Isooctane is produced from isobutylene in a two step process. The first step involves the catalytic dimerization of two isobutylene molecules to produce isooctene, or 2,4,4-trimethyl-1-pentene. The second step is hydrogenation of the isooctene to isooctane. The overall reaction is represented below: 
Conventional oligomerization processes have been used since the late 1930's to produce polymerization gasoline. Strong acid catalysts can be used to accomplish the oligomerization. Phosphoric acid catalysts are the principal catalysts used, particularly in the older units, designed specifically for polymerization gasoline production. However, phosphoric acid-based process typically afford fairly significant quantities of tri- and tetra-isobutylene oligomers. These heavier materials fall outside the boiling range for gasoline.
Now, due to the potential phase-out of MTBE in the U.S., several technology licensors have modified the isooctane process to accomplish the dimerization step utilizing the same catalyst used in the MTBE production process in the same equipment and at essentially the same process conditions. Acidic cation exchange resin catalysts are the common type used in the etherification step. The dimerization reaction is carried out in the presence of a hydrating agent, typically methanol, tertiary butyl alcohol or MTBE, as required by the resin catalyst. It is important to note that due to the presence of other butene compounds besides isobutylene and the selectivity limits inherent in the acid catalysts, the actual product from the isooctane process is a mixture of octane isomers, commonly referred to as isooctane gasoline. Physical properties for a typical isooctane gasoline are presented in the table below along with properties for other common gasoline blend components. AVERAGE PROPERTIES OF TYPICAL GASOLINE BLENDSTOCKS
| | Isooctane Gasoline1 | MTBE | TAME | Ethanol | C4 Alkylate2 | Isomerate3 | Reformate | FCCGasoline |
|---|
| S.G. | 0.72 | 0.75 | 0.77 | 0.79 | 0.68 | 0.70 | 0.78 | 0.75 | | RON | 100 | 116 | 114 | 125 | 95 | 89 | 95 | 90 | | MON | 94 | 96 | 99 | 100 | 92 | 85 | 85 | 80 | | RVP, psI | 2.0 | 7.3 | 2.0 | 17 | 4.5 | 14.5 | 3 | 5 |
1 Isooctane gasoline with 64 vol% Isooctane, 20 vol% trimethylpentanes, 8% dimethylhexanes, and 8% C9+
2 Hydrofluoric Acid process
3 Penex-Molex recycle processThe costs and benefits of retrofitting existing MTBE units with isooctane technology are difficult to quantify on a generic basis. The cost of retrofitting an existing MTBE plant can vary considerably depending primarily on the number and size of reactors and the capacity of the distillation columns and auxiliary equipment such as condensers and reboilers. Much of the equipment used in the MTBE process can be readily converted to dimerization service. However, the hydrogenation section is likely added as a grassroots unit and represents a significant portion of the overall retrofit cost. It is important to note that some refiners may choose not to add a hydrogenation section, thus producing an olefinic gasoline stream, with minimal new investment. However, olefin content of gasoline is expected to undergo further scrutiny with regard to future specifications and may be further reduced from current levels. The judgement to implement isooctane production technology will have to be made on a case by case assessment. Different refiners have different alternatives depending on their particular situation. Factors such as configuration of the existing MTBE unit, availability of existing alkylation capacity, hydrogen availability, and corporate philosophy of investment versus operating flexibility are key parameters that will need to be considered. |
