The production of target compounds from plants as renewable resources gains more and more interest. On the one hand the application of fossil resources might be partially replaced and, on the other hand, some particular complex molecules are only accessible via isolation from the natural plant origin. The projects in this frame are concerned with investigating the potential of crystallization processes for isolation of a certain target compound from plant-based extracts. This is exemplified on 1) separation and purification of pharmaceutically relevant substances (e.g. Artemisinin) and 2) the recovery of particulate lignin from lignocellulose within the biorefinery concept.
Separation and purification of natural products
The product of the plant extraction or organic synthesis commonly represents a multicomponent mixture of the targeted natural compound with a great variety of structurally very similar undesired by-products. In dependence of the quality requirements for the final application, further purification of the target compound is frequently required. In order to provide the target component in the pure and crystalline form the crystallization as a highly selective separation technique is often applied as the final purification step.
Using a profound knowledge of basic solution equilibria as well as of the recrystallization and solid phase behavior of the pure target compound and in the presence of impurities, a crystallization process can be designed. In particular introducing of seeds into the supersaturated solution of the component to be purified (Fig. 1) enables the selective separation of the target compound of desired purity from the complex mixture.
Besides rutin, crystallization-based purification of other pharmaceutical relevant natural products (e.g. artemisinin, curcumin) from plant extract or synthesis solution was studied [1-4].
|||Horosanskaia, E., Nguyen Minh Tan, Vu Dinh Tien, Seidel-Morgenstern, A., Lorenz, H. (2017) Crystallization-Based Isolation of Pure Rutin from Herbal Extract of Sophora Japonica L., Organic Process Research & Development, 21 (11), pp. 1769–1778.|
|||Horosanskaia, E., Lorenz, H., Seidel-Morgenstern, A., Nguyen Minh Tan, Vu Dinh Tien (2016) Method for purifying Rutin. International patent number VN 1-2016-01852 (Application July 2016).|
|||Horvath, Z., Horosanskaia, E., Lee, J. W., Lorenz, H., Gilmore, K., Seeberger, P. H., et al. (2015) Recovery of Artemisinin from a Complex Reaction Mixture Using Continuous Chromatography and Crystallization, Organic Process Research & Development, 19 (6), pp. 624-634.|
|||Horosanskaia, E., Seidel-Morgenstern, A., & Lorenz, H. (2014) Investigation of drug polymorphism: Case of artemisinin, Thermochimica Acta, 578, pp. 74-81.|
Separation of lignin from organic solvent pulping liquors
Through the finiteness of fossil resources for phenolic compounds, alternative and preferred renewable resources have to be acquired in order to supply these compounds for the chemical industry in the future. Lignin is the second most available vegetable compound in the world, as lignin, cellulose and hemicellulose are the main components of the composite material lignocellulose (e.g. wood). Native lignin is built up from randomly branched phenylpropane units (cumaryl-, coniferyl- and sinapyl units) forming an amorphous biopolymer. Therefore, lignin is an attractive source of phenolic compounds. The production potential in the worldwide pulping industry is about 300 million tons lignin per year. At present, about 95% of the produced lignin is combusted for recovering set-in pulping chemicals and energy (e.g. Kraft pulping) . That means, without cutting down more trees, just by using an appropriate pulping method , about 285 million tons of lignin per year could be additionally used as a renewable phenolic feedstock.
A lignocellulose biorefinery (see Fig. 1) that applies an organic solvent pulping process separates lignocellulosic biomass into mainly lignin, C5-sugars and cellulose (pulp). The organosolv pulping process can be briefly described as follows: Lignocellulosic material and a solvent (ethanol, acetone, etc.)/acidified water (pH 1.5-3) mixture is heated to 140 - 180 °C at 15-20 bars pressure for around 90 minutes. Lignin and hemicellulose are solvolyzed, while cellulose remains as pulp. The resulting pulping liquor, which contains mainly the solvent, water, lignin and hemicellulose, is separated from the pulp. The pulp can be further processed and used as material or depolymerized to valuable chemicals. The lignin is precipitated from the pulping liquor. The lignin phase is then separated by filtration or another suitable separation method. The solvent is recycled to the pulping process and the remaining filtrate is further processed to yield aqueous sugar solution, acetic acid and sugar degradation products like furfural.
Challenges in lignin separation
The separation of lignin from the pulping liquor is a critical process step. Lignin separation can be facilitated by decreasing the solvent concentration of the pulping liquor, as lignin has a very low solubility in water. For that, usually the solvent is evaporated from the liquor or the liquor is diluted with water. Both common methods have disadvantages. Evaporation precipitation produces soft lignin particles that stick to the reactor (see Fig. 2a) and have to be dissolved again or mechanically removed after the process. Dilution precipitation produces very small particles that are hard to filter, multiplies the liquor mass stream and shows limited lignin yields (see Fig. 2b). Thus, both process variants are unsuitable for upscaling.
In that frame our work is directed to:
- Development of continuous and economic precipitation process for lab and pilot plant (pilot plant at Fraunhofer CBP in Leuna, Germany)
- Improvement of lignin yield
- Improvement of filterability of lignin dispersion
- Prevention of lignin incrustation on reactor internals (yield)
- Prevention of dilution (water addition)
- Implementation of process analytic technology to control sensitive process parameters and product quality
- Improvement of solvent recycling
The phase behavior is the basis for separation process development. Thus, the (pseudo-)ternary phase diagram of lignin, ethanol and water was determined. Two miscibility gaps were found in the system and the one between lignin and water is relevant for the process.
Taking into account the lignin solubility and softening behavior, a continuous lignin separation process was developed and patented [3-6]. In Fig. 3 the process flowsheet of an experimental setup for continuous lignin precipitation is shown. Spent liquor is continuously fed into the stirred double wall reactor and is instantaneously diluted by a start-up lignin dispersion with low ethanol content made from water and spent liquor. Lignin precipitates as it is very low soluble in the dispersion. The ethanol fed with the spent liquor is evaporated at low pressure and concentrated by rectification. In this way, the ethanol content of the lignin dispersion is kept constant. The ethanol content was followed by an ATR-FTIR probe and controlled by regulation of spent liquor feed and/or heat input. The lignin particle size and shapes were monitored by inline analytical techniques.
The achievements made within the performed studies are as follows :
- Practically 100 % of non-water soluble lignin is yielded in product dispersion (very low lignin incrustation)
- Filterability of lignin particles is improved by controlled particle agglomeration
- Solvent is recycled by rectification and no water has to be added (except pilot plant because of missing rectification column)
- Successful upscaling of the continuous lab process to pilot plant scale at Fraunhofer CBP in Leuna, Germany, including process analytic technology to control sensitive process parameters (infrared spectroscopy for solvent content monitoring) 
|||G. Cazacu, M. Capraru, V. Popa, Advances Concerning Lignin Utilization in New Materials (2013) in: S. Thomas, P.M. Visakh, A.P. Mathew (Eds.) Advances in Natural Polymers, Springer Berlin Heidelberg, pp. 255-312.|
|||E.K.L. Pye and J. H. Lora (1991), The Alcell Process - a Proven Alternative to Kraft Pulping. Tappi Journal 74(3): pp. 113-118.|
|||P. Schulze, H. Lorenz, A. Seidel-Morgenstern, M. Leschinsky and G. Unkelbach (2014), Method for precipitating lignin from organosolv pulping liquors. Patent WO2016062676A1.|
|||P. Schulze, H. Lorenz, A. Seidel-Morgenstern, M. Leschinsky, G. Unkelbach (2016), Advanced process for precipitation of lignin from ethanol organosolv spent liquors. Bioresource Technology, 199, pp.128-134.|
|||P. Schulze, H. Lorenz, A. Seidel-Morgenstern (2016), Continuous separation of lignin from ethanol-water pulping liquors. In EWLP 2016 - 14th European Workshop on Lignocellulosics and Pulp: Proceedings for Poster Presentations, pp. 161-165.|
|||P. Schulze (2018), Lignin Separation from Ethanol Water Pulping Liquors. PhD thesis, Otto von Guericke University, Magdeburg, doi: 10.13140/RG.2.2.24072.26887.|