Rheology of Lignocellulose Suspensions and Impact of Hydrolysis: A Review

Abstract

White biotechnologies have several challenges to overcome in order to become a viable industrial process. Achieving highly concentrated lignocellulose materials and releasing fermentable substrates, with controlled kinetics in order to regulate micro-organism activity, present major technical and scientific bottlenecks. The degradation of the main polymeric fractions of lignocellulose into simpler molecules is a prerequisite for an integrated utilisation of this resource in a biorefinery concept. The characterisation methods and the observations developed for rheology, morphology, etc., that are reviewed here are strongly dependent on the fibrous nature of lignocellulose, are thus similar or constitute a good approach to filamentous culture broths. This review focuses on scientific works related to the study of the rheological behaviour of lignocellulose suspensions and their evolution during biocatalysis. In order to produce the targeted molecules (synthon), the lignocellulose substrates are converted by enzymatic degradation and are then metabolised by micro-organisms. The dynamics of the mechanisms is limited by coupled phenomena between flow, heat and mass transfers in regard to diffusion (within solid and liquid phases), convection (mixing, transfer coefficients, homogeneity) and specific inhibitors (concentration gradients). As lignocellulose suspensions consist of long entangled fibres for the matrix of industrial interest, they exhibit diverse and complex properties linked to this fibrous character (rheological, morphological, thermal, mechanical and biochemical parameters). Among the main variables to be studied, the rheological behaviour of such suspensions appears to be determinant for process efficiency. It is this behaviour that will determine the equipment to be used and the strategies applied (substrate and biocatalysis feed, mixing, etc.). This review provides an overview of (i) the rheological behaviour of fibrous materials in suspension, (ii) the methods and experimental conditions for their measurements, (iii) the main models used and (iv) their evolution during biocatalytic reactions with a focus on enzymatic hydrolysis.

Using in-situ viscosimetry and morphogranulometry to explore hydrolysis mechanisms of filter paper and pretreated sugarcane bagasse under semi-dilute suspensions

“…The mean viscosities of FP suspension decreased from 0.249 Pa s at 100 min −1 to 0.183 Pa s at 125 min −1 . This behavior is largely reported in literature [33,39]. Fig.…”

“…Thousands of individual chord lengths are typically measured each second to produce the number-weighted CLD, which is the fundamental measurement provided by FBRM ® . The conversion from number-weighted CLD into volume-weighted PSD is possible under severe assumptions [33], but they are inappropriate for FP and SCB substrates because they contain particles of various shapes and dispersed sizes. Consequently, the number-weighted CLD, E n (lc), and the average number of chords counted per second, N c , will be used as indicators of population evolution.…”

“…The minimal viscosity is attributed to the presence of small particles between the larger ones in bimodal suspensions. In that way, at a given concentration, if the fiber length grows, the interaction probability between particles increases, especially due to entanglement, and this generates an increase in viscosity [33]. Suspension viscosity is controlled by particle concentration, but also by parameters such as morphological aspects (aspect ratio, shape, size, etc.…”

“…Suspension viscosity is controlled by particle concentration, but also by parameters such as morphological aspects (aspect ratio, shape, size, etc. ), agglomerates and particle entanglement, surface properties, swelling propensity, and deformation capability [33,[35][36][37][38] These numerous parameters lead to an ambiguous and hardly quantifiable definition of particle volume fraction. In this context, results reported by Dasari and Berson [13] indicated correlation between particle size and suspension viscosity with sawdust without considering this complexity.…”

“…This illustrates the fact that parameters such as particle morphology and/or particle interactions greatly influence rheological behavior [33,[35][36][37][38]. Moreover, the reason for E v (d SE ) × v evolution needs to be examined and assumptions may be postulated.…”