Nitrogen

Personal opinion: The catalyst itself contains a certain amount of water, and the carrier it uses, especially gamma-alumina, has a high water absorption capacity. Removing this water from the catalyst at 120 degrees Celsius helps to prevent damage to the catalyst’s structure. As for the temperature of 250 degrees Celsius, it generally depends on the drying curve provided by the catalyst manufacturer; temperatures between 150 and 250 degrees Celsius are acceptable. During this period, the heating rate can be increased slightly, as most of the water has already been removed at a constant temperature of 120 degrees Celsius. This is done to ensure that all the water is removed completely, with the heating rate not exceeding 20 degrees Celsius per hour

Completely remove the water from the catalyst

Further remove free water and crystalline water to prevent catalyst damage during rapid temperature rise.

The purpose of drying the catalyst with nitrogen in two steps at 120°C and 250°C is to remove all free water on the catalyst surface as well as the crystalline water within the catalyst carrier. Crystal water: It occupies fixed positions within the crystal lattice of a substance and serves as a structural unit; it exists in the form of neutral water molecules and is part of the chemical composition of the substance. The number of water molecules is in a simple proportion to that of other components. For example, gypsum, CaSO4·2H2O, consists of calcium sulfate along with 2 water molecules. Blue vitriol, CuSO4·5H2O, consists of copper sulfate along with 5 water molecules. Due to being bound by the lattice, crystal water is firmly attached to the substance, and high temperatures (usually 200–500 degrees Celsius or higher) are required to remove it from the substance. When a substance loses its water, its structure is completely destroyed and its atoms reorganize to form a new substance. Free water: Free water exists only on the surface of a substance or in its cracks; it does not participate in the crystal lattice and exists in the form of water molecules. The amount of free water in a substance is not fixed. When the temperature reaches 100–110 degrees Celsius, the free water completely escapes from the substance.

Upstairs, I’ve learned it, thanks a lot!