Method for producing concentrated carbonate aqueous solution

Abstract

This specification discloses a method for producing a concentrated aqueous carbonate solution. The present invention relates to a method for producing a concentrated aqueous carbonate solution, comprising a step of dehydrating an aqueous bicarbonate solution using a salt barrier membrane to produce a concentrated aqueous bicarbonate solution. The concentrated aqueous bicarbonate solution obtained in the aforementioned step is heated to thermally decompose the bicarbonate into carbonate, carbon dioxide, and water, while simultaneously evaporating the water to produce a concentrate of the aqueous carbonate solution.

Description

Technical Field

[0001] This patent application claims priority under the Paris Convention based on Japanese Patent Application No. 2019-168092 (filing date: September 17, 2019), which is hereby incorporated by reference into this specification in its entirety.

[0002] The present invention relates to a method for producing a concentrated carbonate aqueous solution, and more particularly, to a method for producing methionine using the method. Background Art

[0003] A known method for producing methionine, a compound useful as a feed additive, includes hydrolyzing 5-(2-methylthio)hydantoin with an alkali metal carbonate and water to produce an aqueous solution of an alkali metal salt of methionine. Carbon dioxide is then added to this solution to separate the solution into solid methionine and a mother liquor containing bicarbonate. Another known method involves heating the separated bicarbonate-containing filtrate to thermally decompose the bicarbonate into carbonate, carbon dioxide, and water, while simultaneously evaporating the water to obtain a concentrated carbonate aqueous solution (carbonate concentration step). This concentrated carbonate aqueous solution is then used for the hydrolysis of hydantoin (see Patent Document 1). However, in the carbonate concentration step, due to the high latent heat of evaporation of water, evaporation by heating is not always satisfactory in terms of energy efficiency.

[0004] Membrane separation technologies such as RO membranes are commonly known as energy-saving dehydration technologies, and membrane elements are commercially available. However, since their applicable range is generally below pH 9, the pH of the concentrate exceeds 9 in carbonate concentration processes (for example, the carbonate concentration process in the methionine production process). Therefore, the application of membranes to carbonate concentration as an industrial production method may cause problems.

[0005] Prior art literature

[0006] Patent Literature

[0007] Japanese Patent Application No. 2008-506520. Summary of the Invention

[0008] Problems to be solved by the invention

[0009] The present invention provides a method for concentrating carbonates with improved energy efficiency by using membrane separation, and in particular provides a method for concentrating a carbonate aqueous solution suitable for the production process of methionine.

[0010] Means for solving problems

[0011] The present invention includes the following aspects.

[0012] 1. A method for producing a concentrated carbonate aqueous solution, comprising the steps of dehydrating a bicarbonate aqueous solution using a salt barrier membrane to produce a concentrated bicarbonate aqueous solution, wherein the concentrated bicarbonate aqueous solution obtained in the aforementioned step is heated to thermally decompose the bicarbonate into carbonate, carbon dioxide, and water, while simultaneously evaporating the water to obtain a concentrate of the carbonate aqueous solution.

[0013] 2. The production method according to item 1, wherein the bicarbonate aqueous solution used for dehydration using the salt barrier membrane is a potassium bicarbonate aqueous solution or a sodium bicarbonate aqueous solution.

[0014] 3. The production method according to item 1 or 2, wherein the bicarbonate aqueous solution used for dehydration using the salt barrier membrane is a 1 to 34 wt % bicarbonate aqueous solution.

[0015] 4. The production method according to any one of items 1 to 3, wherein the salt barrier membrane is an RO membrane or a NF membrane.

[0016] 5. The production method according to any one of items 1 to 4, wherein the salt barrier film is an organic film.

[0017] 6. The production method according to any one of items 1 to 5, wherein the bicarbonate aqueous solution is a bicarbonate aqueous solution obtained from a methionine production process.

[0018] 7. The production method according to item 6, wherein the bicarbonate aqueous solution is a bicarbonate aqueous solution obtained by filtering out methionine.

[0019] 8. A method for producing methionine, comprising:

[0020] 1) Process for producing 5-(2-methylmercaptoethyl)-hydantoin by reacting 3-mercaptopropionaldehyde, hydrogen cyanide, ammonia and carbon dioxide or ammonium carbonate

[0021] 2) Hydrolyzing the generated 5-(2-methylmercaptoethyl)-hydantoin with potassium carbonate and water to obtain the potassium salt of methionine

[0022] 3) The process of introducing carbon dioxide into the hydrolysis reaction solution to precipitate methionine

[0023] A method for producing methionine, comprising:

[0024] 4) a step of filtering out methionine from the mixture of the potassium bicarbonate aqueous solution and methionine produced in the above step, and dehydrating the obtained potassium bicarbonate aqueous solution using a salt barrier membrane,

[0025] 5) heating the membrane-concentrated potassium bicarbonate aqueous solution to thermally decompose the potassium bicarbonate into potassium carbonate and carbon dioxide while evaporating water to obtain a concentrated potassium carbonate aqueous solution.

[0026] 6) The concentrated potassium carbonate aqueous solution obtained is further subjected to the above-mentioned step 2).

[0027] Effects of the Invention

[0028] According to this method, the concentrated carbonate aqueous solution and by-produced carbon dioxide managed to a predetermined concentration can be recovered with better energy efficiency than before. DETAILED DESCRIPTION

[0029] A method for producing a concentrated aqueous carbonate solution is described. The method includes a step of dehydrating an aqueous bicarbonate solution using a salt barrier membrane to produce a concentrated aqueous bicarbonate solution. The concentrated aqueous bicarbonate solution obtained in the aforementioned step is heated to thermally decompose the bicarbonate into carbonate, carbon dioxide, and water, while simultaneously evaporating the water to obtain a concentrate of the aqueous carbonate solution.

[0030] Bicarbonate is an amphoteric substance (HCO3 - Relative to CO3 2- Since hydrogen carbonate is an acid relative to hydrogen (and a base relative to hydrogen dioxide), the theoretical pH is calculated using the dissociation constant, mass balance, and charge balance: (pKa1 + pKa2) / 2 = 8.4. Furthermore, since the theoretical pH formula does not include a concentration term, the pH is independent of bicarbonate concentration. Therefore, concentration using membranes does not increase the pH, and the concentration rate is not constrained by this increase.

[0031] 1) Dehydration process using membrane

[0032] Considering the solubility of bicarbonate at the membrane's optimal operating temperature (typically 45°C or below), the concentration of the bicarbonate solution concentrated using a salt barrier membrane is preferably 34 wt% or less. Examples of bicarbonate include sodium bicarbonate and potassium bicarbonate, and examples of thermally decomposed carbonates include sodium carbonate and potassium carbonate. The pH of the bicarbonate solution used for dehydration using a salt barrier membrane is typically approximately 9 or below. Concentration using the membrane is preferably performed at 50°C or below, more preferably 45°C or below. The dehydration rate using the salt barrier membrane is preferably 5% or above, more preferably 10% or above. The membrane supply pressure is preferably 1 MPaG or above, more preferably 3 MPaG or above.

[0033] Examples of salt-removing membranes include liquid separation membranes such as RO membranes and NF membranes, which selectively allow the solvent to permeate by applying a pressure greater than the osmotic pressure difference between the solutions across the membrane, which allows the solvent to pass but not the solute, on the high-concentration side. Examples of membrane structures include polymer membranes such as asymmetric membranes and composite membranes. Examples of salt-removing membranes include organic membranes, made of, but not limited to, polyamide-based materials such as aromatic polyamides, aliphatic polyamides, and composites thereof, and cellulose-based materials such as cellulose acetate. Furthermore, the module format is not particularly limited, and examples include tubular membrane modules, flat membrane modules, spiral membrane modules, and hollow fiber membrane modules. Commercially available RO and NF membrane elements include SU-820FA (Toray Industries, Inc.) and CPA5-LD (Nitto Denko Corporation). Among them, SU-600, NTR-729HF, NTR-7250, and NTR-7450 can be exemplified.

[0034] 2) Carbonate concentration process

[0035] The membrane-concentrated potassium bicarbonate aqueous solution is heated to thermally decompose the bicarbonate into carbonate, carbon dioxide, and water while simultaneously evaporating the water, producing a concentrate of the potassium carbonate aqueous solution. To facilitate water evaporation, the pressure is preferably 0.5 MPaG or less, more preferably 0.1 MPaG or less. Heating and gas-liquid separation can be performed by pouring the solution into a gas-liquid separator after heating, or using a distillation column. Concentration to the target concentration can be performed in a single stage, or multiple stages can be combined, such as in a multi-effect tank.

[0036] As an example of the bicarbonate aqueous solution, an aqueous bicarbonate solution obtained from a methionine production process can be exemplified. In the following scheme, the case where the bicarbonate is composed of an alkali metal, a typical example of which is potassium, is shown as an example.

[0037]

[0038] Here, as shown in formula (2), the hydantoin compound is hydrolyzed with a carbonate aqueous solution, and then, as shown in formula (3), carbon dioxide is blown into the reaction system to neutralize the base, and methionine is filtered out as a solid from the mother liquor containing the carbonate aqueous solution.

[0039] The filtered bicarbonate aqueous solution is concentrated through a membrane and then decomposed by heat treatment such as steam heating and then evaporated to obtain a concentrated carbonate aqueous solution concentrated to a predetermined potassium ion concentration, as shown in the following flow. The concentrated carbonate aqueous solution can be recycled to the step of formula (2), and the generated carbon dioxide can be recycled to the step of formula (3).

[0040]

[0041] For the production process including the above formulae (1), (2), (3), and (4), reference can be made to the descriptions of, for example, US2006016334 and US5770769.

[0042] As a method for producing methionine including the step of producing the concentrated carbonate aqueous solution, the following aspects can be exemplified.

[0043] A method for producing methionine, comprising:

[0044] 1) Process for producing 5-(2-methylmercaptoethyl)-hydantoin by reacting 3-methylmercaptopropionaldehyde, hydrogen cyanide, ammonia and carbon dioxide or ammonium carbonate

[0045] 2) Hydrolyzing the generated 5-(2-methylmercaptoethyl)-hydantoin with potassium carbonate to obtain the potassium salt of methionine

[0046] 3) Step of introducing carbon dioxide into the hydrolysis reaction solution to precipitate methionine

[0047] A method for producing methionine, comprising:

[0048] 4) a step of filtering out methionine from the mixture of the potassium bicarbonate aqueous solution and methionine produced in the previous step, and dehydrating the obtained potassium bicarbonate aqueous solution using a salt barrier membrane,

[0049] 5) heating the membrane-concentrated potassium bicarbonate aqueous solution to thermally decompose the potassium bicarbonate into potassium carbonate, carbon dioxide, and water, while evaporating the water to obtain a concentrated potassium carbonate aqueous solution.

[0050] 6) The concentrated potassium carbonate aqueous solution obtained is further subjected to the above-mentioned step 2). Example

[0051] Hereinafter, the present invention will be described with reference to Examples, but the present invention is not limited thereto.

[0052] Example 1

[0053] The bicarbonate aqueous solution (hereinafter referred to as the stock solution) used for membrane dehydration was a 10 wt% potassium bicarbonate and 90 wt% water aqueous solution at 25°C under atmospheric pressure. The energy required to concentrate the potassium ion concentration to twice that of the stock solution through 1) membrane dehydration and 2) carbonate concentration was calculated. Although it does not contribute to energy efficiency, for ease of calculation, the bicarbonate aqueous solution flow rate of the stock solution was set at 100 kg / h (10 kg / h potassium bicarbonate, 90 kg / h water).

[0054] 1) Dehydration process using membranes (dehydration of bicarbonate aqueous solution using membrane separation)

[0055] The dehydration rate was set to 10%. Since the raw liquid needed to be pressurized to a specified pressure, the pressurization energy was calculated using Aspen Plus (v10), and the pump efficiency was calculated using the default value of 71.27% in the same software. The membrane supply pressure required for dehydration was increased to the osmotic pressure difference at the membrane outlet, which had the highest osmotic pressure difference. The osmotic pressure was calculated using the ELECNRTL model in Aspen Plus (v10).

[0056] 2) Carbonate concentration process (thermal decomposition of bicarbonate and evaporation of water)

[0057] After reducing the pressure of the concentrated bicarbonate aqueous solution to atmospheric pressure, heating energy is applied to thermally decompose the bicarbonate into carbonate, carbon dioxide, and water, while simultaneously evaporating the water to produce a concentrate of the carbonate aqueous solution. During this process, heating energy is applied to achieve a predetermined potassium ion concentration in the concentrated carbonate aqueous solution. Specifically, the concentrated bicarbonate aqueous solution is supplied to an atmospheric pressure still (stage 1). The conditions for achieving a target potassium ion concentration (doubled) in the bottoms liquid are determined, and the required heating energy is defined as the thermal energy applied by the distillation column reboiler when these conditions are met.

[0058] (Calculation results)

[0059] The energy required for concentration using the above-mentioned steps 1), 2), and 3) was 85% of that in Comparative Example 1.

[0060] Example 2

[0061] In the method of Example 1, the same calculation was performed except that the dehydration rate by the membrane was set to 20% instead of 10%.

[0062] (Calculation results)

[0063] The energy required for concentration was 68% of that in Comparative Example 1.

[0064] Example 3

[0065] In the method of Example 1, the target concentration rate of potassium ion concentration was calculated by setting it to 3 times instead of 2 times.

[0066] (Calculation results)

[0067] The energy required for concentration was 88% of that in Comparative Example 2.

[0068] Example 4

[0069] In the method of Example 3, the same calculation was performed except that the dehydration rate by the membrane was set to 20% instead of 10%.

[0070] (Calculation results)

[0071] The energy required for the concentration is 75% of that in Comparative Example 2.

[0072] Comparative Example 1

[0073] The aqueous bicarbonate solution was thermally decomposed and concentrated without membrane concentration, and the potassium ion concentration was concentrated to twice that of the original solution.

[0074] Comparative Example 2

[0075] The aqueous bicarbonate solution was thermally decomposed and concentrated without membrane concentration, and the potassium ion concentration was concentrated to 3 times that of the original solution.

[0076] The results of Examples 1 to 4 and Comparative Examples 1 and 2 are summarized in Table 1.

[0077] [Table 1]

[0078]

[0079] Compared with the previous methods, the process of the present invention is shown to be an energy-saving process.

[0080] Industrial Applicability

[0081] A concentrated carbonate aqueous solution can be obtained efficiently.

Claims

1. A method for producing a concentrated carbonate aqueous solution, comprising: a step of dehydrating the bicarbonate aqueous solution using a salt barrier membrane to produce a concentrated bicarbonate aqueous solution, and a step of heating the concentrated bicarbonate aqueous solution obtained in the aforementioned step to thermally decompose the bicarbonate into carbonate, carbon dioxide, and water while evaporating the water to obtain a concentrate of the carbonate aqueous solution. The salt barrier membrane is an RO membrane or a NF membrane.

2. The manufacturing method according to claim 1, wherein The bicarbonate aqueous solution used for dehydration using the salt barrier membrane is a potassium bicarbonate aqueous solution or a sodium bicarbonate aqueous solution.

3. The manufacturing method according to claim 1 or claim 2, wherein: The bicarbonate aqueous solution used for dehydration using the salt barrier membrane is a 1 to 34 wt % bicarbonate aqueous solution.

4. The manufacturing method according to claim 1 or claim 2, wherein: Salt barrier membranes are organic membranes.

5. The manufacturing method according to claim 1 or claim 2, wherein: The bicarbonate aqueous solution is a bicarbonate aqueous solution obtained from a methionine manufacturing process.

6. The manufacturing method according to claim 5, wherein: The bicarbonate aqueous solution is a bicarbonate aqueous solution obtained by filtering out methionine.

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