Method for starting an air separation apparatus by cryogenic distillation

Abstract

Title: Starting Process for an Air Separation Apparatus by Cryogenic Distillation In a starting process for an air separation apparatus by cryogenic distillation, a flow of liquid oxygen (32) is sent from an external source (1, 30) to a low-pressure column (21) of a double column at a point below a vaporizer (20), while no air is simultaneously sent to the medium-pressure column (19). The flow of liquid oxygen to the low-pressure column is stopped as long as the liquid level around the vaporizer rises above a threshold, and then air to be separated is sent to the medium-pressure column. Abstract Figure: Figure 2

Description

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

[0001] 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.

[0002] The columns can be arranged with the second column above the first column, or alternatively, the two columns can be side by side. The columns are located inside a thermally insulated enclosure called a cold box.

[0003] One object of the invention is to have a rapid automatic start-up, regardless of the starting temperature of the cold box of an air separation device and regardless of the liquid levels inside the columns of the device.

[0004] Another objective is that one can choose between a feed (addition of cryogenic liquid directly into the column) with liquid nitrogen or with liquid oxygen using the same piping between the storage of the desired liquid and the cold box.

[0005] Another object of the invention is to implement safe liquid oxygen feeding, knowing that the oxygen used contains at least one impurity (for example at least one of CO2, N2O, C3H8) which can concentrate during the cooling of the device.

[0006] A cryogenic distillation air separation apparatus typically comprises a CO2 and water purification unit, at least one first column operating at a first pressure and a second column operating at a second pressure, lower than the first pressure, the vessel of the second column being thermally connected to the vessel of the first column. According to the normal operation, purified air is sent to the purification unit at a first column operating at a first pressure, an oxygen-enriched liquid and a nitrogen-enriched liquid are sent from the first column to the second column, and heat is supplied to the vessel of the second column through a vaporizer heated with a nitrogen-enriched gas flow from the first column.

[0007] This makes it possible to produce gaseous oxygen in the tank of the second column by extracting it directly in gaseous form from the column and gaseous nitrogen at the top of the second column.

[0008] The present invention has the specific feature of sending air into the second column before replenishing the liquid level in the second column's tank vaporizer, in certain cases. It is recommended to initially send air only into the second column and to send air into the first column only once a liquid level threshold is reached around the vaporizer.

[0009] Another advantage of the invention is having a single start-up procedure regardless of the initial temperature of the air separation device.

[0010] The invention consists of sending liquid oxygen to the tank of the second column, below the vaporizer. The nominal level of the vaporizer is established by priming with liquid oxygen from an external source, before starting a compressor that produces air at the initial pressure, which is then sent to the first column. This ensures safe operation of the vaporizer in its nominal thermosiphon regime.

[0011] Furthermore, care is taken to ensure that the oxygen-rich gas generated during cooling does not go to a circuit not intended for the "oxygen service", typically the waste gas circuit, i.e. a pipe connected to the upper part of the second column. DETAILED DESCRIPTION

[0012] Current design: State of the art

[0013] The liquid oxygen for pre-cooling is generally sent to the tank of the second column once the apparatus has cooled sufficiently, i.e., when there is already a small level of liquid in the tank of the second column, to limit the risk of impurity concentrations. The cooling has been previously produced by the air sent to the first column, which is expanded in a Claude turbine or blower to the second column. Furthermore, the distillation is also initiated (presence of liquid reflux), which ensures that an oxygen-rich gas is not sent to a circuit not intended for oxygen service.

[0014] Design according to the invention

[0015] According to one aspect of the invention, a method is provided for starting up an air separation apparatus by cryogenic distillation comprising a CO2 and water purification unit, a heat exchanger, at least one 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 in which, according to the 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 or second pressure, cooled, and the purified air is sent to the purification unit. and cooled in the first column operating at the first pressure, an oxygen-enriched liquid and a nitrogen-enriched liquid are sent from the first column to the second column, heat is supplied to the tank of the second column through a vaporizer heated with a nitrogen-enriched gas flow from the first column characterized in that to start the apparatus while the first and second columns as well as the vaporizer are at ambient temperature or below 0°C, i. A flow of liquid oxygen containing at least one hydrocarbon and / or CO2 and / or N2O is sent from an external source to the second column at a point below the vaporizer, while no air is simultaneously sent to the first column, and the flow of liquid oxygen to the second column is stopped if a liquid level around the vaporizer rises above a threshold and ii. after step i) when the liquid threshold level is reached, air to be separated is sent to the first column and preferably no more liquid oxygen is sent to the second column.

[0016] According to other optional aspects:

[0017] to start the apparatus when the first and second columns and the vaporizer are at room temperature or below 0°C, before step i), air is sent at the second pressure at a temperature above or below 0°C to the second column but neither cryogenic liquid is sent from an external source to the first and second columns nor air to the first column. • before step i), air is sent at the second pressure at a temperature above or below 0°C from the compressor to the purification unit, the air that has passed through the purification unit is divided into at least two parts, one part is sent to the second column and another part is sent to a heater and from the heater to the purification unit to pass through an adsorption bed of the purification unit, during the regeneration of this adsorption bed. • the other part is sent to the heater while it is running and the other part is heated there • the other part is sent to the heater while it is not running and the other part does not heat up there • before step i), air is sent at the second pressure at a temperature above or below 0°C from the compressor to the purification unit and part of the air is sent to the atmosphere through a pipe, connected to the hot end of the heat exchanger, which in normal operation is used to transport nitrogen-enriched gas from the second column. During normal operation, the air flow sent to the first column has a flow rate D, and during startup, after step i), the air flow sent to the first column is initially less than D / 10, between D / 10 and D / 25, or even less than D / 25. before step i), no air is sent to the first or second column and no cryogenic liquid is sent from an external source to the first or second column. during step i), a flow of gas is drawn from the lower part of the second column and sent to the air, the molar flow rate of liquid oxygen sent below the vaporizer being less than the molar flow rate of gas put to the air. during startup, when the first and second columns and the vaporizer are at a temperature below -50°C, or even below -100°C, during step i), a flow of liquid oxygen containing at least one hydrocarbon and / or CO2 and / or N2O is sent to the second column at a point below the vaporizer, while no air is simultaneously sent to the first column, and the flow of liquid oxygen to the second column is stopped if a liquid level around the vaporizer rises above a threshold and after step i) if the liquid level is above the threshold, air is sent to be separated to the first column and preferably no more liquid oxygen is sent to the second column. to start the device when the first and second columns and the vaporizer are at a temperature below -50°C, or even below -100°C before step i), air is sent at the second pressure to the second column but no cryogenic liquid is sent from an external source to the first or second column or air to the first column. before step i), the air passes through the second column without separating and part of the air exits the second column through a pipe connected to the head of the second column and / or at least part of the air exits the second column through a pipe connected to the tank of the second column. before step i), the air passes through the second column without separating, entering through the head of the column or an intermediate region of the column Oxygen-enriched gas contains less than 98.5% mol O2, or even less than 96% mol O2. In normal operation, a first compressor compresses the air to the second pressure, the air is substantially purified at the second pressure, and a portion of the purified air is sent to a blower. which compresses the portion of the purified air up 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 supercharger. • During startup, compressed and purified air is sent to the second column, arriving at an intermediate region of the second column and exiting through a pipe connected to the head of the second column and through a pipe connected to the tank of the second column • During start-up, compressed and purified air is sent to the top of the second column and exits through a pipe connected to the tank of the second column • During startup, compressed and purified air is sent to the top of the second column through a pipe which, in normal operation, is used to supply nitrogen to the top of the column and exits through a pipe connected to the tank of the second column • 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 oxygen is sent to a column and the process is kept cold by expansion of at least one fluid intended for or from one of the columns in at least one turbine. • the heat exchanger and / or the first column and / or the second column of the device to be started is at a temperature above 0°C. • the heat exchanger and / or the first column and / or the second column of the device to be started is at a temperature below -100°C, or even -170°C.

[0018] The invention is described below on two types of apparatus: i. An air 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 with air purification at the first pressure ii. 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 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).

[0019] The invention will be described in more detail with reference to the figures, in which:

[0020] [Fig. 1] represents a first-type air gas separation apparatus with first-pressure purification, capable of being started according to a method of the invention

[0021] [Fig.2] represents a second-type air gas separation apparatus with second-pressure purification, capable of being started according to a method of the invention

[0022] The invention will now be described for [Fig. 1]. An air gas separation apparatus 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 tank of the second column being thermally connected to the tank of the first column by a vaporizer 4.

[0023] In normal operation, a flow of air 7 purified of CO2 and water in a purification unit at the pressure of the first column 3 is cooled in a heat exchanger 6 to an intermediate temperature, then divided into two parts. One part 11 is expanded in a blower turbine 10 and sent to the second column 5 for separation. The other part is cooled to the cold end of the exchanger 6 and sent to the tank of the first column 3 in gaseous form.

[0024] A tank liquid and a top liquid from the first column 3 are expanded and sent to the second column 5 in the usual manner. A flow of nitrogen gas 9 is drawn off at the top of the second column 5 and heated in the heat exchanger 6. The line for transferring this flow from the column to the exchanger is designed to transfer a nitrogen-rich flow and is therefore not designed to transfer a flow with a high oxygen content, for example, more than 23.5% oxygen. A flow of oxygen gas 8 is drawn off at the bottom of the second column 5 and heated in the heat exchanger 6. Thus, the flows 8 and 9 cool the air to be separated 7.

[0025] In normal operation, the cold required for the process is supplied by the expansion in the blower turbine 10. The vaporizer 4 is heated by nitrogen from the top of the first column 3 to vaporize the oxygen-rich tank liquid from the tank of the second column 5.

[0026] This type of column is obviously well known. Alternatively, the blowing turbine can be replaced by a Claude turbine which supplies the gaseous air to the first column 3.

[0027] To start the device, the sequence of steps is as follows: i. In a first step, send liquid oxygen into the tank of the second column below the vaporizer, without sending air into the first or second columns ii. In a second step, only once the liquid around the vaporizer reaches a sufficient level, send air to the first column and expand air in an insufflation turbine (if present) or a Claude turbine (if present)

[0028] To start the apparatus of [Fig. 1], it is possible in the second step to send a portion of the compressed and purified air to the top of the second column 5 to circulate within the column and exit at the bottom of the column, for example, through the line used to discharge gaseous or liquid oxygen during normal operation. The air preferably enters the column through a line used to discharge gaseous nitrogen from the column during normal operation.

[0029] The invention will now be described with reference to [Fig. 2]. An air gas separation apparatus comprises a first column 19 operating at a first pressure and a second column 21 operating at a second pressure, lower than the first 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). A vaporizer 20 heated by nitrogen gas from the first column 19 heats the tank of the second column 21.

[0030] In nominal (or normal) operation, the 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.

[0031] Part of the air purified at the second pressure is sent directly into the exchanger 15, then to the first column (low pressure column) 21 where it is separated, without having been compressed or expanded downstream of the compressor 2.

[0032] Another part of the purified air at the second pressure is compressed in the compressor 8, then cooled in the exchanger 9, then in the exchanger 15, then is sent partly to the turbine 10, the other part being sent to separate in the first column (medium pressure column) 19. The part expanded in the turbine 10 is sent to the first column 21 in gaseous form at an intermediate level to be separated.

[0033] The tank-rich liquid from the first column 19 is cooled in the subcooler 22, then expanded in the valve 24 and then sent to an intermediate section of the second column 21.

[0034] The lean liquid at the top of the first column 19 is cooled in the subcooler 22, then expanded in the valve 23 and then sent to the top of the second column 21.

[0035] The reflux of the first column 19 and the reboiling of the second column 21 are ensured by the vaporizer-condenser 20.

[0036] Gaseous oxygen 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).

[0037] At the top of the second column 21, residual nitrogen is produced and heated in the heat exchanger 22, then in the heat exchanger 15. Part of the heated residual nitrogen is used for regeneration of the purification process 4, passing through the heater 6, which can be electric and / or steam and / or hot water, for example. The remainder is released into the atmosphere via the valve 13. It can also be partially sold.

[0038] Following a shutdown of the unit, the device can be in two main states: • Warm start: following defrosting, for example, all the equipment in the cold storage unit is at ambient temperature • Cold start: following a short stop, all equipment is at cryogenic temperature, for example below -100°C, or even below -170°C, and a majority of the cryogenic liquids have been kept, in particular in the tanks of the first and second columns.

[0039] In reality, one can find oneself in a whole continuum between these two extreme states.

[0040] For example, the device may be at a temperature of -170°C but no longer contain cryogenic liquid. After a shutdown of at least 48 hours, the cryogenic liquids must be purged for safety reasons.

[0041] 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.

[0042] The feeding of liquid oxygen 32 containing at least one hydrocarbon and / or CO2 and / or N2O is done from a liquid oxygen storage 30, the liquid passing through a pressure-reducing and regulating valve 31, then being injected under the vaporizer 20. The advantage of sending the cryogenic liquid under the vaporizer is to limit the thermal shock, if the latter is hot: it will first see cold gas, before seeing cryogenic liquid.

[0043] 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 may include at least some of the following steps in the order mentioned: 1. The air compressor 2 is started with a flow rate between 70 and 100% of the nominal flow rate, and the air purification system 4 is also started, 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 compressed air through Compressor 2, up to the second pressure, supplies a circuit where residual nitrogen circulates during normal operation, ensuring a regeneration flow for the purification 4, which circulates during start-up, passing through the heater 6. Excess air from the bypass circuit is vented via valve 13. The remaining air compressed by compressor 2 to the second pressure is sent through the exchanger 15 to the second column 21. No air flow is sent to the first column 19, and no feeding liquid is sent to either the first or second column. 2. Automatic regulation of the oxygen production venting 40 to its nominal flow rate or a value close to its nominal flow rate, for example between 70% and 100% of the nominal flow rate 3. The feeding valve 2 is opened via an automatic regulator to maintain a constant level of liquid oxygen at the vaporizer 20. The feeding flow rate 32 is limited to always be lower than the oxygen venting flow rate 40. Thus, all the gas generated during the cooling and vaporization of the liquid oxygen in the second column tank exits through the oxygen production venting circuit. Part of the airflow at the second pressure arriving in the second column 21 flows in the opposite direction to the oxygen production circuit, which is a line connected to the column 21 tank; the other part exits through the waste circuit, which is a line connected to the top of column 21. This ensures that the waste circuit does not receive oxygen-enriched fluid.The air sent to column 21 is not cold enough to be distilled and exits as a gas through the lines connected to the tank and the top of column 21. Sending air to the line connected to the tank prevents the oxygen heated at the column's inlet from reaching it. Proper operation can be verified with an O2 analysis of the residual material, for example, at the outlet of heat exchanger 15. 4. Automatic regulation of the return of rich liquid 24, opening of the lean liquid valve 23 to a very low value, for example between 3 and 10% 5. When the nominal level in the tank of the second column 21 is reached (100% submersion of the vaporizer 20), the compressor 8 starts at a very reduced flow rate (between D / 10 and D / 25) compared to the compressor flow rate. This ensures safe operation of the vaporizer in its nominal thermosiphon mode. The flow limitation on the MP 8 compressor ensures that all the gas generated by the vaporizer 20 (when (the one that will be primed) will be evacuated by venting the oxygen production, avoiding a risk of over-oxygenation in the residual circuit. 6. Starting up the purging of vaporizer 20 and / or the analysis of impurities contained in the bath of vaporizer 20 (this analysis can be done directly by a device connected to the bath of vaporizer 20, or to the purging or even to the purging vaporized instantly.) 7. Turbine 10 Start-up 8. When the nominal liquid level in the tank of the first column 19 is reached, the lean liquid valve 23 switches to automatic regulation at its nominal value and the compressor regulation 8 is set to its nominal value 9. Close the feeding valve 2 when the 100% submersion level is again reached on the vaporizer 20 or has stabilized for a few minutes, with the turbine operating at its nominal capacity. 10. Oxygen production begins when the required content is reached

[0044] Alternatively, step 6 can take place after step 3 and before step 5, or even before step 4, to monitor the purity of the liquid bath during its filling.

Claims

1. Demands A method for starting up an air separation apparatus by cryogenic distillation comprising a CO2 and water purification unit (4), a heat exchanger (15), at least one 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 tank of the second column being thermally connected to the tank of the first column in which, according to the 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 or second pressure, cooled, purified air is sent from the purification unit and cooled to the first column operating at the first pressure, an oxygen-enriched liquid and a nitrogen-enriched liquid are sent from the first column to the second column,Heat is supplied to the second column vessel through a vaporizer (4, 20) heated with a nitrogen-enriched gas flow from the first column, characterized in that to start the apparatus, when the first and second columns and the vaporizer are at ambient temperature or below 0°C, i) a flow of liquid oxygen (32) containing at least one hydrocarbon and / or CO2 and / or N2O is sent from an external source (1, 30) to the second column at a point below the vaporizer, while no air is simultaneously sent to the first column, and the flow of liquid oxygen to the second column is stopped if a liquid level around the vaporizer rises above a threshold; ii) after step i), when the liquid threshold level is reached, air to be separated is sent to the first column, and preferably,No more liquid oxygen is sent to the second column, and to start the apparatus when the first and second columns (3, 5, 19, 21) as well as the vaporizer are at ambient temperature or below 0°C, before step i), air at the second pressure at a temperature above or below 0°C is sent to the second column (5, 21), but neither cryogenic liquid from an external source (1, 30) is sent to the first and second columns nor air to the first column.

2. A method according to claim 1 wherein before step i), air at the second pressure at a temperature above 0°C is sent from the compressor (2, 4) to the purification unit, the air having passed through the purification unit is divided into at least two parts, one part is sent to the second column (21) and another part is sent to a heater (6) and from the heater to the purification unit (4) to pass through an adsorption bed of the purification unit, during the regeneration of this adsorption bed.

3. A method according to claim 1 or 2 wherein before step i), air at the second pressure at a temperature above 0°C is sent from the compressor (2) to the cleaning unit (4) and a portion of the air is sent to the atmosphere through a pipe (13), connected to the hot end of the heat exchanger (15), which in normal operation is used to transport nitrogen-enriched gas from the second column.

4. A method according to any one of the preceding claims 1 to 3 wherein during normal operation, the air flow sent to the first column (3, 19) has a flow rate D, and during start-up, after step i), the air flow sent to the first column is initially less than D / 10, or even D / 25.

5. A method according to any one of the preceding claims wherein during step i), a flow of gas is withdrawn from the lower part of the second column (5, 21) and sent to the air, the molar flow rate of liquid oxygen (31) sent below the vaporizer (4, 20) being less than the molar flow rate of gas sent to the air.

6. A method according to any one of the preceding claims wherein during a start-up while the first and second columns (3, 5, 19, 21) and the vaporizer (4, 20) are at a temperature below -50°C, during step i), a flow of liquid oxygen (31) containing at least one hydrocarbon and / or CO2 and / or N2O is sent to the second column at a point below the vaporizer while no air is sent simultaneously to the first column and the flow of liquid oxygen to the second column is stopped if a liquid level around the vaporizer goes above a threshold and after step i) if the liquid level is above the threshold, air is sent to be separated to the first column (3, 19) and preferably, no more liquid oxygen is sent to the second column (5, 21).

7. Method according to claim 6 wherein to start the apparatus when the first and second columns (3,5, 19, 21) and the vaporizer (4, 20) are at a temperature below -50°C, before step i), air is sent at the second pressure to the second column but no cryogenic liquid (31) is sent from an external source (1, 30) to the first or second column or air to the first column.

8. A method according to any one of the preceding claims wherein before step i), the air passes through the second column (5, 21) without separating there and a part of the air exits the second column through a pipe connected to the head of the second column and / or at least a part of the air exits the second column through a pipe connected to the tank of the second column.

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