(594a) Optimization of Chromatographic Separation for Production of Bioethanol From Lignocellulose by Concentrated Acid Hydrolysis

Bioethanol production by converting lignocellulose (wood) to fermentable monosaccharides has received considerable attention during the last decade. Concentrated acid hydrolysis results in a high yield of sugars, but requires large volumes of the acid. In addition, the pH of the hydrolysate must be increased before the fermentation. Since overliming is neither economically feasible nor ecologically sustainable, chromatographic separation of the acid and sugars has been recognized as an attractive option, and has been studied to some extent in the academia and in the industry.

When strong acid ion exchange resins are used for acid-sugar separation, the separation is a result of three retention mechanisms. Electrolytes (sulphuric acid and salts) are partly excluded from the resin due to the Donnan potential, and thus are the least retained component in the system. Large molecules are excluded from the resin due their size (size exclusion), whereas small neutral species (e.g. monosaccharides and fermentation inhibitors) penetrate into the particles, and are retained by adsorption.

Compromises are inevitable when choosing the stationary phase: electrolyte exclusion is stronger with highly cross-linked resins, whereas a low cross-link density is beneficial for sugar sorption. In addition, low cross-link density results in large variations in the bed height.

In this work, recovery of the hydrolysis acid and fermentable sugars, as well as removal of fermentation inhibitors, were investigated both experimentally and by numerical simulations. Lignocellulose hydrolysates were prepared by stepwise treating wood chips (spruce and birch) with 70 wt-% and 30 wt-% sulphuric acid solution in a 1 L reactor at 353 K. The influence of ion exchange resin properties (cross-link density, particle size) and column loading on the performance of separation process (productivity, eluent consumption, concentrations of the product fractions) was studied. Interaction between the adsorbate molecules was found to give rise to unexpected retention behaviour of the sugars.

The single column chromatographic data were correlated with a mathematical model. Simplified competitive adsorption isotherms were derived and found to describe the retention behaviour of all components with reasonable accuracy. Optimization of the separation process was carried out for each performance parameter separately and for various combinations of the performance parameters.