DEVICE AND METHOD FOR AIR SEPARATION BY LOWER TEMPERATURE DISTILLATION

Description

[0001] The present invention relates to an air separation apparatus by cryogenic distillation comprising at least three separation columns, two of which are concentric, and a corresponding air separation method according to the preambles of claims 1 and 8 respectively. Such an apparatus and such a method are known from DE3709588A1.

[0002] It is known to separate air in a double distillation column comprising a first column operating at a first pressure and a second column operating at a second pressure lower than the first. Typically, the vessel of the second column is thermally connected to the head of the first column. A third column can be connected to the second column and possibly to the first column as well.

[0003] This third column can be of the Etienne column type, operating at a pressure between the first and second pressures and fed by an oxygen-enriched liquid from the first column's tank. An oxygen-enriched fluid and a nitrogen-enriched fluid produced in the third column can feed the second column at different levels.

[0004] While the first and second columns form a structure called a double column, the third column is independent of this structure.

[0005] In a standard configuration, the second column is stacked on top of the first column and has a larger diameter. The cross-sectional area of ​​the cold box that must house the two columns is therefore dictated by the diameter of the second column. Adding an additional column, such as the third column described above, further increases the cross-sectional area of ​​the cold box, or even necessitates the addition of another box. Other configurations besides this one are known, such as those described in EP1078212B1, but these still require either an enlarged cold box or a separate cold box.

[0006] It is therefore advantageous to utilize the available space beneath the second, larger-diameter column to house the additional column. Conveniently, it will be concentric with the first column.

[0007] Since the available space (and therefore the thickness of this concentric column) is limited, this configuration is particularly well-suited for adding a column that handles a gas charge (which, to a first approximation, determines the column's cross-sectional area) that is small compared to the air flow rate supplying the system composed of the first and second columns. This is the case for the third column, either an Etienne column, which handles approximately 40% mol of the total feed air flow rate at an intermediate pressure, or a mixing column, which handles approximately 30% mol of the total feed air flow rate, for example, at the first pressure.

[0008] The choice of packing for this concentric column (and the first and second columns) can be judiciously adapted so that its capacity is such that it is not necessary to have an outside diameter of the concentric column greater than that of the second column.

[0009] DE3709588A1 discloses a double column arranged inside another double column, thus forming four distillation compartments, the two double columns having a common vessel.

[0010] Research Disclosure for "Air Separation Low Purity Oxygen Production" proposes placing the mixing column inside the high-pressure column, the opposite of what is proposed in the present invention. It suggests using concentric columns, but given the basic premise, the high-pressure column would be the annular column.

[0011] FR2776206 describes concentric columns but with sections that operate in series. Thus, the distillation that takes place in the inner column of the Figure 1 is the continuation of that which began in the annular column that surrounds it.

[0012] According to one object of the invention, an apparatus according to claim 1 is provided.

[0013] According to other optional aspects: No pipe is connected to supply air to the third column. The second column is connected to receive overhead liquid from the third column. The apparatus does not include a gap between the outer wall of the upper section of the first column and the inner wall of the third column; the outer wall of the first column is the inner wall of the third column. The apparatus is designed to operate with a pressure difference between the third column and the first column of less than 3.5 bar, preferably less than 2 bar. The tank of the third column is located below the tank of the first column. The first column is entirely contained within the third column. The apparatus includes a thermally insulated enclosure containing the first, second, and third columns. The second column has a larger diameter than the first column.The second column does not contain a partition. The first column has a closed tank. The third column has a closed tank. The second and third columns have a common ferrule. The first column is shorter than the third column. The first and third columns have a common roof. The roof of the first column forms at least part of the tank of the second column.

[0014] According to another aspect of the invention, a method according to claim 8 is provided

[0015] Preferably the system includes only the first distillation column capable of operating at a first pressure having an upper section and a lower section, the second distillation column capable of operating at a second pressure lower than the first pressure and having a diameter and the third separation column.

[0016] Preferably, the entire space within the second column operates at a single pressure. It is obvious that there will be minor pressure differences between the top and bottom of the column. However, at a given height of the second column, all mass and heat exchange devices operate at the same pressure throughout the column cross-section at that height.

[0017] In preference to any elevation of the second column, any means of mass and heat exchange operates at the same pressure across the column section at that elevation.

[0018] The second column is not necessarily supplied with air.

[0019] According to the invention, the third column is an annular column arranged around the upper part of the first column.

[0020] Preferably the operating pressure of the annular column differs by less than 3.5 bars from the operating pressure of the first column.

[0021] Preferably the operating temperature of the annular column (third column) differs by less than 5 °C from the operating temperature of the first column.

[0022] Thus the temperature differential is small enough to prevent a heat transfer significant enough between the first and third columns to disrupt the distillation carried out in each of the columns.

[0023] If the column temperatures are too different, a thermal insulation system, such as a gas wedge, is necessary. The third column consists of an outer cylindrical wall and an inner cylindrical wall. The gas wedge is formed by an annular space within the space enclosed by the inner wall and in contact with it. Lacking any means of mass and heat exchange, this space is open at the bottom and / or top to isolate the third column from the first column, but does not allow for the bypassing of the gas to be distilled.

[0024] According to the invention, a liquid withdrawal and optionally at least one gas withdrawal or another liquid withdrawal is carried out from the portion of the first column surrounded by the third column. This withdrawal, connected to the first column, passes through the third column in a zone devoid of mass and heat exchange means. The configuration of the packed sections of the third column K3 is preferably established such that any withdrawal (liquid or gas) from the first column takes place in a zone devoid of packing in the third column.

[0025] The invention will be described in more detail with reference to the figure.

[0026] [ Fig. 1 ] schematically shows a three-column apparatus, the third column operating at a pressure intermediate between those of the other two columns.

[0027] [ Fig. 1] illustrates an air separation apparatus by cryogenic distillation comprising a first column K1 capable of operating at a first pressure having an upper section and a lower section and a second column K2 capable of operating at a second pressure lower than the first pressure and having a diameter.

[0028] The second column K2 is positioned above the first column K1, forming a unique structure.

[0029] An air flow 1 is sent as feed gas into the tank of the first column K1 where it separates by distillation into an oxygen-enriched liquid in the tank and a nitrogen-enriched gas at the top.

[0030] The first column K1 is also supplied by a two-phase air flow 3 which in the figure passes through a duct through the packings. A passage outside the packings would be preferable.

[0031] A third column K3 of annular shape, having an internal diameter and an external diameter, is arranged around the first column K1; in the illustrated case, the third column is higher than the first column, so that the tank of the third column K3 is below the tank of the first column K1.

[0032] The operating pressure of the annular column K3 differs by less than 3.5 bars from the operating pressure of the first column K1. The first column can, for example, operate at 6 bars and the third K3 at a pressure between 3.5 and 4 bars.

[0033] The first column, K1, comprises three sections of padding, one above the other, separated by gaps; the third column, K3, has two. Obviously, the number of sections can vary depending on the required separations. The sections in column K1 have a circular base, and those in column K3 have an annular base.

[0034] In this case, the outer wall of the first column K1 forms the inner wall of the third column K3. It is also possible for the first column to have a separate outer wall and the third column a separate inner wall, with a space between the two walls. This space would be open at one end to potentially allow for the installation of an insulating gas layer.

[0035] The second column K2 has the same external diameter as the third column K3. In this way, columns K2 and K3 can have a shell of the same diameter, or even a common one. The top of the third column K3 is separated from the vessel of column K2 by a frustoconic barrier 19, preventing the passage of any fluid.

[0036] The nitrogen gas formed at the top of the first column is drawn off through a pipe passing through a space above the packings of the third column K3. The nitrogen 5 is sent to a heat exchanger E1 where it condenses to form a liquid.

[0037] Liquid nitrogen 15 is drawn through a pipe passing through a space above the packings of the third column K3. It is mixed with condensed nitrogen 5 from the exchanger E1.

[0038] Oxygen-enriched liquid 17 is drawn from the tank of the first column K1 through the third column K3 into an unpacked space. The liquid 17 is optionally subcooled in a heat exchanger E2 and sent to feed the third column K3 where it separates.

[0039] The nitrogen-enriched gas formed at the top of column K3 is withdrawn from the column and condensed in heat exchanger E3 before being sent partly to the top of column K3 as reflux, and partly to the top of the second column as reflux.

[0040] The oxygen-enriched liquid from tank 7 of the third column K3 is divided in two. One portion 9 is vaporized in the heat exchanger E1 and feeds the tank of the third column K3. The remainder 11 is expanded through a valve to a pressure close to that of the second column and sent as a cold source to the heat exchanger E3, where it is mostly vaporized. The vaporized fraction and possibly a remaining liquid fraction are sent as feed to the second column.

[0041] The second column K2, which is not illustrated in detail, corresponds to a usual low-pressure column of a double column.

[0042] Thus the fraction(s) of the vaporized liquid 11 which feed(s) the second column is / are separated to form nitrogen at the top of the second column K2 and an oxygen-rich liquid in the tank of the second column K2.

[0043] The second column K2 is connected to receive head liquid 15 from the third column K3. The apparatus can therefore produce gaseous and / or liquid oxygen from the second column K2 and liquid and / or gaseous nitrogen from the first column.

[0044] The third column K3 corresponds to a well-known Etienne column in art.

[0045] The flow rate 17 supplying the third column K3 is between 30% and 50% (molar base) of the total air flow rate supplying the first, second and third columns (flow rates 1 and 3).

[0046] If the pressure of column K3 differs from that of the first column by less than 3.5 bars, the presence of a gas blade will not be necessary.

[0047] The third column can surround only part of the first column, for example the top part or the bottom part.

[0048] The second column has a diameter greater than that of the first column and / or equal to or greater than the outside diameter of the third column.

[0049] The largest diameter of a column is considered its diameter. For example, if the second column includes an upper section, often called a minaret, with a smaller diameter than the main part of the column, the diameter of the main part is considered the diameter. The fillings of the first column K1 are structured fillings.

[0050] The fillings in the third column K3 are bulk fillings.

[0051] Preferably the pressure of the third column differs from that of the first column by at least 1 bar.

Claims

1. An apparatus for separating air by cryogenic distillation comprising a column system including a first distillation column (K1) capable of operating at a first pressure having an upper section and a lower section, a second distillation column (K2) capable of operating at a second pressure lower than the first pressure and having a diameter, and a third separation column (K3), for example for distillation, of annular section, having an internal diameter and an external diameter, disposed around the first column, the external diameter of the third column being at most equal to that of the second column, the apparatus also comprising a conduit for sending air (1, 3) to the first column, a conduit for sending an oxygen-enriched fluid to an intermediate point of the second column, optionally a reflux conduit (15) connected to an intermediate level of the upper section of the first column for withdrawing a nitrogen-enriched liquid, the reflux conduit being connected to the top of the second column and passing through a region of the third column devoid of mass exchange means, and a conduit (15, 17) for withdrawing a liquid from the first column, the conduit being connected to another column (K2, K3) of the column system and passing through a region of the third column devoid of mass transfer means, the third column (K3) is connected to receive sump liquid (17) from the first column (K1), characterized in that the second distillation column contains no other distillation column and in that the apparatus comprises a conduit for feeding the third column (K3) with sump liquid (17) from the first column (K1).

2. The apparatus according to claim 1, wherein the second column (K2) is connected to receive top liquid from the third column (K3).

3. The apparatus according to one of claims 1 or 2, not comprising a space between the outer wall of the upper section of the first column (K1) and the inner wall of the third column (K3), the outer wall of the first column preferably being the inner wall of the third column.

4. The apparatus according to one of the preceding claims, designed to operate with a pressure difference between that of the third column (K3) and that of the first column (K1) of less than 3.5 bar, preferably less than 2 bar.

5. The apparatus according to one of the preceding claims, wherein the sump of the third column (K3) is disposed below the sump of the first column (K1).

6. The apparatus according to one of the preceding claims, wherein the first column (K1) is entirely contained within the third column (K3).

7. The apparatus according to one of the preceding claims, comprising a thermally insulated enclosure containing the first, second and third columns.

8. A process for separating air by cryogenic distillation in a column system including a first distillation column (K1) capable of operating at a first pressure having an upper section and a lower section, a second distillation column (K2) capable of operating at a second pressure lower than the first pressure and having a diameter, and a third separation column (K3), for example for distillation, of annular section, having an internal diameter and an external diameter, disposed in an annular manner around the first column, the external diameter of the third column being at most equal to that of the second column, wherein air (1, 3) is sent to the first column, an oxygen-enriched fluid is sent to an intermediate point of the second column, the third column is fed with oxygen-enriched liquid (17) from the first column (K1), optionally a nitrogen-enriched liquid (15) is withdrawn via a reflux conduit connected to an intermediate level of the upper section of the first column, the reflux conduit being connected to the top of the second column and passing through a region of the third column devoid of mass exchange means, and an oxygen-enriched liquid (17) is withdrawn from the first column via a conduit connected to another column of the column system and passing through a region of the third column devoid of mass exchange means, characterized in that the second distillation column contains no other distillation column, and the flow rate feeding the third column being comprised between 30% and 50% molar basis of the total feed flow rate of the first, second and third columns9. The process according to claim 8, wherein at a given elevation of the second column (K2), any mass and heat exchange means operates at the same pressure across the section of the column at that elevation.

10. The process according to claim 9, wherein at any elevation of the second column (K2), any mass and heat exchange means operates at the same pressure across the section of the column at that elevation.