Effectiveness of CoMo and NiMo catalysts on co-hydroprocessing of heavy atmospheric gas oil–waste cooking oil mixtures

Fuel 125 (2014) 129–136

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Effectiveness of CoMo and NiMo catalysts on co-hydroprocessing of heavy atmospheric gas oil–waste cooking oil mixtures Stella Bezergianni ⇑, Athanasios Dimitriadis, Georgios Meletidis Laboratory of Environmental Fuels and Hydrocarbons – LEFH, Chemical Process and Energy Resources Institute - CPERI, Centre for Research and Technology Hellas – CERTH, 6km Charilaou-Thermi, Thessaloniki, Greece

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 HDS/HDN efficiency depends on

catalyst type and lipid content of feedstock.  Diesel selectivity and saturation increases for increasing WCO content.  NiMo catalyst HDS/HDN effectiveness is not limited by WCO.  CoMo catalyst effectiveness is limited by increasing WCO and temperature.  NiMo deactivation is slower than CoMo for WCO containing feedstocks.

a r t i c l e

i n f o

Article history: Received 3 January 2014 Received in revised form 3 February 2014 Accepted 6 February 2014 Available online 19 February 2014 Keywords: Co-processing Catalyst Biofuels Hybrid fuels Hydrotreating

a b s t r a c t Co-hydroprocessing of fossil fractions with lipids is an alternative pathway for integrating biomass in the transportation sector. This work involves the evaluation of two commercial hydrodesulfurization (HDS) catalysts in terms of their effectiveness and suitability for hydroconversion of heavy atmospheric gas oil (HAGO) and waste cooking oil (WCO) mixtures. As the most common catalysts for conventional gas oil hydroprocessing are CoMo and NiMo over Al2O3, this work focused on comparing a CoMo/Al2O3 and a NiMo/Al2O3 catalyst with respect to the resulting diesel selectivity and quality. Both catalysts were investigated for three feedstocks including pure HAGO, a low WCO content (10% v/v) HAGO/WCO and a higher WCO content (30% v/v) HAGO/WCO mixture under three different reactor temperatures (330 °C, 350 °C and 370 °C). All the experiments were performed at constant pressure 812 psig, liquid hourly space velocity (LHSV) 1 h1 and H2/Oil ratio 505.9 nl/l. The results have shown that the catalyst HDS efficiency depends primarily upon the reaction temperature and HAGO to WCO ratio, but is quite different for both catalyst types. The HDS effectiveness of the NiMo catalyst is not affected by the addition of WCO, even in the lowest temperature (330 °C), while the one of the CoMo catalyst is strongly affected by WCO. The presence of WCO in the feedstock was proven favorable for both diesel yield and saturation, for both catalysts, but affected strongly the deactivation rate of the CoMo catalyst. Based on the experimental results obtained via this study, it was evident that NiMo type catalysts are more suitable for co-hydroprocessing of petroleum fractions with lipid containing feedstocks. Ó 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +30 2310 498 315; fax: +30 2310 498 380. E-mail address: [email protected] (S. Bezergianni). http://dx.doi.org/10.1016/j.fuel.2014.02.010 0016-2361/Ó 2014 Elsevier Ltd. All rights reserved.

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1. Introduction In 2010 the worldwide biofuels (mainly ethanol and biodiesel) production reached 105 billion liters accounting for 2.7% of the global road transportation fuels demand, showing a 17% increase since 2009. According to the International Energy Agency, biofuels have the potential to meet more than a quarter of the world’s demand for transportation fuels by 2050 [1]. Based on that expectancy, not only biofuels producers but also oil companies have already started to investigate options for using existing petroleum refineries infrastructure to convert biomass-derived feedstocks into fuels and chemicals including catalytic hydroprocessing and catalytic pyrolysis [2], as widespread technologies in conventional petroleum refineries. Catalytic hydroprocessing is specifically envisioned as a promising conversion process for liquid biomass (vegetable oils, algal oil, residual oils) or bio-based intermediates (pyrolysis oil). Catalytic hydroprocessing refers to the catalytic addition of hydrogen, aiming to increase hydrogen-to-carbon ratio, heteroatom (S, N and O) and metals removal, as well as saturation, while reducing the average molecular weight. Catalytic hydroprocessing of vegetable and waste oils has been extensively applied in pilot and industrial scale for the production of renewable diesel fuels, commonly known as Hydrotreated Vegetable Oils (HVOs) [3]. However, the investment cost of a standalone hydroprocessing unit for vegetable or waste oils is very high, which drove research activity to the alternative of co-processing with petroleum-derived raw materials. This solution allows utilization of existing refinery technology and equipment, rendering it a more economically attractive solution. As it is the case for standalone hydrotreatment [4], catalyst selection is of outmost importance and affects both product yields and quality. The most commonly-used catalysts for standalone lipid hydrotreatment applications are sulphided molybdenum catalysts on alumina supports with nickel or cobalt as a promoter metal [19]. Often these catalysts may contain modifier elements such as phosphorous, boron, fluorine or chloride. The concentration of the metals is usually 1–4 wt% for Ni and Co and 8–16 wt% for Mo [20]. Other types of catalysts such as Rh/Al2O3, Pd/SiO2, Pd/C and Pt/C have also been proposed, aiming to avoid presulfiding which leaves traces of sulfur in the end products [21], which are yet in experimental stage. In the case of co-hydroprocessing of lipids with various petroleum fractions, several studies have recently emerged, utilizing either NiMo or CoMo catalysts, without though comparing the results of the two catalyst types. NiMo-based catalysts have been explored for co-processing studies, both targeting to hydrotreating and hydrocracking reactions. Sankaranarayanan et al. showed that the hydrogenation of vegetable oil with gas–oil mixtures reduced the HDS efficiency compared to the one obtained for pure gas–oil [5], which indicated that oxygen removal takes place on the same active centers of the catalyst employed for desulphurization reactions [6]. Furthermore, Tiwary et al. have used a hydrocracking (sulfide NiAW/SiO2AAlO3) and a hydrotreating catalyst (sulfide NiAMo/Al2O3) to convert waste soya–oil mixtures with refinery-oil into saturated hydrocarbons, showing increased kerosene (140°–250 °C) selectivity for the hydrocracking catalyst and high diesel (250°–380 °C) selectivity [7]. Simacek and Kubicka employed a NiMo catalyst for hydrocracking a pure petroleum vacuum distillate with 5 wt% of rapeseed oil at 400° and 420 °C and under a hydrogen pressure of 18 MPa [8], providing a gas–oil product with similar properties with the one obtained from pure vacuum distillate hydrocracking, including reasonable cold flow properties (cloud point: -23 °C, CFPP: 24 °C). Good efficiency was also observed for rape oil hydroprocessing at 10 or 20 vol.% with light gas oil with NiMo/

Al2O3 (diesel fuel fraction) at 350°–380 °C reaction temperatures and 5 MPa pressures [9]. Similarly, a series of experimental studies was performed with CoMo-type catalysts. Templis et al. observed a decrease in the HDS rate of the gas–oil sulfur-bearing molecules with an up to 5% addition of palm oil was observed, which though leveled out at higher vegetable oil blending ratios (5–10%) [10]. Toth et al. tested light gas oil and sunflower oil mixtures, showing high yields under favorable operating conditions and excellent product properties with the exception of CFPP [11], which they have shown that it can be overcome by low-level additives or by an additional hydro-isomerization step [12]. Co-hydrocracking of heavy vacuum gas oil (HVGO) (both hydrotreated and non-hydrotreated) with vegetable oil, in two different concentrations 90/10 and 70/30 HVGO/vegetable oil, was studied in an authors’ previous work, indicating that higher vegetable oil content favors hydrocracking product yields and qualities [13]. As vegetable oil feedstocks are associated a high carbon footprint and with the food vs. fuel debate, non-edible and residual lipid feedstocks were also explored as alternative bio-based feedstocks for co-processing applications. Furthermore, the authors have explored co-hydrotreating heavy gas–oil (HGO)–waste cooking oil (WCO) mixtures with emphasis on the temperature effect over NiMo/Al2O3 catalyst [14], showing that WCO favors conversion, which verified the results of their previous work [13]. The selection of a hydroprocessing catalyst is a critical step defining the hydrotreating products’ yields and their corresponding quality as well as the expected run-length of the process [15]. Especially in the case of co-processing of two feedstocks, the catalyst selection must accommodate both feedstock characteristics and conversion targets, as well as processing requirements. This work focused on comparing a CoMo/Al2O3 and a NiMo/Al2O3 catalyst with respect to the resulting diesel selectivity and quality, as these two types are the most commonly utilized in commercial hydrotreating applications. In particular, the basis of this study was the catalytic co-hydrotreatment of a heavy refinery fraction, i.e. Heavy atmospheric gas–oil (HAGO), with a residual lipid feedstock, i.e. Waste cooking oil (WCO). Several parameters were considered for evaluating the two catalysts’ effectiveness at different temperatures, including heteroatom removal (sulfur, nitrogen and oxygen), saturation and catalyst decay rate. All the experiments took place in the Centre for Research and Technology Hellas (CERTH) in the Chemical Process and Energy Resources Institute (CPERI).

2. Materials and methods 2.1. Feeds and catalysts For this study, two types of feedstock were used; heavy atmospheric gas oil (HAGO) and waste cooking oil (WCO). HAGO was supplied by the Hellenic Petroleum refinery in Greece, as a lower grade diesel blending component that could be a good candidate for co-hydroprocessing. WCO was collected from local restaurants and households as the residual lipid feedstock to be co-hydroprocessed with HAGO, after food residues were mechanically filtered. The key properties of HAGO and WCO are summarized in Table 1. Regarding the fatty acid composition of WCO, included 7.46% palmitic acid (C16:0), 2.97% stearic acid (C18:0), 33.52% oleic acid (C18:1) and 54.79 linoleic acid (C18:2), as presented in the authors’ previous work [4]. Three HAGO/WCO mixing ratios were studied: (a) pure HAGO (no WCO), (b) 90/10 v/v HAGO/WCO and (c) 70/ 30 v/v HAGO/WCO, none of which contained any additives. The two blends (90/10 and 70/30 v/v) were produced at the Centre

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S. Bezergianni et al. / Fuel 125 (2014) 129–136 Table 1 Basic feedstock properties, including pure HAGO and WCO, as well as HAGO/WCO blends 90/10 and 70/30 v/v. Pure feedstocks

HAGO/WCO blends

Properties

Units

HAGO

WCO

90/10

70/30

Density Sulfur Nitrogen Hydrogen Carbon Oxygen Bromine index

g/ml wppm wppm wt% wt% wt% NA

0.8469 12450.0 473.57 12.64 86.99 0 1200

0.8947 20.3 16.28 11.05 77.00 11.95 49,100

0.8841 11260.0 438.20 13.05 85.68 0.10 1500

0.8937 8340.0 407.19 13.57 82.78 2.77 5600

Distillation IBP 5% 10% 20% 30% 40% 50% 60% 70% 80% 90% 95% FBP

°C °C °C °C °C °C °C °C °C °C °C °C °C

157 251.4 287.6 322.4 345 359.6 372.2 383.6 394.6 406 421 432.8 468.6

449 543 594 605.8 609.8 612.4 614.6 616.4 618.2 619.6 621 625.8 731.4

159.4 250.6 289.2 325.4 347.2 362.8 375.8 388 400.4 415.2 443.6 604.8 610.6

161.6 268.2 303.2 339.2 360 376.6 392.4 408.8 440.2 604.6 610.4 612.4 703.6

for Research and Technology Hellas in Greece, just before being loaded in the experimental apparatus described in the next section. According to Table 1 summarizing the HAGO and WCO properties, both feedstocks are heavy but incorporate different hydrotreating challenges. WCO has a higher density (0.8947 g/ml) compared to HAGO (0.8469 g/ml), both exceeding the maximum diesel specification (0.82–0.845 g/ml) according to EN590. The large molecular size of both feed types is confirmed by their elevated distillation curves (Table 1), particularly that of WCO. As far as the heteroatom content is concerned, WCO has a very low sulfur (20.3 wppm) and nitrogen (16.28 wppm) but high oxygen (11.95 wt%) content in contrast to HAGO (sulfur 12,450 wppm, nitrogen 473.57 wppm, and negligible oxygen). Therefore, HAGO/ WCO hydrotreatment should focus on sulfur and nitrogen removal of HAGO molecules as well as cracking and oxygen removal of WCO molecules. For this study two commercial catalysts were compared, a commercial hydrotreating NiMo/Al2O3 and a commercial hydrotreating CoMo/Al2O3 catalyst, which are currently being employed by the Hellenic Petroleum refinery in Thessaloniki for HAGO hydrotreatment, limiting the possibility to provide detailed information about the consistency and structure of the catalysts. 2.2. Experimental apparatus and analyses A small-scale hydroprocessing pilot plant (VB01) of Centre for Research and Technology Hellas (CERTH) in the Chemical Process and Energy Resources Institute (CPERI) was utilized for this work. This pilot plant consists of a liquid feed system, a gas (hydrogen) feed system, a fixed-bed continuous flow reactor and a high pressure low temperature separation system, as it is described via an authors’ previous study [4]. For the evaluation of the liquid product, daily samples were collected and analyzed in the CPERI/CERTH analytical laboratory. The liquid product primarily contains an organic phase and an immiscible aqueous phase, which can be separated via sedimentation. Several analyses were performed in the organic phase product, as well as in the corresponding feed samples. Density was determined via ASTM D-4052, while the distillation curve was estimated according to the Simulated Distillation ASTM

D2887 procedure. Sulfur concentration was determined by ASTM D5453-93, and nitrogen concentration by ASTM D-4629, for feeds and liquid products. Hydrogen and carbon content were determined via the ASTM D-5291 method, and the bromine number was calculated via ASTM 2719 method. Once total carbon, hydrogen, sulfur, and nitrogen content (wt%) were determined, the oxygen concentration was indirectly calculated assuming negligible concentration of all other elements in the sample. Finally, the gaseous product was analyzed offline by a Gas Chromatograph equipped with a flame ionization detector (FID) and a thermal conductivity detector (TCD). The gases that included in gas analysis are hydrogen, methane, ethane, ethylene, propane, propylene, isobutene, n-butane, 1-butane, isobutylene, trans-2-butene, cis-2butene, isopentane, n-pentane, C5+, C6+, carbon dioxide, carbon monoxide, oxygen, nitrogen and H2S. 2.3. Hydrotreating effectiveness parameters The two catalysts were compared in terms of desulfurization (HDS), denitrogenation (HDN), saturation and catalyst decay rate. One of the main goals of hydrotreating is to remove the undesirable heteroatoms, primarily sulfur, nitrogen and oxygen. Sulfur is removed as hydrogen sulfide (H2S) via hydrodesulfurization reactions (HDS), and nitrogen is removed as ammonia (NH3) via hydrodenitrogenation reactions (HDN). Oxygen, which is introduced when biomass is (co)processes, is removed as water (H2O), carbon monoxide (CO) and carbon dioxide (CO2) via decarboxylation, decarbonylation and deoxygenation reactions. The heteroatom content of all products is given in the comprehensive Tables 2 and 3, which summarize the hydrotreating effectiveness for each catalyst. Based on the feed and product S and N content, the HDS (Sulfurfeed–Sulfurproduct/Sulfurfeed) and HDN (Nitorgenfeed– Nitrogenproduct/Nitrogenfeed) effectiveness is determined. In order to analyze the effectiveness of hydrotreating reactions, conversion and diesel selectivity are evaluated for each test. Conversion represents the percent of heavy molecules (boiling point >360 °C) that have been converted to lighter ones, as defined in an authors’ previous work [4]. Diesel selectivity is based on the boiling point range and is defined by the following equation:

Diesel selectivity ð%Þ ¼

Product180360  Feed180360  100 Feed360þ  Product360þ

where Feed360+ and Product360+ are the weight percent of the feed and product, respectively, which have a boiling point higher than 360 °C, while Feed180–360 and Product180–360 are the weight percent of the feed and product, respectively, which have a boiling point between 180° and 360 °C (diesel molecules). Based on the above definition, diesel selectivity may render values higher than 100% and lower than 0%. The saturation reaction effectiveness can be measured by bromine index variation (%) decrease, as described in the following equation:

Bromine Indexð%Þ ¼

Bromine Indexfeed  Bromine Indexproduct  100 Bromine Indexfeed

where Bromine Indexfeed and Bromine Indexproduct represent bromine index (wppm) in the feed and product, respectively. Finally the catalyst decay rate was also examined in terms of HDS rate. The deactivation study of the NiMo catalyst was combined with two successive experiments evaluating the low (HAGO/WCO 90/10 v/v) and high (HAGO/WCO 70/30 v/v) WCO content feeds, each at three different reaction temperatures (330°, 350° and 370°C). As each condition lasted 4–5 days (time necessary for reactions to reach steady state), each feedstock was

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Table 2 Products characteristics with NiMo catalyst. Pure HAGO

90/10 HAGO/WCO

70/30 HAGO/WCO

Properties

Unit

330 °C

350 °C

370 °C

330 °C

350 °C

370 °C

330 °C

350 °C

370 °C

Density Sulfur Nitrogen Hydrogen Carbon Oxygen HDS HDN Density variation Bromine index

g/ml wppm wppm wt% wt% wt% % % % NA

0.8664 1350.0 199.65 12.350 84.54 2.9550 89.15 57.84 – 267

0.8637 316.7 85.12 13.818 85.95 0.1918 97.45 82.02 – 641

0.8582 94.1 11.29 13.876 84.94 1.1735 99.24 97.61 – 199

0.8565 893.1 126.78 14.066 85.45 0.3800 92.06 71.06 3.12 770

0.8508 366.6 36.21 14.048 85.95 0 96.74 91.73 3.76 444

0.8469 88.8 71.78 14.122 86.96 0 99.21 83.61 4.20 579

0.8404 676.3 113.44 14.200 84.21 1.5100 91.89 72.14 5.96 604

0.8384 143.0 34.97 14.247 85.60 0.1352 98.28 91.41 6.18 440

0.8367 72.6 5.68 14.296 86.07 0 99.12 98.60 6.37 581

Distillation IBP 5% 10% 20% 30% 40% 50% 60% 70% 80% 90% 95% FBP

°C °C °C °C °C °C °C °C °C °C °C °C °C

158.6 244 277 312.8 334 351.4 365 377.2 389 401.6 416.8 429 458.8

155.8 239 273.2 309.6 332 349.6 363.6 376 388 400.8 416.2 428.6 458.6

128.8 219.4 260.2 302.4 326.2 345.8 360.2 373.4 385.8 399 415 427.8 462.4

161.6 248 279.2 304.8 321.2 342.4 358.2 371.4 384.2 397.8 414.2 426.8 457.6

152.2 240.2 273.8 304.4 320.2 342.6 358.8 372.8 386 400 416 429 462.6

140.4 219.6 260.8 301.2 317.6 334.8 354.6 370 383.2 397.8 414.4 427.8 464.2

164.4 255.4 281.4 304.4 310.8 320.6 340 358.8 375.4 391.8 410.4 423.8 460

148.4 246.2 274.4 303.6 307.8 319.6 338.8 358.4 375.4 392 411.6 426.2 474

137.8 226 269.2 301.2 305.6 318.6 334.6 356.6 373.8 391.2 411.4 426.8 481.8

Table 3 Products characteristics with CoMo catalyst. Pure HAGO

90/10 HAGO/WCO

70/30 HAGO/WCO

Properties

Unit

330 °C

350 °C

370 °C

330 °C

350 °C

370 °C

330 °C

350 °C

370 °C

Density Sulfur Nitrogen Hydrogen Carbon Oxygen HDS HDN Density variation Bromine index

g/ml wppm wppm wt% wt% wt% % % % NA

0.8647 624.5 85.02 13.755 86.65 0.0793 95.00 82.04 – 179

0.8554 144.7 4.14 13.950 86.75 0.0251 98.83 99.12 – 441

0.8484 51.1 3.80 13.986 85.42 1.4545 99.58 99.19 – 144

0.8572 1450.0 134.10 13.846 86.66 0 87.12 69.39 3.04 910

0.8489 171.9 9.47 14.008 86.19 0 98.47 97.83 3.98 439

0.8423 57.9 6.81 14.065 86.77 0 99.48 98.44 4.72 703

0.8429 1590.0 161.51 14.120 85.99 0 80.93 60.33 5.68 1040

0.8411 461.7 40.75 14.171 86.53 0 94.46 89.99 5.88 482

0.8324 68.1 6.32 14.266 86.31 0 99.18 98.44 6.86 540

Distillation IBP 5% 10% 20% 30% 40% 50% 60% 70% 80% 90% 95% FBP

°C °C °C °C °C °C °C °C °C °C °C °C °C

155.8 240.4 275.8 311.4 332.8 350.4 364.2 376.2 388 400.8 416.4 428.8 463.4

129 209 251 295 320.4 342 358 371.2 383.8 397.4 413.8 426.4 458

105.2 182 223.8 276.2 307.8 331.6 351 367 381.4 394.6 412.8 425.4 463.8

156.6 243.2 276.8 303.8 319.6 341.2 357.2 370.2 383 396.6 413 426 457.4

121.8 210.8 253.8 296.8 315 330.2 349.6 365.6 380 393.2 411.6 424.4 460.8

81.4 181.6 227.8 279.8 304.4 320 344 361.2 377 392.8 411.2 424.6 469

155.4 253.6 283.4 304.4 314.6 320.6 343.8 361.8 378 393.2 412.6 426.8 469

137.4 240 271.8 303.4 308.2 319.6 341 359.8 376.4 392.4 411.8 425.8 468.2

66.6 190.6 241.6 288.2 305 313 323 348.2 369.4 387.4 408.4 423.4 469.6

evaluated for 12–15 days (evaluation included three temperatures). The catalyst decay rate was evaluated based on the HDS activity dependence on time, which is determined by the days on stream (DOS) that the feedstock is in contact with the catalyst. More specifically the HDS activity was assessed in the beginning of the experiment (DOS = 4) where the catalyst is fresh, in the middle of the experiment (DOS = 12) as the catalyst is experiencing some aging and in the end of the experiment (DOS = 26) where the catalyst aging is the highest, considering in all cases the same feedstock (pure HAGO) and the same operating conditions (T = 350 °C). If the product sulfur is not significantly changing from the initial days of the experiment, the normalized value is close to

one. Exactly the same procedure was also followed to evaluate the CoMo decay rate. 2.4. Experimental procedure The main premise of this study is the experimental comparison of two common hydrotreating refining catalysts (NiMo and CoMo) in order to compare their effectiveness and suitability for coprocessing petroleum and lipid streams. For this experimental study three different reactor temperatures were investigated for two catalyst types and three different feedstocks. The reactor temperature range employed for the experiments was from 330°

S. Bezergianni et al. / Fuel 125 (2014) 129–136

to 370 °C. In order to maintain the desired liquid hourly space velocity (LHSV), a particular catalyst amount is employed, which was diluted with an inert material (silicon carbide) to ensure even dispersion along the reactor bed for enabling good heat and mass transfer, while enabling feed channeling. The presulfiding process for both catalysts was a standard procedure defined by the catalyst manufacturers. Each experiment (condition) was considered complete when the reactions reached steady state, usually after 3–4 days on stream (DOS) (determined by monitoring the product density and sulfur content in the liquid product). The total liquid product collected during the last day of each experiment was analyzed in detail (see Section 2.2), representing that particular condition. The gas product collected on the last day (after steady state was achieved) was chromatographically analyzed offline. The three aforementioned feedstocks were studied separately under three different temperatures 330 °C, 350 °C and 370 °C, while the other parameters were maintained constant (pressure = 812 psig, LHSV = 1 h1 and H2/Oil ratio = 3000 scfb or 505.9 nl/l). 3. Results As the main premise of this study is the suitability of NiMo and CoMo catalyst on co-processing of HAGO/WCO mixtures, the two catalysts were compared in terms of heteroatom removal, saturation and catalyst decay rate. 3.1. Heteroatom removal Hydrodesulfurization (HDS) effectiveness is a primary concern for all hydrotreatment catalysts, as the final product sulfur specifications are becoming particularly strict. Sulfur removal (%) for 100% HAGO, 90/10 HAGO/WCO and 70/30 HAGO/WCO under three reaction temperatures (330°–370 °C) is shown in Fig. 1, as achieved via NiMo(a) and CoMo(b) tested catalysts. Apparently, HDS via both catalysts is more effective at higher temperatures, reaching up to 99.5% in the case of pure HAGO. Nevertheless, the two catalysts show a different dependence on temperature and presence of WCO in the feedstock. In particular, as far as the NiMo catalyst is concerned, sulfur removal shows a stronger dependence on temperature. Catalytic hydrotreatment of pure HAGO achieves sulfur removal from 89.15% at 330 °C and rises up to 99.24% at 370 °C, which is in agreement with the author’s previous study [14]. However, the NiMo effectiveness is not strongly affected by the addition of WCO, even in the lowest temperature (330 °C), where sulfur removal is 89.15%, 92.06% and 91.89% for 0%, 10% and 30% WCO in the feedstock respectively (see Fig. 1). In fact WCO addition appears to have a positive effect in HDS in the case of NiMo catalyst. As WCO is a sulfur free feedstock component, the HDS efficiency could be theoretically expected to be improved in co-processing of WCO with a sulfur containing feedstock such as

(a)

133

HAGO, assuming there are no other competitive reactions or catalyst deactivation due to WCO hydroprocessing by-products (CO, CO2, H2O). The competitive reactions of deoxygenation/decarboxylation/decarbonylation are not limiting HDS as H2 excess was introduced in the feed system, sufficient to accommodate for all heteroatom (S, N, O) removal reactions. Furthermore, as no HDS activity loss is observed with the addition of WCO, it can be assumed that the WCO hydrotreating byproducts do not affect the HDS effectiveness of the NiMo catalyst (see Fig. 1). The HDS profile of the CoMo catalyst is, however, significantly different over that of the NiMo. HAGO sulfur removal ranges from 95.00% to 99.58% considering temperatures between the 330° and 370 °C, and the HDS effectiveness is particularly affected by the WCO addition. In the case of the lowest temperature (330 °C), sulfur removal drops to 87.12% and 80.93% with the addition of 10% and 30% WCO in the feedstock respectively (see Fig. 1), while a significant effectiveness loss is also observed at 350 °C. In the case of HAGO/WCO mixtures, the HDS effectiveness loss indicates that in the case of the CoMo catalyst, the HDS reactions compete with the deoxygenation ones as they occur in the same active sites. Thus the HDS of CoMo catalyst is inhibited by the oxygen contained in the WCO triglycerides, even though WCO containing feedstocks have less sulfur [10]. In general, two possible reaction pathways for sulfur removal have been identified for the most challenging [24] sulfur containing molecules (dibenzothiophenes and alkylated dibenzothiophenes): (a) hydrogenolysis or direct extraction of the sulfur atom from the molecule, and (b) hydrogenation which leads to saturation of aromatics allowing the sulfur atom extraction. CoMo catalysts enable primarily hydrogenolysis, while NiMo catalysts exhibit a higher selectivity for hydrogenation route [23]. As a result, the choice between a CoMo and a NiMo catalyst depends upon the given feedstock. Indicatively, CoMo catalysts generally perform better than NiMo catalysts on HDS of feeds containing cracked stocks [23]. Moreover, CoMo catalysts show higher HDS effectiveness at lower temperatures while NiMo catalysts have a superior performance at higher temperatures for HAGO type feedstocks, which was attributed to the preferred hydrogenolysis (direct extraction) HDS route of the CoMo catalysts over the hydrogenation pathway of the NiMo catalysts which is limited by thermodynamic equilibrium [24]. The above analysis justifies the HDS comparison results for pure HAGO feedstocks. Hydrodenitrogenation (HDN) is also a key reaction that is targeted. Nitrogen removal (%) for 100% HAGO, 90/10 HAGO/WCO and 70/30 HAGO/WCO under three reaction temperatures (330°– 370 °C) for both tested catalysts (CoMo and NiMo) is shown in Fig. 2. The two catalysts show a different dependence on temperature and presence of WCO in the feedstock. In particular, as far as NiMo catalyst is concerned, nitrogen removal shows a strong dependence on temperature. Catalytic hydrotreatment of pure HAGO achieves from 57.84% nitrogen

(b)

Fig. 1. HDS (%) of pure HAGO, 90/10 HAGO/WCO and 70/30 HAGO/WCO from catalytic hydrotreating at 330° (j), 350° () and 370 °C (N) employing (a) NiMo and (b) CoMo catalysts. All experiments were performed at pressure = 812 psig, LHSV = 1 h1, H2/Oil = 505.9 nl/l.

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(a)

(b)

Fig. 2. HDN (%) of HAGO, 90/10 HAGO/WCO and 70/30 HAGO/WCO from catalytic hydrotreating at 330° (j), 350° () and 370 °C (N) employing (a) NiMo and (b) CoMo catalysts. All experiments were performed at pressure = 812 psig, LHSV = 1 h1, H2/Oil = 505.9 nl/l.

removal at 330 °C to 97.61% at 370 °C. Moreover, the NiMo effectiveness is also strongly affected by the addition of WCO, in 330 °C and 350 °C the addition of WCO improved HDN ability of the catalyst; on the other hand at 370 °C a small decrease of 83.6% occurred for 10% of WCO which however, reach 98.6% with 30% WCO. This decrease for the 10% WCO could be attributed to competing reaction, most likely conversion reactions occurring for increasing WCO content feedstocks. It is obvious from Fig. 2, that the addition of WCO appears to have a positive effect in HDN reactions. As WCO is nitrogen free feedstock component, the HDN efficiency could be theoretically expected to be improved in co-processing of WCO with a nitrogen containing feedstock such as HAGO, assuming there are no other competitive reactions or catalyst deactivation due to WCO hydroprocessing by-products (CO, CO2, H2O). The competitive reactions of deoxygenation/decarboxylation/decarbonylation are not limiting HDN as H2 excess was introduced in the feed system, sufficient to accommodate for all heteroatom (S, N, O) removal reactions. Furthermore, as no HDN activity loss is observed for the addition of WCO, it can be assumed that the WCO hydrotreating byproducts do not affect the HDN effectiveness of the NiMo catalyst. In contrast to NiMo, the HDN profile of CoMo catalyst is significantly different. HAGO nitrogen removal ranges from 82.04% to 99.19% between the 330 °C and 370 °C. However, the addition of WCO strongly affected the HDN ability of CoMo catalyst for the lower reactor temperature 330 °C and 350 °C with the exception at 370 °C where the nitrogen removal remains constant at 98%, these results come in agreement with the results of Rasmus et al. [24] In the case of the lowest temperature (330 °C), nitrogen removal drops to 69.39% and 60.33% with the addition of 10% and 30% WCO in the feedstock respectively. Following up the theoretical explanation provided for the NiMo catalyst, the HDN loss of the CoMo catalyst can only be explained by the limiting effect that the WCO hydrotreatment byproducts might have on the catalyst. The addition of WCO in HAGO has a negative effect for the CoMo catalyst, while the NiMo one is unaffected. This was verified by Rasmus et al. [24] who studied how the HDS and HDN activities of CoMo and NiMo type catalysts respond to co-processing of rapeseed oil. Based on their analysis the formation of CO from hydrodeoxygenation reactions inhibits the catalyst effectiveness of CoMo catalysts, indicating that in the case of co-hydroprocessing of lipid-with fossil-based feedstocks NiMo types catalysts are preferred. 3.2. Conversion and diesel selectivity HAGO is a heavy petroleum fraction that consist of large molecules with density at 0.8469 g/ml. Similarly WCO is a heavy feedstock with an even higher density (see Table 1). Besides heteroatom removal, catalytic hydrotreatment enables cracking of the larger molecules into diesel-range molecules (180°–360 °C).

As far as conversion obtained from the NiMo catalyst (Fig. 3a), the pure HAGO feedstock exhibits the lowest conversion (10%), which though increases with the addition of WCO. This favorable effect is expected as the triglycerides contained in WCO are more easily converted into light hydrocarbons than pure HAGO contained moelecules [10,11]. It should be noted that the effect of temperature on the conversion efficiency is not significant as shown in Fig. 3a. The presence of WCO in the feedstock has also a favorable effect for the case of CoMo catalysts as shown in Fig. 3b. A maximum conversion of 55% is obtained for the feedstock containing 30% of WCO, which can be attributed to the faster/easier conversion of triglycerides vs. HAGO molecules [10,11]. In the case of CoMo catalyst, the hydrotreating temperature has a more explicit effect on conversion than that of NiMo catalyst. For pure HAGO, the highest conversion is observed at the highest temperature (370 °C). Nevertheless, the presence of WCO in the feedstock changes the temperature dependence. This can be verified for the feedstock containing 30% of WCO, exhibiting the highest conversion at the lowest temperature (330 °C), which reveals other competing reactions (most probably heteroatom removal) which are favored by temperature. Diesel selectivity is an important parameter indicating the effectiveness of converting a certain feedstock into diesel. Both catalysts exhibit good diesel selectivity, which increases for higher WCO containing feedstocks and decreases with temperature (Fig. 4). In the case of the NiMo catalyst (Fig. 4a), WCO content favors diesel selectivity, as the triglycerides contained in WCO can be easily cracked to their corresponding free fatty acids [4,14], yielding diesel range paraffins (C15AC18) [17,18]. Nevertheless, as temperature increases, diesel selectivity decreases which is due to the increased cracking activity leading to the formation of lighter molecules. As a result, the maximum diesel selectivity is obtained at the lowest hydrotreating temperature (330 °C). Another important observation is that diesel selectivity is relatively stable for WCO containing feedstocks at all temperatures studied. This can be explained as NiMo catalysts exhibit a higher selectivity for hydrogenation route [23], enabling heteroatom removal and not conversion reactions. The different observations for NiMo and CoMo also indicate different acidity of support (c-Al2O3), which though cannot be verified due to the proprietary rights binding the two catalysts. The diesel selectivity profile of the CoMo catalyst is significantly different over the one of NiMo (Fig. 4b). Apparently temperature has a more significant effect on diesel selectivity for both pure HAGO and HAGO–WCO feedstocks, ranging from 75% to 96%. Furthermore, the effect of WCO on diesel selectivity is much more dominant than what it was observed in the case of the NiMo catalyst. This diesel selectivity variability is due to the hydrogenolysis reactions, which are the main focus of CoMo catalysts [23], favoring conversion of larger molecules to diesel-range molecules.

S. Bezergianni et al. / Fuel 125 (2014) 129–136

(a)

135

(b)

Fig. 3. Conversion (%) at 360 °C of pure HAGO, 90/10 HAGO/WCO and 70/30 HAGO/WCO from catalytic hydrotreating at 330° (j), 350° () and 370 °C (N) employing (a) NiMo and (b) CoMo catalysts. All experiments were performed at pressure = 812 psig, LHSV = 1 h1, H2/Oil = 505.9 nl/l.

(a)

(b)

Fig. 4. Diesel selectivity (%) 180°–360 °C of pure HAGO, 90/10 HAGO/WCO and 70/30 HAGO/WCO from catalytic hydrotreating at 330° (j), 350° () and 370 °C (N) employing (a) NiMo and (b) CoMo catalysts. All experiments were performed at pressure = 812 psig, LHSV = 1 h1, H2/Oil = 505.9 nl/l.

3.3. Saturation Saturation of double bonds is another key hydrotreating reaction, especially for highly olefinic feedstocks. In fact, as WCO triglycerides contain unsaturated fatty acids, it is essential to achieve pure saturation to avoid further fuel oxidation and polymerization. The natural triglycerides contain a large number of double bonds, which can induce an oxidative instability of hydrocarbon products. These drawbacks of the bio-derived molecules can be overcome by the saturation of the double bonds using molybdenum catalysts, showing superior performances in hydrogenation as well as hydrodeoxygenation. The variation of bromine index shows the effectiveness of the catalysts on saturation reactions. The bromine index is the fraction of reactive unsaturated compounds (mostly C@C double bonds) in hydrocarbons encountered in the petrochemical industry. The lower the bromine index, the more effective is the catalyst on saturation reactions. As it is indicated in the feedstock properties in Table 1, the WCO addition into HAGO increases the bromine index of the WCO/HAGO feedstocks due to the increased olefinic bonds present in WCO [9]. Based on the bromine index comparison in Fig. 5, both catalysts achieved the best saturation of double bonds at the higher reaction temperature (370 °C).

(a)

More specifically, saturation reactions over the NiMo catalyst indicate that they are not significantly affected by reaction temperature and not limited by WCO. The highest saturation decrease was observed with 30% of WCO which reached 95% for all three temperatures. Similarly for the CoMo catalyst, the increasing olefinic bonds of the triglycerides contained in WCO are effectively saturated, as indicated by the bromine index variation in Fig. 5(b). However the temperature effect was more explicit in the case of CoMo catalyst. Once again, this difference can be attributed to the primary reactions that the two catalysts types focus, i.e. hydrogenation for NiMo and hydrogenolysis for CoMo. 3.4. Catalyst decay rate Besides the product quality provided by co-hydrotreating HAGO/WCO mixtures, the expected catalyst decay rate was also studied, as an important parameter for determining the suitability of hydrotreating catalysts in industry. The catalyst deactivation rate is very important for an industrial unit, as it is associated with the overall refinery planning and has significant economic impact. The deactivation rate is evaluated in terms of the HDS efficiency that is lost for each catalyst with increasing days of experiment, i.e. days on stream (DOS).

(b)

Fig. 5. Bromine index variation (%) of pure HAGO, 90/10 HAGO/WCO and 70/30 HAGO/WCO from catalytic hydrotreating at 330° (j), 350° () and 370 °C (N) employing (a) NiMo and (b) CoMo catalysts. All experiments were performed at pressure = 812 psig, LHSV = 1 h1, H2/Oil = 505.9 nl/l.

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Normalized HDS (1 equals to DOS=4)

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Fig. 6. Normalized HDS rate Sprod(DOS)/Sprod(4) for NiMo (black d) and CoMo (gray ). Values normalized based on the initial HDS rate at DOS = 4. The three desulphurization rates were calculated by experiments performed at T = 350 °C, P = 812 psig, LHSV = 1 h1, H2/Oil = 505.9 nl/l.

In Fig. 6 the NiMo and CoMo catalyst decay rate are compared in terms of their HDS efficiency loss vs. the days on stream (DOS). It should be noted that the deactivation study of the NiMo catalyst was combined with two successive experiments evaluating the low (HAGO/WCO 90/10 v/v) and high (HAGO/WCO 70/30 v/v) WCO content feeds, indicated by the two test periods 90/10 and 70/30 in Fig. 6 between the catalyst deactivation data points. Normalized HDS values are presented, based on the sulfur content of the product obtained from the pure HAGO feedstock in the first days of the experiment (DOS = 4) where the catalyst is presumably fresh (Normalized desulfurzation = 1 for DOS = 4). For product sulfur higher than the one obtained in DOS = 4, i.e. for decreased HDS rates, the corresponding normalized value is greater than one. It is easily observed from Fig. 6 that the deactivation rate of NiMo is less than that of CoMo for the same DOS. The low sulfur content in the bio-derived feedstock reduces the sulfided Ni or Co to their metal state, resulting in catalyst deactivation [22]. The better controlled deactivation of NiMo renders it a better choice for lipid containing feedstocks. This is even expected as NiMo catalysts are more hydrogenating and able to keep a greater fraction of the acid sites free of coke [16]. Furthermore, the CoMo catalyst, which is a typical HDS catalyst, requires vacancies as active sites, which readily adsorb O-containing compounds in the case of co-processing. These compounds have less adverse effect on NiMo catalyst with predominantly brim type sites. 4. Conclusion Hydrotreating of petroleum fractions with lipids feedstocks offers a unique opportunity to produce a sustainable diesel fuel completely compatible with existing fuel infrastructure and engine technology. In this research two different types of catalyst were evaluated (NiMo and CoMo) regarding their suitability and effectiveness on co-hydroprocessing of HAGO and WCO. The evaluation was based in terms of the efficiency of heteroatom removal, saturation reactions and catalyst decay rate. The results have showed that the presence of WCO in the feed has a negative effect on the HDS and HDN reactions rates for CoMo catalyst, while the opposite holds for the NiMo catalyst. Moreover, the addition of WCO favors conversion, diesel selectivity and saturation for both catalysts, however the effectiveness of these reactions is strongly affected by reaction temperatures in the case of the CoMo catalyst. Besides the superiority of NiMo catalyst regarding the product quality of co-hydrotreating HAGO/WCO, the NiMo catalyst also indicated a more attractive decay rate over the CoMo catalyst. Therefore, even though the CoMo catalyst was

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