Chemical Reactions on Solid Surfaces - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 455

surface. High temperature and high electrolyte content favor the oligomers to be dissolved
in the liquor phase.
As an example, we shall examine the hot water extraction or hydrolysis of wood in the
following. The mechanism for hydrolysis can be very complicated when mass transfer is
coupled with the chemistry. However, when viewed on each component, one can deduce that:

1. Transport of hydrogen ion and/or small strong hydrogen bond-forming molecules from
the bulk liquor to the surface of the solid particle;
2. Chemisorptions (hydrogen bond formation, for example) occur on the solid surface;
3. Surface reaction between the hydrogen ion (proton) and xylan;
4. Desorption or cleavage of the dissoluble xylooligomer;
5. Diffusion and transport of the dissolved xylooligomer to the bulk liquor.

which is typical of LHHW kinetics. However, the solid phase is not acting as a catalyst. The
solid phase contains both the reactant and the desired product.
Under normal conditions, transport steps 1) and 5) are probably unimportant, especially
when the effective thickness of the wood chips is less than the critical thickness, i.e. 2 e 3 mm.
However, the particle size still poses a strong influence on the reaction as only the active components on
the surface can be reacted. These transport steps reach a negligible extent to the overall observed
extraction rate. Step 3, the surface reaction step, can be complicated in nature. However, there
exists a controlling substep in the chemistry of xylan hydrolysis: breakage of a glycosidic
bond. Therefore, step 3 can be simplified to an elementary step representing the glycosidic
bond breakage. The chemical reaction involved is said to be the rate-controlling step. In
this case, the overall reaction rate is dependent on steps 3 e 5 directly. In this paper, we are
interested in the intrinsic kinetics, i.e. steps (3) through (5), only.
Under autocatalytic conditions, the hydrogen ion concentration is initially very low.
However, acetyl groups are present in wood as they are associated with, extractives, lignin
as well as hemicellulose. Hydration of the acetyl groups lead to the acidification of the liquor
and thus formation of hydrogen ions.
According to these considerations, the extraction-hydrolysis involving solid woodchips
can be represented by a simplistic model:

H 2 O H 2 O (9.146)

%

H 2 O % R-X n OH $ H 2 O ð

ð Þþ Þ (9.147)

R-X n OH s s

R-X n OH $ H 2 O ð s Þ / R-X m OH ð s Þþ HX s OH ð aq Þ (9.148)

H 2 O ð R-POAc ð s Þþ aq Þ % R-POAc $ H 2 O ð s Þ (9.149)

R-POAc $ H 2 O ð s Þ % R-POH ð s Þþ HOAc ð aq Þ (9.150)

R-POAc $ H 2 O

ð Þ % ð Þþ ð Þ (9.151)

s R-OH s HPOAc aq

H þ ð OAc ð HOAc ð aq Þ % aq Þþ aq Þ (9.152)

H þ ð R-X n OH $ H þ ð R-X n OH ð s Þþ aq Þ % s Þ (9.153)

R-X n OH $ H þ ð R-X m OH $ H þ ð s Þþ H 2 O ð aq Þ / s Þþ HX s OH ð aq Þ (9.154)