Article pubs.acs.org/EF
Trace Elements in Vegetable Oils after Hydroconversion into Advanced-Generation Biocomponents Łukasz Jęczmionek,*,† Marek Kozak,† Xymena Badura,† Zbigniew Stępień,† Leszek Ziemiański,† Grazẏ na Ż ak,† Wojciech Krasodomski,† and Grzegorz Wcisło‡ †
Oil and Gas Institute - National Research Institute, Lubicz 25A, 31-503 Kraków, Poland Faculty of Production Engineering and Power Technologies, University of Agriculture, 31-120 Kraków, Poland
‡
ABSTRACT: Direct hydroconversion of vegetable oils (by a one-stage process) and hydroconversion of vegetable oil zeoformates (by a two-stage process) lead to hydroraffinates that contain trace elements. The concentrations of these trace elements must be below the inductively coupled plasma optical emission spectrometry determination limit, in accordance with the recommendations and requirements specified in the relevant legislation (Worldwide Fuel Charter). In this work, it was found that some trace elements (Na, Ca, Mg, and P) are deposited on the catalyst surface during the initial zeoforming of vegetable oils. At the same time, other ions (Al) can be removed by elution from the surface of the zeolite catalyst. During hydroconversion, most of the trace elements present in vegetable oils and zeoformates pass into the aqueous phase.
1. INTRODUCTION Several factors can contribute to the presence of trace elements in vegetable oils. These elements may be the components of chemical compounds present in oils/fats (e.g., phospholipids and arsenolipids). The sources of trace elements may be solid or liquid contaminants with different solubilities. The concentrations of trace elements are highly variable, depending upon the origin of feedstock and the environment of its genesis (soil composition, humidity, variety, time of vegetation, fertilization, etc.).1 The contents of trace elements in vegetable oils or animal fats can also change significantly as a result of the oil usage, for example, in frying or baking.1 In vegetable oils, phosphorus is present as a component of phospholipids.2 In addition, fatty acids containing halogens, arsenic, nitrogen, and sulfur may be found in vegetable oils.3−16 Fuels derived from crude oils are typically characterized by low levels of trace elements (less than 1 mg/kg).17,18 These levels, however, can increase dramatically when using biocomponents.19 Trace element contents have an adverse effect on the fuel intake system (nozzles and valves) as well as the exhaust system (exhaust gas catalyst deactivation).20 In addition, trace elements in the fuel contribute to the formation of ash during the combustion process. Detailed information about the impact of various trace elements on the fuel and exhaust system of the internal combustion engine is described in ref 17. In this paper, it was found that the nature of the inactivation process depends upon its mechanism; either the poison acts on the catalyst active phase or mechanical blocking of its pores occurs, limiting the access of the exhaust gas to the active zone. Requirements for the contents of trace elements in diesel fuel are not specified in the EN 590 standard. Such requirements, however, have been described in other documents addressing the issue of quality of fuels, biofuels, and biocomponents. According to the Worldwide Fuel Charter (WWFC), the sum of trace elements in diesel fuel should not exceed 1 mg/kg.21 Restricted elements include metallic elements, such as copper, © 2017 American Chemical Society
iron, manganese, sodium, lead, and zinc, a semi-metallic element, such as silicon, and non-metallic elements, such as phosphorus, sulfur, and chlorine. The above-mentioned recommendations apply equally to fuel in all markets, regardless of local requirements.21 The recommendations given in the WWFC are not obligatory. Requirements for trace element contents in biocomponents of first-generation fuels intended for the production of diesel fuel are given in the standard EN 14214 for fatty acid methyl esters (FAMEs).22 There are very few documented regulations for trace elements in hydrocarbon biocomponents of advancedgeneration fuels intended for diesel production. No material of this type is addressed in the basic document CEN/TS 15940:2012,23 which includes quality requirements for synthetic paraffinic fuels obtained in the gas to liquids (GTL), coal to liquids (CTL), or biomass to liquids (BTL) processes as well as paraffin hydrotreated vegetable oil (HVO) biocomponents obtained from the hydroconversion of vegetable oils and animal fats. Limits for the contents of trace elements in biocomponents (biohydrocarbons) intended for the production of diesel fuel are sometimes given in local acts (e.g., in the Regulation of the Minister of Economy of Poland).24 The presence of trace elements in second-generation biocomponents (biohydrocarbons) is a relatively new issue. This problem has been highlighted by a few authors.25−28 The author of ref 25 proposed the initial demetallization of natural oils before their hydroconversion. A two-stage hydroconversion process, including the initial zeoforming (isomerization) of vegetable oils, has been described in previous papers.29−32 Received: May 31, 2016 Revised: December 16, 2016 Published: January 5, 2017 1536
DOI: 10.1021/acs.energyfuels.6b01310 Energy Fuels 2017, 31, 1536−1543
Article
Energy & Fuels
catalysts (changes in the concentrations of trace elements on the surface). A lab reactor OL-150, consisting of a reactor with a capacity of 100 mL, was used to carry out the catalytic converter test. The apparatus was equipped with a feed pump, a separator, and a cooling system for the received liquid fraction, which is the product. The schematic diagram of the apparatus is shown in Figure 1.
The present work describes the determination of residual trace elements in one- and two-stage (with the preliminary zeoforming of vegetable oil) hydroconversion processes.
2. EXPERIMENTAL SECTION 2.1. Experimental Approaches. The test procedure adopted in this study involved one- and two-stage (with pretreatment by zeoforming of raw materials) hydroconversion of various vegetable oils. The contents of trace elements were determined at each stage of the process: in the feedstock, in the intermediates (zeoformates), and in the final products (hydroraffinates), as well as on the surface of the catalysts, to determine the residual elements at the end of the hydroconversion process. 2.2. Materials and Feedstock. 2.2.1. Feedstock. Two unrefined and degummed rapeseed oils intended for FAME production were used (DRO1 and DRO2) and waste vegetable oils: (a) waste oil after frying pork meat [waste rapeseed oil (WRO)] and (b) waste oil after frying of meals in a restaurant [waste palm oil (WPO)]. 2.2.2. Catalysts. To achieve zeoforming of vegetable oil, zeolite material consisting of a hydrogen form of ZSM-5 was used. This material is characterized by the following properties: surface area of 150 m2/g, acidity of 0.25 mmol/g, according to temperatureprogrammed desorption of ammonia (NH3-TPD), an extrudate circular cross section with a diameter of 2 mm and a length of 3−5 mm, silicon module (80−90), and porosity of 0.6 cm3/g (including 0.5 cm3/g meso- and macropores). An industrial catalyst, NiMo supported on Al2O3, was used for the hydroconversion of the zeoformates obtained from vegetable oils. The vegetable oil was preheated through the inert filling of the reactor (glass beads with a cross section of 1−2 mm). The catalysts were obtained from Aditen, Ltd. 2.2.3. Other Materials. Hydrogen with a purity of 99.99% was used in the hydroconversion process. Raw materials (zeoformates and nonzeoformed natural oils) for the hydroconversion process were spiked with dimethyl disulfide (DMDS) to protect the surface of the hydroconversion catalyst (NiMo/Al2O3) from inactivation. DMDS was added in such a way that the sulfur content in each material was approximately 600 mg/kg. 2.3. Analytical Techniques. Inductively coupled plasma optical emission spectrometry (ICP−OES) was applied for the determination of trace elements using an optical emission spectrometer with an inductively coupled plasma SPECTRO ARCOS SOP (Spectro Analytical Instruments, Germany). The operating conditions for the spectrometer were chosen according to the literature33 and the recommendations of the manufacturer. Trace elements in the natural oils were determined after dilution with kerosene and the addition of an internal standard. Calibration with an internal standard was used to correct physical interference effects caused by the differences in the viscosities of samples and standard solutions. The standard oil S-21 + K + Li + Sr (CONOSTAN, Champlain, NY, U.S.A.) was used for multi0element analysis, while an internal standard of yttrium (CONOSTAN, Champlain, NY, U.S.A.) was used for single-element analysis. For the determination of trace elements in the aqueous phase obtained in the hydroconversion of degummed rapeseed oil and zeoforming of rapeseed oil, samples and standard solutions were acidified with nitric acid (V). The aqueous multi-element standards QC standard 1 (AccuStandard, New Haven, CT, U.S.A.) and QC Standard N 2 (AccuStandard, New Haven, CT, U.S.A.) were used for the preparation of aqueous standard solutions. The calibrations were carried out using the standard series method. Sulfur was determined by X-ray diffraction according to PN-EN ISO 20884, using a Sindie-7039 apparatus. The pH of the aqueous phase (co-product of vegetable oil hydroconversion) was determined according to the PN-89/C-04963 test method. X-ray fluorescence combined with scanning electron microscopy (EDAX, Nova Nano SEM apparatus) was used to test the surface of
Figure 1. Schematic diagram of the zeoforming and hydroconversion apparatus. Oils used as feedstocks (DRO1, DRO2, WRO, and WPO) were treated via two methods: (1) In the single-stage experiment, vegetable oils (feedstock) were subjected to direct hydroconversion under the conditions described above. (2) In the two-stage experiment, the first stage of catalytic converter tests was the zeoforming of the vegetable oils. A zeoforming catalyst was dried under a nitrogen stream at 150 °C and atmospheric pressure. This was followed by pressurization to 1.7 MPa (with a nitrogen atmosphere) and next by treatment with the feedstock [liquid hourly space velocity (LHSV) = 3.0 h−1], while the temperature was simultaneously increased to 250 °C. Because gases were formed during the zeoforming process, there was no additional flow of gas (hydrogen) through the reactor. Zeoforming was carried out at a temperature of 320 °C. After stabilization of the temperature, zeoforming was continued for several hours, to obtain a sufficient amount of sample for further testing. The second stage of the experiment was the hydroconversion of the zeoformed (after zeoforming) and the non-zeoformed waste natural oils. The tests were carried out under the following conditions: volumetric application of the feedstock at a rate of 0.5 h−1, addition of hydrogen in a H2/feedstock ratio of 2000 Nm3/m3, pressure of 4.5 MPa, and a temperature of 360 °C for the hydrotreating process. The obtained hydroraffinates were purged for 1 h with hydrogen to remove dissolved gases. The complete conversion of triglycerides in hydroraffinates was confirmed by gas chromatography. Trace element contents were determined in the feedstock, in the zeoformed vegetable oils (in the two-step experiment), and in the final hydroraffinates, as well as in the aqueous phase formed during the hydroconversion process. The elemental composition of the catalyst surfaces was examined: in the fresh zeoforming catalyst and used catalysts in each of the feedstocks, as well as the hydrotreating catalyst used for hydroconversion.
3. RESULTS AND DISCUSSION One of the most important problems in vegetable oil hydroconversion seems to be the typical presence of pnictogens in vegetable oils (mainly phosphorus and arsenic). For example, even pre-refined rapeseed oil may contain a few hundred milligrams per kilogram of phosphorus.26 Arsenic is present in certain fish oils in concentrations up to approximately 10 mg/ kg.12−14 The level of trace elements is indeed very low in waste 1537
DOI: 10.1021/acs.energyfuels.6b01310 Energy Fuels 2017, 31, 1536−1543
Article
Energy & Fuels
3.2. Two-Stage Process. The contents of trace elements in the feedstock, the hydroraffinates from the two-stage process, and the zeoformates and hydroraffinates of the zeoformates from the two-stage process are shown in Tables 2 and 3. The EDAX analyzes for the surface of the zeoforming catalyst (ZSM-5), fresh and used (in zeoforming of waste natural oil and degummed natural oil), are shown in Figure 2. Scanning electron microscopy (SEM) images of the tested catalyst surfaces are shown in Scheme 1. The results of the EDAX analysis of the surface of the zeoforming catalyst (NiMo/Al2O3), fresh and used (in hydroconversion of zeoformates degummed natural oil), are shown in panels A and B of Figure 3, respectively. SEM images of the tested catalyst surfaces are shown in Scheme 2. A small amount of the aqueous phase (several tens of milliliters with respect to 1 L of hydroraffinate, 70 mL of H2O/ 1 L of DRO1, and 62 mL of H2O/1 L of WRO) was obtained during the hydroconversion of vegetable oils and their zeoformates. The water was accumulated at the bottom and on the walls of the receiver (a glass flask with a capacity of 1000 mL). The entire contents of the flask (aqueous phase and hydrocarbon phase) were poured into a separate funnel and separated into the individual phases. The aqueous phase was clear and colorless and showed no signs of sludge. The phase was slightly acidic (pH 6.0−6.2), which may indicate the presence of hydrolyzed salts. The results of trace elemental analysis using ICP−OES are shown in Table 4. The aqueous phase revealed the presence of elements introduced with the raw material (vegetable oil): Na, Ca, K, Mg, and P. For vegetable oil zeoformates, Al, Cr, Fe, Mn, Ni, V, and Zn were also introduced into the hydroconversion reactor in addition to Na, Ca, K, Mg, and P. On the basis of the results shown in Tables 1 and 2, it was found that the direct (single-stage) hydroconversion of waste and degummed vegetable oils leads to hydroraffinates with trace element levels below the detection limit, thus fulfilling the recommendations and requirements specified by the relevant legislation in this regard.21−24
vegetable oils (used for frying) because they are mainly used in the kitchen for cooking. The feedstock for highly refined oils contains a small percentage.27 Trace elements, including arsenic or phosphorus, can accumulate on the hydrotreating catalyst during the hydroconversion of vegetable oils,25 thus decreasing the likelihood of the transfer of these undesirable elements into the final fuel. However, this creates another problem, namely, the loss of or change in the activity and selectivity of the catalyst. According to some results,25 during the hydroconversion process of a feedstock containing 5 mg/kg of phosphorus and at LHSV = 1 h−1, more than 43 kg of phosphorus may condense on the catalyst bed per year per ton of catalyst. 3.1. One-Stage Process. The contents of trace elements in the feedstock, the hydroraffinates from the one-stage process, and the zeoformates and hydroraffinates from the two-stage process are shown in Tables 1 and 2. Table 1. Contents of Trace Elements Determined for the Products and Substrates in the Direct Hydroconversion (One-Stage Process) of Waste Vegetable Oils (WPO and WRO)
element P S Ag, Ba, Ca, Cd, Cr, Cu, Fe, Li, Mg, Mn, Mo, Na, Ni, Sr, Ti, V, and Zn Al, B, K, Pb, and Si Sn a
waste rapeseed oil (WRO) (mg/kg)
waste palm oil (WPO) (mg/kg)
hydroraffinates (mg/kg)
2.0 ± 0.8 21
