Focused revamp increases diesel and HVGO recovery
Refiners have the best opportunity to maximise return through identification of creative solutions during a focused capital revamp.
Scott Golden, Tony Barletta and Steve White. Process Consulting Services
Ben C Miller. CITGO
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CITGO Petroleum Corporation’s Lake Charles Refinery started up its Crude Vacuum 1 (CV1) unit revamp in 2020. The revamp recovered more than 11 kBPD of incremental diesel from FCC feed and reduced vacuum residue (VR) yield by 5 kBPD while concurrently increasing the capability to process refinery gasoil (RGO) and excess atmospheric residue (AR) from other crude units in the refinery. Vacuum column HVGO product TBP cutpoint increased from 960 to 1,060°F by lowering vacuum column operating pressure, increasing heater outlet temperature, and improving stripping section efficiency. No additional fuel gas was needed for atmospheric or vacuum heaters, and steam consumption decreased by more than 37,000 lb/hr. No new equipment services were added; exchanger services were repurposed to allow column heat balances to be adjusted to yield higher value product and increase heat recovery to oil feed streams.
CV1 is an integrated crude vacuum unit. It was originally designed to process heavy bitumen crudes with deepcut vacuum unit operation. The vacuum unit was sized to process excess refinery AR, which was being processed in the coker, as well as other crude unit RGO to improve overall refinery diesel yield (see Figure 1). When the unit was built, the vacuum system, column, and associated equipment were some of the largest in the world. World-class equipment sizes often introduce another layer of design complexity. For example, designing a fractionation section liquid distributor in a 38 ft-diameter column section is very different mechanically compared with a typical distributor used for more frequently encountered diameters. Uniformly distributing an inherently low liquid rate (less than 1.0 gpm/ft2) to fractionation beds is essential to maximise vacuum diesel yield, but realising it is difficult at world scale.
Prior to the revamp, the vacuum column operating pressure would increase to more than 50 mmHg absolute during the summer. Even though the pressure was lower during cooler months, it was still above original design pressure, and HVGO recovery suffered. A thorough vacuum system troubleshooting effort was done to identify the root cause. Poor first-stage inter-condenser performance was the root cause of high operating pressure. As part of a baseline unit evaluation, profitability improvement opportunities were framed for CITGO’s refinery leadership team to consider. Once this work was completed and opportunities prioritised, a feasibility study was completed to better develop scope options and firm up the cost/benefit analysis. Ultimately, a process design package (PDP) detailed any process flow scheme changes and equipment design requirements for a focused capital revamp.
Focused capital revamps require a different engineering approach than highly structured large capital projects that often focus on rigid process design basis development, strict project scope, milestones, and schedule controls. During the conceptual and feasibility design phases, the refiner has the best opportunity to maximise return through identification of creative solutions. Project scope is shaped by practical alternatives developed and investment scope prioritised. During these phases, experienced process design engineers should focus on constructible solutions and avoid high-cost solutions that cannot be implemented during a turnaround or within a constrained capital environment. Some solutions are simply too expensive and do not warrant consideration, whereas others are clearly more constructible and lower cost. It is essential for process design engineers to have equipment expertise so that practical, cost-effective solutions can be the focus. Little or no time should be wasted on unrealistic options. When conceptual and feasibility work is done properly, scope growth is minimised, with late-stage cost cutting rarely required. The following discussion demonstrates how creative solutions can lead to focused revamps, which often result in big unit improvements.
Identifying opportunities and challenges
CV1 is a modern integrated crude and vacuum distillation unit commissioned in 2005 to process very heavy crude oils.1 The vacuum column was designed for deepcut operation to maximise gasoil recovery from vacuum resid. The column was built with a diesel recovery section to capture diesel from vacuum gasoil. The other crude units in the refinery produce HGO products with large amounts of diesel boiling range material. The other vacuum units do not have diesel recovery sections, and the LVGO streams contain roughly 50% diesel. CV1 was originally designed to process HGO and LVGO streams from these other units in its vacuum unit to recover incremental diesel. The other vacuum units in the refinery are an older design, lacking stripping steam and overflash recycle. With the lack of VR stripping, their HVGO product cutpoints are much lower than CV1’s. These units produce more coker feed and less gasoil.² CV1 vacuum unit was designed to process excess atmospheric residue (AR) to improve RGO recovery from coker feed.
Over time between turnarounds, as the vacuum system deteriorates and operating pressure increases, HVGO cutpoint suffers. As VR production increased, less RGO and excess AR could be processed. It was readily apparent that reducing the vacuum column pressure would increase HVGO cutpoint, reduce the VR rate, and give flexibility to increase the amount of heavy crude in the blend. In addition, more RGO and excess AR could be processed. Fixing the vacuum was a major opportunity to improve overall refinery diesel and gasoil recoveries (see Figure 2).
Preliminary engineering work identified other equipment shortcomings that would need to be addressed to maximise gasoil and diesel recoveries while increasing RGO and excess AR processing. The atmospheric and vacuum heaters were operating at maximum capacity, and the duties could not be increased. This was a hard constraint that needed a workaround.
Increasing the heater outlet temperature would require a higher inlet temperature as major retrofitting to the existing heaters or building new heaters was infeasible within the turnaround window. Therefore, engineering work focused on opportunities to improve preheat train utilisation. The atmospheric column was originally designed with a heavy gasoil (HGO) pumparound, which provided high-temperature heat for crude preheat and steam generation. The downside to an HGO pumparound is that it reduces light gasoil (LGO) product (atmospheric diesel) yield due to reduced reflux between the two cuts (see Figure 3).3 Shifting heat up the atmospheric column presented an opportunity to increase LGO up against heater constraints.
Likewise, incremental LVGO (vacuum diesel) could be produced by shifting heat up the vacuum column. Shifting heat to the LVGO PA section increases condensing capacity, allowing maximum LVGO yield, but it also has a heat recovery penalty because MVGO PA draw is 500°F whereas LVGO PA draw is 275°F. The majority of the LVGO PA duty is rejected to air-cooled and water-cooled exchangers.
CV1 is more complicated than most crude units because it processes external feed streams in addition to crude oil. The excess AR is cold and needs to be heated. This complicates preheat train heat recovery because a portion of the preheat train dduty must be used to preheat external AR and is not available for crude preheating. Increasing excess AR processing and shifting heat up to lower draw temperature pumparounds in the atmospheric and vacuum columns were challenges to increasing the heater inlet temperatures.
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