Pressure Relief, Displacement of Cooking Liquor, and Discharge

Determination of the end-point of the cook is based on a combination of empirical

cooking models and color analysis of the cooking liquor. The empirical models

used for sulfite pulping are called either the S- or the H-factors [3]. The S-factor

includes both the temperature and the partial pressure of SO2. It is generally

accepted that the rate of delignification is proportional to the ion product of

[H+].[HSO3

– ]n, with n most likely being 0.75, and the rate of cellulose degradation

(equals viscosity loss) to [H+], both being proportional to the partial pressure of

SO2 [3]. Thus, the S-factor (SF) is developed from the following expression:

430 4 Chemical Pulping Processes

rL _ _

dL

dt _ kL _ _ L a _ pSO 2 n

_174_

where L is the lignin concentration, kL the rate constant, pSO2 the partial pressure

of SO2, and a and n are constants, with a assumed to be unity. The SF calculates

to the expression:

SF _ _

tfinal

tT _100_ C

dL

L _

tfinal

tT _100_ C

Exp

EA _ L

R _ 373 _

EA _ L

T _ __ pSO 2 n

_ dt _175_

The SF also correlates with the viscosity, provided that the activation energy is

adjusted to a value determined for the carbohydrate degradation, EA,C.

The energy of activation for delignification, EA,L, has been found to be 67 kJ mol–1

in the beginning of delignification, and 95 kJ mol–1 at the final phase [4]. The energy

of activation for the dissolution of the carbohydrates, EA,C, changed only

slightly in the course of cooking from about 80 kJ mol–1 in at the start of the cook to

90 kJ mol–1 at the end of the cook [4]. For cellulose degradation during acid sulfite

pulping, higher values for EA,C (e.g., 125 kJmol–1 and 176 kJmol–1) havebeenreported,

respectively [5,6]. Pressure regulation clearly has an impact on the velocity of cellulose

degradation, and thus on the calculated value for the activation energy.

According to Eq. (175), the partial pressure of SO2 must be considered, though

this is barely measurable. To estimate a value for the partial pressure of SO2 it has

been assumed that the total digester pressure, ptot, is primarily a combination of

the partial pressure of SO2 and the partial pressure of water, pH2O, at the specified

temperature [Eq. (176)][7].

pSO 2 __ ptot _ pH 2 O _ _176_

It is, however, common practice that pressure and temperature are adjusted to

preset values during the cooking phase (and deviate from the preset values only

during the heating-up period), which therefore would maintain a rather constant

partial pressure of SO2 when calculated according to Eq. (176). In view of this situation,

a simple H-factor concept in combination with a color analysis of the cooking

liquor would be sufficient for correct end-point determination. The activation

energy for cellulose degradation, EA,C, during acid sulfite pulping of beech wood

with pressure control at a level of 8.5 bar, has been calculated by nonlinear regression

analysis using the following approximation for H-factor determination [8].

For practical reasons, cellulose degradation is measured as loss in intrinsic viscosity.

HS _ C _

tF

t 0

Exp _

EA _ C

R _ __

T _

373_15 _ _ ___ dt _177_

4.3 Sulfite Chemical Pulping 431

where HS,C is the H-factor for cellulose degradation during acid sulfite pulping.

Based on a total of 155 laboratory cooks, an activation energy for cellulose degradation,

EA,C, of 110 kJ mol–1 has been determined. Inserting this activation energy

leads to the following expression for the HS,C:

H _

tfinal

tT _100_ C

Exp 35_47 _

T _ _ dt _178_

The dissolving pulp viscosity cannot be adjusted with sufficiently high precision

by only using H-factor control. Cellulose degradation is additionally influenced by

the composition of the cooking liquor – for example, the amounts of combined

and free SO2 and the liquor-to-wood ratio. H-factor control is, however, well-suited

for the precalculation of cooking times which enables the optimization of digester

sequencing, steam supply and thus the prediction of production output. The real

end-point determination of a sulfite cook – particularly a sulfite-dissolving cook –

is very difficult for two main reasons. The first reason is that, to date, there is no

capability of analyzing a representative sample from the entire cook to determine

the target pulp properties. Examples include the pulp viscosity of a dissolving

pulp or the residual lignin content (kappa number) for a paper grade-pulp, to be

assessed either within a very short time or even on-line, such that the process

operator is still in a position to adjust the process conditions accordingly. The second

reason is that a sulfite cook accelerates towards the end of the process, and

reactions cannot be stopped immediately at a predetermined time. Consequently,

the whole process of terminating the cook including the relief of digester pressure

and cold displacement – must be controlled with regard to the viscosity (or kappa

number) development. Currently, only cooking liquor analysis is applied to monitor

the reaction medium of the cook towards the end of the process. Although

they are only indirect methods, cooking liquor tests have the advantage that the

samples – which preferably are removed from the liquor circulation – represent

the entire digester content, and the analysis can be carried out rapidly and even

recorded on-line. Among a wide variety of possible methods listed in Table 4.56,

color determination of the cooking liquor is the most important parameter for

end-point determination, at least for dissolving pulp production.

Absorbance at 280 nm, which is characteristic for the lignin and furfural concentrations

of the liquor, changes during the final period of dissolving pulp cook

due to condensation reactions. Therefore, absorbance at this wavelength is not

well-suited to measure the lignin concentration of the cooking liquor. However,

absorbance in the visible region – preferably between 400 and 500 nm – correlated

well with the acidity prevailing in the cooking liquor. The liquor color, which converts

from light yellow to brown and finally to dark-brown, most likely originates

from condensation reactions involving carbonyl groups from lignin structures

induced by a lack of hydrogen sulfite ion concentration and the development of

acidity [11]. In industrial practice, the color is measured at 430 nm against pure

water. Development of the color is carefully monitored during the whole final

432 4 Chemical Pulping Processes

cooking phase (from the beginning of the pressure relief until the blow). Thus,

absorbency at 430 nm shows a reasonably good correlation to pulp viscosity

(Fig. 4.158). [13].

Tab. 4.56 Cooking liquor analysis methods used for end-point

determination of acid sulfite cooks.

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