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Protolytic cracking in Daqing VGO catalytic cracking process
JOURNAL OF FUEL CHEMISTRY AND TECHNOLOGY Volume 36, Issue 6, December 2008 Online English edition of the Chinese language journal Cite this article as: J Fuel Chem Technol, 2008, 36(6), 691695
RESEARCH PAPER
Protolytic cracking in Daqing VGO catalytic cracking process GONG Jian-hong, LONG Jun, XU You-hao* Research Institute of Petroleum Processing, SINOPEC, Beijing 100083, China
Abstract: The protolytic cracking of Daqing vacuum gas oil (VGO) catalytic cracking process was studied by conducting multiple series of experiments from the low reaction conversion to high conversion over acid catalyst in a small fixed fluid bed (FFB) unit. A concept of the secondary protolytic cracking is proposed for VGO catalytic cracking process. The reactants of the secondary protolytic cracking are mainly paraffins in naphtha that are readsorbed on the acid catalyst and then cracked subsequently. The primary reason that causes the secondary protolytic cracking is the change of selective adsorption of the intermediate products on catalysts at the late reaction stage. The secondary protolytic cracking is not very sensitive to the reaction temperature. The portion of the secondary protolytic cracking in the whole protolytic cracking process is 60% for Daqing VGO at 500qC. Keywords: VGO; catalytic cracking; reaction mechanism; protolytic cracking
Based on the carbenium ions theory[1] developed by Whitemore et al, bimolecular catalytic cracking mechanism was proposed by Greensfelder[2] and Thomas et al[3] in 1940s. The bimolecular catalytic cracking mechanism has long been regarded as essentially correct so that in 1983 Hansford declared that “it seems unlikely that a better theory as applied to catalytic cracking will ever replace it[4].” However, due to its simplicity, the bimolecular catalytic cracking mechanism is hard and insufficient to account for the complex product distribution during catalytic cracking. In addition, the initiation of catalytic cracking (how the first carbenium ion is formed) cannot be explained by the bimolecular mechanism. Until 1984, Haag and Dessau[5] and Corma et al[6] presented a protolytic cracking theory by linking petroleum refining chemistry and Olah[7]’s hydrocarbon chemistry in super acids. They postulated that the alkane cracking over solid acid catalyst was initiated by Brönsted acids by directly attacking C—C and C—H bonds and protonating alkane to give pentacoordinated carbonium ions, which were transitional states in cracking[8]. The carbonium ions were subsequently collapsed to give a light alkane or H2, and corresponding tricoordinated carbenium ions. The protolytic cracking theory makes the whole catalytic cracking mechanism perfect. The protolytic cracking reaction has drawn great attention as it is the initiation step of the alkane cracking. But present studies on protolytic cracking were just limited to use pure
hydrocarbons as feed[9,10], and researchers seldom investigated the protolytic cracking reaction in a heavy oil complex system over acid catalysts. Long et al[11] attempted to study the formation mechanism of dry gas during vacuum gas oil (VGO) catalytic cracking by applying quantum chemistry theory. Because the protolytic cracking was treated as the initiation of cracking, much attention was focused on the low conversion and researchers seldom cared about whether there was protolytic cracking at the high conversion, especially for heavy oil used as feedstock. Therefore, this article investigated the protolytic cracking that occur at different conversion stages during heavy oil cracking over acid catalysts.
1
Experimental
Daqing VGO was used as raw material. The density(20qC) of VGO was 858.6 kg/cm3, and the other main properties were listed in literature[12]. MLC-500 catalyst, manufactured by Qilu catalyst company, was steam-deactivated for 17 h at 800qC with 100% steam before use, and its properties were also listed in literature[12]. Heavy oil’s catalytic cracking was conducted in a small fixed fluidized bed (FFB) unit, in which reaction and regeneration stages were operated intermittently. The unit flow and experimental procedure were listed in literature[13].
Received: 19-May-2008; Revised: 03-Aug-2008 * Corresponding author. E-mail: [email protected] Foundation item: Supported by the Major State Basic Research Development Program of China (973 Program, 2006CB202501). Copyright 2008, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.
GONG Jian-hong et al. / Journal of Fuel Chemistry and Technology, 2008, 36(6): 691695
The liquid product was separated into gasoline, light cycle oil (LCO), and slurry by the American Society for Testing and Materials (ASTM) simulated distillation methods, whereas, the gas product was determined by an Angilent 6890 gas chromatograph. Conversion is defined as the sum of liquid products boiling below 493 K, gases, and coke. The variation of conversion was achieved by changing the catalyst/oil ratio. Before catalytic cracking experiments, parallel experiments with quartz sands as inert medium were conducted in the FFB unit at 460qC, 480qC, and 500qC. Results show that no obvious reactions occurred at 460qC, 480qC, and 500qC. Therefore, it is concluded that thermal cracking can be neglected at and below 500qC. Molar yield is applied to calculate the product yields in this article. The molar yield is defined as the molar amounts of product produced per gram of Daqing VGO. The unit of the molar yield is mmol/g.
2 2.1
Results and discussion Protolytic cracking and conversion
According to the reaction mechanism of light alkane, the protolytic cracking was favored at low conversion as the initial cracking reaction, whereas bimolecular cracking was favored at high conversion. It should be pointed out that during catalytic cracking, dry gas denotes the sum of hydrogen, methane, ethane, and ethylene. Corma et al[14] believe that in a real fluid catalytic cracking (FCC) unit one of the characteristics of bimolecular catalytic cracking differing from the protolytic cracking is the dry gas yield. High dry gas yield will be obtained by protolytic cracking. On the other hand, bimolecular catalytic cracking will give low dry gas. Therefore, the dry gas can be used as the characteristic product to investigate the protolytic cracking. The variation of selectivity of dry gas with conversion is used to examine the relation between protolytic cracking and conversion in this article. Fig. 1 shows the variation of selectivity of dry gas with the conversion at different reaction temperatures. It can be seen that after the catalytic cracking reaction is initiated, the selectivity of dry gas is high at the low conversion and then it decreased gradually with the increase of conversion. These results are in accordance with the protolytic cracking results of light alkane described earlier. However, there is another noteworthy phenomenon in Fig. 1 that shows that at the high conversion the selectivity of dry gas increases instead of decreasing further. Therefore, for heavy oil catalytic cracking over acid catalysts, the protolytic cracking is predominant at the initial stage of reaction and with the increase of conversion, the protolytic cracking decreases gradually.
However, when the conversion is high enough, the protolytic cracking reaction boosts up again. This result is totally different from the conventional protolytic cracking results of light alkane over acid catalysts. In addition, it can be seen from Fig. 1 that the selectivities of dry gas are high at the higher temperatures when the catalytic cracking is at low or moderate conversions, which also conforms to the conclusion inferred from pure hydrocarbon. However, the difference from that of pure hydrocarbon is that the temperature has little effect on the selectivity of dry gas at the high conversion. In other words, the protolytic cracking is not sensitive to the reaction temperature at the high conversion. The reasons for protolytic cracking enhancing again at the high conversion for heavy oil catalytic cracking over acid catalysts should be discussed. According to the traditional FCC knowledge it can be interpreted by gasoline (intermediate product)’s “overcracking” at high severity, which means the gasoline products are again overcracked at the high conversion. Thereby, the following will investigate further the behavior of protolytic cracking at the high conversion from the point of reaction chemistry of “overcracking”. In combination with the knowledge of “overcracking”, the repeat protolytic cracking enhancement at the high conversion indicates that during catalytic cracking many paraffinic molecules (including saturated cycloparaffins and alkyl-aromatics) in gasoline readsorbed over acid catalysts to form pentacoordinated carbonium ions and then collapsed subsequently. According to theoretical analysis, the normal alkanes, iso-alkanes, cycloparaffins, and alkyl-aromatics in gasoline can all have the protolytic cracking possibility to form pentacoordinated carbonium ions. Further investigation is done to study what molecules or which kinds of molecules together cause the increment of the selectivity of dry gas. Figure 2 shows the variation of the characteristic product (hydrogen, methane, ethane, and ethylene) of protolytic cracking with conversion when Daqing VGO is cracked at 500qC over acid catalysts.
Fig. 1 Variation of the dry gas selectivities with conversion : 500qC; : 480qC; : 460qC
GONG Jian-hong et al. / Journal of Fuel Chemistry and Technology, 2008, 36(6): 691695
Fig. 2 Variation of the characteristic products of protolytic
Fig. 3 Variation of the yields of n-paraffins in naphtha and
cracking with conversion
dry gas with conversion
: H2; : CH4; 0: C2H6; : C2H4
: n-paraffins in naphtha; : dry gas
It can be seen from Fig. 2 that the molar yield of methane obviously exceeds that of hydrogen at high conversion, which shows that the breakage rates of C—C bonds are more than those of C—H bonds. This result conforms to that of protolytic cracking of light alkane. Therefore, it can be concluded that the increase of the selectivity of dry gas at high conversion may be caused by the protolytic cracking of paraffinic molecules (lower carbon numbers) in gasoline product. To prove the above hypothesis, the variation of the molar yield of dry gas and the components in gasoline product with conversion needs to be investigated further. Fig. 3 shows the variation of the molar yields of normal paraffins in naphtha and dry gas with conversion when Daqing VGO is catalytically cracked at 500qC. It can be found that at high conversion the sharp increment of the dry gas molar yield corresponds well with the reduction of normal paraffins in FCC naphtha. It can be further concluded that the protolytic cracking of the normal paraffins in naphtha at high conversion at least contribute to a part of the sharp increment of dry gas. The molar yield of isoparaffins in naphtha does not correspond well with the dry gas yield, however, at high conversion, when the dry gas yield increases sharply the increasing trend of isoparaffin molar yield with conversion ascends gently. Therefore, the possibility of isoparaffins in naphtha taking part in protolytic cracking cannot be excluded. Are the cycloparaffins in FCC naphtha also involved in protolytic cracking? Corma et al[15] examined the reaction chemistry of cycloparaffin over acid catalyst and found the molar selectivity of hydrogen was higher than that of methane in the products, which indicates that the selectivity of hydrogen is greater than that of methane during protolytic cracking of cycloparaffins in naphtha. In fact, from Fig. 2 it can be seen that at high conversion the molar yield of methane is higher than that of hydrogen, whereas the molar yield of hydrogen is almost constant. Thereby, it can be
concluded that at high conversion the cycloparaffins in naphtha do not participate in protolytic cracking. Do alkyl-aromatics (another component in FCC naphtha) contribute to the enhancement of the selectivity of dry gas through its protolytic cracking? The aromatics in FCC naphtha consist mainly of multiple methyl-aromatics at high conversion (about 80%) when Daqing VGO is used as raw material[16], and it is difficult to carry on protolytic cracking for methyl-aromatics. Therefore, the possibility of protolytic cracking of the alkyl-aromatics in naphtha can also be excluded. It can be drawn from the above discussion that for heavy oil cracking over acid catalysts, the protolytic cracking is predominant as initial reaction at early reaction stage, although it decreases gradually with the increasing conversion. At high conversion the protolytic cracking is also prominent, which is totally different from that of light alkanes used as raw material. During heavy oil catalytic cracking the paraffinic molecules in FCC naphtha (intermediate product), especially normal paraffinic molecules, readsorb over acid catalysts to form pentacoordinated carbonium ions and then collapse subsequently. To distinguish from the classical protolytic cracking, the protolytic cracking that occurred at high conversion is defined as secondary protolytic cracking during heavy oil catalytic cracking. Combined with Fig. 1, it is found that the secondary protolytic cracking is not sensitive to reaction temperature, which is totally different from literature data on the relation between protolytic cracking and reaction temperature. 2.2
Cause of secondary protolytic cracking
It is difficult to accept the conclusion that the secondary protolytic cracking is caused by paraffinic molecules in FCC naphtha product, especially normal paraffinic molecules,
GONG Jian-hong et al. / Journal of Fuel Chemistry and Technology, 2008, 36(6): 691695
which readsorb over acid catalysts to form pentacoordinated carbonium ions and then collapse subsequently. According to the reaction mechanism of catalytic cracking, once the feed molecules is initiated by protolytic cracking the produced carbenium ions can easily participate in bimolecular reaction including hydride transfer and so on, and the catalytic cracking reaction are stopped as soon as the carbenium ions are desorbed. Therefore, the cracking mechanism of hydrocarbon shows that if there are any olefin molecules (easily to be protonated forming the carbenium ions over Brönsted acid sites) in the intermediate product, the paraffinic molecules are prone to be consumed by bimolecular cracking reaction. In fact, the presence of olefins, either as product or added in the feed, resulting in bimolecular cracking reaction are more important contributors to feed conversion[17]. The following further investigates the question of why the paraffinic molecules in naphtha are not completely consumed through bimolecular hydride transfer with carbenium ions formed from olefins, but some of them are reacted through the secondary protolytic cracking. The secondary protolytic cracking occurs at high conversion when large quantities of coke are deposited on catalysts. Cummings et al[18] observed that coke molecules saturated the produced olefins through the hydrogen transfer reaction. Reyniers et al[19] also found that the coke molecules could not be treated as inertial product; on the contrary, it could participate in reaction as a hydrogen donor. Reyniers et al. also presented that the coke molecules could compete with the paraffinic molecules to participate in the bimolecular cracking reactions. This demonstrates that the production of coke can affect paraffins inducing it to take part in bimolecular cracking reactions. Although it is discovered that coke molecules have negative effect on the bimolecular cracking reaction of paraffinic molecules due to its competing with paraffins to donate hydrogen, it represents just one aspect and only illustrates that the percentage of paraffinic molecules participating in bimolecular cracking may be reduced by the production of coke molecules during catalytic cracking. More important is that even assuming that no produced paraffinic molecules participate in any bimolecular cracking due to competition, it just demonstrates that there are more paraffinic molecules in the end product. The essence of competition still cannot explain why the produced paraffinic molecules participates in the secondary protolytic cracking. Though at high conversion large quantities of coke molecules deposited on catalyst can participate in some kinds of reaction as hydrogen donator, at the same time it can deactivate the catalyst rapidly by adsorbing on the active sites, thereby leading to the acid density of catalyst decreasing gradually with the increasing conversion. Besides
the coke molecules, the multiple-ring aromatics produced in cracking reaction or in the feed with higher adsorption constant can deactivate the catalysts quickly[20], which also cause the acid density of catalyst decreasing continually. A decrease in acid density means that active sites nearby reduce the force to a certain active site, which accordingly cause the acid strength increase of the specific active site[21]. Corma et al[22] investigated the influence of the level of steam dealumination on the selective adsorption of olefins and paraffins during catalytic cracking on the ultra stable (US)Y zeolites. The results showed that the highly dealuminated USY zeolite have less selective adsorption of olefins with respect to paraffins. Based on these, the cause of secondary protolytic cracking can be well explained. As heavy oil catalytic cracking proceeds, coke and multiple ring aromatics are produced continually with increasing conversion. Because the channel of Y zeolite is so large that some multiple ring aromatics in the feed (or product) and coke molecules can enter the zeolite channel and deposit on the active sites, causing the acid density of catalyst to decrease greatly. This process is similar to the dealumination of zeolite by steam treatment to some extent, so as to cause the remaining active sites in zeolite channel enhance the selective adsorption of paraffin. At the latter stage of the heavy oil cracking reaction, some paraffins in naphtha are selectively readsorbed to form pentacoordinated carbonium ions and are then consumed through secondary protolytic cracking. Abbot et al[23] also reported that for normal paraffins the protolytic cracking reaction is easier than bimolecular cracking reactions through hydride transfer and Escission. 2.3
Ratio of secondary protolytic cracking
Protolytic cracking plays a very important role for being the initial reaction during the heavy oil catalytic cracking, but at the same time, it has a negative effect on the selectivity to the product of interest due to the dry gas generated in the reaction. The secondary protolytic cracking should be especially avoided because its feed mainly consisted of naphtha paraffins in industrial production. Therefore, it is very necessary to further investigate the ratio of the secondary protolytic cracking during the whole catalytic cracking process. To distinguish from the secondary protolytic cracking defined earlier, the protolytic cracking that occurs at the early reaction stage as the initial reaction is named as the first protolytic cracking in this article. During catalytic cracking, it is difficult to define the limit between the first protolytic cracking and secondary protolytic cracking, because the limit is closely related to feed properties, catalyst properties, and even operating parameters. For Daqing VGO’s cracking over
GONG Jian-hong et al. / Journal of Fuel Chemistry and Technology, 2008, 36(6): 691695
Y zeolite based catalyst at 500qC, the conversion rate of 75% is used to divide these two protolytic crackings. The protolytic cracking that occurs at below the conversion of 75% is treated as the first protolytic cracking, whereas the protolytic cracking that occurs at above the conversion of 75% is viewed as the secondary protolytic cracking. Assuming a conversion of 90% as the highest value achieved in these experiments, it is easy to calculate the ratio of the secondary protolytic cracking (defined as a parameterK) by examining the molar yield of dry gas.
[3] Thomas C L. Chemistry of Cracking Catalysts. Ind Eng Chem, 1949, 41(11): 2564–2573. [4] Hansford R C. Development of the theory of catalytic cracking. In: Davis B H, Hettinger W P. Heterogeneous Catalysis Selected American Histories, ACS Symposium Series 222. Washington DC: American Chemical Society, 1983: 247–252. [5] Haag W O, Dessau R M. Duality of Mechanism for Acid-Catalyzed Cracking. In: Basel V C. Proceedings of the 8th International Congress on Catalysis, Vol. 2. Frankfurt am Main: Dechema, 1984: 305–316. [6] Corma A, Planelles J, Sanchez-Marin J, Tomas F. The role of different types of acid site in the cracking of alkanes on zeolite catalysts. J Catal, 1985, 93(1): 30–37.
where n 90 denotes the molar yield of dry gas at conversion dy of 90% and n 75 is the molar yield of dry gas at conversion dy of 75%. Based on the above-mentioned equation, the ratio of the secondary protolytic cracking can be calculated for Daqing VGO cracking over Y zeolite-based catalyst at 500qC. K= 60% Then the ratio of the first protolytic cracking equals to 40%.
3
Conclusions
Based on the reaction mechanism of pure hydrocarbon cracking, the protolytic cracking of Daqing VGO cracking over acid catalysts has been studied by conducting multiple series of experiments from low conversion to high conversion. By studying the dependence of the protolytic cracking of heavy oils on conversion, it is found that the protolytic cracking rate decreases gradually from the reaction initiation to the stage of moderate conversion. However, the protolytic cracking is different from the reaction mechanisms of pure paraffinic molecules. The rate of the protolytic cracking reaction of heavy oils is enhanced apparently again at the late reaction stage. Therefore, the distinctive protolytic cracking at the late reaction stage is defined as the secondary protolytic cracking. The origin, cause, relationship with reaction temperature, and the ratio of the secondary protolytic cracking relative to the first protolytic cracking are investigated. The study on the secondary protolytic cracking is very helpful for commercialization of the catalytic cracking process.
References [1] Whitmore F C, Church J M. The isomermers in diisobutylene III. Determination of their structure. J Am Chem Soc, 1932, 54(9): 3710–3714. [2] Greensfelder B S, Voge H H, Good G M. Catalytic and Thermal Cracking of Pure Hydrocarbons: Mechanisms of Reaction. Ind Eng Chem, 1949, 41(11): 2573–2584.
[7] Olah G A, Schlosberg R H. Chemistry in super acids: I Hydrogen exchange and polycondensation of methane and alkanes in FSO3H-SbF5 (“magic acid”) solution. Protonation of alkanes and the intermediacy of CH5+ and related hydrocarbon ions. The high chemical reactivity of “paraffins” in ionic solution reactions. J Am Chem Soc, 1968, 90(10): 2726–2727. [8] Lercher J A, Van Santen R A, Vinek H. Carbonium ion formation in zeolite catalysis. Catal Lett, 1994, 27(1): 91–96. [9] Babitz S M, Willams B A, Miller J T. Monomolecular cracking of n-hexane on Y, MOR, and ZSM-5 zeolites. Appl Catal A, 1999, 179(1): 71–86. [10] Corma A, Miguel P J, Orchilles A V. On the limitations to establish the contribution of the different reaction mechanisms from selectivity data, during cracking of long-chain linear paraffins. Ind Eng Chem Res, 1997, 36(8): 3400–3415. [11] Long J, Wei X L. Study on the catalytic mechanism of dry gas formation in catalytic cracking reactions. Acta Petrolei Sinica( Petroleum Processing Section ), 2007, 23(1): 1–7. [12] Gong J H, Long J, Xu Y H. Different reaction characteristics of hydride transfer and hydrogen transfer in catalytic cracking. Chinese Journal of Catalysis, 2007, 28(1): 67–72. [13] Gong J H, Yang Y N, Xu Y H, Zhang J S, Long J. Effect of temperature on gasoline olefin cracking reaction s over acid catalyst. Petrochemcal Technology, 2005, 34(10): 938–942. [14] Corma A, Orchille´s A V. Current views on the mechanism of catalytic cracking. Microporous Mesoporous Mater, 2000, 35–36: 21–30. [15] Corma A, Mocholi F, Orchille´s V, Koermer G R, Gerald S, Madon Rostam J. The cracking chemistry of naphthenes. Prepr Am Chem Soc Div Pet Chem, 1989, 34(4): 681–685. [16] Wei X L. Mechanistic study on hydrocarbon conversion in the 1st reaction zone in MIP technology. Beijing: Research Institute of Petroleum Processing, 2005. [17] Williams B A, Ji W, Miller J T, Snurr R Q, Kung H H. Evidence of different reaction mechanism during the cracking of n-hexane on H-USY zeolite. Appl Catal A, 2000, 203(2): 179–190. [18] Cumming K A, Wojciechowski B W. Hydrogen transfer, coke formation, and catalyst decay and their role in the chain
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USY-zeolite. Appl Catal A, 2000, 202(1): 49–63.
paraffins and its implication on hydrogen transfer reactions
[20] Corma A, Ortega F J. Influence of adsorption parameters on catalytic cracking and catalyst decay. J Catal, 2005, 233(2):
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RESEARCH PAPER
Protolytic cracking in Daqing VGO catalytic cracking process GONG Jian-hong, LONG Jun, XU You-hao* Research Institute of Petroleum Processing, SINOPEC, Beijing 100083, China
Abstract: The protolytic cracking of Daqing vacuum gas oil (VGO) catalytic cracking process was studied by conducting multiple series of experiments from the low reaction conversion to high conversion over acid catalyst in a small fixed fluid bed (FFB) unit. A concept of the secondary protolytic cracking is proposed for VGO catalytic cracking process. The reactants of the secondary protolytic cracking are mainly paraffins in naphtha that are readsorbed on the acid catalyst and then cracked subsequently. The primary reason that causes the secondary protolytic cracking is the change of selective adsorption of the intermediate products on catalysts at the late reaction stage. The secondary protolytic cracking is not very sensitive to the reaction temperature. The portion of the secondary protolytic cracking in the whole protolytic cracking process is 60% for Daqing VGO at 500qC. Keywords: VGO; catalytic cracking; reaction mechanism; protolytic cracking
Based on the carbenium ions theory[1] developed by Whitemore et al, bimolecular catalytic cracking mechanism was proposed by Greensfelder[2] and Thomas et al[3] in 1940s. The bimolecular catalytic cracking mechanism has long been regarded as essentially correct so that in 1983 Hansford declared that “it seems unlikely that a better theory as applied to catalytic cracking will ever replace it[4].” However, due to its simplicity, the bimolecular catalytic cracking mechanism is hard and insufficient to account for the complex product distribution during catalytic cracking. In addition, the initiation of catalytic cracking (how the first carbenium ion is formed) cannot be explained by the bimolecular mechanism. Until 1984, Haag and Dessau[5] and Corma et al[6] presented a protolytic cracking theory by linking petroleum refining chemistry and Olah[7]’s hydrocarbon chemistry in super acids. They postulated that the alkane cracking over solid acid catalyst was initiated by Brönsted acids by directly attacking C—C and C—H bonds and protonating alkane to give pentacoordinated carbonium ions, which were transitional states in cracking[8]. The carbonium ions were subsequently collapsed to give a light alkane or H2, and corresponding tricoordinated carbenium ions. The protolytic cracking theory makes the whole catalytic cracking mechanism perfect. The protolytic cracking reaction has drawn great attention as it is the initiation step of the alkane cracking. But present studies on protolytic cracking were just limited to use pure
hydrocarbons as feed[9,10], and researchers seldom investigated the protolytic cracking reaction in a heavy oil complex system over acid catalysts. Long et al[11] attempted to study the formation mechanism of dry gas during vacuum gas oil (VGO) catalytic cracking by applying quantum chemistry theory. Because the protolytic cracking was treated as the initiation of cracking, much attention was focused on the low conversion and researchers seldom cared about whether there was protolytic cracking at the high conversion, especially for heavy oil used as feedstock. Therefore, this article investigated the protolytic cracking that occur at different conversion stages during heavy oil cracking over acid catalysts.
1
Experimental
Daqing VGO was used as raw material. The density(20qC) of VGO was 858.6 kg/cm3, and the other main properties were listed in literature[12]. MLC-500 catalyst, manufactured by Qilu catalyst company, was steam-deactivated for 17 h at 800qC with 100% steam before use, and its properties were also listed in literature[12]. Heavy oil’s catalytic cracking was conducted in a small fixed fluidized bed (FFB) unit, in which reaction and regeneration stages were operated intermittently. The unit flow and experimental procedure were listed in literature[13].
Received: 19-May-2008; Revised: 03-Aug-2008 * Corresponding author. E-mail: [email protected] Foundation item: Supported by the Major State Basic Research Development Program of China (973 Program, 2006CB202501). Copyright 2008, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.
GONG Jian-hong et al. / Journal of Fuel Chemistry and Technology, 2008, 36(6): 691695
The liquid product was separated into gasoline, light cycle oil (LCO), and slurry by the American Society for Testing and Materials (ASTM) simulated distillation methods, whereas, the gas product was determined by an Angilent 6890 gas chromatograph. Conversion is defined as the sum of liquid products boiling below 493 K, gases, and coke. The variation of conversion was achieved by changing the catalyst/oil ratio. Before catalytic cracking experiments, parallel experiments with quartz sands as inert medium were conducted in the FFB unit at 460qC, 480qC, and 500qC. Results show that no obvious reactions occurred at 460qC, 480qC, and 500qC. Therefore, it is concluded that thermal cracking can be neglected at and below 500qC. Molar yield is applied to calculate the product yields in this article. The molar yield is defined as the molar amounts of product produced per gram of Daqing VGO. The unit of the molar yield is mmol/g.
2 2.1
Results and discussion Protolytic cracking and conversion
According to the reaction mechanism of light alkane, the protolytic cracking was favored at low conversion as the initial cracking reaction, whereas bimolecular cracking was favored at high conversion. It should be pointed out that during catalytic cracking, dry gas denotes the sum of hydrogen, methane, ethane, and ethylene. Corma et al[14] believe that in a real fluid catalytic cracking (FCC) unit one of the characteristics of bimolecular catalytic cracking differing from the protolytic cracking is the dry gas yield. High dry gas yield will be obtained by protolytic cracking. On the other hand, bimolecular catalytic cracking will give low dry gas. Therefore, the dry gas can be used as the characteristic product to investigate the protolytic cracking. The variation of selectivity of dry gas with conversion is used to examine the relation between protolytic cracking and conversion in this article. Fig. 1 shows the variation of selectivity of dry gas with the conversion at different reaction temperatures. It can be seen that after the catalytic cracking reaction is initiated, the selectivity of dry gas is high at the low conversion and then it decreased gradually with the increase of conversion. These results are in accordance with the protolytic cracking results of light alkane described earlier. However, there is another noteworthy phenomenon in Fig. 1 that shows that at the high conversion the selectivity of dry gas increases instead of decreasing further. Therefore, for heavy oil catalytic cracking over acid catalysts, the protolytic cracking is predominant at the initial stage of reaction and with the increase of conversion, the protolytic cracking decreases gradually.
However, when the conversion is high enough, the protolytic cracking reaction boosts up again. This result is totally different from the conventional protolytic cracking results of light alkane over acid catalysts. In addition, it can be seen from Fig. 1 that the selectivities of dry gas are high at the higher temperatures when the catalytic cracking is at low or moderate conversions, which also conforms to the conclusion inferred from pure hydrocarbon. However, the difference from that of pure hydrocarbon is that the temperature has little effect on the selectivity of dry gas at the high conversion. In other words, the protolytic cracking is not sensitive to the reaction temperature at the high conversion. The reasons for protolytic cracking enhancing again at the high conversion for heavy oil catalytic cracking over acid catalysts should be discussed. According to the traditional FCC knowledge it can be interpreted by gasoline (intermediate product)’s “overcracking” at high severity, which means the gasoline products are again overcracked at the high conversion. Thereby, the following will investigate further the behavior of protolytic cracking at the high conversion from the point of reaction chemistry of “overcracking”. In combination with the knowledge of “overcracking”, the repeat protolytic cracking enhancement at the high conversion indicates that during catalytic cracking many paraffinic molecules (including saturated cycloparaffins and alkyl-aromatics) in gasoline readsorbed over acid catalysts to form pentacoordinated carbonium ions and then collapsed subsequently. According to theoretical analysis, the normal alkanes, iso-alkanes, cycloparaffins, and alkyl-aromatics in gasoline can all have the protolytic cracking possibility to form pentacoordinated carbonium ions. Further investigation is done to study what molecules or which kinds of molecules together cause the increment of the selectivity of dry gas. Figure 2 shows the variation of the characteristic product (hydrogen, methane, ethane, and ethylene) of protolytic cracking with conversion when Daqing VGO is cracked at 500qC over acid catalysts.
Fig. 1 Variation of the dry gas selectivities with conversion : 500qC; : 480qC; : 460qC
GONG Jian-hong et al. / Journal of Fuel Chemistry and Technology, 2008, 36(6): 691695
Fig. 2 Variation of the characteristic products of protolytic
Fig. 3 Variation of the yields of n-paraffins in naphtha and
cracking with conversion
dry gas with conversion
: H2; : CH4; 0: C2H6; : C2H4
: n-paraffins in naphtha; : dry gas
It can be seen from Fig. 2 that the molar yield of methane obviously exceeds that of hydrogen at high conversion, which shows that the breakage rates of C—C bonds are more than those of C—H bonds. This result conforms to that of protolytic cracking of light alkane. Therefore, it can be concluded that the increase of the selectivity of dry gas at high conversion may be caused by the protolytic cracking of paraffinic molecules (lower carbon numbers) in gasoline product. To prove the above hypothesis, the variation of the molar yield of dry gas and the components in gasoline product with conversion needs to be investigated further. Fig. 3 shows the variation of the molar yields of normal paraffins in naphtha and dry gas with conversion when Daqing VGO is catalytically cracked at 500qC. It can be found that at high conversion the sharp increment of the dry gas molar yield corresponds well with the reduction of normal paraffins in FCC naphtha. It can be further concluded that the protolytic cracking of the normal paraffins in naphtha at high conversion at least contribute to a part of the sharp increment of dry gas. The molar yield of isoparaffins in naphtha does not correspond well with the dry gas yield, however, at high conversion, when the dry gas yield increases sharply the increasing trend of isoparaffin molar yield with conversion ascends gently. Therefore, the possibility of isoparaffins in naphtha taking part in protolytic cracking cannot be excluded. Are the cycloparaffins in FCC naphtha also involved in protolytic cracking? Corma et al[15] examined the reaction chemistry of cycloparaffin over acid catalyst and found the molar selectivity of hydrogen was higher than that of methane in the products, which indicates that the selectivity of hydrogen is greater than that of methane during protolytic cracking of cycloparaffins in naphtha. In fact, from Fig. 2 it can be seen that at high conversion the molar yield of methane is higher than that of hydrogen, whereas the molar yield of hydrogen is almost constant. Thereby, it can be
concluded that at high conversion the cycloparaffins in naphtha do not participate in protolytic cracking. Do alkyl-aromatics (another component in FCC naphtha) contribute to the enhancement of the selectivity of dry gas through its protolytic cracking? The aromatics in FCC naphtha consist mainly of multiple methyl-aromatics at high conversion (about 80%) when Daqing VGO is used as raw material[16], and it is difficult to carry on protolytic cracking for methyl-aromatics. Therefore, the possibility of protolytic cracking of the alkyl-aromatics in naphtha can also be excluded. It can be drawn from the above discussion that for heavy oil cracking over acid catalysts, the protolytic cracking is predominant as initial reaction at early reaction stage, although it decreases gradually with the increasing conversion. At high conversion the protolytic cracking is also prominent, which is totally different from that of light alkanes used as raw material. During heavy oil catalytic cracking the paraffinic molecules in FCC naphtha (intermediate product), especially normal paraffinic molecules, readsorb over acid catalysts to form pentacoordinated carbonium ions and then collapse subsequently. To distinguish from the classical protolytic cracking, the protolytic cracking that occurred at high conversion is defined as secondary protolytic cracking during heavy oil catalytic cracking. Combined with Fig. 1, it is found that the secondary protolytic cracking is not sensitive to reaction temperature, which is totally different from literature data on the relation between protolytic cracking and reaction temperature. 2.2
Cause of secondary protolytic cracking
It is difficult to accept the conclusion that the secondary protolytic cracking is caused by paraffinic molecules in FCC naphtha product, especially normal paraffinic molecules,
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which readsorb over acid catalysts to form pentacoordinated carbonium ions and then collapse subsequently. According to the reaction mechanism of catalytic cracking, once the feed molecules is initiated by protolytic cracking the produced carbenium ions can easily participate in bimolecular reaction including hydride transfer and so on, and the catalytic cracking reaction are stopped as soon as the carbenium ions are desorbed. Therefore, the cracking mechanism of hydrocarbon shows that if there are any olefin molecules (easily to be protonated forming the carbenium ions over Brönsted acid sites) in the intermediate product, the paraffinic molecules are prone to be consumed by bimolecular cracking reaction. In fact, the presence of olefins, either as product or added in the feed, resulting in bimolecular cracking reaction are more important contributors to feed conversion[17]. The following further investigates the question of why the paraffinic molecules in naphtha are not completely consumed through bimolecular hydride transfer with carbenium ions formed from olefins, but some of them are reacted through the secondary protolytic cracking. The secondary protolytic cracking occurs at high conversion when large quantities of coke are deposited on catalysts. Cummings et al[18] observed that coke molecules saturated the produced olefins through the hydrogen transfer reaction. Reyniers et al[19] also found that the coke molecules could not be treated as inertial product; on the contrary, it could participate in reaction as a hydrogen donor. Reyniers et al. also presented that the coke molecules could compete with the paraffinic molecules to participate in the bimolecular cracking reactions. This demonstrates that the production of coke can affect paraffins inducing it to take part in bimolecular cracking reactions. Although it is discovered that coke molecules have negative effect on the bimolecular cracking reaction of paraffinic molecules due to its competing with paraffins to donate hydrogen, it represents just one aspect and only illustrates that the percentage of paraffinic molecules participating in bimolecular cracking may be reduced by the production of coke molecules during catalytic cracking. More important is that even assuming that no produced paraffinic molecules participate in any bimolecular cracking due to competition, it just demonstrates that there are more paraffinic molecules in the end product. The essence of competition still cannot explain why the produced paraffinic molecules participates in the secondary protolytic cracking. Though at high conversion large quantities of coke molecules deposited on catalyst can participate in some kinds of reaction as hydrogen donator, at the same time it can deactivate the catalyst rapidly by adsorbing on the active sites, thereby leading to the acid density of catalyst decreasing gradually with the increasing conversion. Besides
the coke molecules, the multiple-ring aromatics produced in cracking reaction or in the feed with higher adsorption constant can deactivate the catalysts quickly[20], which also cause the acid density of catalyst decreasing continually. A decrease in acid density means that active sites nearby reduce the force to a certain active site, which accordingly cause the acid strength increase of the specific active site[21]. Corma et al[22] investigated the influence of the level of steam dealumination on the selective adsorption of olefins and paraffins during catalytic cracking on the ultra stable (US)Y zeolites. The results showed that the highly dealuminated USY zeolite have less selective adsorption of olefins with respect to paraffins. Based on these, the cause of secondary protolytic cracking can be well explained. As heavy oil catalytic cracking proceeds, coke and multiple ring aromatics are produced continually with increasing conversion. Because the channel of Y zeolite is so large that some multiple ring aromatics in the feed (or product) and coke molecules can enter the zeolite channel and deposit on the active sites, causing the acid density of catalyst to decrease greatly. This process is similar to the dealumination of zeolite by steam treatment to some extent, so as to cause the remaining active sites in zeolite channel enhance the selective adsorption of paraffin. At the latter stage of the heavy oil cracking reaction, some paraffins in naphtha are selectively readsorbed to form pentacoordinated carbonium ions and are then consumed through secondary protolytic cracking. Abbot et al[23] also reported that for normal paraffins the protolytic cracking reaction is easier than bimolecular cracking reactions through hydride transfer and Escission. 2.3
Ratio of secondary protolytic cracking
Protolytic cracking plays a very important role for being the initial reaction during the heavy oil catalytic cracking, but at the same time, it has a negative effect on the selectivity to the product of interest due to the dry gas generated in the reaction. The secondary protolytic cracking should be especially avoided because its feed mainly consisted of naphtha paraffins in industrial production. Therefore, it is very necessary to further investigate the ratio of the secondary protolytic cracking during the whole catalytic cracking process. To distinguish from the secondary protolytic cracking defined earlier, the protolytic cracking that occurs at the early reaction stage as the initial reaction is named as the first protolytic cracking in this article. During catalytic cracking, it is difficult to define the limit between the first protolytic cracking and secondary protolytic cracking, because the limit is closely related to feed properties, catalyst properties, and even operating parameters. For Daqing VGO’s cracking over
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Y zeolite based catalyst at 500qC, the conversion rate of 75% is used to divide these two protolytic crackings. The protolytic cracking that occurs at below the conversion of 75% is treated as the first protolytic cracking, whereas the protolytic cracking that occurs at above the conversion of 75% is viewed as the secondary protolytic cracking. Assuming a conversion of 90% as the highest value achieved in these experiments, it is easy to calculate the ratio of the secondary protolytic cracking (defined as a parameterK) by examining the molar yield of dry gas.
[3] Thomas C L. Chemistry of Cracking Catalysts. Ind Eng Chem, 1949, 41(11): 2564–2573. [4] Hansford R C. Development of the theory of catalytic cracking. In: Davis B H, Hettinger W P. Heterogeneous Catalysis Selected American Histories, ACS Symposium Series 222. Washington DC: American Chemical Society, 1983: 247–252. [5] Haag W O, Dessau R M. Duality of Mechanism for Acid-Catalyzed Cracking. In: Basel V C. Proceedings of the 8th International Congress on Catalysis, Vol. 2. Frankfurt am Main: Dechema, 1984: 305–316. [6] Corma A, Planelles J, Sanchez-Marin J, Tomas F. The role of different types of acid site in the cracking of alkanes on zeolite catalysts. J Catal, 1985, 93(1): 30–37.
where n 90 denotes the molar yield of dry gas at conversion dy of 90% and n 75 is the molar yield of dry gas at conversion dy of 75%. Based on the above-mentioned equation, the ratio of the secondary protolytic cracking can be calculated for Daqing VGO cracking over Y zeolite-based catalyst at 500qC. K= 60% Then the ratio of the first protolytic cracking equals to 40%.
3
Conclusions
Based on the reaction mechanism of pure hydrocarbon cracking, the protolytic cracking of Daqing VGO cracking over acid catalysts has been studied by conducting multiple series of experiments from low conversion to high conversion. By studying the dependence of the protolytic cracking of heavy oils on conversion, it is found that the protolytic cracking rate decreases gradually from the reaction initiation to the stage of moderate conversion. However, the protolytic cracking is different from the reaction mechanisms of pure paraffinic molecules. The rate of the protolytic cracking reaction of heavy oils is enhanced apparently again at the late reaction stage. Therefore, the distinctive protolytic cracking at the late reaction stage is defined as the secondary protolytic cracking. The origin, cause, relationship with reaction temperature, and the ratio of the secondary protolytic cracking relative to the first protolytic cracking are investigated. The study on the secondary protolytic cracking is very helpful for commercialization of the catalytic cracking process.
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