Side Relief and SO2-Dosage

Complete filling of the digester results in a rather high liquor-to-wood ratio (4–5:1

for softwoods, 3–5:1 for hardwoods) which would cause an excessive steam consumption

for heating to maximum temperature, and in addition an unnecessarily

high dilution of the spent liquor. Hence, cooking liquor is withdrawn through a

side relief until a target liquor-to-wood ratio is obtained, provided that liquor circulation

is ensured. Side relief of up to 30% is possible, and this decreases the

liquor-to-wood ratio to about 3.5:1 for softwoods and to 2.5:1 for dense hardwoods.

The following example illustrates the specific amounts of free and enclosed cooking

liquor prior and after the side relief:

Wood source Fagus sylvatica (beech)

Dry beech wood density 0.68 g cm–3

Dry solid density 1.53 g cm–3

Chip packing density 0.28 t m–3 digester

Liquor-to-wood ratio after side relief 2.5:1

Bulk volume in digester Vb = 1/0.28= 3.57 m3 odt w–1

Volume of dry chip Vc = 1/0.68= 1.54 m3 odt w–1

Free volume between chips Vs = (3.57 – 1.54) = 2.03 m3 odt w–1

Enclosed cooking liquor

at full impregnation Vel = (1/0.68 – 1/1.53) = 0.82 m3 odt w–1

Liquor-to-wood ratio

prior to side relief Vl,0 = 2.03 + 0.82 = 2.85 m3 odt w–1

Free volume after side relief Vl,1 = (2.5 – 0.82) = 1.68 m3 odt w–1

As soon as the target liquor-to-wood ratio is reached, the precalculated amount

of liquid SO2 is added into the digester. By keeping the liquor-to-wood ratio constant,

the charge of liquid SO2 controls the total amount of SO2 (and also the

amount of free SO2), whereas the amount of combined SO2 is given by the

amount of active base present in the clarified raw acid.

A typical example illustrates the adjustment of final cooking liquor:

The composition of the cooking liquor prior SO2 charge including the dilution

with water from wood and steam:

Total SO2 45 g L–1

MgO 8 g L–1

Free SO2 19.6 g L–1 [(45 – 8). 3.179]

A target of total SO2 charge of 140 kg odt w–1 corresponds to a concentration of

56 g L–1 total SO2 in case of a liquor-to-wood ratio of 2.5:1, which requires a dosage

of (56 – 45). 2.5 = 27.5 kg liquid SO2 odt w–1. Thus, the final cooking liquor composition

after SO2 charge yields:

Total SO2 56 g L–1

MgO 8 g L–1

Free SO2 30.6 g L–1

Due to the low temperature and the low amount of free SO2 prior to liquid SO2

charge, almost no reactions occur up to this stage of the cook. At this point, the

cooking process can be started by increasing the temperature.