Integrated Renewable Production of Sorbitol and Xylitol from Switchgrass

Abstract

This work deals with the design of integrated facilities for the production of xylitol and sorbitol from lignocellulosic biomass. Xylitol can be obtained from xylose via fermentation or catalytic hydrogenation. Sorbitol is obtained from glucose, but preferably from fructose, and also via fermentation or catalytic hydrogenation. Fructose can be obtained from glucose via isomerization. Thus, a superstructure of alternatives is formulated to process switchgrass, corn stover, miscanthus, and other agricultural and forestry residues. Different pretreatments, such as dilute acid or ammonia fiber explosion (AFEX), for the fractionation of the biomass are evaluated. Next, after hydrolysis, the C5 and C6 sugars are processed separately for which a catalytic or a fermentation stage are considered. Glucose has to be isomerized before it can be processed. Finally, crystallization in a multistage evaporator system is used for purification. The optimization of the system is done by the use of dilute acid and the catalytic system. A system of 3 crystallizers is selected. For a facility that produces 145 kt/yr of xylitol and 157.6 kt/yr of sorbitol, the investment adds up to 120.74 M€ for a production cost of 0.28 €/kg products. The inverse engineering of biomass was also performed resulting in a composition of 15% water, 20% cellulose, 40% hemicellulose, 15% lignin, and 5% ash. The closest biomass corresponds to Sargassum (brown algae), which is capable of producing 230.5 kt/yr of xylitol and 116 kt/yr of sorbitol with investment and production costs of 120.5 M€ and 0.25 €/kg products, respectively.

Figure 1

Superstructure for the renewable production of xylitol and sorbitol.

Figure 2

Schematic of AFEX pretreatment.

Figure 3

Schematic of dilute acid pretreatment.

Figure 4

Details of the fermentation pathway.

Figure 5

Catalytic pathway.

Figure 6

Xylitol purification.

Figure 7

Sorbitol purification.

Figure 8

Detailed productions costs and investment costs for AFEX-fermentation (a and b), AFEX-catalytic hydrogenation (c and d), dilute acid-fermentation (e and f), and dilute acid-catalytic hydrogenation (g and h).

Figure 9

Dilute acid-catalytic hydrogenation free composition: productions costs (a) and investment costs (b).