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 relates in particular to processes and apparatus for producing oxygen and/or nitrogen at elevated pressure.

Gaseous oxygen produced by air separation plants are usually at elevated pressure about 20 to 50 bar. The basic distillation scheme is usually a double column process producing oxygen at the bottom of the low-pressure column operated at 1.4 to 4 bar. The oxygen must be compressed to higher pressure either by oxygen compressor or by the liquid pumping process. Because of the safety issues associated with the oxygen compressors, most recent oxygen plants are based on the liquid pumping process. In order to vaporize liquid oxygen at elevated pressure there is a need for an additional motor-driven booster compressor to raise a portion of the feed air or nitrogen to higher pressure in the range of 40-80 bars. In essence, the booster replaces the oxygen compressor.

One of the goals in the development of new process cycles is to decrease the power consumption of an oxygen plant. In the effort to decrease this power consumption, it is desirable to introduce all feed air streams to the columns at a temperature close to the column temperature at the location where the stream is fed in order to reduce the thermodynamic irreversibility of the system. Unfortunately, this is not achievable with a "classical" pump cycle.

An illustration of this prior art is presented in Figure 1.In Figure 1 as described in FR-A-2777641 , in an air separation unit 1 , a double column 2 is used, comprising a high pressure column 8 and a low pressure column 9, thermally connected by a reboiler/condenser 10. All the feed air is compressed in compressor 6 to the pressure of the high pressure column 8, purified in purification unit 7 and divided into three.

One stream 502 is sent to a booster compressor 503, cooled in a water cooler (not represented) and further cooled in the heat exchanger 5 and then

expanded in a turbine 501 , coupled to the booster compressor 503. The expanded air 502 is sent to the low pressure column.

Another part of the air is sent to the heat exchanger 5 at substantially the same pressure as the high pressure column 8. The third stream is compressed in a compressor 230 and sent to the heat exchanger where it condenses. The liquefied air is divided between the high pressure column 8 and the low pressure column 9.

An oxygen enriched liquid stream LR is expanded and sent from the high- pressure column to the low-pressure column. Nitrogen enriched liquid stream LP is expanded and sent from the high-pressure column to the low-pressure column. Pure liquid nitrogen NLMP is produced from high-pressure column, further cooled in heat exchanger 24 and expanded in valve 143 and sent to a storage 144. High- pressure gaseous nitrogen 39 is removed from the top of the high-pressure column and warmed in the heat exchanger to form a product stream 40. Liquid oxygen OL is removed from the bottom of the low pressure column 9, pressurized by a pump 37 and sent in part as stream 38 to the heat exchanger 5 where it vaporizes by heat exchange with the pressurized air to form gaseous pressurized oxygen. The rest of the liquid oxygen 52 is removed as a liquid product. A top nitrogen enriched gaseous stream NR is removed from the low-pressure column 9, warmed in the heat exchanger 5 as stream 33.

Argon is produced using impure argon column 3 and pure argon 4. The impure argon column is fed by stream 16 from the low pressure column 9. A liquid stream 17 is sent from the base of the impure argon column 3 to the low pressure column 9. Rich liquid is sent to the top condenser 12 of the column 3 via valve 26 and is evaporated to form stream 27 which is returned to the low pressure column. A product stream 19 is sent to condenser 20 and thence forms stream 19. Stream 19 is condensed in heat exchanger 20 and divided into stream 48 which is sent to the waste stream 33 at intersection point 50 and a further stream. The further stream is sent via valve 21 to the column 4.

The pure argon column 4 produces a product stream 45. The top condenser 13 of the pure argon column 4 is fed by nitrogen rich liquid LP from the high pressure column via valve 34 and the vaporized nitrogen is removed via valve 35 as stream 33 and cooled in subcooler 24. The bottom reboiler 14 of the pure argon column is heated using air and the liquefied air 23 is sent to the high pressure column.

A purge stream 46 is also removed.

The condenser 20 is fed by nitrogen rich liquid LP via valve 31 and the vaporized liquid is sent via valve 32 to the waste stream 33. Some different versions of the cold compression process were also described in prior art as in US-A-5379598, US-A-5475980, US-A-5596885, US-A- 5901576 and US-A-6626008.

In US-A-5379598 a fraction of feed air is further compressed by a booster compressor followed by a cold compressor to yield a pressurized stream needed for the vaporization of oxygen. This approach still has at least two compressors and the purification unit still operates at low pressure.

A cold compression process as described in US-A-5,475,980 provides a technique to drive the oxygen plant with a single air compressor. In this process, air to be distilled is chilled in the main exchanger then further compressed by a booster compressor driven by an expander exhausting into the high-pressure column of a double column process. By doing so, the discharge pressure of the air compressor is in the range of 15 bar which is also quite advantageous for the purification unit. One inconvenience of this approach is the increase of the size of the main exchanger due to additional flow recycling which is typical for the cold compression plant. One can reduce the size of the exchanger by opening up the temperature approaches of the exchanger. However, this would lead to inefficient power usage and higher discharge pressure of the compressor, therefore increasing its cost.

In US-A-5596885, a fraction of the feed air is further compressed in a warm booster whilst at least part of the air is further compressed in a cold booster.

Air from both boosters is liquefied and part of the cold compressed air is expanded in a Claude expander.

US-A-5901576 describes several arrangements of cold compression schemes utilizing the expansion of vaporized rich liquid of the bottom of the high- pressure column, or the expansion of high-pressure nitrogen to drive the cold compressor. In some cases, motor driven cold compressors were also used. These processes also operate with feed air at about the high-pressure column's pressure and in most cases a booster compressor is also needed.

US-A-6009723 describes a cold compressor process in which oxygen is vaporized at a single pressure and then compressed in a cold compressor.

US-A-6626008 describes a heat pump cycle utilizing 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 a booster compressor are also typical for this kind of process. Therefore it is a purpose of this invention to resolve the inconveniences of these processes, in particular by introducing all feed air streams to the columns at a temperature close to the column's temperature at the location where the stream is fed in order to reduce the thermodynamic irreversibility of the system. The overall product cost of an oxygen plant can therefore be reduced. The main improvement in power consumption is due to the reduction in the cold compressor flow by using essentially latent heat instead of specific heat. The other improvement is related to the use of a cold compressor driven by an expansion turbine. As a cold compressor is extracting refrigeration from the oxygen plant, it is necessary to increase the cold production by approximately the same amount and one solution consists of an expansion turbine. For some process cycles, it is possible to expand either air or nitrogen at low pressure without affecting the oxygen recovery too much. In other words, this expansion is "free". In this case, the use of a cold compressor is a good solution to use this "free" refrigeration. All percentages listed are molar percentages.

According to the present invention, there is provided a process for separating air by cryogenic distillation in a column system comprising a high pressure column and a low pressure column comprising the steps of: i) compressing all the feed air in a first compressor to a first outlet pressure at most two bars higher than the pressure of the high pressure column ii) sending a first part of the air at the first outlet pressure to a second compressor and compressing the air to a second outlet pressure iii) cooling and condensing at least part of the air at the second outlet pressure in a heat exchanger iv) removing liquid from a column of the column system, pressurizing the liquid and vaporizing the liquid by heat exchange in the heat exchanger at a first temperature v) at least partially vaporizing an auxiliary fluid in the heat exchanger at a second temperature lower than the first temperature, possibly further warming said auxiliary fluid in the heat exchanger, sending at least part of this auxiliary fluid to a third compressor to be compressed to a third outlet pressure, introducing at least part of said auxiliary fluid at said third outlet pressure in the heat exchanger, wherein the inlet of the third compressor is at a subambient temperature.

According to further optional features, the process comprises: - compressing all the feed air in a first compressor to a first outlet pressure of at most 8 bars abs;

- the latent heat of vaporization of the auxiliary fluid is at least 10%, preferably at least 30% of the latent heat of vaporization of the liquid vaporized at the first temperature; - cooling said auxiliary fluid after introducing it into the heat exchanger, at least partially liquefying said auxiliary fluid, removing said auxiliary stream from the heat exchanger and expanding it to a fourth pressure level before reintroducing it in the heat exchanger for the afore mentioned at least partial vaporization step;

- warming the said at least part of the auxiliary fluid after introducing it into the heat exchanger.

The auxiliary fluid may be oxygen rich, nitrogen rich or argon rich. In particular, the argon rich auxiliary fluid may be derived from an argon purification column fed from the column system.

According to further optional features:

- air at the second outlet pressure is expanded in a turbine and sent to the high pressure column and the turbine is coupled to the third compressor;

- air at the first outlet pressure is expanded in a turbine and sent to the low pressure column and the turbine is coupled to the third compressor;

- a nitrogen rich stream is expanded in a turbine and the turbine is coupled to the third compressor;

- the process comprises withdrawing a liquid stream from the column system, vaporizing at least part of the liquid stream, sending at least part of the vaporized stream as the auxiliary fluid to the third compressor;

- the process comprises increasing the pressure of the liquid stream upstream of the vaporization step;

- the process step comprises vaporizing the auxiliary fluid to be sent to the third compressor in the heat exchanger; - part of the air stream at the first outlet pressure is cooled in the heat exchanger and sent to the high pressure column without being compressed or expanded;

- the liquid stream is the liquid stream of step iv);

- the third compressor is a single stage machine coupled with a single stage expansion turbine;

- the rotating speeds of the third compressor and the expansion turbine are equal and/or the third compressor and the expansion turbine are on the same shaft;

- the inlet temperature of the third compressor is above the vaporization temperature of the liquid vaporized in the heat exchanger;

- the inlet temperature of the third compressor is below 20°C, preferably below -40 ⁰ C, still more preferably below -140 ⁰ C.

According to a further aspect of the invention, there is provided an apparatus for separating air by cryogenic distillation in a column system comprising a high pressure column and a low pressure column further comprising: i) a first compressor for compressing the feed air to a first outlet pressure at most two bars higher than the pressure of the high pressure column ii) a second compressor and means for sending a first part of the air at the first outlet pressure to the second compressor to compress the air to a second outlet pressure iii) a heat exchanger wherein at least part of the air at the second outlet pressure is cooled and condensed iv) means for removing liquid from a column of the column system, means for pressurizing the liquid, means for sending the pressurized liquid to the heat exchanger and means for removing vaporized liquid from the heat exchanger v) a third compressor, means for sending an auxiliary fluid to the heat exchanger, means for sending at least part of this auxiliary fluid to the third compressor to be compressed to a third outlet pressure and means for introducing at least part of said auxiliary fluid at said third outlet pressure in the heat exchanger, wherein the inlet of the third compressor is at a subambient temperature characterized in that it comprises means for removing the auxiliary fluid to be sent to the third compressor from a column of the column system in liquid form. The invention will now be described in greater detail with reference to Figures 3, 5, 6 and 7 which are process flow diagrams representing cryogenic air separation processes according to the invention and Figure 4 which is a heat exchange diagram.

In the embodiment of Figure 3, in an air separation unit 1 , a double column

2 is used, comprising a high pressure column 8 and a low pressure column 9, thermally connected by a reboiler/condenser 10. All the feed air is compressed in

compressor 6 to the pressure of the high pressure column 8, purified in purification unit 7 and divided into three.

One stream 502 is sent to a booster compressor 503, cooled in a water cooler (not represented) and then further cooled in the heat exchanger 5 and then expanded in a turbine 501 , coupled to the booster compressor 503. The expanded air 502 is sent to the low pressure column.

Another part 507 of the air is sent to the heat exchanger 5 at substantially the same pressure as the high pressure column 8.

The third stream 505 is compressed in a compressor 230 and sent to the heat exchanger where it condenses. The liquefied air is divided between the high pressure column 8 and the low pressure column 9.

An oxygen enriched liquid stream LR is expanded and sent from the high- pressure column to the low-pressure column. Nitrogen enriched liquid stream LP is expanded and sent from the high-pressure column to the low-pressure column. Pure liquid nitrogen NLMP is produced from the high pressure column 8, further cooled in heat exchanger 24 and expanded in valve 143 and sent to storage 144. High-pressure gaseous nitrogen 39 is removed from the top of the high-pressure column and warmed in the heat exchanger to form a product stream 40. Liquid oxygen OL is removed from the bottom of the low pressure column 9, pressurized by a pump 37 and sent in part as stream 38 to the heat exchanger 5 where it vaporizes by heat exchange with the pressurized air to form gaseous pressurized oxygen. The rest of the liquid oxygen 52 is removed as a liquid product. A top nitrogen enriched gaseous stream NR is removed from the low-pressure column 9, warmed in the heat exchanger 5 as stream 33. Argon is produced using impure argon column 3 and pure argon 4. The impure argon column is fed by stream 16 from the low pressure column 9. A liquid stream 17 is sent from the base of the impure argon column 3 to the low pressure column 9. Rich liquid is sent to the top condenser 12 of the column 3 via valve 26 and is evaporated to form stream 27 which is returned to the low pressure column. A product stream 19 is sent to condenser 20 and thence forms stream 19. Stream

19 is condensed in heat exchanger 20 and divided into stream 48 which is sent to the waste stream 33 at intersection point 50 and a further stream. The further stream is sent via valve 21 to the column 4.

The pure argon column 4 produces a product stream 45. The top condenser 13 of the pure argon column 4 is fed by nitrogen rich liquid LP from the high pressure column via valve 34 and the vaporized nitrogen is removed via valve 35 as stream 33 and cooled in subcooler 24. The bottom reboiler 14 of the pure argon column is heated using air and the liquefied air 23 is sent to the high pressure column. A purge stream 46 is also removed.

Nitrogen rich liquid 43 is collected via valve 143 in storage 144. The condenser 20 is fed by nitrogen rich liquid LP via valve 31 and the vaporized liquid is sent via valve 32 to the waste stream 33.

An auxiliary fluid mixture of argon (80%) and oxygen (20%) is introduced in heat exchanger 5 where it is vaporized and slightly warmed after vaporization to yield a cold auxiliary gaseous stream 107. The proportions of argon and oxygen are variable and the mixture may also include other components. This vaporization occurs in heat exchanger 5 at a temperature around -143°C, as can be seen in Figure 4, which is lower than the oxygen vaporization temperature of around - 126°C. At least a portion of this cold auxiliary stream 107 is sent to a cold brake compressor 108 at subambient temperature T1 to be compressed to raise its pressure (stream 109). Temperature T1 is preferably above and most preferably slightly above the oxygen vaporization temperature which is in this case around - 126°C. Stream 109 is then sent back to the exchanger 5 at temperature T2 which is greater than T1 and cooled in exchanger 5 to condense to form a liquefied auxiliary stream (stream 10), which is expanded in a valve 116 to form stream 107. This condensation occurs at a temperature slightly above the oxygen vaporization temperature. In this case, this condensation is at a temperature around -124°C. A phase separator could be added if the expanded stream is a two-phase fluid, the liquid phase being introduced in heat exchanger 5 and the vapor phase mixed with

stream 107. The term "condensing" covers condensation from a vapor to a liquid or partially liquid state. It also covers the pseudo-condensation of a supercritical fluid when cooled from a temperature above the supercritical temperature to a temperature below the supercritical temperature. Figure 4 shows the exchange diagram corresponding to the process of

Figure 3.

A variant of this process is shown in Figure 5: part of the air from compressor 230 is removed at an intermediate temperature from the exchanger 5 and sent to expander 121. The expanded air stream is at the pressure of the high pressure column 8 and is sent thereto mixed with air stream 507.

The expander 121 is coupled to a cold compressor 108. The cold compressor 108 is fed by a vaporized liquid argon stream. The liquid argon 105 is derived from the production of column 4 and is vaporized, at a lower temperature than the vaporization temperature of stream 38, in exchanger 5 to form stream 107. The compressed stream 109 is sent to the exchanger 5 and may be either cooled to be recycled in the cold compressor cycle or warmed and removed as a product or waste stream 120. In addition an argon stream 119 may be removed as part of the vaporized stream 105 which is not sent to the cold compressor 108. If it were chosen to use a mixture of oxygen and argon as auxiliary stream, fluid 105 could be extracted from column 3. In case of a mixture of nitrogen and argon, fluid 105 could be extracted from an intermediate location of column 4.

A variant of Figure 5 is shown in Figure 6 where the air turbine 121 is replaced by a high pressure nitrogen turbine 121 fed by part of the stream 39 of high pressure nitrogen removed from the top of column 8. Another variant of this process (not illustrated) is to replace the air turbine 121 by a low pressure air turbine expanding part of the air delivered by compressor 6 to the low pressure column.

In Figure 7, the cold compressor 108 is fed by vaporized liquid oxygen. The liquid oxygen is pumped in pump 37 to form a liquid product 52 and a liquid stream 38 sent to the exchanger 5 wherein it vaporizes. A further part of the liquid

oxygen is expanded in valve 116 and sent to the exchanger 5 to form vaporized stream 107, vaporized at a lower temperature than stream 38. Stream 107 is compressed in compressor 108 and sent as stream 109 to mix with vaporized stream 38. The process may be modified to vaporize pumped liquid nitrogen as an additional stream or as a stream replacing the pumped oxygen stream. In this context, part of the pumped nitrogen stream could be expanded in the valve and compressed in the cold compressor 108 before being mixed with the principal vaporized nitrogen stream. In all the examples the latent heat of vaporization of the auxiliary fluid 109 is at least 10%, preferably at least 30% of the latent heat of vaporization of the liquid 38 vaporized at the first temperature.

The illustrated processes show double column systems but it will be readily understood that the invention applies to triple column systems. It could be also used with process cycles producing low purity oxygen (typically 95% O ₂ instead of 99.5% O ₂ ) such as "dual vaporizer" process cycles.

In the case where the double or triple column systems operate at elevated pressures, some of the low pressure nitrogen may be expanded in an expander

18.