Catalytic Reforming
The catalytic reforming is a key conversion process in the petroleum refining, precisely in the production of gasoline.
It is a gasoline refining process but at the same time it provides a significant part of the petrochemical industry’s supply of aromatic hydrocarbons.
Catalytic reforming help convert low octane oil cuts (40 to 60): Naphtha, to high octane fuel.
The aim is to transform, as much as possible, hydrocarbons with low octane to hydrocarbons to high octane, increasing the aromatic content and thus improving the octane number of the petroleum fractions by conversion.
This improvement in octane number is mainly the result of a sharp increase in the aromatics content. But what is an octane number?
Briefly, an octane rating or an octane number is a standard measure of a fuel’s ability to resist “knocking” or “pinging” during combustion, caused by air/fuel mixture detonating prematurely in the engine.
“Knocking/pinging” means an abnormal combustion resulting in resonance of the explosion on the walls of the combustion chamber and the piston in the engine: this is one phenomenon to absolutely avoid when running an engine.
This is why, since the origin of the engines, we have designed and produced gasoline/petrol that eliminates any risk of knocking: The higher the octane number, the more the car manufacturer will be able to increase the compression ratio and optimize the combustion process without the risk of knocking and here comes the role of catalytic reforming, to optimize this number in order to obtain a good quality petrol that can be used as a fuel.
The reforming process also uses a catalyst, it is likely to activate the desired reactions and make the process commercially feasible. All current catalysts are derived from platinum on chlorinated alumina: the catalyst is therefore composed of a base or support which is alumina (Al₂O₃) and certain metals that are catalytically active in reforming reactions, in our case Platinum (Pt).
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The reforming catalyst is bifunctional monometallic catalyst Pt/Al₂O₃ as it consists of a metal deposited on an acid support; the metal content is of the order of only 1% to ensure a good dispersion of the metal particles on the surface and in the pores of the support. The acidity of the support is maintained by injecting a small quantity of chlorinated compounds with the initial feed.
A bifunctional bimetallic – Iridium or rhenium catalyst Pt-Re/ Al₂O₃ could also be used; the rhenium helps stabilize the platinum on the surface of the acid support, and this kind has shown better performance with naphtha as the metal content significantly affects the performance of catalysts in terms of aromatic selectivity and catalytic activity.
How does it work?
The catalytic reforming unit consists of multiple sections that allow the feed preparation, the reaction and the effluents separation…:
1-Hydrotreating section
2- Magnaforming section
3-Fractionnation section
4-Absorption section
It takes place at a temperature of 490-540°C and a pressure of 0.53 MPa.
The low pressure here, favors the formation of aromatics at the same time, it accelerates the formation of coke.
First, the feed “naphtha” enters the hydrotreating section in order to eliminate the catalyst-damaging compounds from the feedstock: sulfur, nitrogen and metals, it is a hydrodesulfurization which takes place in the presence of catalysts based on molybdenum and cobalt or nickel and molybdenum on alumina at a temperature of 320 to 380°C under a partial pressure of hydrogen of about 0.5 to 0.8 MPa (This Hydrogen pressure is obtained by recycling part of the Hydrogen produced).
Second and after being desulfurized, the treated naphtha is sent to the Magnaforming section, passes over a catalyst bed where it meets a stream of Hydrogen H₂ for the main purpose to avoid coke formation on the surface of the catalyst and crosses a series of three reactors with intermediate furnaces where generally dehydrogenation occurs in the first reactor, Isomerization in the second and hydrocracking in the third.
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After that the reactions products are separated in a two-phase separator: a liquid mixture and a gas rich in hydrogen, the gas is drawn off from the top of the separator, purified and sent to be re-used in the process and other parts of the refinery while the liquid mixture from the bottom enters a stabilization column (is similar to the atmospheric distillation column except that it does not have side streams). The effluent recovered at the column head enters a separation drum where liquefied petroleum gases LPG (C₃, C₄) and other combustible gases are obtained while at the bottom of the column, we have our “reformate” containing gasoline with an octane number of 95-101 and is rich of aromatics BTX.
The “reformate” is further separated where aromatics are extracted by solvents, and then aromatics need to be separated individually through another series of extraction to get: Benzene, Toluene, Xylenes, ethylbenzene…
Now, during the whole process of catalytic reforming and the multiple reactions operating on each and every reactor something always go wrong and that is the coke deposit on the surfaces of the catalyst, positioning on the active centers of the catalyst as it is bifunctional.
Coking is the most complex reaction. Metals alone or acids alone cause coking in the presence of hydrocarbons at high temperatures. The acid function alone causes rapid coking but the presence of Platinum limits this type of coking. Therefore the regeneration of the catalyst is an important step of the process.
It takes place by burning the coke using Oxygen, several times in a regenerator.
Usually, it operates within a time line where the catalyst gets deactivated, a period of six months to a year and it requires that the plant has to shut down; that is why continuous catalyst regeneration is more likely to be chosen lately as the plant does not have to shut down and it implies less time for the unit and more production.
In the end, the catalytic reforming works fine from both the gasoline industry and the petrochemical industry as it provides high octane gasoline and a whole range of aromatics BTX which will be further processed in other industries.