Residence time distribution analysis in the transport and compressing screws of a biomass pretreatment process

Abstract The raise of environmental concerns in the past decades consequently increased the need of obtaining cleaner sources of energy. Among the studied alternatives, second generation biofuels (produced from non-food resources) are one of the most promising solutions and nearly reached industrialization. The production of cellulosic bioethanol is one of the possibilities of second generation biofuels, and the studies involving the use of cellulosic compounds to produce bioethanol recently increased. Its production involves four dependent steps: pretreatment, enzymatic hydrolysis, fermentation and distillation. This work considered the pretreatment stage, aiming at modifying the structure of the lignocellulosic biomass so that cellulose becomes more accessible to enzymatic hydrolysis step. This work particularly focused on the biomass flow characterization in the transport and compression screws involved in the pretreatment step of an industrial-scale process. The main challenge was firstly to perform measurements on an industrial-scale device working under harsh conditions. Residence time distribution (RTD) experiments were thus performed using a novel methodology adapted to these working conditions. Sodium carbonate was selected as a tracer. Due to the reaction with the acidified biomass, both electrical conductivity and pH were monitored at the exit of the screws. A chemical model was developed, allowing the determination of tracer concentrations from the measured data. The measurements obtained were compared with three optimized models: a combination of plug flow and continuous stirred tank reactor in series (PFR-CSTR), plug flow with axial dispersion (AD) and a model based on the Zusatz function. The results of this work pointed out the non-plug flow behavior of these screws in their standard working conditions. In accordance with the physical motion of the tracer inside the screws, the use of the PF-CSTR is recommended for representing RTD inside screws in conditions in which backflow is likely to occur.