DEHYDROGENATION OF OLEFIN-RICH HYDROCARBON MIXTURES

Abstract

The object of the invention is to specify a process for dehydrogenating alkanes in which such feedstock mixtures may be used having a high proportion of olefins, i.e. approximately 1% by weight to 10% by weight. Specifically, alkenes having two to five carbon atoms should be generated from alkanes having the same chain length and therefore the number of carbon atoms should not be changed by the dehydrogenation. The process is intended to be feasible on an industrial scale. A basic concept of the invention consists of hydrogenating alkenes present in the feedstock mixture to the corresponding alkanes before they come into contact with the dehydrogenation catalyst. An undesired coke deposit is thus avoided. The hydrogenation is effected by minimal addition of hydrogen (80% to 120% of the stoichiometrically required amount). The hydrogenation is effected either over a hydrogenation catalyst specifically provided therefor, which differs from the dehydrogenation catalyst, or over the dehydrogenation catalyst itself.

Claims

1. Process for preparing alkenes by dehydrogenation of alkanes having the following steps: a) providing a liquid feedstock mixture at a pressure between 0.1*10.sup.5 Pa and 6.0*10.sup.5 Pa, wherein the feedstock mixture comprises alkanes having two to five carbon atoms and alkenes having two to five carbon atoms, and wherein the part by mass of alkenes in the feedstock mixture based on the total mass thereof is 1% by weight to 10% by weight; b) evaporating the feedstock mixture by increasing the temperature; c) adding hydrogen to the evaporated feedstock mixture such that the molar ratio of hydrogen to the alkenes present in the feedstock mixture is between 0.8:1 and 1.2:1; d1) contacting the evaporated, hydrogen-containing feedstock mixture with a solid catalyst at a temperature between 450 C. and 760 C. and a pressure of 0.1*10.sup.5 Pa to 6.0*10.sup.5 Pa to obtain a product mixture, wherein the part by mass of the alkenes having two to five carbon atoms in the product mixture based on the total mass thereof is 30% by weight to 70% by weight; or d2) contacting the evaporated, hydrogen-containing feedstock mixture with a first solid catalyst and a pressure between 0.1*10.sup.5 Pa and 6*10.sup.5 Pa to obtain an intermediate, wherein the part by mass of the alkenes in the intermediate based on the total mass thereof is 0% by weight to 1% by weight and wherein the temperature of the evaporated, hydrogen-containing feedstock mixture and/or of the intermediate is increased; and e) contacting the intermediate with a second solid catalyst at a temperature between 450 C. and 760 C. and a pressure of 0.1*10.sup.5 Pa to 6.0*10.sup.5 Pa to obtain a product mixture, wherein the part by mass of the alkenes having two to five carbon atoms in the product mixture based on the total mass thereof is 30% by weight to 70% by weight.

2. Process according to claim 1, characterized in that the first catalyst and the second catalyst are identical.

3. Process according to claim 2, characterized in that the catalyst comprises a support material and at least one element from groups 8, 9 and 10 of the Periodic Table of the Elements according to IUPAC convention.

4. Process according to claim 2, characterized in that the catalyst comprises a support material and at least tin and/or zinc.

5. Process according to claim 3, characterized in that the support material is silicon dioxide or aluminium oxide or a mixture of silicon dioxide and aluminium oxide.

6. Process according to claim 3, characterized in that the support material is an aluminate which has been formed from aluminium oxide and an alkaline earth metal.

7. Process according to claim 3, characterized in that the support material is hydrotalcite.

8. Process according to claim 1, characterized in that the first catalyst and the second catalyst are different.

9. Process according to claim 8, characterized in that the first catalyst comprises a support material and at least one element applied thereto selected from the group consisting of nickel, platinum and palladium.

10. Process according to claim 8, characterized in that the support material is silicon dioxide or aluminium oxide or a mixture of silicon dioxide and aluminium oxide.

11. Process according to claim 1, characterized in that the feedstock mixture has the following composition adding up to 100% by weight: Propane: 0% by weight to 50% by weight; Isobutane: 0% by weight to 100% by weight; n-Butane: 0% by weight to 100% by weight; Propene: 0% by weight to 10% by weight; Isobutene: 0% by weight to 10% by weight; n-Butene: 0% by weight to 10% by weight; sum of other substances: 0% by weight to 5% by weight.

12. Process according to claim 1, conducted in an apparatus having a heating zone and a reaction zone, wherein the first catalyst is arranged in the heating zone and the second catalyst is arranged in the reaction zone, and wherein the feedstock mixture or the intermediate is heated in the heating zone so that it enters the reaction zone at a temperature between 450 C. and 760 C.

13. Process according to claim 1, characterized in that it is carried out isobarically.

14. Apparatus for carrying out a process according to claim 12, comprising an intake for the feeding of a liquid feedstock mixture, which is under a pressure between 0.1*10.sup.5 Pa and 6.0*10.sup.5 Pa, an evaporator for evaporating the feedstock mixture by increasing the temperature thereof, a component for metering hydrogen into the evaporated feedstock mixture, a heating zone for heating the evaporated feedstock mixture or an intermediate resulting therefrom, means of heating the heating zone, a reaction zone for contacting the intermediate with a second solid catalyst in which the second solid catalyst is arranged and means of heating the reaction zone to a temperature between 450 C. and 760 C., characterized in that, in the heating section, a first solid catalyst is arranged and that the entire apparatus is designed for a pressure between 0.1*10.sup.5 Pa and 6.0*10.sup.5 Pa.

Description

[0067] The invention will now be explained in more detail by reference to a simplified process flow diagram.

[0068] FIG. 1: Process flow diagram of the process according to the invention.

[0069] The feedstock mixture Feed is supplied in liquid form specifically at the pressure level of the subsequent dehydrogenation.

[0070] In an evaporator 1, the feedstock mixture is evaporated. This is accomplished by heating. In a component 2, hydrogen H.sub.2 is metered into the evaporated feedstock mixture, specifically as precisely as possible the molar (stoichiometric) amount corresponding to the alkenes present in the feedstock mixture. The gaseous feedstock mixture enriched with hydrogen is then brought into contact with a first solid catalyst 3. Here, the alkenes present in the feedstock mixture Feed are hydrogenated with the hydrogen H.sub.2 fed in to give the corresponding alkanes. Here, an intermediate Intermediate is obtained whose alkene proportion is now below 1% by weight.

[0071] Subsequently, the intermediate is now brought to a temperature level (450 C. to 760 C.) required for the dehydrogenation in a heat exchanger 4. It is also possible, however, to already preheat during the hydrogenation over the first catalyst 3 as long as the equilibrium reaction is not thereby shifted in the direction of dehydrogenation. It should also be noted that heat of reaction is already released by the exothermic hydrogenation which flows into the intermediate.

[0072] Since the temperature of the feedstock mixture or the intermediate resulting therefrom is already increased by the metered addition of hydrogen in component 2 until in the heat exchanger 4, this region is also interpreted as a heating zone.

[0073] The intermediate is now subjected to a dehydrogenation by contact over a second catalyst 5 such that a product mixture Product is formed, which comprises again a high proportion of alkenes (30 to 70% by weight). This is accomplished in a reaction zone of the process which follows the heating zone. The boundary between heating zone and reaction zone is at the point where the temperature is high such that dehydrogenation is favoured thermodynamically over hydrogenation. Since the dehydrogenation is endothermic, appropriate means to heat the reaction zone are required; a gas burner for example (not shown).

[0074] The first catalyst 3 and the second catalyst 5 may be different or identical. The heating zone and reaction zone may be separate apparatuses or be integrated. In an isobaric procedure, they are distinguished by the temperature.

LIST OF REFERENCE SYMBOLS

[0075] 1: evaporator [0076] 2: component [0077] 3: first solid catalyst [0078] 4: heat exchanger [0079] 5: second solid catalyst [0080] Feed: feedstock mixture [0081] Intermediate: intermediate [0082] Product: product mixture