WO2000013768A1

WO2000013768A1 - Supported glass membrane for gas separation

Info

Publication number: WO2000013768A1
Application number: PCT/US1999/020913
Authority: WO
Inventors: George Gavalas; Huanting Wang
Original assignee: California Institute Of Technology
Priority date: 1998-09-09
Filing date: 1999-09-09
Publication date: 2000-03-16

Abstract

An element for separating gases is disclosed. A porous alumina tube (100) is coated with a mixed glass particle layer (102). The layer is annealed to cause phase separation. The phase separated glass layer (110) is then acid-leached to remove one of the phases.The remaining phase is porous, and used as a supported gas separation membrane.

Description

SUPPORTED GLASS MEMBRANE FOR GAS SEPARATION

Background

Membranes can be used to separate gases. Membranes have certain advantages when used for gas separation, because of their low energy consumption and their suitability for small volume as well as large volume applications. At present, the commercially-used gas separation membranes are polymeric in hollow fiber or spiral wound geometries.

Inorganic gas separation membranes have certain additional advantages including the ability to operate at elevated temperatures. This ability is essential in certain integrated membrane reactor assemblies. These membranes are also capable of higher separation selectivity than polymeric membranes for certain gas mixtures.

Inorganic membranes are made in unsupported form or supported form. Unsupported membranes are tubes, hollow fibers, or plates wherein the entire thickness of the membrane possesses the separation property. Supported membranes, on the other hand, consist of a thin layer of the material that provides the separation property deposited on a porous ceramic support. The support can be, for example, a tube or plate that provides the mechanical strength. The ceramic support is often porous alumina, although other materials have been used. The supported form provides high productivity and mechanical strength. The cost of the ceramic support and the steps of depositing the layer often increase the cost of supported inorganic membranes as compared with the polymeric membranes .

Two key measures of membrane performance are productivity and selectivity. Membrane productivity is quantified by the permeance Q, which for a gas i is defined as

where Qi is the flow rate through the membrane in mols per unit time, Δpi is the partial pressure difference of component I between the feed and permeate sides and A is the membrane surface area. The membrane surface required for a given production rate is essentially inversely proportional to the membrane permeance. Hence, permeance has a direct influence on capital cost.

Membrane selectivity refers to the relative permeation rate of two or more gases. The ratio of the permeances of two gases, Qi/Qj , measured one gas at a time, is often called the ideal selectivity or ideal separation factor. More important is the permeance ratio measured for two gases in a mixture, for which we will use the term mixture selectivity, or simply, selectivity, to distinguish it from the ideal selectivity. The mixture selectivity is the relevant property in regard to actual applications. The function of a membrane is to separate the feed gas into two streams. One stream, usually called the permeate gas, consists of the gas that permeated through the membrane and is enriched in the components that have higher selectivity. The other stream, usually called the retentate gas, is enriched in the components that have lower selectivity. Membrane properties generally depend on temperature, total pressure and mixture composition. Under conditions of sufficiently low pressure and/or sufficiently high temperature, the permeance and selectivity become independent of total pressure and composition. Permeation is then said to take place in the Henry's Law regime.

Inorganic membranes can be classified as dense or microporous . Dense membranes made of palladium and other metals are very selective to hydrogen permeation. Other dense membranes are made of ion conducting perovskite or other complex oxides and can be selective to oxygen permeation. Dense silica membranes selective to hydrogen permeation have also been fabricated. Microporous silica, carbon and zeolite membranes have been developed that are suitable for a variety of separations, important examples of which are carbon dioxide - methane, olefin-para fin and branched alkanes-straight chain alkanes. Carbon membranes are also suitable for oxygen-nitrogen separation.

Microporous silica membranes have been studied extensively because of their wide applicability. A common type of supported microporous silica membrane is made by coating a mesoporous support having pore diameter of about 50nm with a polymeric silica solution followed by drying and calcination.

A type of unsupported microporous silica membranes is made in the shape of hollow fibers that are typically about 50-100 μm outside diameter and 5-25μm wall thickness. Membranes of this type are described in U.S. Patent Nos . 4,842,620 and 4,853,001. These microporous glass fibers are made by extrusion of a borosilicate glass melt containing Si0₂, B₂0₃, and Na₂0 with possibly smaller quantities of other oxides like A1₂0₃ and Zr0₂. After cooling, the fibers are leached with an acid solution which removes much of the B₂0₃ and Na₂0 contents leaving behind a microporous silica fiber with low levels of B₂0₃ and other oxides. U.S. Patent 4,853,001 describes a glass membrane with room temperature C0₂:CH₄ selectivity of 425 for a C0₂-CH₄ mixture. Summary The present system teaches a new type of inorganic membranes that have some special separation properties. In contrast to other membranes reported in the literature, the membranes of the present invention have very low permeance for carbon dioxide relative to certain other gases and are, therefore, suitable for the separations requiring C0₂ to largely remain in the retentate gas. In particular, the membranes of the present invention provide very high selectivity for CH₄, and low selectivity for C0₂ in the separation of CH₄-C0₂ mixtures, in contrast with the membranes described in patents 4,842,620 and 4,853,001, and other membranes reported in the technical literature, that have high selectivity for carbon dioxide. The membranes of the present invention is especially useful in separating methane from natural gas that contains large quantities of carbon dioxide where it is desired that methane is removed as the permeate gas and most of carbon dioxide remains in the retentate gas . Brief Description of the Drawings Figures 1A -IE show steps of forming the gas separation membrane;

Figures 2 and 3 show an elemental composition profile; and Figure 4 shows a graph of gas selectivity based on a function of sintering temperature.

Description of the Preferred Embodiment Certain glass compositions form a single phase at sufficiently high temperatures, but upon cooling to some intermediate temperature and kept at that temperature for a certain length of time (annealing) separate into two intertwined phases. The sizes of the domains of the phases depend on the glass composition and the temperature and duration of annealing. One of the two phases is soluble in acid solutions, while the other phase is insoluble.

The phase-separated glass is leached in an acid solution to remove the acid-soluble phase. This leaves behind a porous glass that is only of the insoluble phase. Increasing the temperature and duration of annealing increases the size of the domains of the two phases, and therefore, increases the pore size of the glass obtained after leaching. This technique is used in the Corning Vycor process typically to produce porous glass with pore diameters of 3-10nm. Diffusion in such pores follows approximately the Knudsen relation, where the permeation rate is inversely proportional to the square root of the molecular weight. The, resulting selectivity is low and has not been of practical interest in gas separations like those of 0₂-N₂ and CH₄-C0₂ mixtures . To improve the selectivity, it is necessary to decrease the pore size.

The present application describes a new type of glass membrane which has small pore size and modified chemical composition. The membrane has unique separation properties described in the Summary. Since these membranes are supported, they have good mechanical strength.

FIGs. 1A-1E show the steps of the process for forming the preferred system of the present application. FIG. 1A shows the porous alumina tube 100 that is used as the membrane support. A glass of a phase-separable composition is first ball-milled in a solvent to form a suspension of micron-sized particles. A thin layer of these glass particles 102 is formed on one of the two cylindrical surfaces (inside or outside) of the tube 100 by dip coating. FIG. IB shows the glass particle layer 102 deposited on the inside surface. The suspension viscosity and content of the glass, and speed of withdrawal can be used to adjust the thickness of the particle layer.

After drying, the coated tube is inserted for a few minutes into a furnace that is preheated to a temperature between 750 and 1200DC. This heating treatment is used to sinter or melt the particle coating into a substantially continuous layer. The particles melt or sinter, depending on temperature, into a smooth non-porous coating of glass. The furnace temperature is preferably sufficiently high to cause sintering of the particles but not as high as to form a low viscosity melt that will penetrate into the pores by capillarity. Raising the sintering temperature also increases the extent of alumina dissolution into the melt.

The supported glass layer is then removed from the furnace and left to cool in ambient air. Fig. 1C shows an optional step of heating/annealing the glass in order to enhance phase separation of the glass. This heating/annealing step is carried out at 500-600DC for a specified length of time. At this point the coating is the phase-separated glass layer 110 shown in FIG. ID. It has been found that increasing the annealing temperature and time increases the pore size of the final membrane. A smaller pore size is obtained when there is no annealing step whatsoever. The device is then leached with an acid solution shown in FIG. IE to remove some of the soluble components, principally boron oxide and sodium oxide. This removal makes the coating porous . The extent of removal of boron oxide and sodium oxide during the leaching step can be reduced by one of the following methods, singly or in combination: eliminating the annealing step, reducing the severity of leaching, or introducing alumina into the melt.

The severity of leaching is determined by the temperature, duration and acid strength employed. Longer leaching, stronger acid, and higher temperature cause more extensive removal of boron oxide and sodium oxide and vice versa.

Introducing aluminum oxide or certain other oxides in the glass melt hinders the removal of sodium oxide and boron oxide during leaching. Alumina can be introduced by adding alumina powder in the original particle suspension. Alumina that is dissolved in the glass coating during the melting/sintering step has the same effect, namely to restrict the removal of sodium oxide and boron oxide solubilization during leaching.

Decreasing the removal of sodium oxide and boron oxide during leaching results in a decrease of the pore size and a change in the chemical composition of the pore surface. By decreasing the membrane pore size, the ability to separate gases is improved. More significantly, the residual sodium oxide content of the membrane sharply lowers the permeance of carbon dioxide and is, therefore, essential for the high methane selectivity in methane-carbon dioxide separation. The low permeance of carbon dioxide is likely due to strong binding with basic sites formed on the membrane pore surface in the presence of sodium oxide. Other alkali oxides such as potassium oxide and lithium oxide and rare earth oxides like calcium oxide or magnesium oxide are expected to have a similar effect.

After leaching and drying, the membranes were tested in a permeation cell. One of the two sides, "the feed side", of the membrane is exposed to the feed gas mixture, e.g. CH₄-C0₂. The other side, "the permeate side" is exposed to a flow of helium gas. The permeate gas is channeled to a gas chromatograph with a thermal conductivity detector. EXAMPLE 1

A glass, having composition 9%Na₂O-30%B₂O₃-61%SiO₂, by weight, was ball-milled in ethanol containing 0.7 wt%

polyvinylbutyral (PVB) to a suspension of particles below 10 μm

(micrometers). The particle loading of the suspension was 10%.

A porous α-Al₂0₃ tube (ID 6mm, OD 9mm, mean pore diameter 0.2μm)

was dip-coated in the suspension. After drying, the coated tube was inserted for 15 minutes in a furnace preheated to 1100DC. The tube was then cooled in ambient air and leached for 1 hour at 90DC in IN HC1. The leached tube was dried and mounted in the permeation cell and tested for separation of CH₄-C0₂ mixtures. The results with a CH₄-C0₂ mixture are presented in Table 1. This membrane was also tested in the separation of C0₂- N₂ mixtures and N₂-CH₄ mixtures. For a 60-40 molar C0₂-CH₄ feed gas, the N₂ permeance was 1 MPU and the N₂:C0₂ selectivity was 30, at room temperature. For a 50-50 molar N₂-CH₄ mixture the N₂ permeance was 1 MPU and the N₂:CH₄ selectivity was 18, also at room temperature.

EXAMPLE 2 A support tube was coated with the same suspension as in Example 1. After drying the tube was inserted for 15 minutes in a furnace preheated to 1100DC and then cooled in ambient air. The tube was subsequently annealed at 400DC for 2 hours and then leached for 3 hours at 90DC in 1 N HCl solution. The permeation results obtained with this membrane are also listed in Table 1.

EXAMPLE 3

A support tube was coated with the same suspension as in Example 1. After drying it was inserted in a furnace preheated to 1000DC for 30 minutes and then cooled in ambient air. The tube was then leached for 2 hours at 90DC in 1 N HCl solution. The permeation results obtained for this tube are included in Table 1.

EXAMPLE 4

A support tube was coated with the same suspension as in Example 1. After drying it was inserted in a furnace preheated to 1100DC for 30 minutes and then cooled in ambient air. The tube was then leached for 1.5 hours at 90DC in 1 N HCl solution. The permeation results obtained for this tube are included in

Table 1.

EXAMPLE 5

A support tube was coated with a suspension containing 5% glass particles but otherwise being identical to that of Example 1. After drying it was inserted in a furnace preheated to 1150DC for 15 minutes and then cooled in ambient air. The tube was then leached for 1 hour at 90DC in 1 N HCl solution and subsequently kept in water for 10 hours. The permeation results obtained for this tube are included in Table 1.

EXAMPLE 6

A support tube was coated with a suspension containing identical to that of example 4. After drying it was inserted in a furnace preheated at 800°C for 15 minutes and then cooled in ambient air. The tube was then leached for 15 minutes in I N HCl solution and subsequently kept in water for 10 hours. The permeation results are included in Table 1. Effect of Sintering Tempera ture on Residual Sodium and Boron Content

The elemental composition of the cross sections of glass membranes prepared by sintering at 800°C or at 1100°C and leached at 90°C in 1 N HCl for 15 minutes or 1 hour was measured by electron microprobe analysis. The elemental composition is plotted in Figures 2 and 3 in terms of the boron to silicon and sodium to silicon molar ratios versus distance from the outer surface of the membrane. The results show that the boron and sodium contents increase with distance and are higher at the higher sintering temperature. These results are explained by the effect of dissolved alumina which increases as the glass support interface is approached (increasing distance from the surface) and is also higher at the higher sintering temperature.

Effect of Sintering Tempera ture and Leaching Time on CH₄ : Cθ₂ Selectivi ty

A series of membranes were prepared according to the procedure of Example 6 except that the leaching time and sintering temperature was varied. The results are shown in Figure 4. As shown in the figure at sintering temperatures below 900°C, a lower leaching time is needed to achieve high selectivity .

Effect of Feed Gas Composition

The permeance and selectivity of the supported glass membranes were observed to depend on the composition of the feed gas. FIG. 5 shows the permeance and selectivity of the membrane of Example 4 versus composition of the feed gas at a total feed gas pressure of 1 atm. The selectivity is very high at low CH₄ content and declines with increasing CH₄ content.

To examine whether dissolved alumina is necessary for the unique permeation properties of the supported glass membranes of the present invention the experiment described in example 8 was performed.

EXAMPLE 7

A membrane was prepared according to the procedure of example 6 except that instead of using an alumina tube, a porous Yttria-stabilized Zirconia disk was used. For a 60-40 C0₂-CH₄ mixture the CH₄:C0₂ selectivity at ambient temperature and at

150°C was 30 and 110 respectively. Thus alumina is not a necessary ingredient of the membranes and other materials can be used as membrane supports.

Table 1. CH -CO₂ separation by supported glass membranes. The feed mixture was 60%CH-ι- 40%CO by volume, the feed pressure was 1.4 bar, and the permeation temperature was 25- 30°C.

* m.UVeaU.suU-re.dU b1/y weighing the tube before leaching

** lMPU=10^"8mol/m^2"-s-Pa

Other modifications are contemplated.

Claims

What is claimed is:

1. A method of forming a membrane, comprising: obtaining a glass composition which has two separate but intertwined phases, one of which is more soluble in acid solutions and the other of which is less soluble in acid solutions; using an acid to remove the more soluble acid phase to form a porous glass; and controlling parameters to control the size and adsorptive properties of pores in said porous glass to provide selectivity to a desired component.

2. A method as in claim 1 further comprising forming said porous glass on a porous supporting tube.

3. A method as in claim 2 wherein said porous tube is formed of alumina.

. A method as in claim 1 further comprising forming said glass by ball milling a phase separable composition of glass to form a suspension of particles.

5. A method as in claim 2 further comprising forming said glass by ball milling a composition of glass to form a suspension of particles and further comprising dip coating or slip casting the glass particles on the tube.

6. A method as in claim 2 further comprising coating the tube with glass particles and heating the glass particles to melt or sinter the coating into a substantially continuous layer .

7. A method as in claim 6 wherein said heating is between 750 and 1200 C.

8. A method as in claim 6 further comprising annealing the glass to enhance phase separation.

9. A method as in claim 1 wherein said glass includes a boron oxide and an alkali metal oxide as the acid soluble components .

10. A method as in claim 9 wherein said alkali metal oxide component is sodium oxide.

11. A method according to claim 9 where the glass composition includes Si0₂, B₂0₃, Na₂0 in weight percentages 56- 75%, 20-35%, 4-12%, respectively.

12. A method according to claim 11 where the glass composition is 58-64% Si02, 25-33% B203 , and 6-10% Na20, by weight .

13. A method according to claim 12 where the glass composition includes also quantities of Al₂0₃ and Zr0₂.

14. A method as in claim 1 wherein said porous sizes are optimized to separate C02-containing mixtures where C02 is selectively obtained in the retentate gas.

15. A method for forming a microporous glass membrane supported on a porous support comprising:

(a) providing an inorganic support element having an outside and an inside surface and a thickness therebetween which define a support wall with pores

extending through the support wall between the outside

and the inside surfaces;

(b) forming a suspension of small particles of a phase- separable glass in a solvent;

(c) coating at least one the surfaces of the support element with a thin layer of the suspension;

(d) removing the solvent from the coated support by drying and then heating and maintaining for a specified length of time at a sufficiently high temperature to

cause melting or sintering of the particles to a continuous glass layer; and

(e) leaching the annealed glass-coated support in an acid solution at a specified temperature for a specified length of time to render the support porous.

16. The method of claim 15, where the glass particles contained in the coating suspension have a phase separable composition microporous glass fibers are made by extrusion of a borosilicate glass melt containing Si0₂, B₂0₃, and Na₂0 with other oxides .

17. The method of claim 16, where the glass particles

contained in the coating dispersion are smaller than 10 ╬╝m.

18. The method of claim 15, where the coating suspension consists of water, ethanol, acetone or some other solvent.

19. The method of claim 18, where the coating suspension contains a dispersant.

20. The method of claim 16, where the glass particles consist of a Vycor-brand glass containing Si0₂, B₂0₃, Na₂0 in weight percentages 56-75%, 20-35%, 4-12%, respectively, and which may contain in addition A1₂0₃.

21. The method of claim 15, further comprising annealing the glass-coated support at some specified temperature for a specified length of time.

22. The method of claim 15, where annealing is carried out at a temperature 300-600DC for at least 5 minutes to as long as 5 hours .

23. The method of claim 15, where said leaching is carried out in an aqueous solution of a strong acid of 0.1-5 mol per liter concentration, temperature from ambient to 100DC for a time between 1 minute and 10 hours.

24. The method of claim 15, where the inorganic support is a porous ceramic tube or plate made of one of the following:

alumina, mullite, zirconia, titania, with pore size 0.06-2╬╝m

throughout the thickness of the wall of the support.

25. The method of claim 15, wherein said phase separable glass includes an alkali metal oxide is one of Na₂0, Li₂0, or K₂0.

26. The method of claim 15, where the inorganic support wall consists of at least two layers of decreasing thickness and decreasing pore size with the smallest pores as small as 3nm and where the different layers may consist of different ceramic materials which is one of alumina, mullite, zirconia, or titania .

27. A method for the separation of CH₄-C0₂ mixtures, comprising separating CH₄ as a faster permeating gas from the mixture .

28. The method of claim 27, where the CH₄-C0₂ mixture also contains other gases such as C1-C4 hydrocarbons, H₂S, and N₂.

29. A method for the separation of mixtures of C0₂-N₂, comprising separating N₂ as the faster permeating gas.

30. A method for the separation of mixtures of N₂-CH₄, comprising separating N₂ as the faster permeating gas.

31. A method of claim 29 where the mixture also contains other gases including C1-C4 hydrocarbons.

32. A method for separating gases, comprising: providing an inorganic porous support element, forming a suspension of small particles of a phase- separable glass in a solvent, coating at least one the surfaces of the support element with a thin layer of the suspension, phase separating the suspension after said coating, and leaching the annealed glass-coated support in an acid solution at a specified temperature for a specified length of time to render the support porous; and using the porous support to separate gases.