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Reforming and Isomerization
How to Increase Naptha Octane Rating
Matt Anderson
August 15, 2023
Reforming and Isomerization are two common methods for increasing naphtha octane rating. This concept is sometimes called “upgrading of light hydrocarbons”.
The general principle for increasing octane requires rearranging straight chain hydrocarbons (often called n-paraffins) into branched and iso-paraffins. Both reforming and isomerization change molecular structure using energy and catalysts.
Reforming
Product is called “reformate”
Hydrogen is produced as a byproduct. The hydrogen is used for other refining processes, such as hydrotreating.
Octane increases through production of isoparaffins and aromatics. The formation of isoparrafins occurs through the chemical process of isomerization (which is confusing as the other refining process for increasing naphtha octane is called ‘Isomerization’).
Feed to Reforming Units must be hydrotreated as sulfur will poison the catalyst. This also means reformates have very low sulfur which is beneficial for gasoline blendstock.
UOP commercialized reforming and built the first reformer unit, called a “Platformer”, in 1949. [1]
Isomerization
Product is called “isomerate”.
Feed is typically C5 and C6, which is lighter than reformer feed. Straight-chain pentane (n-pentane) has a Research Octane Number (RON) of about 60, while isopentane has a RON of 90.
“Penex” is the most common commercial technology for isomerization of C5 and C6 molecules. It was developed and is licensed by UOP. [2]
Uses a platinum catalyst (not cheap).
Consumes hydrogen.
Saturates olefins if they are present. [2]
Feed must be hydrotreated to remove sulfur to avoid poisoning catalyst.
Why it matters: Straight-run* naphtha does not have a high enough octane rating to be used effectively in gasoline blending.
Go Deeper:
87 is a common octane rating for regular gasoline in the United States. The vast majority of gasoline in the U.S. is 90% blendstock from refineries mixed with 10% ethanol. This is called “E10”.
Ethanol is an octane booster. Blendstock without ethanol typically has an 83-84 octane rating. Only after it is mixed with ethanol will the finished gasoline have an 87 octane.
At this point, it is very helpful to have some basic chemistry background. I made a video (only 3 minutes) that walks through simplified chemistry concepts that are needed for understanding how refineries operate. LINK TO VIDEO.
When crude enters a refinery, it is first separated by boiling points in a distillation column. Lighter molecules, that are used for gasoline, are called naphtha. Heavier molecules are used for jet fuel and diesel. Even heavier molecules are called gasoil and can be used for fuel oil blending.
The naphtha coming off the crude distillation tower can have a range of octane values. A reasonable RON estimate is 50-70. The heavier molecules cannot be used in gasoline.
Upgrading light hydrocarbons
At this point, it is very reasonable to think, “then how do we get gasoline blendstock with 83-84 octane?”
This is the value of refineries. The general strategy is to rearrange molecules to increase the octane value. Reforming and Isomerization are the two most common technologies for upgrading naphtha. The higher octane of reformate and isomerate make them ideal for gasoline blending.
Vocab Word of the Week:
*Straight-Run = Intermediate hydrocarbon coming off the crude distillation tower. This means that the hydrocarbon stream has not gone through any upgrading processes or cracking processes such as coking or catalytic cracking. This also means it will be relatively low in olefins and aromatics (especially compared to intermediates that have been cracked).