PTQ Q4 2022 Issue

Water

High pressure steam

Air

Flue gasses

Regenerator

CO boiler

Fuel gas

Catalyst

Gases concentration area

Reactor/ separation vessel

Main fractionator

Feed

Pre-heating

LPG

Light cycle oil (LCO)

Decanted oil

Cracked naphtha

Figure 2 Schematic process flow for a typical fluid catalytic cracking unit

Petroleum needle coke route Production of ultra-premium needle cokes based on heavy petroleum residuals is limited to FCCDO residuals as a precursor. Fluidised catalytic cracking (FCC) is a carbon rejection conversion technology using heavier residuals to produce lower molecular weight distillates. While FCC pro- cess conditions are not usually altered to enhance residual (FCCDO) quality, it may be utilised as a bunker fuel viscos- ity cutter stock or as a needle coke precursor. FCC technology is one of the main processes which give higher operational flexibility and profitability to refiners. The catalytic cracking process has been widely studied over the last decades and has become the principal and most employed process dedicated to converting heavy oil fractions into higher economic value streams. Installation of FCC units allows refiners to process heavier crude oils and, consequently, cheaper, raising the refining margin, mainly in higher crude oil price scenarios or in geo- politics crises that can become difficult to access to light oils. The typical FCC unit feed stream is gas oils from the vacuum distillation process. However, variations are found in some refineries, like sending heavy coker naphtha, coker gas oils, and deasphalted oils from deasphalting units to processing in the FCC unit. The catalyst normally employed in FCC units is a solid constituted of small particles of alumina (Al₂O₃) and silica

(SiO₂) (zeolite). Due to the catalyst characteristics and the operational conditions in the catalytic cracking process (temperature higher than 500°C), the process is inefficient for cracking aromatic compounds. Higher paraffinic con- tent in the feed stream correlates to higher unit conversion. Figure 2 presents a process scheme for a typical FCC unit. In a conventional scheme, the catalyst regeneration pro- cess consists of the carbon partial burning deposited over the catalyst, according to the chemical reaction below:

C + ½ O₂  CO

The carbon monoxide is burned in a boiler capable of generating higher pressure steam that supplies others pro- cess units in the refinery. The principal operational variables in an FCC unit are reaction temperature, normally considered the temperature at the top of the reactor (called a riser), feed stream tem- perature, feed stream quality (mainly carbon residue), feed stream flow rate, and catalyst quality. Feedstock quality is especially relevant, bkut this variable is a function of the crude oil processed by the refinery. For example, aromatic feedstocks with high metals content are refractory to crack- ing and lead to quick catalyst deactivation. An important variation of FCC technology is residue FCC (or RFCC). In this case, feedstock to the process is basically residue from the atmospheric distillation column due to the high carbon residue and contaminants (metals, sulphur, nitrogen). It is necessary for some adaptations in the unit, such as catalyst with higher resistance to metals and nitro- gen and catalyst coolers. Furthermore, it is necessary to apply materials with the most noble metallurgy due to higher temperatures reached in the catalyst regeneration step (due to higher coke quan- tity deposited on the catalyst). This significantly raises the capital investment in the unit installation. Nitrogen is a strong contaminant to the FCC catalyst because it neu- tralises the catalyst acid sites, which are responsible for the cracking reactions. Typically, the average yield in FCC units is 55% cracked naphtha by volume and 30% LPG. Figure 3 presents a scheme for the FCC main fractionator with the principal product streams. The decanted oil stream contains the heavier products with high aromatic content. It is common for this stream to be contaminated with catalyst fines and is normally used

Light cracked naphtha, LPG, f uel gas, water

Heavy cracked naphtha

Light cycle oil (LCO)

Reactor euent

Decanted oil

Figure 3 Main fractionator scheme for a typical FCC unit

PTQ Q4 2022