Gas separation – Zigzag – corrugated – or conical
Reexamination Certificate
2001-12-03
2003-04-22
Spitzer, Robert H. (Department: 1724)
Gas separation
Zigzag, corrugated, or conical
C055S523000, C055S524000, C055SDIG005
Reexamination Certificate
active
06551369
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to the field of filter and separation technology and ceramics and concerns a ceramic flat membrane, for example, in the form of a multichannel element for use in fluid and gas filtration, and a process for producing this membrane. Multi-channel elements of this kind are preferably used in the form of filter modules which are comprised of a number of multi-channel elements.
2. Discussion of Background Information
There are ceramic membranes for filtration applications, which have flat geometry and tubular geometry. These membranes have as high as possible a ratio of surface area to volume. In membranes with a tubular geometry, both single-channel and multi-channel elements are possible. With tubular multi-channel elements (e.g., 7 or 19 channels), a considerably higher ratio of surface area to volume is achieved than with single-channel elements. By increasing the number of channels to 37, for example, the usable filtration surface area can be increased further. With a constant outer diameter of the tube, however, the number of channels cannot be increased to arbitrarily high numbers because otherwise the wall thicknesses become too thin and consequently, the mechanical stability is no longer sufficiently assured. In addition, with an increasing number of channels, it becomes more difficult to transport the permeate from the inner channels, through the ceramic toward the outside. For tubular geometries, the ratio of surface area to volume can also be increased by changing the conventional concentric channel cross-sectional forms into a cloverleaf or butterfly shape.
The conventional tubular single-channel and multi-channel elements are produced in one piece by using extrusion.
Centrifuging is a new technique for producing single-channel elements which have a very low roughness and are therefore well-suited for membrane coating (Nijmeijer, A., et al, Am. Ceram. Soc. Bull. 77(4), pp. 95-98, 1998).
The actual separation membrane is applied in a subsequent step, e.g., using a single or multiple wetting of the tube with a suspension containing ceramic powders or with a precursor solution. Independent of the chemistry and composition of the precursor, the separation membrane is disposed on the inside of the tubular channel or channels.
In addition to membranes using tubular geometry, there are also flat membranes. Flat membranes of this kind are disclosed in WO 98/28060 A1, DE 36 12 825 A1, and DE 196 24 176 C2. All of these flat membranes are characterized by a planar surface. In addition, in fluid filtration, they must be very thin in order to assure a sufficient filtrate flow. Due to the relatively low mechanical stability of thin ceramic membranes, a porous support is required whose thickness is more than 100 times greater than the thickness of the membrane layer.
DE 43 35 122 A1 describes a ceramic flat membrane which is comprised of two flat membrane films which are separated from each other by a wave-shaped spacer that is made of the same material, but in an absolutely impervious form. The side of the membrane against which the medium (feed) to be separated flows has a flat, i.e., non-structured, surface. By contrast, DE 43 29 473 C1 describes a flat membrane whose geometry is embodied in such a way that tensile stresses do not occur at any point in the membrane. To this end, there are grooves on the filtrate side which have the approximate cross section of a semicircular arc.
WO 90/15661 describes a self-supporting ceramic membrane which is comprised of two layers containing one coarse powder and one fine powder. The manufacture is carried out by means of the superposed casting of the layers by means of tape casting technology. The support layer bond thus produced is fired on in one step. The above document also discloses that, in the unfired state, the membrane is flexible and can be embossed (e.g., channels).
U.S. Pat. No. 5,766,290 describes a ceramic filter comprised of specially structured plates (zigzag pattern), which are stacked in an offset manner and glued to each other, which produces parallel channels in this instance. The plates have a uniform, porous structure and are comprised of fragments of ceramic fibers. They are used as high temperature-stable and pressure-stable filters for lateral flow filtration of gases (flue gas, natural gas).
The disadvantages of this are the limited geometric design potential and the uniform porous structure, which stands in opposition to a broad application.
The disadvantages of the prior flat membranes as well as those in tubular geometries is that the membrane surface areas cannot be increased to arbitrarily high levels and that, with an increase of this kind, the mechanical stability decreases.
As is known, known membranes are primarily comprised of polymers. Despite having better properties (higher chemical, thermal, and mechanical stability), ceramic membranes have not yet been able to catch on due to higher production costs.
The prior technology for producing ceramic flat membranes is comprised of the following steps:
1. Production of the support
Mixing of narrowly fractionated powders in the magnitude range of a few &mgr;m, with known shaping agents,
Shaping,
Sintering.
2. Production of a partial layer on the support
Suspension production out of fine powders that are likewise fractionated as narrowly as possible, with grain sizes that are smaller than that of the support,
Coating of the support by means of dip coating,
Drying,
Sintering, multiple repetitions of the last three steps
3. Production of another partial layer on the first partial layer
Suspension production out of fine powders that are fractionated as narrowly as possible, with grain sizes that are smaller than those of the previously applied partial layer, or sol production,
Coating of the first sintered partial layer by means of dip coating,
Drying,
Sintering, multiple repetitions of the last three steps
4. Production of the modules
Sealing of the end pieces (sealing)
Assembly.
It is clear from this description that the primary disadvantage of the known manufacturing process is the expensive repetition of the application, drying, and sintering of the partial layers. These manufacturing steps are particularly cost-intensive.
Research has already been done toward improving this manufacturing process by integrating the coating into the shaping process. This was attempted particularly in conjunction with coextrusion. In this connection, strand-shaped masses are extruded over a dome using a piston extruder (DE 22 08 767 C1).
However, the research was not continued and has not been used in actual practice. In any case, it is not always possible to produce multilayered elements in this manner.
SUMMARY OF THE INVENTION
The object of the invention is to provide a ceramic flat membrane in which an increase of the effective membrane surface area and thereby an improvement in the ratio of surface area to volume is achieved while maintaining the mechanical stability of the membranes, and these membranes can be produced using a more reasonably priced process.
The present invention is directed to a ceramic flat membrane, comprising at least two layers of equal thickness, each of said at least two layers being composed of at least two differently porous partial layers; at least one of said at least two layers having a non-planar geometry; and said at least two layers being attached to each other at least at points and through material adhesion forming a ceramic flat membrane having channels whose cross-sections have arbitrary geometries.
The present invention is also directed to a process for producing ceramic flat membranes, comprising producing ceramic suspensions from at least two ceramic powders having different particle sizes, or from at least one ceramic powder and at least one solution, which contains precursors of a ceramic material, through addition of additive, sintering agent, shaping agent, bonding agent, or dispersion agent, where particle size of the additives is not greater that the grea
Adler Jörg
Lenk Reinhard
Richter Hans-Jürgen
Stockmann Regina
Stroh Norbert
Fraunhofer-Gesellschaft zur Forderung der Angewandten Forschung
Spitzer Robert H.
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