Modelling and simulation of a Holm oak fractionation process in a packed bed with hot pressurized water

Abstract

Biomass has been proposed as a possible solution to the energetic problem related with the end of petrol age. This proposal is based on the fact that biomass is formed by two polysaccharides, cellulose and hemicellulose, and lignin. Therefore, sugars could be extracted in order to convert them in liquid fuels. This idea has been study thoroughly. However, it has not been developed yet a model able to reproduce the experimental behaviour considering the main involved physical phenomena, like porosity variations or solid-liquid mass transfer. Thus, the aim of this work was to obtain a realistic model for biomass fractionation. Holm oak was selected because it is one of the most common trees in Spain. The experimental device was a tubular reactor charged with 5 g of biomass and fed with water. The range of operating temperatures was between 175ºC and 207ºC in order to enhance hemicellulose extraction. The operating pressure was fixed at 100 bar to maintain water as liquid. Four different liquid flow rates (5ml/min, 10ml/min, 20ml/min and 40 ml/min) and two particle diameters (3mm and 6mm) were used to assess how they affect the process. The higher the flow was, the bigger the extraction was and the lower the particle diameter was, the smaller the extraction was. The developed reaction pathway establishes that cellulose and hemicellulose start their decomposition in the solid, breaking in a water-soluble oligomer, which decomposes in the last oligomer before sugar formation by hydrolysis. In addition, deacetylation reactions were considered for both of them. So, acetic acid would be produced and solved in water, which allows the hydrolysis in water. An insoluble oligomer production form cellulose was introduced too. Regarding kinetics, their reaction orders were one respect to oligomers and protons in liquid phase. In solid phase, an autocatalytic kinetic to cellulose and hemicellulose was used to the first soluble oligomer formation reaction. The model was validated by fitting TOC, acetic acid concentration and pH profiles with absolute deviation ranges between 16%-56%, 47%-87%, 6%-11% respectively. Besides, the calculated solid mass had deviations lower than 30%.