Fischer-Tropsch Syncrude

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Prologue

It is unlikely that transportation fuel will change from hydrocarbon based motor-gasoline and diesel in the foreseeable future. Most transportation fuel is refined from crude oil, which is considered to be a non-renewable (finite) resource and is likely to reach its global peak production in the near future. However, fuel supply is already under pressure, not due to limited oil production, but due to the refining infrastructure that is not capable of processing the increasing volume of heavy crude oils being produced. The declining number of refineries aggravates the situation and contributes to a sense of energy insecurity. Energy insecurity is partly responsible for the renewed interest in Fischer-Tropsch (FT) technology, since FT provides a way to convert coal and gas into synthetic crude oil. Yet, despite the increasing body of literature dealing with Fischer-Tropsch synthesis, little literature deals with the refining of Fischer-Tropsch products, which is equally important for the ultimate conversion of coal and gas into transportation fuel. Conventional refining technology has to be adapted to deal with syncrude feed peculiarities and imposing a crude refining methodology on a FT facility can lead to an unwieldy and expensive refinery configuration. There is consequently a need to study Fischer-Tropsch refining as a topic in its own right.

Refining of Fischer-Tropsch products

The output from a Fischer-Tropsch process is not a final product, but a synthetic crude, often called syncrude to emphasise the similarity with crude oil. Like crude oil, syncrude has to be refined to produce useful products. If one has a look at the basic flow diagrams of the commercial HTFT fuel refineries,(52) it seems that dealing with HTFT syncrude requires a number of processing steps over and above the normal separation steps involved in fuels refining. This is in stark contrast to the refinery flowscheme associated with an LTFT process, like the SMDS,(53)(54) which employs a hydrocracker and a hydrotreater as only refinery conversion units. A similar process scheme is used in the Oryx GTL plant in Qatar, but consists of only a single conversion unit, namely a hydrocracker. The considerable difference in refinery complexity between commercial HTFT and LTFT facilities would explain the focus on LTFT technology and the virtual absence of interest in HTFT technology. However, a fact that is conveniently glossed over, is that the products from LTFT refining cannot directly be sold as transportation fuel. Despite the excellent performance of LTFT diesel,(55) it does not meet Euro-4 specifications, nor does it comply with any of the fuel categories in the World-wide Fuel Charter.(56) Furthermore, the naphtha, which is in the motor-gasoline boiling range, has to be sold as paraffins in the chemicals market, or as cracker feedstock,(57) due to its poor transportation fuel properties. The refining of Fischer-Tropsch products to yield on specification fuels is seemingly quite complex and belies the assumption that is often made in Fischer-Tropsch literature, namely that Fischer-Tropsch syncrude can be refined in an analogous way to crude oil.
This view is further strengthened by the fact that the commercial FT refineries have been built with conventional refining technology. The tacit assumption that is erroneously made is that conventional refining technology can be used without modification. In practice crude oil refining technologies must be adapted to deal with syncrude feed peculiarities. Furthermore, imposing crude oil refining methodology on a syncrude refinery comes at a high cost, as can be seen from the recent upgrade of the Sasol Synfules (Secunda) refineries. To comply with the 2006 South African fuel specifications, which is not yet on par with Euro-4 specifications, Sasol had to spend 13 000 000 000 SA Rand.q,(58) This is more than the cost of a brand new crude oil refinery of similar capacity. This view has been challenged and it has been shown that a refinery design optimised for the refining of HTFT syncrude can be less complex, cheaper and more environmentally friendly than a crude oil refinery of similar capacity.(59)(60) This view was reinforced by the success of the syncrude specific contingency plans implemented at Sasol Synfuels,(61) which actually demonstrated that the 2006 South African fuel specificationsr could met without much capital. There is consequently a need to study FT syncrude refining, as opposed to crude oil refining, as a topic in its own right.

Fuel Specifications

The technical, environmental and political origins for motor-gasoline (petrol), diesel fuel and aviation turbine fuel (jet fuel) specifications are discussed. Some general trends were found and used to predict likely future specifications. Motor-gasoline specifications are moving towards unleaded, low sulphur (≤10 μg·g -1), low benzene (≤1%), high-octane fuel, with a likely reduction in olefin content to 10%. The oxygenate composition and content of motor- gasoline is mostly driven by politics and significant country specific differences are expected. Diesel specifications are focussed on meeting emission standards by reducing sulphur (≤10 μg·g -1). Other directional changes are to increase cetane, lower polynuclear aromatics, decrease the T95 boiling point and narrowing the density and viscosity ranges. The mandatory inclusion of biodiesel as blending component is likely. Little change in jet fuel specifications are expected, although a reduction in naphthalene and sulphur content might be seen in future, as jet fuel evolves synergistically with the changes in motor-gasoline and diesel specifications.

Introduction

Fuel specifications have both technical and political origins. Most vehicle owners know little or nothing about fuel and assume that the stuff they put in their car or truck will allow the engine to start and to keep it running despite the variable demands imposed on the engine by different driving conditions. Moreover, they want it to be cheap and readily available. Vehicle manufacturers want their engines to live up to these expectations, so that they can sell more vehicles and keep their business thriving.
The general population, irrespective of whether they own a vehicle or not, cares about the cost of transport. Traffic conditions may also bring other aspects of fuel performance and properties to people’s attention, like air pollution, soil and water contamination and they might even be convinced by the popular press that CO2 induced global warming is a reality. Then there are the refiners, who are not charity organisations as most people would want them to be, but are in the business of producing transportation fuel to make money. Transportation fuel is therefore constantly under scrutiny for both technical reasons, like engine performance and protection, as well as political reasons, like pollution control, environmental protection, cost and the opportunity to gain free publicity. Presently the main drivers for specification changes are environmental protection and health.(1) The problem faced by refiners and vehicle manufacturers, who are at the technical end of the equation, is that they have to make the marriage between transportation fuel and engine performance work.
This in itself is a source of many changes in the quality of transportation fuels.(2) These changes have to happen while obliging some political rulings that are based on political expediency,a,(3) like the banning of methyl tert-butyl ether (MTBE),(4)(5)(6) the former sweetheart of politicians and public alike, or the inclusion of bio-derived feed materials such as ethanol,(7) which is not necessarily a good thing.(8) The technical merits of some political decisions can be questioned, but such political decisions are defining the reality within which refiners must operate. Nevertheless, fuel specifications protect the public, vehicle manufacturers and refiners, since it sets the ground rules of the transportation fuel business. Irrespective of the political influences, there are some fuel properties that are necessary to meet the technical demands of engine performance and emission control too. These properties are embodied in the fuel specifications and have a rational basis that will be explored.
Based on the fundamentals of fuel chemistry, combustion and engine design, predictions can be made about future fuel specifications. This is important, since any text dealing with refining has to look well beyond the present. Changes in fuel specifications create challenges for refiners, because modifying refinery infrastructure to comply with new fuel specifications not only takes time to plan and implement, but also comes at a cost, which can be quite substantial. Predictions are not reality and the political drivers are often even more important than technical drivers in determining the legislation that will ultimately govern fuel specifications. Fuel specifications will be explored in such a way that the rationale behind it becomes clear. Only the three main transportation fuels produced by refineries will be dealt with in detail, namely motor-gasoline (petrol), diesel fuel and aviation turbine fuel (jet fuel). The fuel properties will be dealt with in general terms, but specific reference will be made to its impact on Fischer-Tropsch derived fuel where applicable.

Table of Contents :

• Chapter I – Prologue
  • 1. Introduction
  • 1.1. Crude oil for transportation fuel
  • 1.2. Energy insecurity
  • 1.3. Fischer-Tropsch technology
  • 1.4. Refining of Fischer-Tropsch products
  • 2. Justification
  • 2.1. Academic justification
  • 2.2. Industrial justification
  • 3. Aim and scope
  • 3.1. Thesis part 1: Background
  • 3.2. Thesis part 2: Refining technology selection and refinery design
  • 4. Literature cited
• Chapter II – Fuel Specifications
  • 1. Introduction
  • 2. Motor-gasoline (petrol)
  • 2.1. International motor-gasoline specifications
  • 2.2. Motor-gasoline properties
    • 2.2.1. Octane number
    • 2.2.2. Volatility
    • 2.2.3. Density
    • 2.2.4. Oxygenate content
    • 2.2.5. Olefin content
    • 2.2.6. Aromatics content
    • 2.2.7. Sulphur content
    • 2.2.8. Metal content
    • 2.2.9. Oxidation stability and gum formation
  • 3. Diesel
  • 3.1. International diesel specifications
  • 3.2. Diesel properties
    • 3.2.1. Cetane number
    • 3.2.2. Flash point
    • 3.2.3. Density and viscosity
    • 3.2.4. Aromatics content
    • 3.2.5. Sulphur content
    • 3.2.6. Lubricity
    • 3.2.7. Cold flow properties
  • 4. Aviation turbine fuel (jet fuel)
  • 4.1. International specifications
  • 4.2. Aviation turbine fuel properties
    • 4.2.1. Heat of combustion
    • 4.2.2. Density and viscosity
    • 4.2.3. Aromatic content and smoke point
    • 4.2.4. Sulphur content
    • 4.2.5. Freezing point
    • 4.2.6. Volatility
    • 4.2.7. Thermal stability
  • 5. Trends for future fuel specifications
  • 5.1. Future motor-gasoline
  • 5.2. Future diesel
  • 5.3. Future aviation turbine fuel
  • 6. Literature cited
  • A. Origin of the European Euro-3 and Euro-4 fuel specifications
  • B. Theoretical limitations and implications of petrol specifications
• Chapter III – Crude Oil
  • 1. Introduction
  • 2. Crude oil composition
  • 2.1. Hydrocarbons
  • 2.2. Sulphur containing compounds
  • 2.3. Oxygen containing compounds
  • 2.4. Nitrogen containing compounds
  • 2.5. Metal containing compounds
  • 3. Crude oil physical properties
  • 3.1. Density
  • 3.2. Pour point
  • 3.3. Viscosity
  • 3.4. Vapour pressure
  • 3.5. Distillation
  • 4. Literature cited
• Chapter IV – Crude Oil Refineries
  • 1. Introduction
  • 2. Separation processes
  • 3. Conversion processes
  • 3.1. Residue upgrading
  • 3.1.1. Conversion of residue to fuels
  • 3.1.2. Non-fuels application of residues
  • 3.2. Diesel and jet fuel upgrading
  • 3.3. Naphtha and gas upgrading
  • 4. Future crude oil refineries
  • 4.1. Change drivers in crude oil refining
  • 4.2. Changes in crude oil refining
  • 4.3. Future crude oil refineries
  • 5. Literature cited
• Chapter V – Fischer-Tropsch Syncrude
  • 1. Introduction
  • 2. Fischer-Tropsch catalysis
  • 2.1. Catalyst properties
    • 2.1.1. Probability of chain growth
    • 2.1.2. Hydrogenation activity
    • 2.1.3. Water gas shift activity
  • 2.1.4. Sensitivity to promoters
  • 2.2. Influence of operating conditions
    • 2.2.1. Synthesis gas composition
    • 2.2.2. Pressure
    • 2.2.3. Temperature
  • 3. Syncrude composition
  • 3.1. Hydrocarbons
  • 3.2. Oxygenates
  • 3.3. Metal containing compounds
  • 4. Properties of commercial syncrudes
  • 5. Comparison of crude oil and syncrude
  • 6. Literature cited
• Chapter VI – Fischer-Tropsch Refineries
  • 1. Introduction
  • 2. German technology (1930-1940’s)
  • 2.1. Normal-pressure cobalt Fischer-Tropsch synthesis
  • 2.2. Refining of normal-pressure syncrude
  • 3. United States technology (1940-1950’s)
  • 3.1. Hydrocol Fischer-Tropsch synthesis
  • 3.2. Refining of Hydrocol syncrude
  • 4. Sasol 1 technology (1950’s)
  • 4.1. Kellogg Fischer-Tropsch synthesis
  • 4.2. Arge Fischer-Tropsch synthesis
  • 4.3. Sasol 1 gas loop
  • 4.4. Sasol 1 refinery
    • 4.4.1. Sasol 1 tar work-up
    • 4.4.2. Sasol 1 Kellogg oil work-up
    • 4.4.3. Sasol 1 Arge oil work-up
    • 4.4.4. Sasol 1 chemical work-up
  • 5. South African Sasol 2 and 3 technology (1970-1980’s)
  • 5.1. Sasol Synthol Fischer-Tropsch synthesis
  • 5.2. Sasol 2 and 3 gas loops
  • 5.3. Sasol 2 and 3 refineries
    • 5.3.1. Sasol 2 and 3 tar work-up
    • 5.3.2. Sasol 2 and 3 condensate and oil work-up
    • 5.3.3. Sasol 2 and 3 chemical work-up
  • 6. Mossgas gas-to-liquids technology (1980-1990’s)
  • 6.1. Mossgas Fischer-Tropsch synthesis
  • 6.2. Mossgas gas loop
  • 6.3. Mossgas refinery
    • 6.3.1. Mossgas oil and condensate work-up
    • 6.3.2. Mossgas chemical work-up
  • 7. Shell gas-to-liquids technology (1980-1990’s)
  • 7.1. Shell Bintulu Fischer-Tropsch synthesis
  • 7.2. Shell Bintulu gas loop
  • 7.3. Shell Bintulu refinery
  • 8. Sasol gas-to-liquids technology (2000’s)
  • 8.1. Oryx GTL Fischer-Tropsch synthesis
  • 8.2. Oryx GTL gas loop
  • 8.3. Oryx GTL refinery
  • 9. Evolution of Sasol Fischer-Tropsch refineries
  • 9.1. Evolution of Sasol
  • 9.2. Evolution of Sasol 2 and
  • 10. Future Fischer-Tropsch refineries
  • 10.1. Change drivers in Fischer-Tropsch refining
  • 10.2. Design of future Fischer-Tropsch refineries
  • 11. Literature cited
• Chapter VII – Refining technologies evaluated in Fischer-Tropsch context
  • 1. Introduction
  • 2. Olefin conversion
  • 2.1. Double bond isomerisation
  • 2.2. Oligomerisation
  • 2.3. Olefin skeletal isomerisation
  • 2.4. Etherification
  • 2.5. Aliphatic alkylation
  • 2.6. Aromatic alkylation
  • 2.7. Metathesis
  • 3. Hydrogen addition
  • 3.1. Hydrotreating
  • 3.2. Hydroisomerisation
  • 3.3. Hydrocracking
  • 4. Carbon rejection
  • 4.1. Fluid catalytic cracking
  • 4.2. Coking
  • 5. Hydrogen rejection
  • 5.1. Thermal cracking
  • 5.2. Catalytic reforming
  • 5.3. Aromatisation
  • 5.4. Alcohol dehydration
  • 6. Discussion
  • 7. Literature cited
• Chapter VIII – Refinery design
  • 1. Introduction
  • 2. Conceptual refinery design
  • 3. Real-world refinery design
  • 3.1. Refinery type
  • 3.2. Products and markets
  • 3.3. Feedstock
  • 3.4. Location
  • 3.5. Secondary design objectives
  • 3.6 Other issues
  • 4. Literature cited
• Chapter IX – Conceptual Fischer-Tropsch refinery designs
  • 1. Introduction
  • 2. Modelling details
  • 2.1. Conceptual design
  • 2.2. Refinery economics
  • 3. Motor-gasoline refineries
  • 3.1. HTFT motor-gasoline refinery development
    • 3.1.1. HTFT paraffinic motor-gasoline
    • 3.1.2. HTFT aromatic motor-gasoline
    • 3.1.3. HTFT olefinic motor-gasoline
    • 3.1.4. HTFT oxygenated motor-gasoline
  • 3.2. HTFT motor-gasoline refinery flowschemes
    • 3.2.1. Flowscheme
    • 3.2.2. Flowscheme
    • 3.2.3. Flowscheme
  • 3.3. LTFT motor-gasoline refinery development
    • 3.3.1. Catalytic cracking of LTFT wax
    • 3.3.2. Hydrocracking of LTFT wax
  • 3.4. LTFT motor-gasoline refinery flowschemes
    • 3.4.1. Flowscheme
    • 3.4.2. Flowscheme
    • 3.4.3. Flowscheme
  • 4. Jet fuel refineries
  • 4.1. HTFT jet fuel refinery development
  • 4.2. HTFT jet fuel refinery flowschemes
    • 4.2.1. Flowscheme
    • 4.2.2. Flowscheme
    • 4.2.3. Flowscheme
  • 4.3. LTFT jet fuel refinery development
  • 4.4. LTFT jet fuel refinery flowschemes
  • 4.4.1. Flowscheme
  • 5. Diesel fuel refineries
  • 5.1. HTFT diesel fuel refinery development
  • 5.2. HTFT diesel fuel refinery flowschemes
    • 5.2.1. Flowscheme
    • 5.2.2. Flowscheme
    • 5.2.3. Flowscheme
  • 5.3. LTFT diesel fuel refinery development
  • 5.4. LTFT diesel fuel refinery flowschemes
    • 5.4.1. Flowscheme
    • 5.4.2. Flowscheme
    • 5.4.3. Flowscheme
  • 6. Literature cited
  • A. Design basis for conceptual refinery development
• Chapter X – Conclusion
  • 1. Introduction
  • 2. What has been achieved?
  • 2.1. Thesis part 1: Background
  • 2.2. Thesis part 2: Refining technology and refinery design
  • 3. Prospects for future study

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