Process and apparatus for the separation of air by cryogenic distillation

The present invention relates to a process and apparatus for the separation of air by cryogenic distillation. It is frequently desirable to have an air separation unit which produces at least mainly gas during at least one period when electricity costs are higher and at least mainly liquid during at least one period when electricity costs are lower. The present invention allows the apparatus to function with optimal power consumption during both modes of operation. The apparatus includes an air separation column system which may be of any known type, but may particularly be a double column system including a high pressure column and a low pressure column, thermally coupled via a reboiler condenser at the bottom of the low pressure column wherein nitrogen from the top of the high pressure column is condensed. All pressures mentioned in this text are absolute pressures.

According to an object of the invention, there is provided a process for the production of at least one liquid product and at least one gaseous product by cryogenic distillation of air under a first mode of operation and a second mode of operation, the process producing more liquid as final product during the second mode than during the first mode wherein in all modes of operation, compressed and purified gaseous air is cooled in a heat exchange line and sent to at least one column of a column system, a liquid stream enriched in a component of air is removed from a column of the column system and vaporised in the heat exchange line, air at an elevated pressure is sent to the heat exchange line, condensed and sent to the column system and part of the feed air is sent to one of at least two expanders and thence to a column of the column system wherein i) according to the first mode, at least part of the feed air is removed from an intermediate point of the heat exchange line, compressed at a cryogenic temperature in a cold compressor and sent to the heat exchange line to be further cooled and sent to the column system and part of the feed air is sent to the first expander and ii) according to the second mode, all of the feed air is compressed to a high pressure at least 20 bars higher than the highest column pressure of the

column system in a second compressor , cooled in the heat exchange line and sent in part to a column system, another part of the high pressure air being sent to the second expander.

According to optional aspects of the invention: - according to the first mode, part of the feed air at the outlet pressure of the cold compressor is cooled and sent to the first expander;

- the cold compressor is coupled to the first expander;

- the second compressor is coupled to the second expander;

- air treated in the second compressor in the second mode and in the cold compressor in the first mode is subsequently sent to a common transfer means upstream of the column system;

- in the first mode the air is sent from the cold compressor to the heat exchange line via a conduit and in the second mode the air is sent from the second compressor to the second expander via the same conduit; - in the first mode the air is sent from the cold compressor via a passage of the heat exchange line to the cold end thereof and in the second mode the air is sent from the second compressor to the cold end of the heat exchange line via the same passage.

According to a further aspect of the invention, there is provided an apparatus for the separation of air by cryogenic distillation comprising: a) a column system b) a heat exchange line c) a main compressor d) a cold compressor connected to the outlet of the main compressor e) a second compressor connected to the outlet of the main compressor f) first and second expanders g) means for sending air from the cold compressor to the first expander h) means for sending air from the second compressor to the second expander i) means for sending air from the first and second expanders to the column system j) means for sending air from the cold compressor and the second compressor to the column system via the heat exchange means without traversing either of the first and second expanders.

According to further optional aspects:

- the means for sending air from the cold compressor to the first expander and means for sending air from the second compressor to the second expander include a common section of conduit; - the means for sending air from the cold compressor and the second compressor to the column system via the heat exchange means without traversing either of the first and second expanders includes at least one common passage in the heat exchange means;

- the apparatus includes a third expander and means for sending air from the column system to the third expander and thence to the heat exchange means.

The invention will be described in more detail with reference to the Figures. Figures 1 , 2 and 3 show air flow diagrams for an air separation unit according to the invention. The dashed lines indicate couplings between a compressor and a turbine.

The process of Figure 1 uses a double column system in which a high pressure column 65 is placed underneath a low pressure column 67 and thermally coupled thereto via a reboiler condenser 69.

In all the modes of operation, cooled, purified and compressed gaseous air is fed to the high pressure column 65. Reflux streams (not shown) are sent from the high pressure column to the low pressure column 67 as is well known in the art. In addition, in all modes, gaseous nitrogen 61 is removed from the top of the low pressure column 67 and warmed in exchanger 19 whilst waste nitrogen 59 is removed from lower down the low pressure column 67 and warmed in exchanger 19 before being used to regenerate the purification unit 8.

In Figure 1 , all the air is compressed to 15.5 bars in compressor 1 and cooled in cooler 4 to form stream 3. Following further cooling in cooler 6, the air is purified in purification unit 8. The outlet of compressor 1 is connected to the inlet of compressor 5 and to the heat exchanger 19. When the apparatus functions under gas mode, none of the air from compressor 1 is sent to compressor 11 as stream 5. All the air is sent to the warm end of the heat exchange line 19, via open valve 13 as stream 7. The air 7 is cooled to an intermediate temperature of the heat exchange line 19 and is compressed to 26 bars in cold compressor 37. Valve 21 being open, all of the

compressed air is then sent back to the heat exchange line 19 via conduit 23, further cooled in conduit 43 and divided in two at an intermediate temperature lower than the inlet temperature of cold compressor 37. One part is cooled completely in the heat exchange line 19 as stream 41 , whilst the rest 33 is sent via valve 35 to turbine 39 coupled to cold compressor 37. The expanded air is then sent to a column of the column system. In this example, the air is sent as stream 45 to the high pressure column 65, forming the sole gaseous feed to the high pressure column.

In this mode, liquid oxygen is removed from the bottom of the low pressure column 57 and pressurised as stream 55 in pump 57 before being vaporised in the heat exchanger 19 to form product high pressure gaseous oxygen (HP GOX).

When the apparatus functions under liquid mode, all the air from compressor 1 is sent to compressors 11 , 17 as stream 5 and is compressed to 34 bars, valve 15 being open and valve 13 being closed. The high pressure air 5 is then further compressed to 47 bars in compressor 17 and sent to the warm end of the heat exchange line 19. Once the stream 5 is partially cooled, it is divided in two, one part 41 being cooled completely in the heat exchange line 19 via conduit 43 and the rest 31 being sent to turbine 29 via conduit 23 and valve 21. The expanded air stream 45 is sent to a column of the column system, in this case the high pressure column 65. The compressor 17 is coupled to expander 49 which expands air 47 removed from the high pressure column. The air 51 from the expander 49 is sent to the heat exchange line 19 and warmed therein before being rejected to the atmosphere. Compressor 12 is coupled to expander 29.

During this liquid mode, liquid oxygen LOX 53 and liquid nitrogen LIN 69 are removed from the low pressure and high pressure columns respectively. In addition, liquid oxygen is removed from the bottom of the low pressure column 57 and pressurised as stream 55 in pump 57 before being vaporised in the heat exchanger 19 to form product high pressure gaseous oxygen (HP GOX).

It will be appreciated that a number of conduits fulfil different purposes depending on which mode is used. The cooling section 43 receives air at 47 bars which is then cooled by passing through the whole heat exchange line during the liquid mode and receives air at 26 bars coming from the cold

compressor 37 during the gas mode. In addition, section 28 sees a reversal of flow between the two modes, air flowing in one direction from the heat exchange line 19 to the turbine 29 in liquid mode and in the other direction from the cold compressor 37 to the heat exchange line 19 in gas mode. It will be appreciated that a small amount of liquid may be produced during the gas mode and that gas is produced during the liquid mode.

Optionally in all the modes of operation of Figure 1 , the nitrogen 61 is compressed to a higher pressure in compressor 63.

In Figure 2, all the air is compressed to 15.5 bars in compressor 1 and cooled in cooler 4 to form stream 3. Following further cooling in cooler 6, the air is purified in purification unit 8. The outlet of compressor 1 is connected to the inlet of compressor 5 and to the heat exchanger 19.

When the apparatus functions under gas mode, none of the air from compressor 1 is sent to compressor 11 as stream 5. All the air is sent to the warm end of the heat exchange line 19, via open valve 13 as stream 7. Valve 15 is closed. The air 7 is cooled to an intermediate temperature of the heat exchange line 19 and is compressed to 26 bars in cold compressor 37. Valve 21 being open, all of the compressed air is then sent back to the heat exchange line 19 via conduit 23, further cooled in conduit 43 and divided in two at an intermediate temperature lower than the inlet temperature of cold compressor 37. One part is cooled completely in the heat exchange line 19 as stream 41 , whilst the rest 33 is sent via valve 35 to turbine 39 coupled to cold compressor 37. The expanded air is then sent to a column of the column system. In this example, the air is sent as stream 45 to the high pressure column 65, forming the sole gaseous feed to the high pressure column.

In this mode, liquid oxygen is removed from the bottom of the low pressure column 57 and pressurised as stream 55 in pump 57 before being vaporised in the heat exchanger 19 to form product high pressure gaseous oxygen (HP GOX). When the apparatus functions under liquid mode, all the air from compressor 1 is sent to compressors 11 ,17 as stream 5 and is compressed to 34 bars, valve 15 being open and valve 13 being closed. The high pressure air 5 is then sent to the warm end of the heat exchange line 19. Once the stream 5 is partially cooled, it is divided in two, one part 41 being cooled completely in the

heat exchange line 19 via conduit 43 and the rest 31 being sent to turbine 29 via conduit 23 and valve 21. The expanded air stream 45 is sent to a column of the column system, in this case the high pressure column 65.. Compressor 17 is coupled to expander 29. During this liquid mode, liquid oxygen LOX 53 and liquid nitrogen LIN 69 are removed from the low pressure and high pressure columns respectively. In addition, liquid oxygen is removed from the bottom of the low pressure column 57 and pressurised as stream 55 in pump 57 before being vaporised in the heat exchanger 19 to form product high pressure gaseous oxygen (HP GOX). It will be appreciated that a number of conduits fulfil different purposes depending on which mode is used. The cooling section 43 receives air which is then cooled by passing through the whole heat exchange line during the liquid mode and receives air at 26 bars coming from the cold compressor 37 during the gas mode. In addition, section 23 sees a reversal of flow between the two modes, air flowing in one direction from the heat exchange line 19 to the turbine 29 in liquid mode and in the other direction from the cold compressor 37 to the heat exchange line 19 in gas mode.

It will be appreciated that a small amount of liquid may be produced during the gas mode and that gas is produced during the liquid mode. Optionally in all the modes of operation of Figure 2, the nitrogen 61 is compressed to a higher pressure in compressor 63.

In a simplified version of Figure 1 , as shown in Figure 3, only two air turbines are used.

The process of Figure 3 may use a double column system as shown and described for Figure 1 , in which a high pressure column 65 is placed underneath a low pressure column 67 and thermally coupled thereto via a reboiler condenser 69.

In all the modes of operation, as in the case of Figure 1 , cooled, purified and compressed gaseous air is fed to the high pressure column 65. Reflux streams (not shown) are sent from the high pressure column to the low pressure column 67 as is well known in the art. In addition, in all modes, gaseous nitrogen 61 is removed from the top of the low pressure column 67 and warmed in exchanger 19 whilst waste nitrogen 59 is removed from lower down

the low pressure column 67 and warmed in exchanger 19 before being used to regenerate the purification unit 8.

In Figure 3, all the air is compressed to 15.5 bars in compressor 1 and forms stream 3. Following further cooling (not shown), the air is purified in a purification unit (not shown). The outlet of compressor 1 is connected to the inlet of compressor 5 and to the heat exchanger 19.

When the apparatus functions under gas mode, none of the air from compressor 1 is sent to compressor 11 as stream 5. All the air is sent to the warm end of the heat exchange line 19, via open valve 13 as stream 7. The air 7 is cooled to an intermediate temperature of the heat exchange line 19 and is compressed to 26 bars in cold compressor 37. Valve 21 being open, all of the compressed air is then sent back to the heat exchange line 19 via conduit 23, further cooled in conduit 43 and divided in two at an intermediate temperature higher than the inlet temperature of cold compressor 37. One part is cooled completely in the heat exchange line 19 as stream 41 , whilst the rest 33 is sent via valve 35 to turbine 39 coupled to cold compressor 37. The expanded air is then sent to a column of the column system. In this example, the air is sent as stream 45 to the high pressure column 65, forming the sole gaseous feed to the high pressure column. In this mode, liquid oxygen is removed from the bottom of the low pressure column 57 and pressurised as stream 55 in pump 57 before being vaporised in the heat exchanger 19 to form product high pressure gaseous oxygen (HP GOX).

When the apparatus functions under liquid mode, all the air from compressor 1 is sent to compressor 11 as stream 5 and is compressed to 34 bars, valve 15 being open and valve 13 being closed. The high pressure air 5 is then further compressed to 47 bars in compressor 17 and sent to the warm end of the heat exchange line 19. Once the stream 5 is partially cooled, it is divided in two, one part 41 being cooled completely in the heat exchange line 19 via conduit 43 and the rest 31 being sent to turbine 29 via conduit 23. The expanded air stream 45 is sent to a column of the column system, in this case the high pressure column 65.. The compressor 17 is coupled to expander 29. During this mode, liquid oxygen LOX 53 and liquid nitrogen LIN 69 are removed from the low pressure and high pressure columns respectively. In addition,

liquid oxygen is removed from the bottom of the low pressure column 57 and pressurised as stream 55 in pump 57 before being vaporised in the heat exchanger 19 to form product high pressure gaseous oxygen (HP GOX).

It will be appreciated that a number of conduits fulfil different purposes depending on which mode is used. The cooling section 43 receives air at 47 bars which is then cooled by passing through the whole heat exchange line during the liquid mode and receives air at 26 bars coming from the cold compressor 37 during the gas mode. In addition, section 23 sees a reversal of flow between the two modes, air flowing in one direction from the heat exchange line 19 to the turbine 29 in liquid mode and in the other direction from the cold compressor 37 to the heat exchange line 19 in gas mode.

For all embodiments, other modes of operation may exist besides the gas mode and liquid mode mentioned.