Steam
ubble Formation.
EFFECTS OF HEATING SURFACE AND U S E OF ANTIFOAMS Arthur L. Jacoby and Lawrence C. Bischmann National Aluminate Corporation, Chicago, I l l . T h e authors present a photographic study of the formation of steam bubbles at atmospheric pressure from smooth heated surfaces and surfaces coated with a calcium carbonate scale. Film boiling from a nonwettable surface is also shown. I n studying the effect of a polyamide antifoam on a boiling foaming solution, the action of the antifoam was shown to begin at the heating surface, causing not only colIapse of the foam but also coalescence of the otherwise small bubbles before their detachment from the heating surface. Similar experiments at pressures of 70 and 250 pounds per square inch are also reported.
S
EVERAL studies have been reported in which the nature
of steam bubble formation a t a heated suifnce was investigated, but most of these deal primarily with the important subject of heat transfer. The effect of the nature of the healed surface has received much less attention. The present lvork was done im a n effort t o investigate the latter factor further and, by the employment of a solution containing certain salts known to be severe foam producers, also t o investigate the role of an antifoam upon steam bubble formation in such a solution. APPARATUS
The investigations made at atmospheric pressure and reported herein were carried out in a vertical Pyrex tube of approximately 66-mm. inside diameter and 4 feet in length, as shown in Figure 1. The 500-watt electric heating element was steel cased, 19 mm. in diameter and 150 mm. in length. Current was supplied to it by a variable voltage transformer so t h a t the heat input could be controlled. A11 photographs shown were taken a t 0.001-second exposure.
around boiling from surfaces coated by various inorganic “scales,” a comparison was made by boiling distilled water from a clean, smooth element and from an element that had been coated n-ith calcium carbonate. Figure 2 shows distilled watcr boiling from the clean element with a heat transfer rate of approximately 17,000 B.t.u. per square foot per hour. Bubble formation occurs at a relatively few isolated points on f i e surface and the bubbles rising in the liquid readily coalesce. Figure 3 shows distilled water boiling from an element covered with a thin layer of calcium carbonate scale. By comparison with Figure 2 it will be seen that boiling is more evenly distributed over the surface of the heating element, and the initially released bubbles are somemhat smaller. Figure 4 shows the heating element coated with a heavy calcium carbonate scale. The coating was applied to the heating clement by the prolonged boiling of a solution containing 85 p.p.m. of calcium chloride and 170 p.p,m. of sodium bicarbonate in distilled water. The boiling was conveniently carried out in the same apparatus; distilled water lvas added from time to time to restore the evaporation losses. Figure 5 shows distilled mater boiling from the element heavily scaled with calcium carbonate (Figure 4) and indicates a further development of the tendencies noted in Figure 3. The points of bubble evolution are so close together t h a t considerable coalescence of the bubbles occurs before their release from the heating surface.
NATURE O F THE SURFACE
30 CM FROM BOTTOM
I ! it
.PYREX TUB& 65MM l D,
I I
500 WATHEATER
/QX /50h4M
Figure 1. Apparatus
~
“Kucleate boiling,” the most usually obscrvrd mode, is influenced markedly by the nature of the heated surface. Jalrob ( 1 1 ) observed the differcnces occurring at suyfaccs t,hat were polished, surfaces t,hat, had been roughened by sand-blasting, and surfaces provided with uniform square i n d e n t a t i o ns. Because much interest wnters
Figure 2 ( L e f t ) . Distilled Water Boiling from a Clean, Smooth Steel Surface Figure 3 (Right). Distilled Water Boiling from a Surface Covered by a Thin Calcium Carbonate Scale
1360
August 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
1361
Certain polyamides have recently been shown (10) t o be particularly effective in reducing such foaming. O u t s t a n d i n g among these are certain N ,AT‘-d i s u b s t i t u t e d dibasic acid amides of high molecular weight (7) and polyacylated polyamines (1, 2, 8, 9). To investigate the mechanism of their action further, a study was made of t h e boiling of a foaming salt solution, with and without the use of antifoam. For this work a water was employed containing 2560 p.p.m, of dissolved solids consisting of 37% sodium hydroxide, 37.5% sodium carbonate, a n d 25.5% Figure 4 ( L e f t ) . Heating Element sodium sulfate (12). Figures 6 and 7. Film Boiling from a Heavily Scaled with Calcium Carbonate Figure 8 shows this Smooth Steel Surface Rendered NonFigure 5 ( R i g h t ) . Distilled Water Boilwettable by Adding 5 P.P.M. of Oleic solution boiling from the ing from a Surface Covered by a Heavy Acid to Distilled Water clean, smooth steel heatScale of Calcium Carbonate ing element. At the heating surface itself the apAlthough the type of boiling observed from the clean, smooth pearance of the boiling is very similar to t h a t of distilled water surface might, at higher heat transfer rates, cause a tendency for (Figure 2), but the steam bubbles t h a t detach themselves from the violent ebullition or bumping, the smooth boiling from the metal surface show almost no tendency t o coalesce. This absence rough surfaces, coupled with t h e ready coalescence of the rising of coalescence resulted in t h e formation of a condition known to steam bubbles, represents a condition approaching the ideal boiler operators as ‘light water,” and a foani layer some 8 inches from the standpoint of preventing foaming, priming, and. steami n depth was formed on t h e surface of t h e solution in the tube. blanketing. The addition of 1 p.p.m of diolevlpiperaeine immediately colThe extreme case of coalescehce on the’heating surface results lapsed the foam layer (Figure Q ) , and caused the ascending steam in “film boiling.” I n film boiling, the metal and the liquid are completely separated by a vapor film. This film behaves a s a n effective insulating agent, and as sOon as film boiling begins t h e rate of heat transfer drops rapidly. Normally, film boiling is associated with large temperature differences between t h e heating surface and the liquid @), differences of sufficient magnitude t o burn out the heating element used i n these experiments. It has been shown (IS),however, t h a t the critical temperature difference, a t which the transition from nucleate boiling t o film boiling occurs, is considerably ‘Figure 8 ( L e f t ) . Foaming Solution less when t h e metal heating surface is not wettable by the b d i n g Boiling from a liquid. Figures 6 and 7 provide graphical conflrmatian of this, Clean, Smooth Steel when compared t o Figure 2. They both show distilled water Surface containing 5 p.p.m. of oleic acid boiling from a clean, smooth Figure 9 ( R i g h t ) heating element. The heat input in these experiments was no Tube Shown in Figure 8 Immediately greater than t h a t of the previously described tests, but the oleic after A d d i t i o n o f acid rendered the metal surface nonwettable by the boiling water Antifoam and film boiling resulted. Jakob reported similar effects on the application of oil t o the heating surface, but these effects were transitory, presumably because of the lesser affinity of the oil for the metal than in the case of oleic acid. I n actual boiler operation, where oil contamination of the feed water might be continuous and in substantial amount, such effects as here shown, even though occurring to a lesser degree, might conceivablv contribute to the overheating of the boiler metal. bubbles t o coalesce t o almost as great a degree as was previEFFECT OF ANTlFOAM ously observed with distilled water. The antifoam was added Foaming of boiling liquids-for example, the foaming of boiler in the form of a solution in a small volume of dioxane: i t had water-is frequently due in large part t o the high concentration been previously determined t h a t the same amount of dioxane of various inorganic salts and alkalies. without the antifoam had no visible effect.
1362
INDUSTRIAL AND ENGINEERING CHEMISTRY
The fact' that in the boiling salt solution the steam bubbles tend to retain their individuality rather t,han coalesce is explained by the balanced-layer thedry of film formation developed by Foulk ( 5 ) . The observation that the antifoam not only destroys the foam layer but' also causes ready coalescence of the ascending steam bubbles indicates the pronounced surfact, act'ivity of t'he polyamide, whereby it becomes st,rongly adsorbed a t each new steam-water inkiface created by the formation of a steam bubble.
Other diamides test,ed and found to exert, a qualitatively similar antifoani behavior in this saling solut'ion included dipalmitylpiperazine, 5,,V'-dihexadecyladipamide, and S,S'-dioctadecylsebacamide. A solution exhibiting even more pronounced foaming tendencies and on which many experiments !yere carried out, consisted of dist.illed water in which were dissolved 8550 p.p.m. of sodiuni sulfate, 8550 p,p.ni, of sodium h j droside and 85 p.p.m. of sodium lignin sulfonate. Addition to t,his mixture of 1 p.p,m. of any of the four diamides described above resulted in completc lmockdown of tmhefoam and the initial release from thc hcating surface of relatively large steam bubbles. While all the experiments described here m r e carried out at atmospheric pressure, for convenience of observation and photography, the authors have performed comparable experiments a t pressures of 70 pounds per square inch. These were carried out, in an apparatus simi1a.r in design to that used at, at,mosphcric pressure, except, t,hat the glass tube Tvas a section of standard Pyrex industrial piping. This \vas closed a t the t,op and provided with valves, gage, etc., but because of the protective shields uscd, no attempt was made t o obt,ain photographs. The rcsults observed, horn-ever, were almost identical as regards t,he action of the antifoam. Man>- experinients a t pressures of 250 pounds per square inch have also been conducted in a laboratory boiler of thc type shown in Figure 12. This picture, made wit,h a portion of t,he brickwork of t,he furnace removed, s h o m part,icularly the long eight glass in the front of the vertical boiler drum. By mcans of this glass and another similar one in back through which light, is directed, t,he conditions in the boiler in a zone several inchoy above and below the normal wat,er level ran be readily observed while the boi1t.r is i n oporation.
Figure 10 ( L e f t ) . Foaming Solution Boiling from a Surface Covered b~ a HeaTy Scale of Calcium Carbonate Figure 11 ( R i g h t ) . Tube Shown in Fipure 10 Immediate11 after Addition of Antifoarn
A more striking effect of the antifoam was found when a watc,i, of the same composition was boiled from the heavily scaled hcating element. As shown in Figure 10, the bubbles formed are much smaller than in any of t,he other experiments and, although these myriads of small bubbles are closely packed together wvhilc ascending, very little coalescence is seen t,o occur. The formation of smaller &am bubbles during the boiling of waters of high saline content has been report,ed in the past, not,ably by Foulk and eo-workers (4, 6). The conditions of t,his experiment are comparable to thosc. normally encountered in many boiler operations, in so far as w e , are dealing with a scale-coated heating surface and a water eontaining dissolved solids tending to produce a foaming condition. Here again we have a good example of t,he production of the condit,ion known as light, water, a phenomenon frequent,ly encountered simultaneously with foaming. The foam layer produced in this experiment was approximately 16 inches in depth. The addition as before of 1 p.p.m, of dioleylpiperazine to this boiling solution immediat.ely collapsed the foam layer (Figure 11) and caused coalescence of the ascending bubbles of steam; in addition, a third effect, was visible-namely, the coalescence of the bubbles on the heat,ing surface itself, even before their detachment, which resulted in the init,ial release of relat,ivcly large steam bubbles. The authors are not aware that such a n effect' of the polyaniide type anti foams has been previously reported, .and believe that, this effect upon the bubbles a t the instant, that they are formed is undoubtedly as significant as the more familar effect on the upper foam blanket.
Vol. 40, No. 8
Figure 12. Laboratory Boiler
August 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
1363
I n a typical series of experiments a feed water was used having the following composition, in parts per million:
ously, showed the markedly larger steam bubbles and the destruction of the foam layer caused by the diamide.
Calcium hardness (as CaC03) 154.0 Magnesium hardness (as CaC03) 154.0 Alkalinity (methyl orange) (as CaCOs) 726.0 Sodium chloride (as NaC1) 85.5 Sodium sulfate (as NalSO4) 718.0 Tannin extract, dry 34.2 Antifoam 0.05-1.0 This water is gradually concentrated in the boiler by evaporation until the dissolved solids reach such a value that they overcome the antifoam and cause carry-over of boiler water with the steam. Use of any of the four diamides described permits the attainment of dissolved solids concentrations of approximately 10,000 t o 20,000 p.p.m. and higher before carry-over due to foam occurs. Visual observations made through the boiler sight glasses during such tests, and also under test conditions similar, except that antifoam was introduced periodically instead of continu-
(1) Bird and Jacoby, Canadian Patent 433,431 (March 5, 1946). ( 2 ) Bird and Jacoby, U. S. Patent 2,428,776 (Oct. 14, 1947). (3) Drew and Mueller, Trans. Am. Inst. Chem. Engrs., 33, 449-73 (1937). ENG.CHEM.,25, 800-3 (1933). (4) Foulk and Groves, IND. (5) Foulk and Miller, Ibid., 23, 1283-8 (1931). (6) Foulk and Ry-mar, Ibid., 31, 722-5 (1939). (7) Imperial Chemical Industries, British Patent 568,318 (March 29, 1945). (8) Ibid., 568,510 (April 9, 1945). (9) ,Jacoby, U. S. Patent 2,428,801 (Oct. 14, 1947). Jacoby and Thompson, Proc. Ann. Water Conf., Enprs. SOC. West. Penna., 7th Ann. Conf., 1947, pp. 31-41. (11) Jakob, iMech. Eng., 58, 643-60 (1936). (12) Leaf et al., Proc. Am. R y . Eng. Assoc., 45, 58 (1944). ENG.CHEM.,30, 1401-6 (1938). (13) Rhodes and Bridges, IXD.
LITERATURE CITED
,
RECEIVED September 4, 1047.
olyamide Foam Inhibitors MECHANISM OF FOAM INHIBITION IN STEAM GENERATORS
L. 0. Gunderson and W. L. Denman Dearborn Chemical Company, Chicago, I l l .
A new approach is made to an explanation of the mechanism of boiler foam formation and foam inhibition. The fundamental difference between foaming boiler water and a foam-inhibited boiler water resides primarily in the great difference i n the number of steam bubble nuclei. Numerous nuclei produce myriads of permanently small bubbles and a mass of foam, whereas a limited number of preferentially activated nuclei produce relatively few and very large bubbles that do not form a foam. Differentiation is made between dynamic films of coalescing bubbles in foam-inhibited solutions and the substantially static films of noncoalescing bubbles in a foaming solu-
T
HE foaming of steam boilers, particularly locomotive boilers has always been a serious engineering problem, causing train delays and power interruptions and greatly increasing maintenance costs of turbines, steam engines, and appurtenances. Until recently, castor oil emulsion antifoam was the only partial remedy, and its usefulness was limited by its ready saponification to form sodium soap in the boiler water. New, highly efficient foam-inhibiting organic compounds are represented by the polyalkylene polyamides, which are substantially nonsaponifiable under boiler conditions and have practically replaced castor oil as a n antifoam. DEFINITIONS AND APPLICATION TO PROBLEM
Boiler foaming may be defined as the result of steam generation incidental to the formation of numerous, small bubbles rising through the boiler water, which causes the expansion of the water in the form of a bubble mass t h a t overflows into the steam outlet. Inhibition of boiler foaminginvolves steam generation coincident with the production of relatively few and relatively large, rapidly growing, unstable steam bubbles rising through a superheated
tion. Recently developed allrylene polyamide compounds are exceedingly potent foam inhibitors that produce relatively few, very,large bubbles on the heating surface; the size of the bubbles is assumedly proportional to the degree and distribution of superheat in the superjacent boiler water through which the formed bubbles rise and freely coalesce. The structural and chemical characteristics of foam inhibitor molecules are discussed and data regarding preparation and testing of the representative polyamides are given. Theoretical mechanisms of boiler foam formation and foam inhibition based on the above fundamental concepts are offered.
boiler water; this resplts in only a slight increase in volume of the water. The foregoing definitions may appear a n oversimplification of complex phenomena t h a t have plagued steam engineers for generations. However, the authors believe that this approach t o the problem is necessary in order t o circumvent the mass of data and discussion centered around various secondary factors t h a t influence various phases of the phenomena but are in no way controlling in steam generating systems. Initially, it is important to differentiate between boiler water foaming and foam formation in aqueous or nonaqueous solutions where steam generation is not a consideration. I n other words, foam development and inhibition incidental t o steam generation comprise specific mechanisms not encountered in other foam-producing systems. To illustrate, in steam generation, temperature considerations are important, particularly under nonequilibrium states-e.g., superheat developed in the boiler water under certain conditions produces rapid growth of bubbles and results in dynamic surface films surrounding the bubbles, facilitating coalescence. Such films have special properties not possessed by films of bubbles in systems where steam is not generated.
