April 1954
INDUSTRIAL AND ENGINEERING CHEMISTRY
Finally, the function used here, A H / A Z against T , is fairly sensitive and it is recommended for testing the consistency and smoothness of values tabulated for saturation properties of pure substances. LITERATURE CITED
(1) Albright, L. F., and Martin, J. J., IND.ENO. CHEW,44, 188 (1952). (2) Am. SOC.Refrig. Engrs., New York, N. Y., "Refrigerating Data Book," pp. 78-9, 1943. (3) Barkelew, C. H., Valentinc, J. L., and Hurd, C. O., Chem. Eng. Progr., 1, No. 1; Trans. Am. Inst. Chem. Engrs., 43, 25 (1947). (4) Canjar,'L. N,, Goldman, M., and Marchman, H., IND. ENG. C H E M . , 1186 ~ ~ , (1951). (5) Joffe, J., J . Am. Chem.SOC., 69,1216 (1947).
(6) Matthem, C. S., and Hurd, C. O., Trans. Am. Inst. Chem. Engrs., 42,55 (1946). (7) Meyers, C. H., Cragoe, C. S.,and Mueller, E. F., J . Research Natl. Bur. Standards, 39,507 (1947). (8) O'Brien, L. J., and Alford, W.'J., IND.ENG.CHEM.,43, 506 (1951).
763
(9) Organick, E. I., and Studhalter. W.R.. Chem. E ~ QProgr., . 44, 847 (1948). (10) Osborne, N. S., Stimson, H. F., and Ginnings, D. C., J . Research Natl. Bur. Standards, 23, 261 (1939). (11) Plank, R., and Riedel, L., Texas J . Sci., 1, 86 (1949). (12) Prengle, H. W., Jr., Greenhaus, L. R., and York. R., Jr., Chem. Eng. Progr., 44, 863 (1948). (13) Rynning, D. F., and Hurd, C. O., Trans. Am. Inst. Chem. Engrs., 41,265 (1945).
(14) Sage, B. H., and Lacey, W. N., "Thermodynamic Properties of the Lighter Paraffin Hydrocarbons and Nitrogen," pp. 716, New York, American Petroleum Institute, 1950. (15) Smith, J. M., Chem. Eng. Proor., 44, 521 (1948). (16) Stearns, W. V., and George, E. J., IND.ENG. CHEY.,35, 602 (1943). (17) Stuart, E. B., Yu, K. T., and Coull, J., Am. Doc. Inst., Document 2753; supplement to Chem. Eng. Progr., 46, 311 (1950). Ex'G.CHEM.,42,1514 (1950). (18) Thodos, G , IND. (19) Walters, C. J., and Smith, J. -M., Chem. Eng. Progr., 48, 337 (1952). (20)
York, R., Jr., and White, E. F., Jr., Trans. Am. Inst. Chem. Engrs., 40,227 (1944).
RECEIVED for review April 13, 1953.
ACCEPTEDDecember 12, 1953.
Explosive Properties of Sugar Dusts R. L. MEEKIAND J. M. DALLAVALLE Georgia Institute of Technology, Atlanta, Ga.
M "
to a standard source of ignition (3, 7, 8); the ignition temperaTRES of flammable gases and air will produce an tures of dust clouds (1, 2, IO); and maximum pressures and explosion if two basic conditions &resatisfied-a suitable rates of pressure rise from explosions of dust cloud8 of various mixture of the gases and an ignition source, electrical or thermal, concentrations. There have also been several studies on the sufficiently intense to ignite the mixture. The gas molecules in prevention and venting of various kinds of dust explosions ( 9 , l J ) . contact with the heat source ignite, and they in turn ignite the gas Geck ( 4 )has recently reviewed the history of sugar dust explosions surrounding them. This process continues and a flame propagain German sugar factories, but has given few details regarding tion results which, under proper conditions, leads to a rapid inthe conditions under which the explosions took place. crease of pressure in the form of an explosion. As the proportion The present work constitutes an attempt t o obtain a better of the flammable gas to the supporting gas (usually air or oxygen) understanding of the fundamental nature of dust explosions. It is decreased, a lower limit of explosiveness is reached a t which concerns a study of the explosibility of three different sugarspoint it can be imagined that the combustible gas molecules are dextrose (CeHlzOe), sucrose (C12H22011), and raffinose ( C18H3201e). too widely separated to support the rapid flame propagation Sugars were selected rather than other materials because they needed for an explosion. could be obtained with a high degree of purity, and because they I n the case of a dust explosion, every condition necessary for a gaseous explosion must be satisfied. The dust concentration represented a series of chemically related substances. Thus, for example, the effect of the number of carbon atoms in three sugars must exceed a minimum explosive limit, and there must be a suitable ignition source to initiate flame propagation and cause st8 related to maximum explosion pressure could he studied. The the explosion. Particle size or surface must also be considered. properties of the sugars investigated are shown in Table I. I n addition to the three basic factors affecting dust explosions, numerous others affect the ease of ignition of the dust as well as the speed and TABLE I. EXPLOSIVE PROPERTIES OF SUGARS INVESTIGATED effectiveness of the flame propagation through Max. Max. R a t e Min. Explos. Opt. Press. the dust cloud. Among these are various chemSpecific Explos. Press., Explos. Increase ical and physical factors such as atmospheric Surface Conon. Lb./Sq. Inch Conon Lb./Sq. Inbh/ humiditv, moisture of the dust, dust and gas Sugar Sq. Cm./'c. G./Cu. Mkter Gage G./Cu. M h e r Sea. " , composition, and heat of combustion of the dust. Hartmann (6) and his coworkers have presented a comprehensive study of the effects of these "secondary" factors. Most of the previous work done on dust explosiveness has been primarily concerned with studies of the relative flammability of dusts-i.e., the percentage by weight of inert dust, usually calcined fuller's earth, required in a mixture of flammable dust t o prevent ignition and flame propagation when a dust cloud of the mixture is exposed Present address, Research Division, Lion Oil Co., E l Dorado, Ark. 1
Dextrose
Sucrose
4500 2930 2350 1480 1330 3830 2950 2350 1800
1320 3700 2980 2270 1980 Dextrose Sucrose ~afinose
...
... ...
20 110 220 300
... 40 120
180
320
...
150 330 500
...
72 68 65
57
0 60 56 52 42 0 41 37 33 0
Extrapolated Optima 87 70
... ... ...
50
310 360 400 440
245 220 200 160
240 280 320 450
155 160 125
...
...
350 470 550
90 70
...
... 55
...
60 ...
180 150 100
300 240 120
I N D U S T R I A L AND E N G I N E E R I N G CHEMISTRY
764
Vol. 46, No. 4
EXPERIMENTAL PROCEDURE
The three sugars teated were carefully screened with a cloqe series of U.S. Standard sieves in order to obtain the maximum number of narrow size fractions, particularly in the range from 140- t o 325-mesh. As the materials are all somewhat hygroscopic, they were dried in a vacuum oven and rescreened before the tests of expiosibility were made. The specific surface of each size fraction was determined by means of the Blaine permeabilitv apparatus (6).
Threoded to fit bottom of ionh
Figure 3.
U
(b)
Figure 1. Explosion Chamber and Dispersion Gone
The chamber used for making explosion tests of the dusts was a cylindrical, pressed-steel tank with a hemispherical bottom. The tank was 14 inches in diameter by 23 inches in depth. Its removable top was fitted n4th the ignition system used to initiate the explosions. This ignition system consisted of two brass electrodes inch apart and connected t o a high-voItage transformer (5000 volts). The test chamber is shown schematically in Figure 1,a.
0.1sec.
H
Figure 2. TypicaI Time-Pressure Record The effect of the size and geometry of an explosion chamber, on both the maximum pressures generated and the rate of pressure propagation, is not too well known. I n selecting the size and design used in the experiments described in this paper, consideration was given to factors which would assure as uniform a concentration of dust as possible and provide a sufficient volume to permit an analysis of the gaseous products of combustion. Before the tests were initiated, changes were made in the location and design of the ignition electrodes and dust-dispersing mechanism to
Pressure Surface (Dextrose)
assure dust cloud uniformity and consistent, reproducible results, 811 data presented in this paper were reproducible to within a few per cent. The dust clouds to be tested were created by instantaneou releasing a blast of dry air from a small steel bomb, thereby blowing the dust sample out of a polished brass funnel upward into the tank. The dispersion cone shown in Figure l , b , was drsigned so that a blast of air through a l/a&ch annulus a t the apex would completely clear the funnel of the dust Eample and, a t the same time, produce a reasonably uniform dust cloud, which in turn could be ignited by the high-voltage arc. The dust cloud formed could be observed through a peephole in the upper portion of the chamber. Adjustments \!ere made with the disperqing mechanism until the cloud appeared uniform. The pressures resulting from the explosions were measured with a compensated Bourdon tube gage which rvas equipped with a very thin bamboo stylus. The trackings of the stylus were made on a soot-blackened cylinder driven by a synchronous motoi. From these time-pressure records it was possible to determine the maximum pressures and also the rates of pressure increase drveloped by the explosions as functions of the type of sugar, the dust concentration, and the specific surface. All other variables were held constant. The choice of a pressure-measuring device proved troublesome The problem was discussed with varioue individuals interested in the measurement of explosion rates and on their recommendation, based chiefly on the relatively low explosion intensities arid pressure propagation of ignited sugar dust, it was concluded that the form of device described above gave sufficiently accurate results The calibration of the gage was checked a t frequent intervals and proved satisfactory throughout the entire series of teste. To obtain test data, a weighed sample of dust was placed in the dispersion cone and the tank was sealed. A dry atmosphere w a ~ then provided by evacuating the chamber and then alloning diied air to fill the chamber until atmospheric pressure mas reached. The kymograph motor and the high voltage x e r e turned on, eo that a zero line would be tracked on the soot-blackened drum. A blast of dry air was then released from a 2-cubic-inch bomb ( 2 i 5 to 300 pounds per square inch gage) b y means of a solenoid valve. The cloud thus created was ignited and a time-pressure record obtained on the kymograph drum. Carbon monoxide, carhon dioxide, oxygen, and hydrogen were determined in the gaseous products of combustion.
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
April 1954
765 Pressure Specific surface
Specific surface
Concentration
Concentration
I
0
200
400
800
600
Figure 5.
to00
Figure 4. Pressure Surface (Sucrose)
The dust clouds generated in the chamber were ignited by use
of a high-voltage electric spark between two pointed electrodes. The high voltage necessary for ignition was obtained by means of a 5000-volt, 150-ma. transformer. A neon tube in series with the spark gave an indication of spark operation. THEORY OF DUST EXPLOSIONS
No comprehensive theory on the initiation and propagation of dust explosions exists a t the present time. The problem is one of considerable complexity; in fact, the mechanism of the dust explosion itself is not fully understood. There are a t the present time two theories which attempt to explain the explosion phenomena; neither is wholly satisfactory. One mechanism proposed for carbonaceous materials is a twostep process whereby the dust is first partially gasified and then ignited by the heat source. A chain-type reaction results which causes propagation of the explosion. The second mechanism proposes that the initiation and propagation of dust explosions in the early stages take place in the same manner as in the later stages-by rapid oxidation of the dust particles themselves. This, in turn, sets up the chain reaction needed for the explosion (12). Some theoretical work has been done on flame propagation and the kinetics of oxidation of dusts (11). Most of this work has been concerned with open or only slightly restricted combustion rather than rapid explosive combustion in a closed space. A reasonably satisfactory basis for estimating rate of pressure build-up in a closed container can be developed along the following lines. Let US suppose that the rate mechanism of the explosion is of the first order and that it is affected by the pressure increase in such a way that dpldt = a p
- $(PI
where po is the maximum pressure and to the time corresponding to the inflection point of the logistic curve. Figure 2 shows a typical time-pressure kymograph record of exploding sugar dust in the experimental chamber. It can be seen that the rate of pressure increase is relatively slow. A11 the sugars investigated gave similar curves. When the results were plotted on a logistic grid in accordance m7it.h the method given by Wilson (14), a single straight line was obtained for all the timepressure data secured in this investigation. Values of O( = 0.95 reciprocal second and to = 0.5 second were obtained, It would appear, therefore, that in so far as sugars are concerned, the rate of pressure increase is adequately described by Equation 2. RESULTS
MAXIMUM EXPLOSIVE CONCENTRATION. The main results of the investigation are contained in Figures 3,4, and 5, which show the relation among maximum pressure, dust concentration, and 80
60
40
20
ln
'0
6
12
18
'0
6
12
18
24
60
40m 40
20
'0
6
12
18
NO. OF CARBON ATOMS IN MOLECULE
fits reasonably well. We then have dp/dt = a p - b p z
(2)
This is the equation of the logistic and the solution is
+ exp I - 4 t
24
00
d p ) = bp2
P / P O = (1
i
Pressure Surface (Raffinose)
(1)
where p is the pressure at any time 1, a is a constant, and + ( p ) is some function of pressure. The negative sign has been inserted because the pressure increase is assumed to be deterred by a resisting force proportional to the pressure. Various assumptions may be made as to the form of + ( p ) , but it appears from several forms attempted that
I
- t0)11-~
(3)
Figure 6.
Relation of Number of Carbon Atoms to Explosibility of Sugar Dusts
8
4000 sq. cm. per gram 3000 sq. om. per gram
0 2000 eq. cm. per gram
24
766
INDUSTRIAL AND ENGINEERING CHEMISTRY
apecific surfaces-Le., different degrees of fineness of the three sugars studied. I n general, the maximum pressure developed during an explosion decreases as the number of carbon atoms in the sugar increases, other variables being fixed. There are both minimum and optimum concentrations a t which explosions take place for all the sugars. The essential explosive properties of the sugars tested are shomn in Table I. The lower explosive limit for a dust explosion may be defined as that point a t which there is barely enough material suspended in air to support combustion and propagate flame. The minimum explosive concentrations for dust clouds cannot be as sharply defined as those for gases, because it is difficult t o produce and maintain an absolutely uniform dust cloud, whereas a homogeneous gas dispersion is easily obtained. The lower explosive limits were carefully checked by observation through the peephole provided in the explosion chamber of Figure 1. The values shown in Figures 3 to 5 and in Table I are averages of ~everal experiments. O m I b f u b f EXPLOSIVE CONCENTRATION. The optimum explosive concentration is that concentration a t which the maximum explosion pressure is developed from the ignition of a dust cloud-that is, for each specific surface of each sugar there is one particular dust concentration from which a higher explosion pressure will be developed than vould result from an explosion of the same dust a t any other concentration. The curves in Figures 3 to 5 illustrate that the optimum explosion pressure increases with specific surface (or decreasing particle size) and that i t tende t o shift toward lower dust concentrations. I n other words, as the particle size decreases, a smaller dust concentration is required t o produce the optimum explosion pressure; in turn, these pressures tend to increase as the particle size diminishes. EFFECTOF SPECIFIC SURFACE A N D SUGAR STRUCTURE O S EXPLOSIBILITY. The effect of specific surface on the explosibility of
Vol. 46, No. 4
sugar dusts is shown in Figures 3, 4, and 5 . For each of the sugars the maximum pressure developed from an explosion of a fixed concentration of dust increases a8 the specific surrace increases. The diagrams show that the maximum pressure in each case decreases as the concentration increases. This is as ~vouldbe expected, as in each case the concentration exceeds the optimum explosive concentration of each sugar. i i s the number of carbon atoms in the sugar molecules increases, the maximum explosive pressure decreases as shown in Figure 6. LITERATURE CITED
(1) Brown, H. R., U. S.Bur. Mines, Inform.Circ. 7148 (1941). (2) Ibid., 7183 (1941). (3) DallaValle, J. XI., “Encyclopedia of Chemical Technoiogy,” by R. E. Kirk, and D. F. Ot’hmer, Vol. 5, p. 309, New York, Interscience Encyclopedia, h e . , 1950. (4) Geck, W.H., Zucker, 4 , 3 1 (1951). (5) Government Printing Office, “Federal Standard Stock Catalog,” SS-C-l58b, Section I\‘, Washington, D. C., 1946. (6) Hartmann, I., IND.ENG.CHEAT., 40, 752 (1948). (7) Hart,mann, I., Cooper, A. R., and Jacobson, M., U. 8. Bur. Mines, Rept. Innest. 4725 (1950). (8) Hartmann, I., and Nagy, J., Ibid., 3751 (1944). (9) Ibid., 3924 (1946). (10) Hartmann, I., Nagy, J., and Jacobson, At., Ibid., 4835 (1951). (11) Lewis, B., “Third Symposium on Combustion, Flame, and Explosion Phenomena,” pp. 185, 466, Baltimore, Md., Williams & Wilkins Co., 1949. (12) Meek, R. L., Ph.D. thesis, Georgia Institute of Technology, 1952. (13) Nagy, J., Zeilinger, J. E., and Hartmann, I., U. S. Bur. Mines, R e p t . Invest. 4636 (1950). (14) Wilson, E. B., Proc. Xatl. Acad. Sci., 11, 451 (1925).
RECEIVED f o r review December 18. 1QZ2.
ACCEPTED January 4 . 1954.
Stearato C R. IC. ILER Grasselli Chemicals Department, Experimental Station, E . I . d u Pont de Nemours & Go., Inc., Wilmington 98, Del.
S’”
3 BRATO chromic chloride (Quilon, trade-mark of E. I. du Pont de Kemours & Co., Inc.) is a water-soluble organoinorganic complex compound having the unusual property of acting as a surface active agent when in solution, but as a hydrophobing agent when adsorbed on a polar surface such as cellulose or glass. This material has found application as a hydrophobing agent for cellulosic, proteinaceous, siliceous, and other negatively charged surfaces. For example, it is currently being employed for treating ice bags, meat mapping paper, and exterior gypsum sheathing, as well as an ingredient in the production of a flame retardant and water-repellent paper, I n addition, this surface active complex is being successfully applied to men’s fur felt hats, as a finish on glass fiber draperies and curtains, as a size on industrial glass fiber, and also for treatment of wool and blends of wool with certain synthetic fibers. The oriented stearato groups also impart antisticking characteristics to treated surfaces; this property has been utilized in surface treatment of bakery pan liners, the backs of pressure-sensitive tapes and labels, and other miscellaneous applications nThere adhesion is to be avoided. Figure 1shows the water repellency of a treated section of test paper.
COMPOSITION
The possibility that basic chromic chloride might react with a long-chain fatty acid to form a n-ater-soluble surface active cat-
ionic complex was first recognized in an investigation of the hydrophobing action of chromyl chloride on paper (3, 6). It was postulated that a hydrophobic basic chromic complex must have been formed by the reduction of chromyl chloride to a basic chromic chloride, which then combined with organic acid;: formed by oxidation of traces of greaselike impurities on the surface of paper. I n order to prepare such a complex of basic chromic chloride with a long-chain organic acid, chromyl chloride was reduced with lauryl alcohol dissolved in carbon tetrachloride. When the reaction product, recovered by evaporation of the carbon tetrachloride, was dissolved. in alcohol, then diluted vAt,li water and applied to paper and dried, the paper became very water repellent. This indicated that the long-chain fatty acid formed by oxidation of the alcohol had combined with the trivalent chromium to form a water-soluble cationic surface active complex which had become adsorbed on the paper ( 6 ) . While the combination of short-chain carboxylic acids with chromium to yield complex cations has been demonstrated by Werner and Pfeiffer (Q),the possibility of forming water-soluble complex chromic salts in which long-chain fatty acids were coordinated with t’he chromium cation had not been previously appreciated. I n order to determine the composition of the complex responsible for the hydrophobing effect on paper, various ratios of chromic chloride, Jodium hydroxide, and stearic acid were combined in
