METHOD AND FACILITY FOR PURIFYING A FEED GAS STREAM COMPRISING AT LEAST 90% CO2

Abstract

A process for purifying a feed gas stream containing at least 90% of CO.sub.2, at least 20% RH and at least one impurity chosen from chlorinated, sulfur-bearing, nitrated or fluorinated compounds is provided. The process includes a) subjecting the feed gas stream to catalytic oxidation producing a stream containing at least one of HCl, NOx, SOx or hydrofluoric acid; b) maintaining the temperature of the gas stream above the highest value between the dew points of water and the acid(s) contained in the gas; c) removing at least a part of the acid impurities by bringing the gas stream into contact with a corrosion-resistant heat exchanger to condense the acid compounds while regulating the temperature of the gas stream exiting below the dew point of water; and d) separating the acid condensates with a corrosion-resistant separator in such a way as to produce a CO.sub.2-enriched gas stream.

Claims

1.-12. (canceled)

13. A process for purifying a feed gas stream comprising at least 90% of CO.sub.2 on a dry basis, at least 20% relative humidity and at least one impurity chosen from chlorinated, sulfur-bearing, nitrated or fluorinated compounds, comprising the following successive steps: a) subjecting the feed gas stream to catalytic oxidation in such a way as to produce a gas stream comprising at least one acid impurity chosen from HCl, NOx, SOx or hydrofluoric acid; b) maintaining the temperature of the gas stream from step a) above the highest value between the dew point of water and the dew point of the acid(s) contained in the gas downstream of the catalytic process; c) removing at least a part of the acid impurities by bringing the gas stream from step b) into contact with at least one corrosion-resistant heat exchanger in such a way as to condense the acid compounds while regulating the temperature of the gas stream exiting below the dew point of water; and d) separating the acid condensates from the gas stream from step c) by means of a corrosion-resistant separator in such a way as to produce a CO.sub.2-enriched gas stream.

14. The process as claimed in claim 13, further comprising, after step d), the following successive steps: e) liquefying the CO.sub.2-enriched gas stream, f) drying the liquefied stream, and g) sending the dried stream to a cryogenic unit.

15. The process as claimed in claim 13, wherein the heat exchanger and the separator comprise materials chosen from austenitic steel, glass or a composite resistant to nitric, sulfuric or hydrochloric acid.

16. The process as claimed in claim 13, wherein the catalytic oxidation is carried out by means of a catalytic oxidation unit resistant to sulfur and to chlorine.

17. The process as claimed in claim 13, wherein the feed gas stream subjected to the catalytic oxidation is at a temperature of at least 300 C.

18. The process as claimed in claim 13, wherein the feed gas stream subjected to the catalytic oxidation is at a pressure of at least 1 bar absolute.

19. The process as claimed in claim 13, wherein the feed gas stream comes from a monoethylene glycol unit, from washing of CO.sub.2 by absorption, from a monoethylene glycol synthesis unit, from a biofermentation or any other process generating a CO.sub.2-rich stream.

20. The process as claimed in claim 13, wherein, in step c), water is used as refrigerant within the heat exchanger.

21. A facility for the purification of a feed gas stream comprising at least 95% of CO.sub.2, at least 20% relative humidity and at least one impurity chosen from chlorinated, sulfur-bearing, nitrated or fluorinated compounds, comprising, in the direction of circulation of the gas stream: a) a catalytic oxidation unit configured to subject the gas stream to catalytic oxidation in such a way as to produce a gas stream comprising at least one acid impurity chosen from HCl, NOx and SOx; b) a means for maintaining the temperature of the gas stream exiting the catalytic oxidation unit above the acid dew point; c) a corrosion-resistant heat exchanger making it possible to condense the acid compounds of the gas stream; and d) a corrosion-resistant separator making it possible to separate the acid compounds from the gas stream in such a way as to produce a CO.sub.2-enriched gas stream.

22. The facility as claimed in claim 21, wherein said facility comprises, downstream of the separator and in the direction of circulation of the CO.sub.2-enriched gas stream: e) a liquefier making it possible to liquefy the CO.sub.2-enriched gas stream; f) a dryer making it possible to dry the liquefied gas; and g) a cryogenic unit.

23. The facility as claimed in claim 21, wherein the heat exchanger and the separator comprise materials chosen from austenitic steel, glass or a composite resistant to nitric, sulfuric or hydrochloric acid.

24. The facility as claimed in claim 21, wherein the catalytic oxidation is carried out by means of a catalytic oxidation unit resistant to sulfur and to chlorine.

Description

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0030] A solution of the present invention is a process for purifying a feed gas stream comprising at least 90% of CO.sub.2, preferably at least 95% of CO.sub.2, at least 20% relative humidity and at least one impurity chosen from chlorinated, sulfur-bearing, nitrated or fluorinated compounds, comprising the following successive steps:

[0031] a) a step of subjecting the feed gas stream to catalytic oxidation in such a way as to produce a gas stream comprising at least one acid impurity chosen from HCl, HNO.sub.3, NOx, SOx, H.sub.2SO.sub.4 and HF;

[0032] b) a step of maintaining the temperature of the gas stream from step a) above the highest value between the dew point of water and the dew point of the acid(s) contained in the gas downstream of the catalytic process (the term critical dew point will be used);

[0033] c) a step of removing at least a part of the acid impurities by bringing the gas stream from step b) into contact with at least one corrosion-resistant heat exchanger in such a way as to condense the acid compounds while regulating the temperature of the gas stream exiting below the dew point of water; and d) a step of separating the acid compounds from the gas stream from step c) by means of a corrosion-resistant separator in such a way as to produce a CO.sub.2-enriched gas stream.

[0034] Step b) makes it possible to avoid any creation of acid liquid that would corrode the piping and other standard materials. Care will be taken never to go below the critical dew point via suitable operating conditions but also by virtue of thermal insulation or even an external temperature maintenance system (electric or steam) preventing the creation of cold spots (temperature locally below the critical dew point) on the equipment.

[0035] In step c), if the composition of the feed gas for the catalytic process varies, the outlet temperature of the condensation exchanger in indirect contact, of plate or shell-and-tube type, will optionally be regulated via the flow rate of cooling liquid/gas feeding it or by the temperature thereof.

[0036] As the case may be, the process according to the invention can exhibit one or more of the following features: [0037] said process comprises, after step d), the following successive steps:

[0038] e) a step of liquefying the CO.sub.2-enriched gas stream,

[0039] f) a step of drying the liquefied stream, and

[0040] g) a step of sending the dried stream to a cryogenic unit.

[0041] The drying is generally carried out via a reversible adsorption unit which makes it possible to achieve water contents compatible with the cryogenic temperature in question (<10 ppmv and preferentially <1 ppmv), [0042] the heat exchanger and the separator consist of materials chosen from austenitic steel, glass or a composite resistant to nitric, sulfuric or hydrochloric acid, [0043] the catalytic oxidation is carried out by means of a catalytic oxidation unit, the catalyst of which is tolerant to sulfur and to chlorine, [0044] the feed gas stream subjected to catalytic oxidation is at a temperature of at least 300 C., preferably at least 425 C., [0045] the feed gas stream subjected to catalytic oxidation is ata pressure greater than 1 bar absolute, [0046] the feed gas stream can come from various sources, such as monoethylene glycol manufacturing units or biofermenters, [0047] in step c), water or a water/glycol mixture is preferentially used as a refrigerant within the heat exchanger.

[0048] In order to improve the exchange surface while remaining on standard exchanger sizes or in order to carry out condensation at 2 temperature levels, it will be possible to use several exchangers in parallel or in series respectively with a common separator vessel downstream or a separator vessel associated with each of the exchangers.

[0049] Preferably, the water heated in the heat exchanger by the gas stream is in a closed circuit and is cooled in a second, ammonia/water heat exchanger.

[0050] A subject of the present invention is also a facility for the purification of a feed gas stream comprising at least 95% of CO.sub.2, at least 20% relative humidity and at least one impurity chosen from chlorinated, sulfur-bearing, nitrated or fluorinated compounds, comprising, in the direction of circulation of the gas stream:

[0051] a) a catalytic oxidation unit making it possible to subject the gas stream to catalytic oxidation in such a way as to produce a gas stream comprising at least one acid impurity chosen from HCl, NOx and SOx;

[0052] b) a means for maintaining the temperature of the gas stream exiting the catalytic oxidation unit above the acid dew point;

[0053] c) a corrosion-resistant heat exchanger making it possible to condense the acid compounds of the gas stream;

[0054] d) a corrosion-resistant separator making it possible to separate the acid compounds from the gas stream in such a way as to produce a CO.sub.2-enriched gas stream.

[0055] Preferably, in step d), a corrosion-resistant drum separator will be used.

[0056] Preferably in step d), the separator is equipped with an automatic emptying system.

[0057] Depending on the case, the facility according to the invention may have one or more of the features below: [0058] said facility comprises, downstream of the separator and in the direction of circulation of the CO.sub.2-enriched gas stream:

[0059] e) a liquefier making it possible to liquefy the CO.sub.2-enriched gas stream;

[0060] f) a dryer making it possible to dry the liquefied gas; and

[0061] g) a cryogenic unit, [0062] the heat exchanger and the separator consist of materials chosen from austenitic steel, glass or a composite resistant to nitric, sulfuric or hydrochloric acid. By way of example, mention will be made of 254SMO and 904L steels, nickel-based alloys or titanium. Preferably, when chlorinated compounds are present in the feed stream, the heat exchanger is made of 254SMO stainless steel for the parts in contact with the process gas, the remainder being made of SS 316L steel, specially designed for strong corrosion created by HCl, [0063] the catalytic oxidation is carried out by means of a catalytic oxidation unit resistant to sulfur and to chlorine.

EXAMPLE

Example: Purification of a Stream Containing Chlorinated Molecules

[0064] Stream from a unit for producing monoethylene glycol containing 1 ppmv of chlorinated compounds:

TABLE-US-00001 Name Crude gas Molar flow Sm.sup.3/h 4456 Flow by weight kg/h 8462 Temperature C. 39 Pressure bar a 1.823 Vapor fraction CO.sub.2 molar fraction 0.933453 Oxygen molar fraction 0.000000 Acetaldehyde molar fraction 0.000250 Methane molar fraction 0.002496 Water molar fraction 0.039930 Ethane molar fraction 0.000032 i-Butane molar fraction 0.000480 Benzene molar fraction 0.000029 Ethylene molar fraction 0.021506 Carbon dioxide molar fraction 0.001823 ClC.sub.2 molar fraction 0.000001 HCl molar fraction 0.000000

[0065] The stream is brought into contact with a catalytic bed containing a first layer of platinum catalyst and a second layer of palladium catalyst, making it possible to convert most of the hydrocarbons to water and to carbon dioxide. At the outlet of the catalytic reactor, the gas stream at a temperature of greater than 300 C. and containing HCl is cooled to a temperature above the dew point of water (43 C.), in order to prevent condensation.

[0066] In practice, a minimum temperature of 55 C., i.e. 12 C. above the theoretical temperature, will be maintained, thus dispensing with measurement inaccuracies and equipment insulation faults.

TABLE-US-00002 Name Skid entry Molar flow Sm.sup.3/h 4819 Flow by weight kg/h 8999 Temperature C. 55 Pressure bar a 1.19 Vapor fraction 1 CO.sub.2 molar fraction 0.910992 Oxygen molar fraction 0.011299 Acetaldehyde molar fraction 0.000000 Methane molar fraction 0.000036 Molar fraction of H.sub.2O 0.077672 Ethane molar fraction 0.000000 i-Butane molar fraction 0.000000 Benzene molar fraction 0.000000 Ethylene molar fraction 0.000001 Carbon dioxide molar fraction 0.000000 ClC.sub.2 molar fraction 0.000000 HCl molar fraction 0.000001

[0067] The fluid, at the temperature of 55 C., will then be conveyed in a skid comprising a heat exchanger made of hydrochloric acid-resistant material such as the steel grade 254SMO. The cold will be brought into the exchanger by means of a heat transfer fluid such as water, at a temperature below 43 C., preferentially below 10 C., in order to minimize the size of the exchanger.

TABLE-US-00003 Name Coo/mg water feeding the exchanger Molar flow Sm.sup.3/h 9953 Flow by weight kg/h 8000 Temperature C. 7 Pressure bar a 3.95 Vapor fraction 0

[0068] In this example, the condensation of water combined with a substantial exchange surface area alone will ensure the removal of the HCl contained in the gas phase due to the very high solubility of HCl in water (720 g/l at 20 C.)

[0069] The separation of the liquid droplets containing the acid molecules will be ensured by a separator vessel (installed directly downstream of the exchanger) made of a material resistant to corrosion by chlorine, such as the steel grade 254 SMO or more simply by a material made of polymer resin.

[0070] The process gas at the outlet of the separator vessel and the condensed liquid, discharged at the bottom of the separator vessel will have the following compositions:

TABLE-US-00004 Separator vessel Separator vessel Name gas outlet liquid outlet Molar flow Sm.sup.3/h 4688 131 Flow by weight kg/h 8893 106 Temperature C. 35 35 Pressure bar a 1.14 1.14 Vapor fraction 1 0 CO.sub.2 molar fraction 0.936535 0.000055 Oxygen molar fraction 0.011615 0.000000 Acetaldehyde molar 0.000000 0.000000 fraction Methane molar fraction 0.000037 0.000000 H.sub.2O molar fraction 0.051811 0.999942 Frac (Ethane) 0.000000 0.000000 i-Butane molar fraction 0.000000 0.000000 Benzene molar fraction 0.000000 0.000000 Ethylene molar fraction 0.000001 0.000000 Carbon dioxide molar 0.000000 0.000000 fraction ClC.sub.2 molar fraction 0.000000 0.000000 HCl molar fraction 0.000000 0.000003

[0071] 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.