Sustainable aviation fuel: Pathways to fully formulated synthetic jet fuel via Fischer–Tropsch synthesis

Fully formulated synthetic jet fuel is an aviation turbine fuel that does not contain petroleum‐derived kerosene and comprises the hydrocarbon compound classes n‐alkanes, isoalkanes, cycloalkanes, and aromatics. When the aim is to produce sustainable aviation fuel, one potential process pathway is by indirect liquefaction via Fischer–Tropsch synthesis. Fischer–Tropsch synthesised paraffinic kerosene plus aromatics (FT SPK/A) is a product that is fully formulated and can in principle be qualified as Jet A‐1. The synthetic jet fuel must ultimately meet all of the Jet A‐1 specifications. However, there are still hurdles on the path toward global approval of fully formulated synthetic jet fuel. In this study, several different refining pathways are shown that can be employed to produce FT SPK/A. The refining pathways have the desirable attribute of being generally useful and not limited to a specific refining technology. A case study is also presented in which FT SPK/A was produced, characterised and compared to Jet A‐1 specification requirements. It illustrated that it was practical to produce a fully formulated jet fuel via Fischer–Tropsch refining.


| INTRODUCTION
Fully formulated synthetic jet fuel is aviation turbine fuel produced only from nonpetroleum derived feed materials.In the context of sustainable aviation fuel (SAF), the implication is that the kerosene is produced from resources that are not only renewable, but also sustainable and therefore replenished at the same rate as they are consumed.Chuck 1 provided an extensive overview of jet fuel produced from such feed materials, which include biomass resources and organic wastes.
One of the potential pathways for converting nonpetroleum resources into SAF is by indirect liquefaction via Fischer-Tropsch synthesis.The front-end engineering design of such a process was previously outlined. 2It dealt with spoke-and-hub supply logistics, conversion of the raw materials into oil, synthesis gas production, and synthesis gas cleaning.In the present study, the focus is on the back-end engineering design of the process.At the back-end of the process is the Fischer-Tropsch refinery where the material is refined to synthetic jet fuel.
Different approaches to large-scale Fischer-Tropsch refinery design to emphasise synthetic jet fuel production in combination with motor-gasoline and diesel fuel have been reported. 3,4The refinery designs were for the production of transport fuels that complied with European standards for motor-gasoline, jet fuel, and diesel fuel.
When considering the production of SAF in a global context, there are two technical aspects that are different, compared to the aforementioned refinery designs.The first difference is that of capacity.At least initially, it is likely that the capacity of facilities for the production of SAF will be small compared to that of petroleum refineries.The second difference is that of the approval of fully synthetic jet fuel as global commodity.At the time of writing there is no approved pathway to produce fully synthetic jet fuel under the American Society for Testing and Materials (ASTM) specifications in ASTM D7566. 5n this work the steps required for future approval of fully formulated synthetic jet fuel under ASTM D7566 are summarised, as well as their implications for the refining processes to produce fully formulated synthetic jet fuel from Fischer-Tropsch synthesis.Then, different approaches to the refining of Fischer-Tropsch products for the production of fully formulated synthetic jet fuel will be discussed.Lastly, a case study of a Fischer-Tropsch refinery design to produce fully formulated synthetic jet fuel together with the characterisation of the products will be presented.

| APPROVAL OF FULLY SYNTHETIC JET FUEL
For a synthetic jet fuel to be a drop-in alternative to petroleum derived jet fuel, it must ultimately comply with the same fuel specifications as petroleum derived jet fuel.This means that in terms of the property and performance range of petroleum jet fuel, synthetic jet fuel should be indistinguishable.In the parlance of the approval process in the United States, ASTM D7566 allows a synthetic jet fuel to be re-identified as Jet A-1 under the ASTM D1655, 6 which governs jet fuel specifications for commercial use.
The ASTM D7566 has refining pathway specific requirements for different synthetic blend components.These requirements are codified in the ASTM D7566 annexes.In this respect, the ASTM approval process is different to the Defence Standard (DEF STAN) of the United Kingdom Ministry of Defence, DEF STAN 91-091. 7The DEF STAN 91-091 imposes additional property requirements on jet fuel that contains synthetic hydrocarbons, but it does not regulate or restrict refining processes that may be employed in the production of the jet fuel.The approval process under ASTM is therefore more onerous, because each new refining pathway has to be approved using the process outlined in ASTM D4054. 8nce the kerosene from the new refining pathway has been found to be an acceptable blend component, a new annex to ASTM D7566 is created, or an existing annex is amended.
This is an important point to bear in mind when developing a process to produce SAF.Even when a kerosene has near identical properties and composition to a kerosene that is obtained from a refining pathway that is described in one of the ASTM D7566 annexes, the kerosene from the new refining pathway has to be approved following ASTM D4054 if the new pathway is not covered by the description given in the standard.The approval is therefore not only dependent on the properties and performance of the fuel, but is also linked to the specifics of the refining pathways as described in the ASTM D7566 annexes.Differently put, ASTM D7566 not only regulates fuel properties, but also regulates refinery design.
In the future, to enable fully synthetic jet fuel qualification under ASTM D7566, it is necessary to: (i) identify synthetic blend components that are fully formulated jet fuels, (ii) approve the use of those synthetic blend components as fully synthetic jet fuel, and (iii) check that the descriptions of refining pathways are generally useful.

| Fully formulated synthetic blend components
The main compound classes found in jet fuel are summarised in Table 1, together with the compound class composition of approved synthetic blend components.It can be seen from Table 1 that only two of the synthetic blend components, A4 and A6, are fully formulated to contain all of the major compound classes.These two fully formulated blend components have the potential to be approved for use as fully synthetic jet fuel.
In practice, jet fuel also contains alkenes (olefins) and bicyclic compounds that include binuclear aromatics (naphthalenes) and naphthenoaromatics (indans and tetralins).The maximum amount of alkenes is not directly regulated, but indirectly regulated through thermal stability.Typically the alkene content of petroleum derived jet fuels is <2.5 vol%. 9The maximum bicyclic content is not regulated, but the maximum de KLERK ET AL.
| 395 naphthalene content is directly regulated to be <3 vol%, 6 or indirectly regulated through the smoke point specification.Thus, neither alkenes or bicyclic compounds are required for a fully formulated jet fuel, but both may be present at low concentration.
The present study is limited to refining pathways via Fischer-Tropsch synthesis.The relevant blend components are Fischer-Tropsch synthesised paraffinic kerosene (FT SPK) and Fischer-Tropsch synthesised paraffinic kerosene plus aromatics (FT SPK/A).To produce fully formulated synthetic jet fuel from refined Fischer-Tropsch material only, the kerosene blend must contain FT SPK/A.

| FISCHER-TROPSCH REFINING PATHWAYS TO FULLY SYNTHETIC JET FUEL
When the primary aim is to produce fully formulated jet fuel via Fischer-Tropsch synthesis, then it is instructive to look at the molecular requirements of the product in relation to the feed.The feed material from Fischer-Tropsch synthesis comprises hydrocarbons and oxygenates, with the hydrocarbons consisting of mainly n-alkanes and n-1-alkenes. 3,10The material spans a wide boiling range, with up to half of the product being atmospheric residue (>360°C).The major compound classes required in the product are n-alkanes, isoalkanes, cycloalkanes, and aromatics (Table 1).Straight run material from Fischer-Tropsch synthesis is therefore deficient in three of the four required compound classes in fully formulated jet fuel.
The types of refining processes that one would require are those that produce branched and cyclic hydrocarbons from linear hydrocarbons, skeletally isomerise linear hydrocarbons to achieve the same and a method to produce aromatics.The added requirement is that the products must be in the kerosene boiling range, approximately 160-260°C; the minimum boiling point is determined by flash point and density requirements and the maximum boiling point is 300°C.
Several refining technologies are useful for this purpose.Light alkene oligomerisation produces a heavier branched hydrocarbon product, which may also contain cyclic hydrocarbons depending on the type of oligomerisation technology employed.Cracking of heavy alkanes concomitantly isomerises the material and produces a lighter branched hydrocarbon product.Aromatisation of naphtha, as exemplified by catalytic naphtha reforming, produces aromatics.If the aromatics are lighter boiling than kerosene, Friedel-Crafts type alkene-aromatic alkylation with light alkenes can increase the boiling point of the aromatic products.Thus, there are refining processes that can be used to convert material from Fischer-Tropsch synthesis to produce all of the required compound classes in fully formulated jet fuel.

| Sasol pathway
The origin of ASTM D7566 Annex 4, is the development of a fully synthetic jet fuel blend by Sasol.The same company was instrumental in the initial development of synthetic jet fuels, 11 and it was the driving force behind the DEF STAN 91-091 specifications for synthetic jet fuel, as well as the work that led to ASTM D7566 Annex 1.
The two synthesised paraffinic kerosene blend components derived from Fischer-Tropsch synthesis, FT SPK and FT SPK/A, are closely related.The main difference between FT SPK and FT SPK/A is the presence of aromatics in the latter (see Table 1).Since the objective of this study is to look at the refining pathway, it is instructive to look at both the origin of the aromatics, and how the aromatics were refined to produce FT SPK/A.
Fischer-Tropsch synthesis can be performed at different temperatures.In an iron-catalysed process the synthesis temperature can range from 220°C to 340°C depending on the technology used. 10In a cobaltcatalysed process the temperature range is less, typically <240°C.The operating temperature of Fischer-Tropsch synthesis affects the composition of the product.Aromatic compounds are secondary products formed during Fischer-Tropsch synthesis, which become more abundant at higher synthesis temperature.The product from high-temperature Fischer-Tropsch synthesis therefore contains some aromatics, but the aromatics concentration is low and the aromatics are most abundant in the naphtha (around 5% of naphtha).
Although the development of FT SPK/A was conducted with material from high-temperature Fischer-Tropsch synthesis, the kerosene blend used did not employ straight run Fischer-Tropsch aromatics.Neither did it employ aromatics synthesised from the Fischer-Tropsch product.The aromatics used to produce FT SPK/A were obtained from the associated coal tar refinery (Figure 1) as light naphtha cut rich in benzene.
The primary refining step involved in the production of FT SPK/A is alkene oligomerisation with an aromatics co-feed, which enables combined alkene oligomerisation and Friedel-Crafts alkene-aromatic alkylation over the same acid catalyst.However, the need to qualify the pathway in ASTM D7566 (see Section 2) means that FT SPK/A can only be produced using coal tar as a feed material.
One may ask why Sasol did not employ aromatics produced from the Fischer-Tropsch synthesis for this purpose?The first part of the answer to this question is related to the need for aromatics as blending material for motor-gasoline. 3The second part of the answer to this question centres on benzene specifically.
Benzene is a high octane number species present in the coal tar naphtha.Due to its carcinogenic nature, the amount of benzene that is allowed in motor-gasoline is restricted by regulation.][14] On the other hand, coal tar naphtha is a coal pyrolysis product and it contains benzene.Coal tar is a source of benzene for the petrochemical industry.In the specific case of Sasol that employs moving bed coal gasifiers to produce the synthesis gas for Fischer-Tropsch synthesis, a tar refinery is co-located with the Fischer-Tropsch refinery. 3The transport fuels are blends of material refined from coal tar and Fischer-Tropsch synthesis and having a high benzene content in the naphtha for motorgasoline blending is inevitable.Due to anticipated changes in legislation that would reduce the amount of benzene allowed in the motor-gasoline, it became necessary to reduce the benzene content in the naphtha.Furthermore, the benzene content had to be reduced in a way that would not cause the octane number contribution of benzene to be lost.
F I G U R E 1 Block flow diagram of Sasol refining pathway for ASTM D7566 Annex 4 to produce synthesised paraffinic kerosene plus aromatics (FT SPK/A) using high-temperature iron-catalysed Fischer-Tropsch synthesis and hydrogenated coal tar naphtha.
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| 397
It was demonstrated in the laboratory and shown in the refinery how the benzene concentration could be reduced without losing its octane number contribution by co-feeding the hydrogenated coal tar naphtha to alkene oligomerisation unit where the benzene was alkylated to produce alkylbenzenes. 15,16In parallel, it was shown how the product from combined alkenearomatic alkylation and oligomerization could be used to produce a fully formulated synthetic jet fuel. 17At refinery level, it enabled the production of kerosene containing a mixture of SPK that formed the basis for ASTM D7566 Annex 1, but that now also contained alkylbenzenes.This in turn enabled the production of sufficient volume of kerosene with aromatics that was necessary for the ASTM D4054 qualification process of SPK/A. 18The kerosene product from the combined oligomerisation of Fischer-Tropsch derived alkenes and alkene-aromatic alkylation with aromatics from the hydrotreated coal tar naphtha (refining pathway shown in Figure 1) is the material that is described in ASTM D7566 Annex 4.
Sasol also showed how fully formulated jet fuel could be produced by blending of a hydrogenated coal tar kerosene stream and a synthesised paraffinic kerosene stream from low-temperature Fischer-Tropsch synthesis. 19The resulting kerosene blend meets the property specifications of Jet A-1, but because the hydrogenated coal tar is directly obtained from the coal tar refinery and did not pass through the oligomerisation process, it is not a refining pathway approved by ASTM D7566.

| Pathway employing aromatisation of Fischer-Tropsch light product
There are several aromatisation technologies that can be considered for the production of aromatics from Fischer-Tropsch products. 3,20The major aromatisation processes are listed in Table 2, organised based on the type of catalysts employed, rather than by specific technology suppliers.The most commonly used of these processes employ Pt on chlorinated alumina as acid catalyst support, which is the catalyst class used for catalytic naphtha reforming. 21Catalytic naphtha reforming processes are found in all petroleum refineries that produce transport fuels.
One potential strategy is to employ an aromatisation process to produce kerosene range aromatics that are suitable for direct blending to jet fuel.Producing sufficient kerosene range aromatics in this way proves to be difficult, because aromatisation processes (see Table 2) are characterised by operating conditions leading mainly to the production of benzene, toluene, and xylenes (BTX).The BTX aromatics are C 6 -C 8 aromatics and are in the naphtha boiling range, not kerosene.Some C 9 and heavier kerosene range aromatics are co-produced, but these heavier aromatics are not major products.
Considering the availability of several aromatisation technologies to produce BTX aromatics, it is just easier to employ an aromatisation process to produce naphtha range aromatics first.Those naphtha range aromatics can then be alkylated to produce kerosene range aromatics.The same conversion chemistry and catalysis involved in the production of SPK/A can be practised by using aromatics that are obtained by aromatisation of Fischer-Tropsch products, instead of aromatics obtained from coal pyrolysis.
When the primary aim is to produce synthetic jet fuel, while keeping the production of other final fuels in mind, it is claimed that the least complex Fischer-Tropsch fuels refinery design that would enable >60 wt% of the product to be refined to jet fuel, requires five conversion units. 22The core refining units that are needed to make on-specification motorgasoline, jet fuel, and diesel fuel are hydrocracking, oligomerisation, aromatisation, alkene-aromatic alkylation, and hydrotreating.It was pointed out that of these units, the oligomerization and alkene-aromatic alkylation units could be combined in a single conversion unit, 22 thereby reducing the requirement to only four conversion units (Figure 2).The block flow diagram shown in Figure 2 is generic and can be used in conjunction with any Fischer-Tropsch technology and any aromatisation technology (Table 2).However, it stands to reason that the distribution of flow rates and composition of the individual streams will be different for different Fischer-Tropsch technologies.The technology selection for each of the blocks will affect how the refinery can be operated to maximise the production of fully formulated kerosene without undermining the quality of the other transport fuels.
In the context of SAF, new Fischer-Tropsch based facilities may not necessarily aim to produce a full suite of transport fuels.In terms of the refinery design, it is less onerous when it is not necessary to produce all liquid fuels as on-specification transport fuels.At the same time, if it is necessary to produce fully formulated jet fuel from Fischer-Tropsch products then aromatics would have to be co-produced.Importantly, by doing so, it would avoid the need for an external source of aromatics, such as coal tar naphtha (see Section 3.1).
The generic design shown in Figure 2 has the benefit of enabling many different combinations of technologies without changing the description of the refining pathway.It shows how a fully formulated jet fuel that is equivalent to SPK/A can be produced using only Fischer-Tropsch derived materials.Thus, it could form the basis for modifying the existing ASTM D7566 Annex 4 to enable the production of SPK/A without being unnecessarily restrictive about the origin of the aromatics.Thereby it would meet the requirement of a refining pathway is generally useful for producing a fully formulated jet fuel.

| Pathway employing catalytic cracking of Fischer-Tropsch heavy product
Catalytic cracking in current refinery practice mostly employs fluid catalytic cracking (FCC) technologies. 23he most common FCC catalysts are amorphous silicaalumina and variants of H-Y (FAU zeolite framework type).When more aromatics are required, these cracking catalysts are used in combination with H-ZSM-5 (MFI zeolite framework type).In the extreme case of deep catalytic cracking, the concentration of aromatics in the naphtha can approach half and the overall yield of these aromatics from a typical petroleum feed is around 13 wt%. 24he yield and composition of Fischer-Tropsch heavy products from high-temperature and low-temperature synthesis are quite different.The carbon yield of atmospheric residue (>360°C boiling material) from high-temperature Fischer-Tropsch synthesis is only around 3 wt%, 10 and conversion of this material to kerosene by cracking is only of academic interest.6][27][28] Over zeolite catalysts, wax FCC is characterised by high wax conversion, low cokemake, and about half of the product is naphtha.The concentration of aromatics in the naphtha depends on both the catalyst and the operating conditions, but is roughly 10%-15%. 25,28It is therefore possible to make use | 399 of FCC to convert the Fischer-Tropsch wax to a product that contains aromatics and that could be used to produce fully formulated jet fuel.A generic refinery design for such a process is shown in Figure 3.
The stoichiometry of hydrogen disproportionation during catalytic cracking of Fischer-Tropsch wax, inherently limits the alkene and aromatic content of the FCC product.Acid catalysts are poor at desorbing molecular hydrogen (H 2 ) and instead, acid catalysts transfer the hydrogen to alkenes to produce alkanes.Alkanes with a lower boiling point temperature than the boiling range of kerosene cannot readily be converted to additional aromatics using the design shown in Figure 3.It highlights the value of aromatisation using a catalyst that is capable of desorbing H 2 (top three rows in Table 2).
One potential modification of the cracking process involves the use of a metal-and-acid catalyst capable of aromatisation.In essence this is a bifunctional hydrocracking catalyst, but employed in a way to combine aromatisation and catalytic cracking.When a bifunctional hydrocracking catalyst is operated at a lower hydrogen partial pressure than used in conventional hydrocracking processes, aromatics can be co-produced.It was claimed that such catalysts, reduced Ni on H-ZSM-5 in particular, were useful for converting Fischer-Tropsch products for the production of synthetic jet fuel. 29,30The operating range indicated was 290-425°C, 0.8-5.5 MPa absolute, and liquid hourly space velocity 0.5-5 h −1 .It was also noted that the Fischer-Tropsch feed material had to be hydrotreated before it was employed as feed to the Ni/H-ZSM-5 catalysed cracking process.With slight modification to Figure 2, one can see how the hydrocracker can be operated under conditions that would also produce aromatics.Similarly, one could substitute the catalytic cracker in Figure 3 with a hydrocracker operated at lower pressure, using a hydrocracking process such as claimed in the patents by Milstein and Stein. 29,30he generic design shown in Figure 3, like that in Figure 2, can be envisioned with different combinations of technologies without changing the description of the refining pathway.Both refining pathways can be described in terms of the production of SPK/A derived from Fischer-Tropsch products, meeting the requirement of a refining pathway is generally useful for production of fully formulated jet fuel.

| Greenfield global pathway
Further intensification of the concepts previously described is possible.The intensified refinery design is shown in Figure 4. 31 The combined oligomerisation, aromatisation, and alkene-aromatic alkylation process does not require an aromatic co-feed.Instead of co-feeding aromatics that were produced externally to the Fischer-Tropsch refinery (Figure 1) or co-feeding of aromatics that were produced in a different unit in the Fischer-Tropsch refinery (Figures 2 and 3), the aromatics are produced within the same unit that also performs the oligomerisation and alkylation.
The combined oligomerisation, aromatisation, and alkene-aromatic alkylation process exploits three properties of acidic silica-alumina catalysts, exemplified by H-ZSM-5, that intensifies and simplifies the design simultaneously.(i) It is capable of converting oxygenates that are present in straight run Fischer-Tropsch materials. 3,32ii) It is capable of performing the conversion of light F I G U R E 3 Block flow diagram of Fischer-Tropsch refinery design using catalytic cracking to convert the heavier product to produce aromatics for fully formulated synthetic jet fuel.The cut-point for heavier product, C 15 and heavier, can be also be increased to produce more gas oil for diesel fuel.
alkenes in the presence of unconverted synthesis gas. 33iii) It is capable of producing aromatics. 34he hydrocracking process is also modified with respect to typical wax hydrocracking operation.It is known that wax hydrocracking can be performed at low H 2 partial pressure, with stable operation demonstrated at 3.5 MPa. 35It is also known that wax hydrocracking can be performed with high conversion at milder conditions, 324-372°C and 3.5-7 MPa, compared to conventional petroleum hydrocracking, 350-430 and 10-20 MPa. 36or the process shown in Figure 4, the intent is to operate at even lower pressure to exploit the unique metal catalysis of Pt in a Pt on acidic silica-alumina hydrocracking catalyst. 37,38Although the intended change in hydrocracker operation does not affect the description of the refining pathway, the kerosene range product from lower pressure operation includes a higher concentration of cycloalkanes in the kerosene.
The benefit of having cycloalkanes for the formulation of jet fuel is known, 39 and cycloalkanes are particularly useful in the formulation of synthetic jet fuel. 3The benefit is related to the higher density of cycloalkanes compared to isoalkanes, without degrading properties such as low freezing point, high smoke point, and thermal stability.By increasing the cycloalkane content in the hydrocracked and hydroisomerised kerosene, it is possible to produce isoparaffinic kerosene with higher density than the density that is usually obtainable by FT SPK.The benefit of this in the context of fully formulated synthetic jet fuel is that the amount of aromatics needed is less.The concentration of aromatics can be closer to the lower limit of minimum 8 vol% aromatics, while still meeting the minimum density specification of 775 kg/m 3 at 15°C.
Despite the specific advantages of the technology choices that were described in relation to Figure 4, the refining pathway can be more generally applied.The only caveat is that not all acidic catalysts that are able to perform alkene oligomerisation and alkenearomatic alkylation are capable of also performing aromatisation.

| CASE STUDY
The objective with the case study is to show how both semi-synthetic and fully synthetic jet fuels can be produced using only materials derived from Fischer-Tropsch synthesis.The refining pathway in Figure 4 formed the basis for the case study.This pathway was chosen over the other pathways presented, because it is a less complex refinery design and it avoids processes operating >400°C, such as aromatisation and fluid catalytic cracking.It was also the pathway employed for previous case studies on front-end design decisions affecting production of SAF. 2 Packed bed flow reactors operated at kg/day scale were employed to represent the different refinery conversion units.The reactors and how they were operated, were analogous to that reported in previous work. 33,37The products were batch distilled in narrow cut fractions before the fractions were blended to produce the kerosene blends that were characterised.The hydrotreated petroleum derived kerosene was similarly fractionated and characterised.The prepared kerosene blends were blended conservatively with respect to distillation range.The initial and final boiling points of the kerosene blends were not at the limit of specifications such as flash point, density, or viscosity and there was no blending optimisation in an attempt to maximise the amount of material that could be included in the kerosene.The kerosene blends characterised and reported in this case study serve as illustrations of the physical and compositional nature of the material produced by the refining pathway in Figure 4.

| Refinery operating modes
In refining practice, it is possible to change the operation of the refinery to increase or decrease the relative amount of each fuel type that is being produced, which is a decision that is based on the prevailing economic situation. 23This type of flexibility in a refinery design is valuable.In a | 401 narrower sense it was anticipated that the production of SAF would also require flexibility in terms of the type of kerosene blend components that are produced.
It is hoped that in future the ASTM D7566 will change to enable fully formulated synthetic jet fuel to be designated Jet A-1, as is already possible under DEF STAN 91-091.Until that happens, it is a business risk to develop a process that can only produce kerosene as a product that is not covered by the process descriptions in the ASTM D7566 annexes.Conversely, in anticipation of future amendments to the ASTM D7566, it would be beneficial to have a refinery design capable of producing fully formulated kerosene for jet fuel.The refinery design shown in Figure 4 was developed with the flexibility to operate in two different operating modes for the aforementioned reasons.
The first operating mode is illustrated in Figure 5 and it produces only FT SPK as described in ASTM D7566 Annex 1.In this mode, the oligomerisation process is operated at a lower temperature, which causes it to perform alkene oligomerisation only.This operating mode can produce only semi-synthetic jet fuel and requires blending with petroleum derived kerosene to produce on-specification Jet A-1.
The second operating mode is illustrated in Figure 6 and it produces FT SPK as described in ASTM D7566 Annex 1, as well as FT SPK/A that is compositionally comparable to kerosene as described in ASTM D7566 Annex 4. In this mode, the oligomerisation process is operated at a higher temperature as described in Section 3.4 to enable aromatisation.As aromatics are formed, alkene-aromatic alkylation also takes place.This operating mode enables the production of fully formulated synthetic jet fuel.

| Semi-synthetic jet fuel
The first mode of operation that is shown in Figure 5 formed the baseline for this study.It was important to demonstrate that the refining pathway could be used in a way that produced kerosene compliant with ASTM D7566 Annex 1 and that it could be blended with petroleum derived kerosene to meet Jet A-1 specification requirements.
The two FT SPK kerosenes that were produced by oligomerisation followed by hydrotreating and produced by hydrocracking, were of comparable composition and physical properties.The nature of the FT SPK was such that it comprised mainly of n-alkanes and isoalkanes, with the amount of cycloalkanes being low.As a consequence, the density of the kerosene was low compared to that of petroleum derived kerosene (Figure 7), because cycloalkanes are the only paraffinic compound class that can increase the density of FT SPK.It was clear that both FT SPK products benefit from blending with petroleum derived kerosene to fall within the density range of jet fuel.
The low temperature flow properties of FT SPK obtained from wax hydrocracking was investigated and compared to petroleum derived kerosene. 40For the materials used in this case study, it was found that the low temperature viscosity of the FT SPK was lower than that of the petroleum derived kerosene.
A semi-synthetic jet fuel blend was prepared using FT SPK from oligomerisation, FT SPK from hydrocracking, and petroleum derived kerosene as shown in Figure 5, but no hydrotreated straight run kerosene.The blended product contained the maximum amount of synthesised paraffinic kerosene allowed by ASTM D7566 Annex 1.The blend consisted of 25 vol% of the 160-260°C distillation fraction of the hydrotreated oligomerization product, 25 vol% of the 160-240°C distillation fraction of the hydrocracked product, and 50 vol% of the commercially obtained petroleum derived kerosene, which had a boiling range of 140-260°C.
F I G U R E 5 Operating mode 1 of refining pathway in Figure 4 to produce FT SPK for blending with petroleum derived kerosene to produce semi-synthetic jet fuel.The unit at the top of the block flow diagram is operated at sufficiently low temperature to perform mainly alkene oligomerisation.
F I G U R E 6 Operating mode 2 of refining pathway in Figure 4 to produce FT SPK/A and FT SPK for blending to produce a fully formulated synthetic jet fuel.The unit at the top of the block flow diagram is operated at sufficiently high temperature for combined alkene oligomerisation, aromatisation, and alkene-aromatic alkylation.
This semi-synthetic jet fuel blend was submitted for characterisation to an external laboratory that was accredited for transport fuel quality control testing.The electric conductivity of the kerosene was 11 pS/m at 20°C and 1 mg/L static dissipater was added to the kerosene before testing.The static dissipater used, Stadis® 450, is an additive certified for this use. 5,41The results from the characterisation of the semi-synthetic jet fuel blend with static dissipater are shown in Table 3.
The semi-synthetic jet fuel blend was 50:50 vol/vol blend with petroleum derived kerosene and the process steps employed in its production conformed to the description in the ASTM D7566.The results in Table 3 demonstrated that the material from the first operating mode shown in Figure 5 was capable of producing a semi-synthetic jet fuel that met the specification criteria for re-designation as Jet A-1 and could be re-designated as Jet A-1 under the current specification.

| Fully formulated synthetic jet fuel
In the second mode of operation, which is shown in Figure 6, there is no petroleum derived kerosene to blend.In this operating mode, the hydrocracker is still producing FT SPK.The FT SPK/A that is produced by the combined oligomerisation, aromatisation, and alkene-aromatic alkylation process must have sufficient aromatics and density to enable blending to a kerosene product that will have the properties that are anticipated for Jet A-1.Although some hydrotreated straight run kerosene could also be blended into this product, this was not done in the present case study.
For the purpose of the case study, the blending ratio was 40 wt% (38 vol%) of the 160-240°C fraction of the FT SPK/A and 60 wt% (62 vol%) of the 160-240°C fraction of the FT SPK.In this blend, the FT SPK/A exceeded the cycloalkane content requirement of ASTM D7566 Annex 4 as an individual synthetic paraffinic kerosene blend component.Nevertheless, the aim was to produce a kerosene blend that would be a fully formulated synthetic jet fuel.
This fully synthetic jet fuel blend was submitted for characterisation to an external laboratory that was accredited for transport fuel quality control testing.The neat conductivity was 10 pS/m at 20°C and 1 mg/L static dissipater had to be added.The results from the characterisation of the fully formulated synthetic jet fuel blend with static dissipater are shown in Table 4.
The comparison with Jet A-1 specifications does not imply that the fully synthetic jet fuel blend can at the time of writing be re-designated and qualified as a Jet A-1, because it cannot.The purpose of the comparison with Jet A-1 is to indicate how well the properties of the fully synthetic jet fuel blend compare with the property requirements that will ultimately have to be met once there is a way to re-designate this material as Jet A-1.
The fully synthetic jet fuel blend was within the specification range for all of the characteristics of a Jet A-1 reported in Table 4.There are two shortcomings of the work presented for the case study that should be noted.Due to the limited capacity scale at which the kerosene products were produced, there was insufficient product for the thermal stability test.Additionally, the distillation range of the fully formulated kerosene was narrow and with hindsight, a broader distillation range should have been used for the blend.
The results in Table 4 nevertheless demonstrated that it is possible to produce a fully formulated synthetic jet fuel using the refining pathway shown in Figure 6.

| Prescreening in preparation for ASTM D4054
Prescreening 42 was performed as the first step toward the qualification of the kerosene product.This is a useful first step for any candidate kerosene that is produced by a refining pathway that is not described in one of the ASTM D7566 annexes, and before full ASTM D4054 qualification starts.The benefit of this approach is that only a small amount of material is required for Note: The Jet A-1 requirements are provided as point of reference only and without implying that the test fuel was re-designated as Jet A-1.
Abbreviation: ASTM, American Society for Testing and Materials.a Value used in combination with naphthalene content.
b Insufficient kerosene to complete this test.
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| 405 prescreening, which is normally all that is available when a process is still at the early stages of development.
As part of the prescreening, the kerosene composition was determined using two-dimensional gas chromatography. 43This technique enables the determination of compound class distribution by carbon number.A summary of the compound class composition of the fully formulated synthetic jet fuel from the refining pathway in Figure 6 is given in Table 5.To facilitate comparison, FT SPK/A from Sasol using the refining pathway in Figure 1 and petroleum derived kerosene from a Merox™ sweetening process are also given in Table 5. 18 T A B L E 5 Compound class composition of fully formulated synthetic jet fuel in the case study, compared to FT SPK/A from Sasol and an example of sweetened petroleum derived kerosene It is noteworthy that the binuclear aromatic content of both synthetic kerosenes was below the quantification limit in the analysis presented in Table 5.In terms of the cycloalkanes and mononuclear aromatics concentration, the kerosene prepared for the case study was closer in composition to the petroleum derived kerosene than that of the Sasol FT SPK/A.The differences are numeric, rather than meaningful in terms of the compositional variance for use as jet fuel.There is naturally large compositional variance within the compound classes for petroleum derived Jet A-1. 9 The distribution of the different compound classes with carbon number, as surrogate for boiling point, is given in Figure 8.The distribution shows that the compound classes are distributed over the distillation range of the kerosene and that the refining pathway did not lead to a product with an overly high concentration of any specific species.
Additional characterisation performed as part of the prescreening is presented in Table 6.As expected, these results corroborated the characterisation shown in Table 4, but also included additional property measures.Notably the analyses included the low temperature viscosity and the kinematic viscosity at −40°C was 8.2 mm 2 /s (cSt), which was below the 12 mm 2 /s maximum.

| DISCUSSION
Aviation is a global business.This means that aviation turbine fuels across the globe must conform to the same specification standards.Although country specific fuel specifications may be contained in local standards, those standards refer to leading specifications, such as ASTM D1655 and DEF STAN 91-091. 6,7Although the Jet A-1 specifications are similar, the standards deal differently with jet fuel that contains 'synthesised' hydrocarbons, which refers to material that was not obtained by conventional petroleum refining.
In this respect, there is a notable difference between the standards when dealing with kerosene containing 'synthesised' hydrocarbons.The DEF STAN 91-091 imposed additional fuel specifications that must be met.These are an aromatics content in the range 8-25 vol%, wear scar diameter <0.85 mm, thermal stability test temperature of 325°C (as opposed to 260°C) and minimum distillation slopes of T50-T10 ≥ 10°C and T90-T10 ≥ 40°C.Apart from these additional requirements the kerosene can be used as fully synthetic jet fuel as long as it meets the requirements for Jet A-1.Conversely, before kerosene that contains 'synthesised' hydrocarbons is eligible to be considered under ASTM D1655, it first has to be re-designated as petroleumequivalent kerosene by complying with ASTM D7566.Importantly, ASTM D7566 is sensitive to the origin of the 'synthesised' hydrocarbons and the refining pathway employed.
This means that SAF is not yet a fuel type that has a globally accepted pathway for production and use, unless it is blended with petroleum derived kerosene.Differently put, despite the societal desire for aviation turbine fuel that is not produced from fossil fuels, at the time of writing SAF on its own is disqualified from commercial use in parts of the world.In terms of the aspirations for SAF producers that are interested in producing Jet A-1 with no added fossil fuels, there are important implications following from the qualification pathway discussed in Section 2.
The processing pathways for the transformation of nonpetroleum raw materials into liquid products are broadly classified into two groups, direct liquefaction and indirect liquefaction. 44Of relevance to the present discussion is that the liquid products from direct liquefaction will retain some of their original raw material identity.Thus, one cannot expect to obtain comparable liquid products from the pyrolysis of woody biomass, 45 and the pyrolysis of waste polyethylene plastic. 46Even when the liquid products obtained from different raw materials are subjected to hydroprocessing, the refined products after hydroprocessing could be compositionally quite different.The implication is that when different raw material types are employed as feed materials to a direct liquefaction process and subsequent refining, each of the kerosene products would have to be qualified separately following ASTM D4054.This is illustrated by looking at the ASTM D7566 annexes, where kerosenes from different sources that are all hydroprocessed materials are compositionally different (Table 1) and each require separate qualification.
When an indirect liquefaction process is employed, the raw material is converted to synthesis gas and it loses all of its original molecular identity.The synthesis process determines the molecular identity of the feed material to the refinery.For this purpose the products from Fischer-Tropsch synthesis would be considered different from methanol synthesis, but the variations within the product composition of different Fischer-Tropsch technologies are considered sufficiently minor that iron-based and cobalt-based Fischer-Tropsch processes are viewed as equivalent by ASTM D7566. 5lthough it may still be necessary to evaluate the products from the different refining pathways described in Section 3 to enable inclusion in ASTM D7566, the properties of the FT SPK/A products produced using Figures 1-4 are likely to be compositionally comparable.
The goal of producing SAF that is a fully formulated synthetic jet fuel via Fischer-Tropsch synthesis using the refining pathways in Figures 2-4 are technically attainable.Although the case study presented in Section 4 employed only the refining pathway in Figure 4, it was de KLERK ET AL.
| 407 demonstrated that the kerosene product was fully formulated and compositionally comparable to FT SPK/ A in ASTM D7566 Annex 4.
Despite differences in the choice of refining process to produce aromatics that may be subjected to alkenearomatic alkylation for the production of the FT SPK/A (see Table 2), on a molecular level the aromatics are indistinguishable between different processes.For example, toluene from conventional catalytic naphtha reforming, or fluid catalytic cracking, or aromatisation of alkenes is still the same species, toluene.
The work showed that it is possible to produce kerosene from the refining of only Fischer-Tropschderived material that is a fully formulated synthetic jet fuel.The work further showed that several generally useful refining pathways could be considered to achieve this objective.There are consequently several different ways to produce SAF via Fischer-Tropsch synthesis.The compositional similarity of FT SPK/A products from different refining pathways and petroleum-derived Jet A-1 were indicated in Tables 4 and 5.There is the hope that in future all of the FT SPK/A materials produced by Figures 1-4 could be incorporated in ASTM D7566 and qualified for re-designation as Jet A-1 as 100% synthesised kerosene, thereby avoiding the need to be blended with petroleum derived kerosene.

| CONCLUSIONS
The work set out to determine how SAF could be produced as a fully formulated synthetic jet fuel with no need for blending with petroleum derived kerosene.For this purpose, a process pathway via Fischer-Tropsch synthesis was selected.The following conclusions were drawn: 1. Fully formulated jet fuel comprises four compound classes, namely, n-alkanes, isoalkanes, cycloalkanes, and aromatics.Straight run material from Fischer-Tropsch synthesis is deficient in three of these four compound classes.To produce fully formulated kerosene that is suitable for use as jet fuel, the Fischer-Tropsch refinery must produce isoalkanes, cycloalkanes, and aromatics.2. Different refining pathways were shown to illustrate how a Fischer-Tropsch refinery could produce fully formulated kerosene.The refining pathways have the desirable attribute of being generally useful and not limited to a specific refining technology.The type of kerosene produced by all of these refining pathways fits the compositional description of Fischer-Tropsch synthesised paraffinic kerosene plus aromatics (FT SPK/A).
3. Through a case study it was shown how FT SPK/A was prepared and characterised.The FT SPK/A met the specification requirements for Jet A-1, illustrating that it was practical to produce a fully formulated jet fuel via Fischer-Tropsch refining.4. At present it is possible to qualify FT SPK/A on its own as Jet A-1 in jurisdictions using the DEF STAN 91-091 specification.No equivalent qualification pathway exists in jurisdictions using ASTM D1655, which requires kerosene containing synthesised hydrocarbons to be re-designated as Jet A-1 by ASTM D7566.ASTM D7566 does not yet enable the qualification of fully synthetic kerosene, although ASTM D7566 includes annexes describing kerosene that is fully formulated.

F
I G U R E 2 Block flow diagram of Fischer-Tropsch refinery design focused on producing fully formulated kerosene for synthetic jet fuel by aromatisation of part of the light product.Use of oxygenates is indicated by a dashed line, because only some catalysts can co-process oxygenates.deKLERK ET AL.

F I G U R E 4
Block flow diagram of Greenfield Global refining pathway to produce fully formulated synthetic jet fuel.The cut-points for the straight run Fischer-Tropsch materials can be changed to facilitate different refinery operating modes.deKLERK ET AL.
Major compound classes in jet fuel and approved synthetic jet fuel blend components Aromatisation processes that could be considered for producing aromatics from Fischer-Tropsch products a Noncatalytic, which is no longer considered competitive with catalytic processes.
Density of kerosene range distillation fractions with respect to average atmospheric equivalent boiling point temperature de KLERK ET AL.Properties of semi-synthetic jet fuel containing 50 vol% FT SPK from oligomerisation and hydrocracking Abbreviation: ASTM, American Society for Testing and Materials.a IBP, initial boiling point; Tnn, boiling point at which nn vol% was distilled; FBP, final boiling point.b Value used in combination with naphthalene content.c Test temperature of 260°C.
Properties of fully formulated synthetic jet fuel produced from FT SPK/A and FT SPK