TECHNOLOGY
New Wave of Methane Conversion Technology Expected Soon Surplus natural gas resources stimulate the development of sophisticated techniques for the direct conversion of methane to transport fuels Joseph Haggin, C&EN Chicago
A recent analysis by the Merrill Lynch investment firm predicts that the natural gas supply bubble has all but burst and that gas prices should begin rising as supplies fall. The firm even suggests that there might be tight supplies of natural gas this winter. However, a number of energy industry engineers disagree with this analysis. Despite dire predictions about the natural gas market from Wall Street analysts, the energy industry is proceeding on the assumption that natural gas will form
an increasingly important adjunct to energy from oil, primarily by means of the conversion of methane to transport fuels. At the recent spring national meeting of the American Institute of Chemical Engineers in New Orleans, there was ample evidence that the industry is preparing for a new wave of methane conversion technology. Although some observers still look upon the prospects for success of these new techniques with skepticism, a lot of money and effort are currently being expended on their development. In assessing the general outlook for conversion of natural gas to transport fuels, N. Wayne Green and Ram V. Ramanathan, engineers with Fluor Daniel of California, note that a major reason for the interest in methane conversion is surplus natural gas resources, particularly in some remote regions. Surplus gas resources have usually been associ-
ated with crude oil exploration. However, there are also some standalone natural gas resources that might become very attractive to investors if some of the new conversion techniques currently under development become available. Green estimates that these largely untapped resources contain as much as 29 trillion cubic meters of natural gas. At the present rate of world natural gas consumption, and factoring in limitations imposed by realistic recovery costs, these reserves would satisfy gas supply needs for about 450 years. To convert all this gas to transport fuels would require an estimated 681 conversion plants, each producing a nominal 15,000 barrels per day of transport fuels for a period of at least 30 years. According to Green, the fuel thus produced would constitute a very significant fraction of current world crude oil production. With present technology, conver-
Texaco syngas process uses high-pressure natural gas and oxygen High-pressure steam Oxygen
k
Natural gas
Water
>•-[)• Steam
Carbon dioxide
i
Deaerator
Steam Scrubber
r
Particulate-free synthesis gas
to
Heat exchanger
—¥
Generator
Waste heat boiler
Heat exchanger
Soot/water recycle
Major reactions in generator: Oxidation
CH4 + 1/2 0 2 *C2H6 + 0 2 Steam reforming H 2 0 + CH4 +=±: Water gas shift H 2 0 + CO « » Steam cracking C2H6 + H 2 0 ^—iHydrocracking C2H6 + H2 « * Soot formation CH4 *- 1/2 C + C2H6 ** CH4 +
2H2 + CO 3H2 + 2CO 3H2 + CO H2 + C0 2 2H2 + CH4 + CO 2CH4 H2 + 1/2 CH4 C + H2
May 9, 1988 C&EN 45
Technology sion of natural gas to transport fuels generally requires prior conversion of the natural gas to synthesis gas, either by partial oxidation or reforming. The syngas is then converted to products either directly, using Fischer-Tropsch (F-T) chemistry, or indirectly, using methanol production. One world-scale plant for gasoline production via methanol using the Mobil MTG (methanolto-gasoline) process is already on stream in New Zealand, and Green suggests that other plants using both direct and indirect technology will be operating by the mid-1990s. The direct conversion of methane to hydrocarbon liquids—eliminating the syngas step—has been a continuing challenge to catalytic chemists. Much R&D is now under way around the world to this end and is almost universally pictured as "promising." Certainly, there is more than enough economic incentive today for development of direct conversion of methane to transport fuels, and there is still more incentive for direct conversion of methane to methanol. Green and Ramanathan suggest that direct conversion of methane could replace the more conventional syngas-based technology after the year 2000. At present, gas conversion is only marginally competitive with crude oil processing, and few projects are under active consideration. As always, a major reason is economic. A crude oil threshold price of about $25 per bbl seems to be a critical value in deciding in favor of proceeding with direct methane conversion. If the crude price were to rise to $40 per bbl, it would be economic to convert almost all available natural gas with costs of up to $2.00 per gigajoule. However, if there were genuine progress in the development of direct methane conversion technology, the crude oil threshold price could drop to $20 per bbl. That would change matters entirely. Although economics may dominate decisions on whether to proceed with direct methane conversion, other considerations also can be decisive. National energy selfsufficiency becomes more prominent in the thinking of many countries. The MTG plant in New Zealand is 46 May 9, 1988 C&EN
an example of this. There is also a growing awareness that having alternative technology available at purchase contract time may exert considerable influence on contract negotiations. Other influences include balance of payments and utilization of domestic gas reserves that would otherwise be ignored. Green noted that all the conventional methane conversion technologies do involve syngas production. Up to 60% of the capital cost of a conversion plant involves gas cleaning and desulfurization, and generation and compression of syngas. Among the options for generating syngas from natural gas are partial oxidation, autothermal reforming, steam reforming in tubular reactors, and a number of specialty processes. Seldom do any of these produce the desired hydrogen/ carbon monoxide ratio in the syngas to be used for either F-T processing or methanol production. Considerable technical effort is devoted to adjusting hydrogen/carbon monoxide ratios, and that sometimes involves a combination of more than one syngas generation system. Of the two procedures for producing hydrocarbon products from syngas, the oldest, F-T chemistry, had all but been written off when crude oil prices dropped several years ago. In the F-T processing system, syngas reacts in the presence of a promoted transition metal (iron, cobalt, ruthenium) catalyst to produce a mixture of predominantly aliphatic products with a wide molecular weight distribution. The
molecular weight distribution generally follows Schultz-Flory polymerization kinetics, which is characterized by a chain length parameter depending strongly on temperature and catalyst composition. In general, high-temperature F-T processing leads to a lighter product distribution, and vice versa. The lowtemperature products also are generally more straight-chain (as opposed to branched) distillates and wax. Today indirect syngas conversion technology is almost restricted to the Mobil MTG process. Mobil also has developed variations of the basic MTG process in which methanol is converted to olefins (MTO) and olefins are converted to gasoline and distillate (MOGD). Several integrated processes also have been developed to optimize the technology. Mobil has further developed a fluid-bed version of the MTG process that is expected to appear in a commercial version in the near future. Up to a dozen methods of direct methane conversion are mentioned by Green to be in various stages of development. Those furthest along include catalytic pyrolysis, catalytic direct oxidation, and catalytic indirect oxidation, all of which produce C2+ compounds. Catalytic pyrolysis also produces aromatics. Cold flame oxidation is noncatalytic and produces methanol. Catalytic oxychlorination with zeolites produces aromatics almost exclusively. Biological conversion of methane, particularly to methanol, has had only
Shell SMDS process leads to kerosine, gas oil Tubular catalytic synthesis reactor
Natural gas " Oxygen 1
Syngas converter
Fuel gas, including liquefied petroleum gas
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Hydrogen •
T Heavy paraffin conversion
• Tops/naphtha > Kerosine - Gas oil
Distillation
modest success thus far. Protonation of alkanes, notably methane, with strong acid catalysts, and the organometallic functionalization of methane are also being investigated in several laboratories, but this work is still in the formative stages. The economics of direct conversion of methane are still speculative. However, general estimates indicate that, on an overall gas-togasoline basis, the cost of a direct conversion system would be only 80 to 90% of the cost of conventional conversion systems now being considered. This would reduce gasoline prices at the pump and, more important, considerably decrease capital investment in processing plants. For remote locations that point could be decisive. If direct conversion of methane obviates the need for large syngas producers, much of the effort for developing improved syngas technology will have been for naught. However, a considerable body of opinion suggests that present technology will be utilized long before direct converters come on stream. One of the well-known syngas processes is that d e v e l o p e d by Texaco Chemical Co. and Texaco Development Co. Texaco's syngas generator was developed in the 1940s for natural gas conversion and then adapted to coal and naphtha. Over the years, more than 100 of the gen-, erators have been licensed. Texaco has now revived its interest in making syngas from natural gas and has developed a new version of the gasifier for use in the partial oxidation of natural gas. The Texaco synthesis gas generation process (TSGGP) is claimed by Texaco's Pierre J. Osterrieth and Manuel E. Quintana to be very competitive with steam reforming, especially when there is a very tight specification on the composition of the syngas. The TSGGP uses high-pressure natural gas and oxygen. Moderators such as carbon dioxide or steam also may be used where needed. A small amount of recycled process water, carrying recycled soot, is normally injected into the feed to allow total carbon conversion. Because of short residence times in the reactor, no complex hydrocarbons are
formed. The product synthesis gas can be cooled by quenching or by heat exchange to produce highpressure steam. The hot synthesis gas exiting the generator contains mostly hydrogen and carbon monoxide, with small amounts of carbon dioxide, steam, unreacted methane, nitrogen compounds, and, if the feed gas contains sulfur, sulfur compounds. The methane content of the synthesis gas must be strictly controlled in the generator. Unreacted methane cannot be separated from the carbon monoxide by the purification processes now in use. The actual methane content of the synthesis gas will depend on several factors, including the temperature of the gas generator and the residence time of the synthesis gas produced in the generator. The residual steam originates primarily in the "tuning" steam that may be injected with the feed gas. Some may also result from unreacted water vapor in the shift reaction. Water concentrations also must be carefully monitored. Reinforcing the observation of Green and Ramanathan that intermediate processes between conventional and direct methane conversion will probably appear first, Shell International has announced that it is ready to go commercial with its newly developed middle distillate synthesis (SMDS) process. H. Voetter and M. J. v. d. Burgt of Shell's development laboratories in The Hague, Netherlands, described the SMDS process as a response to the demand for reduction in the cost of syngas. Shell's analyses indicated that long-term emphasis should be placed on middle distillate products. Shell regards these products as dominant needs for the next several years. Natural gas would become the primary raw material. The goal is to make kerosine and gas oil from natural gas. The production of saturated hydrocarbons requires syngas with a hydrogen/ carbon monoxide ratio of about 2. The synthesis step in the SMDS process is a highly modernized version of F-T chemistry, emphasizing high yields and favorable catalyst performance. The product actually produced is waxy and can be hydro-
isomerized and hydrocracked to give the appropriate yields of distillate products. Syngas production and the actual synthesis of end products are both big energy producers, but there is a large energy sink in the oxygen plant. In the process, oxygen is used rather than air to avoid nitrogen buildup in the system. The syngas is produced by noncatalytic, autothermal partial oxidation and can be used either for middle distillate products or for methanol. For the F-T synthesis, any sulfur present in the syngas is removed upstream from the gasifier. The reaction in the gasifier follows conventional Schultz-Flory polymerization kinetics. The new catalyst used is proprietary and is not specified by Shell. However, Shell claims that it overcomes the problems of traditional F-T catalysts and is robust enough to be used in a fixed-bed pipe reactor at high temperatures. To convert the heavy, waxy material produced in the SMDS process, the effluent from the heavy paraffin step is passed through a trickle bed flow reactor under mild conditions. The catalyst used—one of Shell's commercial catalysts— provides the isomerization in the heavy paraffin conversion. The hydrogen demand is not great. Products from the SMDS process are highly paraffinic and free of nitrogen and sulfur. Shell says they are easily blendable with conventional refinery streams. In developing the SMDS process, the largest pilot plant was a fully integrated unit with a throughput of 2 bbl per day. Shell says there are no unusual environmental problems. All effluents can be handled with conventional treatment to make discharges acceptable. That the process is a net producer of water might be an advantage in arid remote regions. Catalyst life is claimed to be several years, and spent catalyst should be returnable for metals recovery. In 1986 dollars, the capital cost of a 10,000 bbl-per-day plant using the SMDS process in a developed country would be about $300 million. In an undeveloped country, the cost would be about twice that. • May 9, 1988 C&EN
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