Gas separation: processes – Solid sorption – Including reduction of pressure
Reexamination Certificate
2002-07-24
2003-09-09
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Solid sorption
Including reduction of pressure
C095S119000, C095S139000, C095S902000, C502S067000, C502S079000
Reexamination Certificate
active
06616732
ABSTRACT:
DESCRIPTION
1. Field of the Invention
The field of the invention is that of zeolite adsorbents for the purification of gas flows contaminated by carbon dioxide, in particular for the purification of air before N
2
/O
2
separation stages.
2. Background of the Invention
The production of pure gases, in particular N
2
and O
2
, from atmospheric air is an industrial operation carried out on a large scale and can make use either of cryogenic processes or of adsorption processes based on the principle of pressure swing adsorption (PSA), that of temperature swing adsorption (TSA) or a combination of the two (PTSA). Furthermore, many gases resulting from industrial processes comprise significant amounts of carbon dioxide, which is often advisable to remove.
The production of N
2
or O
2
from air requires a purification prior to the separation stage proper. This is because, in carrying out cryogenic processes, water or carbon dioxide present in the feed air can result In blockages of the equipment due to the fact that these operations are carried out at temperatures far below the freezing points of these impurities. In the adsorption processes, water and carbon dioxide are more strongly adsorbed than nitrogen and result, in the long run, in poisoning of the adsorbent, the consequence of which is a decrease in the expected life time.
In these processes, a zeolite of faujasite type (13X, the Si/Al ratio of which is greater than 1.2) is very generally employed to Provide for the removal of the carbon dioxide, the trapping of the water generally being carried out on an alumina bed place upstream of the bed of zeolite adsorbent. The regeneration of the adsorbent is of PTSA type, that is to say that a slight rise in temperature to approximately 150° C. is Combined with a reduction in pressure. During the stage, a fraction of purified gas produced which comprises N
2
, O
2
and approximately 1% by volume of argon, is conveyed to the beds of adsorbents for the purpose of regenerating them by desorbing CO
2
and H
2
O.
It is has been known for a long time that zeolite X is a better adsorbent for carbon dioxide than silica gel or active charcoal (U.S. Pat. No. 2,882,244). This patent also teaches that the selectivity with respect to various adsorbents varies with the temperature and the pressure.
U.S. Pat. No. 3,885,927 teaches that the adsorption of CO
2
can be carried out on a zeolite X exchanged to more than 90% with barium: under these conditions, the CO
2
content of the gas to be purified does not exceed 1000 ppm and the temperature can be between −40° C. and 50° C.
EP 294 588 teaches that a zeolite X exchanged with strontium, preferably to 70%, can also be used to carry out this purification.
The influence on CO
2
adsorption of the number of exchangeable cations on the zeolite has been studied by Barrer et al. in “Molecular Sieves” (Soc. Chem. Ind., London, 1968), p. 233, and by Coughlan et al. in “J. C. S. Faraday”, 1, 1975, 71, 1809. These studies show that the adsorption capacity of the zeolite for CO
2
increases as the Si/Al ratio decreases, up to a limit of 1.2, the lower range not having been explored.
Zeollte X, the Si/Al ratio of which is close to 1.25 and which is commonly used, is very selective for CO
2
, this selectivity increasing as the temperature falls. At temperatures in the region of ambient temperatures, the efficiency decreases greatly as a result of the competition with nitrogen, which is present in much greater molar proportions. The N
2
/CO
2
ratio in ambient air (with CO
2
~300/400 vpm) is of the order of 3000.
U.S. Pat. No. 5,531,808 discloses the teaching that CO
2
can be very efficiently adsorbed by means of a zeolite of X type having an Si/Al ratio of less than 1.15 and preferably equal to or very close to 1, referred to in the continuation of the account as zeolite LSX (Low Silica X). The advantage with respect to the conventional zeolite X (Si/Al>1.2) lies in the fact that it is no longer necessary to decrease the temperature at the decarbonatation stage by means of a cold unit as the efficiency of the zeolite is such that the selectivity for CO
2
with respect to nitrogen remains high, even up to 50° C.
The Applicant Company has found that the CO
2
adsorption capacity of a zeolite NaLSX increases with the degree of exchange with sodium but also that the increase in efficiency begins to reach a ceiling when degrees of exchange with sodium are achieved which are of the order of 90% for relatively high CO
2
partial pressures. On the other hand, the Applicant Company has shown, in WO 99/46031, that a very substantial increase in efficiency can be obtained for The decarbonatation under low CO
2
partial pressures, of the order of 2 mbar, with zeolites LSX having a degree of exchange with sodium (defined as the molar ratio of the sodium ions to the aluminium atoms in the tetrahedral position, the remainder being potassium) of at least 98%.
DESCRIPTION OF THE INVENTION
A subject-matter of the present invention is a novel family of zeolite adsorbents comprising a mixture of 5% to 95% and preferably of 50 to 90% by weight of at least one zeolite X with an Si/Al ratio equal to 1.25 and of 95 to 5% and preferably of 50 to 10% by weight of at least one zeolite LSX with Si/Al=1 for which
either at least 80% of the sum of the exchangeable cationic sites of all of the zeolites of the mixture are occupied by sodium cations,
or at least 70% of the sum of the exchangeable cationic sites of all of the zeolites of the mixture are occupied by strontium cations, it being possible for the remainder of the exchangeable sites to be occupied by cations chosen from Groups IA, IIA and IIIA of the Periodic Table or trivalent ions from the rare earth or lanthanide series.
Mention will very particularly be made, among preferred adsorbents, of those with an overall degree of exchange with sodium of greater than 90% and advantageously greater than 98%. Mention will also be made of mixtures of zeolite adsorbents as defined above exchanged to at least 70% with strontium, the majority of the remaining cationic sites of which are occupied by sodium ions.
These novel zeolite adsorbents can be provided in the form of a powder but they can also be agglomerated in the form of beads or extrudates with 5 to 25, preferably 5 to 20, parts by weight of an inert agglomeration binder (amorohous material with a cohesive nature which has very little tendency to adsorb carbon dioxide) per 100 parts by weight of mixture of zeolite X and zeolite TSX and of binder.
The agglomerates are particularly well suited to industrial uses insofar as their handling during loading and unloading operations in an industrial unit limits the pressure drops with respect to adsorbents in the pulverulent form.
Another subject-matter of the present invention is the process for the preparation of the adsorbents as defined above.
When the adsorbents are provided in the pulverulent form, they can be obtained by simple mixing of zeolite X and zeolite LSX powders.
Synthetic zeolite X and zeolite LSX powders generally exhibit a degree of exchange with sodium of 100% and 77% respectively, the remainder of the cationic sites being essentially potassium ions.
These powders can be subjected to one or more optional cationic exchanges, either separately (i.e. prior to the intimate mixing thereof) or subsequent to the mixing stage.
These cationic exchanges consist in bringing the powders into contact with saline solutions of the cation or cations which it is desired to partially or completely insert in the zeolite structure or structures in place of the exchangeable cations already present.
Degrees of exchange are generally obtained in the conventional manner by carrying out successive exchanges with the saline solution or solutions of cations.
When the powders comprise a mixture of cations, the exchange can be carried out either via a mixed solution comprising salts of several cations or by successive exchanges of individual saline solutions, in order to insert the cations one after the other.
When the adsorbents ar
Grandmougin Marie-Therese
Mayolet Francis
Rouet Jacques
Ceca S.A.
Smith , Gambrell & Russell, LLP
Spitzer Robert H.
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