Environmental Engineering Reference
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occurring through the vascular tissue [11]. In untreated biomass, the pits do not
appear to support significant transport. It is probable that these pits disintegrate and
open up during pretreatment allowing fluid to flow through [40]. Thus, new path-
ways for catalyst penetration are formed either during the drying process that creates
fractures in plant tissues or after some degree of biomass degradation.
The primary major barrier to fluid transport into native dry plant tissue appears
to be air entrained in the cell lumen. Simple exposure of tissues to high tempera-
ture fluids is insufficient to achieve catalyst distribution to all parts of the biomass
[11]. The primary escape route for the intracellular air is most likely through pits.
However, the small pit openings (approx 20 nm) could be blocked due to cell wall
drying and water surface tension may prevent movement through these narrow open-
ings. Forced air removal by vacuum provides additional driving force for the bulk
fluid mobility necessary to enhance liquid and catalyst penetration into tissues as
demonstrated by Viamajala and coworkers [11]. Heating dry biomass can minimize
the amount of entrained air (due to expansion of air by heat) and assist in drawing
liquid into the cells by contraction of the entrained air when cooled by immersion
in catalyst-carrying liquid. Thus, bulk transport, rather than diffusive penetration, is
the dominant mass transfer mechanism into dry biomass.
Although movement of fluids is associated with catalyst transport, the primary
goal of catalyst distribution is to deliver the catalyst to cell wall surfaces con-
taining fuel-yielding carbohydrates, rather than to empty cytoplasmic space in
dry tissues. In fact, entrainment of fluids in the biomass bulk can be detrimen-
tal to small time-scale dilute acid or hot water pretreatments, as the presence of
excess water increases the net heat capacity of the material, increasing the heating
time needed to achieve desired pretreatment temperatures. Data shown in Fig. 2
support this hypothesis. In this set of experiments, un-milled sections of corn stems
Fig. 1.2 Effect of preimpregnation of corn stover stalks with dilute acid and particle size reduction
on ( a ) pretreatment and ( b ) subsequent enzymatic hydrolysis
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