The extracted SPK sample was analyzed by GC-MS and 31 components were identified. The components were grouped as families according to their carbon number. The total wt.% distribution for the mixture is given in Fig. 2, which shows the distribution of identified iso- and normal-paraffinic compounds according to their carbon number. About 10% by weight of the components of the SPK cut could not be identified.
Table 8 gives the test results of the key properties for the extracted SPK fuel cut. The density of the SPK fuel cut is lower than the minimum value set by the ASTM D7566 specification and the wear scar diameter is higher than the maximum allowed value. On the other hand, the flash point, viscosity, freezing point, and heat content are all within the limits of the industry specifications.
The high percentage of normal paraffins (Fig. 2) explains the low density of the fuel cut and its high heat content. Normal paraffins (n-paraffins) have lower densities and a higher heat content when compared to branched paraffins with the same carbon number [5]. Additionally, the higher paraffinic content of the extracted SPK compared to Jet A1, results in the SPK fuel with a high H/C as reported in Table 8.
In general, fuels with high n-paraffinic content tend to have low ignition delay time compared to fuels with high content of iso-paraffins and aromatics [30]. In addition to fuel composition, the fuel’s volatility has a significant impact on delay times as the injected fuel forms droplets which evaporate before ignition. Low-boiling fuel tends to ignite faster, thus reducing the time delay. The distillation curve of the extracted SPK fuel is illustrated in Fig. 3, which shows initial and final boiling points of 163 °C and 241 °C, respectively. The curve satisfies the industry standards as the first 10% of volume was recovered at a temperature lower than 205 °C. Distillation residue and distillation losses were 1.4% and 1.3%, respectively, both within the acceptable limits specified in ASTM D7566.
Table 8
Test results for key properties of SPK fuel cut
Tested Property | ASTM Method | Unit | SPK |
Density | D4052 | g/cm3 | 0.7442 |
Lubricity | D5001 | µm | 900 |
Viscosity at -40 °C | D7042 | cSt | 6.4085 |
Flash Point | D56 | °C | 52 |
Freezing Point | D5972 | °C | -41.5 |
Net Heat of Combustion | D240 | MJ/kg | 44.29 |
Smoke Point | D1322 | Mm | No smoke |
MW | - | kg/mole | 160.46 |
H/C | - | - | 2.19 |
Since the extracted SPK fuel has high content of n-paraffinic compounds and no aromatics, it is expected to have low delay times and preferable combustion behavior, especially when considering the initial boiling point of the fuel. Since aromatics and naphthenes form cokes more easily than paraffins during the cracking reaction [19], it is expected that the extracted SPK used in this study would have a lower tendency to form coke deposits at elevated temperatures and with lower cracking rates at high temperature. Thus, this makes SPK a safer fuel compared to its conventional counterpart.
Quinoline initially dissolves easily in the SPK; however, but it tends to precipitate out of the SPK after sometime creating two phases. This phenomenon indicates that quinoline has a low stability in the SPK fuel. No similar trends were observed in the linoleic acid or ethyl oleate samples.
3.1. Lubricity trends of the fuel samples
The wear scar diameter (WSD) of the extracted SPK was found to be 900 µm and did not meet the ASTM D5001 specification of a jet fuel, i.e., 850 µm. Upon addition of linoleic acid as an additive in the extracted SPK, a significant improvement in the lubricity was observed as the WSD of the SPK dropped to 450 µm from 900 µm. The lubricity results for linoleic acid mixed SPK are shown in Fig. 4a. It can be observed that the concentration of linoleic acid used in this study was sufficient to improve the lubricity of the SPK fuel. Similarly, both quinoline and ethyl oleate addition to the SPK resulted in ad substantial improvement on the lubricity value of the fuel samples as depicted in the Fig. 4b. However, linoleic acid has a pronounced effect on improving the lubricity value of the SPK samples compared to the quinoline and ethyl oleate. Only 0.001–0.01(vol%) of linoleic acid was necessary to meet ASTM D5001 specification of lubricity compared to 0.5–10 (vol%) of quinolone and ethyl oleate. Referring to Fig. 4b, it is clear that the performance of ethyl oleate as a lubricity improver is superior to its quinoline counterpart at the same concentration. Analyzing the lubricity trends of all the fuel samples, we propose an exponential correlation (Eq-3.1) between additive concentration and fuels as given in Eq. 3.1.
Where =459.82, , and
Comparison of the experimental results and the model predictions is shown in Fig. 4a. It can be observed that the model predicted values agree well with the experimental results for the linoleic acid samples. However, the model fails to predict the lubricity value of the extracted SPK fuel which has no additives. The variation in the model prediction was about 33% from the experimental result for the extracted SPK. This could be attributed to the fact that the model used was developed for a conventional jet fuel but not for a SPK fuel.
In the case of quinoline and ethyl oleate samples, model predictions are in good agreement with the experimental data as given in Fig. 4b. However, a noticeable deviation in the model predictions was observed when the additive concentrations were varied between 2–4 (vol%). The empirical model used to calculate the lubricity was developed for low-sulfur conventional diesel fuel using a standard method. The standard method for determining lubricity of diesel fuel was performed using the HFRR (High Frequency Reciprocating Rig) according to ASTM D6079. However, the test method used in this study utilized the BOCLE (Ball on Cylinder Lubricity Evaluator), following ASTM D5001 test method. The ASTM D5001 test is conducted at room temperature, while ASTM D6079 is performed at 60 °C, and with a lower load than the ASTM D5001 test.
As evident from the lubricity results, all the additives used in this study successfully improved SPK’s lubricity to meet the ASTM D7566 specification of a Jet fuel. However, it is also important to ensure that these additives do not offer significant trade-offs when it comes to other critical fuel properties such as density, viscosity, freezing point, flash point and heat content which may lead to catastrophic consequences.
3.2. Density trend of the fuel samples
Density profiles of all the fuel samples are highlighted in Fig. 5a and 5b. The density tests results revealed that all the tested fuel samples failed to meet the ASTM D7566 specification, which is specified as a minimum of 775 g/cm3. As discussed in the Sect. 3.1, a very small concentration of linoleic acid was sufficient to improve the SPK fuel lubricity; however, there is no observable variation in the density profiles of the linoleic acid fuel samples. Linoleic acid has a higher density value of (900 g/L) compared to the extracted SPK. Nevertheless, a very small concentration of it as an additive in the fuel was not sufficient to cause any apparent increase in the density of the sample.
The model predicts a negligible change in the density value for the linoleic acid samples which is the same trend found in the experimental result as shown in Fig. 5a. This could be attributed for the same reason: the small contractions of linoleic acid added were not sufficient to cause an increase in density of the sample.
Density predictions for ethyl oleate and quinolone samples correspond well with experimental results. The differences between the model prediction and measured densities do not exceed 2%. However, there is a subtle difference in the density trend predicted by the model compared to the trend obtained in the experiments, which warrants fine tuning of the model parameters.
By extrapolating the density trend for quinolone sample, we estimate that a concentration beyond 25 vol% will meet the ASTM specification for density and lubricity. Similarly, ethyl oleate will require a much higher concentration to meet the ASTM specification for density and lubricity.
3.3. Freezing point trend of the fuel
The freezing point is one of the most crucial properties of jet fuel, which ensures the proper operation of the fuel system at an elevated altitude and thus has severe implications when it comes to the safe operation of the aircraft. It is important to note that any additive used to boost a particular property of the fuel should not negatively affect the freezing point. The ASTM-D7566 specification for maximum freezing point is -40 °C for jet fuels. Mixing linoleic acid with SPK has no significant effect on the freezing point of the SPK samples, as shown in Fig. 6a. This was further confirmed in the modelling part. The linear mixing rule was used to estimate the freezing point, since the concentration of the added linoleic acid was small. Figure 6a illustrates that model predictions and experimental results are within the margin of error for the device.
Quinoline and ethyl oleate, on the other hand, resulted in a sample failing to meet ASTM D7566 specification for freezing point at concentrations beyond 2.5 vol%, as illustrated in Fig. 6b.
The model predictions for freezing point at a lower additive concentration below 2.5 vol% agrees well with experimental value, as seen in Fig. 6b. At concentrations beyond 2.5% of the sample volume, model predictions deviated significantly. Both quinolone and ethyl oleate have higher melting points of -15 °C and − 32 °C than the extracted SPK, respectively. The comparison shows that a correction term for the higher concentration needs to be added to the linear mixing rule as at higher concentrations, the effect of composition on the freezing point is non-ideal. Therefore, dissolution is expected below their melting points. As the concentration is increased beyond 2.5 vol%, there is a probable dissolution of the additives in the fuel sample resulting in a non-homogeneous mixture when the temperature is below subzero conditions [7]. On the other hand, the freezing point model used in this study does not account for the solubility characteristics of the additive in the fuel, which is more pronounced when the temperature drops to subzero levels, thus explaining the deviation between the model predictions and experimental data.
3.4. Flash point trends of fuel samples
The flash point of the extracted SPK fuel was measured to be 52 oC which is well above the minimum required limit by ASTM D1655 and D7566 of 38 oC. Figures 7a and 7bshow that all the tested samples have flash points in the range of 51oC to 54.5oC. No clear relationship between the additive concentration and the flash point could be established; especially given that the variation in the temperature for each additive was small and fairly insignificant. This result is in agreement with a previous study by Elmalik et al. [5].
Utilizing the linear mixing rule for estimating the flash point for linoleic acid samples, it can be in Fig. 7a how small is the variation between the mathematical and experimental data; the small difference is due to the margin of error of the Pensky-Martens SETA PM-93 device. The model predicted a linear trend for the flash point of the tested fuel samples. In the case of ethyl oleate and quinoline samples, the mathematical model used to predict the flash points used a group contribution method reported by Kalakul et al. [31]. Later Choudhury et al. [17] successfully reported a flash point prediction method of surrogate diesel using the same model with a maximum error of 28.34%. This study reports the maximum error in the model prediction for flash point to be ~ 15%, which could due to the assumption of the activity coefficient to be unity. Therefore, an improvement in the model predictions could be achieved by considering the activity coefficient of each of the constituent components in the fuel sample.
3.5. Viscosity trends of fuel samples
The extracted SPK has a viscosity of 6.40845 cSt at -40 °C, which meets the ASTM D7566 specification. As per ASTM D7566 maximum viscosity at -40 °C is 12 cSt. None of the additives used in this study have changed the viscosity of the samples under test to an unacceptable level of ASTM D7566 specifications. The viscosity value of all the fuel samples are depicted in Figs. 8a and 8b. Adding linoleic acid to the extracted SPK had no significant effect on its viscosity as the difference between the viscosities of extracted SPK and the additized fuel samples are small. The model predictions for the viscosity of the linoleic acid samples are in excellent agreement with the data obtained in the laboratory as depicted in Fig. 8a. As mentioned in Sect. 3.3, the linoleic acid is added in part parts million (ppm) to the SPK and the subtle variation in the concentration is beyond the detection limit of the instrument used to measure the viscosity value.
The effect of ethyl oleate on the viscosity value was the highest compared to both quinolone and linoleic acid. However, the maximum concentration of the ethyl oleate used in this study (10 vol%) did not increase the viscosity beyond its acceptable limits set by ASTM D7566. Therefore, all the tested fuel samples met the industry standards, despite the fact that the additives did increase the viscosity marginally. Model predictions of viscosity for quionoline and ethyl oleate are in good agreement with the experimental results as depicted in the Fig. 8b.
3.6. Net heat content trends of fuel samples
The extracted SPK fuel meets the target for net heat content as specified in ASTM D7566 (min. of 42.8 MJ/kg). All the tested samples were found to have a lower heat content than the extracted SPK fuel. Figure 9 depicts the net heat profiles for ethyl oleate and quinoline samples. The decrease in the net heat content of the fuel was more noticeable with the increase in their concentration in the fuel samples. It can be observed that the heat contents follow a linear correlation with the additive concentration, which is in agreement with previous studies in literature [16–17].
The model predicted net heat content values are compared with the experimental data and are shown in Fig. 9. It is clear from Fig. 9 that the model predictions are in good agreement with the experimental data.
It is expected that the addition of the ethyl oleate and quinoline to the SPK would result in an increase in the delay time of the fuel, since both are considered highly stable compounds; especially quinoline with its stable rings. On top of that, their addition to the SPK increases its viscosity, therefore the SPK would have larger droplets when injected into the engine. It is worth noting that the H/C ratio of ethyl oleate and quinoline samples are lower than those of the SPK as depicted Fig. 10. Therefore, these fuel samples will tend to form more coke as a result of cracking reactions at elevated temperatures.
3.7. Optimization Results
The MINLP optimization model predicted that the optimal mixture which would satisfy ASTM D7655 specification is the SPK-ethyl oleate sample with 5.6 vol% of ethyl oleate as given in Table 9. The model predicted an optimum fuel sample that closely resembles an FCL009 composition, which is a sample of extracted SPK and ethyl oleate. The physicochemical properties, which have been predicted for the optimum sample, are in good agreement with the properties of the FCL009 fuel sample which has the best lubricity characteristics among all other tested fuels. The Optimization model developed in this study demonstrated a promising potential for future studies in designing future new generation ultra-clean fuels.
Table 9
Optimization results of the MINLP model
Properties | Optimization Model | Lab testing data (FCL009) |
Additives | Ethyl oleate | Ethyl oleate |
Fuel | SPK | Extracted SPK |
Concentration (Vol%) | 5.6 | 5 |
Lubricity (µm) | 493.48 | 417 |
Density (g/cm3) | 0.749 | 0.75 |
Freezing point (℃) | -42.2 | -38.9 |
Flash point (℃) | 58.8 | 53.5 |
Viscosity (cSt) | 7.3 | 7.06 |
Gross Heat Content (MJ/kg) | 44.02 | 43.78 |