Polymer induced flocculation and separation of particulates from extracts of lignocellulosic materials

Biofuels from lignocellulosic materials like wood are renewable and sustainable alternatives to petroleum and other fossil fuels. Wood can be grown and harvested without adding to the carbon load of the atmosphere and thus can be part of the solution to the problem posed by global climate changes. Recently much interest has developed on the concept of the forest product biorefinery, where wood is hydrolyzed prior to conventional pulping and papermaking processes and the hydrolyzate consisting of hemicellulose sugars are to be used as a feedstock for biofuels or bioplastics. The purification of the hydrolyzate stream and the separation of fermentable sugars from it thus constitutes an important step in biorefinery processes.The separation of particulate material from wood hydrolyzates is considered in this paper. Sugar maple hardwood was extracted with hot water at 160 °C. The extracts contain hemicelluloses (primarily xylooligomers, xylose and xylan), acetic acid and smaller amounts of lignin. The colloidal stability of the extracts plays a critical role in the separation and purification of the wood extracts. Here, we report the size and charge of the particles in the extract measured using standard instruments based on light scattering and microelectrophoresis. Particles were found to be in the size range from ∼220 nm to 270 nm. Zeta potential measurements showed them to be negatively charged. By treating the extracts with a cationic flocculating agent poly-DADMAC, it was possible to preferentially precipitate out the colloidal fraction containing lignin and lignin derived compounds. Upon the addition of poly-DADMAC the turbidity of the suspension reduced from 920 NTU to 4 NTU in a 24 h period and particulates sedimented from the extract. The lignin concentration was reduced in the supernatant, while the sugar content remained unchanged. The addition of an indifferent electrolyte hindered the effectiveness of the polyelectrolyte. The optimum pH for the effectiveness of the polymer was found to be around 4.5. In order to accelerate the sedimentation of the particles, Kaolin was added to attach to the flocs after adding the polyelectrolyte. Kaolin helped accelerate the separation of the particulates by almost an order of magnitude. This work shows that flocculation with a cationic polyelectrolyte followed by separation by sedimentation, centrifugation or microfiltration can be an effective technique to purify biorefinery wood extracts.