Ft Diesel,Past Present New Print

Diesel Production from Fischer-Tropsch: The Past, the Present,and New Concepts

Dieter Leckel*

Fischer-Tropsch Refinery Catalysis, Sasol Technology Research and DeVelopment, P.O. Box 1,Sasolburg 1947, South Africa

ReceiVed January 22, 2009. ReVised Manuscript ReceiVed March 11, 2009

Fischer-Tropsch synthesis is technically classified into two categories, the high-temperature Fischer-Tropsch(HTFT) and the low-temperature Fischer-Tropsch (LTFT) processes. The criterion for this classification isthe operating temperature of the synthesis, which ranges between 310-340 °C for the HTFT process and210-260 °C for the LTFT process. A Fischer-Tropsch facility can be divided into roughly three sections,synthesis gas (syngas) generation, FT synthesis, and refining of the synthetic crude (syncrude). Fischer-Tropschrefineries differ regarding the product upgrading, and both transportation fuels and chemicals can be produced.Regarding the FT refinery history, the configuration of each refinery also reflects the requirements of the fuelspecification at that time. This paper gives a condensed overview of how Fischer-Tropsch facilities changedduring the last 70 years and focuses in particular on the diesel fuel produced. Some conceptual flow schemesare additionally presented with emphasis on the combined upgrading of the high boiling part of the FT productspectrum with liquids derived from coal pyrolysis.

1. Introduction

Commercialization of the Fischer-Tropsch (FT) technologybegan in 1934 when Ruhrchemie A.-G. undertook the industrialdevelopment of the Fischer-Tropsch process. Laboratory, pilot-plant, and semicommercial investigations were followed by theconstruction of several commercial plants that used coal assyngas source and applying the low-temperature FT (LTFT)technology. The first Fischer-Tropsch industrial plant went intooperation in Germany in 1936, and in the 1940s over 1 milliontons of FT liquids were produced yearly.1 A typical processscheme of those plants operating in Germany during the years1939-1945 are presented together with representative dieselfuel characteristics of those facilities. Diesel oil was a byproductof those plants, and high cetane numbers (>80) but low densitywere reported.1,2

The Carthage Hydrocol Fischer-Tropsch plant was anexample of an earlier iron-based high-temperature (HTFT)process developed in the United States of America in the1940-1950s.3,4 The processing scheme was laid out to maxi-mize gasoline production based on natural gas, and thehydrogenated distillate byproduct had compared to a typicalcrude-derived distillate high cetane numbers (above 70).

Construction of the first coal-based Sasol FT plant (Sasol 1)at Sasolburg, South Africa, began in 1952, and the facility washighly complex, combining the two variants of the Fischer-Tropsch process.5 The Sasol 1 refinery consisted of four differentrefineries that were grouped together, namely, tar workup, HTFToil workup, LTFT oil workup, and chemical workup. The design

had very little refinery integration, apart from a common aqueousproduct workup and a transfer of the C3-C4 fraction from theLTFT oil workup to the HTFT oil workup section. The benefitof the integration was more apparent in the gas loop and theblending opportunities of the unique characteristics of both typesof Fischer-Tropsch products. The fuel specification during thoseyears were not very demanding and the distillate producedneeded little refining. The Sasol 1 plant design changed duringthe years, and the last change occurred in 2004 when coalgasification was replaced by natural gas reforming, transformingthe site essentially into a gas-to-liquids (GTL) plant producingwaxes and chemicals.

The formation of OPEC and the oil crisis in the early 1970sprompted Sasol to construct another coal-based HTFT plant atSecunda, South Africa (Sasol 2); of which construction startedin 1976.6 However, due to increases in oil price two plants werebuilt, with Sasol 2 coming on stream in 1980 and the Sasol 3plant being commissioned in 1983. The design of the Sasol 2and Sasol 3 refineries were similar, except for the addition of ahigh pressure distillate hydrogenation section to the tar refineryof Sasol 3, which processed gasification-derived coal pyrolysisliquids from both facilities. The original HTFT circulatingfluidized bed (CFB) synthesis reactors installed at Secunda wereimproved and scaled-up versions of those operating at Sasol 1,Sasolburg. These reactors were replaced in the late 1990s bySasol Advanced Synthol (SAS) reactors. The Sasol 2 and 3plants each consisted of four different refineries, namely tarworkup, condensate workup, oil workup, and chemical workup.The two HTFT plants are, to date, so interlinked that the SasolSecunda facility was renamed Sasol Synfuels. The facilityproduces 160 000 bpd of products that are mainly motor-gasoline and diesel, but also includes chemicals.

Another FT facility that is based on the Sasol iron-catalyzedHTFT Synthol technology is operated by PetroSA at Mossel

* Phone: +27 16 960-3830. Fax: +27 11 522-3975. E-mail: [email protected].

(1) Underwood, A. J. V. Ind. Eng. Chem 1940, 32 (4), 449–454.(2) Weil, B. H.; Lane, J. C. The Technology of the Fischer-Tropsch

Process; Constable: London, 1949.(3) Schulz, H. Appl. Catal., A 1999, 186, 3–12.(4) Keith, P. C. Oil Gas J. 1946, 102–112.(5) Dry, M. E. Catal. Today 2002, 71, 227–241. (6) Steynberg, A. Stud. Surf. Sci. Catal. 2004, 152, 1.

Energy & Fuels 2009, 23, 2342–23582342

10.1021/ef900064c CCC: $40.75 2009 American Chemical SocietyPublished on Web 04/16/2009

Bay, South Africa, since 1993.7 This plant is supplied withnatural gas and integrates the refining of natural gas liquids withthat of the synthetic hydrocarbons from the HTFT synthesis.The process scheme is primarily designed to produce gasolineand smaller amounts of distillates.

The first commercial GTL facility based on cobalt-catalyzedFT synthesis started production in 1993 in Bintulu, Malaysia.On the basis of the Shell middle distillate synthesis (SMDS)technology, this plant typically produces a hydrocarbon spectrumthat includes naphtha, kerosene, and distillates.8 Paraffin andwax production is optional.

The Sasol Oryx GTL plant using Sasol proprietary cobalt-catalyst-based LTFT technology was commissioned in 2006 inLas Raffan, Qatar.9 This technology uses the Sasol slurry phasedistillate (Sasol SPD) process to produce mainly diesel fuel andnaphtha as byproduct. First products were produced in 2007.10

The products from the 34 000 bpd Oryx GTL plant are similarto those from the SMDS process, since both use a similarrefining concept to process a similar FT syncrude. The largestGTL facility (named Pearl) is being constructed by Shell in LasRaffan, Qatar, and will be based on the SMDS process usingcobalt-based fixed-bed LTFT technology to produce mainlydistillate and base oils. Pearl will produce 140 000 barrels aday (bpd) of GTL products and 120 000 boe/d of condensate,liquefied petroleum gas and ethane. An annual output of 90 mbarrel of combined products is estimated.11

In the following paragraphs the flow schemes of the above-mentioned facilities are presented and, particularly, the distillateproduction and characteristics are discussed. Specific issuesregarding conceptual Fischer-Tropsch fuels refining schemes12

for maximum diesel fuel production are considered; finally,refinery concepts are presented that specifically address thecombined distillate refining of FT heavy ends and coal pyrolysistar oil heavy ends.

2. Cobalt-based Low-Temperature Fischer-Tropsch(LTFT) Plants in Germany, 1935-1945

During World War II, Germany operated 12 high-pressurecoal hydrogenation plants together with 9 LTFT plants toproduce 4.5 million tons per annum of motor fuels, 90% ofGermany’s consumption at that time (Figure 1). The FT dieselwas obtained by distillation of the light hydrocarbons from theFT reactors without further hydroprocessing, and as a conse-quence it contained olefins and oxygenates. However, the FTprocess was at that time of secondary importance for the Germanfuel economy, producing 9.1% of the total German oil supply.13

The FT gasoline was of poor quality, which could not competewith gasoline derived from coal pyrolysis or coal hydrogenationprocesses. The outstanding good ignition and sulfur-free qualityof the FT diesel oil was, in the German point of view, not

incentive enough to develop an engine that could fully exploitthe outstanding 90-100 cetane number.

The diesel oil from the FT plants was used to upgrade thediesel fraction from coal tar with cetane numbers of 10-15 todiesel oil having cetane numbers ranging from 40-50. Theproduct quality of a straight-run distillate produced by a cobalt-based FT catalyst is presented in Table 1.14 The diesel fuel wascomposed largely of linear paraffins (88%), about 10% olefins,and 2% polar compounds. The high cetane number of 80 is theresult of the high n-paraffin content of the fuel. The polarfraction contained mainly oxygenates such as carbonyls, alcoholsand acids. Table 2 lists German WWII diesel fuel specifications

(7) (a) Terblanche, K. Hydroc. Eng. 1997, 2 (2), 2–4. (b) Steyn, C. Therole of Mossgas in Southern Africa. 2nd Subsaharan Africa Catal Symp.;Swakopmund, Namibia, 2001.

(8) (a) Sie, S. T. ReV. Chem. Eng 1998, 14 (2), 109. (b) Tim, P. J. A.;Van Wechem, H. M. H.; Senden, M. M. G. The Shell Middle DistillateSynthesis project - new opportunities for marketing natural gas; AlternateEnergy ’93 Conference, Colorado Springs, USA, April 27-30, 1993.

(9) Daya, A. In vogue. Pet. Economist 2006, 73 (4), 27.(10) Anon, Oil Gas J 2007, 105 (5), 10.(11) Pearl GTL- a gem of a project, so long as it works. Pet. Economist;

April 8-11, 2008.(12) De Klerk, A. Fischer-Tropsch Refining. Ph.D. Thesis, Dep. Chem.

Eng., University of Pretoria, South Africa, February, 2008.(13) U.S. DOD. U.S. Naval Technical Mission in Europe Technical

Report No. 248-45. The synthesis of hydrocarbons and chemicals fromCO and H2 1945, p 8.

(14) Dancuart, L. P.; de Haan, R.; de Klerk, A. Processing of PrimaryFischer-Tropsch Products. In Studies in Surface Science and Catalysis 152;Steynberg, A., Dry, M. Eds.; Elsevier B.V: 2004; p 515.

(15) Ward, C. C.; Schwartz, F. G.; Adams, N. G. Ind. Eng. Chem 1951,43 (5), 1117–1119.

(16) U.S. Navy Technical Mission in Europe, Technical Report No.187-45, German Diesel Fuels, August 1945, p 3.

Figure 1. Typical process scheme of the FT plants in 1939-1945Germany.13

Table 1. Quality of a World War II German FT Diesel14

property units value

density @ 20 °C kg m-3 768.1cetane No. 80cloud point °C 0flash point °C 78bromine no. g Br2 /100 g 6.9aniline point °C 86.5H wt % 15.0C wt % 84.9O wt % 0.14

ASTM D-158 distillation °CIBP 19310% 21850% 24890% 291FBP 311

Table 2. German WWII Diesel Fuel Specifications16

specification

Wehrmachtdiesel

Kraftstoff

Sonderdiesel

KraftstoffLuftwaffe

K1Kriegsmarine

Treibstoff

density @ 15 °C, kg m-3 810-865 810-865 810-865 880 maxcetane No., min 45 45 50 nonecloud point, °C -10 -10pour point, °C, max -30 -30 -45flash point, °C 55 21 50 55sulfur, wt % 1.0 1.0 1.0 1.0

ReViews Energy & Fuels, Vol. 23, 2009 2343

that were achieved by blending fuel oil fractions obtained fromthe hydrogenation of coal, coke-oven tar or brown-coal tar oils.15

3. Iron-based High-Temperature Fischer-Tropsch(HTFT) Plants (USA)

Hydrocarbon Research Inc. developed a HTFT process thatwas operated during the years 1951-1957, known as theHydrocol process, which produced hydrocarbons mainly boilingin the petrol range.3,4 The octane rating of the straight-rungasoline was a RON of 62 with a MON of 68.17 By-productswere high-grade diesel oil and oxygenated compounds such asalcohols. A 5000 bpsd FT plant operated briefly in Brownsville,Texas. The processing scheme used natural gas, and thecondensable natural gas was directly blended with the FT-derived material (Figure 2). The fixed fluidized bed (FFB)reactors operated at about 2.1-4.5 MPa and 305-345 °C,meaning the process applied was a HTFT process.

The distillate fraction was not upgraded any further andreportedly18 had a cetane number of 45-50 and a pour point ofless than -9 °C.

The products from the iron-based HTFT Hydrocol processconsisted mainly of hydrocarbons boiling in the C5 to 204 °C(naphtha) boiling range (Table 3).17 The hydrocarbons were richin olefins, specifically R-olefins, while lower amounts of oxygenatesand aromatics were also part of the hydrocarbon products.

Distillates from the HTFT Fluidized-bed HydrocolProcess. The hydrogenated liquid products were separated intonaphtha, diesel fuel, and a bottoms product. The cut points wereselected to give maximum diesel yield with a flash point above66 °C and a pour point of about -4 °C. Light hydrocarbonsboiling below 204 °C were removed from the hydrogenateddistillate, which resulted in a distillate density of 806.3 kg m-3

at 15 °C (Table 4.)Hydrogenation of this distillate fraction reduced significantly

the olefin and oxygenate content but increased the pour point.Nevertheless, a distillate with a high cetane number of 71 wasobtained. It has to be noted that the cetane number of thehydrogenated iron-based HTFT distillate is well below that ofthe cobalt-based LTFT unhydrotreated distillate, which had acetane value of 78 and indicated a high paraffinicity of the latterdistillate.

4. The Coal-based Sasol 1 Integrated HTFT-LTFTPlant

The first Fischer-Tropsch facility of Sasol at Sasolburg,South Africa, was, compared to the Hydrocol processing

(17) Bruner, F. H. Ind. Eng. Chem 1949, 41 (11), 2511–2515.(18) Tilton, J. A.; Smith, W. M.; Hockberger, W. G. Ind. Eng. Chem

1948, 40 (7), 1269–1273.

Figure 2. Integration of natural gas in the Carthage Hydrocol HTFT refinery (1950-1953).

Table 3. Composition of Synthesis Product from the HydrocolProcess

fraction % olefinicity mass %

liquid petroleum gas, LPG 82 32naphtha (C5-204 °C) 85-90 56distillate 75-85 8residue 4

Table 4. Straight-run and Hydrogenated (20.7 MPa) DistillateFuel Fractions by the Hydrocol FT Synthesis18

iron-catalyst-based HTFT

property straight-run hydrogenatedcobalt-catalyst-based

LTFT straight-run

density @ 15 °C, kg m-3 787.9 806.3 769.4cetane No. 56 (estd.) 71 (detd.) 78pour point, °C -9.4 -1.1 -1.1flash point, °C

(Pensky-Martens)71

bromine No., g Br2/100 g 47 2 1sulfur, µg g-1 200 80oxygen, wt %

(by difference)0.30 0

Distillation10% distilled 204 227 22150% 232 260 24990% 304 327 288

2344 Energy & Fuels, Vol. 23, 2009

scheme, more complex. It was the first integrated plant thatcombined two variants of the Fischer-Tropsch process, theM.W. American Kellog Co. HTFT process, using circulatingfluidized bed (CFB) reactors at 2 MPa and 340 °C andGermany’s fixed bed Arbeits-Gemeinschaft Lurgi and Ruhrche-mie (ARGE) LTFT process, operating at 2.7 MPa and 230 °C.

Coal was gasified by Lurgi pressure gasifiers using steamand oxygen, with the latter supplied by a Linde oxygen plant.The integration was done to maximize the benefits of the uniquecharacteristics of the FT products. Figure 3 shows the original

concept of the integrated nature and the products obtained fromthe facility operating in the late 1950s.19 Regarding the ARGEproducts, a rough separation of the FT synthesis product waseffected between high- and low-boiling product fractions bypartial or stepwise condensation of the reactor products. Thereaction product consisted of hydrocarbons ranging frommethane to extra-hard paraffin waxes. The produced hydrocar-bons condense as the temperature drops in the several units of

(19) Hoogendoorn, J. C.; Salomon, J. H. British Chem. Eng. 1957, 238-244, 308-312, 368-373, 418-419.

Figure 3. Sasol 1 integrated HTFT-LTFT plant in Sasolburg, South Africa (late 1950s).


the cooling train of each reactor and are collected separately.By this way of partial condensation a rough separation in boilingranges was achieved. From the reactor bottom and wax separatora hard wax, called reactor condensate, was obtained. The heatexchanger supplied the hot condensate medium-boiling hydro-carbons and the condenser supplied the cold condensate, thelighter hydrocarbon oils, reaction water, and water-solublealcohols.

Figure 4 shows the principal block flow diagram of the ARGEseparation and refinery section of the integrated facility, theprocess scheme of which included 8 separate units. The objectivewas to produce fuels such as petrol and diesel, but productionof LPG and hydrogenated waxes was also included. Thenecessary synthetic naphtha quality was achieved by fixed-bedrefining at 400 °C, thereby converting the oxygenates to paraffinsand shifting the double bond of the olefins to the internalposition. This improved fuel stability and octane number. Thediesel was a blend of straight-run FT products and cracked stockfrom the thermal cracker, which diesel therefore containedolefinic and oxygenated hydrocarbons. The wax streams werehydrogenated in fixed-bed reactors over a nickel catalyst tosaturate the small amounts of olefins and oxygenates present.

The Kellogg HTFT products recovery section can broadlybe divided into two subsections, the hydrocarbon and thechemicals recovery section.19 The gasoline, distillate, and fueloil fractions are obtained by essential flash distillations.Oxygenates and specifically the acids were intermediatelyremoved by routing the liquid streams boiling <343 °C over areactor containing silica-alumina beds, which at the same timeachieved isomerization of the hydrocarbons and imparted ahigher octane rating to the petrol portion. The heavy oil wasessentially unaffected by this oil-treating process.

During cooling of the gas from coal gasification, a largequantity of condensate separated out. This stream contains gasliquor, light aromatic oil, and tar. The ratio of water to tar inthe condensate stream was high, and the composition wastypically 98.3% water, 0.3% oil, and 1.4% tar. The gas liquorcontained on average 0.85% ammonia and 0.15% tar acids.Those tar acids, after residual tar and oil were separated bygravity and sand filtration, were recovered by the phenolsolvan

process using a butyl acetate extraction.20 The tar and the lightoil were worked up in a tar refinery. The overheads, water andoil, from a first atmospheric distillation unit (ADU) wereseparated, and the oil passed over a hydrogenation unit primarilyto desulfurize the oil fraction. The bottoms of the ADU wererouted to a vacuum tower that fractionated the ADU bottomsinto an overhead and side stream of creosote and road tar prime,respectively, and a bottoms product of pitch. The creosote, tar,and pitch were sold as such.

The aromatic-rich raw gas naphtha from the Rectisol gaspurification, separated from the prewash methanol, was routedtogether with the overheads from the ADU from the tar refineryto a naphtha hydrotreater. Gum-forming material and phenol,but also sulfur, were removed in this gas-phase hydrotreater.The hydrogenated naphtha was, after an alkali and water wash,fractionated in a petrol grade fraction, a fraction containingbenzol, toluol, and xylenol, a neutral creosote fraction, and aheavy naphtha fraction; those latter fractions were sold assolvents (Figure 3).

In the following years, the Kellogg HTFT process and thecatalyst were improved by Sasol, which finally resulted in theSasol HTFT Synthol process. Table 5 compares the straight-run FT products from the ARGE and Synthol process andhighlights some properties of the fuel fractions.21,22 The olefi-nicity of individual carbon numbers is much higher with regardsto the HTFT product, and the carbon number distributions arealso different. HTFT products are more branched and containaromatics, whereas no aromatics are found in LTFT products.Because of the basic FT catalytic growth mechanism it isinevitable that a wide spectrum of carbon numbers is alwaysobtained with the commercially applied FT synthesis. Straight-run distillate yield was ranging between 10 and 20 mass %,and in order to maximize the overall distillate yield, secondaryprocesses were applied such as hydrocracking and C3/C4 olefinoligomerization. The olefin content of the FT synthesis product

(20) Hoogendoorn, J. C.; Salomon, J. H. British Chem. Eng. 1957, 368–373.

(21) (a) Hoogendoorn, J. C.; Salomon, J. H. British Chem. Eng. 1957,418–419. (b) Jager, B.; Kelfkens, R. C.; Steynberg, A. P. A slurry bedreactor for low temperature Fischer-Tropsch. In: Natural Gas ConVersionII; Hyde, H. E., Howe, R. F. Eds.;Elsevier: 1994; 419-425.

(22) Dry, M. E. Hydrocarbon Process. 1982, 121–124.

Figure 4. The LTFT ARGE separation and refinery section of the original Sasol 1 integrated FT plant in 1957; the figure has been directly reproducedfrom ref. 12.


is high, specifically that of the HTFT process, which makes itobvious to apply olefin oligomerization to increase the dieselpool. In the case of the LTFT process, waxes are the majorproduct fraction, therefore wax hydrocracking is the appropriateprocess to maximize diesel yields.

In addition to these two FT synthesis processes, slurry bedreactors systems can be applied. Kolbel et al.23 of the TechnicalUniversity of Berlin did considerable work in this field in the1950s to the late 1970s. Sasol’s work on slurry started in theearly 1980s, and in 1993 a slurry bed reactor (5 m nominaldiameter) was commissioned using an iron-based catalyst thatreplaced the CFB units. The slurry bed reactor (SBR) is operatedsuch that its product slate closely matches that of the fixed-bedreactor; however, differences can be noted (Table 6). Thecomposition is typical but not necessarily representative ofprocess conditions optimal for conversion or wax selectivity orquality. However, Table 6 shows that the LTFT slurry synthesisproduct is much more olefinic and contains less paraffinicmaterial than the fixed-bed product. If a cobalt-based catalystwere used instead of the iron-based catalyst, then the olefinicityof the product would be even lower. The process schemechanged once more in 2004 when coal gasification was changedby natural gas reforming, thus transforming the Sasol 1 plantat Sasolburg essentially into a GTL plant, producing waxes andchemicals rather then synthetic fuels.

Maximum LTFT Diesel Fuel Production Schemes ofthe 1980s. Production schemes to maximize FT diesel fuel were

reported in the early 1980s.24,25 The straight-run diesel fuel froma LTFT operation (slurry and fixed-bed) typically has a cetanenumber of 75-80, whereas the diesel hydrocracked from LTFTwax has a cetane number of about 70-75. Figure 5 depicts aprocess scheme26 that targets to maximize diesel fuel productionbased on LTFT synthesis. Almost two-thirds of the final dieselcomes from the wax hydrocracker distillate when the bottomsfraction is recycled to extinction. About one-third of the dieselwould come from the fractionation of the hydrotreated LTFTcondensate fraction. The inclusion of an oligomerization unitfor the conversion of light olefins to fuel boiling rangehydrocarbons can add some 7% of distillate to a scheme wherethis option is excluded. The distillate derived from sucholigomerization using, for example, C3-C6 olefins is highlybranched and consequently has a low cetane number.

Advantageous is that all distillates are essentially free ofnitrogen and sulfur and only small amounts of aromatics (<0.5wt %) are present in the distillate from the wax hydrocracker.The final diesel density is, however, low due to its highparaffinicity and ranges between 760-780 kg m-3 at 15 °C.The benefit of the high cetane number of those FT distillates isthat other low-quality diesel fuels can be blended into the dieselpool. The highly branched distillates produced from the short-chain olefin oligomerization have low cetane numbers (about30) and are therefore an ideal blending component for the FTdistillates that have high cetane numbers of over 70. Thoseblends still maintain the advantage of being sulfur-, nitrogen-and almost aromatic-free, having high cetane numbers (about60-65) but lacking density, which is the only diesel fuelspecification those blends will not achieve.

5. HTFT Plants and Diesel Producing Schemes, 1980 toPresent

Figure 6 shows the simplified flow scheme of the 2004 coal-based Sasol HTFT Synfuels plant in Secunda, South Africa,which was built in the late 1970s and early 1980s. Several majordifferences involve product separation and workup units. Thetail gas is cryogenically separated into various components thatenable C2 and C3 extraction. Unlike the original Sasol 1 plantat Sasolburg, where petrol was directly blended into petrolderived from crude oil, the Synfuels naphtha is reformed afterremoval of oxygenates and olefins over a platformer for octaneimprovement. The Sasol Synfuels Secunda process scheme wasconceived to maximize petrol production and includes a C3/C4

oligomerization unit to increase the petrol pool. After 1990,recovery of linear C6 to C8 R-olefins was introduced. This waslater followed by the extraction of heavier R-olefins, which arepurified for detergent alcohol production using the hydrofromy-lation reaction. Diesel production is part of the FT refinery andis mainly achieved by hydroprocessing.

Because FT syncrude products are free of sulfur and nitrogen,the main upgrading reactions involved are olefin saturation,hydrodeoxygenation (HDO), and to a minor extent hydrodearo-matization (HDA). As is shown in Figure 6, the coal-basedHTFT Synfuels plant requires, in addition to the FT refinery, atar refinery to process the pyrolysis products from the fixed-bed dry bottom (FBDB) gasification.

(23) Kolbel, H.; Ralek, M. Catal. ReV.-Sci. Eng. 1980, 21 (2), 225–174.

(24) Dry, M. E. The Sasol Fischer-Tropsch Processes. In: AppliedIndustrial Catalysis; Leech, B. E. Ed.; 1983; Vol. 2, Ch. 5, pp 167-213.

(25) Dry, M. E. ChemSA 1981, 286–288.(26) Dancuart, L. P.; Steynberg, A. P. Fischer-Tropsch based GTL

Technology: a New Process? In: Studies in Surface Science and Catalysis;Davis, B. H., Ocelli, M. L. Eds.; Elsevier: 2007, Vol. 163, pp 379-399.

Table 5. Selectivity on Carbon Basis for Straight-run FTProducts from the LTFT ARGE and HTFT Synthol Process21

and Selected Properties of the Naphtha (C5-C10) and DieselCut (C11-C18)22

product fraction,mass %

ARGE fixed-bed,LTFT

Sasol SyntholCFB, HTFT

C1 4 7C2-C4 olefins 4 24C2-C4 paraffins 4 6C5-C10 (naphtha) 18 36C11-C22 (diesel) 19 12C23+ (heavy oil and wax) 48 9water-soluble oxygenates 3 6

Naphthaolefins 32 65paraffins (n-paraffins) 60 (95) 14 (55)aromatics 0 10alcohols 7 6ketones 0.6 6acids 0.4 2

Dieselolefins 25 73(60)paraffins (n-paraffins) 65 (93) 10aromatics 0 10alcohols 6 4ketones <1 2acids 0.05 2cetane No. 75 55

Table 6. Typical C5-C12 and C13-C18 Product Spectra(mass %) for LTFT (fixed-bed, slurry) and HTFT (Synthol)12

fixed-bed slurry HTFT

compounds C5-C12 C13-C18 C5-C12 C13-C18 C5-C12 C13-C18

paraffins 53 65 29 44 13 15olefins 40 28 64 50 70 60aromatic 0 0 0 0 5 15oxygenates 7 7 7 6 12 10n-paraffins 95 93 96 95 55 60


As listed in Tables 5 and 6, the olefin content of the HTFTsyncrude is far higher compared to that of the LTFT synthesis;therefore, olefin oligomerization plays a major role in a HTFTplant. The olefins can be processed according to three processes:thermal polymerization via a free radical mechanism, acid-catalyzed processes using amorphous catalysts, and acid-catalyzed processes using zeolite-based catalysts. The thermalprocess produces a distillate that is fairly linear with cetanenumbers around 50-60. However, compared to the catalyticprocesses, higher temperature and pressure and also a lowerspace velocity have to be applied.

The distillate from a zeolite-based (i.e., ZSM-5 zeolite)oligomerization process has, despite being more branched, acetane number of about 50.25,27 The molecules are mainlymethyl-branched due to the catalyst’s pore structure, whichresults in a fairly high viscosity product (about 3.0 cSt at 40°C). Acid catalysts such as phosphoric acid on kieselguhr(“CatPoly”), originally developed by UOP, produce a highlybranched distillate (C10-C14) with low cetane number valuesof about 30 (34 after hydrogenation) by recycling the gasolineback to the reactor. Overall distillate yield can be as high as75%. This CatPoly distillate has, because of its highly branchednature, a low viscosity (1.8 cSt at 40 °C), but its cold flowproperties are outstanding. The amount of this diesel that canbe blended to achieve a cetane number of, for example, 51,depends very much on the quality of the other distillate blendcomponent. The FT process itself produces predominantlystraight-chain products, therefore the quality of the distillate ishigh. The cetane number of the straight-run HTFT distillate isabout 53.

The Sasol HTFT Synfuels plant is geared to maximize petrolvia short chain olefin oligomerization rather than maximizing

distillates. This original HTFT plant design was the result ofgeopolitical- as well market-related circumstances. The diesel-to-petrol ratio can vary in the range of 35/65 to 40/60. Fuelspecifications in the 1980s were not very demanding; therefore,the refinery design does not reflect the prevailing fuel specifica-tions. The complex CFP reactors were replaced in the 1990sby more energy efficient Sasol advanced Synthol (SAS) reactorsdeveloped by Sasol. Advantages over the CFB reactors are welldocumented.28 The product selectivities29 (Tables 7 and 8) fromthe SAS reactors are similar to those from the CFB reactors(Tables 1 and 2) due to the fact that the same catalystcomposition is used. The more isothermal operation of the SASreactors results in slightly lower methane selectivity.

Although more and more olefins are extracted at the HTFTSynfuels refinery, most of the production volume to date stillgoes into fuels. The composition of the primary HTFT products(Table 7) highlights what is typical of HTFT refining. The oilproduct low in heteroatoms such as sulfur and nitrogen is richin olefins (predominantly R-olefins); the oil is poor in aromaticsand naphthenic hydrocarbons but contains percentage levels ofoxygenates. The oil product is mainly linear and follows aSchulz-Flory distribution heavily weighted toward lighterhydrocarbons as opposed to a LTFT synthesis product.

6. Refining Fischer-Tropsch Syncrude Heavy Ends andPyrolysis Tar Oil in a Coal-based HTFT Synfuels Plant

FT Distillate Refining. The distillate (C11-C22) and residue(>360 °C boiling material) fractions of the HTFT syncrude makeup about 10% of the total products from the Fischer-Tropschsynthesis. The residue fraction is consequently quite small(3-4% of the total primary product), and an atmospheric

(27) Dry, M. E. The Sasol Route to Chemicals and Fuels. In: MethaneConVersion; Bibby, D. M., Chang, C. D., Howe, R. F., Yurchak, S.Eds.;Elsevier B.V.: Amsterdam, 1988; pp 447-456.

(28) Jager, B.; Dry, M. E.; Shingles, T.; Steynberg, A. P. Catal. Lett.1990, 7, 293.

(29) Steynberg, A. P.; Espinoza, R. L.; Jager, B.; Vosloo, A. C. Appl.Catal., A 1999, 186, 41–54.

Figure 5. Maximum LTFT diesel producing scheme.


distillation unit (ADU) is all that is required for primary productseparation. A separate vacuum distillation unit (VDU) isincluded in the refinery design of the Sasol HTFT Synfuelsfacilities (Figures 6 and 7) to avoid potential problems that areassociated with thermal cracking in the reboiler of the ADU.

The olefins and oxygenates present in the HTFT syncrude aresusceptible to thermal decomposition when the reboiler tem-perature exceeds 300-320 °C.

The distillate and residue fractions are contained in twostreams from the HTFT synthesis, namely the stabilized lightoil (SLO) and the decanted oil (DO). The DO is the heaviestfraction, and it consists mainly of residue material and somecatalyst fines that have not been removed by the cyclones in

Figure 6. Coal-based HTFT facility with fixed-bed dry bottom (FBDB) gasification and separate FT and tar refinery (Sasol Synfuels Refinery 2004,Secunda, South Africa); CTN: coal tar naphtha, DHT: distillate hydrotreater, DU: distillation unit, HP: high pressure, HT: hydrotreater.

Table 7. Selectivity on Carbon Basis for the SAS Reactors12

product %

methane 7C2-C4 olefins 24C2-C4 paraffins 6gasoline 36middle distillate 12heavy oils and waxes 9watersoluble oxygenates 6

Table 8. Selectivity (Hydrocarbon Type) for the SAS Reactor12

product, % C5-C10 C11-C14

paraffins 13 15olefins 70 60aromatics 5 15oxygenates 12 10


the Fischer-Tropsch synthesis reactors. This product is knockedout after the feed-product heat exchanger of the synthesisreactors, which partially condenses some of the heavier products.The SLO is recovered after further cooling and contains distillateand residue. The SLO and DO fractions are both flashseparations and should be viewed as rough splits that result fromthe design of Fischer-Tropsch product cooling, rather thanintentional product separation steps.

The HTFT distillate and residue cuts are different incomposition from the predominantly linear paraffins and waxesthat are found in the equivalent LTFT cuts. It contains asignificant amount of aromatics (but little polynuclear aromat-ics), oxygenates, and olefins, but as in the LTFT syncrude, it isalmost sulfur and nitrogen-free. In the case of HTFT distillate,it is olefins and oxygenates that are the major compound classesto be saturated. Less severe hydrotreating conditions arerequired, although olefins and oxygenates are capable of highlyexothermic reactions. An important consideration in HTFTdistillate refining is the product density, which is too low, onaccount of the low aromatic content of HTFT distillate. Theseand other important aspects will be highlighted during the furtherdiscussions.

The HTFT residue fraction is comparatively clean, apart fromsome iron that is present as iron carboxylates. The HTFT residuecan be hydrocracked to distillate and naphtha, or it can bedewaxed to produce base oils;30 however, hydro-dewaxing isthe preferred process in the present HTFT Secunda Synfuelsfacility. Because the HTFT distillates have an inherently lowdensity, preservation of aromatics in the Synfuels facility forinclusion in the final diesel pool is highly desirable.

Refining of FBDB Gasification Derived Pyrolysis TarOil. The feed to the tar distillation unit (Figure 8) consists ofphenolic pitch, after phenol extraction in a phenosolvan unit,crude tar oil from a coal tar filtration unit and the recycle stream(residue oil) from the coal tar naphtha hydrogenation. A typicalproduct distribution from tar distillation consisted of sixfractions, namely, light naphtha (10%), heavy naphtha (8%),medium creosote (25%), heavy creosote, (13%), residue oil(5%), and pitch (39%).

The light naphtha, heavy naphtha, and Rectisol naphtha fromthe Rectisol unit are combined to serve as feed for the coal tarnaphtha (CTN) hydrogenation unit. In this unit the feed is

preheated and flashed to separate the naphtha range materialfrom the heavier boiling material at around 210 °C. The heaviermaterial, called residue oil, is recycled to the tar distillation unit,while the naphtha vapor is hydrotreated at 5 MPa pressure. Thehydrogenated naphtha consists of mainly paraffins and aromat-ics, but with low octane number (typically around 80), and it isused as a fuel-blending component. The medium creosote, heavycreosote, and residue oil from the tar distillation unit arecombined and hydrogenated at high pressure (18.5 MPa) in acreosote hydrotreater that consists of four reactors in series. Theproduct is a mixture of hydrogenated naphtha and distillate. Thecreosote distillate is an important blending component, since itis the main source of diesel density in the HTFT Synfuelsrefinery.29

The pyrolysis liquids have a low hydrogen-to-carbon ratioand high nitrogen, oxygen, and polynuclear aromatic concentra-tions. Similar compositions are found in liquids derived fromdirect coal pyrolysis.31 Due to the high levels of nitrogen,phenolic compounds, and polynuclear aromatics, severe hydro-processing conditions are necessary, and consequently, hydrogenconsumption in these coal tar naphtha and creosote hydrogena-tion units is typically high. The high hydrogen pressures andlow space velocities applied for the hydroprocessing of thedistillate boiling coal pyrolysis liquids favor heteroatom re-moval.32 Nitrogen (<15 ppm), sulfur (<1 ppm), and phenolicslevels (<100 ppm) obtained in the final tar oil distillate of theSasol HTFT Synfuels plant are low; however, density remainsrelatively high (870-880 kg m-3).32

Properties of Hydrotreated HTFT and Pyrolysis TarOil Distillates. The hydroprocessing of straight-run HTFTdistillate over sulfided NiMo/Al2O3 catalysts yields productcharacteristics that are shown in Table 9. The moderate pressuresof 5.0 MPa does not favor monoaromatic saturation and moresevere process conditions are required, specifically higheroperating pressures and temperatures combined with low spacevelocities to achieve the hydrogenation of monoaromatics.33

The products from HTFT straight-run distillate hydrotreatingare naphtha and mainly diesel. The naphtha product results fromthe hydrodeoxygenation reaction, which shifts the oxygenates

(30) Leckel, D. Energy Fuels 2009, 23, 38–45.

(31) Teo, K. C.; Watkinson, A. P. Fuel 1990, 69 (10), 1211–1218.(32) (a) Leckel, D. Energy Fuels. 2006, 20, 1761–1766. (b) Leckel, D.

Energy Fuels. 2008, 22, 231–236.(33) (a) Cooper, B. H.; Donnis, B. B. L. Appl. Catal., A 1996, 137,

203. (b) Girgis, M. J.; Gates, B. C. Ind. Eng. Chem. Res. 1991, 30, 2021.

Figure 7. Operation of the HTFT heavy-end syncrude conversion and separation units in the Sasol Synfuels facilities at Secunda, South Africa(2008). The vacuum gas oil, in practice, comprises of two fractions, a light vacuum gas oil (LVGO) and a heavy vacuum gas oil (HVGO) that arecombined for further hydroprocessing.


initially boiling in the distillate range into the naphtha range.The naphtha with a final boiling point below 190 °C has a RONin the range of 35-40 and has therefore to be further upgraded,typically by catalytic reforming.

The hydrotreated HTFT distillate has a cetane number in therange of 55-65, mainly as a result of its high linear paraffincontent and low concentration of polycyclic aromatics. The bulkof aromatics are predominantly long chain alkylbenzenes that,due to their high paraffinicity, also have a high cetane number.HTFT distillates are clean fuels, low in sulfur and nitrogen (<1ppm), but due to its paraffinicity lack density compared to crude-oil-derived diesel.

Distillates derived from HTFT synthesis have a lower densitycompared to gasification-tar-derived distillate despite the samelevel of total aromatics (Table 10). The cetane number of thepyrolysis tar oil distillate is low compared to the FT distillate.33

The FT distillate is mainly paraffinic with the aromatics in thedistillate being predominantly monoaromatics. Those aromaticsstructures are particularly alkylated benzenes carrying a longalkyl side chain and hydroaromatics in the form of indans andtetralins. The levels of naphthalenes or anthracenes in ahydrotreated HTFT distillate are generally below 0.50 mass %.The hydrotreated pyrolysis tar oil distillate is mainly naphthenic(hydroaromatics).

Blending Opportunity of HTFT and Pyrolysis Tar OilDistillates. The complementary nature of the properties of thehydrotreated HTFT and FBDB gasification-derived tar distillatesis advantageous. The tar distillate redresses adequately the mainshortcoming of the HTFT distillate, namely density (Figure 9).

Figure 9 illustrates the blending of a HTFT distillate with agasification-tar-derived distillate, both distillates hydroprocessed

Figure 8. Tar workup section at Sasol’s HTFT Synfuels plant at Secunda, South Africa.

Table 9. Selected Properties of HTFT Distillate Hydrotreatedover a NiMo/Al2O3 Catalyst at 5.0 MPa30

properties

hydrotreatedHTFT

distillate

EN590-Euro 4diesel fuel

specification

cetane No. 63 51 mindensity at 15 °C (kg ·m-3) 804.1 820-845viscosity at 40 °C (cSt) 2.29 2.0-4.5lubricity, HFRR (µm) 546 460 maxflash point (°C) 74 62 mincloud point (°C) -1 a

cold filter plugging point (°C) -2 a

T95 distillation point (°C) 378 360 maxfinal boiling point (°C) 381acid content (mg KOH/g) 0.004olefin content (g Br2/100 g) 1.18N (µg g-1) <1S (µg g-1) <1 10 maxtotal aromatic by HPLC (mass %) 22.44mononuclear 22.20dinuclear 0.24polynuclear not detected 11 max

a Dependent on climate.

Table 10. Comparison of Properties of Hydrotreated PyrolysisTar Oil Distillate and HTFT Distillate30

property pyrolysis tar oil distillate HTFT distillate

density at 15 °C, kg m-3 887.3 804.1viscosity at 40 °C, cSt 2.33 2.23HPLC-aromatics, mass % 25.50 22.45monoaromatics 24.0 22.2diaromatics 1.20 0.24polycondensed aromatics 0.30 <0.01cetane number 38 63CFPP, °C -7 -2cloud point, °C -2 -1phenolics, mg kg-1 58 <1nitrogen, mg kg-1 6 <1sulfur, mg kg-1 <1 <1


at the Sasol Synfuels refinery at Secunda, South Africa. Thisexample shows that a HTFT distillate of 804 kg m-3 densitycan be blended with about 20 vol % of a gasification tar deriveddistillate with a density of 887 kg m-3 to achieve the EN590-Euro 4 minimum density diesel fuel specification. Table 11shows typical properties of a final HTFT Synfuels diesel blendcommercially marketed by Sasol in South Africa, which includes71 vol % of a HTFT light distillate fraction, 25 vol % of agasification-tar-derived distillate, and 4 vol % of a HTFT heavydistillate fraction.30 The final blended diesel fuel propertiescomply with the EN590-Euro 4 diesel fuel specificationsmeeting a summer grade B cold filter plugging point (CFPP).The integration, therefore, of a tar refinery into an FT plantcreates opportunities for fuel processing and in downstream fuelblending.

7. The PetroSA HTFT Gas-to-Liquids Plant

The PetroSA facility at Mossel Bay, South Africa, is basedon the HTFT Synthol technology developed by Sasol and hasbeen in operation since 1993. This plant is supplied with naturalgas (NG) and integrates the refining of natural gas liquids (NGL)with that of the synthetic hydrocarbons from the HTFTsynthesis. The process scheme is primarily designed to producegasoline and smaller amounts of distillates. The syngas for theFT synthesis using a fused iron catalyst is produced from natural

gas coming from an off-shore gas field. There are two mainrefinery sections, the oil workup section taking care of the allhydrocarbons and the aqueous phase workup section refiningthe water-soluble oxygenates. Integration features of the plantare the combined upgrading of the natural gas liquid derivednaphtha with the synthetic FT naphtha and the combined refiningof the natural gas condensates with the synthetic FT distillates(Figure 10).12,26

A key unit for distillate production in the refinery is theoligomerization unit that uses the conversion of olefins todistillate (COD) process that was specifically developed for theupgrading of FT olefins to diesel. The process was designed inSouth Africa by the Central Energy Fund (CEF) andSud-Chemie using a ZSM-5 zeolite catalyst (Sud-Chemie COD-9).34 The mixture of olefins is converted in three reactors. Onereactor can be taken off-line for in situ catalyst regeneration bycontrolled carbon burnoff. Distillate of high quality (cetanenumbers of 48-54) but low density (800 kg m-3 at 15 °C) isproduced.28,35 The distillate hydrotreater (DHT) hydrogenatesthe COD distillate together with the distillate of the SLOdistillation to produce diesel and kerosene. The straight-rundistillate from the NGL recovery is combined with the hy-drotreated product to produce a final diesel. The PetroSA dieselfuel formulations have CFPP values typically in the range of-20 °C and lower due to the large portion of iso-paraffinspresent. Aromatic content generally is less than 18%, with thebulk of the aromatics being single ring monoaromatics.

8. The Shell LTFT Gas-to-Liquids Plant(Bintulu, Malayia)

Shell’s first commercial GTL facility based on the Shellmiddle distillate synthesis (SMDS) technology was the 12 500bpd plant commissioned in Bintulu, Sarawak, Malaysia in 1993.It was partly destroyed by an explosion in the air separationunit in December 1997. A new air separation unit wasconstructed, and the plant was recommissioned in May 2000.It has been in operation without problems since then. Shelldeveloped its LTFT technology based on the SMDS process.The process consists of three major steps, namely, the synthesisgas generation from natural gas, heavy paraffin synthesis (HPS)using the generated syngas, and heavy paraffin conversion(HPC) to middle distillates.36 Maximum distillate productionis achieved by first using the Fischer-Tropsch reaction toproduce heavier products (high R-value), which then is selec-tively cracked to the desired carbon number range. The ShellCo LTFT catalyst development resulted in a catalyst producingan R-value of around 0.90 and higher, while synthesis gasconversion between 80-95% with a C5+ product selectivityin the range 85-95% have been reported.37-39

A simplified flow scheme of the SMDS plant in Bintulu isshown in Figure 11. It is a two-stage process convertingsynthesis gas into middle distillates. The hydrocracker in theHPC step is operated in the range of 300-350 °C and 3-5MPa with a Shell proprietary noble metal hydrocrackingcatalyst.38 In the HPC stage the olefins and oxygenates areremoved by hydrogenation, and the resulting paraffinic hydro-

(34) Minnie, O. R.; Petersen, F. W.; Samadi, F. R. The effect of 1-hexeneextraction on the COD process conVersion of olefins to distillates. SouthAfrican Chem. Eng. Congr.: Sun City, South Africa, 2003.

(35) Knottenbelt, C. Catal. Today 2002, 71, 437.(36) Sie, S. T.; Senden, M. M. G.; Van Wechem, M. M. M. Catal. Today

1991, 8, 371–394.(37) Dry, M. E.; Steynberg, A. Stud. Surf. Sci. Catal. 2004, 152, 406.(38) Smith, R.; Asaro, M. Fuels of the future. Technology Intelligence

for Gas to Liquids Strategies; SRI: Menlo Park, CA, 2005.

Figure 9. Blending a HTFT distillate with density of 804.1 kg m-3

with a FBDB gasification-derived tar oil distillate having a density of887 kg m-3. EN590-Euro 4 minimum diesel density specification at15 °C is achieved at a blend ratio of 20% tar oil to 80% HTFT distillate.

Table 11. Selected Fuel Properties of a Typical Final SynfuelsHTFT Diesel Blend Commercially Marketed by Sasol in

South Africa30

analysis units methodcommercialfinal diesel

density @ 15 °C kg m-3 ASTM D4052 829distillation IBP °C ASTM D86 178T10 °C 200T50 °C 240T95 °C 361FBP °C 374flash point °C ASTM D93 70viscosity @ 40 °C cSt ASTM D445 2.23CFPP °C IP 309 -1sulfur mg kg-1 ASTM D5453 <1cetane No. ASTM D613 53oxidation stability mg (100 mL)-1 ASTM D2274 0.4HFRR WSD µm ASTM D6079 567HFRR WSD @ 250 µg/g

lubricity improverµm 307

net heating value MJ kg-1 ASTM D240 42.8MJ l-1 35.3

total aromatics mass % ASTM D6591 36.8


carbons are hydroisomerized and hydrocracked into the desiredboiling range. The product is fractionated and the unconvertedwax-fraction can either be sold as a wax or recycled to thehydrocracker.

The product selectivity can be influenced by varying thesingle-pass conversion.39 Depending on the hydrocrackingseverity, the product composition can be varied from 15% lightsand naphtha, 25% kerosene, and 60% gas oil (distillate) in gasoil mode, to 25% lights and naphtha, 50% kerosene, and 25%gas oil in kerosene mode. Some typical product properties aregiven in Table 12.40,41 Although the wax, paraffins, and LPGcan be sold as final products, the marketing of the fuel products

depend on Shell’s ability to blend it with the products fromtheir crude oil refineries to meet fuel specifications.

9. The Sasol Oryx LTFT Gas-to-Liquids Plant in Qatar

The 34 000 bpd Oryx GTL plant at Las Raffan, Qatar, usesLTFT technology that is based on the Sasol SPD process. TheSPD process uses a Co-based Sasol proprietary catalyst typicallyoperating at 230 °C and 2.5 MPa.

The basic refinery design (Figure 12) is very similar to thatof the Shell Bintulu refinery. The refinery section receives twofeed streams, namely wax and condensate that are combined toserve as feed to the hydrocracker. The hydrocracker uses

Figure 10. Oil work-up at PetroSA refinery, Mosselbay, South Africa.

Figure 11. Simplified flow scheme of the Shell LTFT GTL plant at Bintulu, Malaysia.


ChevronTexaco isocracking technology with a Chevron pro-prietary hydrocracking catalyst, which is a commercially avail-able sulfided base metal catalyst on an acidic support. Typicaloperating conditions are 350 °C and 7 MPa, with the temperaturebeing adjusted to maintain a per pass conversion of around 60%.The product from hydrocracking is distilled to produce LPG(3-7%), naphtha (20-30%), and distillate (65-75%), with theunconverted >360 °C waxy product being recycled back to thehydrocracker.43 The reaction water is separated by distillationinto an alcohol-rich overheads product that is incinerated and acarboxylic-acid-containing water product that is biologicallydegraded to purify the water. No oxygenates are recovered fromthe reaction water. The distillate is sold as transportation fuelblend stock, whereas the naphtha can be marketed as steamcracker feed since it has a poor octane value RON (50-55).44

Typical properties of a Sasol SPD distillate are listed in Table13.45 The distillates contain virtually no heteroatoms and havean almost zero aromatic hydrocarbon content. This results in ahigh cetane number (above 70) but a low density, which is themajor reason that LTFT-derived distillate is not able to meetdiesel fuel specifications. These excellent fuel propertiescontribute to the clean combustion of the LTFT diesel but tothe loss of volumetric heating value. The distillate can exhibita range of cold flow properties, depending on the hydrocrackingseverity. Lubricating properties are poor, but with addition of

lubricity improvers an acceptable level of lubricity performancecan be achieved.

10. Conceptual Fischer-Tropsch Refinery Designs

Some conceptual FT refinery designs to maximize diesel fuelproduction were developed by A. de Klerk.12 Both high-temperature and low-temperature Fischer-Tropsch technologywere considered. Distillates from HTFT and LTFT are es-sentially sulfur-free and have a low aromatic content and a highstraight-run cetane number.39,46,47 It was demonstrated that LTFTor GTL diesel compared to CARB or Swedish EC1 dieselproduced the lowest regulated and unregulated exhaust emis-sions.48 The excellent fuel properties of a LTFT diesel contributeto a clean combustion but also to a loss of volumetric heatingvalue.49

There is, however, the one diesel fuel specification that isnot easily met by FT diesel, and that is density (820-845 kgm-3 at 15 °C). The density of a hydrocracked LTFT distillatetypically ranges between 760 and 780 kg m-3 due to onlynegligible amounts of aromatics and high paraffinicity.45 Ahydrotreated HTFT distillate30 contains, on the other hand, somearomatics, mainly monoaromatics, which results in a densitybetween 804 and 813 kg m-3.

With regards to maximum diesel production in a LTFTfacility, cetane number (>70) or volume (due to the high R-value

Table 12. Typical Kerosene and Gas Oil Properties of the ShellLTFT GTL Plant in Malaysia40,41 and Properties of a GTL Test

Fuel G100 (Shell Int. Gas Ltd.)42

property kerosene gas oil G100

density @ 15 °, kg m-3 738 776 779boiling range, °C (ASTM D86) 155-191 184-357 310 (at 90%)total aromatics, wt % <0.1 <0.05 1.4cetane index 58 76 75 (cetane No.)sulfur, wt % 0.0003lower heating value, MJ kg-1 43.6smoke point, mm >50flash point, °C 42freezing point, °C -47

Figure 12. Simplified process flow diagram of the Sasol Oryx GTL plant at Las Raffan, Qatar.

Table 13. Typical Properties of a Hydrocracked Distillate of theSasol SPD Process

property distillate

density @ 15 °C, kg m-3 769boiling range, °C 151-334cetane No. 73CFPP -19flash point, °C 58total aromatics, wt % 0.1viscosity @ 40 °C, cSt 2.0lubricity, HFRR WSD, µm 617sulfur, mg kg-1 <1Net heating value, MJ kg-1(MJ L-1) 43.79 (33.65)


in the FT synthesis step) is not the issue but rather density. Theconstraint in a HTFT facility targeting maximum diesel produc-tion is the volume (due to the lower R-value in the FT synthesisstep) rather than density or cetane. The distillate fraction in thestraight-run HTFT syncrude is about 8%.

This property relationship regarding the LTFT and the HTFTdistillate is referred to by De Klerk in his thesis12 as the “density-cetane-yield triangle”. It is possible to meet any two of thementioned three properties without too much refining effort,but meeting all three is very difficult. The conceptual FT refinerystudies confirmed that it is possible to produce on-specificationmotor-gasoline and jet fuel in a Fischer-Tropsch refinery, butthat Fischer-Tropsch syncrude is not suited for the productionof on-specification diesel fuel. It was concluded by de Klerkthat HTFT and LTFT syncrude is on a molecular level unsuitedfor maximizing Euro 4 type diesel fuel production, but is bestachieved by blending the syncrude-derived distillate withmaterial form other sources such as coal pyrolysis products orcrude oil.

11. Integrated Refining of FT and FBDBGasification-Derived Heavy Fractions

Revisiting the complexity of a Sasol HTFT Synfuels facilityit is obvious that reducing the number of processing units couldreduce complexity and could also reduce capital costs of analternative type of Synfuels refinery design. Instead of refiningFT and gasification-derived pyrolysis tar oil heavy endsseparately, the combined upgrading of those heavy ends was

(39) Eilers, J.; Posthuma, S. A.; Sie, S. T. Catal. Lett. 1990, 7, 253.(40) Tijm, P. J. A. Prepr. Pap.-Am. Chem. Soc., DiV. Fuel Chem 1994,

39 (4), 1146.

(41) Schrauwen, F. J. M. Shell Middle Distillate Synthesis (SMDS)process. In: Handbook of Petroleum Refining Processes; Meyers, R. A. Ed.;Mc-Graw-Hill: New York, 2004; pp 15-25.

(42) Wu, T.; Huang, Z.; Zhang, W.-G.; fang, J.-H.; Yin, Q. Energy Fuels2007, 21 (4), 1908–1914.

(43) Dancuart, L. P.; De Haan, R.; De Klerk, A. Stud. Surf. Sci. Catal.2004, 152, 482.

(44) Dancuart, L. P.; Mayer, J. F.; Tallman, M. J.; Adams, J. Prepr.Pap.-Am. Chem. Soc., DiV. Pet. Chem 2003, 48 (2), 132.

(45) Lamprecht, D.; Dancuart, L. P.; Harrilall, K. Energy Fuels 2007,21 (5), 286–2852.

(46) Ward, C. C.; Schwartz, F. G.; Adams, N. G. Ind. Eng. Chem. Res.1951, 43 (5), 1117–1119.

(47) Cookson, D. J.; Smith, B. E. Energy Fuels 1992, 6 (5), 581–585.(48) Fanick, E. R.; Schubert, P. F.; Russell, B. J.; Freeks, R. L.

Comparison of emission characteristics of convetional and, hydrotreatedand Fischer-Tropsch diesel fuels in a heavy duty diesel engine; SAE Tech.Pap. Ser. 2001, 2001-01-3519.

Figure 13. Coal-based HTFT plant with FBDB gasification and integrated FT and tar heavy end refining; SDA: solvent deasphalting.


considered. This processing scheme is, to date, not commerciallypracticed but could potentially be incorporated in future FTplants producing on-specification diesel fuels.

Figure 13 is a proposal of such a less complex refineryscheme. Since heavy ends from the HTFT synthesis andgasification derived pyrolysis tar oils might potentially containsolids from the FT synthesis catalyst and the coal gasificationprocess (i.e., ash or char), the heavy end upgrading unit of thoseproposed FT facilities would typically employ an ebullated orslurry bed hydrocracker.

Slurry bed hydrocracking studies to convert the heavy coalpyrolysis fraction boiling above 360 °C into liquid fuel fractionswere conducted at Sasol in the years 1984 to 1987 at Sasol’sresearch and development pilot plant facilities at Sasolburg. Itwas established that those heavy fractions could successfullybe converted to liquid fuel fractions such as naphtha anddistillates using the spent iron-based Sasol HTFT catalyst. Thefuel fractions (C4-400 °C) had to be further hydroprocessed indedicated naphtha and diesel hydrotreaters to adjust the het-eroatom content to targeted fuel specifications.

Decanted oil, the heaviest fraction of the HTFT synthesis,contains mostly long-chain linear paraffins and the major partof the aromatics produced by the HTFT synthesis reaction.Those aromatics are typically alkylated benzenes and tetralinsand contain smaller portions of alkylated naphthalenes. Initialtests in batch autoclaves at Sasol have shown that the heavyfraction of pyrolysis tar oils can be hydroprocessed togetherwith the heavy end from the HTFT synthesis. This workindicated that a scheme as provided in Figure 13 could presenta viable alternative option of a CTL facility applying FBDBgasification and FT synthesis. The FT synthesis could either bebased on the high-temperature but also on the low-temperaturetechnology, which would then result in a flow scheme aspictured in Figure 14. The LTFT heavy ends are in this caseupgraded together with the FBDB gasification-derived pyrolysisheavy fraction in a dedicated hydrocracker.

12. Recent Global Fischer-Tropsch CTL Developments

The International Energy Agency (IEA) foresees that fossilfuels will continue to dominate energy supplies in 2030 withthe transportation sector expected to account for two-thirds ofthe growth, and global daily oil demand will possibly reach

(49) Schaberg, P. W.; Myburgh, I. S.; Botha, J. J.; Roets, P. N.; Viljoen,C. L.; Dancuart, L. P.; Starr, M. E. Diesel exhaust emissions using SasolSlurry Phase Distillate Process Fuels. SAE Tech. Pap. Ser. 1997, 972898.

Figure 14. Coal-based LTFT plant with FBDB gasification and integrated FT and tar heavy end refining; HC: hydrocracker.


115 million barrels per day in 2030.50 The currently knownreserves of coal exceed those of crude oil by a factor of 25,with the United States, Russia, and China having the largestcoal reserves in the world.5 China and India together accountfor almost three-quarters of the recent increase in coal demandin developing countries and two-thirds of the increase in worldcoal demand.

As the United States seeks out solutions toward energyindependence, liquefaction technologies such as the Fischer-Tropsch process, that can produce liquid fuels from coal, arebecoming more and more attractive, specifically at high oilprices. Concepts consist of independent, stand-alone CTLfacilities but also CTL options involving cositing of a CTLfacility with an integrated gasification combined cycle (IGCC)power plant, where syngas from the IGCC is routed to the FTplant. Both concepts produce distillate and naphtha.

The only commercial-scale indirect coal liquefaction (ICL)processes producing liquid synthetic fuels and currently inoperation are those operated by Sasol in South Africa. Theseplants currently produce about 37% of South Africa’s trans-portation fuel requirements.

The United States has two commercial facilities that convertcoal to fuels and chemicals by indirect liquefaction. The GreatPlains Synfuels Plant, located in Beulah, North Dakota, producessynthetic natural gas (SNG) from North Dakota lignite by LurgiFBDB gasification and methanation. Operated by the DakotaGasification Company (DGC), the plant converts lignite topipeline quality gas in 14 Lurgi gasifiers.

Indirect coal liquefaction projects are currently being studiedor planned in China, Philippines, Germany, Netherlands, India,Indonesia, Australia, Mongolia, Pakistan, and Canada. In theUnited States, indirect coal liquefaction projects are beingconsidered in Alaska, Arizona, Colorado, Illinois, Indiana,Kentucky, Louisiana, Mississippi, Montana, North Dakota, Ohio,Pennsylvania, Texas, West Virginia and Wyoming. Table 14lists51 in more detail various FT CTL projects under consider-ation in the United States of America.

The prospects for a commercial FT CTL industry in theUnited States remain presently unclear with three major hurdlesblocking the way forward, namely, the uncertainty about thecosts and performance of CTL plants, the uncertainty about thefuture course of world oil prices, and the significance ofgreenhouse gas emissions (CO2) reduction from such CTLplants. CTL plants with carbon dioxide capture and sequestrationcan produce fuels with life-cycle greenhouse gas emissionprofiles that are as good or better than that of petroleum-derived

production; however, the technical and overall challenge is toreduce the immense CO2 emissions.

13. Conclusions

A brief overview was given on the development ofFischer-Tropsch plants and their refinery flow schemes, span-ning from the first commercial Fischer-Tropsch plant in 1930sGermany until the newest conceptual studies. Special emphasiswas placed on the distillate upgrading and the relevant fuelproperties.

The oldest Fischer-Tropsch facility still in operation is Sasol1, which is now over 50 years old, although not producing fuelsanymore. Sasol Synfuels at Secunda, South Africa, previouslySasol 2 and 3, is still producing predominantly fuels but is nowalso coproducing chemicals.

The Fischer-Tropsch syncrude composition and its distillateyields are mainly determined by the FT catalyst type, iron- orcobalt-based, and the reaction conditions. The carbon numberdistribution of a HTFT and a LTFT synthesis product isdifferent. HTFT syncrude has a high naphtha yield and lowcontents of material boiling above 360 °C, wheres a synthesisproduct from the LTFT process consists of a considerableamount of higher boiling hydrocarbons. The final distillate yieldis substantially larger in case of the LTFT process.

Fuel specifications and environmental legislations havechanged during the decades and affected the refineries, whichhad to alternate their processing schemes to meet the newproduct specifications and the required emission standards.These changes are most of the time accompanied by large capitalinvestments.

The HTFT facilities are, with regards to the refinery flowschemes, far more complex than the LTFT facilities. The latterflow schemes are simple and consist mainly of a hydroprocess-ing step and a fractionation step to produce naphtha and middledistillate. The drawback of LTFT refineries is that they are notable to produce final fuels rather than fuel blending stocks. TheHTFT syncrude is better suited to diesel fuel production thanLTFT syncrude since the distillate boiling range materialcontains aromatics, hydroaromatics, and naphthenes, resultingin a diesel density that is closer to the required dieselspecification.

Blending with coal pyrolysis products or crude-oil-deriveddistillates or otherwise distillates derived from direct coalliquefaction are ways to produce a final diesel complying withfuel specification. Alternative ways for production of on-specification diesel fuel, as highlighted in the provided con-ceptual flow schemes, could be the combined hydroprocessingof heavy boiling fractions derived from FT synthesis and coalpyrolysis or liquefaction.

Important diesel fuel properties are cetane number, density,sulfur, polycyclic aromatic content, viscosity, cold flow, lubric-

(50) Lloyd Wright, T. Hydrocarbon Process. 2007, (Supplement), 4.(51) Smith, R.; Asaro, M.; Naqvi, S. Fuels of the Future. Technology

Intelligence for Coal to Liquids Strategies; CRI Consulting: July 2008.

Table 14. Selected Indirect FT Coal-to-Liquids Plants under Consideration in the United States

project lead project partner location status capacity, bpd cost

American Clean Coal Fuels none cited Oakland, IL feasibility 25 000 N/ASynfuels Inc. GE, Haldor Topsoe, NACC,

ExxonMobilAscensuion, Parish, LA feasibility N/A $5 billion

DKRW Advanced Fuels Rentech, GE Medicine Bow, WY design (2011) 13 000 $1.4 billionAIDA ANRTL,CPC Cook Inlet, AK feasibility 80 000 $5-8 billionWMPI Sasol, Shell, DOE Gilberton, PA design 5000 $612 millionRentech/Peabody N/A Southern IL, Soutwest IN,

Western KYfeasibility 1800 N/A

Rentech Adams County Natches, MS feasibility 35 000 $650-750 millionRentech Baard Energy Wellsville, OH feasibility 10-50 000 $4 billionHeadwaters NACC, GRE, Falkirk AZ feasibility 40 000 N/A


ity, flash point, and distillation range. The cetane number, themain quality criterion for a diesel fuel, is a measure of ignitionand combustion ability of the diesel fuel, and the higher thenumber the better the fuel quality. Cetane numbers for FT dieselare generally exceeding the required diesel fuel specifications,since Fischer-Tropsch molecules are mainly linear.

Density is an important factor for fuel consumption, whereasthe other properties mentioned have an effect on emissions.Although density is not a problem for aromatic-containing HTFTdiesel, LTFT diesel contains virtually no aromatics and theresulting density of 770-780 kg m-3 poses a refining challenge.However, synthetic LTFT distillate is increasingly blended intotop quality diesel fuels to reduce emissions in modern high-performance diesel engines.

Fischer-Tropsch syncrude, be it HTFT or LTFT syncrudederived, has unique properties and lends itself to a refining that

is different from upgrading crude-oil-derived material. Fischer-Tropsch specific refining technologies or catalysts have not beendeveloped in the past, and the FT refiners therefore adoptedtechnologies that were commercially offered for crude oilrefining. There are, however, exceptions, and Axens is nowoffering the Gasel technology (a Eni/IFP/Axens combineddevelopment) within their technology portfolio, which providesXTL LTFT technology with integrated product upgrading byhydrocracking.52

Acknowledgment. The author appreciates the permission ofSasol Technology to publish this work.

EF900064C

(52) (a) Fedou, S.; Douziech, D.; Caprani, E. J. PetroTech Soc. 2008,26–31. (b) Caprani, E. Gasel: the XTL technology suite for the conVersionof syngas to diesel; OAPEC-IFP Joint Seminar, June 18, 2008.


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