Cooking (Heating-up, Maintaining at Cooking Temperature)

Modern sulfite digesters practice indirect heating; this involves circulation of the

cooking liquors through external heat exchangers with the aid of circulation

pumps, which draw off cooking liquor through strainers in the digester walls and

return the heated liquor at appropriate inlets. Circulation systems ensure a more

even temperature profile as compared to direct heating systems. In acid sulfite

cooking, the rate of heating should be low to allow a homogeneous distribution of

active cooking chemicals within the wood structure (see Section 4.3.3, Impregnation).

Thus, heating rates should be kept in the range between 0.2 °C and

0.4 °C min–1. On increasing the temperature, the pressure increases, until top gas

relief is started at a preset pressure level, approximately 2–3 bar below the design

pressure of the digester. Top gas relief is typically adjusted at a pressure of about

8.5 bar abs (see Fig. 4.157), and consists of vapor containing H2O, SO2, CO2, O2,

N2, and volatile organic compounds depending on the wood source (hardwoods –

acetic acid, furfural, etc.; softwoods – p-cymene from a-pinene). Due to the high

content of SO2, the top gas relief is recycled to the hot accumulator acid, where it

fortifies the raw cooking acid. The pressure determines the cooking liquor composition

and therefore the rate of cooking. A high pressure maintains a high sulfur

dioxide concentration and results in a rapid sulfite cook. An appropriate cooking

control is more important for a sulfite cook as compared to a kraft cook. Although

a large amount of the cooking chemicals are consumed, the acidity of the cooking

liquor increases toward the end of the cook due to progressive formation of strong

acid anions, [A– ], and the simultaneous consumption of combined SO2, [HSO3

– ].

Hence, the concentration of hydrogen ions increases and the rate of the carbohydrate

hydrolysis accelerates. The final cooking phase is very important for the production

of high-purity, low-viscosity dissolving pulps. Since hypochlorite bleaching

has been replaced by chlorine-free bleaching stages (e.g., ozone, hydrogen peroxide),

viscosity control is predominantly carried out during the final cooking

phase.