February 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
composition solids and ( b ) rate of flow of a coating composition to give specified uniform film thicknesses has been given. Although no extensive plant experiments have been conducted on this method of interiorly coating objects, it is believed that, in certain cases, a production line setup could be established to accomplish unique results by this method, particularly where it is desired to coat certain sections of the containers without resorting to expensive masking operations.
OEM-sr-796. Permission to publish tjhe resultts of this gation is gratefully acknowledged.
301 iirvrrti-
LITERATURE CITED
(1) Chemical Warfare Service, unpublished work, Edgewood Ameiial Edgewood, Md. (2) Graves, Stuart, IND. ENG.CHEM., ANAL.ED., 16, 599-602 (1944, (3) Marvel, C. S., unpublished work, Univ. of Ill., Uibana, Ill. (4) Payne, H. F., IND. E m . CHEM.,ANAL.ED.,15, 48-56 (1943)
ACKNOWLEDGMENT
This article is based upon work performed for the Office of Srirntifir Research and Development under Contract No.
RECEIVED July 17, 1946. Presented before the Division of Paint, Varnish. and Plaatica Chemistry a t the 110th Meeting of the AMERICAN CHEMroai SOCIETY,Chicago, Ill.
STABLE RED PHOSPHORUS M. S. SILVFXSTEIN, G. F. NORDBLOM, C. W. DITTRICH, AND J. J. JAKABCIN Frankford Arsenal, Philadelphia, Pa.
ED phosphorus, or amorphous phosphorus, as the commercial product is sometimes called, is notcd for its dcterioration when cxposed to air a t normal temperaturcs and humidities. The acidic, hygroscopic, and poisonous propertics of the products of oxidation and the spontaneous combustian hazards of commercial red phosphorus provcd highly objectionable in certain applications of this material and resulted in a wartime develop ment of a stable type of red phosphorus. This article summarizes the more important aspects of this invcatigation. The oxygcn and water vapor of the air combinc with red phosphorus to form a mixture of phosphorus acids-namcly, phosphoric, phosphorous, and hypophosphorous acids and a small amount of phosphine. The mole fractions of tho acids vary slightly with oxidizing conditions and duration of oxidation, and are approxirnatcly 0.40: 0.55: 0.05, respectivcly. Thcsc acids are hygroscopic and are rcsponsible for the sticky, nonflowing character of tho present commercial product after cxposure. The amount of phosphine formcd in the oxidation of red phosphorus is rclativcly small, thc ratio of phosphorus combined in phosphorus acids to that in phosphine being approximately 13 to 1. Neverthcless, phosphine formation is important, for the gas is quite poisonous, and care must bc exercised where largc quantities of red phosphorus are cxposed or where relatively small quantities are exposed for long periods of time. Henderson and Haggard' state that the symptoms of phosphine poisoning resemble those of food poisoning. The maximum concentration of this gas that can be inhaled for one hour without serious results is given as 100 to 200 parts of phosphine pcr million parts of air. The handling of large quantities of red phosphorus raises the problem of spontaneous combustion. I t is to he cxpected that the exothermic oxidation which occurs when rod phosphorus is exposed to air will result in a tempcrature gradient with ita highest point toward the center laycr of red phosphorus, since the surrounding phosphorus acts as a heat insulator. However, the higher the temperature the greatcr the rate of oxidation; conscquently more heat is produccd, and this causes the tomperature to rise still further. Considering a laycr of red phosphorus with a cross-sectional area much greater than the thickness, therc is a maximum thickness of laycr below which this temperature rise is brought to a halt by the accompanying increase in rate of hcat transfcr before the ignition tempcrature is rcachcd. But with thickncsses greater than this crilical laycr thickness the temperature mounb in a pyramiding fashion and the vapor 1 Renderaon and Haggard, "Noxious Gama," A.C.S. Monograph 86. Chemical Catalog Go., 1927.
above the mass begins to glow until the whole mass of phosphorus bursts into flame. The critical thickness of the layer is a function of the ambient tcmperaturc, heat transfer coefficient, and rate of generation of heat. This function takes the form:
where Y = critical thickness of layer, cm., above which spontencous ignition occurs X = distance from plane of maximum temperature, cm K = hcat transfcr coefficient, cal./cm./' C./sec. To = autogcnous temperaturk, C. 2'. = ambienttemperature, C. Q = hcat of reaction, cal./cc./sec. Equation 1 shows that the thickness of layer above which spontaneous ignition occurs is inverscly proportional to the square root of the rate of generation of heat, which is directly proportional to the oxidation rate of the red phosphorus. Thus it is evident that the principle disadvantugcous charaateristics of commercial red phosphorus-namcly, its acidic, hygroscopic, poisonous, and spontancous combustion propertiesare a result of, and incrcase with, its rate of oxidation. The problem, thcrcfore, consisted of stabilizing red phosphorus-that is, drastically reducing its oxidation rate. Thc solution of the problem was resolved into the identification and classification of the accclerators and inhibitors of the oxidation, followed by the development of methods for removing the principal acccleratom and methods for applying the best inhibitor. The effects of a number of classes of inorganic and organic substanccs on the oxidation rate were investigatcd. I t w a ~ found that the most important effects are obtained with certain mctals, in clcmentary form or as the oxides or salts. The effect on the oxidation rate a t 60" C. and 90% relative humidity of samples of rcd phosphorus containing 5% powdercd metals ie prcsented in Table 1. Table I shows that copper, bismuth, silver, iron, and nickel increase the oxidation rate drastically, that cadmium and tin increase the rate moderately, that lead and chromium have very littlc effcct on the rate, and that aluminum and zinc decrease the rate. Thus, the mctals present in red phosphorus are an extremely important stability consideration. Of all the matcrials tested, the best inhibitors of oxidation were found to be the hydroxides of the inhibiting metals of Table I, of which aluminum hydroxide w&s by far superior. Tho effect on
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as in the case of iron, Copper can be removcd from red phouphorus by treatment with alkaline cyanide solutions, the optiPHOSPHORUS AT 60' c. mum conditione being to treat the phosphorus with a boiling Phosphorus Acids/Gram Phosphorus after solution of 5% sodium cyanide and 1.5% sodium hydroxide for -?;lillimoles Metal 200 hr. 400 hr. 600 hr. SOmF0.5 hour and filteiing and washing the residue. I n this way the Aluminum 0.100 0.148 0.197 0.048 copper content is reduced t o about 3 parts per million. Since it 0.215 0.340 0.462 Zinc 0,088 0.475 0.75 0.265 Chromium 0.132 n-as found that acid treatment for iron removal does not affect 0.286 0,482 0.80 0.129 Lead 0,486 0.82 0.114 None 0.290 copper content, one would expect the order of acid treatment and Tin 0.246 0.73 ... 0.488 cyanide treatment to be immaterial. However, analyses demon> 1.4 0.74 Cadmium 0.397 0.55 > 1.4 * ~ . Nickel strate that the order is important, and that a sample which is 0.71 Iron > 1.4 ... I.. Silver >1,4 0.85 ..* acid-treated first and then cyanide-treated has a lower copper Bismuth >1.4 ._. ... .,, content than when the reverse order is applied. This difference Copper > 1.4 . , ... may be due to tho impurity of the acid used for the acid treatTABLE 11. EFFECTOF 5% ALCUINCM, ZISC, AXD CHROMIUM ment. If copper is present in the acid and a sample is first acidtreated, the copper is removed from the phosphorus by the HYDROXIDES ON OXIDATIOX RATEOF REDPHOSPHORL-s AT 63" c. and 90% REL.4TIVE HUMIDITY cyanide treatment rhich follon7s; however, if a sample is first Millimoles of Phosphorus Acids per Gram cyanide-treated and then acid-treated the copper from the acid Phosphorus after Metal is retained on the phosphorus. After copper is removed from red 400 hr. 800 hr. 1200 hr. 1600 hr. Hydroxide phosphorus, the exposure of the phosphorus to large quantities of Aluminum hydroxide 0,015 0,015 0.015 0.026 Zinc hydroxide 0,058 0.117 0,177 0,240 water should be kept at a minimum, for low-copper red phosChromium hydroxide 0.112 0.278 0.527 >0.7 phorus takes up copper from water containing traces of copperNone 0.393 b0.7 ... ... that is, in the order of a few parts of copper per billion parts of water. This fact has becn used in the collection of small quantities of copper and similar metals for analysis. che oxidation rate a t 60" C. and 90% relative humidity of samThe effect on the oxidation rate of removing iron and copper ples of red phosphorus containing 5 % of the hydroxides is prefrom commercial red phosphorus is presented in Figure 1. aented in Table 11. It was previously mentioned that of all the materials tested the Of the accelerating metals, those present as impurities in combest inhibitor of the oxidation of red phosphorus was found to be mercial red phosphorus are iron and copper. These two metals alumina hy'drate. Of the methods tested, this inhibitor can besl ere responsible for practically all of the oxidation that occurs be applied by precipitation of aluminum hydroxide from sodiurri with this material. Iron and copper are present in the usual bicarbonate and aluminum sulfate solutions in the presence of commercial red phosphorus t o the extent of about 250 and 30 the red phosphorus. The ratio of alumina to phosphorus used i v parts per million, respectively. The iron is distributed almost dependent upon the specific red phosphorus and its intended use entirely on the surface. This was established by comparing iron The stability of the aluminated red phosphorus increases with content per unit with specific surface. If the iron is concentrated alumina content up to about 7% alumina [calculated as A1 (0H)sl mainly on the surface in a given sample, the ratio of the iron conand thereafter additional alumina has no effect on stabilitj tent to the surface area shodd be constant. This was found However, as little as 0.5% alumina stabilizw the material conto be the case (Table 111). It should be noted that, whereas siderably. The effccts of varying quantities of alumina are prr the surface distribution of iron occurs in most of the red phossentcd in Figure 2. I n Figure 2 the oxid'ation rates at 100" C . phorus on the market, there is some oommercial red phosphorus 100% relative humidity, and 50 pounds per square inch initial manufactured by minor producers wherein considerable iron is oxygen pressure are presented in place of the previously mendistributed throughout the mass of the particles. tioned rates a t 60" C., 60% relative humidity, and atmospheric pressure. This is necessary because of the high degree of stability of aluminated red phosphorus, which requires excessively TABLE111. DISTRIBUTION OF IRONIN REDPHOSPHORUS long p e r i o d s of Ratio, Iron Surface Area, Content t o Iron Content, time for an appreSample P.P. kr . Sq. Cm./Gram Surface Area ciable amount EFFECT OF REMOVING IRON B COPPER A 15 535 0 I028 1075 0.032 FROM COMMERCIAL RED PHOSPHORUS ON of o x i d a t i o n A 34 23.50 0,037 A 86 to take place 3 OXIDATION RATE IN AIR AT 60°C 8 90"1,RH B 5 1430 0.0035 6 7070 0,0038 B 27 a t B O O C . , 907~ a relative humidity, and atmospheric The surface distribution of iron facilitates its removal. I n pressurc. A 1s o 5 v general, such iron can be removcd by an acid treatment. One t h c a m o u n t of B oxidtition in the variation is to promote the oxidation by exposure to elcvated temperatures and humidity, In this way the phosphorus acid morc drastic test products of ttic oxidation leach the iron from the phosphorus, is measurcd as deand the solution is washed from the phosphorus. A more efficrease i n oxygen cient and convenient method is to treat the red phosphorus with a pressure. T h e hot solution of a strong mineral acid. The preferred procedure is preferred proceto treat rcd phosphorus vith boiling 5% sulfuric acid solution for dure for duminat4 to 6 hours, then filter off and wash the phosphorus. I n this ing red phosp h o r u s is as way the iron content may be reduced to approximately 5 parts per million. follows: Add the Coppcr is not removed from red phosphorus by this procedure, a t oichiometric and the strtltility of red phosphorus aftcr such an acid treatment quantity of s e 0 IO0 zoo 300 dium bicarbonate decreases sharply with increasing copper content. As in the case HOURS of iron, copper is distributed almost entirely on the surface in the in the form of a usual red phusphorus. This was determined in the same manner lOyo s o l u t i o n Figure 1
TABLE I. EFFECT O F 5vc
I f E T h L S ON OXIDATION R A T E O F RED AND 90% RELATIVE HUMIDITY
I..
~.. ... I
.
H
~
February 1948
INDUSTRIAL A N D ENGINEERING CHEMISTRY
o
HOURS
Figure 2
heated to 55-60' C. to the red phosphorus then add thc stdchiometric quantity of aluminum sulfate in the form of a 10% solution heated to 55-60' c.,agitate the mixture for about an hour, Blter, and dry. Since copper and iron are mainly found on the surface of rcd phosphorus, and since the rate of oxidation increases with specific surface, it is advantageous to remove as much of the fines as the use of the particular red phosphorus will permit. In the irwesti-
303
gation described, the particles less than 10 microns in diameter were kept below a maximum of about 1% by weight. This can be accomplished by sedimentation, contrifuging, elutriation, or any other similar procedure. In conclusion, by the processes of size classification, removal of iron and coppcr, and the addition of hydrated alumina, a stable red phosphorus can be produced. Such a material has been produced on a pilot plant scale in cooperation with the Oldbury Electrochemical Company and the Chemical Engineering Department of the Tennessee Valley Authority. This new product remains free-flowing for a t least five years when exposed to normal atmospheric conditions. Furthermore, the disadvantages of the dsual commercial red phoaphorus-namely, the generation of acidic, hygroscopic, and poisonous oxidation products and the hazards of spontaneous combustion-have been minimized to the point where they are, for practical purposes, nonexistent. ACKNOWLEDGMENT
The authors wish to express their appreciation to C. C. Fawcett, E. R. Reehel, and J. W. Mitchell of the Frankford Arsenal Ordnance Laboratory for their helpful suggestions and cooperation, and to the Ordnance Department for permission to publish this paper. REOEIVED January 8,1947.
Contact Times of Continuous-Flow Reacting Systems with Volume Change STUART R. BRINKLEY, JR. Central Experiment Station,
U. S. Bureau of Mines, Pittsburgh, Pa.
A
general relation between contact time and space velocity is obtained by comparing the solutions of the Euler and Lagrange forms of the hydrodynamical steady-state equations for the composition of a flowing system which undergoes chemical reaction with accompanying volume change. It is assumed that the reactor is isothermal and isobaric and that the flow is one-dimensional. This relation is applied to the calculation of contact times for particular rate laws. An approximateformula is developed for the estimation of contact times when the rate law is unknown, and the resulting error is obtained for several cases by a direct numerical comparison with the correct expressions. The application of these methods to reactions on a granular catalyst is discussed.
S
TUDIES of reactions taking place in continuous-flow sys-
tems, which are conducted with a v:ew t o the determination of the governing rate law, are most appropriately correlated in terms of thc contact time, since the rate law in terms of this quantity has the same form as in the static case. If the reaction proceeds without volume change, the contact time for one-dimensional flow with negligible diffusion is simply the reciprocal of the space velocity based on the feed and on the free volume of the
reactor. However, if a volume change accompanies the reap tion, the relation between contact time and space velocity is somewhat more complicated, and involves the degrce of reaction in a manner determined by the rate law. The hydrodynamical equations for continuous-flow system in which chemical reaction occurs have been obtained by Eckart (1). These relations have been employed by Hulburt ( X ) in s discussion of the kinetics of continuous flow reacting systems. Instcad of the contact time, Hulburt employs as independent variable the distance from the inlet of the reactor, and he d i e cusscs in detail thc application of the resulting relations to typical systems. The use of contact time as independent variable leade to an alternative description of continuous-flow systems which is simpler in some particulars than that provided by the use of distance (or space velocity), In the present communication a relation is developed between the space velocity and the contact time for systcms in which a volume change accompanies the reaction. Examples are given of the application of this general relation to three simple types of reaations, two of which have been previously considered in detail by Hulbkrt. An approximate formula is developed for the estimation of contact times where the rate law is unknown, and the usefulness of the approximate relation is illustrated by numerical comparison with the exact results for the three simple types of reactions.
