RU2069293C1 - Cryogenic method of producing nitrogen from air - Google Patents

Abstract

FIELD: cryogenic engineering; plants for separation of air by low-temperature rectification method. SUBSTANCE: compressed purified air is cooled and is divided into two parts; one part of air is introduced into high-pressure column for separation into first fractions enriched with nitrogen and oxygen, second part of air is condensed, evaporated and is fed to low-pressure column where it is separated into second fractions enriched with nitrogen and oxygen. Evaporation of part of condensed air is effected by indirect heat exchange with first fraction enriched with nitrogen; first fraction enriched with oxygen is directed to low-pressure column for separation. EFFECT: enhanced efficiency. 10 cl, 4 dwg, 1 tbl

Description

 The present invention relates to the field of air separation processes and, in particular, to a method and apparatus for producing nitrogen, oxygen and / or argon from air, in which liquefied air is used as a heat transfer medium of a condenser of a high pressure column, thereby ensuring the energy efficiency of the process.

 Existing methods of cryogenic separation of air include filtering the supplied air to remove fine particles, after which air is compressed to communicate the energy necessary for separation. Typically, the supplied air is then cooled and passed through adsorbents to remove contaminants and, in particular, carbon dioxide and water vapor. The resulting air stream is then subjected to cryogenic distillation.

 Cryogenic distillation or separation of air involves supplying high pressure air to one or more separation columns operating at cryogenic temperatures, where the components of the air and, in particular, oxygen, nitrogen, argon and noble gases can be separated by distillation.

 The cryogenic separation process, including the contact of the liquid and vapor phases, depends on the pressure difference of the saturated vapors of the corresponding components of the air. Components having a higher saturated vapor pressure, which means more volatile components having a lower boiling point, tend to concentrate in the vapor phase. Components having a lower saturated vapor pressure, which means less volatile components having a higher boiling point, tend to concentrate in the liquid phase.

 The separation process, which involves heating the liquid mixture to concentrate volatile components in the vapor phase and less volatile components in the liquid phase, is distillation. Partial condensation is a separation process in which the vapor mixture is cooled in order to concentrate the volatile component or components in the vapor phase and at the same time to concentrate the less volatile component or components in the liquid phase.

 A process in which successive partial vapors and condensations are combined, in which countercurrent treatment of the vapor and liquid phases is carried out, is called rectification (or the term continuous distillation is sometimes used). The interaction in the counterflow of the vapor and liquid phases is adiabatic and may include integral or differential contact between the phases.

 The device used to carry out the separation process, which uses the principles of distillation to separate mixtures, is often called a distillation column, distillation column or fractionation column.

 In the text of the present description and claims, the term "column" means a distillation or fractionation column or zone. Such a device can also be characterized as a contacting column or zone where the liquid and vapor phases are brought into contact in countercurrent to separate the fluid mixture. As an example, we can talk about the contact of the vapor and liquid phases on a sequence of plates vertically located in space, which are often made with holes or curved in the form of a wave and which extend across the column perpendicular to its central axis. At the locations of the plates, filler materials may be used to fill the column.

 As used below, the term "double column" refers to a higher pressure column in the upper part of which heat is exchanged with a lower part having a reduced pressure.

 The expression "standard method and device for air separation", used below, refers to the method and device described above, as well as to other methods of air separation known to specialists in this field of technology.

 In the present description and claims, the expression "indirect heat transfer" means bringing into a state of heat exchange two fluxes of fluids without any physical contact or stirring.

 Historically, nitrogen, oxygen and / or argon was obtained by one of two main technological methods, which involved the use of production technology with one column or with two columns.

 As for nitrogen, the single-column method allows to obtain a sufficiently high-quality gaseous and liquid nitrogen at pressures ranging from 6 to 10 barrels. The production of nitrogen is limited by the equilibrium state that occurs at the bottom of the column. Typically, the method allows you to get nitrogen in an amount of about 50 to 60 percent of the nitrogen content in the air supplied to the process.

 In a two-column process, nitrogen is obtained at a pressure in the range of one to four barrels. This technology is more effective than the technology with a single column and allows you to get up to 90 percent of nitrogen and even more from the amount of nitrogen contained in the air supplied to the process. As a rule, the columns are combined with a condenser-evaporator separating the two columns. Since the described method allows to obtain nitrogen at a relatively low pressure, it is often necessary to further compress nitrogen, which increases the cost of production.

 In a known two-column process, air is separated by cryogenic distillation or sublimation to obtain a nitrogen rich stream, i.e. nitrogen-rich fractions in the upper part of the high-pressure column and a stream of oxygen-rich in the lower part. The nitrogen rich stream is directed to the top of the low pressure column to form a reverse flow in this column. The bottom stream rich in oxygen is fed to the low pressure column for further separation.

 In the low-pressure column, the feed stream is further separated by cryogenic distillation into an oxygen rich stream or a fraction containing oxygen in the bottom and a stream rich in nitrogen or a nitrogen fraction in the upper part. The upper stream can then be selected already as the final nitrogen product. In the double column, the high and low pressure columns are thermally connected through a condenser-evaporator. Thus, in the known two-column process method, the nitrogen fraction exiting the high-pressure column condenses upon evaporation of the oxygen-containing fraction in the low-pressure column.

 At a given pressure in the low-pressure column, the pressure of the air supplied to the high-pressure column is determined by the composition of the vaporized stream containing oxygen, the temperature difference between the condenser of the high-pressure column and the evaporator of the low-pressure column, and to some extent the composition of the condensed nitrogen-rich stream, sufficiently purified from other components.

 Other well-known technological schemes are variations of the above processes in a single column or in a double column, and various additional features, for example, an additional top condenser or bottom evaporator, may be present.

 The present invention is a method effective from the point of view of energy consumption, the production of nitrogen, oxygen and argon.

 The essence of the invention is the use of liquefied and evaporated air as a heating and cooling medium between high and low pressure columns. Previously, nitrogen was used for this.

 The invention is further described in detail in relation to the production of nitrogen, however, it should be understood that the invention is equally applied to the production of oxygen and argon. It will be apparent to those skilled in the art how to optimize temperature, pressure and other operating parameters to optimize the production of oxygen and / or argon as target products. A particular advantage of using air as a heating and cooling medium is that it requires less energy to condense than to condense a nitrogen-rich gas. Since the main energy expenditures are associated with the compression of gases, the lower pressure required to condense air at a given temperature also provides a lower cost than nitrogen condensation. For example, nitrogen condenses at a pressure of 7 barrels and a temperature of minus 180 degrees Celsius. In contrast, air condensation only requires a pressure of 6 barrels at a temperature of minus 178 degrees Celsius. Thus, the temperature difference of two degrees Celsius and the pressure difference of one barrels provide energy savings and, accordingly, costs for it in the proposed method.

 In known methods, when nitrogen is used as a heating and cooling medium between high and low pressure columns, it is necessary to compress the supply air to a higher supply pressure required for nitrogen. Thus, the initial energy savings are due to lower costs for compression of the supplied air.

 The proposed method makes it possible to obtain highly pure nitrogen in amounts of more than 90 percent of the amount of nitrogen contained in the supplied air. Nitrogen can be obtained at pressures ranging from 3 to 15 barrels, and nitrogen can be obtained at both high and low pressures. High and low pressure nitrogen can be obtained both simultaneously and separately. The proposed method provides energy savings in comparison with known methods.

 In accordance with the proposed method, the supplied air, purified from moisture and impurities such as carbon dioxide and methane by passing through molecular sieves, alumina, silica gel and similar filters, is compressed and fed to a heat exchanger for heat exchange with selected products.

 In one embodiment, the feed air is divided into two fractions, one fraction is fed to the bottom of the high pressure column, and the other fraction is fed to a condenser-re-evaporator located at the base of the low pressure column. Good results were obtained using two equal fractions of the supplied air, although other proportions can be used.

 In accordance with another embodiment, the supplied air is divided into three parts. Two parts of the supplied air are supplied to the high-pressure column and the condenser-re-evaporator at the base of the low-pressure column, as described above. The third fraction is expanded to cool the entire plant and then fed to a low pressure column for cryogenic separation.

 The first part of the supplied air is separated by cryogenic distillation in a high pressure column into a first fraction enriched with nitrogen and a first liquid fraction enriched with oxygen. The oxygen-rich liquid fraction is discharged from the base of the high pressure column and fed to the low pressure column. The second part of the supplied air, which is supplied to the condenser-re-evaporator located at the base of the low-pressure column, is condensed by heat exchange with the liquid fraction enriched with oxygen in the bottom of the low-pressure column, which, therefore, evaporates. The liquefied air thus obtained in the condenser-re-evaporator is then fed to the upper condenser of the high-pressure column, where it is evaporated by indirect heat exchange with the first nitrogen-rich vapor fraction obtained in the high-pressure column. Due to this, nitrogen condenses.

 In one embodiment of the invention, a portion of the condensed nitrogen enriched fraction in the high pressure column is separated and fed to the low pressure column to create an additional stream. At the same time, the second part of the supplied air, which was evaporated during indirect heat exchange with nitrogen in the upper condenser of the high pressure column, is then fed to the low pressure column for cryogenic separation.

 In the low-pressure column, the second part of the supplied air, as well as part of the fraction enriched with oxygen, from the high-pressure column are separated into a secondary stream enriched with oxygen.

 In a further embodiment, a portion of the secondary nitrogen-rich stream can be diverted as high pressure nitrogen to be produced, and the remaining part is used to provide the flow in the low pressure column.

 In a further embodiment, a portion of the high pressure exhaust nitrogen can be expanded to cool the entire plant and then added to the low pressure exhaust nitrogen stream.

 The secondary, oxygen-enriched stream entering the bottom of the low-pressure column undergoes evaporation during indirect heat exchange with the incoming second part of the supplied air, which condenses. In yet another embodiment, the secondary oxygen enriched fraction may also include a third of the supplied air, which undergoes expansion before being fed to the low pressure column.

 A portion of the secondary, oxygen-enriched stream is fed to the upper condenser of the low-pressure column, where it evaporates during heat exchange with rising nitrogen, which thus condenses. The secondary oxygen-enriched stream thus evaporated can be removed from the upper condenser as an exhaust gas and heated in an additional cooler during indirect heat exchange with the streams flowing into the installation and the supplied air.

 If desired, the exhaust oxygen can be expanded to provide cooling for the installation. Waste oxygen, having a purity of about 70 percent, can also be used as a standalone product for applications in which there is no need to use high purity oxygen.

 A device for implementing the above method is also provided. The proposed device includes, in combination, an air compressor for compressing air from an external source, a cleaning device for removing carbon dioxide and water vapor from the compressor compressed air, and a heat exchanger for cooling the compressed air to a cryogenic temperature. The first distillation column is provided, equipped with an upper column or upper evaporator-condenser, designed for cryogenic separation of part of the air coming from the heat exchanger.

 A second distillation column is also provided, equipped with an upper condenser and a lower evaporator, designed to separate, by fractionation, at least a portion of the cooled compressed air supplied after passing through the lower evaporator of the second distillation column and the upper condenser of the first distillation column together with at least part an oxygen-enriched liquid phase coming from the first distillation column into a second, oxygen-enriched fraction, and a second, azo-enriched th fraction.

 A device is provided for outputting liquid oxygen, located at the base of the second distillation column, for subsequent supply to the upper condenser of the second distillation column to provide indirect heat exchange with vapors ascending in the second distillation column.

 An expander is provided for expanding the compressed air before being fed to the second distillation column, for expanding the oxygen discharged from the upper condenser of the second distillation column, and / or for expanding the resulting nitrogen to cool it.

 FIG. 1 is a flow chart in a device that implements the method according to the invention and is designed to produce nitrogen under low pressure.

 FIG. 2 is a flow chart in a device implementing the method of the invention and a similar device shown in FIG. 1 with the difference that air compression is provided instead of expansion.

 FIG. 3 is a flow chart in a device that implements the method according to the invention, which provides nitrogen at low and high pressures.

 FIG. 4 is a flow diagram in a device that implements the method according to the invention, similar to that shown in FIG. 3, with the difference that part of the high pressure nitrogen expands into the low pressure nitrogen system.

 In the flow diagram depicted in FIG. 1 shows that the supplied compressed air, cleaned of impurities, flows through a pipe 20 to the heat exchanger 30. The air is preferably supplied to the heat exchanger 30 under a pressure in the range of 5 to 20 barrels, and the air temperature is reduced to a cryogenic level by indirect heat exchange with outgoing side flow and the flow of the finished product.

 Further, the supplied air is divided into two parts. Positive results were obtained with equal flows of supplied air, however, other ratios can be used. The first supply air stream is directed to the high pressure column 32 via lines 22 and 62, and the second part of the supply air is directed to the evaporator 58 of the low pressure column 34 via lines 22 and 60.

 In the high pressure column 32, the pressure is maintained in the range from 5 to 20 barrels.

 The first supply air stream enters the bottom of the column 32 below the bottom distillation tray, as shown at 36. Here, the first supply air stream is divided into a first nitrogen-rich vapor fraction that rises to the top of the column 32 and a first oxygen-rich liquid fraction which remains in the bottom of the column 32.

 At least a portion of the oxygen-enriched liquid is discharged from the bottom of the high-pressure column at the location indicated at 38. In this case, 35 to 40 percent of oxygen is discharged, which is approximately the same proportion as that provided for by known techniques.

 The first oxygen-enriched liquid discharged from the bottom of the high-pressure column 32 through line 54 is passed through a pre-cooler 46, where its temperature is further reduced by indirect heat exchange with nitrogen obtained as the final product, which is discharged from the upper part of the low-pressure column 34 on line 48, as well as off-gas discharged through line 52 from the upper condenser-evaporator 70 of the low pressure column 34.

 Initially, the cooled oxygen is discharged from the pre-cooler 46 in the form of a liquid and then supplied to the low pressure column 34 above the bottom plate after passing the expansion from valve 76.

 The second stream of supplied air entering the evaporator condenser 58 in the lower part of the low pressure column 34 condenses during indirect heat exchange with the oxygen-enriched liquid in the bottom of the low-pressure column 34, while the second stream of supplied air condenses and the oxygen-enriched liquid evaporates.

 The condensed second supply air stream is discharged from the condenser-evaporator 58 of the low pressure column 34 through line 82 and supplied to the pre-cooler 46. The liquefied air is discharged from the pre-cooler 46 through line 84 and expanded through valve 44 to the condenser / evaporator 40 of the high pressure column 32. If necessary, part of the condensed second supplied air stream may enter the low pressure column 34 via line 90 after expansion through valve 92 to control the air balance between nnami high and low pressure.

 The first nitrogen-enriched vapor fraction rises to the upper part of the high-pressure column 32, where it enters the condenser-evaporator 40. Here, the nitrogen vapors are indirectly exchanged with the condensed second part of the supplied air, which enters through the valve 44 from the condenser-evaporator 58 of the column low pressure 34. In this case, the liquefied air evaporates, and the nitrogen vapor condenses. Figures 3 and 4 show that partially or fully condensed nitrogen is returned to the high pressure column 32 to provide the desired flow.

 Nitrogen vapors that remained non-condensed during indirect heat exchange with the condensed second part of the supplied air can be selected in the form of nitrogen under high pressure by withdrawing from the upper part of the high pressure column 32, for example, along line 67, as shown in Fig. 3.

 Partially condensed nitrogen may be directed to the low pressure column 34 to create an additional stream if the high pressure nitrogen stream is sufficiently small or not needed. This portion of the condensed nitrogen is discharged from the top of the high pressure column 32 through line 68, as shown in Figures 1 and 3. Condensed nitrogen is then passed through pre-cooler 66, where it is brought into indirect heat exchange with the exhaust nitrogen, which is the product, and the exhaust gases. From precooler 58, condensed nitrogen is passed through the extension of line 68 and fed to the low pressure column 34 after expansion through valve 78.

 At the same time, the vaporized air leaving line 56 from the condenser-evaporator 40 in the upper part of the high pressure column 32 is separated by supplying the low pressure column 34 through line 64 to approximately the same level of supply of the first oxygen-enriched liquid that flows through line 54.

 The first oxygen-enriched liquid discharged from the base of the column 32 and the vaporized air discharged from the condenser-evaporator 40 in the upper part of the high-pressure column 32 via line 56 are further separated in the column 34 into a secondary, nitrogen-rich vapor fraction, and a secondary, enriched oxygen fraction.

 The secondary, nitrogen-enriched vapor fraction rises to the top of the low pressure column 34, while the secondary, oxygen-enriched fraction descends to the bottom of this column.

 Part of the secondary oxygen-enriched liquid fraction in the bottom of the low-pressure column 34 is discharged through line 74 and passed through the first pre-cooler 46. Here, the secondary oxygen-enriched liquid is then cooled by indirect heat exchange with nitrogen gas removed from the upper part of the low-pressure column 34 line 48, as well as the effluent discharged through line 52 from the upper condenser 70 of the low pressure column 34.

 A secondary oxygen-enriched liquid is passed by extending line 74 to a second pre-cooler 66 for further cooling by indirect heat exchange with nitrogen gas discharged from the upper part of the high pressure column 32 via line 68, as well as a stream of oxygen discharged from the upper condenser 70 on line 52.

 The secondary oxygen-enriched liquid resulting from cooling is discharged along the line 74 and is fed to the upper condenser 70 in the upper part of the low-pressure column 34 after expansion through valve 72 to further cool the secondary oxygen-enriched stream.

 Most of the nitrogen-rich secondary stream is taken as the final product from the top of the low-pressure column 34 via line 48. The nitrogen gas stream is heated as it passes through pre-coolers 46 and 66 and the heat exchanger 30 before exiting the system.

 The remaining part of the secondary nitrogen-rich stream in the volume of the low-pressure column 34 is condensed by heat exchange with the secondary oxygen-rich liquid in the upper evaporator-condenser 70 of the low-pressure column 34, which leads to the evaporation of the secondary oxygen-rich liquid. Nitrogen condensation provides a reverse flow in the low pressure column 34. Oxygen-enriched vapors leave the upper evaporator-condenser 70 via line 52 and heat up significantly when passing through pre-coolers 66 and 46 and heat exchanger 30.

 After heating in the heat exchanger 30, the off-stream of oxygen is passed through a pipe expander 79, where the stream expands for overall cooling of the installation.

 It can be seen that in the above process, air is used as a heating and cooling medium between the high and low pressure columns. Typically, in the known methods, a nitrogen-rich stream was used to transfer heat to the bottom of a low pressure column. It should be noted that for a given nitrogen performance, i.e. to obtain an oxygen-enriched stream of the same composition, more energy is required to condense the nitrogen-enriched stream than to condense air. This means that for a given nitrogen performance when using air as a heat transfer medium, a high-pressure column can operate at a pressure lower than the pressure used in known processes. In other words, for the same pressure in the high pressure column in the system according to the invention, the low pressure column can operate at a higher pressure.

 Table 1 below shows the possible results when implementing the proposed method in the system depicted in FIG. 1, upon receipt of nitrogen as the final product.

 When implementing the embodiments depicted in Figures 3 or 4, an absolute supply pressure of 21 barrels of pressure will create an absolute pressure of about 20 barrels absolute in the volume of the high pressure column 32 and a pressure of about 14 barrels absolute in the volume of the low pressure column 34.

 Various modifications of the proposed method and device are apparent to those skilled in the art that can be implemented without departing from the spirit and scope of the invention as defined in the following claims.

Claims (10)

 1. A cryogenic method for producing nitrogen from air by cooling compressed air purified from moisture and impurities and separating it into two parts, the first of which is introduced into a high-pressure column for separation by cryogenic distillation into the first fractions enriched with nitrogen and oxygen, with subsequent withdrawal of at least parts of these fractions from the column, and the second part of the air is condensed in the condenser of the low pressure column, evaporated in the condenser of the high pressure column and fed to the low pressure column, in which into the second nitrogen and oxygen enriched fractions, the second nitrogen enriched fraction being discharged as the final product, and the oxygen enriched fraction being fed to the evaporator of the low pressure column, evaporated by indirect heat exchange with the second nitrogen enriched fraction and withdrawn as waste, the second part of the air is condensed by indirect heat exchange with a second oxygen-enriched fraction, characterized in that the evaporation of the second part of the condensed air is carried out by indirect heat BMENA a first nitrogen-rich fraction and a first oxygen-rich fraction is sent to separation in the low pressure column.  2. The method according to claim 1, characterized in that a portion of the condensed first nitrogen-rich fraction is withdrawn from the high pressure column as a high-pressure nitrogen product.  3. The method according to claims 1 and 2, characterized in that a portion of the condensed first nitrogen-rich fraction is introduced into the low pressure column.  4. The method according to claims 1 to 3, characterized in that a part of the cooled and purified air is separated, expanded, and fed to a low pressure column.  5. The method according to p. 1, characterized in that the waste enriched oxygen fraction is further expanded.  6. The method according to claim 2, characterized in that the high-pressure nitrogen product is expanded and mixed with a second nitrogen-rich fraction.  7. The method according to PP.1 to 6, characterized in that the cooling of the air is carried out by indirect heat exchange with waste and product streams, and the air is compressed to provide pressure in the high pressure column from 5 to 20 barrels.  8. The method according to PP.1 to 7, characterized in that the cooling of the air is carried out by indirect heat exchange with waste and product streams.  9. The method according to PP.1 to 8, characterized in that the first part of the cooled and purified air is fed into the lower half of the high pressure column, and the first oxygen-enriched fraction is removed from the base of this column.  10. The method according to PP.1 to 9, characterized in that the first oxygen-enriched fraction is introduced into the lower half of the high pressure column, and the second oxygen-enriched fraction is removed from the base of this column.

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