Synergistic Process for Coker Gas Oil Catalytic Cracking and Gasoline ...

Article pubs.acs.org/EF

Synergistic Process for Coker Gas Oil Catalytic Cracking and Gasoline Reformation Jinhong Zhang,† Honghong Shan,† Wenjing Liu,† Xiaobo Chen,† Chunyi Li,*,† and Chaohe Yang*,† †

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266555, China S Supporting Information *

ABSTRACT: The most critical problem of processing coker gas oil (CGO) is its high nitrogen content, especially the basic nitrogen compounds, which limits its cracking performance in the fluid catalytic cracking (FCC) process. For enhancing the conversion of CGO, three processing schemes were evaluated in a pilot-scale riser FCC unit. Four indexes (thermal cracking index, dehydrogenation index, hydrogen transfer coefficient, and isomerization reaction index) were used to investigate the effects of operating conditions on the reactions of CGO cracking. Results show that the optimal operating conditions for CGO cracking are high reaction temperature and large catalyst-to-oil ratio with a short residence time. Therefore, we proposed a synergistic process by selectively recycling light FCC gasoline (LCG) from the upper position of the riser reactor, which can provide a high-severity reaction zone for CGO cracking and a low-severity reaction zone for gasoline upgrading. To further investigate the mutual effect of the two feeds, different recycle ratios of LCG were tested. Results indicate that the conversion of CGO significantly increased with the LCG recycle ratio. When the recycle ratio reached 50 wt %, the gasoline could be upgraded at a higher efficiency. To ensure the optimal recycle ratio and improve the gasoline quality, a two-stage synergistic (TSS) process was proposed. The simulated experiments of the TSS process show that the higher conversion and more desired products can be achieved, even though under a high processing ratio of CGO to conventional feeds.

1. INTRODUCTION In recent years, coker gas oil (CGO) has become one of the main feedstock for fluid catalytic cracking units (FCCU) due to the decline of crude quality and the trend of processing heavier crudes which are beneficial to refineries.1 The increasing demand for light fuel oil calls for more FCC feeds. At the same time, more and more coker gas oils are available for the FCCU as the delayed coking is an important residue upgrading process in the current refining scenario, and CGO is a main product of this process, which accounts for 20−30 wt % in the product distribution.2 However, for several decades, the ratio of CGO that can be blended into VGO as FCC feedstock is limited mainly due to the poisoning effect of basic nitrogen compounds.3−10 Also, the retardation effect of nonbasic nitrogen compounds and polynuclear aromatics has been investigated recently.11 So far, several approaches have been developed to pretreating CGO, both hydrodenitrogenation2,12−14 and non-hydrodenitrogenation (solvent refinement,15,16 adsorption,17 and complexing method,18 etc.). Even though they are useful for processing CGO, several problems are introduced, such as high cost, low yield, and waste disposal.19 Thus, the most beneficial and convenient method is to improve the capability of FCC units to process CGO, which can be conducted from both catalyst and technology means. Several basic nitrogen resistant catalysts had been used in commercial FCC units;20 however, they can only improve the conversion of CGO slightly. Despite the large quantities of CGO required to be upgraded in China, there are a few FCC processes presented for processing the feedstock with higher content of basic nitrogen compounds, for example, the denitrified catalytic cracking (DNCC) process21 and the twostage riser (TSR) FCC process.22 However, they all significantly increase the quantity of heavy cycle oil (HCO); therefore, more © 2013 American Chemical Society

attention should be focused on increasing the once-through conversion of the CGO. Recently, Li et al.23 proposed a divisional fluid catalytic cracking (DFCC) process where a separate reaction zone was added to process CGO, but the poisoning catalyst still can influence the cracking of conventional feedstock in the mixed reaction zone, which leads to a limited processing ratio of CGO, thus processing CGO in a separated reactor will be a better method. In China, about 75−80% of the gasoline pool comes from FCC gasoline, and the olefin content of FCC gasoline is usually as high as 40−60 vol %, which cannot meet the gasoline standards.24,25 Moreover, it will cause serious air pollution. Therefore, immediate measures should be taken to reduce the olefin content of FCC gasoline. Due to the increasing trend of light fuel need, the losses of gasoline yield in the process should be minimized. Several FCC processes have been developed. Both the flexible dual-fluid catalytic cracking (FDFCC) process26 and the subsidiary riser FCC (SRFCC) process all add a secondary parallel riser for upgrading gasoline separately.27 For reducing the undesirable products, Wang et al.28 proposed a dual-reactions mutual control (DMC) process where measures were taken to reduce the temperature difference between gasoline and regenerated catalyst; however, little attention has been focused on reducing the losses of gasoline yield. In the maximizing isoparaffins (MIP) process, the temperature-lowered and partly coked catalyst is used to upgrade gasoline to avoid overcracking.29 However, the long-time upgrading process for all the products may lead to the overcracking of light oil as catalytic Received: September 11, 2012 Revised: January 13, 2013 Published: January 24, 2013 654

dx.doi.org/10.1021/ef3017442 | Energy Fuels 2013, 27, 654−665

Energy & Fuels

Article

Table 1. Properties of Daqing AR and Qilu CGO SARA analysis, wt % feeds

density (20 °C), kg/m3

AR CGO feeds

885 930 C, wt %

AR CGO a

CCRa, wt % 4.39 0.56 H, wt %

86.71 85.86

12.98 12.09

basic nitrogen, μg/g

saturates

− 2178

68.13 55.34

S, wt %

N, wt %

0.22 1.27

0.35 0.66

aromatics 27.99 27.54 Ni, μg/g

resins

asphaltenes

3.49 16.43 V, μg/g

0.39 0.69 H/C

0.17 0.18

1.80 1.69

3.94 0.17

CCR is the Conradson carbon residue.

Table 2. Properties of Catalysts particle size distribution, vol %

a

2

3

catalysts

microactivity

surface area, m /g

pore volume, cm /g

apparent density, kg/m

Aa Bb

72 77

153 170

0.16 0.20

840 830

3

0−20 μm

20−40 μm

40−110 μm

>110 μm

0.1 0.1

11.9 12.4

68.8 67.5

19.2 20.0

Provided by Daqing refinery. bThe mixture of catalyst A and the fresh one at the ratio of 9:1.

Figure 1. Schematic of the pilot-scale riser FCC unit.

compared. Then, different recycle ratios of light gasoline were tested and evaluated from the aspects of CGO conversion, olefin reduction of gasoline, and the losses of gasoline yield. Finally, a combination with the TSR FCC process was discussed, and then a two-stage synergistic (TSS) process was proposed and tested.

cracking is a parallel-series reaction. Therefore, a more effective process for gasoline upgrading needs to be developed. With the aim of enhancing the once-through conversion of CGO with high basic nitrogen content and reforming the FCC gasoline with lower cracking loss, we proposed a synergistic process which could resolve the two problems in a riser reactor. In the first work, three processing schemes (high reaction temperature with large catalyst-to-oil ratio, long residence time, and high catalyst activity) for enhancing the once-through conversion of CGO were tested in a pilot-scale FCC unit, the reaction process, the product distribution, and product quality

2. EXPERIMENTAL SECTION 2.1. Feedstock and Catalyst. The CGO was chosen as the main experimental feed, which was obtained from the Qilu refinery, and the atmospheric residue (AR) was used in the final experiment of this work as the feed of the first-stage riser in the 655


Energy & Fuels

Article

Figure 2. Effect of reaction temperature on product distribution, conversion, and selectivity.

TSR and TSS FCC processes, which was provided by the Daqing refinery. Their properties are shown in Table 1. The nitrogen content of CGO is far higher than that in AR, and its basic nitrogen content is up to 2178 μg/g. Associating with the SARA analysis and hydrogen content, the inferior crackability of the CGO can be inferred. The light gasoline used in the experiments was produced by the Daqing FCC unit. The detailed PIONA analysis of the gasoline is listed in Table S1 (see the Supporting Information). The commercial equilibrium catalyst A was obtained from the Changqing refinery, and the catalyst B is the mixture of catalyst A and the fresh one at the ratio of 9:1. The properties of the catalysts are given in Table 2. 2.2. Experimental Apparatus. The research was performed in a pilot-scale riser FCC unit (shown in Figure 1). The pilot plant unit includes five sections: a feed and steam injection system, a reactor−regenerator system, a product condensing and measurement system, a pneumatic control system, and a computer control system (which shows the main operation parameters and can be used to adjust the unit conveniently). This unit can be operated continuously similar to the commercial units; moreover, the experimental results are consistent with the industrial production, and it has afforded the design data for more than five commercial plants. Along the riser reactor, there are two feed nozzles at different heights, so the residence time can be adjusted in a wider range. In the conventional experiments only one feed injection point is used, while in the synergistic process both of them are used: the lower one is for CGO injection and the upper one for light FCC gasoline injection. During the tests, the mass losses were all less than 3 wt %. In the study, the conversion was defined as the yield sum of dry gas, liquefied petroleum gas (LPG), gasoline, diesel, and coke. In addition, the light oil referred to the sum of gasoline and diesel

yields, and the liquid products were the yields of LPG and light oil. The selectivity was defined as the ratio of product yield to conversion multiplied by 100. 2.3. Product Analysis. The compositions of gas products and flue gas were analyzed by a Varian CP-3800 gas chromatograph, while the liquid products collected were weighted and then analyzed for the simulated distillation by another Varian CP-3800 GC according to the ASTM-2887-D procedure. The boiling point range of gasoline was defined from IBP to 204 °C; diesel 204−350 °C; heavy cycle oil (HCO) above 350 °C. Combining the volumes of cracking gas and flue gas, the product distribution can be computed. The collected liquid products were then fractionated by the true boiling point distillation, so the gasoline, diesel, and heavy oil can be obtained, and then their properties can be tested. PIONA analyses for both gaseous and liquid products were carried out using a third Varian CP-3800 GC equipped with a CP-Sil PONA CB silica column connected to a FID detector.

3. RESULTS AND DISCUSSION 3.1. Methods for Enhancing CGO Conversion. When processing CGO with high nitrogen content, the operation parameters should be adjusted for increasing the conversion of CGO. In the study, three methods (high reaction temperature with large CTO, long residence time, and high catalyst activity) were investigated, and their product distribution and product quality were compared. To further investigate the effects of operating conditions on the reactions of CGO cracking, four indexes from the open literatures were introduced. The first one is the thermal cracking index (TCI), which is defined as the weight ratio of the sum of C1 and C2 yields to the sum of isobutane and isobutene yields {(C1+C2)/(i−C4o+i−C4=)}, the higher the value means the ratio of thermal cracking is larger.30,31 656


Energy & Fuels

Article

The hydrogen is mainly produced by thermal cracking and the dehydrogenation reactions. On the basis of the hypothesis that H2, CH4, and C2 hydrocarbons are generated at random during thermal cracking, the dehydrogenation index (DHI), which is defined as the volume ratio of H2 to the sum of C1 and C2 yields {H2/(C1+C2)}, can be used to describe the degree of dehydrogenation.32,33 The hydrogen transfer reaction and the isomerization reaction are very important for improving the quality of gasoline.34 The hydrogen transfer coefficient (HTC), defined as the volume ratio of butanes to butenes, can be used to describe the strength of hydrogen transfer reactions,35,36 which are helpful for reducing the olefin content in gasoline. The characteristic products of the isomerization reaction are isoparaffins and isoolefins; therefore, the degree of isomerization reaction can be described by the isomerization reaction index, ISO {(i−C4o+i−C4=)/(ΣC4)} (volume ratio).32 3.1.1. High Reaction Temperature with Large Catalyst-toOil Ratio. Increasing the reaction temperature can decrease the adsorption of nitrogen on the catalyst acid sites and reduce the poisoning effects of nitrogen compounds.4 The effect of catalystto-oil ratio (CTO) on the conversion of CGO is significant as well.8,22 However, in the previous studies, the two parameters were studied separately which did not conform to the industrial operation which is determined according to the heat balance. In commercial FCC units, when increasing the reaction temperature, the CTO would rise, so they should change together. Considering this, in the work, we fixed the temperature of the regenerator catalyst at 690 °C and computed the CTO at different reaction temperatures. For reducing the ratio of secondary cracking reactions, a short reaction time was chosen at 0.8 s. The reaction temperatures were set at 500 °C, 520 °C, 540 °C, 555 °C, 568 °C, and 580 °C, respectively, and the corresponding CTO were 5.9, 7.4, 9.4, 11.2, 13.3, and 15.7, respectively. The CGO reaction performance over the commercial equilibrium catalyst A is shown in Figure 2. As the reaction temperature increased from 500 to 580 °C, the feed conversion showed a dramatic rise by approximately 20 wt %, and the yields of gasoline, light oil, and liquid products increase monotonically as well. On the other hand, the selectivity of light oil presented an opposite trend as the decrease of diesel yield. This is because catalytic cracking is a complex parallel-series reaction system, and the diesel is the intermediate product; the higher operating severity could lead to the further cracking of diesel. Because the activation energy of thermal cracking is much higher than that of catalytic cracking, increasing the temperature would increase the ratio of thermal cracking. By contrast, larger CTO can introduce more active centers for catalytic cracking reactions, which would lower the value of TCI. As can be seen from Figure 3, in the test, all the values of TCI were lower than 0.6, which indicated that catalytic cracking was the major reaction. Despite significantly raising the reaction temperature, the rate ratio of thermal cracking to catalytic cracking only slightly increased. This is because larger CTO intensifies the reaction of catalytic cracking at the same time. As the increase of feed conversion, the value of DHI shows a significant drop, which is different from the research of Wang et al.33 This may be due to the higher conversion in our study, thus only part of the curve can be observed. Besides that, the influence of reactors should also be considered. A lower DHI value also indicates that the usage of hydrogen is more efficient, as more hydrogen atoms remain in desired products not in H2 gas. The

Figure 3. Effect of reaction temperature on reactions.

DHI can be greater than unity because DHI is a volumetric (rather than weight) ratio. From the opposite variation trend of coke yield and DHI value, we suggest that the DHI value cannot be used to reflect the degree of the dehydrogenation condensation reaction that leads to coke accumulating. Hydrogen transfer, being an exothermic reaction with a slower reaction rate, is disfavored by a higher reaction temperature and shorter reaction time, while being a bimolecular reaction is favored by a higher acid density (which can be achieved by increasing the CTO).37,38 Overall, the value of HTC changed little and reached the peak at 540 °C. The ISO values keep stable; this may be due to isomerization being a monomolecular reaction, which is dependent mainly on the strength of the acid site37 and the much faster rate of double-bound isomerization of olefins.39 Thus, in our study range, the influence of operating conditions can be neglected. 3.1.2. Re-evaluate Residence Time Impact. In the conventional FCC process, the common operating conditions are 500 °C reaction temperature, 6 CTO, and 3 s residence time.31 However, from the experimental results (shown in Table S2 in the Supporting Information) of group A we discover that when processing CGO with a high nitrogen content, lengthening the residence time cannot enhance the conversion of heavy oil significantly but aggravates the product distribution. The cracking of heavy oil mainly happened during the initial contact of oil vapor and high temperature catalyst. During the cracking process of CGO, the nitrogen compounds will preferentially adsorb on the catalyst surface, and then cokes are formed.33,40 Because most of the catalyst acid sites on the matrix were covered by the cokes, the left heavy oil could not be cracked but on the contrary forms cokes by dehydrogenation reactions. When the residence time was extended from 0.8 to 3.2 s, approximately 70% of the increase in conversion was due to the increase in coke yield. The heavy oil is difficult to further crack into light oil; however, the gasoline fraction still could enter the pore and then be cracked into LPG; as a result, the light oil yield decreased significantly. In order to enhance the conversion of heavy oil, the common option is increasing the reaction temperature. Therefore, the influence of residence time under higher operating severity was investigated. In the experiments of group B, the reaction temperature increases to 520 °C, and the catalyst-to-oil ratio rises as well. Because of more available active centers, more heavy oil was converted compared to group A. When the residence time lengthened, over 3% of heavy oil were further converted, but nearly 60 wt % of the increase in conversion were attributable to 657


Energy & Fuels

Article

Figure 4. Effect of catalyst activity on product distribution and selectivity.

the increase in coke yield. An excessively long residence time also led to the loss of light oil yield (mainly gasoline yield). The results indicate the reason why blending CGO into conventional FCC feedstock would aggravate the product distribution not only is because of the poisoning effects of basic nitrogen compounds but also due to the residence time in the conventional FCC process being far too long. A long residence time cannot further crack the heavy oil efficiently but cause the overcracking of light oil and coking of polynuclear aromatics. Consequently, when processing CGO or other high nitrogen content inferior feedstocks, the reaction temperature should be enhanced, and the residence time should be shortened. 3.1.3. High Catalyst Activity. The third method to increase the conversion of CGO is enhancing the activity of the catalyst, which can be conveniently operated by adding more fresh catalyst due to its higher activity, larger surface area, and micropore volume, which are favorable to converting highnitrogen feedstocks.41,42 These properties are the major object of nitrogen-resistant FCC catalysts as well. Thus, here, the effects of adding fresh catalyst into commercial equilibrium catalyst were investigated from the aspects of product distribution, selectivity, and the reaction process. The reaction temperatures (used Cat. B) were 500 °C, 540 °C, 555 °C, 565 °C, and 575 °C, respectively, and the corresponding CTO were 5.6, 8.5, 10.1, 11.4, and 13.0, respectively. A short residence time was chosen at 0.8 s also. The conditions for tests with catalyst A can be found in section 3.1.1. Figure 4 shows that, at constant conversion, an increase in catalyst activity results in a decrease in dry gas and gasoline yields and an increase in LPG yield. This can be explained if one considers that, in order to reach a close conversion with catalyst A as that with catalyst B, the severity of the reaction has to be increased, and as the increase in acid sites of catalyst decreases the poisoning effect of the nitrogen bases, more gasoline fractions

were cracked into LPG. The liquid products yield was constant when at constant conversion, which indicates that the additional LPG is formed at the expense of gasoline. The plots also show that, at constant conversion, the yields of coke, diesel, and liquid products were constant regardless of the blending of fresh catalyst. The increase of active sites in catalyst B enhanced the conversion of CGO, making it possible to convert high-nitrogen feedstocks at less severe reaction conditions. However, it also resulted in sharp decreases of light oil yield and selectivity. Figure 5 shows the effect of catalyst activity on thermal cracking, dehydrogenation, hydrogen transfer, and isomerization

Figure 5. Effect of catalyst activity on reactions.

reactions. The plots show that at constant conversion, the value of TCI obtained with catalyst B is lower than that with catalyst A. This is mainly due to the reaction severity on catalyst A is higher than that on catalyst B in order to reach a close conversion. Furthermore, with an increase in catalyst activity, the ratio of catalytic cracking reactions would be increased. Consequently, 658


Energy & Fuels

Article

ation reactions. The excess of residence time would lead to the loss of light oil and significant rise of coke yield. The CGO conversion can be enhanced at conventional reaction temperature by increasing the activity of the catalyst, which resulted in a lower dry gas yield (compare to processes II and III); however, the strong acid sites on fresh catalyst also easily lead to the overcracking of gasoline. Figure 6 shows a comparison of the product selectivity of different CGO processing schemes. In the three enhancing

the relative proportion of thermal cracking decreased. After adding fresh catalyst, the DHI values significantly decreased. This may be due to the increasing of acid strength and density of catalysts, which would change the pathways of protolytic cracking proposed by Haag and Dessau.43 Therefore, the mechanism should be studied further. As for hydrogen transfer and isomerization reactions, as the increase of acid density and strength, both of them were enhanced at some extent due to their reaction mechanisms.37 3.1.4. Comparison of Different CGO Processing Schemes. To further determine the optimal operating conditions for CGO conversion, the product distribution and product quality were compared among the four processing schemes: conventional FCC process, high reaction temperature and large CTO with short residence time (high temperature process), long residence time at proper reaction temperature (long time process), and high catalyst activity (high activity process). In Table 3, compared to the conventional FCC process, the three enhancing processes (II, III, and IV) significantly increased Table 3. Comparison of Product Distributions in Different Processing Schemes items temperature, °C CTO, kg/kg residence time, s Product Distribution, wt % dry gas LPG gasoline diesel heavy oil coke light oil yield liquid products yield conversion Index HTC ISO

I

II

III

IV

500 5.8 3.2

555 11.2 0.8

540 8.9 2.6

500 5.6 0.8

1.57 15.71 22.11 19.49 35.46 5.67 41.60 57.31 64.54

2.37 16.79 33.93 18.62 21.91 6.38 52.55 69.34 78.09

2.04 20.01 30.90 18.30 20.41 8.34 49.20 69.21 79.59

1.77 20.45 30.34 18.22 22.21 7.01 48.56 69.01 77.79

0.129 0.469

0.193 0.472

0.156 0.481

0.338 0.536

Figure 6. Comparison of product selectivity of different CGO processing schemes (I−conventional FCC process; II−high temperature process; III−long time process; IV−high activity process).

processes, the high temperature process has the highest selectivity of all in gasoline, light oil, and liquid products, with 43.45%, 67.29%, and 88.79%, respectively. On the other hand, it has the lowest selectivity in undesired products (dry gas and coke). The followed high activity process has similar selectivity in undesired products and liquid products, but due to the overcracking of gasoline, both the selectivity of gasoline and light oil are much lower than the high temperature process. As expected, the long time processing scheme is the worst one among the three enhancing processes. Overall, both from the desired products yield and selectivity, the high temperature processing scheme is the best one. The optimal operating conditions for high-nitrogen CGO conversion are high reaction temperature, large catalyst-to-oil ratio, and short residence time. The quality of the products especially the olefin content of gasoline has been a concern for decades in China due to the FCC gasoline accounts for approximately 80% of the motor gasoline pool, and the olefin content of FCC gasoline is usually much higher than the gasoline standard. Therefore, the compositions of gasoline under different processing schemes were analyzed. As can be seen from Figure 7, when processing the CGO under conventional FCC operating conditions, the olefin content of gasoline was as high as 58.3 wt %. Increasing the reaction temperature, the exothermic hydrogen transfer and isomerization reactions will be restrained, while the endothermic aromatization reactions are enhanced.45 A higher catalyst-to-oil ratio can afford more active sites for the reaction of hydrocarbons; therefore, all of the hydrogen transfer, isomerization, and aromatization reactions are strengthened. Coupling the effects of temperature and CTO, the high temperature process produced the gasoline with higher contents of isoparaffins (17.4 wt %) and aromatics (20.7 wt %) and lower olefin content (53.9 wt %) compared to the conventional FCC process. This can also be proved by the indexes of HTC and ISO shown in Table 3. Lengthening the residence time also can decrease the olefin

the feed conversion with approximately 14 wt %. At close conversion, the liquid products yields of the three enhanced processes were all around 69 wt %; however, in the high temperature process, more light oil could be obtained (more than 3 wt %). Even though the higher reaction temperature would lead to the generation of more dry gas, less nitrogen compounds would absorb on catalyst acid sites which resulted in lower coke yield. Furthermore, the average carbon deposition on catalyst must be lower in the high temperature process as the much higher catalyst-to-oil ratio; thus, the poisoning effect on the catalyst might be lighter. In addition, the H/C atomic ratio of coke is lower when reacting at higher temperature, which means that more hydrogen can be redistributed into desired products.44 Lengthening the residence time can increase the contact probability of oil vapor and catalyst and then increase the conversion of heavy oil. However, according to the previous studies, most of the reactions occur during the initial time; after then, the pore channels of catalyst are blocked, and only the light hydrocarbons such as gasoline fractions can be further cracked; while the polycyclic aromatics are adsorbed on the external surfaces of catalyst and finally form cokes through dehydrogen659


Energy & Fuels

Article

contacting with gasoline is much lower than the regenerated catalyst; therefore, the thermal cracking reactions can be restrained.31 Second, the catalyst which gasoline reacts over is partly coked. As the nitrogen compounds are preferentially adsorbed on the strong acid sites, the acid strength on a “semispent” catalyst is more mild and the catalyst activity is lowered, thus, more intermediates can be maintained. On the other hand, the bimolecular hydrogen transfer reactions, as the main reaction for gasoline reformation, still need enough active centers. Therefore, the coke content of a “semispent” catalyst should be appropriate. As expected, this problem would be solved by increasing the catalyst-to-oil ratio. Finally, in the process, only the light fraction gasoline is recycled. According to the previous studies, the pentene and hexene are difficult to crack at 500 °C, while the heptene is very easily cracked even though in a very short reaction time.49,50 Therefore, recycling the light gasoline selectively is conductive to increasing gasoline yield. Moreover, more than 80 wt % of olefins are in the light fraction gasoline; thus, only upgrading the high olefin content fraction can improve the efficiency of gasoline reformation. Furthermore, it can reduce the load of the fractionation system due to the light gasoline which only accounts for approximately half of the gasoline. In addition, after the reaction of CGO, the catalyst may suffer from pore mouth blockage,51 but the small molecule such as pentene and hexene still can enter the pore and react over the inner acid sites. 3.2.2. Recycle Ratios of Light Gasoline. According to the heat balance, with an increase in the recycle ratio of light FCC gasoline (LCG), more heat should be provided by the catalyst, thus requiring a larger catalyst circulation. The higher the catalyst circulation means a larger catalyst-to-oil ratio and a higher severity in the CGO reaction zone. The product distributions for total feeds at different recycle ratios of light gasoline are listed in Table S3 in the Supporting Information. On the basis of the hypothesis that no reactions take place at the moment of catalyst contacting with oil, the catalystto-oil ratio can be calculated by the local heat balance at the riser inlet when the catalyst−oil mixing temperature is detected.31 With an increase in the recycle ratio, the catalyst-to-oil ratio for CGO shows a dramatic rise from 5.8 to 15.9, while the temperature of the riser outlet remains at 500 °C. In contrast, the CTO for LCG decreased from 30.9 to 13.4. At a low recycle ratio, the operation severity may not be enough for CGO conversion, while under a higher recycle ratio, the injection of a large quantity of gasoline is likely to compete for active centers with heavy oil. For gasoline reformation, similar problems also exist. When the CTO is small, the lack of acid sites is unfavorable for bimolecular hydrogen transfer reactions, while a too large CTO may lead to the overcracking of gasoline, thereby reducing the yield of gasoline. Considering the CGO conversion and gasoline reformation, there might be an optimal range of recycle ratio of LCG to CGO. However, from the total product distribution, we cannot obtain the information. According to our previous studies and the studies by Wang et al.,28 in the process of gasoline reformation, no heavy oil would be generated. Therefore, the yield of heavy oil for CGO (YHCO, CGO) can be calculated by

Figure 7. Comparison of gasoline composition of different CGO processing schemes (I−conventional FCC process; II−high temperature process; III−long time process; IV−high activity process).

content of gasoline, but the effect is limited. When increasing the activity of the catalyst, both of the hydrogen transfer and isomerization reactions were significantly enhanced, which can be seen from the marked increase of HTC and ISO values. As a consequence, the gasoline produced by the high activity processing scheme has the highest isoparaffins and lowest olefin content, with 25.4 and 44.8 wt %, respectively. 3.2. Synergistic Process. 3.2.1. Synergistic Process Proposed. On the basis of the above experiments, the optimizing operating conditions for cracking CGO are high reaction temperature, large catalyst-to-oil ratio, and short residence time. However, it is difficult to directly implement in commercial FCC units due to the long average residence time of oil gases in the disengager leading to the overcracking of light oil in a high temperature environment.46 The olefin content of cracked gasoline is substandard as well, which needs to be efficiently reformed. Krishna47 proposed a split feed injection mode which divided the riser reactor into two reaction zones to enhance the gasoline octane. Tiscornia et al.48 used this mode to process visbreaker naphtha by injecting it before the conventional feedstock and found that the high-severity regime can significantly improve the blending properties of the gasoline. However, this mode would cause a significant loss of gasoline. Therefore, a synergistic process (a novel multiple injection mode) was proposed. The proposed scheme is shown in Figure 1: the CGO was injected from the lower nozzle, first contacted with the high temperature regenerated catalyst, while the light fraction gasoline was recycled from the upper nozzle and reacted on the temperature-lowered and partly coked catalyst. According to the heat balance, the injection of light gasoline needs the catalyst to afford more heat for maintaining the outlet temperature of the riser, thus the circulation of catalyst will be enlarged, which means a larger catalyst-to-oil ratio can be obtained. In the process, there are two reaction zones along the riser reactor: the CGO reaction zone and the light gasoline reaction zone. The CGO reaction zone is operated at high reaction temperature, large CTO, and short residence time, which is beneficial to the CGO cracking. After a short reaction time, the temperature of the catalyst is lowered, and its activity is appropriately decreased as well. Then the light gasoline is injected, cooling the catalyst sharply; thus, the overcracking reactions of middle distillate oil are controlled. To reduce the loss of gasoline yield, in the process, three measures are applied. First, the temperature of catalyst

YHCO,CGO = YHCO,total·Ftotal /FCGO

(1)

where YHCO, total is the yield of heavy oil for total feeds, Ftotal is the total feed rate, and FCGO is the feed rate of CGO. Thus, the conversion for CGO can be calculated and shown in Figure 8. The data show that the optimal LCG recycle ratio for CGO 660


Energy & Fuels

Article

reserving the gasoline and diesel fractions. Since the reaction temperature is cooled sharply, the thermal cracking reactions decreased and less dry gas and coke generated. For the reformation of gasoline, the temperature-lowered and partly coked catalyst is helpful for minimizing the dry gas and coke which form via thermal cracking. Compared with the calculated results (calculated by a weighted average method at the same ratio as that in the synergistic process), as expected, the yield of dry gas and coke decreased by 0.52 wt % despite under a higher conversion, while the light oil yield increased more than 7 wt %, which mainly due to the secondary cracking of gasoline was restrained significantly. Even though the reaction conditions of CGO in the separate process were severer than that in the synergistic process, its conversion is still lower than the latter. Thus it can be referred to that in the synergistic process the heavy oil still can be further cracked in the gasoline reaction zone. First, it may be due to the longer residence time in the gasoline reaction zone, which can increase the contact opportunity between hydrocarbon molecules and catalyst acid sites, but it may also lead to higher dry gas and coke yield and lower gasoline yield, according to our previous studies. Second, it can be explained by fluid dynamics. The injection of light gasoline may intensify the turbulent diffusion in the gasoline injection zone, thus the contact efficiency of oil gas and catalyst increased, which is helpful for the conversion of heavy oil. Finally, it may be caused by the “gasoline stripping” which is similar to the steam stripping. Since some heavy hydrocarbons are stripped from the outer surface of catalysts, some acid sites are freed as well, especially the strong Brønsted acid sites;51 therefore, the heavy hydrocarbons can be cracked again rather than forming cokes. 3.2.4. Gasoline Reformation in the Synergistic Process. In the process, the gasoline reformed in the gasoline reaction zone can be divided into two parts: one is the recycling light gasoline and the other is that generated by CGO cracking. However, the yield of gasoline created by CGO cannot be directly obtained, but the approximate value can be obtained from the separate process of CGO cracking. Since the olefin content of the gasoline can be obtained by the combination of true boiling point distillation and PIONA analysis, the olefin content of the mixed gasoline before reformation (OM) can be calculated by

Figure 8. Effect of the LCG recycle ratio on the conversion of CGO.

conversion is approximately 75 wt %. The conversion of CGO increased 17.6 wt %, which is much higher than the DFCC process proposed by Li (about 4 wt %, a better feedstock was used).23 3.2.3. Reaction Performance of the Synergistic Process. The reaction performance of the synergistic process was analyzed at the optimal LCG recycle ratio of 75 wt % for CGO conversion. In Table 4, the synergistic process was compared with the separate Table 4. Comparison of FCC Product Distributions between the Synergistic Process and the Separate Process items

synergistic process

LCG recycle ratio, 74.8 wt % rea-temp. of CGO, 568 °C rea-temp. of LCG, 500 °C CTO (CGO/LCG), 12.5/16.7 kg/kg res-time (CGO/ 0.5/1.6 LCG), s Product Distribution, wt % dry gas 1.69 LPG 12.65 gasoline 57.95 diesel 11.95 heavy oil 10.23 coke 5.53 light oil yield 69.90 liquid products yield 82.55 conversion 89.77 a

CGO

LCG

calculation

Δa

2.09 18.03 51.24 11.42 11.56 5.65 62.66 80.69 88.44

−0.40 −5.38 6.71 0.53 −1.33 −0.12 7.24 1.86 1.33

568 500 13.3

14.8

0.8

1.5

2.42 17.32 34.66 18.06 20.21 7.33 52.72 70.04 79.79

1.65 18.99 73.41 2.55 0.00 3.40 75.96 94.95 −

OM = OCGO,g ·YCGO,g /(YCGO,g + RLCG) + OLCG ·RLCG/(YCGO,g + RLCG)

where OCGO, g is the olefin content of gasoline generated by CGO, YCGO, g is the yield of gasoline generated by CGO cracking separately, RLCG is the recycle ratio of LCG, and OLCG is the olefin content of light gasoline (here is 63.99 wt %). Further, the reduction of gasoline olefins before and after reformation (OR) can be calculated, and the olefin conversion (OC) is defined as the ratio of the converted olefin content to the olefin content before reformation.52 From the data which are shown in Table S4 in the Supporting Information, we can discover the optimal recycle ratio of LCG for gasoline reformation is approximately 50 wt %, which reduced the content of olefins in gasoline by around 22 wt %. When the recycle ratio of LCG is low, take 28.8 wt % for example, the upgrading effect of gasoline is far from satisfactory. The CTO for CGO is too low in this case, which may cause an obvious catalyst deactivation that is unfavorable for the hydrogen transfer reactions in the gasoline reaction zone. After passing the optimal value, as the increase of the LCG recycle ratio, the CTO for

Item Δ is the values of synergistic process minus that of calculation.

process (the CGO and LCG separately reacted at the conditions similar to that in the synergistic process), and the calculation values were calculated by YC, i = YCGO, i·R CGO + YLCG, i·RLCG

(3)

(2)

where YC, i is the calculated product−yield; YCGO, i and YLCG, i are the product−yield when CGO or LCG reacted separately; and RCGO and RLCG are the percentage of CGO and LCG in the total feeds. For example, RCGO is 42.8 wt %, while RLCG is 57.2 wt %. In the synergistic process, the CGO is cracked at high severity; the injection of light FCC gasoline can terminate the high temperature reactions of CGO in time, which is favorable for 661


Energy & Fuels

Article

gasoline decreased, and the temperature of the “semispent” catalyst increased as well. These are not beneficial to the hydrogen transfer reactions; therefore, the performance of gasoline reformation declined. 3.2.5. Estimation of the Gasoline Loss. The proposed synergistic process mainly aims to resolve three problems, increasing the conversion of CGO and reforming the FCC gasoline that have been previously discussed, and the last one is to reduce the gasoline loss in the reformation process. Therefore, the loss of gasoline was estimated based on the hypotheses that the gasoline yields generated by CGO in the synergistic process were similar to that in the separate process at similar reaction conditions. Thus, the quantity of gasoline (FCGO, g) which stem from CGO can be calculated by FCGO,g = FCGO·YCGO,g

The two-stage riser (TSR) FCC technology was previously proposed by our research group, and it has been applied in more than 10 commercial FCC units.54−56 Thus a two-stage synergistic (TSS) FCC process was proposed to combine the synergistic process with the TSR FCC process, which is shown in Figure 9. In the TSS process, three reaction zones were included:

(4)

where FCGO is the feed rate of CGO, and YCGO, g is the yield of gasoline which is generated by CGO cracking separately. Then the total gasoline loss (Ltotal, g) can be estimated by L total,g = FCGO,g + FLCG − Ftotal ·Ytotal,g

(5)

where FLCG is the feed rate of LCG, Ftotal is the total feed rate of CGO and LCG, and Ytotal, g is the yield of gasoline in the synergistic process. Due to the reformed gasoline consisting of two parts the gasoline loss of the two parts should be calculated respectively. Based on the hypotheses that in the reformation process the loss rates of gasoline and light gasoline are equal,53 thus the loss rate of gasoline (Rloss) can be calculated by R loss = L total,g /(FCGO,g + FLCG)

Figure 9. Schematic of the proposed TSS FCC process and the TSR FCC process.

(6)

the first riser reaction zone for the conventional feedstock, the CGO reaction zone at the bottom of the second riser, and the light gasoline reaction zone in the upper part of the second riser. Therefore, the problem caused by lack of LCG can be resolved, as more LCG can be generated by the conventional feedstock. Compared to the conventional TSR FCC process which is shown in Figure 9, the TSS process not only adds the LCG reaction zone but also changes the feeding ways significantly. The CGO which used to be added to the conventional feeds is processed in the second stage riser separately; thus, the CGO can be upgraded without affecting the conversion of conventional feeds. Due to the significant increase of feed conversion, the CGO processing capacity of the FCC unit rises up as well. Moreover, the olefin content of gasoline generated by the TSS process will be reduced. 3.3.2. Simulation Experiments of the TSS Process. In order to investigate the effects of the TSS FCC process on CGO conversion and gasoline reformation, two comparison experiments were carried out in the pilot-scale FCC unit using Daqing AR, Qilu CGO, and Daqing LCG as feedstocks. In the comparative test, the AR was fed from the first-stage riser at the condition of CTO = 6, 490 °C as the reaction temperature and 1.2 s as the residence time, while the CGO was injected from the second-stage riser. Because of the inferior crackability of CGO, the reaction conditions were set at a higher operating severity with CTO = 6, 500 °C at the riser outlet and residence time 3.2 s. In addition, the feed rate of AR and CGO was set as 2:1. In the pilot-scale unit, most of the LCG fractions exist in the cracking gas; the LCG cannot be obtained from the cracked gasoline that AR and CGO generated. Therefore, the LCG used in the experiment was taken from the FCC commercial unit of

Table S4 shows that with an increase in the recycle ratio (before 75 wt %), the loss rate of gasoline remained stable at around 7.4 wt % (include LPG), which was affected by the temperature and activity of “semispent” catalysts and the catalystto-oil ratio. When at a low recycle ratio, the temperature and activity of “semispent” catalysts are relatively low which are helpful for maintaining gasoline yield, but the effect of large CTO is opposite. When the recycle ratio of LCG was as high as 96.6 wt %, the loss rate climbed to approximately 11 wt %; this is because of the “semispent” catalyst with higher temperature and lower coke blockage leading to the overcracking of gasoline. Even so, the loss rate is far lower than that reported in the literature,28 which was higher than 17 wt %. For describing the efficiency of gasoline reformation, the reforming efficiency (e) can be defined as the ratio of the olefin reduction to the gasoline loss rate (e=OR/Rloss), the higher the value means the gasoline can be upgraded at less expense of gasoline yield. It can be seen from Table S4 that the reforming efficiency of gasoline reaches the top of 2.93 when the recycle ratio of LCG is around 50 wt %, which is higher than the separately upgrading process of LCG (about 1.6) and that reported in the literature (between 1.6−1.7).28 3.3. Combination with the TSR FCC Technology. 3.3.1. Two-Stage Synergistic (TSS) Process. From the above studies, the conclusion can be inferred that in the synergistic process the optimal recycle ratio of light FCC gasoline is 50−75 wt %, where 50 wt % is the optimal value for gasoline reformation and 75 wt % is that for CGO conversion. However, this large quantity of LCG cannot be obtained from the product of CGO cracking. Therefore, it should be combined with other FCC processes for reaching the optimal performance. 662


Energy & Fuels

Article

LCG recycle ratio for gasoline reformation is around 50 wt % which can reduce the olefin content of gasoline with approximately 22 wt % at less expense of gasoline yield. A two-stage synergistic process was proposed. The simulation tests show that the conversion of heavy oil (AR:CGO = 2:1) increased significantly as well as the olefin content of the final gasoline decreased by 13.5 wt %. The TSS process could offer oil refineries alternative approaches for processing inferior feedstocks, such as CGO, vacuum residue, or other feedstocks with high nitrogen content, as well as improve the quality of FCC gasoline.

the Daqing refinery. The recycle ratio of LCG was determined by the product distributions of AR and CGO cracking, which accounted for 17.7% and 7.1%, respectively (24.8% total). The results of the comparative test and the simulated TSS process were calculated by a weighted average method and are listed in Table S5 in the Supporting Information. From Table S5, it can be seen that the product distribution of the TSS process is superior to that of the comparative test. As the severer operating conditions in the CGO reaction zone, the conversion of total feeds in the TSS process was much higher than that in the comparative test with approximately 5 wt %. The yields of LPG, gasoline, and diesel increased in varying degrees as well. However, the higher severity also resulted in more dry gas and coke products. In the TSS process, the selectivities of LPG and gasoline showed small decreases; this is because of the increase of feed conversion. In fact, the LPG and gasoline yields increased more than 3 wt %. The decrease of diesel selectivity is because of the increase of reaction temperature and CTO leading to the further cracking of diesel fraction. Because the crackability of HCO is worse than the fresh feedstock, the increase of oncethrough conversion would lead to higher selectivities of dry gas and coke (but improving the FCC process could control the degree). Moreover, the LCG upgrading process would also cause small increases of dry gas and coke yields. Table S5 also shows that the gasoline composition of the TSS process is different from that of the comparative test. The isoparaffins, naphthenes, and aromatics were both higher in the TSS process than those in the comparative test, while n-paraffins were slightly lower. The content of olefins decreased 13.5 wt % which were mostly converted into aromatics (the increment is 10.85 wt %). Recycling the light fraction gasoline selectively can keep the low olefin content heavy gasoline fractions from being cracked. In addition, the higher catalyst-to-oil ratio can not only boost the heavy oil cracking into desired products but also strengthen the hydrogen transfer, isomerization, and aromatization reactions, which are the key reactions for upgrading gasoline. Therefore, it can be deduced that the TSS process can not only enhance the conversion of CGO with an idea product distribution but also can produce gasoline with lower olefin content without losing much of the gasoline yield.

■

ASSOCIATED CONTENT

S Supporting Information *

Compositions of Daqing FCC light gasoline, influence of residence time and LCG recycle ratio on product distribution, estimate of olefin reduction in gasoline and the gasoline loss, and the comparison of different processing schemes. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Phone: +86-532-86981169. Fax: +86-532-86981718. E-mail: [email protected] (C.Y.). Phone: +86-532-86981862. Fax: +86-532-86981718. E-mail: [email protected] (C.L.). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors acknowledge the financial support provided by the National Basic Research Program of China (Grant 2012CB215006) and the National Natural Science Foundation for Young Scholars (Grant 21206198). We also thank the reviewers for some valuable comments leading to a better manuscript.

■

REFERENCES

(1) Sawarkar, A. N.; Pandit, A. B.; Samant, S. D.; Joshi, J. B. Petroleum residue upgrading via delayed coking: A review. Can. J. Chem. Eng. 2007, 85 (1), 1−24. (2) Meng, X. H.; Xu, C. M.; Gao, J. S. Hydrofining and catalytic cracking of coker gas oil. Pet. Sci. Technol. 2009, 27 (3), 279−290. (3) Mills, G. A.; Boedeker, E. R.; Oblad, A. G. Chemical characterization of catalysts. I. Poisoning of cracking catalysts by nitrogen compounds and potassium ion. J. Am. Chem. Soc. 1950, 72 (4), 1554−1560. (4) Fu, C. M.; Schaffer, A. M. Effect of nitrogen compounds on cracking catalysts. Ind. Eng. Chem. Prod. Res. Dev. 1985, 24 (1), 68−75. (5) Scherzer, J.; McArthur, D. P. Tests show effects of nitrogen compounds on commercial fluid cat cracking catalysts. Oil Gas J. 1986, 84 (43), 76−81. (6) Corma, A.; Fornes, V.; Monton, J. B.; Orchilles, A. V. Catalytic cracking of alkanes on large pore, high SiO2/Al2O3 zeolites in the presence of basic nitrogen compounds. Influence of catalyst structure and composition in the activity and selectivity. Ind. Eng. Chem. Res. 1987, 26 (5), 882−886. (7) Caeiro, G.; Costa, A. F.; Cerqueira, H. S.; Magnoux, P.; Lopes, J. M.; Matias, P.; Ribeiro, F. R. Nitrogen poisoning effect on the catalytic cracking of gasoil. Appl. Catal., A 2007, 320, 8−15. (8) Wang, G.; Liu, Y. D.; Wang, X. Q.; Xu, C. M.; Gao, J. S. Studies on the catalytic cracking performance of coker gas oil. Energy Fuels 2009, 23 (4), 1942−1949. (9) Li, Z. K.; Wang, G.; Shi, Q.; Xu, C. M.; Gao, J. S. Retardation effect of basic nitrogen compounds on hydrocarbons catalytic cracking in

4. CONCLUSIONS The optimal operating conditions for converting CGO were high reaction temperature, large catalyst-to-oil ratio, and short residence time, which could enhance the conversion of CGO without significantly increasing the thermal cracking. The dehydrogenation reactions were restrained as well. Meanwhile, no significant changes happened to hydrogen transfer and isomerization reactions. In the conventional FCC process, the blending of CGO will aggravate the product distribution not only because of the poisoning effects but also due to the unsuitable residence time. Even for inferior feedstocks, taking a long residence time is not a good choice for increasing the feed conversion. Increasing the catalyst activity can reach a high conversion at low operating severity at the expense of gasoline yield. A synergistic process was proposed, which can process CGO and LCG at different operating severity. The results show that the optimal LCG recycle ratio for converting CGO is 75 wt %. Compared to the conventional FCC process, the once-through conversion of CGO increased by 17.6 wt %. Compared with the separate process, the dry gas and coke yields decreased slightly, while the yield of light oil rose by 7.2 wt %. The optimal value of 663


Energy & Fuels

Article

coker gas oil and their structural identification. Ind. Eng. Chem. Res. 2011, 50 (7), 4123−4132. (10) Young, G. W. Fluid catalytic cracker catalyst design for nitrogen tolerance. J. Phys. Chem. 1986, 90 (20), 4894−4900. (11) Li, Z. K.; Gao, J. S.; Wang, G.; Shi, Q.; Xu, C. M. Influence of nonbasic nitrogen compounds and condensed aromatics on coker gas oil catalytic cracking and their characterization. Ind. Eng. Chem. Res. 2011, 50 (15), 9415−9424. (12) Dorbon, M.; Ignatiadis, I.; Schmitter, J.-M.; Arpino, P.; Guiochon, G.; Toulhoat, H.; Huc, A. Identification of carbazoles and benzocarbazoles in a coker gas oil and influence of catalytic hydrotreatment on their distribution. Fuel 1984, 63 (4), 565−570. (13) Schmitter, J.-M.; Ignatiadis, I.; Dorbon, M.; Arpino, P.; Guiochon, G.; Toulhoat, H.; Hue, A. Identification of nitrogen bases in a coker gas oil and influence of catalytic hydrotreat ment on their composition. Fuel 1984, 63 (4), 557−564. (14) Bej, S. K.; Dalai, A. K.; Adjaye, J. Comparison of hydrodenitrogenation of basic and nonbasic nitrogen compounds present in oil sands derived heavy gas oil. Energy Fuels 2001, 15 (2), 377−383. (15) Wang, G.; Li, Z. K.; Huang, H.; Lan, X. Y.; Xu, C. M.; Gao, J. S. Synergistic process for coker gas oil and heavy cycle oil conversion for maximum light production. Ind. Eng. Chem. Res. 2010, 49 (22), 11260− 11268. (16) Kodera, Y.; Ukegawa, K.; Mito, Y.; Komoto, M.; Ishikawa, E.; Nakayama, T. Solvent extraction of nitrogen compounds from coal liquids. Fuel 1991, 70 (6), 765−769. (17) Luan, X. L.; Li, C. Y.; Chen, W. Y.; Wang, H. Y. Study on removal of basic nitrogen in coker gas oil using adsorption method. J. Petrochem. Univ. 1999, 12, 15−18. (18) Guo, L. Y.; Wan, S. B.; Zhao, G. H.; Qin, L. H. Study on complexing denitrogenation of coker gatch used as FCC mixed feedstock. Pet. Process. Petrochem. 2008, 39 (10), 18−21. (19) Yu, D. Y.; Xu, H.; Que, G. H. Progress in technologies of petroleum non−hydrodenitrogenation. Chem. Ind. Eng. Pro. 2001, 10, 32−35. (20) Cao, B.; Gao, J. S.; Xu, C. M. Comments on technical measures to improve coker gas oil blending rate in FCC feed. Pet. Petrochem. Today 2003, 11, 37−40. (21) Zhang, R. C.; Shi, W. Y. Denitrified catalytic cracking (DNCC) technology for coker gas oil processing. Pet. Process. Petrochem. 1998, 29, 22−27. (22) Yuan, Q. M.; Wang, Y. L.; Li, C. Y.; Yang, C. H.; Shan, H. H. Study on conversion of coker gas oil by two−stage riser fluid catalytic cracking. J. China Univ. Pet., Ed. Nat. Sci. 2007, 31, 122−126. (23) Li, Z.; Wang, G.; Liu, Y.; Wang, H.; Liang, Y.; Xu, C.; Gao, J. Catalytic cracking constraints analysis and divisional fluid catalytic cracking process for coker gas oil. Energy Fuels 2012, 26 (4), 2281− 2291. (24) Li, D.; Li, M.; Chu, Y.; Nie, H.; Shi, Y. Skeletal isomerization of light FCC naphtha. Catal. Today 2003, 81 (1), 65−73. (25) Fan, Y.; Bao, X. J.; Shi, G. Hβ/HZSM−5 composite carrier supported catalysts for olefins reduction of FCC gasoline via hydroisomerization and aromatization. Catal. Lett. 2005, 105 (1), 67− 75. (26) Meng, F. D.; Wang, L. Y.; Hao, X. R. Technology for reducing olefin in cracked naphtha−FDFCC process. Pet. Process. Petrochem. 2004, 35, 6−10. (27) Gao, J. S.; Xu, C. M.; Bai, Y. H.; Lu, C. X.; Liu, Y. F. Chinese Patent. 1,458,226A, 2002. (28) Wang, G.; Yang, G. F.; Xu, C. M.; Gao, J. S. A novel conceptional process for residue catalytic cracking and gasoline reformation dual− reactions mutual control. Appl. Catal., A 2008, 341 (1−2), 98−105. (29) Song, H. T.; Da, Z. J.; Zhu, Y. X.; Tian, H. P. Effect of coke deposition on the remaining activity of FCC catalysts during gas oil and residue cracking. Catal. Commun. 2011, 16 (1), 70−74. (30) Gao, Y. C.; Zhang, J. S. Thermal cracking and catalytic cracking in fluid catalytic cracking process. J. Chem. Ind. Eng. 2002, 53, 469−472. (31) Wang, G.; Lan, X. Y.; Xu, C. M.; Gao, J. S. Study of optimal reaction conditions and a modified residue catalytic cracking process for

maximizing liquid products. Ind. Eng. Chem. Res. 2009, 48 (7), 3308− 3316. (32) Xu, Y. H.; Gong, J. H.; Zhang, J. S.; Long, J.; Xu, H. Experimental study on ″two reaction zone″ concept connected with MIP process. Acta Pet. Sin., Pet. Process. Sect. 2004, 20, 1−5. (33) Wang, G.; Li, Z. K.; Liu, Y. D.; Gao, J. S.; Xu, C. M.; Lan, X. Y.; Ning, G. Q.; Liang, Y. M. FCC−catalyst coking: sources and estimation of their contribution during coker gas oil cracking process. Ind. Eng. Chem. Res. 2012, 51 (5), 2247−2256. (34) Scherzer, J. Octane−enhancing, zeolitic FCC catalysts: scientific and technical aspects. Catal. Rev.: Sci. Eng. 1989, 31 (3), 215−354. (35) Chen, J. W.; Cao, H. C. Catalytic Cracking Technology and Engineering, 2nd ed.; SINOPEC Press: Beijing, China, 2005; pp 154− 155. (36) De Jong, J. I. Hydrogen transfer in catalytic cracking. In Ketjen Catalyst Symposium, Scheveningen, The Netherlands, Paper F−2, 1986. (37) Cheng, W. C.; Rajagopalan, K. Conversion of cyclohexene over Y−zeolites: A model reaction for hydrogen transfer. J. Catal. 1989, 119 (2), 354−358. (38) Corma, A.; Miguel, P. J.; Orchilles, A. V. The role of reaction temperature and cracking catalyst characteristics in determining the relative rates of protolytic cracking, chain propagation, and hydrogen transfer. J. Catal. 1994, 145 (1), 171−180. (39) Buchanan, J. S.; Santiesteban, J. G.; Haag, W. O. Mechanistic considerations in acid−catalyzed cracking of olefins. J. Catal. 1996, 158 (1), 279−287. (40) Qian, K.; Tomczak, D. C.; Rakiewicz, E. F.; Harding, R. H.; Yaluris, G.; Cheng; Zhao, X.; Peters, A. W. Coke formation in the fluid catalytic cracking process by combined analytical techniques. Energy Fuels 1997, 11 (3), 596−601. (41) Scherzer, J.; McArthur, D. P. Catalytic cracking of high−nitrogen petroleum feedstocks: effect of catalyst composition and properties. Ind. Eng. Chem. Res. 1988, 27 (9), 1571−1576. (42) Corma, A.; Mocholí, F. A. New silica−alumina−magnesia FCC active matrix and its possibilities as a basic nitrogen passivating compound. Appl. Catal., A 1992, 84 (1), 31−46. (43) Haag, W. O.; Dessau, R. M. In Duality of Mechanism in Acid Catalyzed Paraffin Cracking, The Eighth International Congress on Catalysis, Berlin, Germany, 1984; Verlag Chemie: Berlin, Germany, 1984; pp 305−315. (44) Wang, H. L.; Wang, G.; Shen, B. J.; Xu, C. M.; Gao, J. S. Upgrading residue by carbon rejection in a fluidized−bed reactor and its multiple lump kinetic model. Ind. Eng. Chem. Res. 2011, 50 (22), 12501−12511. (45) Ouyang, F. S.; Pei, X.; Zhao, X. H.; Weng, H. X. 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. Energy Fuels 2009, 24 (1), 475−482. (46) Lan, X. Y.; Xu, C. M.; Wang, G.; Wu, L.; Gao, J. S. CFD modeling of gas−solid flow and cracking reaction in two−stage riser FCC reactors. Chem. Eng. Sci. 2009, 64 (17), 3847−3858. (47) Krishna, A. S. Gasoline octane enhancement in fluid catalytic cracking process with split feed injection to riser reactor. US 4,869,807, 1989. (48) Tiscornia, I. S.; de la Puente, G.; Sedran, U. Recycling of low− value hydrocarbon cuts by means of multiple injections to FCC units. Ind. Eng. Chem. Res. 2002, 41 (24), 5976−5982. (49) Yuan, Y. X.; Yang, C. H.; Shan, H. H.; Zhang, J. F.; Han, Z. X. Study on the reaction mechanism of olefins on FCC catalyst. J. Fuel Chem. Technol. 2005, 435−439. (50) den Hollander, M. A.; Wissink, M.; Makkee, M.; Moulijn, J. A. Gasoline conversion: reactivity towards cracking with equilibrated FCC and ZSM−5 catalysts. Appl. Catal., A 2002, 223 (1−2), 85−102. (51) Cerqueira, H. S.; Sievers, C.; Joly, G.; Magnoux, P.; Lercher, J. A. Multitechnique characterization of coke produced during commercial resid FCC operation. Ind. Eng. Chem. Res. 2005, 44 (7), 2069−2077. (52) Yang, G. F.; Wang, G.; Gao, J. S.; Xu, C. M. Coke formation and olefins conversion in FCC naphthaolefin reformulation at low reaction temperature. J. Fuel Chem. Technol. 2007, 35 (5), 572−577. 664


Energy & Fuels

Article

(53) Bai, Y. H.; Gao, J. S.; Xu, C. M. Study on reaction rules of different processed for decreasing FCC gasoline olefin content. Pet. Refin. Eng. 2004, 34, 7−10. (54) Shan, H. H.; Dong, H. J.; Zhang, J. F.; Niu, G. L. Experimental study of two−stage riser FCC reactions. Fuel 2001, 80 (8), 1179−1185. (55) Yang, C. H.; Shan, H. H.; Zhang, J. F. Two−stage riser FCC technologies. Pet. Refin. Eng. 2005, 35 (3), 28−33. (56) Li, C. Y.; Yang, C. H.; Shan, H. H. Maximizing propylene yield by two−stage riser catalytic cracking of heavy oil. Ind. Eng. Chem. Res. 2007, 46 (14), 4914−4920.

665
