(136a) Continuous Precipitation and Agglomeration of Lignin from Organosolv Pulping Liquors at Pilot Scale

Lignin is the largest natural and renewable resource of aromatics on earth. It is one of the three main constituents of lignocellulosic (LC) biomass. However, lignin is still underexploited in the worldwide pulping industry, where lignin is mainly combusted to release its heating value and only a small part is utilized as chemicals and materials [1]. One reason for the current situation is that the conventional pulping processes (e.g. Kraft pulping) were originally not designed to recover lignin as product, although processes for lignin recovery in those mills have been developed [2-5].

Alternative organosolv (organic solvent + water) pulping processes that are designed to recover all the LC biomass constituents as products (pulp, lignin and hemicellulose) are well known for decades [6]. However, commercial organosolv pulping mills, realized in the 1980s and 90s, were shut down again because of problems in downstream processing (solvent cycle closure and lignin recovery) and other issues [7].

In this contribution, a novel continuous process for combined lignin separation and solvent recycling from organosolv pulping liquors is presented [8, 9]. The “LigniSep” process was developed in lab scale at the MPI DCTS and upscaled to a pilot plant at the Fraunhofer CBP in Leuna, Germany. The process is based on the phase behavior of lignin in the solvent/water system (solubility and softening) investigated before [10]. Lignin yield is increased in comparison to dilution precipitation processes by simultaneous precipitation at low solvent concentrations and solvent evaporation. Controlled lignin softening systematically prevents lignin incrustations and triggers lignin particle agglomeration by carefully arranged process conditions. The lignin agglomerates are much better filterable than the precipitated primary particles. The process is monitored and controlled by inline analytical techniques, like infrared spectrometry for solvent content measurement and particle size measurement and microscopy to follow lignin particle formation. A quantitative dynamic model of the pilot plant process was formulated to predict process conditions for given start and boundary conditions. The model was used for the planning of save experiments in the pilot plant. Pulping liquors from batches with different biomass and pulping conditions have been processed in 600kg scale, each. The pulping was performed in another pilot plant right next to the lignin and solvent recovery plant. Results from several experimental campaigns (e.g. process trends, mass balances and yields) will be presented.

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