METHOD OF AND DEVICE FOR PRODUCING NITROGEN OF HIGH PURITY

Abstract: Material gas in a cooled state is fed into a main fractionating column so as to be rectified, liquid nitrogen is taken out at a position lower than the uppermost step in the main fractionating column, the liquid nitrogen thus taken out is fed into the sub-rectifier to be rectified, and nitrogen as a product is taken out from the sub-rectifier at a position lower than the position at which liquid nitrogen is introduced.

Description: BACKGROUND ART The present invention relates to a method and system for producing high purity nitrogen according to the features of the preamble of claim 1, resp. claim 9. Such a method and system is known, for example, from JP-A-62-14 14 85. In the known method the rectification column is used to separate high-boiling components such as oxygen, monoxide etc from the nitrogen. When the nitrogen contains low-boiling components, these components cannot be separated in the following subrectification column and no ultra high purity nitrogen can be obtained. Fig. 4 is a diagram showing an example of conventional system for producing high purity nitrogen. In this system, after passing through such processing as compression and cooling, the air including nitrogen and serving as material (hereinafter referred to as material gas) is transported to a molecular sieves adsorption unit 10. Impurities included in the material gas are removed by being adsorbed in the molecular sieves adsorption unit 10. Subsequently, the material gas is transported to a heat exchanger 14 provided in a cold box 12. At this stage, the material gas is free from water, carbon dioxide (CO₂), and the like. However. such impurities as hydrogen. helium, and neon are still included in the material gas. In this state, the material gas is directly introduced to a bottom portion of a fractionating column 16 to be rectified therein. Oxygen and hydrocarbon such as methane, which have high boiling points, are removed from the material gas by the rectification effected in the rectification column 16. Consequently, the nitrogen is extracted from the rectification column 16 as product. Impurity gas such as hydrogen, helium, and neon has a boiling point lower than that of nitrogen (hereinafter referred to as low boiling gas.) The low boiling gas ascends in the rectification column 16 without liquefying therein, and are stored in a condenser 161 provided at an uppermost stage of the rectification column 16. If left in the condenser 161, the low boiling gas, which is noncondensable gas, impairs the efficiency of the condenser 161. In addition, there is a likelihood that the noncondensable gas adversely affects the purity of the produced nitrogen to be extracted from the rectification column 16. In view of this, the following method has been adopted: The noncondensable gas stored in the condenser 161 is purged with the use of nitrogen. Further, the nitrogen is extracted from a stage lower than the one where the condenser 161 is provided, maximally preventing the likelihood that the noncondensable gas is mixed into the produced nitrogen. This causes the purity of the produced nitrogen to be greatly improved. However, the foregoing conventional method presents the following problems. As described above, with the advance of the recent semiconductor production technology, there is a great demand for nitrogen having higher purity. Presently, it is beginning to be required that the low boiling gas, such as hydrogen, helium, and neon. be also removed as impurities from the material gas. However, having a boiling point lower than that of the nitrogen, the low boiling gas is abundantly included in the gas ascending in the rectification column 16 even after introduced thereto. Specifically, concentration of the low boiling gas in the ascending gas is substantially equal to that in the material gas when introduced to the bottom portion of the rectification column 16. Accordingly, the conventional method, by which the noncondensable gas stored in the condenser 161 is purged, and produced nitrogen is extracted from the stage lower than the one where the condenser 161 is provided, may be sufficiently effective in preventing the purity of the produced nitrogen from being reduced due to the noncondensable gas. However, such a conventional method experiences difficulty in substantially reducing the concentration of the low boiling gas in the produced nitrogen. Concerning the hydrogen, one kind of the low boiling gas, it has been known that if the hydrogen is caused to react with the oxygen in the material gas in the presence of catalyst, the hydrogen concentration in the material gas can be reduced to as low as the order of 0.1 ppm. However, the remaining 0.1 ppm hydrogen cannot be removed from the material gas, and therefore the hydrogen concentration cannot be reduced any further. DISCLOSURE OF THE INVENTION It is an object of the invention to provide such a method and a system for producing high purity nitrogen as to solve the foregoing problems. According, a method of the invention comprises the steps as defined in claim 1. According to the above method. the gas ascending in the main rectification column includes low boiling gas abundantly. However, only a tiny quantity of low boiling gas is included in the liquid nitrogen extracted from the main rectification column. In addition, the liquid nitrogen is introduced into the subrectification column to be further rectified therein. The low boiling gas included in the liquid nitrogen is heated to evaporate, and the evaporated low boiling gas oozes upward in the subrectification column. Accordingly, produced nitrogen is extracted from a position lower than the position from which the liquid nitrogen is introduced into the subrectification column, thus produced nitrogen has ultra-high purity. Further, a system according to the invention for carrying out the above method comprises the features as claimed in claim 9. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram showing an entire construction of an exemplary system for producing high purity nitrogen according to the present invention; Fig. 2 is a diagram showing an entire construction of another system for producing high purity nitrogen according to the present invention; Fig. 3 is a diagram showing an entire construction of still another system for producing high purity nitrogen according to the present invention; and Fig. 4 is a diagram showing an explanatory conventional system for producing high purity nitrogen. BEST MODE FOR CARRYING OUT THE INVENTION Fig. 1 is a diagram showing an exemplary system for carrying out a method of the present invention. The system is Provided with a molecular sieves adsorption unit 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 subrectification column. Condensers 161, 201 are provided at uppermost stages of the main rectification column 16 and the subrectification column 20 respectively. Next, there will be described a method for producing high purity nitrogen carried out in thus constructed system. First, the air is passed through the molecular sieves adsorption unit 10 so as to remove impurities from the material gas. Subsequently, the gas is passed through the main heat exchanger 14 through a air gas inlet passage 11. The expansion turbine 18 works to adiabatically expand waste gas from the rectification column 16 and the subrectification column 20. The adiabatically expanded waste gas is also passed through the main heat exchanger 14. The air is cooled to about -170 °C through the heat-exchange with the waste gas in the heat exchanger 14. The air is then introduced to the main rectification column 16 with being cooled at this temperature. In the rectification column 16, flowing down liquid and ascending vapors come into contact with each other, whereby rectification is performed. Low boiling components become more concentrated as ascending to the top of the rectification column 16. On the other hand, high boiling components become more concentrated as descending to the bottom of the rectification column 16. Any low boiling gas (impurity such as hydrogen, helium, and neon) included in the material gas has a boiling point lower than that of nitrogen. Accordingly, such low boiling gas is abundantly included in the ascending vapors and concentrated at the top of the column 16. However, only a tiny quantity of low boiling gas is included in the liquid nitrogen flowing down the rectification column 16. In view of this, the liquid nitrogen is extracted from a stage lower than the uppermost one of the rectification column 16. The extracted liquid nitrogen is transported to a middle stage of the subrectification column 20 through a liquid nitrogen transport passage 22. The liquid nitrogen is rectified in the subrectification column 20, and the low boiling gas included in the liquid nitrogen in only small quantities is caused to ascend by the heating. Nitrogen gas is extracted from the subrectification column 20 from a position lower than the one from which the liquid nitrogen is introduced thereinto. As a result, it is made possible to produce exceedingly high purity nitrogen. The produced nitrogen is transported to the main heat exchanger 14 through a produced nitrogen outlet passage 24. The produced nitrogen is heated in the heat exchanger 14 so as to reach a substantially normal temperature through the heat-exchange with the material gas supplied from outside, then brought out of the system. In the foregoing process, it is preferable that the heat of the nitrogen gas present in the top of the rectification column 16 is utilized as heat source to heat the bottom portion of the subrectification column 20, i.e., heat source for a reboiler of the column 20. More specifically, the liquid nitrogen in the bottom portion of the subrectification column 20 is transported to the condenser 161 of the main rectification column 16 through a passage 26. Nitrogen gas evaporated in the condenser 161 is transported to the subrectification column 20 through a passage 27. The passages 26 and 27 constitute a liquid nitrogen circulating passage. Liquid nitrogen which is not evaporated in the condenser 161 may be transported back to the subrectification column together with the evaporated nitrogen gas. In this way, the liquid nitrogen exchanges heat with the nitrogen gas at the top portion of the column 16, thereby condensing the nitrogen. The liquid nitrogen from the subrectification column 20 is evaporated in the condenser 161 by the heat of condensation. The evaporated nitrogen gas transported back to the subrectification column 20 serves to heat the low boiling gas therein. In this way, the rectification in the column 20 can be effectively performed. In order to smoothly effect the aforementioned heat exchange, it is appropriate that the operating pressure in the subrectification column 20 be set lower than that in the main rectification column 16 by about 0.6 kg/cm². Accordingly, liquid nitrogen has the pressure thereof reduced while being transported to the subrectification column 20 from the main rectification column 16 through the liquid nitrogen transport passage 22. This causes the saturation temperature of the liquid nitrogen to be reduced. Also, in the above process, a large amount of nitrogen gas is ascending together with low boiling gas in the subrectification column 20. Accordingly, it is preferable to liquefy the nitrogen gas at the upper portion of the subrectification column 20 and recover it as liquid nitrogen. In this case, liquid deposited at the bottom of the main rectification column 16 may be transported to the upper portion of the subrectification column 20 through a liquid transport passage 28 with the pressure thereof being reduced. Thereby, the upper portion of the subrectification column 20 can be cooled, enabling efficient liquefaction and recovery of the nitrogen gas. Similar effects can be obtained in the case where liquid present in the middle of the main rectification column 16 is transported to the upper portion of the subrectification column 20 through a passage similar to the liquid transport passage 28 with the pressure thereof being 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 one thereof. The extracted liquid nitrogen is then introduced into the subrectification column 20. Consequently, the produced nitrogen is extracted from the subrectification column 20 from a position lower than the liquid nitrogen introducing position. Accordingly, low boiling gas in the material air, such as hydrogen, helium, and neon, can be effectively removed from the air, thereby enabling production of nitrogen having a greatly improved purity. Table-1 below shows 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 material gas according to the conventional method described with reference to Fig. 4. Gas 3 is nitrogen produced from the air according to the present method. The concentration of hydrogen in the air is reduced to 0.1 ppm in advance with the use of catalyst. As apparent from Table-1, the conventional method can remove almost no low boiling gases present in the air. On the contrary, the present method can effectively remove the respective low boiling gases. The present invention is not limited to the foregoing embodiment, but can be embodied as follows. (1) In the process of the foregoing embodiment, helium gas extracted from the respective rectification columns 16, 20 is directly brought out of the system as shown in Fig. 1. However, the helium gas may be transported to the main heat exchanger 14 from the rectification columns 16, 20 respectively through helium gas transport passages 31, 32. and thereafter recovered. The above operation enables the low temperature helium gas to greatly serve as cooling medium for the main heat exchanger 14, thereby improving overall heat efficiency of the system. Further, as shown in Figs. 1 and 2, the gas taken from the upper portion of the main rectification column 16 may be transported to the expansion turbine 18 (expansion means) through a gas transport passage 36. The gas is adiabatically expanded by the expansion turbine 18, and further transported as it is to the main heat exchanger 14. In this way, the gas in the upper portion of the main rectification column 16 can serve as cooling medium for the main heat exchanger. It should be noted that the gas in the upper portion of the subrectification column 20 can also be used for the same purpose. Fig. 3 shows a system for cooling the system with the use of latent heat of evaporation of liquid nitrogen introduced into the upper portion of the main rectification column 16 through a liquid nitrogen inlet passage 34. In this system, the gas in the upper portion of the column 16 may be transported to the main heat exchanger 14 through the gas transport passage 36, and can be consequently served as cooling medium for the main heat exchanger 14 similar to the above examples. Also, the liquid nitrogen may be supplied to the subrectification column 20 instead of the main rectification column 16 so as to take helium gas therefrom. It may also be appropriate that the liquid nitrogen be supplied to both rectification columns 16, 20. (2) The present method can be applied not only to an air separator, but also to any system for producing high purity nitrogen from material gas including nitrogen and low boiling gases. EXPLOITATION IN INDUSTRY As described above, according to the method and the system of the present invention, liquid nitrogen having high purity is extracted from a main rectification column from a position lower than an uppermost stage thereof, and introduced into a subrectification column. The liquid nitrogen is further heated so as to evaporate low boiling gases therein in the subrectification column. Consequently, produced nitrogen is extracted from the subrectification column from a position lower than the position from which the liquid nitrogen is introduced thereinto. Accordingly. the low boiling gases can be effectively removed from the material gas, greatly contributing to production of nitrogen having ultra-high purity.

Claims: A method for producing high purity nitrogen comprising the steps of: introducing air into a main rectification column (16) in a cold state; extracting liquid nitrogen from the main rectification column (16) from a position lower than an uppermost stage thereof; introducing the extracted liquid nitrogen into a sub rectification column (20); characterized in that: in the main rectification column (16), nitrogen and component having a boiling point lower than nitrogen are separated from component having a boiling point higher than nitrogen; in the subrectification column (20), the component having a boiling point lower than nitrogen is separated from the nitrogen; and separated nitrogen is extracted from the subrectification column (20) from a position lower than a position into which the liquid nitrogen is introduced. A method as defined in claim 1 further comprising the steps of: introducing liquid nitrogen deposited at a bottom portion of the subrectification column (20) into a condenser (161) provided at a top portion of the main rectification column (16); and introducing evaporated nitrogen gas from the condenser into the subrectification column (20). A method as defined in claim 1 further comprising the step of introducing liquid nitrogen deposited at a bottom portion of the main rectification column (16) into a condenser (161) provided at a top portion of the subrectification column (20). A method as defined in claim 1 further comprising the step of introducing liquid present in a middle portion of the main rectification column (16) into a condenser (201) provided at a top portion of the subrectification column (20). A method as defined in any of claims 1 to 4 further comprising the steps of: exchanging heat between the air and the produced nitrogen by using a heat exchanger (14); and taking gas present in a top portion (161, 201) of at least one of the main rectification column (16) and the subrectification column (20) therefrom; and passing the gas through the heat exchanger (14). A method as defined in claim 5 further comprising the step of expanding the gas while transporting the same from the top portion of at least one of the rectification columns (16, 20) to the heat exchanger (14). A method as defined in claim 5 further comprising the step of introducing liquid nitrogen into the top portion (161, 201) of at least one of the rectification columns (16, 20) so as to cool the gas therein before taking the gas therefrom. A method as defined in any of claims 1 to 4 further comprising the steps of: exchanging heat between the air and the produced nitrogen with the use of a heat exchanger (14); taking helium gas from at least one of the main rectification column (16) and the subrectification column (20); and passing the taken helium gas through the heat exchanger (14). A system for producing high purity nitrogen comprising: a main rectification column (16) and a subrectification column (20) for effecting rectification therein; an inlet passage (11) for introducing air into the main rectification column (16): a liquid nitrogen transport passage (22) for transporting liquid nitrogen from the main rectification column (16), to the sub rectification column (20), one end of the passage (22) being connected to the main rectification column (16) at a position lower than an uppermost stage thereof and the other end thereof being connected to the subrectification column (20); and a produced nitrogen outlet passage (24) for extracting produced nitrogen from the sub rectification column (20); characterized in that: the main rectification column (16) separates nitrogen and component having a boiling point lower than nitrogen from component having a boiling point higher than nitrogen; the subrectification column (20) separates the component having a boiling point lower than nitrogen from the nitrogen; and one end of the outlet passage (24) is connected to the subrectification column (20) at a position lower than a position into which the liquid nitrogen is introduced. A system as defined in claim 9 further comprising: a condenser (161) provided at a top portion of the main rectification column (16); and a circulating passage (26) for transporting liquid nitrogen deposited at a bottom portion of the subrectification column (20) to the condenser and a passage (27) for transporting nitrogen gas evaporated in the condenser (161) to the subrectification column (20). A system as defined in claim 9 further comprising: a condenser (201) provided at a top portion of the subrectification column (20); and a liquid transport passage (28) for transporting liquid deposited at a bottom portion of the main rectification column (16) to the condenser (201). A system as defined in claim 9 further comprising: a condenser (201) provided at a top portion of the subrectification column (20); and a liquid transport passage for transporting liquid deposited at a middle portion of the main rectification column (16) to the condenser (201). A system as defined in any of claims 9 to 12 further comprising: a heat exchanger (14) for exchanging heat between the air and the produced nitrogen; and a gas outlet passage (32, 36) for passing gas from top portions of the respective rectification columns (16, 20) through the heat exchanger (14), one end of the outlet passage being connected to the top portions (161, 201) of the respective rectification columns and the other end thereof extending through the heat exchanger (14). A system as defined in claim 13 further comprising expansion means (18) provided in a specified position along the gas outlet passage between the heat exchanger (14) and the top portions (161, 201) of the respective rectification columns, the expansion means (18) adapted for expanding the gas passing therethrough. A system as defined in claim 13 further comprising liquid nitrogen inlet passages (34) for introducing liquid nitrogen to the top portions (161, 201) of the respective rectification columns (16, 20). A system as defined in any of claims 9 to 12 further comprising: a heat exchanger (14) for exchanging heat between the air and the produced nitrogen; and a helium gas outlet passage (31, 32) for passing helium gas from the respective rectification columns (16, 20) through the heat exchanger (14), one end of the helium gas outlet passage being connected to the respective rectification columns and the other end thereof extending through the heat exchanger (14).

Inventors: FUJITA HIDETO - DANJYOCHO -CHO, TANAKA MASAYUKI - - SAKURADAI, FUJITA, HIDETO, 8-20, DANJYOCHO 3-CHOME, TANAKA, MASAYUKI, 15-1-404, SAKURADAI 6-CHOME
Applicants: KOBE STEEL LTD, KABUSHIKI KAISHA KOBE SEIKO SHO
Application year: 1990
Main classification: F25J3
IPC codes: F25J3