A method for producing biodiesel

Abstract

The present disclosure relates to a method for preparing biodiesel, which comprises: feeding fatty acid raw material and saturated monohydric alcohol into a fixed bed reactor in a continuous feeding mode, and reacting with a solid catalyst; the solid catalyst comprises an acidic molecular sieve and an inert binder, the acidic molecular sieve comprises one or more of ZSM-5 molecular sieve with a silicon-aluminum ratio of 100 or more, ZSM-48 molecular sieve with a silicon-aluminum ratio of 100 or more, beta molecular sieve with a silicon-aluminum ratio of 20 or more, X-type molecular sieve with a silicon-aluminum ratio of 1 or more, and NaY molecular sieve with a silicon-aluminum ratio of 2 or more, wherein the silicon-aluminum ratio is the molar ratio of SiO2 to Al2O3 in the acidic molecular sieve. The present disclosure packs the solid catalyst in the fixed bed reactor, and feeds the fatty acid raw material and the saturated monohydric alcohol in a continuous feeding mode, the product is free of catalyst, the acid value is low, and no catalyst separation is required after the reaction, and the operation steps are simple.

Description

Technical Field

[0001] This disclosure relates to a method for preparing biodiesel. Background Technology

[0002] Energy is a key factor in my country's sustained and stable economic development. my country is poor in oil resources, and since the 21st century, its dependence on imported oil has exceeded 60%. Biomass energy, represented by renewable biodiesel, is an important energy supplement. Bio-based free fatty acids and fatty acid-based raw materials mainly come from rancid palm oil in Southeast Asia, containing over 80% to 95% palmitic acid by weight. This palmitic acid reacts with conventional liquid alkali in a saponification reaction, making it impossible to directly produce fatty acid methyl esters using the alkali-catalyzed transesterification method. Furthermore, waste kitchen oils are subject to complex storage and collection processes, sometimes undergoing hydrolysis, resulting in free acid weights exceeding 60% to 80% in these inexpensive raw materials, which are mainly triglycerides. Due to their high acid values, direct conversion and utilization of these bio-based raw materials present significant challenges.

[0003] Numerous studies have explored the use of solid catalysts to catalyze the reaction of fatty acids with methanol to produce fatty acid methyl esters. However, the catalysts typically used are solid superacids, which are frequently involved in dehydroxylation, cracking, olefin breakage, or polymerization reactions during esterification. Without a catalyst, harsher conditions are generally employed. For example, CN101638609A describes a supercritical method for biodiesel production, where both esterification and transesterification reactions are conducted at high temperatures and pressures. This method is effective for high-acid-value oils with acid values ​​ranging from 3-200 mg KOH / g. The pre-esterification stage is carried out at temperatures above 220°C, with a methanol-to-oil molar ratio of 10, a temperature of 240°C, and a pressure of 9 MPa. Under catalyst-free conditions, the acid value can be reduced to 1.0-2.0 mg KOH / g. The product contains no acidic or alkaline catalyst residue. Yang Dongyuan et al. [Grains and Oils, 2009, (6) 18-20] proposed using arginine as a catalyst to catalyze the preparation of biodiesel from waste oil under supercritical methanol conditions. However, the operating conditions are harsh, the reaction pressure exceeds 8 MPa, the investment in the reaction equipment is high, and the technical and economic efficiency is poor.

[0004] To date, conventional high-acid esterification and pre-esterification reactions of acidic biological waste oils mostly employ homogeneous acidic catalysts and intermittent operation, resulting in catalyst residues in the products, complex subsequent treatment, and high emissions of waste gas, wastewater, and solid waste; or they are carried out under supercritical conditions, with high temperature and high pressure operation, which requires high corrosion and pressure resistance of the equipment and has poor technical and economic efficiency. Summary of the Invention

[0005] The purpose of this disclosure is to provide a method for preparing biodiesel, which produces a product with low acid value, no catalyst residue, and simple operation steps.

[0006] To achieve the above objectives, this disclosure provides a method for preparing biodiesel, the method comprising: feeding fatty acid feedstock and saturated monohydric alcohol into a fixed-bed reactor in a continuous feeding manner, and reacting them with a solid catalyst;

[0007] The solid catalyst comprises an acidic molecular sieve and an inert binder. The acidic molecular sieve comprises one or more of the following: ZSM-5 molecular sieve with a silicon-to-aluminum ratio of 100 or more, ZSM-48 molecular sieve with a silicon-to-aluminum ratio of 100 or more, β molecular sieve with a silicon-to-aluminum ratio of 20 or more, X-type molecular sieve with a silicon-to-aluminum ratio of 1 or more, and NaY molecular sieve with a silicon-to-aluminum ratio of 2 or more. The silicon-to-aluminum ratio is the molar ratio of SiO2 to Al2O3 in the acidic molecular sieve.

[0008] Optionally, the acidic molecular sieve includes one or more of the following: ZSM-5 molecular sieve with a silica-to-alumina ratio of 100-200, ZSM-48 molecular sieve with a silica-to-alumina ratio of 100-200, β molecular sieve with a silica-to-alumina ratio of 20-100, X-type molecular sieve with a silica-to-alumina ratio of 1-1.5, and NaY molecular sieve with a silica-to-alumina ratio of 2-30.

[0009] The inert binder includes one or more of boehmite, graphite binder, and silica sol;

[0010] Optionally, the content of the acidic molecular sieve is 75-98% by weight and the content of the inert binder is 2-25% by weight relative to the total weight of the solid catalyst.

[0011] Optionally, the content of the fatty acid is 80-100% by weight relative to the total weight of the fatty acid raw material;

[0012] The acid value of the fatty acid raw material is above 180 mg KOH / g;

[0013] The fatty acids include one or more of lauric acid, myristic acid, palmitic acid, oleic acid, linoleic acid, and linolenic acid.

[0014] Optionally, the solid catalyst is spherical with a diameter of 0.5-3.0 mm; or,

[0015] The solid catalyst is in strip shape and has a length of 3-8 mm.

[0016] Optionally, the saturated monohydric alcohol includes one or more of saturated monohydric alcohols having 1-3 carbon atoms, preferably methanol and / or ethanol.

[0017] Optionally, the conditions for the contact reaction include: a temperature of 180-280°C and a pressure of 0.5-3.0 MPa;

[0018] Based on the total weight of the fatty acid raw materials, the feed mass hourly space velocity is 0.1-1.0 h⁻¹. -1 The weight ratio of the fatty acid raw material to the saturated monohydric alcohol is 3:(1-9).

[0019] Optionally, the method includes: feeding the fatty acid raw material into the fixed-bed reactor in a segmented feeding manner to carry out the contact reaction;

[0020] Optionally, the fixed-bed reactor includes multiple spaced solid catalyst beds, the fatty acid feedstock is divided into 2-5 streams, the saturated monohydric alcohol is mixed with the first stream of fatty acid feedstock and then enters the fixed-bed reactor from the top and reacts with the first solid catalyst bed, and the remaining streams of fatty acid feedstock enter between two adjacent solid catalyst beds respectively.

[0021] The weight ratio of the saturated monohydric alcohol to the first fatty acid raw material is (1-9):1, and the weights of the remaining fatty acid raw materials are the same.

[0022] Optionally, the method further includes: preheating the fatty acid raw material and the saturated monohydric alcohol before introducing them into the fixed-bed reactor, wherein the preheating temperature is 160-280°C.

[0023] Optionally, the method further includes: flash evaporation of the mixture obtained from the contact reaction to obtain a flash vapor phase and a flash liquid phase containing biodiesel, condensing the flash vapor phase, and returning the resulting condensate as a supplementary saturated monohydric alcohol to the fixed-bed reactor to continue the contact reaction;

[0024] The flash evaporation conditions include: a temperature of 100-180℃ and a pressure of 10... 4 -10 5 Pa.

[0025] Optionally, the method further includes: removing low-boiling-point impurities by flash distillation of the liquid phase to obtain a biodiesel product.

[0026] Optionally, the acid value of the biodiesel product is less than 2.0 mg KOH / g.

[0027] Through the above technical solution, this disclosure loads a solid catalyst containing a specific acidic molecular sieve into a continuous operation reactor, and feeds in fatty acid raw materials and saturated monohydric alcohol in a continuous feeding manner to prepare biodiesel. The above method can achieve online water separation without the use of a dehydrating agent, and can obtain biodiesel products with low acid value. There is no catalyst residue in the product, and no catalyst separation is required after the reaction. The operation steps are simple.

[0028] Other features and advantages of this disclosure will be described in detail in the following detailed description section. Attached Figure Description

[0029] The accompanying drawings are provided to further illustrate the present disclosure and form part of the specification. They are used together with the following detailed description to explain the present disclosure, but do not constitute a limitation thereof. In the drawings:

[0030] Figure 1 This is a reaction flow diagram of Embodiment 3 of this disclosure.

[0031] Explanation of reference numerals in the attached figures

[0032] 1. Fatty acid feed pipeline; 2. Methanol feed pipeline; 3. Mixer; 4. Heat exchanger; 5. Segmented fixed-bed reactor; 6. Flash tank; 7. Distillation tower; 8. Segmented fatty acid feed pipeline; 9. Reaction product water outlet; 10. Methanol circulation pipeline; 11. Biodiesel primary product outlet; 12. Biodiesel product outlet; 13. Methanol and water outlet; 14. Methanol storage tank. Detailed Implementation

[0033] The specific embodiments of this disclosure will be described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are for illustration and explanation only and are not intended to limit this disclosure.

[0034] This disclosure provides a method for preparing biodiesel, the method comprising: feeding fatty acid feedstock and saturated monohydric alcohol into a fixed-bed reactor in a continuous feeding manner, and reacting them with a solid catalyst;

[0035] The solid catalyst comprises an acidic molecular sieve and an inert binder. The acidic molecular sieve comprises one or more of the following: ZSM-5 molecular sieve with a silicon-to-aluminum ratio of 100 or more, ZSM-48 molecular sieve with a silicon-to-aluminum ratio of 100 or more, β molecular sieve with a silicon-to-aluminum ratio of 20 or more, X-type molecular sieve with a silicon-to-aluminum ratio of 1 or more, and NaY molecular sieve with a silicon-to-aluminum ratio of 2 or more. The silicon-to-aluminum ratio is the molar ratio of SiO2 to Al2O3 in the acidic molecular sieve.

[0036] This disclosure utilizes a solid acid catalyst containing specific acidic molecular sieves to achieve continuous fatty acid esterification in a fixed-bed reaction to produce biodiesel. Because it avoids the use of homogeneous organic and inorganic acids such as sulfuric acid and benzenesulfonic acid, the product contains no metal or conventional catalyst acid residues, eliminating the need for subsequent alkali neutralization and product washing processes, and preventing the generation of acidic wastewater. The solid catalyst itself is hydrophobic, allowing water to be carried from the catalyst surface into the gas phase during the reaction, improving conversion through phase equilibrium transfer. A product with a low acid value can be obtained in a single pass. The overall process is highly efficient and environmentally friendly, safe and simple to operate, highly controllable, and adaptable to various raw materials. Furthermore, the use of a fixed-bed reactor offers higher production efficiency compared to traditional reactors.

[0037] In this disclosure, the fatty acid raw material is mainly derived from rancid palmitic acid; the acid value of the fatty acid raw material is above 180 mg KOH / g; the fatty acid content is 80-100% by weight relative to the total weight of the fatty acid raw material; the fatty acid includes fatty acids with 12-24 carbon atoms, preferably including one or more of lauric acid, myristic acid, palmitic acid, oleic acid, linoleic acid and linolenic acid.

[0038] According to one embodiment of this disclosure, the acidic molecular sieve includes one or more of the following: ZSM-5 molecular sieve with a silica-to-alumina ratio of 100-200, ZSM-48 molecular sieve with a silica-to-alumina ratio of 100-200, β molecular sieve with a silica-to-alumina ratio of 20-100, X-type molecular sieve with a silica-to-alumina ratio of 1-1.5, and NaY molecular sieve with a silica-to-alumina ratio of 2-30. For example, the silica-to-alumina ratio of ZSM-5 molecular sieve can be 120 or 200; the silica-to-alumina ratio of ZSM-48 molecular sieve can be 150, 180, or 200.

[0039] According to one embodiment of this disclosure, the type of inert binder is conventional in the art, including one or more of boehmite, graphite binders, and silica sol.

[0040] According to one embodiment of this disclosure, the content of acidic molecular sieve is 75-98% by weight and the content of inert binder is 2-25% by weight relative to the total weight of the solid catalyst.

[0041] According to one embodiment of this disclosure, the solid catalyst can be of any shape. Preferably, the solid catalyst is spherical with a diameter of 0.5-3.0 mm; or, the solid catalyst is strip-shaped with a length of 3-8 mm. The cross-section of the strip-shaped catalyst can be of any shape, such as circular, rectangular, or irregular. The strip-shaped catalyst can be cylindrical with a length of 3-8 mm and a diameter of 1-3 mm.

[0042] According to one embodiment of this disclosure, the solid catalyst is formed in a conventional manner in the art, such as ball forming, extrusion forming or tablet forming, and the specific steps are not described in detail here.

[0043] According to one embodiment of this disclosure, the saturated monohydric alcohol includes one or more of saturated monohydric alcohols having 1-3 carbon atoms, preferably methanol and / or ethanol.

[0044] According to one embodiment of this disclosure, the contact reaction conditions include: a temperature of 180-280°C and a pressure of 0.5-3.0 MPa; and a feed mass hourly space velocity (WHSV) of 0.1-1.0 h⁻¹, based on the total weight of the fatty acid feedstock. -1 The weight ratio of fatty acid feedstock to saturated monohydric alcohol is 3:(1-9); the contact reaction temperature can be any temperature between 180-280℃, such as 200℃, 210℃, 220℃, 240℃, 250℃, etc.; the contact reaction pressure can be any pressure between 0.5-3.0MPa, such as 0.8MPa, 1.0MPa, 1.2MPa, 1.5MPa, 2.5MPa, etc.; the feed mass hourly space velocity can be 0.1-1.0 h⁻¹. -1 Any mass space velocity between these values, for example, could be 0.3h. -1 0.4h -1 0.5h -1 1.0h -1 wait.

[0045] In this disclosure, "feeding fatty acid feedstock and saturated monohydric alcohol into a fixed-bed reactor in a continuous feeding manner" can mean feeding fatty acid feedstock and saturated monohydric alcohol into the fixed-bed reactor in a continuous feeding manner, or feeding fatty acid feedstock and saturated monohydric alcohol into the fixed-bed reactor in a continuous feeding manner after mixing them.

[0046] According to one embodiment of this disclosure, the method further includes: preheating the fatty acid raw material and the saturated monohydric alcohol before introducing them into a fixed-bed reactor, wherein the preheating temperature is 160-280°C; specifically, the fatty acid raw material and the saturated monohydric alcohol are respectively introduced into a heat exchanger for preheating, then mixed through a mixer, and then introduced into the fixed-bed reactor.

[0047] According to one embodiment of this disclosure, the method further includes: flash evaporation of the mixture obtained from the contact reaction to obtain a flash vapor phase and a flash liquid phase containing biodiesel; condensing the flash vapor phase; and returning the resulting condensate as a supplementary saturated monohydric alcohol to the fixed-bed reactor for continued contact reaction; the flash evaporation conditions include: a temperature of 100-180°C and a pressure of 10... 4 -10 5Pa. The above-described flash evaporation treatment can further improve the conversion rate of saturated monohydric alcohols.

[0048] According to one embodiment of this disclosure, the method further includes: removing low-boiling-point impurities by flash liquid-phase distillation to obtain a biodiesel product; the acid value of the biodiesel product is less than 2.0 mg KOH / g; wherein the low-boiling-point impurities include water, saturated monohydric alcohols, etc.

[0049] According to one embodiment of this disclosure, the method includes: feeding fatty acid raw materials into the fixed-bed reactor in a segmented manner for contact reaction; further, the fixed-bed reactor includes multiple spaced solid catalyst beds, the fatty acid raw materials are divided into 2-5 streams, the first stream of fatty acid raw materials is mixed with saturated monohydric alcohol and then enters the fixed-bed reactor from the top and reacts with the first solid catalyst bed, and the remaining streams of fatty acid raw materials are respectively introduced between two adjacent solid catalyst beds; wherein the weight ratio of saturated monohydric alcohol to the first stream of fatty acid raw materials is (1-9):1, and the remaining streams of fatty acid raw materials have the same weight. The solid catalyst beds are separated by a filter screen. Using a segmented feeding method for fatty acid raw materials can reduce local high temperatures and improve the dispersion of fatty acid raw materials, promoting their reaction with saturated monohydric alcohol and further reducing the acid value of the product.

[0050] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way.

[0051] All raw materials used in the examples and comparative examples were obtained commercially and, unless otherwise specified, were of analytical grade.

[0052] The acid value test method is the SH / T 0264 standard method, which involves dissolving the sample in an ethanol-water solution and then titrating the acid amount with KOH.

[0053] Example 1

[0054] NaY molecular sieve with a silica-to-alumina ratio of 2.6 was extruded with silica sol as an inert binder to obtain cylindrical catalysts with a diameter of φ1.5mm × 3-6mm. The content of NaY molecular sieve was 85% by weight, and the content of the inert binder was 15% by weight relative to the total weight of the catalyst. Fatty acid feedstock with an acid value of 188 mg KOH / g and methanol were pumped separately into a heat exchanger and preheated to 200℃. Then, the feedstock entered a fixed-bed reactor with an inner diameter of 12mm. The catalyst loading was 15.0g, the feed method was top-in, bottom-out, the reaction temperature was 240℃, the reaction pressure was 1.0 MPa, and the feed mass hourly space velocity (WHSV) was 0.4 h⁻¹ based on the total weight of the fatty acid feedstock. -1The fatty acid feedstock and methanol were processed in a 3:2 weight ratio, passing through a single-pass reactor and undergoing conversion to obtain a mixture. This mixture was then subjected to flash evaporation to produce a flash vapor phase and a flash liquid phase containing biodiesel. The flash vapor phase was condensed, and the resulting condensate was returned to the fixed-bed reactor as a supplementary saturated monohydric alcohol for further contact reaction. The flash evaporation temperature was 110°C, and the pressure was 10... 4 Pa, the flash liquid phase was distilled to remove low-boiling-point impurities to obtain biodiesel product. The product is divided into upper and lower phases. The acid values ​​of the upper and lower phases were titrated and found to be 1.70 mg KOH / g and 1.55 mg KOH / g, respectively. The main component of the upper material is fatty acid methyl ester.

[0055] The fatty acid raw material contains 95% by weight of fatty acids, and the fatty acid is palmitic acid.

[0056] Example 2

[0057] ZSM-48 molecular sieve with a silica-to-alumina ratio of 200 and boehmite as an inert binder were extruded to obtain cylindrical catalysts with a diameter of 1.5 mm and a length of 3-6 mm. The content of ZSM-48 molecular sieve was 88% by weight, and the content of inert binder was 12% by weight. Fatty acid feedstock with an acid value of 200 mg KOH / g and methanol were pumped into a heat exchanger separately using two pumps and preheated to 200 °C. Then, they were fed into a fixed-bed reactor with an inner diameter of 12 mm and a catalyst loading of 15.0 g. The feed method was top-in and bottom-out. The reaction temperature was 240 °C, the reaction pressure was 1.2 MPa, and the feed mass hourly space velocity (WHSV) was 0.5 h⁻¹ based on the total weight of the fatty acid feedstock. -1 The fatty acid feedstock and methanol are in a 1:1 weight ratio. The mixture is passed through a single-pass reactor and converted to obtain a mixture. This mixture is then flash-distilled to obtain a flash vapor phase and a flash liquid phase containing biodiesel. The flash vapor phase is condensed, and the resulting condensate is returned to the fixed-bed reactor as a supplementary saturated monohydric alcohol for further contact reaction. The flash temperature is 110°C and the pressure is 10... 5 Pa, the flash liquid phase was distilled to remove low-boiling-point impurities, and the biodiesel product was obtained. The product was divided into two phases, upper and lower. After being stirred evenly, the acid value was titrated and the acid value was 1.65 mg KOH / g.

[0058] The fatty acid raw material contains 99% by weight of fatty acids, and the fatty acid is lauric acid.

[0059] Example 3

[0060] use Figure 1 The apparatus shown uses fatty acid raw material with an acid value of 188 mg KOH / g and methanol as raw materials, with a weight ratio of fatty acid raw material to methanol of 9:6.

[0061] At the reactor inlet, 6.0 parts by weight of fatty acid feedstock and 6.0 parts by weight of methanol are added to mixer 3 and heat exchanger 4 from fatty acid feedstock inlet line 1 and methanol feedstock line 2 respectively using two pumps. In heat exchanger 4, the mixture is preheated to 200°C and then enters segmented fixed-bed reactor 5. The reactor has four spaced solid catalyst beds. A segmented feed port is provided on the side wall between adjacent solid catalyst beds. Each feed port corresponds to 1.0 part of fatty acid feedstock, which is pumped into the reactor and mixed with the product from the previous stage before entering the next stage reactor, for a total of 3.0 parts of fatty acid feedstock.

[0062] In this process, NaY molecular sieves with a silica-to-alumina ratio of 2.6 and pseudoboehmite as an inert binder were extruded to obtain cylindrical catalysts with a diameter of φ1.5mm × 5mm. The NaY molecular sieve content was 85% by weight, and the inert binder content was 15% by weight relative to the total catalyst weight. The fixed-bed reactor had an inner diameter of 45mm, with a total catalyst loading of 600g, and each solid catalyst bed containing 150g of catalyst. The reaction temperature was 240℃, the reaction pressure was 1.2MPa, and the feed mass hourly space velocity (WHSV) was 0.5h based on the total weight of the fatty acid feedstock. -1 After the reaction, the product enters flash tank 6, where water and methanol are distilled off. The flash temperature is 110℃ and the pressure is 10. 4 After separation of methanol and water, water is discharged from the reaction product outlet 9, and unreacted methanol flows into the methanol storage tank 14 from the methanol circulation pipeline 10. The primary biodiesel product enters the distillation column 7 from the primary biodiesel product outlet 11, where water and methanol are distilled off from the methanol and water outlet 13. The biodiesel product flows out from the biodiesel product outlet 12. The product obtained from the lower end of the segmented fixed-bed reactor 5 is stirred to homogeneity and then titrated to determine an acid value of 0.92 mg KOH / g. The product obtained from the biodiesel product outlet 12 has a titrated acid value of 0.75 mg KOH / g.

[0063] The fatty acid raw material contains 95% by weight of fatty acids, and the fatty acid is palmitic acid.

[0064] Comparative Example 1

[0065] ZNP-type acidic molecular sieves with a silica-to-alumina ratio of 15 were extruded with boehmite as an inert binder to obtain cylindrical molecular sieves with a diameter of 1.5 mm and a length of 3-6 mm. The content of ZNP-type acidic molecular sieves was 85% by weight, and the content of inert binder was 15% by weight. Raw material oil with an acid value of 188 mg KOH / g and methanol were pumped into a heat exchanger using two separate pumps and preheated to 200°C. Then, the mixture was fed into a fixed-bed reactor with an inner diameter of 12 mm and a catalyst loading of 15.0 g. The feed method was top-in, bottom-out. The reaction temperature was 240°C, the reaction pressure was 1.0 MPa, and the mass hourly space velocity (HHSV) was 0.4 h⁻¹ based on the total weight of the raw material oil. -1 The raw material oil and methanol are in a weight ratio of 3:2. The mixture is passed through a single-pass reactor and converted to obtain a mixture. This mixture is then subjected to flash evaporation to obtain a flash vapor phase and a flash liquid phase containing the mixed oil. The flash vapor phase is condensed, and the resulting condensate is returned to the fixed-bed reactor as a supplementary saturated monohydric alcohol for further contact reaction. The flash evaporation temperature is 110℃ and the pressure is 10... 3 Pa, the mixed oil after single-pass conversion is divided into upper and lower phases, with acid values ​​of 20.6 mgKOH / g and 15.7 mgKOH / g, respectively. The main component of the upper layer material is fatty acid methyl ester.

[0066] The fatty acid raw material contains 99% by weight of fatty acids, and the fatty acid is oleic acid.

[0067] The above data demonstrate that the biodiesel products prepared using the method disclosed herein have low acid values.

[0068] The preferred embodiments of this disclosure have been described in detail above with reference to the accompanying drawings. However, this disclosure is not limited to the specific details of the above embodiments. Within the scope of the technical concept of this disclosure, various simple modifications can be made to the technical solutions of this disclosure, and these simple modifications all fall within the protection scope of this disclosure.

[0069] It should also be noted that the various specific technical features described in the above embodiments can be combined in any suitable manner without contradiction. To avoid unnecessary repetition, this disclosure will not describe the various possible combinations separately.

[0070] Furthermore, various different embodiments of this disclosure can be combined in any way, as long as they do not violate the spirit of this disclosure, they should also be regarded as the content disclosed in this disclosure.

Claims

1. A method for preparing biodiesel, characterized in that, The method includes: feeding fatty acid feedstock and saturated monohydric alcohol into a fixed-bed reactor in a continuous feed manner, reacting them with a solid catalyst to obtain the biodiesel; The conditions for the contact reaction include: a temperature of 180-280℃ and a pressure of 0.5-3.0MPa; The weight ratio of the fatty acid raw material to the saturated monohydric alcohol is 3:(1-9). The fixed-bed reactor includes multiple spaced solid catalyst beds. The method includes: dividing the fatty acid raw material into 2-5 streams, mixing the saturated monohydric alcohol with the first stream of fatty acid raw material and then entering the fixed-bed reactor from the top and reacting with the first solid catalyst bed, and allowing the remaining streams of fatty acid raw material to enter between two adjacent solid catalyst beds respectively. The weight ratio of the saturated monohydric alcohol to the first fatty acid raw material is (1-9):1, and the weights of the remaining fatty acid raw materials are the same. The solid catalyst comprises an acidic molecular sieve and an inert binder. The acidic molecular sieve is a ZSM-48 molecular sieve with a silicon-to-aluminum ratio of 100 or higher, wherein the silicon-to-aluminum ratio is the molar ratio of SiO2 to Al2O3 in the acidic molecular sieve. The solid catalyst has hydrophobic properties.

2. The preparation method according to claim 1, wherein, The acidic molecular sieve is ZSM-48 molecular sieve with a silica-alumina ratio of 100-200; The inert binder is one or more of boehmite, graphite binder, and silica sol.

3. The preparation method according to claim 1, wherein, The content of the acidic molecular sieve is 75-98% by weight relative to the total weight of the solid catalyst, and the content of the inert binder is 2-25% by weight.

4. The preparation method according to claim 1, wherein, The fatty acid content is 80-100% by weight relative to the total weight of the fatty acid raw materials. The acid value of the fatty acid raw material is above 180 mgKOH / g; The fatty acid is one or more of lauric acid, myristic acid, palmitic acid, oleic acid, linoleic acid and linolenic acid.

5. The preparation method according to claim 1, wherein, The solid catalyst is spherical with a diameter of 0.5-3.0 mm; or, The solid catalyst is in strip shape and has a length of 3-8 mm.

6. The preparation method according to claim 1, wherein, The saturated monohydric alcohol includes one or more of the saturated monohydric alcohols having 1-3 carbon atoms.

7. The preparation method according to claim 1, wherein, The saturated monohydric alcohol is methanol and / or ethanol.

8. The preparation method according to claim 1, wherein, The method further includes: before the contact reaction, preheating the fatty acid raw material and the saturated monohydric alcohol before introducing them into the fixed-bed reactor, wherein the preheating temperature is 160-280°C.

9. The preparation method according to claim 1, wherein, The method further includes: flash evaporating the mixture obtained from the contact reaction to obtain a flash vapor phase and a flash liquid phase containing biodiesel, condensing the flash vapor phase, and returning the resulting condensate as a supplementary saturated monohydric alcohol to the fixed-bed reactor to continue the contact reaction; The flash evaporation conditions include: a temperature of 100-180℃ and a pressure of 10... 4 -10 5 Pa; The method further includes: removing low-boiling-point impurities by flash distillation of the liquid phase to obtain biodiesel products.

10. The preparation method according to claim 9, wherein, The acid value of the biodiesel product is less than 2.0 mg KOH / g.

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