Abstract:

 The fractionator of FCC unit distillates the reaction oil and gas, and then draws out several streams as the products and sets up multiple middle sections to recover the heat of the reaction oil and gas. In the early stage, due to the low catalyst activity and high recycle ratio of the unit, a separate mid-pumparound for the fractionator would result in high liquid load in the trays near the middle section, thus increasing the diameter of the fractionator. In order to reduce the diameter of the fractionator and the investment, many existing units in operation are equipped with two mid-pumparounds, that is, light cycle oil (LCO) and heavy cycle oil (HCO). The simulation results of unit A in Case 1 and Case 2 show that the diameter of the fractionator can be reduced by about 300 mm after setting the HCO. In recent years, with the improvement of catalyst activity and the change in product plan of the unit, the recycle ratio of most FCC units has been reduced to about 0.1. At this point, it is possible to discuss canceling the HCO and reducing the operation difficulty of fractionator. The simulation results show that the diameters of fractionator in Case 1 and Case 3 are the same, which indicates that the existing fractionator can be used under the conditions of canceling the HCO and reducing the recycle ratio at the same time. Considering that such modification is usually carried out during overhaul and the time is limited, the utilization of the existing fractionator greatly shortens the construction period. At the same time, about 95% of the heat is used as the heat source of reboiler in the reasonably optimized heat exchange process of the LCO. In addition, the heat recovery ratio of the slurry system of fractionator is increased by about 10%. The above two measures can improve the energy utilization efficiency of the fractionator. This paper provides a reasonable and alternative modification idea for similar units.

Key words: catalytic cracking/FCC, recycle ratio, fractionating tower, heavy cycle oil