Ind. Eng. Chem. Res. 2003, 42, 5003-5006
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A Chemical Alternative to Natural Gas Flaring Michael Golombok* Shell International Exploration and Production, Volmerlaan 8, 2288 GD Rijswijk, The Netherlands
Wendy Teunissen† Avantium Technologies, Zekeringstraat 29, 1012 TX Amsterdam, The Netherlands
A novel chemical liquefaction conversion process for one-step synthesis of natural gas to a methanol/acetic acid mixture is described. Previously reported homogeneous catalytic systems for carrying out methane coupling to CO2 are shown to work equally effectively with just methane. 13 C labeling studies show that both carbons in the acetic acid originate from the methane. The reaction is therefore one of auto-oxidative coupling. The true yield is lower than previously claimed because the solubility of methane in the solvent needs to be taken into account. Introduction Oil companies are under pressure to reduce the flaring of methane. This occurs primarily at oil wells where “associated” gas is considered a byproduct of the principle hydrocarbon liquid production (i.e., crude oil). The customary definition of an associated field is that the gas-to-oil ratio is less than 1000 scf/bbl, or in volume gas to volume oil ratios around 200. Below this ratio, the gas is usually not considered worth capturing or liquefying and is thus typically vented or flared, with the justification for the latter process being that CO2 has a lower greenhouse effect than methane by a factor of around 20. This is particularly the case when the hydrocarbons are “stranded”, i.e., far from a market. Natural gas liquefaction is only profitable when one is dealing with large gas fields rather than oil fields with associated gas production. A chemical conversion is attractive in situations where no pipeline transport network is available, and one such example that has recently come into industrial-scale application is a twostage process involving a conversion to synthesis gas followed by Fischer-Tropsch coupling to make paraffin waxes.1,2 Like gas liquefaction, this process is only practicable for reasonably large scales and thus not for associated gas. In flaring/venting scenarios, the possibility of simultaneously eliminating carbon dioxide and methane would be particularly interesting. The application of such a process would thus not just be confined to the flaring of associated gas. There are many stranded gas fields that contain large amounts of CO2 (i.e., not associated with oil; gas is the main hydrocarbon produced). They are labeled “stranded” because, although they may be close to market, the production is blocked by the unacceptable cost of “sweetening” the gas, i.e., removing contaminating components such as CO2, H2S, or nitrogen. A gas process that absorbed methane and CO2 to form a liquid would be of interest in these cases. Although one can consider a large number of reactions to couple methane and CO2 on paper, thermodynami* To whom correspondence should be addressed. Tel.: 31 70 447 2327. Fax: 31 70 447 3366. E-mail: michael.golombok@ shell.com. † Present address: Shell International, P.O. Box 336, 2501 CH The Hague, The Netherlands.
cally all are extremely unfavorable. Only coreactants of CO2 with a high enthalpy have a favorable equilibrium, such as the reaction of propylene oxide with carbon dioxide to make the solvent propylene carbonate. However, the reaction with methane is thermochemically much more problematic. For example, the theoretical coupling of methane and CO2 to make acetic acid
CH4 + CO2 T CH3CO2H is very unfavorable; equilibrium constants are on the order of 10-7. Systems based on carbonylation as well as carboxylation3 use vanadium catalysts that can also stabilize a controlled methane oxidative coupling4 or even, in a more complex form, fix methane.5 In this work, Fujiwara et al. have shown that methane can also be converted to acetic acid even if no CO or CO2 reactant is used. A process apparently conforming to the above reaction equation using a simpler homogeneous catalysis for methane and CO2 was also recently described by Taniguchi et al. in a batch reactor.6,7 Over a 20-h run at 80 °C, they achieved conversions of up to 97% methane. Commercial application of such a process would appear to be hindered by the fact that, in addition to a catalyst VO(acac)2 in a rather corrosive solvent trifluoroacetic acid (TFA), the oxidant potassium persulfate (K2S2O8) was consumed and would thus appear to be involved as either a radical initiator or a stoichiometric partial oxidizer. Despite these drawbacks, such a system would be very interesting for commercial development if some modifications could be made while maintaining such an apparently high conversion. One other factor that needs to be assessed here is to confirm that CO2 is indeed consumed. This was by no means self-evident from the earlier work. In fact, in reports of these experiments 80% of the reactor volume is filled with TFA solvent. Carbon dioxide, and to a lesser extent methane, is soluble in the liquid so that a large amount of the input gas in the system does not contribute to pressure in the gas cap but is dissolved in the liquid, an effect not apparently considered in previous work, which we discuss below. This would have the effect of reducing the yield of acetic acid defined by
Y ) nacid/nCH4
10.1021/ie0303808 CCC: $25.00 © 2003 American Chemical Society Published on Web 09/09/2003
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Figure 1. Conversion to acetic acid and the total conversion as a function of the partial pressure of methane.
because, in the original work, the denominator term appears to have been calculated from a measurement of the partial pressure of methane in the gas cap above the solvent in the batch reactor vessel. Our initial experimental study therefore had two goals: (1) ascertain whether CO2 is actually fixed into a liquid product and (2) check for liquid products other than acetic acid in the reaction. Experiment The experiments were carried out in a set of four parallel batch reactors equipped with overhead stirring, temperature measurement and control (including calorimetric heat compensation operation), mass flow controllers, and pressure measurement and control. The total volume of the Hastalloy C276 autoclave reactor is 62 mL. TFA was added, followed by the addition of weighed amounts of K2S2O8 and VO(acac)2. The reactor was closed tightly to avoid any leaks. The reactor was flushed for 5-10 min, with the gas added first to remove air present in the headspace. The reactor was pressurized to the required pressure at room temperature with each gas while stirring. To maximize the amount of gases added to the reactor, we filled the reactors at room temperature while stirring. After pressurization, the temperature was raised to 80 °C and the reaction ran for 20 h at a constant temperature. The pressure was monitored during the reaction. 1H NMR analysis was performed on the liquid mixture after completion of the reaction. Calibrations with reference solutions were carried out so that concentrations could be assessed. In some of the experiments, a combination of 13C labeled CH4 and/or CO2 along with “normal” reactant components was used. The aim was to map the uptake of these components in different segments of the product species. For these experiments, 13C NMR analyses were performed on the liquid mixture after completion of the reaction. Results We found that the principal products in this reaction were acetic acid and methanol. Figure 1 shows the yield of acetic acid (involving presumably CO2 take-up) compared to the total yield of liquid product (i.e., acetic acid + methanol) as a function of the partial pressure of methane. For each partial pressure of methane, a range of CO2 partial pressures was used, but this did not seem to significantly affect the yields as shown. We observe that the conversion is greatest at lower partial pressures of methane and that the selectivity to acetic
Figure 2. Pressure observed in 5-mL gas volume as a function of methane added. The value “gas cap” refers to the assumption of no dissolution in the TFA liquid, and the “solubilizing” allows for it, as described in the text.
acid formation is a function of that methane partial pressure, with increasing preference for methanol formation at higher pressures. We will see below that this is consistent with a mechanism where methanol is an intermediate, which subsequently couples to another methane-derived species. Thus, at low pressures, a higher fraction of the total methane is dissolved in TFA and is available to react. In the higher pressure case, TFA is already saturated, there is a fast reaction to methanol, and it appears that methane transport from the gas cap into the liquid is limited, inhibiting further oxidative coupling to acetic acid. A possible reason for the decreasing overall conversion of methane at higher pressures is that increasing pressure is simply adding to the gas phase above the liquid that is already saturated with the gas. Thus, we are simply increasing the denominator of eq 1. This solubility limit on access to chemical conversion is now discussed. We observe that our yields are considerably lower than those observed by Taniguchi. This effect can be explained as follows: if we allow for solubility in TFA at pressure rather than assume that the partial pressure of methane in the gas cap reflects the total number of input moles of methane, then we obtain the plot in Figure 2. For our solubility calculations, we used our Shell in-house extended cubic equation of state model of the Soave-Redlich-Kwong type with pure-component parameters fitted to vapor pressures and liquid densities along with a composition-dependent mixing rule. The plot in Figure 2 shows that for a particular pressure then the actual number of moles of methane input into the solubilizing system is typically a factor of 4 higher than would be obtained if it is assumed that all gas remains trapped above the liquid. We would then expect the yield based on eq 1 to be a factor of 4 lower, and indeed the yields of Figure 1 confirm this. An interesting feature of Figure 1 is that it encompasses a range of partial CO2 pressures ranging from 0 to 20 bar. To study the influence of CO2 on the formation of acetic acid and ethanol, the yields were plotted against the partial CO2 pressure at a partial CH4 pressure of 4 bar, with that pressure corresponding to a high conversion. Figure 3 shows the yield of acetic acid as the partial pressure of CO2 is varied for constant methane. The partial pressure of CO2 has no influence on the formation of acetic acid or on the formation of methanol. We deduce from this observation that the process thus is not really coupling of CO2 with methane but simply some form of auto-oxidative coupling by which methane is partially oxidized and then couples
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Figure 3. Yields of total liquid and acetic acid component as a function of the partial CO2 pressure. The methane pressure is 4 bar.
carboxylate group on acetic acid is clearly from the 13Clabeled methane. The simplest mechanism is that a methyl radical has combined with a partially oxidized C1 species. A small amount of 12C-labeled carboxylate could arise from TFA, as has been postulated recently by another group working in this area.8 Not shown in Figure 4, because it is basically a null result, is the effect of using purely normal methane with 13C-labeled CO with 12C methane; we found that 2 predominantly the products were all normal 12C materials, proving again that CO2 is not significantly involved in this reaction. If, however, very high partial pressures of CO2 are used, then some 13C-labeled carbon atoms are found in the products, indicating involvement in the reaction. This could again be the effect of naturally occurring isotopes in TFA. One other possible source would be whether the species that actually bonds to a a CH3 radical is CO, which also originates from methane. In that case, a disproportionation reaction of the form 12
CO + 13CO2 f 12CO2 + 13CO
Figure 4. Isotopic selectivities for the formation of acetic acid and methanol from labeled methane and carbon dioxide reactants. Quantities in brackets indicate the pressure in bar of the components above the liquid.
with itself. We may thus postulate two reactive paths. First there is methane conversion to methanol: [ox]
CH4 98 CH3OH This is followed by further partial oxidation to an intermediate oxidized moiety. [ox]
CH3OH 98 CHxOy This then reacts with a methyl radical generated from another methane to yield an acetic acid molecule.
would explain the result. To test this, it would be necessary to carry out gas sampling in the reactor, and this is currently being studied. In conclusion, we have shown that externally added carbon dioxide is not involved significantly in the production of acetic acid in the single-step conversion of methane. The carboxyl group in the acetic acid is generated from methane itself. There does appear to be some marginal take-up, but this may be due to disproportionation of the oxidized methane with CO2 in the TFA solvent. At any rate, the reaction would only be of industrial interest with an aerobic oxidant. For that reason we never examined what happened to K2S2O8, which one would never use in an industrial process. Finally, the process should not be accompanied by oxidation of methane to CO2 on a large scale; i.e., there has to be a reasonably high carbon selectivity to liquid products. We are currently working to ascertain whether this is the case. Literature Cited
[ox]
•
[ox]
(CH4 98) CH3 + CHxOy 98 CH3CO2H One other possibility that was recently raised was that the carboxyl group in the acetic acid product arises from detachment from the TFA solvent.8 To address both of these points (the role of externally added CO2 and the role of the solvent), we turn to the results of our 13C labeling experiments. Only 32 scans were used so that only 13C-enriched species could be observed, except for the resonances relating to the solvent TFA because it is present in such large excess. When the information from the 1H and 13C NMR spectra, as well as the 1H/13C coupling, are combined, the ratio of the four combinations of acetic acid could be calculated in addition to the ratios of 12C/13C methanol. Figure 4 shows a typical and significant result for these labeling experiments. For the indicated inputs, we calculate the selectivity of each of the four possible combinations of acetic acid and the two possibilities for methanol. It can be seen that the vast bulk of the
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5006 Ind. Eng. Chem. Res., Vol. 42, No. 20, 2003 (7) Kurioka, M.; Nakata, K.; Jintoku, T.; Taniguchi, Y.; Takaki, K.; Fujiwara, Y. Palladium catalyzed acetic acid synthesis from methane and carbon monoxide or dioxide. Chem. Lett. 1995, 244. (8) Wilcox, E. M.; Roberts, G. W.; Spivey, J. J. Thermodynamics of light alkane oxidation. Appl. Catal. A 2002, 226, 317-318.
Received for review May 1, 2003 Revised manuscript received August 13, 2003 Accepted August 25, 2003 IE0303808
