JP7600597B2 - How ammonia is produced - Google Patents

Description

The present invention relates to a method for producing ammonia, which includes a membrane reactor in which an ammonia synthesis catalyst and an ammonia separation membrane that selectively separates ammonia gas from a mixed gas of hydrogen/nitrogen/ammonia 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, and a method for producing ammonia water.

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, it is considered preferable to perform the reaction under high pressure and low temperature conditions. However, in reality, it is necessary to ensure the reaction rate of the catalyst, and high temperatures of 400 to 600°C and high pressures of 10 to 20 MPa are generally required.

Traditionally, the Haber-Bosch process has used catalysts whose main component is iron, but research has been reported on the development of 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 are the raw gases of 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 the equilibrium constraints of the ammonia production reaction, and therefore the cooling efficiency is poor in the ammonia recovery process, resulting in 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 to the ammonia synthesis reactor as raw material gas, and the recycled gas containing the unreacted hydrogen and nitrogen separated from the ammonia by cooling must be heated again to the reaction temperature, consuming further energy.
Therefore, because the ammonia production process consumes a large amount of energy due to the pressurization and cooling, there has been a demand for the development of 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 generated gas by cooling and condensing 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 in which a separation membrane is used to selectively allow hydrogen gas and/or nitrogen gas to permeate from the mixed gas, and 2) a method in which a separation membrane is used 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 generated in a multi-stage system to separate and recover ammonia. On the other hand, in the latter method of using a separation membrane, a method of selectively permeating ammonia gas from a mixed gas of hydrogen gas, nitrogen gas, and ammonia gas by using, for example, a zeolite membrane has been developed (Patent Documents 2 and 3). By utilizing the acidic sites of zeolite and the adsorption due to the basicity of ammonia, it has been found that ammonia can be selectively separated stably even under high temperature conditions.

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

Patent Document 5 proposes a reaction system that produces ammonia more efficiently by using an adsorbent that selectively adsorbs ammonia in a reactor, and by adsorbing the ammonia produced by an ammonia synthesis catalyst onto the adsorbent, the equilibrium reaction is shifted to the production side.

JP 2008-247654 A 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 I, Maruzen Co., Ltd. (2014), p. 641

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

In the process using an ammonia separation membrane as in Patent Document 1, a step of separating ammonia gas from a high-concentration hydrogen mixed gas containing ammonia gas that has permeated through the first-stage separation membrane and a mixed gas of nitrogen gas and ammonia gas that has not permeated through the first-stage separation membrane is essential. In other words, this method is a complicated process in which ammonia gas is separated from a mixed gas of hydrogen gas, nitrogen gas, and ammonia gas in at least two stages, and further requires a step of recovering ammonia from both the mixed gas that has permeated through the first stage and the mixed gas that has not permeated through the first stage, making it a complicated process. In order to essentially solve these problems, 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, but such a process is not productive and is not realistic.

In Patent Document 3, a zeolite membrane capable of selectively separating ammonia is used, 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 permeated 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 the power required can be reduced by developing a new process for synthesizing ammonia from water and nitrogen using electrolysis, but there are issues to be overcome in terms of long-term stability, such as the development of corrosion-resistant structural materials, in order to put this technology into practical use.

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

The present invention has been 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 with less energy.

In order to solve the above problems, the inventors have 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 equilibrium constraints in the reaction, a membrane reactor is used in combination with a separation membrane that selectively separates ammonia from the reaction system, and ammonia is recovered from a gas containing a high concentration of ammonia that has permeated the separation membrane, and the unreacted raw material gas is recycled or purged from the system, thereby providing an ammonia production process with low energy consumption, and have completed the present invention.

In the ammonia production process using the membrane reactor of the present invention, since the ammonia production reaction is an equilibrium reaction, it is possible to suppress the reverse reaction of ammonia by combining an ammonia synthesis catalyst with an ammonia separation membrane that selectively separates ammonia. As a result, ammonia can be obtained at a higher conversion rate than in a normal reaction with equilibrium constraints. In addition, since the ammonia production reaction is an exothermic reaction, the higher the reaction conversion rate, the more heat is generated, and energy can be saved by recovering this heat and using it as energy. Furthermore, since 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, comprising the steps of:
a reaction and separation step of synthesizing ammonia from the raw material by 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;
a recycling step of recycling unreacted raw material gas;
A method for producing ammonia comprising:
[2] A method for producing ammonia using hydrogen and nitrogen as raw materials, comprising the steps of:
a reaction and separation step of synthesizing ammonia from the raw material by 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;
a purging step of purging unreacted raw material gas out of the system;
A method for producing ammonia comprising:
[3] The method for producing ammonia according to [1] or [2], wherein in the reaction and separation step, an ammonia concentration in a 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, a 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 a gas supply side and a permeation side of an ammonia separation membrane of a 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 reaction and 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 reaction and 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 ammonia water using hydrogen and nitrogen as raw materials, comprising the steps of:
a reaction and separation step of synthesizing ammonia from the raw material by a membrane reactor using an ammonia synthesis catalyst and an ammonia separation membrane that selectively separates ammonia from a mixed gas of hydrogen/nitrogen/ammonia;
an ammonia absorption step of supplying water to the permeation 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 ammonia water using hydrogen and nitrogen as raw materials, comprising the steps of:
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 mixed gas of hydrogen/nitrogen/ammonia;
an ammonia absorption step of supplying water to the permeation 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, an ammonia concentration in a 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 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.
[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 in an equilibrium reaction can be obtained, so that the gas that permeates through the membrane in the membrane reactor contains a high concentration of ammonia, making it possible to construct a process that consumes less energy.

Furthermore, the process configuration of the present invention requires a small amount of recycled gas, and the size of equipment such as boosters and reactors can be reduced, making it possible to make the process more compact. In other words, it makes it possible to produce ammonia using a small-scale process that is difficult to achieve using the Haber-Bosch process, which recycles much of the unreacted gas, and it makes it possible to provide an on-site ammonia production process that can produce the required amount at the required location, making it highly 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 a membrane reactor is recycled and unliquefied gas is recycled in an ammonia recovery process. 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 process for producing aqueous ammonia according to a conventional technique. 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. 2 is a flow diagram showing an ammonia absorption step in the ammonia water production process.

One embodiment 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 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 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 embodiment 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 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 ammonia contained in a 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 the outside of the system.

The ammonia synthesis reactor used in the present invention is a reactor equipped with an ammonia synthesis catalyst without using 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, JP 2005-58823 A can be referred to as a selectively permeable membrane reactor, and the selectively permeable membrane reactor is a reactor that has both catalytic action and selective permeability, and is equipped with a catalyst for promoting a chemical reaction and a selectively permeable membrane having selective permeability for a specific component.

In the reaction and separation step in the ammonia production process of the present invention, the ammonia concentration in the separated gas is preferably 1 vol% or more, from the viewpoint that the separation performance of the ammonia separation membrane used in the membrane reactor is such that the higher the ammonia concentration, the easier it is to separate ammonia. Therefore, the membrane reactor in the reaction and separation step is usually configured to have a reaction and separation step after the catalytic reaction step so that the ammonia concentration in the gas separated by the ammonia separation membrane is 1 vol% or more, but for example, a reactor designed to integrate the ammonia synthesis reactor and the membrane reactor, place the ammonia synthesis catalyst upstream of the ammonia separation membrane, and have the ammonia concentration in the gas separated by the membrane be 1 vol% or more may be used.

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 the membrane 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, and 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. Therefore, the amount of heat generated when producing ammonia is greater than in the conventional Haber-Bosch process, and since there is less unreacted gas, it has the advantage of requiring less energy for cooling and compression in the ammonia recovery process described below, as well as less energy for recycling.

(Liquefied ammonia production process)
In the catalytic reaction process, hydrogen and nitrogen as raw materials are reacted 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 the 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 any one of pressure swing adsorption (PSA), temperature swing adsorption (TSA), and pressure temperature swing adsorption (PTSA), which use multiple adsorption towers arranged in parallel to separate and recover nitrogen by varying (swinging) the pressure, temperature, or pressure and temperature, in terms of power and installation costs. Among these, pressure swing adsorption (PSA) is the most suitable. In these separation methods, the nitrogen may contain a small amount of argon from the air, but the concentration of argon can be avoided by purging in the ammonia production process, etc.

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.

It is preferable to use hydrogen as a raw material in a hydrogen-containing gas having 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 particularly preferably 99.5 to 100 mol % hydrogen.

The gas contained other than hydrogen may be any gas that is inert to the reaction and does not affect the catalytic activity, such as argon, carbon dioxide, methane, etc. 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 can be carried out under conditions such as temperature, pressure, catalyst amount, space velocity, linear velocity, and raw 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 of the ammonia synthesis reactor are preferably high pressure conditions in order to ensure the reaction rate, and are preferably 0.5 MPa or more, more preferably 1 MPa or more, even more preferably 2 MPa or more, particularly preferably 3 MPa or more, especially preferably 4 MPa or more, and most preferably 5 MPa or more. On the other hand, since increasing the pressure requires pressure increase energy, which 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, especially 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 a gas with a high ammonia concentration to be obtained by the ammonia production reaction. On the other hand, in order to increase the reaction rate, a higher temperature condition is better, and a temperature of 100° C. or higher is preferable, 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 in the ammonia separation membrane in the membrane reactor of the reaction separation process arranged in the latter stage, separation of ammonia and hydrogen is usually more difficult than separation of ammonia and nitrogen, so that the ammonia concentration in the gas that has permeated the ammonia separation membrane is higher in a gas with a low hydrogen concentration. 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, it 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. (Ooka's memo: Added a lower limit)

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 supporting Ru, but a catalyst containing Ru is preferred.

The ammonia synthesis reactor may be combined upstream of the membrane reactor, and may be one 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 in the outlet gas of the ammonia synthesis reactor 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, 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 during the production of ammonia in the ammonia synthesis reactor can be removed by installing a heat recovery system inside the ammonia synthesis reactor, or it can be recovered 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 reaction rate of the catalyst, the pressure conditions on the gas supply side of the ammonia separation membrane in the membrane reactor are usually preferably high, 0.5 MPa or more is preferable, 1 MPa or more is more preferable, 2 MPa or more is even more preferable, 3 MPa or more is particularly preferable, 4 MPa or more is especially preferable, and 5 MPa or more is most preferable. On the other hand, to increase the pressure, it is necessary to increase the pressure using a booster, which consumes energy. Therefore, 20 MPa or less is usually preferable, 15 MPa or less is more preferable, 12 MPa or less is even more preferable, 10 MPa or less is especially preferable, 8 MPa or less is especially preferable, and 6 MPa or less is most preferable.

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

The pressure applied to the permeation side of the ammonia separation membrane of 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, the higher the pressure applied to the permeation side of the ammonia separation membrane, the easier it is to condense ammonia by cooling, so that 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 less, more preferably 500°C or less, particularly preferably 450°C or less, and most preferably 400°C or less, because a lower temperature can reduce energy consumption and produce gas with a high ammonia concentration. On the other hand, higher temperatures are better for accelerating the reaction rate, and a temperature of 100°C or more is preferable, more preferably 200°C or more, even more preferably 300°C or more, and particularly preferably 350°C or more.

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 catalyst carrying Ru, but a catalyst containing Ru is preferred.

The ammonia separation membrane used in the membrane reactor may be any membrane capable of separating ammonia, such as a synthetic resin separation membrane, a carbon separation membrane, a porous silica membrane, a carbon membrane, etc. The membrane reactor is preferably a zeolite membrane, which is heat-resistant since the membrane is used under high-temperature conditions, and which is capable of selectively separating ammonia from a mixed gas of hydrogen/nitrogen/ammonia. For example, the zeolite membrane described in Patent Document 3 is an example.
The zeolite membrane contains zeolite of aluminosilicate. The zeolite constituting the zeolite membrane is preferably an aluminosilicate. The aluminosilicate is mainly composed of oxides of Si and Al, but may contain other elements. The SiO ₂ /Al ₂ O ₃ 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 the amount of Al as an impurity, and the SiO ₂ /Al ₂ O ₃ 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, determined by X-ray photoelectron spectroscopy under the following measurement conditions, of 0.01 to 4. The molar ratio of Si atoms to Al atoms is preferably 2.0 to 10. 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 radiation, 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 a zeolite membrane can separate ammonia gas more selectively and efficiently than existing silica membranes, the rates of change in thermal shrinkage at 200°C and 300°C relative to 30°C change 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 separation performance of ammonia gas decreases, particularly in the temperature range exceeding 200°C, leaving room for improvement. This is presumably because the thermal contraction of the zeolite causes cracks to form in the zeolite grain boundaries, allowing gas to permeate through the cracks. However, even in the case of RHO-type zeolite, which has a rate of change in thermal contraction rate from 30°C to 200°C of 1.55%, which is significantly higher than CHA-type zeolite and shows nonlinear thermal expansion/contraction behavior with temperature, if the rate of change in thermal expansion rate at 300°C is around 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 of the thermal expansion coefficient of the zeolite membrane at 300°C relative to the thermal expansion coefficient at 30°C is within ±0.25%, and the rate of change of the thermal expansion coefficient at 400°C relative to the thermal expansion coefficient at 30°C is within ±0.35%. It is even more preferable that the rate of change of the thermal expansion coefficient at 300°C relative to the thermal expansion coefficient at 30°C relative to the thermal expansion coefficient at 400°C relative to the thermal expansion coefficient at 30°C is within ±120%.

As for the separation performance of the ammonia separation membrane, the higher the ammonia/nitrogen or ammonia/hydrogen permeability coefficient ratio, the less likely unreacted hydrogen or nitrogen will permeate the membrane, which is preferable. 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 the better the separation performance, the higher the concentration of ammonia will be obtained, so no 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 the better the separation performance, the higher the concentration of ammonia will be obtained, so no upper limit is set.
In addition, 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 the permeability coefficient ratio of ammonia/nitrogen is generally higher than the permeability coefficient ratio of ammonia/hydrogen.

In order for the ammonia separation membrane used in the membrane reactor to exhibit the ability to separate ammonia, 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, in order to increase the ammonia concentration, it is necessary to increase the ammonia synthesis catalyst, which reduces the economic efficiency due to the catalyst cost. 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 more, more preferably 50 vol% or more, even more preferably 60 vol% or more, particularly preferably 65 vol% or more, especially preferably 70 vol% or more, and most preferably 80 vol% or more. 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 may be 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 to the outside of the system or recycled to the inlet of the membrane reactor. In addition, the non-permeated gas may be combined with the gas that has permeated the membrane of the membrane reactor to be used as the product gas.
In the process of recycling the non-permeated side of a membrane reactor 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 part with poor gas permeability, or may be merged 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 multiple types.
The membrane reactor may be a single stage or a multi-stage reactor. 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 separation step is carried out after the catalytic reaction step, but this catalytic reaction step and reaction separation step may be repeated multiple times before the ammonia recovery step 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 high-concentration 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, and the more energy required for cooling can be reduced. Therefore, the ammonia concentration contained in the gas before cooling in the recovery process is preferably 50 vol% or more, more preferably 60 vol% or more, even more preferably 65 vol% or more, particularly preferably 70 vol% or more, and most preferably 80 vol% or more. And it is most preferable that the ammonia concentration is 100 vol%.

In the ammonia recovery process, the gas generated in the membrane reactor is pressurized. The pressure at that time is determined by the recovery rate of liquefied ammonia and the energy for pressurization. In other words, if the pressure is lowered to suppress the energy for pressurization, the recovery rate decreases and the purge amount (recycle amount in the recycle process) increases, resulting in poor economic efficiency. Therefore, usually, 1 MPa or more is preferable, 2 MPa or more is more preferable, 3 MPa or more is even more preferable, 4 MPa or more is particularly preferable, and 5 MPa or more is most preferable. On the other hand, if the pressure is increased to increase the recovery rate of ammonia, the energy for pressurization increases. Therefore, usually, 20 MPa or less is preferable, 15 MPa or less is more preferable, 10 MPa or less is even more preferable, 8 MPa or less is particularly preferable, and 6 MPa or less is most preferable.
As for the temperature, appropriate conditions may be determined in consideration of the ammonia recovery rate and cooling energy in the same manner as described above, but generally, the temperature is 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, the unliquefied gas, i.e., the gas containing the unreacted raw material gas, may be recycled and used as the raw material for the ammonia synthesis reactor. When recycling, a portion of the gas may be purged from the system to prevent the concentration of impurities. When 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 and 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 synthesis catalyst and an ammonia separation membrane that selectively separates ammonia from the mixed gas; an ammonia absorption step of supplying water to a permeable gas that has permeated through the ammonia separation membrane to produce ammonia water, and recovering the ammonia water; and a recycling step of recycling unreacted raw material gas.
Further, still another embodiment of the present invention is 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 and 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 ammonia; an ammonia absorption step of supplying water to a permeable gas that has permeated through 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.

5) Ammonia Absorption Step In the method for producing ammonia water, 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. Usually, 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 an absorption tower that absorbs 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, it is preferably 60°C or lower, more preferably 50°C or lower, even more preferably 40°C or lower, particularly preferably 30°C or lower, especially preferably 25°C or lower, and most preferably 20°C or lower. On the other hand, since cooling energy is required to lower the temperature, it is preferably -20°C or higher, more preferably -15°C or higher, even more preferably -10°C or higher, especially preferably -5°C or higher, and most preferably 0°C or higher.

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.
The lower the ammonia concentration in the unrecovered gas in the absorption tower, the greater the amount of ammonia 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 the efficiency of the process.
In another embodiment, the method includes a purging step in which the 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 suppress the concentration of impurities.

Unrecovered gas that contains water due to contact with water in the absorption tower can be recycled as is, or if water will have an effect on subsequent processes, a dehydration process can be provided to remove water from the recycled gas before returning it to the catalytic reaction process. The dehydration process can be performed using a known method, and if water is dehydrated using an adsorbent, the adsorbent is not particularly limited 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 non-permeated gas in the membrane reactor in the reaction separation process and purging in the recycle process when recycling unliquefied gas in the ammonia recovery process are performed 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 raw material impurity concentration. Therefore, in this calculation, it was assumed that there were no impurities in the raw material, and the energy calculation was performed without performing the purging required to remove impurities.

<Examples 1 to 5, Comparative Examples 1 to 3>
Figures 2 and 3 show typical process configurations in 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 in which an ammonia synthesis catalyst is placed, and a reaction separation process having a membrane reactor in which an ammonia synthesis catalyst and an ammonia separation membrane are placed. 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 and introduced into the ammonia synthesis process.
In FIG. 3, the unliquefied gas containing unreacted hydrogen and nitrogen in FIG. 2 is purged rather than recycled.
FIG. 4 shows an application example of FIG. 2 in which the gas from 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 from 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 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 gas (40°C → 400°C), pressurization (0.1MPa → 5MPa), pressurization of gas that has permeated the membrane of the membrane reactor (0.1MPa → 5MPa), 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),

The (i) hydrogen/nitrogen amount ratio and (ii) recycled amount ratio in Table 1 were calculated as follows.
(i) Hydrogen/nitrogen amount ratio: 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) Recycle amount ratio: 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 energy consumption required to produce 1 kg of ammonia
The ratio of "(A) the total energy consumption described in the process for synthesizing ammonia only in the catalytic reaction process" to "(B) the total energy consumption described in the process for synthesizing ammonia in the catalytic reaction process and the reaction separation process" (b) Comparison of the energy obtained or recovered when producing 1 kg of ammonia
(c) Comparison of the energy required to produce 1 kg of ammonia: The ratio of "(B) the total energy recovered described in the process for synthesizing ammonia in a catalytic reaction step and a reaction separation step" to "(A) the total energy recovered described in the process for synthesizing ammonia in only a catalytic reaction step" to "(A) the total energy consumed minus the total recovered energy described in the process for synthesizing ammonia in only a catalytic reaction step" to "(B) the total energy consumed minus the total recovered energy described in the process for synthesizing ammonia in a catalytic reaction step and a reaction separation step"

The energy calculations for this process were performed using the process simulator Aspen
The calculation was performed using Aspen Plus Version. 11, a product of Aspen Tech.

As conditions for calculating the energy in the processes of the Examples and/or Comparative Examples, (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 are as follows.
(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.

The ammonia synthesis reaction in the ammonia synthesis reactors in the examples and comparative examples was set to the equilibrium value of the reaction 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 described 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 in the processes of the examples and comparative examples.

In Example 1, the energy calculation was performed using an ammonia separation membrane with a permeability coefficient ratio of 40 for both NH ₃ /N ₂ and NH ₃ /H ₂ in the membrane reactor in the process of FIG. 2. In this case, the ammonia concentration contained in the generated gas that permeated the ammonia separation membrane in the membrane reactor was 62%, and the amount of recycled gas was greatly reduced due to the improvement in hydrogen conversion rate. As a result, the energy consumption required to produce 1 kg of ammonia was reduced by 37% compared to Comparative Example 1, but on the other hand, the amount of recovered energy obtained when producing 1 kg of ammonia was reduced by 40% due to the effect of reducing the amount of gas circulated by recycling and reducing the amount of heat recovered by cooling the outlet gas of the membrane reactor. However, the effect of reducing energy consumption was large, and it was found that the energy required to produce 1 kg of ammonia in this process can be reduced by about 10% compared to Comparative Example 1.

In Example 2, energy calculation was performed using an ammonia separation membrane with a permeability coefficient ratio of 40 for both NH ₃ /N ₂ and NH ₃ /H ₂ in the membrane reactor in the process of FIG. 3. The hydrogen conversion rate in the membrane reactor was improved, and the ammonia concentration in the generated gas permeated through the ammonia separation membrane was 62%. In this process, the raw material is 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 is large, 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 is no gas to circulate due to recycling, the amount of heat recovered by cooling in the membrane reactor outlet gas is 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 is large, and it was found that this process is a process in which the energy required to produce 1 kg of ammonia can be reduced 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 ammonia concentration and hydrogen conversion rate contained in the product gas that permeated the ammonia separation membrane in the membrane reactor were higher than those in Example 1 due to the improved separation performance of the membrane. Therefore, it was found that the same effects as in Example 1 could be obtained, and therefore the energy required to produce 1 kg of ammonia in this process can be reduced by 90% compared to Comparative Example 1.

In Example 4, energy calculations were performed using an ammonia separation membrane with _NH3 / _N2 and _NH3 / _H2 permeability coefficient ratios of 100 in the membrane reactor in the process of Figure 3. In this case, too, the ammonia concentration and hydrogen conversion rate contained in the product gas that permeated the ammonia separation membrane in the membrane reactor were further increased due to the improved separation performance of the membrane. Since the same effects as in Example 2 can be further obtained, it was found that the energy required to produce 1 kg of ammonia in this process can be reduced by 95% compared to Comparative Example 1.

In Example 5, energy calculation was performed using an ammonia separation membrane with a permeability coefficient ratio of 10,000 for both NH ₃ /N ₂ and NH ₃ /H ₂ in the membrane reactor in the process of Figure 3. In this case, the ammonia concentration contained in the product gas that permeated the ammonia separation membrane in the membrane reactor was 99.5%, which means that it contained almost no unreacted nitrogen or hydrogen, and there was no need to recycle. In addition, the reduction effect of the energy consumption required to produce 1 kg of ammonia was large, and the energy obtained or recovered when producing 1 kg of ammonia exceeded the energy consumption required to produce 1 kg of ammonia, and it was found that energy was generated 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, but the pressure was reduced by permeating through the membrane, so energy was required to increase the pressure again, resulting in an increase in energy. As a result, the energy required to produce 1 kg of ammonia increased by about 18 times compared to Comparative Example 1, and it was found that this process consumed a large amount of energy.

(Ammonia water production process)
In the method for producing ammonia using a membrane reactor of the present invention, when liquefied ammonia is produced, the pressure is reduced by permeating through the membrane in the membrane reactor, so the product gas at the outlet of the membrane reactor is pressurized again 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 pressurizing, 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, compared to when producing liquefied ammonia.

<Examples 6 to 12, Comparative Example 4>
Fig. 6 shows an ammonia water production process of a comparative example, and Figs. 7 and 8 show a typical process configuration of the present invention. In Fig. 7, ammonia is synthesized using hydrogen and nitrogen as raw materials in a catalytic reaction process having an ammonia synthesis reactor in which an ammonia synthesis catalyst is arranged, and a reaction separation process having a membrane reactor in which an ammonia synthesis catalyst and an ammonia separation membrane are arranged. In the specification, the catalytic reaction synthesis 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 has permeated the membrane is then absorbed into water in an ammonia absorption tower in an ammonia absorption process and recovered as ammonia water, and the unliquefied raw material 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 from 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 from the non-permeation side of the membrane reactor in the reaction separation step is recycled to the membrane reactor.

Purging of the non-permeated gas in the membrane reactor in the reaction separation process and purging in the recycle process when recycling unliquefied gas in the ammonia absorption process are performed 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 raw material impurity concentration. Therefore, in this calculation, it was assumed that there were no impurities in the raw material, and the calculation was performed without performing 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 gas (40°C → 400°C), pressurization (0.1MPa → 5MPa), freezing in the ammonia absorption tower (40°C → 0°C), heating of recycled gas (0°C → 40°C) and pressurization (0.1MPa → 5MPa) 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) Processes for synthesizing ammonia through a catalytic process and a reaction separation process (processes of Examples 6 to 12):
Energy consumption: Heating the raw gas (40°C → 400°C), pressurizing (0.1MPa → 5MPa), freezing the second tower of the ammonia absorption tower (40°C → 0°C), heating the recycled gas (0°C → 40°C) and pressurizing (0.1MPa → 5MPa) only when recycling
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)

The (iii) hydrogen/nitrogen amount ratio and (iv) recycled amount ratio in Table 2 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 recycled amount in the process of synthesizing ammonia in the membrane catalytic 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 2 were calculated as follows:
(d) Comparison of the energy consumption required to produce 1 kg of ammonia
The ratio of "(D) the total energy consumption described in the process for synthesizing ammonia in a catalytic reaction step and a reaction separation step" to "(C) the total energy consumption described in the process for synthesizing ammonia in only a catalytic reaction step"
(e) Comparison of the energy recovery obtained when producing 1 kg of ammonia
The ratio of "(D) the total energy recovery described in the process for synthesizing ammonia in a catalytic process and a reaction separation process" to "(C) the total energy recovery described in the process for synthesizing ammonia in only a catalytic reaction process"
(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 for synthesizing ammonia only through the catalytic reaction process" to "(D) the total energy consumed minus the total energy recovered in the process for synthesizing ammonia through 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 are as follows.
(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 at 40°C, 2nd tower at 0°C (see Figure 11 for the layout of the absorption towers)

The ammonia synthesis reaction in the ammonia synthesis reactors in the examples and comparative examples was set to 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 described 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 regarding the ammonia water synthesized by the process configurations according to Examples 6 to 12 and Comparative Example 4.

In Example 6, energy calculation was performed using an ammonia separation membrane with a permeability coefficient ratio of 10 for both NH ₃ /N ₂ and NH ₃ /H ₂ in the membrane reactor in the process of FIG. 7. In this case, the ammonia concentration contained in the generated gas that permeated the ammonia separation membrane in the membrane reactor was 35%, and the amount of recycled material was greatly reduced due to the improvement in hydrogen conversion rate. 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 was reduced by 23%, but the effect of reducing energy consumption was large, and it was found that the energy required to produce 1 kg of ammonia in this process can be reduced by about 77% compared to Comparative Example 4.

In Example 7, energy calculation was performed using an ammonia separation membrane with a permeability coefficient ratio of 10 for both NH ₃ /N ₂ and NH ₃ /H ₂ in the membrane reactor in the process of FIG. 8. The hydrogen conversion rate in the membrane reactor was improved, and the ammonia concentration in the generated gas permeated through the ammonia separation membrane was 35%. In this process, the raw material is lost due to purging of unreacted gas in the ammonia absorption step, so the amount of hydrogen and nitrogen raw material needs to be increased compared to Comparative Example 4. However, the effect of not recycling is large, and the energy consumption required to produce 1 kg of ammonia was reduced by 52% compared to Comparative Example 4. Furthermore, the improvement in 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 is a process in which the energy required to produce 1 kg of ammonia can be reduced by 94% compared to Comparative Example 4.

In Example 8, 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 7. In this case, the ammonia concentration and hydrogen conversion rate contained in the product gas that permeated the ammonia separation membrane in the membrane reactor were higher than those in Example 6 due to the improved separation performance of the membrane. 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 consumption required to produce 1 kg of ammonia, and it was found that energy is produced along with ammonia in this process.

In Example 9, 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 8. In this case, the ammonia concentration and hydrogen conversion rate contained in the product gas that permeated the ammonia separation membrane in the membrane reactor were higher than those in Example 7 due to the improved separation performance of the membrane. 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 consumption required to produce 1 kg of ammonia, and it was found that energy is produced along with ammonia in this process.

In Examples 10, 11, and 12, membranes with even higher separation performance were used, which allowed the above-mentioned effects to be further achieved, and therefore further energy savings could be achieved.

The process according to the present invention consumes less energy per kg of ammonia, making it possible to provide an environmentally friendly ammonia production process with less _CO2 emissions. In addition, because there is less unreacted hydrogen and nitrogen, it is possible to reduce the size of each device by reducing the amount of recycling, making it possible to provide it as an on-site small-scale process.
In particular, in the ammonia production method including the membrane reactor of the present invention, by using an ammonia separation membrane having a permeability coefficient ratio of _NH3 / _H2 and/or _NH3 / _N2 of 20 or more in the membrane reactor, a conversion rate higher than the equilibrium constraint of the ammonia production reaction is achieved, the amount of heat generated by ammonia production is increased, and further, the energy required for separating ammonia in the ammonia recovery process and the energy required for recycling unreacted raw materials are reduced, and ammonia can be produced with less energy than the energy required for ammonia production in the conventional technology. In addition, when _NH3 / _H2 and/or _NH3 / _N2 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 process.
Furthermore, when producing ammonia water, since the steps of pressurization and cooling are not required, a sufficiently economical process can be constructed by using an ammonia separation membrane having a _NH3 / _H2 and/or _NH3 / _N2 permeability coefficient ratio of preferably 10 or more, more preferably both of 10 or more.

Claims (5)

A method for producing ammonia using hydrogen and nitrogen as raw materials, comprising the steps of:
a reaction and separation step of synthesizing ammonia from the raw material by 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;
a recycling step of recycling unreacted raw material gas;
Equipped with
the ammonia separation membrane is an RHO type zeolite membrane, and the zeolite constituting the RHO type zeolite membrane is an aluminosilicate;
In the reaction and separation step,
The temperature of the membrane reactor is 300° C. or more and 600° C. or less,
The pressure difference between the supply side and the permeation side of the ammonia separation membrane is 1 MPa or more and 15 MPa or less, and
The ammonia separation membrane has an ammonia/hydrogen permeability coefficient ratio of 40 or more and an ammonia/nitrogen permeability coefficient ratio of 40 or more at 400° C. and the pressure difference of 5 MPa. A method for producing ammonia using hydrogen and nitrogen as raw materials, comprising the steps of:
a reaction and separation step of synthesizing ammonia from the raw material by 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;
a purging step of purging unreacted raw material gas out of the system;
Equipped with
the ammonia separation membrane is an RHO type zeolite membrane, and the zeolite constituting the RHO type zeolite membrane is an aluminosilicate;

In the reaction and separation step,
The temperature of the membrane reactor is 300° C. or more and 600° C. or less,
The pressure difference between the supply side and the permeation side of the ammonia separation membrane is 1 MPa or more and 15 MPa or less, and
The ammonia separation membrane has an ammonia/hydrogen permeability coefficient ratio of 40 or more at 400° C. and 5 MPa (pressure difference between the gas supply side and the permeation side) and an ammonia/nitrogen permeability coefficient ratio of 40 or more.
A method for producing ammonia. The method for producing ammonia according to claim 1 or 2, wherein the ammonia concentration in the permeation gas that has permeated the ammonia separation membrane in the reaction and separation process is 50 vol% or more. The method for producing ammonia according to any one of claims 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. The method for producing ammonia according to any one of claims 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.

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