DEVICE AND METHOD FOR AIR SEPARATION BY CRYOGENS DISTILLATION

Description

[0001] The present invention relates to an apparatus and a method for separating air by cryogenic distillation.

[0002] In particular, it relates to an air separation apparatus comprising a double column with a first column operating at a first pressure and a second column operating at a second pressure, lower than the first pressure. The top of the first column produces a gas which condenses in a reboiler of the second column.

[0003] It is generally a goal of air separation devices to seek the lowest possible energy consumption.

[0004] Air purification is generally carried out at a pressure equal to or greater than the initial pressure. This allows for a reduction in the volume of the purification unit.

[0005] It is nevertheless known from US4964901 to purify part of the air at the first pressure and the rest at the second pressure, using two parallel purification units. The air purified at the second pressure is sent directly to the second column, while the air purified at the first pressure is split in two: one part is sent directly to the first column, and the rest is over-pressurized, cooled in a heat exchanger, expanded in a turbine coupled to the blower, and sent to the second column. Thus, the turbine used is a blow turbine, and the low-pressure column receives air that has been purified at two different pressures.

[0006] The process of US5934105 purifies the air to a pressure above the second pressure but below the first pressure, then the air destined for the first column is compressed and the air destined for the second column is expanded.

[0007] JPH11063810 and EP1050730 are similar to US5934105.

[0008] If all the flow going to the second column is expanded in the turbine, as in the prior art, to maximize energy gain, the air flow going to the first column is approximately 66% of the total purified flow, for example, to produce 96% oxygen. This means that 34% of the air flow must be passed at a relatively low pressure in the turbine.

[0009] According to the present invention, between 6 and 8% of the air is expanded in an air turbine, therefore the turbine according to the prior art is at least 4 to 5 times larger due to the volume flow rate.

[0010] Since the cooling capacity of the prior art process is fixed and remains low because the process does not produce a liquid final product, this means that the turbine expansion rate is very low, resulting in an inefficient turbine and in any case not at all standardized, or even non-existent among cryogenic turbine suppliers.

[0011] In cases where the goal is to impose a specific airflow rate to the first column to maximize energy efficiency, according to the prior art, during operation, the cooling capacity cannot be regulated by reducing the turbine flow rate. Instead, it must be regulated by adjusting the pressure upstream of the turbine, i.e., the scrubbing pressure and ultimately the blower pressure. This significantly complicates the regulation process and necessitates sizing the scrubbing system for the lowest possible pressure achievable with a lower cooling capacity than nominal or during a transient phase. According to the invention, the scrubbing pressure is designed to be very close to the second pressure.

[0012] The invention provides a process which consumes 1% less energy (2% less if we consider a turbine efficiency reduced by 5%pt) compared to the prior art (for example, according to EP1050730); according to the process of EP1050730, the purification is carried out at a pressure between the first and second pressure.

[0013] The expansion ratio of the EP1050730 process is low, between 1.2:1 and 3.8:1, preferably between 1.4:1 and 2.5:1, whereas conventional cryogenic turbines are in an expansion ratio range of between 4:1 and 10:1. The invention uses an expansion ratio that remains at the lower limit of this range, thus avoiding a significantly degraded turbine efficiency.

[0014] In EP1050730, the inlet pressure of the purification unit is typically 2.5 bara (instead of approximately 1.3 bara according to the invention). This process uses a first multi-stage compressor, typically two stages, with cooling between the two stages. According to the invention, the compressor that compresses all the air has only one stage, therefore no cooling between the two stages.

[0015] The device produces an oxygen-enriched gas flow with a particularly low energy consumption.

[0016] US5666824 describes a process according to the preamble of claim 1, but wherein the first flow is at least partially condensed in an intermediate condenser of the second column. If a gas is formed, it is itself condensed in another intermediate condenser of the second column, and the liquid thus formed is sent to the top of the second column. Thus, the first flow is not sent directly to distillation.

[0017] According to one object of the invention, a device according to claim 1 is provided.

[0018] According to other optional aspects: The means for producing cooling include a turbine for expanding a nitrogen-rich gas from the first column and / or means for supplying a cryogenic liquid from an external source to the double column. The means for supplying air to the purification unit at the third pressure do not include any compression means other than a single-stage compressor. The apparatus does not include any means for compressing the first flow rate.

[0019] According to another aspect of the invention, a method according to claim 5 is provided.

[0020] According to other optional aspects: The entire first flow is sent to the second column. The first flow is sent to the second column at a level lower than or equal to the inlet level of the oxygen-enriched liquid. The process does not produce any liquid product as an end product and / or no liquid flow is withdrawn from the double column to serve as an end product. The portion of air expanded in the turbine represents between 6 and 15% vol, preferably between 6 and 8% of the purified air. All the air is purified at a pressure not exceeding 1.5 bara, or even 1.3 bara. The first pressure does not exceed 6 bara. The second pressure does not exceed 1.5 bara. The oxygen-enriched gas contains at least 80 mol% oxygen. The oxygen-enriched gas contains at least 90 mol% oxygen. The oxygen-enriched gas contains less than 98 mol% oxygen. The first flow rate represents between 20 and 30% of the volume of the purified air flow rate. The second flow rate represents between 70 and 80% of the volume of the purified air flow rate.An oxygen-enriched gas and / or a nitrogen-enriched gas are drawn from the double column and heated in the heat exchanger to form a process product by introducing them at the cold end of the heat exchanger. An oxygen-enriched liquid is drawn and heated in the heat exchanger to form a process product. The oxygen-enriched liquid is pressurized before being vaporized either in a dedicated vaporizer or in the heat exchanger. The oxygen-enriched liquid is vaporized by heat exchange with a portion of the second flow or with a third flow of pressurized air at a pressure greater than the fifth pressure. The first air flow is subcooled between the heat exchanger and the second column. The portion of air expanded in the turbine is subcooled between the turbine outlet and the second column.

[0021] The invention will be described in more detail with reference to the figure.

[0022] There figure 1 represents a process for separating air by cryogenic distillation according to the invention.

[0023] A cryogenic distillation air separation apparatus comprises a double column with a first column K3 operating at a first pressure and a second column K4 operating at a second pressure, lower than the first pressure, the second column having a tank reboiler M. The second column K4 does not contain an intermediate condenser.

[0024] In this example the first pressure is 4.5 bara and the second pressure is 1.13 bara.

[0025] A nitrogen-enriched gas is sent from the top of the first column to the tank reboiler M and at least a part of the condensed nitrogen-enriched gas from the tank reboiler is sent to the top of the first column.

[0026] Air at atmospheric pressure is filtered in a filter A, compressed by a single-stage blower B to a pressure of at most 1 bar, preferably at most 0.5 bar, above atmospheric pressure, cooled by a cooling means C, and purified of water and carbon dioxide in a single purification unit D into which the air 4 enters at a third pressure higher than atmospheric pressure by at most 1 bar, preferably at most 0.5 bar. The purification unit comprises two beds of adsorbent used alternately to purify the air, one bed purifying the air while the other is regenerated.

[0027] The purified air in unit D is divided into two to form two flow rates 6,8. Air 8 is neither compressed nor expanded and is at a pressure that differs from the second pressure by a pressure equal to the pressure losses in the pipes and heat exchanger G.

[0028] Preferably the first flow rate 8 represents between 20 and 30% of the volume of flow rate 4 and the second flow rate 6 represents between 70 and 80% of the volume of flow rate 4.

[0029] Thus, air 8 is sent directly from the purification unit to the second column K2 for separation, entering the column in a completely gaseous state. Air 8 is cooled in the heat exchanger G to a temperature at least 5°C above its dew point.

[0030] The air 6 is compressed in a blower E, cooled in a cooler F, and sent to the heat exchanger G. The blower E compresses the air 6 to a fifth pressure between the first pressure and 1 bar above the first pressure. The air 6 is split into two parts 30 and 32 at an intermediate level in the exchanger. The air 30 exits the exchanger at an intermediate temperature, for example -125°C, is expanded in a turbine 28 to the second pressure, and re-enters as a gas, mixed with the air 8, to be separated in the second column K4.

[0031] Flow rate 30 can represent between 6 and 15% vol, preferably between 6 and 8% of air 4.

[0032] Air 32 is cooled to the cold end of the heat exchanger G and is sent to the tank of the first column K3 in essentially gaseous form for separation. Air 8 is cooled in the heat exchanger G to a temperature at least 5°C above its dew point.

[0033] An oxygen-enriched liquid flow 34 is drawn from the first column's tank and sent to a level in the second column that is above the air inlet. Alternatively, air can enter the second column at the same level as the liquid 34 inlet.

[0034] The expanded liquid 34 can be separated in a phase separator: the liquid from the phase separator is sent to column K4 and the vapor phase can be mixed at the air inlet 8.30 in column K4.

[0035] A flow of liquid nitrogen 35 is drawn from the top of the first column and sent to the top of the second column.

[0036] Nitrogen gas 36 is drawn off at the top of the second column K4 and heated in the subcooler S and then in the exchanger G. Part 14 of this gas is used to regenerate the purification unit D.

[0037] Oxygen gas 29 is withdrawn from the tank of the second column K4. Flow 29 preferably contains at least 80% mol oxygen, or even at least 90% mol oxygen, but preferably less than 98% mol oxygen.

[0038] It should be noted that the process does not produce any liquid flow as an end product. The process does not produce any liquid flow to be vaporized to form a final gaseous product, possibly under pressure. However, it is possible to produce a small quantity of final gaseous product in this way, which can then be mixed with the main gaseous product.

[0039] In addition, a small flow of liquid could be produced.

[0040] Alternatively, air 8 and / or air 30 can be subcooled in subcooler S and then introduced into the second column K4. Otherwise, the mixture of flow rates 8 and 30 can be subcooled in subcooler S and then introduced into the second column K4.

[0041] In the example described, flow 29 is a flow of gaseous oxygen that is heated in the heat exchanger G from the cold end of the exchanger G. Alternatively, flow 29 can be a flow of oxygen-rich liquid pressurized to a pressure above that of the second column K4. Liquid 29 vaporizes either in a dedicated vaporizer (not shown) or in the heat exchanger G. Liquid 29 can be vaporized by heat exchange with all of the air 32 to partially condense the air 32, which is then sent to the tank of the first column K3. Alternatively, liquid 29 can be vaporized by heat exchange with a portion of the air 32 to completely condense that portion of the air 32. The condensed air is then sent to the tank of the first column K3 or to an intermediate point in the first and / or second column.

[0042] Alternatively, part of the purified air can be supercharged in a supercharger to a pressure higher than that of the first column K3 to vaporize the liquid 29.

Claims

1. An air separation apparatus comprising a double column with a first column (K3) configured to operate at a first pressure and a second column (K4) configured to operate at a second pressure, lower than the first pressure, the second column having a bottom reboiler (M), means for sending a nitrogen-enriched gas from the top of the first column to the bottom reboiler and means for sending at least a part of the nitrogen-enriched gas condensed in the bottom reboiler to the top of the first column, a heat exchanger (G), a purification unit (D), means (B) for sending air to the purification unit at a third pressure greater than atmospheric pressure by at most 1 bar, a conduit for sending a first air stream (8), purified in the purification unit, to the heat exchanger at a fourth pressure greater than the second pressure by at most 1 bar, a conduit for introducing the first purified air stream cooled in the heat exchanger into the second column to be separated therein, a booster compressor (E), a conduit for sending a second air stream (6) purified in the purification unit to the booster compressor, a conduit for sending the entire second stream compressed by the booster compressor up to a fifth pressure between the first pressure and 1 bar above the first pressure to the heat exchanger, refrigeration production means (28), comprising at least one expansion turbine (28) for expanding a part (30) of the second stream (6) from the fifth pressure to the second pressure, the turbine being connected to the second column (K4) to send the expanded air thereto, a conduit for withdrawing at least one oxygen- or nitrogen-enriched fluid (29) from a column of the double column connected to the heat exchanger and a conduit for exiting at least one oxygen- or nitrogen-enriched fluid from the heat exchanger as a product, the apparatus comprising no expansion means for the first stream and comprising only a single purification unit, means for exiting the part of the second stream (6) intended for the turbine from the heat exchanger (G) at an intermediate temperature of the heat exchanger, the inlet of the turbine (28) being at the intermediate temperature of the heat exchanger and means for sending another part (32) of the second stream, which is cooled in the heat exchanger down to its cold end, to the first column, characterized in that the second column does not comprise an intermediate condenser, the conduit for introducing the first purified air stream being connected to the interior of the second column to allow the first stream to participate in the distillation.

2. The apparatus according to claim 1, in which the refrigeration production means comprise an expansion turbine for a nitrogen-rich gas coming from the first column (K3) and / or means for sending a cryogenic liquid from an external source to the double column (K3, K4).

3. The apparatus according to claim 1 or 2, in which the means for sending air to the purification unit at the third pressure comprise no compression means other than a single-stage compressor (B).

4. The apparatus according to claim 1, 2 or 3, comprising no means for compressing the first stream (8).

5. A method of air separation by cryogenic distillation using a double column with a first column (K3) operating at a first pressure and a second column (K4) operating at a second pressure, lower than the first pressure, the second column having a bottom reboiler (M), in which: i) air containing water and carbon dioxide is sent to a single purification unit (D) at a third pressure greater than atmospheric pressure by at most 1 bar, ii) the purified air is separated into two, iii) a first air stream (8) purified in the purification unit is sent to a heat exchanger (G) at a fourth pressure greater than the second pressure by at most 1 bar, iv) the first purified air stream cooled in the heat exchanger is sent to the second column (K4), without having been expanded, v) a second air stream (6) is boosted to a fifth pressure between the first pressure and 1 bar above the first pressure, the second stream is sent at the fifth pressure to the heat exchanger and a part of the second stream is sent to the first column in gaseous form, vi) refrigeration is supplied to maintain the process cold, vii) a nitrogen-rich gas from the first column is at least partially condensed in the reboiler and at least a part of the condensed nitrogen is returned to the first column, viii) a nitrogen-enriched liquid (35) and an oxygen-enriched liquid (34) are sent from the first column to the second column, ix) an oxygen-enriched gas (29) or a nitrogen-enriched gas is withdrawn from the double column and is reheated in the heat exchanger to form a product of the process x) the process being kept cold by expansion of a part (30) of the second stream (6) in a turbine (28) from the fifth pressure to the second pressure, the entire second stream (6) is cooled in the heat exchanger (G) to an intermediate temperature of the heat exchanger, the inlet of the turbine (28) being at the intermediate temperature of the heat exchanger and the part (32) of the second stream sent to the first column being cooled in the heat exchanger down to its cold end. xi) the first air stream and / or the part of the second stream intended for the first column is cooled in the heat exchanger to a temperature at least 5°C above its dew point characterized in that the first air stream is sent directly into the second column to be separated therein without having been condensed in a condenser.

6. The method according to claim 5, in which the first stream (8) is sent to the second column (K4) at a level lower than or equal to the arrival level of the oxygen-enriched liquid (34).

7. The method according to one of claims 5 or 6, in which the part of the air expanded in the turbine represents between 6 and 15% by vol, preferably between 6 and 8% of the purified air.

8. The method according to one of claims 5 to 7, in which all the air (4) is purified at a pressure not exceeding 1.5 bara, or even not exceeding 1.3 bara.

9. The method according to one of claims 5 to 8, in which the oxygen-enriched gas (29) contains at least 80 mol% oxygen, or even at least 90 mol% oxygen, but preferably less than 98 mol% oxygen.

10. The method according to one of claims 5 to 9, in which the first stream (8) represents between 20 and 30% by vol of the purified air stream.

11. The method according to one of claims 5 to 10, in which the second stream (6) represents between 70 and 80% by vol of the purified air stream.

12. The method according to one of claims 5 to 11, in which an oxygen-enriched gas (29) and / or a nitrogen-enriched gas is withdrawn from the double column and is / are reheated in the heat exchanger (G) to form a product of the process by introducing it or them at the cold end of the heat exchanger.