OLIGOMERIZATION OF ETHENE IN SUPERCRITICAL MODE

Abstract

The invention is concerned with the oligomerization of supercritical ethene. An essential aspect of the invention is that of mixing ethene with an inert medium and setting the conditions in the reaction such that both ethene and inert medium are supercritical. This is because the solubility for ethene in the inert medium is greater in the supercritical state, such that more ethene is dissolved in the supercritical inert medium than in a liquid solvent. The process regime in the supercritical state therefore enables the use of a much higher proportion of ethene in a homogeneous mixture of ethene and inert medium than is possible on the basis of the thermodynamic solubility restriction in a purely liquid hydrocarbon stream. In this way, the space-time yield is distinctly enhanced. Since a greater amount of ethene can be passed into the reactor, it is possible as a result to better exploit the apparatus volume compared to a liquid phase process. The inert medium used may, for example, be isobutane.

Claims

1. A process for oligomerizing ethene, in which a mixture comprising ethene and an inert medium contacts a solid catalyst, characterized in that the proportion by weight of the inert medium in the mixture is greater than the proportion by weight of ethene in the mixture, the inert medium is an alkane or a cycloalkane, where the alkane or cycloalkane has not less than three and not more than seven carbon atoms, and pressure and temperature of the mixture are chosen with respect to the proportion of ethene in the mixture such that ethene and inert medium are in the supercritical state.

2. The process according to claim 1, wherein the mixture is at first provided in the supercritical state and then contacted with the solid catalyst, where the provision of the mixture in the supercritical state is affected either according to alternative a) or according to alternative b): a) inert medium is converted to the supercritical state by increasing the pressure and/or temperature and ethene is metered into the supercritical inert medium to obtain the supercritical mixture; b) inert medium and ethene are mixed to give the mixture, and the mixture is converted to the supercritical state by increasing the temperature and/or pressure.

3. The process according to claim 1, wherein the inert medium is one of the following substances: propane, isobutane, n-butane, isopentane, n-pentane, n-hexane, n-heptane, cyclopentane, cyclohexane, 2-methylpentane, 3-methylpentane, 2-methylhexane, 3-methylhexane, methylcyclopentane, methylcyclohexane.

4. The process according to claim 3, where the inert medium is isobutane, wherein the proportion of ethene in the mixture on commencement of contact with the catalyst is between 4% by weight and 30% by weight, the pressure of the mixture is adjusted to a value between 25*10.sup.5 Pa and 100*10.sup.5 Pa, and the temperature of the mixture is adjusted to a value between 90° C. and 200° C., with the proviso that both ethene and isobutane are in the supercritical state.

5. The process according to claim 4, wherein the mixture is provided as reactant mixture together with at least one accompanying substance, where the reactant mixture on commencement of contact with the catalyst has the following composition that adds up to 100% by weight: Isobutane: 70% by weight to 96% by weight; Ethene: 4% by weight to 30% by weight; Sum of all accompanying substances: more than 0% by weight to a maximum of 5% by weight.

6. The process according to claim 4, wherein a product mixture having the following composition that adds up to 100% by weight is drawn off from the catalyst: Isobutane: 70% by weight to 96% by weight; Ethene: 0% by weight to 2% by weight; Olefins having four carbon atoms: 2.3% by weight to 21% by weight; Olefins having six carbon atoms: 0.9% by weight to 7.2% by weight; Olefins having eight carbon atoms: 0.1% by weight to 6.3% by weight; Olefins having ten carbon atoms: 0% by weight to 3% by weight; Olefins having twelve carbon atoms: 0% by weight to 2.7% by weight; Sum of all other constituents: 0% by weight to 5% by weight.

7. The process according to claim 3, where the inert medium is n-hexane, wherein the proportion of ethene in the mixture on commencement of contact with the catalyst is between 5% by weight and 30% by weight, the pressure of the mixture is adjusted to a value between 25*10.sup.5 Pa and 100*10.sup.5 Pa, and the temperature of the mixture is adjusted to a value between 90° C. and 250° C., with the proviso that both ethene and n-hexane are in the supercritical state.

8. The process according to claim 1, wherein the catalyst is in the solid state and comprises at least two components, where the first component comprises at least one element selected from Ni, Cr, Fe, Ti which is in metallic and/or oxidic and/or hydridic form, and where the second component comprises at least one metal oxide selected from Al.sub.2O.sub.3, SiO.sub.2, TiO.sub.2, or ZrO.sub.2.

9. A combination of a process according to claim 1 with a process for regeneration of the catalyst, having the following features: a) the oligomerization is effected in an oligomerization operation in which inert medium and ethene, each in the supercritical state, are contacted with the catalyst; b) the regeneration is effected in a regeneration operation in which the catalyst in the absence of ethene, hydrogen and oxygen is purged with a liquid purge medium; c) operation is interchanged over time between the oligomerization operation and the regeneration operation, in such a way that a time-limited oligomerization operation is followed by a time-limited regeneration operation, and the latter in turn is followed by a time-limited oligomerization operation; d) the catalyst is always at the same location, which is the reason why both oligomerization operation and regeneration operation take place at this location; e) the location of the catalyst is supplied with positive or negative thermal energy in order to impose a set temperature thereon, it being entirely possible for the actual temperature at the catalyst to deviate from the set temperature in a time- and space-limited manner; f) the set temperature in regeneration operation is lower than the set temperature in oligomerization operation; and g) the pressure at the location of the catalyst is lower in regeneration operation than in oligomerization operation.

10. The combination according to claim 9, where the purge medium and inert medium used is the same substance, with the proviso that this substance is in the supercritical state in oligomerization operation and in the liquid state in regeneration operation.

11. The combination according to claim 10, where the purge medium and inert medium used is isobutane, wherein the pressure in oligomerization operation is adjusted to a value between 45*10.sup.5 Pa and 55*10.sup.5 Pa and the temperature in oligomerization operation is adjusted to a value between 125° C. and 155° C., and pressure and temperature are lowered in regeneration operation, namely to a pressure between 25*10.sup.5 Pa and 35*10.sup.5 Pa and to a temperature between 105° C. and 125° C.

12. The process according to claim 2, wherein the inert medium is one of the following substances: propane, isobutane, n-butane, isopentane, n-pentane, n-hexane, n-heptane, cyclopentane, cyclohexane, 2-methylpentane, 3-methylpentane, 2-methylhexane, 3-methylhexane, methylcyclopentane, methylcyclohexane.

13. The process according to claim 5, wherein a product mixture having the following composition that adds up to 100% by weight is drawn off from the catalyst: Isobutane: 70% by weight to 96% by weight; Ethene: 0% by weight to 2% by weight; Olefins having four carbon atoms: 2.3% by weight to 21% by weight; Olefins having six carbon atoms: 0.9% by weight to 7.2% by weight; Olefins having eight carbon atoms: 0.1% by weight to 6.3% by weight; Olefins having ten carbon atoms: 0% by weight to 3% by weight; Olefins having twelve carbon atoms: 0% by weight to 2.7% by weight; Sum of all other constituents: 0% by weight to 5% by weight.

14. The process according to claim 2, wherein the catalyst is in the solid state and comprises at least two components, where the first component comprises at least one element selected from Ni, Cr, Fe, Ti which is in metallic and/or oxidic and/or hydridic form, and where the second component comprises at least one metal oxide selected from Al.sub.2O.sub.3, SiO.sub.2, TiO.sub.2, or ZrO.sub.2.

15. The process according to claim 3, wherein the catalyst is in the solid state and comprises at least two components, where the first component comprises at least one element selected from Ni, Cr, Fe, Ti which is in metallic and/or oxidic and/or hydridic form, and where the second component comprises at least one metal oxide selected from Al.sub.2O.sub.3, SiO.sub.2, TiO.sub.2, or ZrO.sub.2.

16. The process according to claim 4, wherein the catalyst is in the solid state and comprises at least two components, where the first component comprises at least one element selected from Ni, Cr, Fe, Ti which is in metallic and/or oxidic and/or hydridic form, and where the second component comprises at least one metal oxide selected from Al.sub.2O.sub.3, SiO.sub.2, TiO.sub.2, or ZrO.sub.2.

17. A combination of a process according to claim 2 with a process for regeneration of the catalyst, having the following features: a) the oligomerization is effected in an oligomerization operation in which inert medium and ethene, each in the supercritical state, are contacted with the catalyst; b) the regeneration is effected in a regeneration operation in which the catalyst in the absence of ethene, hydrogen and oxygen is purged with a liquid purge medium; c) operation is interchanged over time between the oligomerization operation and the regeneration operation, in such a way that a time-limited oligomerization operation is followed by a time-limited regeneration operation, and the latter in turn is followed by a time-limited oligomerization operation; d) the catalyst is always at the same location, which is the reason why both oligomerization operation and regeneration operation take place at this location; e) the location of the catalyst is supplied with positive or negative thermal energy in order to impose a set temperature thereon, it being entirely possible for the actual temperature at the catalyst to deviate from the set temperature in a time- and space-limited manner; f) the set temperature in regeneration operation is lower than the set temperature in oligomerization operation; and g) the pressure at the location of the catalyst is lower in regeneration operation than in oligomerization operation.

18. A combination of a process according to claim 3 with a process for regeneration of the catalyst, having the following features: a) the oligomerization is effected in an oligomerization operation in which inert medium and ethene, each in the supercritical state, are contacted with the catalyst; b) the regeneration is effected in a regeneration operation in which the catalyst in the absence of ethene, hydrogen and oxygen is purged with a liquid purge medium; c) operation is interchanged over time between the oligomerization operation and the regeneration operation, in such a way that a time-limited oligomerization operation is followed by a time-limited regeneration operation, and the latter in turn is followed by a time-limited oligomerization operation; d) the catalyst is always at the same location, which is the reason why both oligomerization operation and regeneration operation take place at this location; e) the location of the catalyst is supplied with positive or negative thermal energy in order to impose a set temperature thereon, it being entirely possible for the actual temperature at the catalyst to deviate from the set temperature in a time- and space-limited manner; f) the set temperature in regeneration operation is lower than the set temperature in oligomerization operation; and g) the pressure at the location of the catalyst is lower in regeneration operation than in oligomerization operation.

19. A combination of a process according to claim 4 with a process for regeneration of the catalyst, having the following features: a) the oligomerization is effected in an oligomerization operation in which inert medium and ethene, each in the supercritical state, are contacted with the catalyst; b) the regeneration is effected in a regeneration operation in which the catalyst in the absence of ethene, hydrogen and oxygen is purged with a liquid purge medium; c) operation is interchanged over time between the oligomerization operation and the regeneration operation, in such a way that a time-limited oligomerization operation is followed by a time-limited regeneration operation, and the latter in turn is followed by a time-limited oligomerization operation; d) the catalyst is always at the same location, which is the reason why both oligomerization operation and regeneration operation take place at this location; e) the location of the catalyst is supplied with positive or negative thermal energy in order to impose a set temperature thereon, it being entirely possible for the actual temperature at the catalyst to deviate from the set temperature in a time- and space-limited manner; f) the set temperature in regeneration operation is lower than the set temperature in oligomerization operation; and g) the pressure at the location of the catalyst is lower in regeneration operation than in oligomerization operation.

20. The combination according to claim 17, where the purge medium and inert medium used is the same substance, with the proviso that this substance is in the supercritical state in oligomerization operation and in the liquid state in regeneration operation.

Description

[0070] The process is now to be elucidated in detail with reference to examples. The corresponding graphs show:

[0071] FIG. 1: Ethylene oligomerization under supercritical conditions (for Example 1)

[0072] FIG. 2: Product distribution from ethylene oligomerization under supercritical conditions (for Example 1)

[0073] FIG. 3: Ethylene oligomerization under supercritical conditions and in situ regeneration by means of liquid isobutane in alternation (for Example 2)

[0074] FIG. 4: Product distribution from ethylene oligomerization under supercritical conditions and in situ regeneration by means of liquid isobutane in alternation (for Example 2)

[0075] FIG. 5: Example of ethylene oligomerization under supercritical conditions (for Example 3)

[0076] FIG. 6: Product distribution from ethylene oligomerization under supercritical conditions (for Example 3)

[0077] FIG. 7: Ethylene oligomerization under supercritical conditions and in situ regeneration by means of liquid isobutane in alternation (for Example 4)

[0078] FIG. 8: Product distribution from ethylene oligomerization under supercritical conditions and in situ regeneration by means of liquid isobutane in alternation (for Example 4)

[0079] FIG. 9: Ethylene oligomerization below the critical pressure of the feed mixture (for Counterexample 5)

[0080] FIG. 10: Product distribution from ethylene oligomerization below the critical pressure of the feed mixture (for Counterexample 5)

[0081] FIG. 11: Calculated critical data of a binary mixture of isobutane and ethene.

EXAMPLE 1: ETHYLENE OLIGOMERIZATION IN ISOBUTANE UNDER SUPERCRITICAL CONDITIONS

[0082] 12.6 g of a heterogeneous catalyst based on nickel, titanium dioxide and silica-alumina (prepared according to WO9514647A1) were introduced into an externally oil-heated and -cooled tubular reactor of length 1 m and internal diameter 6 mm. Subsequently, a mixture of 4% by mass to 15% by mass of ethene and 85% by mass to 96% by mass of isobutane, after the pressure had been increased to 50 bar, was run continuously over the catalyst at a total flow rate of 120 g/h at a temperature of 140° C. (weight hourly space velocity WHSV=9.5 h.sup.−1 based on the overall feed). The pressure was kept constant at 50 bar (50 bar corresponds to 50*10.sup.5 Pa). After a time of approximately 23 hours, a state in which there was no further change in conversion had been attained.

[0083] FIG. 1 shows the conversion plot including the proportion of ethylene fed in as a function of the run time of the reaction. The ethene conversion is marked by the solid dot •, the starting concentration of ethene by a dash -, and the temperature by a triangle Δ.

[0084] Over the course of 600 h, a relatively constant ethylene conversion of about 95% was recorded. Only in the case of an ethylene content of more than 12% in the feed was there minimal deactivation, and this progressed ever further, such that, after a further run time of nearly 300 h, the conversion dropped to 91% and the regeneration of the catalyst was commenced.

[0085] In FIG. 2, the selectivities achieved were plotted. The diamond ⋄ marks the selectivity for the butenes, the triangle Δ the selectivity for the hexenes, the cross X the selectivity for the octenes, and the circle ∘ the selectivity for the C.sub.10+ olefins.

[0086] As apparent from FIG. 2, over the entire run time of the experiment, a slightly falling butene selectivity was observed, commencing at 65%. It reached a value of about 57% after a run time of about 1000 h. The further product fractions, by contrast, showed a slight rise in their selectivity of about 2 to 4 percentage points in each case.

EXAMPLE 2: ETHYLENE OLIGOMERIZATION UNDER SUPERCRITICAL CONDITIONS AND IN SITU REGENERATION BY MEANS OF THE LIQUID INERT MEDIUM ISOBUTANE IN ALTERNATION

[0087] For regeneration, the experiment with the catalyst from Example 1 was continued. The ethene feed was stopped and the total flow rate was increased to 167 g/h to accelerate the purging operation. The temperature was lowered to 120° C. and the pressure to 30 bar (30*10.sup.5 Pa). After a respective regeneration interval of 75 h to 98 h, the fine filter that was kept at room temperature and 30 bar beyond the plant was checked and changed if required. Then ethylene was mixed in again, and the total flow rate was lowered to 120 g/h, while the temperature was increased to 140° C. and the pressure to 50 bar. This operation was repeated several times as soon as the ethylene conversion had dropped noticeably after a run time of 130 h to 188 h in each case, or the installed filter suggested polymer deposits by the indication of an increase in the pressure differential. Overall, 94 mg of polyethylene were recovered from the filter, mainly as fine powder, over the course of the experiment, which accounts for much less than 0.01% of the total amount of ethylene used.

[0088] FIG. 3 shows the regeneration intervals described in the liquid phase (intervals at 120° C.) in alternation with intervals of ethene oligomerization under supercritical conditions (intervals at 140° C.). The ethene conversion is again marked by the solid dot •, the starting concentration of ethene by a dash -, and the temperature by a triangle Δ.

[0089] The selectivities achieved were plotted in the graph of FIG. 4. The diamond 0 marks the selectivity for the butenes, the triangle Δ the selectivity for the hexenes, the cross X the selectivity for the octenes, and the circle ∘ the selectivity for the C.sub.10+ olefins. FIG. 4 shows that the regeneration in conjunction with the supercritical mode of operation enables restoration of the initial selectivities from Example 1. The butene selectivity increases again over the course of the run time from 56% to 63%, while the selectivities for the hexene and octene fractions each fall by 2 percentage points. The advantage of the supercritical mode of operation becomes particularly clear in the C.sub.10+ fraction; the selectivity value falls considerably from 6% to 1%.

Comparison of Examples 1 and 2

[0090] Examples 1 and 2 show that it is possible to conduct ethene oligomerization under supercritical conditions with an ethylene content up to 15% over a long period at very high conversions of 80% to 99% continuously over a notable run time of more than 2000 h when the reaction is effected alternately with regeneration intervals in the liquid phase, the regeneration time and frequency depending on the ethylene content in the mixture and being about 14% of the total run time in the present example. As shown, the regeneration can be implemented without the necessity for dismantling of the reactor, but in a simple manner in situ by purging with liquid inert medium after the ethylene has been shut down and pressure and temperature have been lowered. Moreover, it is apparent from Example 2 that the supercritical mode of operation in combination with regular regeneration is capable of restoring and maintaining the very good initial selectivities.

EXAMPLE 3: ETHYLENE OLIGOMERIZATION IN ISOBUTANE UNDER SUPERCRITICAL CONDITIONS

[0091] 19.3 g of a heterogeneous catalyst based on nickel and silica-alumina (prepared according to U.S. Pat. No. 2,581,228) were introduced into a tubular reactor which had a length of 1 m and an internal diameter of 6 mm and whose temperature was controlled from the outside by means of oil. Subsequently, a mixture of 4% by mass to 17% by mass of ethene and 83% by mass to 96% by mass of isobutane, after the pressure had been increased to 50 bar, was run continuously over the catalyst at a total flow rate of 125 g/h at a temperature of 140° C. at a weight hourly space velocity (WHSV) of 6.5 h.sup.−1 based on the overall feed. The pressure was kept constant at 50 bar.

[0092] FIG. 5 shows the conversion plot as a function of the run time of the reaction and the proportion of ethylene fed in the reactor. The ethene conversion is marked by the solid dot •, the starting concentration of ethene by a dash -, and the temperature by a triangle Δ.

[0093] After slight initial deactivation of the catalyst, a relatively constant ethylene conversion of about 95% was recorded over 860 h. Only in the case of an ethylene content of more than 13% in the feed was there minimal deactivation, and this progressed ever further, such that, after a further run time of nearly 144 h, the conversion dropped to 92% and the regeneration of the catalyst was commenced.

[0094] In FIG. 6, the selectivities achieved in Experiment 3 were plotted. The diamond 0 marks the selectivity for the butenes, the triangle Δ the selectivity for the hexenes, the cross X the selectivity for the octenes, and the circle ∘ the selectivity for the C.sub.10+ olefins. It is apparent from FIG. 6 that the product selectivities are at a comparable level to Example 1. The evolution thereof over the run time is also virtually identical.

EXAMPLE 4: ETHYLENE OLIGOMERIZATION UNDER SUPERCRITICAL CONDITIONS AND IN SITU REGENERATION BY MEANS OF THE LIQUID INERT MEDIUM ISOBUTANE IN ALTERNATION

[0095] For regeneration, the experiment with the catalyst from Example 3 was continued. The ethylene feed was stopped and the total flow rate was increased to 167 g/h to accelerate the purging operation. The temperature was lowered to 120° C. and the pressure to 30 bar. After a respective regeneration interval of 50 h to 60 h, the fine filter that was kept at room temperature and 30 bar beyond the plant was checked and changed if required. Then ethylene was mixed in again, and the total flow rate was lowered to 120 g/h, while the temperature was increased to 140° C. and the pressure to 50 bar. The mixture varied between 15% by mass and 25% by mass of ethene and 75% by mass to 85% by mass of isobutane. The regeneration operation was repeated several times as soon as the ethylene conversion had dropped noticeably after a run time of 50 h to 160 h in each case, or the installed filter suggested polymer deposits by the indication of an increase in the pressure differential. Overall, 350 mg of polyethylene were recovered from the filter, mainly as fine powder, over the course of the experiment, which accounts for much less than 0.01% of the total amount of ethylene used.

[0096] FIG. 7 shows the regeneration intervals described in the liquid phase (intervals at 120° C.) in alternation with intervals of ethylene oligomerization under supercritical conditions (intervals at 140° C.). The ethene conversion is marked by the solid dot •, the starting concentration of ethene by a dash -, and the temperature by a triangle Δ.

[0097] FIG. 8 shows the corresponding product distribution between the regeneration cycles. The diamond 0 marks the selectivity for the butenes, the triangle Δ the selectivity for the hexenes, the cross X the selectivity for the octenes, and the circle ∘ the selectivity for the C.sub.10+ olefins. As apparent, it is possible by means of the combination of supercritical mode and regeneration to keep the product distribution at least constant even in the case of very high ethylene contents in the feed.

Discussion of Examples 3 and 4

[0098] Examples 3 and 4 show that it is likewise possible to conduct ethylene oligomerization under supercritical conditions with an ethylene content of more than 15% and of up to 23% over a long period at very high conversions of 80% to 99% and constant product selectivities continuously over a notable run time of more than 1550 h when the reaction is effected alternately with regeneration intervals in the liquid phase, the regeneration time and frequency of the regeneration depending on the ethylene content in the mixture and being about 4% of the total run time in the present example. As shown, the regeneration can be implemented without the necessity for dismantling of the reactor, but in a simple manner in situ by purging with liquid inert medium after the ethylene has been shut down and pressure and temperature have been lowered.

COUNTEREXAMPLE 5: ETHYLENE OLIGOMERIZATION OUTSIDE THE SUPERCRITICAL RANGE AS A RESULT OF TOO HIGH AN ETHYLENE CONCENTRATION

[0099] After a regeneration phase for 50 h under the same conditions as in the previous examples, the experiment was continued with the catalyst from Examples 3 and 4. Then ethylene was mixed in again, and the total flow rate was lowered to 120 g/h, while the temperature was increased to 140° C. and the pressure to 50 bar. The feed mixture contained 25% by mass of ethene and 75% by mass of isobutane. The conversion fell significantly to 82% within 5 h. This rapid deactivation is the result of the pressure being below the critical pressure, which is about 54 bar with this proportion of ethylene in the mixture with isobutane. Moreover, within this very short period of time, in parallel with the conversion, the selectivity for butenes also dropped from 60% to 56%, while there was a distinct increase in the C.sub.10+ fraction from 6% to 9%. It was thus shown that compliance with supercritical conditions is essential for high activity, stability and selectivity of the process. FIGS. 9 and 10 show the ethene oligomerization interval described from a run time of 1590 h outside the critical phase. The marking symbols were chosen in accordance with the other graphs.

Discussion of Counterexample 5

[0100] Counterexample 5 shows that it is crucial to conduct ethylene oligomerization under supercritical conditions of the feed mixture. Specifically when the pressure of the mixture is below the critical pressure, the catalyst is deactivated rapidly with simultaneous significant loss of selectivity for the short-chain desired products having four to eight carbon atoms within a few hours.

[0101] With increasing ethene content in the feed mixture, there is a rise in the critical pressure of the mixture with a maximum around 50% by weight of ethene. This relationship is also apparent in FIG. 11. The critical pressures (diamond ⋄) and critical temperatures (solid circle ) were calculated with the Aspen Properties V7.3 software from Aspen Technology. The thermodynamic properties of ethene and isobutane that are required for the calculation come from the APV73 material database with PURE25, based on the release of the DIPPR database (January 2010).