Ammonia converter is the equipment used to catalyze the reaction of nitrogen and hydrogen for ammonia synthesis under high pressure and high temperature.Ammonia converter is the heart of ammonia plant and a reactor with complex structure.
Ammonia synthesis converter is a device used to catalyze the reaction of nitrogen and hydrogen for ammonia synthesis under high pressure and high temperature.
At present, ammonia synthesis in industry is carried out at a pressure of 15.2~30.4MPa and a temperature of 400~520 ℃. In order to prevent the corrosion of hydrogen to steel under high pressure and high temperature, the ammonia synthesis tower consists of a high pressure resistant head, an outer cylinder and a high temperature resistant internals installed in the cylinder.
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In industry, ammonia synthesis is carried out at a pressure of 15.2~30.4MPa and a temperature of 400~520 ℃. In order to prevent the corrosion of steel by hydrogen under high pressure and high temperature, the ammonia synthesis tower is composed of a high pressure resistant head, an outer cylinder and high temperature resistant internals installed in the cylinder.
There is a thermal insulation layer outside the internals. During operation, the cold gas entering the tower flows through the annular space between the inner and outer cylinders, so as to avoid excessive temperature of the outer cylinder.
In this way, the outer cylinder can only bear high pressure and can be made of low-alloy high-strength steel.
Although the internals are operated at high temperature, they only bear the pressure difference between the inlet and outlet of the ammonia converter, and can be made of heat-resistant nickel chromium alloy steel.
The internals include two main parts: catalyst basket and heat exchanger. The basket is filled with iron catalyst, where the ammonia synthesis reaction is carried out.
The temperature of hot gas coming out of the catalyst basket is usually above 460 ℃, and the temperature of cold gas entering the ammonia converter may be 20~30 ℃ or 140 ℃ or more depending on the process.
In order to heat the gas entering the ammonia synthesis tower to the reaction temperature and cool the gas after reaction, a heat exchanger is also set in the tower.
The heat exchanger has tube and tube type, spiral plate type and corrugated plate type, of which tube and tube type is the most widely used.
Ammonia synthesis catalystIt must be restored before starting (seeAmmonia synthesis）Reduction needs to provide a certain amount of heat. For this reason, small and medium-sized ammonia converters are equipped with electric heaters inside, while large ammonia converters are equipped with start-up heaters outside the tower.
Under a given iron catalyst and pressure, the reaction rate varies with the temperature of ammonia synthesis.
For a certain ammonia content, the temperature at which the ammonia synthesis reaction speed is maximum is called the optimal temperature, which decreases with the increase of ammonia content.
As ammonia synthesis becomes an exothermic reaction, the temperature of the catalyst bed will continuously increase with the reaction.
In order to ensure that the ammonia synthesis reaction can be carried out at close to the optimal temperature, measures need to be taken to remove the excess heat.
In industry, the types of catalyst baskets are distinguished according to the heat transfer mode.
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Internal heat exchange type
It is also called continuous heat exchange type.It is characterized by that a cooling pipe is set in the catalyst bed to conduct indirect heat exchange of cold and hot airflow in the bed through the cooling pipe, so as to achieve the purpose of adjusting the bed temperature.
The cooling pipes can be divided into single pipe, double pipe and three pipe. According to the similarities and differences of the gas flow direction in the catalyst bed and the cooling pipe, there are also counter flow and parallel flow cooling pipes.
Schematic diagram of ammonia converter
Taking the parallel flow double sleeve ammonia converter as an example, the gas enters from the top of the tower, goes down along the tower wall in the annular space, passes through the shell side of the heat exchanger, goes to the gas distribution box, and is dispersed to the internal cooling tubes of each double sleeve, goes to the top of the tube and folds to the external cooling tubes. The gas is preheated to the active temperature of the iron catalyst (usually about 400 ℃), and then flows through the central tube with an electric heater.
Through the catalyst bed from top to bottom, nitrogen and hydrogen react here and then exit the catalyst basket. The temperature is reduced through the tube side of the heat exchanger and exit the synthesis tower.
Ammonia converter
In order to control the temperature of the catalyst bed not to be too high, a small amount of gas enters the tower from the cold gas bypass pipe and directly mixes with the preheated gas without passing through the shell side of the heat exchanger.
Intermittent heat exchange
The main feature is that the reaction and heat exchange are conducted intermittently.
The catalyst bed is divided into several sections, and the unpreheated nitrogen hydrogen mixture introduced between the sections is used for direct cooling, which is called multi-layer direct cooling ammonia converter.
According to the different flow directions of gas in the bed, it can be divided into axial ammonia converter flowing along the central axis and radial ammonia converter flowing along the radial direction.
They have simple structure, few vulnerable parts and relatively stable internals, and are mostly used by large ammonia plants.
