Engineering and
Drying Granular Solids in Superheated Steam LEONARD WENZEL UNIVERSITY
AND
ROBERT R. WHITE
During the constant rate period, the rate of drying is limited only by the rate a t which heat is transferred to the wet surface, the heat producing evaporation of the water. The coefficient of heat transfer as defined by Equation 1
& (2)2’3(7) 18.8 LG
development
OF MICHIGAN, ANN ARBOR, MICH.
T h i s investigation was undertaken to provide a quantitative basis for comparing steam and air as drying media for granular materials that are not sensitive to temperature. The rates at which sand was dried in trays by superheated steam were measured and correlated as convection heat transfer coefficients which range from 2.4 to 17.8 B. t .u./(hour)(sq.,foot)( F.) according to the equation =
pOCeSS
may be calculated from the rate of heat transfer per unit area of drying surface, -,dq the temperature of the drying medium, to, Ad8 and the surface temperature, ts. The rate of heat transfer per unit area of drying surface is related to the rate of drying as determined by sample weight through Equation 2.
-0.65
The experimental conditions covered temperatures from 300” to 380” F., pressures from 25 to 100 pounds per square inch absolute, superheat to 100’ F., and mass velocities from 200 to 500 lb./(hour)(sq. foot). Drying beds from 1 to 3 inches deep were used, and particle sizes included 20 to 28, 80 to 100, and 115 to 170 Tyler screen mesh. The results indicate that higher drying rates and thermal efficiencies are possible when drying with superheated steam rather than air. The use of steam for drying must be justified by savings in operating costs as compared to greater capital expenditure. The degree of confinement of the drying medium as it passes over the material being dried is an important variable.
d p = - A H - A& Ad0 dW Gas temperature, to, is directly measurable, but surface temperature, ts, being an interfacial condition, is almost impossible t o measure directly. It may be closely estimated, however, through combination of the mass transfer relation
- -dWBdO
-
k(P*
- Pa)
(3
with Equations 1and 2
S
UPERHEATED steam is an attractive drying agent for ma-
terials which are not temperature sensitive. The water removed from the solid during the drying process becomes, on heating, a part of the drying medium whereas in air drying, the saturated air must ultimately be replaced by fresh air heated to a suitable operating temperature level. The advantages of superheated steam relative to air from a thermodynamic point of view have been discussed (6, I S ) . The purpose of this investigation was to determine the rates of drying of a granular solid in superheated steam in typical equipment in order to provide a quantitative basis for comparing steam and air drying. The rates of drying are expressed as heat transfer coefficients during the
5
SO/lOO
1.255
1-Inch Bed
I
0 WATER IN I-INCH AND 3-INCH PANS
0
1.275 0,590
RATE
Effect of Channel Boundaries on Flow Pattern Around an Obstruction
o 0
S-36
EVAPORATING WATER INTO AIR
1
NUMBER, (LG/p)XlO-'l
Figure 8. Correlation of h, with Steam Properties for Evaporation of Water i n Steam
2
5 IO 20 REYNOLD'S NUMBER, LG//LX 10-4
Figure 9.
50
100
Correlation of h, with Steam Properties
INDUSTRIAL AND ENGINEERING CHEMISTRY
1836 1
0
04
02
0
data, as there are no available data on critical moisture content obtained when drying in superheated steam; the available data on critical points found in air drying are contradictory (a,9,1 0 , I W). Falling Rate Drying Period. It has been stated in the literature (4, 1 4 ) that, the drying rate-moisture content curve during the falling rate period closely approximates a straight line connecting the first critical point and the origin. The data presented here fit this approximation remarkably well, as shown in Figure 11where the drying data are plotted as sample weight, versus time on semilogarithmic coordinates. Here the constant rate period appears as a curve and the falling rate period as a straight line. A closer scrutiny of these data could not be justified because of the large percentage error in the data obtained a t low drying rates.
I-INCH BED
08
06
I2
IO
1.4
I6
CONCLUSIOIS
Figure 10. Critical hIoisture Contents When Drying Reds of Ottawa Sand in Superheated Steam
face of the uninsulated pan, assuming the liquid motion in the pan is by nat#ural convection. The resulting values of
( y )' I 3
Vol. 43, No. 8
(".> CPG
X
are far above the rest of the esperimental data.
Critical Moisture Content. .Is discussed previously (2, 10, I W) t h e moisture content at the first critical point. would be expected t o be a function of the thickness of the solid being dried, the drying rat.e during the constant rate period, and t,he properties of the drying stock as it influences t,he effective radii of the steam-water menisci, the permeability of the solid, and the constant in the capillary flow equation. The data obtained on critical moisture content when drying beds of Ottawa sand in superheated steam have been collected in Table V. Figure 10 shows these critical moisture contents plotted against the drying rate during the constant rate period. Three parallel lines result-one for each sample bed depth studied. The particle size of the sand used had no effect on the critical moisture content over the size range studied. These results cannot, be checked by comparison with previously published
The use of steam rather than air as a medium for drying granular solids does not alter the general characteristics of the drying process. The drying rate during the constant rate period may be espressed in terms of the heat transferred to the solid t o supply the requisite heat of vaporization. This heat is transferred to the surface of the solid by convection from the flowing steam, by radiation from the steam and from surrounding surfaces, and by conduction through the edges and bottom of the sample pan. I n the tests reported here, the heat transferred by radiation amounted t o 7.5 to 31% of the total heat transferred, and the heat transferred by conduction through the walls of the sample pan amounted to 6 t o 57% of the total heat transferred. The convective heat transfer coefficients resulting from these calculations range from 2.36 to 17.83 B.t.u./(hour) (square foot) ( ' F . ) and are correlated with the steam properties by
These results indicate that the convection coefficient for heat transfer from the flowing gas to the sample surface is dependent on the degree of confinement of the sample in the duct; the value of this coefficient increases and its dependence on rate of fluid flow decreases a s the passage around the sample becomes narrower. This effect warrants further investigation. The moisture content in the solid a t the end of the period of constant rate drying, for the sand beds used in this work, depends on the thickness of the bed and on the drying rate during the constant rate period. These critical moisture contents varied f r o m 0.02 to 0.095 pound of water per pound of dry sand. The data on the drying rate during the falling rate period are adequatelv represented by
where c is evaluated from a knowledge of the moisture content a t the critical point. Higher drying rates and greater thermal efficiencies are possible when drying with superheated steam rather than with air. The use of steam for drying heat-insensitive solids must depend on the savings in operating costs as compared to the greater capital espenditure required by the higher temperature and pressure ranges. ACKNO W LEDGiM EYT ~
2
3
4
5
6
7
Figure 11. Typical 3Ioisture Content z's. Time Plots
This investigation was sponsored by the Owens-Illinois Glass Co., Toledo, Ohio.
August 1951
NOMENCLATURE
drying area, sq. ft. heat capacity, B.t.u./(lb.) ( O F.) diffusivity,.sq. ft./hour mass velocity, Ib./(hr.)(sq. ft.) H = enthalpy, B.t.u./lb. L = length, feet hl = molecular weight P = total pressure, lb./sq. ft. W = weight,Ib. = heat transfer coefficient, B.t.u./(hr.) (sq. ft.) ( O F . ) h = factor defined by Equation 6 ( 3 ) j = mass transfer coefficient, Ib./(hr.) (sq. ft.) (atm.) k ka = thermal conductivity, B.t.u./(hr.) (sq. ft.) ( O F./ft.) p = partial pressure, atm. Q = heat transferred, B.t.u. = resistance t o heat transfer ( l / h ) r t = temperature, “ F . = absorptivity in radiant heat transfer 01 = emissivity in radiant heat transfer e p = density, lb./cu. ft. = viscosity, lb./(ft.) (hr ) p e = time, hours y = shape factor
A C, DO G
1837
INDUSTRIAL AND ENGINEERING CHEMISTRY
= = = =
LITERATURE CITED
(1) Broughton, D. B., IND.ENG.CHEM.,37, 1184 (1945). (2) Ceaglske, N. H., and Hougen, 0. A., Ibid., 29, 805 (1937). (3) Colburn, A. P., Trans. Am. I n s t . Chem. Engrs., 29, 174-210 (1933). (4) Gilliland, E. R., IND.ENG.CHEY.,30,506 (1938). (5) Govier, G. W., Sc.D. thesis, University of Michigan (1948). (6) Hausbrand, E., “Das Trocknen mit Luft und Dampf,” Berlin, Julius Springer, 1908. (7) Hottel, H. C., and Egbert, R. B., Trans. Am. Inst. Chem. Engrs., 38, 531 (1942). (8) Hougen, 0. A., McCauley, H. J., and Marshall, W. R., Jr., Ibid., 36, 183 (1940). (9) MoCready, D. W., and McCabe, W. L., Ibid., 29, 131 (1933). (10) Shepherd, C. B., Hadlock, C., and Brewer, R. C., IND.ENG. CHEM.,30,388 (1938). (11) Sherwood, T. K., Ibid., 21, 976 (1929). (12) Sherwood, T. K., and Comings, E. W., Ibid., 25, 311 (1933). (13) Ungewitter, C., “Science and Salvage,” London, Crosley Lockwood and Son, 1944. (14) Walker, W. H., Lewis, W. K., McAdams, W. H., and Gilliland, E. R., “Principles of Chemical Engineering,” New Yolk, McGraw-Hill Book Co., 1937. (15) Washington, L., and Marks, W. M., IND. ENG.CHEM.,29, 337 (1937). RECEIVED Septeiiiber 7, 1950.
Pressure Drop in Flow of Dense Coal-Air Mixtures
EngFnyring Process development
P.
SIMONS, AND L. D. SCHMIDT BUREAU OF MINES, U. S. DEPARTMENT OF THE INTERIOR, AND ENGINEERING EXPERIMENT STATION, WEST VIRGINIA UNIVERSITY, MORGANTOWN, W. VA.
C. W. ALBRIGHT, J. H. HOLDEN, H.
T h i s investigation was undertaken to obtain a method of predicting the pressure drop in a pneumatic system used to feed pulverized coal to a generator in which the coal reacts with oxygen and steam to form synthesis gas (CO Hz). The feeding system operates by causing a partially settled mixture of coal and air to flow through a tube from a fluidized bed of pulverized coal. The results have been corrklated in the form of two empirical equations which give the pressure drop for the flow
of mixtures of air and coal (90% through 200-mesh) through horizontal tubes and in the ratio of approximately 200 pounds of coal per pound of air. So far as is known, these are the first pressuredrop measurements on mixtures using such a high solid-gas ratio, and the equations developed represent the best knowa method of determining the pressure drop for the flow of coal-air mixtures in the ratio of about 200 pounds of coal per pound of air.
A
pressure drop of the coal-air mixture flowing through the tube in terms of the weight rate of coal flow, the ratio of coal t o conveying air, and the tube diameter. Information in the literature on the flow of solid-gas mixtures is very limited, and no information has thus far been found concerning the flow of mixtures at the high solid-gas ratios utilized by the feeder. Therefore, measurements of the pressure drop have been undertaken for four tube diameters and for various coal-flow rates and coal-air ratios. The results of these measurements and an empirical correlation are presented.
+
S PART of the synthetic liquid fuels program, a feeder has been developed by the U. S. Bureau of Mines at Morgantown, W. Va., in cooperation with West Virginia University, t o deliver coal at a uniform rate to a pulverized coal gasifier for making synthesis gas. The feeder, which has been described previously ( 1 , Q), operates by forming a fluidized bed of coal in a vertical cylindrical container and by causing a mixture of coal and air t o flow through a tube to the gasifier, Part of the air used to fluidize the coal acts as the conveying gas, while the rest is vented from the top of the container. The entrance t o the coal-delivery tube is located near the bottom of the fluidized bed of coal and is protected from the direct action of the fluidizing gas by a shield that greatly improves the uniformity of the coal-air mixture. Tests (3)on the uniformity of the coal-air mixture flowing through the delivery tube have shown that the variation in the volumetric ratio of coal to air is small over time intervals of about 0.1 second. To predict the rate of coal delivery, it is necessary to know the
EXPERIMENTAL METHODS
Two different pieces of experimental apparatus were used to obtain the data. The first was a small scale model of the feeder ( I ) ; the test section was a horizontal 12-foot length of a/le-inch (0.0097-foot inside diameter) tubing equipped with pressure taps. The pressure measurements in this apparatus were inaccurate in spite of
