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Renewable Diesel Production from the Hydrotreating of Rapeseed Oil with Pt/Zeolite and NiMo/Al2O3 Catalysts Rogelio Sotelo-Boyas,*,† Yanyong Liu,‡ and Tomoaki Minowa‡ † ‡
Instituto Politecnico Nacional, Mexico ESIQIE, Mexico D.F. 07738, Mexico Biomass Technology Research Center-National Institute of Advanced Industrial Science and Technology, Chugoku, 2-2-2 Hirosuehiro, Kure, Hiroshima 737-019, Japan ABSTRACT: As an alternative way to produce diesel hydrocarbons, the hydrocracking of rapeseed oil was studied on three different types of bifunctional catalysts: Pt/H-Y, Pt/H-ZSM-5, and sulfided NiMo/γ-Al2O3. Experiments were carried out in a batch reactor over a temperature range of 300-400 °C and initial hydrogen pressures from 5 to 11 MPa. The reaction time was limited to 3 h to prevent a high degree of cracking. The Pt-zeolite catalysts had a strong catalytic activity for both cracking and hydrogenation reactions, and therefore a higher severity was required to reach a relatively high oil conversion into liquid hydrocarbons. With dependence on the activity of the acid sites of the catalysts, the results show a trade-off between the yield of green diesel and the degree of isomerization, which had a direct effect on the cold properties of the diesel. Among the three catalysts, hydrocracking on Ni-Mo/γ-Al2O3 gave the highest yield of liquid hydrocarbons in the boiling range of the diesel fraction, i.e., green diesel, containing mainly n-paraffins from C15 to C18, and therefore with poor cold flow properties. While for both zeolitic catalysts, hydrotreating of rapeseed oil produced more iso- than n-paraffins in the boiling range of C5 to C22, which included significant amounts of both green diesel and green gasoline. The gas chromatography (GC) analysis of the gaseous phase revealed the presence of mainly CO2, CO, propane, and remaining hydrogen. It was observed that both pressure and temperature play an important role in the transformation of triglycerides and fatty acids into hydrocarbons.
1. INTRODUCTION Climate change has been shown to be directly influenced by the increasing emission of greenhouse gases.1 As the combustion of transportation fuels contributes significantly to the emission of CO2, it is indispensable to research for neutral carbon technologies that lead to the production of clean and renewable gasoline and diesel. Hydroprocessing is a well-known technology in the petroleum refining industry, which can be carried out either by hydrocracking technology or by the less severe hydrotreating technology. When they are applied to oxygenated hydrocarbons, the removal of oxygen can be carried out by decarboxylation, decarbonylation, or hydrodeoxygenation.2-4 Hydroprocessing of vegetable oil leads to the production of hydrocarbons in the boiling range of diesel, and therefore it is commonly called green diesel or renewable diesel. The basic idea is to transform by effect of high pressure, high temperature, and a bifunctional catalyst (e.g., sulfided NiMo/γ-Al2O3) the triglycerides in the vegetable oil into high cetane hydrocarbons (mainly n-C17H36 and n-C18H38). The MoS2 phase of the catalyst and a high hydrogen pressure contribute to the saturation of the side chains of the triglycerides. The acid function of the catalyst contributes to the cracking of the C-O bound and to the isomerization of the n-olefins formed, which after hydrogenation are transformed into isoparaffins. Compared with biodiesel (fatty acids of methyl esters), green diesel in general has a higher oxidation stability, lower specific gravity, higher cetane number, and when it is blended with petroleum diesel it has much better cold flow properties. In addition, green diesel is totally compatible with petroleum diesel, thus changes or maintenance to the engine are not required. Green r 2010 American Chemical Society
diesel is also environmentally competitive, as its use may produce fewer greenhouse gases than petroleum diesel, biodiesel, and fossil-derived syndiesel (without carbon sequestration).5 Trying to optimize the process variables that lead to a highquality green diesel, a number of studies on the hydroprocessing of vegetable oils have been carried out.6-18 Hydrotreating of model compounds derived from vegetable oil has also been studied by Murzin et al.4,19 and Krause et al.20 Several oil companies have also developed commercial hydrotreating processes by considering the cofeeding of vegetable oil and vacuum gas oil (VGO).21-27 The cofeeding with VGO has also been studied by some researchers.8,11,13,28 The main vegetable oils used in the above studies have been sunflower,8,18 rapeseed,13,17 cottonseed,11 jatropha,10 soybean,22 castor,21 and palm24 oils. Animal fat has also been considered26 as a viable feedstock. In general these studies have shown that hydroprocessing vegetable oil contributes to the production of a renewable diesel with a premium quality as a fuel. Among the most significant factors in the conversion of triglycerides into diesel hydrocarbons stands out that of the activity of the catalyst. Conventional hydrotreatment catalysts such as NiMo or CoMo supported on γ-Al2O3 have been mostly used for hydrotreating vegetable oils. Zeolites have also been considered in the processing of vegetable oils, however most of these studies have been focused on catalytic cracking to produce Special Issue: IMCCRE 2010 Received: April 5, 2010 Accepted: August 25, 2010 Revised: August 18, 2010 Published: September 23, 2010 2791
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Industrial & Engineering Chemistry Research gasoline29,30 rather than on hydrocracking to produce diesel. The moderate acidity of alumina has been preferred due to its reduced cracking activity, which leads to a high yield of green diesel, containing mainly normal C17 and C18 paraffins and therefore with a high cetane number. The cold flow properties of green diesel are however poor and it would preferably be used blended, i.e., with petroleum diesel. To enhance the cold flow properties of green diesel, it is important to research for improved hydroisomerization activity of n-C17 and n-C18 paraffins, for instance by using catalysts with more acidic sites than those of alumina. Traditional hydrocracking catalysts such as faujasite zeolites have been shown to produce up to 30 wt % of isoparaffins in the range from C7 to C17.31 The hydrotreating of long chain n-paraffins on Pt/zeolite catalyst has been extensively studied by Froment et al.32-35 who have investigated the mechanisms of hydroisomerization and hydrocracking of long n-paraffins on different types of zeolites and have modeled the single-event kinetics of the various elementary steps by considering alkylcarbenium ions as key reaction intermediates. The understanding of the chemistry of the hydrocracking process and of the catalyst structure lead to a better understanding of the isomerization/cracking selectivity of the catalyst, which finally may lead to selection of the best catalyst for the hydroprocessing of vegetable oils. Thus, in this work, the effect of different active sites and supports is investigated by comparing the hydroconversion of rapeseed oil on NiMo/γ-Al2O3 and on zeolites Pt/H-Y and Pt/HZSM-5. The scope of the present work is the study of the optimal conditions at which both types of catalysts can efficiently hydroconvert rapeseed oil, providing thus insights that allow for a future optimal operation and industrial scaling. Experiments were conducted on a batch reactor, and the yield of green diesel was monitored.
2. METHODS 2.1. Materials. The rapeseed oil used in the experiments corresponded to a commercial type sold in Japan. Table 1 shows the total fatty acid composition of the oil. This was determined by derivation of the corresponding fatty acid methyl esters (FAME). A modified method AOAC 969.33 by Lee et al.36 was used to obtain the FAME. A GC-2014 Shimadzu was used. Table 2 shows some physical properties of the oil. The CHNS composition of the oil was determined by using an elemental analyzer (CE instruments EA1110); the oxygen content was obtained by balance. The density was determined at 20 °C using a density/ specific gravity meter (Kyoto Electronics DA-130N). The viscosity of the oil was determined at 20 and 40 °C using a vibroviscometer (A&D Co. Lim. Japan, SV-10). The acid value of rapeseed oil was obtained by titration with a KOH solution (0.1 N). 2.2. Catalysts. 2.2.1. NiMo/γ-Alumina. The catalyst precursor consisting in a mixture of NiO and MoO supported in γalumina corresponded to a commercial type (CDS-R25NQ) and was supplied by Catalyst and Chemicals Ind. Co. This catalyst is used in refining operations for hydrotreating of gas oil and atmospheric residue. It contains a high desulfurization activity. The catalyst was crushed in particles of about 0.3 mm of diameter. The activation of the catalyst was done in situ with elemental sulfur. In a typical procedure, the elemental sulfur and the NiMo/Al2O3 catalyst (0.8 g of S/g of catalyst) were added to the vegetable oil inside the autoclave reactor. Then the reactor was tightly closed and purged with hydrogen at room temperature.
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Table 1. Fatty Acid Composition of Rapeseed Oil structurea
wt %
palmitic
C16:0
3.65
palmitoleic
C16:1
0.18
stearic
C18:0
1.65
oleic
C18:1
63.72
fatty acid
linoleic
C18:2
15.42
linolenic
C18:3
14.28
cis-11-eicosenoic
C20:1
1.10
a
Cx:y, where x is the number of carbon atoms; y is the number of double bonds.
Table 2. Physicochemical Properties of Rapeseed Oil property elemental composition (wt %) C H
77.903 11.689
N
0.041
S
0.000
O
10.367
density at 20 °C, g cm-3
63.0
viscosity at 40 °C, mPa s
28.1
acid value, mg of KOH/g of oil FFA contenta, wt % a
0.89
viscosity at 25 °C, mPa s
0.55 0.28
On the basis of oleic acid.
As the reactor was heated up to temperatures close to 300 °C, the elemental sulfur reacted with hydrogen to form H2S, which sulfided and activated the NiMo/Al2O3 catalyst in the reactor for the hydrotreatment of vegetable oil. 2.2.2. Pt/H-ZSM-5 Catalyst (2 wt % Pt). The precursor NH4H-ZSM-5 was obtained by ionic exchange between Na-ZSM-5 and ammonium chloride (NH4Cl). This precursor was dried for 24 h at 110 °C and then calcined at 550 °C in air for 3 h to form H-ZSM-5. The SiO2/Al2O3 molar ratio in H-ZSM-5 was 30. Pt/ H-ZSM-5 catalyst was then prepared using an incipient wetness method by impregnating H-ZSM-5 particles in a 1 wt % H2PtCl6 aqueous solution. For a catalyst containing 2% of platinum, 1.5 g of H-ZSM-5 powder and 7.95 g of H2PtCl6 aqueous solution were used. About 20 mL of water were also added. The mixture was heated at 30 °C with stirring for 1 h, and then it was heated at 95 °C to evaporate all the water until a solid sample was obtained. Subsequently, the solid sample was dried at 110 °C for 24 h and then calcined at 400 °C in air for 3 h. Finally, the catalyst sample was reduced at 350 °C for 2 h in a tubular reactor with an internal diameter of 10 mm. and a hydrogen flow rate of 50 mL (STP)/min. 2.2.3. Pt/H-Y Zeolite Catalyst (2 wt % Pt). H-Y zeolite corresponded to a synthetic HS-320 catalyst and was purchased from Waco Chemical Co. (I.D. 325-27765). The SiO2/Al2O3 molar ratio in H-Y was 5.5. Pt/H-Y catalyst was prepared using an impregnation method similar to Pt/H-ZSM-5. H-Y particles were impregnated in an aqueous solution of H2PtCl6, and the water was then evaporated at 95 °C to obtain a solid sample. The solid sample was then dried at 110 °C for 24 h, calcined in air at 400 °C for 3 h, and finally reduced in hydrogen flow at 350 °C for 2 h. The loading of platinum in Pt/H-Y catalyst was 2 wt %. 2792
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Industrial & Engineering Chemistry Research 2.3. Experimental Procedure. Experiments were conducted in an 80 mL-batch reactor with an internal diameter of 20 mm and equipped with a mechanical stirrer. The operative limits of the reactor were 30 MPa and 900 °C. For all experiments, the stirrer speed was kept constant at 350 rpm, and the temperature control was (2 °C. The feed consisted in rapeseed oil and catalyst in a ratio of 3 wt % cat/wt of oil. The reaction conditions for hydrotreating experiments were a temperature range of 300-400 °C and initial hydrogen pressures between 5 and 11 MPa. Before carrying out the reaction, the reactor was purged several times with nitrogen and then with hydrogen at room temperature. The reactor was then heated up to the final temperature and kept at this condition for 3 h (reaction time). Initial experiments from 1 to 6 h at 350 °C were carried out to determine the optimal reaction time. A time of 3 h was found to be appropriate to avoid an excessive cracking of the main components. Furthermore, after 3 h the yield of main components did not increase significantly, indicating there was enough time to reach equilibrium. After cooling down the reactor, the gas and liquid products were recovered. The liquid phase containing the catalyst was filtered. When solid product was obtained, this was diluted prior to its analysis with 1,2,3,4-tetra hydronaphthalene. Both gas and liquid phases were analyzed by gas chromatography. The produced gases were analyzed by a GC 323 (GL Sciences) equipped with a thermal conductivity detector and two columns, one being a Pora-Q capillary column (30 m, 0.53 mm i.d.) for determination of CO2 and the other one a packed column (MS-5A) for determination of H2, O2, N2, and CO. Light hydrocarbons (C1C4) were analyzed by a FID GC-353 B (GL Sciences) equipped with a RT-QPLOT capillary column. C5þ hydrocarbons were analyzed by an Agilent 6890 N FID-GC equipped with an UADX capillary column. Fatty acids were analyzed by an Agilent 6890N FID-GC equipped with a HP-624 capillary column. To verify the presence in the liquid product of important compounds, such as remaining carboxylic acids, isoparaffins, cyclic paraffins, and aromatics, a detailed analysis by mass spectrometry was performed.
3. RESULTS AND DISCUSSION 3.1. Rapeseed Oil Composition. The composition of the vegetable to be hydrotreated has a direct effect on the consumption of hydrogen, which is needed to saturate the double bonds in the side chains of triglycerides, as well as the unsaturated carboxylic acids, and olefins that are formed by hydrocracking. As shown in Table 1, the rapeseed oil used in this work has a high content of unsaturated carboxylic acids, i.e., oleic, linoleic, and linolenic acids, which is an indication that rapeseed oil contains mostly triolein, trilinolein, and trilinolenin. As these are unsaturated triglycerides, the hydrotreating of rapeseed oil needs a higher consumption of hydrogen during the hydrotreating process than the one that would need the hydrotreating of less unsaturated oils, e.g., palm oil, which is rich in palmitolein. In Table 2, it is also possible to notice that this commercial rapeseed oil does not contain a significant amount of free fatty acids (FFA), as it is given by the acid value. The low content of FFA, characteristic of commercial oils, is desirable since FFA could eventually be transformed into a more unsaturated FFA, increasing the number of double bonds and therefore increasing the consumption of hydrogen and the operative costs.
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Figure 1. Product distribution yields for paraffins after 3 h with NiMo/ Al2O3 as the catalyst at 350 °C and an initial PH2 = 8 MPa.
Figure 2. Yields of the C7-C24 iso-paraffins in the liquid fraction after 3 h with NiMo/Al2O3 as the catalyst at 350 °C and the initial PH2 = 8 MPa.
3.2. Hydrotreating with NiMo/Al2O3 Catalyst. In this study, we used a gas chromatography-mass spectrometry (GC-MS) analysis to determine the composition of the different products, which allowed us to present the results on the basis of carbon number intervals. All yields in this paper are reported on a weight percent basis with respect to the weight of the oil. Figure 1 shows the paraffinic product yields for hydrotreating rapeseed oil on NiMo/Al2O3 from an initial pressure of 8 MPa and a final temperature of 350 °C for 3 h. At these conditions, the rapeseed oil is converted into a liquid phase product that contains mainly n-paraffins and isoparaffins in between C15 and C24 hydrocarbons. The total yield of liquid hydrocarbon was 86.3 wt %. The main components were n-heptadecane with 32.7 wt % and n-octadecane with 18.4 wt %. As the cetane number of n-C17 and n-C18 is higher than 100, the cetane number of the green diesel will be very high and probably much higher than that of biodiesel or that of petrodiesel. Figure 2 shows the distribution of isoparafins in the liquid phase. It was observed that the main isoparaffins were single-branched C17 and C18 paraffins, e.g., 1-methyl hexadecane and 2-methyl heptadecane. Compared to the yield of n-paraffins, the global yield of isoparaffins is very low, which is thought to be related to the moderate acidity of the alumina. The same reason can explain the nonsignificant yield of cracked 2793
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Scheme 1. Main Reactions of Stearic Acid under Hydrogenation Conditions
Figure 3. Yields of the components in the gas phase after 3 h with NiMo/Al2O3 as the catalyst at 350 °C and the initial PH2 = 8 MPa.
products, as it is noticed in Figure 1 by the yield of the fraction corresponding to C5-C12 hydrocarbons (gasoline), which only accounts for 3.2 wt %. By GC-MS analysis, a very low content of cycloalkanes and aromatics was also detected. The latter ones were more predominant in the products obtained with zeolitic catalysts. This may be due to the stronger acids sites in the zeolites that promote isomerization, cracking, and cyclization. As a basis for the subsequent discussion of results and to get a better understanding of the reaction mechanism, Scheme 1 shows the transformation of stearic acid into hydrocarbons by the three main pathways: n-C17 is the main product by the decarbonylation and decarboxylation pathways, while n-C18 is the main product by the hydrodeoxygenation pathway (water is a byproduct). The gas product distribution from the hydrotreating of rapeseed oil is shown in Figure 3. The remaining hydrogen (∼40% mol) is not included in this figure. The total yield of CO2 predominates over that of CO, which may indicate that decarboxylation (loss of the carboxylic group as CO2) is favored over decarbonylation (loss of the carboxylic group as CO, via hydrogenation) at the above conditions. This is because the former reaction does not need hydrogen as the latter does, and since this study was carried out in a batch reactor, the hydrogen pressure is continuously decreasing and also the rate of decarbonylation. It is important however to notice that the presence of CO2 could also be due to the water gas shift reaction, in which CO and water are transformed into CO2 plus hydrogen. As CO and water are formed by decarbonylation (viz. Scheme 1), they are probably transformed into CO2 and hydrogen by means of the water
gas shift reaction. Thus, with a decrease in the partial pressure of steam (H2O) and CO, the equilibrium of decarbonylation reaction would favor a higher production of n-C17H36. The higher yield of n-C17H36 than that of n-C18H38 indicates that at the above-mentioned conditions decarboxylation and decarbonylation are favored over hydrodeoxygenation. An important byproduct in the gas phase is propane, viz. Figure 3. Its formation is believed to be mainly due to the cracking of the carboxylic C17- and C18-side chains of the triglycerides, with consequent transformation of the glycerol backbone into propane. In a large scale operation, propane production will be economically very beneficial. In addition, carbon monoxide could be steam-reformed to generate the hydrogen needed for producing green diesel,27 and by doing so the process could be economically more viable. To reduce the impact of CO2 as a green house gas, it could be sequestrated through enhanced oil recovery, which can also bring an economic profit for oil producers.37 3.3. Effect of Initial Hydrogen Pressure. As the hydrogen consumption is one of the important limiting economical factors in hydrotreating, the effect of the initial hydrogen pressure in the reactor was studied. To evaluate this effect, several experiments were performed in an interval of 5-11 MPa. When using NiMo/ Al2O3 as a catalyst, initial hydrogen pressures of 8-10 MPa were found to be more appropriate. At pressures below 8 MPa, the product was partially solid and a high content of saturated carboxylic acids were detected, i.e., palmitic and stearic acids primarily. With zeolitic catalysts, even higher pressures were required to obtain a liquid product. In this section, only the results with NiMo/Al2O3 as the catalyst are shown. In Section 3.5, the results with zeolitic catalysts are described. Figures 4-7 show the global yields for the different fractions obtained when using NiMo/Al2O3, a reaction time of 3 h at 350 °C, and different initial hydrogen pressures. As observed in Figure 4, the yield of C23þ fraction (wax) decreased, indicating that catalytic cracking of these compounds into lighter hydrocarbons is favored as pressure increases. In the interval from 8 to 10 MPa, the global yield of green diesel is slightly affected by the pressure, as it only varies from 74.8 to 78.5 wt %, viz. Figure 5. In the same interval, the yield of n-C17 was higher than that of n-C18, as it can be observed in Figure 6. In the gas phase, there was also observed in all cases a higher amount of CO2 than CO. These two observations confirm that decarboxylation is the main reaction pathway at the conditions in the batch reactor. In Figure 6, it can also be observed that the effect on increasing the initial hydrogen pressure is slightly more 2794
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Figure 4. Influence of initial PH2 on the global production of the main fractions: gas (C1-C4), gasoline (C5-C12), and wax (C23þ). T = 350 °C on NiMo/Al2O3.
Figure 5. Influence of initial PH2 on the global production of the main fraction: green diesel (C13-C22). T = 350 °C on NiMo/Al2O3.
Figure 6. Influence of initial PH2 on the production of major products: n-heptadecane and n-octadecane. T = 350 °C on NiMo/Al2O3.
pronounced on the yield of n-octadecane than on the yield of n-heptadecane. Thus, the relative rate of decarboxylation and decarbonylation versus hydrodeoxygenation decreases as the hydrogen pressure increases, as the latter reaction requires more hydrogen, viz. Scheme 1. Therefore, with an increase in the pressure, hydrodeoxygenation could be promoted and a higher yield of n-C18 would be reached. This reaction however produces water as a byproduct, which may deactivate the catalyst and reduce the yield of green diesel. Previously, Senol et al.38 on their study of hydrodeoxygenation of aliphatic esters on sulphided NiMo/γ-Al2O3 have observed a decrease in the yields of hydrocarbons when the amount of water was increased.
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Figure 7. Influence of initial PH2 on the global production of carboxylic acids. T = 350 °C on NiMo/Al2O3.
Figure 8. Influence of temperature on the production of the main fractions: gas (C1-C4), gasoline (C5-C12), and wax (C23þ). Initial PH2 = 9 MPa; NiMo/Al2O3 as the catalyst.
Figure 7 shows the global yield of carboxylic acids at different initial hydrogen pressures. At 350 °C and 10 MPa, the product still contained fatty acids, mainly stearic and oleic acids. To totally convert these fatty acids into hydrocarbons, it was necessary to increase the temperature, as it is described in the next section. 3.4. Effect of Temperature. The effect of temperature on the hydrocracking of rapeseed oil on NiMo/Al2O3 was studied by performing experiments from 300 to 400 °C and with an initial hydrogen pressure of 9 MPa. Global yields of the different fractions obtained at different temperatures can be observed in Figures 8 and 9. The yields of the main products are shown in Figure 10. At temperatures below 350 °C, the product was partially solid. Only a small amount of liquid was obtained. Therefore only those results obtained at temperatures at 350 °C and above are presented here. At temperatures above 350 °C there is an increase in the degree of cracking of n-paraffins in the boiling range of the diesel fraction, as it can be observed by the decreasing yield trend of the major n-paraffins, i.e., n-C17 and n-C18, in Figure 10. Consequently, the yield of the C5 to C12 fraction (gasoline) increases, as shown in Figure 8. In Figure 9, the maximum yield (78 wt %) of the green diesel (C13-C22 fraction) is observed at 350 °C; at 375 °C the yield of this fraction is slightly reduced to about 76 wt %, but when the temperature is increased above 380 °C (viz. Figure 10), there is a significant reduction in the yield of green diesel to about 52 wt %. The small effect of increasing the reaction temperature from 350 to 375 °C on the yield of green diesel was observed to be due to the increase in the yield of C13 to C16 hydrocarbons with 2795
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Figure 9. Influence of temperature on the production of green diesel (C13-C22). Initial PH2 = 9 MPa; NiMo/Al2O3 as the catalyst.
Figure 10. Influence of temperature on the production of major products. Initial PH2 = 9 MPa; NiMo/Al2O3 as the catalyst.
temperature, probably by means of cracking of C17 to C22 hydrocarbons and the consequent production of lighter hydrocarbons. The yield of i-C17 was also observed to slightly increase in this temperature range. The global yield of C5 to C22 hydrocarbon products was near 81 wt % at 375 °C (viz. Figures 8 and 9), which indicates that almost all C-CO bonds were cracked at 375 °C. When the temperature was increased from 375 to 400 °C, the yields of green diesel and wax significantly decreased and the amount of light hydrocarbons greatly increased. This indicates that temperature plays an important role in the cracking of the intermediate carbenium ions when the temperature is higher than 375 °C. This work considers that the transformations of hydrocarbons follow a bifunctional mechanism. After n-C17H36 and n-C18H38 were formed by the hydrotreatment of vegetable oil on NiMo sites, the corresponding carbenium intermediates were formed on the acid sites of Al2O3. The carbenium intermediates underwent cracking to form light hydrocarbons and isomerization to form isoparaffins. The formed isoparaffins were also hydrocracked in the reactor. These caused that the products from the hydrotreatment of vegetable oil contained different paraffins although n-C17H36 and n-C18H38 were the main components when the temperature was lower than 375 °C. From Figure 10, it is possible to observe the influence of temperature on the yields of n-C17 and n-C18. The n-C17/n-C18 yield ratios at 350, 375, and 400 °C were 2.2, 1.6, and 1.5, respectively. This indicates that by using a batch reactor in which the hydrogen pressure decreases with time, the relative rate of decarbonylation plus decarboxylation (n-C17 production) versus hydrodeoxygenation (n-C18 production) decreases as the reaction temperature increases. This result is contrary to the ones
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Figure 11. Influence of temperature on the global yield of fatty acids. Initial PH2 = 9 MPa; NiMo/Al2O3 as the catalyst.
reported by Huber et al.8 and Simacek et al.,17 which have performed hydrotreating of similar vegetable oils on a continuous reactor, in which the pressure is kept constant and there is enough hydrogen to promote the hydrodeoxygenation reaction. Thus, these other authors have found that n-C18 yield is higher than that of n-C17. In Figure 11, the global yield of fatty acids in the product obtained at different temperatures is also observed. These results indicate that an enough high temperature (∼375 °C) is required to hydrocrack the carboxylic acids. At 350 °C, the product analysis revealed the presence of carboxylic acids, mainly stearic acid, which is solid at room temperature. The total transformations of carboxylic acids lead to a purely hydrocarbon-based product. This product without carboxylic acids is liquid at room temperature. Thus, the hydrocracking of carboxylic acids can be considered the determining step in the conversion of vegetable oils into gasoline and diesel hydrocarbons. It is important to mention that in a previous study by Simacek et al.17 using a continuous reactor and a similar catalyst to the one used in this work have shown that a temperature close to 310 °C was enough to hydrocrack carboxylic acids from rapeseed oil into liquid products. Those authors have used a high-pressure flow system in which the hydrogen pressure was kept at 7 MPa with a surplus flow rate of hydrogen to the raw material (1000:1). In the present work, we have used a batch reactor in which hydrogen is continuously decreasing and therefore a high temperature is needed to hydrocrack the carboxylic acids. With the use of the autoclave reactor, a large amount of hydrogen has been saved. Some other differences with the previous work are the activation method and the catalyst amounts used in the experiments. 3.5. Hydrotreating with Pt/Zeolite Catalysts. The high content of C15 to C18 n-paraffins assures that the liquid produced with NiMo/Al2O3 as catalyst is a high cetane number diesel fuel containing mainly n-C17 and n-C18. However the pour point of these n-paraffins is relatively high. Although it is not likely that green diesel will be used anytime soon unblended, it could be desirable to improve its cold properties. The best option is to transform these n-paraffins into their corresponding isoparaffins, which have a very low pour point. As the isomerization reaction is promoted by the acids sites of the catalyst, the use of strong acid catalyst, such as zeolites, can be appropriate. Two types of zeolitic catalysts were used. The first is a platinum catalyst containing H-ZSM-5 zeolite, which belongs to the family of so-called 10-ring zeolites (having micropores with windows circumscribed by 10 oxygen atoms).35 The second catalyst was platinum containing H-Y zeolite, which is acknowledged 2796
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Figure 12. Global yields obtained with the operation of the two zeolitic catalysts used in the experiments. Initial PH2 = 11 MPa; final T = 380 °C for 3 h.
Figure 13. Yields of the gas phase components obtained with the operation of the two zeolitic catalysts used in the experiments. Initial PH2 = 11 MPa; final T = 380 °C for 3 h.
to have a larger size of intersections between micropore channels and a greater hydrogen transfer capacity than H-ZSM-5.39 Figure 12 compares the yields of the major fractions obtained with both catalysts when operating with an initial hydrogen pressure of 11 MPa and 380 °C during 3 h. At these conditions, both catalysts gave the highest yield of diesel. The hydrotreating of rapeseed oil on Pt/H-ZSM-5 produced a higher yield of gasoline (C5-C12 fraction) than diesel (C13-C22 fraction). The contrary happened with Pt/H-Y. The higher production of cracked products with the 10-ring zeolite suggests that the acid sites of Pt/H-ZSM-5 are stronger than those of Pt/H-Y. Similar results have been found by Souverijns et al.35 In Figure 13, the higher yield of propane produced by hydrocracking with Pt/ H-ZSM-5 reveals also its strong cracking activity by means of the β-scission mechanism. The presence of methane and ethane in the gas product with Pt/H-ZSM-5 could be due to hydrogenolysis of long paraffins on the platinum sites. As with the NiMo/Al2O3 catalyst, there is a higher content of CO2 than CO in the gas phase produced by the hydrocracking of rapeseed oil with both Pt/zeolites, which could be an indication that decarboxylation is the main reaction pathway at the conditions in which the experiments were carried out. Another characteristic found for the three catalysts used in this study was the ratio of n-paraffins to isoparaffins in the interval of C5 to C22 paraffins. At 375 °C and initial pressure of 9 MPa, the ratio was
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5.5 for NiMo/Al2O3; while at 380 °C and at initial pressure of 11 MPa, the ratios n-P/i-P in the product for Pt/H-ZSM-5 and Pt/HY were 0.527 and 0.744, respectively. The acid sites of the zeolite clearly promoted the isomerization of n-paraffins. Because of its stronger acid sites and probably due to the shape selectivity that are known to have the 10-ring zeolites, Pt/H-ZSM-5 was the one which gave the highest yield of isoparaffins. This is in line with the results found by Park et al.40 and Souverijnis et al.35 who have studied the hydroisomerization of long chain paraffins in different Pt/zeolite catalysts. The shape selectivity of the H-ZSM-5 zeolite may have prevented the cracking of multibranched paraffins by limiting the introduction of these molecules into the inner micropores of the zeolite. Previous studies35 have shown that the H-Y catalyst promotes the formation of multibranched isoparaffins that have the advantage of lowering the pour point but the disadvantage of being susceptible to cracking, resulting in a yield loss of diesel hydrocarbons. Compared with the yield of diesel obtained with NiMo/Al2O3 (from 70-80%), the yield of green diesel with Pt/zeolite catalysts is very low (from 20 to 40%). Although hydrotreating of rapeseed oil with both zeolitic catalysts produce a green diesel with a relatively high concentration of isoparaffins and hence with better cold flow properties than that produced with NiMo/ Al2O3, they require a higher severity, i.e., higher temperature, which increases the cracking of long paraffinic chains and produces middle molecules in the range of the gasoline fraction. Thus, both thermal and catalytic cracking reduce the yield of diesel hydrocarbons. Among both zeolitic catalysts, the Pt/H-Y catalyst seems to be more appropriate to produce a high yield of green diesel, while Pt/H-ZSM-5 is more appropriate for producing green gasoline, though with this catalyst there are also more light ends and CO2 produced, viz. Figure 13. On the other side, in some countries, gasoline would be more profitable than diesel and thus its production could be of great interest for refiners as well. In Figure 13, it is also possible to notice that the total amount of CO plus CO2 is very different for both catalysts. A possible reason could be that Pt/H-Y favors reduction, while Pt/H-ZSM5 favors decarbonylation and decarboxylation from the viewpoint of the reaction mechanism. As compared with the Pt/H-Y catalyst, Pt/H-ZSM-5 catalyst possesses stronger acid sites but the acid amount in H-ZSM-5 is less than that in H-Y because the SiO2/Al2O3 molar ratio was 5.5 in H-Y, while in H-ZSM-5 was 30.The acid strength and acid amount in zeolites probably influenced the hydrocracking of carboxylic acids and hence the yield of CO þ CO2 in the products. It is important to mention that for most of experiments with both zeolite catalysts, there was also observed the presence of about a 10-20% yield of carboxylic acids in the products, mainly stearic, oleic, and palmitic acids. To increase their conversion into hydrocarbons, it was necessary to increase the severity. However, with an increase in temperature, e.g., higher than 380 °C, a high degree of cracking was observed. After 3 h of reaction, the pressure in the reactor was slightly decreased for Pt/H-Y and slightly increased for Pt/H-ZSM-5, with respect to the initial pressure. This is probably due to the high content of light hydrocarbons that are formed because of cracking. In addition, thermal and catalytic cracking produce olefins which are then saturated, and therefore, as the degree of cracking increases, the consumption of hydrogen increases as well. Thus, as the reaction proceeds, the hydrogen pressure eventually is not enough to convert the fatty acids into hydrocarbons. 2797
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Industrial & Engineering Chemistry Research In summary, the results obtained with this study indicate that both pressure and temperature play an important role in the transformation of triglycerides and mainly carboxylic acids. A high enough hydrogen pressure is needed to hydrogenate the unsaturated chains of triglycerides and carboxylic acids as well as those of the olefins formed by cracking. Another important role played by hydrogen is keeping the catalytic activity while impeding coke formation. Increasing temperature also favors the cracking of the double bond CdO and that of the bond R— CO in the carboxylic acids, thus favoring the production of C18 and C17 n-paraffins, respectively.
4. CONCLUSIONS The hydrotreating of vegetable oils on bifunctional catalysts makes possible the production of liquid hydrocarbons known as “green diesel” containing mostly n-heptadecane and n-octadecane, which are mainly formed by decarboxylation/decarbonylation and hydrodeoxygenation reactions, respectively. Because of their strong acid sites, both Pt/zeolites-supported catalysts have a strong cracking activity. The less acidity of the PtHY catalyst led to a larger production of green diesel than green gasoline. The yield of diesel with these catalysts is lower than that obtained when using NiMo/Al2O3 as a catalyst. Even though the NiMo/Al2O3 catalyst effectively promotes the hydrocracking of triglycerides and carboxylic acids at lower temperature and pressure, its moderate acidity does not largely contribute to the production of isoparaffins, which are also important due to their lower pour points than those of their corresponding n-paraffins. The process to produce diesel with zeolitic catalysts requires a higher severity than NiMo/Al2O3 and therefore a higher operative cost. The strong acid sites of zeolites favor the production of isoparaffins, which are desirable for the diesel to have a low pour point. It is therefore necessary to moderate the acidity of the zeolite catalysts to increase the isomerization activity while keeping a moderate cracking. Thus, a large amount of isoparaffins could be obtained. This work has shown the conditions at which two zeolitic catalysts can produce green diesel as well as green gasoline. Further research is needed to determine the appropriate acidity of zeolite catalysts to improve the hydroisomerization selectivity while keeping an effective hydrocracking of triglycerides and fatty acids. The transformation of carboxylic acids is the most important step for the production of diesel hydrocarbons from rapeseed oil. The quality of the diesel product depends mainly on the pressure, temperature, and on the balance and strength of metal-acid sites of the catalyst used. Because of its large concentration of n-C17 and n-C18 hydrocarbons, the green diesel as obtained with NiMo/Al2O3 probably has a high cetane number even larger than that of the typical biodiesel composed by FAME. Green diesel can be used directly as transportation fuel, although at the current time it could be used rather as an excellent additive for increasing the cetane number of petroleum diesel. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Funding Sources
The Japanese International Cooperation Agency (JICA) is acknowledged for the financial support to carry out the present study.
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