Article pubs.acs.org/IECR
Cite This: Ind. Eng. Chem. Res. 2018, 57, 11843−11851
Alternative Non-Food-Based Diesel Fuels and Base Oils András Holló,†,‡ Annett Wollmann,§ Ferenc Lónyi,∥ József Valyon,∥ and Jenő Hancsók*,‡ †
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Downstream Research and Development, MOL Hungarian Oil and Gas Ltd., 2 Olajmunkás street, H-2440 Százhalombatta, Hungary ‡ MOL Department of Hydrocarbon and Coal Processing, University of Pannonia, 10 Egyetem street, H-8200 Veszprém, Hungary § CUTEC-Institut GmbH, 23 Leibnizstraße, D-38678 Clausthal-Zellerfeld, Germany ∥ Institute of Materials and Environmental Chemistry, Research Centre for Natural Sciences, Hungarian Academy of Sciences, P.O. Box 286, H-1519 Budapest, Hungary S Supporting Information *
ABSTRACT: The importance of aliphatic fuel and base oil blending components produced on a nonpetroleum basis is increasing. Therefore, the possibility to produce these environmentally friendly products in new catalytic ways was studied. High molecular weight Fischer−Tropsch waxes synthesized from lignocellulose-based synthesis gas, waste fatty acids, and different triglycerides were converted to diesel fuels and base oils over NiMoP/Al2O3 and Pt/SAPO-11 catalysts. The optimal operating parameters of high-quality products were determined. The activity of a special Pt/SAPO-11 catalyst, synthesized by a novel method, was compared with a conventionally synthesized Pt/SAPO-11 one in bio-alkane hydrocracking/isomerization. The novel catalyst had higher activity and selectivity than the conventionally manufactured one. Results of present study show that reactions can be directed to get mainly either monobranched or multibranched alkanes by selecting appropriate process parameters. The properties and the quality of the products could be controlled in a wide range to satisfy the quality demands.
1. INTRODUCTION Global demand for transportation energy carriers is expected to increase significantly in the world by the middle of the century. However, the future demand of liquid fuels will stagnate or decrease, e.g., in the countries of Organization for Economic Co-operation and Development (OECD), by 2040 based on different predictions.1,2 The reasons for the decreasing demand are the new environmental protection legislations, the utilization of more efficient and new powertrains, and changes of consumer behavior that contribute to greenhouse gas (GHG) reduction in mobility. However, the role of high quality jet and diesel fuels will be still significant or higher in the future in different transportation areas, e.g., in air transportation, heavy duty road transportation, and inland or seaborne shipping.2,3 In these segments the non-food-based biomass or waste-based diesel fuels (advanced biofuels) having low greenhouse gas intensities are especially favored by future legislation, e.g., in the European Union, too, setting the minimum targets for advanced biofuels (3.6% in energy terms in 2030). On the other hand, the blending limit of food-based or feed-crop-based biofuels would be gradually reduced to 3.8% (in energy terms) by 2030. Sustainability criteria for biofuels will be also amended, requiring that advanced biofuels must emit at least 70% less GHG than fuels of fossil carbon origin.4 © 2018 American Chemical Society
Another aspect is that the automotive industry prefers environmentally friendly, oxygen-free biobased diesel fuels. In the fifth edition of the World Wide Fuel Charter (WWFC), the utilization of high quality synthetic fuels containing bio-alkane (gas-to-liquids, coal-to-liquids, biomass-to-liquids, and wasteto-liquids) and hydrotreated triglycerides is especially preferred.5 Conventional fatty acid methyl ester (FAME) is proposed to be phased out from markets with highly advanced requirements for emission control and fuel efficiency. Additionally, the blending of FAME into crude oil based diesel fuel can result in lower storage and oxidation stability, worse cold flow properties, higher tendency to produce deposits on engine parts, some seal incompatibilities, hygroscopic effect, water sensitivity, and higher biodegradability compared to crude based fuels of compression ignition engines.5,6 Since aromatics, polyaromatics, olefins, and oxygen-containing components are limited by the fuel standards and the different categories of WWFC, too, there is only one option to produce environmentally friendly biomass or waste based blending components. These are the alkanes (isoalkanes, normal alkanes, and Received: Revised: Accepted: Published: 11843
May 23, 2018 August 7, 2018 August 8, 2018 August 8, 2018 DOI: 10.1021/acs.iecr.8b02295 Ind. Eng. Chem. Res. 2018, 57, 11843−11851
Article
Industrial & Engineering Chemistry Research
Figure 1. Feasible and most economical ways of converting waste and biomass to diesel-range and base-oil-range alkanes.
tion of biomass-based FT wax using known and new catalytic systems. This study also deals with the conversion of non-foodbase or waste-based fatty acids/triglycerides to alkanes by hydrodeoxygenation, and to upgrade of the alkane mixture by hydrocracking and hydroisomerization over different catalysts. Without the necessity of completeness, we point out that, in the industry, the hydrocracking of middle and heavy distillates of crude oil is mainly carried out, e.g., in fixed bed reactors (multiple split beds), intermediate cooling with one or more stages, or in ebullated bed technologies. As catalysts, metalremoving, heteroatom-removing, prehydrocracking, and major hydrocracking and stability-increasing post hydrogenation catalysts are placed in the reactors. Metal, sulfur, nitrogen, and oxygen removal catalysts with different activities are used. The main process parameters are the following: pressure, 80− 200 bar; temperature, 340−400 °C; liquid hourly space velocity (LHSV), 1−2 h−1; hydrogen:hydrocarbon ratio, 800− 1000.6,17 For hydroisomerization, a split bed reactor or two reactors are used with fewer catalyst splits. In general, two types of catalysts are used for middle distillates: a selective hydrocracking one and a hydroisomerization one. The catalyst used in the first case has larger acidity. This has higher cracking activity of the hydrocarbon chain and has lower product yield. For selective hydroisomerization, catalysts with less acidic strengths are used; the main reaction on the latter is the chain rearrangement and only a small degree of cracking. Main process parameters are the following: pressure, 40−50 bar; temperature, 300−360 °C; LHSV, 1−2 h−1; hydrogen:hydrocarbon molar ratio, 0.5−2.6,17 For oxygen-containing, long carbon chain compounds (triglycerides from different sources), usually a prehydrogenation reactor is applied to saturate the olefin double bonds followed by the oxygen removal reactor. The formed isoparaffin mixture can be produced by isomerization of the normal paraffin. For oxygen removal of the feedstocks (fatty acids and fatty acid esters from different sources) with high oxygen content (5−15%) CoMo, NiMo catalysts of different compositions on Al2O3 supports are applied. When oxygen is removed from natural triglycerides, propylene cleavage and its fast hydrogenation to propane take place. The resulting nparaffin mixtures are generally selectively isomerized (e.g., on Pt/H-ZSM-22, Pt-conventional SAPO-11, etc.). Main process parameters are the following: pressure, 40−80 bar; temperature, 300−360 °C; LHSV, 1−2 h−1; hydrogen:hydrocarbon ratio, 400−600.6,17
cycloalkanes), which have higher hydrogen and lower carbon contents than the above-mentioned compounds. Based on the aforesaid reasons, our research target was to identify, select, and develop economical routes to manufacture non-food biomass based or waste based, practically oxygen-free and sulfur-free hydrocarbon products having high hydrogen content in the molecule. There are numerous routes for biomass to fuel conversion for internal combustion engines (purified vegetable oil, FAME, bio-methane, bio-alcohols, biohydrogen, synthetic hydrocarbons, etc., including bio-alkanes, too).6 However, only high isoalkanes and n-alkanes containing diesel blending components produced from waste or non-food based biomass meet the above requirements. The most promising and feasible methods to produce non petroleum based diesel-range and/or base-oil-range alkanes are (i) the synthesis of bio-alkane from lignocellulose/starch/sugar feedstocks through sugar molecules with their multistep conversion;7−9 (ii) liquefaction of lignocellulose, and then further hydrogenation;10−14 (iii) the oxygen-free fast pyrolysis of lignocellulose feedstocks, and then further deoxygenation;15−21 (iv) the production of n-alkane and isoalkane mixture from biomass through synthesis gas and Fischer−Tropsch (FT) synthesis, e.g., with the hydrocracking and isomerization of heavy FT waxes;22−26 (v) the hydrogenation and a possible further isomerization of natural and/or waste triglycerides/ fatty acids from non-food or waste based feedstocks resulting bio-alkane products.27−33 These important routes of bio-alkane production are summarized in Figure 1. Options i−iii producing bio-alkane seem to be too complex and expensive, and further extensive research is needed to clarify all uncertainties. Various catalysts are proposed, e.g., for the isomerization of various molecular weight paraffins. In the case of gas oil boiling range fractions, mainly different types of SAPO-116,17,34−36 and H-ZSM-22 bifunctional catalyst are proposed6,17,37−39 containing Pt, Co, or Ni metals. However, experiments are usually performed with model compounds and there are few experimental results with realistic (industrial) feedstocks. In the meantime, short-term solutions are needed for fuel manufacturers to produce high quality, less expensive biofuels having lower carbon intensity (higher hydrogen content) from waste or non-food biomass to meet near-future environmental protection legislation. That is the reason for our research to find appropriate catalyst(s), process parameters, and new feedstocks for advanced diesel fuel and in some cases for lubricating base oil production. One aspect of our work, reported here, was to find favorable process parameters for the hydrocracking and hydroisomeriza11844
DOI: 10.1021/acs.iecr.8b02295 Ind. Eng. Chem. Res. 2018, 57, 11843−11851
Article
Industrial & Engineering Chemistry Research
2. EXPERIMENTAL SECTION 2.1. Catalysts. Two SAPO-11 catalysts were prepared according to a conventional method (SAPO-11C) and a new method (SAPO-11N). SAPO-11 material has a monodimensional channel system, which consists of nonintersecting elliptical 10-membered-ring pores of diameter 0.39 × 0.63 nm. According to the conventional synthesis of SAPO-11, water-diluted phosphoric acid, aluminum isopropylate, silica gel, and di-n-propylamine (DNPA) template are used.40,41 The synthesis takes place at 170−200 °C temperature, and it lasts 4−5 days. The crystalline product is calcined in air. In this work, the SAPO-11N material was prepared according to a new method. We used a buffer during the crystallization. The buffer was formed from a weak acid and base pair, dissolved in the medium of the synthesis. Namely, the acid was acetic acid and the base was the template DNPA. Instead of acetic acid it was found advantageous to use aluminum acetate or basic aluminum acetate, Al(OH)(CH3COO)2, in the synthesis mixture. The amount of the latter corresponded to the stoichiometric equivalent of the aluminum isopropylate, used according to the conventional synthesis method. The aluminum hydroxyl acetate hydrolyzes in the presence of aqueous phosphoric acid solution in the reacting mixture. The amount of acetic acid continuously increases as the aluminum hydroxide builds into the SAPO11N crystals. The acid made forms a salt with a portion of the basic template used in large stoichiometric excess. The salt of the acid suppresses the dissociation of the weak acid. Thus, the acid-salt system behaves as a buffer providing relatively constant pH during the synthesis. The elemental composition of the product obtained by the new synthesis method was the same as that of the conventional preparation because we used the same molar amount of aluminum in the two synthesis processes. An advantage of the new method is that pressure, developed during the synthesis, is lower than the pressure of the conventional synthesis that applies aluminum isopropylate. The reason behind this is that during the hydrolysis of the latter compound propane is also formed. Thus, with the new method the pressure increase in the closed synthesis reactor was only around 3 bar, whereas it was 20−25 bar in the convention synthesis. Besides that, crystallization takes place at lower temperature and shorter time. Typical temperature and time are 130−140 °C and 3−4 days, respectively. The buffer and template were burned out during a post synthesis calcination treatment. The product of the novel synthesis method showed the same X-ray diffraction (XRD) pattern as the conventionally synthesized SAPO-11C. By the incipient wetness method 0.5% Pt was loaded on both SAPO-11 supports in the form of tetraammine platinum(II) hydroxide hydrate precursor. Then catalysts were dried and activated in situ in the reactor by a reduction in hydrogen flow. Commercial NiMoP/alumina catalyst in sulfide form was also applied in the hydrodeoxygenation/cracking of fatty acids and triglyceride feedstocks to alkanes. 2.2. Properties of Catalysts. The ammonium form of the catalysts was prepared from the air-dry and template-free catalysts using conventional, exhaustive, liquid phase ion exchange with aqueous ammonium chloride solution. The filtered and washed NH4,Pt/SAPO-11 samples were heated at a rate of 10 °C min−1 to 600 °C in a flow of inert gas. The released NH3 was made to absorb in water that was
continuously titrated with HCl. The amount of desorbed ammonia was determined as a function of the desorption temperature. Desorption temperature was considered to characterize the acid strength of the sorption site, whereas the amount of desorbed ammonia was considered to be equivalent with the amount of the acid sites. More details about the properties and characterization techniques of the Pt/ SAPO-11 samples are summarized elsewhere.42 The two catalysts prepared (Table 1) contained similar amounts of acid sites; however, the catalyst prepared using the Table 1. Main Properties of Pt/SAPO-11 Catalysts property platinum content, % total acidity, mmol of NH3/g average pore diameter, nm BET surface area, m2/g total pore volume, cm3/g mesopore volume, cm3/g a
Pt/SAPO-11C (conventional)
Pt/SAPO-11N (new synthesis)
0.5 0.65 (0.28/0.37)a
0.5 0.60 (0.41/0.19)a
0.63 × 0.39
0.63 × 0.39
91 0.13
106 0.18
0.08
0.12
Weak acid sites (100−250 °C)/strong acid sites (250−420 °C).
new method (SAPO-11N) showed lower acid strength than the catalyst made according to the traditional method (SAPO11C).43 The platinum content of the catalysts was determined according to the UOP-274 standard. Nitrogen adsorption− desorption measurements were performed on a Micromeritics ASAP 2020 instrument at −196 °C after the samples degassed at 350 °C under vacuum. The specific surfaces areas of the samples were calculated by the Brunauer−Emmett−Teller (BET) method, while micropore volumes were estimated using the de Boer t-plot method and mesopore volumes were estimated by the Barrett−Joyner−Halenda (BJH) method. The crystalline structure of the catalyst was examined by X-ray diffraction (Bruker AXS D8 Advance X-ray diffractometer with Cu Kα radiation; 40 kV, 40 mA). 2.3. Feedstocks. One of the feedstocks was an alkane mixture produced from biomass-based synthesis gas with Fischer−Tropsch synthesis. It was produced and supplied by CUTEC-Institut GmbH.44 The main properties of the FT wax are the following: n-alkane content (C18−C55), 99.4%; pour point, 69 °C; sulfur content, 200−360 °C), base oil fraction, and residue (>360−470 °C boiling point; unconverted C33+). The reason for this was that the separation of the gasoline and diesel fraction (20−360 °C boiling range) from the base oil fraction is necessary to carry out the detailed evaluation of the results. Figure 4 presents the yield of diesel
Table 2. Quality and Yield Data of Diesel Fuel/Base Oil Boiling Range Fractions from FT Wax Conversion over Pt/ SAPO-11Na process params and product properties reaction temperature, °C LHSV, h−1 total yield of valuable products,b % C33+ conversion, % isoparaffin content, % isoparaffins with a single branch, % diesel fuel/base oil yields, % cetane number/viscosity index CFPPc/pour point, °C
diesel fuel
base oil
340 0.75−1.0 98.1−96.5
360 1.5 97.3
96.7−95.3 78.9−76.9 73.1−68.4
95.9 70.4 63.5
38.7−36.1/36.5−43.1 52−56/125−129 −26 to −18/−21 to −13
33.8/38.9 67/137 −11/−16
a
Pressure was 60 bar and H2/hydrocarbon ratio, Nm3/m3, was 700 in all cases. bRelative to the amount of feed. cCold filter plugging point.
a very low CFPP (Arctic grade diesel fuel) and still proper cetane number satisfying the cetane limit of, e.g., the EU diesel fuel standard (EN 590). This result can provide significant flexibility of product quality for oil refineries. Of course, the quality features of the products are fundamentally determined by the hydrocarbon composition produced. A very low CFPP and moderate cetane number of a gas oil fraction were obtained at 340 °C temperature, 60 bar pressure, and 0.75 h−1 LHSV over both Pt/SAPO-11 catalysts. The most favorable CFPP values in the case of Pt/SAPO-11N can be explained with the formation of multibranched isoalkanes on this catalyst. The freezing points of these multibranched isoalkanes are substantially lower than those of the monobranched ones and especially lower than those of the n-alkanes having the same carbon number. Cetane numbers of these multibranched alkanes are substantially smaller than those of single-branched ones, or n-alkanes. The sulfur content of the gas oil fractions obtained was less than 1 mg/kg on both catalysts. The same tendency can be estimated for the obtained base oil fractions for the selected operating parameter combinations (Table 2). The difference is that in this case the two basic product-rating characteristics are the viscosity index, which indicates the extent of variation in viscosity and to some extent lubricity with variation of temperature, as well as the pour point. We emphasize that all products are practically sulfur free that are produced during the catalytic conversion of FT wax made from biomass based synthesis gas. This can be a significant contribution (in addition to the high hydrogen content in the molecule structures) to environmentally friendly mobility. Since the crystal structure of the novel SAPO-11 is the same as that of the conventionally synthesized SAPO-11, the pore size is the same. Nevertheless, the accessible pore volume and the specific surface area of the new preparation are higher. Therefore, the higher number of accessible channels having the same size provides shape selectivity in the same way (inhibiting the formation of multibranched isomers) as the SAPO-11 product produced by conventional methods. Due to the larger number of channels, the conversion of more molecules can take place simultaneously with the desired selectivity. Therefore, the non shape selective side reactions on the outer surface of the Pt/SAPO-11N crystallites, producing multibranched isomers and cracked products, are less likely. As a result of the
Figure 4. Yield of diesel fuel + base oil fractions of FT wax hydrocracking as a function of temperature on 0.5% Pt/SAPO-11N catalyst.
fuel plus base oil fractions as a function of reaction temperature. The yield of the targeted products increased up to 340−360 °C, except when the lowest liquid hourly space velocity (LHSV) was applied. Beyond the temperature range, the yield of the targeted products was lower; however, it remained higher than 90% even at 380 °C. (The yield of a product was simply the ratio of the amount of product actually obtained to the amount of feed.) With the reduction of LHSV the yield of fuel plus base oil fractions clearly increased. The largest value was 98.1% (340 °C, 40−60 bar, and 0.75 h−1). The amount of residue was 0.3−0.4%, and the amount of gas product was 1.6−1.5% with these parameters. At higher LHSV, higher temperatures (350−360 °C) were needed to get conversions comparable to those above. Pressure has a minor effect on the conversion of the feedstock, especially at 380 °C. Table 2 and Table S3 show typical examples of quality and yield data of gas oil and base oil boiling range fractions obtained over Pt/SAPO-11N and Pt/SAPO-11C at advantageous process parameters. The data in Table 2 clearly illustrate that it is possible to select parameter combinations where highquality blending components being in the diesel fuel boiling range can be produced with relatively high yield (33.8−38.7%) over Pt/SAPO-11N; these data are larger than the yields obtained on the Pt/SAPO-11C catalyst. This is due to the higher concentrations of acidic sites in the commercial catalyst, resulting in more gas and liquid products having lower boiling points than the gasoline boiling point. With expediently chosen operating parameters, it is possible to produce a diesel fuel blending component having a extremely high cetane number and still appropriate cold flow properties (cold filter plugging point, CFPP), or a product with 11847
DOI: 10.1021/acs.iecr.8b02295 Ind. Eng. Chem. Res. 2018, 57, 11843−11851
Article
Industrial & Engineering Chemistry Research Table 3. Favorable Results of Fatty Acid/Triglycerides Conversion on Commercial NiMoP/Al2O3 feedstock process paramsa and product properties
fatty acid mixture
high erucic acid rapeseed oil
used cooking oil
waste lard
temperature, °C LHSV, h−1 hydrogen/hydrocarbon ratio, Nm3/m3 theoretical yield,b % approach of theoretical yield,c % n-alkane content, % isoalkane content, % aromatics and cycloalkane content, % oxygen-containing compounds, % cetane number
320 1.2 500 87.3−84.2 96.4 96.4 3.6
