Pulping operations cooking

Paper Products. Paper (qv) products account for about 2% of sulfur demand. The largest single segment of demand is in the manufacture of wood pulp by the sulfite process (see Pulp). In this process, the main sulfur intermediate is sulfur dioxide, which is generally produced at the plant site by burning elemental sulfur. Some sulfur dioxide, however, is produced as a by-product at smelter operations, purified andUquefied, and shipped to the pulp mills. The sulfur dioxide is converted to sulfurous acid, and the salt of this acid is a principal component of the cooking Hquor for the sulfite process. 

The increasing demands on the pulp and paper industry to reduce emissions from bleaching operations has lead to modifications of the cooking process to provide a pulp that requires a decreased amount of bleaching. This is accomplished by decreasing the amount of residual... 

Sulfite Process. The sulfite process uses a cooking liquor of sulfurous acid and a salt of the acid. Although calcium was the most widely used base at one time, it has been supplanted by sodium, magnesium, and ammonia. Lignin will react with the bisulfite ion (HSO3") under acidic conditions to form lignosulfonates that are soluble in water. For many years this was the preferred process because it produced pulps of light color that could easily be bleached, it used cheap chemicals in fairly limited amounts so that no recovery was necessary, and it was a relatively simple process to operate. 

The long warm up period contributes little to the H factor because reaetion kinetics dictate that the rate of delignification more than doubles for every 10°C rise in temperature. The equation is useful as it allows mill operators to determine when to stop the cook even though the operating temperatures have differed from normal, for example because of fluctuations in steam supply to the digesters. Operators merely have to ensure that they cook to a constant H factor (Figure 13.12b), as pulps eooked to the same H factor have essentially the same properties, i.e. yield, lignin eontent (kappa number). Results can be compared provided the initial alkali and sulphidity eoneentrations are the same. 

Unless an especially coarse or hard graininess of texture is desired, flavor is likely to be best in fully matured and well-ripened fruit. Quite a bit of the volatile flavor that maturation and ripening develop is lost in cooking by a sort of steam-distillation of the pulp. This involves a heat transfer-efficient direct contact of steam with finely-cut apple, followed by centrifugal pulping through a screen while still very hot. However, a well-made sauce should retain some aroma if these operations are effectively enclosed. 

There are three different techniques to be observed in tapa-making. The most sophisticated method consists of three steps. In the first, small strips of bast are cut and cooked for several hours in suds of wood ash. This cooking is very similar to the basic operation of our alkaline pulping. Then the strips are rinsed, placed together on a wooden board and beaten with a mallet, thus forming a small sheet on the board. The third step consists of drying and smoothing. 

---