Chemistry of Catalytic Reforming
The general categories of the desired reactions in catalytic reforming are identified in the list below, along with the catalysts used in the process. Considering that the main purpose of the process is to increase the octane number of heavy naphtha, conversion of naphthenes to aromatics and isomerization of n-paraffins to i-paraffins are the most important reactions of interest. Under the right reaction conditions, aromatics in the feed, or those produced by dehydrogenation naphthenes, should remain unchanged. The reforming reactions produce large quantities of hydrogen, and one should remember that the dehydrogenation catalysts used in reforming can also catalyze hydrogenation and hydrocracking of aromatics during catalytic reforming. It is, therefore, important to keep these side reactions to a minimum by controlling the reactor conditions such as temperature and hydrogen pressure, as discussed in more detail later in this section.
The catalysts used in reforming contains platinum (Pt), palladium (Pd), or, in some processes, bimetallic formulations of Pt with Iridium or Rhenium supported on alumina (Al2O3).
Catalytic Reforming Reactions and Catalysts
Reactions of Interest
- naphthenes → aromatics
- paraffins are isomerized
- aromatics are unchanged
Platinum catalyst on metal oxide support (platforming)
Bimetallic – Iridium or Rhenium
The information above shows the ranges of composition for feedstock heavy naphtha and the reformate product (high-octane gasoline). Comparing the compositions of the feedstock and the product, one can see that the largest change in feedstock composition is a substantial increase in the aromatics content of the feedstock, with attendant decreases in naphthene and paraffin contents to constitute the product.
Catalytic Reforming Feedstock and Product
Feedstock: Heavy Naphtha
Paraffins ⇒ 45-55%
Naphthenes ⇒ 30-40%
Aromatics ⇒ 5-10%
Product: High Octane Gasoline
Paraffins ⇒ 30-50%
Naphthenes ⇒ 5-10%
Aromatics ⇒ 45-60%
Low severity (relatively low octane) → low paraffin conversion
High severity → high paraffin conversion
Lean naphtha → high n-paraffinic content - difficult to process
Rich naphtha → low n-paraffinic (high naphthene) content - easy to process
The information above also defines some specific terms for catalytic reforming related to the feedstock composition (lean, or rich naphtha), or to the extent of n-paraffin conversion in the process (low-, or high-severity). One could conclude from these terms that reforming of heavy naphtha that contains higher n-paraffin content requires more severe conditions in the reactor.