JPS6113152B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to removing carbon dioxide from a reversible heat exchanger at the time of startup of an air separation apparatus employing an aluminum plate type reversible heat exchanger.

In air separation equipment, easily condensable impurities such as moisture and carbon dioxide are contained in the compressed raw air to be separated, and in the process of liquefying the raw air,
Since there is a defect in which these impurities precipitate and solidify, clogging the feed air passage, generally, these precipitated impurities can be removed by sublimation into the separated product gas at the same time as the feed air is cooled.
An aluminum plate type reversible heat exchanger (hereinafter simply referred to as a reversible heat exchanger) is used, which has a structure in which corrugated fins and flat plates are alternately laminated and soldered to have a large number of minute passages.

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 pressurized to approximately 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 side by side and each passage is connected by a header, during reduced air volume operation below the rated air volume, that is, in the process of transitioning from the start of the device to the settling operation state, The flow rates of feed air and separated product gas flowing through the reversible heat exchanger are significantly reduced.
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 must be installed as shown in Figure 1.
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 2 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. 1, when the cooling stage of the reversible heat exchanger 2 from room temperature 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-be-e in Fig. 2, 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.

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.

The present invention provides an air separation device in which a connecting pipe is provided from the low-temperature compressed air passage to the separated product gas passage in order to prevent uneven flow 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 the two rectification towers, 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 closed. The temperature of the cooled gas, which has been reduced by the throttle expansion of the 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 2. 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 expansion of the shutoff valve of the known communication passage. By mixing the air whose temperature has decreased with the high temperature air of 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 temperature is reduced at startup. In this case, carbon dioxide gas is completely sublimated and removed using a reversible heat exchanger.

Hereinafter, embodiments of the present invention will be described in detail with reference to FIGS. 3 and 4.

First, an embodiment of the present invention will be described 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 raw material air passage 1 and the separation product gas passage 3 are automatically switched periodically (every 15 to 20 minutes) by a switching valve (not shown) and a check valve 4. During steady state operation, reversible heat exchanger 2
The raw air, which has been cooled to near the saturation temperature and from which impurities have been removed, is mostly led to a rectification column 6 through a low-temperature compressed air passage 5. The remaining low-temperature compressed air passes through passage 7 and becomes reheated air in reversible heat exchanger 2, where it is heated to about -120°C and joins with the low-temperature compressed air that has passed through passage 8. death,
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 is blown into the rectification column 6 through a passage 10. On the other hand, the gas separated and produced in the rectification column 6 is returned to the reversible heat exchanger 2 through the separated product gas passage 13. 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 saturated temperature (approximately -193°C) of the separated product gas at the outlet of the rectification column 6 into a supercooler 14 and a liquefier 15. The temperature is raised to a temperature (approximately -177°C) that allows carbon dioxide gas to be removed by sublimation.

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 12 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. Therefore, even at the time of startup, the above-mentioned known communication passage 16 is provided so that a flow rate similar to that during steady-state operation flows through the reversible heat exchanger 2, and a part of the low-temperature compressed air is mixed into the separated product gas passage 13 by bypass. has been taken so far.

However, this method has no problem when the temperature of the low-temperature compressed air passage 5 is relatively high, but as the reversible heat exchanger 2 is cooled, the temperature of the low-temperature compressed air passage 5 also decreases, and during steady-state operation. When the compressed air drops to near the saturation temperature (-172°C), it is throttled and expanded by the shutoff valve 17, and the air whose temperature has decreased (approximately -182°C) due to the Juul-Thomson effect is mixed into the separated product gas passage 13. , the temperature of the separated product gas returning to the reversible heat exchanger 2 becomes below the specified value (-177°C), and as shown at point e in Figure 2, the carbon dioxide gas is completely removed at the cold end of the reversible heat exchanger 2. sublimation cannot be removed.

The present invention provides a low temperature compressed air passage 13 and a supercooler 14.
A communication passage 18 and a shutoff valve 19 are provided which connect the separated product gas passage 13 on the inlet side, and the low-temperature compressed air passage 1
3 becomes below a certain temperature, the low-temperature compressed air that had been bypassed through the communication passage 16 and the shutoff valve 17 is transferred to the communication passage 18 and the shutoff valve 1.
9 side, the temperature of the cold whose temperature has decreased due to the throttle expansion of the shutoff valve 19 is recovered by the subcooler 14 and the liquefier 15, and the separated product gas passage 13 returns to the reversible heat exchanger 2.
The temperature is returned to the specified value (-177℃).

In the present invention, two communication passages 16 and 1 are provided from the low-temperature compressed air passage 5 to the separated product gas passage 13.
However, if the supercooler 14 and the liquefier 15 are heat exchangers that will not be clogged even when air containing impurities flows during startup, the communication passage 16 can be omitted and the communication passage A similar effect can be obtained with just one system of 18.

However, if the supercooler 14 and liquefier 15 are of the aluminum plate heat exchanger type, the low-temperature compressed air passage 5 at startup contains moisture and carbon dioxide gas, so the communication passage 18 Bypassing the air is undesirable because it results in clogging of the heat exchangers 14 and 15. In such a case, as shown in Figure 3, connecting passage 1 of the two series
It is necessary to provide 6 and 18.

Further, as an embodiment of the present invention, the communication passage 18
is connected to the inlet side of the supercooler 14 of the separated product gas passage 13, but the same effect can be obtained even if it is connected between the supercooler 14 and the liquefier 15.

Next, another embodiment of the present invention will be explained with reference to FIG. In order to prevent the flow rate imbalance of the reversible heat exchanger 2 at the time of startup, a communication passage 16 and a shutoff valve 1 are provided from the low temperature compressed air passage 5 to the separated product gas passage 13.
7 is provided as in the embodiment shown in FIG. 3 above. At startup, cooling of the reversible heat exchanger 2 progresses, and the separated product gas passage 1 is transferred from the low-temperature compressed air passage 5.
Air whose temperature has decreased due to expansion and restriction through the shutoff valve 17 enters the reversible heat exchanger 2 through the communication passage 16.
When carbon dioxide gas cannot be removed by sublimation at the cold end of
Through the communication passage 20 connecting the reheated air outlet passage 7 of the reversible heat exchanger 2 of the present invention and the separated product gas passage 13, the necessary amount of reheated air with a high temperature (approximately -120°C) is passed through the shutoff valve 21. Mix it into the separated gas,
The temperature of the separated gas returning to the reversible heat exchanger 2 is adjusted to a temperature at which carbon dioxide gas can be sublimed and removed.

In the embodiment shown in FIG. 4, the reheated air of the reversible heat exchanger 2 is taken out from the middle, but the reheated air is taken out from the warm end of the reversible heat exchanger 2, and the reheated air brought to room temperature is passed through the passage 20. A similar effect can be obtained even if a bypass is mixed.

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 This eliminates the problem of dry ice brought into the system, and allows the air separation device to be started up very smoothly.

[Brief explanation of the drawing]

Figure 1 is a system diagram of an air separation device that uses a conventional aluminum plate type reversible heat exchanger, Figure 2 is an explanatory diagram of the temperature difference curve and carbon dioxide removal limit line of the reversible heat exchanger, and Figure 3 is a diagram of the present invention. FIG. 4 is a system diagram of an air separation device showing one embodiment of the invention. FIG. 4 is a system diagram of an air separation device showing another embodiment of the invention. 1, 3, 8, 10... passage, 2... reversible heat exchanger, 4... check valve, 5... low temperature compressed air passage, 6
... Rectification column, 7 ... Reheated air outlet passage, 9 ... Expander, 11 ... Three-way valve, 12 ... Bypass circuit,
13...Separated product gas passage, 14...Supercooler,
15...Liquifier, 16, 18, 20... Communication passage, 17, 19, 21... Closing valve.

Claims (1)

[Scope of Claims] 1. In an air separation device that employs an aluminum plate type reversible heat exchanger to cool compressed raw air to be separated and remove impurities such as moisture and carbon dioxide, A communication passage connecting the passage of low-temperature compressed air sent from the rectifier to the rectification column in the separation process and the passage of the separated product gas returned from the rectification column to the reversible heat exchanger from the liquefier outlet to the reversible heat exchanger. An air separation device characterized in that a valve is provided, and a communication passage connecting the passage for the low-temperature compressed air and the supercooler or liquefier inlet passage for the separated product gas and a shutoff valve are provided. 2. An aluminum plate reversible heat exchanger is used to cool the compressed raw air to be separated and remove impurities such as moisture and carbon dioxide.
An air separation device equipped with a communication passage and a shutoff valve that connects the passage of low-temperature compressed air sent from the reversible heat exchanger to the rectification column in the separation process and the passage of the separated product gas returned from the rectification column to the reversible heat exchanger. An air separation device characterized in that a communication passage connecting the reheated air outlet passage of the reversible heat exchanger and the separation product gas passage and a shutoff valve are provided.