Calcium containing feedstock processing

Development of a FCC catalyst/additive combination with high tolerance to calcium contamination from lower cost feedstock

Reliance Industries Limited

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Article Summary

Refiners continually find ways to improve refining margin by processing cheaper feedstocks, such as heavy oil, resid and opportunity crudes in their FCC units. Whenever processing of resid or some cheaper feedstock is increased in the FCC unit, plant operating severity is adjusted. Both metals and basic nitrogen compounds, which are known to poison FCC catalysts, are concentrated in the heavier end of gas oils and resids. These poisons, mostly present in the heavier hydrocarbon molecules, deposit on the catalyst during cracking reactions.

Each type of poison affects the FCC catalyst differently. Refiners are conversant with the detrimental effects of vanadium, basic nitrogen, nickel and their treatments. Vanadium deactivates Y-zeolite by the formation of vanadic acid in the presence of steam at high temperature and sodium facilitates the dealumination reaction by forming a low eutectic with the rare earth metals present in Y zeolite. Nickel promotes dehydrogenation reactions leading to high fuel gas make and coke formation and hence reduces the selectivity of desired products. Iron and calcium metals deposit on the catalyst surface and cause a loss of diffusivity, which leads to a loss in conversion and an increase in coke and bottoms.

Deposition of iron on FCC catalyst reduces accessibility to the catalyst pore and consequently reduces the catalyst’s activity. Researchers have studied the effect of calcium and iron on coke formation over ultra-stable Y-zeolite catalyst in the absence and presence of nickel and vanadium metal.1 Different zeolite samples are prepared by impregnating nickel and vanadium on ultra-stable Y-zeolite, previously exchanged with calcium. The catalyst samples are used for cracking n-hexane at 500°C. The study showed that catalyst containing calcium in combination with nickel and vanadium reduces coke formation significantly and increases the olefin to paraffin ratio.

There is plenty of reported information on the effects of contaminant nickel, vanadium, sodium and other metals in the FCC.2 Guthrie et al described passivating the reactivity of contaminant metals, such as nickel and vanadium, which deposited on a catalytic cracking catalyst, by adding to a cracking catalyst a mixture of a calcium containing material and a magnesium containing material in a separate reactor in the presence of steam.2 The preferred calcium containing material was dolomite and the preferred magnesium containing material was sepiolite. The cheaper feedstock contains metal contaminants, which contribute to lower conversion and the production of more fuel gas and coke. The higher fuel gas yield often touches the reactor cyclone velocity limits, which results in lower severity operation in the FCC unit, such as lower riser temperature. Similarly, higher coke yield leads to a higher regenerator temperature that lowers unit conversion.

However, there have not been many studies focused specifically on calcium contaminants and their effect on the performance of FCC catalysts/additives. Some crude samples have a higher concentration of calcium, alone or with other conventional contaminant metals (Na, Ni and V). Hence, a study was undertaken on the effects of calcium on FCC catalyst/additive on their performance in a fixed fluidised bed reactor. The catalyst and additive were optimised with high zeolite and matrix components to resist the effects of contaminants like calcium in the feed. The performance results are correlated with physico-chemical characterisations to gain a better understandings of the effect of calcium on catalyst and additive.

Catalyst preparation

FCC catalyst and additive were prepared according to procedu res mentioned in earlier patent disclosures.3 ZSM-5 additive was prepared by mixing the required quantities of kaolin clay, boehmite alumina, phosphate salt, ZSM-5 zeolite and colloidal silica with a suitable dispersant to obtain a free flowing slurry, which was then subjected to spray drying to form catalyst microspheres. Two kinds of phosphorus source, H3PO4 and monoammonium hydrogen phosphate were used to introduce phosphorus to the ZSM-5 additive formulation. The composition details of the catalyst and additive are shown in Table 1. The obtained spray dried microsphere particles were calcined at 500°C for one hour prior to hydrothermal deactivation.

The FCC catalyst was prepared by mixing the required quantities of kaolin clay, peptised boehmite alumina, USY-zeolite and colloidal silica with a suitable dispersant to obtain a free flowing slurry, which was then subjected to spray drying to form catalyst microspheres. The spray dried catalyst was exchanged with lanthanum nitrate (a rare earth salt) and then the sample was calcined at 500°C for one hour prior to hydrothermal deactivation.

A modified Mitchell method4 was followed for the impregnation of calcium on catalyst and additive separately, followed by calcination at 590°C for three hours. The calcium content varied from 0 ppm to 10 000 ppm using calcium naphthenate as the source of calcium precursor since calcium is present in crude mostly in the form of calcium naphthenate.

All calcium doped catalyst and additive samples were then hydrothermally deactivated at 800°C for 20 hours using 100% steam at atmospheric pressure before the cracking reaction took place.
BET surface area and pore volume measurements
Nitrogen gas adsorption/desorption isotherms were obtained using a Micromeritics ASAP 2020 unit. The catalysts were degassed for two hours at 300°C prior to adsorption. Nitrogen gas was dosed very precisely for both adsorption and desorption processes to generate highly accurate isotherm data. The BET surface area was determined by considering relative pressure (P/P0) between 0.05 to 0.20 and pore volume at 0.98 relative pressure. The t-plot was used to calculate the external surface area of the catalyst particles (calculated as the surface area of pores larger than micropores).

NH3-TPD measurements
Ammonia temperature programmed desorption (TPD) experiments were carried out on a Micrometrics Autochem 2920 unit equipped with a thermal conductivity detector (TCD). The sample was pre-treated at 600°C under a flow of helium gas for an hour. The sample was saturated with ammonia at 120°C for 30 minutes and ammonia was flushed out subsequently at the same temperature in a helium flow for an hour to remove weakly adsorbed ammonia. TPD analysis was carried out from 100°C to 600°C at a heating rate of 10°C/min. The desorbed ammonia was quantified with TCD and the signal was plotted against time/ temperature.

Cracking reaction
The advanced cracking evaluation (ACE) unit is a fixed fluid bed reactor for the evaluation of catalysts and additives, feedstocks and process development. It is downflow with respect to feed injection, an isothermal tubular reactor equipped with a central thermowell to measure temperature in the catalyst bed. The reactor is heated by an electric furnace with a minimum of three separate heating sections, which allow fine control for isothermal operation. It includes control system hardware and software that enables accurate multiple runs without operator intervention.5 Cracking reactions in the ACE reactor take place in conditions that simulate a commercial FCC riser (see Table 2). The conversion is varied by changing the catalyst loading at constant feed rate.

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