Process For Automatic Control Of Air Separation Apparatus

Abstract

In an air separation apparatus which comprises a reversing heat exchanger, an air liquefier, a single column rectifier provided with a condenser-evaporator and a cold generation device and wherein air is cooled in the reversing heat exchanger and liquefied in the air liquefier, the liquefied air is rectified in the single column rectifier to separate into liquid air abundantly containing oxygen and highly pure nitrogen gas, the liquid air being subjected to heat exchange in the condenser-evaporator, the resulting gasified air being subjected to heat exchange in the air liquefier and sent through the reversing heat exchanger to the cold generation device and the resultant liquefied air being sent through the air liquefier and the reversing heat exchanger to release, a process for controlling the generation of cold which is characterized in that a by-pass channel is provided for communicating a position on a passage between the condenser-evaporator and the air liquefier and a position on a passage between the cold generation device and the air liquefier, a control valve is provided on the by-pass channel and the opening degree of the control valve is automatically controlled so as to regulate appropriately the flow volume of the gaseous air passing through the by-pass channel whereby the level of the liquid air in the condenser-evaporator is kept constant and the rectification of the liquefied air in the single column rectifier is carried out under stable conditions.

Description

The present invention relates to a process for automatic control of an air separation apparatus. More particularly, it relates to a process for automatically controlling the rectifying conditions in a single column rectifier and the cold generating conditions of a cold generation device with the stabilized operation of an air separation apparatus.

In an air separation apparatus comprising a single column rectifier provided with a condenser-evaporator and a cold generation device such as an intermediate pressure expansion turbine, the starting material, i.e., air, is liquefied using the cold generated in the cold generation device, the liquefied air is rectified in the single column rectifier and the waste gas formed in the condenser-evaporator is sent to the cold generation device for utilization as the source of cold. Among various factors for generation of cold in the cold generation device such as the pressures at the inlet and the outlet, the temperature and the flow volume, the pressure at the inlet and the flow volume are closely related to the pressure on the evaporation side and the evaporation volume in the condenser-evaporator of the single column rectifier. The over-and-under rate of the cold in the entire air separation apparatus is judged on whether the liquid level of the evaporation side in the condenser-evaporator is in an ascending direction or in a descending direction. When the cold has run short and accordingly the liquid level begins to show a descending tendency, the pressure at the inlet of the cold generation device is required to rise or the flow volume in the cold generation device must be increased. When the cold is in excess and hence the ascending tendency of the liquid level is recognized, the pressure at the inlet of the cold generation device is required to fall or the flow volume in the cold generation device should be decreased. In order to carry out rectification under stabilized conditions, however, the pressure and the volume of air being treated in the single column rectifier should not be abruptly changed. As stated above, the waste gas formed in the condenser-evaporator provided on the single column rectifier is supplied to the cold generation device. Therefore, the raising or lowering of the pressure at the inlet of the cold generation device results in raising or lowering of the pressure at the evaporation side of the condenser-evaporator. Such variations in the pressure will inevitably cause a considerable change in the pressure and the volume of air being treated in the single column rectifier so that stabilized conditions for rectification can not be assured.

If the variation of the pressure on the evaporation side of the condenser-evaporator is permitted, the maintenance of the pressure on the condensation side constant will result in a change of the temperature difference between the condensation side and the evaporation side whereby the heat exchange is increased or decreased, i.e., the volume of air being treated in the single column rectifier will change. If the volume of air being treated is kept constant, then the pressure on the condensation side must be varied so as to make constant the temperature difference between the condensation side and the evaporation side.

The above facts can be expressed in terms of concrete numerical values by way of an example where the operation is supposed to be according to the following conditions:

volume of air: W = 1,000 Nm.sup.3 /h;

Pressure on the condensation side: 6.4 atg (condensation temperature of gaseous nitrogen: -173.5.degree.C);

Pressure of liquefied air containing 49 percent oxygen on the evaporation side: 3.4 atg (evaporation temperature: -175.5.degree.C);

Temperature difference between the condensation side and the evaporation side of the condenser-evaporator: .DELTA.T = 2.degree.C;

Rate of heat transfer: Q = 33,200 Kcal/h.

Under the above conditions, a change of the pressure on the evaporation side by 0.2 kg/cm.sup.2 corresponds to a change of the evaporation temperature of liquid air on the evaporation side by 0.5.degree.C. Thereupon,

a. Pressure on the condensation side being constant:

The temperature difference of the condenser-evaporator is .DELTA.T = 2 .+-. 0.5.degree.C. In the equation Q = UA.DELTA. T wherein U is the overall heat transfer coefficient and A is the heat transfer area, A is constant and U is also approximately constant, and the rate of heat transfer Q is varied. Accordingly, the volume of air treated will become W = 1,000 .+-. 250 Nm.sup.3 /h and thus the rate of variation of treated air will come to .+-. 25 %.

b. Volume of treated air being constant (i.e. heat exchange being constant)

Since Q is constant, the pressure on the condensation side is changed so that T will become constant and also the temperature on the condensation side changes by .+-.0.5.degree.C, pursuant to the temperature change of 0.5.degree.C on the evaporation side. The change of pressure corresponding to the afore-mentioned temperature will come to 0.35 kg/cm.sup.2 and, in turn, with the magnitude of pressure being 6.4 .+-. 0.35 atg, the rate of variation of pressure on the condensation side becomes .+-. 5.5 percent.

As will be understood from the above explanation, the pressure variation on the condensation side of the condenser-evaporator exerts a great influence on the rectifying conditions in the single column rectifier. For carrying out the rectification under stabilized conditions, it is necessary to preclude variations of the pressure and the volume of air being treated in the rectifier as much as possible. On the other hand, it is indispensable to regulate the pressure on the evaporation side, i.e., the inlet pressure of the cold generation device, when the heat balance of the entire apparatus is taken into consideration.

According to the present invention, there is provided a method for controlling the generation of cold in an air separation apparatus comprising a single column rectifier provided with a condenser-evaporator, a cold generation device and a passage connecting the condenser-evaporator and the cold generation device, the passage having a by-pass channel connected to the outlet side of the cold generation device, where the starting air is liquefied with the cold generated in the cold generation device, the liquefied air is rectified in the single column rectifier, and the waste gas formed in the condenser-evaporator is partly supplied to the cold generation device through the passage and the remaining waste gas to the by-pass channel, which is characterized in that the flow volume of the waste gas to the by-pass channel is controlled by the variation of the liquid level on the evaporation side of the condenser-evaporator.

The present invention will be illustrated in detail by way of an example referring to the accompanying drawings wherein

FIG. 1 is a flow sheet showing a nitrogen producing apparatus as a preferred embodiment of the invention and

FIG. 2 is a diagram with curves showing an example of the controlling of the liquid level and the pressure on the evaporation side of the condenser-evaporator by the opening of a control valve provided on the by-pass channel.

In FIG. 1, the starting air is supplied through tube 1 and compressed in a compressor 2. When the production of gaseous nitrogen alone is carried out, for instance, compression can be up to around 6 to 7 kg/cm.sup.2. When the simultaneous production of liquid nitrogen is desired, compression up to around 8 to 9 kg/cm.sup.2 can be effected. The compressed air goes through a conduit tube 3 to the higher temperature portion 4 and the lower temperature portion 5 of a reversing heat exchanger where heat exchange with returning gas is performed. The air thus cooled nearly to the liquefying temperature is sent through a check valve 6 and a conduit tube 7 and enters into the bottom portion of a single column rectifier 8 where the air is liquefied and separated into liquefied air containing an abundant amount of oxygen and gaseous air containing nitrogen in a high concentration. The gaseous air taken out through a conduit tube 10 is heat exchanged with returning gas in an air liquefier 9 and the resulting liquefied air is returned through a conduit tube 11 to the bottom portion of the rectifier 8.

The liquid air at the bottom portion of the rectifier 8 is sent through a conduit tube 12 to a filter 13 where impurities such as CO.sub.2 and hydrocarbons are absorbed and eliminated. Then, the filtered liquid goes on through a flow control valve 14 where the pressure is reduced to around 3 to 4 kg/cm.sup.2 and enters into the evaporation side 15 of a condenser-evaporator 16. In the condenser-evaporator 16, the liquid air is heat exchanged with the nitrogen gas of high purity from the top portion of the rectifier 8 through a conduit tube 17 whereby the nitrogen gas is liquefied. The liquefied nitrogen flows through a conduit tube 18 to the top portion of the rectifier 8. On the other hand, the liquid air is vaporized as the result of the heat exchange and goes on through a conduit tube 19 for utilization as returning gas.

A major portion of the above vaporized air flowing through the conduit tube 19 is sent through a conduit tube 20 to the liquefier 9 and flows through a conduit tube 21 where it is separated into two portions, one of them going through a control valve 22 and a conduit tube 23 to the lower temperature portion 5 for heat exchange with the starting air coming from the compressor 2 to liquefy CO.sub.2 therein and then flowing through a conduit tube 24 and a control valve 25 to an expansion turbine 26. The other portion is sent from the conduit tube 21 through a control valve 27 and combined with the above returning gas. The combined gas is expanded in the expansion turbine 26 nearly to atmospheric pressure whereby any thermo-dynamic exterior work is performed and the temperature is lowered remarkably to generate the cold sufficient to fulfill the demand for the apparatus. The expanded gas flows through conduit tubes 28 and 29 to the liquefier 9 and then goes through a conduit tube 30 and the check valve 6 to the reversing heat exchanger. The gas is warmed to room temperature as the result of the heat exchange with the starting material air and then discharged through a conduit tube 31.

A minor residual portion of the vaporized air flowing through the conduit tube 19 flows through a by-pass channel 33 and a control valve 32 and, after being combined with the gas from the conduit tube 28, goes on through the conduit tube 29. The control valve 32 regulates the flow volume and the pressure at the inlet of the expansion turbine 26 and thus controls the generation of cold. When the liquid level of the liquid air on the evaporation side 15 of the condenser-evaporator 16 drops lower than a set level, the control valve 32 moves toward the closed position whereby the flow volume in the expansion turbine 26 is increased and the level of the liquid air in the condenser evaporator increases toward the set level. When the liquid level goes higher than the set level, the control valve 32 is operated to move toward the open position whereby the flow volume in the expansion turbine is decreased and the level of the liquid air decreases toward the set level.

It should be noted that, when the control valve 32 is moved toward the closed position in such a manner that the pressure on the evaporation side 15 of the condenser-evaporator 16 is increased abruptly, there is a danger of disturbing the stabilized rectifying conditions of the single column rectifier; and besides, there will be a considerable time lag from the action of the control valve 32 to the actual return to the set liquid level.

For preventing the overrunning of the control valve 32, it is recommended to provide a mechanism which permits the pressure on the evaporation side 15 of the condenser-evaporator 16 to deviate within certain upper and lower limits from the pressure level before the control valve 32 starts extreme corrective action.

In FIG. 2 which shows the operation of the apparatus under the control of such a mechanism, there are shown the relative variations of the liquid level L of the liquid air on the evaporation side 15 of the condenser-evaporator 16, the pressure P on the evaporation side 15 and the degree of opening V of the by-pass control valve 32 with the time t. In the case where the liquid level varies, starting from the time t.sub.1, to drop lower than the set limit value Lo, the pressure on the evaporation side 15 of the condenser-evaporator 16 is raised from the set valve Po upwards by .theta.', which corresponds to the degree of level displacement .theta.; and at the same time, the control valve 32 is activated to move toward the closed position so as to increase the flow volume in the expansion turbine 26. While .theta.'/.theta. is optionally adjustable, the controlling arrangement should be such that a drastic pressure variation does not occur with respect to the liquid level. When the control valve 32 is moved toward the closed position, if the liquid level restores itself as indicated at e in the diagram and, along with it, the pressure P as well as the degree of valve opening V also respectively move up to the initial values Po and Vo, then the control is deemed satisfactory. In the event, however, that the liquid level lowering still continues and does not cease and, in turn, the pressure raises to P.sub.H, then a control measure is taken to maintain the degree of valve opening at V.sub.L which is the degree of opening for pressure P.sub.H. An appropriate value for P.sub.H may be Po + 0.05.about. 0.1 kg/cm.sup.2. By establishing this condition, the cold balance is successfully achieved without allowing the pressure on the evaporation side 15 to go through a large change in the time lag from the start of movement of the control valve 32 to the actual recovery of the liquid level; in other words, the cold balance can be maintained without impairing or disturbing the rectifying conditions. If during the time the degree of valve opening is maintained at V.sub.L, if the liquid level shows a tendency to be restored as shown at d in the diagram, then the pressure which has been maintained at P.sub.H until the recovery of the liquid level to Lo will start to return towards Po as the valve starts opening. If the liquid level still continues to fall, then a control measure is taken to close the valve further so that the pressure P goes up at the time when the liquid level has dropped to L.sub.L. If the liquid level still continues to fall and the pressure also continues to rise almost reaching the set limit in spite of the above valve actuation, such trend is detected by means of an alarm device or the like so that the initial set value of the pressure or the value of .theta.'/.theta. can be appropriately changed to achieve the restoration of the liquid level. When the liquid level has once gone beyond the set limit degree of value L.sub.L and come back to the same, the valve opening is kept at a constant so as to maintain the pressure at the value at the moment that the liquid level has reached L.sub.L ; afterwards, when the liquid level reaches the initial set value L.sub.O or a certain period of time elapses, the degree of valve opening is allowed to change freely again and the regulation is continued depending on the variation of the liquid level at that moment. The above developments are shown by way of the symbols a, b and c in the diagram.

When the apparatus is in a stable stage of operation, the liquid level generally restores itself at the time t.sub.2 or t.sub.3 and further supplementary control is seldom needed unless the heat balance is disturbed by some external factor. But, the set value may be appropriately changed depending on the temperature change with the seasons after the time t.sub.3.

The above illustration is for the case where the liquid level falls but the same may be applied to the case where the liquid level ascends.

Claims

What is claimed is:

1. In an air separation apparatus which comprises a reversing heat exchanger, a single column rectifier connected to said reversing heat exchanger for receiving cooled air therefrom, said rectifier having a condenser-evaporator, an air liquifier connected to said rectifier for receiving gaseous air therefrom and returning it to said rectifier after cooling it, a vaporized air line from the rectifier on the evaporater side of said condenser evaporator to said air liquifier, a cold generation device coupled to said air liquifier for receiving air from the air liquifier through the reversing heat exchanger and expanding it, and an expanded air line from said cold generation device to said air liquifier, the liquified air from the air liquifier being rectified in the rectifier to separate into liquid air rich in oxygen and highly pure nitrogen gas, the liquid air being subjected to heat exchange in the condenser-evaporator and the resulting vaporized air being subjected to heat exchange in the air liquifier and cooled in the cold generation device and again being subject to heat exchange in the air liquifier, and then being sent through the reversing heat exchanger and discharged, that improvement comprising means for controlling the cold generating device comprising a by-pass line connected between the vaporized air line downstream of said rectifier and the expanded air line from the cold generation device, a control valve in said by-pass line and control means connected between said control valve and the rectifier and responsive to the liquid air level in the condenser-evaporator portion of said rectifier for moving said control valve toward the open position when said level rises and moving said control valve toward the closed position when said level falls, whereby the level of liquid air in the condenser evaporator is kept constant and the rectification of the liquified air in the rectifier is carried out under stable conditions.

2. The improvement as claimed in claim 1, wherein the control means stops the control valve at the time when the pressure on the evaporation side of the condenser-evaporator has varied to a certain extent from a set value so as to control the time lag during recovery of the liquid level.

3. A method for automatic control of an air separation apparatus having a single column rectifier provided with a condenser-evaporator which comprises flowing vaporized air from the evaporator side of said condenser-evaporator through an air liquifier, a heat exchanger and an expansion type cold generation device and then back through said air liquifier, by-passing a portion of the vaporized air through a control valve from the evaporator condenser to the output side of the cold generation device, changing the degree of opening of the control valve in response to the level of liquid air on the evaporation side of the condenser-evaporator for controlling the flow of the bypassed part of the vaporized air to adjust the flow ratio of said part relative to that of the other part of said vaporized air which is passed through a cold generating device and fixing the amount of opening of the control valve when the pressure on the evaporation side has varied to a predetermined extent from the pressure existing before the change of the degree of opening of the valve.