Investigating the effect of temperature, pressure, and reactor wall in non-catalytic ethylene oligomerization

Investigating the effect of temperature, pressure, and reactor wall in non-catalytic ethylene oligomerization | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Investigating the effect of temperature, pressure, and reactor wall in non-catalytic ethylene oligomerization Ghazal Azadi, Eduardo Lins de Barros Neto, Federico Galli, Jean-Michel Lavoie This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6770942/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract The aviation industry contributes to greenhouse gas (GHG) emissions, driving interest in sustainable aviation fuel (SAF) production. A promising route is the bioethanol-to-jet process, where bioethylene undergoes non-catalytic (thermal) oligomerization to form longer-chain hydrocarbons. However, limited research exists on this process at high temperatures and pressures. This study examines the influence of temperature (200–650°C), pressure (5 and 15 bar), and reactor material (SS316 and Inconel) on ethylene conversion and product distribution. Higher temperature (600–650°C) and pressure (15 bar) significantly enhanced conversion, reaching 94% in both reactors, while at lower pressure (5 bar), conversion of 30–58% was reached. Increased temperature and pressure promoted C4 hydrocarbon formation, with heavier products appearing at a lower temperature in the Inconel reactor (450°C) compared to SS316 (500°C). Additionally, liquid product yield was higher in the Inconel reactor, producing up to 2 g of liquid, compared to 1.71 g in SS316, which showed a greater proportion of heavier hydrocarbon distribution. Liquid product analysis showed that C6-C8 hydrocarbons were predominant, while heavier fractions (C9-C12 and C12+) were more abundant at lower pressures. These results highlight Inconel’s potential for improving ethylene oligomerization efficiency, offering insights for optimizing SAF production. SAF ethylene oligomerization wall effect parametric study Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 Figure 14 Figure 15 Figure 16 1. Introduction The aviation industry is responsible for about 2% of global human-caused GHG emissions and is expected to grow notably by 2050, this potentially will increase CO₂ emissions. In 2019, aviation fuel consumption reached 350 million metric tons, which emitted an estimated 1.09 Gigatons of CO₂ [ 1 ]. In order to meet the goal of the Paris Agreement to limit the global temperature increase to below 1.5°C, and according to the latest UN environmental report, which emphasizes the need to reduce greenhouse gas emissions by 43% by 2030 [ 2 ], shifting to cleaner energy production methods, including sustainable aviation fuels, is essential to reducing this sector's environmental impact [ 1 ]. Conventional aviation fuel is derived from kerosene that is produced from crude oil, which is fossil-based [ 3 ]. The alternative, known as SAF (Sustainable Aviation Fuel), refers to jet fuel produced from renewable sources that are, in principle, nondepletable. SAF has compounds similar to conventional jet fuel but is produced from renewable sources, so it has a high potential to reduce greenhouse gases. Based on the International Civil Aviation Organization (ICAO) report, SAF could meet 100% demand for jet fuel by 2050 and could reduce CO 2 emissions by 63% [ 1 ]. While Fischer-Tropsch (FT) and Hydro-processed Esters and Fatty Acid (HEFA) methods were the first alternative aviation fuels approved to be used in this industry, companies such as Byogy Renewables Inc. reported the possibility of producing jet fuel from bioethanol in 2009 [ 4 ]. One promising pathway to produce SAF is Alcohol-to-Jet (ATJ) where alcohols are converted to jet fuel through dehydration, oligomerization, hydrogenation, isomerization, and distillation [ 5 ]. Among these, the dehydration of light alcohols such as ethanol, produces light-chain olefins like ethylene, an important raw material with a wide range of applications in the petrochemical, polymer, and surfactant industries. Currently, ethylene comes from fossil fuels such as petroleum steam reforming and natural gas [ 6 ]. However, the environmental concerns about fossil fuels, emphasize the importance of renewable ethylene through bioethanol dehydration [ 6 ], [ 7 ]. Bioethanol production from sugar grains experienced rapid growth in the 1980s and provided the opportunity for bioethylene production. Such an approach represents an alternative to fossil-based ethylene production with fewer environmental footprints [ 7 ]. Ethylene can then undergo oligomerization to generate longer-chain hydrocarbons (α-olefins), which are precursors to produce polyethylene, lubricants, and fuels such as gasoline, diesel, and jet fuel blends [ 7 ]. Ethylene oligomerization is catalyzed by either transition metal catalysts (e.g., nickel, iron, and cobalt) or solid mesoporous acidic catalysts (such as H-ZSM-5 [ 8 ] [ 9 ], [ 10 ], [ 11 ], and [ 12 ]). However, these approaches have drawbacks and limitations such as catalyst deactivation, regeneration, and co-catalyst requirement. In contrast, thermal ethylene oligomerization, which has been employed in refineries since the 1920s, could minimize coking and deactivation issues. However, it requires higher temperature and pressure than catalytic ethylene oligomerization [ 13 ]. Despite the advances in catalytic oligomerization, research on thermal ethylene oligomerization is limited specifically to elevated temperature and pressure ranges. Pease [ 14 ] investigated non-catalytic ethylene polymerization from 450 to 650°C, and reported that at temperatures above 525°C, the formation of methane, ethane, and liquid hydrocarbons increased. More methane formation at higher temperatures suggested the secondary product evolution. Since this study has been done at atmospheric pressure, there were limited insights into higher pressure. Boyd et al. [ 15 ] extended this work, studied on thermal reactions of ethylene in a quartz cylinder reactor in temperatures between 500 to 600°C, and concluded that the impact of temperature on reaction rates and product distribution is significant. While the main products were ethane, propylene, and butene, higher temperatures led to the production of smaller hydrocarbons as a result of favoring secondary reactions. More recently, Conard et al. [ 16 ] and [ 13 ] investigated thermal ethylene oligomerization at a temperature range of 320 to 500°C and pressure of 1.5 to 43.5 bar, with the main focus on reaction pathways and DFT studies. It has been reported that higher temperatures favored lighter hydrocarbon production while lower temperatures stabilized the production of longer-chain hydrocarbons. Their studies also concluded that higher pressure improved ethylene conversion to liquid products. Thus, the ethylene conversion and liquid product composition remained almost unclear for the same and higher operational conditions. The lack of broad knowledge, in addition to the issues mentioned for catalytic oligomerization, emphasizes the importance of more research on thermal oligomerization. It could be possible that by going more in-depth into the thermal ethylene oligomerization aspects, engineering catalysts that make this reaction happen in milder operational conditions become possible. The effect of the reactor wall is another important aspect that has been investigated. Paese et al. [ 14 ] studied the effect of reactor walls in non-catalytic ethylene oligomerization by doing experiments in empty and packed Pyrex tubes. He concluded that there is no significant enhancement in reaction rate by using packed tubes; this also ruled out the catalytic effect of the reactor wall. Boyd et al. [ 15 ] also investigated the reactor wall effect by conducting experiments in quartz reactors and concluded that the reaction is mainly homogeneous with no notable contribution of the reactor wall catalytic effect in thermal ethylene oligomerization. In Conrad’s first research work on thermal ethylene oligomerization in quartz and stainless-steel reactors, it was verified that reactor walls did not have a significant effect on product distribution or reaction rates [ 13 ]. These results were consistent for both reactor materials that have been used in this research and reinforced the negligible effect of the reactor wall. In another similar study, Conrad et al. [ 16 ] concluded the same negligible effect of the reactor wall on product distribution and reaction rates by increasing the reactor surface area. Conversely, studies on other high-temperature, high-pressure reactions, such as supercritical water gasification of glycerol, were done by Salierno et al. [ 17 ] and Zhu et al. [ 18 ] have concluded that reactor materials such as Inconel and Hastelloy can improve reactions such as hydrocarbon production and water-gas shift. These studies suggested that the reactor wall material can impact reaction outcomes under certain circumstances. In another recent study, Bezerra et al. [ 19 ] investigated the effect of reactor material on syngas reforming and found that an Inconel reactor could significantly improve CH 4 and CO 2 conversion compared to a stainless-steel reactor, which emphasizes the potential catalytic effect of high Nickel content material. Although the reactor wall effect is less emphasized in catalytic ethylene oligomerization due to the predominant role of the catalyst, some studies have indirectly addressed it. Jan[ 20 ] studied ethylene oligomerization over Ni-Hβ catalyst, in which the role of the reactor’s interaction in the catalyst deactivation mechanism was indirectly addressed. Panpain[ 21 ] worked on catalytic ethylene oligomerization using Ni/Al-KIT-6, implying that a stainless-steel fixed bed reactor minimized the interactions between reactants and the reactor wall, with more focus on catalyst activity and reaction efficiency rather than the reactor wall effect. Although these studies focused on optimizing catalyst and reactor design, they rarely did an in-depth investigation of the role of reactor walls. The primary goal of this research is to investigate the effect of temperature, pressure, and the wall effect on the non-catalytic (thermal) ethylene oligomerization. To achieve these goals, a series of experiments were performed under controlled conditions. To evaluate the product distribution, ethylene conversion, and produced oligomers have been analyzed at a temperature gradient from 200 to 650°C and pressures of 5 and 15 bar. This research also aims to explore the potential influence of the reactor wall (SS316 and Inconel) on the thermal oligomerization of ethylene. While the previous studies [ 13 ], [ 14 ], [ 15 ], and[ 16 ] ruled out the notable catalytic effect of the reactor wall, these conclusions were obtained in the operating conditions that may not fully represent the elevated temperature and pressure conditions in this field. Thus, this aims to push further previous findings in the field by expanding temperature and pressure ranges. it will also utilize Inconel for such conditions and will investigate whether it has any influence on ethylene conversion or product distributions, especially at higher temperatures and pressures, where the surface interactions could be more significant. It will also help to extend the understanding of non-catalytic systems, where the absence of a catalyst highlights the potential role of the reactor wall as a significant variable. 2. Methodology The experiments were performed in a continuous reaction system equipped with an SS316 fixed bed reactor (12 ″ L, 0.37″ W, 0.065″ T), heated by an electric furnace (Thermcraft split tube furnace 1″ ID X 10″ L) used for temperature control. Nitrogen (99.999% purity from Linde) and ethylene (C 2 H 4 , 99.999% purity from Linde) were fed to the system through two Mass Flow Controllers (MFC) (Brooks Instrument), keeping the ethylene-nitrogen ratio at 95:5 with a total flow of 88 ml/min. The reaction temperature was monitored by two thermocouples that were placed into the furnace wall to measure the temperature outside of the reactor and inside the reactor, down to the middle, to measure the temperature inside. The reactor pressure was adjusted and maintained using a Back Pressure Regulator (BPR) (Swagelok, KBP1J0A4A5A20000, 0-500 psi), which was used to regulate the experiments performed at 15 and 5 bar. The system also contained a Pressure Relief valve (PRV) (Swagelok, SS-4R3A1-EP) for safety purposes. A depiction of the reaction system is shown in Fig. 1 . At the beginning of each run, 88 ml/min nitrogen was passed through the reactor while the temperature and pressure were increased to the desired value. Once temperature and pressure were stabilized, nitrogen flow was decreased to 4.4 ml/min, and ethylene flow was increased to 83.6 ml/min. The reaction time started from the moment ethylene was injected into the reactor. The gas phase was analyzed offline (every 30 min) by gas chromatography (GC-456 Natural Gas Analyzer, Bruker) to determine and quantify the produced gas, to calculate conversion, and verify product distribution. The data processing program was Compass CDC, using external standards. Liquid samples were collected after the reaction for GC-FID analysis (Shimadzu GC-FID) in order to quantify the hydrocarbon composition. As for the GC program, the injector was set at 275°C, He was used as the carrier gas, at a column flow of 1.30 ml/min, and using a 1:20 split. The Oven program started at 40°C and was held for 1 min, after which it was fixed at 300°C at a 10°C/min increment, after which it was held for 1 min. Finally, the FID temperature was set at 325°C. The effect of temperature on ethylene oligomerization was studied from 200 to 650°C with 100°C temperature increments up to 400°C and 50°C increments up to 650°C. The temperature range and increment amounts were chosen based on previous studies on thermal ethylene oligomerization [ 13 ], [ 16 ]. According to the recent study done by Conrad et al. [ 16 ], since the thermal ethylene oligomerization was reported to happen mainly in temperatures higher than 400°C, increments were set to be smaller (50°C). The reactor wall effect was examined by replacing the SS316 reactor with an Inconel reactor (1/2″ OD × 0.065″ wall thickness, McMaster Carr) with the same dimensions. The effect of pressure was investigated by decreasing it from 15 bar to 5 bar in the higher temperature range, where more conversion was expected. Data processing and calculations are shown in equations 2 − 1 to 2–3, where F t is total flow (inlet/outlet), F x is gas species (N 2 /C 2 H 4 ) flow, y x is gas species (N 2 /C 2 H 4 ) volume fraction, and X C2H4 is the ethylene conversion. \(\:{F}_{t,out}=\frac{{F}_{t,in}*{y}_{N2,\:in}}{{y}_{N2,out}}\) (Eq. 2 − 1) \(\:{F}_{C2H4,out}={F}_{t,out}*{y}_{C2H4,out}\) (Eq. 2–2) \(\:Ethylene\:conversion\:\left({\%X}_{C2H4}\right)=\:\frac{{F}_{C2H4,\:in}-{F}_{C2H4,out}}{{F}_{C2H4,\:in}}*100\) (Eq. 2–3) Hydrocarbon distribution in the liquid phase has also been calculated based on the GC-FID results and the peak area comparison with aliphatic hydrocarbon standard calibration mix (C5-C12) (UST157 from Sigma-Aldrich) that was analyzed under similar conditions to liquid products. The Sigmoid function model was used to fit ethylene conversion data and improve visualization of the conversion trend with respect to temperature. The model is characterized by an S-shape and captures the transition from low to high temperatures as the temperature rises. Using this model results in a smoother interpretation of the experimental data, depicting conversion more clearly across different conditions. Curve fitting was performed using nonlinear regression in Python, and the resulting sigmoid model parameters (maximum conversion, inflection point, and slope) are reported beside the experimental data. The Sigmoid function that was used is shown by Eq. 2–4 where y is ethylene conversion (%), x is temperature (°C), L is maximum conversion (plateau), x 0 is inflection point where the curve becomes more steep, k is the slope to determine how steep the curve is. \(\:y=\frac{L}{1+{e}^{-k(x-{x}_{0})}}\) (Eq. 2–4) The nonlinear least squares optimization method uses a function to find values for L, x₀, and k that minimize the difference between the experimental data and the curve. The x and y values are available from the experimental data. The initial values for L, x₀, and k are 100% (the maximum conversion is 0.94), 550°C (the midpoint at which conversion increased significantly), and 0.1 (the average steepness), respectively. 3. Results 3.1. The effect of temperature on ethylene conversion and product distribution The ethylene conversion showed an increasing trend as temperature increased from 200 to 650°C. The conversion increased significantly from 66% at 550°C to 94% at 650°C, which is presented in Fig. 2 , and the mass balance error was all ± 5%. To better visualize the trend, a sigmoid curve was fitted to the data. This curve describes the low-conversion plateau at lower temperatures and the saturation behavior at higher temperatures. As shown in Figs. 2 and 3 , the curve closely matches the experimental data, indicating a sharp increase in conversion between 500 and 600°C. The Inconel reactor showed a similar trend as the SS316 overall, however, the ethylene started its conversion at 450°C, as compared to the SS316 reactor, and a more noticeable increase at temperatures higher than 500°C, which is shown in Fig. 3 . Ultimately, the conversion reached 94% after 5 hours of reaction. The ethylene conversion comparison between the SS316 and the Inconel reactor is shown in Fig. 4 , where the blue line and red line represent the SS316 and the Inconel reactors, respectively. In both SS316 and Inconel reactors, methane and propylene began to be produced after 550°C. The C3 volume fraction at 550°C was 16.1% and 15.9% for the SS316 and Inconel reactors, respectively, and these fractions showed a decreasing trend until the end of reaction at 650°C. In the meantime, methane started to be produced at the same temperature as propylene, with the opposite trend in volume fraction in the outlet gas. It increased from 2.12–35.14% in the SS316 reactor and from 2.42–34.59% in the Inconel reactor when changing the temperature from 550 to 650°C. The emergence of new, unidentified peaks in the chromatogram at 550°C (in addition to those attributed to methane and propylene) suggested the beginning of ethylene conversion to heavier hydrocarbons. The peak areas and intensity started to rise (even slightly) as the temperature increased, aligning with Conrad et al. [ 13 ],[ 16 ] that reported about the possibility of thermal ethylene oligomerization at temperatures above 465°C. Following the calibration of the gas chromatogram equipment for C4 hydrocarbons (butane and cis/trans butene), a distinct peak appeared at 1.29 minutes on the chromatogram, corresponding to these hydrocarbons. Due to the similarities of the corresponding factor and retention time for both butane and butene, this peak was collectively designated as C4 for identification. The C4 peak area and intensity were increased by elevating the temperature to 600°C before declining thereafter. This could be because of the production of new compounds in the outlet gas, which showed up as a new peak on the gas chromatography report at a further retention time, such as 3.55 min. It indicates a shift in the product distribution toward heavier and more complex molecules at higher temperatures. Figure 5 shows the C4 peak area changing trend in the outlet gas at different temperatures in both SS316 and Inconel reactors. Notably, the C4 peaks began to be identified by the GC-FID column at 450°C in the Inconel reactor, whereas no such signal was observed under the same conditions in the SS316 reactor. At 650°C, the SS316 produced a smaller amount of C4 compared to Inconel. This lower C4 presence, coupled with the appearance of new peaks at longer retention times, implies that SS316 may exhibit more selectivity toward heavier hydrocarbons in the gas phase. The GC-FID column data for the outlet gas shows a clear correlation between temperature and retention time of the detected peaks in both the SS316 and Inconel reactors, which is depicted in Figs. 6 and 7 . By increasing the temperature from 500 to 650°C, new peaks appeared at longer retention times, indicating the formation of heavier hydrocarbons. This trend aligns with the observations by Egloff et al. [ 22 ], who showed that higher temperatures favor the thermal oligomerization of light hydrocarbons to longer-chain molecules. The gas phase experimental data are shown in Figs. 6 and 7 , clearly demonstrating this correlation as peaks that appeared after 1.5 minutes were associated with the longer chain hydrocarbons, while peaks that appeared at shorter retention times (less than 1.39 minutes) corresponded to lighter hydrocarbons. This trend was consistent in all the experimental runs. While both Inconel and SS316 reactors showed shifting toward heavier hydrocarbons by increasing temperature, the key difference was observed in the onset temperature of hydrocarbon formation. In the Inconel reactor, the detectable hydrocarbon peaks started at 450°C, whereas in the SS316 reactor, they first appeared at 500°C. This earlier hydrocarbon production onset in the Inconel reactor highlights the potential effect of reactor material on improving ethylene conversion. Additionally, the Inconel reactor indicated slightly higher peak areas at certain retention times, indicating increased hydrocarbon formation under similar operating conditions. This suggests that Inconel may promote ethylene oligomerization, likely due to its material composition. As reported by Zhu et al.[ 18 ] and Bezerra et al. [ 19 ], materials that contain nickel can have a catalytic effect under elevated temperature and pressure by facilitating bond formation and improving product yields. However, for the retention times that correlate mainly with lighter hydrocarbons (e.g., 1.29–1.39), similar areas were observed for both reactors, which points out the minimal difference in lighter hydrocarbon production. 3.2. Effect of pressure on ethylene conversion and product distribution Since higher conversions were observed at a higher temperature range (450 to 650°C), this interval was selected to check the effect of pressure. Experiments were done at 5 bar using both SS316 and Inconel reactors, following the same methodology as the high-pressure (15 bar) runs to maintain consistency. Figures 8 and 9 present the comparison of ethylene conversion at 5 and 15 bar in the SS316 and Inconel reactors. A clear correlation between pressure and ethylene conversion was observed for both reactors in the same temperature range, where ethylene conversion dropped significantly as pressure decreased. In contrast, at 5 bar, ethylene conversion had a significant decrement, specifically at higher temperatures such as 600°C. This showed that lower pressure fails to provide sufficient ethylene density to maintain the oligomerization process effectively, thereby leading to significantly reduced conversion When comparing the SS316 and Inconel reactors under identical reaction conditions, ethylene conversion was consistently higher in the Inconel reactor. For instance, at 650°C and 5 bar, the Inconel reactor reached a slightly higher conversion than the SS316 reactor. This observation suggests the potential catalytic effect of the reactor wall material, likely due to Inconel's higher nickel content, which could facilitate the secondary reactions. A similar effect has been reported by Zhu et al. [ 18 ], who demonstrated that Ni-containing materials can improve hydrocarbon conversion specifically at elevated pressure and temperature. As shown in Figs. 8 and 9 , the effect of pressure on ethylene conversion was particularly significant at temperatures above 550°C. Under these conditions, the conversion rates at 15 bar were notably higher than those observed at 5 bar. Higher pressure not only maximized the ethylene conversion up to 94% at 600 and 650°C but also contributed to reaction stability, as shown by the consistently high conversion values across multiple temperature points. These observations align with Egloff et al. [ 22 ] study, who indicated that high pressure and temperature improve the efficiency of thermal ethylene oligomerization. When comparing the SS316 and Inconel reactors, ethylene conversion started at a lower temperature (500°C) in the Inconel reactor compared to the SS316 reactor (550°C). This also aligns with the observation of new peaks at 1.29 min associated with C4 hydrocarbons according to the primary calibration. These compounds appeared for the first time at 500°C for the Inconel reactor and at 550°C for the SS316 reactor, further supporting the point that reactor material may influence the conversion behavior under identical conditions. The same pattern has happened using the Inconel reactor, showing a notably higher conversion at higher pressure, as Fig. 10 shows this trend. Figures 10 and 11 present the GC-FID peak areas in different retention times for both SS316 and Inconel reactors at 5 bar. Across all retention times, the peak areas are notably lower compared to those observed at 15 bar. This pattern was consistent in both SS316 and Inconel reactors and directly reflects the reduced ethylene conversion at lower pressure. This observation aligns with Conrad et al. [ 16 ] and Boyd et al. [ 15 ] findings regarding the improving effect of elevated pressure on ethylene conversion and product distribution. At 5 bar, the product distribution seemed to be more toward lighter hydrocarbons in the gas phase, indicated by larger areas at lower retention times (1.29 to 1.34 min) for both reactors. This trend suggests that lower pressure limits oligomerization reaction due to reduced ethylene density and fewer effective molecular collisions. Consistent with the previous observation on the earlier onset of ethylene conversion in the Inconel reactor (550°C) compared to the SS316 reactor (500°C ) at both pressures, it highlights the potential effect of reactor material on improving ethylene conversion and product distribution. 3.3. Liquid product analysis The quantity of liquid product increased with temperature for both reactor materials, as shown in Fig. 12 (a-d). In the SS316 reactor, experiments from 200 to 550°C led to liquid production that was visible, although it was not enough to measure its weight accurately. In contrast, changing to the Inconel reactor under similar conditions yielded approximately 0.08 g of liquid, demonstrating slightly greater production efficiency compared to the SS316 reactor. Raising the temperature from 450 to 650°C at 15 bar increased the quantity of liquid products significantly: approximately 1.71 g was collected from the SS316 reactor, and about 2 g from the Inconel reactor. However, decreasing pressure from 15 bar to 5 bar under the same temperature condition led to the production of a lower amount of liquid production in both reactors, averaging about 0.53 g. These results are illustrated in Fig. 13 (a and b) and demonstrate the strong influence of pressure on reaction production yield. The quantity of liquid product clearly illustrates the effect of temperature on the non-catalytic ethylene oligomerization. The increased liquid yield at higher temperature ranges in both reactors is consistent with the rise in ethylene conversion, highlighting the role of elevated temperature in improving oligomer formation. In addition, the production of liquid serves as an indicator of heavier hydrocarbon generation. The production of heavier hydrocarbons such as C6, C7, C8, and C10 during thermal ethylene oligomerization was also reported in Conrad et al.[ 13 ], [ 16 ]. Their experiments were carried out in SS316 and quartz reactors at a low pressure of 1.5 bar and a high pressure of 43.5 bar and elevated temperatures, which similarly led to the production of middle to long chain hydrocarbons in the liquid phase. The effect of pressure on the liquid production is clearly demonstrated by comparing Fig. 12 (c and d) and Fig. 13 (a and b), which represent the experiments conducted at a similar temperature range and different pressures. The lower liquid yield is consistent with the decline observed in ethylene under lower pressure. This reinforces the influence of pressure in both conversion and heavier hydrocarbon formation. The hydrocarbons distribution in the liquid phase for both SS316 and Inconel reactors at 200 to 650°C and 15 bar is shown in Fig. 14 . As presented, C6-C8 are the majority of products in the liquid phase composition, accounting for over 93% in both reactors. These observations are consistent with Conrad et al.[ 16 ] study, who reported that C6-C9 were the majority of product distribution up to 450°C and 14 bar in thermal ethylene oligomerization. However, Inconel resulted in a slightly higher share of C9-C12 (4.81%) compared to SS316 (3.26%). This suggests the potential influence of the reactor wall on improving the formation of middle-chain oligomers. On the other hand, SS316 demonstrated a higher share of C12+ (3.06%) compared to the Inconel reactor (1.13%), possibly showing that the formed heavier hydrocarbons in the Inconel reactor went through more thermal cracking, or the SS316 reaction environment was more suitable for preserving longer chains. 4. Discussion Regarding the effect of temperature, the ethylene conversion above 450°C aligns with the research work done by Conard et. al [ 16 ] at similar temperature and pressure. Literature reports indicate that at temperatures below 400°C, ethylene conversion is extremely low (less than 1%), demonstrating that without a catalyst, this reaction is highly unlikely at lower temperatures and only improves significantly as the temperature increases. S.M. Al-Salem [ 23 ] studied polyethylene pyrolysis between 500 to 800°C in an Inconel-700 reactor and showed an increasing trend in gas production from 500 to 800°C, which was more noticeable between 500 to 600°C. Obtaining lighter hydrocarbons (C2-C4) as the dominant species rather than C5-C10 at higher temperatures supports the effect of elevated temperature on decomposition reactions. Increasing temperature also enhanced the C2 to C4 yield up to 79% when reaching 800°C. It has been found by Conard et al.[ 13 ], [ 16 ] that although even carbon chains such as ethylene, butene, hexene, octene, etc. are the most probable species that are produced in oligomerization reactions, non-oligomer compounds such as methane, propylene, pentene, etc. can also be produced in large amounts. In addition, it has been reported by Egloff et al. [ 24 ] that at higher temperatures (e.g., 600–650°C), decomposition reactions lead to lighter hydrocarbon production. The increased peak areas at longer retention time, shown in Figs. 6 and 7 , additionally confirm that rising heavier hydrocarbon concentration rises with increasing temperature. These conclusions are supported by the work of Arey et al [ 25 ] asserting that in gas chromatography, retention time correlates with molecular weight, in which longer retention times are a sign of the occurrence of heavier hydrocarbons in the mixture. Using retention time as an approximation of molecular weight was verified by Zellner et al. [ 26 ], who described the retention index principles. According to this study, longer retention time shows the elevated interactions between analytes and the stationary phase, which aligns with the behavior of heavier hydrocarbons. This is also reinforced by the predictive model presented by Katritzky et al.[ 27 ] linking retention indices to hydrocarbon chain length. At temperatures above 550°C, secondary reactions such as recombination and cracking are likely to cause the appearance of new peaks. Considering the similar pattern reported by Conrad et al. [ 16 ], the higher temperatures and pressure in non-catalytic ethylene oligomerization caused the formation of secondary products in the form of a mixture of lighter and heavier hydrocarbons. This mechanism also helps explain why the methane volume fraction increased as the temperature rose. In the meantime, a higher intensity and a larger number of peaks at longer retention times can be related to ethylene oligomerization and subsequent reactions that favor the formation of heavier hydrocarbons. Additionally, the relative contribution of oligomerization and cracking is clearly reflected in the peak area distribution shown in Figs. 6 and 7 . Peaks that appeared at shorter retention times correspond to lighter hydrocarbons such as methane and ethylene, while peaks that were detected at higher retention times reflect the formation of heavier compounds. The product distribution pattern showed an obvious shift toward heavier hydrocarbons as temperature increased. This conclusion also aligns with Boyd et al. [ 15 ] and Conrad et al. [ 13 ], [ 16 ] who reported that higher temperatures improved the production of longer-chain hydrocarbons. However, they also noted that elevated thermal conditions promote cracking reactions at the same time, resulting in a broader mixture of both light and heavy products. As for the effect of pressure, ethylene conversion increased proportionally with temperature at 15 bar, reaching approximately 94% in both the SS316 and Inconel reactors as indicated in Figs. 8 and 9 . This observation aligns with previous studies such as Conrad et al. [ 16 ], which emphasized that elevated pressure favors ethylene oligomerization by shifting the reaction equilibrium toward product formation. Additionally, higher pressure increases ethylene density in the system, improving the frequency of molecular collisions and facilitating bond formation as reported by Boyd et al. [ 15 ]. Also, increasing pressure limits secondary cracking reactions, which results in higher conversion by producing lighter hydrocarbons. A similar conclusion was drawn by Egloff et al. [ 22 ], who highlighted the role of pressure in improving ethylene conversion and limiting by-product formation. In a related study, Egloff et al. [ 24 ] demonstrated that higher pressure could improve the formation of heavier hydrocarbons, whereas lower pressure could cause lighter hydrocarbon production due to fewer molecular collisions. However, it is important to note that studies such as Conrad et al. [ 16 ] have mainly ruled out the significant catalytic role of reactor material in non-catalytic reactions. Thus, the higher conversion observed in the Inconel reactor could be due to the minor surface interactions or differences in heat distribution and thermal conductivity between the two materials. Overall, the results point out the critical role of pressure in non-catalytic ethylene oligomerization. While the reactor material seemed to have some influence, its effect was secondary compared to the major impact of elevated pressure. These findings are consistent with broader research on thermal ethylene oligomerization that emphasises the importance of the effect of operational conditions on product distribution [ 13 ], [ 16 ], [ 22 ], [ 27 ]. The use of sigmoid curve fitting further validated these trends, showing consistent inflection points and saturation behavior aligned with theoretical expectations for temperature-dependent conversion in non-catalytic oligomerization. In terms of the liquid products, it was observed that the liquid produced in the Inconel reactor across all temperature and pressure conditions was clearer compared to what was produced using the SS316 reactor, which is presented in Figs. 12 and 13 . To assess the possible effect of the reactor wall on the liquid composition more accurately, further analysis using gas chromatography with flame ionization detection (GC-FID) was necessary. According to a study by Cui et al. [ 28 ], GC-FID enables to provide detailed data on hydrocarbon mixture composition, including liquid oligomers. However, this method does have limitations, particularly in resolving all individual components due to the restricted capacity of the separation column to distinguish between closely eluted hydrocarbons. This limitation is especially relevant when analyzing complex hydrocarbon mixtures such as those derived from ethylene oligomerization. Although the quantification and characterization of the indicative hydrocarbons by GC-FID faced some challenges, the analysis nonetheless demonstrated the likely presence of heavier hydrocarbons such as C6 to C12+. This conclusion was based on the peak areas achieved in the liquid samples from oligomerization and their comparison with the standard calibration olefin mixture from C5 to C12, as illustrated in Fig. 14 . Using GC-FID peak area correlation with hydrocarbon molecular weight was explored in a couple of studies. Cui et al. [ 28 ] developed a gasoline composition model based on GC-FID results, indicating the possibility of linking peak area distributions to specific hydrocarbons. Durand et al. [ 29 ] studied weight determination and distribution in crude oil and condensate by GC analysis and confirmed the correlation between compound weight and peak area. Product distribution variation with pressure is shown in Figs. 15 and 16 for SS316 and Inconel reactors, respectively. Decreasing pressure from 15 to 5 bar reduced C6-C8 in both reactor materials, while interestingly, it resulted in heavier hydrocarbon fractions such as C9-C12 and C12+. Although higher pressure is known as a parameter to enhance ethylene conversion and form lighter hydrocarbons because of cracking, observing more heavier hydrocarbons, specifically using Inconel, may be due to the subtle balance between stabilization and propagation occurrence before fragmentation. This could mean that if longer chain hydrocarbons stabilized rapidly because of reaction conditions such as time and reactor wall material, heavier hydrocarbons such as C9-C12 may remain. This observation aligns with Conrad et al.[ 16 ] study on thermal ethylene oligomerization, where they reported that pressure and temperature affect chain growth and cracking tendencies in a complex way. The pressure effect was more significant in the Inconel reactor, where the C9-C12 hydrocarbon fraction was almost tripled, and C12 + increased. This may be an indication of the effect of the nickel-based wall of Inconel in a way that it stabilized and promoted the formation of longer-chain hydrocarbons, which was reported by Zhu et al.[ 18 ] as well. These observations highlight both the pressure effect complexity and the reactor wall material on thermal ethylene oligomerization. 5. Conclusion This study explored the effect of temperature, pressure, and reactor wall material on ethylene conversion and product distribution in the non-catalytic oligomerization (thermal) of ethylene. Higher temperatures notably enhanced ethylene conversion, reaching 94% at 650°C in both SS316 and Inconel reactors. At lower pressure (5 bar), ethylene conversion decreased significantly and favored lighter hydrocarbons in the gas phase. This highlights the reduced oligomerization activity under low-pressure conditions. Elevated temperature and pressure both improved ethylene conversion and liquid production in both reactors, with the Inconel reactor resulting in more and clearer liquid. Hydrocarbon distribution analysis in the liquid phase at 200 to 650°C and 15 bar demonstrated that C6-C8 were the primary produced hydrocarbons in both SS316 and Inconel reactors. Earlier observation of ethylene conversion point at 450°C and more C9-C12 hydrocarbons formation in the Inconel reactor compared to SS316, suggesting the reactor wall material may facilitate oligomerization and affect chain growth. Both reactors showed a shift toward heavier hydrocarbons (C9-C12 and C12+) at 5 bar compared to 15 bar, specifically in the Inconel reactor. This suggests that lower pressure may result in fewer secondary cracking reactions, allowing longer-chain hydrocarbons to form and be more stable. Overall, the results from this work show that optimizing temperature, pressure, and reactor material (particularly Inconel) can improve both the gas-phase and liquid-phase products in non-catalytic ethylene oligomerization, facilitating the way for more efficient, sustainable aviation fuel production. Declarations Acknowledgements The authors would like to acknowledge the Biomass Technology Laboratory (BTL) for providing the required infrastructure for this study, as well as the Biomass, Bioproduct, and Bioprocess Analysis Laboratory (LAB) and Mr. Maxime Lessard for the analysis performed. Further acknowledgment goes to the sponsor of this research work, Greenfield Global. Finally, the authors would like to express gratitude to Mr. Henry Gauvin (Université de Sherbrooke) for his assistance in assembling the reaction system. This project is financially supported by the Natural Sciences and Engineering Research Council of Canada (NSERC - ALLRP 571672-2021), Consortium de recherche et innovations en bioprocédés industriels au Québec (CRIBIQ − 2022-065-C87), and Greenfield Global. Conflict of Interest : The authors declare no conflict of interest. References Gonzalez-Garay, A., et al.: Jun., Unravelling the potential of sustainable aviation fuels to decarbonise the aviation sector, Energy Environ Sci , vol. 15, no. 8, pp. 3291–3309, (2022). 10.1039/d1ee03437e Olhoff, A., et al.: Emissions Gap Report 2024: No more hot air … please! With a massive gap between rhetoric and reality, countries draft new climate commitments. United Nations Environ. Programme. (2024). 10.59117/20.500.11822/46404 Ng, K.S., Farooq, D., Yang, A.: Global biorenewable development strategies for sustainable aviation fuel production, Oct. 01, Elsevier Ltd . 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(1989). 10.1002/jhrc.1240120408 Supplementary Files floatimage1.jpeg Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-6770942","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":492828373,"identity":"3a43466d-672b-499e-8fb3-220b1de8b17a","order_by":0,"name":"Ghazal Azadi","email":"","orcid":"","institution":"Université de Sherbrooke: Universite de Sherbrooke","correspondingAuthor":false,"prefix":"","firstName":"Ghazal","middleName":"","lastName":"Azadi","suffix":""},{"id":492828374,"identity":"c0abd6fc-3aba-4d22-b205-95a6d56116c8","order_by":1,"name":"Eduardo Lins de Barros Neto","email":"","orcid":"","institution":"UFRN: Universidade Federal do 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7","display":"","copyAsset":false,"role":"figure","size":109975,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of temperature on the gas products GC-FID peaks at different retention times for the Inconel reactor operated at 15 bar\u003c/p\u003e","description":"","filename":"Picture7.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6770942/v1/0914bc72637fc27886c6d370.jpg"},{"id":88236541,"identity":"b8bc74ea-04cb-4055-941a-094acb37ab8a","added_by":"auto","created_at":"2025-08-04 10:24:03","extension":"jpg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":115644,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of pressure on ethylene conversion at 550 to 650 °C in the SS316 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12","display":"","copyAsset":false,"role":"figure","size":99053,"visible":true,"origin":"","legend":"\u003cp\u003eLiquid products obtained at 15 bar under different temperature ranges: 200–550 °C in SS316 (a) and Inconel (b), and 450–650 °Cin SS316 (c) and Inconel (d).\u003c/p\u003e","description":"","filename":"12.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6770942/v1/a5ab525083e05f30b18b0baa.jpg"},{"id":88236974,"identity":"1ec10df4-6ab1-4ba5-ab06-fab29469a819","added_by":"auto","created_at":"2025-08-04 10:32:04","extension":"jpg","order_by":13,"title":"Figure 13","display":"","copyAsset":false,"role":"figure","size":79769,"visible":true,"origin":"","legend":"\u003cp\u003eLiquid products at 5 bar and 450 -650 °C in SS316 (a) and Inconel (b) 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Introduction","content":"\u003cp\u003eThe aviation industry is responsible for about 2% of global human-caused GHG emissions and is expected to grow notably by 2050, this potentially will increase CO₂ emissions. In 2019, aviation fuel consumption reached 350\u0026nbsp;million metric tons, which emitted an estimated 1.09 Gigatons of CO₂ [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. In order to meet the goal of the Paris Agreement to limit the global temperature increase to below 1.5\u0026deg;C, and according to the latest UN environmental report, which emphasizes the need to reduce greenhouse gas emissions by 43% by 2030 [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e], shifting to cleaner energy production methods, including sustainable aviation fuels, is essential to reducing this sector's environmental impact [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eConventional aviation fuel is derived from kerosene that is produced from crude oil, which is fossil-based [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. The alternative, known as SAF (Sustainable Aviation Fuel), refers to jet fuel produced from renewable sources that are, in principle, nondepletable. SAF has compounds similar to conventional jet fuel but is produced from renewable sources, so it has a high potential to reduce greenhouse gases. Based on the International Civil Aviation Organization (ICAO) report, SAF could meet 100% demand for jet fuel by 2050 and could reduce CO\u003csub\u003e2\u003c/sub\u003e emissions by 63% [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. While Fischer-Tropsch (FT) and Hydro-processed Esters and Fatty Acid (HEFA) methods were the first alternative aviation fuels approved to be used in this industry, companies such as Byogy Renewables Inc. reported the possibility of producing jet fuel from bioethanol in 2009 [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eOne promising pathway to produce SAF is Alcohol-to-Jet (ATJ) where alcohols are converted to jet fuel through dehydration, oligomerization, hydrogenation, isomerization, and distillation [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. Among these, the dehydration of light alcohols such as ethanol, produces light-chain olefins like ethylene, an important raw material with a wide range of applications in the petrochemical, polymer, and surfactant industries. Currently, ethylene comes from fossil fuels such as petroleum steam reforming and natural gas [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. However, the environmental concerns about fossil fuels, emphasize the importance of renewable ethylene through bioethanol dehydration [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e], [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eBioethanol production from sugar grains experienced rapid growth in the 1980s and provided the opportunity for bioethylene production. Such an approach represents an alternative to fossil-based ethylene production with fewer environmental footprints [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. Ethylene can then undergo oligomerization to generate longer-chain hydrocarbons (α-olefins), which are precursors to produce polyethylene, lubricants, and fuels such as gasoline, diesel, and jet fuel blends [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eEthylene oligomerization is catalyzed by either transition metal catalysts (e.g., nickel, iron, and cobalt) or solid mesoporous acidic catalysts (such as H-ZSM-5 [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e] [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e], [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e], [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e], and [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]). However, these approaches have drawbacks and limitations such as catalyst deactivation, regeneration, and co-catalyst requirement. In contrast, thermal ethylene oligomerization, which has been employed in refineries since the 1920s, could minimize coking and deactivation issues. However, it requires higher temperature and pressure than catalytic ethylene oligomerization [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eDespite the advances in catalytic oligomerization, research on thermal ethylene oligomerization is limited specifically to elevated temperature and pressure ranges. Pease [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e] investigated non-catalytic ethylene polymerization from 450 to 650\u0026deg;C, and reported that at temperatures above 525\u0026deg;C, the formation of methane, ethane, and liquid hydrocarbons increased. More methane formation at higher temperatures suggested the secondary product evolution. Since this study has been done at atmospheric pressure, there were limited insights into higher pressure. Boyd et al. [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e] extended this work, studied on thermal reactions of ethylene in a quartz cylinder reactor in temperatures between 500 to 600\u0026deg;C, and concluded that the impact of temperature on reaction rates and product distribution is significant. While the main products were ethane, propylene, and butene, higher temperatures led to the production of smaller hydrocarbons as a result of favoring secondary reactions.\u003c/p\u003e\u003cp\u003eMore recently, Conard et al. [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e] and [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e] investigated thermal ethylene oligomerization at a temperature range of 320 to 500\u0026deg;C and pressure of 1.5 to 43.5 bar, with the main focus on reaction pathways and DFT studies. It has been reported that higher temperatures favored lighter hydrocarbon production while lower temperatures stabilized the production of longer-chain hydrocarbons. Their studies also concluded that higher pressure improved ethylene conversion to liquid products.\u003c/p\u003e\u003cp\u003eThus, the ethylene conversion and liquid product composition remained almost unclear for the same and higher operational conditions. The lack of broad knowledge, in addition to the issues mentioned for catalytic oligomerization, emphasizes the importance of more research on thermal oligomerization. It could be possible that by going more in-depth into the thermal ethylene oligomerization aspects, engineering catalysts that make this reaction happen in milder operational conditions become possible.\u003c/p\u003e\u003cp\u003eThe effect of the reactor wall is another important aspect that has been investigated. Paese et al. [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e] studied the effect of reactor walls in non-catalytic ethylene oligomerization by doing experiments in empty and packed Pyrex tubes. He concluded that there is no significant enhancement in reaction rate by using packed tubes; this also ruled out the catalytic effect of the reactor wall. Boyd et al. [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e] also investigated the reactor wall effect by conducting experiments in quartz reactors and concluded that the reaction is mainly homogeneous with no notable contribution of the reactor wall catalytic effect in thermal ethylene oligomerization.\u003c/p\u003e\u003cp\u003eIn Conrad\u0026rsquo;s first research work on thermal ethylene oligomerization in quartz and stainless-steel reactors, it was verified that reactor walls did not have a significant effect on product distribution or reaction rates [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. These results were consistent for both reactor materials that have been used in this research and reinforced the negligible effect of the reactor wall. In another similar study, Conrad et al. [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e] concluded the same negligible effect of the reactor wall on product distribution and reaction rates by increasing the reactor surface area.\u003c/p\u003e\u003cp\u003eConversely, studies on other high-temperature, high-pressure reactions, such as supercritical water gasification of glycerol, were done by Salierno et al. [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e] and Zhu et al. [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e] have concluded that reactor materials such as Inconel and Hastelloy can improve reactions such as hydrocarbon production and water-gas shift. These studies suggested that the reactor wall material can impact reaction outcomes under certain circumstances. In another recent study, Bezerra et al. [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e] investigated the effect of reactor material on syngas reforming and found that an Inconel reactor could significantly improve CH\u003csub\u003e4\u003c/sub\u003e and CO\u003csub\u003e2\u003c/sub\u003e conversion compared to a stainless-steel reactor, which emphasizes the potential catalytic effect of high Nickel content material.\u003c/p\u003e\u003cp\u003eAlthough the reactor wall effect is less emphasized in catalytic ethylene oligomerization due to the predominant role of the catalyst, some studies have indirectly addressed it. Jan[\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e] studied ethylene oligomerization over Ni-Hβ catalyst, in which the role of the reactor\u0026rsquo;s interaction in the catalyst deactivation mechanism was indirectly addressed. Panpain[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e] worked on catalytic ethylene oligomerization using Ni/Al-KIT-6, implying that a stainless-steel fixed bed reactor minimized the interactions between reactants and the reactor wall, with more focus on catalyst activity and reaction efficiency rather than the reactor wall effect. Although these studies focused on optimizing catalyst and reactor design, they rarely did an in-depth investigation of the role of reactor walls. The primary goal of this research is to investigate the effect of temperature, pressure, and the wall effect on the non-catalytic (thermal) ethylene oligomerization. To achieve these goals, a series of experiments were performed under controlled conditions. To evaluate the product distribution, ethylene conversion, and produced oligomers have been analyzed at a temperature gradient from 200 to 650\u0026deg;C and pressures of 5 and 15 bar. This research also aims to explore the potential influence of the reactor wall (SS316 and Inconel) on the thermal oligomerization of ethylene. While the previous studies [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e], [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e], [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e], and[\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e] ruled out the notable catalytic effect of the reactor wall, these conclusions were obtained in the operating conditions that may not fully represent the elevated temperature and pressure conditions in this field.\u003c/p\u003e\u003cp\u003eThus, this aims to push further previous findings in the field by expanding temperature and pressure ranges. it will also utilize Inconel for such conditions and will investigate whether it has any influence on ethylene conversion or product distributions, especially at higher temperatures and pressures, where the surface interactions could be more significant. It will also help to extend the understanding of non-catalytic systems, where the absence of a catalyst highlights the potential role of the reactor wall as a significant variable.\u003c/p\u003e"},{"header":"2. Methodology","content":"\u003cp\u003eThe experiments were performed in a continuous reaction system equipped with an SS316 fixed bed reactor (12 \u0026Prime; L, 0.37\u0026Prime; W, 0.065\u0026Prime; T), heated by an electric furnace (Thermcraft split tube furnace 1\u0026Prime; ID X 10\u0026Prime; L) used for temperature control. Nitrogen (99.999% purity from Linde) and ethylene (C\u003csub\u003e2\u003c/sub\u003eH\u003csub\u003e4\u003c/sub\u003e, 99.999% purity from Linde) were fed to the system through two Mass Flow Controllers (MFC) (Brooks Instrument), keeping the ethylene-nitrogen ratio at 95:5 with a total flow of 88 ml/min. The reaction temperature was monitored by two thermocouples that were placed into the furnace wall to measure the temperature outside of the reactor and inside the reactor, down to the middle, to measure the temperature inside. The reactor pressure was adjusted and maintained using a Back Pressure Regulator (BPR) (Swagelok, KBP1J0A4A5A20000, 0-500 psi), which was used to regulate the experiments performed at 15 and 5 bar. The system also contained a Pressure Relief valve (PRV) (Swagelok, SS-4R3A1-EP) for safety purposes. A depiction of the reaction system is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eAt the beginning of each run, 88 ml/min nitrogen was passed through the reactor while the temperature and pressure were increased to the desired value. Once temperature and pressure were stabilized, nitrogen flow was decreased to 4.4 ml/min, and ethylene flow was increased to 83.6 ml/min. The reaction time started from the moment ethylene was injected into the reactor. The gas phase was analyzed offline (every 30 min) by gas chromatography (GC-456 Natural Gas Analyzer, Bruker) to determine and quantify the produced gas, to calculate conversion, and verify product distribution. The data processing program was Compass CDC, using external standards. Liquid samples were collected after the reaction for GC-FID analysis (Shimadzu GC-FID) in order to quantify the hydrocarbon composition. As for the GC program, the injector was set at 275\u0026deg;C, He was used as the carrier gas, at a column flow of 1.30 ml/min, and using a 1:20 split. The Oven program started at 40\u0026deg;C and was held for 1 min, after which it was fixed at 300\u0026deg;C at a 10\u0026deg;C/min increment, after which it was held for 1 min. Finally, the FID temperature was set at 325\u0026deg;C.\u003c/p\u003e\u003cp\u003eThe effect of temperature on ethylene oligomerization was studied from 200 to 650\u0026deg;C with 100\u0026deg;C temperature increments up to 400\u0026deg;C and 50\u0026deg;C increments up to 650\u0026deg;C. The temperature range and increment amounts were chosen based on previous studies on thermal ethylene oligomerization [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e], [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. According to the recent study done by Conrad et al. [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e], since the thermal ethylene oligomerization was reported to happen mainly in temperatures higher than 400\u0026deg;C, increments were set to be smaller (50\u0026deg;C).\u003c/p\u003e\u003cp\u003eThe reactor wall effect was examined by replacing the SS316 reactor with an Inconel reactor (1/2\u0026Prime; OD \u0026times; 0.065\u0026Prime; wall thickness, McMaster Carr) with the same dimensions. The effect of pressure was investigated by decreasing it from 15 bar to 5 bar in the higher temperature range, where more conversion was expected.\u003c/p\u003e\u003cp\u003eData processing and calculations are shown in equations 2\u0026thinsp;\u0026minus;\u0026thinsp;1 to 2\u0026ndash;3, where F\u003csub\u003et\u003c/sub\u003e is total flow (inlet/outlet), F\u003csub\u003ex\u003c/sub\u003e is gas species (N\u003csub\u003e2\u003c/sub\u003e/C\u003csub\u003e2\u003c/sub\u003eH\u003csub\u003e4\u003c/sub\u003e) flow, y\u003csub\u003ex\u003c/sub\u003e is gas species (N\u003csub\u003e2\u003c/sub\u003e/C\u003csub\u003e2\u003c/sub\u003eH\u003csub\u003e4\u003c/sub\u003e) volume fraction, and X\u003csub\u003eC2H4\u003c/sub\u003e is the ethylene conversion.\u003c/p\u003e\u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{F}_{t,out}=\\frac{{F}_{t,in}*{y}_{N2,\\:in}}{{y}_{N2,out}}\\)\u003c/span\u003e\u003c/span\u003e (Eq.\u0026nbsp;2\u0026thinsp;\u0026minus;\u0026thinsp;1)\u003c/p\u003e\u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{F}_{C2H4,out}={F}_{t,out}*{y}_{C2H4,out}\\)\u003c/span\u003e\u003c/span\u003e (Eq.\u0026nbsp;2\u0026ndash;2)\u003c/p\u003e\u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:Ethylene\\:conversion\\:\\left({\\%X}_{C2H4}\\right)=\\:\\frac{{F}_{C2H4,\\:in}-{F}_{C2H4,out}}{{F}_{C2H4,\\:in}}*100\\)\u003c/span\u003e\u003c/span\u003e (Eq.\u0026nbsp;2\u0026ndash;3)\u003c/p\u003e\u003cp\u003eHydrocarbon distribution in the liquid phase has also been calculated based on the GC-FID results and the peak area comparison with aliphatic hydrocarbon standard calibration mix (C5-C12) (UST157 from Sigma-Aldrich) that was analyzed under similar conditions to liquid products.\u003c/p\u003e\u003cp\u003eThe Sigmoid function model was used to fit ethylene conversion data and improve visualization of the conversion trend with respect to temperature. The model is characterized by an S-shape and captures the transition from low to high temperatures as the temperature rises. Using this model results in a smoother interpretation of the experimental data, depicting conversion more clearly across different conditions. Curve fitting was performed using nonlinear regression in Python, and the resulting sigmoid model parameters (maximum conversion, inflection point, and slope) are reported beside the experimental data.\u003c/p\u003e\u003cp\u003eThe Sigmoid function that was used is shown by Eq.\u0026nbsp;2\u0026ndash;4 where y is ethylene conversion (%), x is temperature (\u0026deg;C), L is maximum conversion (plateau), x\u003csub\u003e0\u003c/sub\u003e is inflection point where the curve becomes more steep, k is the slope to determine how steep the curve is.\u003c/p\u003e\u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:y=\\frac{L}{1+{e}^{-k(x-{x}_{0})}}\\)\u003c/span\u003e\u003c/span\u003e (Eq.\u0026nbsp;2\u0026ndash;4)\u003c/p\u003e\u003cp\u003eThe nonlinear least squares optimization method uses a function to find values for L, x₀, and k that minimize the difference between the experimental data and the curve. The x and y values are available from the experimental data. The initial values for L, x₀, and k are 100% (the maximum conversion is 0.94), 550\u0026deg;C (the midpoint at which conversion increased significantly), and 0.1 (the average steepness), respectively.\u003c/p\u003e"},{"header":"3. Results","content":"\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e3.1. The effect of temperature on ethylene conversion and product distribution\u003c/h2\u003e\u003cp\u003eThe ethylene conversion showed an increasing trend as temperature increased from 200 to 650\u0026deg;C. The conversion increased significantly from 66% at 550\u0026deg;C to 94% at 650\u0026deg;C, which is presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, and the mass balance error was all \u0026plusmn;\u0026thinsp;5%. To better visualize the trend, a sigmoid curve was fitted to the data. This curve describes the low-conversion plateau at lower temperatures and the saturation behavior at higher temperatures. As shown in Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e and \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, the curve closely matches the experimental data, indicating a sharp increase in conversion between 500 and 600\u0026deg;C.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe Inconel reactor showed a similar trend as the SS316 overall, however, the ethylene started its conversion at 450\u0026deg;C, as compared to the SS316 reactor, and a more noticeable increase at temperatures higher than 500\u0026deg;C, which is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. Ultimately, the conversion reached 94% after 5 hours of reaction. The ethylene conversion comparison between the SS316 and the Inconel reactor is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, where the blue line and red line represent the SS316 and the Inconel reactors, respectively.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eIn both SS316 and Inconel reactors, methane and propylene began to be produced after 550\u0026deg;C. The C3 volume fraction at 550\u0026deg;C was 16.1% and 15.9% for the SS316 and Inconel reactors, respectively, and these fractions showed a decreasing trend until the end of reaction at 650\u0026deg;C. In the meantime, methane started to be produced at the same temperature as propylene, with the opposite trend in volume fraction in the outlet gas. It increased from 2.12\u0026ndash;35.14% in the SS316 reactor and from 2.42\u0026ndash;34.59% in the Inconel reactor when changing the temperature from 550 to 650\u0026deg;C.\u003c/p\u003e\u003cp\u003eThe emergence of new, unidentified peaks in the chromatogram at 550\u0026deg;C (in addition to those attributed to methane and propylene) suggested the beginning of ethylene conversion to heavier hydrocarbons. The peak areas and intensity started to rise (even slightly) as the temperature increased, aligning with Conrad et al. [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e],[\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e] that reported about the possibility of thermal ethylene oligomerization at temperatures above 465\u0026deg;C. Following the calibration of the gas chromatogram equipment for C4 hydrocarbons (butane and \u003cem\u003ecis/trans\u003c/em\u003e butene), a distinct peak appeared at 1.29 minutes on the chromatogram, corresponding to these hydrocarbons. Due to the similarities of the corresponding factor and retention time for both butane and butene, this peak was collectively designated as C4 for identification. The C4 peak area and intensity were increased by elevating the temperature to 600\u0026deg;C before declining thereafter. This could be because of the production of new compounds in the outlet gas, which showed up as a new peak on the gas chromatography report at a further retention time, such as 3.55 min. It indicates a shift in the product distribution toward heavier and more complex molecules at higher temperatures.\u003c/p\u003e\u003cp\u003eFigure \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e shows the C4 peak area changing trend in the outlet gas at different temperatures in both SS316 and Inconel reactors. Notably, the C4 peaks began to be identified by the GC-FID column at 450\u0026deg;C in the Inconel reactor, whereas no such signal was observed under the same conditions in the SS316 reactor. At 650\u0026deg;C, the SS316 produced a smaller amount of C4 compared to Inconel. This lower C4 presence, coupled with the appearance of new peaks at longer retention times, implies that SS316 may exhibit more selectivity toward heavier hydrocarbons in the gas phase.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe GC-FID column data for the outlet gas shows a clear correlation between temperature and retention time of the detected peaks in both the SS316 and Inconel reactors, which is depicted in Figs.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e and \u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e. By increasing the temperature from 500 to 650\u0026deg;C, new peaks appeared at longer retention times, indicating the formation of heavier hydrocarbons. This trend aligns with the observations by Egloff et al. [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e], who showed that higher temperatures favor the thermal oligomerization of light hydrocarbons to longer-chain molecules. The gas phase experimental data are shown in Figs.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e and \u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e, clearly demonstrating this correlation as peaks that appeared after 1.5 minutes were associated with the longer chain hydrocarbons, while peaks that appeared at shorter retention times (less than 1.39 minutes) corresponded to lighter hydrocarbons. This trend was consistent in all the experimental runs.\u003c/p\u003e\u003cp\u003eWhile both Inconel and SS316 reactors showed shifting toward heavier hydrocarbons by increasing temperature, the key difference was observed in the onset temperature of hydrocarbon formation. In the Inconel reactor, the detectable hydrocarbon peaks started at 450\u0026deg;C, whereas in the SS316 reactor, they first appeared at 500\u0026deg;C. This earlier hydrocarbon production onset in the Inconel reactor highlights the potential effect of reactor material on improving ethylene conversion.\u003c/p\u003e\u003cp\u003eAdditionally, the Inconel reactor indicated slightly higher peak areas at certain retention times, indicating increased hydrocarbon formation under similar operating conditions. This suggests that Inconel may promote ethylene oligomerization, likely due to its material composition. As reported by Zhu et al.[\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e] and Bezerra et al. [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e], materials that contain nickel can have a catalytic effect under elevated temperature and pressure by facilitating bond formation and improving product yields. However, for the retention times that correlate mainly with lighter hydrocarbons (e.g., 1.29\u0026ndash;1.39), similar areas were observed for both reactors, which points out the minimal difference in lighter hydrocarbon production.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003e3.2. Effect of pressure on ethylene conversion and product distribution\u003c/h2\u003e\u003cp\u003eSince higher conversions were observed at a higher temperature range (450 to 650\u0026deg;C), this interval was selected to check the effect of pressure. Experiments were done at 5 bar using both SS316 and Inconel reactors, following the same methodology as the high-pressure (15 bar) runs to maintain consistency. Figures\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e and \u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e present the comparison of ethylene conversion at 5 and 15 bar in the SS316 and Inconel reactors. A clear correlation between pressure and ethylene conversion was observed for both reactors in the same temperature range, where ethylene conversion dropped significantly as pressure decreased.\u003c/p\u003e\u003cp\u003eIn contrast, at 5 bar, ethylene conversion had a significant decrement, specifically at higher temperatures such as 600\u0026deg;C. This showed that lower pressure fails to provide sufficient ethylene density to maintain the oligomerization process effectively, thereby leading to significantly reduced conversion\u003c/p\u003e\u003cp\u003eWhen comparing the SS316 and Inconel reactors under identical reaction conditions, ethylene conversion was consistently higher in the Inconel reactor. For instance, at 650\u0026deg;C and 5 bar, the Inconel reactor reached a slightly higher conversion than the SS316 reactor. This observation suggests the potential catalytic effect of the reactor wall material, likely due to Inconel's higher nickel content, which could facilitate the secondary reactions. A similar effect has been reported by Zhu et al. [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e], who demonstrated that Ni-containing materials can improve hydrocarbon conversion specifically at elevated pressure and temperature.\u003c/p\u003e\u003cp\u003eAs shown in Figs.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e and \u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e, the effect of pressure on ethylene conversion was particularly significant at temperatures above 550\u0026deg;C. Under these conditions, the conversion rates at 15 bar were notably higher than those observed at 5 bar. Higher pressure not only maximized the ethylene conversion up to 94% at 600 and 650\u0026deg;C but also contributed to reaction stability, as shown by the consistently high conversion values across multiple temperature points. These observations align with Egloff et al. [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e] study, who indicated that high pressure and temperature improve the efficiency of thermal ethylene oligomerization.\u003c/p\u003e\u003cp\u003eWhen comparing the SS316 and Inconel reactors, ethylene conversion started at a lower temperature (500\u0026deg;C) in the Inconel reactor compared to the SS316 reactor (550\u0026deg;C). This also aligns with the observation of new peaks at 1.29 min associated with C4 hydrocarbons according to the primary calibration. These compounds appeared for the first time at 500\u0026deg;C for the Inconel reactor and at 550\u0026deg;C for the SS316 reactor, further supporting the point that reactor material may influence the conversion behavior under identical conditions.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe same pattern has happened using the Inconel reactor, showing a notably higher conversion at higher pressure, as Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e shows this trend.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFigures \u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e and \u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e present the GC-FID peak areas in different retention times for both SS316 and Inconel reactors at 5 bar. Across all retention times, the peak areas are notably lower compared to those observed at 15 bar. This pattern was consistent in both SS316 and Inconel reactors and directly reflects the reduced ethylene conversion at lower pressure. This observation aligns with Conrad et al. [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e] and Boyd et al. [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e] findings regarding the improving effect of elevated pressure on ethylene conversion and product distribution.\u003c/p\u003e\u003cp\u003eAt 5 bar, the product distribution seemed to be more toward lighter hydrocarbons in the gas phase, indicated by larger areas at lower retention times (1.29 to 1.34 min) for both reactors. This trend suggests that lower pressure limits oligomerization reaction due to reduced ethylene density and fewer effective molecular collisions. Consistent with the previous observation on the earlier onset of ethylene conversion in the Inconel reactor (550\u0026deg;C) compared to the SS316 reactor (500\u0026deg;C ) at both pressures, it highlights the potential effect of reactor material on improving ethylene conversion and product distribution.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\u003ch2\u003e3.3. Liquid product analysis\u003c/h2\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe quantity of liquid product increased with temperature for both reactor materials, as shown in Fig.\u0026nbsp;12 (a-d). In the SS316 reactor, experiments from 200 to 550\u0026deg;C led to liquid production that was visible, although it was not enough to measure its weight accurately. In contrast, changing to the Inconel reactor under similar conditions yielded approximately 0.08 g of liquid, demonstrating slightly greater production efficiency compared to the SS316 reactor. Raising the temperature from 450 to 650\u0026deg;C at 15 bar increased the quantity of liquid products significantly: approximately 1.71 g was collected from the SS316 reactor, and about 2 g from the Inconel reactor. However, decreasing pressure from 15 bar to 5 bar under the same temperature condition led to the production of a lower amount of liquid production in both reactors, averaging about 0.53 g. These results are illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e13\u003c/span\u003e (a and b) and demonstrate the strong influence of pressure on reaction production yield.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe quantity of liquid product clearly illustrates the effect of temperature on the non-catalytic ethylene oligomerization. The increased liquid yield at higher temperature ranges in both reactors is consistent with the rise in ethylene conversion, highlighting the role of elevated temperature in improving oligomer formation. In addition, the production of liquid serves as an indicator of heavier hydrocarbon generation. The production of heavier hydrocarbons such as C6, C7, C8, and C10 during thermal ethylene oligomerization was also reported in Conrad et al.[\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e], [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. Their experiments were carried out in SS316 and quartz reactors at a low pressure of 1.5 bar and a high pressure of 43.5 bar and elevated temperatures, which similarly led to the production of middle to long chain hydrocarbons in the liquid phase.\u003c/p\u003e\u003cp\u003eThe effect of pressure on the liquid production is clearly demonstrated by comparing Fig.\u0026nbsp;12 (c and d) and Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e13\u003c/span\u003e (a and b), which represent the experiments conducted at a similar temperature range and different pressures. The lower liquid yield is consistent with the decline observed in ethylene under lower pressure. This reinforces the influence of pressure in both conversion and heavier hydrocarbon formation.\u003c/p\u003e\u003cp\u003eThe hydrocarbons distribution in the liquid phase for both SS316 and Inconel reactors at 200 to 650\u0026deg;C and 15 bar is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e14\u003c/span\u003e. As presented, C6-C8 are the majority of products in the liquid phase composition, accounting for over 93% in both reactors. These observations are consistent with Conrad et al.[\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e] study, who reported that C6-C9 were the majority of product distribution up to 450\u0026deg;C and 14 bar in thermal ethylene oligomerization.\u003c/p\u003e\u003cp\u003eHowever, Inconel resulted in a slightly higher share of C9-C12 (4.81%) compared to SS316 (3.26%). This suggests the potential influence of the reactor wall on improving the formation of middle-chain oligomers. On the other hand, SS316 demonstrated a higher share of C12+ (3.06%) compared to the Inconel reactor (1.13%), possibly showing that the formed heavier hydrocarbons in the Inconel reactor went through more thermal cracking, or the SS316 reaction environment was more suitable for preserving longer chains.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e"},{"header":"4. Discussion","content":"\u003cp\u003eRegarding the effect of temperature, the ethylene conversion above 450\u0026deg;C aligns with the research work done by Conard et. al [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e] at similar temperature and pressure. Literature reports indicate that at temperatures below 400\u0026deg;C, ethylene conversion is extremely low (less than 1%), demonstrating that without a catalyst, this reaction is highly unlikely at lower temperatures and only improves significantly as the temperature increases.\u003c/p\u003e\u003cp\u003eS.M. Al-Salem [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e] studied polyethylene pyrolysis between 500 to 800\u0026deg;C in an Inconel-700 reactor and showed an increasing trend in gas production from 500 to 800\u0026deg;C, which was more noticeable between 500 to 600\u0026deg;C. Obtaining lighter hydrocarbons (C2-C4) as the dominant species rather than C5-C10 at higher temperatures supports the effect of elevated temperature on decomposition reactions. Increasing temperature also enhanced the C2 to C4 yield up to 79% when reaching 800\u0026deg;C. It has been found by Conard et al.[\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e], [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e] that although even carbon chains such as ethylene, butene, hexene, octene, etc. are the most probable species that are produced in oligomerization reactions, non-oligomer compounds such as methane, propylene, pentene, etc. can also be produced in large amounts. In addition, it has been reported by Egloff et al. [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e] that at higher temperatures (e.g., 600\u0026ndash;650\u0026deg;C), decomposition reactions lead to lighter hydrocarbon production.\u003c/p\u003e\u003cp\u003eThe increased peak areas at longer retention time, shown in Figs.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e and \u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e, additionally confirm that rising heavier hydrocarbon concentration rises with increasing temperature. These conclusions are supported by the work of Arey et al [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e] asserting that in gas chromatography, retention time correlates with molecular weight, in which longer retention times are a sign of the occurrence of heavier hydrocarbons in the mixture.\u003c/p\u003e\u003cp\u003eUsing retention time as an approximation of molecular weight was verified by Zellner et al. [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e], who described the retention index principles. According to this study, longer retention time shows the elevated interactions between analytes and the stationary phase, which aligns with the behavior of heavier hydrocarbons. This is also reinforced by the predictive model presented by Katritzky et al.[\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e] linking retention indices to hydrocarbon chain length.\u003c/p\u003e\u003cp\u003eAt temperatures above 550\u0026deg;C, secondary reactions such as recombination and cracking are likely to cause the appearance of new peaks. Considering the similar pattern reported by Conrad et al. [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e], the higher temperatures and pressure in non-catalytic ethylene oligomerization caused the formation of secondary products in the form of a mixture of lighter and heavier hydrocarbons. This mechanism also helps explain why the methane volume fraction increased as the temperature rose. In the meantime, a higher intensity and a larger number of peaks at longer retention times can be related to ethylene oligomerization and subsequent reactions that favor the formation of heavier hydrocarbons.\u003c/p\u003e\u003cp\u003eAdditionally, the relative contribution of oligomerization and cracking is clearly reflected in the peak area distribution shown in Figs.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e and \u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e. Peaks that appeared at shorter retention times correspond to lighter hydrocarbons such as methane and ethylene, while peaks that were detected at higher retention times reflect the formation of heavier compounds.\u003c/p\u003e\u003cp\u003eThe product distribution pattern showed an obvious shift toward heavier hydrocarbons as temperature increased. This conclusion also aligns with Boyd et al. [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e] and Conrad et al. [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e], [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e] who reported that higher temperatures improved the production of longer-chain hydrocarbons. However, they also noted that elevated thermal conditions promote cracking reactions at the same time, resulting in a broader mixture of both light and heavy products.\u003c/p\u003e\u003cp\u003eAs for the effect of pressure, ethylene conversion increased proportionally with temperature at 15 bar, reaching approximately 94% in both the SS316 and Inconel reactors as indicated in Figs.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e and \u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e. This observation aligns with previous studies such as Conrad et al. [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e], which emphasized that elevated pressure favors ethylene oligomerization by shifting the reaction equilibrium toward product formation. Additionally, higher pressure increases ethylene density in the system, improving the frequency of molecular collisions and facilitating bond formation as reported by Boyd et al. [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. Also, increasing pressure limits secondary cracking reactions, which results in higher conversion by producing lighter hydrocarbons. A similar conclusion was drawn by Egloff et al. [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e], who highlighted the role of pressure in improving ethylene conversion and limiting by-product formation. In a related study, Egloff et al. [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e] demonstrated that higher pressure could improve the formation of heavier hydrocarbons, whereas lower pressure could cause lighter hydrocarbon production due to fewer molecular collisions.\u003c/p\u003e\u003cp\u003eHowever, it is important to note that studies such as Conrad et al. [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e] have mainly ruled out the significant catalytic role of reactor material in non-catalytic reactions. Thus, the higher conversion observed in the Inconel reactor could be due to the minor surface interactions or differences in heat distribution and thermal conductivity between the two materials.\u003c/p\u003e\u003cp\u003eOverall, the results point out the critical role of pressure in non-catalytic ethylene oligomerization. While the reactor material seemed to have some influence, its effect was secondary compared to the major impact of elevated pressure. These findings are consistent with broader research on thermal ethylene oligomerization that emphasises the importance of the effect of operational conditions on product distribution [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e], [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e], [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e], [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]. The use of sigmoid curve fitting further validated these trends, showing consistent inflection points and saturation behavior aligned with theoretical expectations for temperature-dependent conversion in non-catalytic oligomerization.\u003c/p\u003e\u003cp\u003eIn terms of the liquid products, it was observed that the liquid produced in the Inconel reactor across all temperature and pressure conditions was clearer compared to what was produced using the SS316 reactor, which is presented in Figs.\u0026nbsp;12 and \u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e13\u003c/span\u003e. To assess the possible effect of the reactor wall on the liquid composition more accurately, further analysis using gas chromatography with flame ionization detection (GC-FID) was necessary. According to a study by Cui et al. [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e], GC-FID enables to provide detailed data on hydrocarbon mixture composition, including liquid oligomers. However, this method does have limitations, particularly in resolving all individual components due to the restricted capacity of the separation column to distinguish between closely eluted hydrocarbons. This limitation is especially relevant when analyzing complex hydrocarbon mixtures such as those derived from ethylene oligomerization. Although the quantification and characterization of the indicative hydrocarbons by GC-FID faced some challenges, the analysis nonetheless demonstrated the likely presence of heavier hydrocarbons such as C6 to C12+. This conclusion was based on the peak areas achieved in the liquid samples from oligomerization and their comparison with the standard calibration olefin mixture from C5 to C12, as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e14\u003c/span\u003e. Using GC-FID peak area correlation with hydrocarbon molecular weight was explored in a couple of studies. Cui et al. [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e] developed a gasoline composition model based on GC-FID results, indicating the possibility of linking peak area distributions to specific hydrocarbons. Durand et al. [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e] studied weight determination and distribution in crude oil and condensate by GC analysis and confirmed the correlation between compound weight and peak area.\u003c/p\u003e\u003cp\u003eProduct distribution variation with pressure is shown in Figs.\u0026nbsp;\u003cspan refid=\"Fig14\" class=\"InternalRef\"\u003e15\u003c/span\u003e and \u003cspan refid=\"Fig15\" class=\"InternalRef\"\u003e16\u003c/span\u003e for SS316 and Inconel reactors, respectively. Decreasing pressure from 15 to 5 bar reduced C6-C8 in both reactor materials, while interestingly, it resulted in heavier hydrocarbon fractions such as C9-C12 and C12+. Although higher pressure is known as a parameter to enhance ethylene conversion and form lighter hydrocarbons because of cracking, observing more heavier hydrocarbons, specifically using Inconel, may be due to the subtle balance between stabilization and propagation occurrence before fragmentation. This could mean that if longer chain hydrocarbons stabilized rapidly because of reaction conditions such as time and reactor wall material, heavier hydrocarbons such as C9-C12 may remain.\u003c/p\u003e\u003cp\u003eThis observation aligns with Conrad et al.[\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e] study on thermal ethylene oligomerization, where they reported that pressure and temperature affect chain growth and cracking tendencies in a complex way. The pressure effect was more significant in the Inconel reactor, where the C9-C12 hydrocarbon fraction was almost tripled, and C12\u0026thinsp;+\u0026thinsp;increased. This may be an indication of the effect of the nickel-based wall of Inconel in a way that it stabilized and promoted the formation of longer-chain hydrocarbons, which was reported by Zhu et al.[\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e] as well. These observations highlight both the pressure effect complexity and the reactor wall material on thermal ethylene oligomerization.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e"},{"header":"5. Conclusion","content":"\u003cp\u003eThis study explored the effect of temperature, pressure, and reactor wall material on ethylene conversion and product distribution in the non-catalytic oligomerization (thermal) of ethylene. Higher temperatures notably enhanced ethylene conversion, reaching 94% at 650\u0026deg;C in both SS316 and Inconel reactors. At lower pressure (5 bar), ethylene conversion decreased significantly and favored lighter hydrocarbons in the gas phase. This highlights the reduced oligomerization activity under low-pressure conditions. Elevated temperature and pressure both improved ethylene conversion and liquid production in both reactors, with the Inconel reactor resulting in more and clearer liquid. Hydrocarbon distribution analysis in the liquid phase at 200 to 650\u0026deg;C and 15 bar demonstrated that C6-C8 were the primary produced hydrocarbons in both SS316 and Inconel reactors. Earlier observation of ethylene conversion point at 450\u0026deg;C and more C9-C12 hydrocarbons formation in the Inconel reactor compared to SS316, suggesting the reactor wall material may facilitate oligomerization and affect chain growth. Both reactors showed a shift toward heavier hydrocarbons (C9-C12 and C12+) at 5 bar compared to 15 bar, specifically in the Inconel reactor. This suggests that lower pressure may result in fewer secondary cracking reactions, allowing longer-chain hydrocarbons to form and be more stable.\u003c/p\u003e\u003cp\u003eOverall, the results from this work show that optimizing temperature, pressure, and reactor material (particularly Inconel) can improve both the gas-phase and liquid-phase products in non-catalytic ethylene oligomerization, facilitating the way for more efficient, sustainable aviation fuel production.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAcknowledgements\u003c/h2\u003e\u003cp\u003eThe authors would like to acknowledge the Biomass Technology Laboratory (BTL) for providing the required infrastructure for this study, as well as the Biomass, Bioproduct, and Bioprocess Analysis Laboratory (LAB) and Mr. Maxime Lessard for the analysis performed. Further acknowledgment goes to the sponsor of this research work, Greenfield Global. Finally, the authors would like to express gratitude to Mr. Henry Gauvin (Universit\u0026eacute; de Sherbrooke) for his assistance in assembling the reaction system. This project is financially supported by the Natural Sciences and Engineering Research Council of Canada (NSERC - ALLRP 571672-2021), Consortium de recherche et innovations en bioproc\u0026eacute;d\u0026eacute;s industriels au Qu\u0026eacute;bec (CRIBIQ \u0026minus;\u0026thinsp;2022-065-C87), and Greenfield Global.\u003c/p\u003e\u003cp\u003e\u003cb\u003eConflict of Interest\u003c/b\u003e: The authors declare no conflict of interest.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eGonzalez-Garay, A., et al.: Jun., Unravelling the potential of sustainable aviation fuels to decarbonise the aviation sector, \u003cem\u003eEnergy Environ Sci\u003c/em\u003e, vol. 15, no. 8, pp. 3291\u0026ndash;3309, (2022). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1039/d1ee03437e\u003c/span\u003e\u003cspan address=\"10.1039/d1ee03437e\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eOlhoff, A., et al.: Emissions Gap Report 2024: No more hot air \u0026hellip; please! 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(1989). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1002/jhrc.1240120408\u003c/span\u003e\u003cspan address=\"10.1002/jhrc.1240120408\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"SAF, ethylene oligomerization, wall effect, parametric study","lastPublishedDoi":"10.21203/rs.3.rs-6770942/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6770942/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe aviation industry contributes to greenhouse gas (GHG) emissions, driving interest in sustainable aviation fuel (SAF) production. A promising route is the bioethanol-to-jet process, where bioethylene undergoes non-catalytic (thermal) oligomerization to form longer-chain hydrocarbons. However, limited research exists on this process at high temperatures and pressures. This study examines the influence of temperature (200\u0026ndash;650\u0026deg;C), pressure (5 and 15 bar), and reactor material (SS316 and Inconel) on ethylene conversion and product distribution. Higher temperature (600\u0026ndash;650\u0026deg;C) and pressure (15 bar) significantly enhanced conversion, reaching 94% in both reactors, while at lower pressure (5 bar), conversion of 30\u0026ndash;58% was reached. Increased temperature and pressure promoted C4 hydrocarbon formation, with heavier products appearing at a lower temperature in the Inconel reactor (450\u0026deg;C) compared to SS316 (500\u0026deg;C). Additionally, liquid product yield was higher in the Inconel reactor, producing up to 2 g of liquid, compared to 1.71 g in SS316, which showed a greater proportion of heavier hydrocarbon distribution. Liquid product analysis showed that C6-C8 hydrocarbons were predominant, while heavier fractions (C9-C12 and C12+) were more abundant at lower pressures. These results highlight Inconel\u0026rsquo;s potential for improving ethylene oligomerization efficiency, offering insights for optimizing SAF production.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e","manuscriptTitle":"Investigating the effect of temperature, pressure, and reactor wall in non-catalytic ethylene oligomerization","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-08-04 10:23:43","doi":"10.21203/rs.3.rs-6770942/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"8f16d7e2-307a-49b7-ac4e-285e3ceaac68","owner":[],"postedDate":"August 4th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2026-03-26T21:43:59+00:00","versionOfRecord":[],"versionCreatedAt":"2025-08-04 10:23:43","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-6770942","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6770942","identity":"rs-6770942","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}