Water injection in hydrocracking
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Why is water injected in hydrocracking? At what temperatures and pressures does crystallization occur?
The purpose of injecting water into the reaction system is to remove the ammonium salts that form as by-products of the reaction. These ammonium salts not only affect the heat exchange efficiency but also block the pipelines, leading to an increase in system pressure and posing a risk to the safe operation of the equipment. The ammonium salts precipitated from the reaction products fall into two types: one is NH4Cl, whose crystallization temperature is 180–200°C, and the other is NH4HS, whose crystallization temperature is 150°C. Due to different crystallization temperatures, the sites where precipitation occurs in the system differ. NH4Cl generally precipitates in the last two heat exchangers, while NH4HS generally precipitates in the high-pressure air cooler. To this end, the locations and quantities of water injection are determined based on the different areas where ammonium salts precipitate; generally, water injection points are installed at the inlet of the high-pressure air cooler and at the inlets of the tubes in the last two high-pressure heat exchangers
At that reaction temperature and pressure, the nitrogen, hydrogen, chlorine, and sulfur elements present in the raw materials combine to form ammonium chloride and ammonium hydrosulfide, which then result in the formation of ammonium salt crystals within the system’s pipelines.
Diluted salt crystals, mainly ammonium chlorides and ammonium hydrogen sulfides