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Jul-2014

Bio-isobutanol: a versatile molecule

Bio-isobutanol has many valuable characteristics that allow it to:
- Be used as is, as either a solvent or as a gasoline blendstock
- Be readily converted, through known processes, to a variety of hydrocarbons for use in the petrochemical and/or refining industries
- Be efficiently and effectively used in existing production, distribution, marketing and end user assets.

Richard kolodziej
Wood Group Mustang

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Article Summary

The pathway to make bio-isobutanol is via fermentation, paired with an integrated separation technology to optimise production. In May 2012, Gevo announced that it had started up the world’s first commercial bio based isobutanol production plant in Luverne, Minnesota, a planned 18 million gal/y facility.

Bio-isobutanol fermentation is quite similar to the existing ethanol process; ethanol plants can be modified and repurposed to make isobutanol relatively easily and cost effectively. And this was the case for Luverne.

One of the main reasons that converted plants have such good projected economics is that bio-isobutanol is so versatile as a platform molecule. In the chemicals arena, it can be: sold as solvent product itself (for paints) and/or through dehydration to isobutylene, converted into materials such as butyl rubber and paraxylene and other derivatives for use in market applications such as tyres, plastic bottles, carpets and clothing. For fuels applications, isobutanol can be blended in as a low vapour pressure gasoline component and/or used as feedstock to make other transportation fuels (for instance, iso-paraffinic kerosene for use as bio-jet) or other renewable products (including renewable heating oil).

Bio-isobutanol’s properties as a gasoline blendstock can best be understood by comparing some of its blending properties to those of ethanol and alkylate. Compared to ethanol, isobutanol has a much lower RVP and about 30% more energy content.  The blend octane of isobutanol is high as well (although slightly lower than ethanol). Isobutanol also has less oxygen content than ethanol, so more isobutanol can be blended into gasoline for a given oxygen content. And more blend volume plus more energy content means more renewable identification number (RIN) generation.

Isobutanol overcomes the regulation ‘blend wall’ limitation of ethanol blending. It is ‘substantially similar’ to gasoline at a 2.7 vol% oxygen content (or up to 12.5 vol% blend). This is a conservative first step for blending for refiners, and generates 16.25 RINs per gallon of finished product. E10 has 3.5 vol% oxygen, which is the currently accepted limit of oxygen content by automobile engine manufacturers. For this same 3.5 vol% oxygen, a US EPA 211(b) waiver exists that would allow isobutanol blending to 16.1 vol%, yielding 20.93 RINs, more than twice the RINs as E10 for the equivalent oxygen content.

Unlike ethanol that is fully miscible in water, isobutanol has very limited water solubility (about 8.5%). Isobutanol also does not cause stress corrosion cracking in pipelines. These factors result in major advantages in terms of blending logistics. Isobutanol can be blended as a drop-in renewable fuel at the refinery and shipped in pipelines to fuel terminals via existing infrastructure which prospectively eliminates the need for segregated tankage or pipelines. This also affords refiners the opportunity to once again produce a finished spec gasoline rather than a sub-octane blendstock for oxygenate blending. 

Taking the bio-isobutanol and processing it further to iso-paraffinic kerosene (IPK) bio-jet has now been demonstrated at Gevo’s hydrocarbon plant in Silsbee, Texas.

To make IPK bio-jet from bio-isobutanol involves three sequential steps:
1. Dehydration of the renewable isobutanol to mainly isobutylene
2. Oligomerisation of the isobutylene to mostly trimers/tetramers to produce C12 and C16 molecules
3. Hydrogenation of olefins to iso-paraffinic kerosene (IPK) bio-jet

These processes present opportunities for retrofits of existing under-utilised refining/petrochemical assets in some cases and commercialisation and integration into an existing process plant should be straightforward.

Depending upon economics, the overall process also has the flexibility to make more or less iso-octene and/or iso-octane byproduct streams (which make good renewable gasoline blending components). It should be noted that both renewable gasoline blendstocks (isobutanol and iso-octene) are not tied to crude oil processing, so these are not likely to carry crude oil price volatility effects.

This bio-jet process has been demonstrated in a small 10,000 gal/month (feedstock design capacity) unit for many months. The alcohol-to-jet (ATJ) product has been sold to the US Air Force as part of the Alternative Fuel Certification Division (AFCO) process.
Again, isobutylene, iso-octene and iso-octane are side products that can also be drawn off as products and used as feedstock for production into other renewable petrochemical products such as renewable paraxylene, which now has also has been demonstrated at Gevo’s site at Silsbee.

For more information:  Richard.kolodziej@mustangeng.com


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