Environmental Engineering Reference
In-Depth Information
In simplest terms, biomass slurries can be described as non-Newtonian pseudo-
plastic (shear-thinning) fluids [27, 61, 62]. Whereas the exact mechanism leading
to pseudoplasticity in biomass slurries is unknown, a possible explanation of the
behavior can be ascribed to formation of three dimensional network structure of
the fibrous particles and subsequent breakdown of this structure under shear [63].
Previous studies show that while free water is present, apparent viscosity values
under continuous shear increase with increasing solid concentrations. These mea-
sured apparent viscosities can be modeled with simple Casson, Bingham or Power
Law models [27, 61, 62]. Thick slurries with little or no free water do not exhibit
a further increase in apparent viscosity with increasing solid concentrations under
continuous shear [27]. Other viscoelastic properties, such as storage and loss mod-
ulii could continue to change; however, these measurements have not yet been
reported for biomass slurries.
The relatively sparse data and lack of fundamental understanding of rheologi-
cal properties of biomass slurries makes calculations on mixing requirements for
biomass conversion processes uncertain. Also, transport properties within biomass
slurries, such as convective/conductive heat transport and convective/diffusive mass
transport, and their effects on conversion are hard to discern or estimate. For exam-
ple, Fig. 6 shows enzyme digestibility data obtained during digestion of pretreated
corn stover at high solids pretreatments (>15% solids). Each data point was gen-
erated as a single measurement from triplicate reactors after 5 days of digestion.
As can be seen from Fig. 6a, conversion of cellulose to glucose decreases steadily
as solids concentrations increase suggesting inhibition of enzymes, possibly due
to poor mass transfer resulting in localized accumulation of sugars as suggested
by Hodge and coworkers [22]. Clearly, slurry properties will play a major role in
determining these transport parameters that are crucial to determine optimal process
performance across multiple scales. As another example, Fig. 7 shows experimental
data from tests performed to evaluate heating time in a closed reactor containing
biomass slurries of varying concentrations. These data show significant retarda-
tion of heat transfer, even with the moderate density slurries containing 10% solids
(w/w). Simple heat transfer simulation models have been developed for biomass
slurries assuming conductive heat transfer and a one-dimensional system; however,
their validity has not been verified with experimental data [64, 65]. In unsatu-
rated biomass slurries containing discrete aggregates, the accurate determination
and prediction of transport properties might be a challenging exercise.
6 Outlook for Challenges Associated with Transport Processes
in Biochemical Conversion of Lignocellulosic Biomass
Significantly greater research and development effort in the conversion of ligno-
cellulosic biomass, spurred by economic, national security and climate change
concerns over the past few years have led to significant strides in development
of a fundamental understanding of transport processes that could appreciably
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