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Jul-2011

Naphtha to diesel via novel routes

An unconventional, synthetic approach to converting LPG and naphtha components could boost diesel production

David Gibbons, Foster Wheeler

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Article Summary

Increasingly, refiners are having to look for alternative ways to dispose of naphtha. Traditional markets such as petrochemical naphtha sales are suffering competition from Middle Eastern gas-based steam crackers. Also, given the considerable amount of spare gasoline production capacity in the world refining system, opportunities to export regular gasoline to markets such as the US will continue to decline and reduce the opportunity to incorporate naphtha into the gasoline pool.

Rebalancing of the refinery product slate away from naphtha/gasoline towards diesel is required to meet the projected global diesel demand. Dieselisation is a term commonly applied in the oil refining industry to the practice of shifting yields towards diesel by a combination of crude selection, low-severity fluid catalytic cracker (FCC) operation, and increased hydrocracking and residue upgrading.

More radical options, which are explored in greater detail in this article, include the conversion of naphtha to heavier distillates via steam reforming and subsequent middle distillate synthesis, or polymerisation (oligomerisation) of cracked naphtha material. Case studies based on an average European refinery configuration and crude feed indicate the potential that could be achieved.

Outlook
Growth in demand for diesel is outpacing that of gasoline, and the global diesel balance appears to be tightening. So, if refiners cannot export gasoline, how about naphtha? Naphtha is also under pressure and, with significant world-scale ethane-based steam crackers continuing to come on-stream, the market for export naphtha is expected to reduce. European naphtha-based steam crackers have also become less competitive.1
All of these drivers come together and give increased impetus to the shift towards diesel production. However, rather than looking at ways to increase diesel only, this article looks at ways to achieve both an increase in diesel output and a decrease in naphtha and gasoline production.

Average European refinery
Before looking at some of the options, let us consider an average European refinery2 based on average crude and downstream process unit capacities. This refinery has both an FCC unit and a hydrocracker, with a visbreaker for residue upgrading. This is a simplification, of course, since most European refineries have only one main vacuum gas oil (VGO) upgrading unit, but there are a handful of sites that do, in fact, have such a configuration.

Assuming an average crude slate of North Sea Brent and Middle East Arab Light, and with the chosen process units, there is a good match between this average feedstock and the average refinery, based on a simple generic refinery model with typical product cut points (see 
Table 1).
With this configuration, an average 135 000 b/d refinery already produces more diesel than gasoline, although the total of LPG + naphtha + gasoline is about the same as diesel (see Table 2). Production of jet/kerosene is minimised.

Using average 2010 crude and product prices (based on a typical value of $80/bbl crude), there is around a $20/bbl differential in the price of diesel above that of gasoline.3 If the average refinery can achieve an extra 10% of diesel production, this could be worth around $40 million annually. However, this gives us only around $200 million as a reasonable investment (assuming a generous five-year simple payback). Therefore, the challenge is to find the appropriate simple, low-cost technologies to achieve the desired increase in the diesel-to-naphtha plus gasoline ratio.

Dieselisation: conventional (easier) options
Some of the easier options that refineries can consider are crude selection, cut points, operating mode and catalysts.

Crude selection can make a large difference, but there are other constraints on a refinery that can limit the choice. Refineries typically produce diesel material from the jet/gas oil boiling range of 180–360°C TBP cut point, but can effectively go a little higher with upgrading facilities, such as a mild or even a full hydrocracker processing VGO, or a visbreaker, for example, processing atmospheric or, more usually, vacuum residue.
With the average refinery configuration as before, it might be possible to achieve up to 10% more diesel output by the appropriate choice of crudes, but, again, the configuration limits the ability to process varying yields of intermediate products and their sulphur contents, while maintaining crude throughput.

A number of process units are designed with some ability to switch from gasoline to diesel, such as the FCC unit, or from naphtha to diesel, such as a hydrocracker. However, there is a limit to what refineries can achieve and how far they can go with operational changes. Catalyst additives or complete unit catalyst inventory changes are often required.

If we consider the FCC unit, refiners can drop conversion and increase light cycle oil (LCO) yield, but this is a poor-quality component with many qualities that need upgrading. A typical LCO cetane number is around 20, compared to a range of 40–60 for straight-run and hydrocracked distillates, which is much closer to the finished product specification of 51 or above, and sulphur content is usually far above 10 ppm for the finished product. While the FCC unit contributes around 20 vol% to the gasoline pool, LCO is only 5% of the diesel pool, and with operational changes a refiner might achieve an increase in overall diesel production of perhaps only another 5% overall.

Dieselisation: conventional options
 — bottoms-up approach

If an existing refinery has already achieved as much as possible in terms of crude selection and operation in diesel mode, traditionally refiners would take a bottoms-up approach. For instance, they would look at technologies to break or crack larger, higher boiling point components into smaller, lower boiling point components.

If a refiner already has a visbreaker and a hydrocracker, they might typically look at adding a coker to give more conversion than a visbreaker, or a residue desulphurisation (RDS) unit to produce additional distillate.


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