An atmospheric crude tower revamp
Overall gas oil yield was maintained with heavier crudes due to a crude tower revamp. The revamp allowed for a crude heater outlet temperature reduction
Daryl W Hanson and Tony Barletta, Process Consulting Services
John V Bernickas, CITGO Petroleum Corporation
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In 2002, CITGO Petroleum Corporation revamped an atmospheric crude tower resulting in improved atmospheric distillate yield and quality, while permitting the atmospheric crude heater outlet temperature to be lowered by approximately 10ºF, with a simultaneous reduction in the vacuum unit feed rate. These results were achieved even though the crude became heavier as more heavy sour crudes were processed. Typically, atmospheric and vacuum tower distillate cut points decrease when crude blends get heavier, because they are more difficult to vapourise. Over the next ten years, US refiners will process higher percentages of heavier sour crude oils due to increasing output from the Orinoco River basin bitumen upgraders in Venezuela, as well as increased production from western Canada and the tar sands in northern Alberta.
When processing heavy crude oils, the atmospheric crude column internals perform a significant role in the overall unit performance. While fractionation improvements may be visibly apparent, other less obvious effects play a critical role too. Since improved atmospheric column stripping allows a reduction in the crude heater outlet temperature at a constant atmospheric distillate yield and lowers the quantity of 650ºF minus material in the vacuum tower feed, overall crude unit performance is optimised. Better stripping increases the recovery of diesel from the FCC feed and allows a lower vacuum column operating pressure due to a lower ejector system load. A lower vacuum column flash zone pressure can result in a higher heavy vacuum gas oil (HVGO) true-boiling point (TBP) cut point, even when processing heavier crudes. Conversely, poor stripping efficiency requires a higher heater outlet temperature to achieve the same atmospheric distillate yield, and lighter components feeding the vacuum unit can load the vacuum column ejectors. CITGO was able to reduce the crude heater outlet temperature by approximately 10ºF while improving the atmospheric gas oil (AGO) product yield. Figure 1 shows how the vacuum tower flash zone pressure was reduced from 24–28 mmHg to approximately 20 mmHg following the revamp.
Processing heavy crudes
The CITGO refinery processes a relatively high percentage of heavy crude oil, containing high quantities of sulphur, naphthenic acid, vanadium and microcarbon residue (MCR). As the crude blend gets heavier, it becomes increasingly difficult to vapourise the oil in the atmospheric and vacuum heaters. Without any operating variable changes, process flow scheme modifications or improved equipment design, the atmospheric tower bottoms (ATB) and vacuum tower bottoms (VTB) TBP cut points will always decrease as crudes get heavier. Table 1 summarises the changes needed in the crude and vacuum units to maintain or improve the atmospheric and vacuum distillate cut points as the feed gets heavier.
Heavy crude blends contain fewer atmospheric distillates and, in the case of very heavy Venezuelan crude oils such as Merey and BCF-17, ATB yields as high as 70 vol% on the whole crude are common. Achieving high diesel and AGO product recoveries becomes increasingly difficult.1 Heavy crude oils contain more vacuum distillates, so it is often necessary to increase the AGO product cut point to stay within the existing vacuum column diameter limit. As the AGO product TBP cut point increases, the vacuum unit feed gets heavier, resulting in a higher VTB yield unless the vacuum heater temperature is increased, the pressure decreased or the efficiency of the stripping section is improved in the VTB.
Most refiners cannot increase the vacuum heater outlet temperature because heater run lengths are reduced due to the high rate of coke formation. Also, the amount of cracked gas produced increases, raising the vacuum tower operating pressure. Although CITGO was able to reduce the vacuum tower operating pressure, many refiners are already operating at the capacity factor limit above where high VTB entrainment occurs. When processing heavy crude oils, the AGO product cut point should be optimised so that the HVGO yield is maximised within the vacuum tower limits, thereby maximising the HVGO product cut point.
Maintaining the HVGO cut point is a significant challenge when crude blends become heavier, as refiners may lose 20–80°F in the HVGO product cut point. It is not uncommon for a crude oil 3º API gravity decrease to result in a 50ºF reduction in the HVGO cut point. Maintaining the cut point requires a combination of lower operating pressure, higher heater outlet temperature, lower flash zone oil partial pressure (more heater coil steam) or improved VTB stripping.
The CITGO atmospheric tower has three pumparounds and several fractionation sections, as shown in Table 2. Prior to the upgrade, all tower sections were trayed, and pumparounds were withdrawn from active trays. The new draw tray designs were optimised to minimise leakage, which reduced problems associated with pumparound and product draws. The AGO product draw and wash zone were also redesigned and the stripping section design optimised.
All of the column internals were revamped too. The draw sumps on each of the three pumparound draws were modified by adding true draw sumps. This aided de-aeration of the downcomer liquid and reduced problems with withdrawing pumparound liquid from the column. When crude unit feed rates are pushed, it is common to have pumparound draw rate limitations because of the draw system initial design. Ideally, collector trays should be used, but the existing tower external piping configuration prevented their installation. Adding a proper draw sump allowed the top pumparound to be maximised to the pump limit, permitting a higher return temperature, thereby reducing the potential for tower tray salt formation.
The tray metallurgy was changed to the proprietary duplex stainless steel AL6XN, which is more resistant to chloride corrosion. In all the trayed sections above the flash zone, fixed valve trays were used, except at the pumparound and product draw trays. At these locations, moveable valve trays were used to reduce leakage through the tray decks, which minimised draw problems during normal operations and throughout unit start-ups.
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