Demonstration of a selective process to transform biomass into sugars by ultrafast hydrolysis in supercritical water

Resumen

To accomplish the challenge of a chemical industry based on biorefineries, research efforts must be focused on developing sustainable processes able to maximize profit out of each biomass fraction with the lowest cost. Hydrothermal processes are an attractive technology to produce valuable products from biomass. However, this technology is still under development to reduce operational costs and at the same time, provide high yields and selectivity towards desired products. To overcome those limitations, the aim of this PhD thesis is to develop an efficient technology to transform agrofood biomass into sugars by the developed FASTSUGARS technology (ultrafast hydrolysis in supercritical water), moving from laboratory to pilot plant scale. To do so, a continuous laboratory scale plant was initially used. That experimental unit, developed in a previous thesis, is able to work with reactor temperatures up to 400 ºC, pressures up to 30 MPa and reaction times between 0.004 and 5 s with a maximum total flow up to 8 kg/h (max. 3 kg/h of biomass). Chapters 1 and 2 were developed working with this facility. Then, the process was scaled up as the main focus of this thesis, moving from laboratory to pilot plant scale. The continuous pilot plant developed in the present work, able to work with reactor temperatures up to 400 ºC, pressures up to 30 MPa and reaction times from 0.07 s, was designed to operate with total flows up to 30 kg/h (up to 10 kg/h biomass). Chapters 3 and 4 were carried out using this experimental setup. In Chapter 1, the effect of inlet concentration on cellulose hydrolysis in supercritical water was evaluated in the laboratory scale plant. To do so, the experiments were carried out at 400 ºC and 25 MPa with reaction times between 0.07 and 1.57 seconds and suspensions concentrations between 5 and 20 % w/w (corresponding to 1.5 – 6 % w/w at reactor inlet). It was found necessary to increase the reaction time to achieve total cellulose conversion when using highly concentrated suspensions. However, increasing reaction time also favored the degradation of glucose. Therefore, cellulose could be selectively hydrolyzed to sugars by using short reaction times and low concentrations of biomass. Indeed, the best result for sugars production (79 % w/w) was obtained working with a cellulose inlet concentration of 1.5 % w/w and 0.07 s reaction time. On the other hand, if the desired products were glucose derivatives, such as glycolaldehyde, higher reaction times (>1s) were needed. For glycoaldehyde production, the best result (42 % w/w) was obtained working with 6 % w/w cellulose inlet concentration and 1.57 s reaction time. It was also found that the inlet concentration of biomass affects the conversion rate of cellulose in supercritical water. Increasing the inlet concentration up to 4 % w/w at reactor inlet, the cellulose solubility in supercritical water was lower so that the reaction occurred in a heterogeneous media where the mass transfer resistances limited the reaction rate. In Chapter 2, sugar beet pulp was hydrolyzed in supercritical water for sugars recovery, operating at 390 ºC, 25 MPa and reaction times between 0.11 and 1.15 s in the laboratory scale plant. Sugar beet pulp is the major by-product in sugar industry and to make profit out of this undervalued residue, the FASTSUGARS process was proposed as a competitive alternative, combining the advantages of supercritical water as hydrolysis medium with very short reaction times. It was possible to achieve a selective and simultaneous recovery of both cellulose and hemicellulose fractions as C-6 and C-5 sugars. In this way, a liquid effluent suitable for further conversion into ethylene glycol and sorbitol was obtained. On the other hand, the solid product obtained could be used as additive for composites production. The highest yields of C-6 and C-5 sugars (61 and 71 % w/w, respectively) were obtained at 0.11 s with the lowest yield of degradation products. The solid product obtained at 0.14 s was thoroughly analyzed by TGA and FTIR analysis to prove its enhanced thermal properties and aromaticity. In Chapter 3, the FASTSUGARS process for sugars’ recovery from agricultural biomass was scaled up from laboratory to pilot plant scale. System performance was evaluated by comparing the results obtained from sugar beet pulp and wheat bran in both laboratory and pilot plants. When comparing the performance of these biomasses, similar trends were found, as selectivity to sugars decreased with reaction time and then, conversion and degradation yield increased with reaction time. Differences between the results obtained for each biomass were due to composition and reactor conditions. To bring the FASTSUGARS process closer to industrial applications, a bigger particle size was used in the pilot plant (250 μm) compared to the laboratory scale plant (≤ 150 μm). It was found that the particle size acted as a mass transfer resistance, slowing down the hydrolysis of biomass, providing lower conversion and therefore reducing sugars’ degradation (degradation yield was always lower than 15 % in the pilot plant). In that way, higher selectivity to sugars was obtained, reaching values around 90 % for both sugar beet pulp and wheat bran in the pilot plant. Therefore, this slowing down effect in the pilot plant resulted to be positive, since selectivity was increased and at the same time, the degradation was remarkably reduced. In Chapter 4, the hydrolysis of commercial inulin with a polymerization degree comparable to fructooligosaccharides (FOS) was hydrolyzed in supercritical water to evaluate the reaction mechanisms. The hydrolysis reactions were performed in the pilot plant at 385 ºC, 25 MPa and reaction times between 0.12 and 0.74 s. It was observed that the conversion of fructose to glyceraldehyde, 5-HMF and furfural was slower than the subsequent production of pyruvaldehyde and formic acid. As it happened for cellulose, it was also found that reaction time affects selectivity, since as reaction time increased, the sugars production decreased due to their degradation into further products (mainly pyruvaldehyde and formic acid). On the other hand, it was demonstrated that increasing the inlet concentration, the conversion of inulin was reduced, providing higher sugars yield and lower degradation rate. Jerusalem artichoke was selected as an inulin-rich biomass for the production of fructo-sugars via supercritical water hydrolysis. It was observed that the hydrolysis of Jerusalem artichoke was similar to that of FOS at high concentration, yielding up to 68 % w/w of sugars. It was concluded that lower conversion was achieved compared to FOS because initial degree of polymerization was higher and acted as a limitation for the dissolution step. Then, results from Jerusalem artichoke were also compared to those of lignocellulosic substrates obtained in Chapter 3 (sugar beet pulp and wheat bran). Higher conversion was achieved in the case of Jerusalem artichoke due to its composition, since its main constituent was inulin, which was much more easily converted than cellulose under the selected conditions.