Method for starting up a cryogenic distillation air separation apparatus

Abstract

In a method for starting up a cryogenic distillation air separation apparatus, the apparatus comprising a first column (3) operating at a first pressure and a second column (5) operating at a second pressure that is lower than the first pressure, the second column having a bottom vaporiser (4), connected so as to be heated by a nitrogen-enriched gas originating from the first column and means for discharging an oxygen-enriched fluid at the bottom of the second column that are connected to a heat exchanger (6) to ensure the heating of the oxygen-enriched fluid, in order to start up the apparatus, air is sent to the first column and then liquid nitrogen is sent underneath the vaporiser from an external source (1).

Description

[0001] Method for starting an air separation apparatus by cryogenic distillation

[0002] Description

[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 medium pressure column, before starting to send liquid nitrogen to 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 medium-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 for the low-pressure column, before the columns are cooled by another means. The liquid nitrogen thus provides the cooling for the columns and then helps to restore the liquid levels within them. The liquid nitrogen will eventually create a small liquid level around the tank vaporizer of the low-pressure column and prime the vaporizer, initially at a low flow rate and primarily by maintaining the pressure in the medium-pressure column. The liquids can then be drawn up from the medium-pressure column, and distillation can begin.So, gradually, we enrich the bath with oxygen: little by little, the level will rise while continuing to enrich the bath, and when the level reaches 100% submersion, the liquid bath will have a concentration quite close to the nominal oxygen level, so the machine will be close to its nominal capacity. This allows us to start without failure.

[0014] • According to one object of the invention, a method is provided for starting an air separation apparatus by cryogenic distillation for the production of gaseous oxygen, the 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 second column having 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 which, in normal operation, air is compressed in a compressor, purified into water and carbon dioxide in the purification unit at a pressure substantially equal to the first, cooled and sent at least in part 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 is drawn from the lower part of the second column, which vaporizes against the air, which cools to form the gaseous oxygen product. To start the device, when the device is at a temperature above 0°C or below 0°C, purified air is first sent to the first column, but not to the second column. Then, liquid nitrogen is sent from an external source below the vaporizer. The flow of liquid nitrogen below the vaporizer is stopped once the liquid level around the vaporizer has risen to a threshold. After starting to send liquid nitrogen below the vaporizer, or even after the threshold is reached,An air turbine, powered by compressed and purified air from the compressor, is started to maintain the cooling process, and the air from the turbine is sent directly to the second column.

[0015] Sending air from the turbine to the second column via a heat exchanger to modify its temperature is considered a direct supply to the column. According to other optional objects:

[0016] • Liquid nitrogen is sent from an external source below the vaporizer after the compressor has started.

[0017] • An air turbine, powered by compressed and purified air from the compressor, is started to keep the process cool after or during the injection of liquid nitrogen below the vaporizer.

[0018] • the device to be started is at a temperature above 0°C.

[0019] • Liquid nitrogen is sent below the vaporizer while the device to be started is at a temperature above 0°C

[0020] • the device to be started is at a temperature below -100°C, or even below -170°C.

[0021] • Liquid nitrogen is sent below the vaporizer while the device to be started is at a temperature below -100°C, or even below -170°C

[0022] • 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.

[0023] • 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.

[0024] • the oxygen-enriched gas contains less than 98.5% mol O2, or even less than 96% mol O2.

[0025] • During startup, the sending of liquid nitrogen below the vaporizer is triggered after the first compressor starts.

[0026] • During startup, a transfer of liquid from the first column tank to the second column is triggered after 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.

[0027] • 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.

[0028] • 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.

[0029] • 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.

[0030] 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.

[0031] Following a shutdown of the unit, one can be in one of two main states:

[0032] • Warm start (above 0°C): following defrosting, for example, all the equipment in the cold storage unit is at ambient temperature

[0033] • 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.

[0034] 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 first 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).

[0035] The invention will be described in more detail with reference to the figure, in which:

[0036] [FIG.1] represents an air gas separation apparatus, capable of being started according to a method of the invention

[0037] The invention is described below in more detail with reference to [FIG.1] which schematically represents an apparatus with first pressure purification operating according to the process of the invention.

[0038] In the case of Figure 1, it is a simplified apparatus for separating air gases to produce impure oxygen (i.e., with a purity of less than 98.5%, or even 96% mol O2). The process according to the invention can easily be extrapolated to other cases.

[0039] The apparatus includes 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 5 having a tank vaporizer 4, connected to be heated by a nitrogen-enriched gas from the first column 3.

[0040] In nominal operation, air at an intermediate pressure 7 is compressed in a compressor and cleaned to remove water and carbon dioxide. The air 7 is cooled in the heat exchanger line 6 and then sent to the first column 3, which operates at the intermediate pressure. A portion 11 of the air 7 is partially cooled in the heat exchanger 6 and then expanded in a turbine 10 to produce cold, before being sent to an intermediate position in the second column 5, which operates at a low pressure. A liquid rich in the tank, or a liquid lean at the top, is drawn from column 3, subcooled in a subcooler, and then sent to an intermediate position, or the top, of the second column 5. This is not shown in the figure.

[0041] Vaporizer 4 ensures the reflux of the first column 3 and the reboiling of the second column 5.

[0042] Oxygen gas 8 is drawn from the tank of the second column 5, above the vaporizer 4, heated in the exchanger 6, and then sold as product 8. Alternatively, the oxygen gas can be used as a waste gas. The invention also applies to the case where the oxygen is drawn as a liquid and then vaporized in the exchanger 6.

[0043] Residual nitrogen 9 is drawn from the head of the second column 5, heated in the subcooler, then the exchanger 6, and then used at least in part 9 as a regeneration fluid for the air purification system.

[0044] Following a shutdown of the unit, one can be in one of 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] Since the only start-up step taking place before the arrival of liquid nitrogen under the vaporizer is the sending of air into the first column, it will be understood that the device has not cooled before the sending of liquid nitrogen under the vaporizer and it is the arrival of the feeding liquid that starts to cool the columns, then the rest of the equipment in the cold box.

[0052] The automatic start-up is the same regardless of the initial state of the cold box, in terms of temperature and presence of cryogenic liquids, and includes at least some of these steps in this order (except for steps 4 and 5), see below):

[0053] 1. Start-up of the air compressor and air purification system 7, sending compressed, purified and cooled air directly to the first column, no compressed, purified and cooled air directly to the second column, no cryogenic liquid sent from an external source to the first or second column

[0054] 2. Automatic regulation of the return of rich liquid (liquid from the first column tank) from the first column to the second column, opening of the lean liquid valve (liquid from the top of the first column) to its nominal value

[0055] 3. Automatic regulation of oxygen production venting 8

[0056] 4. Opening of the feed valve 2 to send liquid nitrogen below the vaporizer 4 via automatic regulation to maintain a constant level at the vaporizer 4, and simultaneously sending compressed, purified, and cooled air directly to the first column, but no air to the second column.

[0057] 5. Start-up of turbine 10 to send compressed, purified and cooled air directly to the second column, simultaneously sending compressed, purified and cooled air directly to the second column and liquid nitrogen below the vaporizer, or optionally, start-up of turbine 10 once a liquid level threshold is reached in the vaporizer.

[0058] 6. When the liquid in the second column tank has risen to a nominal level in the second column tank, the lean liquid valve switches to automatic regulation.

[0059] 7. Closure of the feed valve 2 when a threshold above the nominal level is reached, for example the 100% submersion level, is reached on the vaporizer 4, at the same time, sending compressed, purified and cooled air directly to the first column and compressed, purified and cooled air directly to the second column from the turbine 10. 8. Start-up of the vaporizer 4 purge and at the same time, sending compressed, purified and cooled air directly to the first column and compressed, purified and cooled air directly to the second column from the turbine 10 and / or start-up of the analysis of the impurities contained in the bath of the vaporizer 4 (this analysis can be done directly by a device connected to the bath of the vaporizer 4, or to the purge or even to the purge vaporized instantaneously.)

[0060] 9. Oxygen production begins when the required concentration is reached, and simultaneously, compressed, purified, and cooled air is sent directly to the first column and compressed, purified, and cooled air is sent directly to the second column from the turbine.

[0061] Apart from a start-up, the vaporizer level regulation can be ensured by the liquid nitrogen feed valve 2 in case of failure of the turbine 10. Sending a small quantity of liquid nitrogen into the liquid oxygen bath, typically between 2 and 8% of the molar production of gaseous oxygen in normal operation, 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.

[0062] 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 from steps 1 to 10 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 a cryogenic distillation air separation apparatus 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 delivering an oxygen-enriched fluid to 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 first 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 first pressure is first sent to the first column, but not to the second column; then liquid nitrogen is sent from an external source (1, 30) below the vaporizer; the sending of liquid nitrogen below the vaporizer is stopped (4,20) Once the liquid level around the vaporizer has risen to a certain threshold, and after liquid nitrogen has been introduced below the vaporizer, or even after the threshold is reached, an air turbine (10) is started, supplied with compressed air from the compressor and purified to keep the process cool, and the air from the turbine is sent directly to the second column.

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. Method according to claim 1 or 2 wherein sending liquid nitrogen below the vaporizer starts the cooling of the separation apparatus.

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 claim 4 wherein liquid nitrogen is sent below the vaporizer while the device to be started is at a temperature above 0°C.

6. A method according to any one of the preceding claims 1 to 3 in which the device to be started is at a temperature below -100°C, or even below -170°C.

7. A method according to claim 6, wherein liquid nitrogen is sent below the vaporizer while the apparatus to be started is at a temperature below -100°C, or even below -170°C.

8. A process for separating air by cryogenic distillation comprising a start-up step according to any 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).

9. 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 the second column in at least one turbine (10).

10. 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.

11. A method according to any one of the preceding claims, wherein the start-up takes place when all the equipment of the apparatus which is to operate at cryogenic temperature in normal operation is at least 0°C.

12. A method according to any one of the preceding claims 1 to 8, wherein the start-up takes place while all the equipment of the device The temperature ranges required to operate at cryogenic temperatures during normal operation are below -100°C.

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