Gas separation: processes – Solid sorption – Including reduction of pressure
Reexamination Certificate
2000-10-26
2003-02-11
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Solid sorption
Including reduction of pressure
C095S102000, C095S130000, C096S108000, C096S130000, C137S240000
Reexamination Certificate
active
06517609
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to an installation and to a method for the cyclic treatment of fluid by adsorption using, during part of the fluid treatment cycle, a pressure below atmospheric pressure, particularly a method of the VSA type for separating and/or purifying a gaseous stream essentially containing oxygen and nitrogen, such as air.
BACKGROUND OF THE INVENTION
It is known that gases and gaseous mixtures find many applications on an industrial plane. Thus, the gases in the air, such as oxygen and nitrogen in particular, are commonly used in various fields, such as, in particular, the field of electronics, the field of combustion, the field of medicine, the field of foodstuffs, the field of welding, etc.
At the present time, one of the techniques used for producing or purifying gases, particularly the gases in the air, is the so-called PSA (Pressure Swing Adsorption) technique, that is to say adsorption using variations in pressure.
According to this PSA technique, when the gaseous mixture that is to be separated is, for example, air and the component that is to be recovered is oxygen, for example, the oxygen is separated from the gaseous mixture that the ambient air constitutes using preferential adsorption of at least the nitrogen onto one or more materials which has a preference to adsorb at least the nitrogen and which are subjected to given pressure cycles in one or more separation zones, generally one or more adsorbers.
The oxygen which is adsorbed little or not at all is recovered at the outlet of the separation zone or zones with a purity generally higher than 90%.
In other words, the adsorption technique is commonly employed to separate the various constituents of a gaseous mixture, so as to produce directly, in the case of air, relatively pure oxygen and/or nitrogen or, as the case may be, to eliminate from it certain impurities, for example water vapour, carbon dioxide, oxides of nitrogen, any traces of hydrocarbons, etc, whose presence may detract from the correct operation of the items of equipment located downstream, such as air fractionation units operating cryogenically, for example.
More generally, a PSA method for separating a gaseous mixture comprising a first component which is preferentially adsorbed onto an adsorbent material and a second component which is adsorbed onto the adsorbent material less preferentially than the first component, with a view to producing the second component, comprises, cyclically;
a stage of preferential adsorption of at least the first component onto the adsorbent material, at an adsorption pressure known as the “high pressure”, accompanied by the recovery of at least some of the second component thus produced;
a stage of desorbing the first component thus trapped by the adsorbent, at a desorption pressure which is lower than the adsorption pressure and known as the “low pressure”;
a stage of recompressing the separation zone containing the adsorbent by swinging from the low pressure to the high pressure.
In other words, in industrial installations based on the adsorption technique, a gaseous stream that it is to be treated flows through one or more adsorbent beds. After a certain running time, the adsorbent materials are saturated and are therefore no longer able to fix further molecules of gas.
It is then necessary to regenerate the adsorbent bed or beds. To do this, there are a number of regeneration or desorption techniques that can be used, for example an increase in temperature, a lowering in pressure, elution using a sweeping gas, etc, it being possible for these various techniques to be used jointly or in succession, if necessary.
At the present time, numerous methods employ a regeneration pressure below atmospheric pressure, that is to say below 1 bar (10
5
PA). A technique such as this which constitutes a special case of the PSA method is commonly known as the VPSA (Vacuum Pressure Swing Adsorption) method or technique, or more simply known as a VSA (Vacuum Swing Adsorption) method, that is to say adsorption with a variation in pressure under vacuum.
By way of example, mention may be made of:
document U.S. Pat. No. 5,042,994 which describes a method of separating the gases of the air comprising a desorption pressure below atmospheric pressure intended for the joint production of high-purity nitrogen and medium-purity oxygen,
document U.S. Pat. No. 5,156,657 which relates to a PSA method for removing the water and CO
2
which are present in the stream of air, with use of a vacuum purge stage,
document U.S. Pat. No. 5,395,427 relating to a PSA method with two adsorption stages for the production of high-purity oxygen with the two adsorption stages placed under vacuum during a production cycle,
document U.S. Pat. No. 5,463,869 which attains to an incorporated adsorption/cryogenic distillation method for separating air, using a cycle of the VSA type during the separation, and
document U.S. Pat. No. 5,785,740 which relates to a transatmospheric production cycle, particularly for the production of oxygen.
It would therefore seem, as shown by these documents, that the use, in a PSA method, of stages which are placed under vacuum, that is to say at a pressure below atmospheric pressure, so as to obtain a VSA method, is known and conventional.
At the present time, materials of the zeolite type are the adsorbents most often used in installations for the separation or purification of gas using a method of the PSA or VSA type. Such zeolites are described, in particular, in documents EP-A-486384, EP-A-606848, EP-A-589391, EP-A-589406, EP-A-548755, EP-A-109063, EP-A-827771 and EP-A-760248.
However, these adsorbents, particularly zeolites, are highly sensitive to contaminants of any nature likely to contaminate and deactivate them, particularly the contaminants present in atmospheric air.
It is thus known that, in air treatment units, traces of water considerably reduce the performance of the zeolites used either for stopping the CO
2
prior to cryogenic separation of air, or for stopping the nitrogen when the desire is to produce oxygen.
This is why there is generally, at the inlet to adsorbers used for separating air, a layer of an adsorbent designed to remove water, which layer is followed by one or more layers of one or more adsorbents intended more specifically to stop the gaseous component that is to be eliminated, for example the CO
2
, the nitrogen or other gaseous components, according to the particular case, as explained hereinabove. Conventionally, the layer of adsorbent designed to eliminate the water is, for example, a layer of activated alumina, possibly doped, or silica gel, or even a zeolite which has a high affinity for water and can be regenerated under the normal operating conditions of the unit.
It is also known practice for periodic regeneration to be performed by raising the temperature and also for so-called exceptional regeneration operations to be performed at a temperature higher than the nominal regeneration temperature so as to eliminate traces of moisture which are likely to build up in the adsorbent, that is to say in the zeolite, over time, and in spite of the periodic regeneration phases.
In general, these regeneration operations are performed at a lower regeneration rate, thus making use of the installed power of the heater to obtain a higher outlet temperature, for example 250° C. instead of 150° C. in normal operation, and, after sweeping the bed of adsorbent, to achieve a very low residual moisture content allowing optimum use of the zeolite.
In the same field, document FR-A-9409162 discloses an anti-pollution device that makes it possible to avoid any ingress of moisture into zeolites during the phases in which the adsorption units are not in operation.
To do this, during stoppages, each adsorber is isolated and its inlet is placed in free communication with the atmosphere.
Furthermore, each adsorber, which is under vacuum during each stoppage, is brought back up to atmospheric pressure by introducing a practically dry gas into it, preferably using the production gas.
Thu
Belot Jean-Marc
Monereau Christian
L'Air Liquide Societe Anonyme A Directoire et Conseil de Su
Spitzer Robert H.
Young & Thompson
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