Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2003-06-03
2004-11-16
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S446000, C428S426000, C428S701000, C428S304400, C428S307300, C502S004000, C502S064000, C502S070000, C210S651000, C210S653000, C210S500210, C095S045000, C095S902000, C264S045100, C264S045500, C423S700000
Reexamination Certificate
active
06818333
ABSTRACT:
This invention relates to the field of supported zeolite membranes that are used in separation.
More particularly, it has as its object a supported zeolite membrane, a process for its preparation and its use in separation.
Various processes for developing zeolite membranes have already been described. To date, it appears difficult to obtain in a controlled and reproducible manner zeolite membranes whose layer that contains the zeolite is continuous and thin. The thinness and the continuity of such a layer are essential parameters for obtaining a membrane material that exhibits advantageous properties that can be used in industrial separation processes. In particular, it is particularly difficult to control the preparation of zeolite membranes: the production processes involve several stages, and it is often necessary to reproduce the crystallization stage on several occasions to obtain, following stages that are time-intensive, high in operating costs, chemical products and energy, a continuous layer that can be used in separation. Furthermore, the thermal and mechanical stability of these inorganic membranes is crucial. Actually, the inorganic materials can in general be used at relatively high temperatures, for example higher than the organic polymer membranes that generally operate at a temperature of less than 100° C. It is then essential, for an industrial and commercial application, to use a membrane that can remain stable during operations and uses at high temperatures, and even high pressures. The hydrothermal path that involves porous substrates exhibits the advantage of stabilizing the zeolite crystals in the pores of a porous matrix (alumina, stainless steel, for example) and also on the surface of the latter.
In Patent Application EP-A-0 778 075, a process for developing zeolite membranes that are supported by porous glass is described. Patent U.S. Pat. No. 5,429,743 and International Patent Application WO-A-95/29751 describe operating procedures for obtaining composite membranes that are supported by an inorganic macroporous matrix. Reference can also be made to documents U.S. Pat. No. 4,099,692, WO-A-93/19840, U.S. Pat. No. 5,567,664 and WO-A-96/01683. In International Patent Application WO-A-00/33948, a process for obtaining composite membranes of zeolite supported on optionally multi-channel tubular solids is described. All of these composite membrane materials with a zeolite base are formed by a zeolite phase that is deposited on a substrate. A series of recent patents (U.S. Pat. Nos. 5,871,650, 5,968,366, 6,090,289, 6,074,457, WO-A-00/53297, and WO-A-00/53298) describes the preparation of membranes whose MFI zeolite phase is found on the outside surface of a porous substrate. The crystallization of the zeolite is generally carried out by multiple hydrothermal treatments of a mixture that contains the precursors of the zeolite phase, which increases the effective thickness of the separating layer. When the crystallization stage of the zeolite is reproduced on several occasions, the synthesis is reproduced after an optional return of the material to ambient temperature, washing and drying of said material.
The repetition and the succession of identical operations for the preparation of zeolite membranes allow the deposition of successive layers and/or the formation of zeolite crystals that fill the interparticulate spaces, which allows the production of a continuous layer for the separation. This method of synthesis in several stages, if it leads to the production of a continuous layer, also leads to the production of thick zeolite layers that run the risk of cracking during the calcination of the membrane (Vroon, Z. A. E. P., Keizer, K., Burggraaf, A. J., Verweij, H., J. Membr. Sci. 144 (1998) 65-76) from bringing the membrane separation unit into steady operation or from use at high temperature. Furthermore, the increase in thickness can considerably limit the transfer of material through the membrane during the separation operation and thus can reduce the technical and economic advantage of the membrane separation operation, due to a reduction in productivity of said separation stage. In addition, a membrane whose separating layer is thick will require using large surface areas of said membrane material to treat a flow of feedstock of the mixture to be separated, which is reflected by high investments. In addition, this method of synthesis in several stages requires a large amount of precursors of the zeolite phase, which increases in particular the cost of raw materials and precursors used. It also exhibits the drawback of extending the period for obtaining the membrane material and increasing the operating cost of the separation.
One of the difficulties linked to the preparation of zeolite-based membranes resides in particular in the monitoring of the crystallization of the zeolite so as to obtain zeolite crystals that are well linked to the substrate, located primarily in the pores of the substrate, thus forming a continuous and thin zeolite/substrate composite layer (obtained by obstructing the empty voids of the substrate by zeolite phase crystals) so as to limit the resistance of transfer through the membrane material. Placing most, and preferably all, of the zeolite phase in the pores of the substrate imparts very good thermal and mechanical resistance to the membrane material. It cannot be ruled out, however, that a minority portion of the zeolite phase be located on the outside surface of the substrate.
One of the essential objects of this invention is to provide a supported zeolite membrane in which the zeolite phase, crystallized by a single hydrothermal treatment, exhibits the characteristics set forth above. In particular, said zeolite phase, which is active in separation, i.e., selective compared to the compounds to be separated, is thin and also exhibits a very high crystallinity.
The supported zeolite membrane according to the invention comprises a zeolite/substrate composite layer and is characterized in that it exhibits, in the n-butane/isobutane separation, a permeance of n-butane of at least 6.10
−7
mol/m
2
.s.Pa and a selectivity of at least 250 at the temperature of 180° C.
Recall that the permeance of a gas, expressed in mol/m
2
.s.Pa, is, by definition, the molar flow rate (mol/s) of this gas related to the unit membrane surface area (m
2
) and related to the partial-pressure difference of this gas between the upstream (where the feedstock circulates) and the downstream (where the permeate is recovered). The permeance of a gas is therefore the molar flow rate of this gas that passes through the membrane per unit of surface area and pressure. Selectivity &agr; (called permselectivity) is, in the case of measurements of permeation of pure elements, the ratio of the permeances of these pure elements. Within the scope of this invention, the selectivity is therefore the ratio of the permeances of n-butane and isobutane.
The supported zeolite membrane according to the invention preferably exhibits, in the n-butane/isobutane separation, a permeance of n-butane of at least 8.10
−7
mol/m
2
.s.Pa and very preferably at least 10.10
−7
mol/m
2
.s.Pa at the temperature of 180° C. The high permeance of n-butane of the membranes according to the invention demonstrates the small thickness of the zeolite phase as well as that of the zeolite/substrate composite layer that exhibits a thickness that is less than 2 &mgr;m and preferably less than 1 &mgr;m and very preferably less than 0.5 &mgr;m.
The permeance is measured as follows: the membrane is inserted in a permeating device (permeation measurement module) with carbon joints that keep the measurement module sealed. The unit (module/membrane) is placed in a gaseous permeation unit, and the material is treated in advance at 350° C. under a flow of cover gas, such as helium, that makes it possible to eliminate all traces of adsorbable gas on the outside surface and in the inside pores of the membrane material. During the gas permeation measurements, the membrane is subjected to a
Chau Christophe
Le Dred Ronan
Sicard Mickaël
Institut Francais du Pe'trole
Jones Deborah
Millen White Zelano & Branigan P.C.
Xu Ling
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