Patent Application
20070051238
This invention relates to the purification of streams containing carbon monoxide and more particularly to the removal of low molecular weight hydrocarbons (e.g., methane) from a carbon monoxide stream by adsorption at cryogenic temperatures.
Carbon monoxide (CO) is a major building block for the chemical industry. Besides use as an intermediate in the production of acetic acid, formic acid, and dimethyl formamide to name a few, CO is also a key raw material in the production of phosgene. Phosgene is a key intermediate in many chemical industries, namely polycarbonates, polyurethanes, agricultural chemicals and fine chemicals (pharmaceutical). During the production of phosgene, a CH4 concentration in the CO of more than 100 ppm is detrimental to the overall process from a standpoint of purity, recovery and environmental emissions. Current industry/customer purity requirements are for a methane concentration around 20 ppm or less.
The production of carbon monoxide involves conventional techniques such as steam methane reforming, partial oxidation of hydrocarbons, methanol cracking, and CO2 reforming. In the steam reforming process, hydrocarbons such as methane are converted to syngas, a mixture of carbon monoxide, carbon dioxide, hydrogen and water, through the reaction of hydrocarbons with steam. In the partial oxidation step, hydrocarbons are reacted with oxygen to give syngas, a mixture of carbon monoxide, hydrogen, carbon dioxide and water. The product from both steam reforming and partial oxidation steps, as well as the other methods, can contain additional impurities such as unreacted hydrocarbons and unreacted oxygen. Amounts of low molecular weight hydrocarbons, such as methane, ethane, etc., in the product from steam methane reforming or the partial oxidation steps can range between 0.1 to 5.0 mol %. In order to isolate the CO from the syngas, the syngas stream undergoes various purification steps (e.g., amine absorption, temperature swing adsorption, vacuum swing adsorption, membrane separation or cryogenic distillation) in order to achieve the desired final CO product purity.
During the vacuum swing adsorption purification step, water is removed first using a temperature swing adsorption step. The dry gas mixture containing carbon dioxide, hydrocarbons, hydrogen and carbon monoxide is then sent to an adsorbent bed containing a carbon monoxide selective adsorbent. High purity carbon monoxide is produced during evacuation of the adsorbent beds. Since carbon monoxide comes out at low pressure, it needs to be compressed before it can be sent to the end user's process. Due to the complexity of vacuum swing adsorption process and compression needs after the process, this approach can become quite expensive, particularly when the amount of carbon monoxide produced is small.
For the final purification using cryogenic distillation, the gas mixture exiting the steam methane reforming or the partial oxidation step is purified in a temperature swing adsorption step or amine wash column wherein both water and carbon dioxide are removed. The gas mixture is then cooled to cryogenic temperatures and impurities such as hydrocarbons, hydrogen and nitrogen are removed in a series of cryogenic distillation columns. Such processes are described in U.S. Pat. Nos. 6,062,042 and 6,073,461, and German patent 19,541,339.
In addition to temperature swing adsorption or amine wash, gas separation membranes can also be used for partial removal of impurities such as water, carbon dioxide and hydrogen prior to further processing by cryogenic distillation. Combination of membranes and cryogenic distillation is described in German patent DE 4,325,513 and Japanese patent JP 63-247582. Due to high capital and power requirements, cryogenic distillation processes are limited to high carbon monoxide product flows (>2,000 Nm3/hr product CO).
Because of the increasing need for carbon monoxide supplies containing low levels of hydrocarbons, continuous efforts are being made to develop inexpensive and efficient processes for the removal of hydrocarbons from carbon monoxide streams. The present invention provides such a process.
High purity carbon monoxide, i.e., carbon monoxide containing no more than about 100 ppm by volume of hydrocarbons, is produced by subjecting a carbon monoxide stream containing hydrocarbon impurities to cryogenic temperature swing adsorption (TSA). The adsorption is generally carried out in the gaseous phase at temperatures between the dew point of the carbon monoxide-hydrocarbon mixture at the pressure prevailing in the adsorption vessel and about −75° C. Operating pressures are in the range of about 1.0 to 40.0 atmospheres, absolute. In preferred embodiments the adsorption is carried out at temperatures in the range of about −175° to −125° C.
The hydrocarbons that are preferably removed from the carbon monoxide-containing gas stream are selected from the group consisting of methane, ethane, ethylene, propane and propylene.
The adsorption is conducted in a bed comprising an adsorbent, which preferentially adsorbs hydrocarbons from the carbon monoxide stream. Suitable adsorbents for use in the process of the invention include adsorbents selected from activated carbon and modified activated carbon, pillared clays, carbon molecular sieve, clinoptilolites and modified clinoptilolites, small pore mordenites and mixtures thereof, and in preferred embodiments, an adsorbent selected from activated carbon, modified activated carbon and pillared clays.
The adsorption is preferably carried out in a battery of two or more adsorption beds arranged in parallel and operated out of phase, so that at least one bed is undergoing adsorption while another is undergoing regeneration. The process of the invention is effective for the removal of up to about 5% total by volume of one or more hydrocarbons from the carbon monoxide product stream.
Upon completion of the adsorption step, flow of the feed gas through the adsorption bed is terminated and the bed is regenerated by passing a warm hydrocarbon-free purge gas therethrough. The purge gas preferably is at a temperature of about −20° to 250° C. The preferred purge gas is gaseous N2 with the high purity carbon monoxide being produced during the adsorption step being used as a final purge gas.
In a different embodiment of the invention, a carbon monoxide-containing stream is produced by either a reforming process or a partial oxidation process. This stream is successively purified in gas separation membrane units, and a further process unit which is selected from the group consisting of a Deoxo unit for the removal of oxygen (O2 reacts catalytically with either H2 or CO), a methanizer for the removal of Hydrogen (H2 is converted to CH4 by reaction with CO), an ambient temperature carbon dioxide removal unit, and combinations of these process units. The stream exiting the carbon dioxide removal unit is cooled to a temperature in the range of about −175° to −125° C. and this carbon monoxide-enriched stream is subjected to a temperature swing adsorption process to remove hydrocarbons, thereby producing a high purity carbon monoxide product stream, i.e., a carbon monoxide stream containing not more than about 100 ppm of hydrocarbons and preferably not more than 20 ppm hydrocarbons.
In a further embodiment of the present invention, a stream containing carbon monoxide is purified in a temperature swing adsorption unit and a vacuum swing adsorption unit to produce a carbon monoxide-rich stream which is then compressed and sent to additional, optional units for the removal of oxygen, hydrogen and carbon dioxide impurities and finally to a cryogenic adsorption unit for the removal of hydrocarbon impurities.
In yet another embodiment of the present invention, a stream containing carbon monoxide is purified in a temperature swing adsorption unit to remove water and carbon dioxide impurities. Hydrocarbon impurities are removed in a cryogenic adsorption unit either before or after a cryogenic distillation system which is used to remove light impurities such as hydrogen and nitrogen.
The apparatus aspects of the invention comprise a cryogenic temperature swing adsorption system either alone or in combination with a membrane separation unit, a DeOxo/methanizer unit and a carbon dioxide adsorption unit. Other apparatus embodiments include cryogenic adsorption unit after a vacuum swing adsorption unit or cryogenic adsorption unit either before or after a cryogenic distillation unit.
While the invention is described primarily with the reference to the purification of CO, it is equally applicable to the purification of streams containing CO and H2. Various H2 removal steps such as membranes and methanizer can be omitted for this case.
In any of the apparatus alternatives the adsorption means contains an adsorbent selected from activated carbon and modified activated carbon, pillared clays, carbon molecular sieve, clinoptilolites and modified clinoptilolites, small pore mordenites and mixtures thereof, and in preferred embodiments, an adsorbent selected from activated carbon, modified activated carbon and pillared clays.
The invention is illustrated in the drawings, in which:
Like characters designate like or corresponding parts throughout the several views. Auxiliary valves, lines and equipment not necessary for an understanding of the invention have been omitted from the drawings.
A carbon monoxide-enriched gas stream containing hydrocarbon impurities is passed through a bed of adsorbent which preferentially adsorbs hydrocarbons from the carbon monoxide-enriched gas stream at cryogenic temperatures, thereby removing substantial quantities of the hydrocarbons from the gas stream. The adsorption process operates on a temperature swing adsorption (TSA) cycle. This aspect of the invention can be carried out in the apparatus illustrated in
Adsorbers A and B are identical and each is packed with a bed of particulate adsorbent which adsorbs hydrocarbons in preference to carbon monoxide. Adsorbers A and B include adsorbents selected from activated carbon and modified activated carbon, pillared clays, carbon molecular sieve, clinoptilolites and modified clinoptilolites, small pore mordenites and mixtures thereof, and in preferred embodiments, an adsorbent selected from activated carbon, modified activated carbon and pillared clays.
In the adsorption system illustrated in
The operation of the adsorption system will first be described with bed A in the adsorption mode and bed B in the regeneration mode. In this half of the cycle, valves 8, 10, 16 and 20 are open and valves 6, 12, 14 and 22 are closed. Feed gas enters the adsorption system through line 2, passes through valve 10 and enters adsorber A. As the gas passes through adsorber A, hydrocarbons are preferentially adsorbed therefrom. The hydrocarbon-depleted carbon monoxide stream, now usually containing no more than about 100 ppm by volume of hydrocarbons, passes through valve 20 and leaves the adsorption system through line 24. In the embodiment illustrated in
While high purity carbon monoxide is being produced in adsorber A, the bed of adsorbent in adsorber B is being regenerated. During regeneration, a warm purge gas is introduced into adsorber B through line 18 and open valve 16. Initial bed heating can be accomplished with a dry gas such as argon, carbon monoxide or nitrogen. It is preferred to use high purity carbon monoxide as the final purge gas to avoid contaminating the adsorption beds. The preferred regeneration direction is countercurrent to adsorption direction. Part of the product leaving the system through line 24 can be used as the regeneration purge gas or the purge can be supplied externally. The warm purge gas passes through bed B, thereby desorbing and sweeping hydrocarbons therefrom. The desorbed hydrocarbons are removed from the system through open valve 8 and line 4. This gas may be completely vented to the atmosphere, used as a fuel or a part of it can be reintroduced into the system to recover the carbon monoxide used as purge gas.
During the course of the adsorption step, the adsorbed gas front in adsorber A progresses toward the outlet end of this unit. When the front reaches a predetermined point in the bed, the first half of the cycle is terminated and the second half is begun.
During the second half of the adsorption cycle, adsorber B is put into adsorption service and the bed in adsorber A is regenerated. During this half of the cycle valves 6, 12, 14 and 22 are open and valves 8, 10, 16 and 20 are closed. Feed gas now enters the adsorption system through line 2 and passes through adsorber B through valves 12 and 22 and line 24. Meanwhile, the bed in adsorber A is being regenerated. During regeneration of the bed in adsorber A, the warm purge gas passes through the adsorber A via line 18, valve 14, valve 6 and line 4. When the adsorption front in the bed in adsorber B reaches the predetermined point in this bed, the second half of the cycle is terminated, and the cycle is repeated.
The feed to adsorbers A and B is typically at a temperature between the dew point of carbon monoxide at the prevailing pressure and about −75° C., and preferably at a temperature in the range of about −175° and −125° C. While the feed to the adsorbers will typically be at its dew point or warmer in some cases the feed can contain small amounts (up to 10%) of liquid to overcome the heat losses and heat of adsorption. The prevailing pressure in adsorbers A and B during the adsorption step is generally in the range of about 1.0 to 40.0 atmospheres, absolute. The rate of flow of the regeneration gas through the system is typically between 5 and 15% of the feed flow rate. The regeneration gas temperature is in the range of about −20° and 250° C. The concentration of hydrocarbon impurities in the feed gas is between 100 ppm and 5%. Prior to the initial start of carbon monoxide purification, the beds in adsorbers A and B are heated to temperatures up to 300° C. to remove any residual moisture contained therein. This step is not repeated during the regular operation. A sample cycle for the process is given in Table I.
In a different embodiment of the present invention illustrated in
Membrane units 38 and 42 include conventional gas separation membranes made from polymers such as polysulfones, polycarbonates, polyimides, cellulose acetates and their modified forms. These membranes are more permeable to gases such as hydrogen, water, carbon dioxide and oxygen in comparison to carbon monoxide.
The carbon monoxide-enriched stream exiting membrane unit 42 is heated in a heater 46 to temperatures between 100 and 400° C. and sent to a DeOxo/methanizer unit 48. In unit 48 any remaining oxygen is removed by reaction with hydrogen or carbon monoxide over a noble metal (platinum, palladium, rhodium or ruthenium) or a base metal (nickel, copper) or copper/manganese oxide catalyst. Hydrogen is also removed in unit 48 by reaction of hydrogen with either carbon monoxide or carbon dioxide over a methanation catalyst such as nickel. The stream exiting unit 48 contains hydrocarbons such as methane and ethane and carbon dioxide as main impurities.
The stream leaving unit 48 is cooled to close to near ambient temperature in a heat exchanger 50 using a coolant 52. The stream exiting unit 50 is sent to a temperature swing adsorption unit 54 wherein carbon dioxide is removed by adsorption on zeolites which are preferably 5A and 13X type zeolites. Typical adsorption times for this unit will range between 4 and 16 hours and the regeneration is carried out by heating the beds with a carbon dioxide-free stream at temperatures between 100 and 250° C. followed by cooling to close to ambient temperatures. Two or more beds are used for close to continuous operation.
The stream exiting unit 54 is cooled to cryogenic temperatures, between −175° and −125° C., in heat exchanger 58 using liquid or gaseous oxygen, nitrogen, argon, mixed hydrocarbons or mixtures thereof as cryogens. The stream exiting unit 58 is purified in the cryogenic adsorption unit 30 to produce high purity carbon monoxide stream 60 containing less than 100 ppm total hydrocarbons. The operation of unit 30 has been described earlier in connection with
If a stream containing CO and H2 is to be purified then membrane units 38 and 42 can be omitted. Unit 48 will contain a catalyst only for reaction of O2 with CO or H2 and unit 54 will contain adsorbents for both water and carbon dioxide removal.
A further embodiment of the present invention is shown in
Gas stream exiting unit 62 is sent to a vacuum swing adsorption unit 64. Unit 64 contains one or more beds wherein carbon monoxide is preferentially adsorbed. The adsorbents in unit 64 typically contain Cu+ (copper of valance one) on zeolites such as Y zeolite or other adsorbents such as activated alumina and activated carbon. The adsorption is typically carried out at temperatures between about 20 and 100° C. and at pressures between about 0.5 to 10 bara. High pressure product from unit 64 is removed as stream 66 and contains hydrogen, carbon dioxide, hydrocarbons and some carbon monoxide. Carbon monoxide product 70 is obtained during evacuation of the adsorbent beds using vacuum pump 68 at pressures between 0.05 to 0.3 bara and may contain impurities such as hydrocarbons, carbon dioxide and hydrogen at low levels. Recovery of carbon monoxide from unit 64 increases as the impurity level in stream 70 increases.
Carbon monoxide product stream 70 is typically at a pressure close to atmospheric and is compressed using compressor 72 to pressures between 5 and 20 bara. If needed, impurities such as hydrogen and carbon dioxide from this stream can be removed by heating the gas mixture in heater 46 removing hydrogen in unit 48 cooling the stream in unit 50 and removing carbon dioxide in unit 54. Operation of these units has been described in more detail during discussion of
A further embodiment of the invention is shown in
Carbon monoxide stream 80, with or without hydrocarbons, is sent to a distillation column system 82 wherein an overhead product containing light impurities such as hydrogen and nitrogen are removed as stream 84 and a carbon monoxide stream 86 is produced as the bottoms product. If the hydrocarbons have been previously removed in unit 30 prior to cryogenic distillation unit 82 stream 86 is high purity carbon monoxide product and can be sent to a downstream process or to storage. If the hydrocarbons have not been removed prior to distillation stream 86 containing hydrocarbon impurities is sent to a cryogenic adsorption unit 30 and the stream 60 exiting this unit is the high purity carbon monoxide product.
The invention is further exemplified by the following examples, in which parts, percentages and ratios are on a volume basis, unless otherwise indicated.
Commercially available 40×60 mesh activated carbon was loaded in a 3 mm diameter adsorbent bed of 10 ft length. The total weight of adsorbent was about 5.4 gms. A feed stream containing 1% methane and 99% carbon monoxide was passed through this bed at −173° C., 10 psig and at a flow rate of 0.1 std liters/min. Methane concentration at the bed outlet was monitored using a total hydrocarbon analyzer. Methane concentration at the bed outlet remained below 1 ppm for a period of about 343 minutes after which methane concentration started rising quickly.
The column of Example I was used and the experiment was run at a feed pressure of 50 psig. The rest of the conditions were same as in Example I. Methane concentration in the bed outlet remained below 1 ppm for a period of 340 minutes.
The vessel with an internal diameter of about 1″ was filled with about 250 grams of 6×8 mesh commercially available activated carbon. The feed contained 1% methane in carbon monoxide and was sent to the bed at a flow rate of 5 std liters/min at 50 psig and −173° C. Methane concentration at the bed outlet was monitored and methane concentration of less than 1 ppm was seen for a period of about 217 minutes.
These examples illustrate that fairly high hydrocarbon adsorption capacities can be obtained by adsorbing these impurities from carbon monoxide at cryogenic adsorption temperatures.
Although the invention is described with reference to specific examples, the scope of the invention is not limited thereto. For example, the feedgas containing carbon monoxide can come from processes other than steam-methane reforming and partial oxidation. Such processes include catalytic partial oxidation, carbon dioxide reforming, methanol cracking and other waste streams from various chemical processes. Also, the feed gas may contain significant quantities of hydrogen as would be typical for a syngas feed. The scope of the invention is limited only by the breadth of the appended claims.
This application claims priority from U.S. Provisional Patent Application Ser. No. 60/714,561 filed Sep. 7, 2005.
| Number | Date | Country | |
|---|---|---|---|
| 60714561 | Sep 2005 | US |
