Jan-2009

Maximising diesel recovery from crude

The CDU/VDU process flow scheme is reviewed, including equipment design and operating fundamentals used to maximise straight-run diesel recovery

Scott W Golden, Process Consulting Services

Article Summary

Increased recovery of straight-run (SR) diesel from crude oil improves refinery profitability. Low crude/vacuum unit (CDU/VDU) SR diesel recovery is caused by existing unit process flow schemes, equipment designs and operating conditions. Since many FCC or hydrocracker feeds contain 25–35% or more diesel boiling-range material, there is significant opportunity to improve recovery. Low or moderate capital investments have increased refinery ULSD product yields by more than 5% on whole crude.

CDU/VDU diesel recovery
Refiners in the US have until recently targeted maximum gasoline production with little focus on CDU/VDU SR diesel recovery. Not surprisingly, few US refiners achieve good SR diesel recovery. This is because most of the existing process flow schemes and equipment were not designed to maximise the SR diesel yield from crude, nor were the operating variables associated with these flow schemes optimised for maximum SR diesel recovery. Most US refiners produce SR diesel only from their atmospheric crude columns. Even some of the new CDU/VDUs have not been designed for maximum recovery, because many major E&Cs (and some refiners) continue to believe that diesel should be produced only from the atmospheric column. Even though diesel margins are very strong, there are still many misunderstandings about maximising CDU/VDU diesel production.

Many non-US refiners have designed their CDU/VDU to maximise diesel recovery. Their atmospheric crude columns have 10–14 trays between the flash zone and the diesel product draws. They do not produce an atmospheric gas oil (AGO) product as FCC or hydrocracker feed. Moreover, the CDU/VDUs have been designed or revamped to produce diesel from the vacuum column’s top side-draw. Conversely, many US refiners have only two to five fractionating trays between the diesel and AGO product draws, and operate with very little reflux below the diesel product draw. Furthermore, it is not unusual for a US refiner’s vacuum column feeds to contain 8–10% diesel, with only a few VDUs producing a vacuum diesel product. It is also not unusual for a US refiner’s top vacuum column side-draw product to contain 70–90% 650°F (343°C) minus diesel boiling-range material. In most cases, this diesel ends up in the FCCU.

Refiners can quickly determine their CDU/VDU performance by evaluating the amount of 650°F (343°C) minus diesel material in the CDU/VDU streams feeding the FCC or hydrocracker. Even though hydrocrackers may recover some of this diesel boiling-range material, it may not be the most cost-effective place to process it. A well-designed CDU/VDU produces FCC or hydrocracker feed streams containing less than 5 vol% diesel boiling-range material. 

Process flow scheme
Most US refiners produce diesel product only from the atmospheric crude column (Figure 1). Changes to the CDU process flow scheme and atmospheric column equipment design and operating changes can improve recovery. However, diesel recovery is limited by process and distillation fundamentals in the atmospheric column. It can be optimised, but it has fundamental limitations that constrain recovery. Only a few US and many non-US refiners produce both atmospheric and vacuum diesel products (Figure 2). Fractionation is inherently better in a vacuum column, therefore diesel recovery is greatly improved. Fractionation is driven by column internal efficiency and the fractionation section’s liquid-to-vapour (L/V) ratio. The top section of the vacuum column has a much higher L/V ratio than the atmospheric column, hence vacuum columns fractionate better than atmospheric columns no matter how good the CDU design. Better fractionation reduces the vacuum diesel product 95% to endpoint tail compared to 
atmospheric column diesel. Additionally, the combined atmospheric and vacuum diesel products have better cold flow properties due to improved fractionation. The CDU/VDU process flow scheme has the largest influence on SR diesel recovery. The CDU/VDU must have a vacuum diesel product draw to maximise recovery.

It is common for US refiners to produce an AGO product (Figure 1). This stream is often combined with the vacuum gas oil (VGO) products feeding an FCC or hydrocracker. Since atmospheric crude column fractionation is inherently poor, AGO product contains 30–70% or more diesel boiling-range material. To maximise diesel recovery, no AGO product should be produced or it should be fed into the upper section of the vacuum column (Figure 3), where it can be fractionated into vacuum diesel and VGO products. Overall CDU/VDU diesel product recovery and energy consumption are improved when atmospheric column AGO is fed to the vacuum column. The amount of diesel in the AGO product will determine incremental recovery from processing AGO in the vacuum column.

When revamping or designing a new CDU/VDU to improve diesel recovery, selecting the most economic process flow scheme should also consider energy efficiency and VGO product yield. Energy optimisation may set the proper amounts of diesel from the atmospheric crude and vacuum columns. Atmospheric column diesel product is withdrawn at 525–575°F (273–302°C), depending on operating pressure and diesel product distillation. This energy can be used to preheat crude, whereas the same diesel boiling-range material produced from the vacuum column is withdrawn at only 250–300°F (121–149°C), making it impossible to cost-effectively recover the heat. Balancing diesel production between the atmospheric and vacuum columns saves energy, especially with light and moderately heavy crude oils.

Refiners processing heavy and extra-heavy crudes like Merey or BCF 17 must balance atmospheric and vacuum column distillate yields against VDU VGO product yield objectives. Atmospheric residues from heavy crudes are so difficult to vapourise even with the best VDU design that atmospheric column distillate yields must be constrained.  Increasing the amount of diesel in the atmospheric residue increases the VGO product yield.  Conversely, as atmospheric residue gets heavier, VGO product decreases and vacuum residue production increases. Since maximising the VGO product yield increases the overall refinery liquid volume yield and reduces coke production, balancing atmospheric and vacuum column diesel yields is crucial when processing heavy and extra-heavy crudes.

Atmospheric crude fundamentals
Understanding atmospheric and vacuum column distillation fundamental principles is crucial to optimising CDU/VDU economics. The atmospheric column internal reflux rate below the diesel product draw and fractionation efficiency determines the diesel yield (Figure 4). The column design requires the correct number of trays or amount of packing efficiency and it also needs optimum internal reflux (L). Fractionation efficiency alone will not produce maximum diesel product. Fractionation section L/V ratio is one of the fundamental principles that determines product yield. The vapour rate into the fractionation section is set by the vapour leaving the flash zone and the column heat balance.