A method and system for producing n-alkanes

Abstract

The present disclosure relates to a method and system for producing normal paraffin. Isobutane raw material is contacted with an isobutane conversion catalyst in an isobutane conversion unit to perform a normal paraffin reaction, wherein the isobutane conversion catalyst has high catalytic activity and does not contain chlorine elements, is safe, non-toxic, and reduces corrosion loss to equipment; the present disclosure introduces the obtained reaction product containing normal paraffin into a membrane separation unit for membrane separation treatment, and high-efficiency separation of normal paraffin and isomeric paraffin can be achieved through a membrane separation element including a molecular sieve membrane, which is high in separation efficiency and simple and easy to implement; the method provided by the present disclosure is simple in process flow, mild in reaction conditions, high in isobutane utilization efficiency, and high in normal paraffin yield; the isobutane raw material is efficiently converted into normal paraffin which has high added value and is in urgent need of an ethylene device, not only realizing value-added utilization of isobutane resources, but also widening the raw material source of the ethylene device.

Description

Technical Field

[0001] The present disclosure relates to the technical field of light hydrocarbon utilization, and in particular, to a method and system for producing normal alkanes. Background Art

[0002] Isobutane is a key byproduct of oil extraction and refining. Oilfield butane typically contains 20-40% isobutane by mass, and liquefied petroleum gas (LPG) produced as a by-product of catalytic cracking accounts for approximately 20% by mass. In recent years, with the improvement of my country's refining capacity and the continuous expansion of catalytic cracking units, isobutane production has also increased rapidly, leading to increasing attention to its deep processing and efficient utilization.

[0003] Normal alkanes are high-quality feedstocks for ethylene cracking, offering high olefin yields and low methane byproducts. In recent years, with the rapid development of my country's national economy and the continuous improvement of people's living standards, the growing demand for ethylene downstream derivatives has driven a rapid increase in ethylene demand. Million-ton-class ethylene plants have been commissioned in my country, necessitating an urgent need to expand the feedstock supply for these plants. Converting isobutane into normal alkanes for use as a feedstock for ethylene cracking can alleviate the feedstock shortage problem for ethylene plants and achieve high-value utilization of isobutane resources, offering promising development prospects. Therefore, the efficient conversion of isobutane into normal alkanes has become a key research topic in the field of light hydrocarbon utilization in recent years.

[0004] US Patent No. 4191845 discloses a method for converting unsaturated C4 hydrocarbons into n-butane. The method first feeds the unsaturated C4 hydrocarbons into a hydrogenation unit to convert them into C4 alkanes. The C4 alkanes are then fed into a separation unit, where the separated n-butane is used as a feedstock for ethylene cracking. The separated isobutane is fed into a normalization unit for reaction, where a portion of the isobutane is converted into n-butane. The normalization product containing n-butane and isobutane is then returned to the separation unit for separation. This method uses a distillation column to separate n-butane and isobutane, and uses a catalyst containing AlCl3 or AlBr3 as the catalyst for the normalization of isobutane. The reaction can be carried out in the presence of hydrogen, but the feed ratio of hydrogen to the normalization feedstock is not specified.

[0005] Patent CN104892339A discloses a method for producing n-butane from isobutane. This method involves first feeding a feedstock containing at least 80% isobutane into a normalization reaction zone for reaction. The normalization reaction product is then fed into a hydrogenation saturation zone for catalytic hydrogenation. The hydrogenated product is then condensed to a temperature of 0-40°C to produce a gaseous product and a liquid product. Two distillation zones are provided for separation of the liquid product: the first fractionation zone separates the low-boiling point components in the liquid product, and the second fractionation zone separates the n-butane from the isobutane. The acidic component of the normalization catalyst used in this method is chloride, and the reaction is carried out in the presence of hydrogen. The molar ratio of hydrogen to isobutane in the feedstock is 1-3:1.

[0006] Patents CN107285977A and CN107285978A disclose a system and method for producing n-butane by converting isobutane. The system comprises a light-removing column, a butane column, a heavy-removing column, and an isobutane normalization reactor. The system is also equipped with a circulating hydrogen compressor for hydrogen circulation during the normalization reaction. This n-butane production method uses a C4 alkane as the raw material, which is first fed into a light-removing column to remove light components. The raw material is then fed into a butane column to separate isobutane. The isobutane is then fed into a normalization reactor for reaction. The reaction product is then fed into a heavy-removing column to remove heavy components, yielding the n-butane product. The catalyst in the normalization reactor comprises a support, an active component, and an additive. Both the support and additive are oxides, and the active component is a Group VIII transition metal. The normalization reaction temperature is 400-550°C, and the reaction pressure is 3-4.5 MPa. Since the normalization reaction temperature is relatively harsh, the products of the normalization reaction of isobutane include isobutylene, propylene, 1-butene, 2-butene, etc. in addition to isobutane, and they need to be hydrogenated and saturated before separation.

[0007] Patent CN108530254A discloses a method for preparing n-butane from mixed C4. This method involves hydrogenating the mixed C4 to saturation, then sending it to a distillation tower for desulfurization. The mixture is then separated in a rectification tower, with propane collected from the top, n-butane from the bottom, and isobutane from the middle. The collected isobutane is then normalized over a catalyst to produce n-butane. The reaction temperature is 400-600°C and the pressure is 2.5-3.5 MPa.

[0008] Currently, normalization is used to convert isobutane to normal butane. However, to achieve complete conversion of isobutane, multiple distillation towers are required to separate the unreacted isobutane from the normal butane in the product, as well as the byproducts from the target product, normal butane. Due to the small difference in boiling points between light hydrocarbons such as normal butane and isobutane, distillation separation requires high energy consumption and a complex process. Existing processes have not yet provided a separation material that can effectively separate isobutane from normal butane, nor have they achieved the goal of effectively separating normal butane and isobutane through simple operation using a separation material.

[0009] Furthermore, the isobutane normalization reaction processes disclosed in existing technologies are all very demanding. Some use chlorine-containing catalysts, which impose strict restrictions on raw material impurities. The presence of chlorine also increases the risk of equipment corrosion, significantly increasing operating and maintenance costs. Although some methods do not use chlorine-containing catalysts, the normalization reaction must be carried out at temperatures exceeding 400°C, which leads to increased side reactions such as dehydrogenation and cracking. The reaction products must then be hydrogenated to remove olefins and byproducts such as methane and ethane, resulting in complex procedures and high costs.

[0010] CN110358142A discloses a catalyst for isobutane normalization reaction, which uses MCM-41 molecular sieve as a carrier and Pt / SO4 2- / ZrO2 is the active component, the normalization reaction temperature is 150-350℃, the pressure is 1.5-4.5MPa, and the mass space velocity of isobutane is 0.5-3.0h -1 The volume ratio of hydrogen to isobutane feed is 200:1. This method uses solid super acid catalyst Pt / SO4 2- / ZrO2 is used as the active component for the isobutane normalization reaction. Although the reaction conditions are relatively mild, the isobutane conversion effect is not ideal (under the condition of n-butane selectivity higher than 90%, the isobutane conversion rate does not exceed 21%, and the n-butane yield is lower than 22.1%). Summary of the Invention

[0011] The present invention aims to provide a method and system for producing normal alkanes, which uses isobutane or C4 alkanes as raw materials and converts them into normal alkanes mainly composed of normal butane and a certain amount of propane, thereby realizing the value-added utilization of isobutane and expanding the raw material source for ethylene plants; and also provides a molecular sieve membrane separation material that can effectively separate normal butane and isobutane with simple operation.

[0012] To achieve the above-mentioned objectives, the present disclosure provides, in a first aspect, a method for producing normal alkanes, comprising the following steps: S1, allowing an isobutane feedstock and hydrogen to enter an isobutane conversion unit, where the feedstock is contacted with an isobutane conversion catalyst to perform an isobutane conversion reaction, thereby obtaining a reaction product comprising normal alkanes; wherein the isobutane conversion catalyst comprises a solid superacid catalyst; S2, allowing the reaction product to enter a membrane separation unit for membrane separation treatment, thereby obtaining a first material comprising isobutane and a second material comprising normal alkanes; wherein the membrane separation element of the membrane separation unit comprises a molecular sieve membrane; and S3, returning the first material to the isobutane conversion unit for continued reaction.

[0013] Optionally, the solid superacid catalyst comprises a first carrier and a first active metal; based on the total weight of the solid superacid catalyst, the solid superacid catalyst comprises 0.05 to 2.0 mass% of the Group VIII metal and 98.0 to 99.95 mass% of the first carrier;

[0014] Preferably, the first carrier contains zirconium, silicon, aluminum and sulfur, respectively calculated as zirconium oxide, silicon oxide, aluminum oxide and SO3, based on the total weight of the first carrier, the first carrier includes 30-90 mass% of zirconium oxide, 0.5-10 mass% of silicon oxide, 5-40 mass% of aluminum oxide and 0.5-10 mass% of SO3;

[0015] Further preferably, the first carrier comprises 40-80 mass% of zirconium oxide, 1-5 mass% of silicon oxide, 10-33 mass% of aluminum oxide and 1-8 mass% of SO3;

[0016] The first active metal is a Group VIII metal, preferably one or both selected from Pt and Pd.

[0017] Optionally, in step S1, the conditions for the isobutane conversion reaction include: a temperature of 150-300°C, a pressure of 0.5-4.0 MPa, a mass space velocity of the isobutane raw material of 0.5-20 h -1 , calculated based on the isobutane in the isobutane raw material, the hydrogen-to-hydrocarbon molar ratio is (0.03-0.2):1.

[0018] Optionally, in step S1, the purity of the isobutane raw material is higher than 80% by mass, preferably higher than 90% by mass, and more preferably higher than 95% by mass; the impurity content in the isobutane raw material includes: olefin content lower than 1% by mass, preferably lower than 0.5% by mass; C5 + The content of hydrocarbons and above is less than 1.5% by mass, preferably less than 1.0% by mass; the water content is less than 5 ppmw, preferably less than 3 ppmw; and the sulfur content is less than 10 ppmw, preferably less than 5 ppmw.

[0019] Optionally, the molecular sieve membrane is selected from at least one of an A-type molecular sieve membrane and a molecular sieve membrane having an MFI structure, preferably a NaA molecular sieve membrane;

[0020] Optionally, the membrane separation element further comprises a second carrier, and the molecular sieve membrane is supported on the second carrier; optionally, the second carrier is in the form of a sheet, a tube with a capped end, or a hollow fiber, preferably a sheet-shaped second carrier; further preferably, the second carrier comprises one or more of alumina, silica, and zirconia, preferably alumina; optionally, the average pore diameter of the second carrier is 100 to 3000 nm, and the pore volume is 0.01 to 0.1 ml / g;

[0021] Preferably, the membrane separation element comprises 90-99.95% by mass of the second carrier and 0.05-10% by mass of the molecular sieve membrane;

[0022] Preferably, the n-butane permeate flow index of the membrane separation element is 5×10 -7 ~3×10 -5 g·m -2 ·s -1 ·Pa -1 ; The n-butane permeate flow ratio is (5~150):1.

[0023] Optionally, the membrane separation element is prepared by the following steps:

[0024] a. mixing an aluminum source, a silicon source, an alkali metal hydroxide, a template and water to obtain a crystallization precursor solution;

[0025] b. placing the second carrier in the crystallization precursor solution for crystallization to obtain a crystallized product; or placing the second carrier in a NaA molecular sieve suspension for immersion; and then placing the second carrier in the crystallization precursor solution for crystallization to obtain a crystallized product;

[0026] c. Washing, drying and calcining the crystallized product.

[0027] Optionally, in step a, the crystallization precursor solution contains aluminum, silicon, an alkali metal, a template, and water, wherein the aluminum, silicon, and alkali metal are calculated as Al2O3, SiO2, and alkali metal oxide, respectively, and the molar ratio of Al2O3:SiO2:alkali metal oxide:template:H2O is (1-5):(2-5):(1-4):(0.05-0.7):(100-200);

[0028] Optionally, the aluminum source is selected from one or more of sodium metaaluminate, aluminum sulfate and aluminum isopropoxide; the silicon source is silica sol, and optionally, the silica sol comprises 5-50% by mass of SiO2 and 0.01-1% by mass of an alkali metal oxide; the alkali metal hydroxide comprises one or both of sodium hydroxide and potassium hydroxide; the template is selected from one or more of tetrapropylammonium hydroxide, tetrabutylammonium hydroxide and tetrapropylammonium bromide;

[0029] In step b, the weight ratio of the NaA molecular sieve in the NaA molecular sieve suspension to the second carrier is (0.001-0.1):1; preferably, the average particle size of the NaA molecular sieve in the NaA molecular sieve suspension is 0.1-5 μm;

[0030] Preferably, in step b, the second carrier is immersed in the NaA molecular sieve suspension at a liquid-to-solid mass ratio of 5 to 100:1, and the immersion time is 5 to 30 minutes;

[0031] Preferably, the crystallization temperature in step b is 80-120° C., and the crystallization time is 30-80 h;

[0032] In step c, the calcination temperature is 400-650° C., and the calcination time is 3-12 hours.

[0033] Optionally, the method further comprises: allowing the C4 alkane feedstock and the optional reaction product from the isobutane conversion unit to enter the membrane separation unit;

[0034] Separating the obtained products through the membrane separation element to obtain a third material containing isobutane and a fourth material containing normal alkanes;

[0035] The third material is used as the isobutane raw material and enters the isobutane conversion unit together with hydrogen;

[0036] Optionally, the C4 alkane feedstock is a C4 alkane produced from one or more of catalytic cracking, hydrocracking, ethylene unit, isobutane dehydrogenation or Fischer-Tropsch synthesis processes; the C4 alkane feedstock contains 0.1 to 99.9% by mass of isobutane and 0.1 to 99.9% by mass of n-butane.

[0037] Optionally, the separation conditions in the membrane separation unit include: a temperature of 30 to 250° C. and a pressure difference of 10 to 600 kPa.

[0038] Optionally, in step S2, the separated first material contains 95-99 mass% of isobutane and less than 5 mass% of isopentane; and the second material contains 90-95 mass% of n-butane.

[0039] According to a second aspect of the present disclosure, a system for producing normal alkanes is provided, which comprises an isobutane conversion unit and a membrane separation unit; wherein the isobutane conversion unit is provided with a reaction raw material inlet for introducing isobutane raw material and hydrogen, a catalyst bed and a reaction product outlet; the catalyst bed comprises an isobutane conversion catalyst, and the isobutane conversion catalyst comprises a solid superacid catalyst; the membrane separation unit is provided with a material to be separated inlet, a retentate outlet and a permeate outlet, and a membrane separation element is further provided inside the membrane separation unit, and the membrane separation element comprises a molecular sieve membrane; the material to be separated inlet is connected to the reaction product outlet of the isobutane conversion unit, and the retentate outlet is connected to the reaction raw material inlet of the isobutane conversion unit via a second pipeline.

[0040] Optionally, the system further comprises a C4 alkane raw material source, and the C4 alkane raw material source is connected to the raw material to be separated inlet of the membrane separation unit.

[0041] By the technical scheme, the isobutane raw material is contacted with an isobutane conversion catalyst in an isobutane conversion unit to perform a normalization reaction, the isobutane conversion catalyst has high catalytic activity and does not contain chlorine element, is safe and non-toxic, and reduces corrosion loss to equipment; the obtained reaction product containing normal alkanes is introduced into a membrane separation unit to perform membrane separation treatment, efficient separation of normal alkanes and isomeric alkanes can be realized through a membrane separation element including a molecular sieve membrane, the separation efficiency is high and the separation method is simple and easy to implement; the method provided by the present disclosure has simple process flow, mild reaction conditions, high isobutane utilization efficiency, and high normal alkane yield; the isobutane raw material is efficiently converted into normal alkanes with high added value and in urgent need of an ethylene device, not only realizing value-added utilization of isobutane resources, but also widening the raw material source of the ethylene device, providing a solution and technical support for refining enterprises to adapt to the national policy, conform to the transformation trend of oil refining to chemical industry, and improve quality and efficiency.

[0042] Other features and advantages of the present disclosure will be described in detail in the following specific embodiments section. BRIEF DESCRIPTION OF DRAWINGS

[0043] The accompanying drawings are included to provide a further understanding of the present disclosure and constitute a part of the specification, and together with the following specific embodiments, serve to explain the present disclosure but do not constitute a limitation on the present disclosure. In the drawings:

[0044] Figure 1 A process flow diagram for producing normal alkanes using isobutane raw material provided by the present disclosure.

[0045] Figure 2 A process flow diagram for producing normal alkanes using carbon four alkane raw material provided by the present disclosure.

[0046] Figure 3 SEM photo (4000 times magnification) of the separation element (NaA molecular sieve membrane loaded on the second carrier) in Example 2-1 of the present disclosure.

[0047] Figure 4 XRD spectrum of the separation element (NaA molecular sieve membrane loaded on the second carrier) in Example 2-1 of the present disclosure. DETAILED DESCRIPTION

[0048] The specific embodiments of the present disclosure are described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are only used to illustrate and explain the present disclosure, and are not used to limit the present disclosure.

[0049] Referring to Figure 1 The first aspect of the present disclosure provides a method for producing normal alkanes, comprising the following steps:

[0050] S1, introducing isobutane raw material and hydrogen into an isobutane conversion unit to perform an isobutane conversion reaction with an isobutane conversion catalyst to obtain a reaction product containing normal alkanes; wherein the isobutane conversion catalyst comprises a solid superacid catalyst;

[0051] S2, introducing the reaction product into a membrane separation unit to perform membrane separation treatment to obtain a first material containing isobutane and a second material containing normal alkanes; wherein the membrane separation element of the membrane separation unit comprises a molecular sieve membrane;

[0052] S3, returning the first material to the isobutane conversion unit for continuous reaction.

[0053] The present disclosure contacts isobutane raw material with isobutane conversion catalyst in an isobutane conversion unit to perform normalization reaction, wherein the isobutane conversion catalyst has high catalytic activity and does not contain chlorine element, which is safe, non-toxic and reduces corrosion loss to equipment; the present disclosure introduces the obtained reaction product containing normal alkanes into a membrane separation unit to perform membrane separation treatment, and high-efficiency separation of normal alkanes and isomeric alkanes can be achieved through a membrane separation element comprising a molecular sieve membrane, which has high separation efficiency and simple separation method; the method provided by the present disclosure has simple process flow, mild reaction conditions, high isobutane utilization efficiency and high normal alkane yield; the isobutane raw material is efficiently converted into normal alkanes with high added value and in urgent need of ethylene devices, which not only realizes the value-added utilization of isobutane resources, but also widens the raw material sources of ethylene devices, and provides a solution and technical support for refining enterprises to adapt to the national policy and the trend of refining to chemical transformation and quality improvement and efficiency increase.

[0054] In the present disclosure, the isobutane raw material can be an additional isobutane raw material, or a material containing isobutane separated from a membrane separation unit in the system, or a mixture of the two. The material entering the membrane separation unit for membrane separation can include the reaction product of the isobutane conversion unit, or a mixture containing n-butane and isobutane, such as a carbon four alkane material.

[0055] In a specific embodiment, the purity of the isobutane raw material is higher than 80 mass%, preferably higher than 90 mass%, and more preferably higher than 95 mass%; the content of impurities in the isobutane raw material includes: olefin content is lower than 1 mass%, preferably lower than 0.5 mass%; C5 +and above hydrocarbon content is less than 1.5% by mass, preferably less than 1.0% by mass; water content is less than 5 ppmw, preferably less than 3 ppmw; sulfur content is less than 10 ppmw, preferably less than 5 ppmw. In the present disclosure, the isobutane raw material can be derived from a mixed C4 hydrocarbon raw material. The mixed C4 hydrocarbon raw material can be refined and separated to obtain an isobutane raw material that meets the above requirements, wherein the refining and separation can be carried out by conventional methods selected in the art. The mixed C4 hydrocarbon raw material can come from catalytic cracking, hydrocracking, C4 as a by-product of an ethylene unit, isobutane dehydrogenation, or C4 as a by-product of a Fischer-Tropsch synthesis process. Furthermore, if the impurities in the isobutane raw material exceed the standard, it is necessary to take corresponding measures to remove them so that the impurity content meets the requirements. The impurity removal methods are all conventional methods well known to those skilled in the art. For example, olefins in the raw material can be removed by hydrogenation, C5 and above hydrocarbons in the raw material can be removed by distillation, water in the raw material can be removed by molecular sieve drying and dehydration, and sulfides in the raw material can be removed by adsorption desulfurization or hydrodesulfurization.

[0056] In one embodiment, the solid superacid catalyst comprises a first carrier and a first active metal; based on the total weight of the solid superacid catalyst, the solid superacid catalyst comprises 0.05-2.0 mass% of the Group VIII metal and 98.0-99.95 mass% of the first carrier;

[0057] Preferably, the first carrier contains zirconium, silicon, aluminum and sulfur, respectively calculated as zirconium oxide, silicon oxide, aluminum oxide and SO3, based on the total weight of the first carrier, the first carrier includes 30-90 mass% of zirconium oxide, 0.5-10 mass% of silicon oxide, 5-40 mass% of aluminum oxide and 0.5-10 mass% of SO3;

[0058] Further preferably, the first carrier comprises 40-80 mass% of zirconium oxide, 1-5 mass% of silicon oxide, 10-33 mass% of aluminum oxide and 1-8 mass% of SO3;

[0059] The first active metal is a Group VIII metal, preferably one or both selected from Pt and Pd. The composition of the first carrier of the solid superacid catalyst disclosed in the present invention is obtained by X-ray fluorescence testing.

[0060] The solid superacid catalyst disclosed herein can be prepared by methods known in the art.

[0061] In a specific embodiment, the solid superacid catalyst can be prepared by the following method:

[0062] a. contacting a soluble zirconium salt with an alkaline solution to form a zirconium hydroxide precipitate, then subjecting the precipitate to a hydrothermal treatment (temperature 70-180° C., time 0.5-100 hours), filtering, and drying the solid product to prepare zirconium hydroxide;

[0063] b. Impregnating the obtained zirconium hydroxide with an aqueous solution of sulfuric acid, ammonium sulfate or ammonium bisulfate (concentration 0.1-2 mol / L) (impregnation time 0.5-24 hours), and drying the solid (drying temperature 80-150° C.) to prepare sulfated zirconium hydroxide;

[0064] c. Mixing sulfated zirconium hydroxide with silica-alumina powder, adding an appropriate amount of inorganic acid (such as hydrochloric acid, sulfuric acid, nitric acid) as a peptizer, and performing extrusion molding, followed by drying and calcining to obtain a mixed oxide support (i.e., the first support);

[0065] d. Use the impregnation method to add the first active metal component (the first active metal water-soluble salt, the concentration is 5×10 -6 ~1×10 - 4 mol / L, the weight ratio of the first active metal water-soluble salt solution to the first carrier is (0.3 to 3): 1) introducing a mixed oxide carrier, drying and calcining to obtain a solid super acid catalyst, wherein the calcination temperature can be 400 to 720°C, preferably 450 to 680°C, and the calcination time is 0.5 to 24 hours, preferably 1 to 12 hours. The amount of each raw material added can be adjusted according to the content of each component of the solid super acid catalyst. The process parameters not limited in the present disclosure can adopt the parameters conventionally selected in the art, or be adjusted according to the required performance of the prepared solid super acid catalyst.

[0066] In one embodiment, in step S1, the conditions for the isobutane conversion reaction include: a temperature of 150 to 300°C, a pressure of 0.5 to 4.0 MPa, a mass space velocity of the isobutane raw material of 0.5 to 20 h -1 , based on the mole of isobutane in the isobutane raw material, the hydrogen-to-hydrocarbon molar ratio is (0.03-0.2):1; preferably, the temperature is 180-250°C, the pressure is 1.0-3.5 MPa, and the raw material mass space velocity is 1.0-10.0 h -1 , the molar ratio of hydrogen to hydrocarbon is (0.03~0.15):1.

[0067] In one embodiment, the molecular sieve membrane in the membrane separation unit of the present disclosure is selected from at least one of a type A molecular sieve membrane and a molecular sieve membrane having an MFI structure, and is preferably a NaA molecular sieve membrane.

[0068] In one embodiment, the membrane separation element further includes a second support on which the molecular sieve membrane is supported. The present disclosure supports the molecular sieve membrane on a suitable second support to ensure membrane strength. The second support may be in the form of a sheet or a tube with a capped end. Alternatively, the second support may be in the form of a hollow fiber, which can further increase the separation area of ​​the molecular sieve membrane.

[0069] It is further preferred that the second carrier comprises one or more of alumina, silica, and zirconia; preferably alumina; optionally, the second carrier has an average pore diameter of 100 to 3000 nm and a pore volume of 0.01 to 0.1 ml / g;

[0070] Preferably, the membrane separation element comprises 99.5-99.99% by mass of the second support and 0.01-0.5% by mass of the molecular sieve membrane. The inventors of the present disclosure have discovered through experiments that by growing a molecular sieve membrane having a NaY molecular sieve structure on a second support (e.g., alumina), the resulting membrane separation element can excellently separate n-butane and isobutane, and the separation process is simple, which helps to reduce the difficulty of separating the isobutane conversion reaction products in actual processes.

[0071] In one embodiment, the n-butane permeate flow index of the membrane separation element is 5×10 -7 ~3×10 - 5 g·m -2 ·s -1 ·Pa -1 ; The n-butane permeate flow ratio is (5~150):1.

[0072] In the present disclosure, the n-butane permeate flow index means: the n-butane permeate flow index of a membrane separation element is determined by measuring the rate (g / s) at which substantially pure n-butane permeates the membrane separation element at 80° C. under a retentate-side absolute pressure of 300 kPa and a permeate-side absolute pressure of 250 kPa. The n-butane permeate flow index is expressed in units of mass (g) of n-butane permeated per square meter of retentate-side membrane surface area per second.

[0073] In the present disclosure, the n-butane permeate flow ratio represents the ratio of the n-butane permeate flow index to the isobutane permeate flow index, wherein the isobutane permeate flow index is measured in the same manner as that of n-butane.

[0074] In a preferred embodiment, the NaA molecular sieve membrane is prepared by the following steps:

[0075] a. mixing an aluminum source, a silicon source, an alkali metal hydroxide, a template and water to obtain a crystallization precursor solution;

[0076] b. placing the second carrier in the crystallization precursor solution for crystallization to obtain a crystallized product; or placing the second carrier in a NaA molecular sieve suspension for immersion; and then placing the second carrier in the crystallization precursor solution for crystallization to obtain a crystallized product;

[0077] c. Washing, drying and calcining the crystallized product.

[0078] The present disclosure places the second carrier in the NaA molecular sieve suspension, which can introduce crystal seeds onto the surface of the second carrier, and is beneficial to the subsequent preparation of the NaA molecular sieve membrane.

[0079] In one embodiment, in step a, the crystallization precursor solution contains aluminum, silicon, alkali metal, template and water, wherein the aluminum, silicon and alkali metal are calculated as Al2O3, SiO2 and alkali metal oxide, respectively, and the molar ratio of Al2O3:SiO2:alkali metal oxide:template:H2O is (1-5):(2-5):(1-4):(0.05-0.7):(100-200);

[0080] Optionally, the aluminum source is selected from one or more of sodium aluminate, aluminum sulfate, and aluminum isopropoxide; the silicon source is silica sol, and optionally, the silica sol comprises 5-50% by mass of SiO2 and 0.01-1% by mass of an alkali metal oxide; the alkali metal hydroxide comprises one or both of sodium oxide and potassium hydroxide; the template is selected from one or more of tetrapropylammonium hydroxide, tetrabutylammonium hydroxide, and tetrapropylammonium bromide;

[0081] In step b, the weight ratio of the NaA molecular sieve in the NaA molecular sieve suspension to the second carrier is (0.001-0.1):1; preferably, the average particle size of the NaA molecular sieve in the NaA molecular sieve suspension is 0.1-5 μm;

[0082] Preferably, in step b, the second carrier is immersed in the NaA molecular sieve suspension at a liquid-to-solid mass ratio of 5 to 100:1, and the immersion time is 5 to 30 minutes;

[0083] Preferably, the crystallization temperature in step b is 80-120° C., and the crystallization time is 30-80 h;

[0084] In step c, the calcination temperature is 400-650° C., and the calcination time is 3-12 hours.

[0085] In one embodiment, the second carrier for supporting the molecular sieve membrane is in the form of a sheet, and the second carrier is composed of more than 99% by mass of aluminum oxide. The specific preparation steps of the sheet-shaped second carrier include: using pseudo-boehmite powder, adding 1 to 10% by mass of methyl cellulose and nitric acid based on the weight of the pseudo-boehmite powder, and adding a certain amount of water, wherein the weight ratio of nitric acid: water: pseudo-boehmite powder is (0.005 to 0.1): (0.1 to 0.5): 1, pressing the sheet under a pressure of 15 to 30 MPa, and then calcining at 1000 to 1500°C for 4 to 8 hours to produce the sheet-shaped second carrier.

[0086] In one embodiment, in step S2, the conditions for the membrane separation treatment include: a temperature of 30 to 250°C and a pressure difference of 10 to 600 kPa; preferably, a temperature of 40 to 220°C and a pressure difference of 15 to 500 kPa. The present disclosure can achieve efficient separation of isobutane and normal alkanes under relatively mild separation conditions. The pressure difference of the membrane separation treatment in the present disclosure is the pressure difference between the retentate side (e.g., isobutane) and the permeate side (e.g., normal butane) of the membrane separation element in the membrane separation unit.

[0087] In one embodiment, in step S2, the first material obtained by separation contains 95% to 99% by mass of isobutane and less than 5% by mass of isopentane; the second material contains 90 to 95% by weight of normal butane. In the present disclosure, membrane separation treatment can separate the isoparaffins (retentate, first material) and other products (permeate, second material) in the isobutane conversion reaction product. The first material containing isobutane can be recycled back to the isobutane conversion reactor as a raw material. The second material is mainly composed of normal butane, with a content of 90 to 95% by mass, and also contains 2 to 4% by mass of propane, 0.5 to 2% by mass of methane and ethane, 0.5 to 2% by mass of normal pentane and a small amount of hydrogen. When the content of normal paraffins (normal butane) in the separated second material exceeds 93% by mass, the second material can be directly used as an ethylene cracking raw material. Since the hydrogen-to-hydrocarbon molar ratio in the isobutane conversion reaction is relatively low, the hydrogen content in the reaction product is also low. Therefore, there is no need to add a separation tower and compressor to recover and recycle the hydrogen. The process flow can be simplified and the operating costs can be greatly reduced.

[0088] See also Figure 2 In a preferred embodiment, the method further comprises: allowing a C4 alkane feedstock and an optional reaction product from the isobutane conversion unit to enter the membrane separation unit; preferably, the weight ratio of the C4 alkane feedstock to the isobutane conversion reaction product is (0.1-10):1;

[0089] Separating the obtained products through the membrane separation element to obtain a third material containing isobutane and a fourth material containing normal alkanes;

[0090] The third material is used as the isobutane raw material and enters the isobutane conversion unit with hydrogen. In this embodiment, the C4 alkane raw material is introduced into the membrane separation element for membrane separation treatment to obtain the isobutane raw material for producing normal paraffins, thereby expanding the raw material source.

[0091] In a specific embodiment, the C4 alkane feedstock is a C4 alkane produced from one or more of catalytic cracking, hydrocracking, ethylene unit, isobutane dehydrogenation or Fischer-Tropsch synthesis processes; the C4 alkane feedstock contains 0.1 to 99.9% by mass of isobutane and 0.1 to 99.9% by mass of n-butane.

[0092] In one embodiment, the conditions for membrane separation of the C4 alkane feedstock in the membrane separation unit are the same as the separation conditions for the reaction product obtained by the isobutane conversion reaction, specifically including: a temperature of 30 to 250° C. and a pressure difference of 10 to 600 kPa; preferably, the temperature is 40 to 220° C. and the pressure difference is 10 to 500 kPa.

[0093] In the present disclosure, the third material obtained after the C4 alkane feedstock is separated by the membrane separation unit is used as the isobutane feedstock, and the content of each component thereof should also meet the component content range of the isobutane feedstock in the present disclosure.

[0094] The second aspect of the present disclosure provides a system for producing normal alkanes, such as Figure 1 As shown, the system includes an isobutane conversion unit and a membrane separation unit; wherein the isobutane conversion unit is provided with a reaction raw material inlet for introducing isobutane raw material and hydrogen, a catalyst bed and a reaction product outlet; the catalyst bed contains an isobutane conversion catalyst, and the isobutane conversion catalyst includes a solid superacid catalyst;

[0095] The membrane separation unit is provided with an inlet for a material to be separated, an outlet for a retentate, and an outlet for a permeate. A membrane separation element is further provided inside the membrane separation unit, and the membrane separation element includes a molecular sieve membrane; the inlet for the material to be separated is connected to the reaction product outlet of the isobutane conversion unit; and the retentate outlet is connected to the reaction raw material inlet of the isobutane conversion unit through a second pipeline.

[0096] Specifically, the present disclosure adopts Figure 1The process flow of the system shown includes: when using isobutane as a feedstock, the isobutane feedstock and hydrogen are introduced into the isobutane conversion unit through the reaction feed inlet of the isobutane conversion unit, contacted with the isobutane conversion catalyst in the catalyst bed of the isobutane conversion unit, and an isobutane conversion reaction is carried out to generate a reaction product containing normal alkanes, which is then withdrawn from the isobutane conversion unit through the reaction product outlet; the reaction product is introduced into the membrane separation unit through the material to be separated inlet, and subjected to membrane separation treatment by a membrane separation element, wherein the membrane separation element divides the membrane separation unit into a retentate side and a permeate side, the material to be separated inlet and the retentate outlet are located on the retentate side, and the permeate outlet is located on the permeate side. Therefore, the first material containing isobutane obtained by the membrane separation treatment of the separated product by the membrane separation element cannot pass through the membrane separation element and is retained on the retentate side. It is returned to the reaction feed inlet of the isobutane conversion unit through the retentate outlet and a second pipeline to continue conversion together with fresh isobutane feedstock; while the second material containing normal alkanes in the reaction product can pass through the membrane separation element into the permeate side and be withdrawn through the permeate outlet.

[0097] The isobutane conversion unit in the present disclosure may adopt a device conventionally selected in the art, such as a fixed bed reactor.

[0098] In a preferred embodiment, Figure 2 As shown, the system further includes a C4 alkane feedstock source, which is connected to the feedstock inlet for separation of the membrane separation unit. Specifically, when using a C4 alkane feedstock, the C4 alkane feedstock is first introduced into the membrane separation element through the feedstock inlet for separation of the membrane separation unit and then subjected to membrane separation treatment (it can also be introduced into the membrane separation unit together with the isobutane conversion reaction product from the isobutane conversion unit for membrane separation treatment), wherein the third material (retentate) containing isobutane cannot pass through the membrane separation element and is retained on the retentate side, and will be returned to the reaction feed inlet of the isobutane conversion unit through the retentate outlet and the second pipeline for further conversion (it can be subjected to conversion reaction together with the fresh isobutane feedstock); the fourth material (permeate) containing normal alkanes can pass through the membrane separation element into the permeate side and then be drawn out through the permeate outlet.

[0099] The present invention is further described below by way of examples, but the present invention is not limited thereto.

[0100] In the following examples and comparative examples, the carrier composition was determined using an XRF-1800 wavelength-dispersive X-ray fluorescence spectrometer. After the powder sample was pressed into tablets, the sample was scanned from B to U on the XRF spectrometer. Based on the intensity of the elements present in the sample obtained by the scans and the instrument's sensitivity to pure substances for these elements, the content of each element in the sample was determined through theoretical calculation and mathematical correction.

[0101] For Cl in solution - Silver nitrate was used for detection;

[0102] The content of active metals loaded on the support was determined using a Lambda 35 UV-visible spectrophotometer;

[0103] A JSM-6701F cold field emission scanning electron microscope (SEM) from Japan Electron Optics Corporation was used to observe the microstructure of the membrane separation element with a resolution of 1.0 nm (15 kV) and 2.2 nm (1 kV).

[0104] The crystal structure of the prepared membrane separation element was analyzed and detected using a Japanese Rigaku D / MAX-3A diffractometer (XRD). The analysis conditions were: X-ray source Cu / Kα target, Ni filter, scanning range 10° to 70°, step width 0.02°;

[0105] The composition of fresh isobutane and C4 alkane raw materials was analyzed by gas chromatography using an Agilent 7890B gas chromatograph with nitrogen as carrier gas and an FID detector.

[0106] The composition of the conversion product at the outlet of the isobutane conversion reactor was analyzed by gas chromatography using an Agilent 7890B gas chromatograph with nitrogen as the carrier gas, an FID detector for hydrocarbon composition, and a TCD detector for hydrogen content.

[0107] The low-temperature nitrogen static capacity adsorption method (BET method) was used to analyze and detect the pore structure of the carrier or molecular sieve membrane. The instrument used was the ASAP2400 specific surface area meter produced by Micromeritics Instrument Company. The pretreatment conditions were: 250°C, 1.3 Pa, treatment for 4 hours, and the adsorption amount at p / p0=0.98 was taken as the sample pore volume.

[0108] In the following examples and comparative examples, Series 1 is an example of preparing an isobutane conversion catalyst, Series 2 is an example of preparing a membrane separation element, and Series 3 is an example of producing normal paraffins.

[0109] Example 1-1

[0110] This example is used to prepare solid superacid catalyst A.

[0111] (1) Preparation of zirconium hydroxide

[0112] Weigh 200g of zirconium oxychloride octahydrate (ZrOCl2·8H2O) and dissolve it in 1500ml of deionized water. Add ammonia water (mass concentration 0.23g / ml, amount added 96ml) dropwise to the solution under stirring to obtain a precipitate. Filter the precipitate and wash it with deionized water until it is neutral and no Cl is detected in the filtrate. -The filter cake was dried at 120 °C for 12 h, and the dried product was hydrothermally treated at 110 °C for 24 h. The hydrothermally treated product was filtered and washed with deionized water until no Cl was detected in the filtrate. - The filter cake was dried at 120°C for 12 h to obtain zirconium hydroxide powder.

[0113] (2) Preparation of sulfated zirconium hydroxide

[0114] Weigh 80.0 g of the zirconium hydroxide powder obtained in step (1), add 50.0 g of a 0.5 mol / L sulfuric acid aqueous solution thereto, stir evenly to form a slurry, and dry the obtained slurry at 120° C. for 24 h to obtain sulfated zirconium hydroxide.

[0115] (3) Preparation of mixed oxide support (first support)

[0116] 60.0 g of the sulfated zirconium hydroxide (80.0 mass % residual value on calcination) obtained in step (2) and 17.6 g of alumina-silica powder (69.1 mass % alumina, 5.9 mass % silica) were mixed uniformly, 2 ml of 10 mass % nitric acid and an appropriate amount of deionized water were added to adjust the liquid-solid ratio to 0.8:1, and the mixture was kneaded uniformly before extrusion into strips. The wet strips were dried at 120° C. for 12 h and calcined at 650° C. for 4 h to produce a mixed oxide support. X-ray fluorescence analysis revealed that the first support contained 72.9 mass % zirconium oxide, 1.52 mass % silica, 17.8 mass % alumina, and 5.57 mass % SO₃.

[0117] (4) Preparation of solid superacid catalyst by introducing Pt

[0118] 50.0 g of the first carrier prepared in step (3) was treated with an aqueous solution containing a calculated amount of chloroplatinic acid (concentration 2.56×10 - 5 mol / L) impregnation with a liquid-to-solid mass ratio of 0.5:1 for 12 hours. The resulting solid was dried at 120°C for 12 hours and calcined at 550°C for 4 hours to produce a solid superacid catalyst, designated as Catalyst A. The Pt content of Catalyst A was determined by colorimetry to be 0.25% by mass.

[0119] Comparative Example 1-1

[0120] A solid superacid catalyst D-1 was prepared by the following method, and X-ray fluorescence analysis showed that the mixed oxide support of D-1 consisted of 71.0% by mass of zirconium oxide and 29.0% by mass of aluminum oxide. The specific preparation process included: first preparing sulfated zirconium hydroxide according to the method of Example 1-1, uniformly mixing 60.0 g of the prepared sulfated zirconium hydroxide and 26.1 g of SB powder (purchased from Sasol, with a residual value on ignition of 75.0% by mass), adding 1.8 ml of 10% by mass nitric acid and an appropriate amount of deionized water to a liquid-solid ratio of 0.77:1, kneading the mixture uniformly, and then extruding the mixture into strips. The wet strips were dried at 120° C. for 12 h and calcined at 650° C. for 4 h to obtain a mixed oxide support, and then preparing the solid superacid catalyst D-1 according to the method of step (4) of Example 1-1.

[0121] Comparative Example 1-2

[0122] Solid superacid catalyst D-2 was prepared by the following method, and X-ray fluorescence analysis showed that the mixed oxide support of D-2 was composed of 70.0% by mass of zirconium oxide and 30.0% by mass of silicon oxide. The specific preparation process included: first, sulfated zirconium hydroxide was prepared according to the method of Example 1-1, 60.0g of the sulfated zirconium hydroxide prepared and 68.6g of silica sol (the silica sol had a SiO2 content of 30% by mass and a Na2O content of 0.05% by mass) were mixed uniformly, 2.5ml of 12% by mass nitric acid and 5ml of deionized water were added, and the mixture was thoroughly kneaded and then extruded into strips. The wet strips were dried at 120°C for 12h and calcined at 650°C for 4h to obtain a mixed oxide support. Then, solid superacid catalyst D-2 was prepared according to the method of step (4) of Example 1-1.

[0123] Example 1-2

[0124] This example is used to prepare solid superacid catalyst B.

[0125] (1) Preparation of zirconium hydroxide

[0126] Weigh 200g of zirconium oxychloride octahydrate (ZrOCl2·8H2O) and dissolve it in 1500ml of deionized water. Add ammonia water (mass concentration 0.23g / ml, amount added 96ml) dropwise to the solution under stirring to obtain a precipitate. Filter the precipitate and wash it with deionized water until it is neutral and no Cl is detected in the filtrate. - The filter cake was dried at 120 °C for 12 h, and the dried product was hydrothermally treated at 110 °C for 24 h. The hydrothermally treated product was filtered and washed with deionized water until no Cl was detected in the filtrate. - The filter cake was dried at 120°C for 12 h to obtain zirconium hydroxide powder.

[0127] (2) Preparation of sulfated zirconium hydroxide

[0128] 80.0 g of the zirconium hydroxide powder obtained in step (1) was weighed, 50.0 g of a 0.5 mol / L sulfuric acid aqueous solution was added thereto, and the mixture was stirred to form a slurry. The obtained slurry was dried at 120° C. for 24 h to obtain sulfated zirconium hydroxide.

[0129] (3) Preparation of mixed oxide support (first support)

[0130] 80.0 g of sulfated zirconium hydroxide (80% by mass on calcination residue) obtained in step (2) and 8.0 g of alumina-silica powder (69.1% by mass of alumina and 5.9% by mass of silica) were mixed uniformly, 1.7 ml of 10% by mass nitric acid and an appropriate amount of deionized water were added to adjust the liquid-solid ratio to 0.8:1, and the mixture was kneaded uniformly before extrusion into strips. The wet strips were dried at 120° C. for 12 h and calcined at 650° C. for 4 h to obtain a mixed oxide support. X-ray fluorescence analysis showed that the first support contained 84.4% by mass of zirconium oxide, 0.66% by mass of silica, 7.84% by mass of alumina, and 6.41% by mass of SO₃.

[0131] (4) Preparation of solid superacid catalyst by introducing Pt

[0132] 50.0 g of the first carrier prepared in step (3) was treated with an aqueous solution containing a calculated amount of chloroplatinic acid (concentration 2.56×10 - 5 mol / L) impregnation with a liquid-to-solid mass ratio of 0.5:1 for 12 hours. The resulting solid was dried at 120°C for 12 hours and calcined at 550°C for 4 hours to obtain a solid superacid catalyst, designated as Catalyst B. The Pt content of Catalyst B was determined by colorimetry to be 0.25% by mass.

[0133] Example 2-1

[0134] This example is used to prepare the membrane separation element M1.

[0135] (1) Preparation of alumina support (second support)

[0136] 200g of SB powder (purchased from Sasol) was weighed, 10.0g of methylcellulose was added, and the mixture was thoroughly mixed. 50ml of deionized water and 5ml of 5% by mass dilute nitric acid were then added and stirred. A flaky support was prepared at a pressure of 20 MPa. The support was then calcined in a muffle furnace at 1100°C for 4 hours to produce a secondary alumina support. The secondary support had an average pore diameter of 500nm and a pore volume of 0.06ml / g.

[0137] (2) Preparation of membrane separation elements

[0138] Sodium aluminate, silica sol (SiO2 content is 24.8% by mass, Na2O content is 4.3% by mass, and the rest is water), sodium hydroxide, deionized water and tetrapropylammonium hydroxide are mixed to form a crystallization precursor solution. The molar ratio of each element in the crystallization precursor solution is Al2O3:SiO2:Na2O (calculated based on all sodium elements in the crystallization precursor solution): template:H2O=1:2:2:0.5:120.

[0139] The flaky alumina second carrier prepared in step (1) is placed in a crystallization precursor solution with a liquid-solid mass ratio of 50:1, and crystallized at 100°C for 48 hours. After the crystallization is completed, the carrier surface is washed with deionized water, then dried at 120°C for 12 hours, and calcined at 550°C for 4 hours to obtain a membrane separation element in which the NaA molecular sieve membrane is loaded on the second carrier, which is recorded as M1. M1 includes 1% by mass of the molecular sieve membrane and 99% by mass of the second carrier (the mass content of the two is obtained by X-ray fluorescence (XRF) characterization results). The SEM electron microscope photograph of the membrane separation element M1 is shown in the figure. Figure 3 The XRD characterization results of the membrane separation element are shown in Figure 4 As shown. Figure 3 It can be seen that the membrane separation element prepared in this embodiment is in sheet form; Figure 4 It can be seen that the membrane separation element prepared in this example has a NaA molecular sieve structure.

[0140] The membrane separation element M1 was tested for n-butane permeate flow index and n-butane permeate flow rate. The n-butane permeate flow index of the membrane separation element was 6×10 -6 g·m -2 ·s -1 ·Pa -1 , the n-butane permeate flow rate is 46.

[0141] Example 2-2

[0142] This example is used to prepare the membrane separation element M2.

[0143] (1) Prepare an alumina support (second support).

[0144] 180g of SB powder (purchased from Sasol) was weighed, 10.0g of methylcellulose was added, and the mixture was thoroughly mixed. 48ml of deionized water and 5ml of 5% by mass dilute nitric acid were then added and stirred. A flaky support was prepared at a pressure of 20 MPa. The support was then calcined in a muffle furnace at 1300°C for 4 hours to produce a secondary alumina support. The secondary support had an average pore diameter of 520nm and a pore volume of 0.05ml / g.

[0145] (2) Preparation of membrane separation elements

[0146] Sodium aluminate, silica sol (SiO2 content is 40.0 mass%, Na2O content is 4.5 mass%, and the rest is water), sodium hydroxide, deionized water and tetrapropylammonium hydroxide are mixed to form a crystallization precursor solution. The molar ratio of each element in the crystallization precursor solution is Al2O3:SiO2:Na2O (calculated based on all sodium elements in the crystallization precursor solution): template: H2O = 1:2:2:0.3:120.

[0147] The flaky alumina second carrier prepared in step (1) was placed in a NaA molecular sieve suspension (the weight ratio of NaA molecular sieve to water in the suspension was 0.02:1; the NaA molecular sieve was purchased from the Changling Branch of Sinopec Catalyst Company, with an average particle size of 0.6 μm) and soaked for 10 minutes. Seed crystals were introduced onto the second carrier, wherein the weight ratio of NaA molecular sieve to the second carrier in the NaA molecular sieve suspension was 0.04:1. The soaked second carrier was then placed in a crystallization precursor solution with a liquid-to-solid mass ratio of 60:1 and crystallized at 100°C for 24 hours. After crystallization, the carrier surface was washed with deionized water, then dried at 120°C for 12 hours and calcined at 550°C for 4 hours to obtain a membrane separation element with NaA molecular sieve membrane supported on the second carrier, which was recorded as M2. M2 included 0.8% by mass of the molecular sieve membrane and 99.2% by mass of the second carrier.

[0148] The membrane separation element was tested for n-butane permeate flow index and n-butane permeate flow rate. The n-butane permeate flow index of the membrane separation element was 1×10 -5 g·m -2 ·s -1 ·Pa -1 , the n-butane permeate flow rate is 60.

[0149] Comparative Example 2-1

[0150] The flaky alumina second carrier prepared in step (1) of Example 2-2 was placed in a NaA molecular sieve suspension (the weight ratio of NaA molecular sieve to water in the suspension was 0.02:1; the NaA molecular sieve was purchased from the Changling Branch of Sinopec Catalyst Company, with an average particle size of 0.6 μm) and soaked for 30 minutes. The soaked solid was then dried at 120°C. The soaking was repeated twice to ensure that the NaA molecular sieve content in the product was close to that of the molecular sieve membrane M2 of Example 2-2. The obtained product was recorded as DM-1.

[0151] The n-butane permeate flow index and n-butane permeate flow rate of DM-1 were tested. The n-butane permeate flow index of the membrane separation element was 1.5×10 -5 g·m -2 ·s -1 ·Pa -1 , the n-butane permeate flow rate is 1.4.

[0152] By comparing the above Examples 2-1 to 2-2 with Comparative Example 2-1, it can be seen that the membrane separation elements provided by Examples 2-1 to 2-2 of the present disclosure have good permeation effects on n-butane, and the permeation effects on isobutane are quite different from those on n-butane, and can be used for the separation of n-butane and isobutane; the n-butane permeate flow rate of DM-1 prepared in Comparative Example 2-1 is small, that is, the permeation effect of DM-1 on isobutane is not significantly different from that on n-butane, and is not suitable for separating the two substances.

[0153] In the following examples and comparative examples, the isobutane conversion reactor was a fixed bed reactor (reactor height 80 cm, inner diameter 10 mm), and the catalyst filling amount was 15 ml.

[0154] In the following examples and comparative examples, the membrane separation unit is a cylindrical container (container height 8 cm, diameter 5 cm), the membrane separation element is set to a size of 3 cm in diameter and 1.5 mm in thickness, and the membrane separation element is set in the cylindrical container in a conventional manner.

[0155] Example 3-1

[0156] This example is used to illustrate the effect of the conversion reaction on fresh isobutane raw material.

[0157] according to Figure 1 The process method shown in the figure converts isobutane into normal butane. The isobutane content of the fresh isobutane raw material used is higher than 98% by mass. The impurities include: olefin content is lower than 0.1% by mass, C5 + The content of hydrocarbons and above is less than 0.1% by mass, and the water content is less than 5 ppmw. The fresh isobutane feed is mixed with the isobutane recycled feed (first material) separated by the membrane separation unit, mixed with hydrogen, and sent to the isobutane conversion unit, where it is contacted with the catalyst A prepared in Example 1-1 to react, so that part of the isobutane is converted into normal alkanes. The isobutane conversion reaction conditions are: temperature 190°C, pressure (gauge pressure) 2.5 MPa, mass space velocity of the isobutane feed (including recycled feed and fresh feed) 2.0 h -1 The feed molar ratio of hydrogen to isobutane in the isobutane raw material was 0.07. Samples were taken from the outlet of the isobutane conversion reactor and the results are listed in Table 1.

[0158] The isobutane conversion reaction product was fed into a membrane separation unit and treated using the membrane separation element M1 prepared in Example 2-1. The membrane separation conditions were: a temperature of 80°C and a pressure differential of 50 kPa. After separation, an isobutane recycle feed (the third feed, comprising 95% by mass of isobutane and less than 3% by mass of isopentane) and a normal alkane product (the fourth feed, comprising 94% by mass of normal butane) were obtained.

[0159] Comparative Example 3-1

[0160] Isobutane was converted to normal alkanes according to the method of Example 3-1, except that the solid superacid catalyst used in the isobutane conversion reaction was Catalyst D-1 prepared in Comparative Example 1-1. Samples were taken from the outlet of the isobutane conversion reactor for product composition analysis, and the results are listed in Table 1.

[0161] Comparative Example 3-2

[0162] Isobutane was converted into normal alkanes according to the method of Example 3-1, except that the reaction conditions for isobutane conversion were the same as those of Example 4 in patent CN110385142A, i.e., reaction temperature 310°C, pressure 2.0 MPa, and isobutane mass space velocity 1.0 h -1 The volume ratio of hydrogen to isobutane was 200.0, the catalyst used was Catalyst A, the loading amount was 15 g, and the conditions of the separation unit were the same as those in Example 3-1. Samples were taken from the outlet of the isobutane conversion reactor for product composition analysis, and the results are listed in Table 1.

[0163] Example 3-2

[0164] This example is used to illustrate the effect of the conversion reaction on fresh isobutane raw material.

[0165] Isobutane was converted to normal alkanes according to the method of Example 3-1, except that the membrane separation element used in the membrane separation unit was M2 in Example 2-2. Samples were taken from the outlet of the isobutane conversion reactor for product composition analysis, and the results are listed in Table 1.

[0166] Comparative Example 3-3

[0167] Isobutane was converted to normal alkanes according to the method of Example 3-1, except that the membrane separation element used in the membrane separation unit was DM-1 in Comparative Example 2-1. Samples were taken from the outlet of the isobutane conversion reactor for product composition analysis, and the results are listed in Table 1.

[0168] The membrane separation treatment conditions are: temperature 80°C, pressure difference 50kPa. After separation, a recycle material (the third material, containing 0.5% by mass of methane and ethane, 1.1% by mass of propane, 62.4% by mass of isobutane, 35.1% by mass of n-butane, and less than 0.9% by mass of pentane) and a product (the fourth material, containing 0.9% by mass of methane and ethane, 3.2% by mass of propane, 52.6% by mass of isobutane, 43.1% by mass of n-butane, and less than 0.2% by mass of isopentane) are obtained. It can be seen that the recycle material separated by DM-1 includes 62.4% by mass of isobutane and 35.1% by mass of n-butane; the fourth material contains 43.1% by mass of n-butane, and DM-1 is unable to effectively separate the isobutane and n-butane in the reaction product.

[0169] Example 3-3

[0170] This example is to illustrate the effect of conversion reaction on isobutane fresh feedstock.

[0171] Isobutane was converted to n-alkanes according to the method of Example 3-1 except that the solid superacid catalyst used in the isobutane conversion reaction was the catalyst B prepared in Example 1-2. The product composition was analyzed from the outlet of the isobutane conversion reactor and the results are listed in Table 1.

[0172] Example 3-4

[0173] This example is to illustrate the effect of conversion reaction on C4 alkane feedstock.

[0174] C4 alkane was converted to n-alkanes according to the method shown in the flow chart of Figure 2 C4 alkane was converted to n-alkanes according to the method shown in the flow chart of -1 The isobutane conversion rate and n-butane yield in Table 1 were calculated by the following formula (1)~(3):

[0175] Comparative Example 3-4

[0176] C4 alkane was converted to n-alkanes according to the method of Example 3-4 except that the solid superacid catalyst used in the isobutane conversion reaction was the catalyst D-2 prepared in Comparative Example 1-2. The product composition was analyzed from the outlet of the isobutane conversion reactor and the results are listed in Table 1.

[0177] The isobutane conversion rate and n-butane yield in Table 1 were calculated by the following formula (1)~(3):

[0178] Isobutane conversion rate = 100% - mass fraction of isobutane in product Formula (1);

[0179] n-Butane yield = isobutane conversion × n-butane selectivity (2);

[0180]

[0181] Table 1

[0182]

[0183]

[0184] The hydrogen gas volume fraction is the hydrogen gas volume fraction in the isobutane normalization product.

[0185] As can be seen from the above table, when Example 3-1 is compared with Comparative Example 3-1 (using D-1 catalyst), and when Example 3-4 is compared with Comparative Example 3-4 (using D-2 catalyst), the solid superacid catalyst provided by the present disclosure can obtain higher isobutane conversion rate, n-butane yield and n-butane selectivity;

[0186] Comparison of Example 3-1 and Comparative Example 3-2 shows that the isobutane conversion experiment using the reaction conditions provided in the present disclosure can achieve higher isobutane conversion rate, n-butane yield and n-butane selectivity;

[0187] Comparison of Example 3-1 with Comparative Example 3-3 demonstrates that the membrane separation element disclosed herein can separate and obtain a normal alkane product primarily composed of n-butane. In Comparative Example 3-3, due to the use of membrane separation element DM-1, the recycled material obtained in the separation unit contains both isobutane and n-butane, resulting in a complex composition. Therefore, it is impossible to calculate the isobutane conversion rate, n-butane yield, and n-butane selectivity of the recycled material in Comparative Example 3-3 when it is recycled to the conversion unit for conversion reaction.

[0188] By comparing Example 3-1 with Example 3-3, it can be seen that when the composition of the solid superacid catalyst prepared satisfies "the first carrier includes 40-80 mass% of zirconium oxide, 1-5 mass% of silicon oxide, 10-33 mass% of aluminum oxide and 1-8 mass% of SO3" (i.e., Catalyst A), the catalyst can obtain a higher isobutane conversion rate, n-butane yield and n-butane selectivity.

[0189] The preferred embodiments of the present disclosure are described in detail above in conjunction with the accompanying drawings. However, the present disclosure is not limited to the specific details of the above embodiments. Within the technical concept of the present disclosure, various simple modifications can be made to the technical solutions of the present disclosure, and these simple modifications all fall within the scope of protection of the present disclosure.

[0190] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. To avoid unnecessary repetition, the present disclosure will not further describe various possible combinations.

[0191] In addition, the various embodiments of the present disclosure may be arbitrarily combined, and as long as they do not violate the concept of the present disclosure, they should also be regarded as the contents disclosed by the present disclosure.

Claims

1. A method for producing normal alkanes, characterized in that: The following steps are involved: S1. An isobutane feedstock and hydrogen are introduced into an isobutane conversion unit, where they are contacted with an isobutane conversion catalyst to carry out an isobutane conversion reaction, thereby obtaining a reaction product comprising normal alkanes; wherein the isobutane conversion catalyst is a solid superacid catalyst; the solid superacid catalyst comprises a first carrier and a first active metal; the first carrier comprises zirconium, silicon, aluminum, and sulfur, respectively, calculated as zirconium oxide, silicon oxide, aluminum oxide, and SO3, and based on the total weight of the first carrier, the first carrier comprises 30-90% by mass of zirconium oxide, 0.5-10% by mass of silicon oxide, 5-40% by mass of aluminum oxide, and 0.5-10% by mass of SO3; the first active metal is a Group VIII metal; and the conditions for the isobutane conversion reaction include: a hydrogen-to-hydrocarbon molar ratio of (0.03-0.2):1, based on the moles of isobutane in the isobutane feedstock; S2. The reaction product is passed into a membrane separation unit for membrane separation treatment to obtain a first material containing isobutane and a second material containing normal alkanes; wherein the membrane separation element of the membrane separation unit comprises a molecular sieve membrane; the membrane separation element further comprises a second carrier, and the molecular sieve membrane is supported on the second carrier; the n-butane permeate flow index of the membrane separation element is 5×10 -7 ~3×10 -5 g·m -2 ·s -1 ·Pa -1 ;The n-butane permeate flow ratio is (5~150):1; S3. Returning the first material to the isobutane conversion unit to continue reaction.

2. The method according to claim 1, characterized in that Based on the total weight of the solid superacid catalyst, the solid superacid catalyst includes 0.05-2.0 mass % of the Group VIII metal and 98.0-99.95 mass % of the first carrier.

3. The method according to claim 1, characterized in that The first carrier includes 40-80 mass % of zirconium oxide, 1-5 mass % of silicon oxide, 10-33 mass % of aluminum oxide and 1-8 mass % of SO 3 .

4. The method according to claim 1, wherein The first active metal is selected from one or both of Pt and Pd.

5. The method according to claim 1, wherein In step S1, the conditions for the isobutane conversion reaction include: temperature of 150-300°C, pressure of 0.5-4.0 MPa, and mass space velocity of the isobutane raw material of 0.5-20h -1 .

6. The method according to claim 1, characterized in that In step S1, the purity of the isobutane raw material is higher than 80% by mass; the impurity content in the isobutane raw material includes: olefin content is lower than 1% by mass; C5 + The content of hydrocarbons and above is less than 1.5% by mass; the water content is less than 5ppmw; and the sulfur content is less than 10ppmw.

7. The method according to claim 6, characterized in that In step S1, the purity of the isobutane raw material is higher than 90% by mass; the impurity content in the isobutane raw material includes: olefin content is lower than 0.5% by mass; C5 + The content of hydrocarbons and above is less than 1.0% by mass; the water content is less than 3ppmw; and the sulfur content is less than 5ppmw.

8. The method according to claim 7, characterized in that In step S1, the purity of the isobutane raw material is higher than 95% by mass.

9. The method according to claim 1, characterized in that The molecular sieve membrane is selected from at least one of an A-type molecular sieve membrane and a molecular sieve membrane having an MFI structure.

10. The method according to claim 9, characterized in that The molecular sieve membrane is a NaA molecular sieve membrane.

11. The method according to claim 1, wherein The second carrier is in the form of a sheet, a tube with one end sealed, or a hollow fiber.

12. The method according to claim 11, characterized in that The second carrier is a sheet-shaped second carrier.

13. The method according to claim 1, wherein The second support includes one or more of alumina, silica, and zirconia.

14. The method according to claim 13, wherein: The second carrier is alumina.

15. The method according to claim 1, wherein The average pore diameter of the second carrier is 100-3000 nm, and the pore volume is 0.01-0.1 ml / g.

16. The method according to claim 1, wherein The membrane separation element comprises 90-99.95 mass % of a second carrier and 0.05-10 mass % of a molecular sieve membrane.

17. The method according to claim 1, wherein The membrane separation element is prepared by the following steps: a. mixing an aluminum source, a silicon source, an alkali metal hydroxide, a template and water to obtain a crystallization precursor solution; b. placing the second carrier in the crystallization precursor solution for crystallization to obtain a crystallized product; or placing the second carrier in a NaA molecular sieve suspension for immersion; and then placing the second carrier in the crystallization precursor solution for crystallization to obtain a crystallized product; c. Washing, drying and calcining the crystallized product.

18. The method according to claim 17, characterized in that In step a, the crystallization precursor solution contains aluminum, silicon, an alkali metal, a template, and water. The aluminum, silicon, and alkali metal are calculated as Al2O3, SiO2, and alkali metal oxide, respectively. The molar ratio of Al2O3:SiO2:alkali metal oxide:template:H2O is (1-5):(2-5):(1-4):(0.05-0.7):(100-200). In step b, the weight ratio of the NaA molecular sieve in the NaA molecular sieve suspension to the second carrier is (0.001-0.1):

1.

19. The method according to claim 17, wherein The average particle size of the NaA molecular sieve in the NaA molecular sieve suspension is 0.1-5 μm.

20. The method according to claim 17, wherein In step b, the second carrier is immersed in the NaA molecular sieve suspension at a liquid-to-solid mass ratio of 5 to 100:1, and the immersion time is 5 to 30 minutes.

21. The method according to claim 17, wherein The crystallization temperature in step b is 80-120° C., and the crystallization time is 30-80 h; In step c, the calcination temperature is 400-650° C., and the calcination time is 3-12 hours.

22. The method according to claim 17, wherein The aluminum source is selected from one or more of sodium metaaluminate, aluminum sulfate and aluminum isopropoxide; and the silicon source is silica sol.

23. The method according to claim 22, characterized in that The silica sol includes 5-50 mass% of SiO2 and 0.01-1 mass% of alkali metal oxide; the alkali metal hydroxide includes one or both of sodium hydroxide and potassium hydroxide; and the template is selected from one or more of tetrapropylammonium hydroxide, tetrabutylammonium hydroxide and tetrapropylammonium bromide.

24. The method according to claim 1, wherein The method further includes: allowing a C4 alkane feedstock and an optional reaction product from the isobutane conversion unit to enter the membrane separation unit; Separating the obtained products through the membrane separation element to obtain a third material containing isobutane and a fourth material containing normal alkanes; The third material is used as the isobutane raw material and enters the isobutane conversion unit together with hydrogen.

25. The method according to claim 24, characterized in that The C4 alkane feedstock is a C4 alkane produced from one or more of catalytic cracking, hydrocracking, ethylene unit, isobutane dehydrogenation or Fischer-Tropsch synthesis processes; the C4 alkane feedstock contains 0.1-99.9% by mass of isobutane and 0.1-99.9% by mass of normal butane.

26. The method according to any one of claims 1 to 25, characterized in that The separation conditions in the membrane separation unit include: a temperature of 30-250° C. and a pressure difference of 10-600 kPa.

27. The method according to claim 26, characterized in that In step S2, the separated first material contains 95-99% by mass of isobutane and less than 5% by mass of isopentane; the second material contains 90-95% by mass of n-butane.

28. The method according to claim 1, wherein The system used in the method for producing normal paraffins includes an isobutane conversion unit and a membrane separation unit; wherein the isobutane conversion unit is provided with a reaction raw material inlet for introducing isobutane raw material and hydrogen, a catalyst bed, and a reaction product outlet; the catalyst bed contains an isobutane conversion catalyst, and the isobutane conversion catalyst includes a solid superacid catalyst; The membrane separation unit is provided with an inlet for a material to be separated, an outlet for a retentate, and an outlet for a permeate. A membrane separation element is further provided inside the membrane separation unit, and the membrane separation element includes a molecular sieve membrane; the inlet for the material to be separated is connected to the reaction product outlet of the isobutane conversion unit; the retentate outlet is connected to the reaction raw material inlet of the isobutane conversion unit through a second pipeline.

29. The method according to claim 28, characterized in that The system further comprises a C4 alkane raw material source, which is connected to the raw material inlet to be separated of the membrane separation unit.

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