852
INDUSTRIAL AND ENGINEERING CHEMISTRY DEFIYDROCALORIYATIONS
CHLORIN~TF S DO Y R COIL. ~ ~ The sample n-as placed in a flask connecttd to an efficient trap. After evacuation of the system to 3 mm. or lesc, steam from ~tgcncrator operating at the same pressurr was paised through the oil while it was heated to the desired temperature for the allotted time. Table IV gives the results of several dehydrochlorinations. The percentage of new conjugated double bonds formed by dehydrochlorination was calculated as follows: The total number of conjugated double bonds per 100 fat acid radicals before dchydrochlorination ~ a found 5 by taking the sum of twice the percentage of diene conjugation, three times the percentage of triene conjugation, and four times the percentage of tetiaene conjugation. This sum was then deducted from the corresponding sum for the oil after dehydrochlorination. I n order to protect the mechanical pump used in these experiments a train of absorption tubes packed with calcium oxide and calcium chloride o as connected between the apparatus and the pump. After each use the pump Tyas drained and flushed. Hydrogen chloride seldom reached the pump; most of it wag retained by the steam caught in the trap and the remainder was renioved by the absorption tubes COKTINUOUS DICHYDROCHLORISAT~OX. The apparatus consisted of a Liebig condenser 50 cm. long, having an inner tube 10 mm. in diameter and a shell 17 mm. in diameter. The outer shell was wound with Nichrome ribbon. Spacing of the windnq \vas uniform except at the upper or feed end where spacing was gradually increased in order to provide a substantial temperature gradient. I n this way oil entering the column was brought bomewhat less suddenly up to temperature, and spattering $$asthus reduced. Temperature was measured with thcrmocouplcs inserted through the water inlets Thc conden+Pr wa- mounted a t an angle of about 30°, the upper end was equipped with a dropping funnel (Z), and the lower end was connected to vacuum through a conventional fraction cutter. The tube wa5 evacuated
Vo!. 41, No. 4
to a pressure of 3 mm. or less, and was heat,ed to the desired t,enipcrature with a variable voltage transformer. Oil was then passed down the tube at a rate which provided about 30-second coiit'act time in the hot tube. Table V gives results obtained by t)his method. ACKh-OWLEDGRIENT
The authors express their appreciation t o t,he .halyticiil arid Physical Chemical Division of this laboratory for supplying thc spectrophotometric analyses. The experiment'son continuous tlchydrochlorination were conducted by J. E. .Ta ckson. LITERA'KKE CITED
(1) Ashburn, G., and Frank. R. F., IND. Esa. CHEM., ANAL.ED., 16, 418 (1944).
(2) Bergstrom, S., S a f u r e , 156, 717 (1945). (3) Bolland, J. I,., and Koch, I T . P., J . C h c m hoc., 1945, 445--7. (4)Chattaway, F.D., and Backeberg, 0 . G., I b i d . , 1923, 299!3r3603. ( 5 ) Emling, B. L., T'ogt. R . R.,and Henriiom G. F., J . Arn.-Chem. SOC.,63, 1634-5 (1941,. (6) Farmer. E. H . , Trans. Famriny Soc., 42, 228-36 (1946). (7) Gardner, €1. A., U. 6 . Patent 1,452,553 ( A p d 24, 1923'1. (8) Harford, C. G., I b i d . , 2,054,814 (Sept. 22, 1936). (9) I b i d . , 2,107,789 (Feb. 8,1938). (10) I b X , 2,179,787 (NOT.14, 1939). (11) I b i d . , 2,207,983 (Jul>- I O , 1940). (12) Irwin, C . F., and Henilion. G. I:., J . Am. ('hem. SOC..63, 858-60 (1941). (13) Kcniier, J . , Nature, 15F, 370 (1945).
(14) Radlove, S.B., Teeter., H . A I . , Bond, IT. FI., CoTTvan, J. C.,m r i Kass, J. P.. IKD. EXG.CHEX.,38, 998 (1946). (15) Ross, J..Gebhart, A. I.. and Gerecht, J. F., J . Am. i'hem. S o c . , 68, 1373-6 (1946). (16) Sandmeyer, T., Her.. 18, 1767-89 (1835). (17) I b i d . , 19, 857-61 (1886,. (18) Ziegler, K., Spat,h, A%., Scliaaf, E., Schumaiin, W., a.nd Winkc!mann, E., Ann., 351, 80-119 (1942).
RECEIVED October 31, 1947. Presented before t l ~ ePaint and I'rotcotive Coatings Group a t the 15th west Regional Meeiing, A x x ~ r c . 4C~I I I ; M ~ C A L S o r I m Y , Kansas City, 110.
Dielec
a SILICON DIQXIDE
ROBERT V. JELINEM, P-IENKY B. LISFQRD, E. IC. McMAHON, AND PHILIP W. SCHUTZ' Colunabia University, .Yew York 27, 3.. Y .
A
N EARLIER paper by Schutz and AlcMahon (4) was the
first published work on the dielectric heating of brds of granular solids, and y a s concerned mainly with alumina, although some data on silica were also presented. The work discussed below is a continuation and expansion of t,he experiments with silica, and i t is hopcd that the data prcsented will help to clarify this problem. The earlier work has shown definitely that for both alumina and silica there exists a variation of heating rate with grain size. For alumina this variation was established as an apparently linear increase of heating rate with particle diameter over the range 0.103 to 2.04 mm., as shown in Figure 1 of this paper, a reproduction of the lower curve of Figure 6 of ( 4 ) . The earlier results for silica were less complete than those for alumina, and it was concluded merely that a general increase in heating rate with grain size exists for silica over the particle size range 0.664 to 3.53 mm. That this latter statement is not entirely true is 1
Deceased.
pointed out in this paper. However, since experimental t.rchniquc in t,he earlier work ivith silica ha,s now been establialictl as faulty, the new data are a clarification rather than a contradiction of thosc previously presented. THEORY
By means of conventional electrical relationships, tJlic usual equation for power input into a dielectric (1, 6) should hc:
where q,. = power input per unit volume (watts per ml.)
E/d
voltage gradient acrojs dielectric (volt3 prr em.) frequency (cvcles per sec.) K = dielectric constant tan 6 = dissipation factoi. K(tan 6) = loss factor =
f=
April 1949
INDUSTRIAL AND ENGINEERING CHEMISTRY
T h e relationship bet*een heating rate and particle diameter has been investigated i n the dielectric heating of crushed fused silica, over the particle size range 0.235 to 3.53 mm. Different sizes of the material were placed i n the three compartments of a rectangular polystyrene cell and subjected to approximately 1250 volts per cm. a t 16.5 megacycles. From observed temperature rises, heating rates were calculated and expressed as ratios of the heating rate of a n arbitrarily chosen standard size (0.542 mm.). A maximum heating rate was found a t 1.41 m m . ; this rate was 3.17 times that of the standard size. A minimum heating rate is indicated i n the vicinity of 0.42 mm. A t a voltage gradient of 1230 volts per cm., the 1.41-mm. particles were found to heat a t the rate of 2.8" C. per minute. These results are compared with those of earlier work with alumina and silica. The phenomenon of change of heating rate with particle size is discussed.
From this it is evident that the dielectric heating rates of two materials subjected to the same conditions of frequency and voltage gradient will be proportional to their loss factors. Regardless of the method of heating,.the rate of heat input per unit volume may be expressed as follows: q. = 7c p c A z ' ) y (0.0697)
where
C, = heat capacity of material heated (cal. per g. per a C.) AT = temperature rise ( c.) t' = time of heating (min.) y = bulk density of material heated (e;. per ml.) 0.0697 = conversion factor (watt-min. per cal.) in order to have qy expressed in watts per ml. as above
853
EXPERIMENTAL
MATERIAL.The granular silica used in this work is a pure grade of fused silica obtained from the Thermal Syndicate, Ltd., and referred to as B.T.S. silica. This material is made by fusing a very pure grade of sand (99.8% minimum silicon dioxide) in an electric furnace and forming i t into tubes, slabs, and other desired shapes. The samples used in this work are crushed chips and scraps obtained from such operations. The material is amorphous in structure, but is not optically clear because of the presence of minute air bubbles. This may be a n important characteristic affecting its electrical properties, and should be kept in mind in any consideration or possible application of the data presented in this paper. According to the manufacturer, B.T.S. silica conforms to the specific heat relationship evaluated for silica glass by White (6). The crushed material used in this work was carefully sized with Tyler Standard wire mesh screens, using a mechanical shaker, a method which is recommended as standard and reliable ( 3 ) . It was then washed with a mixture of concentrated nitric and hydrochloric acids and thoroughly rinsed with water. Residual undissolved impurities were carefully picked out. Drying was done first a t 110" to 130" C. for 8 to 12 hours, followed by 250" to 300" C. for 3 t o 6 hours. The samples were stored in sealed glass jars until used. Some of the sizes which were exposed to the air considerably during the work were redried from time to time. I n arriving at an average particle size for each mesh range, the arithmetic mean of the two mesh openings which formed the boundaries of the range was used. The 28- to 32-mesh size (average particle diameter 0.542 mm.) was arbitrarily chosen as the standard against whose heating rate those of the other sizes were compared. APPARATUS. Figure 2 s h o w details and dimensions of the three-compartment polystyrene cell used in the heating runs and of the smaller cylindrical cell used for &-meter measurements.
If two materials of the same heat capacity-Le., two grain sizes of the same substance-are heated under similar conditions for the same time interval, their heating rates will be in the same proportion as the products ( AT) y. Thus if two (or more) different particle sizes of the same solid are heated by dielectric heating under the same voltage gradient at the same frequency, relative heating rates obtained by measuring temperature rises should be identical with relative loss factors obtained a t the same frequency, or
THERMOMETER
VI /
This fact was demonstrated experimentally in the detailed study of the heating rates of alumina (4, Figures 5 and 6).
u/
POLYSTYRENE
MATERIAL SHEET, JOINED W I T H POLYSTYRENE CEMENT.
.a
HEATING C & POLYSTYRENE ADJUSTABLE.
*A
3.41" I D
&0.875 ' SECTION A.A
Figure 1.
Relative Heating Rates of Granular Alumina a t 20 Mc.
Q-METER CELL Figure 2.
Cell Details
L
A
INDUSTRIAL AND ENGINEERING CHEMISTRY
854
VcX. 41, No. 4
here because the very lorn loss factors characteriitic of silica made &-Meter readings unreliable. However, a set of measurements has bccn made for the 10- to 14-mcsh matcrial and is included in the results as a general indication of the magnitude. of the electrical properties of granular silica. RE su L E
t
i
/
AVERAGE 1.0
Figure 3.
I
PhYT C L t I DIAMETER ( r m )
20
30
Relative Heating Rates of B.T.S. Silica at 16.5 M c .
The electrodes were two 0.25-inch steel plates, 7 by 9.5 inches in size. The cell was sealed to the lower plate with polystyrene cement; the upper plate mas removable for filling and emptying the cell. The generator used in the heating runs is a oommercial oscillator of the modified Hartley type; the fre uency was kept substantially at 16.5 megacycles throughout &e work. The apparatus used for measuring loss factor is a Boonton type 1 6 0 4 &-Meter. Thin-wire (KO. 30 gage) copper-constantan thermocouples were used for measuring temperature rises of the silica during heating. A Leeds & Northrup model 8662 potentiometer waq used for measuring thermocouple e.m.f. values. PROCEDURE. The procedure used on alumina and in the earlier work with silica ( 4 ) was unsatisfactory for this work, and it was modified by substituting polystyrene for Mycalex in the cell walls, and by using removable thermocouples instead of fixed thermometers for measurement of temperature rises. These changes were necessary because the heat produced in the Mycalex wall was of comparable magnitude to that produced in the silica, although negligible in compaiison to that of alumina. Also, a distorting effect of the metallic thermometer columns produced serious eleotrical aberrations in the silica bed that were inconsequential in the alumina studies. Detailed data on the thermometer effect are available in an unpublished thesis on file in the library of Columbia University (!). The general effect of metallic inserts on dielectric heating is discussed by Dakin and Auxier ( 1 ) . The procedure employed in obtaining the data reported here was the following. Each compartment of the rectangular cell was filled level (without packing) with a different particle size. The standard size (28 to 32 mesh) was present in the cell during every run. A thermocouple was inserted into the center of each compartment through small holes drilled in the top plate for this purpose, and starting temperatures were read. Thermocouplek were withdrawn, power was applied for 5 minutes, and the couples were reinserted as soon as power was turned off. A cooling curve was then plotted for each compartment over a period of 15 to 20 minutes, and by extrapolation of these curves the value of the temperature in each compartment a t the time of power cutoff was estimated. The difference between this and the starting temperature was then used to calculate a relative temperature rise (relative t o that of the standard size) for each material in each run. Relative heating rates were then obtained from relative temperature rises and bulk densities (as explained above), Several runs were made with each rombination of grain sizes, and the various materials were shiftrd around from one compartment to another periodicall>- to eliminate any packing or geometric effects. Before each series of runs, after t h r grain sizes had been c h a n g d and the cell compartments refilled, the circuit was tuned to a maximum value of approximately 6000 to 6500 volts across the cell (about 1200 to 1300 volts per em.). Because no adjustments were found to be necessary during a run, and only sniall adjustments were made from run to run, the voltmeter was not kept in the circuit during runs. Small changes in voltage from one run to another should make no difference in relative heating rates, as the standard size was present in the cell during every run. Correlation of heating rates with loss factor data obtained from Q-Meter readings (as was done for alumina) proved impractical
Figure 3 shon s graphically the relationship betn een heating rate and particle size for silica. Table I lists the values used to plot the graph. Table I1 indicates the relation between particle size and bulk density for B.T.S. silica. Table I11 lists the electrical characteristics of the material with the greatest observed heating rate (1.41-mm. average diameter) and summarizes the observed heating rate data for this material, Table I V lists the heating rates obtained previouslv (4)io1 alumina, converted to relative rates based on the standard size of silica (28 to 32 mesh). Figure 4 is a plot of Tables I and I\-, all data expressed on the standard m e silica basis. The quantity u presented with the results in Tables I and 11 is an indication of the degree of reproducibility of the value with which it is associated. As defined by Worthing and Geffner (7), c is the "standard deviation for a single reading from the arithmetic mean." It has been calculated by the method suggested by Worthing and Geffner. DISCU5510N
The results show that there is a definite maximum in the heating rate of granular silica occurring at t,he particle size corresponding to an average diameter of 1.41 mm. This rate is 3.17 times that of the standard size. There also appears to be a minimum occurring in the vicinity of 0.42 mm. and having the value of
TABLE 1. HEATING RESULTSFOR B.T.S. SILICA Mesh Size 4-8 8-10 10-14 16-20 20-28 28-32
Particle Diameter, ivm. 3.53 2.01 1.41 0.91 0.711 0.542
Relative Heating Rate 0.61 2.33 3.17 1.86 1.57 1.OO
32-42 42-48 42-80
0.423 0.323 0.235
0.79 0.91 1.04
0'
0.11 0.23 0.27 0.23 0.16 Standard size 0.07 0.10 0.06
TABLE 11. BULKDENSITIES OF B.T.S. SIIdC-4 Mesh Size 4-8 8-10 10-14 14-16 16-20 20-28
28-32
TABLE
Bulk Density G./LlL Relative 1.084 1.1787 1.079 1.1735 1 063 1.1562 1,020 1,1095 1.055 1,1476 1.008 1.0958 1.. 000 1.0874 ~
F O E 10- T O 1 4 - x E S H (1.41 hf\f.) B.T.S 111. RESULTS SILICAAT 16.5 Nc.
Q-Meter Xearurements Dielectric constant ( K ) Dissipation factor (tan S) Loss factor ( K tan 8 ) Heating R a t e Summary Voltage gradient Average rate of temperature rise" Average rate of heat input a Experimental. 6 Calculated from
tion 2.
a
0 028 0.027 0 025 0.025 0 034 O102B Standard size 0,022 0.028 0.021 0.021
1,54 0.0031 0.0048 L230 voltg/om. 2.8' C./min: 0 . 0 6 6 wattsiml. 0 . 9 5 cal./min. mi.
average rate of temperature rise, b) means of fCyi~aC., from Whltr ( 8 ) . Mean Cp (25-40° C.) = 0.2951 cal./g.
April 1949
855
INDUSTRIAL AND ENGINEERING CHEMISTRY
0.79 times that of the standard size. The largest material investigated (3.53 mm.) exhibits a relative heating rate of 0.61. For all the sizes listed in Table I, the value of u is approximately 10% of the relative heating rate with which it is associated, indicating fairly good reproducibility of data. These results differ somewhat from the results for silica reported earlier (d), when it was stated that a general increase in heating rate was observed over the size range 0.66 to 3.53 mm. However, because of the two basic improvements in procedure discussed above (replacement of the Mycalex cell with one made of polystyrene, and removal of thermometers from tbe cell during heating), the results presented in this paper are believed t o be more reliable than the earlier ones. If the apparently linear relationship obtained previously for alumina (Figure 1) were to continue indefinitely in both directions of change of grain size, this would demandan infinite heating rate for very large particles and a negative heating rate for very small particles. The existence of such conditions is highly improbable. Accordingly it is logical to conclude that the relation obtained for alumina is only a part of a more general curve, and that if larger and smaller sizes were investigated, this relation between heating rate and grain size must of necessity depart from linearity and go through a maximum or limiting value for large sizes and a minimum or limiting value for very small sizes. For silica, the situation appears to be such that the suspected maximum and minimum both occur within the size range investigated. Reference t o Figure 3 and Table I shows that the occurrence of a maximum heating rate a t 1.21 mm. is definite and cannot be discounted as an experimental accident. The existence of the minimum a t 0.42 mm. is iridicated, although not definitely, as the values of heating rate are spaced rather closely a t this end of the curve. Nevertheless, the curve does indicate conclusively that heating rate does not continue to fall off steeply in the smaller size range. Above the maximum, the curve falls off with increasing particle size less rapidly than it had risen below the maximum. It is believed that if larger sizes are investigated (above 3.53 mm.) a limiting value of heating rate will be found beyond which little or no apparent change with size will be detected. In addition t o pointing out the great numerical difference in the heating rates of corresponding sizes of alumina and silica, Figure 4 shows that when plotted on this larger scale, the last two points for alumina (1.71 and 2.04 mm.) show a slight dropoff from the linear relationship previously presented (Figure 1). In view of the above discussion, this is logical and appears t o indicate that a maximum heating rate may occur at a size not far beyond the range investigated. Because of their different physical and chemical nature, silica and alumina cannot be expected to behave identically under the effects of a high frequency
RESULTSFOR ALUMINA TABLE IV. HEATING Alumina Relative Heating Rate Particle Alumina . Silic? Diameter, basis basis Mm. 1.00 201.3 2.04 1.71 183.3 0.91 1.21 125.0 0.62 0.730 68.5 0.34 58.6 0.29 0.610 50.4 0.25 0.620 0,370 34.2 0.17 0.03 0,103 6.1 a Heating rate = 1.00 for 0.542 mm. silica (same basis as Table I ) . Method of oonversion from alumina t o silica basis: loss faotor 2 04 mm. alumina = 0.305 ( 4 ) ; loss factor, 1.41 mm. silica = 0.0048 (Tabli I I b . relative heating rate 1.41 mm. silica = 3.17 (Table I). loss factor 0.54i mm. silica = 0.0048/3.17 = 0.00151 (this value, mdltiplied by ;elative heating rates from Table I, yields loss factors for all of the sizes of silica investigated) : relative heating rate, 2.04 mm. alumina = 0.305/0.00151 = 201.3: and relative heating rate, 1.71 mm. alumina = 201.3 X 0.91 = 183.3.
L
.I
/
t
1
/
m: Heating
rate.1 00
far 0 542 mrn Silico (some os Fig 3 )
4 SIL1CL n /
00
Figure 4.
;
