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Meeting production targets for ultra low sulphur transportation fuels

Increasingly stringent sulphur regulations and diesel price premiums are prompting refiners to draw a broad process envelope for yield, quality and economic assessment

Bob Leliveld
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Article Summary
Are increasingly stringent specifications for transport fuel sulphur a challenge or an opportunity? The answer depends on many variables, including refinery configuration, operating strategy and catalyst technology. Even with the latest high activity catalyst technologies, refiners face accelerated catalyst deactivation and thus more frequent catalyst changeouts. This leads to shorter unit run lengths and higher operating costs. Although meeting new transport fuel sulphur regulations will add to their costs for most refiners, the application of new catalyst technologies may help to lessen the impact and to give some of them a competitive advantage.

The amount of sulphur allowed in transport fuels has been reducing across the world. For example, the United States Environmental Protection Agency’s Tier 3 specifications, which come into effect in 2017, will reduce gasoline sulphur content to ≤10 wppm from the Tier 2 maximum of ≤30 wppm. At the same time, demand for diesel transport fuel is increasing, as light-duty diesel vehicle sales increase to meet Corporate Average Fuel Economy standards.

For refiners without FCC naphtha post-treatment capabilities, the greatest impact of Tier 3 and similar ultra low sulphur gasoline (ULSG) regulations will be on FCC-PT unit operations. These refiners are likely to experience significant reductions in FCC-PT unit cycle lengths and have higher operating costs. This is because the more severe processing required for the heavy FCC-PT feed will result in accelerated catalyst deactivation and more frequent catalyst changeouts.

FCC gasoline typically contains 5–7% of the sulphur content of the hydrotreated FCC vacuum gas oil (VGO) feed. Thus, if the VGO product from the FCC-PT unit contains 1200–1500 wppm sulphur, the FCC naphtha that results generally contains 60–100 wppm sulphur (see Figure 1). For a refinery meeting the Tier 2 gasoline specifications of ≤30 wppm, this implies that the FCC naphtha going to the gasoline pool can be 30–50% of the overall pool. Most FCC-PT units operating in constant hydrodesulphurisation (HDS) mode (about 70% of refineries worldwide) can achieve these VGO product sulphur limits easily and many achieve cycle lengths of two and a half years or more. Consequently, many FCC‑PT units undergo two turnarounds for each FCC unit turnaround.

To meet Tier 2 gasoline specifications with the FCC feed and naphtha product sulphur allowances cited, many refiners without FCC naphtha post-treatment capabilities must achieve 90–95% HDS in their FCC-PT units. To meet the Tier 3 ULSG specifications, the allowable sulphur content of FCC naphtha will need to be 20–35 wppm. This will generally mean that the VGO feed to the FCC unit should contain 400–600 wppm sulphur, which translates into 97–98% HDS for the same FCC-PT unit.

Reduced unit cycle lengths
Albemarle has used its proprietary hydroprocessing catalyst (HPC) process model to simulate units operating under three different pressure regimes to predict the likely impacts of Tier 3 ULSG regulations on different types of FCC-PT unit operation. These screening studies assumed 100% hydrotreatment of FCC feed and were for illustrative purposes only. The actual impacts of the Tier 3 specifications will depend on a refinery’s specific feed compositions, unit capabilities and constraints, and FCC-PT operation strategies.

For units operating in constant HDS mode at low to moderate hydrogen partial pressure (40–70 bar), meeting Tier 3 gasoline specifications is projected to shorten the FCC-PT cycle length by 20–40%, even if very high-activity CoMo or NiCoMo VGO HDS catalysts are used (see Figure 2). The hydrogen consumption at the start of run is likely to increase by roughly the same amount because the weighted average bed temperature will have to be higher and will begin close to the maximum aromatics saturation temperature for the feed and operating pressure.

For moderate to high pressure FCC-PT units currently operating in maximum aromatics saturation mode, the effects of the new specifications are harder to quantify. Initial simulations suggest that the decrease in cycle length could be 50% or more and that the operation may have to switch to constant HDS mode early on to achieve and maintain greater than 98% HDS (see Figure 3).

What are the options?
In assessing their options for meeting Tier 3 ULSG regulations, refiners will need to draw a broad process envelope for yield, quality and economic assessments to account for the overall effects on the bottom line. Specifically, as a minimum, the assessments will need to include the FCC-PT and FCC units’ performance. Refiners with FCC naphtha post-treatment capabilities will also need to include these units in their process envelope.

As a result of the higher severity of FCC-PT operations, most refiners will face increased challenges when trying to coordinate FCC-PT operating cycles with FCC unit turnarounds. The present convenient fit of two FCC-PT operating cycles for each FCC unit turnaround will no longer be possible. At the same time, many refiners will be trying to increase their output of ultra low sulphur diesel. Faced with this dual challenge, it seems likely that many operators will investigate alternative FCC-PT operating strategies to help them optimise their overall operations and product slates.

Those refiners with the flexibility to implement alternative modes of FCC-PT operation, such as mild hydrocracking (MHC), may opt to evaluate alternative operating strategies, and their projected impacts on overall refinery yields and qualities of intermediate and finished product streams. This option may be especially important for refiners that are trying to increase their ultra low sulphur diesel yields.

Refiners with FCC naphtha post-treatment capabilities may find that operating their FCC-PT unit at a higher severity to lower the sulphur content of FCC naphtha helps them to meet Tier 3 ULSG regulations with similar octane losses to those incurred in meeting Tier 2 specifications.

In general, refiners will incur higher costs for their FCC-PT units to meet Tier 3 sulphur regulations. Loading higher activity catalysts, such as ultra high HDN activity KF 861 NiMo catalyst, will help to lengthen cycles compared with using current catalysts. However, more frequent changeouts, even with these new catalysts, are likely to result in annualised fill costs up to 30% higher than those currently incurred. Shorter cycles mean that the cost of turnarounds and spent FCC-PT catalyst disposal will also increase.

Additionally, higher hydrogen consumption for the early portion of the operating cycle (until the 
maximum aromatics saturation temperature is reached) will increase the operating costs for that part of the cycle. It is also more critical that there is full hydrogen availability at the start of cycle to avoid hydrogen starvation of the catalyst. The overall impacts of these factors will be highly dependent on a given refinery’s specific supply and unit considerations.

Taking advantage of low cost hydrogen through volume swell
Increased gas production from hydraulic fracturing shale formations in North America has led to a substantial fall in natural gas and hydrogen prices. These prices are likely to remain relatively stable over the coming years, which makes hydrotreating an attractive option for boosting ULSD volumes.

Adding hydrogen to aromatic distillate feedstocks significantly increases liquid volume and improves product quality. The largest gains in liquid volume come from increased hydrogen addition to the most aromatic distillate streams, such as light crude oil (LCO), but, for a moderate to high pressure ULSD unit, gains are possible even when processing typical feedstocks.
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