Fractionation process of surplus biomass by autohydrolysis in subcritical water obtaining added value products

Resumen

In the last years, the challenge is a society based on the concept of bioeconomy. This term refers to the sustainable production and conversion of biomass into a range of food, health, fibre, industrial products and energy developed in futures biorefineries. In order to reach this challenge, new policies have been promoted to use the renewable resources and new policies will be necessary to develop a decentralized local-scale production to produce bio-products according to the biomass availability in each area. The decentralized local-scale production results in a major flexibility and versatility in the process due to the use of different biomass generating diversification of products. However, it is required a better knowledge of chemical and structural properties of biomass which would be obtained from research works in science and technology of biomass. The hydrothermal process has been proposed to develop sustainable and efficient conversion of natural resources into fuels and chemicals. This technology is a promising alternative to perform the fractionation of biomass because the reaction medium allows the transformation of the different fractions of biomass by choosing the appropriate conditions. Furthermore, water is a clean, safe and environmentally benign solvent. The use of semicontinuous reactors has been proposed to study the behaviour of hydrothermal processes being the results obtained ¿acceptable¿ compared with the investment and equipment required. The aim of this PhD thesis is to develop a process capable of obtaining added value products from lignocellulosic biomass using the fractionation hydrolysis process and using subcritical water as solvent. A semicontinuous reactor was designed and constructed. The maximum working temperature is 400ºC and pressure is 25 MPa. In Chapter 1, the hydrothermal hydrolysis of grape seeds focused in the production of bio-oil was studied. The grape seeds composition in terms of lignin, sugars, ash, extractives and bio-oil was determined. The composition of grape seeds was: 17.0% wt. of extractives; 36.8% wt. of sugars (hemicellulose and cellulose); 43.8% wt. of lignin and 2.4% wt. of ash. The grape seeds were hydrothermally treated using three different temperatures: 250ºC, 300ºC and 340ºC employing a semi-continuous reactor. The solid residue varied from 25.6 - 35.8% wt. depending on the hydrolysis temperature. The maximum yields of Light (15.7% wt.) and Heavy Bio-oil (16.2% wt.) were achieved at 340ºC. The Arrhenius parameters for the kinetic of grape seeds hydrolysis in our system were k0 = 0.995 g·min-1 and Ea=13.8 kJ·mol-1. The increment of the flow rate favoured the mass transfer in the system and so, the hydrolysis rate. However, the maximum hydrolysis rate was found at a water surface velocity of 2.3 cm·min-1. In Chapter 2, the fractionation of grape seeds as a model biomass was studied using a combination of two processes: solvothermal extraction and hydrothermal fractionation-hydrolysis process in a semicontinuous reactor. First, grape seeds were subjected to an extraction process with ethanol/water (70/30% wt.) at 90ºC during 60 min obtaining ca. 13.0% wt. of oil and extractable components with 4.46% wt. of polyphenols (66% of the maximum). Afterwards, the solvent was water and the biomass was treated in steps at different temperatures (150ºC to 340ºC). During the hydrolysis the pH decreased from 5.5 down to 3.0 due to acetyl group liberation. The total quantity of recovered sugars varied around 20.0 to 23.1% wt. The best experimental condition for obtaining the maximum amount of pentoses + hexoses + oligosaccharides was 180ºC (45min) + 250 to 265ºC (45 min) + 330 to 340ºC (45 min). In Chapter 3, the hydrolysis of Holm oak (Quercus ilex) using subcritical water conditions was investigated. The experiments were carried out in a semicontinuous reactor using different temperatures (from 175 to 207ºC), flow rates (from 3 to 34 ml·min-1) and particle sizes (3 and 6 mm). The behaviour of pH, Total Organic Carbon and sugars by HPLC were measured and studied. The current results provided an interesting relation between these parameters. The minimum pH was located at the same time as the Total Organic Carbon and sugars by HPLC presented a maximum. The pH can be used to follow on-line the hydrolysis process reducing the analytical and time expenditures to the minimum possible and at the same time understanding the behaviour of the system. In Chapter 4, the hydrothermal treatment of Holm oak (Quercus ilex) separating a liquid fraction rich in hemicelluloses was studied. The experiments were carried out using a five reactors connected in series to form a cascade reactor. The temperatures were between 130 and 170ºC. The effects of temperature and reaction time on the conversion and molar mass of hemicelluloses were investigated. The results show that the maximum hydrolysis rate of hemicelluloses depends strongly on the temperature and the biomass used. The maximum conversion (approximately 60%) was obtained at 170ºC during 20 min. After this time, the decreases of conversion can be attributed to the presence of degradation products. On the contrary, the conversion at 130ºC and 140ºC did not exhibit a maximum value indicating that the reaction time was not long enough for complete the hydrolysis. The major component extracted at lower temperatures was glucose (130 and 140ºC) and at higher temperatures (150, 160 and 170ºC) was xylose. The deacetylation was accompanied by a reduction in the molar mass. The average molar mass of the carbohydrates from hydrolysis of Holm oak decreased with increasing reaction temperature. The average molar mass decreased from 12.9 to 1.75 kDa at 170 ºC during 60 min of hydrolysis. At higher temperatures the hemicelluloses had a pronounced lower average molar mass after a few minutes of reaction. Compared to Norway spruce (softwood), the average molar mass in Holm oak (hardwood) was lower under the same reaction conditions suggesting that the deacetylation is higher due to a higher content of acetyl groups. In Chapter 5, subcritical water was employed to fractionate woody biomass into carbohydrates and lignin. Nine urban trees species (hardwood and softwood) from Spain were studied. The experiments were carried out in a semi-continuous reactor at 250ºC for 64 min. The hemicellulose and cellulose recovery yields were between 30% wt. and 80% wt. while the lignin content in the solid product ranged between 32% wt. and 92% wt. It was observed that an increment of solubilized lignin disfavored the hydrolysis of hemicelluloses. It was determined that the maximum extraction of hemicellulose was achieved at 20 min of solid reaction time while the extraction of celluloses not exhibited a maximum value. The hydrolysis of hemicellulose and cellulose would be governed by the hydrolysis kinetic and the polymers accessibility. In addition, the extraction of hemicellulose was negatively affected by the lignin content in the raw material while cellulose hydrolysis was not affected by this parameter.