Study on in Situ Sulfur Removal from Gasoline in Fluid Catalytic

May 15, 2012 - active site; consequently, it affected the conversion of sulfur impurities in naphtha. Therefore, an intrinsic conflict existed between...
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Study on in Situ Sulfur Removal from Gasoline in Fluid Catalytic Cracking Process Yaoshun Wen, Gang Wang,* Chunming Xu, and Jinsen Gao State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing, 102249, China ABSTRACT: On the basis of the cracking desulfurization of naphtha and the subsidiary riser in the fluid catalytic cracking (FCC) process for gasoline reformation, the in situ sulfur removing from naphtha in the FCC unit was investigated and a conceptual process was proposed. The relevant experiments were conducted in a confined fluidized bed reactor (CFB) and a technical pilot scale riser (TPSR) FCC apparatus. The effects of operation conditions such as temperature, catalyst-to-oil ratio, and weight hourly space velocity on product distribution and sulfur reduction of the recycled FCC naphtha were researched in the CFB reactor, and the optimal conditions were determined. The hydrogen transfer reaction aiming for sulfur removal was quantitatively analyzed. Results indicated that higher hydrogen transfer activity and proper cracking ability was beneficial to reduce the sulfur content of gasoline. The TPSR experimental results showed that sulfur concentration of the naphtha in the conceptual process decreased from 426 to 139 μg·g−1, with a 67.37% reduction compared to that of the routine FCC process. No obvious changes in light oil yield and liquid product yield were observed except a small increase in coke yield.

1. INTRODUCTION The refining industry is under constant environmental pressure to achieve more rigorous standards on sulfur content in transport fuel. The limits of 10 μg·g−1 sulfur compounds in gasoline has been in effect since 2009 in European Union and Japan. Because sulfur impurities will not only increase SOx emissions but will also lead to irreversible deactivation the ternary catalyst in automotive catalytic converters.1−3 Fluid catalytic cracking (FCC) naphtha, which accounts for 80 wt % of the total gasoline pool in China, is by far the major sulfur contributor in gasoline, up to 85−95 wt %. There are three primary commercial alternatives available for the reduction of sulfur of FCC naphtha, such as the pretreatment of FCC feedstock, post-treatment of the FCC naphtha,4−6 and in situ sulfur removal through FCC units.7−9 Owing to operation flexibility, low investment, and hydrogen free, gasoline desulfurization through the FCC process has raised wide attention for one of the economical options for low sulfur gasoline production. Many studies focused on the synthesis and development of high active desulfurization catalysts or additives10−12 and the optimization of operation conditions.13 However, the effectiveness of this approach in commercial application is still under question, and industry data demonstrated that the degree of sulfur reduction is limited, no more than 20−30 wt %. The originality and distribution of refractory sulfur in FCC naphtha were extensively investigated.14−16 Generally, FCC naphtha contained hydrogen sulfide, thiols, disulfide, thiophene, and alkyl thiophenic derivatives. Among them, thiophene and its alkyl derivatives accounted for more than 60 wt % of total sulfur compounds in FCC naphtha and 70 wt % in residue FCC naphtha, respectively.17 Because of the aromatics character, thiophene and its alkyl derivatives are relatively stable under the real FCC environment.18 Herein, enhancing the conversion of thiophene and its alkyl derivatives © 2012 American Chemical Society

is the key target for in situ sulfur removal through the FCC process. Valla19 concluded that reprocessing naphtha in the FCC unit can considerably reduce the total sulfur concentration of gasoline. A two fluidized bed system, including a fluidized-bed reactor and a fluidized-regenerator was proposed for FCC gasoline catalytic desulfurization by Jaimes,20,21 who studied the thermodynamics and kinetics about thiophene conversion over ZSM-5 catalyst under the FCC environment. However, model compounds were used as feedstock for the desulfurization test, and the desulfurization ability of this approach for real FCC naphtha needs further investigation to verify. Moreover, significant coke formed during this approach may affect the gasoline yield. Coke and cracking gas are the low valuable products. Therefore, the desulfurization activity and product distribution should be both taken into consideration during the cracking desulfurization process. Corma et al.22 systemically investigated the reaction behavior of sulfur compounds under the real FCC condition and proposed a reaction scheme. According to the scheme, thiophenic compounds converted to tetrahydrothiophene by hydrogen transfer reaction from a hydrogen donor and then cracked into olefin and H2S. Shan23 suggested that hydrogen transfer and cracking were the two principal elementary reaction steps for thiophene reduction via cracking. However, hydrogen transfer reaction is restrained under a classic FCC process, which is a complex parallel-series reaction system.24 To obtain maximum middle product yield and decrease the formation of coke and dry gas, a short residence time between the hydrocarbon molecule and spent catalyst at the second half of riser is adapted to prevent an undesired secondary reaction. The rate of the cracking reaction is much faster than that of Received: March 23, 2012 Revised: May 14, 2012 Published: May 15, 2012 3201

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hydrogen transfer. Herein, a long residence time is favorable to enhance the latter. Moreover, the catalytic cracking reaction is endothermic, while the hydrogen transfer reaction is exothermic. A proper low temperature is thermodynamic for the hydrogen transfer reaction. In addition, the heavy oil molecule was preferentially adsorbed on the acid site and retarded the accessibility of sulfur compounds on the catalytic active site; consequently, it affected the conversion of sulfur impurities in naphtha. Therefore, an intrinsic conflict existed between the heavy oil cracking and in situ gasoline desulfurization in the conventional FCC process. Recently, several modified FCC processes have been developed in China and successfully applied to commercial plants. Meng et al.25 investigated a flexible dual-fluid catalytic cracking (FDFCC) to decrease the olefin content of naphtha and increase propylene. Xu and co-workers26,27 proposed a novel FCC process for maximizing iso-paraffin (MIP) in cracked gasoline. Two reaction zones were designed in one riser reactor. The principal reaction for the first reaction zone was cracking heavy oil into light oil, while the second reaction zone improved the quality of gasoline by hydrogen transfer reactions. Shan et al.28 researched a two-stage residue catalytic (TSRFCC) to improve the light oil yield, and the pilot experimental results demonstrated the light oil yield increased 3−4 wt %. Gao et al.29,30 developed a subsidiary riser FCC process (SRFCC) to upgrade naphtha, and the commercial data indicated the olefin content of naphtha was reduced below 35 vol % without affecting the process capacity. These processes all provide the independent reaction zone for naphtha upgrading in FCC units, and the sulfur concentrations of naphtha are usually lower than that of conventional FCC naphtha. Xu et al.31 analyzed the MIP series technologies in reducing sulfur content of FCC naphtha, suggesting that the strong hydrogen transfer reaction trend in the second MIP reaction zone and a low olefin content of naphtha are responsible to lower sulfur concentration. However, the main purpose of the approaches is to reduce the olefin content of naphtha or obtain more light oil. The systematic investigation on the optimal reaction environment and a suitable reactor type for gasoline desulfurization is scarce. In the present paper, the effects of reaction conditions on gasoline desulfurization performance were performed in a confined fluidized bed (CFB) reactor employing commercial FCC naphtha and the optimum condition was obtained. Sulfur removal oriented hydrogen transfer reaction was quantitatively analyzed. On the basis of the analysis of reaction behavior, a novel conceptual process for in situ gasoline sulfur reduction was proposed and the simulated experiment was also conducted on the technical pilot scale riser (TPSR) apparatus.

Table 1. Properties of Commercial FCC Naphtha properties

commercial FCC naphtha

density (20 °C) (kg·m−3) sulfur content (μg·g−1) PIONA analysis (wt %) paraffin iso-paraffin olefin naphthene aromatic distillation (°C) IBP/5%a 10/30% 50/70% 90%/FBPb

737.4 281 6.17 30.83 40.75 9.64 12.61 38/56 64/86 112/141 175/213

a

IBP = Initial Boiling Temperature. bFBP = Final Boiling Temperature.

Table 2. Properties of Heavy Oil FCC Feedstock properties

70 wt % VGO + 30 wt % VR

−3

density (20 °C) (kg·m ) viscosity (80 °C) (mm2·s−1) average molecular weight Conradson carbon residue (wt %) elemental composition (wt %) C H S N nickel (μg·g−1) vanadium (μg·g−1) SARA analysis (wt %) saturates aromatics resins asphaltenes (n-heptane)

904.9 10.64 389 4.30 86.55 12.54 0.68 0.23 7.0 26.8 61.76 22.46 15.08 0.70

as Cat-1, and the other one was a commercial FCC catalyst LRC-99 denoted as Cat-2 as a contrast. Both catalysts were aged at 800 °C for 10 h with 100% steam; therefore, the activity and stability are very stable in our tests after pretreatment. The properties of the catalysts are illustrated in Table 3.

Table 3. Properties of FCC Catalysts properties microactivity index pore volume (cm3·g−1) specific surface area (m2·g−1) packing density (g·cm−3) particle size distribution (μm) 0−65 65−105 105−160 >160

2. EXPERIMENTAL SECTION 2.1. Feedstock and Catalysts. Typical commercial FCC naphtha was employed as feedstock for gasoline desulfurization upgrading. The properties of commercial naphtha are shown in Table 1. The sulfur concentration and olefin content were 281 μg·g−1 and 40.75 wt %, respectively. A paraffin-based feedstock spiked with 70 wt % vacuum gas oil (VGO) and 30 wt % vacuum residue (VR), which was provided by the CNPC Company, was used as the heavy oil FCC feedstock. The basic properties of heavy FCC feedstock are detailed in Table 2. Moreover, FCC naphtha derived from FCC experiments in TPSR apparatus was also used for the desulfurization upgrading test for simulating the conceptual process. Two catalysts were employed in this study for comparison. One was a specially designed desulfurization catalyst prepared by Lanzhou Petrochemical Research Institute coded

Cat-1

Cat-2

69 0.19 136.42 0.80

67 0.18 138.77 0.78

53.19 42.81 2.82 1.18

54.21 42.16 2.34 1.29

2.2. Apparatus. In present study, the reaction behavior of FCC naphtha cracking desulfurization was carried out in a confined fluidized bed reactor (CFB), and the detailed description of the reactor can be viewed elsewhere.32,33 The FCC reaction performance of heavy oil and the simulated the conceptual process were carried out in the technical pilot scale riser apparatus (TPSR). The schematic diagram of the TPSR apparatus is depicted in Figure 1. It can be operated with a 3202

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Figure 1. Schematic of the technical pilot scale riser (TPSR) FCC apparatus. slurry (>350 °C). Second, the liquid product was fractionated by a small-scale distillation experimental apparatus, and then the basic properties of the gasoline were analyzed. The sulfur concentration of liquid product was determined on an ANTEK 7000NS sulfur−nitrogen analyzer through an ultraluminescence method. The individual sulfur compounds containing in gasoline before and after upgrading was identified by a GC CP-3800 equipped with a pulsed flame photometric detector (PFPD). The TPSR apparatus and the CFB reactor had superior repeatability for fluid catalytic cracking and FCC naphtha upgrading, and the mass balances of all tests were between 97 and 100 wt %. For the residue FCC tests, the conversion (Cr) was defined as the weight percentage of feedstock converted into dry gas, liquid petroleum gas (LPG), gasoline, and coke. The conversion for upgrading naphtha (Cg) was referred to as the total weight percentages of dry gas, LPG, and coke.

continuous cracking reaction and catalyst regeneration, similar to industry FCC units. It is comprised of a feed injection system, a two reaction system (On the left is a riser plus a fluidized reactor, and on the right is a 6.3 m high riser reactor), two disengagers, two strippers, a regenerator, and a product recovery system. The hot regenerated catalyst flows through a standpipe from the regenerator to the bottom of the right riser, where a finely atomized hydrocarbon feedstock is injected onto the high-temperature catalyst through a casing nozzle with steam as the atomization medium. The hydrocarbon molecule catalytic cracking reaction occurs and lifts the catalyst to the top of the riser. The hydrocarbon vapors are quickly separated from the catalyst in the disengager to reduce the unwanted secondary reaction, then flowed into the product recovery system, and cooled into liquid oil and cracking gas. The deactivated catalyst is sent into the stripper, in which the steam is employed to remove entrained hydrocarbons in a countercurrent dense phase stripper bed. After that, the spent catalyst is flowed through a plug valve into the regenerator, where it is regenerated by burning off the coke in the excess air. The operation limits of TPSR can be seen in reference.34 It is necessary to note that the TPSR apparatus can perform residue FCC experiments and reprocess FCC naphtha in an independent reaction zone. Moreover, the naphtha feed can be injected at the bottom of the left riser or at the bottom of the fluidized bed reactor to operate at different residence times (Figure 1). 2.3. Product Analysis. The cracking gas was monitored by a flowmeter and analyzed by an Agilent 6890 gas chromatography (GC) equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID). Chemical station software was used to determine the volume percentage of H2, N2, and C1−C6 hydrocarbons in the gas sample. The equation of state for the ideal gas was employed to convert the volume percentage into the mass percentage. The coke deposited on the catalyst during the naphtha cracking in CFB reactor was determined using a high-frequency infrared analyzer (HIR 914). While the CO2 formed in the regenerator from TPSR apparatus was online monitored and quantified by an IR cell and the volume was measured through a flowmeter, then the coke formed during catalytic cracking in the TPSR apparatus was calculated. The liquid product was weighed and analyzed by two methods. First, simulated distillation of the liquid sample was carried out on an Agilent 6890N GC according to the ASTM-D2887 method with a FID to quantify the mass percentage of gasoline (IBP−205 °C), diesel (205−350 °C), and

Cr = (dry gas + LPG + gasoline + coke)/feedstock

(1)

Cg = (dry gas + LPG + coke)/feedstock

(2)

3. RESULTS AND DISCUSSION 3.1. Reaction Performance of FCC Naphtha Cracking Desulfurization. To investigate the reaction behavior of hydrocarbon and sulfur compounds during naphtha reprocessing desulfurization, commercial FCC naphtha was cracked in a CFB reactor over the Cat-1 catalyst. The effects of the operation conditions such as temperature, catalyst-to-oil ratio (CTO), and weight hourly space velocity (WHSV) on product distribution and desulfurization ability were discussed. Generally, reprocessing FCC naphtha decreases sulfur content at the expense of coke and dry gas formation, which are low valuable products. A coefficient of desulfurization was proposed to characterize desulfurization selectivity (Ss), and it was defined as follows Ss = Sc/Cg

(3)

where Sc was defined as sulfur conversion, 3203

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}

Sc = (Sfeed − S liquid product)/Sfeed × 100%

naphtha by cracking should be operated at a mild operation severity. One should note that even at a low temperature such as 360 °C, sulfur conversion was achieved at 48.46 wt %, indicating that reprocessing FCC naphtha can significantly reduce the sulfur concentration. This is in agreement with previous research.19 In addition, the sulfur selectivity (Figure 3b) for all tests is larger than 1 and demonstrated that sulfur compounds exhibit a higher affinity with catalyst than that of hydrocarbons such as olefin or aromatics. The sulfur atom in the thiophenic compound exists as two lone pair electrons with the electronic configuration 1s22s22p63s23p4. One pair of electrons is in the six-system conjugated π bond, and the other lies in the plane of the ring. The electron densities of thiophene and its derivatives are higher than that of aromatics or olefin because of the electron donor heteroatom. Moreover, sulfur selectivity declined linearly as the gasoline conversion increased, demonstrating that the enhancement cracking activity is not always favorable to sulfur reduction. Can and co-workers elucidated the reactivity and interaction of sulfur with sulfur reduction additives of naphtha by in situ and operando infrared spectroscopy. They pointed out that the sulfur removal additives reduce thiophenic compounds by an efficient deposition of the tetrahydrothiophene produced by hydrogen transfer.7 The results in our experiment indicated that the ratecontrolling step of sulfur removal during the in situ FCC process transferred at different operation severities. 3.1.2. Effect of CTO. The CTO varied from 4 to 10 was studied under the condition of a temperature of 450 °C and WHSV of 20 h−1. The products distribution and sulfur conversion are outlined in Figure 4. The LPG yield increased gradually from 4.71 to 8.38 wt % as CTO varied from 4.15 to 7.54 and then increased slowly. Similarly, the amount of coke formation rose from 2.31 wt % at CTO 4.15 to 4.10 wt % at CTO 9.32, with a 77.49% increase. A higher CTO enhanced the catalytic cracking reaction of hydrocarbons, but more catalytic coke formed.35 The yield of dry gas changed a little at the condition of the experimental range. The sulfur conversion went up with the CTO increase (Figure 4b). This can be explained that the higher CTO provided more catalytic active sites, which is beneficial to a bimolecular hydrogen transfer reaction36 and promoted sulfur reduction. However, more cracking active sites lead to hydrocarbon molecules cracked and formed lower value products such as coke. Taking both product

(4)

3.1.1. Effect of Temperature. The reaction temperature is a key operation parameter in commercial FCC units. The influence of temperature on desulfurization performance was investigated in the CFB reactor over Cat-1 at a wide range from 360 to 500 °C with CTO 5 and WHSV 20 h−1. Experimental results on the products distribution are illustrated in Figure 2. It

Figure 2. Effect of reaction temperature on products distribution (CTO = 5, WHSV = 20 h−1, Cat-1).

can be clearly seen that the gasoline yield gradually decreased from 95.46 to 90.86 wt % when the temperature increased from 360 to 450 °C, and then it reduced sharply with the temperature further increasing. Similarly, the LPG yield went up slowly before 450 °C and soared significantly while the temperature was higher than 450 °C. However, the formation of dry gas and coke almost remained unchanged at the temperature range discussed, except a slight increase from 360 to 380 °C. It indicated that naphtha mainly converted into LPG at a higher temperature. For the sake of maintaining the gasoline yield, the reaction temperature should be less than 450 °C. The sulfur conversion and selectivity are shown in Figure 3. Interestingly, sulfur conversion first increased remarkably with gasoline conversion and then increased slightly while gasoline conversion was higher than 10 wt %. The sulfur removal percentage was up to 61.43 wt %, while the gasoline loss was 8.34 wt %. Sulfur conversion was not increased linearly with gasoline conversion, suggesting that desulfurization of FCC

Figure 3. Effect of conversion on sulfur conversion and selectivity: (a) sulfur conversion and (b) sulfur conversion selectivity (CTO = 5, WHSV = 20 h−1, Cat-1). 3204

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Figure 4. Effect of CTO on products distribution and sulfur conversion: (a) product distribution and (b) sulfur conversion (T = 450 °C, WHSV = 20 h−1, Cat-1).

Figure 5. Effect of WHSV on product distribution and sulfur conversion: (a) product distribution and (b) sulfur conversion (T = 450 °C, CTO = 5, Cat-1).

distributions and sulfur conversion into consideration, the optimum CTO was determined at 5. 3.1.3. Effect of WHSV. The WHSV determines the residence time of hydrocarbon and sulfur impurities on the catalyst. The effects of WHSV on desulfurization performances were conducted in the range 10−40 h−1 by changing the rate of feedstock injection under conditions of temperature at 450 °C and CTO of 5. The product yields and sulfur reduction are depicted in Figure 5. As expected, the LPG yields steadily declined when the WHSV increased from 10 to 20 h−1 and then changed slowly. The dry gas and coke formation decreased slightly. Results showed that the residence time had a major effect on LPG production rather than dry gas and coke formed. Interestingly, the sulfur conversion decreased a little in the range 10−20 h−1 and then reduced remarkably when the WHSV higher than 20 h−1. So, the optimization WHSV was determined at 20 h−1 in our experiment. 3.2. Analysis of Hydrogen Transfer on Desulfurization Performance. According to the reaction network of sulfur compounds under FCC conditions,22 there are three primary pathways that can be summarized for in situ reduction of the sulfur impurities of naphtha during the FCC process, which are illustrated in Figure 6. First, the hydrogen transfer reaction was enhanced to form more tetrahydrothiophene and then cracked into inorganic sulfur such as H2S. Second, heavy sulfur molecules such as benzothiophene or alkyl benzothiophene were formed by alkylation and cyclization, and the refractory sulfur went into the diesel fraction and reduced by distillation.

Figure 6. Schematic of the desulfurization pathway during reprocessing FCC naphtha.

Third, the recombination of thiophene or its derivatives from H2S and olefins14,37 was restricted to decrease the sulfur content. Therefore, the hydrogen transfer reaction plays a key role in the sulfur-removal during the FCC process. A lot of effort was devoted to investigation of the hydrogen transfer reaction during the heavy fluid catalytic cracking process and the relationship to the quality of FCC naphtha.38−40 However, the quantifying characterization of the hydrogen transfer reaction for the gasoline sulfur reduction oriented FCC process in the open literature is rare. A coefficient of the hydrogen transfer parameter (CHT) was proposed to the quantitative analysis of the degree of the hydrogen transfer 3205

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reaction.24 CHT is defined as the ratio of the weight percentage between paraffin and olefin in LPG. C HT = (C30 + C40)/(C3= + C4=)

(5)

The principal hydrogen transfer reactions are described as eqs 6−9. CnH2n and CmH2m represent typical olefin and naphthene molecules in the gasoline range, respectively. Equations 6 and 7 are the two representative hydrogen transfer reactions related to olefin molecules, and eqs 8 and 9 are involved with thiophenic compound reduction. It should be noted that the reduction of olefin content is mainly attributed to reactions 6 and 7 because the concentration of thiophene compounds is quite lower than that of the olefin components. 3CnH 2n + CmH 2m → 3CnH 2n + 2 + CmH 2m − 6

(6) Figure 8. Effect of temperature on hydrocarbon group composition (CTO = 5, WHSV = 20 h−1, Cat-1).

CnH 2n + polycyclic aromatic → 3CnH 2n + 2 + coke precursor/coke

3C4 H4S + 2CmH 2m → 3C4 H8S + 2CmH 2m − 6

(7)

that the cyclization reaction took place. It should be noted that a considerable hydrogen transfer reaction occurred during the desulfurization process. The content of olefin decreased from 40.76 to 25.19 wt % after upgrading and iso-paraffin and aromatics with a 6.68 wt % and 11.02 wt % increase, respectively. Cracking, hydrogen transfer, and aromatization are the three main reactions that occurred during FCC gasoline upgrading desulfurization. 3.3. Conceptual Process for in Situ Sulfur Removal of Naphtha Proposed. According to the reaction behavior of naphtha upgrading desulfurization, a condition including that low temperature, large CTO, and proper long residence time were determined for considerably reducing the sulfur content of naphtha. A new conceptual approach for in situ sulfur removal of gasoline through the FCC unit is developed. The schematic of the conceptual process and the routine FCC process are illustrated in Figure 9. In the conceptual process, the residue FCC feedstock is injected in the main riser and operated at classical reaction conditions. The cracked products flow into a main fractionating column to be cut into cracking gas, naphtha, diesel, cycle oil, and slurry. The naphtha is pumped into an

(8)

C4 H4S + polycyclic aromatic → C4 H8S + coke precursor/coke

(9)

Compared to the state-of-art FCC process with a straight-run feedstock without olefin, the real meaning of CHT in reprocessing FCC naphtha is quite different due to plenty of olefin contained in the naphtha. As shown in Figure 7, the CHT

Figure 7. Effect of temperature on hydrogen transfer coefficient (CTO = 5, WHSV = 20 h−1, Cat-1).

coefficient is higher than one unity and showed that the hydrogen transfer reaction was enhanced. The CHT first remained unchanged and then declined remarkably when the temperature was higher than 480 °C. It could be explained as follows. On the one hand, the rate of the cracking reaction is greatly strengthened by increasing the temperature. On the other hand, the hydrogen transfer is thermodynamically restrained at the elevated temperature because of the exothermic reaction. Moreover, the PIONA compositions of gasoline after upgrading could also indicate the degree of hydrogen transfer. Figure 8 presented the group composition of gasoline after cracking desulfurization. The aromatics contents gradually increased, while iso-paraffin contents decreased as the temperature increased. No obvious changes were found in the contents of olefin, naphthene, and n-paraffin as the temperature varied. The content of naphthene changed a little and showed

Figure 9. Schematic of the routine FCC and the conceptual FCC processes. 3206

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The light oil yield was 59.06 wt % compared to 56.82 wt %, while the liquid product yield was almost the same. The sulfur concentrations in gasoline of Cat-1 and Cat-2 were 288 and 426 μg·g−1, respectively, which was reduced about 32.39%. However, the coke formation of Cat-1 was 8.45 wt %, which was much higher than that of the commercial catalyst. The increase of conversion leads to more coke formation and favors reducing the sulfur content of gasoline. As seen from Table 3, the microactivity index of the two catalysts was 69 and 67, respectively. Higher microactivity strengthens the sulfur removing capability by enhancing the cracking of tetrahydrothiophene but significantly increases the undesired coke yield. To improve the product distribution and reduce the unwanted coke, the operation parameters of residue FCC should be optimized. Moreover, the really sulfur removing capability in naphtha of the catalysts should be contrasted at the same feed conversion. As we know, the residence time considerably affects the feed conversion, and the catalytic coke yield is strongly dependent on the reaction time during the FCC process.24 The content of atomization steam was changed to adjust the residence time of hydrocarbon vapors in the riser varied from 2.80 to 1.62 s, while keeping the temperature and CTO constant. Table 5

independent naphtha reactor to reprocess for further reduction sulfur compounds at an optimized condition. The combined reactor contains a riser and a fast bed reactor to provide a long residence time. Moreover, the fluidized bed could increase the catalyst density of the reaction zone, and more catalytic actives are available. A separate disengager and an individual fractionating column are employed to avoiding the mixing with the high sulfur content naphtha from the residue riser. It is necessary to note that a catalyst cooler is added to reduce the temperature of regenerated catalyst, which could increase the CTO at proper low-reaction temperature conditions. In addition, the decrease of the temperature difference between catalyst and naphtha can reduce the amount coke and gas.34 The catalyst regenerated in the regenerator is separated into two parts: one stream flows into the main riser for cracking residue feed, and the other stream enters the naphtha reaction zone. One should point out that the temperature, CTO, and residence time could be adjusted independently in the two reaction zones. The heavy oil cracking and naphtha upgrading process employ a common catalyst, which was specially designed with a high desulfurization activity. In addition, aiming to maximize the reduction of sulfur compounds of FCC naphtha, the heavy catalytic cracking reaction zone and naphtha upgrading reaction zone should be optimized separately. Moreover, the synergistic effect of the two reaction zones should also be taken into consideration. 3.4. Simulated Experimental of the Conceptual Process. 3.4.1. Residue FCC Process. As a synergistic process, the conceptual approach combined the residue catalytic cracking and naphtha desulfurization upgrading. The performance of heavy oil FCC does not only effect the product distribution but also influences the properties of naphtha and sulfur contents. To elucidate the naphtha sulfur reduction ability and reaction behavior during the residue catalytic cracking process over a special desulfurization catalyst, two catalysts Cat-1 and Cat-2 were compared in TPSR apparatus at the conditions of reaction temperature 500 °C, residence time 2.80 s, and CTO 6.0. The product distributions are illustrated in Table 4. It can be clearly seen that Cat-1 has a remarkable ability to crack heavy hydrocarbon to light oil compared to Cat-2, with conversions of 84.19 and 78.36 wt %, respectively. The former yielded 47.22 wt % of gasoline and increased nearly 6% than that of the latter.

Table 5. Effect of Residence Time on Product Distribution of the Residue FCC Process (T = 500 °C, CTO = 6.0, Cat-1) residence time, (s) production distribution (wt %) dry gas LPG gasoline diesel slurry coke conversion (wt %) light oil yield (wt %) liquid products yield (wt %) sulfur content in gasoline (μg·g−1)

catalyst

Cat-1

Cat-2

500 5.97 2.80

500 5.93 2.80

2.11 26.41 47.22 11.83 3.98 8.45 84.19 59.05 85.46 288

2.03 28.68 41.35 15.47 6.17 6.30 78.36 56.82 85.50 426

2.29

1.80

1.62

2.11 26.41 47.22 11.83 3.98 8.45 84.19 59.05 85.46 288

2.01 26.26 46.10 12.57 4.82 8.24 82.61 58.67 84.93 380

1.79 24.91 45.73 14.03 6.07 7.47 79.90 59.76 84.67 428

1.65 24.34 43.74 14.69 9.04 6.54 76.27 58.43 82.77 371

demonstrates the product distributions according to the reaction time. Gasoline and LPG yields decreased slowly with the residence time reduction from 2.80 to 1.62 s, while the diesel and slurry gradually increased. The undesired coke formation reduced sharply. The coke yield was 6.54 wt % when the residence time was 1.62 s, which is similar with that of the state-of-the-art catalyst at the approximate feed conversion. This is in agreement with previous research results.41 The dry gas production changed slightly over the varying range of residence times. The sulfur compounds went into the gasoline fraction and first increased from 288 to 428 μg·g−1 with the reaction time shortening from 2.80 to 1.80 s. After that, the sulfur content decreased to 371 μg·g−1 when the residence time further shortened to 1.62 s. This could be explained as follows: When the residence time shortened from 2.80 to 1.80 s, the sulfur concentration in the naphtha fraction increased because the hydrogen transfer reaction was suppressed, which affected the sulfur reduction of FCC naphtha to form H2S. Tang and coworkers42 found that the conversion depth of the feedstock determined the sulfur distribution in products during FCC units, and higher conversion lead to more sulfur converted into H2 S. However, when the time further shortened, the

Table 4. Effect of Catalysts on Products Distribution of the Residue FCC Process

reaction temperature (°C) CTO residence time (s) production distribution (wt %) dry gas LPG gasoline diesel slurry coke conversion (wt %) light oil yield (wt %) liquid products yield (wt %) sulfur content in gasoline (μg·g−1)

2.80

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conversion of heavy hydrocarbon decreased sharply and the cracking of long chain aromatic sulfur molecules leading sulfur ending up in the gasoline was also restrained.43 Sulfur impurities ending up in diesel increased simultaneously. The effect of residence time on hydrocarbon group composition in FCC naphtha was also analyzed in Figure 10

Table 6. Products Distribution of Gasoline Reprocessing Process in TPSR Apparatus (T = 450 °C, CTO = 5.0, τ = 3.0 s, Cat-1) item production distribution (wt %) dry gas LPG gasoline diesel coke conversion (g) (wt %) light oil yield (wt %) liquid products yield (wt %) sulfur content in gasoline (μg·g−1) sulfur reduction (%)

data 0.76 6.29 87.86 3.07 2.02 9.07 90.93 97.22 139 50.53

the upgrading process, with a 50.53 wt % reduction compared to untreated feedstock, which was lower than China National III regulations of commercial gasoline. The gasoline yield was 87.86 wt %, and about 3.07 wt % diesel was formed by dimerization of olefin molecules or the alkylation reaction. The product yield was similar with that of CFB experimental results. It should be noted that the coke formation was 2.02 wt %, less than that of the CFB reactor with a coke yield of 2.70 wt %. Interestingly, the hydrogen formation of the TPSR experimental result is higher than that of CFB results in our study. This may be attributed to the difference of the reactor type and the residence time of the hydrocarbon over the catalyst.44 It should be noted that it appears there is a lack of economics for the refinery if about 2.78 wt % of gasoline is converted into dry gas and coke when the sulfur content decreases to 50.53 wt %. For some refineries, especially in China, the FCC naphtha takes up 85−90 wt % of the total gasoline pool. The highoctane number gasoline such as alkylates and reformates only takes up a small percentage. The loss of the research octane number (RON) is quite large if the high olefin content naphtha directs desulfurization by a conventional hydrorefining process. Upgrading the naphtha for desulfurization in FCC units not only decrease the sulfur content but also converts olefin components into aromatics or iso-paraffin, which improves the RON of the gasoline. The naphtha after reprocessing has a low olefin content, in which the RON loss during the hydrodesulfurization process is quite small. Therefore, the in situ sulfur removal of gasoline from the FCC process has been attractive for these kind of refineries. To elucidate the sulfur removal mechanism during the naphtha reprocessing, the contents of individual sulfur compounds before and after upgrading were analyzed in Figure 11. Naphtha derived from residue FCC contained thiols, thiophene, alkyl thiophene, and tetrahydrothiophene. Benzothiophene and its alkyl derivatives were not detected because of the lower distillation temperature of gasoline feedstock. The content order of group composition was as follows: C1 thiophene > C2 thiophene > thiophene. After the upgrading process, the thiols concentration reduced from 38.77 to 6.62 μg·g−1, with an 82.92% reduction. The reduction percentage of C3 thiophene was 81.21 wt %, which was almost equal to that of thiols, indicating that thiols and long chain alkyl thiophenic molecules are easy to be removed from naphtha during the reprocessing process. The conversion of C1 thiophene and C2 thiophene was 76.40% and 76.98%, respectively. The content of thiophene compounds were 71.86 and 33.64 μg·g−1 before and

Figure 10. Effect of residence time on gasoline hydrocarbon group composition in the residue FCC process (T = 500 °C, CTO = 6.0, Cat-1).

to discuss the hydrogen transfer behavior. As shown in Figure 10, the increasing residence time strengthened the hydrogen transfer reaction, while the amount of iso-paraffin and aromatics soared steadily and the content of olefin declined sharply. This proved that the appropriate hydrogen transfer condition also promotes thiophene derivates decomposition under a heavy oil cracking process. As mentioned above, naphtha reprocessing will produce unwanted coke and gas and reduce the yield of gasoline. To improve the product distribution, the heavy catalytic cracking process should improve the yield of gasoline at a higher conversion. At optimized conditions of temperature 495 °C, CTO 8.0, and residence time 1.80 s, the conversion and gasoline yield were 81.49 and 46.31 wt %, respectively, which were 3.13 wt % and 3.82 wt % higher than that of the routine FCC process. Large CTO and short residence time are favorable to promote the naphtha yield during the heavy oil catalytic cracking. 3.4.2. Reprocess FCC Naphtha in TPSR Apparatus. Overall, in the consideration of the product distribution of the residue FCC process, a relatively short residence and a large CTO were determined as the optimum operation conditions for residue catalytic cracking of the conceptual process. Compared to the conventional FCC process using a commercial catalyst, the conceptual process has a higher conversion and naphtha selectivity. Larger gasoline production could improve the product distributions of the conceptual FCC process. The liquid product of the optimization conditions was fractionated into gasoline, diesel, and slurry by a small distillation apparatus. The naphtha derived from the residue FCC process was used as a feedstock for the upgrading process in the TPSR apparatus. The upgrading experiment was conducted over the Cat-1 catalyst at temperature of 450 °C, CTO 5, residence time 3.0 s, with a feedstock injection rate of 2.0 kg·h−1. The product distribution of reprocessing naphtha is illustrated in Table 6. As shown in Table 6, the sulfur compounds ending up in gasoline decreased to 139 μg·g−1 after 3208

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of the total sulfur impurities. As mentioned in Figure 6, sulfur reduction by forming heavy benzothiophene or alkyl benzothiophene through alkylation took up an important position, at least, as much as hydrogen transfer desulfurization during the naphtha reprocessing process was observed. These findings are not consistent with the data reported by Jaimes et al.,20,45 who observed that thiophene mainly converted into coke deposited on the catalyst during thiophene and n-octane feed cracking over the ZSM-5 catalyst. A possible reason was as follows: the pure model compounds such as n-octane rather than real FCC naphtha may change the pathway of sulfur conversion. Moreover, the properties of the catalyst will also play a role. It is necessary to note that these results are different from the traditional alkylation desulfurization of FCC naphtha over a solid acid catalyst,46,47 which usually operated at a low temperature range (130−150 °C). 3.4.3. Comparison of the Conceptual Process and the Routine FCC Process. The conceptual process includes residue catalytic cracking in a main riser reactor and FCC naphtha reprocessing in a combined reactor. The comparison of product distributions of the conceptual process and the routine FCC process is demonstrated in Figure 13. As shown in Figure 13,

Figure 11. Individual sulfur compound distributions before and after upgrading desulfurization over TPSR apparatus (T = 450 °C, CTO = 5.0, τ = 3.0 s, Cat-1).

after desulfurization and the reduction percentage was 53.19%, which was remarkably lower than that of thiols or alkyl thiophene. The results indicated that the thiophene molecule was the most unreactive during the recracking of FCC naphtha. After cracking desulfurization, the percentage of thiophene compounds increased from 18.26 to 31.16% and also provided indirect evidence. The tetrahydrothiophene is an intermediate species formed by the hydrogen transfer reaction and easy to crack into H2S. Moreover, the C1-tetrahydrothiophene molecule was identified in the product and also proved that the sulfur was removed by hydrogen transfer and the cracking pathway. Heavy refractory sulfur such as C4 thiophene and benzothiophene were characterized in product gasoline, showing that the thiophenic compounds formed long chain alkyl thiophene or alkyl benzothiophene into the diesel fraction by alkylation and cyclization reactions. The total sulfur mass balance of naphtha upgrading in TPSR equipment was also calculated in Figure 12. Results showed

Figure 13. Comparison of main products distribution between the conceptual and the routine processes.

the conceptual process had a higher LPG yield and a lower gasoline yield, while the diesel and dry gas changed slightly compared to the routine FCC process. The former exhibited a higher conversion with 80.07 wt %, which was 1.71 wt % larger than that of the latter. The slurry yields were 4.18 and 6.17 wt %. Meanwhile, the coke yields were 8.05 and 6.30 wt %, respectively. The coke formed during the reprocessing naphtha accounted for the reduction of gasoline production. Interestingly, the conceptual process could reduce the sulfur content from 426 to 139 μg·g−1 at the optimizing conditions, with 67.37% reduction compared with the conventional FCC process. It should be noted that the light oil yield and liquid product yield of the two processes were almost identical. Higher conversion and larger LPG yield are the characteristics of the conceptual FCC process for in situ sulfur removal. The properties of gasoline between the novel process and the state-of-the-art FCC process were illustrated in Table 7. The naphtha of the former featured a high content of aromatics and iso-paraffin and a quite low olefin concentration with less than 10 wt %, while the traditional FCC naphtha exhibited high olefin contents. Moreover, the research octane number of the former was higher than that of the latter because of the high octane aromatic components.

Figure 12. The sulfur mass balance during upgrading process over TPSR apparatus (T = 450 °C, CTO = 5.0, τ = 3.0 s, Cat-1).

that about 26.41 wt % of sulfur ended up in gas and coke. Because of analysis limitations, the individual content of H2S and sulfur decomposed to S in coke cannot be determined separately. Interestingly, approximately 31.09 wt % of sulfur converted into heavy sulfur such as benzothiophene or alkyl benzothiophene in the diesel. The sulfur compounds enriched in the diesel fraction were investigated; the diesel only took up 3.07 wt % of total product, but it concentrated about one-third 3209

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Table 7. Properties of Naphtha between the Two Different FCC Processes item density (20 °C) (kg·m−3) sulfur content (μg·g−1) RON PIONA analysis (wt %) paraffin iso-paraffin olefin naphthene aromatic

conceptual

routine

737.7 139 93.4

733.2 426 93.1

5.27 39.80 8.76 5.34 40.83

4.61 30.97 27.75 6.69 29.98

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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4. CONCLUSION The present study developed a conceptual process for in situ sulfur reduction of naphtha through FCC units based on the subsidiary riser FCC process for gasoline reformation. The related experiments were systematically investigated over the CFB reactor and TPSR apparatus. From the experimental results, several important conclusions could be drawn. Taking product distribution and sulfur removal rate into account, the optimized operation condition for naphtha cracking desulfurization was at 450 °C, CTO 5, and WHSV 20 h−1. At this condition, the gasoline yield and sulfur reduction were 90.86 wt % and 61.43 wt %, respectively. The hydrogen transfer reaction was quantitatively analyzed during the cracking process of naphtha. A condition favorable to decrease the refractory sulfur of gasoline, including a higher hydrogen transfer and proper cracking ability, was investigated. The products yield of the conceptual process can be optimized by independently adjusting the residue cracking process and naphtha upgrading process. To improve the product distribution, high conversion, and high gasoline selectivity, operating conditions were adapted during heavy oil cracking in the conceptual process. The simulation experiment was conducted in TPSR apparatus. The sulfur content of the conceptual process was 139 μg·g−1 with a 67.37% reduction compared with the routine FCC process. No considerable differences in light oil yield and liquid product yield were found, except 0.66 wt % gasoline yield reduction. Results showed that undesired coke formation is inevitable during the cracking desulfurization of naphtha. Interestingly, a sulfur decrease was observed by the alkylation reaction and formation of heavy sulfur compounds beyond the gasoline fraction that played a key role during upgrading naphtha for in situ sulfur removal in FCC units.

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ACKNOWLEDGMENTS

The authors acknowledge the financial support provided by the National Natural Science Foundation (Grant 21176252) and the National Science Foundation for Young Scholars of China (Grant 20906103). 3210

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