FR2896860A1 - Air separation method for use in purification installation, involves separating part of purified air in medium pressure column into oxygen and nitrogen enriched liquids, and sending liquid from column to low pressure column - Google Patents

Abstract

The method involves purifying a compressed air flow in an absorption unit with absorbers (R1, R2), and transmitting the air to a column system with low and medium pressure columns (BP, MP). A part of the purified air is separated in the column (MP) into oxygen and nitrogen enriched liquids. Liquid from the column (MP) is sent to the column (BP) after a trigger and during the stage of depressurizing an absorber. A liquid flow sent from the column (MP) to the column (BP) is reduced relative to liquid flow sent to the column (BP) during the stage of absorption or regeneration of the absorber.

Description

The present invention relates to a method of separating air by

  cryogenic distillation. For cryogenic distillation, the treated air must be dry and decarbonated to prevent the formation of ice in the cold box. In order to purify the compressed air in a main compressor, an adsorption gas treatment unit is commonly used, of the type in which at least two adsorbers are used, each of which follows, in offset, the same cycle in which one succeeds one after the other. adsorption phase, at a high cycle pressure, and a regeneration phase with depressurization, which ends with a repressurization of the adsorber, the process comprising a step of paralleling the adsorbers during which the total flow of treated gas is obtained by both the treatment of a first gas flow rate by at least one adsorber ending its adsorption phase, and the treatment of a second gas flow rate to be treated by at least one other adsorber starting its phase of adsorption.

  Figure 1 shows a purification plant 1 of an air distillation unit. This installation 1 is adapted to remove impurities, in particular water and CO2, contained in a stream of compressed atmospheric air at a pressure of between 3 and 36 bars and brought by a line 2, to supply via a line 3 , a main heat exchange line and then an air distillation unit. The latter elements are not shown so as not to overload Figure 1. The air distillation unit can be, for example, a double air distillation column producing liquid oxygen which, after pumping, is vaporized. in the heat exchange line by condensation of a part of the compressed air. The installation 1 comprises two substantially identical adsorbers R1 and R2. Each adsorber R1, R2 comprises a bottle 4, 5 each containing a material, for example molecular sieve with optionally alumina, capable of adsorbing water and CO2 contained in the air. The installation 1 furthermore comprises a certain number of connection pipes and valves whose arrangement will now appear when describing the process implemented in the installation 1. This process is obtained by repeating a cycle, illustrated in FIGS. 2 and 3, the adsorber R1 according to the cycle shown in FIG. 2 and the adsorber R2 running in parallel with the same time-offset cycle, shown in FIG. 3. In FIGS. 2 and 3, where the times t are plotted on the abscissa and the absolute pressures P on the ordinate, the lines marked by arrows indicate the motions and destinations of the gaseous currents, and, in addition, the flow direction in the adsorbers R1 and R2 respectively: when an arrow is in the direction of increasing ordinates (towards the top of the diagram), the current is said to cocurrent, in the adsorber. If the upward arrow is below the line indicating the pressure in the adsorber, the current enters the adsorber through the inlet end of the adsorber; if the upwardly directed arrow is above the pressure line, the stream exits the adsorber through the outlet end of the adsorber, the inlet and outlet ends being respectively those of the adsorber gas to be treated and gas withdrawn during the production phase; when an arrow is in the direction of decreasing ordinates (towards the bottom of the diagram), the current is said against the current, in the adsorber. If the downward arrow is below the line indicating the pressure of the adsorber, the stream exits the adsorber through the inlet end of the adsorber; if the downward arrow is above the pressure line, the current enters the adsorber through the outlet end of the adsorber, the inlet and outlet ends always being those of the gas at the outlet. process and gas withdrawn during the production phase. It should be noted that the origin of the times is identical in FIGS. 2 and 3, thus allowing simultaneous reading of the two figures for the same given instant. The description of the cycle will mainly be based on FIG. 3, it being understood that FIG. 2 represents the same cycle shifted by one cycle half-cycle.

  The cycle of FIG. 3, whose period T is, for example, equal to 360 minutes for an adsorption pressure substantially equal to 6 bars, comprises four successive stages I to IV. During step I, that is to say from t = 0 to t1 = 5 minutes, the adsorber R1 is in the adsorption phase, while the main adsorber R2 is in the regeneration phase. The air of the line 2 is then brought by an open valve 10 in the adsorber R1. The purified air, that is to say desiccated and decarbonated, is then returned via a valve 11 open to the line 3.

  Meanwhile, the adsorber R2 is reduced to a pressure substantially equal to the atmospheric pressure by opening a valve 12, thus airing the adsorber R2 via a line 6. The valve 12 remains open up to 11 = 5 minutes, that is to say until the pressure inside the bottle 5 of the adsorber R2 is substantially equal to atmospheric pressure. Then, the valve 12 is closed and the residual nitrogen of the distillation unit channelized by a line 7, coming for example from the head of the low pressure column of the double column, feeds the adsorber R2 via a valve 13 opened.

  This waste nitrogen circulates in the adsorber R2 in the direction of regeneration, that is to say in the opposite direction to adsorption direction, ensuring the regeneration of this saturated adsorber during a previous adsorption phase. The regeneration nitrogen is first heated and then unheated.

  The waste nitrogen carrying the desorbed water and CO2 is then returned from the adsorber R2 via an open valve 14 to an evacuation line 8. This valve 14 remains open until t2 = 155 minutes, where it closes, and the valve 13 to allow repressurization of the adsorber R2. This repressurization of R2 consists, from t2 to t3 = 170 minutes, in bringing the pressure inside the bottle 5 of the adsorber R2 to a value substantially equal to that maintained in the downstream elements. It is carried out by various possible operating arrangements, in particular by the opening of a valve 15 connecting the outlets of the two adsorbers R1 and R2, and possibly by the opening of a valve 15B connected to the outlet end of R2 and partially supplying the latter with oxygen-rich fluid, for example oxygen from the air distillation unit via a line 102. Stage I ends when the pressure of the adsorber R2 has reached the desired value 6 bars. During stage II, that is to say from t3 to t4 = 180 minutes, the adsorber R1 is, as in stage I, adsorbed.

  However, the air of the line 2 is also brought by a valve 16 open in the adsorber R2. The air thus purified is sent through an open valve 17, to the line 3. During stage II, the adsorbers R1 and R2 are both in adsorption phase to purify the air in parallel.

  However, the stream of purified air leaving the adsorber R2 is at a much higher temperature than that at the outlet of the adsorber R1, as previously recalled. The total air flow conveyed by line 3, consisting of the outgoing flow of R1 and the outflow of R2, is therefore at a temperature intermediate that of the outgoing flow of R1 and that of the outgoing flow of R2 substantially following a law. mixtures. This step II of placing the adsorbers in adsorption phase in parallel is continued for a predetermined duration much greater than the switching time of the control valves of the adsorbers R1 and R2 establishing this step, which lasts a few seconds, or even a few tens of seconds, and / or until the intermediate temperature of the total stream is below a predetermined temperature, substantially equal to the temperature of the air entering R1 to which is added a temperature tolerance threshold, equal to 10 in this example. During step III, that is to say from t4 to t7 = 345 minutes, the adsorber R2 is maintained in the adsorption phase, while the adsorber R1 is in the regeneration phase. In a similar manner to R2 in step I, R1 is successively subjected to: - a depressurization of t4 at t5 = 185 minutes by means of the closing of the valves 10 and 11 and the opening of a valve 18 similar function to the valve 12 for R2; a flow of residual nitrogen from t5 to t6 = 335 minutes via the closing of the valve 18 and the opening of valves 19 and 20 of functions similar to the valves 13 and 14 respectively for R2; a repressurization of t6 at t7 = 350 minutes via the opening of the valve 15, and possibly the opening of a valve 15A, of function similar to the valve 15B for R2.

  At the end of stage III, the adsorber R1 is regenerated. During step IV, that is to say from t7 to t8 = 360 minutes, the adsorbers R1 and R2 provide in parallel the purification of the air conveyed by the line 2, these adsorbers being both in the adsorption phase as in stage II.

  Step IV continues for the aforementioned predetermined duration and / or until the temperature of the total flow conveyed by line 3 is below the same prefixed temperature mentioned above, this time substantially equal to the sum the temperature of the air entering R2 and the same temperature tolerance threshold.

  For a total cycle of 120 to 300 minutes, the repressurization stage generally lasts between 5 and 20 minutes. This time is a function of the additional air flow available for repressurization. In general, between 5 and 10% of the compressed air flow (relative to the nominal flow rate) is used to repressurize. Of course, the sizing of the main air compressor must take into account these repressurization steps. For a unit that will use 100 of air, it is necessary to provide a compressor with a capacity of 105 as shown in Figure 4. One of the aims of the invention is to allow the use of a smaller compressor. Another goal is to allow constant energy consumption by the air compressor.

  The invention applies to all air separation processes with at least one double column (medium pressure column and low pressure column) with a so-called pumped oxygen production: that is to say that liquid oxygen withdrawn at the bottom of the low pressure column is pumped at a pressure above 10 bars, before being vaporized in one or more exchangers. The invention also applies to devices producing impure oxygen by the principle of the mixing column. For this purpose, the subject of the invention is a process for separating air by cryogenic distillation in which a flow of air is compressed in a compressor, the compressed air flow is purified in an adsorption unit comprising at least two adsorbers, each of which follows, in offset, the same cycle in which an adsorption phase is followed at a high cycle pressure, and a regeneration phase with depressurization, which ends with a repressurization of the adsorber, the purified air is sent to a column system comprising a high pressure column and a low pressure column, at least a portion of the purified air separates in the medium pressure column into an oxygen enriched liquid and a nitrogen enriched liquid and less a liquid of the medium pressure column is sent to the low pressure column following expansion and during a repressurization step of an adsorber, at least one flow of liquid from the adsorber is reduced. a medium pressure column at the low pressure column with respect to the flow of the same liquid sent to the low pressure column during the adsorption or regeneration step of the same adsorber. According to other characteristics of the invention: the flow rate of an oxygen-enriched liquid sent from the medium-pressure column to the low-pressure column during the repressurization stage is reduced; the flow rate of a nitrogen enriched liquid sent from the medium pressure column to the low pressure column during the repressurization stage is reduced; a flow of liquefied air sent from the medium pressure column to the low pressure column is reduced during the repressurization stage; - The compressed air flow remains substantially constant during the adsorption, regeneration and repressurization steps for a given adsorber; the purified air flow rate decreases during the repressurization stage with respect to the flow rate during the adsorption and regeneration and repressurization stages for a given adsorber; the level of liquid in the tank of the medium pressure column increases during the repressurization stage of a given adsorber; the level of liquid in the tank of the low pressure column reduced during the repressurization stage of an adsorber; the level of liquid in the tank of the reduced medium pressure column outside the repressurization stages of the adsorbers; the level of liquid in the tank of the low pressure column increases outside the repressurization stages of the adsorbers; - The flow rate of at least one liquid sent from the medium pressure column to the low pressure column is adjusted at least according to the air flow entering the medium pressure column; the flow rate of at least one liquid sent from the medium-pressure column to the low-pressure column is set at least as a function of a signal emitted when a repressurization step is triggered; at least one enriched liquid is withdrawn from an air component of the column system, pressurized and vaporized in a heat exchanger to form a pressurized gas flow; at least one oxygen enriched liquid is withdrawn from the low pressure column, pressurized and sent to a mixing column. The invention also relates to an air separation plant capable of operating according to the method described above comprising a compressor, a purification unit and a column system comprising a medium pressure column and a low pressure column, at the at least one liquid supply line from the medium pressure column to the low pressure column and a device for regulating the flow of liquid in the pipe characterized in that the regulation device is controlled by a detector. The invention will be better understood on reading the description which follows, given solely by way of example, and with reference to the appended drawings, in which: FIG. 5 represents an air separation apparatus operating according to the process of the invention; FIGS. 6A and 6B show the variation of the levels of the low pressure and medium pressure columns respectively of FIG. 5; FIG. 7 shows the flow rates of compressed and purified air during the cycle according to the invention, to be compared with FIG. 4; FIG. 8 shows the set points for the lean liquid and rich liquid levels; FIG. 9 shows the set points for the air charge sent to the cold box and for the lean liquid flow rate; - Figure 10 shows the set points for the flow and the level of rich liquid; - Figure 11 shows the set point for the level of the bath vaporizer. Figure 5 shows an apparatus for separating air by cryogenic distillation using a double column. For simplicity, the means of production of frigories are not shown but it will be easily understood that they can be well known means, such as Claude turbine, blowing turbine, nitrogen turbine ... A flow of air 1 is compressed in a compressor C to form a flow of compressed air 3 at the pressure of the medium pressure column. The compressor C is driven by an electric motor M which can be replaced by a steam turbine or any other known means.

  The air is then purified in the absorbers R1, R2 in a conventional manner, for example as described above. The purified air 5 is divided in two. Part 7 is sent to a booster where it is pressurized to a high pressure. It liquefies in exchanger E and is sent to the medium pressure column MP in liquid form. Part of the air 9 cools in the exchanger and is sent to the medium pressure column in gaseous form. The apparatus comprises the conventional liquid lifts: a rich liquid line (enriched with oxygen) 11 expanded in a valve V11, a pipe 13 of liquid air expanded in a valve V13, a lower liquid line poor (enriched in nitrogen ) Expanded in a valve V15 and a higher lean liquid line (enriched in nitrogen) 17 expanded in a valve V17. These liquids are sent from the medium pressure column to the low pressure column to provide reflux. From the low pressure column are withdrawn a flow 19 of pure nitrogen, a flow of residual nitrogen (not shown) and a flow of liquid oxygen 21. The liquid oxygen 21 is pressurized in a pumped and then vaporizes in the exchanger E.

  The invention consists in using the liquid capacities that one has in the double column, to remove a large part of the over-capacity of the air compressor. During all stages other than re-pressurization, a slight excess of air is sent, less than 1%. This excess makes it possible to constitute a pool of oxygen molecules in the bottom of the LP low pressure column. The level of liquid oxygen in the bottom of the low pressure column increases slightly as can be seen in Figure 6A, outside the periods t2-t3 and t6-t7. Meanwhile, the level in the bottom of the medium pressure column decreases slightly MP (Figure 6B), in order to meet the refrigeration budget. During the few minutes (t2-t3 and t6-t7) of the re-pressurization step, the flow of compressed air by the compressor C (or more compressors C in parallel) remains constant. On the other hand, since part of the air is used for re-pressurization, the flow of purified air sent to the cold box decreases accordingly. The flow rates are then adjusted inside the cold box, in order to keep both the oxygen production rate and the oxygen content produced.

  The manipulated flow rates are the liquid lifts: liquid air 13, poor liquid (lower 15 & upper 17), rich liquid 11 (called very rich liquid in the case of a double column with mixing column).

  The opening of each valve V11, V13, V15, V17 is controlled by a flow controller FIC. In this way, the flow rate sent to the low pressure column can be reduced during the repressurization steps and increased outside of these steps. To adjust the open loops, one can either use the measurement of the air flow actually entering the cold box, or a signal from the sequence of the overhead purification (eg, opening the valve 15, 15A; 15B; re-pressurization), a combination of both. For an air separation unit with low flexibility (operating permanently close to its nominal), it will be preferred to adjust the flow rates based on the logic of the sequence. For a unit with great flexibility, a combination of logic and airflow will be used to adapt the changes to the load of the unit. During the re-pressurization step, the oxygen molecules are released, this results in a drop in the level of liquid oxygen in the bottom of the low pressure column, and an increase in the level in the medium pressure column makes it possible to observe the overall refrigeration balance of the air separation unit. In Figure 6A, the change in the oxygen-rich liquid level of the NBP low-pressure column vessel is shown with time during the purification cycle. It can be seen that during the stages t2-t3, t6-T7, where one of the adsorbers is repressurized, the level of the liquid decreases because the liquid returns to the low pressure column have been reduced. During the rest of the cycle, the level of the tank increases from a low level N1 to a high level N2.

  In Figure 6B, the variation of the oxygen enriched liquid level of the medium pressure NMP column with time is shown during the purification cycle. It can be seen that during the stages t2-t3, t6-t7, where one of the adsorbers is repressurized, the level of the liquid increases from a low level N1 to a high level N2 because the rising of liquid at the low pressure column have been reduced. During the remainder of the cycle, the tank level reduces from the high level N2 to the low level N1.

  The advantage of the invention is the reduction of the size of the compression system. For a unit that will use 100 air, instead of providing a compressor with a capacity of 105, a compressor of 101 will suffice. As shown in Figure 7, the excess air required for repressurization is distributed throughout the cycle, which significantly reduces the cost of the air compression system. According to the invention, as can be seen in FIG. 8A, the air load shown in abscissas varies between a maximum value and a minimum value. At the maximum value of the air charge, the set point SPLLL of the lean liquid level, on the ordinate, is at a maximum value and it reduces to a minimum value for the minimum value of the charge. 'air. The same is true for the set point SPRLL of the rich liquid level as shown in Figure 8B. In Figure 9A, the variation of the air load sent to the cold box is shown on the ordinate and the time on the abscissa. It can be seen that the charge does not vary during substantially the entire cycle except during pressurization periods P when it drops by 25 kNm3 / h. Likewise, as illustrated in FIG. 9B, during the pressurization P, the SPLLF set point of the lean liquid flow rate decreases by 15 kNm3 / h to return to its normal value just after the end of the pressurization stage.

  As illustrated in FIG. 10A, during the pressurization P, the SPRLF set point of the rich liquid flow rate drops by 17 kNm3 / h to rise to its normal value just after the end of the pressurization step. In contrast, the SPRLI setpoint of the rich liquid level of Figure 10B declines just prior to pressurization, increases during pressurization, and then slowly reduces to the next pressurization stage. Figure 11 shows the variation of the SP BVL set point of the bath vaporizer level. During the entire cycle outside the pressurization steps, the level rises to a maximum value. During the first half of the pressurization, the level drops to a minimum value to maintain this level during the second half of the pressurization.

  The compressed flow rate in the air compressor remains constant, its energy consumption remains constant, which can be an additional advantage: • When the compression energy comes from a relaxation of water vapor, the steam consumption remains constant (no disturbance on the steam network, so no risk of dropping the pressure of the steam collector). • When the compressor is driven by an electric motor, it is much easier to predict the power consumption of the unit, and thus optimize the bill (especially if the cost of energy is based on a fixed part and a variable part).

Claims (14)

  A method of separating air by cryogenic distillation in which a flow of air is compressed in a compressor, the flow of compressed air is purified in an adsorption unit comprising at least two adsorbers (R1, R2), which each follow, in offset, the same cycle where follow an adsorption phase, a high cycle pressure, and a regeneration phase with depressurization, which ends with a repressurization of the adsorber, purified air is sent to a column system comprising a high pressure column and a low pressure column, at least a portion of the purified air separates in the medium pressure column into an oxygen enriched liquid and a nitrogen enriched liquid and at least one liquid the medium pressure column is sent to the low pressure column following expansion and during a repressurization stage of an adsorber, at least one flow of liquid sent from the medium pressure column is reduced to the low pressure column with respect to the flow of the same liquid sent to the low pressure column during the adsorption or regeneration step of the same adsorber.   2. Process according to claim 1, characterized in that the flow rate of an oxygen-enriched liquid sent from the medium-pressure column to the low-pressure column during the repressurization step is reduced.   3. Process according to claim 1 or 2, characterized in that the flow rate of a nitrogen enriched liquid sent from the medium pressure column to the low pressure column during the repressurization step is reduced.   4. Process according to one of the preceding claims, characterized in that a liquefied air flow rate from the medium pressure column to the low pressure column is reduced during the repressurization stage.   5. Method according to one of the preceding claims, characterized in that the compressed air flow remains substantially constant during the adsorption, regeneration and repressurization steps for a given adsorber.   6. Method according to one of the preceding claims, characterized in that the flow of purified air decreases during the repressurization step relative to the flow during the adsorption and regeneration steps and repressurization for a given adsorber.   7. Method according to one of the preceding claims, characterized in that the liquid level in the tank of the medium pressure column increases during the repressurization step of a given adsorber.   8. Method according to one of the preceding claims, characterized in that the liquid level in the tank of the low pressure column reduced during the repressurization step of an adsorber.   9. A method according to one of the preceding claims, characterized in that the liquid level in the bottom of the reduced pressure medium column outside the repressurization steps of the adsorbers.   10. Process according to one of the preceding claims, characterized in that the liquid level in the bottom of the low pressure column increases outside the repressurization stages of the adsorbers.   11. Method according to one of the preceding claims, characterized in that the flow rate of at least one liquid sent from the medium pressure column 10 to the low pressure column is adjusted at least as a function of the air flow entering the column. medium pressure.   12. Method according to one of the preceding claims, characterized in that the flow rate of at least one liquid sent from the medium pressure column to the low pressure column is set at least according to a signal emitted at the time of triggering. a step of repressurization.   Method according to one of the preceding claims, characterized in that at least one enriched liquid is withdrawn into an air component of the column system, pressurized and vaporized in a heat exchanger to form a pressurized gas flow.   14. Process according to one of the preceding claims, characterized in that at least one oxygen enriched liquid is withdrawn from the low pressure column, pressurized and sent to a mixing column. 20