Reactions of Naphtha Catalytic Reforming Process

The main purpose of the catalytic naphtha reforming process is to increase the octane number of heavy naphtha and to produce large quantities of hydrogen. The reforming process and expected product yields are dependent on the concentrations of paraffins, naphthenes, and aromatics in naphthas. In catalytic reforming, naphthenic naphtha feeds produce higher reformate, aromatics, and hydrogen products relative to those for paraffinic naphtha.

The five major reactions are naphthene dehydrogenation, isomerization of paraffins, paraffin dehydrocyclization, hydrocracking of paraffins, and hydrodealkylation of alkyl aromatics. In addition, some reviews are provided of the contributions of feed hydrocarbons and contaminants, and process conditions that promote coking reactions and higher rates of catalytic coke depositions on catalytic reforming catalysts.

1. Naphthene Dehydrogenation

Naphthene Dehydrogenation is a key reaction that produces most of the hydrogen and aromatics in a catalytic reformer. Naphthene dehydrogenation reactions are rapid, endothermic reactions that convert naphthenes to aromatics and hydrogen. The reactions occur with a net increase in the moles of gaseous products.

Thermodynamically, the reaction is highly endothermic and is favoured by high temperature, low pressure and metallic function of the catalyst. In addition, higher carbon number cyclohexanes yield higher aromatics production at equilibrium.

Since it produces aromatics with a high octane rating, so this is the most desirable reaction. Since naphthene dehydrogenation reactions are rapid and consume substantial amounts of heat, heaters are required between reactors to supply the necessary heat for sustaining hydrocarbon reforming reactions.

2. Paraffins Isomerization

The acid-promoted isomerization reaction is important to convert linear bowls of paraffin into higher octane-branched paraffins. These reactions are fast and slightly exothermic, and the number of carbon atoms does not change. The octane boost associated with paraffins isomerization reactions is high.

The thermodynamic equilibrium of isoparaffins to normal paraffins depends mainly on the temperature (low temperature favourable) and increases with the carbon number of the paraffin. The pressure does not affect the equilibrium. Paraffin isomerization reactions are less selective than naphthene isomerization reactions, as significant undesirable reactions such as paraffin hydrocracking and hydrogenolysis occur.

3. Naphthenes Isomerization

Naphthene isomerization reactions are critically important in the formation of aromatics. The isomerization of an alkyl cyclopentane into an alkyl cyclohexane involves a ring rearrangement and is desirable because of the subsequent dehydrogenation of the alkyl cyclohexane into an aromatic. Due to the difficulty of the ring rearrangement, the risk of ring opening resulting in paraffin is high.

The reaction is slightly exothermic. At a typical reforming operating temperature (~ 500 °C) thermodynamics limits the alkylcyclohexane formation. However, the subsequent dehydrogenation of the alkylcyclohexane into an aromatic is very fast and shifts the reaction toward the desired direction.

This type of reaction is also easier for higher carbon number molecules. The octane number increase is significant when considering aromatics as an end product. The naphthenes isomerization reactions are promoted by the acid functions of the catalyst and are slightly dependent on pressure.

4. Parrafins Dehydrocyclization

A key objective of the catalytic reforming process is the high conversion of paraffins via aromatization reactions to increase the aromatics concentrations of reformates and increase the RON of reformate.

Paraffin dehydrocyclization or paraffin aromatization involves the conversion of paraffins over both the hydrogenation/ dehydrogenation (platinum) and acidic sites of catalytic reforming catalysts to produce aromatics.

Paraffin dehydrocyclization reactions are extremely slow and the most difficult reactions to proceed. Further, lighter paraffins limit the equilibrium of conversions. Heavier paraffins make the conversion process much easier.

One of the key factors in the design of the reactors is the need to provide sufficient residence time so a high percentage of slow paraffin aromatization reactions are enhanced and completed. These reactions are favoured by high temperature and low pressure, under both acid and metal functions of the catalyst.

5. Parrafins Hydrocracking

Hydrocracking is the reaction of straight-chain paraffinic hydrocarbons with hydrogen to produce methane, ethane, propane, and butane. Hydrocracking is not a desirable reaction, as it leads to liquid product losses. Essentially, there is a significant loss of reformates and aromatics depending on the extent of paraffin hydrocracking reactions.

Paraffin hydrocracking is moderately fast and the reactions are faster than those of paraffin dehydrocyclization. Parrafin Hydrocracking reactions are favoured by high temperature and high pressure and are catalyzed by an acidic function of the catalyst.

Reactions are exothermic and, if they occur in the lead reactors, lead to significant declines in the lead reactor temperature drop. Paraffin hydrocracking reactions typically occur mostly in the last two reactors. The conversion of paraffin into cracked lighter products consumes hydrogen and reduces the reformate product yield.

6. Hydrodealkylation of Nephthenes and Aromatics

Hydrodealkylation of naphthenes and aromatics is a desirable reaction for increasing C6 through C8 aromatic compound yields and reducing the rate of coke formation in catalytic reformers. Cracking of bulky alkyl side chains from naphthene intermediates in the paraffin aromatization mechanism leads to a lower rate of coke formation and higher C5+ liquid yields.

Depending on the metal and acidic functionalities of the catalysts and process conditions of the catalytic reformers, alkyl aromatic compounds react with hydrogen to produce benzene, toluene, xylenes and alkanes.

7. Demethylation of Parrafins and Aromatics

This reaction involves the removal of methyl molecules from Paraffins and Aromatics groups. The reactions are favoured by high temperature and high pressure at the metal function of the catalyst.