Study shows gasoline pre-blending in ethanol production could cut energy requirements of separation by 17-40%
Researchers at the University of Witwatersrand and the University of South Africa are proposing replacing the final purification steps of conventional bio-ethanol production with a simple gasoline-blending step.
In a paper published in the ACS journal Energy & Fuels, they show that gasoline pre-blending results in a spontaneous liquid phase split which produces a viable fuel with desirable ethanol content and high recovery of ethanol; reduces the energy requirements of separation by between 17 and 40%; reduces operating costs of the process; and also eliminates capital expenses.
The most prevalent use of bioethanol is not as a fuel in its pure form but rather as an additive to petrol. Conventional processes, however, fully purify bioethanol prior to blending it in petrol. This is an energy-intensive and costly process and is typically achieved through distillation. This paper examines the possibility of blending partially purified fermentation products directly into petrol, allowing for the spontaneous liquid-phase separation to eliminate the bulk of the remaining water without the addition of separation energy.
… The nature of the fermentation process dictates that fermentation products are dilute, comprised primarily of water. For bioethanol to be used as a fuel, this water must be eliminated. This separation is conventionally achieved using distillation. Because distillation requires the evaporation of liquids, it tends to require substantial energy inputs. This is exacerbated in this case by the high heat capacity of water and the existence of a binary azeotrope in the ethanol/water mixture, resulting in a particularly energy-intensive separation process. This paper examines the possibility that some of this energy consumption can be alleviated by the use of simple flowsheet improvement.—Stacey et al.
Conventional fermentation processes to produce ethanol make use of either one or two distillation columns that produce an azeotropic mixture (a mixture of liquids that has a constant boiling point because the vapor has the same composition as the liquid mixture) of water and ethanol. This precludes the use of simple distillation to produce pure ethanol. Azeotropic distillation—the introduction of another component (an entrainer) to produce two immiscible liquid phases—delivers the final purification.
The South African researchers are proposing replacing the final purification step with a simple liquid-liquid phase split, thereby eliminating between 17.3 and 40.8% of the total separation energy, potentially saving between 0.916 and 2.04 MJ/L of ethanol produced.
In their study, they found that blending 8.75 L of gasoline for each liter of ethanol in the azeotropic mixture yielded an enriched gasoline stream with an ethanol content of 10% while achieving an ethanol recovery of 97.5%.
The process can also be modified slightly to cater to different objectives in terms of ethanol composition. For example, a two-stage blending process with a blending ratio of 48 delivered an ethanol content of 2% and an ethanol recovery of 99.9%.
It is important to put this result into context. Globally, over 20 billion gallons of bioethanol are produced per year. Most of that bioethanol is purified for use in fuels, using energy-intensive azeotropic distillation methods. Converting even a fraction of those existing processes to instead use the direct blending method would result in energy savings on the order of terajoules per year, cutting global carbon dioxide emissions significantly while reducing the cost of renewable fuels. The content of this paper sets out the groundwork for developing processes to achieve this goal.
It must also be noted that this approach is equally applicable to the separation of other alcohols for fuel usage. Ongoing work is underway to develop a similar process for biobutanol separation, with the expectation of even larger energy savings, owing to the fact that butanol is a less polar molecule and will therefore tend to dissolve into the fuel phase more preferentially than ethanol, allowing for a lower purity mixture to be blended while achieving the desired recovery and alcohol content.—Stacey et al.
Neil T. Stacey, Aristoklis Hadjitheodorou, and David Glasser (2016) “Gasoline Preblending for Energy-Efficient Bioethanol Recovery” Energy & Fuels doi: 10.1021/acs.energyfuels.6b01591