FR2776940A1 - Pressure swing adsorption for purifying gas containing impurities - Google Patents

Abstract

Each adsorber goes through a cycle comprising: (a) production mainly at high cycle pressure with introduction of the impure gas and recovery of the pure gas; (b) depressurization to a balance zone; (c) elution with an elution gas; (d) first recompression with gas from the balance zone; and (e) second recompression with part of the pure gas produced by another adsorber at the first stage. The adsorption system has at least two adsorbers each containing at least one adsorption bed. In (b), the depressurization is carried out until a low pressure is reached, preferably atmospheric pressure, in forward or back flow, preferably forward flow. (c) is carried out at low pressure, preferably atmospheric pressure, in forward or back flow, preferably back flow, using a residual gas from an air separation unit, preferably impure nitrogen from the low pressure column. The first recompression (d) precedes the second recompression (e) and is in back flow. The pure gas produced in (a) is sent to a cryogenic separation unit. The high pressure is 3 x 10<5> and 100 x 10<5> Pa. An Independent claim is included for an apparatus for the above process, with a PSA air purification unit the pressure of which can be alternated, comprising a compressor for atmospheric air, at least two adsorbers in parallel, each containing at least one adsorbent bed, at least one balance capacity connected to the adsorbers, a channeling device for the purified air from the adsorbers to at least one cryogenic separation unit, and a cryogenic separation unit for the purified air produced.

Description

The aim of the present invention is to provide a process for the pretreatment or purification of atmospheric air before cryogenic separation of said air, in particular by cryogenic distillation.

It is known that atmospheric air contains compounds which must be eliminated before the introduction of said air into the heat exchangers of the cold box of an air separation unit, in particular the compounds of carbon dioxide (C02) and vapor. 'water (ho).

In fact, in the absence of such a pre-treatment of the air to remove its impurities CO2 and water, there is condensation and solidification of these impurities to ice when the air is cooled to cryogenic temperature, from which it can result problems of clogging of the equipment, in particular the heat exchangers, of the distillation columns ...

In addition, it is also customary to remove the hydrocarbon impurities likely to be present in the air in order to avoid their excessive concentration at the bottom of the distillation column (s), thus to mitigate any risk of explosion.

Currently, this pre-treatment of the air is carried out, as appropriate, by the TSA process (Temperature
Swing Adsorption) or by the PSA process (Pressure Swing
Adsorption); by PSA process is meant the actual PSA processes, the VSA processes (Vacuum Swing
Adsorption), VPSA processes and the like.

Conventionally, a TSA process cycle comprises the following steps:
a) air purification by adsorption of impurities at super-atmospheric pressure and room temperature,
b) depressurization of the adsorber to atmospheric pressure or below atmospheric pressure,
c) regeneration of the adsorbent at atmospheric pressure, in particular by the waste gases or waste gases, typically impure nitrogen coming from an air separation unit and heated to a temperature usually between 100 and 2000C by means of one or more heat exchangers,
d) cooling the adsorbent to ambient or subambient temperature, in particular by continuing to introduce therein said waste gas coming from the air separation unit, but not reheated,
e) repressurization of the adsorber with purified air obtained, for example, from another adsorber in the production phase.

Usually, a PSA process cycle comprises, for its part, substantially the same steps a), b) and e), but differs from a TSA process by an absence of reheating of the waste gas or gases during the step of. regeneration (step c)), therefore the absence of step d) and, in general, a shorter cycle time than in the TSA process.

The removal of CO2 and water vapor is usually carried out on several adsorbent beds, namely a first adsorbent intended to preferentially stop the water, for example a bed of activated alumina, of silica gel or of zeolites, and a second bed of adsorbent to preferentially stop the CO 2, for example a zeolite. Mention may in particular be made of documents US-A-5531808, US-A-5587003 and US-A-4233038.

Generally, the air pretreatment devices comprise two adsorbers, operating alternately, that is to say that one of the adsorbers is in the production phase, while the other is in the regeneration phase.

Such PSA air purification processes are in particular described in documents US-A-3738084, FR-A7725845 and JP-A-63107720.

More precisely, the cycle described by JP-A-63107720 breaks down into an adsorption step of 15 min, a depressurization step of 2 min, a regeneration step of 8 min and a repressurization step of 5 min. The purification pressure or high pressure is equal to 7.10 Pa, which makes it possible to obtain purified air containing less than 1 ppm of CC2 and H2O impurities.

In addition, Tomomura et al., Kagaku Kogaku Ronbunshu, 13 (5), 1987, p. 548-553, “Application to an air separation plant”, describes an air separation unit supplied at a flow rate of 2700 Nm3 / hr with purified air under pressure. The air purification is carried out in a PSA installation with two adsorbers each containing an adsorption bed consisting 40% of activated alumina beads and 60% of zeolite beads. In the PSA cycle, this PSA installation consumes 6.3% additional energy from the compressor compared to operation in the TSA cycle, which is explained by a loss of compressed air during the adsorber depressurization phase.

In fact, the compressor must compensate for said losses of compressed air, during each phase of repressurization of the adsorbers, which has a duration, here, corresponding to approximately one third of that of the cycle time.

However, this requires that the compressors deliver approximately 19% of additional compressed air during said pressurization phase, in order to be able to maintain a flow of purified air substantially constant to the air separation unit placed downstream, hence a overconsumption of energy.

However, given the high frequency of these flow peaks, namely every 10 minutes approximately, it is desirable to modify the PSA installation so as not to risk damaging the compressor and / or disturbing the cryogenic distillation process. subsequent.

In fact, the amount of compressed air lost is likely directly attributable to the adsorbent beds.

In fact, it is known that zeolite, for example, has a strong affinity for the gases of the air and, consequently, a bed of zeolite will retain a large quantity of compressed air, thus generating non-pressure drops. -negligible. In this regard, the following documents may be cited: US-A-4,711,645, DE-A-3702190, DE-A3045451 and JP-A-55152517.

In order to try to minimize these pressure drops due to the use of an adsorbent of zeolitic type, certain documents, such as US Pat. No. 5560763, EP-A-453202, JP-A63107720, US-A-5656064 and EP- A-766991, recommend replacing all or part of the zeolite bed with an activated alumina bed, given that activated alumina has the particularity, in the PSA cycle, of adsorbing much less air than zeolite, while leading to at least equivalent purification performance.

Thus, document US Pat. No. 5560763 specifies that a given quantity of zeolite adsorbs about three times more air than the same quantity of activated alumina.

However, this at least partial replacement of the zeolite bed by an activated alumina bed only makes it possible to reduce the air losses by 1.5 to 2.5% relative to the total volume of compressed air.

Moreover and alternatively, similar results can be obtained by reducing the size of the adsorbent bed and therefore the pressure drops.

However, this reduction in the size of the adsorbent bed is often made to the detriment of the efficiency of purification and / or of the investments in materials which then have to be implemented.

Indeed, documents EP-A-449474 and US-A-5156657 recommend using an additional vacuum pump to complete the regeneration of the adsorbent bed at the end of the step of elution of said adsorbent by the waste gases. .

Furthermore, document US-A-5560763 describes the implementation of a PSA process with at least three adsorbers, so as to increase the time available for the regeneration and repressurization of the adsorbers, to cancel or minimize the peak of very unfavorable flow rate for the compressor, to improve the regeneration of the adsorbers, therefore to reduce the quantity of adsorbent required and, thereby, to halve the pressure drops and to achieve an energy gain.

However, the increase in the number of adsorbers inevitably leads to an increase in the complexity of the installation, namely in particular at least one additional adsorber, several valves, an additional valve control system, etc. PSA unit.

The aim of the present invention is to provide a process and a PSA air purification installation:
- does not have the drawbacks of known installations and methods;
- which make it possible to greatly reduce the losses of compressed air occurring during the repressurization phase, for example a reduction of these losses of air from 20 to 458;
- which do not require a significant increase in the complexity of the installation, and therefore in material costs;
- which lead to significant energy gains,
- which allow accelerated recompression of the adsorbers from the low pressure of the cycle to the high pressure, so as to increase the time available for their regeneration; and
- which lead to a clear relief of the compressors, given that 20% to 45% of the repressurization is carried out by means of a buffer capacity.

The present invention therefore relates to a PSA process for purifying a gas containing impurities, using an adsorption system comprising at least two adsorbers each containing at least one adsorption bed, each adsorber being subjected to an adsorption cycle. pressure swing adsorption, comprising:
a) a stage of production substantially at the high pressure of the cycle with introduction of the impure gas in cocurrently into the adsorber and recovery of the pure gas thus produced,
b) a depressurization step towards at least one buffer zone,
c) an elution step with an elution gas,
d) a first recompression step with gas coming from the buffer zone,
e) a second recompression step with part of the pure gas produced by another adsorber in step a).

Depending on the case, the method of the invention may comprise one or more of the following characteristics:
- In step b), the depressurization is continued to substantially the low pressure of the cycle, preferably at approximately atmospheric pressure;
- in step b), the depressurization is carried out co-current or against the current, preferably co-current;
- step c) of elution is carried out at substantially the low pressure of the cycle, preferably at approximately atmospheric pressure;
- step c) of elution is carried out against the current or cocurrently, preferably against the current;
- In step c), the elution is carried out by means of a waste gas obtained from an air separation unit, preferably impure nitrogen obtained from the low pressure column;
the first recompression step d) precedes the second recompression step e);
- step d) of first recompression is carried out against the current;
- the pure gas produced in step a) is sent to a cryogenic separation unit;
- the high pressure is between 3.105 Pa and 5 100.10 Pa;
- impure gas is impure air and pure gas is purified air.

The invention further relates to an atmospheric air treatment installation, characterized in that it comprises
a PSA unit for purifying atmospheric air capable of implementing an adsorption process at alternating pressure according to one of claims 1 to 11, comprising
a) means for compressing atmospheric air,
b) at least two adsorbers operating in parallel and each containing at least one adsorption bed, and
c) at least one buffer capacity connected to said adsorbers,
- means for conveying the purified air produced by said adsorbers to a cryogenic separation unit,
- a cryogenic separation unit for the purified air produced.

The buffer capacity may contain an adsorbent to reduce its volume. Thus, if said buffer capacity is filled with zeolite particles, the latter will have a volume three times smaller than if it is empty.

However, it is nevertheless preferable to use an empty buffer capacity, that is to say without adsorbent inside.

The invention will now be described in more detail with the aid of the appended figures, given by way of illustration but not limitation.

FIG. 1 represents a PSA unit according to the prior art consisting of 3 adsorbers 1, 2, 3 operating alternately.

The air to be purified is conveyed, after compression to the adsorber in the production phase, via the feed gas line 5.

The gas produced, that is to say the purified air, is evacuated to its site of use via the product gas line 6.

A regeneration gas line 7 makes it possible to supply the adsorber in the elution phase with a regeneration gas, the eluted gas being discharged through line 8.

As can be seen, this device of the prior art comprises a large number of valves, namely 15 valves (4).

FIG. 2 represents a device according to the prior art similar to that of FIG. 1, but which comprises, this time, two adsorbers (11, 12) and only 9 valves (4).

However, as previously explained, this type of device generates high energy consumption due to air losses during decompression, despite the savings made, by reducing the number of valves 14 and adsorbers.

FIG. 3, on the other hand, represents an air purification unit PSA, before cryogenic distillation of said air, in accordance with the present invention.

More precisely, the device according to the invention is similar to that of FIG. 2, but also comprises a buffer capacity 33 to which the gases are sent during the step of depressurization of the adsorber 31 or 32 in phase. of depressurization.

It is further noted that despite the incorporation of a buffer zone or buffer capacity 33, the PSA unit according to the invention (FIG. 3) comprises the same number of valves 20 to 29 as the PSA unit according to prior art shown in Figure 2, namely 9 valves.

The operation of the adsorbers 31 and 32 of Figure 3 has been shown schematically in Figure 4.

Thus, it can be clearly seen that, when the adsorber 31 is in the production phase TP, the adsorber 32 is in the regeneration phase TR, and vice versa.

In addition, each production phase TR is followed by a phase of depressurization Dp of the adsorber 31 or 32, and each regeneration phase TR is followed by an elution / recompression phase Rp.

More precisely, it can be seen that part of the gas leaving the adsorber 32 during the depressurization phase Dp is sent to the buffer capacity (CT) 33, which then fills up during a filling time RPCT-
Then, the buffer capacity 33 is no longer called upon for the entire duration of the regeneration phase TR of the adsorber 32, which corresponds to the major part of the duration of the production phase TR of the adsorber 31.

After the regeneration and elution phase TR of the adsorber 32, the latter is subjected to a recompression phase Rp during which the gas stored in the buffer capacity 33 is sent to the adsorber 32, said recompression phase being completed by subsequent introduction into said adsorber 32 of gas produced by the adsorber 31 during its production phase 31.

The buffer capacity 33 then undergoes a new phase of filling or recompression with the decompression gas from the adsorber 31, which in turn enters its decompression Dp and regeneration phases.
TR; the adsorbers 31 and 32 operating alternately.

The following table indicates an opening (O) and closing (F) sequencing of the valves 20 to 29 of FIG. 3 during the implementation of the method according to the invention.

BOARD
I CHECKING THE VALVES

<tb> valves <SEP> 21 <SEP> T <SEP> 22 <SEP> 1 <SEP> 23 <SEP> | <SEP> 24 <SEP> 1 <SEP> 25 <SEP> | <SEP> 26 <SEP> 27 <SEP> - <SEP> 1 <SEP> 28 <SEP> T <SEP> 29 <SEP> 1 <SEP> 20 <SEP>
<tb>
Time in minutes

<tb> 0-1 <SEP> O <SEP> F <SEP> F <SEP> F <SEP> O <SEP> O <SEP> F <SEP> F <SEP> O <SEP> F
<tb> 1-2 <SEP> 0 <SEP> F <SEP> F <SEP> F <SEP> F <SEP> F <SEP> F <SEP> O <SEP> O <SEP> F
<tb> 2-11 <SEP> O <SEP> O <SEP> F <SEP> F <SEP> O <SEP> F <SEP> F <SEP> O <SEP> O <SEP> F
<tb> 11-12 <SEP> O <SEP> F <SEP> F <SEP> F <SEP> O <SEP> O <SEP> F <SEP> F <SEP> O <SEP> F
<tb> 12-15 <SEP> O <SEP> F <SEP> F <SEP> O <SEP> O <SEP> F <SEP> F <SEP> F <SEP> O <SEP> F
<tb> 15-16 <SEP> F <SEP> F <SEP> | <SEP> O <SEP> | <SEP> O <SEP> F <SEP> O <SEP> F <SEP> F <SEP> F <SEP> O
<tb> 16-17 <SEP> F <SEP> F <SEP> O <SEP> F <SEP> F <SEP> F <SEP> O <SEP> F <SEP> F <SEP> O
<tb> 17-26 <SEP> F <SEP> O <SEP> O <SEP> O <SEP> F <SEP> F <SEP> O <SEP> F <SEP> F <SEP> O
<tb> 26-27 <SEP> F <SEP> F <SEP> O <SEP> O <SEP> F <SEP> O <SEP> F <SEP> F <SEP> F <SEP> O
<tb> 27-30 <SEP> F <SEP> F <SEP> O <SEP> O <SEP> O <SEP> F <SEP> F <SEP> F <SEP> F <SEP> O
<tb>"O" = valve open, "F" = valve closed.

In addition, the capacity 33 can contain an adsorbent bed, which makes it possible to reduce its volume.

The pressure in the capacity converges towards a regime with two pressure levels, the quantity of compressed air recovered at the end of each phase of depressurization of an adsorber depending on the volume of the capacity 33.

In FIG. 5, the high and low pressures are shown and being established within the buffer capacity 33, as a function of the volume of this buffer capacity, when it is filled with a molecular sieve of the zeolite type.

It can be seen that the greater the volume of the buffer capacity, the less it works under pressure, that is to say that the pressure difference between the high pressure phase and that at low pressure decreases.

FIG. 6 shows diagrammatically the quantity of compressed air recovered by the capacity 33 and standardized with respect to the quantity of compressed air lost in a simple two-bottle PSA process, such as that shown in FIG. 2.

In addition, it can be seen, when the investment is compared with the energy saved, that the buffer capacity has a maximum efficiency when its volume is between 60% and 100% of the volume of an adsorbent. In other words, further increasing the volume of the buffer capacity is unnecessary or even undesirable from an economic point of view, since the energy savings are then negligible.

Claims (12)

1. PSA process for purifying a gas containing impurities, using an adsorption system comprising at least two adsorbers each containing at least one adsorption bed, each adsorber being subjected to an adsorption cycle at alternating pressure , comprising: a) a stage of production substantially at the high pressure of the cycle with introduction of the impure gas in cocurrently into the adsorber and recovery of the pure gas thus produced, b) a depressurization step towards at least one buffer zone, c) an elution step with an elution gas, d) a first recompression step with gas coming from the buffer zone, e) a second recompression step with part of the pure gas produced by another adsorber in step a). 2. Method according to claim 1, characterized in that in step b), the depressurization is continued until substantially the low pressure of the cycle, preferably at approximately atmospheric pressure. 3. Method according to one of claims 1 or 2, characterized in that in step b), the depressurization is carried out cocurrently or countercurrently, preferably cocurrently. 4. Method according to one of claims 1 to 3, characterized in that step c) of elution is carried out at substantially the low pressure of the cycle, preferably at approximately atmospheric pressure. 5. Method according to one of claims 1 to 4, characterized in that step c) of elution is carried out against the current or co-current, preferably against the current. 6. Method according to one of claims 1 to 5, characterized in that in step c), the elution is carried out by means of a waste gas from an air separation unit, preferably impure nitrogen from the low pressure column. 7. Method according to one of claims 1 to 6, characterized in that step d) of first recompression precedes step e) of second recompression. 8. Method according to one of claims 1 to 7, characterized in that step d) of first recompression is carried out against the current. 9. Method according to one of claims 1 to 8, characterized in that the pure gas produced in step a) is sent to a cryogenic separation unit. 10. Method according to one of claims 1 to 9, characterized in that the high pressure is between 3.105 Pa and 100.105 Pa. 11. Method according to one of claims 1 to 10, characterized in that the impure gas is impure air and the pure gas is purified air. 12. An atmospheric air treatment installation, characterized in that it comprises a PSA unit for purifying atmospheric air capable of implementing an adsorption process at alternating pressure according to one of claims 1 to 11, comprising a) means for compressing atmospheric air, b) at least two adsorbers operating in parallel and each containing at least one adsorption bed, and c) at least one buffer capacity connected to said adsorbers, - means for conveying the purified air produced by said adsorbers to a cryogenic separation unit, - a cryogenic separation unit for the purified air produced.