Environmental Engineering Reference
In-Depth Information
(~ 1 inch long) were saturated to various degrees with dilute sulfuric acid (2% w/w)
and pretreated in 15 mL of the same acid solution at 150
◦
C for 20 min. Milled
corn stems (-20 mesh) pretreated under identical conditions served as controls.
All pretreatments were performed in 22 mL gold coated Swage-Lok (Cleveland,
OH) pipe-reactors, heated in an air-fluidized sand bath [42]. After pretreatment,
whole stem sections were air-dried, milled and enzymatically digested for 120 h
with a 25 mg/g of cellulose loading of a commercial
T. reesei
cellulase preparation
(Spezyme CP, Genencor International, Copenhagen, Denmark) supplemented with
an excess loading (90 mg/g of cellulose) of commercial
Aspergillus niger
cellobiase
preparation (Novo 188, Novozymes Ltd., Bagsvaerd, Denmark) using procedures
described previously [47]. Milled stover pretreated as controls in this experiment
was dried and digested similarly, but without any further comminution.
In Fig. 2a, dry internodes pretreated without pre-impregnation of catalyst were
poorly pretreated as evidenced by the high amounts of xylan remaining in the
biomass after reaction. Stem sections pre-impregnated to achieve 20% satura-
tion showed better reactivity and xylan removal and this trend continued when
stem sections pre-impregnated to 50% saturation were pretreated. However, when
completely saturated (100%) stem sections were pretreated, xylan conversion was
observed to be lower. Milled materials with and without pre-impregnation of
catalyst - conditions that would have lowest mass transfer limitations, showed com-
parable pretreatment performance with each other as well as with the 50% saturated
stem sections. These results confirm that only limited catalyst penetration and pre-
treatment is achieved when air remains entrapped in cytoplasmic spaces such as in
dry internodes. Enhanced catalyst distribution and transport dramatically enhances
pretreatability up to a certain point, after which excess fluid impedes pretreatment.
Similar conclusions on the negative impacts of poor bulk transfer on biomass pre-
treatability can be inferred from other reported studies also. Tucker and coworkers
[10] observed poor pretreatability of biomass during steam explosion of corn stover
when materials were not pre-wetted with dilute acid and ascribed their results to
mass transport limitations. In another study Kim and coworkers [13] observed poor
pretreatment of biomass when the biomass was pressed prior to pretreatment and
hypothesized that the mechanical compression of biomass caused pore structure
collapse resulting in formation of material that was relatively impervious to heat
and mass transfer.
Enzymatic digestion results corresponding to pretreatments shown in Fig. 2a,
are presented in Fig. 2b. As expected, release of monomeric sugars from pretreated
whole stem sections was proportional to the degree of pretreatment they experi-
enced. Unmilled biomass that was 50% saturated with acid before pretreatment
showed better digestibility than the sections that were pre-saturated to lower or
higher levels. Milled biomass, however, digested best, demonstrating the importance
of enhanced enzyme transport - an outcome of the more thorough and uniform
pretreatment of milled materials. With woody feedstocks, milling to fine parti-
cle sizes may be impractical and pre-impregnation of biomass with catalyst, as
practiced in the pulp and paper industry [48], might need to be utilized to improve
conversion efficiencies.
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