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Apr 12, 2016 - ABSTRACT: Light cycle oil (LCO) from fluid catalytic cracking. (FCC) was treated by selective hydrogenation and then cracked in a...
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Synergistic Process for FCC Light Cycle Oil Efficient Conversion To Produce High-Octane Number Gasoline Nan Jin, Gang Wang,* Libo Yao, Miao Hu, and Jinsen Gao State Key Laboratory of Heavy Oil Processing, College of Chemical Engineering, China University of Petroleum, Beijing 102249, China S Supporting Information *

ABSTRACT: Light cycle oil (LCO) from fluid catalytic cracking (FCC) was treated by selective hydrogenation and then cracked in a FCC apparatus. Compared with LCO, hydrogenated LCO (hydroLCO) exhibited remarkable FCC performance, recording with that 50.83 wt % hydro-LCO was converted into gasoline fraction. This is attributed to the reduction of aromatics in hydro-LCO, especially for the multiring aromatics. After hydrogenation, the amount of multiring aromatics significantly decreased from 63.2 to 9.5 wt %, while naphthenoaromatics (including indans, tetralin, and indenes) increased from 8.8 to 34.2 wt %. In accordance with the experimental results and theoretical analysis of LCO reaction characteristics, a synergistic process for LCO efficient conversion to high octane number gasoline was proposed, and simulation experiments were carried out. The results show that, compared with routine FCC, 20 wt % higher conversion and 16 wt % more gasoline could be obtained. Moreover, gasoline from synergistic process exhibited decreased sulfur and olefins, but increased aromatics, and thereby improved octane number. These findings indicate that the proposed synergistic process could be an effective option for producing gasoline with high octane number.

1. INTRODUCTION

octane number (ON) will be generated. In conclusion, upgrading LCO into commercial diesel is not an efficient way. The most abundant constituent in LCO is aromatic compounds, which are unfavorable components in diesel, but favorable ones in high ON gasoline. This provides a new idea to produce high ON gasoline through transferring aromatics from LCO to gasoline. Transformation of aromatics during FCC process is depicted in Figure 1.9 The monoaromatics crack directly into gasoline fraction through breaking long side chains and/or opening naphthenic rings. Meanwhile, concentration of aromatics improves ON of gasoline. For multiring aromatics,

Fluid catalytic cracking (FCC) is one of the most important processes in converting heavy oils into high-value light products, such as gasoline and light cycle oil (LCO). Generally in China, FCC gasoline and LCO are used as blend composition of commercial gasoline and diesel, respectively.1 As a result, FCC units provide over 70% commercial gasoline and 30% commercial diesel.2 However, as the feedstocks are becoming heavier and poorer and the operating conditions becoming stricter, the quality of FCC products is becoming more deteriorative, especially for LCO. One of the typical characteristics of LCO is its abundant aromatic content, especially multiring aromatics,3 which directly induces its low cetane number (CN).4 Therefore, the direct introduction of LCO into the diesel fuel lowers the overall quality of diesel, and hence, LCO is not an ideal blend composition for clean and high grade diesel. Several approaches have been proposed to improve the quality of LCO, including deep hydrotreating,5,6 aromatic saturation,7 hydrocracking,8 and selective ring-opening.4 No matter which kind of hydrogenation process is taken, inadequacy always exists. To produce ultralow sulfur diesel, harsh reaction conditions are needed, which will lead to oversaturation of aromatics, accompanying by low utilization of hydrogen and limited increase of CN. When aiming at increasing CN, a considerable amount of gasoline with low © XXXX American Chemical Society

Figure 1. Transformation of aromatics during FCC process. Received: January 25, 2016 Revised: March 20, 2016 Accepted: April 12, 2016

A

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2.2. Apparatus. 2.2.1. Hydrogenation. Hydrogenation experiments were carried out in a fixed-bed reactor. Figure S1 shows a diagram of the experimental system. The system is comprised of seven sections, including gas supply, pressure control, oil input, reaction zone, product separation and collection system, temperature control, and exhaust gas measuring system. The hydrogenation feeds are mixed with H2 and then preheated in the preheating furnace, followed by contacting with catalysts in the reactor. After reaction, oil gas flows into the product separation. Most of product vapors are condensed in the high-pressure separator and the left are condensed in the low-pressure separator. The noncondensable gas is collected and measured by the exhaust gas measuring system. 2.2.2. Fluid Catalytic Cracking. The FCC experiments were performed in a TPSR FCC apparatus with a throughput of 2.0 kg/h and a catalyst holdup of 12 kg, as shown in Figure S2. The plant consists of five sections, including feed injection, riser reactor, catalyst stripper with disengaging section, regenerator, and product recovery system. The details of this system have been described in the previous publications.18,19 During all tests, the mass balances were all between 97 and 100 wt % of injected feed. The conversion is defined as the sum of the percentage of dry gas (including H2, C1, and C2), liquefied petroleum gas (LPG, including C3 and C4), gasoline, and coke produced.

the remaining aromatic nucleus cannot continue to crack into gasoline fraction. Nonetheless, they are easily saturated into 1ring aromatics under hydrogenation conditions, then the generated 1-ring aromatics could be used to produce high ON gasoline according to Figure 1(a). In other words, LCO is a potential and inexpensive resource for high ON gasoline through hydrogenation-FCC process. Some technologies adopted this idea to increase production of gasoline or light aromatic hydrocarbons. In the early 1980s, Ashland Oil, Inc.10 proposed combination technology of hydrogenation and FCC to produce gasoline which was rich of aromatics. However, the operating conditions used in hydroLCO FCC process were not optimized; thereby, there is no increase in ON of gasoline because of substantial decrease in olefin content. ExxonMobil Research and Engineering Company11−13 also pointed out that hydrogenating LCO first, and followed by FCC, could realize enhancing aromatic and olefin content. Unfortunately, application results have not been published in literature. Despite hydrogenation of LCO potentially increasing gasoline yield, there is still room for improvement of gasoline quality. After hydrogenation, hydro-LCO contains a considerable amount of naphthenoaromatics such as tetralin, which are superior hydrogen donors that have been demonstrated by many studies.14−16 When contacting with heavy oil, these hydrogen donors would rather induce hydrogen transfer reactions and generate 2-ring aromatics into LCO fraction than open naphthenic rings and generate alkylbenzenes into gasoline fraction.17 Therefore, if LCO is directly mixed and hydrogenated with residue or gas oil, and then returns back to FCC unit together, conversion of hydro-LCO would still be low. In the meantime, overwhelmingly strong hydrogen transfer reactions lead to remarkable decline of olefin content in LPG and gasoline, and thereby decrease of gasoline ON. Given this, a new synergistic process for FCC LCO efficient conversion to produce high ON gasoline was proposed. The synergistic process consisted of two sections: hydrogenation and FCC. In the hydrogenation unit, multiring aromatics in LCO were saturated into 1-ring aromatics moderately and orientedly. The hydro-LCO which was rich in monoaromatics then entered into the FCC unit, in which hydro-LCO and heavy oil were fed separately in order to decrease hydrogen transfer reaction and increase ring opening as much as possible. Eventually, LCO was efficiently transformed into high ON gasoline or light aromatic hydrocarbons. The work presented in this article analyzed reaction mechanism from theoretical and experimental angles to evaluate the processability of LCO. In accordance with the above results, simulation experiments of the synergistic process were then carried out to verify the feasibility and the effect of the process in a technical pilot scale riser (TPSR) FCC apparatus.

conversion =

dry gas + LPG + gasoline + coke (%) feed (1)

Selectivity is defined as product yield to conversion, as shown in eq 2. selectivity =

Yi (%) conversion

(2)

2.3. Product Analysis. The gas products were analyzed using an Agilent 6890 gas chromatograph, which equipped with a flame ionization detector and a thermal conductivity detector. N2 and H2 were used as the carrier gases to analyze H2 and other components, respectively. The temperature program consisted of an isothermal step at 50 °C for 3 min, followed by a ramp of 5 °C/min to reach 100 °C, then a ramp of 10 °C/ min to reach 180 °C, and a final isothermal step at 180 °C for 3 min. After the volume percentage of H2, N2, and C1−C6 hydrocarbons were measured, the equation of state for an ideal gas was used to convert the data to mass percentages. Collected liquid products were weighed and then analyzed by simulated distillation on another Agilent 6890 gas chromatograph (according to the ASTM D-2887 method) to determine the yields of gasoline (IBP to 204 °C), LCO (205−350 °C) and heavy cycle oil (HCO, above 350 °C). The coke generated from the reaction in the TPSR was determined by a CO and CO2 analyzer at the flue gas outlet, and the flue gas volume was measured by a flowmeter. To thoroughly investigate detailed properties of hydrocarbons in FCC products such as gasoline, LCO and HCO, the liquid products were cut by a true boiling point (TBP) distillation method. The LCO was analyzed by a gas chromatography−mass spectroscopy (GC MS, according to the ASTM D-2425 method) to obtain the hydrocarbon composition of LCO, including paraffins, naphthenes and aromatics.

2. EXPERIMENTAL DETAILS 2.1. Feedstock and Catalysts. A kind of paraffinintermediate base heavy oil was used as the experimental feedstock, which was obtained from a China National Petroleum Corporation (CNPC) refinery. Its properties are listed in Table S1, from which we can see that the heavy oil contains a considerable amount of saturates, which are most prone to cracking. These two kinds of catalysts, which were designed for hydrogenation and FCC process, were used in the experiments. Their properties are detailed in Tables S2 and S3, respectively. B

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Figure 2. Reaction scheme for catalytic cracking of tetralin and decalin.

3. REACTION MECHANISM ANALYSIS AND PROPOSITION OF NEW PROCESS 3.1. Crackabilitiy Comparison of Ring Compounds. As mentioned earlier, LCO contains abundant multiring aromatics, among which diaromatics, such as naphthalene, are the most predominant constituents in consideration of boiling range of LCO. It is scarcely possible for naphthalene to open its aromatic rings in the FCC circumstance, which is precisely why naphthalene is concentrated in LCO. Compared with strong C−C bonds in the aromatic rings, C−C bonds in naphthenic rings are easier to be broken. That is, if the aromatic rings could be hydrogenated into naphthenic rings, then the original naphthalene can be transformed into gasoline fraction. However, hydrogenation of naphthalene is a typical sequential reaction and mainly produce tetralin and decalin, which correspond to mild and deep hydrogenation, respectively.20 Figure 2 depicts the reaction scheme for the cracking of tetralin and decalin cited from literature.21−23 During the cracking of decalin, gaseous molecules and naphthenics were produced in large amounts, all of which are not ideal components of high ON gasoline. By contrast, when cracking tetralin, 1-ring aromatics accounted for almost 80% of the total conversion. Such products are ideal components of high ON gasoline. Obviously, hydrogenation of multiring aromatics could increase production of light fractions, but deep hydrogenation would produce much low value products. Selective hydrogenation of multiring aromatics for naphthenoaromatics not only reduces the consumption of H2, but also obtains desired products. However, it is important to note in Figure 2a that hydrogen transfer reaction led tetralin to dehydrogenate into naphthalene, followed by condensation into coke. Such a reaction pathway is not ideal and needs to be suppressed. In other words, abundant hydrogen transfer reactions during FCC process would result hydrogenation of multiring aromatics in vain. Therefore, polyaromatics are needed to be partially saturated and then cracked in the FCC process, in which saturated structures are shape-selectively cracked on the premise that hydrogen transfer reactions could be lessened as much as possible. The integration of selective hydrogenation and FCC can take full advantages of both and thereby obtain high gasoline yield. 3.2. Cracking Mechanism Analysis of Naphthenoaromatics. To have a better understanding about how to reduce hydrogen transfer reaction during the FCC of naphthenoaromatics, cracking mechanism of naphthenoaromatics based on

quantum mechanics was analyzed taking tetralin as an example. Molecular simulation of chemical structure and transformation mechanism of tetralin were performed using Materials Studio 5.5. The calculation procedures are as follows: (1) building structure models for tetralin and its products, (2) optimizing their structures by DMol3 module based on density function theory, (3) seeking transition states by LST/QST method, (4) verifying transition states by frequency analysis, (5) calculating bond energies and energy barriers. Before modeling reaction, chemical structural characteristics of reactants are needed to have a better understanding. Bond energy is an important indication for strength of chemical bond, which is directly relative to reaction difficulty. Therefore, characteristics of C−C and C−H bonds were first explored from the perspective of bond energy. Figure 3 illustrates the C−H and C−C bond energy of tetralin. From Figure 3a, it can be seen that the C−H bond

Figure 3. Bond energy for (a) C−H and (b) C−C bonds in tetralin, unit: kJ/mol.

energies in benzene ring were about 485 kJ/mol, while those in naphthenic ring were lower than 450 kJ/mol. This indicates that the C−H bonds on benzene ring are much more stable and hence difficult to participate in FCC reaction. For the C−H bonds on naphthenic ring, bond energy of C−H bond adjacent to the benzene ring (α C−H) was 371.6 kJ/mol, and that of C−H bond far from the benzene ring (β C−H) was 431.8 kJ/ mol, implying that the α C−H bond has better reactivity and is easy to be broken. Similar to C−H bonds, C−C bonds on benzene ring also show inferior reaction characteristics in Figure 3b. C−C bond energies on naphthenic ring also varied largely in different positions, among which the β C−C bonds were clearly quite weak (328.2 kJ/mol), and the α C−C bonds had the highest bond energy (427.7 kJ/mol). In terms of bond energy, α C−H and β C−C bonds have higher reaction propensity. C

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Figure 4. Formation and transformation of tetralin carbonium ions.

Figure 5. Schematic of routine FCC and the synergistic process.

The first step for tetralin to involve into FCC reactions is forming carbonium ions by the attack of a Bronsted site. The exact position at which H+ attacks tetralin depends on density of electron cloud and strength of bond energy. Therefore, tetralin carbonium ions are formed mainly through three ways, as shown in Figure 4a. One of the pathways is that H+ directly attacks tertiary carbon atom which has the highest density of electron cloud. Path (2) and (3) are corresponding to low bond energies for β C−C bond and α C−H bond, respectively. Through calculations, the energy barriers for path (1), (2), and (3) were 191.6, 263.2, and 171.5 kJ/mol, respectively. Obviously, path (3) has the lowest energy barrier and thereby is much easier to induce the subsequent FCC reaction. But what is worth noting is breakage of C−H bond would lead to formation of H2, which is unwanted in the FCC process. The carbonium ions would undergo subsequent β-scission and form alkylbenzenes, also shown in Figure 4b. The energy barriers of β-scission were 183.1 and 204.9 kJ/mol for path (1) and (3), which is relatively high. Therefore, suitable conditions

and/or special catalyst are needed to match in order to favor opening naphthenic rings. As shown in Figure 3b, the α C−H bonds have the lowest bond energy, which means α C−H bonds are much easier to break. This feature leads tetralin to become an effective hydrogen donor, which has been demonstrated by many researchers. When contacting with other carbonium ions, tetralin underwent hydrogen transfer reactions as shown in Figure 4c. The energy barriers of hydrogen transfer reaction in path (4) and (5) were 92.4 and 123.0 kJ/mol. These values are much lower than those of β-scission, suggesting that tetralin underwent hydrogen transfer reaction rather than β-scission when there are other carbonium ions present in the reaction system. That is why strong hydrogen transfer reaction occurs when hydro-LCO is co-processed with heavy oil. Heavy oil is easy to produce massive carbonium ions on the acid catalyst and become hydrogen acceptor of the hydrogen transfer reaction. D

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experiments, mild hydrogenation experiment of LCO was carried out with a supported bimetallic catalyst on the fixed-bed reactor under the condition of reaction temperature of 340− 360 °C, reaction pressure of 6−9 MPa, liquid hourly space velocity of 1.0−2.0 h−1, and hydrogen-to-feedstock volume ratio of 400−600. Table 1 summarizes the properties and composition of LCO and hydro-LCO.

In consequence, higher energy should be provided in order to crack naphthenic structure in the tetralin efficiently. At the same time, contact between tetralin and reactive carbonium ions should be avoided, in case unfavorable hydrogen transfer reactions occur. 3.3. A New Synergistic Process Proposed. Since tetralin-like naphthenoaromatics in hydro-LCO preferentially undergo hydrogen transfer reaction when contacting with other constituents, it is necessary to separate hydro-LCO with routine feedstock during the FCC process. Figure 5 shows the schematic diagram of the routine FCC and synergistic process. In contrast to the routine FCC process, a hydrogenation section for LCO, and a riser for hydro-LCO are added in the synergistic process. 3.3.1. Selective Hydrogenation. Discussion in Section 3.1 has explained that controlling hydrogenation depth of LCO in the moderate extent is the key to subsequent processing. Through theoretical analysis of selective hydrogenation,24 noble metals such as palladium and platinum, and base metals such as nickel and cobalt are suggested as the active center of catalyst, and supported onto carriers with high surface area and pore volume. Operating conditions, such as reaction temperature, reaction pressure, and space velocity, also play important roles in controlling hydrogenation depth. The main purpose of selective hydrogenation is to transform polyaromatics into naphthenoaromatics available for subsequent processing. Therefore, the operating conditions of selective hydrogenation should be milder than those of hydrocracking. At the same time, thermodynamic and kinetic analysis of mild and deep hydrogenation also confirm the above conclusion.25 Therefore, lower reaction temperature, lower reaction pressure and higher space velocity should be adopted on the premise of achieving the basic requirements. 3.3.2. FCC Section. To suppress hydrogen transfer reaction, a separate riser reactor specially for hydro-LCO is added to the existing FCC unit, instead of mixing and co-processing hydroLCO with heavy oil. In addition, differences in chemical composition between heavy oil and hydro-LCO are destine to have the favorable reaction conditions for both differ. Through selective hydrogenation, hydro-LCO contains lots of naphthenoaromatics, which has high energy barriers both for initiation and β-scission as calculated in Section 3.2. Therefore, reaction conditions should be severe, such as a higher temperature and catalyst-to-oil to favor opening naphthenic rings, but a shorter reaction time to avoid secondary reaction of generated alkylbenzenes. However, heavy oil usually contains a large amount of refractory hydrocarbons, resulting in poor crackability but great coking propensity. Severe conditions will induce serious thermal cracking reaction, and thereby high coke yield. In this synergistic process, cracking of heavy oil is meant to maximize liquid yield with low coke yield. During the heavy oil FCC, saturated moieties, such as alkanes and cycloalkanes, are cracked into lighter fractions, while aromatic moieties are retained and used for subsequent selective hydrogenation. Cracking of saturates is easier than opening of naphthenic rings; hence, comparing with hydro-LCO, heavy oil FCC needs milder conditions.

Table 1. Properties and Composition of LCO and HydroLCO items Density (20 °C), g·cm−3 Cetane index Hydrocarbon Composition, wt % Paraffins Naphthenes 1-ring naphthenes 2-ring naphthenes 3-ring naphthenes Aromatics 1-ring aromatics Alkylbenzenes Indans/tetralin Indenes 2-ring aromatics Naphthalenes Acenaphthenes Acenaphthylenes 3-ring aromatics Total

LCO

hydro-LCO

0.9690 17.8

0.8977 24.3

6.3 2.4 1.8 0.5 0.1 91.3 28.1 15.1 8.8 4.2 55.4 38.0 8.4 9.0 7.8 100.0

10.5 24.2 8.6 10.5 5.1 65.2 55.7 13.4 34.2 8.1 8.8 4.2 2.8 1.9 0.7 100.0

It is evident that hydro-LCO had more superior properties than LCO, including lower density and higher cetane number. These differences are ascribed to change of aromatic content. As we all know, aromatics have relatively high density and ultralow cetane number. Table 1 shows that the aromatic content decreased from 91.3 to 65.2 wt % after hydrogenation, leading to density decrease from 0.9690 to 0.8977 g·cm−3 and cetane number increase from 17.8 to 24.3. Additionally, the hydrogen content increased from 8.79 to 11.42 wt %. After further analyzing aromatic types in the LCO, it can be found that the 1-ring aromatic hydrocarbons, including alkylbenzenes, indans, tetralin and indenes, accounted for about 30% of total aromatics; and 2-ring aromatic hydrocarbons, mainly including naphthalenes, acenaphthenes, acenaphthylenes, occupied 60% of total aromatics, while the 3-ring aromatic hydrocarbons, including phenanthrenes and anthracenes, only made up 10% of total aromatics. Such a distribution strongly depends on differences of boiling point among them. Compared with 2-ring aromatics, most of 1-ring aromatics have lower boiling point, and thereby exist in gasoline fraction. Similarly, most of 3-ring aromatics exist in HCO fraction. In addition, the aromatic distribution implies that the 2-ring aromatics are key converting constituents. After hydrogenation, the amount of total aromatics decreased from 91.3 to 65.2 wt %, which means that 26.1 wt % aromatics are saturated into paraffins and naphthenes. Among the aromatics, the content of multiring aromatics was reduced by 53.7 wt %, while 1-ring aromatics increased by 27.6 wt %, accounting for 51% of the decrement of multiring aromatics. This result reveals that over 50% multiring aromatics are transformed into 1-ring aromatics during the moderate

4. RESULTS AND DISCUSSION 4.1. Properties of LCO before and after Hydrogenation. On the basis of the above analysis and previous E

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reaction to generate smaller molecules, while aromatics generally experience breaking of side alkyl chains and opening of naphthenic rings, and then remain the aromatic nucleus. Among the aromatics, a big difference between 1-ring and multiring lies in the attribution of remaining aromatic nucleus. Clearly, 1-ring aromatic nucleus belong to gasoline fraction, while multiring aromatic nucleus belong to cycle oil and may further condense into coke. In conclusion, high amounts of paraffins, naphthenes and 1-ring aromatics contribute to high gasoline yield, high feed conversion and low coke selectivity. 4.3. Comparison between Routine FCC and Synergistic Process. After obtaining basic data about chemical composition and reaction performance of LCO before and after hydrogenation, it is necessary to evaluate application effect of the proposed synergistic process. Therefore, simulation experiments of synergistic process were carried out on the TPSR and fixed-bed reactor. For our experiments, heavy oil was first converted by FCC at a temperature of 490−510 °C, catalyst-tooil weight ratio of 6−8, and residence time of 2.5−3.0 s. The LCO from above FCC experiment was then subjected to moderate hydrogenation with reaction temperature of 340−360 °C, reaction pressure of 6−9 MPa, liquid hourly space velocity of 1.0−2.0 h−1, and hydrogen-to-feedstock volume ratio of 400−600. Finally, the hydro-LCO was used as feedstock for the subsequent re-FCC. The reaction conditions were temperature of 540−560 °C, catalyst-to-oil weight ratio of 9−12, and residence time of 1.0−2.0 s. 4.3.1. Product Distribution. Table 3 shows the comparison of product distribution for the routine FCC and synergistic

hydrogenation. The amount of 1-ring aromatics increased from 28.1 to 55.7 wt %, of which alkylbenzenes slightly decreased, while cyclic hydrocarbyl benzenes increased greatly from 13 to 42.3 wt %. This observation indicates that above transformed multiring aromatics are mainly hydrogenated into cyclic hydrocarbyl benzenes without ring opening under the investigated condition. 4.2. FCC Performance of LCO before and after Hydrogenation. To obtain FCC reaction performance of hydro-LCO, FCC experiment of hydro-LCO was conducted on TPSR apparatus under the operating conditions of temperature of 540−560 °C, catalyst-to-oil weight ratio of 9−12, and residence time of 1.0−2.0 s. For comparison, FCC reaction performance of LCO was also investigated under the same condition. Table 2 displays the experimental data. Table 2. FCC Product Distribution of LCO and Hydro-LCO feedstock Product Distribution, wt % Dry gas LPG Gasoline Cycle oil Coke Loss Conversion, wt % Selectivity, % Dry gas LPG Gasoline Coke

LCO

hydro-LCO

2.67 5.73 24.05 61.26 5.79 0.50 38.24

2.45 7.15 50.83 35.36 3.65 0.55 64.09

6.98 14.98 62.89 15.14

3.82 11.16 79.31 5.70

Table 3. FCC Product Distribution of Routine FCC and Synergistic Process process

During the FCC process, only 38 wt % LCO was converted. The resulted low conversion is mainly due to its composition. As previously mentioned, after primary reaction in which a large amount of saturated moieties are cracked, LCO contains abundant short chain aromatic compounds. This kind of compounds is difficult to convert into light products, accordingly resulting in low conversion of LCO. Even so, LCO still contains 36.8 wt % saturates and 1-ring aromatics, both of which have decent crackabilities. Therefore, 24 wt % gasoline was produced. The yield of cycle oil was high up to 61.26 wt % attributed to over 60 wt % multiring aromatics in the LCO. As expected, these compounds are refractory and hence remain in the cycle oil fraction. In addition, high selectivity of coke is also because of substantial amount of multiring aromatics. Compared with product distribution of LCO, it is clearly found that hydro-LCO had superior FCC reaction performance. Given the same reaction condition, hydro-LCO yielded improved production, recording that gasoline increased from 24.05 to 50.83 wt % and cycle oil decreased from 61.26 to 35.36 wt %. The feed conversion of LCO was only 38.24 wt %, much lower than that of hydro-LCO with 64.09 wt %. After hydrogenation, the selectivity of desired product (LPG + gasoline) was as high up as 90% and that of byproduct (dry gas + coke) declined by 12.6%. The difference of hydrocarbon composition between LCO and hydro-LCO in Table 1 could explain these changes. Compared with saturates and 1-ring aromatics, multiring aromatics have poorer crackabilities. During FCC process, paraffins and naphthenes mainly undergo decomposition

routine FCC

Product Distribution, wt % Dry gas 3.46 LPG 18.33 Gasoline 42.95 LCO 21.25 HCO 4.73 Coke 8.79 Loss 0.49 Conversion, wt % 73.53

synergistic process

difference

4.26 20.96 59.33 1.57 3.80 9.58 0.50 94.13

+0.80 +2.63 +16.38 −19.68 −0.93 +0.79 +20.60

process. Much higher conversion and superior product distribution were observed for the synergistic process. Compared with routine FCC, conversion of synergistic process increased by 20.6 wt %, leading to the ultimate conversion as high up as 94.13 wt %. The gasoline yield increased by 16.38 wt %, while that of LCO decreased by almost 20 wt %. The yields of dry gas and coke also increased slightly by 0.80 wt % each. These results are attributed to efficient recracking of LCO. As shown in Table 1, the LCO contains a large amount of aromatics, which are refractory in the routine FCC process. Hence, the yield of LCO during the FCC is relatively high. In the synergistic process, most of the refractory compounds in the LCO are hydrogenated into naphthenoaromatic compounds in the hydro-LCO, which are easier to be cracked into gasoline fraction in the further re-FCC process. Therefore, the overall conversion of heavy oil and yield of whole gasoline increase greatly, and the yield of LCO decreases accordingly. However, the hydro-LCO still contains almost 10 wt % multiring aromatics, which have serious coking propensity. F

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Industrial & Engineering Chemistry Research Inevitably, extra coke would be formed during the re-FCC process, leading to the overall yield of coke increases slightly. On the whole, the synergistic process is beneficial to converting heavy oil into desired products, especially gasoline. 4.3.2. Gasoline Property. The synergistic process increases not only feed conversion and gasoline yield, but also gasoline quality. A comparison of gasoline qualities obtained from different process is shown in Table S4. Gasoline produced from the synergistic process exhibited decreased amount of sulfur compounds and olefins, but increased amount of aromatics. These parameters are in line with those of high quality gasoline. More importantly, gasoline from synergistic process had higher octane number. In fact, gasoline from synergistic process consists of gasoline produced from FCC of heavy oil, named as primary gasoline, and gasoline produced from FCC of hydro-LCO, named as secondary gasoline. Obviously, the FCC gasoline is also the primary gasoline. In that case, all the differences of gasoline between routine FCC and synergistic process are derived from the secondary gasoline. If the quality of secondary gasoline is superior to that of primary gasoline, the gasoline from synergistic process will be superior to FCC gasoline as well, and vice versa. The differences of qualities between primary and secondary gasoline are ascribed to the differences of properties between heavy oil and hydro-LCO. Comparing Table S1 with Table 1, hydro-LCO has much lower sulfur and higher aromatic content than heavy oil, then so does their products. High quality of secondary gasoline would lead to high quality of overall gasoline from synergistic process. In addition, hydrogenation makes a portion of olefins in the hydro-LCO saturate, resulting in lower content of olefins in the secondary gasoline. Even so, increased aromatics still make the octane number increase for the gasoline from synergistic process. These findings indicate that the proposed synergistic process could be an effective option for producing gasoline with high octane number.

Application of the proposed synergistic process was investigated. Compared with routine FCC, 20 wt % higher conversion and 16 wt % more gasoline were obtained for the synergistic process. Gasoline from the synergistic process exhibited decreased sulfur and olefins, but increased aromatics, and thereby improved octane number. These findings indicate that the proposed synergistic process is an effective option for producing gasoline with high octane number.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b00360. Properties of feedstock (Table S1), properties of hydrogenation catalysts (Table S2), properties of FCC catalysts (Table S3), gasoline properties of routine FCC and synergistic process (Table S4), schematic diagram of the fixed-bed reactor (Figure S1), and schematic diagram of technical pilot scale riser FCC apparatus (TPSR) (Figure S2) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 8610-8973-3085. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

■

REFERENCES

The authors acknowledge the financial support provided by the State Key Program of National Natural Science Foundation of China (21336011), National Natural Science Foundation of China (21476259), Science Foundation of China University of Petroleum-Beijing (No.2462015YQ0310) and the Program for New Century Excellent Talents in University of China (NCET13-1029).

5. CONCLUSIONS LCO is a main byproduct of FCC unit; therefore, its conversion directly influences efficiency of FCC process. On the basis of theoretical analysis of LCO reaction characteristics, a new synergistic process was proposed. Through partial hydrogenation of LCO and then shape-selectively cracking of hydroLCO, increased production of high ON gasoline was achieved. Compositional changes of LCO before and after hydrogenation were analyzed. LCO was characterized by high aromatic content and thereby low cetane number. Aromatics took up 91.3 wt % of LCO, among which multiring aromatics accounted for 63.2 wt %. After moderate hydrogenation, the amount of multiring aromatics decreased greatly by 53.7 wt %, whereas 1-ring aromatics increased by 27.6 wt %. These changes indicate that over 50% multiring aromatics are translated into 1-ring aromatics. Among the 1-ring aromatics, cyclic hydrocarbyl benzenes increased by 29.3 wt %, revealing that above translated multiring aromatics are mainly hydrogenated into cyclic hydrocarbyl benzenes without ring opening. FCC reaction behaviors of LCO and hydro-LCO were compared. Poor crackability of aromatics resulted in only 38.24 wt % FCC conversion of LCO, while decrease of multiring aromatics in hydro-LCO directly improved its FCC performance, recording with 64.09 wt % conversion and 50.83 wt % gasoline yield.

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DOI: 10.1021/acs.iecr.6b00360 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX