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Shell's resid FCC technology reflects evolutionary development
Commercial operating experience has been gained on conversion of long residues by fluid catalytic cracking (FCC) in a long-residue catalytic cracking unit at Shell U.K. Ltd.'s Stanlow refinery in Ellesmere Port, U.K.
The unit design is the result of many evolutionary developments in Shell's FCC technology since its beginnings in the 1940s.
Shell's first long residue FCC unit at the Stanlow refinery was started up in May 1988. The unit has operated at its design capacity of 9,500 metric tons/day (66,000 b/d), processing feedstocks with a Conradson carbon content of up to 5 wt %. With these heavy feeds, the gasoline yield is about 45 wt % (55 vol %).
The produced butane/butylene stream goes to C[sub.4] solvents and a new alkylation unit. About 5 wt % (9 vol %) of the production consists of high-grade propylene, whereas the ethylene will be used to produce ethylbenzene.
The unit has proven to be extremely flexible. It includes Europe's largest power recovery train, capable of generating 21,000 kw.
Specific design features include a proprietary liftpot and feed injection system, a compact reactor design, a staged catalyst stripper, a unique two-stage, but single-vessel regenerator, controllable catalyst coolers on the regenerator vessel, and a catalyst-handling system for continuous addition and withdrawal.
A high priority is given to energy efficiency of the units. Most new Shell FCC designs and revamps incorporate the Shell swirl tube separator and power recovery technology.
Not only does this proven technology greatly reduce plant operating costs by the recovery of energy from the flue gas through an expansion turbine, but it also makes long-residue FCC's net power exporters.
New developments will enable processing of increasingly heavier residual feedstocks.
Early Shell work
Today, some 30 Shell or Shell-advised FCC units are in operation worldwide, with capacities ranging from 1,000 to 11,000 metric tons/sd (7,000 to 80,000 b/sd). A wide variety of feedstock types is processed, ranging from clean vacuum gas oil to straight-run atmospheric residues.
Coprocessing of many other unconventional feed types is practiced, such as flashed distillates from visbreaking/ thermal cracking and lube oil extracts.
Since the early 1940s, the technology of catalytic cracking has made very significant progress with respect to catalyst as well as hardware efficiency.
Fig. 1 shows the trends in the catalyst developments. The effects of the improvements are reflected in terms of yield breakdowns in Fig. 2 showing, mainly as a result of the catalyst developments, a pronounced shift from residual components to gasoline.
The simultaneous hardware developments have enabled the FCC technology to maintain its prominent role in conversion. Gross margins of $30-70 US/metric ton of feed have resulted in a primary emphasis on maintaining high throughput and onstream time.
The high differential values between distillates and residual material (fuel) have led to the inclusion of more and more residue in FCC feedstocks. The problems to be solved, from intitial experience, include high coke-forming tendency, catalyst deactivation by metals and consequently poorer yields, particularly high gas yields, and coke depositon in the reactor section.
Much progress has been made, including that by catalyst manufacturers, to overcome, or at least contain, the harmful effects of residue in feed. As a consequence, in the past 10 years Shell units have seen a gradual increase in feed residue content.
Currently, feedstocks with about 5 wt % Conradson carbon and about 10 wt ppm nickel (Ni) + vanadium (V) are being processed in Shell units. The latest Shell long-residue FCC units, in Singapore and Geelong, Australia, are designed for feedstocks containing 6-7 wt % Conradson carbon and about 20 wt ppm Ni + V.
The Singapore unit was started in the first quarter of 1990. Progress in feed heaviness is shown in Fig. 3.
The developments in FCC catalysts have led to significant adaptations in the designs of the reactor/regenerator/stripper sections. Fig. 4 shows units which typify Shell's FCC technology over the decades since the 1940s. All units are drawn on a "same intake/same scale" basis to make comparisons realistic.
The design of the first FCC units was largely dictated by the quality of catalyst available at the time. Amorphous, silica-alumina catalysts, by today's standards, …