Supported porous ceramic membrane

Description: TECHNICAL FIELD [Detailed description of the invention]

【0001】

[Industrial application field]

The present invention relates to the general technical field of porous ceramic membranes, and more particularly to methods of forming supported microporous ceramic membranes and products manufactured by such methods.

【0002】

[Conventional technology]

The porous ceramic film is a durable inorganic thin film material having a porous property. At the microscopic level, such a thin film material is conceptualized as a series of nearly uniform spherical particles arranged in a tightly packed model, in which adjacent portions between these particles are integrally melted. The result is a durable, inorganic, homogeneous amorphous to liquid crystal material. In this substance, the pores are relatively uniformly distributed, and the pores are determined by the size (particle size) of the particles forming the film. The smaller the particle size, the smaller the pores formed between the particles when they are filled and melted.

【0003】

The properties of thin film materials are determined by the general procedure for making such films. There is a sol-gel method as a general method for producing such a film. In the sol portion of this method, hydrolysis of the metal alkoxide or metal salt generally results in the formation of a dilute colloidal solution or suspension of metal oxide particles. If a small pore size is desired for the membrane, care must be taken to prevent large particles from adhering and growing at this stage. The solvent is then removed from the solution under very tightly controlled conditions to form a xerogel or semi-solid phase known as a gel. Generally, this gel is a translucent or transparent semi-solid material, which can retain its shape but is still easily deformed. Once the solvent removal and gel sintering are complete, a durable hard ceramic material is formed. If the temperature is too high, the pores will be destroyed, so there are various restrictions on the heating of the sintering process. However, within a wide range, porous ceramic materials can be formed as supported or unsupported membranes.

【0004】

Some attention has been paid to the formation of porous ceramic films with extremely small pore sizes. An example of such a study is disclosed in US Pat. No. 5,006,248. A similar description is written by Anderson et al., "Journal of Membrane Science". (1988, No. 39, pp. 243 to 458). The method described in the US patent can form a porous ceramic film with a small pore size as a supported or non-supported material. Practical ceramic films require large, thin, crack-free surfaces, but the brittleness of ceramic materials makes it difficult to reliably manufacture such ceramic films in an unsupported form. Is. Therefore, for most applications, supported membranes are more practical. It turns out that the traditional method of adhering, or layering, very small ceramic particles onto a porous support is by no means a waste of effort. Since such particles are irregularly adherently grown or precipitated on the support, the thickness tends to be non-uniform. The pores of the support that the microporous membrane must crosslink (span) are much larger than the colloidal particles that form the membrane itself. Also, the surface topography of the support. And the electrochemical properties adversely affect the precipitation of particles on the accumulation film on the support. Since the purpose of depositing such a film on the porous support is to make a material that can be used for filtration, the pores of the resulting porous ceramic film have a highly uniform size distribution. It is desired that the film is thin and has a uniform thickness.

【0005】

For catalytic purposes, ceramic films of metal oxides made of transition metals are also used. U.S. Pat. No. 5,035,784 describes how such materials can be used in the presence of UV light, which degrades polychlorinated organic chemicals. Doping can be used in mixed membrane materials to increase conductivity for the purposes of various catalysts. U.S. Pat. No. 5,028,568 describes a method of doping a titanium film containing niobium to obtain high conductivity.

【0006】

[Problems to be Solved by the Invention]

An object of the present invention is to provide a reliable and convenient structure of a microporous metal oxide ceramic membrane effective for very strict filtration operations such as ultrafiltration, reverse osmosis and molecular sieving. is there. The membrane produced by the present invention can also be used in ceramic membrane reactors and catalyst devices.

【0007】

One of the features of the present invention is that a microporous metal oxide ceramic film effective for the above object can be efficiently reproduced and reliably produced. Another object of the present invention is to provide a method that does not require difficult and costly equipment and is easily applicable to most manufacturing processes.

【0008】

[Means for solving problems]

The present invention can be summarized as follows. That is, in the microporous film of the present invention, a dilute colloidal suspension of metal oxide particles is passed through one side (side surface) of the porous support, and is passed through the opposite side (side surface) of the porous support. The solvent is evaporated from the suspension by a gas stream to precipitate the particles in the suspension as a gel between the opposite pores, and then the gel is carefully dried to form a xerogel sintered body. , Obtained by forming a porous metal oxide ceramic film.

【0009】

Other objects, advantages and features of the present invention will become apparent from the following description described in connection with the accompanying drawings.

【0010】

[Example]

The method of the present invention is a method of forming a microporous ceramic film made of metal oxide particles on the surface of a porous support layer. In the present specification, a method of precipitating particles forming a microporous ceramic film on a support will be referred to as "permformation". The term "palm formation" is used to describe a method of precipitating particles that ultimately form a film onto a support. It is a coined word that combines "(formation)" and "permeation (transparency)". Under its most common conditions, a colloidal suspension or sol of metal or metal oxide particles is passed through a porous support. On the opposite surface of the porous support, the gas flow causes particles to precipitate in the pores of the support and the solvent to evaporate. As the solvent evaporates, capillarity causes the colloidal suspension to be continuously aspirated into the support. As a result, a layer of colloidal particles is deposited in and / or on the surface of the porous support adjacent to the interface where the gas stream causes evaporation of the solvent. This method of selective precipitation can directly control the thickness of the obtained gel thin film by controlling the temperature and relative humidity of the dry gas to change the evaporation rate of the solvent or changing the concentration of the sol. To. If the structure of the porous support is isotropic, the thickness of the gel will be uniform in all dry planes and the thickness can be further controlled by varying the length of time this method is performed. The gel thin film particles deposited in the plane of the porous support are then carefully dried and the gel burned in a reproducible, reliable and efficient manner to achieve uniform thickness and uniform thickness. A crack-free film having a porosity can be produced.

【0011】

Figure 1 shows the general concept of palm formation. A dilute sol is placed on one side surface of the porous support shown by No. 10 in FIG. In the first embodiment described here, the porous support 10 is a hollow cylinder. Since FIG. 1 is a cross-sectional view of the porous support 10, both facing portions of the support 10 are shown, a sol is installed on the outer surface of the support 10, and a gas flow is vertical in the hollow central portion. Passed in the direction. The dilute sol is aspirated through the porous support 10 by the capillary flow indicated by arrow 12. A gas stream is passed through the opposite side of the porous support 10, that is, the central portion thereof, as shown by No. 14. The gas stream can be one of a dry gas, such as an air stream, or an inert gas, such as nitrogen, to prevent an unavoidable reaction, or a rare gas. Also, the gas flow is H₂S or NH₃It may be a flow of reactive gases such as (these gases will also cause precipitation of metal particles and evaporation of solvent). Vapors from the gas stream and solvent are expelled and residual particles from the dilute sol are deposited in the porous support 10 and on the opposite surface. As shown in FIG. 1, which is a cross-sectional view of the porous tube used as the support 10, a film can be deposited on both inner surfaces of the cylindrical porous support 10.

【0012】

FIG. 2 is a detailed schematic intended to explain the conceptual situation of a palm formation ceramic membrane product made by the present invention and used for a particular support. This method is intended to deposit a microporous membrane on one surface of an already porous support. The cylindrical porous support itself used in the following examples is formed of several layers made of granular material, and these layers are sintered into a single material. In this example, the supporting material is made of alumina particles slip-cast into a series of particle layers of varying particle size. The particular porous support used is made up of three layers of alpha alumina of different particle size ranges. This porous support is shown in FIG. 2 with number 20. This porous support 20 is commercially available from Alcoa as a cylindrical assembly. The alpha alumina support 20 is formed of three layers having various particle sizes and pore diameters. The largest layer has a thickness of 1.6 mm and a pore size of 10-15 microns. This largest layer is called the support and is indicated by number 22. The second layer, called the first intermediate layer 24, is about 0.02 mm. It has a thickness of 0.8 micron pore size with a porosity of 40%. The third layer, referred to here as the second intermediate layer 26, is the innermost layer of the tubular support and is formed by a 0.006 mm layer consisting of particles precipitated so that 0.2 micron pore size is formed between the particles. Has been done. The porosity of this layer 26 is about 35%. An object of the present invention is to deposit a flat ultrathin layer made of a microporous ceramic material on such a support. The microporous ceramic membrane layer is indicated by number 28 in FIG. The microporous ceramic film layer 28 is formed within the second intermediate layer 26 and probably extends to the surface of the layer. The microporous membrane (microporous ceramic membrane layer) 28 is considered to be a continuum of fine particles precipitated as a matrix, that is, a web, in the pores inside the second intermediate layer 26. Therefore, layer 28 is 0.8 between the particles. It has both particles of a support with micron pores and particles of a microporous ceramic film deposited in the pores, reducing the average pore size to the range of 5 to 50 angstroms. I'm letting you. It is this ultrafiltration layer, or reverse osmosis layer 28, that is precipitated by the method used in the present invention. In general, such microporous layers 28 are routinely produced to have an average pore size from less than 100 to 3-5 angstroms.

【0013】

Although the three-layer alpha alumina support below is particularly good for use within the practices of the present invention, other porous supports can also be used. Other porous supports that can be easily adapted for use in palm formation methods include stainless steel supports, sintered metal supports, porous glass (such as "Vycor"), fibrous mats, or "Anotec". There is a ceramic filter commercially available under the trade name of. Therefore, the porous support itself does not have to be formed of sintered particles. Although a cylindrical support is used in one embodiment of the present invention, a porous support having many other physical shapes such as a flat plate can be used in another embodiment. The device that implements the method of the invention must be modified according to the shape of the support so that the sol passes through one side of the support and the gas flow passes through the other side.

【0014】

There is considerable flexibility in the chemical composition of the sol used in the palm formation methods described below. Both aqueous sol and alcohol sol can be used in the palm formation method described here. In addition, a wide range of metals and metal oxides can be used. Metal oxide ceramic films can be made of titania, zirconia and other transition metal oxides, as well as silica, alumina and iron oxides. Colloidal metal particles such as tungsten or silver can also be used. The particle size of the colloidal particles is an important factor in determining the pore size of the palm formation film (the film formed by the palm formation). The thickness of the palm formation film is determined by the particle size and the length of working time of the method. The Huckel mode of the electric double layer "thickness" can be used to estimate the effective particle size of the particles as charge, spheres and water. From this effective particle size and from the knowledge of the number of metal oxide ions in the sol, the thickness of the xerogel and the film obtained can be theoretically estimated.

【0015】

In order to maintain preferential precipitation of colloidal particles at the mouth of the pores of the support, the interaction between the particles and the support wall and the interaction between the particles until the particles reach the desired surface. Must be kept to a minimum. The reason why the use of dilute sol is preferable is that the interaction between particles is minimized. This goal can be achieved if it is sufficiently dilute and the average spacing between the particles is very large compared to the size (particle size) of the particles themselves. If the orthorhombic shape (8 nearest neighbor particles) is used as a distribution model of the sol particles in the sol, the average separation distance between the nearest neighbor particles can be determined. For example, the molar concentration required to achieve the required separation distance between particles is calculated in Table 1 below. The particle spacing factor is indicated by "n" and the molar concentration required to achieve n of 1, 5 or 10 is disclosed for two particle sizes. These results are given with respect to the molar concentration required to achieve the desired particle separation to avoid these interactions.

【0016】

[table 1]

table 1 Molar concentration of sol to achieve particle separation pH particle size (nm) u = 1 n = 5 u = 10 8 25 1.4 0.011 0.0014 2 12 12 0.098 0.012 FIG. 3 shows the device of the first embodiment effective for carrying out the palm formation according to the present invention. In FIG. 3, the reaction vessel is indicated by number 30. The gas stream flows in through the inlet port 32 and flows out from the outlet port 34 together with the vapor of the solvent. The stopper 36 seals the inside of the reaction vessel 30 with respect to the atmosphere. Within the reaction vessel 30, the cylindrical porous support numbered 38 is connected to a suitable tube leading to the inlet port 32 and the outlet port 34. A U-shaped glass fitting 40 is arranged at the bottom of the device and is connected to the inlet port 32 and the outlet port 34 by the Tygon tube piece indicated by the number 42. The Tygon tube 42 is a convenient material, but any tube will suffice as long as it is not permeable to solvent. The reaction vessel 30 is filled with an appropriate amount of sol up to its neck. To operate the reaction vessel 30, a gas stream is continuously supplied from the inlet port 32 and discharged from the outlet port 34. As a result, the solvent evaporates from the reaction vessel 30, and the sol slowly dries. As the solvent evaporates, particles of metal oxide are deposited on the inner surface of the porous support 38. In this embodiment, the outer surface of the cylindrical support 38 acts as the sol side of the support, and the inner surface of the support 38 acts as the precipitation side.

【0017】

Following this palm formation step, the precipitated gel-like colloidal particles must be dried to form a xerogel. This is done by drying very slowly, preventing the xerogel from cracking and removing the residual solvent contained within the xerogel. The form of the palm formation can be reversed in order to reduce the drying rate in the bore (ie, gel surface) of the support 38. The glass U-tube 40 at the bottom of the device is filled with solvent, which seals the inlet port 32 and the outlet port 34. This procedure is intended to create a 100% relative humidity environment inside the drying loop. The sol reservoir is then emptied and exposed to atmospheric humidity conditions. The relative humidity gradient thus generated in the direction across (passing) the wall of the support 38 is opposite to the relative humidity gradient generated during the palm formation process. This relative humidity gradient, which occurs in the direction of passing through the wall of the support 38, causes the meniscus of the sol to recede toward the outer surface of the support 38. Typical drying time is 1-2 days.

【0018】

The dried xerogel is then burned in atmospheric conditions. Generally, the burning condition of the supported membrane is a relatively slow heating rate of 2 ° C per minute until the maximum temperature of 400 ° C is reached. Previous experiments have demonstrated that the typical combustion temperature range is 400-600 ° C, but for some membranes combustion temperatures up to 600 ° C can be used. Tube 38 is maintained at maximum combustion temperature for a period of time (typically 4 hours) and then cooled again to room temperature at a controlled rate of decline of approximately 2 ° C per minute.

【0019】

As a result of such a method, a microporous metal oxide ceramic film deposited on the support is formed, which imparts great strength and rigidity to the material of the support. The microporous membrane is actually deposited in the pores of the support and will probably be present throughout the precipitation side of the support. The material thus formed is particularly suitable for microfiltration operations such as ultrafiltration, reverse osmosis and molecular sieving. Since the size of the pores can be easily manipulated within a narrow range by tightly controlling the particle size of the particles used to form the film, the palm formation film can be designed and created according to the desired specifications. Such materials can be used for gas separation, liquid filtration, and separation of substances from solvents such as desalination of seawater. The material can also be used in catalytic membrane reactors for general catalytic action.

【0020】

Other embodiments of devices that implement this method are also possible. For example, as a modification of the reactor of FIG. 3, when the cylindrical porous support 38 is arranged horizontally instead of vertically, the pressure drop difference of the entire fluid support is minimized. FIG. 4 also shows another device that implements the same method, which has a flat disk-shaped porous support. In the device of FIG. 4, the reaction vessel is indicated by number 130. The input gas flow flows in from the inlet port 132 and flows out from the outlet port 134. The porous support (in this case the porous clay-ceramic disc) is indicated by number 138. The input gas stream passes through the flared tube 140 until it exits through the glass frit 142 so that the glass frit 142 is diffused over the top surface of the support 138. The sol is placed in the reaction chamber 130, which is filled until the bottom of the support 138 comes into contact with the sol. The graduated leveling chamber 144 allows measurement of sol levels and provides an inlet for sol to be added when needed.

【0021】

When activated, the device of FIG. 4 functions similarly to the device of FIG. The input airflow comes into contact with the top surface or precipitation side of the support 138. The sol is in contact with the lower surface of the support 138, that is, the side surface of the sol, and is sucked into the support 138 by capillary action. The air stream evaporates the solvent on the precipitation side of the support 138, which causes colloidal particles to precipitate as a gel in the voids of the support 138.

【0022】

In some cases, proper manipulation of the porous support 138 and / or gas flow is intended to achieve good membrane formation at the desired location. Since the sol enters the support 138 from the sol side, care must be taken to prevent the particles from precipitating until they reach the precipitation side. Therefore, the charge attraction between the support 138 and the particles must be minimized. This problem can also be controlled by further diluting the sol. Care must be taken to ensure that the desired precipitation occurs on the precipitation side once the charge-attracting force between the support and the particles in the support 138 is minimized. Phosphate treatment on the precipitation side will promote the formation of a charged attractive force on the surface (precipitation side). As a gas stream, H changes the pH of the sol on the precipitation surface and thus accelerates the precipitation of particles.₂S or NH₃And other reactive gases can be used. It is also possible to heat the gas stream to dynamically destabilize the particles in the colloid and induce precipitation. All or any of these techniques aids in preferentially favorably precipitating particles on the precipitating side of the support.

【0023】

Example Formation of iron sol The production of supported microporous ceramic membranes began with the matching of colloidal solutions of metal oxides or sol. The production of a ceramic film of iron oxide was started with goethite synthesized from chemicals for ACS reagents and Milli-Q deionized water. To synthesize goethite, a solution of ferric nitrate (125 ml, 0.83 M) is passed through a glass fine fiber filter to remove dust and undissolved particles. Next, NaOH (41.6 ml, 5M) is added to ferric nitrate and stirred rapidly to partially neutralize. The OH / Fe ratio is calculated to be 2.0. After some initial precipitation, the ferric nitrate solution was resolubilized after about 30 minutes. The ferric nitrate solution was then aged in a glass container shaker at 25 ° C. for 60 hours. The pH of the solution after aging is 1.4 Met. The partially neutralized ferric nitrate solution was then hydrolyzed by the addition of NaOH (30.2 ml, 5M). NaOH was added in a polypropylene container for 3 minutes with strong stirring with a Teflon impeller. This caused the pH of the iron solution to rise to 12.6 in about 3 minutes. The hydrolyzed iron solution was then aged in a shaker at 60 ° C. for 6 days. Initially, the color of the iron solution was dark reddish brown, but after 24 hours during the aging period, the color changed to light tan. This indicates the formation of goethite (FeOOH) particles. Excess electrolyte was then removed from the goethite sol by repeated washing with precipitated and decanted Milli-Q water. Washing continued until it was detected that the conductivity of the supernatant was no longer increased. The goethite sol thus obtained can be used in the palm formation method.

【0024】

Example 2 Formation of silica sol Aqueous silica sol was synthesized from ACS reagent chemicals in Milli-Q deionized water. In this method, first, the mixture is rapidly stirred and NH.₄It was started with 4.5 ml of tetraethyl orthosilicate (TEOS) added to OH solution (31 ml, 0.5 M). Initially a two-phase mixture was formed, but after stirring for 1 hour, the solution became a homogeneous silica sol. The sol was carried to a dialysis membrane (3500 molecular weight cutoff) to remove ammonium ions and ethanol formed during hydrolysis. The sol was dialyzed against Milli-Q water until its pH was below 9. The purified sol was then filtered through glass fine fiber paper to remove any dust or particles. The aqueous silica sol thus obtained can be used in the palm formation method.

【0025】

Membrane formation Both a silica film and an iron film were formed in the device of FIG. A sol was placed inside the reaction vessel 30. An assembly having an inlet port 32, an outlet port 34, a porous support 38, a U-shaped pipe joint 40, and a tiegon tube 42 is arranged as a unit in the reaction vessel 30, and a stopper 36 seals the reaction vessel 30 against the atmosphere. doing. A seal was formed between the Tygon tube 42 and the porous ceramic (porous support) 38 using epoxy resin. This epoxy sealant is not used when applying glass glaze to both ends of the ceramic support 38.

【0026】

A nylon thread of a certain length is inserted between the stopper 36 and the cylinder at the neck so that the pressure becomes equal when the sol level drops. In order to minimize the loss of steam passing through the neck of the reaction vessel 30 resulting from this, the joint with the stopper 36 was surrounded by a paraffin film. High-purity nitrogen gas was used as the drying medium. The nitrogen cylinder of the regulator was attached to the inlet port 32 via a tiegon tube of constant length. Two humidity display cards were provided at the outlet port 34 and used as a rough estimate of gas flow humidity related to atmospheric conditions.

【0027】

The length of the palm formation operation is determined by measuring the decrease in sol level over a period of time. The average sol evaporation rate was calculated as a volume change for each time of a given length. Approximate film thickness was calculated based on a model of the pore structure of the support and filling of colloidal particles during gelation. Tables 2 and 3 below show the results obtained when the silica sol was precipitated by the palm formation method. The first operation was performed using an extremely dilute sol (separation coefficient: 20). This operation lasted for 27 hours and was intended to form a film with a thickness of 3 microns. The second run was intended to form a film with a thickness of 8 microns using a sol with a higher concentration than before (separation factor: 10).

【0028】

[Table 2]

Table 2 Dilute silica sol Time Sol level (mm) Precipitation rate (ml / hour) Thickness (μm) 0.0 42 16.5 25 0.84 2.1 20.75 20.5 0.86 2.6 26.5 15.0 0.78 3.3 [0029]

[Table 3]

Table 3 Thick silica sol Time Sol level (mm) Precipitation rate (ml / hour) Thickness (μm) 0 39 ---- 0 2 37 0.82 0.5 22 25 0.49 3.6 24 23.5 0.61 4.0 32.5 19 0.43 5.2 45 12.5 0.42 6.8 51.5 9 0.44 7.7 After the palm formation step, the gelled colloidal particles were dried into xerogels and further burned to form a sintered porous ceramic film. The drying process must be performed carefully to avoid gel cracking caused by evaporative stress. The palm former configuration was reversed to reduce the drying rate in the bore of the support. The glass tube was filled with water and the inlet port 32 and outlet port 34 were sealed. As a result, a 100% humidity environment was formed inside the drying loop. The sol reservoir was then emptied and exposed to atmospheric humidity conditions. The relative humidity gradient that occurs through the walls of the support causes the meniscus of the sol to recede towards the bore surface of the support. The typical drying time was 1-2 days. After the membrane was dried, the end seal of the support was removed using a diamond saw.

【0030】

The resulting dried xerogel was burned in the atmosphere. The combustion conditions were controlled so that the heating and cooling gradients were 2 ° C per minute and the maximum combustion temperature of 400 ° C was maintained for 4 hours. One reason for the successful precipitation of small colloidal particles on the porous support is to reduce the flow rate (flow velocity) of the sol through the support for a period of time. The flow rate of the sol (expressed as the amount of solvent evaporation) has been found to decrease throughout the palm formation. Table 4 below shows the measured decrease in flow rate for the silica sol deposited on the cylindrical gamma-alumina support.

【0031】

[Table 4]

Table 4 Cumulative operation Sol level Incremental evaporation rate Overall evaporation rate Decrease in hours (minutes) (ml) (ml / hour) (ml / hour) 0 0.15 ---- ---- 14 0.43 1.20 1.20 29 0.76 1.32 1.26 60 1.23 0.91 1.08 109 1.80 0.70 0.91 133 2.00 0.50 0.83 225 2.60 0.39 0.65 386 3.52 0.34 0.52 438 3.80 0.32 0.50 The microporous ceramic membrane is considered to have an average pore size adjustable within the range of 5-100 angstroms. Since the membrane is formed in the pores of the support, the overall porosity is low, generally 30% or less. Microporous membranes with pores of 100 angstroms or less can be used for ultrafiltration, and microporous membranes with pore sizes of 5-30 angstroms can be used for reverse osmosis and molecular sieving. Due to the durability of the ceramic material, the membrane can withstand large pressure drops and is effective for various industrial applications.

[Simple explanation of drawings]

[Figure 1]

It is the schematic which shows the concept of the method of this invention.

[Figure 2]

It is another schematic showing the concept of the method of this invention.

[Fig. 3]

It is a drawing which shows an Example of the apparatus which can be used for carrying out the method of this invention.

[Fig. 4]

It is a drawing which shows the other embodiment of the apparatus which can be used for practicing the method of this invention.

[Explanation of symbols]

10 Porous support 12 Capillary flow 14 gas flow 20 Porous support 22 Maximum layer (support) 24 2nd layer (1st intermediate layer) 26 3rd layer (2nd intermediate layer) 28 Microporous ceramic membrane layer (microporous membrane, ultrafiltration layer, reverse osmosis layer) 30 Reaction vessel 32 entrance port 34 Exit port 36 Stopper 38 Porous support 40 U-shaped glassware 42 Tigon tube 130 Reaction vessel 132 entrance port 134 Exit port 138 Porous support 140 Flared tube 142 glass frit 144 Leveling Chamber