Method for concentrating low-concentration formic acid at low temperature

Abstract

The invention provides a method for concentrating low-concentration formic acid at low temperature, and belongs to the technical field of chemical production. The method comprises the following steps: cooling and crystallizing a low-concentration formic acid aqueous solution to obtain crystal mush; and filtering and separating the obtained crystal mush to obtain concentrated formic acid. According to the method, water in the formic acid aqueous solution is preferentially solidified and separated out by utilizing the freezing point difference of formic acid and water, and then the part of solid water is removed to obtain concentrated formic acid, so that the formic acid concentration is improved at low cost and low energy consumption under mild conditions, heating and pressurization are not needed, the overall separation cost is effectively reduced, and the energy consumption and operation difficulty in the formic acid-water separation process are reduced; the requirement on equipment tolerance is low; according to the method, no solvent is used, so that solvent residues and a solvent post-treatment process are avoided.

Description

Technical Field

[0001] This invention relates to the field of chemical technology, and in particular to a method for concentrating low-concentration formic acid at low temperatures. Background Technology

[0002] Formic acid is an important chemical raw material widely used in organic synthesis and chemical industries such as leather and printing and dyeing. The methyl formate hydrolysis method is currently a commonly used industrial process for producing formic acid; however, due to chemical equilibrium limitations, the hydrolysis process cannot yield high-concentration formic acid, and the product obtained from the hydrolysis stage is a formic acid solution containing a large amount of water.

[0003] Patent CN201010140283.1 discloses a process for producing formic acid by pressure swing distillation. The method involves separating a mixture after catalytic hydrolysis by distillation, resulting in an aqueous formic acid solution containing a small amount of formic acid and a large amount of water in the bottom of a distillation column. This aqueous formic acid solution is then processed through two distillation columns, one pressurized and one depressurized, to obtain two high-concentration formic acid products of different purities. Because formic acid and water exhibit an azeotropic reaction, and the azeotropic temperature is higher than the boiling points of both formic acid and water, the pressurized column section is used to evaporate excess water, bringing the raw material composition to the azeotropic composition required for the operating pressure. This pressurized distillation process consumes a large amount of steam in formic acid production and places higher demands on the durability and safety of the equipment materials, thus increasing the difficulty of the process operation.

[0004] Patent CN92106327.X discloses a process for preparing high-concentration formic acid at low temperature. The method involves freezing and solidifying a high-concentration formic acid solution (formic acid content ≥80wt%), followed by thawing. Because aqueous formic acid has a low freezing point, it melts preferentially, yielding a high-concentration formic acid product. Compared to distillation processes, this method offers milder production conditions and saves energy and organic solvents, but it requires obtaining a high-concentration formic acid solution with a formic acid content ≥80wt% beforehand.

[0005] Therefore, a low-energy-consumption, easy-to-operate, and mild-reaction method is needed to concentrate low-concentration formic acid in aqueous solution. Summary of the Invention

[0006] The purpose of this invention is to provide a method for concentrating low-concentration formic acid at low temperatures. This method does not require heating, pressurization, or the use of solvents, effectively reducing energy consumption and operational difficulty in the formic acid-water separation process, and has low requirements for equipment tolerance.

[0007] To achieve the above-mentioned objectives, the present invention provides the following technical solution: This invention provides a method for concentrating low-concentration formic acid at low temperature, comprising the following steps: A low-concentration formic acid aqueous solution was cooled and crystallized to obtain a crystal slurry. The slurry was filtered and separated to obtain concentrated formic acid; The formic acid content in the low-concentration formic acid aqueous solution is ≤41 mol%; The endpoint temperature for the cooling crystallization is -48.5℃ to 0℃.

[0008] Preferably, the cooling crystallization is performed using programmed cooling, and the cooling rate of the programmed cooling is 0.05~0.2℃ / min.

[0009] Preferably, the cooling crystallization is carried out under stirring conditions, and the stirring speed is 30~200 rpm.

[0010] Preferably, the filter used for filtration has a filtration accuracy of 10~50μm; the filtration device used for filtration has a heat preservation device, and the filtration temperature is -48.5℃~0℃ and is consistent with the endpoint temperature of the cooling crystallization.

[0011] Preferably, the filtration medium includes at least one of organic polymer, inorganic non-metallic material and metallic material.

[0012] Preferably, the organic polymer includes nylon, polytetrafluoroethylene, or ordinary filter paper; The inorganic non-metallic material includes ceramic or glass fiber filter paper; The metal material includes 316L sintered metal filter sheets.

[0013] The beneficial effects of this invention are: This invention involves a programmed cooling crystallization process using a low-concentration formic acid aqueous solution with a formic acid content ≤41 mol%. The resulting crystal slurry is then filtered to obtain concentrated formic acid. Utilizing the difference in freezing points between formic acid and water, water in the formic acid aqueous solution preferentially solidifies and precipitates out. This solid water is then removed to obtain concentrated formic acid. This method employs cooling crystallization, increasing formic acid concentration under mild conditions with low cost and energy consumption. It eliminates the need for heating and pressurization, effectively reducing overall separation costs, energy consumption, and operational difficulty in the formic acid-water separation process, and lowering equipment tolerance requirements. Furthermore, this method does not use solvents, avoiding solvent residue and post-treatment processes. Attached Figure Description

[0014] Figure 1 This is a flowchart of the method for concentrating low-concentration formic acid at low temperature according to the present invention. Detailed Implementation

[0015] This invention provides a method for concentrating low-concentration formic acid at low temperature, comprising the following steps: A low-concentration formic acid aqueous solution was cooled and crystallized to obtain a crystal slurry. The slurry was filtered and separated to obtain concentrated formic acid; The formic acid content in the low-concentration formic acid aqueous solution is ≤41 mol%; The endpoint temperature for the cooling crystallization is -48.5℃ to 0℃.

[0016] In this invention, unless otherwise specified, all raw materials required for preparation are commercially available products well known to those skilled in the art.

[0017] like Figure 1 As shown, the present invention preferably adds a low-concentration formic acid aqueous solution to a crystallization vessel and performs cooling crystallization under stirring conditions to obtain a crystal slurry.

[0018] In this invention, the formic acid content in the low-concentration formic acid aqueous solution is preferably ≤41 mol%, more preferably ≤14.36 mol; since the formic acid aqueous solution forms the lowest freezing point mixture when the formic acid content is 41 mol%, the formic acid content in the formic acid water obtained by this method cannot exceed 41 mol.

[0019] In this invention, the cooling crystallization preferably employs programmed cooling, and the cooling rate of the programmed cooling is preferably 0.05~0.2℃ / min, more preferably 0.1℃ / min.

[0020] In this invention, the preferred endpoint temperature for cooling crystallization is -48.5℃ to 0℃. This temperature is lower than the crystal precipitation temperature of the corresponding low-concentration formic acid aqueous solution and higher than the solid-phase transition temperature of the corresponding low-concentration formic acid aqueous solution. The solid-phase transition temperature is the temperature at which the solution completely transforms into a solid state. Outside this temperature range, the solution is either entirely liquid or entirely solid, making the separation process impossible. Since formic acid and water are miscible in any proportion, the crystal precipitation temperature and freezing point in the solution will vary with the formic acid content, and there is a minimum co-freezing point of -48.5℃. Under the cooling crystallization temperature conditions specified in this invention, the difference in freezing points between formic acid and water is utilized to allow the water in the formic acid aqueous solution to form a formic acid-water mixture. The solid water of the compound solidifies and precipitates out. The water content in the precipitated formic acid-water mixture is greater than that in the original low-concentration formic acid aqueous solution. This solid water is then removed to enrich the formic acid in the low-concentration formic acid aqueous solution. The crystal precipitation temperature and the solid-phase transition temperature vary with the formic acid content in the initial low-concentration formic acid aqueous solution. When the formic acid aqueous solution concentration is 0 (i.e., pure water), it precipitates at 0°C, which is also the solid-phase transition temperature at that concentration. 41 mol% formic acid precipitates at -48.5°C, which is also the solid-phase transition temperature. The composition of the precipitated crystals varies with the formic acid content in the initial low-concentration formic acid aqueous solution and the set endpoint temperature for cooling crystallization, and is not pure water or pure formic acid.

[0021] In this invention, the cooling crystallization is preferably carried out under stirring conditions, and the stirring speed is preferably 30~200 rpm, more preferably 100~200 rpm.

[0022] like Figure 1As shown, the present invention preferably uses a filter medium to filter and separate the above-mentioned crystal slurry to obtain filtrate and filter residue, wherein the filtrate is concentrated formic acid.

[0023] In this invention, the filtration accuracy of the filter used for filtration is preferably 10~50μm, more preferably 20~40μm; the filtration device used for filtration preferably has a heat preservation device, and the filtration temperature is preferably -48.5℃~0℃ and consistent with the endpoint temperature of the cooling crystallization.

[0024] In this invention, the filtration medium preferably includes at least one of organic polymer, inorganic non-metallic material and metallic material; the organic polymer preferably includes nylon, polytetrafluoroethylene or ordinary filter paper; the inorganic non-metallic material preferably includes glass fiber filter paper or ceramic; the metallic material preferably includes 316L sintered metal filter sheet.

[0025] The technical solutions provided by the present invention will be described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0026] Example 1

[0027] like Figure 1 As shown, 500g of a 14.36mol% formic acid aqueous solution was added to a crystallization vessel, and the solution was cooled at a rate of 0.1℃ / min while stirring at 100rpm until the solution temperature reached -20℃, resulting in a crystal slurry. At -20℃, the crystal slurry was filtered and separated using a 400-mesh nylon filter cloth to obtain a filtrate and a filter residue. The filtrate was concentrated formic acid, with a formic acid content of 20.69mol% and a yield of 65%.

[0028] Example 2

[0029] like Figure 1 As shown, 500g of a 14.36mol% formic acid aqueous solution was added to a crystallization vessel, and the solution was cooled at a rate of 0.1℃ / min while stirring at 100rpm until the solution temperature reached -18℃, resulting in a crystal slurry. At -18℃, the crystal slurry was filtered and separated using a 400-mesh nylon filter cloth to obtain a filtrate and a filter residue. The filtrate was concentrated formic acid, with a formic acid content of 19.34mol% and a yield of 64%.

[0030] Example 3

[0031] like Figure 1As shown, 500g of an aqueous solution of formic acid with a content of 11.54mol% was added to a crystallization vessel, and the solution was cooled at a rate of 0.1℃ / min while stirring at a speed of 100rpm until the solution temperature reached -12℃, thus obtaining a crystal slurry. At -12℃, the obtained crystal slurry was filtered and separated using a 400-mesh nylon filter cloth to obtain a filtrate and a filter residue. The filtrate was concentrated formic acid, with a formic acid content of 14.35mol% and a yield of 37%.

[0032] Comparative Example 1

[0033] like Figure 1 As shown, 500g of a 14.36mol% formic acid aqueous solution was added to a crystallization vessel and cooled at a rate of 0.1℃ / min. The stirring speed was 100rpm, and the solution temperature was stirred until it reached -25℃, resulting in a slushy solid. At -25℃, the obtained powder was filtered and separated using a 400-mesh nylon filter cloth, yielding only a filter residue with a formic acid content of 14.36mol% at the initial concentration.

[0034] Comparing Examples 1, 2, and 3, it is evident that a 14.36 mol% formic acid aqueous solution begins to precipitate at -16°C, while an 11.54 mol% formic acid aqueous solution begins to precipitate at -10°C. Comparing Examples 1 and 2 with Comparative Example 1, it is evident that a 14.36 mol% formic acid aqueous solution completely transforms into a solid phase at -25°C. This demonstrates that the concentration of the obtained concentrated formic acid is affected by the final cooling temperature: in a formic acid-water solution of a fixed composition (i.e., the initial raw material), when the final cooling temperature is below the crystal precipitation temperature, a small amount of formic acid and a large amount of water precipitate as a formic acid-water mixed solid with a concentration lower than that of the initial solution, and the remaining liquid forms the concentrated formic acid product. As the final cooling temperature decreases, the formic acid concentration in the concentrated formic acid continuously increases, and more formic acid precipitates as a formic acid-water solid. However, due to the unique properties of water's transition from liquid to solid, some of the concentrated formic acid is trapped and adsorbed within the crystals. As the final temperature decreases, the number and size of the crystals increase, resulting in more formic acid being retained on the filter cake, leading to a rapid decrease in the yield of concentrated formic acid. Furthermore, after cooling to a certain point, the system transforms into a slushy solid (corresponding to the solid-phase transition temperature, as shown in Comparative Example 1), making it impossible to obtain the filtrate, rather than the concentrated solution at the corresponding eutectic temperature of 41 mol%. In other words, as the final temperature decreases, the content of concentrated formic acid increases and the yield decreases.

[0035] As can be seen from the above embodiments, the present invention provides a method for concentrating low-concentration formic acid at low temperature. A low-concentration formic acid aqueous solution with a formic acid content ≤41mol% is subjected to programmed cooling crystallization, and the resulting crystal slurry is filtered and separated to obtain concentrated formic acid. The method of the present invention is simple to operate, saves energy in concentrating low-concentration formic acid, and reduces the cost of formic acid concentration.

[0036] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for concentrating low-concentration formic acid at low temperature, characterized in that, Includes the following steps: A low-concentration formic acid aqueous solution was cooled and crystallized to obtain a crystal slurry. The slurry was filtered and separated to obtain concentrated formic acid; The formic acid content in the low-concentration formic acid aqueous solution is ≤41 mol%; The endpoint temperature for the cooling crystallization is -48.5℃ to 0℃.

2. The method according to claim 1, characterized in that, The cooling crystallization process employs programmed cooling, with a cooling rate of 0.05~0.2℃ / min.

3. The method according to claim 2, characterized in that, The cooling crystallization is carried out under stirring conditions, and the stirring speed is 30~200 rpm.

4. The method according to claim 1, characterized in that, The filter used for filtration has a filtration accuracy of 10~50μm; the filtration device used for filtration has a heat preservation device, and the filtration temperature is -48.5℃~0℃, which is consistent with the endpoint temperature of the cooling crystallization.

5. The method according to claim 4, characterized in that, The filtration medium includes at least one of organic polymers, inorganic non-metallic materials, and metallic materials.

6. The method according to claim 5, characterized in that, The organic polymer includes nylon, polytetrafluoroethylene, or ordinary filter paper; The inorganic non-metallic material includes ceramic or glass fiber filter paper; The metal material includes 316L sintered metal filter sheets.

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