Knock, an abnormal combustion phenomenon, is the primary constraint on the efficiency of spark ignition (SI) engines. As engine designers strive for higher efficiency SI engines, pressure will build up to increase the anti-knock or octane quality of gasolines in the future. This poses challenges for refineries and high-octane blending components will enable the fuel manufacturers to provide suitable fuels for future engines. Oxygenates such as ethanol and methyl tertiary butyl ether (MTBE), are known for their resistance to knock and are used as gasoline blending components to increase octane number In addition, ethanol can be made from bio sources and is considered as a renewable fuel component. It is mandated for use in gasoline in many countries. MTBE is made from methanol and isobutylene-methanol itself can be produced from natural gas or coal while isobutylene is derived either from natural gas or from by-products of fluid catalytic and steam crackers.

… In this paper, we introduce a new high octane gasoline blending component, SuperButol, which is mainly a mixture of different isomers of butanol with small amounts of di-isobutylenes (DIB) and is made from mixed butenes which have low economic value in the refinery and low octane rating for the engine. This blending component could be used on its own or with other octane boosters such as MTBE. In fact, recent studies have demonstrated the potential of using alternative octane boosters, such as SuperButol in less processed fuels in order to reach the same anti-knock properties as a commercial high-octane fuel.

—Kalamaras et al.

Butenes are olefinic hydrocarbon gases; currently, refineries use mixed butenes resulting from cracking operations as a feedstock to produce intermediates for chemicals. Some of these techniques for processing mixed butenes are costly, and corrosive, the team notes.

By contrast, the SuperButanol production process—“Butenes to Butanols” (BTB)—is simple and safe, and was successfully demonstrated in a large capacity pilot plant.

Hydration and dimerization reactions occur in a fixed bed reactor at 10–70 bar and 100–170 ˚C using a sulfonated ion exchange resin catalyst with a water-to-hydrocarbon ratio in the range of 1–10 vol%. Overall single pass conversion is 14 vol%. Unreacted butenes are recycled to the reactor until all the butenes are converted to the isomers of butanols and iso-octane.

Process flow diagram for Butenes to Butanols (BTB) pilot plant. Product stream from the reactor is sent to the high pressure and low pressure separators to remove the majority of unreacted water to be recycled after azeotropic distillation. The organic phase from the separator vessels is sent to the debutenizer column to separate SuperButol product from unreacted butenes that would be recycled to ensure complete butenes conversion. Click to enlarge.

The SuperButol product as obtained mainly comprises 2-Butanol (63.9%) and tert-Butanol (29.0%). The Saudi Aramco team assessed the blend properties of SuperButol by blending it into gasoline at different concentrations.

This product has very good anti-knock qualities and can be used to increase both RON and sensitivity of a gasoline. It has higher volumetric energy content and leads to lower vapor pressure in blends compared to those containing either ethanol or MTBE, which increases blending flexibility. SuperButol can also increase the refinery revenues by enabling the blending of low value pentanes into the gasoline pool at an acceptable vapor pressure. Its oxidation stability, gum and deposit formation tendencies are similar to those of MTBE. In terms of haze formation and water extractability performance, it is better than ethanol and butanol and should pose no handling problems provided suitable precautions are taken. As a result, SuperButol could provide another practical and affordable option to produce higher octane gasolines in the future.

—Kalamaras et al.

Resources

• Christos Kalamaras, Kareemuddin Shaik, Gautam Kalghatgi, Hassan Babiker, Abdulkarim Alsamah, David McLeary, Wei Xu (2017) “SuperButol – A novel high-octane gasoline blending component,” Fuel, Volume 195, Pages 165-173 doi: 10.1016/j.fuel.2017.01.062