FR2750888A1 - Oxygen production from air by adsorption using centrifugal pump - Google Patents

Abstract

The treatment of a gas mixture, in particular air comprises variable pressure adsorption (PSA) using at least one adsorber and operating the following cycle in each: (a) an adsorption phase in which the mixture is introduced at one end (inlet) of the adsorber, circulated in the adsorber to the other end (outlet) and from this outlet comes the less adsorbable gas; (b) a desorption phase with pumping backflow to a low pressure (P'm) below ambient; and (c) a repressurisation phase returning the adsorber to its cycle high pressure. At least part of the pumping stage uses a centrifugal vacuum pump, and for a given adsorber, the final extraction pressure (P'F) of the centrifugal pump is close to the initial extraction pressure (PI). Also claimed is the application of the above process to the production of oxygen from air.

Description

The present invention relates to a process for treating a gaseous mixture, in particular of air, by pressure variation adsorption, of the type in which at least one adsorber is used and, in or in each adsorber, works a cycle comprising successively
(a) an adsorption phase in which the mixture is introduced through one end, called the inlet, of the adsorber, it is made to circulate in the adsorber in a direction called co-current to the other end, called outlet, from the adsorber, and a weakly adsorbable gaseous fraction is extracted from this outlet;
(b) a desorption phase comprising a step of pumping against the current up to a low pressure of the cycle below atmospheric pressure; and
(c) a phase of repressurization of the adsorber to a pressure close to the high pressure of the cycle.

 The invention applies in particular to the production of oxygen from atmospheric air.

 The pressures discussed below are absolute pressures.

 The pumping step is located at least in the final part of the desorption phase. It is carried out by means of one or more pumping systems and can be accompanied by an elution of the adsorber by injection against the current of a gaseous fraction containing the least adsorbable constituents.

 The repressurization phase is carried out by cocurrent with the mixture to be treated and / or against the current with production gas resulting from the adsorption stage (a), and / or with gas originating from internal recycling. , in particular with gas extracted from the adsorber during a first co-current depressurization step preceding the pumping step, or during the start of this same pumping step.

 The number N of adsorbers can be arbitrary, or N 2 1.

 By "pumping system" is meant either a pump proper, or a reversible machine operating alternately as a compressor / blower, in particular during the adsorption phase (a), and as a vacuum pump.

 In order to carry out vacuum pumping in the type of cycle considered, commonly used positive displacement compressors of the "Roots" type. These are robust machines accepting constantly changing operating conditions and, in particular, sudden variations in suction pressure when switching from one adsorber to another.

 However, these machines have the following drawbacks.

 On the one hand, their maximum available size limits the unit capacity of PSA devices and leads either to installing several units in parallel, or to putting several machines in parallel on the same unit when the pumping capacity required exceeds the possibilities offered by suppliers. of these machines.

 On the other hand, the pumping energy efficiency of Roots machines is poor and decreases appreciably when the pressure difference between the inlet and the outlet of these machines increases, which generally leads to installing two or even three stages in series to reduce energy consumption.

 There are machines that are more energy efficient and work with much higher flow rates, namely centrifugal pumps or pumping systems. However, their use in PSA units poses serious difficulties.

 Indeed, the efficiency of such a machine remains close to its maximum value - in practice greater than 80% - only within a limited operating range. In particular, in the use made of it as a vacuum pump, its performance is very sensitive to the suction pressure: above a limited range of suction pressures, there is a sharp drop in yield, while below this range, one enters the unstable operating zone of the machine.

 On the other hand, the change of adsorber at the end of the pumping stage creates an instantaneous variation in the suction pressure of the centrifugal machine which suddenly modifies the hydraulic conditions of this machine. This phenomenon, which is repeated with great frequency, can lead to rapid destruction of the machine.

 A certain number of devices (speed variator, mobile blades, variable throttles) mentioned below allow to extend the optimal operating zone when the suction pressure varies, however they do not allow to eliminate the harmful effects of the changes snapshots of the operating conditions of the pump.

 The object of the invention is to allow efficient use of centrifugal pumping systems in PSA units, in particular avoiding the problem mentioned last.

To this end, the invention relates to a process of the aforementioned type, characterized in that
at least part of the pumping step (b2) is carried out by means of a centrifugal type vacuum pump, and
- for a given adsorber, the final suction pressure of the centrifugal pump is close to its initial suction pressure.

The method according to the invention may include one or more of the following characteristics
- The difference between said final and initial pressures is less than approximately 100 mb, and preferably less than approximately 50 mb;
- in a final part of the pumping step, an elution gas rich in weakly adsorbable constituents is sent against the current in the adsorber with a regulated flow rate to obtain a pumping end pressure close to the start pressure pumping the centrifugal pump;
the elution rate is obtained through at least one all-or-nothing type valve having a manually adjustable mechanical opening limiter;
- the elution flow rate is obtained through at least one valve of the positioner type controlled from a preset opening ramp or automatically adjustable from the cycle parameters;
- Said gas is, at least in part, gas of first co-current decompression of an adsorber at the start of the desorption phase;
- Said gas is, at least in part, taken from the production of an adsorber in the adsorption phase;
- At least part of said gas elutes in a substantially isobaric counter-current at the low pressure of the cycle;
- At least part of said gas elutes against the current at variable flow;
- the efficiency of the centrifugal pump is optimized, during its operation relative to a given adsorber, by varying its speed of rotation and / or by varying the inclination of the blades of the pump and / or by a variable throttling of the flow that she processes; and
- Is used, during the pumping step, first a positive displacement pump, then said centrifugal pump.

Examples of implementation of the invention will now be described with reference to the accompanying drawings, in which
- Figure 1 schematically shows an installation with three adsorbers intended for the implementation of the method according to the invention;
- Figure 2 is a diagram which illustrates on the one hand a conventional cycle, on the other hand a cycle according to the invention, implemented with the installation of Figure 1;
- Figure 3 is a diagram similar to the
Figure 2 but relating to the implementation of the invention with an installation with two adsorbers; and
- Figure 4 is a diagram similar to the
Figure 2 but relating to the implementation of the invention with an installation with five adsorbers.

 The installation shown in Figure 1 is intended to produce air enriched in oxygen, or impure oxygen, with a content preferably between approximately 90 and 95%, from unpurified atmospheric air. It comprises three adsorbers 1 to 3, an intake line 4 equipped with a fan or a blower 5, a pumping line 6 equipped with a vacuum pump 7 of the centrifugal type discharging into the open air, and a line 8 for the production of enriched air. Each adsorber has a cylindrical configuration and has a lower inlet 1A to 3A and an upper outlet 1B to 3B. The adsorbers are filled with an adsorbent which preferentially adsorbs nitrogen with respect to oxygen and argon, in particular with a zeolite of type A or X. Optionally, each adsorber can have at its base a layer of 'another adsorbent having a drying action, in particular of alumina or silica gel.

 Line 4 is connected to the inlet of each adsorber via a respective valve 9-1 to 9-3. Likewise, line 6 is connected to the inlet of each adsorber via a respective valve 10-1 to 10-3. Line 8 is connected to the output of each adsorber via a respective valve 11-1 to 11-3. A line 14, via valves 15-1 to 15-3, allows gas exchanges between the adsorbers during desorption and repressurization.

 The suction of the pump 7 can also be connected to the open air via a line 12 fitted with a valve 13.

 By means of this installation, a cycle is carried out for each adsorber, for the nominal production of enriched air, which has been illustrated in FIG. 2 with reference to adsorber 1. If T denotes the duration of the cycle, the operation of the adsorber 2 is deduced therefrom by time shift of T / 3 and that of the adsorber 3 by time shift of 2T / 3. In the example illustrated, one can choose for example a cycle time from 1 minute to a few minutes. Machines 5 and 7 are continuously driven.

 In FIG. 2, where the times t are plotted on the abscissa and the absolute pressures P on the ordinate, the lines oriented by arrows indicate the movements and destinations of the gas streams; when the arrows are parallel to the ordinate axis, they also indicate the direction of circulation in an adsorber: when an arrow is in the direction of increasing ordinates (towards the top of the diagram), the direction of the current in the adsorber is co-current; if the arrow pointing upwards is located below the line indicating the pressure in the absorber, the current enters the adsorber through the inlet end of the absorber; if the arrow, directed upwards, is located above the line indicating the pressure, the current leaves the adsorber through the outlet end of the adsorber, the inlet and outlet ends being respectively those of the gas to be treat with the adsorber under consideration and gas withdrawn from this same adsorber in the adsorption phase; when an arrow is in the direction of decreasing ordinates (down the diagram), the direction of the current in the adsorber is against the current.

If the downward pointing arrow is located below the line indicating the pressure of the adsorber, the current leaves the adsorber through the inlet end of the adsorber; if the downward pointing arrow is located above the line indicating the pressure, the current enters the adsorber through the outlet end of the adsorber, the inlet and outlet ends always being those of the gas to be treated and gas withdrawn during the adsorption phase. On the other hand, the gas streams which relate exclusively to an adsorber are indicated in solid lines and the gas streams coming from or towards other adsorbers in dotted lines.

The complete cycle will now be described for an adsorber, for example adsorber 1, with reference to
Figures 1 and 2. In the example of Figure 2, the cycle evolves between two extreme pressures, namely a high or maximum pressure PM between atmospheric pressure and 2 bars and more generally between 1 and 1.6 bar, and a low or minimum pressure Pm between 200 and 500 mb, which explains the use of the blower 5 and the vacuum pump 7.

 Two cycles have been illustrated in FIG. 2 which can be implemented by means of the installation in FIG. 1. These two cycles have a common part, from time t = 0 to time tl, between T / 3 and 2T / 3, which will be defined later. From this instant tl, the lower curve A relates to a conventional cycle, while the upper curve B represents a cycle according to the invention.

The classic cycle includes
(a) From t = 0 to T / 3, an adsorption phase in the vicinity of the high pressure PM. During this phase, the atmospheric air to be treated is admitted to the inlet of the adsorber and circulates cocurrently therein, while production gas (air enriched in oxygen) is withdrawn at the outlet of the adsorber.

(b) From T / 3 to 2T / 3, a desorption phase, which is subdivided into two stages
(bl) from T / 3 to tl, a co-current decompression step, during which gas is drawn off at the outlet of the adsorber and transferred to the outlet of another adsorber at the start of the recompression phase.

At time tl, an intermediate pressure PI is reached which is for example substantially equal to 0.9 bar. And
(b2) from tl to 2T / 3 (curve A), a purging step by pumping against the current up to the low pressure of the cycle Pm, in which the inlet of the adsorber is connected to the suction of pump 7.

(c) from 2T / 3 to T (curve A), a recompression phase, which is subdivided into two stages
(cl) from 2T / 3 to t2, with t2 - 2T / 3 = tl
T / 3, a phase of first recompression / elution with pumping, in which the adsorber receives by its output gas from the output of another adsorber in co-current decompression step (bl), while a lower gas flow rate is sucked up against the current by the pump 7. At time t2, the pressure reached PF is much lower than PI and is for example of the order of 0.4 bar.

And
(c2) from t2 to T, a step of final recompression against the current with gas taken from the outlet of another adsorber in step (a) of adsorption.

As explained above, such a cycle can hardly be adopted industrially with a centrifugal machine. Indeed, the pump is in operation on the adsorber in question from tl to t2, ie for a third of the period (T / 3). The machine therefore passes directly from one adsorber to another, and it sees its aspiration suddenly pass from the pressure PF to the pressure
PI when it is switched, at time t2, from the adsorber 1 to the absorber which then goes to step (b2).

 To resolve this problem and, consequently, avoid the risk of rapid deterioration of the pump, the cycle is modified as follows between the instants tl and T (upper curve B).

 (1) The pressure Pm is noted up to a value P8m> PmZ preferably by reducing the size ratio of the vacuum pump / sieve volume as explained below.

 (2) When the pressure P 'is reached, at an instant t3 such that tl <t3 <2T / 3, a third desorption step (b3) consisting of a counter elution is introduced from t3 to 2T / 3 -current substantially isobaric by means of production gas taken from another adsorber in the adsorption phase (a), with simultaneous pumping against the current.

 Thus, at time 2T / 3, the adsorber is at the pressure P 'm and it is easy, during step (cl), to reach a final pumping pressure P'F substantially equal to PI. As a result, the suction of the pump 7 remains at substantially the same pressure when the pump is switched to the next adsorber.

 From t2 to T, the cycle ends as before by a final recompression against the current, using production gas, to a pressure close to the high pressure PM.

 In practice, a pressure difference of at most 50 mb, or even 100 mb, between the P'F and PI values is acceptable.

 It should be noted that, to compensate for the increase in the low pressure of the cycle, and therefore the decrease in efficiency of regeneration by vacuum, the mass of adsorbent will preferably be increased, which will make it possible to retain the same amount impurities per cycle.

 It will be noted in this regard that the share of energy in the cost of oxygen produced from large capacity units (greater than 50 to around 75 t / d) is preponderant before the share relating to investment, and that the additional cost linked to the increase in the absorbent mass will be largely offset by the gain in energy consumed due to the use of a high-efficiency centrifugal pumping machine.

 Furthermore, an additional effect of raising the low pressure Pm can be obtained by shortening the purge time, while remaining within the acceptable limits vis-à-vis the kinetic effects of desorption.

 Figure 3 illustrates a cycle for implementing the invention on an installation similar to that of Figure 1 but comprising only two adsorbers. In this case, the curve is shifted by T / 2 from one adsorber to another. As previously, a known cycle and a cycle according to the invention have been illustrated. The two cycles are identical from t = 0 to tl and from t3 to t4, these times being defined below.

From tl to t3, the upper curve Al corresponds to the known cycle and the lower curve B1 corresponds to the cycle according to the invention. On the contrary, from t4 to T, the lower curve A2 corresponds to the known cycle while the upper curve B2 corresponds to the cycle according to the invention.

The classic cycle includes
- from t = 0 to tl: a co-current adsorption phase, analogous to the previous phase (a);
- from tl to t2: a first co-current decompression step, analogous to the previous step (bl);
- from t2 to t3: a step of second decompression against the current up to the pressure PI, which is substantially equal to atmospheric pressure. In this step, the inlet of the adsorber is placed in communication with the atmosphere;
- From t3 to t4: a final decompression step by pumping, by means of the centrifugal pump 7, until the low pressure Pm of the cycle, this step being analogous to the previous step (b2);
- From t4 to t5: a step of roughly isobaric elution by means of production gas with, simultaneously, pumping against the current by means of the pump 7, this step being analogous to the previous step (b3);
- from t5 to t6: a first step of first counter-current repressurization with simultaneous counter-current pumping, this step being analogous to the previous step (cl);
- From t6 to t7, second recompression against the current using production gas taken from the outlet of the other adsorber; and
- from t7 to T, final co-current recompression using air to be treated, up to a pressure close to the high pressure of the PM cycle.

 In this example, we have t6-t2 = T / 2. Consequently, to make the pump 7 operate continuously, its suction is connected to the atmosphere from t2 to t3, as shown schematically in Figure 3.

 In this conventional cycle, the pump 7 is connected to the inlet of the adsorber until time t6, at which the pressure PF is significantly lower than the pressure PI at the start of pumping of the adsorber.

 Consequently, as before, switching the pump from one adsorber to another is accompanied by a large variation in the suction pressure, with the corresponding risks of deterioration of the pump.

 According to the invention, the cycle is modified in the following manner.

(1) The production gas flow sent to the adsorber from t4 to t5 is increased, which raises the pressure at time t5; and
(2) from tl to t2, the flow of gas leaving the co-current from the adsorber is increased. This has the effect of transferring, at the same time, a larger quantity of gas into the other adsorber during the first countercurrent recompression, and therefore increasing the slope of the curve from t5 to t6.

In this way, as before, it is possible to reach, at the instant t6 at the end of pumping, a final suction pressure P'F substantially equal to the pressure
PI pumping start, which is, in this example, substantially equal to 1 bar.

 Figure 4 illustrates the implementation of the invention by means of an installation with five adsorbers.

The cycle according to the invention comprises
- From t = 0 to T / 5, a first co-current adsorption step, during which part of the production gas is sent against the current in another adsorber in the final recompression phase;
- from T / 5 to 2T / 5: another co-current adsorption step, in which part of the production gas is used as counter-current elution gas in another adsorber in the final decompression phase / elution.

 For these first two stages, the pressure is close to the high pressure PM of the cycle and is substantially equal to atmospheric pressure.

 - From 2T / 5 to 3T / 5, a step of first decompression by pumping against the current, by means of an additional pump 14 of the volumetric type, in particular Roots.

 - from 3T / 5 to 4T / 5, a final decompression / elution phase. During this stage, the adsorber receives at its outlet production gas taken off at the outlet of another adsorber in the second co-current adsorption phase, while its outlet is connected to the suction of the pump. centrifugal 7.

 During this step, the elution gas flow rate and / or the pumping flow rate are variably adjusted so that, at instant 4T / 5, the final suction pressure P'F is substantially equal to the initial suction pressure PI corresponding to time 3T / 5.

 Following the curve indicated by solid lines in Figure 4, the variable setting has the effect that, for approximately half of this fifth of the period, the pressure begins by decreasing by PI at the low pressure of the PmS cycle and then increases appreciably until PI.

 However, as indicated in phantom, one can freely choose the shape of this section of curve, for example a shape close to a horizontal line.

 - from 4T / 5 to T, a final recompression step against the current using production gas taken from the outlet of another adsorber during the first adsorption phase.

Claims (12)

 1 - Process for treating a gaseous mixture, in particular of air, by pressure variation adsorption (PSA), of the type in which at least one adsorber is used and, in or in each adsorber, a cycle comprising successively  (a) an adsorption phase in which the mixture is introduced through one end, called the inlet, of the adsorber, it is made to circulate in the adsorber in a direction called co-current to the other end, called outlet, from the adsorber, and a weakly adsorbable gaseous fraction is extracted from this outlet;  (b) a desorption phase comprising a step (b2) of pumping against the current to a low pressure (Pm) of the cycle below atmospheric pressure; and  (c) a phase of repressurization of the adsorber to a pressure close to a high pressure of the cycle, characterized in that  at least part of the pumping step (b2) is carried out by means of a vacuum pump (7) of the centrifugal type, and  - for a given adsorber, the final suction pressure (P'F) of the centrifugal pump is close to its initial suction pressure (PI).  2 - Process according to claim 1, characterized in that the difference between said final (P'F) and initial (PI) suction pressures is less than approximately 100 mb, and preferably less than approximately 50 mb.  3 - Process according to claim 1 or 2, characterized in that in a final part of the pumping step, an elution gas rich in weakly adsorbable constituents is sent against the current in the adsorber with a flow rate adjusted for obtain a pumping end pressure (P'F) close to the pumping start pressure (PI) of the centrifugal pump (7).  4 - Process according to claim 3, characterized in that the elution rate is obtained through at least one valve of the all or nothing type having a manually adjustable mechanical opening limiter.  5 - Method according to claim 3, characterized in that the elution rate is obtained through at least one valve of the positioner type controlled from a preset opening ramp or automatically adjustable from the cycle parameters.  6 - Method according to any one of claims 3 to 5, characterized in that said gas is, at least in part, gas of first decompression co-current of an adsorber at the start of desorption phase (b).  7 - Method according to any one of claims 3 to 6, characterized in that said gas is, at least in part, taken from the production of an adsorber in phase (a) adsorption.  8 - Method according to claim 7, characterized in that at least a portion of said gas performs a substantially isobaric counter-current elution at the low pressure (higher) of the cycle (Figure 2).  9 - Method according to claim 7, characterized in that at least a portion of said gas elutes against the current with variable flow (Figure 4).  10 - Process according to any one of claims 1 to 9, characterized in that the efficiency of the centrifugal pump (7) is optimized, during its operation relating to a given adsorber, by variation of its speed of rotation and / or by varying the inclination of the blades of the pump and / or by a variable throttling of the flow rate it treats.  11 - Method according to any one of claims 1 to 10, characterized in that one uses, during the pumping step (b2), first a positive displacement pump, then said centrifugal pump (7).  12 - Application of a method according to any one of claims 1 to 11 to the production of oxygen from atmospheric air.