Device and method for filtering a colloidal suspension

Abstract: Separation of selected components from a colloidal suspension utilizes a vessel capable of holding a colloidal suspension (14). A membrane (40) permeable to selected components of a colloidal suspension is sealed over a support (44) to form a leaf element (12). The leaf element (12) includes an outlet (24) for the selected components of the colloidal suspension (14) and is extended into the colloidal suspension. The leaf element (12) is controllably vibrated (18), advantageously simultaneously with application of a negative (32) or positive pressure which is used to motivate permeation of the membrane (40) by selected components of the colloidal suspension (14).

Description: The present invention relates to an apparatus for separating a component from a suspension, formed of a liquid and particles having a size less than 50 micrometers, and to the use of such an apparatus. Permeable membranes have been used to separate ions, molecules, and solids from the liquid portion of the colloidal suspension. Although filtration has been employed in this regard there is an ever present problem of plugging or fouling of the filter membrane. Methods of enhancing membrane permeate rates (dewatering rates) are found in the prior art. Such methods include the shearing of liquid slurry across the membrane in tangential flow i.e. crossflow filtration. Such method uses a pump to force the feed slurry to flow tangent to the dewatering membrane. The resulting shear causes concentrated material, usually in the form of a filter cake, to be removed from the face of the membrane. Thus, the rate of liquid removal through the membrane is increased. Unfortunately, the provision of pumps to force the feed slurry in this manner requires expensive and bulky equipment and creates serious problems in the sealing of the vessel holding the colloidal suspension. United States Patent 4,253,962 proposes the use of sonic vibration, created by ultrasonic transducers, to produce cavitation at the face of the membrane. United States Patent 4,526,688 proposes a shock-type system where the membrane support structure and a filtration apparatus are periodically banged to induce the filter cake to drop from the membrane. United States Patent 4,545,969 employs a shearing plate which is oscillated parallel to a fixed membrane. Further, United States Patent 3,970,564 teaches a system where a membrane is mechanically vibrated in a direction normal to the membrane. United States Patents 4,062,768 and 4,446,022 show screening and sieving devices used in dry mineral and wet powder classification in which the screen is vibrated parallel to the face of the screen to induce the powder to fall through the pores of the screen. None of these devices are suitable for separation of the components of a colloidal suspension with the application of a negative or positive pressure vessel. The CX ultrafilter manufactured by Milipore Products Division of Bedford, MA. shows a system intended for separating proteins from aqueous solutions. The system utilizes a cylindrical probe which is inserted into a test tube containing the solution to be separated. The cylindrical wall of the probe is formed in part from a membrane material and the proteins pass through the filter from the solution occupying the annular volume between the probe and the test tube wall. The cylindrical probe is reciprocated over a small amplitude (less than 0.01 centimeters) and at 60 Hz. The shear created between the opposed walls of the probe and the test tube is partially effective in reducing plugging of the membrane by the proteins. In general, the technique of cross-flow microfiltration and ultrafiltration is limited since shear rates above 20,000 sec.⁻¹ of intensity are difficult to achieve. Such high intensities require a massive amount of power and the provision of entrance pressures which are uneconomical. Also, such membranes are often arrayed in a rectangular pressure vessel such as a plate and frame cross-flow device. The transmembrane pressure drop is limited by the inherently weak vessel walls. A membrane filtration device which is able to produce a large shear intensity on the exterior or face of the membrane simultaneously with the application of a large pressure drop across the membrane to create high permeate rates would be a great advance in the art of filtration and component separation. DE-A-24 49 817 describes methods and apparatuses for filtration of suspensions and ultrasonically filtering solutions of large molecular weight. The filter cake on the filtering layer is fluidized by vibrations and is mixed by stirring into the suspension. In one embodiment disclosed, there are a plurality of parallel, plate-shaped filter elements which are reciprocally vibrated in a fixed vessel holding the suspension. By the present invention a useful apparatus for concentrating, dewatering, or separating suspensions having particles of less than 50 micrometers, particularly colloidal and molecular slurries through the use of membrane filtration, is provided. The apparatus of the invention is set out in claim 1. A colloidal particle is generally defined as a particle possessing a size such that the dominant force influencing the particle motion in a liquid are surface forces; eg: surface charge interaction, Van der Wals forces, and the like. This normally occurs below a particle size of 50 microns. Practically, this definition includes finely divided clays, protein molecules, and ions. The vessel holding the suspension may be a pressure vessel and may be formed into a cylindrical or spherical body. A filtration membrane having interior and exterior surfaces is sealed around a support to form a leaf element. The interior surface of the membrane may be connected to the support of the leaf element for movement therewith. The leaf element also includes an outlet for conduction of the selected components of the suspension permeating the membrane (permeates). Such outlet may be formed as part of the support member. The membrane may also be laminated to a drainage material, such as a open weave cloth, which is placed between the interior surface of the membrane and support. The vibrating means may include a vibrator and a rod or shaft which is connected to the support of the leaf element. Such rod may be hollow and thus conduct the permeates from the interior of the leaf element, outwardly from the vessel. The leaf element is fixed rigidly to a containment vessel and vibration is applied to both the pressure vessel and the attached leaf elements. Vibration is applied torsionally approximately parallel to the leaf element to produce a shearing between the leaf element and the liquid slurry or colloidal suspension in the vessel. The leaf element or elements may be single-sided or double-sided. In the latter case, the support within the leaf member could be shaped to accommodate a plurality of faces on the membrane. In this regard, the support may be solid or of open construction i.e. a rigid screen. Means is also included for applying a pressure influence to motivate or to urge permeation of the membrane by the permeates. Such pressure may be in the form of a vacuum communicating with the outlet of the leaf element or leaf elements; in such format the pressure vessel would be open to atmospheric pressure. On the other hand, the vessel may be constructed as a pressure vessel to accept a positive pressure which presses on the slurry or colloidal suspension and, in turn, on the membrane to increase permeate transfer to the interior of the leaf element or elements. Advantages attainable by embodiments of the present invention include separating selected components from a colloidal suspension with a very high permeate flow rate; easy sealing for use with negative or positive pressure environments; vibration of permeable membrane at varying frequencies commensurate with the shear characteristics of the colloidal suspension being separated by the membrane; capability for employing a plurality of leaf elements in a pressure vessel holding the colloidal suspension; improved filtering rates of colloidal suspensions by preventing plugging of the filter membrane in such filtration process; and use of vibration imparting torsional motion. Specific embodiments of this invention will now be described, by way of example, with reference to the accompanying drawings, in which: FIG. 1 is an axial sectional view of a first embodiment of the present invention utilizing multiple membrane leaf elements. FIG. 2 is an axial sectional view of a second embodiment of the present invention utilizing multiple membrane leaf elements. FIG. 3 is a sectional taken along line 13-13 of FIG. 2. FIG. 4 is a schematic view depicting the operation of the second embodiment depicted in FIGS. 2 and 3. FIG. 1 shows embodiment 10C of the present invention in which a plurality of circular leaf elements 144 are connected to a central shaft 145. For example, disk-shaped element 146, similarly constructed to each of the plurality of leaf elements 144, includes a support structure 148 and a pair of membranes 150 and 152 laminated thereto. A shaft 154 possesses a passageway 156 which conducts permeate from opening 158 through support 148. Passageway 156 leads to a central passage 160 through shaft 145. A plurality of circular disk leaf elements 144 are each connected to central shaft 145 inside a cylindrical vessel 162. A slurry or colloidal suspension 164 is poured inside vessel 162 to contact plurality of leaf elements 144. It should be noted that shaft 145 is rigidly attached to cylindrical vessel 162 at the base 166 by suitable fastening means such as welding, gluing, and the like. Of course, such attachment of 145 to vessel 162 would seal the escape of slurry 164 through opening 168 in vessel 162. Cylindrical vessel 162 is also connected to rigid drive shaft 170. Drive shaft 170 includes an L-shaped passageway 172 which exits shaft 170 at nipple 174. Shaft 170 is driven by torsional vibrator 176. Flexible tube 178 sealingly engages nipple 174 to complete the path of permeate from slurry 164 to outlet 180 according to directional arrow 182. In other words, vacuum pump 185 aids in the withdrawal of permeate from slurry 164 from open vessel 162, through central passage 160 of shaft 145, opening 168 in vessel 162, passage 172 of drive shaft 170, and through flexible tube 178 to outlet 180. With reference to FIG. 2, another embodiment 10D of the present invention is depicted in which a plurality of disc-shaped leaf elements 184 are shown inside a cylindrical pressure vessel 186. With reference to FIG. 3, a detail of disc-shaped leaf element 188 is depicted. The support member 190 periphery terminates in a quartet of tabs 192, 194, 196, and 198. Each of the tabs engage a corresponding groove, such as grooves 200, 202, 204, and 206, which are machined into the side wall 208 of pressure vessel 186. A cylindrical support member 210A found within pressure vessel 186 permits the pre-stacking of plurality of leaf elements 184 to assure accurate fitting of the same within pressure vessel 186. Returning to FIG. 3, it may be seen that leaf element 188 is shaped as an annulus with a pair of membranes 210 and 212 heat sealed to outside rim 214 and inside rim 216 surrounding opening 218. It should be noted that membrane 212 is sealed in the same manner to the side opposite that shown in FIG. 3 of leaf element 184. Permeate removal tube 220 is inserted within leaf element 184 via opening 222 through pressure vessel 186 and passage 224 through support member 190. Permeate from slurry or colloidal suspension 224 exits through tube 220 and passes to manifold 226 acording to directional arrow 228. Pressure type bushing 231 seals any leakage through passage 222 outside of pressure vessel 186. The plurality of leaf elements 184 are similarly constructed to leaf element 188. Returning to FIG. 2, it may be observed that pressure vessel 186 is formed of a cylindrical body 230 and a pair of plates 232 and 234. A multiplicity of long bolts 236 extend from top plate 232 to bottom plate 234, where threading engagement occurs. Means 238 for pressure motivating the separation of slurry 225 may takes the form of a positive pressure pump 240 which applies pressure to slurry 225 and eventually to plurality of leaf elements 184. Bleed valve 242 permits the removal of concentrated material from pressure vessel 186. With reference to FIG. 3 it should be noted that each of the plurality of leaf elements 184 includes a gap 244A which permits circulation of slurry 225 around plurality of leaf elements 184. Of course, slurry 225 does not extend into the grooves machined into pressure vessel 186 to engage a quartet of tabs found in each of the plurality of leaf elements 184. Turning to FIG. 4, the pressure vessel 186 is schematically depicted, and is assumed to contain plurality of stacked leaf elements 184 as detailed in FIGS. 2 and 3. Pressure vessel 186 attaches to a torsion spring 244 which is itself rigidly attached to a large seismic mass 246. Torsional force transducer 248, which may be a brushless permanent magnet motor having a low inertia rotor, attaches to the top of pressure vessel 186 via shaft 250. Alternating current runs source 252 drives force transducer 248 at the natural frequency of the torsional resonating system created by the heretofore described elements. It should be noted that torsional spring 244 may be a coil spring, torsion bar, or similar torsion spring element. In operation, each of the embodiments of the present invention 10C and 10D are vibrated by vibration means such as torsional vibrator 176, and torsional force transducer 248, respectively. Respective leaf element or elements permit permeate to flow from each slurry containing vessel. In certain cases the flow of permeate is motivated by pressure means such as a vacuum pump or a positive pressure pump. For example, in embodiments 10C and 10D, vacuum pump 185 and positive pressure pump 240 are used respectively in this way. Each separation system of the present invention provides a high shear intensity on the outer surface or faces of the leaf element membranes without resort to cross-flow pumping of the colloidal suspension or slurry held in a particular vessel. Vibration may be induced in both the membrane leaf elements and the surrounding pressure vessel. For example, in embodiments 10C and D, a simultaneous application of high shearing and high transmembrane negative or positive pressure can be achieved. As an example, vibration frequencies of about 70 herz (Hz), with a displacement amplitude of 10 centimeters peak to peak, in combination with a transmembrane pressure drop of several hundred PSI have been produced by using a steel pressure vessel mounted to a torsion spring. The resulting higher permeate flow rate is achieved at an lower cost than cross flow systems. It is believed that the device and method employed in the present invention will produce shear intensities on the particular membrane leaf elements greatly in excess of those typically achieved by cross-flow devices or any of the vibrating filtration devices described in the prior art. It has been found that the particular vibration induced on the leaf element produces the same motion of liquid on the face or outer surface of the membrane leaf element being employed. In this regard, vibration is always applied to cause shearing between the membrane and the slurry or colloidal suspension in which the membrane is immersed. Vibration is therefore applied tangentially to the face or outer surface of a particular leaf element. Liquid contacting the face of the membrane moves at exactly the velocity of the membrane due to the no-slip boundary conditions obeyed by all liquids. As one travels away from the face or outer surface of the membrane, fluid velocity amplitude decays exponentially as the envelope of a propagating shear wave. The decay length for the shear wave velocity amplitude may be expressed as follows. L s =(µ/ρω) 1/2 where µ is the slurry viscosity, ρ is the slurry density and ω is the frequency of vibration in radians per second. It should be noted that where the frequency of vibration f is expressed in Hz then ω = 2πf Within 3 or 4 decay lengths, (typically a fraction of a millimeter), the shear intensity is essentially zero. At this point the liquid is stationary. If the vibratory motion of the membrane leaf element is sinusoidal, the velocity of the membrane surface (u) can be written as: u ∼ δω sin ωt where δ is the peak displacement of the membrane leaf away from its average position. The shear intensity (S) on the face of the membrane can be calculated to be approximately S = 157 f 1.5 δ in c. g. s. units. Thus, a vibratory filter in which the frequency of vibration is 10 Hz and peak displacement amplitude is 10 centimeters, will produce shearing on the face of the membrane of 49,000 seconds⁻¹. Equivalently, a membrane leaf element vibrating at 50 Hz with a displacement amplitude of 1 centimeter would produce a shear intensity of 55,000 sec.⁻¹. Many combinations of frequency and displacement amplitude would produce the same level of shearing. It has been determined experimentally that the permeate rate often increases proportionally to the square root of the shear intensity as defined herein, fixing the transmembrane pressure drop. The permeate rate also increases proportionally to the square root of the transmembrane pressure drop, fixing the shear intensity. If both shear intensity and transmembrane pressure are increased, the permeate rate increases proportional to the square root of the product of shearing times pressure. This implies that the highest permeate rate increase can be achieved by producing simultaneously a very intense shearing and a very large transmembrane pressure drop. Ultimately, the maximum obtainable shear intensity obtainable with the present invention depends to a large degree on the strength of material used in constructing the membrane leaf elements. The force acting on the leaf elements is proportional to the acceleration times the mass of the leaf elements: F=ma The peak acceleration is proportional to the frequency squared times the displacement. Thus, force acting on the leaf elements increases proportionally to frequency squared. In the preferred embodiments, the preferred range of frequency induced by the vibratory devices described hereinabove ranges between 5 Hz and 300 Hz. It has been found that low frequency operation, although producing low "g" forces on the membrane leaf elements requires high displacement amplitudes. In certain cases such amplitudes may be difficult to control. Also, if the frequency induced on the membrane leaf elements is too low, the distances between parallel leaf elements stacked as depicted in FIGS. 1 and 2 must be increased. In other words, the zone of shear must be allowed to extend a greater distance from the outer surface or face of the membranes of the membrane leaf elements. It has also been found that increasing the frequencies of the embodiments depicted in the drawings produces a rather small increase shear intensity and a rather large increase in "g" forces above a frequency of 300 Hz. However, other materials and arrangements may extend the heretofore described perferred frequency range. Nethertheless, for the embodiments depicted, normal operation would take place between 20 Hz and 150 Hz. The low end of this range, at or about 20 Hz, may be employed to separate low viscosity slurries, especially those which are sensitive to shear, such as cellular suspensions. Conversely, the high end of the range, at or about 150 Hz, may be used to separate molecules in ultrafiltration and reverse osmosis applications. Also, such higher frequency may be employed to produce very high shear intensities in non-shear sensitive materials having high viscosity, such as in an application known as microfiltration of mineral clays. Commonly, the operating frequency of the preferred embodiments may range between 40 Hz and 70 Hz. Likewise, the displacement amplitude of the vibration induced in any of the systems above described may be varied depending on whether the materials to be filtered are shear sensitive. Many living cells and molecules of biological importance are sensitive to shearing. In this case, the vibration displacement amplitude and/or operating frequency would have to be limited to produce a shearing of less than 10,000-20,000 second⁻¹. Non-shear sensitive material such a mineral clays can be processed at very high shear intensities of 500,000 second⁻¹ or more. It is particularly advantageous in such application to simultaneously apply high shearing intensity and high transmembrane pressure to achieve a very high permeate flow rate. In general, the method and devices described above improved the filtering of colloidal suspensions by avoiding the plugging of the filter membrane. It is believed that applying a rigorous shear flow at the interface between the filter and membrane and the suspension removes the concentrated polarization layer primarily responsible for plugging of the filter membrane.

Claims: Apparatus for separating a component from a suspension formed of a liquid and particles having a size less than 50 micrometers comprising a. a vessel (162,186) capable of holding the suspension b. a plurality of leaf elements (146,188) located in a parallel arrangement within the vessel, each of said elements comprising a filtration membrane (150,152,210,212) which is permeable to selected components of the suspension, sealed around an underlying support (148,190), the membranes having an exterior surface for contact with the suspension within the vessel and an interior surface facing the support, and the support including an outlet (154,224) for the permeate c. means (185,240) for applying a pressure difference across the membranes characterized in that d. said leaf elements (146,188) are rigidly fixed to the vessel (162,186) e. means (176,248) are provided for torsionally vibrating the vessel and all the leaf elements together to induce shearing forces parallel to the exterior surface of the membranes, and f. an apparatus having stirring means is excluded. Apparatus according to claim 1 in which the means for applying a pressure difference comprises a vacuum source (185) acting on an outlet of each of the leaf elements. Apparatus according to claim 1 in which the means for applying a pressure difference comprises a positive pressure source (240) acting on the filtration surface of the membranes. Apparatus according to any one of claims 1 to 3 in which said means for torsionally vibrating said vessel (186) includes: a. a torsion spring (244) mechanically linked to said vessel; b. a seismic mass (246), said torsion spring also being mechanically linked to said seismic mass; c. a motor (248) having a shaft (250), said motor shaft being mechanically linked to said leaf elements; and d. means for electrically driving said motor. Apparatus according to any one of the preceding claims, wherein said leaf elements (188) are annular and are arranged axially spaced apart in a stack extending axially of said vessel (186), and have their central apertures aligned for passage of liquid therethrough and their outer peripheries fixed to the vessel with gaps (244A) between them and the vessel wall for passage of the liquid. Use of the apparatus of any preceding claim for filtration of a component from a suspension formed of liquid and particles having a size of less than 50 micrometer. Use according to claim 6 wherein the torsional vibration of the vessel is operative to produce a displacement amplitude of the leaf elements of at least 5 millimeters. Use according to claim 7 wherein the displacement amplitude of the leaf elements is between 5 and 50 millimeters. Use according to any one of claims 6 to 8 wherein the torsional vibration has a frequency in the range 5 to 300 Hz. Use according to claim 9 wherein said torsional vibration frequency is in the range 20 to 100 Hz. Use according to claim 10 wherein said torsional vibration frequency is in the range 40 to 70 Hz. Use according to any one of claims 6 to 11 wherein the torsional vibration induces shearing intensity at said exterior surfaces of said membranes of a magnitude of at least 5000 s⁻¹.

Inventors: CULKIN JOSEPH BRADLEY, CULKIN, JOSEPH BRADLEY
Applicants: CULKIN JOSEPH B, CULKIN, JOSEPH BRADLEY
Application year: 1989
Main classification: B01D61
IPC codes: B01D63, B01D61