Effect of Impregnation on the Uniformity of Delignification

The effects of chip size have been evaluated by several research groups. In a study

of kraft pulping of pine chips, Backman [57]stated that above a thickness of 1 mm

the reaction rate is at least partially controlled by the transport steps, while below

this thickness the rate is probably controlled by the rate of the chemical reactions

involved. Hartler and Ostberg identified that the Roe number remained constant

when the thickness was 3 mm or less [58], and Larocque and Maass found essentially

the same effect of chip size [59]. Both the chip size and the uniformity of

chip dimensions are very important criteria for pulp properties. In particular, chip

thickness is a critical dimension which strongly controls the extent of delignification,

the amount of rejects, and even strength development. Chip length and

width have been shown to have a minimal influence on delignification [12], with

4.2 Kraft Pulping Processes 159

thick chips showing very steep delignification gradients. Wood is overdelignified

at the surface, while chip centers are almost undelignified. Gullichsen et al. [8]

reported a kappa number gradient from 14 on the surface to more than 120 in the

chip center for a 8 mm-thick chip of a Scots pine cook with an average screened

kappa number of 23.4 [8]. The same authors also noted that only chips with a

thickness <2 mm can be uniformly delignified under conventional cooking conditions

[8]. Uniform thin chips without knots and reaction wood can be produced

either by efficient screening or by applying an innovative chipping technique.

Another approach involves the application of chip pretreatments and optimization

of impregnation conditions, aimed at improving penetration and efficient diffusion.

Recently, the effects of chip steaming and increased pressure impregnation during

the hot black liquor stage on the kappa number distribution inside handmade

pine chips (Pinus silvestris) were investigated by using reflection Fourier transmission

infra-red (FTIR) spectroscopy (equipped with a microscope which enables a

lateral resolution of an area of approximately 100. 100 lm) [60]. Two scenarios

with different impregnation conditions have been compared with regard to the

uniformity of delignification. Scenario A represents very poor impregnation conditions

involving no pre-steaming and applying only 5 bar overpressure during

the hot black liquor stage. Scenario D, a very efficient mode of impregnation,

combines intensive pre-steaming (30 min, 105 °C) with a high-pressure treatment

(9 bar overpressure) during the hot black liquor stage. The subsequent cooking

steps and conditions were identical for the two scenarios investigated (Superbatch

technology, 17% EA on wood, 40% sulfidity, 170 °C, 890 H-factor in cooking

stage). The handmade chips were cut to a length of 34 mm (longitudinal direction

in wood), a width of 14 mm (tangential direction in wood) and a thickness of

8 mm (radial direction in wood). To analyze the uniformity of delignification,

cooked heartwood chips were cut across the thickness dimensions at distances of

2 mm. Infrared spectra were measured along the chip length and along the chip

width from the middle to the edge, with steps of 2 and 1 mm, respectively. The

kappa number profiles within heartwood chips for cooking scenarios A and D are

illustrated in Fig. 4.19.

In cooking scenario A, the middle part of the 8 mm-thick chip (4 mm deep) was

clearly undercooked. There was a gradual transition along the chip length from

the edge kappa number of 40 to the undercooked regions with kappa number

over 90. In the tangential direction (along the chip width), kappa number rise

was, however, very abrupt close to the chip edge, which confirmed the limited

mass transfer in this direction (not seen).

In scenario D, with the application of pre-steaming and higher pressure profile,

the uniformity of delignification of heartwood chips was significantly improved,

though some gradient was present in the middle (4 mm deep) layer of the chip.

However, the undercooked region was much narrower than in scenario A. These

results confirmed the beneficial effect of reinforced impregnation conditions with

regard to the uniformity of delignification.

160 4 Chemical Pulping Processes

1 4 7 10 13 16

surface, scenario A surface, scenario D

4 mm deep, scenario A 4 mm deep, scenario D

Kappa number

Distance from the chip edge along the length, mm

Fig. 4.19 Delignification profiles within the pine heartwood of handmade chips [60].

An alternative method has recently been introduced to determine the alkali and

lignin concentration profiles in the free and entrapped liquor as a function of chip

thickness [61]. Eucalyptus globulus chips with dimensions of about 30. 30. 1 mm

and 30. 30. 6 mm were used as raw material for the impregnation and cooking

studies. Impregnation trials were carried out at 5 °C and 80 °C using a cooking

liquor with an effective alkali (EA) concentration of 19.3–22.6% on dry wood, a

sulfidity of 25–30%, and an initial liquor:wood ratio of 6:8. Cooking experiments

were conducted at 165 °C using only the 6-mm chips. At the end of each trial the

chips were immediately separated from the remaining free liquor and the excess

liquor at the surface was carefully removed with sorption paper. The chips were

then pressed to 350 bar for 2–3 min to release the entrapped liquor that had been

collected in a previously inertized flask and cooled in a similar ice bath. The free

and entrapped liquors were analyzed for EA and lignin concentrations according

to standard methods [62,63]. The concentration profiles for EA in both free and

entrapped liquors are illustrated for experiments with 1- and 6-mm chips at 80 °C,

and with 6-mm chips at 165 °C (Fig. 4.20). The results confirmed the remarkably

high difference between the entrapped and free liquor concentrations, especially

at the beginning of the reaction. For the thin chips of only 1 mm thickness, the

EA concentrations in both liquors were similar at about 60 min impregnation

time. However, for the 6-mm chips, even after 300 min, there was no equalization

of EA concentrations. Performing these experiments at a cooking temperature of

165 °C yielded a more significant decrease in the concentration of EA in the free

liquor, but this was not followed by a higher increase of EA concentration in the

entrapped liquor. These findings clearly indicate that higher consumption rates of

4.2 Kraft Pulping Processes 161

0 100 200 300

0,0

0,2

0,4

0,6

0,8

1,0

* normalized

EL 6 mm chips, 80 °C FL 1 mm chips, 80 °C

FL 6 mm chips, 80 °C EL 6 mm chips, 165 °C

EL 1 mm chips, 80 °C FL 6 mm chips, 165 °C

effective alkali concentration [g/l]*

Time [min]

Fig. 4.20 Effective alkali concentrations (normalized) in the

free (FL) and entrapped (EL) liquid phases versus time for

1- and 6-mm chips at 80 °C and 165 °C.

alkali occur when chemical reactions are conducted at temperatures above 80 °C.

Moreover, the results reveal that, in such a heterogeneous process, the alkali concentration

profiles in the free and entrapped liquors do not necessarily appear as

image and mirror image.

The consumption of EA does not correspond to the alkali concentration in the

bulk liquor because the alkali inside the chip is not totally consumed by the wood

components. It is clear that the alkali concentration in the entrapped liquor determines

the dissolution of wood components, and this must be considered in the

development of a heterogeneous delignification kinetic model.

The concentration profiles of dissolved kraft lignin in both entrapped and free

liquors depend heavily on the reaction temperature. At 80 °C, the concentration of

dissolved lignin in the bulk liquor remains very low, indicating only a small

degree of delignification at this temperature. Inside the chips, the concentration

rises quickly to a constant low level. When the temperature is increased to 165 °C,

the dissolved lignin concentration in the entrapped liquor reaches a maximum

after about 80 min. Continuing the cooking process leads to a decrease in dissolved

lignin concentration in the enclosed liquor due to a slow-down of the

delignification rate, and this results in an overall enhanced mass transfer of lignin

to the free liquor. The whole mass balance of lignin calculated from lignin concentration

profiles in the entrapped and free liquors and the lignin content in the

wood chips is illustrated in Fig. 4.21.

162 4 Chemical Pulping Processes

0 100 200 300

0,0

0,2

0,4

0,6

0,8

1,0

1,2

* normalized

Dissolved (80 °C) Dissolved (165 °C)

Residual (80 °C) Residual (165 °C)

Total (80 °C) Total (165 °C)

Amount of Lignin [g]*

Time [min]

Fig. 4.21 Mass balance of lignin during impregnation at 80 °C

and kraft cooking at 165 °C using Eucalyptus globulus chips

with a thickness of 6 mm.

The data shown in Fig. 4.21 confirm that at a temperature of 80 °C, which is a

typical temperature for impregnation, only negligible delignification occurs. At

165 °C, the expected pattern of residual lignin as a function of time can be observed.

The total normalized mass of lignin increases up to 10% of its initial value

being attributed to experimental errors as dissolved extractives contribute to lignin

concentration using UV detection at 280 nm.

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