Method for producing concentrated carbonate aqueous solution

Abstract

The present specification discloses a method for producing a concentrated carbonate aqueous solution. The present invention relates to a method for producing a concentrated carbonate aqueous solution, comprising a step of dewatering an aqueous solution of hydrogen carbonate with a salt blocking film to produce a concentrated aqueous solution of hydrogen carbonate, wherein the concentrated aqueous solution of hydrogen carbonate obtained in the above step is heated to thermally decompose the hydrogen carbonate into a carbonate, carbon dioxide, and water, and water is evaporated to obtain a concentrate of the carbonate aqueous solution.

Description

Technical Field

[0001] This patent application claims priority under the Paris Convention with respect to Japanese Patent Application No. 2019-168092 (filing date: September 17, 2019), the entire disclosure of which is incorporated herein by reference in its entirety. The present invention relates to a method for producing a concentrated aqueous carbonate solution, and more particularly to a method for producing methionine using the method.

Background Art

[0002] As a method for producing methionine, which is a compound useful as a feed additive, 5-(2-methylthio)hydantoin is hydrolyzed with an alkali metal carbonate and water to form an aqueous solution of an alkali metal salt of methionine, carbon dioxide is added to this solution, and the mixture is separated into solid methionine and a mother liquor containing bicarbonate. A method including this step is known. The filtrate containing the separated bicarbonate is heated to thermally decompose the bicarbonate into carbonate, carbon dioxide, and water, and at the same time, water is evaporated to obtain a concentrate of the aqueous carbonate solution (carbonate concentration step), and this concentrated aqueous carbonate solution is known to be used for the hydrolysis of hydantoin (see Patent Document 1). However, in the carbonate concentration step, water evaporation by heating is not always satisfactory in terms of energy efficiency because the latent heat of vaporization of water is large.

[0003] As a general energy-saving dehydration technique, membrane separation techniques such as RO membranes are known, and membrane elements are also commercially available. However, since the applicable range is generally pH 9 or less, in the carbonate concentration step (for example, the carbonate concentration step in the methionine production process), the pH value of the concentrated solution exceeds 9, so the application of the membrane to carbonate concentration may cause problems as an industrial production method.

Prior Art Documents

Patent Documents

[0004] Japanese Patent Publication No. 2008-506520

Summary of the Invention

[0005] The present invention provides an energy-efficient method for concentrating carbonates, particularly an aqueous carbonate concentration method suitable for the methionine production process, by using membrane separation. [Means for solving the problem]

[0006] The present invention encompasses the following embodiments. 1. A process to produce a concentrated bicarbonate aqueous solution by dehydrating an aqueous bicarbonate solution using a salt-blocking membrane. and The concentrated bicarbonate aqueous solution obtained in the above step is heated to thermally decompose the bicarbonate into carbonate, carbon dioxide, and water, and the water is evaporated to obtain a concentrated carbonate aqueous solution. process including A method for producing a concentrated carbonate aqueous solution. 2. The manufacturing method according to item 1, wherein the bicarbonate aqueous solution subjected to dehydration by a salt-blocking membrane is an aqueous solution of potassium bicarbonate or an aqueous solution of sodium bicarbonate. 3. The manufacturing method according to item 1 or 2, wherein the bicarbonate aqueous solution subjected to dehydration by a salt-blocking membrane is a 1-34 wt% bicarbonate aqueous solution. 4. The manufacturing method according to any one of items 1 to 3, wherein the salt-blocking membrane is an RO membrane or an NF membrane. 5. The manufacturing method according to any one of items 1 to 4, wherein the salt-blocking membrane is an organic membrane. 6. The manufacturing 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. 7. The manufacturing method according to item 6, wherein the bicarbonate aqueous solution is an aqueous bicarbonate aqueous solution obtained by filtering off methionine. 8.1) A process to produce 5-(2-methylmercaptoethyl)-hydantoin by reacting 3-methylmercaptopropionaldehyde, hydrogen cyanide, ammonia, and carbon dioxide or ammonium carbonate. 2) A step in which the generated 5-(2-methylmercaptoethyl)-hydantoin is hydrolyzed with potassium carbonate and water to obtain the potassium salt of methionine. 3) A step in which carbon dioxide is introduced into the hydrolysis reaction solution to precipitate methionine. A method for producing methionine, which includes, 4) A step of dehydrating the potassium bicarbonate aqueous solution obtained by filtering off methionine from the mixture of potassium bicarbonate aqueous solution and methionine produced in the above step using a salt-blocking membrane. 5) A step of heating the membrane-concentrated potassium bicarbonate aqueous solution to thermally decompose the potassium bicarbonate into potassium carbonate and carbon dioxide, while evaporating the water to obtain a concentrated potassium carbonate aqueous solution. 6) The process of further subjecting the obtained concentrated potassium carbonate aqueous solution to step 2). A method for producing methionine containing [the substance]. [Effects of the Invention]

[0007] This method makes it possible to recover concentrated carbonate aqueous solution and by-product carbon dioxide at a predetermined concentration with greater energy efficiency than conventional methods. [Modes for carrying out the invention]

[0008] This document describes a method for producing a concentrated carbonate solution, which involves dehydrating a bicarbonate aqueous solution using a salt-blocking membrane to produce a concentrated bicarbonate aqueous solution, and then heating the concentrated bicarbonate aqueous solution obtained in the first step to thermally decompose the bicarbonate into carbonate, carbon dioxide, and water, while evaporating the water to obtain a concentrated carbonate aqueous solution.

[0009] Bicarbonates are amphoteric substances (HCO3) - is CO3 2-Since it is an acid to and a base to H2CO3, the pH of the solution can be calculated from the dissociation constant, mass balance, and charge balance, resulting in a theoretical pH of approximately 8.4 (pKa1 + pKa2) / 2. Furthermore, because the theoretical pH formula does not include a concentration term, the pH is independent of the bicarbonate concentration, and since no pH increase occurs during membrane concentration, the concentration rate is not constrained by pH increases.

[0010] 1) Dehydration process using a membrane The concentration of the bicarbonate aqueous solution concentrated with the salt-blocking membrane is suitable for application to concentrations of 34 wt% or less, considering the solubility of bicarbonate at the membrane's preferred operating temperature (usually 45°C or below). Examples of bicarbonate include sodium bicarbonate and potassium bicarbonate, and examples of carbonates after thermal decomposition include sodium carbonate and potassium carbonate. The pH of the bicarbonate aqueous solution subjected to dehydration by the salt-blocking membrane is typically about pH 9 or below. Concentration by the membrane is preferably carried out at 50°C or below, and more preferably at 45°C or below. The dehydration rate by the salt-blocking membrane is preferably 5% or more, and more preferably 10% or more. The membrane supply pressure is preferably 1 MPaG or more, and more preferably 3 MPaG or more.

[0011] Examples of salt-blocking membranes include liquid separation membranes such as RO membranes and NF membranes, which selectively permeate the solvent by applying a pressure greater than the osmotic pressure difference between solutions on the higher concentration side, thereby allowing the solvent to pass through but not the solute. Examples of membrane structures include polymer membranes such as asymmetric membranes and composite membranes. Examples of salt-blocking membranes include organic membranes, and their materials include, but are not limited to, polyamide materials such as aromatic polyamides, aliphatic polyamides, and composites thereof, and cellulose materials such as cellulose acetate. There are no particular restrictions on the module form, and examples include tubular membrane modules, planar membrane modules, spiral membrane modules, and hollow fiber membrane modules. Examples of commercially available elements for RO membranes and NF membranes include SU-820FA (manufactured by Toray Industries, Inc.) and CPA5-LD (manufactured by Nitto Denko Corporation). Among these, SU-600, NTR-729HF, NTR-7250, and NTR-7450 can be cited as examples.

[0012] 2) Carbonate concentration process The potassium bicarbonate aqueous solution concentrated by the membrane is heated to thermally decompose the bicarbonate into carbonate, carbon dioxide, and water, while the water is evaporated to produce a concentrated potassium carbonate aqueous solution. To facilitate the evaporation of water, the pressure is preferably 0.5 MPaG or less, and more preferably 0.1 MPaG or less. Heating and gas-liquid separation may be performed by heating followed by a gas-liquid separator, or a distillation column may be used. Furthermore, concentration to the target concentration may be performed in a single stage, or multiple stages may be combined, such as in a multi-effect boiler.

[0013] An example of a bicarbonate aqueous solution is one obtained from the methionine production process. In the following scheme, the bicarbonate is composed of an alkali metal, with potassium being a typical example.

[0014] TIFF0007850557000001.tif70147

[0015] Here, as shown in formula (2), after hydrolyzing the hydantoin compound with an aqueous carbonate solution, carbon dioxide gas is blown into the reaction system as shown in formula (3) to neutralize the alkali, and methionine is filtered off as a solid from the mother liquor containing the aqueous carbonate solution. The aqueous potassium hydrogen carbonate solution obtained by filtration is, after membrane concentration, obtained as a concentrated aqueous carbonate solution concentrated to a predetermined potassium ion concentration through decomposition by heat treatment such as steam heating and evaporation of water shown in the following scheme. The concentrated aqueous carbonate solution can be recycled to the step of formula (2), and the generated carbon dioxide can be recycled to the step of formula (3).

[0016] TIFF0007850557000002.tif13123

[0017] For the manufacturing processes including the above formulas (1), (2), (3) and (4), reference can be made to, for example, the descriptions in US2006016334 and US5770769.

[0018] As a method for producing methionine including the step of producing the above concentrated aqueous carbonate solution, for example, the following embodiments can be exemplified. 1) A step of reacting 3-methylmercaptopropionaldehyde, hydrogen cyanide, ammonia and carbon dioxide or ammonium carbonate to produce 5-(2-methylmercaptoethyl)-hydantoin 2) A step of hydrolyzing the produced 5-(2-methylmercaptoethyl)-hydantoin with potassium carbonate to obtain a potassium salt of methionine 3) A step of introducing carbon dioxide into the hydrolysis reaction solution to precipitate (precipitate) methionine A method for producing methionine, including 4) A step of dehydrating the aqueous potassium hydrogen carbonate solution obtained by filtering methionine from the mixture of the aqueous potassium hydrogen carbonate solution and methionine produced in the previous step with a salt-blocking membrane, 5) A step of heating the membrane-concentrated aqueous potassium hydrogen carbonate solution to thermally decompose potassium hydrogen carbonate into potassium carbonate, carbon dioxide and water, and evaporate the moisture to obtain a concentrated aqueous solution of potassium carbonate 6) The process of further subjecting the obtained concentrated potassium carbonate aqueous solution to step 2). A method for producing methionine containing [the substance]. [Examples]

[0019] The invention will be described below with reference to examples, but the present invention is not limited to these examples.

[0020] Example 1 The bicarbonate aqueous solution used for membrane dehydration (hereinafter referred to as the stock solution) was an aqueous solution of potassium bicarbonate: 10 wt% and water: 90 wt% at atmospheric pressure and 25°C. The energy required to concentrate the potassium ion concentration to twice that of the stock solution was calculated through 1) a membrane dehydration process and 2) a carbonate concentration process. Although it does not contribute to energy efficiency, for the sake of calculation convenience, the flow rate of the bicarbonate aqueous solution of the stock solution was set to 100 kg / h (potassium bicarbonate: 10 kg / h, water: 90 kg / h).

[0021] 1) Dehydration process using membranes (dehydration of bicarbonate aqueous solution by membrane separation) The dehydration rate was set to 10%. Since the stock solution needed to be pressurized to a predetermined pressure, the pressurization energy was calculated using Aspen Plus (v10), and the pump efficiency was set to the default value of 71.27%. The membrane supply pressure required for dehydration was increased to the osmotic pressure difference relative to the membrane outlet, where the osmotic pressure difference was highest. Osmotic pressure was calculated using the ELECNRTL model in Aspen Plus (v10).

[0022] 2) Carbonate concentration process (thermal decomposition of bicarbonate and evaporation of water) The concentrated bicarbonate aqueous solution is reduced in pressure to atmospheric pressure, and then heated to decompose the bicarbonate into carbonate, carbon dioxide, and water, while simultaneously evaporating the water to obtain a concentrated carbonate aqueous solution. During this process, heating energy is applied so that the potassium ion concentration in the concentrated carbonate aqueous solution reaches a predetermined concentration. Specifically, the concentrated bicarbonate aqueous solution is supplied to an atmospheric pressure distiller (stage 1), and the conditions under which the potassium ion concentration in the bottom output reaches the target concentration ratio (2 times) are determined. The thermal energy supplied by the distillation column reboiler to satisfy these conditions is defined as the required heating energy. (calculation result) The energy required for concentration by steps 1), 2), and 3) above is 85% of that required in Comparative Example 1.

[0023] Example 2 In the method of Example 1, the dehydration rate by the membrane was changed from 10% to 20%, and the same calculations were performed. (calculation result) The energy required for concentration is 68% of that required in Comparative Example 1.

[0024] Example 3 In the method of Example 1, the target concentration ratio of potassium ions was set to 3 times instead of 2 times for the calculation. (calculation result) The energy required for concentration is 88% of that required for Comparative Example 2.

[0025] Example 4 In the method of Example 3, the dehydration rate by the membrane was changed from 10% to 20%, and the same calculations were performed. (calculation result) The energy required for concentration is 75% of that required for Comparative Example 2.

[0026] Comparative Example 1 Without membrane concentration, the bicarbonate solution is thermally decomposed and concentrated to double the potassium ion concentration of the original solution.

[0027] Comparative Example 2 Without membrane concentration, the bicarbonate solution is thermally decomposed and concentrated to increase the potassium ion concentration to three times that of the original solution.

[0028] The results for Examples 1-4 and Comparative Examples 1 and 2 are summarized in Table 1.

[0029] [Table 1]

[0030] The process of the present invention has been shown to be an energy-saving process compared to conventional methods. [Industrial applicability]

[0031] This method allows for the efficient acquisition of concentrated carbonate aqueous solutions.

Claims

1. A process for producing a concentrated bicarbonate aqueous solution by dehydrating an aqueous bicarbonate solution using a salt-blocking membrane, and A step in which the concentrated bicarbonate aqueous solution obtained in the above step is heated to thermally decompose the bicarbonate into carbonate, carbon dioxide, and water, and the water is evaporated to obtain a concentrated carbonate aqueous solution. A method for producing a concentrated carbonate aqueous solution, which includes The pH of the bicarbonate aqueous solution subjected to dehydration by a salt-blocking membrane is 9 or less. Salt-blocking membranes are RO membranes or NF membranes; manufacturing method.

2. The manufacturing method according to claim 1, wherein the bicarbonate aqueous solution subjected to dehydration by a salt-blocking 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 subjected to dehydration by a salt-blocking membrane is a 1 to 34 wt% bicarbonate aqueous solution.

4. The manufacturing method according to any one of claims 1 to 3, wherein the salt-blocking membrane is an organic membrane.

5. The manufacturing method according to any one of claims 1 to 4, wherein the bicarbonate aqueous solution is a bicarbonate aqueous solution obtained from a methionine production process.

6. The manufacturing method according to claim 5, wherein the bicarbonate aqueous solution is an aqueous bicarbonate aqueous solution obtained by filtering off methionine.

7. 1) A process to produce 5-(2-methylmercaptoethyl)-hydantoin by reacting 3-methylmercaptopropionaldehyde, hydrogen cyanide, ammonia, and carbon dioxide or ammonium carbonate. 2) A step in which the generated 5-(2-methylmercaptoethyl)-hydantoin is hydrolyzed with potassium carbonate and water to obtain the potassium salt of methionine. 3) A step in which carbon dioxide is introduced into the hydrolysis reaction solution produced to precipitate methionine. A method for producing methionine, which includes, 4) A step of dehydrating the potassium bicarbonate aqueous solution obtained by filtering off methionine from the mixture of potassium bicarbonate aqueous solution and methionine produced in the above step using a salt-blocking membrane, wherein the pH of the bicarbonate aqueous solution subjected to dehydration by the salt-blocking membrane is 9 or less, and the salt-blocking membrane is an RO membrane or an NF membrane. 5) A step of 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. 6) The process of further subjecting the obtained concentrated potassium carbonate aqueous solution to step 2). A method for producing methionine containing [the substance].

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