Plug-Flow Reactor for Continuous Hydrolysis of Glucans and Xylans from Pretreated Corn Fiber

Pressure cooking of corn fiber in liquid water at 160 °C and a pH maintained at 4−7 produces an aqueous stream of dissolved glucans, xylans, proteins, phenolics, and minerals. We report hydrolysis of these oligosaccharides to glucose and xylose in a fixed-bed reactor packed with a macroreticular strong cation exchanger. The aqueous stream is first contacted with the cation exchanger at room temperature where proteins, phenolics, minerals, and other catalyst fouling components are removed. The material is then passed over a packed-bed of the same catalyst at 130 °C to give 88% hydrolysis for a space time of 105 min. Comparison of cation exchanger in a plug-flow versus a batch reactor for hydrolysis of oligosaccharides as well as for hydrolysis of the disaccharide cellobiose shows that yields at 110−160 °C are greatest for a plug-flow reactor. Maximum glucose yield increases as hydrolysis temperature increases and reaches 90% at 160 °C, which was the highest temperature tested in this study. A model of reactor performance based on first-order kinetics with diffusion resistance fit the data for cellobiose with an observed hydrolysis yield of 90% at a residence time of 3.5 min at 160 °C. A preliminary economic analysis shows 1 lb of catalyst that generates 1000 lb of glucose will give incremental costs of between $0.01 and $0.18/gal of ethanol, depending on catalyst cost. Further improvements in catalyst life and selectivity could result in an alternative or complimentary approach to enzyme hydrolysis for biomass pretreatment processes that generate water-soluble glucans and xylans from corn fiber and other cellulosic residues. Ultimately a sequential, continuous pretreatment and hydrolysis system is envisioned that has the added benefit of minimizing reactor volumes in large-scale cellulose to ethanol plants.