Patent Application
20240011706
This application claims the benefit of priority under 35 U.S.C. § 119 (a) and (b) to French patent application No. FR2206870, filed Jul. 5, 2022, the entire contents of which are incorporated herein by reference.
The present invention relates to a method for regulating a device for separating air by cryogenic distillation.
The field of application of the present invention is that of air separation units provided with a cryogenic argon production system, that is, with an impure argon column (which allows the separation of argon and oxygen) and a pure argon column (also known as a denitrogenation column, which makes it possible to remove the nitrogen and thus obtain pure argon).
The air separation devices to which this invention relates comprise a double column, comprising a first column operating at a first pressure and a second column operating at a second pressure, lower than the first pressure. The second column is fed with a nitrogen-enriched liquid and an oxygen-enriched liquid coming from the first column.
Because of the differences in relative volatility between argon, nitrogen and oxygen, virtually pure nitrogen is formed at the top of the second column, virtually pure oxygen is formed at the bottom of the second column, and argon-rich gas is formed in the middle of the second column around a point known as the argon bulge or argon bubble.
Typically, the argon bulge has a maximum argon content of 15% and a nitrogen content of 8 to 10%. A gas sent to the third column is taken several theoretical stages below this maximum point, where the argon content is only 10%, but where the nitrogen content is reduced to 0.1%.
This argon-enriched gas, often known as crude argon, withdrawn from the second column at the argon bulge, is sent to an argon separation column having a top condenser. The crude argon is rectified to give an oxygen-rich reflux at the bottom of the third column (which is subsequently returned to the second column) and a very argon-rich stream (often known as the argon mixture) which now contains virtually no oxygen (the oxygen content in the argon mixture is typically less than 3 ppm of oxygen) at the top of the third column. This argon mixture is sent to a denitrogenation column in order to remove the nitrogen by reboiling. At the bottom of the denitrogenation column, pure argon is withdrawn in liquid form and is sent to a liquid argon storage facility.
The argon separation column (argon mixture column) can be in two parts in order to reduce the height of the cold box.
In the context of the development of more flexible methods for producing gas from air that make it possible to incorporate intermittent energy resources, the regulation of devices for separating air by cryogenic distillation during rapid mode changes is crucial.
JP-A-2000-1800049 describes an air separation device in which, if the flow rate of air feeding the device drops, the flow rate of rich liquid feeding the low-pressure column reduces more quickly than the flow rate of air. However, the flow rate of liquid nitrogen sent to the low-pressure column reduces less quickly than the air.
A device for separating air by cryogenic distillation (ASU) with argon production is much more difficult to regulate, and a person skilled in the art knows that the mode change speed of such an ASU is very limited, with a feed air flow rate increase or reduction value of around 0.3%/min, at most 0.5%/min.
It is particularly important to ensure the stability of the argon bulge in the low-pressure column in order to prevent the argon column from stopping. If a nitrogen rush is generated in the argon column, this rush will cool the vaporiser and stop the operation thereof. The argon column stopping often means a loss of argon production (at least 12 hours and up to several days to restart) during which the ASU must operate in degraded mode.
The strategies for regulating the argon bulge currently used are essentially based on correcting the feed air flow rate as a function of the oxygen content measured at the argon bulge or at an intermediate point of the argon mixture column.
In certain embodiments, the present invention relates to a regulation strategy applicable in the context of a very rapid mode change (for example, an increase or reduction of at least 1% per minute, for example 1.6%/min, of the flow rate of air taken in by the main compressor) in the operating speed of an ASU. This strategy makes it possible in particular:
In certain embodiments, the strategy uses at least one lead-lag filter on at least two of the liquid flow rates entering the second column (rich liquid reflux (RL), liquid air reflux). When the flow rate of air decreases, a lead is applied to the response of the flow rate of rich liquid entering the low-pressure column, for example. The flow rate of RL is not therefore decreased in proportion to the decrease in the flow rate of air, but is decreased at a higher speed, creating a hold-up in the bottom of the first column.
This can be combined with the management of the difference in argon hold-up between two steady states in the argon column.
During the ramp-down, the quantity of argon naturally released by the column can either:
According to one object of the invention, a method for regulating a device for separating air by cryogenic distillation is provided 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 cooled, purified gaseous air is sent to the first column, a first, oxygen-enriched, liquid is sent from the bottom of the first column to the second column, a second liquid that is liquefied air or that is withdrawn from an intermediate level of the first column is sent to the second column, a third nitrogen-enriched liquid and optionally a fourth nitrogen-enriched liquid from an upper part of the first column is/are sent to the second column, an oxygen-enriched fluid is withdrawn from the second column, an argon-enriched gas is sent from the second column to the third column for separation therein, and a fluid that is enriched in argon compared to the argon-enriched gas that feeds the third column is withdrawn at the top of the third column, and the device operates 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, greater than the first flow rate:
According to other optional aspects:
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:
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
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.
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.
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.
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.
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.
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.
The value n is equal to or greater than 1 in both cases.
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.
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.
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.
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.
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.
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.
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.
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.
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·(βs+1)(αs+1 (1))
where β 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/α·(k·u(t)−x(t)) (2)
y(t)=β/α·(Ku(t))+(1−β/α·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
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.
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.
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.
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).
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.
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.
The argon-enriched fluid 15, 21 withdrawn from the third column contains at least 97 mol % argon.
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 α and β. It must be noted that as a function of the values of β an overshoot can be obtained at the end of the up or down ramps of the flow rate of rich liquid 2.
This effect is also applied for the second liquid 4.
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·u(t)−y(t) (4)
where yRR corresponds to an inverse response compared to the response obtained with a lead-lag
with the formulation of (1), (2), (3). In
The curve for α=β=1 corresponds to the unmodified signal. An example of an inverse response (RR)′ is also shown (longer-dashed curve).
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”, [
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%) ([
In
A flow 9 is sent from the column 1 to the double column.
In
In
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.
Here, the second flow rate changes from above 220,000 Nm3/h to approximately 145,000 Nm3/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 Nm3/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 Nm3/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.
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 92.3±0.6%. In addition, they make it possible to reduce the nitrogen content at the top of the argon column to approximately 1×10 3%, which ensures that the condenser of the third column, and therefore the distillation in the column, continue to operate during the mode change.
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).
With the regulation according to the invention, the oxygen content remains at 92.3±0.6 mol % (point A). The nitrogen content does not exceed 6% (peak at 6 hours in
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.
The singular forms “a”, “an” and “the” include plural referents, unless the context clearly dictates otherwise.
“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.
“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.
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.
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| FR 2206870 | Jul 2022 | FR | national |
