Many recent developments in the FCC area have focused on reducing the contact time between catalyst and hydrocarbon vapor to improve product selectivity: lower delta coke on catalyst, and higher selectivity to gasoline and light olefins. Nevertheless, very special conditions should be used to benefit from such short reaction times, as clearly indicated in pilot tests carried out at PETROBRAS.
The downflow reactor provides significant selectivity advantages, mainly in terms of coke and olefins, due to its much more uniform gas-solids flow pattern. However, in downflow reactors, the catalyst residence time and catalyst holdup is lower, considering the same hydrocarbon residence time used in a riser. Thus, for obtaining the adequate conversion levels, higher catalyst to oil ratio, higher reaction temperature or more active catalyst would be necessary. Fortunately, the continuing development of new catalyst technologies has made possible to work with downflow systems without any harm on conversion levels, using regular FCC operating conditions.
Experimental studies carried out at a pilot downflow unit, using a pilot riser unit as reference, showed advantages in conversion, LPG and propylene. Following these studies, PETROBRAS will revamp one of its FCC units into a downflow catalytic cracker.
Historically, the fluid catalytic cracking process (FCC) has been geared towards the production of maximum gasoline. This traditional scenario has changed dramatically in the last years. There is a growing demand of diesel fuel in EU and some other countries. Even more important is the strong tendency to increase production of light olefins, which are used as building blocks for petrochemicals, for instance. They can be also used for alkylates production, suitable to be added as octane booster in the gasoline pool.
PETROBRAS has to face an additional challenge: to match the increasing demand of light olefins to the increasing supply of Brazilian heavy crudes from Campos basin, like Cabiúnas and Marlim, which are heavier than the normal imported ones, resulting in a higher vacuum residue yield: approximately 30 – 33 wt%. Moreover, these crudes contain higher amounts of nitrogen compounds, asphalthenes and metals. Consequently, the percentage of 570°C+ fraction in the feedstocks processed in the FCC surpass 20 vol%. Typical API density is below 20, while Concarbon Residue is above 2 wt%.
Many recent developments in the FCC area have focused on rapid separation devices to reduce post-riser cracking, reducing thermal cracking and dilute phase catalytic cracking. Dilute phase cracking increases dry gas production and causes additional coke formation on the surface of the BLOCK 2 - - FORUM 13 523 ULTRA-SELECTIVE FCC PROCE