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SURGE DRUM
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JU CHIN CHU, STANLEY FINELT, WILLIAM HOERRNER, and MIN-SHUEY LIN hnic Institute of Brooklyn, Brooklyn 1,
N.Y.
Drying with Superheated Steam-Air Mixtures Design of continuous drying equipment has a substantial influence on drying rates for air and superheated steam
COMMERCIAL
use of superheated-vapor drying has been limited (2, 77) because suitable equipment (74) was lacking. The operation. conducted under pressure, was batchwise and uneconomical. The relative merits of using superheated vapor as a drying medium have been described by various early investigators (3, 6, 77, 72, 74,27, 23). However, no investigation has been made in which air and a superheated vapor are used to dry solids in the same equipment, nor have investigations been made using an air and steam mixture as drying medium.
Equipment The equipment consisted of a boiler,
superheater, drying chamber, and condenser connected in a continuous flow system (figure above). The details of the equipment diagram are given elsewhere
(5). Boiler. Steam was generated in a 40-gallon copper boiler equipped with a pressure gage, 0 to 15 p.s.i. in 0.5pound graduations; a thermometer, 0 O to 110' C. with 0.1' C. graduations; and a u-tube manometer, reading up to 5 inches of mercury. The safety valve was set at 10 p.s.i.; a sight glass indicated water level in the boiler. Steam was generated by electrical immersion heaters andlor high pressure steam (up to 70 p.s.i.) through internal coils of the boiler. Three 5-kw. General
Electric immersion heaters were set into the boiler from the top. The Variac setting was calibrated against steam generation rate and gave a rough estimate of steam rate to the dryer. Boiler rate using immersion heaters alone was about 40 pounds per hour. Addition of high pressure steam, controlled with a Taylor pressure controller, raised the rate to about 90 pounds per hour. In addition to internal heating with high pressure steam, a bypass on the line allowed "live" high pressure steam to be added to the boiler. Superheater. The superheater was a 3.5-inch stainless steel U-shaped tube equipped with one 2.5-kw. and two 1.5kw. Watlow heaters. Temperature conVOL. 51, NO. 3
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MARCH 1959
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trol \vas provided, and 500' F. steam could be obtained at the superheater exit ming all three heaters. Drying Section. At the inlet to the drying section, a steam trap removed any entrained water in the superheated steam. .4bypass line permitted circulation of superheated steam through the system lvithout passing over solids in the dr>-er. Steam \vas superheated to the desired temperature before allowing it to start drying the solids. An orifice and thermocouple were located in the bypass line to permit frequent checks of steam condition. .An orifice \vas also located in the main line to the drying section. Past this orifice was a 6-foot calming section made of 3.5-inch stainless steel tubing, cut longitudinally to form a semicircular cross section. T h e drying chamber consisted of an aluminum plate support on Lvhich rested a n aluminum drying pan insulated Lvith Teflon strips. Under the support plate a 0.25-inch coupling \vas provided for the aluminum rod connecting the support plate to the plate on the scale. T h e scale ivas a Toledo scale w.ith a range of 2 pounds in 0.005-pound divisions. Thus, tveight loss of the solids could be measured. All weighing \vas done from below the pan, and no obstructions were placed in the gas flow, as would be required with a n overhead weighing arrangement. T o prevent gas from escaping and by-
I-
-
-
,
02
91
MO S T W E C ~ ~ Y T E Y T - , B S / ~ B ,
Figure 1. Data in the falling rate period show the linear relationship between moisture content and drying rate with air Air Mass Flow Rate, Lb./Hr. (sq. F t )
A e
AS-4 AS-] 1
276
620 a50
Bulk density of the sand \vas 108 pounds per cubic foot, Lvith a saturated water content of 0.220 pound of water per pound of dry sand. Because of the sand used, results of this investigation are comparable to the data of Shepherd (78) and LVenzel (23) Yvho used the same sand composition. Experimental conditions for the three types of drying are:
D r j ing Medium
Mass Flon Rate Lb / ( H r ) (Sq F t )
Temp Range
Air (AS) Air-steam (ASS) Steam ( S S )
400-850 920-2150 240-3300
140-280 315-350 240-370
inch on each side. The whole assembly \vas then securely fasrened on the scale. Thermocouple wires \Yere passed up through the aluminum tubing and into the solids to obtain the temperature at different points Ivithout having thermocouple wires extend into the gas stream from 01-erhead. 111 addition to solids temperature, other temperatures measured in the dr)ing section were: inlet to calming section, inlet to drying chamber, exit from drl-ing chamber. and inlet to condenser (exit from entire drying section). Thermocouples used \\-ere 20-gage iron-Constantan nire shielded with stainless steel to prevent corrosion. All the thermocouple leads ivere connected to a 10-point switch served by a Rubicon potentiometer, Model No. 2374. Accuracy of the potentiometer was f 0 . 5 O F. for these operations. When air was used as a drying medium, an inlet port was added to the superheater. Compressed air was first sent through a surge tank containing a filter to remove impurities and entrained substances. Inlet \vet and dry bulb temperatures of the air were measured. Air passed through the system in the same manner as the steam. Mixing point for air-steam mixtures \\*as at the superheater inlet, and the mixture traveled the indicated path. Manometers used to measure rates \Yere all equipped with steam traps to prevent any condensate from getting into the manometer tubes. Results
l 0
passing the solids, an overfloiv cup \vith an attached overflow tube was fixed to the aluminum rod and mated \vith a projection heloiv the drl-ing. chamber. \Vhen steam condensed in the chamber and ran into the cup. i t overflo\ved and was collected. The cup \vas sealed with \vater at the start of a run. The pan and support were set into the chamber xvith a clearance of
A total of 63 runs were made using air. air-steam mixtures, and superheated steam for drying sand which had the following screen analysis. U S Mesh Screen Size
n-7.
- 3 0 f 40 -40 50 -50 60 -60 70 -70 80
42.4 34.8 12.3 7.4 3.1 __ 100.0
+ + +
Av. Temp.,
F.
+
161
282
INDUSTRIAL AND ENGINEERING CHEMISTRY
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%
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%
C~lniing Length I n
0 20-60 100
71 71 0, 71
Steam,
Details of experimental results, along \vith calculation procedures, are given elsewhere ( 5 ) . Air. Data in the falling. rate period (Figure 1) shoiv a linear relationship between moisture content and drying rate. Badger and Banchero ( 7 ) note that equilibrium moisture content for sand is essentially zero and independent of humidity and temperature. For Figure 1, if the linear relationship \cere extrapolated to zero drying rate, an equilibrium moisture content \vould be obtained, but, if equilibrium moisture content is zero, a distinct break in the linear relationship must carr!' the curve to the origin of the axis. Accurate data in this region are difficult to obtain because small measurement errors yield large discrepancies in results. Hoivei-er, several investigators (8, 79) have observed this second falling rate period leading to zero equilibriuni moisture content and brenzel (23) found zero equilibrium moisture content with steam drying. In run .&S-ll (Figure l ) , the second falling rate period begins a t a moisture content of about 0.043 pound of water per pound of sand. The air used had an average humidity of 0.160 pound of Lvater per pound of drl- air for three temperature ranges. Lurie and Michailoff (75). using air Ivith absolute humidity of 0.014 pound of ivater per pound of dry air, did similar \\-ark lvith a rough solid: corresponding to an irregular surface, in a long dr!.ing chamber. Their results are in good agreement with those of this investigation. Figure 2 compares data of this investigation with that of Shepherd (78). Good agreement a t higher G values and poor agreement as G decreases can be accounted for by the different Reynolds numbers for Shepherd's G values. Low G values here are in the laminar floiv region, whereas Shepherd's work was solely in the turbulent zone. Changing the floiv pattern accounts for better agreement at high G values.
+
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SUPERHEATED STEAM-AIR D R Y I N G Figure 4
2.
The
change
in
flow pattern during air drying accounts for better agreement a t high mass flow rates
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Figure 3. Drying curves with superheated steam show distinct constant rate and fulling rate periods
I 10
3cc
500
700
1000
9 0
I500
MASS FLJW RATE - L3S/ h R-SQ fT.
Superheated Steam. Wenzel's {cork on superheated steam drying showed no change in the characteristic drying curve for sand. a conclusion supported by this investigation. Typical drying curves obtained \sit11 superheated steam (Figure 3) show that distinct constant rate and falling rate periods are evident. Comparing data in the falling rate period with that of air drying substantiates an equilibrium moisture content of zero. During the constant rate drying period. surface temperature remains essentially constant. As critical moisture content is approached, rate of moisture movement to the surface derreases, and eventually the surface becomes dry. Temperature of the surface begins to rise and approach the drying medium temperature. Likewise, a temperature gradient develops through the bed of solids. Kunz (73) has shoirn such relationship for drying blotting paper with air. Data for superheated steam drying show a constant surface temperature of about 212' F. during the constant rate period, corresponding to the saturation temperature of steam a t 1 atm. After critical moisture content is reached, surface temperature rises sharply to approach temperature of superheated steam. Drying rate during the constant rate period is correlated Lrirh mass flo\v rate in Figure 4. Fluctuation from average is about f 15' F.: accounting for the scattering of some points. .Although at low vapor flotv rates superheat temperature does not influence drying rate, it has a marked effect as mass velocity increases. A small increase in steam temperature at high flo\v rares results in a large increase in the drying rate. This agrees with Chu and coirorkers ( 3 ) , \rho found that a high superheat steam temperature is necessary to obtain economical drying operation. Data for air drying a t 280' F. are included in Figure 4 as a preliminary comparison of steam and air drying. Over 900 pounds per hour per square
---..+
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foot, superheated
steam gives faster drying rates even a t a lower temperature. However, even a t the same values of mass velocity, the Reynolds number of steam is higher than that for air because of the lower viscosity of steam. Hence, steam data may be in the turbulent region of flow thus accounting for higher dryine; rates. Estimating critical moisture content of a stock during the drying operation is a very difficult operation. LVenzel (23), bvho has covered variables affecting the critical moisture content, found that drying rate during the constant rate period can be correlated against the critical moisture content (Figure 5). Wenzel's line was obtained by extrapolating his data to the 0.75-inch bed thickness used here. The excellent agreement 'verifies the as-
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0
0
01
Air, 280'
A
1
N
l
02
sumption that critical moisture content is directly related to drying rate. Mixtures of Air and Steam. -4s a possibility for drying. mixtures of air and superheated steam were used to dry the sand. Othmer (76) has shown that a very small percentage of air in steam greatly reduces over-all heat transfer coefficient during heat exchange. HoLrever, no previous investigations have been made using mixtures as drying mediums. This investigation used mixtures ranging frclm 20 to 70yc steam to observe the effects in the middle range of mixtures rather than a t the extremes. Figure 6 shows two drying curves obtained with steam-air mixtures. The
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MOISTURE COPITENT -L3,/'LB,
1 T Figure 4. The temperature of superheated steam has a marked effect on drying rate as mass velocity increases
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F.
Steam, 250' F.
0 Steam, 31 0 ' 0 Steam, 350'
F. F.
VOL. 51, NO. 3
MARCH 1959
277
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X Y NG QATE--BS/bR-SQ'-4 Figure 5. Agreement with Wenzel's data shows that critical moisture content is ;elated to drying rate A SSI runs 0 SSlll runs SSll runs
characteristic shape has been maintained. The high steam content of ASS-2 indicated d rather sharp change in falling rate data if a n equilibrium moisture content of zero is to be obtained. The effective dr)ing abilit., of air during the falling rate period has been decreased by the addition of steam. Likewise, the abilitv of superheated steam to dry during the falling rate period has been decreased by the addition of air. Thus, equal mixtures of air and steam are poor mediums for drying in the falling rate period because of the resulting high equilibrium moisture con-
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tent. If a zero equilibrium moisture content is assumed, a radical second falling rate period is present for sand when air-steam mixture is used as a drying medium. A qualitive relationship bettveen percentage of steam in air-steam mixtures and drying rate was plotted in Figure 7. Experimental points for 070 steam were estimated from extrapolated data; other points are within the range of average mass rate of flow and temperature. Drying rate increases rapidly with addition of steam up to about 50% and thereafter tends to become somewhat independent of steam concentration. Qualitatively, i t would appear that steam will help the drying rate when mixed with air. However, the amount is limited to about 50%. Under the average conditions chosen steam alone is the better drying medium than air; this effect could be due to the higher Re>nolds number of steam for the same mass velocity and temperature.
Correlation of Results
J!t
Several ways are available for correlating data. Considering heat transfer, Colburn (4)has suggested the form: ( h / C j G ) ( C j ~ j k )= ~ ' a(LG/pIn ~
0
02
0
MOISTURE CONTENT- BS/L 9.
Figure 6. Although the characteristic shape of drying curves was maintained with air-steam mixtures, the drying abilities of air and steam were decreased in the falling rate period
%
Mass Flow Rate, Lb./(Hr.) (Sq. Ft.)
19.9
1100
42.3
1050
Steam,
0
A
ASS-1 ASS-2
278
Av. Temp.,
F. 350 360
(1)
circular cross sections. They concluded that De should yield a good correlation when dealing with noncircular cross sections. The calming section of the dryer was semicircular in cross section. To compare results of this investigation with previous data, both L and D , were used in the correlation. Figure 8, A , correlates forced beat transfer coefficient, in terms of j h , with Reynolds number. Design of the equipment has permitted a transitional zone. O n the basis of heat transfer, Colburn's correlation (4)should show a transition zone between laminar flow and turbulent flow. The nature of the transition zone is difficult to analyze. Hoivever, j h will increase with Reynolds number over this range. Also, L I D , length of approach to length of drying pan, is a parameter in the correlation. Should insufficient calming section be present, floiv over the drying pan will always be turbulent and no transitional flow will be observed. Paquill (77) has given this as the reason investigators have been unable to compare their results with others. Drying rate is a function of the nature of vapor flow and hence a function of equipment used. It has been recommended that a minimum of 40 be allowed for the ratio of calming section to path length. The transitional zone starts a t about ReD, = 1900 (Figure 8, A ) and fully turbulent f l o ~ va t ReD, = 4400. According to Washington and Marks ( 2 2 ) , the transitional zone falls between 3400 and 14,000 in pressure drop correlations. T h e difference between values can probably be explained by the work of Jakob and Dow (70) who found that the critical Reynolds number value depends on the shape of the leading edge of the plate. T h e more "streamline" or smoother the leading edge, the lower will be the critical Revnolds number.
This form has been used by previous investigators (3; 23). There is some question concerning the length term to use for Reynolds number. Badger and Banchero ( 7 ) recommend length of the drying pan, L , which has been used (70) in experiments on heat transfer to flat plates. Wenzel used this term, most likely because of the rather irregular shape of his drying chamber. I t is common practice in heat transfer problems involving annuli to use the equivalent diameter, D,: De
=
4
(cross-sectional area of flow) (2) wetted perimeter 2%
0
N E F T ZER :EtT
LVashington and Marks (22) used D e in their work on pressure drop through channels of irregular shapes. This procedure compared well with correlations of pressure drop through
INDUSTRIAL AND ENGINEERING CHEMISTRY
STEPII'
Figure 7. Addition of steam improves drying rate only up to about 50% Mass flow rate, 1 1 25 Ib./(hr.)(sq. ft.); temperature, 340' F.
vapor
SUPERHEATED STEAM-AIR DRYING
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5 v
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7
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3k103
6YIC3
REYNOL2S NO.-(DG/IL)
1x104
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Fiaure 8. Correlation of heat transfer coefficient with Reynolds number based on Reynolds number based on L ( E ) showed no significant difference Steam,
0
AS
SSI-SSIV ASS
In this experiment the leading edge of the pan was out of the flow stream, and essentially little or no edge protruded, corresponding to almost streamline flow over the pan and accounting for the lower critical ReD, obtained. Using the method of least squa
