Catalytic Hydroisomerisation of Fischer–Tropsch Waxes to Lubricating Oil and Investigation of the Correlation between Its Physical Properties and the Chemical Composition of the Corresponding Fuel Fractions

: Due to environmental concerns, the role of renewable sources for petroleum-based products has become an invaluable research topic. One possibility of achieving this goal is the Fischer–Tropsch synthesis (FTS) based on sustainable raw materials. Those materials include, but are not limited to, synthesis gas from biomass gasiﬁcation or hydrogen through electrolysis powered by renewable electricity. In recent years, the utilisation of CO 2 as carbon source for FTS was one main R&D topic. This is one of the reasons for its increase in value and the removal of its label as being just exhaust gas. With the heavy product fraction of FTS, referred to as Fischer–Tropsch waxes (FTW), being rather limited in their application, catalytic upgrading can help to increase the economic viability of such a process by converting the waxes to high value transportation fuels and lubricating oils. In this paper, the dewaxing of FTW via hydroisomerisation and hydrocracking was investigated. A three phase ﬁxed bed reactor was used in combination with a zeolitic catalyst with an AEL (SAPO-11) structure and 0.3 wt% platinum (Pt). The desired products were high quality white oils with low cloud points. These products were successfully produced in a one-step catalytic dewaxing process. Within this work, a direct correlation between the physical properties of the white oils and the chemical composition of the simultaneously produced fuel fractions could be established.


Introduction
Environmental trepidations are an excessively discussed topic and the demand for fossil free energy supply is increasing. Current legislative measures, such as the taxation of carbon dioxide (CO 2 ) as proposed in Germany will inevitably push prices of fossil fuels and carbon sources to novel highs [1,2]. The most common suggestions are the increased usage of electric power in sectors such as transportation and commutation. While an energy supply via battery is viable for cars and light commercial vehicles, it reaches its limits with air and sea travel due to the low energy density of batteries [3]. This problem has reignited the demand for renewable liquid energy carriers, such as middle distillates from Fischer-Tropsch (FT) synthesis. Baseline for this approach is the usage of carbon monoxide (CO) and hydrogen (H 2 ) as synthesis gas. The original feedstock for FT synthesis, which is coal, can be replaced by renewable sources such as bio mass [4]. Another possibility is the usage or reuse of excess CO 2 as carbon source, which has been proven a viable and flexible alternative for CO [5]. Research has also been conducted regarding the use of the FT synthesis in a power-to-liquid (PTL) approach [6]. A general description of the FT synthesis from CO 2 to paraffins can be defined as the following. CO 2 + H 2 → CO + H 2 O n CO + (2n + 1) H 2 → C n H 2n+2 + n H 2 O The main issue with this process is that it will, with certain exceptions [7,8], almost always result in a probability distribution of products according to Schulz-Flory, reaching from methane to heavy paraffinic waxes [9]. The product distribution can be regulated using higher or lower reaction temperatures with lighter products (based on molecular weight) at higher temperatures and heavier products at lower ones [10]. Those processes are known as high temperature FT synthesis (HTFT) and low temperature FT synthesis (LTFT) [11]. The second issue is the paraffinic nature of FT products. Them being free of poly aromatic hydrocarbons (PAH) will generally result in lower emissions and soot formation during combustion [12]. However, the high amount of n-paraffins hinders the application of FT products as fuel due to poor cold flow properties or low octane numbers. A similar issue refers to the wax fraction. With its high melting point, its product applications are rather limited. Examples include industrial coatings and usage in street construction. By lowering the melting point, especially regarding the lubricant sector, more applications can be developed. Those problems can be resolved with a hydroprocessing step. The demand for synthetic lubricants increased as well in recent years due to changing quality demands. Miller et al. reported a shift to lower viscosity oils in order to decrease friction in mechanical stressed areas and lower oil volatility to increase longevity and sustainability [13]. Multiple authors have extensively studied hydroprocessing or hydroconversion of FT products to middle distillates [14][15][16][17][18][19], making the production of liquid transportation fuels from FT waxes a well-established process. The same cannot be stated about the production of lubricants from FT-waxes. The process itself is established and commercially used, for example, at the Shell PEARL GTL plant in Qatar, which utilizes offshore gas to produce FT products and subsequently lubricants such as technical and medicinal grade white oil [20]. The know-how is, with some mentionable exceptions [21][22][23], rather limited to patent literature [24][25][26][27]. Therefore, the aim of this paper is to investigate and describe the lubricant production from FT waxes and the differences of yield, viscosity and cloud point depending on reaction temperature and pressure. A correlation between molecular composition of the simultaneously produced fuel fractions to the corresponding lube properties is also to be established.

Dewaxing
The process of dewaxing describes the removal of n-paraffins from a given hydrocarbon mixture with the purpose of lowering the melting point. There a two main, commercially used dewaxing processes. Solvent dewaxing and catalytic dewaxing. Solvent dewaxing is based on removing wax crystals through precipitation. The untreated oil is usually diluted with an organic solvent such as ketones or aromatics or a mixture of both. It is then cooled below the desired pour point under constant stirring. This results in the formation of wax crystals, which can subsequently be removed by mechanical separation, such as cold filtration [28]. The main advantage of this process is its high yield and high viscosity index during dewaxing of petroleum based feedstock, yet it does require high amounts of extra solvent and additional distillation. Catalytic dewaxing is the selective removal of normal paraffins through cracking or isomerisation. While commercial base stock, such as vacuum gas oil, can be dewaxed by cracking of the long chain paraffins and removal of the lighter fraction via distillation, this is not true for FT-waxes. Those consist almost exclusively of straight chain hydrocarbons. Consequently, cracking would consume the entire wax, transforming it into lighter fuels. To dewax this product, hydroisomerisation is necessary. This will convert the normal paraffins into branched ones by reducing their pour point significantly while mostly maintaining their boiling range [29]. Hsu and Robinson also reported that commercial dewaxing through isomerisation has been available since 1993 when Chevron developed its ISODEWAXING ® process using SAPO catalysts. Similar techniques for hydroisomerisation are employed at the Shell PEARL GTL plant.

Hydroprocessing
Hydroprocessing is an umbrella term for multiple reactions of hydrocarbons under the H 2 atmosphere, including hydrotreating, hydrofinishing, hydrocracking and hydroisomerisation. In this paper, hydroprocessing refers to the latter two. Numerous different mechanisms of hydroisomerisation and hydrocracking were described by Bouchy et al. [15]. An example can be seen in Figure 1. The formation of a new isomer is depicted at (a), while the cracking reaction can be observed at (b). Both require the presence of carbenium ions. The reaction rate of isomerisation is generally higher than the one for cracking, with the notable exception at the scisscion of tertiary carbon ions which can exceed the isomerisation rate.
and Robinson also reported that commercial dewaxing through isomerisation ha available since 1993 when Chevron developed its ISODEWAXING ® process using catalysts. Similar techniques for hydroisomerisation are employed at the Shell P GTL plant.

Hydroprocessing
Hydroprocessing is an umbrella term for multiple reactions of hydrocarbons the H2 atmosphere, including hydrotreating, hydrofinishing, hydrocracking and isomerisation. In this paper, hydroprocessing refers to the latter two. Numerous di mechanisms of hydroisomerisation and hydrocracking were described by Bouchy [15]. An example can be seen in Figure 1. The formation of a new isomer is depicted while the cracking reaction can be observed at (b). Both require the presence of carb ions. The reaction rate of isomerisation is generally higher than the one for cracking the notable exception at the scisscion of tertiary carbon ions which can exceed the is isation rate. Cracking of FT products shifts the yield distribution away from waxy compon the desired yields, usually towards the middle distillate range. Isomerisation will the melting points in contrast to their n-paraffin counterparts [30], which will re better cold flow properties.

Catalytic Mechanism
Hydroprocessing itself is conducted on a bifunctional catalyst, which means are two different active centres on the catalyst surface, with distinct applications: tallic site for hydrogenation and dehydrogenation of the product, as well as an acid for isomerisation and cracking [15]. The acid component can be provided by a zeol rier, which can have varying degrees of acidity. A detailed description of the indi mechanisms is depicted in Figure 2. Cracking of FT products shifts the yield distribution away from waxy components to the desired yields, usually towards the middle distillate range. Isomerisation will lower the melting points in contrast to their n-paraffin counterparts [30], which will result in better cold flow properties.

Catalytic Mechanism
Hydroprocessing itself is conducted on a bifunctional catalyst, which means there are two different active centres on the catalyst surface, with distinct applications: A metallic site for hydrogenation and dehydrogenation of the product, as well as an acidic site for isomerisation and cracking [15]. The acid component can be provided by a zeolite carrier, which can have varying degrees of acidity. A detailed description of the individual mechanisms is depicted in Figure 2.
mechanisms of hydroisomerisation and hydrocracking were de [15]. An example can be seen in Figure 1. The formation of a new while the cracking reaction can be observed at (b). Both require th ions. The reaction rate of isomerisation is generally higher than t the notable exception at the scisscion of tertiary carbon ions whic isation rate. Cracking of FT products shifts the yield distribution away fr the desired yields, usually towards the middle distillate range. the melting points in contrast to their n-paraffin counterparts [ better cold flow properties.

Catalytic Mechanism
Hydroprocessing itself is conducted on a bifunctional cata are two different active centres on the catalyst surface, with dis tallic site for hydrogenation and dehydrogenation of the produc for isomerisation and cracking [15]. The acid component can be p rier, which can have varying degrees of acidity. A detailed desc mechanisms is depicted in Figure 2.  The saturated n-paraffin is dehydrated on the metallic site and an olefin is formed. The olefin transitions to the acidic site of the catalyst via diffusion. Due to proton uptake on the acidic catalyst site, secondary carbenium ions are formed and those can either crack or branch out to form tertiary carbenium ions [31]. Disintegration of the carbenium ions will result in rehydrogenation and formation of saturated n-paraffins and iso-paraffins.

Synthetic Lubricants from FT Waxes
Lubricating oils can be described as hydrophobic liquids, which have boiling points higher than water and do not crystalize at room temperature. Commercial Lubricants are specified more clearly. They depend on different parameters including the type of their feedstock (Table 1), their density, viscosity, viscosity index, pour point, cloud point, boiling range, volatility, flash point, amount of saturates, naphtenes and aromatics, oxidative resistance, acidity, colour and conradson carbon residue (CCR). A synthetic lubricant from a GTL process is categorised as API III+ [29]. Wax from FT synthesis is a decent feedstock for lubricant production due to its low content of heteroatoms or other impurities. The main issue with FTW is that its melting point is usually above 40 • C. The reason is the high amount of linear paraffins in the mixture. In order to reduce the melting temperature of the wax, the chemical composition has to be altered by hydroisomerisation as explained above. FTW dewaxing has been performed using catalytic dewaxing under hydrogen in either a one-step or two-step approach, occasionally including solvent dewaxing as a final step [22][23][24][25][26][27]. A pretreatment step may necessarily not required. This was stated by Miller et al. [13] who hydroisomerised FT waxes mixed with pyroysed plastics to produce lubricating oils. This results in the conclusion that hydrogenation of pure FT wax prior to hydroisomerisation should be unnecessary. Before hydroprocessing certain wax fractions, what needs to be clarified is whether residual oxygenates can hurt the used catalyst. The two main properties that any lubricating oil will be defined by are its viscosity and its congealing point. The viscosity of hydrocarbon oils is mainly dependent on its average chain length and can only be slightly adjusted by its chemical structure. The kinematic viscosity can even be used to calculate the median molecular weight of the lubricant [32]. The congealing temperature is heavily dependent on the molecular structure, with n-paraffins having the highest melting points in a given carbon range [29].

Cloud Point
The cloud point (CP) of a paraffin mixture defines the temperature when precipitation of wax crystals begins. It is usually at higher temperatures than other cold flow properties [33]. At CP-temperature, the fluidity of the mixture is not yet inhibited, but wax crystals start to form, which can accumulate on cold surfaces and subsequently plug filters [34]. It is slightly above the common cold filter plugging point (CFPP). Due to it being the highest temperature where significant effects on the consistency of the lubricant can be observed, this work has chosen to display the change in cold flow behaviour.

Reactor Setup and Materials Used
In order to apply the processes proposed in the previous section, a three-phase fixed bed reactor was used ( Figure 3). The melted wax was kept in a five litre storage vessel from where it was pumped at 90 • C into the reactor by using a high pressure and high temperature piston pump (Bischoff HPD Pump Multitherm 200 model 3351).
Energies 2021, 14, x FOR PEER REVIEW 5 of 13 In order to apply the processes proposed in the previous section, a three-phase fixed bed reactor was used ( Figure 3). The melted wax was kept in a five litre storage vessel from where it was pumped at 90 °C into the reactor by using a high pressure and high temperature piston pump (Bischoff HPD Pump Multitherm 200 model 3351).  This allows a liquid flowrate in the range from 0.6-300 mL/h. The reactor itself consists of a tube with 14.9 mm inner diameter and a length of 800 mm. The used reaction zone itself consists of 400 mm. A 1/8'' tube is inserted in the middle to provide access for temperature measurements. After the reaction zone, the product mixture enters three consecutive vapour-liquid separation vessels with a volume of 800 mL each and from there it can be removed via a needle valve at the bottom. In the reaction zone, 20 g (100-200 µm) of a commercially available zeolitic catalyst with AEL (SAPO-11) structure and 0.3 wt% platinum as a hydrogenation agent were used. This catalyst was chosen with respect to its high selectivity towards isomerisation [35]. The reaction temperature was set on the outside of the reactor tube. Under reaction conditions, the inner temperature increases approximately between 1-2 °C. The feed was Fischer-Tropsch wax from Sasol with a cloud point, as determined by differential scanning calorimetry (DSC) measurement, of 63.6 °C ( Figure 4). All experiments were conducted at the same wax flow rate of 0.28 mL/min at 90 °C, resulting in a liquid hourly space velocity (LHSV) of 0.75 h −1 . The hydrogen flow of 550 mL/min (at laboratory conditions) was also not changed.

Sample Preparation
The retrieved samples were weighed and separated according to the following atmospheric boiling ranges: gasoline <170 °C, middle distillate 170-340 °C and lubricant This allows a liquid flowrate in the range from 0.6-300 mL/h. The reactor itself consists of a tube with 14.9 mm inner diameter and a length of 800 mm. The used reaction zone itself consists of 400 mm. A 1/8" tube is inserted in the middle to provide access for temperature measurements. After the reaction zone, the product mixture enters three consecutive vapour-liquid separation vessels with a volume of 800 mL each and from there it can be removed via a needle valve at the bottom. In the reaction zone, 20 g (100-200 µm) of a commercially available zeolitic catalyst with AEL (SAPO-11) structure and 0.3 wt% platinum as a hydrogenation agent were used. This catalyst was chosen with respect to its high selectivity towards isomerisation [35]. The reaction temperature was set on the outside of the reactor tube. Under reaction conditions, the inner temperature increases approximately between 1-2 • C. The feed was Fischer-Tropsch wax from Sasol with a cloud point, as determined by differential scanning calorimetry (DSC) measurement, of 63.6 • C ( Figure 4). All experiments were conducted at the same wax flow rate of 0.28 mL/min at 90 • C, resulting in a liquid hourly space velocity (LHSV) of 0.75 h −1 . The hydrogen flow of 550 mL/min (at laboratory conditions) was also not changed. In order to apply the processes proposed in the previous section, a three-phase fixed bed reactor was used ( Figure 3). The melted wax was kept in a five litre storage vesse from where it was pumped at 90 °C into the reactor by using a high pressure and high temperature piston pump (Bischoff HPD Pump Multitherm 200 model 3351).  This allows a liquid flowrate in the range from 0.6-300 mL/h. The reactor itself con sists of a tube with 14.9 mm inner diameter and a length of 800 mm. The used reaction zone itself consists of 400 mm. A 1/8'' tube is inserted in the middle to provide access fo temperature measurements. After the reaction zone, the product mixture enters three con secutive vapour-liquid separation vessels with a volume of 800 mL each and from there it can be removed via a needle valve at the bottom. In the reaction zone, 20 g (100-200 µm of a commercially available zeolitic catalyst with AEL (SAPO-11) structure and 0.3 wt% platinum as a hydrogenation agent were used. This catalyst was chosen with respect to it high selectivity towards isomerisation [35]. The reaction temperature was set on the out side of the reactor tube. Under reaction conditions, the inner temperature increases ap proximately between 1-2 °C. The feed was Fischer-Tropsch wax from Sasol with a cloud point, as determined by differential scanning calorimetry (DSC) measurement, of 63.6 °C ( Figure 4). All experiments were conducted at the same wax flow rate of 0.28 mL/min a 90 °C, resulting in a liquid hourly space velocity (LHSV) of 0.75 h −1 . The hydrogen flow o 550 mL/min (at laboratory conditions) was also not changed.

Sample Preparation
The retrieved samples were weighed and separated according to the following at mospheric boiling ranges: gasoline <170 °C, middle distillate 170-340 °C and lubrican

Sample Preparation
The retrieved samples were weighed and separated according to the following atmospheric boiling ranges: gasoline <170 • C, middle distillate 170-340 • C and lubricant >340 • C. In order to avoid overheating the samples, separation of middle distillate from the lubricant was conducted under vacuum using a standard laboratory glass distillation apparatus. The required vacuum was generated with a water-jet pump. The remaining wax crystals were removed by centrifugation (Hettich Rotana 460) at 9000 rpm for 10 min.

Analytics
The cloud points (CP) of lubricant fractions and middle distillate were determined with differential scanning calorimetry (Netzsch DSC 214 Polyma) under nitrogen atmosphere. In order to achieve exact measurements, a constant cooling rate and sample weight is necessary [36]. For one analysis, 10 mg of the sample was used and a cooling rate of 10 K/min was applied. Afterwards, the sample was reheated before being removed. The cloud point was identified by extrapolating the tangents of the first detectable peak-onset during cooling. The measured temperature at the intersections of both straight lines was noted (T M ). An exemplary peak onset for a commercial paraffin oil and the used temperature programme can be observed in Figure 5. This sequence was developed based on the methods already presented in the literature [37,38] or slightly adapted from similar ASTM Norms [39,40]. The DSC-device required additional calibration to determine the start of crystallisation during cooling. For this purpose, pure components with various solidification temperatures were chosen (n-dodecane, mesitylene, deuterated chloroform, butanone, tetradecane, octacosane, tetracosane, hexadecane and triacontane). Aditionally two Cloud Point reference diesel fuels (ASTM D 2500 [41]) with CPs at 7.7 ± 0.6 • C and −21.0 ± 2 • C were included. The calibration showed significant deviancy of the crystallisation temperature (T C ) and the measured temperature (T M ), the correctional functions can be reviewed in Appendix A. The initial calibration did not include the reference fuels. A sample of 10 mg each were analysed. The resulting temperatures were 8.29 • C and −19.09 • C which lies within the stated deviation as determined by ASTM D 2500 and also within the expected scope of DSC measurements, described by Claudy et al. [42]. In order to improve accuracy, the samples were included into the calibration, as presented in Appendix A. It needs to be stated that it was not always possible to determine a clear peak with this method. Some samples crystallized homogenously with no detectable CP. Those were not considered in this work.
Energies 2021, 14, x FOR PEER REVIEW 6 of 13 >340 °C. In order to avoid overheating the samples, separation of middle distillate from the lubricant was conducted under vacuum using a standard laboratory glass distillation apparatus. The required vacuum was generated with a water-jet pump. The remaining wax crystals were removed by centrifugation (Hettich Rotana 460) at 9000 rpm for 10 min.

Analytics
The cloud points (CP) of lubricant fractions and middle distillate were determined with differential scanning calorimetry (Netzsch DSC 214 Polyma) under nitrogen atmosphere. In order to achieve exact measurements, a constant cooling rate and sample weight is necessary [36]. For one analysis, 10 mg of the sample was used and a cooling rate of 10 K/min was applied. Afterwards, the sample was reheated before being removed. The cloud point was identified by extrapolating the tangents of the first detectable peak-onset during cooling. The measured temperature at the intersections of both straight lines was noted (TM). An exemplary peak onset for a commercial paraffin oil and the used temperature programme can be observed in Figure 5. This sequence was developed based on the methods already presented in the literature [37,38] or slightly adapted from similar ASTM Norms [39,40]. The DSC-device required additional calibration to determine the start of crystallisation during cooling. For this purpose, pure components with various solidification temperatures were chosen (n-dodecane, mesitylene, deuterated chloroform, butanone, tetradecane, octacosane, tetracosane, hexadecane and triacontane). Aditionally two Cloud Point reference diesel fuels (ASTM D 2500 [41]) with CPs at 7.7 ± 0.6 °C and −21.0 ± 2 °C were included. The calibration showed significant deviancy of the crystallisation temperature (TC) and the measured temperature (TM), the correctional functions can be reviewed in Appendix A-C. The initial calibration did not include the reference fuels. A sample of 10 mg each were analysed. The resulting temperatures were 8.29 °C and −19.09 °C which lies within the stated deviation as determined by ASTM D 2500 and also within the expected scope of DSC measurements, described by Claudy et al. [42]. In order to improve accuracy, the samples were included into the calibration, as presented in Appendix A. It needs to be stated that it was not always possible to determine a clear peak with this method. Some samples crystallized homogenously with no detectable CP. Those were not considered in this work. The dynamic viscosity was measured with a plate rheometer (Anton Paar MCR 302e) at a sheer rate of 50 s −1 and a rotating plate size of 50 mm for five minutes to achieve reliable measurements. The required density to calculate the kinematic viscosity was determined by oscillating tube measurement (Anton Paar DMA 4200). The composition of the gasoline fractions was analysed using a PAC Reformulyzer M4 Hydrocarbon Group The dynamic viscosity was measured with a plate rheometer (Anton Paar MCR 302e) at a sheer rate of 50 s −1 and a rotating plate size of 50 mm for five minutes to achieve reliable measurements. The required density to calculate the kinematic viscosity was determined by oscillating tube measurement (Anton Paar DMA 4200). The composition of Energies 2021, 14, 4202 7 of 14 the gasoline fractions was analysed using a PAC Reformulyzer M4 Hydrocarbon Group Type Analysis. This allowed the measurement of n-/iso-Paraffin content and also detected possible aromatic or naphthenic components. The composition of the produced gas phase was determined by gas chromatography (Agilent Technologies 7890A). Due to calibration problems, the gas phase could only be analysed for one set of parameters (Appendix B) and will not be further discussed in this paper. Determination of the boiling ranges was conducted using a simulated distillation method (Shimadzu Nexis GC-2030).

Boiling Range and Cloud Points
In order to check if the distillation was successful, the boiling range and cloud points of the produced oil had to be measured. This was performed via simulated distillation and an example is given in Figure 6. The individual data points refer to the sum of iso-and n-paraffins within a given carbon number. It was performed in this manner to incorporate response factors. It can be observed that the resulting oil fraction boils entirely above 300 • C and has less than 10 wt% boiling below 340 • C. While the boiling range of the lubricant barely changes, the cloud point could be reduced by 73 • C. With the cloud point being below room temperature and the boiling range indicating no fuel components within the mixture, a lubricating oil was successfully produced. The variation in cloud points was investigated with regards to changing process parameters, in this case the points were reactor temperature and pressure.
Type Analysis. This allowed the measurement of n-/iso-Paraffin content an possible aromatic or naphthenic components. The composition of the prod was determined by gas chromatography (Agilent Technologies 7890A). Du problems, the gas phase could only be analysed for one set of parameter and will not be further discussed in this paper. Determination of the boi conducted using a simulated distillation method (Shimadzu Nexis GC-20

Boiling Range and Cloud Points
In order to check if the distillation was successful, the boiling range a of the produced oil had to be measured. This was performed via simul and an example is given in Figure 6. The individual data points refer to and n-paraffins within a given carbon number. It was performed in this m porate response factors. It can be observed that the resulting oil fractio above 300 °C and has less than 10 wt% boiling below 340 °C. While the b the lubricant barely changes, the cloud point could be reduced by 73 °C. point being below room temperature and the boiling range indicating no f within the mixture, a lubricating oil was successfully produced. The va points was investigated with regards to changing process parameters, points were reactor temperature and pressure. It can be observed that the cloud point decreases rapidly with increasi ( Figure 7a). This indicates higher n-paraffin conversion and higher yiel higher temperatures as expected. Higher pressure results in higher clou 7b). This correlates with the mechanism of hydrocracking, where higher creases carbenium ion formation, while higher pressure inhibits it. The am affins should therefore be higher at low cloud points. A direct measureme in the oil fraction was not possible with the available setup. It can be observed that the cloud point decreases rapidly with increasing temperature (Figure 7a). This indicates higher n-paraffin conversion and higher yield of isomers at higher temperatures as expected. Higher pressure results in higher cloud point (Figure 7b). This correlates with the mechanism of hydrocracking, where higher temperature increases carbenium ion formation, while higher pressure inhibits it. The amount of isoparaffins should therefore be higher at low cloud points. A direct measurement of n-paraffins in the oil fraction was not possible with the available setup.

Yield and Viscosity
Another critical issue is the yield of the potential lubricating oil. The possible liquid yield loss can occur during the reaction itself with the production of high amounts of carbon gasses or during distillation via evaporation to the vacuum pump. The "de facto" yields are calculated in regards to the inlet flow of the FTW and are depicted in Figure 8. The observed trends are similar to the trends witnessed for the cloud points, indicating a dependency on n-paraffin conversion, resulting in the conclusion that low cloud points can only be achieved under loss of yield. Similar effects for the pour point have been reported by Hsu and Robinson [29]. To illustrate this issue, the reduction in cloud point was plotted over the potential lubricant yield (Figure 9). Two additional successfully conducted measurements were added (Appendix C, Sample 8 and 9) to verify the following trends at low yield and cloud points. DSC-CP and Yield decrease simultaneously. Higher quality base oil will therefore come at the cost of lower oil return. This effect is independent from reaction temperature or pressure. This concludes a general opposite trend between yield and quality of the base oil. How this dependency will be afflicted by catalyst choice or LHSV variance will be investigated in future studies. It has been shown that catalysts such as ZSM-5 (MFI) generate less lubricant at similar pour points than a SAPO catalyst would [29].

Yield and Viscosity
Another critical issue is the yield of the potential lubricating oil. The possible liquid yield loss can occur during the reaction itself with the production of high amounts of carbon gasses or during distillation via evaporation to the vacuum pump. The "de facto" yields are calculated in regards to the inlet flow of the FTW and are depicted in Figure 8. The observed trends are similar to the trends witnessed for the cloud points, indicating a dependency on n-paraffin conversion, resulting in the conclusion that low cloud points can only be achieved under loss of yield. Similar effects for the pour point have been reported by Hsu and Robinson [29].

Yield and Viscosity
Another critical issue is the yield of the potential lubricating oil. The possible liquid yield loss can occur during the reaction itself with the production of high amounts of carbon gasses or during distillation via evaporation to the vacuum pump. The "de facto" yields are calculated in regards to the inlet flow of the FTW and are depicted in Figure 8. The observed trends are similar to the trends witnessed for the cloud points, indicating a dependency on n-paraffin conversion, resulting in the conclusion that low cloud points can only be achieved under loss of yield. Similar effects for the pour point have been reported by Hsu and Robinson [29]. To illustrate this issue, the reduction in cloud point was plotted over the potential lubricant yield (Figure 9). Two additional successfully conducted measurements were added (Appendix C, Sample 8 and 9) to verify the following trends at low yield and cloud points. DSC-CP and Yield decrease simultaneously. Higher quality base oil will therefore come at the cost of lower oil return. This effect is independent from reaction temperature or pressure. This concludes a general opposite trend between yield and quality of the base oil. How this dependency will be afflicted by catalyst choice or LHSV variance will be investigated in future studies. It has been shown that catalysts such as ZSM-5 (MFI) generate less lubricant at similar pour points than a SAPO catalyst would [29]. To illustrate this issue, the reduction in cloud point was plotted over the potential lubricant yield (Figure 9). Two additional successfully conducted measurements were added (Appendix C, Samples 8 and 9) to verify the following trends at low yield and cloud points. DSC-CP and Yield decrease simultaneously. Higher quality base oil will therefore come at the cost of lower oil return. This effect is independent from reaction temperature or pressure. This concludes a general opposite trend between yield and quality of the base oil. How this dependency will be afflicted by catalyst choice or LHSV variance will be investigated in future studies. It has been shown that catalysts such as ZSM-5 (MFI) generate less lubricant at similar pour points than a SAPO catalyst would [29].  While there are clear dependencies between the oil yields and reaction param the same could not be observed for the corresponding viscosity. The fluctuation of v ity measured at 20 °C was mostly between 41-50 mm 2 /s. In the above presented tem ture and pressure ranges, barely any trends are observable as depicted in Figure 10. trends are not significant enough to have an impact on the oil quality overall. A sign decrease could only be observed for Sample 8 (Appendix C), which might be due to catalyst activity for it being produced on a recently regenerated catalyst bed. Yet the ured value for the cloud point of the lubricant and middle distillate was consisten the expected values, as it will be presented in Section 4.3.

Determination of Cloud Point Using Composition of the Gasoline Fraction
A major issue in lubrication oil analytics is the determination of molecular com tion in the mixture. The longer the chain of the molecule, the more isomers and oth products are possible. A highly isomerised product might not be separable by ga matography and requires more sophisticated measurements such as thin film chrom raphy or high power liquid chromatography. Even if the analytics are available, one issue is the potential modelling of the fractions with high molecular weight beca While there are clear dependencies between the oil yields and reaction parameters, the same could not be observed for the corresponding viscosity. The fluctuation of viscosity measured at 20 • C was mostly between 41-50 mm 2 /s. In the above presented temperature and pressure ranges, barely any trends are observable as depicted in Figure 10. Those trends are not significant enough to have an impact on the oil quality overall. A significant decrease could only be observed for Sample 8 (Appendix C), which might be due to higher catalyst activity for it being produced on a recently regenerated catalyst bed. Yet the measured value for the cloud point of the lubricant and middle distillate was consistent with the expected values, as it will be presented in Section 4.3. While there are clear dependencies between the oil yields and reaction parameters, the same could not be observed for the corresponding viscosity. The fluctuation of viscosity measured at 20 °C was mostly between 41-50 mm 2 /s. In the above presented temperature and pressure ranges, barely any trends are observable as depicted in Figure 10. Those trends are not significant enough to have an impact on the oil quality overall. A significant decrease could only be observed for Sample 8 (Appendix C), which might be due to higher catalyst activity for it being produced on a recently regenerated catalyst bed. Yet the measured value for the cloud point of the lubricant and middle distillate was consistent with the expected values, as it will be presented in Section 4.3.

Determination of Cloud Point Using Composition of the Gasoline Fraction
A major issue in lubrication oil analytics is the determination of molecular composition in the mixture. The longer the chain of the molecule, the more isomers and other byproducts are possible. A highly isomerised product might not be separable by gas chromatography and requires more sophisticated measurements such as thin film chromatography or high power liquid chromatography. Even if the analytics are available, one major issue is the potential modelling of the fractions with high molecular weight because of unavailable datasets for the produced isomers. On the contrary, the analytics and modelling of products with lower molecular weight has been conducted [43].
The resulting gasoline was analysed regarding its molecular composition and compared to the cloud point of the corresponding lubricant fraction. Figure 11 shows the DSC-CP for lubricant and middle distillate against the n-paraffin and naphthene content within

Determination of Cloud Point Using Composition of the Gasoline Fraction
A major issue in lubrication oil analytics is the determination of molecular composition in the mixture. The longer the chain of the molecule, the more isomers and other by-products are possible. A highly isomerised product might not be separable by gas chromatography and requires more sophisticated measurements such as thin film chromatography or high power liquid chromatography. Even if the analytics are available, one major issue is the potential modelling of the fractions with high molecular weight because of unavailable datasets for the produced isomers. On the contrary, the analytics and modelling of products with lower molecular weight has been conducted [43].
The resulting gasoline was analysed regarding its molecular composition and compared to the cloud point of the corresponding lubricant fraction. Figure 11 shows the DSC-CP for lubricant and middle distillate against the n-paraffin and naphthene content within the conforming gasoline fraction. It can be observed that the cloud point increases simultaneously with n-paraffins in the fuel fraction. The trend is more significant for the lubricants than for the middle distillate. This indicates that the chemical equilibrium of n-paraffins and isoparaffins is comparable within the different boiling ranges. A high n-paraffin content in the gasoline will correspond to a high n-paraffin content in the lube fraction and will therefore begin to crystallise at higher temperatures.
Energies 2021, 14, x FOR PEER REVIEW 10 of the conforming gasoline fraction. It can be observed that the cloud point increases simu taneously with n-paraffins in the fuel fraction. The trend is more significant for the lubr cants than for the middle distillate. This indicates that the chemical equilibrium of n-pa affins and isoparaffins is comparable within the different boiling ranges. A high n-paraff content in the gasoline will correspond to a high n-paraffin content in the lube fractio and will therefore begin to crystallise at higher temperatures. A similar but steeper trend can be observed for the naphthenic components. Hig molecular weight naphthenes are generally decent lubricant components and desirab over n-paraffins for improvement of the cold flow properties.
This method even allows the description of the expected liquid yields (Figure 12). A high n-paraffin content, the highest yields are to be gathered. With the reaction progres ing and conversion of n-paraffins through isomerisation and cracking, the yields for ga oline increases, while the middle distillate mostly stays the same.

Conclusions
In this work, the general production process of base oil from FT waxes was presente A method for the determination of cloud point using differential scanning calorimetr was applied. It was shown that lubricating oil can be produced by minimal reduction boiling ranges, while simultaneously reducing the cloud point and subsequently oth cold flow properties significantly. It was also shown that the viscosity was hardly relian on changing reaction parameters. The dependency of cloud points and base-oil yields o reaction parameters, such as temperature and pressure, was investigated and presente A clear correlation between yield loss and reduction in cloud points could be establishe A similar but steeper trend can be observed for the naphthenic components. High molecular weight naphthenes are generally decent lubricant components and desirable over n-paraffins for improvement of the cold flow properties.
This method even allows the description of the expected liquid yields (Figure 12). At high n-paraffin content, the highest yields are to be gathered. With the reaction progressing and conversion of n-paraffins through isomerisation and cracking, the yields for gasoline increases, while the middle distillate mostly stays the same.
Energies 2021, 14, x FOR PEER REVIEW 10 of 13 the conforming gasoline fraction. It can be observed that the cloud point increases simultaneously with n-paraffins in the fuel fraction. The trend is more significant for the lubricants than for the middle distillate. This indicates that the chemical equilibrium of n-paraffins and isoparaffins is comparable within the different boiling ranges. A high n-paraffin content in the gasoline will correspond to a high n-paraffin content in the lube fraction and will therefore begin to crystallise at higher temperatures. A similar but steeper trend can be observed for the naphthenic components. High molecular weight naphthenes are generally decent lubricant components and desirable over n-paraffins for improvement of the cold flow properties.
This method even allows the description of the expected liquid yields (Figure 12). At high n-paraffin content, the highest yields are to be gathered. With the reaction progressing and conversion of n-paraffins through isomerisation and cracking, the yields for gasoline increases, while the middle distillate mostly stays the same.

Conclusions
In this work, the general production process of base oil from FT waxes was presented. A method for the determination of cloud point using differential scanning calorimetry was applied. It was shown that lubricating oil can be produced by minimal reduction in boiling ranges, while simultaneously reducing the cloud point and subsequently other cold flow properties significantly. It was also shown that the viscosity was hardly reliant on changing reaction parameters. The dependency of cloud points and base-oil yields on reaction parameters, such as temperature and pressure, was investigated and presented. A clear correlation between yield loss and reduction in cloud points could be established.

Conclusions
In this work, the general production process of base oil from FT waxes was presented. A method for the determination of cloud point using differential scanning calorimetry was applied. It was shown that lubricating oil can be produced by minimal reduction in boiling ranges, while simultaneously reducing the cloud point and subsequently other cold flow properties significantly. It was also shown that the viscosity was hardly reliant on changing reaction parameters. The dependency of cloud points and base-oil yields on reaction parameters, such as temperature and pressure, was investigated and presented. A clear correlation between yield loss and reduction in cloud points could be established. A method for the determination of cloud points through analysis of the distilled gasoline fraction