CH687004A5 - Membrane reactor for the conversion of houses on gaseous precursors. - Google Patents

Description

1

CH 687 004 A5

2nd

description

The present invention relates to a process for converting essentially gaseous starting materials into at least one substantially vaporous product, a process for producing methanol from synthesis gas and a device for carrying out the process.

Originally, for example, technical methanol synthesis started as a typical reaction of gaseous starting materials from a synthesis gas obtained from natural gas or naphtha by steam reforming with the main components CO and H2. This synthesis is based on the following balance:

CO + 2HZ - CH3OH.

In the so-called low-pressure process, this classic synthesis is carried out technically in a pressure reactor over a highly active Cu / Zn catalyst at temperatures between 230-280 ° C and pressures of approx. 50-100 bar. Under the specified reaction conditions, e.g. 260 ° C and 50 bar the theoretical maximum conversion, i.e. sales based on carbon usage at 45%. By raising the synthesis pressure to e.g. 100 bar sales can be increased to 65%. However, increasing the pressure is associated with additional costs for gas compression and the design of the pressurized system parts and is therefore not preferred in practice.

In order to make good use of the feedstocks, multiple recirculation of the unreacted synthesis gas is practiced instead. The enriched gas from the reactor is cooled to about 30 ° C, and methanol condenses from the unreacted synthesis gas. The unreacted reactants are recompressed, heated again and returned to the reactor for further reaction. In practice, the total conversion is increased to approximately 88% by recycling the unreacted synthesis gas with a circulating gas ratio of 5 and a recirculation rate of usually 4-6. In this context, reference is made to E. Supp, Chem.Technol. 3 (1973) No. 7, pp. 430-435.

However, methanol synthesis from carbon dioxide and hydrogen is becoming increasingly important. This synthesis is based on equilibrium:

C02 + 3H2 ~ CH3OH + H20

This reaction shows that water is also produced in addition to methanol. Compared to methanol synthesis from carbon monoxide and hydrogen, the conversion from CO2 and H2 is much lower. For 250 ° C and 50 bar e.g. the theoretical maximum conversion is approximately 16%. This means that after one pass through the reactor, at best 16% of the gas mixture (1 part CO2 and 3 parts H2) are converted. To ensure optimal gas utilization, multiple recirculation is also necessary here, with methanol and water now having to be removed from the rich gas.

For both the first-mentioned classic and the second-mentioned alternative methanol synthesis, only significant savings in investment costs, energy and raw materials can be achieved if the implementation per run is improved so that recycling of the unreacted synthesis gas can be avoided. This applies in general to all reactions where gaseous starting materials are converted into vaporous products, as is specifically the case with methanol synthesis.

In this context, K.R. Westerterp, M. Kuczynski, T.N. Bodewes and M.S.A. Vrijland suggest in an article «New converter systems for methanol synthesis», Chem.-Ing.-Tech. 61 (1989) No. 3, pp. 193-199, before continuously separating the products obtained during the synthesis from the reaction mixture. In principle, it is known that the turnover per run can be increased at unchanged pressure and temperature if one is able to separate the products simultaneously and continuously during the synthesis. For this purpose, a reactor concept is proposed, the in situ separation of methanol (from carbon monoxide and hydrogen) by means of adsorption, namely the gas / solid / solid trickle reactor, using a trickling down solid adsorbent in the reactor. As a result, sales of up to 100% are achieved in a simple manner and the need for recirculation is eliminated. However, this proposed method is very complex in terms of process technology.

The object of the present invention is therefore to propose a method generally for reactions of gaseous starting materials for the formation of vaporous products and in particular for methanol synthesis, the conversion or, during the reaction or the synthesis, simultaneous removal of one or more of the products, the conversion or the conversion can be increased. According to the invention, this object is achieved by means of a method according to the wording of claim 1.

In general, a process for converting essentially gaseous starting materials is proposed, which starting materials are converted into at least one essentially vaporous product, the reaction equilibrium being shifted towards product (s) in order to increase the reaction yield by using at least one product in the reaction mixture Membrane permeation is withdrawn.

It is proposed that the at least one product be withdrawn by means of so-called steam permeation.

Furthermore, a method for producing methanol from synthesis gas is proposed, the reaction equilibrium being shifted again towards methanol in order to increase the yield by adding methanol and / or possibly another during the reaction to the reaction mixture

5

10th

15

20th

25th

30th

35

40

45

50

55

60

65

2nd

3rd

CH 687 004 A5

4th

resulting product is removed by membrane permeation.

Membranes are thin layers that can have very different structures, but all have the same property of opposing different resistance to the passage of different substances. It is known to separate individual components from fluid mixtures using membranes. In this regard, reference is made to the article by Y. Cen, K. Meckl and R. N. Lichtenthaler with the title “Nonporous Membranes and Their Applications”, Chem.-Ing.-Tech. 65 (1993) No. 8, pp. 901-913.

The separation of substances by means of, for example, non-porous membranes is based on the differences in the solubility and the rate of diffusion of different mixture components in the membrane material. The mass transfer takes place in three successive steps:

1. sorption of the components from the feed mixture or from the reaction mixture,

2. Diffusion of the adsorbent components through the selective membrane, and

3. Desorption of the components in the permeate phase.

It is known that vapors in suitable polymer membranes have a significantly higher permeability than gases. The essential thing is that the mixture to be prepared or classified contains not only the actual gases but also components that can be condensed under standard conditions (1 bar and 0 ° C). The driving force for mass transport through a so-called non-porous membrane is given by the difference in the chemical potential of the permeating components between the feed and permeate side. With steam permeation, this difference is generated by the fact that the partial pressures of the individual components are much higher on the feed side than on the permeate side. Based on the above observations, it is suggested that in the synthesis of methanol according to the following reaction equation:

XCO2 + (1-X) CO + (2 + X) H2 ^ CH3OH + XH2O (1),

where X can be a value between 0 to 1, the reaction equilibrium is shifted towards methanol using the membrane permeation described above.

In particular, a method for producing methanol from carbon dioxide and hydrogen according to the following reaction equation is proposed:

CO2 + 3H2- CH3OH + H2O (2),

the reaction equilibrium being shifted towards methanol in order to increase the product yield, by withdrawing methanol and / or water from the reaction mixture by means of membrane permeation.

Due to the higher permeability of vapors through suitable polymer membranes, it is proposed according to the invention that a polymer membrane is used as the membrane for the removal of methanol and / or water, which, as mentioned above, has a higher permeability for vapors than for gases. Thus, while the reaction is being carried out, methanol and / or, if appropriate, water are continuously separated off, as a result of which, as required according to the invention, the conversion can be increased significantly at unchanged pressure and temperature.

It is preferably proposed to use a membrane made of a perfluorinated ionomer, such as a perfluorinated cation exchange membrane. Such fluoropolymers with sulfonic acid and / or carboxyl groups are normally used as ionomeric membranes in chlor-alkali electrolysis. In this context, reference is also made to an article by R. S. Yeo entitled “Applications of Perfluorosulfonated Polymer Membranes in Fuel Cells, Electrolyzers, and Load Leveling Devices” in “Perfluorinated lonomer Membranes”, Edit. A. Eisenberg, H.L. Yeager, ACS Symposium, Ser. 180, Washington DC (1982).

According to the invention, it has now been shown that, especially when using perfluorinated ionomers, such as, for example, a perfluorinated polysulfonic acid membrane, a selective separation of methanol and possibly water from the reaction mixture can be achieved. The chemical and physical stability as well as the permeation properties depend on the type of counter ion. According to the invention, it is correspondingly proposed to add lithium ions to the perfluorinated polysulfonic acid membrane, for example by passing a lithium chloride solution over the membrane before carrying out the synthesis.

The perfluorinated polysulfonic acid membrane in lithium form proposed according to the invention has excellent chemical resistance on the one hand and good temperature resistance up to approximately 250 ° C. on the other hand. Accordingly, it is possible to carry out the reaction at temperatures up to about 220 ° C., using the catalysts customary in methanol syntheses, such as copper, zinc, chromium and / or aluminum, or mixtures thereof, or at least partially oxide mixtures thereof can.

The idea of the invention will be explained in more detail, for example, with the aid of the attached figures. It shows:

1 is a schematic diagram of a possible structure of a membrane reactor claimed according to the invention for carrying out the methanol synthesis;

2 shows a membrane module, as was used in a laboratory test arrangement, in order to carry out or check the method according to the invention, and

Fig. 3 shows an example of the design of a technical or industrial reactor for the production of methanol according to the principle of the invention.

5

10th

15

20th

25th

30th

35

40

45

50

55

60

65

3rd

5

CH 687 004 A5

6

1 schematically shows the principle of a membrane reactor according to the invention, suitable for carrying out the methanol synthesis. The reactor comprises a semi-permeable membrane 1 which is coated or surrounded on its outer surface with catalyst particles. The semi-permeable membrane is a so-called non-porous membrane, and perfluorinated polysulfonic acid in lithium form has proven to be the preferred material as the membrane material. As already mentioned above, perfluorinated ionomers have a high selectivity for the transport of water. Perfluorinated polysulfonic acid is available on the market, for example under the “NAFION” brand name, and is manufactured by the Du Pont company. As also mentioned above, the permeation properties of such perfluorinated ionomers depend on the type of counterion. For this purpose, the perfluorinated polysulfonic acid membrane used according to the invention was treated with lithium chloride solution before carrying out the methanol synthesis reaction, as a result of which the counterion is formed by lithium ions. Suitable catalysts are all catalysts usually used in methanol synthesis, such as copper, zinc, chromium, aluminum, mixtures thereof or at least partially oxides of these metals.

When carrying out the reaction, the reactor is charged with synthesis gas 5, which is carbon dioxide and hydrogen gas. Near the surface of the semi-permeable membrane 1, i.e. in the region of the catalyst particles 3, the reaction to methanol and water takes place, these condensable products according to the arrows 7 preferably permeating through the membrane 1 in order to be discharged on the opposite surface of the membrane, for example by means of a gas stream or by means of a vacuum in the direction of the arrow 9 to become. The continuous withdrawal of methanol and / or water through the membrane naturally shifts the equilibrium of the reaction on the product side, as a result of which the conversion of the reaction equation CO2 + 3H2 = CH3OH + H2O can be increased significantly.

On the basis of an exemplary embodiment, it was confirmed that the conversion proposed for the synthesis of methanol can be increased significantly by using a semi-permeable membrane by means of the method proposed according to the invention. The structure of the membrane module used in the exemplary embodiment is shown schematically in FIG. 2. The module 11 used here comprises an outer shell 13 and an inner tubular membrane 15, supplied by Perma Pure Products Inc. in Thom's River, NJ 08754, USA, with an inlet end 17 and an outlet 19. Again, a perfluorinated polysulfonic acid -Membrane used, the membrane area was 0.0122 m2 with a membrane thickness of 3.15 10 ~ 6 m. The inner hose volume was 6.6 10-6 m3. The outer shell 13 is a tubular steel jacket. The membrane separates the tube volume from the jacket volume, so that gas type (medium), pressure, flow velocity and flow direction can be set independently of one another in both parts of the reactor module 11.

The outer tubular steel casing 13 in turn comprises an inlet 20 and an outlet 21.

Before the methanol reaction was carried out, 750 ml of a 1 N lithium chloride solution at about 50 ° C. were passed in parallel through the reactor membrane module 11 with the aid of a peristaltic pump. I.e. the lithium chloride solution was passed both within the tubular membrane 15 and through the jacket. It was then rinsed with 1 liter of distilled water and then dried with compressed air.

7.0 g of catalyst (with a grain size of 500-1000 μm) were filled into the tube volume and the ends were each loosely closed with glass wool. The catalyst entered was based on copper, zinc or aluminum. The catalyst was converted to the active form according to the usual procedures suggested by the catalyst suppliers.

To carry out the methanol synthesis in the membrane reactor 11, a flushing gas flow of 200 ml / min (100% by volume argon) and a synthesis gas flow of 64 ml / were carried out with the aid of mass flow controllers at a pressure of 4.3 bar in the jacket and hose volume. min (76.2 vol% hydrogen, 23.8 vol% carbon dioxide). The purge gas and the synthesis gas were conducted in the opposite direction (countercurrent principle). In other words, the synthesis gas was fed into the interior of the tubular membrane 15 at arrow 17 and discharged at arrow 19. In contrast, the purge gas flow of argon was entered into the jacket at arrow 20 and discharged at arrow 21. The drying cabinet temperature was 200 ° C. while the methanol synthesis was being carried out. The methanol yield was determined integrally by condensing the gases in two successive wash bottles filled with water. The methanol content was determined quantitatively by gas chromatography. The methanol yield based on the carbon dioxide supply was 3.6%.

In comparison, an analog tubular reactor without a membrane was used, the reaction being carried out as in the exemplary embodiment mentioned above. With the same catalyst load, the methanol yield based on the carbon dioxide offered was only 2.5%. In other words, by using the tubular membrane or by means of the process control proposed according to the invention, an increase in yield in the laboratory test shown can be achieved by approximately 50%. Of course, the exemplary embodiment shown is a laboratory test to check and confirm the effectiveness of the method proposed according to the invention. Other reactor designs are of course to be used for the technical or industrial scale, with methanol synthesis reactors per se being well known industrially. After all, it is a question of optimization how the reactor looks5

10th

15

20th

25th

30th

35

40

45

50

55

60

65

4th

7

CH 687 004 A5

8th

must be taken into account, taking into account factors such as process conditions (pressure, temperature), catalyst used, membrane material used, membrane thickness as well as system size, etc.

3 schematically shows an example of a possible design of an industrial methanol synthesis reactor in which, as proposed according to the invention, a membrane is provided for separating methanol and / or water from the reaction mixture.

Starting from a typical reactor in a 1000 t / day methanol plant (as described in: "New converter systems for methanol synthesis"; KR Westerterp, M. Kuczynski, TN Bode-wes and MSA Vrijland; Chem.-Ing.- Tech. 61 (1989) No. 3, pp. 193-199) are 4000 tubes, each filled with approx. 20 kg of catalyst 3. Such a tube 4 is shown in Fig. 3. Each tube 4 is 10 m long and has a diameter of 0.05 m. In such a system, a space-time yield (RZA) of methanol of approx. 0.5 mol / hour * kg cat. Is set.

The following is proposed for a technical implementation: in a tube 4 filled with catalyst 3, the methanol yield per pass can be started from e.g. CO2 and H2 at 30 bar and 220 ° C. can be increased according to the invention if perfluorinated cation exchange membranes 1 are installed in this tube. Here, the continuous product separation 9 is maintained by means of vacuum on the permeate side.

The membrane can be hollow fibers (e.g. 10 m long, 120 µm in diameter and 10 µm in thickness) that withstand the pressure difference (synthesis pressure minus vacuum pressure). However, other constructions in which the membrane is in the form of thin foils (layers) or tubes would also be conceivable. In these cases, a support body could take up the differential pressure.

A total membrane area of around 0.3 m2 per pipe is required to separate the above-mentioned RZA. This means about 80 hollow fibers of the size mentioned above per tube 4, the residual gas is removed in the direction of arrow 6.

Of course, the reactor tube 4 shown in FIG. 3 is only one possible example, which can be modified, modified and supplemented in any desired manner. 3 only serves to show that the reactor type shown schematically or on a laboratory scale in FIGS. 1 and 2 can be transformed to the industrial scale.

In principle, it should also be added that the invention is to be explained, for example, on the basis of methanol synthesis, but is in principle applicable to all chemical reactions in which gaseous starting materials are at least partially used to produce vaporous products.

It is essential to the invention that when gaseous starting materials are converted into at least one vaporous product, the product (s) obtained are / are removed from the reaction mixture by means of a semipermeable membrane, so as to shift the equilibrium towards products and so as to increase sales.

Claims (13)

Claims 1. A process for converting substantially gaseous starting materials into at least one substantially vaporous product, characterized in that the reaction equilibrium is shifted in the direction of product (e) in order to increase the reaction yield by removing at least one product from the reaction mixture by means of membrane permeation. 2. The method according to claim 1, characterized in that the withdrawal of the at least one product takes place by means of steam permeation. 3. The method according to claim 1 for producing methanol from synthesis gas, characterized in that to increase the yield, the reaction equilibrium is shifted towards methanol by withdrawing methanol and / or another product obtained in the reaction by means of membrane permeation from the reaction mixture. 4. The method according to claim 3, characterized in that the synthesis of methanol takes place according to the following reaction equation: XCO2 + (1-x) CO + (2 + X) H2 = CH3OH + XH2O (1), where X can have a value between 0 and 1. 5. The method according to any one of claims 1 to 4, for producing methanol from carbon dioxide and hydrogen according to the reaction equation: CO2 + 3H2 = CH3OH + HzO (2), in order to increase the production yield, the reaction equilibrium is shifted towards methanol by removing methanol and / or water from the reaction mixture by means of membrane permeation. 6. The method according to any one of claims 1 or 5, characterized in that a polymer membrane is used as the membrane for the removal of methanol and / or water, which has a significantly higher permeability for vapors than for gases. 7. The method according to any one of claims 1 to 6, characterized in that the membrane consists of a perfluorinated ionomer. 8. The method according to claim 7, characterized in that at least one perfluorinated cation exchange membrane is used. 9. The method according to any one of claims 7 or 8, characterized in that at least one perfluorinated polysulfonic acid membrane is used, wherein for example the perfluorinated polysulfonic acid membrane is mixed with lithium ions before carrying out the reaction according to formula (1) or (2) is, for example by passing a lithium chloride solution over the membrane. 10. The method according to any one of claims 1 to 5 10th 15 20th 25th 30th 35 40 45 50 55 60 65 5 9 CH 687 004 A5 9, characterized in that the reaction is carried out using a copper, zinc, chromium and / or aluminum catalyst or a mixture thereof or at least partially an oxide mixture thereof, the temperature in the reactor having the value of approx. Should not exceed 220 ° C. 11. The method according to any one of claims 1 to 10, characterized in that the catalyst is preferably arranged near the reaction-side surface of the membrane and that flow-generating means are provided in the reactor in order to achieve optimum flow guidance of the reaction mixture on the membrane surface in order to derive the permeate, consisting of methanol and / or to generate water through the membrane. 12. Device for performing the method according to one of claims 1 to 11, characterized in that it has a non-porous membrane which is suitable for separating vaporous substances from gases. 13. The apparatus according to claim 12, characterized in that there is a methanol synthesis reactor comprising at least one perfluorinated cation exchange membrane, such as a perfluorinated polysulfonic acid membrane, by means of which (which) the actual reaction space for carrying out the methanol synthesis and a space for the stripping separated from methanol and / or water. 5 10th 15 20th 25th 30th 35 40 45 50 55 60 65 6