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Maximising distillate and alkylation feed from the FCC

The tight oil revolution has changed FCC priorities. Operators can maximise profitability by adopting new catalysts to target their yield profile

Albemarle Corporation
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Article Summary
The tight oil revolution, falling gasoline demand, Renewable Fuel Standards requirements and the emergence of the US as an exporter of refining products have changed the economics and operating objectives for many refiners. Distillate and alkylation feed from the FCC unit are often more profitable than gasoline. Where available, cheaper tight oil crudes have reversed the trend towards heavy and sour crudes, thereby creating both opportunities and challenges. These paraffinic sweet crudes make FCC feeds that have lower distillate production, gasoline octanes and C4 olefinicity. Tight oil feeds are generally low in nickel and vanadium, but high in iron, calcium and, often, sodium, which creates significant challenges for FCC catalysts.

Albemarle has developed technologies to meet these changing needs and has introduced catalysts that help FCC unit operators to achieve their yield objectives for distillate and alkylation feed.

The Action FCC catalyst uses a combination of novel zeolite and alumina-based matrix technologies to deliver maximum bottoms cracking upgrading and metals tolerance. These technologies provide the catalyst surface chemistry and pore architecture necessary to selectively process molecules ranging from the paraffins of tight oils to the multi-ring aromatics of heavy residue feeds.

Improved accessibility

Given the short contact times experienced in most FCC unit risers, highly accessible catalysts enable more of the active ingredients and activity to be utilised in the reactor (see Figure 1). Higher accessibility means faster diffusion of feed molecules into the catalyst particle, which results in better bottoms cracking and higher intrinsic activity. Higher accessibility also accelerates the diffusion of reactants out of the catalyst particle, which helps to reduce secondary reactions such as overcracking and hydrogen transfer.

The strong matrix activity and corresponding bottoms upgrading offered by Albemarle’s ADM-20 and ADM-60 matrices provide greater flexibility to balance the contributions of the matrix and zeolite. This enables custom design of catalysts to suit specific unit needs for distillate and alkylation feed production.

The high accessibility of the matrix technology in Action also provides high resistance to iron, calcium and sodium contaminants, which are present in heavy residues and many opportunity feeds and light tight oil-derived feedstocks.1 The use of alumina-based matrices means that Action catalysts maintain maximum activity and bottoms cracking at high levels of iron and calcium contamination. These catalysts are also formulated with ADZT-100, a novel high silica to aluminum ratio zeolite technology that is more resistant to deactivation by sodium or vanadium.

Zeolite rare earth exchange is employed by some catalyst manufacturers to increase zeolite activity and stability in the severe hydrothermal conditions of the FCC unit and to provide resistance to sodium- and vanadium-induced zeolite destruction. However, the activity provided by the matrices in Action catalysts, the inherent metals tolerance and the stability of the high silica to alumina ratio zeolites used mean that Albemarle does not depend on high levels of rare earth to maintain catalyst stability. Catalysts with lower 
rare earth content have lower hydrogen transfer and make more olefinic LPG and higher gasoline octane.

In addition, ADZT-100 technology enhances isomerisation reactions rather than hydrogen transfer reactions. As a result, C4 olefins hydrogenation is averted, which enables Action to achieve higher butylene yields. Furthermore, increasing isomerisation reactions raises octane, which is particularly important when processing tight oils.

Example 1: Vacuum gas oil (VGO) feed application
The large FCC unit in this application is a side-by-side design with recent updates to the riser and regenerator. The feedstock is hydrotreated VGO with low metals and Conradson carbon values well below 1 wt% and often below 0.5 wt%. The daily catalyst replacement rate for this FCC unit is below 1% per day.

The main economic drivers for the refiner are distillate plus gasoline maximisation and bottoms minimisation. Increasing LPG olefins is another major objective and more gasoline octane improves unit profitability, so the refiner often uses ZSM-5 additives. A test comparison of catalyst systems was conducted during extended periods without ZSM-5 additives. The results are shown in Table 1 and discussed below.

Comparison of e-cat phosphorous and Z/M ratio
Phosphorus analyses from the equilibrium catalyst indicate that both catalysts, Action and a competitor’s commercial catalyst, were relatively free of additive. The difference in results reflects the differences in the phosphorous content 
of the cracking catalyst 
and residual amounts from earlier periods of ZSM-5 additive use.

The riser temperature averaged 975°F (524°C) for both periods and varied from 960°F to 985°F (516-530°C). The comparison was conducted at constant operating conditions, catalyst additions and feed properties using KBC’s CatOp FCC model. For the projections, the cut points were fixed at 430°F and 670°F (221°C, 354°C) for gasoline and LCO respectively.

Activity retention and coke selectivity
The feed to this unit is considered quite crackable, but the low catalyst replacement rate and moderately high regenerator temperatures favour a catalyst with good stability and activity retention. Since this unit processes a hydrotreated VGO feed, the competitor’s catalyst relied on zeolite to provide most of the activity. However, the conversion and coke selectivity results showed that Action’s highly active and stable matrix components, combined with the ADZT-100 zeolite technology, proved a more appropriate fit.

High matrix catalysts can offer significant coke selectivity advantages over high Z/M catalysts, which results in higher catalyst circulation rates, higher catalyst to oil ratios and improved yield selectivities and activities.2

LCO/ bottoms ratio and LPG yield
Action delivered a 22% increase in the LCO/bottoms ratio compared with the competitor’s high zeolite catalyst. Model simulations indicated that even if the refiner had lowered the catalyst addition rate with Action to maintain constant conversion, it would still have provided more bottoms cracking and an improved LCO/bottoms ratio.

Refiners can take advantage of superior bottoms upgrading to increase distillate, gasoline or gasoline plus distillate production, depending on their profitability objectives. In this case, Action was formulated to maximise total liquid product (gasoline plus distillate) and LPG yield and olefins. It increased LPG plus gasoline plus diesel from 101% to 104% of feed at constant conditions.
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