(21a) Reducing FCC Main Fractionator Operating Risks

Light Tight Oil (LTO) crude processing in US and foreign refineries has been substantially increased over the past 10 years. LTO is highly paraffinic and has a very high percentage of Kerosene and lighter boiling range material yield per barrel of crude. Domestic production of LTO is very attractive to US refineries as it reduces transportation costs substantially. LTO processing experience is increasing after many processing learning lessons, mainly in blending and Crude Distillation Unit (CDU) and Vacuum Distillation Unit (VDU), have been cited in industry publications.

CDU and VDUs are not the only units that have experienced operating issues. LTO Atmospheric Gas Oil (AGO), Vacuum Gas Oil (VGO) and Atmospheric Tower Bottoms (ATB) are excellent feedstocks for the Fluid Catalytic Cracker (FCC) unit due to its paraffinicity and low metals / con-carbon content. Those feedstocks result in high conversion and excellent product slates, without high catalyst contamination. Excellent catalytic conversion of this feedstock sometimes makes the FCC unit operations challenged because adequate main fractionator operation and run-down goals should be maintained with minimum Slurry (FCC Main Fractionator Bottoms) yield. At high FCC reactor temperature, the LTO feedstock will approach terminal conversion, which places particular challenges on the operation with this LTO feedstock.

Many FCC main fractionator designs are heritages from previous refinery operating strategies. These fractionator designs have sections of the column that pose large operating risks without providing measurable benefits. In the current market, it is important that operating risks be minimized in order to maximize feedstock flexibility while minimizing shutdown risks.

In this case study, the authors will discuss a recent a recent FCC troubleshooting and evaluation. The FCC main fractionator was a traditional design with a Heavy Cycle Oil (HCO) pumparound, HCO wash, and Slurry pumparound sections in the lower half of the fractionator. Successful HCO wash section operation became challenging due to the heat removal requirements while processing LTO feedstocks. HCO wash section performance evaluation showed that the HCO wash section removal had little- to no-effect on the overall column operation. In addition, impacts on the Light Cycle Oil (LCO) / Slurry fractionation section were minimal in order to fully compensate for the impacts of losing the HCO wash section (HCO / Slurry fractionation). Eliminating the HCO wash section reduces the fractionator operating risk as this section has a very high vapor rate, low liquid flux, and very high temperatures which make it prone to coking.