Method for realizing C5 / C6 alkane isomer separation by using multi-channel hollow fiber MFI molecular sieve membrane

Abstract

The invention relates to a method for separating C5 / C6 alkane isomers, in particular to a method for separating C5 / C6 alkane isomers by using a multi-channel hollow fiber MFI molecular sieve membrane. The invention provides a method for realizing C5 / C6 alkane isomer separation by using multi-channel hollow fiber MFI molecular sieve membranes, a fluid containing C5 / C6 alkane isomers is introduced into an MFI membrane assembly for separation so as to obtain n-alkanes and isoalkanes, the MFI membrane assembly comprises a plurality of multi-channel hollow fiber MFI molecular sieve membranes, and the plurality of multi-channel hollow fiber MFI molecular sieve membranes are integrally produced and prepared in batches.

Description

Technical Field

[0001] The invention relates to a method for separating C5 / C6 alkane isomers, and in particular to a method for separating C5 / C6 alkane isomers by utilizing a multi-channel hollow fiber MFI molecular sieve membrane. Background Art

[0002] Light naphtha is a petroleum fraction between 45℃ and 61℃, and its C5 and C6 alkane isomers account for more than 89.3%. Influenced by the international situation and the booming development of the ethylene industry, the separation of light naphtha mixtures has received more and more attention, and the development of energy-saving and efficient hydrocarbon mixture separation technology has become one of the hot issues of concern.

[0003] The average pore size of the MFI molecular sieve membrane is 0.55 nm. Based on molecular sieving, the separation of C5 and C6 alkane isomers (n-pentane / isopentane, n-hexane / 2-methylpentane, n-hexane / 3-methylpentane) can be achieved. In the previous work (invention patent CN113462424A), the inventor also proposed the use of MFI molecular sieve membrane to separate n-isomers of naphtha, but the above work is based on traditional supports with low packing density, which is difficult to meet the actual industrial needs.

[0004] Multi-channel hollow fibers have multiple channels in their inner cores. Compared with traditional supports, they not only have a higher packing density, but also have mechanical strength suitable for industrial applications. The inventors also did a lot of work on multi-channel hollow fibers in their early work, and tried to use them as carriers to prepare MFI molecular sieve membranes for the separation of xylene isomers, and achieved good results. Based on this effect, the inventors tried to batch synthesize multi-channel hollow fibers for the separation of C5 / C6 alkane isomers, but found that the permeability of a single membrane was poor. After research, two main reasons were found. One is that batch preparation requires that multiple carriers be packaged into bundles through glaze, and there is a height difference between the carrier and the sealing glaze, so the membrane layer at the glaze sealing position is easy to break; the second is that in order to meet industrial applications, the carrier length is relatively long, so there is a difference in the crystallization temperature of the upper and lower ends of the carrier during the synthesis process.

[0005] Therefore, there is an urgent need to optimize the existing membrane separation method for C5 / C6 alkane isomer separation to meet the actual needs of industrial applications. Summary of the invention

[0006] In view of the above problems, the present invention proposes to use a multi-channel hollow fiber MFI molecular sieve membrane as a key membrane material to achieve the separation of C5 / C6 alkane isomers, and optimizes the preparation method of MFI molecular sieve membrane in batches using the multi-channel hollow fiber MFI molecular sieve membrane as a carrier, so as to meet the actual needs of industrial applications.

[0007] Specifically, the present invention provides a method for separating C5 / C6 alkane isomers by using a multi-channel hollow fiber MFI molecular sieve membrane. The method comprises passing a fluid containing C5 / C6 alkane isomers into an MFI membrane assembly for separation to obtain normal alkanes and isoalkanes. The method is characterized in that the MFI membrane assembly comprises a plurality of multi-channel hollow fiber MFI molecular sieve membranes, and the plurality of multi-channel hollow fiber MFI molecular sieve membranes are prepared by batch integrated production.

[0008] Preferably, the single multi-channel hollow fiber MFI molecular sieve membrane in the MFI membrane assembly has a n-C6 / 3MP separation factor greater than 70 at a separation temperature of 80°C and a total feedstock partial pressure of 5 kPa, and an n-C6 permeability greater than 1.3×10 -7 mol·m -2 ·s -1 ·Pa -1 .

[0009] Preferably, the multi-channel hollow fiber is a 4-9 channel hollow fiber.

[0010] Preferably, the method for preparing the multi-channel hollow fiber MFI molecular sieve membrane comprises the following steps: 1) several multi-channel hollow fibers are encapsulated into a bundle through glaze, and seed crystals are coated on the surface of the multi-channel hollow fibers and dried; 2) placing the seed-coated multi-channel hollow fiber bundle in a casting solution, performing hydrothermal synthesis at 100-180° C. for 3-5 hours to complete a single synthesis, and taking out, washing and drying after the single synthesis is completed; 3) placing the multi-channel hollow fiber bundle after the first synthesis in the casting solution again, and performing static hydrothermal synthesis at 100-180° C. for 5-10 hours, and then placing the synthesis reactor containing the casting solution on a rotating rack, and continuing the dynamic hydrothermal synthesis for 5-8 hours to complete the secondary synthesis to form a multi-channel hollow fiber MFI molecular sieve membrane, and taking it out after the synthesis is completed, washing it and drying it, and the rotation speed of the rotating rack is 20-60 rpm; 4) The multi-channel hollow fiber MFI molecular sieve membrane synthesized after secondary synthesis is placed in an ozone atmosphere to remove the template agent.

[0011] Preferably, in step 1), the number of fibers is 5-120, the multi-channel hollow fiber has a length of 5-50 cm, an average pore size of 0.3-0.5 μm, a porosity of 30-40%, an outer diameter of 3-5 mm, and a diameter of each channel of 0.8-1.2 mm.

[0012] Preferably, in step 1), the seed crystals are prepared by mixing MFI molecular sieve particles and deionized water into a seed crystal solution with a concentration of 0.2-2 wt%, and the solution is coated on the surface of the multi-channel hollow fiber by vacuum suction or immersion coating.

[0013] Preferably, in step 1), the average particle size of the seed crystals is 60-300 nm.

[0014] Preferably, in steps 2) and 3), the molar ratio of the casting solution is SiO2: TPAOH: H2O = 1: 0.2-0.3: 150-200.

[0015] Preferably, in step 4), the template is removed by heating to 200-300° C. at a heating / cooling rate of 0.5-2° C. / min under an ozone atmosphere and maintaining the temperature for 5-24 hours.

[0016] Preferably, in step 4), the ozone concentration of the ozone atmosphere is 50-2000 mg / L.

[0017] Compared with the prior art, the present invention has the following advantages: Firstly, the present invention provides a method for separating C5 / C6 alkane isomers by using a multi-channel hollow fiber MFI molecular sieve membrane. The method can increase the packing area and has high permeation selectivity and permeation flux.

[0018] Secondly, in response to the problems of "height difference between carrier and sealing glaze" and "temperature field" in the batch synthesis of multi-channel MFI molecular sieve membranes, the present invention attempts to reduce the "height difference between carrier and sealing glaze" through a one-time synthesis, and solves the above problems simultaneously through a secondary synthesis combined with a static and dynamic combination to improve the permeability performance of the batch-produced membrane.

[0019] In addition, the present invention uses a large-pore multi-channel hollow fiber as a carrier, which can improve the permeation flux of the membrane compared to a small-pore carrier, and through the optimization of the parameters of each step, ensures that the membrane has high permeation selectivity and permeation flux.

[0020] Finally, the present invention overcomes the problem of different inner core seed coating uniformity in the seed coating process of the multi-channel hollow fiber by vacuum suction. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 : Schematic diagram of membrane separation device, in which: 1: gas cylinder; 2: pressure reducing valve; 3: bubbling gas; 4: raw material tank; 5: dilution gas; 6: purge gas; 7: oven; 8: buffer tank; 9: membrane assembly; 10: tubular furnace; 11: gas chromatograph; 12: back pressure valve; Figure 2 : Photo of hollow fiber support assembly; Figure 3 : SEM photos of the surface and cross-section of the MFI molecular sieve membrane prepared in Example 1 at different positions, including the top (ab), middle (cd), and bottom (ef); Figure 4 : SEM photos of the surface and cross-section of the MFI molecular sieve membrane prepared in Comparative Example 1 at different positions, including the top (ab), middle (cd), and bottom (ef); Figure 5 : SEM photos of the MFI molecular sieve membrane sealing glaze and the interface between the carrier and the glaze prepared in Comparative Example 1 at different magnifications Figure 6 : Example 1 (a, e) SEM images of MFI seed crystals and their XRD spectra, (b, c, d, f, g) surface images of the outer ring of the component and the center carrier after coating and their cross-sectional images, and (h) SEM image of the carrier surface at the sealing glaze; Figure 7 : SEM images of the cross section and surface of the outermost circle, the secondary circle and the center carrier of the carrier assembly of Example 2 after coating; Figure 8 : Schematic diagram of the bottom of the MFI molecular sieve membrane assembly; Fig. 9 : Long membrane C6 isomer separation performance in the membrane modules of Example 1 and Example 2; Fig.10 : Separation factor and permeability of the membrane assembly prepared in Example 1 for separating C5 / C6 alkane isomers. DETAILED DESCRIPTION

[0022] Example 1 like Figure 1 As shown, the prepared hollow fiber molecular sieve membrane is loaded into an alkane isomer membrane separation device for performance evaluation. The schematic diagram of the process is shown in Figure 3 As shown. The raw material used in the experiment is a mixture of C5 or C6 alkane isomers, wherein: C5 alkane isomers are a mixture of n-pentane (n-C5) and isopentane (i-C5); C6 alkane isomers are a mixture of n-hexane (n-C6), 2-methylpentane (2MP) and 3-methylpentane (3MP). Due to the low boiling point of the alkane mixture, the raw materials are stored in a low-temperature raw material tank (<10°C) to ensure the stability of the feed composition. It is brought into the vaporization chamber at 80°C by bubbling with carrier gas (N2), and then another carrier gas is added for dilution and then transported into the membrane assembly in the electric furnace. Hollow fiber molecular sieve membrane (the actual picture and its cross-sectional photo are shown in Figure 1The raw materials to be separated are carried into the membrane assembly by the carrier gas, and a purge gas (N2) sends the materials that permeate through the membrane to the gas chromatograph (GC, Shimadzu, GC-2014) for detection. The capillary column is SE-30 (30 m) from GLScience. When the total pressure on the permeate side of the membrane assembly is maintained at normal pressure, the partial pressure of the raw material composition is adjusted by changing the flow rate of the bubbling and diluent gases. A back pressure valve is installed at the outlet of the retentate side to adjust the pressure on the feed side.

[0023] The above membrane assembly uses a four-channel hollow fiber MFI molecular sieve membrane, and the preparation method is as follows: 1) Take 17 40 cm alumina hollow fiber carriers (outer diameter 3.55 mm, average pore size 0.40 μm, porosity 36%) and encapsulate them into a bundle to form a hollow fiber carrier assembly (such as Figure 2 As shown); MFI seeds are prepared by hydrothermal synthesis, the initial molar composition of the synthesis solution is SiO2: TPAOH: NaOH: H2O = 1: 0.3: 0.1: 14, and the MFI molecular sieve with an average particle size of 80nm is obtained by hydrothermal synthesis at 60°C for 360 h, and diluted to 0.5 wt.% in deionized water to form a seed solution. The above hollow fiber carrier assembly is immersed in the seed solution, and the seed is coated by vacuum suction, and the vacuum degree is 50kPa; 2) A mixture with a molar ratio of SiO2: TPAOH: H2O = 1: 0.24: 178 was used as the casting solution. The first synthesis was performed at 160°C for 4 hours. After the synthesis was completed, the mixture was taken out, washed and dried. 3) The base film after the primary synthesis is further placed in a casting solution with a molar ratio of SiO2: TPAOH: H2O = 1: 0.24: 178, and subjected to static hydrothermal synthesis at 160°C for 6 h. The synthesis reactor is then placed on a rotating rack and subjected to dynamic hydrothermal synthesis at 160°C for 6 h at a speed of 30 rpm. After the synthesis is completed, the base film is taken out, washed and dried. 4) The MFI molecular sieve membrane assembly was placed in an ozone atmosphere, and the membrane was calcined at 220 °C with a heating / cooling rate of 1 °C / min for 12 h to remove the organic template. The ozone concentration was 110 mg / L.

[0024] Example 2 The difference between this and Example 1 is that the seed coating is carried out by dipping: the carrier is completely immersed in 0.5 wt.% MFI seed solution for 15 s, then taken out and placed vertically for 5 min. After the surface is dried, the direction is reversed and the second seed coating is carried out by the same method.

[0025] Comparative Example 1 The difference between this embodiment and Example 1 is that the hydrothermal synthesis is performed once at 160° C. for 12 h.

[0026] Characterization 1. SEM Characterization (1) The long membranes of Example 1 and Comparative Example 1 were cut into three parts, marked as upper part, middle part and bottom part from top to bottom, and the surface morphology and membrane thickness at different positions of the 40 cm long MFI molecular sieve membrane were characterized by SEM.

[0027] Combination Figure 3-4 It can be seen that the thickness of the MFI molecular sieve membrane prepared by the method of Example 1 is uniform at the top, middle and bottom positions, and there is no obvious film break at the junction of the carrier and the sealing glaze. However, the thickness of the top, middle and bottom film layers of the MFI molecular sieve membrane prepared by the one-time synthesis method are different. The MFI molecular sieve on the top and middle surfaces grows to form a continuous randomly oriented MFI molecular sieve membrane layer, but the MFI molecular sieve crystals on the surface of the membrane layer at each position are of different sizes. Moreover, Figure 5 It is shown that the membrane layer is obviously broken at the junction of the carrier and the sealing glaze of Comparative Example 1. Later, the permeability performance of the single membrane and membrane module of Comparative Example 1 was tested, and the permeability selectivity was very low, which is consistent with the fact that the membrane layer is broken.

[0028] (2) The carriers after seed coating in Example 1 and Example 2 were subjected to SEM characterization. Figure 6 As shown in FIG. 1 , the XRD peak of the seed crystal fits well with the standard XRD peak of the MFI structure, indicating that the prepared seed crystal is pure phase and has high crystallinity. The outermost and centralmost samples of the assembly of Example 1 are obtained by longitudinally cutting the carrier, as shown in FIG. Figure 6 As shown in (b), (c), (d), (f), and (g), the thickness of the outermost and central seed layers are 2~3 μm and 1 μm, respectively. It can be seen from the figure that the seed coverage density on the carrier surface is high. At the same time, the local area of ​​the sealing glaze at the connection between the carrier and the base is characterized by SEM, as shown in Figure 6 As shown in (h), the seed crystals can evenly and continuously cover the joint between the carrier and the sealing glaze. Figure 7 The SEM images of the outermost circle, the second circle and the most central area of ​​Module-1 prepared by the dip-pull coating method are shown. From the cross-sectional image, it can be seen that the seed layer thickness of the outermost circle and the second circle is about 2 μm, while the seed layer thickness of the most central area is less than 1 μm. At the same time, the surface image clearly shows that the seed density of the outermost circle and the second circle is significantly higher than that of the most central area per unit carrier area.

[0029] 2. Performance Characterization Figure 8The bottom schematic diagram of the MFI molecular sieve membrane assembly prepared in Examples 1 and 2, 17 40 cm long molecular sieve membranes are numbered, the outer 10 membranes are numbered 1-10, the middle 6 membranes are numbered 11-16, and the inner layer is numbered 17. The membrane assembly of Examples 1 and 2 was disassembled into 17 single membranes of 40 cm in length, and their C6 isomer separation performance was tested. Fig. 9 The C6 / 3MP separation performance of 17 40 cm MFI molecular sieve membranes of Examples 1 and 2 at a separation temperature of 80 °C and a total feed partial pressure of 5 kPa was demonstrated. The results showed that all 17 MFI molecular sieve membranes in Example 1 exhibited excellent separation selectivity, with n-C6 / 3MP separation factors ranging from 80 to 683, and n-C6 permeability ranging from 1.3 to 3.7×10 -7 mol·m -2 ·s -1 ·Pa -1 The n-C6 / 3MP separation factor of the 17 MFI molecular sieve membranes in Example 2 ranged from 39 to 207, and the n-C6 permeability ranged from 5.5 to 7.0 × 10 - 7 mol·m -2 ·s -1 ·Pa -1 , the separation selectivity of the inner 7 fibers is less than 100. Overall, the performance of the membrane assembly in Example 1 is better than that of the single long membrane in Example 2, indicating that the hollow fiber MFI molecular sieve membrane assembly prepared by the vacuum suction crystal coating method has better performance in separating C6 alkane isomers. The overall permeability of the MFI molecular sieve assembly was tested, as shown in Fig.10 As shown in the figure, the test separation temperature of C5 isomers is 40 ℃, and that of C6 isomers and simulated naphtha mixture is 80 ℃. Within the partial pressure range studied, the flux of binary C5 isomers increases with the increase of total partial pressure, and the separation selectivity increases by 78%. Under the same conditions, increasing the driving force on both sides of the membrane separation makes n-pentane more sensitive to partial pressure and has higher permeation efficiency, resulting in a continuous increase in separation selectivity. For binary C6 isomer mixtures, at a total partial pressure of 10-30 kPa, the adsorption is in the Henry's law region. When the total partial pressure is increased, the adsorption of n-hexane increases and the separation selectivity increases. When reaching 30 kPa (where the partial pressure of n-hexane is 14 kPa), the flux of hexane isomers reaches equilibrium, and the adsorption of the molecular sieve membrane tends to saturation. At a total partial pressure of 60 kPa, a significant decrease in the flux of isohexane can be observed. For C5 / C6 alkane mixtures, at 25-50 kPa, the alkane flux and partial pressure selectivity are weak functions of partial pressure. At a partial pressure of 50 kPa, the flux of n-alkanes is as high as 7.8×10 -4 mol·m -2 ·s -1, and its separation selectivity is 18. It should be noted that the separation performance of the membrane assembly and the single membrane is different, mainly because the two membrane separations are operated in different ways. The single membrane adopts the form of bubbling and sweeping. In this case, the substances to be separated interact less with each other, and normal alkanes are more likely to diffuse through the pores of the molecular sieve membrane. The membrane assembly uses vacuum steam permeation to separate the mixture. This method has a high concentration of the components to be separated and is prone to concentration polarization, which manifests itself in the difference in separation factors. However, judging from the content of normal alkanes on the permeate side, the separation effects of the two operating methods can meet industrial needs (normal alkane content ≥ 80%).

Claims

1. A method for separating C5 / C6 alkane isomers by using a multi-channel hollow fiber MFI molecular sieve membrane, wherein a fluid containing C5 / C6 alkane isomers is passed through an MFI membrane module for separation to obtain normal alkanes and isoalkanes, characterized in that: The MFI membrane assembly comprises a plurality of multi-channel hollow fiber MFI molecular sieve membranes, and the plurality of multi-channel hollow fiber MFI molecular sieve membranes are prepared by batch integrated production.

2. The method according to claim 1, characterized in that The single multi-channel hollow fiber MFI molecular sieve membrane in the MFI membrane assembly has a n-C6 / 3MP separation factor greater than 70 at a separation temperature of 80 °C and a total feedstock partial pressure of 5 kPa, and an n-C6 permeability greater than 1.3×10 -7 mol·m -2 ·s -1 ·Pa -1 .

3. The method according to claim 1, characterized in that The multi-channel hollow fiber is a 4-9 channel hollow fiber.

4. The method according to claim 1, characterized in that The preparation method of the multi-channel hollow fiber MFI molecular sieve membrane comprises the following steps: 1) several multi-channel hollow fibers are encapsulated into a bundle through glaze, and seed crystals are coated on the surface of the multi-channel hollow fibers and dried; 2) placing the seed-coated multi-channel hollow fiber bundle in a casting solution, performing hydrothermal synthesis at 100-180° C. for 3-5 hours to complete a single synthesis, and taking out, washing and drying after the single synthesis is completed; 3) placing the multi-channel hollow fiber bundle after the first synthesis in the casting solution again, and performing static hydrothermal synthesis at 100-180° C. for 5-10 hours, and then placing the synthesis reactor containing the casting solution on a rotating rack, and continuing the dynamic hydrothermal synthesis for 5-8 hours to complete the secondary synthesis to form a multi-channel hollow fiber MFI molecular sieve membrane, and taking it out after the synthesis is completed, washing it and drying it, and the rotation speed of the rotating rack is 20-60 rpm; 4) The multi-channel hollow fiber MFI molecular sieve membrane synthesized after secondary synthesis is placed in an ozone atmosphere to remove the template agent.

5. The method according to claim 4, characterized in that In step 1), the number of fibers is 5-120, the multi-channel hollow fibers have a length of 5-50 cm, an average pore size of 0.3-0.5 μm, a porosity of 30-40%, and an outer diameter of 3-5 mm.

6. The method according to claim 4, characterized in that In step 1), the seed crystals are prepared by mixing MFI molecular sieve particles and deionized water into a seed crystal solution with a concentration of 0.2-2 wt%, and the seed crystals are coated on the surface of the multi-channel hollow fiber by vacuum suction or immersion coating.

7. The method according to claim 4, characterized in that In step 1), the average particle size of the seed crystal is 60-300 nm.

8. The method according to claim 4, characterized in that In steps 2) and 3), the molar ratio of the casting solution is SiO2:TPAOH:H2O=1:0.2-0.3:150-200.

9. The method according to claim 4, characterized in that In step 4), the temperature is raised to 200-300°C at a heating / cooling rate of 0.5-2°C / min under an ozone atmosphere and maintained for 5-24 hours to remove the template.

10. The method according to claim 4, characterized in that In step 4), the ozone concentration of the ozone atmosphere is 50-2000 mg / L.

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