Designing Vacuum Units

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

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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 performance 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

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PTQ Q2 2006 SEPARATIONS designed for minimum temperature and minimum oil residence time. Figure 1 shows major components of the feed system. Lowering the oil residence time and minimising the oil temperature Pass #1 Pass #3 Pass #2 Pass #4 requires relatively high rates of coil steam injection and minimum heater tube size. This increases the system pressure drop, dictating a high charge pump head. Outlet Outlet tubes tubes These systems should be designed with Double fired overall performance in mind. Common Serpentine Double tubes tubes fired practice is to send out a heater bid specification document with the maximum pressure drop based on charge pump hydraulics. Heater vendors base the heater tube size on allowable pressure drop. Vendors minimise the number of passes and make other compromises, because equipment is often purchased solely on low initial cost. This is not the vendor’s fault. If the buying decision is based on price, what choice do they Figure 3 Double-fired serpentine tube layout have? When designing vacuum units, the charge pump head is a consequence Vacuum heater designed vacuum heater will dictate a of the heater design; it should never Fired heaters processing unstable oils or lower outlet temperature and produce determine the heater design. those operating at a very high less VGO product yield, because the oil temperature must be designed to avoid film temperature and oil residence time Vacuum unit operating rapid coke formation rates. Controlling are both high. Properly designed heaters variables the oil film temperature and residence allow much higher heater outlet Process and equipment design will time is essential to minimise coking. temperatures, producing more VGO determine the product yield, product Nevertheless, most vacuum unit fired product for the same heater run length. contaminants and unit run length. heaters are designed by the vendors Critical oil film and oil residence time Since there is little commercial according to the process data given to variables are controlled through heater experience with many of these crudes, them on an API 560 data sheet. As long design and coil steam injection. the process engineers should be cautious as the vendors comply with the API Large single-cell vacuum heaters have when evaluating key operating guidelines, such as minimum burner four to six tube passes. A well-designed variables. Since many of these crudes are spacing from the tubes, their bids are heater minimises heater flux variation, less thermally stable, the heater outlet considered technically acceptable. This although heat flux always varies from temperature should be balanced against approach ensures a substandard heater the combustion zone, where the fuel/air other variables used to meet the targeted is installed when the primary selection mixture is burned, to the flue gas outlet VGO yield. Flash zone pressure, coil criterion is low initial costs. Vacuum from the radiant section. It is lowest at steam rate and column stripping section heater designs should be specified in the floor of the heater, highest where performance all influence the VGO detail, including tube passes, tube size, the greatest amount of combustion yield. Reducing the column operating tube configuration, burner layout, coil occurs, and decreases in the top of the pressure, decreasing the flash zone oil steam injection location and all other radiant section. Each heater pass must partial pressure by adding coil steam, or factors influencing the oil film be designed so the average heat flux is increasing the stripping section temperature and oil residence time. the same for each pass. Low-cost heaters efficiency will all increase the VGO Heater outlet temperature is only one are designed such that heat absorbed per product yield. Adjusting these variables of the variables affecting coke pass varies significantly. The result is a also affects capital investment and formation, and it is a relatively higher heat flux and higher oil film operating costs. unimportant variable. However, many temperatures in some of the passes. Meeting VGO product TBP cutpoints designers assume a low heater outlet Several factors including the number of 950–970°F or higher requires steam in temperature will ensure a heater will not of burners, burner layout, burner flame the heater, and meeting cutpoints at or coke. Bulk oil temperature certainly length, burner-to-tube spacing, firebox above 1000°F requires a stripping contributes to heater coking, but oil height-to-width (L/D) and non-ideal section on the column. Figure 2 shows cracking occurs in the oil film flowing flue gas flow patterns influence heat flux the total vacuum distillate, which on the inside of the tube. A poorly variability from the floor of the heater includes both the heater and stripping to the outlet. Since heat flux always section contributions. Increasing the varies, the amount of radiant section yield from the stripping section “When designing vacuum surface area in the low- and high-flux decreases the heater outlet temperature units, the charge pump zones must be equal. One approach is to needed to meet a targeted VGO product use multiple external jump-overs so the yield. Determining the most cost- head is a consequence of surface area in each pass is the same in effective combination of column the low- and high-flux zones. However, operating pressure, heater outlet the heater design; it should the designer must be able to predict the temperature, coil and stripping steam never determine the location of high- and low-heat flux flow rates is critical for heavy crude zones. Since many factors, including processing, especially at high gas oil heater design” burner operation (as to opposed to cutpoints. design), influence the location of the

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Single fired tube Double fired tube

Peak heat flux equals ϳ 1.8 ϫ average Peak heat flux equals ϳ 1.15 ϫ average

Figure 4 Single-fired tube — high peak heat flux Figure 5 Double-fired tube — much lower peak heat flux

High mass flux reduces oil film temperature

Mass flux /PTIMISETUBETRANSITIONSIZE LOCATIONTOAVOIDLOWMASSVELOCITY Coil steam ANDHIGHOILRESIDENCETIME

Coil steam controls oil residence time

Figure 6 Oil mass flux and coil steam function Figure 7 Heater tube size transition location high- and low-flux zones, using external the tube facing the burners has a high itf = Temperature drop across the oil film jump-overs does not ensure each pass heat flux, while the side facing the = Qlocal Do/Di h i absorbs an equal amount of heat. A = ºF (1) refractory is much lower. For a single- better solution is to use a serpentine pass fired tube (Figure 4) on two-to-one layout that guarantees each pass gets the Equation 1 shows how temperature spacing, the peak flux is approximately same amount of heat in spite of heat drop across the oil film is calculated. 1.8 times the average for the tube. A flux variation (Figure 3). The Do and Di are the outside and few vacuum heaters and most modern Coke forms inside the radiant section inside tube diameters respectively. coker heaters use double-fired tubes tubes, because the oil film flowing along Combustion zone flame radiation, flue (Figure 5) that greatly reduce the the inside of the tube exceeds the gas temperature and single or double peak flux to about 1.15–1.2 times the temperature and residence time needed firing determine the amount of heat tube average. to initiate thermal cracking. Oil film transferred locally (Qlocal). For a given Vacuum units processing low-stability temperature depends on heat flux and localised heat flux (Qlocal), the crudes should use a double-fired design oil mass velocity. Increasing the heat temperature drop through the oil film is to reduce the peak heat flux and the flux raises the oil film temperature at a controlled by process fluid inside the peak oil film temperature. For example, a fixed mass flux, while a higher mass flux film coefficient (h i). Reducing the tube single-fired heater tube with an average lowers the film temperature at a given size increases the oil mass flux (Equation heat flux of 10 000btu/hr-ft2-°F will have heat flux. Oil residence time depends on 2) and heat-transfer coefficient a peak heat flux of 18 000btu/hr-ft2-°F, oil mass velocity, the quantity of coil (Equation 3), decreasing the oil film whereas the double-fired design having steam and the radiant section tube temperature and thereby reducing the the same average heat flux will have a surface area. While increasing the rate of coke formation. peak heat flux of only 12 000btu/hr-ft2- radiant section surface area decreases °F. Hence, peak film temperatures are the heat flux, it also raises the oil G (mass flux) = Mass rate of oil/inside cross- significantly reduced. residence time. sectional area of heater tube Minimising oil residence time in the 2 Bulk oil temperature plus a = lb/sec-ft (2) radiant section reduces the rate of coke temperature rise across the oil film sets formation. Oil residence time depends 0.8 the film temperature. Cracking occurs Inside tube heat (h i) = (0.023) k/D (DG/e) on the oil mass flux and coil steam rate 0.33 0.14 (cpe/k) (e/ew) where the oil film reaches its highest 2 (Figure 6). Increasing the oil mass flux (peak) temperature. Peak oil film Transfer coefficient = Btu/hr-ft -ºF (3) and injecting the coil steam reduces the temperature depends on the peak heat oil residence time, but it also flux and oil mass flux. Designs that Most vacuum heaters are single fired. dramatically increases the heater reduce the localised peak heat flux lower One side-to-tube faces the burners pressure drop, requiring a much higher the oil film temperature, allowing and the other sees only the refractory. pump head. higher heater outlet temperatures Since the fire is on one side, the heat Coking in the heater outlet is a without excessive cracking and high flux around the circumference of the common problem. Outlet tubes have rates of coke formation. tube varies greatly. The portion of several sizes to control the pressure

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PTQ Q2 2006 SEPARATIONS drop, as the oil vapourises to minimise the bulk oil temperature. In addition, two-phase mixed velocity must be kept 3TEAM below critical to avoid a higher operating pressure, further raising the bulk oil temperature. But a larger tube /PTIMISE size decreases the oil mass flux, causing OVERHEAD PRESSURE a higher temperature drop across the oil film. Oil vapourisation in the larger tubes increases the fluid velocity as long as the tube transition location is correct. 6ACUUM DIESEL The increased fluid velocity dramatically reduces the oil residence time, thus avoiding rapid coke formation. However, if the tube size is increased before oil vapourisation begins, the -INIMISE 0 mass flux decreases, oil velocity -6'/ PRODUCT decreases, oil residence time increases and oil film temperature increases, causing rapid coke formation. Tube size (6'/ PRODUCT transition location must be selected so that the coking in the larger tube sizes is not a problem (Figure 7). Column operating pressure Column flash zone pressure, column diameter and ejector size should be -INIMISE TEMPERATURE -AXIMISE optimised to meet the vacuum gas yield STRIPPING target while minimising the heater 3TEAM outlet temperature. Operating pressure, VGO yield and total steam rate set the vacuum column diameter. Since vessel diameter is a large cost issue, optimising the operating pressure is 6ACUUM RESIDUE essential. Ultimately, column top pressure is selected so that the design flash zone pressure allows the unit to Figure 8 Optimise column operating pressure meets its design VGO yield at an acceptable heater outlet temperature to VGO. If the design operating pressure is avoid rapid coking. The coil and lower than necessary, the ejector system stripping steam rate and design column and column diameter are larger than top pressure determine the ejector necessary, raising capital costs. Motive steam system size. These variables must be Conversely, a high operating pressure carefully selected to minimise overall can increase the heater outlet investment. temperature above the thermal cracking Column flash zone pressure depends limit of these crude oils. Coil steam on the design ejector inlet pressure and the pressure drop through the Conclusions Stripping steam column internals (Figure 8). Internals Lessons learned processing heavy should be designed for a minimum Venezuelan crudes should be applied to pressure drop. Since stripping section these new projects that will process performance has a large influence on heavy sour Canadian crude oils. These the heater outlet temperature, its units should not be designed with efficiency should be optimised so that conventional practices used for light the stripping steam rate is minimum. crudes. Heavy oil properties are very Furthermore, the total coil steam and different, and design requirements will stripping steam rate should be raise initial costs. These higher initial optimised so that the booster ejector costs will pay out over the life of the size is minimised (Figure 9). project through increased VGO product Optimising the column operating yield, lower maintenance cost, reduced pressure has major reliability, product vanadium content gas oil and longer Figure 9 Booster ejector steam load yield, product quality and cost run length between decokings. implications. Ideally, column diameter should be minimum. However, if it is Email: [email protected] too small, the heavy vacuum gas Scott W Golden is a chemical engineer Tony Barletta is a chemical engineer with product will be of poor quality due to with Process Consulting Services Inc in Process Consulting Services Inc in Houston, residue entrainment. Since volatile Houston, Texas, USA. He holds a BS Texas, USA. He holds a BS degree in metals are already high in these crudes, degree in chemical engineering from the chemical engineering from Lehigh any entrainment results in poor-quality University of Maine. University. Email: [email protected]

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