(555b) Separation of Lignocellulosic Sugars from Pyrolytic Bio-Oil Using Simulated Moving Bed

Anhydrosugars are the major carbohydrate products of autothermal pyrolysis of lignocellulosic biomass that can be upgraded into biofuels and bio-based chemicals. Our research group has developed a fast pyrolysis process, namely autothermal pyrolysis, which can produce high yields of bio-oil by thermal depolymerization of pretreated biomass. A significant fraction of the heavy ends of bio-oil contain sugars, mainly levoglucosan, but also xylose as the major carbohydrate products. The anhydrosugars in bio-oil are acid-hydrolyzed to produce glucose, which can serve as a fermentation substrate for manufacturing common renewable fuels such as ethanol.

The bio-oil contains some phenolic monomers generated from the thermal depolymerization of lignin during the pyrolysis process. These phenolic monomers end up in the hydrolyzed, neutralized, and filtered water-soluble sugar fraction of the bio-oil due to a moderate solubility of phenol and other substituted phenolic monomers in water. The presence of phenolic species in the aqueous extraction of pyrolytic bio-oil poses significant toxicity challenges in employing fermenting microorganisms downstream and therefore must be removed from the aqueous solution of bio-oil sugars prior to fermentation. Our previous report shows effectiveness of hydrophobic polymeric resins in preferential adsorption of phenolic species compared to levoglucosan and glucose from an aqueous fraction of bio-oil.

In this work, we have developed a simulated moving bed technology (SMB) to employ continuous scale separation of sugars from phenolic species for an aqueous bio-oil fraction using the principle of adsorption chromatography. Sugars were separated from phenolics in the SMB processing a hydrolyzed, neutralized and filtered water-soluble fraction of bio-oil containing 225 g/L glucose, 10 g/L xylose and 21.1 g/L phenolics at the rate of 4 mL/min. Although it is a binary separation, the SMB is modeled to separate two classes of chemicals (sugars and phenolics) rather than two individual chemical species. The SMB uses a 3-1-1-1 configuration of six columns each having 53 cc bed volume and can operate at 0.5-8.0 bed volumes per hour (BV/h) flow rates. The columns are packed with hydrophobic resin Dowex L493 to adsorb the phenolic compounds preferentially on the resin surfaces as they are more non-polar compared to the sugars in aqueous solution. Ethanol is used to desorb the adsorbed phenolic species on the column surfaces while DI water is used to purge any adsorbed sugars on columns into a purified sugar solution. Water is also used to regenerate the columns after each cycle of feeding, desorption and purging before the next cycle of feed flow begins. The entire unit periodically switches the entry point locations of feed, purge, desorption, and regeneration steams to simulate a continuous flow of solid beds of adsorbent resin against the liquid solution of water-soluble sugars. The purified sugar product contained 154 g/L glucose and 0.8 g/L phenolics after 6 h of operation at 4 BV/h using 20 min switch interval time and 1:1 ethanol/water consumption. We have further explored the effects of switch interval time and flow rates of the streams on the separation efficiency of the SMB unit to optimize its design.