JP7816451B2 - Ammonia production method and ammonia water production method - Google Patents

Description

The present invention relates to a method for producing ammonia and a method for producing aqueous ammonia, which include a membrane reactor in which an ammonia synthesis catalyst and an ammonia separation membrane that selectively separates ammonia gas from a hydrogen/nitrogen/ammonia mixed gas are installed, and the ammonia gas produced by the catalyst is separated by the ammonia separation membrane, thereby suppressing the reverse reaction of ammonia and producing ammonia at a higher conversion rate than the equilibrium reaction.

The Haber-Bosch process for producing ammonia is known as one of the most important industrial processes. The ammonia production reaction is an equilibrium reaction, and thermodynamically, high-pressure, low-temperature conditions are considered favorable. However, in reality, it is necessary to ensure the catalytic reaction rate, and high temperatures of 400-600°C and high pressures of 10-20 MPa are generally required.

Traditionally, the Haber-Bosch process has used catalysts primarily composed of iron, but research has been reported to develop highly active catalysts to alleviate the harsh reaction conditions described above, and in recent years, the development of Ru-based catalysts has been active (Non-Patent Document 1).

A conventional ammonia synthesis process is shown in FIG.
A mixed gas of hydrogen and nitrogen, which is the raw material gas for ammonia, is reacted in an ammonia synthesis reactor in a catalytic reaction process, and the ammonia gas contained in the resulting product gas is liquefied in an ammonia recovery process and recovered as liquefied ammonia. In addition to the ammonia product, the product gas also contains unreacted raw materials, hydrogen and nitrogen, and in order to separate these, it is necessary to liquefy and separate the ammonia under high pressure and low temperature. Therefore, the product gas is usually cooled to about -20°C to -5°C to condense and separate the ammonia (Non-Patent Document 2).
In particular, the product gas from which ammonia is produced in the catalytic reaction process contains a large amount of unreacted hydrogen and nitrogen gas due to equilibrium constraints in the ammonia production reaction, which results in poor cooling efficiency in the ammonia recovery process and the consumption of a large amount of energy.In addition, in this process, in order to reduce raw material losses, it is necessary to separate hydrogen gas and nitrogen gas from the product gas and recycle them as raw material gas to the ammonia synthesis reactor, and the recycled gas containing unreacted hydrogen and nitrogen separated from ammonia by cooling must be heated again to the reaction temperature, which actually consumes further energy.
Therefore, since the ammonia production process consumes a large amount of energy due to the pressurization and cooling, there was a need to develop an energy-saving and environmentally friendly process.

In order to avoid such energy-intensive processes, a process has been proposed that replaces the method of recovering ammonia from the produced gas by cooling and condensation separation with a separation method using an inorganic separation membrane, thereby efficiently recovering high-concentration ammonia gas (Patent Document 1).

Methods for separating a mixed gas containing a high concentration of ammonia gas from a mixed gas of hydrogen gas, nitrogen gas, and ammonia gas include: 1) a method using a separation membrane to selectively allow hydrogen gas and/or nitrogen gas to permeate from the mixed gas; and 2) a method using a separation membrane to selectively allow ammonia gas to permeate from the mixed gas.
In the former method of selectively permeating hydrogen gas and/or nitrogen gas, ammonia is produced in a multi-stage system to separate and recover ammonia. On the other hand, in the latter method using a separation membrane, a method has been developed in which ammonia gas is selectively permeated from a mixed gas of hydrogen gas, nitrogen gas, and ammonia gas by using, for example, a zeolite membrane (Patent Documents 2 and 3). It has been found that selective separation of ammonia is possible by utilizing the acidic sites of zeolite and the adsorption of ammonia due to its basicity, and that ammonia can be selectively separated stably even under high-temperature conditions.

Patent Document 4 reports an ammonia production method that reduces the burden on equipment and energy by combining electrolysis with membrane separation or PSA ammonia concentration technology. This method is characterized by the fact that it can achieve power reduction effects even when there is a leakage of about 20% of inert components such as nitrogen into the ammonia side using separation membranes or PSA alone.

Patent Document 5 proposes a reaction system that produces ammonia more efficiently by using an adsorbent in the reactor that selectively adsorbs ammonia, causing the ammonia produced by the ammonia synthesis catalyst to be adsorbed onto the adsorbent, thereby shifting the equilibrium reaction to the production side.

Japanese Patent Application Laid-Open No. 2008-247654 JP 2014-58433 A International Publication No. 2018/230737 International Publication No. 2017/149718 US Patent Application Publication No. 2006/0039847

Nat. Chem. 2012, 4(11): 934-940. Chemical Society of Japan, 7th Edition, Chemical Handbook, Applied Chemistry, Vol. I, Maruzen Co., Ltd. (2014), p. 641

However, in conventional ammonia synthesis processes such as the Haber-Bosch process, equilibrium constraints mean that the produced gas contains large amounts of the raw materials hydrogen and nitrogen in addition to the product ammonia. To separate these, the ammonia must be liquefied or separated using a separation membrane, and energy is required to recycle the hydrogen and nitrogen gases after separation.

In processes using ammonia separation membranes, such as those described in Patent Document 1, a step is required to separate ammonia gas from a high-concentration hydrogen mixed gas containing ammonia gas that has permeated through the first-stage separation membrane, and from a mixed gas of nitrogen gas and ammonia gas that has not. In other words, this method is a complicated process that requires at least two stages to separate ammonia gas from a mixed gas of hydrogen gas, nitrogen gas, and ammonia gas. It also requires a step to recover ammonia from both the mixed gas that has permeated through the first stage and the mixed gas that has not permeated. To fundamentally solve these issues, it becomes necessary to separate hydrogen gas and nitrogen gas from a mixed gas containing a relatively high concentration of hydrogen gas and nitrogen gas and/or a mixed gas containing a relatively low concentration of ammonia gas. However, such a process is not practical and does not allow for a productive ammonia production process.

Patent Document 3 uses a zeolite membrane capable of selectively separating ammonia, making it possible to separate gas containing a high concentration of ammonia from a produced mixed gas containing unreacted hydrogen gas and nitrogen gas. However, in order to condense and separate ammonia from the permeable gas that has permeated the zeolite membrane, it is necessary to re-pressurize the gas, and, as with the Haber-Bosch process, large amounts of unreacted hydrogen gas and nitrogen gas are recycled, so the reduction in energy consumption is limited.

Patent Document 4 claims that power consumption can be reduced by developing a new process for synthesizing ammonia from water and nitrogen using electrolysis, but there are issues with long-term stability, such as the development of corrosion-resistant structural materials, that need to be addressed before it can be put into practical use.

Patent Document 5 describes a process using a reaction-separation reactor that uses an adsorbent to produce ammonia at a yield higher than the equilibrium conversion rate. However, because a process is required to separate the adsorbed ammonia from the adsorbent, the reactor system is complex and difficult to put into practical use.

The present invention was made in consideration of the above-mentioned conventional situation, and aims to provide an ammonia production method and an ammonia water production method that can efficiently produce product ammonia and ammonia water using little energy.

In order to solve the above problems, the inventors discovered that, unlike the conventional Haber-Bosch process in which the product mixed gas contains large amounts of unreacted hydrogen gas and nitrogen gas due to reaction equilibrium constraints, a membrane reactor equipped with a separation membrane that selectively separates ammonia from the reaction system can be used to recover ammonia from gas containing a high concentration of ammonia that permeates the separation membrane, and the unreacted feed gas can be recycled or purged from the system, thereby providing an ammonia production process with low energy consumption, leading to the completion of the present invention.

In the ammonia production process using the membrane reactor of the present invention, the ammonia production reaction is an equilibrium reaction, so by combining an ammonia separation membrane that selectively separates ammonia with an ammonia synthesis catalyst, it is possible to suppress the reverse reaction of ammonia. As a result, ammonia can be obtained at a higher conversion rate than in conventional reactions that are subject to equilibrium constraints. In addition, because the ammonia production reaction is an exothermic reaction, the higher the reaction conversion rate, the more heat is produced. Energy can be saved by recovering this heat and using it as energy. Furthermore, because the amount of unreacted gas recycled is reduced, it is also possible to reduce the energy required to recycle unreacted gas.

The embodiments of the present invention have been achieved based on these findings and provide the following.
[1] A method for producing ammonia using hydrogen and nitrogen as raw materials,
a reaction and separation step of synthesizing ammonia from the raw material using a membrane reactor that uses an ammonia synthesis catalyst and an ammonia separation membrane;
an ammonia recovery step of cooling the ammonia contained in the permeation gas that has permeated the ammonia separation membrane and recovering the condensed liquefied ammonia;
and a recycling step of recycling unreacted raw material gas.
[2] A method for producing ammonia using hydrogen and nitrogen as raw materials,
a reaction and separation step of synthesizing ammonia from the raw material using a membrane reactor that uses an ammonia synthesis catalyst and an ammonia separation membrane;
an ammonia recovery step of cooling the ammonia contained in the permeation gas that has permeated the ammonia separation membrane and recovering the condensed liquefied ammonia;
and a purging step of purging unreacted raw material gas out of the system.
[3] The method for producing ammonia according to [1] or [2], wherein in the reaction and separation step, the ammonia concentration in the permeable gas that has permeated the ammonia separation membrane is 50 vol % or more.
[4] The method for producing ammonia according to any one of [1] to [3], wherein in the reaction and separation step, the pressure on the gas supply side of the ammonia separation membrane is 0.1 MPa or more and 10 MPa or less.
[5] The method for producing ammonia according to any one of [1] to [4], wherein in the reaction and separation step, a pressure difference between the gas supply side and the permeation side of the ammonia separation membrane of the membrane reactor is 0.1 MPa or more and 10 MPa or less.
[6] The method for producing ammonia according to any one of [1] to [5], wherein in the reactive separation step, the ammonia separation membrane has an ammonia/hydrogen permeability coefficient ratio of 20 or more.
[7] The method for producing ammonia according to any one of [1] to [6], wherein in the reactive separation step, the ammonia separation membrane has an ammonia/nitrogen permeability coefficient ratio of 20 or more.
[8] The method for producing ammonia according to any one of [1] to [7], wherein in the reaction and separation step, the ammonia separation membrane is a zeolite membrane.
[9] A method for producing aqueous ammonia using hydrogen and nitrogen as raw materials,
a reaction and separation step of synthesizing ammonia from the raw material using a membrane reactor that uses an ammonia synthesis catalyst and an ammonia separation membrane that selectively separates ammonia from a hydrogen/nitrogen/ammonia mixed gas;
an ammonia absorption step of supplying water to the permeable gas that has permeated the ammonia separation membrane to produce ammonia water, and recovering the ammonia water;
and a recycling step of recycling unreacted raw material gas.
[10] A method for producing aqueous ammonia using hydrogen and nitrogen as raw materials,
a reaction and separation step of synthesizing ammonia from the raw material using a membrane reactor equipped with an ammonia synthesis catalyst and an ammonia separation membrane that selectively separates ammonia from a hydrogen/nitrogen/ammonia mixed gas;
an ammonia absorption step of supplying water to the permeable gas that has permeated the ammonia separation membrane to produce ammonia water, and recovering the ammonia water;
and a purging step of purging unreacted raw material gas out of the system.
[11] The method for producing ammonia water according to [9] or [10], wherein in the reaction and separation step, the ammonia concentration in the permeable gas that has permeated the ammonia separation membrane is 30 vol % or more.
[12] The method for producing ammonia water according to any one of [9] to [11], wherein in the reaction separation step, the pressure on the gas supply side of the ammonia separation membrane is 0.1 MPa or more and 10 MPa or less.
[13] The method for producing ammonia water according to any one of [9] to [12], wherein in the reaction and separation step, a pressure difference between the gas supply side and the permeation side of the ammonia separation membrane of the membrane reactor is 0.1 MPa or more and 10 MPa or less.
[14] The method for producing ammonia water according to any one of [9] to [13], wherein in the reactive separation step, the ammonia separation membrane has an ammonia/hydrogen permeability coefficient ratio of 10 or more. [15] The method for producing ammonia water according to any one of [9] to [14], wherein in the reactive separation step, the ammonia separation membrane has an ammonia/nitrogen permeability coefficient ratio of 10 or more. [16] The method for producing ammonia water according to any one of [9] to [15], wherein in the reactive separation step, the ammonia separation membrane is a zeolite membrane.

According to the present invention, by synthesizing ammonia using a membrane reactor that employs an ammonia separation membrane that selectively separates ammonia in the reaction separation process, a conversion rate higher than that achieved in an equilibrium reaction can be achieved. As a result, the gas that permeates the membrane in the membrane reactor contains a high concentration of ammonia, making it possible to create a process that consumes less energy.

Furthermore, the process configuration of the present invention requires a small amount of recycled gas, allowing for the size of equipment such as boosters and reactors to be reduced, making it possible to compact the process. In other words, it makes it possible to produce ammonia using a small-scale process, which is difficult to do using the Haber-Bosch process, which recycles much of the unreacted gas, and it is possible to provide an on-site ammonia production process that can produce the required amount wherever it is needed, making it extremely valuable.

FIG. 1 is a flow diagram showing a liquefied ammonia production process according to the prior art. FIG. 1 is a flow diagram showing a liquefied ammonia production process in which unliquefied gas is recycled in the ammonia recovery step. FIG. 1 is a flow diagram showing a liquefied ammonia production process in which unliquefied gas is purged in the ammonia recovery step. FIG. 1 is a flow diagram showing a liquefied ammonia production process in which the non-permeated side of the membrane reactor is recycled and unliquefied gas is recycled in the ammonia recovery step. FIG. 1 is a flow diagram showing a liquefied ammonia production process in which unliquefied gas is purged in the ammonia recovery step. FIG. 1 is a flow diagram showing a conventional ammonia water production process. FIG. 1 is a flow diagram showing an ammonia water production process in which unrecovered gas is recycled in an ammonia absorption step. FIG. 1 is a flow diagram showing an ammonia water production process in which unrecovered gas is recycled in an ammonia absorption step. FIG. 1 is a flow diagram showing a process for producing aqueous ammonia in which the non-permeated side of a membrane reactor is recycled and unrecovered gas is recycled in an ammonia absorption step. FIG. 1 is a flow diagram showing a process for producing aqueous ammonia in which the non-permeated side of a membrane reactor is recycled and unrecovered gas is recycled in an ammonia absorption step. FIG. 1 is a flow diagram showing an ammonia absorption step in an ammonia water production process.

One aspect of the present invention is a method for producing ammonia using hydrogen and nitrogen as raw materials, the method comprising: a catalytic reaction step of synthesizing ammonia from the raw material in an ammonia synthesis reactor using an ammonia synthesis catalyst; a reaction and separation step of synthesizing ammonia from a reaction gas containing ammonia obtained in the ammonia synthesis reactor in a membrane reactor using an ammonia synthesis catalyst and an ammonia separation membrane; an ammonia recovery step of cooling ammonia contained in a permeation gas that has permeated the ammonia separation membrane and recovering condensed liquefied ammonia; and a recycling step of recycling unreacted raw material gas.
Another aspect of the present invention is a method for producing ammonia using hydrogen and nitrogen as raw materials, the method comprising: a catalytic reaction step of synthesizing ammonia from the raw material in an ammonia synthesis reactor using an ammonia synthesis catalyst; a reaction and separation step of synthesizing ammonia from the raw material using a reaction gas containing ammonia obtained in the ammonia synthesis reactor in a membrane reactor using an ammonia synthesis catalyst and an ammonia separation membrane; an ammonia recovery step of cooling the ammonia contained in the permeation gas that has permeated the ammonia separation membrane and recovering condensed liquefied ammonia; and a purging step of purging unreacted raw material gas to outside the system.

The ammonia synthesis reactor used in the present invention is a reactor equipped with an ammonia synthesis catalyst but without an ammonia separation membrane, and the membrane reactor is a reactor equipped with an ammonia synthesis catalyst and an ammonia separation membrane.
The catalytic reaction process used in the present invention is a process having an ammonia synthesis reactor, and the reaction separation process is a process having a membrane reactor.
The membrane reactor used in the present invention is called a selectively permeable membrane reactor. For example, see JP 2005-58823 A, which is a reactor equipped with a catalyst for promoting a chemical reaction and a selectively permeable membrane having selective permeability for a specific component, and thus has both catalytic action and selective permeability.

In the reaction and separation step of the ammonia production process of the present invention, the ammonia concentration contained in the separated gas is preferably 1 vol% or higher, from the perspective that the separation performance of the ammonia separation membrane used in the membrane reactor is such that a higher ammonia concentration makes it easier to separate ammonia. Therefore, the membrane reactor in the reaction and separation step is typically configured to undergo a reaction and separation step after a catalytic reaction step so that the ammonia concentration in the gas separated by the ammonia separation membrane is 1 vol% or higher. However, it is also possible to use a reactor that integrates the ammonia synthesis reactor and membrane reactor, places an ammonia synthesis catalyst upstream of the ammonia separation membrane, and is designed so that the ammonia concentration in the gas separated by the membrane is 1 vol% or higher.

The reaction and separation process is a process in which ammonia is synthesized using a membrane reactor equipped with an ammonia synthesis catalyst and an ammonia separation membrane. The membrane reactor is not particularly limited as long as it is equipped with an ammonia synthesis catalyst and an ammonia separation membrane, but an example of such a reactor is the reactor disclosed in Patent Document 3 (WO 2018/230737).

The ammonia synthesis reaction is expressed by the following formula, which uses hydrogen and nitrogen as raw materials. It is an exothermic reaction. Therefore, as the reaction progresses, the heat generated can be recovered as energy.
(Ammonia synthesis reaction)
N ₂ + 3H ₂ ←→ 2NH ₃ ΔH=-91.44 KJ/mol

In the reaction separation process using a membrane reactor, it is possible to suppress the reverse reaction of the ammonia production reaction, and the conversion rate in the membrane reactor is higher than the conversion rate at reaction equilibrium. As a result, the amount of heat generated when producing ammonia is greater than in the conventional Haber-Bosch process, and because there is less unreacted gas, it is characterized by less energy required for cooling, compression, and recycling in the ammonia recovery process described below.

(Liquefied ammonia production process)
In the catalytic reaction process, hydrogen and nitrogen are reacted as raw materials to synthesize ammonia.
1) Raw Material The nitrogen used as the raw material is preferably a nitrogen-containing gas having as high a nitrogen concentration as possible at the raw material stage. Specifically, it is preferable to use a nitrogen-containing gas containing 90 to 100 mol %, preferably 95 to 100 mol %, more preferably 98 to 100 mol %, even more preferably 99 to 100 mol %, and particularly preferably 99.5 to 100 mol % of nitrogen.

The gas contained other than nitrogen can be any gas that is inert to the reaction and does not affect catalytic activity, such as hydrogen, methane, argon, or carbon dioxide.

As a means for obtaining nitrogen-containing gas with an increased nitrogen concentration from air, it is desirable to use one of pressure swing adsorption (PSA), temperature swing adsorption (TSA), and pressure temperature swing adsorption (PTSA) methods, which use multiple adsorption towers arranged in parallel and separate and recover nitrogen by varying (swinging) the pressure, temperature, or both. Among these, pressure swing adsorption (PSA) is the most suitable. In these separation methods, the nitrogen may contain trace amounts of argon from the air, but the concentration of argon can be avoided by purging in the ammonia production process, for example.

Cryogenic air separation units can also be used to obtain ultra-high purity nitrogen gas. Cryogenic air separation units can produce nitrogen that does not contain argon.

The hydrogen used as a raw material is preferably a hydrogen-containing gas with as high a hydrogen concentration as possible at the raw material stage. Specifically, it is preferable to use a hydrogen-containing gas containing 90 to 100 mol%, preferably 95 to 100 mol%, more preferably 98 to 100 mol%, even more preferably 99 to 100 mol%, and especially preferably 99.5 to 100 mol% hydrogen.

The gases contained other than hydrogen can be any gas that is inert to the reaction and does not affect catalytic activity, such as argon, carbon dioxide, or methane. Concentration of these impurities can be avoided by purging during the ammonia production process.

Water electrolysis devices and water photolysis devices can also be used to obtain ultra-high purity hydrogen gas.

The ammonia synthesis reaction using an ammonia synthesis catalyst should be carried out under conditions such as temperature, pressure, catalyst amount, space velocity, linear velocity, and feed gas composition that ensure a sufficient reaction rate.

The reaction raw materials used in the ammonia synthesis reactor may be the above-mentioned hydrogen and nitrogen raw materials plus unreacted recycled gas to carry out the ammonia synthesis reaction. In this case, the recycled gas may contain ammonia.

2) Catalytic Reaction Step In the catalytic reaction step, the reaction conditions in the ammonia synthesis reactor are preferably high pressure conditions in order to ensure a high reaction rate, with 0.5 MPa or more being preferred, 1 MPa or more being more preferred, 2 MPa or more being even more preferred, 3 MPa or more being particularly preferred, 4 MPa or more being especially preferred, and 5 MPa or more being most preferred. On the other hand, since increasing the pressure requires pressure-boosting energy and therefore increases energy consumption, the pressure is preferably 20 MPa or less, more preferably 15 MPa or less, even more preferably 12 MPa or less, particularly preferably 10 MPa or less, particularly preferably 8 MPa or less, and most preferably 6 MPa or less.
The reaction temperature in the ammonia synthesis reactor is preferably 600°C or lower, more preferably 500°C or lower, particularly preferably 450°C or lower, and most preferably 400°C or lower, because a lower temperature reduces energy consumption and enables the production of gas with a high ammonia concentration through the ammonia production reaction. On the other hand, higher temperature conditions are better in order to increase the reaction rate, and a temperature of 100°C or higher is preferred, more preferably 200°C or higher, even more preferably 300°C or higher, and particularly preferably 350°C or higher.

The raw material gas composition is not particularly limited, and the reaction may be carried out under conditions that favor the reaction rate, taking into consideration the order of the reaction rate equation of the catalyst, but since it is usually more difficult to separate ammonia and hydrogen than ammonia and nitrogen in the ammonia separation membrane in the membrane reactor of the reaction separation step arranged in the subsequent stage, a gas with a low hydrogen concentration will have a higher ammonia concentration in the gas that has permeated the ammonia separation membrane. Therefore, the hydrogen/nitrogen molar ratio in the raw material gas composition is usually preferably 3.0 or less, more preferably 2.5 or less, even more preferably 2.0 or less, particularly preferably 1.5 or less, and most preferably 1.25 or less.
On the other hand, since the higher the hydrogen/nitrogen molar ratio, the higher the ammonia concentration in the equilibrium state, the ratio is preferably 0.1 or more, more preferably 0.3 or more, particularly preferably 0.5 or more, particularly preferably 0.8 or more, and most preferably 1.0 or more.

The ammonia synthesis catalyst used in the ammonia synthesis reactor is not particularly limited as long as it is a catalyst that synthesizes ammonia, such as the iron-based catalyst used in the Haber-Bosch process or a catalyst supported by Ru, but a catalyst containing Ru is preferred.

The ammonia synthesis reactor may be combined upstream of the membrane reactor, and may be a single stage or may be divided into multiple stages. When combined upstream of the membrane reactor, the ammonia synthesis reactor and the membrane reactor may be integrated.
The ammonia concentration contained in the outlet gas of the ammonia synthesis reactor is preferably 1 vol% or more, more preferably 3 vol% or more, still more preferably 6 vol% or more, particularly preferably 10 vol% or more, particularly preferably 15 vol% or more, and most preferably 20 vol% or more, from the viewpoint of improving the separation performance of the ammonia separation membrane in the downstream membrane reactor.

The heat generated when ammonia is produced in the ammonia synthesis reactor can be removed by installing a heat recovery system inside the ammonia synthesis reactor, or by installing a heat exchanger downstream of the ammonia synthesis reactor.

3) Reaction and Separation Step In the membrane reactor in the reaction and separation step, ammonia gas is synthesized using as raw materials the unreacted hydrogen and nitrogen gas containing ammonia gas synthesized in the ammonia synthesis reactor in the catalytic reaction step.
The ammonia synthesis reaction in the membrane reactor may be carried out under conditions that satisfy both conditions such as temperature, pressure, catalyst amount, space velocity, and linear velocity that can ensure a sufficient reaction rate, and conditions such as temperature, pressure, catalyst amount, membrane area, space velocity, and linear velocity that can easily permeate and separate ammonia through the ammonia separation membrane.

In order to ensure the catalytic reaction rate, the pressure conditions on the gas supply side of the ammonia separation membrane in a membrane reactor are usually high, preferably 0.5 MPa or higher, more preferably 1 MPa or higher, even more preferably 2 MPa or higher, particularly preferably 3 MPa or higher, especially preferably 4 MPa or higher, and most preferably 5 MPa or higher. On the other hand, increasing the pressure requires a booster, which consumes energy. Therefore, a pressure of 20 MPa or lower is usually preferred, more preferably 15 MPa or lower, even more preferably 12 MPa or lower, especially preferably 10 MPa or lower, especially preferably 8 MPa or lower, and most preferably 6 MPa or lower.

The pressure difference between the gas supply side and permeation side of the ammonia separation membrane in a membrane reactor is usually preferably high to ensure the catalytic reaction rate, with 0.5 MPa or more being preferred, 1 MPa or more being more preferred, 2 MPa or more being even more preferred, 3 MPa or more being particularly preferred, 4 MPa or more being especially preferred, and 5 MPa or more being most preferred. On the other hand, as mentioned above, increasing the pressure requires energy to increase the pressure, which increases energy costs. Therefore, a pressure of 20 MPa or less is usually preferred, with 15 MPa or less being more preferred, 12 MPa or less being even more preferred, 10 MPa or less being especially preferred, 8 MPa or less being especially preferred, and 6 MPa or less being most preferred.

The pressure applied to the permeation side of the ammonia separation membrane in the membrane reactor may be such that a pressure required for condensing ammonia in the subsequent ammonia recovery step remains. In this case, the pressure applied to the permeation side of the ammonia separation membrane is preferably 5 MPa or less, more preferably 4 MPa or less, even more preferably 3 MPa or less, and particularly preferably 2 MPa or less. On the other hand, since a higher pressure applied to the permeation side of the ammonia separation membrane makes it easier to condense ammonia by cooling, the pressure is usually preferably 0.1 MPa or more, more preferably 0.3 MPa or more, even more preferably 0.5 MPa or more, and particularly preferably 1 MPa or more.

The temperature of the membrane reactor is preferably 600°C or lower, more preferably 500°C or lower, particularly preferably 450°C or lower, and most preferably 400°C or lower, because lower temperatures reduce energy consumption and produce gas with a high ammonia concentration. On the other hand, higher temperatures are better for accelerating the reaction rate; temperatures of 100°C or higher are preferred, more preferably 200°C or higher, even more preferably 300°C or higher, and particularly preferably 350°C or higher.

The catalyst used in the membrane reactor in the reaction separation step is not particularly limited as long as it is a catalyst that synthesizes ammonia, such as the iron-based catalyst used in the Haber-Bosch process or a Ru-supported catalyst, but a Ru-containing catalyst is preferred.

The ammonia separation membrane used in the membrane reactor may be any membrane capable of separating ammonia, and examples thereof include separation membranes made of synthetic resins, carbon separation membranes, porous silica membranes, and carbon membranes. Since the membrane reactor is used under high-temperature conditions, a zeolite membrane is preferred, as it is heat-resistant and capable of selectively separating ammonia from a hydrogen/nitrogen/ammonia mixed gas. For example, the zeolite membrane described in Patent Document 3 can be mentioned.
The zeolite membrane contains an aluminosilicate zeolite. Preferably, the zeolite constituting the zeolite membrane is an aluminosilicate. The aluminosilicate is primarily composed of oxides of Si and Al, but may contain other elements. The _SiO2 / _{Al2O3 molar} ratio of the aluminosilicate is not particularly limited, but is usually 2 or more, preferably 3 or more, more preferably 4 or more, more preferably 5 or more, even more preferably 6 or more, even more preferably 8 or more, and particularly preferably 10 or more. The upper limit is usually such that Al is present in an amount comparable to an impurity, and the _SiO2 / _{Al2O3 molar} ratio is usually 500 or less, preferably 100 or less, more preferably 80 or less, even more preferably 60 or less, particularly preferably 40 or less, and most preferably 30 or less.
The zeolite membrane is preferably an MFI type zeolite membrane or an RHO type zeolite membrane, which have high separation performance even at high temperatures, and more preferably an RHO type zeolite membrane.

The zeolite membrane preferably has a molar ratio of nitrogen atoms to Al atoms of 0.01 to 4, as determined by X-ray photoelectron spectroscopy under the following measurement conditions. The molar ratio of Si atoms to Al atoms is preferably 2.0 to 10. Furthermore, the molar ratio of alkali metal atoms to Al atoms is preferably 0.01 to 0.070.
(Measurement conditions)
X-ray source during measurement: monochromatic Al-Kα ray, output 16 kV-34 W
Background determination method for quantitative calculation: Shirley method When the zeolite membrane satisfies the above requirements, ammonia gas can be separated with high efficiency.

Furthermore, upon further investigation into the separation of ammonia gas using a zeolite membrane composite, it was found that, although zeolite membranes can separate ammonia gas more selectively and efficiently than existing silica membranes, the rate of change in thermal shrinkage at 200°C and 300°C relative to 30°C changes monotonically to 0.13% and 0.30% (c-axis direction), and that when a zeolite membrane composite formed from CHA-type zeolite is used, the ammonia gas separation performance decreases, particularly in the temperature range above 200°C, leaving room for improvement. This is presumably because cracks occur at the grain boundaries of the zeolite due to thermal contraction of the zeolite, and gas passes through these cracks. However, even in the case of RHO-type zeolite, in which the rate of change in thermal contraction rate from 30°C to 200°C is 1.55%, which is significantly higher than that of CHA-type zeolite and which exhibits nonlinear thermal expansion/contraction behavior with temperature, if the rate of change in thermal expansion rate at 300°C is about 0.02%, ammonia can be separated efficiently and with high selectivity under high-temperature conditions exceeding 200°C.

Therefore, it is preferable that the rate of change in the thermal expansion coefficient of the zeolite membrane at 300°C relative to that at 30°C be within ±0.25%, and that the rate of change in the thermal expansion coefficient at 400°C relative to that at 30°C be within ±0.35%. It is even more preferable that the rate of change in the thermal expansion coefficient at 300°C relative to that at 30°C, and that the rate of change in the thermal expansion coefficient at 400°C relative to that at 30°C be within ±120%.

In terms of the separation performance of an ammonia separation membrane, a higher ammonia/nitrogen or ammonia/hydrogen permeability coefficient ratio is preferable because unreacted hydrogen and nitrogen are less likely to permeate the membrane. Specifically, the ammonia/nitrogen permeability coefficient ratio is preferably 10 or more, more preferably 20 or more, more preferably 25 or more, particularly preferably 30 or more, and most preferably 40 or more, but since the better the separation performance, the higher the concentration of ammonia that can be obtained, no particular upper limit is set. Similarly, the ammonia/hydrogen permeability coefficient ratio is preferably 10 or more, more preferably 20 or more, more preferably 25 or more, particularly preferably 30 or more, and most preferably 40 or more, but since the better the separation performance, the higher the concentration of ammonia that can be obtained, no particular upper limit is set.
Furthermore, since nitrogen is usually easier to separate than hydrogen, the permeability coefficient ratios of ammonia/nitrogen and ammonia/hydrogen do not need to be the same, and generally the permeability coefficient ratio of ammonia/nitrogen is higher than the permeability coefficient ratio of ammonia/hydrogen.

In order for the ammonia separation membrane used in the membrane reactor to exhibit ammonia separation performance, the ammonia concentration contained in the gas separated by the ammonia separation membrane is preferably 1 vol% or more, more preferably 3 vol% or more, even more preferably 6 vol% or more, particularly preferably 10 vol% or more, especially preferably 15 vol% or more, and most preferably 20 vol% or more. On the other hand, increasing the ammonia concentration requires an increase in the ammonia synthesis catalyst, which reduces economic efficiency due to catalyst costs. Therefore, the ammonia concentration contained in the gas separated by the ammonia separation membrane is preferably 50 vol% or less, more preferably 40 vol% or less, even more preferably 35 vol% or less, especially preferably 30 vol% or less, especially preferably 28 vol% or less, and most preferably 25 vol% or less.

The higher the ammonia concentration at the outlet of the membrane reactor, i.e., the ammonia concentration contained in the permeable gas that has permeated the ammonia separation membrane, the easier it is to condense the ammonia by cooling. Therefore, the ammonia concentration contained in the permeable gas that has permeated the ammonia separation membrane is usually preferably 30 vol% or higher, more preferably 50 vol% or higher, even more preferably 60 vol% or higher, particularly preferably 65 vol% or higher, especially preferably 70 vol% or higher, and most preferably 80 vol% or higher. An ammonia concentration of 100 vol% provides the highest cooling efficiency.

The gas discharged from the membrane reactor may be only the gas that has permeated the membrane, or both the gas that has permeated the membrane and the non-permeated gas. When the non-permeated gas is discharged, the non-permeated gas may be purged from the system or recycled to the inlet of the membrane reactor. Alternatively, the non-permeated gas may be combined with the gas that has permeated the membrane of the membrane reactor to form the product gas.
In a process in which the non-permeated side of a membrane reactor is recycled to the membrane reactor, if the raw material used for ammonia synthesis contains impurities, a part of the impurities may be removed by purging from outside the system to avoid concentration of the impurities. On the other hand, the gas on the non-permeated side may be purged to reduce the membrane surface area of the membrane in the portion with poor gas permeability, or may be combined with the gas that has permeated the membrane to be used as the product gas.

The ammonia separation membrane used in the membrane reactor may be of one type or a combination of several types.
The membrane reactor may be a single stage or may be divided into multiple stages. When the membrane reactor is a multi-stage reactor, the ammonia synthesis catalyst and the ammonia separation membrane used in each membrane reactor may be the same or different.

Typically, a reaction and separation process is carried out after the catalytic reaction process, but this catalytic reaction process and reaction and separation process may be repeated multiple times before the ammonia recovery process is carried out.

The heat generated when ammonia is produced in the reaction separation process can be recovered by installing a heat recovery system inside the membrane reactor, or by installing a heat exchanger downstream of the membrane reactor.

4) Ammonia Recovery Step The highly concentrated ammonia produced in the membrane reactor in the catalytic reaction step is cooled and condensed in the ammonia recovery step and recovered as liquefied ammonia.

In the ammonia recovery process, the higher the ammonia concentration of the gas before entering the recovery process, the easier it is to condense the ammonia, allowing for a reduction in cooling energy. Therefore, the ammonia concentration contained in the gas before cooling in the recovery process is preferably 50 vol% or higher, more preferably 60 vol% or higher, even more preferably 65 vol% or higher, particularly preferably 70 vol% or higher, and most preferably 80 vol% or higher. It is most preferable that the ammonia concentration is 100 vol%.

In the ammonia recovery step, the gas produced in the membrane reactor is pressurized. The pressure at this time is determined by the recovery rate of liquefied ammonia and the energy required to increase the pressure. In other words, if the pressure is reduced to suppress the energy required to increase the pressure, the recovery rate decreases and the purge amount (recycle amount in a recycle process) increases, resulting in poor economic efficiency. Therefore, usually, the pressure is preferably 1 MPa or more, more preferably 2 MPa or more, even more preferably 3 MPa or more, particularly preferably 4 MPa or more, and most preferably 5 MPa or more. On the other hand, if the pressure is increased to increase the recovery rate of ammonia, the energy required to increase the pressure increases. Therefore, usually, the pressure is preferably 20 MPa or less, more preferably 15 MPa or less, even more preferably 10 MPa or less, particularly preferably 8 MPa or less, and most preferably 6 MPa or less.
Similarly to the above, the temperature can be determined appropriately taking into consideration the ammonia recovery rate and cooling energy, but is usually preferably 20° C. or lower, more preferably 10° C. or lower, even more preferably 0° C. or lower, particularly preferably −10° C. or lower, and most preferably −20° C. or lower. The lower limit is preferably −50° C. or higher, more preferably −40° C. or higher, particularly preferably −30° C. or higher, and most preferably −25° C. or higher.
The conditions for liquefying ammonia are determined by the ammonia concentration, temperature, and pressure, so it is sufficient to set the conditions that result in the most energy-efficient process, taking into account the above-mentioned influences.

In one embodiment, unliquefied gas, i.e., gas containing unreacted feed gas, may be recycled and used as feed for the ammonia synthesis reactor. When recycling, a portion of the gas may be purged from the system to prevent the concentration of impurities. If the reaction conversion rate in the membrane reactor is high and the amount of unreacted gas is small, the gas may be purged without being recycled.

Another aspect of the present invention is a method for producing ammonia water using hydrogen and nitrogen as raw materials, comprising: a catalytic reaction step of synthesizing ammonia from the raw material in an ammonia synthesis reactor using an ammonia synthesis catalyst; a reaction separation step of synthesizing ammonia in a mixed gas of hydrogen/nitrogen/ammonia obtained in the ammonia synthesis reactor in a membrane reactor using an ammonia separation membrane that selectively separates the ammonia synthesis catalyst and the ammonia; an ammonia absorption step of supplying water to a permeable gas that has permeated the ammonia separation membrane to produce ammonia water, and recovering the ammonia water; and a recycling step of recycling unreacted raw material gas.
In still another embodiment of the present invention, there is provided a method for producing ammonia water using hydrogen and nitrogen as raw materials, the method comprising: a catalytic reaction step of synthesizing ammonia from the raw material in an ammonia synthesis reactor using an ammonia synthesis catalyst; a reaction separation step of synthesizing ammonia in a mixed gas of hydrogen/nitrogen/ammonia obtained in the ammonia synthesis reactor in a membrane reactor using an ammonia separation membrane that selectively separates the ammonia synthesis catalyst and the ammonia; an ammonia absorption step of supplying water to a permeable gas that has permeated the ammonia separation membrane to produce ammonia water, and recovering the ammonia water; and a purging step of purging unreacted raw material gas to the system.

5) Ammonia Absorption Step In the method for producing aqueous ammonia, the catalytic reaction step is the same as that described in the method for producing ammonia.
The ammonia absorption step is a step of supplying water to the permeable gas that has permeated the ammonia separation membrane to produce ammonia water, and recovering the ammonia water. Typically, the ammonia in the permeable gas is absorbed with water in an absorption tower. The absorption tower is not particularly limited, and a known absorption tower can be used.

In absorption towers that absorb ammonia with water, the heat of dissolution generated when ammonia dissolves in water can raise the temperature inside the tower, reducing the efficiency of ammonia recovery. In such cases, the amount of unrecovered ammonia increases, so the absorption tower can be divided into two or more towers, and the ammonia unrecovered in the first tower stage can be recovered as ammonia water in the second or subsequent absorption towers.

The lower the temperature of the absorption tower, the easier it is to absorb ammonia. Therefore, a temperature of 60°C or lower is preferable, 50°C or lower is more preferable, 40°C or lower is even more preferable, 30°C or lower is particularly preferable, 25°C or lower is especially preferable, and 20°C or lower is most preferable. On the other hand, since lowering the temperature requires cooling energy, a temperature of -20°C or higher is preferable, -15°C or higher is more preferable, -10°C or higher is even more preferable, -5°C or higher is especially preferable, and 0°C or higher is most preferable.

The concentration of aqueous ammonia produced in the absorption tower is preferably 1 vol% or more, more preferably 5 vol% or more, further preferably 10 vol% or more, particularly preferably 15 vol% or more, particularly preferably 18 vol% or more, and most preferably 20 vol% or more.
A lower ammonia concentration in the unrecovered gas in the absorption tower can reduce the amount recycled in the recycle process and the loss of unrecovered ammonia in the purge process. Therefore, the ammonia concentration in the unrecovered gas is preferably 5 vol% or less, more preferably 4 vol% or less, even more preferably 3 vol% or less, particularly preferably 2 vol% or less, particularly preferably 1 vol% or less, and most preferably 0.5 vol% or less.

In one embodiment, the method further includes a recycling step in which unrecovered gas is reused as a raw material, thereby reducing raw material loss and further improving process efficiency.
In another embodiment, the method further includes a purging step in which unreacted raw material gas is purged and discharged to the outside of the system. In the recycling step, a part of the unreacted gas may be purged in order to prevent concentration of impurities.

Unrecovered gas that has come into contact with water in the absorption tower and contains water can be recycled as is, or if the water will have an effect on subsequent processes, a dehydration step can be added to remove water from the recycled gas before returning it to the catalytic reaction step. The dehydration step can be performed using a known method, and if the water is dehydrated using an adsorbent, there are no particular restrictions on the adsorbent, and any known adsorbent can be used.

(Example)
Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Note that the present invention is not limited to these embodiments, and when there are multiple embodiments, the present invention also includes combinations of the embodiments.
Purging of the gas on the non-permeate side of the membrane reactor in the reaction separation process and purging in the recycle process when recycling unliquefied gas in the ammonia recovery process are carried out to suppress the concentration of impurities when they are contained in the hydrogen or nitrogen raw materials for ammonia synthesis, but the amount of purging varies depending on the impurity concentration in the raw materials. Therefore, in this calculation, it was assumed that there were no impurities in the raw materials, and the energy calculation was carried out without carrying out the purging required to remove impurities.

<Examples 1 to 5, Comparative Examples 1 to 3>
Figures 2 and 3 show typical process configurations of the present invention. In Figure 2, ammonia is synthesized using hydrogen and nitrogen as raw materials in a catalytic reaction process having an ammonia synthesis reactor equipped with an ammonia synthesis catalyst, and a reaction separation process having a membrane reactor equipped with an ammonia synthesis catalyst and an ammonia separation membrane. In the specification, the catalytic reaction process and reaction separation process in the figure are collectively referred to as the ammonia synthesis process. The product gas containing a high concentration of ammonia that permeates the membrane is then compressed in a booster and then cooled in a refrigerator, and liquefied ammonia is recovered. The unliquefied gas containing unreacted hydrogen and nitrogen is recycled and mixed with the raw materials to be introduced into the ammonia synthesis process.
In FIG. 3, the unliquefied gas containing unreacted hydrogen and nitrogen in FIG. 2 is purged without being recycled.
FIG. 4 shows an application example of FIG. 2 in which the gas on the non-permeation side of the membrane reactor in the reaction separation step in FIG. 2 is recycled to the membrane reactor.
FIG. 5 shows an application example of FIG. 3 in which the gas on the non-permeation side of the membrane reactor in the reaction separation step is recycled to the membrane reactor.

The energy calculated in the process configuration according to Example 1 is as follows:
(A) Process for synthesizing ammonia only through a catalytic reaction step (process of Comparative Example 1)
Energy consumption: heating of raw material gas (40°C → 400°C), pressurization (0.1MPa → 5MPa), freezing of ammonia synthesis reactor outlet gas (40°C → -22°C), heating of recycled gas only when recycled (-22°C → 40°C),
Recovered energy: Reaction heat of the ammonia synthesis reactor, heat recovery by cooling the outlet gas of the ammonia synthesis reactor (400 ° C → 40 ° C),

(B) Process for synthesizing ammonia in a catalytic reaction step and a reaction separation step (processes of Examples 1 to 5, Comparative Examples 2 to 3)
Energy consumption: heating of raw material gas (40°C → 400°C), pressurization (0.1 MPa → 5 MPa), pressurization of gas that has permeated the membrane of the membrane reactor (0.1 MPa → 5 MPa), freezing (40°C → -22°C), heating of recycled gas only when recycled (-22°C → 40°C)
Recovered energy: Reaction heat of the ammonia synthesis reactor and the membrane reactor, and heat recovery by cooling the outlet gas of the ammonia synthesis reactor (400°C → 40°C),

In Table 1, (i) the ratio of hydrogen to nitrogen amounts and (ii) the ratio of recycled amounts were calculated as follows.
(i) Ratio of hydrogen and nitrogen amounts: The ratio of "the total amount of raw hydrogen and nitrogen in the process of synthesizing ammonia in the catalytic reaction step and the reaction separation step" to "the total amount of raw hydrogen and nitrogen in the process of synthesizing ammonia in the catalytic reaction step only." (ii) Ratio of recycled amounts: The ratio of "the recycled amount in the process of synthesizing ammonia in the catalytic reaction step and the reaction separation step" to "the recycled amount in the process of synthesizing ammonia in the catalytic reaction step only."

The energy comparisons in Table 1 were calculated as follows.
(a) Comparison of the energy consumption required to produce 1 kg of ammonia: The ratio of "(A) the total energy consumption described in the process for synthesizing ammonia using only a catalytic reaction step" to "(B) the total energy consumption described in the process for synthesizing ammonia using a catalytic reaction step and a reaction separation step" (b) Comparison of the energy recovered when producing 1 kg of ammonia: The ratio of "(A) the total energy recovered described in the process for synthesizing ammonia using only a catalytic reaction step" to "(B) the total energy recovered described in the process for synthesizing ammonia using a catalytic reaction step and a reaction separation step" (c) Comparison of the energy required to produce 1 kg of ammonia: The ratio of "(A) the total energy consumption described in the process for synthesizing ammonia using only a catalytic reaction step minus the total recovered energy ..." to "(B) the total energy consumption described in the process for synthesizing ammonia using a catalytic reaction step and a reaction separation step minus the total recovered energy described in the process for synthesizing ammonia using only a catalytic reaction step"

The energy calculation for this process was performed using the process simulator Aspen.
The calculation was carried out using Aspen Plus Version 11, a product of Tech Co., Ltd.

The conditions for calculating the energy in the processes of the Examples and/or Comparative Examples are as follows: (1) the reaction conditions of the ammonia synthesis reactor in the catalytic reaction step, (2) the reaction conditions of the membrane reactor in the reaction separation step, and (3) the ammonia liquefaction conditions in the ammonia recovery step.
(1) Reaction conditions of the ammonia synthesis reactor: composition ratio of raw material gas: hydrogen/nitrogen = 3, 400°C, 5 MPa
(2) Reaction conditions of the membrane reactor: 400°C, 5 MPa (pressure difference between the gas supply side and the permeation side)
(3) Ammonia liquefaction conditions in the ammonia recovery process: 5 MPa, −22° C.

In addition, the ammonia synthesis reaction in the ammonia synthesis reactors in the Examples and Comparative Examples was performed at the equilibrium value determined by the reaction conditions described above.

In order to calculate the energy consumption of the process configurations according to the Examples and Comparative Examples, the following reaction rate equation was calculated based on the catalyst activity evaluation data given in Table 1 of Top. Cat., 1944, 1, 233.
(Reaction rate equation)
R=4.56563Exp(-51/RT)×[N ₂ ]/INH ² −2.31978Exp(+7)×Exp(-76.46/RT)×[H ₂ ] ⁻³ [NH ₃ ] ² /INH ²
INH=1+0.0307Exp(-80/RT)×[H ₂ ] ^-1.5 [NH ₃ ]

Table 1 shows a comparison table of ammonia production using the processes of the Examples and Comparative Examples.

In Example 1, energy calculations were performed in the process of Figure 2 using an ammonia separation membrane in which the _NH3 / _N2 and _NH3 / _H2 permeability coefficient ratios were both 40 in the membrane reactor. In this case, the ammonia concentration contained in the product gas that permeated the ammonia separation membrane in the membrane reactor was 62%, and the improved hydrogen conversion rate significantly reduced the recycled amount. As a result, the energy consumption required to produce 1 kg of ammonia was reduced by 37% compared to Comparative Example 1. However, due to the reduced amount of gas circulated by recycling and the reduced amount of heat recovered by cooling the membrane reactor outlet gas, the recovered energy obtained when producing 1 kg of ammonia was reduced by 40%. However, the effect of reducing energy consumption was significant, and it was found that this process could reduce the energy required to produce 1 kg of ammonia by about 10% compared to Comparative Example 1.

In Example 2, energy calculations were performed using an ammonia separation membrane with _NH3 / _N2 and _NH3 / _H2 permeability coefficient ratios of 40 in the membrane reactor in the process of Figure 3. The hydrogen conversion rate in the membrane reactor was improved, and the ammonia concentration in the product gas that permeated the ammonia separation membrane was 62%. In this process, raw materials are lost due to purging of unreacted gas in the ammonia recovery step, so the hydrogen and nitrogen raw materials need to be increased compared to Comparative Example 1. However, the effect of not recycling was significant, and the energy consumption required to produce 1 kg of ammonia was reduced by 36% compared to Comparative Example 1. On the other hand, since there was no gas circulating due to recycling, the amount of heat recovered by cooling the membrane reactor outlet gas was reduced, and the recovered energy obtained when producing 1 kg of ammonia was reduced by 38%. As a result, the effect of reducing energy consumption was significant, and it was found that this process could reduce the energy required to produce 1 kg of ammonia by 7% compared to Comparative Example 1.

In Example 3, energy calculations were performed using an ammonia separation membrane in which the permeability coefficient ratios of _NH3 / _N2 and _NH3 /H2 were both 100 in the membrane reactor in the process of Figure _2. In this case, the separation performance of the membrane was improved, and the ammonia concentration and hydrogen conversion rate contained in the product gas that permeated the ammonia separation membrane in the membrane reactor were even higher than in Example 1. Therefore, it was found that the same effects as in Example 1 could be further obtained, and the energy required to produce 1 kg of ammonia in this process could be reduced by 90% compared to Comparative Example 1.

In Example 4, energy calculations were performed using an ammonia separation membrane in the membrane reactor in the process of Figure ₃ , with both _NH3 / _N2 and _NH3 /H2 permeability coefficient ratios of 100. In this case, too, the improved separation performance of the membrane further increased the ammonia concentration and hydrogen conversion rate contained in the product gas that permeated the ammonia separation membrane in the membrane reactor. Because the same effects as in Example 2 can be further obtained, it was found that this process can reduce the energy required to produce 1 kg of ammonia by 95% compared to Comparative Example 1.

In Example 5, energy calculations were performed in the process of Figure 3 using an ammonia separation membrane with _NH3 / _N2 and _NH3 / _H2 permeability coefficient ratios of 10,000 in the membrane reactor. In this case, the ammonia concentration contained in the product gas that permeated the ammonia separation membrane in the membrane reactor was 99.5%, meaning that it contained almost no unreacted nitrogen or hydrogen, and therefore no recycling was necessary. In addition, the effect of reducing the energy consumption required to produce 1 kg of ammonia was significant, and the energy obtained or recovered when producing 1 kg of ammonia exceeded the energy consumption required to produce 1 kg of ammonia, demonstrating that energy is produced along with ammonia in this process.

On the other hand, in Comparative Examples 2 and 3, an ammonia separation membrane with a permeability coefficient ratio of 5 for both _NH3 / _N2 and _NH3 / _H2 was used in the membrane reactor. In this case, the ammonia concentration contained in the product gas that permeated the ammonia separation membrane in the membrane reactor was 18%, which was almost the same as in Comparative Example 1. However, since the pressure decreases due to permeation through the membrane, energy is required to increase the pressure again, resulting in an increase in energy consumption. As a result, the energy required to produce 1 kg of ammonia increased by about 18 times compared to Comparative Example 1, demonstrating that this process consumes a large amount of energy.

(Ammonia water production process)
In the ammonia production method using a membrane reactor of the present invention, when liquefied ammonia is produced, the pressure drops as the ammonia permeates through the membrane in the membrane reactor, so the product gas at the outlet of the membrane reactor is re-pressurized and cooled to condense and separate the ammonia. On the other hand, when producing aqueous ammonia, the gas at the outlet of the membrane reactor can be absorbed with water in an absorption tower without pressurization, so there is no need to pressurize it. Therefore, it is possible to provide a process that consumes even less energy when producing aqueous ammonia than when producing liquefied ammonia.

<Examples 6 to 12, Comparative Example 4>
Figure 6 shows a comparative example of an ammonia water production process, and Figures 7 and 8 show typical process configurations according to the present invention. In Figure 7, ammonia is synthesized using hydrogen and nitrogen as raw materials in a catalytic reaction step having an ammonia synthesis reactor equipped with an ammonia synthesis catalyst, and a reaction separation step having a membrane reactor equipped with an ammonia synthesis catalyst and an ammonia separation membrane. In the specification, the catalytic reaction synthesis step and reaction separation step in the figure are collectively referred to as the ammonia synthesis step. The product gas containing a high concentration of ammonia that has permeated the membrane is then absorbed into water in an ammonia absorption tower in an ammonia absorption step and recovered as ammonia water, and the unliquefied feed gas containing unreacted hydrogen and nitrogen that has not been absorbed into water is recycled.
In FIG. 8, the unliquefied gas containing unreacted hydrogen and nitrogen in FIG. 7 is purged without being recycled.
FIG. 9 shows an application example of FIG. 7 in which the gas on the non-permeation side of the membrane reactor in the reaction separation step in FIG. 7 is recycled to the membrane reactor.
FIG. 10 shows an application example of FIG. 8 in which the gas on the non-permeation side of the membrane reactor in the reaction separation step is recycled to the membrane reactor.

Purging of the gas on the non-permeate side of the membrane reactor in the reaction separation process and purging in the recycle process when recycling unliquefied gas in the ammonia absorption process are carried out to suppress the concentration of impurities when they are contained in the hydrogen or nitrogen raw materials for ammonia synthesis, but the amount of purging varies depending on the impurity concentration in the raw materials. Therefore, in this calculation, it was assumed that there were no impurities in the raw materials, and the calculation was carried out without carrying out the purging required to remove impurities.

The energy calculated in the process configurations according to the examples and comparative examples is as follows:
(C) Process for synthesizing ammonia only through a catalytic reaction step (Comparative Example 4)
Energy consumption: Heating of raw material gas (40°C → 400°C), pressurization (0.1 MPa → 5 MPa), freezing in the ammonia absorption tower (40°C → 0°C), heating of recycled gas (0°C → 40°C) and pressurization (0.1 MPa → 5 MPa) in the case of recycling
Recovered energy: Reaction heat in ammonia synthesis reactors and membrane reactors;
Heat recovery by cooling the outlet gas of the ammonia synthesis reactor (400°C → 40°C)

(D) Process for synthesizing ammonia through a catalytic step and a reaction separation step (processes of Examples 6 to 12):
Energy consumption: Heating of raw material gas (40°C → 400°C), pressurization (0.1 MPa → 5 MPa), refrigeration of the second tower of the ammonia absorption tower (40°C → 0°C), heating of recycled gas (0°C → 40°C) and pressurization (0.1 MPa → 5 MPa) only when recycling
Recovered energy: Reaction heat of the ammonia synthesis reactor and membrane reactor, and heat recovery by cooling the ammonia synthesis reactor outlet gas (400°C → 40°C)

In Table 2, (iii) the ratio of hydrogen to nitrogen amounts and (iv) the ratio of recycled amounts were calculated as follows.
(iii) Hydrogen/nitrogen amount ratio: The ratio of "the total amount of raw hydrogen and nitrogen in the process of synthesizing ammonia in the catalytic step and the reaction separation step" to "the total amount of raw hydrogen and nitrogen in the process of synthesizing ammonia in the catalytic reaction step only." (iv) Recycle amount ratio: The ratio of "the amount of recycled in the process of synthesizing ammonia in the membrane catalytic step and the reaction separation step" to "the amount of recycled in the process of synthesizing ammonia in the catalytic reaction step only."

The energy comparisons in Table 2 were calculated as follows:
(d) Comparison of the energy consumption required to produce 1 kg of ammonia: The ratio of "(C) The total energy consumption described in the process for synthesizing ammonia using only the catalytic reaction step" to "(D) The total energy consumption described in the process for synthesizing ammonia using both the catalytic step and the reaction separation step"
(e) Comparison of the recovered energy obtained when producing 1 kg of ammonia: The ratio of "(C) the total recovered energy described in the process of synthesizing ammonia only by the catalytic reaction step" to "(D) the total recovered energy described in the process of synthesizing ammonia by the catalytic step and the reaction separation step"
(f) Comparison of the energy required to produce 1 kg of ammonia: The ratio of "(C) the total energy consumed minus the total energy recovered in the process of synthesizing ammonia using only the catalytic reaction process" to "(D) the total energy consumed minus the total energy recovered in the process of synthesizing ammonia using both the catalytic process and the reaction separation process."

The energy consumption in this process was calculated using a process simulator, Aspen Plus Version 11, a product of Aspen Tech.

The conditions for calculating the energy in the processes according to the Examples and Comparative Examples are as follows: (1) reaction conditions in the ammonia synthesis reactor, (2) reaction conditions in the membrane reactor, and (3) ammonia absorption conditions in the ammonia water absorption step.
(1) Reaction conditions of the ammonia synthesis reactor: composition ratio of raw material gas: hydrogen/nitrogen = 3, 400°C
, 5 MPa
(2) Reaction conditions of the membrane reactor: 400°C, 5 MPa (pressure difference between the gas supply side and the permeation side)
(3) Ammonia absorption tower: 1st tower is 40°C, 2nd tower is 0°C (see Figure 11 for the layout of the absorption towers)

In addition, the ammonia synthesis reaction in the ammonia synthesis reactors in the examples and comparative examples was at an equilibrium value determined by the above reaction conditions.

In order to calculate the energy consumption of the process configurations according to the Examples and Comparative Examples, the following reaction rate equation was calculated based on the catalyst activity evaluation data given in Table 1 of Top. Cat., 1944, 1, 233.
(Reaction rate equation)
R=4.56563Exp(-51/RT)×[N ₂ ]/INH ² −2.31978Exp(+7)×Exp(-76.46/RT)×[H ₂ ] ⁻³ [NH ₃ ] ² /INH ²
INH=1+0.0307Exp(-80/RT)×[H ₂ ] ^-1.5 [NH ₃ ]

Table 2 shows a comparison table of the ammonia water synthesized by the process configurations of Examples 6 to 12 and Comparative Example 4.

In Example 6, energy calculations were performed in the process of Figure 7 using an ammonia separation membrane with _NH3 / _N2 and _NH3 / _H2 permeability coefficient ratios of 10 in the membrane reactor. In this case, the ammonia concentration contained in the product gas that permeated the ammonia separation membrane in the membrane reactor was 35%, and the improved hydrogen conversion rate significantly reduced the recycled amount. As a result, the energy consumption required to produce 1 kg of ammonia was reduced by 62%. On the other hand, the recovered energy obtained when producing 1 kg of ammonia decreased by 23%, but the effect of reducing energy consumption was significant, and it was found that this process could reduce the energy required to produce 1 kg of ammonia by approximately 77% compared to Comparative Example 4.

In Example 7, energy calculations were performed using an ammonia separation membrane with NH ₃ /N ₂ and NH ₃ /H ₂ permeability coefficient ratios of 10 in the membrane reactor in the process of Figure 8. The hydrogen conversion rate in the membrane reactor was improved, and the ammonia concentration in the product gas that permeated the ammonia separation membrane was 35%. In this process, raw materials are lost due to purging of unreacted gas in the ammonia absorption step, so the hydrogen and nitrogen raw materials need to be increased compared to Comparative Example 4. However, the effect of not recycling was significant, and the energy consumption required to produce 1 kg of ammonia was reduced by 52% compared to Comparative Example 4. Furthermore, the improved conversion rate increased the amount of reaction heat recoverable in the membrane reactor, and the recovered energy obtained when producing 1 kg of ammonia increased by 53%. As a result, it was found that this process can reduce the energy required to produce 1 kg of ammonia by 94% compared to Comparative Example 4.

In Example 8, energy calculations were performed using an ammonia separation membrane in the membrane reactor in the process of Figure ₇ , with both _NH3 / _N2 and _NH3 /H2 permeability coefficient ratios of 40. In this case, the improved separation performance of the membrane resulted in a higher ammonia concentration and hydrogen conversion rate contained in the product gas that permeated the ammonia separation membrane in the membrane reactor than in Example 6. Therefore, the same effects as in Example 6 can be further obtained, and it was found that in this process the energy obtained or recovered when producing 1 kg of ammonia exceeds the energy consumed to produce 1 kg of ammonia, and that energy is produced along with ammonia in this process.

In Example 9, energy calculations were performed using an ammonia separation membrane in the membrane reactor in the process of Figure ₈ , with both _NH3 / _N2 and _NH3 /H2 permeability coefficient ratios of 40. In this case, the improved separation performance of the membrane resulted in a higher ammonia concentration and hydrogen conversion rate contained in the product gas that permeated the ammonia separation membrane in the membrane reactor than in Example 7. Therefore, since the same effects as in Example 6 can be further obtained, in this process the energy obtained or recovered when producing 1 kg of ammonia exceeds the energy consumed to produce 1 kg of ammonia, and it was found that in this process energy is produced along with ammonia.

In Examples 10, 11, and 12, membranes with even higher separation performance were used, which further enhanced the above-mentioned effects and therefore enabled greater energy savings.

The process of the present invention, which consumes less energy per kg of ammonia, can provide an environmentally friendly ammonia production process with less CO2 emissions. In addition, since there is less unreacted hydrogen and nitrogen, the size of each device can be reduced by reducing the amount of recycled material, making it possible to provide a small-scale on-site process.
In particular, in the ammonia production method including the membrane reactor of the present invention, by using an ammonia separation membrane preferably having an _NH3 / _H2 and/or _NH3 / _N2 permeability coefficient ratio of 20 or more in the membrane reactor, a conversion higher than the equilibrium constraint of the ammonia production reaction can be achieved, the amount of heat recovered by ammonia production can be increased, and further, the energy required to separate ammonia in the ammonia recovery step and the energy required to recycle unreacted raw materials can be reduced, resulting in the effect of being able to produce ammonia with less energy than the energy required for ammonia production using conventional techniques. Furthermore, when the _NH3 / _H2 and/or _NH3 / _N2 ratios are high, it is possible to construct a highly economical process from the viewpoint of raw material loss even if unreacted gas is purged in the ammonia recovery step.
Furthermore, when producing ammonia water, since the pressurization and cooling steps are not required, a sufficiently economical process can be constructed by using an ammonia separation membrane having a NH ₃ /H ₂ and/or NH ₃ /N ₂ permeability coefficient ratio of preferably 10 or more, more preferably both 10 or more.

Claims (11)

A method for producing ammonia using hydrogen and nitrogen as raw materials, comprising:
a catalytic reaction step of synthesizing ammonia from the raw material in an ammonia synthesis reactor using an ammonia synthesis catalyst to obtain a mixed gas of ammonia gas, hydrogen, and nitrogen;
a reaction and separation step in which ammonia gas is synthesized by reacting hydrogen and nitrogen contained in the mixed gas obtained in the catalytic reaction step in a membrane reactor using an ammonia synthesis catalyst and an ammonia separation membrane, and the ammonia gas is selectively separated by the ammonia separation membrane;
an ammonia recovery step of cooling ammonia gas contained in the permeable gas that has permeated the ammonia separation membrane in the reaction separation step, and recovering condensed liquefied ammonia. 2. The method for producing ammonia according to claim 1, wherein in the reactive separation step, the hydrogen conversion rate of the permeable gas that has permeated the ammonia separation membrane at the outlet of the membrane reactor is 77% or more. 3. The method for producing ammonia according to claim 1 , wherein the mixed gas obtained in the catalytic reaction step has an ammonia concentration of 1 vol % or more. The method for producing ammonia according to any one of claims 1 to 3 , further comprising a step of returning the unliquefied gas obtained in the ammonia recovery step to the catalytic reaction step to use the unliquefied gas as a raw material for ammonia synthesized in the catalytic reaction step. The method for producing ammonia according to any one of claims 1 to 4 , further comprising a step of purging the unliquefied gas obtained in the ammonia recovery step out of the system. 6. The method for producing ammonia according to claim 1 , wherein in the reactive separation step, the ammonia concentration in the permeable gas that has permeated the ammonia separation membrane is 50 vol % or more. 7. The method for producing ammonia according to claim 1 , wherein in the reaction and separation step, a pressure on the gas supply side of the ammonia separation membrane is 0.1 MPa or more and 10 MPa or less. 8. The method for producing ammonia according to claim 1 , wherein in the reaction and separation step, a pressure difference between the gas supply side and the permeation side of the ammonia separation membrane of the membrane reactor is 0.1 MPa or more and 10 MPa or less. 9. The method for producing ammonia according to claim 1 , wherein in the reactive separation step, the ammonia separation membrane has an ammonia/hydrogen permeability coefficient ratio of 20 or more. 10. The method for producing ammonia according to claim 1 , wherein in the reactive separation step, the ammonia separation membrane has an ammonia/nitrogen permeability coefficient ratio of 20 or more. The method for producing ammonia according to any one of claims 1 to 10 , wherein in the reaction and separation step, the ammonia separation membrane is a zeolite membrane.