Fluid Catalytic Cracking Technology – High Quality Derivates and Higher Refining Margin
Fluid Catalytic Cracking (FCC) is one of the main processes which give higher operational flexibility and profitability to refiners. The catalytic cracking process was widely studied over last decades and became the principal and most employed process dedicated to converting heavy oil fractions in higher economic value streams.
The installation of catalytic cracking units allows the refiners to process heavier crude oils and consequently cheaper, raising the refining margin, mainly in higher crude oil prices scenario or in geopolitics crises that can become difficult the access to light oils. The typical Catalytic Cracking Unit feedstream is gas oils from vacuum distillation process. However, some variations are found in some refineries, like sending heavy coke naphtha, coke gas oils and deasphalted oils from deasphalting units to processing in the FCC unit.
The catalyst normally employed in fluid catalytic cracking units is a solid constituted by small particles of alumina (Al2O3) and silica (SiO2) (zeolite). By the catalyst characteristics and the operational conditions in the catalytic cracking process (temperature higher than 500 oC), the process is inefficient to cracking aromatic compounds, therefore, how much more paraffinic is the feedstream, higher is the unit conversion. Figure 1 presents a process scheme for a typical Fluid Catalytic Cracking Unit (FCCU).
In a conventional scheme, the catalyst regeneration process consists in the carbon partial burning deposited over the catalyst, according to chemical reaction below:
C + ½ O2 → CO
The carbon monoxide is burned in a boiler capable of generating higher pressure steam that supplies others process units in the refinery.
Figure 1 – Schematic Process Flow for a Typical Fluid Catalytic Cracking Process Unit (FCCU)
The principal operational variables in a fluid catalytic cracking unit are reaction temperature, normally considered the temperature in the top of the reactor ( called riser), feed stream temperature, feed stream quality (mainly carbon residue), feed stream flow rate and catalyst quality. Feedstock quality is especially relevant, but this variable is a function of the crude oil processed by the refinery, so is difficultly can be changed, but for example, aromatic feedstock’s with high metals content are refractory to cracking and conducting to a quick catalyst deactivation.
An important variation of the fluid catalytic cracking technology is the residue fluid catalytic cracking unit (RFCC). In this case, the feedstock to the process is basically residue from atmospheric distillation column, due to the high carbon residue and contaminants (metals, sulphur, nitrogen, etc.) are necessary some adaptations in the unit like catalyst with higher resistance to metals and nitrogen and catalyst coolers furthermore, it’s necessary apply materials with most noble metallurgy due the higher temperatures reached in the catalyst regeneration step (due the higher coke quantity deposited on the catalyst), that raises significantly the capital investment to the unit installation. Nitrogen is a strong contaminant to the FCC catalyst because they neutralize the acid sites of the catalyst which are responsible for the cracking reactions.
When the residue has high contaminants content, is common the feed stream treatment in hydrotreating units to reduce the metals and heteroatoms concentration to protect the FCC catalyst.
Typically, the average yield in fluid catalytic cracking units is 55% in volume in cracked naphtha and 30 % in LPG. Figure 2 presents a scheme for the main fractionator of the FCC unit with the principal product streams.
Figure 2 – Main Fractionator Scheme for a Typical Fluid Catalytic Cracking Unit
The decanted oil stream contain the heavier products and have high aromatic content, is common that this product are contaminated with catalyst fines and normally this stream is directed to use like fuel oil diluent, but in some refineries, this stream can be used to produce black carbon.
Light Cycle Oil (LCO) has a distillation range close to diesel and normally this stream is directed to treatment in severe hydrotreating units (due to the high aromaticity), after this treatment the LCO is sent to the refinery diesel pool.
Heavy cracked naphtha is normally directed to refinery gasoline pool, however, in scenarios where the objective is to raise the production of middle distillates, this stream can be sent to hydrotreating units for further diesel production.
The overhead products from the main fractionator are still in the gaseous phase and are sent to the gas separation section. The fuel gas is sent to the refinery fuel gas ring, after treatment to remove H2S, where will be burned in fired heaters while the LPG is directed to treatment (MEROX) and further commercialization. The LPG produced by FCC unit have a high content of light olefins (mainly Propylene) so, in some refineries, the LPG stream is processed in a Propylene separation unit to recovery the propylene that has higher added value than LPG.
Cracked naphtha is usually sent to refinery gasoline pool which is formed by naphtha produced by other process units like straight run naphtha, naphtha from the catalytic reforming unit, etc. Due to the production process (deep conversion of residues), the cracked naphtha has high sulfur content and to attend the currently environmental legislation this stream needs to be processed to reducing the contaminants content, mainly sulfur.
The cracked naphtha sulfur removing represents a great technology challenge because is necessary to remove the sulfur components without molecules saturation that gives high octane number for gasoline (mainly olefins).
Over the last decades some technology licensers had developed new processes aiming to reduce the sulfur content in the cracked naphtha with minimum octane number loss, some of the main technologies dedicated for this purpose are technology PRIME G+ ™ from Axens, the processes OCTAGAIN ™ and SCANfining ™ from Exxon Mobil, the process S-Zorb™ from ConocoPhillips and ISAL™ technology from UOP.
Usually, catalytic cracking units are optimized to aiming the production of fuels (mainly gasoline), however, some process units are optimized to maximize the light olefins production (propylene and ethylene). Process units dedicated for this purpose have his project and operational conditions significantly changed once the process severity is strongly raised in this case.
The reaction temperature reaches 600 oC and higher catalyst circulation rate raises the gases production, which requires a scaling up of gas separation section.
Figure 3 presents a typical scheme for a gas separation section for a fluid catalytic cracking unit.
Figure 3 – Basic Process Flow Diagram for a Typical Gas Separation Section from FCC Unit
In several cases, due the higher heat necessity of the unit is advantageous to operate the regenerator with the total combustion of the coke deposited on the catalyst, this arrangement changes significantly the thermal balance of the refinery once it’s no longer possible to resort the steam produced by the CO boiler.
Over last decades, the fluid catalytic cracking technology was intensively studied aiming mainly the development of units capable of producing light olefins (Deep Catalytic Cracking) and to process heavier feedstocks. The main licensers for fluid catalytic cracking technology nowadays are the companies KBR, UOP, STONE & WEBSTER, AXENS, CB&I and FOSTER WHEELER.
Despite the great operational flexibility which fluid catalytic cracking technology give for the refineries, some new projects have dismissed these units in the refining scheme, mainly when the new refinery objective is to maximize middle distillates products (Diesel and Kerosene) once this is not the focus of the fluid catalytic cracking unit.
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