Table IX.
a
Summary of Corrosion Rates of Experimental Alloy 20" Corrosion Rate, Solution MilslMonth 0.6 65 wt. " 0 8 2.0 4 M H2SO4 4M H2S04-20 grams/liter dissolved 0.9 0.8 8.4 22 0.5 5 M HNOj-0.5M Fe(s08)a See Tables ZZ-VZZZjor test conditions.
tained with commercial alloys, the optimum alloy composition for a single general-purpose power reactor fuels dissolver corresponds to Experimental Alloy 20 (see Table I). Additional corrosion resistance might be obtained by further increasing the nickel and chromium content. However! impaired workability limits the maximum permissible chromium content. Corrosion rates obtained with this alloy in the solutions in question are summarized in Table I X . The corrosion resisttance in 65 wt. yo HXOa, 0.6 mil per month, is as good or better than that of the better 300 series stainless steels. The corrosion rate was 2 mils per month in 4M H 2 S 0 4and about 1 mil per month when dissolved stainless steel was present. The corrosion rate in 6M SH4F solution was about 0.8 mil per month and about 10-fold higher when 0.5M NOa- was present. Stainless steel corrosion rates in these solutions are about 10 mils per month irrespective of the presence or absence of NOS-. Evaluation of as-welded weldments in the neutral fluoride solutions and in the very corrosive acid fluoride
system showed no weld decay and no knifeline or other preferential attack. As with the 300 series stainless steels, attack by 5M HN03-0.5M Fe(iY03)s solution was predominantly intergranular. T h e application of Experimental Alloy 20 need not be restricted to the power fuel reprocessing industry. The alloy appears to be superior to all known commercial alloys of its class, especially regarding weld decay and knifeline attack, and should be usefd in many chemical plant environments involving corrosive pure and impure mineral acids. literature Cited
(1) Culler, F. L., Jr., Blanco, R. E., Goeller, H. E., Watson, C. D.? U. S. At. Energy Comm. ORNL-2265, March 27,1957. (2) Flanary, J. R., Clark, W. E., Goode, J. H., Kibbey, A. H., Zbid.. ORNL-2461. March 30. 1959. (3) Gens, T. A,, Baiid, F. G., Zbid., ORNL-2713, Dec. 22, 1959. (4) Hyman, M. L., Zbid.,ORNL-2239, Oct. 9, 1957. (5) Kitts, F. G., Clark, W. E., Zbid., ORNL-2712, June 7, 1962. (6) Maness, R. F., Zbid., HW-61662, Aug. 26, 1959. (7) Zbid., HW-66884, Sept. 23, 1960. (8) Zbid., HW-68426, Feb. 1961. (9) Peterson, C. L.. Drennen, D. C., Langston, M. E., Hall, A. M., Boyd, W. K., Zbid., BMI-1459, Aug. 8, 1960. (10) Peterson, C . L., Miller, P. D., Jackson, J. D., Fink, F. W., Zbid.,BMI-1375, April 28, 1959. (11) Schulz, W. W., Duke, E. M., Zbid., HW-62086, Sept. 15, 1959. (12) Stevenson, C. E., Zbid., IDO-14443, Sept. 15, 1958. (13) Swanson, J. L., Zbid., HW-49633, April 15, 1957. RECEIVED for review September 10, 1962 ACCEPTED February 11, 1963 Division of Industrial and Engineering Chemistry, 141st Meeting, ACS, Washington, D. C., March 1962. Melting, fabrication, and welding of experimental alloys, as well as part of the corrosion investigations, conducted at Battelle Memorial Institute under AEC Contract W-7405-eng-92. Balance of investigation conducted at Hanford Atomic Products Operation under Contract NO. AT(45-1)-1350.
ALKYL AMINES A S FILM FORMERS IN CONDENSING STEAM SYSTEMS Effect on Over-All Heat Transj5er Coeficient M A L V E R N F . O B R E C H T A N D C L Y D E C. D E W I T T
Department of Chemical Engineerinz. Michigan State Univs,sity, East Lansing, Mich. Long-chain amines and their salts, such as octadecylamine acetate, are widely used for corrosion control in steam condensing systems. Increases in heat transfer rates, which apparently cannot b e attributed to reduction o f corrosion o r removal of fouling layers, have been observed in certain heat transfer systems. Data are presented showing the influence o f the carbon chain length o f the various normal alkyl amines on heat transfer with condensing steam. Over-all heat transfer coefficients calculated from experimental data are correlated. Of the normal alkyl amines studied, dodecylamine i s responsible for a higher heat transfer coefficient than that shown b y octadecylamine, while octylamine actually hindered heat transfer.
A comparison of octadecylamine acetate with stearic acid i s made. Both materials gave about a 10% increase in over-all coefficient. However, the mode of condensate formation and behavior were radically different. CTADECYLANISE
A N D ITS SALTS
are being used as corrosion
0 inhibitors for protecting steam and condensate return
systems (3, 9, 73). These amines function by forming a film, with the long-chain polar ends of the molecules attached to the metal surface. The resulting film is believed to be essentially monomolecular to form a nonwettable surface. Field Present address, Leisure City, Fla.
tests made in paper mills and other industrial installations using octadecylamine acetate for corrosion protection indicated a simultaneous increase in plant efficiency (73). In certain cases the increase apparently could only be attributed to the effect the material has on mode of steam condensation and its behavior. The purpose of this study was to determine the effect of octadecylamine acetate and the influence of carbon VOL. 2
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JUNE 1963
167
chain length of various normal alkyl amines on the over-all heat transfer coefficient of steam condensing on uncontaminated surfaces. When a saturated steam vapor contacts a surface with a temperature below the dew point of the saturated vapor, condensation results. Normal film-wise condensation does not occur on a nonwettable metallic surface (2, 7-70, 73). There are two extreme types of condensation which are dependent on the condition of the condensing surface. The normal type of condensation encountered on a wettable metallic surface is called “filmlvise condensation” and actually is a continuous layer of liquid condensate. This continuous layer introduces a resistance to heat transfer ( A , Figure 1). The other extreme of condensate formation generally occurs when a metallic surface is rendered nonwettable. The vapor condenses on the nonwettable surface in minute drops which gradually grow to a critical size, the critical size depending upon surface tension and density of the liquid, angle of inclination of the surface, and the contact angle of the drop (6). T h e drop then sweeps the surface, collecting other droplets of condensed vapor, and moves downward leaving a bare strip on the surface. This cycle thus reduces resistance to heat transfer of the condensate layer shown in B, Figure 1. Xote that any corrosion product layer \vi11 add to heat transfer resistance of a given system. I t has been observed in the field that the fouling layer is greatly reduced when filming amines are employed.
IEA
SIDE TEMP
-
A Filmwise Condensation B Dropwlse Condensotlon (Inhibitor treated)
Distance
Figure 1 . transfer
Apparatus. The finger-type condenser (Figure 2) used for this investigation, was similar to that used by Drew, Nagle, and Smith ( 4 ) ,Emmons (5),and Spoelstra ( 7 7 ) . The cooling surface was a 5.5-inch piece of 0.5-inch copper tubing, sealed a t one end, inside of which was placed a finger of 0.25-inch copper tubing. The open end of the inside tube was 1/16 inch from the sealed end of the outer tube. T h e inside tube was held
54 . O 53.8 53.5 52.9 54.1 53 .O
Tem4.. ‘F. Outlct Reference Blanka 64.4 60.9 58 .O 56.3 56.4 54.7
53.9 53.6 53.6 53.4 53.5 53.0
Stearic Acidb 66 . 0 60.3 57.4 56.3 56.1 55 . O
Inlet
16 27 31 36
1.484 2.172 3.484 4.565 7.126 9.406
40 46 51 59 63 69
1.500 2.750 4.406 5.906 6.626 8.782
4 9
-
ond Dirt
Me1oII1c Woll
Woler Film Filming Corroiion Inh!b!lor
L
Effect of filming corrosion inhibitors on heat
Determination of Heat Transfer Coefficients
Water Rate, Lb. / M i n .
Run No.
Condensing Sleom I Corrosion
rigidly and centered in place by a 0.5 X 0.25 inch reducing fitting. An outlet was made by drilling a hole in the outside tube and soldering a short section of 0.25-inch tubing in place. T h e coolant passed downard through the inside tube, through the annulus. and then to the discharge. The outer tubing had a 0,500-inch outer diameter and 0.435-inch inner diameter. The inside tube had a 0.250inch outer diameter and 0.190-inch inner diameter. The actual length of the outside tube exposed to condensate was 3.5 inches. A self-contained system was provided by the use of a boiling chamber fitted with a reflux condenser (Figure 3). This
Experimental
Table 1.
B
A
J
,Steam
Water Velocztv, Ft./Sec.
Heat Transfer Coeficient ( U ) , B.t.u./Hr./Sq. Ft.1” F.
208.0 207.5 207.5 207.8 208.2 207.3
0.574 0.840 1.348 1.801 2.756 3.638
163.0 161.2 162.8 162.4 167.9 163.8
209 . O 208.6 208.2 208,5 208.5 209.5
0.580 1.604 1.704 2.284 2.563 3.397
191.4 191 . O 171.8 175.3 176.2 177.5
I
Octadrcylamine AcetateC 0.852 182.4 208,O 61.9 2.203 54 .O 71 1.130 185.1 208.5 60.1 76 2.922 54.0 1.692 180.4 208.5 54 .O 58.0 89 4.375 1.874 180.5 208.0 57.9 54.3 93 4.846 2.333 186.3 208.5 57.3 54.3 98 6.032 3.022 177.2 208.0 56.6 54.4 7.812 100 3,035 177.5 208.5 56.7 7.846 54.5 102 rurface highly polished, no promofer, estimated over 90% Jilmwise condensation. b Conditions: su?jace highly polished, good dropwise a Conditions: condensation observed. Conditzons: surface highly polished, rstimated 4570 dropwise condensation excdlent run-off obserud.
168
l & E C P R O D U C T RESEARCH A N D D E V E L O P M E N T
COOLING WATER INLET
/--
R E F L U X CONDENSER ETER WATER
INLET
INLET 5 W AATER TER OUTLET
COOLING WATER OUTLET
COPPER T U B E
I
0 . 2 5 0 . 0.0. 0.190 T.D.
0.500' O.D. 0.I SO* I . D .
FLASK
BO I L I N G DlSTlLLE D WATER o
o
0
0
W
Figure 2.
Finger-type condenser
apparatus allowed clear, accurate visual observation a t all times. T h e reflux condenser kept the material under study a t nearly constant concentration and permitted long operating periods. The stopper was treated to extract the natural resins and covered with a metal foil. Temperatures were measured by Beckmann thermometers. Prior to each test run, the finger-type condenser was polished with four successively finer grades of emery cloth, and finally polished with crocus cloth. The condenser, flask, foil. reflux condenser, and all contact parts were scrubbed with a commercial scouring agent and thoroughly rinsed. This procedure was repeated until the distilled water formed a liquid layer on all parts of the system. Procedures. Reference runs were made using distilled water. Cooling water rate was held constant, and the system was allowed to reach equilibrium before data were taken. Inlet and outlet cooling water temperatures, flow rate. steam temperature, and flask level were measured. T h e water velocity was then varied and the procedure repeated. Mechanism and behavior of condensation were observed. The procedures for studying octadecylamine, dodecylamine, octylamine, octadecylamine acetate, and stearic acid were the sdme as that used for distilled water. except for introduction of the promoting agent. T h e copper surface was treated by rubbing with the desired agent. This technique was particularly necessary for stearic acid since it does not vaporize. An additional portion of the agent was added to the distilled water in the flask.
Figure 3. Complete test apparatus with finger-type condenser assembled in place
The duration of a test run was approximately 3 hours. except when a test was made in which the promoting agent was removed by distillation or by filtering the reflux. This was done to determine the effect of complete removal of additive from the system and established the maximum increase in heat transfer for each material listed. These values are reported herein. Stearic acid po\vder was used because a literature search ( 7 , 4, 6 ) indicated that this material gives the type of nonwettable surface necessary for the promotion of "pure" dropwise condensation. The general technique used by the earlier investigators \vas to rub the stearic acid on the surfaces.
Results Data obtained by the above procedure were used to determine the over-all heat transfer coefficients as a function of water velocities. A method devised by \$%on (74) for correlating the over-all coefficient, U (B.t.u.,'hr. '2.1. ft.," E.) with the ivater velocity, V (ft./sec.), is used for reporting the effect of n-alkyl amine chain length. This entails plotting 1 / P * us. 1/U. T h e factor l/VO.P demands that cooling Ivater flow in turbulent fashion. All data reported in the graphs correspond to Reynolds numbers greater than 2500, thus indicating turbulent Aow. Results were plotted using the method of least squares. Figure 4 shows the comparison tests in octylamine, dodecylamine, and octadecylamine with distilled water. Figure 5 shows the comparison of stearic acid and octadecylamine with VOL. 2
NO. 2
JUNE 1 9 6 3
169
distilled water. Distilled water produced "pure" filmwise condensation. The value of the intercept corresponds to the over-all resistance to heat transfer a t infinite velocity. Since the resistance caused by water flowing a t infinite velocity is zero, the over-all coefficient a t this point is the sum of the resistances on the steam side of the metallic wall and the resistance resulting from any fouling on the surface. Figure 4 indica-es that the amine with a chain length of eight carbon atoms increased the resistance to heat transfer, compared with a clean surface condensing distilled water. O n the other hand, octadecylamine and dodecylamine showed a decreased resistance compared with the same reference. The amine with 12 carbon atoms produced a greater effect in the reduction of resistance to heat transfer. Table I shows typical data obtained on the finger type apparatus with distilled water, stearic acid, and octadecylamine acetate. Velocities rvere too low to use a Wilson plot. Table I1 summarizes some hundred tests on these three materials; it shows a 10,85y0 increase in heat (ransfer for stearic acid and a 10.80% increase in heat transfer for octadecylamine acetate. both over distilled water. I t is interesting that although both materials gave approximately the same increase in the over-all hea transfer coefficient, there was observed a different mode of condensation behavior. In the case of the stearic acid, good dropwise condensation \+as observed over most of the surface. with a heavy layer of condensate near the lower part of the finger condenser. The tests using octadecylamine acetate had less surface area in dropwise condensation, but the run-off of the condensate layer was much faster, It gave the appearance of a thin frictionless ring of condensate Lvhich slipped down the surface rapidly. This same behavior was observed to some degree with the other long-chain amines.
Table II. Effect of Promoter on Heat Transfer Coefficient Au. Mean Heat ~~.
Conditions No promoter Stearic acid Octadecylamine acetate
Au. No. of Runs
38 31 32
Transfer Coefficient ( U ) , Incrsase B.t.u./Hr./ over Sq. Ft./' F. Blank, 70 163.1 180.80 Sj 180.68 10.80
10:
This phenomenon of different mode of condensation behavior appeared important enough to study. Figare 6 shows various stages of the dropuise condensation cycles. Figurc 6 A shows the formation of tiny droplets on the bare surface. These droplets grew in size until conditions shown in Figure 6B were obtained. These drops continued to grow, and upon reaching a critical size, rolled down the condensing surface. Drops rolling down the surface coalesced with drops vertically below and left a bare path behind them. Figure 6C shows a subsequent similar condensation run-off cycle. Figure 6 D shows the complete filmwise condensation obtained with the blank run using distilled lvater. Figures 6E and 6F show the typical mixed condensation obtained with the long chain amines and the better run-off of the condensate. To check the removal of materials. to establish a maximum increase for a minimum film thickness, and to see that blanks were re-established, long-time tests were run. A packed absorption column was inserted between the flask and the reflux condenser. These tests showed a re-establishment of blank conditions after various lengths of time. They also showed that the film forming materials would have to be replaced continuously to assure the heat transfer improvement.
T
F \
. : ci
4
2z \
1a.
s
L
ha
lu
$ k
80 e
4
3
z
h
ZI a -1 4
h3 0
3 . 6 ~
J
.)
9
. \
k 3
0.1
\
\
0
01
0:2 I/ V y (
0.3
v --
0.4
0.5
1 t / a. c . )
Figure 4. Over-all coefficients v5. velocity for octylamine, octadecylamine, and dodecylamine 170
I & E C P R O D U C T RESEARCH A N D DEVELOPMEN1
0.2
0.3
0.4
0.5
0
I / VO.!(
v = t t . / 8.C.
)
Figure 5. Over-all coefficients vs. velocity stearic acid and octadecylamine
for
A
B
D c Figure 6. Dropwise condensation cycles
A-C. Typical condensation pattern.
D. Distilled water. E, F.
Long-chain amines
Discussion
T h e factors which control the occurrence or inhibition of dropwise condensation are clearly the result of surface phenomena and should he studied as such, Drew and Nagle (4) suggested that the formation of tiny stable droplets or the establishment of a uniform film of condensate depends on molecular forces of attraction between two condensate malecules and between condensate and surface molecules. If cohesion between the condensate molecules is great and adhesion between the condensate and surface is sufficiently small, stable droplets will form, since the condensate would be unstable as a film. O n this basis, a relationship between interfacial tensions and the wettability of a solid surface was developed. It appears that the above conditions are a result of an interaction of a more fundamental nature, namely, molecular orientation of the promoter molecules. I n order that dropwise condensation may occur, the molecules of the promoting agent either must he absorbed upon the condensing surface and not washed away with the condensate, or, if they are removed, they must be replaced by new agent molecules carried with the steam. This required the molecules to be composed of two parts: an active group which is capable of being absorbed upon the surface and a nonpolar part which has only a small affinity for the condensate. As the length of the hydrocarbon chain of the amines increases, the solubility becomes very small (I), but the energy of adhesion remains nearly the same (72). T h e -MI group for the amines and the -COOH group for the stearic acid were responsible for the affinity between the promoter molecules and the metallic surface. Figure 7 shows the orientation of the molecular layer
of promuter molecules.
A valid theoretical explanation for the corrosion protection of the filming amines in steam condensate system? is based on the fullowing assumptions: The metal surface is completely covered by the active agent. The molecular head of the molecules of active substance takes up the full area of a circle made by the inscribed molecular diameter.
PROMOTER
MOLECULES
E
METALLIC SURFACE
/
i'i'os
INERT CHAW
I
Figure 7. Orientation of prc molecules on metallic surface There is no liability of molecular shape or form to the molecule or to the surface. Thus, the area occupied hy the absorbed active group of the promoter molecules is circular, and a top view of the orientation would he as shown in Figures and 9 , Figure represents the closest possible orientation with the least molecules, while Figure 9 shows the closest stable orientation with the maximum VOL. 2
NO. 2 J U N E 1 9 6 3
171
T h e size relationships between condensate molecules and long chain amine polar groups are thought to be the basis for control of surface wettability and, therefore, offer a partial criterion for predicting the mode of condensation, as well as the degree of heat transfer improvement and corrosion protection possible with the promoter. Physical imperfections of metal surfaces are very large compared with the size of the polar molecule, and, therefore, there is nothing to keep the straight chain parts of the molecules from aligning parallel to each other. The almost perfect corrosion inhibition obtained with octadecylamine a t critical concentrations (73) tends to indicate the validity of condensed parallel close packing of the amine molecules. T h e decrease in heat transfer of octylamine may be attributed to the reaction between the metal surface and the octylamine which causes a n increase in roughness or resistance to conFigure 8. Minimum molecular spacing of densate run-off. In general, the amines of shorter chain circular groups with minimum number length are more water soluble and offer less protection as filmab = 5.28 A. ac = 4 . 5 7 A. a0 = 3.04 A. or = 0.40 A. forming corrosion inhibitors. The increased heat transfer Area of circle = 0.50 A. * of dodecylamine over octadecylamine suggests the possibility of more than a monomolecular layer of octadecylamine I t would appear that there is better slippage of the water molecules in run-off from the dodecylamine than from a n octadecylamine-filmed metal surface. However, both of these I + materials give substantial increases in heat transfer over the nontreated steam condensate system. An extensive research program involving laboratory, pilot plant, and field tests is now in progress. The laboratory phase is concerned with studying the effects of the various n-alkyl amines on the steam side coefficient directly and the effect of the condensate run-off in a large vertical heat transfer apparatus of special design. Pilot plant work is being conducted on evaporators, dryers, and heat exchangers using octadecylamine and its salts. These results are being correlated with long term field test data. Acknowledgment Figure 9. Minimum molecular spacing of circular groups with maximum stable number ac = 7.48 A. rs = 2.20 A. Area of circle = 3.80 A.I
ab = 5.28 A.
The authors are grateful for data obtained by D. D. Squire and W. D. Erickson in the research program on filming amines a t Michigan State University. Literature Cited
number of molecules; both show the area of the metallic surface the active groups cover while contacting the surface. T h e area taken up by the -COOH group of stearic acid is 22 A . 2 (72). The corresponding diameter of this group was calculated to be 5.28 A. The largest circle which can be inscribed between the molecules was calculated for each case and shown in Figures 8 and 9. The largest diameters for the inscribed circles for the minimum and maximum spacing are 0.80 and 2.20 A., respectively. The areas occupied by some “contaminating” molecules are: COZ = 14.1 A.Z. 0 2 = 12.1 A4.2, and Nz = 13.8 A.2 (72). Since the areas enclosed by the inscribed circles are 0.50 .4.2and 3.80 A4.2, respectively, it is not possible for any of the above molecules to reach the metallic surface for the orientations under consideration, unless a much more open arrangement of adsorbed molecules is postulated. I t is interesting that a water molecule occupies a greater surface area than do molecules of Sz. 0 2 , or COS and, therefore, could not be oriented on the metallic surface exposed in a filming amine-treated system. The corrosion protection against COz and oxygen attack in steam condensing systems afforded by the filming amines is already due to a n orientation approaching that given in Figures 8 and 9. 172
I & E C P R O D U C T RESEARCH A N D D E V E L O P M E N T
(1) Armour and Co.. Chicago. Ill.. “The Chemistry of Fatty (2) Brown, G. G., et al., “Unit Operations,” pp. 448-53, Wiley,
New York, 1950. W. L. (to Dearborn Chemical Co.), U , S. Patent 2,400,543;May 2, 1946. (4) Drew, T. B., Nagle, W.M., Smith, W.Q., Chem. Eng. Progr. 31. 605-21 (1935). --(j)Emmons, H. W’., Zbid., 35, 109-22 (1939). (6) Fatica, N., Katz, D. L., Zbid., 45, 661-74 (1949). (7) Hampson, H., “Proceedings of the General Discussion on Heat Transfer,” pp. 58-61, Inst. of Mech. Engrs., London, Am. SOC.Mech. Engrs., New York, 1951. (8) Jakob, M.: “Heat Transfer,” pp. 693-6, Wiley, New York, 1949. (9) Obrecht, M. F., Proc. 46th Ann. Mtg. Natl. District Heating Assoc., Pittsburgh, Pa., May 1955. (10) Shea, F. L., Krase, N. W., Chem. Eng. Progr., 36, 463-90 ’ (1940). (11) Spoelstra, H . J., Arch. Suzkerind. 39,111, 905-56 (1931). (12) Taylor, H. S., Glasstone, S., “A Treatise on Physical Chemistry,” 3rd ed., pp. 583-8, 605, Van Nostrand, New York, 1955. (13) Wilkes, J. W.,Denman, W. L., Obrecht, M. F., Proc. 17th 4nn. Am. Power Conf., Chicago, Ill., April 1955. (14) Wilson, E. E., Trans. A m . Soc. Mech. Engrs. 37, 47-82 (1915). (3) Denman, \
,
I
~
RECEIVED for review December 26, 1962 ACCEPTEDMarch 11, 1963 Presented in part, Division of Water and Waste Chemistry, 128th Meeting, ACS, Minneapolis, Minn., September 1956.
