The Kraft Pulping Process

Kraft pulping may be earried out in either bateh or eontinuous digesters, the use of which has been described earlier. The traditional bateh operation is simpler to describe operationally. 

C) In the absence ot sulphur, l e soda pulping, alkali stable structures are formed without cleavage at the P-carbon atom 

The H faetor was derived from the kinetics of kraft pulping by Vroom (1957) as follows. Under eonditions of eonstant alkali concentration (high liquor-to-wood ratio) kraft delignifieation obeys first order kinetics  

When the values for the relative delignification rate are plotted against eooking time (in minutes), the area under the curve corresponds to the total amount of delignifieation that has oeeurred and this is expressed by a term ealled the H factor  

The H factor is important as it combines the variables of temperature and time as a single number. A normal eook to low kappa number requires an H factor of 1500 to 1800 units of which heating to temperature contributes only about 180 units. 

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The composition of common fats and oils are found in Table 1. The most predominant feedstocks for the manufacture of fatty acids are tallow and grease, coconut oil, palm oil, palm kernel oil, soybean oil, rapeseed oil, and cottonseed oil. Another large source of fatty acids comes from the distillation of cmde tall oil obtained as a by-product from the Kraft pulping process (see Tall oil Carboxylic acids, fatty acids from tall oil). 

Tall oil fatty acids (TOFA) consist primarily of oleic andlinoleic acids and are obtained by the distillation of crude tall oil. Crude tall oil, a by-product of the kraft pulping process, is a mixture of fatty acids, rosin acids, and unsaponiftables (1). These components are separated from one another by a series of distillations (2). Several grades of TOFA are available depending on rosin, unsap oniftable content, color, and color stabiUty. Typical compositions of tall oil fatty acid products are shown in Table 1 (see Tall oil). 

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. 

Fig. 6-11. Schematic diagram of the kraft pulping process (6). 1, digester 2, blow tank 3, blow heat recovery 4, washers 5, screens 6, dryers 7, oxidation tower 8, foam tank 9, multiple effect evaporator 10, direct evaporator 11, recovery furnace 12, electrostatic precipitator 13, dissolver, 14, causticizer 15, mud filter 16, lime khn 17, slaker 18, sewer.

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. 

Tall oil rosin is obtained from crude tall oil obtained from the Kraft (sulphate) pulping of various coniferous trees in the paper manufacturing industry. During the Kraft pulping process the fatty acids and the resin acids from the coniferous wood are saponified by the alkaline medium. On concentration of the resulting pulping liquor, the sodium soap of these mixed acids rises to the surface from where they are skimmed out. By acidification of this material with sulphuric acid, the crude tall oil is obtained. Fractional steam distillation of the crude tall oil allows the separation of the tall oil fatty acids and the tall oil rosins [21]. 

Ibbie 8.1 Data for the Gaseous Emission of the Kraft Pulping Process ... 

Consid - the Kraft pulping process shown in Fig. 8.8 (Dunn and El-Halwa, 1993). The first step in the process is digestion in which wood chips, containing primarily lignin, cellulose, and hemicellulose, are cooked" in white liquor (NaOH, Na2S, Na2C03 and... 

Figure 8.8 A simplified flowsheet of the Kraft pulping process (Dunn and ETHalwagi, 1993).

Black liquor is 13 to 17% strength, rinsed extract from washed and cooked woodchip pulp, produced in the Kraft pulping process. This... 

Lignins are most usually employed as various types of sodium lig-nosulfonate and are products of sulfite pulping. (Kraft lignins from the Kraft pulping process are also available.) They are used as dispersants, binders, sequestrants, and emulsifiers. Sodium lignosulfonate product variables include color, MW, and the degree of carboxylation and sulfonation on the lignin backbone. 

The raw materials for the manufacture of soap, the alkali salts of saturated and unsaturated C10-C20 carboxylic acids, are natural fats and fatty oils, especially tallow oil and other animal fats (lard), coconut oil, palm kernel oil, peanut oil, and even olive oil. In addition, the tall oil fatty acids, which are obtained in the kraft pulping process, are used for soap production. A typical formulation of fats for the manufacture of soap contains 80-90% tallow oil and 10-20% coconut oil [2]. For the manufacture of soft soaps, the potassium salts of fatty acids are used, as are linseed oil, soybean oil, and cottonseed oil acids. High-quality soap can only be produced by high-quality fats, independent of the soap being produced by saponification of the natural fat with caustic soda solution or by neutralization of distilled fatty acids, obtained by hydrolysis of fats, with soda or caustic soda solutions. Fatty acids produced by paraffin wax oxidation are of inferior quality due to a high content of unwanted byproducts. Therefore in industrially developed countries these fatty acids are not used for the manufacture of soap. This now seems to be true as well for the developing countries. 

Another natural source for fatty acids (mostly CI8) is tall oil, which is a byproduct of the kraft pulping process. 

Foster, R., et al., Optimization of the Chemical Recovery CVcle of the Kraft Pulping Process, Laboratory for Applied Industrial Control, Report 54. Purdue University, West Lafayette, IN, 1973. 

FIGURE 21.4 The kraft pulping process (with chemical recovery). (Taken from U.S. EPA, Profile of the Pulp and Paper Industry, 2nd ed., report EPA/310-R-02-002, U.S. EPA, Washington, November 2002.)... 

During the kraft pulping process, the first step in making hundreds of paper products, crude tall oil is obtained from the alkaline material by acidifying it with sulfuric acid. The crude tall oil is then converted through dehydration, dry distillation, and finally the fractionation of the vaporized tall oil compounds. Fatty acids, rosin acids, and neutral materials make up tall oil. 

Dimethyl Sulfoxide, Dimethyl sulfoxide is manufactured from dimethyl sulfide (DMS), which is obtained either by processing spent liquors from the kraft pulping process or by the reaction of methanol or dimethyl ether with hydrogen sulfide,... 

The oleoresinous exudate or "pitch of many conifers, but mainly pines, is the raw material for the major products of the naval stores industry. The oleoresin is produced in the epithelial cells which surround the resin canals. When the tree is wounded the resin canals are cut. The pressure of the epithelial cells forces die oleoresin to the surface of die wound where it is collected. The oleoresin is separated into two fractions by steam distillation. The volatile fraction is called gum turpentine and contains chiefly a mixture of monoterpenes but a smaller amount of sesquiterpenes is present also. The nonvolatile gum rosin 5 consists mainly of llie dilerpenuid resin acids and smaller amounts of esters, alcohols and steroids. Wood turpentine, wood rosin and a fraction of intermediate volatility, pine oil are obtained together by gasoline extrachon of the chipped wood of old pine stumps. Pine oil is largely a mixture of the monoterpenoids terpineol. borneol and fenchyl alcohol. Sulfate turpentine and its nonvolatile counterpart, tall oil, 5 are isolated as by-products of the kraft pulping process. Tall oil consists of nearly equal amounts of saponified fatty acid esters and resin acids. 

Lime is also used in the kraft pulping process for the purpose of regenerating caustic soda (sodium hydroxide). 

There were good intentions when a small chemical terminal in Savannah, Georgia, that supplied the paper and pulp industry decided to change their product mix. Their terminal was to be converted from handling all nonflammable chemicals to storing large quantities of flammable crude sulfate turpentine (CST). Crude sulfate turpentine is an impure form of turpentine produced as a byproduct of the Kraft pulping process. It is classified as a Class IC flammable liquid. 

Dimethyl sulfide (DMS) is recovered from the waste black liquor in the kraft pulping process by flash drying of the liquor after the DMS content has been increased by additions of sulfur. 

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