Heat Transfer to Boiling Methanol Effect of Added Agents

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A. J. LOWERY, Jr.l/ and J. W. WESTWATER University of Illinois, Urbana, 111.

Heat Transfer to Boiling Methanol Effect of Added Agents

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Nucleation, not interfacial tension, may explain why additives or foreign particles change boiling curves for liquids L I T E R A T U R E IS CONTRADICTORY about the role of surface tension during boiling and correlations obtained from dimensional analysis vary greatly. Some experimenters have reported that surface active agents in water increase heat flux at a given temperature-difference driving force, while others report a decrease. If an exponential relationship is assumed for heat transfer coefficient, h and surface tension, u, so that haa", at least a dozen values are published for n. For example, Jakob and Linke (2) give -2.5; Stroebe, Baker, and Badger (77), -2.0; McNelly ( 7 ) , -1.0; Kutateladze ( 5 ) , [email protected]; and Nakagawa and Yoshida ( 8 ) , +1.275. Included in a summary of 35 reports ( 6 )are several authors who used quenching tests to study the film region of boiling, and one author who studied a limited portion of the transition region. All others were concerned with nucleate boiling. Theoretically, surface tension is an important variable. Rate of formation of vapor nuclei in a hot liquid is proportional to e--03. Thus, small decreases in u should cause large incqeases in the number of nuclei. The cavitation theory

Present address, Shell Oil Co., Wood River, Ill.

predicts that the force required to rupture a liquid i n tension is proportional to u3j2. Thus, liquids with large surface tensions should be difficult to fracture. Detailed discussions of these theories are available ( 7 3 ) . On the other hand, experimental proof of the actual effect of surface tension on boiling heat transfer is not convincing. Traces of foreign materials are exceedingly important during such nucleation phenomena as crystallization and condensation; presumably they trigger nucleation, and may be thought of as catalysts which speed reactions that might otherwise require a long time. For boiling water then, although a trace of soap lowers liquid-vapor surface tension and may alter vapor formation rate, it could also alter rate of nucleation even if surface tension remained constant. But with boiling water, results may be confusing, because its surface tension is sensitive to traces of foreign material. For most organic liquids, however, this is not true for small amounts of additives; therefore, these liquids should be useful in determining the role of additives. Boiler Construction

The boiler, 9 X 9 x 4 inches and built of stainless steel, contained flat

borosilicate glass windows, 7 x 7 inches, in front and back. Neoprene gaskets prevented leakage at the windows. A 20-gage copper heater tube, having a '/(-inch outside diameter and secured in neoprene stoppers a t the ends of the boiler, passed parallel to the windows. Steam was fed to the tube a t pressures from 0 to 88 pounds per square inch gage. T o facilitate drainage of steam condensate, the tube was inclined at a 2-degree angle with the horizontal. The steam came from the laboratory line, passed through a pneumatic pressure controller, and then through the heater tube. A trap ensured freedom from drips in the supply going to the tube. A sight glass, downstream from the tube, was operated manually as a steam trap for metering steam condensed in the boiler tube. Methanol vapors, formed in the boiler, passed up an entrainment separator and into a condenser. Condensate passed back to the boiler or was diverted for metering. A manometer connected to the liquid-return line served as a pressure indicator and safety blowoff should unexpected pressure build up. The system's pressure was regulated by adjustments in the condenser water flow. This was a sensitive control method.

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In this boiling equipment, pressure is sensitively controlled by regulating condenser water flow

In this wiring system, the heater tube is used as a resistance thermometer VOL. 49, NO. 9

SEPTEMBER 1957

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Heater Tube Temperature The average temperature of the steam tube was found by using the tube itself as a resistance thermometer. The method is a variation of that used by Jeffery ( 3 ) and by Tobias and Stoppel (72) for tubes in condensers. A 25-ampere direct current was passed through the tube and then through a 0.000333-ohm Manganin standard resistance. Voltage drops across the tube and the standard were measured to within 10-6 volt. Current passed through an 11-inch length of the tube, 8.0 inches of which were immersed during boiling. Voltage drop was measured for 7.9 inches of the immersed portion. Voltage leads passed into the boiler through slits in the neoprene stoppers, and the endsweresoldered to the tube surface. The measured voltage drop was about 0.009 volt. This means that electrical heat generation was less than 0.3 watt. a negligible quantity. Calibration was done with the tube immersed in tanks of three boiling liquids -methanol, water, and ethylene glycol. External heat was applied and no steam was run through the tube. The calibration data, temperature LIS. the ratio of standard e.m.f. to the tube e.m.f., are shown in Figure 1. In polishing the tube before each run, some metal was removed ; therefore, after 40 runs, a second calibration was made and after 80 runs, a third. Each cleaning caused a shift of 0.1' F., which agreed well with 0.11lo F. found for another tube polished in the same manner ( 9 ) . This was taken into account in calculating test results.

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Figure 1. Calibration curve for electrical resistance of heater tube

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The tube was routinely cleaned by 50 axial strokes with a damp vinyl sponge sprinkled with Cameo copper cleaner (Cameo Corp., Chicago), followed by rinsing with water and rubbing down with a dry cloth.

Procedure

After the boiIer and connecting lines were cleaned with water and with methanol, the boiler was assembled and about 1 gallon of methanol with a known amount of additive was put in. Flow of steam was begun, and steady state was reached in about an hour. The heat input (condensing steam) and heat output (methanol boil-up) both were determined. A prior calibration of heat losses from the equipment served to close the heat balance. The average heat unaccounted for was 67, and the maximum 15y0. Heat fluxes in this report are averages of inputs and outputs. Electric current was passed through the tube while heat data were recorded, and readings were taken with the current flowing first in one direction and then in the other. The reversal measurements proved the absence of stray e.m.f. generation in the equipment, Maximum error in the tube temperature from calibration errors \vas 1.6' F.; the reproducibility of the indicated temperature at a fixed heat flux was within 0.25O F., thus. uncertainty in the final tube temperature was at the most 3' or 4' F. Temperatures found by this electrical scheme are values averaged both radially and longitudinally, as discussed by Jeffrey. From these and heat flux, surface temperature of the tube could be easily calculated. Added agents were commercial materials highly surface active with water, hut not necessarily with other liquids. The nonionic agent was Span 20, Lot 1759C, sorbitan monolaurate, having a molecular M-eight of about 340 (.4tlas Powder Co.). The cationicagentwasHyamine 1622, Lot 379A, diisobutyl phenoxyethoxyethyl dimethyl benzyl ammonium chloride having a molecular weight of about 466 (Rohm Bi Haas Co.). The anionic agent was Aerosol OT, Lot A6839, dioctyl sodiumsulfosuccinate having a molecular weight of about 444 (American Cyanamid Co.), Surface tension of pure methanol in air, measured a t 60" F. with a du Nouy tensiometer, was 23.0 dynes per cm. The three agents did not alter this value significantly when present in 0.01 or 170 by weight; they gave values ranging from 22.6 to 23.0 dynes per cm. At 126' F., similar results occurred. Pure methanol gave 19.9 dynes per cm. while its solutions gave 19.9 to 20.4. Therefore, these agents probably

INDUSTRIAL AND ENGINEERING CHEMISTRY

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Figure 2. Effect of additives boiling curve for methanol A. 8.

C.

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Cationic agent Anionic agent Nonionic agent

do not change surface tension a t the boiling point, 148' F. Foaming during methanol boiling occurred for the 1% cationic agent onIy. The foam, about 2 inches high, was unstable and easily broken. With boiling water, all the agents are definite foam formers. At the end of each run, the tube was examined for fouling. No visible deposits formed from the lowest concentrations, but faint deposits of the added agents, occurred when 0.1% concentrations were used. At 1% a lacelike, fine white deposit was observed. Boiling heat fluxes were measured for both pure methanol and that containing 0.001, 0.01, and 0.1 weight yo of each agent. Additional runs were made with 0.5 and cationic agent. The metal - to methanol temperature driving force M-as varied from 49' to 175' F. (Figure 2). For the curve representing pure methanol, based on 25 runs, data points are omitted to reduce confusion in the graphs; its scatter, however, is similar to that for some of the curves shown-e.g., 0.0170 cationic agent. In Figure 2 each point represents one test run. Replicate symbols

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ADDITIVES IN BOILING LIQUIDS at a given AT show reproducibility of results obtained from check runs. Changes i n Boiling Curve

Adding 0.01% or more of any of the three agents caused a pronounced change in the boiling curve; even O . O O l ~ o was sufficient for the cationic and nonionic agents. Increasing concentration of the additives generally increased the heat flux at a particular AT. I n Figure 2,A, the peak heat flux increases smoothly from 154,000 B.t.u. per hour per square foot (pure methanol) to 214,000 for the highest concentration (1%). Also, the corresponding critical AT increases from 64 to 95' F. This shift in critical data is also shown in Figure 3 . Figure 2,A, shows a startling comparison for a fixed AT of 95' F . ; the heat flux with 1% agent is nearly six times that for pure methanol, and at a AT of 160' F., this ratio is even greater. The shape of the boiling curve for this concentration is surprisingly flat. Steepness, typical of nucleate boiling at small 4 T values and of transition regions of boiling at medium AT values, is absent. At 1% concentration, any AT between 50' and 160' F. gives a heat flux greater than that attainable with pure methanol. Visually, all the runs with this percentage of agent were intense nucleate-boiling ; nothing resembling normal film-boiling was seen.

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Figure 4. The center bubble, in methanol containing 1% of cationic additives growing on the bottom of the tube, has a contact angle of 60 degrees

The sound of the boiling was unusually loud and a t a dominant low frequency which, during warmup, was sufficient to shake the supporting framework. Anionic and nonionic agents show two interesting anomalies. For 0.1% anionic agent during nucleate boiling (Figure 2,B), data appear to the right of the expected abscissa values, but the remainder is similar to Figure 2 , A . Secondly (Figure 2,C), the critical AT and maximum heat flux for the 0.001 and 0.01% of nonionic agent are smaller than expected from Figure 2,A. The entire data for 0.1% nonionic agent, as well as data for 0.01 and 0.001% nonionic agent a t values of AT beyond 80' F., are normal. Considering these anomalies, effect of added agents on nucleate boiling is not predictable and depends on the agent selected. For film boiling, however, the qualitative effect is predictable-heat flux increases. For the transition region, added agents cause a decided decrease in the steepness of the boiling curve. Surface Tension Not a n Explanation

Because the surface tensions of all agents in all concentrations were the same, changes in boiling characteristics cannot be explained by ordinary surface tension. Also, changes in other physical properties, such as viscosity and density, were too slight for significance.

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Figure 3. Changes in maximum heat flux and critical AT caused by added agents

Corty and Foust (7) suggested that the contact angle of the liquid-vaporsolid system could be changed by adding test agents. When Freon 113 was boiled on nickel, these workers noted that boiling curves for old and new surfaces were different and concluded that a change in contact angle could be involved. Their angles for growing bubbles of boiling pentane, measured by a photographic method, were 60 and 90 degrees for new and old nickel surfaces, respectively. T o test the contact angle approach, contact angles were measured for both pure liquid methanol and methanol containing 0.1 weight yo of each additive. Two techniques were tried. First, equilibrium contact angles were measured for liquid drops on a flat copper plate in contact with air saturated with methanol vapor a t room temperature. The apparatus produced a projected image of the drop profile, 20 times normal size. The test liquids spread rapidly on clean copper and the contact angle measured through the liquid, was essentially zero for pure methanol and each of the three solutions. This is not surprising. Pockels (70) reports spreading of this type of ethyl alcohol on copper and zinc and that ether and benzene give small contact angles on the solids such as zinc, copper, platinum, and glass. In the second technique, dynamic contact angles were determined for

Table 1. Contact Angles of Growing Bubbles (Comparison of pure methanol with that containing additives) Pure Additive, 1% Methanol Span OT No. of bubbles measured 16 10 10 Contact angle, degrees 46 46 50 Mean Min. 30 43 36 Max. 68 59 60 Standard deviation 11.8 7.7 7.1 Major diameter, mm. Mean 1.80 2.31 1.73 Min. 1.0 1.0 1.0 Max. 2.9 3.8 2.7 Standard deviation 0.66 1.09 0.48

Ryamine 8 44 28 61 8.1 1.98 1.1 3.6 0.86

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bubbles of methanol vapor growing on the copper heating tube during actual boiling. These contact angles need not be the same as for the equilibrium situation. The AT was made just enough (15’ or 20’ F.) to produce a small population of bubbles which were measured from photographs made with 10-6 second exposure. Images were enlarged to five times actual size and bubbles assumed suitable for measurement (Figure 4), were those on the exact bottom of the tube, whose reflection on the tube was visible and in contact with the bubble a t the exact bottom of the tube. Approximately 50 photographs Tvere made containing over 5000 bubbles, of which 44 met these conditions. Tangents were drawn and the angles measured with a protractor. The accuracy of this method is not known; however, the angles are probably correct to about *5 degrees. The difficulty in measuring the contact angle of a bubble on a curved surface lies in selecting the true points of contact. iMajor diameters of the bubbles, measured also, are accurate within lt0.05 mm. Contact angles. neither zero nor constant for growing bubbles (Table I ) ranged from 28 to 68 degrees. HOLVever ranges in mean values for the aifferent test liquids \vex much smaller44 to 50 degrees. Application of statistics, Student’s t test, to the contact angles shows no significance in differences between the test liquids. I t is concluded. therefore, that effect of additives on boiling heat transfer is not caused by change in contact angles on the solid heat source.

Nucleation Theory a Possible Explanation

Effects of additives may be explained, however, by nucleation. For a vapor bubble to form in an absolutely pure liquid is difficult and great liquid superheat is required. Wismer and others ( 4 ) found that pure methanol a t 1 atm. can be superheated 205’ F. beyond its boiling point in the liquid state. However, in such superheated liquids. spontaneous nucleation eventually occurs and the liquid explodes. Spontaneous nuclei are considered as regions where random fluctuations of density have caused vapor clusters of “correct” density and “correct” size. Such clusters are unstable thermodynamically and can grow spontaneously; this critical size can be computed from theory. For methanol, a critical nucleus is about 100 A. in diameter (assuming a sphere) as calculated by using M’ismer’s data in the La Place equation, Ab = 2a/R. \$-hat happens if a synthetic nucleus is put in a boiling liquid? If an organic molecule having a molecular weight

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of 400 is a slender rod, its length is roughly 25 A.. and if spherical, its diameter is about 5 A. Covering either of these shapes with a molecular layer of vapor can require much less energy than creating a vapor cluster of the same over-all size. Such a vapor-covered particle is well on its way of becoming a nucleus of critical size. I t is necessary for the vapor to wet the particle-Le., the contact angle (that through the liquid) for the particleliquid-vapor s) stem must be greater than that for the heat source liquidvapor system. Thus, two other interfacial tensions are as important as the ordinary liquid-vapor tension which by itself cannot be used to predict effect of an added material on boiling. The actual particle sizes of agents used in these tests are not known. Clumping of the molecules into micelles is definitely possible. The nucleation argument is particularly attractive for so-called nucleate regions of boiling, wherein vapor is generated a t specific points on the hot solid. If foreign nuclei are effective, they must be located on or close to the hot solid. The nucleation approach has an appeal also for the transition region of boiling, where vapor is formed by random, visible explosive bursts (74). Presumably the liquid, moving close to the source of heat, becomes superheated. Foreign nuclei, somewhere in the thin layer of superheated liquid, could reduce the amount of superheat required. Lastly, for film boiling, it is more difficult to imagine the role of foreign nuclei. Particles on the hot solid surface are of no consequence, because no vapor is formed there, but those a t or close to the liquid-vapor interface which surrounds the hot solid could have a n effect. For particles in this interface to be effective, they must influence either rate of heat conduction across the interface or rate of mass diffusion across the interface in the reverse direction. Particles in the liquid could be effective nucleating agents if the liquid were superheated. The usual description of film boiling assumes that the liquid is not superheated, even a t the liquidvapor interface; but this has not been proved experimentally. Thus, over-all effects for small amounts of additives on boiling curves depend on size and shape of additive molecules and on whether the additive is present as single molecules or micelles. I t is important whether the particles are distributed evenly through the liquid or become concentrated in a preferred location which has greatest importance when on or very near the hot solid. A few extraneous tests demonstrated that impurities can cause the same results as added agents. Bits of wood.

INDUSTRIAL AND ENGINEERING CHEMISTRY

black rubber stoppers, and gasket ccment caused nonreproducibility. Unless cleanliness is meticulously observed, test liquids will contain abundant forcign nuclei. In these tests, 10 p.p.m. of some foreign materials were ample. Acknowledgment

Assistance was given by Thomas Dunskus, W. H. Lowden, C. D. Nelson, A. S. Perkins, and B. J. Sutker. Partial support was furnished by the National Science Foundation. Test additives were given by American Cyanamid Co.. Atlas Powder Co., and Rohm & Haas Co. Nomenclature

h

= boiling liquid heat transfer coef-

ficient, B.t.u. per hour per square foot per degree Fahrcnheit n = arbitrary exponent Afi = difference in pressure, p , - PI, po = pressure of vapor inside bubble p L = external pressure on liquid bulk Q / A = heat flux, B.t.u. per hour per square foot R = bubble radius A 7 = temperature driving force, mctalto-liquid, O F. = ordinary surface tension, vapora liquid or air-liquid Literature Cited (1) Corty, Claude, Foust, A . S., Chenz. Eng. Prozr. Symposium Series 51, No. 17, l(1955). ( 2 ) Jakob, M., Linke, W., Ph>.sik. %. 36,

267 (1935).

( 3 ) Jeffrey, J. O., Cornel1 Univ. Expt. Station, Ithaca, N. Y . , Bull. 21,

1936. (4)Kendrick, F. B., Gilbert, C. S., Wismer: K. L., J . Phys. Chem. 2 8 , 1297 (1924). ( 5 ) Kutateladze, S. S., Itvest. Akad. N a u k S.S.S.R., Otdel. Tekh. 2Vauk 1951, 529-36. (6) Lowery, .4. J., Jr., M.S. thesis in chem. eng., Univ. of Illinois, Urbana, Ill., 1955. (7) Mcn‘elly, M. J., J. Imp. Coll. Chem. Eng. SOC.7, 18 (1953). ( 8 ) h-akagawa, Y., Yoshida: ‘ I ~ . , Soc., Cheni. Engrs. ( J a p a n ) 86, No. 3, 6 119321. ( 9 ) Peikins:’ A. S., Westwater, .J. M.‘., A.2.Ch.E. Journal 2, 471 (1956). (10) Pockels, Agnes, Physik. %. 15, 39 (1914). (11) Stroebe, G. W., Baker, E. M., Badger, \I:L.. . IND.EKG. CEIEM. 31. 200 (1939). (12) Tobias, M., Stoppel, A. E.; Zbid., 46, 1450 (1954). (13) Westwarer, J. LA,.: “Advanccs in Chemical Engineering,” vol. I, Chap. I, Academic Press, Yew Yorlr, 1956. (14) Westwater, J. W., Santangelo, J. G., IND. ENG. CEIEXI. 47, 1605 (1955). RECEIVED for review December 2 7 > 1955 .4CCEPTED February 23, 1957 Division of Industrial and Enginccring Chemistry, 129th Meeting, ACS, Dallas, Tex., April 1956.