Dehydration, bioethanol

Glnco-amylase enzyme converts the starch into D-glucose. The enzymatic hydrolysis is then followed by fermentation, distillation and dehydration to yield anhydrous bioethanol. Com (60-70% starch) is the dominant feedstock in the starch-to-bioeth-anol industry worldwide. 

Another problem with fermentation products is often the limited outlet. The primary fermentation products such as alcohols require chemical transformations to convert them into species acceptable by the chemical industry as intermediates. This can normally occur through dehydration reactions [77]. For example, ethanol may need to be dehydrated into ethylene, isopropanol into propylene and n-butanol into n-butylene. These reactions are reversed petrochemical reactions and normally lead to products that have a lower selling price than the starting materials under the present structure of the chemical industry. For this reason, bioethanol is still used unchanged as an oxygenated gasoline additive. 

Chen, G., Li, S., Jiao, F. and Yuan, Q. 2007. Catalytic Dehydration of Bioethanol to Ethylene Over Ti02/y-Al203 Catalysts in MicroChannel Reactors. Catal. Today, 125, ill-119. 

This reaction (Equation 3.3), which occurs at 250—300 °C with almost total yield (187), might find renewed interest in the future to convert bioethanol produced by fermentation into bioethylene (375,376) in the frame of a new industrial organic chemistry based on renewables. Ethanol dehydration has also been used recently as a test reaction for the investigation of the surface properties of aluminas (187,377—379). At low conversions, ethanol can be converted into diethylether with high selectivitiy. IR spectra show that ethanol adsorbs in the form of ethoxy groups, which are formed either by dissociation on Lewis acid—base pairs or by substitution of hydroxyl groups (187). 

Biobutanol is also an attractive biofuel with superior fuel properties but this application has not yet been established in the market. Biobutanol fits the existing fuel infrastructure it has a higher energy density (similar to petrol/gasoline) and better performance than bioethanol. In addition, butanol can be dehydrated to 1-butene and catalyzed into longer-chain oligomers for jet-fuel applications. Biobutanol can substitute for both ethanol and biodiesel in the biofuel market estimated to be 247 billion by 2020 [177]. 

Degradation of the sucrose framework Reactions or reaction cascades, including dehydration/rehydration, ring opening/closure, bond scission/ formation lead to structures being of known value from petrochemically derived compounds, but are based on the renewable feedstock sucrose, the most prominent representatives being bioethanol and 5-hydroxymethyl furfural (HMF). 

A. A. Kiss and D. Suszwalak, Enhanced bioethanol dehydration by extractive and azeotropic distillation in dividing-wall columns, Sep. Pur if. Technol. 86, 70 (2012). 

A. A. Martinez, J. Saucedo-Luna, J. G. Seqovia-Hernandez, S. Hernandez, R I. Comez-Castro, and A. J. Castro-Monteya, Dehydration of bioethanol by hybrid process liquid-liquid extrac-tion/extractive distillation, Ind. Eng. Chem. Res. 51, 5847-5855 (2012). 

So far, the following building blocks can be produced microbially for polymerization purposes hydroxyalkanoic acids with many structural variations, lactic acid, succinic acid, (i )-3-hydroxypropionic acid, bioethylene produced from dehydration of bioethanol, 1,3-propanediol, and c/ s -3,5-cyclohexadiene-I,2-diols from microbial transformation of benzene and other chemicals. They have been successfully used for making various bacterial plastics. 

The fermentation of starchy or sugar crops to bioethanol is performed using a series of different processes which are dependent on the raw material used. A general hioethanol process includes milling, liquefaction, saccharification, fermentation, distillation and dehydration, as shown in Figure 6.1. 

By inversion of the catalytic ethylene hydrolysis (Chapter 4.2) fuel-grade anhydrous bioethanol is rather easily dehydrated to ethylene at elevated temperature using, for example, a silicoaluminophosphate, HZSM-5 zeolite, or a heteropolyacid catalyst in a fixed bed or fluidized bed reactor (Figure 4A.25)." " ... 

Bioethanol (20 vol% in aqueous solution) has also been converted into hydrocarbons over Ce-modified ZSM-5 zeolites." The reaction was carried out in a fixed-bed reactor under atmospheric pressure, at 400 C. Various reactions, such as dehydration, oligomerization, cracking, hydrogen transfer, dehydrocyclization, aromatiza-tion, and dehydrogenation occurred to produce ethene, diethyl ether, and acetaldehyde as primary products. The secondary products formed from ethene. The presence of cerium in the HZSM-5... 

In the short term the choice of ethanol instead of methanol is obvious as bioethanol is available on a large scale today while there is hardly any production of biomethanol. Another advantage is that the composition of the crude ethylene flow from the reactor is much simpler than that obtained by dehydration of methanol. Mainly it contains water and ethylene with some nonreacted ethanol and minor traces of other substances. Consequently, there is no need for distillation equipment leading to a lower investment. Table 6.1 gives the composition of the crude ethylene stream from the reactor and the composition after water washing, compression and cooling as stated in Table II in the Braskem patent [29], The ethylene to be sent for flnal purification is approximately 99.4%, with ethane (0.15%) and butene-1 (0.27%) as the most abundant by-products. 

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