About Pengfei

About Pengfei

Synthetic ammonia


Introduction

Synthetic ammonia refers to ammonia directly synthesized from nitrogen and hydrogen in the presence of high temperature, high pressure and catalyst, which is a basic inorganic chemical process. In modern chemical industry, ammonia is the main raw material of chemical fertilizer industry and basic organic chemical industry.

The synthetic ammonia industry was formed in the early 20th century and began to use ammonia as a raw material for the explosive industry to serve the war. After the end of the First World War, it turned to serve agriculture and industry. With the development of science and technology, the demand for ammonia is increasing.

catalytic mechanism

Thermodynamic calculations show that low temperature and high pressure are beneficial to the reaction of synthetic ammonia, but without catalyst, the activation energy of the reaction is very high and the reaction hardly occurs. When an iron catalyst is used, the reaction proceeds at a significant rate due to a change in the reaction history, lowering the activation energy of the reaction. The mechanism of ammonia synthesis reaction, the first is the chemical adsorption of nitrogen molecules on the surface of the iron catalyst, so that the chemical bond between nitrogen atoms is weakened. Then, the chemical adsorption of hydrogen atoms continuously interacts with nitrogen molecules on the surface, gradually generating-NH,-NH2 and NH3 on the surface of the catalyst, and finally ammonia molecules are desorbed on the surface to generate gaseous ammonia. The above reaction pathway can be simply expressed:

xFe N2→FexN

FexN [H]吸→FexNH

FexNH [H]吸→FexNH2

FexNH2 [H] FexNH3xFe NH3

In the absence of catalyst, the activation energy of the ammonia synthesis reaction is very high, about 335kJ/mol. After addition of the iron catalyst, the reaction proceeds in two stages to form nitrides and nitrogen hydrides. The activation energy of the first stage is 126kJ/mol ~ 167kJ/mol, and the activation energy of the second stage is 13kJ/mol. Due to the change of the reaction pathway (the formation of unstable intermediate compounds), the activation energy of the reaction is reduced, so the reaction rate is accelerated.

The catalytic ability of a catalyst is generally referred to as catalytic activity. Some people think that because the chemical properties and quality of the catalyst before and after the reaction are unchanged, once a batch of catalysts are made, they can be used continuously. In fact, during the use of many catalysts, their activity gradually reaches the normal level from small to large, which is the maturity period of the catalyst. Subsequently, the catalyst activity remains stable for a period of time and then declines again until it ages and can no longer be used. The time that the activity remains stable is the life of the catalyst, and its length varies with the preparation method and the conditions of use of the catalyst.

During the stable activity of the catalyst, the activity is often significantly decreased or even destroyed due to contact with a small amount of impurities. This phenomenon is called catalyst poisoning. It is generally believed that the active centers on the surface of the catalyst are occupied by impurities and cause poisoning. Poisoning is divided into temporary poisoning and permanent poisoning two kinds. For example, for an iron catalyst in the ammonia synthesis reaction, O2, CO, CO2, water vapor, and the like can poison the catalyst. However, when the pure hydrogen and nitrogen mixed gas passes through the poisoned catalyst, the activity of the catalyst can be restored, so this poisoning is temporary poisoning. In contrast, compounds containing P, S, and As can permanently poison the iron catalyst. After the catalyst is poisoned, it often loses its activity completely. At this time, even if it is treated with pure hydrogen and nitrogen mixed gas, the activity is difficult to recover. Catalyst poisoning will seriously affect the normal production. In industry, in order to prevent catalyst poisoning, the reactant raw materials should be purified to remove the poison, thus increasing the equipment and increasing the cost. Therefore, the development of a new type of catalyst with strong anti-toxic ability is an important issue.

Process flow

(1) Raw material gas preparation Raw materials such as coal and natural gas are made into crude raw material gas containing hydrogen and nitrogen. For solid raw materials coal and coke, gasification is usually used to produce synthesis gas; residual oil can be used to obtain synthesis gas by non-catalytic partial oxidation; for gaseous hydrocarbons and naphtha, two-stage steam conversion is used to produce synthesis gas in industry.

Ammonia plant internal structure

(2) Purification The crude feed gas is purified to remove impurities other than hydrogen and nitrogen, mainly including the conversion process, desulfurization and decarbonization process and gas refining process.

① Carbon monoxide conversion process In the production of synthetic ammonia, the raw material gas prepared by various methods contains CO, and its volume fraction is generally 120. The two components required for synthetic ammonia are H2 and N2, so it is necessary to remove CO from the synthesis gas. The shift reaction is as follows: CO H2O → H2 CO2 ΔH =-41.2kJ/mol
Since the CO conversion process is a strong exothermic process, it must be carried out in stages to facilitate the recovery of reaction heat and control the residual CO content at the outlet of the conversion section. The first step is high-temperature conversion, which converts most of the CO into CO2 and H2; the second step is low-temperature conversion, which reduces the CO content to 0.3. Therefore, the CO shift reaction is not only the continuation of raw gas production, but also the process of purification, creating conditions for the subsequent decarburization process.

② The crude feed gas produced from various raw materials in the desulfurization and decarbonization process contains some sulfur and carbon oxides. In order to prevent the catalyst from poisoning in the ammonia production process, it must be removed before the ammonia synthesis process. The steam conversion method using natural gas as raw material, the first process is desulfurization to protect the conversion catalyst, and the partial oxidation method using heavy oil and coal as raw materials, the location of desulfurization is determined depending on whether a sulfur tolerant catalyst is used for the carbon monoxide shift. There are many types of industrial desulfurization methods, usually using physical or chemical absorption methods, commonly used are low-temperature methanol washing method (Rectisol), polyethylene glycol dimethyl ether method (Selexol) and so on.
After the crude feed gas is converted by CO, in addition to H2, there are CO2, CO and CH4 components in the converted gas, of which CO2 content is the most. CO2 is not only a poison of ammonia synthesis catalyst, but also an important raw material for the manufacture of urea, ammonium bicarbonate and other nitrogen fertilizers. Therefore, the removal of CO2 in the shift gas needs to take into account these two requirements.
Generally, CO2 is removed by solution absorption method. According to the performance of the absorbent, it can be divided into two categories. One is the physical absorption method, such as low temperature methanol washing method (Rectisol), polyethylene glycol dimethyl ether method (Selexol), propylene carbonate method. One is the chemical absorption method, such as hot potash method, low heat consumption Benfield method, activated MDEA method, MEA method, etc.

③ The raw gas after CO conversion and CO2 removal in the gas refining process still contains a small amount of residual CO and CO2. In order to prevent poisoning of the ammonia synthesis catalyst, it is stipulated that the total content of CO and CO2 should not exceed 10 cm3/m3 (volume fraction). Therefore, before the raw gas enters the synthesis process, the final purification of the raw gas must be carried out, that is, the refining process.
In industrial production, the final purification method is divided into cryogenic separation method and methanation method. The cryogenic separation method is mainly liquid nitrogen washing method, which uses liquid nitrogen to absorb and separate a small amount of CO under the condition of deep freezing (<-100 ℃), and can also remove methane and most argon. In this way, hydrogen-nitrogen mixture containing only inert gas below 100 cm3/m3 can be obtained. The cryogenic purification method is usually combined with air separation and low-temperature methanol washing. Methanation is a purification process that reacts a small amount of CO, CO2 and H2 to produce CH4 and H2O in the presence of a catalyst. It is required that the content (volume fraction) of carbon oxides in the inlet feed gas should generally be less than that. 0.7 methanation can remove the content of carbon oxides (CO CO2) in the gas to less than 10 cm3/m3, but it needs to consume the effective component H2 and increase the content of inert gas CH4. The methanation reaction is as follows:

CO 3H2→CH4 H2O=-206.2kJ/mol0298HΔ

CO2 4H2→CH4 2H2O=-165.1kJ/mol0298HΔ

(3) Ammonia synthesis The pure hydrogen and nitrogen mixture is compressed to high pressure, and ammonia is synthesized under the action of a catalyst. Ammonia synthesis is the process of providing liquid ammonia products and is the core part of the entire synthetic ammonia production process. The ammonia synthesis reaction is carried out under the condition of higher pressure and the presence of a catalyst. Since the ammonia content in the gas after the reaction is not high, there is generally only 100, so the process of unreacted hydrogen and nitrogen circulation is used. Ammonia synthesis reaction is as follows:

N2 3H2→2NH3(g)=-92.4kJ/mol

Main use

Ammonia is one of the important inorganic chemical products and occupies an important position in the national economy. About 80% of ammonia is used to produce chemical fertilizers and 20% is raw material for other chemical products. Ammonia is mainly used in the manufacture of nitrogen fertilizers and compound fertilizers, such as urea, ammonium nitrate, ammonium phosphate, ammonium chloride and various nitrogen-containing compound fertilizers, all of which are made from ammonia. Ammonia as industrial raw materials and ammoniated feed, the amount of about 1/2 of the world's production.

Nitric acid, various nitrogen-containing inorganic salts and organic intermediates, sulfa drugs, polyurethanes, polyamide fibers and nitrile rubber are directly required to ammonia as raw materials.

Liquid ammonia is often used as a refrigerant, and part of the storage and transportation of commercial ammonia is transported to the field by the manufacturer in liquid form.

In addition, in order to ensure the balance between supply and demand between the synthetic ammonia and ammonia processing plants in the manufacturing plant and to prevent production from being stopped due to short-term accidents, a liquid ammonia warehouse is required. Liquid ammonia storage according to the size of the different capacity, there are three types of non-frozen, semi-frozen and fully frozen. The transportation modes of liquid ammonia are sea transportation, barge transportation, pipeline transportation, tank truck transportation and truck transportation.