Gas separation: processes – Solid sorption – Including reduction of pressure
Reexamination Certificate
2000-07-31
2002-04-30
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Solid sorption
Including reduction of pressure
C095S103000, C095S115000, C095S117000, C095S119000, C095S129000, C095S142000, C095S143000, C095S900000
Reexamination Certificate
active
06379430
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the use of an activated alumina having a specific surface area of less than 299 m
2
/g, as adsorbent intended for removing the CO
2
and/or H
2
O present in a gas stream, particularly a stream of air or of a synthesis gas, the adsorbent being employed in an adsorption process of the PSA, TSA or similar type.
BACKGROUND OF THE INVENTION
Before being able to be used for industrial purposes, many gases have to be pretreated, in particular purified, in order to remove therefrom all or some of the impurities that may be present, in varying amounts, in them.
In this regard, mention may especially be made of the atmospheric air and synthesis gases. For example, it is known that the atmospheric air contains impurities of the carbon dioxide (CO
2
) and/or water vapour (H
2
O) type which have to be removed before any cryogenic separation of the air, that is to say prior to the air being introduced into the heat exchangers of the cold box of an air separation unit.
This is because, in the absence of such an air pretreatment, condensation and solidification of these impurities in the form of ice may occur when cooling the air to a cryogenic temperature, which may result in problems of the cryogenic separation unit or equipment, especially the heat exchangers, distillation columns, etc., becoming blocked and, consequently, damaging this unit or equipment.
By way of indication, the CO
2
content of the atmospheric air is usually between 200 vpm and 600 vpm, typically about 350 to 450 vpm.
To avoid these problems, it is common practice to pretreat the air that has to be cryogenically separated prior to this cryogenic separation. This pretreatment of the air is generally called “front” purification since it is carried out upstream of the cryogenic separation unit.
This pretreatment of the air is carried out by the TSA (Temperature Swing Adsorption) process or by the PSA (Pressure Swing Adsorption) process, depending on the case.
Conventionally, a TSA process cycle comprises the following steps:
a) purification of the air by adsorption of the impurities at a superatmospheric pressure, e.g. greater than 1 bar;
b) depressurization of the adsorber down to atmospheric pressure or below atmospheric pressure, e.g. at about or less than 1 bar;
c) complete regeneration of the adsorbent at approximately atmospheric pressure using a hot gas, especially by the residual gases or waste gases, typically impure nitrogen coming from an air separation unit and warmed by means of one or more heat exchangers;
d) cooling of the adsorbent, especially by continuing to introduce the waste gas coming from the air separation unit, but not warmed, into the adsorbent;
e) repressurization of the adsorber with purified air coming, for example, from another adsorber which is in production phase.
Similarly, as regards a PSA process cycle, this comprises substantially the same steps a), b) and e), but is distinguished from a TSA process by the waste gas or gases not being warmed during the regeneration step (step c)), and therefore by the absence of step d), and, in general, by a shorter cycle time than in the TSA process.
Preferably, the air pretreatment devices comprise at least two adsorbers, operating in parallel, that is to say operating alternately, one of the adsorbers being in production phase while the other is in regeneration phase.
Such TSA or PSA air purification processes are described, for instance, in documents U.S. Pat. No. 3,738,084, U.S. Pat. No. 5,531,808, U.S. Pat. No. 5,587,003 and U.S. Pat. No. 4,233,038.
Furthermore, it is known that, when adsorbent particles are used for prepurifying the air before separating it by cryogenic distillation, it is common practice to adjust (by water cooling) the temperature of the compressed air from a temperature normally of at least 80° C., or even higher, down to the ambient temperature and then to precool the air before it is introduced into the adsorber or adsorbers; this is usually carried out by a refrigeration unit taking the air from the ambient temperature down to a temperature below the ambient temperature. This is, moreover, clearly explained by the document
Industrial Gases
&
Cryogenics Today, IOMA Broadcaster, Air Purification for cryogenic air separation units
, Jan.-Feb. 1984, p. 15 et seq. or by document EP-A-438,282.
This is because it is recommended to precool the air before subjecting it to an adsorption separation step since, as is known by those skilled in the art, the lower the adsorption temperature the more effective the adsorption of the impurities. In other words, the effectiveness of the air prepurification is markedly favoured in the case of low temperatures, that is to say temperatures close to 5° C.
Next, after it has passed through the purification zone, that is to say through the adsorber or adsorbers, the air stripped of all or some of its harmful impurities, particularly the CO
2
and H
2
O impurities, is conventionally cooled to a cryogenic temperature, i.e. generally to a temperature of less than approximately −120° C., before being taken to the cryogenic distillation unit and introduced into one or more distillation columns for the purpose of being separated therein, for recovering nitrogen, oxygen and/or argon.
Similarly, the synthesis gases or syngases usually contain impurities that have to be removed prior to any cryogenic separation of the mixture of hydrogen (H
2
) and carbon monoxide (CO) of which the syngas is composed. For example, the synthesis gases coming from an amine scrubbing unit conventionally contain from 1 to 600 vpm, preferably from 10 to 500 vpm, of CO
2
-type impurities.
In general, it is normal to use an adsorbent of the zeolite type, for example an X or A zeolite, to remove the CO
2
impurities and to employ an adsorbent of the alumina, zeolite or silica-gel type in order to adsorb the water vapour (H
2
O) contained in a gas to be purified. For example, reference may be made to document EP-A-718,024 which describes a process for removing the CO
2
contained in a stream of air at ambient temperature by means of an adsorbent of the X zeolite type having an Si/Al ratio of less than 1.15, preferably about 1. It should be noted that a similar process is disclosed by document EP-A-284,850.
Although certain documents refer to the possibility of using alumina for removing CO
2
, alumina is recommended more for removing water since it has a good mechanical strength, a high water adsorptivity and a high water affinity, it causes only very little coadsorption of the other gases and, in addition, is easy to regenerate.
On the other hand, alumina is reputed to be a less effective adsorbent for removing CO
2
and, in order to overcome this lack of effectiveness, it is common practice to follow the alumina bed with one or more beds of another adsorbent, such as a zeolite, as described, for example, by documents U.S. Pat. No. 5,137,548, U.S. Pat. No. 4,541,851 and U.S. Pat. No. 5,689,974.
In order to try to solve this problem, certain documents suggest the use of modified aluminas, particularly aluminas doped with metal cations, such as sodium and/or potassium cations.
For example, document EP-A-766,991 teaches a PSA (Pressure Swing Adsorption)-type process for adsorbing the CO
2
present in a gas, in which process the gas is brought into contact with particles of an impregnated alumina, which alumina is obtained by impregnating it with a basic solution having a pH of greater than 9, and then dried, but without thereafter being calcined. In other words, the particles of activated alumina are formed and then impregnated with a suitable solution and finally dried at a temperature of about 120° C. In this case, the impregnation treatment therefore takes place on already formed particles of activated alumina.
Alternatively, document U.S. Pat. No. 4,433,981 describes a process for removing the CO
2
from a gas stream by bringing it into contact, at a temperature ranging up to 300° C., with alumina particles prepared by impregnating a porous alumina with a solution o
L'Air Liquide, Societe Anonyme a Directoire et Conseil de S
Spitzer Robert H.
Young & Thompson
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