Note: Descriptions are shown in the official language in which they were submitted.
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Title: "Process for the production of methanol~'.
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DESCRIPTION
The present invention concerns a process to produce
methanol from synthesis gas, as well as a device for
carrying out the process.
Originally, t-he technical synthesis of methanol as typical
reaction of gaseous reagents was realized from a synthesis
gas obtained by vapor reforming of natural gas or naphtha,
having as principal components CO and H2. At the basis of
this synthesis we have the following equilibrium:
CO+2H2 c----~ CH30H.
In the so-called low pressure process this classic
synthesis is obtained technically in a pressure reactor
through a very active Cu/Zn catalyst at temperatures
between 230-280 C and pressures of about 50-100 bar. At
the indicated reaction conditions, for instance at 260 C
and 50 bar, the theoretically maximal conversion, that is
the conversion related to the carbon charge, is of 45%. If
the synthesis pressure is raised at for instance 100 bar,
the conversion can be increased up to 65%. The pressure
rise is however connected with higher costs due to the gas
compression and the covering of the plant parts which are
under pressure, and therefore in practice it is not
preferred.
Instead of this, in order to exploit in the best way the
employed materials, a multiple recirculation of the
unreacted synthesis gas is carried out. The enriched gas
coming from the reactor is cooled at about 30 C and
methanol is condensed from the unreacted synthesis gas. The
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unreacted reagents are again compressed and heated up and
recirculated in the reactor for the successive conversion.
In practice, by bringing back the unreacted synthesis gas,
with a recirculated gas ratio of 5 and a recirculation rate
of normally 4-6, the overall conversion is increased to
about 88~. In this connection we refer to E. Supp, Chem.
Technol. 3 (1973) Nr 7., pages 430-435.
The methanol synthesis from carbon dioxide and hydrogen is
becoming more and more important. At the basis of this
synthesis there is the following equilibrium:
C2 + 3H2 ~ -> CH30H + H20
This reaction indicates that besides methanol there appears
also water. In comparison with the methanol synthesis from
carbon monoxide and hydrogen the conversion from C02 and H2
is essentially lower. For instancé, at 250 C and 50 bar
the theoretically maximal conversion is of about 16%. That
means that, after a passage through the reactor, in the
best case there is reacted 16% of-the gas mixture (1 part
of C2 and 3 parts of H2). In order to obtain an optimal
utilization of the gas it is necessary also in the present
case of a multiple recirculation, wherein methanol and
water must be separated from the enriched gas.
Both for the classic methanol synthesis first mentioned, as
well as for the above mentioned methanol synthesis, it is
possible to substantially save in the investment costs as
well as in energy and raw materials, only if the conversion
pro passage is modified such as to avoid a recycling of the
unreacted synthesis gas. This is generally valid for all
reactions where gaseous reagents are reacted in products in
vapor phase, as it happens in particular in the methanol
synthesis.
In this connec~ion, K. R. Westerterp, M. Kuczynski, T. N.
.
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Bodewes and M. S. A. Vrijland propose in an article "Neue
Konvertersysteme fur die Methanol-Synthese", Chem.-Ing.-
Tech. 61 (1989) Nr. 3, at the pages 193-199, to separate
continuously from the reaction mixture the products which
appear during the synthesis. In principle, it is well known
that the conversion pro passage, at the same temperature
and pressure, can be increased, if it is possible to
separate simultaneously and continuously the products
during the synthesis. To this purpose it is proposed a
reactor concept, in which the in-situ separation of
methanol can be carried out (from carbon monoxide and
hydrogen) by means of adsorption, that is the
gas/solid/solid fluidized reactor, where moreover it is
employed a solid adsorbent which runs downwards in the
reactor. In this way it is possible to obtain conversions
up to 100~ with a quite simple process and method, and the
recirculation is no more necessary. Nevertheless this
process is very expensive from the technical point of view.
The problem underlying the present invention relates
therefore to a process for the methanol synthesis, wherein
during the synthesis the conversion can be increased by
simultaneous removal of one or several products. According
to the invention this problem is solved by a process
according to claim 1.
Generally, the invention concerns a process for the
conversion of essentially gaseous raw materials which are
transformed in at least one product in vapor phase,
wherein, in order to increase the reaction yield, the
reaction equilibrium is shifted in the direction of the
product(s), in which at least one product is removed from
the reaction mixture by membrane permeation.
Preferably, the removal of at least one product is carri^d
out by means of the so-called vapor permeation.
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In particular, it is proposed a process for the production
of methanol from synthesis gas, wherein, in order to
increase the yield, the reaction equilibrium is shifted in
the direction of methanol as a consequence of the removal
of methanol and/or of another product of the reaction from
the reaction mixture by membrane permeationand in
particular vapor permeation.
As membranes there are indicated thin layers which may have
very different structures, but which have all in common the
characteristic of opposing a different resistance to the
passage of different materials. It is well-known how to
separate using membranes single components from flowing
mixtures. In this connection we refer to the article of Y.
Cen, K. Meckl and R. N. Lichtenthaler with the title
"Nichtporose Membranen und ihre Anwendungen'l, Chem.-Ing.-
Tech. 65 (1993) Nr. 8, at the pages 901-913.
The separation of materials by means for instance of non
porous membranes is based on the difference in the
solubility and in the diffusion speed of different mixture
components in the membrane material. Here the transport of
material occurs basically in three successive steps:
1. sorption of the components from the feed mixture and
from the reaction mixture, respectively,
2. diffusion of the adsorbed components through the
selective membrane, and
3. desorption of the components in the permeate phase.
It is well-known that vapors have a much higher
permeability than gases in suitable polymeric membranes.
The essential here is that the mixture to be prepared and
to be classified may contain, besides gases, also
components which are condensable at the standard conditions
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(1 bar and 0 C). The driving force for the transport of
materials through the so-called non porous membrane is
given by the difference of the chemical potential of the
components to be permeated, between the feed side and the
permeate side. By the vapor permeation this difference is
due to the fact that the partial pressures of the single
components on the feed side are much higher than those on
the permeate side. In consideration of the above mentioned
observations it is proposed that, during the synthesis of
methanol according to the following reaction equation:
XC02 + (l-X)C0 + (2 + X)H2 ~----~ CH30H + XH20 (1),
in which X can have a value between 0 and 1, the reaction
equilibrium is shifted in the direction of methanol by
employing the above described membrane permeation.
In particular, it is proposed a process for the production
of methanol from carbon dioxide and hydrogen according to
the following reaction equation:
C2 + 3H2 ~ CH30H + H20 (2),
in which to increase the production yield the equilibrium
of the reaction is shifted in the direction of methanol, by
removing methanol and/or water from the reaction mixture by
membrane permeation.
Because of the higher permeability of vapors through
suitable polymer membranes it is proposed, according to the
invention, to employ as membrane for the removal of
methanol and/or water a polymer membrane, which, as above
mentioned, comprises an higher permeability to vapors than
to gases. In this way, methanol and/or water are
continuously removed during the reaction and consequently,
as required by the invention, the conversion can be
substantially increased at unchanged temperature and
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pressure.
Preferably, it is proposed to employ a membrane from a
perfluorinated ionomer, as for instance a perfluorinated
cation exchange membrane. Such fluorinepolymers with
sulfonic acid and/or carboxylic groups are usually employed
as ionomer membranes in the chlorine-alkali electrolysis.
In this connection reference is also made to an article of
R. S. Yeo with the title "Applications of
Perfluorsulfonated Polymer Membranes in Fuel Cells,
Electrolyzers and Load Leveling Devices" in "Perfluorinated
Ionomer Membranes", Edit. A. Eisenberg, H. L. Yeager, ACS
S~ymposium, Ser. 180, Washington D.C. (1982).
According to the invention it results therefore that
especially by the use of perfluorinated ionomers, as for
instance of a perfluorinated polysulfonic acid membrane, we
can obtain a selective separation of methanol and
eventually of water from the reaction mixture. Here, both
the chemical and physical stability as well as the
permeation characteristics depend on the type of the so-
called counter ion. According to the invention it isproposed to dope the perfluorinated polysulfonic acid
membrane by means of lithium ions, for instance by
contacting the membrane, before effecting the synthesis,
with a solution of lithium chloride.
The perfluorinated polysulfonic acid membrane in lithium
form presents, according to the invention, on one side an
excellent stability to chemical products and on the other
side also a good temperature stability up to about 250 C.
Accordingly, it is also possible to carry out the reaction
at temperatures up to about 220 C, so that it is possible
to employ the usual catalysts for the methanol synthesis,
such as copper, zinc, chrome and/or aluminium, mixtures
thereof or at least in part oxide mixtures thereof.
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The inventive idea is better explained by the enclosed
figures, which show:
Fig.1: a draft of a possible structure of a membrane
reactor as claimed in the present invention for carrying
out the methanol synthesis;
Fig. 2: a membrane module as employed in a laboratory
experimental arrangement in order to carry out and
respectively to test the process according to the present
invention, and
Fig. 3: an example of representation of a technical,
respectively industrial reactor for the methanol production
according to the inventive principle.
In Fig. 1 it is represented schematically the principle of
a membrane reactor suitable for carrying out the methanol
synthesis according to the invention. Here, the reactor
comprises a semi-permeable membrane 1, which is coated and
respectively surrounded on its external surface by catalyst
particles. The semi-permeable membrane is a so-called non
porous membrane, and the best material for this membrane
has resulted to be perfluorinated polysulfonic acid in
- lithium form. As already mentioned above, perfluorinated
ionomers have an high selectivity for the water transport.
Perfluorinated polysulfonic acid can be obtained on the
market for instance under the mark name "NAFION" and it is
produced by the company Du Pont. As also already mentioned
above, the permeation characteristics of such
perfluorinated ionomers depend on the type of the counter
ion. To this purpose, before carrying out the methanol
synthesis reaction, the perfluorinated polysulfonic acid
membrane, according to the invention, has been treated with
a solution of lithium chloride, so that the counter ion is
formed through lithium ions. As catalysts there can be used
all the catalysts usually employed in the methanol
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synthesis, such as copper, zinc, chrome, aluminium,
mixtures thereof, or at least in part oxides of these
metals.
While carrying out the reaction, the reactor is fed with
synthesis gas 5, that is carbon dioxide and hydrogen. Near
the surface of the semi-permeable membrane 1, that is in
the area of the catalyst particle 3, there occurs the
reaction into methanol and water, and here these
condensable products permeate preferably through the
membrane 1, as indicated by the arrow 7, in order to be
discharged on the opposite surface of the membrane, for
instance by means of a gas flow or a vacuum in the
direction of arrow 9. With the continuos removal of
methanol and/or water through the membrane the reaction
equilibrium is obviously shifted on the products side, and
consequently the yield of the reaction equation CO2 + 3H2
~----~ CH30H + H2O can be substantially increased.
On the base of an example of realization it can be better
explained that, by means of the process according to the
invention, the conversion during the methanol synthesis can
be clearly increased by the use of a semi-permeable
membrane. The structure of the membrane module used in the
example of realization is represented schematically in Fig.
2. Here, the employed module 11 comprises an external shell
13, as well as an internal tubular membrane 15, which has
been supplied by the company Perma Pure Products Inc. in
Thom's River, N.J. 08754, USA, with an inlet 17 and an
outlet 19. It is again used a perfluorinated polysulfonic
acid membrane, whose surface is of 0,0122 m2, with a
membrane thickness of 3,15 10~6m. The internal tubular
volume is of 6,6 10~6m3. The external shell 13 is made of a
tubular steel coating. The membrane separates the tubular
volume from the shell volume, so that gas type (medium),
pressure, flow speed and flow direction in both parts of
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the reactor module 11 can be adjusted independently one
from the other.
The external tubular steel shell 13 comprises moreover an
inlet 20 and an outlet 21.
Before carrying out the methanol reaction, 750 ml of an
about 50C warm solution of lN lithium chloride was flown
through the reactor membrane module 11 for 90 minutes by
means of a tubular pump in parallel. This means that the
lithium chloride solution was flown both within the tubular
membrane 15 as well as through the coating shell.
Successively, it was flushed with 1 litre of distilled
water and dried with compressed air.
In the tubular volume there were charged 7,0 g of catalyst
(with a grain size of 500-1000 ~m) and the ends were
loosely closed with glass wool. The introduced catalyst had
a basis structure of copper, zinc or aluminium,
respectively. The catalyst was transformed into the active
phase according to the usual processes recommended by the
catalyst suppliers.
For the carrying out of the methanol synthesis in the
membrane reactor 11, a flushing gas flow of 200 ml/min (100
Vol~ Argon) and a synthesis gas flow of 64 ml/min (76,2
Vol~ of hydrogen, 23,8 Vol~ of carbon dioxide) was
regulated in the coating and tubular volume by means of
mass flow regulators at a pressure of 4,3 bar. The flushing
gas and the synthesis gas were conducted in opposite
directions (counter flow principle). In other words the
synthesis gas was fed at arrow 17 into the inside of the
tubular membrane 15 and it was extracted at arrow 19. On
the other side, the flushing flow of Argon was introduced
into the coating shell at arrow 20 and it was discharged at
arrow 21. The temperature of the drier during the carrying
out of the methanol synthesis was of 200 C. The methanol
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yield was determined integrally through the gas condensed
in two wash bottles filled with water and operated one
after the other. The quantity of the methanol content was
defined with gas chromatography. The methanol yield
referred to the carbon dioxide was of 3,6%.
For a comparison, there was employed a similar tube reactor
without membrane, wherein the reaction was carried out as
in the above referred example. With the same catalyst
charge the methanol yield referred to the carbon dioxide
was only of 2,5~. In other words, through the employment of
the tubular membrane, respectively, with the process
according to the invention, we can obtain an yield increase
of 50~ in the above described laboratory experiment.
Obviously, the above example concerns a laboratory
- 15 experiment which has the purpose of Px~mi n;ng and
explaining the efficiency of the process according to the
invention. For the industrial and technical scale it is
obviously necessary to use other reactor design, wherein
methanol synthesis reactors are per se very well known in
the industry. It is in fact a question of optimization of
how the reactor must be build up, wherein factors as
process conditions (pressure, temperature), the employed
catalyst, the employed membrane material, the membrane
thickness, as well as the plant size, and so on should be
considered.
In Fig. 3 it is represented schematically, on the base of
an example, a possible design of an industrial methanol
synthesis reactor, in which, as proposed in the invention,
there is employed a membrane for the separation of methanol
and/or water from the reaction mixture.
Starting from a typical reactor in a methanol plant of 1000
t/day (as described in: "Neue Konvertersysteme fur dle
Methanol-Synthese"; K. R. Westerterp, M. Kuczynski, T. N.
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Bodewes and M. S. A. Vrijland; Chem.-Ing.-Tech. 61 (1989)
Nr. 3, pages 193-199) there are filled 4000 tubes each with
about 20 kg of catalyst 3. Such a tube 4 is represented in
fig. 3. Each tube 4 is long 10 m and has a diameter of 0.05
m. In this plant there is regulated space/time yield (STY)
of methanol of about 0,5 mol/hour*kg catalyst.
For a technical realization it is proposed the following:
in a tube 4, filled with catalyst 3, the methanol yield pro
passage, starting for instance from CO2 and H2 at 30 bar
and 220 C, can be increased according to the invention if
in this tube there are inserted perfluorinated cations
exchange membranes 1. Here, the continuous product
separation 9 iæ maintained by a vacuum on the permeate
side.
As to the membrane, it can be used hollow fibers (each for
instance 10 m long, with a diameter of 120 ~m and a
thickness of 10 ~m) which resist to the pressure difference
(synthesis pressure less vacuum pressure). Other
embodiments are also possible, in which the membrane is in
the form of thin layers or tubes. In these cases a support
body could withheld the pressure difference.
For the separation of the above mentioned STY, it is
necessary to employ an overall membrane surface of about
0,3 m2 pro tube. This means about 80 hollow fibers with the
above mentioned size pro tube 4, the rem~;n;ng gas is
discharged in direction of arrow 6.
Obviously, the tubular reactor represented in fig. 3 is
only a possible example, which may be modified and
completed in many other ways. The figure 3 has only the
purpose to indicate that the reactor type represented
schematically and respectively in laboratory scale in fig.
1 and 2, can be modified on industrial scale.
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It is also to be underlined that the invention as described
with reference to the methanol synthesis can also be
basically applied to all chemical conversions in which from
gaseous reagents there are produced products in vapor
S phase.
For the invention it is essential that in the conversion of
gaseous reagents into at least one product in vapor phase,
the product(s) obtained is(are) removed from the reaction
mixture by-a semi-permeable membrane, in order to shift the
reaction equilibrium in the direction of the products and
to obtain in this way an increase of the conversion.
