Fractionating tower, reflux, product quality control
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In the coking fractionation tower, the top stream, diesel, middle fraction, wax oil, and heavy wax oil are drawn from the oil collection tank; after being pressurized by pumps, there is a downward reflux stream, while another portion flows upward after heat exchange. What are the respective functions of the upward reflux and downward reflux streams?
Reflux serves mainly the following purposes: it provides cold reflux on the tray, removes excess heat from within the tower, and maintains thermal balance inside the tower； Cold fluid is provided on the tray, where the gas and liquid phases come into contact in opposite directions. The heavier components in the rising gas condense, while the lighter components in the descending liquid absorb heat and vaporize. This repeated process of condensation and vaporization further improves the precision of product separation. The so-called top reflux, or tower top circulating reflux, refers to the liquid drawn from the lower trays at the top of the tower; after being cooled, this liquid is returned to the topmost tray of the tower. In this way, each of the lower trays also has internal reflux, thereby satisfying the requirement for a liquid-phase reflux within the tower. In addition to recovering waste heat and ensuring a uniform gas-liquid load in the distillation column, the top reflux of the coking distillation column is also responsible for bringing the dry point of the crude gasoline to the required level.