Why is it necessary to fill the system with ammonia before heating and reducing the ammonia synthesis catalyst?

To prevent the water formed through heating and reduction from freezing and damaging the equipment, in case the ammonia cooler drops to 0 degrees Celsius. 1# Yan Qiusheng

To prevent freezing and blockage in the ammonia cooler behind the ice starter

The main reason for filling the system with ammonia before heating and reducing the ammonia synthesis catalyst is to prevent the water generated from condensing at low temperatures and damaging the equipment, pipes, valves, etc.

To enable the water generated by catalytic reduction to be separated in a timely manner and to prevent low-temperature condensation from freezing, thereby accelerating the reduction process and shortening the reduction time.

During catalytic heating for reduction, the water produced is not entirely in liquid form under normal conditions. To reduce the vapor concentration, ammonia is used to lower the temperature to below -10 degrees. To prevent the water produced by the catalyst from freezing, ammonia is added to the system in order to avoid damage to the equipment, as well as blockages in the system’s pipes and valves caused by ice formation.

During the heating process, the catalyst first releases some physical water. As long as the product in the early stages of reduction is water, the temperature of the gas exiting the ammonia cooler during this reduction process must be below 0°C (typically around -10°C, to ensure that the water produced condenses after passing through the ammonia cooler). ), The purpose of adding ammonia is to create an ammonia solution of sufficient concentration in the water, thereby preventing the water from freezing inside the tubes of the ammonia cooler or in the pipes, which could otherwise lead to blockages of those tubes or pipes; in severe cases, the tubes might even crack from freezing.

Because physical water is generated during catalyst reduction, and the cooling temperature of ammonia during this process must be kept below -10 degrees; at such temperatures, ordinary water freezes, which can lead to pipe blockages or damage to the equipment, resulting in equipment failures and preventing the reduction process from proceeding. By adding a certain amount of ammonia before reduction and mixing it with water to form ammonia water, the freezing point is lowered, so that it does not freeze at the ammonia-cooled temperature required for reduction; this prevents pipe blockages and damage to the equipment.

To prevent the separated water from freezing in the ammonia cooler, the concentration of ammonia water should be maintained above 15%. At the initial stage of catalyst reduction, less ammonia is produced; to avoid a too low concentration of ammonia water, ammonia must be added to the circulating gas so that its ammonia content reaches 0.5%-1.0%, which results in an ammonia water concentration of over 25%

“The Q&A on Production Operations in Small and Medium-sized Ammonia Synthesis Plants” provides the following answer: The functions of filling the system with ammonia are two. (1) Ammonia solution has a very low freezing point, so it does not freeze at low temperatures. To reduce the water vapor concentration in the reducing gas, the ammonia cooling temperature is generally kept below -10°C at the beginning of the reduction process. At this temperature, if no ammonia is added to the system, the water vapor generated during reduction will cause freezing within the system. By adding a certain amount of ammonia, the activated water produced during reduction can be converted into an ammonia solution with an ammonia content of over 25%, whose freezing point is below -45°C, thereby preventing freezing from occurring. (2) Ammonia water has a low vapor pressure, which can reduce the concentration of water vapor in the reducing gas. The higher the ammonia concentration, the lower the water vapor concentration. After ammonia is introduced into the system, the activated water becomes an ammonia solution with an ammonia content of over 25%; after separation, the vapor concentration can be reduced to below 1.0 g/m3 (standard), thereby protecting the catalyst.