Free SO2

[moI L–1]

Base

[moI L–1]

[H+]

[moI L–1]

pH-Value [SO2.H2O]

[moI L–1]

[HSO3

– ]

[moI L–1]

[SO3

2– ]

[moI L–1]

[H+]*[HSO3

– ]

0.78 0.00 1.04·10–1 0.98 6.77·10–1 1.04·10–1 1.00·10–7 1.07·10–2

0.58 0.20 3.65·10–2 1.44 5.44·10–1 2.37·10–1 6.48·10–7 8.63·10–3

0.39 0.39 1.47·10–2 1.83 3.76·10–1 4.05·10–1 2.76·10–6 5.96·10–3

0.28 0.50 8.49·10–3 2.07 2.72·10–1 5.09·10–1 5.99·10–6 4.32·10–3

0.08 0.70 1.79·10–3 2.75 7.92·10–2 7.02·10–1 3.92·10–5 1.26·10–3

0.00 0.78 3.94·10–5 4.40 1.93·10–3 7.77·10–1 1.97·10–3 3.06·10–5

–0.02 0.80 3.97·10–6 5.40 1.91·10–4 7.62·10–1 1.92·10–2 3.02·10–6

–0.22 1.00 2.57·10–7 6.59 9.10·10–6 5.62·10–1 2.19·10–1 1.44·10–7

–0.42 1.20 8.64·10–8 7.06 1.97·10–6 3.62·10–1 4.19·10–1 3.13·10–8

–0.62 1.40 2.62·10–8 7.58 2.67·10–7 1.62·10–1 6.19·10–1 4.24·10–9

–0.74 1.52 5.68·10–9 8.25 1.51·10–8 4.20·10–2 7.39·10–1 2.39·10–10

–0.78 1.56 3.58·10–11 10.45 3.04·10–13 2.79·10–4 7.81·10–1 1.00·10–14

The relative concentrations of sulfur dioxide, hydrogen sulfite, and sulfite are

determined by the pH of the aqueous solution. Figure 4.152 shows that sulfur

dioxide is present predominantly as SO2.H2O and hydrogen sulfite ions at pH 1–

2, typical for acid sulfite cooking. With increasing pH, the proportion of hydrogen

sulfite ion increases significantly, and in the pH range characteristic for magnefite

cooking (3–5), sulfur dioxide is present almost exclusively in the form of hydrogen

sulfite ions. Above this pH level, the sulfite ions start to become the dominating

ionic species in the sulfite cooking liquor.

400 4 Chemical Pulping Processes

0 2 4 6 8 10

SO

.H

O HSO

- SO

2-

mol percentage

pH-value

Fig. 4.152 Relative molar percentage of SO2·H2O, hydrogen

sulfite and sulfite ions as a function of the pH at 25 °C. Data

based on information in Tab. 4.54.

Due to the decrease in the acid dissociation constant of hydrated sulfur dioxide,

Ka,1, with increasing temperature, the pH level of the acid sulfite cooking liquor is

shifted to higher values at cooking temperature (Fig. 4.151, Tab. 4.51). This must

be considered in acid calcium sulfite pulping by increasing the proportion of the

free sulfur dioxide concentration to avoid the formation of insoluble calcium sulfite.

The ionic product, [H+].[HSO3

– ], is said to be proportional to the rate of delignification

in the course of sulfite pulping [6]. According to Tab. 4.54, this ionic product

increases exponentially with decreasing pH, which is equal to an increase in

the free SO2 concentration. This result corresponds well with industrial experience.

Increasing the proportion of free SO2 in the cooking acid continuously

reduces the cooking time at given process conditions.

Moreover, the presence of free SO2 largely determines the vapor pressure of the

cooking acid at the prevailing temperature. Figure 4.153 illustrates the development

of the partial pressure of SO2 of a cooking acid with a total SO2 concentration

of 0.78 mol L–1 containing two different amounts of free SO2, 0.39 mol L–1

(50% of total) and 0.23 mol L–1 (30% of total), respectively, at varying temperature

levels.

The inter-relation of partial SO2 pressure, and free and total SO2 is exemplified

in Fig. 4.154 for two temperatures, 100 °C and 140 °C, the latter being typical for

the cooking phase.

4.3 Sulfite Chemical Pulping 401

0 50 100 150

0.78 mol/l ÓSO

/l; 0.39 mol/l free SO

0.78 mol/l ÓSO

/l; 0.23 mol/l free SO

partial pressure of SO

[bar]

Temperature [. C]

Fig. 4.153 Development of partial pressure of

SO2 of a cooking acid comprising two different

proportions of free SO2, 50% and 30% of total

SO2 concentration (0.78 mol L–1), as a function

of temperature. The equilibrium conditions

were simulated by means of ASPEN-PLUS [10]

based on the pioneering studies of Hagfeldt et

al. [11].

0,1 0,3 0,5 0,7

pH value

pSO2: 100 °C 140 °C

partial pressure of SO2 [bar]

Free SO2 [mol/l]

Total SO2: 0.78 mol/l

pH: 100 °C 140 °C

Fig. 4.154 Development of partial pressure of

SO2 and pH of the three-component system

magnesium oxide-sulfur dioxide-water as a

function of free SO2 concentration for two different

temperatures, 100 °C and 140 °C,

respectively, while keeping the total SO2 concentration

constant at 0.78 mol L–1. The equilibrium

conditions were simulated by means of

ASPEN-PLUS [10]based on the pioneering studies

of Hagfeldt et al. [11].

402 4 Chemical Pulping Processes

The total pressure of sulfite cooking acids containing large quantities of free

SO2 is largely determined by the partial pressures of SO2, water and, in the case of

hardwood pulping, also by considerable amounts of volatile carbonic acids (e.g.,

acetic acid, furfural, etc.) and carbon dioxide. Digester pressures are usually limited

to 8–10 bar, which means that gas must be released through the relief pressure

valve during the entire cooking phase. New cooking digesters are designed to

operate at higher pressures (>12 bar), which is an effective measure to further

reduce cooking time.

4.3.3

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