Pulp Yield as a Function of Process Parameters

Pulp yield is a very decisive economical factor, as the wood cost dominates the

total production cost of a kraft pulp. Consequently, the knowledge of the relationship

between process conditions and pulp yield is an important prerequisite for

economical process optimization. Based on the numerous published reports on

conventional kraft pulping, it is known that the pulp yield generally increases by

0.14% per increase of one kappa unit for softwood in the kappa number range of

30 to 90, and by 0.16% for hardwood in the kappa number range of 10 to 90,

respectively [1]. In the higher and lower kappa number range, the influence on

yield is slightly more pronounced. Kappa numbers below 28 should be avoided

when using conventional kraft pulping technology, because the yield and the viscosity

losses increase considerably. The pulp yield is also influenced by the effective

alkali charge (EA). It is reported that in pulping of softwood an increase in the

EA charge of 1% NaOH on wood, will decrease the total yield by 0.15% [2]. The

small overall drop in yield is explained by two oppositely directed effects, namely

an increase in the retention of glucomannan and a decrease in xylan due to

increased peeling reactions. The influence of EA charge is much more pronounced

in case of hardwoods due to the very small amounts of glucomannans

present. An increase of 1% EA charge results in a total yield loss of about 0.4%

(Fig. 4.37) [3].

4.2 Kraft Pulping Processes 229

©2006 WILEY-VCHVerlag GmbH&Co.

Handbook of Pulp

Edited by Herbert Sixta

0 20 40 60 80 100 120 140 160

Southern Pine: 20% EA charge Southern Pine: 15% EA charge

Mixed Hardwood: 20% EA charge Mixed Hardwood: 15% EA charge

Total Yield, % on wood

Kappa number

Fig. 4.37 Total pulp yield in kraft pulping of southern pine

and southern mixed hardwoods as a function of kappa number

(according to [1]).

Sulfidity exerts a significant influence on pulp yield for softwood and hardwood

at sulfidity values below 15%. Compared to a pure soda cook, the addition of

sodium sulfide to achieve a sulfidity of 15% enables a yield increase of approximately

2.8% for softwood and 2.4% for hardwood, respectively [4]. A further

increase of the sulfidity to 40% means an additional yield increase of about 1% for

softwood and only about 0.2% for hardwood. Yield is also affected by the chip

dimension [5]. A reduction in chip thickness improves the uniformity of pulping,

which leads indirectly to a slight increase in pulp yield. The better uniformity of

pulping in case of thin chips makes it possible to reduce the EA charge which in

turn results in an improved pulp yield at a given kappa number (see Chapter

4.2.3, Impregnation).

In conventional cooking, the EA concentration profile follows an exponential

decrease with increasing cooking intensity measured as H-factor (see Fig. 4.36;

see also Fig. 4.38). In the initial phase of hardwood (birch) kraft pulping, about

8% xylan can be dissolved in the cooking liquor, depending on the EA concentration

[1]. Part of the dissolved xylan can be adsorbed onto the surface of the wood

fibers in the final cooking phase as soon as the pH falls below 13.5 [6]. In the

pulping of birch, a yield increase of 1–2% has been observed due to the reprecipitation

of dissolved xylan [7]. The effect on yield is reported to be about half for softwood

(pine) as compared to birch due to the lower amount of xylan present in

both wood and cooking liquor.

Conventional kraft pulping in batch digesters is a very simple process and comprises

the following steps:

230 4 Chemical Pulping Processes

_ Chip filling.

_ Chip steaming.

_ Introduction of an aqueous solution containing the cooking

chemicals in the form of white liquor, or a mixture of white liquor

and black liquor from a preceding cook.

_ Heating the digester to a cooking temperature of about 170 °C by

direct steam or by indirect heating in a steam/liquor heat exchanger.

_ In case of indirect heating, the cooking liquor is circulated

through a heat exchanger to even out temperature and chemical

concentration gradients within the digester.

_ Cooking is maintained until the target H-factor is reached. The

pressure is controlled by continuously purging volatile substances

being released during the cooking process.

_ Condensable gases are partly recovered as wood by-products,

such as turpentine.

_ Digester content is blown by digester pressure to a blow tank.

The pulp from the blow tank is then washed and screened before it enters the

bleach plant.

The performance of conventional kraft pulping is predominantly dependent on

the wood species, the wood quality, the EA charge, the ratio of hydrogen sulfide

ion to hydroxide ion concentration, the time–temperature profile, the H-factor,

and the terminal displacement and pulp discharge procedure. Laboratory trials

according to the description in Chapter 3 (see Section 4.2.5.3.6. Reaction kinetics:

Validation and application of the kinetic model) were conducted to investigate the

influence of sulfidity, cooking temperature and H-factor. A mixture of industrial

pine (Pinus sylvestris) and spruce (Picea abies) in a ratio of about 50:50 was used as

raw material. The time–temperature and time–pressure profiles correspond to a

conventional batch cooking procedure, characterized by a long heating-up time

(see Fig. 4.34). Approximately 80% of the EA is consumed already during the heating-

up time, which corresponds to an H-factor of about 180 (Fig. 4.38). This leads

to the conclusion that 80% of the alkali-consuming reactions occur in the course

of only 15% of the total cooking intensity (180 H-factor versus 1200 H-factor to

obtain a kappa number of about 25).

At the start of bulk delignification, the hydroxide concentration reaches a value

of about 0.45 mol L–1. From the viewpoint of delignification kinetics, the course of

hydroxide ion concentration in a conventional batch cook – with a high [OH– ]dur -

ing the initial and a low [OH– ]during bulk and residual delignification – is very

unfavorable. Moreover, delignification efficiency and selectivity are impaired due

to the increasing concentration of dissolved solids in the late stages of cooking.

An increase in sulfidity, even only by 3% from 35% to 38%, shows a significant

improvement in delignification selectivity characterized as viscosity–kappa number

relationship. The reduction in cooking temperature from 170 °C to 155 °C

reveals a further slight improvement in delignification selectivity (Fig. 4.39).

4.2 Kraft Pulping Processes 231

80 100 120 140 160 180

0.0

0.3

0.6

0.9

1.2

1.5

T = 170.C, S = 35 - 38%

H-Factor

EA-concentration [mol/l]

Temperature [. C]

Fig. 4.38 Course of effective alkali concentration

during conventional kraft cooks as a

function of cooking temperature and H-factor

(according to [8]). Raw material was a mixture

of industrial pine (Pinus sylvestris) and spruce

(Picea abies) in a ratio of about 50:50. The

EA-charge was kept constant at 19% on ovendry

wood, sulfidity varied from 35 to 38%,

liquor-to-wood-ratio 3.7 L kg–1. (See also

Fig. 4.36.)

Lowering the cooking temperature additionally improves the screened yield,

mainly because of more homogeneous delignification reactions resulting in a

lower amount of rejects (Fig. 4.40).

20 25 30 35 40 45

170.C, S = 35% 170.C, S = 38% 155.C, S = 38%

Viscosity [ml/g]

Kappa number

Fig. 4.39 Selectivity plot (viscosity–kappa number relationship)

of pine/spruce conventional kraft cooking (according to

[8]). Influence of sulfidity: [HS– ]= 0.28 versus 0.31 mol L–1

and cooking temperature: 170 °C versus 155 °C. The EAcharge

was kept constant at 19% on o.d. wood, liquor-towood-

ratio 3.7 L kg–1.

232 4 Chemical Pulping Processes

20 25 30 35 40 45

170.C, S = 35% 170.C, S = 38%, 155.C, S = 35%

Screened Yield [%]

Kappa number

Reject [%]

Fig. 4.40 Pine/spruce conventional kraft cooking. Screened

yield and amount of rejects as a function of kappa number

(according to [8]). Influence of sulfidity: [HS– ]= 0.28 versus

0.31 mol L–1 and cooking temperature: 170 °C versus 155 °C.

The EA-charge was kept constant at 19% on o.d. wood, liquorto-

wood-ratio 3.7 L kg–1.

The effect of sulfidity and cooking temperature on the processability and selectivity

of conventional batch cooking is illustrated for a kappa number 25 softwood

kraft pulp in Tab. 4.25.

Increasing sulfidity and lowering the cooking temperature to 155 °C improves

the viscosity of the unbleached kappa number 25 pulp by 60 units, and the

screened yield by more than 2%. The yield increase can be attributed to a lower

amount of rejects and higher contents of arabinoxylan and cellulose (Tab. 4.25).

Moreover, a significant lower amount of carboxylic groups of the unbleached pulp

derived from high-sulfidity and low-temperature conditions is noticeable. It can

be speculated that this pulp contains a lower amount of hexenuronic acid, as it is

reported that a low cooking temperature leads to a lower hexenuronic acid content

at a given kappa number [9–11]

Despite the yield and viscosity advantages, the reduction of cooking temperature

from 170 °C to 155 °C results in an extension of the cooking time by approximately

200 min (Tab. 4.25). The cover-to-cover time of a conventional batch cook

would thus increase from about 265 min to 465 min, which is totally unacceptable

from an economic point of view. At a given digester volume, the prolongation of

the cooking cycle due to a reduction in cooking temperature would reduce the production

capacity by 43% (1–465–1/265–1). On the basis of conventional batch cooking,

technology improvements in the pulping efficiency and selectivity are very

4.2 Kraft Pulping Processes 233

limited. The progressive knowledge on pulping reactions and delignification

kinetics finally led to the development of modified kraft cooking concepts.

Tab. 4.25 Production of unbleached softwood kraft pulps with

kappa number 25 using a conventional batch cooking

procedure. Comparison of three different cooking conditions:

(a) low sulfidity (S), high cooking temperature (T); (b) high

sulfidity, high cooking temperature; (c) high sulfidity and low

cooking temperature, according to [8].

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