GB2159729A - Apparatus for controlling diffusion of selected fluid components - Google Patents

Abstract

Apparatus for controlling the diffusion and/or separation of fluids comprises a porous ceramic structure 20, generally in tubular shape, that has a central channel 22 and a plurality of parallel channels 24. At least one of said ceramic structures can be embodied into a suitable manifold 40a providing means for proper fluid management. The said ceramic structure contains of plurality of pore sizes providing an asymmetric monolithic structure especially well suited for performing the controlled diffusion and/or separation of fluids. In a suitable configuration this invention represents a significantly improved asymmetric ceramic filter. <IMAGE>

Description

SPECIFICATION Apparatus for controlling diffusion of selected fluid components BACKGROUND OF THE INVENTION Field of the Invention This invention relates to equipment used for selectively separating or combining components of fluid phases. The invention is particularly suited for use in continuous processes for such separation or combination of fluid components. Control is achieved by causing the fluids or the components thereof to diffuse through porous ceramic bodies of a particular type to be described more fully below.

Description of the Prior Art The need to separate components of fluid streams is a very common one in continuous chemical procesing of many types and in other engineering operations. One of the most common techniques is to use semipermeable membranes of polymer materials which have the property of substantially different permeability to different components of the stream to be separated. The differences in permeability may be based on physical or chemical interactions between the component(s) of the membrane and of the fluid. More commonly, it is based simply on the presence in the membrane of small pores which are open to passage of one component of the fluid having relatively small melecules or particles but effectively barred to another component with larger molecules or particles.

It has been found in practice that most separations of commercial importance require membranes with rather small pores, which have a correspondingly low diffusion rate even for the component which passes through them most rapidly. The rate can be increased by pressure on the fluid, but this leads to a requirement for mechanical strength of the membrane to resist the pressure to prevent compaction, deformation or rupturing. Such mechanical strength normally is achieved by thickening the membrane, but this reduces the diffusion rate and thus is at least partially self-defeating.

The use of high pressure is not practical with some fluids, particularly biological ones such as blood. The pressure can lead to damage to cellular structures. Even non-biological colloidal suspensions, such as many of the common latex polymers, may be subject to similar damage by pressurization.

Some past use has been made of porous ceramics for separation of fluid components, primarily in filtration processes in which finely divided solid components of a fluid are removed. Because of their susceptability to fracture, and the difficulty to make thin ceramic layers with tightly controlled pore size, ceramic filters normally must be made thicker than polymeric membranes. Thus, they have even slower separation rates than polymeric membranes. This has limited the practical use of ceramics as separation devices. Until recently, as illustrated in French application 81 06340 filed March 30, 1981, a layered ceramic filtering structure was disclosed.

Means for mixing components into a fluid at controlled rates are also useful in industrial and medical processes, although they are probably less often used than continuous separation means.

BRIEF DESCRIPTION OF THE INVENTION According to this invention there is provided an apparatus for controlling diffusion of selected fluid components, comprising: (a) a porous ceramic body having at least one first hollow channel therethrough; (b) means for continuously introducing at least one first fluid having at least two distinct chemical components into said hollow channel and causing said first fluid at least in part to flow continuously through the pores of said porous ceramic body; (c) at least one first zone of said porous ceramic body extending athwart the entire path of flow of fluid through the pores of said body and having pores of sufficiently small size that one component of said first fluid diffuses through said zone substantially more rapidly than at least one other component of said first fluid; and (d) means for separately collecting (i) the portion of said first fluid which flows through the pores of said porous ceramic body and (ii) the portion of said first fluid which flows through the hollow channel through said porous ceramic body.

It has been found that superior ceramic bodies for controlling the diffusion of fluid components can be manufactured by sintering certain cermic powders to provide ceramic bodies of highly uniform pore size and high mechanical strength. Preferably, powders of at least two sizes are used, thereby achieving a multilayer structure, in which a thin layer with relatively small pores is joined with a least one thicker layer with much larger pores. The thick layer provides mechanical strength while the thin layer provides selectivity combined with reasonable speed of operation.

The ceramic bodies of this invention can be utilized most effectively when incorporated into structures which are different from the conventional flat sheet or single tubes or hollow fibers most commonly used for particulate removal from liquids and gases of micromolecule fractionation. The product disclosed by this invention comprises a plurality of cylindrical spaces walled with bilayers of the type described herein in a single separation element, generally of cylindrical form and extended length. Manifolds or similar devices on one or both ends of the elements provide for introduction and collection of the input and output fluids respectively.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a fragmentary perspective view of a portion of a presently preferred embodiment of a porous ceramic element for use in the present invention.

Figure 2 is longitudinal cross-sectional view of the element of Fig. 1 taken along line 2-2 of Fig. 1 but drawn to a larger scale. This cross sectional view shows the bilayer structure, comprised in this case of fine pore layer 28 and coarser pore layer 26. In this particular embodiment, the central cylindrical space 22 is not lined with a finer pore layer, because it is used as a permeate collecting space rather than a separation means. This layer can be on all surfaces of the devices if needed.

Figure 3 is a perspective view of apparatus incorporating a separation element such as shown in Figs. 1 and 2, together with a housing and piping to connect the separation element to a continuous process stream.

Figure 4 is a longitudinal cross-sectional view of the apparatus of Fig. 3 taken along line 4-4 of Fig. 3. Figs. 5, 6 and 8 are crosssectional views of other possible embodiments for use in the invention, while Fig. 7 is a perspective view of one of these embodiments, showing line 8-8 along which the cross-sectional view of Fig. 8 was taken.

With proper selection of fluid management these structures represent a signfiicant improvement over the prior art structure for microfiltration or ultrafiltration e.g. the structure described in the aforesaid French patent application. This is primarily due to the increase ratio of filtering surface per wt. volume.

DESCRIPTION OF THE PREFERRED EMBODI MENTS The apparatus of this invention includes a body of sintered ceramic material, having substantially uniform pore size, good mechanical strength, and corrosion resistance. A wide variety of known ceramic powders, such as the oxides of aluminum, silicon, zirconium, titanium, chromium, or magnesium; the carbides of silicon, titanium, or tungsten; or naturally occuring minerals such as cordierite, mullite, or the like, are suitable for construction of the devices. Mixtures of the foregoing may also be used.

The thicker the diffusion control element, the slower will be the permeation rate through the element. It is therefore economically desirable to keep the thickness as low as possible consistent with adequate strength and durability. A wall thickness for the diffusion control element of at least 1 micron is needed if the entire wall contains pores of uniform thickness and the ceramic material alumina, is a preferred ceramic.

The operation of the invention can be further understood in conjunction with the drawings. Figs. 1 and 2 show a presently preferred element for use in the invention, generally designated by reference number 20, suitable for separting fluid components continuously.

A plurality of channels 24 extend throughout the length of element 20. Each channel 24 is walled by a thin layer of fine pore size ceramic 28, which in turn is surrounded by larger pore size ceramic 26. A larger central channel 22 extends through the length of the element.

It will be appreciated that the dimensions of the embodiment 20 can be varied considerably. The porosity of layers 28 and 26 can be separately controlled, by use of approprite ceramic powders for sintering, to have pores primarily in a size range from about 0.01 micron to 20 microns or more. Typically, the pore volume will be from 35 to 55% of the total volume of these layers, with generally at least 95% of the pores being interconnected to form a tortous flow path. For most applications, it is now preferred that the device 20 have a diameter in the range of from about 2 millimeters (mm) to about 20 mm; that central channel 22 will have a diameter from about 0.5-10 mm and that peripheral channels 24 will have a diameter from about 0.2-5 mm, with diameter no more than 2 mm most preferred. Layer 28 would normally have a thickness between 1 and 15 microns.

The space between the pores has to be controlled such as being 0.1 mm to 2 mm. Any convenient length within the bounds set by ceramic sintering technology may be used, with one meter being an approximate practical upper limit according to present experience.

In-normal practice, element 20 is used in conjunction with other equipment shown in Figs. 3 and 4. A fluid-impermeable housing surrounds the element 20 and together with end caps 34a and 34b and resilient connector members 38 prevents escape of the fluid to be separated. Fluid is advantageously introduced into the plurality of channels 24 via an optional manifold 40a that is provided with a corresponding plurality of feed tubes 42a that are passed through respective apertures 44 in end cap 34a and then pressed into the ends of the respective resilient connector members 38. Apertures 44 are advantageously provided with O-rings 46 so as to seal the apertures against fluid leakage. A connector 48a is provided on manifold 40a to connect the manifold to a fluid supply source not shown. End caps 34 are slightly spaced from the ceramic element 20 and the latter is properly positioned within housing 32 by means of spacers 36. Connectors 50a and 50b give access to the connected space including the central channel 22 of the ceramic element and the space 52 between the ceramic element and the housing 32.

All the auxiliary structures used in conjunction with element 20 should be constructed from a fluid-impermeble material which is inert chemically with respect to the fluid to be conveyed.

The embodiment shown in Figs. 1-4 may be used in a variety of ways. One of the simplest is a process for separating from a fluid F a component A which diffuses through a fine pored ceramic more rapidly than any other constituent of F. The fluid F could be introduced through connector 48a into the manifold 40a and thence into channels 24 of the ceramic element 20. Connector 50a would be sealed off. A vaccuum might be maintained through connector 50b, or the fluid F might be introduced at greater than atmospheric pressure into connector 48a. In either case, the fluid F will be split by its passage through the apparatus into a stream Fd depleted in component A and a stream Fe enriched in constituent A. Stream Fd may be drawn off through connector 48b, while stream Fe may be withdrawn through connector 50b.

More complex processes making use of embodiment 30 are readily conceivable to those skilled in the art. For example, a second fluid G could be introduced into connector 50b, while the fluid F is introduced into connector 48a as before. As the fluids pass countercurrently through the element, comonent A diffusses into fluid G. Thus the output streams will consist of Fd, a variation of F depleted in A as before, and Get a variation of G enriched in component A, which may be withdrawn from connector 50a.

A particular practical example of such use of embodiment 30 for countercurrent exchange of fluid components is the artificial oxygenation of blood, a process well known as an important part of some medical treatments. For this purpose, oxygen would be introduced through connector 48a and blood containing high partial pressures of carbon dioxide but little oxygen would be introduced through connector 50b. When the pore size of layer 28 is fine enough, none of the components of blood other than oxygen and carbon dioxide can pass through the layer, so that reoxygenated blood could be withdrawn through connector 50a and a mixture of oxygen and carbon dioxide withdrawn through connector 48b.This process can be accomplished with little or no gross pressure difference between the blood and oxygen fluid streams, because the difference in partial pressures of oxygen and carbon dioxide across the composite structure 28/26 will provide sufficient driving force for the exchange to occur.

A typical example of the use of such an embodiment 30 is for cell entrapment in tissue culture, cell propagation, cell harvesting, cell fractionation and cell recycling etc., while nutrients will diffuse through fine pore layers.

It will be readily appreciated by those skilled in the art that multiple embodiments of the type illustrated in Figs. 1-4 could advantageously be combined in tandem or in parallel to achieve greater degrees of separation or higher fluid flow volume with the same degree of separation respectively. Also, it is not necesary to effect separations of the same type or degree in successive multiple embodiments. For example, carbon dioxide could be stripped from blood by a vacuum in a first embodiment of style 30, while oxygen could be introduced into the gas depleted blood in a separate such embodiment. In this way, independent control of the carbon dioxide and oxygen levels could be achieved. Such independent control might not be possible in a single embodiment operating with cross diffusion of these two gases.

One of the advantages of the particular embodiment of style 20 is the provision of a high surface area of contact between the two fluid streams. Another advantage is that the tubular shape of the entire element contributes to good mechanical strength, even when the wall thicknesses are relatively small.

Many different embodiments are also possible. For example, Fig. 5 illustrates an embodiment generally designated by number 120 having two separate groups of channels 124 and 125 within ceramic substrates 126 and 127 respectively. A distinct region 128 separates the two groups of channels. The pore sizes of regions 126, 127, and 128 may all be independently varied, as may the diameters of the two groups of channels and of the central channel. Three or more different fluid streams could be introduced into an embodiment of style 120. Fig. 6 illustrates an embodiment 220 having two channels 223 and 224, neither of which is cylindrical or axial. Embodiment 220 is provided with a non-porous ceramic coating 60, an alternative to the separate housing 32 provided for embodiment 20. Although not shown in Fig. 6, a thin layer of fine pore ceramic could be provided as the innermost part of the wall of channels 222 and 223. With suitable fluid connectors, cross diffusion between these two channels could be achieved, or a single fluid could be introduced into both channels and a permeate recovered from the ends after passage through the porous bulk of embodiment 220. Still another variation is shown in Figs.

7 and 8. This device 320 is much like embodiment 20 of Figs. 1-4, except that the central channel 322 is extended beyond the end of the peripheral channels 324. Such a configuration will assist in separating fluid within channel 322 from fluid in channels 324 by appropriate use of gaskets. Embodiments could also be constructed so as to provide helical flow, or other flow paths tending to minimize the presence of "dead zones" in the flow channel(s).

Claims (14)

1. Apparatus for controlling diffusion of selected fluid components, comprising: (a) a porous ceramic body having at least one first hollow channel therethrough; (b) means for continuously introducing at least one first fluid having at least two distinct chemical components into said hollow channel and causing said first fluid at least in part to flow continuously through the pores of said porous ceramic body: (c) at least one first zone of said porous ceramic body extending athwart the entire path of flow of fluid through the pores of said body and having pores of sufficiently small size that one component of said first fluid diffuses through said zone substantially more rapidly than at least one other component of said first fluid; and (d) means for separately collecting (i) the portion of said first fluid which flows through the pores of said porous ceramic body and (ii) the portion of said first fluid which flows through the hollow channel through said porous ceramic body.

2. Apparatus according to Claim 1 having in said porous ceramic body at least one second hollow channel so situated as to receive at least part of the flow of that portion of said first fluid which flows through the pores of said porous ceramic body.

3. Apparatus according to Claim 2 having a plurality of said first hollow channels arranged peripherally around said second hollow channel.

4. Apparatus according to Claim 3 wherein said hollow channels and the ceramic body are substantially cylindrical in shape.

5. Apparatus for controlling the diffusion of a selected component of one fluid into another fluid, comprising: (a) a porous ceramic body having at least one first hollow channel and one second hollow channel, each of said first and second hollow channels extending through said porous ceramic body, and having a continuous tortuous path through the pores of said porous ceramic body for flow of fluids between said first and said second hollow channels; (b) means for continuously introducing a first fluid having at least two distinct chemical components into one end of said first hollow channel; (c) means for continuously introducing a second fluid into one end of said second hollow channel;; (d) at least one first zone of said porous ceramic body extending athwart the entire path of flow of fluid through the pores of said porous ceramic body between said first and said second hollow channels, said first zone having pores of suifficiently small size that one component of said first fluid diffuses through said first zone substantially more rapidly than at least one other component of said first fluid; (e) means for collecting from the end of the said first hollow channel opposite the end of introduction of said first fluid a third fluid which is depleted, relative to said first fluid, in the most rapidly diffusing component of said first fluid; and (f) means for collecting from the end of said second hollow channel opposite the end of introduction of said second fluid a fourth fluid which is enriched, relative to said second fluid, in the most rapidly diffusing component of said first fluid.

6. Apparatus according to Claim 5 having a plurality of said first hollow channels arranged peripherally around a single said second hollow channel.

7. Apparatus according to Claim 5 having a plurality of said first hollow channels arranged peripherally around a plurality of said second hollow channels.

8. first and said second hollow channels and said ceramic body are substantially cylindrical in shape.

9. A process for depleting a fluid, having at least two distinct chemical components, of the most rapidly diffusing of its components, by introducing it as the first fluid into an apparatus according to Claim 1 and collecting the depleted fluid at the end of said hollow channel opposite the end of introduction of said fluid.

10. A process for depleting a first fluid, having at least two distinct chemical components, of the most rapidly diffusing of its components while simultaneously enriching a second fluid in the most rapidly diffusing of the components of the first fluid, comprising introducing said first fluid and said second fluid into apparatus according to Claim 5, collecting the depleted variation of said first fluid from the end of said first hollow channel opposite the end of introduction into said first hollow channel, and collecting the enriched variation of said second fluid from the end of said second hollow channel opposite the end of introduction into said second hollow channel.

11. A process according to Claim 10, wherein said depleted variation of said first fluid is enriched in a component of said second fluid.

12. An improved ceramic filtering structure for cross flow microfiltration and ultrafiltration for particulate removal from gases and fluids, micromolecular fractionation, cell recycling or cell harvesting comprising: a) a porous ceramic body having at least first hollow channel therethrough; b) means for introducing a first fluid containing particulate matter to be separated and causing said first fluid to cross flow continuously through the pores of said porous ceramic body; c) means for introducing a second fluid and causing it to flow through said body countercurrently against the direction of the flow of said first fluid; and d) means for separately collecting (i) the portion of said first fluid and (ii) the second fluid.

13. An apparatus for controlling diffusion of selected fluid components substantially as herein described with reference to and as illustrated in each of the embodiments of the accompanying drawings.

14. A process for depleting a fluid, having at least two distinct chemical components substantially as herein described with reference to and as illustrated in each of the embodiments of the accompanying drawings.