Series Cooking process Xylan additiona)

[time interval, min]

Relative composition [%]

Glu Xyl Man Ara Gal

CB Conventional batch 84.6 8.5 6.4 0.5 0

CLF-early Continuous liquor flow 70–130 86.4 7.1 5.9 0.4 0.2

CLF-late Continuous liquor flow 90–150 85.6 7.4 6.2 0.5 0.3

CLF-ref Continuous liquor flow no addition 87.8 6.1 5.7 0.2 0.2

a) Xylan concentration 3.5 g L–1.

There appears to be no significant difference in the observed xylan content between

the series of early (CLF-early) and late xylan addition (CLF-late), the latter

simulating a conventional batch cooking system. As expected, the xylose content

is highest in pulps from the conventional batch cooks, and lowest in pulps from

the continuous liquor flow cooks without xylan addition (CB and CLF-ref). Pulps

from pilot plant trials using a Lodgepole pine/spruce mixture as raw material

indicate that the xylan content of a rapid displacement heating (RDH) pulp with a

kappa number of 26.7 is slightly lower (7.9% on unbleached pulp) as compared to

a pulp from a conventional batch cook with a kappa number 33.5 (8.6% on

unbleached pulp) [60].

Cellulose degradation was monitored during both conventional kraft and modified

continuous kraft (MCC) pulping of Pinus radiata using gel permeation chromatography

(GPC) measurement of isolated holocellulose fractions [61]. The molecular

mass of the cellulose fraction was determined by assuming that all the cellulose

molecules eluted until a certain elution volume was reached. The results

suggest that, in both the conventional and modified processes, cellulose degradation

takes place only at pulp yields lower than 75%. From this result it can be concluded

that cellulose degradation becomes apparent only after initial delignification

has completed.

The extent of cellulose chain scissions during bulk and residual delignification

is thus more pronounced for conventional kraft pulping as compared to MCC

pulping (Fig. 4.56).

The results are in line with the experience from practice that MCC pulping is more

selective with respect to cellulose degradation than conventional kraft pulping.

4.2 Kraft Pulping Processes 255

40 60 80 100

Conventional Kraft MCC Kraft

DP

W

Pulp Yield [%]

Fig. 4.56 Degree of polymerization (DPw) of

the cellulose fraction from conventional kraft

and MCC pulps (according to [61]). Holocellulose

was isolated according to the method of

Holmes and Kurth [62]from the pulps prior to

GPC measurement. The weight average molecular

weight was determined from the tricarbanilate,

considering only the high molecularweight

peak and assuming that no cellulose is

eluted with the second peak at the higher elution

volume.

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