US3497418A - Method for drying fibrous masses - Google Patents

Description

Feb. 24,1970 A, THALE ETAL 3,497,418

' METHOD FOR DRYING FIBROUS MASSES 7 Filed Aug 21, 1967 United States Patent "ice 3,497,418 METHOD FOR DRYING FIBROUS MASSES Allan Thale, Kokkedal, and Olav Bjorn Hovstad, Farum,

Denmark, and Lennart Ivnas, Skarblacka, Sweden, assignors to Aktieselskabet Niro Atomizer, Soborg, Denmark, and Fiskeby Alrtiebolag, Norrkoping, Sweden Filed Aug. 21, 1967, Ser. No. 661,889 Claims priority, application Sweden, Aug. 22, 1966, 11,317/ 66 Int. 'Cl. D21d /00 US. Cl. 162-100 2 Claims ABSTRACT OF THE DISCLOSURE After preconcentration to a content of maximum 65 percent dry matter the cellulose pulp is fiuffed to such a degree that the modified Stoke diameter of the particle when dried to an air dry condition does not exceed 2,0-" g./cm. and preferably does not exceed 1.7-10 g./cm.

This invention relates to a method of drying fibrous pulps, such as groundwood, sulphate, sulphite or semichemical pulps where, subsequent to dehydration to a maximum dry-matter content of 65 percent, the pulp is treated in a device for fiuifing the pulp and then dried, for example by pneumatical means.

Such method forms, by way of example, a stage in the treatment of fibrous material used for the manufacture of paper. Particularly in this case it is of importance to avoid the formation of knots or nodules (in the following the term nodules will be used) in the fibrous material to the highest possible degree, as such nodules will appear as spots or so-called fish eyes in the finished paper.

The formation of nodules is due to many different reasons, mechanical as well as chemical, and before as well as after the flufiing process, and various methods have been suggested for reducing the effect of the various reasons, or for wholly or partially removing said reasons, as well as for removing or dissolving nodules already formed. Since it is not possible to observe with any certainty the nodules until the paper is finished, it is difficult to safely judge the individual reasons for the formation of nodules, or the effects of the measures taken with a view to avoiding such formation of nodules.

It is assumed that during the fluffing process nodules already formed may to some extent be broken down. Observations made in conjunction with the development of this invention seem, however, to indicate that this is not true, or, at any rate, that this is not the primary effect of the fluffing on the number of nodules, as it rather seems to be a fact that the fluffing process may be a very essential reason for the formation of nodules.

This invention is based on the recognition that the particle size and density produced during the fiuffing process are of decisive importance for the formation of nodules, not only directly but also indirectly during the subsequent drying process, as well as during the possible subsequent storing process which, as is known, involves certain ageing processes, during which nodules may develop in the pulp, probably as a consequence of an equalization of a certain irregularity in the distribution of moisture.

The characteristic of the method according to the invention consists in the fiuifing process being carried out in such a way that the modified Stoke diameter d of the particles produced by the fluffing process, is for all 3,497,418 Patented Feb. 24, 1970 of the particles subsequent to drying not more than 2.0- 10- preferably not more than 17-10" g./cm.

Experiments carried out with various types of pulps have shown that in this manner a very low number of nodules is attained in comparison with the number attained with the previously known methods.

Generally, the two factors of the product which in the .usual manner is deignated as the modified Stoke diameter, can hardly be determined in a simple way, but by means of well-known formulae it can be seen that the stated limit of the product means that the terminal falling velocity of the particles must be about 600 cm./ see. as a maximum, preferably 500 cm./sec. as a maximum.

Below, a few examples will be given for illustrating the invention is more detail, but first it should, however, be explained how the determination of the number of nodules can be carried into effect.

In this connection drying of paper pulp will be considered, in that quantitatively paper pulp is the most important product that may come into consideration.

The presence of such nodules in the pulp involves deterior'ation of the quality of the paper manufactured. However, the effect is to a very high degree dependent on the type of paper, notably the thickness of the paper. Dependent on the dimensions of the nodules, the defects may vary from slight blemishes to the complete destruction of the functional qualities of the paper. For practical purposes a nodule may be defined as an accumulation of fibres which, structurally, differs from the surrounding fibres and is visible in the paper manufactured. However, since the effect, as said above, is dependent on the type of paper, this definition is not applicable for unequivocably characterizing the quality of the pulp, not least, because part of the nodules in the pulp may be broken down during the further processing, so that the remaining number of nodules in the finished paper varies with the processing.

With a view to attaining an unequivocal characterization, the following method has in the experiments reported below, been used for determining the number of nodules.

The pulp was heat-treated at C. for 12 hours, enclosed in a plastic bag, whereby the building up of latent nodules was completed. After the heat-treatment the pulp was slushed in a defibrator in the standardized way, and subsequently a sheet of paper was produced and the number of nodules determined. Fibre nodules visible to the eye in a sheet of paper so produced are then defined in an unequivocal manner.

EXAMPLE 1 Wet sulphate pulp was pressed to contain 50 percent of dry matter, whereupon the pulp was fluffed in a mechanical shredder which was so set as to deliver five different densities and particle sizes. The five different samples were later dried in the laboratory in a standardized way to contain percent of dry matter. Next, the terminal falling velocity was determined at room temperature in respect of the five samples, and the velocity was found to be 7.3 m./sec., 6.7 m./sec., 6.0 m./sec., 5.6 m./sec. and 4.9 m./sec. In the dried pulps nodules were so determined that subsequent to the heat-treatment they were slushed in a standard defibrator at 3 percent pulp concentration and, next, laboratory sheets were produced from the pulps, the number of particles not broken dow =nodules was determined. For the different terminal falling velocities and corresponding values of d s, the following nodules numbers were obtained:

Sulphate pulp which was given such density that after drying the terminal falling velocity was 7.5 m./sec. (d :2.5-10- was before drying fiuifed in a pneumatical flutter so that the terminal falling velocity of dried pulp was 3.9 m./sec. (d =1.30-10- g./cm.). The nodule number per 100 grammes of pulp fell from 9,900 to 100.

EXAMPLE 3 In a flash drying plant having a capacity of 10 t./24 h., the shredding and the fiufling were varied so that dried sulphate pulps, having terminal falling velocities of between 3.4 and 6.3 m./sec. were obtained, corresponding to the value of d between 1.13-10- and 2.10-10 g./ cm. The nodule number per 100 grammes of pulp was determined, and conditions similar to those of Example 1 Were obtained. If an acceptable quality of paper is to be regarded as obtained, the nodule number should by way of experience not exceed 700 nodules per 100 grammes of pulp. For the terminal falling velocity of 5.0 m./sec. (ci =l.66-l g./cm.), this nodule limit was obtained, and the number of nodules fell with decreasing terminal falling velocities.

EXAMPLE 4 Experiments similar to those of Example 1 were carried out with sulphite pulp, and the following values were obtained:

For sulphite pulp the same investigations were made as in Example 3 to find out at which maximum terminal falling velocity an acceptable pulp was obtained. It was found out that in this case the terminal falling velocity should not exceed 4.8 m./sec. (d =l.60- g./cm.) to make it possible to produce an acceptable pulp quality.

From the above examples it clearly appears that according to the invention it is possible to produce a fully acceptable pulp quality without being compelled to add expensive chemicals, and that it is possible to dry widely different pulp qualities in one and the same installation.

Further, it will be seen from the examples that relatively uniform results have been obtained, though some of the experiments have been carried out on a laboratory scale and others on an industrial scale, and that it has exerted no influence on the results that in some cases mechanical fiufiing has been used and in others pneumatical fluffing. But the only requirement regarding the fluffing device, is, that it is of such a type as to be adjustable to efiect the defibration in the desired manner. As an example of a mechanical fiutfer suitable for the purpose, reference can be made to the one forming the object of our co-pending patent application No. 647,088, filed on June 19, 1967, under the title Machine for Shredding Fibrous Pulp and Method for Operating Such Machine. A suitable pneumatical fiuffer can be designed as a tube, wherein mechanical obstacles, such as inclined plates or screens are arranged in spaced-apart relationship, or the tube may be provided with restrictions. With such a fiuffer it is possible when applying the required air velocity to obtain a satisfactory defibration of the pulp, without any resulting compression of the fibres, in such a Way that the requirements made in conjunction with the present invention can without difficulty be complied with.

In the following text the invention is explained by way of an illustrated example by means of the drawing which gives a diagrammatic representation of a plant for the drying of cellulose pulp.

Cellulose pulp in the form of a 3.5 percent suspension is fed through conduit 1 to a dewatering press 2 at a rate of 160,000 kg. per hour corresponding to 6,250 kg. of air-dry matter per hour. Press 2 may be of any appr0- priate type, as for instance the well-known Impco press manufactured by Messrs. Improved Machinery Inc., Nashua, NH. The pulp is dewatered or preconcentrated to a concentration of 48 percent in press 2. The concentrated pulp leaves the press through pipe 3 delivering the pulp to a shredder or flufier 4 in which the pulp is shredded or fiuffed so as to obtain the particle size desired, which means that the particles, when subsequently dried to an air-dry condition, i.e. to a content of dry matter of 90 percent, attain a Stokes diameter not exceeding 2.0- 10 g./cm., preferably not more than 1.7-10 g./cm.

In order to obtain such particles, a shredder such as described in our co-pending patent application mentioned above can be used, by means of which any desired particle size, within practical limits, can be obtained by adjusting the depth of engagement between the toothed rollers and their speed of rotation. However, it is also possible to employ an ordinary two-stage device in which the suspension is subjected to a coarse shredding in a shredder following by fiufiing in a fiuffer.

From shredder 4 the pulp is conveyed through pipes 5 and 7 by means of a conveying fan or blower 6, which draws in conveying air through a pipe 8 connected to the pipe 5 at a point between the shredder 4 and the fan 6. The air passes through pipe 8 at a rate of 36,000 kg. per hour, moreover, a certain amount of air is drawn in through the shredder, so that the rate of the air conveying the pulp through pipe 7 amounts to about 40,000 kg. per hour.

Through pipe 7 the suspension is fed to a cyclone 9 in which it is separated from the conveying air which leaves the cyclone 9 through the pipe 10. From the bottom of the cyclone 9 the pulp is, in conjunction with drying air from a pipe 12, fed into a drying duct 11 by means of a fan 13.

The drying duct 11 is a vertical tube having e.g. a diameter of 2 metres and a height of 10 metres. The drying air is supplied at a rate of 60,000 kg. .per hour and at a temperature of 150 C. In the drying duct the pulp is dried to a concentration of approximately 55 percent and is separated from the air in a cyclone 14. The air and the vapour leave the cyclone 14 through a pipe 15 at a temperature of C.

The concentrated pulp is removed from the cyclone 14 through an air lock 16 by means of drying air supplied at a rate of 60,000 kg. per hour and at a temperature of 350 C. and passes through a pipe 17 and a fan 18 to a second drying duct 19 which is similar to the first drying duct 11.

In the second drying duct 19' the pulp is dried to a concentration of percent corresponding to what is termed air-dry.

The air-dry matter is separated from the drying air in a cyclone 20 from which the air and vapour leave through the above-mentioned pipe 12 leading to the first drying duct 11.

The air-dry matter is removed from the cyclone 20 through an air lock 21 by means of conveying air supplied by a fan 22 and fed through a conveying tube 23 to a cyclone 24. The air locks 16 and 21 may be of the type described in U.S. patent specification No. 3,195,241.

In said cyclone 24 the air-dry matter is separated from the air and removed through the bottom of the cyclone from which it drops into for instance a baling press arrangement which is not shown.

The air leaves the cyclone 24 through the above-mentioned pipe 8.

It should be understood that the invention is not confined to the use of the arrangement or details thereof described above, since the method, according to the invention, may be used in any cellulose drying plant in which shredding or flufiing of cellulose pulp is performed.

What We claim is:

1. A method of drying fibrous pulps comprising ground- Wood, sulphate, sulphite, or semichemical pulps compris- (a) dehydrating the pulp to a maximum dry-matter content of 65 percent;

(b) fiufling the thus dehydrated pulp in a fluffing device of the kind permitting an adjustment of the particle size;

(c) drying the thus fluffed pulp to obtain air-dry particles;

(d) determining the terminal falling velocity of said air-dry particles;

(e) adjusting the particle size of the fluffed pulp by adjusting said flufling device until said terminal falling velocity for all particles is not more than 600 cm./ sec.

2. A method according to claim 1 wherein said adjusting of said fiufling device is performed until said terminal falling velocity is for all particles not more than 500 cm./ sec.

References Cited UNITED STATES PATENTS 2,474,314 6/1949 Koehne 24128 X 2,627,375 2/1953 Grondal et a1 241 28 X 3,055,795 9/1962 Eberhardt 162100 3,195,241 7/1965 Hohne 34-10 3,316,141 4/1967 Bergholm et a1. 34--10 X 3,364,101 1/1968 Eek 162100 X 3,414,469 12/1968 Brown et al 162'100 X S. LEON BASHORE, Primary Examiner FREDERICK FREI, Assistant Examiner U.S. c1. X.R. 34-10; -24128

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