DE4428343A1 - Prepn. of ethane and/or ethylene@ - Google Patents

Abstract

A method of the prepn. of ethane and/or ethylene by coupling methane comprises the radiolytic formation of methyl radicals and subsequent thermal reaction. A device for carrying out the method, comprising an ionising radiation field through which the methane is passed, which is linked to a thermal reaction zone, is also claimed. Pref. radiolysis is achieved using electron beam radiation (eg. from an electron accelerate, psi radiation, beta radiation, (hard) X-ray radiation or microwave radiation. The subsequent thermal reaction is carried out at 300-800(600) deg C. Ionisation is carried out in a tube reactor with entry windows for the ionising radiation.

Description

The invention relates to a method for the coupling of methane to ethane and / or Ethylene. Such methods are interesting because they are a cheap commodity that is in the over shot is present (methane CH₄), in a valuable product (ethane C₂H₆ and / or ethylene C₂H₄) convert.

The known methods work with metal oxide catalysts (eg., Sm₂O₃ / CaO / Ag or Li / MgO) with the addition of a much less than stoichiometric amount of O₂. The latter is necessary to a total oxidation of reactant (methane, C₁) and in particular the Pro products (ethylene, ethane: C₂) to prevent. At the same time this measure leads to very low conversions per reactor passage. Typical process temperatures are in the range of approx. 700 to 900 ° C.

In a conventional catalytic fixed bed reactor, therefore, only very small Methane conversions and C₂ yields achieved. An increase in conversion (over the setting of the methane / oxygen ratio in the feed) automatically has a Ver reduction of C₂-selectivity result. For example, a high C₂ selectivity of about 96% only at a methane conversion of about 3% (about 2.9% yield) achieved. Conversely, one reached about 16% methane conversion only with a decrease in the C₂-Selek productivity to 70%, corresponding to a yield of approx. 11%. The reason for this Ver is due to the fact that the C₂ products can be easily further oxidized as the educt. Therefore, the O₂ partial pressure must be kept low, but what au automatically reduces sales.

In the literature

• Y. Jiang, I.V. Yentekakis, C.G. Vayenas, Methanes to Ethylene with 85 Percent Yield in a Gas Recycle Electrocatalytic Reactor Separator ", Science 264, 10 June 1994, 1563-1566 (hereinafter D1) and
• A.L. Tonkovich, R.W. Carr, R. Aris, "Enhanced C₂ Yields from Methane Oxidative Coupling by Means of a Separative Chemical Reactor ", Science 262, 8 October 1993, 221-223 (hereinafter D2)

this problem is solved by the desired educts (ethane and ethylene) be separated quickly from the reactants (oxygen and methane). In D1 ge  this happens in a combination of electro-catalytic reactor and downstream molecular sieve adsorber for ethylene, whereas in D2 a simulated countercur Rent moving bed chromatographic reactor "(SCMCR) is used the reaction products are each gaschromatogra after a reaction step phically separated from the reaction mixture.

Both solutions yield high yields with high selectivities (eg D1: 88% C₂ selectivity at 97% methane conversion = 85% yield). Both concepts is the Disadvantage in common that the respective separation step of the C2 products at essential lower temperatures than the reaction step. This, together with the fact that large excesses of inert gas (eg N₂) with through the system must be driven, prevents implementation in an economic industriel les procedure, since large amounts of gas must be repeatedly heated and cooled (in D1: reaction temperature = 835 ° C, absorber temperature = 30 ° C, desorption of C₂H₄ from the absorber molecular sieve at 400 ° C).

Both approaches require a very elaborate process control, one wirt difficult to transfer to technical scale. By the estate oxygen is the formation of CO₂ not even with optimal process control avoid. Carbon dioxide, however, lowers the C₂ formation rate, probably over a par tielle poisoning of the catalyst. The relatively high temperatures increase the Nei for coking of catalyst and apparatus.

The object of the invention is to avoid the disadvantages listed in order to one economic technical process.

This object is achieved in that the methyl radicals (CH₃) Radiolytic be generated and then continue to react thermally. The procedure will According to the invention conducted so that the methyl radicals after their formation, the radiation leave field, so that subsequently the subsequent reactions can take place thermally, and under conditions (pressure, temperature), the maximum yield of C₂- Guarantee products. Since the products are made in an area that the ionizing radiation is not exposed, they are deprived of radiolysis.

The radiation source is preferably electron radiation from an electron accelerator. However, the method also works with gamma radiation, beta frame (hard) X-rays or microwave radiation. The thermal Weiterreak  tion takes place at temperatures between 300 and 800 ° C, preferably at 600 ° C. exporting ments of the invention and devices for carrying out the invention are counter states of subclaims.

The invention has the following advantages:

• - It is achieved a very high C₂ selectivity at high methane conversions.
• - The system is simple: the reactor contains only hydrocarbon species. Heteroatoms such. As oxygen and associated side reactions omitted.
• - Education kinetics and product distribution can be set within wide limits by varying the process parameters. These parameters are in particular:
  Initial concentration of methane, pressure, temperature, dose rate of the radiation source, beam geometry and radiant energy.
• - No catalyst is needed.
• - Compared to the oxidative coupling ge at relatively low temperatures works, which reduces the risk of coking.
• - The system is technically very simple and requires no complex control such as the SCMCR system.
• - The kinetics of sales is almost exclusively of homogeneous gas phase senreaktionen determined. This makes the process easy to calculate. uncertainties in the arithmetical interpretation, such. B. varying catalyst activities, entfal len.

The invention will be explained in more detail with reference to two figures.

Fig. 1 shows schematically above a tubular reactor according to the invention for the implementation of the method and below the concentrations of starting material and product over the length of the tubular reactor.

Fig. 2 shows the concentrations of various substances over the reaction time.

Fig. 1 shows the top of a tubular reactor R, which is from the left with methane at pressures of a few millibar to a few 10 bar (preferably 1 bar) is flown. This may be pure methane or else methane in inert gas (eg argon or nitrogen), it being possible for the concentration to be varied within a wide range (typically 0.1-100%, preferably 100%). The methane then enters a radiation field S, which originates from windows in the tubular reactor R from external sources. There is any ionizing radiation in question, z. As gamma radiation, beta radiation, hard X-rays or - preferably - electron radiation from an industrial electron accelerator. Likewise, the use of microwave radiation is possible. The embodiment shows in the figure, two opposite Elektronenbe accelerator E, which produce a strong radiation field S with low spatial extension in the direction of the reactor axis. This is so that the substances are only briefly in the radiation regime. If they were there too long, the C-C bonds of the product substances would start again. It is therefore always very short to adjust the irradiation time in relation to the time of further reaction. As electron accelerator E eig nen z. B. 800 to 1000 keV linear cathode accelerator. The arrangement and the operating parameters (beam current, electron energy) are chosen so that the largest possible G values (ie number of initiating C-H bond fractions per 100 eV of absorbed energy) are achieved.

Preferred values for a large-scale plant are z. B .:

Beam current: 100 to 350 mA Electron energy: 900 keV Installed capacity: about 90 kW Absorbed dose: 0.5-2.0 megarad

The formation of methyl radicals from methane in the radiation zone S takes place via reac tions of the primary radical cation or - preferably - on the fragmentation tion of the "superexcited states", which arise from the interaction of the radiation with the methane molecules. These primary processes take place on a time scale in the typical picosecond to nanosecond range (10 ^-12 to 10 ^-9 s).

When leaving the radiation field S get the CH₃ radicals in the thermal reaction zone T, which is maintained at temperatures of 300 to 800 ° C, preferably 600 ° C. Here the radical follow-up reactions take place, which will be explained in the following example. Under suitable conditions, such as flow rate, pressure and temperature, predominantly ethane is obtained as a direct recombination product of the methyl radicals. Fig. 1 shows schematically below the decrease of the methane concentration and the increase of the ethane concentration over the length of the tubular reactor R. The time scale for the recombination can be adjusted within wide limits by means of methane concentration, jet current of the electron accelerator E and the reaction temperature.

Fig. 2 shows the results of a calculation example. Both partial figures show the temporal concentration gradients of different substances. The calculation starts with pure methane at a pressure P _ges = P _ethane = 1 atm with C₀ (methane) = 1.4 × 10 ^-5 mol / cm³ as feed and a reactor temperature of 600 ° C.

The most important reactions of this system can be formulated as follows:

Reaction (1) takes place according to the invention in a radiation-induced manner, while steps (2) to (10) extend substantially thermally downstream of the radiation field S. Because the kinetic parameters for these elementary reactions with reasonable tolerances be can be, the product formation rates and the product yields for ver calculate different pressures, temperatures and educt concentrations.

The elementary reactions (1) - (12) result in a system of ordinary differential equations of the first order, which can be solved for various technically interesting initial values. The result of the calculation is shown in FIG . The concentration time profiles shown there (the time axis can be translated directly into a length axis in the flow system) show a rapid decrease of the methane concentration even in the radiation regime, accompanied by an ethane formation almost to the initial concentration of methane (here: 1.4 × 10 ^{- 5} moles per cc). Ethylene is produced under these conditions only to a much smaller extent. However, product distribution and time scale of product formation can be varied within wide limits by adjusting the temperature, pressure, initial concentration of methyl radicals (electron accelerator flow) and initial methane concentration. The residence time in the radiation field is in the calculation example at 1 microsecond. The time range between 10 nanoseconds to 100 microseconds. is sufficient for the radiation-induced reaction, but on the other hand short enough not to destroy the resulting C₂ products by radiation-induced bond breaks again. As a total reaction time can in Fig. 2 z. B. the period 1 × 10 ^-4 seconds are taken. As shown in FIG. 2 above, after this time virtually all of the methane used has been converted into ethane. After this time, as shown in FIG. 2 below, the methyl radicals are also practically degraded.

In any case, the statement that the ent standing hydrogen atoms do not interfere with the reaction process. much more promote z. B. by the abstraction reaction (2) rather the formation of ge Wanted C₂ products.

Instead of using a flow system with a narrow radiation field, as shown in FIG. 1, the application of a pulse radiolysis in a batch reactor is possible.

Here, the gas (pure or diluted methane) at a temperature in the order of 600 ° C and z. B. a pressure of 1 atm in a closed container exposed to a radiation pulse. This can z. B. again come from a Elektronenstrah development cannon and typically have the duration of about 1 to 100 microseconds. Subsequently, the product can be withdrawn, since the further reaction is completed after milliseconds. For large-scale operation but recommends a Strö determination reactor as shown in Fig. 1, because of the continuous operation.

Claims (5)

1. A process for the coupling of methane to ethane and / or ethylene, characterized by radiolytic generation of methyl radicals and thermal Weiterreakti on. 2. The method according to claim 1, characterized in that the radiolysis by Electron radiation, z. B. from an electron accelerator, gamma radiation, Beta radiation, (hard) X-rays or microwave radiation triggered becomes. 3. The method according to claim 1 or 2, characterized in that the thermal Further reaction at temperatures of 300 to 800 ° C, preferably by 600 ° C, by to be led. 4. An apparatus for coupling methane to ethane and / or ethylene, gekennzeich net by an ionizing radiation field (S) through which the methane is passed and a subsequent thermal reaction zone (T) for further reaction. 5. Apparatus according to claim 4, characterized by a tubular reactor (R) with Entry windows for the ionizing radiation.