(748a) Separation of Lignin from Organosolv Spent Liquor- from Phase Behavior to Continuous Processing

Worldwide pulping mills combust several hundred million tons of lignin every year in their recovery boilers to recycle the pulping chemicals and provide process heat [Cazacu et al. 2013]. The most applied Kraft and Sulfite pulping techniques are focused on pulp production whereas lignin is utilized as cheap energy provider.

Lignin is build up from phenylpropane monomers with hydroxyl and methoxy functional groups, making it a valuable renewable resource for the chemical and material industry. Lignocellulose biorefineries are supposed to sustainably provide renewable platform chemicals for the future by fractionating woody biomass with appropriate processes. Organosolv pulping techniques can fractionate lignocellulosic biomass into mainly pulp, lignin and (oligo-) sugars by using an organic solvent and water as pulping liquor at temperatures near 200 °C. The resulting pulp is of reasonable quality [Zhang et al. 2016] and the dissolved lignin can be separated from the spent liquor as precipitate by removing the organic solvent, which is recycled to pulping liquor. The (oligo-) sugar fraction remains dissolved in the aqueous phase. Organosolv lignin is known to be of high quality with low ash, sulfur and carbohydrate contents. The separation processes in a lignocellulose biorefinery utilizing organosolv pulping are essential for the economic viability [Viell et al. 2013].

The Fraunhofer Center for Chemical-Biotechnological Processes CBP in Leuna, Germany, operates a lignocellulose pilot plant consisting mainly of a batch forced circulation reactor for ethanol organosolv pulping, an enzymatic digestion unit for cellulose to sugar hydrolysis, as well as ethanol evaporation and lignin precipitation facilities. Fraunhofer CBP and MPI DCTS initiated a cooperation for investigation and optimization of the lignin separation process especially in terms of energy consumption, fouling prevention and downstream processability in filtration and drying.

In this contribution the path from fundamental phase behavior determination to an advanced continuous lignin precipitation process is presented [Schulze et al. 2016]. Systematic solubility measurements in the ternary lignin-ethanol-water system at different temperatures have been performed in order to elucidate attractive process possibilities. Thereby lignin was found to form a liquid phase under certain conditions as result of a decreased glass transition temperature. The liquid lignin phase potentially causes fouling on the reactor internals during separation processes if the process conditions are not carefully adjusted. On the other hand, softened lignin particles can agglomerate and coalesce to form bigger particles. A compromise between particle enlargement and incrustation formation has to be made in selection of process conditions. Enlarged lignin particles contribute to a better filterability. A (semi-) continuous process for simultaneous lignin precipitation, ethanol recovery and particle enlargement was developed on the knowledge from phase behavior. Experimental results from lab scale continuous processing of spent liquors produced at different pulping conditions indicate the influence of lignin properties (like molecular weight distribution) on the separation process. Semi-continuous precipitation experiments were performed in pilot scale confirming the scalability of the process to the next level. Inline process analytic techniques (Mettler Toledo FBRM®, PVM and ATR-FTIR) were utilized for monitoring and controlling lignin particle size distribution, lignin particle shape and aqueous phase composition. Agglomeration kinetics were derived from the monitored data. Filtration experiments with the produced lignin dispersions provided filter cake resistance data to assess filterability.

References:

Cazacu, G., Capraru, M., Popa, V. (2013). Advances concerning lignin utilization in new materials. In: Thomas, S., Visakh, P.M., Mathew, A.P. (Eds.), Advances in Natural Polymers. Springer, Berlin Heidelberg, pp. 255â??312.

Schulze, P., Lorenz, H., Seidel-Morgenstern, A., Leschinsky, M., Unkelbach, G. (2014). Verfahren zur Fällung von Lignin aus Organosolv-Kochlaugen. DE priority patent application filed in 20th October 2014.

Schulze, P., Lorenz, H., Seidel-Morgenstern, A., Leschinsky, M., Unkelbach, G. (2016). Advanced process for precipitation of lignin from ethanol organosolv spent liquors. Bioresource Technology, 199, 128â??134.

Viell, J., Harwardt, A., Seiler, J., Marquardt, W., (2013). Is biomass fractionation by Organosolv-like processes economically viable? A conceptual design study. Bioresource Technology, 150, 89â??97.

Zhang, K., Pei, Z., Wang, D. (2016). Organic solvent pretreatment of lignocellulosic biomass for biofuels and biochemicals: a review. Bioresource Technology, 199, 21â??33.