Vacuum unit design for high metals crudes
Vacuum unit design can influence vacuum gas yield, product quality and run length. A wet vacuum unit with residue stripping achieves the highest cutpoint
Scott W Golden, Tony Barletta and Steve White, Process Consulting Services
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Several coker projects are in the planning phase or being built to process heavy sour crudes from Canada, Mexico, Venezuela and the deepwater Gulf of Mexico. These projects include new or revamped vacuum units. Although large heavy sour-sweet crude price differentials are the major economic driver today, vacuum gas oil (VGO) yield also influences refinery profitability. Feeding VGO to a coker reduces refinery liquid yields because it converts 10–15% of this material into coke, with the total value of the remaining coker products worth considerably less than the VGO feed. Processing recoverable VGO through the coker may be necessary due to poor vacuum unit design, but the assumption that VGO flows through the coker unchanged is wrong. Increasing the vacuum unit cutpoint from 1000°F (538°C) or less to 1050°F (566°C) (Figure 1) or higher increases the refinery liquid volume yield by at least 0.6–0.7 vol%, depending on conversion unit performance.
Previous heavy oil projects
In the last 20 years, grassroots and revamped crude unit heavy oil projects have been designed for Venezuelan heavy and extra-heavy Maya crudes. Canadian heavy sour has been processed by some refiners, but few have consistently run high blend percentages. The few that did operated low cutpoint vacuum units, feeding 6–8 vol% of whole crude gas oil boiling-range material to their coker unit. Prior to the mid-1980s, refiners outside the producing countries had little experience of processing these crudes in high blend percentages. Consequently, few process engineers had any experience of designing vacuum units for these heavy and extra-heavy crudes. Furthermore, the gas oil cutpoints for the producing countries’ vacuum units were less than 1000°F (538°C), and whole crude 1000–1150°F (538–621°C) TBP boiling-range metals distribution data were generally not available. Hence, volatile nickel and vanadium’s influence on gas oil product quality was poorly understood. With all these unknowns, early heavy crude projects had problems meeting their design goals other than the crude charge rate.
Low product yield, high metals gas oil product and short heater run lengths were common. At least one project has been operating with a gas oil product cutpoint 100°F (56°C) lower than design since startup at a loss of 6 vol% in gas oil yield, resulting in increased coke production and lower refinery liquid volume yields. Another unit was designed for a heater outlet temperature of 800°F (427°C), but actual temperature had to be reduced to 775°F (404°C) to increase the run length to two years between decoking. Nearly every vacuum column operating above a 730–740°F (388-393°C) flash zone temperature has coked the wash section packing in less than a four-year run. In several instances, the run length has been less than a year. The lessons learned from these earlier projects should be incorporated into new projects.
Unfortunately, many heavy oil projects today are focusing mainly on the large heavy sour-sweet price differentials, with minimal concern for refinery liquid yields. Yet, during most of the past 20 years, the heavy sour-light crude price differential has been much lower, making refinery liquid yields a major factor in refinery profitability. Achieving a high gas oil product yield on heavy crudes requires the right type of vacuum unit and correctly designed the equipment. Low-cost dry-vacuum unit designs that work well for Arab Light or other easy- to-refine crudes should not be used for heavy crudes. The refiner’s decision to use a dry (Figure 2), wet with no stripping (Figure 3) or wet with stripping (Figure 4) vacuum unit will determine whether these future heavy oil projects meet all their goals. Fundamentally sound equipment design is critical, because deep-cut vacuum units must operate at a high temperature throughout the run to achieve yield. In today’s environment, operators are requiring four- to six-year crude runs.
Heavy crudes are difficult to vapourise, contain large amounts of volatile metals and their properties vary. Many conventional heavy and oil sands-based crudes are blends of condensate or upgrader products and very heavy conventional crude or bitumen, or they consist of several crudes such as Western Canadian Select (WCS). Evaluating critical operating variables over a range of feedstock qualities avoids surprises when the unit begins operation. Capital investment needs to be balanced against the reduced profits from a short run length, low product yield or high metal in the product.
Limited crude assay information is common for some Canadian heavy sour Dilbit or Synbit crudes that have limited or no current production. A few Canadian oils sands producers currently operate vacuum units with cutpoints of around 900°F (482°C), but refiners have little experience of processing these crudes at high blend percentages. Without commercial experience, laboratory data have to be used to evaluate crude properties. For instance, cracking in the laboratory at low temperatures may not reflect the true cracking tendencies in the vacuum heater, but these data do show directionally important stability differences between conventional heavy crude oils and some of the oils sands-based crudes such as Cold Lake. Ultimately, the process designer will need to consider this information when selecting the type of vacuum unit or equipment design to meet the realities of crude variability, thermal stability problems or other peculiarities of these new crudes.
Vacuum unit type
The three broad categories of vacuum unit are dry, wet (damp) without stripping and wet with stripping. Dry units are the least expensive, but they are the most prone to short heater run lengths or low heater outlet temperatures, resulting in low gas oil product yields. Wet units with coil steam only allow higher heater outlet temperatures, while maintaining four- to six-year run lengths (Figure 2) and a somewhat higher gas oil yield than a dry unit, but they are more costly. Wet units with stripping produce the best quality gas oil at a given TBP cutpoint and permit the lowest heater outlet temperature for a given gas oil cutpoint. Wet designs with residue stripping produce the highest revenue by far, but they are also the most costly to build. Coil and stripping steam must be optimised according to capital constraints, product values and operating costs. Once vacuum unit type is selected, equipment design considerations determine the product yield, product quality and run length. For example, unstable crudes require coil steam to reduce the oil residence time and double-fired heaters to minimise the oil film temperature.
The type of vacuum unit is the single most critical choice that will influence a heavy oil project over its lifetime. The wrong decision results in low gas oil product yield, high metals, microcarbon and short run length.
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