There are two main processes used in cooking - sulphate and sulphite, each designed to produce pulp with specific characteristics.
Both processes are basically the same. In both cases the wood chips are impregnated with cooking liquor (known as white liquor), heated under pressure to 130-170 °C and held at that temperature for a specified period. Then the pressure and consequently the temperature are lowered and the pulp is blown or washed out of the digester.
There are two main types of process equipment for cooking – the continuous digester and the batch digester. The cooking process for both types of digesters is very similar and while each type has its advantages, most new pulp plants around the world are now built with continuous digesters.