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Apr-2009

Continued gains in FCC pretreating: part II

Case studies show the application of MHC systems in FCC pretreaters to increase middle distillate yields, while achieving reasonable conversion and cycle length. These systems offer stable operations at high temperatures and moderate pressures using low-quality feed

Desiree J deHaan, Andy Shivaram and Kevin D Carlson
Criterion Catalysts & Technologies
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Article Summary
Part I of this article, published in Catalysis 2009, used case studies to highlight where users are achieving significant gains in system capability, flexibility and economics. Also discussed were catalyst system loadings that reduced the need for low-activity demetallisation catalysts traditionally required to process heavier feeds. Part II features additional case studies that demonstrate the application of mild hydrocracking (MHC) systems in FCC pretreaters to increase middle distillate yields.

Case study 3
Processing heavier feeds with 
more contaminants
Of the various streams treated in an FCC pretreat unit, one of the most difficult feeds processed is deasphalted oil (DAO). DAO feeds typically contain high levels of both Conradson carbon and contaminant metals. In order 
to take maximum advantage of the margin of upgrading heavy fuel oil 
into lighter products, one operation variable that has been leveraged by refiners is the degree of “lift” employed at the deasphalting unit. Lift is the measure of recovered DAO from the produced feed residue stream. As lift in the deasphalting unit increases, the level of contaminants in the DAO product rises. This increases both feed severity and the catalyst deactivation rate in the FCC pretreat unit processing the DAO stream.

Processing heavy sour crude and using both delayed coking and deasphalting to upgrade the bottom of the barrel, the refinery produces a large amount of difficult-to-process heavy gas oil, which requires pretreating prior to upgrading in the refinery FCCU. Post-treatment of the FCC naphtha allows flexibility in targeting the FCC feed sulphur.

The refinery has a large FCC feed hydrotreater (Unit 15), which operates at a relatively low pressure and high space velocity. Due to the economics of run length and this unit’s design configuration, Unit 15 has typically operated on a fixed cycle while attempting to maximise metals removal from the FCC feed.

In FCC pretreat hydrotreaters that load high levels of contaminant metals on catalyst, rapid deactivation can occur due to catalyst pore mouth plugging, which prematurely blocks off access to active catalyst sites, preventing full utilisation of available catalyst activity. An early adoptee of the Ascent technology, the refinery utilised a stacked NiMo/CoMo catalyst system of DN-3551 and DC-2551. Early on, these catalysts showed excellent metals uptake capacity, allowing for stable operations within the required operating window of the facility. This allowed the facility to optimise upstream operations to achieve a higher benefit from this asset.

Table 1 outlines the typical conditions and feed used in the previous cycle of operation. In previous operations, Unit 15’s combined feed included a DAO component produced at a 30% lift. Cycle duration was chosen to achieve a high level of metals removal SOR HDS performance in the 85–89% range. The facility had understood the benefit of increased lift, but previous operations at 30% lift had experienced rapid deactivation, indicative of pore mouth plugging, resulting in unplanned performance shortfalls.

With the first cycle of operation 
with the Ascent system showing excellent stability and high metals uptake, the facility was able to further increase deasphalting lift to 40+%, raising DAO recovery from heavy fuel oil by 3000–4000 bpd. While a shorter cycle was targeted for this mode of operation, significant increases in feed contaminants were observed without indications of pore mouth plugging, demonstrating the potential for further increases in unit severity and accompanying increases in clean fuels production from the facility.

The Ascent system has demonstrated that the improved contaminant uptake allows for more rapid accumulation of contaminants without pore mouth plugging impacting performance. In addition, it can be demonstrated that normalised HDS WABT continues to track very closely to contaminant metals accumulation on catalyst, even with the high concentration of feed contaminants.

The Ascent system has allowed for reliable and predictable operations with a more challenging feed. This has permitted a significant recovery of DAO from heavy fuel oil into the clean fuels pool, resulting in a large economic gain.

Case study 4
Hydrogen consumption minimised
At another facility, the operator has chosen to utilise the Ascent DC-2551 catalyst system to provide a high level of HDS performance, while reducing both hydrogen consumption and catalyst deactivation rates. In this case, the Ascent catalyst system that best met the refinery’s constraints was utilised.

Case study 5
Refiner co-produces ULSD and 
low-sulphur FCC naphtha
In reviewing the options available for producing clean fuels, one refiner found their optimal solution by adding refinery capability and shifting the service of existing assets. Utilising Ascent DN-3551 has been critical in achieving both ULSD and Tier II gasoline production from the FCC pretreat/FCC complex.

Previously operating as a cracking refinery with residue hydrotreating capability, the addition of a delayed coking unit eliminated the need for resid hydrotreating. This allowed a high-pressure hydrotreater, previously utilised as a residue HDS unit, to be used as an FCC feed pretreater. Working together with Criterion, it was shown how this existing asset could be operated at a high level of performance and, utilising Ascent DN-3551, provide both direct production of ULSD from the unit and low-sulphur gasoline production from the downstream FCC.

Operating for more than a year, the FCC pretreat unit has processed a mixed feed of vacuum gas oil (VGO), coker gas oil and coker naphtha. During this period, high levels of feed contaminants have been successfully managed, while producing very high-quality FCC feed. 
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