Method for starting a cryogenic distillation air separation unit

Abstract

The invention relates to a method for starting a cryogenic distillation air separation unit, wherein the unit comprises a first column (3) operating at a first pressure and a second column (5) operating at a second pressure lower than the first pressure, the second column having a tank reboiler (4), connected so as to be heated by a nitrogen-enriched gas coming from the first column, and means for discharging an oxygen-enriched fluid from the tank of the second column which are connected to a heat exchanger (6) so as to ensure the heating of the oxygen-enriched fluid; in order to start the unit, air is sent to the first or second column and then liquid nitrogen is sent below the reboiler from an external source (1).

Description

[0001] Description

[0002] Title of the invention: Method for starting an air separation apparatus by cryogenic distillation

[0003] The present invention relates to a method for starting up an air separation apparatus by cryogenic distillation. The separation apparatus comprises a first column operating at a first pressure, called medium pressure, and a second column operating at a second pressure, lower than the first pressure, called low pressure. The head of the first column is thermally connected to the vessel of the second column.

[0004] The columns can be arranged with the second column above the first, or alternatively, the two columns can be side by side. The columns are located inside a thermally insulated enclosure called a cold box.

[0005] One aim of the invention is to have a rapid automatic start regardless of the starting temperature of the cold box and the liquid levels of an air separation apparatus containing the distillation columns.

[0006] Another objective of the invention is to be able to start the device by adding liquid nitrogen directly into the column using piping between the storage and the column intended for sending liquid oxygen.

[0007] It is known to add liquid nitrogen to a cryogenic distillation air separation column. In the case of a double column, the nitrogen is added at the top of the low-pressure column (SU832273) or at the top of the medium-pressure column (FR2578532). The liquid nitrogen is generally introduced at the top of the low-pressure column, at the same level as the upper lean liquid, which has a similar concentration.

[0008] When liquid nitrogen is used continuously for cooling (for example if the turbine is out of order), this prevents disruption to the distillation of the low-pressure column.

[0009] In some cases, liquid oxygen is sent into the tank of the low-pressure column from an external source (US4732595, FR1169625, US3039274).

[0010] Some processes involve the use of liquid oxygen and liquid nitrogen from external sources, but the two liquids are injected into different locations within the column according to their concentrations (FR2699992, US4853015). According to the invention, liquid nitrogen from an external source is injected into the tank of the low-pressure column, below the vaporizer. Thus, the liquid nitrogen is injected into a location where, under steady-state operation, liquid oxygen accumulates.

[0011] During the start-up sequence, the main air compressor is started first to send air to the low-pressure column, before starting to send liquid nitrogen into the low-pressure column tank.

[0012] If the startup process were initiated by adding liquid nitrogen from an external source, and thus supplying the tank vaporizer of the low-pressure column (second column) solely with liquid nitrogen, the vaporizer would operate completely outside its operating range when the air compressor supplying the medium-pressure column (first column) started. This is because the surrounding liquid concentration is not that of the stable operating range. The resulting pressure in the medium-pressure column would be very low (possibly lower than that intended for the low-pressure column), and a very high flow rate of air, and subsequently nitrogen, would condense in the vaporizer. It would therefore become impossible to start the apparatus, particularly due to insufficient pressure to pump the reflux liquids from the medium-pressure column back to the low-pressure column.

[0013] According to the invention, in one embodiment, the process begins by starting the air compressor, which sends gaseous air to the low-pressure column. The air compressor will reach the top of its operating curve (maximum high pressure and slightly reduced flow rate), and its regulator will vent the unused air to the atmosphere, bypassing the column. Liquid nitrogen will then be sent to the tank vaporizer in the low-pressure column, eventually creating a small liquid level around the tank vaporizer and priming the vaporizer, initially at a low flow rate and primarily maintaining the pressure of the medium-pressure column. The liquids can then be drawn up from the medium-pressure column, and distillation can begin. Thus, the bath is gradually enriched with oxygen: little by little, the level will rise while the bath continues to be enriched, and when the level reaches 100% submersion,The liquid bath will have a concentration quite close to the nominal oxygen content, so the machine will be close to its nominal capacity. This allows for a trouble-free start-up. According to one aspect of the invention, a method is provided for starting an air separation apparatus by cryogenic distillation for the production of gaseous oxygen. The apparatus comprises a first column operating at a first pressure and a second column operating at a second pressure lower than the first. The second column has a tank vaporizer, connected to be heated by a nitrogen-enriched gas from the first column, a heat exchanger, a purification unit, and means for delivering an oxygen-enriched fluid from the tank of the second column, connected to the heat exchanger to ensure the heating of the oxygen-enriched fluid to serve as a product. In normal operation, air is compressed in a compressor.It is purified into water and carbon dioxide in the purification unit at a pressure substantially equal to the first or second pressure, it is cooled and at least partially sent to the first column; an oxygen-enriched liquid is sent from the first column to the second column and an oxygen-enriched gas is drawn from the lower part of the second column as a gaseous oxygen product, or an oxygen-enriched liquid from the lower part of the second column which vaporizes against the air which cools to form the gaseous oxygen product and to start the apparatus, while the apparatus is at a temperature above 0°C or below 0°C, purified air is first sent to the second column, preferably to the second column but not to the first column, and then liquid nitrogen is sent from an external source below the vaporizer.

[0014] According to other optional objects:

[0015] • Liquid nitrogen is sent from an external source below the vaporizer after the compressor has started.

[0016] • We stop sending liquid nitrogen below the vaporizer once the liquid level around the vaporizer has risen to a threshold and after the threshold is reached, we start an air turbine powered by compressed air in the compressor and purified to keep the process cold.

[0017] • an air turbine is started, supplied with compressed air from the compressor and purified to keep the process cold before or during the sending of liquid nitrogen below the vaporizer • the device to be started is at a temperature above 0°C.

[0018] • the device to be started is at a temperature below -100°C, or even -170°C.

[0019] • a cryogenic distillation air separation process comprising a start-up step as described above and a stable operating step in which no flow of liquid nitrogen is sent to the second column and the process is kept at least partially cold by expanding at least one fluid intended for or from one of the columns in at least one turbine.

[0020] • a cryogenic distillation air separation process comprising a start-up step as described above and a stable operating step in which liquid nitrogen is sent to the second column and optionally the process is also kept cold by expansion of at least one fluid intended for or from one of the columns in at least one turbine.

[0021] • the oxygen-enriched gas contains less than 98.5% mol O2, or even less than 96% mol O2.

[0022] • In normal operation, a first compressor compresses the air to the second pressure, the air is substantially purified at the second pressure and part of the purified air is sent to a blower which compresses the part of the purified air to the first pressure and the supercharged air is sent partly to the first column and part of the purified air at the second pressure is sent to the second column and during start-up, the first compressor is started before the blower.

[0023] • During start-up, the sending of liquid nitrogen below the vaporizer is triggered after the start of the first compressor and the blower.

[0024] • During startup, the sending of liquid nitrogen below the vaporizer is triggered once the first and second columns are supplied with air.

[0025] • During startup, a transfer of liquid from the first column tank to the second column is triggered before the transfer of liquid nitrogen to the second column tank. • During startup, preferably only during startup, a portion of the purified air is sent to regenerate the purification unit.

[0026] • In normal operation, a first compressor compresses the air to the first pressure, the air is purified at the first pressure, part of the air at the first pressure is sent to the first column and part of the air at the first pressure is expanded in a turbine and sent to the second column and during start-up, the first compressor is started before triggering the sending of liquid nitrogen below the vaporizer.

[0027] • the start-up takes place when all the equipment of the device which must operate at cryogenic temperature in normal operation is at least 0°C.

[0028] • the start-up takes place when all the equipment of the device which must operate at cryogenic temperature in normal operation is below -100°C.

[0029] The present invention can be used for both cold and hot starts. It also has the advantage of comprising the same steps whether the start is hot or cold, which simplifies automation.

[0030] Following a shutdown of the unit, one can be in one of two main states:

[0031] • Warm start (above 0°C): following defrosting, for example, all the equipment in the cold storage unit is at ambient temperature

[0032] • Cold start (below -100°C, or even below -170°C): following a short stop, all equipment is at cryogenic temperature and a majority of cryogenic liquids have sometimes been kept, particularly in the tanks of the first and second columns.

[0033] The invention is described below for an air gas separation apparatus comprising a first column operating at a first pressure and a second column operating at a second pressure, lower than the first pressure, the tank of the second column being thermally connected to the tank of the first column, the apparatus comprising air purification at the second pressure, i.e. a low pressure, to produce impure oxygen drawn off in gaseous or liquid form (i.e. with a purity of less than 98.5% mol, or even less than 96% mol O2).

[0034] The invention will be described in more detail with reference to the figure, in which:

[0035] [FIG.1] represents an air gas separation apparatus with second pressure purification, capable of being started according to a method of the invention. The invention is described below in more detail with reference to [FIG.1] which schematically represents an apparatus with second pressure purification operating according to the method of the invention.

[0036] An air separation apparatus comprises a first column 19 operating at a first pressure and a second column 21 operating at a second, lower pressure. The tank of the second column is thermally connected to the tank of the first column, with purification at the second pressure, i.e., a low pressure, to produce impure oxygen (i.e., with a purity of less than 98.5 mol%, or even less than 96 mol O2). The oxygen can be withdrawn in gaseous form or, failing that, in liquid form, and is then vaporized by heat exchange with air, possibly after pumping. A vaporizer 20, heated by nitrogen gas from the first column 19, heats the tank of the second column 21.

[0037] In nominal (or normal) operation, ambient air is filtered in filter 1, then compressed in compressor 2 to the second pressure, then cooled in exchanger 3. It is then purified in purification 4 to the second pressure.

[0038] Part of the air purified at the second pressure is sent directly into the exchanger 15, then to the second column (low pressure column) 21 where it is separated, without having been compressed or expanded downstream of the compressor 2.

[0039] Another portion of the purified air at the second pressure is compressed in compressor s, then cooled in heat exchanger 9, then in heat exchanger 15, and then partly sent to turbine 10, the other part being sent to separate in the first column (medium pressure column) 19. The portion expanded in turbine 10 is sent to the second column 21 in gaseous form at an intermediate level for separation. The tank-rich liquid from the first column 19 is cooled in subcooler 22, then expanded in valve 24 and then sent to an intermediate section of the second column 21.

[0040] The lean liquid at the top of the first column 19 is cooled in the sub-cooler 22, then expanded in the valve 23 and then sent to the top of the second column 21.

[0041] The reflux of the first column 19 and the reboiling of the second column 21 are ensured by the vaporizer-condenser 20.

[0042] Oxygen gas is produced in the tank of the second column 21, which is heated in the exchanger 15, and then sold as product 40. This is impure oxygen (i.e. with a purity of less than 98.5% mol, or even less than 96% mol O2).

[0043] At the top of the second column 21, residual nitrogen is produced which is heated in the exchanger 22, then the exchanger 15. Part of the heated residual nitrogen is used for the regeneration of the purification 4, passing through the heater 6. The rest is released into the atmosphere via the valve 13. It can also be partly sold.

[0044] Following a shutdown of the unit, the device can be in two main states:

[0045] • Warm start (above 0°C): following defrosting, for example, all the equipment in the cold storage unit is at ambient temperature

[0046] • Cold start (below -100°C, or even below -170°C): following a short stop, all equipment is at cryogenic temperature and a majority of cryogenic liquids have been kept, particularly in the tanks of the first and second columns.

[0047] In reality, one can find oneself anywhere along a continuum between these two extreme states.

[0048] For example, the device may be at a temperature as low as -170°C but no longer contain cryogenic fluid. After a shutdown of at least 48 hours, the cryogenic fluids must be purged for safety reasons.

[0049] Or, after a week's breakdown, there may no longer be any liquid in the cryogenic device and the temperature may have started to rise in the cold box through the thermal inlets, for example to have an average temperature of -100°C or -50°C if the shutdown is much longer, without necessarily defrosting.

[0050] The addition of liquid nitrogen from an external source, called "feeding," is carried out from a liquid nitrogen storage tank 1, which constitutes the external source. The feed liquid passes through a pressure-reducing and regulating valve 2 and is then injected below the vaporizer 4. The advantage of injecting the cryogenic liquid below the vaporizer is to limit thermal shock; if the vaporizer is hot, it will initially come into contact with cold gas before coming into contact with the cryogenic liquid.

[0051] The automatic start-up of a device of the second type is the same, regardless of the initial state of the cold box, for example in terms of temperature and cryogenic liquids present, and may include at least some of the following steps in the order mentioned, except for steps v) and vi): i) Start-up of the air compressor BP 2 and the air purification system 4, while the compressor 8 is not started and no liquid oxygen or liquid nitrogen is sent to the tank of the second column 21 or air to the first column 19. The bypass valve 5 is used to send a portion of the air compressed by the compressor 2 up to the second pressure to a circuit where residual nitrogen circulates during normal operation, so as to have a regeneration flow for the purification 4 which circulates during start-up,passing through heater 6. Excess air from the bypass circuit is vented to the air via valve 13. The other part of the air compressed by compressor 2 up to the second pressure is sent through exchanger 15 to the second column 21. No air flow is sent to the first column 19 and no feeding liquid is sent to the first or second column 19, 21. ii) Start of compressor 8 and sending of compressed and purified air to the first and second columns 19, 21 iii) Automatic regulation of the return of rich liquid (tank liquid from the first column) from the first column to the second column, opening of the lean liquid valve (head liquid from the first column) to its nominal value iv) Automatic regulation of the venting of the oxygen production 40 to its nominal flow rate v) Opening of the feeding valve 31,vi) Start of turbine 10 vii) When the liquid level in the tank of the first column 19 rises to a nominal liquid level in the tank, viii) Closure of the feed valve 31 when a threshold, for example the 100% submersion level, higher than the nominal level, is reached by the increase in liquid level, on the vaporizer 20 ix) Start-up of the vaporizer 20 purge and / or analysis of the impurities contained in the vaporizer bath 20 (this analysis can be done directly by a device connected to the vaporizer bath 20, or to the purge, or even to the instantaneously vaporized purge.) x) Start-up of oxygen production when the required concentration is reached. Steps v) and vi) can be reversed or simultaneous.

[0052] Apart from a start-up, the vaporizer level regulation can be ensured by the liquid nitrogen feed valve 31 in case of failure of the turbine 10. Sending a small quantity of liquid nitrogen, typically a molar flow rate of between 2 and 8% of the molar production of gaseous oxygen, into the liquid oxygen bath has little disturbance to the distillation and very little effect on the energy consumption to maintain the required gaseous oxygen content, the latter being impure, i.e. with a purity of less than 98.5%, or even 96% mol O2.

[0053] If the device is started from cold but the oxygen-rich liquid level around the vaporizer has been maintained, it is not strictly necessary to take the precautions described above: the vaporizer will operate close to its nominal capacity (i.e., with an acceptable pressure in the first column). However, according to the invention, it is recommended to follow the procedure described below in steps i) to x) to ensure a single, consistent startup program, regardless of the device temperature or the cryogenic liquid levels within the device.

Claims

Demands 1. A method for starting up an air separation apparatus by cryogenic distillation for the production of gaseous oxygen (8, 40), the apparatus comprising a first column (3, 19) operating at a first pressure and a second column (5, 21) operating at a second pressure lower than the first pressure, the second column having a tank vaporizer (4, 20), connected to be heated by a nitrogen-enriched gas from the first column, a heat exchanger (6, 15), a purification unit and means for removing an oxygen-enriched fluid from the tank of the second column connected to the heat exchanger to ensure the heating of the oxygen-enriched fluid to serve as a product in which, in normal operation, air is compressed in a compressor (2), purified into water and carbon dioxide in the purification unit at a pressure substantially equal to the second pressure,It is cooled and at least partially sent to the first column; an oxygen-enriched liquid is sent from the first column to the second column, and an oxygen-enriched gas is drawn from the lower part of the second column as a gaseous oxygen product, or an oxygen-enriched liquid from the lower part of the second column, which vaporizes against the air, which cools to form the gaseous oxygen product. To start the apparatus, while the apparatus is at a temperature above 0°C or below 0°C, purified air at the second pressure is first sent to the second column, preferably to the second column but not to the first column, and then liquid nitrogen is sent from an external source (1, 30) below the vaporizer.

2. Method according to claim 1 in which liquid nitrogen is sent from an external source (1, 30) below the vaporizer (4, 20) after the compressor has started.

3. A method according to claim 1 or 2 wherein liquid nitrogen is stopped being sent below the vaporizer (4, 20) once the liquid level around the vaporizer has risen to a threshold and after the threshold is reached, an air turbine (10) supplied with compressed air from the compressor and purified to keep the process cold is started.

4. A method according to any one of the preceding claims wherein the device to be started is at a temperature above 0°C.

5. Method according to any one of the preceding claims 1 to 3 wherein the device to be started is at a temperature below -100°C, or even -170°C.

6. A process for separating air by cryogenic distillation comprising a start-up step according to one of the preceding claims and a stable operating step in which no flow of liquid nitrogen is sent to the second column and the process is kept at least partially cold by expanding at least one fluid intended for or from one of the columns in at least one turbine (10).

7. A process for separating air by cryogenic distillation comprising a start-up step according to any one of the preceding claims 1, 2, 4 or 5 and a stable operating step in which liquid nitrogen is sent to the second column (5, 21) and optionally the process is also kept cold by expanding at least one fluid intended for or from one of the columns in at least one turbine (10).

8. A process according to any one of the preceding claims wherein the oxygen-enriched gas (8, 40) contains less than 98.5% mol O2, or even less than 96% mol O2.

9. A method according to any one of the preceding claims wherein in normal operation, a first compressor (2) compresses the air to the second pressure, the air is substantially cleaned at the second pressure and part of the cleaned air is sent to a blower (8) which compresses the part of the cleaned air to the first pressure and the blower air is sent partly to the first column (19) and part of the cleaned air at the second pressure is sent to the second column (21) and during start-up, the first compressor is started before the blower.

10. Method according to claim 9 wherein during start-up, the sending of liquid nitrogen below the vaporizer (20) is triggered after the start-up of the first compressor and the blower.

11. Method according to claim 9 or 10 wherein during start-up, a transfer of liquid from the first column tank to the second column is triggered before the transfer of liquid nitrogen to the second column tank.

12. A method according to any one of the preceding claims, wherein the start-up occurs when all the equipment of the apparatus intended to operate at cryogenic temperature during normal operation is at least 0°C.

13. A method according to any one of the preceding claims, wherein the start-up occurs when all the equipment of the apparatus intended to operate at cryogenic temperature during normal operation is below -100°C.

Images