Production of Renewable Hydrocarbons by Thermal Cracking of Oleic

Aug 2, 2017 - The results showed that distribution of the different classes of compounds was mainly dependent on the reaction temperature. In the abse...
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Production of Renewable Hydrocarbons by Thermal Cracking of Oleic Acid in the Presence of Water Mehdi Omidghane, Ehsan Jenab, Michael Chae, and David C. Bressler* Department of Agricultural, Food and Nutritional Science, University of Alberta, Edmonton, Canada T6G 2P5 ABSTRACT: The thermal cracking of oleic acid, a model fatty acid, in the temperature range of 390−450 °C was studied in the presence and absence of water. The effect of water on conversion yield and product distribution was determined. Analysis of the pyrolysis product by gas chromatography and mass spectrometry revealed the presence of expected compounds such as alkanes, alkenes, cyclic hydrocarbons, aromatics, and fatty acids in the liquid product. The results showed that distribution of the different classes of compounds was mainly dependent on the reaction temperature. In the absence of water, the products generated from pyrolysis of fatty acids at 430 and 450 °C yielded lower volumes of liquid product and higher amounts of aromatic compounds and solid residues than at 390 °C. However, these reactions still displayed high conversion and low fatty acid content. In the presence of water, an increase in liquid product yield was measured compared to pyrolysis conducted in the absence of water. However, the presence of water resulted in a remarkable drop in conversion (from 91.2 ± 1.0% to 74.2 ± 0.4%) at 390 °C. At 430 °C, the conversion did not change significantly in the presence of 5% water. At 450 °C, the conversion decreased slightly from 99.7 ± 0.1 to 98.1 ± 1.1 and to 98.2 ± 0.8 in the presence of 5 and 10% water, respectively. Despite the lower conversion at 450 °C, the reduction in absolute amount was very small and thus the conversion rate was still close to 100%. At 450 °C, a decrease in solid residue yield and aromatics was also observed in the presence of water, while cyclic compounds increased at both 430 and 450 °C. The plausible underlying mechanisms have been discussed in this work.

1. INTRODUCTION The growing concerns about global warming have resulted in a quest for alternative sources of energy. The emission of greenhouse gases associated with the current technologies of energy production has been linked to climate change and other environmental issues. As a result, scientists have investigated alternative routes to produce energy from renewable sources such as biomass, plant oils, and animal fats. Many studies devoted to this field can be found in the literature, focusing on green alternatives to conventional fuels and chemicals. These works reported on possible conversion methods to produce hydrocarbons (typically liquid fuels) from renewable materials.1−9 Various technologies have been developed to produce renewable fuels with several advantages and disadvantages.10,11 One of the most promising techniques to produce transportation fuels is thermochemical liquefaction or pyrolysis.12−14 In this process, organic matter in feedstocks are decomposed to useful products through a series of thermochemical reactions occurring at a relatively high temperature and in the absence of oxygen. The literature on pyrolysis is vast due to its enormous applications.15−17 The composition of the pyrolysis product, however, depends on the operating conditions and type of feedstock used; for example, pyrolysis of lignocelluslosic biomass and triglyceride materials results in a liquid that is very complex and highly oxygenated.18 The products of pyrolysis often require upgrading, a catalytic process requiring additional expense and capital intensity.19−22 Although the catalytic conversion of all sources of biomass is possible and has been reported commonly in the literature,23−25 it is very cost intensive, and thus a direct and noncatalytic pathway is preferred. In this context, a noncatalytic pyrolysis of free fatty acids was © XXXX American Chemical Society

developed to produce liquid hydrocarbons with excellent compatibility with existing petroleum-based fuels and associated infrastructure.26−29 Vegetable oils, plant lipids, animal fats, and other widely available and low cost feedstocks such as waste oils and fats (e.g., brown and yellow grease) consist of mostly triglycerides. These compounds are made up of 1 mol of glycerol and 3 mol of fatty acids. The fatty acids are typically of various chain length and degrees of unsaturation. Previous studies showed that direct pyrolysis of triglycerides resulted in highly oxygenated products;18 however, the fatty acids could be released through various hydrolysis techniques and then used in pyrolysis.11,30,31 It has been reported that the product obtained from pyrolysis of free fatty acids was dominated mostly by aliphatic hydrocarbons with some aromatic compounds, and unlike the product from other methods, upgrading was not required.26,27,29 The performance of the pyrolysis process of free fatty acids was investigated in the presence of some reagents. As an example, Asomaning et al.32 investigated the pyrolysis of fatty acids in light unsaturated hydrocarbon gas atmospheres to improve conversion and liquid yield. However, pyrolysis of fatty acids in the presence of water, and in the absence of a catalyst, has not been reported.4,33,34 The presence of water may influence the pyrolysis reaction. Furthermore, feedstocks such as yellow and brown grease, which are mixtures of used cooking vegetable oils and animal fats, may contain some amounts of water.31 Therefore, it is important to investigate the negative or Received: April 6, 2017 Revised: July 5, 2017 Published: August 2, 2017 A

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and then submerged into a bucket of water at room temperature to terminate the reaction. They were then cleaned and dried using compressed air. All experiments were replicated three times. 2.3. Gas Product Analysis. The microreactors were weighed before and after releasing the gas product to determine the mass of gas generated. After each experiment, a sample of gas product was taken using a gastight syringe for gas chromatography (GC) analysis. The collected gas sample was transferred and maintained in 5 mL vacutainer tubes (BD Biosciences, Franklin Lakes, NJ, USA) before further analysis. Two sets of analyses were conducted on the gas product: (1) N2, CO, CO2, and CH4 analysis in the gas samples was carried out by injecting 100 μL aliquots of gas samples into an Agilent 19091P-MS4 packed column in a gas chromatographer (7890A, Agilent Technologies, Fort Worth, TX, USA) equipped with a thermal conductivity detector (TCD). The injector and detector temperatures were set at 170 °C. The GC oven temperature program was set at 40 °C for 2 min and then increased to 170 °C at a rate of 10 °C min−1, and then held for 4 min; it was then increased to 180 °C in 1 min and held for 4 min to achieve a total run of 25 min. The carrier gas for analysis of N2, CO, and CO2 was helium with a constant pressure of 20 psi; it was switched to argon for the analysis of H2. (2) To quantify hydrocarbons in the gas fraction, a gas chromatographer (7890A, Agilent) equipped with a flame ionization detector (FID) was used. A sample of 100 μL of the gas fraction was injected onto a CP-Al2O3/Na2SO4 Varian capillary column (50 m × 320 μm × 5 μm; Varian Inc., Lake Forest, CA, USA). The injector and detector temperatures were set at 170 and 230 °C, respectively. Helium was used as the carrier gas at a flow rate of 1.6 mL min−1. The initial oven temperature was held at 70 °C for 0.67 min, increased to 170 °C at a rate of 3 °C min−1, and then held for 26 min to achieve a total run time of 60 min. Compounds in the gas product were identified by comparing their retention times with pure standards. All the hydrocarbons in the gas products were quantified using methane as a standard. A standard curve for methane for hydrocarbon quantification was available by injections of 2.5, 10, 40, 70, and 100 μL triplicates at the same conditions. 2.4. Liquid Product Analysis. After venting, the microreactors were opened and the liquid products were extracted with pentane as follows. After removing the reactor heads, 10 mL of internal standard solvent solution (130 mg of methyl nonadecanoate aka nonadecanoic acid methyl ester (C19 fatty acid methyl ester) in 100 mL of pentane) was added to each microreactor to dissolve and dilute the product. The content was thoroughly mixed with a glass agitator. After mixing, the microreactor was left for 15 min. The contents were poured into sample vials and capped with Teflon lined screw caps, and then stored at 4 °C prior to analysis. Any solid materials left in the reactor were considered pentane insoluble residues. To measure the amount of solid residue, the microreactor was left in a fumehood overnight until all the solvent had evaporated. The weight of the microreactor was measured and then it was thoroughly cleaned with solvent. The weight difference before and after cleaning was considered as an estimate of the weight of solid residue. Prior to GC analysis, the liquid sample obtained from pyrolysis of oleic acid in the presence of water was first dehydrated by mixing with 1 g of sodium sulfate. It was then centrifuged to separate the sodium sulfate. To analyze the liquid fraction, a gas chromatographer (6890N, Agilent) equipped with an FID in split mode was used. Prior to GC analysis, 150 μL of the diluted sample was added to a dram vial containing 1 mL of diazomethane to convert any fatty acids into their methyl esters for GC analysis.39 The Agilent 7683 series autosampler was used to inject 1 μL of each sample onto a HP-MS5 column (30 m × 250 μm × 0.25 μm; Agilent). Helium was used as the carrier gas at a flow rate of 1 mL min−1. The injector and detector temperatures were set at 310 and 320 °C, respectively. The initial oven temperature was set at 35 °C, which was held for 0.1 min, increased to 280 °C at 10 °C min−1, and then held for 5.4 min to reach a total time of 30 min.

positive effects of water on the performance of the pyrolysis of fatty acids. It is worth mentioning that there are significant studies regarding the influence of water on the pyrolysis of lignocellulosic biomass to bio-oil,35−38 but such an impact on the pyrolysis of free fatty acids has not been reported. Due to the differences in the chemistry of the reactions, the effect of water on the pyrolysis of fatty acids requires investigation. This report focuses on the pyrolysis of oleic acid as a model fatty acid to study the effect of water in production of renewable hydrocarbons. Oleic acid is a mono-unsaturated free fatty acid that is commonly found in large quantities in plant lipids and oils. Thus, it is commonly used as feedstock to produce liquid hydrocarbons such as fuels and chemicals.11

2. MATERIALS AND METHODS 2.1. Materials. Oleic acid as an unsaturated C18 fatty acid (≥99%) and pentane (≥99%) and the internal standard (methyl nonadecanoate) for gas chromatography (GC) of the liquid products were purchased from Sigma-Aldrich (St. Louis, MO, USA). Carbon monoxide and gaseous alkanes and alkenes standards for GC analysis of the gas products were also purchased from Sigma-Aldrich. Diazald (N-methyl-N-nitroso-p-toluenesulfonamide) used for diazomethane preparation was purchased from TLC PharmaChem Inc. (Concord, ON, Canada). Diazomethane for derivatization of fatty acids was prepared using a Diazald kit (Sigma-Aldrich) following the manufacturer’s procedures. Air, N2, H2, He, Ar, and CH4 gases were purchased from Praxair (Praxair Inc., Danbury, CT, USA). Milli-Q water (Etobicoke, ON, Canada) was used in the experiments. 2.2. Pyrolysis Reactions. 2.2.1. Pyrolysis under Nitrogen Atmosphere. The experimental procedure is described in detail elsewhere.26−28 Briefly, pyrolysis reactions were conducted in a 15 mL batch microreactor made from 3/4 in. stainless steel Swagelok fittings, and tubing. The microreactor was first washed with acetone to ensure that it was clean and free of any material. Approximately 1 g of oleic acid was loaded into a clean microreactor. The microreactor was sealed and then connected to a nitrogen source. It was pressurized to 500 psi and tested for leaks. It was purged three times with nitrogen. The microreactor was then heated in a Techne model SBS-4 fluidized sand bath with a Techne TC-8D temperature controller (Burlington, NJ, USA) at the desired temperature (390, 430, or 450 °C) for a period of 2 h. 2.2.2. Pyrolysis in the Presence of Water. Another set of experiments was carried out under the same conditions as in section 2.2.1, except that varying amounts of Milli-Q water were added to 1 g of oleic acid in a 15 mL microreactor. Here, 45 μL (5% by volume) of extra water was added for the pyrolysis reactions conducted at 390, 430, and 450 °C, while 90 μL (10% by volume) of extra water was only examined at 450 °C. The reason we tested 10% water only at 450 °C was that, at the lower end of the temperature range (390 °C), the conversion dropped significantly with 5% water such that further investigation with 10% water did not seem necessary. However, at higher temperatures, the decline in conversion was not huge, thus we continued to investigate the effect of 10% water at the upper end of the range (450 °C). To obtain the optimum amount of water at each temperature, one should implement further optimization tests that were out of the scope of this study. After loading the feedstock, the reactor was then sealed and purged with nitrogen at 500 psi. The microreactor was then heated in the sand bath at the desired temperature for 2 h. 2.2.3. Pyrolysis under Hydrogen Atmosphere. The pyrolysis of oleic acid was carried out at 450 °C under hydrogen atmosphere. A 1 g amount of oleic acid was loaded into a microreactor. The reactor was sealed and purged with nitrogen. Hydrogen was then added to the microreactor such that the pressure was 100 psig. The microreactor was then placed in a sand bath at 450 °C for 2 h. 2.2.4. Postpyrolysis Processing. In all three conditions mentioned above, the microreactors were removed from the sand bath after 2 h B

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Energy & Fuels The same GC system described above, but coupled to an Agilent 5975B inert XL EI/CI MSD operated in electron ionization (EI) mode, was used to carry out mass spectrometry (MS). GC-MS analysis was used to identify peaks in the chromatogram based on retention times by matching to the National Institute of Standards and Testing (NIST) mass spectral library and recognition of the fragmentation pattern of the mass spectra. The peaks with a quality match of 90 and higher were considered as identified peaks. The FID chromatograms were then matched with their related MS spectra. After matching the mass spectra with their related FID chromatograms, the areas of the peaks were obtained and they were used to determine quantification of compounds based on the area of the internal standard with known mass.

conversion/% =

3. RESULTS AND DISCUSSION The pyrolysis of oleic acid in two different scenarios was studied: (1) oleic acid alone and (2) oleic acid in the presence of 5% water (at 390, 430, and 450 °C) or 10% water (only at 450 °C). All reactions resulted in the formation of pyrolytic products with three fractions: gas, liquid, and solid residue. The products of pyrolysis in the above-mentioned scenarios were analyzed to investigate the influence of water. 3.1. Effect of Water on Conversion Rate of Reactions. Pyrolysis of oleic acid resulted in the formation of a liquid product under all the conditions tested. The oleic acid feed was converted to lighter hydrocarbons and the extent of feedstock conversion is defined as follows:

mass of C18 fatty acid in feed − mass of C18 fatty acid in product × 100 mass of C18 fatty acid in feed

3.2. Liquid Product Yield. As mentioned earlier, pyrolysis of oleic acid resulted in the formation of a liquid product under all conditions examined. The liquid product yield was calculated by measuring the mass of liquid product as a percentage of mass of oleic acid feed. Figure 1 shows the liquid

Note that the feed material was oleic acid (C18:1). However, other 18-carbon fatty acids may also be identified in the product such as stearic acid (C18:0), in addition to oleic acid (C18:1). Since the goal is to convert all C18 fatty acids, the other forms of 18-carbon fatty acids were also considered as unreacted materials for this calculation. Table 1 shows the conversion at different temperatures and amounts of water. Regardless of whether water was present or Table 1. Conversion Rate at Different Temperatures and in the Presence of Different Amounts of Watera temp

% water

390 390 430 430 450 450 450

0 5 0 5 0 5 10

conversion, % 91.2 74.2 99.2 98.1 99.7 98.1 98.2

± ± ± ± ± ± ±

1.0C 0.4D 0.6AB 0.9B 0.1A 1.1B 0.8B

a The numbers are the average of three replicates for each treatment ± of standard deviation. Values sharing the same superscripts are not significantly different at the 95% confidence level.

Figure 1. Yield of liquid product from oleic acid pyrolysis at different temperatures and amounts of water. The yield was calculated as the mass percentage of the oleic acid feed. The bars are the average of three replicates for each treatment ± standard deviation. The bars with the same superscripts are not significantly different at a 95% confidence level.

absent during pyrolysis, oleic acid conversion increased significantly when the temperature was raised from 390 to 430 °C; it reached nearly 100% at 430 °C, and thus a further increase in temperature to 450 °C did not significantly improve the feedstock conversion rate. The improvement in conversion caused by a temperature increase from 390 to 430 °C is attributed to the fact that the severity of cracking reactions increase with temperature.26,27 The effect of water on the conversion is described as follows: At 390 °C, the presence of water resulted in a significantly lower conversion. The conversion decreased from 91.2 ± 1.0% in the absence of water and to 74.2 ± 0.4% in the presence of water. At 430 °C, however, the presence of water did not significantly change the conversion. At 450 °C, the conversion decreased slightly from 99.7 ± 0.1 to 98.1 ± 1.1 and to 98.2 ± 0.8 in the presence of 5 and 10% water, respectively, though there was not a significant difference in conversion between 5% and 10% water. Despite the lower conversion at 450 °C, the reduction in absolute amount was very small and thus the conversion rate is still close to 100%.

yield at different temperatures (390, 430, and 450 °C) and amounts of water. The liquid yield was found to decrease with increasing temperature. This phenomenon was likely due to an increase in cracking of molecules at high temperature, which resulted in elevated amounts of gaseous products. This effect will be discussed later in this study. The reduction in liquid product yield would typically make the pyrolysis undesirable at 450 °C in a batch reactor system. In the presence of water, however, pyrolysis of oleic acid resulted in higher liquid product yields at all temperatures. It is worth mentioning that, under all conditions tested, the liquid product yield increased more than the percentage of water added. The liquid yield was calculated based on the amount of oleic acid loaded into the microreactors. Thus, if we assumed that water did not contribute to the reaction, if we add 5% water to the microreactor, the yield of liquid product (weight percent of oleic acid feed) should increase by about 5% more liquid. However, adding 5% water (at 450 °C) increased C

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therefore decarbonylation was the dominant pathway of fatty acid conversion. At 450 °C, the amount of CO2 increased while the amount of CO decreased suggesting that decarboxylation and decarbonylation both participated similarly in deoxygenation. The decarboxylation reaction is thermodynamically more favorable at high temperature; the reason for this temperature dependence is the difference in activation energy between the decarbonylation and decarboxylation, resulting in a larger Gibbs free energy of decarboxylation compared to decarbonylation reaction.42 With regard to the effect of water, it is known from literature that decarbonylation results in the formation of water;27,43 thus the presence of water in such a system would suppress the decarbonylation reaction. This could be a possible reason why, at 390 and 430 °C, the presence of water caused a decrease in the amount of carbon monoxide. The same effect was seen at 450 °C when 10% water was added. A statistically significant difference was not seen at 5% water at 450 °C. This is because the effect of temperature is quite large from 430 to 450 °C, and thus the large change caused by the temperature increase probably masked the more subtle changes elicited by adding 5% water. By adding 10% water, the effects could be more easily seen (i.e., more significant compared to the increase in temperature). Another reason for CO reduction might be the conversion of carbon monoxide into carbon dioxide as it was observed that carbon dioxide increased at 390 and 430 °C when 5% water was added and also at 450 °C when adding 10% water. The CO2 increase could be possibly attributed to the water gas shift reaction. Asomaning et al. suggested the forward reaction of the water gas shift as a plausible explanation for the increase in CO2 and hydrogen in pyrolysis of fatty acids.27,28 Kubatova et al. also mentioned a reverse reaction of water gas shift as a reason for the production of hydrogen in pyrolysis of triacylglycerols (canola and soybean oil).40 Figure 3c shows that a relatively higher amount of hydrogen was actually detected at 390 and 430 °C in the presence of 5% water (compared to the no water samples) and also at 450 °C in the presence of 10% water. Therefore, it was hypothesized that a higher amount of hydrogen could be possibly due to a reaction between water and carbon monoxide (water gas shift reaction). To examine this hypothesis, a microreactor test was conducted under the exact same condition of a pyrolysis reaction at 450 °C, except that no oleic acid was involved. The microreactor reactor was loaded with 100 mg of water, and then purged with nitrogen and filled with carbon monoxide (100 psig). Figure 4 shows the GC chromatograms of the gas sample before and after reaction. The results showed that a significant amount of carbon monoxide was converted into hydrogen. Thus, we can conclude that, during pyrolysis at 450 °C, the water gas shift reaction does occur, which results in the production of hydrogen gas. In addition to carbon dioxide and carbon monoxide and hydrogen, the gas fraction contained a significant amount of short-chain hydrocarbons, such as C1−C4 alkanes and alkenes (Figure 3d). The presence of C1−C4 hydrocarbon gases in the product can be attributed to decomposition of C−C bond cleavage of hydrocarbon molecules as well as the long-chain hydrocarbon radicals. With a temperature increase in the absence of water, the amounts of short-chain hydrocarbons increased as a result of a higher degree of cracking. At high temperature, polymerization can also occur that results in a decrease in low molecular hydrocarbon formation, especially in the presence of free radicals. The olefins might form polymeric

the liquid product yield by 16% (weight percent of oleic acid feed), suggesting that water in fact participated in the reaction toward the formation of more liquid hydrocarbons. Similarly, the addition of 10% water at 450 °C resulted in a liquid product yield that was 28% higher (weight percent of oleic acid feed). Additionally, the increased liquid product yield could be due to hydrogen addition reactions from water to pyrolysis products. The mechanism will be discussed below with regard to the formation of gases and residues. 3.3. Gas Fraction Products. The pyrolysis of oleic acid in the presence and absence of water also resulted in a gas fraction. The amount of gas produced at different temperatures and amounts of water, with respect to initial mass of oleic acid, is shown in Figure 2. The results showed that, in the presence

Figure 2. Amounts of gas produced from pyrolysis of oleic acid in the presence and absence of water at temperatures of 390, 430, and 450 °C. The bars are the average of three replicates for each treatment ± standard deviation. The bars sharing the same superscripts are not significantly different at a 95% confidence level.

of water, at a given temperature, the amount of the gas fraction decreased. However, the amount of gas produced increased with temperature in both the presence and absence of water. As discussed earlier, the conversion increased by increasing temperature from 390 to 430 °C in both the absence and presence of water, and thus, a greater amount of feed was converted to the products including gases. The conversion rate, however, was identical between 430 and 450 °C as it had already reached the maximum. Therefore, the thermal energy provided was used to increase the cracking of the liquids to gaseous products. The temperature and presence of water may also influence the composition of the gas phase. Figure 3 shows the compositions of the gas fractions from various reaction conditions. Carbon dioxide, carbon monoxide, hydrogen, and light hydrocarbons (including methane, ethane, ethene, propane, and propene) were detected in the gas fraction. The relative distribution of carbon monoxide and carbon dioxide formation is indicative of the dominant pathway of deoxygenation involved in any pyrolysis reaction of fatty acids. Fatty acid thermal deoxygenation occurs through decarbonylation or decarboxylation. The CO production during thermal decomposition can be attributed to decarbonylation of the feedstock, while CO2 generation during the pyrolysis process can be attributed to decarboxylation reactions.40,41 It was observed that the relative distribution of CO and CO2 depended on the temperature. In the absence of water, at 390 and 430 °C the amount of CO was much greater than CO2, and D

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Figure 3. Mass percentages of (a) carbon monoxide, (b) carbon dioxide, (c) hydrogen, and (d) C1−C4 hydrocarbons present in the gas product at different temperatures and amounts of water. The bars are the average of three replicates for each treatment ± standard deviation. The bars sharing the same superscripts are not significantly different at a 95% confidence level.

Figure 4. GC chromatograms of a gas sample taken from a 15 mL microreactor, which was initially loaded with 100 mg of water and carbon monoxide with pressure equal to 100 psig (a) before and (b) after being in a sand bath at 450 °C for 2 h. Panels a and b have the same scale on the yaxis.

important because it provides insights into its properties and suitability for use as transportation fuels. To examine the composition, the liquid product obtained through pyrolysis at different temperatures and amounts of water were analyzed using gas chromatography and mass spectrometry. The liquid product compositions obtained using different temperatures and amounts of water were categorized into four classes of compounds: (1) alkanes and alkenes (linear and branched); (2) cyclic hydrocarbons; (3) aromatic compounds; and (4) fatty acids. (Figure 5). The compounds reported have molecular weight higher than n-hexane because the peaks lower than or equal to n-hexane could not be integrated due to solvent peak overlap. In the pyrolysis product, the alcohols or aldehydes and ketones were not detected. Except for the pyrolysis reaction at 390 °C where the conversion dropped significantly, the results showed that alkenes and alkanes were

materials that could not be analyzed by GC. However, the rate of cracking was found to be higher than the rate of polymerization as most of the free radicals were converted to the saturated aliphatic hydrocarbons through saturation reactions. This increase in short-chain hydrocarbons with temperature has been already recognized in the literature.26,27 The presence of water, however, did not significantly change the total amount of C1−C4 hydrocarbons in the gas fraction at 430 and 450 °C, but at 390 °C, in the presence of water, the total amount of C1−C4 hydrocarbons in the gas fraction increased significantly; this is likely because at 390 °C the feed conversion decreased in the presence of water, and thus the thermal energy was used to crack the hydrocarbons to short chains. 3.4. Liquid Fraction Composition. The chemical composition of the liquid product resulting from pyrolysis is E

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Figure 5. Mass percentages of (a) C7−C17 alkanes and alkenes, (b) aromatics, (c) cyclic compounds, and (d) C4−C18 fatty acids in the liquid fraction of oleic acid pyrolysis at different temperatures and amounts of water. The bars are the average of three replicates for each treatment ± standard deviation. The bars sharing the same superscripts are not significantly different at a 95% confidence level.

amounts of short-chain hydrocarbons increased by increasing temperature. It was observed that C17 hydrocarbons were the most abundant products at 390 °C. This was likely due to the cleavage of the C−C bond adjacent to the carboxylic group in oleic acid.26 While there was a big portion of C17 hydrocarbons in the liquid product at 390 °C, short-chain hydrocarbons were the most abundant compounds at 450 °C. The same trends were reported by the previous studies of pyrolysis of fatty acids.26,29 It was also observed that the amounts of C7 and C8 hydrocarbons were relatively higher than the amount of C10− C16 hydrocarbons in the absence of water at 430 and 450 °C. This behavior can be attributed to the fact that the cracking of C−C bonds might occur in the proximity of unsaturated bonds in oleic acid.29 As shown in Figure 6, the effect of water on the distribution of individual hydrocarbons mostly depended on the reaction temperature. At 390 °C, the amounts of C17 hydrocarbons decreased significantly in the presence of water. This behavior can be explained by the effect of water on the conversion at this temperature. As mentioned earlier, at 390 °C, the presence of water resulted in a significantly lower conversion. The conversion decreased from 91.2 ± 1.0% in the absence of water to 74.2 ± 0.4% in the presence of water. As a result, the amounts of C7−C9 hydrocarbons were also decreased significantly. On the other hand, at 430 and 450 °C, the presence of water did not significantly affect the individual distribution of hydrocarbons. The aromatic compounds produced in pyrolysis reactions of oleic acid increased significantly with temperature in the

the most abundant class of compounds found in the products (Figure 5a). The fraction of alkanes and alkenes increased with a temperature change from 390 to 430 °C, possibly due to the conversion improvement at 430 °C (Table 1). At this temperature, a larger amount of oleic acid was cracked compared to at 390 °C because a greater amount of cracking energy was provided. However, a temperature increase from 430 to 450 °C did not result in an increase in the total amounts of alkanes and alkenes. This trend can likely be explained by the fact that conversion of oleic acid was nearly 100% at 430 °C, and a further increase to 450 °C could not improve conversion rates and therefore could not result in a greater production of alkanes/alkenes. These data also reveal that a temperature change from 430 to 450 °C did not result in significant thermal breakdown of alkanes/alkenes. Regarding the effect of water, the results showed that, at 390 °C, in the presence of water the fraction of identified alkanes and alkanes decreased significantly to almost half of that in the absence of water (Figure 5a). This behavior could be attributed to the negative impact of water on the conversion at 390 °C. In contrast, at 430 °C in the presence of 5% water and at 450 °C in the presence of 5 or 10% water, no significant difference in the fraction of identified alkanes and alkenes was observed relative to the no water samples, perhaps because the conversion remained near 100% in those cases. The distribution of the individual alkanes and alkenes compounds in the liquid product is also shown in Figure 6. This figure shows that, in the absence or presence of water, the F

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to the no water sample treated at the same temperature. However, at 450 °C the amounts of aromatics were reduced to 8.76 ± 0.8% and 8.5 ± 0.1% in the presence of 5% and 10% water, respectively. As discussed earlier (section 3.3), a higher amount of hydrogen was likely formed during pyrolysis in the presence of water due to the reaction between water and carbon monoxide. The hydrogen produced would hinder dehydrogenation reactions that are known to be responsible for the formation of aromatics from cyclic compounds.18 Since these reactions are temperature dependent, it is likely that, at 430 °C, the amount of hydrogen produced through addition of water (i.e., the water gas shift reaction) was not large enough to significantly impact generation of aromatics at this temperature. Nevertheless, at 430 °C the amount of cyclic compounds increased slightly upon addition of water (Figure 5c). When adding 10% water at 450 °C, the amount of hydrogen was large enough to cause a significant decrease in aromatics and increase in cyclic compounds. To confirm if a hydrogen-rich atmosphere could result in a significant reduction in the dehydrogenation of cyclic compounds, thereby suppressing formation of aromatics, pyrolysis of oleic acid at 450 °C was carried out under hydrogen atmosphere as described in section 2.2.3. The results of this process were compared to those obtained under nitrogen atmosphere. Under hydrogen atmosphere, the total amount of aromatic compounds was significantly less than that under nitrogen atmosphere, while the total amount of cyclic compounds was significantly higher (Figure 7). The amounts of

Figure 6. Mass percentages of alkanes and alkenes in the liquid fraction at different temperatures and amounts of water as a function of the carbon number. Percentages are uncorrected for the amount of unidentified peaks and nonvolatile materials present, and the bars are the average of three replicates of each treatment ± standard deviation. The bars sharing the same superscripts are not significantly different at a 95% confidence level.

Figure 7. Mass percentages of C7−C17 alkanes and alkenes, aromatics, cyclic compounds, and total C4−C18 fatty acids in the liquid fraction of the product obtained from oleic acid pyrolysis at 450 °C for 2 h under hydrogen atmosphere. The bars are the average of three replicates of each treatment ± standard deviation. The bars sharing the same superscripts are not significantly different at a 95% confidence level.

absence of water (Figure 5b). The liquid product acquired from reactions at 390 °C were characterized as having almost no aromatics, whereas it contained a considerable amount of aromatics at 430 °C (5.1 ± 1.0%); a temperature change from 430 to 450 °C increased the aromatics to an even higher level (10.4 ± 0.4%). This showed that increasing temperature promoted aromatization reactions. This effect of temperature on the amount of aromatic compounds is reported in the literature.27,29 Since a temperature of 390 °C was not high enough to favor aromatization reactions in the absence of water, it is not surprising that the amount of aromatics observed at this temperature upon addition of 5% water were also very low (Figure 5b). The addition of 5% water at 430 °C did not significantly change the amount of aromatics produced, relative

alkanes and alkenes, as well as fatty acids, were not significantly different between the two conditions examined (Figure 7). The results show that, at 450 °C, the hydrogen available in the reaction atmosphere would decrease the rate of the dehydrogenation of cyclic compounds, resulting in a reduction of aromatics present in the pyrolysis product, while increasing the amount of cyclic compounds. This trend was similar to what was observed in the pyrolysis reaction conducted at 450 °C in the presence of 10% water. Therefore, these data suggest that the hydrogen generated through a water gas shift reaction at this condition can in fact influence the pyrolysis performance as described here. G

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the amount of residue produced at both 390 and 430 °C. The reason why the amount of solid residues was significantly lower in the presence of water at 450 °C could be explained as follows. The solid residues are formed mainly due to polymerization of olefins and aromatic hydrocarbons.18,27,29 Polyaromatic hydrocarbons are the intermediates in the formation of solid residues that are formed by elimination of hydrogen from aromatics.18 As discussed earlier, pyrolysis of oleic acid in the presence of water resulted in formation of hydrogen gas due to water gas shift reactions at 450 °C. An atmosphere rich in hydrogen would likely hinder dehydrogenation of hydrocarbons, and subsequent formation of polymers and solid residues. As described earlier (section 3.4), the pyrolysis of oleic acid at 450 °C was also carried out under hydrogen atmosphere to examine the effect of hydrogen. During pyrolysis under hydrogen atmosphere, the solid residue was also measured in smaller quantities compared to those obtained under nitrogen atmosphere (Figure 9). This trend is similar to what was observed in the pyrolysis reaction conducted at 450 °C in the presence of 5 and 10% water (Figure 8).

The other major group of compounds identified in the liquid product was fatty acids (Figure 5d). This group consisted of a range of long-chain fatty acids (e.g., unreacted oleic acid) to short-chain fatty acids. As seen in Figure 5d, the amount of total fatty acids in the pyrolysis product at 430 °C was significantly lower than that at 390 °C in the absence of water (6.7 ± 1.6% compared to 17.6 ± 1.6%). The presence of water, however, increased the fatty acids significantly at 390 °C. This is mostly due to the conversion decline as described earlier in section 3.1. In contrast, the presence of 5% water did not change the amount of fatty acids at 430 and 450 °C. However, 10% water increased the total amount of fatty acids slightly at 450 °C, compared to the sample with no water. The increase of these fatty acids, especially short-chain fatty acids, in the product acquired from pyrolysis at 450 °C and 10% water showed again that the decarboxylation and decarbonylation reactions were likely hindered by the presence of water vapors, whereas the temperature was high enough to increase the cleavage of the C−C bonds. At 430 and 450 °C, the temperature was likely sufficiently high, such that the addition of 5% water was probably not enough to significantly impact the decarboxylation and decarbonylation reactions. This is consistent with the data reported for conversion in Table 1. Based on this observation, the reduction in carbon monoxide at 430 °C when adding water (as discussed in section 3.3) was most probably due to its conversion into carbon dioxide. At 450 °C, however, a significant amount of carbon dioxide was already released due to the shift from decarbonylation to decarboxylation; this extra CO2 would result in less conversion of CO into CO2, thus preventing a significant difference in CO, CO2, and H2 when adding 5% water. 3.5. Solid Residues. A solid residue was observed in the pyrolysis reaction. The amount of solid residue was measured for each experiment performed at different temperatures and amounts of water. The data are presented in Figure 8. The

Figure 9. Mass percentage of solid residue produced in pyrolysis of oleic acid at 450 °C under nitrogen and hydrogen atmosphere. The bars are the average of three replicates for each treatment ± standard deviation. The bars do not share the same superscripts and therefore are significantly different at a 95% confidence level.

4. CONCLUSIONS The pyrolysis of oleic acid as a model free fatty acid in the presence of 5% water at 390, 430, and 450 °C, as well as 10% water at 450 °C, was studied. The effect of water on pyrolysis was determined by analyzing the reaction contents following pyrolysis. It was found that the additional water decreased the conversion at 390 °C, but at 430 and 450 °C with 5% water, the conversion was not statistically changed. At 450 °C and 10% water, the conversion decreased slightly from 99.7 ± 0.1 to 98.2 ± 0.7. The presence of water, however, increased the liquid yield at all conditions tested, even after correcting for the amount of liquid added to the reaction. It was also shown that a higher amount of hydrogen was detected in the presence of water at 390 and 430 °C. While there was no significant difference in hydrogen percentage at 450 °C when 5% water was added, the hydrogen increased in the presence of 10% water. This hydrogen increase could be a plausible reason for reduction in aromatics formation in the presence of 10% water at 450 °C. The amounts of cyclic compounds were also increased at 430 °C (5% water) and 450 °C (10% water). The solid residues were found to be decreased at 450 °C in the presence of water.

Figure 8. Mass percentage of solid residue produced at different temperatures and amounts of water. The bars are the average of three replicates for each treatment ± standard deviation. The bars sharing the same superscripts are not significantly different at a 95% confidence level.

results showed that adding water to oleic acid in the pyrolysis reaction at 390 and 430 °C did not result in a significant difference in the amount of solid residue. At 450 °C, on the other hand, pyrolysis in the presence of water (at both 5% and 10%) reduced the solid residue from 6.8 ± 1.1% in absence of water to 3.4 ± 0.3% and 2.7 ± 0.4 in the presence of 5% and 10% water, respectively. Note that by adding water at 450 °C, the amount of residues was reduced to a value comparable to H

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Mehdi Omidghane: 0000-0002-6566-257X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support provided by the Natural Sciences and Engineering Research Council of Canada (NSERC), the Biorefining Conversions Network (BCN) at the University of Alberta, Alberta-Innovates Bio Solutions (AI-Bio), BioFuelNet Canada, Forge Hydrocarbons Inc., and Mitacs Canada.

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