Patent Application
20180021728
The invention relates to a method for separation of a mixture by a SAPO-34 molecular sieve membrane, especially a method for the pervaporation (pervaporative separation) and vapor-permeation separation of a gas-liquid mixture or a liquid mixture by a SAPO-34 molecular sieve membrane prepared in a dry-gel process.
Dimethyl carbonate (DMC), which has a molecular formula of CO(OCH3)2, is a good solvent, has low volatility and similar toxicity values to anhydrous ethanol, and is completely biodegradable. It is an environmental-friendly chemical; its molecules have an oxygen content of 53%, which is three times higher than that of methyl tert-butyl ether (MTBE). It can be used as an additive in gasoline to enhance octane number and to suppress emission of carbon monoxide and hydrocarbons. It is very active in terms of chemistry, and it is an important intermediate and starting material for organic synthesis. Dimethyl carbonate finds extensive applications in the fields of pharmaceutical, chemical engineering and energy etc, and is receiving increasing attention. It has been rapidly developed and it is known as a new foundation of organic synthesis.
The industrial methods for producing DMC mainly include methods of oxidative carbonylation, transesterification, or phosgenation of methanol [Applied Catalysis A: General, 221(2001) 241-251]. No matter which method is used, a mixture of methanol (MeOH) and DMC was always obtained from the reaction. At normal pressure, MeOH and DMC would form a binary azeotrope (70 wt % MeOH and 30 wt % DMC), whose azeotropic temperature is 64° C. Therefore, it is a necessary to separate and recover DMC from the azeotrope. Currently, methods for separation of the MeOH/DMC azeotrope mainly include low temperature crystallization, adsorption, extractive distillation, azeotropic distillation and pressure distillation. All of these methods possess the disadvantages and shortcomings that energy consumption is high, it is difficult to select the appropriate solvent, it is difficult to operate and the safety has deficiencies. In contrast, a pervaporation method possesses advantages of low energy consumption, high efficiency and flexible operation conditions.
The pervaporation is a new membrane technology for separation. It uses the differential chemical potentials of a component on both sides of the membrane as a driving force. The membrane can be used to achieve selective separation of different components in feed liquids according to different affinity and mass transfer resistance of the components. Currently, the membranes used for pervaporation mainly include polymeric membrane, inorganic membrane and composite membrane. Rencetly, some progress has been made in studies on pervaporation separation of MeOH/DMC mixtures. Most of the studies focused on the polymeric membranes. The researchers found that materials such as polyvinyl alcohol (PVA), polyacrylic acid, chitosan or the like can be prepared into pervaporation membranes which preferentially remove methanol and have good separation performance.
Wooyoung et al. used a cross-linked chitosan membrane for pervaporation separation of MeOH/DMC and investigated the influences of operation temperature and feed composition on the separation factor and flux and received a good result [Separation and Purification Technology 31 (2003) 129-140]. Wang et al. prepared a polyacrylic acid (PAA)/polyvinyl alcohol (PVA) mixed membrane, wherein a mixed membrane containing 70 wt % PPA has a separation factor of 13 and a permeation flux of 577 g/(m2 h) [Journal of Membrane Science 305 (2007) 238-246]. Pasternak et al. tested the performance of a polyvinyl alcohol (PVA) membrane for the separation of MeOH/DMC; for a feed composition of 70/30 MeOH/DMC, a methanol solution of 93-97 wt % concentration is produced on the permeate side and the flux is 110-1130 g/(m2 h) [U.S. Pat. No. 4,798,674 (1989)]. Chen et al. prepared a hybrid membrane of chitosan and silica through cross-linking chitosan with aminopropyl triethoxy silane. Separation factor of 30 and permeation flux of 1265 g/(m2 h) were achieved at 50° C. for a 70/30 MeOH/DMC mixture.
The polymeric membranes have an advantage of low cost. However, they are also suffering from the disadvantages such as low chemical and thermal stability, easy to swell during the process of separation, and incapability of being used for separation at high pressure; all of which would influence the separation performance of the membranes. On the other hand, the inorganic membranes can well solve these issues because the inorganic membranes have a uniform pore size and high chemical and thermal stability. Therefore, the inorganic membranes can be used for the separation in an environment under harsh conditions and they are also suitable for separation under high pressure. Currently, the main application of inorganic zeolite molecular sieve membranes is dehydration of organics. Applications of molecular sieve membrane in the separation of MeOH/DMC were rarely reported. Li et al. prepared a ZSM-5 molecular sieve membrane on porous alumina support and used the same for the separation of a water/acetic acid mixture [Journal of Membrane Science 218 (2003) 185-194]. Pina et al. synthesized a NaA molecular sieve membrane on Al2O3 support and used the NaA molecular sieve membrane to separate a water/ethanol mixture by pervaporation, in which the separation factor can reach 3600 and the permeation flux of water reaches 3800 g/(m2 h) [Journal of Membrane Science 244 (2004) 141-150]. Hidetoshi et al. prepared NaX and NaY membranes on supports and systemically studied the pervaporation separation performance of the membranes. It was found that the membranes have very high selectivity to alcohols and benzene. They also studied the selectivity of these membranes for MeOH/DMC separation, and as a result, separation factor of 480 and permeation flux of 1530 g/(m2 h) were achieved while the feed composition was 50/50 [Separation and Purification Technology 25 (2001) 261-268].
The technical problem to be solved by the present invention is to provide 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 prepared in a dry gel process. The inventive method achieves very high methanol (MeOH) selectivity and permeation flux. It also reduces the cost of molecular sieve membrane synthesis and reduces environmental pollution.
To resolve the issues mentioned above, the invention provides a method for the pervaporation or vapor-permeation separation of a gas-liquid mixture or a liquid mixture (e.g., separation of a methanol-containing mixture) by a SAPO-34 molecular sieve membrane prepared in a dry gel process, which method includes the following steps:
1) synthesis of SAPO-34 molecular sieve seeds (crystal seeds)
mixing and dissolving an Al source, tetraethylammonium hydroxide (TEAOH, a template agent), water, a Si source and a P source to make a reaction liquor for seeds, which is then subjected to crystallization for 2-72 h by heating at 120-230° 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 Al2O3: 1-2 P2O5: 0.3-0.6 SiO2: 1-3 TEAOH: 55-150 H2O;
2) coating (e.g., uniformly coating) the SAPO-34 molecular sieve seeds on a porous support;
3) preparation of a mother liquor for dry gel synthesis of SAPO-34 molecular sieve membrane uniformly mixing an Al source, a P source, a Si source, tetraethyl ammonium hydroxide (TEAOH) and water to form a mother liquor for dry gel synthesis of SAPO-34 molecular sieve membrane; wherein the molar ratio of the Al source, P source, Si source, tetraethyl ammonium hydroxide and all water in the mother liquor for dry gel synthesis is: 1 Al2O3: 1-2 P2O5: 0.1-0.6 SiO2: 1-8 TEAOH:30-1000 H2O;
4) supporting the mother liquor for dry gel synthesis on the porous support coated with SAPO-34 molecular sieve seeds prepared in step 2), and drying, to form a porous support having a dry gel layer;
5) placing the porous support prepared in step 4) into a reaction vessel, adding a solvent (no direct contact between the solvent and the membrane tube), performing crystallization of the dry gel;
6) calcining at a high temperature of 400-600° C. for 4-8 h for removal of the template agent, so as to obtain a SAPO-34 molecular sieve membrane;
7) using the SAPO-34 molecular sieve membrane obtained in step 6) to perform separation of a gas-liquid mixture or a liquid mixture by a process of pervaporation separation or vapor-permeation separation. The gas in the gas-liquid mixture includes common gases, for is selected from inert gas, hydrogen gas, oxygen gas, CO2 or gaseous hydrocarbon, and the liquid in the gas-liquid mixture includes common solvents such as water, alcohol, ketone or aromatics;
In addition, in step 7), in the separation of the liquid mixtures by the SAPO-34 molecular sieve membrane, said liquid mixture is a mixture of methanol and a liquid other than methanol, said liquid other than methanol is selected from dimethyl carbonate, ethanol, methyl tert-butyl ether.
In the step 1), the detailed procedures for making the reaction liquor for seeds are: adding the Al source to the tetraethyl ammonium hydroxide solution, and after full hydrolysis, adding the Si source and P source, and stirring for 12-24 h to form the reaction liquor for seeds.
In the steps 1) and 3), the Al source is selected from one or more of aluminium isopropoxide, aluminium hydroxide, elemental aluminium, an aluminium salt, alumina, and hydrated alumina; wherein the aluminium salt is selected from one or more of aluminium nitrate, aluminium chloride, alumimium sulfate, aluminium phosphate; the Si source is selectged from one or more of silica sol, silicate ester, silica aerosol and sodium silicate; and the P source is phosphoric acid.
In the step 1), the heating is preferably microwave heating.
In step 1), the size of the SAPO-34 molecular sieve seeds is 50-1000 nm.
In the step 2), the shapes of the porous support are selected from single-channel tube, multi-channel tube, flat plate, hollow fiber tube; the material of the porous support is selected from ceramics, stainless steel, alumina, titania, zirconia, silica, silicon carbide, or silicon nitride; the pore size of the porous support is 5-2000 nm.
In the step 2), the coating method is selected from brush coating, dip-coating, spray coating or spin coating; wherein the procedure in case of dip-coating is: uniformly coating a 0.01-1 wt % (mass percentage) solution of SAPO-34 molecular sieve seeds in ethanol on the porous support.
In the step 3), the specific procedure of forming the mother liquor for dry gel synthesis of SAPO-34 molecular sieve membrane comprises: adding the Al source to the P source and after full hydrolysis, adding the Si source and tetraethyl ammonium hydroxide and stirring for 12-24 h, to obtain the mother liquor for dry gel synthesis of SAPO-34 molecular sieve membrane.
In the step 4), the supporting method is selected from dip-coating, spray coating or spin coating; wherein the specific procedure in the case of dip-coating comprises: immersing the porous support coated with SAPO-34 molecular sieve seeds in the mother liquor for dry gel synthesis for 1 s to 24 h, and then drying for 1 min to 48 h at room temperature to 120° C.
In the step 4), the molar composition of the dry gel layer is preferably 1 Al2O3: 1-2 P2O5: 0.1-0.6 SiO2: 1-8 TEAOH: 1-300 H2O.
In the step 5), the solvent is selected from one or more of water, ammonia water, the mother liquor for dry gel synthesis, an organic solvent; the solvent is used in an amount of 0.001-0.1 g per mL volume of the reaction vessel; moreover, in step 5), when in liquid state, the solvent does not directly contact the dry gel layer, but when the crystallization is performed at high temperature, the vapor produced after the vaporization of the solvent comes into direct contact with the dry gel layer.
In the step 5), the temperature for crystallization of the dry gel is 120-230° C. and the crystallization time is 2-72 h, preferably 4-7 h.
In the step 6), 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 is not higher than 2 K/min.
In the step 6) the membrane thickness of the SAPO-34 molecular sieve membrane is 1-2 microns.
In the step 7), the conditions for the process of pervaporation separation or vapor-permeation separation are as follows: the methanol concentration of the feed is 1-99 wt %, the feed flow rate is 1-500 mL/min, the separation operation temperature is room temperature to 150° C., and the pressure on the permeate side is 0.06 to 300 Pa.
The invention provides a ultrathin SAPO-34 molecular sieve membrane in a dry gel process. Said membrane has a high flux, small thickness (can be as low as about 1 micron), so that the mass transfer resistance is low, and the permeation rate is high; when used for pervaporation separation of a gas-liquid mixture or a liquid mixture such as a MeOH/DMC mixture, it shows high MeOH selectivity and permeation flux. For example, when the SAPO-34 molecular sieve membrane is used for separation of a methanol/dimethyl carbonate mixture (in a mass ratio of 90/10) at a temperature of 120° C., the permeation flux reaches 10 kg·m−2·h−1, the separation factor is above 500 and the methanol concentration on the permeate side is above 99.98 wt %.
On the other hand, in terms of the preparation of the molecular sieve membrane, the present invention greatly reduces consumption of starting materials and template agent for the synthesis and thus reduces the synthesis cost and reduces the environmental pollution. Furthermore, the prepared SAPO-34 molecular sieve membrane is very thin in thickness, thereby the mass transfer resistance is greatly decreased and the methanol flux through the membrane is improved. Therefore, the invention will be of great importance in industrial application because it provides a high-efficiency, environmental-friendly and economic method for separation of MeOH/DMC mixture
In addition to the separation of a methanol/dimethyl carbonate mixture, the SAPO-34 molecular sieve membrane of the present invention can also be applied for the pervaporation or vapor-permeation separation of a mixture of methanol and other liquid, such as methanol-ethanol, methanol-methyl tert-butyl ether or the like.
In addition, the SAPO-34 molecular sieve membrane of the present invention can also be used for the pervaporation or vapor-permeation separation of a gas-liquid mixture.
The invention will be explained in further detail by taking the appended figures and the detailed implementation.
Step 1. 2.46 g of DI water were added into 31.13 g of tetraethylammonium hydroxide solution (TEAOH, 35 wt %), and then 7.65 g of aluminum isopropoxide were added thereto, and the resultant was stirred for 2-3 h at room temperature. Then 1.665 g silica sol (40 wt %) was added dropwise and the resultant was stirred 1 h. Finally, 8.53 g phosphoric acid solution (H3PO4, 85 wt %) were slowly added dropwise. The resulting solution was stirred overnight (e.g. stirred for 12 h). Then crystallization is 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 XRD pattern and the SEM image of the seeds are shown in
A porous ceramic tube (a porous ceramic tube having an inner diameter of 7 mm, an outer diameter of 10 mm and a length of 600 mm, the shape of the porous support being single-channel tube, and its material being 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 Teflon tape. Then the SAPO-34 molecular sieve seeds are coated on the inner surface of the porous ceramic tube by brush coating.
Step 2. 9.07 g of aluminium isoproxide were added to 5.12 g of phosphoric acid solution (85 wt %) and 23.33 g of DI water. After full hydrolysis, 1.00 g of silica gel (40 wt %) and 9.34 g of tetraethyl ammonium hydroxide (35 wt %) were added in turn. The resultant was stirred overnight (e.g., stirred for 12 h) to form a mother liquor for dry gel synthesis of molecular sieve membrane.
The resulting mother liquor had a molar composition of 1.0 Al2O3: 1.0 P2O5: 0.3 SiO2: 1TEAOH: 77 H2O.
Step 3. The porous ceramic tube coated with SAPO-34 molecular sieve seeds prepared in step 1 was soaked in the mother liquor for dry gel synthesis for 10 min, taken out and dried at room temperature for 10 min, thereby to have a dry gel layer formed. Then, the porous ceramic tube having a dry gel layer was placed in a 100 mL reaction vessel, and then 1 mL of DI water was added. The dry gel was crystallized for 5 h at 220° C. After washing and drying, a SAPO-34 molecular sieve membrane tube was obtained.
The membrane thickness of the SAPO-34 molecular sieve membrane prepared in Example 1 is around 1 micron (
Step 4. The SAPO-34 molecular sieve membrane tube obtained in step 3 was calcined under vacuum at 400° C. for removal of the template agent (the temperature increasing rate and the temperature decreasing rate were both 1 K/min), to get a ultrathin SAPO-34 molecular sieve membrane prepared via dry gel process.
Step 5. The ultrathin SAPO-34 molecular sieve membrane prepared in step 4 was used to separate a methanol/dimethyl carbonate (i.e., DMC/MeOH) mixture by a process of pervaporation separation, wherein the feed temperature (i.e., separation operation temperature) is 120° C., the composition of the feed MeOH/DMC is 90/10 (mass ratio), the feed flow rate is 1 mL/min, the system pressure is 0.3 MPa and the permeate side pressure is 100 Pa.
The separation factor is calculated from: α=(w2m/w2d)/(w1m/w1d), where w2m is the mass concentration of methanol on the permeate side, w2d is the mass concentration of dimethyl carbonate on the permeate side, w1m is the mass concentration of methanol in the feed and w1d is the mass concentration of dimethyl carbonate in the feed.
The permeation flux equation is J=Δm/(sxt), where Δm is the mass (unit g) of a product collected on the permeate side, s is the area (m2) of the molecular sieve membrane and t is the collection time (h).
It can be seen from Table 1 that the separation factor for the methanol/dimethyl carbonate of the ultrathin SAPO-34 molecular sieve membrane prepared by dry-gel process is lower than that of the SAPO-34 molecular sieve membrane prepared by hydrothermal synthesis, which was reduced from to 790. But the flux of the former one is increased by 4 times, i.e. from 2.1 kg/(m2·h) to 10.6 kg/(m2·h). It is apparent that the decreasing of the membrane thickness greatly reduces the mass transfer resistance of the membrane, thereby resulting in a great enhancement of the flux. In comparison, although the separation factor of the ultrathin SAPO-34 molecular sieve membrane prepared by dry-gel process decreases to some extent, a separation factor of 790 is high enough because the concentration of methanol in the permeate is higher than 99.9 wt %.
All steps in this Example are the same as in Example 1 except that the porous support coated with SAPO-34 molecular sieve seeds was soaked in the mother liquor for dry gel synthesis for 2 h in step 3.
It can be seen from Table 2 that in the dry-gel synthesis method, the ultrathin SAPO-34 molecular sieve membrane prepared by immersion in the mother liquor for 2 h has a methanol selectivity of 350, but the flux is higher than 13 kg/(m2·h) (Table 2).
The surface and cross section SEM images of the SAPO-34 molecular sieve membrane prepared in Example 2 are shown in
Example 3 differs from Example 1 in that: in step 2), 8.46 g of aluminium hydroxide were added to 10.24 g of phosphoric acid solution (85 wt %) and 31.82 g of DI water. And after adequate hydrolysis, 4.00 g of silica sol (40 wt %) and 33.62 g of tetraethyl ammonium hydroxide (35 wt %) were added in turn. The resultant was stirred overnight (e.g., stirred for 12 h) to form a mother liquor for dry gel synthesis of molecular sieve membrane. The resulting mother liquor had a molar composition of 1 Al2O3: 1 P2O5: 0.6 SiO2: 1.8 TEAOH: 77H2O. Other steps of Example 3 are the same as that of Example 1.
It can be seen from Table 3 that when a dry gel synthesis method is used and the mother liquor has a molar composition of 1 Al2O3: 1 P2O5: 0.6SiO2: 1.8TEAOH: 77H2O, the synthesized ultrathin SAPO-34 molecular sieve membrane has a methanol selectivity of 128 and a flux higher than 12 kg/(m2·h).
All steps in this Example are the same as in Example 1 except that in step 3), the porous support coated with SAPO-34 molecular sieve seeds was soaked in the mother liquor for dry gel synthesis for 40 min and 2 mL DI water was added to the 100 mlLreaction vessel.
It can be seen from Table 4 that when a dry gel synthesis method is used and 2 g of DI water were added to the bottom of the 100 mL hydrothermal reaction vessel, the synthesized ultrathin SAPO-34 molecular sieve membrane had a methanol selectivity of 305 and a flux higher than 12 kg/(m2·h).
In addition, the pervaporation process of Examples 1-4 is shown in
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; and the liquid of the gas-liquid mixture may be one of water, methanol, acetone or benzene or the like.
| Number | Date | Country | Kind |
|---|---|---|---|
| 201510054314.4 | Feb 2015 | CN | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2016/052208 | 2/2/2016 | WO | 00 |
