Bioconversion of hybrid poplar to ethanol and co-products using an organosolv fractionation process: Optimization of process yields

Abstract

An organosolv process involving extraction with hot aqueous ethanol has been evaluated for bioconversion of hybrid poplar to ethanol. The process resulted in fractionation of poplar chips into a cellulose-rich solids fraction, an ethanol organosolv lignin (EOL) fraction, and a water-soluble fraction containing hemicellulosic sugars, sugar breakdown products, degraded lignin, and other components. The influence of four independent process variables (temperature, time, catalyst dose, and ethanol concentration) on product yields was analyzed over a broad range using a small composite design and response surface methodology. Center point conditions for the composite design (180 degrees C, 60 min, 1.25% H(2)SO(4), and 60% ethanol), yielded a solids fraction containing approximately 88% of the cellulose present in the untreated poplar. Approximately 82% of the total cellulose in the untreated poplar was recovered as monomeric glucose after hydrolysis of the solids fraction for 24 h using a low enzyme loading (20 filter paper units of cellulase/g cellulose); approximately 85% was recovered after 48 h hydrolysis. Total recovery of xylose (soluble and insoluble) was equivalent to approximately 72% of the xylose present in untreated wood. Approximately 74% of the lignin in untreated wood was recovered as EOL. Other cooking conditions resulted in either similar or inferior product yields although the distribution of components between the various fractions differed markedly. Data analysis generated regression models that describe process responses for any combination of the four variables.

Cellulolytic enzymes on lignocellulosic substrates in solid state fermentation by Aspergillus niger

“…Pretreatment process may improve substrate utilization by the microbes and enhance enzyme yields 25,26 . In the present study, only native lignocellulosic substrates without pretreatment were used.…”

Economic Analysis of an Organosolv Process for Bioethanol Production

“…As discussed in the previous paper (Kautto et al 2013), the carbohydrate recovery was assumed to be 84% based on Pan et al (2006), being considerably lower than that in the NREL study (Humbird et al 2011) (approximately 98%). Mabee et al (2006) reported a higher recovery in the organosolv cooking of softwood (90% for both glucose and hemicelluloses), suggesting that a higher carbohydrate recovery could be achievable.…”

“…Pan et al (2006) presented how low lignin content organosolv pulps exhibit significantly better response to enzymatic hydrolysis than higher lignin content steam-exploded wood. With approximately the same enzyme loading (20.9 mg cellulase and 5.7 mg β-glucosidase per g cellulose) and same hydrolysis time of 36 h (but with a significantly lower solids content), Pan et al (2006) reported hybrid poplar pulp prepared at the same conditions adopted in this study to exhibit a conversion efficiency of cellulose to glucose of approximately 96%. The enzymatic conversion of organosolv-cooked hybrid poplar might therefore be higher than that of dilute acid-pretreated corn stover of the NREL study (Humbird et al 2011) (cellulose to glucose conversion of 90%).…”

“…The effect of improved enzymatic hydrolysis on the economics of the organosolv process was studied by setting sugar conversion yield 6 percentage points higher for all carbohydrates. On the other hand, due to enzyme inhibition and deactivation caused by soluble sugars, furfural, hydroxymethyl furfural, organic acids, and phenolics at higher solids contents (see Kim et al 2011), the conversion rates in industrial scale enzymatic hydrolysis could be significantly lower than those reported by Pan et al (2006). Therefore, a conversion of both cellulose and hemicelluloses to sugars of 80% was adopted as a more conservative scenario for the enzymatic hydrolysis.…”

“…On the other hand, the fermentation of xylose and other hemicellulosic sugars has generally proven to be challenging (see Aden et al 2002). In addition, due partially to relatively high temperature and long residence time in organosolv cooking (assumed to be 180 °C and 60 minutes (Kautto et al 2013), based on Pan et al (2006)), and the need to close the cooking mass balance, the aqueous stream from cooking was assumed to contain initially relatively high amounts of sugar degradation products (see discussion in the companion paper (Kautto et al 2013)). Although the amount of these and other inhibitory compounds were assumed to be decreased to a level low enough for fermentation (Kautto et al 2013), residual inhibitory compounds might affect the fermentation, and no experimental data were available on the fermentability of such a stream.…”