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Jul-2013

Additives provide flexibility for FCC units and delayed cokers

A proprietary additive applied to a novel coker process increases the liquid yield from delayed coking

ALAN KRAMER and RAUL ARRIAGA
Albemarle Corporation

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Article Summary

Flexibility is a primary advantage in the ever-changing refining industry. Successful refiners achieve profitability by quickly reacting to market conditions. And catalyst additives have long been one method refiners have turned to when reacting to volatile market dynamics. Albemarle has long been a manufacturer of FCC additives, which are used for environmental compliance, propylene maximisation and overall yield improvement. Recent opportunities in this market have highlighted the benefits of using its Bottoms Cracking and Metals Tolerance (BCMT) additives to drive up FCC unit profitability. The company has expanded its focus on refinery additives by partnering with OptiFuel Technology Group (OFTG) to develop a technology utilising a proprietary additive supplied by Albemarle that increases the liquid yield from delayed coker units.1 The benefit can be around 3.7 $/bbl to refiners that implement this process, with additional profit possible in cases where increased unit throughput results.

Fluid cracking catalyst additives
Refiners have leveraged the benefits of BCMT additives in many ways. For example, some have used it to process less expensive opportunity feeds. Others have lowered total catalyst expenditures by purchasing fresh distressed FCC catalyst stocks at a discount from closed refineries then enhancing that catalyst with this additive in order to make it usable in their refinery. Several are using BCMT to correct for non-
optimised base catalyst formulations because the refiner is either locked into a supply agreement with a supplier that cannot formulate a catalyst with enough bottoms cracking activity to meet their needs, or the immediate economics provide an opportunity to increase profitability by changing the product mix temporarily.

Table 1 shows recent use of BCMT-500 and the lower rare-earth BCMT-500-LRT2 at well over 30 refineries globally.

BCMT additives are built upon Albemarle’s Topaz 
alumina-gel, high-accessibility catalyst technology.3 However, these additives differ from cracking catalysts because they have been formulated to provide extreme accessibility 
and metals tolerance in a highly concentrated form since the additive makes up only a small fraction of the FCC unit’s circulating inventory.

The mechanism behind BCMT has been discussed in detail,4,5,6,7,8 but can be summarised as follows. These additives enhance the diffusional architecture of the catalyst system, thereby improving accessibility and allowing feed molecules to selectively pre-crack before entering the zeolite pores. They will also trap deleterious metals, such as nickel, reducing hydrogen make while improving coke selectivity and overall catalyst performance. Vanadium poisoning is effectively dealt with through the use of vanadium-tolerant zeolites and matrix bottoms cracking components. The additive also improves overall performance in units with high iron and calcium in part via its high accessibility, as will be shown in one of the following case studies.

FCC additive case studies
The following are recent examples of BCMT’s performance.

Unit A is a FCC unit that processes a near equal mix of hydrotreated VGO and atmospheric resid. The combined feed has a Conradson carbon residue of 2.5 and gravity of 24 °API. Equilibrium catalyst metals range from 3000-4000 ppm nickel and 5000-7000 ppm vanadium. The unit is limited by regenerator temperature, and the refiner has a desire to maximise LCO production. This refinery currently adds BCMT-500 at 20% of the total catalyst addition rate.

Figure 1 shows that the LCO/bottoms ratio increased by over 30% at constant relative conversion with 20% BCMT at steady state. These results become more impressive when one takes into account that, for the period shown, the fraction of atmospheric resid in the feed was 20 relative per cent higher than the baseline and the riser temperature was on average 9°C lower.

Since this refiner adjusted operating conditions to maximise LCO yield during BCMT usage, Albemarle normalised the raw operating data from the trial period back to the baseline conditions using a FCC simulator model. At normalised conditions of constant riser and feed preheat temperatures, the regenerator temperature is 13°C lower. This improvement in delta coke is due in part to the metals passivation activity of BCMT-500.

Unit B is a larger FCC unit processing a partially hydrotreated VGO feed with 0.2% Conradson carbon residue. Metals on the equilibrium catalyst are moderate to low at 900 ppm nickel and 300 ppm vanadium. The major driver for BCMT-500 usage at this refinery was to temporarily increase heavy naphtha and light cycle oil yield without reformulating the base catalyst, which was geared towards maximum gasoline yield. The additive was introduced into the unit at a 20% addition rate, replacing an equal amount of base catalysts. The unit reached 11% BCMT in inventory before ceasing additions and reverting back to maximum gasoline operations. The riser temperature remained constant during the period of usage at 510°C and the regenerator temperature dropped an average of 2°C at the peak of BCMT-500 turnover.

The yield shifts with 11% additive determined from the refinery operating data have been summarised in the middle column of Table 2. The right-hand column shows model corrected yield shifts, which are more telling of the true performance of the additive because the as-produced gasoline required cut-point adjustments.

As can be seen from the data, the introduction of 11% additive into the FCC unit greatly increased LCO yield by 
increasing bottoms activity significantly and adjusting the unit’s selectivities towards the desired yield pattern.

Unit C is a small, independent refiner struggling to process a tough 6 Conradson carbon residue feed that contains up to 16 ppm nickel and 6.5 ppm vanadium. The feed also contains significant amounts of calcium and iron that together deposit an additional 1.2 wt% on the equilibrium catalyst. Organic iron from the feed is known to deposit on the surface of cracking catalyst, creating a barrier that prevents the diffusion of large feed molecules into the catalyst.9 Together, iron and calcium can act to form low melting point eutectic compounds, further reducing catalyst accessibility.10 The deleterious effects of metals coupled with low catalyst replacement rates resulted in this refiner’s baseline e-cat activity ranging in the upper 50s to low 60s.


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