Designing Vacuum Units
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SEPARATIONS Designing vacuum units When designing vacuum units for processing heavy Canadian crudes, reliability costs can be high if the feedstock’s thermal instability is not fully appreciated. Process design considerations affecting VGO yield and quality are discussed Scott W Golden and Tony Barletta Process Consulting Services Inc ew vacuum unit capacity is production or whether there is sufficient crude will dictate whether metallurgy currently being added at several blendstock for the bitumen. upgrades will pay off. NNorth American refineries to Applying conventional design process higher amounts of heavy practices to new units that will process Crude composition variability Canadian crude. Many other refiners are heavy Canadian crudes will result in Conventional and oil sands heavy evaluating revamps or new units. many unwanted surprises. Oil sands- Canadian crudes are blends of Ultimately, coker unit capacity will based crude oils are generally less condensate or upgrader products and determine the maximum amount of thermally stable than conventional very heavy crude or bitumen. Many are heavy Canadian crude refiners can crudes, some are very unstable, most have currently produced in limited volumes. process. Nonetheless, maintaining a high volatile vanadium and nickel Thus, it is prudent to assume crude high vacuum gas oil (VGO) yield content in the VGO boiling range, many composition will likely be different than throughout the run will reduce the coker have high solids content and several have the current assays when commercial charge rate, decrease the amount of coke very high total acid number (TAN) in the production begins. The same has been produced per barrel of crude, and permit VGO boiling range. Rapid equipment true of extra-heavy Venezuelan blends; maximum processing of heavy crudes. coking and asphaltene precipitation are there have been significant variations in Heavy Canadian crude oil properties major problems facing refiners. In some compositions. Lower API gravity crude influence vacuum unit reliability, VGO instances, heater outlet temperatures forces a higher heater outlet yield, gas oil quality and run length. Few have been reduced below 700°F to obtain temperature to achieve the same VGO refiners currently process high reasonable heater run lengths at the product cutpoint. Even though the percentages of these crudes in their expense of a low gas oil cutpoint. crude source is the same, gravity crude blend. Furthermore, oil sands- Crude processing flexibility will variability requires operating flexibility based crudes are less thermally stable largely depend on equipment to achieve the same distillate cutpoint. than conventional crudes, with metallurgy. Some oil sands-based crude In addition, laboratory data show asphaltene precipitation and vacuum oils produce heavy vacuum gas oil that a 10°F change in temperature in the heater tube coking just two of the many (HVGO) product with TAN values of 6–8. laboratory distillation device dramat- challenges. Operating at a low gas oil Processing high percentages requires ically alters the rate of thermal cracking cutpoint is one solution, but this 317L SS in many of the circuits. Since the on some of these crudes. Even though approach reduces the VGO yield, crude supply situation is still developing, laboratory devices do not reflect true increases the coker charge rate and raises metallurgical selection may be the cracking tendencies in the vacuum coke production per barrel of crude. difference between processing low-cost heater, they do show directionally that crudes and having to compete with other there is an important stability difference Heavy Canadian crude oils refiners for the low-TAN feedstocks. If between conventional heavy crude oils The implications of crude source on most new units are built to process only like Cold Lake and some of the oil vacuum unit design cannot be low-TAN crudes to minimise investment, sands-based crudes. overstated. Heavy Canadian oils consist crude flexibility will be limited or high Typical vacuum unit design assumes of conventional heavy and oil sands- corrosion rates in some circuits will be crude quality is fixed based on a given based crude oils. Cold Lake and Western the consequence of processing high-TAN assay. An alternate approach assumes Canadian Select are examples of crudes. Price differentials between low- large quality variability during the conventional heavy crude. Oil sands- and high-TAN heavy sour Canadian design phase and determines the based crudes include Albian Heavy, operating flexibility and capital cost to Christina Lake, McKay River and others maintain product yield, quality and run that have not yet come into production. “Maintaining a high VGO length. Evaluating critical operating Total conventional heavy Canadian yield throughout the run will variables and estimating the VGO yield crude production today is approximately over a range of feedstock qualities is 500Mbpd, which is expected to drop reduce the coker charge rate, prudent. Refiners processing heavy and slowly until 2010 and then sharply decrease the amount of coke extra-heavy Venezuelan and Maya crude decline. Conversely, oil sands-based oils have experienced significant production will increase from about produced per barrel of crude, performance differences between the 1000Mbpd today to about 2500Mbpd or and permit maximum design and actual operation. Many of more by 2015, depending on how many these early heavy oil projects were of the planned upgrader projects are processing of heavy crudes” designed at a benchmark 1050°F TPB actually built, when they reach full cutpoint, yet several achieved less than 105 PTQ Q2 2006 www.e ptq.com SEPARATIONS decisions based solely on a design case heat and material balance and low Atmospheric column initial investment cost, ignoring the need for operating flexibility. While initial cost is important, building units that actually meet their design gas oil yield, quality and run length should be part of the investment decisions. Yet, many cost/performance decisions are Consequence Low Steam much more complicated and involve high ⌬P pressure drop many more unknowns than looking at only initial cost. It is easy to eliminate Vacuum equipment or purchase the lowest-cost tower Transfer line vacuum heater. However, these low-cost units will have a much lower return over High head Vacuum the life of the unit than one that has pump Coil heater been designed properly. steam Many difficult questions must be answered. Is there a run length benefit to installing a well-designed heater vs a Figure 1 Vacuum unit charge system low-cost one when asphaltene precipitation and poor thermal stability are known problems? Is it prudent to design a double-fired heater with $ISTILLATE $ISTILLATE balanced-pass heat flux, low oil FROM FROM HEATER STRIPPER residence time and high oil mass velocity, and pay 20% more for the heater? Should the vacuum ejector (EATEROUTLET system be designed for 7mmHg absolute TEMPERATURE pressure or 10mmHg, and what effects will this have on product yield and run length as well as cost? Should vacuum column wash zone internals be designed 6ACUUM for high efficiency to reduce VGO HEATER vanadium content, knowing this -AXIMISESTRIPPING increases the coking tendency? True MINIMISESHEATER value engineering should be performed OUTLETTEMPERATURE 3TEAM continuously as part of the process design effort and should encompass all critical decisions influencing perfor- mance and cost, not simply initial cost. During value engineering, someone will generally ask why a dry vacuum 6ACUUM RESIDUE unit design is not being used. Dry units have much smaller vacuum ejectors, use less cooling water and steam, have a Figure 2 Minimising heater outlet temperature lower heater pressure drop and require a lower charge pump head, but they also 1000°F when commissioned. These Canadian crude oils. And these few produce less VGO product and more units produced 3 vol% less VGO and operate at TBP cutpoints below 950°F. In coker charge. First and foremost, higher coker feed rates than design. In some instances, major equipment vacuum units processing heavy oils some cases, the actual crude charge rate design changes were needed to reliably must be designed with steam. Designing was reduced by 8–10% to stay in balance produce high volumes of acceptable- heaters without steam dramatically with coker capacities. quality VGO for four-to-five-year run increases the rate of coke formation and Reducing crude oil cost is the major lengths. In several cases, vacuum heater decreases the run length. Those refiners incentive driving these new crude and and column wash sections coked in less who have dry units have had to reduce vacuum unit projects. Yet the reality is than one year. One refiner had less than their heater outlet temperature to that higher capital cost designs are one year’s run on the gas oil maintain a reasonable run length when needed to process these crudes reliably hydrotreater due to HVGO product processing heavy crude oils. A lower over a four-to-five-year run length. vanadium content of 10ppmw or more. heater outlet temperature reduces the Designing a low-cost unit will reduce Additional reactors were added to VGO yield. There are places to reduce project revenues, because product yields remove the high metals prior to the initial investment costs; however, are lower and run lengths are shorter desulphurisation reactors so that building a dry vacuum unit is a poor between turnarounds. reasonable run lengths were achieved in economic decision. Past experience is the hydrotreater. being ignored. Vacuum unit reliability During the design phase, it is Fundamental principles drive process Few refiners have experience of common to perform value engineering. and equipment design. For example, processing high volumes of either Typical value engineering approaches because many of these crudes are conventional or oil sands-based heavy make design and equipment selection thermally unstable, equipment must be 106 PTQ Q2 2006 SEPARATIONS designed for minimum temperature and minimum oil residence time. Figure 1 shows major components of the feed system.