Mar-2011

Dieselisation screening studies

Catalyst and process studies address refiners’ requirements to respond to a changing market for diesel and other fuels

Kenneth Stippich, UOP, A Honeywell Company
Eelko Brevoord, Albemarle

Article Summary

Diesel demand is expected to keep growing over the long term. However, external factors such as economic activity and weather cause this demand to fluctuate. These ebbs and flows in demand create volatility, which requires refineries to be flexible. This article discusses options to increase diesel production by running the FCC pretreater in conversion mode, the FCC unit in maximum light cycle oil (LCO) mode, and debottlenecking the distillate hydrotreater.

Early in 2008, prices for ultra-low sulphur diesel soared, resulting in attractive margins for those refiners who could increase diesel production. Many refiners responded by operating their FCC units at lower conversion levels, adjusting hydrocracker conditions to shift towards more diesel and, where possible, processed crudes containing more diesel-range feeds. In 2009, the situation reversed and most refiners changed their FCC operations back to maximum conversion mode.

Based on projections of economic growth in many emerging economies, it is expected that demand for diesel will continue to grow and that margins for producing diesel will once again be attractive. In addition, efforts to regulate sulphur emissions from bunker fuels will likely require the substitution of marine diesel for high-sulphur bunker fuel in some regions to meet the specifications, further increasing the demand for diesel. Renewable fuel standards requiring increased ethanol blending, together with requirements for increased vehicle efficiency, will likely result in reduced demand for petroleum-derived gasoline in the US. Refiners in the US and those regions supplying the US market will be forced to seek other viable economic uses for their refining assets in order to stay in business.

Given the increased demand for diesel, refiners will look at adjusting those assets to produce diesel when the economic drivers are favourable, while retaining the flexibility to produce gasoline during those times when economics favour this product. Consequently, product flexibility is vital to successful refining business.

To help refiners address this need for flexibility and establish a strategy for maximum profitability, Albemarle and UOP offer dieselisation solutions through the Hydroprocessing Alliance, which the two companies formed in 2006. These solutions include dieselisation screening studies to help refiners anticipate future market scenarios and strategies designed to optimise diesel yield, minimise fuel oil production and retain product flexibility.

Dieselisation screening studies
A dieselisation screening study focuses on the catalytic processes and hydrogen issues that directly affect diesel yield and transportation fuel flexibility. The areas considered are:
• FCC unit feedstock pretreatment
• FCC unit
• Middle distillate hydrotreating
• Hydrocracking: mild, partial and full conversion
• Hydrogen management and purification.

To provide refiners with the flexibility to produce more diesel, a dieselisation screening study evaluates the constraints of existing technologies and their suitability for rapid implementation for this project. The approach is to find low-cost options, such as the application of different catalysts, different operating modes and equipment modifications. The most attractive scenarios are determined via a cost-benefit analysis.

Technology options
The evaluation of options available  to increase distillate production within a refinery requires a comprehensive understanding of the current refinery configuration and the technical options available for meeting the refiner’s desired goals. The basis for this review, in order to present a varied number of scenarios, will be an FCC-based refinery that produces transportation fuels. Attention will be paid to those units that produce or process distillate-range fuel. These include the units that are in the FCC complex, the distillate hydrotreating unit and the naphtha hydrocracking unit.

FCC complex
Many refineries include FCC technology as their conversion unit. The FCC unit generally is operated to maximise gasoline production. The distillate-range LCO that is produced traditionally has been blended off into fuel oil and the diesel pool. While new LCO-selective catalysts, along with modifications to process conditions, allow the refiner to increase the amount of LCO produced by the FCC unit, existing and upcoming regulations are making it difficult to continue to blend off the LCO.

Figure 1 represents an FCC-based refinery with a catalytic feed hydro-treating (CFH) unit and a delayed coking unit (DCU) for bottoms upgrading. Feed to the CFH unit consists of atmospheric bottoms and vacuum gas oil (VGO) from the crude distillation unit (CDU) and coker gas oil from the DCU. The product from the CFH unit includes naphtha and lighter material, distillate and hydrotreated feedstock for the FCC unit. The FCC unit produces naphtha and lighter material, LCO and unconverted slurry oil.

Distillate-range material from the CFH unit and DCU, along with straight-run distillate from the crude unit and LCO, is processed in the distillate hydrotreater (DHT) to meet refinery distillate fuel specifications. The naphtha streams from the DCU, CFH and FCC units are sent for further processing to produce naphtha for gasoline blendstock and other uses.

Operating the FCC unit in maximum LCO mode
LCO yield from the FCC unit can be increased by reducing conversion. Reducing conversion can be achieved by lowering the FCC reactor temperature and/or catalyst activity. In most cases, this is not a very attractive operation, as the slurry oil yield also increases.¹

In a case where the slurry oil has good quality, one could consider recycling it back to the reactor riser. Although recycling has been practised in many refineries, it is usually not very attractive, as it takes up unit capacity, increases delta coke and deteriorates performance.