JP2009509120A - Method and apparatus for separating air by cryogenic distillation. - Google Patents

Abstract

A method for separating air by cryogenic distillation in a column system comprising a high pressure column and a low pressure column comprises compressing all feed air to a first outlet pressure in a first compressor (1), and said first outlet Sending a first portion of the air at pressure to a second compressor (3), compressing the air to a second outlet pressure, and heat exchanging at least a portion of the air at the second outlet pressure Cooling in a vessel (5), liquefying at least part of the air at the second outlet pressure, and sending the liquefied air to at least one tower of the tower system, the tower system At least 50% of the liquefied air sent to the air is compressed in the second compressor, and the second portion (12) of the air at the first outlet pressure is cooled in the heat exchanger. The above At least a portion of the second portion of gas is expanded in the expander (13) from the first outlet pressure to the pressure of one column (30, 31) of the column system, and the expanded air is expanded into the column The auxiliary fluid (6) is at least partially vaporized, and finally the auxiliary fluid is heated in the heat exchanger, and at least part of the auxiliary fluid is transferred to the third compressor. (8) to a third outlet pressure, introducing at least a portion (9) of the auxiliary fluid at the third outlet pressure into the heat exchanger, cooling the auxiliary fluid, and Liquefying the auxiliary fluid and removing the auxiliary stream (10) from the heat exchanger and passing it into the heat exchanger where it is partially vaporized as described above. 4th pressure level before reintroducing it In comprising a inflating, and taking out liquid (20) from one column of the column system (31), and vaporizing the liquid by heat exchange in the heat exchanger.
[Selection] Figure 2

Description

  The present invention relates to a method and apparatus for separating air by cryogenic distillation. In particular, it relates to a method and apparatus for the production of oxygen and / or nitrogen at elevated pressure.

  The gaseous oxygen produced by the air separation plant is usually at an elevated pressure of about 20-50 bar. The basic distillation scheme is a double column process that produces oxygen at the bottom of a low pressure column, usually operated at 1.4 to 4 bar. Oxygen needs to be compressed to high pressure by either an oxygen compressor or a liquid pump process. Due to safety issues associated with oxygen compressors, modern oxygen plants are based on a liquid pump process. In order to vaporize liquid oxygen at elevated pressures, an additional motor driven booster compressor is required that raises a portion of the supply air or nitrogen to a higher pressure in the range of 40-80 bar. In essence, the booster has replaced the oxygen compressor.

  In an effort to reduce the complexity of the oxygen plant, it is desirable to reduce the number of motor driven compressors. Significant cost savings can be achieved if the booster can be removed without significantly adversely affecting plant performance in terms of power consumption. Furthermore, an air purification unit conceived for a conventional oxygen plant will operate at about 5-7 bar, which is substantially the pressure of the high pressure column, and this pressure will be increased to make the device smaller and cheaper. It is also desirable to raise it to a higher level.

  The cold compression process described in US Pat. No. 5,475,980 provides a technique for driving an oxygen plant with a single air compressor. In this process, the air to be distilled is further compressed by a booster compressor driven by an expander that cools in the main exchanger and then exhausts into the high pressure column of the double column process. By doing so, the discharge pressure of the air compressor is in the range of 15 bar, which is also quite convenient for the purification unit. One inconvenience of this approach is the increase in main exchanger size due to the additional flow recirculation typical of cold compression plants. By pioneering the exchanger temperature approach, the size of the exchanger can be reduced. However, this will result in inefficient power utilization of the compressor and higher discharge pressure, thereby increasing its cost. An illustration of this prior art is shown in FIG. 1, where an oil brake has been added to the system, wasting the power required for refrigeration. In larger plants, the compressor and / or generator can replace the oil brake.

  In FIG. 1, all the supply air is compressed in the compressor 1, purified in the purification unit 2 and sent as stream 11 to the warm end of the heat exchanger 5. All supply air is cooled to an intermediate temperature, removed from the heat exchanger as stream 7 and compressed in cold compressor 8. The compressed stream 9 is sent back to the heat exchanger at a higher intermediate temperature, cooled to a temperature lower than the temperature at the inlet of the cold compressor 8 and divided in two. Stream 15 is sent to Claude expander 13 which is braked by compressor 8 and oil brake. The remaining air 10 is liquefied in a heat exchanger and divided into two parts, one part being sent to the high pressure column 30 and the remaining 34 being sent to the low pressure column 31.

  The oxygen-enriched liquid stream 28 is expanded and sent from the high pressure column to the low pressure column. The nitrogen-enriched liquid stream 29 is expanded and sent from the high pressure column to the low pressure column. High pressure gaseous nitrogen 14 is withdrawn from the top of the high pressure column and warmed in a heat exchanger to produce a product stream 24. The liquid oxygen 20 is taken from the bottom of the low pressure column 31 and pressurized by the pump 21 and sent to the heat exchanger 5 as a stream 22 where it is vaporized by heat exchange with the pressurized air 10. To generate pressurized oxygen 23 in the form of gas. A nitrogen-rich gas stream 25 at the top of the column is withdrawn from the low pressure column 31 and warmed in the heat exchanger 5, thereby producing a stream 26.

  Several different variations of the cold compression process are also described in the prior art, such as US Pat. No. 5,379,598, US Pat. No. 5,596,885, US Pat. No. 5,901,576 and US Pat. No. 6,626,008.

  In US Pat. No. 5,379,598, a portion of the supply air is further compressed by a booster compressor followed by a cold compressor to obtain the pressurized flow required for oxygen vaporization. This approach still has at least two compressors and the purification unit still operates at low pressure.

  In US Pat. No. 5,596,885, a portion of the supply air is further compressed in a warm booster while at least a portion of this air is further compressed in a cold booster. The air from both boosters is liquefied and a portion of the cold compressed air is expanded in a Claude expander.

  U.S. Pat.No. 5,901,576 describes several arrangements of cold compression schemes that utilize cold rich liquid expansion at the bottom of the high pressure column or high pressure nitrogen expansion to Drive the compressor. In some cases, motor driven cold compressors were also used. These processes also operate with feed air at approximately the pressure of the high pressure column, and in most cases a booster compressor is also required.

  U.S. Pat. No. 6,626,008 describes a heat pump cycle that utilizes a cold compressor to improve the distillation process for the production of low purity oxygen for a double vaporizer oxygen process. . Low air pressure and booster compressors are also typical for this type of process.

  It is therefore an object of the present invention to solve the inconveniences of conventional processes by providing a solution to simplify the compressor row and reduce the size of the purification unit. . This can also be achieved with good power consumption. Thereby, the overall production cost of the oxygen plant can be reduced. The main improvement in power consumption is due to the cold compressor flow being reduced by essentially using latent heat instead of specific heat.

  All percentages stated are molar percentages.

According to the present invention, a method for separating air by cryogenic distillation in a column system comprising a high pressure column and a low pressure column, comprising:
i) compressing all supply air to the first outlet pressure in the first compressor;
ii) sending a first portion of the air at the first outlet pressure to a second compressor and compressing the air to a second outlet pressure;
iii) cooling at least a portion of the air at the second outlet pressure in a heat exchanger to produce cooled compressed air at the second outlet pressure, wherein the air at the second outlet pressure Liquefying at least a portion and sending the liquefied air to at least one tower of the tower system;
iv) cooling the second part of the air at the first outlet pressure in the heat exchanger, and at least part of the second part of the air in the expander from the first outlet pressure to the tower system Expanding to the pressure of one tower and sending the expanded air to the tower;
v) removing liquid from one tower of the tower system, pressurizing the liquid, and evaporating the liquid in the heat exchanger by heat exchange;
vi) At least partially vaporize the auxiliary fluid and eventually warm the auxiliary fluid in the heat exchanger and send at least a portion of the auxiliary fluid to the third compressor to a third outlet pressure Compressing and introducing at least a portion of the auxiliary fluid at the third outlet pressure into the heat exchanger, cooling the auxiliary fluid and at least partially liquefying the auxiliary fluid; And removing it from the heat exchanger and expanding it to a fourth pressure level before reintroducing it into the heat exchanger where it is vaporized as described above. The

According to any feature of the invention,
-Additional air at the first pressure is liquefied in the heat exchanger.
- the third compressor, the following _{gases: He, H 2, Ne,} N 2, CO, Ar, O 2, CH 4, Kr, NO, Xe, CF 4, HCF 3, C 2 H 4, C _The auxiliary fluid containing at least one of ₂ H ₆ , C ₂ F ₆ , C ₃ F ₈ , N ₂ O, and CO ₂ is compressed.

The third compressor compresses the auxiliary fluid whose main component contains at least one of Ar, O ₂ , CH ₄ and Kr;

According to another aspect of the invention, an apparatus for separating air by cryogenic distillation comprising:
a) a tower system;
b) first, second and third compressors;
c) an expander;
d) a conduit for sending air to the first compressor to produce compressed air at a first outlet pressure;
e) a conduit that delivers a first portion of the air at the first outlet pressure to the second compressor to generate air at a second outlet pressure;
f) a heat exchanger and a conduit that delivers at least a portion of the air at the second outlet pressure to the heat exchanger to generate cooled compressed air at the second outlet pressure;
g) a conduit for removing liquefied air at the second outlet pressure from the heat exchanger and delivering the liquefied air to at least one tower of the tower system;
h) a conduit for removing a second portion of the air at the first outlet pressure from the heat exchanger and delivering at least a portion of the second portion of the air to the expander;
i) a conduit for sending air expanded in the expander to at least one tower of the tower system;
j) a conduit for removing liquid from one tower of the tower system, means for pressurizing at least a portion of the liquid to produce a pressurized liquid, and at least a portion of the pressurized liquid to the heat exchanger. A conduit to send,
k) a refrigeration cycle comprising the third compressor and second expander (16), a conduit for sending auxiliary fluid from the third compressor to the heat exchanger, and the auxiliary fluid from the heat exchanger. A conduit for sending the auxiliary fluid to the second expander, a conduit for sending the auxiliary fluid from the second expander to the heat exchanger, and a conduit for sending the auxiliary fluid from the heat exchanger to the third compressor There is provided an apparatus comprising:

  According to a further optional aspect of the invention, the device may comprise a further expander and means for sending nitrogen from the tower system or outside air to the further expander.

  In this case, one of the second and third compressors may be connected to the expander, and the other of the second and third compressors may be the further compressor. It may be connected to.

  At least one of the second and third compressors is connected to an air expander.

  Preferably, the conduit for delivering the first portion of the air at the first outlet pressure to the second compressor is connected to an intermediate position of the heat exchanger.

  Preferably, the second and third compressors are connected in series.

  The expander is selected from the group comprising an air expander having an outlet connected to a high pressure tower, an air expander having an outlet connected to a low pressure tower, a high pressure nitrogen expander, and a low pressure nitrogen expander. May be.

  The apparatus is selected from the group comprising an air expander having an outlet connected to the high pressure column, an air expander having an outlet connected to the low pressure column, a high pressure nitrogen expander, and a low pressure nitrogen expander. Additional expanders may be included.

  Preferably, the further expander is connected to one of the second and third expanders.

  Here, FIGS. 2, 3, 5 and 6 which are process flow diagrams showing a low-temperature air separation method according to the present invention, FIG. 4 which is a heat exchange diagram, and a connecting system of a compressor and an expander in the method according to the present invention. The present invention will be described in more detail with reference to FIG.

  In the embodiment of FIG. 2, atmospheric air is compressed by the air compressor 1 and purified in the purification unit 2 to obtain an air stream (stream 11). This stream 11 is free of impurities such as moisture and carbon dioxide that can be frozen in a cryogenic device. This first portion of air is compressed in booster brake compressor 3 to further increase its pressure. This pressurized first portion (stream 4) is then cooled and condensed in the main exchanger 5 to produce a liquefied air stream (stream 27). This stream 27 is fed to at least one of the distillation columns after expansion at the valve. This air can liquefy inside or downstream of the main exchanger depending on the pressure used. Auxiliary fluid mixture 6 consisting of krypton (90%) and oxygen (10%) is introduced into heat exchanger 5 when it is vaporized and slightly warmed after vaporization to provide a cold intermediate temperature T1. A flow of auxiliary gas is obtained. At least a portion of this cold auxiliary stream (stream 7) is sent to a cold brake compressor 8 at temperature T1, where it is compressed to increase its pressure (stream 9). Next, stream 9 is sent back to a heat exchanger at a temperature T2 higher than T1, where it is cooled and condensed in exchanger 5 to produce a liquefied auxiliary stream (stream 10). Stream 10 is expanded at valve 16 to produce stream 6. If stream 6 is a two-phase fluid, a phase separator may be added, the liquid phase is introduced into heat exchanger 5 and the vapor phase is mixed with stream 7. The second part of stream 11 (stream 12) is cooled in exchanger 5 to obtain stream 15. Stream 15 is sent to expander 13 at the inlet temperature of T3 for expansion into the high pressure column. The power generated by the expander 13 is preferably used to drive the booster brake compressor 3. The remainder of stream 12 is liquefied as stream 33 and sent to high pressure column 30. Nitrogen rich gas 14 can be removed from high pressure column 30 and heated in exchanger 5 to produce stream 17. Next, the stream 17 is expanded in an expander 18 having an inlet temperature T4. The output of the expander 18 can preferably be used to drive the cold booster brake compressor 8. The exhaust gas of expander 18 (stream 19) is then returned to the cold end of exchanger 5 and reheated to near ambient temperature to produce stream 24. The pump 21 raises the pressure of the liquid oxygen product 20 taken at the bottom of the low pressure column 31 to the desired pressure, and then sends the pressurized oxygen stream 22 to the exchanger 5 for evaporation and heating. Thus, the oxygen product 23 is obtained. Double column systems are a traditional type of double column process, as described in numerous patents or papers on air separation technology, and are thermally coupled at the bottom of the low pressure column by a reboiler-condenser. The high pressure column 30 and the low pressure column 31 are provided. An argon column (not shown) can be used with a double column system to provide a stream of concentrated argon.

  The above temperatures T1, T2, T3 and T4 are provided in a preferred arrangement. Arranging the above from the highest temperature to the lowest temperature results in T2, T5, T1, and T3. Depending on the pressure of the vaporized oxygen and the pressure of the column system, the order of these temperatures can be changed to optimize process performance.

  The booster brake compressor 3 is a one-stage compressor and is usually provided as part of an expander booster package, and therefore its construction is much simpler than a stand alone or motor driven booster compressor. It is beneficial to realize that and its cost structure is very low. However, if necessary, the compressor 3 may be a stand-alone or motor-driven booster compressor. The compressor 8 may be either a stand alone or motor driven booster compressor having one to four stages depending on the pressure of the stream 4 and the stream 23. In order to optimize the performance of the booster and expander, it may be driven directly by the expander 18 (or expander 13) at the same speed or using a transmission.

The range of process variables for the embodiment of FIG. 2 is as follows:
Pressure of stream 11: about 9 to 17 bar a
Pressure of stream 4: about 16 to 50 bar a
Stream 9 pressure: about 5 to 20 bara for krypton rich mixture
T1: about −110 ° C. to −165 ° C.

  The flow compressed by the booster brake compressor 8 can be reduced by optionally taking some of the stream 12 as a liquefied air stream 33. By doing so, little power is required to drive the booster brake compressor 8, and some power savings can be achieved. The amount of liquefied air at the first pressure should be no more than 50%, preferably no more than 40%, more preferably no more than 35% of the liquefied air sent to the tower system.

  Replacing a nitrogen expander with an air expander is common practice in air separation technology. The embodiment of FIG. 3 illustrates such an arrangement: after the first compressor, a part 12 of the stream 11 is cooled in the exchanger 5 and a part of this stream is withdrawn to produce a stream 50. Get. Stream 50 is sent to expander 52 for expansion into low pressure column 31. The output of the expander 52 is preferably used to drive the cold compressor 8. Choose to split stream 12 in front of exchanger 5 and send the corresponding air stream to the separation path in exchanger 5, then cool it and expand it toward the tower in expander 52 It is beneficial to realize that they can also be FIG. 4 shows an exchange diagram corresponding to the process of FIG.

  The above technique can be modified slightly as described in FIG. 5; a portion 53 of the air in the exhaust stream 54 of the expander 13 is warmed in the exchanger 5 and then in the low pressure column. Can be sent to the expander 52 for expansion. If some condensation occurs in stream 54, the gas supplied to expander 52 can be withdrawn by adding a gas-liquid separator, or better still, a high pressure column sump can be used as a separator. In this case, the gas supplied to the expander is withdrawn at the sump of the high pressure column.

  In many cases requiring a significant amount of nitrogen rich gas product at elevated pressure, it is no longer economical to utilize the nitrogen rich gas expander 18. Instead, as shown in FIG. 6, the nitrogen-rich gas 14 can be extracted directly from the high pressure column 30 to produce a nitrogen product 41. In such a case, the power output of the expander 13 can be increased by choosing to increase the pressure of the compressor 1 to compensate for the lack of cooling due to the exclusion of the nitrogen expander. To further simplify the placement of the expander and booster brake compressor, the series of expanders and booster brakes can be mechanically integrated into one row: the output of the expander 13 is 2 The compression brakes 3 (1 stage) and 8 (2 stages) are driven. In addition, the motor and / or generator 60 can extract mechanical power or add to the system depending on the performance and output expected from the plant at a given time. Depending on the flow and pressure of the expander and booster brake compressor, a transmission (transmission) can be used to optimize system performance. A diagram of the arrangement using the transmission is shown in FIG. Additional expanders 18, 52 can also be added to the system.

  This process may be modified to vaporize the pumped liquid nitrogen as an additional stream or as a stream that replaces the pumped oxygen stream.

  The illustrated process shows a double column system, but it will be readily understood that the present invention applies to a triple column system.

  Some of the low pressure nitrogen may be expanded in the expander 18 when the double column or triple column system operates at elevated pressure.

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Claims (8)

A method for separating air by cryogenic distillation in a column system comprising a high pressure column and a low pressure column,
i) compressing all supply air to the first outlet pressure in the first compressor (1);
ii) sending a first portion (4) of the air at the first outlet pressure to a second compressor (3) and compressing the air to a second outlet pressure;
iii) cooling at least a portion of the air at the second outlet pressure in a heat exchanger (5);
iv) cooling the second part (12) of the air at the first outlet pressure in the heat exchanger, and at least part of the second part of the air in the expander (13) Expanding from the outlet pressure to the pressure of one tower (30) of the tower system and sending the expanded air to the tower;
v) removing liquid (20) from one tower (31) of the tower system, pressurizing the liquid, and vaporizing the liquid by heat exchange in the heat exchanger;
vi) The auxiliary fluid is at least partially vaporized in the heat exchanger, and finally the auxiliary fluid is heated in the heat exchanger, and at least a part of the auxiliary fluid is transferred to the third compressor (8). And compressing to a third outlet pressure, introducing at least a portion (9) of the auxiliary fluid at the third outlet pressure into the heat exchanger, cooling the auxiliary fluid, and A fourth pressure before partially liquefying, removing the auxiliary stream from the heat exchanger, and reintroducing it into the heat exchanger for at least the partial vaporization step described above. Expanding to a level.   The method of claim 1, wherein at least a portion of the first portion of the air is cooled upstream of the second compressor (3).   The method of claim 2, wherein at least a portion of the first portion of the air is cooled in the heat exchanger (5) upstream of the second compressor (3).   The method of claim 2, wherein at least a portion of the first portion of the air is cooled upstream of the second compressor using a refrigeration unit in the heat exchanger.   The method according to claims 1 to 4, wherein additional air (27, 33) at at least one of the first and second pressures is liquefied in the heat exchanger. The third compressor has the following gases: He, H ₂ , Ne, N ₂ , CO, Ar, O ₂ , CH ₄ , Kr, NO, Xe, CF ₄ , HCF ₃ , C ₂ H ₄ , C _{2. H 6, C 2 F 6,} C 3 F 8, N 2 O, claims 1 to 5 method according to compress an auxiliary fluid chosen from the group comprising at least one of CO _2. The method according to claim 6, wherein a main component of the auxiliary fluid is at least one of Ar, O ₂ , CH _4, and Kr. An apparatus for separating air by cryogenic distillation,
a) tower system (30, 31);
b) first, second and third compressors (1, 3, 8); c) first expander (13);
d) a conduit for sending air to the first compressor to produce compressed air at a first outlet pressure;
e) a conduit that delivers a first portion of the air at the first outlet pressure to the second compressor to generate air at a second outlet pressure;
f) a heat exchanger (5) and a conduit for delivering at least a portion of the air at the second outlet pressure to the heat exchanger to produce cooled compressed air at the second outlet pressure;
g) a conduit for removing liquefied air at the second outlet pressure from the heat exchanger and delivering the liquefied air to at least one tower of the tower system;
h) removing a second portion of the air at the first outlet pressure from the heat exchanger and removing at least a portion of the second portion of the air from the expanded air in at least a tower system; A conduit leading to the expander conduit leading to one tower;
i) a conduit for removing liquid from one tower of the tower system, means for pressurizing at least a portion of the liquid to produce a pressurized liquid, and at least a portion of the pressurized liquid to the heat exchanger And a conduit to send
j) a refrigeration cycle comprising the third compressor and the second expander (16), a conduit for sending auxiliary fluid from the third compressor to the heat exchanger, and the auxiliary fluid from the heat exchanger. A conduit for sending the auxiliary fluid to the second expander, a conduit for sending the auxiliary fluid from the second expander to the heat exchanger, and a conduit for sending the auxiliary fluid from the heat exchanger to the third compressor A device comprising:

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