Kraft process

Fig. 11. Flow diagram for the beater additive process. Kraft represents the kraft process wood pulp and NC is nitrocellulose used as starting materials...

The beater additive process starts with a very dilute aqueous slurry of fibrous nitrocellulose, kraft process woodpulp, and a stabilizer such as diphenylamine in a felting tank. A solution of resin such as poly(vinyl acetate) is added to the slurry of these components. The next step, felting, involves use of a fine metal screen in the shape of the inner dimensions of the final molded part. The screen is lowered into the slurry. A vacuum is appHed which causes the fibrous materials to be deposited on the form. The form is pulled out after a required thickness of felt is deposited, and the wet, low density felt removed from the form. The felt is then molded in a matched metal mold by the appHcation of heat and pressure which serves to remove moisture, set the resin, and press the fibers into near final shape (180—182). 

Kraft Process. The dominant chemical wood pulping process is the kraft or sulfate process. The alkaline pulping Hquor or digesting solution contains about a three-to-one ratio of sodium hydroxide, NaOH, and sodium sulfide, Na2S. The name kraft, which means strength in German,... 

PolysuWde Process. One modification to the kraft process being appHed commercially is the polysulfide process (38). Under alkaline conditions and relatively low temperature (100—120°C), polysulfides oxidize the active end group of the polysaccharide polymer to an alkaH-stable aldonic acid. This reaction, known for many years (39), was not produced on a commercial scale until the development of an efficient method for in situ generation of the polysulfide in kraft white Hquor. 

A high yield chemical pulp, eg, 52—53% bleached yield from softwoods, can be obtained, but strength properties ate inferior to those obtained from the kraft process. If a protector, eg, potassium iodide, is added, an additional 2—3% yield is obtained, as is an improvement in all strength properties. The gas penetration problem can be minimized if ftbetization is accompHshed before treatment with oxygen. Oxygen treatment of virtually all types of semichemical and mechanical pulps has been explored (55). Caustic, sodium bicarbonate, and sodium carbonate have been used as the source of base (56,57). In all cases, the replacement of the kraft by these other processes has not been justified over the alternative of pollution abatement procedures. 

P. Sandstrom, H. Lundberg, and A. Teder, Development of Modified Kraft Process with a Mathematical Modelfor Continuous Digesters, SCAN Eorest Report No. 441, Swedish Eorest Products Lab, Stockholm, Sweden, 1985 A. Teder and P. Sandstrom, TappiJ. 68(1), 94—95 (fan. 1985). 

Kraft Process. Wood derived from softwood sources is generally heated under pressure with a 10—20% mixture of NaOH and Na2S for only... 

FIG. 22-22 Schematic diagram of the Kraft process for producing decaffeinated coffee using supercritical carbon dioxide (McHugh and Ktukonis, op. cit.). 

The most widely used pulping process is the kraft process, as shown in Fig. 6-11, which results in recovery and regeneration of the chemicals. This occurs in the recovery furnace, which operates with both oxidizing and reducing zones. Emissions from such recovery furnaces include particulate matter, very odorous reduced sulfur compounds, and oxides of sulfur. If extensive and expensive control is not exercised over the kraft pulp process, the odors and aerosol emissions will affect a wide area. Odor complaints have been reported over 100 km away from these plants. A properly controlled and operated kraft plant will handle huge amounts of material and produce millions of kilograms of finished products per day, with little or no complaint regarding odor or particulate emissions. 

The kraft process has become the dominant process for pulp production throughout the world, primarily because of the recovery of the pulping chemicals. A schematic diagram of the kraft pulping process, with the location of atmospheric emission sources, is shown in Fig. 6-11. 

Three major sources in the kraft process are responsible for the majority of the H2S emissions. These involve the gaseous waste streams leaving the recovery furnace, the evaporator and the air stripper, respectively denoted by R), R2 and R3. Stream data for the gaseous wastes are summarized in Table 8.8. Several candidate MSAs are screened. These include three process MSAs and three external MSAs. The process MSAs are the white, the green and the black liquors (referred to as Si, S2 and S3, respectively). The external MSAs include diethanolamine (DBA), S4. activated carbon, Sj, and 30 wt% hot potassium carbonate solution, S6. Stream data for the MSAs is summarized in Table 8.9. Syndiesize a MOC REAMEN that can accomplish the desulfurization task for the three waste streams. 

We focused our attention on Tall oil, a by-product of the paper industry, whenever this is prepared according to the KRAFT process. Said material consists of a mixture of highly unsaturated fatty acids (many of which with conjugated diene systems) and terpene derived rosin acids. The rosin acids have the molecular formula C20H30O2 and thus belong to the diterpenes (pimaric and abietic acids). Tall Oil has an iodine number equal to approximately 170 gl2/100 g. 

Highly bleached and purified kraft process wood pulp suitable for conversion into products such as rayon, viscose, acetate, and cellophane... 

Bleached or unbleached kraft process wood pulp usually converted into paperboard, coarse papers, tissue papers, and fine papers such as business, writing and printing... 

The kraft process evolved from the soda process. The soda process uses an alkaline liquor of only sodium hydroxide (NaOH). The kraft process has virtually replaced the soda process due to the economic benefits of chemical recovery and improved reaction rates (the soda process has a lower yield of pulp per pound of wood furnish than the kraft process). 

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