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can easily be increased by the addition of more vessels and it is a simple matter to take it down and remove it to another location if desired. Limitations to Use Stoneware plant for storing corrosive liquids has the advantage of absolute permanence, but a handicap to its more general use has been the somewhat small units in which it is possible to manufacture stoneware storage vessels as compared with containers made of other structural materials that are otherwise less desirable. Stoneware storage vessels up to 1600 gallons capacity are manufactured, but the hazard in making these large shapes (Figure 9) in one piece of clayware -9 feet high, 7 feet in diameter, and weighing about 2 tonsis considerable and the price per unit volume is correspond-
Vol. 22, No. 2
ingly greater. We have found that the largest size that can be put into quantity production with no more than the normal loss in manufacture is 525 gallons, or 2000 liters. These vessels are 6 feet 6 inches high, 4 feet 10 inches in diameter, and weigh 1370 pounds each. The present installation a t Enka consists of thirty-seven of these vessels as well &s several of similar design but smaller capacity. Acknowledgment The storage equipment that has been described was designed, manufactured, and installed for the American Enka Corporation by the General Ceramics Company under the supervision of Lockwood Greene Engineers, Inc. The writer wishes to thank bhese corporations for permission to publish this description.
The Drying of Solids-111’ Mechanism of the Drying of Pulp and Paper T. K. Sherwood WORCESTER POLYTECHXIC INSTITUTE, WORCESTER, MASS.
The explanation of the general mechanism of drying as given in two previous papers is summarized. The drying operation is in general divisible into a constant-rate period and a falling-rate period. During the constant-rate period the surface of the solid is wet with liquid and the rate of evaporation is not a function of t h e water content; the major factors affecting the rate of drying in this period are the air velocity past the surface, heat conduction from adjoining dry surfaces, and radiation from the surroundings. The rate of drying falls off during the falling-rate period either because the wetted surface decreases or because the rate of internal liquid diffusion becomes controlling. Either of these mechanisms of drying or each in turn may prevail during the falling-rate period. When
internal liquid diffusion is controlling, there is a tendency for the evaporation to occur not a t the surface, but a t points within the solid structure. Data on the drying of pulp show t h a t this material dries according to the general mechanism of drying as outlined. Internal liquid diffusion is shown to control throughout the falling-rate period for the case of thick pulp slabs, and evaporation is shown to take place at points within the pulp structure. For the case of thin sheets of pulp or paper, the mechanism of drying during the falling-rate period is shown to be t h a t of unsaturated drying. A n empirical equation for the relation between water content and time is found to fit the data well for the drying of pulp and of a fiber wallboard.
..... . H E drying of solids by vaporization of the water content is not only a problem of supplying the necessary heat of vaporization of the water and sufficient air for removal of the vapor formed, but involves the more fundamental processes of liquid and vapor diffusion by which the water in the solid travels to the surface and thence out into the air. Two previous papers (6, 7) have presented an explanation of the general mechanism of drying, supported by experimental data on the drying of a number of different solids. It is the purpose of this paper to summarize this picture of the general mechanism of drying and further to give data on the drying of pulp and pulp products in support of the application to the drying of such materials of the hypothesis proposed. General Mechanism of Drying
T
I n the initial stages of the drying of a very wet solid the surface is completely wet with water, and the drying process is similar to the evaporation of water from a free liquid surface. As long as the surface is wholly wet the rate of evaporation is 1 Received December 26, 1929. Presented under the title “The Drying of Pulp and Paper” a t the meeting of the Americin Institute of Chemical Engineers, Asheville, N. C., December 2 t o 4, 1929. Material from a thesis submitted in partial fulfilment of the requirements for the degree of doctor of science in chemical engineering a t the Massachusetts Bnstitute of Technology, 1928.
not a function of the water content of the solid and, under constant drying conditions, the rate of drying continues constant. However, a t some definite water content the rate of drying begins to decrease, and the range from there to dryness is called “the falling-rate period.” The water content at the end of the “constant-rate period” and the beginning of the “falling-rate period” is termed the “critical water content.” When dried for a very long time the water content of the solid approaches an ultimate value which depends on the humidity of the air used, and is called the “equilibrium water content.” In the adiabatic evaporation of water from a free liquid surface the water assumes the wet-bulb temperature of the air. The heat flow through the surface air film from air to liquid corresponds exactly to the latent heat of vaporization of the water which evaporates and diffuses back through the air film out into the air. However, if heat is received by the water in any other way than by conduction through the surface air film, the nice balance between temperature difference and partial-pressure difference is upset, the temperature difference tending to be decreased and the partial-pressure difference increased. For example, if the liquid receives heat by radiation frqm surroundings a t a higher temperature, the liquid temperature may be increased considerably above the wet-bulb temperature of the air, and the rate of vaporization
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February, 1930
increawl. This principle has been employed to speed up the rate of drying in a comniercial paper drier by thci use of electric radiant heaters. h second important manner in which heat ma> be supplied to the liquid contained by a yery r e t solid is by conduction through the dry edges or other dry surfaces of the material. -4special esample of this is a sheet of paper on a paper drier, which receiyes heat by conduction from the steam-heated drum. In the caw of a block or slab of material, one or more faces of n-hich are (covered or otherwise xaterproofed, the heat inflow by conduction through these dry edges has a similar effect in incrrasing the temperature of e\ aporation of the liquid and so increasing the rate of drying.
0
PERCENT FREE WATER IN SOLID.
Figure 1-Diagrammatic
Rate-of-Drying Curves
Besides the effect of radiant heat and of conduction from dry edges, the velocity of the air past the wet surface of the material being dried is a third important factor affecting the rate of drying in the constant-rate period. High air velocities tend to reduce the thickness of the air film of the surface of the liquid, and so speed up the rate of vaporization, as has been shown (7) for the case of drying a wet block of sulfite pulp. The extensive experiments of Hinchley and Himus ( 3 ) on the rate of evaporation of water from a shallow pan using different wind velocities may also be noted. Frequently the desired final water content is greater than the critical water content, so that the constant-rate period constitutes the whole of the drying process. -4n initial adjustment period is sometimes of importance, during which the material is warmed or cooled to the equilibrium temperature which is to prevail during the constant-rate period. With some slow-drying materials the initial water content may be less than the critical, in which case the falling-rate period constitutes the m-hoLe of the drying process. The falling-rate period is in general divisible into tlyo secondary periods or zones, which from the mechanisms of drying prevailing in each may be called the zone of unsaturated surface drying and the zone where internal liquid diffusion controls. The former follows immediately after the critical point; the decrease in the rate of drying in this zone is due t o a decrease in the 15-ettedsurface of the material. The surface is no longer completely wetted, but dry portions of the solid jut out into the air film, so that the rate of evaporation per unit of total surface is reduced. The mechanism of drying is essentially the same as during the constant-rate period, so that the rate is independent of the thickness of the material being dried. During the zone of unsaturated surface drying, water diffuses to the surface as fast as it is evaporated, and it may be said that the resistance to internal liquid diffusion is small compared with the resistance to diffusion of vapor through the surface air film. The maximum rate of diffusion of water to the surface, however, decreases with the water content of the
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material, so that a second critical is reached beyond which the resistance to internal liquid diffusion is greater than the surface resistance to vapor removal. During this second zone of the falling-rate period the rate of internal liquid diffusion controls the rate of drying. Obyiously, if the initial water content is less than this second critical JTater content, internal liquid diffusion will control throughout the drying process. The diffusion of liquids through solids obeys the same fundaniental diffusion laws as hold in the case of the diffusion of heat. The Fourier equations of heat conduction have been qhown (6) to hold for the drying of solids slabs when internal liquid diffusion controls, and the integrated equation given for the case of a slab. This equation brings out the fact that, with internal liquid diffusion controlling, the time required to dry to a given water content is proportional to the square of the slab thickness. Since during this second zone of the falling-rate period the rate of arrival of water to the surface is less than the rate a t which evaporation could take place at the surface, the water in the solid near the surface tends to be depleted, and the plane or locus of evaporation tends to retreat from the surface. This probably takes place only in porous or fibrous solids, such as pulp, and not in colloidal materials, such as clay. When evaporation occurs within the solid, the vapor formed must diffuse, not only through the surface air film, but through the relatively dry surface layer of solid. The actual evaporation takes place a t such a distance from the surface that the rate of diffusion of vapor through the solid and air film resistances is equal t o the rate of internal liquid diffusion.
2
FREE WATER,
Figure 2-Drying
DRY BASIS.
of a 1.70-cm. SulEte-Pulp Slab
Figure 1 illustrates the possible drying curves which may be followed in drying a given material. The rate of drying, as weight of water per unit surface area per unit time, is plotted against the water content of the solid, the horizontal line CA representing the rate of evaporation from the wholly wet surface under the drying Conditions prevailing-i. e., the rate of drying during the constant-rate period. The three curves HO, KGO, LEO represent the maximum rates of diffusion of liquid to the surface for three thicknesses of the material, HO being for the thinnest. The straight line OC represents the relation between rate of drying and water content for the conditions of unsaturated surface drying for the particular material. Then, since the rate of drying is governed by the slowest part of the diffusion process, the resulting rate curve for any thickness will be a composite curve made up of the lowest curves representing the rates in the three
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different zones. For the sample of intermediate thickness, starting with a water content at A , the rate of drying will follow AC to the critical point C, where the zone of unsaturated surface drying commences; follow CO to the second critical point D, where internal liquid diffusion becomes controlling, and then along DGO. The thicker sample will dry a t a rate following AC to B , after which internal diffusion will control and no zone of unsaturated surface drying will appear. For the case of the thinnest sample the rate will follow AC to the critical point C, thence along CO to dryness, the material being so thin that a t no point does the rate of internal diffusion control. In this case the falling-rate period .~
HOUR5 2.6
60
~
..
CM. FROM SURFACE
Figure 3-Moisture
Gradients in Pulp Blocks during Drying
a second critical point, the points falling on a smooth curve concave upward. This shape of rate-of-drying curve is obtained when internal liquid diffusion controls the rate of drying, and the results therefore suggest that internal liquid diffusion is controlling throughout the falling-rate period for the case of the thick pulp slab. That this is actually the case may be shown in several ways. For example Tu (8) has shown that the velocity of the air past the surface has no appreciable effect on the rate of drying during the falling-rate period, in the case of a pulp slab 0.82 cm. thick, which would not be true were the mechanism that of unsaturated surface drying. Collins and Fisher ( 1 ) have shown that the rate of drying during the falling-rate period for the case of thick slabs of pressed pulp is a function of the slab thickness, which again mould not be true were the mechanism in this period that of unsaturated surface drying. Furthermore, data on the coefficients of heat flow from air to pulp show that the actual evaporation occurs within the solid, the retreat from the surface of the locus of evaporation being caused by the slowness of arrival of the water from the interior. The oTer-all coefficient of heat flow, H , is seen to remain constant during the constant-rate period, but to decrease rapidly after the critical moisture is reached. Since during the constant-rate period the evaporation takes place a t the surface of the very wet pulp, the coefficient H corresponds to the resistance to heat flow offered by the air film on the surface. The decrease in H during the falling-rate period corresponds to an increase in the over-all thermal resistance, which must be due to the occurrence of the actual evaporation of the water a t points beneath the surface of the solid structure. The heat must therefore flow, not only through the surface air film, but through the surface layer of relatively dry solid through which the rapor formed must diffuse. Thus the decrease in the over-all coefficient H is an excellent criterion of the occurrence of evaporation within the solid structure.
consists only of the zone of unsaturated surface drying. The rate of drying during the constant-rate period is influenced by the several factors discussed above; the location of the internal diffusion curves is fixed by the nature and dimensions of the material being dried.
"
Experimental Data on Drying of Pulp
Figure 2 shows the plotted results of a representative test on the drying of a slab of hydraulically pressed sulfite pulp. In this particular test the sample was 6.0 by 14.9 cm. and 1.70 cm. thick, having a dry weight of 82.0 grams. The edges were covered with collodion to prevent drying from any surface but the two faces, and the slab was soaked in water 19 hours before the start of the experiment. The sample was then placed in the drying cabinet, through which air a t about 39" C. was flowing continuously a t a low velocity (slightly less than 1 meter per second). Eight thermocouples in series imbedded in the pulp half way between the faces were used to obtain the temperature in the interior of the pulp, using a recording pyrometer. A recording balance was used, making it possible to obtain the rate of drying with good accuracy by measuring the slope of the curve drawn by the pen. From the measured temperature difference between sample and air, it is possible to calculate the over-all coefficient of heat flow from air to pulp. Figure 2 shows the calculated values of the over-all coefficient of heat flow, H , and also the rate of drying both plotted against the water content of the pulp on the dry basis. The rate-of-drying curve indicates a critical water content of about 58 per cent, which is about the average of the values obtained for sulfite-pulp slabs in a large number of tests. The rate curve in the falling-rate period shows no evidence of
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* I 2
'
O
i
k;/ I
I20
).
k
% ,
WATER,
Figure 4-Shrinkage
DRY
BASIS
Data on Pulp Slabs
In every one of a number of similar tests the over-all heat flow coefficient started to decrease a t a water content very close to the critical water content. The intercept of the H curve with the ordinate scale corresponds to the over-all thermal resistance when dry, which is the air-film resistance plus the resistance of half the pulp slab. The intercepts are therefore higher for the thinner than for the thicker samples. In the case of the test described the intercept is approximately 0.00010, corresponding to an over-all resistance of 10,000. The resistance of the air film is I/O.OOOSS or 1790, so that the thermal resistance of the 0.85 cm. thickness of dry pulp is 8210, corresponding to a value of 0.000103 for the
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thermal contlucti\-ity of the pulp. This compare- reasonably well with the value of 0.00015 found by Lees arid Chorlton (4) for blotting paper. Having established the fact that evaporation occurs within the pulp structure, it is of interest to inquire whether all the evaporation at a giren inqtant takes place a t the same, or
Figure 5-Drying
of Thin Pulp Sheets
nearly the qame, distance from the surface, or whether the evaporation is distributed through a layer of relatively dry pulpinear the surface; that is, whether evaporation takes place a t a plane continually retreating from the surface during the falling-rate period, or whether there is an evaporation zone of appreciable thickness. I n order to throw light on this point data were obtained on the actual moisture gradients existing in pulp slabs at different stages in the drying process.
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repre.;entiiig the center lilies of the slices, the different curves being for the different blocks. The highest curve, representing the block sliced after drying 2.6 hours, shows a small gradient toward the back of the sample, due t o the fact that the pulp, soaked in water after the cement was applied, was not completely wetted, so that the moisture distribution at the start was not entirely uniform. The double a r r o w represent the positions of the backs of the blocks, and 50 indicate the block thicknesses. If all the evaporation were taking place a t a plane a definite distance from the surface in each case, a discontinuity would lie expected in the gradient curve a t an abscissa repreienting the location of such a plane. However, since air present in the pulp would be nearly dry a t the pulp surface, and approach saturation near the plane of evaporation, the water content of the pulp should be nearly zero at the pulp surface and near the plane of evaporation should approach that water content in equilibrium with saturated air. For the pulp used the water content in equilibrium with saturated air was about 13.5 per cent, and it is therefore evident that the discontinuity in the gradient curves described above should appear at ordinates of about 13.5 per cent water. From the curves of Figure 3 it is apparent that no such discontinuities occurred; in fact, not even a moderate increase in the slopes of the gradient curves m s observed. It may therefore be concluded that the data obtained on the moisture gradients in pulp tend to eliminate the possibility that all the evaporation occurs at a retreating plane, or even in a narrow zone. The structure of the pulp is non-homogeneous, the sizes of the individual fibers varying greatly, as do the sizes of the spaces between the fibers. It seeins reasonable to suppose, therefore, that even when the inner limit of the evaporation zone has retreated a considerable distance into the pulp slab, a small aniount of evaporation is still taking place at points very near the pulp surface, probably from the surfaces of the small water wedges between the bmaller fibers. Shrinkage data on pulp slabs being dried give additional information regarding the manner in which the water leaves the pulp. Figure 4 shows data obtained by Hawes ( 2 ) on the change of thickness during drying in air of two slabs of sulfite pulp-one hydraulically pressed when wet, the other formed by hand. Practically no shrinkage occurs until the
100
1.00
09 08
0.9 0.8
07
0.7
06
0.6
05
a5
04
04
d T W O THICKNESSES
E'
E
03
03
02
02
0.1
0
Z
4 6 HOURS
Figure 6-Semi-Log
8 IO I2 14 16 F A L L I N G RATE P E R I O D
18
20
Plot of Drying Data on Pulp
Small pulp blocks nere coated on the edges and one face with waterproof cement and dried from the remaining face. After different lengths of time in the drier, the blocks were cut into thin slices parallel to the face. I n Figure 3 the average total moisture content of the slices is plotted against abscissas
0.1 0
PO0
Figure 7-Semi-Log
400
600
TIME-
MINUTES
800
1000
Plot of Drying Data on Celotex
critical moisture content is reachecl, when shrinkage i 5 heen to commence abruptly. The hand-pressed pulp shows continued shrinkage from this point t o dryness, but the hydraulically pressed pulp is seen to shrink t o a minimum at about 15 per cent water and then expand. This surprising result may
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I-VDUSTRIAL AND ENGINEERISG CHEMISTRY
probably be explained as follows: The shrinkage is due to the retreat of the water planes into the smaller water wedges, the fibers being pulled together by the capillary action of the water as two glass. plates with water between are pulled together; when the water is wholly gone the tension is no longer acting and, if the structure is strong and firm, as in the case of the hydraulically pressed pulp, the fibers may spring apart, causing the reverse of shrinkage. Since the shrinkage is negligible during the constant-rate period, the water lost must be replaced by an equal volume of air. However, since no appreciable moisture gradient is set up during the constantrate period the large amount of air entering the pulp must be fairly evenly distributed throughout the mass. It would therefore seem probable that the larger capillary openings throughout the pulp are emptied during the constant-rate period. I n the drying of relatively thick pulp slabs, internal liquid diffusion has been shown to be controlling throughout the falling-rate period. I n drying sheets of paper or pulp of the thickness ordinarily dried on pulp driers, internal liquid diffusion is probably never of importance, the mechanism of drying during the entire falling-rate period being that of unsaturated surface drying. This has been shown in a previous paper (7) to be the case for newsprint by a comparison of the rate curves of two samples, one a single thickness and the other a double thickness of ordinary newspaper. The curves of loss in weight per unit time per unit surface area for the two samples dried under identical drying conditions were found to coincide exactly. Since the rate of drying was therefore independent of the sheet thickness, internal liquid diffusion could a t no point be controlling. Similar data were obtained on the drying of pulp lap. Small samples 5.09 by 12.7 by 0.094 cm. thick or ordinary sulfite-pulp lap were used, the fist test being run with a single sheet and the second with two sheets sewed together. The two samples were wet with water and dried wliile suspended from the arm of a chemical balance in a drying cabinet maintained a t 25’ C. and approximately constant humidity, using no forced air circulation. The data obtained are shown on Figure 5 , and it will be seen that the curve for the single thickness falls 7 to 30 per cent higher than the curve for the double thickpess. This difference is small compared with 100 per cent deviation, which would be expected if internal liquid diffusion were controlling, so it may be concluded that for such thin pulp sheets, as in the case of the newsprint, unsaturated surface drying prevails during the entire fallingrate period. Approximate E q u a t i o n for Falling-Rate Period
I n practical drying problems it is important to have some equation representing the relation between moisture content and time of drying. Such an equation has been given (6) for the case of drying with internal liquid diffusion controlling, which, however, is useful only in cases where unsaturated surface drying is not important. A very useful equation of a simple form can be derived by assuming that the rate of drying during the falling-rate period is directly proportional to the free-water content. Although this assumption is only a rough approximation to the facts, it leads to an equation which has been shown (7)to fit the data surprisingly well for the case of drying whiting slabs. The integrated equation may be written log E‘ = -K e’ (1) where E’ is the ratio of the free-water content at any time 0‘to the free-water content a t the critical point; and 0’ is the time after the start of the falling-rate period. The constant K depends on the air velocity and other variables which may be termed “drying conditions,” and varies inversely with the slab thickness.
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Equation 1 indicates that for the drying of any one sample E’ vs. 8’should give a straight line on semi-logarithmic coordinate paper. Figure 6 shows the data obtained during the falling-rate period in the drying of several pulp slabs of different thicknesses, plotted as E’ vs. 8’as suggested by Equation 1. The points are seen to fall on approximately straight lines, thus supporting the applicability of Equation 1 to the drying of such materials. The failure of the lines to fall exactly in order of the slab thicknesses is no doubt due t o differences in air velocity, humidity, ratio of dry to wetted surface, etc. Figure 7 shows a similar plot of data obtained by Sanford (5) on the drying of a commercial fiber wallboard (Celotex). The two curves represent the results obtained using one and two thicknesses, respectively, of 1.ll-cm. board. I n this case the initial water contents (129 and 144 per cent) were less than the critical, and no constant-rate period appeared. The plot is consequently of E’ vs. 8‘ on semilogarithmic paper and excellent straight lines result. Moreover, the ratio of the slopes of the two lines is 2.4 as compared with the thickness ratio of 2.0. When internal diffusion is controlling, the time required to reach a given value of E’ varies inversely as the square of the thickness of the slab, as pointed out above, and it may be concluded, therefore, that in the drying of this wallboard surface evaporation is controlling throughout the falling-rate period, the mechanism of drying being that of unsaturated surface drying. a plot of
Literature Cited (1) Collins and Fisher, Massachusetts Institute of Technology Undergraduate Thesis, 1927. (2) Hawes, Massachusetts Institute of Technology Chemical Engineering Thesis, 1928. (3) Hinchley and Himus, Trans. Inst. Chem. Eng. (British), 2, 57 (1924); Chemistry b‘ Industry, 48, 840 (1924). (4) Lees and Chorlton, Refrigeraling Eng., 10, 259 (1924). (6) Sanford, Worcester Polytechnic Institute Undergraduate Thesis, 1928. (6) Sherwood, IND.END.CHEM., 21, 12 (1929). (7) Sherwood, Ibid., 21, 976 (1929). (8) Tu, Massachusetts Institute of Technology Undergraduate Thesis, 1928.
Safety Measures at Plants Manufacturing Natural Gasoline Although the natural-gasoline industry has recently made great strides in reducing accidents, the industry remains a hazardous one because of the very nature of the raw materials handled, the process of extracting gasoline from natural gas, and the products obtained, according to the United States Bureau of Mines. Owing to the facts that gasoline is extracted under pressure and that gas and gasoline are highly inflammable, the risk of fire and explosion always attends the recovery of gasoline from natural gas. The serious explosions and fires that have occurred recently demonstrate the need of increased efforts to prevent recurrence of similar disasters. Intelligent instruction and education have been the greatest contributors to the progress that has been made in attaining safer conditions in the natural gasoline industry. Decreased height of towers, better design of stills, automatic regulator controls, and proper venting have improved operating equipment and increased the safety of operation. Improvements in equipment and the introduction of new devices and methods have resulted also in the manufacture of better grades of gasoline. In designing a natural-gasoline plant the following factors bear directly upon its safe operation: selection of the site; the plan of the plant and equipment with reference to the surface contour; arrangement of the hazardous equipment to reduce, so far as possible, the risk of fire; provision for future construction without congestion; adequate strength of all structures and equipment; and specifications for materials. In this industry accident prevention is inseparably related t o fire prevention. Although every accident hazard is not necessarily a fire hazard, every fire hazard is an accident hazard. Details regarding safety measures which should help in the reduction of accidents is given in Bureau of Mines Technical Paper 462, “Safety a t Natural-Gasoline Plants,” copies of which may be obtained from the Bureau of Mines, Washington, D. C.
