318
INDUSTRIAL AND ENGINEERING CHEMISTRY
condensation assumption is entirely out of line; according to it the dephlegmator should have given as much fractionation as or more than the entire column actually did. The dephlegmator was equivalent to about one-half a theoretical plate. It is entirely possible, however, that a dephlegmator could be equivalent to more than one theoretical plate, in which case both the first two assumptions (no fractionation and simple condensation) would give plate efficienciestoo high. The dephlegmator efficiency in this particular case appears to be dependent on both the operating conditions and the system used. It increased somewhat with increasing reflux ratio, as did the plate efficiency in the methanol-water distillations. Both plate efficiency and dephlegmator efficiency were lower for the acetone-water distillation, however. It is evident that the efficiency of dephlegmators in general will be dependent on a large number of factors such as construction, operating conditions, materials being rectified, mechanism of the process, etc. It is not possible a t present to predict the efficiency of a dephlegmator with any degree of confidence because of lack of data on all of these factors. The method of testing them as outlined in this article offers a means of observing the effect of these variables on the dephlegmator efficiency. Despite the opinion of many investigators that the dephlegmator is uneconomical as a means of fractionation, it is widely used and more data should be available for its proper design.
VOL. 30, NO. 3
Nomenclature
D V
distillate, Ib. moles/hr. ascending va or, lb. moles/hr. L reflux, lb. moyes/hr. ZD = mole fraction of more volatile component in product z~ = mole fraction of more volatile component in reflux z1, ZZ, . . zn= mole fraction of more volatile component in liquid on plates 1, 2, . . n YD = mole fraction of more volatile component in vapor leaving dephlegmator y1, ya, . . . yn = mole fraction of more volatile component in vapor above plates 1, 2, . . . n yl* = composition of vapor in equilibrium with z1 y ~= * composition of va or in equilibrium with zg R = reflux ratio = ~ / f ; = = =
.
.
Literature Cited Badger, W. L., and MoCabe, W. L., “Elements of Chemical Engineering,” pp. 344, 360 (1936). Fischer, W., Arch. Wtirmewirt., 14,217-19 (1933). Hirsch, A., J. IND. ENQ.CHEM.,2, 409 (1910). Kirsohbaum, E., Chem. Fabrik, 3,181,189 (1930);7,109 (1934) Lewis, W.K.,J. IND. ENQ.CHBM.,1, 522 (1909). MoCabe, W. L.,and Thiele, E. W., Ibid., 17, 605 (1925). Murphree, E. V.,Ibid., 17,747,960 (1925). Perry, J. H., Chemical Engineer’s Handbook, p. 1202, New York, McGraw-Hill Book Co., 1934. Underwood, A. J. V., Trans. Inst. Chem. Engrs., 10,141 (1932) Young, Sydney, “Distillation Principles and Processes,” pp. 272, 309,403 (1922). RECEIVED August 14,1937.
BLEACHED SULFATE PULP HERVEY J. SKINNER Skinner & Sherman, Inc., Boston, Mass.
T
HE bleaching of kraft or sulfate pulp has been carried on in European and, to some extent, in North American mills for about fifteen or twenty years. The products from these mills have been relatively low in color and not comparable with the usual grades of bleached sulfite pulp. In recent years considerable research and development have been carried on both in this country and abroad with a view to producing a bleached sulfate pulp of a color that will permit it to be used in white papers and therefore to compete with bleached sulfite pulp.
Wood Pulp Processes Wood pulp may be separated into two general classifications-namely, mechanical or ground wood pulp and chemical wood pulp. The former, as the name implies, is made by the purely mechanical process of grinding the wood by holding it under pressure against specially constructed grindstones. The raw material is spruce or other coniferous wood, and the ground wood pulp is used in newsprint and other products where permanence is not a factor. Chemical wood pulp is classified according to the process by which the wood is treated-namely, sulfite, soda, or sulfate. The sulfite process is the only acid process and consists of cooking the wood in the form of chips with a liquor consisting of bisulfites of lime and magnesia held in solution by an excess of sulfur dioxide. It is used on the coniferous woods such as spruce, hemlock, and pine. The wood pulp made by this process may be used as unbleached sulfite, which is somewhat straw colored, or it may be bleached and used as bleached sulfite.
The soda process involves cooking the chips with a solution of caustic soda and is used almost exclusively on the broadleaf woods such as poplar, cottonwood, etc. The process employs relatively large amounts of caustic soda which must be recovered by evaporation of the cooking liquor, conversion of the alkali to carbonate by burning, and causticizing with lime, and the loss of caustic soda is made up with soda ash. The product is almost universally sold in bleached form. The sulfate process is a modification of the soda process and is similar to it except that the loss of soda is made up by adding salt cake or sodium sulfate instead of the more expensive soda ash. The sulfate is reduced during the recovery process so that the resultant cooking liquor consists of a mixture of caustic soda and sodium sulfide. The process is particularly adapted for the reduction of the very resinous woods.
Sulfate Process The sulfate process was developed in Europe and made rapid headway in the early part of the present century in Sweden, Norway, and Finland, where large quantities of sawmill refuse formerly used as fuel were available. The resulting pulp was dark in color and resisted the action of the ordinary bleaching agents. The pulp had excellent strength due to its being somewhat undercooked, and came to be known as kraft pulp, from the Swedish word kraft meaning strength. Because of its strength and the difficulty of bleaching, the use of sulfate or kraft pulp has been chiefly as the basis of kraft wrapping paper and for container board. The
MARCH, 1938
INDUSTRIAL AND ENGINEERING CHEMISTRY
process was introduced into Canada in 1907 and subsequently in the United States where its use was rapidly extended. The terms “kraft” and “sulfate” have been used more or less synonymously, although somewhat incorrectly, when the bleached product is considered. In the bleaching of pulp made by the sulfate process, it is necessary to modify the original kraft cooking process so that there is a difference between the kraft pulp made for wrapping and container board, and the sulfate pulp produced for bleaching purposes. ‘(Bleached kraft” is therefore somewhat of a misnomer, and “bleached sulfate” would be a more proper designation. However, because of the similarity of the terms “bleached sulfite” and “bleached sulfate,” the latter will probably continue to be referred to as “bleached kraft,” which is less likely to be confused with sulfite. Kraft pulp can be semi-bleached by the conventional method used in the,paper industry of bleaching with calcium hypochlorite or ordinary bleaching powder. Such treatment results in a pulp which is slightly inferior to unbleached sulfite in color and retains most, if not all, of the strength of unbleached kraft. I n order to improve the color further, it is necessary to modify the bleaching process and employ two or more stages, the first stage being direct chlorination. Kraft pulp bleached in this manner results in a product which in color, cleanliness, and strength, and also cost of production, is superior to unbleached sulfite. However, there has been considerable research on the bleaching of these kraft or sulfate pulps to a degree of whiteness which would compare with bleached sulfite and still retain the strength of the sulfate. The bleaching of kraft to higher whites which are comparable with bleached sulfite involves several stages in bleaching, in fact, it becomes necessary to go back to the cooking process. The requirements of cleanliness in the case of the brown kraft paper products are relatively low, with the result that . little attention has been paid to the condition of the wood in regard to dirt, decay, freedom from bark, etc. I n the manufacture of a bleached pulp, cleanliness and uniformity are vitally important. Only sound wood, as free from knots as possible, and thoroughly freed from bark, should be used. The chips should be uniform and every precaution should be taken to keep dirt and foreign particles from entering the digester. Iron is particularly troublesome since any rust from the digester or other equipment is converted to sulfide by the cooking liquors, and in the bleaching process is changed to ferric compounds which fix themselves to the fibers, producing a yellowish tint difficult to remove. The brown color of pulp obtained by the sulfate process is due to a certain class of dyes formed from the tannins and accentuated by the sulfides used in the cooking process. These dyes are fairly easily bleached to a light straw color but resist with considerable tenacity further action of the bleaching agent. However, they are soluble to some extent in alkali. I n the ordinary kraft cook the caustic concentration is allowed to drop towards the end of the cooking period, with the result that these dyes are reabsorbed by the fibers. In preparing sulfate pulp for bleaching, it is essential to adjust the concentration of the cooking liquors so that after cooking, a relatively high concentration of caustic and a low concentration of digestion products will be obtained.
Bleaching When the product is to be bleached, the cooking process is carried out a t a lower temperature and for a longer period,
and in order to maintain the proper caustic concentration, a greater quantity of caustic is required. This means that increased digester capacity over that normally used is required. After being cooked, the pulp must be completely
319
defibered, since fiber bundles are cliflicult to bleach. The pulp must be thoroughly washed and screened with finer cut screen plates than are necessary with ordinary kraft pulp. The bleaching of the sulfate or modified kraft pulp involves at least three stages. The first stage is chlorination and involves passing chlorine into the stock. The chlorine reacts with the lignin rather rapidly and sets free hydrochloric acid. Chlorine also acts as an oxidizing or bleaching agent in the presence of water, and after the first rapid reaction, a secondary reaction takes place which proceeds a t a slow rate and results in a partial bleaching of the color. It has been found that, by increasing the addition of the chlorine progressively, the bleaching process is less drastic and results in less deterioration of the strength of the pulp. Sometimes lime is added to control the pH of the pulp so that any unconsumed chlorine will be converted to hypochlorite. The products of this chlorination are not very soluble in water but are readily soluble in dilute caustic alkali. The next step, therefore, is to extract with caustic soda. The third stage is a true hypochlorite bleach, using the conventional methods employed in the pulp industry, although in order to obtain a pulp of the desired physical and chemical properties, it is important to maintain a proper pH control. Thorough washing of the pulp after each stage of the bleaching process is very essential since there is a tendency for sulfate pulp to lose its brightness of color on drying and in storage. Inasmuch as the stock from the last stage is alkaline and the last traces of alkali are difficult to remove by washing, it is necessary to adjust the pH of the stock to less than 7.0 and to rewash after a soaking period. Chlorine water has been found valuable for this pH adjustment, since it provides the acid and also maintains an oxidizing action until all the products have been removed. The bleaching process naturally lowers the strength of the raw or kraft pulp, but if properly carried out, the resulting product should be stronger than other pulps of equal brightness. The cost of producing unbleached kraft is only slightly less than that of unbleached sulfite, and, in order to obtain a product comparable in color to the unbleached sulfite, the kraft must be a t least semi-bleached. This additional process offsets any cost advantage of the kraft process so that in many localities the only justification for bleaching is to obtain a pulp of superior strength.
Sulfate Pulp in the South In the southern part of the United States the situation is quite different, and the bleaching of sulfate pulp is of especial importance. The principal raw materials of this section are the several species of pine which are not easily reduced by the sulfite process. Because of their resinous nature they are best treated by the sulfate process, and the pulp has been used almost entirely for the manufacture of kraft products. The manufacture of pulp and paper from the southern pines began over thirty years ago. Like all pioneer developments, the progress was slow, many mistakes were made, and much experience had to be gained. For a long time kraft made from southern pines was considered inferior to that made from the northern species; but in recent years the southern product has been so much improved that expansion on a large scale has taken place, and the industry has grown from practically nothing in 1912 to an estimated mill capacity for 1938 of 1,500,000 tons. Within the last year or so, ten new pulp mills have been put into service. By the end of 1938 there will have been added in two years, mills capable of making 1,200,000 tons of kraft or sulfate pulp from southern pine wood. The greater part of this increased tonnage will be used in the manufacture of bag and wrapping paper and for liners.
INDUSTRIAL AND ENGINEERING CHEMISTRY
320
for corrugated container board, but a portion of it will be diverted to bleached products. The relatively low cost of wood and labor in the South give this section a distinct advantage over the producer of sulfite pulp in other sections, except possibly the Pacific Coast, where mills are a t a disadvantage in eastern markets because of transportation costs. Semi-bleached sulfate pulps made in the South can therefore compete with unbleached sulfite from the northern mills, and their manufacture has already begun. Until recently, bleached sulfite has been used almost exclusively in the white grade of paper, such as book and writing papers. The development of processes for bleaching sulfate to a satisfactory color for these grades of paper brings it into direct competition with unbleached sulfite and offers the additional feature of greater strength. The development of these processes for bleaching sulfate pulp will enable the southern mills to extend their operations and permit them to manufacture pulp of high color, a field which has heretofore been denied them.
VOL. 30, NO. 3
about 260,000 in 1937. Several large mills for the manufacture of bleached sulfate have either started operations recently or will do so in the near future. One of these is located on the Pacific Coast, one in the Lake States, and three in the Southern States. The three southern mills will have a yearly production of 150,000 tons, or an increase of nearly 50 per cent over the 1936 imports. The South has a definite cost advantage over the foreign mills, and the main incentive in erecting these new mills has been to make the paper industry of the United States independent of the more expensive imported kraft or sulfate pulp. In 1925 the amount of bleached sulfate used in this country was slightly over 5 per cent of the total amount of bleached pulp (sulfite and sulfate). In 1936 this had increased to approximately 15 per cent. This trend is shown by the following tonnage figures: Year 1925 1936
--Bleached Produced 612,576 1,150,000
SulfiteImported 321,413 512,168
--Bleached Produced 31,797 156,800
SulfateImported 19,509 102,375
Production Statistics According to the statistics of the United States Pulp Producers? Association, the production of bleached sulfate pulp in the United States has increased from about 30,000 tons in 1925 to 156,800 in 1936. During the same period the imports of bleached sulfate have increased from 19,509 to 102,375 tons. The consumption of bleached sulfate in the United States a t the end of 1936 was therefore approximately 260,000 tons. A survey of the mill capacity for this grade of pulp estimates an increase from around 170,000 tons in 1935 and 1936 to
The extent to which the southern woods can extend this competition will depend in some measure on further chemical research in improving existing pulping and bleaching methods. Not so many years ago the manufacture of white paper from kraft pulp would have been considered impossible, yet today it is being done. It may not be too optimistic to expect that chemical research will develop improvements that will enable bleached sulfate to replace other types of wood pulp in the manufacture of cellulose products other than paper. RHlCEIVBD November 23, 1937.
Line Coordinate Chart for Vapor Pressures of Organic Solvents D. S. DAVIS Wayne University, Detroit, Mich.
ARDNER and Brewer' presented valuable data on the
G
vapor pressures of eleven commercial high-boiling organic solvents. The samples represented technical products in actual use and were riat specially purified. Their values are in agreement with fragmentary data which exist in the literature, and it appears worth while to present the new data in a compact and readily usable line coordinate chart. As required by the integrated Clausius-Clapeyron equation, the data plot as straight lines on paper ruled log P ws. 1/T so that the equation for each set is of the form log P = A
- B/T
suggesting the diagram shown in Figure 1 on the opposite Page. Values of constants A and B were determined by the method of averages, discarding occasional pairs of values which appeared to be in error. Table I lists the constants. The line coardinate chart was prepared from these constants, and its use is illustrated as follows: What is the 1 IND.
ENQ.CEBM.,29, 179 (1937).
vapor pressure of benzyl acetate a t 90" C.? Connect 90 on the temperature scale with point 1 and note the intersection with the pressure scale a t 9.2 mm., the desired value. SimiIarIy, what is the boiling point of Hexalin? Connect 760 on the pressure scale with Hexalin point 8 and note the intersection with the temperature scale a t 157" C. TABLEI Organic Solvent Benzyl acetate Benzyl alcohol But 1 carbitol Cargitol Deaalin
A 8.557 9,133 9.684 9.336 8.049
1000 E 2.770 2.965 3.328 3.001 2.389
Organic Solvent Dibutyl phthalate Dimethyl phthalate Hexalin Terpenyl acetate Terpineol Te tralin
A 6.455 9-209 9.337 9.408 9.432 8.235
1000 E 2.662 3.452 2.779 3.224 3.137 2.567
The legend a t the bottom of Figure 1 lists the solvents, their identification numbers? and the limits of the temperature ranges. RECEIVE~D June 1, 1937.
