INDUSTRIAL AND ENGINEERING CHEMISTRY
March 1948
't
/loo
O F RUBBER CEMENT BY ETHYLENE TABLE VII. GELATION
MERCAPTAN
Experiment No. 1
?
,O 600
3
Ia
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a \
\
Iiniperox 60.
Addition
Observations Remains fluid Thickens slowly. gelation occurs after 5' days' exposure t o afternoon sun Ethylene dimarcaptan plus Similar to (2) hut gelled peroxideo after 3 days
None Ethylene dimercaptan
I"
W
# 400
reaction as a feasible step in sultur vulcanization as proposed in the first two papers of this series.
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UNAGED
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I
511
c5
I
I 3 3 % POLYMETHYLENE D I M E R C A P T A N
,
Figure 7. Elongation as Function of Concentration of Polymethylene Dimercaptans, 45 Minutes at
144-195" C.
ACKNOWLEDGMENT
The writer acknowledges with thanks the suggestions of 0. D. .Cole of the Firestone Tire and Rubber Company and C. E. Boord of The Ohio State University. They also wish t o thank Mrs. Fred M. Ernsberger for helping with the analytical work. LITERATURE CITED
(1) Adams, Roger, and Marvel, C. S., Org. Syntheses, Coll. Vol.
RUBBERCEMENTS. The rate of gelling as well as the rate of mill scorching is much slower for rubber than for GR-S. This is shown in TRble VI1 for a 10% solution of rubber in xylene. DISCUSSION AND CONCLUSIONS
The expected vulcanizing activity of dimercaptans has been demonstrated. It seems likely that mill scorching results from peroxide-catalyzed mercaptan addition (6); milling a t higher temperaturcs would tend to inhibit reaction by destroying peroxides. The development of considerable tensile strength iri stocks containing dimercaptans and the gelling of cements by dimercaptans give strong support to the mercaptan addition
I,
504 (1921). (2) Bourgeois, E.,Rec. trav. chim., 18,444(1899). (3) Hall, W. P., and Reid, E. E., J. Am. Chem. SOC.,65, 1466-8 (1943). (4) Hull, C.M.,Olsen, S. R., and France, W. G., IND.ENG.CHEM. 38,1282 (1946). (5) Mayo, F. R., and Walling, Cheves, Chem. Revs., 27, 387-94 (1940). (6) Nicolet, B.H., J . Am. Chem. SOL,57,1098 (1935). (7) Olsen, $. R., Hull, C. M., and France, W. G., IND. ENG.CHEM., 38,1273 (1946). (8)'Tucker, N. B.,and Reid, E. E., J . Am. Chem. SOC.,55, 775-81 (1933). RECEIVED June 10, 1947. Presented before the Division of Rubber Chemistry at its meeting in Cleveland. Ohio, May 26 to 28, 1947, as a contribution from The Ohio State University Research Foundation Firestone Tire and Rubber Company Projeot.
Evaluation of Engine Coolants TECHNIQUES FOR TESTING SMALL QUANTITIES '
G. A. HAWKINS AND C. 32. WARNER Purdue University, Lafayette, Ind.
As a part of a development program dealing with new engine coolants, it is desirable from time to time to compare quickly the heat transfer characteristics of new fluid samples with coolants such as water, ethylene glycol, and alcohol. Because of the cost of preparing large samples and the time required, it is desirable to limit the amount of material to very small quantities-for example, 200-cc. samples. Apparatus and techniques which have been used in an experimental program, are briefly described for comparing small samples. T " E P roblem of evaluating and comparing new fluids for use in liquid-cooled aircraft engines is complicated by the many variables involved. The heat developed in the engine and removed by the coolant aq it flows through the channels must be later dissipated to the air by means of a radiator. In any attempt to evaluate a new fluid, its behavior in the cylinder jacket and radiator as well as the pumping characteristics must be given serious consideration. In addition to the heat transfer and pumping
characteristics, such factors as corrosion, flammability, and boiling point must be considered. Because of the cost of preparation, and the difficult chemical technique required, it is desirable t o be able to evaluate new fluids as coolants by use of very small samples. When large quantities are available full scale engine tests may be conducted; thus many of the uncertain factors discussed in this paper may be eliminated. The following remarks are based on the assumption that only very small quantities of proposed liquids are available (200 cc. maximum) for evaluation purposes. This is a critical limitation and should be kept in mind 1,hroughout the discussion. The major factors t o be considered in the evaluation of a liquid for aircraft coolants are as follows: (a) the transfer of heat from the cylinder wall to the fluid in the cylinder jacket; ( b ) the transfer of heat from the coolant to the air by means of a suitable radiator; (c) the resistance of the fluid to flow through the various hydraulic circuits; and) ( d ) chemical and miscellaneous factors, such as weight of the fluid, corrosive action, cost, flammability, and availability.
518
INDUSTRIAL AND ENGINEERING CHEMISTRY
A research program was initiated a t Purdue University which dealt with the evaluation of very small quantities of new fluids for use as engine coolants. Although the progress made was considered satisfactory in some phases of the work, changes in the program will result in marked improvements in the methods and techniques used. The primary objective of this review of the work is to provoke thought and discussion among interested engineers, which will eventually lead to a more satisfactory method for the evaluation of small quantities of new fluids not only for llquid-cooled aircraft engines but for other devices using liquids for the transfer of heat. HEAT TRANSFER FROM CYLINDER WALL TO COOLANT
Results from actual tests of liquid-cooled aircraft engines indicate that metal surfaces of the cylinder exposed to the coolant attain temperatures as high as 480" F., and heat transfer rates reach an upper limit of 130,000 B.t.u. per hour per square foot. The large temperature differences which exist between the metal surface of the cylinder wall and the coolant have caused local boiling to occur in some cases. T h e vapor thus formed is transferred t o the main body of fluid where it condenses. The effect of local boiling on the total heat exchange should be considered together with the transfer of heat in the jacket chamber by F i g u r e 1. Schematic Diagram forced convection. of Wire a n d Liquid The problems associated with evaluating small quantities of new fluids on the basis of transfer of heat in the cylinder jacket are complicated. As a first attempt t o evaluate the fluids it was decided t o study the heat transfer from a heated wire submerged in the fluid, and t o consider the type of condensation and evaporation associated with the fluid, While it is recognized that data resulting from tests on the heat transfer from a heated wire submerged in the test liquid do not exactly represent results of the transfer of heat from the cylinder wall to the coolant in actual engine tests, the method had many advantages from the experimental standpoint. By means of the apparatus, high rates of heat transfer were obtained between the heated wire and very small quantities of fluid. In case of failure the wires could be replaced a t low cost without affecting 'the constants of the apparatus. I n general the apparatus consists of an iron wire heated electrically and submerged in the test liquid. The fluid under test was placed in a mica-lined copper box 1.1 inches wide; 6 inches deep, and 11 inches long. The top of the container was open, and the sides and bottom were jacketed so that cooling water could be circulated around the test container. The 200 cc. of liquid under test formed a prism 11 inches long with a cross section approximately 1 inch square when in the test chamber. The iron wire was located as shown in Figure 1 relative to the cross-section area of the liquid. A rectangular-shaped fluid container was selected so that a long single wire could be used to provide for a relatively large mire surface area. A study of the influence of wire length and chamber size was made using different wire lengths in a given chamber and one wire length in different chamber arrangements. The coefficients were calculated from test data obtained using various wire lengths
Vol. 40, No. 3
and chambers but with water. The coefficients thus obtained agreed within =t5%for a given liquid temperature and temperature difference between wire surface and ambient liquid temperature. No. 30 iron wire was selected for the following reasons: The size limited the current to a value in keeping with the precision meters available ; the desired energy release, 130,000 B.t.u. per hour per square foot of surface, could be maintained; the heat loss by conduction from the wire to the supporting electrodes was reduced t o a minimum; and S o . 30 wire proved more durable during operation than did smaller sizes. The iron wire was tightly stretched between two cocper electrodes which were held in the desired position by means of adjustable clamps. The electrodes were designed so that the heat loss through them was reduced to a negligible amount. Lengths of No. 30 wire having effective heating lengths of 10 inches were used for the tests. The iron wire served as a resistance thermometer as well as the source of heat transferred to the fluid. The wire was calibrated as a resistance thermometer by determining its resistance on a Wheatstone bridge while the wire was immersed in an oil bath maintained a t a constant temperature. After calibration the wire was cleaned and placed in the coolant test chamber. The bulk temperature of the coolant was determined by mercury thermometers suspended in the liquid, The problem of temperature measurement was complicated by the fact that the test liquid was cooled a t the surfaces in contact with the walls of the liquid chamber, which produced a temperature gradient from the center outward. It was finally decided that, for the purpose of the investigation, the temperature as determined by a calibrated mercury-in-glass thermometer would be sufficient. It was found that the temperatures as indicated by the thermometer did not vary more than 370 a t positions along the length of the wire. A position midway between the two electrodes and l / s inch from the wire was finally chosen. Although the temperature thus obtained might be subject to question for quantitative analysis, it proved entirely satisfactory for comparative purposes. Electric energy, voltage, and related quantities were measured with standard instruments-ammeter, wattmeter, variable voltage transformer, slide-wire rheostat for controlling the iron wire current, electrodes, and liquid test chamber. At the start of a test the voltage was adjusted until the energy dissipated by the wire submerged in the liquid was 40 watts.
0
20
1
I
I
I
I
40
60
80
100
120
TEMPERATURE DIFFERENCE BETWEEN Liauin
140
AND
HEATING SURFACE.'F
Figure 2. H e a t Dissipated by Iron Wire 10 Inches Long Immersed i n Water
March 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
When liquid temperature reached 150" F., the cooling water was regulated to maintain a constant fluid temperature of 150" F. Approximately 30 minutes were necessary to establish equilibrium conditions after the energy input was changed. After steady conditions had been maintained, the various instrument readings were recorded. The energy t o the wire was increased 20 watts and the cooling water readjusted so that the coolant temperature remained a t 150" F. This procedure was continued until the maximum safe value of energy supply was reached. Above this value the wire would fail. After completion of a series of measurementsat one fluid temperature the energy input was reduced to 40 watts and a new coolant temperature established. The coefficients for the transfer of heat between the wire surface,and the liquid were calculated using the equation
where h = heat transfer coefficient, B.t.u./hr./sq. ft./' F. p = heat dissipated by wire as determined by measured energy input to the wire, B.t.u./hr. A = wire surface area, sq.ft. At = temperature difference betwzen wire surface and ambient liquid temperature, F. The results obtained were plotted t o give energy input against temperature difference for the various coolant bulk'temperatures. Figure 2 shows a typical set of test data. Coolants having the lowest temperature difference for a given power input were judged t o have the best heat transfer characteristics from the standpoint of energy transfer in an aircraft engine cylinder jacket. Every attempt was made to acquire data from actual engine tests in order to determine the reliablity of the results obtained by use of the hot wire tests. Actual published engine test data suitable for comparative purposes were practically nonexistent. Results obtained using a single cylinder (C.F.R.) engine and several different coolants were compared with data obbained pn the hot wire apparatun The fluids used in the hot wire tests were samples of those used in the actual engine tests. Both test results conclusively showed that aqueous glycol solutions were superior t o pure glycol but inferior t o water as coolant fluids under comparable conditions. Four coolants, A , B, C, and D, were found to be less desirable from the standpoint of heat transfer characteristics according t o the engine test results, in the given order of decreasing desirability. The results of the hot wire tests (Figure 3 ) showed the order as B, C, D , and A , the difference being in the position of fluid A . No explanation has been established for tho disagreement in the order; however, the wide ranges in the boiling points for these fluids made the analysis difficult. The two sets of tests conchsively showed these fluids
519
to be inferior t o pure glycol. It was found that the relative standing of the liquids did not change appreciably with bulk temperature; however, the difference between the liquids decreased sharply as the liquid bulk temperature increased. Plans are now being made t o obtain extensive engine test data for use in determining the reliability of the hot wire tests. Another factor which was considered as a possibility for use in evaluating new fluids was the type of condensation associated with the fluid. The type of condensation for the various fluids was observed visually by allowing the vapor to condense on the surface of a water-cooled coppq tube. The copper tube was supported above the surface of the liquid inside a glass container. The condensing surface was cleaned by means of emery cloth and sulfuric acid-dichromate cleaning solution. Due t o the fact that film and dropwise condensation usually appeared together in varying degrees as mixed condensation, it was not possible t o evaluate the results. No significance was placed on the results obtained from these tests. HEAT TRANSFER IN RADIATOR
Evaluating the coolant from the standpoint of the heat transfer in the radiator on the basis of experimental data was not possible because of the very small quantity of each fluid which was available. It was decided to compute heat transfer coefficients from dimensionless ratios and t o compare these results. The dimensionless relation for the heat transfer coefficient for turbulent flow of fluids through tube banks was wed t o determine one coefficient. The reliability of the results is unknown since experimental values for the thermal conductivity of t h e new fluids were not available. The J. F. D. Smith equation for computing thermal conductivity values was employed to obtain theoretical values for use in the calculations. Published values for water and glycol were used in calculations dealing with tpese fluids. The equation developed by Colburn ( I ) and later presented by McAdams (a) was utilized in the study. This equation was developed for the flow of a fluid perpendicular t o a tube bank:
For a given set of conditions the heat transfer coefficients vere found t o be as follows: Fluid Water 60% gIyco1-40% water by wt. Glycol
Heat Transfer Coefficient, % of That for Water 100
.
57 26
The fluids were rated in the same relative order by the hot wire test data. The relation developed by Parsons and Graffncw (3) was also used in attempting t o evaluate the liquids. This particular relation expresses the heat transfer coefficient for turbulent flow inside a pipe in terms of the physical properties of the fluid flowing and the power loss due t o fluid friction. The equation is expressed as follows:
Although this equation is not strictly applicable t o an aircraft cooling system, it was used t o determine the results that might be expected, basing the work on the assumption of equal energy input for circulating the fluid through a cooling system. The relative standings of the three fluids previously cited follow: TEMPERATURE DIFFERENCEBETWEEN
HEATIN5
SURFACE At40
LIQUID, 'F
Figure 3. Heat Dissipated by Iron Wire Immersed in Various Liquids at Liquid Temperature of 150" F.
Fluid
Heat TranRfer Coefficient, % of That for Water
Water 60% glyc01-40% water by wt. Glycol
100 42
19
INDUSTRIAL AND ENGINEERING CHEMISTRY
520
FLUID FRICTION IN HYDRAULIC CIRCUITS
In evaluating a fluid as a coolant some attention should be given t o the flow characteristics of the fluid. Fluids requiring a large amount of pump work t o circulate them through the cylinder jacket, radiator, and auxiliary piping should not be rated so high as those requiring less work. For this reason careful consideration should be given t o the changes in viscosity with temperature, especially a t the lower temperatures. Since the quantities of fluid available were very small, it was not possible to study the fluid flow characteristics by experimental means. In the analysis pressure drop computations were made and compared for turbulent flow of fluids through straight tubes. CHEMICAL AND MISCELLANEOUS REQUIREMENTS
The corrosive action of the test fluid on aluminum alloys, brass, copper, solder, and steel wa,s studied. The corrosion apparatus consisted of a rotating sample holder, which was arranged so that the samples under test were rotated in the fluid. An electrical heater was provided to maintain the temperature of the fluid a t a given value. The apparatus was provided with a water-cooled cover t o reduce the evaporation loss from the apparatus a t elevated temperatures. The corrosion was determined by noting the loss between initial and final weights of the samples during a test. Becawe of limited time the samples were allowed to remain in the apparatus for 120 hours.
Vol. 40, No. 3
The test results obtained using commercial coolants were not the same as the results in actual engine tests using the same fluids. However, by using a commercial coolant as a standard, it was possible t o grade the corrosive action of the test fluids with respect t o the standard. In this way it was possible t o eliminate the fluids which were more corrosive than standard commercial coolants. Factors such as flash and fire points, spontaneous ignition temperature, freezing point, and molecular weight Kere also given consideration in determining the suitability of the fluid for use as a coolant for liquid-cooled aircraft engines. Other factors which are normally considered in the evaluation of a fluid are cost, availability, toxicity, and the effect of fluid on packing and gasket materials. These factors were not considered in the investigation described here. The final evaluation of the small quantities of new coolants was based on the results of (a) the hot wire tests, (b) theoretical computations for the heat transfer in the radiator, (c) flow characteristics of the fluid, and (d) corrosion tests. In general each fluid tested was compared with water as the basic fluid. LITERATURE CITED
(1) Colburn, A. P., Trans. Am. Inst. Chem. Engrs., 29, 174-210 (1933). (2) McAdams. W. H., “Heat Transmission,” 2nd ed., John Wiley & Sons, Inc., 1942. (3) Parsons, P. W., Trans. Am. Inst. Chem. Engrs., 40, 655-73 (1944).
RECBIVED October 3, 1946.
Heat Stability of Molybdena-Alumina Dehydrocyclization Catalysts ALLEK S. RUSSELL AND JOHN J. STQKES, JR. Aluminum Research Laboratories, New’Kensington, P a . T h e heat stability in dry air of molybdena-alumina catalysts has been measured as a function of alumina type, alumina area, and molybdena concentration. Activity of these catalysts for the dehydrocyclization of nheptane to toluene increased at calcination temperatures of 600” to 700” C., but decreased at higher temperaturesr Activated alumina of the H type was more stable than activated alumina of the F type. Stability decreased with increase of molybdena concentration. For molybdenaimpregnated activated alumina F and low-silica H, activity was stable towards loss of area on calcination until the area was reduced to a value just sufficient to accommodate the molybdena in a monolayer and thereafter activity decreased linearly with further loss of area.
M
OLYBDENA-alumina dchydrocyclization catalysts for conversion of n-heptane to toluene lose activity in use. One type of loss is caused by the deposition of coke on the active surface during the reaction. This loss is reversible, and the catalysts can be restored by appropriate burning and reduction. A second type of activity loss, and the one with which this report is concerned, is irreversible and is caused by structural changes in the catalyst; these changes are accelerated a t the temperatures produced during coke burning. The ability to maintain activity after high temperature treatment is an important advantage gained for molybdena by its impregnation onto
alumina, but not all aluminas impart the same heat stability to the composite catalyst. In a previous report (3) the initial activity of molybdena-alumina dehydrocyclization catalysts was discussed. The results indicated that activity increased linearly with the amount of “impregnated area”-that is, the amount of alumina area covered with molybdena. The highest activity resulted from impregnation of a large quantity of molybdena on high area alumina; neither a large quantity of molybdena on low area alumina nor large area alumina without adequate molybdena was effective. This report gives data on the effects of alumina type, alumina area, and molybdena concentration on the heat stability of molybdena-alumina catalysts. The change in activity on high temperature treatment is correlated with the extent of the impregnated area. MATERIALS, APPARATUS, AND ANALYSIS
The activated aluminas were the same purified products of the Aluminum Company of America, Chemicals Division, which have been described (3) except for the H alumina, low-silica, whoso analysis was: 37, loss on ignition, 0.4y0 NazO, 0.3% SiOz, 0.1% FezOa,0.3y0 CaO, 0.2% MgO, and 0.3y0SO3. Unless otherwise designated, the catalysts vxre prepared by the regular procedure of impregnating alumina with ammonium molybdate. “Coprecipitated” molybdena-aluminas were prepared by coagulating, with ammonia and ammonium molybdate, an alumina sol result-
