Oligomerization process implementing the recycling of the gas headspace

1. A process for the oligomerization of ethylene, carried out at a pressure of between 0.1 and 10 MPa, at a temperature of between 30 and 200° C., in a sequence of at least two gas/liquid reactors placed in series, said process comprising the following steps:

a) a step of introducing a catalytic oligomerization system comprising at least one metal precursor, optionally at least one activating agent and optionally at least one additive into at least the first reactor of the sequence comprising a reaction chamber comprising a liquid phase in a lower zone and a gas phase in an upper zone,

b) a step of introducing gaseous ethylene into the lower part of the reaction chamber of at least the first reactor of the sequence,

c) for each reactor, a step of withdrawing a fraction of liquid phase from the lower part of the reaction chamber of the reactor, the liquid fraction being separated into two streams: a first stream corresponding to a first portion of the liquid fraction, termed main liquid fraction, which is sent to the cooling step c); a second stream corresponding to the second portion of the liquid fraction which constitutes the liquid feed of the reactor, located downstream in the sequence, unless the liquid fraction withdrawn is from the final reactor of the sequence, and for the final reactor of the sequence, the second stream corresponds to the effluent obtained at the end of the oligomerization process,

d) a step of introducing said second portion of the liquid fraction withdrawn from the reactor located upstream into the reaction chamber of the reactor located downstream in the direction of flow,

e) a step of cooling said main liquid fraction withdrawn from the reactor located upstream in step c) by passing said first portion of the liquid fraction into a heat exchanger in order to obtain a cooled liquid fraction,

f) a step of introducing said liquid fraction cooled in step e) at the top of the reaction chamber of said reactor, steps a) to f) being carried out for each reactor of the sequence, and

g) a step of recycling a gas fraction, withdrawn from the upper zone of the reaction chamber and introduced at the lower part of said reaction chamber into the liquid phase, said recycling step g) being carried out in at least the first reactor of the sequence.

2. The process as claimed in claim 1 , wherein the sequence of reactors uses from 2 to 10 gas/liquid reactors placed in series.

3. The process as claimed in claim 1 , wherein the gas phase withdrawn in step g) is introduced as a mixture with the gaseous ethylene introduced in step b).

4. The process as claimed in claim 1 , wherein the rate of withdrawal of the gaseous fraction in step g) is between 0.1 and 100% of the flow rate of gaseous ethylene introduced in step b).

5. The process as claimed in claim 1 , wherein the gaseous fraction withdrawn in step g) is introduced at the lateral lower part of the reaction chamber.

6. The process as claimed in claim 1 , wherein the rate of withdrawal of the gaseous fraction in step g) is controlled by the pressure within the reaction chamber.

7. The process as claimed in claim 1 , wherein a second purge gas stream is withdrawn from the gas phase.

8. The process as claimed in claim 7 , wherein the flow rate of the second gaseous stream is between 0.005 and 1.00% of the flow rate of ethylene introduced in step b).

9. The process as claimed in claim 1 , using a solvent and having a content by weight of solvent introduced into the sequence of reactors of between 0.5 and 10.0.

10. The process as claimed in claim 1 , wherein a gaseous hydrogen stream is introduced in step b) into the reaction chamber, with a flow rate representing 0.2 to 5.0% by weight of the flow rate of incoming ethylene.

11. The process as claimed in claim 1 , wherein the concentration of metal precursor in the catalytic system is between 0.1 and 50.0 ppm by weight of atomic metal relative to the reaction mass.

12. The process as claimed in claim 1 , wherein the catalytic oligomerization reaction is carried out continuously.

13. The process as claimed in claim 1 , wherein the oligomerization makes it possible to obtain linear olefins comprising from 4 to 20 carbon atoms.

14. A reaction device for implementing the process as claimed in claim 1 , wherein each of the reactors comprises the following elements:

a reaction chamber i), of elongate shape along the vertical axis, a lower zone and an upper zone above the lower zone,

an optional means ii) for introducing gaseous ethylene into said reaction chamber, the first reactor of the sequence always comprising said means ii) for introducing the gaseous ethylene,

an optional means iii) for introducing the catalytic system, the first reactor of the sequence always comprising said means iii) for introducing the catalytic system,

a recirculation loop iv) comprising a withdrawing means at the base of the reaction chamber for withdrawing a liquid fraction and a heat exchanger, and a means for introducing said cooled liquid into the upper part of the lower zone of the reaction chamber,

a gas phase recycle loop v), comprising a means for withdrawing a gas fraction at the level of the upper zone of the reaction chamber, and a means for introducing said withdrawn gas fraction into the lower zone of the reaction chamber,

with the exception of the first reactor, at least one means vi) for feeding a second portion of the liquid fraction withdrawn from the upstream reactor in the sequence,

at least the first reactor of the sequence always being fed with catalytic system and with gaseous ethylene.

15. The device as claimed in claim 14 , wherein the means for introducing the withdrawn gas fraction into the recycle loop corresponds to the means ii) for introducing the gaseous ethylene.

16. The device as claimed in claim 14 , wherein said one or more feed means is a pipe directly feeding the reaction chamber.

17. The device as claimed in claim 14 , wherein said one or more feed means is a pipe joining the recirculation loop used for the cooling step e).