Zeoforming of Triglycerides Can Improve Some Properties of

Nov 18, 2014 - Rapeseed vegetable oil was initially zeoformed in the temperature range of 200–300 °C and at a pressure of 1.7 MPa using a catalyst ...
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Zeoforming of Triglycerides Can Improve Some Properties of Hydrorefined Vegetable Oil Biocomponents Łukasz Jęczmionek,*,† Zygmunt Burnus,† Grazẏ na Ż ak,† Leszek Ziemiański,† Michał Wojtasik,† Wojciech Krasodomski,† Zbigniew Stępień,† Małgorzata Rutkowska,‡ and Agnieszka Węgrzyn‡ †

Oil and Gas Institute, National Research Institute, Ulica Lubicz 25A, 31-503 Kraków, Poland Faculty of Chemistry, Jagiellonian University, Ulica Ingardena 3, 30-060 Kraków, Poland

‡

ABSTRACT: Rapeseed vegetable oil was initially zeoformed in the temperature range of 200−300 °C and at a pressure of 1.7 MPa using a catalyst containing ZSM-5, and the obtained zeoformates were subsequently converted into hydrocarbons [hydrorefined vegetable oil (HVO)] through the process of hydroconversion. The resulting hydroraffinates (HVO fuel biocomponents) contained n-paraffins, isoparaffins, and up to 15% aromatic compounds. It has been established that hydroraffinates containing aromatic compounds have good low-temperature properties [cold filter plugging point (CFPP) of approximately −12 °C] and a density of 825 kg/m3. The hydroraffinate obtained over the catalyst at the highest applied temperature (300 °C) was characterized by a decreased initial boiling point of distillation (IBP) of 174 °C (the IBP for the nonzeoformed oil hydroraffinate was 284 °C) and an increased distillation final boiling point (FBP) of approximately 379 °C, which was higher than that of the non-zeoformed hydroraffinate (337 °C). Investigation of the obtained hydroraffinate properties led to the conclusion that the preliminary zeoforming process may cause the coupling (oligomerization) of fatty acid chains and the creation of aromatic structures containing aliphatic functional groups. In comparison to the studies described above1−7 in the area of the implementation of porous materials for vegetable oil decomposition, triglyceride zeoforming is connected with the creation of precursors of hydrocarbon fuel biocomponents, which are formed in the hydroconversion stage of the process.8−11 The purpose of such a zeoforming process is partial isomerization and modification of triglycerides, leading to the formation of branched and cyclic structures. It should be noted that the term “triglyceride zeoforming” was borrowed from, by analogy, the process of producing aromatic gasoline components of light paraffin fractions, which is rarely used in practice.12 Subsequent hydroconversion of modified (zeoformed) oil resulted in obtaining a hydrocarbon fraction in which the n-paraffins still represent a significant part of the product, thereby maintaining a relatively high cetane number.8−11 Isoprafins and cyclic compounds obtained during zeoforming have a positive effect on selected properties (e.g., low-temperature properties and density characteristics)12,13 of the obtained second-generation biocomponents. Currently used processes for the preparation of hydrorefined vegetable oil (HVO) biocomponents are generally two-stage processes. The first stage consists of the hydroconversion of the raw material (vegetable oil and fat) leading to n-paraffins, and the second stage is for the isomerization of n-paraffins. The aim of the isomerization is to make the product that will meet lowtemperature requirements for fuel biocomponents.11 The NExBTL process (Neste Oil) or Ecofining (UOP) are examples of such technologies. The use of a partial isomer-

1. INTRODUCTION Placing natural oils and fats in contact with zeolite catalysts is a widespread research topic with the aim of decomposing the natural products into fuel fractions or partially finished products that could be used in further stages for the production of such fractions.1−11 There are many published studies describing the catalytic application of the zeolite ZSM-5 for the decomposition of vegetable oils to obtain the hydrocarbon fractions relevant for the boiling range of gasoline.1 Additionally, the processes for the decomposition of rapeseed oil into hydrocarbons using various cracking catalysts have been broadly studied.2 Other vegetable oils, such as palm oil3 and algae oil,4 have also been used as raw materials. The decomposition process of waste oils of biological origin into carbohydrate components5 has been researched as well. A detailed overview of the natural triglyceride decomposition process using zeolite catalysts can be found in the literature.6 The authors of this work state that the most frequently used catalyst in these types of processes is the zeolite ZSM-5. It enhances the formation of high-octane hydrocarbon fractions containing aromatic compounds. On the other hand, mesoporous molecular sieves from the MCM family are effective for obtaining hydrocarbons that have a linear structure. Similar results can be obtained when using activated alumina and sodium carbonate. The resulting aliphatic hydrocarbons can be useful as diesel-fuel-blending components, whereas the aromatic fractions (mainly obtained with the ZSM5 catalyst) can be used as gasoline components. In these studies, natural zeolites, such as clinoptilolite, faujasite, heulandite, and analcime, were used.6 There are also known processes for the pyrolysis of natural oils into hydrocarbons using porous materials, such as activated carbon, which run at relatively high temperatures (>500 °C).7 © 2014 American Chemical Society

Received: September 18, 2014 Revised: November 17, 2014 Published: November 18, 2014 7569

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fractions,17,18 also for glyceride zeoforming. The HZSM-5 catalyst used showed a relatively high activity in the cyclization process of linear structures, which was also observed in earlier studies in relation to zeoforming of medium fractions from the Fischer−Tropsch synthesis12 and light paraffinic fractions.17,18 In contrary to previous studies,8−11 the hydroconversion was conducted with pure (100%) vegetable oil without adding any other fractions.

ization of the glyceride structures in a zeoforming process as a first stage, before being subjected to hydroconversion, is an innovative approach when it comes to obtaining biocomponents.14 Recently, the results of the studies on the effect of isomerization of vegetable oils over a zeolite beta on the properties of fatty acid methyl ester (FAME) were published.15 Zeolite beta was used for the purpose of these studies. It was found that the initial isomerization and hydroisomerization of fatty acid glycerides and fatty oils leads to improvement of lowtemperature properties of FAME. Other work16 presents the results of isomerization of rapeseed oil using a ZSM-5 catalyst and the impact of this process on the properties of FAME. In this case, there was no positive effect of improving the lowtemperature properties of FAME, and there has been a deterioration of certain parameters, e.g., increase in the water content. Previous studies8−11 present results on the positive impact of initial vegetable oil zeoforming on the low-temperature properties of the second generation of fuel biocomponents obtained by hydroconversion. Several studies8,9,11 describe experiments with fresh rapeseed oil. In ref 10, the utilization of waste rapeseed oil (used previously for pork frying) was described. In every case, an improvement was detected in the low-temperature properties of the hydroraffinates obtained from the zeoformed oils in comparison to the hydroraffinates obtained from the nonzeoformed oils. This was exhibited by a lowering of their cloud point temperature (CP), pour point (PP), and cold filter plugging point (CFPP). It has therefore been concluded that the technology of oil zeoforming to improve its performance parameters before the main hydroconversion may potentially be useful in the production of diesel biocomponents.8−11 Thus far, this type of study has been performed with one type of material, the H-ZSM-5 zeoforming catalyst, for which the hydroconversion of the zeoformed vegetable oil was realized using the co-processing variant.8−11 The use of co-processing (processing of vegetable oil in a mixture with a hydrocarbon fraction) effectively hinders interpretation of the obtained results11 because the final product contains a quantity of identical hydrocarbons derived from natural oils and present in petroleum. The difficulty is to assess the impact of the initial modification of the vegetable oil on its performance parameters, such as its distillation characteristics, because the petroleum fraction, which is a major part of the mixture in the coprocessing, determines its properties to a large extent. In the paper,8 the issue of catalyst coking during the process of vegetable oil zeoforming was discussed. It has been observed that the effect of catalyst coking in this case is much weaker than during classical zeoforming. Most likely, this is due to the relatively low temperatures of triglyceride zeoforming relative to those for the classical process of zeoforming. This paper presents the results of research on the hydroconversion of zeoformed rapeseed oil. Initial zeoforming of rapeseed oil was realized over the H-ZSM-5 catalyst characterized by high activity toward the formation of cyclic structures.13 The purpose of this study was to determine the effect of initial zeoforming of rapeseed oil over a catalyst-comprising hydrogen form of ZSM-5 (HZSM-5) on the properties of the biocomponents obtained by hydroconversion. The zeoforming catalyst was specially chosen for this research to determine the applicability of relatively mild conditions (low pressure and temperature), typically used in zeoforming of light paraffinic

2. EXPERIMENTAL SECTION 2.1. Analytical Methods. Esterification of the selected samples of the zeoformates was performed 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 edition) were used. The following properties of the liquid fractions obtained (hydroraffinates) were determined: a visual evaluation (color, clarity, and uniformity), the sulfur content (EN ISO 20884 standard), the density (EN ISO 12185), the acidity (according to EN 14104), the content of unreacted glycerides (EN 14105), the distillation characteristics (according to DIN EN ISO 3405), the pour 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). In the determination of the density of the hydroraffinates obtained at the lower temperatures of the tested range, it was found that several had pour points above +15 °C. In these cases, their densities were established at +25 and +50 °C, and the density at +15 °C was determined by extrapolation. The density values of these hydroraffinates are indicated. They should be considered as approximate results. 2.2. Feedstock and Catalyst. In the tests, edible, refined, and commercially available rapeseed oil produced by the Kruszwica ZPT Company was used with a composition of fatty acids in the triglycerides, as indicated in Table 1.

Table 1. Content of Fatty Acids in the Rapeseed Oil Triglycerides Used in the Research (wt %) fatty acid content in the rapeseed oil triglycerides used in test (wt %)

fatty acid myristic palmitic palmitoleic stearic oleic linoleic linolenic arachidic eikozenoic behenic erucic lignoceric nervonic unidentified

C14:0 C16:0 C16:1 C18:0 C18:1 C18:2 C18:3 C20:0 C20:1 C22:0 C22:1 C24:0 C24:1

0.1 4.3 0.2 1.9 63.2 18.5 7.4 0.6 1.6 0.4 1.4 0.2 0.1 0.1

The hydrogen used in the hydroconversion process was of 99.99% purity. The raw materials (zeoformed or non-zeoformed rapeseed oils) were mixed with dimethyl disulfide (DMDS) (methyldisulfanyl)methane to protect the sulfided surface of the NiMo/Al2O3 catalyst from deactivation.11 The DMDS was applied such that the sulfur content in each of the raw materials was approximately 600 mg/kg. To implement the vegetable oil zeoforming, commercial zeolite material (delivered by Aditen Company, Poland) was used along with the hydrogen form of the ZSM-5 catalyst. The catalyst was characterized by the silicon module of approximately 80−90, specific 7570

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280 and 300 °C were used. These results were consistent with previous results.8−11 The high acidity level of the obtained zeoformates (up to more than 10 mg of KOH/g) was detected. This allows us to assume that the decomposition of the glyceride structures and the formation of free acids may take place. Observed reactions seem unusual because there was no additional hydrogen feeding during the zeoforming. Further studies are required to understand the exact mechanism of this process. For the zeoformate of rapeseed oil obtained at 300 °C, a fatty acid content analysis was performed. It was found that this zeoformate contained approximately 10.6 wt % unsaponifiable compounds (Table 2). The fatty acid composition clearly

surface area (SBET) of 360 m2/g, and total pore volume of 0.6 cm3/g (0.5 cm3/g meso- and macropore volume). The acidity [by temperature-programmed desorption of ammonia (NH3-TPD)] was 0.25 mmol/g. This zeolite material used had the form of circular extrusions: 2 mm in diameter and 3−5 mm in length. Similar materials have previously been used in studies on the zeoforming of light paraffinic fractions.14,15 In previous studies,8−11 alternative ZSM-5 zeolite material was used, characterized by the silicon module 70−80 and comparable to the other parameters listed above, however, with a significantly reduced volume of meso- and macropores. In this work, a NiMo catalyst on an Al2O3 carrier was used for the hydroconversion of zeoformates and fresh vegetable oil.8 The catalyst has been developed for deep hydrotreating of middle distillates, especially for deep hydrodesulfurization. 2.3. Experimental Setup. The experiment was conducted in two stages. In the first stage, the rapeseed oil was zeoformed under the following conditions: a raw material feeding relative to the catalyst volume [liquid hourly space velocity (LHSV)] of 1.0 h−1, a pressure of 1.7 MPa, and temperatures of 200, 220, 240, 260, 280, and 300 °C. The first step of the test was drying the catalyst at 150 °C in a stream of nitrogen at atmospheric pressure. This was followed by an increase in the nitrogen pressure to 1.7 MPa and the initiation of the raw material feeding at a rate of 100 mL/h (LHSV = 1.0 h−1) relative to the volume of the catalyst (100 mL, 86 g) in the catalytic bed. At the same time, the temperature was increased to 200 °C. No use of a supplementary flow of gas (hydrogen) through the reactor was applied. After stable conditions were reached for a given process, the process itself was conducted for 6 h, allowing for the sampling of zeoformates in the amounts necessary for further studies. The rapeseed oil zeoforming was performed using an OL-105 apparatus containing a 100 mL flow reactor. The device was equipped with a raw material pump, a separator, and a system for the collection of the converted fractions, both liquid and gas. Detailed description of the apparatus is given in ref 8. The feed rate was controlled by an automatic measuring system. Only selected properties of the zeoformates were determined. In the second part of the experiment, the zeoformates obtained were subjected to hydroconversion under fixed conditions. A flow reactor with a 100 mL capacity was used for this purpose and equipped as necessary. The NiMo catalyst (100 mL, 88 g) employed in this step was sulfided in accordance with the typical research procedure using diesel fuel with the addition of 2% sulfur (in the form of DMDS).11 After the process of the catalyst sulfurization was finished, the test system was flushed with the middle petroleum fraction that had a low sulfur content for 24 h to rinse the system. The hydroconversion conditions used were the following: a temperature of 340 °C, a LHSV of 0.5 h−1, a pressure 4.5 MPa, and a hydrogen feed of 1500 Nm3/m3. The raw materials were prepared according to the test procedure described in section 2.2. The hydroconversion process was conducted for 5 h for each type of input raw material. The process stabilization periods (2 h) were applied between the measurements (the collection of the converted product). For comparison, the hydroconversion of a non-zeoformed rapeseed oil was also performed. The obtained hydroraffinates (liquids) were purged with nitrogen for 1 h (gas stripping) to remove dissolved gases.

Table 2. Fatty Acid Content in Rapeseed Oil Triglycerides Zeoformed at 300 °C over a ZSM-5 Catalyst (wt %) fatty acid myristic palmitic palmitoleic stearic oleic linoleic linolenic arachidic eikozenoic behenic erucic lignoceric nervonic unidentified unsaponifiable compounds

content (wt %) C14:0 C16:0 C16:1 C18:0 C18:1 C18:2 C18:3 C20:0 C20:1 C22:0 C22:1 C24:0 C24:1

0.1 6.1 0.5 2.3 53.4 21.9 1.2 0.3 0.3 0.4 0.5 0.1 0.1 2.2 10.6

changed in comparison to that of the raw material used (see Tables 1 and 2). There was also an increase in the content of fatty acids with undetermined structures. The hydroraffinates obtained from the zeoformed natural oils had the form of clear liquid fractions of 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 consistent with what was predicted because water is one of the co-products of triglyceride hydroconversion.11,16−18 The obtained samples, after stripping (purging) them with nitrogen (for 1 h), were poured into a separation unit, and then the aqueous phase was separated. In all of the hydroraffinates obtained, the sulfur content determined was less than 5 mg/kg (ppm). For the obtained hydroraffinates, the density was 786−825 kg/m3 (at 15 °C), but this parameter clearly increased with the zeoforming temperature. The lowest density corresponded to the hydroraffinate obtained from the rapeseed oil that was not subjected to zeoforming. The increased density could be caused by the presence of hydrocarbon compounds other than paraffins during zeoforming and then hydroconversion. Traces of unreacted (unbroken) glycerides were found in the hydroraffinates obtained from the natural oil, reaching below 0.05 wt % for monoglycerides, below 0.10 wt % for diglycerides, and below 0.1 wt % for triglycerides. The low-temperature properties of the products were considered to be important parameters because of their potential use as biocomponents of engine fuels (diesel fuel).

3. RESULTS AND DISCUSSION The rapeseed oil zeoformates (the liquid fraction) obtained in the first part of the planned experiment were homogeneous and clear, with a consistency similar to the raw material. The zeoformates were darker in color than the input oil, and a darker appearance corresponded to a higher process temperature, with colors ranging from yellow−brown to orange−dark brown. While the process of the rapeseed oil zeoforming was conducted, the gas formation of 1−3 dm3/h was detected with the highest amounts when the highest process temperatures of 7571

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Table 3. Selected Properties of Hydroraffinates Obtained from Zeoformed and Non-zeoformed Rapeseed Oils hydroraffinate of rapeseed oil non-zeoformed zeoformed at 200 °C 220 °C 240 °C 260 °C 280 °C 300 °C a

CFFP (°C)

density (kg/m3)

sulfur content (mg/kg)

+22

782a