CA1084421A

CA1084421A - Recovery of gases from gaseous mixtures

Info

Publication number: CA1084421A
Application number: CA234,075A
Authority: CA
Inventors: Heinrich Munzner; Hugo Horbel; Karl Knoblauch; Werner Korbacher; Werner Peters; Harald Juntgen
Original assignee: Bergwerksverband GmbH
Current assignee: Bergwerksverband GmbH
Priority date: 1974-08-29
Filing date: 1975-08-25
Publication date: 1980-08-26
Also published as: FR2283094A1; NL181559C; ZA755356B; BE831821A; DE2441447C3; JPS5150298A; JPS5417595B2; DE2441447A1; AU8436875A; BR7505509A; NL181559B; NL7510234A; DE2441447B2; AU503999B2; IT1040717B; FR2283094B1; GB1480866A

Abstract

ABSTRACT OF THE DISCLOSURE

A process and and arrangement for converting air into a gaseous mixture enriched with nitrogen includes passing a first stream of air through an adsorber which preferentially adsorbs oxygen from the admitted first stream and thereby dis-charges a second stream of a gaseous mixture having a higher pro-portion of nitrogen as compared with the first stream. Control means are positioned in the second stream to measure the oxygen content of the continuously discharging second stream and termi-nates the discharge of the same when the oxygen content thereof exceeds a predetermined value. Residual amounts of gaseous mix-ture are then evacuated from the adsorber. An additional adsorber is connected with the first-mentioned adsorber to form a first stage with each adsorber being alternately charged with air in a cyclic-al operation. Additional stages are connected in series so that the effluent nitrogen-enriched gaseous mixture can be further en-riched to a final desired value.

Description

~ 10~421 Gaseous mixtures enriched in nitrogen are widely used in industry for their inert propertles, and particularly as a protect-ive gas in preventing explositions. -The invention relates generally to the conversion of a gaseous mixture having two components into a mixture having an in-creased proportion of one of the components, and more particularly the nitrogen-enrichment of air by selective adsorption of oxygen.
In the prior art, nitrogen is obtained by the fractional distillation of liquid air. This process is very costly and is generally undesirable. The equipment is very expensive to operate and uses a relatively great expenditure of energy.
It has also already been proposed to recover nitrogen- ~ -enriched air utilizing an adsorption process employing siliceous or carbon-containing adsorption agents and involving the use of ~ -relatively high temperature or pressure changes during adsorption and desorption. Certain silicates, for example, zeolites, are effective for preferably adsorbing nitrogen from its mixture with oxygen so that, by conducting air through a zeolite-filled adsorber, ~ -the remaining gas is effectively enriched as regards its nitrogen content. me regeneration of the zeolite, however, requires a con-siderable outlay for energy and apparatus. Additionally, the zeolites are completely effective only when used with dry air since they are hydrophilic.
Accordingly, the present invention overcomes disadvantages of the prior art by providing an economical process and an ar~ange-ment for converting a gaseous mixture having an initial proportion of a first component into an enriched mixture having an increased proportion of the first component.

,~ _ _ 10~44Zl Further the yield of nitrogen from air is hereby increased in a si~,ple and efficient manner.
Additionally the proportion of contaminants such as water is reduced from the enriched mixture.
In keeping with these advantages and othe~swhich will -become apparent hereinafter, one feature of the invention is to pass a first stream of a gaseous mixture having an initial pro-portion of a first component through an adsorber filled with a carbon-containing molecular sieve material such as molecular sieve coke, hereinafter referred to as M-coke. ;
The M~coke preferentially adsorbs a second component from the admitted first stream so that a second stream discharges from the adsorber which contains a higher proportion of the first compon-ent. The effluent second stream continues to discharge from the ad-sorber while its proportion of the second component continues to increase as the capacity of the M~coke to adsorb additional amounts of second co~,ponent in the incoming gaseous mixture decreases. When the second component exceeds a predetermined proportion, the dis-charge of the second stream is terminated and the residual gaseous mixture in the adsorber is evacuated.
In the particular case where the starting gaseous mixture is air, which contains 79% by volume of nitrogen and 21% by volume of oxygen, the oxygen will be preferentially adsorbed by the M-coke and the effluent second stream will be nitrogen-enriched. me pro-cess and the arrangement for converting a gaseous mixture into an enriched mixture will be described with reference to the recovery of nitrogen from air. It is to be explicitly understood, however, that the description is not intended to limit the invention in any nanner.
Accordingly, the present invention provides a process for converting a gaseous mixture including an initial proportion ~ 3 ~o~4~Z~ ~:

of nitrogen and oxygen into an enriched gaseous mixture having an increased `
proportion of nitrogen, comprising the steps of passing a first stream of the gaseous mixture throu h an adsorber having a filling of a carbon-contain-ing molecular sieve material, the first stream being conveyed at a space velocity of at least 0.01 and at most 0.04 volumes of gaseous mixture per volume of molecular sieve material per second over a time period of at least 40 and at most 80 seconds; adsorbing the oxygen preferentially from the admitted first stream so that a second stream discharges from said adsorber which contains a higher proportion of nitrogen and at most a predetermined proportion of oxygen; continuing the discharge of said second stream from said adsorber until the oxygen in said second stream exceeds said predetermined proportion; thereupon terminating the discharge of said second stream; and evacuating residual gaseous mixture from said adsorber.
In another aspect, the present invention provides an arrangement for converting a gaseous mixture including an initial proportion of nitrogen and oxygen into an enriched gaseous mixture having an increased proportion of nitrogen, comprising adsorbing means including a filling of molecular sieve material; admitting means for passing a first stream of the gaseous mixture through said adsorbing means at a space velocity of at least 0.01 and at most 0.04 volumes of gaseous mixture per volume of molecular sieve material per second over a time period of at least 40 and at most 80 seconds said adsorbing means adsorbing the oxygen preferentially from the admitted first stream so that a second stream discharges from said adsorbing means which contains a hi~her proportion of nitrogen and at most a predetermined proportion of oxygen; means for continuing the discharge of said second -stream from said adsorbing means until the oxygen in said second stream exceeds said predetermined proportion; means for thereupon terminating the discharge of said second stream; and means for evacuating residual gaseous mixture from said adsorbing means.
Thus, the air has been strongly reduced, resulting in an effluent nitrogen-enriched gaseous mixture whose final proportion of nitrogen can be increased if desired, as will be explained herein. The effluent:mixture ~ - 4 -~' ~ , - 10844Zl also surprisingly contains other contaminants as well, such as unadsorbed traces of oxygen, carbon dioxide, argon and moisture. However, through well-known fine cleaning techniques, virtually 100% pure nitrogen can be reclaimed from the mixture.
The arrangement and process for converting the gaseous mixture into an enriched mixture thus overcomes the prior art drawbacks and achieves the aforementioned objects in a novel manner. The nitrogen is recovered from the starting nitrogen-oxygen gaseous mixture with great efficiency and economy of operation.
The M-coke, referred to above and described in our copending Canadian application serial no. 191,472 (now Canadian Patent 1,033,342) and in United States Patent No. 3,801,513, is capable of adsorbing oxygen molecules even from a relatively fast flowing stream of air. It is especially preferable if the average pore size of the M-coke be from at the least 0.2 and at most 0.4 millimlcrons (2-4 Angstroms) and particularly 0.3 millimicrons. The stream passing throu~h the M-coke should flow at a speed so that the air takes about one minute to travel through an adsorbing chamber. To test the effectiveness of the coke, air flowing at a speed of about 3 cm/sec should be passed through a one liter sample of the M-coke. Thereupon, the residual mixture remc~ining in the test sample is evacuated and should contain an oxygen content of at least 35% by volume.
The feature of terminating the discharge of the second stream when the proportion of oxygen increases to a predetermined - 4a -4Z~

1 value is very advantageous in improving the nitrogen yield. As pre-viously noted the oxygen content of the effluent gaseous mixture may increase from at least 0.02% by volume to 10% or more by volume.
Therefore, limiting values of oxygen ~ by volume are preselected.
Depending upon the degree of nitrogen enrichment desired, limiting values of 0.02, 2, 4, etc. by volume of oxygen may be selected.
Another feature of the invention is that the pressures --required for evacuating the M-coke from an adsorption chamber are not critical. Commercially available vacuum pumps capable of generat-ing pressures down to 100 torr may be used. Pressures in the neigh- ~ -borhood of 70-20 torr are easily obtainable by such pumps, and are therefore especially recommended.
Another feature of the invention is to admit the starting air (under overpressures of 2-5 bar) into the adsorption chamber containing the M-coke at an inlet portion, and to evacuate the re-sidual gases remaining therein after termination of the flow of the effluent nitrogen-enriched mixture from the same inlet portion where the starting air is admitted into the adsorption chamber but in the opposite direction. It has been found that the moisture in the start-ing air tends to congregate at the inlet portion of the chamber sothat, by evacuating the chamber so that residual gases are removed in a direction opposite to the incoming air flow, the accumulated moisture is for the most part eliminated. This overcomes the prior art problem of removing the residual gases so that the moisture is conducted all the way through the M-coke.
The feature of pressurizing the air flow to overpressures of 2- 5 bar is also advantageous in increasing the adsorption capa-city of the M-coke. It has been found that lower or higher pressures decrease the effectiveness of the M-coke, although these lower or higher pressures are not intended to be excluded from possible use.

~()8~42~

1 To further improve the adsorption effectiveness of the M-coke, the velocity of the air stream must be controlled. We have found it especially advantageous if the gaseous mixture has a space velocity of 0.01-0.04 volumes of gas per volume of M-coke per second over a time period of at least 40 and at most 80 seconds. The velo-city may be controlled by adjustable nozzles in conventional manner.
It is further advantageous to partially or entirely pre-fill a previously partially or entirely evacuated adsorber with a gas having a nitrogen component so as to prevent an excessively turbulent reaction when the air first passes through the adsorber.
The gas may come from an external source, or preferably the residual~` -gases remaining in the pores or interstices of the M-coke after termination of the discharging of the second nitrogen-enriched stream of an additional adsorber may be used.
To achieve this object, equilibrium valves located be-tween the adsorbers and a vacuum pump which creates an underpress-ure are used to conduct the residual gases having a nitrogen com-ponent from one of the adsorbers into a freshly evacuated adsorber.
The two adsorbers are connected in parallel and operate in alternating fashion to produce a continuous effluent stream. Of course, other pairs of adsorbers can be connected in parallel like the first-mentioned pair to improve the nitrogen yield and the continuity of operation.
Still another feature of the invention is to connect two pairs of adsorbers in series so that the effluent nitrogen-enriched stream of the first pair or stage is used as the starting gas for the second pair or stage. Thus, the proportion of nitrogen component by volume in the final end product will increase.
In addition, residual gases from the second stage in their entirety may be fed back to initially load the adsorbers of the first ~08~f~

l stage, since the residual gases have been surprisingly found to have less oxygen than the residual gases remaining in the inter-stices of the respective adsorbers of the first stage. Typically, : 10%-20% of the residual gases remaining in the interstices of the adsorber are replaced by the residual gases of the second stage.
The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims.
The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying j drawing.
FIG. 1 is a diagrammatic view of a pair of adsorbers ac-cording to the present invention; and FIG. 2 is a diagrammatic view of two pairs of adsorbers according to the present invention.
Inasmuch as the recovery of nitrogen from air is of such great technical importance, the invention will be described with reference to such enrichment. However, it is to be explicitly under-stood that this is not intended to limit the invention in any mannerand that the invention is generally applicable to the conversion of a gaseous mixture having at least two components so as to obtain an enriched gaseous mixture having an increased proportion of one of the components.
Discussing jointly the method and the apparatus illustrat-ed in FIG. l for obtaining an enriched gaseous mixture having an in-creased proportion of one of the components, it will be seen that reference numeral l identifies an adsorber which is at least partial- -ly filled with a carbon-containing molecular sieve, and particularly molecular sieve coke, hereinafter referred to as M-coke.

10844Zl 1 Such a sieve is capable of preferentially adsorbing that component of a gaseous mixture which has the smaller mole-cular size. With reference to air, the oxygen molecules will be preferentially adsorbed since they have a smaller molecular size than nitrogen molecules. Details of a process for the production of a carbon-containing molecular sieve capable of separating different gases, particularly nitrogen and oxygen, are known in the art.
Admitting means is provided for passing a first stream of the gaseous mixture (e.g. air) through adsorber 1. The admitting 10 means comprises a blower 3 and an inlet valve 7.
Discharging means is provided for issuing a second stream from the adsorber 1. The first stream has already undergone adsorp-tion, and the thereby resulting second stream has a higher proportion of one of the components (nitrogen). The discha-ging means comprises an outlet valve 9.
Control means 12a is provided in the path of the dis-charging second stream. The control means 12a measures the propor-tion of the adsorbed component (oxygen) therein. As will be explained herein, the control means is operative to terminate the discharge of 20 the second stream whenever the quantity of adsorbed component (oxygen) exceeds a predetermined value.
Reference numeral 2 identifies an analogous additional adsorber containing M-coke and~ together with adsorber 1, comprises a first stage. The interiors of adsorbers 1 and 2 are in communica-tion at their respective upper portions by equilibrium valve 12 and at their respective lower portions by equilibrium valve 11.
~ he admittlng means communicates w1th adsorber 2 by means .: . ............................. :
, , 10844Zl ~ :

1 of inlet valve 8; the discharging means also permits the second i~ stream issuing from adsorber 2 by means of outlet valve 10 to be measured by control means 12a.
Evacuating means comprise a vacuum pump 4 and evacuation valves 5 and 6 associated with adsorbers 1 and 2, respectively. The evacuating means is operative for removing or desorbing residual gaseous mixture from the adsorbers 1 and 2.
The alternating cycle operation of adsorbers 1 and 2 will now be described with particular reference to FIG. 1: Firstly, eva-cuation valve 5 is opened and all remaining valves 6 - 12 are closed~.
Upon activation of pump 4, the interior of adsorber 1 is evacuated and maintained at an underpressure. Next, equilibrium valves 11 and 12 are opened so that the residual gaseous mixture contained in the interstices or pores of the M-coke in adsorber 2 will flow into ad-sorber 1. The equilibrium valves are preferably kept open for a time period of at least 2 and at most 3 seconds. After this time period is over, equilibrium valves 11 and 12 are closed.
Next, the inlet valve 7 and other valve 9 are opened and blower 3 is actuated to pass a first stream of air through adsorber 1 to be adsorbed therein. The resulting adsorbed second stream con- -tains a higher proportion of nitrogen as compared with its initial proportion in the air. The preferred time period for the first stream to pass through adsorber 1 is about one minute, or about 1 cubic meter air per 1 cubic meter of M-coke.
As the M-coke within adsorber 1 becomes more and more loaded with oxygen, its effectiveness decreases. The escaping second !
~ stream will, at first, have a very small percentage by volume of :~
oxygen mixed with the desired nitrogen gas and, subsequently, the percentage by volume of oxygen will increase. The control means 12a ~` 30 is an oxygen-measuring device which is located in the discharging :'~

_9_ ~, , ............................................... .
7~

~~
10844Zl J 1 second stream to measure when the quantity of oxygen exceeds a predetermined value. In one application, the initial percentages '~ of oxygen is about 0.5% by volume and the upper limit is about 4 by volume. When this upper predetermined value is reached, inlet - valve 7 and the outlet valve 9 are closed, thus terminating the ; discharge of the second stream from adsorber 1.
During this time between the aforementioned opening and closing of the inlet value 7 -- that is, during the loading of ad-sorber 1 and the obtaining of a first fraction of gaseous mixture enriched with nitrogen -- the evacuation valve 6 is opened and pump 4 reactuated to a final pressure of about 35 torr so that the interior of adsorber 2 is evacuated and maintained at underpressure.
Moreover, evacuation valve 6 is then closed, and equilibrium valves 11 and 12 opened so as to permit the residual gaseous mixture con-tained in the interstices of the M-coke in adsorber 1 (after valves 7 and 9 have been closed) to flow into adsorber 2. Equlibrium valves 11 and 12 are then closed and inlet and outlet valves 8 and ~ 10 opened so as to permit the admitting means to pass another : stream of air through adsorber 2.
As described above for adsorber 1, adsorber 2 discharges a second stream of nitrogen-enriched gaseous mixture towards control means 12a. As before, when the upper predetermined value is obtained, inlet valve 8 and outlet valve 10 are closed, thus terminating the discharge of the second stream from adsorber 2. Thereupon, evacua-tion valve 5 is opened, and inlet valve 7 and outlet valve 9 are closed so that the above-described cycle will beging again.
In this manner, adsorbers 1 and 2 alternately discharge a nitrogen-enriched gaseous mixture. They may work independently of each other, or together in the above-described cyclical fashion, or with other non-illustrated adsorbers so as to produce a continuous i, . ' , -10- . -. ' , ' . ,, : .. .. ~ . , : :, , . .. :
~ : . :. :
.. .. . .

lV844Zl 1 effluent stream.
Turning now to FIG. 2, the first stage of adsorbers 1 and 2 is connected in series with a second stage of adsorbers 21 and 22, that is the effluent nitrogen-enriched stream is used as the starting gas in the subsequent second stage. Except as noted below, the second stage is exactly identical with the first in that equilibrium valves 31 and 32 correspond to valves 11 and 12, inlet valves 27 and 28 correspond to valves 7 and 8, outlet valves 29 and 30 correspond to valves 9 and 10, evacuation valves 25 and 26 cor-respond to valves 5 and 6, pump 24 corresponds to pump 4, and con-trol means 32a corresponds to control means 12a.
A gasometer or gas storage container 13 is located inter-mediate the stages to collect the effluent gaseous mixture discharged ; from the adsorbers of the first stage and supply the second stage. -~
In order to further increase the yield, the residual gas of the second stage which contains less oxygen as a rule than the residual gases respectively remaining in the interstices of the adsorbers 1 and 2 is fed back to the first stage by means of feedback valves 14 and 15. Another gasometer 33 is used to collect the residual gases of , 20 the second stage alternately evacuated from adsorbers 21 and 22.
The operation of the two stages is as follows: As before, adsorber 1 is evacuated, equilibrium valves 11 and 12 are opened, ` residual gas remaining within the interstices of adsorber 2 flows into adsorber 1, and equilibrium valves 11 and 12 are closed after about 2 or 3 seconds. Feedback valve 14 and outlet valve 9 are then opened, and the residual gas collected in gasometer 33 is conveyed to and through adsorber 1 for a time period of about 4 - 6 seconds.
Feedback valve 14 is then closed, and inlet valve 7 is opened so that the blower 3 passes an additional air stream through adsorber 1. The additional air stream pushes the already nitrogen-enriched .
, ` -11-., .
' : ' 10844Zl -.

1 residual gas out of the interstices of adsorber 1. The additional air stream is discharged from adsorber 1 and is collected down-stream in gasometer 13. As soon as the proportion of oxygen in the - effluent stream exceeds a predetermined value, e.g., the proportion rises from 0.5 to 4% by volume, the inlet and outlet valves 7 and 9 are closed.
As before, during this time, adsorber 2 has been evacuat-ed so that now by opening equilibrium valves 11 and 12, any residual gas remaining in adsorber 1 is conveyed to adsorber 2. Then, feed-back valve 15 and outlet valve lO are opened, and the remainder of the residual gas collected in gasometer 33 is conveyed to and through adsorber 2. Then, feedback valve 15 is closed, and inlet valve 8 is opened so that the blower 3 forces an additional stream of air through adsorber 2 past control means 12a towards gasometer 13. In this manner, adsorbers 21 and 22 are alternately loaded with a first fraction of effluent nitrogen-enriched mixture and, in a manner analogous to the first stage, generate a second fraction of effluent mixture even ~ore greatly enriched than the first fraction. The control means 32a is therefore set to measure a lower proportion of oxygen content than control means 12a. For example, the second frac-tion may rise from 0.1 to 1% by volume of oxygen before the dis-charging operation is terminated by closing the inlet and outlet valves 27, 29, or 28, 30 respectively. The second fraction further contains traces of moisture, carbon dioxide, carbon monoxide, and argon.
The two stages operate with continuously driven gas and vacuum pumps and valves. The actuation of the pumps and valves is directly controlled by the control means 12a and 32a in convention-al manner. Instead of such control means, the valves and pumps can -be operated by a timing device which actuates the devices by a pre-" ~

~ -12- ~ ~

.: . :: . : - :
, : .. : .
.

1084gZl 1 selected time sequence program.
Since both stages have gasometers which have a variable volume capacity, both stages may be operated independently of each other. Since the effluent gas of the first stage is already nitro-gen-enriched, the required volume of M-coke in the second stage is considerably less. As diagrammatically shown in FIG. 2, adsorbers 21 and 22 are smaller than adsorbers 1 and 2 in order to illustrate that the entire volume of M-coke required in the second stage is ~-about 2/3 to 1/2 of the volume required in the first stage (e.g.
10 10 cubic meters of M-coke). The primary supply source upstream of the first stage should have a capacity which corresponds at least to the full capacity of the adsorbers of the first stage.
The following Examples are given in order to more fully illustrate the invention but are in no manner to be considered as limiting the scope thereof.
Example 1 Two adsorbers filled with M-coke, and each having a capa-s city of 0.5 m3, are alternately charged with air(21% by volume of oxygen, 450 ppm carbon dioxide, dew point 16C) in a cyclic opera-20 tion. Each adsorber is in turn evacuated and the time to achieve equilibrium between the adsorbers is 2.5 seconds. Pressure is built up in 4 seconds, and the time for the air to alternately pass through each adsorber is 53.5 seconds. Each adsorber is evacuated or desorb-ed in 57.5 seconds, and the adsorption pressure at the end of the adsorption phase is 770 torr. The effluent gaseous mixture at the end of the adsorption phase has an oxygen content of 3% by volume.
At the end of the evacuation of the adsorbers, a vacuum of 35 torr is obtained. For loading the adsorbers, 57.5 m of air at NTP is required. Flowing out of each adsorber, 25 m3 of nitrogen-enriched 30 mixture at NTP having an average content of 1.6% by volume of oxygen, . . .

'' .
.
.. . .

i0844Zl :

1 10 ppm carbon dioxide, and a dew point of - 35C is generated.
From the desorption or evacuation phase of operation, 32.5 m3 of gaseous mixture having an average oxygen content of 36.4% is re-moved. This evacuated mixture also contains traces of carbon di-oxide and moisture.
Example 2 Two adsorbers filled with M-coke, and each having a ca-pacity of 0.5 m , are alternately loaded and unloaded with air ; under an adsorption pressure of 2 bar. At the end of the evacua-tion or desorption phase, an end vacuum pressure of 57 torr is realized. For loading purposes, 112.6 m3 of air at NTP are required.
The predetermined end limting value of the effluent nitrogen-en-riched mixture is 1.5% by volume of oxygen. The capacity of the effluent mixture is 42.3 m3 at NTP with an average oxygen content of 0.9% by volume of oxygen, 13 ppm carbon dioxide, dew point - 36C. The capacity of the evacuated mixture is 76.8 m3 at NTP
with an average oxygen content of 32.9% by volume.
Example 3 ; Four adsorbers filled with M-coke, and each having a capacity of 0.25 m3 comprise two pairs of adsorber stages. The i first pair of adsorbers is alternately loaded and unloaded with ; air; the second pair is connected in series with the first pair so that the adsorbers of the second pair are alternately loaded and unloaded with the effluent nitrogen-enriched gaseous mixture of the first pair by means of a gasometer placed between the two pairs and serving as a storage place. Both pairs or stages are controlled in synchronism. Equilibrium time between adsorbers is 2.5 seconds.
Pressure is built up in 4 seconds, and the time that the adsorbers are loaded with gas is 53.5 seconds. Evacuation time is 57.5 seconds.
;` 30 The adsorption pressure at the end of the adsorption phase in the .' '~. .
:~ ., . .: .:
.. - . ,. . . . ~ ~ .

. 10~44Z~ :
. ~.,.
l first stage is 780 torr; the final adsorption pressure in the second stage is 765 torr. In both stages, a 34 torr vacuum is created at the end of the evacuation phase. The end limiting value for the oxygen content in the effluent gas of the first stage is ., .
; 7% by volume; the end value for the oxygen content of the effluent gas of the second stage is 0.4% by volume. For loading the first stage, 24.2 m of air at NTP are required. The residual gaseous mixture of the first stage amounts to 13.7 m at NTP with an oxygen ,~
content of 37.1%. The effluent enriched gaseous mixture of the second stage, that is the end product, has an oxygen content of 0.1% by volume and an overall volume of 10.5 m at NTP.
Example 4 Two adsorbers filled with M-coke, and each having a ca-pacity of 0.5 m3, are alternately charged and unloaded with an ~,~ oxygen-nitrogen mixture. For purposes of loading 50 m3 of boiler exhaust gases which contains 93% by volume of nitrogen and 7% by volume of oxygen are used. The effluent nitrogen-enriched gaseous ~, mixture has a capacity of 14 m at NTP with an average oxygen con-tent of 0.7% by volume. From the desorption step, 36 m3 of residual gases at NTP are removed with an oxygen content of 8.8% by volume.
The end limiting value of oxygen is 1.2% by volume. ;~
It will be understood that each of the elements described :,.~, . .
.~ above, or two or more together, may also find a useful application in other types of processes and arrangements differing from the ;~ types des~ribed above.

~ While the invention has been illustrated and described as ,r.;
embodied in a process and an arrangement for the conversion of a gaseous mixture, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present , , . - ~ , -`` 1084421 invention .

`: :

. . ~,. .

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for converting a gaseous mixture including an initial proportion of nitrogen and oxygen into an enriched gaseous mixture having an increased proportion of nitrogen, comprising the steps of passing a first stream of the gaseous mixture through an adsorber having a filling of a carbon-containing molecular sieve material, the first stream being conveyed at a space velocity of at least 0.01 and at most 0.04 volumes of gaseous mixture per volume of molecular sieve material per second over a time period of at least 40 and at most 80 seconds; adsorbing the oxygen prefer-entially from the admitted first stream so that a second stream discharges from said adsorber which contains a higher proportion of nitrogen and at most a predetermined proportion of oxygen; con-tinuing the discharge of said second stream from said adsorber until the oxygen in said second stream exceeds said predetermined pro-portion; thereupon terminating the discharge of said second stream;
and evacuating residual gaseous mixture from said adsorber.

2. A process as defined in claim 1, wherein the step of passing said first stream through said adsorber is performed at a pressure of at least 2 and at most 5 bar.

3. A process as defined in claim 1, wherein the step of adsorbing is performed by utilizing molecular sieve coke as the sieve material.

4. A process as defined in claim l, wherein the step of evacuating said residual gaseous stream from said adsorber is performed at a pressure of at most 70 and at least 20 torr.

5. A process as defined in claim 1, wherein the step of evacuating residual gaseous mixture is performed by removing the latter in a direction opposite to the flow of said first stream passing through said adsorber.

6. A process as defined in claim l, wherein the step of terminating the discharge of the second stream includes the step of monitoring the percentage by volume of the oxygen in said sec-ond stream.

7. A process as defined in claim l, and further compris-ing repeating said steps with an additional adsorber, said steps of passing said first stream, adsorbing, discharging said second stream and terminating the discharge thereof being performed with said first mentioned adsorber while said step of evacuating is simul-taneously performed with said additional adsorber and vice versa.

8. A process as defined in claim 7; and further compris-ing the step of connecting said first-mentioned and additional ad-sorber in parallel to form a first stage, said adsorbers having a common inlet and a common outlet.

9. A process as defined in claim 8; and further compris-ing the step of establishing equilibrium between said adsorbers of said first stage after one of said adsorbers has been evacuated.

10. A process as defined in claim 8, wherein the step of terminating the discharge of the second stream includes the step of monitoring the oxygen in said second stream being dis-charged from said common outlet, said monitoring step including measuring at least 0.5 and at most 4% by volume of the oxygen prior to terminating the discharge thereof.

11. A process as defined in claim 8; and further comprising the step of connecting another pair of adsorbers constituting a second stage in series with said first stage, said other pair having a common inlet which is connected to said common outlet of said first stage.

12. A process as defined in claim 11; and further comprising the step of feeding residual gaseous mixture of said second stage back to said common inlet of said first stage.

13. A process as defined in claim 11, wherein the step of terminating the discharge of the second stream includes the step of monitoring the oxygen in said second streams being re-spectively discharged from said common outlets of said first and second stages, said monitoring steps including measuring at least 4 and at most 10% by volume of the oxygen in said common outlet of said first stage prior to terminating the discharge thereof and measuring at least 0.1 and at most 5% by volume of the oxygen in said common outlet of said second stage prior to terminating the discharge thereof.

14. A process as defined in claim 11; and further comprising the step of dimensioning said other pair of adsorbers to contain about one half of the volumetric capacity of said first stage.

15. An arrangement for converting a gaseous mixture including an initial proportion of nitrogen and oxygen into an enriched gaseous mixture having an increased proportion of nitrogen, comprising adsorbing means including a filling of molecular sieve material; admitting means for passing a first stream of the gaseous mixture through said adsorbing means at a space velocity of at least 0.01 and at most 0.04 volumes of gaseous mixture per volume of molecular sieve material per second over a time period of at least 40 and at most 80 seconds, said adsorbing means adsorbing the oxygen preferentially from the admitted first stream so that a second stream discharges from said adsorbing means which contains a higher proportion of nit-rogen and at most a predetermined proportion of oxygen; means for continuing the discharge of said second stream from said ad-sorbing means until the oxygen in said second stream exceeds said predetermined proportion; means for thereupon terminating the discharge of said second stream; and means for evacuating residual gaseous mixture from said adsorbing means.

16. An arrangement as defined in claim 15, wherein said admitting means passes said first stream through said ad-sorbing means under a pressure of at least 2 and at most 5 bar.

17. An arrangement as defined in claim 15, wherein said material comprises molecular sieve coke.

18. An arrangement as defined in claim 15 wherein said evacuating means removes said residual gaseous stream from said adsorbing means at a pressure of at most 70 and at least 20 torr.

19. An arrangement as defined in claim 15, wherein said evacuating means removes said residual gaseous mixture in a direction opposite to the flow of said first stream passing through said adsorbing means.

20. An arrangement as defined in claim 15, wherein said terminating means includes means for monitoring the per-centage by volume of the oxygen in said second stream.

21. An arrangement as defined in claim 15, wherein said adsorbing means comprises a pair of adsorbers; said means for admitting, continuing the discharge of said second stream and for terminating the discharge thereof being performed with one of said adsorbers while said evacuating means is simultaneously evacuating the other of said adsorbers and vice versa.

22. An arrangement as defined in claim 21; and further comprising connecting means for connecting said adsorbers in parallel to form a first stage, said pair of adsorbers having a common inlet and a common outlet.

23. An arrangement as defined in claim 22; and further comprising equilibrium means for establishing equilibrium between said pair of adsorbers of said first stage after one of said adsorbers has been evacuated.

24. An arrangement as defined in claim 22, wherein said means for terminating the discharge of the second stream includes means for monitoring the oxygen in said second stream being discharged from said common outlet, said monitoring means including means for measuring at least 0.5 and at most 4% by volume of the oxygen prior to terminating the discharge thereof.

25. An arrangement as defined in claim 22; and further comprising additional connecting means for connecting another pair of adsorbers constituting a second stage in series with said first stage, said other pair having a common inlet which is connected to said common outlet of said first stage.

26. An arrangement as defined in claim 25; and further comprising means for feeding residual gaseous mixture of said second stage back to said common inlet of said first stage.

27. An arrangement as defined in claim 25, wherein said means for terminating the discharge of the second stream includes means for monitoring the oxygen in said second streams being respect-tively discharged from said common outlets of said first and second stages, said monitoring means including means for measuring at least 4 and at most 10% by volume of the oxygen in said common outlet of said first stage prior to terminating the discharge thereof and means for measuring at least 0.1 and at most 4% by volume of the oxygen in said common outlet of said second stage prior to terminating the discharge thereof.

28. An arrangement as defined in claim 25, wherein said other pair of adsorbers contains about one half of the volumetric capacity of said pair of adsorbers of said first stage.