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The ZSM-5 additive is only able to crack C7 to C10 olefins to LPG.  Consequently, most of the C5 and C6 cut are not converted by ZSM-5 in the main riser.  In order to convert this cut, it has been confirmed that the optimum catalytic system is a recycle in a separate riser operating under more severe conditions –a PetroRiserTM.

Indeed, recycling with the feed does not allow the conversion of this light naphtha as the temperature is too low in the main riser.  If the naphtha recycle is injected before the feed in a zone where the catalyst temperature is above 700°C, the production of fuel gas is very high due to thermal cracking as well as detrimental side reactions that occur in these conditions.  In addition, injection of light naphtha below the main feed alters the riser conditions at the point of injection of the main residue feed resulting in a less than optimum performance.
The conclusion of the R&D work is that recycling light naphtha to a separate riser at a temperature higher than the main riser allows cracking of the C5 and C6 olefins and also paraffins to attain more LPG and less C5-70°C naphtha.
PetroRiser has been licensed in Abu Dhabi for the largest RFCC unit (127000 BPSD).
PetroRiser maximixes LPG olefins and minimizes naphtha olefins leading to a high aromaticity of the naphtha produced.
There is an additional benefit of the second high severity riser when processing residue. The added heat load in the PetroRiser acts as a “chemical” catalyst cooler. Due to the very low coke produced when cracking light naphtha, the added heat demand on the overall system provides a synergy for residue cracking.

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