FCC feed preparation for improved quality
The benefits of hydrotreating heavy gas oil FCC feedstocks at different levels of severity are discussed in this article, particularly the operating success of a combination of processes at the Ultramar refinery in California
Henry Chung and Susan Kolbush, Ultramar Incorporated
Emiliano de la Fuente and Preben Christensen, Haldor Topsoe
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When refiners first began using FCC pretreating, in most cases they did not expect to attain benefits other than compliance with environmental regulations on the sulphur emissions from the regenerators. For this reason, hydrotreating of the FCC feed was done at the lowest possible pressure that would allow refiners to meet environmental regulations and achieve the desired catalyst cycle length. In this manner the required investment and hydrogen consumption were reduced. These units were typically designed for pressures of 800–1000psig.
The present environmental trend worldwide is to have more stringent specifications for motor fuels, particularly in the sulphur levels. FCC units are one of the major producers of gasoline in a refinery, and FCC gasoline is usually the major contributor to the sulphur levels in the gasoline pool. In addition, the light cycle oil (LCO) produced is a very poor blending component for diesel because of its high sulphur content and low cetane number.
FCC feed pretreatment is an excellent tool to meet required sulphur levels in both gasoline and diesel, and to reduce the amount of light cycle oil produced and improve its quality. Furthermore, it allows processing of more difficult feeds such as cracked stocks or heavier cuts and optimum combinations of catalysts and operating conditions can be utilised to design an FCC pretreating unit that meets the refinery objectives.
The trend in FCC pretreater design is therefore to apply higher pressures and lower space velocities to increase the levels of denitrogenation and saturation of poly-nuclear aromatics (PNA). Such units will only require a marginally higher investment, but the added benefits and flexibility can easily justify this.
In addition to reducing the sulphur emissions from the regenerator of the FCC unit, one of the main benefits of FCC feed pretreatment is that it makes the products more environmentally friendly.
The pretreater can be designed to reduce the sulphur level in the feed so that when the products from the FCC unit are blended with other refinery streams, the blend can meet the most stringent sulphur specifications. This is particularly important in many areas of the world, such as California, where the gasoline sulphur specification is 40ppm. Direct desulphurisation of the FCC gasoline to the required blending levels would reduce the octane number significantly
Nitrogen poisons the active sites of FCC zeolitic catalyst reducing its activity and making it less selective towards valuable products. Most refiners like to process feeds with nitrogen content at approximately 1000ppm level. This level cannot be attained without hydrotreatment when the FCC unit is processing gas oils derived from certain crudes, or when the feed contains a large portion of cracked gas oil.
Aromatic compounds are not easily cracked in an FCC unit, and the limited amount of cracking achieved produces a large amount of coke. However, by saturating the PNA compounds present in typical FCC feeds and converting them into naphthenes, they are easily cracked into valuable products.
Nickel promotes dehydrogenation reactions and increases coke and gas production at the expense of more valuable products. Vanadium destroys the zeolite in the catalyst. Since these metals are present in the heavier portions of the gas oils, having the capability to remove metals from the feedstock allows the refiner to process heavier feeds and reduce catalyst consumption.
Conradson carbon reduction
Reducing the Conradson carbon of the FCC feed lowers the amount of coke on the spent catalyst going to the regenerator. This reduction directionally provides better catalyst selectivity with lower deactivation. Since less carbon has to be burned, it unloads the regenerator, permitting higher throughputs and/or conversions.
This feature, as well as metals removal, is more important for FCC units processing resids, where both of these contaminants are present in substantially higher quantities than in typical heavy gas oil feeds.
Establishing the operating conditions for the pretreater has to be based on the results expected from the FCC operation. Pressure, space velocity, recycle gas rate and quench requirements are based on the properties of the feed to be processed, the required severity of the operation to achieve the product objectives, the purity of the make-up hydrogen, the required catalyst cycle length, and the catalyst to be used.
In turn, the catalyst to be used depends on the product objectives, with Co/Mo being the preferred catalyst when the primary objective is desulphurisation, and Ni/Mo catalyst being the preferred choice when denitrogenation and aromatics saturation are the main goals. In most modern FCC pretreaters Ni/Mo catalysts are being used.
Ultramar feed consists of a blend of approximately 25 per cent heavy coker distillates and 75 per cent straight-run vacuum gas oil having the properties shown in Table 1. The catalyst selected by Ultramar for this operation was Topsoe TK-555 to achieve a minimum cycle of three years at a throughput of 48000bpsd. Product targets were 875ppm nitrogen and 400ppm sulphur in the 450°F+ product. Before this unit was commissioned in March 1996, Ultramar had a pretreater that operated at a nominal 1000psig pressure, mostly to comply with sulphur emissions from the FCC regenerator. The new unit operates at a pressure that is approximately 75 per cent higher than the old unit, achieving deeper desulphurisation and denitrogenation and an increase in the saturation of PNAs.
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