Second generation biofuels are those produced from lignocellulosic biomass or crops that do not compete with food production. Cellulose and hemicellulose in plants can be transformed to building blocks, pentoses and hexoses, by hydrolysis. However, most processes require a physic-chemical pretreatment to remove lignin, which acts as a barrier for chemicals during hydrolysis. In fact, pretreatment for removal of lignin or lignin byproducts formed during hydrolysis can be considered as limiting steps for subsequent sugar transformation processes. Alternatively, industrial cellulosic residues, in which lignin and most of hemicellulose have been previously removed through chemical treatment, could be used as raw material for the production of reducing sugars without the drawbacks related to pretreatment and purification steps. Such is the case for diaper industry, which generates a polyacrylate-cellulose dust (POCEL), which cannot be recycled to the process. POCEL is comprised by around of 60 wt. % cellulose and 40 wt. % super absorbent polymer sodium polyacrylate. Due to the high content of cellulose in comparison to lignocellulosic biomass, in which the cellulose content usually oscillate between in the range 20 to 40 wt.%, POCEL could be used as a low cost raw material for glucose production, that can be subsequently fermented to produce bioethanol.
In this work, experimental results of POCEL suspensions hydrolysis under subcritical water conditions in a continuous laboratory scale unit are presented. POCEL suspensions in water at 1.2 wt. % were prepared by adding a few drops of hydrochloric acid up to pH 2 to prevent sodium polyacrylate jellification with water. The hydrolysis unit in subcritical water at continuous laboratory scale consisted of a high pressure pump, preheater, a tubular reactor immersed in an electrical oven, heat exchanger and needle valve to regulate system pressure. Experiments were carried out in the temperature range 350 to 420 °C, pressures of 100 to 136 atm and suspension flowrates of up to 6.4 mL/min. Reactor effluent samples were characterized by means of dinitrosalicylic acid (DNS) colorimetric method. The highest concentration of reducing sugars was obtained at 390 °C and a water flowrate of 1.9 mL/min, which corresponds to a residence time of 126.7 s in the reactor. The estimated productivity of sugar production based on a cellulose content of 60 wt.% in POCEL was 85.4%, which indicates this kind of cellulosic waste could be used to obtain high reducing sugars concentration in hydrolysates through a continuous subcritical water process.