INDUSTRIAL AND ENGINEERING CHEMISTRY
1196
ture in a fluidized bed, and in cases where heat is being transferred into and out of the bed, there may be significant radial differences. The data in Figure 13 do not alloiT one t o determine which of the two methods of inixing is the more effective. Heat is undoubtedly carried down to the lower section by solid flow down along the walls, but there are no data available to indicate the magnitude of the flov-.
100
80
P -&
Vol. 41, No. 6
60
2
L
SUMMARY
I
2 40
T h e experimental data on gas mixing indicate that the backmixing of gas in small fluidized beds with high L I D ratios is relatively small and in the case of most reactions not particularly detrimental. On the basis of these data, it is recommended t h a t reaction rate studies conducted in such units be correlated on the basis of piston flow, neglecting mixing. The data on the heat flow in the fluidized bed indicate t h a t solid mixing is relatively rapid and t h a t the sensible heat carried by the solid can serve to maintain relatively constant temperature throughout the bed, despite a wide vaiiation in rate of heat release.
2
?. PO
__----
COOi€D
_/---
0
Figure 13.
5
-+-H€A7tU.4
20 25 /NCVLS FROM BOTTOM OF COLUMN 10
15
30y'
Radial T e m p e r a t u r e Differences Heated -Cooled C o l u m n
in
Filtrol, superficial air velocity 1.2 feet per second
It is intcrcsting to consider the mechanism by which the heat is transferred from the upper to the lower section. T h e thermal eddy diffusivity for the gas only JT-as estimated from the gas mixing eddy diffusivity, and a n estimate of the heat transfer from the upper to the loner section on this basis accounted for less than 3% of the total. Thus i t appears that the heat must be mainly transferred by the mixing of the solids This downflow of heat corresponds t o a relatively rapid mixing of the solid, which could be of tlyo types. First, the mixing could be due simply t o general turbulence, which progressively transmitted the heat to a lo\yer level, or it could be the dovnflow pattern similar t o that suggested above-Le., the solid would flow up the center and down the \$all. This would not imply that there Kas not considerable random inixing but simply that such ciiculation x a s superimposed upon the vertical pattern. I n order to investigate the mixing pattern, temperatures Tyere measured by a thermocouple approximately 0.1 inch from the wall along the length of the reactor. T h e data are given in Figure 13 as the difference between this temperature and the center-line temperature. A t the bottom of the unit this temperature difference 1s relatively small but it rises to a fairly high value a t the top of the cooling section. This indicates t h a t temperature gradients along the renter line may be misleading as to the constancv of tempera-
NOMENCLATURE
B C CJ Cy
=
= = = = =
C, D E = k =
8' L n
R 77
-V
e
=
= = = = =
=
constant concentration concentration leaving reactor concentration leaving reactor, no gas mixing concentration entering reactor diameter eddy diffusivity first-order reaction rate constant apparent first-order reaction rate constant length of reaction zone quantity material transferred per unit area radius superficial gas velocity distance time L I T E R A T U R E CITED
(1) Ciboron-ski, J. IT., 10-90 Report, Chem. Eng., Massachusetts Institute of Technology, 1947. ( 2 ) Kennel. W. E., S.M.thesis in chemical engineering, Massachusetts Institute of Technology, 1946. (3) ,McAdams, W. H., "Heat Transmission," 2nd ed., p. 186, New York, McGraw-Hill Book Co., 1942. (4) Sweeney, G. C., Jr., S.M. thesis in chemical engineering, Massachusetts Institute of Technology, 1948. KECEITEDJanuary 19, 1049.
ue to Dust Particles in a Gas Stream A few experiments were m a d e i n a n effort t o evaluate some o f t h e basic factors of erosion t h a t can arise in gas-solid flow systems of h i g h velocity. In essence theexperim e n t a l method was one of sandblasting targets a t different j e t velocities a n d angles of i m p i n g e m e n t w i t h different entrained dusts. T h e a m o u n t of erosion was determined f r o m t h e change i n weight of t h e targets. R. L.STOKER1 WESTERN PRECIPITATION CORPORATION, LOS ANGEI-ES, C A L I F .
E
ROSIOK can occur v h e n small solid particles carried in a
fast-moving gas stream impinge upon a solid surface. In sandblasting operat'ions such as cleaning castings and et'ching glass this eroding action is emphasized. Considering how long sandblasting has been used, one would expect to find literature describing all phases of the art, and technical and experimental 1
r.08
Present address, Department of Engineering, University Angeles, Calif.
of
California,
inforination on the essential factors, and the relationships existing between them, that are responsible for this type of erosion. Although much has been written on the qualitative nature of sandblasting, quantitative, basic information is surprisingly scarce. Current erosion studies a t high gas velocities have been reported by Fisher and Davis (1). T h e experiments described here were exploratory and preliminary steps taken a fen. vwrq a g o in an attempt to solve certain
Iune 1949
INDUSTRIAL AND ENGINEERING CHEMISTRY
PLW COCK OR/F/C€
1 Figure 1.
*
VENT ~ O Z Z L E
Schematic Diagram of Experimental Apparatus
erosion problems associated Kith fluid type cat,alytic cracking plants. Only black iron and gypsum plaster were used as target materials in these experiments. The dusts were a fairly fine silica sand and t w o grades of a commercial synthetic catalyst. Air at ordinary conditions served as the gaseous vehicle in all these tests. The air velocity was varied from approximately 100 to 600 feet per second for the iron targets and from about 60 to 150 feet per second for the plaster ones. Several angles of impingement mere tried in the case of the black iron. In spite of the limited scope of these experiments, the results have proved very useful in interpreting and correcting erosion difficulties in engineering practice. EXPERIMENTAL METHOD
The experimental apparatus used is indicated schematically ill Figure 1; allhough the setup was not designed specifically for these erosion studies, it served this purpose satisfactorily. Atmospheric air was drawn through a metering orifice by a small positive displacement blower, and after passing through a @urgechamber to smooth the pulsating flow, was discharged from a nozzle onto the specimen being eroded. The air flow was varied by means of a valve in the by-pass line of the blower. The dust from the reservoir flowed through an orifice in a vertical pipe in the manner that sand flows in an hourglass. The dust then joined and mixed with the air stream a few inches above the nozzle. The pressure above the dust bed in the dust reservoir and that below the dust orifice were made equal by means of a pressure-equalizing line. The valve in the equalizing line and the one below the dust reservoir were opened at the start of a test. The specimen to be eroded was mounted on a holder which could be rotated about a horizontal axis to ,provide several different angles of impingement of the jet. The chamber containing the specimen holder and the nozzle had a vent through which the air and some of the fines in the dust passed to the atmosphere. A hopper was provided below to collect the remaining dust. Fresh dust \vas used for each test. The black iron specimens were prepared from hot rolled, commercial quality, 16-gage sheet metal and were cut into squares approximately 2 inches on a side. After acid etching to remove all mill scale, each specimen was weighed by means of an analytical balance and then stored in a dry place until needed. By positioning the specimen on the holder so that its center was displaced slightly from the nozzle axis, four tests on each side of the specimen could be made without overlapping of any eroded areas. All the specimens were given an eroFiion test a t one set of conditions
1197
and only those that experienced an erosion to within =t3% of a previously chosen value were used in further tests. Thus after meeting this acceptability test, each specimen was used for seven additional distinct tests. The specimen mas weighed before and after each test and the change in weight was the measure of the erosion. The plaster specimens were all made from one batch of commercial gypsum plaster, which was well mixed with a specified amount of water and poured into rectangular molds to set. Later the plaster casts were sawed and sanded into 2-inch squares about 0.125 inch thick and cemented to thin metal backings to facilitate handling and mounting in the holder, as vias done \vith the black iron targets. Only three testd besides the acceptability test could be made on each plaster specimen. In all the tests reported here the amount of material eroded was kept to a minimum consistent with reliable determinations or neight change. Thus the target surface remained a plane surface in so far as was discernible. Iron was found to withstand erosion slightly better after prolonged erosion tests, probably because of the cold working effect on the surface, resulting from the continued impacts. However, all results reported here are for tests of ninch less duration than would cause this effect to become noticeable. Three dusts were employed in the black iron erosion tests: fine, washed, silica sand, and two grades of fresh commercial synthetic catalyst (A and B) such as that used in fluid type catalytic cracking plants (Table I). The A catalyst was typical of that normally used, whereas the B grade vias a bit coarser. The angle of impingement tests were made TTith the silica sand and iron targets. All other tests were with the jet impinging perpendicularly to the target. The plaster erosion runs were made with catalyst A only.
Table I.
Particle Size Distribution of Dust"
Silica Sand (Del Monte Regular) Mesh gr,
t 20
30 40
0.1 1.6 29.6
50 70 100 100
40.3 25.7 2.6 0.1
Catalyst A Mesh 7%
+I4000
200
207:;
Catalyst B-Mesh % + 30 0 0
39.0
60 100
8 0 15.0 8.0 4.0
- 200
2.3
79.0
Microns
a
74-40 40-20 20-10
10-0
1.50 200
4 0
7 5 7.0
These analyses are representative clmsifications of the three dusts used.
The nozzle used had a conical inlet and was i/g2 inch in inside diameter and about 1 inch in length. The nozzle exit was located approximately 0.5 inch above the target. The inside surface of the nozzle 'Fvasso hard that any change in diameter was imperceptible. The velocity of the air discharging from the nozzle was computed from the metered rate of air flow, cross-sectional area of the nozzle, and air temperature and pressure. The air temperature in the jet was measured without dust flow before and after tests. The pressure surrounding the jet was atmospheric. The rate of flow of dust through the dust orifice was constant for any given dust; consequently the resulting dust loading varied inversely with the air flow or the air velocity, The silica sand flowed at the rate of approximately 25 grams per minute, which was about two or three times that of the two catalysts. Thus the dust loadings corresponding to an air velocity of 200 feet per second were approximately 125 grains per cubic foot of air for the sand and about 50 grains per cubic foot for the catalysts. The dust concentrations employed in these experiments fell in the usual range encountered in the "disperse phase" that exists above the fluidized bed in the reactor and regenerator of fluid type cracking installations.
INDUSTRIAL AND ENGINEERING CHEMISTRY
1198
0
1
I
I
100
200
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AIR Figure 2.
I 400
V€LOClT)(
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528
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FUS€C
Erosion of Black Iron
Jets of various dusts directed perpendicularly t o surface of test
0
100
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AIR V€LOClTK Figure 3.
Vol. 41, No. 6
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NO
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FT/SEC.
Erosion of Black Iron
Data in Flsure 2 expressed as r a t i o of erosion of black i r o n a t various j e t velocities t o eroslon at 200 feet Der second
specimen
i n general, the erosive power of a given amount of dust dispersed in a gas stream depends partly upon the ratio of dust to gas. However, as the dust concentration is loxered the individual dust, particles will interfere with each other less and less during impingement upon solid boundaries and thus for very loiv dust loadings the erosion should be independent of the concentration. It was thought that results were not appreciably affected by variations in the low concentrations used in these experiments. Consequently, the amount of erosion in thcso esperiments was espresed in terms of the quantity of dust, used in each run without, further consideration of the concentration. An amount of dust equal t o 500 grams was conimonly used in these test's, but the lolvest velocity test on iron required several thousand grams to produce an adequate change in the target weight.
would be abuut one tenth as large as at, 200 feet per second. Should an experimental extension of this curve down to still lower velocities continue to follow the trend indicated in Figure 3, t,he change in erosion with a change of air velocity would be even more pronounced. Thus it appears, when operating along certain portions of the curve of erosion m.velocity, that a very powerful way to reduce erosion is to lover rhe velocity. I n Figure 4 the results of blasting black iron with silica sand at various angles of impingement are shown. The air velocity was 540 feet per second for all the angles investigated. A portion of the curve is shown by a broken line because insuficient data were available to establish t,he curve definitely in this int>erval. The
E X P E R I M E N T A L RESULTS
The result,s obtained from hlusting black iron perpendicularly to its surface wit,h silica sand m d the t,xo grades of synthetic catalysts are plotted in Figure 2. The data from the two catalysts can be represented by a single curve that indicates the erosion with the catalysts to be a little more than one half of that caused by the sand. I n Figure 3, all the data in Figure 2 are found t o lie on a single curve if the relative erosion rather than the actual erosion is plotted. The relative erosion lor any given velocity was arbitrarily defined as the actual erosion at the given velocity divided by the actual erosion that occurred with a velocity of 200 feet per second. Alt,hough the results from the three dusts in these experinients lie on a single curve, this does not justify the conclusion that all dusts and targct mat)erial combinations would do likewise. Probably t,he most interesting aspect of the curve in Figure 3, from the practical standpoint of alleviating erosion, is the strong variation in erosion with the gas velocit'y, particularly at, the lower velocities. For inst'ance, if the curve is extrapolated to a velocity of 100 feet' per second it is seen that there the erosion
0 30 60 ANGL€ OF /MP/NGEM€NT Figure 4.
90 DEGREES
Comparative Erosion of Black Iron
Various angles of impingement of silica sand in 540 feet per second air j e t
INDUSTRIAL AND ENGINEERING CHEMISTRY
June 1949
1199
P r,
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9
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/o0
/50
200
AIR V€LOC/T): FT/SEC. Figure 6. Volume Ratio of Piaster Erosion t o I r o n Erosion Catalyst A w l t h 90° jet Impingement angle
40 Figure 5.
6 0 80109 150 ZOO
AIR V€LOCI T yi f /r/S€C. Erosion of Gypsum Plaster
The conditions that must theoretically be satisfied in order to predict erosion of one material from that of another may appear rather awkward-but so is erosion testing in the prototype. Assuming that plaster can be used in this manner, a single day’s test with it would be equivalent to a run of several months’ duration with iron.
Catalyst A dlspersed in jet implnging perpendicularly t o surface of t e s t specimen
maximum erosion occurred a t about 20” and was approximately twice as large as when the impingement was perpendicular to the target’s surface. The erosion of gypsum plaster with catalyst 9dispersed in the air jet is shown for various velocities and at a 90” impingement angle in Figure 5 . This curve was drawn as a straight line on the full logarithmic plot, as the experimental points do not warrant a more refined graphical treatment. The line drawn indicates that this erosion varies approximately as the cube of the air velocity. The corresponding erosion curve for iron in Figure 2 would plot as a curve on a log-log plot, but a t 150 feet per second its slope would be very nearly that of Figure 5. The volume ratio of plaster erosion to iron erosion for catalyst A a t 90’ impingement for the velocity range between 100 and 200 feet per second is shown in Figure 6. This ratio of erosion was obtained from the erosion by weight of Figures 2 and 5 by taking into account the relative densities of iron and the plaster. It would be very useful if the erosion in metal installations could be predicted fIom the erosion of systems made up of easily eroded materials. These experiments with gypsum plaster, although incomplete, were prompted by this considefation. If two gassolid flow systems are identical except for the material of which their boundaries are constructed, in order that the erosion in one system may be simply a constant times that in the other, it is sufficient that the erosion ratio of the two systems be the same at all flow rates. I n a special case the erosion in each system is proportional to some corresponding velocity of the two systems raised to the same power. Among these experiments are flow systems that are identical except that the target is iron in one case and plaster in the other. In Figure 6 i t is seen that in the velocity range of 100 to 200 feet per second the volume-erosion ratio does not vary by a large percentage. One might then conclude that between these two velocities it should be possible to predict the erosion of iron fairly well from data on plaster. This would be true for systems in which all particles impinge perpendicularly. T o predict for more general ftows would require t h a t the plaster have a curve of erosion vs. impingement angle that differs from one for iron by a constant factor over the above velocity range.
CONCLUSIONS
Although these experimental results are limited in scope and application, some significant factors relative t o erosion are indicated. Thus the important role that velocity may have, the criticalness of the angle of impingement a t low angles, and the possibility of using plaster models for predicting the erosion life of more durable materials all appear to be very significant and should be investigated more exhaustively. ACKNOWLEDGMENTS
The writer appreciates the permission granted by the Western Precipitation Corporation to present these data, and is grateful for helpful discussions with N. M. McGrsne and D. 8.Lundy. LITERATURE CITED
(1) Fisher, M. A., and Davis, E. F., “Studies on Fly Ash Erosion,” annual meeting, Am. Soc. Mech. Engrs., Paper 48-A-53 (1948) ; Mech. Eng., 70, 1016 (1948) (digest).
RECEIVED January 3, 1949
COURTE8Y LITHGOW CORPORATION
Sandblast Testing
