Conversion of ethanol fermentation stillage into aliphatic ketones by two-step process of hydrothermal treatment and catalytic reaction
Graphical abstract
Highlights
► Proteins of the fermentation stillage were converted into heavy components ► Carbohydrates of the fermentation stillage were converted into ketones, carboxylic acids, and esters ► Catalytic reaction of carboxylic acids and esters produced aliphatic ketones
Introduction
In recent years, there has been increasing worldwide interest in shifting energy dependence away from petroleum (a nonrenewable carbon resource) to renewable biomass resources, because the price of petroleum continues to rise sharply and also for effective management of greenhouse gas emissions. Therefore, ethanol is currently being used as an automotive fuel in many countries [1].
The advantages of ethanol as a renewable, carbon-neutral resource have motivated many countries to produce and utilize ethanol, primarily from biomass by fermentation. The predominant feedstock for this process is corn, although small amounts of wheat, sorghum, and barley are also used [2], [3], [4], [5]. Fermentation of cereal grains to make ethanol also yields a protein-rich by-product (stillage) that remains after the ethanol distillation [6].
The stillage can be used as a valuable resource for fertilizers, animal feed, or methane gas [6], [7], [8]. Methods for utilizing stillage include concentration and drying. The solid and liquid fractions are separated using centrifuges or pressing devices. The solid phase is then dried (dried distillers grain), while the liquid phase is concentrated up to a 30–40% suspended solid content (condensed distillers solubles) is achieved. When the condensed distillers solubles are dried, dried distillers solubles are obtained. In some distilleries, the dried distillers solubles are mixed with dried distillers grain, and the mixture obtained is termed dried distillers grain with solubles (DDGS) [7].
In this study, the DDGS was used as the feed for aliphatic ketones production. We converted DDGS by a two-step process, hydrothermal treatment followed by a catalytic reaction. Hydrothermal treatment was used to convert DDGS into oxygen containing tar (liquid tar) using water as the reaction medium. Then, a catalytic reaction was carried out to upgrade the liquid tar into aliphatic ketones. In the catalytic reaction, ZrO2-FeOX catalyst was used since it has been successfully utilized to upgrade pyroligneous acid and oxygen-containing tars derived from sewage sludge and coliform-fermented residue to form ketones [9], [10], [11].
Section snippets
Composition of dried distillers grain with solubles (DDGS)
The DDGS feed for hydrothermal treatment was produced from fermentation of irregular wheat for ethanol production. The DDGS contained 49 wt.% carbon based on elemental analysis. This value of carbon concentration was used as the calculation basis of the total carbon yield after hydrothermal treatment. Table 1 shows the compositions of the irregular wheat and DDGS.
Carbohydrates, such as glucose, in the irregular wheat were converted into ethanol by yeast; however, some of the carbohydrates,
Hydrothermal treatment of DDGS
Fig. 2(a) shows the effect of hydrothermal temperature on the product yield. Hydrothermal treatment resulted in products consisting of gas, liquid tar, heavy tar (paste), char and carbonaceous residue. The main component of the gas product was CO2 with trace amounts of light hydrocarbons [C1, C2, C3, and C4] for hydrothermal treatment at 200–250 °C. As the hydrothermal temperature was increased to 300–350 °C, formation of CO2 decreased and the formation of C1-C4 light hydrocarbons increased.
In
Conclusions
The conversion of DDGS (dried distillers grains with solubles) into aliphatic ketones was carried out by a two-step process of hydrothermal treatment followed by a catalytic reaction. During hydrothermal treatment, yields of gas and light components were influenced by temperature. At lower temperatures (200–250 °C), the main composition of gas was CO2, however, by increasing temperature, the formation of gas was replaced by light hydrocarbons (C1-C4). In higher temperature, higher yield of light
Acknowledgements
This work was supported by the Global COE Program (Project No. B01: Catalysis as the Basis for Innovation in Materials Sciences) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.
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