Hemicellulose production using hotpressurized water: from lab to pilotscale

Resum

The climate change, the decline in fossil resources and the growing demand for energy are some of the main problems of our society. These issues could be cushioned with a greater and responsible use of renewable resources. In addition, it is imperative to optimize the use of resources, creating high benefits and lower amounts of waste. The concept of lignocellulosic biorefinery is based on this idea, with the objective of using agricultural and forest waste to produce energy, fuel and chemicals. An ideal biorefinery, in addition to using renewable raw materials and “green” solvents, would not produce any harmful products for health or the environment. A promising, clean and cheap way to extract biocompounds and biopolymers consists of treating biomass with liquid pressurized water above 100 ºC. Depending on the compound to extract, different temperatures are needed. The selective removal of hydrosoluble extractives requires temperatures around 100 °C, the extraction of hemicellulose between 140 and 190 °C, while the extraction of cellulose requires temperatures above 240 ºC. In the liquid phase, the extracted compounds undergo a hydrolysis and degradation process in which, depending on temperature and residence time, oligosaccharides are depolymerized to monosaccharides and then to degradation products. Temperature and residence time are therefore fundamental parameters to increase the selectivity towards certain products than others. The aim of this thesis is to investigate the possibility of bringing the liquid hot water process, for hemicellulose extraction and transformation into high value compounds, closer to the industrial level. The route started with investigating the process at a laboratory scale, studying the fractionation of wood chips from Eucalyptus globulus with a flow through reactor. This setup has been chosen due to its many advantages over batch systems and continuous systems. Respect to batch, mass transfer is improved in flow through reactors, thanks to the continuous supply of fresh water to the system; moreover, the rapid removal of products from the system, prevents their degradation. Unlike in continuous reactors, flow-through reactors do not require solid pumping and extreme milling of biomass. The process was studied at temperatures ranging from 140 to 285 ºC, and liquid flow-rates between 2.5 mL/min and 20 mL/min, identifying the optimal conditions for extracting hemicellulose without obtaining degradation products. In the second step of our research, we investigated the possibility of degrading selectively the extracted products, to obtain high value chemicals. Flow through reactor was connected in series with a continuous reactor: a flow of hot pressurized water extracted biocompounds from biomass in the first unit, and subsequently it was mixed with a stream of supercritical water in the second unit, where the reaction time was precisely controlled, and extracted compounds were hydrolysed and degraded. In the flow-through reactor, fractionation of holm oak wood chips was performed in two stages: optimizing the hemicellulose extraction, at 180 ºC, and subsequently extracting cellulose and hemicellulose stronger associated to cellulose, remaining in the biomass structure at 260 ºC. Hydrolysis of extracted compounds was performed in the continuous reactor by a water stream with temperatures between 350 and 400 ºC. The temperature, pressure and reaction time were modified to tune the selectivity of the reaction. In addition to defining the operating conditions to optimize hydrothermal hydrolysis, it is important to identify which types of biomass are best candidates to obtain high product yields. In this thesis we chose wood as feedstock, due to its high and non-seasonal availability, its low content in inorganic compounds and its ease cultivation with respect to crops. However, woods from different species have different compositions and different structures, which allows different products to be obtained. In the third part of this path, we analysed the composition of wood from 10 different species of tree, typical of the Castilla y Leon region. Hydrothermal extraction of hemicelluloses contained in the different woods, was performed at 160 ºC using a batch-wise cascade reactor, located in Åbo Akademi (Finland). The objective was to seek the wood species that allowed to obtain a high concentration, yield and/or molecular weight of hemicelluloses. Finally, by analysing the structure of the raw materials through TGA analysis, we identified a relationship between composition, structure of the raw material and final yield in hemicelluloses. The experience and knowledge accumulated during the first period of the thesis, brought us closer to the main objective, which was to scale the hydrothermal hydrolysis process and approach it to an industrial level. The fourth step of this route was therefore to scale-up the laboratory scale flow-through reactor, to design and build a pilot plant. A one pilot reactor system, with a volume 72 times larger than the laboratory scale was built, and subsequently upgraded to become a manifold system composed of 5 flow-through reactors capable of working in series. This novel apparatus permitted a continuous operability, minimizing downtime when replacing the biomass during loading and unloading phases, fundamental characteristics in an industry. To verify the homogeneous working of the plant, a study was performed to determine the evolution of the composition and molecular weight of the extracted solution, by varying the temperature (140, 150, 160 and 170 ºC) and the residence time of the liquid phase and the solid phase within the system. After checking the effectiveness of the pilot plant's operation, we identified the features and advantages that distinguish it from existing installations with the same purpose. The fifth step in our work was the writing of a patent to protect our technology and increase its value at an industrial level. Finally, as sixth and final step, we explored the potential commercialization of one of the possible products that can be obtained by the processing of the effluent obtained with our technology. The idea is to produce a sweetener based on xylitol, obtained through the hydrolysis and hydrogenation of the hemicellulose extracted with hot pressurized water, from lignocellulosic biomass. A business plan was made, based on the selling of this sweetener to industries producing confectionery, soft drinks and candies. Mutual benefits are considered: farmers, can sell their waste, otherwise destined to be burned, while customers can get sugar-free products with benefits on health and environment.