To mitigate global warming, humankind has been forced to develop new efficient energy solutions based on renewable energy sources. Hydrothermal liquefaction (HTL) is a promising technology that can efficiently produce bio-oil from several biomass sources. The HTL process uses sub- or supercritical water for producing bio-oil, water-soluble organics, gaseous products and char. Black liquor mainly contains cooking chemicals (mainly alkali salts) lignin and the hemicellulose parts of the wood chips used for cellulose digestion. This review explores the effects of different process parameters, solvents and catalysts for the HTL of black liquor or black liquor-derived lignin. Using short residence times under near- or supercritical water conditions may improve both the quality and the quantity of the bio-oil yield. The quality and yield of bio-oil can be further improved by using solvents (e.g., phenol) and catalysts (e.g., alkali salts, zirconia). However, the solubility of alkali salts present in black liquor can lead to clogging problem in the HTL reactor and process tubes when approaching supercritical water conditions.

The majority of platform chemicals are currently provided through crude oil processes. Nevertheless, the substitution of the crude oil with biomass should be the ecological aim. Lignin, an aromatic macromolecule, may play an important role in that exchange, as it is the only bio based source of aromatic compounds. For instance, it could be a source of bifunctional aromatic molecules, like the monocyclic compounds catechol or guaiacol, or bifunctional oligomers. However, no process for the production of aromatics from lignin in technical scale has been established until now. Hence, the focus of this work is to clarify the chemical degradation mechanism under hydrothermal conditions, to liquefy lignin delivering high functional molecules and to increase the yield and selectivity of the cleavage towards bifunctional molecules like catechol. The combination of fast hydrolysis, thermal degradation reactions and hydrogenation drives the hydrothermal liquefaction; this gives the possibility to narrow down the product spectrum in comparison to other "dry" cleavage methods, towards a higher yield of e.g. catechols.

Biorefineries, which are using mostly unused side streams of other existing processes like bark or lignin, have a huge potential to open new resources, for example, chemicals. But with new resources new challenges will be met along the way. These challenges must be addressed and discussed to build a solid and far‐sighted process. This work focuses on the formation of monocyclic compounds like catechol as a valuable product during the hydrothermal liquefaction of beech wood bark as well as Kraft lignin from pine wood like Indulin AT. The focus is to get a better knowledge of the behavior of bark during hydrothermal liquefaction for depolymerization aiming at the production of aromatic building blocks for chemicals. Therefore, the influence, for example, of temperature and reaction time, the chemical reaction pathways, and the therefore necessary analytics need to be understood. Several limitations and challenges of common analytical methods are discussed and compared for bark and Kraft lignin, which is relatively well investigated and can act as a reference material to build a common ground and make it possible to build standards for all bioeconomic processes. Hydrothermal conditions increase the yield and selectivity toward bifunctional molecules like catechol. With rising temperatures and longer retention times, the catechol mass yields get lower. At temperatures above 350°C, nearly no catechol could be found any more. Different types of wood deliver different lignin compositions in terms of the monomeric units. However, it can be observed that different lignins show the same trends in regard to the catechol yield concerning temperatures and reaction time dependence, but overall a different product spectrum.

For better understanding and optimization of multiphase flow in miniaturized devices, micro-computed tomography (μCT) is a promising visualization tool, as it is nondestructive, three-dimensional, and offers a high spatial resolution. Today, computed tomography (CT) is a standard imaging technique. However, using CT in microfluidics is still challenging, since X-ray related artifacts, low phase contrast, and limited spatial resolution complicate the exact localization of interfaces. We apply μCT for the characterization of stationary interfaces in thin capillaries. The entire workflow for imaging stationary interfaces in capillaries, from image acquisition to the analysis of interfaces, is presented. Special emphasis is given to an in-house developed segmentation routine. For demonstration purposes, contact angles of water, liquid polydimethylsiloxane, and air in FEP, glass, and PMMA are determined and the influence of gravity on interface formation is discussed. This work comprises the first steps for a systematic 3D investigation of multiphase flows in capillaries using μCT.

Droplet generation in microfluidic devices has emerged as a promising approach for the design of highly controllable processes in the chemical and pharmaceutical industry. However, droplet generation is still not fully understood due to the complexity of the underlying physics. In this work, micro-computed tomography is applied to investigate droplet formation in a circular channel in a co-flow configuration at different flow conditions (Ca < 0.001). The application of an in-house developed scanning protocol assisted by comprehensive image processing allows for the time-resolved investigation of droplet formation. By tracking different droplet parameters (length, radii, volume, surface, Laplace pressure) the effect of flow conditions on droplet progression is determined. As characteristic for the squeezing regime, final droplet size was nearly independent of Ca for higher Ca tested. For lower Ca, the final droplet size increased with decreasing Ca, which points to the leaking regime that was recently introduced in the literature.

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