Pulp yield

Wood Delignijication. The production of wood pulp (qv) for the paper (qv) industry consists of removing lignin (qv) from wood chips, thus freeing the ceUulose fibers. An aqueous solution containing 30—70 wt % sulfolane efficiently extracts the lignin from aspen. Western hemlock, and Southern pine wood chips. Pulp yields are from 50—75% (43,44). 

Autoclave extraction of sugar beet pulp yields gel-forming pectic hairy regions... 

Removal of Proteinaceous Clouds. With the unchanged method of Averell and Norris (1) the final colored solution of 50.0 ml. may contain as much as 20% ethyl alcohol. Many plant materials, particularly apple pulp, yield proteinaceous substances to the benzene during extraction if the aliquot of extract represents more than 40 to 50 grams of sample, these substances will be present in this final solution as a cloud of solid particles. These substances, whatever their nature, appear to be completely soluble in 60% ethyl alcohol. Thus, the authors recommend the addition of 50 ml. more of ethyl alcohol at this stage, the addition of more hydrochloric acid to maintain pH, and final dilution to 100.0 ml. with water. The resulting solution will be optically clear except for oily or waxy substances, which are removed in the next step. 

Several pulp mills now use AQ on a commercial basis (Oloman 1996). According to Oloman, the addition of AQ to pulping liquor in an amount of 1 kg AQ per ton of pulp increases the pulp yield from starting woods by abont 1%. This complementary action gives rise to increased revenue of about 2 million per year for a pnlp mill with a daily prodnctivity of 10001. 

Wood is converted to pulp by mechanical and chemical actions, which constitute the pulping process. Their selection depends upon the type of wood supply available and the pulp qualities desired. Pulps can be characterized on the basis of the unbleached pulp yields achieved by the pulping process used, i.e.. the yield of oven-drv (OD) pulp obtained from oven-dry debarked wood. 

Extractives sometimes prevent or inhibit the utilization of wood. For example, woods containing phenolic type extractives cannot be pulped via the sulfite process. The so-called "pitch troubles" in the pulp and paper industry result from the tendency of the resin type extractives to coagulate and adhere to metal and fibrous surfaces. Also the presence of extractives result in a higher consumption of pulping chemicals and in lower pulp yields. 

The actual mechanisms of Kraft delignification are highly complex, revolving around the ionization of acid phenolic units in lignin by OH and nucleophilic displacement of lignin units with HS. The chemistry of delignification is reviewed in detail elsewhere. It is sufficient to note here the conditions specified above and the pulp yields typically 45-55% of the dry weight of wood furnish is produced as Kraft pulp. 

If a papermill produced paper with a basis weight of 65 g/m2 (40 lb/3000 ft2) at a level of 1000 tonnes per day, a paper machine 7 m wide would have to run at speeds in excess of 1500 m/min. (almost 60 miles per hour) to produce this output. Most mills of this size would have more than one paper machine. Any upsets in the papermaking process are very costly. Any incremental improvements made in pulping yield must be matched by increasing the paper machine speeds to utilize the extra furnish because paper machines are too expensive to replace. 

Method pH range "Base" alternatives Active reagents Max. temp. (°C) Time at max. temp, (hr) Softwood pulp yield (%)... 

The increase in pulp yield may reach 8% on dry wood basis, but requires high pressures (> 1000 kPa) and a large excess of hydrogen sulfide (ca. 10% of wood). Only a fraction of the hydrogen sulfide (1 -2% of wood) is consumed and the rest is recoverable. Table 7-7 illustrates the influence of some oxidizing and reducing agents on the carbohydrate yield of kraft pulp. Stabilization with anthraquinone is dealt with in Section 7.3.7. 

Process Acronym Chemical Mechanical Pulp Yield (%)... 

In summary, modern pulping processes may be considered to be chiefly mechanical, mechanochemical (semichemical), and chemical processes. The classification in terms of pulp yield is depicted in Figure 1. 

Figure 1. Modern pulping process vs. pulp yield...

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