Modelling electrochemical lignin depolymerization into value-added products
- Modellierung der elektrochemischen Lignin-Depolymerisation in Mehrwertprodukte
Bawareth, Bander Abubaker; Weßling, Matthias (Thesis advisor); Palkovits, Regina (Thesis advisor)
Dissertation / PhD Thesis
Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2019
Lignocellulosic material is a very important alternative source to fulfill the increment in the global energy demand. The potential aim to start up new biorefineries has been restricted by the low economical value of these plants. Therefore, several high value products must be produced to increase the revenue of the biorefinery. For aromatic feedstock, lignin has the highest abundance of aromatic content in nature. Lignin valorisation via electrochemical depolymerization is a promising approach for commercial application due to its moderate reaction conditions. However, the electro-oxidation is a non-selective reaction and causes over-oxidation of the products. A membrane reactor can selectively remove the desired products of lignin depolymerization. As a result, further degradation of the products can be avoided. The scope of this thesis is to develop a conceptual study of an electrochemical membrane reactor process. The study consists of reaction kinetics modelling and membrane process modelling. Conventional reaction kinetics equations are inadequate when used for lignin degradation because of the limited kinetics information with respect to the reaction mechanism and wide product spectrum. We suggested to use population balance equations to predict the evolution of molecular weight distribution of lignin with time, based on the chain scission mechanisms (random and chain-end scissions). Solving the low molecular weight (MW) population balance equations was carried out discretely whereas a continuous solution was implemented for the high MW. Additionally, the model accounted for a recombination reaction for the depolymerized species. The model is capable of predicting the molecular weight distribution of lignin as a function of electrochemical processing time. Experimental results were used to extract kinetics constants for different Kraft lignin samples. Different process parameters were examined, in an electrochemical membrane reactor process, via sensitivity analysis. Through the analysis, the objective was to manipulate the process parameters in order to maximize the aromatics production yield. These parameters are considering, in general, the membrane characteristics, in terms of membrane pore diameter and area, and process parameters, in terms of trans-membrane pressure and reaction residence time. The sensitivity study revealed that a membrane with a molecular weight cut-off (MWCO) of 750 Da and pore diameter of 1 nm is an optimum nano-filtration membrane for a continuous membrane reactor. The aromatic product yield could be increased from 0.01\%, in the batch reactor, to 11\%, in the membrane reactor. Diluted lignin concentration was found to be a process drawback with regard to the product recovery from the permeate. The presented model unravelled the electrochemical depolymerization of lignin in a macro perspective. Combining the findings of this study with an efficient and integrable process design, can contribute to bring the biorefinery concept closer to commercial realization.