Batch Reactor - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 163

In chemical pulping, this temperature T R is usually taken as 100 C (or 373.15 K) and the
resulting t R is called H -factor. Once the activation energy is known, the effect of temperature
and time can be easily lumped together with this single parameter, t R ,or H -factor. This
turned out to be smart idea as operators can control the reaction effectively even the temper-
ature varied during the process. Frequently quoted values of the constants are of those of
Vroom (1957) for kraft pulping of woodchips as shown below.

Z t d

16115 K
T

HF ¼ exp 43:2 t (E4-6.12)

0

As temperature rises, more solvent evaporates to the vapor phase. As pressure increases,
more solvent returns to the liquid phase. Therefore for liquid or liquid e solid reactions at
high temperature, the effective reaction volume V changes with temperature and pressure
if not with concentration or composition. The constant volume assumption is only
approximate. The complexity further arises as the reaction kinetics is not the same when
temperature is increased causing different components in the plant biomass to be susceptible
to reaction. Thus, the H -factor treatment is only an approximation of the reaction system. One
variant of the H -factor is the severity factor. Equation (E4-6.11) can also be written as

Z t T T R

RT R T=E

d

t R ¼ exp t (E4-6.13)

0

If one approximates

RT R T
E

z T E

(E4-6.14)

as a constant, then Eqn (E4-2.13) can be written as

Z t

exp T T R
T E

t ¼ t exp T T R
T E

t R z d (E4-6.15)

0

The severity factor is defined as the log of t R , that is

t exp T T R
T E

¼ log t þ T T R
T E ln10

SF ¼ log (E4-6.16)

is 14.75 C or 14.75 K to be
precise (Overend & Chornet 1987; Montane et al. 1994; Kabel et al. 2006). This is equivalent
to E

For hot water extraction of woody biomass, the frequently used T

E

R ¼

10800 K for hot water extraction of woody biomass or hemicellulose reaction.

Example 4-7. Hot water extraction of woodchips. Hot water extraction is an excellent tech-
nique to remove hemicelluloses from wood. Hemicellulose extracted this way can be con-
verted to valuable products at low cost. Kate carefully prepared a batch of woodchips to
conduct hot water extraction experiment. She loaded 500 g of sugar maple chips and