Method for the Pervaporation and Vapor-Permeation Separation of Gas-Liquid Mixtures and Liquid Mixtures by SAPO-34 Molecular Sieve Membrane

Abstract

The present invention discloses a method for the pervaporation and vapor-permeation separation of a gas-liquid mixture or a liquid mixture by a SAPO-34 molecular sieve membrane, which comprises: 1) mixing an Al source, tetraethyl ammonium hydroxide, water, a Si source and a P source, and subjecting the resultant to hydrothermal crystallization, then centrifuging, washing and drying to get SAPO-34 molecular sieve seeds; 2) coating the SAPO-34 molecular sieve seeds onto the inner surface of a porous support tube; 3) synthesis of a SAPO-34 molecular sieve membrane tube; 4) calcining the obtained SAPO-34 molecular sieve membrane tube to obtain a SAPO-34 molecular sieve membrane; 5) using the SAPO-34 molecular sieve membrane obtained from step 4) to perform separation of a gas-liquid mixture or a liquid mixture via a process of pervaporation separation or vapor-permeation separation. The invention has the advantages of very high methanol selectivity and permeation flux, and provides an efficient and energy-saving separation way via pervaporation or vapor-permeation separation.

Claims

1. A method for pervaporation separation of a gas-liquid mixture or a liquid mixture by preparing and using a SAPO-34 molecular sieve membrane, characterized in that the method comprises: 1) mixing and dissolving an Al source, tetraethyl ammonium hydroxide TEAOH, water, a Si source and a P source to make a reaction liquor for seeds, which is then subjected to crystallization for 4-7 h by heating at 170-210 C., then centrifuging, washing and drying to get SAPO-34 molecular sieve seeds; wherein the molar ratio of the Al source, P source, Si source, tetraethylammonium hydroxide and all water in the reaction liquor for seeds is 1 Al.sub.2O.sub.3: 1-2 P.sub.2O.sub.5: 0.3-0.6 SiO.sub.2: 1-3 TEAOH: 55-150 H.sub.2O. 2) coating the SAPO-34 molecular sieve seeds onto the internal surface of a porous support tube to get a porous support tube coated with SAPO-34 molecular sieve seeds; 3) synthesizing a SAPO-34 molecular sieve membrane tube by: A. uniformly mixing an Al source, a P source, a Si source, tetraethylammonium hydroxide TEAOH, di-n-propyl amine DPA, water and a fluoride to form a mother liquor for molecular sieve membrane synthesis; wherein the molar ratio of the Al source, P source, Si source, tetraethylammonium hydroxide, di-n-propyl amine and all water in the mother liquor for molecular sieve membrane synthesis is 1 Al.sub.2O.sub.3: 0.5-3.5 P.sub.2O.sub.5: 0.05-0.6 SiO.sub.2: 0.5-8 TEAOH: 0.1-4.0 DPA: 0.01-1F.sup.: 50-300 H.sub.2O; B. placing the porous support tube coated with SAPO-34 molecular sieve seeds obtained from step 2) in the mother liquor for molecular sieve membrane synthesis and after aging for 2-8 h at room temperature 80 C., crystallizing for 3-24 h at 150-240 C. to synthesize the SAPO-34 molecular sieve membrane tube; 4) calcining the SAPO-34 molecular sieve membrane tube obtained in step 3) at 370-700 C. for 2-8 h, to get a SAPO-34 molecular sieve membrane; 5) using the SAPO-34 molecular sieve membrane obtained in step 4) to perform the separation of a liquid mixture via a process of pervaporation separation; wherein the liquid mixture is a mixture of methanol and a liquid other than methanol, wherein said liquid other than methanol includes one of dimethyl carbonate, ethanol, methyl t-butyl ether.

2. The method according to claim 1 characterized in that in steps 1) and 3), the Al source includes one or more of aluminum isopropoxide, Al(OH).sub.3, elemental aluminum, an Al salt; wherein said Al salt includes one or more of aluminum nitrate, aluminum chloride, aluminum sulfate, and aluminum phosphate; in steps 1) and 3), the P source includes phosphoric acid; and in steps 1) and 3), the Si source includes one or more of tetraethyl orthosilicate, tetramethyl orthosilicate, silica sol, silica, sodium silicate, and water glass.

3. The method according to claim 1, characterized in that in step 1), the heating comprises microwave heating; and the size of the SAPO-34 molecular sieve seeds is 50-1000 nm.

4. The method according to claim 1 characterized in that in step 2), the porous support tube includes a porous ceramic tube; wherein the pore size of the porous ceramic tube is 5-2000 nm; and the material of the porous ceramic tubes is selected from Al.sub.2O.sub.3, TiO.sub.2, ZrO.sub.2, SiC or silicon nitride.

5. The method according to claim 1 characterized in that the coating of the seeds in step 2), is performed according to the following procedure, sealing the two ends of the porous support tube with glaze, washing and drying, sealing the outer surface, and then coating the SAPO-34 molecular sieve seeds onto the inner surface of the porous support tube; wherein the coating method includes brush coating or dip coating.

6. The method according to claim 1 characterized in that in step 3), the fluoride includes one or a mixture of HF, and a fluoride salt; wherein the fluoride salt includes ammonium fluoride, a fluoride salt of a main-group metal or a fluoride salt of a transition metal.

7. The method according to claim 6, characterized in that the fluoride salt includes one or more of potassium fluoride, sodium fluoride, and ammonium fluoride.

8. The method according to claim 1, characterized in that in step 3), the operation procedures of forming the mother liquor for molecular sieve membrane synthesis are as follows, mixing the Al source, P source and water, stirring for 1-5 h; then adding the Si source, stirring for 0.5-2 h; then adding tetraethyl ammonium hydroxide, stirring for 0.5-2 h; then adding di-n-propyl amine, stirring for 0.5 h; then adding the fluoride, stirring for 12-96 h at room temperature60 C., thereby to get a homogeneous mother liquor for molecular sieve membrane synthesis.

9. The method according to claim 1 characterized in that in step 4) the atmosphere for calcination is selected from inert gas, vacuum, air, oxygen gas, or diluted oxygen gas in any ratio; and in the calcination, the temperature increasing rate and the temperature decreasing rate are not higher than 2K/min.

10. The method according to claim 1 characterized in that in step 5), the conditions for the process of pervaporation separation are: a methanol concentration in the feed of 1-99 wt %, a permeation operation temperature ranging from 20C. to 150 C., a feed pressure ranging from atmospheric pressure to 20 atms, a pressure on the permeate side ranging from 0.06 Pa to 2000 Pa, and a feed flow rate ranging from 1-500 mL/min;

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] The invention will be explained in further detail by taking the appended figures and the examples.

[0038] FIG. 1. is a SEM (Scanning Electron Microscopy) image of SAPO-34 seeds of Example 1.

[0039] FIG. 2. is an XRD (X-ray diffraction) pattern of SAPO-34 seeds of Example 1.

[0040] FIG. 3. is a surface SEM image of SAPO-34 molecular sieve membrane of Example 1 (prepared by adding 0.1 mol HF).

[0041] FIG. 4. is a cross sectional SEM image of SAPO-34 molecular sieve membrane of Example 1 (prepared by adding 0.1 mol HF).

[0042] FIG. 5. is a schematic diagram of a pervaporation separation process, wherein 1 denotes feed liquid, 2 denotes peristaltic pump, 3 denotes molecular sieve membrane assembly and heat source, 4 denotes stop valve, 5 denotes cold trap, 6 denotes vacuum gauge, 7 denotes vacuum pump.

[0043] FIG. 6. is a surface SEM image of SAPO-34 molecular sieve membrane of Example 4 (prepared by adding 0.1 mol NH.sub.4F).

[0044] FIG. 7. is a cross sectional SEM image of SAPO-34 molecular sieve membrane of Example 4 (prepared by adding 0.1 mol NH.sub.4F).

EXAMPLES

Example 1

Separation of Methanol/Dimethyl Carbonate at Different Feed Composition by SAPO-34 Molecular Sieve Membrane

[0045] Step1: 2.46 g of DI water were added to 31.13 g of tetraethyl ammonium hydroxide solution (TEAOH, 35 wt %) . Then 7.56 g of aluminum isopropoxide were added thereto, and the resultant was stirred for 2-3 h at room temperature. Then 1.665 g of silica sol (40 wt %) were added dropwise and the resultant was stirred for 1 h. Finally, 8.53 g of phosphoric acid solution (H.sub.3PO.sub.4, 85 wt %) were added slowly dropwise and the resultant was stirred overnight (e.g., stirred for 12 hours). Then crystallization was performed at 180 C. for 7 h by using microwave heating. The obtained product was taken out from the reactor, centrifuged, washed, dried, to obtain SAPO-34 molecular sieve seeds. The SEM image of the seeds is shown in FIG. 1 and the XRD pattern of the seeds is shown in FIG. 2. From the SEM image, it can be seen that the size of the seeds is around 300 nm*300 nm*100 nm. Moreover, the XRD pattern indicates that the seeds are pure SAPO-34 phase, and are well crystallized with no impure phase.

[0046] Step 2: A porous ceramic tube (material: alumina) with 5 nm pore size was used as a support. The two ends of the support were sealed with glaze. After washing and drying, the out surface of the support was sealed (covered) by PTFE tape. Then the SAPO-34 molecular sieve seeds were coated onto the inner surface of the ceramic tube by brush coating method. Thus, a porous ceramic tube coated with SAPO-34 molecular sieve seeds was obtained.

[0047] Step 3: 4.27 g of phosphoric acid solution (H.sub.3PO.sub.4, 85 wt %) were mixed with 43.8 g of DI water, and the resultant was stirred for 5 min. Then 7.56 g of aluminum isopropoxide were added, and the resultant was stirred for 3 h at room temperature. 0.83 g of silica sol (40 wt %) were added, and the resultant was stirred for 30 min at room temperature. Then, 7.78 g of tetraethyl ammonium hydroxide solution (TEAOH, 35 wt %) were added dropwise, and the resultant was stirred for 1 h at room temperature. Finally, 3.0 g of di-n-propylamine were added thereto, and after the resultant was stirred for 30 min at room temperature. 0.045 g of hydrofluoric acid (HF, 40 wt %) were added, and the resultant was stirred overnight (e.g., stirred for 12 hours) at 5 C., getting a uniform mother liquor for synthesis of SAPO-34 molecular sieve membrane. The porous ceramic tube coated with SAPO-34 molecular sieve seeds, which was prepared in the above step 2, was placed in a reaction vessel, and the mother liquor for synthesis of SAPO-34 molecular sieve membrane was added. The reaction vessel was closed and aging was performed for 3 h at room temperature. Then hydrothermalsynthesis was performed at 22 C. for 5 h. After taken out from the reaction vessel, the product was thoroughly rinsed and dried in an oven. Thus, a SAPO-34 molecular sieve membrane tube was obtained.

[0048] Step 4: The SAPO-34 molecular sieve membrane tube obtained in step 3 was calcined in vacuum for 4 h to remove the template agent (the temperature increasing rate and temperature decreasing rates were 1 C./min, respectively), thereby to obtain an activated SAPO-34 molecular sieve membrane.

[0049] The surface and cross sectional SEM images of the SAPO-34 molecular sieve membrane (prepared by addition of 0.1 mol HF) are respectively shown in FIGS. 3 and 4. It can be seen that the support surface is completely covered by square lamellar SAPO-34 crystals which are perfectly cross-linked therebetween. The crystal size is 4-7 microns, and the molecular sieve membrane surface is flat. The cross sectional image shows that the thickness of the membrane is about 5-6 microns.

[0050] Step 5. A methanol/dimethyl carbonate (i.e., DMC/MeOH) azeotrope was separated by pervaporation separation process at a permeation operation temperature of 120 C., a feed pressure of 0.3 MPa, a feed flow rate of 1 mL/min, a pressure on the permeate side of 100 Pa, with a composition (in mass ratio) of the MeOH/DMC feed being 90/10, 70/30, 50/50, 30/70 and 10/90, respectively. The schematic diagram of the pervaporation process is shown in FIG. 5.

[0051] The separation factor is calculated from: =(w.sub.2m/w.sub.2d)/(w.sub.1m/w.sub.1d), where w.sub.2m is the mass concentration of methanol on the permeate side, w.sub.2d is the mass concentration of dimethyl carbonate on the permeate side, w.sub.1m is the mass concentration of methanol in the feed and w.sub.1d is the mass concentration of dimethyl carbonate (DMC) in the feed.

[0052] The permeation flux equation is J=m/(st), wherein m is the mass (g) of a product collected on the permeate side, s is the molecular sieve membrane area (m.sup.2) and t is the collecting time (h).

TABLE-US-00001 TABLE 1 The pervaporation separation test results of MeOH/DMC in Example 1. Feed Methanol concentration in composition Permeation flux J Separation the permeated product MeOH/DMC [g/(m.sup.2 .Math. h)] factor (wt %) 10/90 94 5600 99.840 30/70 168 2620 99.911 50/50 384 5000 99.980 70/30 806 8600 99.995 90/10 1498 5300 99.998

[0053] It can be seen from Table 1 that at different feed compositions, the SAPO-34 molecular sieve membranes synthesized from the fluoride-containing system have very high methanol selectivity.

[0054] The separation factor reaches a minimum of 2620 when the feed has a composition of 30-70 wt %, and reaches a maximum of about 8600 when the feed has a composition of 70-30 wt %. The methanol concentration in the permeate is at least 99.84 wt %. With the increase of methanol concentration in the feed, the permeation flux gradually increases, which is caused by the increasing of methanol partial pressure.

Example 2

Separation of Methanol/Dimethyl Carbonate by SAPO-34 Molecular Sieve Membrane at Different Operation Temperatures

[0055] All steps in this Example are the same as in Example 1 except that in step 5, the feed composition of MeOH/DMC is 90/10, and the operation temperature is 100 C., 110 C., 120 C., 130 C., 140 C., respectively.

TABLE-US-00002 TABLE 2 The vapor-permeation separation test results of MeOH/DMC in Example 2. Operation temperature Methanol permeation flux J Separation C. [kg/(m.sup.2 .Math. h)] factor 100 0.71 1300 110 0.92 1330 120 1.10 5300 130 1.80 3800 140 2.00 3450

[0056] It can be seen from Table 2 that at different operation temperatures (100-140 C.), the SAPO-34 molecular sieve membranes synthesized from the fluoride-containing system have very high methanol selectivity. With the increase of operation temperature, the permeation flux of methanol gradually increases, which is due to the increase of methanol partial pressure.

Example 3

Separation of Methanol/Dimethyl Carbonate by SAPO-34 Molecular Sieve Membrane at Different Feed Pressures

[0057] All steps in this Example are the same as in Example 1 except that in step 5, the feed composition of MeOH/DMC is 90/10, and the feed pressures are 0.3 MPa, 0.4 MPa, 0.5 MPa, 0.6 MPa, respectively.

TABLE-US-00003 TABLE 3 The pervaporation separation test results of MeOH/DMC in Example 3. Feed pressure Methanol permeation flux J Separation MPa [kg/(m.sup.2 .Math. h)] factor 0.6 1.65 3050 0.5 1.68 2720 0.4 1.30 3100 0.3 1.10 5300

[0058] It can be seen from Table 3 that at different feed pressures, the SAPO-34 molecular sieve membrane synthesized from the fluoride-containing system have very high methanol selectivity. With the increase of system pressure, the permeation flux increases gradually. When the pressure reaches 0.5 MPa, the methanol permeation flux becomes constant.

Example 4

Separation of Methanol/Dimethyl Carbonate by SAPO-34 Molecular Sieve Membrane Synthesized by Addition of Different Fluoride

[0059] All steps in this Example are the same as in Example 1 except that in step 3, 0.037 g of sodium fluoride, 0.033 g of ammonium fluoride are added respectively, and in step 5, the feed composition of MeOH/DMC is 90/10, and the feed pressure is 0.3 MPa.

TABLE-US-00004 TABLE 4 The pervaporation separation test results of MeOH/DMC in Example 4. Methanol permeation flux J Separation Fluoride [kg/(m.sup.2 .Math. h)] factor NaF 1.03 4200 NH.sub.4F 1.14 3900

[0060] It can be seen from Table 4 that the SAPO-34 molecular sieve membranes synthesized from the system containing a different fluoride have very high methanol selectivity and high permeation flux. Thus, in case of addition of ammonium fluoride and sodium fluoride, a high-performance SAPO-34 molecular sieve membranes can also be prepared.

[0061] The surface and sectional SEM images of the SAPO-34 molecular sieve membrane (prepared by adding 0.1 mol NH4F) are respectively shown in FIGS. 6 and 7. It can be seen that the support surface was completely covered by square lamellar SAPO-34 crystals which are perfectly cross-linked therebetween. The crystal size is 4-7 microns, and the molecular sieve membrane surface is flat. The images of the cross section show that the thickness of the membrane is about 5-6 microns.

[0062] In addition, the SAPO-34 molecular sieve membranes prepared as above can also be used for the pervaporation or vapor-permeation separation of a gas-liquid mixture, wherein the gas of the gas-liquid mixture may be one of nitrogen gas, hydrogen gas, oxygen gas, carbon dioxide or methane or the like. The liquid of the gas-liquid mixture may be one of water, methanol, acetone or benzene or the like.