Carbohydrate solubility in water as a function of temperature is important in a wide range of applications in food, pharmaceutical, as well as biofuels technology. Both experimental solubility data along with thermodynamic modeling are required for optimizing the processing of materials containing, e.g., to optimize sugar oligomer-monomer production in biomass conversion processes. This study presents an extensive experimental study on the measurement of high temperature aqueous solubilities of different carbohydrates (glucose, maltose, xylose, etc.) between 20 and 200°C. Sugar solubility data in the subcritical water region above the boiling point of water is of particular interest since carbohydrate polymers in various biomass (cellulose, hemicellulose) are frequently hydrolyzed under subcritical water conditions. Sugar solubilities were measured by a modification of the Miller-Hawthorne [1] continuous flow technique in which the sugar is saturated at various temperatures in a stream of flowing hot water. HPSEC was used to measure sugar solubility in diluted samples taken during the "saturation" region of solubility measurement technique. Due to the large increase in sugar solubility with temperature - particularly above the boiling point of water - adjustments in the size and amount of solute in the saturation cell as well as adjustment of the dilution water rate from a second pump were necessary. The obtained sugar solubility values compare favorably with existing data below the boiling of water - there is no data available for comparison at higher temperatures. Above 160°C, some degradation of the sugars occurs resulting in an apparent lowering of sugar solubility, but this can be alleviated by making the adjustments given described in the experimental section. The measured solubility data have been correlated using A-UNIFAC model (which takes into consideration association effects between carbohydrates and water by introducing a specific term in the original UNIFAC equation based on Wertheim associating theory)[2] - and with data from the literature to predict polysaccharide solubilities as a function of temperature and molecular structure.
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