Two main challenges of the ABE production by fermentation are low yield and contamination by aerobic and acid producing anaerobic bacteria. The low yield is being addressed by continuous separation of ABE from the bioreactor, and the toxicity tolerance is being addressed by genetic improvement in the microorganism.

The ABE mixture can become a compatible fuel if it is transformed into a drop-in fuel for the replacement of gasoline, diesel, or jet fuel. For upgrading ABE to be used in automotive engines, dehydration or deoxygenation of the ABE mixture to produce a higher energy content hydrocarbon chain is the most promising route. Dehydration or deoxygenation can be aided by inorganic catalysts containing acid sites. However, to date, not much work has been done on the conversion of the ABE mixture.

...This work examines the dehydration of the ABE mixture. In this work, the mixture of acetone, n-butanol, and ethanol has been taken in a mass ratio of 3.7:8:1, which implies a composition of 29.3, 62.9, and 7.8 wt %, respectively, a typical composition obtained from genetically modified clostridium bacteria. The main objective of this work is to dehydrate the ABE mixture to long chain hydrocarbons.

—Nahreen and Gupta

In their experiments, they preheated a feed of the ABE mixture and pumped it through a catalytic packed bed tubular reactor in a continuous process at pressures of 3−13 bar. Using different operating temperatures and feed flow rates, they observed the effect on the dehydration products, which are mixtures of three phases:

1. a gas phase consisting of light hydrocarbons and carbon dioxide;
2. an organic liquid phase consisting of heavy hydrocarbons; and
3. an aqueous phase with dissolved oxygenated hydrocarbons.

The dehydration products from the ABE mixture are mostly unsaturated hydrocarbon chains in the range of C₂−C₁₆.

They analyzed the products and compared them to those from the dehydration of pure n-butanol, acetone, and ethanol feedstocks. On the basis of the higher heating values (HHV) of the liquid products and infrared spectra of the gas products, they concluded that the products from the ABE feedstock are different from those from the individual components, which suggests a cross reactivity of the components during the reaction.

Catalytic dehydration of the ABE mixture reveals an interesting synergy of the reacting components. The product from the ABE mixture is not a simple sum of the products from the individual feed components, acetone, 1-butanol, and ethanol. The inter-reactivity of the components contributed to a liquid product with a high heating value.

Out of the conditions studied here, γ-Al₂O₃-catalyzed dehydration at 400 °C produced the highest amount of useful hydrocarbon products in terms of butene and high HHV organic liquid.

—Nahreen and Gupta

Resources

• Shaima Nahreen and Ram B. Gupta (2013) Conversion of the Acetone–Butanol–Ethanol (ABE) Mixture to Hydrocarbons by Catalytic Dehydration. Energy & Fuels doi: 10.1021/ef302080n