The digester is a special piece of machinery used in the process of papermaking. Its purpose is to cook small wood chips for several hours in order to soften them. These softened chips are then passed to a machine that whitens them to the desired shade. The digester itself can be horizontal, upright, revolving, spherical, or cylindrical.
Before the wood is ran through the digester, it must undergo certain preparations. First, the harvested trees are cut into logs. Generally, these logs are anywhere from four to eight feet in length. The logs are then sent to a debarking drum, which is very large and in the horizontal position. This debarking drum rotates and strips all of the bark away from the logs. The bare logs are then fed to a chipper. Here, the logs are reduced to handheld sized chip pieces.
After the logs are made into wooden chips, they are placed inside the digester where they are chemically cooked for several hours in order to make them softer. There are two types of digesters: sulphite and sulphate. The sulphite digester utilizes calcium acid sulphite to aid in the process, which is usually referred to as the acid process. The sulphate process, also known as the Kraft process, uses an alkaline system instead. The Kraft process is newer than the sulphite process, but it is used by more companies because it is less corrosive and more efficient.
Pulp and paper companies continue to work on more efficient digester processes. Therefore, they are looking at what is called the “total fiber management” approach. With this approach, more additives are included in the digester during the cooking process. The purpose of these additives is to attempt to reduce the number of rejected wood chips and to reduce the Kappa number, which indicates how much bleach is necessary to achieve the desired whiteness in the paper.
The goal of total fiber management is to also reduce the amount of time needed to cook the wood chips in the digester, as well as to lower the cooking temperature. The ultimate goal is to reduce production time as well as the amount of more expensive materials needed to produce paper. Current research has indicated that surfactant-based digester additives may be helpful in reaching these goals.
Surfactant-based digester additives improve the efficiency at which cooking liquor penetrates wood. As a result, the chips are defibered more quickly and the cooking liquor is diffused more efficiently into the chips. This is accomplished by reducing the surface tension between the wood and the liquor, which helps the chips become more thoroughly wetted by the chemicals. This results in a more uniform cooking of the chips and an increase in the amount of useable pulp produced.
The digester is an integral machine in the making of paper. Without it and the chemicals used within it, paper would not be able to be produced as quickly or as efficiently. In addition, the quality of the paper would suffer without the benefit of being cooked and treated in the digester.