Diesel and VGO recovery
Fundamental design considerations influencing diesel and vacuum gas oil (VGO) recovery when revamping existing or designing grassroots crude units. By increasing vacuum column diesel recovery, crude throughputs via major conversion units may also be increased
Tony Barletta, Process Consulting Services
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Good fractionation is basic to achieving maximum return on every barrel of charged crude. This is so obvious that it is puzzling why some refiners ignore innovative changes to traditional crude unit design, which can increase the yield of higher value streams, unload the FCC, hydrotreater and coker, permit more efficient use of energy, and allow higher crude charge rates. Yet, in many cases, all this can be accomplished with relatively modest investment.
Specifically, two increasingly important crude unit design considerations are diesel and FCC feedstock recovery. Large amounts of 650°F (343°C) -minus diesel boiling range hydrocarbons in the FCC feed and 1100°F (593°C)-minus VGO boiling range material in the coker feed unit consume capacity of these units. Because often one or both of these units operates at maximum capacity, low recovery can limit crude throughput or the amount of heavy crude in the crude blend.
Typically, US refiners produce diesel and a small amount of FCC feed as atmospheric gas oil (AGO) from the crude unit’s atmospheric column. The remainder of the FCC feed comes from the vacuum column (Figure 1). Most non-US refiners, on the other hand, produce diesel product from both the atmospheric and vacuum crude columns (Figure 2) and no FCC or hydrocracker feedstocks from the atmospheric column. Because US refiners have historically focused on gasoline production, they have not been too concerned about diesel recovery, but outside the US the primary motor fuel is diesel, hence recovery is generally high. The result is that many US refiners produce FCC feedstock containing 20–25 vol% diesel, whereas non-US refiners’ FCCU feed contain less than 5 vol% 650°F (343°C)-minus material. Improving diesel recovery can unload the FCC or hydrocracker, allowing VGO recovery to be increased to fill these conversion units.
VGO recovery varies widely and depends on the design and operation of the vacuum unit as well as the type and contaminant levels of crude oils processed. Refiners running lighter crudes tend to have higher heavy vacuum gas oil (HVGO) cutpoints because the vacuum unit feed is easier to vapourise. Conversely, it is not uncommon for refiners processing heavy crudes to have HVGO TBP cutpoints of less than 950°F (510°C). In these cases the coker unit feed contains as much as 5–7 vol% of the crude charge as recoverable gas oil.
When revamping or designing grassroots crude units, diesel and VGO recovery are important design considerations that impact crude charge rate or the amount of heavy opportunity crude that can be processed within downstream unit capacity limitations. Furthermore, as new ULSD specifications go into effect, being able to sharply fractionate diesel from the FCC feed will have significant economic benefits by increasing ULSD hydrotreater run length and capacity.
As noted, non-US refiners have designed their crude units to maximise diesel recovery. Their atmospheric crude columns have 8–12 trays between the flash zone and heavy diesel draw and have heat integration schemes that allow the units to be operated at 4–6 vol% overflash. Moreover, these units have been designed to produce diesel from the vacuum column top product draw or have been revamped to produce a vacuum diesel stream. On the other hand, many US refiners have only 3–6 trays between the diesel and AGO product draws and little reflux below the diesel product draw resulting in AGO product containing more than 50–60 vol% diesel. Furthermore, it is not unusual for US refiners’ vacuum column feeds to contain 8–10% diesel with only a very few vacuum units producing a diesel product. Thus, it is not unusual for the light vacuum gas oil (LVGO) product to contain 70–90% 650°F (343°C)-minus diesel boiling range material. In most cases this diesel ends up in the FCC unit or the hydrocracker.
Atmospheric column diesel recovery is inherently difficult because the molar liquid-to-vapour (L/V) ratio in the fractionation section is typically less than 0.1, whereas in the vacuum unit it is 0.3–0.5. It is simply impossible to achieve high recovery without a diesel product draw on the vacuum column. Fractionation basics favour this solution. Thus it is not surprising that crude units are designed differently where diesel product is the primary motor fuel and economics favour high recovery.
Heat recovery and VGO product recovery determine optimum split when producing diesel from the atmospheric and vacuum columns. High atmospheric column diesel recovery makes the feed heavier to the vacuum unit, reducing VGO product yield. VGO yield decreases as atmospheric distillate yield increases at a fixed vacuum heater outlet temperature and column flash zone pressure. Heat recovery and crude preheat depend on atmospheric diesel yield. Diesel draw temperature is 530°F (276°C) or higher in the atmospheric column compared to 250°F (121°C) from the vacuum column. Consequently, all the heat duty needed to condense vacuum diesel is lost to air and water, whereas all the condensing heat and approximately half the product cooling heat are recoverable to crude oils from the atmospheric column.
There are wide variations in VGO recovery, depending on several factors. Some units have high recovery but many still operate at low or moderate TBP cutpoints. Low pressure and high temperature, and residue steam stripping are variables used to adjust yield. Flash zone pressure should be minimised to the column diameter limitation. Vacuum heater outlet temperature should be maintained as high as possible while achieving acceptable heater run lengths. Refiners processing medium to low API gravity crude oils must use steam in the heater coils and design the column with a stripping section to achieve reasonable recovery. HVGO product TBP cutpoints can be increased by up to 80°F when designing the unit with a proper stripping section.
Improved diesel recovery
In a typical US refinery, higher diesel recovery requires yielding more atmospheric column diesel or modifying the vacuum unit to produce diesel. Raising atmospheric column diesel yield demands better stripping, higher flash zone temperature, lower column operating pressure, higher diesel/AGO fractionation section reflux or more fractionation efficiency. In some instances this is the most practical and cost-effective solution. Yet in other cases modifying the atmospheric column is very high-cost with relatively small yield improvement. Each case must be treated individually.
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