Process and apparatus for separating carbon dioxide from a waste gas of a fluid bed catalytic cracking (FCC) installation

Abstract

In a process for separating carbon dioxide from a waste gas (3) of a fluid bed catalytic cracking installation (1) containing carbon dioxide, nitrogen and possibly carbon monoxide, the waste gas (3) is separated by adsorption to form a gas enriched in carbon dioxide and depleted in nitrogen (29) and a gas rich in nitrogen and depleted in carbon dioxide (31), and at least a portion of the gas enriched in carbon dioxide and depleted in nitrogen is separated in a separation device (30) 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 (35) and a fluid depleted in carbon dioxide (37).

Claims

1. A process for separating carbon dioxide from a waste gas of a fluid bed catalytic cracking installation containing carbon dioxide, nitrogen and carbon monoxide, wherein: i) at least a portion of the carbon monoxide of the waste gas is converted into carbon dioxide to form a flow enriched in carbon dioxide, ii) the waste gas, or the flow enriched in carbon dioxide from step i), is separated 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, the process comprising: separating at least a portion of the gas enriched in carbon dioxide and depleted in nitrogen in a separation device 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, wherein a portion of the fluid rich in carbon dioxide is sent to step ii) as a rinsing gas.

2. The process according to claim 1, wherein the waste gas or the flow enriched in carbon dioxide is compressed upstream of step ii) to a pressure of between 2.5 and 10 bar abs.

3. The process according to claim 1, wherein the gas rich in nitrogen and depleted in carbon dioxide contains less than 5 mol % CO.sub.2.

4. The process according to claim 1, wherein the gas enriched in carbon dioxide and depleted in nitrogen contains more than 45 mol % CO.sub.2.

5. The process according to claim 1, wherein the fluid depleted in carbon dioxide contains at most 15% CO.sub.2.

6. The process according to claim 1, wherein the fluid depleted in carbon dioxide is compressed and mixed with the flow enriched in carbon dioxide sent to step ii).

7. The process according to claim 1, wherein the gas rich in nitrogen and depleted in carbon dioxide is expanded in a turbine and is sent to the atmosphere.

8. The process according to claim 7, wherein the gas rich in nitrogen and depleted in carbon dioxide is heated by a fluid originating from the fluid bed 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 by at least a portion of the flow enriched in carbon dioxide.

9. The process according to claim 1, 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).

10. The process according to claim 9, 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.

11. The process according to claim 1, 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 device.

12. The process according to claim 1, wherein at least a portion of the carbon monoxide of the waste gas is converted into carbon dioxide to form a flow enriched in carbon dioxide by combustion of the carbon monoxide in air and in the presence of a fuel.

13. The process according to claim 1, wherein no part of the gas enriched in carbon dioxide and depleted in nitrogen and/or of the fluid rich in carbon dioxide is sent to the fluid bed catalytic cracking installation.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) 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:

(2) FIG. 1 is a schematic representation of a process according to one embodiment of the invention.

(3) FIG. 2 is a schematic representation of a process according to one embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(4) In FIG. 1, 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.

(5) 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.

(6) 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. The bottom liquid of the tower 47 is sent to a particulate separator 57. The functioning of the unit 1 is well known in the art.

(7) 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 close 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 air 15. The conversion process also produces steam.

(8) In certain operating modes of the unit 1, it is possible to produce a waste gas containing very little carbon monoxide. In this case, the step of conversion in unit 13 is not necessary.

(9) This produces a gas 17 at atmospheric pressure and around 120° C. containing 17% CO2, 3% oxygen and 80% 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.

(10) 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).

(11) 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.

(12) 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.

(13) 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 CO2 and the electrical consumption of the unit 27 may be lower.

(14) The gas 29 containing between 50% and 60% carbon dioxide and between 40% and 50% 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 45% CO2, or at least 50% CO2, or at least 80% CO2.

(15) The fluid 35 contains at least 70%, and preferentially at least 95%, carbon dioxide in liquid or gaseous form. A portion of the fluid 35 may be sent to the adsorption unit 27 as a rinsing gas. At least part of the fluid 35 serves as product.

(16) The fluid 37 contains between 15% and 25% 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 gas 25, the fluid 37 can be expanded in a valve or a turbine.

(17) The fluid depleted in carbon dioxide 37 is optionally separated in a membrane to produce a permeate enriched in CO2. The permeate may be sent to the adsorption unit 27 as feed gas for separation. The residue can be expanded in a turbine and/or mixed with the gas rich in nitrogen and depleted in carbon dioxide 31.

(18) The membrane may optionally separate the fluid 37 at a temperature of less than −30° C.

(19) 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 device 30. It thus contributes to the production of the required frigories.

(20) 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.

(21) In FIG. 2, the main difference from FIG. 1 is that the expanded gas 11 is sent to a conversion unit 13 where the carbon monoxide in the gas 11 is converted into carbon dioxide, at least partially by combustion with oxygen 41. The oxygen 41 contains at least 90% oxygen and at most 10% nitrogen. It may contain at least 99.5% oxygen. In this way, the gas produced contains between 20% and 30% CO.sub.2, for example 26% CO.sub.2. The gas 29 is richer in CO.sub.2 (67%) and the gas 37 is also richer in CO.sub.2 (22%). A portion 39 of the gas 29 produced by the PSA 27 can be recycled to the conversion unit 13 at 1.05 bar and containing 67% CO.sub.2. At least a portion of the fluid 39 is mixed with oxygen to form the oxidant used by the conversion unit 13. In addition or alternatively, a portion of the fluid 35 and/or a portion of the fluid 37 may be sent to the conversion unit 13. Thus, it is possible that no part of the fluid 35 is sent to the conversion unit 13.

(22) The FCC 1 continues to be fed with optionally oxygen-enriched air 49 (as in FIG. 1).

(23) In addition to producing a waste 17 which is richer in carbon dioxide, the process of FIG. 2 makes it possible to increase the temperature of the flame in the converter 13, 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.

(24) The unit 27 and the unit 30 of FIG. 2 will be smaller than those of FIG. 1.