Apparatus and method for maintaining or increasing liquid production of an air separation unit

Abstract

An apparatus and method for operating an air separation unit (ASU) to provide variable liquid and gas product production. The apparatus includes a main air compressor (MAC), a booster air compressor (BAC), and a cold box containing a cryogenic distillation column system. A controller is configured to switch the ASU between a first mode and a second mode of operation. In the second mode, liquid production is increased by reducing the outlet pressure of the MAC to a level below the operating pressure of a higher-pressure distillation column within the cold box. This pressure differential causes air to flow from the cold box back to the inlet of the BAC, effectively recycling process air to increase the volumetric flow through the BAC and an associated expansion turbine. The method allows for shifting capacities between gaseous and liquid products without requiring increased MAC flow rates or significant capital expenditures.

Description

RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Application Ser. No. 63 / 742,884 filed on Jan. 8, 2025 and U.S. Provisional Application Ser. No. 63 / 742,887 filed Jan. 8, 2025, both of which are hereby incorporated by reference in their entireties.TECHNICAL FIELD OF THE INVENTION

[0002] The present invention generally relates to a method and apparatus for efficiently operating an air separation plant that is configured to produce variable amounts of liquid and gas products.BACKGROUND OF THE INVENTION

[0003] Air separation plants separate atmospheric air into its primary constituents: nitrogen and oxygen, and occasionally argon, xenon and krypton. These gases are sometimes referred to as air gases.

[0004] A typical cryogenic air separation process can include the following steps: (1) filtering the air in order to remove large particulates that might damage the main air compressor; (2) compressing the pre-filtered air in the main air compressor and using interstage cooling to condense some of the water out of the compressed air; (3) passing the compressed air stream through a front-end-purification unit to remove residual water and carbon dioxide; (4) cooling the purified air in a heat exchanger by indirect heat exchange against process streams from the cryogenic distillation column; (5) expanding at least a portion of the cold air to provide refrigeration for the system; (6) introducing the cold air into the distillation column for rectification therein; (7) collecting nitrogen from the top of the column (typically as a gas) and collecting oxygen from the bottom of the column as a liquid.

[0005] In a typical air separation unit (ASU), it is common to use a process configuration utilizing an internal compression (pumping) cycle. As a non-limiting example, this includes pumping the liquid oxygen produced from the lower-pressure column from low pressure to a higher pressure and then vaporizing the pressurized oxygen within the heat exchanger, most commonly against a high pressure air stream coming from a booster air compressor (“BAC”) or from the main air compressor (“MAC”). As used herein, a booster air compressor is a secondary air compressor that is located downstream of the purification unit that is used to boost a portion of the main air feed for purposes of efficiently vaporizing the product liquid oxygen stream.

[0006] Many industrial processes have fluctuating oxygen needs that can be met by cryogenic air separation plants designed to adjust their oxygen production rates. In these plants, liquid oxygen is stored when demand is low, while liquid nitrogen is stored when demand is high. During periods of high demand, the plant vaporizes the stored liquid oxygen to produce gaseous oxygen, while condensing the gaseous nitrogen it generates to provide liquid nitrogen. However, this method suffers from inefficiencies and requires separate equipment for the storage of liquid products.

[0007] In other embodiments of the prior art, the flow rate of process air (feed air) that is supplied by the MAC may be controlled by modulating the MAC inlet guide vanes (IGV). After passing through a precooling and purification process, a portion of feed air can directly feed the cold box as “medium pressure air” (MPair). The remaining feed air can be further compressed by the BAC to supply “high-pressure air” (HPair). Similar to the MAC, HPair flow rate can be controlled by modulating the BAC IGV.

[0008] HPair can also be split into two streams: 1. Liquid air for high pressure oxygen vaporization; 2. Turbo expansion for liquid production. ASU separation capacity, specifically oxygen production rate, can be determined by the amount of feed air, while liquid capacity is set by the amount of HPair.

[0009] The ratio between HPair and feed air usually varies anywhere from 0.5 to 1 depending on liquid production. However, in some cases when high pressure gaseous oxygen (HPGOX) demand is low and / or liquid demand is high, the required HPair flow can be higher than the required feed air flow. A conventional way of achieving this is to artificially increase feed air at least to the amount or higher than required by HPair, and consequently, the extra O2 product must be vented. Doing so inevitably increases MAC power consumption, and ultimately, the cost per unit produced also increases.

[0010] Therefore, it would be advantageous to provide a method and apparatus that operated in a more efficient and less costly manner.SUMMARY OF THE INVENTION

[0011] The present invention is directed to a method and apparatus that satisfies at least one of these needs.

[0012] In one embodiment, the invention can include a method for adjusting the liquid production of the air gases (e.g., nitrogen and oxygen) on demand, thereby reducing power consumption and / or increasing liquid production when desired.

[0013] In one embodiment, an apparatus for the production of air gases by the cryogenic separation of air is provided. In this embodiment, the apparatus may include: a main air compressor configured to compress air to a pressure suitable for the cryogenic rectification of air to produce a compressed humid air stream, the compressed humid air stream having a first pressure Po; a front end purification system configured to purify the compressed humid 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 humid air stream; a booster compressor in fluid communication with the front end purification system, wherein the booster compressor is configured to compress a first portion of the dry air stream to form a boosted air stream, the boosted air stream having a first boosted pressure PB1; a second booster compressor in fluid communication with an outlet of the booster compressor, wherein the second booster compressor is configured to further compress the boosted air stream; a cold box comprising a main heat exchanger, a system of columns having a double column composed of a lower-pressure column and a higher-pressure column, a condenser disposed at a bottom portion of the lower-pressure column, a liquid oxygen pump, a cold turbine, wherein the cold box is configured to receive the boosted air stream under conditions effective to separate air to form an air gas product, wherein the air gas product is selected from the group consisting of oxygen, nitrogen, and combinations thereof; and a controller configured to switch between a first mode of operation and a second mode of operation, wherein the second mode of operation results in increased liquid production as compared to the first mode of operation, wherein during the second mode of operation, the controller is configured to cause an outlet pressure of the main air compressor to be lower than an operating pressure of the higher-pressure column, such that air flows from the cold box to an inlet of the booster air compressor.

[0014] In optional embodiments of the apparatus:

[0015] the process controller is further configured to access process conditions selected from the group consisting of spot pricing data for electricity, local liquid inventories, and combinations thereof;

[0016] during the second mode of operation, the process controller is configured to adjust inlet guide vanes of the main air compressor and inlet guide vanes of the booster air compressor;

[0017] during the second mode of operation, the process controller is configured to adjust flow rates of gaseous stream withdrawn from the lower-pressure column; and / or

[0018] the cold turbine and the second booster compressor share a common shaft.

[0019] In another embodiment, a method for the production of air gases by the cryogenic separation of air is provided. In this embodiment, the method can include the steps of:

[0020] a) compressing air to a pressure suitable for the cryogenic rectification of air to produce a compressed humid air stream, the compressed humid air stream having a first pressure Po;

[0021] b) purifying the compressed humid air stream of water and carbon dioxide within a front end purification system to produce a dry air stream having reduced amounts of water and carbon dioxide as compared to the compressed humid air stream;

[0022] c) compressing a first portion of the dry air stream in a booster compressor to form a boosted air stream, the boosted air stream having a first boosted pressure PB1;

[0023] d) further compressing the boosted air stream in a second booster compressor to form a second boosted air stream;

[0024] e) introducing a second portion of the dry air stream and the boosted air stream to a cold box under conditions effective to separate air to form an air gas product, wherein the air gas product is selected from the group consisting of oxygen, nitrogen, argon, and combinations thereof;

[0025] f) withdrawing the air gas product from the cold box, the air gas product having a first product pressure PP1;

[0026] wherein the method further comprises an increased gas mode and an increased liquid mode, wherein during the increased liquid mode, the method further comprises the step of:

[0027] g) increasing liquid production from the cold box by reducing a flow rate of the second portion of the dry air stream introduced to the cold box.

[0028] In optional embodiments of the method:

[0029] the cold box comprises a main heat exchanger, a system of columns having a double column composed of a lower pressure column and a higher pressure column, a condenser disposed at a bottom portion of the lower pressure column, and a liquid oxygen pump;

[0030] the air gas product is oxygen; and / or

[0031] the air gas product is nitrogen.

[0032] In yet another embodiment, a method for the production of air gases by the cryogenic separation of air is provided. This method can include a first mode of operation and a second mode of operation, wherein during the first mode of operation, the method can include the step of: sending a first air stream and a second air stream to a cold box under conditions effective for cryogenically separating the first and second air streams to form gaseous oxygen, liquid oxygen, gaseous nitrogen, and liquid nitrogen using a system of columns disposed within the cold box, wherein the first air stream is at a higher pressure than the second air stream when entering the cold box; wherein during the second mode of operation, the method includes the step of: increasing liquid production from the cold box by reversing the flow of the second air stream, such that the first air stream has an increased flow rate, which results in increased liquid production from the cold box.BRIEF DESCRIPTION OF THE DRAWINGS

[0033] 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.

[0034] FIG. 1 provides a simplified process flow diagram of an embodiment of the present invention in a first mode of operation.

[0035] FIG. 2 provides a simplified process flow diagram of an embodiment of the present invention in a second mode of operation

[0036] FIG. 3 provides another embodiment of the present invention providing additional detail for the cold box in the first mode of operation.

[0037] FIG. 4 provides another embodiment of the present invention providing additional detail for the cold box in the second mode of operation.DETAILED DESCRIPTION

[0038] 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.

[0039] Due to product demand fluctuation, often times it is economically beneficial for an ASU to be able to exchange the production rates between liquid and gaseous products. In certain embodiments, the invention concerns a method and apparatus for varying production rate of a product in exchange of production rates of other products, more specially, high pressure gaseous oxygen product (HPGOX) in exchange of liquid products, such liquid oxygen (LOX), liquid nitrogen (LIN) and liquid argon (LAR) in an air separation unit (ASU). In certain embodiments, this can be achieved by shifting the capacities between main air compressor (MAC) and booster air compressor (BAC) by recycling a portion of feed air from the cold box while reducing the flow through the MAC. This may be achieved by reducing the MAC flow and reducing the pressure at the outlet of the MAC. By doing this, some of the MPair flowing from the cold turbine will naturally flow back to the inlet of the BAC, thereby increasing the volumetric flow rate through the BAC, which allows for increased liquid production without increasing MAC flow rates or pressure. In short, liquid production can be adjusted without greatly increasing OPEX or CAPEX costs.

[0040] Now turning to FIG. 1, which represents simplified embodiment configured to operate in a first mode (e.g., regular production). Air 2 is introduced into main air compressor 10 and compressed, preferably to a pressure of at least 55 psig to 75 psig (or around 5 psig higher than the pressure of the higher pressure column) as measured by first pressure indicator PI1. The resulting compressed humid air stream 12 is then purified of water and CO2 in front end purification system 20, thereby producing dry air stream 22. This dry air stream is then split into a first portion 24, and a second portion 26, with both portions being sent to the cold box. In the embodiment shown, first portion of dry air stream 24 is sent to booster air compressor 30 and further compressed to form boosted air stream 32 before being further compressed in turbobooster 70, which is preferably powered by a cold turbine (see 90 of FIG. 2), cooled in cooler 75 prior to being introduced to cold box 40. While the embodiment of FIG. 1 shows booster air compressor 30 as a single compressor, those of ordinary skill in the art will recognize that booster air compressor 30 can be more than one physical compressor. Additionally, booster air compressor 30 can also be a multi-stage compressor.

[0041] Within cold box 40, the air is cooled and cryogenically treated in order to separate the air into air gas product 42. First liquid air gas product 44 and / or second liquid air gas product 48 can also be removed from cold box 40. The flow rate of first liquid air gas product 44 can be measured by flow indicator FI2, the flow rate of second liquid air gas product 48 can be measured by flow indicator FI3, and the flow rate of air gas product 42 can be measured by flow indicator FI4.

[0042] In one embodiment, the various pressure and flow indicators / sensors are configured to communicate (e.g., wirelessly or wired communication) with process controller 55, such that the various flow rates and pressures can be monitored by process controller 55, which is configured to adjust various settings throughout the process based on the measured flows and pressures.

[0043] While the figures show direct lines of communication from the various pressure and flow indicators to the process controller 55, embodiments of the invention should not be so limited. Rather, those of ordinary skill in the art will recognize that embodiments of the invention may include instances in which certain indicators communicate directly with a related pressure controller.

[0044] FIG. 2 provides a more detailed view of cold box 40. In this embodiment, cold box 40 also includes heat exchanger 80, turbine 90, valve 100, double column 110, higher pressure column 120, auxiliary heat exchanger 130, lower pressure column 140, condenser / reboiler 150, and liquid oxygen pump 160. Turbine 90 can be attached to turbobooster 70 via a common shaft. Just like in FIG. 1, air 2 is introduced into main air compressor 10 and compressed, preferably to a pressure of at least 55 psig to 75 psig (or around 5 psig higher than the pressure of the higher-pressure column). The resulting compressed humid air stream 12 is then purified of water and CO2 in front end purification system 20, thereby producing dry air stream 22. A first portion of dry air stream 24 is sent to booster air compressor 30, with the remaining portion of dry air stream 26 entering cold box 40, wherein it is fully cooled in heat exchanger 80 before being introduced via line 94 to higher pressure column 120 for separation therein.

[0045] Following pressurization in booster air compressor 30, boosted air stream 32 is preferably further compressed in secondary booster 70 and then cooled in after cooler 75. The resulting stream is then split into first boosted stream 74 and second boosted stream 72. Second boosted stream 72 undergoes partial cooling in heat exchanger 80, wherein it is withdrawn from an intermediate section of heat exchanger 80 and then expanded in turbine 90, thereby forming expanded air stream 92, which can then be combined with second portion of dry air stream 26 before introduction to higher pressure column 120

[0046] First boosted stream 74 is preferably fully cooled in heat exchanger 80 and then expanded across valve 100, before being introduced into a bottom portion of higher pressure column 120 for separation therein.

[0047] Higher pressure column 120 is configured to allow for rectification of air within, thereby producing an oxygen-rich liquid 122 at the bottom and a nitrogen-rich gaseous stream 126 at the top. Oxygen-rich liquid 122 is withdrawn from the bottom of higher pressure column 120 before exchanging heat with low pressure waste nitrogen 114 and low pressure nitrogen product 112 in auxiliary heat exchanger 130, and then expanded across a valve and introduced into lower pressure column 140.

[0048] As is well known in the art, higher pressure column 120 and lower pressure column 140 are part of double column 110, and the two columns are thermally coupled via condenser / reboiler 150, which condenses rising nitrogen rich gas from higher pressure column 120 and vaporizes liquid oxygen that has collected at the bottom of lower pressure column 140. In the embodiment shown, two nitrogen-rich gas streams 126, 128 are withdrawn from higher pressure column 120, exchange heat with low pressure nitrogen product 112 and low pressure waste nitrogen 114, subsequently expanded across their respective valves 226, 228, and then introduced into lower pressure column 140. Medium pressure nitrogen product 129 can also be withdrawn from higher pressure column 120 and then warmed in heat exchanger 80.

[0049] Liquid oxygen collects at the bottom of lower pressure column 140 and is withdrawn and pressurized to an appropriate pressure by liquid oxygen pump 160 to form liquid oxygen 162. Liquid oxygen 162 is then vaporized within heat exchanger 80 to form air gas product 42. The pressure and flow rate of air gas product 42 can be measured via second pressure sensor PI2 and FI4, respectively. Liquid oxygen product 44 from liquid oxygen pump 160 is delivered to the storage (not shown). Liquid nitrogen product 48 from top of lower pressure column 140 is delivered to the storage (not shown).

[0050] FIG. 3 represents a simplified process flow diagram for when liquid demand (e.g., LIN, LOX, and / or LAR) increases. In this embodiment, the goal is to make the flow of feed air 2 through MAC 10 to be lower than the flow of air through the BAC 30. This can be achieved by ensuring that the pressure of the high-pressure column 120 is greater than the pressure at the outlet of the MAC 10.

[0051] There are multiple ways of achieving this effect. For example, the MAC IGV can be adjusted in order to reduce the flow rate of feed air 2 processed by MAC 10. Additionally, the outlet pressure of the MAC (as measured by first pressure indicator PI1), is made to be lower than the operating pressure of the higher-pressure distillation column, which forces MPair to flow from the cold box 40 back to BAC 30. In short, the flow of air through line 26 is reversed, thereby allowing for the BAC to have a higher flow rate of air as compared to the MAC.

[0052] In this mode of increased liquid production, the BAC flow rate has been increased to allow for an increased flow of the second boosted stream 72 through the turbine 90 (which increases liquid production). Potentially, the BAC flow 32 can be raised higher than the MAC flow 12 if the pressure of the higher-pressure column is greater than the pressure at first pressure indicator PI1 (i.e., the MAC outlet and BAC inlet). This naturally forces some flow from the discharge of the turbine 90 to flow to the inlet of the BAC (i.e., reversed or recycled flow through the MP air passage 26 in the heat exchanger). In certain embodiments, the column pressures can be adjusted higher by adjusting gas flows from the lower pressure column 140 (e.g., adjusting flow rates of streams 114 and 112). In certain embodiments, the gas flows can be controlled via valves 212 and / or 214, which are preferably in communication with controller 55 (not shown in order to reduce unnecessary clutter).

[0053] In certain embodiments, the amount of flow being recycled can be controlled by controlling the flow difference between the MAC and BAC via controlling of the inlet guide vanes for each of the MAC and the BAC. In certain embodiments, controller 55 can communicate (A, B) with MAC 10 and BAC 30 to adjust their respective inlet guide vanes, with the respective flow rates being monitored by their respective flow indicators FI1, FI5.

[0054] FIG. 4 provides a more detailed version of increased liquid production shown in FIG. 3. In this mode of increased liquid production, the BAC flow rate has been increased to allow for an increased flow of the second boosted stream 72 through the turbine 90 (which increases liquid production). Potentially, the BAC flow 32 can be raised higher than the MAC flow 12 if the pressure of the higher-pressure column is greater than the pressure at first pressure indicator PI1 (i.e., the MAC outlet and BAC inlet). This naturally forces some flow from the discharge of the turbine 90 to flow to the inlet of the BAC (i.e., reversed or recycled flow through the MP air passage 26 in the heat exchanger). In certain embodiments, the column pressures can be adjusted higher by adjusting gas flows from the lower pressure column 140 (e.g., adjusting flow rates of streams 114 and 112). In certain embodiments, the gas flows can be controlled via valves 212 and / or 214, which are preferably in communication with controller 55 (not shown in order to reduce unnecessary clutter).

[0055] In certain embodiments, the amount of flow being recycled can be controlled by controlling the flow difference between the MAC and BAC via controlling of the inlet guide vanes for each of the MAC and the BAC. In certain embodiments, controller 55 can communicate (A, B) with MAC 10 and BAC 30 to adjust their respective inlet guide vanes, with the respective flow rates being monitored by their respective flow indicators FI1, FI5.

[0056] By reversing the flow of air through line 26, the BAC 30 suction pressure increases, thereby increasing the available capacity of the BAC either in term of flow rate or discharge pressure, which in turn enables an increased flow and / or pressure across turbine 90. This ultimately results in increased liquid production due to increased refrigeration capacity (i.e., lower resulting temperature and increased flow rate of stream 92). At the same time, the available capacity of the MAC 10 will decrease due to the increase in back pressure, which causes a separation capacity (i.e., HPGOX production 42) to decrease.

[0057] Although this disclosure discloses the embodiments with respect to GOX product as an example, the concept can easily be applied to any other product (for example, high pressure gaseous nitrogen) that is produced by internal compression process

[0058] In another embodiment, process controller 55 can be configured to access spot pricing data (or the user can input data into the controller), such that process controller 55 can be configured to optimize / adjust the amount of increased LIN and / or LOX based upon the current spot pricing data. Similarly, process controller 55 can also be configured to keep track of local inventories of LIN and / or LOX, and make adjustments to the production of LIN and / or LOX based on this additional data.

[0059] In another embodiment, process controller 55 can determine whether to operate in power savings mode or additional liquid production mode based upon certain conditions. For example, if electricity is cheaper than normal, saving power might not be of great importance, and therefore, process controller 55 can make a determination to switch to liquid production mode. In a preferred embodiment, process controller 55 makes these decisions automatically based on input conditions. In another embodiment, process controller 55 can include a manual override.

[0060] 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.

[0061] 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.

[0062] The singular forms “a”, “an” and “the” include plural referents, unless the context clearly dictates otherwise.

[0063] “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.

[0064] “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.

[0065] 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.

[0066] 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.

[0067] 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.

Examples

Embodiment Construction

[0038]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.

[0039]Due to product demand fluctuation, often times it is economically beneficial for an ASU to be able to exchange the production rates between liquid and gaseous products. In certain embodiments, the invention concerns a method and apparatus for varying production rate of a product in exchange of production rates of other products, more specially, high pressure gaseous oxygen product (HPGOX) in exchange of liquid products, such liquid oxygen (LOX), liquid nitrogen (LIN) and liquid argon (LAR) in an air separation unit (ASU). In certain embodiments, this can be achieved by shifting the capacities between main air compressor ...

RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Application Ser. No. 63 / 742,884 filed on Jan. 8, 2025 and U.S. Provisional Application Ser. No. 63 / 742,887 filed Jan. 8, 2025, both of which are hereby incorporated by reference in their entireties.TECHNICAL FIELD OF THE INVENTION

[0002] The present invention generally relates to a method and apparatus for efficiently operating an air separation plant that is configured to produce variable amounts of liquid and gas products.BACKGROUND OF THE INVENTION

[0003] Air separation plants separate atmospheric air into its primary constituents: nitrogen and oxygen, and occasionally argon, xenon and krypton. These gases are sometimes referred to as air gases.

[0004] A typical cryogenic air separation process can include the following steps: (1) filtering the air in order to remove large particulates that might damage the main air compressor; (2) compressing the pre-filtered air in the main air compressor and using interstage cooling to condense some of the water out of the compressed air; (3) passing the compressed air stream through a front-end-purification unit to remove residual water and carbon dioxide; (4) cooling the purified air in a heat exchanger by indirect heat exchange against process streams from the cryogenic distillation column; (5) expanding at least a portion of the cold air to provide refrigeration for the system; (6) introducing the cold air into the distillation column for rectification therein; (7) collecting nitrogen from the top of the column (typically as a gas) and collecting oxygen from the bottom of the column as a liquid.

[0005] In a typical air separation unit (ASU), it is common to use a process configuration utilizing an internal compression (pumping) cycle. As a non-limiting example, this includes pumping the liquid oxygen produced from the lower-pressure column from low pressure to a higher pressure and then vaporizing the pressurized oxygen within the heat exchanger, most commonly against a high pressure air stream coming from a booster air compressor (“BAC”) or from the main air compressor (“MAC”). As used herein, a booster air compressor is a secondary air compressor that is located downstream of the purification unit that is used to boost a portion of the main air feed for purposes of efficiently vaporizing the product liquid oxygen stream.

[0006] Many industrial processes have fluctuating oxygen needs that can be met by cryogenic air separation plants designed to adjust their oxygen production rates. In these plants, liquid oxygen is stored when demand is low, while liquid nitrogen is stored when demand is high. During periods of high demand, the plant vaporizes the stored liquid oxygen to produce gaseous oxygen, while condensing the gaseous nitrogen it generates to provide liquid nitrogen. However, this method suffers from inefficiencies and requires separate equipment for the storage of liquid products.

[0007] In other embodiments of the prior art, the flow rate of process air (feed air) that is supplied by the MAC may be controlled by modulating the MAC inlet guide vanes (IGV). After passing through a precooling and purification process, a portion of feed air can directly feed the cold box as “medium pressure air” (MPair). The remaining feed air can be further compressed by the BAC to supply “high-pressure air” (HPair). Similar to the MAC, HPair flow rate can be controlled by modulating the BAC IGV.

[0008] HPair can also be split into two streams: 1. Liquid air for high pressure oxygen vaporization; 2. Turbo expansion for liquid production. ASU separation capacity, specifically oxygen production rate, can be determined by the amount of feed air, while liquid capacity is set by the amount of HPair.

[0009] The ratio between HPair and feed air usually varies anywhere from 0.5 to 1 depending on liquid production. However, in some cases when high pressure gaseous oxygen (HPGOX) demand is low and / or liquid demand is high, the required HPair flow can be higher than the required feed air flow. A conventional way of achieving this is to artificially increase feed air at least to the amount or higher than required by HPair, and consequently, the extra O2 product must be vented. Doing so inevitably increases MAC power consumption, and ultimately, the cost per unit produced also increases.

[0010] Therefore, it would be advantageous to provide a method and apparatus that operated in a more efficient and less costly manner.SUMMARY OF THE INVENTION

[0011] The present invention is directed to a method and apparatus that satisfies at least one of these needs.

[0012] In one embodiment, the invention can include a method for adjusting the liquid production of the air gases (e.g., nitrogen and oxygen) on demand, thereby reducing power consumption and / or increasing liquid production when desired.

[0013] In one embodiment, an apparatus for the production of air gases by the cryogenic separation of air is provided. In this embodiment, the apparatus may include: a main air compressor configured to compress air to a pressure suitable for the cryogenic rectification of air to produce a compressed humid air stream, the compressed humid air stream having a first pressure Po; a front end purification system configured to purify the compressed humid 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 humid air stream; a booster compressor in fluid communication with the front end purification system, wherein the booster compressor is configured to compress a first portion of the dry air stream to form a boosted air stream, the boosted air stream having a first boosted pressure PB1; a second booster compressor in fluid communication with an outlet of the booster compressor, wherein the second booster compressor is configured to further compress the boosted air stream; a cold box comprising a main heat exchanger, a system of columns having a double column composed of a lower-pressure column and a higher-pressure column, a condenser disposed at a bottom portion of the lower-pressure column, a liquid oxygen pump, a cold turbine, wherein the cold box is configured to receive the boosted air stream under conditions effective to separate air to form an air gas product, wherein the air gas product is selected from the group consisting of oxygen, nitrogen, and combinations thereof; and a controller configured to switch between a first mode of operation and a second mode of operation, wherein the second mode of operation results in increased liquid production as compared to the first mode of operation, wherein during the second mode of operation, the controller is configured to cause an outlet pressure of the main air compressor to be lower than an operating pressure of the higher-pressure column, such that air flows from the cold box to an inlet of the booster air compressor.

[0014] In optional embodiments of the apparatus:

[0015] the process controller is further configured to access process conditions selected from the group consisting of spot pricing data for electricity, local liquid inventories, and combinations thereof;

[0016] during the second mode of operation, the process controller is configured to adjust inlet guide vanes of the main air compressor and inlet guide vanes of the booster air compressor;

[0017] during the second mode of operation, the process controller is configured to adjust flow rates of gaseous stream withdrawn from the lower-pressure column; and / or

[0018] the cold turbine and the second booster compressor share a common shaft.

[0019] In another embodiment, a method for the production of air gases by the cryogenic separation of air is provided. In this embodiment, the method can include the steps of:

[0020] a) compressing air to a pressure suitable for the cryogenic rectification of air to produce a compressed humid air stream, the compressed humid air stream having a first pressure Po;

[0021] b) purifying the compressed humid air stream of water and carbon dioxide within a front end purification system to produce a dry air stream having reduced amounts of water and carbon dioxide as compared to the compressed humid air stream;

[0022] c) compressing a first portion of the dry air stream in a booster compressor to form a boosted air stream, the boosted air stream having a first boosted pressure PB1;

[0023] d) further compressing the boosted air stream in a second booster compressor to form a second boosted air stream;

[0024] e) introducing a second portion of the dry air stream and the boosted air stream to a cold box under conditions effective to separate air to form an air gas product, wherein the air gas product is selected from the group consisting of oxygen, nitrogen, argon, and combinations thereof;

[0025] f) withdrawing the air gas product from the cold box, the air gas product having a first product pressure PP1;

[0026] wherein the method further comprises an increased gas mode and an increased liquid mode, wherein during the increased liquid mode, the method further comprises the step of:

[0027] g) increasing liquid production from the cold box by reducing a flow rate of the second portion of the dry air stream introduced to the cold box.

[0028] In optional embodiments of the method:

[0029] the cold box comprises a main heat exchanger, a system of columns having a double column composed of a lower pressure column and a higher pressure column, a condenser disposed at a bottom portion of the lower pressure column, and a liquid oxygen pump;

[0030] the air gas product is oxygen; and / or

[0031] the air gas product is nitrogen.

[0032] In yet another embodiment, a method for the production of air gases by the cryogenic separation of air is provided. This method can include a first mode of operation and a second mode of operation, wherein during the first mode of operation, the method can include the step of: sending a first air stream and a second air stream to a cold box under conditions effective for cryogenically separating the first and second air streams to form gaseous oxygen, liquid oxygen, gaseous nitrogen, and liquid nitrogen using a system of columns disposed within the cold box, wherein the first air stream is at a higher pressure than the second air stream when entering the cold box; wherein during the second mode of operation, the method includes the step of: increasing liquid production from the cold box by reversing the flow of the second air stream, such that the first air stream has an increased flow rate, which results in increased liquid production from the cold box.BRIEF DESCRIPTION OF THE DRAWINGS

[0033] 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.

[0034] FIG. 1 provides a simplified process flow diagram of an embodiment of the present invention in a first mode of operation.

[0035] FIG. 2 provides a simplified process flow diagram of an embodiment of the present invention in a second mode of operation

[0036] FIG. 3 provides another embodiment of the present invention providing additional detail for the cold box in the first mode of operation.

[0037] FIG. 4 provides another embodiment of the present invention providing additional detail for the cold box in the second mode of operation.DETAILED DESCRIPTION

[0038] 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.

[0039] Due to product demand fluctuation, often times it is economically beneficial for an ASU to be able to exchange the production rates between liquid and gaseous products. In certain embodiments, the invention concerns a method and apparatus for varying production rate of a product in exchange of production rates of other products, more specially, high pressure gaseous oxygen product (HPGOX) in exchange of liquid products, such liquid oxygen (LOX), liquid nitrogen (LIN) and liquid argon (LAR) in an air separation unit (ASU). In certain embodiments, this can be achieved by shifting the capacities between main air compressor (MAC) and booster air compressor (BAC) by recycling a portion of feed air from the cold box while reducing the flow through the MAC. This may be achieved by reducing the MAC flow and reducing the pressure at the outlet of the MAC. By doing this, some of the MPair flowing from the cold turbine will naturally flow back to the inlet of the BAC, thereby increasing the volumetric flow rate through the BAC, which allows for increased liquid production without increasing MAC flow rates or pressure. In short, liquid production can be adjusted without greatly increasing OPEX or CAPEX costs.

[0040] Now turning to FIG. 1, which represents simplified embodiment configured to operate in a first mode (e.g., regular production). Air 2 is introduced into main air compressor 10 and compressed, preferably to a pressure of at least 55 psig to 75 psig (or around 5 psig higher than the pressure of the higher pressure column) as measured by first pressure indicator PI1. The resulting compressed humid air stream 12 is then purified of water and CO2 in front end purification system 20, thereby producing dry air stream 22. This dry air stream is then split into a first portion 24, and a second portion 26, with both portions being sent to the cold box. In the embodiment shown, first portion of dry air stream 24 is sent to booster air compressor 30 and further compressed to form boosted air stream 32 before being further compressed in turbobooster 70, which is preferably powered by a cold turbine (see 90 of FIG. 2), cooled in cooler 75 prior to being introduced to cold box 40. While the embodiment of FIG. 1 shows booster air compressor 30 as a single compressor, those of ordinary skill in the art will recognize that booster air compressor 30 can be more than one physical compressor. Additionally, booster air compressor 30 can also be a multi-stage compressor.

[0041] Within cold box 40, the air is cooled and cryogenically treated in order to separate the air into air gas product 42. First liquid air gas product 44 and / or second liquid air gas product 48 can also be removed from cold box 40. The flow rate of first liquid air gas product 44 can be measured by flow indicator FI2, the flow rate of second liquid air gas product 48 can be measured by flow indicator FI3, and the flow rate of air gas product 42 can be measured by flow indicator FI4.

[0042] In one embodiment, the various pressure and flow indicators / sensors are configured to communicate (e.g., wirelessly or wired communication) with process controller 55, such that the various flow rates and pressures can be monitored by process controller 55, which is configured to adjust various settings throughout the process based on the measured flows and pressures.

[0043] While the figures show direct lines of communication from the various pressure and flow indicators to the process controller 55, embodiments of the invention should not be so limited. Rather, those of ordinary skill in the art will recognize that embodiments of the invention may include instances in which certain indicators communicate directly with a related pressure controller.

[0044] FIG. 2 provides a more detailed view of cold box 40. In this embodiment, cold box 40 also includes heat exchanger 80, turbine 90, valve 100, double column 110, higher pressure column 120, auxiliary heat exchanger 130, lower pressure column 140, condenser / reboiler 150, and liquid oxygen pump 160. Turbine 90 can be attached to turbobooster 70 via a common shaft. Just like in FIG. 1, air 2 is introduced into main air compressor 10 and compressed, preferably to a pressure of at least 55 psig to 75 psig (or around 5 psig higher than the pressure of the higher-pressure column). The resulting compressed humid air stream 12 is then purified of water and CO2 in front end purification system 20, thereby producing dry air stream 22. A first portion of dry air stream 24 is sent to booster air compressor 30, with the remaining portion of dry air stream 26 entering cold box 40, wherein it is fully cooled in heat exchanger 80 before being introduced via line 94 to higher pressure column 120 for separation therein.

[0045] Following pressurization in booster air compressor 30, boosted air stream 32 is preferably further compressed in secondary booster 70 and then cooled in after cooler 75. The resulting stream is then split into first boosted stream 74 and second boosted stream 72. Second boosted stream 72 undergoes partial cooling in heat exchanger 80, wherein it is withdrawn from an intermediate section of heat exchanger 80 and then expanded in turbine 90, thereby forming expanded air stream 92, which can then be combined with second portion of dry air stream 26 before introduction to higher pressure column 120

[0046] First boosted stream 74 is preferably fully cooled in heat exchanger 80 and then expanded across valve 100, before being introduced into a bottom portion of higher pressure column 120 for separation therein.

[0047] Higher pressure column 120 is configured to allow for rectification of air within, thereby producing an oxygen-rich liquid 122 at the bottom and a nitrogen-rich gaseous stream 126 at the top. Oxygen-rich liquid 122 is withdrawn from the bottom of higher pressure column 120 before exchanging heat with low pressure waste nitrogen 114 and low pressure nitrogen product 112 in auxiliary heat exchanger 130, and then expanded across a valve and introduced into lower pressure column 140.

[0048] As is well known in the art, higher pressure column 120 and lower pressure column 140 are part of double column 110, and the two columns are thermally coupled via condenser / reboiler 150, which condenses rising nitrogen rich gas from higher pressure column 120 and vaporizes liquid oxygen that has collected at the bottom of lower pressure column 140. In the embodiment shown, two nitrogen-rich gas streams 126, 128 are withdrawn from higher pressure column 120, exchange heat with low pressure nitrogen product 112 and low pressure waste nitrogen 114, subsequently expanded across their respective valves 226, 228, and then introduced into lower pressure column 140. Medium pressure nitrogen product 129 can also be withdrawn from higher pressure column 120 and then warmed in heat exchanger 80.

[0049] Liquid oxygen collects at the bottom of lower pressure column 140 and is withdrawn and pressurized to an appropriate pressure by liquid oxygen pump 160 to form liquid oxygen 162. Liquid oxygen 162 is then vaporized within heat exchanger 80 to form air gas product 42. The pressure and flow rate of air gas product 42 can be measured via second pressure sensor PI2 and FI4, respectively. Liquid oxygen product 44 from liquid oxygen pump 160 is delivered to the storage (not shown). Liquid nitrogen product 48 from top of lower pressure column 140 is delivered to the storage (not shown).

[0050] FIG. 3 represents a simplified process flow diagram for when liquid demand (e.g., LIN, LOX, and / or LAR) increases. In this embodiment, the goal is to make the flow of feed air 2 through MAC 10 to be lower than the flow of air through the BAC 30. This can be achieved by ensuring that the pressure of the high-pressure column 120 is greater than the pressure at the outlet of the MAC 10.

[0051] There are multiple ways of achieving this effect. For example, the MAC IGV can be adjusted in order to reduce the flow rate of feed air 2 processed by MAC 10. Additionally, the outlet pressure of the MAC (as measured by first pressure indicator PI1), is made to be lower than the operating pressure of the higher-pressure distillation column, which forces MPair to flow from the cold box 40 back to BAC 30. In short, the flow of air through line 26 is reversed, thereby allowing for the BAC to have a higher flow rate of air as compared to the MAC.

[0052] In this mode of increased liquid production, the BAC flow rate has been increased to allow for an increased flow of the second boosted stream 72 through the turbine 90 (which increases liquid production). Potentially, the BAC flow 32 can be raised higher than the MAC flow 12 if the pressure of the higher-pressure column is greater than the pressure at first pressure indicator PI1 (i.e., the MAC outlet and BAC inlet). This naturally forces some flow from the discharge of the turbine 90 to flow to the inlet of the BAC (i.e., reversed or recycled flow through the MP air passage 26 in the heat exchanger). In certain embodiments, the column pressures can be adjusted higher by adjusting gas flows from the lower pressure column 140 (e.g., adjusting flow rates of streams 114 and 112). In certain embodiments, the gas flows can be controlled via valves 212 and / or 214, which are preferably in communication with controller 55 (not shown in order to reduce unnecessary clutter).

[0053] In certain embodiments, the amount of flow being recycled can be controlled by controlling the flow difference between the MAC and BAC via controlling of the inlet guide vanes for each of the MAC and the BAC. In certain embodiments, controller 55 can communicate (A, B) with MAC 10 and BAC 30 to adjust their respective inlet guide vanes, with the respective flow rates being monitored by their respective flow indicators FI1, FI5.

[0054] FIG. 4 provides a more detailed version of increased liquid production shown in FIG. 3. In this mode of increased liquid production, the BAC flow rate has been increased to allow for an increased flow of the second boosted stream 72 through the turbine 90 (which increases liquid production). Potentially, the BAC flow 32 can be raised higher than the MAC flow 12 if the pressure of the higher-pressure column is greater than the pressure at first pressure indicator PI1 (i.e., the MAC outlet and BAC inlet). This naturally forces some flow from the discharge of the turbine 90 to flow to the inlet of the BAC (i.e., reversed or recycled flow through the MP air passage 26 in the heat exchanger). In certain embodiments, the column pressures can be adjusted higher by adjusting gas flows from the lower pressure column 140 (e.g., adjusting flow rates of streams 114 and 112). In certain embodiments, the gas flows can be controlled via valves 212 and / or 214, which are preferably in communication with controller 55 (not shown in order to reduce unnecessary clutter).

[0055] In certain embodiments, the amount of flow being recycled can be controlled by controlling the flow difference between the MAC and BAC via controlling of the inlet guide vanes for each of the MAC and the BAC. In certain embodiments, controller 55 can communicate (A, B) with MAC 10 and BAC 30 to adjust their respective inlet guide vanes, with the respective flow rates being monitored by their respective flow indicators FI1, FI5.

[0056] By reversing the flow of air through line 26, the BAC 30 suction pressure increases, thereby increasing the available capacity of the BAC either in term of flow rate or discharge pressure, which in turn enables an increased flow and / or pressure across turbine 90. This ultimately results in increased liquid production due to increased refrigeration capacity (i.e., lower resulting temperature and increased flow rate of stream 92). At the same time, the available capacity of the MAC 10 will decrease due to the increase in back pressure, which causes a separation capacity (i.e., HPGOX production 42) to decrease.

[0057] Although this disclosure discloses the embodiments with respect to GOX product as an example, the concept can easily be applied to any other product (for example, high pressure gaseous nitrogen) that is produced by internal compression process

[0058] In another embodiment, process controller 55 can be configured to access spot pricing data (or the user can input data into the controller), such that process controller 55 can be configured to optimize / adjust the amount of increased LIN and / or LOX based upon the current spot pricing data. Similarly, process controller 55 can also be configured to keep track of local inventories of LIN and / or LOX, and make adjustments to the production of LIN and / or LOX based on this additional data.

[0059] In another embodiment, process controller 55 can determine whether to operate in power savings mode or additional liquid production mode based upon certain conditions. For example, if electricity is cheaper than normal, saving power might not be of great importance, and therefore, process controller 55 can make a determination to switch to liquid production mode. In a preferred embodiment, process controller 55 makes these decisions automatically based on input conditions. In another embodiment, process controller 55 can include a manual override.

[0060] 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.

[0061] 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.

[0062] The singular forms “a”, “an” and “the” include plural referents, unless the context clearly dictates otherwise.

[0063] “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.

[0064] “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.

[0065] 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.

[0066] 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.

[0067] 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.

Examples

Embodiment Construction

[0038]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.

[0039]Due to product demand fluctuation, often times it is economically beneficial for an ASU to be able to exchange the production rates between liquid and gaseous products. In certain embodiments, the invention concerns a method and apparatus for varying production rate of a product in exchange of production rates of other products, more specially, high pressure gaseous oxygen product (HPGOX) in exchange of liquid products, such liquid oxygen (LOX), liquid nitrogen (LIN) and liquid argon (LAR) in an air separation unit (ASU). In certain embodiments, this can be achieved by shifting the capacities between main air compressor ...

Claims

1. An apparatus for the production of air gases by the cryogenic separation of air, the apparatus comprising:a) a main air compressor configured to compress air to a pressure suitable for the cryogenic rectification of air to produce a compressed humid air stream, the compressed humid air stream having a first pressure Po;b) a front end purification system configured to purify the compressed humid 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 humid air stream;c) a booster compressor in fluid communication with the front end purification system, wherein the booster compressor is configured to compress a first portion of the dry air stream to form a boosted air stream, the boosted air stream having a first boosted pressure PB1;d) a second booster compressor in fluid communication with an outlet of the booster compressor, wherein the second booster compressor is configured to further compress the boosted air stream;e) a cold box comprising a main heat exchanger, a system of columns having a double column composed of a lower-pressure column and a higher-pressure column, a condenser disposed at a bottom portion of the lower-pressure column, a liquid oxygen pump, a cold turbine, wherein the cold box is configured to receive the boosted air stream under conditions effective to separate air to form an air gas product, wherein the air gas product is selected from the group consisting of oxygen, nitrogen, and combinations thereof; andf) a controller configured to switch between a first mode of operation and a second mode of operation, wherein the second mode of operation results in increased liquid production as compared to the first mode of operation,wherein during the second mode of operation, the controller is configured to cause an outlet pressure of the main air compressor to be lower than an operating pressure of the higher-pressure column, such that air flows from the cold box to an inlet of the booster air compressor.

2. The apparatus as claimed in claim 1, wherein the process controller is further configured to access process conditions selected from the group consisting of spot pricing data for electricity, local liquid inventories, and combinations thereof.

3. The apparatus as claimed in claim 1, wherein during the second mode of operation, the process controller is configured to adjust inlet guide vanes of the main air compressor and inlet guide vanes of the booster air compressor.

4. The apparatus as claimed in claim 1, wherein during the second mode of operation, the process controller is configured to adjust flow rates of gaseous stream withdrawn from the lower-pressure column.

5. The apparatus as claimed in claim 1, wherein the cold turbine and the second booster compressor share a common shaft.

6. A method for the production of air gases by the cryogenic separation of air, the method comprising the steps of:a) compressing air to a pressure suitable for the cryogenic rectification of air to produce a compressed humid air stream, the compressed humid air stream having a first pressure Po;b) purifying the compressed humid air stream of water and carbon dioxide within a front end purification system to produce a dry air stream having reduced amounts of water and carbon dioxide as compared to the compressed humid air stream;c) compressing a first portion of the dry air stream in a booster compressor to form a boosted air stream, the boosted air stream having a first boosted pressure PB1;d) further compressing the boosted air stream in a second booster compressor to form a second boosted air stream;e) introducing a second portion of the dry air stream and the second boosted air stream to a cold box under conditions effective to separate air to form an air gas product, wherein the air gas product is selected from the group consisting of oxygen, nitrogen, argon, and combinations thereof;f) withdrawing the air gas product from the cold box, the air gas product having a first product pressure PP1;wherein the method further comprises an increased gas mode and an increased liquid mode, wherein during the increased liquid mode, the method further comprises the step of:g) increasing liquid production from the cold box by reducing a flow rate of the second portion of the dry air stream introduced to the cold box.

7. The method as claimed in claim 6, wherein the cold box comprises a main heat exchanger, a system of columns having a double column composed of a lower pressure column and a higher pressure column, a condenser disposed at a bottom portion of the lower pressure column, and a liquid oxygen pump.

8. The method as claimed in claim 7, wherein the air gas product is oxygen.

9. The method as claimed in claim 7, wherein the air gas product is nitrogen.

10. A method for the production of air gases by the cryogenic separation of air, the method comprises a first mode of operation and a second mode of operation, wherein during the first mode of operation, the method comprises the step of:sending a first air stream and a second air stream to a cold box under conditions effective for cryogenically separating the first and second air streams to form gaseous oxygen, liquid oxygen, gaseous nitrogen, and liquid nitrogen using a system of columns disposed within the cold box, wherein the first air stream is at a higher pressure than the second air stream when entering the cold box;wherein during the second mode of operation, the method comprises the step of:increasing liquid production from the cold box by reversing the flow of the second air stream, such that the first air stream has an increased flow rate, which results in increased liquid production from the cold box.

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