This application is a §371 of International PCT Application PCT/EP2007/056085, filed Jun. 19, 2007.
1.Field of the Invention
The present invention relates to an air separation process and apparatus using cryogenic distillation. In particular, it relates to the production of nitrogen using a single column kept refrigerated by liquid injection (the sending of liquid nitrogen coming from an external source into the top of the column). The aim of the invention is more particularly to meet moderate and variable demands (typically 100 to 2000 Sm3/h) of high-purity nitrogen, that is to say nitrogen containing typically less than 0.1% oxygen. In the present specification, the flow rates in question are mass flow rates.
High-purity nitrogen is usually obtained cryogenically. For low consumptions, the construction of a conventional autonomous production unit represents a prohibitive level of investment in the case of automated installations, and a more limited level of investment but high labour costs in the opposite case. This always amounts to a high cost price of the nitrogen.
A more economical solution consists in using an evaporator, that is to say a liquid nitrogen tank of large capacity, for example several tens of thousands of liters, from which liquid nitrogen is withdrawn and vaporized. This solution is not very satisfactory from the energy standpoint, since the refrigeration energy contained in the liquid nitrogen is lost and, furthermore, it requires the presence relatively nearby of a liquid nitrogen production unit in order for the cost of replenishing the evaporator by a tanker lorry to remain moderate.
2. Related Art
Sometimes a gaseous nitrogen generator with liquid injection is installed with an emergency delivery system consisting of an evaporator, which makes it possible either to deliver gas to the customer if the apparatus is defective or to produce more gaseous nitrogen if the customer consumes more than the nominal production of the apparatus. The liquid from the emergency delivery system is generally vaporized in an atmospheric heater as may be seen in EP-A-0452177.
When there is a peak in consumption by the customer, liquid nitrogen from the storage tank is vaporized in an atmospheric exchanger (or a water pool) in order to top-up with nitrogen molecules, which nitrogen will be mixed with the nitrogen product output by the cryogenic apparatus. The refrigeration power of the liquid is therefore lost.
Liquid nitrogen also serves to keep the apparatus cold, by liquid injection. The amount of liquid nitrogen sent to the apparatus under steady operating conditions is about 3% of the nitrogen flow produced by the apparatus.
The invention proposes to recover some of the refrigeration power of the liquid that has been vaporized when the emergency delivery system is used for a peak in consumption.
According to the invention, when there is a peak in consumption, all or some of the liquid, which according to the prior art had to be vaporized in an atmospheric heater in order to top up with molecules, is sent to the distillation column via the liquid injection line.
In the distillation column, this inflow of liquid increases the level of reflux into the column. For a constant air throughput, it is then possible to extract more nitrogen than the nominal amount from the apparatus, by increasing the level of extraction. This increase in output makes it possible to have virtually one additional gas molecule per liquid molecule added. The column therefore acts as a “vaporizer” for liquid coming from the storage tank.
The consequence of increasing the reflux of the column is an excess production of column bottoms liquid rich relative to the nominal production. This excess results in the recovery of the refrigeration of the liquid coming from the storage tank. This excess will be stored either in the bottom of the column or in a dedicated container.
This mode of operation of the apparatus stops when the peak in demand is over or when the LR (rich liquid) storage capacity is reached. The level of extraction returns to its nominal value and the apparatus produces its nominal capacity (
The stored rich liquid will be used to keep the apparatus cold, instead of conventional liquid nitrogen injection. Depending on the customer consumption profile, it is even conceivable for there to be enough autonomy between two peaks in consumption to completely dispense with liquid injection.
There is therefore a not insignificant reduction in operating costs, by reducing or even eliminating the consumption of liquid nitrogen.
According to a first aspect of the invention, a process according to Claim 1 is provided.
The liquid injection flow is considered as being essentially stopped if it does not exceed 10%, or even 5%, of the liquid injection flow sent during the first operation. The most advantageous situation is obviously when the flow is stopped.
According to other optional aspects:
According to another aspect of the invention, a cryogenic distillation air separation apparatus is provided which comprises:
i) an exchanger;
ii) a distillation column;
iii) a line for feeding compressed, purified and cooled air to the exchanger and from the exchanger to the column;
iv) a line for feeding gaseous nitrogen from the column to the exchanger in order to warm it as product;
v) an overhead condenser for condensing nitrogen at the top of the column;
vi) a liquid nitrogen feed line coming from an external source, the liquid nitrogen feed line being connected to the top of the column; and
vii) means for detecting the bottoms level of the column, said means being connected to a liquid nitrogen feed line,
characterized in that the means for detecting the bottoms level of the column are capable of stopping the flow of liquid nitrogen being sent to the column if the bottoms level reaches a high threshold and/or of restarting the flow of liquid nitrogen sent to the column if the bottoms level reaches a low threshold.
One example of the implementation of the invention will now be described in conjunction with the appended drawings, in which:
The unit 7 shown in
The tank 8 may also be inside the cold box or even form a structure integrated into the column 10.
The line 16 runs into a use line 17 equipped with a buffer tank 18 and, downstream of the latter, with a pressure sensor 19.
The operation of the unit 7 will now be described with regard to
The nominal operation DN for which the column is designed will firstly be addressed.
In this operation (corresponding to t<t1 in
At time t1 it will be assumed that the gaseous nitrogen consumption (or demand) starts to increase, reaching a fixed value D′ above the nominal flow (
The flow D of injected liquid nitrogen is equal to 15% of the nominal flow, in order to increase the production of the column, i.e. a value of B+x. Some of the liquid serving for the peak in consumption will be injected via the liquid injection line, to be “vaporized” in the distillation column. The refrigeration power is therefore recovered in the form of rich liquid in the bottom of the column, where it is stored. This store can then be used to keep the apparatus cold, instead of injecting liquid nitrogen.
The benefit of the invention is that it saves on liquid nitrogen, hence a reduction in operating costs.
If the level of rich bottoms liquid of the column 10 reaches a high value L2 (
Over a given period t2-t3, the apparatus may continue to produce the nominal flow, without liquid injection, by using the stored rich bottoms liquid to provide the refrigeration. Obviously, this lowers the level of rich liquid, and when a level L1 is reached it is necessary to restart sending liquid nitrogen into the column.
When the gaseous nitrogen consumption resumes at a value above the nominal flow (time t3) the pressure drops and the solenoid valve 30 opens. This solenoid valve 30 is designed, in the open position, to let through a flow of liquid nitrogen at least equal to 15% of the nominal flow DN. Here again, the valve remains open until time t4, when the consumption drops to the nominal flow or until the liquid level LR reaches the value L2.
After t4, the liquid injection is stopped. The stored rich liquid alone provides the refrigeration for the distillation, and liquid injection is resumed only at time t5 when the level LR reaches its minimal value L1. At this moment, the liquid injection amounts to 3% of the nominal flow in order to ensure nominal production of the apparatus.
It may be seen that during the periods t2-t3 and t4-t5, the liquid injection flow is zero, which represents an appreciable saving of liquid nitrogen.
In
In the case of
At time t1′ it will be assumed that the gaseous nitrogen consumption (or demand) starts to increase, reaching a fixed value above the nominal flow (
The flow D of injected liquid nitrogen is equal to 15% of the nominal flow, in order to increase the production of the column. Some of the liquid serving for the peak in consumption will be injected via the liquid injection line, to be “vaporized” in the distillation column. The refrigeration power is therefore recovered in the form of rich liquid in the bottom of the column, where it is stored. This store can then be used to keep the apparatus cold, instead of injecting liquid nitrogen.
The benefit of the invention is that it saves on liquid nitrogen, hence a reduction in operating costs.
If the level of rich bottoms liquid of the column 10 reaches a high value L2, by closing the valve 30 liquid nitrogen is stopped being sent to the top of the column from the line 20, and the production of the column is returned to its nominal value. Over a given period t2′-t3′, the apparatus may continue to produce the nominal flow, without liquid injection, by using the stored rich bottoms liquid to provide the refrigeration.
Since in this case the consumed flow C remains at its high value, it is not possible to operate with liquid injection after t1′, the column bottoms level having reached the threshold L2. Here the top-up for the consumption is made through additional vaporization of liquid nitrogen (
When the liquid level LR reaches a value L1 at t3′, the solenoid valve 30 opens. This solenoid valve 30 is designed, in the open position, to let through a flow of liquid nitrogen at least equal, in molar terms, to 15% of the nominal flow DN. Here again, the valve remains open until time t4′, when the liquid level LR reaches the value L2. After time t4′, the liquid injection is stopped. It may be seen that during the period t2′-t3′ and after t4′, the liquid injection flow is zero, thereby representing an appreciable saving of liquid nitrogen.
In certain cases, the maximum liquid injection flow is insufficient to meet the entire increase in production required right from the start of the increase. In this case, part of the additional production comes from the column fed with an increased liquid injection flow and the remainder is produced by vaporizing liquid nitrogen in the emergency vaporizer.
In
In the variant of
Firstly, the customer consumes at a nominal value (or less). The rich liquid level is regulated at the low threshold L1 with a conventional liquid injection flow D with a value B.
Next, the customer consumes more then the nominal value (C=150). The liquid injection is increased to B+x and therefore the production by the column increases correspondingly, in order to reach the high threshold L2 for the rich liquid LR (if there is time to reach it, depending on the duration of customer over-consumption).
Thereafter, a conventional liquid injection flow D of value B is used.
The consumption C by the customer drops to the nominal value (or less): the level of LR slowly drops down to L1 without liquid injection, and then the level of LR is regulated at the low threshold L1 with a conventional liquid injection flow D of value B.
According to the prior art, the liquid injection flow remains constant outside the start-up, as may be seen in
As already described in the prior art, the single nitrogen production column may be combined with an oxygen production column fed with an oxygen-enriched fluid coming from the single column.
It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above.
Number | Date | Country | Kind |
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06 52793 | Jul 2006 | FR | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP2007/056085 | 6/19/2007 | WO | 00 | 5/15/2009 |
Publishing Document | Publishing Date | Country | Kind |
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WO2008/003585 | 1/10/2008 | WO | A |
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4698079 | Yoshino | Oct 1987 | A |
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5355680 | Darredeau | Oct 1994 | A |
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6155078 | Miyashita et al. | Dec 2000 | A |
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6357259 | Higginbotham et al. | Mar 2002 | B1 |
Number | Date | Country |
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0 538 857 | Apr 1993 | EP |
0538857 | Apr 1993 | EP |
Entry |
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PCT Search Report for PCT/EP2007/056085. |
Number | Date | Country | |
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20090314031 A1 | Dec 2009 | US |