Gas separation: processes – Solid sorption – Inorganic gas or liquid particle sorbed
Reexamination Certificate
1998-12-02
2001-03-27
Dunn, Tom (Department: 1754)
Gas separation: processes
Solid sorption
Inorganic gas or liquid particle sorbed
C095S902000
Reexamination Certificate
active
06206952
ABSTRACT:
FIELD OF INVENTION
The invention generally relates to separating and removing hydrogen contaminant from hydrogen-containing process streams and more particularly to selectively removing hydrogen from such process streams using Cd-exchanged zeolite rho compositions.
BACKGROUND OF THE INVENTION
Zeolites are widely used as sorbents in many applications that use the zeolite's ability to entrap liquids and gases. One potential application is the development of zeolite storage materials for gases. In addition, zeolites offer the possibility of selective separation of gases from mixed streams.
Zeolites are crystalline aluminosilicates with framework structures. The framework structure contains channels and cages of molecular dimensions. Cations and small molecules can reside on the face of the framework or within the pores, also called cages. Zeolite rho is a typical example, with a 3-dimensional network of alpha-cages (cubo-octahedra) which are connected to each other by octahedral prisms, or in other words, a body centered cubic structure of alpha cages. Selective blocking of pores in zeolites can be achieved by ion-exchanging the zeolites with different sized cations, thereby altering the sorption properties of the zeolite.
There have been several attempts to encapsulate hydrogen gas in various metal-exchanged zeolites. For example, Yoon and Heo (J. Phys. Chem., Vol. 96, pp. 4997-5000, 1992) studied encapsulation in Cs
2.5
-zeolite A at pressures ranging from 10-129 atm (1.01-13.1 MPa) and temperatures ranging from 100-350° C., and achieved a maximum amount of 871 &mgr;mol/g of H
2
encapsulation.
Weitkamp et al. (
Proc.
9
th Intl. Zeolite Conf.,
Montreal 1992, Eds. Ballmoos et al., Butterworth-Heinemann Pub., vol. 2, pp. 11-19) used various metal-exchanged zeolites including zeolite-rho. However, the largest amount Of H
2
that they were able to encapsulate was 410 &mgr;mol/g using zeolite NaA at 300° C. and 10.0 MPa after 15 min. They were also only able to encapsulate 22.3 &mgr;mol/g of H
2
using zeolite H-rho under the same conditions.
Efstathiou et al. (J. of Catalysts, vol. 135, pp. 135-146, 1992) studied B
2
encapsulation in Cs, Ni, and Eu-exchanged zeolite A at 1 atm (0.1 MPa) and 37-300° C. They achieved the largest amount of H
2
encapsulation, 3.50 &mgr;mol/g, using NaA at 300° C.
Takaishi et al. (U.S. Pat. No. 4,466,812) disclose a hydrogen encapsulating zeolite material composed of a Na zeolite A ion-exchanged with cesium and a divalent metal. H
2
encapsulation was performed at 300° C. or less at pressures of 97 atm (9.8 MPa). No specific examples are provided for a composition containing Cd, although Cd is generally disclosed as one of the divalent metals.
What are needed are zeolite compositions capable of encapsulating hydrogen in larger amounts and at lower pressure than the prior art. In addition, a method of separating and selectively removing hydrogen from hydrogen-containing process streams using said zeolite compositions is also needed. Other objects and advantages of the present invention will become apparent to those skilled in the art upon reference to the detailed description of the invention which hereinafter follows.
SUMMARY OF THE INVENTION
The present invention provides a method of separating and removing hydrogen from a hydrogen-containing process stream comprising contacting a hydrogen-containing process stream with a Cd-exchanged zeolite rho composition of the formula:
Cd
w
M1
x
M2
y
M3
z
Si
a
Al
b
O
96
wherein: M1, M2 and M3 are each selected from the group consisting of Li, Zn, Rb, Sr, H, NH
4
, Ca, K, Na, Mg, Ba and Cs;
w is from about 1 to about 6;
x, y and z are each 0-6;
a is 36-48; and
b is in excess of 0 and up to 12, with the proviso that a+b=48.
Preferably, w is about 5 or about 6 in the formula. Typically, at least one of x, y, or z is greater than zero and the Si:Al molar ratio is from about 3:1 to about 5:1.
The Cd-exchanged zeolite rho compositions can surprisingly separate and remove hydrogen from process streams in relatively large amounts (e.g., at least 150 &mgr;mol/g of hydrogen) even at lower pressures like 1 atmosphere. Preferably, the hydrogen-containing process stream contains hydrogen contaminant in concentrations of from about 0.0001 to about 15 wt. %. Such streams-could also contain other gases in addition to hydrogen such as, for example, HCl, HF, HBr, HI, Cl
2
, N
2
, CO, CO
2
, Ne, Ar, Kr, Xe, He, NH
3
, CH
4
, air and H
2
O. The use of the above-defined Cd-exchanged zeolite rho compositions allows for the selective removal of hydrogen when other gases are also present.
As used herein, “hydrogen” means elemental hydrogen (e.g., gaseous H
2
) as well as isotopes thereof, including, for example, deuterium and tritium.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Zeolites can be generically described as complex aluminosilicates characterized by three-dimensional framework structures enclosing cavities occupied by ions and water molecules, all of which can move with significant freedom within the zeolite matrix. In commercially useful zeolites, the water molecules can be removed from or replaced within the framework structures without destroying the zeolite's geometry.
Zeolites can be generally represented by the following formula: M
2
O.Al
2
O
3
.xSiO
2
.yH
2
O; wherein M is a cation of valence n, x≧2, and y is a number determined by the porosity and the hydration state of the zeolite, generally from 2 to 8. In naturally-occurring zeolites, M is principally represented by Na, Ca, K, Mg and Ba in proportions usually reflecting their approximate geochemical abundance. The cations M are loosely bound to the structure and can frequently be completely or partially replaced with other cations by conventional ion exchange.
The dimensions which control access to the interior of the zeolite are determined not only by the tetrahedra forming a pore opening, but also by the presence or absence of ions in or near the pore. In the case of zeolite A, for example, access can be restricted by monovalent ions, such as Na
+
or K
+
, which are situated in or near 8-ring openings as well as 6-ring openings. Access is enhanced by divalent ions, such as Ca
2
+which are situated only in or near 6-rings. Thus, KA and NaA exhibit effective pore openings of about 0.3 nm and 0.4 nm respectively, whereas CaA has an effective pore opening of about 0.5 nm.
Zeolite rho, the zeolite species employed in the method of the present invention, is a small-pore synthetic zeolite. One non-limiting, illustrative formula which describes zeolite rho is:
(Na,Cs)
12
Al
12
Si
36
O
96
.44H
2
O.
The structure and synthesis of this zeolite rho are described by Robson et al., “Synthesis and Crystal Structure of Zeolite Rho—A new Zeolite Related to Linde Type A”, Advances in Chemistry Series 121 (American Chemical Society 1973), and Robson, U.S. Pat. No. 3,904,738. The crystal structure of zeolite rho is also characterized, for example, by U.S. Pat. No. 4,806,689. The combined disclosures of these references are incorporated herein by reference.
Other cationic species can be exchanged for the Na
+
and Cs
+
present in rho zeolites using a variety of techniques. The number of cationic species that can be present in the zeolite is dependent on the valence of the cationic species. The total positive charge of the cations must equal the total anionic charge of the AlO
2
−
units present; in other words the metal cations present must be in such stoichiometric amounts to balance the electrostatic charge present.
The applicants have found that the above-described Cd-exchanged zeolite rho compositions can be used in the separation and selective removal of hydrogen from hydrogen-containing process streams (e.g., in chemical and nuclear plants). This use would result in an inexpensive method for separating and selectively removing hydrogen gas from a mixture with other gases, something that is needed in the art.
To help illustrate some of the possible applications, several examples of industrial processes are de
Corbin David Richard
Felix Vinci Martinez
Dunn Tom
E. I. Du Pont de Nemours and Company
LandOfFree
Method of separating and selectively removing hydrogen... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method of separating and selectively removing hydrogen..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method of separating and selectively removing hydrogen... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2515279