CRYOGENIC AIR SEPARATION PLANT AND STANDBY METHOD FOR CRYOGENIC AIR SEPARATION PLANT

Abstract

To provide an air liquefaction separation device which can increase a frequency of low temperature stand-by of the air liquefaction separation device without damaging a heat exchanger installed at the air liquefaction separation device by heat stress and enables reduction of a startup time of the device.SOLUTION: An air liquefaction separation device 100 includes: a heat exchanger 5 which cools raw material air; rectification towers which liquefy, rectify, and separate the raw material air; and a cold box 4 which houses the heat exchanger 5 and the rectification towers. The air liquefaction separation device 100 includes a supply path 11 of a dry gas at a normal temperature in a hot fluid path at a hot end side of the heat exchanger 5 and includes a discharge path 12 for discharging the dry gas, which has been subject to heat exchange by the heat exchanger 5, from the cold box 4 to an outside at a cold end side of the heat exchanger 5.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a cryogenic air separation unit and a standby method for the cryogenic air separation unit. [Background technology]

[0002] When an air separation unit is shut down, the cryogenic liquefied gas remaining in the unit must be released to prevent the internal pressure from rising above the design pressure due to evaporation caused by heat intrusion. However, releasing gas from the room-temperature gas system to adjust the pressure results in a flow of cold fluid without any warm fluid entering the heat exchanger, causing the warm end of the heat exchanger to cool and eventually reach the temperature of the cryogenic liquefied gas itself. However, room-temperature gas system piping is generally made of carbon steel, and flowing gas equivalent to the temperature of the cryogenic liquefied gas will cause low-temperature embrittlement and break. Therefore, standby of the air separation unit must be stopped before the warm end temperature of the heat exchanger falls below -10°C, the allowable temperature for carbon steel piping, and the liquefied gas must be released from the system.

[0003] Furthermore, after releasing the liquefied gas out of the system, the cryogenic air separation unit must first be completely returned to room temperature and operated to remove moisture and carbon dioxide (full heating operation), and then cooled from the room temperature state to start operation of the unit (room temperature start-up), which requires enormous amounts of energy and time.To solve this problem, it has been proposed to provide a liquid backflow prevention device, a gas release valve, etc. (See, for example, Patent Documents 1 and 2).

[0004] Furthermore, in a conventional air cryogenic separation unit (for example, the unit shown in FIG. 2 and disclosed in Patent Document 2) 100a, when operation is stopped (low-temperature standby state), the heat exchanger 5 is naturally stopped, and gas supply to the heat exchanger 5 is stopped. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 2782355 [Patent Document 2] Japanese Patent Application Publication No. 2019-178816 Summary of the Invention [Problem to be solved by the invention]

[0006] However, as described in Patent Documents 1 and 2, if gas is not flowing through the heat exchanger, the temperature distribution inside the heat exchanger will decrease over time due to heat transfer through the metal that makes up the heat exchanger, with the temperature near the hot end decreasing and the temperature near the cold end increasing over time. When the cryogenic air separation unit is subsequently restarted, room-temperature feed air is introduced through the hot end, and gas close to the saturation temperature of the liquefied gas stored in the unit is introduced through the cold end. If gas is introduced into the heat exchanger at a temperature close to the rated temperature under these conditions, the temperature difference between the gas and the metal parts of the heat exchanger will increase, resulting in localized excessive stress within the heat exchanger. Repeated occurrences of this condition could, in the worst case scenario, result in damage to the heat exchanger. To avoid damage to the heat exchanger, the gas introduction rate can be adjusted to prevent a sudden temperature change in the heat exchanger during the initial cooling phase, but this operation requires a longer start-up time. Furthermore, a longer start-up time results in longer periods of wasted power consumption, making the system less efficient.

[0007] In view of the above, an object of the present invention is to provide a cryogenic air separation unit and a standby method therefor that can increase the frequency of low-temperature standby of the cryogenic air separation unit without damaging the heat exchanger due to thermal stress and further shorten the startup time of the unit. [Means for solving the problem]

[0008] The first aspect of the present invention is Water and carbon dioxide were excludedAn air liquefaction separation device comprising a heat exchanger for cooling raw material air, a rectification column for liquefying and rectifying the raw material air, and a cold box for housing the heat exchanger and the rectification column, wherein a supply path for dry gas at normal temperature is provided in a warm fluid path on the warm end side of the heat exchanger, and a discharge path for discharging the dry gas heat-exchanged by the heat exchanger from the cold box to the outside is provided on the cold end side of the heat exchanger.

[0009] A second invention of the present invention is characterized in that, in the first invention, the supply source of the dry gas is obtained by vaporizing and pressure-feeding the liquefied gas in a liquefied gas storage tank.

[0010] A third invention of the present invention is characterized in that, in the first invention, a compressed cold fluid gas flowing from the cold end side to the warm end side of the heat exchanger is introduced into the supply path of the dry gas.

[0011] A fourth invention of the present invention is a standby method for the air liquefaction separation device according to any one of the first to third inventions. In the standby state when the operation of the device is stopped, the gas generated by evaporation of the liquefied gas in the rectification column is introduced into the cold end side of the heat exchanger, and the dry gas is introduced into the warm end side of the heat exchanger. The amount of dry gas introduced is adjusted by monitoring at least one of the temperature of the fluid discharged from the hot end of the heat exchanger and the temperature of at least one surface thermometer installed on the hot end of the heat exchanger. It is characterized by the above.

[0012] A fifth invention of the present invention is characterized in that, in the fourth invention, the temperature distribution inside the heat exchanger is maintained in the same state as during normal operation of the device.

Effect of the Invention

[0013] According to the air liquefaction separation device and the standby method of the air liquefaction separation device of the present invention, since the dry gas is introduced from the warm end side of the heat exchanger, it is possible to significantly reduce the thermal fatigue applied to the heat exchanger during standby, and the life of the heat exchanger can be improved.

Brief Description of the Drawings

[0014] [Figure 1] It is a figure which shows one form example of the air liquefaction separation apparatus to which the standby method of this invention is applicable. [Figure 2] It is a figure which shows the conventional air liquefaction separation apparatus (the apparatus described in the said patent document 2).

MODE FOR CARRYING OUT THE INVENTION

[0015] Fig. 1 shows the air liquefaction separation apparatus 100 of this form example. The air liquefaction separation apparatus 100 includes, as main equipment, an air filter 1, a raw material air compressor 2, a pretreatment facility 3, a cold box 4, a heat exchanger 5, an expansion turbine 6, a medium-pressure rectification column 7, a condenser / reboiler (condenser) 8, a low-pressure rectification column 9, a subcooler 10, a dry gas supply path 11, a heat exchanger cold-end side discharge path 12, and a heat exchanger warm-end side discharge path 13. Here, the medium pressure is about 3 bar (absolute pressure) to about 15 bar (absolute pressure), and the low pressure means a pressure relatively lower than the medium pressure. Also, in the following description, high temperature and low temperature indicate relative temperature differences and do not specify a temperature range.

[0016] The air filter 1 is a filter for filtering dust in the raw material air. The raw material air compressor 2 is a device for compressing the raw material air and supplying it to the pretreatment facility 3 described later. The pretreatment facility 3 is a device that performs pretreatment such as adsorptive removal of H2O, CO2, etc. contained in the compressed air supplied from the raw material air compressor 2.

[0017] The cold box 4 houses the heat exchanger 5, the medium-pressure rectification column 7, the low-pressure rectification column 9, the subcooler 10, etc. described later, and is provided to suppress heat intrusion from the outside and maintain a low temperature.

[0018] The heat exchanger 5 is a device for pre-cooling the raw material air by exchanging heat with a product gas or the like. The expansion turbine 6 is a device for further cooling the raw material air pre-cooled by the heat exchanger 5 by adiabatic expansion.

[0019] The medium-pressure rectification column 7 and the low-pressure rectification column 9 are rectification columns for liquefying and rectifying the cooled raw material air, and liquefied nitrogen, liquefied oxygen, etc. are separated by the liquefaction and rectification separation.

[0020] The condenser 8 is a condenser that liquefies the medium-pressure nitrogen gas separated at the upper part of the medium-pressure rectification column 7 by heat exchange with the low-pressure liquefied oxygen separated at the bottom of the low-pressure rectification column 9 inside the low-pressure rectification column 9, and generates liquefied nitrogen and oxygen gas.

[0021] The subcooler 10 is a device that exchanges heat between the liquefied gas derived from the bottom of the medium-pressure rectification column 7 and the gas derived from the upper part of the low-pressure rectification column 9.

[0022] The dry gas supply path 11 is provided in the warm fluid path on the warm end side of the heat exchanger 5, which is the secondary side of the raw material air introduction valve 51a in the raw material air introduction path 51, and is a path for introducing dry gas (a normal temperature gas containing no H2O or CO2) from the outside. The dry gas introduced through the path 11 is cooled to a low temperature by heat exchange with the low-temperature gas. At this time, dry gas containing no H2O or CO2 is used to prevent the gas from solidifying and blocking the piping equipment.

[0023] The heat exchanger cold end side discharge path 12 is a path for discharging the dry gas cooled by heat exchange in the heat exchanger 5 and the gas (cold fluid gas) generated by the evaporation of the liquefied gas in the low-pressure rectification column 9 when the operation of the device is stopped (low-temperature standby state, etc.) from the cold end side of the heat exchanger 5.

[0024] The dry gas is discharged from a path branched from the low-temperature raw material air path 52 through the cold end side discharge valve 12b, and the cold fluid gas is discharged from a path branched from the crude pure nitrogen extraction path 59 through the cold end side discharge valve 12a. This is done to supply cold fluid gas with a heat quantity corresponding to the heat quantity of the dry gas introduced into the heat exchanger and appropriately maintain the temperature distribution of the heat exchanger.

[0025] The heat exchanger hot-end discharge path 13 is a path for discharging the cold fluid gas heated by heat exchange to the outside via the hot-end discharge valves 13a-13d, and is used to discharge gas when the device is stopped (low-temperature standby state). The cold fluid gas is not only discharged from the crude pure nitrogen collection path 59 mentioned above, but also from the low-pressure nitrogen discharge path 58, the product oxygen collection path 61, and the medium-pressure nitrogen collection path 62.

[0026] The normal operation of the cryogenic air separation unit 100 configured as above will now be described.

[0027] First, dust is filtered from raw air by air filter 1, and then the raw air is compressed in raw air compressor 2. Thereafter, H2O, CO2, etc. are adsorbed and removed in pretreatment device 3, and the air is purified.

[0028] The purified feed air passes through feed air inlet passage 51 and is introduced as a warm fluid into the warm end of heat exchanger 5 in cold box 4 via feed air inlet valve 51a, where it exchanges heat with liquefied gas, which is a cold fluid, and is cooled (liquefied) to a predetermined low temperature state. Thereafter, for the purpose of further cooling the feed air, it may be supplied to expansion turbine 6 via expansion turbine passage 65 and cooled by adiabatic expansion. However, since the pressure of the feed air after adiabatic expansion has decreased, it is supplied to low-pressure rectification column 9 via post-expansion passage 66.

[0029] When the feed air passes only through the heat exchanger 5 without passing through the expansion turbine 6, the temperature of the feed air after heat exchange is low and it is being liquefied, and it is introduced into the medium-pressure rectification column 7 through the low-temperature feed air line 52. The low-temperature feed air is introduced into the bottom of the medium-pressure rectification column 7 and rises within the column, and the rectification operation within the column separates medium-pressure nitrogen gas at the top of the column and oxygen-enriched medium-pressure liquefied air at the bottom of the column. The liquefied air at the bottom of the medium-pressure rectification column 7 is extracted into the liquefied air outlet line 53, cooled to a subcooled state in the subcooler 10, passed through the liquefied air introduction line 54, and reduced in pressure by the pressure reducing valve 54a to a low-pressure state corresponding to the pressure of the low-pressure rectification column 9, and introduced into the middle stage of the low-pressure rectification column 9 as a descending liquid.

[0030] Furthermore, the medium-pressure nitrogen gas at the top of the medium-pressure rectification column 7 is liquefied in condenser 8, and then a portion of it is introduced as a descending liquid into the top of the medium-pressure rectification column 7, while the remainder passes through liquefied nitrogen line 57 and subcooler 10, is reduced in pressure by pressure reducing valve 57a, and is then introduced as a descending liquid into the top of the low-pressure rectification column 9. At this time, liquefied nitrogen may be introduced from the outside through liquefied nitrogen introduction line 63 equipped with liquefied nitrogen introduction valve 63a, if necessary. Furthermore, liquefied oxygen at the top of the medium-pressure rectification column 7 is extracted as product liquefied oxygen from liquefied oxygen extraction line 60 equipped with liquefied oxygen extraction valve 60a.

[0031] In the low-pressure rectification column 9, low-pressure nitrogen gas is separated at the top of the column by the rectification operation within the column, and low-pressure liquefied oxygen is separated at the bottom of the column. The low-pressure nitrogen gas at the top of the column is extracted into low-pressure nitrogen discharge line 58, passes through subcooler 10, and is further heated by heat exchange with feed air in heat exchanger 5, and is then extracted as product low-pressure nitrogen gas from a low-pressure nitrogen extraction line equipped with a low-pressure nitrogen extraction valve 58a.

[0032] Meanwhile, the liquefied oxygen at the bottom of the low-pressure rectification column 9 is vaporized by heat exchange with medium-pressure nitrogen gas in the condenser 8, and then extracted through the product oxygen collection line 61. The temperature of the liquefied oxygen is increased by heat exchange with the feed air in the heat exchanger 5, and the product oxygen gas is then collected from the product oxygen collection line 61 equipped with a product oxygen collection valve 61a.

[0033] In addition, the nitrogen gas (medium-pressure nitrogen) generated from the medium-pressure rectification column 7 is extracted from the medium-pressure nitrogen sampling line 62, heated through the heat exchanger 5, and then sampled as product medium-pressure nitrogen gas from the medium-pressure nitrogen sampling line 62 equipped with a medium-pressure nitrogen sampling valve 62a.

[0034] Furthermore, low-purity nitrogen (crude nitrogen) is extracted from the upper middle stage of the low-pressure rectification column 9 through a crude nitrogen extraction line 59 as an exhaust gas that serves as a fluid for cooling the feed air, passes through the subcooler 10, and is heated through the heat exchanger 5. After that, a portion of the exhaust gas is extracted through the crude nitrogen extraction line 59 equipped with a crude nitrogen extraction valve 59a, and the remainder passes through an off-gas extraction line 64 equipped with an off-gas extraction valve 64a and is used as a regeneration gas for the pretreatment facility 3.

[0035] Next, in the air liquefaction separation apparatus 100 of the above embodiment, a standby method during the operation stop (low-temperature standby state) of the apparatus will be described.

[0036] In the air liquefaction separation apparatus 100 of the above embodiment, in the low-temperature standby state, the liquefied gas held in the apparatus (inside the rectification column) evaporates, and the evaporated gas is introduced as a cold fluid to the cold end side of the heat exchanger 5. Therefore, as a counteracting warm fluid, dry gas is introduced to the warm end side of the heat exchanger 5 from the dry gas supply path 11 through the dry gas supply valve 11a. At this time, while monitoring the state of the heat exchanger 5, by adjusting the introduction amount of the warm fluid, the temperature distribution inside the heat exchanger 5 can be maintained in the same state as during the normal operation of the apparatus 100. Regarding the state monitoring of the heat exchanger, the temperature of the fluid introduced into and discharged from the heat exchanger 5 is measured by thermometers installed on the warm end side and the cold end side of the heat exchanger 5, or the surface temperature of the heat exchanger 5 is measured by one or more surface thermometers provided in the height direction of the heat exchanger 5, so that the state of the heat exchanger 5 and the temperature distribution of the heat exchanger 5 can be grasped.

[0037] Since the dry gas supplied through the dry gas supply path 11 is used only for maintaining the temperature distribution inside the heat exchanger 5, a supply amount much smaller than that during the normal operation of the apparatus is sufficient. Therefore, as a supply source of the dry gas, it is conceivable to supply raw air, nitrogen gas, etc. from another air liquefaction separation apparatus (not shown), or to supply it by a vaporization booster (not shown) of liquefied gas from another liquefied gas storage tank.

[0038] Also, in order to further reduce the supply amount of the dry gas, it is also possible to provide a compressor (not shown) for the cold fluid gas on the warm end side of the heat exchanger 5 and use the gas obtained by heating and compressing the cold fluid as the warm fluid.

[0039] In any case, since the supplied dry gas becomes low temperature at the outlet of the heat exchanger 5, it is discharged to the outside from the low-temperature part of the cold box 4 on the cold end side through the cold end side discharge path 12.

[0040] In the low-temperature standby state, the feed air and crude pure nitrogen can be released to the outside from cold-end release valves 12a and 12b branched off from the paths. Also, the product gases (medium-pressure nitrogen, low-pressure nitrogen, oxygen) and crude pure nitrogen can be released to the outside from hot-end release valves 13a to 13d branched off from the respective collection paths.

[0041] By doing as described above, the temperature distribution inside the heat exchanger 5 (for example, the hot end is at room temperature and the cold end is at -160°C) is maintained even in low-temperature standby mode, so even if a volume of gas close to the rated capacity is supplied to the heat exchanger 5 from the start of low-temperature startup of the air cryogenic separation unit 100 (when operation returns from low-temperature standby mode), a sudden change in the temperature inside the heat exchanger 5 can be prevented, and thermal fatigue inside the heat exchanger 5 can be minimized even with frequent startups and shutdowns. This makes it possible to significantly increase the number of low-temperature standby modes. Furthermore, the next startup can also be performed from a low-temperature state, significantly shortening the startup time.

[0042] If necessary, a dedicated air compressor for dry gas and a purifier for removing H2O and CO2 may be provided in the dry gas supply path 11 in order to purify the dry gas. After the cold fluid in the heat exchanger 5 reaches room temperature in the heat exchanger 5, it is released into the atmosphere through each release path, but the release amount must be adjusted to a smaller gas amount than the normal gas flow rate. Therefore, a separate dedicated release path may be provided for low-temperature standby to improve controllability.

[0043] In the above embodiment, the liquefaction separation of air is carried out using a two-column system consisting of a low-pressure rectification column 9 and a medium-pressure rectification column 7. However, depending on the conditions of the liquefaction separation, it is also possible to use rectification columns of a type other than those described above, such as a three-column system consisting of low-pressure, medium-pressure, and high-pressure columns or a two-column system consisting of low-pressure and high-pressure columns.

[0044] Furthermore, in the above embodiment, it is shown that the temperature inside the heat exchanger is maintained at the same state as during normal operation of the device. However, it is not necessary to maintain the temperature at the same state as during normal operation of the device. If a drop in temperature can be prevented by introducing dry gas into the hot end side so that the heat exchanger is not damaged by thermal stress, the effect of the present invention can be fully achieved. [Explanation of symbols]

[0045] 1 Air filter, 2 Feed air compressor, 3 Pretreatment equipment, 4 Cold box, 5 Heat exchanger, 6 Expansion turbine, 7 Medium pressure rectification column, 8 Condenser reboiler (condenser), 9 Low pressure rectification column, 10 Subcooler, 11 Dry gas supply line, 12 Cold end heat exchanger discharge line, 13 Hot end heat exchanger discharge line, 51 Feed air introduction line, 51a Feed air introduction valve, 52 Low temperature feed air line, 53 Liquefied air discharge line, 54 Liquefied air introduction line, 54a Pressure reducing valve, 57 Liquefied nitrogen line, 57a ··Pressure reducing valve, 58··Low pressure nitrogen discharge line, 58a··Low pressure nitrogen sampling valve, 59··Crude pure nitrogen sampling line, 59a··Crude pure nitrogen sampling valve, 60··Liquefied oxygen sampling line, 60a··Liquefied oxygen sampling valve, 61··Product oxygen sampling line, 61a··Product oxygen sampling valve, 62··Medium pressure nitrogen sampling line, 62a··Medium pressure nitrogen sampling valve, 63··Liquefied nitrogen introduction line, 63a··Liquefied nitrogen introduction valve, 64··Off gas discharge line, 64a··Off gas discharge valve, 65··Path for expansion turbine, 66··Post-expansion path, 100··Cryogenic air separation unit, 100a··Conventional Cryogenic air separation unit

Claims

1. A heat exchanger for cooling raw air from which water and carbon dioxide have been excluded, a rectification column for liquefying and rectifying the separation of the raw air, a cold box housing the heat exchanger and the rectification column, a supply path for dry gas at normal temperature in the warm fluid path on the warm end side of the heat exchanger, and a discharge path for discharging the dry gas heat-exchanged by the heat exchanger from the cold box to the outside on the cold end side of the heat exchanger A standby method for an air liquefaction separation device comprising: In the standby state when the operation of the device is stopped, introducing the gas generated by the evaporation of the liquefied gas in the rectification column to the cold end side of the heat exchanger, introducing the dry gas to the warm end side of the heat exchanger, adjusting the introduction amount of the dry gas by monitoring at least one of the temperature of the fluid derived from the warm end side of the heat exchanger or the temperature of at least one surface thermometer installed on the warm end side of the heat exchanger. A standby method for an air liquefaction separation device.

2. The standby method for an air liquefaction separation device according to Claim 1, characterized in that the temperature distribution inside the heat exchanger is maintained in the same state as during normal operation of the device.

3. The supply source of the dry gas is obtained by vaporizing and pressure-feeding the liquefied gas in the liquefied gas storage tank The standby method for an air liquefaction separation device according to Claim 1 or 2, characterized in that.

4. The standby method for an air liquefaction separation device according to Claim 1 or 2, characterized in that a compressed cold fluid gas flowing from the cold end side to the warm end side of the heat exchanger is introduced into the supply path of the dry gas. ​

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