The use of a cold box having a circular cross-section is a state of the art practice that is particularly interesting as it enables a drastic reduction in the structural steel quantity for a given side area of the cold box, thanks to improved resistance to internal overpressure in this case, compared to a more traditional square cross-section cold box. Orders of magnitude in our industry show that structural weight savings could be as high as 20% when using a circular cross-section cold box.

It is standard practice in the ASU industry that two physical columns are installed inside of the cold box, for cryogenic production of oxygen, nitrogen and/or argon.

One configuration is to have high pressure and low pressure columns as separate physical columns side by side in the same cold box. Alternatively, one may have two large columns comprising the main column (composed of high pressure and low pressure columns, stacked one on top of the other, with the main heat vaporizer in between of the two columns) and the crude argon column with a crude argon condenser on top of it.

With current state of the art technology, the two large columns (main column and crude argon column) have similar diameters. This means that the ratio between the cross-sectional diameters of the columns is generally between 1 and 2. At least one additional column forming part of the air separation unit may have a diameter smaller than the diameter of the main column and/or the crude argon column. This additional column could be, for example, the pure argon distillation column whose diameter is typically at most half that of the crude argon column. It is a common target in the industry to increase the footprint occupation ratio, defined as the ratio between the overall cold box area and the area occupied by the columns, as a basic cost optimization rule (the cost of the structural work of the cold box is high, mainly due to the large height of the cold box).

It is instructive to compare the footprint occupation ratio for a double column with a high pressure placed below the low pressure and an argon column, all the columns being placed within a single cold box. For obvious geometrical reasons, the footprint occupation ratio will be lower for a circular cross-section cold box than it would be for a square or rectangular cross-section cold box. A circular cross-section cold box with a double column and an argon column would have a footprint occupation ratio around 10% less than that obtained with a rectangular cross-section cold box

The present invention uses a round shaped cold box, with its inherent savings in structural steel, whilst achieving a high enough footprint occupation ratio so as to obtain a cost-optimized cold box.

The storage tank placed within the cold box may serve to store liquid oxygen, liquid nitrogen, liquid argon or oxygen-enriched liquid.

The storage tank may be connected to the outside of the cold box so as to permit delivery of a liquid product at a cryogenic temperature. This avoids the use of an external tank and reduces insulation costs and piping costs.

The storage tank may be connected to a vaporizer, which may be the main heat exchanger of the air separation unit or may be a dedicated vaporizer, warmed using steam or air. In this way a back up production of vaporized liquid can be provided when the distillation is not operating or cannot provide sufficient product.

The storage tank may be connected to the double column or the argon column so as to receive a liquid to be stored.

The storage tank may be connected to the double column or the argon column so as to send the stored liquid to the column. This set-up may be used to allow the air separation unit to react more quickly to a product increase requirement or to reduce start-up time. In this case, oxygen enriched liquid, for example, may be taken from the bottom of the double column, stored in the storage tank and sent back to the column when required.

The storage tank may be placed within a security enclosure, made, for example of concrete, and enclosing only the storage tank.

According to an object of the invention, there is provided a cryogenic enclosure for use in the separation of air, the cryogenic enclosure being cylindrical and having a substantially circular cross-section, a circular roof, a central axis, a cylindrical wall in contact with the atmosphere, a base and an enclosure diameter, the cryogenic enclosure containing first and second distillation columns, the first distillation column being a double column comprising a column adapted to operate at a lower pressure and a column adapted to operate at a higher pressure, the lower pressure column being placed above the higher pressure column and being contiguous therewith, the double column having a maximum diameter denoted as a first diameter, the second distillation column having a second diameter and being connected to the column adapted to operate at a lower pressure to receive, in use, an argon enriched stream, said first and second distillation columns containing heat and mass exchange elements, a cylindrical storage tank, the storage tank being devoid of any means for mass transfer and having a circular cross-section and a third diameter, the sum of the cross-sections of the first and second distillation columns and the storage tank being equal to at least 40% of the cross-section of the cryogenic enclosure and preferably less than 90% of the cross-section of the cryogenic enclosure, each of the first and second columns and the storage tank having each a vertical axis, any space around the columns and storage tank within the enclosure being filled with a particulate insulating material.

The cross-sections of the enclosure, the first and second columns and the storage tank are the maximum cross-sections, taken at the point where the enclosure, the column or tank has its maximum diameter, in the case where that diameter is not uniform.

The second column, the enclosure and the storage tank may each have a uniform diameter.

According to other optional features:

- the sum of the cross-sections of the first and second distillation columns and the storage tank being equal to at least 60% of the cross-section of the cryogenic enclosure - the sum of the cross-sections of the first and second distillation columns and the storage tank being equal to at least 70% of the cross-section of the cryogenic enclosure

- the enclosure contains a further distillation column having a fourth diameter connected to receive argon rich fluid from the second distillation column, the second diameter being greater than the fourth diameter.

-the storage tank is connected to receive liquid from the higher pressure column or the lower pressure column or the second distillation column or the further distillation column

- the storage tank is connected to send liquid to the higher pressure column or the lower pressure column or the second distillation column or the further distillation column

- the further distillation column is placed above the storage tank.

- the only distillation columns contained with the enclosure are the first and second distillation columns and optionally the further distillation column.

- the height of the storage tank is at least one third of the height of the first, second or further columns.

- the ratios between the first diameter and the second diameter and between the first diameter and the third diameter are both between 1 and 2.

- the ratio between the enclosure diameter and at least one, preferably each, of the first, second and third diameters is not more than 2.

- the storage tank is the only storage tank within the enclosure having a diameter with a ratio to all of the first, second and possibly fourth diameters between 1 and 2.

- the storage tank is the only storage tank within the enclosure.

- the storage tank is connected to one of the first and second distillation columns to receive cryogenic liquid from the column.

- the height of the enclosure and/or the first distillation column and/or the second distillation column is at least 20m, or even at least 30m.

- the height of the storage tank is at most equal to 50% of the height of the enclosure.

- the bottom of the storage tank is positioned adjacent to the bottom of the enclosure.

According to a further object of the invention, there is provided a cryogenic air separation unit comprising at least one enclosure according to any preceding claims, a compressor, a purification unit, a heat exchanger, a conduit for sending air from the compressor to the purification unit, a conduit for sending air from the purification unit to the heat exchanger, a conduit for sending air from the heat exchanger to the first distillation column of at least one enclosure.

According to a further object of the invention, there is provided an installation including at least three cryogenic enclosures according to one of Claims 1 to 13 wherein the storage tanks for each cryogenic enclosure is a cryogenic liquid oxygen storage tank, wherein there are means for sending liquid oxygen from the lower pressure column for each enclosure and wherein each of the storage tank is connected to a single customer supply line, the single customer not being connected to any common storage tank supplied by at least two lower pressure columns.

The invention will now be described in greater detail with relation to the figures. Figures 1 and 3 show an overhead view of a cryogenic enclosure according to the invention, Figure 2 shows a side view of the cryogenic enclosure, the enclosure itself being represented by a series of rings and Figure 4 shows an assembly of several cryogenic enclosures according to the invention.

In Figure 1 , a cryogenic enclosure CB, seen from above, is a cylindrical body of circular cross-section, placed with its major axis in a vertical position. The enclosure is solidly anchored with its base on the ground and has a diameter of 8m. The enclosure contains a storage tank S for a cryogenic liquid with a diameter of 3.2m, an argon separation column AR with a diameter of 2.7m and an air separation column which a double column comprising a higher pressure column MP placed below a lower pressure column BP, the top of the higher pressure column being thermally linked with the bottom of the lower pressure column. The higher pressure column MP has a smaller diameter than the greater part of the low pressure column BP. The topmost part of the low pressure column is composed of a reduced diameter section M, having a smaller diameter than the low pressure BP and the higher pressure column MP. The present of the reduced diameter section M is not an essential feature of the invention. The low pressure column has a maximum diameter of 2.8m.

The low pressure column produces an oxygen enriched fluid and a nitrogen enriched fluid, either or both of which may serve as a final product. The low pressure column or the high pressure column may be used to produce a nitrogen liquid product and the low pressure column may be used to produce an oxygen liquid product.

The argon separation column AR has a smaller diameter than the lower pressure column BP and is connected to the lower pressure column to receive argon enriched gas therefrom and to send bottom liquid thereto. The argon separation column AR is used to produce an argon enriched fluid which may be sent back to the lower pressure column BP, removed as a product or removed as a waste gas. An argon enriched liquid may be produced by the argon separation column AR.

The cryogenic storage tank S is used to store a cryogenic liquid coming from one of the other columns MP, BP or AR and/or to be sent to one of these columns. The storage tank S may for example contain an oxygen rich product liquid to be vaporized in the case of reduced or zero output of oxygen rich gas from the double column. It may alternatively contain a nitrogen rich liquid or an argon rich liquid, which may be removed as a product. The tank may even contain an oxygen enriched liquid from the column MP. The storage tank is not designed to be used as a distillation vessel.

The storage tank S has a larger diameter than the argon column AR and the lower pressure column BP. The height of the storage tank S is at least one third of the height of the first, second or further columns MP,BP,AR. The storage tank S is the only storage tank within the enclosure having a diameter with a ratio to all of the first, second and possibly fourth diameters between 1 and 2. The storage tank is in fact the only storage tank within the enclosure CB. The height of the storage tank S is at most equal to 50% of the height of the enclosure CB.

The space between the columns MP, BP, AR may contain at least one subcooler and/or at least one conduit and/or at least one pump and/or at least one turbine and/or at least one phase separator. The remaining space between the elements is filled with insulating material, for example perlite P. The enclosure CB does not normally contain any other distillation column or any heat exchanger intended to cool the feed air from the ambient temperature.

The advantage of the configuration according to the invention is that, as confirmed by the basic geometry, the footprint occupation ratio will be higher, for an enclosure CB containing three vessels S, AR, BP of similar size. The columns and storage tank are considered to be vessels of similar size if the maximum cross-sectional diameters of any two vessels have a ratio of at most 2 and at least 1 .

The idea is therefore to insert a single relatively large cryogenic storage tank inside the round cold box containing two columns of similar size (as defined previously) so as to attain a higher or equal occupation ratio than the one that would have been achieved with a rectangular cold box. By doing this, not only is the lay out occupation ratio of the cold box improved, (i.e. the wasted volume of the cold box is minimized) but also the cost of the separate storage for the cryogenic liquid is drastically reduced since no separate insulation is used. This storage can be considered to be a very simple cryogenic drum typically used in all air separation units.

The sum of the cross-sections of the first and second distillation columns and the storage tank is equal to at least 40% of the cross-section of the cryogenic enclosure and preferably less than 90% of the cross-section of the cryogenic enclosure. The ratios between the first diameter (the maximum diameter of the low pressure column) and the second diameter (the diameter of the argon column) and between the first diameter and the third diameter (the diameter of the storage tank) are both between 1 and 2

Figure 2 shows a side view of the set-up of Figure 1 where we see that the air separation column AR and the double column composed of columns MP and BP are set on the ground, as is the storage tank S. The storage tank S is considerably shorter in height than the columns.

Figure 3 shows a different set up in which the argon column is in two sections of equal diameter AR1 , AR2. The section AR1 is placed with its base at the bottom of the cold box as for Figure 2. However the section AR2 is placed above the storage tank S. The remaining elements are disposed as shown in Figures 1 and 2 and have the same diameters as for Figure 1 . The sum of the cross-sections of the first and second distillation columns and the storage tank is equal to at least 40% of the cross-section of the cryogenic enclosure and preferably less than 90% of the cross-section of the cryogenic enclosure. Detailed work shows that for a given plant, the addition of a relatively large storage tank within the round cold box enables the footprint occupation ratio to be increased by at least 10% as summarized in the following table:

First quotations on given plants confirm that important savings are achievable with the proposed configuration.

Last, in some configurations, and due to different ratios of vessel diameters between columns and storage tank, the footprint occupation ratio in the proposed configuration could become very close to the one in the base rectangular solution. Related estimates have shown that, even in this case, the proposed solution would be globally more competitive owing to the savings linked to the change of technology in the cryogenic storage tank, since rather than using a separate vacuum insulated storage tank, a much simpler drum can be enclosed within the particulate insulation of the cold box.

In Figure 4, an air separation installation is illustrated in which a plurality of at least three air separation units supplies oxygen to a single customer. Each of the at least three air separation units includes a cryogenic enclosure according to the invention. Within the enclosure, for each air separation unit, are a double column for air separation by cryogenic distillation, a column by separating an argon stream by distillation and an oxygen storage tank. The oxygen storage tank may be supplied with liquid oxygen from the storage tank and provides liquid oxygen to the customer or to a vaporizer which provided gasified liquid oxygen to the customer. In this way, the customer receives oxygen from the storage tanks of each of the at least three cryogenic enclosures, when one of the double columns is not functioning or when the customer's requirements exceed what can be produced, from an physical or economic point of view. There is no central liquid oxygen tank which is supplied from at least two of the air separation units. This leads to considerable simplification of the system for transporting the liquid oxygen and gives an additional degree of freedom for the positioning of the air separation units.