Hydrocracking of vacuum gas oil-vegetable oil mixtures for biofuels production

Bioresource Technology 100 (2009) 3036–3042

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Hydrocracking of vacuum gas oil-vegetable oil mixtures for biofuels production Stella Bezergianni *, Aggeliki Kalogianni, Iacovos A. Vasalos Chemical Process Engineering Research Institute – CPERI, Center for Research and Technology Hellas – CERTH, 6th klm Harilaou-Thermi Rd., Thermi-Thessaloniki 57001, Greece

a r t i c l e

i n f o

Article history: Received 30 October 2008 Received in revised form 12 January 2009 Accepted 13 January 2009 Available online 23 February 2009 Keywords: Hydrocracking Vegetable oil Biofuels

a b s t r a c t Hydrocracking of vacuum gas oil (VGO) - vegetable oil mixtures is a prominent process for the production of biofuels. In this work both pre-hydrotreated and non-hydrotreated VGO are assessed whether they are suitable fossil components in a VGO-vegetable oil mixture as feed-stocks to a hydrocracking process. This assessment indicates the necessity of a VGO pre-hydrotreated step prior to hydrocracking the VGO-vegetable oil mixture. Moreover, the comparison of two different mixing ratios suggests that higher vegetable oil content favors hydrocracking product yields and qualities. Three commercial catalysts of different activity are utilized in order to identify a range of products that can be produced via a hydrocracking route. Finally, the effect of temperature on hydrocracking VGO-vegetable oil mixtures is studied in terms of conversion and selectivity to diesel, jet/kerosene and naphtha. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction Biofuels are becoming a prominent source of transportation energy, especially since their production process ensures sustainability and economic growth. In Europe vegetable oils are the primary feedstock for biofuels (i.e. biodiesel) production (Panorama of Energy, 2007) and after 2010 they are expected to play an even more significant role in meeting the goal of a 5.75% in the transportation fuels. Currently, vegetable oils are the main feedstock for biodiesel production via the trans-esterification process. However, this biofuels production route has several economic considerations mainly attributed to the price and availability of the main byproduct glycerin. Another drawback is the demand for large biodiesel production units requiring large investments (Knothe et al., 2005). An alternative use of vegetable oils, as feedstock for biofuels production, is the hydroprocessing of vegetable oils, an option that utilizes the existing infrastructure of petroleum refineries (Huber and Corma, 2007; Stumborg et al., 1996). Hydroprocessing incorporates hydrotreating i.e. heteroatom removal such as sulfur and nitrogen and hydrocracking, i.e. saturation and breakage of C–C bonds in order to produce high quality gasoline and diesel molecules (Scherzer and Gruia, 1996). Scientists early realized the potential of hydroprocessing of vegetable oils for the production of biofuels. Gusmão et al. (1989) studied the hydrocracking of soyabean and palm oil for the production of high quality hydrocarbons over a sulfided NiMo/c-Al2O3 catalyst. This team employed a batch reactor at high temperatures 350–400 °C and hydrogen partial pressures (10–200 bar) and identified the optimal conditions that completely converted these two vegetable oils into n-alkanes. da * Corresponding author. Tel.: +30 2310 498315; fax: +30 2310 498380. E-mail address: [email protected] (S. Bezergianni). 0960-8524/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2009.01.018

Roch Filho et al. (1992), after performing similar experiments with unsaturated vegetable oils, showed that hydrocracking reactions produced not only normal alkanes but also cycloalkanes, aromatics and carboxylic acids. The same group investigated further the hydrocracking reactions (decarboxylation, decarbonylation and reduction) that take place and the way these reactions are influenced by the vegetable oil composition and by the reaction temperature and pressure (da Roch Filho et al., 1993). Kubicˇková et al. (2005) studied the deoxygenation reactions of vegetable oils over carbon supported metal catalysts, employing model compounds in a batch reactor. Their results indicated that the production of aromatic compounds is suppressed at higher hydrogen pressures. Hydrotreating of vegetable oils has already commercial application with a process developed by Neste Oil (Neste Oil Corporation, 2007). Hydrotreated vegetable oils have better fuel properties than biodiesel produced via trans-esterification and their use improves engine fuel economy (Huber and Corma, 2007). Huber et al. (2007) have explored hydrotreating of vegetable oils and vegetable oil-heavy vacuum oil (HVO) mixtures at standard hydrotreating conditions (300–450 °C) over conventional hydrotreating catalysts (sulfided NiMo/Al2O3). Their work identified a reaction pathway which results mainly in C15–C18 n-alkanes and iso-alkanes. As far as hydrotreating of vegetable oil-HVO mixtures is concerned, the yields of normal C15–C18 alkanes increase with increasing concentration of vegetable oil, as long as the reaction temperature is lower than 350 °C. However there are some concerns regarding catalyst deactivation due to the presence of water in the catalyst. This paper investigates hydrocracking of vacuum gas oil (VGO) and vegetable oil mixtures and the key parameters affecting product yield and quality. Firstly, the effects of pre-hydrotreatment of VGO as well as the mixing ratio with vegetable oil are considered.

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Secondly, the choice of catalyst is another important parameter, which is studied via the three commercial catalysts employed. Finally the effect of reactor temperature as the predominant operating parameter is also studied. 2. Experimental The CPERI hydroprocessing unit is a small-scale pilot plant unit which is employed for hydrotreating (HDS, HDN) and hydrocracking of various feedstocks, varying from light gas oil to heavy vacuum gas oil. This small-scale pilot plant consists of a feed system, a reactor system and a product separation system, as schematically depicted in Fig. 1. The feed system effectively maintains constant feed quality and H2-to-oil ratio via a liquid feed pump and a gas flow controller. The reactor system consists of a single fixedbed reactor (L = 70 cm, ID = 14.7 mm) with six independent heating zones, which sustain the desired temperature profile within the reactor. The reactor product passes through the product separation system, where it is first cooled via a cooling zone and then flashed via a high pressure low temperature (HPLT) separator. The gas product flow enables the system pressure control via a pneumatic control valve. The liquid product flow is controlled via a separator level control system through a second pneumatic valve right after the HPLT separator. The range of operating parameters is given in Fig. 1. The total liquid product is collected and several analyses can take place in the analytical laboratory of CPERI. The simulated distillation curve is determined via an Agilent 6890N-GC according to the ASTM D-7213 procedure. The density of the total liquid product is measured via an Anton-Paar density/concentration meter DMA 4500 according to ASTM D-1052. The concentration of sulfur and nitrogen is measured via an Antek 5000 system, according to ASTM D5453-93 and ASTM D4629 procedures respectively. Total carbon concentration is measured via a CHN LECO 800 analyzer. Finally, hydrogen is measured via an Oxford Instruments NMR MQA 7020. Once total carbon, hydrogen, sulfur and nitrogen wt% are determined, the oxygen concentration is indirectly determined assuming it’s the only significant element contained in the product. The aforementioned analysis is also performed for the feedstocks. The reaction gases are analyzed offline via a Hewlett–Packard 5890 Series II-GC equipped with two detectors, a Thermal Conductivity Detector (TCD) and a Flame Ionization Detector (FID). The TCD is used for the analysis of H2, CO, CO2, O2, N2 and H2S while the FID is used for CH4 and C2–C6, hydrocarbons. For all experiments, commercial hydrocracking catalysts were employed. The catalysts were pre-sulphided according to each catalyst provider’s recommended procedure. Furthermore, in order to maintain constant catalyst activity, DMDS (Di-Methyl-Di-Sulfide) and TBA (Tetra-Butyl-Amine) were added to achieve a specific sulfur and nitrogen concentration in each feedstock. An experiment was considered complete when the reactions reached steady state,

usually after 5–6 days on stream. This was examined by monitoring the product density daily. Once the product density was stabilized, the individual effects of each experiment were considered stable and the study complete. The product collected during the last day of each study was analyzed in detail, as it represented that particular condition. 3. Results For this study a series of experiments were conducted aiming to identify the effects of different feedstocks as well as of different catalysts. In total three different feedstocks are employed: one bio-based and two fossil based. The first bio-based feedstock is raw, unprocessed sunflower oil collected by the Agriculture department of the University of Thessaly (Greece). The remaining two fossil based feedstocks are vacuum gas oils (VGO). The first is a straight run VGO, and the second is a hydrotreated (HDT) VGO as it is normally done in conventional refineries for heteroatoms removal (mostly S and N). The properties of all 3 feedstocks are given in Table 1. Throughout the studies presented in this paper, three different catalysts were employed. Catalyst A is a mild-hydrocracking catalyst which allows small conversion (<30%) and is normally employed under moderate pressures. Catalyst B is a severe hydrocracking catalyst which maximizes naphtha production. Finally catalyst C is another severe hydrocracking catalyst which maximizes diesel production. All the aforementioned catalysts are commercial hydrocracking catalysts obtained from three different major catalyst manufacturers.

Table 1 Properties of different feedstock bases.

Density (kgr/m3) S (wppm) N (wppm) H (wt%) C (wt%) O (wt%) IBP (°C) 5% (°C) 10% (°C) 20% (°C) 30% (°C) 40% (°C) 50% (°C) 60% (°C) 70% (°C) 80% (°C) 90% (°C) 95% (°C) FBP (°C)

Variable Liquid Flow Gas Flow H2/oil Pressure Reactor Temperatures

0-60ml/h (0-0.183⋅10-6 m3/s) 0-70l/h (0-0.194⋅10-3 m3/s) 0-1600nl/l (0-1600nm3/m3) 0-150 atm (0-15198.75kPa) 25-450oC

Reactor Volume Catalyst Volume

114ml (114⋅10-6 m3) 6.5-50ml (6.5⋅10-6-50⋅10-6 m3)

Raw sunflower oil

Non-HDT VGO

HDT VGO

0.8912 0.4 5.35 11.64 77.43 10.93 382.2 532.2 596.4 598.4 603.0 603.6 604.2 604.4 604.8 605.0 605.4 608.2 638.6

0.8936 23570 1053 11.94 83.67 1.94 204.6 286.0 319.8 362.6 392.6 416.0 435.2 452.4 469.6 488.8 514.0 533.0 574.8

0.8501 166.4 29.04 13.08 85.12 1.79 165.0 242.2 276.0 320.8 354.6 381.2 404.4 425.0 445.0 467.0 495.2 516.8 563.8

Range Gas Product

H2 Oil RX

HPLT

Fig. 1. Simplified schematic diagram of CPERI/CERTH hydroprocessing pilot plant.

Liquid Product

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3.1. Effect of pre-hydrotreatment of VGO on mixtures with vegetable oil

100 Sulphur Oxygen

The first experimental studies of hydrocracking VGO-vegetable oil mixtures were at a fixed temperature 350 °C, which was evaluated as an effective temperature according to previous studies (Huber et al., 2007), and at moderate pressure 1000 psig (6894.7 kPa). These experiments were conducted at constant LHSV = 1.5 h1 and H2/oil = 6000 scfb (1068 nm3/m3), and they employed the mildhydrocracking catalyst A. The vegetable oil employed was raw sunflower oil, the properties of which are given in Table 1. The sunflower oil was mixed separately with each of the two types of VGO (hydrotreated and non-hydrotreated VGO) in order to evaluate the effect of pre-hydrotreatment in the final products. Two different mixing ratios were studied, 70/30 v/v and 90/10 v/v of VGO/ vegetable oil, generating 4 different feedstocks that were studied. The corresponding 4 products were collected and analyzed according to the procedures and analytical equipment described in Section 2. For the four experiments, the concentration of heteroatoms sulphur, nitrogen and oxygen for all four feedstocks and their corresponding products are presented in Table 2. In the same Table the concentration of sulphur, nitrogen and oxygen of pure hydrotreated VGO and its hydrocracked product over the same catalyst A is also compared. These results indicate that the sulphur and nitrogen removal is inhibited in the presence of raw vegetable oil, as the extent of heteroatom removal is lower over hydrotreated VGO/vegetable oil mixtures than over pure hydrotreated VGO. The extent of nitrogen removal in particular, exhibits a very low extent for all feedstocks. Especially when the extent of nitrogen removal of the four feedstock mixtures is compared with the one of the pure hydrotreated VGO (Table 2), then it is clear that nitrogen removal is inhibited in the presence of vegetable oil. The extent of sulphur and oxygen removal is compared in Fig. 2, and particularly the percentage of the sulphur and oxygen contained in the feed which have been removed during hydroprocessing. It is evident that the mixtures of hydrotreated VGO - vegetable oil allow a higher extent of sulphur and oxygen removal compared to the non-hydrotreated VGO - vegetable oil feedstocks. Regarding oxygen removal it is clearily higher for non-hydrotreated VGO vegetable oil mixtures. This is possibly due attributed to the absorption of large sulphur compounds contained in the nonhydrotreated VGO on the catalyst active sites, causing the catalyst to lose part of its activity by blocking them from enabling the oxygen removal. Furthermore for the feedstocks with smaller percentage of vegetable oil (90/10) oxygen is removed at a larger extent, which is expected as they contain smaller amounts of oxygen (contained as triglycerides). This is consistent both for the feedstocks containing hydrotreated VGO and non-hydrotreated VGO. In the case of sulphur removal the results are not as straightforward. For hydrotreated VGO - vegetable oil mixtures, the higher the VGO content the higher the sulphur removal. It should be noted that the sulphur contained in these two feedstocks is artifi-

Heteroatom removal (%)

80 60 40 20 0 -20 HDT-VGO / VegOil [70/30]

HDT-VGO / VegOil [90/10]

nonHDT-VGO / VegOil [70/30]

nonHDT-VGO / VegOil [90/10]

Fig. 2. Heteroatom (sulfur and oxygen) removal% via mild-hydrocracking of two hydrotreated VGO - vegetable oil mixtures (70/30 and 90/10) and two nonhydrotreated VGO - vegetable oil mixtures (70/30 and 90/10).

cially added in the form of DMDS, as the HDT-VGO contains an insignificant amount of sulphur (see Table 1). Regarding the two non-hydrotreated VGO - vegetable oil mixtures, the opposite is observed, i.e. the higher the VGO content the lower the sulphur removal. For these feedstocks no DMDS was used as the nonHDTVGO contained significant amount of sulphur (see Table 1). The results are expected as the nonHDT-VGO contains natural sulphur compounds such as thiophenes, di-benzothiophenes etc, which are large molecules that need the particular action of a hydrotreating catalyst in order to be removed. Moreover, these sulphur containing molecules have also to compete with the vegetable oil molecules which make the process even more challenging. The negative sulphur removal percentage for nonHDT VGO - vegetable oil 90/10 mixture is not necessarily an erroneous analysis result, since there is almost no sulphur removal and during hydrocracking some of the feedstock material is removed as gases (CH4, C2–C6, CO, CO2 etc) and therefore the product weight is lower than the feedstock weight. Finally, the extent of the sulphur removal for feedstocks containing HDT-VGO is significantly higher than for feedstocks containing nonHDT-VGO, since the sulphur contained in the first two feedstocks is artificially added DMDS which can easily react and be removed as H2S. The effectiveness of hydrocracking reactions is also measured with conversion (%) which is defined as the percentage of the heavy fraction of feed which has been converted to lighter products during hydrocracking. Conversion is calculated using the distillation curve data of the feed and product and is defined by the following equation:

Conversionð%Þ ¼

Feed360þ  Product360þ  100 Feed360þ

ð1Þ

where Feed360+ and Product360+ are the wt% of the feed and product respectively which have a boiling point higher than 360 °C.

Table 2 Heteroatom (sulfur, nitrogen and oxygen) removal% via mild-hydrocracking of two hydrotreated VGO - vegetable oil mixtures (70/30 and 90/10), two non-hydrotreated VGO vegetable oil mixtures (70/30 and 90/10) and of pure hydrotreated VGO.

Feed

Product

a

S (wppm) N (wppm) O (wt%) S (wppm) N (wppm) O (wt%)

HDT-VGOa (70/30)

HDT-VGOa (90/10)

nonHDT-VGO (70/30)

nonHDT-VGO (90/10)

HDT-VGOa (100%)

16910 532 4.010 2180 614 1.904

18600 662 1.329 1027 649 0.384

16030 793 3.545 14910 839 2.518

19810 999 1.951 20700 1028 0.680

25390 739 NA 101.9 12.81 NA

The hydrotreated feedstocks contain DMDS and TBA.

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20 18 Conversion (%)

16 14 12 10 8 6 4 2 0

HDT-VGO / VegOil [70/30]

HDT-VGO / VegOil [90/10]

nonHDT-VGO / VegOil [70/30]

nonHDT-VGO / VegOil [90/10]

Reactor Temperature (deg C) Fig. 3. Conversion comparison of mild-hydrocracking of two hydrotreated VGO vegetable oil mixtures (70/30 and 90/10) and two non-hydrotreated VGO vegetable oil mixtures (70/30 and 90/10).

For the four products the conversion is given in Fig. 3. Even though the catalyst employed for this study was a mild hydrocracking catalyst and the conversions were not significant, it is clear that the larger the vegetable oil percentage in the feed, the higher the conversion, which is in agreement with literature (Huber et al., 2007). Furthermore, in the presence of mild hydrocracking catalyst, the conversion of hydrotreated VGO - vegetable oil mixtures is higher than the conversion of non-hydrotreated VGO - vegetable oil mixtures, indicating the necessity of a pre-treatment step of VGO prior to the co-hydroprocessing with vegetable oil. 3.2. Effect of catalyst The choice of catalyst is a key parameter in defining the effectiveness of hydrocracking as well as the yields of different products that can be produced (naphtha, kerosene/jet, diesel etc). In comparing different catalysts one should perform identical experiments as much as possible i.e. utilize same feedstock and maintain constant operating conditions (mainly reactor temperature and pressure, LHSV and H2/oil ratio). The catalyst effectiveness is quantified with conversion, which was defined in Eq. (1). Furthermore selectivity is another measure, which indicates the catalysts effectiveness in enhancing the production of a desired product instead of the other products. The selectivity of diesel production and naphtha production is defined in the following equations:

Product180360  Feed180360  100 Feed360þ  Product360þ Product170270  Feed170270  100 Kero=Jet selectivity yð%Þ ¼ Feed360þ  Product360þ Product40200  Feed40200  100 Naphtha selectivity yð%Þ ¼ Feed360þ  Product360þ Diesel selectivity yð%Þ ¼

ð2Þ ð3Þ ð4Þ

where Feed360+ and Product360+ are the wt% of the feed and product respectively which have a boiling point higher than 360 °C, Feed180360 and Product180360 are the wt% of the feed and product, respectively which have a boiling point between 180–360 °C (diesel molecules), Feed170270 and Product170270 are the wt% of the feed and product, respectively which have a boiling point between 170–270 °C (kerosene/jet molecules) and Feed40200 and Product82200 are the wt% of the feed and product, respectively which have a boiling point between 40–200 °C (naphtha molecules). In order to examine the effect of the catalyst choice on the effectiveness of hydrocracking, all three catalysts were utilized in

various experiments. For these series of experiments a single type of feedstock was employed consisting of hydrotreated VGO and raw vegetable oil at 70/30 v/v mixing ratio. Moreover, all these experiments were conducted at reactor temperature T = 350 °C, LHSV = 1.5 h1 and H2/oil = 6000 scfb (1068 nm3/m3). However, two different reactor pressures were utilized. As catalyst A normally requires moderate pressures, it was compared with catalyst B via two experiments (one for each catalyst) at 1000 psig (6894.7 kPa) and at the aforementioned operating conditions. Catalysts B (max naphtha) and C (max diesel) were compared via two additional experiments, which were conducted at 2000 psig (13789.5 kPa). In Table 3 the product quality analysis of the four experiments is summarized. The first comparison among catalysts A and B was performed at a moderate pressure (1000 psig), according to the mild-hydrocracking catalyst A operating range. The comparison of conversion as well as diesel, kerosene/jet and naphtha selectivities is shown in Fig. 5. In terms of conversion, catalyst B is a far superior catalyst as it can convert 58% of the heavy molecules contained in the feed into lighter and more useful molecules, compared with the 17% that catalyst A offers. This is of course expected as any severe hydrocracking catalyst would give higher conversion rates over mild-hydrocracking catalysts. However, when diesel selectivity is considered as defined in Eq. (2), catalyst A appears a more attractive option over catalyst B, as over 100% of the heavy molecules of the feed are converted into diesel molecules. The opposite is inferred by comparing kerosene/jet and naphtha selectivies of the two catalysts, which shows that even at moderate pressure catalyst B can still offer significant kerosene/jet as well as naphtha yields. Catalyst A exhibits negative naphtha selectivity which implies that naphtha molecules in the product are less than the ones in the feed, possibly due to cracking of the feed naphtha molecules to lighter ones and also due to minimum if no production of new naphtha molecules. Therefore, if moderate pressure should be employed, catalyst A would offer higher diesel yields than catalyst B, but catalyst B would offer a significant amount of kerosene/jet and naphtha. The second comparison was conducted between catalysts B and C, based on experiments performed under high pressure (2000 psig). The comparison of conversion as well as diesel and naphtha selectivities is shown in Fig. 6. Catalyst B shows a higher conversion (64%) than catalyst C (37.5%) indicating its superiority of converting heavy feed molecules into lighter. Moreover, catalyst

Table 3 Quality comparison of hydrocracking products of catalysts A, B and C from experiments performed at T = 350 °C, LHSV = 1.5 h1, H2/oil = 6000 scfb (1068 nm3/ m3) and [1] 1000 psig (6894.7 kPa) or [2] 2000 psig (13789.5 kPa).

Density (kgr/m3) S (wppm) N (wppm) H (wt%) C (wt%) O (wt%) IBP (°C) 5% (°C) 10% (°C) 20% (°C) 30% (°C) 40% (°C) 50% (°C) 60% (°C) 70% (°C) 80% (°C) 90% (°C) 95% (°C) FBP (°C)

Catalyst A [1]

Catalyst B [1]

Catalyst B [2]

Catalyst C [2]

0.8421 2180 613.7 13.12 84.7 1.90 158.8 249.8 287 315.6 343.6 368.6 381 410.8 437.8 468.2 503 528.8 619.6

0.7962 1034.4 43.4 13.89 85.73 0.28 100.6 143.6 177 224.2 273.6 304 308.2 321.4 368.2 417.4 464.6 494.2 565.4

0.7692 688.1 5.8 14.33 84.95 0.65 73.2 111.4 135.6 177.4 217.8 271.4 304.4 317.6 343.2 402 455.4 486.4 550

0.8183 617.6 2.47 13.79 84.99 1.16 150.6 218.4 267.4 303.4 311.6 319.6 356.2 392 422.6 451.2 481.8 507 561.4

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700 A-1000psi B-1000psi B-2000psi C-2000psi

600

BP (deg C)

500

400

300

200

100

0 0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Fig. 4. Simulated distillation of hydrocracking products of catalysts A, B and C from experiments performed at T = 350 °C, LHSV = 1.5 h1, H2/oil = 6000 scfb (1068 nm3/m3) and P = 1000psig (6894.7 kPa) and/or 2000 psig (13789.5 kPa).

120 Catalyst A Catalyst B

Conversion / Selectivities (%)

100

80

60

40

20

0 Conversion -20

Diesel Selectivity

Kero/Jet Selectivity

Naphtha Selectivity

Fig. 5. Comparison of catalysts A and B during hydrocracking experiments of VGO vegetable oil 70/30 at T = 350 °C, P = 1000 psig (6894.7kPa), LHSV = 1.5 h1 and H2/oil = 6000 scfb (1068 nm3/m3).

B appeared superior over catalyst C in terms of kerosene/jet and naphtha selectivities indicating that over 50% of the heavy molecules that were converted were transformed into naphtha mole100 Catalyst B

Conversion / Selectivities (%)

90

Catalyst C

80 70 60 50 40 30 20 10 0

Conversion

Diesel Selectivity

Kero/Jet Selectivity

Naphtha Selectivity

Fig. 6. Comparison of catalysts B and C during hydrocracking experiments of VGO vegetable oil 70/30 (v/v) at T = 350 °C, P = 2000 psig (13789.5 kPa), LHSV = 1.5 h1 and H2/oil = 6000 scfb (1068 nm3/m3).

cules. This is of course expected, as catalyst B is a max-naphtha hydrocracking catalyst. Catalyst C was better only regarding diesel selectivity which exceeded 80%, certifying its ability to promote diesel than kerosene/jet or naphtha products. From this study it is evident that according to the catalyst choice, a VGO - vegetable oil mixture may give diesel-type product or gasoline-type product, with significant yields. Investigating further the products of this analysis (Table 3), it is clear that in all cases almost all contained vegetable oil is converted to lighter molecules. Sunflower oil consists of heavy large molecules, 95% of which have a boiling point over 530 °C (Table 1). The distillation analysis of the products (Table 3, Fig. 4) indicates that 95% of all the products boil below 530 °C; therefore almost all vegetable oil is converted. Furthermore, catalyst B appears more effective in reducing oxygen content over the other two catalysts. However, regarding sulfur and nitrogen removal, catalyst C appears more effective over the other two catalysts. 3.3. Effect of reactor temperature Temperature has been identified as a key parameter for catalyst effectiveness and catalyst life. Previous hydrotreating studies (Huber et al., 2007) concluded that 350 °C (662 F) is an optimal temperature which maximizes product yield. In this work a study of product quality and yields was performed on hydrocracking of a VGO - vegetable oil 70/30 v/v mixture at three different temperatures 350°, 370° and 390 °C. The remaining operating conditions were P = 2000 psig (13789.5 kPa), LHSV = 1.5 h1 and H2/ oil = 6000 scfb (1068 nm3/m3). For this study catalyst C was employed. The analysis of the hydrocracking products obtained at the three reactor temperatures as well as of the feed are summarized in Table 4. As expected, product density is decreased with increasing reactor temperature. The feed sulphur and nitrogen, added as DMDS and TBA, respectively, are significantly removed. Moreover, higher temperatures favour hydrogen concentration as well as oxygen content. Nitrogen specifically is systematically decreasing with increasing reactor temperature. However, product sulphur is higher at higher reactor temperatures and thus hydrodesulphurization was studied further. The actual feedstock contains sulphur both in the form of sulphur species (only from HDT-VGO portion) and DMDS. The concentration of the sulphur species contained in the HDT-VGO is given in Table 5, and is compared with the concentration of sulphur species

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S. Bezergianni et al. / Bioresource Technology 100 (2009) 3036–3042 Table 4 Effect of reactor temperature on product quality. All experiments were performed at P = 2000 psig (13789.5 kPa), LHSV = 1.5 h1 and H2/oil = 6000 scfb (1068 nm3/m3), utilizing catalyst C.

Density (kgr/m ) S (wppm) N (wppm) H (wt%) C (wt%) O (wt%) IBP (°C) 5% (°C) 10% (°C) 20% (°C) 30% (°C) 40% (°C) 50% (°C) 60% (°C) 70% (°C) 80% (°C) 90% (°C) 95% (°C) FBP (°C)

Feed

350 °C

370 °C

390°

0.865 21320 569.2 12.50 81.99 3.33 159 253.6 294 345.6 382.6 412.6 439.2 468.8 515.8 604.2 607.6 608.4 619.6

0.8183 617.6 2.47 13.79 84.99 1.16 150.6 218.4 267.4 303.4 311.6 319.6 356.2 392 422.6 451.2 481.8 507 561.4

0.8085 1000.5 2.39 13.99 85.39 0.52 102.8 181.6 214.2 276.6 303.6 315.4 329.8 369 406.2 440 474.2 500.8 558.4

0.7846 1330 0.5 14.28 84.78 0.81 86 129.6 158.6 196.6 235.6 273.8 301.4 310.2 331 376 429.6 464.8 535.8

Table 5 Effect of reactor temperature on product sulfur compounds. All experiments were performed at P = 2000 psig (13789.5 kPa), LHSV = 1.5 h1 and H2/oil = 6000 scfb (1068 nm3/m3), utilizing catalyst C. Name of compound/group

Molecular HDTType VGO

350 °C 370 °C 390°

Alkylthiophenes (wt%) Benzothiophenea (wt%) Monoalkyl-benzothiophenes (wt%) Di-alkyl-benzothiophenes (wt%) Trialkyl-benzothiophenes (wt%) Tetraalkyl-benzothiophenes (wt%) Di-beznothiophenea (wt%) 4-Methyl-dibenzothiophenea (wt%) Monoalkyl-di-benzothiophenesb (wt%) 4,6-Dimethyl-di-benzothiophenea (wt%) Dialkyl-dibenzothiophenesc (wt%) 2,4,6-Trimethyl-dibenzothiophenea (wt%) Trialkyl-dibenzothiophenesd (wt%) Heavy (wt%) Others (wt%) Total (wt%)

C8H12S C8H6S C9H8S C10H10S C11H12S C12H14S C12H8S C13H10S C13H10S C14H12S

0.43 0 0 0 0 0 0 0 0.36 33.37

0 0 0 0 6.87 3.2 0 0 3.02 83.21

0 0 0 0 0 0 0 0 0 89.7

0 0 0 0 0 0 5.19 0 0 83.74

C14H12S C15H14S

5.4 0.72

0 0

0 0

0 0

C15H14S – – –

2.28 55.36 2.09 100

0 0 3.7 100

0 0 10.3 100

0 0 11.07 100

a b c d

Compound. Except 4-methyl-dibenzothiophene. Except 4,6-dimethyl-dibenzothiophene. Except 2,4,6-Trimethyl-dibenzothiophene.

of the products obtained at three different reactor temperatures. Regarding the products it is clear that in all cases over 80 wt% of the sulphur in the final product is attributed to 4,6-DimethylDibenzothiophene. From this Table it is evident not only that the removal of this molecule is inhibited, but also that it is formed during the reactions. The reaction mechanism for the formation of this compound is yet to be determined. Therefore in the case of VGO vegetable oil mixtures, as the hydrocracking catalyst activity increases, the hydrodesulphurization action decreases or becomes less intense, which is in agreement with literature for hydrotreating reactions (Huber et al., 2007). At this point it should be noted that these studies were performed using only hydrocracking catalyst, aiming to study the hydrocracking reactions and all associated secondary reactions such as heteroatom removal. Nevertheless, in actual industrial applications, sulphur removal is achieved by incorporating sec-

90 80

Conversion/Selectivities (%)

3

Conversion Diesel Selectivity Kero/JetSelectivity Naphtha Selectivity

100

70 60 50 40 30 20 10 0

350

355

360

365

370

375

380

385

390

o

Reactor Temperature ( C) Fig. 7. Effect of reactor temperature on conversion, diesel selectivity and naphtha selectivity.

tions of hydrotreating catalyst at the first or last section (bed) of the reactor. Consecutively the remaining sulphur contained in the total liquid product is not a limiting factor for this technology. The effect of temperature on the conversion and selectivities, as calculated from the distillation data (Table 4) is shown in Fig. 7. It is evident that conversion increases as temperature increases. At the highest temperature (390 °C) conversion approaches 70% indicating significant cracking of both VGO and vegetable oil molecules into lighter products. However when diesel selectivity is examined, it is evident that temperature has a negative effect, as it contributes not only to the cracking of the heavier molecules (i.e. with boiling point above 360 °C) but also to the cracking of the diesel molecules into lighter ones. As a result the naphtha selectivity increases with increasing temperature. 4. Conclusions Hydrocracking of VGO - vegetable oil mixtures is a prominent process for the production of hybrid biofuels. This work considers various VGO - sunflower oil mixtures as well as hydrocracking catalysts and temperatures. The results identified the necessity of a VGO pre-treatment step prior to hydrocracking, since straight run (nonHDT) VGO - vegetable oil mixtures exhibited both poor conversion and heteroatom (S and N) removal. Furthermore, poor conversion and heteroatom removal is observed for smaller VGO - vegetable oil ratios, indicating that the increase of vegetable oil portions in the mixture increases hydrocracking activity. Besides the VGO - vegetable oil ratio and VGO type employed, the effect of catalyst type was also studied by comparing three different commercial catalysts: a mild-hydrocracking, a max-naphtha hydrocracking and a max-diesel hydrocracking catalyst. The maxnaphtha catalyst exhibited the highest conversion (65%) as well as the highest naphtha selectivity (50%), as expected. In all cases it was evident that over 95% of the contained vegetable oil was converted into lighter and more useful molecules (naphtha, gasoline and diesel range). Therefore not only diesel but also gasoline can be produced from vegetable oil - VGO mixtures via the hydrocracking route. Finally the reactor temperature is also an important parameter for the hydrocracking product yield and quality. In accordance to literature, conversion increases with increasing reactor temperature, as the hydrocracking catalyst becomes more active. Interestingly, diesel selectivity substantially decreases with increasing reactor temperature, even though the max-diesel catalyst was

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employed for this study, while naphtha selectivity increases, indicating that a portion of diesel molecules are converted into naphtha. Therefore higher reactor temperatures are more attractive if gasoline production is of interest, while moderate reaction temperatures are more suitable if diesel production is more important. Acknowledgements The assistance of personnel from the Laboratory of Environmental Fuels and Hydrocarbons (LEFH) of the Center for Research and Technology Hellas (CERTH), especially Dr. Spyros Voutetakis and Mrs Georgia Tsioni is gratefully acknowledged. The authors also wish to express their appreciation for the financial support provided by the program MOHLOS, funded partially from the European Regional Development Fund by 75% and from the Greek General Government by 25%, in conjunction with the Measure 1.3, Action 1.3.1 ‘‘Development of Research Infrastructure supporting Innovation and Entrepreneurship with the Participation of Users. Development of Infrastructure supporting Innovation and Entrepreneurship in the Public Research Sector MOHLOS” of the Central Macedonia Regional Operational Program (ROP) - 3rd Community Support Program.

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