Gas separation: processes – Solid sorption – Including reduction of pressure
Reexamination Certificate
1998-07-17
2003-11-25
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Solid sorption
Including reduction of pressure
C095S130000, C095S140000, C095S902000, C502S068000, C502S079000, C502S080000, C502S085000
Reexamination Certificate
active
06652626
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to adsorbents for the non-cryogenic separation of industrial gases and more particularly for the separation of nitrogen by adsorption in gas flows, such as air, and the purification of hydrogen by adsorption of CO and/or N
2
.
BACKGROUND OF THE INVENTION
The separation of nitrogen from gas mixtures is the basis for several non-cryogenic industrial processes, among which the production of oxygen from air by a PSA process (Pressure Swing Adsorption: adsorption under modulated pressure) is one of the most important. In this application, air is compressed and conveyed through an adsorbing column having a marked preference for the nitrogen molecule. Oxygen, at approximately 94-95%, and argon are thus produced during the adsorption cycle. After a certain period of time, the column is reduced in pressure and then maintained at the low pressure, during which period the nitrogen is desorbed. Recompression is subsequently provided by means of a portion of the oxygen produced and the cycle continues. The advantage of this process with respect to cryogenic processes is the greater simplicity of the plants and their greater ease of maintenance. The quality of adsorbent used is the key to an efficient and competitive process. The performance of the adsorbent is related to several factors, among which may be mentioned: the nitrogen adsorption capacity, which will be determining in calculating the ideal column sizes, the selectivity between nitrogen and oxygen, which will condition the production yield (ratio between the oxygen produced and oxygen entered), and the adsorption kinetics, which will enable the cycle times to be optimized and the productivity of the plant to be improved.
PRIOR ART
The use of molecular sieves as selective adsorbents for nitrogen is a well-known technology. The family of zeolites having a pore diameter of at least 0.4 nm (4 Å) has been provided by McRobbie in U.S. Pat. No. 3,140,931 for the separation of oxygen
itrogen mixtures. The comparative performance of the various ionic forms of zeolites was examined by McKee in U.S. Pat. No. 3,140,933, in particular that of the lithium form presented as the most efficient in terms of selectivity. The advantage of this zeolite has remained limited due to the difficulty in exchanging the faujasite structure into a lithium form. It is known from Chao (U.S. Pat. No. 4,859,217) that the potentialities of such an adsorbent are fully displayed at high degrees of exchange, typically greater than 88%.
Exchange by means of the calcium ion being easier, efforts have been directed towards calcium-exchanged faujasite structures or towards faujasite structures exchanged by means of two divalent ions, calcium plus strontium (see, for example, Patents U.S. Pat. No. 4,544,378 from Coe and U.S. Pat. No. 4,455,736 from Sircar). In the disclosure by Coe, it is indicated that the state of hydroxylation of the exchanged ions is particularly important with respect to the performances and that this state can be obtained by a specific thermal activation.
The purification of hydrogen by adsorption is also an industrial process of great importance. It relates to the recovery of hydrogen from a mixture of several constituents originating from the catalytic reforming of natural gas, plants for the production of ammonia or ethylene units. The principle of pressure swing adsorption (PSA) is applied in order to obtain hydrogen of high purity. The impurities contained in hydrogen are generally composed of CO
2
, NH
3
, N
2
, CO, CH
4
and C
1
-C
4
hydrocarbons, at contents ranging from a few ppm to a few percent. In practice, use is made of a bed composed of alumina or of silica gel, for retaining water, of active charcoal, for retaining CO
2
and CH
4
, and of molecular sieve, for trapping CO and N
2
.
The first industrial plant, which dates from 1967, is disclosed by UCC in U.S. Pat. No. 3,430,418 and, until now, the zeolitic adsorbent used is a molecular sieve of 5A type.
L'Air Liquide has disclosed, in WO 97/45363, a process for the separation of hydrogen contained in a gas mixture contaminated by CO and containing at least one other impurity chosen from the group consisting of CO
2
and saturated or unsaturated, linear, branched or cyclic, C
1
-C
8
hydrocarbons, as well as nitrogen, which comprises bringing the gas mixture to be purified into contact with the bed of a first adsorbent selective for at least carbon dioxide and C
1
-C
8
hydrocarbons, then with the bed of an adsorbent specific to nitrogen (capable of adsorbing most of the nitrogen present in the gas mixture), such as zeolite 5A, and, finally, the bed of a third adsorbent which is a zeolite of the faujasite type exchanged to at least 80% with lithium and in which the Si/Al ratio is less than 1.5, in order to remove the carbon monoxide.
In the light of the importance of non-cryogenic processes for the separation of industrial gases employing molecular sieves, the discovery of increasingly high performance adsorbents is an important objective, both for companies which produce gases and for companies which supply molecular sieves.
SUMMARY OF THE INVENTION
The present invention deals with agglomerated adsorbents. Conventionally, agglomerated adsorbents are composed of a zeolite powder, which constitutes the active component, and of a binder intended to ensure the cohesion of the crystals in the form of grains. This binder has no adsorbing property, its function being to give the grain sufficient mechanical strength for it to withstand the vibrations and movements to which it is subjected during pressurization and pressure-reduction operations of the column.
Various means have been provided for overcoming this disadvantage of the binder being inert with respect to adsorbing performances, including the conversion of the binder, in all or part, into zeolite. This operation is easily carried out when use is made of clays from the kaolinite family, calcined beforehand at temperatures of between 500° C. and 700° C. An alternative form consists in manufacturing pure kaolin grains and in converting them to zeolite: its principle is explained in “Zeolite Molecular Sieves” by D. W. Breck, John Wiley and Sons, New York. The technology in question has been applied with success to the synthesis of grains of zeolite A or X, composed up to 95% by weight of the zeolite itself and of an unconverted residual binder (see, to this end, Howell, U.S. Pat. No. 3,119,660). Other binders belonging to the kaolinite family, such as halloysite, have been converted into zeolite, the addition of a silica source being recommended when it is desired to obtain a zeolite X (“Zeolite Molecular Sieves”, Breck, p. 320).
Kuznicki and coworkers have shown (U.S. Pat. No. 4,603,040) that it is possible to convert a kaolin agglomerate into zeolite X with an Si/Al ratio equal to 1. The reaction, in order to be virtually complete, that is to say in order to result in the formation of a grain composed of approximately 95% zeolite X, requires some 10 days at 50° C., however, which makes the operation unfeasible industrially. It is carried out by combining a maturing period of 5 days at 40° C. with a consecutive crystallization at a higher temperature.
JP-05163015 (Tosoh Corp.) teaches that grains of zeolite X with a low Si/Al ratio, of the order of 1.0, can be formed by mixing a zeolite X powder, with an Si/Al ratio of 1, with kaolin, potassium hydroxide, sodium hydroxide and carboxymethylcellulose. Shaping is carried out by extrusion. The grains thus obtained are dried, calcined at 600° C. for 2 hours and then immersed in a sodium hydroxide and potassium hydroxide solution at 40° C. for 2 days.
By following the teachings of these two documents, it is possible to prepare mechanically strong solids mainly composed of zeolite X, the Si/Al ratio of which is substantially less than that of zeolites X conventionally manufactured by the gel route, the Si/Al ratio of which is between 1.1 and 1.5. These processes are inelegant and suffer either from an excessive reaction time or
CECA S.A.
Smith , Gambrell & Russell, LLP
Spitzer Robert H.
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