Abstract
Diffusion of large molecules throughout the porous microstructure of wood pretreated with steam explosion was investigated by using the lattice Boltzmann method for simulations. Wood samples were investigated with high-resolution X-ray tomography to effectively reconstruct an accurate geometry of the structural changes that ensue after pretreatment. Samples of approximately 1 mm3 with voxel sizes from 0.5 to 1 μm were examined with X-ray imaging. These large volumes, relative to what reasonably can be simulated, were divided into sub-volumes and were further reconstructed into geometries suited for the LBM simulations. The transient development of the concentration was investigated, and the effective diffusion coefficient at steady state was computed. Diffusion rates were found to increase significantly in the transversal direction due to the steam explosion pretreatment. The increase was observed both in the time needed for solutes to diffuse throughout the pores and in the effective diffusion coefficient. A shorter diffusion pathway and a higher connectivity between pores were found for the pretreated samples, even though the porosity was similar and the pore size distribution narrower than the native sample. These results show that local mass transport depends not only on porosity but also, in a complex manner, on pore structure. Thus, a more detailed analysis of pore space structure using tomography data, in combination with simulations, enables a more general understanding of the diffusional process.
Highlights
Diffusion of large molecules in the porous microstructure of wood is important for understanding the limiting steps of an efficient material-driven biorefinery
In steam explosion, pressurized steam is introduced to wood chips for a short duration of time, after which the pressure is rapidly decreased to atmospheric conditions, allowing the steam to expand inside the wood and a pressure gradient to develop between the wood and its surroundings
In a material-driven biorefinery setting, the mass transport of large molecules, such as enzymes and polysaccharides, in wood structure is of fundamental importance for understanding limiting steps of future biorefineries
Summary
Diffusion of large molecules in the porous microstructure of wood is important for understanding the limiting steps of an efficient material-driven biorefinery. In a material biorefinery (Mattsson et al 2017), it is desirable to obtain high molecular weight fractions of the three main constituents of the wood cell wall, namely cellulose, hemicellulose, and lignin, that are used as precursors for value-added bioproducts (Gomes et al 2014). To separate these constituents from the recalcitrant cell wall while retaining high molecular weight requires milder process conditions in the upstream pretreatment steps to reduce hydrolysis and still facilitate accessibility. In steam explosion, pressurized steam is introduced to wood chips for a short duration of time, after which the pressure is rapidly decreased to atmospheric conditions, allowing the steam to expand inside the wood and a pressure gradient to develop between the wood and its surroundings
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