CA1049419A - Method and apparatus for separating gases - Google Patents
Method and apparatus for separating gasesInfo
- Publication number
- CA1049419A CA1049419A CA221,952A CA221952A CA1049419A CA 1049419 A CA1049419 A CA 1049419A CA 221952 A CA221952 A CA 221952A CA 1049419 A CA1049419 A CA 1049419A
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- CA
- Canada
- Prior art keywords
- gas
- zone
- compartment
- inlet
- separation section
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
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Classifications
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
- B01D53/02—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols by adsorption, e.g. preparative gas chromatography
- B01D53/04—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols by adsorption, e.g. preparative gas chromatography with stationary adsorbents
- B01D53/047—Pressure swing adsorption
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
- C01B23/00—Noble gases; Compounds thereof
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D2256/00—Main component in the product gas stream after treatment
- B01D2256/18—Noble gases
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D2256/00—Main component in the product gas stream after treatment
- B01D2256/24—Hydrocarbons
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D2257/00—Components to be removed
- B01D2257/50—Carbon oxides
- B01D2257/504—Carbon dioxide
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D2257/00—Components to be removed
- B01D2257/80—Water
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D2258/00—Sources of waste gases
- B01D2258/06—Polluted air
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D2259/00—Type of treatment
- B01D2259/40—Further details for adsorption processes and devices
- B01D2259/40011—Methods relating to the process cycle in pressure or temperature swing adsorption
- B01D2259/40058—Number of sequence steps, including sub-steps, per cycle
- B01D2259/40062—Four
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D2259/00—Type of treatment
- B01D2259/40—Further details for adsorption processes and devices
- B01D2259/40011—Methods relating to the process cycle in pressure or temperature swing adsorption
- B01D2259/40058—Number of sequence steps, including sub-steps, per cycle
- B01D2259/40067—Seven
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02C—CAPTURE, STORAGE, SEQUESTRATION OR DISPOSAL OF GREENHOUSE GASES [GHG]
- Y02C20/00—Capture or disposal of greenhouse gases
- Y02C20/40—Capture or disposal of greenhouse gases of CO2
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
- Y02P20/00—Technologies relating to chemical industry
- Y02P20/151—Reduction of greenhouse gas [GHG] emissions, e.g. CO2
Abstract
A B S T R A C T
In the purification of a stream of gas having a number of components it is known to remove unwanted components by adsorption in a pressure swing adsorber. However a pressure swing adsorber is inefficient in that the spent adsorbate is usually regenerated using a counter flow of purified feed gas.
The present invention overcomes this drawback by providing a method and an apparatus for pressure swing adsorption where in the adsorbate is regenerated by separation and evacuation of one of a series of zones in which the spent adsorbate is located.
In the purification of a stream of gas having a number of components it is known to remove unwanted components by adsorption in a pressure swing adsorber. However a pressure swing adsorber is inefficient in that the spent adsorbate is usually regenerated using a counter flow of purified feed gas.
The present invention overcomes this drawback by providing a method and an apparatus for pressure swing adsorption where in the adsorbate is regenerated by separation and evacuation of one of a series of zones in which the spent adsorbate is located.
Description
1~99L19 This invention relates to a method and apparatus for separating gas mixtures and in particular to a method and apparatus using the technique known as pressure swing adsorption.
The pressure swing adsorption method of separating a gaseous mixture involves passing the mixture at an elevated pressure through a bed of adsor-bent, usually in the form of solid particles, which is designed to selectively adsorb one or more components of the mixture, these components generally com-prising contaminants of the desired product.
After treatment of the mixture to adsorb the adsorbable component or components ~contaminants) therefrom, the flow of gaseous mixture through the bed is interrupted and the bed of adsorbent is then generally regenerated for re-use by purging it of the adsorbed material by passing through the bed, generally in the opposite direction to the direction of flow taken by the gaseous mixture, a purge - or regeneration - gas stream which generally com-prises a portion of the purified product at a lower pressure.
Pressure swing adsorption apparatus usually includes at least two beds of adsorbent so that while one is being purged (regenerated) at the low pressure another can be adsorbing at the elevated pressure. In some embodi-ments, more than two beds are used so thatJ for example, while one bed is adsorping, a bed that has just completed its adsorption period is being de-pressurized ready for purging, a third bed is undergoing purging and a fourth -bed which has just been purged is undergoing re-pressurisation prior to com-mencing a further period of adsorbing.
The purge or regeneration gas stream recovered from the bed of 't adsorbent, which will contain quantlties of contaminants desorbed ~rom the bed of adsorbent, is generally regarded as having no value other than possibly as a low grade fuel gas. Consequently it is usually vented to atmosphere or burned, depending on its constitution. Accordingly, it is desirable to reduce it as much as possible in ordeT to achieve a high yield of purif:led product It is also desirable to maximise the utilisation of the adsorbent so as ~o ' ~9~
minimise the volumeo~ adsorbent required to treat a given volume of gas mix-ture.
Many proposals have been put forward for achieving one or both of these objectives. In general these proposals have involved the use of as many as four or even more beds of adsorben~ only one of which is utilized for separation at any one ~ime and complicated arrangements for using the beds in rotation involving as many as eight or even more individual steps in each cycle and as many as four different gas pressures.
This invention provides a method of reducing the consumption of purge gas and hence increasing the yield of purified product. It also provides a means of reducing the amount of adsorbent required for the process and per-mits the recovery of the adsorbed constituents in a higher concentration in the purge stream thereby providing the possibility of upgrading the purge stream from a waste stream to a valuable by-product stream.
When a gaseous mixture is passed at an elevated pressure through a bed of adsorbent which is inert to one or more of the components which to-gether constitute the desired product but adsorbs one or more other of the components, that portion of the bed which is close to the inlet of the gas mixture rapidly becomes saturated with adsorbate while that part of the bed which is remote from the gas inlet remains initially free of adsorbed material.
As a result, the gas traversing the initial portion of the bed and which is in a condition close to equilibrium with the adsorbate will contain a relative-ly high concentration of the adsorbable constituents, while the gas close to , the bed outlet will initially be virtually free of adsorbate. There is thus formed a saturation "front" or, in the case of several adsorbable constituents, , a plurality of saturation fronts since in general these constituents will -~ have different adsorption characteristics.
As the process of adsorption continues, so the amount of adsorbable material ta~en up by the bed will increase so that the "front" or "fronts" of saturation will travel towards the outlet end of the bed. After a certain
The pressure swing adsorption method of separating a gaseous mixture involves passing the mixture at an elevated pressure through a bed of adsor-bent, usually in the form of solid particles, which is designed to selectively adsorb one or more components of the mixture, these components generally com-prising contaminants of the desired product.
After treatment of the mixture to adsorb the adsorbable component or components ~contaminants) therefrom, the flow of gaseous mixture through the bed is interrupted and the bed of adsorbent is then generally regenerated for re-use by purging it of the adsorbed material by passing through the bed, generally in the opposite direction to the direction of flow taken by the gaseous mixture, a purge - or regeneration - gas stream which generally com-prises a portion of the purified product at a lower pressure.
Pressure swing adsorption apparatus usually includes at least two beds of adsorbent so that while one is being purged (regenerated) at the low pressure another can be adsorbing at the elevated pressure. In some embodi-ments, more than two beds are used so thatJ for example, while one bed is adsorping, a bed that has just completed its adsorption period is being de-pressurized ready for purging, a third bed is undergoing purging and a fourth -bed which has just been purged is undergoing re-pressurisation prior to com-mencing a further period of adsorbing.
The purge or regeneration gas stream recovered from the bed of 't adsorbent, which will contain quantlties of contaminants desorbed ~rom the bed of adsorbent, is generally regarded as having no value other than possibly as a low grade fuel gas. Consequently it is usually vented to atmosphere or burned, depending on its constitution. Accordingly, it is desirable to reduce it as much as possible in ordeT to achieve a high yield of purif:led product It is also desirable to maximise the utilisation of the adsorbent so as ~o ' ~9~
minimise the volumeo~ adsorbent required to treat a given volume of gas mix-ture.
Many proposals have been put forward for achieving one or both of these objectives. In general these proposals have involved the use of as many as four or even more beds of adsorben~ only one of which is utilized for separation at any one ~ime and complicated arrangements for using the beds in rotation involving as many as eight or even more individual steps in each cycle and as many as four different gas pressures.
This invention provides a method of reducing the consumption of purge gas and hence increasing the yield of purified product. It also provides a means of reducing the amount of adsorbent required for the process and per-mits the recovery of the adsorbed constituents in a higher concentration in the purge stream thereby providing the possibility of upgrading the purge stream from a waste stream to a valuable by-product stream.
When a gaseous mixture is passed at an elevated pressure through a bed of adsorbent which is inert to one or more of the components which to-gether constitute the desired product but adsorbs one or more other of the components, that portion of the bed which is close to the inlet of the gas mixture rapidly becomes saturated with adsorbate while that part of the bed which is remote from the gas inlet remains initially free of adsorbed material.
As a result, the gas traversing the initial portion of the bed and which is in a condition close to equilibrium with the adsorbate will contain a relative-ly high concentration of the adsorbable constituents, while the gas close to , the bed outlet will initially be virtually free of adsorbate. There is thus formed a saturation "front" or, in the case of several adsorbable constituents, , a plurality of saturation fronts since in general these constituents will -~ have different adsorption characteristics.
As the process of adsorption continues, so the amount of adsorbable material ta~en up by the bed will increase so that the "front" or "fronts" of saturation will travel towards the outlet end of the bed. After a certain
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time, which depends inter alia on the length of the bed, the flow rate of the gas, the concentration of adsorbable components in the gas, the mass of adsorb-ent and the adsorption capacity of the adsorbent~ the gas leaving the bed will begin to contain appreciable amounts of one or more of the adsorbable con-stituent or constituents. At this point of time, or preferably just before this point of time, it is customary to interrupt the flow of gas through this bed, switch the gas flow to another bed, and begin purging the first bed with purified gas at low pressure.
The point at which the gas flow is interrupted will usually depend upon the concentration of adsorbable constituent or constituents that can be tolerated in the product gasO This in turn is generally dependent upon such factors as the use to which the product is to be put. In any event, in prac-tice it is usual for the interruption to be effected at such a point in time as to ensure that the concentration of adsorbable constituent or constituents in the product gas does not exceed a predetermined value which may be an over- ~`
all value or a value for each particular adsorbable constituent.
The point at which interruption should be effected can be determined, for example, by analysis of the product gas e.g. by means of a katharometer.
However, where the constitution of the feed gas to the bed of adsorbent can be accepted as being reasonably uniform over protrac~ed time periods, the interruption çan be effected by a time switch the setting of which can be determined by calculation from knowledge of such parameters of the process as are adumbrated above, or by empirical means.
We have now found that by operating in this way, utilisation of the bed as a whole is below optimum. This is because when the concentration of ~j adsorbable constituent or constituents reaches the level at which interruption of the flow through the bed of adsorbent has to be effected, ~he total adsorp-tion capacity of the bed has not been reached.
Generally, the most readily available, or only immediately available source of the purge gas for regenerating the adsorbent in each zone after the `I 3 , ' .
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ZQne has been removed from the separation section is the product gas. In this case~ a portion of the product gas is separated from the remainder, re-duced to the second or purge pressure, and used as the purge gas. Such use, however, represents a net loss of product gas.
Generally, the greater the difference between the separation pres-sure and the purge pressure, the smaller the quantity of purge gas required but it is not normally desirable to reduce the pressure of the purge gas below atmospheric pressure because of the need to install vacuum equipment.
I have now found that the loss in product gas through the use of a portion thereof for purging may be avoided by regenerating the adsorbent in the removed zone by evacuation rather than by using a purge gas.
Accordingly, in the process of the present invention the step of regenerating the adsorbent in each zone after it has been removed from the separation SectiQn is effected by evacuation of the removed zone.
This not only avoids the loss of a part of the product gas as purge gas but also reduces the amount of product gas required for repressurisation since it has been found that evacuation effects more efficient regeneration than the use of a purge gas so that regeneration, and hence also repressurisa-tion, are required less frequently. This saving of product gas through the reduction in frequency of repressurisation more than offsets the increase in volume of repressurisation gas that may be required per repressurisation be-cause of any increased pressure difference between adsorption pressure and regeneration pressure as a result of regenerating by evacuation rather than the use of a purge gas at or above atmospheric pressure.
Accordingly? the present provides a method of separating a purified gas from a mi~ture thereof with at least one gaseous contaminant, said method comprising the steps of (a) providing an adsorption unit comprising x zones of adsorbent mater-:;
-~ ial which selectively adsorbs said gaseous contaminant, x being a whole number ~ 30 of at least three;
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(b) passing the gas mixture at a first pressure through y of the zones of said adsorption unit arranged in series with respect to the flow of the gas mixture therethrough, y being a whole number of from 2 to x - 1 and saicl y zones constituting a separation section of said adsorption unit;
(c) recovering from the last zone of said separation section a puri-fied gas in which the concentration of the contaminant does not exceed a pre-determined value which is below the concentration of contaminant in the gas mixture;
(d) before the concentration of the contaminant in said purified gas rises above said predetermined value, but after the concentration of said contaminant in the gas stream leaving the zone forming the penultimate zone of said separation section has exceeded said predetermined value, adding another zone of said adsorption unit to the end of said separation section, expressed in terms of the direction of flow of said gas mixture through the separation section whereby said added zone now forms the last zone of said , separation section;
(e) removing from said separation section the zone forming the first zone of the section whereby the zone next to it in the separation section now becomes the first zone;
(f) regen~rating the adsorbent in said removed zone by evacuation of said zone;
(g) periodically repeating steps ~d), (e) and (f) whereby each zone l of said unit is in successive periods incorporated into the separation section ;' as the last zone thereof, subsequently periodically progressed along said section untll it becomes the first zone thereof, and then treated to regener-ate the adsorbent therein ready for re-incorporation into the separation section in a subsequent period.
Apparatus suitable for use with this invention comprises an adsorp-tion unit comprising a plurality of adsorption zones for adsorbing said con-taminant from said mixture and means for regenerating adsorbent, said adsorb-, : :
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bent, said adsorption unit including at least three compartments for adsorbent, each having a gas inlet and a gas outlet for gas to be passed through adsorb- :
ent in the compartment, each compartment forming a separately isolatable adsorption zone, an inlet for the gas mixture to be separated in said unit, an outlet for purified gas, an inlet manifold connected to the inlet for the gas mixture, a purified gas outlet manifold connected to the outlet for the purified product, said regenerating means comprising means for evacuating the adsorption zones, first conduit means for connecting the inlet of each com-partment to the inlet manifold, with an inlet valve in the conduit between each compartment inlet and the inlet manifold, second conduit means for con-necting the outlet from each compartment to the purified gas manifold, with ~:
; an outlet valve in the conduit between each compartment outlet and the outlet manifold, third conduit means for connecting the interior of each compartment to said evacuating means, there being a valve between each compartment interior and the evacuating means, and fourth conduit means adapted to connect the compartments together for flow of gas therethrough with the outlet of each : .
compartment connected to the inlet of the next compartment, there being a valve in the conduit between the outlet of each compartment and the inlet of the next compartment9 said valves being arranged such that at least a first and a second of said compartments are connected in series to provide a separation section between the inlet manifold and the outlet manifold for flow of gas therethrough and in that order with respect to the gas flow, and at least a third compartment is connected to said evacuating means for regeneration of the adsorbenttherein,and automatic valve control means adapted periodically to switch the valves to disconnect said third compartment from said evacuating means and connect it in series with said second compartmentin said separation section to form the last compartment in said section, expressed in terms of I flow of gas therethrough and disconnect said first compartment from said 'i separation section and connect it to said evacuating means for regeneration . 30 of the adsorbent therein.
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It has also been observed that during the depressurization re-quired for regenera~ion of the adsorbent whe~her the regeneration is by evacuation or by means of a purge gas, the composition of the initial exit gas from the zone being regenerated is similar to that of the feed gas mixture fed to the zone while that zone was part of the separation section, and initially during the depressurization remains substantially constant.
It is believed that this is because during the initial stages o~f depres- -surization, the exit gas is largely gas that has been retained in the in-terstitial voids in the adsorbent bed in the zone (hereinafter reerred to as interstitial gas), and that very little o~ the contaminant adsorbed by the bed is being desorbed. Since the adsorbent in the zone is substantially saturated when the zone is removed from the separation section for regener-ation, this interstitial gas will have virtually the same composition as the feed gas, and where the bed of adsorbent contains a large voidage, a substantial volume of this interstitial gas is involved. As the pressure is reduced further, however, the adsorbed material commences to be desorbed and this is reflected in a sharp rise in concentration of this material in the exit gas.
Further in accordance with the present invention, therefore, re-clamation of this interstitial gas is effected by recycling the gas initially exiting from the zone during depressurization to the inlet of the separation section to be passed through said separation section at substantially the adsorption pressure, said recycle being continued until not later than the occurrence of a rapid increase in the concentration of adsorbable material in the exit gas.
By the ~;'adsorption pressure" is meant the pressure at the inlet to the separation section.
Thus, where regeneration of the adsorbent is by evacuation, the method of reclaiming interstit~al gas includes ~he step of recycling the gas ini~ially exi~ing from said removed zone during the evacuation procedure :
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of step (f) to the inlet o the separation section to be passed there-through at substantially said first pressureJ said recycle heing continued until not later than the occurrence of a rapid increase in concentration of said contaminant in said exit gas.
Where the regeneration is by means of purge gas, the me~hod which provides for reclama~ion of interstitial gas comprises th0 steps o:E (a) pro-viding an adsorption unit co~prising x zones of adsorbent material which selectively adsorbs said gaseous contaminant, x being a whole numbeT of at least three; (b) passing the gas mixture at a firs~ pressure through y of the zones of said adsorption unit arranged in series with respect to the flow of the gas mixture therethrough, y being a whole number of from 2 to x - 1 and said y zones constituting a separation section of said adsorption unit; (c) recovering from the.last zone of said separation section a purified gas in which the concentration of the contaminant does not exceed a predetermined value which is below the concentration of contaminant in the :~
gas mixture; (d) before the concentration of the con~aminant in said purified gas rises above said predetermined value, but after the concentration of said contaminant in the gas stream leaving the zone forming the penultimate zone of said separation section has exceeded said predetermined value, adding another zone of said adsorption unit to the end of said separation section, expressed in terms of the direction.of flow of said gas mixture through the separation section whereby said added zone now forms the last zone of said separation section; (e) removing from said separation section the zone forming the first zone of ~he section whereby the zone next to it in ~he separation section now becomes the first zone; ~f) reducing the pressure in said removed zone and purging the adsorbent in said removed zone of adsorbed material by passing a purge gas through it at a second pressure which is lower than the first pressure whereby ~o prepare said zone for re-use in the ,~, .
adsorption section commencing as the last zone of said section: (g) period~
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. 30 lcally repeating steps (d), ~e) and (f) whereby each zone of said unit is in : .
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~ - 8 -~, ' ~94~L9 successive periods incorporated into the separation section as the last zone thereof, subsequently periodically progressed along said section until it becomes the first zone thereof, and then purged ready for re-incorporation into the separation section in a subsequent period; and (h) recycling the gas initially exiting from said removed zone during the step of reducing the pressure in said zone to the inlet of the separation section to be passed therethrough a~ subs~antially said first pressure, said recycle being con-tinued until not later *han the occurrence of a rapid increase in concen-tration of said contaminant in said exit gas.
An adsorp~ion unit for separating a purified gas from a mixture thereof with at least one gaseous contaminant and wherein regeneration of the adsorbent is by evacuation, can be adapted for reclamation of interstitial gas from the compartment being regenerated by including valved conduit means for directing gas from the interior of said third compartment to said ~ , evacuating means or to the inlet of a gas compressor whose outle~ communicates with the inlet to said separation section, the apparatus also including valve control means adapted to direct said gas in an initial phase to said compressor and in a subsequent phase to said evacuating means which exhaust the gas to waste, said initial phase terminating not later than the occurrence of a rapid increase in the concentration of the adsorbable material in said gas.
Where regeneration is by means of purge gas, apparatus adapted for recovering interstitial gas from the compartment being regenèrated will com-prise a pressure swing adsorption unit comprising a plurality of adsorption zones for selectively adsorbing said contaminant from said mixture and means for purging adsorbent of adsorbed contaminant, said adsorption unit including at least three compartments for adsorbent~ each having a gas inlet and a gas .: outlet ~or gas to be passed through adsorbent in the compartment, each com-partment forming a separately isolatable adsorption zone, an inlet for the gas mixture to be saparated in said unit, and outlet for purified gas, an .
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inlet manifold connected to the inlet for the gas mixture, a purified gas outlet manifold connected to the outlet for the purified product, a purge gas manifold, a waste gas manifold, first conduit means for connecting the inlet of each compartment to the inle~ manifold, with an inlet valve in the conduit between each compartment inlet and the inlet manifold, second conduit means for connecting the outlet from each compartment to the purified gas manifold, with an outlet valve in the conduit between each compartment outlet and the outlet manifold, third conduit means for connecting the purge gas manifold to the outlet from each compartment, with a purge gas inlet valve in the conduit between each compartment outlet and the purge gas manifold, four~h conduit means for connecting ~he inlet to each compartment to the waste gas manifold, there being a waste gas outlet valve between each compart~
ment inlet and the waste gas manifold, and fifth condui~ means adapted to connect the compartments together for flow of gas therethrough with the out-let of each compartment connected to the inlet of the next compartment, there being a valve in the conduit between the outlet of each compartment and the inlet of the next compartment, said valves being arranged such that at least a first and a second of said compartments are connected in series to provide a separation section be~ween the inlet manifold and the outlet mani-fold for flow of gas therethrough and in that order with respect to the gas flow, and at least a third compartment is connected for gas flow there-through from the purge gas manifold to the waste gas manifold for purging, and automatic valve control means adapted periodically to switch the valves to disconnect said third compartment from said purge gas manifold and said waste gas manifold and connect it in series with said second compartment in said separation section to form ~he last compartment in said section, ex-pressed in terms of flow o~ gas therethrough, and disconnect said first compartm0nt from said separation section and connect it between the purge gas manifold and the waste gas manifold for purging, said apparatus further including valved conduit means for passing gas from the third compartment :
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to said waste gas manifold or to the inlet of a gas compressor whose outlet communicates with the inlet to the separation section, and valve control means adapted to direct said gas in an initial phase to said compressor and in a subsequent phase to waste, said initial phase termina~ing not later than the occurrence of a rapid increase in the concentration of adsorbable material in said gas.
The composition of the gas exiting from the third compartment can be readily monitored using known gas analysis tec]miques, e.g. katharometry, and therefore a rapid increase in the concentration of the adsorbable material can be readily determined.
For any given gas mixture and adsorption conditions such as of temperature and pressure, this rapid increase in concentration of the adsorbable material will generally occur at or near a given value for the concentration.
Thus, in accordance with one method of carrying out the recycle process of the invention, the gas exiting from the zone during the initial s~ages of depressurization may be recovered and recycled until the concen-tration of adsorbable material in said gas reaches a predetermined level which is not greater than the level which corresponds to the occurrence of the above-mentioned rapid increase. Preferably this predetermined level is -substantially equal ~o the level corresponding to the rapid increase. -In apparatus for carrying out this embodiment~ the valve control means suitably comprise sensor means for sensing the level of concentration of contaminant in the gas from the third compartment and means responsive to said sensor means and adapted to terminate said initial phase when said sensed level exceeds a predetermined level.
The pressure at which this occurs will vary depending on the nature of the material to be desorbed from the adsorber bed but experience shows that in general it will be found to be equivalent to about 15% to 30%
of the total drop in pressure from the adsorption pressure to the pressure .
1049~9 of the purge gas where a purge gas is used, or about 10% ~o 30% of the adsorption pressure expressed as absolute pressure where regeneration is effected by evacuation.
Thus in accordance with another method of carrying out the recycle process of the invention, the gas exiting from the zone during the initi~l stages of depressurization may be recovered and recycled until its pressure falls to a predetermined pressure below the adsorption pressure.
This may be achieved for example by sensing the pressure of the gas exiting from the zone and stopping the recycling when this pressure reaches said predetermined pressure. Alternatively, the recycling may be stopped after the elapse of a period of time after commencement of depres-surization which is known, e.g. from trial and experiment or from calculation, to correspond to said pressure!
In apparatus for use with the first alternative, the valve control means may suitably comprise sensor means for sensing the pressure of the gas in the third compartment and means responsive to said sensor means and adapted to terminate said initial phase when the sensed pressure falls be- ~
low a predetermined level. ~ ;
In apparatus for use with the second alternative, the valve control means may suitably comprise timing means and means responsive to said timing means adapted to terminate said initial phase at a predetermined time after the commencement of evacuation of said third compartment.
The pressure of the gas that is recycled to the separation section is restored to the adsorption pressure by m~ans of the compressor. The same compressor may also be used to compress ~he feed gas mixture to the adsorption pressure where such compression is required.
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The use of evacuation to regenerate the adsorbent and the recycling of the interstitial gas to the separation section are illustrated by means of the following example and with the aid of the accompanying drawings in which: -: ~ ' `' ~
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Figure 1 is a schematic representation of a method of operating a pressure swing absorp~ion unit with four zones of adsorbent;
Eigure 2 is a schematic flow sheet of a modification of the four zone pressure swing adsorption unit illustra~ed in Figure 1 when in the mode identified as STAGE 1 in Figure 1, and wherein regeneration of the adsorbent is effected by evacuation in place of purging;
Figure 3 is a schematic flow sheet of another modification of the four zone pressure swing adsorption unit illustrated in Figure 1, again when in the mode identified as STAGE 1 in Figure 1, wherein provision is made for recycle of the interstitial gas recovered during depressurisation; and E:igure 4 is a schematic flow sheet of a modification of the apparatus of Figure 2 wherein provision is made for recycle of the interstitial gas recovered during depressurisation.
Referring to Figure 1, the pressure swing adsorption unit comprises four zones of adsorbent A, B, C and D. During the first stage ~STAGE 1 in the drawing), the compressed gaseous mixture FG to be separated, which for the purposes of simplicity is regarded as containing one component that is adsorb-ed by the adsorbent, is passed through a separation section comprising zones A and B in series and a purified product gas PE is recovered. During this same ~0 period, zone C, which has previously been purged at low pressure, is repressur-ised with repressurisation gas RG which is taken from the stream of purified product PE leaving zone B, and zone D is depressurised and purged by passing `; through it in the opposite direction to that of the compressed gas flow a ; purge gas PG which comprises a further portion of the purified gas from B
` which has been expanded through valve V to a lower pressure. The gas DG carry-ing the material desorbed from zone D is removed from the system as shown.
At the point in time when the concentration of the adsorbable com-ponent of the feed gas mixture FG is detected as having reached a prede~er-; mined level in the product gas PE leaving zone B, this level being usually the maximum permitted level of the adsorbable component in the product gas, or '' _ 10 -. , -~49~
after a predetermined period of time has elapsed which is determined by em-pirical means or calculated from available data to correspond to this point in time. Zone A will be substantially fully saturated with adsorbable material while zone B has been utilised only to about 20%.
In accordance with the invention, at this point in time, zone C, which has previously been purged and repressurised to the pressure of the feed gas mixture, is added to the separation section to form the last zone thereof (expressed with reference to the direction of flow of the feed gas mixture through the separation section) and zone A, which during STAGE 1 formed the first zone of the separation section (expressed with reference to the direc-tion of flow of the feed gas mixture through the separation section), is removed from the separation section and commences depressurisation and purg-ing. Thus zone B now becomes the first zone of the separation section and zone C becomes the second or las~ zone. At the same time zone D, which during STAGE 1 has been undergoing depressurisation and purging, commences repressuri-sation.
The situation is then as shown in STAGE 2 of Figure ~, with the feed gas mixture FG being passed through a separation section comprising zones B
~ and C in that order, zone D being repressurised with a portion RG of the product gas and zone A being depressurised and purged with purge gas PG. As the zone C has been previously purged, the concentration of adsorbable compon-ent in the product gas leaving the separation section will thus fall away.
When the point in time is reached when the concentration of the adsorbable component of the feed gas mixture FG is detected as having again , reached the a~orementioned predetermined level in the product gas (or when a further period of time has elapsed corresponding to reaching this point in ~ time) zone B will now be substantially fully saturated while zone C is only ;1 partly saturated. At this point in time, previously purged and re-pressurised zone D is now added to the separation section as the last zone thereof and zone B is removed and commences depressurisation and purging. Thus zone C now becomes the first zone of the separation section and zone D becomes the s0cond .
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(or last) zone. At the same time, zone A which during STAGE 2 has been under-going depressurisation and purging, commences re-pressurisation.
The situation is then as shown in STAGE 3 of Figure 1 with feed gas mixture FG being passed through a separation section comprising zones C and C in that order, zone A being re-pressurised with a portion RG of the product gas and zone B being depressurised and purged with purge gas PG.
: At the end of STAGE 3, determined as described above for STAGES 1 and 2, freshly pressurised zone A is added to the separation section as the last zone and zone C, which formed the first zone of the separation section during STAGE 3, is removed from the separation section for depressurisation and purging so that zone D becomes the first zone.
The situation is then as shown in STAGE 4 of Figure 4 with the feed gas mixture FG being passed through a separation section comprising zones D
and A in that order, zone C being depressurised and purged and zone B being re-pressurised.
At the end of STAGE 4, again determined as described above for STAGES 1, 2 and 3, fully pressurised zone B is added to the separation section as the last zone, and zone D, which formed the first zone of the separation section during STAGE 4, is removed from the separation section for depressurisa-tion and purging so that zone A becomes the first zone9 and æone C commences re-pressurisation. The situation is then as shown in STAGE 1 of Figure 1 and the cycle commences all over again.
The complete cycle may be represented as follows : -_ ZONE STAGE
j 1 2 3 4 _ . _ A S1 D~P RP S2 B S2 Sl D~P RP
C RP S2 Sl D~P
D D~P RP S2 S
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where Sl and S2 are the first and second beds respectively in the separation section ~expressed in terms of the direction of flow of the ieed gas mixture through the separation section) D~P represents depressurisation and purging, and RP represents re-pressurisation.
The above illustration has been described for simplicity with refer-ence to one adsorbable component in the feed gas mixture FG but the arrange-ment may also be used for feed gas mixtures containing three or more components two or more of which are adsorbed by the adsorbent. In this case, the signal for changing from one stage to the next may be, for example, the concentration of one of the adsorbed components reaching a predetermined value in the pro-duct stream, or the total concentration of a combination of two or more of the components reaching a predetermined value in the product stream, or the first o any of the adsorbed components to reach a predetermined concentration in the product stream for that component. Alternatively, the changeover may be effected after a predetermined time interval which, may be calculated or deter-mined empirically. -Referring now to the arrangement illustrated in Figure 2, the adsor-ber illustrated differs from that of Figure l in that the regeneration is effected by evacuation, no purge gas being used. Thus, in the arrangement illustrated in Figure 2, the feed gas for separation is fed at elevated pres-sure in pipeline 302 ~or is fed in pipeline 303 to be compressed to elevated pressure in compressor 304 and fed via pipeline 306) to pipeline 308 leading to the inlet of the separation section 310 of the adsorber, consisting, as before, of zones A and B connected in series. Product gas substantially free of the adsorbable material leaves the separation section in pipeline 312 but in this case it is divided into two portions only, one of which is recovered in pipeline 314 as product, and the other of which is fed via pipeline 316 to ~one C to repressurise this zone to adsorption pressure. Zone D is under-going regeneration by evacuation through pipelines 324 and 328 by means ~, . . ..
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of vacuum pump 340. The evacuated material is passed for venting or flaring.
The cycle of operation of the apparatus corresponds to that of the apparatus shown in Figure 1.
Figure 3, in which the features which are common with Figure 2 have the same reference numerals or letters, is again a flow sheet of a four zone adsorber which operates with the same cycle as that illustrated in Figure 1 and is shown in the period of the cycle corresponding to STAGE 1 in Figure 1.
As in Figure 1, the product gas leaving the separation zone is divided into three portions. The first portion is recovered through pipeline 314 as product gas. The second portion is fed via pipeline 316 to adsorber zone C to re-pressurise this zone to adsorption pressure. A third portion is fed via pipeline 318 to expansion valve 320 where its pressure is reduced, and is then passed to adsorber zone D as purge gas. In the adsorber of Figure 3, however, provision is made for recycling the gas exiting from zone D, which is under-going depressurisation and purging, to the separation section inlet. Thus, pipeline 324 is connected to two pipelines, 326 and 32~, each of which is fitted with a valve, 330, 332 respectively. Pipeline 326 connects pipeline 324 to pipeline 303 and the inlet to compressor 304. Pipeline 328 connects pipeline 324 to the flare or vent to atmosphere. A pressure sensing device 334 is located in pipeline 324 to sense the pressure of the gas therein and is ; connected to a valve control means 336 in the form of a pressure controller which is adapted to control valves 330 and 332. The controller is adapted so that in operation it closes valve 330 and opens valve 332 when the sensed pres-sure of the gas in pipeline 324 falls below a predetermined level, the posi-tions of these valves being reversed when the sensed pressure of the gas in pipeline 324 is above that level. In operation, the pressure controller is arranged so that it operates to close valve 330 and open valve 332 at a press-sure which is at or slightly above the pressure at which the adsorbed contamin-ant commences to be desorbed from the adsorbent in 7one D, resulting in a rapid increase in the concentration o~ contaminant in the gas in pipeline 24.
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1~4~ 9 Figure 4, in which the features common with Figures 2 and 3 have the same reference numerals or letters, in a flow sheet of the four zone adsorber of Figure 2 but modified, as in the adsorber oE Figure 3, to provide for recycling of gas exiting from the zone D to the separation section inlet.
EXAMPLE
A. In a first experiment, a feed gas mixture consisting of 94 mol % ~le and 6 mol % of adsorbable impurities was separated using the adsorber arrange-ment of Figure 1 using coconut shell activated carbon as the adsorbent.
The pressure in the separation section was 350 psig, the length of each period of the cycle was 10 minutes, and the purge gas pressure was 5 pSlg.
With a feed gas rate of 149.5 SCFH the total product recovered from the separation section was 140.5 SCFH of 99.995% pure helium of which 15 SCFH
was used as purge gas and 40.5 SCFH was required for repressurisation since each zone required 6.75 SCF of gas for repressurisation to 350 psig and six zones required repressurisation each hour.
Thus, the hourly recovery of helium was [140.5 - ~15 + 40.5)] x 0.99995 and the yield of helium, expressed as percentage of theoretical was ~140.5 - (15 + 40.4~] x 0.99995 = 60.5%
149.5 x 0.94 B. In a second experiment, the same feed gas mixture was separated in the adsorber of Figure 2 using the same volume of the same adsorbent as in the first experiment~ to produce a product gas of the same 99.995% purity.
It was ound that using the same adsorption pressure of 350 psig and the same rate of feed of 149.5 SCFH, as a result of the increased effi-ciency of regeneration by use of vacuum rather than purging, the length of each period of the cycle could be increased safely to about 15 minutes dura-tion so that only 4 zones required regeneration in each hour instead of 6.
However, as it was now necessary to repressurise each zone from vacuum to ;i 30 350 psig, the quantity of product gas required to repressurise each zone was , :
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found to have incr~ased to 7.05 SCF. On the other hand, no product gas was lost aspurge gas.
Thus, the yield of He was 0.99995 [140.5 - (4 x 7.05)] = 80%
149.5 x 0.94 C. In a third experiment, the same feed gas mixture was separated in the adsorber of Figure 3 using the same volume of the same adsorbent as in the ~-first experiment, to produce a product gas of the same 99~995% purity.
The pressure controller 336 of Figure 3 was set to close valve 330 in each period after the pressure of the gas exiting from the zone undergoing regeneration had dropped to 50 psig.
The pressure and total rate of feed of the gas mixture to the separation section were maintained the same as the first experiment at 350 psig and 149.5 SCFH, as were the purge gas pressure (5 psig) and the length of each period of the cycle (10 minutes).
Since the adsorption and purge gas pressures were the same as for Experiment A, the volume of gas required to repressurise each zone remained the same at 6.75 SCF.
It was found that under these conditions 5.55 SCF of interstitial gas was recovered and recycled from each zone during depressurisation.
Thus, the net flow of fresh feed to the adsorber was 149.5 -~6 x 5.55) = 116.2 SCFH.
The yield of He was thus [140.5 - (15 ~ 40.5) ] x 0.99995 = 78%
116.2 x 0.94 D. In the fourth experiment, the same feed gas mixture was separat-ed in the adsorber of Figure 4 using the same volume of the same adsorbent as in all the other experiments, again to produce a product gas of 99.995% purity.
Total feed rate to the separator s0ction was 149.5 SCFH at 350 psig.
Since, as in the second experiment, regeneration was effected by ;30 evacuation, there was no purge gas stream and~ as in said second experiment, :~ , .
,, ; .- ., - ,, . -- . . . . .
g~9 each period could be safely extended to 15 minutes duration. Again, as in the second experiment, since repressurisation had to be effected from vacuumJ
7.05 SCF of gas was required to repressurise each zone.
The pressure controller 336 was set so that in each period the valve 330 closed and valve 332 opened after 5.2875 SCF o:E interstitial gas had been recovered and recycled from each zone during evacuation.
Thus the net flow of fresh feed to the adsorber is [149.5 - (4 x 5.2875)] = 128.35 SCFH.
The net product flow is 140.5 - (4 x 7.05 x 0.99995)= 112.3 SCFH.
Thus~ the yield of He is 112.3 = 87.5%
128.35 ", . . .
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time, which depends inter alia on the length of the bed, the flow rate of the gas, the concentration of adsorbable components in the gas, the mass of adsorb-ent and the adsorption capacity of the adsorbent~ the gas leaving the bed will begin to contain appreciable amounts of one or more of the adsorbable con-stituent or constituents. At this point of time, or preferably just before this point of time, it is customary to interrupt the flow of gas through this bed, switch the gas flow to another bed, and begin purging the first bed with purified gas at low pressure.
The point at which the gas flow is interrupted will usually depend upon the concentration of adsorbable constituent or constituents that can be tolerated in the product gasO This in turn is generally dependent upon such factors as the use to which the product is to be put. In any event, in prac-tice it is usual for the interruption to be effected at such a point in time as to ensure that the concentration of adsorbable constituent or constituents in the product gas does not exceed a predetermined value which may be an over- ~`
all value or a value for each particular adsorbable constituent.
The point at which interruption should be effected can be determined, for example, by analysis of the product gas e.g. by means of a katharometer.
However, where the constitution of the feed gas to the bed of adsorbent can be accepted as being reasonably uniform over protrac~ed time periods, the interruption çan be effected by a time switch the setting of which can be determined by calculation from knowledge of such parameters of the process as are adumbrated above, or by empirical means.
We have now found that by operating in this way, utilisation of the bed as a whole is below optimum. This is because when the concentration of ~j adsorbable constituent or constituents reaches the level at which interruption of the flow through the bed of adsorbent has to be effected, ~he total adsorp-tion capacity of the bed has not been reached.
Generally, the most readily available, or only immediately available source of the purge gas for regenerating the adsorbent in each zone after the `I 3 , ' .
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ZQne has been removed from the separation section is the product gas. In this case~ a portion of the product gas is separated from the remainder, re-duced to the second or purge pressure, and used as the purge gas. Such use, however, represents a net loss of product gas.
Generally, the greater the difference between the separation pres-sure and the purge pressure, the smaller the quantity of purge gas required but it is not normally desirable to reduce the pressure of the purge gas below atmospheric pressure because of the need to install vacuum equipment.
I have now found that the loss in product gas through the use of a portion thereof for purging may be avoided by regenerating the adsorbent in the removed zone by evacuation rather than by using a purge gas.
Accordingly, in the process of the present invention the step of regenerating the adsorbent in each zone after it has been removed from the separation SectiQn is effected by evacuation of the removed zone.
This not only avoids the loss of a part of the product gas as purge gas but also reduces the amount of product gas required for repressurisation since it has been found that evacuation effects more efficient regeneration than the use of a purge gas so that regeneration, and hence also repressurisa-tion, are required less frequently. This saving of product gas through the reduction in frequency of repressurisation more than offsets the increase in volume of repressurisation gas that may be required per repressurisation be-cause of any increased pressure difference between adsorption pressure and regeneration pressure as a result of regenerating by evacuation rather than the use of a purge gas at or above atmospheric pressure.
Accordingly? the present provides a method of separating a purified gas from a mi~ture thereof with at least one gaseous contaminant, said method comprising the steps of (a) providing an adsorption unit comprising x zones of adsorbent mater-:;
-~ ial which selectively adsorbs said gaseous contaminant, x being a whole number ~ 30 of at least three;
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(b) passing the gas mixture at a first pressure through y of the zones of said adsorption unit arranged in series with respect to the flow of the gas mixture therethrough, y being a whole number of from 2 to x - 1 and saicl y zones constituting a separation section of said adsorption unit;
(c) recovering from the last zone of said separation section a puri-fied gas in which the concentration of the contaminant does not exceed a pre-determined value which is below the concentration of contaminant in the gas mixture;
(d) before the concentration of the contaminant in said purified gas rises above said predetermined value, but after the concentration of said contaminant in the gas stream leaving the zone forming the penultimate zone of said separation section has exceeded said predetermined value, adding another zone of said adsorption unit to the end of said separation section, expressed in terms of the direction of flow of said gas mixture through the separation section whereby said added zone now forms the last zone of said , separation section;
(e) removing from said separation section the zone forming the first zone of the section whereby the zone next to it in the separation section now becomes the first zone;
(f) regen~rating the adsorbent in said removed zone by evacuation of said zone;
(g) periodically repeating steps ~d), (e) and (f) whereby each zone l of said unit is in successive periods incorporated into the separation section ;' as the last zone thereof, subsequently periodically progressed along said section untll it becomes the first zone thereof, and then treated to regener-ate the adsorbent therein ready for re-incorporation into the separation section in a subsequent period.
Apparatus suitable for use with this invention comprises an adsorp-tion unit comprising a plurality of adsorption zones for adsorbing said con-taminant from said mixture and means for regenerating adsorbent, said adsorb-, : :
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bent, said adsorption unit including at least three compartments for adsorbent, each having a gas inlet and a gas outlet for gas to be passed through adsorb- :
ent in the compartment, each compartment forming a separately isolatable adsorption zone, an inlet for the gas mixture to be separated in said unit, an outlet for purified gas, an inlet manifold connected to the inlet for the gas mixture, a purified gas outlet manifold connected to the outlet for the purified product, said regenerating means comprising means for evacuating the adsorption zones, first conduit means for connecting the inlet of each com-partment to the inlet manifold, with an inlet valve in the conduit between each compartment inlet and the inlet manifold, second conduit means for con-necting the outlet from each compartment to the purified gas manifold, with ~:
; an outlet valve in the conduit between each compartment outlet and the outlet manifold, third conduit means for connecting the interior of each compartment to said evacuating means, there being a valve between each compartment interior and the evacuating means, and fourth conduit means adapted to connect the compartments together for flow of gas therethrough with the outlet of each : .
compartment connected to the inlet of the next compartment, there being a valve in the conduit between the outlet of each compartment and the inlet of the next compartment9 said valves being arranged such that at least a first and a second of said compartments are connected in series to provide a separation section between the inlet manifold and the outlet manifold for flow of gas therethrough and in that order with respect to the gas flow, and at least a third compartment is connected to said evacuating means for regeneration of the adsorbenttherein,and automatic valve control means adapted periodically to switch the valves to disconnect said third compartment from said evacuating means and connect it in series with said second compartmentin said separation section to form the last compartment in said section, expressed in terms of I flow of gas therethrough and disconnect said first compartment from said 'i separation section and connect it to said evacuating means for regeneration . 30 of the adsorbent therein.
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It has also been observed that during the depressurization re-quired for regenera~ion of the adsorbent whe~her the regeneration is by evacuation or by means of a purge gas, the composition of the initial exit gas from the zone being regenerated is similar to that of the feed gas mixture fed to the zone while that zone was part of the separation section, and initially during the depressurization remains substantially constant.
It is believed that this is because during the initial stages o~f depres- -surization, the exit gas is largely gas that has been retained in the in-terstitial voids in the adsorbent bed in the zone (hereinafter reerred to as interstitial gas), and that very little o~ the contaminant adsorbed by the bed is being desorbed. Since the adsorbent in the zone is substantially saturated when the zone is removed from the separation section for regener-ation, this interstitial gas will have virtually the same composition as the feed gas, and where the bed of adsorbent contains a large voidage, a substantial volume of this interstitial gas is involved. As the pressure is reduced further, however, the adsorbed material commences to be desorbed and this is reflected in a sharp rise in concentration of this material in the exit gas.
Further in accordance with the present invention, therefore, re-clamation of this interstitial gas is effected by recycling the gas initially exiting from the zone during depressurization to the inlet of the separation section to be passed through said separation section at substantially the adsorption pressure, said recycle being continued until not later than the occurrence of a rapid increase in the concentration of adsorbable material in the exit gas.
By the ~;'adsorption pressure" is meant the pressure at the inlet to the separation section.
Thus, where regeneration of the adsorbent is by evacuation, the method of reclaiming interstit~al gas includes ~he step of recycling the gas ini~ially exi~ing from said removed zone during the evacuation procedure :
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of step (f) to the inlet o the separation section to be passed there-through at substantially said first pressureJ said recycle heing continued until not later than the occurrence of a rapid increase in concentration of said contaminant in said exit gas.
Where the regeneration is by means of purge gas, the me~hod which provides for reclama~ion of interstitial gas comprises th0 steps o:E (a) pro-viding an adsorption unit co~prising x zones of adsorbent material which selectively adsorbs said gaseous contaminant, x being a whole numbeT of at least three; (b) passing the gas mixture at a firs~ pressure through y of the zones of said adsorption unit arranged in series with respect to the flow of the gas mixture therethrough, y being a whole number of from 2 to x - 1 and said y zones constituting a separation section of said adsorption unit; (c) recovering from the.last zone of said separation section a purified gas in which the concentration of the contaminant does not exceed a predetermined value which is below the concentration of contaminant in the :~
gas mixture; (d) before the concentration of the con~aminant in said purified gas rises above said predetermined value, but after the concentration of said contaminant in the gas stream leaving the zone forming the penultimate zone of said separation section has exceeded said predetermined value, adding another zone of said adsorption unit to the end of said separation section, expressed in terms of the direction.of flow of said gas mixture through the separation section whereby said added zone now forms the last zone of said separation section; (e) removing from said separation section the zone forming the first zone of ~he section whereby the zone next to it in ~he separation section now becomes the first zone; ~f) reducing the pressure in said removed zone and purging the adsorbent in said removed zone of adsorbed material by passing a purge gas through it at a second pressure which is lower than the first pressure whereby ~o prepare said zone for re-use in the ,~, .
adsorption section commencing as the last zone of said section: (g) period~
.
. 30 lcally repeating steps (d), ~e) and (f) whereby each zone of said unit is in : .
.'',' -:
~ - 8 -~, ' ~94~L9 successive periods incorporated into the separation section as the last zone thereof, subsequently periodically progressed along said section until it becomes the first zone thereof, and then purged ready for re-incorporation into the separation section in a subsequent period; and (h) recycling the gas initially exiting from said removed zone during the step of reducing the pressure in said zone to the inlet of the separation section to be passed therethrough a~ subs~antially said first pressure, said recycle being con-tinued until not later *han the occurrence of a rapid increase in concen-tration of said contaminant in said exit gas.
An adsorp~ion unit for separating a purified gas from a mixture thereof with at least one gaseous contaminant and wherein regeneration of the adsorbent is by evacuation, can be adapted for reclamation of interstitial gas from the compartment being regenerated by including valved conduit means for directing gas from the interior of said third compartment to said ~ , evacuating means or to the inlet of a gas compressor whose outle~ communicates with the inlet to said separation section, the apparatus also including valve control means adapted to direct said gas in an initial phase to said compressor and in a subsequent phase to said evacuating means which exhaust the gas to waste, said initial phase terminating not later than the occurrence of a rapid increase in the concentration of the adsorbable material in said gas.
Where regeneration is by means of purge gas, apparatus adapted for recovering interstitial gas from the compartment being regenèrated will com-prise a pressure swing adsorption unit comprising a plurality of adsorption zones for selectively adsorbing said contaminant from said mixture and means for purging adsorbent of adsorbed contaminant, said adsorption unit including at least three compartments for adsorbent~ each having a gas inlet and a gas .: outlet ~or gas to be passed through adsorbent in the compartment, each com-partment forming a separately isolatable adsorption zone, an inlet for the gas mixture to be saparated in said unit, and outlet for purified gas, an .
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inlet manifold connected to the inlet for the gas mixture, a purified gas outlet manifold connected to the outlet for the purified product, a purge gas manifold, a waste gas manifold, first conduit means for connecting the inlet of each compartment to the inle~ manifold, with an inlet valve in the conduit between each compartment inlet and the inlet manifold, second conduit means for connecting the outlet from each compartment to the purified gas manifold, with an outlet valve in the conduit between each compartment outlet and the outlet manifold, third conduit means for connecting the purge gas manifold to the outlet from each compartment, with a purge gas inlet valve in the conduit between each compartment outlet and the purge gas manifold, four~h conduit means for connecting ~he inlet to each compartment to the waste gas manifold, there being a waste gas outlet valve between each compart~
ment inlet and the waste gas manifold, and fifth condui~ means adapted to connect the compartments together for flow of gas therethrough with the out-let of each compartment connected to the inlet of the next compartment, there being a valve in the conduit between the outlet of each compartment and the inlet of the next compartment, said valves being arranged such that at least a first and a second of said compartments are connected in series to provide a separation section be~ween the inlet manifold and the outlet mani-fold for flow of gas therethrough and in that order with respect to the gas flow, and at least a third compartment is connected for gas flow there-through from the purge gas manifold to the waste gas manifold for purging, and automatic valve control means adapted periodically to switch the valves to disconnect said third compartment from said purge gas manifold and said waste gas manifold and connect it in series with said second compartment in said separation section to form ~he last compartment in said section, ex-pressed in terms of flow o~ gas therethrough, and disconnect said first compartm0nt from said separation section and connect it between the purge gas manifold and the waste gas manifold for purging, said apparatus further including valved conduit means for passing gas from the third compartment :
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to said waste gas manifold or to the inlet of a gas compressor whose outlet communicates with the inlet to the separation section, and valve control means adapted to direct said gas in an initial phase to said compressor and in a subsequent phase to waste, said initial phase termina~ing not later than the occurrence of a rapid increase in the concentration of adsorbable material in said gas.
The composition of the gas exiting from the third compartment can be readily monitored using known gas analysis tec]miques, e.g. katharometry, and therefore a rapid increase in the concentration of the adsorbable material can be readily determined.
For any given gas mixture and adsorption conditions such as of temperature and pressure, this rapid increase in concentration of the adsorbable material will generally occur at or near a given value for the concentration.
Thus, in accordance with one method of carrying out the recycle process of the invention, the gas exiting from the zone during the initial s~ages of depressurization may be recovered and recycled until the concen-tration of adsorbable material in said gas reaches a predetermined level which is not greater than the level which corresponds to the occurrence of the above-mentioned rapid increase. Preferably this predetermined level is -substantially equal ~o the level corresponding to the rapid increase. -In apparatus for carrying out this embodiment~ the valve control means suitably comprise sensor means for sensing the level of concentration of contaminant in the gas from the third compartment and means responsive to said sensor means and adapted to terminate said initial phase when said sensed level exceeds a predetermined level.
The pressure at which this occurs will vary depending on the nature of the material to be desorbed from the adsorber bed but experience shows that in general it will be found to be equivalent to about 15% to 30%
of the total drop in pressure from the adsorption pressure to the pressure .
1049~9 of the purge gas where a purge gas is used, or about 10% ~o 30% of the adsorption pressure expressed as absolute pressure where regeneration is effected by evacuation.
Thus in accordance with another method of carrying out the recycle process of the invention, the gas exiting from the zone during the initi~l stages of depressurization may be recovered and recycled until its pressure falls to a predetermined pressure below the adsorption pressure.
This may be achieved for example by sensing the pressure of the gas exiting from the zone and stopping the recycling when this pressure reaches said predetermined pressure. Alternatively, the recycling may be stopped after the elapse of a period of time after commencement of depres-surization which is known, e.g. from trial and experiment or from calculation, to correspond to said pressure!
In apparatus for use with the first alternative, the valve control means may suitably comprise sensor means for sensing the pressure of the gas in the third compartment and means responsive to said sensor means and adapted to terminate said initial phase when the sensed pressure falls be- ~
low a predetermined level. ~ ;
In apparatus for use with the second alternative, the valve control means may suitably comprise timing means and means responsive to said timing means adapted to terminate said initial phase at a predetermined time after the commencement of evacuation of said third compartment.
The pressure of the gas that is recycled to the separation section is restored to the adsorption pressure by m~ans of the compressor. The same compressor may also be used to compress ~he feed gas mixture to the adsorption pressure where such compression is required.
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The use of evacuation to regenerate the adsorbent and the recycling of the interstitial gas to the separation section are illustrated by means of the following example and with the aid of the accompanying drawings in which: -: ~ ' `' ~
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Figure 1 is a schematic representation of a method of operating a pressure swing absorp~ion unit with four zones of adsorbent;
Eigure 2 is a schematic flow sheet of a modification of the four zone pressure swing adsorption unit illustra~ed in Figure 1 when in the mode identified as STAGE 1 in Figure 1, and wherein regeneration of the adsorbent is effected by evacuation in place of purging;
Figure 3 is a schematic flow sheet of another modification of the four zone pressure swing adsorption unit illustrated in Figure 1, again when in the mode identified as STAGE 1 in Figure 1, wherein provision is made for recycle of the interstitial gas recovered during depressurisation; and E:igure 4 is a schematic flow sheet of a modification of the apparatus of Figure 2 wherein provision is made for recycle of the interstitial gas recovered during depressurisation.
Referring to Figure 1, the pressure swing adsorption unit comprises four zones of adsorbent A, B, C and D. During the first stage ~STAGE 1 in the drawing), the compressed gaseous mixture FG to be separated, which for the purposes of simplicity is regarded as containing one component that is adsorb-ed by the adsorbent, is passed through a separation section comprising zones A and B in series and a purified product gas PE is recovered. During this same ~0 period, zone C, which has previously been purged at low pressure, is repressur-ised with repressurisation gas RG which is taken from the stream of purified product PE leaving zone B, and zone D is depressurised and purged by passing `; through it in the opposite direction to that of the compressed gas flow a ; purge gas PG which comprises a further portion of the purified gas from B
` which has been expanded through valve V to a lower pressure. The gas DG carry-ing the material desorbed from zone D is removed from the system as shown.
At the point in time when the concentration of the adsorbable com-ponent of the feed gas mixture FG is detected as having reached a prede~er-; mined level in the product gas PE leaving zone B, this level being usually the maximum permitted level of the adsorbable component in the product gas, or '' _ 10 -. , -~49~
after a predetermined period of time has elapsed which is determined by em-pirical means or calculated from available data to correspond to this point in time. Zone A will be substantially fully saturated with adsorbable material while zone B has been utilised only to about 20%.
In accordance with the invention, at this point in time, zone C, which has previously been purged and repressurised to the pressure of the feed gas mixture, is added to the separation section to form the last zone thereof (expressed with reference to the direction of flow of the feed gas mixture through the separation section) and zone A, which during STAGE 1 formed the first zone of the separation section (expressed with reference to the direc-tion of flow of the feed gas mixture through the separation section), is removed from the separation section and commences depressurisation and purg-ing. Thus zone B now becomes the first zone of the separation section and zone C becomes the second or las~ zone. At the same time zone D, which during STAGE 1 has been undergoing depressurisation and purging, commences repressuri-sation.
The situation is then as shown in STAGE 2 of Figure ~, with the feed gas mixture FG being passed through a separation section comprising zones B
~ and C in that order, zone D being repressurised with a portion RG of the product gas and zone A being depressurised and purged with purge gas PG. As the zone C has been previously purged, the concentration of adsorbable compon-ent in the product gas leaving the separation section will thus fall away.
When the point in time is reached when the concentration of the adsorbable component of the feed gas mixture FG is detected as having again , reached the a~orementioned predetermined level in the product gas (or when a further period of time has elapsed corresponding to reaching this point in ~ time) zone B will now be substantially fully saturated while zone C is only ;1 partly saturated. At this point in time, previously purged and re-pressurised zone D is now added to the separation section as the last zone thereof and zone B is removed and commences depressurisation and purging. Thus zone C now becomes the first zone of the separation section and zone D becomes the s0cond .
1~9~1~
(or last) zone. At the same time, zone A which during STAGE 2 has been under-going depressurisation and purging, commences re-pressurisation.
The situation is then as shown in STAGE 3 of Figure 1 with feed gas mixture FG being passed through a separation section comprising zones C and C in that order, zone A being re-pressurised with a portion RG of the product gas and zone B being depressurised and purged with purge gas PG.
: At the end of STAGE 3, determined as described above for STAGES 1 and 2, freshly pressurised zone A is added to the separation section as the last zone and zone C, which formed the first zone of the separation section during STAGE 3, is removed from the separation section for depressurisation and purging so that zone D becomes the first zone.
The situation is then as shown in STAGE 4 of Figure 4 with the feed gas mixture FG being passed through a separation section comprising zones D
and A in that order, zone C being depressurised and purged and zone B being re-pressurised.
At the end of STAGE 4, again determined as described above for STAGES 1, 2 and 3, fully pressurised zone B is added to the separation section as the last zone, and zone D, which formed the first zone of the separation section during STAGE 4, is removed from the separation section for depressurisa-tion and purging so that zone A becomes the first zone9 and æone C commences re-pressurisation. The situation is then as shown in STAGE 1 of Figure 1 and the cycle commences all over again.
The complete cycle may be represented as follows : -_ ZONE STAGE
j 1 2 3 4 _ . _ A S1 D~P RP S2 B S2 Sl D~P RP
C RP S2 Sl D~P
D D~P RP S2 S
` .
::
94~
where Sl and S2 are the first and second beds respectively in the separation section ~expressed in terms of the direction of flow of the ieed gas mixture through the separation section) D~P represents depressurisation and purging, and RP represents re-pressurisation.
The above illustration has been described for simplicity with refer-ence to one adsorbable component in the feed gas mixture FG but the arrange-ment may also be used for feed gas mixtures containing three or more components two or more of which are adsorbed by the adsorbent. In this case, the signal for changing from one stage to the next may be, for example, the concentration of one of the adsorbed components reaching a predetermined value in the pro-duct stream, or the total concentration of a combination of two or more of the components reaching a predetermined value in the product stream, or the first o any of the adsorbed components to reach a predetermined concentration in the product stream for that component. Alternatively, the changeover may be effected after a predetermined time interval which, may be calculated or deter-mined empirically. -Referring now to the arrangement illustrated in Figure 2, the adsor-ber illustrated differs from that of Figure l in that the regeneration is effected by evacuation, no purge gas being used. Thus, in the arrangement illustrated in Figure 2, the feed gas for separation is fed at elevated pres-sure in pipeline 302 ~or is fed in pipeline 303 to be compressed to elevated pressure in compressor 304 and fed via pipeline 306) to pipeline 308 leading to the inlet of the separation section 310 of the adsorber, consisting, as before, of zones A and B connected in series. Product gas substantially free of the adsorbable material leaves the separation section in pipeline 312 but in this case it is divided into two portions only, one of which is recovered in pipeline 314 as product, and the other of which is fed via pipeline 316 to ~one C to repressurise this zone to adsorption pressure. Zone D is under-going regeneration by evacuation through pipelines 324 and 328 by means ~, . . ..
:` :
. ~ .
~49~
of vacuum pump 340. The evacuated material is passed for venting or flaring.
The cycle of operation of the apparatus corresponds to that of the apparatus shown in Figure 1.
Figure 3, in which the features which are common with Figure 2 have the same reference numerals or letters, is again a flow sheet of a four zone adsorber which operates with the same cycle as that illustrated in Figure 1 and is shown in the period of the cycle corresponding to STAGE 1 in Figure 1.
As in Figure 1, the product gas leaving the separation zone is divided into three portions. The first portion is recovered through pipeline 314 as product gas. The second portion is fed via pipeline 316 to adsorber zone C to re-pressurise this zone to adsorption pressure. A third portion is fed via pipeline 318 to expansion valve 320 where its pressure is reduced, and is then passed to adsorber zone D as purge gas. In the adsorber of Figure 3, however, provision is made for recycling the gas exiting from zone D, which is under-going depressurisation and purging, to the separation section inlet. Thus, pipeline 324 is connected to two pipelines, 326 and 32~, each of which is fitted with a valve, 330, 332 respectively. Pipeline 326 connects pipeline 324 to pipeline 303 and the inlet to compressor 304. Pipeline 328 connects pipeline 324 to the flare or vent to atmosphere. A pressure sensing device 334 is located in pipeline 324 to sense the pressure of the gas therein and is ; connected to a valve control means 336 in the form of a pressure controller which is adapted to control valves 330 and 332. The controller is adapted so that in operation it closes valve 330 and opens valve 332 when the sensed pres-sure of the gas in pipeline 324 falls below a predetermined level, the posi-tions of these valves being reversed when the sensed pressure of the gas in pipeline 324 is above that level. In operation, the pressure controller is arranged so that it operates to close valve 330 and open valve 332 at a press-sure which is at or slightly above the pressure at which the adsorbed contamin-ant commences to be desorbed from the adsorbent in 7one D, resulting in a rapid increase in the concentration o~ contaminant in the gas in pipeline 24.
', "
:.
1~4~ 9 Figure 4, in which the features common with Figures 2 and 3 have the same reference numerals or letters, in a flow sheet of the four zone adsorber of Figure 2 but modified, as in the adsorber oE Figure 3, to provide for recycling of gas exiting from the zone D to the separation section inlet.
EXAMPLE
A. In a first experiment, a feed gas mixture consisting of 94 mol % ~le and 6 mol % of adsorbable impurities was separated using the adsorber arrange-ment of Figure 1 using coconut shell activated carbon as the adsorbent.
The pressure in the separation section was 350 psig, the length of each period of the cycle was 10 minutes, and the purge gas pressure was 5 pSlg.
With a feed gas rate of 149.5 SCFH the total product recovered from the separation section was 140.5 SCFH of 99.995% pure helium of which 15 SCFH
was used as purge gas and 40.5 SCFH was required for repressurisation since each zone required 6.75 SCF of gas for repressurisation to 350 psig and six zones required repressurisation each hour.
Thus, the hourly recovery of helium was [140.5 - ~15 + 40.5)] x 0.99995 and the yield of helium, expressed as percentage of theoretical was ~140.5 - (15 + 40.4~] x 0.99995 = 60.5%
149.5 x 0.94 B. In a second experiment, the same feed gas mixture was separated in the adsorber of Figure 2 using the same volume of the same adsorbent as in the first experiment~ to produce a product gas of the same 99.995% purity.
It was ound that using the same adsorption pressure of 350 psig and the same rate of feed of 149.5 SCFH, as a result of the increased effi-ciency of regeneration by use of vacuum rather than purging, the length of each period of the cycle could be increased safely to about 15 minutes dura-tion so that only 4 zones required regeneration in each hour instead of 6.
However, as it was now necessary to repressurise each zone from vacuum to ;i 30 350 psig, the quantity of product gas required to repressurise each zone was , :
, ~
- - \
~4~
found to have incr~ased to 7.05 SCF. On the other hand, no product gas was lost aspurge gas.
Thus, the yield of He was 0.99995 [140.5 - (4 x 7.05)] = 80%
149.5 x 0.94 C. In a third experiment, the same feed gas mixture was separated in the adsorber of Figure 3 using the same volume of the same adsorbent as in the ~-first experiment, to produce a product gas of the same 99~995% purity.
The pressure controller 336 of Figure 3 was set to close valve 330 in each period after the pressure of the gas exiting from the zone undergoing regeneration had dropped to 50 psig.
The pressure and total rate of feed of the gas mixture to the separation section were maintained the same as the first experiment at 350 psig and 149.5 SCFH, as were the purge gas pressure (5 psig) and the length of each period of the cycle (10 minutes).
Since the adsorption and purge gas pressures were the same as for Experiment A, the volume of gas required to repressurise each zone remained the same at 6.75 SCF.
It was found that under these conditions 5.55 SCF of interstitial gas was recovered and recycled from each zone during depressurisation.
Thus, the net flow of fresh feed to the adsorber was 149.5 -~6 x 5.55) = 116.2 SCFH.
The yield of He was thus [140.5 - (15 ~ 40.5) ] x 0.99995 = 78%
116.2 x 0.94 D. In the fourth experiment, the same feed gas mixture was separat-ed in the adsorber of Figure 4 using the same volume of the same adsorbent as in all the other experiments, again to produce a product gas of 99.995% purity.
Total feed rate to the separator s0ction was 149.5 SCFH at 350 psig.
Since, as in the second experiment, regeneration was effected by ;30 evacuation, there was no purge gas stream and~ as in said second experiment, :~ , .
,, ; .- ., - ,, . -- . . . . .
g~9 each period could be safely extended to 15 minutes duration. Again, as in the second experiment, since repressurisation had to be effected from vacuumJ
7.05 SCF of gas was required to repressurise each zone.
The pressure controller 336 was set so that in each period the valve 330 closed and valve 332 opened after 5.2875 SCF o:E interstitial gas had been recovered and recycled from each zone during evacuation.
Thus the net flow of fresh feed to the adsorber is [149.5 - (4 x 5.2875)] = 128.35 SCFH.
The net product flow is 140.5 - (4 x 7.05 x 0.99995)= 112.3 SCFH.
Thus~ the yield of He is 112.3 = 87.5%
128.35 ", . . .
~' ~ 17 ' ~' . `. . . . . . , . .- . . . .
Claims (18)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A method of separating a purified gas from a mixture thereof with at least one gaseous contaminant, said method comprising the steps of (a) providing an adsorption unit comprising x zones of adsorbent material which selectively adsorbs said gaseous contaminant, x being a whole number of at least three; (b) passing the gas mixture at a first pressure through y of the zones of said adsorption unit arranged in series with respect to the flow of the gas mixture therethrough, y being a whole number of from 2 to x - 1 and said y zones constituting a separation section of said adsorption unit;
(c) recovering from the last zone of said separation section a purified gas in which the concentration of the contaminant does not exceed a predetermined value which is below the concentration of contaminant in the gas mixture; (d) before the concentration of the contaminant in said purified gas rises above said predetermined value, but after the concentration of said contaminant in the gas stream leaving the zone forming the penultimate zone of said separat-ion section has exceeded said predetermined value, adding another zone of said adsorption unit to the end of said separation section, expressed in terms of the direction of flow of said gas mixture through the separation section whereby said added zone now forms the last zone of said separation section; (e) removing from said separation section the zone forming the first zone of the section whereby the zone next to it in the separation section now becomes the first zone; (f) regenerating the adsorbent in said removed zone by evacuation of said zone; (g) periodically repeating steps (d), (e) and (f) whereby each zone of said unit is in successive periods incorporated into the separation section as the last zone thereof, subsequently periodically pro-gressed along said section until it becomes the first zone thereof, and then treated to regenerate the adsorbent therein ready for re-incorporation into the separation section in a subsequent period.
(c) recovering from the last zone of said separation section a purified gas in which the concentration of the contaminant does not exceed a predetermined value which is below the concentration of contaminant in the gas mixture; (d) before the concentration of the contaminant in said purified gas rises above said predetermined value, but after the concentration of said contaminant in the gas stream leaving the zone forming the penultimate zone of said separat-ion section has exceeded said predetermined value, adding another zone of said adsorption unit to the end of said separation section, expressed in terms of the direction of flow of said gas mixture through the separation section whereby said added zone now forms the last zone of said separation section; (e) removing from said separation section the zone forming the first zone of the section whereby the zone next to it in the separation section now becomes the first zone; (f) regenerating the adsorbent in said removed zone by evacuation of said zone; (g) periodically repeating steps (d), (e) and (f) whereby each zone of said unit is in successive periods incorporated into the separation section as the last zone thereof, subsequently periodically pro-gressed along said section until it becomes the first zone thereof, and then treated to regenerate the adsorbent therein ready for re-incorporation into the separation section in a subsequent period.
2. A method as claimed in claim 1 further including the step of recycl-ing the gas initially exiting from said removed zone during the evacuation procedure of step (f) to the inlet of the separation section to be passed therethrough at substantially said first pressure, said recycle being con-tinued until not later than the occurrence of a rapid increase in concentrat-ion of said contaminant in said exit gas.
3. A method as claimed in claim 2 which includes sensing the level of concentration of said contaminant in said exit gas and terminating the recycle of said exit gas when the level of contaminant rises to a predetermined level which is not greater than that which corresponds to the occurrence of said rapid increase.
4. A method as claimed in claim 2 which includes sensing the pressure of said exit gas and terminating the recycle of said exit gas when said pressure drops to a predetermined pressure which is not lower than the pres-sure at which said rapid increase in concentration of contaminant occurs.
5. A method as claimed in claim 2 in which said recycle is terminated at the end of a predetermined time interval after the commencement of the evacuation procedure of step (f), the length of said time interval being at most the length of the time interval at which said rapid increase of concen-tration of contaminant occurs.
6. A method of separating a purified gas from a mixture thereof with at least one gaseous contaminant by pressure swing adsorption, said method comprising the steps of (a) providing an adsorption unit comprising x zones of adsorbent material which selectively adsorbs said gaseous contaminant, x being a whole number of at least three; (b) passing the gas mixture at a first pressure through y of the zones of said adsorption unit arranged in series with respect to the flow of the gas mixture therethrough, y being a whole number of from 2 to x - 1 and said y zones constituting a separation section of said adsorption unit; (c) recovering from the last zone of said separation section a purified gas in which the concentration of the contamin-ant does not exceed a predetermined value which is below the concentration of contaminant in the gas mixture; (d) before the concentration of the con-taminant in said purified gas rises above said predetermined value, but after the concentration of said contaminant in the gas stream leaving the zone forming the penultimate zone of said separation section has exceeded said predetermined value, adding another zone of said adsorption unit to the end of said separation section, expressed in terms of the direction of flow of said gas mixture through the separation section whereby said added zone now forms the last zone of said separation section; (e) removing from said sep-aration section the zone forming the first zone of the section whereby the zone next to it in the separation section now becomes the first zone; (f) reducing the pressure in said removed zone and purging the adsorbent in said removed zone of adsorbed material by passing a purge gas through it at a second pressure which is lower than the first pressure whereby to prepare said zone for re-use in the adsorption section commencing as the last zone of said section; (g) periodically repeating steps (d), (e) and (f) whereby each zone of said unit is in successive periods incorporated into the separat-ion section as the last zone thereof, subsequently periodically progressed along said section until it becomes the first zone thereof, and then purged ready for re-incorporation into the separation section in a subsequent period; and (h) recycling the gas initially exiting from said removed zone during the step of reducing the pressure in said zone to the inlet of the separation section to be passed therethrough at substantially said first pressure, said recycle being continued until not later than the occurrence of a rapid increase in concentration of said contaminant in said exit gas.
7. A method as claimed in claim 6 which includes sensing the level of concentration of said contaminant in said exit gas and terminating the recycle of said exit gas when the level of contaminant rises to a predeter-mined level which is not greater than that which corresponds to the occurrence of said rapid increase.
8. A method as claimed in claim 6 which includes sensing the pressure of said exit gas and terminating the recycle of said exit gas when said pres-sure drops to a predetermined pressure which is not lower than the pressure at which said rapid increase in concentration of contaminant occurs.
9. A method as claimed in claim 6 in which said recycle is terminated at the end of a predetermined time interval after the commencement of the evacuation procedure of step (f), the length of said time interval being at most the length of the time interval at which said rapid increase of concen-tration of contaminant occurs.
10. Apparatus for separating a purified gas from a mixture thereof with at least one gaseous contaminant, said apparatus comprising an adsorption unit comprising a plurality of adsorption zones for adsorbing said contaminant from said mixture and means for regenerating adsorbent, said adsorption unit including at least three compartments for adsorbent, each having a gas inlet and a gas outlet for gas to be passed through adsorbent in the compartment, each compartment forming a separately isolatable adsorption zone, an inlet for the gas mixture to be separated in said unit, an outlet for purified gas, an inlet manifold connected to the inlet for the gas mixture, a purified gas outlet manifold connected to the outlet for the purified product, said regen-erating means comprising means for evacuating the adsorption zones, first conduit means for connecting the inlet of each compartment to the inlet mani-fold, with an inlet valve in the conduit between each compartment inlet and the inlet manifold, second conduit means for connecting the outlet from each compartment to the purified gas manifold, with an outlet valve in the conduit between each compartment outlet and the outlet manifold, third conduit means for connecting the interior of each compartment to said evacuating means, there being a valve between each compartment interior and the evacuating means, and fourth conduit means adapted to connect the compartments together for flow of gas therethrough with the outlet of each compartment connected to the inlet of the next compartment, there being a valve in the conduit be-tween the outlet of each compartment and the inlet of the next compartment, said valves being arranged such that at least a first and a second of said compartments are connected in series to provide a separation section between the inlet manifold and the outlet namifold for flow of gas therethrough and in that order with respect to the gas flow, and at least a third compartment is connected to said evacuating means for regeneration of the adsorbent therein, and automatic valve control means adapted periodically to switch the valves to disconnect said third compartment from said evacuating means and connect it in series with said second compartment in said separation section to form the last compartment in said section, expressed in terms of flow of gas therethrough and disconnect said first compartment from said separation section and connect it to said evacuating means for regeneration of the adsorbent therein.
11. Apparatus as claimed in claim 10 in which valved conduit means are provided for directing gas from the interior of said third compartment to said evacuating means or to the inlet of a gas compressor whose outlet communicates with the inlet to said separation section, the apparatus also including valve control means adapted to direct said gas in an initial phase to said compressor and in a subsequent phase to said evacuating means which exhaust the gas to waste, said initial phase terminating not later than the occurrence of a rapid increase in the concentration of the adsor-bable material in said gas.
12. Apparatus as claimed in claim 11 wherein said valve control means comprise sensor means for sensing the level of concentration of contaminant in the gas from the third compartment and means responsive to said sensor means adapted to terminate said initial phase when the sensed level exceeds a predetermined level.
13. Apparatus as claimed in claim 11 wherein said valve control means comprise sensor means for sensing the pressure of the gas from the third compartment and means responsive to said sensor means and adapted to terminate said initial phase when the sensed pressure falls below a pre-determined level.
14. Apparatus as claimed in claim 11 wherein said valve control means comprise timing means and means responsive to said timing means adapted to terminate said initial phase at a predetermined time after the commencement of depressurisation of said third compartment.
15. Apparatus for separating a purified gas from a mixture thereof with at least one gaseous contaminant, said apparatus comprising a pressure swing adsorption unit comprising a plurality of adsorption zones for selectively adsorbing said contaminant from said mixture and means for purg-ing adsorbent of adsorbed contaminant, said adsorption unit including at least three compartments for adsorbent, each having a gas inlet and a gas outlet for gas to be passed through adsorbent in the compartment, each com-partment forming a separately isolatable adsorption zone, an inlet for the gas mixture to be separated in said unit, an outlet for purified gas, an inlet manifold connected to the inlet for the gas mixture, a purified gas outlet manifold connected to the outlet for the purified product, a purge gas manifold, a waste gas manifold, first conduit means for connecting the inlet of each compartment to the inlet manifold, with an inlet valve in the conduit between each compartment inlet and the inlet manifold, second con-duit means for connecting the outlet from each compartment to the purified gas manifold, with an outlet valve in the conduit between each compartment outlet and the outlet manifold, third conduit means for connecting the purge gas manifold to the outlet from each compartment, with a purge gas inlet valve in the conduit between each compartment outlet and the purge gas manifold, fourth conduit means for connecting the inlet to each com-partment to the waste gas manifold, there being a waste gas outlet valve between each compartment inlet and the waste gas manifold, and fifth con-duit means adapted to connect the compartments together for flow of gas therethrough with the outlet of each compartment connected to the inlet of the next compartment, there being a valve in the conduit between the out-let of each compartment and the inlet of the next compartment, said valves being arranged such that at least a first and a second of said compartments are connected in series to provide a separation section between the inlet manifold and the outlet manifold for flow of gas therethrough and in that order with respect to the gas flow, and at least a third compartment is connected for gas flow therethrough from the purge gas manifold to the waste gas manifold for purging, and automatic valve control means adapted periodically to switch the valves to disconnect said third compartment from said purge gas manifold and said waste gas manifold and connect it in series with said second compartment in said separation section to form the last compartment in said section, expressed in terms of flow of gas therethrough, and disconnect said first compartment from said separation section and connect it between the purge gas manifold and the waste gas manifold for purging, said apparatus further including valved conduit means for passing gas from the third compartment to said waste gas manifold or to the inlet of a gas compressor whose outlet communicates with the inlet to the separation sec-tion, and valve control means adapted to direct said gas in an initial phase to said compressor and in a subsequent phase to waste, said initial phase terminating not later than the occurrence of a rapid increase in the con-centration of adsorbable material in said.gas.
16. Apparatus as claimed in claim 15 wherein said valve control means comprise sensor means for sensing the level of concentration of said adsorbable material in the gas from the third compartment and means responsive to said sensor means and adapted to terminate said initial phase when the sensed level exceeds a predetermined level.
17. Apparatus as claimed in claim 15 wherein said valve control means comprise sensor means for sensing the pressure of the gas from the third compartment and means responsive to said sensor means and adapted to terminate said initial phase when the sensed pressure falls below a predetermined level.
18. Apparatus as claimed in claim 15 wherein said valve control means comprise timing means and means responsive to said timing means adapted to terminate said initial phase at a predetermined time after the commencement of depressurization of said third compartment.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB11000/74A GB1506161A (en) | 1974-03-12 | 1974-03-12 | Method and apparatus for separating gases |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1049419A true CA1049419A (en) | 1979-02-27 |
Family
ID=9978209
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA221,952A Expired CA1049419A (en) | 1974-03-12 | 1975-03-12 | Method and apparatus for separating gases |
Country Status (3)
| Country | Link |
|---|---|
| AU (1) | AU504098B2 (en) |
| CA (1) | CA1049419A (en) |
| GB (1) | GB1506161A (en) |
-
1974
- 1974-03-12 GB GB11000/74A patent/GB1506161A/en not_active Expired
-
1975
- 1975-03-12 CA CA221,952A patent/CA1049419A/en not_active Expired
- 1975-03-13 AU AU79033/75A patent/AU504098B2/en not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| GB1506161A (en) | 1978-04-05 |
| AU7903375A (en) | 1976-09-16 |
| AU504098B2 (en) | 1979-10-04 |
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