Ind. Eng. C h e m . Res. 1995,34, 4238-4243
4238
Modern State of Direct High Pressure Partial Oxidation of Natural Gas to Methanol Vladimir S. Arutyunov,*JValentin Ya. Basevich, and Vladimir I. Vedeneev Semenov Znstitute of Chemical Physics, Russian Academy of Science, Kosygin st. 4, Moscow 11 7334, Russia
The survey of existing experimental results on direct partial oxidation of methane to methanol (DMTM) is followed by theoretical analysis of the process. Special attention is paid to the role of the pressure. The cause of failure of many attempts to promote a high pressure DMTM process by means of catalysts is elucidated. The modern industrial significance and future prospects of the process and the most important directions of future investigations are discussed.
Introduction The direct partial oxidation of methane to methanol (DMTM)is one of the most promising routes of natural gas conversion into more easily transportable fuels and valuable chemicals. The process may be accomplished with both catalytic and homogeneous conditions. In the last case high pressures exceeding 50 atm are necessary for appropriate product yield (Newitt and Huffner, 1932). After Newitt and co-workers (Newitt and Huffher, 1932;Newitt and Szego, 1934;Newitt, 1937)had shown that high pressure methane oxidation can provide a sufficiently high yield of methanol, there were permanent attempts to increase the methanol yield and to elaborate the industrial scale process. Several plant scale attempts were accomplished before and during the war, but now there are no such plants. Some years ago interest in this process was renewed due to the rising role of natural gas (NG) as the dominant energy source in the near future and the severe need of industrial countries for ecologically clean motor fuels. Undoubtedly, this interest was enhanced by the work of Gesser et al. (1985)in which very high selectivities of methanol formation were obtained. Since then some other investigations of the gas phase process were published (Table l),including those with and without catalyst (Burch et al., 19891, pilot scale experiments (Budymka et al., 19871, and theoretical kinetic simulations (Vedeneevet al., 1988). On the basis of these results and the theoretical analysis of this process, some conclusions about its kinetic mechanism, modern industrial significance, and future prospects may be done. Discussion of the most important directions of future investigations in this field is made.
Principal Experimental Results Pressure Dependence of the Products Yield. It is well-known that pressure reduces reaction time (Melvin, 1966) or reaction temperature (Burch et al., 1989) and promotes methanol formation. But the cost of gas compression is one of the most prominent factors in the total product cost (Edwards and Foster, 19861, so it is very important to determine the optimum pressure range. There were also some indications of a possible fall-off in methanol yield at pressures above 180 atom (Boomer and Thomas, 1937) or methanol selectivity even at pressures above 100 atm (Gesser and Hunter, 1992). FAX: (7-095)938-2156;E-mail: KINEWglas.apc.org 0888-5885/95/2634-4238$09.00lO
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[Oz]= 2.8%.
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Figure 2. Pressure dependence of the main product concentrations C in liquid product with T = 400 "C and LO21 = 2.8%: (1) methanol; (2) formaldehyde; (3) ethanol; (4)acetone; (5) organic acids.
Figure 1(Arutyunov et al., 1994) shows that the total liquid product yield rises monotonously with pressure up to 200 atm, but the most dramatic changes occur at pressures below 100 atom. Concentrations of alcohols and acetone in this liquid product really exhibit a maximum in the vicinity of 150 atm, and that of formaldehyde constantly drops with pressure (Figure 2). But the yield of formaldehyde remains approximately constant in this pressure range due to enhancement of the total liquid product yield (Figure 3). And the methanol yield reaches maximum values at pressures slightly above 100 atm. So there are no serious reasons to use in this process pressures above 100 atm, and in some cases even pipeline pressure (75 atm) will be sufficient. Carbon monoxide is the main gas phase product of the reaction. Its concentration in output gases reaches 0 1995 American Chemical Society
Ind. Eng. Chem. Res., Vol. 34, No. 12, 1995 4239 Table 1. Results on Gas Phase, Fast Flow, Partial Oxidation of Methane N 1 2 3 4 5 6 7
authors Newitt and Szego Budymka et al. (100 tondyear pilot plant) Onsager et al. Burch et al. Rytz and Baiker Arutyunov et al. Vedeneev et al. (kinetic simulations) 2.00
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Figure 3. Experimental methanol (1) and formaldehyde (2) yield vs pressure: T = 400 "C;[ 0 2 1 = 2.8%. [CH,OHl/ICH#l lot
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20
70
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Figure 5. Methane/oxygen ratio effect on liquid products yields with T = 400 "C and P = 100 atm: (1)total liquid product; (2) water; (3) methanol; (4) formaldehyde. 0
100
200
P, atm Figure 4. Pressure dependence of the methanoVformaldehyde ratio: T = 400 "C;[OZ] = 2.8%.
1.5% at an initial oxygen content of -3% and remains approximately constant in the pressure range from 30 t o 230 atm. That of carbon dioxide is several times lower and increases with pressure, leading to a prominent drop in the carbon monoxide/carbon dioxide ratio (Arutyunov et al., 1994). The rise of carbon dioxide concentration with pressure is, probably, a t any rate, partly connected with formic acid formation, followed by its decay to carbon dioxide. The mechanism of this process was suggested by Vedeneev et al. (1992). Another important component of output gases is hydrogen. Some work was done with very high pressures up t o 3400 atm (Tripathy, 1975) and even higher (Lott and Sliepcevich, 1967) but no obvious advantages of using such pressures were obtained. It is worth noting that it is possible t o change the liquid product composition, in particular the methanol to formaldehyde ratio, by means of a pressure change (Figure 4) (Arutyunov et al., 1994). Influence of the Composition of the Reacting Mixture on the Process. The process is very sensitive to the methane/oxygen ratio; methanol selectivity decreases with the reduction of this ratio (Newitt and Szego, 1934; Budymka et al., 1987; Onsager et al., 1989; Yarlagadda et al., 1988; Bistolfi et al., 1991;Arutyunov et al., 1995a). Figure 5 (Arutyunov et al., 1995a) shows the dependence of the main product yields from this parameter. Due to these investigations the optimum oxygen concentration in the reacting mixture is approximately 3%, which determines the low methane conversion per pass. It makes recycling of reacting gases very desirable after separation of liquid products. But the presence
in output gases of such easily reacting and difficulty separating admixtures as hydrogen and carbon monoxide may cause some problems due t o their ability to accumulate at recycling. Fortunately, it occurs that if their concentrations do not exceed 4-5%, the drop in methanol and formaldehyde yield is low (Arutyunov et al., 1995a). So, if measures are taken t o prevent these output gases accumulations above this level, recycling is possible. Dilution of the reacting mixture by nitrogen and, apparently, other nonreactive compounds does not influence the process while the methane/oxygen ratio remains constant (Burch et al., 1989; Arutyunov et al., 1995a). In some cases the use of cheaper air instead of pure oxygen in the process is possible. The influence of small impurities of higher hydrocarbons on the process is well established. Their presence in natural gas lowers the necessary reaction temperature without essential changes in product yield (Burch et al., 1989; Boomer and Thomas, 1937; Hunter et al., 1990),but there are no systematic studies on high ('3%) admixtures of other hydrocarbons t o methane. Role of Surface, Catalysts, and Promoters. Although there were many attempts t o improve the methanol yield and selectivity in DMTM by means of various catalysts (Gesser and Hunter, 1992;Pitchai and Klier, 1986; Krylov, 19921, now it is obvious that there are no advantages in using catalysts at high pressures (Burch et al., 1989; Gesser and Hunter, 1992; Hunter et al., 1990). It was also shown that there are few, if any, advantages in using gas phase promoters in the process (Hunter et al., 1990). The reason for this will be discussed below. The influence of various packing and wall surface materials on the process was also studied (Burch et al., 1989; Morton et al., 1991). Quartz and stainless steel are the preferable materials, but a t real operating pressures (above 50 atm) and reaction times (below 1 s) the influence of the surface on the process is negli-
4240 Ind. Eng. Chem. Res., Vol. 34, No. 12, 1995
gible. Relatively low output temperatures (below 850 K at initial oxygen content -3%) and a low yield of organic acids (Figure 2) cause no essential problems with reactor materials. Influence of Temperature and Residence Time. If a high enough temperature for complete oxygen conversion is reached, then further temperature increases are undesirable because they may cause a fall in methanol and formaldehyde selectivities (Burch et al., 1989). The same is valid for residence time, although the effect of this parameter after complete oxygen consumption is very weak (Burch et al., 1989; Arutyunov et al., 1994). Attempts were made to take advantage of the rapid cooling of the reaction products (Onsager et al., 19891, but results are very close to those obtained without cooling (Table 1). Methanol and Formaldehyde Selectivities. Table 1 shows a summary of the most reliable results of gas phase, fast flow, high pressure, partial oxidation of methane to methanol. It is evident that methanol selectivities up t o 50% and that of formaldehyde up to 6-9% may be achieved a t a wide enough range of conditions. In this paper we, naturally, mentioned very high selectivities of slow flow experiments (Gesser et al., 1985; Yarlagadda et al., 1988). But, because of the absence of a satisfactory explanation of the purposes that lead to such dramatic changes in methanol selectivity in comparison with the usual fast flow experiments, at present we must regard these results as a perspective goal for further investigations.
Theoretical Analysis of the Process The real understanding of a complex process such as the partial high pressure oxidation of methane is impossible without relevant kinetic modeling. A special kinetic model for high pressure methane oxidation at moderate temperatures was elaborated by Vedeneev et al. (1988) and proved by simulation of available data for a very wide range of experimental conditions presented in the works of Newitt and Huffner (1932), Newitt and Szego (1934), Budymka et al. (19871, Burch et al. (1989), Onsager et al. (1989), Rytz and Baiker (19911, and Arutyunov et al. (1994): static and flow, both laminar and turbulent; isothermal and nonisothermal; laboratory scale and pilot plant scale. It should be stressed that no changes in the chemical part of the model were made to describe every particular case of this variety of experiments, only those dealing with reactor dimensions, gas flow, and heat transfer conditions. Of course, some improvements were made since the publication of the model due to new data on some elementary reactions, but they were not of great importance. Other groups successfully used this model for describing their own experimental results (Thomas et al., 1992). Naturally, a good coincidence of calculations with experimental data cannot be considered as a theoretical ground of the model. Some predictions were made on the basis of this model and then proved experimentally. Below will be given the main conclusions of the kinetic analysis of high pressure partial methane oxidation. 1. The process has two phases, distinctly differing by their time scales. The very short initial stage of the process has a chain-branched mechanism, which is very similar to the mechanism of hydrogen oxidation (Vedeneev et al., 1992). This stage is very sensitive to initial conditions, especially pressure. Any reactions of radi-
Time.
8
Figure 6. Calculated kinetics of methanol formation with T = 447 "C and P = 100 atm: (1)LO21 = 2%;(2) LO21 = 3%;(3) E021 = 4%.
cals with intermediate products in this phase are unimportant. This initial autoaccelerating phase is completed by a subsequent quasistationary phase which is characterized by the approximate equality of the rates of branching and radical recombination. The mechanism of this second phase of the reaction may be considered as a degenerate chain-branched mechanism. Chain-branching in the second phase is connected with intermediate products. The most important branching reactions in this phase are interaction of methyl peroxide and hydrogen peroxide radicals with methane, methanol, formaldehyde, and hydrogen peroxide. 2. The formation of the main products-methanol and formaldehyde-proceeds simultaneously and independently. Only a relatively small part of formaldehyde forms from the oxidative and thermal decomposition of methanol. The difference in their concentrations is only due to the difference in their stability t o further oxidation. The main part of methanol forms at the end of the reaction just before total oxygen consumption (Figure 6). It explains the very high sensitivity of the methanol yield from residence time and temperature at incomplete oxygen conversion. 3. The increase of the oxygen concentration not only decreases the selectivity of methanol formation but also the whole rate of the process (Figure 6). This theoretical prediction was lately confirmed experimentally. 4. There exist two stationary regimes of the process with a difference in radical concentrations and in the process velocities of over 4 orders of magnitude. There also exists a critical pressure for the transition between these two regimes (Vedeneev et al., 1994). The calculations conducted for the mixture [CH41:[021 = 9:l at temperatures of 600-750 K and pressures from 1 to 10 atm have shown that transition via critical pressure changes the reation time (Figure 71, the rate of oxygen consumption (Figure 81, and the quasistationary CH3O0' radicals concentration (Figure 9) at the initial stage by over 4 orders of magnitude and their approaches to a quasistationary concentration. The value of the critical pressure decreased with the increase of the initial temperature. For 600 K the calculated critical pressure lies between 6 and 7 atm, whereas at 700 K i t is between 1and 2 atm (Vedeneev et al., 1994). The existence of this critical phenomena may be explained in terms of an initial phase of the process which has been thoroughly analyzed by Vedeneev et al. (1987). Below the critical pressure the initial phase is a relatively slow chain reaction, usually with surface termination of the radicals. But above the critical pres-
Ind. Eng. Chem. Res., Vol. 34, No. 12, 1995 4241
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Time,
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l
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Figure 8. Calculated kinetics of oxygen consumption a t T = 377 "C: (a)P = 3 atm; (b) P = 4 atm.
sure it turns to a chain-branching self-accelerating process. The concentration of the radicals in this case is 103-104 times higher (Figure 91,and quasistationarity is provided only by means of their gas phase selfrecombination a t these very high concentrations. 5. In the high pressure region above the critical pressure the initial chain-branched phase provides a very high velocity of radical generation, so any catalyst, promoter, or other source of radicals can hardly compete with it. Usually their influence on the process is negligible. It explains the failure of numerous attempts to improve the high pressure process by means of a catalyst. 6. The theoretically calculated selectivity of methanol formation is 45-50% and that of formaldehyde is 6-8%. These values are very similar to those obtained in fast flow, high pressure experiments (Table 11, and there is no theoretical evidence to expect higher selectivities in these two known and described above quasistationary regimes.
b
2.0
Time, s
Time, s
Industrial Significance and Future Prospects of the Process
Figure 9. Calculated kinetics of methylperoxy radicals a t T = 377 "C: (a)P = 3 atm; (b) P = 4 atm.
Almost all analytical reviews predict a sharp rise in methanol demand in the near future, mainly for methyl tert-butyl ether (MTBE) and other ether production for clean motor fuels. But on the other side there is not much construction of new methanol plants, especially in industrial countries. To our mind, it is connected with the common feeling that conventional technology via syngas with its high complexity and capital costs and very high energy consumption is not appropriate for meeting this future demand; therefore no one wants to make a long-term investment in this technology. The only real challenge to this technology now is the DMTM process. Some economic evaluations of the conceptual DMTM process were made in the past few years (Edwards and Foster, 1986; Kuo et al., 1989; Geerts et al., 1990). It was shown that a t methane to methanol selectivities of higher than 77% the catalytic DMTM process will be preferable against the conventional route via syngas (Edwards and Foster, 1986). According t o our estimates, the real gas phase process will be preferable at selectivities no higher than 70%due to the absence of catalyst and carbon dioxide removal. This
level has not been reached yet on the pilot plant scale, but those selectivities to methanol and formaldehyde (higher than 60%)that have already been reached open a wide area for practical use of the process. The most obvious area for such plants is connected with the necessity to meet internal demands by the gas and oil industry for methanol and motor fuels, especially in remote areas where main gas and oil fields are located. For instance, the Russian gas industry consumes more than 0.3 million tons of methanol per year to prevent solid gas hydrates formation in gas wells and pipelines, and this amount will rise to 0.5 million tons by the end of the century (Arutyunov et al., 1993). Also the real cost of production of this methanol means little if the transportation component for these remote areas exceeds by several times the market price of the product. The DMTM process permits the construction of relatively simple modular factory-made installations with a desirable capacity from hundred tons t o hundreds of thousand tons per year just to meet this local needs (Arutyunov et al., 1992; Arutyunov et al., 1993). The very important advantage of the DMTM process over
4242 Ind. Eng. Chem. Res., Vol. 34, No. 12, 1995
the traditional one is that the cost of the final product depends very slightly on its capacity. So building a number of low scale DMTM plants just at sites of methanol consumption instead of one conventional large scale plant may solve the transportation problem. Also by means of the well-known Mobil process, produced rough methanol may be converted into high quality motor fuel for local motor vehicles. If oxygen is used as the oxidizing agent, then the DMTM process almost does not add uncombustible components to output gases, so unreacted gas may be returned back to the pipeline. In this way the problem of low natural gas conversion per pass may be overcome. Such installations, especially those without a catalyst, are very flexible for inputting gas composition, capacity, and other local conditions and may be easily moved from site to site. This will also let us use gas from small gas fields and other sources which cannot be connected t o pipelines (Arutyunov et al., 1992; Arutyunov et al., 1993). In this case recycling of output gases is desirable. Up to 30% savings may be achieved if air is used instead of pure oxygen (Edwards and Foster, 1986). Such a process is possible if, for instance, output gases are used for the combustion in a power station. We believe that the real economic figures for the DMTM process will be even better, since on the basis of investigations described above some improvements of the process have been suggested, including changes in reactor construction. The tests of a suggested new method were carried out on a 100 ton per year pilot plant and showed approximately a 30% increase in productivity. For the needs of the Russian gas industry, the project of a DMTM pilot plant with productivity 10000 tons of methanol per year, which takes into account these recent achievements, was worked out.
Discussion on Future Investigations The obtained results (Table 1)are good enough for some practical cases, but a further increase in product selectivity is desirable for a more successful DMTM process in competing with a conventional large scale process. In some works (Gesser et al., 1985;Yarlagadda et al., 1988) very high selectivities were obtained, but there were some doubts whether these selectivities may be reproduced in an industrial scale process. So it is important to analyze if there exist theoretical possibilities for enhancing methanol selectivity. The kinetic model of the process (Vedeneev et al., 1988) is highly nonlinear due to the very important chain-branching and chain-terminating reactions between highly reactive intermediates. Under nonlinearity we understand the nonlinear character of differential equations describing the chemical system. This nonlinearity explains chain-branching explosions, flame propagation, temperature oscillations, cool flames, and other nonlinear phenomena a t methane oxidation (Gray et al., 1994). So apart from the two stable regimes below and above the critical pressure, which were described above, one can expect the existence of other unknown regimes. These new regimes may be both stable and unstable, but even some unstable regimes may be stabilized artificially in some cases. The search for such regimes must be the main goal of further investigations, starting with theoretical studies. It is obvious that the yield of main products in these new regimes may be very different from those obtained under conventional experimental conditions.
The typical phenomena connected with nonlinear mechanisms of the oxidation of hydrocarbons are so called cool flames. They are known for many hydrocarbons (Shtern, 1964), but only the work of Vanpee (1956) can be considered as real evidence of cool flames for methane. We recently confirmed the results of Vanpee (Sokolov et al., 1995), discovered the existence of a region of negative temperature coefficient at methane oxidation at atmospheric pressure (Arutyunov et al., 1995b), and theoretically showed the possibility of such phenomena both a t low and at high pressures (Basevich et al., 1995). Another very important direction of future investigations is the study of high pressure partial oxidation of hydrocarbon gases with a high content of higher hydrocarbons (ethane, propane, etc.). There are several reasons for using such gases in the DMTM process: There is a large amount of such gases in Russia, both natural and industrial. More than 30% of Russian natural gases are ethane containing gases (Staroselskii, 1992). And their share constantly increases due to the working out of dry gases. Russia’s proved reserves of gases containing ethane and hydrogen sulfide as estimated by Jan 1, 1991, are 18.5 trillion m3, including 12.1 trillion m3 in West Siberia. Proved reserves of 125 gas fields contain 1085.7 million tons of ethane, 560.8 million tons of propane, and 351.2 million tons of butane (Staroselskii, 1992). But in spite of such large reserves of higher hydrocarbons, the share of natural gas used as chemical feedstock in Russia does not exceed 2.02.5% and only 0.5% is used as motor fuel. High pressure partial oxidation of higher hydrocarbons provides good yields of higher alcohols (ethanol, propanol, etc.) (Shtern, 1964). So ethane rich gases will produce with the DMTM process alcohol mixtures with a large content of higher alcohols. Such alcohol mixtures may be used directly as an octane admixture to low quality gasoline instead of MTBE (Hohlein et al., 1991)with the obvious advantage that in this case there is no need of scarce butene. The high content of ethane and propane in hydrocarbon gas makes conditions for DMTM more mild than for pure methane. It was shown at pilot investigations that 3% of ethane in natural gas lowered the initial operating temperature approximately by 100 degrees. If ethane and propane concentrations will reach a dozen percents, a sufficient lowering in operating pressures may be expected. Thus, an additional savings up to 10% (Edwards and Foster, 1986) may be achieved. The cost of natural gas itself amounts to 50% of the DMTM production cost (Edwards and Foster, 1986).But if casing-head or hydrocarbon processing gases cannot be used on site, then these gases had to be burned or, what is more preferable, may be used in low scale DMTM plants; the economic figures for the process will be very high.
Conclusions The results on the high pressure gas phase DMTM process achieved now on a pilot plant scale (up to 60% yield of methanol and formaldehyde at 3-4% methane conversion) are good enough for industrial use of this process, especially for remote gas and oil producing regions in Russia. For more successful competition of DMTM with the conventional route via syngas, it is desirable to enhance the product yield. It may be achieved by studying other unknown regimes of the process.
Ind. Eng. Chem. Res., Vol. 34, No. 12, 1995 4243 Some important advantages for the DMTM process may be achieved by wide use of ethane and propane containing gases.
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Received for review March 28, 1995 Accepted J u n e 14,1995@
IE940425S Abstract published in Advance A C S Abstracts, September 15, 1995. @
