Fast-tracking an FCC revamp
The design, detailed engineering and installation of one revamp took just
Michael Whatley, Navajo Refining Company
Scott Golden and Tony Barletta, Process Consulting Services
Viewed : 4280
In 2003, Navajo Refining Company revamped its FCCU to increase capacity from 18–25 Mbpd. During the previous turnaround, the reactor-regenerator section had been modified to meet the 25.Mbpd feed rate, but downstream equipment had not been upgraded. There were only four-and-a-half months from engineering kick-off to unit start-up. Everything had to be done on a fast track. Consequently, identifying modified or new long lead-time items quickly, such as compressor rotor modifications or vessels, was a priority. Normal linear engineering practices prior to issuing bids could not be followed. These included finalised simulations, heat and material balances, and equipment specification. In spite of the challenges, the unit started up on schedule. The unit has been able to exceed its design feed rate, and increase gasoline and LCO product recovery. Payout was less than six months.
It is not unusual in today’s business climate for revamp projects to take two to three years to engineer and construct. But with a dedicated project team and experienced revamp group, it is possible to complete fast-track work on schedule and within budget. When this project was executed, refinery margins were tight and capital was scarce. Hence, the decision to invest was made as near as possible to the upcoming turnaround.
However, fast track does not mean wasting money. The proper execution of fast-track revamps avoids the excessive engineering costs associated with studying options that are not practical. In this case, options that did not make sense were eliminated by discussions with process, project, mechanical and operating personnel with a vested interest in the successful outcome. Other time-consuming activities such as the approval of drawings were done in one or two days versus weeks by an appropriate working-level team. Avoiding bureaucratic project execution processes eliminates waste, unnecessary costs and scheduling delays.
Developing a complete scope of work was key to preparing a good estimate and controlling costs. Fast-track revamps are challenging, because engineering activities need to be prioritised around long lead-time equipment, and standard engineering practices often have to be ignored as not being necessary. Major equipment must be specified in sufficient detail to get an accurate quote, but details that are not needed can wait until after the critical activities are complete. For example, when buying a new vessel, process nozzle sizes can be estimated based on preliminary simulations and finalised after the vessel manufacturer has been selected and plate ordered. While there is risk of cost escalation if a process nozzle changes from, say, 8–10in, waiting until everything is finalised will at best ensure a premium is paid for the steel plate or, at worse, the schedule cannot be met. In many instances, standard engineering practices dictate the equipment specification development time, not truly practical requirements that ensure equipment deliveries are met, costs are contained and ultimately the unit operates properly.
Since the schedule was short, sufficient process simulations and equipment modelling were done to assess the major system limits such as the wet gas compressor, condenser system, main column and feed hydraulics. In parallel, field pressure and temperature measurements were gathered to identify problem areas. Proper simulation and equipment modelling are important, but accurate measurements are essential to quickly identify problem areas. In this case, field pressure measurements showed a 15 psi pressure drop from the reactor to the wet gas compressor inlet. The reactor effluent line had a 5 psi pressure drop due to coke build-up at the main column inlet flange. The main column pressure drop was 3 psi and the main fractionator-to-wet gas compressor inlet was 7 psi. Measured pressure was only 3 psig at the suction of the wet gas machine. But maintaining a 3 psig compressor inlet pressure would have required a new compressor and motor. Revamp economics, and capital and schedule constraints eliminated this option. Another solution had to be found.
Process engineering focused on developing major scope items, not finalising the simulations, heat and material balances, and finally equipment specifications. During every FCC revamp, the wet gas compressor, main column heat removal, main column capacity, gas plant capacity and reactor-to-wet gas compressor pressure drop are critical systems that must be evaluated. These are always the focus. For example, preliminary process simulation and equipment modelling showed the wet gas compressor suction pressure needed to be increased to 10 psig to keep wet gas production within the compressor capacity. Furthermore, additional condenser capacity was needed to reduce the receiver temperature to less than 105°F to stay within the compressor size. Preliminary simulation and equipment modelling were accurate enough to identify these constraints.
A higher FCC feed rate (and higher conversion) increases the amount of heat entering the main column, so heat removal must increase. Yet heat removed at a given location in the column determines product quality and product recovery. For example, increasing the slurry pumparound duty increases the total heat removal, but the liquid/vapour ratio throughout the column drops. Fractionation decreases, thereby increasing the amount of gasoline in the LCO and LCO in the slurry. Moreover, sulphur species in the products change, making sulphur specifications difficult to meet in downstream units. Before the FCC revamp was complete, a new gas oil hydrotreater needed to be put in service. Its design basis called for additional hot feed to the FCC. As long as there was capacity to remove this heat elsewhere, it was acceptable. But cold feed was the LCO and HCO pumparounds’ major heat sink. Main fractionator pumparound heat removal was therefore a significant constraint that needed to be addressed.
Preliminary simulations showed that the gas plant debutaniser and, to a lesser extent, the stripper column diameters were too small to process all the FCC gasoline. Even using high-capacity trays in the debutaniser required a large percentage of the gasoline to be produced as main fractionator heavy naphtha product to reduce gas plant liquid loading. Producing heavy naphtha reduces the main fractionator overhead temperature, lowering the condenser system driving force temperatures, reducing the main fractionator overhead vapour temperature and lowering the heavy naphtha pumparound draw temperature. These all make heat removal more difficult. In addition, the main fractionator overhead temperature could drop to 210°F in the worst case, resulting in salt formation on column internals. Even though final simulations and equipment evaluations were not complete, many unit constraints were becoming apparent.
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