METHOD AND APPARATUS FOR SEPARATING CARBON DIOXIDE FROM A RESIDUAL GAS IN A FLUIDISED BED CATALYTIC CRACKING PLANT (FCC)

Abstract

A process for separating carbon dioxide from a waste gas of a fluid catalytic cracking installation including converting at least a portion of the carbon monoxide of the waste gas into carbon dioxide to form a flow enriched in carbon dioxide, separating at least a portion of the flow enriched in carbon dioxide to form a gas enriched in carbon dioxide and depleted in nitrogen and a gas rich in nitrogen and depleted in carbon dioxide, and at least a portion of the gas enriched in carbon dioxide and depleted in nitrogen is separated by way of separation at a temperature of less than 0° C. to form a fluid rich in carbon dioxide and a fluid depleted in carbon dioxide and sending a gas containing at least 90% oxygen to combustion.

Claims

1.-14. (canceled)

15. A process for separating carbon dioxide from a waste gas of a fluid catalytic cracking installation containing carbon dioxide, nitrogen and carbon monoxide, the process comprising: i) converting at least a portion of the carbon monoxide of the waste gas into carbon dioxide to form a flow enriched in carbon dioxide by combustion, ii) separating at least a portion of the flow enriched in carbon dioxide from step i) by adsorption to form a gas enriched in carbon dioxide and depleted in nitrogen and a gas rich in nitrogen and depleted in carbon dioxide, and at least a portion of the gas enriched in carbon dioxide and depleted in nitrogen is separated in a separation apparatus by way of separation at a temperature of less than 0° C. by partial condensation and/or by distillation to form a fluid rich in carbon dioxide and a fluid depleted in carbon dioxide, and iii) sending a gas containing at least 90% oxygen to the combustion of step i) as a gas containing at least 40% carbon dioxide, consisting either of a product of the separation of the flow enriched in carbon dioxide, or of a portion of the flow enriched in carbon dioxide.

16. The process as claimed in claim 15, wherein the gas rich in nitrogen and depleted in carbon dioxide contains less than 5 mol % CO.sub.2.

17. The process as claimed in claim 15, wherein the fluid rich in carbon dioxide contains more than 45 mol % CO.sub.2.

18. The process as claimed claim 15, wherein the fluid depleted in carbon dioxide is compressed and mixed with the flow enriched in carbon dioxide sent to step ii).

19. The process as claimed in claim 15, wherein the gas rich in nitrogen and depleted in carbon dioxide is expanded in a turbine and is sent to the atmosphere.

20. The process as claimed in claim 19, wherein the gas rich in nitrogen and depleted in carbon dioxide is heated by a fluid originating from the catalytic cracking installation or by at least a portion of the gas enriched in carbon dioxide and depleted in nitrogen or by at least a portion of the waste gas, or at least a portion of the flow enriched in carbon dioxide.

21. The process as claimed in claim 15, wherein the waste gas is expanded upstream of step i) in a turbine and the flow enriched in carbon dioxide is compressed in a compressor driven by the turbine upstream of step ii).

22. The process as claimed in claim 21, wherein an electricity generator and/or a motor is mounted on the same shaft or the same speed-increasing gearing as the compressor for the flow enriched in carbon dioxide upstream of step ii) and the turbine for the waste gas.

23. The process as claimed in claim 15, wherein the fluid depleted in carbon dioxide and/or the gas rich in nitrogen and depleted in carbon dioxide is expanded in a turbine to a temperature of less than 0° C. and greater than −100° C. in a chamber containing the separation apparatus.

24. The process as claimed in claim 15, wherein at least a portion of the gas rich in nitrogen and depleted in carbon dioxide and/or at least a portion of the fluid rich in carbon dioxide and/or at least a portion of the fluid depleted in carbon dioxide is/are sent to the combustion of the carbon monoxide.

25. The process as claimed in claim 15, wherein the waste gas leaving a regenerator of the unit contains more than 50% nitrogen.

26. An apparatus for separating carbon dioxide from a waste gas of a fluid catalytic cracking installation containing carbon dioxide, nitrogen and carbon monoxide, comprising: a converter configured to convert at least a portion of the carbon monoxide of the waste gas into carbon dioxide to form a flow enriched in carbon dioxide by combustion, a separation unit for separating at least a portion of the flow enriched in carbon dioxide, comprising an adsorption separation unit configured to form a gas enriched in carbon dioxide and depleted in nitrogen and a gas rich in nitrogen and depleted in carbon dioxide, a separation apparatus for separating at least a portion of the gas enriched in carbon dioxide and depleted in nitrogen by separation at a temperature of less than 0° C. by partial condensation and/or by distillation, to form a fluid rich in carbon dioxide and a fluid depleted in carbon dioxide. a means for sending a gas containing at least 90% oxygen to the converter to take part in the combustion and a means for sending to the converter a gas containing at least 40% carbon dioxide, consisting either of a product of the separation of the flow enriched in carbon dioxide, or of a portion of the flow enriched in carbon dioxide.

27. The apparatus as claimed in claim 26, wherein the adsorption separation unit is a PSA or a VPSA.

28. The apparatus as claimed in claim 26, further comprising a fluid catalytic cracking unit comprising a reactor and a regenerator.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0043] For a further understanding of the nature and objects for the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein:

[0044] FIG. 1 represents a process according to the invention.

[0045] FIG. 2 represents a process according to the invention.

[0046] FIG. 3 represents a process according to the invention where a gas depleted in carbon dioxide or enriched in carbon dioxide produced by separating a gas originating from a carbon monoxide converter is recycled to this converter.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0047] The unit 1 is a fluid catalytic cracking unit of a refinery, It comprises a reactor 45 and a regenerator 43. In this process, the catalyst flows continuously between the reactor 45 and the regenerator 43 into which combustion air 49 is blown, and then returns to the reactor 45 after having been freed of the coke which has accumulated on the catalyst during the reaction. The reactor 45 is fed with steam 53.

[0048] The air 49 may possibly be enriched in oxygen so as to contain at most 30% oxygen, without significantly modifying the structure of the unit 1 and the processes performed in the unit 1.

[0049] After passage of the feedstocks 51 into the reactor 45, the effluents are guided to the main fractionating tower 47 in order to form the products 55 of the FCC.

[0050] The bottom liquid of the tower 47 is sent to a particulate separator 57. The functioning of the unit 1 is well known per se and for more details reference may be made to “Fluid Catalytic Cracking Technology and Operations” by Wilson, 1997, or “Fluid Catalytic Cracking Handbook” by Sadeghbeigi, 2000.

[0051] The waste gas 3 extracted from the regenerator 43 contains carbon monoxide, carbon dioxide and nitrogen, and also dust. After separation of the dust in the filter 5, a purified gas 7 is produced containing for example 80% nitrogen, 12.5% carbon dioxide and 7.5% carbon monoxide. This gas 7 is at 3.5 bar and 650° C. and is expanded in a turbine 9 or in a valve to a pressure dose to atmospheric pressure and a temperature of approximately 450° C. This expanded gas 11 is sent to a conversion unit 13 referred to as a “CO boiler”, where the carbon monoxide in the gas 11 is converted into carbon dioxide, at least partially by combustion with oxygen 41. The gas flow 41 contains at least 90% oxygen, or even at least 99.5% oxygen and can come from an air distillation separation apparatus.

[0052] The oxygen 41 is preferably mixed upstream of the combustion with a gas flow 39 containing at least 40% carbon dioxide and/or with a recycle of flue gas (=waste at the output of the CO converter). In FIG. 1, this flow constitutes a portion of the gas 29 produced by the adsorption unit 27 and intended for the separation unit 30. In this example, the flow 39 is at 1.05 bar and contains 67% CO.sub.2. At least a portion of the fluid 39 is mixed with oxygen 41 to form the oxidant used by the conversion unit 13.

[0053] Alternatively, another flow containing at least 40% carbon dioxide derived from the flow 17, 21 can replace the flow 39 or be mixed therewith,

[0054] In addition or alternatively, a portion of the fluid 35 and/or at least a portion of the fluid 37 may be sent to the conversion unit 13.

[0055] In FIG. 2, the flow 39 is a recycle which consists of a portion of the gas 17 taken at the outlet of the converter 13 intended for the separation unit 27. In this example, the flow 39 is at 1.05 bar and contains 20% CO.sub.2. A portion 39 of the fluid 17 is mixed with oxygen 41 to form the oxidant used by the conversion unit 13.

[0056] The conversion process also produces steam.

[0057] In addition to producing a waste 17 which is richer in carbon dioxide than a converter according to the prior art, the process makes it possible to increase the temperature of the flame in the converter 13 (while having an oxidant which is richer in O.sub.2), which makes it possible to produce steam at a higher pressure and/or at a higher temperature while at the same time increasing the efficiency of the production of steam. The unit 27 and the unit 30 of FIG. 2 will be all the smaller as the waste gas is enriched in carbon dioxide.

[0058] Thus, produced in FIG. 2 is a gas 17 at atmospheric pressure and around 120° C. containing 26% CO.sub.2, 3% oxygen and 71% nitrogen. The gas 17 is possibly filtered and then compressed in a compressor 19 coupled to the turbine 9 to produce a compressed gas 21.

[0059] An electricity generator and/or a motor may also be provided on the same shaft as the compressor 19 and the turbine 9, The stages of compression and expansion may be mounted on an integrated speed-increasing gearing (“integrally geared” centrifugal device).

[0060] The compressed gas 21 is then compressed in a compressor 23 to between 2.5 and 10 bar, for example at least 8 bar and at least 30° C. as gas 25. The gas 25 feeds a unit for separation by pressure swing adsorption 27, generally known under the acronym PSA. There, it is separated to form a gas enriched in carbon dioxide and depleted in nitrogen and oxygen 29 (constituting a tail gas) and a gas rich in nitrogen, enriched in oxygen and depleted in carbon dioxide 31 (constituting the product gas). The gas 31 at approximately 8 bar is expanded (possibly after preheating) in a turbine 33 coupled to the compressor 23 and is released to the atmosphere with a composition of 97% nitrogen and 3% carbon dioxide. The gas 31 comprises at most 5% carbon dioxide, or at most 3% carbon dioxide, indeed even at most 1.5% carbon dioxide.

[0061] This expansion in the turbine 33 can be done after heating the gas 31. It may be heated with a hot fluid from the catalytic cracking installation 1 and/or by exchange with a fluid exiting a compression stage upstream of the adsorption unit 27 or upstream of the unit 30. It may even be possible to have two turbine stages in series with intermediate heating in order to maximize the recovery of energy at the shaft of the turbine.

[0062] The unit 27 may be a unit for separation by vacuum pressure swing adsorption, generally known under the acronym VPSA. In this case, the gas 21 is compressed less, but the unit comprises vacuum pumps. The purity of the gas 29 will be higher in CO.sub.2 and the electrical consumption of the unit 27 may be lower.

[0063] The gas 29 containing between 65% and 70% carbon dioxide and between 30% and 35% nitrogen and around 1% oxygen is compressed to a pressure of greater than 15 bar abs and preferentially between 20 and 30 bar abs (the compressor is included in the unit 30), dried and then cooled in a separation apparatus 30 to a temperature of less than 0° C. by partial condensation and/or by distillation to form a fluid rich in carbon dioxide 35 and a fluid depleted in carbon dioxide 37. The gas 29 may contain at least 60% CO.sub.2, or at least 70% CO.sub.2, or at least 80% CO.sub.2.

[0064] The fluid 35 contains at least 70%, and preferentially at least 95%, carbon dioxide in liquid or gaseous form. The fluid 37 contains between 18% and 28% carbon dioxide and also nitrogen and oxygen and is recycled upstream of the adsorption unit 27 to join the gas 25 as feed flow. Before being mixed with the flow 25, the fluid 37 can be expanded in a valve or a turbine.

[0065] Optionally, the fluid depleted in carbon dioxide 37 is separated in a membrane to produce a permeate enriched in CO.sub.2. The permeate can be sent to the adsorption unit 27 as a feed gas to be separated. The residue can be expanded in a turbine and/or mixed with the gas rich in nitrogen and depleted in carbon dioxide 31 and/or used as a regeneration gas for step ii).

[0066] The membrane may optionally separate the fluid 37 at a temperature of less than −30° C.

[0067] The fluid depleted in carbon dioxide 37 and/or the gas rich in nitrogen and depleted in carbon dioxide 31 may be expanded in a turbine to a temperature of less than 0° C. and greater than −100° C. in a chamber containing the separation apparatus 30. It thus contributes to the production of the required frigories.

[0068] A dryer for the flow 25 may be installed upstream of the unit 27. A separation unit utilizing activated carbon may be installed on the flow 29 upstream of the unit 30 or on the flow 25 upstream of the unit 27 in order to remove impurities. In all the figures, the flow enriched in carbon dioxide 17 can be separated by distillation and/or by partial condensation and/or by adsorption and/or by absorption to produce the product containing at least 40% carbon dioxide recycled to the converter 13. it will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above.