METHOD FOR THE PRODUCTION OF AIR GASES BY THE CRYOGENIC SEPARATION OF AIR WITH IMPROVED FRONT END PURIFICATION AND AIR COMPRESSION

Abstract

A method and apparatus for the production of air gases by the cryogenic separation of air with front end purification and air compression can include using an available compressed dry gas such as nitrogen, oxygen, stored purified air, or synthetic air to repressurize the adsorber without diverting any of the purified air just exiting the currently on-line adsorber or changing the flow rate of the main air compressor or air sent to the cold box. This enables the main air compressor (MAC) to operate at a relatively constant flow rate while also sending a relatively constant air flow to the cold box during this repressurization step, thereby reducing the risks of process upsets and minimizing capital expenditures related to the MAC and other warm-end equipments.

Claims

1. A method for reducing process disturbances during pressurization of an adsorber in an air separation unit, wherein the air separation unit comprises a front end purification unit, a main air compressor, a cold box having a main heat exchanger and a distillation column system disposed therein, and an air buffer tank, wherein the front end purification unit comprises a first adsorber and a second adsorber, the method comprising the steps of: pressurizing the first adsorber while the second adsorber operates in an adsorption cycle, wherein the step of pressurizing the first adsorber further comprises the steps of withdrawing a pressurized air stream from the air buffer tank and introducing the pressurized air stream to the first adsorber until the first adsorber is at a target pressure, wherein the air buffer tank is in fluid communication with an outlet of the front end purification unit, wherein the method further comprises the step of continually sending a first portion of purified air flow from the front end purification unit to the air buffer tank.

2. The method as claimed in claim 1, wherein the first portion of purified air flow from the front end purification unit is between 0.3% to 1% of total air flowing into the booster air compressor.

3. The method as claimed in claim 1, wherein the first portion of purified air flow is first compressed in a booster air compressor prior to being sent to the air buffer tank.

4. A method for reducing process disturbances during pressurization of an adsorber in an air separation unit, wherein the air separation unit comprises a front end purification unit, a main air compressor, a cold box having a main heat exchanger and a distillation column system disposed therein, a booster air compressor, and an air buffer tank, wherein the front end purification unit comprises a first adsorber and a second adsorber, the method comprising the steps of: compressing an air stream in a main air compressor to form a compressed main air stream; purifying the compressed main air stream in the front end purification unit to remove water and carbon dioxide to form a dry main air stream; sending a first portion of the dry main air stream to the cold box for cooling and rectification therein; boosting a second portion of the dry main air stream to a higher pressure P.sub.H in the booster air compressor to produce a boosted air stream; and sending the boosted air stream to the air buffer tank, wherein each of the adsorbers of the front end purification unit undergoes a processing cycle comprising a regeneration cycle, a pressurization cycle, and an adsorption cycle, wherein during the pressurization cycle, a pressurized air stream is withdrawn from the air buffer tank and introduced to the adsorber that is undergoing pressurization.

5. The method as claimed in claim 4, wherein the pressurized air stream is only withdrawn from the air buffer tank and introduced to the adsorber during the pressurization cycle.

6. The method as claimed in claim 4, wherein the flow rate of the boosted air stream sent to the air buffer tank is between 0.3% to 1% of the flow rate of the dry main air stream.

7. The method as claimed in claim 4, wherein the boosted air stream is sent to the air buffer tank at a constant rate during the entire processing cycle of the front end purification unit.

8. An apparatus for reducing process disturbances during pressurization of an adsorber in an air separation unit, wherein the apparatus comprises: a main air compressor configured to compress an air stream to form a compressed main air stream; a front end purification unit configured to purify the compressed main air stream of water and carbon dioxide to form a dry main air stream, wherein the front end purification unit comprises two adsorbers, wherein each adsorber is configured to operate with an adsorption cycle, a regeneration cycle, and a pressurization cycle; a booster air compressor in fluid communication with the front end purification unit, wherein the booster air compressor is configured to boost a second portion of the dry main air stream to a higher pressure PH to form a boosted air stream; an air buffer tank having an air inlet in fluid communication with an outlet of the booster air compressor, wherein the air buffer tank is configured to receive the boosted air stream, wherein an outlet of the air buffer tank is in fluid communication with the front end purification unit; a cold box in fluid communication with the front end purification unit, wherein the cold box is configured to receive a first portion of the dry main air stream, wherein the cold box houses a main heat exchanger and a distillation column system, wherein the main heat exchanger is configured to cool the first portion of the dry main air stream to a cryogenic temperature suitable for rectification of air, wherein the distillation column system is configured to receive the first portion of the dry main air stream from the main heat exchanger after cooling, wherein the distillation column system is configured to separate the dry main air stream into nitrogen and oxygen; wherein during a pressurization cycle of each adsorber, a valve located between the air buffer tank and the front end purification unit is configured to open to allow for dry air to flow from the air buffer tank to the adsorber, wherein the valve is configured to close, thereby stopping the flow of dry air to the adsorber, once the pressurization cycle is completed.

9. The apparatus as claimed in claim 8, further comprising means for regulating the flow rate of the boosted air stream sent to the air buffer tank.

10. The apparatus as claimed in claim 9, wherein the flow rate of the boosted air stream sent to the air buffer tank is between 0.3% to 1% of the flow rate of the dry main air stream.

11. An apparatus for the production of air gases by the cryogenic separation of air with front end purification and air compression, the apparatus comprising: a main air compressor configured to compress atmospheric air to a pressure suitable for the cryogenic rectification of air to produce a compressed wet air stream; a front end purification system in fluid communication with the main air compressor, such that the front end purification system is configured to receive the compressed wet air stream from the main air compressor and purify the compressed wet air stream of water and carbon dioxide to produce a dry air stream having reduced amounts of water and carbon dioxide as compared to the compressed wet air stream, wherein the front end purification system comprises a first vessel and a second vessel configured in a permutable fashion, wherein the first vessel comprises a first adsorber and the second vessel comprises a second adsorber, wherein the first and second adsorbers operate in alternating cycles such that while the first adsorber is in an adsorption cycle, the second adsorber is in a regeneration cycle and while the second adsorber is in the adsorption cycle, the first adsorber is in the regeneration cycle; a cold box in fluid communication with the front end purification system, such that the cold box is configured to receive the dry air stream from the front end purification system and separate the dry air stream into a nitrogen enriched stream and an oxygen enriched stream, wherein the regeneration cycle of each vessel of the front end purification system is configured to: 1) depressurize the vessel from an adsorption pressure to a regeneration pressure that is sufficiently low to release water and carbon dioxide from a surface of an adsorbent material within the vessel; 2) regenerate the adsorbent material using a first dry gas; and 3) pressurize the vessel to the adsorption pressure using a second gas; and an absence of a flow means configured to transfer a portion of the dry air stream from the first adsorber directly to the second adsorber when the second adsorber is in its pressurization cycle.

12. The apparatus as claimed in claim 11, wherein the main air compressor is configured to operate at substantially the same flow rate during all cycles of the front end purification system, and wherein the cold box is configured to operate at substantially the same flow rate of the dry air stream dry air during all cycles of the front end purification system.

13. The apparatus as claimed in claim 11, further comprising a compressed air storage tank configured to source the second gas, wherein the compressed air storage tank is in fluid communication with the front end purification system, such that the compressed air storage tank is configured to receive a portion of the dry air stream exiting the front end purification system prior to the dry air stream being introduced to the cold box.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0060] These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, claims, and accompanying drawings. It is to be noted, however, that the drawings illustrate only several embodiments of the invention and are therefore not to be considered limiting of the invention's scope as it can admit to other equally effective embodiments.

[0061] FIG. 1 provides an embodiment of the prior art in which vessel A is adsorbing and vessel B is being regenerated.

[0062] FIG. 2 provides an embodiment of the prior art in which vessel A is adsorbing and vessel B is being pressurized.

[0063] FIG. 3 provides an embodiment of the present invention in which vessel A is adsorbing and vessel B is being regenerated.

[0064] FIG. 4 provides an embodiment of the present invention in which vessel A is adsorbing and vessel B is being pressurized.

[0065] FIG. 5 provides an embodiment of the present invention in which both vessels are adsorbing.

[0066] FIG. 6 provides a graphical comparison between embodiments of the current invention and that of the prior art.

[0067] FIG. 7 provides a graphical representation of the main air compressor flow rate for an embodiment of the prior art.

[0068] FIG. 8 provides a graphical representation of the main air compressor flow rate for an embodiment of the present invention.

[0069] FIG. 9 provides a graphical representation of the flow rate of dry air sent to the cold box for an embodiment of the prior art.

[0070] FIG. 10 provides a graphical representation of the flow rate of dry air sent to the cold box for an embodiment of the present invention.

DETAILED DESCRIPTION

[0071] While the invention will be described in connection with several embodiments, it will be understood that it is not intended to limit the invention to those embodiments. On the contrary, it is intended to cover all the alternatives, modifications and equivalence as may be included within the spirit and scope of the invention defined by the appended claims.

[0072] FIGS. 3 and 4 represent embodiments of the present invention. As in FIG. 1, FIG. 3 represents a flow diagram of front end purification unit 10, in which first adsorber A is in its adsorption cycle and second adsorber B is in the cleaning stage of its regeneration cycle. In this setup, valves 3A, 4A, 5B and 6B are open, while valves 3B, 4B, 5A, 6A, 7, 8A, 8B, 9A, and 9B are closed. As shown in FIG. 3, the flow of gases (air and regeneration gas) during adsorption and cleaning stage of regeneration are largely unchanged. The primary difference between the setups of FIG. 1 and FIG. 3 is the presence of pressurizing gas line 42 and valves 8A and 8B. Bypass valves 9A and 9B are also included, but they are optional, as will be explained in more detail. In a preferred embodiment, pressurizing gas line 42 contains a dry pressurizing gas. However, a wet pressurizing gas can also be used.

[0073] Another optional element is purified air slip stream 28 and compressed air storage 30. In this optional embodiment, a small portion of purified air can be compressed 29 sent to the compressed air storage 30, such that the process can have a storage tank of air, which can be useful for various purposes. In embodiments including compressed air storage 30, not all of the air compressed by the MAC and purified by the adsorbers will be sent to the cold box 20. In one embodiment, purified air slip stream 28 can be a continuous flow, preferably at a constant flow rate. In one embodiment, the percentage flow of purified air slip stream 28 as compared to the flow rate of compressed wet air 2 can be based on the amount of dry gas used during the repressurization vis-à-vis the total adsorber cycle (i.e., adsorption, depressurization, regeneration), and therefore, can be of the order of less than 2% of the flow rate of compressed wet air 2, preferably less than 1%, preferably less than 0.5%, more preferably about 0.05-0.5%.

[0074] Similarly to FIG. 2, FIG. 4 represents a flow diagram of front end purification unit 10 in which first adsorber A is still in an adsorption cycle and second adsorber B is in a repressurization phase (pressurizing back to normal adsorption pressure (e.g., high pressure) to get ready for its adsorption phase). However, instead of using purified air coming from first adsorber A to pressurize second adsorber B by opening valve 7, second adsorber B can be pressurized by introducing a dry, pressurized gas from an external source 40. It is preferable that the gas from the external source 40 is dry; however, in certain embodiments the pressurized gas can be wet.

[0075] In the embodiment shown, 5 moles of pressurizing gas via line 42 pass through valve 8B and into second adsorber B. This allows the flow rate coming from the MAC to remain unchanged (100 moles in this example) while also keeping the flow rate of purified, dry gas 12 sent to the cold box 20 to also remain constant.

[0076] In one embodiment, valves 3A, 3B, 4A, 4B, 5A, 5B, 6A, and 6B, are on/off valves, meaning that they are configured to either be opened or closed, and do not partially open. Valves 7, 8A, 8B, 9A, and 9B are preferably configured to control the flow rates through their respective lines and can be set to any point between closed and fully open.

[0077] In one embodiment, the dry, pressurizing gas 42 can be a pressurized nitrogen rich product stream from the cold box. In another embodiment, the pressurizing gas 42 can come from the same source as the regeneration gas 14, such as a waste nitrogen stream coming from the cold box (e.g., stream 44 by passing through valves 21 and 25 and then pressurized to an appropriate pressure via a pressurizing means not shown). Other sources for pressurizing gas 42 can include nearby pipelines (e.g., a pressurized nitrogen pipeline), pressurized gas vaporized from a liquid storage tank (e.g., nitrogen and/or oxygen storage tank). In yet another embodiment, the pressurizing gas 42 can be introduced to the top of the adsorbers A, B by reversing flow through line 44 and going through valves 5A, 5B as appropriate.

[0078] In an alternate embodiment, pressurizing gas 42 can be a synthetic air stream made of nitrogen 22 (e.g., product and/or waste nitrogen) and oxygen 26 coming from the cold box 20 in respective amounts that simulate the composition of air. Valves 21 and 23 can be configured to control the flow rates of the two gasses appropriately. This preferably allows the pressurization gas to have the same composition as that of air without using any of the purified air coming out of the adsorber vessels A or B. Alternatively, in embodiments with compressed air storage 30, compressed air 32 can be used as the pressurizing gas 42.

[0079] In one embodiment, front end purification unit can include bypass valves 9A and 9B. In embodiments in which the pressurizing gas 42 has a composition different from air, the process can include a switch cycle in which both first adsorber A and second adsorber B receive wet air 2 and operate in a parallel adsorption cycle. This allows for the composition within second adsorber B to slowly equilibrate to that of air while also maintaining the composition of the dry air stream sent to the cold box, thereby reducing any potential process upsets associated with sending a significantly different composition to the cold box.

[0080] As shown in FIG. 5, valves 3A, 4A, 4B and 9B are set to open, while valves 3B, 5A, 5B, 6A, 6B, 7, 8A, 8B, 7, 9A are set to closed. In one embodiment, valve 9B is only slightly open and then gradually opens more and more until the composition within second adsorber B is that of air. In one embodiment, the process can include a step of measuring the composition of the purified gas exiting one or both of the adsorbers, and once they match or are within the tolerance range (e.g., approximately 78% nitrogen), valve 3B can be set to fully open before closing valve 9B, while also closing valves 3A and 4A, thereby placing second adsorber B in the adsorption cycle. In another embodiment, the opening and closing of valves 3A, 3B, 4A and 9B can be set to a predetermined time delay. In one embodiment, this time delay can be based on the time needed to allow adsorber B to approach a composition similar to that of air. Following the switchover, an appropriate depressurizing valve (not shown) would be opened to despressurize adsorber A to essentially atmospheric pressure and then valves 5A and 6A would be opened and the depressurizing valve (not shown) would be closed in order to start the regeneration cycle of first adsorber A.

[0081] Alternatively at the beginning of the parallel run, valve 3A can be closed while valve 9A can be set to fully open. Valve 9A can then subsequently be adjusted in step with the gradual adjustment of valve 9B in order to provide additional fine tuning.

[0082] In one embodiment, gas analyzers 50, 55 can be in electric communication with a controller (not shown) that is configured to adjust the biases of valves, for example valves 3A, 3B, 9A and 9B, based on the composition of the purified dry gas exiting the first and second adsorbers. Those of ordinary skill in the art will also recognize that flow indicators could be included in order to follow the flow rates of various streams. However, these flow indicators have been left off in order to provide more clarity for the figures.

[0083] For illustrative purposes, in one embodiment, the adsorption pressure can be at least 4 bar, preferably between 5-6 bar. Regeneration pressure is preferably just above atmospheric pressure; however, those of ordinary skill in the art will recognize that the pressure can be anything lower than the adsorption pressure that is still effective for removing the adsorbed impurities. In another embodiment, adsorption temperature can be around 55° F.±15° F., while the heating cycle of the regeneration phase can be approximately 300° F. Again, those of ordinary skill in the art will recognize that the temperature can be adjusted in order to improve adsorption or regeneration conditions. In a preferred embodiment, the adsorption cycle can last between 1.5 to 4 hours, with the regeneration cycle also being between 1.5 to 4 hours. In a preferred embodiment, the regeneration cycle can include a depressurization step lasting less than 10 minutes, a heating and subsequent cooling step, a pressurization step of about 10 minutes, and then a switching step of about 10 minutes. In one embodiment, the heating step can last about 36 to 84 minutes and the cooling step can last about 54 to 126 minutes. In a preferred embodiment, the cooling step is approximately 50% longer than the heating step.

[0084] FIG. 6 provides a comparison view of the theoretical advantage of embodiments of the present invention as compared to the methods of the prior art. The top line shows that embodiments of the current invention can flow at approximately 100% of the available MAC flow continuously. The bottom portion of the graph shows that the methods of the prior art have to run at approximately 95% of the available MAC capacity and have periods of 100% available MAC flow only during the pressurization phase.

WORKING EXAMPLES

[0085] FIGS. 7-10 provide actual data for setups of the prior art as compared to an embodiment of the present invention. FIG. 7 provides a graphical representation of MAC air flow rate versus time for methods of the prior art. As can be seen, during periods of pressurization, the MAC air flow rate drastically rises and falls during this period. FIG. 8 provides the same graphical representation of MAC air flow rate as a function of time for an embodiment of the present invention. As can be clearly seen, the MAC air flow rate is much smoother and does not have drastic spikes. Not only does this allow the MAC to be sized in a more efficient manner, it can also help to extend the life of the MAC as it is not experiencing drastic fluctuations throughout its lifecycle.

[0086] FIGS. 9 and 10 provide graphical representations of the air flow sent from the front end purification system to the cold box for the methods of the prior art and methods in accordance with an embodiment of the present invention, respectively. While not as pronounced as the MAC air flow rate, embodiments of the present invention advantageously provide a smaller standard deviation to the cold box (5.72 vs. 4.60). This can be attributed to less disruptions during the process caused by the spiking of the MAC.

[0087] The terms “nitrogen-rich” and “oxygen-rich” will be understood by those skilled in the art to be in reference to the composition of air. As such, nitrogen-rich encompasses a fluid having a nitrogen content greater than that of air. Similarly, oxygen-rich encompasses a fluid having an oxygen content greater than that of air. The term “dry” as it pertains to gases will be understood by those skilled in the art to encompass a gas that has reduced amounts of water vapor as compared to the contaminated wet air (i.e., local atmospheric air).

[0088] While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step.

[0089] The singular forms “a”, “an” and “the” include plural referents, unless the context clearly dictates otherwise.

[0090] “Comprising” in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing (i.e., anything else may be additionally included and remain within the scope of “comprising”). “Comprising” as used herein may be replaced by the more limited transitional terms “consisting essentially of” and “consisting of” unless otherwise indicated herein.

[0091] “Providing” in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary.

[0092] Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.

[0093] Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range.

[0094] All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited.