DSpace at VNU: Experimental investigation of the effects of cycloparaffins and aromatics on the sooting tendency and the freezing point of soap-derived biokerosene and normal paraffins

Fuel 185 (2016) 855–862 Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel Full Length Article Experimental investigation of the effects of cycloparaffins and aromatics on the sooting tendency and the freezing point of soap-derived biokerosene and normal paraffins Long H Duong a,d, Osamu Fujita b,⇑, Iman K Reksowardojo a, Tatang H Soerawidjaja c, Godlief F Neonufa c a Combustion Engines and Propulsion Systems Laboratory, Faculty of Mechanical and Aerospace Engineering, Institut Teknologi Bandung, Bandung 40132, Indonesia Division of Mechanical and Space Engineering, Hokkaido University, Sapporo 060-8628, Japan Department of Chemical Engineering, Faculty of Industrial Technology, Institut Teknologi Bandung, Bandung 40132, Indonesia d Department of Automotive Engineering, Faculty of Transportation Engineering, Ho Chi Minh City University of Technology, Ho Chi Minh City 70350, Viet Nam b c a r t i c l e i n f o Article history: Received May 2016 Received in revised form 10 August 2016 Accepted 12 August 2016 Available online 19 August 2016 Keywords: Soap-derived biokerosene Sooting tendency Freezing point Normal paraffins Cycloparaffins Aromatics a b s t r a c t The effects of cycloparaffin and aromatic hydrocarbons when blended with soap-derived biokerosene (SBK) and normal paraffins (n-paraffins) on the sooting tendency and the freezing point are quantified to determine a method for improving the properties of SBK and n-paraffin fuels In this study, SBK was derived from the saponification and dercarboxylation of coconut oil, and consists predominantly of n-paraffins with carbon chain lengths from C7 to C17 Dodecane, butylcyclohexane and butylbenzene were chosen as surrogate components for n-paraffins in SBK, cycloparaffins and aromatics, respectively The total soot volume was measured from the light extinction at ambient conditions in a wick-fed laminar diffusion flame The measured smoke point of the fuel was correlated with the required sooting tendency according to the jet fuel standard The freezing point was measured using the JIS K2276 test method The results show that butylcyclohexane affects the sooting tendency much lesser than butylbenzene when blended with SBK or dodecane In contrast, butylcyclohexane decreases the freezing point more, as compared to butylbenzene, when blended with dodecane Butylcyclohexane and butylbenzene have a similar trend of effect on the freezing point when blended with SBK or dodecane Blending SBK or dodecane with butylcyclohexane matches the requirements of both smoke point and freezing point for jet fuel specified by ASTM D1655 Conversely, blending SBK or dodecane with butylbenzene does not meet these requirements Therefore, given the tradeoff between sooting tendency and freezing point, cycloparaffins are considered more promising than aromatics for blending with SBK or n-paraffin fuels Ó 2016 Elsevier Ltd All rights reserved Introduction Air transportation in the modern world is rapidly growing in popularity due to an increasing demand for business and leisure travel As a result, the worldwide commercial jet fleet is expected to increase by approximately 102% [1], and the estimated annual average growth rate of world traffic is 4.6% for the next 20 years (2014–2034) [2] Consequently, the jet fuel demand and the subsequent exhaust gas emissions will increase Currently, jet fuel prices fluctuate as they depend not only on the availability of crude oil obtained from fossil fuels but also on many societal, economical, and, especially, political factors These fluctuations in the price of jet fuels create many serious problems for airline companies, ⇑ Corresponding author at: Division of Mechanical and Space Engineering, Hokkaido University, Kita 13 Nishi 8, Kita-ku, Sapporo 060-8628, Japan E-mail address: ofujita@eng.hokudai.ac.jp (O Fujita) http://dx.doi.org/10.1016/j.fuel.2016.08.050 0016-2361/Ó 2016 Elsevier Ltd All rights reserved because the fuel cost represents up to 30% of an airline’s operating cost and it is expected to increase [3] In addition, the airline sector is currently responsible for approximately 3% of the total global greenhouse gas (GHG) emissions Although this value represents a small fraction of the total GHG emissions, aircraft emissions continue to increase and are expected to constitute nearly 5%, even could reach up to 15% of global GHG emissions by 2050 [4] In an effort to reduce GHG emissions and meet the European environmental goals for 2020 and beyond, the European Union has implemented the Directive 101/2008/EC [5] since 2012 According to this legislation, all aircrafts flying within or into the European Economic Area must either decrease their GHG emissions or purchase CO2 allowances Therefore, the commercial aviation industry faces billions of US dollars in cost to pay for the carbon emission tax [6] To reduce GHG emissions, some efforts have been applied such as improvement of fuel consumption efficiency by increasing engine 856 L.H Duong et al / Fuel 185 (2016) 855–862 efficiency, aircraft structure designing, and optimizing air traffic management Along with those methods, using biofuel is increasingly considered by airline companies and governments because biofuel is a renewable resource and significantly reduces the lifecycle CO2 emissions [7,8] Besides, using aviation biofuel may reduce the pollutant emissions, such as carbon monoxide, unburned hydrocarbon, nitrogen oxides, and soot emission, from engines [11–15] Currently, certain types of aviation biofuels have been approved by the American Society for Testing Materials (ASTM) for blending with conventional jet fuel to use in aviation gas turbine engines These include synthesized paraffinic kerosine from hydroprocessed esters and fatty acids, Fischer–Tropsch hydroprocessed synthesized paraffinic kerosine from biomass, synthesized iso-paraffins from hydroprocessed fermented sugars, synthesized kerosene with aromatics derived by alkylation of light aromatics from non-petroleum resources, and alcohol-to-jet synthesized paraffinic kerosene The detailed requirements of their composition and properties are presented in ASTM D7566 [9] Tropical countries, in general, and Indonesia, in particular, are the ideal location for developing ground-used and aviation biofuel because of the abundance of plant oil resources, the vast rural areas, and the low labor cost Moreover, the Indonesian government approved a plan to use up to 2%, 3%, and 5% by volume of aviation biofuel during flights by 2016, 2018, and 2025, respectively [8] However, Indonesia faces significant challenges to produce aviation biofuel due to the limitation in technology and investment Therefore, production of aviation biofuel based on simple technologies at a low cost, and leveraging the available national advantages is a foreseeable solution The production of soap-derived biokerosene (SBK) meets the above criteria, because it primarily uses a simple production process as shown in Fig SBK production comprises of two main steps: (1) the saponification process, through which plant oil fatty acids and triglycerides are converted into basic-soap and (2) the subsequent thermal decarboxylation process reacted at 275 °C and ambient pressure, through which the basic-soap is transformed into normal-paraffins (n-paraffins) Thereby, SBK production is simpler and less energy consumption than the hydtrotreating processes, which are reacted at high temperature and pressure with the presence of catalyst and hydrogen, in the common aviation biofuel productions (Fig 2) to convert the plant oils or animal fats into n-paraffins [10] Furthermore, in common aviation biofuel production pathways [12,30,31], because the fuel used in aviation gas turbine engines is required to have a very low freezing point [16,17], the n-paraffins are generally isomerized into branched paraffins (iso-paraffins), which have a significantly lower freezing point Besides, in order to improve the other properties of aviation biofuels, especially, distillation, the cracking process is also implemented to break the long carbon chain that exceeds the jet range into shorter chain length paraffins A typical production process of this type of aviation biofuel is presented in Fig However, the isomerization and cracking processes are complicated and costly [18–20] Hence, substitution with other appropriate solutions based on the feedstock conditions, the available technology, and the socioeconomic situation in Indonesia is encouraged Thereby, to avoid the cracking process in the SBK production, the plant oils that have carbon chain length of fatty acids dominantly in jet rang (C8–C16 [16,17]) are selected as feedstock Besides, mixing SBK with other bio-derived components that have low freezing points is a potential alternative to the isomerization Coconut Oil Saponification process Basic Soaps process to reduce the freezing point of the biofuel Mixed biofuels with products of different production processes is also promising [21] because it helps to optimize the utilization of flexible feedstock and suitable facilities to produce biofuels, thus maximizing the national potential to develop a sustainable aviation biofuel in each country Currently, there are many approaches to produce aromatics or cycloparaffins (naphthenes) from bio-feedstocks [23–27] Because cycloparaffins are commonly produced through hydrogenation of aromatics, their price may be higher than that of aromatics [22] However, if new production processes or feedstocks are established in the future, then it would be simpler and cheaper to produce cycloparaffins One promising production process for producing cycloparaffins in Indonesia is the hydrogenation of turpentine oil obtained from pine tree (Pinus merkusii), as proposed by Hudaya et al [28] Although in their study they proposed a method for producing iso-paraffins, the same method can theoretically be used to produce cycloparaffins Cycloparaffins and aromatics can be mixed with SBK to decrease its freezing point since they have a lower freezing point than that of n-paraffins with same carbon number However, they exhibit a very different sooting tendency The jet fuel standard (ASTM D1655) specifies that the maximum volume fraction of aromatics present in commercial jet fuels such as Jet A, Jet A-1, and Jet B is 25%, whereas the fraction of cycloparaffins is not limited [17] Soot formation in a gas turbine combustor is problematic because it generates high radiation heat and deposits, which can damage the combustion chamber or the turbine blade, thus reducing the engine’s life [29,32] In addition, the unburned soot particles emitted with the exhaust gas from a gas turbine engine are dangerous for human health and the environment [33] Therefore, the sooting tendency plays an important role for evaluating a potential component to mix with SBK This research focuses on comparing the effects of cycloparaffins and aromatics on the sooting tendency and the freezing point when mixed with SBK and n-paraffin fuels Butylcyclohexane and butylbenzene were chosen as the surrogate components for cycloparaffin and aromatic hydrocarbons, respectively Because dodecane has the same carbon number with the average SBK molecule and with jet fuel [33], it was used to represent for n-paraffins in SBK and jet range n-paraffin fuels Based on the results of this work, the potential of cycloparaffins and aromatics for mixing with SBK and n-paraffin fuels is assessed with regard to the tradeoffs between the sooting tendency and the decrease of the freezing point Experimental setup and procedures 2.1 Total soot volume of laminar diffusion wick-fed flame and smoke point measurement The experimental setup used for determining the total soot volume and measuring the smoke point is shown in Fig The total soot volume of the laminar diffusion wick-fed flame was determined by employing a light extinction method As shown in Fig 3, the burner comprised two closely fitting concentric brass tubes (outer tube diameter = 0.8 cm, inner tube diameter = 0.5 cm) following the methods proposed by Olson et al [34] and Watson et al [35] The outer and inner tubes belonged to the fuel tank Decarboxylation process Fig Soap-derived biokerosene (SBK) production process Normal paraffins 857 L.H Duong et al / Fuel 185 (2016) 855–862 Plant oils, animal fats Hydrotreating process Isomerization & cracking process Normal paraffins (n + iso) paraffins Fig Hydroprocessed esters and fatty acids production process [12,30,31] [37,38] attributed to the work of Dalzell and Sarofim [42] The total soot volume within the flame was calculated using Eq (2): Chimney Z Vs ¼ Burner Interference filter lens Light source Digital Camera Honeycomb Fuel tank socket Dry air Valve Flow meter Fuel tank Fuel refill & level Wick height control Fig Schematic of the experimental setup socket and fuel tank, respectively By rotating the wick-height control nut, the fuel tank could shift up or down inside the fixed socket to expose more or less of the wick, respectively, thus controlling the flame height ASTM specification cotton wick was used to install to the fuel tank A 30-cm-long, and 9-cm-outer-diameter glass tube covered the burner as a chimney Dry air was supplied to the chimney at a constant flow rate of 30 L/min, or an equivalent air velocity of 7.86 cm/s, for all measurements Three aluminum honeycombs were used to generate laminar flow inside the chimney To refill the fuel and measure the fuel consumption, a 1-mminner-diameter clear glass tube was connected to the fuel tank through a silicon pipe If kept parallel and vertical, the level of the fuel in the fuel tank remained the same as that in the fuel refill tube The fuel mass was measured with a Shimadzu UX2200H digital balance at a resolution of 0.01 g The duration of burning was timed with a stopwatch The approximate flame height was determined with a ruler and then precisely determined by analyzing the flame image recorded by a Canon VIXIA HF S21 camera To derive the total soot volume, a Panasonic HDC-TM750 camera with an interference filter lens was used This filter lens only allowed light with 540 nm wavelength from a backlight system The recorded backlight image with and without the flame was used as input to a MATLAB program to calculate the total soot volume of the flame The BouguerÀLambertÀBeer theory was applied to evaluate the soot volume fraction in the Rayleigh wavelength range (particle diameter/light wavelength (1) using Eq (1), as the method used in Jeon et al [36] The light scattering by the particles was assumed to be negligible; this practice was commonly used by other studies as well [36–41] fv ¼ k ln II0 m 1ị 6pLIm m ỵ2ị 1ị where fv is the soot volume fraction, I0 is the initial light intensity, I is the transmitted light intensity, k is the wavelength, L is the optical path length, and m is the optical refractive index of the soot particles In this study, the optical refractive index of the soot particles was m = 1.57–0.56i, in accordance with many previous studies Hf Z R f v ðr;zÞ 2prdrdz ð2Þ where Hf and R are the flame height and radius, respectively The smoke point of the fuel, which is defined as the maximum height (in millimeters) of a smoke-free laminar diffusion flame of fuel burned in a wick-fed lamp [43], was derived through the method proposed by Olson et al [34] and Watson et al [35] These studies proposed a method of using the relationship between the mass fuel consumption and the flame height to obtain a more accurate smoke point of the fuel, as compared to the direct visual observation presented in ASTM D1322 [43] However, the experimental system in this work has some slight modifications compared with that of these studies Instead of placing the balance under the fuel tank to determine the fuel mass, a constant volume of fuel measured based of its mass just before filling to the fuel tank was used for determining the duration of the burn Theoretically, because the temperature of the fuel tank was nearly same throughout a measurement, the same volume of fuel has the same mass Two standard samples proposed by ASTM D1322 mixed with toluene and iso-octane (2,2,4-trimethylpentane) with volume fractions of 40/60 and 10/90, respectively, were measured to compare with the smoke point given in ASTM D1322 The results indicated that the smoke points in this study for the 40/60 and 10/90 samples differed from that of the ASTM D1322 standard by 2.04% (15 mm versus 14.7 mm) and 0.66% (30.0 mm versus 30.2 mm), respectively Therefore, this method was considered reliable for measuring the smoke point of other fuel samples In order to investigate the sooting tendency, several mixtures of SBK/butylcyclohexane, and SBK/butylbenzene were used to measure on the total soot volume and the smoke point Because the composition of SBK consists predominantly of n-paraffins that exhibit a carbon chain length within the jet range, some typical n-paraffins in this range including decane, dodecane, and hexadecane were applied to measure on the total soot volume SBK was produced in-house from coconut oil The composition of SBK was analyzed by using gas chromatograph equipment named Shimadzu 2010 The weight fraction of hydrocarbon types in SBK are listed in Table The chemical compounds were purchased from SigmaAldrich Corp and Tokyo Chemical Industry Co., Ltd The properties of SBK and the chemical compounds are listed in Table 2.2 Freezing point measurement The freezing points of the fuels were measured by JFE TechnoResearch Corporation in Japan using the JIS K2276 test method, which is equivalent to ASTM D2386 or IP16 [44] The freezing Table Composition of soap-derived biokerosene Hydrocarbon type wt.% Normal paraffins Olefins Aromatics Branched paraffins 67.19 21.77 9.06 1.93 858 L.H Duong et al / Fuel 185 (2016) 855–862 Table Properties and information of soap-derived biokerosene (SBK), and hydrocarbon compounds Fuel Code Soap-derived biokerosene Decane Dodecane Hexadecane Butylcyclohexane Butylbenzene Toluene 2,2,4-Trimethylpentane a b c d SBK DEC DOD HEX BCH BBZ Molecular formula C7-C17 C10H22 C12H26 C16H34 C10H20 C10H14 C7H8 C8H18 CAS no 124-18-5 112-40-3 544-76-3 1678-93-9 104-51-8 108-88-3 540-84-1 Density (kg/m3) Purity (%) a F.P d 810 730 750 774 800 860 865 692 >99.0 >99.0 >99.0 >99.0 >99.0 >99.0 >99.0 M.W 160.64 142.28 170.33 226.44 140.27 134.22 92.14 114.23 b (°C) 0.5 À30 À9.6 18 À78 À88 À93 À107 S.P c (mm) 52.5 99.5 94 87.5 55.5 8.5 43 M.W: molecular weight F.P: freezing point S.P: smoke point Obtaining by estimation method points of several mixtures of SBK/butylcyclohexane, dodecane/ butylcyclohexane, and dodecane/butylbenzene were measured Results and discussion 3.1 Total soot volume Fig shows that SBK has a significantly greater total soot volume compared to the n-paraffins in the range of C10–C16 This is due to the fraction of olefins, especially, aromatics present in SBK (Table 1) Considering the production process (Fig 1), the product’s composition includes unsaturated hydrocarbons such as olefins, which are formed from the unsaturated fatty acid chains in coconut oil The olefins have a greater sooting tendency than the n-paraffins with similar carbon number [34,48,49] Besides, SBK produces greater soot mainly due to its aromatic fraction In decarboxylation, some fractions of the unsaturated hydrocarbon bonds in basic-soap are converted into aromatic hydrocarbons [50,51] Aromatics are known to have a very high soot formation [34,48,49], and that is confirmed by the results of the present work The total soot volume as measured for a mixture of 90% dodecane and 10% butylbenzene (DOD90BBZ10) by volume Dodecane was selected to represent the n-paraffins in SBK because it is the closest with the average molecule of SBK By volume, 10% butylbenzene was mixed with dodecane since it represents the approximate fraction of the aromatics found in SBK (Table 1) As shown in Fig 4, the 4.5 total soot volume of SBK is very close to that of DOD90BBZ10 at several flame heights Fig also shows that butylcyclohexane has slightly less total soot volume than SBK while butylbenzene has the highest total soot volume Therefore, blending butylcyclohexane with SBK produces less total soot volume; however, the difference is very small, as indicated in Fig Conversely, the butylbenzene significantly increases the total soot volume when blended with SBK, as shown in Fig From Figs and 6, the butylbenzene clearly leads to greater formation of soot compared to butylcyclohexane when they are mixed with SBK To obtain a clearer quantitative comparison regarding to the jet fuel specification, the smoke point of these blends was measured and presented in the following section 3.2 Smoke point Fig shows a comparison of the effect of butylcyclohexane and butylbenzene on the smoke point when blended with SBK Butylcyclohexane slightly increases the smoke point because its smoke point is a little higher compared with that of SBK (55.5 mm versus 52.5 mm), whereas, butylbenzene strongly decreases the smoke point of these blends because it has a significantly lower smoke point than SBK (8.5 mm versus 52.5 mm) The lower smoke point indicates a greater tendency to form soot The distribution of carbon chains in aviation biofuel simulates that of conventional jet fuel as much as possible Thus, the carbon chain length of the current aviation biofuels commonly lay within the jet range (C8–C16; and average at C12 [33]) The results on the DEC 4.5 DOD 3.5 HEX 3.0 BCH 2.5 SBK 2.0 BBZ 1.5 DOD90BBZ10 1.0 0.5 Total soot volume (10-3 mm3) Total soot volume (10-3 mm3) 4.0 4.0 SBK 3.5 SBK75BCH25 3.0 SBK50BCH50 2.5 BCH 2.0 1.5 1.0 0.5 0.0 0.0 10 15 20 25 30 35 40 45 50 55 Flame height (mm) Fig Total soot volume as a function of flame height of SBK, pure hydrocarbons and mixture 10 15 20 25 30 35 40 45 50 55 Flame height (mm) Fig Total soot volume vs flame height of the mixtures: SBK (100% SBK); SBK75BCH25 (75% SBK + 25% butylcyclohexane); SBK50BCH50 (50% SBK + 50% butylcyclohexane); BCH (100% butylcyclohexane) 859 L.H Duong et al / Fuel 185 (2016) 855–862 Total soot volume (10-3 mm3) 4.5 4.0 SBK 3.5 SBK75BBZ25 3.0 SBK50BBZ50 2.5 BBZ 2.0 1.5 1.0 0.5 0.0 10 15 20 25 30 35 40 45 50 55 Flame height (mm) Fig Total soot volume vs flame height of the mixtures: SBK (100% SBK); SBK75BBZ25 (75% SBK + 25% butylbenzene); SBK50BBZ50 (50% SBK + 50% butylbenzene); BBZ100 (100% butylbenzene) 60 Smoke point (mm) 50 SBK/BCH 40 SBK/BBZ 30 ASTM D1655 20 composition Fig indicates that both butylcyclohexane and butylbenzene decrease the smoke point when blended with dodecane However, butylbenzene has a much greater effect on the smoke point than butylcyclohexane Figs and also show that the volume fraction of butylcyclohexane blended with SBK and dodecane could reach up to 100%, satisfying the requirement on the smoke point of the jet fuel specified by ASTM D1655 (min 25 mm) In contrast, the volume fraction of butylbenzene to blend with SBK and dodecane is limited by a certain amount that is can be estimated as shown in Fig From Figs and 8, it becomes clear that when butylcyclohexane or butylbenzene is blended with SBK, the smoke points of the mixtures are significantly lower than those of their mixtures with dodecane, when using the same volume fractions This is because the smoke point of SBK (52.5 mm) is lower than that of dodecane (94.0 mm) The lower smoke point of SBK is due to the fractions of included olefins and aromatics, as shown in Table Fig also shows that the mixture of dodecane and butylbenzene with volume fraction of 90/10 exhibits a smoke point very close to that of SBK (52.5 mm) This observation is consistent with the results on the total soot volume shown in Fig Based on the results shown in Figs 4, and 8, it could conclude that the mixture of DOD90BBZ10 is suitable as a surrogate for SBK while dodecane can be used as a surrogate component for the jet range nparaffins to evaluate the sooting tendency Fig shows the correlation between the volume fraction of butylcyclohexane or butylbenzene and the reversed smoke point of the fuel mixtures This correlation is consistent with the ones proposed by Van Treuren [52] and Li and Sunderland [53] as shown in Eq (3) LSP;mix ¼ 10 ti À1 ð3Þ LSP;i where LSP,mix, LSP,i, and ti represent the smoke point of the mixture, the smoke point of the component i and the volume fraction of the component i, respectively Van Treuren proposed this correlation for 0 X 10 20 30 40 50 60 70 80 90 100 110 Fraction of butylcyclohexane or butylbenzene (vol%.) 100 Fig Smoke point as a function of the butylcyclohexane and butylbenzene content in the mixture with SBK Smoke Point (mm) total soot volume in Fig shows that the differences in the total soot volume among the n-paraffins with carbon chain length from C10 to C16 are insignificant Dodecane has the same carbon number with the average carbon chain length of jet fuel Thus, it is a good candidate for a surrogate component of biofuel for aviation alternative fuels containing of n-paraffins, as well as n-paraffin hydrocarbon class in the jet fuel On the other hand, regarding the production process of SBK, n-paraffins in SBK are expect to have a carbon chain length shorter by one than that of the fatty acids in coconut oil; the latter lays almost within the jet range and predominantly at C12 [45,46] (Table 3) Therefore, the experimental results on dodecane are not only useful for studies on fuels that consist of n-paraffins in the jet range, but also provide a good reference for comparison with SBK to confirm its quality and 90 80 70 60 50 DOD/BCH 40 DOD/BBZ 30 ASTM D1655 20 10 0 10 20 30 40 50 60 70 80 90 100 110 Fraction of butylcyclohexane or butylbenzene (vol.%) Fig Smoke point as a function of the butylcyclohexane and butylbenzene content in the mixture with dodecane Table Fatty acids profile of coconut oil Oil Coconut oil [45] Coconut oil [46] Fatty acids composition (wt.%) 8:0 10:0 12:0 14:0 16:0 18:0 18:1 18:2 18:3 4.9–8.7 4.6–9.5 4.3–6.5 4.5–9.7 42.3–53.1 44–51 17.2–19.8 13–20.6 7.4–10.8 7.5–10.5 2.0–3.4 1–3.5 4.7–8.9 5–8.2 0.7–3.5 1–2.6 0–0.2 0–0.2 860 L.H Duong et al / Fuel 185 (2016) 855–862 SBK/BCH 0.12 SBK/BBZ 0.11 DOD/BCH -10 0.10 DOD/BBZ -20 0.09 ASTM D1655 0.08 0 y = 0.001x + 0.0199 R² = 0.99964 0.07 0.06 y = 0.0011x + 0.0087 R² = 0.9986 0.05 0.04 y = -1E-05x + 0.019 R² = 0.94648 0.03 0.02 y = 7E-05x + 0.0104 R² = 0.99299 0.01 0.00 Fraction of butylcyclohexane or butylbenzene (vol.%) 10 0.13 10 20 30 40 50 60 70 80 90 100 110 Fraction of butylcyclohexane or butylbenzene (vol.%) Fig Inversed smoke point as a function of the butylcyclohexane and butylbenzene content in mixture with SBK and dodecane the mixture of fossil-derived liquid fuels [52], then Li and Sunderland [53] examined it for several mixtures of hydrocarbons A parameter that equals to a constant multiply by the inversed smoke point was proposed as the first definition on the sooting tendency for hydrocarbon fuels [48,54,55] However, quantifying the sooting tendency of hydrocarbon fuels by using this parameter remained some shortcomings because it did not take into account the effect of fuel molecular size on flame height To consume a unit volume of fuel, the fuel with higher molecular weight needs more volume of oxygen to diffuse into the flame, thus increasing the flame height [56] Consequently, Threshold Sooting Index (TSI) was established by Calcote and Manos [57] to quantify the sooting tendency for hydrocarbon fuels to resolve this issue Recently, Barrientos et al [58] proposed the use of the Oxygen Extend Sooting Index (OESI) to extend the same concept to oxygenated fuels Currently, TSI and OESI are more commonly used than the inversed smoke point for evaluating and comparing on sooting tendency of the fuels [34,55–62] However, there is no requirement on TSI or OESI specified by standard for jet fuel Therefore, to find the maximum allowable volume fraction of butylcyclohexane or butylbenzene to blend with SBK or dodecane in order to satisfy the requirement on the smoke point of jet fuel, the correlation on inversed smoke point of hydrocarbon mixture (Eq (3)) was used in this study Thereby, the linear functions in Fig suggest that approximately 20% and 28% are the maximum allowable volumes of butylbenzene to blend with SBK and dodecane, respectively, in order to satisfy the minimum smoke point required for jet fuel (25 mm) However, because SBK contains some fractions of aromatics, it should be noted that the maximum allowable volume fraction of aromatics in jet fuel is 25% according to ASTM D1655 This figure also indicates that no limitations exist in the fraction of butylcyclohexane that hinder its blending with SBK or dodecane to satisfy the smoke point requirement of jet fuel 3.3 Freezing point The freezing point is one of the most critical properties of jet fuel ASTM D1655 requires that the maximum freezing point is À47 °C for Jet A1, which is the commercial jet fuel widely used worldwide The n-paraffins, which have a carbon chain length Freezing point (ºC) [Smoke Point]-1 (mm-1) 0.14 10 20 30 40 50 60 70 80 90 100 -30 -40 -50 -60 -70 SBK/BCH DOD/BCH DOD/BBZ -80 -90 ASTM D1655 max ASTM D1655 max vol frac -100 Fig 10 Freezing point as a function of the butylcyclohexane and butylbenzene content in the mixture with SBK and dodecane equal to the fatty acid chains in plant oils or animal fats, normally have a much higher freezing point than À47 °C because these fatty acids present in almost all plant oils and animal fats have carbon chain lengths predominantly between C16 and C18 [45–47] Therefore, to decrease the freezing point of these n-paraffins, further processing is required Two of the most typical such processes are cracking and isomerization However, these are both complicated and expensive [18–20] Mixing SBK or n-paraffin fuels with cycloparaffins or aromatics may also reduce the freezing point Fig 10 shows that the effect on the freezing point of butylcyclohexane when blended with SBK and dodecane is almost similar The freezing point decreases with increasing volume fraction of butylcyclohexane in blend with SBK or dodecane The slightly lower freezing points of dodecane/butylcyclohexane blends are due to the lower freezing point of dodecane (À9.6 °C) compared to SBK (0.5 °C) This figure also indicates that butylcyclohexane decreases the freezing point more when mixed with dodecane than butylbenzene, although the freezing point of butylcyclohexane (À78 °C) is higher than that of butylbenzene (À88 °C) This might due to the improved solubility of the butylcyclohexane, as compared to butylbenzene, when blended with dodecane, since the freezing point of a liquid mixture is related to solubility [63] Furthermore, Fig 10 indicates that mixing butylcyclohexane with SBK or dodecane can reduce the freezing point of the blend up to À47 °C, thus matching the freezing point required by ASTM D1655 for Jet A1 In contrast, because the maximum allowable volume fraction of butylbenzene in a blend is limited to 25%, which is insufficient to achieve a freezing point of less than À47 °C when mixed with dodecane, and definitely when blended with SBK Conclusions The total soot volume, smoke point, and freezing point of SBK, dodecane, and their mixtures with butylcyclohexane and butylbenzene were measured in this study In addition, some n-paraffins with carbon number within the jet range, such as decane, dodecane, and hexadecane, were also examined with regard to their total soot volume to compare them with SBK The freezing point is a critical property of jet fuel and the most challenging requirement of aviation biofuel Blending SBK or n-paraffins fuel with cycloparaffin and aromatic hydrocarbons, which have low freezing points is a potential solution to reduce the freezing point of the fuel However, the requirement regarding L.H Duong et al / Fuel 185 (2016) 855–862 the formation of soot might limit the fraction of these hydrocarbons that can be mixed with SBK or n-paraffins This work was done to assess this issue, and the following conclusions were reached: [13] [14] (1) Butylcyclohexane and butylbenzene produce reverse effects on the sooting tendency and the freezing point when blended with SBK or dodecane Regarding the sooting tendency, butylcyclohexane produces a significantly smaller effect compared to butylbenzene In contrast, butylcyclohexane decreases the freezing point more than butylbenzene although the freezing point of butylcyclohexane is higher than that of bultylbenzene (2) SBK or dodecane can be blended with butylcyclohexane to reduce the freezing point of the mixtures to meet the requirements on both the smoke point and the freezing point specified for jet fuel In contrast, because of the requirements on the smoke point and/or the maximum allowable volume fraction of aromatics, blending SBK or dodecane with butylbenzene is infeasible to satisfy the freezing point (3) The differences in the total soot volume among n-paraffins with carbon number range from C10 to C16 are insignificant Therefore, the fuel that consists of n-paraffins in this carbon number range, especially, when dominated by C12 can use dodecane as a surrogate for evaluating the sooting tendency SBK includes some fractions of olefins and aromatics, thus a mixture of 90% dodecane and 10% butylbenzene by volume is suitable as a surrogate of SBK to simulate the total soot volume and the smoke point Considering the tradeoff between the sooting tendency and the decrease of the freezing point, cycloparaffins are better for blending with SBK or n-paraffins fuel as compared to aromatics [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] Acknowledgments We gratefully acknowledge the ASEAN University Network Southeast Asian Engineering Education Development 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the freezing point when mixed with SBK and n-paraffin fuels Butylcyclohexane and butylbenzene... surrogate of SBK to simulate the total soot volume and the smoke point Considering the tradeoff between the sooting tendency and the decrease of the freezing point, cycloparaffins are better for