JPS6323468B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a reversible heat exchanger that removes moisture and carbon dioxide from feed air by switching channels, which is used in an air liquefaction cryogenic separation device. The present invention relates to a flow path switching method for a vessel.

[Background of the invention]

Conventionally, in air separation equipment that uses a reversible heat exchanger that sublimates and removes moisture and carbon dioxide from the feed air by switching the flow path, separation is performed between the feed air flow path of the reversible heat exchanger and the rectification column. As a method for switching the waste gas flow path, a switching device is provided that includes two air-side switching valves, two waste gas-side switching valves, and two check valves that operate together. The flow path is being switched. This switching device employs a method in which several sets are installed together, with one set as a unit, depending on the capacity of the air separation device.

Furthermore, one set of switching devices is provided with at least one reversible heat exchanger or a plurality of reversible heat exchangers depending on the size of the device.

In order to prevent the flow path from being blocked by moisture and carbon dioxide in the feed air, the switching flow path of a reversible heat exchanger generally switches between the feed air and the waste gas separated in the rectification column every 10 to 20 minutes. By switching the flow through the switching channel, the ice or dry ice that has condensed on the heat transfer surface of the switching channel is sublimated and removed by the waste gas. The conventional flow path switching method will be explained in more detail with reference to FIG.

After the raw air is sent to compressor 1 and pressurized,
It is directly cooled by the cooling water sent from the water pump 4 in the water washing cooling tower 3 via the conduit 2, and then from the conduit 5 to the flow path switching type reversible heat exchanger 8 with an air side switching valve 6A,
6B.

When the raw air introduced into the reversible heat exchanger 8 passes through one of the switching channels 9A and 9B in the heat exchanger, it passes through the other switching channel and separates the waste air. It is cooled from a temperature of about 25°C to -172°C by gas and product gas through other non-switched flow paths and reheated air. On this occasion,
Moisture and carbon dioxide contained in the raw air are condensed and deposited on the heat transfer surfaces of the switching channels 9A and 9B as they cool. Switching channels 9A and 9B are 10 to 20
It is automatically switched every minute via a switching device by switching valves 6A, 6B, 7A, 7B and check valves 10A, 10B.

By switching the flow path, the moisture and carbon dioxide condensation deposits brought in by the feed air are sublimated when the waste gas flows due to the difference in vapor pressure of the impurities present in the feed air and waste gas. The heat transfer surface of the flow path is restored to its original clean surface by the next switching cycle. This periodic flow switching allows the reversible heat exchanger to operate continuously for a long period of one year or more without being blocked by impurities.

The feed air cooled and purified by the reversible heat exchanger 8 passes through the conduit 11 through the check valves 10A and 10B, and most of it is blown into the lower part of the lower column 13 of the rectification column through the conduit 12. A portion of the remaining raw material air is branched from the conduit 11 through the conduit 14 and transferred to the reversible heat exchanger 8.
The reheated air is guided from the conduit 15 to the expander 16 through the reheated air flow path and its bypass flow path, where it performs external work by adiabatic expansion and becomes low temperature itself to compensate for the coldness of the equipment. There is. The expanded air is blown into the middle part of the upper column 18 of the rectification column through the conduit 17. The raw air blown into the lower column 13 is separated into liquid nitrogen in the upper part and liquid air in the lower part by rectification separation. The separated liquid nitrogen passes from the upper part of the lower column 13 through a conduit 19, is reduced in pressure to the pressure of the upper column 18 by an expansion valve 20, and is supplied as a reflux liquid to the upper column 18 through a conduit 21. The liquid air is passed from the lower part of the lower column 13 through the conduit 22 and the heat exchanger 23 to the upper column 1.
The liquid is subcooled by heat exchange with the separated gas separated in step 8, and is reduced in pressure through a conduit 24 to the pressure of the upper column 18 by an expansion valve 25, and is supplied as a reflux liquid to the middle part of the upper column 18 through a conduit 26.

In the upper column 18, oxygen is stored in the lower part of the upper column 18 as liquid oxygen by rectification separation. Nitrogen is removed from the top of the upper column 18. Product oxygen is supplied to the main condenser 35 provided at the lower part of the upper column 18, and then to the lower column 1.
A part of the rising gas obtained by evaporating the liquid oxygen with nitrogen in step 3 is taken out from the conduit 27, passes through the heat exchanger 23, and is passed through the conduit 28 through a flow path without switching of the reversible heat exchanger 8, where it is heat exchanged with the raw air. When the temperature reaches room temperature, the conduit 29
More oxygen is sent to the end where it is used. Nitrogen gas taken out from the upper part of the upper tower 18 passes through the heat exchanger 23 through a conduit 30, and then passes through the check valves 10A and 10B through a conduit 31.
through the switching channels 9A, 9B opposite to the feed air of the reversible heat exchanger 8, exchanging heat with the feed air to raise the temperature, and adhering to the heat transfer surfaces of the switching channels 9A, 9B. Impurities such as moisture and carbon dioxide are removed by sublimation, and the waste gas is discharged into the atmosphere from the conduit 32 through either one of the waste gas side switching valves 7A and 7B. In addition, when a part of nitrogen is taken out as a product gas, a flow path without switching can be provided in the reversible heat exchanger 8, and the nitrogen can be taken out in a branched manner.

In order to sublimate and remove moisture and carbon dioxide in the feed air with waste gas in the reversible heat exchanger 8, a certain allowable temperature is required in each zone of the feed air and waste gas switching channels 9A and 9B during heat exchange. It is necessary to keep it within the difference. If this temperature difference is not maintained, ice and dry ice will accumulate in the switching flow path without being sublimated due to the lack of partial pressure of impurities in the coal waste gas, and over a long period of time ice and dry ice will accumulate. This results in the entire switching flow path being blocked. For this reason, conventionally, when a plurality of reversible heat exchangers 8 are arranged, temperature control of the heat exchangers has been difficult, and many improvements have been made so far. However, all of these methods are based on the concept of equalizing the temperature difference among the plurality of reversible heat exchangers 8 as a whole, or the idea of equalizing the temperature difference between the reversible heat exchangers 8 and the switching flow paths 9A and 9B of the reversible heat exchanger 8 at the time of flow path switching. The high pressure raw material air accumulated in either one of the waste gas side switching valves 7A,
A bypass switching valve 33 is provided in the switching flow paths 9A and 9B to reduce the purge loss of raw material air released into the atmosphere through either one of the waste gas side switching valves 7B and 7B.
Before discharging from A, 7B, the bypass switching valve 33 is opened, and the other switching valves 6A, 6B, 7A, 7B are
The idea is to completely close all of the switching valves 6A and 9B and recover the high-pressure feed air that has entered one of the switching channels 9A and 9B of the reversible heat exchanger 8 to the other one. , 6B, 7A, and 7B are instantaneously switched, moisture and carbon dioxide are brought into the reversible heat exchanger 8 due to large flow rate fluctuations.
Until now, no idea had been proposed to essentially reduce it.

For this reason, the heat transfer area of the conventional reversible heat exchanger 8 not only needs to be considerably larger than the required heat transfer area obtained from chemical engineering calculations, but also due to large flow fluctuations in a short period of time during switching. The introduced moisture and carbon dioxide gas were not completely sublimated, causing adverse effects such as the switching channels 9A and 9B gradually becoming clogged during long-term operation.

[Purpose of the invention]

The object of the present invention is to improve such drawbacks, to control the amount of feed air introduced into the reversible heat exchanger to be almost constant even when switching channels, and to completely remove impurities by sublimation. An object of the present invention is to provide a flow path switching method for a type heat exchanger.

[Summary of the invention]

In conventional reversible heat exchangers, flow path switching was performed by rapidly closing either the air-side switching valve or the waste gas-side switching valve and quickly opening the other. is the amount of raw material air that flows into the reversible heat exchanger through the switching valve by operating the opening degree of the air side switching valve in two or more steps to introduce the raw material air into the reversible heat exchanger. is controlled to an amount close to that during steady state even when switching.

Generally, an air separation device is designed to minimize the flow path resistance of each part in order to minimize power consumption. For this reason, the flow path resistance of the switching valve is usually around 0.01Kg/ ^cm2 ,
Generally, the gas flow velocity is limited to 10 to 20 m/sec. On the other hand, the switching speed of the switching valve must be 1 to 2 seconds. If the opening/closing speed is too slow, the air discharged from the compressor will accumulate in the flow path from the outlet of the compressor to the air side switching valve on the raw air side, causing the pressure to rise rapidly. If the pressure rises too much, a surging phenomenon may occur if a centrifugal blower is used for the compressor, and there is a risk of damage to the machine.

The same applies to the waste gas side switching valve; if the opening/closing speed is slow, gas will stagnate in the flow path from the upper tower to the waste gas side switching valve, increasing the pressure and reducing the amount of gas rising in the rectification tower. Therefore, it becomes impossible to maintain the purity of the product gas. Therefore, it is necessary to increase the opening and closing speed of the switching valve to the full mechanical capacity of the valve.

As described above, there are special conditions for flow path switching in a reversible heat exchanger, and for this reason, conventionally, the idea of reducing the effect of switching on the rectification column side has been prioritized.
Large fluctuations in the amount of air introduced into the reversible heat exchanger during switching were considered to be a phenomenon that cannot be avoided as a characteristic of the reversible heat exchanger.

In such a conventional method, when the raw air is flowed through the air side switching valves 6A, 6B to the switching channels 9A, 9B, the pressure in the conduit 5 and the switching channel 9 during steady operation are
The pressure difference between the air flow paths of either A or 9B is
There is only the flow resistance of the air side switching valves 6A and 6B,
The raw material air flows due to this resistance difference of approximately 0.01Kg/cm ² . However, when switching the flow path, high-pressure raw material air is passed through the low-pressure waste gas flow path, so the resistance difference of the switching valve at this time is about 5 kg/cm ² . Due to this large pressure difference, almost 10 times as much raw material air as in the steady state momentarily flows into the switching channels 9A, 9B of the reversible heat exchanger 8 through the air side switching valves 6A, 6B. When such a large amount of air passes through the reversible heat exchanger 8, the heat transfer area becomes insufficient and the air passes through without being completely cooled. is brought in and accumulated without being completely sublimated.

The present invention analyzes conventional flow path switching methods and compares actual operation data to achieve a constant flow of feed air to reversible heat exchangers during flow path switching, which was previously thought to be impossible. The amount is controlled to be approximately the same as always.

In the present invention, regarding the flow rate of raw material air passing through the air side switching valve immediately after switching the flow path, the pressure in the waste gas flow path before switching is increased by the flow of raw material air through the switching valve, and the switching valve Focusing on the fact that the flow rate is proportional to the diameter of the switching valve until the pressure on the secondary side reaches the critical pressure (approximately 1/2 of the inlet pressure), we determined the opening degree of the switching valve so that the secondary pressure reaches the critical pressure. The valve opening is locked for about 2 seconds until the valve is switched, and then the valve is opened to full open. If the opening is set just enough for the amount to flow, the discharge pressure of the compressor will not increase while one side of the switching flow path is pressurized, and even if the pressurization time is longer than before, the pressure will actually flow to the rectification column side. It was confirmed that there was no impact.

Another method to achieve the same effect as locking the opening of the switching valve in the middle is to add a bypass slave switching valve to the conventional air-side switching valve to allow the discharge amount of the compressor to flow. However, there is a method in which the slave valve switching valve is suddenly opened at the same time as the switching command is issued, and then the conventional air side switching valve is opened after the time period when the secondary pressure reaches the critical pressure has elapsed.

[Embodiments of the invention]

Hereinafter, an example of the case where the flow path switching method according to the present invention is applied to a total low pressure air separation device that uses one reversible heat exchanger and one set of switching valves and check valves and extracts oxygen as a product gas. An example will be explained with reference to FIG.

In FIG. 2, the same parts as in FIG. 1 are indicated by the same symbols, and their explanation will be omitted.

Reference numerals 6Aa and 6Bb are bypass slave switching valves attached to the air side switching valves 6A and 6B.
Assuming that the waste gas is flowing to the 9B side, at this time,
The air side switching valves 6A, 6Aa and the waste gas side switching valve 7B are fully open, and the air side switching valves 6B, 6Bb and the waste gas side switching valve 7A are fully closed.

However, due to the switching command of the reversible heat exchanger,
The switching valves are temporarily fully closed to prevent air from being released from the waste gas switching valve 7B, and immediately after being fully closed, the switching valves shift to the next valve opening degree. That is, the bypass slave switching valve 6Bb of the air side switching valve 6B and the waste gas side switching valve 7A are fully opened, and the other valves remain fully closed. The bypass slave valve switching valve 6Bb is designed to flow into the low-pressure switching flow path an amount greater than the amount of raw air discharged from the compressor 1 when fully open, so it prevents the pressure increase in the discharge flow path of the compressor 1 from increasing. In addition, the amount of raw air flowing into the switching flow path 9B of the reversible heat exchanger 8 can be suppressed to the steady flow rate. When the pressure in the switching flow path 9B of the reversible heat exchanger 8 rises and reaches approximately 1/2 of the discharge pressure of the compressor 1, the air side switching valve 6B starts to open, and the bypass slave switching valve 6Bb starts to open. Since the difference becomes too small and the flow rate is insufficient, this amount is flowed by the air side switching valve 6B. When the pressure in the switching channel 9B rises and becomes higher than the pressure in the lower column 13 of the rectifying column, it is cooled by the reversible heat exchanger 8, and moisture and carbon dioxide condense and adhere to the heat transfer surface of the switching channel 9B. , the purified raw material air passes through the check valve 10B to the lower column 13 and the expander 1.
6 and returns to the original steady state.

This switching cycle is performed every 10 to 20 minutes, and continuous operation is performed for more than one year.

〔Effect of the invention〕

As described above, according to the present invention, even when switching the flow paths of a reversible heat exchanger, the amount of air flowing through the switching flow path can be controlled to be almost the same as during steady state, so , it is no longer necessary to design according to the instantaneous maximum flow rate at the time of switching, and the heat transfer area of the reversible heat exchanger can be kept to the minimum necessary. Even if a heat transfer area is provided, it is not possible to sufficiently cool the amount of air that is nearly 10 times the steady state during switching.
Although it has not been possible to prevent moisture and carbon dioxide gas from entering the low-temperature zone during switching, the present invention can completely prevent these harmful effects and has a significant effect on stable, long-term continuous operation of air separation equipment. can get. Furthermore, the change in the discharge pressure of the compressor can be made smaller than in the conventional method, and there is a great effect that entrainment of water from the water washing cooling tower can be prevented.

[Brief explanation of the drawing]

Figure 1 is a system diagram of a total low-pressure air separation device that uses a conventional reversible heat exchanger, and Figure 2 shows an example of a total low-pressure air separation device that uses a reversible heat exchanger that implements the method of the present invention. It is a system diagram. 2, 5, 11, 12, 14, 15, 17, 1
9, 21, 22, 24, 26-32... conduit, 1...
Air compressor, 3...Water cooling tower, 4...Water pump, 6
A, 6B... Air side switching valve, 6Aa, 6Bb... Bypass child valve switching valve, 7A, 7B... Waste gas side switching valve,
8... Reversible heat exchanger, 9A, 9B... Switching flow path, 1
0A, 10B...Check valve, 13...Rectification column lower column, 16
... Expander, 18... Rectification column upper column, 20, 25... Expansion valve, 23... Heat exchanger, 33... Bypass switching valve, 3
5...Main condenser.

Claims (1)

[Claims] 1. In order to remove moisture and carbon dioxide from the feed air, the flow path of the feed air of the heat exchanger and the flow path of the waste gas separated in the rectification column are periodically connected using a switching valve. A reversible heat exchanger that sublimates and removes moisture and carbon dioxide that precipitates and condenses on the heat transfer surface of the heat exchanger when used as a high-pressure raw air flow path. In air liquefaction cryogenic separation equipment using an exchanger, when the flow path of a reversible heat exchanger is switched, the air side is switched when the feed air on the high pressure side is introduced through the switching valve into the low pressure side waste gas flow path before switching. An air separation device using a reversible heat exchanger, characterized in that the amount of feed air introduced per unit time through a valve into a waste gas flow path before switching is controlled to be almost constant throughout the switching period. flow path switching method. 2. In claim 1, the reversible heat exchanger is provided with one or more air-side switching valves installed in the feed air inlet flow path of the reversible heat exchanger in parallel for each flow path. When switching the flow path, when changing the air side switching valve from the fully closed position to the fully open position, in the first step, the secondary pressure of the high pressure side raw material air flowing through the air side switching valve reaches the critical pressure. (1/of inlet absolute pressure
2), the opening of the air side switching valve is fixed to a cross-sectional area that allows a flow rate close to the raw material air flow rate during steady operation, and then when the secondary pressure rises to near the critical pressure. , at least two or more steps are performed to increase the cross-sectional area from the fixed cross-sectional area at the first step to the fully open opening so that the flow rate flowing through the air side switching valve becomes close to the raw material air flow rate during steady operation. A flow path switching method for an air separation device using a reversible heat exchanger, characterized in that an air side switching valve is actuated by operation.