JPS6257915B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an air separation technique for separating oxygen, argon, and nitrogen by rectification at low temperatures (cryogenic method), and particularly to a method for efficiently separating argon.

In an air separation device using cryogenic separation, cooling and impurities in the air are removed by a self-cooling method that indirectly exchanges heat between the low-temperature gas fluid generated within the system and the feed air using a switching heat exchanger. Thereafter, the air is fed to the bottom of the double rectifier medium pressure column. A double rectification column consists of a medium pressure column that operates at a medium pressure of 5 to 6 ata, and a
A low pressure column operating at a low pressure of ~1.4 ata is connected by a main condenser. The medium-pressure column liquefies the supplied air and roughly separates the air, which is extracted as a liquid from the medium-pressure column and used as the reflux liquid of the low-pressure column. In the low pressure column, the main condenser is used as a reboiler to generate oxygen vapor, and the reflux liquid from the low pressure column is brought into gas-liquid contact within the low pressure column to perform rectification separation. In the low-pressure column, the oxygen concentration gradually decreases toward the upper trays, but for argon, which is an intermediate component between nitrogen and oxygen, a tray where the argon concentration is maximum occurs in the low-pressure column. Argon-containing vapor is extracted from appropriate trays near the tray where the argon concentration is at its maximum and is used as a feed gas for the crude argon column. In the crude argon column, crude argon is obtained from the top of the crude argon column mainly by crude separation of oxygen and argon. Note that, in order to obtain product argon, crude argon is further purified by conducting a purification process. The bottoms extracted from the bottom of the crude argon column is returned to the crude argon column feed gas extraction plate of the low pressure column. The operating principle of such a conventional argon recovery method will be explained with reference to FIG.

The raw air 100 is pressurized to a pressure of 5 to 6 atata by the compressor 7 and is led to the primary exchange heat exchanger 8 as a stream 107. Here, the air is indirectly exchanged with product oxygen, nitrogen gas, and impure nitrogen gas extracted from the low-pressure column, and water, carbon dioxide, hydrocarbons, etc. are removed and cooled, and the stream 104 is transferred to the lower part of the medium-pressure column 1. supply to. Further, a part of the air flowing through the switching heat exchanger 8 is guided from the flow 116 to the expansion turbine 9, where it is adiabatically expanded and supplied with the amount of refrigeration required by the system. Adiabatically expanded and cooled air is supplied from stream 117 to the middle stage of low pressure column 3. The air supplied to the medium pressure column 1 is liquefied in the main condenser 2, and purified by rectification separation within the medium pressure column. Stream 10 from medium pressure column 1
5,106,108. A stream 109 obtained by expanding the stream 108 withdrawn from the upper part of the medium pressure column 1 by the expansion valve 6 is supplied to the upper part of the low pressure column as a reflux liquid. On the other hand, a stream 106 withdrawn from the middle stage of the intermediate pressure column 1 passes through the expansion valve 6 and is supplied as a reflux liquid from a stream 107 to the tray in the upper region of the low pressure column. The liquid withdrawn from the bottom of the medium pressure column 1 in stream 124 is split into two streams, stream 125 is fed through expansion valve 6 from stream 126 to the upper region of the low pressure column. The other stream 105 is fed through the expansion valve 6 from stream 118 to the primary side of the condenser 5 as a refrigerant to the condenser 5 of the crude argon column 4, and the secondary stream 12
0 is fed to the upper region of the low pressure column. In the low-pressure column 3, the main condenser 2 is used as a reboiler to generate oxygen vapor, which is brought into gas-liquid contact with the reflux liquid from the medium-pressure column 1 to perform rectification separation. The oxygen separated in the low-pressure column is withdrawn as a gas from the lower part of the low-pressure column as stream 110 and passed through the switching heat exchanger 8 to extract the product as stream 111. On the other hand, the product nitrogen gas is extracted from the upper part of the low-pressure column, converted into a stream 112, passed through the switching heat exchanger 8, cooled and recovered, and then extracted from the system as a stream 113. In addition, impure nitrogen gas flows from the upper region of the low-pressure column.
4, and after cold recovery through the switching heat exchanger 8, it is discharged as a stream 115 to the outside of the system.

When rectification separation is performed in a low pressure column, argon, which is an intermediate component between nitrogen and oxygen, is produced in a tray where the argon concentration is at its maximum. Argon-containing vapor is extracted from a suitable stage near this stage and supplied to the crude argon column 4 from stream 121. The vapor supplied to the crude argon column 4 is liquefied in the condenser 5 installed at the top of the crude argon column, and descends between the plates as an internal reflux liquid.
The gas-liquid comes into contact with the steam rising inside the column, and rectification separation is performed. The crudely separated argon is withdrawn as a crude argon top stream 123. On the other hand, the bottoms extracted from the bottom of the crude argon column is stream 122.
The crude argon column feed gas is returned to the extraction tray 10 of the low pressure column.

When attempting to recover argon from the air by such cryogenic separation, selection of the crude argon column reflux ratio is important. From the viewpoint of the operation of the crude argon column, it is preferable to select a large reflux ratio for the crude argon column.
Nitrogen may be mixed into the feed gas of No. 21, which may adversely affect the rectification separation function of the low pressure column in various ways. This is because when recovering argon, the materials in the low-pressure column and the crude argon column have a close influence on the relationship between receiving and receiving heat, but considering the rectification separation function of the low-pressure column, The ratio cannot be made larger than necessary. On the other hand, at the bottom of the crude argon column in this argon recovery method, approximately argon,
It is operated in a pseudo-pinch state where the oxygen content does not change. Considering the material balance around the crude argon column in this pinch state, as shown in equation (1), the argon concentration y _∞i of the crude argon column supply gas is the crude argon purity x _Di , the crude argon column reflux ratio R, and the pinch If the gas-liquid equilibrium constant of the state is K _∞i , then y _∞i = x _Di・K _∞i / (R+1)K _∞i −R
...(1) becomes. That is, from equation (1), it is clear that x _Di depends on the argon concentration in the crude argon column feed gas y _∞i , assuming that R is constant. That is, in order to obtain a high argon recovery rate with this argon recovery method, it is necessary and important to select a small reflux ratio of the crude argon column and to increase the argon concentration of the supplied gas. However, in the conventional argon recovery method and apparatus described above, the argon concentration in the gas supplied to the crude argon column is restricted due to various constraints.

An object of the present invention is to provide an argon recovery method that increases the argon concentration in the gas supplied to the crude argon column and substantially increases the argon recovery rate. The main points of the present invention will be explained below. At the bottom of the crude argon column, there is a pseudo-pinch state as described above, and the gas supplied to the crude argon column and the composition of the bottoms are approximately in a vapor-liquid equilibrium relationship. In the conventional argon recovery method, the bottoms from the crude argon column were returned to the same tray as the extraction tray for the crude argon column feed gas in the low-pressure column. However, in the internal composition of the low-pressure column in the argon recovery method, the plate where argon is at its maximum is always located above the crude argon column feed gas extraction plate. However, near the plate where argon is at its maximum, nitrogen is mixed in to the extent that it will impede argon purity and recovery rate if it is introduced as a feed gas into a crude argon column. For this reason, it is possible to increase the argon concentration of the gas supplied to the crude argon column by improving the rectification separation function near the plate where the argon concentration of the low-pressure column is at its maximum and separating nitrogen, which is an inhibiting factor, from the argon component. It is. In the present invention, when returning the bottoms from the crude argon column to the low pressure column, one
~By returning to the upper stage of the 4th stage and increasing the gas-liquid flow rate ratio (referred to as L/V value) of the upper stages 1 to 4 of the supply gas extraction stage, which influences the argon concentration of the supply gas, to improve the rectification separation efficiency. It is something to be solved.

Next, one embodiment of the present invention will be described. FIG. 2 shows a typical working principle diagram of the present invention. In FIG. 2, argon-containing vapor is extracted from the crude argon column feed gas extraction stage 10 of the low pressure column 3 of the double rectification column 121. Stream 121 becomes the feed gas for purified argon column 4. The supply gas is led to the lower part of the crude argon column, liquid air is supplied from the medium pressure column 1 to the crude argon column condenser as a refrigerant, the vapor rising to the condenser 5 is liquefied, and a part of the liquid is stored inside the crude argon column. It becomes a reflux liquid. In the column, the rising vapor and reflux liquid come into contact with gas and liquid and are separated by rectification, and argon is concentrated in the upper part of the crude argon column. Concentrated argon is withdrawn from stream 123. On the other hand, the bottoms of the crude argon column 4 is stream 1
22 and is returned to the return stage 20 for the crude argon column bottoms of the low pressure column 3 of the double rectification column. In the conventional method, the bottoms (stream 122) of the crude argon column is returned to the same tray 10 as the crude argon column feed gas extraction stage of the low pressure column, but in the method of the present invention, as shown in FIG. Crude argon column feed gas extraction stage 1
0 to the upper return step 20, and the embodiment shown in FIG. 2 shows the case of returning to the upper step of 4 steps. FIG. 3 shows an example of the specific effects of the present invention. In Fig. 3, the tray for extracting the crude argon column supply gas from the low-pressure column is fixed, and in order to return the bottoms of the crude argon column to the low-pressure column, it is moved from the extraction tray to the upper tray and the lower tray. , and the relationship between the argon concentration in the crude argon column supply gas and the argon concentration in the crude argon extracted from the upper part of the crude argon column is also shown. As a result, it is recognized that the argon concentration of the feed gas becomes higher as the bottoms return stage is returned to an upper stage than the feed gas extraction stage. However, the effect tends to be saturated even if the return stage is moved four stages or more from the supply gas extraction stage. In addition, from the supply gas extraction stage, 5
If the return stage is moved further than the stage, the nitrogen concentration becomes high enough to inhibit the argon recovery rate in the crude argon column. On the other hand, the argon concentration of the crude argon extracted from the upper part of the crude argon column tends to increase as the argon concentration in the supplied gas increases.

As described above, in the present invention, by returning the bottoms of the crude argon column to the 1st to 4th stages above the stage from which the crude argon tower supply gas is extracted from the low pressure column, the The V value increases, and as a result, the rectification separation efficiency improves.

The above-described effects of the present invention are also clear from a comparison of FIGS. 4 and 5. That is, Figure 4 shows the distribution of argon and nitrogen components near the supply gas extraction stage and the crude argon column bottoms return stage in the low-pressure column in the conventional method, and Figure 5 shows the same distribution in the present invention method. The composition inside the column is shown. A comparison of both shows that the present invention is more effective than the conventional method. That is, when comparing the method of the present invention and the conventional method with respect to the L/V value of the extraction stage of the low-pressure column that extracts the crude argon column supply gas and the return stage of the bottoms of the crude argon column, it is found that L'/V is lower than that of the conventional method. Therefore, in the method of the present invention, the internal reflux liquid volume L _A of the crude argon column becomes (L'+L _A )/V, and in the method of the present invention, L/V
The value can be increased by (L'+ _LA )/ _LA . Here, the comparison with the conventional method was made under the same conditions such as product oxygen, nitrogen, and crude argon column reflux ratio, but with the method of the present invention, the product oxygen recovery rate was also higher than the conventional method due to the improved argon recovery rate. It became clear that it was going to be expensive.

As an application example of the present invention, the effects of the present invention can be expected even if the crude argon column supply gas is removed from the low pressure column in multiple stages, or the bottoms of the crude argon column are returned to the low pressure column in multiple stages. can. Also,
In the examples cited for the effects of the present invention, the product was collected using gas, but the effects of the present invention can also be exhibited even if the product is extracted from the system as a liquid.

As a result of the above, according to the method of the present invention, even though argon is to be obtained by cryogenic separation, argon-containing steam is vaporized from the tray near the tray where the argon concentration in the low pressure column of the double rectification column is at its maximum. In order to extract argon, roughly separate the argon in an attached crude argon column, and return the bottoms of the crude argon column to the low pressure column of the double rectification column, 1 to By returning the gas to the upper shelf of the fourth stage, the L/V value, which controls the rectification separation efficiency, is increased, thereby increasing the argon concentration in the supplied gas, increasing the argon recovery rate. The effect of doing so can be obtained. An additional effect is that the oxygen recovery rate also increases, and the oxygen power consumption rate, which is an indicator of running costs from the perspective of the entire plant, can be lowered.

[Brief explanation of the drawing]

Fig. 1 is a flow sheet diagram showing a conventional example of an air separation device, Fig. 2 is an operating principle diagram showing an argon recovery method in an air separation device which is an embodiment of the present invention, and Fig. 3 is a flow sheet diagram showing a conventional example of an air separation device. A diagram of the relationship between the liquid return stage to the upper column, the argon concentration in the feed gas, and the argon concentration in the crude argon, and Figures 4 and 5 show the argon and nitrogen concentrations around the feed gas extraction stage in the low-pressure column, respectively. It is a distribution map. 1...Double rectification column medium pressure column, 2...Main condenser, 3
...Double rectification column low pressure column, 4...Crude argon column, 5
...crude argon column condenser, 6...expansion valve, 7...
Compressor, 8... Switched heat exchanger, 9... Expansion turbine, 100-126... Flow number.

Claims (1)

[Claims] 1 Air compressed and cooled by a switching heat exchanger is compressed and cooled by a switching heat exchanger, and a combination of a rectification tower that operates at low and intermediate pressures is connected by a main condenser, and a crude argon tower that operates at low pressure, converts the air into oxygen and nitrogen. , in which argon is cryogenically separated, argon-containing vapor is extracted from the tray near the tray where the argon concentration is maximum in the low-pressure column of the double rectification column and where the nitrogen concentration is low, and the vapor is transferred to the crude argon column. After rough argon separation in the crude argon column, the bottoms extracted from the bottom of the crude argon column are passed through one or more stages above the tray from which the crude argon column feed gas of the double rectification column low-pressure column is extracted. A method for recovering argon by cryogenic separation, characterized in that argon is returned to the upper shelf of four stages.