GB2336325A

GB2336325A - Producing nitrogen enriched air

Info

Publication number: GB2336325A
Application number: GB9908614A
Authority: GB
Inventors: Ravi Kumar; Noberto O Lemcoff; Debajyoti Chatterji
Original assignee: BOC Group Inc
Current assignee: Messer LLC
Priority date: 1998-04-17
Filing date: 1999-04-15
Publication date: 1999-10-20
Also published as: GB9908614D0

Abstract

Nitrogen-enriched air that is dry and free of carbon dioxide and which typically contains at least 5% oxygen is produced from pressurised air feed by pressure swing adsorption in an apparatus which contains, in series, a bed or layer 2 of moisture-selective adsorbent, a bed or layer 6 of oxygen-selective adsorbent and a bed or layer 10 of carbon dioxide-selective adsorbent.

Description

2336325 PRODUCTION OF NITROGEN-ENRICHED AIR This invention relates to the

production of nitrogen-enriched air, and more particularly to the production of a gaseous nitrogen product containing at least 5% by volume oxygen. and preferably not more than about 1 ppm carbon dioxide.

Carbon dioxide-free nitrogen is generally produced by cryogenic distillation of air when large quantities are required. However, due to the considerable capital and operating costs of cryogenic distillation plants, it is not commercially feasible to produce nitrogen by cryogenic distillation in small quantities, for example, in quantities not greater than about 200 tons per day. In such cases, it is generally preferable to produce the nitrogen product by alternate methods, such as, for example, by nitrogen PSA of air using an oxygen-selective adsorbent.

Atmospheric air generally contains 350 or more parts per million (ppm) of carbon dioxide. Since it is often desirable or necessary to produce carbon dioxide-free nitrogen, measures may need to be taken to ensure the removal of substantially all of the carbon dioxide from the air or the nkrogen product gas. This is commonly accomplished by reversing heat exchange, wfth condensation or freezing of the carbon dioxide or by adsorption, for example temperature swing adsorption (TSA) or pressure swing adsorption (PSA), using a carbon dioxide-selective adsorbent.

When nitrogen is produced from air by PSA, R is preferably to remove carbon dioxide from the air by PSA rather than reversing heat exchange or TSA, since this makes it possible to layer the carbon dioxide-selective adsorbent and the oxygen -selective adsorbent in a single adsorption vessel, which results in a reduction of the cost of the adsorption system and simplifies regeneration of the adsorbents. Thus, conventional commercial-scale nitrogen PSA processes generally include an air prepurification step, in which one or more layers of water vapour- and carbon dioxide-selective adsorbent are used to substantially reduce the concentration of water vapour and carbon dioxide in the air feed, and an oxygen-removal step, in which the prepurified air is passed through an oxygen-selective adsorbent. In many cases, such as when the nitrogen:rich product gas is to be liquefied, it is essential that the nitrogen PSA plant product gas be substantially moisture-free and substantially carbon dioxide-free, to prevent freezing of these components during the liquefaction process. This is accomplished by making the water vapour- and carbon dioxide-selective adsorbent layer(s) large enough to effect the necessary air prepurification.

Currently practised commercial PSA processes for the adsorption of oxygen from air use carbon molecular sieve (CMS) or zeolite 4A. The separation of oxygen from nitrogen using these adsorbents is a kinetic process, i. e. , it is rate dependent. Nitrogen and oxygen are both adsorbed by these adsorbents, but oxygen is adsorbed at a much faster rate; thus, oxygen can be separated from nitrogen using these adsorbents in processes with short air-adsorbent contact times. On the other hand, the adsorption of carbon dioxide from air by conventional carbon dioxide-selective adsorbents such as alumina or 13X zeolite is equilibrium-based. Commonly used oxygen -selective adsorbents, such as CIVIS, are capable of adsorbing carbon dioxide from air when the carbon dioxide is present at moderate concentrations, but these adsorbents are not very efficient for reducing the carbon dioxide to very low levels. Therefore, when it is desirable or necessary to remove substantially all water vapour and carbon dioxide from air by the above-described conventional nitrogen PSA processes, all carbon dioxide removal must be accomplished in the prepurification unit, and no advantage can be taken of the ability of oxygen-selective adsorbent to adsorb carbon dioxide.

When it is desired to produce a substantially moisture-free and carbon dioxide-free nitrogen product containing less than about 5% by volume of oxygen from atmospheric air, this can be accomplished by nitrogen PSA with preliminary PSA-based moisture and carbon dioxide adsorption and subsequent oxygen adsorption. If, however, it is desired to produce a nitrogen product gas containing 5% by volume or more of oxygen from atmospheric air using the same equipment, the air is passed through the adsorbent at a higher flow rate. Unfortunately, as the flow rate of air through the adsorption beds increases, not only does the percentage of oxygen in the product gas increase, but also the percentage of carbon dioxide in the product increases.

Traditionally, this problem can be resolved by, for example, changing the relative sizes of the carbon dioxide- and oxygen-selective adsorption beds. However, this procedure does not take advantage of the carbon dioxide-adsorbing ability of oxygen-selective adsorbents.

Convenient methods of producing nitrogen having very low levels of moisture and carbon dioxide but varying levels of oxygen from atmospheric air are continually sought. The present invention provides a flexible method and adsorption system which can be easily adapted to the production of nitrogen products having different oxygen concentrations. This is accomplished by positioning a carbon dioxide-selective adsorbent downstream of the oxygen-selective adsorbent, thereby taking advantage of the carbon dioxide-adsorbing ability of oxygen-selective adsorbents.

The invention relates to apparatus including a set of adsorption beds including, in general, a first bed of water vapour-selective adsorbent, a second bed of oxygenselective adsorbent and a third bed of carbon dioxideselective adsorbent and a method suitable for producing dry, substantially carbon dioxide-free nitrogen enriched air by subjecting air to pressure swing adsorption (PSA) in the set of adsorption beds.

According to the invention there is provided apparatus for producing nitrogenenriched air comprising:

(a) at least one set of adsorbent beds, each set comprising, in series, a bed of moisture-selective adsorbent, a bed of oxygen-selective adsorbent and a bed of carbon dioxide-selective adsorbent, the bed of oxygenselective adsorbent being in fluid communication with the bed of moistureselective adsorbent and the bed of carbon dioxideselective adsorbent; (b) means for introducing air at superatmospheric pressure into the bed of moisture-selective adsorbent; (c) means for removing nitrogen-enriched air from the bed of carbon dioxide-selective adsorbent; and (d) means for removing nitrogen-depleted air from the bed of moisture- selective adsorbent.

Preferably, each set of adsorbent beds is contained in an adsorption vessel, i. e., the set of beds is arranged as a series of layers in a single vessel.

Preferably, the bed of moisture-selective adsorbent comprises activated alumina, silica gel, zeolite 3A or combinations of these. Preferably, the bed of oxygenselective adsorbent comprises carbon molecular sieve, zeolite 4A or combinations of these. Preferably, the bed of carbon dioxide selective adsorbent comprises activated alumina, zeolite 13X, zeolite SA, activated carbon or combinations of these.

More preferably, the bed of moisture-selective adsorbent comprises activated alumina, silica gel, zeolite 3A or combinations of these; the bed of oxygen -selective adsorbent comprises carbon molecular sieve, zeolite 4A or combinations of these; and the bed of carbon dioxide selective adsorbent comphsing activated alumina, zeolite 13X, zeolite SA, activated carbon or combinations of these.

Most preferably, the moisture-selective adsorbent comprises activated alumina, the bed of oxygen -selective adsorbent comphses carbon molecular sieve, and the bed of carbon dioxide selective adsorbent comprises activated alumina.

Preferably, the apparatus according to the invention further comprises means for purging the at least one set of adsorbent beds with gas that is depleted in water vapour, carbon dioxide and oxygen.

Preferably, the apparatus according to the invention comprises a battery of sets of adsorbent beds arranged in parallel and adapted to be operated out of phase, each set of adsorbent beds comprising a bed of waterselective adsorbent, a bed of oxygen -selective adsorbent and a bed of carbon dioxide-selective adsorbent.

The invention also provides a method of producing nitrogen-enriched air by pressure swing adsorption (PSA) comprising the steps:

(a) passing air at superatmospheric pressure through at least one set of adsorption beds, each set of beds including, in series, a bed of moistureselective adsorbent, a bed of oxygen-seiective adsorbent and a bed of carbon dioxide-selective adsorbent, thereby producing nitrogen-enriched air that is substantially moisture-free, substantially carbon dioxidefree and oxygen-depleted; and (b) depressurizing the at least one set of adsorption beds, thereby regenerating the adsorbent in each of said beds.

The method according to the invention may be carried out in a system in which each set of adsorbent beds is contained in an adsorption vessel, i. e., the beds of each set are arranged as a series of layers in a single vessel.

Preferably, moisture is removed from the air by passing the air through a bed of moisture-selective adsorbent comprising activated alumina, silica gel, zeolite 3A or combinations of these. Preferably, the resulting dry air is depleted of oxygen by passing it through a bed of oxygen-selective adsorbent comprising carbon molecular sieve, zeolite 4A or combinations of thew. Carbon dioxide is removed from the resulting dry, oxygen- depleted air by passing it through a bed of carbon dioxide selective- adsorbent comprising activated alumina, zeolite 13X, zeoUte 5A, activated carbon or combinations of these.

More preferably, moisture, oxygen and carbon dioxide are removed from the passing it through a bed of moisture-selective adsorbent comprising activated alumina, silica gel, zeolite 3A or combinations of these; then through a bed of oxygen -selective adsorbent comprising carbon molecular sieve, zeolite 4A or combinations of these; and then through a bed of carbon dioxide selective adsorbent comprising activated alumina, zeolite 13X, zeolite 5A, activated carbon or combinations of these.

-6 Most preferably, the air is passed through a bed of moisture-selective adsorbent comprising activated alumina, then through a bed of oxygen selective adsorbent comprising carbon molecular sieve, and then through a bed of carbon dioxide selective adsorbent comprising activated alumina.

The method according to the invention is well suited to producing nitrogen-enriched air which contains about 5 to about 18% oxygen.

Preferably, each set of adsorbent beds is purged with purge gas that is depleted in water vapour, carbon dioxide and oxygen during or subsequent to its regeneration. If desired, the nitrogen-enriched air produced by the method according to the invention may be used as purge gas. Altematively, the n itrogen-en rich ed air may be liquefied, thereby producing nitrogen-enriched liquid air and nitrogen-enriched waste gas, and the nitrogen-enriched waste gas used to purge the beds.

Preferably, the method according to the invention is carded out in at least two sets of adsorption beds arranged in parallel and operated out of phase, such that at least one set of adsorption beds is undergoing step (a) while at least one other set of adsorption beds is undergoing step (b). Preferably, the method is carried out in at least one pair of sets of adsorption beds operated 1809 out of phase.

The method according to the invention is generally carded out at a temperature in the range of about 0 to about 1002 C, is preferably carried out at a temperature in the range of about 5 to about 452 C and is most preferably carried out at a temperature in the range of about 10 to about 402 C.

Step (a) of the method according to the invention is generally carried out at a pressure in the range of about 2 to about 50 bara (bar absolute), and is preferably carried out at a pressure in the range of about 3 to about 20 bara.

Step (b) of the method according to the invention is carried out at a pressure in the range of about 0.15 to about 5 bara, and is preferably carried out at a pressure in the range of about 0.2 to about 2 bara. During step (b), the set of adsorbent beds can be evacuated using vacuum means, when necessary or desirable.

As used herein, the term "nitrogen-endched aie' means air that contains less oxygen than does atmospheric air; the terms "carbon dioxide-free and "substantially carbon dioxide-free", when used to describe a gas, means that the gas contains no more than about 1 ppm carbon dioxide; and the terms "dry', "moisture-free", "substantially moisture-free", "water vapour-free" and "substantially water vapour-free", are variously used herein to describe a gas that contains no more than about 1 ppm water.

The method and apparatus according to the invention takes advantage of the carbon dioxide-adsorbing ability of oxygen-selective adsorbent by enabling the oxygenselective adsorbent of the system to remove carbon dioxide from the feed air. Since the carbon dioxide-selective adsorbent follows the oxygen -selective adsorbent in the system, some of the carbon dioxide-removing duty can be performed by the oxygenselective adsorbent before the air being processed reaches the carbon dioxideselective layer. Because of this, the total quantity of carbon dioxide-selective adsorbent necessary to effect carbon dioYide removal to a specified purity will be less than is required in conventional systems. Thus, the overall size of a system required to achieve the desired result can be reduced.

The following example shows one way that a currently used nitrogen PSA system comprising a preliminary layer of activated alumina for adsorbing both water vapour and carbon dioxide and a main layer of CMS as the oxygen-selective adsorbent, can be modified to provide the benefits according to the invention. Assume that the preliminary layer of activated alumina is just adequate to remove substantially all water vapour and carbon dioxide from the air feed. Next, reduce the size of the activated alumina layer to the extent that a moderate amount of carbon dioxide ( for example, more than about 10 ppm) passes through the activated alumina layer. When air is now passed through the system under the same PSA conditions as before, the product gas exiting the layer of CIVIS will contain more carbon dioxide than it did before activated alumina was removed from the preliminary layer. Next, insert into the system, downstream of the layer of CIVIS, a layer of activated alumina of sufficient size to just reduce the concentration of carbon dioxide in the product gas to the level that it was originally, i. e., before activated alumina was removed from the preliminary layer. It will now be observed that when the system is operated under the same PSA conditions as before, the total quantity of activated alumina in the preliminary layer and the downstream layer needed to achieve the same results is less than that used in the system as originally operated. Reduction of the total amount of activated alumina required is possible because some of the carbon dioxide will be removed from the air as it passes through the layer of CIVIS. Incidentally, reducing the size of the preliminary layer will not affect water vapour removal, since water vapour is more strongly adsorbed by activated alumina than is carbon dioxide; thus, the upstream end of the preliminary layer of activated alumina will adsorb substantially all water vapour in the feed air.

This example can also be practised by using as the preliminary layer a water vapourselective adsorbent that does not adsorb carbon dioxide, such as, for example, zeolite 3A. The CIVIS and the downstream layer of carbon dioxide-selective adsorbent can then easily remove all carbon dioxide from the product stream.

The method and apparatus according to the above example can be used to produce nitrogen-enriched air that is substantially free of carbon dioxide and water vapour and which contains amounts of oxygen varying from the low ppm range to just under the amount of oxygen normally contained in atmospheric air. The invention is particularly useful for producing substantially carbon dioxide-free and substantially moisture- free nitrogen-enriched air that contains about 5 to about 18 mole percent of oxygen. The oxygen content of the product gas can be increased by increasing the flow rate of air through the adsorption system.

The adsorption system used in the above example can comprise a single adsorption unit or a battery of adsorption units operated in phase, or a plurality of adsorption units or batteries of adsorption units operated out of phase, whichever is desired. When a system comprising a single adsorption unit or a battery of units all of which are operated in phase is used, the adsorption step must be periodically stopped to permit regeneration of the adsorbent bed(s), whereas when a plurality of adsorption units are employed in parallel and operated out of phase, one or more units can be in the adsorption stage, adsorbing the components that it is desired to remove from the air stream fed into the system, while one or more other units are undergoing regeneration to desorb the adsorbed gas components. Operation of the adsorption systems according to the invention is preferably cyclical. In preferred adsorption systems, cycles are repeatedly carried out in a manner such that production of the desired product gas is substantially continuous. In preferred embodiments according to the invention, the adsorption processes are carried out in plural or multi-bed systems which comprise a battery of adsorption vessels arranged in parallel and operated out of phase to provide continuous production of nonadsorbed and desorbed components. In the particular system illustrated in the drawing, the system comprises a twin bed system designed to be operated 1802 out of phase, so that one bed is in the adsorption mode while the other bed is undergoing regeneration.

The method and apparatus according to the invention will now be described by way of example with reference to Figure 1 of the accompanying drawing which is a schematic diagram of an air separation unit.

Referring to the drawing, the illustrated apparatus includes, as major equipment units, adsorption vessels A and B to which are connected various process lines. Vessels A and B are substantially identical and contain water vapour-selective layers or beds 2 and 4, respectively, oxygen selective layers or beds 6 and 8, respectively, and carbon dioxideselective layers or beds 10 and 12, respectively. Layers 2 and 4 can be any water vapour-selective adsorbent or combination of adsorbents, such as activated alumina, silica gel or zeolite 3A, all of which have a strong affinity for moisture but do adsorb significant amounts of oxygen or Mrogen. Activated alumina is an especially preferred adsorbent for layers 2 and 4, because of its ability to adsorb significant quantities of carbon dioxide, in addition to water vapour.

Layers 6 and 8 are positioned downstream of layers 2 and 4, respectively, and comprise oxygen -selective adsorbents. The preferred adsorbents for use in layers 6 and 8 are CMS and zeolite 4A, and the most preferred adsorbent is CMS.

Carbon dioxide-selective layers are positioned downstream of layers 6 and 8 in vessels A and B. these adsorbents may be any substances which more strongly adsorb carbon dioxide than oxygen and nitrogen. Typical carbon dioxide-seiective adsorbents suitable for use as layers 10 and 12 are activated alumina, zeolite 13X, zeolite 5A, activated carbon. The preferred carbon dioxide-selective adsorbent is activated alumina, including alkali-washed activated alumina, such as the activated aluminas sold by Alcoa Industrial Chemicals under the trademark Selexsorbo COS.

The system illustrated in the drawing includes air feed line 14, which is connected to feed gas manifold 16. Manifold 16 is connected via valves 18 and 20 to adsorption vessel feed lines 22 and 24, respectively. Lines 22 and 24 are connected to the inlets of vessels A and B, respectively. Vent gas manifold 26 is connected to lines 22 and 24 by valves 28 and 30, respectively. Vent line 32 joins manifold 26 at a point between valves 28 and 30.

Connected to the nonadsorbed outlet end of vessels A and B are discharge lines 34 and 36, respectively. Cross-connection line 38, fitted with valve 40 joins lines 34 and 36. Lines 34 and 36 are connected to product gas manifold 42, which is fitted with valves 44 and 46. Product line 48 joins manifold 42 at a point between valves 44 and 46. Purge manifold 50 is connected to lines 34 and 36 via valves 52 and 54, respectively. Purge gas feed line 56 joins manifold 50 at a point between valves 52 and 54.

The method according to the invention will be described as it applies to the production of nitrogen-enriched air using an adsorption cycle which includes a feed pressurization step, a production step, a bed equalisation step and a bed regeneration step. Layers 2 and 4 are activated alumina, layers 6 and 8 are CMS and layers 10 and 12 are activated alumina.

In the following description, vessel A will be initially in the adsorption mode and vessel B initially in the bed regeneration mode. During the first phase of the process, valves 18, 30 and 54 are open and all other valves are closed. Atmospheric air is compressed, cooled and filtered (in equipment not shown) and introduced into vessel A through line 14, manifold 16 and fine 22. When the pressure in vessel A reaches the desired adsorption pressure, valve 44 is opened and air flows through vessel A at the adsorption pressure. As the feed air passes cocurrently (in the direction from the feed entrance towards the nonadsorbed gas outlet of the vessels) through layer 2, substantially all water vapour and some of the carbon dioxide contained in the air are adsorbed. The dried air passes out of layer 2 and next passes through layer 6, which removes oxygen and some carbon dioxide from the air. The dry oxygen- depleted air leaves layer 6 and next passes through layer 10, and as it does so, substantially all of the remaining carbon dioxide is removed from the air. The dry, carbon dioxide-free nitrogen-enriched air leaves vessel A via its nonadsorbed gas outlet and then passes through line 34, manifold 42 and line 48 and is sent to product storage or a downstream application.

Meanwhile, regeneration of the adsorbents in vessel B is underway. During regeneration, vessel B depressurizes as gas contained in this vessel flows therefrom through line 24, manifold 26 and line 32. During regeneration, low pressure purge gas that is substantially free of water vapour and carbon dioxide, and which is depleted in oxygen enters the system through line 56. As noted above, the regeneration gas may be a nitrogen-enfiched product gas produced in vessel A in the current half cycle or in vessels A and/or B in earlier cycles, or it may be a waste gas from a downstream nitrogen-enriched air liquefying plant. The regeneration gas passes through manifold 50 and line 36, and then flows countercurreffily (in the direction opposite to the flow of feed gas through the vessels) through the layers of adsorbent in vessel B. As it passes through layer 12, it desorbs carbon dioxide contained in this layer. The purge gas then flows through layer 8, where it desorbs oxygen and carbon dioxide from the CMS adsorbent. The purge gas next passes through layer 4, and as it does so, it desorbs water vapour therefrom. The purge gas and the gas components desorbed from the adsorbents in vessel B, pass out of vessel B and leave the system through line 24, manifold 26 and line 32.

As the PSA adsorption step in vessel A proceeds, the adsorption fronts in layers 2, 6 and 10 advance toward the outlet ends of these layers. When a selected front reaches a predetermined point in one of these layers, the adsorption step of the first half-cycle of the process is terminated. This is accomplished by closing valves 18, 30, 44 and 54. The particular front whose advance determines the end of the adsorption cycle will depend upon the relative lengths of the beds in vessels A and B. In any event, it is preferred to terminate the adsorption step prior to substantial breakthrough of any of the three adsorbed components from vessel A. At or prior to this point in the cycle, vessel B will have completed its bed regenerati on step.

The next step of the cycle is bed equalisation. To initiate this step, valve 40 is opened and all other valves remain in the closed position. Gas from vessel A now passes cocurrently out of vessel A, through lines 34, 38 and 36 and countercurrently into vessel B. The flow of gas continues until the pressure in the two vessels is the same, or nearly is the same. This step is then terminated by closing valve 40. This ends the first phase of the adsorption cycle.

During the second phase vessel B, which has completed its adsorbent regeneration phase, is put into adsorption service and the adsorbents in vessel A are regenerated. The changeover is accomplished by opening valves 20, 28 and 52. All other valves remain closed. Compressed and cooled atmospheric air is now introduced into vessel B through line 14, manifold 16 and line 24. When the pressure in vessel B reaches the desired adsorption pressure, valve 46 is opened and air flows through vessel B at the adsorption pressure. As the feed air passes cocurrently through vessel B, substantially all water vapour and carbon dioxide and the desired amount of oxygen are removed therefrom, in the manner described above. The dry, carbon dioxide-free nitrogen-endched air leaves vessel B via its nonadsorbed gas outlet and then passes through line 36, manifold 42 and line 48 and is sent to product storage or a downstream application.

Meanwhile, regeneration of the adsorbents in vessel A is underway. During regeneration, vessel A depressurizes as gas contained in this vessel flows therefrom through line 22, manifold 26 and line 32. During regeneration, low pressure purge gas that is substantially free of water vapour and carbon dioxide, and which is depleted in oxygen enters the system through line 56. The regeneration gas passes through manifold 50 and line 34, and then flows countercurrently through the layers of adsorbent vessel A. As it passes through vessel A, it desorbs carbon dioxide, oxygen and water vapour from the adsorbent in this vessel. The purge gas and desorbed contaminants pass out of vessel A through line 22, manifold 26 and line 32.

As the PSA adsorption step in vessel B proceeds, the adsorption fronts in layers 4, 8 and 12 advance toward the outlet ends of these layers. When the selected front reaches the predetermined point in the determining layer, the adsorption step of the second half-cycle of the process is terminated. This is accomplished by closing valves 20, 28, 46 and 52. At this point, vessel A will have completed its bed regeneration step.

The final step of the cycle is bed equalisation with gas flowing from vessel B to vessel A. To initiate this step, valve 40 is opened and all other valves remain in the closed position. Gas from vessel B now passes cocurrently out of vessel B, through lines 36, 38 and 34 and countercurrently into vessel A. The flow of gas continues until the pressure in the two vessels is the same, or nearly is the same. This step is then terminated by closing valve 40. This ends the second phase of the adsorption cycle. The process continues with the next cycle, wherein vessel A is again in adsorption service and the adsorbents in vessel B are being regenerated.

The adsorption cycle used in the process according to the invention is not critical to the success according to the invention. The cycle may exclude some of the abovementioned steps, or it may include steps other than those described above. For example, it may exclude the bed equalisation step or, when the system comprises more than two adsorption vessels arranged in parallel, it may have multiple equalisation steps. Furthermore, it mayinclude as a pressurization step in place of or following the bed equalisation step, a product backfill step.

The duration of the complete cycle of the pressure swing adsorption process can vary, depending upon the size of the various adsorbent beds in the system, the particular adsorbents used and the operating temperature and pressures. In general, for a two-bed system it is usually in the range of about 60 to about 1200 seconds, and is preferably in the range of about 180 to about 600 seconds.

It will be appreciated that it is within the scope of the present invention to utilize conventional equipment to monitor and automatically regulate the flow of gases within the system so that it can be fully automated to run continuously in an efficient manner.

The invention is further illustrated by the following example in which, unless otherwise indicated, parts, percentages and ratios are on a volume basis.

EXAMPLE

An adsorbent apparatus similar to the apparatus illustrated in the drawing is used to produce liquid air. The adsorption vessels used in the system have an intemal diameter of 6 feet and a total height of 8.7 feet and are packed with a bottom layer of activated alumina 0.4 feet high, a middle layer of CIVIS 6.3 feet high and a top layer of activated alumina 2 feet high. Atmosphedc air is compressed to 9.8 atmospheres and subjected to PSA in the above system using a cycle which includes the following steps and durations: production - 101 secs; bed equalisation - 4 secs; countercurrent depressurisation - 15 secs; countercurrent purge 101 secs; and feed repressudsation 15 secs. The purge step is conducted at a pressure of 1.2 atmospheres using waste gas from the liquefaction plant. Liquid air product containing 17% oxygen and 1 ppm carbon dioxide is produced at the rate of 190 tons per day.

Although the invention has been deschbed with particular reference to specific equipment arrangements, PSA cycles and specific experiments, these features are merely exemplary according to the invention and variations are contemplated. For example, the adsorption system may include layers of adsorbent which remove hydrocarbons and layers of catalytic agents to remove carbon monoxide and hydrogen from the air.

Claims

1. Apparatus for producing n itrogen-en rich ed air comprising:

(a) at least one set of adsorbent beds, each set comprising, in series, a bed of moisture-selective adsorbent, a bed of oxygen -selective adsorbent and a bed of carbon dioxide-seiective adsorbent, said bed of oxygenselective adsorbent being in fluid communication with said bed of moisture-selective adsorbent and said bed of carbon dioxideselective adsorbent; (b) means for introducing air at superatmospheric pressure into said bed of moisture-selective adsorbent; (c) means for removing nitrogen-enriched air from said bed of carbon dioxide-selective adsorbent; and (d) means for removing nitrogen-depleted air from said bed of moisture- selective adsorbent.

2. Apparatus as claimed in claim 1, wherein each set of adsorbent beds is contained in an adsorption vessel.

3. Apparatus as claimed in claim 1 or claim 2, wherein said bed of moisture- selective adsorbent comprises activated alumina, silica gel, zeolite 3A or combinations of these.

Apparatus as claimed in any one of the preceding claims, wherein said bed of oxygen -selective adsorbent comprises carbon molecular sieve, zeolite 4A or combinations of these.

5.

Apparatus as claimed in any one of the preceding claims, wherein said bed of carbon dioxide selective adsorbent comprises activated alumina, zeolite 13X, zeolite 5A, activated carbon or combinations of these.

6. Apparatus as claimed in any one of the preceding claims, further comprising means for purging said at least one set of adsorbent beds with gas that is depleted in water vapour, carbon dioxide and oxygen.

Apparatus as claimed in any one of the preceding claims, comprising a battery of said sets of adsorbent beds arranged in parallel and adapted to be operated out of phase.

8. A method of producing nitrogen-enriched air by pressure swing adsorption comprising the steps:

(a) passing air at superatmospheric pressure through at least one set of adsorption beds, each set of beds including, in series, a bed of moistureselective adsorbent, a bed of oxygen-selective adsorbent and a bed of carbon dioxide-selective adsorbent, thereby producing nitrogen-enriched air that is substantially moisture-free, substantially carbon dioxidefree and oxygen-depleted; and (b) depressurizing said at least one set of adsorption beds, thereby regenerating the adsorbent in each of said beds.

9. A method as claimed in claim 8, wherein each set of adsorption beds is contained in an adsorption vessel.

10. A method as claimed in claim 8 or claim 9, wherein said bed of moistureselective adsorbent comphses activated alumina, silica gel, zeolite 3A or combinations of these.

11. A method as claimed in any one of claims 8 to 10, wherein said bed of oxygen-selective adsorbent comprises carbon molecular sieve, zeolite 4A or combinations of these.

12. A method as claimed in any one of claims 8 to 11, wherein said bed of carbon dioxide selective adsorbent comphses activated alumina, zeolite 13X, zeolite SA, activated carbon or combinations of these.

13. A method as claimed in any one of claims 8 to 12, further comprising purging each set of adsorbent beds with purge gas that is depleted in water vapour, carbon dioxide and oxygen dudng or subsequent to its regeneration.

14. A method as claimed in any one of claims 8 to 13, carried out in at least two sets of adsorption beds arranged in parallel and operated out of phase, such that at least one set of adsorption beds is undergoing step (a) while at least one other set of adsorption beds is undergoing step (b).

A method as claimed in claim 14, carried out in at least one pair of sets of adsorption beds operated 1802 out of phase.

16. A method as claimed in any one of claims 8 to 15, carried out at a temperature in the range of 0 to 500 C.

17. A method as claimed in any one of claims 8 to 16, wherein step (a) is carried out at a pressure in the range of 2 to 50 bara.

18. A method as claimed in any one of claims 8 to 17, wherein step (b) is carried out at a pressure in the range of 0. 15 to 5 bara.

19. A method as claimed in any one of claims 8 to 18, wherein said nitrogenenriched air is used as purge gas.

20. A method as claimed in any one of claims 8 to 18, further comprising liquefying said nitrogen-enfiched air, thereby producing liquid nitrogenenriched air and nitrogen-enriched waste gas, and using said nitrogenenriched waste gas as purge gas.

21. A method as claimed in any one of claims 8 to 20, wherein the nitrogenenriched air contains from 5 to 18% by volume of oxygen.

22. A method of producing nitrogen-enriched air by pressure swing adsorption substantially as herein described with reference to the accompanying drawing.

23. Apparatus for producing n itrogen-en rich ed air substantially as herein described with reference to the accompanying drawing.