasoline olefin content.
conditions of the catalysts to reduce the content of olefins in FCC gasoline.
3. Results and Discussion 3.1. Effects of Reaction Temperature and Catalyst/Oil Ratio on the Content of Olefins in FCC Gasoline. In this section, the influence of reaction temperature (490-520 °C), catalyst-to-oil ratio (cat/oil ratio, as described below, in the range of 5-8), and the weight hourly space velocity (WHSV, as described below, 30 h-1) on the olefin content of FCC gasoline was investigated. The reaction results are shown in Figure 1. Figure 1 illustrates that (i) the content of olefins in FCC gasoline is significantly increased for a given cat/oil ratio as the reaction temperature increases and (ii) within the reaction conditions, the content of olefins in FCC gasoline for GOR-Q will increase by ∼1-5 percentage units (absolute) for each 10 °C increase in reaction temperature. However, for the MLC-500 catalyst, the olefin content of gasoline will increase by ∼0.2-2 percentage units (absolute). Because of the fact that the cracking reaction is dominant reaction in the FCC, the macromolecular hydrocarbons are cracked and produce plenty of olefins, and, at high temperatures, the cracking reaction rate is faster than that of the hydrogen
2. Experimental Section Feedstock oils, the experimental device, and the analysis measures for products used in this study have been described in ref 6. The main compositions and properties of the catalysts used in this study are listed in Table 1. The hydrocarbon composition in the gasoline product was analyzed by gas chromatography, combined with RIPP-GCAS software (Version 2.1), which enables the contents of almost all hydrocarbons (∼300 components) in FCC gasoline to be calculated and then classified by their hydrocarbon groups (PONA), such as n-alkane isoparaffins, olefins, cycloalkanes, and aromatics, because the experimental equipment used in this laboratory is a small-scale and fixed-fluid-bed reactor in which the residence time of the products is longer than the riser reactor widely used in the FCC process. In addition, the degree of backmixing among the products and the catalyst in the fixed fluidized bed is more intense than that of the riser reactor, which is beneficial to hydrogen transfer reactions for decreasing olefin. Therefore, in the laboratory, the olefin content of gasoline is often lower than that of the industrial riser reactor. 476
Energy Fuels 2010, 24, 475–482
: DOI:10.1021/ef900857n
Ouyang et al.
Figure 3. Effect of reaction temperature on the distribution of gasoline olefins.
rare-earth content, resulting in an increase in hydrogen transfer activity. All of these are conducive to bimolecular reactions, which will lead to the decrease of the olefin content in gasoline. However, the function of the MLC-500 catalyst is to increase the selectivity of diesel by decreasing the activity of secondary cracking and keeping the activity of the large molecules.7,8 The fundamental difference in the developing ideals between the two catalysts has resulted in the distinctive performances for two catalysts. Figure 2 shows that the decreased amplitude of the olefin content in gasoline for the MLC-500 catalyst is higher than that of the GOR-Q catalyst when the cat/oil ratio is enhanced from 5 to 8, but the olefin content of gasoline for the MLC-500 catalyst is still higher than that of the GOR-Q catalyst. However, it is not feasible for the MLC-500 catalyst, in practical operations, to attain the same olefin content in gasoline for the GOR-Q catalyst by further increasing the cat/oil ratio, because it is bound to cause a large enhancement of coke yield. Therefore, to reduce the content of olefins in FCC gasoline effectively, a new type of catalyst must be developed and appropriate operation conditions must be adopted. 3.2. Effects of Reaction Temperature and Catalyst/Oil Ratio on Olefin Distribution in Gasoline. Figures 3 and 4 show the effects of reaction temperature and cat/oil ratio, respectively, on the carbon number distribution of gasoline olefins using the GOR-Q catalyst. The two figures show that the olefins in FCC gasoline consist mostly of C5-C7, with these accounting for more than 75% of the olefins. The reduction of olefin content is mainly achieved by a decrease of C5-C7 olefin molecules. This is because large alkane molecules are cracked into a smaller alkane molecule and a smaller olefin molecule, and the cracking rates of olefins are higher than other hydrocarbons with the same carbon number in the FCC process. In addition, the cracking rates of olefin molecules are quickened with the increase of the carbon chain length and a larger olefin molecule will be decomposed into two small olefin molecules with good chemical stability. Accordingly, the content of C5-C7 olefins will increase. On the other hand, the GOR-Q catalyst can provide good hydrogen transfer activities and store a certain amount of active hydrogen to ensure the saturation effects of more H atoms toward olefins and also controls the growth of the carbonium-ion chain. Thus, the selectivity of hydrogen
Figure 2. Comparison in gasoline olefin content between the two catalysts at different cat/oil ratios.
transfer reaction. Because the hydrogen-transfer reactions are exothermic, a higher temperature is advantageous to cracking reactions but is disadvantageous to the hydrogen transfer reactions, which are the main reactions for reducing the content of olefins in FCC gasoline. As a result, the content of olefins in FCC gasoline will become higher when the reaction temperature increases. Thus, it is necessary to take the lower reaction temperature only from the view of reducing the olefin content of gasoline. However, the conversion rate decreases as the reaction temperature decreases. In this study, effect of cat/oil ratio on the content of olefins in FCC gasoline was investigated at each of three different reaction temperatures (490, 500, and 520 °C) and a constant WHSV of 30 h-1. The results are shown in Figure 1. Figure 1shows that the content of olefins in FCC gasoline has a tendency to decrease as the cat/oil ratio increases. For the GOR-Q catalyst, a one-unit increase in cat/oil ratio will produce a reduction in FCC gasoline content by ∼1-3 percentage units (absolute). An increase in the cat/oil ratio means an increase in the catalytic cracking and hydrogen transfer activity of the catalyst. However, the cracking rate of olefin is larger than that of other equal carbon-number hydrocarbons. Some larger molecular olefins in gasoline then are cracked into two smaller molecular olefins. As a result, some smaller molecular olefins generated from these reactions enter into the air. Therefore, some prior generated olefins are saturated for hydrogen transfer reactions and the content of olefins in FCC gasoline decreases. Figure 1b shows that the content of olefins in FCC gasoline has a tendency to decrease as the cat/oil ratio increases for the MLC-500 catalyst, and that one-unit increase in the cat/oil ratio will produce a reduction of the olefin content in gasoline by ∼2-4 percentage units. Changes of the olefin content in gasoline for two catalysts were compared in Figure 2. The graph shows that the decrease in the olefin content in gasoline for the GOR-Q catalyst (G) is significantly less than that of the MLC-500 catalyst (M) under the same experimental conditions. The reason is that the GOR-Q catalyst contains the ultrastable Y-zeolite through special treatment and an increase in 477
Energy Fuels 2010, 24, 475–482
: DOI:10.1021/ef900857n
Ouyang et al.
Figure 5. Gasoline olefins distribution with two catalysts at different cat/oil ratios. Figure 4. Effect of cat/oil ratio on the distribution of gasoline olefins.
transfer reactions for decreasing olefins in gasoline is controlled. In addition, the catalyst can promote the selective cracking and isomerization of olefins with more than seven C atoms to form small molecule olefins and convert some large olefins into isoparaffin and aromatics in gasoline. This is because the modified ZRP is a shape-selective catalysis and shows superior isomerization and aromatization properties. As a result, large-molecule olefins in gasoline will be reduced substantially, but not small-molecule olefins, thereby concentrating olefins mainly in the C5-C7 range. In this way, reduction of the olefin content and restoration of the loss of octane number of gasoline is achieved. Figure 3 shows that the content of small-molecule olefins in gasoline is increased and large molecules is decreased for a given cat/oil ratio as the reaction temperature increases. This is because large olefin molecules become more unstable than small ones, so they become more likely to be cracked into two small ones, and their reaction rates will be enhanced with the increase of the carbon number in the olefin molecule. Figure 4 shows that, at a constant temperature, because the cracking and hydrogen transfer activities of the catalysts are increased as the cat/oil ratio increases, the content of each of the olefins in gasoline declines. In view of this, the decrease of olefin content in gasoline under decreasing reaction temperature is performed by decreasing the amount of small-molecule olefins. Studies have shown that decreasing the amount of small-molecule olefins is advantageous to environmental protection, because C5 olefins in gasoline account for 67%-78% of the damage to the ozone layer that is caused by olefins and the effectiveness of the removal of C5 olefins is 2.5-5 times higher than that of C6 olefins.9 A sharp decrease in small-molecule olefins in gasoline is bound to cause a larger loss of octane number, because the octane number of small-molecule olefins is higher than that of large-molecule olefins. The contents of olefins with various carbon numbers in gasoline, overall, will be decreased as the cat/oil ratio increases. The loss of gasoline octane number caused by the increase in cat/ oil ratio is clearly less than that which is caused by the decrease in reaction temperature.
Figure 6. Gasoline olefins distribution with two catalysts at different reaction temperatures.
Figures 5 and 6 list the effects of different cat/oil ratios and reaction temperatures on the carbon number distribution of olefins in gasoline for GOR-Q and MLC-500 catalysts. The results show the following: (1) At the same cat/oil ratio, the content of C5-C7 olefins in FCC gasoline accounts for 80% of the total olefins, using the MLC-500 catalyst. However, the content of olefins with different carbon numbers using the GOR-Q catalyst is all less than that observed using the MLC-500 catalyst. This shows that the GOR-Q catalyst can effectively reduce the olefin content of FCC gasoline. (2) For the MLC-500 catalyst, the amounts of olefins with different carbon numbers are reduced overall as the cat/ oil ratio increases, but the reduction of total olefin content is mainly caused by the reduction of C5-C9 olefins when the reaction temperature decreases. However, for the GOR-Q catalyst, the decrease in total olefin content is mainly caused by the reduction of C5-C7 olefins. 478
Energy Fuels 2010, 24, 475–482
: DOI:10.1021/ef900857n
Ouyang et al.
Figure 7. Effect of operation conditions on content of isoparaffin and aromatics.
the influence of operation conditions on the contents of n-alkanes and cycloalkanes in FCC gasoline. Figure 8a shows that, for the two catalysts, the n-alkane content does not change more than 1 percentage unit and it only accounts for 3%-4% of the gasoline, despite the increase in cat/oil ratio (from 5 to 8) and reaction temperature (from 490 °C to 520 °C). This finding indicates that the catalysts and operation conditions have little effect on the total n-alkane content. n-Alkanes are mainly generated from the cracking reaction and the hydrogen transfer of olefins. Olefins are intermediates in secondary reactions, so they are inclined to receive a proton to become primary carbonium ions. However, these ions will soon be transformed to more-stable secondary carbonium ions or tertiary carbonium ions, because of their instability. These carbonium ions then will easily be transformed to isoparaffins instead of n-alkanes through hydrogen transfer reaction. Moreover, large-molecule n-alkanes can be cracked more easily.10 Therefore, the content of n-alkanes in gasoline is relatively small, resulting in a small effect on the octane number of gasoline. Figure 8b shows that, for the MLC-500 catalyst, the cycloalkane content in gasoline changes by no more than 1 percentage unit, accounting for 6.2%-7.2% of the gasoline at reaction temperatures of 490-520 °C and the cat/oil ratio changes from 5 to 8. However, for the GOR-Q catalyst, the cycloalkanes content changed by no more than 1 percentage unit and they only account for 5.4% to 7.2% of gasoline under the same reaction temperature or cat/oil ratio. The formation mechanism of cycloalkanes is similar to that of alkanes, but cycloalkanes have high reaction ability to cracking, because they have many secondary and tertiary C atoms. Therefore, the distribution of cycloalkanes in the cracking products is more similar to that of the olefins. In addition, the H atoms in cycloalkanes can be easily
3.3. Effects of Operation Conditions on the Content of Aromatics and Isoparaffin in Gasoline. Aromatics and isoparaffin have higher octane numbers, similar to olefin; the decrease in gasoline olefin content is bound to affect the content of aromatics and isoparaffin in gasoline and the octane number of gasoline. In this paper, the effect of operation conditions on the content of aromatics and isoparaffin was investigated to reflect the influence of operation conditions on the octane number of gasoline. Figure 7 illustrates the influence of cat/oil ratio and reaction temperature on the content of aromatics and isoparaffin in gasoline for the GOR-Q and MLC-500 catalysts. The results show that, at the same cat/oil ratio, the aromatics content in gasoline for the catalysts increases for the endothermic aromatization reaction and the isoparaffin content in gasoline decreases for the exothermic hydrogen transfer and isomerization as the reaction temperature increases. At the same reaction temperature, the content of aromatics in gasoline for the GOR-Q catalyst increases as the cat/oil ratio increases, but the isoparaffins content decreases. However, for the MLC500 catalyst, the amounts of both increase. Under the same conditions, the aromatics content in gasoline for the GOR-Q catalyst is significantly higher than that of the MLC-500 catalyst, because the aromatization function of the GOR-Q catalyst is strengthened. Also, Figure 7shows that aromatics and isoparaffins in gasoline have a greater effect on the octane number of gasoline, because the content of the former is 20%-30% and that of the latter is 30%-40%. 3.4. Effects of Operation Conditions on the Contents of n-Alkanes and Cycloalkanes in Gasoline. The octane number of n-alkane in gasoline is the lowest one of the hydrocarbons, and that of cycloalkane is moderate. Therefore, exploration of the influence of operation conditions on the contents of n-alkanes and cycloalkanes would contribute to understanding the changes in gasoline octane number. Figure 8 shows
(10) Chen, J. W.; Cao, H. C. Technology and engineering of catalytic cracking. China Petrochem. Press 1995, 112–146.
479
Energy Fuels 2010, 24, 475–482
: DOI:10.1021/ef900857n
Ouyang et al.
Figure 8. Effect of reaction temperature and cat/oil ratio on the contents of n-alkanes and cycloparaffins for the two catalysts (MLC-500 (denoted as “M” in the legend) and GOR-Q (denoted as “G” in the legend)).
Figure 9. Effect of reaction temperature and cat/oil ratio on the hydrogen transfer coefficient (HTC) for the different catalysts: (a) GOR-Q and (b) MLC-500.
transferred, which can promote the following reactions:11 cycloalkanes þ olefins f aromatics þ alkanes
coefficient (which is abbreviated as HTC and is expressed as the ratio of butanes to butylenes content in the gas, by mass) is used to describe the degree of hydrogen-transfer reaction activity.10 The larger HTC value will lead to the higher hydrogen transfer activity. The effects of the operation conditions on the HTC are shown in Figure 9. This figure shows that high temperature results in a low HTC value, whereas a high HTC value is obtained at high cat/oil ratios. Figure 9a also shows that the increase in the HTC value by reducing the reaction temperature is more remarkable than increasing the cat/oil ratio when the cat/oil ratio is 7 and reaction temperatures of 490-500 °C, the HTC value increases significantly, but the increase in the HTC value becomes less obvious at a temperature of 520 °C. Therefore, to reduce the content of olefins in FCC gasoline when using the GOR-Q catalyst, the cat/oil ratio and the reaction temperature should be controlled to be >7 and olefins > cycloalkanes > alkanes That is to say, reducing the reaction temperature to reduce the olefin content of gasoline will lead to a simultaneous decline in octane number.
Figure 10. Effect of reaction temperature and cat/oil ratio on the gasoline research octane number (RON).
Figure 10 also shows that, at a cat/oil ratio of 5 and a reaction temperature of 500 or 520 °C, the RON value of gasoline for the GOR-Q catalyst is higher than that for the MLC-500 catalyst. This is because, although the content of olefins for the GOR-Q catalyst is significantly lower than that for the MLC-500 catalyst, the contents of isoparaffins and aromatics in gasoline for the GOR-Q catalyst are significantly higher than that for the MLC-500 catalyst (see Figure 7). As a result, the RON value of gasoline for the GOR-Q catalyst is higher than that of the MLC-500 catalyst. When the cat/oil ratio increases, the increase in gasoline octane number that is caused by the increase in aromatics content for the GOR-Q catalyst is difficult to compensate for the loss of gasoline octane number caused by the remarkable decline of isoparaffins and olefins content. As a result, the octane number decreases. Meanwhile, for the MLC-500 catalyst, it is easy to compensate for the loss in octane number, because the reduction of olefins in gasoline is much lower than that of the GOR-Q catalyst. As a result, the octane number of gasoline for the MLC-500 catalyst increases. Thus, when the cat/oil ratio is >7, the RON value of gasoline for the MLC-500 catalyst is higher than that for the GOR-Q catalyst. The development of olefinreducing catalysts for FCC gasoline clearly must be focused not only on the olefin-reducing reactions through hydrogen transfer reactions but also on improving the functions of the catalyst for aromatization and isomerization. Figure 10 illustrates that the decrease of gasoline RON value for the GOR-Q catalyst was only 1.1-1.7 units when the cat/oil ratio was enhanced from 5 to 8 and the reaction temperature was 500-520 °C. This especially reminds us that, although a substantial drop in the olefin content in gasoline occurred, it is important to maintain the gasoline octane number during the application of the GOR-Q catalyst. The factors that affect FCC gasoline composition are very complicated. Therefore, to decrease the olefin content in gasoline while reducing the loss in gasoline octane number, new process technologies, appropriate catalysts, suitable operation conditions, etc. should be applied, but one should not rely on measurements only.
(12) Peng, P.; Lu, W. Z. Foreign Analysis Method for Calculating Octane Number. Pet. Refin. Ind. 1986, 17 (3), 39–41. (13) Chen, G. Z.; Zou, N. Z. Determination of Gasoline Octane Numbers by High Resolution Gas Chromatography. Pet. Refin. Ind. 1988, 19 (10), 14–19. (14) Chen, G. Z. Study of the Relationship of the Composition and Octane Number of FCC Gasoline Gas Chromatography. Petrol. Refin. Ind. 1990, 21 (7), 49–56. (15) Wu, X. B. Determination of Reformed-Gasoline Octane Numbers by Gas Chromatography. Anqing Petrochem. 1998, 20 (4), 60–62. (16) Li, Y. H.; Zhang, Z. X.; Zhou, J. L. Octane Number Determination for MFF Gasoline by Capillary Gas Chromatography. J. Instrum. Anal. 1999, 18 (6), 73–75. (17) Zhang, C. C.; Yang, MA X.; Rapid, W. F. Determination of Octane Number of Gasoline by High-Resolution Gas Chromatography. J. Yunnan Univ. 1999, 21 (4), 291–293. (18) Liang, Y. H.; Li, G. J.; Ma, P. S. Estimating the Octane Number of Hydrocarbons. Petrochem. Technol. 2000, 29, 432–435.
4. Conclusions (1) Ther GOR-Q catalyst is obviously effective in reducing the content of olefins in fluidized catalytically cracked (FCC) gasoline. Low reaction temperature and high catalyst/oil (cat/ oil) ratio are beneficial to decreasing the content of olefins in FCC gasoline. Olefins in FCC gasoline consist mostly of 481
Energy Fuels 2010, 24, 475–482
: DOI:10.1021/ef900857n
Ouyang et al.
reaction temperature at 520 °C and a cat/oil ratio of >7.0 are effective in decreasing the olefin content. (4) The decline of the content of olefins in FCC gasoline that is caused by decreasing the reaction temperature will lead to a decrease in the gasoline octane number. For the GOR-Q catalyst, the research octane number (RON) value of FCC gasoline will decrease as the cat/oil ratio increases. However, for the MLC-500 catalyst, it will increase. The decrease of FCC gasoline for the GOR-Q catalyst is only 1.1-1.7 units when the cat/oil ratio increases from 5 to 8 and reaction temperature is 500-520 °C. To reduce the loss in octane number of FCC gasoline, the development of olefin-reducing catalysts for FCC gasoline must be focused not only on the olefin-reducing reactions through hydrogen transfer reactions but also on improving the functions of the catalyst for aromatization and isomerization.
C5-C7 hydrocarbons. For the GOR-Q catalyst, the reduction of the olefin content of gasoline is achieved by the decrease in the amount of small-molecule olefins through increasing cat/oil ratio and decreasing reaction temperature. Compared to the MLC-500 catalyst, the contents of olefins with various carbon numbers in FCC gasoline will have an overall decrease. (2) For both catalysts, the content of aromatics in gasoline increases and the content of isoparaffins in gasoline decreases as the reaction temperature increases. As the cat/oil ratio increases, the content of aromatics in gasoline increases. For the GOR-Q catalyst, the content of isoparaffin in gasoline decreases, whereas for the MLC-500 catalyst, it increases. Under the same conditions, the content of aromatics in gasoline for the GOR-Q catalyst is significantly higher than that for the MLC-500 catalyst. Cycloalkanes and alkanes account for a small percentage of gasoline, and reaction temperature and cat/oil ratio have little influence on their content. (3) According to the influence of the reaction temperature and the cat/oil ratio on the hydrogen transfer coefficient (HTC), the HTC value is consistent with the influence of reaction temperature and cat/oil ratio on the olefin content of gasoline. For the GOR-Q catalyst, it also indicates that a
Note Added after ASAP Publication. The second paragraph of section 3.5 was modified in the version of this paper published ASAP October 19, 2009; the corrected version published ASAP December 7, 2009.
482
