What Is Ammonia Synthesis?

Ammonia synthesis refers to the process of synthesizing ammonia raw material gas (nitrogen-hydrogen mixed gas) directly into ammonia in the presence of high temperature, high pressure and catalyst. The reaction is a reversible reaction that exotherms and shrinks the volume. The temperature and pressure have an influence on the chemical equilibrium of the reaction.

Chinese name: Ammonia synthesis
Foreign name: ammonia synthesis

Definition: Process for synthesizing ammonia from nitrogen-hydrogen mixed gas
Separation and recovery method: Condensation and absorption

Ammonia Synthesis Overview

Definition of ammonia synthesis

Ammonia synthesis: The English name (ammonia synthesis) refers to the process of synthesizing ammonia raw material gas (nitrogen-hydrogen mixed gas) directly into ammonia in the presence of high temperature, high pressure and catalyst.

The reaction equation is: N ₂ (g) + 3H ₂ (g) 2NH ₃ (g)

Ammonia synthesis process

The ammonia synthesis reaction is a reversible reaction that exotherms and shrinks the volume. Temperature and pressure have an influence on the chemical equilibrium of the reaction. When the molar ratio of hydrogen to nitrogen in the mixed gas is 3, the equilibrium ammonia concentration increases with decreasing temperature and increasing pressure. However, at lower temperatures, the reaction rate of ammonia synthesis is very slow, and a catalyst is required to accelerate the reaction. Due to the limitation of the activity of the catalyst used, the temperature cannot be too low. Therefore, in order to increase the ammonia content in the gas after the reaction, ammonia synthesis should be performed under high pressure. When iron catalysts are used industrially, the pressure is mostly selected from 15.2 to 30.4 MPa (150 to 300 atm). Even when operating under such pressure conditions, only a part of the nitrogen and hydrogen reacts to ammonia each time, so the ammonia concentration in the outlet gas of the ammonia synthesis tower Usually 10% to 20% (volume). The main factors determining the reaction are the activity of the iron catalyst, the separation of ammonia, nitrogen, and hydrogen produced by the reaction, and the recycling of nitrogen and hydrogen.

Catalyst for ammonia synthesis reaction

Iron catalyst activity Ammonia synthesis uses an iron catalyst with a cocatalyst added. The components of the co-catalyst include potassium oxide, aluminum oxide, magnesium oxide, calcium oxide, and cobalt oxide. The main component of the iron catalyst before the start of the operation is ferric tetroxide (see metal oxide catalysts), which has no catalytic effect on the ammonia synthesis reaction. Before being put into use, it must be reduced to hydrogen with hydrogen to be active. The catalytic activities obtained under different reducing conditions are very different. During normal production, iron catalysts often reduce their activity due to the small amount of sulfur compounds, carbon oxides, and other gases in the nitrogen-hydrogen mixed gas. Generally, it is required that carbon monoxide and carbon dioxide not exceed 10 ppm (volume). The life of the iron catalyst (see catalyst life) is closely related to its manufacturing quality and use conditions. The short is 1 to 2 years, and the long is 8 to 9 years.

Separation of ammonia to ammonia

The single-pass conversion of ammonia synthesis is not high. In order to obtain the product ammonia, the ammonia gas needs to be separated from the outlet gas of the ammonia synthesis reactor. There are two methods commonly used in industry ^[1] :

Condensation method Utilizing the characteristic that the critical temperature of ammonia is higher than that of nitrogen and hydrogen, only the ammonia-containing mixed gas needs to be cooled, and the ammonia in it can be condensed from a gaseous state to a liquid state. The lower the temperature, the more ammonia is condensed. Industrial production uses the product liquid ammonia as a refrigerant. In order to save the amount of refrigerant, the mixed gas is first cooled with water.

Absorption method Uses the property that ammonia gas has greater solubility in water than nitrogen and hydrogen, and absorbs it with water under high pressure to make concentrated ammonia water. The production of liquid ammonia from concentrated ammonia water still needs to go through steps such as ammonia water rectification and ammonia gas condensation. It consumes a lot of heat energy and has been rarely used in industry.

Ammonia synthesis cycle

In order to enable the nitrogen and hydrogen in the outlet gas of the ammonia synthesis reactor to continue to be recycled after the ammonia gas is separated, a circulation process is adopted in order to return to the ammonia synthesis reactor. The guarantee conditions of this law are:

Continuously replenish fresh nitrogen-hydrogen mixed gas into the circulation loop;

Separate ammonia from the outlet gas of the ammonia synthesis reactor;

Set up a circulating gas compressor to compensate the gas pressure loss of the circulating circuit;

Recycling the heat of ammonia synthesis reaction;

In order to avoid a small amount of methane, argon and other inert gases in the fresh nitrogen-hydrogen mixed gas from accumulating in the circuit, an appropriate amount of circulating gas must be discharged.

To this end, a variety of loop processes are designed. In the early ammonia synthesis processes, reciprocating cycle compressors were often used, and the reaction heat was not fully utilized. The gas at the outlet of the reactor is cooled by water, and a part of ammonia is condensed before entering the ammonia separator. In order to reduce the content of methane and argon in the circulating gas, the gas after the ammonia separator is vented in small quantities, and most of it enters the circulating gas compressor. After the pressure is added, it enters the oil filter.

The reactor outlet gas is exchanged with the circulating gas after the liquid ammonia is separated through the upper heat exchanger inside the cold exchanger, and then cooled to below 0 ° C in the ammonia condenser, so that most of the ammonia condenses and returns to the lower part Separator where liquid ammonia is separated. The gas after separating ammonia goes to the upper heat exchanger, is heated to 20-30 ° C and enters the ammonia synthesis tower, thereby completing a cycle.

Ammonia synthesis modern ammonia synthesis process

The modern ammonia synthesis process uses a centrifugal cycle compressor. The reactor outlet gas first passes through the boiler feed water preheater, and recovers a part of the heat energy before passing through the heat exchanger to heat the reactor inlet gas to 130-140 ° C. Then pass the water cooler, cold exchanger, the first and second ammonia condensers to cool below 0 ° C, and most of the ammonia condenses down. After the liquid ammonia is separated in the ammonia separator, the circulating gas enters the compressor through the cold exchanger, is mixed with the fresh gas, is then heat-exchanged, and finally enters the ammonia synthesis reactor. This cycle operation is carried out to carry out production. In order to recover the hydrogen released from the circulating gas, in recent years, three methods of membrane separation, pressure swing adsorption and cryogenic separation have been developed in industry ^[2] , and some ammonia plants have already adopted it.