Method for producing oxygen by VPSA

Abstract

A method for producing oxygen by adsorbing a stream of atmospheric air, using a VPSA, including at least one adsorber, each adsorber undergoing a single pressure cycle including the following steps: a) producing a first stream of gas having an oxygen content T1 while loading the adsorber of the stream of atmospheric air upstream; b) producing a second stream of gas including an oxygen content T2<T1: c) producing a third stream of gas including an oxygen content T3<T2<T1 while simultaneously extracting a nitrogen-enriched residual stream; d) eluting the adsorber, from which the three streams of gas produced in steps a), b), and c) are taken with the second stream of gas produced in step b); e) repressurizing the adsorber consecutively with at least two streams, first and second repressurizing streams, with increasing oxygen content.

Claims

1. A process for the production of oxygen by adsorption of a stream of atmospheric air employing VPSA comprising at least one adsorber, each adsorber being subjected to the same pressure cycle comprising the following stages: a) production of a first gas stream comprising an oxygen content C1 while charging upstream the adsorber with the stream of atmospheric air, b) production of a second gas stream comprising an oxygen content C2<C1, c) production of a third gas stream comprising an oxygen content C3<C2<C1 with simultaneous extraction of a waste stream enriched in nitrogen, d) elution of the adsorber, from which have emerged the three gas streams produced in stages a), b) and c), with the second gas stream produced in stage b), e) repressurization of the adsorber which has been subjected to the elution of stage d) with successively at least two streams, a first and a second repressurization stream, having an increasing oxygen content, the first repressurization stream being the third gas stream produced in stage c) and the second repressurization stream being the second gas stream produced in stage b).

2. The process of claim 1, wherein the production of the second gas stream in stage b) is carried out by cocurrentwise depressurization.

3. The process of claim 1, wherein the production of the second gas stream in stage b) is carried out in two steps, a first step during which the production is carried out by cocurrentwise depressurization and a second step during which the production is carried out by a cocurrentwise depressurization combined with a countercurrentwise depressurization.

4. The process of claim 3, wherein the countercurrentwise depressurization is carried out by a valve.

5. The process of claim 1, wherein the repressurization stage e) is carried out with successively 3 streams, a first, a second and a third repressurization stream, having an increasing oxygen content, the first repressurization stream being the third gas stream produced in stage c), the second repressurization stream being the second gas stream produced in stage b) and the third repressurization stream being the first gas stream produced in stage a).

6. The process of claim 1, wherein, in the repressurization stage e), the repressurization with the first repressurization stream is carried out simultaneously with the introduction of air cocurrentwise on the feed side of the adsorber for at least a portion of this stage.

7. The process of claim 1, wherein, in the repressurization stage e), the repressurization with the second repressurization stream is carried out simultaneously with the introduction of air cocurrentwise on the feed side of the adsorber for at least a portion of this stage.

8. The process of claim 1, wherein, in the repressurization stage e), the repressurization with the third repressurization stream is carried out simultaneously with the introduction of air cocurrentwise on the feed side of the adsorber for at least a portion of this stage.

9. The process of claim 1, wherein, after the production stages a), b) and c) and before the elution stage d), the adsorber is subjected to a vacuum pumping stage.

10. The process of claim 1, wherein: the pressure at the end of stage a) is between 1.75 and 1.25 bar, the end of stage b) is between 1.5 and 1.0 bara, the pressure at the end of stage c) is between 1.0 and 0.7 bara, and the low pressure of the pressure cycle is between 0.25 and 0.45 bara.

11. The process of claim 1, wherein the duration of the pressure cycle is less than 60 seconds.

12. The process of claim 1, wherein the VPSA comprises N adsorbers or N groups of adsorbers with each adsorber n or each group of adsorbers n following the pressure cycle with an offset of a phase time with respect to the pressure cycle of the adsorber n1 with nN.

13. The process of claim 12, wherein N is between 1 and 4.

Description

DESCRIPTION OF PREFERRED EMBODIMENTS

(1) The solution proposed here is more simple than the solutions of the prior art as it results only in the production of three streams of decreasing purity and furthermore it differs with regard to the use of these streams in the cycle. The elution is carried out entirely with a portion of the stream produced in stage b), whereas the repressurization is carried out by successively using the streams produced of increasing purity.

(2) The cycle proposed in the context of the present invention is thus characterized by the sequences i, i+1 and i+2, during which the unit produces 3 successive streams rich in oxygen and of decreasing purity. Stage i corresponds to stage a) and thus to the production proper with a mean purity C1 which generally corresponds to the specification requested by the client; let us take, by way of example, 93 vol % O.sub.2. During this stage, the adsorber is fed with air via an air compression unit (C-air).

(3) The stage referenced i+1 corresponds to stage b) and thus to the production of a second stream rich in oxygen but with a mean purity C2 lower than the preceding one; let us take, by way of example, 91 vol % O.sub.2. This fraction can be produced with or without introduction of air at the other end of the adsorber. More specifically, the adsorber can be isolated on the air side and the oxygen fraction is produced by cocurrentwise decompression, or air can be introduced during only a fraction of this stage or throughout the duration of the stage, at nominal or reduced flow rate. It is also possible to simultaneously withdraw, during all or a part only of the stage, by countercurrentwise depressurization, a stream rich in nitrogen.

(4) The stage referenced i+2 corresponds to stage c) and thus to the production of a third stream rich in oxygen with a mean purity C3 lower than the preceding one; let us take 89 vol % O.sub.2. This stream is obtained by a cocurrentwise decompression simultaneous with a countercurrentwise decompression intended to simultaneously extract, from the adsorber, a stream rich in nitrogen. According to the pressure level, this countercurrentwise decompression can be carried out via a valve and/or via a vacuum pump. In practice, it will concern at least at the stage end a vacuum pumping and, for simplicity, the representation [VP] has been adopted to mean these different possibilities, VP being taken for vacuum pumping and the presence of square brackets meaning that the vacuum pumping stage is not obligatory.

(5) The management in the cycle of these different oxygen fractions produced is characterized by the linking together of the following sequences:

(6) Stage j corresponds to a stage of simple vacuum pumping in order to extract nitrogen. The production side of the adsorber is isolated.

(7) Stage j+1 corresponds to a stage of elution with pumping. A gas rich in oxygen is introduced on the production side simultaneously with the pumping. The presence of oxygen facilities the desorption of the nitrogen.

(8) Stages j+2, j+3 and j+4 are stages of repressurization of the adsorber.

(9) The elution of stage j+1 is carried out solely with the gas resulting from stage b), corresponding to stage i+1 which was defined above.

(10) The repressurization is carried out, for its part, by successive stages with streams of increasing oxygen purity: a first repressurization stream which is the third gas stream produced in stage c), a second repressurization stream which is the second gas stream produced in stage b) and optionally a third repressurization stream which is the first gas stream produced in stage a). According to the operating conditions, the contribution of the third repressurization may be weak and can be avoided, for simplicity. It is this which is meant here by the presence of square brackets: [C1].

(11) There exist several ways of representing, in summarizing fashion, the cycles of a unit of PSA type.

(12) Use will be made here of the tables method, in which each individual stage appears in one square. The cycle can be defined by describing all of the stages which an adsorber performs during a cycle. Preferably, the description of the different phases which the different adsorbers follow simultaneously are represented one under the other. If it is desired to be exhaustive, the operation of each adsorber is described one under the other. Each square defines a stage by an abbreviated title (Prod, Eq, Purge, and the like). This representation is very compact and very practical. However, since an incoming or exiting stream has several uses, this method loses clarity as it becomes difficult to simply define the corresponding stage. Nevertheless, this remains the method currently most widely used.

(13) Use has been made here of an intermediate method in the form of a table where, for each stage, the incoming and exiting streams of the adsorber are defined. This method has already been used in a somewhat different form. Thus, for example:

(14) TABLE-US-00001 Prod 1 C- air
means that the adsorber is in stage 1 and that it receives as feed a stream resulting from a compressor (C-air), this stream being, in the context of the invention, atmospheric air. The stream corresponding to the production (Prod) exits at the opposite end of the adsorber.

(15) TABLE-US-00002 X X j VP
means that stage j is a stage of simple placing under vacuum via a vacuum pump connected to the feed end while the production side is closed (X).

(16) The two top or bottom squares are left empty, if what takes place respectively on the production side or feed side is not formally defined or is not to be defined at this moment for a satisfactory understanding of the cycle, that is to say, for example, that the fact that there is an extraction, an injection of gas or that the end is isolated is not characteristic of the stage in question and that all the cases, indeed even their combination, for example an injection followed by an extraction, are possible.

(17) The family of cycles relating to the present invention can then be characterized by the following table, the columns left free meaning that, besides the 8 stages described, there may be additional stages, such as those corresponding to a change of vacuum pump, a final repressurization simply with air, and the like.

(18) TABLE-US-00003 C1 [C1] (Prod) C2 C3 X C2 C3 C2 [Prod] X i i + 1 i + 2 j j + 1 j + 2 j + 3 j + 4 C-air [VP] VP VP

(19) According to one embodiment, the production of a second gas stream rich in oxygen according to stage b) is carried out by simple cocurrentwise depressurization, corresponding to the following characteristic stages:

(20) TABLE-US-00004 C1 [C1] (Prod) C2 C3 X C2 C3 C2 [Prod] X i i + 1 i + 2 j j + 1 j + 2 j + 3 j + 4 X C-air X VP VP VP

(21) According to another embodiment, the production of the second gas stream rich in oxygen according to stage b) is carried out in two substages, first by simple cocurrentwise depressurization and then still by cocurrentwise depressurization but simultaneously with a countercurrentwise depressurization, preferably toward the atmosphere via a valve. The latter operation corresponds to the following characteristic substages k-a and k-b relating to the production of the stream of purity Pur2, which substages replace stage i+1 in the table above.

(22) TABLE-US-00005 C2 k-a k-b X X ATM

(23) According to another preferred embodiment, the initial repressurization of the adsorber with the third gas stream produced in stage c) is carried out simultaneously with the cocurrentwise introduction of air on the feed side during all or part of this stage. As the adsorber is under vacuum, this introduction of air can be carried out directly from the atmosphere via a valve. It can be an all or nothing valve or a valve whose opening changes throughout the stage. The opening may only be carried out the course of the stage with regard to a time delay or a pressure threshold. This opening is one of the parameters to be optimized. The most effective simulation software makes it possible to determine the tendencies to be observed. Onsite adjustments can make it possible to refine the opening characteristics. As it is not obligatory to pass through the vacuum pump and as it is possible for this injection of air to be reduced, indeed even zero, the representation [ATM] has been adopted to represent these operating possibilities, hence the representative stages:

(24) TABLE-US-00006 C1 [C1] (Prod) C2 C3 X C2 C3 C2 [Prod] X i i + 1 i + 2 j j + 1 j + 2 j + 3 j + 4 X C-air X [VP] VP VP [ATM]

(25) According to another preferred embodiment, the repressurization of the adsorber with the second gas stream produced in stage b) is carried out simultaneously with the cocurrentwise introduction of air on the feed side during all or part of this stage and preferably throughout the entire stage. As the adsorber is under vacuum, the bulk of the repressurization with air can be carried out directly from the atmosphere via a valve. It can be an all or nothing valve or a valve whose opening changes throughout the stage. The opening may only be carried out the course of the stage with regard to a time delay or a pressure threshold. This opening is one of the parameters to be optimized. As said above, the air is preferably introduced throughout this stage and the corresponding representation is then as follows:

(26) TABLE-US-00007 C1 [C1] (Prod) C2 C3 X C2 C3 C2 [Prod] X i i + 1 i + 2 j j + 1 j + 2 j + 3 j + 4 X C-air X [VP] VP VP [ATM] ATM

(27) According to a variant, the repressurization of the adsorber with the third gas stream produced in stage a) is carried out simultaneously with the cocurrentwise introduction of air on the feed side. In view of the pressure cycle, this optional repressurization takes place around atmospheric pressure or entirely above atmospheric pressure. It is then necessary to use a compression means (C-air) in order to introduce the atmospheric air into the adsorber.

(28) TABLE-US-00008 C1 C1 (Prod) C2 C3 X C2 C3 C2 (Prod) X i i + 1 i + 2 j j + 1 j + 2 j + 3 j + 4 X C-air X [VP] VP VP [ATM] ATM C-air

(29) The production of oxygen (stage i) then immediately follows the repressurization by the stream rich in oxygen (stage j+4). In practice, it may justifiable to have there a reversal of the direction of circulation of the oxygen when the pressure in the adsorber exceeds the pressure of the oxygen circuit.

(30) According to a preferred embodiment, the cycle additionally comprises a final repressurization of the adsorber with solely introduction of air on the feed side. This stage then precedes the stage of production of oxygen with the purity C1. The duration of this stage is determined by a time delay or by a pressure threshold. The air is introduced from the air compressor C-air as the adsorber is at a pressure greater than atmospheric pressure at least at the stage end. This stage takes place after the repressurization stage using the stream of purity C2 or after the optional stage of repressurization with gas of purity C1, resulting from the production.

(31) The following two variants are then respectively obtained:

(32) TABLE-US-00009 C1 (Prod) C2 C3 X C2 C3 C2 X X X i i + 1 i + 2 j j + 1 j + 2 j + 3 j + 4 X C-air X [VP] VP VP [ATM] ATM C-air
and

(33) TABLE-US-00010 C1 [C1] X (Prod) C2 C3 X C2 C3 C2 [Prod] X X i-1 i i + 1 i + 2 j j + 1 j + 2 j + 3 j + 4 X C-air C-air X [VP] VP VP [ATM] ATM C-air

(34) The choice of the high pressure of the cycle, while remaining within a relatively restricted range of pressures, can depend on the use which is made of the oxygen. If it is possible to use the production directly, that is to say without the addition of a compression device, it will be seen to that the oxygen is available at the correct pressure. Otherwise, it is by a VSA O.sub.2 unit/O.sub.2 compression means optimization that the best pressure will be determined, which will generally remain within the range extending from 1.25 to 1.75 bara. The choice of the low pressure, still within a limited pressure range, will depend both on the choice of the pumping device(s) and on the capital expenditure/energy economic optimization. A slightly lower pressure tends to reduce the volume of adsorbent to be brought into play but this is to the detriment of the energy consumption. The range envisaged in the context of the invention extends from 0.25 to 0.45 bara.

(35) The intermediate pressures between the high and low pressures are determined by optimization, once the pressure cycle and the linking together of the stages have been selected.

(36) Thus, according to an embodiment selected which comes within the context of the invention, the pressure at the end of stage a) is between 1.25 and 1.75 bara, the pressure at the end of stage b) is between 1.5 and 1.0 bara, the pressure at the end of stage c) is between 1.0 and 0.7 bara and the low pressure of the cycle is between 0.25 and 0.45 bara.

(37) According to a preferred embodiment, the pressure at the end of stage a) is in the vicinity of 1.5 bara, the pressure at the end of stage b) is in the vicinity of 1.25 bara, the pressure at the end of stage c) is in the vicinity of 0.85 bara and the low pressure of the cycle is in the vicinity of 0.35 bara.

(38) The term in the vicinity is understood to mean to plus or minus 50 mbar and preferably to plus or minus 25 mbar.

(39) As was said above, it is possible to carry out this cycle with a priori any number of adsorbers but the most appropriate units will comprise 1, 2, 3 or 4 adsorbers.

(40) According to one embodiment, the VPSA unit thus comprises 1 adsorber which follows the 8-stage cycle below:

(41) TABLE-US-00011 C1 [C1] (Prod) C2 C3 X C2 C3 C2 [Prod] X 1 (i) 2 (i + 1) 3 (i + 2) 4 (j) 5 (j + 1) 6 (j + 2) 7 (j + 3) 8 (j + 4) X C-air X [VP] VP VP [ATM] ATM C-air

(42) Preferably, stage b), that is to say 2 in the above table, will comprise a first step 2-i during which production is carried out by cocurrentwise depressurization and a second step 2-ii during which production is carried out by a cocurrentwise depressurization combined with a countercurrentwise depressurization.

(43) The corresponding cycle can be represented as follows:

(44) TABLE-US-00012 C1 (Prod) C2 C3 X C2 C3 C2 X X X 1 (i) 2-i (i + 1) 2-ii (i + 1) 3 (i + 2) 4 (j) 5 (j + 1) 6 (j + 2) 7 (j + 3) 8 (j 1) X C-air X ATM VP VP VP ATM ATM C-air

(45) This cycle, like the preceding one, comprises 3 holding tanks, corresponding to each of the purities.

(46) This cycle differs very substantially from the other cycles proposed for a unit having just one adsorber. The document U.S. Pat. No. 6,132,496 in particular describes a unit comprising one adsorber, a single device acting as compressor and vacuum pump and comprising 3 tanks, like the cycles according to the present invention. Elution is carried out in both cases by the gas of second purity C2 but the repressurization on the oxygen side is carried out exclusively with the gas resulting from the production C1; there is neither gas of 3rd purity nor gradual repressurization with gases of increasing purity. All the other cycles of monoadsorber type operate a priori with one or two holding tanks only and thus employ different streams.

(47) According to another embodiment, the VPSA unit comprises 2 adsorbers.

(48) In order to make the cycle simpler to follow, the start of the repressurization has been chosen as first stage. Nevertheless, it would be possible to begin with another stage without changing the principle thereof.

(49) In the same spirit, each phase comprises the same number of stages, in this instance 5. This is not obligatory and, for example, stages 7 and 8, on the one hand, and 9 and 10, on the other hand, might be combined together into a single stage.

(50) TABLE-US-00013 C1 C3 C2 [C1] (Prod) C2 C3 X X C2 C2 X X 1 (i + 2) 2 (i + 3) 3 (j + 4) 4 (i) 5 (i + 1) 6 (i + 2) 7 (j) 8 (j) 9 (j + 1) 10 (j + 1) (Atm) Atm C-air C-air [C-air] VP VP VP VP VP

(51) The unit comprises an air compressor which can be in continuous operation (stages 1 to 5) or noncontinuous operation.

(52) The vacuum pump is in continuous operation (stages 6 to 10).

(53) The cycle requires at least 2 holding tanks in order to manage the production stream (C1) and the stream of second purity (C2). A portion of the streams which are produced will advantageously be used simultaneously, only the unused portion being stored.

(54) The preferential cycle is characterized in that there is no introduction of air in the stage of production of the stream of oxygen of second purity; as explained above, the advantage of the introduction of oxygen during the final repressurization stage (stage 3) will partially depend on the type of adsorbers which are used. It may be unnecessary for a cylindrical adsorber having a vertical axis (or having a cluster of adsorbers of this type) exhibiting a low oxygen dead space and on the contrary, have a positive effect in the case of a radial adsorber comprising a high oxygen dead space, for example 30% of the volume of adsorbent.

(55) With our conventions, the corresponding cycle for such an adsorber is represented as follows:

(56) TABLE-US-00014 C1 C3 C2 C1 X (Prod) C2 C3 X X X C2 C2 X X X X 1 (j + 2) 2 (j + 3) 3 (j + 4) 4 (i 1) 5 (i) 6 (i + 1) 7 (i + 2) 8 (j) 9 (j) 10 (j) 11 (j + 1) 12 (j + 1) X (Atm) Atm C-air C-air C-air X VP VP VP VP VP VP

(57) The reader is reminded that, during stage 1, the repressurization with the oxygen stream of purity Pur3 can be accompanied by a cocurrentwise repressurization with atmospheric air, which repressurization can begin in the course of the stage.

(58) According to another embodiment, the VPSA unit comprises 3 adsorbers.

(59) A cycle characteristic of the invention is represented below:

(60) TABLE-US-00015 C1 [C1] (Prod) C2 C3 X C2 C3 C2 [Prod] X 1 2 (i) 3 4 5 (j) 6 7 8 9 (i + 2) (i + 3) (j + 1) (j + 2) (j + 3) (j + 4) X C-air C-air X VP VP VP [ATM] ATM C-air

(61) Cycles corresponding to 1, 2 and 3 adsorbers have been described in detail but it is easy for a person skilled in the art, starting from the instructions given, to adapt this cycle to any number of adsorbers, for example for 6 adsorbers, starting from the final table by thus combining together the stages: 9+1/2/3+4/5/6/7+8, which constitutes only one of the possibilities.

(62) For the cycles comprising 1 to 4 adsorbers, particulate adsorbents (beads, rods, crushed materials) are preferably used. And, for the cycles comprising more than 4 adsorbers, structured adsorbents are preferably used.

(63) It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above.