CA2186222A1

CA2186222A1 - Diaphragm reactor for converting gaseous base materials

Info

Publication number: CA2186222A1
Application number: CA002186222A
Authority: CA
Inventors: Ruud Struis; Samuel Stucki; Michael Wiedorn
Original assignee: Individual
Current assignee: Casale SA
Priority date: 1994-04-08
Filing date: 1995-04-04
Publication date: 1995-10-19
Also published as: CH687004A5; AU2065595A; CN1147241A; WO1995027690A1; EP0754172A1; MX9604577A; JPH09511509A

Abstract

For converting substantially gaseous raw materials into at least one product which is substantially in a vapor phase and for increasing the yield of the reaction, the invention proposes shifting the equilibrium of the reaction in the direction of the product(s) by removing at least one product from the reaction mixture by membrane permeation. A typical example thereof is the synthesis of methanol which generally occurs according to the following equation: XCO2 + (1-X)CO + (2 + X)H2 <----> CH3OH + XH2O (1), in which X can have a value between 0 and 1. In this respect, the gaseous reagents are converted into methanol in the vapor phase and possibly water. The resultant products are removed by membrane permeation in order to shift the equilibrium of the reaction in the direction of the product(s).

Description

~ i - 1 2 1 8 6222 Title: "Process for the production of methanol~'.

*** * ***

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 ~ - 2`- 2186222 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.
.

~ _ 3 _ 21 ~6222 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.

~ 4 _ 21 B6222 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 (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 ~ - 6 - 2~86222 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.

.

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 ~ - 8 - 2 1 86222 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 ~, 9 2 1 86222 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 ~ - 10 - 2186222 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.

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.

~ . 2tB6222 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.

Claims

1. Process for the production of methanol from synthesis gas, characterized by the fact that to increase the yield the equilibrium of the reaction is shifted in the direction of methanol, by removing from the reaction mixture methanol and/or another product occurring in the reaction by membrane permeation.

2. Process, in particular according to claim 1, characterized by the fact that the methanol synthesis is carried out according to the following reaction equation:
XCO2 + (1-X)CO + (2 + X)H2 <----> CH3OH + XH2O (1), in which X can have a value between 0 and 1.

3. Process, in particular according to one of the claims 1 to 2, for the production of methanol from carbon dioxide and hydrogen according to the following reaction equation :
CO2 +3H2 <----> CH3OH + H2O (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.

4. Process, in particular according to one of the claims 1 or 3, characterized by the fact that as membrane for the removal of methanol and/or water there is employed a polymer membrane, which shows a much higher permeability to vapors than to gases.

5. Process, in particular according to one of the claims from 1 to 4, characterized by the fact that the membrane consists of a perfluorinated ionomer.

6. Process, in particular according to claim 5, characterized by the fact that there is employed at least one perfluorinated cation exchanged membrane.

7. Process, in particular according to one of the claims 5 or 6, characterized by the fact that there is employed at least one perfluorinated polysulfonic acid membrane, in which for instance, before carrying out the reaction according to the equation (1) or (2), the perfluorinated polysulfonic acid membrane is doped by lithium ions, for instance by contacting it with a lithium chloride solution.

8. Process, in particular according to one of the claim 1 to 7, characterized by the fact that the reaction is carried out by employing respectively a copper, zinc, chrome and/or aluminium catalyst, or a mixture thereof, or at least in part an oxide mixture thereof, wherein the temperature in the reactor should not be higher than ca.
220°C.

9. Process, in particular according to one of the claims 1 to 8, characterized by the fact that the catalyst is preferably piaced near the reaction side of the membrane surface, and that in the reactor there are provided flow generating means, in order to obtain an optimal flow control of the reaction mixture at the membrane surface and to obtain a possibly high removal of the permeate, consisting of methanol and/or water, by the membrane.

10. Device for the carrying out of the process according to one of the claims 1 to 9, characterized by the fact that it comprises a non porous membrane, which is suitable for the separation of substances in vapor phase from gases.

11. Device, in particular according to claim 10, characterized by the fact that there is employed a methanol synthesis reactor, which comprises at least one perfluorinated cations exchange membrane, as for instance-one perfluorinated polysulfonic acid membrane, by which the real reaction space for carrying out the methanol synthesis and a space for the removal of methanol and/or water are separated one from the other.

12. Use of a perfluorinated polysulfonic-acid cationic exchanged membrane for the conversion of gaseous raw materials in at least one product in vapor phase, at temperatures between 200°C and 250°C.

13. Use of a membrane according to claim 12, in lithium form to selectively separate alcohols and eventually water from a reaction mixture.

14. Use of a membrane according to claim 13 for the synthesis of methanol.