Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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PROCESS ~OR THE PRODUCTION OF METHYL TERT.-BUTYL ETHER.
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The invention relates to a process for the
production of methyl tert.-butyl ether from a mixture of
light hydrocarbons which contain butane or butanes.
In the petroleum-exporting countries,
considerable quantities of gaseous paraffin
hyodrocarbons are formed during the refining of the
crude oil by distillation. Frequently, no possibility
exists for the exploitation of these products on the
spot. Since the transportation of butane to the
consuming countries is costly and has so far not been
done on a relatively large scale, at the present time
considerable quantities of butane are burnt off.
It is known from German Offenlegungsschrift
2620011 to process a stream of n-butane formed in the
- petroleum refinery into methyl tert.butyl ether. In
accordance therewith, the n-butane is partially
isomerised to form isobutane, the n-butane/isobutane
20- mixture is partially dehydrogenated, and n-butenes are
also formed as well as isobutene. The dehydrogenation
product, still containing n-butane, is then etherified
with excess alcohol, more especially with methanol, the
isobutene formed in the dehydrogenation stage being
converted to methyl tert.-butyl ether. The excess
methanol is extracted with water from the product
mixture and the remaining C4 hydrocarbons are
separated from the ether by distillation and returned
into the distillation stage. Due to the presence of
n-butane, which is necessary with this process, it is
necessary to have correspondingly relatively large
dimensions of the installation Eor a given production
output. Moreover, due to the presence of n-butane,
butadiene is also formed in the dehydrogenation stage,
more especially with return of C4 hydrocarbons which
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contain n-butene. This is undesirable, since butadiene
has a tendency to form a resin. Finally, the separation
of methanol from the etherified mixture by extraction
with water is also disadvantageous, because thereafter
both a methanol/water separation and also drying of the
ether are necessary.
The present invention seeks to convert the
butane fraction of the light hydrocarbons being
liberated with the refining of petroleum oil into methyl
tert.-butyl ether and in this way to transform this
fraction, which in many cases can only be exploited with
difficulty, into a high-quality benzine additive, the
transporting of which to the petroleum-importing
countries is substantially more economic than the
transporting of butane. More particularly, the
conversion of the n-butane into the high-octane methyl
tert.-butyl ether is to be carried out substantially
completely and the formation of butadiene is to be
largely avoided.
The invention is accordingly based on a process
for the production of methyl tert.-butyl ether from a
mixture of light hydrocarbons which contain butane or
butanes, by separation of the butane fraction from the
mixture, isomerisation of n-butane with formation of
isobutane, dehydrogenation of the isobutane down to an
isobutene/isobutane ratio in the ranye of from 0.4 to
2:1; advantageously l.0 to 1.5:1, etherification of the
isobutane contained in the dehydrogenation mixture with
methanol, with formation of methyl tert.-butyl ether,
separation of the isobutane from the etherification
mixture and return of the isobutane into the
dehydrogenation stage.
According to the invention, this process is
characterised in that the butane fraction and the isomer
obtained with the isomerisation of the n-butane are
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separated by distillation into an n-butane fraction and
an isobutane fraction, the n-butane fraction is supplied
to the isomerisation stage, the isobutane fraction,
including the returned isobutane with a content of 70 to
100~ by weight, advantageously 90 to 99% by weight of
isobutane, is dehydrogenated, a mixture consisting
substantially of isobutene and isobutane is recovered
from the dehydrogenation mixture by absorption and
desorption and, after the etherification, first of all
the isobutane and thereafter possibly the unreacted
methanol are separated from the etherification product
by distillation.
In the case where the gases are liberated with
the working up of a crude petroleum by distillation, the
butane-containing mixture of light hydrocarbons
consists, for example, of 5~ by volume methane, 10% by
- volume ethane, 30% by volume propane, 35% by volume
butanes and 20% by volume pentanes and higher
hydrocarbons. The butane fraction can be separated by
intense cooling or other known measures from the crude
oil distillation waste gas. Obviously, the butane
fraction used in the process according to the invention
can also contain butenes, if the basic hydrocarbon
mixture comprises cracking gases or other
olefin-containing refinery gases.
The isomerisation of the n-butane is expediently
carried out on a platinum-containing solid bed catalyst
in the presence of hydrogen in a temperature range from
150 to 205C, advantageously 150 to 180C. The
operation takes place at super-atmospheric pressure,
advantageously in the pressure range from 14 to 28 bar.
The isomerisation product which is obtained consists,
for the major part, advantageously of 55 to 60% of
isobutane and for the remainder substantially of
n-butane. The isomerisation conditions, i.e.
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temperature, space velocity and pressure, are so
adjusted that an extensive approximation to the
isomerisation equilibrium is provided. The product
consisting of isobutane and n-butane is then rectified
together with the butane fraction used in the process,
which fraction may likewise be a mixture of n-butane and
isobutane and possibly butenes. The separation of these
two streams or flows into an isobutane stream and an
n-butane stream is effected, for example, in a column
having 50 to 100 plates at pressures in the range from 8
to 14 bar, advantageously 9 to 12 bar, and temperatures
in the range from 60 to 90C, advantageously 70 to 80C.
While the n-butane fraction is supplied to the
isomerisation stage, the isobutane fraction passing out
at the top i-s combined with the isobutane separated from
the etherification product. The combined isobutane
stream has an isobutane content of at least 70 and
advantageously at least 90~ by weight, the remainder
consisting essentially of n-butane and possibly butenes.
In general, the isobutane content of the stream supplied
to the dehydrogenation stage is higher than 95% by
weight.
The dehydrogenation of the isobutane can be
effected by strictly thermal of catalytic procedures.
Dehydrogenation is advantageously effected catalytically
by a solid bed process in several reactors, which are
alternatively charged with the isobutane stream. The
dehydrogenation temperature is in the range from 538 to
649C and the pressure is preferably in the range from
0.2 to 0 9 bar absolute. The dehydrogenation
catalyst generally consists of active aluminium oxide,
which is impregnated with 18 to 20~ chromium oxide and
has the form of cylindrical pellets. The heat of
reaction is mainly supplied by the heat of combustion
of a small quantity of coke, which is deposited during
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the reaction period on the catalyst. This coke is burnt
in the regeneration periods as a result of which a rise
in temperature is prod~ced in the reactor. Furthermore,
the stream of isobutane is preheated before being
introduced into the reactor, so that it vaporises
spontaneously with the expansion to the reaction
pressure. With the process according to the invention
the isobutane, depending on the operating conditions in
the reactor, can be more or less largely dehydrogenated
to isobutene. The isobutene/isobutane ratio in the
exhaust stream of the reactor is advantageously in the
range from l.0 to 1.5. The proportion of the secondary
products, as for example propane, is small. The exhaust
flow from the dehydrogenation reactor is generally
cooled by contact with cold oil, compressed to a
pressure in the range from 8 to 15 bar with intermediate
cooling, in which case a part of the C4 fractions
already condenses. The uncondensed gases are absorbed
with absorption oil for the purpose of recovering
isobutane/isobutene. The residual gases can be used in
the production of the methanol required for the
following etherification stage. After desorption of the
C4 hydrocarbons from the absorption oil and being
united with the condensed C4 hydrocarbons from the
compression stage, propane can be separated out in a
stabilisation column. The C4 hydrocarbons as thus
obtained consist essentially only of isobutane and
isobutene. They are more especially free from butenes
and butadiene.
The isobutane/isobutene mixture is then
catalytically etherified with methanol, the isobutene
being almost quantitatively converted to methyl ~`
tert.-butyl ether, while the isobutane experiences no
conversion and leaves the reactor unchanged. Generally
serving as catalyst are sulphonated ion exchanger resins
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arranged as a solid bed, such as those which have
frequently been described for this reaction, for
example, in the Offenlegungsschrift 26 20 011 already
mentioned. Generally, the etherification takes place in
the liquid phase at temperatures in the range from 30 to
100C, advantageously 60 to 90~C and pressures from 2 to
24 and advantageously 10 to 22 bar. The pressure only
serves the purpose of keeping the components in the
reaction mixture in liquid phase during the reaction.
The molar ratio between methanol and isobutene is
generally in the range from 1 to 2:1, advantageously in
the range from 1.1 to 1.4:1.
The exhaust flow from the etherification reactor
consists essentially of methyl tert.-butyl ether,
isobutane and excess methanol. It is an essential
advantage of the process according to the invention that
the isobutane, and also the methanol, are capable of
being easily separated from the etherification mixture
and used again in the process. For this purpose, the -~
product mixture discharging ~rom the etherification
reactor is supplied to a pressure distillation column,
in which the isobutane is distilled off at the top,
while the methanol/ether mixture is extracted from the
sump. The isobutane is combined with the top or head
product coming from the separation column of the
isomerisation stage and supplied again to the
dehydrogenation stage. The ether/methanol mixture is `
distilled in a second pressure distillation column, an
ether/methanol azeotrope with a composition dependent on
the distillation prèssure passing out at the top and
pure methyl tert.-butyl ether remaining in the sump.
The azeotrope can be returned into the etherification
stage. With this separation of the methanol from the
etherification product, it is not essential to distil
down to pure methyl tert.-butyl ether. If desired, a
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small proportion, for example, up to 10~ by volume, of
methanol can remain in the ether. The operational costs
or the return of the azeotrope are thereby reduced. If
the isobutene has been etherified only with a small
methanol excess, the methanol separation may be
completely omitted.
The dehydrogenation is advantageously operated
with a charge having s~ch a composition and under such
reaction conditions that the butadiene content of the
dehydrogenation mixture remains below 0.5% by weight.
Due to the fact that the charge in the dehydrogenation
stage is substantially free from n-butane and n-butenes,
the formation of butadiene remains small. A selective
hydrogenation of the dehydrogenation mixture for the
purpose of removing butadiene is consequently
unnecessary.
According to the preferred embodiment of the
process according to the invention, the
isobutene/isobutane mixture is etherified with methanol,
which has been produced by reforming light hydrocarbons
with steam and catalytic synthesis under a pressure in
the range from 40 to 100 bar. Consequently, the same
mixture of light hydrocarbons, from which the isobutene/
isobutane mixture for the etherification has been
obtained, is used as starting material for the methanol
synthesis. The same mixture is expediently used for the
methanol synthesis, after the butane fraction for the
production of the isobutene,/isobutane mixture has been
separated out.
Hydrogen is expediently separated from the
product gas of the dehydrogenation and/or the process
gas and/or the exhaust gas of the methanol synthesis and
this hydrogen is used with the isomerisation and
possibly other hydrogen-consuming processes. The ~-
separation of the hydrogen from the accompanying gases,
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more especially the light hydrocarbons, can for examp]e
be effected by adsorption or by low-temperature cooling
and condensation of the accompanying gases. The
hydrogen which is thus available can be used more
especially for the hydro-desulphurising of condensed
hydrocarbon fractions.
The invention is also directed to an additive
for improving the octane n~mber and the volatility
properties (front end volatility) of benzines, which is
characterised by a content of 60 to 99.5% by weight,
advantageously 97 to 99% by weight, of methyl
tert.-butyl ether.
The invention is hereinafter more fully
described by way of example by reference to the drawing,
in which is represented the flow diagram of an
installation by which the process of the invention may
be carried into effect.
A mixture of light hydrocarbons, suitably a gas
mixture which forms on separation by distillation from
crude oil, is supplied through a pipe 1 to a rectifier
column 2~ The mixture is separated by distillation into
a Cl_3 stream and a C4+ stream. The
Cl_3 stream leaving at the top serves as initial
material for the methanol synthesis and is supplied by
way of pipe 3 to the reforming stage of a methanol plant
27. The bottom fraction, consisting of C~ and heavier
hydrocarbons, passes through a pipe 4 to a column 5, in
which the C4 fraction is distilled off at the top and
Cs and higher hydrocarbons are removed as bottom
product through a pipe 6. The C4 fraction, which
generally consists of butane, passes through a pipe 7 to
the de-isobutanising column 8, in which is effected a
separation of the total charge into isobutane and
n-butane. The n-butane is extracted as bottom product
through a pipe 9 and supplied to a catalytic
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isomerisation reactor 10, in which the n-butane is
partially changed to isobutane. The mixture of
isobutane and n-butane is removed from the reactor 10 by
way of pipe 11 and, a~ter separation of lighter
hydrocarbons in a column (not shown), is again supplied
to the de-isobutanisation column 8.
The high-percentage isobutane is heated in the
heat exchanger 13 and, after expansion and being
combined with isobutane returned by way of pipe 35, is
supplied to one of the catalytic hydrogenation reactors
14a or 14b. The two reactors 14a, 14b are alternately
charged with the stream of isobutane, the reactor
switched off at any time being regenerated with hot air.
The exhaust gas of the dehydrogenation reactor consists
essentially of an isobutene/isobutane mixture and passes
by way of pipe 15 to a quenching toT~er 16, in which it
is quenched by direct contact with cold oil. The
mixture then flows through a pipe 17 to a multi-stage
compressor 18 with intermediate cooling, by which the - `
pressure is for example raised to 10 bar. The gas
mixture then passes through a pipe 19 into an absorption
column 20, in which isobutene and isobutane are washed
out of the gas stream with absorption oil. Hydrogen,
and light hydrocarbons formed as secondary product of
the dehydrogenation, remain in the gas phase and leave
the installation through pipe 21. The hydrogen can be
recovered from this secondary product and can, for
example, be used in connection with the isomerisation
and/or with the hydro-desulphurisation of condensed
30 hydrocarbons. The cold absorption oil, charged with `
C4 hydrocarbons, passes through a pipe 22 into the
desorbing unit 24, in which the C4 hydrocarbons are
driven off by heating the absorption solution. The
regenerated absorption oil flows back by way of pipe 23 `
to the absorber 20. The gas mixture, consisting
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essentially of isobutene and isobutane, leaves the
desorbing unit by way of pipe 25.
The isobutene/isobutane mixture as thus obtained
is fed, together with methanol introduced by way of a
pipe 28 from the methanol plant 27 and me~hanol/ether
mixture returned by way of pipe 33, and after preheating
(not shown) into the catalytic etherifying reactor 26.
The reactor 26 contains a solid bed catalyst and is
provided with an internal cooling system (not shown).
In the reactor, the isobutene introduced by way of pipe
25 is reacted with methanol to form methyl tert.-butyl
ether. A mixture consisting substantially of methyl
tert.-butyl ether, isobutane and excess methanol leaves
the reactor 26 and is supplied through a pipe 29 to a
first pressure column 30. In the column 30, the
isobutane is distilled at the top and is combined by way
of a pipe 35 with the charging flow for the
dehydrogenation reactors 14a, 14b. The sump product of
the column 30 is a mixture of methyl tert.-butyl ether
and methanol and is supplied by way of pipe 31 to a
second pressure column 32, in which an azeotrope
consisting of methanol and methyl tert.-butyl ether is
distilled at the top end, this mixture being returned
by way of pipe 33 into the etherification reactor 26.
The methyl tert.-butyl ether is extracted as product
f rom the sump of the column 32 at 34.
Example
A desulphurised stream of liquid gas, resulting
from the processing of petroleum or from a petroleum gas
reservoir, is processed in an installation corresponding
substantially to the installation which is shown in the
Figure, the said stream consisting substantially of 14
parts by weight of Cl-C3 hydrocarbons, 44 parts by
weight of C4 hydrocarbons (essentially n-butane and
35 isobutane; 60-99~ nC4 and 1 40% iC4 ) and 22 parts by
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weight of Cs aQd higher hydrocarbons. In the
distillation column 1, the liquid stream is separated
into 15 parts by weight of Cl-C3 hydrocarbons with a
low butane content and into C4 and higher
hydrocarbons. The Cl-C3 hydrocarbons are converted
by reforming with steam and low-pressure synthesis t~ 18
parts by weight of methanol. The C4~ hydrocarbons
are split up in column 5 into 22 parts by weight of
Cs+ hydrocarbons as bottom product, which can for
example be used, inter alia, for admixture with benzine,
production of petrochemicals, and 43 parts by weight of
top product, consisting essentially of n-butane and some
isobutane. The top or head product is supplied to a
de-isobutanisation column, which is combined with a
catalytic isomerisation plant for isomerising n-butane
to isobutane. The isomerisation reactor is operated at
170C~ a pressure of 20 bar, an hourly liquid space
velocity of 4 h~l and with a molar ratio between
hydrogen and charging liquid of 0.3:1. Supplied to the
isomerisation reactor i5 1 part by weight of gas with
80% hydrogen (secondary product of the dehydrogenation
or methanol synthesis), together with return hydrogen.
For the purpose of removing propane and lighter gases,
the isomer is stabilised and is then split up, as a
result of which there are obtained 42 parts by weight of
98 to 99% isobutane and also an n-butane fraction with
small quantities of Cs+ hydrocarbons.
This isobutane product is supplied to the
dehydrogenation reactors i4, possibly together with 28
parts by weight oE returned raffinate from the
etherification stage, after it has been heated in the
preheater 13. The dehydrogenation takes place in a
cyclic, adiabatic, catalytic solid bed process with
three or more reactors. The heat of reaction necessary
for the 10-minute dehydrogenation periods is
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substantially equal to the heat of combustion of the
coke deposited on the catalyst, which heat is generated
during the catalyst regeneration with pre-heated air.
The entire period for reaction, flushing and
regeneration is 20 minutes. The catalyst is chromium
oxide in an amount of 18 to 20~ on aluminium oxide with
admixed inert particles of high heat capacity, so as to
achieve the necessary heat capacity of the bed. The
dehydrogenation takes place at 540C, a pressure of 300
mmHg absolute and an hourly space velocity of the liquid
of 2.5. The yield of isobutene from isobutane amounts
to 75%. The dehydrogenation product from the reactors
is quenched by direct contact with circulating oil and
is compressed in the compressors to 18 to 12 bar. The
total dehydrogenation product, in an amount of 70 parts
by weight, is then split up by absorption and desorption
and the liquid as thereby recovered is stabilised for
the removal of propane and lighter gases. There are
obtained 59 parts by weight of a mixture which consists
of at least 97% of isobutene and isobutane in the molar
ratio of 1.15, and also a small quantity of n-butane,
other butenes and traces of higher hydrocarbons. In
addition, there are formed 9 parts by weight of p~ropane
and lighter gases, including hydrogen. 'rhe Cs+
hydrocarbons and losses amount to 2 parts by weight.
Eighteen parts by weight of methanol are mixed with 59
parts by weight of isobutene/isobutane and possibly with
unreacted methanol from column 32 and supplied to the
etherification reactor 26. The etherification occurs at
about 60C in the liquid phase on a sulphonated r
strongly acid, macroporous, organic ion exchanger resin
with a molar ratio between methanol and isobutene in the
range from 1:1 to 2:1, advantageously 1.2:1 to 1.5:1.
The etherified mixture is split up in the de-butanising
35 column 30 into 28 parts by weight of isobutane, with a ~
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small content of other unreacted hydrocarbons, as top
product, and into a bottom product, which is separated
by distillation in the column 32 into 49 parts by weight
of methyl tert.-butyl ether and unreacted methanol. The
methyl tert.-butyl ether product contains about 98~ of
methyl tert.-butyl ether, 1.0% of methanol and 1.0% of
other compounds.
The methyl tert.-butyl ether product as produced
has the following mixing or blending properties: 110 to
135 RON and 98 to 110 MON, depending on the nature of
the benzine components.
The product mixture of methanol, methyl
tert.-butyl ether and unreacted hydrocarbon can also be
directly admixed with a large quantity o~ benzine,
without hydrocarbons and methanol having to be separated
out beforehand. The concentration of the methyl
tert.-butyl ether in the mixture is then in the region
of 60%. This methyl tert.-butyl ether product o~ low
concentration is added to the benzine in the amount as
permitted by the volatility properties and the required
behaviour of the benzine.
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