METHOD AND APPARATUS FOR AIR SEPARATION BY CRYOGENS DISTILLATION

Description

[0001] According to the present invention, a method and apparatus for separating air by cryogenic distillation are provided. The method is carried out using a double air distillation column, which is well known in itself, combined with an argon separation column.

[0002] Typically in an air separation device, purified and cooled air is sent to a first column operating at a cryogenic temperature to be separated into a nitrogen-enriched gas and an oxygen-enriched liquid.

[0003] The liquid is drawn from the first column and sent to a second column operating at a lower pressure than the first column, after expansion in a valve.

[0004] Air separation units often include an argon separation column in addition to the double column. This argon separation column can obviously be used to produce argon, but in some cases, it is installed primarily to increase oxygen yield and / or increase nitrogen production at high pressure and / or to allow for the expansion of a large amount of air destined for the second column in order to increase cooling capacity and therefore liquid production, or to improve energy efficiency.

[0005] One aim of the present invention is to improve the energy performance of air separation units, with the presence of the argon separation column.

[0006] In this case, even if argon is not produced and / or the column contains very few stages, the invention aims to reduce the additional cost associated with the presence of this column. Thus, the energy performance gain provided by the invention can be achieved, in whole or in part, at a lower cost.

[0007] A method and apparatus according to the characterizing parts of claim 1 and claim 5 respectively are known to JP-A-5412662.

[0008] Another air separation process by air distillation is known from EP-A-0860670. In this process, the liquid feeding the argon column's top condenser does not come directly from the first column, but has first undergone partial vaporization to condense the argon mixture. As the liquid becomes more concentrated in oxygen, its vaporization temperature increases. The temperature difference in the argon column condenser is then extremely small and requires a very large heat exchanger volume. This results in a larger cold box.

[0009] According to one object of the invention, an air separation process is provided according to claim 1.

[0010] Depending on other optional aspects that can be combined with each other: The portion of oxygen-enriched liquid sent to the upper condenser of the third column has not undergone oxygen enrichment. The third column is located inside the second column, and at least some of the oxygen-enriched liquid is vaporized by heat exchange with the argon-enriched gas inside the second column. The argon-enriched gas sent to the heat exchanger has a condensation temperature higher than the vaporization temperature of the oxygen-enriched liquid in the heat exchanger. Some of the oxygen-enriched liquid is sent from the first column tank to the heat exchanger where it is at least partially vaporized, and some of the oxygen-enriched liquid is sent from the first column tank to the second column without passing through the heat exchanger.

[0011] All the oxygen-enriched liquid sent to the heat exchanger vaporizes there. According to another object of the invention, an air separation device according to claim 5 is provided.

[0012] According to other optional aspects: The apparatus includes means for sending the produced steam to the second column, where it is mixed with the expanded flow in the turbine. The third column is arranged inside the second column. The apparatus includes means for vaporizing the oxygen-enriched portion of the liquid by heat exchange with the argon-enriched gas inside the second column. The third column contains fewer than 50, or even fewer than 10, theoretical stages.

[0013] The invention will be described in more detail with reference to the figures: [ Fig. 1 composed of figures 1a et 1b represents comparative processes. Fig. 2 ] represents a method according to the invention. Fig. 3 ] represents a comparative process. Fig. 4 ] represents a variant of Figures 2 And 3 which is not covered by the claims of the invention.

[0014] In the Figure 1a We see a double air separation column comprising a first column K1 operating at a first pressure and a second column K2 operating at a second pressure, lower than the first pressure. The two columns are thermally connected, for example by a condenser-reboiler C which vaporizes the tank oxygen of the second column K2 by exchanging heat with the nitrogen gas of the first column K1.

[0015] A nitrogen-rich liquid 11 is sent from the top of the first column K1 to the top of the second column K2. The first column is supplied with gaseous air by a flow of air 1 that has been cooled and purified of water and CO2. Air can also be supplied to the second column K2.

[0016] An oxygen-enriched liquid is drawn from the tank of the first column K1 and divided in two. Part 3 is sent to the heat exchanger E where it is completely vaporized to form a gas 5. Gas 5 is expanded in a turbine T and sent to an intermediate point in the first column K1. The cooling generated at a very low temperature by this expansion therefore results in a reduction in the unit's energy consumption, compared to what would be consumed without this expansion.

[0017] The remaining 10 of the oxygen-enriched liquid withdrawn from the tank is expanded in a valve and sent as a flow rate 12 above the arrival points of the flow rates 5 and 9.

[0018] The exchanger E, contained in a chamber B, also serves to liquefy an intermediate gas flow 7 from the second column K2. This gas 7 will be drawn off at a position such that its condensation temperature (bubble point) will be higher than the vaporization temperature of the oxygen-enriched liquid 3 in the exchanger E. Typically, its composition will be that of the feed gas of an argon production column.

[0019] After condensing in E, this flow is then sent, possibly by means of a pump P, to at least one point above its point of withdrawal and below the expanded gas inlet of the turbine T.

[0020] An oxygen-rich liquid 15 is withdrawn from the tank of the second column K2 and a nitrogen-enriched overhead gas 13 is withdrawn from the head of the same column.

[0021] Alternatively, as illustrated in the Figure 1b All the tank liquid can be sent to the heat exchanger E where it partially vaporizes. The partially condensed flow is separated in a phase separator 8 to produce a gas 5 and a liquid 100 enriched in oxygen relative to liquid 3. The gas formed 5 is expanded in a turbine T and the remaining liquid 10 is expanded and sent to the column as fluid 12. In this case, the liquid enters column K2 at a level above the gas in turbine T since it has been enriched in oxygen. Figure 1B illustrates only a modified part of the Figure 1a .

[0022] These diagrams do not include an argon separation column, unlike the Figures 2 And 3 .

[0023] In the Figure 2 , variant of the Figure 1 , the oxygen-enriched liquid 3 is divided into three parts 3,17,19.

[0024] Part 17 is sent directly to the second column K2 in liquid form.

[0025] Part 3, as for the Figure 1 , heat exchange with an argon-enriched flow 7 which is part of the argon-enriched gas withdrawn from the second column, the remainder of the 7A gas being sent directly to feed the argon separation column K3.

[0026] Part 3 is vaporized to form the gas flow 5 at 2.1 bar, then expanded in the turbine T and sent to the column K2. The flow 7 condenses in the exchanger E contained in a chamber B, and the liquid formed 9 feeds the column K3, preferably a few stages above the gas inlet 7A.

[0027] The enclosure B is preferably positioned above the point of arrival of the liquid 9 in the column K3.

[0028] Part 19 of the oxygen-enriched liquid feeds the top condenser N of column K3 without having been oxygen-enriched and vaporizes there to form a gas 23. The gas 23 is mixed with the gas expanded in the turbine T to form a gas 25 which feeds the second column K2.

[0029] Thus the oxygen-enriched liquid feeds the exchanger E and the head condenser N in parallel.

[0030] The argon yield is around 80%, if the oxygen-purified argon (flow rate 21) is recovered as a product. If flow rate 21 is not recovered as a pure product, the K3 column can be very small, containing only a few dozen theoretical stages (< 50), or even fewer than 10 theoretical stages.

[0031] In the Figure 3 The oxygen-enriched liquid is divided into only two parts 3,3A. Part 3A feeds column K2 and part 3 is partially vaporized in heat exchanger E. The remaining liquid 3B feeds the top condenser N of column K3 and the gas formed 23 in the condenser feeds column K2.

[0032] The 7A gas formed in the exchanger E feeds the turbine T at an inlet pressure of 2.7 bars.

[0033] The argon yield is around 75 to 76%, if the argon is recovered (flow rate 21). In the case of Figures 2 And 3The argon column has a liquid feed in addition to the usual gaseous feed. Therefore, the diameter of the K3 column can be reduced by approximately 20% in the section above the liquid inlet 9, thus reducing its cost. Since the argon column is the tallest column in the unit, it is important to be able to reduce its volume and thereby reduce the dimensions of the cold box that houses it (not shown).

[0034] Alternatively, column K3 of Figures 2 And 3 It can be located inside column K2, arranged concentrically with the collar of column K2. Column K3 can contain structured packings or loose packings.

[0035] The gas rising in column K2 will pass either into column K3 or into the annular part surrounding column K2.

[0036] In this case, the top condenser N of column K3 will be used to heat a liquid bath located halfway up column K2. The gas from the top of column K3 will pass through a pipe into the top condenser N via a barrier forming a tank halfway up column K2, and the liquid condensed in condenser N will similarly pass through another pipe via the barrier back to column K2. A valve can regulate the amount of liquid returned from condenser N to column K2.

[0037] Column K3 is surrounded by an annular section of column K2 containing packing. The gas separated at the top of the annular section is sent to the K2 section by passing through the barrier in a pipe or is sent outside the column below the barrier to re-enter the column above the barrier. The tank liquid that accumulates above the barrier is sent to the top of the annular section either through a pipe passing through the barrier or through a pipe connected to the outside of the column.

[0038] In this case, the heat exchanger E in its enclosure B is always located outside column K2 and outside column K3. In this case, the flow rate 7 is drawn directly from column K2, without being divided, since the flow rate equivalent to 7A rises directly in column K2 towards column K3.

[0039] Similarly, liquid 3B is injected into column K2 to be directed towards condenser N.

[0040] For a concentric column K3 inside another column K2, since the fluid mixtures are not of identical composition on both sides of the inner column K3, there will be heat exchange through the wall of column K2 between the interior of column K2 and the annular portion. This heat exchange favors distillation at the top of column K2, while at the bottom, it does not.

[0041] It is therefore recommended to improve the exchange in the upper part of the K3 column by increasing the heat exchange surface by adding fins to the shell of the upper part of the K3 column.

[0042] Alternatively, a metal with better thermal conductivity can be used for the upper part of the shell than for the lower part (for example, aluminum at the top of the K3 column shell and stainless steel at the bottom). Another possibility is to use a K3 shell made entirely of aluminum and apply a coating to the lower section to reduce heat transfer.

[0043] It has been proposed in the past to have an argon separation column with an overhead condenser in a second column (low-pressure column). One possibility is to position the overhead column so that the gas from the argon column condenses partly in the overhead condenser of the argon column and partly in an overhead condenser of the low-pressure column by exchanging heat with oxygen-rich liquid from the tank of the first column (medium-pressure column). The liquid formed in the overhead condenser of the second column is sent to the top of the second column, and the vaporized liquid is sent to a level above the overhead condenser of the argon column. The overhead condenser can be a film vaporizer.

[0044] There Figure 4 illustrates yet another variant of Figures 2 And 3where the oxygen-enriched liquid 3 from the tank of the first column is oxygen-enriched in an Etienne K5 column whose tank reboiler E corresponds to the exchanger E of the previous figures.

[0045] Thus the reboiler E is heated by a gas flow 7 enriched in argon coming from the second column.

[0046] The liquid flow produced 9 serves as a second feed to the argon K3 column in addition to the gaseous feed.

[0047] Liquid 3, expanded through a valve, descends the stages of column K5 and becomes enriched with oxygen to produce an oxygen-rich flow 53 (75% oxygen), a tank flow, and a head gas containing only 16% oxygen. Flow 53 feeds column K2 and provides a 3% increase in argon yield.

Claims

1. A method of air separation by cryogenic distillation, wherein: a) Cooled and water-purified air (1) is sent to a first column (K1) operating at a first pressure where it is separated into a nitrogen-enriched gas and an oxygen-enriched liquid (3), b) A nitrogen-enriched liquid (11) relative to air is withdrawn from the first column and sent to the top of a second column (K2) thermally connected to the first column and operating at a second pressure lower than the first pressure, c) The oxygen-enriched liquid (3) relative to air is withdrawn from the first column and optionally a first part (10, 17) of the oxygen-enriched liquid is sent to an intermediate level of the second column, optionally after having undergone a partial vaporization step which has enriched it in oxygen, d) An argon-enriched gas (7) relative to air is withdrawn from the second column, e) A liquid derived from at least a part of the oxygen-enriched liquid by enriching it in oxygen in a stripping column (K5) is at least partially vaporized by heat exchange with the argon-enriched gas, and the vaporized oxygen-enriched liquid (5, 53) is sent to an intermediate level of the second column, f) At least a condensed part (9) of the argon-enriched gas is returned to a third column (K3) which is also fed with an argon-enriched gas stream (7A) from the second column (K2), an argon-enriched stream (21) is withdrawn from the top of the third column, and an argon-depleted liquid is returned from the third column to the second column, g) A part (19) of the oxygen-enriched liquid is sent to a top condenser (N) of the third column (K3), without having been reheated against the argon-enriched gas stream (7), h) The oxygen-enriched liquid sent to the top condenser is vaporized therein and the produced vapor (23) is sent to the second column characterized in that the vaporized oxygen-enriched liquid (5) is at a pressure of at least 1 bar higher than the pressure of the second column (K2), and is expanded in a turbine (T) then sent to an intermediate level of the second column.

2. The method according to claim 1, wherein the produced vapor (23) is sent to the second column, being mixed with the stream expanded in the turbine (T).

3. The method according to claim 1, wherein the third column (K3) is disposed inside the second column (K2) and the at least one part of the oxygen-enriched liquid is vaporized by heat exchange with the argon-enriched gas inside the second column.

4. The method according to any one of the preceding claims, wherein the argon-enriched gas (7), sent to the heat exchanger (E) where the heat exchange takes place, has a condensation temperature higher than the vaporization temperature of the oxygen-enriched liquid (3) in the exchanger.

5. An apparatus for air separation by cryogenic distillation comprising a first column (K1) operating at a first pressure, a second column (K2) thermally connected to the first column and operating at a second pressure lower than the first pressure, a heat exchanger (E), means for sending cooled and water-purified air (1) to the first column operating at the first pressure where it is separated into a nitrogen-enriched gas and an oxygen-enriched liquid, means for withdrawing a nitrogen-enriched liquid (11) relative to air from the first column, means for sending the nitrogen-enriched liquid to the top of the second column, means for withdrawing an oxygen-enriched liquid relative to air from the first column, optionally means for sending a first part (10, 17) of the oxygen-enriched liquid to an intermediate level of the second column, optionally after having enriched it in oxygen, means for withdrawing an argon-enriched gas (7) relative to air from the second column, means for sending a part (3) of the oxygen-enriched liquid to the heat exchanger to be at least partially vaporized by heat exchange with the argon-enriched gas and means for sending the vaporized oxygen-enriched liquid (5) from the heat exchanger to an intermediate level of the second column, a third column (K3) comprising a top condenser (N), means for sending at least a condensed part (9) of the argon-enriched gas in the heat exchanger to the third column and means for sending an argon-enriched gas stream (7A) from the second column to the third column, means for withdrawing an argon-enriched stream (21) from the top of the third column, means for returning an argon-depleted liquid from the third column to the second column, means for sending a part (19) of the oxygen-enriched liquid to the top condenser (N) of the third column (K3), connected directly to the first column without passing through the heat exchanger (E) and means for sending the vapor (23) produced by vaporizing the oxygen-enriched liquid in the top condenser to the second column, characterized in that it comprises a turbine (T) connected to an intermediate level of the second column (K2) fed by the vaporized oxygen-enriched liquid (5).

6. The apparatus according to claim 5, comprising means for sending the produced vapor (23) to the second column (K2), being mixed with the stream expanded in the turbine (T).

7. The apparatus according to one of claims 5 or 6, wherein the third column (K3) is disposed inside the second column (K2) and comprising means for vaporizing the at least one part of the oxygen-enriched liquid by heat exchange with the argon-enriched gas inside the second column.

8. The apparatus according to one of claims 5 to 7, wherein the third column (K3) contains fewer than 50 theoretical trays.

9. The apparatus according to claim 8, wherein the third column (K3) contains fewer than 10 theoretical trays.