Environmental Engineering Reference
In-Depth Information
biomass to acid or alkaline catalysts at temperatures ranging from 120 to 200
◦
C.
Pretreated slurries (the hydrolysate liquor containing soluble sugars, oligosaccha-
rides, and other released solubles plus the residual solids) are then enzymatically
digested at 40-60
◦
C to release sugars from the polysaccharides and oligomers
remaining after pretreatment [1-9]. In both of these steps, adequate heat, mass, and
momentum transfer is required to achieve uniform reactions and desirable kinetics.
Plant cell walls, which make up almost all of the mass in lignocellulosic biomass,
are highly variable both across and within plant tissue types. At the macroscopic
scale, such as within a stem or leaf, uneven distribution of catalyst (chemical or
enzyme) due to the different properties of different tissues results in heterogeneous
treatment, with only a fraction of the plant material exposed to optimal conditions
[10-13]. Tissues that do not get exposed to sufficient amounts of catalyst during
pretreatment are incompletely processed, resulting in decreased overall enzymatic
digestibility of pretreated biomass [6]. When pretreatment severity is increased, by
increasing temperature, catalyst concentration, or time of reaction, areas of biomass
readily exposed to catalyst undergo excessive treatment leading to sugar degra-
dation and formation of toxic by-products (furfural, hydroxymethyl furfural, and
levulinic acid) that inhibit downstream sugar fermentation and decrease conversion
yields [1]. This problem continues at a microscopic scale due to the compositional
and structural differences between middle lamella, primary cell wall, and secondary
cell wall. At even smaller scales, intermeshed polymers of cellulose, hemicellulose,
lignin, and other polysaccharides present another layer of heterogeneity that must
be addressed during bioconversion of plant cell walls to sugars.
Milling to fine particle sizes improves some of these mass transfer limitations,
but can add significant costs [14, 15]. Size reduction, however, may not overcome
heat transfer limitations associated with short time-scale pretreatments that employ
hot water/steam and/or dilute acids. When such pretreatments are carried out at high
solids loading (>30% w/w) to improve process efficiency and increase product con-
centrations, heat cannot penetrate quickly and uniformly into these unsaturated and
viscous slurries. It is thought that steam added to high-solids pretreatments can con-
dense on particle surfaces impeding convective heat transfer. Depending on particle
and slurry properties, the condensed steam can form temperature gradients within
biomass aggregates, resulting in non-uniform pretreatment.
Besides limiting heat transfer rates, biomass slurries can pose other process-
ing challenges. At high solids concentrations, slurries become thick, paste-like,
and unsaturated. Limited mass transfer within these slurries can cause localized
accumulation of sugars during enzymatic hydrolysis, decreasing cellulase and hemi-
cellulase activity through product inhibition [16-23]. In addition, slurry transport
through process unit operations is challenging at full scale. As solid concentrations
increase, hydrodynamic interactions between particles and the surrounding fluid as
well as interactions among particles increase. At high solids concentrations “dense
suspensions” are formed and the resulting multiple-body collisional or frictional
interactions and entanglement between particles creates a complex slurry rheology
[24-26]. A further complicating aspect is water absorption by biomass, causing the
bulk to become unsaturated at fairly low insoluble solids concentrations (~ 30-40%
Search WWH ::
Custom Search