DE69023141T2 - METHOD AND DEVICE FOR PRODUCING HIGH PURITY NITROGEN. - Google Patents

Description

Background of the invention

The present invention relates to a method and a plant for the production of high-purity nitrogen according to the features of the preamble of claim 1 and claim 9 respectively.

Such a process and a corresponding plant are known, for example, from JP-A-62-141485. In the known process, a rectification column is used to separate the high-boiling components, such as oxygen, monoxide, etc., from nitrogen. If the nitrogen contains low-boiling components, the components cannot be separated in the following lower rectification column and ultra-pure nitrogen cannot be obtained.

Fig. 4 is a diagram showing an example of a conventional plant for producing high-purity nitrogen. After such treatment as compression and cooling are carried out in this plant, the air including nitrogen and serving as a substance (hereinafter referred to as a gaseous substance) is transported to a molecular sieve adsorption plant 10. The impurities included in the gaseous substance are removed by being adsorbed in the molecular sieve adsorption plant 10. Then, the gaseous substance is transported to a heat exchanger 14 provided in the cold box 12. At this stage, the gaseous substance is free of water, carbon dioxide (CO₂) and the like. Such impurities as hydrogen, helium and neon are nevertheless included in the gaseous substance. In this state, the gaseous substance is directly introduced into the lower section of a fractionating column 16 in which rectification is carried out. Oxygen and hydrocarbon such as methane, which have high boiling points, are removed from the gaseous substance by rectification in the rectification column 16. Consequently, the nitrogen is extracted from the rectification column 16 as a product.

Gaseous impurities such as hydrogen, helium and neon exhibit a boiling point lower than that of nitrogen (hereinafter referred to as low boiling gas). The low boiling gas rises up the rectification column 16 without being liquefied therein and is stored in a condenser 161 provided in the uppermost stage of the rectification column 16. If it remains in the condenser 161, the low boiling gas, which is a non-condensable gas, impairs the efficiency of the condenser 161. In addition, the non-condensable gas is likely to adversely affect the purity of the produced nitrogen to be extracted from the rectification column 16.

In view of this, the following procedure was adopted:

The non-condensable gas stored in the condenser 161 is discharged when nitrogen is used. In addition, the nitrogen is extracted from a stage lower than that in which the condenser 161 is present, thereby preventing to the greatest extent the possibility of the non-condensable gas being mixed into the nitrogen produced. This brings about a great improvement in the purity of the nitrogen produced.

However, the conventional method mentioned above brings with it the following problems.

As described above, with the advancement of the latest technology for semiconductor production, there is a great demand for nitrogen with a higher purity. At present, there is a requirement that the low boiling point gas such as hydrogen, helium and neon should also be removed as an impurity from the gaseous substance.

However, since it has a boiling point lower than that of nitrogen, the low boiling point gas is included in abundance in the gas rising up the rectification column 16 just after it is introduced therein. Specifically, the concentration of the low boiling point gas in the rising gas is substantially the same as that of the gaseous substance when it is introduced into the lower portion of the rectification column 16. Accordingly, the conventional method in which the non-condensable gas stored in the condenser 161 and the nitrogen produced is extracted from the stage below that in which the condenser 161 is present, have sufficient efficiency to prevent a reduction in the purity of the nitrogen produced due to the non-condensable gas. However, such a conventional method has a difficulty in substantially reducing the concentration of the low boiling point gas in the nitrogen produced.

As for hydrogen, a type of low boiling point gas, it is known that if hydrogen is made to react with oxygen in the gaseous substance in the presence of a catalyst, the concentration of hydrogen in the gaseous substance can be reduced to the order of only 0.1 t/million. However, the remaining 0.1 t/million of hydrogen cannot be removed from the gaseous substance and therefore no further reduction in the concentration of hydrogen can be achieved.

Disclosure of the invention

It is an object of the invention to provide such a process and a plant for the production of high-purity nitrogen in order to solve the above problems.

Accordingly, the method of the invention comprises the steps defined in claim 1.

According to the above-mentioned method, the gas rising up the main rectification column includes low boiling point gas in abundance. However, only a very small amount of low boiling point gas is included in the liquid nitrogen extracted from the main rectification column. In addition, the liquid nitrogen is introduced into the sub-rectification column where further rectification is carried out. The low boiling point gas included in the liquid nitrogen is heated to vaporization, and the vaporized low boiling point gas slowly exits upward in the sub-rectification column. Accordingly, the produced nitrogen is extracted from a position lower than the position from which the liquid nitrogen is introduced into the sub-rectification column, whereby the produced nitrogen has the highest purity.

Furthermore, a system according to the invention has the features claimed in claim 9.

Short description of the drawings

Fig. 1 is a diagram showing the entire structure of a typical plant for producing high-purity nitrogen according to the present invention;

Fig. 2 is a diagram showing the entire structure of another plant for producing high-purity nitrogen according to the present invention,

Fig. 3 is a diagram showing the entire structure of still another plant for producing high-purity nitrogen according to the present invention; and

Fig. 4 is a diagram showing a conventional plant for the production of high-purity nitrogen for explanatory purposes.

Suitable method for carrying out the invention

Fig. 1 is a diagram showing a typical plant for carrying out the process of the present invention. The plant is provided with a molecular sieve adsorption plant 10 and a cold box 12. The cold box 12 comprises: a main heat exchanger 14, a main rectification column 16, an expansion turbine 18 and a sub-rectification column. The condensers 161, 201 are present on the uppermost stages of the main rectification column 16 and the sub-rectification column 20, respectively.

Next, a process for the production of high-purity nitrogen is described, which is carried out in the plant thus constructed.

First, air is passed through the molecular sieve adsorption unit 10 to remove the impurities from the gaseous substance. Then, the gas is passed through the main heat exchanger 14 through an air inlet channel 11. The expansion turbine 18 functions to remove the exhaust gas from the rectification column 16 and the sub-rectification column 20. The adiabatically expanded exhaust gas is also passed through the main heat exchanger 14. The air is cooled to about -170 ºC by the heat exchange with the exhaust gas in the heat exchanger 14. The air is then introduced into the main rectification column 16, where it is cooled to this temperature.

In the rectification column 16, the liquid flowing downward and the vapors rising upward come into contact with each other, whereby rectification occurs. The low boiling point components become more concentrated as they rise upward in the rectification column 16. On the other hand, the high boiling point components become more concentrated as they move toward the bottom of the rectification column 16. Any low boiling point gas (an impurity such as hydrogen, helium and neon) included in the gaseous substance has a boiling point lower than that of nitrogen. Accordingly, such a low boiling point gas is included in abundance in the vapors rising upward and concentrated at the top of the column 16. However, only a very small amount of the low boiling point gas is included in the liquid nitrogen flowing downward in the rectification column 16. In view of this, the liquid nitrogen is extracted from a stage located below the uppermost stage of the rectification column 16. The extracted liquid nitrogen is transported to a middle stage of the lower rectification column 20 through a liquid nitrogen transport channel 22.

The liquid nitrogen is rectified in the sub-rectification column 20, and the low-boiling gas included in the liquid nitrogen in only small amounts rises upward due to heating. The nitrogen gas is extracted from the sub-rectification column 20 from a position lower than that from which the liquid nitrogen is introduced. As a result, it is possible to produce nitrogen with an excessively high purity. The produced nitrogen is transported to the main heat exchanger 14 through a produced nitrogen discharge passage 24. The produced nitrogen is heated in the heat exchanger 14 so that a substantially normal temperature is reached by heat exchange with the gaseous substance supplied from the outside, and thereafter it is discharged from the plant.

In the above-mentioned method, it is preferable that the heat of the nitrogen gas present at the top of the rectification column 16 is used as a heat source to heat the lower portion of the sub-rectification column 20, that is, as a heat source for a distillation vessel of the column 20. More specifically, the liquid nitrogen in the lower portion of the sub-rectification column 20 is transported to the condenser 161 of the main rectification column 16 through a channel 26. The nitrogen gas vaporized in the condenser 161 is transported to the sub-rectification column 20 through a channel 27. The channels 26 and 27 form a circulation channel for the liquid nitrogen. The liquid nitrogen not vaporized in the condenser 161 can be transported back to the sub-rectification column together with the vaporized nitrogen gas. In this In this way, the liquid nitrogen exchanges heat with the nitrogen gas in the upper section of the column 16, whereby the nitrogen condenses. The liquid nitrogen from the sub-rectification column 20 is evaporated in the condenser 161 by the heat of condensation. The evaporated nitrogen gas, which is transported back to the sub-rectification column 20, serves to heat the low-boiling gas therein. In this way, the rectification in the column 20 can be carried out effectively.

In order to effect the above-mentioned heat exchange evenly, it is appropriate to set the operating pressure in the sub-rectification column 20 to be about 0.6 kg/cm² lower than that in the main rectification column 16. Accordingly, the pressure of the liquid nitrogen is reduced while it is transported from the main rectification column 16 to the sub-rectification column 20 through the liquid nitrogen transport channel 22. This causes a reduction in the saturation temperature of the liquid nitrogen.

In the above-mentioned method, a large amount of nitrogen gas will also rise up the lower rectification column 20 together with the low-boiling gas. Accordingly, it is preferable to liquefy the nitrogen gas in the upper section of the lower rectification column 20 and recover it as liquid nitrogen. In this case, the liquid separated on the bottom of the main rectification column 16 can be transported through a liquid transport channel 28 to the upper section of the lower rectification column 20, the pressure of which is reduced. This allows the upper portion of the sub-rectification column 20 to be cooled, thereby enabling efficient liquefaction and recovery of nitrogen gas. Similar effects can be achieved in the case where the liquid present in the middle of the main rectification column 16 is transported through a channel similar to the liquid transport channel 28 to the upper portion of the sub-rectification column 20, the pressure of which is reduced.

As described above, according to the present method, high purity liquid nitrogen is extracted from the main rectification column 16 from a stage lower than the uppermost stage thereof. The extracted liquid nitrogen is then introduced into the sub-rectification column 20. Consequently, the produced nitrogen is extracted from the sub-rectification column 20 from a position lower than the position of introducing the liquid nitrogen. Accordingly, the low boiling point gas in the air such as hydrogen, helium and neon can be effectively removed therefrom, thereby enabling the production of nitrogen with a greatly improved purity.

The following Table 1 shows the concentrations of helium, neon and hydrogen in gases 1, 2 and 3 respectively.

In Table 1, gas 1 is air. Gas 2 is nitrogen produced from the gaseous substance according to the conventional method described with reference to Fig. 4. Gas 3 is nitrogen produced from the air according to the The concentration of hydrogen in the air is reduced to 0.1 ppm when the catalyst is used.

As can be seen in Table 1, the conventional method can almost not remove low boiling point gases present in the air. In contrast, the present method can effectively remove the corresponding low boiling point gases. Table Helium Neon Hydrogen Gas (air) (conventional) (invention) (T./million)

The present invention is not limited to the foregoing embodiment, but can be represented as follows.

(1) In the method of the foregoing embodiment, the helium gas extracted from the respective rectification columns 16, 20 is directly discharged from the plant as shown in Fig. 1. However, the helium gas may be transported from the rectification columns 16 and 20 through the helium transport channels 31, 32 to the main heat exchanger 14 and then recovered. The foregoing method enables the low-temperature helium gas to be used largely as a coolant. for the main heat exchanger 14, thereby improving the overall thermal efficiency of the system.

Furthermore, as shown in Figures 1 and 2, the gas taken from the upper section of the main rectification column 16 can be transported to an expansion turbine 18 (expansion device) through a gas transport channel 36. The gas is expanded by the expansion turbine 18 and further transported in its state to the main heat exchanger 14. In this way, the gas in the upper section of the main rectification column 16 can serve as a coolant for the main heat exchanger. It is to be noted that the gas in the upper section of the sub-rectification column 20 can also be used for the same purpose.

Fig. 3 shows a device for cooling the plant using the heat of vaporization of the liquid nitrogen introduced into the upper section of the main rectification column 16 through a liquid nitrogen inlet channel 34. In this device, the gas in the upper section of the column 16 can be transported to the main heat exchanger 14 through the gas transport channel 36 and can thus serve as a coolant for the main heat exchanger 14 as in the previous examples. The liquid nitrogen can also be supplied to the sub-rectification column 20 instead of the main rectification column 16 so that helium gas is taken out therefrom. It can also be suitable to supply the liquid nitrogen to both rectification columns 16, 20.

(2) The present process may be applied not only to an air separator but also to any plant producing high purity nitrogen from a gaseous substance comprising nitrogen and low boiling gases.

Exploitation in industry

As described above, according to the method and apparatus of the present invention, the liquid nitrogen of high purity is extracted from the main rectification column from a position lower than the uppermost stage thereof and introduced into a sub-rectification column. The liquid nitrogen is further heated to vaporize the low-boiling gases therein in the sub-rectification column. Consequently, the produced nitrogen is extracted from the sub-rectification column from a position lower than the position from which the liquid nitrogen is introduced therein. Accordingly, the low-boiling gases can be effectively removed from the gaseous substance, which greatly contributes to the production of ultra-pure nitrogen.

Claims (16)

1. A process for the production of high purity nitrogen comprising the following steps: Introducing air into a main rectification column (16) in the cold state; Extracting liquid nitrogen from the main rectification column (16) from a position below the uppermost stage thereof; Introducing the extracted liquid nitrogen into a sub-rectification column (20); characterized in that: in the main rectification column (16) the nitrogen and the component having a lower boiling point than the nitrogen are separated from the component having a higher boiling point than the nitrogen; in the lower rectification column (20) the component with a lower boiling point than the nitrogen is separated from the nitrogen; and the separated nitrogen from the sub-rectification column (20) is extracted from a position which is below the position at which the liquid nitrogen is introduced. 2. The method of claim 1, further comprising the following steps: Introducing the liquid nitrogen separated in the lower section of the sub-rectification column (20) into a Condenser (161) located in the upper section of the main rectification column (16); and Introducing the vaporized nitrogen gas from the condenser into the sub-rectification column (20). 3. A process according to claim 1, further comprising the step of introducing liquid nitrogen separated in the lower section of the main rectification column (16) into a condenser (161) provided in the upper section of the sub-rectification column (20). 4. A process according to claim 1, further comprising the step of introducing the liquid present in the middle section of the main rectification column (16) into a condenser (201) present in the upper section of the sub-rectification column (20). 5. Method according to one of claims 1 to 4, further comprising the following steps: exchanging heat between the air and the generated nitrogen by using a heat exchanger (14); and Extracting the gas present in the upper section (161, 201) in at least one of the main rectification column (16) and the sub-rectification column (20); and Passing the gas through the heat exchanger (14). 6. The method of claim 5, further comprising the step of expanding the gas while transporting the gas from the upper section of at least one of the rectification columns (16, 20) to the heat exchanger (14). 7. A process according to claim 5, further comprising the step of introducing the liquid nitrogen into the upper section (161, 201) of at least one of the rectification columns (16, 20) to cool the gas therein before it is removed therefrom. 8. Method according to one of claims 1 to 4, further comprising the following steps: Exchanging heat between the air and the nitrogen produced using a heat exchanger (14); Removing the helium gas from at least one of the main rectification column (16) and the sub-rectification column (20); and Passing the extracted helium gas through the heat exchanger (14). 9. Plant for the production of high purity nitrogen, comprising: a main rectification column (16) and a sub-rectification column (20) for rectification; an inlet channel (11) for introducing the air into the main rectification column (16); a liquid nitrogen transport channel (22) for transporting the liquid nitrogen from the main rectification column (16) to the sub-rectification column (20), one end of the channel (22) being connected to the main rectification column (16) at a position below the uppermost stage thereof and the other end of the channel being connected to the sub-rectification column (20); and an outlet channel (24) for the generated nitrogen to extract the generated nitrogen from the sub-rectification column (20); characterized in that: the main rectification column (16) separates the nitrogen and the component with a lower boiling point than nitrogen from the component with a higher boiling point than nitrogen; the sub-rectification column (20) separates the component with a lower boiling point than nitrogen from the nitrogen; and one end of the outlet channel (24) is connected to the sub-rectification column (20) in a position which is below the position in which the liquid nitrogen is introduced. 10. The system of claim 9, further comprising: a condenser (161) provided in the upper section of the main rectification column (16); and a circulation channel (26) for transporting the liquid nitrogen separated in the lower section of the sub-rectification column (20) to the condenser and a channel (27) for transporting the nitrogen gas evaporated in the condenser (161) to the sub-rectification column (20). 11. Plant according to claim 9, further comprising: a condenser (201) provided in the upper section of the sub-rectification column (20); and a liquid transport channel (28) to transport the liquid separated in the lower section of the main rectification column (16) to the condenser (201). 12. The system of claim 9, further comprising: a condenser (201) provided in the upper section of the sub-rectification column (20); and a liquid transport channel to transport the liquid separated in the middle section of the main rectification column (16) to the condenser (201). 13. Installation according to one of claims 9 to 12, which also comprises : a heat exchanger (14) for the heat exchange between the air and the nitrogen produced; and a gas outlet channel (32, 36) for guiding the gas from the upper sections of the respective rectification columns (16, 20) through the heat exchanger (14), one end of the outlet channel being connected to the upper sections (161, 201) of the respective rectification columns and the other end extending through the heat exchanger (14). 14. Plant according to claim 13, further comprising an expansion device (18) present in the prescribed position along the gas outlet channel between the heat exchanger (14) and the upper sections (161, 201) of the respective rectification columns, the expansion device (18) being designed to expand the gas flowing therethrough. 15. Plant according to claim 13, further comprising inlet channels (34) for the liquid nitrogen to introduce the liquid nitrogen into the upper sections (161, 201) of the respective rectification columns (16, 20). 16. Installation according to one of claims 9 to 12, which further comprises: a heat exchanger (14) for the heat exchange between the air and the nitrogen produced; and an outlet channel (31, 32) for the helium gas to guide the helium gas from the corresponding rectification columns (16, 20) through the heat exchanger (14), one end of the outlet channel for the helium gas being connected to the corresponding rectification columns and the other end extending through the heat exchanger (14).