WO1988000084A1

WO1988000084A1 - Method and installation for increasing the content of a desired constituent in a gas mixture

Info

Publication number: WO1988000084A1
Application number: PCT/NL1987/000014
Authority: WO
Inventors: Drewes Egbert Hielema
Original assignee: Hielco
Priority date: 1986-06-25
Filing date: 1987-06-23
Publication date: 1988-01-14
Also published as: NL8601664A

Abstract

Method of increasing the content of at least one desired constituent in a gas containing a plurality of constituents, an installation for applying a method of this kind, and air enriched with oxygen by the application of this method. A pressure swing adsorption process and installation for carrying out said process are described. In the process according to the invention a temperature gradient in one chamber, formed in a plurality of cycles, is compensated by a gradient in an adjoining chamber by appropriate regulation of the flow in each chamber. At a certain moment in time the flows in two adjoining chambers will be countercurrent if in both chambers adsorption or desorption takes place and co-current if the processes in both chambers are different, i.e. one adsorption and one desorption. Also an installation embodying above process is described.

Description

Title :

Method and installation for increasing the content of a desired constituent in a gas mixture

The invention relates to a method of increa sing the content of at least one desired constituent in a gas containing a plurality of constituents by the application of pressure swing adsorption.

A method of this kind is known from American Patent Specification No. 3,323,288.

In general upon adsorption of gaseous substances on solid adsorption media latent heat of adsorption is released. This heat of adsorption leads to a rise in temperature in the adsorption medium and the unadsorbed gas.

The heat released in the adsorption stage is transported in the direction of the end of the adsorption medium bed by the flow of unadsorbed gas, and the gas enriched with oxygen is discharged there. During the following desorption heat is taken from the bed and, since during that step the directi of the gas flow is reversed in relation to its direc tion during the adsorption, the temperature at the end of the bed (that is to say the end of the bed . where in the adsorption step the gas was introduced) will be lower than that at the point where the gas was introduced in the desorption step. Continuation of the alternation of the adsorption and desorption steps thus has the consequence that the temperature of the adsorption medium at the product discharge end is always higher and at the untreated gas inlet end is always lower, so that a relatively high tern- perature gradient over the length of the bed results.

In view of the fact that the desorption properties of the adsorption medium are poorer at lower temperatures, this has the consequence that the adsorbed constituents at the feed end of the bed are poorly desorbed on continued operation of the installation. On the other hand, because of the high temperature at the product discharge end of the bed the adsorption properties of the adsorp- tion medium are poorer. Because of these two cir¬ cumstances the efficiency of the adsorption bed diminishes in the course of time.

Through the introduction of hot untreated gas or cold regeneration gas the temperature gradient caused by the adsorption and desorption can be partly reduced and even reversed. However, temperature equalization is not possible, because the zones situated at the inlet and outlet openings are strong¬ ly influenced, whereas the zones situated further inwards in the adsorption bed are influenced scarcely at all.

In the above me t oned US-A-3,323,288 a process is described f_or separating a gas mixture essentially using two axially extending adsorption zones which are in heat exchange contact by a common side wall. While in one compactment an adsorption step takes place the gas to be treated flowing in one direction; in the adjoining compart¬ ment a desorption takes place simultaneously, during which desorption the gas flows countercurrent to the gas in the first compartment.

The adsorption time needed is relatively long (total cycle in the order of 6 minutes) and a complete exchange of the heat developed in one adsorption step to the heat withdrawn in one des¬ cription step in an adjoining compartment is aimed for. This way of operating has as a main dis¬ advantage that the separation efficiency varies with time and that therefore no product with constant composition may be withdrawn from the installation. The present invention has as a goal to provide a process not having such a drawback in which high temperature gradients over the adsorption bed no longer occur and whereby during lengthy use of the bed its efficiency is not substantially changed. According to the invention the method is for this purpose characterized in that ^"one or more adsorption units are used, each of which comprises a plurality of chambers, adjoining chambers being in heat exchange contact with each other, while the gas flow in each of the chambers is adjusted in such a manner that the temperature gradients occurring in adjoining chambers in the course of a plurality of cycles in each chamber,as the con¬ sequence of heat of adsorption and desorption,are mutually opposite; the adsorption time per cycle being essentially shorter than 15 seconds.

By ensuring that in an adsorption unit temperature gradients formed in adjoining chambers in the course of a plurality of cycles are always mutually opposite and by assisting heat exchange through the nature of the contact, the effect is achieved in the application of the method according to the invention that a very high degree of temperature equality is attained over the adsorp- tion medium beds disposed in the chamber, which leads to constant efficiency. It is particularly important in the process according to the invention that the temperature gradients formed during a plurality of cycles are equalized between adjoining compartments; a heat flow during simultaneous pro¬ ceeding process steps will be very low as a result of the short adsorption and desorptions cycle time concerned. In particular, in the method according to the invention a plurality of adsorption units are connected to one another in heat exchange con¬ tact in such a manner that the temperature gradients formed in adjoining adsorption units are mutually opposite. In this way it is ensured that any gradients occurring at the boundaries of the adsorption units will as far as possible be prevented by connecting together a plurality of adsorption units. In this connection it is conceivable for a plurality of adsorption units to be placed one against the other with their side walls in heat exchange contact.

In the method according to the invention provision is advantageously made to ensure that in adjoining chambers in which the same step or different steps of the pressure swing adsorption process takes or take place, which are both carried out in the adsorption direction or are both carried out in the desorption direction, the gas flows are countercurrent, while cocurrent flow is used when one of the same or different steps is carried out in the adsorption direction and the other in the desorption direction. In this case it is conceivable for the gas being treated to be allowed to flow in the longitudinal direction of the different cham¬ bers, while if the adsorption step is thus carried out in two adjoining chambers the gas flows in the two chambers should be oppositely directed; if, on the other hand, adsorption is effected in one chamber and desorption in the other chamber the direction of flow of the gas should in that case be the same in both cases.

The present invention also relates to an. installation for increasing by pressure swing adsorp- tion the content of at least one desired constituent of a gas containing a plurality of constituents, which installation comprises one or more adsorption units. According to the invention this installa¬ tion is characterized in that each of the adsorption units forming part of the installation comprises chambers which are separated by heat-conducting partition walls, and that means are provided for adjusting in the desired manner the gas flow in each of the chambers.

In one particular form of construction of the installation according to the invention each of the chambers of an adsorption unit forming part of the installation is divided by a heat-conducting dividing wall into two halves, which are in communi¬ cation with one another at one end of the chamber via an opening in said dividing wall. By providing in each of the chambers a dividing wall by which the chamber is divided into two halves, the length of the chamber is doubled and the effect is achieved that the high-temperature side and the low-temperature side of one and the same chamber come to lie one against the other, whereby it is made possible for a substantial equali¬ zation of temperatures to be achieved.

In an installation according to the inven¬ tion it is expedient for one or more valves to be installed in each of the chambers of the adsorption unit for the purpose of adjusting the gas flow.

In particular, in the installation accor¬ ding to the invention each of the chambers of an adsorption unit and, if a plurality of adsorption units are used, all of the adsorption units are connected to a central rotary valve. Through the use of one central valve for controlling all gas movements, the effect is achieved that an important saving of installation costs is possible, together with simplified control by means of one single valve, as compared with the control and possible energiza¬ tion which are necessary when a system comprising a large number of valves is used. A central rotary valve of this kind can, for example be composed of a casing which is provided with a gas inlet opening and in which two parts provided with openings are sealingly connected together, the movement of the two parts relative to one another enabling openings in the two parts to be made to coincide in order to form connections. In that case at least one of the two parts will be provided with connection members for connecting together the ope- nings in that part. Through the operation of the abovedescribed central valve the effect can be achie¬ ved that in each of the chambers filled with adsorp¬ tion medium an adsorption process and a desorption process will take place alternately, while in addi- tion, as is customary, the pressure in one chamber is always raised by the use of an excess of gas at sufficiently high pressure which is available in another chamber.

Both in the case of the use of valves co- operating with each of the chambers and in the case of the use of one central rotary valve, it is of course necessary for the movement of the valves to take place in accordance with a determined pro¬ gram. It is particularly advantageous to provide for this purpose a control unit for the programmed operation of the valve or valves associated with the adsorption unit. In this case the valves should of course be of a type suitable for remote control. Valves of this kind are widely known.

In accordance with safety regulations it must be ensured that metals showing to tendency to form sparks are used for parts of the installa¬ tion which come into contact with the gas being treated. In the case of the enrichment of air with oxygen, mention may be made of copper or copper alloys.

In order to reduce friction between surfaces sealingly connected together in the valve or valves forming part of the installation, use will be made of a coating of a suitable plastics material, for example glass fibre reinforced polytetrafluoroethylene, on one of the surfaces. The other surface can then be made of one of the previously mentioned metals. When use is made of a central flat valve, it is advantageous for an elastomer coating to be applied between the plastics coating on one of the surfaces and the other surface. The sealing of the surfaces is further improved by this means.

The invention will now be explained with the aid of the drawing, in which:

Figure 1 shows schematically an installa- tion according to the invention.

Figure 2 shows in perspective an adsorption unit according to the invention, comprising four chambers.

Figure 3 shows schematically an adsorption unit according to the invention, in which the adsorp¬ tion step is carried out in one part of the chambers, and desorption is effected in another part.

Figure 4 shows a form of construction of an apparatus according to the invention, in which each of the chambers is divided into two compart¬ ments by a dividing wall,

Figure 5 is a graph showing a curve of pressure plotted against time in an adsorption cham¬ ber. In Figure 1 the general reference numeral

1 designates an adsorption unit according to the invention, while 2 designates the chambers which are filled with adsorption medium.

This adsorption medium may consist of a material known for such purposes; for the purpose of enriching air with oxygen use is advantageously made of zeolite type molecular sieves, on which nitrogen, water vapour and carbon dioxide can be selectively adsorbed, reference numeral 3 indicates a schematically repres r.-ed adsorption medium bed, while sieve plates 4 separate the adsorption medium and the gas inlet and cutlet spaces. 5 and © designate respectively the feed gas inlet and the product outlet.

As indicated by the arrows, the directions of flow in adjoining chambers 2 are mutually oppo¬ site, thus achieving opposite temperature gradients. Reference n r.eral 1* designates a desorp¬ tion unit according ro the invention which has a similar structure as unit 1_r the only difference being that in the unit the material adsorbed in a previous number of cycles is now desorbed. Optionally units 1 and 1' may be positioned such that two side walls contact each other; in that case the flowing direction of gas in each of the chambers of one of the units should be reversed. Figure 2 shows on a slightly larger scale how four chambers are connected together in an ad¬ sorption unit,in which appropriate selection of the connections ensure opposite flows in adjoining chambers. The connection between the various chambers is made by heat-conductive partition walls 7, while the chambers may be joined together by permanent connection methods, such as welding, adhesive bon¬ ding, and the like, although a detachable construc¬ tion is of course also possible.

Figure 3 shows the situation in which gas to be treated is supplied at 10 and distributed over three chambers. The product passes out via 9; a part 11 of the product output is taken off and supplied as flushing gas to the remaining chambers, which are regenerated in this manner. During this regeneration, therefore, desorption of the gases adsorbed on the adsorption material takes place. The gas mixture released during the regeneration is discharged via 8. The gas flow in adjoining cham¬ bers is in the same direction in this case. In Figure 4 four chambers 12, 13, 14 and 15 are shown, each chamber being divided by a divi¬ ding wall 16 into two compartments. At the end of each chamber the dividing wall has an opening, so that the gas can pass from one half of the chamber into the second half. In this example chambers 12, 13 and 14 are in use for an adsorption step or a pressure reducing step which takes place in the same direction of flow as the adsorption step. Cham- ber 15 is being regenerated.

Figure 4 shows the state of the process as represented in step 1 in Table A (this table will be discussed more fully later on). Chamber 12 in Figure 4 is operating in the adsorption step with air pressing (RF) and (REH); chamber 13 is supplying product (P) and supplying gas for pressure equalization (DEH). Chamber 14 is delivering gas for flushing (DPP), and chamber 15 is being purged under vacuum (VP). In view of the fact that the steps (DEH), (DEL) and (DPP) take place in the same direction as the desorption, the adjoining parts of the chambers 12, 13 and 14 operate in countercur¬ rent, while co-current flow exists between the right- hand half of chamber 14 and the left-hand half of chamber 15.

Table A shows the course of the process for an adsorption medium system consiting of four adsorption chambers, the process steps carried out in the different chambers being shown in dependence on time. At the time 0 seconds the pressure equali¬ zation takes place in adsorption chamber 1 at raised pressure with the aid of surplus air at adequate pressure from adsorption chamber 2, while at the same time feed air is pressed in. After three seconds a sufficiently high pressure is reached and discharge of product can start. This discharge of product continues to 10 seconds from the commencement of the cycle, whereupon tempering equalization takes place, the air thereby released being fed to adsorp- _

tion chamber 4, as indicated by an arrow in the table. At the time 16 seconds air at adequate pressure is still present for pressure equalization at low pressure for adsorption chamber 3, as indicated by the arrow. This pressure equalization has ended at the time 20 seconds, after which tempering with purging proceeds. The air thereby released is fed to adsorption chamber 2, which at that moment is at a sufficiently low absolute pressure. This tempe- ring with purging continues to the time 26 seconds, at which moment the system is brought to a greatly reduced pressure,- so that the desorption of adsorbed gases can be carried out. This reduction of pressure continues to the time 30 seconds, whereupon vacuum purging is started. During the vacuum purging gas at relatively high pressure is fed to chamber 1; the capacity of the vacuum pump is sufficiently great to ensure good desorption of adsorbed consti¬ tuents. The vacuum purging continues to the time 36 seconds, whereupon the pressure in adsorption chamber 1 is gradually built up by the introduction of gas at adequately high pressure from adsorption chamber 3. At the time 40 seconds the pressure in adsorption chamber 1 is once again approximately atmospheric, and thereupon the cycle outlined above for adsorption chamber 1 can proceed again in the sequence described above. The pressure/time proces¬ ses for the other three adsorption chambers are also shown in the table; the table is self-explana- tory.

It is clear from the table that for the raising of the pressure in one of the compartments a surplus of gas at adequately high pressure, asso¬ ciated with the desired reduction of pressure in another compartment, is always used. In this way a considerable saving of expense for pumping is achieved.

The pressure/time cycle for adsorption chambers 1, 2, 3 and 4, as described above with _

the aid of Table A, is shown again in the form of a graph in Figure 5. In this graph the steps from the time 0 to the time 26 are carried out in de adsorption direction, and the steps from the time 26 to the time 40 in the desorption direction.

It is further observed that the construc¬ tion of the chambers 2 of the adsorption medium system is extremely simple in view of the filling with zeolite type molecular sieves which permits support of the walls during the vacuum part of each cycle, so that the construction of the chamber' walls can be particularly light, which leads directly to a considerable saving of investment costs.

By the application of the method according to the invention it is for example possible to achieve the enrichment of air with oxygen as indicated in the table below.

Constituent Feed air Product

0₂ 20.95 vol.% 80-96 vol.% , 78.08 vol.% remainder

Argon 0.93 vol.% 3.5-4.5 vol.%

C0₂ 0.03 vol.% none

H-0 saturation or less none

Hydrocarbons traces none In the example given above it is assumed that the method is applied to the enrichment of air with oxygen. The method may however be equally successfully employed for enriching with one or more constituents a gas containing a plurality of constituents, provided that there is a difference in affinity between the desired constituent or con¬ stituents and the other constituents.

Claims

1. Method for increasing the content of at least one desired constituent in a gas containing a plurality of constituents by the application of pressure swing adsorption, characterized in that one or more adsorption units (1) are used, each of which comprises a plurality of chambers (2), adjoi¬ ning chambers (2) being in heat exchange contact with each other, while the gas flow in each of the chambers (2) is adjusted in such a manner that tempera¬ ture gradients accurring in adjoining chambers (2) in the course of a plurality of cycles in each chamber, as the consequence of heat of adsorption and desorp- tion,are mutually opposite; the adsorption time per cycle being essentially shorter than 15 seconds.

2. Method according to Claim 1, characterized in that a plurality of adsorption units (1) are con¬ nected to one another in heat exchange contact in such a manner that the temperature gradients formed in adjoining adsorption units (1) are mutually oppo¬ site.

3. Method according to Claim 1, characterized in that in adjoining chambers in which the same step or different steps of the pressure swing adsorption process takes or take place, which are both carried out in the adsorption direction or are both carried out in the desorption direction, the gas flows are countercurrent, while co-current flow is used when one of the same or different steps is carried out in the adsorption direction and the other in the desorption direction.

4. Installation for increasing by pressure swing adsorption the content of at least one desired constituent of a gas containing a plurality of consti¬ tuents, which installation comprises one or more adsorption units (1) , characterized in that each of the adsorption units (1) forming part of the instal- lation comprises chambers (2) which are separated by heat-conducting partition walls (7), and that means are provided for adjusting in the desired manner the gas flow in each of the chambers.

5. Installation according to Claim 4, charac¬ terized in that each of the chambers of an adsorption unit forming part of the installation is divided by a heat-conducting dividing wall (16) into two halves (2', 2"), which are in communication with one another at one end of the chamber via an opening in said dividing wall.

6. Installation according to Claim 4 or 5, characterized in that for the adjustment of the gas flow each of the chambers (2) of an adsorption unit (1) is provided with one or more valves.

7. Installation according to Claim 4 or 5, characterized in that each of the chambers (2) of an adsorption unit (1) and, where applicable, all the adsorption units (1) are connected to one central rotary valve.

8. Installation according to Claim 6 or 7, characterized in that a control unit is provided for the programmed operation of the valve or valves associated with the apparatus.

9. Air enriched with oxygen, characterized in that said air is obtained by application of the method according to one or more of Claims 1 to 4 and by using an adsorption medium in the form of zeolite type moleculair sieves.