Feed and process effects on the in situ reduction of sulfur in FCC gasoline

Applied Catalysis A: General 276 (2004) 75–87 www.elsevier.com/locate/apcata

Feed and process effects on the in situ reduction of sulfur in FCC gasoline J.A. Vallaa, A.A. Lappasb,*, I.A. Vasalosa, C.W. Kuehlerc, N.J. Gudded a

Department of Chemical Engineering, University of Thessaloniki, P.O. Box 361, 57001 Thermi, Thessaloniki, Greece b Chemical Process Engineering Research Institute (CPERI), 6th klm Charilaou Thermi, P.O. Box 361, 57001 Thermi, Thessaloniki, Greece c AKZO NOBEL Catalysts LLC, Houston, TX 77058, USA d BP Oil Technology Centre, Sunbury on Thames, Middlesex TW16 7LN, UK Received in revised form 8 June 2004; accepted 26 July 2004 Available online 11 September 2004

Abstract In this study we investigated the effects of various types of fluid catalytic cracking (FCC) feedstocks (VGO, FCC gasoline and FCC gasoline cuts) on sulfur compound distribution in the gasoline produced from FCC process. A bench scale short contact time microactivity test unit (SCT-MAT) and an FCC pilot plant unit were found to be satisfactory for the gasoline sulfur studies. Based on these studies reaction mechanisms are proposed for the formation of sulfur compounds during FCC process. Hydrogen transfer reactions play an important role on gasoline sulfur and thus the cracking temperature affects gasoline sulfur removal. As expected the FCC feed sulfur strongly affects the gasoline sulfur. FCC feeds with higher sulfur give gasoline with higher sulfur concentration. However, the percent feed sulfur ending up in gasoline increases with decreasing sulfur in feed content. The cracking of various FCC gasoline cuts, enriched in specific sulfur compounds, was used to indicate reaction networks through which these compounds are desulfurized in the FCC environment. The most interesting result of this study is the observation that re-cracking the gasoline product in the FCC unit significantly reduces total sulfur. This reduction is mainly due to the cracking and/or cyclization of long chain alkyl-thiophene compounds. # 2004 Elsevier B.V. All rights reserved. Keywords: Sulfur reduction; FCC process; Feed effect on gasoline sulfur

1. Introduction Environmental restrictions in Europe and USA regarding transportation fuels are currently important issues facing the production of traditional fuels from petroleum refineries. The primary goal of the recently proposed regulations is to reduce the sulfur content in gasoline and diesel because sulfur degrades the performance of catalytic converters. For this reason the European refiners are required to meet a 50 ppmw sulfur limit by the year 2005 [1–3]. Beyond that, there are proposals to move to a 10 ppmw sulfur limit. Approximately 90% of the sulfur in the gasoline pool comes from the fluid catalytic cracking unit (FCCU), thus * Corresponding author. Tel.: +30 2310 498305; fax: +30 2310 498380. E-mail address: [email protected] (A.A. Lappas). 0926-860X/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.07.042

FCC gasoline is the main focus of many refineries’ gasoline sulfur treatment efforts [4–6]. The two primary ways to reduce the sulfur in FCC gasoline pool are FCC feed hydrotreating or FCC product post-treatment. Because severe pretreatment of the FCC feed is very expensive, most refiners are investigating options for post-treatment. Key issues are operational cost and minimising gasoline octane loss [1,2]. Recently catalytic advances appear effective for reduction of the gasoline sulfur [1]. There are several studies that have been reported, involving the use of selective additives and catalysts specifically designed for the reduction of sulfur species [4–11]. While this solution should be the most economical and easiest to implement for refinery operations, its effectiveness has not yet been adequate to support widespread application [12]. There are also several novel processes for the reduction of sulfur in FCC gasoline. Most

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of them involve post-treatment of FCC gasoline either by reforming [13], by transforming the sulfur compounds of the gasoline range to higher boiling point products [14] or by selective hydrogenation [15]. In order to proceed with the development of more effective approaches for in situ FCC gasoline sulfur removal, recent studies have been focused in more fundamental issues. While the study of the sulfur distribution in FCC products has been described some years ago [16], renewed interest in FCC sulfur reduction driven by regulatory demands has encouraged a more detailed analysis and a deeper investigation of the parameters that affect the sulfur distribution in FCC products. Typically an FCC non-hydrotreated feed consists of 0.5– 3 wt.% sulfur. Alkylated thiophenes, benzothiophenes and dibenzothiophenes predominate. The aromatic character of the thiophenic ring makes these compounds refractory under the industrial FCC conditions [17]. According to the FCC pilot plant results reported by Collet et al. [18] the majority of the feed sulfur is found in the flue gases as H2S (33–55 wt.%) and in liquid streams, namely gasoline (2–5 wt.%), diesel (18–30 wt.%) and HCO (9–30 wt.%). The sulfur in cracked gasoline distributes roughly as light mercaptans and disulfides (20 wt.%), thiophene and alkyl-thiophenes (50 wt.%) and benzothiophenes (30 wt.%) depending on the feed sulfur. Although hydrotreated feeds contain less sulfur, a proportion ends up in gasoline boiling range [18]. The overall distribution of feed sulfur in cracked products using both FCC pilot plant and MAT units are reported by Ng et al. [19] for three FCC feedstocks with different sulfur content. Their results showed that there is a clear relationship between the sulfur content in feed and in LCO but not for gasoline. Fundamental studies have been conducted to elucidate the mechanism that the specific sulfur compounds follow to decompose in the FCC process, e.g., Corma et al. [12]. In this work it was concluded that the saturated compounds are decomposed mainly to H2S, the short chain alkyl-thiophenes undergo mainly dealkylation and isomerization reactions, while for the long chain alkyl-thiophenes the cyclization/ dehydrogenation reactions are more important. The effect of the presence of highly reactive sulfur species on gasoline sulfur has been studied recently by Beltran et al. [20]. They suggest that thiols and mercaptans could be responsible for the formation of thiophenic species in gasoline. A similar conclusion was reached by Leflaive et al. as well [21]. They studied the reactivities of thiophene derivatives and their possible precursors showing that thiophene and benzothiophene are very stable molecules, while H2S in combination with the presence of olefins in the FCC unit can be responsible for the formation of thiophene derivatives. The objective of the present study is to obtain knowledge on the distribution of sulfur compounds in the gasoline range obtained during the FCC process. This will contribute to a better knowledge on the transformation mechanism of sulfur compounds under FCC conditions and might suggest new approaches for the in situ reduction of FCC gasoline sulfur.

For this reason mechanistic studies were performed, cracking various FCC gasoline cuts enriched in specific sulfur compounds as well as full boiling range FCC gasoline, obtained in either FCC pilot plant or commercial tests.

2. Experimental 2.1. Feeds and catalysts tested 2.1.1. Feed For this study two FCC feedstocks coded as feeds A and B were used. Feed A is a VGO with high sulfur content (S = 2.78 wt.%). Feed B is a blend, obtained by the high sulfur feed (feed A) and a hydtrotreated feed (feed C, S = 0.08 wt.%) in a relative proportion 20/80. The properties of these three feedstocks are given in Table 1. 2.1.2. Catalysts Two equilibrium catalysts coded for the needs of the study as Ecat(B) and Ecat(C), were used. Ecat(B) has high metals content and was provided by AkzoNobel Catalysts, while Ecat(C) was a conventional FCC equilibrium catalyst with low metals content supplied by BP. The catalyst properties are given in Table 2. 2.2. Experimental units For the present study experiments were performed in a short contact time-microactivity test unit (SCT-MAT), and an FCC pilot plant. The SCT-MAT is a fully automated unit where the catalytic reaction was at 560 8C and the run time duration 12 s. The catalyst to oil ratio varied by changing the catalyst amount. The gaseous products were analyzed by a Refinery Gas GC Analyzer (HP-5890), equipped with four columns and two detectors (TCD and FID). Simulated distillation procedures (GC: Varian 3400 with MEGAPORE OV-101 column) were applied for measuring the conversion of liquid products. The hydrocarbons in gasoline range were analyzed by a Capillary Gas Chromatographer (HP 5880 A), while an Elemental Analyzer (LECO CHN-800 model) was used for measuring the weight of coke deposited on the catalyst. Table 1 Feedstock properties Code name

Feed A

Feed B

API (8C) Density (g/cm3, 15.56 8C) Carbon aromatics (NMR) Sulfur (wt.%) Total nitrogen (wt.%) Basic nitrogen (wt.%) Micro carbon residue (wt.%) Molecular weight Refractive index (70 8C) Average boiling point (8C)

18.7 0.94158 21.2 2.78 0.132 0.044 0.6 415 1.5054 455

24.31 0.90735 0.71 0.045 0.059 1.4823

Feed C 26.5 0.87478 10.3 0.08 0.001 0.013 375 1.4734 412

J.A. Valla et al. / Applied Catalysis A: General 276 (2004) 75–87 Table 2 Catalyst properties

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Table 3 Comparison of FCC pp and SCT-MAT results using feed A (S = 2.78 wt.%)

Code names 2

Total surface area (m /g) Zeolite surface area (m2/g) Matrix surface area (m2/g) Z/M ˚) UCS (A Ni (ppmw) V (ppmw)

Ecat(B)

Ecat(C)

121.3 40.7 80.6 0.5 24.36 3030 4100

178.4 58.5 119.9 0.49 24.26 150 367

The FCC pilot plant, operated in Chemical Process Engineering Research Institute (CPERI) [22], was also used in the present study. The unit operates in a fully circulating mode under the following conditions: Liquid feed rate (g/min) Riser temperature (8C) Regenerator temperature (8C) Stripper temperature (8C)

15 Isothermal at 525 700 520

The composition of gases was also determined by GC. The liquid products were collected and weighed on line in a special collection vessel, while the coke on catalyst were analyzed in a LECO CHN analyzer. The total sulfur in gasoline as well as the individual sulfur compounds contained in the total liquid product (TLP) collected from the SCT-MAT tests and FCC pilot plant were identified by a GC (HP 6890 plus) connected to a Sulfur Chemiluminescence Detector-SCD (Sievers model 355). The % product yields, as well as the concentration of the sulfur products in gasoline at constant 65 wt.% conversion were determined by interpolation. The % relative standard deviations of the above SCT-MAT components at 65 wt.% are: gasoline 1.3, coke 6.0, dry gases 9.0, LPG 4.0, total sulfur in gasoline 3.5, thiophene 11.0, C1-thiophenes 5.0, C2-thiophenes 11.0, C3-thiophenes 4.0, C4-thiophenes 7.0, and benzothiophene 9.0.

3. Results and discussion 3.1. Gasoline sulfur distribution in the SCT-MAT unit 3.1.1. Comparison of the SCT-MAT and the FCC pilot plant experimental results Previous studies on catalysts for reducing the sulfur in gasoline [4,5] mainly utilized laboratory units such as SCTMAT. Limited results have been reported concerning FCC pilot plant testing [18,19]. One of the main goals of the present work is to understand the applicability of a laboratory unit for the evaluation of different catalysts on gasoline sulfur reduction capability. For this reason we conducted a comparison of the SCT-MAT and the FCC pilot plant using two different catalysts (Ecat(B) and Ecat(C)) and the feed A. The FCC pilot plant operating in CPERI has been

Conversion (wt.%) Operating conditions Temperature (8C) WHSV (1/h) Cat/Oil Product yields (wt.%) H2 Dry gases LPG Gasoline Coke Gasoline S (ppm) Mercaptans Sulfides Disulfides Unknown Tetrahydrothiophene Thiophene C1-thiophene C2-thiophene C3-thiophene C4-thiophene Benzothiophene

Ecat(B)

Ecat(C)

FCC pp

SCT-MAT FCC pp

65 526 72 12.2 0.29 2.40 11.8 43.3 7.50 1730 19.80 2.50 3.40 44 35 95 260 345 257 188 480

Gasoline S (mg/g S in feed) 26610 Mercaptans 305 Sulfides 38 Disulfides 54 Unknown 698 Tetrahydrothiophene 540 Thiophene 1480 3980 C1-thiophene C2-thiophene 5350 C3-thiophene 3980 C4-thiophene 2800 Benzothiophene 7385

65 560 110 2.85 0.5 2.22 11.62 46.0 5.16 1780 5.1 1.0 0.7 3.0 25 100 325 340 245 160 575 29450 82 30 12 50 413 1654 5377 5626 4054 2647 9514

65 526 70 12.5 0.076 2.25 14.54 43.15 5.06 2510 16 9 1.8 0.0 80 107 350 505 495 365 585 40000 275 155 25 0.0 1275 1800 5900 8500 6600 6200 9800

SCT-MAT 65 560 112 2.72 0.19 2.18 14.07 45.6 3.15 2700 4.5 2 3.75 15 50 120 425 525 410 345 800 44290 75 33 62 246 800 1950 6920 8500 6650 5500 13100

described in literature and it has been proven that simulates the commercial units successfully [23]. In Table 3, at standard conversion 65 wt.% for both catalysts and units the following are reported: (a) the wt.% product yields, (b) the sulfur compound concentration in gasoline and (c) the feed sulfur that ends up in gasoline. With the SCT-MAT, the catalyst with the high metals level Ecat(B) produced more coke and dry gases than Ecat(C) due to metal promoted dehydrogenation reactions. The sulfur concentration in gasoline product of the Ecat(B) was also lower. The positive effects of the presence of metals Ni and V on sulfur in gasoline is well known and have been discussed in several studies [6,24]. The sulfur compounds distribution in gasoline appeared to be the same for both catalysts (Table 4). The concentration of saturated sulfur compounds is the lowest. Thiophenes are in the range of 4– 6 wt.%, while benzothiophenes are in the range of 25– 30 wt.% of the total sulfur in gasoline. The C2-thiophenes constitute the higher percentage of alkyl-thiophenes for both

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Table 4 Sulfur compounds distribution (%) in FCC pp and SCT-MAT at 65 wt.% conversion (feed A, S = 2.78 wt.%) Ecat(B)

Sulfur compounds Mercaptans Sulfides Disulfides Tetrahydrothiophene Thiophene C1-thiophene C2-thiophene C3-thiophene C4-thiophene Benzothiophene

Ecat(C)

FCC pp

SCT-MAT

FCC pp

SCT-MAT

0.1 0.014 0.019 0.20 5.6 15.0 19.9 14.8 10.86 27

0.28 0.056 0.04 1.4 5.62 18.26 19.10 13.76 8.98 32

0.7 0.4 0.1 3.3 4.4 14.5 21.0 20.5 15.1 24.3

0.2 0.1 0.1 1.9 4.4 15.7 19.4 15.2 12.8 29.6

catalysts (19–21%). The difference in the alkyl-thiophenes distribution between the two catalysts is attributed to the effect of the catalytic properties (high UCS, high metals content) on desulfurization [7,25]. For the catalysts with the higher UCS and the high metals level, Ecat(B), dealkylation, hydrogen transfer and dehydrocyclization reactions result to lower alkythiophenes and higher proportion of thiophenes and benzothiophenes. The FCC product yields trends for both catalysts are in accordance with these of the SCT-MAT. Ecat(B) produces more coke and dry gases than Ecat(C). More coke and less gasoline is produced in the FCC pilot plant, relative to the SCT-MAT unit. It must be pointed out that the differences between the two units in terms of sulfur in the produced gasoline are minor. The total sulfur concentration in gasoline produced by the FCC pilot plant is slightly lower than that produced by the SCT-MAT unit. For the Ecat(B) the SCT-MAT unit gives about 6% higher sulfur concentration in gasoline than the FCC pilot plant. The corresponding value for Ecat(C) is 11%. On a feed basis (mg/g S in feed), it appears that more feed sulfur ends up in the SCT-MAT gasoline than in the FCC pilot plant gasoline. The main differences are observed in the C1-thiophene and the benzothiophene. About 15% more C1-thiophene is produced in the SCT-MAT in relation to the FCC pilot plant while for benzothiophene the difference is 25%. Thiophene is also slightly higher in the SCT-MAT unit (about 5%). The differences in the C2- and C3-thiophene between the two units are almost negligible. The FCC pilot plant produces more C4thiophene than the SCT-MAT. It must be noted that not only the trends but also the absolute differences between the two units are similar for both catalysts. The above differences in the individual gasoline sulfur compounds are reflected to the distribution of these compounds in the gasoline. FCC pilot plant gasoline contains about 25–28% benzothiophene, while SCT-MAT gasoline 33–35%. The distribution of the other compounds is similar as indicated in Table 4. The above differences are attributed to the different extent of the secondary hydrogen transfer reactions in the

two units. As we have discussed in a previous study [24] hydrogen transfer reactions favor the reduction of gasoline sulfur probably through the saturation of the aromatic character of thiophenes to produce tetrahydrothiophenes which subsequently crack to H2S and hydrocarbons. These reactions are favored in the FCC pilot plant relative to the SCT-MAT unit. This occurs because the injector design of the FCC pilot plant favors backmixing, while in the SCTMAT unit an annular bed is used that favors plug flow conditions. In order to further investigate the effect of hydrogen transfer and cracking reactions on gasoline sulfur and verify how these reactions affect the gasoline sulfur compounds we performed SCT-MAT experiments increasing the reaction temperature by 20 8C. The results are discussed in the following section. 3.1.2. Effect of SCT-MAT reaction temperature The effect of the cracking temperature variation was investigated in this study by increasing the reaction temperature in the SCT-MAT Unit from 560 to 580 8C. For this purpose the Ecat(C) and the high sulfur feed A were used. It was observed that increasing the reaction temperature reduces the gasoline yield while increasing dry gas and LPG yields. In Fig. 1 the experimental results for the total sulfur content at 560 and 580 8C are indicated. The total sulfur in gasoline was increased almost by 19 wt.% (from 2700 to 3215 ppm) at standard 65 wt.% conversion, when the reaction temperature increased by 20 8C. All the individual sulfur compounds, except from disulfides and tetrahydrothiophenes, tend to increase. These results are in accordance with previously reported FCC pilot plant data [18], where increasing the temperature form 515 to 545 8C resulted to an increase in gasoline sulfur of 16.5%, while increasing the reactor temperature from 485 to 535 8C the sulfur content in FCC gasoline increased by 20%. Increasing reaction temperature strongly influences the relative rates of cracking and hydrogen transfer and as discussed earlier, these reactions play an important role in thiophene reduction [26]. Increasing hydrogen transfer

Fig. 1. The effect of reaction temperature on gasoline sulfur (*) T = 560 8C, (^) T = 580 8C.

J.A. Valla et al. / Applied Catalysis A: General 276 (2004) 75–87

increases sulfur removal. Hydrogen transfer is exothermic and high temperature reduces the reaction. Thus, what we observe when increasing the reaction temperature is an overall increase of all thiophenic compounds. On the other hand, cracking is an endothermic reaction and high temperature promotes the reaction [27]. So, what we observed was an even greater increase of thiophenes and benzothiophenes, compared to the other thiophenic compounds, which may be attributed to the dealkylation of alkylthiophenes and -benzothiophenes, respectively [28]. Moreover, increasing of temperature may cause an increase in the cracking rate of highly reactive sulfur species such as mercaptans and sulfides to produce H2S. This is another possible explanation for the increase in thiophenes, since as it has been reported H2S reacts with olefins to produce thiophenes [21]. 3.2. Feed effects on FCC gasoline sulfur The most important parameter that affects the sulfur in the FCC gasoline is the sulfur in the FCC feed. In an attempt to study how the feed sulfur affects the sulfur in FCC gasoline we performed SCT-MAT experiments using the feeds A and B and the Ecat(B) and Ecat(C). In Figs. 2 and 3 we summarize the feed effects for the catalyst tested at standard conversion 65 wt.%. As expected, for both catalysts the sulfur concentration in gasoline range is lower using the lower sulfur feed (Fig. 1). However, it seems that for the low sulfur feed, the fraction of feed sulfur that ended up in gasoline was higher than with the high sulfur feed (Fig. 2). Hernandez-Beltran et al. [29] also studied the feed effects on the gasoline sulfur using a high and a lower non-hydrotreated sulfur feed. They showed that with the high sulfur feed the percent of feed sulfur ending up in naphtha, is higher than with the lower sulfur feed. Collet et al. [18] studied several feedstocks with different sulfur content regarding their effect on sulfur in the different FCC products. They showed that for some feeds there was a trend of decreasing gasoline sulfur concentration with decreasing the sulfur content in feed while the percent feed sulfur that ends up in gasoline increases.

Fig. 2. The effect of sulfur in feed on gasoline sulfur concentration at standard 65% conversion – (&): Ecat(C), (*): Ecat(B).

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Fig. 3. The effect of sulfur in feed on % sulfur that ends up in gasoline range at standard 65% conversion – (&): Ecat(C), (*): Ecat(B).

The SCT-MAT experimental results of the present study for both catalysts and feeds A and B are reported in detail in Table 3, Table 5, respectively. For both catalysts using the feed B, the % gasoline yield is higher, while coke, dry gases and LPG are lower. Regarding the sulfur concentration in gasoline, when the low sulfur feed was used compared to the result obtained by the high sulfur feed, the results are quite clear. The low sulfur feed, feed B, is a mixture of 20% high Table 5 SCT-MAT experimental results at 65 wt.% conversion using feed B (S = 0.71 wt.%) Ecat(B)

Ecat(C)

Conversion (wt.%) Cat/Oil

65 1.4

65 1.56

Product yields (wt.%) H2 Dry gases LPG Gasoline Coke

0.235 1.05 10.2 52.2 1.55

0.08 1.07 13.1 49.8 1.03

Gasoline S (ppm) Mercaptans Sulfides Disulfides Unknown Tetrahydrothiophene Thiophene C1-thiophene C2-thiophene C3-thiophene C4-thiophene Benzothiophene Gasoline S (mg/g S in feed) Mercaptans Sulfides Disulfides Unknown Tetrahydrothiophene Thiophene C1-thiophene C2-thiophene C3-thiophene C4-thiophene Benzothiophene

475 1.3 0.4 1.5 3 14 24 79 98 60 44 150 34440 90 30 105 215 980 1720 5700 7000 4400 3200 11000

695 2 0.5 1.7 8.0 18 30 105 135 100 90 205 48750 140 35 120 143 1260 2105 7365 9470 7015 6310 14380

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sulfur feed (2.78 wt.% S) and 80% hydrotreated feed (0.08 wt.% S). Consequently, the sulfur concentration in gasoline product is lower. However, the fraction of sulfur that ends up in gasoline range is higher, with the low sulfur feed for both catalysts tested. Therefore with Ecat(C) and the low sulfur feed almost 10% more sulfur ends up in gasoline compared with the same catalyst and the high sulfur feed. Using Ecat(B) their difference is even higher, almost 17%. If we consider the individual sulfur compounds, it seems that all are in somewhat higher proportion on sulfur in feed using the low sulfur feed. Major differences exist for the saturated sulfur compounds, e.g., mercaptans, sulfides, disulfides and tetrahydrothiophene. We suggest that there are three main factors that affect the feed sulfur, which ends up in gasoline product when feedstocks with different sulfur content is used. First, it is generally accepted that hydrotreating the feedstock leads to a lower degree of feed sulfur being converted to H2S, with a higher proportion in the liquid products and coke. This has been also validated from commercial data which showed that the ratio (S in product):(S in feed) is 10–20% lower with a high sulfur feed than that with a lower sulfur feed. This happens because the sulfur species in hydrotreated feeds are more difficult to be removed under FCC conditions, since they are concentrated in the more aromatic and stable molecules. On the other hand, considering that in the present study the low sulfur feed is composed by 80% of a hydrotreated feed meaning a highly saturated environment, we should

expect a decrease of aromatic sulfur compounds due to the enhanced hydrogen transfer reactions. This saturation of the aromatic thiophenic compounds should lead either to the formation of saturated sulfur compounds like mercaptans, sulfides and tetrahydrothiophenes, or to their elimination, e.g., H2S production. What we observed was indeed a very high increase of the proportion of the saturated sulfur compounds in gasoline product, when the low sulfur feed was tested. However we have not observed the expected decrease of the aromatic thiophenic compounds. On first examination, one could conclude that hydrogen transfer reactions are not a strong factor for achieving the hydrogenation of a large part of heavy aromatic sulfur compounds. All the results correspond to constant feed conversion 65 wt.%. In order to achieve this target conversion with the low sulfur feed much lower catalyst to oil ratio was used compared with the high sulfur feed. Comparing the results at catalyst to oil ratio 2.85 the low sulfur feed would achieve 75 wt.% conversion where the results are quite different. In this case the effect of hydrogen transfer reactions is clear. The saturated sulfur compounds are also much higher in proportion with the low sulfur feed than with the high sulfur feed. The short chain alkylthiophenic compounds, e.g., C1- and C2-thiophenes, are almost in the same proportion for both feeds, while the longer chain alkyl-thiophenes like C3- and C4-thiophenes are much lower with the low sulfur feed. It has been reported that the alkylated thiophenes should transfer hydrogen much faster than the non-alkylated, such as thiophenes and benzothio-

Table 6 Sulfur compound concentration and distribution in various gasoline cuts used as FCC feeds

Total sulfur (ppm) Mercaptans Sulfides Disulfides Tetrahydrothiophenes Thiophenes C1-thiophenes C2-thiophenes C3-thiophenes C4-thiophenes Benzothiophenes Unknown Alkyl-benzothiophenes Total Sulfur (mg/g S in feed)  106 Mercaptans Sulfides Disulfides Tetrahydrothiophenes Thiophenes C1-thiophenes C2-thiophenes C3-thiophenes C4-thiophenes Benzothiophenes Unknown Alkyl-benzothiophenes

C5-71 8C

71–110 8C

110–138 8C

138–166 8C

166–199 8C

199–216 8C

C5–216 8C

376 51.9 10.4 – – 307 – – – – – – –

1192 5.4 5.7 – 77 244 861 – – – – – –

2317 112 – – 138 10 704 1352 – – – – –

2517 11 – 23 6.4 3.4 11 1118 1258 29 – – 13

2443 16 – – 11 2.9 27 63 736 1188 290 – 12

6823 – – – – – – – – 854 4210 321 1438

2014 11 0.5 3.8 56 99 307 297 216 274 563 – 175

1.0 0.138 0.0277 – – 0.817 – – – – – – –

1.0 0.0045 0.0048 – 0.064 0.2046 0.7219 – – – – – –

1.0 0.048 – – 0.0597 0.0043 0.3039 0.5836 – – – – –

1.0 0.0043 – 0.009 0.0025 0.0014 0.0044 0.4444 0.4999 0.0117 – – 0.0053

1.0 0.0026 – – 0.0045 0.0018 0.0112 0.0259 0.3013 0.4864 0.1187 0.0 0.0051

1.0 – – – – – – – – 0.064 0.611 0.047 0.2107

1.0 0.0057 0.0003 0.0019 0.0278 0.0492 0.1524 0.1475 0.107 0.136 0.2795 0.0041 0.0869

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3.3. Cracking of FCC gasoline cuts in the SCT-MAT unit – mechanistic studies

gasoline cuts obtained by the CPERI FCC pilot plant, following TBP distillation of the total liquid product. These FCC pilot plant experiments were performed using the high sulfur feed A and the Ecat(B). The sulfur compounds of the gasoline cuts and the full boiling range gasoline are given in Table 6. It is clear that each cut has different distribution of sulfur compounds.

Improvements in gasoline sulfur reduction technology will be facilitated if the chemistry of sulfur compounds under cracking conditions is better elucidated [12,17,21,30]. For this reason, and in order to get an insight into the reaction mechanism of sulfur compounds, model compound studies were carried out. For this purpose, gasoline cuts enriched in specific sulfur compounds were used. The main reason for using gasoline cuts was to maintain similar hydrocarbon partial pressure, to the FCC environment. The mechanistic studies were performed in the SCTMAT unit using as feedstock pure FCC gasoline and FCC

3.3.1. Cracking of C5-71 8C cut The first gasoline fraction is mainly composed of thiophene with low concentration of mercaptans and sulfides. Cracking of this fraction under FCC conditions (560 8C, 1 atm) reduces gasoline by 15.5 wt.%, while sulfur in gasoline fraction is reduced by 20 wt.% at a standard catalyst to oil ratio of 2 (Tables 6 and 7). If we exclude the gasoline dilution effect then the total S in gasoline is reduced by 30 wt.% per g S in feed. Sulfur due to thiophene is reduced by 17 wt.%, while mercaptans and sulfides are completely decomposed. The sulfur in liquid products of

phenes. In addition, thiophene and especially benzothiophenes are 15 and 35% higher relative to the higher sulfur feed. This is an effect of hydroteated feed which consists of heavy and stable sulfur aromatic molecules.

Table 7 Product yields (wt.% on feed), sulfur compounds concentration and distribution (ppmw) in gasoline and LCO produced from cracking various FCC gasoline cuts at standard cat/oil = 2 (SCT-MAT experiments using Ecat(B)) Product yields (wt.%) Dry gases LPG Gasoline LCO HCO Coke Total Sulfur in gasoline (ppm) Mercaptans Sulfides Disulfides Tetrahydrothiophenes Thiophenes C1-thiophenes C2-thiophenes C3-thiophenes C4-thiophenes Benzothiophenes Unknown

Feed C5-71 8C

Feed 71–110 8C

Feed 110–138 8C

Feed 138–166 8C

Feed 166–199 8C

0.62 8.2 85.5 3.25 1.4 0.95

0.64 12.5 81 3.25 1.8 0.83

0.38 13 81.5 2.55 2.1 0.55

0.54 9.4 84 3.2 2.2 0.65

0.73 8.17 81.8 6.5 1.95 0.85

308 – – – 3.7 298 7 2.5 – – – –

Total Sulfur in LCO (ppm) Alkyl-benzothiophenes

1095 1.4 – – 60 262 710 47 15 – – –

1130 4.6 – – 60 37 490 485 55 5 – –

1160 5.1 0.1 – 5.8 44 138 540 370 55 0.3 –

675

280

970 0.9 0.3 0.4 9.8 37 115 150 250 230 180 –

0.9 7.8 64.3 24.7 1.1 1.2 4015 – – – – 32 100 100 70 110 3600 – 6700

Feed C5-216 8C 0.67 8.8 81 2.0 6.3 0.75 1480 1.0 0.0 2.7 30 115 300 240 140 110 530 –

–

–

Total Sulfur in gasoline (mg/g S in feed)  10 Mercaptans Sulfides Disulfides Tetrahydrothiophenes Thiophenes C1-thiophenes C2-thiophenes C3-thiophenes C4-thiophenes Benzothiophenes Unknown

0.70 – – – 0.0084 0.675 0.016 0.06 – – – –

0.74 0.0009 – – 0.0407 0.178 0.48 0.04 0.010 – – –

0.40 0.0017 – – 0.0211 0.013 0.1723 0.1706 0.0193 0.0018 – –

0.39 0.0017 – – 0.0019 0.0147 0.046 0.18 0.123 0.0184 0.0001 –

0.32 0.0003 0.0001 0.0001 0.0033 0.0124 0.0039 0.0502 0.0837 0.0770 0.0603 –

0.38 – – – – 0.0030 0.0094 0.0094 0.0066 0.0104 0.339 –

0.6 – – 0.0011 0.012 0.0463 0.12 0.097 0.056 0.04 0.2192 0.0004

Total Sulfur in LCO (mg/g S in feed)  106 Alkyl-benzothiophenes

0.0

0.0

0.007

0.0035

0.0319

0.2425

0.1265

6

1200

Feed 199–216 8C

12250

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Fig. 4. Transformation of thiophene after cracking (C5-71 8C gasoline cut).

thiophene cracking is presented in Table 7. The results indicate that these products are mainly C1-, C2-thiophenes and tetrahydrothiophene [21,31]. The reaction pathways of thiophene are presented in Fig. 4. The extent of its reaction was calculated taking into account the moles of each sulfur compounds before and after cracking. Regarding this fraction it was considered that initially, the only sulfur compound in the gasoline fraction was thiophene. The extent of thiophene decomposition to H2S and coke was calculated by the difference of the total thiophene reduction and its conversion to the other sulfur compounds. These results indicate that a possible reaction pathway for thiophene under FCC conditions is alkylation. However, the extent of thiophene alkylation reactions is limited. A second major pathway is hydrogen transfer to produce the saturated tetrahydrothiophene ring and finally H2S [26] or coke [21]. It must also be pointed out that according to the literature one possible reaction also is the formation of thiophene by the sulfides or by H2S and olefins or diolefins through the formation of tetrahydrothiophene [12]. So we should not exclude the possibility of thiophenes generation under this environment rich in olefins and diolefins. 3.3.2. Cracking of 71–110 8C cut The second gasoline fraction mainly consists of C1thiophene and thiophene with the balance light mercaptans, sulfides and tetrahydrothiophene. The total sulfur concentration (ppm) in gasoline (Tables 6 and 7) is reduced by 8 wt.%, while gasoline fraction reduction is almost 20 wt.%. Excluding gasoline dilution, total sulfur is reduced by 25 wt.%. In this fraction, sulfur contained in thiophene is

reduced by a lower percentage than in the previous gasoline cut. This observation leads us to consider that C1-thiophene undergoes dealkylation reactions to produce thiophenes [21]. C2- and C3-thiophenes are formed in very small quantities, which indicates alkylation reactions of C1thiophene. The hydrogenation of C1-thiophene through hydrogen transfer reactions (of the very rich in naphthenes fraction) seems to be one of the main reaction routes for the decrease of C1-thiophene. In fact it seems that it is more likely for C1-thiophene to take part to these reactions than thiophene. These results are in line with the findings of Corma et al. [12]. They concluded that the controlling step for cracking thiophene and alkyl-thiophenes is the hydrogen transfer step from a hydride donor molecule. They also expected that 2C1-thiophene, which has the possibility of forming a tertiary carbocation upon protonation, should transfer hydrogen much faster than thiophene. Additionally, it has been reported that methyl-thiophenes have low reactivity, with the main products obtained on fresh FCC catalysts after their cracking being coke [21]. Assuming that thiophene is reduced by 17 wt.% (as to the first fraction) the conversion of C1-thiophene to other sulfur molecules and to H2S and coke can be calculated. The conversion mechanism of C1-thiophene is presented in Fig. 5. 3.3.3. Cracking of 110–138 8C cut This fraction consists mainly of C2-, C1-thiophenes, tetrahydrothiophenes and lighter compounds. The total sulfur in gasoline in this fraction is reduced under FCC conditions by 60 wt.% on sulfur in feed base at standard catalyst to oil 2 (Tables 6 and 7). C2-thiophenes are converted to a very high extent, 71%. The reaction pathways for C2-thiophenes decomposition, is alkylation to C3thiophenes, and C4-thiophenes (much less favored) and dealkylation to thiophenes. Its primary reaction pathway is their conversion to H2S and coke. Unexpected is the higher conversion of C1-thiophene compared to the previous gasoline fraction at standard catalyst to oil. This may indicate recombination reactions between C1- and C2thiophenes and therefore C2-thiophenes are reduced to give

Fig. 5. Transformation of C1-thiophene after cracking (71–110 8C gasoline cut).

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Fig. 6. Transformation of C1- and C2-thiophene after cracking (110–138 8C gasoline cut).

C1-thiophenes and vice versa. This high reduction of C1thiophene could be attributed to the highly naphthenic environment of this gasoline cut. As it was noted previously, the naphthenic compounds are hydrogen donors and promote hydrogen transfer reactions. It is therefore possible that the C1-thiophenes are saturated in a greater extent than in the previous fraction, to tetrahydrothiophenes and consequently to H2S. Furthermore the production of some alkyl-benzothiophenes is not surprising since in this gasoline fraction the alkyl chains of the hydrocarbons are long enough such that high boiling point alkyl-thiophenes produced could be converted to alkyl-benzothiophenes through a cyclization reaction step. The conversion mechanism of this fraction is shown in Fig. 6. 3.3.4. Cracking of 138–166 8C cut The sulfur components in this fraction include: C2-, C3-, C4-thiophenes and traces of C1-benzothiophenes (Table 6). The fact that benzothiophene was not detected, while C1benzothiophenes were detected in traces, does not mean that they are not exist. Possible, benzothiophene exist but their quantity is so minor that cannot be detected. On the other hand, when we refer to C1-benzothiophenes we consider the sum of many peaks of isomers on a sulfur chromatograph at a certain retention time. In this fraction the reaction pathways are very complex. Total sulfur concentration in gasoline is reduced about 54 wt.% but excluding gasoline dilution factor the reduction is 61 wt.% per S in feed for standard cat/oil (Tables 6 and 7). C3-thiophenes conversion is very high, while C2-thiophenes conversion is lower than in the previous fraction establishing that C3-thiophenes are dealkylated to C2-thiophenes. Thiophenes and C1-thiophenes are increased in a high extent too, leading to the above conclusion. Increased C4thiophenes are also produced indicating alkylation reactions of the short chain alkyl-thiophenes. Benzothiophenes and alkylated benzothiophenes appear also in traces. Reviewing the data on cracking the different gasoline fractions reveals that the reactivity of alkyl-thiophenes depends strongly on the degree of alkylation. As we move

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Fig. 7. Transformation of C2- and C3-thiophenes after cracking (138– 166 8C gasoline cut).

to heavier gasoline fractions, therefore to larger sulfur compounds their reactivity is increased and they decomposed to a very high extent [21]. It is obvious from the results that the alkylation and especially the dealkylation reactions are promoted. However, the fact is that these heavy molecules with aromatic character are desulfurized to produce more sulfur in coke and H2S. In Fig. 7 a transformation mechanism of this fraction is proposed. A net sulfur balance across the gasoline fraction indicate that after cracking, 39 wt.% of sulfur in feed remains in gasoline, 0.35 wt.% becomes sulfur in LCO, while 60.65 wt.% becomes H2S and sulfur in coke. 3.3.5. Cracking of 166–199 8C cut This gasoline fraction mainly consists of C4-, C3thiophenes, benzothiophene, and traces of alkylated benzothiophenes. Total sulfur (ppm) in gasoline is reduced by 60 wt.% while excluding gasoline reduction total sulfur is reduced by 68 wt.% per S in feed for standard catalyst to oil 2 (Tables 6 and 7). C4-thiophenes have the higher reduction of all the other sulfur compounds present in the fraction. The reduced conversion of C3-thiophenes and the increased production of C2-, C1-thiophenes and thiophenes reflect the dealkylation reactions that C4-thiophenes undergo. Benzothiophenes are converted by 41 wt.%, through two probable reaction pathways, to other sulfur compounds. The first is the hydrogenation of the aromatic ring and further ring opening to produce alkylated thiophenes, while the second, and more likely, is the alkylation of benzothiophenes to alkylated benzothiophenes. The extended production of the alkylated benzothiophenes is indicative of cyclization reactions. However, we cannot exclude the possibility of forming benzothiophenes and alkylated benzothiophenes via a cyclization of long chain alkyl-thiophenes like C4thiophenes and their dehydrogenation to produce benzothiophenes and alkyl-benzothiophenes [12]. As observed previously the degree of alkylation increases reactivity. Alkylation, dealkylation and cyclization reactions are observed from the results, however the desulfurization reaction producing sulfur in coke and H2S are the predominant reactions. We propose, based on other studies too,

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that the large amounts of coke that alkyl-thiophenes produce reveals that alkyl-thiophenes undergo desulfurization reactions by hydrogen transfer [21]. Alkyl-thiophenes possess at least one tertiary carbon that can produce a carbocation over the catalyst acid surface. This step requires the addition of one proton that could come from coke precursors through hydrogen transfer reactions. Depending on the alkyl substitution, the adsorbed carbocation undergoes either coke formation or successive steps of hydrogen transfer to tetrahydrothiophenes, which are easily crack to produce H2S and light gases [21]. 3.3.6. Cracking of 199–216 8C cut The final fraction consists mainly of benzothiophene (61%), alkylated benzothiophenes (21%) and small quantities of C4-thiophenes (Table 6). Cracking of this fraction leads to 64.3 wt.% gasoline and 24.7 wt.% LCO. Based on sulfur in feed C4-thiophenes are reduced by almost 84 wt.% (Table 7). The dealkylation reactions are obvious from the production of thiophenes C1-, C2- and C3-thiophenes. Surprising is the high reduction of benzothiophene. Although it is not clear from the sulfur concentration, based on sulfur in feed there is a 45 wt.% reduction. In this case benzothiophene can either hydrogenate and be further dealkylated to produce alkyl-thiophenes or be alkylated to produce alkyl-benzothiophenes [12]. The alkylated benzothiophenes are increased by 14 wt%. Since the behavior of alkylated thiophenes did not show a high tendency to produce alkyl-benzothiophenes, we can conclude that an important part of benzothiophenes becomes alkyl-benzothiophenes. A net sulfur balance across this gasoline fraction indicates that 38 wt.% of sulfur in feed ends up in gasoline, 24 wt.% ends up in LCO, while the remaining becomes H2S and sulfur in coke. 3.4. Re-cracking the FCC gasoline In order to observe the differences of cracking each gasoline cut, and therefore the individual sulfur compounds, and the cracking of the same compounds under a real FCC hydrocarbon environment, we performed SCT-MAT experiments using the total gasoline cut as feed. The specific gasoline cut was the blend of the gasoline fractions studied above, as they were obtained by FCC pilot plant experiments using Ecat(B) and the high sulfur feed, feed A. The catalyst used for SCT-MAT experiments was Ecat(B). Although cracking the different gasoline fractions seems to reduce sulfur to a very high extent, when cracking the whole gasoline fraction the results are different. The decomposition of the individual sulfur compounds is lower, perhaps due to the presence of the other sulfur and hydrocarbon compounds. Mercaptans and sulfides almost totally disappear while tetrahydrothiophene is reduced to a large degree. Thiophene reduction in this real gasoline range environment is even lower (6 wt.%) than in the fraction C5-

75 8C (17 wt.%) where it was virtually the only sulfur compound. Moreover benzothiophene’s reduction was quite high via cracking of 199–216 8C fraction (44 wt.%). However when cracking the entire gasoline fraction, benzothiophene was reduced only by 25%. The same results are observed for all the individual sulfur compounds. Their decomposition in the total gasoline cut is much lower than in the individual gasoline cuts. In fact, they appear to be produced in the highly olefinic FCC environment via the reaction of olefins and H2S or light sulfur compounds as sulfides [12,21]. The above results indicate, as other researchers have also proposed [16], that the reactions of some sulfur compounds in the FCC unit are kinetically controlled. The trends are identical in both the gasoline cuts and the full boiling range gasoline. Thiophene is the most difficult thiophenic compound to convert, and the long chain alkyl-thiophenes are reduced to a greater degree than the shorter chain alkyl-thiophenes. The conversion of alkylbenzothiophenes is quite different, since they increased considerably when cracking the total gasoline fraction. That observation leads us to conclude that in the real FCC environment the cyclization/dehydrogenation reactions of long chain alkyl-thiophenes to produce alkyl-benzothiophenes are more pronounced. A net sulfur balance indicates that after cracking, 60 wt.% of sulfur in feed remains in the gasoline product, while 12.6 wt.% ends up as alkyl-benzothiophenes in LCO. The remaining 27.4 wt.% ends up in H2S and S in coke. As was mentioned at the beginning of this section, one of the main objectives of the mechanistic studies was to provide new and novel approaches for reducing the sulfur in gasoline produced by the FCC process. A significant result that came out of this study is that re-cracking the FCC gasoline product in a second step can reduce the gasoline sulfur significantly. Similar approaches have been already reported in patent literature [32]. As it has been proven recracking the whole gasoline product in a second step using the same catalyst as in the first cracking and under the same operating conditions (temperature, pressure) the sulfur can be reduced almost 35 wt.%. Furthermore taking into account the results of re-cracking the individual gasoline cuts, one can assume that it would be even more profitable to re-crack specific gasoline fractions. Thus, it would be of real interest to fractionate the heavy gasoline cut, which contains C3- and C4-thiophenes, and re-crack these reactive compounds. Alternatively, and depending on the objective of a refinery it could be beneficial to re-crack the heavier gasoline cuts, that is, C4-thiophenes and benzothiophenes. As it was shown earlier, the long chain alkyl-thiophenes via re-cracking produce benzothiophenes or/and alkylated benzothiophenes through alkylation and cyclization reactions. Furthermore benzothiophenes are converted to alkylated benzothiophenes too. These alkylated benzothiophenes are heavy sulfur compounds out of the gasoline range.

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Table 8 SCT-MAT results using as feed a commercial light cycle naphtha (Ecat(B), standard cat/oil: 2) Product yields (wt.%) Dry gases LPG Gasoline LCO HCO Coke Total sulfur in gasoline (ppm) Mercaptans Sulfides Disulfides Tetrahydrothiophenes Thiophenes C1-thiophenes C2-thiophenes C3-thiophenes C4-thiophenes Benzothiophenes Alkyl-benzothiophenes Total sulfur in gasoline (mg/g S in feed)  106 Mercaptans Sulfides Disulfides Tetrahydrothiophenes Thiophenes C1-thiophenes C2-thiophenes C3-thiophenes C4-thiophenes Benzothiophenes Unknown Alkyl-benzothiophenes

The combination of cracking the gas oil in the FCC unit using a selective catalyst appropriately designed to produce a low sulfur gasoline in a first stage and re-cracking the gasoline produced or an intermediate or heavier gasoline fraction in a second stage may be proved very promising for industrial application. Furthermore, it is well known that one of the main disadvantages when dealing with desulfurization technologies is their impact on other gasoline specifications. For example, in hydrodesulfurization the sulfur reduction is very high but the gasoline octane number (RON) is reduced. For this reason the applicability of various desulfurization technologies should be evaluated taking into account all requirements for the produced fuels. In order to validate the results obtained by re-cracking the gasoline cuts and to observe the impact of gasoline recracking on the gasoline octane number, two naphtha fractions, a light cracked naphtha (LCN) and a heavy cracked naphtha (HCN) obtained by a Greek Refinery were used as feedstock in SCT-MAT tests. The catalyst used was the base Ecat(B). The results are shown in Tables 8 and 9, respectively, for constant cat/oil. The total sulfur concentration in LCN is 88 ppm and consists of mercaptans to traces of C4-thiophenes and benzothiophenes. After cracking, the gasoline yield is decreased by 13.6 wt.% and sulfur

Light cut naphtha (wt.%)

100

88 19 – – 1.6 11 21 18 13 5.2 1.6 – 1.0 0.212 – – 0.018 0.124 0.233 0.202 0.152 0.059 – – –

Product yields 0.54 7.80 86.4 3.50 0.85 0.87 51 – – – – 10 19 11 7 2.5 – – 0.496 – – – – 0.102 0.187 0.112 0.073 0.025 – – –

concentration is decreased to 51 ppm at standard catalyst to oil. The sulfur reduction in gasoline on sulfur in feed base is almost 50 wt.%. In full accordance with the results described above, tetrahydrothiophenes and saturated sulfur compounds are decomposed very easily, thiophenes appear to have a lower reduction compared with the other thiophenic compounds, while the alkylated thiophenes are decomposed to a higher extent which is increased as the degree of alkylation is increased. C4-thiophenes are decreased 58 wt.%. The HCN consists of 75 wt.% gasoline and 25 wt.% LCO and the total liquid product consists of heavy sulfur compounds in the range of C1- to C2-benzothiophenes. Sulfur in total liquid is 827 ppm. The results before and after cracking are presented in Table 9 and they are in agreement with the corresponding results of the gasoline cuts cracking study. Thiophenes and C1-thiophenes are produced mainly due to dealkylation reactions of the more alkylated thiophenes. C4-thiophenes are reduced to a very high extent and they seem to almost disappear. Benzothiophenes are reduced also. However, another observation that has been already reported by other researchers too [32], was that recracking of the HCN caused an increase in RON yield from 90.6 to 94.8. This increase was caused by an overall increase

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Table 9 SCT-MAT results using as feed a commercial heavy cycle naphtha (Ecat B, Standard cat/oil: 2) Product yields (wt.%) Dry gases LPG Gasoline LCO HCO Coke Total Sulfur in gasoline (ppm) Mercaptans Sulfides Disulfides Tetrahydrothiophenes Thiophenes C1-thiophenes C2-thiophenes C3-thiophenes C4-thiophenes Benzothiophenes Total Sulfur in LCO (ppm)

Heavy cycle naphtha (wt.%)

75 25

388 – – – – – 6 16 26 35 301 2140

Product yields 0.75 7 69.25 20 1.75 1.05 208 – – – – 1.7 4.8 9 8 184 2210

Total Sulfur in gasoline (mg/g S in feed)  106 Mercaptans Sulfides Disulfides Tetrahydrothiophenes Thiophenes C1-thiophenes C2-thiophenes C3-thiophenes C4-thiophenes Benzothiophenes

0.346 – – – – – 0.004 0.014 0.023 0.032 0.273

0.17 – – – – 0.001 0.004 0.008 0.007 0.154

Total Sulfur in LCO (g S/g S in feed)

0.656

0.53

in aromatic compounds. Re-cracking the HCN caused an increase in benzene, toluene, C8 and C9 aromatics, while the olefins decreased.

4. Conclusions The correspondence between the SCT-MAT unit and the FCC pilot plant for prediction of sulfur in gasoline was satisfactory considering the great differences in the reactor configuration and operating principles between the two systems. Compared with the FCC pilot plant, the SCT-MAT unit tends to give higher sulfur in gasoline mainly due to C1thiophenes and benzothiophenes. One of the main conclusions of this study was that the SCT-MAT unit is a reliable tool to evaluate quickly new catalytic materials and to demonstrate new, more effective FCC operating principles for reduction of gasoline sulfur. It is obvious that the higher the sulfur in feed the higher the sulfur concentration in gasoline range. However, we confirmed previous literature results showing that as the sulfur in feed is increased, the fraction of sulfur that ends up in gasoline range is lower. It was suggested that this occurs due to the refractory nature of the sulfur compounds in

hydrotreated feeds to be decomposed, to the saturated hydrocarbon environment of hydrotreated feeds which enforces the hydrogen transfer reactions and the reduced catalyst to oil ratio needed to achieve the desired conversion. The results of the mechanistic studies confirmed that thiophene is a quite stable molecule, its decomposition is difficult, and after cracking produces mainly H2S and S in coke. The short chain alkyl-thiophenes undergo dealkylation and isomerization reactions, although the hydrogen transfer reactions to produce H2S and coke are more pronounced. Moreover, the reactivity of alkyl-thiophenes depends strongly on the degree of their alkylation, so as the degree of alkylation increase, their reduction is also improved. In the case of long chain alkyl-thiophenes, the cyclization/ dehydrogenation reactions to produce benzothiophenes and alkyl-benzothiophenes become significant. Benzothiophene is shown to be more stable than the alkyl-thiophenes. The decomposition of sulfur compounds existing in the gasoline range was also studied under real FCC environment using pure gasoline cut obtained by the FCC pilot plant as well as an LCN and an HCN produced by commercial tests. In this case the results were quite different compared to cracking the gasoline cuts individually, indicating that desulfurization reactions are kinetically controlled. How-

J.A. Valla et al. / Applied Catalysis A: General 276 (2004) 75–87

ever, one of the most significant results of this study was that re-cracking the gasoline or a part of the gasoline can be very effective for sulfur reduction. Octane number is increased, especially when re-cracking the heavy cut naphtha.

Acknowledgements This work was partially funded by the General Secretariat of Research and Technology Hellas (GSRT) under the program AKMON 01.

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