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- Herausgeber: Wiley-VCH
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
Greener Fischer-Tropsch Processes
How can we use our carbon-based resources in the most responsible manner? How can we most efficiently transform natural gas, coal, or biomass into diesel, jet fuel or gasoline to drive our machines? The Big Questions today are energy-related, and the Fischer-Tropsch process provides industrially tested solutions.
This book offers a comprehensive and up-to-date overview of the Fischer-Tropsch process, from the basic science and engineering to commercial issues. It covers industrial, economic, environmental, and fundamental aspects, with a specific focus on “green” concepts such as sustainability, process improvement, waste-reduction, and environmental care.
The result is a practical reference for researchers, engineers, and financial analysts working in the energy sector, who are interested in carbon conversion, fuel processing or synthetic fuel technologies. It is also an ideal introductory book on the Fischer-Tropsch process for graduate courses in chemistry and chemical engineering.
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Veröffentlichungsjahr: 2013
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Contents
Cover
Related Titles
Title Page
Copyright
Preface
List of Contributors
Part One: Introduction
Chapter 1: What is Fischer–Tropsch?
Synopsis
1.1 Feedstocks for Fuel and for Chemicals Manufacture
1.2 The Problems
1.3 Fuels for Transportation
1.4 Feedstocks for the Chemical Industry
1.5 Sustainability and Renewables: Alternatives to Fossil Fuels
1.6 The Way Forward
1.7 XTL and the Fischer–Tropsch Process (FTP)
1.8 Alternatives to Fischer–Tropsch
References
Part Two: Industrial and Economics Aspects
Chapter 2: Syngas: The Basis of Fischer–Tropsch
Synopsis
2.1 Syngas as Feedstock
2.2 Routes to Syngas: XTL (X = Gas, Coal, Biomass, and Waste)
2.3 Water-Gas Shift Reaction (WGSR)
2.4 Synthesis Gas Cleanup
2.5 Thermal and Carbon Efficiency
2.6 The XTL Gas Loop
2.7 CO2 Production and CO2 as Feedstock
References
Chapter 3: Fischer–Tropsch Technology
Synopsis
3.1 Introduction
3.2 Industrially Applied FT Technologies
3.3 FT Catalysts
3.4 Requirements for Industrial Catalysts
3.5 FT Reactors
3.6 Selecting the Right FT Technology
3.7 Selecting the FT Operating Conditions
3.8 Selecting the FT Catalyst Type
3.9 Other Factors That Affect FT Technology Selection
References
Chapter 4: What can we do with Fischer–Tropsch Products?
Synopsis
4.1 Introduction
4.2 Composition of Fischer–Tropsch Syncrude
4.3 Syncrude Recovery after Fischer–Tropsch Synthesis
4.4 Fuel Products from Fischer–Tropsch Syncrude
4.5 Lubricants from Fischer–Tropsch Syncrude
4.6 Petrochemical Products from Fischer–Tropsch Syncrude
References
Chapter 5: Industrial Case Studies
Synopsis
5.1 Introduction
5.2 A Brief History of Industrial FT Development
5.3 Industrial FT Facilities
5.4 Perspectives on Industrial Developments
References
Chapter 6: Other Industrially Important Syngas Reactions
Synopsis
6.1 Survey of CO Hydrogenation Reactions
6.2 Syngas to Methanol
6.3 Syngas to Dimethyl Ether (DME)
6.4 Syngas to Ethanol
6.5 Syngas to Acetic Acid
6.6 Higher Hydrocarbons and Higher Oxygenates
6.7 Hydroformylation
6.8 Other Reactions Based on Syngas
References
Chapter 7: Fischer–Tropsch Process Economics
Synopsis
7.1 Introduction and Background
7.2 Market Outlook (Natural Gas)
7.3 Capital Cost
7.4 Operating Costs
7.5 Revenues
7.6 Economics and Sensitivity Analysis
References
Part Three: Fundamental Aspects
Chapter 8: Preparation of Iron FT Catalysts
Synopsis
8.1 Introduction
8.2 High-Temperature Fischer–Tropsch (HTFT) Catalysts
8.3 Low-Temperature Catalysts
8.4 Individual Steps
References
Chapter 9: Cobalt FT Catalysts
Synopsis
9.1 Introduction
9.2 Early German Work
9.3 Support Preparation
9.4 Addition of Cobalt and Promoters
9.5 Calcination
9.6 Reduction
9.7 Catalyst Transfer
9.8 Catalyst Attrition
9.9 Addendum Recent Literature Summary
References
Chapter 10: Other FT Catalysts
Synopsis
10.1 Introduction
10.2 Ni Catalysts
10.3 Ruthenium Catalysts
10.4 Rhodium Catalysts
10.5 Other Catalysts and Promoters
References
Chapter 11: Surface Science Studies Related to Fischer–Tropsch Reactions
Synopsis
11.1 Introduction: Surfaces in Catalysts and Catalytic Cycles
11.2 Heterogeneous Catalyst Characterization
11.3 Species Detected on Surfaces
11.4 Theoretical Calculations
References
Chapter 12: Mechanistic Studies Related to the Fischer–Tropsch Hydrocarbon Synthesis and Some Cognate Processes
Synopsis
12.1 Introduction
12.2 Basic FT Reaction: Dissociative and Associative Paths
12.3 Some Mechanisms-Related Experimental Studies
12.4 Current Views on the Mechanisms of the FT-S
12.5 Now: Toward a Consensus?
12.6 Dual FT Mechanisms
12.7 Cognate Processes: The Formation of Oxygenates in FT-S
12.8 Dual Mechanisms Summary
12.9 Improvements by Catalyst Modifications
12.10 Catalyst Activation and Deactivation Processes
12.11 Desorption and Displacement Effects
12.12 Directions for Future Researches
12.13 Caveat
References
Part Four: Environmental Aspects
Chapter 13: Fischer–Tropsch Catalyst Life Cycle
Synopsis
13.1 Introduction
13.2 Catalyst Manufacturing
13.3 Catalyst Consumption
13.4 Catalyst Disposal
References
Chapter 14: Fischer–Tropsch Syncrude: To Refine or to Upgrade?
Synopsis
14.1 Introduction
14.2 Wax Hydrocracking and Hydroisomerization
14.3 Olefin Dimerization and Oligomerization
References
Chapter 15: Environmental Sustainability
Synopsis
15.1 Introduction
15.2 Impact of FT Facilities on the Environment
15.3 Water and Wastewater Management
15.4 Solid Waste Management
15.5 Air Quality Management
15.6 Environmental Footprint of FT Refineries
References
Part Five: Future Prospects
Chapter 16: New Directions, Challenges, and Opportunities
Synopsis
16.1 Introduction
16.2 Why Go Along the Fischer–Tropsch Route?
16.3 Considerations against Fischer–Tropsch Facilities
16.4 Opportunities to Improve Fischer–Tropsch Facilities
16.5 Fundamental Studies: Keys to Improved FT Processes
16.6 Challenges for the Future
16.7 Conclusions
References
Glossary
Index
Related Titles
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Preface
And what is a man without energy? Nothing. Nothing at all
(Mark Twain)
Energy and persistence conquer all things
(Benjamin Franklin)
This book on Fischer-Tropsch is a study of aspects of energy: how it is produced and transformed today, with special reference to liquid fuels such as those used to drive cars, buses, planes, and other forms of transportation.
We still live in an era of relatively plentiful and cheap fuel, mostly derived from the fossilized organic materials: coal, oil, and natural gas.
New supplies are being discovered all the time and brought into use in quite surprising ways. A good example is natural gas for which it is now estimated that, because of the emergence of techniques such as fracking, the world's reserves may well be enough for around 200 years. This is close to being on a par with coal and much greater than our oil reserves. However, our assets of fossil fuels are limited and, in fairness to the next generations, we must not squander them.
We must learn to use them to buy time until a better and really sustainable source of energy becomes available.
The advantages of natural gas are considerable in comparison to those of coal or oil: it is much easier to clean and much easier to transport from where it occurs in nature to where it is required for work, warmth, and recreation. Compared to oil or coal, the main disadvantage of natural gas is that since it has a large volume for the equivalent energy content, a good pipeline infrastructure or the equivalent is needed.
For deposits that are small, in remote locations, or accumulations that are far from consumers, transportation by pipeline may not be economical. It is for these situations that the Fischer–Tropsch technology is particularly useful, since it enables the conversion into liquid products.
For coal the position is different. Although coal can be transported more simply than gas, cleaning it is a major task and ultimately it must also be converted into a refineable liquid product, before it can be turned into transportation fuels or chemicals. Fischer–Tropsch conversion is again a useful way to achieve this goal.
A considerable problem with all carbon-based fuels is that they produce carbon dioxide when burned. Atmospheric carbon dioxide is a “greenhouse gas,” which when present in large quantities is widely believed to have serious consequences for the climate of our planet.
It can be argued that one should not consider carbon-based fuels and chemicals technology for the future. Unfortunately, at present we have few viable alternatives to fossil fuels on the scale that is required to meet the energy needs of a world population that is already at around 7 billion and still increasing rapidly. Although most of our energy comes from the sun, the direct use of solar power to produce biofuels or to generate hydrogen on industrial scales is still a long way off. In the meantime, we will have to continue to rely on the power of the sun indirectly, via fossil fuels. The question then becomes: even if it is only an interim measure, how can we use our carbon-based resources in the most responsible manner?
The immediate challenge is the efficient transformation of one form of fossil fuel energy into another; in other words, how can we most efficiently transform natural gas, coal, or oil into say diesel or gasoline that we can harness to drive our machines. Even this is a vast task, but it is one that is being tackled very effectively through the Fischer-Tropsch process. That is what this book is about, an up to date review of the fundamental chemical, industrial, economic and environmental aspects of the Fischer-Tropsch process.
Acknowledgments
We have had a lot of help in producing the book and we thank our many friends and colleagues, including Paul Arwas, Norman Basco, Gian Paolo Chiusoli, Allen Hill, Brian James, Tom Lawrence, Kenichi Maruya, Peter Portious, Marco Ricci, Sally Maitlis, Julia Weinstein, and Valerio Zanotti, for reading parts and making helpful comments Above all, we give our warmest thanks to Marion, Peter Maitlis's wife, and Chèrie, Arno de Klerk's wife, for the help, patience, understanding, and love they showed while we worked on this book
Universityof Sheffield, UK
University of Alberta, Canada
October 2012
Peter M. MaitlisArno de Klerk
List of Contributors
Letizia Bua
Research Center for Non-Conventional Energy
Eni Istituto Donegani
via Fauser, 4
28100 Novara
Italy
Vincenzo Calemma
Eni S.p.A. – Refining & Marketing Division
via Felice Maritano, 26
20097 S. Donato Milanese
Milan – Italy
Lino Carnelli
Research Center for Non-Conventional Energy
Eni Istituto Donegani
via Fauser, 4
28100 Novara
Italy
Burtron H. Davis
University of Kentucky
Center for Applied Energy Research
2540 Research Park Drive
Lexington
KY 40511
USA
Arno de Klerk
University of Alberta
Chemical & Materials Engineering
9107 – 116 Street
Edmonton
Alberta T6G 2V4
Canada
Yong-Wang Li
Chinese Academy of Science
Institute of Coal Chemistry
Beijing
China
Peter M. Maitlis
University of Sheffield
Department of Chemistry
Sheffield S3 7HF
UK
Roberta Miglio
Eni SPA – Exploration & Production Division
San Donato Milanese
20097 Milan
Italy
Alessandra d'Arminio Monforte
Research Center for Non-Conventional Energy
Eni Istituto Donegani
via Fauser, 4
28100 Novara
Italy
Julius Pretorius
Alberta Innovates Technology Futures
250 Karl Clark Road
Edmonton
Alberta T6N 1E4
Canada
Cecilia Querci
Research Center for Non-Conventional Energy
Eni Istituto Donegani
via Fauser, 4
28100 Novara
Italy
Marco Ricci
Research Center for Non-Conventional Energy
Eni Istituto Donegani
via Fauser, 4
28100 Novara
Italy
Roberto Zennaro
Eni S.p.A. – Exploration & Production Division
via Emilia, 1
20097 San Donato Milanese
Milan – Italy
Part One
Introduction
1
What is Fischer–Tropsch?
Peter M. Maitlis
Synopsis
Some of the fundamental and most frequently used terms are explained. Fischer–Tropsch (FT) technology involves the conversion of syngas (a mixture of CO and H2) into liquid hydrocarbons. It is a key element in the industrial conversion processes X-To-Liquids (XTL), where X = C, coal; G, natural gas; B, biomass; or W, organic waste. For example, a gas-to-liquids (GTL) process converts natural gas into syncrude, a mixture mainly of long-chain hydrocarbons. The conversion reactions are usually catalyzed by metals (iron, cobalt, and sometimes ruthenium) often carried on oxide supports such as silica or alumina. The liquid hydrocarbons are important sources of transportation fuels and of specialty chemicals. Syngas is now mainly obtained from coal, oil, or natural gas, but will in future be increasingly made from renewable sources such as biomass or organic waste. Since the available reserves of fossil fuels are diminishing, the renewables should provide more sustainable feedstocks in the long term.
1.1 Feedstocks for Fuel and for Chemicals Manufacture
Syngas, the name given to a mixture of carbon monoxide and hydrogen, is the lifeblood of the chemicals industry and helps to provide a lot of our energy. It can be made from many sources, including coal, natural gas, organic waste, or biomass. The Fischer–Tropsch (FT) process converts syngas catalytically into organic chemicals, mainly linear alkenes and alkanes, which are used as both liquid fuels and feedstocks for making further useful chemicals. Some oxygenates can also be formed (chiefly methanol and ethanol) (see Chapters 4 and 6).
Alkene and alkane formation in the FT-Hydrocarbon Synthesis can be summarized as follows:
(1.1)
(1.2)
Energy has been said to be “the single most important scientific and technological challenge facing humanity in the twenty first century” [1], and we agree. There is the global requirement for more energy, especially as transportation fuels, as populations increase in number and sophistication. In addition, there is also a more specific need for new feedstocks for chemicals manufacture. As we will see, these two needs have features in common. And above all, we recognize the imperative now demanded by Society to produce both fuel and feedstocks in an environmentally acceptable and preferably sustainable manner. We also aim to correct some of the erroneous beliefs and myths present in the energy and chemicals sectors in order that our students, who will be tomorrow's academic and industrial leaders, have reliable foundations on which to build.
Mankind literally lives off energy. Most of it comes from the sun, indirectly via plants that use carbon dioxide and water to grow. Eventually they die and decay and, very slowly, over geological timescales, are turned into the fossil fuels (coal, oil, natural gas) that we extract and combust to provide heat, light, and other forms of power [2].
1.2 The Problems
There are two main problems with fossil fuels: the reserves are finite and slowly running out and, since all fossil fuels contain combined carbon, their combustion (oxidation) produces carbon dioxide, which accumulates in the atmosphere and which is likely to have serious consequences for the climate of our planet. Combustion also generates other materials that can harm mankind and the environment, such as CO, oxides of sulfur and nitrogen, and metallic oxide ashes, arising from incomplete oxidation and from impurities in the fuel.
For some end-uses there are many alternatives to fossil fuels, such as hydroelectric and nuclear power and others that are being developed commercially, including solar, wind, tidal, and geothermal power. The latter technologies will play their very important role mainly by providing electric power via large fixed installations. However, they will not have a direct part in providing more liquid transportation fuels or new feedstocks for the chemicals industry.
Why should Fischer–Tropsch be the approach to replace or supplement crude oil as a source of transportation fuels, gasoline (in the United States), or petrol and diesel (in the United Kingdom)? Today transportation fuels from crude oil must undergo extensive cleaning to remove materials containing heteroatoms (N, S, metals, etc.) from the raw feedstocks; if these materials are not removed, the impurities will quickly spoil and deactivate the catalyst. The amounts of hydrogen and energy needed for this cleaning have steadily increased as the crude oils have become heavier (i.e., more impure) over the years. Today, about 15–20% of the energy in the oil is required to produce environmentally acceptable transportation fuels, and the percentage can only increase as the crude becomes heavier. Thus, the energy advantage of crude oil over other fossil fuels is becoming narrower as time passes. Even today (2012), one is able to convert coal (a very “dirty” material) into transportation fuels in a Fischer–Tropsch process at a cost that is competitive with crude oil.
The environmental properties of the FT-synthesized transportation fuels meet or usually exceed those of crude oil-derived fuels. There are of course a number of other approaches that can be used for converting coal into transportation fuels. For example, the Exxon-Mobil methanol to gasoline process is able to convert coal first into syngas, then methanol, and then gasoline; however, the gasoline obtained by this process is high in aromatics and essentially no diesel range fuels are produced. Another variation converts the coal to low molecular weight alkenes and then further to gasoline and diesel range fuels; however, the diesel that is produced will be multiple branched and have a lower cetane number than the FT diesel.
Environmental concerns today cause governments to provide subsidies to allow renewable fuels to be utilized, as, for example, ethanol in the United States. Even without this subsidy, FT fuels are competitive with the subsidized renewables in some areas. In addition, improvements in gasification procedures are allowing fuels to be obtained from a mixture of renewables and coal so that the FT oil will have the environmental advantage over crude oil.
1.3 Fuels for Transportation
1.3.1 Internal Combustion Engines
The form in which the energy is available is important. Although it has been done (e.g., in wartime), it is unrealistic to try and run cars, trucks, or planes on coal, wood, or natural gas. Wikipedia has estimated that there were over 1 billion cars and light trucks on the road in 2010. As motor vehicles are now manufactured in many countries, developed as well as developing, the total must exceed 1.1 billion (109) quite soon. Almost all of them run on liquid hydrocarbons and it has been estimated that they burn well over 1 billion cubic meters (1 Bcm, 260 billion US gallons, or 8.5 × 108 tons) of fuel each year. The engineering has been well worked out so that the internal combustion engines are now extremely efficient for the appropriate fuel. The optimum gasoline has a high proportion of branched chain alkanes (giving a high octane number), while the best diesel has a high component of linear alkanes (with a high cetane number). It should be remembered that it will be necessary to continue to provide fuel for all the (older) vehicles at present on our roads, as well as those currently being built and planned.
1.3.2 Electric Cars
There is considerable interest in using electricity for transportation and most manufacturers are making electric cars, as they are perceived to cause less pollution in their immediate neighborhoods. However, there are some serious disadvantages. Some of the problems as well as the benefits of the electric car have been amusingly illustrated by Jeremy Clarkson, the presenter of the BBC TV's very popular car show “Top Gear,” when he reviewed the projected Mini E being built by BMW [5]. This car works well but requires 5088 lithium ion batteries (weighing 260 kg) and even then has a range of only 104 miles, after which it requires charging for 4.5 h. Eventually, the batteries will need replacing, the cost of which does not bear thinking about. The wide acceptance of electric cars depends on the availability of inexpensive and high-power batteries and also on the availability of national networks of fast-charging stations, which are at present hardly on the drawing board. To get round the problems, many manufacturers add on a liquid hydrocarbon fuel motor to extend both the range and the convenience of electric cars. There are many now available or coming on to the market, for example, the hybrid (electric–gasoline) Toyota Prius or the Chevrolet Volt or Ampera.
There are several serious snags on the way to commercially viable electric cars. Not only are the batteries costly and heavy, but also the lithium they require is difficult to source. The provenance of the electricity for recharging them must also be considered. Thus, the US Energy Information Agency estimates that two-thirds of world electricity is generated from fossil fuels (coal 42%, natural gas 21%, and oil 4%), 14% from nuclear and only 19% from renewables. Furthermore, it has been estimated that the average CO2 output for electric cars is 128 g/km compared to an average of 105 g/km for hybrids such as the Toyota Prius, when the emissions from coal- and oil-fired electricity-generating stations are included [6]. If we want to minimize CO2 production by diminishing the use of fossil fuels, given the technology available at present (2012), the nuclear option currently seems the choice for generating sustainable electricity. But that also has serious problems as the disasters at the Chernobyl, Fukushima, and Three Mile Island nuclear plants showed.
1.3.3 Hydrogen-Powered Vehicles
Hydrogen is a very attractive source of power as the only product of combustion is water; unfortunately, large-scale commercial applications are further in the future, even though the science is well known and hydrogen is easily made by splitting water, for example, by electrolysis or solar heating. However, the cost of doing so, in terms of the energy required, makes it very expensive.
Currently, hydrogen is produced mainly by gasification/reforming; thus, hydrogen should be considered a by-product of the petrochemicals industry in the formation of carbon monoxide, for example, from hydrocarbons:
(1.3)
(1.4)
The water-gas shift reaction (WGSR) is then employed to increase the proportion of hydrogen, but this in turn produces carbon dioxide:
(1.5)
Thus, the conventional production of hydrogen today is always associated with the production of CO2.
Perhaps the development of hydrogen-powered fuel cells for cars is a promising direction [7].
One requirement for viable electric or hydrogen-powered transportation systems is the availability of widespread national grids for recharging, the setting up of which will be a mammoth and vastly expensive task. And if the electricity for the grid comes from burning fossil fuels, we have not addressed the sustainability problem – merely moved it sideways to another area.
1.4 Feedstocks for the Chemical Industry
The raw materials for the organic chemicals industry are largely carbon based; in the eighteenth century, the pyrolysis of wood provided useful chemicals. In the nineteenth century, coal tar was exploited as the source of many materials, especially aromatics; while in the twentieth and twenty-first centuries, the feedstocks for many organic chemicals have been derived from oil. To that extent therefore, the supply of feedstocks for chemicals and of fuel for transportation currently run parallel and both depend on nonrenewable resources.
1.5 Sustainability and Renewables: Alternatives to Fossil Fuels
It has been estimated that more solar energy strikes the Earth in 1 h (4.3 × 1020 J) than is currently consumed by all mankind in a year (4.1 × 1020 J). That even allows a great expansion of use as there would be more than enough. Thus, there is a continuing search for usable sources of energy that are either from renewable “biofuels,” and thus will not deplete our reserves, or that utilize sunlight more directly and do not involve organic intermediates, for example, some form of hydrogen generation by splitting water. The main biorenewables are fast-growing plants, trees, or algae, for example, that can be harvested and burned, directly or indirectly, with the carbon dioxide produced going back to feed more plants.
1.5.1 Biofuels
The best-known commercial example of biofuel manufacture is in Brazil where sugarcane grown on a very large scale is harvested and thereby sugar is extracted and fermented into alcohol that is distilled to be sold in filling stations (as bioethanol) to power motor vehicles. Brazil, with a population close to 200 million, has plentiful sunlight, cheap labor, and some government assistance. Prior to the discovery of large offshore oil and gas deposits, it also had the additional stimulus of a lack of home-produced oil fuel. It therefore turned to ethanol to power internal combustion engines, and most Brazilian cars are now able to run on either gasoline or alcohol. Currently, the home-produced ethanol takes care of some 13% of the country's motor fuel needs; the comparable figure is about 4% for the United States [8].
Large amounts of bioethanol, made from maize (corn), are produced in the United States, and ethanol commonly makes up 10% of the fuel at the pump (designated E10). However, it is now recognized that there are major problems with such agriculturally produced fuels. One is that the acreage of arable land needed to grow plants to power transport can seriously hinder the growing of food. This in turn impinges on the cost of food. The energy balance is also more complex than it may appear at first sight since, in addition to sunlight, considerable energy derived from fossil fuels is required to produce the ethanol. Much water is also required, and since water is also a scarce commodity, it must be conserved and recovered, which will also require energy.
It has been calculated that irrespective of crop, one acre of land, pond, or bioreactor can annually yield enough amount of biomass to fuel one motor vehicle or meet the calorific requirement of several people. This amount of biomass therefore makes only a very small contribution to our present road transport requirements and yet can contribute significantly to global food shortages and rising prices [9, 10]. New technology to make ethanol based on lignocellulose, and which does not depend on food crops, is being actively pursued. Thus, while biomass is used as a renewable fuel, it is not yet the cure-all the world is seeking.
Other forms of biofuels are also known, such as biodiesel made from waste fats (long-chain esters); however, this has not been promoted to the extent of bioethanol and is likely to remain a minor source of energy for transportation.
1.5.2 Other Renewable but Nonbio Fuels
The production of energy by such means that do not involve biointermediates is a very active area of science research. There are many ways to harness solar energy: using photovoltaic cells or solar furnaces, it can be turned directly into electricity. Wind and tidal power can also be similarly harnessed; however, all these sources have the disadvantage that the energy is not continuously produced and the electricity must be stored and cabled to the site where it is needed. Although the technology to mass-produce solar cells has improved and in some countries (Germany, Japan, Spain, and Israel) electricity from such devices is beginning to make a significant contribution to the national grid, the cost of solar power is currently estimated to be between 10 and 20 times that of power from burning coal. Storage on the scale needed to ensure that power is available nationally even during hours of darkness has also lagged behind. Because fossil fuels are still abundant and inexpensive, non-biorenewables are not likely to play a large role in primary power generation until technological or cost breakthroughs are achieved, or environment-driven carbon taxes are brought in.
1.6 The Way Forward
So, where do we go? If the large-scale use of electric and hydrogen-powered cars is only over the horizon and renewable biofuels will supply a small fraction of our needs for transportation, we must make the best of what we have by improving our tools to deal with our present resources. Since major discoveries of oil and gas and coal are still being made, exact numbers are imprecise, but current best estimates indicate that our planet has enough reserves of oil for about 50 years and of natural gas for perhaps 150–200 years at current consumption levels. Coal is more plentiful and some 100–200 years supply may be available. However, the important factor is how difficult (i.e., how expensive) it will become to extract these fuels: cost is very likely to determine the uses to which fossil fuels will be put in future. The other side of the argument is of course the growth in carbon dioxide. The EIA estimates that annual CO2 emissions will rise from the 2007 level of about 29.7 billion tons to around 42.4 billion tons by 2035. This 43% increase is likely to have a significant effect on many aspects of our lives, in particular through changes in our climate.
For the twin reasons of conserving our fossil fuels and curbing the increase in CO2 levels, our primary concern should be in using our resources better and more efficiently. One way to do that is to improve the conversions of the raw materials into conveniently usable fuels and/or chemicals. Doing that is not necessarily straightforward or obvious. Taking natural gas (which is largely methane) as an example, while direct approaches such as partial oxidation of methane to methanol or to higher alkanes may become commercially viable in the future, the best way currently is to reform the natural gas into syngas (CO + H2) and then to build on that. The engineering needed for reforming is well established and there are many well worked out reactions making useful products from syngas. One of these is of course the Fischer–Tropsch hydrocarbon synthesis in which the syngas is converted into linear hydrocarbons that can be used either as fuel (diesel) or as chemical feedstocks. Our thesis therefore is that improvements to Fischer–Tropsch are desirable, possible, and necessary and should be developed as soon as practicable. Some other paths that are being followed are outlined in Section 1.7.
1.7 XTL and the Fischer–Tropsch Process (FTP)
The Fischer–Tropsch Process (FTP) is a key part of the technology that is needed to convert one type of carbon-based fuel into another. This in turn allows industry to choose which feedstock and which technique is most suitable for a given purpose. A number of composite technologies known as XTL have been developed: CTL (coal-to-liquids), GTL (gas-to-liquids), BTL (biomass-to-liquids), and WTL (waste-to-liquids). Thus, for example, GTL reforms natural gas (mainly methane) by partial oxidation into syngas:
(1.6)
(1.7)
Alternatively, CTL, for example, makes syngas from coal:
(1.8)
The CO: H2 ratio is adjusted by the catalytic Water-Gas Shift Reaction (WGSR):
(1.5)
and the gases are then led to another reactor where they are contacted with a different metal catalyst (usually iron or cobalt) in the Fischer–Tropsch reaction. For cobalt and other metals, the catalytically active metal is generally deposited as nanoparticles on an oxide such as silica or alumina that was classically thought to act simply as an inert support. Iron-catalyzed reactions are generally carried out over the unsupported (massive) metal. Details of the various XTL processes are given in Chapters 2 and 5.
The product distribution of hydrocarbons formed during the FT process follows an Anderson–Schulz–Flory distribution, expressed as W/N = (1 − α)2αn−1 where W is the weight fraction of hydrocarbon molecules containing N carbon atoms and α is the chain growth probability [11]. This can be visualized by plotting log (W/N) against N, and shows a monotonic decrease from lower to higher molecular mass products, indicative of a step-growth polymerization of a C1 species (see Figure 12.3).
Methane is always the largest single product; however, by bringing α close to one, the total amount of methane formed can be minimized and the formation of long-chain hydrocarbons is increased. Very long-chain hydrocarbons are waxes, which must be cracked in order to produce liquid transportation fuels.
Although the FT process has been applied on a large scale, its universal acceptance has been hampered by high capital costs, high operation and maintenance costs, and environmental concerns. In practice, FT liquid fuels compete with natural gas that can be supplied by conventional gas pipelines and liquefied natural gas (LNG) technology. Thus, FT gas as a feedstock becomes economically viable as a supply of “stranded gas,” in other words, a source of natural gas that is impractical to exploit as it is far from major conurbations.
1.7.1 Some History
The history of the FTP is a classical example of the stepwise development so characteristic of science: there was really no single “Eureka” moment. It also illustrates how closely advances in science and technology are coupled to economic and political circumstances.
In 1902, Sabatier and Senderens reported that a reaction occurred between carbon monoxide and hydrogen to give methane over a nickel catalyst; then, in 1910 Mittasch, Bosch, and Haber developed promoted iron catalysts for ammonia synthesis from hydrogen and nitrogen. That was shortly followed (in 1913) by patents issued to BASF for the production of hydrocarbons and oxygenates by the high-pressure hydrogenation of CO over oxide catalysts. In the 1920s, Fischer and Tropsch working at the Kaiser Wilhem Institute in Berlin first made Synthol (containing oxygenates) by hydrogenation of CO over alkalized iron, and then in 1925, they announced the synthesis of higher hydrocarbons at atmospheric pressures over Co and Ni. The interest in the process grew rapidly and workers in England, Japan, and the United States, especially at the US Bureau of Mines, devoted much time and effort to improving the methodology. Considerable engineering work and catalyst development continued in Nazi Germany, especially during the 1939–1945 World War when the process was used to make motor fuel from coal. Germany was acutely short of oil, but had copious reserves of low-quality brown coal (lignite) that could be turned into fuel for the war effort. The development of new reactor designs for the FTP continued after the War as there were fears that petroleum would be in short supply. With the discovery of large new oilfields, interest in Fischer–Tropsch waned somewhat until the 1970s brought a large increase in the price of oil and sanctions on the export of oil to South Africa. This encouraged SASOL (Suid Afrikaanse Steenkool en Olie, the South African Coal and Oil company) to expand its CTL plants in order to become more self-sufficient [12]. Although the economic and political pressures have long since changed, SASOL has actively continued to develop its processes in both CTL and GTL. They are based on FT technology using iron or cobalt catalysts, and SASOL continues to play a major role in developing new plants in other countries (including Qatar, Nigeria, Egypt, etc.). Shell has also built major FT plants (in Malaysia and Qatar) using cobalt catalysts (see Chapters 3, 5 and 9). Total world production of FT hydrocarbons has been estimated at about 10 million tons per year.
In parallel with the Fischer–Tropsch hydrocarbon synthesis, work continued on another reaction based on syngas and originally developed in Germany: the synthesis of methanol. That came to fruition in 1966 when ICI in the United Kingdom brought in the low pressure process, using a copper–zinc oxide catalyst, which still dominates the technology (see Chapter 6) and currently enables methanol production of about 30 million tons annually.
1.7.2 FT Technology: An Overview
FT processes are currently used commercially to make hydrocarbons by passing syngas over supported metal catalysts. Fe, Co, Ru, Rh, and even Ni all have FT activity, though in somewhat different ways. The most active catalyst is Ru, but it is not used commercially because of its high cost. The original and most commonly used catalyst is Fe, though Shell uses a Co catalyst to make long-chain alkanes (waxes) that are then broken down to smaller alkanes. The cobalt catalyst generally consists of very fine particles of the metal supported on an oxide surface such as silica or alumina. These nanoparticles have the advantage of high activity due to their large surface areas; however, this also makes it easy for impurities to be adsorbed that can affect the performance of the catalyst. In some cases the activity can be improved, but many substances will diminish the activity and selectivity.
Two main regimes have been used: low-temperature Fischer–Tropsch (LTFT), usually at 200–250 °C, that gives long-chain molecules, and the high-temperature Fischer–Tropsch (HTFT), at 320–375 °C, that gives shorter chain molecules. It is fairly generally agreed that the primary products of the reaction are 1-n-alkenes, but under harsher conditions (high pressure of hydrogen, higher temperature, or hydrogenating catalysts such as Co) n-alkanes result. The primary alkene products are also further hydrogenated, isomerized, dehydrogenated, cyclized, carbonylated, or even oxidized, under the reaction conditions and thus a wide spectrum of products can be formed.
The best form of the reactors to be used depends on the catalyst, the conditions, and the distribution of products that is desired. HTFT uses iron catalysts in two-phase fluidized bed reactors; LTFT uses either iron or cobalt in three-phase slurry reactors or tubular fixed bed reactors. Much of the skill in running a successful FT plant comes from the use of properly designed reactors [13].
The silica or alumina support was long believed to play little role in the basic FT reaction, though it was significant in the subsequent, secondary reactions. However, studies by surface scientists have shown that the actual FT catalysis usually takes place at the interface between the metal and the oxide, which can be either the support to or a component of the catalyst.
1.7.3 What Goes on?
In progressing from CO that has one carbon atom to an alkene or alkane, quite a complex series of reactions must be occurring. Essentially however, it is a polymerization of C1 units. The question then arises how this occurs on a metal surface. It is only quite recently that surface scientists have had access to the tools that will allow them to begin to answer this riddle. Thus, there have been many attempts to understand the reactions that occur and many theories, the more important of which are summarized in Chapter 12.
1.7.4 CO Hydrogenation: Basic Thermodynamics and Kinetics
As in all chemical transformations, although the rates are governed by the kinetics of the individual steps, it is important to ensure that the thermodynamics of the steps are favorable or if one step of a sequence is unfavorable, it is coupled to a very favorable one.
As Table 1.1 indicates, the formation of hydrocarbons from CO hydrogenation is generally favored overall, but, as shown in a comparison of the free energies (ΔG°), the reaction can be thought of as driven by formation of water. Thus, making methane also involves making 1 mol of water is more favorable, but higher hydrocarbons are less favored. If free water is not formed, then the thermodynamics are much more difficult as is shown by the positive ΔG° for methanol and glycol; only when some water is also formed, as with ethanol, does the reaction become favored.
Table 1.1 Energetics of CO hydrogenation.
(1.9)
(1.10)
(1.11)
(1.12)
(1.13)
(1.14)
(1.5)
1.8 Alternatives to Fischer–Tropsch
Given that at present the best way of using fossil fuels is to reform them into syngas, some of the alternatives to FT are discussed in Chapter 6. In fact, the highest volume use of syngas is the reaction to methanol, which can be used as a fuel additive and which is also a very useful chemical and a C1 feedstock. Examples include the Mobil process that converts methanol into gasoline over an acid zeolite catalyst (HZSM-5), and the Haldor–Topsoe A/S TIGAS process that uses dimethyl ether for the same transformation. Several processes also exist for converting methanol into olefins. These include the UOP/Norske Hydro process (with a pilot plant in Norway and a demonstration plant in Belgium) and Lurgi has a similar methanol-to-propylene (MTP) process. The Institute of Chemical Physics (in Dalian, China) commissioned the first commercial methanol-to-olefin process (DMTO) in the world in 2010. This has a production capacity of 600 000 tons of lower olefins per year (http://english.dicp.cas.cn/ns/es/201008/t20100811_57266.html.).
Other widely practiced alternatives use syngas together with an organic substrate to extend the chain lengths, as, for example, in the hydroformylation of propene to butanal and isobutanal,
(1.15)
A large number of related reactions of olefins with carbon monoxide, for example, giving acids and esters, are known and some of these are important industrially [14]:
(1.16)
(1.17)
Last but not least, the WGSR is used to greatly increase the proportion of hydrogen in the syngas, which can then be separated and used as a nonpolluting fuel or in a hydrogenation plant. Since the WGSR is an equilibrium, the trouble is that by increasing the amount of hydrogen in syngas, it also increases the amount of the very undesirable CO2.
References
1. Chem. Eng. News (August 22, 2005), quoting Nobel Laureate Rick Smalley, in testimony to the US Senate (April 2004).
2. Lewis, N.S. (2007) MRS Bull., 32, 808–820.
3. BP Statistical Review of World Energy (June, 2011)
4. International Energy Outlook (May, 2010) Energy Information Administration, US Department of Energy, Washington, DC, www.eia.doe.gov/oiaf/ieo/index.html.
5. Sunday Times (August 1, 2010), London.
6. Daily Telegraph (September 4, 2010), London, Car Clinic, September 4.
7. Eberle, U. and von Helmolt, R. (2010) Energy Environ. Sci., 3, 689–699.
8. Ritter, S.K. (June 25, 2007) Chem. Eng. News, p. 15.
9. Walker, D.A. (2010) Ann. Appl. Biol., 156, 319–327.
10. Walker, D.A. (2010) J. Appl. Phycol. doi: 10.1007/s10811-009-9446-5.
11. Dry, M.E. (1981) Chapter 4, in Catalysis Science and Technology, vol. 1 (eds J.R. Anderson and M. Boudart), Springer, Berlin.
12. Anderson, R.B. (1984) The Fischer–Tropsch Synthesis, Academic Press Inc., Orlando.
13. Steynberg, A.P., Dry, M.E., Davis, B.H., and Breman, B.B. (2004) Fischer–Tropsch reactors, in Studies in Surface Science and Catalysis, vol. 152 (eds A.P. Steynberg and M. Dry), Elsevier BV, Amsterdam.
14. For further details of these and related reactions, see Maitlis, P.M. and Haynes, A. (2006) Syntheses based on carbon monoxide, in Metal-Catalysis in Industrial Organic Processes (eds G.P. Chiusoli and P.M. Maitlis), RSC Publishing, Cambridge.
Part Two
Industrial and Economics Aspects
2
Syngas: The Basis of Fischer–Tropsch
Roberto Zennaro, Marco Ricci, Letizia Bua, Cecilia Querci, Lino Carnelli, Alessandra d'Arminio Monforte
Synopsis
Syngas (a mixture of carbon monoxide and hydrogen) is normally made industrially from natural gas or coal. The ratio of H2 to CO can be manipulated by the water-gas shift reaction (WGSR):
Since the WGSR is reversible, carbon dioxide and water are also formed. The significance of gas loops in industrial plants is explained, as is the importance of the formation of CO2. Potential uses are being explored.
2.1 Syngas as Feedstock
Synthesis gas (or syngas) is the name commonly given to a mixture of carbon monoxide (CO) and hydrogen (H2); various molecular ratios are used industrially. It can be made from coal (C), natural gas (CH4), biomass (CxHyOz), and other organic materials such as plastic waste by a partial oxidation, often with addition of steam (H2O) to increase the hydrogen content. Syngas has approximately half the energy density of natural gas (methane) and can be used for its heat value in steam cycles, gas engines, fuel cells, or turbines to generate power and heat. Syngas is also an intermediate feedstock for making liquid fuels and a large number of commodity chemicals, including hydrogen, synthetic natural gas (SNG), naphtha, kerosene, diesel, methanol, dimethyl ether (DME), and ammonia.
Syngas is particularly important in refineries as a source of hydrogen, which is required for hydrotreating, removal of impurities, hydrogenating olefins, and other hydroprocessing such as catalytic cracking.
Synthetic natural gas is similar to natural gas (i.e., largely methane), but is produced by gasification of different carbon sources. The gasification processes involve the catalytic reaction of carbon monoxide and/or carbon dioxide with hydrogen to give gases with high methane content, and are frequently known as methanation.
(2.1)
(2.2)
As the methanation reactions of both carbon monoxide and carbon dioxide are highly exothermic, increases in reactor temperatures need to be avoided. This can be accomplished by recycling the reacted gas, by steam dilution, or by using isothermal reactors with indirect cooling. Catalysts with high nickel content are preferred for SNG production, similar to those in reforming catalysts.
As explained in Chapter 1 and elsewhere, in the catalytic Fischer–Tropsch (FT) synthesis, one mole of CO reacts with two moles of hydrogen to form mainly straight-chain 1-alkenes (CnH2n) together with n-alkanes, some internal alkenes, and minor amounts of branched hydrocarbons and primary alcohols. Side reactions are methanation, the Boudouard reaction, and coke deposition.
(2.3)
The product is synthetic crude oil (syncrude) that can be refined to produce excellent diesel fuel, lube oils, and naphtha. The most important catalysts are based on iron (Fe) or cobalt (Co). Cobalt catalysts generally have higher conversion rates, are more effective for hydrogenation, and thus produce fewer olefins and alcohols compared to iron catalysts.
Methanol is also produced by the reaction of carbon monoxide and/or carbon dioxide with hydrogen, but using catalytic systems different to those that lead to hydrocarbons (Chapter 6). Both reactions are exothermic and, somewhat surprisingly, it has been found that the methanol synthesis by hydrogenation of CO largely proceeds via carbon dioxide.
(2.4)
(2.5)
Side reactions can again lead to formation of by-products such as methane, higher alcohols, or dimethyl ether.
Although natural gas is the most widely used carbon source for methanol production, many other feedstocks can be used to produce syngas via steam reforming. Coal is increasingly being used as a feedstock for methanol production, particularly in China.
Dimethyl ether is now produced by methanol dehydration, requiring methanol as starting material; however, a direct production route, combining three reactions (2.6; 2.7 and 2.8) in a single reactor, is planned:
(2.6)
(2.7)
(2.8)
DME is industrially used in the production of the methylating agent dimethyl sulfate and is also used as an aerosol propellant. DME has potential as a fuel and it can be used directly in power generation or in blending with (or as substitute for) LPG or diesel, in particular because of its very high cetane rating. The boiling point of −25 °C allows fast fuel/air mixing, reduces ignition delay, and gives excellent cold starting properties. DME is an efficient alternative to other energy sources for medium-sized power plants, especially in isolated or remote locations where it can be difficult to transport natural gas and where the construction of LNG regasification terminals would not be viable.
Syngas is also the raw material for ammonia production, which is needed for the manufacture of fertilizers. chemicals, plastics, fibers, and explosives. The gas mixture is purified and the hydrogen to nitrogen ratio is adjusted to the stoichiometric 3 : 1 molar ratio needed for ammonia synthesis. The catalytic reaction is carried out at high pressure (100–250 atm) and between 350–550 °C, usually over an iron-based catalyst.
(2.9)
A low (20–30%) once-through conversion is used and a part of the unconverted gas is circulated to increase the total conversion.
Other applications In addition to the mixture of alcohols that are produced as by-products in the FT synthesis, related processes that give mixed alcohols include MAS (methanol plus higher alcohols) technology, developed by Snamprogetti, EniChem, and Haldor Topsoe, which was demonstrated at an industrial level in the 1980s [1, 2]. A further application is the production of carbon monoxide, used for acetyls production (acetic acid, anhydride, etc.) or as an alternative carbon source.
Table 2.1 summarizes the different syngas specifications for the main applications.
Table 2.1 Synthesis gas specification for different applications.
2.2 Routes to Syngas: XTL (X = Gas, Coal, Biomass, and Waste)
When syngas is applied in the Fischer–Tropsch synthesis, the overall process is generally named XTL, X, depending on the carbon source, for example, CTL, coal-to-liquids, GTL (natural gas-to-liquids), BTL (biomass-to-liquids), or WTL (waste-to-liquids), as shown in Figure 2.1.
Figure 2.1 General scheme for XTL.
The main steps [3] that take place in a XTL complex include the syngas generation, followed by syngas cleanup and Fischer–Tropsch synthesis for synthetic fuels production. Combining coal and biomass, in a so-called CBTL process, is another possible route since the cofiring of a biomass and coal mixture is feasible in a modern gasifier.
The main technologies employed in the production of syngas starting from coal, natural gas, biomass, and some kinds of wastes (in particular, wood residues from construction or plastics) are gasification and reforming that may be grouped into two large production processes (Figure 2.2):
Gasification from solid feedstock, such as coal, biomass, and waste.Reforming for gas feedstock.Figure 2.2 Routes to syngas.
The term gasification is applied to the conversion of any carbonaceous fuel to a gaseous product with a useable heating value. This definition excludes combustion, because the product, flue gas, has no useful heating content. The dominant technology is partial oxidation to produce syngas, where the oxidant may be pure oxygen, air, and/or steam. Partial oxidation can be applied to solid, liquid, and gaseous feedstocks, including coal, biomass, residual oils, and natural gas; the last is also included despite the tautology involved in gas gasification. Gasification relates to the transformation of solid or liquid feedstock in an oxygen-poor environment (i.e., less than that needed for complete combustion), whereas reforming is applied to the transformation of natural gas into syngas.
2.2.1 Starting from Gas (GTL)
Syngas production represents the least efficient and most costly step of a GTL plant [4], and hence it has been an area of intense development by a number of technology providers. Six technologies for syngas production from natural gas, which have already been commercialized or are at advanced stages of development, are listed in Table 2.2 and discussed in the following sections.
Table 2.2 Comparison of syngas production technologies [7]
TechnologyAdvantagesDisadvantagesSMR• Most extensive industrial experience • Oxygen not required • Lowest process temperature requirement • Best H2/CO ratio for hydrogen production applications• H2/CO ratio often higher than required • Highest CO2 emissionsATR• Natural H2/CO ratio is often favorable • Lower process temperature requirement than POX • Low methane slip • Syngas methane content can be tailored by adjusting reformer outlet temperatures• Usually requires oxygenPOX• Feedstock desulfurization not required • Absence of catalyst permits carbon formation and, therefore, operation without steam, significantly lowering syngas CO2 content • Low methane slip • Low natural H2/CO ratio is an advantage for applications requiring ratio < 2• Low natural H2/CO ratio is a disadvantage for applications requiring a ratio > 2 • Very high process temperatures • Usually requires oxygen • High-temperature heat recovery and soot formation/handling adds process complexity • Syngas methane content is inherently low and not easily modified to meet downstream processing requirementsCPO• Lower temperatures than ATR • Lower oxygen consumption• Cost of the catalyst (usually a noble metal, in particular Rh)HER• Compact overall size • Application flexibility offers additional options for providing incremental capacity• Limited commercial experience • In some configurations, must be used in tandem with another syngas generation technologyCPR• More compact design • Very high-thermal efficiency • Less catalyst required • Suitable for offshore application• Large number of parallel units for GTL scale(a) Steam Methane Reforming (SMR)
Steam methane reforming is widely applied for hydrogen-rich syngas production, used to make ammonia, and hydrogen itself. The reactants are methane and steam and the chemical conversion, Equation 2.10, takes place over a Ni-based catalyst, with the following stoichiometry:
(2.10)
(2.6)
Operating temperatures and pressures for a conventional SMR unit are commonly 800–900 °C and 20–30 atm at the outlet. The volume percent of methane slippage is typically around 1% on a dry gas basis. The reaction (Equation 2.10) is endothermic and the need to sustain the required reaction temperatures is often a limiting factor in the design of an efficient heat transfer of the SMR reactor system. The SMR reactor is typically designed as an externally fired tubular reactor; such units have a high steam and fuel consumption and need a high capital investment. Excess steam (2.5 : 1–3.5 : 1 molar steam to carbon ratio) is required to prevent coke formation in the reactor tubes, giving a typical SMR H2/CO ratio of 3 : 1. While this is suitable for ammonia and hydrogen production, it is much higher than the 2 : 1 ratio required for Fischer–Tropsch applications and needs to be adjusted by the WGSR (Equation 2.6).
(b) Autothermal Reforming (ATR)
ATR is practiced in current commercial syngas applications by feeding a mixture of steam, methane, and oxygen over a fixed bed Ni-based catalyst. The system is adiabatic because of the presence of oxygen inside the reactor; thus, the heat required for the endothermic SMR reactions (Equations 2.6 and 2.10) is provided by the exothermic oxidation reactions (Equations 2.11 and 2.12).
In addition to the SMR reactions, a number of other reactions also occur:
(2.11)
(2.12)
(2.6)
The oxidation reactions take place directly through a “burner” nozzle in the vapor space above the fixed catalyst bed, where flame core temperatures may exceed 1900 °C. The SMR and CO shift reactions apparently occur sequentially in the catalyst bed, resulting in syngas exit temperatures from the ATR unit of approximately 950–1050 °C. ATR has the advantage to produce a syngas with a H2/CO ratio close to 2 : 1 suitable for FT synthesis. Excess steam is also required to prevent soot formation, but much less than that needed for an SMR (e.g., 0.6 : 1 molar steam to carbon ratio) [5].
Although there is an additional cost needed for a cryogenic oxygen plant or an air compressor for air-blown ATR units, ATR appears to be generally more economical than conventional SMR, in particular for large-scale FT applications. Moreover, a fraction of the FT tail gas is recycled back to the ATR, in a GTL complex, to enhance the thermal efficiency of the whole process (see Section 2.6).
(c) Noncatalytic Partial Oxidation (POX)
In noncatalytic partial oxidation, the oxidation reactions are predominant and require less steam than for an ATR to achieve a similar H2/CO ratio of 2 : 1. The oxidant and the hydrocarbon feedstocks are mixed and the reactions take place in homogeneous phase. The reaction temperatures are generally higher (around 1300 °C) and more oxygen may be required in some cases. All the heat required for the syngas reaction is supplied by the partial combustion of the fuel and no external heat is required. The reactor often consists of a refractory-lined open pressure vessel. As soot formation can occur in the effluent, provision for soot removal by scrubbers is prudent.
Both Shell and Texaco have supplied technology for natural gas conversion by POX gasification for decades, and lately Lurgi is also promoting a multipurpose gasification process (MPG), which is available in a version adapted for natural gas.
(d) Catalytic Partial Oxidation (CPO)
The production of syngas based on heterogeneous catalytic reactions is normally referred to as catalytic partial oxidation (CPO). CPO represents an alternative in syngas production; the principles are similar to that of ATR, with the difference that all the reactions occur in heterogeneous phase and that the temperature is usually lower in CPO than in ATR.
The potential advantages of CPO for large-scale syngas production have been intensively investigated by Conoco-Phillips and other companies. Eni has developed the SCT-CPO (short contact time catalytic partial oxidation) for the production of hydrogen from methane or light hydrocarbons. The technology can be used for both H2 generation in refineries and syngas production directly at the well-site for GTL applications [6]. Compared to ATR or POX, a lower oxygen consumption, essentially no use of steam, and operating temperatures under 1000 °C are among the advantages claimed in the literature for this technology. Catalysts are usually based on noble metals (Pt, Pd, Rh, and Ir). The choice of the catalyst should be driven by its cost, activity, and natural gas conversion. For example, the severe catalyst operating conditions and the importance of suppressing soot formation with minimal steam consumption appear to favor the use of rhodium (Rh)-based monolithic catalysts.
(e) Heat Exchange Reforming (HER)
A two-stage process for methane reforming, heat exchange reforming (HER), is under investigation; this seems a promising approach, but has not yet been commercialized. In the first stage, SMR reactions occur inside the tube side of a heat exchanger filled with catalyst. The second stage typically consists of a conventional ATR. The heat of reaction required by the endothermic SMR reaction is provided by the hot effluent syngas from the ATR second stage. Advantages of the technology are lower capital costs and higher thermal efficiency. The companies that developed this technology (JM-Katalco, Haldor Topsøe, Uhde) initially targeted it for ammonia and methanol plant applications, but it should be suitable for GTL plants too.
A particular type of HER is the GHR (gas-heated reformer), where the reformer is heated by process gas. HERs may be classified in different ways depending on the process concept used (i.e., series or parallel arrangement). Synetix has also commercialized a second-generation HER called advanced gas-heated reformer (AGHR) with a different tube design for a methanol plant in Australia.
