PHYSICAL TESTSON STOCKS USED
TABLE VI. Stock Gravity A. P. I. Reid vabor pressure, lb./ss. in. Lamp sulfur, wt. % Aniline point, F. Acid heat value, O F. A. 8, T . M. distillation, Initial 5 % over 10%
50%
_ .
yo recovered
Vol. 33, No. 12
INDUSTRIAL AND ENGINEERING CHEMISTRY
1560
O
A 65.0
B 56.6
C 59.4
8.5 0.011 135 8
6.8 0.085 90 130
105 134 149 175 194 209 225
109 135 151 179 205 227 249 269 291 318 351 378 401 98.0
D 56.4
E 62.1
F 63.6
G 66.5
H 71.8
9.9 12.0 5.2 7.6 0.054 0.076 1.485 0.066 120 69 101 106 82 203 219 170
6.4 0.005 128 3
7.3 0.004 165 3
105 133 149 176 199 220
118 141 151 166 176 185 2 3 201 209 224 236 256 283 98.0
104 138 156 186 202 214
F.
239 253 273 303 329 355 98.0
E
255 275 298 339 368 393 98.0
92 118 132 164 194 224
250
282 312 344 380 400 402 96.0
125 172 187 203 215 228
240
256 274 291 325 361 399 98.0
98 138 166 217 233 242
251 263 284 346 400
4'3'0 94.0
method is preferred over an alternative method in which the values of lead susceptibility are read for each constituent a t their respective sensitivities and a t the volumetric average sulfur content, and then averaged to obtain the final value. This latter method is sometimes more convenient, however, and can be safely used if the differences in sensitivity are small. Comparisons of observed C. F. R. Motor and C. F. R. gasolines with Research (1939) Octane data On values predicted by Figure 2 are shown in Table 111. Similar
220 224 230 236 246 250 302 98.0
comparisons for leaded octane values are given in Tables IV and V in which the predicted octanes are based on 4 values read from Figure 4 and on the observed clear octanes prior to lead addition. Physical tests on the various stocks are given in Table VI. These tabulations average deviations of about 0.5 and maximum deviations of about 1.5 octane numbers. It should be borne in mind that although a wide variety of gasoline types has been studied, the relations shown are strictly empirical and due caution should therefore be exercised in the application of these correlations to new or unusual gasoline types.
Acknowledgments I
The writer wishes to acknowledge the assistance of Howard Wilson and E. M. Barber in the development of the octane blending relations, and of G. J. Goddard and N. B. Haskell in the development of the lead response relations,
Literature Cited CHEM., 31,850 (1939). Graves, IND. ( 2 ) Hebl. Rendel. and Garton. J . Inst. PetroZeurn Tech.. 18. 187 (1932); I I D . ENO.CHEM., 31,865 (1939). (3) Lovell, Campbell, and Boyd, Ibid., 26, 1105 (1934).
PRESENTED before the Division of Petroleum Chemistry a t t h e 102nd Meeting of t h e American Chemical ~ o c i e t y , ' ~ t i a n tcity, ic N. J.
CALCIUM METAPHOSPHATE Rate of Reaction of Phosphorus Pentoxide
with Rock Phosphate G. L. FREAR AND L. H. HULL Tennessee Valley Authority, Wilson Dam, Ala.
HE process and equipment used in the manufacture of fertilizer-grade calcium metaphosphate by the Tennessee Valley Authority were described in previous publications (1, 2 ) . The experiments comprising the present work included measurements of the rate of absorption of phosphorus pentoxide vapor in hot rock phosphate as influenced by the temperature, phosphorus pentoxide concentration in the gas, velocity of gas, grade of rock phosphate, and presence of water vapor. The method of measurement consisted in exposing previously calcined test pieces, which had been compacted from finely ground rock phosphate, to atmospheres containing phosphorus pentoxide and in determining the weights gained by the test pieces in different periods of exposure under controlled conditions.
T
Materials The phosphorus pentoxide was generated within the apparatus by burning wi.th cylinder-grade oxygen the phosphorus vapor obtained by saturating oxygen-free nitrogen with high-grade yellow phosphorus.
The rock phosphate used as absorbent in most of the experiments was prepared by crushing several large lumps of Tennessee brown rock. The fluorapatite mas a high-grade Canadian mineral, the phosphatic matrix was from Tennessee, and the calcium oxide was National Formulary grade. The composition of the phosphatic absorbing materials, all of which were -150 mesh, is given in Table I as corrected for ignition loss a t 1100" C.
Equipment The principal elements of the apparatus were the means for generating P205*, the absorption chamber, the P20ssampler, and the means for disposal of the exit gas. Phosphorus pentoxide was absorbed by the material under study in the tube assembly shown in Figure 1. The mullite absorption tube, of 4.3-cm. inside diameter and 70-cm. length, was drilled through the side a t a point near its center; this
* Pi05 is used throughout this paper as the simplest formula for t h e oxide of pentavalent phosphorus and is not intended t o represent any particular
molecular species.
December, 1941
INDUSTRIAL AND ENGINEERING CHEMISTRY
1561
a flowmeter to the phosphorus saturator, The trains through which oxygen was delivered to the phosphorus burner and to the preheating chamber were identical to the nitrogen train, except that the deoxidizing furnace was omitted. The condenser and trap for disposal of the phosphorus pentoxide in the waste gases are shown in Figure 1; a spring suspension was provided to minimize the strain on the joint with the absorption tube.
TABLE I. COMPOSITION OF ABSORBINQ MATERIALS Composition, Weight yo,Ignited Basis Absorbin Materiak PI06 cao SiOn ~0208 ALO, F Fluorapatite 38.7 55.9 1.2 02 0.4 3.58 Brownrock 33.2 45.8 7.7 3.7 4.0 2.71b Matrix 25.2 37.0 20.70 6.7 8 0 a Perchloric-acid insoluble. b Determined on ignited sample.
...
The specimen support was a mullite thermocouple sheath, the lower end of which was clad with platinum welded to a snug fit and held by a platinum ring welded in a groove in the mullite. To the lower end of the platinum shield was welded a stirrup of platinum wire for suspending the specimen. The specimen support was centered on the axis of the absorption tube by means of a triangular guide and was held a t any desired elevation by a rubber sleeve on the adapter by which the support was connected to the absorption tube assembly. ~~
FIGURE 1. ABSORPTION FURNACE
opening, as well as the two ends of the tube, was ground to a taper t o receive the various appurtenances. T o the lower end of the absorption tube was connected the phosphorus burner. The body of the burner was constructed of Pyrex glass with two concentric mullite delivery tubes for conducting the phosphorus-nitrogen mixture and the oxygen to the hot portion of the combustion chamber. A grid of fused alumina separated the combustion chamber from the absorption chamber and supported a liner of platinum foil, which was designed to prevent disturbances of the PZOSconcentration in the absorption chamber due to reactions with the mullite tube. The upper portion of the absorption chamber was protected against entry of gases from above by a fused alumina baffle resting on the platinum liner. Above this baffle was a side tube for the exit of the gases from the exposure chamber, together with the oxygen from the preheating chamber. I n the reaction tube above the side arm was mounted coaxially a mullite tube of 2 cm. inside diameter which served as a preheating chamber for the specimens. A continuous stream of oxygen was passed down the preheating chamber to prevent entry of PzOa. The head of the assembly bore a side tube for the admission of this oxygen and was fitted at its upper end with a sleeve of Gooch tubing and a screw clamp to facilitate the introduction of either the specimen support or the phosphorus pentoxide sampling tube. The phosphorus saturator (Figure 1) was constructed of Pyrex glass and contained two fritted glass disks; the first disk was for dispersion of the carrier gas through the phosphorus and the second for removal of entrained phosphorus. The saturator was heated in a thermostat. The delivery tube connecting the saturator to the phosphorus burner was lagged and electrically heated. Cylinder-grade nitrogen was deoxidized with hot copper, dried with anhydrous calcium sulfate, and then passed through
~
In the manufacture of calcium metaphosphate, rock phosphate is treated with PzO, a t elevated temperatures. To study the factors affecting this reaction, individual pellets of rock phosphate of different compositions, of fluorapatite, and of lime were suspended in a tube furnace in a stream of gas containing PzO,. The temperature, gas velocity, PzOl concentration, and water vapor concentration were varied. The gain in weight of the specimen was measured. A t the instant of initial exposure the rate of reaction of PIOl with the pellet was nearly independent of the temperature in the range 700' to llOOo C., varied with the gas velocity, and was directly proportional to the P20s concentration; thus the transfer through the gas was the ratedetermining step. After a period of exposure that varied with the conditions, the rate of the reaction generally decreased, became more dependent upon the temperature and less dependent upon the gas velocity, and ceased to be proportional to the PzOs concentration; here the transfer through the coating of liquid product forming on the pellet was the ratedetermining step. The rate of reaction of PzO, with the different materials decreased in the order lime, fluorapatite, rock phosphate. The observed rates were not appreciably affected by the concentration of water vapor.
The phosphorus pentoxide sampling apparatus (Figure 2) consisted of a Pyrex glass air condenser and trap, a Pyrex wool filter, a platinum tube of 0.6-cm. inside diameter and 31-cm. length for conducting the sample from the exposure
1562
INDUSTRIAL AND ENGINEERING CHEMISTRY
point through the preheating chamber to the condenser, and a gas buret and leveling bulb for withdrawing and measuring the gas. The sampling tube was connected to the absorption tube by an adapter similar to that used for the specimen support. The absorption specimens were made by compressing the - 150-mesh absorbing material into lenticular tablets of approximately uniform weight. Platinum hooks were inserted for their suspension. The tablets were heated to 1100" C. for about one hour and then cooled. The heating caused a noticeable shrinkage of the brown rock and the matrix specimens; the pellets hardened sufficiently to resist crumbling at the edges on handling. The fluorapatite and the calcium oxide specimens were less hard and required cautious handling. The surface area of each specimen was calculated from measurements taken with a micrometer caliper. The measured specimens were reheated to 1100" C. to remove any moisture or carbon dioxide absorbed during handling and were rapidly cooled and immediately placed in individual glass vials for storage. The specimens before and after exposure were generally weighed in the vials under conditions that minimized errors due t o adsorbed moisture. The phosphorus saturator was charged with liquid phosphorus under water. After the phosphorus was frozen with the saturator in an FIGURE 2. PHOSinclined position, the water was disPHORUS PENTOXIDE placed by a stream of oxygen-free SAMPLING TUBE nitrogen and the phosphorus was washed with alcohol. Finally, the phosphorus was melted and heated to about 60" C., and a current of nitrogen was admitted to remove the water and alcohol. A small flow of nitrogen through the saturator was maintained thereafter until the beginning of the experiment, when the flow of nitrogen was adjusted to a predetermined value. When, after operation for an hour or longer, the P205concentration was considered to have been stabilized, the gas stream was sampled for P&, and the absorption measurements were started. Prior t o exposure to the phosphorus pentoxide vapor, the absorption specimen was lowered into the preheating chamber and allowed to reach the absorption temperature as indicated by the thermocouple in the specimen support. The specimen was then rapidly lowered to the exposure point. After exposure, the specimen was withdrawn to the cooler portion of the preheating chamber. The accuracy in timing the exposure was probably within one second. Finally, the specimen was removed from the apparatus and replaced in the vial. I n computing the amount of Pz05absorbed from the weight gained by the pellet, the error introduced by neglecting the weight of fluorine evolved (about 3 per cent) was unimportant in comparing the factors that influence the absorption rate in the phosphatic specimens containing fluorine. Four to six P20ssamples were taken a t approximately regular intervals during each experiment. The P& concentrations determined analytically were usually 95 to 100 per cent of the concentrations calculated from the gas flow and the vapor pressure of phosphorus. At the lowest P&b, concentration and gas velocity, however, analytical values as low
Vol. 33, No. 12
as 80 per cent of the calculated PzOaconcentration were obtained in some experiments. The primary experimental data are given in Table 11. The Pe05concentrations are mean values for each experiment. The gas velocities are mean values expressed as flow per unit area of free path a t the level of the specimen. Since the density of the gaseous mixtures (at standard temperature and pressure) varied only 20 per cent between the extreme P205 and H20contents, the velocities expressed in these terms are roughly proportional t o the mass velocities. The only variable in a given experiment was the exposure period. To indicate the effect of the variables under study, the weight gain per unit area of the specimens was plotted against the exposure period. With the exception of experiments performed a t 700" C. where the weight gains were slight, smooth curves were obtained. Typical curves are shown in Figure 3.
Mechanism of Absorption The mechanism of the absorption process involved transport and subsequent chemical reaction. The relative insensitivity of the observed initial absorption rates to temperature (Table V) indicated that the rate of chemical reaction between the rock phosphate and phosphorus pentoxide was so rapid in the temperature range under study as to be inconsequential in its influence on the over-all absorption rate. The resistance encountered 'by the phosphorus pentoxide in reaching the solid absorbent was apparently the factor determining the absorption rate. At the start of the absorption, the phosphorus pentoxide diffused through gas only. As the absorption proceeded, a coating of liquid product was formed on the surface of the solid absorbent, and additional phosphorus pentoxide could react with the absorbent only after transfer through both the gas and the liquid coating. Data with respect to the PzO5 concentration gradient within the coating, as described in a later section of this paper, indicated that the resistance within the coating was not confined to a thin film
EXPOSURE PERIOD, SECONDS
FIGURE 3. EFFECT OF P,O, CONCENTRATION AT 1100" c.
INDUSTRIAL AND ENGINEERING CHEMISTRY
December, 1941
1563
0.30
regarded as evidence of gas resistance; decrease of the rate with increasing exposure 5 time was considered an indication of liquid resistance. 0.20 Typical curves representing 8 both types of control are N ' shown in Figure 4. Based 5> upon the constancy or the P 0.10 decrease of the absorption rate with increasing exposure as criteria, the experiments were distinguished in which, respectively, the gas resist:a20 ance and the liquid resistance predominated. These experiments are represented b in Figure 5 as functions of the Pz06concentration and the temperature. The 5 tendency for transport within the liquid to be the ratedetermining process presum0 0 IO 20 30 40 10 I5 ably increased as the coating TOTAL ~OsA0SOReED, MG./CM? TOTAL 50, ABSORBED, MG./CM? of product thickened, although a t high temperatures FIQURE 4. DECREASE IN ABSORPTION RATEWITH INCREASING QUANTITY ABSORBED:EFFECT this effect may not have deOB Pz05 CONCENTRATION AT CONSTANT TEMPERATURE veloped owing to flow of the product to the bottom of the specimen. Figure 5 indicates chat increase in temperature on the exterior of the coating but extended throughout the reduced the influence of the resistance within the liquid and depth of the coating. tended to make transport through the gas the controlling The ratio of the transport resistance within the gas to that factor in the absorption. Inability of the transfer within the offered by the liquid coating obviously depended on the opliquid to keep pace with that in the gas a t high PzOa concentraerating conditions. An increase in temperature, for example, tions is also indicated. has relatively little effect on the diffusion coefficient for a gas but increases the rate of diffusion through a liquid, both beEffect of Pz06Concentration cause of the lowered viscosity of the liquid and because of inThe effect of Pz05concentration on the initial absorption creased diffusivity. Constancy of the absorption rate for all rate is shown in Table 111. The rates were derived from the of the exposure periods under a given set of conditions was TEMPERATURE IIOO'C. 3 . 8 ~STWSEWCM? ~. INDICATED Os CONCN. I N MG/LITER,S%F!
GAS VELOCITY,
.
i
~
~
TABLE11. PRIMARY EXPERIMENTAL DATA^ Expt. NO.
18 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42h 434 441 46k 461
Temp., 0
c.
1100 1100 1100 1100 1100 1100 1100 1100 1000 900 850 1000 900 850 1000 850 1000 850 700 700 850 1000 1000 1000 1000 1000 1000
41.7 289 47.7 278 49.5 296 17.0 18.4 18.3 18.8 18.0 17.6 18.3 18.8 275 261
6.1 6.1 6.0
41.4 41.9 43.4 42.3 42.7 42.3 41.3 41.8
0.38 1.03 1.13 3.46 3.76 0.35 0.38 3.82 3.79 3.77 3.77 0.39 0.39 0.38 3.49 3.49 3.89 3.82 3.84 3.90 3.87 3.90 3.85 3.89 3.93 3.93 3.94
...
Weight Gain in Mg./Sq. Cm. after FolIowing Exposure Period in Seoonds: 20 40 80 160 320 640 960 0.6 1.6 2.1 3.0 5.1 9.8 18.2 2.1 4.8 8.6 14.0 22.2 6.1 li:? 23.1 ... 0.1 0.3 1.9 2.6 2.8 5.1 8.9 15.5 26.1 42.5 0.7 1.0 2.0 4.3 8.8 16.8 31.3 2.5 3.3 6.8 12.1 20.3 35.1-34.5 ... 1.6 4.2 7.2-i.9 0 ... 0.7 0.0 0.0 0.8 0.2 0.0 1.0 1.5 2.9 6.1 12.3-12.1 -0.1 0.0 0.6 2.0 4.8 10.4 15.2-15.6 0.6 1.3 2.7 5.1 8.3 11.W 0.7 1.3 2.5 4.5 6.7 8.3 0.0 0.1 0.7 1.4 3.3 8.9 10.4 0.3 0.7 1.6 3.5 6.3 9.1 0.7 0.9 2.0 3.5 6.0 8.0 3.0 3.1 g.+ 10.3 15.9 22.5 1.2
... ... ... ... .(. ...
0.1 0.0 0.0 0.1 0.0 0.6
5 1.2 1.0-1.2 0.1 0.7-1.1 0.1 0.9
...
0.0
...
... ... ... ... ...
1.4
10
...
...
... ...
.., ... ... ...
... ... ... ...
1.3 0.5 0.7 0.4 0.6
0.4
a The absorbing material was brown rock phosphate, except a8 noted. b At standard temperature and pressure. I
660 seoonds. d 1020 seconds. e 1340 seoonds. I 1290.secqnds. 103 theoretjoal Hz0 t o hydrate P ~ O K t o metaphosphortc ao+d.
k 2147 theoretioal Hz0 t o hydrate PzOs t o metaghosphoria acid. 1 Lime. m 335 seoonds.
0
... ...
... ... ...
...
1.7
1.1 0.6 1.3 0.8 1.8
2.8 2.7 3.1 2.1 3.1 3.1
1400 seconds.
5.3 5.9 4.1 5.6 6.1 6.5
h Fluorapatite.
10.0 12.1 6.3 11.0 10.7 12.9,
17.7-17.5 21.3-21.9 9.9-10.2 17.3-17.8 18.4-19.8 22.7-23.1
6 Phosphatio matrix.
... ... ... ... ... ...
1280
... ... ... ... ... ...
... ... ...
13.2 10.1-9.98 13.7; 11.7 9.8-9.6'J
...
... ... ... ... ...
...
INDUSTRIAL AND ENGINEERING CHEMISTRY
1564
initial slopes of the curves for weight gain us. time when extrapolated to the origin. By means of auxiliary curves the observed initial rates were corrected to the nominal conditions of Pz06 concentration and gas velocity indicated in Table 111. Under the experimental conditions the initial absorption rate varied directly with the PZOS concentration, as shown by the approximate constancy of the ratio of the absorption ~ in each group of experiments in rate to the P z O concentration which other conditions remained constant. The PZOs concentration was depleted as much as 28 per cent in some instances in the experiments a t the gas velocity of 0.38 cc. (S. T. P.) per second per sq. em. Hence the data for this velocity were less reliable than those for the higher velocities a t which generally less than 5 per cent of the phosphorus pentoxide input was absorbed.
Vol. 33, No. 12
c
300
3 2 RATE SLIGHTLY AFFECTED RATE N O T A F F E C T E D
OF P ~ OCONCENTRATION S ON INITIAL TABLE 111. EFFECT ABSORPTION RATE
700
-
Gas R = Initial c PI06 Velocitya Absorption Expt. Conon.*, TzmJ.., Cc./Sec./' Rate, ME./ R x 10' No. Mg./L. S a Cm. Sq. Cm./Seo. C 3.8 0.31 1.09 23 285) 1100 3.8 0.048 1.12 24 43 1100 3.8 0.019 1.06 27 18 1100 1.14 0 . 2 5 0.88 21 285 1100 0.037 0.86 1.14 22 43 1100 0.38 0.18 0.63 25 285 1100 0.028 0.65 0.38 18 43 1100 0.38 0.012 0.67 26 18 1100 34 285 1000 3.8 0.28 0.98 1000 3.8 0.046 1.07 41 43 1000 3.8 0.017 0 95 28 1000 36 6 3.8 0.006 1.0 >0.2 >0.7 35 285 850 3.8 0.046 1.07 40 43 850 3.8 0.019 1.06 30 18 850 3.8 0.007 1.2 37 6 850 3.8 a Nominal values a t standard temperature and pressure. b Corresponds t o the P10a Concentration obtained b y burning phosphorus with 207% of the theoretical air.
TABLE IV. EFFECT OF GAS VELOCITYON INITIAL ABSORPTION RATE
v-
Expt. Gas Velocity, Temp No. Cc./Sec./Sq. Cm.* C." 23 3.8 1100 21 1.14 1100 25 0.38 1100 24 3.8 1100 22 1.14 1100 18 0.38 1100 27 3.8 1100 26 0.38 1100 28 3.8 1000 31 0.38 1000 29 3.8 900 32 0.38 900 30 3.8 850 33 0.38 850
.
0
P*Os
Concn.,
Rlg./L.a
285 285 285
43 43 43 18 18 18 18 18 18 18 18
R = Initial Absorption Rate Mg./Sq. Cm./SeE. 0.31 0.25 0.18 0.048 0.037 0.028 0.019 0,012 0.017 0.011 0.016 0.010
0.019 0.012
R/V 0.24 0.24 0.22 0.037 0.036 0.034 0.015 0.015 0.013 0.013 0 012 0.012 0.015 0.015
Effect of Gas Velocity The effect of gas velocity on the initial absorption rate is shown in Table IV. The initial rate varied as the 0.15 to 0.25 power of the gas velocity. F gure 6 (left) shows the effect of gas velocity on the absorption rate throughout the measured course of the absorption a t 1000" or 1100" C. and a t the nominal PzOsconcentration of 18mg. per liter, S. T. P.; the horizontal absorption
1100
900 IO00 TEMPERATURE, 'C
FIGURE 5. EFFECT OF METAPHOSPHATE COATING ON ABSORPTION RATE
rate curves are indicative of resistance to transfer within the gas. The actual effect of gas velocity was probably somewhat less than is indicated by the absorption rate data, for the depletion of the PzOsconcentration in the gas stream was much greater a t the gas velocity of 0.38 cc. (S. T. P.) per second per sq. cm. than a t the higher gas velocities. The sloping rate curves a t 900" C. are indicative of increasing transfer resistance in the liquid. At this temperature the influence of gas velocity became less as the absorption progressed. The curves of Figure 6 (right) show the definite effect of gas velocity on the initial absorption rate at the higher P20. concentration of 285 mg. per liter, S.T. P.; these curves also give qualitative evidence that the rate was affected less by the gas velocity and more by the liquid resistance as the absorption advanced. [Quantitative estimation of this trend was not made from the right-hand curves of Figure 6, because either the 1.14- or the 3.8-cc. experiment seemed to be in error; the results a t 0.38 cc. were checked by running the experiment in duplicate. Figure 6 (right) suggested that the values of the absorption rates measured in the latter portion of the experiment a t the high gas velocity were probably 10 to 30 per cent too high, for it was not clear why this rate curve should depart from parallelism with the intermediate curve after the initial period of absorption.]
TABLEV.
Nominal values a t standard temperature and pressure.
After the liquid coating had formed on the specimens, the absorption rate was proportional to the P20b concentration only in the experiments a t the lowest P20aconcentrations.
800
Expt. No. 23 34 35 24 41 40 27 28 29 80 26 31 32 33 36 37 a
EFFECTOF TEMPERATURE ON INITIAL ABSORPTION RATE Temp., C. 1100 1000 850 1100 1000 850 1100 1000 900 850 1100 1000 900 850 1000 850
PrOr
Concn.a, Mg./L. 285 285 285 43 43 43 18 18 18 18 18 18 18 18 6 6
Gaa Velocity" Cc./Seo./Sq. CI$. 3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8 0.38 0.38 0.38 0.38 3.8 3.8
Initial Absorption Rate Mg./Sq. Cm./Sed. 0.31 0.28 0.2 0.048 0.046 0.046 0,019 0.017 0.016 0.019 0.012 0 011 0.010 0.012
Nominal values a t standard temperature and pressure.
0.006
0.007
December, 1941
INDUSTRIAL AND ENGINEERING CHEMISTRY FIQURE
RATE WITH INCREASINQ UANTITY ABSORBED:EFFECT OF :ONAS VELOCITY AT CONSTANT Pzo~ CONCENTRATION AND CONSTANT
% 0.01 O.O2
5 $
g
6. DECREASE IN ABSORP-
TEMPERATURE
0
0.02 0.01
i o
i
0'02
$
R5
0.01
10 TOTAL QOsABSORBEo MG/CMF
Effect of Temperature
TOTAL
I
I
I
10
20
30
g 0,ABSORBED. MWCM?
1565
while the concentration in the film formed a t 850" C. varied from a maximumof about 89 per cent a t the outer surface to a minimum of 74 per cent adjacent to the underlying rock. These data showed that the P106 concentration in the product was definitely a function of temperature. A gradient in PaOr concentration in the glaze was also in evidence. That this gradient was not confined to a thin film on the surface was indicated by variations in Pz06concentration within most of the samples, corresponding to the inclusion of material from different depths in the coating. The steep gradient in PzOa concentration in the coating formed at 850' C. was attributed to the high viscosity of the product at this temperature.
Effect of Water Vapor
The addition of water vapor to the oxygen supplied for combustion had very little effect on the absorption rate a t 1000" C., as was indicated by comparing experiments 41, 44, and 45 which were performed a t a nominal P106 concentration of 43 mg. per liter (S. T. P.) and a nominal gas velocity of 3.8cc. (S. T.P,) per second per sq. em.
The effect of temperature on the initial absorption rate is given in Table V. At the instant of initial exposure the absorption rate showed little change with temperature. As mentioned in connection with Figure 5, an increase in temperature reduced any tendency for the absorption rate to Effect of Composition decrease with increasing quantity absorbed. This effect is further illustrated in Figure 7. (This figure does not b' rive a The effect of composition of the absorbing material was complete comparison, but it is evident that after a layer of studied to a minor extent. Curves for calcium oxide, Canametaphosphate had formed, the absorption rate at 1100' C. dian fluorapatite containing 38.7 per cent P106, and a matrix was a t least fifty times the rate a t 700' C.) The marked efcontaining 25.2 per cent PIOs as compared with the standard fect of temperature on the later stages of the absorption emrock containing 33.2 per cent PZOSare shown in Figure 8. phasized the importance of maintaining temperatures of the The initial absorption rates for the four materials were similar. order of 1100' C. in the manufacture of calcium metaphosCorrection for the fluorine loss from the fluorapatite did not phate by the absorption process. completely account for the difference between the absorption The effect of the absorption temperature on the PpOs 0.30 concentration in the glaze &Os CONCN.,285MG./LITER, ST.R formed on the rock speciw \ S VELOCITY, 3.8CC. STI?/SEC/CMZ mens was studied by determining the refractive inII I I dices of the glazes on two 0.02 specimens from comparable experiments (Nos. 23 and 35) performed at 1100' and at 850' C. Measurements of 0.0I the refractive indices of four glasses of known PzOscontent prepared by fusing the standard brown rock phos0 phate (Table I) with differ0.OlC ent proportions of phosphoric &Os CONCN.,43MG/LITER, S.T P acid had shown that the reGAS VELOCITY, 3.8CC,S.T P/SEC/Cd fractive index was an apP 5 I I proximate measure of the Q . Il00". P,O6 content. The P,Oscon0.00: 53 0 . 0 5 I centration in the product coating on the specimen prepared at 1100' C. varied from a maximum of 74 per 20 - 30 in 15 cent a t the outer surface to TOTAL &-Os ABSOR8ED, hoWCM! TOTAL %os ABSORBED. MGJCM: a minimum of 68 per cent at the interface between the FIGURE 7. DECREASE IN ABSORPTION RATEWITH INCREASING QUANTITY ABSORBED:EFFECT glaze and the unreacted rock, OF TEMPERATURE AT CONSTANT PnOsCONCENTRATION (S. T. P.)
9&a
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
1566 20
1
I
I/
GAS VELOCIXY JBCC.,S.%WSEC/CM. FREE PAM
250
-0
I
500
E XPOSURE PEROD, SECONDS
EFFECT OF COMPOSITION OF ABSORBING MATERIAL AT 1000° C.
FIQWRE 8.
curves for the fluorapatite and for calcium oxide. The transport resistance within the gas in evidence in the initial stages of the absorption with all four materials persisted throughout the tests with lime and apatite, whereas control by resistance within the liquid manifested itself in the absorption with rock and became predominant in the control of the absorption with matrix. The shape of the curves for rock phosphate and for matrix may be explained by assuming an increase in viscosity of the product resulting from the presence of impurities. Silica is probably the most important impurity in this respect, since experience in preparing highly siliceous metaphosphate melts in the laboratory has shown these compositions to be very viscous.
Conclusions 1. I n the initial stage of the absorption of phosphorus pentoxide by the rock phosphate, the absorption rate is governed by the rate of transfer through the gas surrounding the particles. I n this stage the absorption rate is directly proportional to the P205 concentration in the gas thmughout a range of concentrations corresponding to those which would result from burning elemental phosphorus with two to ninety times the theoretical proportion of air necessary for combustion. As the absorption progresses, the particles of rock become coated with a layer of liquid product. At temperatures below 900” C. and after a few seconds’ exposure to all the gases except those containing the lowest concentrations of PZOs,the rate of transfer through this liquid layer becomes the rate-determining step in the absorption. The absorption rate diminishes rapidly as absorption proceeds, and no proportionality is evident between P205 concentration and absorption rate, although the rate a t a high Pa06 concentration is generally greater than a t a low PZOS concentration. At temperatures of 1000” or 1100” C. also, provided the P206concentration is high, the resistance within the liquid apparently is predominant, but a t lower concentrations of the order of 20 mg. P20sper liter, S. T. P. (corresponding to the concentrations developed by burning phosphorus with twenty to thirty times the theoretical air), no decrease in absorption rate was observed within the duration of the experiments;
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and the proportionality between the P2OS concentration and the absorption rate persisted. 2. The rate of absorption was found to be very dependent upon temperature, except a t the instant of initial exposure which is not of practical significance. After the rock is coated with a metaphosphate film, which is the condition of practical interest, it is estimated that the absorption rate a t 1100” C. is a t least f l t y times the rate a t 700” C. To maintain an absorption rate appropriate for full-scale operation, the temperature of the absorption zone of a metaphosphate unit should be not less than 1000” C. At lower temperatures, the absorption rate is greatly retarded as the layer of liquid product becomes thicker; furthermore, the product formed, particularly in the presence of high concentrations of phosphorus pentoxide vapor, is rich in Pz06and consequently hygroscopic. 3. The absorption rate is influenced by the gas velocity to a minor extent when the liquid resistance predominates. Even when the transport resistance within the gas was determinative, the absorption rate was found to vary only with the 0.15-0.25 power of the gas velocity over the range 0.05 to 0.6 linear foot per second, which is believed to be in the range of viscous flow. 4. The presence of moderate concentrations of water vapor has no appreciable influence on the absorption rate at
loooo c.
5. Although the effects of the different impurities in the rock phosphate have not been studied individually, the results show that the impurities, in general, decrease the absorption rate as absorption proceeds.
Acknowledgment The authors wish to express their appreciation t o R. L. Copson and J. W. H. Aldred for advice and criticism, and to other members of the TVA Chemical Engineering Staff for their cooperation during the progress of the work.
Literature Cited (1) Curtis, H. A., Copson, R. L., and Abrams. A. J., Chem. & Met. Eng., 44, 140-2 (1937). (2) Curtis, H. A., Copson, R. L., Abrams, A. J., and Junkins, J. N.. Ibid., 45, 318-22 (1938).
PRESENTED before the Division of Industrial and Engineering Chemistry st the lOlst Meeting of the American Chemical Society, St. Louis, &Io.
Phenolic Resins for Plywood-Correction It has been called to the attention of the author that the following statement in the article “Phenolic Resins for Plywood” [IND. ENG.CHEM.,33,976 (1941)lmight be misinterpreted: “In the latter part of the decade between 1920 and 1930, active development work took place in the field. There seems t o be little question that the products investigated initially were solutions or dispersions of phenolic resins. One company marketed for some time a dispersion of a phenolic resin in water. The process never became a great success chiefly because of the difficulty of controlling spread and adjusting moisture content at the time of gluing.” Reference is made to the fact that the first water dispersions brought out were entirely unsuccessful in their application for the reasons given in the article. However, it is true that very recently, within the last year or two, water dispersions and solutions of phenolic resins have been considerably improved, with the result that at least two companies are marketing successfully products of this general character for use as a plywood adhesive, particularly in the Douglas fir plywood industry. LOUISKLEIN
