Article pubs.acs.org/EF
Hydroconversion of Vegetable Oils Isomerized over ZSM-5: Composition and Properties of Hydroraffinates Łukasz Jęczmionek* and Wojciech Krasodomski Oil and Gas Institute, National Research Institute, Lubicz 25A, 31-503 Kraków, Poland ABSTRACT: Three vegetable oils (rapeseed oil, palm oil, and soybean oil) were isomerized (the so-called zeoforming process) using two different ZSM-5 catalysts (A or B) at a constant temperature (300 °C) and liquid hourly space velocity (LHSV) (1.0 h−1) and variable pressure (1.7, 2.5, or 4.0 MPa). The obtained isomerized zeoformates were hydrotreated under constant conditions (temperature, 340 °C; LHSV, 0.5 h−1; pressure, 4.5 MPa; and hydrogen/feed ratio, 1500 Nm3/m3). The obtained hydroraffinates exhibited diverse contents of n-paraffins (24−83 wt %), isoparaffins (reaching greater than 50 wt %), aromatics (up to 15.1 wt %), and possibly compounds with alkyl naphthene structures. Hydroraffinates containing high aromatic contents also showed increased final boiling points (FBPs) of distillation, which was most likely related to the oligomerization reactions of fatty acids. The increase in pressure during the zeoforming process reduces the formation of aromatics and isoparaffins. Moreover, under the same temperature and pressure conditions, the zeoforming reactions of saturated fatty acids were more difficult, resulting in smaller yields of isoparaffins and aromatics in the hydroraffinates.
1. INTRODUCTION The isomerization of vegetable oils and/or animal fats using a variety of catalysts, including zeolites, as an initial treatment in their processing into biofuel components is one of the lines of research for improving the performance of such products.1 Modified (isomerized) natural oils or fats can be processed into first-generation biocomponents [fatty acid methyl esters (FAMEs)]1−5 and second-generation biocomponents [hydrotreated vegetable oils (HVOs)].5−8 FAMEs produced from isomerized vegetable oils have better stability and lower pour point compared to unmodified FAMEs.9−11 There are two basic methods for the isomerization of oil.1 The first is the use of solid acid catalysts. The isomerization mechanism involves the formation of carbocations in the fatty acid chain at the site of the double bond and ultimately leads to the formation of methyl or ethyl branching. The second method is hydroisomerization using solid acid catalysts doped with active metals, e.g. Pd-ZSM-5 or Pt-ZSM5.12,13 This method leads to the hydrogenation of multiple bonds in the fatty acid chains. In ref 1, studies on the initial isomerization and hydroisomerization of eight different vegetable oils and animal fats [palm oil, coconut oil, rapeseed oil, corn oil, soybean oil, olive oil, animal fat (beef) and lard] over a beta zeolite doped with platinum are described. Obtained isomerized or hydroisomerized vegetable oils and fats to produce methyl esters (FAMEs) were used, and their cloud points were then determined. The results showed that the use of high hydrogen pressure (4 MPa) decreased the hydroisomerization cloud point (CP) of the obtained FAMEs compared to the product produced with a non-modified oil or fat. This results from the hydrogenation of multiple bonds in the fatty acid chain present in the glycerides along with the formation of saturated fatty acids. FAMEs obtained from oils and fats modified at low pressure (1.5 MPa) exhibit superior low-temperature properties with respect to the product obtained with non-modified materials. The greatest improvement was found in coconut © 2015 American Chemical Society
oil (the CP was decreased by over a dozen degrees Celsius). Reference 1 also highlighted the overlap that is detrimental to the properties of the obtained biocomponent chemical reactions: pyrolysis, cracking, and oligomerization. Reference 5 presents the results of the isomerization of rapeseed oil using ZSM-5 and the impact of the process on the properties of FAMEs. In this case, no improvements in the low-temperature properties of FAMEs were observed and certain parameters were negatively affected (e.g., the water content increased). As stated, the use of ZSM-5 in the isomerization reactions can cause oligomerization reactions of fatty acids. The products of these reactions may be aliphatic branched oligomers (dimers) or cyclic molecules, including aromatic molecules.14,15 Furthermore, decomposition reactions may transform glyceride structures to di- and monoglycerides along with free fatty acids, leading to a drastic increase in acidity. These reactions may directly cause the deterioration of some FAME parameters and negate the positive effects resulting from the isomerization of fatty acids.5 With regard to the second-generation biocomponents derived from vegetable oils and animal fats (HVOs), the isomerization of fatty acids before hydroconversion has been the subject of several research papers.5−8 These works report the positive effects of the initial isomerization (the so-called “zeoforming”) of vegetable oil on the low-temperature properties of the second-generation biocomponents obtained via hydroconversion. Several studies describe experiments with fresh5−7 and waste rapeseed oil (used previously for pork frying).8 In every case, improvements were detected in the lowtemperature properties of the hydroraffinates obtained from the zeoformed oils compared to the hydroraffinates obtained from the non-zeoformed oils. This was exhibited by lowered CP Received: March 20, 2015 Revised: May 22, 2015 Published: May 23, 2015 3739
DOI: 10.1021/acs.energyfuels.5b00582 Energy Fuels 2015, 29, 3739−3747
Article
Energy & Fuels temperature, pour point (PP), and cold filter plugging point (CFPP). It has, therefore, been concluded that the use of oil zeoforming to improve the performance parameters of hydroraffinates before the main hydroconversion may potentially be useful in the production of diesel biocomponents.5−8 In ref 6, catalyst coking during the zeoforming of vegetable oil was discussed. It was observed that the effect of catalyst coking is much weaker than in classical zeoforming, probably because of the use of relatively lower temperatures compared to the classical zeoforming of light n-paraffins. In ref 7, the hydroconversion of zeoformates over NiMo/ Al2O3 was carried out. For a hydroraffinate obtained from zeoformed at 300 °C and 1.7 MPa rapeseed oil, the n-paraffin content was approximately 25% and the aromatic content was approximately 15% (including 0.4% diaromatics and less than 0.1% polyaromatics); the remaining content was isoparaffin. However, the possibility of the presence of the alkyl naphthenes, which may have resulted from the partial hydrogenation of aromatic compounds during hydroconversion [340 °C, 4.5 MPa, liquid hourly space velocity (LHSV) of 0.5 h−1, and hydrogen feed of 1500 Nm3/m3], should be taken into account. The resulting biocomponent showed a higher final boiling point (FBP) of distillation compared to a biocomponent derived from non-zeoformed rapeseed oil because of the overlap already mentioned in the oligomerization reaction of fatty acids during zeoforming. Similar results were presented in ref 16, which reported the distillation characteristics of hydroraffinates derived from zeoformed vegetable oil. It is worthwhile to refer to classical zeoforming, i.e., the processing of the fraction of light paraffinic naphtha to high aromatic fractions.6 In the process of classical zeoforming, mainly n-paraffin hydrocarbons are transformed, while the isoparaffin remains relatively unchanged (zeolite channels are not available for branched hydrocarbons). n-Paraffins are transformed into cyclic and aromatic compounds produced by the recombination of olefins. In this way, even light pentane and hexane may form aromatic rings. To some extent, alkylation reactions also occur. The hydrocarbon conversion mechanism is therefore different from that of typical reforming catalysts containing precious metals supported on acidic centers, for which the basic reaction is the dehydrogenation of naphthenes and paraffins.6 The purpose of this study was to investigate the effect of pressure during the zeoforming of different vegetable oils (rapeseed, palm, and soybean) on the properties of the hydroraffinates obtained via hydroconversion. The ranges of zeoforming process parameters (temperature, pressure, and LHSV) were selected according to the mild classical zeoforming of light paraffinic fractions.6,7
Table 1. Fatty Acid Contents of the Rapeseed Oil, Palm Oil, and Soybean Oil Triglycerides Used in the Research (wt %) triglyceride fatty acid content (wt %) fatty acid capric C10:0 lauric C12:0 mirystic C14:0 palmitic C16:0 palmitoleic C16:1 stearic C18:0 oleic C18:1 linoleic C18:2 linolenic C18:3 arachidic C20:0 eikozenoic C20:1 behenic C22:0 erucic C22:1 lignoceric C24:0 nervonic C24:1 unidentified
soybean
0.08 10.9 0.2 3.1 25.5 53.7 4.8 0.3 0.3 0.4 0.2 0.1 0.42
palm 1.1 2.0 1.8 39.9 4.3 38.6 6.7 0.3 0.3 0.2
0.3
rapeseed
4.5 0.2 1.8 63.0 18.7 7.4 0.6 1.7 0.4 1.3 0.2 0.1 0.1
Catalyst II, ZSM-5 (B): SBET, 340 m2/g; acidity, 0.24 mmol/g by NH3-TPD; extrudate size, 2 mm (circular cross-sectional diameter) and 3−5 mm (length); silicon module, 70−80; and porosity, ∼0.35 cm3/g (including 0.2 cm3/g of meso- and macropores). In this work, a NiMo-type catalyst on an Al2O3 carrier was used for the hydroconversion of zeoformed and fresh vegetable oil. The catalyst was developed for the deep hydrotreatment of middle distillates, especially for deep hydrodesulfurization. The catalysts were delivered by Aditen Company, Poland. The hydrogen used in the hydroconversion process was of 99.99% purity. The raw materials (zeoformed and non-zeoformed rapeseed oil) were mixed with dimethyl disulfide (DMDS) to protect the surface of the NiMo/Al2O3 catalyst with sulfur on it from deactivation. DMDS was applied in such a way that the sulfur content in the raw materials was approximately 600 mg/kg. 2.3. Experimental Procedure. The experiment was conducted in two stages. In the first stage, the rapeseed oil was zeoformed over the ZSM-5 catalyst (A or B) under the following conditions: a raw material feeding in relation to the catalyst volume (LHSV) of 1.0 h−1, a pressure of 1.7, 2.5, or 4.0 MPa, and a temperature of 300 °C. The rapeseed oil zeoforming was performed using an OL-105 apparatus containing a flow reactor with a 100 mL capacity. The device was equipped with a raw material pump, a separator, and a system with receptacles for the converted fractions, i.e., the liquid and gas fractions. The feed rate was controlled by an automatic measuring system. A detailed description of the apparatus is shown in ref 17. After stabilizing the conditions, the process itself was conducted for 6 h, which allowed for the zeoformates to be sampled in the amounts necessary for further studies. In the second part of the experiment, the obtained zeoformates were subjected to hydroconversion under fixed conditions. As in the zeoforming, a flow reactor with a capacity of 100 mL equipped with the necessary equipment was used for the hydroconversion. The NiMo catalyst employed in this step was sulfurized in accordance with the typical research procedure using diesel fuel with 2% sulfur added (in the form of DMDS).16 After finishing the process of catalyst sulfurization, the test system was rinsed with a middle petroleum fraction with a low sulfur content for 24 h to flush the system. The applied hydroconversion conditions were as follows: a temperature of 340 °C, a LHSV of 0.5 h−1, a pressure of 4.5 MPa, and a hydrogen/ feed ratio of 1500 Nm3/m3. The hydroconversion of fresh rapeseed oil (also containing DMDS to stabilize the test conditions) was then conducted for the next 20 h. The hydroconversion process was conducted for 5 h for each type of raw material (zeoformate and fresh rapeseed oil). The 2 h process stabilization periods were applied
2. MATERIALS AND METHODS 2.1. Raw Materials. In the tests, edible, refined, and commercially available rapeseed oil, palm oil, and soybean oil were used as raw materials; the fatty acid compositions of their triglycerides are listed in Table 1. 2.2. Catalysts and Materials. To zeoform vegetable oil, two zeolite catalysts comprising a hydrogen form of ZSM-5 were used. These materials were characterized by the following features: Catalyst I, ZSM-5 (A): specific surface area determined by Brunauer−Emmett−Teller (SBET), 360 m2/g; acidity, 0.25 mmol/g by ammonia temperature-programmed desorption (NH3-TPD); extrudate size, 2 mm (circular cross-sectional diameter) and 3−5 mm (length); silicon module, 80−90; and porosity, ∼0.6 cm3/g (including 0.5 cm3/g of meso- and macropores). 3740
DOI: 10.1021/acs.energyfuels.5b00582 Energy Fuels 2015, 29, 3739−3747
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Energy & Fuels between the points of measurement (collection of the converted product). The obtained hydroraffinates (liquids) were purged with nitrogen for 1 h (gas stripping) to remove the dissolved gases. 2.4. Analytical Methods. The selected oil samples were esterified according to the methodology EN ISO 5509 to determine the fatty acid composition according to the EN ISO 5508 standard. To evaluate the obtained hydroraffinates for their use as fuel biocomponents (diesel fuel), the methods specified in the standard for diesel fuel (EN 590, the latest edition18) were used. The following properties of the obtained liquid fractions (hydroraffinates) were determined: a visual evaluation (color, clarity, and uniformity), the sulfur content (EN ISO 20884 standard), the acidity (according to EN 14104), the content of unreacted glycerides (EN 14105), the distillation characteristics (according to DIN EN ISO 3405), the cloud point (according to EN ISO 3015), the temperature of the CFPP (according to EN 116), the content and distribution of n-paraffins (ASTM D5442), and the aromatic hydrocarbon content (according to EN 12916).
Table 3. Properties of Hydroraffinates Obtained from Zeoformed Rapeseed Oil (Catalyst B) pressure/temperature of zeoforming (MPa/°C) from zeoformed oil
2.5/300 4.0/300 from non-zeoformed oil
pressure/temperature of zeoforming (MPa/°C) from zeoformed oil
from zeoformed oil
1.7/300 2.5/300 4.0/300
from non-zeoformed oil
CP (°C) CFPP (°C) CP (°C) CFPP (°C) CP (°C) CFPP (°C) CP (°C) CFPP (°C)
+16 +15 +16 +15 +16 +15 +18 +18
1.7/300 2.5/300 4.0/300
from non-zeoformed oil
+13 +8 +13 +9 +14 +10 +25 +22
77.7 80.6 83.2 ∼98
low-temperature parameter CP (°C) CFPP (°C) CP (°C) CFPP (°C) CP (°C) CFPP (°C) CP (°C) CFPP (°C)
+14 +10 +14 +11 +15 +13 +23 +20
n-paraffin content (wt %) 82.6 82.7 83.5 ∼98
rapeseed oil, while the weakest influence was observed for palm oil. For catalyst A, the hydroconversion of zeoformates at 1.7 MPa led to a liquid product characterized by improved CP and CFPP parameters compared to hydroraffinates prepared from non-zeoformed (fresh) oils. This effect was much stronger than that observed for catalyst B. In general, under the same process conditions, the hydroraffinates of zeoformates obtained from vegetable oils using catalyst A (Tables 2−4) have much better low-temperature characteristics than hydroraffinates obtained using catalyst B (Tables 5−7 and Figures 1 and 2). Because of identical conditions for the process using both catalysts, the differences in low-temperature properties of hydroraffinates have to be induced by the different catalyst properties. Zeolite properties, such a specific acidity, extrudate size, surface area, and silicon module, were the same or almost the same. The only relevant difference was porosity, including volume of meso- and macropores. The porosity in the case of catalyst A was twice that in the case of catalyst B, and it is probably the main reason for observed differences in product properties 3.5. Content of n-Paraffins. The content of n-paraffinic hydrocarbons in the hydroraffinates of non-zeoformed vegetable oils was determined as approximately 98 wt %. This was due to the presence of residues of unreacted glycerides (approximately 0.5 wt % in total) along with the presence of trace aromatics that sterols present in oils and isoparaffins, which are derivatives of the branched fatty acids that are naturally present in the oils in trace quantities and possible traces of other compounds present in the oils, e.g., hydrocarbons (squalene).20 The content of n-paraffin hydrocarbons in the hydroraffinates of vegetable oil zeoformates obtained using catalyst B was between 76 and 85 wt %. However, the accuracy of the
Table 2. Properties of Hydroraffinates Obtained from Zeoformed Palm Oil (Catalyst B) low-temperature parameter
CP (°C) CFPP (°C) CP (°C) CFPP (°C) CP (°C) CFPP (°C) CP (°C) CFPP (°C)
n-paraffin content (wt %)
Table 4. Properties of Hydroraffinates Obtained from Zeoformed Soybean Oil (Catalyst B)
3. RESULTS AND DISCUSSION 3.1. Sulfur Content. In all of the obtained hydroraffinates, the sulfur content determined was less than 5 mg/kg (ppm). 3.2. Zeoformates of Vegetable Oils. The rapeseed oil zeoformates (the liquid fraction) obtained in the first part of the experiment were homogeneous and clear, with a consistency similar to the raw material. These results were consistent with previous ones.6−8 A high acidity level of the obtained zeoformates (reaching over 10 mg of KOH/g) was detected. Thus, we assume that the decomposition of the glyceride structures and the formation of free acids may have occurred.7 3.3. Hydroraffinates. The hydroraffinates obtained from the zeoformed natural oils were clear liquid fractions with various colors ranging from yellowish to yellowish light orange. The samples were not homogeneous. At the bottom of the hydroraffinate collection vessel (a 1000 mL glass flask), the presence of a water phase was also detected. Its presence was expected because water is one of the co-products of glyceride hydroconversion.17,19 After stripping (purging) the obtained samples with nitrogen for 1 h, they were poured into a separation unit and the aqueous phase was then separated. 3.4. Low-Temperature Properties of Hydroraffinates. For both ZSM-5 catalysts used in the experiments, the lowest CP and CFPP parameters (Tables 2−7) and, therefore, the most preferred hydroraffinates for the preparation of the biofuel components were obtained at the lowest applied pressure (1.7 MPa). The hydroraffinates with the weakest low-temperature properties were obtained under the highest pressure (4.0 MPa). Given the nature of the feedstock (vegetable oil), it should be noted that, for catalyst B, the strongest influence was visible for
pressure/temperature of zeoforming (MPa/°C)
1.7/300
low-temperature parameter
n-paraffin content (wt %) 76.8 78.3 77.9 ∼98
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DOI: 10.1021/acs.energyfuels.5b00582 Energy Fuels 2015, 29, 3739−3747
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Energy & Fuels Table 5. Properties of Hydroraffinates Obtained from Zeoformed Palm Oil (Catalyst A) low-temperature parameter
pressure/temperature of zeoforming (MPa/°C) from zeoformed oil
1.7/300 2.5/300 4.0/300
from non-zeoformed oil
CP (°C) CFPP (°C) CP (°C) CFPP (°C) CP (°C) CFPP (°C) CP (°C) CFPP (°C)
−9 −10 −8 −10 −7 −9 +18 +18
n-paraffin content (wt %)
aromatic content (wt %)
26.6
11.1
27.0
10.3
27.9
9.4
∼98
