Reference Wood

Source

Kappa

Unbleached

R2 Preexponential

Factor

A

Reaction order# EA

[kJ mol–1]

k

[Kappa(q–1) ·

min–1]

Kappa

[calc after

m n q 30 mina]

Hsu & Hsie [10] Southern

pine

29.5 0.94 3.20E+10 0.68 1.28 5.23 97.2 1.90E-07 12.2

Iribarne &

Schroeder [12]

Pinus taeda 20.3–58 0.92 3.00E+06 0.70 0.70 2.00 51.0 1.02E-03 14.2

Argawal et al [3] Southern

hardwood

13.2 0.96 4.42E+07 1.20 0.23 7.08 98.9 9.91E-09 8.8

Argawal et al [3] Southern

hardwood

28.6 0.97 4.42E+07 1.20 0.23 6.00 98.9 9.91E-09 14.4

Argawal et al [3] Southern

hardwood

47.9 n.d. 4.42E+07 1.20 0.23 5.15 98.9 9.91E-09 21.8

a. assuming initial Kappa number, K0 = 25.

0 10 20 30 40 50 60

κ

=13.2, q = 7.08

κ

=28.6, q = 6.00

k = 9.879*10-9 kappa(q-1) min-1

κ

=47.9, q = 5.15

Kappa number

Reaction time [min]

Fig. 7.29 Course of kappa numbers throughout

oxygen delignification at 100 °C, 12% consistency,

0.085 mol L–1 NaOH and 0.0048 mol L–1

dissolved oxygen (equals an oxygen pressure

of 690 kPa) according to Agarwal et al. [3].

Points correspond to the experimental data,

the curves represent the calculated values

using k equal to 9.879·10–9 kappa6,7 min–1.

A good agreement between the experimental data and the model curves can be

obtained by adjusting the apparent rate order q appropriately. The lower apparent

orders q for pulps with the higher initial kappa numbers would mean a higher

proportion of easily eliminated lignin. In terms of Schoon’s model, the fraction of

first-order rate constants with a high delignification rate increases correspondingly.

Unfortunately, this explanation does not account for unbleached pulps of equal

or comparable kappa numbers, but different reactivity of the residual lignin. Zou

et al. have prepared four different hardwood pulps of the same kappa number

(15–16) under different cooking conditions [14]. The extent of a subsequent oxygen

delignification clearly increases for pulps which are cooked with increasing

amounts of effective alkali. According to Agarwal et al., the rate constant k of oxygen

delignification remains constant at given reaction conditions (e.g., 90 °C,

60 min, 0.051mol L–1 [OH– ], 0.005 mol L–1 dissolved oxygen). This implies that

the apparent order q increases parallel to the increasing fraction of easily degradable

lignin to account for the accelerated delignification rate. On the other hand,

the reaction rate coefficient increases while keeping the apparent reaction order q

constant at the (recalculated) average value of 7.44. The results of these calculations

are summarized in Tab. 7.15.

The interpretation of Agarwal et al. that a lower value of q would correlate with

a lower fraction of refractory lignin structures can only be applied for pulps of different

initial kappa numbers [3]. In case the initial kappa number remains

unchanged, a higher extent of delignification during oxygen delignification is

678 7Pulp Bleaching

Tab. 7.15 Kinetic parameters of the power-law rate equation for

oxygen delignification of hardwood kraft pulps of comparable

kappa numbers obtained by different cooking conditions

according to Zou et al. [14]. Rate Eq. (25) is fitted in two ways:

(a) by keeping the reaction rate constant, k, constant; and (b) by

keeping the apparent order, q, constant. The constant reaction

rate, k, is calculated by using the kinetic parameters obtained

by Agarwal et al. [3] and considering the conditions of oxygen

delignification: 90 °C, 0.051 mol L–1 [OH– ] OH, 0.005 mol L–1

dissolved oxygen, 60 min.

Дата добавления: 2015-10-21