As mentioned above, the liquid fraction resulting from lignocellulosic biomass pretreatment can be unified with the cellulose hydrolyzate obtained after the enzymatic treatment of the cellulose contained in the solid fraction coming from the mentioned pretreatment process. The liquid stream produced contains all the soluble sugars derived from the biomass, mostly glucose and xylose. This stream can be fermented by microorganisms able to convert these two sugars into etha­nol. For this, recombinant bacteria can be used. Leksawasdi et al. (2001) employed an engineered strain of Z. mobilis able to co-ferment hexoses and pentoses to
process a solution containing a mixture of glucose and pentose. In addition, they developed and experimentally tested an accurate mathematical description that considers substrate limitation by the two sugars, substrate inhibition by both sug­ars, and ethanol inhibition. For this, it employs the concepts of threshold ethanol concentration for which inhibition of growth begins, and maximum ethanol con­centration for which biomass growth becomes zero. Inhibition constants for the substrates are taken into account for considering glucose and xylose concentra­tions inhibiting both cell growth and ethanol biosynthesis as a result of catabolic repression. Kinetic expressions can be found in the work of Leksawasdi et al. (2001). This description was used by the authors of this book to consider co­fermentation processes during process synthesis procedures. The equations of the model are as follows:

r = [ar, 1 + (1 — a )rx, 2]X

where rx is the overall cell growth rate, rxj and rxj are the cell growth rate from glucose and xylose, respectively; rsj and rsj are the glucose and xylose consump­tion rates, respectively; rp is the overall ethanol production rate; rp1 and rp1 are the ethanol production rates from glucose and xylose, respectively; X, Sj, S2, and P are the concentrations (in g/L) of cell biomass, glucose, xylose, and ethanol,

respectively; a, ^max, j, Amax,2, qs, max, j, qs, max,2, qp, max, j, qp, max,2, Ksx, j, Ksx,2, Kss, j, Kss,2,

K K P P P P P P K K K K K K P

sp 1 sp 2 mx,1 mx,2 ms,1 ms,2 mp,1 mp,2 ix,1 ix,2’ ■ is,1 is,2 ip,1 ip,2 ix,1’

Pix, Pisj, Pis,:2, Ppj, Pip,2 are the kinetic parameters.

Case Study 7.1 Modeling of Co-Fermentation Fermentation

Based on the model of Leksawasdi et al. (2001), the simulation of alcoholic fer­mentation from biomass was performed with initial glucose concentration of 100 g/L and initial xylose concentration of 50 g/L. These concentrations approximately correspond to those of lignocellulosic hydrolyzates. Inhibition of growth rate can be observed from Figure 7.9 due to relatively high amounts of ethanol in the broth, as can be seen after 25 h. The use of more concentrated culture media leads to the underutilization of expensive feedstocks, which cannot be transformed into ethanol despite their availability in the broth. According to the model, when a medium con­taining up to 400 g/L of fermentable sugars is employed, an ethanol concentration of about 71.2 g/L is reached only after 80.5 h of cultivation. An ethanol concentra­tion of 24.7 g/L is attained at 48 h remaining more than 347 g/L of substrate (com­pared with data of Figure 7.9 for a medium with a lower substrate concentration). Therefore, this model proved its suitability for describing the complex inherent phenomena of the co-fermentation of lignocellulosic hydrolyzates.