Reducing FCC main fractionator operating risks
Analysis of an FCC main fractionator upset and follow-up revamp showed that removal of the HCO wash section did not impact operations or product quality.
DARYL HANSON and SOUN HO LEE
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Light tight oil (LTO) crude processing in refineries has increased substantially over the last 10 years. LTO is highly paraffinic and yields a very high percentage of kerosene and lighter boiling range material 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, mainly in blending and in the crude distillation unit (CDU) and the vacuum distillation unit (VDU), as cited in industry publications.1,2
CDUs and VDUs are not the only units to have experienced operating issues. LTO atmospheric gasoil (AGO), vacuum gasoil (VGO) and atmospheric tower bottoms (ATB) are excellent feedstocks for the fluid catalytic cracker (FCC) unit with respect to paraffinicity, low metals and Concarbon content. Those feedstocks result in high conversion and excellent product slates without high catalyst contamination. Catalytic conversion of this feedstock sometimes challenges FCC unit operations because adequate main fractionator operation and run-down goals should be maintained with minimum slurry (FCC main fractionator bottoms) yield. At high FCC reactor temperatures, the LTO feedstock will approach terminal conversion, which places particular challenges on operations.
Many FCC main fractionator designs are inherited from previous refinery operating strategies. These designs have column sections that pose significant operating risks without providing measurable benefits. In the current market, it is important that operating risks be minimised in order to maximise feedstock flexibility while minimising shutdown risks.
In this case study, we discuss recent FCC troubleshooting activity and evaluation. The FCC main fractionator was a traditional design with heavy cycle oil (HCO) pumparound, HCO wash, and slurry pumparound sections in the lower half of the fractionator. Successful operation of the HCO wash section became challenging due to requirements for heat removal when processing LTO feedstocks. Evaluation of the HCO wash section’s performance showed that its removal had little to no effect on the column’s overall operation. In addition, impacts on the light cycle oil (LCO)/slurry fractionation section were minimal in order to fully compensate for the effects of losing the HCO wash section (HCO/slurry fractionation). Eliminating the HCO wash section reduces the fractionator’s operating risk as this section has a very high vapour rate, low liquid flux, and very high temperatures, which make it prone to coking.
Reducing operating risk
Operational risk evaluation and reduction is fundamental to the refining business. Safety has been a benchmark to determining successful refining operations. Reducing risk during start-up/shutdown activities, fixing adverse operating conditions such as flooding, as well as normal operations, is key to minimise overall refinery operating risk. The consequences of insufficient risk management include equipment damage, unplanned outages, and other, more serious consequences.
Efforts to reduce operating risk are evident across the refining complex, especially in heavy oil processing units. In the early 1990s, commonly accepted random packing was replaced by grid style packing in the VDU in order to enhance coking resistance. Spray chamber design with special spray nozzle selection improves the reliability of the coker fractionator wash section without sacrificing washing performance. Large fixed valve trays have been implemented in numerous refinery columns to improve fouling resistance. Nevertheless, refinery column fouling events through LTO processing and incompatible crude blending have increased. Fouling of the atmospheric column stripping section in the CDU is one example. Successful upgrading for fouling resistance and extended unit run length through implementation of a shed deck tray has been previously discussed.3
In this case, the solution to reducing operating risk included eliminating internals from a section of the fractionator that has been historically used in FCC main fractionators. This is the most severe and most prone to coking section of the FCC main fractionator due to its high operating temperature and extremely low liquid to vapour traffic ratio.
Case study: background
The FCC main fractionator under discussion belongs to a refinery that processes a significant amount of LTO. Hydrotreated AGO/VGO from the CDU/VDU are introduced to FCC as feedstocks. FCC reactor effluent is fed to the FCC main fractionator for separation and five product streams are withdrawn from the fractionator: overhead off-gas, light cracked naphtha (LCN), heavy cracked naphtha (HCN), LCO, and slurry. Overhead off-gas is introduced to the primary absorber via the wet gas compressor and high pressure receiver. The FCC main fractionator’s configuration is illustrated in Figure 1.
The LCN and HCN streams are introduced to the primary absorber in order to absorb C3+ materials from the off-gas stream. Sponge absorber lean oil is sourced from the LCO stream, while sponge absorber rich oil is returned to the main fractionator through the LCO pumparound return stream. The HCO pumparound reboils several gas concentration/alkylation unit services and has the provision to yield an HCO product. HCO is not withdrawn from the fractionator because it is not valuable in the current FCC operating mode. The fractionator has four pumparound circuits: HCN, LCN, HCO, and slurry pumparound (pumparound and quench).
FCC main fractionator upset
During a routine FCC step test to optimise performance, the main fractionator bottom liquid level and fractionator heat removal balances became erratic. Increased unit conversion resulted in lower slurry yield than the minimum rate and higher vapour traffic to the fractionator. As a result, pressure drop values across the fractionator sections were erratic and a cycle of pressure drop dumping and building was observed. This unstable operation resulted in numerous temperature excursions and a loss of the fractionator bottom liquid level. Operators used an emergency method of feed injection into the bottom liquid pool to recover the bottom liquid level and re-establish operational stability.
After stability was restored, a column scan was conducted to identify limited areas in the FCC main fractionator. The scan indicated that the sections between the LCO and HCO pumparound draws were flooded. The slurry pumparound section could not be diagnosed through the scan because disc and donut trays were installed in the section and their condition could not be ascertained. The scan result is shown in Figure 2. Each line indicates scan per active area in two pass trays.
Multiple locations for slurry pumparound circuit plugging were identified. These made slurry circulation difficult and prevented adequate bottoms temperature control. External heating to the slurry circuit piping had been attempted in an effort to liquefy highly viscous slurry and free up solidified circuit piping; however, this was not successful and the slurry pumparound circuit was eventually shut down.
After the outage, heat exchanger bundles were pulled. It was found that soft layers of coke had built up significantly in one of the FCC feed/slurry pumparound heat exchangers. The fouled heat exchanger is shown in Figure 3. Upon entry, the internals in the HCO pumparound and wash sections of the fractionator were found to be compromised with solids and coke formation (see Figures 4 and 5).
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