JPH10259988A - Air separation method and device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To utilize the owned capability of a main heat exchanger most effectively by utilizing a line at a lower temperature side than a compressed air blow-off position halfway from a position in a main heat exchanger as the passage line of a return gas. SOLUTION: In an air separation method, a compressed air being sent from a branch pipeline 6b being branched from a compressed air feeding pipeline 6a to the hightemperature side of a main heat exchanger 7 is extracted halfway through the main heat exchanger 7, is heat-insulated and inflated by an inflation turbine 5, is cooled to a cold temperature, and is sent to the upper part of a rectifying column. In this case, in the compressed air being sent to the main heat exchanger 7 through the branch pipeline 6b, only one portion of the high- temperature side is utilized for heat exchange so that it is not cooled excessively in the main heat exchanger 7 and the greater part of it is not utilized. Furthermore, the cold air is collected in the main heat exchanger 7 most effectively when a return gas temperature is set to the lowest temperature region, thus utilizing the unused low-temperature side region in the main heat exchanger 7 effectively.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an air separation method and apparatus, and more particularly to an air separation method of a type in which raw air is cooled by heat exchange with return gas in a main heat exchanger disposed in the air separation apparatus. The present invention relates to a method and an apparatus capable of maximally and effectively utilizing the heat exchange performance of a main heat exchanger and reducing power for air separation.

[0002]

2. Description of the Related Art An air separation method for separating air into nitrogen gas and oxygen gas is used in a wide range of fields such as steelmaking, chemicals, and electronics. Various studies have been conducted on such an air separation method for the purpose of improving separation efficiency, lowering running costs, improving operation stability, and the like.

FIG. 3 is a flow chart showing a molecular sieve type air separation method developed under such a situation.
The raw air passes through an air filter 1, a raw air compressor 2, a cooler 3, and the like, is converted into air having a desired pressure, temperature, and humidity (hereinafter, may be referred to as compressed air), and is guided to a molecular sieve adsorber 6. The molecular sieve adsorber 6 shown in FIG.
In the adsorber 6, moisture, carbon dioxide gas, hydrocarbon gas and the like in the compressed air are almost completely removed by the adsorption of zeolite or the like. The compressed air led out of the adsorber 6 through the pipe 6a is led to the main heat exchanger 7 where it is cooled to near the liquefaction point by heat exchange with return gas to be described later. Is introduced to

The compressed air introduced into the lower tower 8a is
The distillation separation proceeds while being cooled in the process of ascending in the column a, and is taken out from the upper portion of the lower column 8a as a low-boiling nitrogen-rich liquid (liquid nitrogen) 9, while the lower portion is a high-boiling oxygen-rich liquid 9. The liquid 10 is stored (hereinafter sometimes referred to as a coarse distillation step). The upper nitrogen rich gas is led by line 13 to main condenser 8b where it is liquefied and
To return to the upper part of the lower tower 8a. The nitrogen-rich liquid in the upper part of the lower tower 8a passes through the supercooler 12 through the pipe 15 and passes through the upper tower 8c.
To the top of

On the other hand, the oxygen-rich liquid 10 is led to the middle stage of the upper tower 8c via the subcooler 12 by the pipe 25.
From the middle stage of the lower tower 8a, liquid nitrogen in the middle stage of the rough distillation process is led to the upper stage of the upper tower 8c via the supercooler 12 by the pipe line 11. As described above, the low-temperature liquid nitrogen and oxygen-rich liquid 10 introduced from the middle, upper, and top portions of the upper tower 8c and descending in the upper tower 8c undergo mass transfer with the gas rising in the upper tower 8c. The rectification proceeds by being performed.

By repeating these steps, high-purity nitrogen gas is collected at the top of the upper tower 8c, and high-purity liquid oxygen is stored at the lower part of the upper tower 8c. 17, and is returned to the main heat exchanger 7 as the return gas, and heat exchange is performed between the compressed air extracted from the molecular sieve adsorber 6 to utilize the cold. Commercialized as oxygen.

At this time, the molecular sieve adsorber 6
Is branched before being introduced into the main heat exchanger 7, and is compressed by the pressurizer 5 on the inlet side of the expansion turbine 5.
After being pressurized by a, it is introduced into the main heat exchanger 7, is cooled on the high temperature side of the main heat exchanger 7, is extracted from the middle thereof, is returned to the expansion turbine 5, and is adiabatically expanded. After being cooled, it is introduced into the middle stage of the upper tower 8c.
Further, from a position slightly below the upper part of the upper tower 8c, crude nitrogen gas (extracted gas) is extracted through the pipe line 20,
After utilizing cold by heat exchange as a return gas from the subcooler 12 through the main heat exchanger 7, the extracted gas after the heat exchange is supplied to the molecular sieve adsorber 6 through the regeneration heater 29, The excess extracted gas is used for the regeneration of the evaporative cooler 4
After being used for cooling the cooling water used for cooling the cooler 3. After the regenerative heating of the molecular sieve adsorber 6, the extracted gas is supplied to the molecular sieve adsorber 6 after the regenerative heating by switching the valves V ₁ and V ₂ to cool the same, thereby completing the preparation for switching to the adsorption step. . In addition, the extraction gas used for the regeneration of the molecular sieve adsorber 6 is sequentially released to the outside of the system.

The main heat exchanger disposed in such an air separation device is a corrugated fin (a plain fin, a herringbone fin, a perforate fin, a louver fin, a serrate fin, etc.) in order to enhance heat exchange efficiency.
The main heat exchange members are stacked and assembled in multiple stages, and heat exchange is performed by flowing fluids to be mutually heat-exchanged to adjacent units in counterflow.

By the way, in the conventional air separation method as shown in FIG. 3, the compressed air is branched before the main heat exchanger 7 and then sent to the main heat exchanger 7 as described above. It is configured to be extracted from the middle and returned to the expansion turbine 5.
Therefore, the portion of the heat exchange unit through which the compressed air passes is effectively used as a heat exchanger, but the portion of the heat exchange unit after the compressed air is extracted, that is, the area indicated by the dashed line in FIG. Despite having a heat exchange function, it is left unused and not utilized.

[0010]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned circumstances, and has as its object the main heat exchange used for cooling compressed air when performing air separation. It is an object of the present invention to establish a technique that enables the unused portion of the heat exchanger to be effectively used for heat exchange and makes the most of the holding capacity of the main heat exchanger.

[0011]

SUMMARY OF THE INVENTION The air separation method according to the present invention, which can solve the above problems, supplies precooled compressed air to a rectification column via a main heat exchanger, and supplies the precooled compressed air to the rectification column. The return gas is passed through the main heat exchanger to be used for cooling the compressed air, and a part of the compressed air flowing through the main heat exchanger is extracted from the middle of the main heat exchanger and adiabatically expanded. In the air separation method for supplying to the upper tower of the rectification column, a line on the lower side of the compressed air withdrawal position in the main heat exchanger is used as a return gas passage line, whereby the return gas is removed. The point lies in increasing the heat exchange utilization rate of the steel.

[0012] The configuration of the air separation device according to the present invention is as follows.
The pre-cooled compressed air is supplied to the rectification column via the main heat exchanger, and the return gas from the rectification column is passed through the main heat exchanger to be used for cooling the compressed air, and the main heat exchanger A part of the compressed air flowing through the main heat exchanger is withdrawn from the middle of the main heat exchanger, adiabatically expanded, and then supplied to the upper tower of the rectification column. It is characterized in that the return gas line is formed at a lower temperature side than the compressed air withdrawal position in the main heat exchanger from the middle, and is joined to another return gas line of the main heat exchanger at the high temperature side. Have.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As described above, in the present invention, a part of the compressed air sent to the main heat exchanger is changed by using the method and the apparatus as shown in FIG. After introduction from the side, when carrying out the method of withdrawing from the middle of the main heat exchanger and adiabatically expanding and then supplying it to the upper column of the rectification column,
It is noted that, as shown by the dashed line in FIG. 3, the lower side of the main heat exchanger than the compressed air extraction position is not used for heat exchange at all, and this portion can be effectively used for heat exchange. By doing so, the performance inherent in the main heat exchanger 7 can be utilized as effectively as possible without waste.

More specifically, for example, as shown in FIG. 1, the main part of the unused portion of the main heat exchanger 7 is returned gas from the rectification column 8 (crude nitrogen gas). Part of the gas is branched and passed therethrough, so that the cold of the return gas can be recovered more effectively in this part, and the effect brought by it is extremely large as shown below. Bring a great benefit in

The first point of the benefit is that the cold recovery rate by the main heat exchanger 7 is significantly increased. That is, in the method as shown in FIG. 1, the compressed air fed from the branch line 6b branched from the compressed air supply line 6a to the high temperature side of the main heat exchanger 7 is supplied to the main heat exchanger 7 as described above. It is withdrawn from the middle, adiabatically expanded by the expansion turbine 5, cooled to near cold, and then sent to the upper rectification tower 8 c. If it is cooled too much while passing through the main heat exchanger 7, the expansion turbine 5 The compressed air liquefies on the outlet side, which hinders stable operation.

Therefore, in order to ensure the operation stability, the compressed air sent into the main heat exchanger 7 through the branch pipe 6b is not cooled excessively in the main heat exchanger 7 so that the compressed air is not excessively cooled. Some are used only for heat exchange, and most remain unused. Moreover, the main heat exchanger 7
The cold is most effectively recovered in the low temperature area where the return gas temperature is the lowest (lower area in FIG. 1), and the unused portion left in the low temperature area improves the cold recovery rate. It is a big obstacle. From such a point, the effective use of the unused low-temperature side region in the main heat exchanger 7 is extremely effective in improving the cold recovery rate.
A second benefit is that the pressure loss of the low pressure system in air separation is reduced, thereby increasing the power efficiency of air separation.

That is, in carrying out the air separation method,
It has been confirmed that the power of the air compressor immediately becomes the power performance of the device, and the lower the outlet pressure of the air compressor, the lower the power of the air compressor and the performance of the air separation device as a whole improves. I have. The most effective means for lowering the outlet pressure of the air compressor is a low-pressure system in the air separation device, in particular, the supercooler 12, the main heat exchanger 7 from the upper tower 8c shown in FIG. And to reduce the pressure loss of the discharge gas line discharged to the atmosphere via the molecular sieve adsorber 6 as much as possible.

[0018] According to the present inventors have confirmed, that the pressure reducing for example 0.1 kg / cm ² G of the outlet side line is about 0.3 kg / cm ² G decreased outlet pressure of the air compressor Leads to In this, a main condenser 8b is provided in an air separation device, in which heat exchange is performed between oxygen at the bottom of the rectification column and nitrogen at the top of the bottom column, where oxygen-nitrogen is exchanged. This is because the differential pressure in the low-pressure system is related to the differential pressure in the high-pressure system due to the boiling point under each pressure.

For example, the boiling point of oxygen at the bottom of the rectification column is
At about 1.4 kg / cm ² A and at -180 ° C., this oxygen evaporates with nitrogen at 5.4 kg / cm ² A at the bottom of the column at 178.2 ° C. (ie oxygen evaporates with nitrogen ,
Nitrogen condenses), but when the pressure of oxygen at the bottom of the rectification column drops by 0.1 kg / cm ² A to about 1.3 kg / cm ² A, the temperature becomes −180.7 ° C. 5.05 kg
Evaporate with nitrogen at -179 ° C at / cm ² A. That is, if the outlet pressure from the low pressure system, that is, the upper tower is reduced by 0.1 kg / cm ² A, the inlet pressure of the high pressure system, that is, the lower tower, is 0.35 kg.
/ cm ² A (5.4-5.05 kg / cm ² A).

As is clear from this, in order to reduce the outlet pressure of the air compressor when performing the air separation method, the pressure of the high-pressure system (that is, the inlet line to the rectification column) must be reduced. It is more effective to suppress the pressure of the low-pressure system (that is, the extraction line from the upper tower of the rectification column) than to suppress the pressure, that is, to reduce the pressure loss of the low-pressure system as much as possible when performing the air separation method. Greatly contributes to power reduction. The largest pressure loss among the extraction lines of the low pressure system is the crude nitrogen gas discharge line 20 having a regeneration heater 29, a molecular sieve adsorber 6, and the like in the line.
And it is possible to reduce the pressure loss of this line as much as possible.
This is most effective in reducing pressure loss in low-voltage systems.

From such a point, it is preferable to select crude nitrogen gas as the return gas to be branched and sent to the unused portion of the main heat exchanger. By passing the crude nitrogen gas through the unused region, If the area of the discharge passage is enlarged, the pressure loss of the crude nitrogen gas discharge line can be reduced in addition to the effective use of the main heat exchanger, and the effect of two birds per stone can be obtained.

FIG. 2 is a schematic plan view illustrating a heat exchange unit U used as the main heat exchanger 7 when the present invention is carried out. FIG. 2A is a schematic view in which compressed air is fed from the branch passage 6b. 3 shows a heat exchange unit U at a portion extracted from the middle of the heat exchanger 7. Therefore The heat exchange unit U, the high temperature side from the (upper drawing) via the distributor D ₁ is compressed air in the corrugated fin F arranged partial fed, the heat exchange unit (not shown) disposed adjacent thereto Heat exchange is performed with the return gas passing through the inside. After the temperature was lowered to a temperature suitable for adiabatic expansion by (reference numeral 5 in FIG. 1) the expansion turbine is withdrawn from the distributor D ₂ provided on the way to the expansion turbine direction.

In the conventional example, the lower side (ie, the low-temperature side) of the extraction position is left unused, but in the present invention, the lower side (ie, the low-temperature side) of the extraction position is the distributor D. while separated by _3, from the distributor D ₄ provided at the bottom (i.e. lowest temperature portion) of the heat exchange unit U, fed a crude nitrogen gas withdrawn branched from extraction line 20 as described above, Even in this part, the cold held by the nitrogen gas can be recovered by heat exchange.

The crude nitrogen gas extracted from the distributor D ₃ is stacked at another position of the main heat exchanger and merges with the heat exchange unit into which the crude nitrogen gas is introduced from the main pipe of the extraction pipe 20. Then, it is sent to the high temperature side.
That is, the another heat exchange unit U 'has, for example, a such structure is shown in Figure 2B, the external introducer distributor D ₅ is provided at a position corresponding to the distributor D ₄ of the heat exchange unit U , from the distributor D _5,
And the manner back to the main stream by introducing the crude nitrogen gas withdrawn from the distributor D ₃ in FIG. 2A.

The heat exchange units shown in FIGS. 2A and 2B are only examples of typical heat exchange units used in carrying out the present invention. In other words, as long as the configuration is such that branching and merging can be performed smoothly, heat exchange units having various structures other than those shown can be adopted depending on the shape and structure of the main heat exchanger used. Are all included in the technical scope of the present invention.

As described above, according to the present invention, the power of the air separation apparatus as a whole can be reduced by effectively utilizing the main heat exchanger and reducing the pressure loss of the extraction gas line. Still another effective means for reducing the loss is to improve the structure of a distributor provided in the main heat exchanger. Hereinafter, this point will be described.

The ordinary distributor is provided to evenly distribute the fluid entering from the inlet to the heat exchange section HE composed of corrugated fins having a wide width and a fine pitch in each unit. For example, as shown in FIG. 2 etc., a fluid can be distributed and introduced in the width direction of the heat exchange section by a corrugated plate having a coarse pitch, or the fluid can be collected and discharged from the width direction of the heat exchange section. It is configured.

However, in such a conventional main heat exchanger,
The flow direction of the fluid is changed particularly at the outlet side distributor portion of each heat exchange unit, and each fluid flows in a state where the flow path is narrower than the heat exchange portion, so that a pressure loss is inevitable.

Therefore, further research was conducted to reduce the pressure loss generated in the distributor portion of such a corrugated fin heat exchanger as much as possible. It has been confirmed that the pressure loss in the distributor portion can be more effectively reduced by using a structure in which a plurality of columns are assembled at appropriate intervals while leaving a passage space.

That is, the ordinary distributor provided in the corrugated fin type heat exchanger, as described above, converts the fluid entering from the inlet of the heat exchanger into a heat exchange formed of corrugated fins having a wide width and a fine pitch in each unit. It is considered indispensable to distribute evenly to departments. For example, as shown in FIG. 2 and the like, a structure in which a corrugated plate having a coarse pitch is sandwiched between plates and assembled is used. It is inevitable that pressure loss will occur.

However, the inventors of the present invention have conducted various studies and found that in the distributor, if a certain length is secured in the fluid flow direction, the gap between the plates can be maintained without disposing a corrugated plate inside. It was confirmed that the fluid was sufficiently distributed only by keeping the hollow state.
However, when assembling a heat exchanger with a large number of heat exchange units to which the distributor is attached, a large clamping force acts between the plates constituting the distributor, so that a pressure resistance sufficient to withstand the clamping force is applied. It is essential to ensure strength.

That is, it is considered effective to design the distributor focusing on the pressure resistance rather than the fluid distribution function, on the premise that a sufficient fluid passage space is secured. However, in a conventional distributor in which a corrugated plate is sandwiched between plates, the pressure resistance of the corrugated plate is not sufficient. A considerable pressure loss occurred in the corrugated plate.

However, considering the structure of the distributor with a particular emphasis on the improvement of the pressure resistance between the plates required during assembly, for example, as shown in FIG. It was found that it was sufficient to secure the pressure resistance sufficient to withstand the restraining force at the time of assembly by the support posts S. That is, the support S functions as a beam between the plates P disposed at both ends thereof, and can sufficiently support the restraining force applied in the vertical direction of the drawing. And since such a pillar has a superior pressure resistance compared with the conventional corrugated plate, it is possible to take a sufficiently large mounting interval, that is, it is possible to take a sufficiently large fluid passage space. As a result, the pressure loss at the distributor portion can be suppressed as much as possible.

The struts S used here are, in short, assembled at appropriate intervals between the plates P, P to provide pressure resistance, and the struts S may have any shape and structure. In terms of securing the passage space for fluid, it is effective to assemble it as a rod-shaped support with a cross section of a round bar or a rectangular shape. However, in such a support, both ends of the support are brazed between the plates P and P. The joining operation becomes extremely complicated, which not only increases the production cost but also makes mass production difficult. Therefore, in consideration of manufacturing cost and productivity, for example, it is advantageous to use a plate-shaped support S as shown in FIG. 4 and assemble it between the plates P, P by known means such as brazing. . At this time, as shown in FIG. 5, a method of integrally forming one plate P and a plate-shaped support S by a drawing method or the like, and a method of joining another plate P to the other end of the support S are employed. Then, the positioning and fixing of the column S can be performed very easily, so that such a structure can be said to be the most practical one in view of the easiness of manufacture.

However, in these examples, the feature is that the pressure resistance between the plates is increased by the columns as described above, so the specific shape and structure of the columns are not limited, and other shapes such as a rod shape may be used. -Of course, it is also possible to use a structural support. When a plate-shaped support is used, the plate-shaped support is not only linear, but also curved in the fluid flow direction as shown in FIG. 6 to reduce the flow resistance, or as shown in FIG. Drill an arbitrary number of holes W,
It is also possible to allow the fluids to diverge from one another through the holes W.

Although it is advantageous to reduce the pressure loss, it is advantageous to make the mounting interval of the columns S as wide as possible as long as the required pressure-resistant strength can be ensured. However, the present inventors have confirmed that the effect of reducing the pressure loss is effective. In order to demonstrate
If the plate-like supports are assembled at intervals of about 3 to 15 times the fin pitch of the corrugated fins constituting the heat exchange unit in the applied heat exchange unit, it is possible to sufficiently reduce the pressure loss in the heat exchange unit. Do you get it. Thus, if the installation interval is excessively wide exceeding 15 times, the pressure resistance strength tends to be insufficient or the support must be excessively thick. On the other hand, if the support installation interval is less than 3 times, While the compressive strength can be increased sufficiently,
This is because it is difficult to obtain a satisfactory pressure loss reducing effect.

Incidentally, the corrugated fin type heat exchanger
The relationship between design pressure, pitch and plate thickness in the following equation
Can be t_p = P_t √ (P / 200σa) t_p : Separate sheet thickness, P_t : Pitch, P: Pressure
Force, σa: allowable stress of material (normal Al alloy is 2.3) Sheet thickness used for corrugated fin for heat exchanger
Is usually 1 mm and σa is 2.3.
Force 1.0kg / cm^Two Pitch P when G _t On
From the notation, P_t = T_p /√(P/200σa)=1.0/√(1.
0/200 · 2.3) = 21.4 mm.

On the other hand, in order to secure the strength to withstand the load when the heat exchange unit is assembled by brazing, it must satisfy the buckling resistance (P _cr ) calculated by the following equation. During buckling strength ^{_{(P cr) = 4π 2 EI}} / l 2, I = tl 3/12 ( wherein, E: elastic modulus, I: sectional secondary moment, l:
Fin height t: Represents the fin plate thickness) Now, for two kinds of corrugated fins of the same material but of the same fin height and different plate thicknesses (therefore, E and l are the same), the respective buckling strengths P _Cr1 when determining the calculation formula P _Cr2, it becomes as the following equation, when the fin thickness is _{t 1: P Cr1 = [4π} 2 · E · (t 1 · l 3/1
2) / l ^3)] When the fin thickness is _{t 2: P Cr2 = [4π} 2 · E · (t 2 · l 3/1
Than 2) / l ^3)] the above _{equation, P Cr1 / P Cr2 = t} 1 / t 2 is derived. Further, under the condition that the same buckling strength is ensured, there is a substantially proportional relationship between the fin pitch (P) and the fin plate thickness (t), so that t ₁ / t ₂ = P _t1 / P _t2 Holds.

Then, for example, 4.2 m generally used as a corrugated fin of a main heat exchanger for an air separator.
The thickness of the m-pitch fin is 0.4 mm, and the design fin pitch is 21.4 m when the thickness is 1 mm as described above.
m, these values are substituted into the above equation to determine the fin plate thickness for obtaining the buckling strength that can withstand the load at the time of assembling: 4.2 / 0.4 = 21.4 / t, That is, t ≒ 2 mm is derived. As a result, the fin thickness is 2 mm at the 21.4 pitch, but it is possible to secure a larger flow path area than that of the fin thickness of 0.4 mm at the 4.2 pitch.

On the other hand, the fin pitch of a normal corrugated fin used for a heat exchanger is Max: 4.2 m.
Since m and Min are 1.4 mm, considering these values, it is preferable to use the reinforcing column for securing the buckling strength against the load at the time of brazing by the reinforcing column provided in the distributor. When the mounting interval is determined, the minimum value is the Max / min ratio of the fin pitch, that is, 4.2 /
1.4 (= 3 times), and the maximum value is the ratio (ie, 21.4) of the corrugated fin pitch to the set fin pitch (ie, 21.4) with respect to the Min value (1.4).
/1.4=15 times), and the preferable minimum pitch interval of the reinforcing columns = 4.2 / 1.4.
= 3 times, preferred maximum pitch spacing of reinforcement columns = 21.4 / 1.
4 = 15 times. That is, by setting the mounting interval of the reinforcing columns in the distributor portion to be 3 to 15 times the fin pitch in the heat exchange section, sufficient buckling strength can be secured without alienating the fluid flow. Will be.

In the present invention, the pressure loss of the low-pressure line is reduced by devising the return gas flow path of the main heat exchanger as described above, or by further improving the structure of the distributor constituting the heat exchanger as described above. The feature is to reduce the power of the air compressor, thereby reducing the power of the air compressor. Such a feature can be effectively exerted even for an air separation device as shown in FIG. In addition, it is possible to further increase the pressure loss reduction or to obtain other effects by adding a configuration as described below.

For example, FIG. 8 is a partial explanatory view showing another example of the air separation device provided with the main heat exchanger, and shows only the vicinity of the upper and lower rectification towers. The portion may be considered to be the same as in the example of FIG.

In the example shown in FIG. 3, the nitrogen gas roughly separated at the top of the lower column 8a is used to heat the liquid oxygen at the bottom of the upper column 8c of the rectification column, and the liquid oxygen is extracted from the upper side wall of the liquid level. The outputted oxygen-rich gas is taken out from the line 17 as product oxygen gas.

On the other hand, in the present embodiment, as shown in FIG. 8, the liquid air at the bottom of the upper rectification column 8c is withdrawn as liquid from the line 30 and introduced into the evaporator 31. The evaporator 31 includes a heating heat exchanger 32 therein. The heating heat exchanger 32 is provided with compressed air that is cooled by the main heat exchanger 7 and then supplied to the rectification lower tower 8a. It is configured so that a part is supplied in a branched manner. Then, the liquid oxygen extracted from the upper rectification tower 8c is evaporated by heating it in the evaporator 31, and the cold is recovered from the line 33 in the main heat exchanger 7 in the same manner as described above, and then the product oxygen gas is produced. On the other hand, the compressed air that has been extracted and given cooling energy by the heating heat exchanger 32 is sent to the lower part of the lower tower 8a. Benefits brought by adopting this method include the following.

The pressure of the product oxygen gas can be increased. That is, as shown, when the evaporator 31 is provided below the liquid oxygen level L in the upper rectification tower 8c, the evaporator 31
Becomes lower than the liquid level L ₁ of the liquid oxygen is the liquid surface L in the pressure of the liquid oxygen supplied to the evaporator 31, the liquid in the liquid level L of the liquid oxygen in the upper column 8c evaporator 31 liquid head difference between the liquid level L ₁ of oxygen becomes higher by an amount corresponding to a (head difference) Hd. Although the boiling point rises by the pressure rise, the heating heat exchanger 32 in the evaporator 31 is compressed at a higher temperature than the crude nitrogen gas supplied to the main condenser 8b of the upper rectification tower 8c. Since the air is diverted and sent,
The liquid air in the evaporator 31 receives sufficient heat for evaporation and evaporates despite the pressure rise. Then, the pressure of the evaporating oxygen gas is withdrawn at a high pressure corresponding to the liquid head difference (head difference) Hd. In general, product oxygen gas is pressurized by a compressor to produce a product. However, since the product oxygen gas is pressurized through a compressor in the final stage of the extraction line, the pressure of the oxygen gas is increased in this portion. Can be reduced, which can contribute to a reduction in power of the entire equipment.

The purity of the product oxygen gas can be increased. That is, as shown in FIG. 3, when the oxygen gas is extracted from slightly above the liquid level of the liquid oxygen in the upper tower 8c of the rectification tower, the purity of the oxygen gas is determined based on the gas-liquid equilibrium in the upper tower 8c. It is impossible to increase the purity of the oxygen gas to more than 90%. For example, when the oxygen concentration of the liquid oxygen is 90%, the purity of the extracted oxygen gas must be about 87 to 88%. However, as in the present example, a method is employed in which liquid oxygen at the bottom of the upper tower 8c is extracted in a liquid state to the evaporator 31 and then heated and evaporated. It becomes possible to extract the oxygen gas maintaining the liquid oxygen purity at the bottom of 8c as the product gas. That is, the temperature and pressure conditions in the evaporator 31 are controlled, and a gas having a high nitrogen content that is first volatilized is released, so that the oxygen concentration in the gaseous phase becomes higher than that of the liquid oxygen (ie, sent from the upper column 8c). A gas-liquid equilibrium state (for example, a gas phase oxygen concentration of 90% and a liquid phase oxygen concentration of 92%) is obtained, and the liquid oxygen is introduced and the gas oxygen is extracted while maintaining this state. If it is performed continuously, it becomes possible to continuously obtain a 90% concentration oxygen gas as a product gas.

It is possible to reduce the pressure for introducing the compressed air into the lower tower 8a, that is, to reduce the power of the raw material air compressor.
That is, as described above, the fact that the purity of the product oxygen gas is increased by the operation with the evaporator 31 provided means that the purity of the liquid oxygen stored at the bottom of the upper rectification column 8c can be relatively reduced. In other words, the lower the purity of the liquid oxygen is, the lower its boiling point is, and it is possible to evaporate at a lower temperature. Accordingly, the nitrogen gas (gas sent from the top of the lower column 8a to the main condenser 8b) serving as a heating source for evaporating the liquid oxygen in the upper column 8c also has a lower pressure (the lower the pressure, the lower the condensation of nitrogen). Operating temperature will be lower). As a result, an operation in which the pressure of the lower tower 8a is set to be lower becomes possible, and it is possible to contribute to a reduction in power of the raw material air compressor.

The above-described configuration is a description of a preferred configuration that can be added as an additional feature when the present invention is carried out. In the present invention, as described above, return gas is returned to an unused portion of the main heat exchanger. By branching and passing, it has the greatest feature in that the holding capacity of the main heat exchanger can be utilized most effectively, so the remaining configuration, for example, the main heat exchanger is configured The specific structure and assembling structure of the heat exchange unit, as well as other equipment such as air compressors, coolers, adsorbers, the upper and lower rectification towers, and their piping and connection structures are particularly limited. Instead, it is possible to appropriately select and apply equipment and piping / connection methods applied to this type of air separation device. For example, as an adsorber, in addition to a molecular sieve adsorber, it is included in air. Impurities (moisture It is of course possible to use other adsorbers as long as they have a function of removing carbon dioxide gas, hydrocarbon gas, etc.). Of course, the present invention can also be effectively used for a rectification column in which a Raschig ring, a pole ring, a bar saddle, an intercross saddle, and other structural fillers are filled to reduce pressure loss.

[0049]

The present invention is configured as described above, and the part of the main heat exchanger that has been left unused can be effectively used for the cold recovery of the return gas. Energy loss can be reduced.
In addition, if crude nitrogen gas is selected as the return gas used in the unused part, the pressure loss of the crude nitrogen gas in the main heat exchanger can be suppressed, and power consumption for air separation can be reduced. Is preferred.

[Brief description of the drawings]

FIG. 1 is a main part flowchart showing an excerpt of a characteristic part of an air separation method and apparatus according to the present invention.

FIG. 2 is a schematic plan view illustrating the structure of a heat exchange unit used in a main heat exchanger when implementing the present invention.

FIG. 3 is an overall flowchart for explaining a conventional air separation method and apparatus.

FIG. 4 is a schematic partial cross-sectional view illustrating the structure of a distributor preferably provided in a heat exchange unit of a main heat exchanger when carrying out the present invention.

FIG. 5 is a partial cross-sectional view illustrating the structure of another distributor preferably provided in the heat exchange unit of the main heat exchanger when carrying out the present invention.

FIG. 6 is a partial cross-sectional view illustrating the structure of still another distributor preferably provided in the heat exchange unit of the main heat exchanger when carrying out the present invention.

FIG. 7 is a partial cross-sectional view illustrating the structure of still another distributor preferably provided in the heat exchange unit of the main heat exchanger when carrying out the present invention.

FIG. 8 is an essential part flow diagram showing an excerpted portion of a characteristic part of an air separation method and apparatus preferably adopted in the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Air filter 2 Raw material air compressor 3 Cooler 4 Evaporative cooler 5 Expansion turbine 6 Molecular sieve adsorber 7 Main heat exchanger 8 Rectification tower 8a Lower tower 8b Main condenser 8c Upper tower 9 Nitrogen rich liquid 10 Oxygen rich liquid 29 regeneration heater 31 vaporizer 32 heating heat exchanger U, U 'heat exchange unit _{D 1, D 2, D 3} , D 4, D 5 distributor F corrugated fin P plate S strut

Claims (2)

[Claims] 1. A pre-cooled compressed air is supplied to a rectification tower through a main heat exchanger, and a return gas from the rectification tower is passed through the main heat exchanger to be used for cooling the compressed air. In the air separation method, a part of the compressed air flowing through the main heat exchanger is withdrawn from the middle of the main heat exchanger, adiabatically expanded, and then supplied to the upper tower of the rectification column. An air separation method, wherein a line on a lower temperature side than a position for extracting compressed air from is used as a return gas passage line, thereby increasing a heat exchange utilization rate of the return gas. 2. A pre-cooled compressed air is supplied to a rectification tower through a main heat exchanger, and a return gas from the rectification tower is passed through the main heat exchanger and used for cooling the compressed air. In the air separation device, a part of the compressed air flowing through the main heat exchanger is withdrawn from the middle of the main heat exchanger, adiabatically expanded, and then supplied to the upper tower of the rectification column. To form the return gas line at a lower temperature side than the compressed air extraction position in the middle of the main heat exchanger, and join the other return gas line of the main heat exchanger at the high temperature side. An air separation device, comprising: