JPH0371633B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to the removal of carbon dioxide from a reversible heat exchanger during startup of an air separation apparatus employing an aluminum plate type reversible heat exchanger.

[Background of the Invention] In air separation equipment, the compressed feed air to be separated contains easily condensable impurities such as moisture and carbon dioxide, and during the process of liquefying the feed air, these impurities precipitate and solidify. This has the disadvantage that the raw air passage is blocked. For this reason, the air separation device has a structure in which corrugated fins and flat plates are alternately laminated, which have the characteristic of being able to cool the feed air and simultaneously remove these precipitated impurities by sublimating them into the separated product gas.
An aluminum plate type reversible heat exchanger (hereinafter simply referred to as a reversible heat exchanger) that is brazed and configured to have a large number of minute passages is employed.

The function of such a reversible heat exchanger is to exchange heat between the compressed raw material air to be separated and the separated product gas, and to convert the raw material air sublimated to about 5 kg/cm ² G from room temperature to around the saturation temperature (-172 At the same time, the easily condensable impurities in the feed air are solidified and adhered to the fin surface in the heat exchanger, and after a certain period of time, the passage is switched and the low-pressure separated gas is flowed in the opposite direction. It exchanges heat with the raw material air to warm it up to outside temperature, and at the same time, impurities that have solidified and adhered to it are removed by sublimation into the same gas, making long-term continuous operation possible.

However, since reversible heat exchangers generally have a structure in which multiple blocks are arranged in parallel and each passage is connected by a header, the reversible heat exchanger is The flow rates of feed air and separated product gas flowing through the heat exchanger are significantly reduced. Therefore, the flow causes a drift phenomenon, and as a result, the flow resistance of the reversible heat exchanger increases.

In order to prevent such a flow imbalance at startup, the reversible heat exchanger 2 as shown in Fig.
A communication passage 1 that connects a passage 5 for low-temperature compressed air sent to the rectification column 6 in the separation process and a passage 13 for the separated product gas returned from the rectification column 6 to the reversible heat exchanger 2.
6 and a shutoff valve 17 are provided, and at startup, excess air, excluding the air supplied to the expander 9 for cold generation, is sent to the separated product gas passage 13 via the communication passage 16, and is throttled and expanded by the shutoff valve 17. let me go back,
Conventionally, a method has been used to flow a flow rate close to the rated air volume even during startup.

In air separation equipment that uses a reversible heat exchanger, in order to precipitate and sublimate easily condensable impurities such as moisture and carbon dioxide in the feed air using the reversible heat exchanger, the temperature between the feed air and the separated product gas must be adjusted. It is necessary to keep the difference within a certain value.

In general, among the impurities in the feed air, water sublimes easily, so it rarely causes trouble during operation, and the majority of clogging in air separation equipment is caused by trace amounts of carbon dioxide gas. being provoked. Figure 3 shows the temperature difference between the carbon dioxide removal limit line in the reversible heat exchanger and the separated product gas with respect to the raw air temperature in the reversible heat exchanger, and the reversible heat exchanger temperature difference curve in the figure. is the curve a-
As shown in b-c, being always below the carbon dioxide removal limit line is an absolute condition for completely sublimating and removing carbon dioxide gas in the reversible heat exchanger.

However, in the conventional operation method, the communication passage from the low-temperature compressed air passage to the separated product gas passage is
Since there was only one system as shown in Fig. 2, when the temperature of the reversible heat exchanger 2 from room temperature to the cooling stage and the temperature of the low-temperature compressed air passage 5 reached the constant operating temperature (-172°C), the communication passage 16 Due to the throttle expansion of the shutoff valve 17, the so-called Juurt-Thomson effect, approximately 10
The air whose temperature has dropped in °C joins the separated gas passage 13, and at the cold end of the reversible heat exchanger 2, the temperature difference between the raw material air and the separated product gas is as shown in a-b-c in Fig. 3, and the carbonic acid Carbon dioxide gas cannot be completely removed by sublimation in the range of part A from the intersection point d to e with the gas removal limit line.

Until now, since the amount of carbon dioxide in this range was very small, it was unavoidable to operate in an incomplete state at startup, but recently there has been a trend towards larger air separation equipment. Even a small trouble caused by an operational error in such an incomplete state during start-up can have a very large effect in addition to stopping the equipment, and more perfect start-up operations are now required.

Unexamined Japanese Patent Publication No. 53-1989 as a known technique to solve this problem.
There is an invention disclosed in No. 118277. The gist of this content is that in the above-mentioned known air separation equipment in which a connecting pipe is provided from the low-temperature compressed air passage to the separated product gas passage in order to prevent drift in the reversible heat exchanger at the time of startup, A communication passage is provided to the separation product gas passage at the outlet of one rectification column, and when the compressed air temperature at the outlet of the reversible heat exchanger falls below a specified temperature, the communication passage is switched and used. The temperature of the cooled gas whose temperature has decreased due to the throttle expansion of the shutoff valve is recovered by a supercooler and a liquefier, and the temperature of the separated gas returned to the reversible heat exchanger is kept within the carbon dioxide removal limit line shown in Figure 3. The other one is to provide the above-mentioned communication passage and another communication passage from the reheated air outlet passage of the reversible heat exchanger to the separated product gas passage, and to restrict the shutoff valve of the known communication passage. By mixing the air whose temperature has decreased due to expansion with the high-temperature air from the reheated air outlet passage, the temperature of the separated product gas that returns to the reversible heat exchanger is brought within the carbon dioxide removal limit line in Figure 2, and the process is started. The carbon dioxide gas is completely sublimated and removed using a reversible heat exchanger.

However, this technique requires two passages to prevent drifting, and switching between them requires monitoring both flow rate and temperature, which requires advanced technology.

[Purpose of the invention]

In order to meet such demands, the purpose of the present invention is to eliminate the incomplete part of carbon dioxide removal in the reversible heat exchanger at startup, facilitate the startup operation of the air separation device, and reduce the amount of carbon dioxide gas at startup. The goal is to completely eliminate problems.

[Summary of the invention]

In order to prevent uneven flow in the reversible heat exchanger at startup, the present invention provides a communication passage and a shutoff valve connecting the low-temperature compressed air passage and the passage from the expander outlet to the three-way valve, and compresses the air at the outlet of the reversible heat exchanger. When the air temperature falls below the specified temperature, by switching the three-way valve, the cold air, whose temperature has decreased due to the throttle expansion of the shutoff valve, is collected through the communication passage through the supercooler and liquefier, and is used for reversible heat exchange. The temperature of the separated gas returning to the vessel is
The carbon dioxide removal limit line shown in the figure is kept within the carbon dioxide removal limit line, and the reversible heat exchanger completely sublimates and removes carbon dioxide even during startup.

[Embodiments of the invention]

Embodiments of the present invention will be described in detail below with reference to FIG. The raw material air compressed to about 5 kg/cm ² G and cooled to room temperature is led to the reversible heat exchanger 2 through the passage 1, and by exchanging heat with the separated product gas from the rectification column 6, Near the saturation temperature of compressed air (−172
℃). At this time, easily condensable impurities such as moisture and carbon dioxide in the raw air are deposited on the passage 1 in the reversible heat exchanger 2. On the other hand, the separated product gas passes through the passage 3, returns to room temperature in the reversible heat exchanger 2, and is taken out of the apparatus. On this occasion,
Moisture and carbon dioxide deposited on the passage 3 in the reversible heat exchanger 2 are removed by sublimation, and taken out of the apparatus together with the separated product gas. The feed air passage 1 and the separation product gas passage 3 are connected periodically (every 15 to 20 minutes).
is automatically switched by a switching valve (not shown) and check valve 4. During steady-state operation, the feed air that has been cooled to near the saturation temperature in the reversible heat exchanger 2 and from which impurities have been removed is mostly guided to the rectification column 6 through the low-temperature compressed air passage 5 . The remaining low-temperature compressed air passes through the passage 7 and becomes reheated air in the reversible heat exchanger 2, where it is heated to about -120°C and is combined with the low-temperature compressed air that has passed through the passage 8. Warm,
It is guided to the expander 9 at about -150°C, and the equipment is cooled by adiabatic expansion.

The air adiabatically expanded by the expander 9 passes through passages 10 and 12.
It is blown into the rectification column 6 through b. On the other hand, the gas separated and produced in the rectification column 6 is transferred to the separated product gas passage 1.
3 and returned to the reversible heat exchanger 2. A supercooler 14 and a liquefier 15 are provided in the middle of the separated product gas passage 13, and the reversible heat exchanger 2 converts the separated product gas from the saturation temperature (approximately -193°C) at the outlet of the rectification column 6 to Temperature at which carbon dioxide gas can be removed by sublimation (approximately -177
℃).

However, since there is no low-temperature separated product gas when the device is started, the temperature of the air in the low-temperature compressed air passage 5 is close to room temperature, and impurities in the feed air are brought into the device as they are. All of it is guided to the expander 9 without being blown into the rectification column 6, and is used as a cooling source for the equipment.

Since the air that exits the expander 9 similarly contains impurities at the time of startup, it is guided by the three-way valve 11 to the separated product gas passage 13 through the bypass passage 12a at the time of startup, and is passed through the reversible heat exchanger 2. used for cooling. Generally, the capacity of the expander 9 of the air separation device is:
Since the processing capacity is only 15 to 30% of the raw air, the flow rate through the reversible heat exchanger 2 is extremely small at startup, causing flow imbalance due to reduced operation, and impurity removal in the reversible heat exchanger 2 becomes difficult. becomes extremely difficult.

For this reason, a communication passage 16 and a shutoff valve 17 are provided so that a flow rate close to that during steady-state operation flows through the reversible heat exchanger 2 even at the time of startup, and a portion of the low-temperature air is bypassed to the outlet passage 10 of the expander 9.

As mentioned above, when the temperature is high at startup, the air leaving the expander 9 contains impurities and cannot be blown into the rectification column 6, so the three-way valve 11 is placed on the side of the bypass passage 12a at startup. Passageway 1
The air for preventing unbalanced flow from the expander 6 is combined with the air from the expander 9, and is passed through the three-way valve 11, the bypass passage 12a,
It is returned to the reversible heat exchanger 2 through the separated product gas passage 13.

In this state, cooling progresses and when the temperature of the low-temperature compressed air passage 5 reaches approximately -165°C, most of the impurities in the low-temperature compressed air adhere to the passage of the reversible heat exchanger 2 and become clean air, which is blown into the rectification column 6. It becomes possible to include Note that the fluid temperature in the separated product gas passage 13 at this time is about -170°C, which is not below the specified value (-177°C) and is at the point where sublimation removal is possible.

However, after this, cooling progresses further and the fluid temperature in the separated product gas passage 13 reaches the temperature at which sublimation removal is possible (-177°C).
If this happens, close the three-way valve 1 to prevent the drop
1 will be used on the rectification column 6 side. By this operation, the low-temperature combined air from the communication passage 16 and the expander 9 is gradually switched from the bypass passage 12a side to the passage 12b side, passes through the rectification column 6 and passage 13, and then passes through the supercooler 14 and the liquefier 15. The temperature of the separated product gas passage 13, which is recovered by heat and returns to the reversible heat exchanger 2, can be maintained at a specified value.

Note that the operation of the three-way valve 11 is a necessary operation even in the prior art when starting up a plant, and is not a special operation for the present invention.

〔Effect of the invention〕

As described above, according to the present invention, when an air separation device that has conventionally adopted an aluminum plate type reversible heat exchanger is started up, the reversible heat exchanger cannot completely sublimate carbon dioxide gas, and as a result, the carbon dioxide inside the device is The problem of dry ice brought into the air separation system can be solved by using one starting passage, and the starting operation of the air separation device can be performed extremely smoothly.

[Brief explanation of drawings]

Fig. 1 is a system diagram of an air separation device showing an embodiment of the present invention, Fig. 2 is a system diagram of an air separation device employing a conventional aluminum plate type reversible heat exchanger, and Fig.
The figure shows an explanatory diagram of the temperature difference curve and carbon dioxide removal limit line of a reversible heat exchanger. 1, 3, 8, 10... passage, 2... reversible heat exchanger, 4... check valve, 5... low temperature compressed air passage, 6
... Rectifier, 7... Reheated air outlet passage, 9... Expander, 11... Three-way valve, 12a... Bypass passage, 12b... Rectification tower blowing passage, 13... Separated product gas passage, 14... supercooler, 15... liquefier, 16... communication passage, 17... closing valve.

Claims (1)

[Scope of Claims] 1. An aluminum plate type reversible heat exchanger that is switched and used to cool compressed raw air to be separated and remove impurities such as moisture and carbon dioxide, and the reversible heat exchanger. A rectification column that introduces the raw air that has passed through the column through a low-temperature compressed air passage and separates it by rectification, and a temperature at which the gas separated and purified in the rectification column is heated from the saturation temperature to a temperature at which carbon dioxide gas can be sublimated and removed by a reversible heat exchanger. a heating means for raising the temperature to a temperature of In an air separation device equipped with a three-way valve and respective passages, the low-temperature compressed air passage for feed air supplied from the reversible heat exchanger to the rectification column; 1. An air separation device characterized in that a communication passage is provided that connects a passage from an expander outlet to a three-way valve, and a shutoff valve is provided in the communication passage.