METHOD FOR REGULATING A DEVICE FOR SEPARATING AIR BY CRYOGENIC DISTILLATION

Abstract

In a method for regulating a device for separating air by cryogenic distillation, during the change from a low-flow mode to a high-flow mode, two flow rates of reflux sent to the low-pressure column of a double column are increased more rapidly than the flow rate of gaseous air sent to the medium-pressure column of the double column in order to ensure the stability of the oxygen content in the argon bulge of the low-pressure column during the mode change.

Claims

1. A method for regulating a device for separating air by cryogenic distillation in a device comprising a first column operating at a first pressure and a second column operating at a second pressure lower than the first pressure, the bottom of the second column being heated by a gas from the first column, and a third column, wherein sending cooled, purified gaseous air to the first column; sending a first, oxygen-enriched, liquid from the bottom of the first column to the second column; sending a second liquid that is liquefied air or that is withdrawn from an intermediate level of the first column to the second column; sending a third nitrogen-enriched liquid and optionally a fourth nitrogen-enriched liquid from an upper part of the first column to the second column; withdrawing an oxygen-enriched fluid from the second column; sending an argon-enriched gas from the second column to the third column for separation therein; withdrawing a fluid enriched in argon compared to the argon-enriched gas that feeds the third column at the top of the third column; and operating the device in two modes including a first mode in which the gaseous air sent to the first column has a first flow rate and a second mode in which the gaseous air sent to the first column has a second flow rate that is greater than the first flow rate, wherein: i) upon a condition in which the flow rate of gaseous air sent to the first column increases by at least n % per minute when changing from the first mode to the second mode, the flow rate of the first liquid arriving in the second column is increased by at least % per minute, where x is non-zero, and the flow rate of the second liquid arriving in the second column is increased by at least % per minute, where x is non-zero, during at least part of a period during which the increase in the flow rate of air is at least n % per minute, and/or ii) upon a condition in which the flow rate of gaseous air sent to the first column reduces by at least n % per minute when changing from the second mode to the first mode, the flow rate of the first liquid arriving in the second column is reduced by at least % per minute, where y is non-zero, and the flow rate of the second liquid arriving in the second column is reduced by at least % per minute, where y is non-zero, during at least part of a period during which the reduction in the flow rate of air is at least n % per minute, wherein n is equal to or greater than 1.

2. The method according to claim 1, wherein the first, second, third and optionally fourth liquids are sent to the second column without passing through a storage capacity.

3. The method according to claim 1, wherein i) upon a condition in which the flow rate of gaseous air sent to the first column increases by at least n % per minute when changing from the first mode to the second mode, the flow rate of the third liquid arriving in the second column sent from the first column to the second column is reduced by at least % per minute, where w is non-zero, and the flow rate of the fourth liquid arriving in the second column from the first column is optionally reduced by at least % per minute, where w is non-zero, during at least part of the period during which the increase in the flow rate of air is at least n % per minute, and/or ii) upon a condition in which the flow rate of gaseous air sent to the first column reduces by at least n % per minute when changing from the second mode to the first mode, the flow rate of the third liquid sent to the second column from the first column is increased by at least % per minute, where z is non-zero, and the flow rate of the fourth liquid sent to the second column from the first column is optionally increased by at least % per minute, where z is non-zero, during at least part of the period during which the reduction in the flow rate of air is at least n % per minute.

4. The method according to claim 3, wherein for feature i) the flow rate of the third and/or fourth liquid sent to the second column is reduced, by increasing the flow rate of a part of the third and/or fourth liquid produced as a final product, compared to this flow rate during the second mode, or part of the third and/or fourth liquid is discharged as a final product whereas none is produced during the second mode.

5. The method according to claim 3, wherein for feature i), the flow rate of the third and/or fourth liquid withdrawn from the first column is reduced by at least % per minute and at least % per minute respectively.

6. The method according to claim 1, wherein the flow rate of air sent to the first column increases or reduces during a time t between the first and second modes, at least equal to quarter of an hour, and i) the flow rate of the first liquid arriving in the second column is reduced by at least % per minute, where x is non-zero, and/or the flow rate of the second liquid arriving in the second column is reduced by at least % per minute, where x is non-zero, during at least the first quarter of the time t and at most three quarters of the time t, and/or ii) the flow rate of the first liquid arriving in the second column is increased by at least % per minute, where y is non-zero, and/or the flow rate of the second liquid arriving in the second column is increased by at least % per minute, where y is non-zero, during at least the first quarter of the time t and at most three quarters of the time t.

7. The method according to claim 1, wherein while, preferably only while, the reduction in the flow rate of gaseous air sent to the first column is greater than n %/minute, and a. at least part of the argon-enriched fluid is sent to a denitrogenation column with an increased flow rate compared to the flow rate during the second mode and/or b. at least part of the argon-enriched fluid is sent in liquid or gaseous form to the top of the second column or mixed with a nitrogen-enriched gas withdrawn from the second column and/or c. at least part of the argon-enriched fluid is stored in liquid form in a storage capacity attached to the third column and/or d. at least part of the argon-enriched fluid is vented in gaseous form and/or e. at least part of the argon-enriched fluid is drained in liquid form.

8. The method according to claim 7, wherein the argon-enriched fluid is sent after, preferably only after, the flow rates of the first and second liquids have been reduced by at least % per minute and at least % per minute respectively while the reduction in the flow rate of gaseous air sent to the first column is at least n % per minute, wherein n is equal to or greater than 1.

9. The method according to claim 1, wherein the argon-enriched fluid contains at least 97 mol % argon.

10. The method according to claim 1, wherein when changing from the second mode to the first mode, the second flow rate of gaseous air entering the first column being equal to m % of the first flow rate of air entering the first column, m being greater than 100, the flow rate of the first liquid has a value of M % of V during the second mode and has a value of V during the period between the second and first modes, M preferably being greater than m.

11. The method according to claim 1, comprising the steps of: detecting the flow rate of gaseous air sent to the first column; and regulating the flow rate of the first and/or second liquid as a function of the flow rate of air detected upon a condition in which the flow rate of gaseous air sent to the first column detected increases by at least n % per minute when changing from the first mode to the second mode, i) the flow rate of the first liquid arriving in the second column is modified by at least % per minute, where x is non-zero, and/or ii) the flow rate of the second liquid arriving in the second column is increased by at least % per minute, where x is non-zero, during at least part of a period during which the increase in the flow rate of air is at least n % per minute.

12. The method according to claim 1, comprising the steps of detecting the flow rate of gaseous air sent to the first column, and regulating the flow rate of the first and/or second liquid as a function of the flow rate of air detected upon a condition in which the flow rate of gaseous air sent to the first column detected reduces by at least n % per minute when changing from the second mode to the first mode, i) the flow rate of the first liquid arriving in the second column is reduced by at least % per minute, where y is non-zero, and/or ii) the flow rate of the second liquid arriving in the second column is reduced by at least % per minute, where y is non-zero, during at least part of a period during which the reduction in the flow rate of air is at least n % per minute.

13. The method according to claim 1, wherein upon a condition in which the flow rate of gaseous air sent to the first column increases by at least n % per minute when changing from the first mode to the second mode, the flow rate of the first liquid arriving in the second column is increased by at least % per minute, where x is non-zero, and the flow rate of the second liquid arriving in the second column is increased by at least % per minute, where x is non-zero, during at least part of a period during which the increase in the flow rate of air is at least n % per minute.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Other features and advantages of the invention will become further apparent via, on the one hand, the following description and, on the other hand, several exemplary embodiments given by way of non-limiting indication and with reference to the attached schematic drawings, in which:

[0049] FIG. 1 shows the double column of a device operating according to the method of the invention.

[0050] FIG. 2 shows the effect of different values of the time constants and on the dynamics of the flow rate D in kmol/h of reflux liquid with the time T in hours, here during a change of mode from 75% to 100% (at t=1 hour) of the operating speed of the main compressor and from 100% to 75% at t=3 hours.

[0051] FIG. 3 shows the concentration C profiles by packing section PS in the low-pressure column before (left) and during (right) a rapid mode change.

[0052] FIG. 4, made up of FIGS. 4A, 4B and 4C, shows the arrangement of the liquid storage capacities for storing argon during the rapid mode change.

[0053] FIG. 5, made up of FIGS. 5A and 5B, shows the change in the flow rates of medium-pressure air, crude argon production and rich liquid during a rapid mode change between 100% and 75% of the operating speed of the main air compressor.

[0054] FIG. 6 shows the change over time in the oxygen content (solid line on the y axis) in the argon bulge of the second column and in the nitrogen content (solid line on the right of the y axis) at the top of the third column for the two scenarios (according to the prior art and with the regulation strategy according to the invention). The time is on the x axis.

DETAILED DESCRIPTION OF THE INVENTION

[0055] FIG. 1 shows the double column of a device for separating air by cryogenic distillation. The gaseous air A is sent into the bottom of a first column K1.

[0056] The rich liquid RL is a liquid enriched in oxygen compared to the air A generally withdrawn at the bottom of the first column K1 of a double air separation column. The double column comprises a first column K1 operating at a first pressure, known as the medium-pressure column, and a second column K2 operating at the second pressure, lower than in the first column, known as the low-pressure column. The first column K1 is fed with air A, and an oxygen-enriched liquid 2 and nitrogen-enriched liquid 12 are sent from the first column to the second column. A liquefied air flow is also sent to the second column, and this flow can be withdrawn from the first column (flow 4) as illustrated here or can be sent directly to the second column without passing through the first column K1. The bottom of the second column K2 is heated by a gas from the first column. The device also comprises a third column 1 illustrated in FIGS. 4A, 4B, 4C. Cooled, purified gaseous air A is sent to the first column K1, a first oxygen-enriched liquid 2 is sent from the bottom of the first column to the second column, a second liquid 4 is optionally sent from an intermediate level of the first column to the second column, at least one nitrogen-enriched liquid 12 (third and/or fourth liquid) is sent from the top of the first column to the second column, an oxygen-enriched fluid 7 is withdrawn from the second column, an argon-enriched gas 5 is sent from the second column K2 to the third column 1 for separation therein, and an argon-enriched fluid is withdrawn from the top of the third column 1.

[0057] A nitrogen-enriched gas flow 8 leaves the top of the second column K2. Part 10 of the rich liquid from the bottom of the column K1 is sent to a top condenser 3 of the third column 1 to cool it.

[0058] The separation device operates according to at least two different modes including a first mode in which the gaseous air A sent to the first column K1 has a substantially constant first flow rate and a second mode in which the gaseous air A sent to the first column has a substantially constant second flow rate greater than the first flow rate.

[0059] The regulation method according to the invention aims to regulate the periods during which the device is changing from the first mode to the second and/or from the second mode to the first.

[0060] When changing from the first mode to the second mode, the flow rate of gaseous air A sent to the first column is detected. It increases and reaches an increase of at least n % per minute. When this increase of at least n % per minute is reached, as a function of the flow rate A detected, the flow rate of the first liquid 2 arriving in the second column is increased by at least (n+x) % per minute, where x is non-zero, and the flow rate of the second liquid 4 arriving in the second column K2 is increased by at least (n+x) % per minute, where x is non-zero, during at least part of a period during which the increase in the flow rate of air is at least 1% per minute.

[0061] The increase in the first liquid 2 is carried out by increasing the flow rate of liquid withdrawn from the first column K1. This can also be the case for the second liquid 4, in the variants in which it is withdrawn from the first column. Otherwise the second liquid, the liquefied air, arrives directly at the second column without passing through the first column. The absence of a reflux liquid storage capacity must be noted.

[0062] When changing from the second mode to the first mode, the flow rate of gaseous air A is also detected. The flow rate A sent to the first column reduces and reaches a reduction of at least n % per minute. When this rate is reached, as a function of the flow rate A detected, the flow rate of the first liquid 2 arriving in the second column K2 is reduced by at least (n+y) % per minute, where y is non-zero, and the flow rate of the second liquid 4 arriving in the second column K2 is reduced by at least (n+y) % per minute, where y is non-zero, during at least part of a period during which the reduction in the flow rate of air A is at least n % per minute.

[0063] The value n is equal to or greater than 1 in both cases.

[0064] Preferably, when changing from the first mode to the second mode, the flow rate of gaseous air A sent to the first column K1 increases by at least n % per minute, the flow rate of a third liquid 12 taken from the top of the first column arriving in the second column sent from the first column to the second column is reduced by at least (n+w) % per minute, where w is non-zero, and the flow rate of a fourth liquid 14 arriving in the second column K2 from the first column K1 is optionally reduced by at least (n+w) % per minute, where w is non-zero, during at least part of the period during which the increase in the flow rate of air is at least n % per minute.

[0065] This reduction can be carried out by reducing the flow rate of liquid 12 and/or 14 withdrawn at the top of the first column. In this case, the flow rate of the third and/or fourth liquid 12, 14 withdrawn from the first column is reduced by at least (n+w) % per minute and at least (n+w) % per minute respectively.

[0066] Otherwise or in addition, part of the liquid 12 or 14 can be taken as a final product 6, thus reducing the flow rate sent to the second column K2.

[0067] Either the flow rate of the third and/or fourth liquid sent to the second column is reduced, by increasing the flow rate of a part of the third and/or fourth liquid produced as a final product between the second and first modes, compared to this flow rate during the second mode, or part of the third and/or fourth liquid is discharged as a final product between the second and first modes, whereas none is produced during the second mode.

[0068] Preferably, when changing from the second mode to the first mode, the flow rate of gaseous air A sent to the first column K1 reduces by at least n % per minute, the flow rate of the third liquid 12 sent to the second column from the first column is increased by at least (n+z) % per minute, where z is non-zero, and the flow rate of the fourth liquid 14 sent to the second column from the first column is optionally increased by at least (n+z) % per minute, where z is non-zero, during at least part of the period during which the reduction in the flow rate of air is at least n % per minute.

[0069] Preferably, the flow rate of air A increases or reduces during a time t between the first and second modes, at least equal to quarter of an hour.

[0070] The flow rate of the first liquid 2 arriving in the second column is reduced by at least (n+x) % per minute, where x is non-zero, and/or the flow rate of the second liquid arriving in the second column is reduced by at least (n+x) % per minute, where x is non-zero, during at least the first quarter of the time t and at most three quarters of the time t.

[0071] The flow rate of the first liquid arriving in the second column is increased by at least (n+y) % per minute, where y is non-zero, and/or the flow rate of the second liquid arriving in the second column is increased by at least (n+y) % per minute, where y is non-zero, during at least the first quarter of the time t and at most three quarters of the time t.

[0072] The lead-lag compensator modifies an input signal of the gaseous air A feeding the first column according to the following transfer function:

G(s)=K.Math.(s+1)(s+1(1))

where is the lead time constant, a is the lag time constant and K is the steady-state gain. This model uses the following equations for an implementation in the time domain:

dx/dt=1/.Math.(k.Math.u(t)x(t))(2)

y(t)=/.Math.(Ku(t))+(1/.Math.x(t)(3)

where u corresponds to the input signal of the model, x is the state of the model (signal after the lag but before the lead), y is the output signal and t is the time; u0 represents the initial conditions of the model with

y(0)=x(0)=u0

[0073] Various solutions can be used to manage the difference in argon hold-up between the second and first modes in the argon column. During the ramp-down, when the change in the flow rate of air reaches at least n %, the quantity of argon naturally released by the argon column can be sent to a denitrogenation column. This can be a measure used only between the two modes. More usually, the denitrogenation column will be fed by at least part of the argon-enriched fluid 15, 21 during the first and second modes, but the flow rate of fluid 15, 21 will be increased when changing from the second mode to the first mode, to discharge the surplus fluid. It is obviously necessary to stay within the limits of the capacity of the denitrogenation column and this solution can therefore be combined with or replaced by at least one other solution.

[0074] For example, at least part of the argon-enriched fluid 15, 21 can be returned in liquid or gaseous form to the top of the second column K2.

[0075] Otherwise, at least part of the argon-enriched fluid 15, 21 can be returned in the residual nitrogen 8 withdrawn at the top of the second column K2. This makes it possible to recover the cold energy, but not the molecules. The flow mixed with the residual nitrogen can be non-existent during the second mode or can be increased compared to the flow mixed with the residual nitrogen during the second mode.

[0076] Otherwise, at least part of the argon-enriched fluid 15, 21 can be stored in liquid form in a storage capacity attached to the argon mixture column 1 (recovery of the cold energy and the argon molecules).

[0077] Otherwise, at least part of the argon-enriched fluid 15, 21 is vented in gaseous form or drained in liquid form. The drained or vented flow can be non-existent during the second mode or can be increased compared to the drained or vented flow during the second mode.

[0078] The argon-enriched fluid 15, 21 according to one of the solutions described here can be sent after, preferably only after, the flow rates of the first and second liquids have been reduced by at least (n+y) % per minute and at least (n+y) % per minute respectively while the reduction in the flow rate of gaseous air sent to the first column is at least n % per minute, n being equal to or greater than 1.

[0079] The argon-enriched fluid 15, 21 withdrawn from the third column contains at least 97 mol % argon.

[0080] FIG. 2 shows different ways of changing the flow rate D of reflux depending on the composition of the liquid.

[0081] The effect of the lead-lag is used on the dynamics of the flow rate of rich liquid (or first liquid) 2 leaving the bottom of the medium-pressure column K1 during a mode change (from the first mode to the second, here between 75% and 100%, then from the second mode to the first, therefore back to 75% here, of the operating speed of the main compressor) for different values of the time constants and . It must be noted that as a function of the values of an overshoot can be obtained at the end of the up or down ramps of the flow rate of rich liquid 2.

[0082] This effect is also applied for the second liquid 4.

[0083] However, for the third liquid 12 and optionally the fourth liquid 14 during a mode change (from the first mode to the second, here between 75% and 100%, then from the second mode to the first, therefore back to 75% here), the effect applied is an inverse response (yRR) from the lead-lag compensator by applying:

yRR=2.Math.u(t)y(t)(4)

where yRR corresponds to an inverse response compared to the response obtained with a lead-lag

[0084] with the formulation of (1), (2), (3). In FIG. 2, this type of response corresponds to the long-dashed curve, with the overshoot occurring at the start of the up- or down-ramp between the first and second modes.

[0085] FIG. 2 shows the effect of different values of the time constants and on the dynamics of the flow rate of rich liquid during a change of mode from 75% to 100% (at t=1 hour) of the operating speed of the main compressor and from 100% to 75% at t=3 hours.

[0086] The curve for ==1 corresponds to the unmodified signal. An example of an inverse response (RR) is also shown (longer-dashed curve).

[0087] When the operating speed of the main air compressor decreases, the argon content increases all along the argon column, which leads to a drop in the oxygen content in the argon column and also in the vapour withdrawn from the low-pressure column to the argon column (argon shower, [FIG. 3]). The term argon shower means that overall, the argon content in the sections below the withdrawal point to the argon column in the low-pressure column increases during ramp-down. It can be seen on the right in FIG. 3 that during the mode change, the argon content increases towards the 30th packing section. Sending additional liquid argon to the downstream unit (the denitrogenation column), venting pure gaseous argon or storing liquid argon in an external storage capacity helps to maintain the content profile in the column and avoids reducing the O2 content at the argon bulge in the low-pressure column. The volume of argon to be discharged corresponds to the argon hold-up differential in the argon column during a speed reduction.

[0088] FIG. 3 shows the concentration profiles in the low-pressure column K2 before (left) and during (right) a rapid mode change: argon bulge V, argon shower at the vapour withdrawal point 5 (ORG) from the low-pressure column to the argon column 1.

[0089] In order not to lose the argon, it can be stored judiciously in liquid form either in a storage capacity connected at the top of the argon column, or in a storage capacity connected at an intermediate level (where the argon content is greater than 97%) ([FIG. 4]). Liquid argon can be returned into the method either at the same level as the tap (with the help of a pump), or at a lower level (by gravity with a header tank).

[0090] FIG. 4 shows the arrangement of the liquid storage capacities for storing argon during the rapid mode change: [0091] a. storage capacity at the top of the argon column, [0092] b. storage capacity at an intermediate level, [0093] c. storage capacity with recirculation via a pump.

[0094] In FIG. 4A, the argon separation column 1 has a top condenser 5 in an enclosure 3. The enclosure 3 is fed by a flow of rich liquid (not illustrated) that vaporizes therein to form a gas 7. A liquid 9 is withdrawn at the bottom of the argon separation column 1. An attached storage capacity 11 is fed by a liquid 21 coming from the top of the column and stores this liquid. The liquid in the storage capacity 11 can be sent to the column by the pipe 23 or otherwise can be mixed with the feed flow 15 from a pure argon column 13 through the pipe 17. The column 13 is surmounted by a condenser 23.

[0095] A flow 9 is sent from the column 1 to the double column.

[0096] In FIG. 4B, the storage capacity is fed from a lower level of the column 1.

[0097] In FIG. 4C, unlike in FIG. 4A, the bottom liquid 23 from the storage capacity 11 is pressurized by a pump P that returns the bottom liquid to the top of the column 1.

[0098] FIG. 5 shows the change in the flow rates of medium-pressure air, crude argon production (argon mixture) and rich liquid during a rapid mode change between 100% and 75% of the operating speed of the main air compressor. Two scenarios are compared: [0099] FIG. 5A is a pseudo-static base scenario in which the mode change takes place from a steady state at 100% of the operating speed of the compressor to 75%, at a rate of 1.6%/min. The decreases in the flow rate of argon production and the flow rate of rich liquid 2 proportionally follow the decrease in the flow rate of medium-pressure air A feeding the first column K1. [0100] FIG. 5B is a scenario that implements the two regulation strategies described previously: a lead-lag on the liquid flow rates sent to the second column K2 (flow rates of rich liquid RL 2, liquid air 4), and an increase then a decrease (inverse response) (flow rate of impure nitrogen 12, flow rate of pure nitrogen 14, flow rate of argon production (Ar) withdrawn from the argon column).

[0101] FIG. 5 shows the comparison of the change in the flow rate of medium-pressure air (MP Air A), flow rate of crude argon production (Ar 15, 21) and flow rate of rich liquid (RL 2) during a rapid mode change for a scenario without using the invention (FIG. 5A, denoted prior art, although no document in the public domain representing this figure is known to the applicant) and a scenario that uses the regulation strategy according to the invention (FIG. 5B). The lead-lag (Lead-lag on RL) on the rich liquid 2 and the venting (Ar vent) of the argon 15, 21 during the change from the second mode to the first mode can be seen. The flow rate of air reduces by 1.6%/min (therefore n equals 1.6).

[0102] When changing from the second mode to the first mode, the second flow rate of gaseous air A entering the first column K1 is equal to m % of the first flow rate of air entering the first column, m being greater than 100.

[0103] Here, the second flow rate changes from above 220,000 Nm.sup.3/h to approximately 145,000 Nm.sup.3/h. The flow rate of the first liquid 2 has a value of M % of V during the second mode and a minimum value of V during the period between the second and first modes, whereas during the second mode, M is preferably greater than m. Here, the first liquid 2 has a minimum value of V (below 80,000 Nm.sup.3/h) during the period between the second and first modes, whereas during the second mode, the flow rate of first liquid is almost 160,0000 Nm.sup.3/h. The flow rate of the first liquid thus reduces proportionally more than the air to reach a minimum value V just before the start of the first mode.

[0104] FIG. 6 shows the effect of the implementation of the regulation strategy described above on two key indicators of the stability of the ASU, and particularly the distillation columns: the oxygen content F, G in the argon bulge V in the low-pressure column and the nitrogen content D, E at the top of the argon column. These results were obtained using a dynamic simulation model.

[0105] FIG. 6 shows that in the base scenario, during the ramp-down in the ASU, the O2 content in the argon bulge V drops by approximately 3% and the N2 content at the top of the argon column reaches up to 14%, which will have the effect of excessively cooling the condensation side of the head condenser of the argon column, thus stopping its operation (the condensation temperature of the 86% Ar, 14% N2 mixture drops to a value below the evaporation temperature of the rich liquid on the vaporization side).

[0106] Conversely, the implementation of the lead-lag on the flow rates of liquid sent from the first column to the second column (the flow rate of rich liquid 2 withdrawn at the bottom, the flow rate of liquid air 4 taken at an intermediate level or from the feed air) and the venting of the argon hold-up differential make it possible to maintain the oxygen content in the bulge at 92.30.6%. In addition, they make it possible to reduce the nitrogen content at the top of the argon column to approximately 110 3%, which ensures that the condenser of the third column, and therefore the distillation in the column, continue to operate during the mode change.

[0107] FIG. 6 shows the change over time in the oxygen content (solid line on they axis) in the argon bulge of the second column and in the nitrogen content (solid line, on the y axis) at the top of the third column for the two scenarios (base and with the regulation strategy according to the invention). The time is on the x axis.

[0108] Without the regulation of the invention, the nitrogen level (point C) at the top of the third column increases to approximately 14 mol % before 2 hours, causing the head condenser of the third column to stop. At the argon bulge in the second column, the oxygen content drops to almost 89 mol % (point B).

[0109] With the regulation according to the invention, the oxygen content remains at 92.30.6 mol % (point A). The nitrogen content does not exceed 6% (peak at 6 hours in FIG. 6).

[0110] 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.

[0111] The singular forms a, an and the include plural referents, unless the context clearly dictates otherwise.

[0112] 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.

[0113] 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.

[0114] 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.

[0115] 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.