JP2920392B2 - Supercooling method of liquefied nitrogen in air liquefaction separator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for supercooling liquefied nitrogen which is derived from a rectification tower of an air liquefaction separator for liquefying and separating raw air and stored in a liquefied nitrogen storage tank. About.

[Conventional technology]

Conventionally, air as raw material has been compressed, refined, cooled and introduced into a rectification column to perform liquefied rectification and separation, and to produce gas products such as oxygen gas and nitrogen gas and liquefied gas products such as liquefied oxygen and liquefied nitrogen. Harvesting is being done.

FIG. 3 shows a main part of an air liquefaction separator for collecting nitrogen gas and liquefied nitrogen using a single rectification column.

The raw material air A is introduced into the main heat exchanger 2 from the conduit 1 through a well-known pretreatment device, and exchanges heat with the product nitrogen gas PGN and the exhaust gas W, and is cooled to near the liquefaction point, and the lower part of the single rectification 3 Will be introduced. This raw material air A is separated into nitrogen gas GN at the top of the column and oxygen-enriched liquefied air (hereinafter, liquefied air) LA at the bottom of the column by a rectification operation. Nitrogen gas GN at the top of the tower
Is led out from the top of the single rectification column 3 to a conduit 4, and a part thereof is collected as a product nitrogen gas PGN via a conduit 5 and a main heat exchanger 2. The remaining nitrogen gas GN is introduced from the conduit 6 into the condenser 7 and liquefied into liquefied nitrogen LN. This liquefied nitrogen LN
Is partially collected from a pipe 8 through a valve 9 as a product liquefied nitrogen PLN in a liquefied nitrogen storage tank 10. The remaining liquid nitrogen LN
Is introduced into the top of the single rectification column 3 and becomes a reflux liquid of the single rectification column 3.

On the other hand, the liquefied air LA at the bottom of the tower is led out of the sump 3a at the bottom of the tower to the conduit 11, and after the pressure of the condenser 7
Will be introduced. The liquefied air LA is supplied to the condenser 7 by the nitrogen gas.
The GN is liquefied and the gas itself is vaporized to become exhaust gas W and the conduit 13
After being heated in the reheat circuit 2 a of the main heat exchanger 2, it is introduced into the expansion turbine 14. The exhaust gas W that has been cooled by expansion in the expansion turbine 14 is returned to room temperature again through the main heat exchanger 2 and then discharged out of the system from the conduit 15.

The product liquefied nitrogen PLN collected in the liquefied nitrogen storage tank 10 in the air liquefaction / separation apparatus having such a configuration is stored in a saturated state liquefied in the condenser 7, for example, 8 kg / cm ² G, at −172 ° C. Part of the liquefied nitrogen obtained by flash loss is vaporized and lost.

For this reason, as shown in FIG. 4, product liquefied nitrogen PLN liquefied and collected in the condenser 7 is introduced into the subcooler 20, and is introduced into the liquefied nitrogen storage tank 10 in a supercooled state. . That is, the supercooler 20 is arranged in the conduit 8 for collecting product liquefied nitrogen of the air liquefaction separation device shown in FIG.
At the outlet of the subcooler 20, a part of the product liquefied nitrogen PLN is branched to a conduit 21, and the branched liquefied nitrogen BLN is expanded under reduced pressure by a pressure reducing valve 22, for example, from 8 kg / cm ² G to 0.1 kg / cm ² G. Inflated to -19
The temperature is set to 5 ° C., and this is used as a cooling fluid for the subcooler 20.
As a result, the product liquefied nitrogen PLN introduced into the subcooler 20 from the conduit 8 can be brought into a supercooled state of about -192 ° C, and the above-mentioned flash loss can be prevented. The branched liquefied nitrogen BLN used as a subcooling source is vaporized into nitrogen gas B
GN, the conduit 15 after the expansion turbine 14 is led out
And the exhaust gas W is discharged. The other components are the same as those of the air liquefaction / separation device shown in FIG. 3, and therefore, are denoted by the same reference numerals, and detailed description thereof will be omitted.

Also, in an air liquefaction / separation apparatus using a double rectification column, liquefied nitrogen derived from the lower column of the double rectification column or the main condenser is cooled by a subcooler using branched liquefied nitrogen as a subcooling source as described above. Cooling and storage in storage tanks are performed.

[Problems to be solved by the invention]

However, in the supercooler configured as described above, since a part of the product liquefied nitrogen to be taken out from the rectification column is expanded and depressurized and used as a supercooling source, a low-pressure storage tank such as a large flat bottom storage tank is used. There is no particular problem when transferring liquid to a storage tank operated at low pressure, but when storing liquefied nitrogen in a medium-pressure storage tank, for example, a small or medium-sized storage tank that operates to send nitrogen gas at 7 kg / cm ² G Is in a supercooled state due to the operating pressure of the storage tank, the nitrogen gas in the storage tank evaporated for nitrogen delivery is reliquefied, the pressure in the storage tank is reduced, and the pressure of the delivered nitrogen is reduced. There was something.

To cope with this, if the supercooling temperature of the product liquefied nitrogen is adjusted to the saturation temperature of the operating pressure of the storage tank,
The difference in the cold end temperature of the subcooler must be increased, resulting in an operation with a large energy loss. Further, in order to avoid this, when the degree of pressure reduction of the branched liquefied nitrogen, which is a supercooling source, is set to an intermediate pressure, an amount of liquefied nitrogen required for supercooling must be extraly extracted from the rectification column.

As described above, in the case where the branched liquefied nitrogen branched from the system for collecting the product liquefied nitrogen is decompressed and used as a supercooling source, in addition to the various inconveniences described above, the liquefaction exceeding the amount collected as the product is performed. Nitrogen had to be withdrawn from the rectification column.

Therefore, an object of the present invention is to provide a subcooling method capable of setting liquefied nitrogen to a necessary and sufficient supercooling temperature and collecting liquefied nitrogen derived from the rectification column as a product without waste.

[Means for solving the problem]

In order to achieve the above object, the method for supercooling liquefied nitrogen in the air liquefaction and separation apparatus of the present invention, in the case of a single rectification column, liquefied nitrogen derived from the single rectification column, derivation of the liquefied nitrogen The liquefied gas withdrawn from the rectification stage at a position lower than the rectification stage or the liquid reservoir at the bottom of the rectification tower and decompressed to a pressure equal to or higher than normal pressure is supercooled. In the case of a double rectification column, liquefied nitrogen is derived from the lower tower of the double rectification column, and a rectification stage at a position lower than the liquefied nitrogen extraction stage or The liquefied gas is extracted from the liquid reservoir at the bottom of the lower tower, and the liquefied gas is cooled by a gas derived from the upper tower to a supercooled state. Characterized by supercooling the liquefied nitrogen derived from That.

(Operation)

As described above, by extracting an appropriate liquefied gas in the rectification column and reducing the pressure, and using this as a supercooling source of liquefied nitrogen,
It is possible to obtain a supercooling fluid having a temperature and an amount appropriate for the supercooling of liquefied nitrogen, to perform necessary and sufficient cooling, and to reduce a temperature difference at a cold end of the subcooler to reduce energy loss. .

〔Example〕

Hereinafter, the present invention will be described in more detail based on embodiments shown in the drawings.

First, FIG. 1 shows an embodiment in which the present invention is applied to an air liquefaction / separation apparatus using the same single rectification column as in FIGS. 3 and 4. In the following description, the same elements as those in the conventional example shown in FIG.

Similarly to the conventional example, the product liquefied nitrogen PLN liquefied in the condenser 7 and led out to the conduit 8 is introduced into the subcooler 30 and subcooled. The subcooler 30 has a liquid reservoir at the bottom of the rectification tower 3.
Part of the liquefied air LA led out from 3a to the conduit 11 is branched into a conduit 31, and the branched liquefied air BLA is reduced to a pressure equal to or higher than normal pressure by a pressure reducing valve 32 to lower the temperature to a temperature required for supercooling, and It has been introduced as a subcooling source for liquefied nitrogen PLN.

Here, the product liquefied nitrogen P
An air liquefaction / separation device shown in FIG. 1 configured to supercool LN, an air liquefaction / separation device shown in FIG. 3 without supercooling product liquefied nitrogen, and a product liquefied nitrogen With respect to the air liquefaction / separation apparatus shown in FIG. 4 which performs supercooling, the gas-liquid state when operating under substantially the same conditions using raw material air at a pressure of 8 kg / cm ² G and 6200 Nm ³ / h is shown. A description will be given based on the results calculated by the inventor.

First, in the air liquefaction / separation apparatus of this embodiment, the pressure is 8 kg / cm.
² G, 6200 Nm ³ / h of raw air A is introduced into the single rectification column 3 via the conduit 1 and the main heat exchanger 2, and 6141 Nm ³ / h nitrogen gas is supplied from the top of the rectification column 3 via the conduit 4. GN into the reservoir 3a at the bottom of the tower
Liquefied air LA of 5099 Nm ³ / h is led out from the pipe 11 by a conduit 11.
Most of this liquefied air LA is 5090.5 Nm ³ / h
The pressure is reduced to 4 kg / cm ² G and introduced into the condenser 7, where it is vaporized to become exhaust gas W. The liquefied air BL branched to the conduit 31
A8.5 Nm ³ / h is expanded to 4 kg / cm ² G by the pressure reducing valve 32, cooled to −178 ° C., and introduced into the subcooler 30. Condenser 7 and subcooler 30
The liquefied air LA and the branched liquefied air BLA introduced into the pipes are respectively vaporized to become exhaust gas W, and are passed through the pipes 13 and 33 to the pipe 34.
And is introduced into the reheat circuit 2a of the main heat exchanger 2. The exhaust gas W which has been appropriately heated in the reheating circuit 2a is supplied to the expansion turbine 14
The main heat exchanger 2 is expanded after being introduced into the
The raw material air A is cooled to recover the temperature and discharged from the conduit 15.

In addition, part of the nitrogen gas GN derived from the top 795N
m ³ / h is derived from the conduit 5 as product nitrogen gas PGN,
The remaining 5346 Nm ³ / h of nitrogen gas GN is introduced into the condenser 7 through the conduit 6. The nitrogen gas GN is liquefied by heat exchange with the liquefied air LA depressurized by the pressure reducing valve 12, and becomes liquefied nitrogen LN at -172 ° C. Most of this liquefied nitrogen LN is 5040Nm ³ / h,
It is returned to the top of the single rectification column 3 as a reflux liquid, and a part of 306 Nm ³
/ h branches to conduit 8 as product liquefied nitrogen PLN,
Will be introduced.

This product liquefied nitrogen PLN exchanges heat with the branch liquefied air BLA in the subcooler 30, and the operating pressure of the liquefied nitrogen storage tank 10 is 7 kg / c.
The liquid is cooled to the saturation temperature of −174 ° C. at m ² G and sent to the liquefied nitrogen storage tank 10 via the valve 9.

In the present embodiment, the product liquefied nitrogen was
The amount of refrigeration obtained by introducing liquefied air (exhaust gas W) vaporized by bringing the PLN into a supercooled state into the expansion turbine 14 corresponds to 1 Nm ³ / h of liquefied nitrogen.

On the other hand, in the conventional air liquefaction and separation apparatus shown in FIG. 3, if the same amount of product nitrogen gas PGN is collected with the same amount of raw material air A, the nitrogen gas GN derived from the top of the rectification column 3
Is 6147 Nm ³ / h, liquefied air introduced into the condenser 7 from the bottom of the tower
LA is 5096Nm ³ / h, of the liquid nitrogen LN liquefied in the condenser 7, min 309 nm ³ / h of amount that is returned to the fractionator 3 as a reflux liquid is collected 5043Nm ³ / h, as a product liquid nitrogen PLN (-172 ° C). However, in the liquefied nitrogen storage tank 10, about 9 Nm ³ / h
Is lost due to flash loss, and the amount of product liquefied nitrogen actually stored in the liquefied nitrogen storage tank is 300 Nm ³ / h.

Further, in the conventional air liquefaction / separation apparatus provided with the subcooler 20 using the branched liquefied nitrogen BLN shown in FIG. 4, if the same amount of product nitrogen gas PGN is collected with the same amount of raw air A, The nitrogen gas GN derived from the top of the distillation tower ³ is 6150 Nm ³ /
h, the liquefied air LA introduced into the condenser 7 from the bottom of the column is 5090 Nm
³ / h, of the liquid nitrogen LN liquefied in the condenser 7, minute 5040Nm ³ / h returned to the fractionator 3 as a reflux liquid, the product liquid nitrogen PLN introduced into the subcooler 20 is 315 nm ³ / h , 9Nm ³ / h of which is a subcooler 20
It becomes a branched liquefied nitrogen BLN as a supercooling source of 306 Nm ³ / h, which is collected in the liquefied nitrogen storage tank 10 as product liquefied nitrogen PLN
Becomes However, the product liquefied nitrogen PLN stored in the liquefied nitrogen storage tank 10 is in a supercooled state at −192 ° C., which causes the above-described inconvenience when nitrogen is delivered.

 The results are summarized in the table below.

Next, FIG. 2 shows an embodiment in which the present invention is applied to an air liquefaction / separation apparatus using a double rectification column.

The raw material air A is compressed, refined, cooled and introduced into the bottom of the lower tower 40 as in the prior art, and separated into nitrogen gas GN at the top of the tower and liquefied air LA at the bottom of the tower. The liquefied air LA is led out from a liquid reservoir 40a at the bottom of the lower tower to a conduit 41, and is supplied to the first subcooler 4
After being introduced into 2 and slightly branched into a supercooled state, it branches off, and most of the pressure is reduced by the pressure reducing valve 43 and introduced into the middle stage of the upper tower 44. The remaining branched liquefied air BLA is depressurized by the pressure reducing valve 45, cooled down to a temperature suitable for supercooling the product liquefied nitrogen PLA, and introduced into the second subcooler 47 from the conduit 46. After branching liquefied air BLA is led from the second subcooler 47 to the conduit 48,
Merges with the exhaust gas W of the conduit 49 derived from the middle stage of the upper tower,
It is discharged through a first subcooler 42 and a main heat exchanger (not shown).

Further, the nitrogen gas GN at the top of the lower tower exchanges heat with liquefied oxygen LO in the main condensing evaporator 50 disposed at the bottom of the upper tower to become liquefied nitrogen LN, and a part of the liquefied nitrogen PLN is supplied from the conduit 51 to the product liquefied nitrogen PLN.
Is derived as The product liquefied nitrogen PLN is introduced from the first subcooler 42 to the second subcooler 47 via the conduit 52, and after being cooled to a temperature corresponding to the operating pressure of the liquefied nitrogen storage tank 53 by the branched liquefied air BLA, Is stored in a liquefied nitrogen storage tank 53 via a conduit 54 and a valve 55.

On the other hand, due to the rectification in the upper tower 44, high-purity nitrogen gas HN is separated at the top of the upper tower 44, and liquefied oxygen LO is separated at the bottom. The high-purity nitrogen gas HN at the top of the upper tower is led out to a conduit 56, and is collected after the temperature has been recovered by the first subcooler 42 and the main heat exchanger. Although not shown, the liquefied oxygen LO at the bottom of the upper tower is collected as liquefied oxygen or oxygen gas as in the related art.

In this way, even when liquefied nitrogen is collected using a double rectification column, the liquefied gas in the column below the liquefied nitrogen outlet, for example, liquefied air LA is extracted as described above, as in the previous example. The same effect can be obtained by reducing the pressure and using it as a supercooling source of the product liquefied nitrogen PLN.

That is, by extracting an appropriate amount of liquefied gas from below the liquefied nitrogen outlet of the double rectification column lower column, decompressing it to a desired temperature and using it as a supercooling source for liquefied nitrogen, Can be prevented, and the liquefied nitrogen can be prevented from being excessively cooled to a temperature not higher than the saturation temperature at the storage pressure, and the liquefied nitrogen can be stored in an optimal state. Further, the liquefied nitrogen obtained in the rectification column can be stored as product liquefied nitrogen without waste, so that it is not necessary to particularly increase the nitrogen yield in the rectification column.

In both examples, the oxygen-enriched liquefied air at the bottom of the purification tower was used as a supercooling source. However, other liquefied gases can be brought to a predetermined temperature by decompression and obtain a predetermined flow rate. Then, it is possible to use the liquefied gas of each part of the rectification column, and it is also possible to reduce the pressure to a low pressure before introducing the supercooler and discharge the same through the main heat exchanger as it is.

〔The invention's effect〕

As described above, the method for supercooling liquefied nitrogen of the present invention comprises the steps of: Therefore, liquefied nitrogen can be cooled to an optimum temperature according to the storage pressure, and liquefied nitrogen can be stored in an optimum state without flash loss or excessive cooling. Moreover, energy loss can be reduced by reducing the difference in cold end temperature of the subcooler, and efficient operation can be performed. Further, the liquefied nitrogen obtained in the rectification column can be stored as product liquefied nitrogen without waste, so that it is not necessary to particularly increase the nitrogen yield in the rectification column.

In addition, in the case of a single rectification column, since the pressure after decompression of the liquefied gas serving as a supercooling source is set to be equal to or higher than normal pressure, it can be introduced into an expansion turbine to generate cold, and the generated cold can be increased. Can be. In addition, if the pressure after the pressure reduction is equal to or higher than a certain pressure corresponding to the pressure of the expansion turbine processing fluid, it can be combined with the expansion turbine processing fluid after evaporation in the subcooler, so that the expansion turbine is not added, Cold generated can be increased.

Furthermore, in the case of a double rectification column, the liquefied gas serving as a supercooling source is used in a supercooled state with a gas derived from the upper tower first, so that the amount of liquefied gas consumed for subcooling the liquefied nitrogen is reduced. it can.

[Brief description of the drawings]

FIG. 1 is a system diagram of an essential part showing an embodiment in which the present invention is applied to an air liquefaction and separation apparatus using a single rectification column, and FIG. 2 is an air liquefaction and separation apparatus using a double rectification column according to the present invention. FIG. 3 and FIG. 4 show a conventional example, and FIG. 3 is a system diagram of an essential part showing an air liquefaction / separation apparatus having no supercooler. FIG. 4 is a system diagram of a main part showing an air liquefaction / separation apparatus having a subcooler. 3 ... single rectification column, 7 ... condenser, 10 ... liquefied nitrogen storage tank,
14 ... Expansion turbine, 30 ... Supercooler, 32 ... Reducing valve, 40
... lower tower, 42 ... first subcooler, 45 ... pressure reducing valve, 47 ...
2nd subcooler, A ... raw air, BLA ... branched liquefied air, L
A: Liquefied air, LN: Liquefied nitrogen, PLN: Liquefied nitrogen, W: Exhaust gas

Claims (2)

(57) [Claims] 1. A method for supercooling liquefied nitrogen in an air liquefaction / separation apparatus for compressing, purifying, cooling and introducing a raw material air into a single rectification column, and liquefying and separating to collect at least liquefied nitrogen. The liquefied nitrogen discharged from the rectification tower is extracted from the rectification stage at a position lower than the liquefied nitrogen discharge stage or the liquid storage portion at the bottom of the rectification column, and is supercooled by a liquefied gas depressurized to a pressure equal to or higher than normal pressure. A method for supercooling liquefied nitrogen in an air liquefaction / separation apparatus, wherein a liquefied gas vaporized as a supercooling source is introduced into an expansion turbine to generate cold. 2. A method for supercooling liquefied nitrogen in an air liquefaction / separation apparatus for compressing, purifying, cooling and introducing a raw material air into a double rectification column, and liquefying and separating to collect at least liquefied nitrogen. While liquefied nitrogen was led out from the lower tower of the rectification tower, liquefied gas was extracted from the rectification stage at a position lower than the liquefied nitrogen outlet stage or the liquid reservoir at the bottom of the lower tower, and the liquefied gas was led out from the upper tower. Liquefied nitrogen in an air liquefaction / separation apparatus, wherein the liquefied nitrogen is cooled by a gas to a supercooled state, and a part of the liquefied gas in the supercooled state is decompressed to supercool the liquefied nitrogen derived from the lower tower. Super cooling method.