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Meeting diesel specifications at sustained production

04.01.2011  |  Singh, D.,  Shedded Uhde Pty Ltd., West Melbourne, Victoria, Austrailia

New approach rethinks operations of crude and vacuum distillation units to recover more distillate

Keywords: [diesel] [clean fuels] [environment] [distillation] [crude oil] [gas oil]

Refiners will face a tough challenge over the coming years to cope with growing diesel demand at the expense of gasoline, while conforming to the progressively tighter diesel fuel specifications to meet environmental regulations. The more stringent specifications are likely to hurt the overall diesel yield due to possible rejection of heavy ends from the present product spread.

Future diesel will have a narrower density band, lower cloud point, lower T95 (D86) specification and less sulfur. Therefore, aligning the existing refineries with progressively tighter diesel specifications at a sustained diesel yield will be quite challenging. Some recent compliance project experience is reported here; the goal is to identify the most cost-effective and appropriate modifications to an existing plant to achieve future diesel specifications at a sustained production cost and rate. Project constraints prompted applying engineering solutions rather than installing more expensive process technology options that would have required reconfiguring the existing refinery at high cost.

Study refinery.

Finished diesel product in a refinery is a blend of various distillate-grade streams, as shown in Fig. 1. A blend optimization is performed with refinery blending simulation software, utilizing the individual blend streams composition and associated proportions in the pool for maximum diesel production at the required market specifications. In a typical refinery configuration, these blend constituents are produced from the units, as illustrated in Fig. 2, and include:
• Crude distillation unit (CDU). This is the main source of straight-run (high-cetane) saturated distillate-grade material to the pool.
• Hydrocracker unit. The main source for cracked but saturated distillate-grade material in a refinery designed for higher diesel production.
• Fluid catalytic cracking unit (FCCU). This unit produces cracked unsaturated (poor quality diesel) distillate material as a secondary product. The intent of the unit is to produce mainly high-quality gasoline or petrochemicals.
• Coker unit. This unit also produces cracked unsaturated poor-quality distillate-grade material as a secondary product.

 
  Fig. 1. Typical product diagram for a refinery.   


A typical refinery contains either an FCCU or a hydrocracker unit, depending on whether higher gasoline or diesel production is targeted. A review of Figs. 1 and 2 identifies the CDU and vacuum distillation unit (VDU) as potential sources for improving the diesel production by improving fractionation performance. In particular, for older refineries, these may have already been revamped to achieve greater throughput rates. This improved fractionation may enable the operator to compensate fully for the possible loss in diesel production due to tighter future specifications.

 
  Fig. 2. Process flow diagram of refinery.  


CDU upgrades.

It is quite challenging to upgrade the atmospheric distillation column to achieve higher diesel recovery while maintaining the stream cloud-point specification. In principle, such upgrades tend to lift more diesel-grade material by elevating the feed flash-zone temperature or stripping harder in the CDU bottom section. These require the refiner to implement several de-bottlenecking options, either individually or collectively:

• Maximizing utilization of crude-feed heater duty and tactically upgrading the crude preheat train to maximize the heat recovery to increase the crude column feed flash-zone temperature.1

• Rectifying CDU bottom stripping steam rate to maximize diesel-grade material stripping from the atmospheric residue. This may require upgrading the stripping section and atmospheric gasoil (AGO) wash section (section between feed flash zone and AGO pumparound) to avoid any possible fouling or coking.

• Maximizing diesel recovery by minimizing diesel fraction overlap with AGO, kerosine and fractions.

Fig. 3 shows a typical configuration of an atmospheric crude column bottom section. Increasing the CDU feed flash-zone temperature for higher diesel lift also results in heavy constituents’ lift, responsible for diesel cloud-point specification. A typical overloaded existing crude column cannot prevent these heavy tails entering into the diesel draw stream. A similar limitation occurs when the column bottom stripping steam rate is increased to enhance diesel recovery. Further, these operational changes increase the column’s pressure drop due to the increased vapor traffic and suppress the upgrades effectiveness to some extent.

 
  Fig. 3. Configuration of the atmospheric crude column bottom section.  


Therefore, these operational changes should be accompanied by some mechanical modifications in a typical crude column bottom section, as suggested in Fig. 4, to obtain the desired results.

 
  Fig. 4. Modified atmospheric crude column bottom section to increase diesel-product yield (modifications shown in red).  


Controlled and optimum AGO wash rate will prevent heavy tails carryover into AGO and diesel sections that would otherwise occur at the increased feed flash-zone temperature and at the increased stripping steam rate. An optimum AGO wash rate minimizes valuable AGO loss from the wash section. Replacing trays with packed beds will overcome the impact of increased vapor loading on the column pressure drop. The proposed modifications of Fig. 4 can provide several advantages over the existing conventional atmospheric crude column:

• A controlled AGO wash rate ensures uninterrupted wetting of wash-section packing needed for trapping the heavy tails, responsible for off-spec diesel product on cloud point.

• A controlled AGO wash configuration provides flexibility to optimize the stream flow to minimize valuable AGO loss from the wash section. In general, the optimum AGO wash rate is achieved by ensuring that the rate of liquid from the wash section is approximately 3–3.5 mass% of the total column feedrate.

• A guaranteed performance of the wash section enables optimizing stripping steam rate in the column bottom for improving diesel and AGO recovery from the column bottom stream while maintaining diesel cloud-point specification. Depending on COT, 10 lb to 20 lb stripping steam per bbl of the bottom product provides the optimum results.

• Replacing trays with packing increases the number of theoretical stages in the AGO and diesel sections, which helps to minimize the distillation overlap between AGO and diesel fractions.

• Able to control diesel under-reflux flow to the AGO section to control slippage of valuable diesel material in the AGO fraction.

• Increased diesel and AGO recovery improves the crude preheat-train performance (extra heat available for exchange) and also reduces feedrate to the VDU. The reduced VDU feedrate lowers the total ambient heat lost via the vacuum gasoil (VGO) air coolers, thus improving the overall heat efficiency.

• Reduced crude column bottom rate generates spare duty in the VDU feed heater and the vacuum section for minimizing the vacuum residue rate.

The proposed upgrade of the studied tower is able to increase the diesel yield approximately 1.5 vol% to 2 vol% of the total crude feedrate. The proposed upgrades have a potential payback of three years.

VDU upgrades.

Fig. 5 shows a typical dry VDU. A wet VDU (stripping steam in the column bottom) is almost taken over by dry VDU once high vacuum generation and low pressure drop internals became feasible. No stripping steam in the column bottom makes the dry VDU column smaller in diameter and also requires a smaller vacuum section as compared to the wet VDU.

 
  Fig. 5. Typical scheme of a dry VDU.  


The usual intent of a VDU in refinery is to recover the VGO fraction from the CDU bottom stream by separating the fuel-oil grade from the heavy tails. The recovered VGO fractions are processed further in downstream conversion units into a more valuable diesel or gasoline products. VGO is coarsely separated into heavy VGO (HVGO) and light VGO (LVGO) in the column upper section to provide required quality wash oil (HVGO) to HVGO wash section and also to minimize the column top section diameter. Thereafter, LVGO and HVGO combined streams are usually sent to either the hydrocracker or FCC units for further processing.

A typical dry VDU has approximately eight theoretical stages in total, almost equally distributed in all four sections, i.e., the slop-oil wash section, HVGO wash section, HVGO section and LVGO section. The slop-oil wash section is an optional feature considered in some VDUs; it is used to further ensure that the heavy metals are retained in the slop oil, thereby eliminating contamination of the VGO stream.

 
  Fig. 6. Scheme of a dry VDU with modifications to improve fractionation performance (modifications shown in red).  


Traditionally, VDUs are not designed for sharp separation between HVGO and LVGO. This can be clearly seen in a performance data plotted for a typical existing VDU in Fig. 7. This VDU was receiving lighter than the usual feed due to inadequate lift in the CDU feed flash zone, caused by the poor performance of the crude preheat train and a reduced stripping steam rate in the column bottom to maintain the diesel side-draw stream cloud-point specification. The reduced stripping rate had resulted in fouling/coking in the CDU stripping section, thereby causing further deterioration of the overall performance.

Poor performance in the CDU could be compensated by upgrading the downstream VDU for enhanced diesel recovery. For example, an existing VDU modification can be justified if the upstream CDU is upgraded for a higher diesel recovery. Cost-effective modifications of an existing CDU can reduce diesel loss in the bottom stream significantly but not completely. Furthermore, a CDU fractionation efficiency cannot be stretched beyond a certain limit. Therefore, it has been observed that even a reasonably well-performing existing CDU in post-upgrade operation can slip diesel-grade material into the bottoms in sufficient quantity to justify the downstream VDU upgrades to recover the diesel. This upgrade option will not only compensate for the diesel-product loss due to the tighter specifications, but also may improve the refinery’s overall economics by increasing diesel recovery.

 
  Fig. 7. Performance of the existing VDU.   

A study was conducted for upgrading a conventional dry VDU for recovering diesel-grade material from a poorly performing CDU bottom stream. The proposed modifications shown in Fig. 6 shifted VDU fractionation performance from Figs. 7 and 8. The improved separation recovered allowed approximately 80% of the diesel slipping from the CDU. The proposed modifications were able to recover almost 40 vol% of VDU feed as diesel-grade material for hydrotreating into a finished diesel product. In many conventional refinery configurations, this diesel fraction would have been recovered as VGO and would have been routed to either the hydrocracker or FCCU for conversion. The proposed upgrade was able to enhance the overall diesel production by 3 vol%–5 vol% of the CDU total feed in the diesel-based refinery (refinery with hydrocracker unit) and that could be even greater in a refinery operating with an FCCU. The estimated payback period for the proposed modifications is two years.

 
  Fig. 8. Performance of the existing VDU after modification to increase diesel make.  


The VDU upgrades proposed in Fig. 6 (shown in red) can be explained as:

• Tripling HVGO section theoretical stages for sharp separation (reduced overlap) between HVGO and LVGO. In this scenario, LVGO was essentially the diesel-grade material.

• Hot LVGO under-reflux (P1 new) to HVGO top section to maximize the diesel recovery in the LVGO section while maintaining the stream cloud-point specification.

• Doubling of the LVGO section theoretical stages reduces LVGO pumparound (P1) rate by achieving a close temperature approach between the vapor inlet and the liquid leaving the LVGO section.

Options.

This study shows that future diesel specifications can be met at sustained production rates, provided that refiners reorient their distillation operations to improve the diesel separation in CDU and VDU. This concept will allow refiners to improve diesel production while also meeting future specification at the nominal upgrade CAPEX. A similar distillation approach in the existing residual FCC or hydrocracker fractionation sections may further improve overall refinery diesel production. HP

ACKNOWLEDGMENT

Special thanks to Graeme Cox, Uhde Shedden chief technology and sustainability officer; Patrick Cadenhouse-Beaty, Uhde Shedden manager of refining technology; and Richard O’Beirne, Uhde Shedden chief process engineer refining, for reviewing and providing their valuable feedback to shape this article into its present form.

LITERATURE CITED

1 Singh, D. and S. Van Wagensveld,” Redesign crude preheater train for efficiency,” Hydrocarbon Processing, May 2007, pp. 91–94.

The author 

  Dinesh Singh is a principal engineer with Uhde Shedden (Australia) Pty Ltd. He has over 18 years of worldwide process engineering experience in petroleum refining, offshore and onshore oil and gas projects. Mr. Singh holds a BS degree in chemical engineering from the Indian Institute of Technology, Roorkee, India. He is a recognized chartered engineer with IChemE UK. 




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05.23.2011

Thanks for agreeing that Fig-4 needs correction.
Regarding Author's comment that "Diesel under reflux will be very small stream", please note that the Diesel under-reflux would be equal to the entire total of entire AGO + CDU Bottoms. Diesel under-reflux would thus be roughly be 35-40% of the CDU feed (depending on crude source), and nearly same as the diesel production.
Hence, even if we modify the Fig-4 by having only under-reflux going via the stripper, that too is doubling the flow profiles in the diesel stripper - and would certainly require replacement of diesel stripper + pump aswell as replacement of the column overhead system to handle the additional steam + stripped vapor flow.

Based on experience from having perfoemd several CDU/VDU revamps, I would advise this revamp study be checked beyond the study stage. Checking the mechanical adequacy of equipment is best done alongside the simulations, to prevent such scenarios.


05.20.2011

The author's responses to those points:

1. Diesel under reflux will be very small stream to prevent AGO tails (responsible for diesel higher cloud point) carryover in diesel section. As far as diesel pump around is concern, it should be as per Fig-3 (Fig-4 should be corrected accordingly).

2. Answer as above

3. Answer as above

Based on the field experience the flow controlled option is observed to be very effective, especially when refinery throughput is varied.


05.16.2011

Revamp as suggested in the article in Fig-4, would be un-workable on following counts :
1. Figure-4 suggests entire diesel pumparound be re-routed via the stripper & pumped by the stripped diesel pump. Pumparound flow would be many-a-times larger than diesel product flow, and this re-route would require many-a-times bigger diameter stripper than the existing one.
2. Liquid flow increase by several times in the stripper, would require proportionately higher stripping steam flowrates, and simple simulation would reveal that the magnified steam consumption would make the overhead condensor ciruit also insufficient.
3. Stripped diesel product pump would also magnify in size.

Secondly, article's emphasis is on AGO wash rate to be controlled by a controller.
But flow controlling the wash rate doesn't provide the operator any additional flexibility, as the wash rate is simply the balance dependent upon associated parameters like COT, draw rates etc.
Just "measurement" of the wash rate is sufficient, for the operator to know the quantity and do the needful COT or product rate adjustments.


05.10.2011

Good article looking to current enviornment regulations and incressing demand of fuel.

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