GB2255516A - Process for conducting chemical equilibrium reactions - Google Patents

Abstract

The invention provides a process for conducting chemical equilibrium reactions involving gaseous reactants which yield a liquid product using a plurality of fluidised bed catalytic reactors in series and an inert solvent to absorb the reaction product from the reaction mixture. Between at least two consecutive fluidized bed reactors, R1, R2, R3, there are means S1, S2, S3 effecting intimate contact between the inert solvent and the product-containing reaction mixture, which also comprises a means effecting a separation of a liquid reaction product/inert solvent solution and a gaseous phase which passes to the next fluidised bed reactor. Preferably this is a swirl tube. Preferably the process is used to prepare methanol by reaction of a gaseous mixture comprising hydrogen and carbon monoxide. The inert solvent employed may be e.g. water, a higher alcohol, a higher ether or a higher ester. The catalyst composition in the bed may comprise the elements copper and zinc and is optionally promoted with another element. The operating conditions of the reactors are within the temperature range of 100 to 350 DEG C and the pressure within the range from 3 to 35 MPa. The invention also provides an apparatus suitable for carrying out the above process. <IMAGE>

Description

PROCESS FOR CONDUCTING CHEMICAL EQUILIBRIUM REACTIONS The invention relates to a process for conducting chemical equilibrium reactions involving gaseous reactants and a liquid reaction product using a plurality of catalytic reactors in series and an inert solvent to remove the product from the reaction mixture.

Processes for conducting chemical equilibrium reactions involving gaseous reactants and a liquid reaction product are important from a technological point of view and they comprise especially the preparation of compounds like methanol, ethanol, methyl tertiary butyl ether, methyl tertiary amyl ether, ethyl tertiary butyl ether etc. The products are inter alia useful as fuel or fuel additive. Starting materials comprise hydrogen, carbon monoxide, and lower alkenes.

Especially, cheap methanol in very large quantities is a valuable product as a fuel and a starting material for further chemical processing. Therefore, there is a need for an economically attractive industrial bulk manufacturing process, using cheap starting materials and operating under attractive economical, environmental and safe conditions, i.e. using rather simple equipment and resulting in a significant reduction of the methanol cost price. Therefore, considerable research and development efforts have been made for a further improved methanol manufacturing process.

The formation of methanol from hydrogen and carbon monoxide is a strongly exothermic equilibrium reaction, wherein relatively high operating pressures and temperatures are required for acceptable reaction rates, but under such reaction conditions the attainable conversion is strongly limited by the thermodynamic equilibrium.

Finding a satisfactory compromise as to the reaction conditions between reaction rate and conversion percentage is therefore difficult.

In industrially applied processes, in which the catalyst is present in the form of a fixed bed of particles, high gas velocities are applied to promote effective removal of reaction heat and to allow good control of the reaction temperature. Due to these high velocities and the thermodynamic limitations low conversions per pass (i.e. less than 30%) are obtained. To achieve acceptable yields of methanol from synthesis gas it is customary to recompress unconverted synthesis gas and recycle it to the reactor inlet. This requires recycle gas compressors of large capacities, which are costly and have high power consumptions. Processes for the production of methanol by reacting a gaseous mixture comprising hydrogen and carbon monoxide in the presence of a fixed bed catalyst composition have been widely disclosed in the prior art.

Also it is known in the art to use the catalyst composition in the form of a fluidized bed (cf. a lecture by Y.Saito, M.Kuwa and O.Hashimoto during the 1987 Annual Meeting of the American Institute of Chemical Engineers, New York, November 15-20, 1978, entitled "Development: of a fluidized bed methanol synthesis process").

To use the catalyst in the form of a slurry of a heterogeneous catalyst in an inert liquid has been disclosed in Chemical Week, 36 (April 16, 1980), which concerns a process known as Chem. Systems' three-phase process in which an inert liquid was used to fluidize the catalyst and to remove the heat of reaction. Good conversions per pass are claimed. However, the inert liquid caused mass transport problems and affected the reaction rate.

Also there is Chemtech, October 1990, pages 624-629 disclosing a process known as Solvent Methanol Process (SMP) in which methanol is prepared from H2, CO and CO2 containing gas in a series of reactors with fixed catalyst beds and heat exchangers between the reactors and gas-liquid separators. A high boiling inert solvent is introduced into the reactor concurrently with the synthesis gas stream and the solvent absorbs methanol selectively. Product rich solvent is depressurized and flashing yields methanol. Although this process enables an equilibrium shift towards methanol it suffers from the same disadvantages as Chem Systems' three-phase process namely mass transport limitations affecting the reaction rate.The mass transport limitations are caused by the use of a high boiling inert solvent under trickle flow conditions with a catalyst of relatively large particle size in view of the pressure drop. When at least three reactors in series were used the gas recycling step could be eliminated.

Finally there is Ind. Eng. Chem. Res. 1989, 28, 763-771 disclosing a process for the synthesis of methanol using a series of packed bed tubular reactors with interstage absorbers to remove methanol. As the methanol absorbent tetraethyleneglycol dimethyl ether was used. A system comprising at least two sets of a reactor and an absorber and preferably four or more are required to avoid a gas recycle.

Disadvantages of this process are that high-pressure absorbers and low-pressure desorbers are required, while a large volume of solvent had to be depressurized and repressurized. Also capillary condensation in the catalyst is likely to occur.

An object of the present invention is the development of an industrial process for conducting chemical equilibrium reactions involving gaseous reactants and a liquid reaction product with high conversion percentages in relatively simple equipment.

The present invention provides a process for conducting chemical equilibrium reactions involving gaseous reactants using a plurality of fluidized catalyst bed reactors in series and an inert solvent to absorb a reaction product from the reaction mixture wherein between at least two consecutive fluidized bed reactors or reactor sections a facility is present effecting intimate contact between the inert solvent and the product-containing reaction mixture and which facility comprises a means effecting separation of a liquid reaction product/inert solvent solution and a gaseous phase.

After separation of the product/inert solvent solution and the gaseous phase the former is depressurized, and flashing and subsequent distillation yields the product, which is removed at each interstage. The inert solvent recovered is then recirculated for re-use in the reactors.

Because of the small catalyst particle size employed in the fluidized bed technique and the excellent mass transport characteristics of the fluidized bed, mass transfer limitations do not play a role in the process according to the present invention.

Also, by control of process conditions capillary condensation in the catalyst material can be avoided.

In a preferred embodiment of the invention the product removing facility comprises a swirl tube as e.g. disclosed in GB-A-1 194 203 or at least one tray with bubble caps as e.g.

disclosed in EP-A-O 082 301. Further alternatives are a rotating disc contactor or a short column with structured packing.

Combinations of facilities can also be used.

One of the major advantages which can be achieved according to the present invention is that by proper selection of conditions it is possible to avoid substantial contact of liquid with the catalyst, so that catalyst pores remain substantially open and that the occurrence of transport problems and/or diffusion limitations is avoided and a high conversion rate is maintained.

According to the invention between the reactors or reactor sections a product removing facility is present and the gaseous phase separated is fed into the next reactor. Reactors and product removing facilities can be separate pieces of equipment connected by tubing. However, in a further preferred embodiment of the invention at least one of the product removing facilities is integrated in one of the reactors and more preferably all of the product removing facilities and reactors (catalyst beds) are integrated in one big, often tubular, reactor.

In another embodiment the process is used for the preparation of methanol and at least one of the reactors comprises a fluidized bed reactor with upward flow of a gaseous mixture comprising hydrogen and carbon monoxide.

According to the present invention an inert solvent is used.

Suitable solvents are comprised by the group consisting of water, higher alcohols, higher ethers and higher esters. Suitable solvents in the practice of this invention have a boiling point around the area of temperature and pressure where the reaction is carried out.

Tetraethylene glycol dimethyl ether and water are examples of suitable solvents. Preferably the inert solvent is introduced in the product removing facility or in the tubing connecting this facility and the preceding reactor. Evaporation of the solvent withdraws part of the reaction heat and allows temperature control sometimes in combination with a heat exchanger. In case heat needs to be supplied to the reaction the temperature of the solvent fed into the reactor can be increased.

The gaseous mixture fed into the reactors comprises hydrogen and carbon monoxide in a molar ratio H2:CO - 1 to 3:1, preferably between 1.5 to 2.5:1, more preferably around 2:1. Synthesis gas obtained by partial oxidation of methane with oxygen (H2:CO - 2:1), is a recommended starting material, synthesis gas obtained by reforming of methane and/or carbon dioxide (H2:CO - 3:1) can also be used.

The reaction temperature depends on the activity of the catalyst composition employed. In the case of an active catalyst the temperature may be as low as 100 "C, but may be as high as 350 "C. Reaction temperatures from 175 to 300 C are preferred.

The total pressure in the reaction zone is usually in the range of from 3 to 35 MPa and again this is dependent on the activity of the catalyst composition employed and with active catalysts pressures below 10 MPa can be used. According to the present process conditions are chosen so in such a way as to achieve per pass over/through a catalyst bed conversions of at least 50%.

The total number of catalyst beds and product/methanol removing facilities in a specific plant is usually chosen in such a way that high overall conversions are achieved and that the remaining off-gas can be disposed of e.g. by burning so that build-up of contaminants as occurs in feed-back is avoided.

The catalyst compositions employed in the practice of this invention when preparing methanol or ethanol may suitably comprise as active elements e.g. copper/zinc optionally promoted with another element such as aluminium or chromium, a precious metal like palladium. Catalyst materials comprising copper and zinc optionally promoted with another element are preferred. When preparing methyl tertiary butyl ether or other tertiary ethers the catalyst may suitably comprise cation resins, especially exchangers containing sulfonic acid groups as can be obtained by polymerization or co-polymerization of aromatic vinyl compounds followed by sulfonation. Particularly suitable exchange resins are disclosed in German Patent Specification 908 247.

Particle size and distribution of the catalyst material must be suitable for fluidized bed or made so e.g. by grinding and sieving. A particle size below 0.4 millimetre is generally suitable. Also adequate resistance of the catalyst towards attrition is important.

According to the present invention the gaseous phase from which reaction product has been removed is directly fed into the next reactor (without reheating or recompressing).

On a practical scale it is recommended to use a series of reactors in which each next reactor has a smaller capacity than the previous one, alternatively it is feasible to use e.g. groups of parallel reaFtors in series wherein the number of parallel reactors in each next group (and consequently their total capacity) is smaller than in the previous group and where interstage removal of reaction product is effected between the groups of reactors.

According to a preferred embodiment of the invention high pressure steam can be generated by burning of the off-gas and can be used for purposes outside the present process, also it can be used for preheating the incoming gas feed which is preheated, preferably by means of a feed gas pre-heater located in the reactor.

A further advantage of the process according to the present invention due to the fluidized catalyst bed employed is that it is possible to replenish the catalyst while the reaction is operated, catalyst stability requirements can be sacrificed in favour of higher activity so that it is possible to carry out the reaction at an unusually high temperature providing additional room to improve space-time-yield.

The invention also relates to an apparatus for carrying out chemical equilibrium reactions comprising a plurality of reactors with a facility suitable for fluidizing a catalyst material with inlets and outlets for reactants, reaction product and inert solvent and between the reactors a facility permitting good contact between the inert solvent and product containing gas and which facility comprises a means for separation of the liquid reaction product/inert solvent solution and the gaseous phase. In a preferred embodiment at least one of the product removing facilities is integrated in one of the reactors.

The invention is further elucidated by the attached drawing, which represents a flowsheet of a tubular reactor particularly suitable for carrying out the process of the invention and which comprises a number (e.g. 3) of interconnected fluidized bed reactors R1, R2, and R3 in series each containing at least one facility for such a fluidized bed and a number (e.g. 3) product removing facilities S1, S2 and S3 (e.g. swirl tube or tray with bubble caps) in between, R1 has an inlet for incoming gas and R3 has an outlet for reaction mixture and S1, S2 and S3 each have an inlet for the inert solvent and an outlet for methanol/inert solvent solutions, which outlets are connected to depressurizing and flashing units.

Each of the reactors is filled with a suitable fluidizable catalyst and the incoming gasstreams in the reactors are properly distributed as to achieve fluidization of the catalyst and as to obtain a high contact efficiency between reacting gas and catalyst particles. By selecting suitable reaction conditions and an active catalyst, such as H2:CO = 67:32, T - 250-200 "C, P - 8 MPa and a reduced Cu-ZnO-Cr203-catalyst having a particle size below 0.1 millimetre, a degree of conversion of more than 60% per pass through a fluidized bed reactor may be obtained. The reaction is followed by interstage removal of methanol using the inert solvent as outlined above. The conditions (p,T) under which contacting of the reaction mixture with the inert solvent is most efficient are determined by the equilibrium characteristics of such a mixture and solvent.Furthermore the contacting efficiency of mixture and solvent must be high as can be achieved in a swirl tube. The remaining gaseous phase is then passed through another reactor, which reactor could be smaller than the first reactor, the procedure of interstage removal of methanol formed and using the gaseous phase as the feed gas for another reactor is repeated a number of times. After multiple stages an overall yield of at least 90% of the feed gas of the first reactor can thus be obtained. A methanol purity of over 98% can be obtained.

It is clear that in case the incoming feed gas of the first reactor is e.g. synthesis gas containing more than 2 moles of hydrogen per mole of carbon monoxide that after two or more passes through a reactor and interstage removal of methanol a hydrogen rich effluent gas is obtained. By applying membrane separation to this hydrogen rich effluent gas it is possible to obtain chemically pure hydrogen.

Claims (12)

1. A process for conducting chemical equilibrium reactions involving gaseous reactants using a plurality of catalytic reactors in series and an inert solvent to absorb a reaction product from the reaction mixture, wherein between at least two consecutive fluidized bed reactors a facility is present effecting intimate contact between the inert solvent and the product-containing reaction mixture and which facility comprises a means effecting a separation of a liquid reaction product/inert solvent solution and a gaseous phase.

2. A process according to claim 1, characterized in that, the facility comprises a swirl tube.

3. A process according to claim 1 or 2, characterized in that, the facility comprises at least one tray with bubble caps.

4. A process according to any of the preceding claims, characterized in that, at least one of the reaction product removing facilities is integrated in one of the reactors.

5. A process according to any of the claims 1-4, characterized in that, the reaction temperature is controlled by adjusting the flow and/or temperature of the solvent fed into the reactors.

6. A process according to any of the claims 1-5, characterized in that, methanol is prepared by catalytic reaction of a gaseous mixture comprising hydrogen and carbon monoxide.

7. A process according to any of the preceding claims characterized in that the inert solvent is a solvent of the group consisting of water, higher alcohols, higher ethers and higher esters.

8. A process according to any of the preceding claims, characterized in that, the catalyst composition in the bed comprises copper and zinc and is optionally promoted with another element.

9. A process according to any of the claims 6 to 8 characterized in that, the a gaseous mixture comprising hydrogen and carbon monoxide contains carbon monoxide and hydrogen in a molar ratio H2:CO - 1 to 3:1, preferably between 1.5 to 2.5:1.

10. A process according to any of the preceding claims, characterized in that, the operating conditions of the reactors are within the temperature range of 100 to 350, preferably 175 to 300 "C and the pressure within the range from 3 to 35 MPa.

11. An apparatus suitable for carrying out a process according to any of the preceding claims characterized in that, the apparatus comprises a plurality of reactors each having at least one facility suitable for fluidizing a catalyst material and inlets and outlets for reactants, reaction product and inert solvent and between at least two consecutive reactors a facility permitting intimate contact between the inert solvent and the product-containing reaction mixture and which facility comprises a means for separation of a liquid reaction product/inert solvent solution and a gaseous phase.

12. A process or apparatus essentially as herein described before, with particular reference to the Specification and/or Drawing.