Ind. Eng. Chem. Process Des. Dev. $902, 21, 717-721
stitution of Chemlcal Englneers: London, 1969.
Sakata, M.; Yanagi, T. In “Dlstilletion-l979”;Institution of Chemical Engineers Symposium Series No. 56; Institution of Chemical Engineers: Rug
Received for review November 9, 1981 Accepted May 10, 1982
by, 1979, Vol. I , p 3.2121. S k y . F. C.; Keller, 0. J. Chem. Eng. Prog. 1988, 62(1). 66. Silvey. F. C.; Keller, 0. J. in “Distillation-1969”;Institution of Chemical EngC neers Symposlum Series No. 32; Institution of Chemical Engineers: London, 1969; p 4:18. Underwood, A. J. V. Trans. Inst. Chem. Eng. 1932, 70, 112. Zuiderweg, F. J.
717
Presented at the AIChE Spring National Meeting, Symposium No. 44, ‘The Effects of Mass Transfer and Hydraulics on Distillation Tray Performance”,Houston, April 1981.
”RecommendedText Mixtures for Dlstiilation Columns”; In-
A Magnetic Control Valve for Flowing Solids: Exploratory Studies Wang Yang,’
E. Jaraiz M.,2 Octave Levensplel,’
and T. J. Fltzgerald’
Chemical Engineering Department, Oregon State Unlvers&, Corvallis, Oregon 9733 1
This paper reports on preliminary experiments with a new type of valve for shutting off or for controlling the flow of solids In vertical plpes. It can also be used as a distributor plate plus downcomer for fluidized beds. However, the stream of solids must be magnetic or must contain some magnetic particles. The grate arrangement was studied in detail. This device has no moving parts or mechanical action and does not require much power to operate (-30 W for a 6-in. pipe). The results give promise for the development of new types of gas-solid contactors with close to plug flow of solids and for multistage fluidized beds which completely avoid and bypass downcomer problems.
Introduction Suppose an electrical conductor such as a copper rod passes through a crowd of magnetic particles. When an electric current flows through the conductor then from the laws of electromagnetism the particles will become magnetized, will line up, and will also be attracted to the conductor. This is shown in Figure 1. From this principle comes the idea for a device for arresting or controlling the flow of solids in pipes. Suppose we insert a set of copper tubes across the flow path of a stream of raining magnetic solids in a vertical pipe (see Figure 2a). When an electric current is passed through the copper tubes and is high enough, the falling particles will be attracted to the conductors and freeze in place, eventually blocking the flow channel (see Figure 2b). After this the solids will pile up above the blocked zone (see Figure 2c). Thus we have the makings of a shutoff valve for the flow of solids. And if the current is switched on and off at controlled intervals, we have a flow controller for solids, all with no moving parta and no mechanical action. Fitzgerald and Levenspiel (1982) have coined the term magnetic valve for solids (MVS) for this device. If air is passed upward through the frozen solids at a high enough velocity the solids resting on the frozen layer will fluidize (see Figure 2d); thus we have the makings of a distributor plate and of a solids flow controller for a fluidized bed. This is called the magnetic distributordowncomer (MDD). This paper reporta on preliminary studies with the MVS, the different geometries that can be used, the current needed to freeze the particles in place and block the flow channel, called the holding current, and the energy requirement of the device. ‘On leave from The Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, Shanxi, China. On leave from University of Salamanca, Spain. 3TRW, Inc., Redondo Beach, CA 90278. 0196-4305/82/1121-0717$01.25/0
Theory Let us see how the spacing of a set of parallel copper conductors, as shown in Figure 2, influences the holding current of the MVS. For this, consider a single conductor X, with flowing current I , surrounded by its blanket of frozen magnetic particles. Focus attention on the outermost particle Y in the lateral direction, the one which is most weakly held (see Figure 3). This particle is affected by four kinds of forces, namely the interaction F1 and F2 with its magnetic neighbors, attraction to the conductor F,, gravity Fg,and the frictional resistance to sliding Ff. Forces F1and F2 are equal to each other in magnitude but are opposed in direction: hence there is no tendency to move up or down. The frictional force Ff is directly proportional to the force pressing the particle in toward the conductor F, and related to it by the coefficient of friction. Finally, this frictional resistance just overcomes the force of gravity Fgsince this particle is still being held. In mathematical terms the force between particle and conductor is deduced from Jackson (1962) as
where H is the magnetic field intensity. Note that the attractive force is also dependent on the gradient of the field intensity dHf dr. For a straight line conductor the field strength in its vicinity is H = I/2ar (2) where I is the current intensity. Hence (3) -dH/dr = I / 2 a r 2 Combining eq 1, 2, and 3 then gives (4)
The friction force is proportional to F, or Ff= k’F, 0 1982 American Chemical Society
(5)
718
Ind. Eng. Chem. Process Des. Dev., Vol. 21, No. 4, 1982
Table I. Grate Valve Variables spacing of copper tubes (center to center distance d,) 13.0
0.d. of copper tubes
3.2
aperture of the iron wire screen, d,
-in. tube) 5.9 (screen I)
19.5
(
copper wire
-” f
iron filings sprinkled on paper
25.4
31.7 m m
6.4 ( 1/4 -in. tube)
9.5 mm (3/, -in. tube) 11.5 m m (screen 3)
8.1 (screen 2 )
turn on current
f2l-i-P power
Figure 1. Experiment showing that magnetic particles line up in chains and are attracted to the electrical conductor. Where friction is overcome (near the conductor) the particles slide to the conductor.
Orate m a d e of water cooled copper tube
Figure 4. Schematic of the equipment. Table 11. Six Sizes of Particles Used Tyler screen sizea -8
-10 -14 -20 +10 + 1 4 + 2 0 + 4 0 ‘fluidized solids
solids piled up .solids frozen
a
(b)
(0)
(C)
(d)
Figure 2. Action of a magnetic valve (MVS): (a) With no current flowing, the solids rain past the tubes; (b) with high enough current solids freeze to the tubes; (c) the MVS a~ a shut-off valve; (d) the MVS as a distributor plate for a fluidized bed. /\
,/
‘\ \
/
\
/
\
/ /
5 t
\ \
I i
I
/-----
I I
F
\\
i copper
7
tube
Figure 3. Forces which freeze a particle Y onto the blanket of particles surrounding an electrical conductor X.
where k’ is the coefficient of friction. Finally, the particle will just be held in place when F f = Fg (6) where Fg = V,pg. Combining eq 4,5, and 6 gives k’(p - 1)Vpr” V*Pg =
average particle 2.0 diameter, d, ( m m )
P
thus the holding current (7)
where K = constant. Naturally the proportionality constant K varies with particle shape and diameter and particle material. Also
1.4 1.0 0.69
-40 -80 +80 +150 0.30 0.14
The 40 and 80 mesh screens are U.S.standard screens.
it should be mentioned that this is only an approximate analysis for a number of reasons. Firstly, the magnetic permeability is not constant everywhere but changes abruptly at the surface of the tube. Secondly, the outermost particle Y may not have balancing forces Fl and F2 because there may be no particle below it. Finally, eq 2 only applies strictly to a line conductor while we have copper tubes. Nevertheless, this type of analysis shows the form of the relationship to be expected between the variables. We have only made our analysis at the lateral extension from the conductor because it is this position which is pertinent for flow blocking in an MVS. Experimental Section The preliminary experiments reported in this paper were designed to seek favorable arrangements for a magnetic grate, the effect of diameter of the copper conductor, the spacing of conductors, and the effect of adding iron screens next to the conductors. The equipment consisted of an adjustable dc power supply, a digital ammeter, copper grates, and iron screens, as shown schematically in Figure 4. The grates consisted Of ‘/gin. (3.18 mm), ‘/&I. (6.35 mm), and 3/&. (9.53 mm) copper tubes arranged at various spacing. Preliminary calculations suggested that very high currents would be required to freeze the solids. Thus 200 A, 5 V dc power supplies were connected in parallel to give up to 600 A. At these high currents water cooling was needed to keep the temperature, and hence resistance of the grates, unchanging with time. For this reason we used copper tubing with internal water flow in all the experiments. The holding power of the copper tube is independent of voltage hence there was no need for high voltage. Certainly safety considerations recommended low voltages. This led us to use commercially available 5 V power supplies. The first set of runs was made using the copper grates alone; then iron wire (-0.5 mm) screens were added just above and just below the grates. Table I shows the values
Ind. Eng. Chem. particle d p l m m l
tn E
dt=64mm
ta
0
C
P
U
2ooc
-
In
Process Des. Dev., Vol. 21, No. 4, 1982 I
I
particle diameter d,= 2 0 "
I
I
A x d , = 19 5 mm
0
-
14 20
H
c
e
d, = 9 5 mm
-1
&
.-
I
Diameter of Copper G r a t e ,
I
x 0 A 0
I
I
no screen 5.9" 8.1 mm 11.5"
/ /
/
'
o
n
X
1
with e grate
dp = I Omm
I / 0
'-
-
!grate with screens
100 (Grate Spacing, m m
1 200
f5
:d o 1
Figure 6. Iron screens placed just below the copper tubes reduce the holding current, especially for large grate spacing.
of the variables examined in this study and Table I1 shows the six narrow size cuts of soft iron particles used in these experiments. The following procedure was used for all runs. At first the current through the grate was raised to a high value; then particles were poured onto the grate in a 2-in. tube. Some fell through, but soon the whole cross section was covered with frozen particles as shown in Figure 2. Then the current was reduced until particles started falling from the grate. This current was defined as the release current and was the primary measure of the holding ability of that particular arrangement of the magnetic valve. Results and Discussion Figures 5 to 8 display the results of experiments aimed at finding how the variables of the system affected the holding ability of the magnetic valve. These findings are summarized below. Spacing of the Copper Tubes in the Grate, dt. For the grate alone eq 7 suggests that a plot of release current Io vs. d,3f2 should give a straight line through the origin. Figure 5 shows such a plot for all the data taken with the grate made of the middle sized copper tube. Each data point represents the mean of six or more readings, with an average scatter of about 5% on either side of the mean. This figure shows that the predictions of eq 7 well fit the
d,
Figure 7. With or without iron screens the tube diameter plays a small but distinct role. Small tubes require a smaller holding current.
e
screen aperture
I
0
grate alone, do = 19 5 mm
dp1'5
Figure 5. Holding current of a copper grate MVS in the absence of iron screens and for various geometries. Data points and the solid line are for If4-in. copper tubes; dashed lines show the slopes for 'I8and 3/s-in. copper tubes.
screen directly below
y f r z t5 u r e ,, d, = 5 9 m 10m
V
(Grate S p a c i n g ,
grate with
@ / o L o n
E
i
I
3
V CI,
300
719
Y
0
20
10 P a r t i c l e Diameter dp
,
mm
Figure 8. Higher holding currents are needed for fine particles than for coarse particles. All runs with 1/4-in. copper tubes with a grate spacing d, = 19.5 mm.
experimental findings. For other sizes of copper tubes similar relationships hold. Influence of Iron Screens. Typical of all sizes of copper tubes and of particles, Figure 6 shows that iron screens placed directly below the copper tubes sharply reduce the current needed to freeze the particles. Clearly, at large grate spacing the iron screen is much more effective at reducing Io than at small grate spacing. In addition, small apertures in the iron screen are much more helpful at large grate spacing. Overall, the smallest aperture in the flow channel, whether it be of screen or of grate, will be the main factor in determining the holding current of the MVS. This is shown in Figure 6, where the line for small screen aperture is practically horizontal for all grate spacings. Effect of Tube Diameter and Spacing of Grate. All other factors being equal, larger tube diameters mean a weaker magnetic field intensity at given r, and hence a higher holding current, as shown for the grate alone in the three upper curves of Figure 7. When iron screens of small aperture are present they will determine the required holding current; hence the curves for different d, all collapse to the lower curve in Figure 7. Grate spacing becomes unimportant here; however, tube diameter still plays a small but distinct role. The Effect of Particle Diameter. Although frictional effects should vary with particle size, the theoretical analysis leading to eq 7 suggests that the attractive force should be size independent. Let us look at the frictional force. If we refer to the downward discharge of solids from a vertical pipe, we find from Kunii and Levenspiel (1969) that (discharge rate of solids)
d,4.246
Thus slightly higher holding currents would be needed to
720
Ind. Eng. Chem. Process Des. Dev., Vol. 21, No. 4, 1982 n
magnetic feeder or solids
( a ) Prong plus
( b ) Coil plus
iron screen
iron screen
Figure 9. Other geometries for the MVS beside the grate arrangement: (a) the prong design; (b) the coil design.
counter the higher discharge rates for smaller particles. Figure 8 for a particular grate geometry shows that the holding current does in fact decrease slightly with increase in particle size, both with and without iron screens. This graph is typical of all the geometries studied. A more detailed tabulation of the data in this study can be obtained by writing the authors directly. Power Requirement for the Operation of an MVS. The energy consumption for an MVS comes from two mechanisms, namely the resistance loss of the conductor and the hysteresis loss of the iron particles as the current is switched on and off. The resistance loss is given by
As an example of the magnitude of this loss, consider 100 A passing through a grate consisting of 3 m of '/&I. copper tubing placed in a 6-in. vertical flow pipe. Then 1 = 3 m, S = 1.68 X 10" m2,and p = 1.7 X am. Replacing in eq 8 gives P=30W
(9)
The hysteresis loss can be estimated by the Steinmetz equation, from Gilbert (1941)
W = TB" [J/m3 of solids] where in SI units W = hysteresis loss per on-off cycle, J/m3 of iron; 7 = constant for a given iron: for annealed iron 2.25 X lo-''; for hardened steel 3.75 X lO-'O; B = maximum magnetic flux density, and n = 1.6-1.7. For our experimenta take 11 N 3 x W1,B N 1.5 We/m2 and let the iron layer frozen in the 6-in. pipe be 0.02 m thick. Then
W=3
X
lo-'' (1.5)1.7= 6
X
lo-" J/m3
If the current is switched on and off twice a second, the power dissipation is
P = (6 X
lo-")
(1- 0.152) (0.02)
X2 = 4 X
W (10)
The rate of total energy loss is the sume of eq 9 and 10. The above figures show that the power consumption of an MVS is not a serious consideration.
Additional Exploratory Observations The copper grate alone and with iron screens was studied quantitatively here. The results showed that iron screens reduced the holding current significantly. Nu-
slowly
moving
packed bed
current switched on a n d o f f
(0)
(b)
Figure 10. Switching the current off and on controls the flow rate of solids: (a) multistage moving bed; (b) multistage fluidized bed with no downcomers.
merous other arrangements were explored. (a) Iron screens placed above the copper grate were nowhere near as effective as iron screens below, and so this geometry was not pursued further. (b) The prong arrangement with or without screen (see Figure 9a) worked satisfactorily and was well able to freeze the solids. (c) The coiled conductor around the flow tube plus iron screen (see Figure 9b) worked satisfactorily. It looks like an attractive arrangement and will be studied and reported on soon. (d) When air was passed upward through the frozen solid, the solids resting above became fluidized as shown in Figure 2d. Even with the vigorous bumping and slugging of the fluidized iron particles the frozen solids were not dislodged, making the MVS an effective distributor for a fluidized bed. (e) With either the fluidized bed above or with just resting solids (Figures 2c and 2d) pulsing the current on and off allowed the solids to drop from the MVS at a controlled rate. (f) It was also possible to stop the flow of mixtures of magnetic and nonmagnetic particles with this magnetic valve, even in mixtures containing just a small fraction of magnetic particles. Most of the preliminary observations reported here will be explored in detail in the near future.
Summary and Conclusions The findings in this paper can be summarized as follows. (a) An electrically conducting grate consisting of copper tubes placed across the flow channel can be made to act as a flow valve. Reducing the spacing of the tubes (Figure 5), the size of the copper tubes (Figure 7) or increasing the size of the flowing particles (Figure 8) makes the grate
Ind. Eng. Chem. Process Des. Dev. 1002, 21, 721-725
721
d, = particle diameter, m d, = diameter of copper tube making up the grate, m d, = aperture of wire screen made of iron, m F = force on a particle, N H = magnetic field intensity, A/m I = current passing through the copper tubes of the grate, A Io = holding current, that which will just keep the flow channel blocked, A 1 = length of copper tube conductor, m P = power loss in blocking or controlling the flow of solids, W r = center to center distance between the copper tube con-
more efficient in that the holding current is reduced. (b) Iron screens placed below the copper grate reduce the holding current; the smaller the screen aperture the more effective the screen (Figure 6). (c) To stop or control the flow of solids in a 6-in. vertical flow tube, or to serve as a distributor plate for a fluidized bed of iron particles, a reasonable design consisting of copper grate and iron screen requires about 50-100 A. Power dissipation is calculated to be about 30 W. (d) Various alternative geometries will work just about as well as the copper grate plus iron screen arrangement. (e) The attractive feature of the MVS is that it has no moving parts, no mechanical action, and has rapid response. (f) These valves can be the key to the development of a new class of countercurrent gasaolid contactom through which the solids pass in close to plug flow at a controlled rate, as shown in Figure 10. This may be the way of using rapidly deactivating friable catalysts. Acknowledgment This research was done as part of a grant from NSF, No. CPE-8026799. E.J.M. is grateful for a postdoctoral grant received from the Comite Conjunto Hispano Norteamericano para la Cooperacion Cientifica y Tecnologica (Spain). Nomenclature d, = center to center spacing of the grate, m
ductor and a particle, m R = resistance of copper tube conductor, fl S = cross sectional area of copper in the copper tube conductor, m2 V , = volume of particle, m3 Greek Letters p = electrical resistance of the copper tube conductor, fl-m p = effective permeability of packed bed, H/m Literature Cited Fitzgerald, T. J.; Levenspiel, 0. U.S. Patent Applied for, 1982. Gilbert, N. E. "Electricity and Magnetism"; Macmillan: N e w York, 1941; p 210. Jackson, J. D. "Classical Electrodynamics";Wiiey: N e w York, 1962; p 176. Kunii, D.; Levenspiel, 0. "Fluidization Engineering";Wiiey: New York, 1969; p 370.
Received for review November 17, 1981 Accepted April 27, 1982
Fluosilicic Acid Extraction by High-Molecular-Weight Amines Patrlcla A. Tlvnan' and Rlchard J. McCluskey" Department of Chemlcai Engineering, Clarkson College of Technology, Potsdam, New York 13676
The extractlon of fluoslilclc acid from dilute aqueous solutions by high-molecular-weight amines in kerosene has been studied. Extraction equlllbrium curves for various commercial amines are presented for room temperature, an amine concentration of 0.2 M, and aqueous acid concentrations up to 1 M. Formation of stable suspensions, coextraction of water, and aggregation of amine salts are discussed. Amine extraction of fluosilicic acid is compared
with that of other mineral acids.
Introduction Phosphate rock typically contains 3-4% fluorine. In the United States, phosphoric acid, a basic raw material of the fertilizer industry, is produced mostly by the wet processing of phosphate rock, wherein the crushed rock is contacted with sulfuric acid. This treatment causes most of the fluorine to convert to fluosilicic acid, H a s B . Some of this acid fluoride is recovered, but much is discarded. Fluosilicic acid and ita salts are valuable byproducts. They find use in aluminum smelting and water fluoridation. Furthermore, large potential applications exist in the production of high-purity silicon crystals for solar cells and in reducing the importation of fluorine in the form of fluorspar. There is environmental concern over the effects of discharged fluoride on soil fertility, and the concern over fugitive emissions grows as the phosphate mining industry IBM, Essex Junction, VT 05452. 0 198-43051821112 1-072 180 1.2510
expands (Liner0 and Baker, 1978). A 1977 EPA study estimated that the phosphate fertilizer industry was responsible for 14% of all domestic soluble fluoride emissions (EPA, 1977). Thus, there is incentive to find economical means of recovering fluosilicic acid from waste water streams from the viewpoints of both environmental protection and increased yield of a marketable product. Some of the methods that have been proposed for recovery of the fluoride include precipitation as calcium fluoride (Blake and Stickney, 1978; O'Neill, 1980), conversion to concentrated hydrofluoric acid (Becker and Weiss, 1972), precipitation as sodium aluminum hexafluoride (Kriihnan and Mahalingam, 1973), and collection by an anion-exchange resin (Rozycka, 1978). This paper examines the feasibility of extracting fluosilicic acid from dilute waste water streams by contact with high-molecular-weight amines dissolved in kerosene. Some advantages of acid recovery via solvent extraction rather than by contact with a solid anion-exchange resin 0 1982 American Chemical Society
