A device and method for producing high-purity nitrogen by flash exhaust low-temperature supercharging cycle

Abstract

The application discloses a device and method for producing high-purity nitrogen by flash exhaust low-temperature pressurization circulation, which comprises an air compressor, a purification system and a cold box, wherein the cold box comprises a main heat exchanger, an expander, a rectifying tower, a flash tower, a condenser and a pressure reducing valve. The application aims at overcoming the problems of low extraction rate, high energy consumption of single-tower rectification and high investment of double-tower rectification in the prior art, and provides a device and method for obtaining high-purity nitrogen by cryogenic separation, which have the advantages of low energy consumption, low investment, small land occupation and economic rationality by recycling the nitrogen component in the flash exhaust and the compression work of the nitrogen component.

Description

Technical Field

[0001] This invention relates to the field of air separation, and more particularly to an apparatus and method for producing high-purity nitrogen gas by circulating the flash exhaust gas from a flash distillation tower into a distillation tower after it has been pressurized at a low temperature at the pressurization end of an expander. Background Technology

[0002] With the continuous advancement of industrialization, high-purity nitrogen, as a basic industrial product, plays a vital role in various industries. Therefore, the market demand for pure nitrogen equipment is constantly expanding. Nitrogen pressure is generally required to be 3–15 bar, and the ability to simultaneously produce liquid nitrogen is often required. Currently, there are many methods for preparing pure nitrogen on the market, but the methods that can be applied on a large scale are mainly pressure swing adsorption and cryogenic distillation.

[0003] If pressure swing adsorption (PSA) is used for nitrogen production, it is difficult to meet the requirements for high nitrogen purity; the extraction rate is low; liquid products cannot be produced; maintenance costs are high; and multiple sets of equipment are required for large-scale gas use.

[0004] If traditional or existing cryogenic distillation equipment is used, the process organization may seem varied, but it can be broadly categorized into single-tower distillation and double-tower distillation. Traditional single-tower distillation processes have low extraction rates and high energy consumption. Double-tower distillation processes involve high investment and complex operation. Summary of the Invention

[0005] The purpose of this invention is to overcome the problems existing in the prior art. Nitrogen distillation uses a single distillation column with waste gas recirculation. An additional flash distillation column is added, with the flash waste gas containing >80% nitrogen. The flash waste gas is then recovered by low-temperature pressurization at the expansion end and recirculated into the lower part of the distillation column to recover its nitrogen components and compressive work for the production of high-purity nitrogen.

[0006] The technical solution of this invention is implemented as follows: A device for producing high-purity nitrogen from flash evaporation waste gas through low-temperature pressurization and circulation includes an air compressor, a purification system, and a cold box. The cold box includes a main heat exchanger, an expander, a distillation column, a flash evaporation column, a condenser, and a pressure reducing valve. An evaporator is installed in the lower part of the flash evaporation column. The air compressor is connected to the purification system via a first pipe. The gas outlet of the purification system is connected to a second pipe, which is connected to the distillation column after passing through the main heat exchanger. The liquid outlet at the bottom of the distillation column is connected to a third pipe, which is connected to the flash evaporation column. The gas outlet at the top of the distillation column is connected to a fourth, fifth, and seventh pipe. The fourth pipe extends to the outside of the cold box after passing through the main heat exchanger, and the fifth pipe is connected to the condenser. The liquid outlet of the condenser... The sixth pipe connects to the top of the distillation column, and the seventh pipe connects to the lower evaporator of the flash column. The liquid outlet of the evaporator in the flash column is connected to the top of the distillation column via the eighth pipe. The liquid outlet at the bottom of the flash column is connected to the ninth pipe, which, after passing through a pressure reducing valve, connects to the condenser. The gas outlet of the condenser is connected to the tenth pipe, which, after passing through the main heat exchanger, connects to the expansion end of the expander. The outlet of the expansion end of the expander is connected to the eleventh pipe, which, after passing through the main heat exchanger, connects to the purification system. The gas phase outlet of the flash column is connected to the twelfth pipe, which, after passing through the main heat exchanger, connects to the pressure boosting end of the expander. The outlet of the pressure boosting end of the expander is connected to the thirteenth pipe, which, after passing through the main heat exchanger, connects to the lower part of the distillation column.

[0007] The distillation column is a sieve tray column or a structured packed column with a theoretical number of 35-50 trays or a corresponding actual number of 45-65 trays. The flash column is a sieve tray column with a theoretical number of 2-8 trays or a corresponding actual number of 4-10 trays. The expansion compressor uses cryogenic compression at its pressurization end.

[0008] The expander is a turbine expander; the condenser is a plate-fin heat exchanger.

[0009] A method for producing high-purity nitrogen using the above-mentioned apparatus, the process of which is as follows:

[0010] The process is as follows: Compressed and purified air enters the main heat exchanger, is cooled by the reflux gas, and then enters the distillation column. Pure nitrogen is obtained at the top of the distillation column. A portion of this nitrogen product is reheated in the main heat exchanger and sent out of the cold box. The second portion of nitrogen enters the condenser, is liquefied, and then returns to the distillation column as reflux. The third portion of nitrogen enters the flash evaporator, is condensed and liquefied, and then also returns to the distillation column as reflux. The oxygen-rich liquid air at the bottom of the distillation column enters the flash evaporator for flash distillation. The oxygen content of the liquid at the bottom of the flash evaporator is... The nitrogen gas at the top of the flash tower has a high nitrogen content. After being depressurized and throttled by the pressure reducing valve, it enters the condenser as a cold source to liquefy nitrogen. After being partially vaporized, it enters the main heat exchanger for partial reheating and enters the expansion end of the expander as expansion waste gas, providing cooling for the entire unit. After being reheated by the main heat exchanger, the expanded waste gas enters the purification system as regeneration gas for the adsorbent. The top gas phase of the flash tower has a high nitrogen content. After being superheated by the main heat exchanger, it enters the pressurization end of the expander for pressurization. After being cooled by the main heat exchanger, it enters the lower part of the distillation tower for direct recycling of its nitrogen components.

[0011] The distillation column operates at a pressure of 0.4~1.5 MPa.

[0012] The flash tower operates at a pressure of 0.3~1.2 MPa.

[0013] The positive effects of the technical solution of the present invention are as follows:

[0014] This application utilizes the flash gas exiting the top of the flash distillation tower (existing processes directly discharge it into the cold box as waste gas), improving distillation efficiency and reducing energy consumption. Compared with the traditional single-tower cryogenic distillation process, it recovers the nitrogen components and compression work of the flash gas without increasing energy consumption, while obtaining pressurized nitrogen product, thus increasing nitrogen extraction rate by 10-15% and saving energy by 10-15%. The air compressor and purification system are smaller in scale than those in the single-tower process, and the distillation section is more complex, with the overall investment only slightly higher than that of the single-tower cryogenic distillation process.

[0015] Compared to commonly used dual-tower cryogenic distillation processes, its nitrogen extraction rate and energy consumption are quite similar. The number of trays in the distillation tower is the same as that in its high-pressure tower; the flash tower mainly performs flash evaporation, and its number of trays is negligible compared to the low-pressure tower. Therefore, its investment is much lower than that of commonly used dual-tower cryogenic distillation processes.

[0016] Therefore, this process not only minimizes investment and land use, reduces energy consumption and production costs, but also saves human resources and investment, making it scientifically sound and rational. This invention is of great significance for improving economic efficiency and plays a positive role in conserving social resources and creating a low-carbon, environmentally friendly society. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the structure of the device for producing high-purity nitrogen from flash evaporation waste gas at low temperature and pressure.

[0018] The diagram is labeled as follows: 1. Air compressor; 2. Purification system; 3. Main heat exchanger; 4. Turbine expander expansion end; 5. Turbine expander pressurization end; 6. Distillation column; 7. Flash distillation column; 8. Condenser; 9. Pressure reducing valve; 10. Evaporator; 11. Cold box; 101. First pipe; 102. Second pipe; 103. Third pipe; 104. Fourth pipe; 105. Fifth pipe; 106. Sixth pipe; 107. Seventh pipe; 108. Eighth pipe; 109. Ninth pipe; 110. Tenth pipe; 111. Eleventh pipe; 112. Twelfth pipe; 113. Thirteenth pipe. Detailed Implementation

[0019] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0020] Purification systems are very versatile technologies, and there are many types, such as TSA or PSA technologies. This patent does not specify any particular type.

[0021] Example 1

[0022] A device for producing high-purity nitrogen from flash evaporation waste gas through low-temperature pressurization and circulation, such as... Figure 1As shown, the system includes an air compressor 1, a purification system 2, and a cold box 11. The cold box 11 includes a main heat exchanger 3, turbine expanders 4 and 5, a distillation column 6, a flash distillation column 7, a condenser 8, and a pressure reducing valve 9. An evaporator 10 is installed in the lower part of the flash distillation column 7. The air compressor 1 is connected to the purification system 2 via a first pipe 101. The gas outlet of the purification system 2 is connected to a second pipe 102, which is connected to the distillation column 6 after passing through the main heat exchanger 3. The distillation column... The bottom liquid outlet of column 6 is connected to a third pipe 103, which is connected to the flash column 7. The top gas outlet of column 6 is connected to a fourth pipe 104, a fifth pipe 105, and a seventh pipe 107. The fourth pipe 104 extends to the outside of the cold box 11 after passing through the main heat exchanger 3, thereby sending nitrogen products with a purity >99.99% out of the cold box 11. The fifth pipe 105 is connected to the condenser 8. The liquid outlet of the condenser 8 is connected to the top of column 6 through a sixth pipe 106. The seventh pipe 107 is connected to the lower evaporator 10 of column 7. The liquid outlet of the evaporator 10 in column 7 is connected to the top of column 6 through an eighth pipe 108. The bottom liquid outlet of column 7 is connected to a ninth pipe 109, which is connected to the condenser 8 after passing through the pressure reducing valve 9. The top gas outlet of condenser 8 is connected to a tenth pipe 110, which is connected to the turbine expander after passing through the main heat exchanger 3. The expansion end 4 is connected; the outlet of the expansion end 4 of the turbine expander is connected to the eleventh pipe 111, which is connected to the purification system 2 after passing through the main heat exchanger 3; the gas phase outlet of the flash tower 7 is connected to the twelfth pipe 112, which is connected to the pressurization end 5 of the turbine expander after passing through the main heat exchanger 3; the outlet of the pressurization end 5 of the turbine expander is connected to the thirteenth pipe 113, which is connected to the lower part of the distillation tower 6 after passing through the main heat exchanger 3.

[0023] The working process of the above device is as follows:

[0024] The compressed air from air compressor 1 has an exhaust pressure of 0.63 MPaA. After compression and purification, the air contains 20.95% oxygen and 78.118% nitrogen, with the remainder being impurities such as argon. The pressure is 0.615 MPaA, and the temperature is 15°C. It enters the main heat exchanger 3 and is cooled by the reflux gas to near its liquefaction temperature of -172°C before entering the distillation column 6. The distillation column 6 operates at a pressure of 0.6 MPaA, has 50 theoretical plates, and uses a structured packed column. Nitrogen gas with a purity >99.99% is obtained at the top of distillation column 6. A portion of this nitrogen gas is reheated to 13°C in the main heat exchanger 3 and then sent out of the cold box. The nitrogen product has a purity >99.99% and an outlet pressure >0.57 MPaA, and can be further compressed for use as needed. The second portion of nitrogen enters condenser 8 and is liquefied. The third portion of nitrogen enters evaporator 10 of flash tower 7 and is liquefied. Both liquefied nitrogen portions are maintained at a pressure of 0.6 MPaA and a saturated liquid temperature of -176.9℃. After merging, they are used as reflux to supplement distillation tower 6. At the bottom of distillation tower 6, oxygen-enriched liquid air at 0.6 MPaA is obtained, with an oxygen content of 36.2% and a nitrogen content of 62.3%. This oxygen-enriched liquid air enters flash tower 7 for flash evaporation. The operating pressure of flash tower 7 is 0.39 MPaA, which is as high as possible to fully utilize the pressure energy of its flash waste gas. The theoretical number of trays in flash tower 7 is set at 5, using a sieve tray tower to save costs. After flash evaporation, the heavy oxygen components are further enriched at the bottom of flash tower 7. The bottom liquid has an oxygen content of 44.5%, a pressure of 0.392 MPaA, and a saturated liquid temperature of -177.5℃. After being reduced in pressure to 0.245 MPaA and -183.3°C, the liquefied nitrogen gas enters condenser 8 as a cold source. After partial vaporization, it enters main heat exchanger 3 and is partially reheated to -168°C. This reheated gas then enters the expansion end 4 of the turbine expander as expansion exhaust gas, providing cooling for the entire unit. After expansion, the exhaust gas pressure is 0.125 MPaA and the temperature is -183.6°C. After being reheated to 13°C by main heat exchanger 3, it enters purification system 2 as regeneration gas for the adsorbent. The regeneration exhaust gas consists entirely of air components, primarily containing 44.5% oxygen and 53.7% nitrogen. The main desorption medium, carbon dioxide, contains 500 PPM. It is directly discharged into the atmosphere at a safe high point. The flash gas at the top of flash tower 7 contains 81.5% nitrogen, 17.5% oxygen, and the remainder is impurities such as argon, and maintains a relatively high pressure of 0.39 MPaA. The flash gas, with a saturated gas temperature of -178.4℃, is collected from the top of flash tower 7. It is first reheated to -174℃ by main heat exchanger 3 and then pressurized to 0.605 MPaA at the turboexpander pressurization end 5, higher than the operating pressure of the distillation column. The temperature after pressurization is -155.3℃. After pressurization, the gas is cooled to near saturation temperature (-173℃) by main heat exchanger 3 and then directly circulated in the lower part of distillation column 6 to recover its nitrogen components. This process achieves a nitrogen extraction rate of 67%, which is 15% higher than the traditional single-tower distillation extraction rate, and also saves 15% in energy consumption.The product purity is between 99% and 99.995%, and the number of trays in the distillation column and the gas handling capacity of the air compressor can be adjusted according to actual needs.

[0025] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements can be made without departing from the principle of the present invention, and these improvements should also be considered within the scope of protection of the present invention.

Claims

1. A device for producing high-purity nitrogen from flash evaporation waste gas through low-temperature pressurization and circulation, characterized in that, The system includes an air compressor, a purification system, and a cold box. The cold box includes a main heat exchanger, an expander, a distillation column, a flash column, a condenser, and a pressure reducing valve. An evaporator is installed in the lower part of the flash column. The air compressor is connected to the purification system via a first pipe. The gas outlet of the purification system is connected to a second pipe, which is connected to the distillation column after passing through the main heat exchanger. The liquid outlet at the bottom of the distillation column is connected to a third pipe, which is connected to the flash column. The gas outlet at the top of the distillation column is connected to a fourth, fifth, and seventh pipe. The fourth pipe extends outside the cold box after passing through the main heat exchanger. The fifth pipe is connected to the condenser. The liquid outlet of the condenser is connected to the top of the distillation column via a sixth pipe. The seventh pipe... The flash distillation column is connected to the lower evaporator. The liquid outlet of the evaporator in the flash distillation column is connected to the top of the distillation column via the eighth pipe. The liquid outlet at the bottom of the flash distillation column is connected to the ninth pipe, which is connected to the condenser after passing through a pressure reducing valve. The gas outlet of the condenser is connected to the tenth pipe, which is connected to the expansion end of the expander after passing through the main heat exchanger. The expansion end outlet of the expander is connected to the eleventh pipe, which is connected to the purification system after passing through the main heat exchanger. The gas phase outlet of the flash distillation column is connected to the twelfth pipe, which is connected to the pressure boosting end of the expander after passing through the main heat exchanger. The pressure boosting end outlet of the expander is connected to the thirteenth pipe, which is connected to the lower part of the distillation column after passing through the main heat exchanger.

2. The apparatus for producing high-purity nitrogen from flash evaporation waste gas by low-temperature pressurization and circulation according to claim 1, characterized in that: The distillation column is a sieve tray column or a structured packed column, and the theoretical number of trays for the sieve tray column and the structured packed column is 35 to 50 or the corresponding actual number of trays is 45 to 65.

3. The apparatus for producing high-purity nitrogen from flash evaporation waste gas by low-temperature pressurization and circulation according to claim 1, characterized in that: The flash tower is a sieve plate tower with a theoretical number of 2 to 8 trays or a corresponding actual number of 4 to 10 trays.

4. The apparatus for producing high-purity nitrogen from flash evaporation waste gas by low-temperature pressurization and circulation according to claim 1, characterized in that: The expander is a turbine expander; the condenser is a plate-fin heat exchanger.

5. A method for producing high-purity nitrogen using the apparatus according to any one of claims 1 to 4, characterized in that, The process is as follows: Compressed and purified air enters the main heat exchanger, is cooled by the reflux gas, and then enters the distillation column. Pure nitrogen is obtained at the top of the distillation column. A portion of this nitrogen product is reheated in the main heat exchanger and sent out of the cold box. The second portion of nitrogen enters the condenser, is liquefied, and then returns to the distillation column as reflux. The third portion of nitrogen enters the flash evaporator, is condensed and liquefied, and then also returns to the distillation column as reflux. The oxygen-rich liquid air at the bottom of the distillation column enters the flash evaporator for flash distillation. The oxygen content of the liquid at the bottom of the flash evaporator is... The nitrogen gas at the top of the flash tower has a high nitrogen content. After being depressurized and throttled by the pressure reducing valve, it enters the condenser as a cold source to liquefy nitrogen. After being partially vaporized, it enters the main heat exchanger for partial reheating and enters the expansion end of the expander as expansion waste gas, providing cooling for the entire unit. After being reheated by the main heat exchanger, the expanded waste gas enters the purification system as regeneration gas for the adsorbent. The top gas phase of the flash tower has a high nitrogen content. After being superheated by the main heat exchanger, it enters the pressurization end of the expander for pressurization. After being cooled by the main heat exchanger, it enters the lower part of the distillation tower for direct recycling of its nitrogen components.

6. The method for producing high-purity nitrogen according to claim 5, characterized in that, The distillation column operates at a pressure of 0.4~1.5 MPa.

7. The method for producing high-purity nitrogen according to claim 5, characterized in that, The flash tower operates at a pressure of 0.3~1.2 MPa.

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