Note: Descriptions are shown in the official language in which they were submitted.
~27~
PROCESS FOR Ti~E PREPARATION
OF AN ETHYLENE-ETI~NE MIXTURE
The present invention relates to a method for producing
ethylene and ethane by oxidative coupling of methane by means of
oxygen, or an oxygen-containing gas, in the presence of a catalyst.
Methods for producing ethylene, which is used in many
applications in chemical synthesis, are at present based almost
entirely on cracking mineral oil distillates or natural gas con-
centrates of "wet" natural gas (ethane and higher hydrocarbons).
The technical cracking process must be followed by extensive cleaning
steps and gas separating processes, in order to obtain ethylene of
the necessary purity for further processing. Less costly processing
steps are needed if ethylene is produced from ethane, but the availa-
bility of ethane is limited.
Methane on the other hand, is a raw material readily
available in natural deposits. Thus natural gas is more than 90%
methane. An economical method for producing ethylene from methane
by oxidative coupling is therefore a matter of interest.
Many reports have been written upon the production of
lower olefins from methane. Thus in Hydrocarbon Proc., Nov. 1982,
page 117, T. Inui ct al describe a method whereby a mixture of lower
olefins is obtained in three reaction stages by means of synthetic
gas and methanol. ln Hydrocarbon Proc., May 1983, page 88, Y. C. Hu
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describes a two stage method in which the synthetic gas formed in
the first stage is converted, by Fischer-Tropsch reaction, into
a mixture of olefins, paraffins and carbon dioxide. The main disad-
vantages of these methods are that it is necessary to operate under
pressure, coke ormation is observed, and multi-stage syntheses are
necessary.
However, methane has also already been converted, in
the presence of oxygen, at atmospheric pressure, in a one-step
reaction. Thus G. E. Keller and M. M. Bahsin report, in J. Catal.
73, 1982, pages 9-19, a method in which conversion in the presence
of numerous metal oxides as catalysts, at 500 to 1000C was in-
vestigated, the reaction products obtained being a mixture of
ethylene and ethane, in addition to carbon monoxide and carbon
dioxide. The disadvantage of this method, however, is very low
C2-hydrocarbon selectivity. These authors suggest a way of improving
selectivity, but it involves very costly equipment and a very complex
process.
With reference to the disadvantages of this prior
publication, Federal Republic of Germany OS 32 37 079 suggests a
method which, without any costly and complex processing, produces
comparable, and in some cases even better, selectivity and higher
space time yields of C2-hydrocarbons (cf. German OS 32 37 079,
final paragraph on page 2 to paragraph 3 on page 3). This
publication describes a method for producing ethane and/or
ethylene in which methane and oxygen are reacted in the presence of
~%~d~
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a catalyst treated in a fluidized bed or arranged stationarily
in a reactor, at temperatures of between 500 and 900 C, in a
specific range of the partial pressure relationships of methane
and oxygen. The catalyst is to be an oxide of multivalent metals
(cf. German OS 32 37 079, final paragraph on page 3). Preferred
catalysts indicated are products with oxides of lead, antimony,
tin, bismuth, cadmium, thallium and indium, or mixtures thereof,
as active components . However, special preference is given to
lead oxide or a mixture thereof with antimony oxide. The metal
oxides may be used as such or finely divided over the surface of
a carrier material, for example aluminum oxide or silicon dioxide
(cf. claim 6 and paragraph 4 on page 4). However, as shown by the
results of the examples of the embodiment in Table 1, even with
the especially preferred lead oxide, maximal selectivity obtained
for hydrocarbons is only about 52.9% (methane-conversion 7%).
W. Hinson, W. By~n and M. Baerns reported, at the 8th International
Catalysis Congress 1984 in Berlin, upon far reaching endeavours to
increase selectivity. According to Vo. III, pages 581-592 of the
congress book "8th International Congress on Catalysis", Verlag
Chemie, Weinheim 1984, the hydrocarbon selectivity of the lead oxide
catalyst can be increased by appropriate choice of carrier materials
and by the addition of alkali. Carrier materials investigated were:
a-aluminum oxide, ~'-aluminum oxide, titanium dioxide, aluminum
silicate and silica gel; of these, the most suitable was found to
be ~-aluminum oxide with a maximal selectivity of 57.7% (methane-
`~ ~;2'7~
conversion 7.1%). However, if oxidative coupling o methane is
to be used commercially, selectivity and methane conversion will
have to be increased substantially.
It is therefore the purpose of the present invention to
convert methane, by the oxidative coupling method kno~n in principle,
in a catalytic reaction, with increased selectivity and satisfactory
methane conversion, more particularly into C2-hydrocarbons.
In accordance with the invention there is provided a
method for producing ethane and ethylene by oxidizing methane with
oxygen, or an oxygen-containing gas, in the presence of a metal
compound as a ca*alyst, at a reaction temperature of 600 to 1000 C,
the method employing a catalyst selected from chlorides, bromides
and iodides of elements selected from the group consisting of alkali
and alkaline earth elements, elements of the Secondary Group IIIb
of the Periodic System, elements having the atomic number 24 to 30,
lantha~ide elements, silv~r, cadmium, lead ~ bismu~h,.in~ividually
or mixed, the atomic weight of the alkali, alkaline earth and IIIb
Secondary Group elements being not greater than 139.
It was surprising to find that with catalysts from the
2Q metsl halide group, alone or mixed, substantially higher selectivity
and methane conversion are obtained, as compared with those appearing
in prior publications.
Generally speaking, improved selectivity is obtained
even with one of the metal chlorides, bromides or iodides alone,
~1
~ 7~
but in these cases the conversion is much reduced. In order to
achieve satis~actory conversionsJ the halide catalyst is preferably
molten at the operating tempera~ure. Por example, if calcium chloride
is present in the solid state at an operating temperature of 750 C,
increasing the temperature to 800C allows the calcium chloride
to melt and the methane conversion to rise from 5.8 to 14.2%
(catalyst: 0.18 mol CaC12 per 20 g of pumice). It is therefore
desirable to choose an operating temperature slightly above the
melting point of the catalysts. In order to avoid unduly high
operating temperatures, the relevant halogenide catalyst is suitably
used in combination with at least one secondary component, in
order to lower the melting point. The secondary component is
usually also a halogenide of an alkali or alkaline earth metal,
or of an element of Secondary Group IIIb of the Periodic System,
or an element having the atomic number 24 to 40, or a lanthanide
element, silver, cadmium, lead or bismuth.
It is to be understood that the chlorides, bromides
and iodides referred to herein include the oxychlorides, oxybromides
and oxyiodides of the elements mentioned which are capable of forming
such oxyhalides, for example bismuth.
Preferred chlorides, bromides and iodides of elements
having the atomic number 24 to 30, for use as catalysts, are those
of the elements chromium, manganese, iron, copper and zinc.
It has also been found advantageous to use as catalysts
~ "
,, .~.~
--6--
h~log~lide~ ~L ~lth~nide el~nent m~xtL~ , for ex~ le t~ e c~n-
tained in natural rare earth minerals. Among naturally occurring
rare earth minerals, cerite earths or yttrium earth, for example
monazite, are suitable for producing catalysts according to the
invention. Cerite earths consists of mixtures of oxides Df lanthanum,
cerium, praseodymium, neodymium, samarium and europium, whereas
yttrium earths consist of mixtures of oxides of gadolinium, terbium,
dysprosium, holmium, erbium, ytterbium, thulium, lutetium, ytterbium
and thorium, the actual composition varying according to the mineral.
Compositions of various cerite earth and yttrium earth minerals,
especially monazite, appear in Ullmanns Encyclopedia of Technical
Chemistry, Verlag Chemie, Weinheim 1982, Vol. 21, page 238, Table 4.
Generally speaking a satisfactory halogenide catalyst
combma~io~ is obtained by choosing, as the primary component,
an inexpensive, usually high melting-point alkali or alkaline earth
halogenide or mixtures thereof and, as the secondary component, a
halogenide of a heavy metal, or mixtures thereof. Heavy metal
halides, includin~ lanthanide halides, fr~4~1ently act ~s promotors.
to increase the yield of ethylene.
The method according to the invention may be carried out
at temperatures of between 600 to 1000C, the preferred range being
between 650 and 850 C.
Chloride, bromide and iodide catalysts are preferably used
upon a carrier which suitably may consist of pumice, silica gel,
~:7~
kieselguhr, precipitated silicic acid and/or oxides of alkaline
earth elements and/or aluminum oxide, silicon dioxide, zinc oxide,
titanium dioxide, zirconium dioxide and/or silicon carbide. Of
the alkaline earth element oxides, preferance is given to magnesium
and calcium oxide.
The catalysts are preferably used in amounts of 0.004
to 0.~00 mols of catalyst per 100 ml of carrier. Particularly
advantageous is 0.012 to 0.190 mol of catalyst per 100 ml of carrier.
All of the catalysts according to the invention may be
used either in a fluidized bed or in a static bed. Bubbles of
reaction gas may also be passed through the molten halogenides.
The catalysts according to the invention may be produced
as follows:
If the halogenide catalysts are to be used upon carriers,
the active components are suitably dissolved in water or in some
other suitable solvent, the carrier material being suspended in the
resulting solution in the form of pellets.
The solution containing the suspended carrier is concen-
trated in a vacuum until dry and in particular is dried in a vacuum
at about 100 C.
If it is to be used without a carrier, the catalyst may
be used as such, i.c. with no further additives. However, it is
also possible to usc a bonding agent with it. Production of a
catalyst with a bonding agent is described hereinafter:
~..27~
The relevant components are reduced to a paste and
thoroughly kneaded with sodium silicate in a mortar. A certain
amount of concentrated ammonia solution is kneaded in, in order
to initiate polymerization of the sodium silicate to form a
bonding agent. The mass solidifies slowly. A metal plate is
coated with this mass to a height of about 5mm. After this
coating has been dried in a vaccum at about 80 C, the resulting
brittle halogenide catalyst particles are ground to the desired
grain size.
The experiments described hereinafter were carried out
in a quartz tube having an inside diameter of 11.3 mm. After the
catalyst has been introduced into this tube, it is brought to the
reaction temperature in a flow of nitrogen. The tube is heated
to a length of 0.5 m by means of a radiant tube furnace. Methane
and oxygen are introduced into this reactor through a flowmeter
and are passed over the catalyst. The emerging reaction mixture
is held at a temperature in excess of 80C and is passed directly
to the gas chromatograph for determination. This mixture contains,
as products resulting from the oxidizing process, carbon monoxides~
carbon dioxide, water, ethylene, ethane and small amounts of higher
hydrocarbons, especially C3- and C4- hydrocarbons. No soot formation
is observed. Hydrogen halide and halogenide may also be detected
in the emerging mixture of gas, but only in very small quantities
which have practically no ef~ect upon further processing of the
~2~
g
product s .
Associated with this is therefore a certain amount of
deactivation of the catalyst, but this may easily be compensated
for by metering hydrogen halide into the feedline or by constant
regeneration of the catalyst (fluidized bed). If this is necessary,
it is preferable to pass the hydrogen halide, corresponding to the
halogenide catalyst used, simultaneously with the methane, the
oxygen, or the oxygen containing gas, continuously or inter-
mittently over the catalyst. An excess of hydrogen halide does
no harm. It is even unnecessary to separate this excess from the
emerging mixture of gas if the ethylene obtained is further
processed by the usual method in which hydrogen halide must be
added. In this case it is desirable to add the hydrogen halide
in such a manner as to produce, in the resulting mixture of gas,
the proportions of hydrogen halide and ethylene required for the
usual method.
Such a method of operation is particularly suitable for
subsequent oxychlorinating of ethylene with hydrogen chloride if
the preceding oxidative coupling of methane with a chloride catalyst
is carried out.
On the other hand, if a hydrogen halide separating
operation is necessary, the hydrogen halide occurs in the form of
highly diluted hydrohalic acid upon condensation of product water.
In the event of subsequent-vinyl chloride production, the ll~ghly
~1 ~ 7~
-10-
diluted hydrochloric acid then available could be combined with
the product water condensate arising after oxychlorination and
the unreacted hydrogen chloride.
The individual method conditions chosen for numerous
experiments, and the results, appear in Tables 1 - 2.
The ratio of methane and oxygen in the mixture of gas
fed in has a substantial effect upon selectivity and conversion,
as shown in Table l:
Table 1: 30 ml of a catalyst consisting of 0.06 mol of calcium
chloride per 20 g of pumice, 750C.
Feed (l/h) Selectivity (%)
Methane Oxygen C114-Conversion 2 C2H4 C2H6 C3~8 C4Hlo LKW
_
go 10 9.4 9.7 68.6 15.4 3.5 2.8 g0.3
15.5 15.7 69.5 10.5-- 2.1 2.6 84.7
66 3~ 19.9 26.1 60.4 9.3 1.3 2.9 73.9
Thus by lowering the proportion of oxygen, selectivity is
improved, whereas by raising the proportion of oxygen, methane con-
version is increased.
~ 27~
Another way of still further improving selectivity,
and at the same time increasing methane conversion, is a method
involving feeding in the oxygen laterally. In this case, methane
only is fed in at one end of the reactor tube, whereas the total
amount of oxygen is introduced into the tube through a plurality of
laterally arranged inle~ apertures.
The results of experiments with numerous halogenide
catalysts appear in Table 2.
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~7~
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-14-
lt can easily be gathered from the Table which condi-
tions are particularly suitable for specific objectives regarding
end product composition. Thus in a few cases it is possible, by
a suitable choice of catalystsJ to reduce the proportion of
hydrocarbons with more than two carbon atoms, if this be desired.
In other cases, the desired ethylene : ethane ratio can be adjusted
by appropriate conditions (temperature, catalyst). ~enerally
speaking, the highest possible proportion of ethylene is sought.
Above all it may be gathered from the Table that, in
certain cases it is possible to obtain particularly high hydrocarbon
selectivity, in excess of 90%, according to the invention, sometimes
even with very high activities (maximal methane conversion 19.0%).
The method according to the invention is thus superior
to that described hereinbefore and is suitable for economical pro-
duction of ethane and ethylene by oxidative coupling of methane.
To this end, it is desirable to operate the process with
as small a proportion of oxygen in the reactor, as is possible, for
example, with a fluidized bed reactor, a loop reactor, lateral feed
or other means of carrying out concentration.
ln selecting suitable equipment for this method, it
should be borne in mind that the reaction mixture contains corrosive
components in the ~orm of halogen an~ hydrogen halide.
In th~` case of technical processes, the reaction mixture
may be separ;lted b! me;ms of conventional refinery gas technology or,
~76~3~;
-15-
if necessary, it may be converted, without separation, for example
into halogenated products, for example vinyl chloride.
