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Feb-2004

Optimising FCC unit performance

Additive currently used in several FCC units increases conversion and gasoline yield without exceeding coke or gas limits. The resulting operating flexibility provided by the additive at an Australian refinery is discussed

Hugh Niblock, BP
Marius Vaarkamp and Maarten van Vliet, BASF Catalysts

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Article Summary

Refiners continue to face challenges due to fluctuating market demands, varying crude quality and increasingly strict environmental regulations. In response, refiners have placed a premium on operational flexibility to offset these impacts and remain economically profitable. Additives provide refiners with a number of options to respond to these challenges and optimise unit performance.

The Converter additive introduced in 2002 is based on Engelhard’s distributed matrix structures (DMS) technology platform. The DMS platform is a proprietary matrix technology that increases rather than dilutes the overall catalyst system activity. As a result, improvements in activity and selectivity are observed at low inventory concentrations without exceeding coke and gas constraints. These improvements increase unit operational flexibility and catalytic activity enhancement along with coke-selective bottoms upgrading. To provide a more flexible approach to FCC unit optimisation, the additive can be added as required. The objectives may be achieved quickly (that is, without the long lag time typically associated with a change in FCC catalyst formulation).

This DMS-based additive’s high activity allows refiners to counterbalance the dilution effects caused by the use of other additives. It also provides high cracking activity per unit of coke make. This coke selectivity presents numerous possibilities for value creation in an FCCU. A previous article detailed various ways to take advantage of the additive’s catalytic properties, including:1
• Reliable processing of opportunistic feedstocks
• Improved profitability despite unit constraints
• Rapid recovery from activity drops, or the ability to quickly increase unit activity if feed quality improves.

Four refineries that were regenerator temperature limited have taken advantage of the additive by increasing the feed rate, processing additional resid, or simply upping inventory activity and hence conversion. It is compatible with all FCC designs and FCC catalysts. Out of a total of 14 users, seven utilised the Converter additive in combination with an Engelhard base catalyst and the rest another supplier’s base catalyst. The FCC additive delivered improved profitability to a BP refiner located in Kwinana, Australia by increasing conversion and gasoline yield without exceeding the unit’s limits on coke burn or gas make. Other users have observed similar improvements in unit operation.

Technology background
The catalyst pore architecture in the DMS-based FCC catalysts and additives, used in more than 80 commercial units worldwide, combines optimised porosity for heavy feed molecule diffusion with selective zeolite-based cracking to achieve deep bottoms conversion with low coke formation.2

The structure imparted by the DMS matrix is designed to provide enhanced diffusion of the feed molecules to pre-cracking sites located on the external, exposed surface of highly dispersed zeolite crystals. Unlike other FCC catalyst technologies, the feed initially cracks on the external zeolite surface rather than on an active amorphous matrix material. This provides better selectivity with reduced coke formation. The distance the cracked products have to travel to the internal crystalline zeolite surface is also minimised, resulting in less overcracking. The net result is high bottoms conversion with low coke and higher yields of valued gasoline and light olefin products.

The unique DMS structure is apparent in the SEM micrograph of the interior of a catalyst particle. The well-developed pore structure is evident and essentially the entire exposed pore surface is covered with zeolite crystallites. The external surfaces of these crystallites are exposed and accessible to the hydrocarbon feed molecules, which diffuse readily through the controlled pore architecture. While other catalyst technologies may feature similar or even higher total pore volumes, they do not have the architecture and inherent zeolite-based cracking pattern representative of the DMS structure.3

Unit description
Commissioned in 1987, the BP Kwinana FCC unit is a Stone & Webster Engineering Corporation (SWEC) two-stage regenerator residue cat cracker (RCU) with no catalyst cooler. The main fractionator (MF) and gas recovery unit (GRU) were revamped in 1994. The RCU was originally designed for 25 000 BPSD and now typically processes 35 000 BPSD. Feedstocks are a combination of high nickel (Ni), low specific gravity (SG) South and East residues with long back-end distillations to high vanadium (V), high SG Middle Eastern residues with lube oil discards, and vacuum residue. Feed to the RCU has between 0.5 and 5.0 wt% Conradson carbon. The equilibrium catalyst has between 1000 and 4000 ppmwt V and between 4000 and 10 000 ppmwt Ni. The total metals (Ni+V) vary between 5000 and 8000 ppmwt.

Logistics
Even though BP Kwinana does have an injection system for FCC additives, Engelhard and BP agreed to conduct the additive trial by on-demand local off-site blending of the additive and the base catalyst. This allowed for adjustments in the ratio of additive and catalyst every three days. Total powder (catalyst plus additive) additions were lowered by about 10 per cent during the use of the Converter additive to avoid increasing the inventory activity beyond the optimum.

During the two-month trial, BP Kwinana performed several test runs. The results, which are corrected for feed and cut-point changes are presented in Table 1. Conversion increased by more than 5 wt%, which translated into an increase of 3.5 wt% of gasoline, corresponding to a gasoline selectivity of 67 wt%. These results were confirmed by weekly Ecat analysis using a standardised feed. Product stream quality (Table 2) was in line with expectations, the most pronounced change being DCO density.


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