WO1995027690A1

WO1995027690A1 - Diaphragm reactor for converting gaseous base materials

Info

Publication number: WO1995027690A1
Application number: PCT/CH1995/000076
Authority: WO
Inventors: Ruud Struis; Samuel Stucki; Michael Wiedorn
Original assignee: Methanol Casale S.A.
Priority date: 1994-04-08
Filing date: 1995-04-04
Publication date: 1995-10-19
Also published as: CH687004A5; EP0754172A1; AU2065595A; CA2186222A1; JPH09511509A; CN1147241A; MX9604577A

Abstract

For converting substantially gaseous base materials into at least one product which is substantially in a vapour state 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 diaphragm permeation. A typical example thereof is the synthesis of methanol which generally occurs according to the following reaction equation: (1), in which X can have a value of between 0 and 1. In this respect, the gaseous educts are converted into methanol in the vapour state and possibly water. The resultant products are removed by diaphragm permeation in order to shift the equilibrium of the reaction in the direction of the product(s).

Description

Membrane reactor for the conversion of gaseous starting materials

The present invention relates to a process for converting substantially 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.

For example, technical methanol synthesis originally 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 H_. This synthesis is based on the following balance:

C0 + 2H ₂ ^ - = - rCH ₃ 0H.

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, the theoretically maximum conversion is, for example, 260 ° C. and 50 bar, ie the conversion based on the carbon dioxide input is 45%. By increasing the synthesis pressure to, for example, 100 bar, the conversion 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 starting materials, a 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 out of the unreacted synthesis gas. The unreacted reactants are recompressed, heated again and returned to the reactor for further conversion. 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. Techno! , 3 (1973) No. 7, pp. 430-435.

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

C0 ₂ + 3H _2i = ü »CH ₃ 0H + H ₂ 0

This reaction shows that water is also produced in addition to methanol. Compared to the methanol synthesis from carbon monoxide and hydrogen, the conversion from CO- and H- 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, 16% of the gas mixture (1 part CO and 3 parts H ») is converted in the best case. In order to ensure optimal gas utilization, multiple recirculation is also necessary here, methanol and water now having to be removed from the rich gas.

Both for the former classic and for the the second-mentioned alternative methanol synthesis can only achieve substantial savings in investment costs and in energy and raw materials if the conversion per pass is improved in such a way that recycling of the unreacted synthesis gas can be avoided. This situation applies in general to all reactions where gaseous starting materials are converted into vaporous products, as is the case specifically with methanol synthesis.

In this context, K.R. Westerterp, M. Kuczynski, T.N. Bodewes and M.S.A, Vrijland 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 pass 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, in which the in situ separation of methanol (from carbon monoxide and hydrogen) by means of adsorption is the basis, namely the gas / solid / solid-fuel reactor, for which purpose a solid adsorbent trickling down is introduced into the reactor is set. 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 provide a method generally for reactions of gas to propose educts for the formation of vaporous products and in particular for the synthesis of methanol, whereby during the reaction or the synthesis, the conversion or conversion can be increased by simultaneously removing one or more of the products. According to the invention, this object is achieved by means of a method according to the wording of claim 1.

In general, a method is proposed for converting essentially gaseous starting materials, 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 removing at least one product from the reaction mixture by means of membrane permeation.

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 product obtained in the reaction to the reaction mixture is removed by means of membrane permeation.

Thin layers are referred to as membranes, which may have very different structures, but all have the common property of opposing different resistance to the passage of different substances. It is known to separate individual grain to separate components from fluid mixtures. Please refer to the article by Y. Cen, K. Meckl and RN Lichtenthaler entitled "Non-porous membranes and their applications", Che. -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 essentially 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 per eat phase.

It is known that vapors in suitable polymer membranes have a considerably higher permeability than gas. The important thing here is that the mixture to be prepared or classified contains not only the actual gases but also components which, under standard conditions (1 bar and 0 ° C) are condensable. The driving force for the 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. In the case of vapor permeation, this difference is produced in that the partial pressures of the individual components on the feed side are much higher than permeation on the side. Based on the above observations, it is suggested that in the synthesis of methanol according to the following reaction equation:

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:

C0 ₂ + 3H ₂ -5 - = - ^ CH ₃ 0H + H ₂ 0 (2),

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

Because of the higher permeability of vapors due to 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 possibly water, which, as mentioned above, has a higher permeability for vapors exhibits than for gases. Thus, while the reaction is being carried out, methanol and / or water, if appropriate, are continuously separated off, which, as required according to the invention, can significantly increase the conversion with unchanged pressure and temperature. It is preferably proposed to use a membrane made from 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 chlorine-alkali electrolysis. In this context, reference is also made to an article by RS Yeo entitled "Applications of Perf1 uorosulfonated Polymer Membranes in Fuel Cells, Electrolyzers, and Load Leveling Devices" in "Perf1 uorated Iono er Membranes", Edit. A. Eisenberg, HL 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. Both the chemical and physical stability and the permeation properties depend on the type of the so-called 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., the catalysts customary in methanol synthesis such as copper, zinc, Chromium and / or aluminum, or mixtures thereof, or at least partially oxide mixtures thereof, can be used.

The idea of the invention will be explained in more detail, for example, with reference to 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 used in an abnormal test arrangement 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.

1 schematically shows the principle of a membrane reactor according to the invention, suitable for carrying out the methanol synthesis. The reactor includes a semi-permeable membrane 1 which is coated or surrounded on its outer surface by means of catalyst particles. The semi-permeable membrane is a so-called non-porous membrane, with perfluorinated polysulfonic acid in lithium form having 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 brand name "NAFION" and is manufactured by the Du Pont company. As mentioned above, hang the permeation properties of such perfluorinated ionomers of the counter ion type. For this purpose, the perfluorinated polysulfonic acid membrane used according to the invention was treated with lithium chloride solution before the methanol synthesis reaction was carried out, 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 the reaction is carried out, the reactor is charged with synthesis gas 5, which is carbon dioxide and hydrogen gas. Near the surface of the semi-permeable membrane 1, ie in the area 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 in the direction of arrow 9 on the opposite surface of the membrane, for example by means of a gas stream or by means of a vacuum. By continuously withdrawing methanol and / or water through the membrane, the equilibrium of the reaction product is naturally shifted, whereby the conversion of the reaction equation C0 ₂ + 3H - ^ - ^ - ^ CH- _j OH + HO can be increased significantly.

On the basis of an exemplary embodiment, it was confirmed that the method proposed according to the invention can significantly increase the conversion in the methanol synthesis by using a semi-permeable membrane. The structure of the membrane module used in the exemplary embodiment is shown schematically in FIG. 2. There- In the case of the module 11 used, it comprises an outer shell 13 and an inner, also cylindrical, membrane 15, supplied by Per a Pure Products Inc. in Thos River, NJ 08754, USA, with an inlet end 17 and an outlet 19. Again, a perfluorinated poly sulphonic acid membrane is used, the membrane surface

? -6

0.0122 m was 10 m with a membrane thickness of 3.15.

-6 3 The inner hose volume was 6.6 10 m. The outer casing 13 is a steel pipe jacket. The membrane separates the tube volume from the jacket volume, so that the gas type (medium), pressure, flow rate and flow direction can be set independently of one another in both parts of the reactor module 11.

The outer steel 1 tubular casing 1 and 13 in turn comprises an inlet 20 and an outlet 21.

Before the methanol reaction was carried out, 750 ml of a 1 N-Li thi umchlor ori d solution at about 50 ° C. were passed through the reactor membrane module 11 in parallel with the aid of a peristaltic pump within 90 minutes. That 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 into the active form in accordance with the customary methods proposed 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 tube 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 guided at arrow 17 into the interior of the membrane 15 and also discharged at arrow 19. In contrast, the flushing 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 carried out. The methanol yield was determined integrally by condensing the gases in two wash bottles filled with water and connected in series. The methanol content was determined quantitatively by gas chromatography. The methanol yield related to the cabbage endi oxide 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 loading, the methanol yield based on the carbon dioxide 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. For the technical or On an industrial scale, other reactor designs are of course to be used, methanol synthesis reactors per se being very well known industrially. Finally, it is a question of optimization how the reactor is to be designed, taking into account factors such as process conditions (pressure, temperature), catalyst used, membrane material used, membrane thickness and 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, the arrangement of a membrane is provided for the separation of 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 Bodewes 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 about 0.5 mol / hour * kg of 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 increased according to the invention starting from, for example, C0 ₂ and H at 30 bar and 220 ° C. if in perfluorinated cation exchange membrane 1 are installed in this tube. Here the maintain continuous product separation 9 by means of vacuum on the permeate side.

The membrane can be hollow fibers (e.g. each 10 m long, diameter 120 µm and thickness 10 µm) 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.

To separate the above-mentioned RZA, a total mem-

2 branch area of about 0.3 m per tube is required. This means approx. 80 hollow fibers of the above-mentioned size per tube 4, in direction 6 the residual gas is removed.

Of course, the reactor tube 4 shown in FIG. 3 is only one possible example which can be modified, modified and supplemented in any manner. 3 only serves to show that the reactor type shown schematically or in 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, using the 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 in the implementation of gas shaped educts in at least one vaporous product, the product (s) obtained are / are removed from the reaction mixture by means of a semi-permeable membrane, so as to shift the equilibrium towards the products and thus increase them to achieve sales.

Claims

claims

1. A process for converting essentially gaseous starting materials into at least one essentially vaporous product, characterized in that for increasing the reaction yield the reaction equilibrium is shifted in the direction of product (s) by using at least one product in the reaction mixture Membrane permeation is withdrawn.

2. The method, in particular according to claim 1, characterized in that the withdrawal of the at least one product takes place by means of so-called steam permeation.

3. A process for the production of methanol from synthesis gas, characterized in that to increase the yield the reaction equilibrium is shifted in the direction of methanol by withdrawing from the reaction mixture methanol and / or possibly another product obtained in the reaction by means of membrane permeation.

4. The method, in particular according to claim 3, characterized in that the synthesis of methanol takes place according to the following reaction equation:

XC0 ₂ + (lX) C0 + (2 + X) H ₂ ^ = - rCH ₃ 0H + XH ₂ 0 (1),

where X can have a value between 0 and 1.

5. The method, in particular according to one of claims 1 to 4, for the production of methanol from carbon dioxide and hydrogen according to the reaction equation:

C0 ₂ + 3H ₂ ^S - * »CH ₃ 0H + H ₂ 0 (2),

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

6. The method, in particular according to one of claims 1 or 5, characterized in that a polymer membrane is used as the membrane for the removal of methanol and / or possibly water, which has a significantly higher permeability for vapors than for gases.

7. The method, in particular according to one of claims 1 to 6, characterized in that the membrane consists of a perfluorinated ionomer.

8. The method, in particular according to claim 7, characterized ge indicates that at least one perfluorinated cation exchange membrane is used.

9. The method, in particular according to one of claims 7 or 8, characterized in that at least one perfluorinated polysulfonic acid membrane is used, for example the perfluorinated polysulfonic acid membrane before carrying out the reaction according to formula (1) or (2 ) is added by means of lithium ions, for example by passing a lithium chloride solution over the membrane.

10. The method, in particular according to one of claims 1 to 9, characterized in that the reaction using a copper, zinc, chromium and / or aluminum catalyst or a mixture thereof or at least partially one Oxide mixture thereof is carried out, the temperature in the reactor should not exceed the value of about 220 ° C.

11. The method, in particular according to 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 ensure optimum flow guidance of the reaction mixture on the membrane surface ¬ to achieve, in order to generate as large a derivative of the permeate as possible, consisting of methanol and / or possibly 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 device, in particular according to claim 12, characterized in that a methanol synthesis reactor is present, comprising at least one perfluorinated cation exchange membrane, such as a perfluorinated polysulfonic acid membrane, by means of which the actual reaction space for carrying out the Methanol synthesis and a space for the removal of methanol and / or possibly water are separated from one another.