Carbohydrates Lignin Total

Wood (W) 0.0 0.0 0.0

Prehydrolysis (P) 5.6 0.1 5.7

Neutralization (N) 4.8 2.1 6.9

Hot displacement (HD) 3.8 1.9 5.7

Cooking (C) 0.7 0.4 1.1

Cold displacement (CD) 0.6 0.0 0.6

Unbleached pulp (UP) 0.3 0.0 0.3

Total 15.8 4.5 20.3

350 4 Chemical Pulping Processes

0 25 50 75 100

k

= 0.103 min-1

k

= 0.021 min-1

after prehydrolysis untreated

Lignin content, %

Reaction time at 160.C, min

Fig. 4.111 Effect of prehydrolysis on the delignification rates

for pulping of Eucalyptus saligna under comparable conditions

(according to [53]). Prehydrolysis-kraft: P-factor 620, l:s = 10:1,

[OH– ]= 0.44 mol L–1, [HS– ]= 0.076 mol L–1. Kraft: l:s = 10:1,

[OH– ]= 0.44 mol L–1, [HS– ]= 0.063 mol L–1.

Prehydrolysis remarkably influences delignification efficiency during the subsequent

kraft cooking process. The application of a prehydrolysis step greatly facilitates

lignin removal which – in practice – is expressed by both lower H-factors

and lower kappa numbers. Kinetic investigations reveal that the bulk delignification

rate of a non-prehydrolyzed Eucalyptus saligna at 160 °C comprises only onefifth

as compared to a prehydrolyzed wood (Fig. 4.111).

This impressive acceleration of the delignification rate of prehydrolyzed wood

chips may be explained by both an improved permeability of the cell wall due to

increased pore volumes [42]resulting in improved penetration of the cooking

liquor and the (partial) hydrolytic cleavage of lignin structures and lignin–carbohydrate

bonds. Moreover, it can be assumed that the formation of non-lignin chromophoric

structures is impeded because of the lower content of residual carbohydrate

structures. It has been shown that prehydrolyzed kraft pulps contain considerably

lower amounts of non-lignin and HexA structures as part of the kappa

number as compared to paper-grade kraft pulps [54].