Method and apparatus for operating an air separation plant

Abstract

A method and apparatus for the production of air gases with variable liquid production by the cryogenic separation of air can include the steps of sending a purified and compressed air stream to a cold box under conditions effective for cryogenically separating the air stream into an oxygen product and nitrogen using a system of columns, wherein the purified and compressed air stream is at a feed pressure when entering the system of columns; withdrawing the oxygen product at a product pressure; delivering the oxygen product at a delivery pressure to an oxygen pipeline, wherein the oxygen pipeline has a pipeline pressure; wherein during the second mode of operation, the method can include monitoring the pipeline pressure; reducing the difference between the pipeline pressure and the delivery pressure; and adjusting liquid production from the cold box. By operating the method in a dynamic fashion, additional liquid production can be realized in instances in which the pipeline pressure deviates from its highest value.

Description

[0001] The present invention generally relates to a method and apparatus for efficiently operating an air separation plant that feeds at least one of its products to a pipeline.

[0002] 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.

[0003] 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.

[0004] In certain cases, the air separation unit ("ASU") can be used to supply one of its air gases to a nearby pipeline (e.g., an oxygen or nitrogen pipeline) in order to supply one or more customers that are not located immediately near the ASU. In a typical ASU supplying a local pipeline, it is common to use a process configuration utilizing an internal compression (pumping) cycle, which in the case of an oxygen pipeline, means that the liquid oxygen produced from the lower pressure column is pumped from low pressure to a higher pressure than that of the pipeline and vaporized 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.

[0005] Under normal conditions, the ASU feeding oxygen to the oxygen pipeline is designed to produce oxygen at a constant pressure. This is because ASUs operate most efficiently at steady state conditions. However, pipelines do not operate at constant pressures. For example, it is not uncommon for an oxygen pipeline to operate between 27.6 to 41.3 barg (400 and 600 psig) (i.e., about a 13.7 bar (200 psig) pressure variance) during a single day. This can occur due to variable customer demand and / or variable supply to the pipeline. In the prior art known heretofore, it is customary to design the ASU to provide the oxygen gas at a constant pressure that is above the highest pressures expected for the pipeline. In order to address the problem associated with pipeline pressure variance, it is customary to let down the pressure of the gaseous oxygen across a control valve to approximately match the pressure of the pipeline just prior to introducing the oxygen gas to the pipeline. However, this method suffers from inefficiencies anytime the pipeline pressure is below that of the design pressure of the ASU. Therefore, it would be advantageous to provide a method and apparatus that operated in a more efficient manner. When the pipeline pressure falls, it is a natural step to reduce the pressure of the product being sent to the pipeline, so as to avoid the wasteful expansion step described above. This is the approach chosen in US2008 / 047298 where the pressure of the internal compression product is varied as a function of the pipeline pressure (0013-0014). US2008 / 047298 recommends in addition that the heat carrier stream (eg the pressurised air stream) is varied with the pipeline pressure and the amount of cold generated in the process is varied also with the pipeline pressure. Note that US2008 / 047298 does not specifically say that the pressure of the internal compression product is reduced as the pipeline pressures reduces in paragraphs 0013-0014 but simply that the two pressures change in dependence on one another. The present invention includes the step of reducing the air gas pressure when the pipeline pressure reduces but takes a different approach, since the operating conditions of the main air compressor and of the booster air compressor are not changed. Thus when the pipeline pressure drops, the air gas pressure drops but the air intake remains the same. In this case, the refrigeration produced is surplus to the requirements of the process and so an equilibrium can be reached by removing a liquid product from the process, so as to evacuate the surplus refrigeration.Summary of the Invention

[0006] The present invention is directed to a method and apparatus that satisfies at least one of these needs.

[0007] According to the invention, a method is provided for adjusting the production pressure(s) of the air gases (e.g., nitrogen and oxygen) to follow the pressure of the pipeline, thereby increasing liquid production when the pipeline pressure decreases.

[0008] In one embodiment, this inefficiency can be minimized by designing the equipment used in the ASU (e.g., main heat exchanger, liquid oxygen ("LOX") pump, BAC, MAC, etc...) to have sufficient flexibility for being able to deliver gaseous oxygen ("GOX") at different pressure levels based on the pipeline pressure. In another embodiment, the method and apparatus can include a process control strategy to automatically and continuously adjust the GOX product pressure coming out of the main heat exchanger to follow the pipeline pressure.

[0009] In another embodiment, as the GOX product pressure can be adjusted to match the oxygen pipeline, the discharge pressure of the BAC can be kept relatively constant, thereby allowing for additional liquid production. Those skilled in the art will also recognize that if the unit does not use a BAC, then the discharge pressure of the MAC can be kept relatively constant in a similar fashion.

[0010] In certain embodiments of the invention, this inefficiency is eliminated by designing the equipment including main exchanger, LOX Pump, MAC, and BAC, etc. with sufficient flexibility of being able to deliver GOX at different pressure levels according to the pipeline pressure and by implementing a process control strategy to automatically and continuously adjust the GOX product pressure to follow the pipeline pressure. In this particular implementation, the automatic pipeline GOX feed valve can be set at 100% open and GOX flow can be controlled by a flow indicator controller ("FIC") manipulating the LOX pump speed. The lower the GOX pipeline pressure at the delivering point, the lower the GOX pressure from the cold box.

[0011] One efficiency gain that can be realized by lowering the GOX product pressure coming from the cold box is to increase the production of liquid product, either liquid oxygen ("LOX") and / or liquid nitrogen ("LIN") without changing the set point of the operating conditions of the MAC or the BAC. Additional liquid production is realized by an overall increase in available refrigeration. For example, by running the LOX pump at a reduced pressure, the LOX pump will produce less excess heat (due to energy losses associated with vibration, friction, etc...). Additionally, a lowered pressure for the LOX results in less heat of compression. Thirdly, the lower pressure LOX going through the heat exchanger results in a smaller heat loss within the heat exchanger, which results in a gain of additional cold recovery. All three of these factors help to contribute to additional available refrigeration, thereby allowing for an increased liquid production (e.g., liquid nitrogen and / or liquid oxygen). Notably, this increased refrigeration does not require any additional compression or expansion steps, and therefore, the additional liquid production is accomplished without the typical increase in power usage.

[0012] For example, a 1500 st / d O 2 ASU producing GOX at 41.3 barg (600 psig) can produce about 117.5 m 3< / h (4150 scfh) additional liquid nitrogen when the oxygen product from the liquid oxygen pump is reduced to 31 barg (450 psig). The overall stability of an ASU process will not be compromised by this pressure variation due to the fact that ASU process usually has faster dynamics than the pipeline and the pipeline often intrinsically contains a large buffer and pressure variation can only occur slowly.

[0013] Although certain embodiments of the invention have only been described for a GOX product sent to an oxygen pipeline, the concept can easily be applied to any product such as high pressure gaseous nitrogen (GAN) that is produced by internal compression process. The control strategy can easily be implemented using any alternative control scheme that can allow GOX and / or GAN pressure to automatically follow the pipeline. For example, the ASU product pressure can be adjusted to follow the pipeline by controlling the pressure differential across the product control valve to the pipeline. For example, instead of directly measuring the pressure of the gaseous product coming from the cold box, the user can measure the pressure drop across the product control valve, and use the control means to derive a desired set point for the pressure drop across the control valve by adjusting the pressure of the gas coming out of the cold box (e.g., if GOX is the product stream, the liquid oxygen pump could be adjusted until the pressure drop across the product control valve is at or below a desired threshold).

[0014] In one embodiment, the pressure differential across the product control valve is less than 0.34 bar (5 psi), more preferably less than 0.2 bar (3 psi), more preferably less than 0.7 bars (1 psi). In another embodiment, the ASU product pressure is within 0.34 bar (5 psi) of the pipeline pressure, thereby allowing the product control valve to remain fully open, resulting in a minimal pressure loss across the product control valve. In another embodiment, the pressure differential across the product control valve is less than 2%, preferably 1%, more preferably 0.5% of the pipeline pressure. Ideally, the pressure drop across the product control valve approaches zero.

[0015] According to the invention there is provided a method for the production of air gases with variable liquid production according to Claim 1.

[0016] In optional embodiments of the method for the production of air gases with variable liquid production by the cryogenic separation of air: the first boosted pressure PB1 is kept substantially constant during steps h) and i); 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 air gas pump selected from the group consisting of a liquid oxygen pump, a liquid nitrogen pump, and combinations thereof; the air gas product is oxygen and the pipeline is an oxygen pipeline; the liquid oxygen pump pressurizes liquid oxygen from the lower pressure column to the first product pressure P P1 ; the air gas product is nitrogen and the pipeline is a nitrogen pipeline; the liquid nitrogen pump pressurizes liquid nitrogen from the higher pressure column to the first product pressure P P1 ; a process controller, in communication with a plurality of flow indicators, pressure indicators, and control valves, is configured to perform steps g) through i);

[0017] In another aspect of the invention, there is provided an apparatus according to Claim 8.In optional embodiments of the apparatus for the production of air gases by the cryogenic separation of air: the first product pressure P P1 is adjusted such that the difference between the first product pressure P P1 and the first delivery pressure P D1 is below a given threshold; the threshold is less than 0.34 bars (5 psi); the threshold is less than 0.2 bar (3 psi); the air gas product is oxygen and the pipeline is an oxygen pipeline; the liquid air gas pump is a liquid oxygen pump for pressurizing liquid oxygen from the lower pressure column to the first product pressure P P1 ; the air gas product is nitrogen and the pipeline is a nitrogen pipeline. Brief Description of the Drawings

[0018] 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. Figure 1 provides an embodiment of the present invention. Figure 2 provides another embodiment of the present invention. Figure 3 provides a graphical representation of simulation data showing increases in liquid production as a function of gaseous oxygen product pressure.

[0019] 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. Now turning to FIG. 1. Air 2 is introduced into main air compressor 10 and compressed, preferably to a pressure of at least 3.79 to 5.17 barg (55 psig to 75 psig) (or around 0.34 bar (5 psig) higher than the pressure of the MP column). The resulting compressed humid air stream 12 is then purified of water and CO 2 in front end purification system 20, thereby producing dry air stream 22. In one embodiment, not according to the invention, all of dry air stream 22 passes via line 26 into cold box 40. Within cold box 40, the air is cooled and cryogenically treated in order to separate the air into air gas product 42. Air gas product 42 is then removed from cold box 40 and passed through product control valve 50 before entering air gas pipeline 60.

[0020] In a preferred embodiment, the pressure and flow rate of air gas product 42 can be measured by second pressure indicator PI2 and flow indicator FI1, respectively. The pressure of air gas pipeline 60 can be measured by pressure indicator PI3. First liquid air gas product 44 and / or second liquid air gas product 48 can also be removed from cold box 40 in certain modes of operation. The flow rate of first liquid air gas product 44 can be measured by flow indicator FI2, and the flow rate of second liquid air gas product 48 can be measured by flow indicator FI3. In the embodiment shown, control valves 46, 47 can be used to control the flow rates of fluids 44, 48.

[0021] 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.

[0022] Additionally, an embodiment of the present invention includes booster air compressor 30. This embodiment is represented by dashed lines. In this embodiment, a portion of dry air stream 22 is sent to booster air compressor 30 via line 24 and further compressed to form boosted air stream 32 before 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.

[0023] 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.

[0024] Fig. 2 provides a more detailed view of cold box 40 for the booster air compressor 30. 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 booster 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 3.79 to 5.17 bar (55 psig to 75 psig) (or around 0.34 bars (5 psig) higher than the pressure of the MP column). The resulting compressed humid air stream 12 is then purified of water and CO 2 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 to higher pressure column 120 for separation therein. Following pressurization in booster air compressor 30, boosted air stream 32 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.

[0025] Partially boosted air stream 37 is preferably removed from an inner stage of booster air compressor 30 before being further compressed in booster 70 and then cooled in after cooler 75 to form 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.

[0026] Higher pressure column 120 is configured to allow for rectification of air within, thereby producing an oxygen-rich liquid at the bottom and a nitrogen-rich gaseous stream 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. 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 liquid 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, 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.

[0027] 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 FI1, respectively. As in FIG. 1, air gas product 42 flows across product control valve 50 and into air gas pipeline 60. 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). Those of ordinary skill in the art will recognize that both product LOX and LIN can be produced as high pressure at the discharge of the pump or low pressure from the columns either in the HP or LP column for LIN or from the bottom of the LP column for LOX.

[0028] As noted previously, the pressure of air gas pipeline 60 tends to drift over time. In methods known heretofore, this problem was solved by adjusting the openness of product control valve 50 to create the appropriate pressure drop. However, there are inefficiencies in doing this. Instead, embodiments of the present invention adjust the pressure set points within the cold box, for example, the discharge pressure of liquid oxygen pump 160. By reducing this pressure an appropriate amount, product control valve 50 can be left fully open, thereby resulting in minimal expansion losses across product control valve 50. In one embodiment, the appropriate amount yields a difference between PI2 and PI3 to be less than 0.34 bar (5 psi), preferably less than 0.2 bar (3 psi)

[0029] By reducing the pressure of liquid oxygen product 162 and keeping the pressure of the incoming air streams at the same pressure set points (e.g., BAC and MAC maintained at constant set points), additional liquid production can be achieved. For example, for an ASU process that is built to produce gaseous oxygen at 42 barg (610 psig) (e.g., stream 42), approximately 0.4 m 3< / s (51 kscfh) LOX and 0.72 m 3< / s (91 kscfh) LIN can be produced. However, this same process can produce approximately 0.44 m 3< / s (57 kscfh) more LIN or 0.42 m 3< / s (54 kscfh) more LOX if the discharge pressure of the LOX pump is reduced to produce a gaseous oxygen product stream at approximately 27.5 barg (400 psig).

[0030] Tables I-III below show comparative data for various streams with Table I being a base case at 42 barg (610 psig) GOX production, Table II being an embodiment in which LIN production was maximized with GOX production being at 27.5 barg (400 psig), and Table III being an embodiment in which LOX production was maximized with GOX production also being at 27.5 barg (400 psig). While these examples only show LIN and LOX production being maximized, respectively, those of ordinary skill in the art will recognize that embodiments of the invention are not so limited. Rather, embodiments of the invention can also include instances in which both LOX and LIN production could be both increased at the same time. Those of ordinary skill in the art will recognize that in these embodiments, the increase for each LIN or LOX will not be as much individually as is shown in Table II or Table III. Table I: 42 barg (610 psig) GOXStream #Flow (kscfh)Flow (m 3< / s)Pressure Psig)Pressure (barg)Temp (°F)Temp (°C)2807363.7800.007222.212807363.78714.908730.624401031.68694.766417.826332926.30694.766417.832266321.0492863.988730.637134710.6451535.518730.642148711.7561042.067423.344510.40302.07-297-182.848910.7260.41-315-192.872134710.6479054.478730.692134710.64664.55-281-173.994467636.94664.55-248-155.6162148711.7561442.33-287-177.2MP Col------664.55------LP Col------60.41------ Table II: 27.5 barg (400psig) GOX - LIN productionStream #Flow (kscfh)Flow (m 3< / s)Pressure Psig)Pressure bargTemp (°F)Temp (°C)2807363.7800.007222.212807363.78714.908730.624401031.68694.766417.826332926.30694.766417.832266321.0492863.988730.637134710.6451535.518730.642148711.7540027.587523.944510.40302.07-297-182.8481480.4160.41-315-192.872134710.6482656.958730.692134710.64664.55-281-173.994467636.94664.55-252-157.8162148711.7540427.85-289178.3MP Col------664.55------LP Col------60.41------ Table III: 27.5 barg (400 psig) GOX - LOX productionStream #Flow (kscfh)Flow (m 3< / s)Pressure (Psig)Pressure (barg)Temp (°F)Temp (°C)2807363.78007222.212807363.78714.98730.624401031.68694.766417.826332926.30694.766417.832266321.0492863.988730.637134710.6451535.518730.642143311.3140027.57523.9441050.83302.07-297-182.848910.7260.41-315-192.872134710.6482656.98730.692134710.64664.55-281-173.994467636.94664.55-248-155.6162143311.3140427.85-287-177.2MP Col---664.55---LP Col---60.41---

[0031] As is shown in the tables above, when the pipeline pressure changes, the pressure of stream 42 is adjusted to match the pipeline pressure and the flow rates of streams 44 or 48 are changed. The remaining streams remain largely unchanged. As will be readily appreciated, being able to produce additional amounts of liquid can be highly beneficial, particularly since liquid streams are at a premium on the market. Furthermore, this is accomplished without any loss of production in terms of flow rate, without any significant upset to the operating conditions of the double column, and with minimal additional capital expenses.

[0032] In an embodiment in which the air gas product is nitrogen, the embodiment includes withdrawing higher pressure nitrogen product 129 as a liquid from higher pressure column 120, and pressurizing it to an appropriate pressure using a liquid nitrogen pump (not shown) before warming in heat exchanger 80. The resultant warmed nitrogen gas product would then be introduced to a nitrogen pipeline in similar manner as described with respect to the gaseous oxygen product. Alternatively, a liquid nitrogen stream can be removed from the lower pressure column instead of the higher pressure column.

[0033] FIG. 3 presents a graphical representation of liquid production as a function of pressure of the air gas product (e.g., stream 42). As shown in the example, going from a pressure of about 44.8 to 27.6 barg (650 psig to 400 psig) can yield an almost two fold increase in LIN production (went from about 2265 to about 4247 m 3< / h (about 80 to about 150 kscfh)). Similarly, liquid oxygen production was increased from around 1132 to about 2973 m 3< / h ( around 40 to about 105 kscfh). While the graphical representation was developed with the assumption that only one of the liquid products was being adjusted at a time, the invention is not intended to be so limited. In fact, it is perfectly acceptable to increase both liquid products at the same time.

[0034] 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.

[0035] 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.

[0036] As used herein, if the pressure of a stream is kept substantially constant, that is intended to mean that the pressure set point for the process equipment that affects said stream pressure remains unchanged. Normal variance due to typical process conditions is meant to be encompassed by this term.

Claims

1. A method for the production of air gases with variable liquid production by the cryogenic separation of air, the method comprising the steps of: a) compressing air (2) in a main air compressor (10) to a pressure suitable for the cryogenic rectification of air to produce a compressed humid air stream (12), 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 (20) to produce a dry air stream (22) having reduced amounts of water and carbon dioxide as compared to the compressed humid air stream (12); c) compressing a first portion of the dry air stream (24) in a booster compressor (30) to form a boosted air stream (32), the boosted air stream having a first boosted pressure PB1; d) introducing a second portion of the dry air stream (26) and the boosted air stream to a cold box (40) under conditions effective to separate air to form an air gas product (42), the air gas product having a first product pressure PP1, wherein the air gas product is selected from the group consisting of oxygen, nitrogen, and combinations thereof; e) withdrawing the air gas product from the cold box; f) introducing the air gas product to a pipeline (60), wherein the pipeline is configured to transport the air gas product to a location located downstream of the pipeline, wherein the pipeline operates at a pipeline pressure PPL, wherein the air gas product is introduced to the pipeline at a first delivery pressure PD1; g) monitoring the pipeline pressure PPL (PI3) within the pipeline; h) adjusting the first product pressure PP1 within the cold box based on the pipeline pressure PPL.; and i) adjusting liquid production (44, 48) from the cold box based on the first product pressure PP1 adjusted in step h), wherein the liquid production from the cold box is a liquid selected from the group consisting of liquid nitrogen ("LIN"),, liquid oxygen("LOX"), and combinations thereof characterized in that if the pipeline pressure PPL reduces at a delivering point, the first product pressure from the cold box is reduced and the liquid production, is increased without changing a set point of the operating conditions of the main air compressor (10) or the booster air compressor (30).

2. The method as claimed in Claim 1, wherein the first boosted pressure PB1 is kept substantially constant during steps h) and i).

3. The method as claimed in any of the preceding claims, wherein the cold box comprises a main heat exchanger (80), a system of columns having a double column (110) composed of a lower pressure column (140) and a higher pressure column (120), a condenser (150) disposed at a bottom portion of the lower pressure column, and a liquid air gas pump selected from the group consisting of a liquid oxygen pump (160), a liquid nitrogen pump, and combinations thereof.

4. The method as claimed in Claim 3 wherein the air gas product is oxygen and the pipeline is an oxygen pipeline, and wherein the liquid oxygen pump (160) pressurizes liquid oxygen from the lower pressure column (140) to the first product pressure PP1.

5. The method as claimed in Claim 3, wherein the air gas product is nitrogen and the pipeline is a nitrogen pipeline.

6. The method as claimed in Claim 5, wherein the liquid nitrogen pump pressurizes liquid nitrogen from the higher pressure column (120) to the first product pressure PP1.

7. The method as claimed in any of the preceding claims, wherein a process controller (55), in communication with a plurality of flow indicators (FI1, FI2, FI3), pressure indicators (PI1, PI2, PI3), and control valves (46, 47, 50), is configured to perform steps g) through i).

8. Apparatus for the production of air gases with variable liquid production by the cryogenic separation of air including a) a main air compressor (10) 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 (20) 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 (30) 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 cold box (40) comprising a main heat exchanger (80), a system of columns having a double column composed of a lower pressure column (140) and a higher pressure column (120), a condenser (150) disposed at a bottom portion of the lower pressure column, and a liquid air gas pump (160), wherein the liquid air gas is selected from the group consisting of liquid nitrogen ("LIN") and liquid oxygen ("LOX") and combinations thereof,wherein the cold box is configured to receive the boosted air stream and a second portion of the dry 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; e) means for monitoring the pressure of a pipeline (60), wherein the pipeline is in fluid communication with the cold box, such that the pipeline is configured to receive the air gas product from the cold box, the air gas product having a first product pressure PP1; f) means for adjusting the first product pressure PP1based on the monitored pipeline pressure, wherein the first product pressure PP1 corresponds to a discharge pressure of the liquid air gas pump, said means including means for reducing the first product pressure from the cold box in response to a reduction in the pipeline pressure PPL; g) means for adjusting liquid production (44, 48) from the cold box characterized in that the apparatus comprises means for increasing the liquid production of liquid product, being either liquid oxygen and / or liquid nitrogen, in response to a reduction in the pipeline pressure PP1 ,without changing the set point of the operating conditions of the main air compressor (10) or the booster air compressor (30).

9. Apparatus according to Claim 8 comprising a plurality of flow indicators, pressure indicators, and control valves (46,47).wherein the means for adjusting liquid production from the cold box comprises a process controller (55), in communication with the plurality of flow indicators, pressure indicators, and control valves .

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