<span style=”color: #008000;”>by: Souad LOUSDAD
Fluid catalytic cracking (FCC)
it is one of the most important processes in a modern refinery and is of essential economic importance.
Unlike the atmospheric distillation and vacuum distillation which are physical separation processes, The FCC is a chemical conversion process that converts high molecular-weight hydrocarbons to lower molecular-weight products of high value, using both high temperature and a catalyst.
Before the advent of FCC, thermal cracking was the first commercial process for obtaining gasoline and other lighter products.
First, why is it called “Fluid” catalytic cracking?
Fluidization is a method used generally, to keep solid particles floating in an upward direction in a flow of gas or liquid. In our case, the fluid which is of a gaseous format (gases/vapors) passes through a solid granular material which is the catalyst shaped as tiny spheres to suspend the solid and cause it to behave as though it were a fluid.
Second, what type of catalyst is used?
With the knowledge that an acid catalyst could be used to crack petroleum more efficiently, a lot of materials were listed.
The catalyst for the FCC process has the format of a powder or as we mentioned before, a solid granular tiny spheres, when in contact with a fluid (gas or liquid) form a fluidized bed.
The most widely used catalysts were the crystallite alumina-silicates; their acid character helps break the carbon atoms and improvements in yields and selectivity were obtained but then zeolites provided enormous activity and selectivity benefits to the field of catalytic cracking, most likely due to a greater concentration of active sites.
Third, what is the typical feed for the FCC process to give the best gasoline yields?
The feeds that could nourish the FCC are the light and heavy distillates (VGO) from the vacuum distillation and the residue from the atmospheric distillation, but the typical feed going into the FCC is the heavy vacuum gas oil (HVGO), whose initial boiling point is 350-380 °C and end point is approximately 550-560 °C.
The fluid catalytic cracking unit (FCCU) consists of two major parts: The reactor, the “heart” of the cracking reaction with the coke deposits on the catalyst and the regenerator where the coke burning occurs.
The fundamental operating principle of the FCC is based on the thermal equilibrium achieved between the reactor and the regenerator.
Now, how does it work?
At a lower pressure, the feedstock is preheated to the temperature of 150-400 °C then it is injected in the bottom of the riser with a small amount of water vapor, after that the mixture vaporizes when it comes in contact with the catalyst heated to the temperature of 550-750 °C, note that the temperature of the catalyst is higher than the temperature of the feed.
When in contact with the catalyst, the hydrocarbon feed cracks through an endothermic reaction, to form gasoline, middle distillates and lighter products and coke. Then they both, the catalyst and the feed keep their ascent in the riser, due to the higher pressure at the base of the riser and the lower density of the mixture catalyst/vapors.
The residence time of the catalyst and the hydrocarbons vapors (supposing that the catalyst is solid and the vapors have the same time of residence) in the riser is estimated of few seconds.
The temperature at the top of the riser is between 450 and 550°C.
The reactor on top of the riser is a sort of disengagement; it aims to separate the catalyst particles from of the vapors via cyclones which make sure that no catalyst particles are integrated in the vapors.
The recovered vapors/gaseous products enter a fractionator to be separated based on various boiling points into several intermediate products: The lighter products are further processed to separate them into fuel gas, propane, propylene, butane and butene. The FCC gasoline must be desulfurized before blending into finished gasoline, the light gas oil is also desulfurized before blending into finished heating oil or diesel, the heavy gas oil is further cracked in either a hydrocracker or a Coker. Finally Slurry Oil, an aromatics residue steam, is partly recycled at the bottom of the fractionator while the other part is blended into fuel oil.
Meanwhile, the used/spent catalyst flows in the stripper situated at the bottom of the reactor where the heavy hydrocarbons, but yet volatile, on the catalyst grains are extracted by steam injection to avoid an overload that might occur in the regenerator later.
The catalyst is then sent to the regenerator via a transportation line.
The regenerator has actually two main roles:
It reactivates the catalyst by burning the coke and it provides the necessary heat for the endothermic cracking reaction in the riser.
It also consists of two phases: The dense phase and the dilute phase.
In the regenerator, the catalyst is reactivated in the dense phase through the burning of coke deposits on its surface by the injection of air at the bottom of the regenerator.
As the coke burning only occurs in the dense phase, the combustion reactions are typical:
C + O2 → CO2 (1.1)
C + 1 2 O2 → CO (1.2)
The combustion fumes escape to the dilute phase where they meet a couple series of cyclones hanging there to separate the remaining catalyst grains entrained in the fumes, like in the reactor.
In the dilute phase, the combustion reactions are neglected and oxidation takes place:
CO + 1 2 O2 → CO2 (1.3)
The fumes referred to as a flue gas (that contains a lot of carbon monoxide CO2) must meet treatment and energy recovery before being discharged into the atmosphere.
The regenerated catalyst is sent continuously to the riser via another circuit.
That circuit consists of a vessel that controls the flow of the regenerated catalyst, at the same time it maintains the necessary pressure in the regenerator.
The catalyst runs a complete cycle in less than 15 minutes including the cracking reaction, the separation of the vapors from the catalyst, catalyst stripping and regeneration.