WO2007037562A1

WO2007037562A1 - Water vapor selective composite membrane and its preparing method

Info

Publication number: WO2007037562A1
Application number: PCT/KR2005/003216
Authority: WO
Inventors: Kew-Ho Lee; Bongkuk Sea
Original assignee: Korea Research Institute Of Chemical Technology
Priority date: 2005-09-28
Filing date: 2005-09-28
Publication date: 2007-04-05

Abstract

The present invention relates to a composite membrane selective to passage of water vapor and its preparation method, and particularly relate to a composite membrane selective to passage of water vapor, wherein a Ϝ-alumina middle layer and a silica layer of gas molecular sieve are laminated in this order on porous support, and an active layer of Ϝ-alumina for separation of water vapor with thin thickness is coated on the upper surface of the silica layer of gas molecular sieve, thus enabling to serve as a molecular sieve that permeates water vapor among high-temperature gas mixture selectively and efficiently and to be used in the process of removing, separating and purifying a gas mixture comprising water vapor of higher than 200 0C and in the field of a catalytic membrane reactor for dehydration at high temperature, and its preparation method.

Description

WATER VAPOR SELECTIVE COMPOSITE MEMBRANE AND ITS PREPARING METHOD

TECHNICAL FIELD The present invention relates to a composite membrane selective to passage of water vapor and its preparation method, and particularly relates to a composite membrane selective to passage of water vapor, wherein a γ-alumma middle layer and a silica layer of gas molecular sieve are laminated in this order on a porous suppoit, and an active layer of γ-alumma for separation of water vapor with thin thickness is coated on the upper suiface of the silica layer of gas molecular sieve, thus enabling to serve as a molecular sieve that selectively permeates water vapor out of a high-temperature gas mixture with efficiency and to be used in the piocess of removing, separating and purifying a gas mixtui e comprising water vapor ot higher than 200 ⁰C and in the field of a catalytic membrane reactoi for dehydration at high temperature, and a method of its pi epaiation

RELATED PRIOR ART

Si/e and distribution of poi es are major factois that deteimme se paration performance in a poious membrane for sepaiatmg gases For example, majority of gas molecules ma} simultaneously pass through a membrane having pores of dozens of nanometers A membrane having mesopores (i e , pores of several nanometers) preferentially peimeates gas molecules with low molecular weight (not smallei molecules) according to Knudsen diffusion mechanism Theoretical sepaiahon coefficient, which is determined by molecular weight (not by molecule size) aie known to be \ cπ low [H P Hsieh, 'Inoi game Membranes foi Sppaudion and Reaction"

ier, NL, 1996] Thciefoie, the poies in a membrane for gas separation should be conti oiled at the

el of nanometers to prepare a moleculai sie\ e that sepaiates gas molecules based on the molecule size "Moleculai sieving effect", which refers to a separation mechanism based on a molecule size, ma}- be accomplished by a membrane having pores of molecule size A moleculai sieve is ideal with respect to separation coefficient because molecules biggei than pores cannot permeate the membrane In addition to the separation peiformance, permeation rate or permeation flux is one of the important factors to consider before applying it in an industrial piocess Io maintain separation performance and also increase permeation flux, it requnes a control on the size of moleculai -sieve pore and an improvement of chemical affinity between permeates and membrane suiface That is, size control in the iange of smaller than a few nanometers and hydrophilicity of the membrane sui face is necessary for increasing both separation performance and selectivity toward water vapor therefore, uniformly-distributed pores having desired size and chemical affinity between permeates and membrane become serious pioblems to be solved Meanwhile, with regard to water vapor selectiv e membrane, it has been repoi ted mainly about pervaporation membrane, which dehydrates and separates organic mixtuie at below 150 ⁰C Especially, ZSM-5 zeolite membiane is known to ha\ e supenor selective permeation performance as water vapoi selective membrane foi dehydiatmg alcohol mixture, due to its chemical and snuctuial property Despite their uniform cn stalhne structure, pores of these zeolites aie mappiopπate foi sepai ating water λ'apor fiom a gas mixture of such as nitrogen (0 % nm), carbon dioxide (0 V> nm) and methane (0 ^8 nm) because of their pore size of 04-0 5 nm Further, to efficiently iemo\ e water vapor without going through with cooling-heating steps w hile piepaπng methanol and syngas in the petiochemical mdustiλ , it requnes a separation membiane that can be used at high temperature without lowering the temperature of the piocess

Since the size and distribution of pores decisively affect the separation perfoimance of a poious membrane foi sepai ating gases, they iemam as serious technical problems to be resolved to introduce a membrane that can serve as a layer for selective sepaiation because of a controlled pore size with respect to molecule size and umtoim pore distribution Especially, for selectively controlling target materials, the improvement in chemical affinity between permeates and membrane surface are also important Iheiefore, it is m urgent need to develop a novel technique capable of controlling pore size and chemical affinity between permeates and membrane surface

The present inventors have performed extensive researches to develop a membrane that can serve as a molecular sieve and also has superior selectivity and permeation toward water vapoi at high temperature As a result, they finally obtained a composite membrane selective to passage of watei vapor bv performing (i) forming a γ-alumina middle layer having a uniform distribution of mesopores with 2-10 nm size and 2-3 μm thickness on a porous support, (ii) forming a silica layer of gas molecular sieve having a uniform distribution of micropores of 03-0 5 nm on the γ-alumina middle layer, (m) coating an active layei of γ-alumma for separation of water vapoi having a uniform distribution of mesopores with 2-7 nm m size and 02-0 5 of thickness on the upper surface of the silica layer, and (iv) heat-treatment, wherein the active la\ er of γ-alumina for separation of water vapor is further coated between the conventional silica moleculai sieve layei and the alumina layer, and completed the present invention

Therefore, the present invention aims to provide a composite membrane selectively permeable to water vapor which serves as a moleculai sieve -with improved selectivity and permeation toward water vapor at high-temperature

DETAILED DESCRIPTION

In one aspect of the present invention, there is provided a composite membiane selec tive to passage of water vapoi, wherein a γ-alumind middle layer ha\ ing a uniform distribution of mesopores and a silica layer of gas moleculai sie\e having a uniform distribution of micropores are laminated in this order on a porous suppoit, and an active layer of γ-alumina for separation of water vapor , wherein mesopores ai e uniformly distributed and Bronsted acidic sites aie foimed, is coated on the upper surface of the silica layer Hereunder is provided a detailed explanation on this aspect tor superior separation performance, a composite membrane for gas separation should geneially meet the requirements of (i) a high permeation rate caused by a thm membrane, (11) a quality separation layer without defects or pinholes, (in) a uniform pore distribution, (iv) a support having substantially no permeation resistςince, and (v) durability and chemical stability Further, a selective membrane should be chemically compatible with the target material to be separated

Considering the aforementioned aspects, the piesent invention provides a porous composite membrane with hydrophilicity that can function as a molecular

\ apoi from a gas mixture at high-temperature, which compi ises a porous support, a middle layer, an active lay er for separation and a

In drophilic coating layer

Figure 1 is a sectional drawing of a (composite) asymmetric composite membrane having a chemicall₎ modified surface

Korean patent publication no 2003-61146, filed by the present inventors, discloses a poious composite membrane, which compiises a porous suppoit, a silica layei

ing miciopores, and γ-alumma middle layer having a unifoim distribution of mesopores between silica layers having micropores, thus enabling to function as a moleculai sieve and impiove gas permeation rate with the increase of temperature

This membrane, howev er, does not show the selectivity depending on the molecule size which may limit its use m a petiochemical process where i each on \ ield may gieath depend on the water t apor content

In contiast, the composite membiane heiein has the chaiacteiistic of the membiane disclosed in Koiean patent publication no 2003 61146 and also ma\ exhibit superior selectivity and permeation due to the hydrophilicity on the surface The chemical modification of silica surface by alumina ma_j form Si(OH)Al-structured Bronsted acidic site and allow water vapoi to preferentially adsorb on the surface, thus increasing the hydrophihαty The Bronsted acidic site is not a simple composite layer physically formed on the interface between γ-alumma middle layer and silica gas molecular sieve layer When a silica gas molecular layer is introduced on the γ-alumma middle layer, the physical layer lamination can be achieved without causing any chemical or structural change on the surface because the γ-alumma middle lavei requires comparatively high temperatuie foi layer formation On the contrary, when an active layer of γ-alumina for separation of water vapor is introduced on the silica layei of gas molecular sieve, the active la\ cr of γ-alumina for separation of water vapor requires a relatively higher temperature than those of the suπ oundmg layers, the chemical structuie and characteristics on the surface of the silica gas molecular layer The Bionstcd acidic site is foimed m the present invention for this reason That is, a simple physical lamination happens or a surface with a new structure may foi m due to chemical change depending on the temperature of heat-treatment in each layer K4eanwhilc the γ-alumma middle layei is intioduced foi impiovmg mechanical piopeitv and it is not involved in the separation and permeation Further, the γ-alumina middle laver, a three-time coated thick layei, is totally diffeicnt from the actrv e layei of γ-alumina foi separation of water vapor m its structure and function, w hich is introduced for selectπ e pei meation of w ater \ apor

Hei eunder is prov ided a detailed piocess for a composite membrane selectn eh permeable to watei vapoi Fust, a γ-alumma middle layer having a uniform distribution of mesopores

^•w ith 2-10 nm of pore size is formed on the porous support bv sol-gel method

It is important to select a porous support that can minimize shear stress i esulted fiom the diffeience in thermal expansion coeffic ients Examples of the porous support include without limitation alumina 01 Vycor glass with 100-200 nm

To increase selective permeation of upper surface of the porous support, γ-alumina middle layer having mesopores with 2-10 nm ot pore size is introduced by sol-gel method Specifically, 0 5-0 7 mol/ L of alumina sol is prepared and the porous support is immersed in the alumina sol at a rate of 1 0-5 0 mm/ sec for 10-20 sec Then, it is dned for 20-30 hrs in a chamber where temperature and humidity are controlled at 15-25 ⁰C and 35-45%, respectively, followed by heat treatment at 700-750 ⁰C for 30-60 mm The aforementioned conditions m coating, drying and he at- treatment processes are preferred to be optimized because they play an impoitant role m forming a quality mesopore layei An alumina layer with 2-10 nm of poi e size and 2-3 μm of thickness is introduced by repeating the piocesses 2-4 times

Second, a silica layei possessing molecular sieve characteristics was laminated on the γ-alumina middle layer by chemical vapor deposit method followed by foimation of an active lay er of alumina foi selective separation of water \ apor by chemically modifying the laminated silica surface with alumina sol

Silica layer is formed by the same CVD method and device as disclosed in Korean patent publication no 2003-61146 Λ poious suppoit inti oduced with the γ-alumma middle layei was placed inside quaitz tube ieactor and equipped m an electric furnace Silane compound was dried and intioduced m a chamber where the temperature is maintained at 313-333 K, followed by heat treatment at 600-650 ⁰C, thus foi mmg a silica layer with thickness of 0 2-0 5 μm on the surface of the Y alumina middle layei After 0 5-0 7 mol/1 of alumina sol is prepared, the composite membiane is immersed in the porous suppoit/ γ-alumina middle layei/ silica at a rate of 1 0 5 0 mm/sec foi 10-20 sec Then, the membi ane is diicd in a chamber wheie humidity and temperatuie aie maintained at 35-45% and 15 25 ⁰C lespectiveh , toi 20-30 hrs followed bv heat treatment at 700-750 ⁰C for 30-60 mm Because the chemically modified layer ma\ serve as a barrier to gas permeation as it grows thick, the thickness is preferred to be contiolled vwthm 0 2-G 5 μm For lhis puipose, the conditions such as immersing time aie preferred to be controlled as mentioned above Alumma-sihca will not be sufficient to exhibit hvdrophihcity if the thickness of the chemicallv modified layer is below 02 μm whereas gas permeation rate may be decreased if it is above 05 μm

Sihca-alumma selective active layer is formed b) chemical modification of silica surface with alumina sol, specifically by performing the immersing, diymg and heat treatment process under the aforementioned condition The composite membrane herein thus prepared is superior in physical properties such as thermal stability at abov e 200 ⁰C, molecular sieving chai acteπstics and water vapoi selectivity and permeation, thus being effective in selectively permeating water vapoi at high temperature the aforementioned composite membrane has water vapor permeation of 10-100x10⁸ (ITIoI m ² S ¹ Pa ¹) at 200-300 ⁰Q with the selectivity (water vapor/nitrogen) of 10-100 piefeiablv 30-98, thus being very useful m separating 5-50 mol% of water vapor from the gas mixture comprising H2, N2, CO2 and C_nH2n+2

BRIEF DESCRIPTION OF DRAWINGS Figuie 1 is a sectional drawing of a hv diophilic multi layered composite membiane according to one aspect of the present invention

Figui e 2 is a dev ice tor measuπng gas permeation of a membiane accυi dmg to one aspect of the present inv ention

EXAMPLES

The piesent invention is described in greater detail bv the following Examples Howev ei the_\ should not be construed as limiting the scope of the present invention Example 1

A γ-alumina middle layer, a silica layer as a gas molecular sieve and an active layer of γ-alumma for separation of water vapor weie laminated in this order on the α-alumina support tube with the average pore size of 150 ran, thereby preparing a composite membrane

First, a γ-alumma middle layer was formed on an α-alumma suppoit tube by sol-gel method Specifically, sol was prepared by admixing 0 09 mol of aluminum isopiopoxide with 1 L of water, an α-alumma tube (YCF-O I , Dongseo Corp , KR) was immersed in the sol at a rate of 3 0 mm/ sec for 50 sec The alumina tub was taken out of the sol and dried at a chamber where humidity and temperature were maintained at 40% and 18 ⁰C, respectivel) , for 24 hrs, followed by heat treatment at 700 ⁰C for 60 mm A γ-alumina layer with an average pore size of 7 nm and layer thickness of 3 μm was formed bv repeating the immersing, drying and heat treatment foi 4 times

Second, a silica la\ er as a gas molecular sie\e was formed on the upper surface of the γ-alumma middle layei by chemical vapor deposition (CVD) method Specifically, an alumina tube with the γ-alumina middle Ia) er was placed inside quartz tube ieactor (Segi Eng Corp , KR) and equipped in an electric furnace (Yulsan Coip , KR) Further, liquid silanes comprising above 99 9% of tctraethoxy silane was placed in a constant temperature chamber (40 ⁰C), dried by using nitrogen and mtioduced into a reactoi, followed b\ CVD at 600 °C, thereby forming a coating layer on the γ-alumina middle

er After coating for 20 mm, silica layer giew so that the aveiagc pore size is 0 4 nm and the thickness is 0 2 μm Then, the active layer of γ-alumma foi separation of water vapor was foimed on the uppei sui face of the silica layei Specifically, the silica-coated tube was chemicall) modified bv immeising the tube in alumina sol at a iate of 1 0 mm/ sec for 10 sec The tube was taken out of the immersing solution dried in a chamber where humidity and temperature were maintained at 40% and 18 "C, respectively, for 24 hrs, followed by heat treatment at 700 ⁰C for 30 min, thereby forming an active layer ot γ-alumma for separation of water vapor Thus formed active layer of γ-alumma for separation of water vapor was analyzed to have an average pore size 5 of 5 nm and thickness of 02 μm

As mentioned above, a composite membrane where an α-alumma support tube, a γ-alumma middle layer, a silica layer as a gas molecular sieve and an active layer of γ-alumina for separation of water vapor was laminated in this order, was prepared lϋ

Example 2

The experiment was performed same as in Example 1 except that immersing time was 20 sec during the chemical modification foi forming the actn c layer of γ-alumina tor separation of water vapor The active layer of γ-alumma for

15 separation of water vapoi was analyzed to have an average pore size of 5 nm and thickness of 0 4 μm

Comparative Example 1 lhe experiment was performed same as in Example 1 except that the chemical 20 modification for forming the active layer of γ-alumina for separation of water vapor was not earned out

Comparative Example 2

The expeiiment was performed same as m Example 1 except that the chemical 5 modific ation for foimmg the actn e layer of γ-alumina foi separation of v\ ater vapor was not carried out and silica was coated for 60 mm b} CVD on the γ-alumma layer

The silica selective layer was analyzed to have an average poie size of 04 nm and thickness of 02 μm Experimental Example: Gas permeation experiment

Gas permeation experiment was performed by using selective layer prepared in Examples and Comparative Examples

[Experimental Method]

Gas permeation experiment was performed at 250 ⁰C by using It, CO2, N2 and CH4 (purity: above 99.9999%). Permeation flux was measured while allowing a gas to flow at a concentration of 20-50% in a carrier gas. Argon gas was allowed to flow at the tube-side of the selective membrane as a sweep gas and concentration of the permeated gas was measured by using gas chromatography (model: GC-14B, Shimadκu Corp.) equipped with TCD-detector. A molecular sieve (MS-5, 3m) was used as a column, the temperature detected by an oven was 100 ⁰C, and the detector current was 50 mA. The pressures of both the shell-side and the tube-side were maintained at atmospheric pressure. Figure 2 is a gas permeation experiment device. Permeance of i component, F, [mol-m^~2-s^~1-Pa^~1], is defined as in the following Equation 1 :

Equation 1 F₁ = (Q₁)/ [(r_m)(l_mχiVPp ,)] = (J₁)Z(AP₁) wherein Qi is a permeation flux of i component (mol/s), r_m is a radius of a membrane (m), l_m is a length of a membrane (m), P_t> and P_P/1 are partial pressure at shell-side and tube-side, respectively [PaJ, J, is a flow rate of i component (mol/m²-s), and /^Λ P, log-mean partial pressure difference along the membrane length.

Table 1

Permeation

Membrane Conditions* Selectivity (mol-m^-s-ⁱ-Pa-^xlO-⁸ Example 1

Example 2

Comp. Ex. 1

Comp. Ex. 2

' Conditions for preparing final coating layer

Table 1 shows the results of water vapor permeation experiments, which were performed at 250 ⁰C by using composite membranes prepared in Examples 1-2 and Comparative Examples 1-2. All the membranes above showed the molecular sieving effect that permeability depends on the size of each gas molecule: H₂O (0.27 ran), H₂ (0 29 ran), CO₂ (0.33 nm), N₂ (0.36 nm), CH₄ (0.38 nm). Further, the coefficients of CO₂, N₂, CH₄ also exceeded theoretical values (0.6-0.8) calculated by

Knudsen mechanism, which shows that they are selectively permeated based on not the weight of a gas molecule but the size of a gas molecule.

As verified in Example 1 and Comparative Example 1, the chemical modification of a silica coating layer with alumina sol increased water vapor permeation and selectivity by three times, which means the hydrophilicity of the selective membrane surface is increased by the chemical modification. Further, membrane of Comparative Example 2 showed high selectivity with low permeation, while that of Comparative Example 1 has an improved permeation with deteriorated selectivity.

Therefore, it was verified that the membrane of the present invention shows much improved selectivity and permeation. The composite membrane selective to passage of water vapor of the present invention hat, a multi-layered structure where a γ-alumma middle layer and a silica layer as a gas molecular sieve are laminated in this oidei on a porous suppoit, and an active layer of γ-alumma for separation of water vapor with thin thickness is coated on the upper surface of the silica layer as a gas molecular sieve Fuither, it is capable of serving as a molecular sieve that selectively permeates water vapor out of a gas mixture at high-temperature with excellent efficiency to be used m the process of removing, separating and purif} mg a gas mixture comprising water vapor with a tempei ature higher than 200 °C and in the field of a catalytic membrane reactor for dehydiation at high temperature

Claims

What is claimed is:

1 A composite membrane selective to passage of water vapor wherein a γ-alumma middle laver having a uniform distribution of mesopores and a silica layer as a gas molecular sieve having a uniform distribution of micropores are laminated in this order on a porous support, and an active layer of γ-alumma for separation of watei

wherein mesopores are unifoi mly distributed and Bronsted acidic sites are formed, is coated on the upper surface of said silica layer possessing moleculai sieve chai acteπshcs

2 The composite membrane of claim 1, wherein said γ-alurmna middle layer, said silica layei as a gas molecular sieve and said active layer of γ-alumma foi sepaiation of water vapor have pore sizes of 2-10 nm, 0 3-0 5 nm and 2-7 nm, lespecbΛ eh

3 The composite membrane of claim 1, wherein said γ-alumina middle lavcr said silica la}er as a gas mokculai sieve and said active

of γ-alumina foi sepaiation of water vapor are 2-3 μm, 02-05 um and 0 2-0 5 μm thick, respectπ ely

4 The composite membiane of claim 1, having a water \ apor peimeabihty of 10 100 xlO ⁸ 1110I m ² S ¹ Pa ¹ at 200-300 ⁰C and a water vapoi selectπ it\ against niti ogi-n gas of 10-100

5 Λ piocess of preparing a composite membrane selective to passage of water \ apor compiising (a) fra ming a γ-alumina middle layer having a uniform distribution of mesopores on a porous suppoit by sol-gel method,

(b) fo) mmg a silica layer as a gas molecular sieve having a uniform distribution of micropores on said γ-alumma middle layer by chemical vapor deposit method, and

(c) forming an active layer of γ-alumina for separation of water vapor having a uniform distribution of mesopores and Bronsted acidic sites on the upper surface of the silica layer as a gas molecular sieve by chemical modification with alumina sol

6 The piocess of claim 5 wherein said active layci of γ-alumina for separation of water vapor is formed by

(a) immeismg said silica layer as a gas molecular sieve in 0 5-0 7 mol/L of alumina sol at a rate of 0 5-1 0 mm/ sec foi 10-20 sec, thei eby forming a coating layer,

(c) heat-treating at 700 750 ⁰C