Technology
Fischer-TroDSrti: new life for old technology Helping to revive this venerable process is surge of research activity, mainly on mechanisms, catalysts, and reactor design Joseph Haggin C&EN, Chicago
Fischer-Tropsch synthesis may be ripe for revival. But it won't look much like the original. A flurry of R&D activity focused on mechanisms, catalysts, and reactor design is putting new flesh onto the venerable but historically dead-ended technology. Historically, the technology for converting the hydrogen/carbon monoxide synthesis gas from coal was used to produce motor fuels. However, with its new vigor, the technology is being directed toward production of basic hydrocarbons. What could help bring about this Fischer-Tropsch (F-T) revival is a
shift in the raw material base for chemicals from oil and gas to coal, which already has begun. The shift likely will be gradual, perhaps halting from time to time, and may reflect the economics of oil and gas substitution. A key item in the shift to coal will be the development of coal conversion technology. Some chemicals are obtained directly from coal via traditional coal tar chemistry. Others are expected to be obtained from various types of direct liquefaction now under development. Most of the major commercial chemicals, however, probably will come from synthesis gas chemistry, and in the beginning the most likely processes are those that already have been demonstrated to be commercially operable. An obvious choice is F-T synthesis. The F-T synthesis originated in Germany in the 1920's, reached its zenith in Germany during World War II where it provided primarily motor fuels, and survives now only at the Sasol works in South Africa (C&EN, Sept. 17,1979, page 13). Immediately after World War II, most industrial nations had F-T-based projects in
Sasol In South Africa Is only commercial plant using Fischer-Tropsch process 22
C&EN Oct. 26, 1981
progress and these were kept going briefly until cheap oil and gas made chemicals and fuels from coal noncompetitive. By 1960 the world supply of natural gas began to decline, and cheap oil began to be imported from the Middle East. Naphthas and reformed natural gas became the principal sources of feedstocks. Large amounts of propylene, butadiene, and aromatics were coproduced. This development proceeded well into the 1970's despite the Arab oil embargo of 1973 and the subsequent rise in crude oil prices. However, industry's reaction following the 1973 embargo was to seek a new economic equilibrium and this, in essence, was manifested in finding a competitive source for oil and gas. In principle any carbonaceous source can be used, but attention has been centered almost entirely on coal. The reaction of the technical community was to revive those coal conversion projects that had been all but abandoned by 1960. In the case of F-T syntheses, this activity yielded a three-pronged program to convert F-T technology to chemical feedstock production rather than to confine it strictly to motor fuels. One program is aimed at development of more selective catalysts to improve the yield of C2 to C4 olefins. A second program is aimed at the production of a coal-derived naphtha suitable as a cracker feedstock. The third program is aimed at producing C10 to C20 aliphatic, straight-chain hydrocarbons for making detergents. At the beginning of the program most of the available technical data came from the experience of the Sasol plant and it was not encouraging. For example, the original aim was to raise the total olefins production to 50% of the product slate. This required a sharp improvement in catalyst selectivity. Experience had shown that selectivity could be varied slightly by changing process operating conditions, but the 50% olefins goal seemed out of the question without some radically new catalysts. At present, catalyst and process improvements have permitted total olefin production up to 90% in pilot runs. And in at least one laboratory
Fischer-Tropsch reaction: the chemistry experiment a 100% yield of a single olefin, namely propylene, has been achieved. The wide-ranging catalyst improvement programs also have sharply improved the motor fuel potential of the F-T synthesis. The net effect is that whereas the utility of F-T technology has been considerably broadened from motor fuels to include commercial petrochemicals, F-T technology has been made a very general method of commercial synthesis that is being adapted to many local requirements. The case for F-T technology has been summarized in a multiclient study made by Catalytica Associates, a San Francisco research and consulting firm specializing in catalysis. The main factors motivating current industrial catalysis research are rising energy and feedstock costs, decreased petroleum reserves, environmental constraints, and sharply increased investment costs. The use of coalderived synthesis gas overcomes most of these problems. More than 65% of the world's recoverable fossil fuel reserves are coal; oil accounts for only 10%. The U.S. has more than abundant coal reserves. Furthermore, Catalytica's president, James A. Cusumano, believes that chemicals and fuels from coal are likely to become competitive with petroleum-based products because the price of oil is expected to rise faster than the price for coal. Cusumano believes that a coal-based synfuels industry likely will appear in the next decade. That implies a large gasification industry, which is the requisite source of syngas for F-T technology and other Ci-based processes. Another assessment of Ci technology in general and F-T technology in particular has been made by A. Eugene Cover, manager of the synfuels process division of M. W. Kellogg Co. Cover notes a number of projects are moving ahead without government participation. The general development activity is very high and, he predicts, will result in commercial production in the next few years. According to Cover, there are 10 major coal gasification processes, six major liquefaction processes, and five in-situ shale oil processes along with nine more processes for aboveground use now in various stages of development. Of the gasification processes, seven are now commercial. Two of the coal liquefaction processes are commercial. None of the shale oil processes are yet commercial. Cover notes that it is important to
The chemistry of the Fischer-Tropsch synthesis usually is defined by consensus, with many individual reservations depending on viewpoint. The classical F-T synthesis has been described by Robert B. Anderson, formerly supervisory chemist at the Bureau of Mines, as a hydrogenation of carbon monoxide producing higher hydrocarbons and oxygenated organic molecules that have predominantly straight carbon chains, at least in the range C4 to C7. This was considered distinct from higher alcohol synthesis and iso-synthesis. The F-T synthesis is a composite of many reactions, producing a variety of product molecules of different types. The mechanism is not well defined, is extremely complex, and provides at various times and in various environments low- and high-molecular-weight liquids and gases, carbides, oxides, carbonyls, and elemental carbon. A more modern view is that of Jurgen Falbe, executive vice president of West Germany's Ruhrchemie A.G. This view claims that all the reactions in the F-T synthesis can be reduced to two. The first is the actual F-T conversion: CO + 2H2 - * (-CH2-) + H20 (AHR = -39.4 kcal) The second is the water-gas shift reaction, which proceeds easily as a consecutive reaction on iron catalysts: CO + H20 -> H2 + C0 2 (AHR = - 9 . 5 kcal) The net overall reaction can be summed as: 2CO + H2 - * (-CH2-) + C0 2 (AHR = -48.9 kcal) If olefins are formed exclusively in a totally stoichiometric conversion, the maximum yield is 208.5 g per cubic meter of synthesis gas. The shift reaction can be suppressed by operating the system at low temperatures, short residence times, and by recycling dry synthesis gas. Undesirable side reactions include methanation, decomposition of carbon monoxide, and oxidation of the catalyst. Optimal product yields can be obtained only when reactants are provided with a CO/H ratio that is the same as that at which the reactants actually are consumed. Any difference between the two values decreases the yield of the desired products. A more inclusive definition of F-T synthesis, although substantially the same, has been included in a recent study by Oak Ridge National Laboratory. The study limits F-T chemistry at present to the following basic reactions:
(2n + 1)H2 + nCO -> CnH2n+2 + nH20 ( n + 1)H2 + 2nCO ~> CnH2n+2 + nC0 2 2nH2 + nCO — CnH2n + nH20 nH2 + 2nCO — CnH2n + nC02 2nH2 + nCO — CnH2n+1OH + ( n - 1 ) H 2 0 ( n + 1)H2 + ( 2 n - 1)CO~* CnH2n+1OH + ( n - 1 ) C 0 2 Again the water-gas shift reaction is included as a means of adjusting the CO/H ratio in the synthesis gas. Competing reactions, most notably the formation of methane, are suppressed by any means available because they inhibit the formation of longer chain hydrocarbons. Another modification included by Herbert Kolbel of the Chemical Engineering Institute at the Technical University of Berlin is the Kolbel-Engelhardt (K-E) synthesis, which forms hydrocarbons from carbon monoxide and water (steam). Kolbel suggests that this be written as: 3CO + H20 — (-CH2-) + 2C0 2 (AHR = -244.2 kJ) Other competing reactions that can complicate the synthesis include the Boudouard reaction: 2CO — C + C0 2 coke deposition: H2 + CO - * C + H20 and carbide formation: xM + C — MXC Typical carbides are Fe2C, Co2C, and Ni3C. All rather drastically alter the catalytic behavior of the metal species. Nickel catalysts are not usually the catalysts of choice, and both iron and cobalt are considered the "classical" F-T catalysts. Cobalt was first employed by Fischer and Tropsch and from mechanistic considerations was considered primarily an olefin former. For economic reasons iron catalysts replaced cobalt in most applications of F-T technology, particularly in World War II. Like cobalt, the iron catalysts are often regarded as olefin formers. The acceptance of hydrogenation offered no problems before 1973, when the emphasis in F-T research shifted to olefins production. Kolbel claims that in modern processes, olefin content can vary from 0 to 90% with appropriate process management and that about 80 % of the olefins formed are n-alpha-olefins.
Oct. 26, 1981 C&EN 23
Technology Fischer-Tropsch reaction: the mechanism At least 14 mechanisms are known to be published explaining the FischerTropsch synthesis. Most investigators accept the impossibility of stating a definitive mechanism at this time. The main problem, according to Jurgen Falbe, executive vice president of West Germany's Ruhrchemie A.G., is that many of the investigations on which various mechanisms are based were made on model systems, which are far removed from commercial reality. The most widely discussed mechanism, Falbe claims, is carbon chain growth at the chain terminal. Excited molecules result from simultaneous chemisorption of carbon monoxide and hydrogen at the catalyst surface. There they react to form a primary enolic complex. Chain growth begins with the splitting off of water from two complexes and leads to the formation of a C-C bond and the simultaneous release of a carbon atom from the catalyst surface by hydrogenation. The C2-complex splits off water and is released from the metal by hydrogenation. Through condensation and hydrogenation there is stepwise growth, one carbon atom at a time. Eventually the complex desorbs, carrying with it a hydroxyl group, which can, in subsequent reactions, produce aldehydes, acids, alcohols, esters, etc. The hydrocarbons can be formed either from the alcohols by dehydrogenation or by cleavage of the adsorption complex. Chain growth also can start with alcohols and/or aldehydes when these are adsorbed on the catalyst surface in the enol form. Chain growth also can begin with olefins, which Falbe believes are probably bound to the catalyst surface in enolic form after reaction with water. The above mechanism is the most likely, if not the only possibility, to a consensus of F-T investigators. There is much evidence supporting it. The formation of a primary enolic complex is considered supported by the fact that, under otherwise equal conditions, the amount of gas mixture adsorbed is larger than the sum of the individual adsorption components. Similarly, the heat of adsorption for the mixture exceeds the sum of the adsorption heats of the individual components and corresponds to the calculated enthalpy of formation of the primary enolic complex. Formaldehyde has been shown to be the first reaction product by mass spectrometry, and quantum calculations indicate that formation of the primary enolic complex is energetically favored. The most recently suggested F-T
24
C&ENOct. 26, 1981
mechanism has been proposed by Richard S. Sapienza of the catalysis group at Brookhaven National Laboratory. He outlined the mechanism in August at the New York meeting of the American Chemical Society. Sapienza claims that the oxide mechanism encompasses the observations of earlier theorists and, more to the point, is useful in predicting catalyst behavior. The fact that Sapienza's group has, apparently, been successful in developing some new, highly active F-T catalysts, lends credence to the claim. Fundamental to understanding the oxide mechanism is the idea that carbon monoxide chemisorbs on a metal surface and reacts with hydrogen to yield an oxygen rather than a carbon coordinated species. The consequences of this mechanism are considerable. Bond strengths of metal oxides should correspond directly to specific activity of methanation, but should be related inversely to methane selectivity. Oxidation of the catalyst surface should increase both catalytic activity and selectivity. Metals that form nonreducible oxides should react with carbon monoxide and hydrogen, yielding methane and leaving behind a surface oxide coating. This reaction has been observed. Previously attempted catalytic reduction of carbon monoxide suggested that the formation of metal-oxygen bonds may be necessary for synthesis to occur. The very strong bonds of metallic oxides investigated at Brookhaven may explain why these reductions could not be made catalytic. The metal-oxide bond strength also will determine the probable product distribution. Weak oxide formers, such as rhodium, should produce ethylene glycol, whereas a strong oxide former, such as iron or manganese, should yield ethylene. Mixed systems should give good yields of ethanol. The same logic applies, says Sapienza, to the effects of promoters. The addition of a weak oxide former should decrease methane and olefin formation and vice versa. The oxide mechanism is still under scrutiny. One of the concerns voiced by critics of the oxide mechanism is that it involves structures that are unusual and even unprecedented in the literature. Examples are those involving tetravalent oxygen coordinated to the metal. It also is difficult for traditionalists to accept the production of such species as M — 0 = C , M=======CH2, M — 0 = C H 2 , and M—O—CH 3 . Sapienza is well aware of the criticisms and is refining the mechanism to meet objections.
remember that in the area of coal liquefaction only two processes are now available that can be considered truly commercial. Coal liquefaction is a category that includes all coal products other than methane or fuel gases. Those processes are the ICI/ Lurgi/Topsoe methanol process, and the Kellogg/Sasol version of the Fischer-Tropsch process. This is the major reason why so much interest is being centered on F-T processing. In fact, the F-T process is the only extant commercial indirect liquefaction process available. That is not to say that it will remain so, but it at least suggests that F-T processing will become the mainstay of efforts involving Ci chemistry from synthesis gas in the near future. Cover thinks that this is particularly true because the F-T process is the only commercially available process for products other than motor fuels—specifically low-molecular weight olefins. F-T catalysts traditionally have been producers of a broad spectrum of molecular weights. The new specificity exhibited by catalysts in recent years, and the efficacy of some of the newer processing techniques—such as improved slurry reactors and flame-spraying of catalysts on tube surfaces—indicate that F-T processing can be tailored to specific needs and can be made considerably more flexible than in former years. One tabulation reports at least 5000 publications and 5000 patents relating to F-T catalysts. Since 1926, says one F-T chemical expert, "just about everything but Rasputin's beard has been tried as a catalyst." Chemists at West Germany's Ruhrchemie A. G. generalize that the 3d and 4f transition metals and their insertion compounds (carbides, nitrides, etc.) are especially suited for the adsorption of the primary F-T complex. However, from a practical viewpoint, only iron and cobalt are useful commercially. Nickel is all but useless because it favors methanation of the synthesis gas. Platinum group metals show little activity. Rhodium produces oxygenates and osmium favors formation of gaseous products. Ruthenium catalysts seem to favor high molecular weights, and compounds with molecular weights of more than 200,000 have been made. Titanium, vanadium, chromium, and manganese are not effective because of the difficulty in reducing their oxides. Molybdenum is currently under investigation but the results are as yet unknown. The reactants produce some profound changes in the fresh catalysts
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Fischer-Tropsch reaction: the thermodynamics and kinetics A survey of five reviews of F-T technology reveals virtually no thermodynamic information on F-T technology as a composite subject. There is only minimal information on this aspect. The reason for this lack lies in the complexity of F-T chemistry. Thermodynamic calculations of the formation probabilities in an F-T reaction are impossible because there are, theoretically, an infinite number of reaction equations. A review by Ruhrchemie A.G. notes that, in practice, F-T thermodynamics is based on the assumption that individual reactions are independent. There is no synthesis gas composition that corresponds to a stoichiometric starting point for all reactions simultaneously. The inference is that product concentrations will depend strongly on the composition of the feed gas. About the only generalization made in the Ruhrchemie review is that hydrogen-rich feeds favor paraffin formation, and carbon monoxide-rich feeds favor olefin and aldehyde formation. All of the reactions encountered in the
26
C&ENOct. 26, 1981
F-T synthesis are exothermic, some of them highly exothermic. Excepting methanation, all the F-T reactions are thermodynamically not favored above 450 °C. The higher the temperature, the greater the tendency to form methane. Hydrogenation of carbon dioxide is more likely than hydrogenation of carbon monoxide. Paraffins are only slightly more likely in a F-T synthesis than are olefins. The conclusion that seems inescapable is that F-T chemistry, at least of the classical variety, is rather more empirical than researchers might like to admit. The kinetics of F-T chemistry are only slightly better known than the thermodynamics. A generally valid kinetic equation for F-T chemistry cannot be written. Each type of catalyst and each individual process variation contribute to a unique variety of kinetics that, in general, cannot be extrapolated or interpolated. Although mass transfer plays an obvious role in F-T kinetics, the role appears to be restricted to pore diffusion
to a rather shallow depth. Mass transfer through adhering hydrocarbon films is claimed not to be a rate-determining feature. The conclusion is that little is known about gross kinetics (macrokinetics), although there is a considerable body of knowledge concerning the kinetics of individual F-T reactions. A problem is that the data on individual reactions are all but impossible to correlate because of the wide differences in the practices of the individual experimenters. The Oak Ridge study notes that since 1951, three major attempts have been made to organize F-T kinetics. The results have not been encouraging. Despite great numerical differences in the data—usually attributed to differences in catalyst samples—the consensus is that F-T reactions are about first order in hydrogen partial pressure and zero order in carbon monoxide as long as the H/CO ratio is between 1 and 3. The kinetic data on methanation, by contrast, is large in amount, rather more consistent, and reliable.
focus of UOP evaluation Slurry reactor system
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of the iron group that result in the active catalyst phase. The phase diagram for cobalt is not very complicated but that for iron is very complex. This is attributed to iron's ability to form inclusion compounds with nitrogen, boron, carbon, etc. It is known for certain that iron catalysts undergo significant phase changes during reaction. Two types of promoters generally are used in the classical F-T synthesis. Metal oxides are structural promoters that form large surface areas and inhibit recrystallization of the active catalyst. Alumina, thoria, and magnesia are examples of this class. The other class, energetic promoters, as their name indicates, achieve their effects via molecular electronics. Chemically active structures also can function as electronic promoters. The alkali metals, for example, appear to produce promotion of F-T reactions in both ways simultaneously. Alkali carbonates are particularly good electronic promoters for iron catalysts. The general effect of promoters appears to be, simultaneously, the increased chemisorption of the reactants, easier primary complex for-
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mation, and improvement of the rates of all carbon monoxide consuming reactions. The biggest problem facing F-T catalyst developers is the complete change in emphasis from motor fuels to low-molecular-weight olefins as the primary products. The prospect of another 25 years of laborious, somewhat empirical catalyst development is not very enticing. One bright light in the tunnel may be a technique being developed by Richard S. Sapienza of the catalysis group at Brookhaven National Laboratory. Sapienza and his associates had as a goal the development of new, highly active catalyst systems for F-T chemistry. The ultimate purpose was to produce diesel fuel and jet fuel from synthesis gas. Cobalt was the catalyst of choice because it was the most active nonnoble metal available. However, cobalt must be promoted to assist the cobalt oxide reduction. The materials chosen were platinum and palladium, both presenting problems in catalyst formulation. Eventually it was discovered that cobalt carbonyl reacts under certain conditions to deposit metallic cobalt
Operating parameters: Outlet temperature = 335 °C H2/CO feed ratio =2.4 Recycle ratio = 2.3 Basic design by M. IV. Kellogg Co.
on some supports, such as alumina. This runs counter to previous belief, but the resulting C0/AI2O3 catalyst displays good F-T activity. This sort of technique led to a series of highly active F-T catalysts that proved useful in bench trials for motor fuel production. Reflecting the initials of Sapienza and his associates, the catalysts are referred to as SOSS catalysts. To illustrate the improvements that are represented in the new catalyst system, Sapienza cited one SOSS catalyst that has the superficial composition 120Co/5Pt/100Al2Oa. At a temperature of 225 °C the catalyst exhibited an activity of 3000 g of product per kg of metal per hour. At th Product + water
High-boiling products
Low-boiling products
SOSS catalysts have been conducted in a batch slurry reactor. Normally catalysts are prepared in situ under synthesis gas pressure from Co2(CO)s and either supported platinum or palladium components. The usual solvents are cyclohexane, xylene, and tetrahydrofuran. Hydrogen gas alone also can be used for catalyst prepartion, which leads to a new heterogeneous catalyst in the slurried state. The type of solvent plays only a minor role in catalyst performance. SOSS catalysts have been used in the temperature range of 150 to 250 °C, with the exception of one particular sample that was used at 70 °C with good results. Catalyst concentrations of up to 7% by weight in the solvent have been used. Pressures ranged from 500 to 1500 psig, but even lower pressures may be used. The consumption ratio of hydrogen to carbon monoxide has always been near 2:1. Sapienza says that the preferred way to use the SOSS catalysts is in a slurry. The catalysts are heterogeneous, and all the evidence gathered so far suggests that they remain so in use. The catalysts also are magnetic, which is helpful in recovery and reprocessing. So far, all of the experimentation has been in batch. The next step is a small continuous pilot plant that will permit evaluation systems that more closely resemble actual industrial environments. C&EN Oct. 26, 1981
Carbon dioxide
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Another possible new development is associated with work on the fast fluid bed coal gasification process under development by Hydrocarbon Research Inc. According to HRFs Derk T. A. Huibers, manager of new process development, some thought is being given to the possibility of incorporating F-T catalysts of a special type in a gasifier. The inference is that F-T synthesis and synthesis gas production might be achieved in the same unit. Considering the high temperatures in the gasifier, reaching this goal remains a challenge indeed. Among the newer types of F-T catalysts suggested by Mobil Research & Development is the incorporation of an excess volume of ZSM-5 zeolite with the iron catalysts. The selectivity for gasoline thus can be increased to more than 60% of the total hydrocarbon product and, essentially, 100% of the liquids produced. The mechanism has not been determined, but it appears that the zeolite function is to intercept molecular intermediates in the propagation sequence and convert them to forms that are inert to further chain growth. Ultimately the success or failure of F-T technology will be found in the operating system. A study by Oak Ridge National Laboratory notes that seven major types of reactors have been used to date. The original reac-
tor had a fixed bed of pellets that operated at temperatures from 200 to 270 °C and produced liquids that were about half gasoline and half heavier hydrocarbons. Precise temperature control was difficult, and hot spots inevitably developed, which led to catalyst deactivation and carbonization. Fluid-bed reactors first were used with fluidized mill scale for catalysts. These operated at higher temperatures (300 to 330 °C), made a lot of methane, and had low selectivity. The Bureau of Mines developed a reactor that used granules of fused iron or steel lathe turnings circulated through the bed with an oil carrier. Temperature was in the range of 235 to 280 °C, with heat removal via circulated oil and an external heat exchanger. This reactor had a low conversion rate and the catalyst disintegrated with time. A modified fixed-bed reactor also has been designed to recycle hot tail gases over the catalyst pellets for temperature control. This operated at up to 340 °C, and the products were lighter than before. The cost of hot gas recycle was prohibitive. The slurry process uses a precipitated or fused catalyst suspended in a heavy oil. Synthesis gas bubbles pass through the suspension at less than 300 °C. Liquid products are filtered out, and the catalyst is reusable. Liquids may be as much as 80% gasoline, and very little methane is produced. The Arge reactor is an improved fixed-bed unit, with very heavy catalyst loadings and higher space velocity. About 40% of the liquids formed are gasolines; most of the rest are heavier. Entrained-bed Synthol reactors developed by M. W. Kellogg Co. are probably the most popular commercial reactors at present. A fused magnetite catalyst and reactants are mixed at the bottom of the reactor. These travel up through the reaction zone at 300 to 335 °C, where heat is removed by two boilers. Products and catalysts are separated by cyclones and settlers. About 78% of the liquids are naphthas, 7% are heavier oils, and the rest are alcohols, acids, etc. With the Sasol plant being the only commercially operating F-T plant, a first approach to adapting the technology to other places would be duplication of the Sasol installations. Studies of this alternative quickly indicate that duplication of Sasol's plant is economically unsuitable almost everywhere else in the world. The present trend appears to be to
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Technology develop a more modern installation based on a new F-T plant complex. For this purpose, some new reactors or some old reactors in new configuration have been evaluated by UOP Inc. under contract to the Department of Energy. The UOP study evaluated four of the leading F-T systems that might be suitable for use in the U.S. In each case the object was to maximize gasoline production, although it was recognized that the shift in emphasis toward low-molecular-weight olefins was taking place. The four systems included the M. W. Kellogg's Synthol system, the tubewall reactor with a flame-sprayed catalyst, the slurry reactor, and the ebullating bed. The tubewall reactor is essentially a heat exchanger. An active iron catalyst is deposited on one side of the tube. The original work on the tubewall reactor was done at the Pittsburgh Energy Technology Center. It subsequently was used in a conceptual design by Ralph M. Parsons Co. for DOE. The catalyst used in the Parsons design was potassium-promoted taconite. The slurry reactor has been under development in Europe for many years, and is now considered ready for commercial production. The basic idea is to suspend the catalyst particles in a high-temperature liquid, usually slack wax, and then bubble the synthesis gas through the reactor. In effect, it is a three-phase fluidized bed, but the small catalyst particle size eliminates the effect of a solid phase and the slurry functions as a bubbling liquid. The ebullating bed is much like the slurry reactor in that the catalyst particles are suspended in a liquid, and gas is bubbled through the bed. The main difference is that the particles are much larger, and the system is a true three-phase system. This work has been revived recently by Chem Systems Inc. The UOP study aimed at using a reaction engineering approach to identify the inherent strengths and weaknesses of each system. A common basis for evaluation was adopted, including the specification of a plant size, product slate, and reaction mechanism. The mechanism selected was assumed to include a polymerization process, was consistent with the generation of normal paraffins and olefins as final products, reflected the nonselective distribution of the traditional F-T process, and incorporated an equilibrium between the olefins on the catalyst sites and olefins in the surrounding gas phase.
Primary products in the UOP model are derived from two basic reactions. One is the hydrogenation of alkyl chains of random length to form paraffins. There also is an adsorption-desorption series to form olefins. In both cases, the termination step for an active intermediate regenerates a catalyst site, which can again participate in chain growth. This means that there is a constant number of active sites. Inherent in F-T chemistry is the water-gas shift reaction. The UOP study assumed that the shift reaction would not compete for active sites and was not assumed to be proportional to the available F-T sites. The basis for design was a plant that converted about 28 million standard cu ft per hour of synthesis gas. This corresponds to a refinery equivalent of 25,000 barrels per day and is typical for indirect liquefaction plants. Similarly, the operating pressure for the reactors was set at 400 psi, which is considered typical for a modern gasifier. The entrained bed is the only reactor evaluated that used recycle in its operation. Although the basic operational features of all four designs were maintained, UOP made some changes for consistency in evaluation. One point of evaluation was the investment costs for the four systems. Another point of comparison was
annual catalyst cost. The ebullating bed had the highest catalyst cost at about $14.4 million per year. The tubewall reactor was about the same. Lowest catalyst cost was exhibited by the slurry reactor at $760,000 per year. The entrained bed had costs of about $6.72 million per year. Thermal efficiency of the process also was considered. The heat of reaction in an F-T system is about 25% of the heat of combustion of the synthesis gas fed to the system. For economic reasons it is necessary to recover as much of this heat as possible. The total thermal efficiency of the entrained-bed system was estimated to be about 66%, the tubewall reactor system 85%, and the two slurry systems about 91%. The inherently higher efficiency of the slurry and ebullating-bed systems is attributed to more efficient use of the water-gas shift reaction. The total product yield for all four types of reactors is about the same, but the molecular weight distributions are quite different. On the basis of maximum gasoline yield, the slurry and ebullating-bed reactors have a clear edge. However, it has bfeen observed that all four systems are operated at the conditions of the study and that is self-limiting. It is probably true that any of the four systems could be operated at a more desirable product slate, but the comparisons still are considered generally valid.
Brookhaven group proposes oxide mechanism CH2
H
C=0 M + CO + H 2 ~^M
\ /
CH 3
/
H
O
I
M 4- M
M
3
/ H
0
I
- M -
H /
GH3OH
H 'M*
*0
co/ \ H CH4 + C0 2
CH 2 -f-CH / O + O
\
—
-M
/
M—^CH3-CH2-0—-CH3CH2OH
\
M
H20
M
M CH2—CH2
I I
OH
/
CH
C'H2"
OH
*=CH*
O +
OCH 2 CH 3 —•Higher hydrocarbons
M
M
Oct. 26, 1981 C&EN
31
Technology Slurry is least costly F-T system a
Index of investment c:osts Entrained T u b e Ebullailng wall Slurry bed bed
Components
(Number of reactors) Reactors and receivers Other vessels Heat exchangers Pumps TOTAL
(2)
(52)
(18)
(20)
34
189
33
28
30 32 4 100
— 15 4 208
~1 10 2 46
— 21 16 65
a Based on total for entrained bed = 1 0 0 . Source: UOP Inc.
The UOP study clearly shows that the most important product variable in all systems is the temperature, followed closely by the hydrogencarbon monoxide ratio. By all counts, the slurry reactor has the clear advantage under the terms specified by the study. Further investigation suggests that the slurry reactor also may be the reactor of choice for lowmolecular-weight olefins. Designing plants for the production of a single specific product is inherently more risky than for one that is flexible and that can be used for a variety of products. This is one of the problems being faced with the F-T plants now being considered for olefins production. Herbert Kolbel of the chemical engineering institute at the Technical University of Berlin claims that the slurry process does not suffer from this limitation. The slur,ry process, he says, is inherently flexible, a major asset when the cost of the synthesis gas represents about 80% of the operating costs. The variable selectivity of the slurry reactor is achieved by a combination of catalyst selection and the set of operating parameters. Kolbel illustrates flexibility by citing two examples. In the first, a low-molecular-weight olefin-rich product was made containing in excess of 87% hydrocarbons boiling under 200 °C with an olefin content of 80%. The catalyst is a regenerated iron precipitation catalyst with 0.1% potassium carbonate added as a promoter. The operating temperature was 278 °C. An alternative method of producing low-boiling olefins is by recycling a portion of a higher boiling fraction over suitable iron catalysts. An intriguing property of the slurry reactor is the ability to program temperature along its length. This permits higher capacity afforded by higher temperatures without the corresponding production of methane. Methane formation is inhibited 32
C&ENOct. 26, 1981
by lower synthesis gas concentrations as well as low temperatures. To take advantage of these facts, the temperature in the reactor is allowed to rise from 30 to 60 °C with height. High-molecular-weight products are favored by lower operating temperatures, typically 225 °C, and by the proper choice of catalysts. It also can be favored by the accumulation of chain-extending molecules through the removal of recycle filtrate. Oxygenate production can be favored by adding potassium carbonate to the slurry. One experiment yielded about 60% alcohols, mostly C2 to C5, in the product. According to Kolbel, the flexibility of the slurry process can be extended to the Kolbel-Engelhardt synthesis by replacement of the hydrogen by steam. Suitable catalysts are iron, nickel, cobalt, and ruthenium. This process is claimed to be particularly suited for the use of carbon monoxide-rich waste gases. Optimum temperatures lie between 250 and 300 °C with the pressure adjusted such that the carbon monoxide content of the synthesis gas is about 35%. At 20 atm, experiments show that conversion of 90% of the carbon monoxide is possible in a single pass. Most of the products are in the C2 to C10 range, and the olefin content is high because of the absence of hydrogen in the feed. New catalysts are under development for F-T systems in general, but there is particular interest in their development for slurry systems. A West German program of catalyst development has been in full gallop since 1974. One group of catalysts consists of iron modified by additions of titania, vanadia, molybdenum oxide, and oxides of manganese and cobalt. A second group consists of catalysts whose selectivity is achieved by partially poisoning the surface with sulfur. This is supposed to shift product distributions toward the low molecular weights. One specific example is the discovery that iron whiskers exhibit very high selectivity when partially poisoned and when given promoters of potassium, gold, and cobalt. A third group of catalysts is under development at Sasol and is proprietary. A final group consists mostly of manganese catalysts developed by Kolbel and his associates in Berlin. The high performance of these catalysts is attributed to the variability in iron-manganese ratio over a wide range. High iron content leads to more saturated products and vice versa. Selectivity of these catalysts is
claimed to be four or five times those of previously employed materials. The prospect of very large slurry reactors in the near future has prompted a flurry of design studies, and one of the major problems that has emerged is ensuring the stability of very large masses of slurry medium. According to Kolbel, the target for designers of slurry systems is a large-diameter reactor with heights of up to 26 ft. The internal hydraulics are complicated by the minute bubble behavior. Another problem is minimizing the tendency for the slurry to "donut" in flow pattern. All these problems are those normally associated with bubble trays in distillation columns and there is every expectation to believe they will be solved in due course. Numerous attempts at piloting the slurry process have been made since 1944. The oldest example, and the one which still is at work is the pilot plant for the Rheinpreussen-Koppers process, operated at Rheinpreussen A.G. in West Germany. After World War II, the pilot was scaled up 300fold, reportedly without difficulties. The original purpose was to make gasoline, and this was successful. Gasoline with an octane number of 83 was achieved with one pass through the reactor and a total carbon monoxide conversion of 91%. The plant was subsequently used to produce a more olefins-oriented product slate, and this program is still in progress. One of the operational changes in the new program is use of multistage operation, which increases throughput and efficiency simultaneously. This increase is attributed to underestimation of the original pilot reactor capacity. The current belief that F-T synthesis is about to undergo a commercial revival is based on high probability that a large coal gasification industry is still rather close to being instituted and that the gasification industry will constitute the base for provision of synthesis gas. F-T technology already has undergone some rather great changes in character since 1973 and probably will undergo even greater changes in the near future. The acceleration of catalyst development is, apparently, solving much of the problem of traditional F-T synthesis: poor selectivity. The slurry reactor and other new fluidized processing media are improving production rates at minimum costs. The only real impediment, and in the long term the one that really matters, is the economics of oil-based production of fuels and chemicals. •
