A method for separating C5 / C6 alkane isomers by using a multi-channel hollow fiber MFI molecular sieve membrane

Abstract

The present application relates to a method for separating C5 / C6 alkane isomers, in particular to a method for separating C5 / C6 alkane isomers by using multi-channel hollow fiber MFI molecular sieve membrane. The present application provides a method for separating C5 / C6 alkane isomers by using multi-channel hollow fiber MFI molecular sieve membrane, which comprises the following steps: feeding a fluid containing C5 / C6 alkane isomers into a MFI membrane assembly for separation to obtain normal alkane and isomeric alkane, wherein the MFI membrane assembly comprises several multi-channel hollow fiber MFI molecular sieve membranes, and the several multi-channel hollow fiber MFI molecular sieve membranes are prepared by batch integrated production.

Description

Technical Field

[0001] This invention relates to a method for separating C5 / C6 alkane isomers, specifically a method for separating C5 / C6 alkane isomers using a multi-channel hollow fiber MFI molecular sieve membrane. Background Technology

[0002] Light naphtha is a petroleum fraction with a temperature between 45 °C and 61 °C, containing over 89.3% C5 and C6 alkane isomers. Influenced by the international situation and the booming development of the ethylene industry, the separation of light naphtha mixtures has received increasing attention, and the development of energy-efficient and high-performance hydrocarbon mixture separation technologies has become a hot topic.

[0003] The MFI molecular sieve membrane has an average pore size of 0.55 nm, and based on molecular sieving, it can separate C5 and C6 alkane isomers (n-pentane / isopentane, n-hexane / 2-methylpentane, n-hexane / 3-methylpentane). The inventors also proposed using MFI molecular sieve membranes to separate n- and iso-alkanes from naphtha in their previous work (invention patent CN113462424A). However, the above work was based on traditional supports with low packing density, making it difficult to meet practical industrial needs.

[0004] Multichannel hollow fibers, with multiple channels in their core, offer higher packing density and mechanical strength suitable for industrial applications compared to traditional supports. The inventors conducted extensive research on multichannel hollow fibers in their early work, experimenting with their use as carriers to prepare MFI molecular sieve membranes for the separation of xylene isomers, achieving good results. Based on this success, the inventors attempted to mass-produce multichannel hollow fibers for the separation of C5 / C6 alkane isomers. However, they found that the permeation performance of individual membranes was poor. Research revealed two main reasons: First, mass production requires bundling multiple carriers with glaze, resulting in a height difference between the carrier and the sealing glaze, making the membrane layer at the glaze-sealed location prone to breakage. Second, to meet industrial application requirements, the carriers are relatively long, leading to differences in crystallization temperatures between the upper and lower ends of the carrier during synthesis.

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

[0006] To address the aforementioned issues, this invention proposes using multi-channel hollow fiber MFI molecular sieve membranes as the key membrane material for the separation of C5 / C6 alkane isomers. Furthermore, it optimizes the preparation method for the mass production of MFI molecular sieve membranes using multi-channel hollow fiber MFI molecular sieve membranes as a carrier, thereby meeting the practical needs of industrial applications.

[0007] Specifically, the present invention discloses a method for separating C5 / C6 alkane isomers using a multi-channel hollow fiber MFI molecular sieve membrane. The method involves passing a fluid containing C5 / C6 alkane isomers into an MFI membrane module for separation to obtain n-alkanes and isoalkanes. The method is characterized in that the MFI membrane module contains 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 module has a separation factor greater than 70 for n-C6 / 3MP and an n-C6 permeability greater than 1.3 × 10⁻⁶ at a separation temperature of 80 °C and a total feed partial pressure of 5 kPa. -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 preparation method of the multi-channel hollow fiber MFI molecular sieve membrane includes the following steps:

[0011] 1) Several multi-channel hollow fibers are bundled together by glazing, and seed crystals are coated on the surface of the multi-channel hollow fibers and then dried;

[0012] 2) The multi-channel hollow fiber bundle coated with seed crystals is placed in the casting solution and hydrothermally synthesized at 100-180℃ for 3-5 hours to complete one synthesis. After the first synthesis, it is taken out, washed and dried.

[0013] 3) After the first synthesis, the multi-channel hollow fiber bundle is placed in the casting solution again and statically hydrothermally synthesized at 100-180℃ for 5-10 hours. Then, the synthesis vessel containing the casting solution is placed on a rotating rack and dynamic hydrothermal synthesis is continued for 5-8 hours to complete the second synthesis and form a multi-channel hollow fiber MFI molecular sieve membrane. After the synthesis is completed, the membrane is taken out, washed and dried. The rotation speed of the rotating rack is 20-60 rpm.

[0014] 4) The template agent was removed from the multi-channel hollow fiber MFI molecular sieve membrane that had undergone secondary synthesis under an ozone atmosphere.

[0015] Preferably, in step 1), the plurality of fibers is 5-120 fibers, the length of the multi-channel hollow fiber is 5-50cm, the average pore size is 0.3-0.5μm, the porosity is 30-40%, the outer diameter is 3-5mm, and the diameter of each channel is 0.8-1.2mm.

[0016] Preferably, in step 1), the seed crystal is prepared by mixing MFI molecular sieve particles and deionized water to form a seed crystal solution with a concentration of 0.2-2wt%, and then coated onto the surface of the multi-channel hollow fiber by vacuum suction or impregnation coating.

[0017] Preferably, in step 1), the average particle size of the seed crystal is 60-300 nm.

[0018] Preferably, in steps 2) and 3), the molar ratio of the casting solution is SiO2: TPAOH: H2O = 1: 0.2-0.3: 150-200.

[0019] Preferably, in step 4), the template agent is removed by heating / cooling at a rate of 0.5-2℃ / min to 200-300℃ under an ozone atmosphere and maintaining the temperature for 5-24 hours.

[0020] Preferably, in step 4), the ozone concentration of the ozone atmosphere is 50-2000 mg / L.

[0021] Compared with the prior art, the present invention has the following advantages:

[0022] First, this invention provides a method for separating C5 / C6 alkane isomers using a multi-channel hollow fiber MFI molecular sieve membrane. This method can increase the packing area and has high permeation selectivity and permeation flux.

[0023] Secondly, in response to the challenges of "height difference between carrier and sealant" and "temperature field" in the mass synthesis of multi-channel MFI molecular sieve membranes, this invention attempts to reduce the "height difference between carrier and sealant" through a single synthesis, and simultaneously solves the above problems through a combination of static and dynamic synthesis to improve the permeation performance of the mass-produced membranes.

[0024] In addition, the present invention uses large-pore multi-channel hollow fibers as a carrier, which can improve the permeation flux of the membrane compared with small-pore carriers. By optimizing the parameters of each step, the membrane is guaranteed to have high permeation selectivity and permeation flux.

[0025] Finally, the present invention overcomes the problem of uneven coating of different core seeds in multi-channel hollow fibers during the seed coating process by using vacuum suction. Attached Figure Description

[0026] Figure 1 Schematic diagram of a membrane separation device. In the diagram, 1: gas cylinder; 2: pressure reducing valve; 3: bubbling gas; 4: feed tank; 5: dilution gas; 6: purge gas; 7: drying oven; 8: buffer tank; 9: membrane module; 10: tubular furnace; 11: gas chromatograph; 12: back pressure valve.

[0027] Figure 2 Photographs of hollow fiber carrier components;

[0028] Figure 3 SEM images of the surface and cross-section of the MFI molecular sieve membrane prepared in Example 1 at different locations, including the top (ab), middle (cd), and bottom (ef).

[0029] Figure 4 SEM images of the surface and cross-section of the MFI molecular sieve membrane prepared in Comparative Example 1 at different locations, including the top (ab), middle (cd), and bottom (ef).

[0030] Figure 5 SEM images at different magnifications of the MFI molecular sieve membrane sealing glaze and the interface between the support and glaze prepared in Comparative Example 1.

[0031] Figure 6 Example 1: (a, e) SEM images and XRD patterns of MFI seed crystals; (b, c, d, f, g) Surface images and cross-sectional images of the outer ring and central carrier of the component after crystal coating; and (h) SEM image of the carrier surface at the sealing glaze.

[0032] Figure 7 Example 2: SEM images of the outermost ring, second ring, and central carrier of the carrier assembly after crystal coating, showing cross-section and surface details.

[0033] Figure 8 : Schematic diagram of the bottom of the MFI molecular sieve membrane module;

[0034] Figure 9 Separation performance of C6 isomers in long membranes in Example 1 and Example 2 membrane modules;

[0035] Figure 10 Example 1: Separation factor and permeability of the membrane module prepared for separating C5 / C6 alkane isomers. Detailed Implementation

[0036] Example 1

[0037] like Figure 1 As shown, the prepared hollow fiber molecular sieve membrane was loaded into an alkane isomer membrane separation device for performance evaluation. A schematic diagram of the process is shown below. Figure 3As shown. The raw materials used in the experiment were a mixture of C5 or C6 alkane isomers, wherein: the C5 alkane isomers were a mixture of n-pentane (n-C5) and isopentane (i-C5); the C6 alkane isomers were a mixture of n-hexane (n-C6), 2-methylpentane (2MP), and 3-methylpentane (3MP). Because the alkane mixture had a low boiling point, the raw materials were stored in a low-temperature feed tank (<10℃) to ensure the stability of the feed composition. The material was bubbled into an 80℃ vaporization chamber using a carrier gas (N2), and then diluted with another carrier gas before being fed into the membrane module in the electric furnace. Hollow fiber molecular sieve membrane (actual image and cross-sectional photograph shown) Figure 1 The membrane (as shown in the diagram) is sealed within the membrane module. The feedstock to be separated is carried into the membrane module by a carrier gas. A purge gas (N2) sends the permeate through the membrane to a gas chromatograph (GC, Shimadzu, GC-2014) for detection. The capillary column is a GLScience SE-30 (30 m). When the total pressure on the permeate side of the membrane module is maintained at atmospheric pressure, the partial pressure of the feedstock composition is modulated by changing the flow rates of the bubbling and dilution gases. A back pressure valve is installed at the outlet on the permeate side to regulate the pressure on the feed side.

[0038] The above membrane module uses a four-channel hollow fiber MFI molecular sieve membrane, which is prepared as follows:

[0039] 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 with glaze to form a bundle to form a hollow fiber carrier assembly (e.g. Figure 2 (As shown); MFI seed crystals were prepared by hydrothermal synthesis. The initial molar composition of the synthesis solution was SiO2: TPAOH: NaOH: H2O = 1: 0.3: 0.1: 14. Hydrothermal synthesis at 60℃ for 360 h yielded MFI molecular sieves with an average particle size of 80 nm, which were then diluted to 0.5 wt.% in deionized water to form a seed solution. The hollow fiber carrier assembly was immersed in the seed solution, and the seed crystals were coated using a vacuum suction method at a vacuum degree of 50 kPa.

[0040] 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 carried out at 160℃ for 4 hours. After the synthesis was completed, the mixture was taken out, washed and dried.

[0041] 3) The base film after the first synthesis was placed in a casting solution with a molar ratio of SiO2: TPAOH: H2O = 1: 0.24:178 and statically hydrothermally synthesized at 160 °C for 6 h. Then the synthesis vessel was placed on a rotating rack and dynamically hydrothermally synthesized at 160 °C for 6 h at a speed of 30 rpm. After the synthesis was completed, the film was removed, washed and dried.

[0042] 4) The MFI molecular sieve membrane module was placed in an ozone atmosphere and calcined at 220 °C at a heating / cooling rate of 1 °C / min for 12 h to remove the organic template agent. The ozone concentration was 110 mg / L.

[0043] Example 2

[0044] The difference between this and Example 1 is that the seed crystals are coated by immersion coating: the carrier is completely immersed in 0.5 wt.% MFI seed solution for 15 seconds, then taken out and placed vertically for 5 minutes. After the surface dries, the orientation is reversed and the same method is used for a second seed crystal coating.

[0045] Comparative Example 1

[0046] The difference between this and Example 1 is that the hydrothermal synthesis is a single process, conducted at 160 °C for 12 h.

[0047] Characterization

[0048] I. SEM Characterization

[0049] (1) Take the long membranes of Example 1 and Comparative Example 1 and cut them into three parts evenly, labeled as upper part, middle part and bottom part from top to bottom. The surface morphology and membrane thickness of the 40 cm long MFI molecular sieve membrane at different positions are characterized by SEM.

[0050] Combination Figure 3-4 As can be seen, the MFI molecular sieve membrane prepared using the method of Example 1 has a uniform thickness at the top, middle, and bottom positions, and there is no obvious membrane breakage at the interface between the support and the sealing glaze. In contrast, the MFI molecular sieve membrane prepared using the one-step synthesis method has different thicknesses at the top, middle, and bottom. The MFI molecular sieves on the top and middle surfaces grow to form continuous, randomly oriented MFI molecular sieve membrane layers, but the MFI molecular sieve crystals on the surface of each position are of varying sizes. Furthermore, Figure 5 The results show that a clear membrane fracture is visible at the interface between the carrier and the sealing glaze in Comparative Example 1. Subsequent permeation performance tests on the single membrane and membrane module of Comparative Example 1 revealed very low permeation selectivity, which is consistent with the membrane fracture.

[0051] (2) The seed-coated supports from Examples 1 and 2 were subjected to SEM characterization. For example... Figure 6 As shown, the XRD peaks of the seed crystals fit well with the standard XRD peaks of the MFI structure, indicating that the prepared seed crystals are pure phase and have high crystallinity. Samples of the outermost ring and the center of the component were obtained by longitudinally cutting the carrier of the component from Example 1. Figure 6As shown in (b), (c), (d), (f), and (g), the thicknesses of the outermost and central seed layers are 2–3 μm and 1 μm, respectively. The figures show a high seed coverage density on the carrier surface. Furthermore, SEM characterization was performed on a localized area of ​​the sealing glaze at the junction of the carrier and the substrate. Figure 6 As shown in (h), the seed crystals can uniformly and continuously cover the junction between the carrier and the sealing glaze. Figure 7 This image shows SEM images of the outermost, second-order, and central regions of Module-1 prepared by the dip-coating method after crystal coating. The cross-sectional image shows that the seed layer thickness of the outermost and second-order layers is approximately 2 μm, while the seed layer thickness of the central region is less than 1 μm. Furthermore, the surface image clearly shows that the seed density per unit area of ​​the outermost and second-order layers is significantly higher than that of the central region.

[0052] II. Performance Characterization

[0053] Figure 8 The diagram shows the bottom of the MFI molecular sieve membrane module prepared in Examples 1 and 2. Seventeen 40 cm long molecular sieve membranes are labeled: the outer 10 membranes are numbered 1-10, the middle 6 membranes are numbered 11-16, and the inner membrane is numbered 17. The membrane modules of Examples 1 and 2 were disassembled into 17 single 40 cm long membranes, and their C6 isomer separation performance was tested. Figure 9 The separation performance of 17 40 cm MFI molecular sieve membranes from Examples 1 and 2 on C6 / 3MP was demonstrated at a separation temperature of 80 °C and a total feed partial pressure of 5 kPa. 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 between 1.3 and 3.7 × 10⁻⁶. -7 mol·m -2 ·s -1 ·Pa -1 In Example 2, the n-C6 / 3MP separation factor of the 17 MFI molecular sieve membranes ranged from 39 to 207, and the n-C6 permeability ranged from 5.5 to 7.0 × 10⁻⁶. - 7 mol·m -2 ·s -1 ·Pa -1 The separation selectivity of the seven innermost membranes was all below 100. Overall, the membrane module of Example 1 showed superior performance compared to the single long membrane of Example 2, indicating that the hollow fiber MFI molecular sieve membrane module prepared by the vacuum suction crystallization method has better performance in separating C6 alkane isomers. The overall permeation performance of the MFI molecular sieve module was tested, such as... Figure 10As shown, the test temperature for separating C5 isomers was 40 °C, and for C6 isomers and simulated naphtha mixtures, it was 80 °C. Within the studied partial pressure range, the flux of binary C5 isomers increased with increasing total partial pressure, and the separation selectivity improved by 78%. Under the same conditions, increasing the driving force on both sides of the membrane separation made n-pentane more sensitive to partial pressure, resulting in higher permeation efficiency and a continuous increase in separation selectivity. For the binary C6 isomer mixture, adsorption was in the Henry's Law region at a total partial pressure of 10-30 kPa. Increasing the total partial pressure increased n-hexane adsorption and separation selectivity. When the total partial pressure reached 30 kPa (where the partial pressure of n-hexane was 14 kPa), the flux of hexane isomers reached equilibrium, and the molecular sieve membrane adsorption tended to saturate. At a total partial pressure of 60 kPa, a significant decrease in isohexane flux was observed. For C5 / C6 alkane mixtures, at 25-50 kPa, alkane flux and partial pressure selectivity were weak functions of partial pressure. At a partial pressure of 50 kPa, the flux of n-alkanes reaches as high as 7.8 × 10⁻⁶. -4 mol·m -2 ·s -1 Meanwhile, its separation selectivity is 18. It should be noted that the separation performance of membrane modules and single membranes differs, mainly due to the different operating methods. Single membranes use a bubbling and purging method, where the substances to be separated interact less, and n-alkanes diffuse more easily through the molecular sieve membrane channels. Membrane modules, on the other hand, use vacuum vapor permeation for mixture separation. This method involves higher concentrations of the components to be separated, easily leading to concentration polarization, which manifests as a difference in the separation factor. However, considering the n-alkane content on the permeate side, both operating methods can meet industrial requirements (n-alkane content ≥80%).

Claims

1. A process for the separation of C5 / C6 alkane isomers using a multi-channel hollow fiber (MFI) molecular sieve membrane, said process being the separation of a fluid comprising C5 / C6 alkane isomers by passing the fluid through a MFI membrane module to produce normal and isomeric alkanes, 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; the preparation method of the multi-channel hollow fiber MFI molecular sieve membrane comprises the following steps: A plurality of multi-channel hollow fibers are packaged into a bundle by glaze sealing, and seeds are coated on the surface of the multi-channel hollow fibers, and then dried; The multi-channel hollow fiber bundle coated with seeds is placed in a casting solution, and one synthesis is completed by hydrothermal synthesis at 100-180℃ for 3-5h, and then taken out, washed and dried after one synthesis is completed; The multi-channel hollow fiber bundle after one synthesis is completed is placed in the casting solution again, and the second synthesis is completed by static hydrothermal synthesis at 100-180℃ for 5-10h, and then the synthesis kettle containing the casting solution is placed on a rotating rack, and the dynamic hydrothermal synthesis is continued for 5-8h to form a multi-channel hollow fiber MFI molecular sieve membrane, and then taken out, washed and dried after synthesis is completed, and the rotating speed of the rotating rack is 20-60rpm; The multi-channel hollow fiber MFI molecular sieve membrane after the second synthesis is placed in an ozone atmosphere to remove the template agent.

2. The method of claim 1, wherein, The single multi-channel hollow fiber MFI molecular sieve membrane in the MFI membrane assembly has a separation factor of greater than 70 for n-C6 / 3MP at a separation temperature of 80 DEG C and a total raw material partial pressure of 5 kPa, and a n-C6 permeability of greater than 1.3 x 10 -7 mol m -2 s -1 ·Pa -1 .

3. The method of claim 1, wherein, The multi-channel hollow fiber is a 4-9 channel hollow fiber.

4. The method of claim 1, wherein, In step 1), the plurality of multi-channel hollow fibers is 5-120, the length of the multi-channel hollow fiber is 5-50cm, the average pore size is 0.3-0.5μm, the porosity is 30-40%, and the outer diameter is 3-5mm.

5. The method of claim 1, wherein, In step 1), the seeds are prepared by mixing MFI molecular sieve particles and deionized water to prepare a seed solution with a concentration of 0.2-2wt%, and then coated on the surface of the multi-channel hollow fiber by vacuum suction or immersion coating.

6. The method of claim 1, wherein, In step 1), the average particle size of the seeds is 60-300nm.

7. The method of claim 1, wherein, In steps 2) and 3), the molar ratio of the casting solution is SiO2:TPAOH:H2O=1:0.2-0.3:150-200.

8. The method of claim 1, wherein, In step 4), the template agent is removed by heating / cooling at a speed of 0.5-2℃ / min to 200-300℃ and maintaining for 5-24h in an ozone atmosphere.

9. The method of claim 1, wherein, In step 4), the ozone concentration of the ozone atmosphere is 50-2000mg / L.

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