Abstract
A process for hemicelluloses fractionation and purification from wheat straw and bran has been investigated and technical considerations (yields, purity) have been coupled to environmental characterizations (water consumption, carbon dioxide emissions) in order to develop an environment-friendly process. Extraction by twin-screw extrusion gave a yield in arabinoxylans equal to 8.5% (weight of (arabinose+xylose) in the extract after fractionation/dry weight of the destarched bran). The extraction of 86kg of straw and bran (with a ratio 6.2:1) with 5.8kg of NaOH in pellet form resulted in the production of a complex extract containing 1.0kg of arabinoxylan polymer, which required concentration and purification steps. Evaporation (EV) followed by ethanol precipitation (P) and freeze-drying (FD), gave a yield in hemicellulosic powder of 36.5% (dry weight of powder/dry weight of extract after liquid/solid separation) with a total sugar content equal to 48.4% but also used a large amount of ethanol. The other studied purification process was based on a combination of ultrafiltration (UF), anion exchange chromatography (CHR) and spray-drying (SD). It gave a yield in hemicellulosic powders of 24.6% and a total sugar content equal to 28.7%. The technical performances of the second process appear to be less attractive but with a lower energetic and ethanol consumption. Thus secondly the environmental impacts (water consumption and CO2 emission) of the ultrafiltration step were quantified. Life Cycle Assessment data (Ecoinvent) were used to convert materials used for the infrastructure and energy consumed during functioning into carbon dioxide emissions and water consumptions. Results have shown that environmental impacts due to the operating conditions are higher than those relative to raw material involved in the installation. The study showed that this kind of approach allows the determination of optimum conditions for the ultrafiltration step.
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