Principles of Displacement Cooking

The basic idea of displacement cooking is to store process liquors from one cook

and to re-use their heat and cooking chemicals in the next cook. For reasons of

supply and demand, liquors need to be stored in accumulators at various levels of

temperature, pressure, and chemical composition. Based on the original displacement

process, technologies have developed over time which utilize the liquors in

the tank farm for alkali and temperature profiling and for in-digester washing.

A typical displacement batch cooking system consists of digesters, tank farm

and discharge tank, as illustrated in Fig. 4.127. At the beginning of the cooking

cycle, wood chips are fed to a digester. Subsequently, liquors of various temperatures

and concentrations are exchanged between the tank farm and the digester

until the wood has been converted to pulp. At the end of the cooking cycle, the

pulp is discharged from the digester into the discharge tank.

Chips

Pulp

Steam

TANK FARM

DIGESTERS

DISCHARGE

TANK

White liquor

Weak black liquor Wash liquor

Liquors

Fig. 4.127 Outline scheme of a displacement batch cooking system.

Wash filtrate from brownstock washing and cooking chemicals in the form of

white liquor enter the tank farm only, and not the digester. Steam is consumed in

the tank farm but may be as well directed to the digester. The weak black liquor

which is not circulated back to the cooking process is subjected to fiber separation

before being transferred to the evaporation plant.