could be converted to dicarboxylic acids or anhydride by reaction with fumaric acid or maleic anhydride (Parkin et al., 1969). Hydrocarbons obtained might be used as secondary plasticizers for PVC (McSweeney, 1968). The terpenes obtained are limonene (45%) and p-cymene (157~1, terpinolene (12%), with lesser amounts of terpenes, and should find markets through the usual outlets. The residue reacts with pentaerythritol through esterification and transesterification to yield an ester having a softening point almost identical to that of the original residue. Reaction with a stoichiometric amount of zinc carbonate gave a salt having a softening point of 148”C. The dimeric residue, its pentaerythritol ester, and its zinc salt have been shown by Berry et al. (1969) to be promising components in reclaimed rubber-based mastics.
30
25
-
0
-1
w> &?
20
A- DEHYDROABIETIC ACID
IN DlSTl LLATE
literature Cited I5O
20
40
60 TIME
80
- MINUTES
100
I20
Figure 1. Increase in yield of polymeric product and dehydroabietic acid content of distilled rosin from gum rosin at 300°C.
equivalent 337 and softening point 145. The 11% residue yield was somewhat low. The residue had color grade G, neutral equivalent 605, and saponification equivalent 423. Forecuts ranging from 5 to 15% were obtained from all the gum rosin distillations. The tall oil rosin showed no separable forecut under the conditions used, but the rosin cut contained about 6% of decarboxylation products. The forecuts from the distillations other than tall oil rosin contained terpenes (about 22%), decarboxylation products (about 34%), and resin acids (about 44%). These materials were easily separated into terpene and rosin oil fractions by distillation. The rosin oil fraction contained approximately 28.3% cyclic conjugated diene as estimated by ultraviolet absorption. The rosin oil should be salable as conventional rosin oil, or the conjugated dienic material
American Society for Testing Materials, Philadelphia, Pa., E28-58T, (1958). Berry, D. A., Bunk, A. R., Schuller, W. H., Lawrence, R. V., Halbrook, N. J., Division of Organic Coatings and Plastics Chemistry, 157th Meeting, ACS, Minneapolis, Minn., April 1969. Boni, K., Battelle Memorial Institute, Columbus, Ohio, unpublished results, 1969. Harris, G. C., “Encyclopedia of Chemical Technology,” R. E. Kirk, D. F. Othmer, Eds., Vol. 11, p. 799, Interscience, New York, 1951. Johnson, A. E., Lawrence, R. V., Anal. Chem. 27, 1345 (1955). Joye, N. M., Jr., Lawrence, R. V., J . Chem. Erg. Data 12, 279 (1967). McSweeney, E. E., Union Camp Corp., Princeton, N. J., private communication, 1968. Parkin, B. A., Jr., Schuller, W. H., Lawrence, R. V., Division of Organic Coatings and Plastics Chemistry, 157th Meeting, ACS, Minneapolis, Minn., April 1969. Takeda, H., Kanno, H., Schuller, W. H., Lawrence, R. V., IND.ENG.CHEM.PROD. RES. DEVELOP. 7,187 (1968). RECEIVED for review February 10, 1968 ACCEPTED May 21, 1969
EFFECTS OF POLYFLUOROCARBON COATINGS ON SCALING IN
EVAPORATORS WITH CONTINUOUS FEED Cas04 SOLUTIONS D . K O S A N D LUH C . T A O Department of Chemical Engineering, University of Nebraska, Lincoln, Neb. 68508 D E N N I S
SCALES formed on evaporator tubes lower the evaporator capacity and necessitate cleaning, which interrupts an operation. Therefore, they increase both the initial and the operation costs of a process such as desalination by thermal evaporation. In this work, experimental results are discussed to show that thin coatings (less than 0.001 inch) of polyfluorocarbon on tube surfaces deterred scaling to effect significant improvements over uncoated ones in evaporators with continuous feed of CaS04solutions. 306
l&EC PRODUCT RESEARCH A N D DEVELOPMEN7
Scaling is an accumulation of solids on heat transfer surfaces. The solids can be products of corrosion and solute precipitated from the processing liquid. While corrosion can be avoided by choosing a proper construction material, the scale-forming solute is usually the same material that is to be removed by the evaporation operation. The major component of scale-forming solutes in desalination of water is CaS04, mainly because of its inverse solubility. Lu and Fabuss (1968) studied calcium sulfate
Thin coatings of polyfluorocarbon on tube surfaces deterred scale formation and improved the over-all heat transfer coefficient during continuous evaporation of a salt feed in a laboratory evaporator. Results were confirmed in a pilot size evaporator. A larger temperature difference across tube wall effected better improvement. A higher Cas04 concentration in the feed indicated better performance of coated tubes relative to uncoated ones because of higher fouling rate of the latter. Continuous removal of suspended solids by withdrawing evaporator liquor must be maintained to realize this improvement.
scaling in saline water distillation and included a tabulated list of data. The present study uses CaS04 for scaling tests. Effective techniques to reduce scaling are: control the degree of supersaturakion of scale-forming solute; reduce the number of nucleation sites (Young and Hummel, 1965); provide smooth heat transfer surfaces (Hall, 1925); introduce crystal seeds in the feed (Surgi, 1955). The use of a coating having low adhesion to scales, thereby dislodging their depiosit during boiling, has not been studied for evaporating a continuous feed. Herbert and Sterns (1963) coated boiler tubes with polyacrylic acid dissolved in the feed. Githens et al. (1965) developed a Teflon heat exchanger which prevents scaling. Jara et al. (1962) claimed the use of a fluorocarbon coating to avoid the deposition of calcium nitrate in processing fertilizers. Palen and Westwater (1966) coated a flat heating strip with a thin layer of polyfluorocarbon and found that the coating lessened scaling in boiling a small batch of saturated CaS04 solution. Their results were confirmed by Myers (1967), who studied several commercially available coating materials in a small evaporator with copper heating tubes to boil a batch of CaS04 solutions at several concentration levels. His visual observations indicated that scales formed during heating were dislodged by boiling action. Alternate redisposition and redislodging of solids occurred on the heating surface. The most suitable coating material found by Myers is McLube 1700 (McGee Chemical Co., Upper Darby, Pa.). Consequently, it was used for the present study. Detailed descriptions of this work including equipment, procedure, data, and analysis are available elsewhere (Kos, 1968; Myers, 1967). Experimental
Equipment. Most of the data were obtained from a small evaporator made of 1-inch-thick epoxy plate with the approximate interior dimensions of 8 (front) by 5 (width) by 8 (height) inches. A 5 %-inch diameter borosilicate glass window was fitted in the front for visual observation of boiling and scaling. Two %-inch 0.d. copper tubes were mounted horizontally and fed with steam to boil the liquid. Both the condensate and the overflow boiling liquor with solids were returned to a feed tank having mechanical stirring. One boiler tube was coated; the other was bare copper tubing. Each was connected to a steam trap collecting steam condensate for measuring the heat transfer rate of each tube. Some data were also obtained in a pilot scale evaporator used in the Unit Operations Laboratory of the University of Nebraska. This contained 22 l x - i n c h 0.d. horizontal copper tubes fed with steam. Condensate was collected through a steam trap. As in the small boiler arrangement, condensate and the overflow liquor were returned to either
one of two small parallel feed tanks which required frequent adjustment of salt concentration. The volumes and tube areas of the small and the pilot scale evaporators are 0.87 gallon, 0.13 sq. foot, and 16 gallons, 15.3 sq. foot, respectively. The feed tanks were 60 gallons for the small evaporator and two each a t 75 gallons for the pilot scale evaporator. Feed rates were all measured by rotameters. Coating Procedures. McLube 1700 is an emulsion containing about 0.1 gram of polyfluorocarbon per ml. of a volatile solvent. This emulsion was pumped to the tube section of an evaporator, drained, and air-dried. This deposited a thin layer of coating, the thickness of which can be controlled by adjusting the concentration of solvent. This procedure was used to coat the tubes for the pilot scale evaporator after tubes had been cleaned by a combination of mechanical and chemical means. For the small evaporator, tubes were cleaned with 0-grade and 000-grade steel wool, followed by rinsing with distilled water and acetone. McLube 1700 was then poured over the vertical tube surface, which was rotated to ensure complete and uniform coverage. This covered surface was then air-dried a t room temperature for about 30 minutes before testing. Several measurements were made on the coating thickness by using a caliber before and after coating at a chosen location. The coatings had a maximum thickness of 0.001 inch when applied in this manner. The measured thickness agreed with that calculated from the heat transfer data of the same tube with and without the coating in boiling distilled water runs. Test Procedures. Uncoated tubes were first tested in the small evaporator by boiling distilled water with condensate returning to the evaporator. The rate of condensate from heating steam, steam pressure and temperature, and various stream temperatures in the evaporator were measured. In one-half hour, the evaporation operation became steady and the over-all heat transfer coefficient of each tube was then calculated from the measured tube area, temperatures of steam and boiling water, and the time required to collect a preset amount of condensate of the heating steam. The over-all heat balance was checked to be within 5%. After the heat transfer coefficients were determined for both tubes, the condensate of vapor from the evaporator was switched to the salt solution feed tank. In the meantime, salt solution from the feed tank was pumped to the evaporator at a controlled rate measured by a Flowrator. The overflow liquor was drained to the feed tank. Measurements were then made intermittently for heat transfer coefficient calculations for each tube over a span ranging from 4 to 24 hours. During a run, visual observations of the appearances of tubes were made frequently, the salt concentration of the feed tank was monitored and adjusted, and the rate of overflow liquor from the evaporator was measured. VOL. 8 NO. 3 SEPTEMBER 1969
307
Table 1. Heat Transfer Data of Small Evaporator with Continuous Feed B.t.u./(Hr.)(Sq. Ft.)(" F.)" Uncoated Coated AT, F. Gal ./Hr. % CdO, Start End Fed OW@W Hr. Run UP U: U; U,"
Run Group
Run NO.
1
10 8 13
0.12 0.12 0.12
43.3 43.0 43.2
43.3 42.8 43.3
1.75 2.80 4.75
1.10 2.03 4.32
5.25 8.00 9.00
4050 3500 4656
1031 1413 1414
2566 3161 2689
1363 1783 1691
1.32 1.26 1.20
2
7 6 12
0.12 0.12 0.12
59.2 59.7 59.5
59.7 60.3 59.2
4.75 6.40 8.00
3.38 5.48 6.12
8.00 9.97 7-00
4994 4448 5068
1082 956 1235
3186 3591 3361
1891 2103 1788
1.75 2.20 1.45
3
17 15 18
0.24 0.24 0.24
59.5 59.7 64.8
57.0 61.2 68.9
4.75 6.30 7.80
4.38 5.81 7.18
5.00 5.00 5.00
4868 4536 5006
593 601 742
4075 4067 3212
1280 1293 1315
2.16 2.15 1.77
4
21
0.31
59.5
59.0
4.75
4.36
4.00
5325
360
3941
1275
3.54
5
23 25
0.12 0.12
62.1 62.8
67.1 66.2
low low
low low
5.00 15.00
5938 5883
1445 1013
4891 4679
1105 887
0.76 0.88
6
24 26
0.12 0.12
61.3 63.3
65.8 68.0
4.75 4.75
4.15 4.15
22.00 24.00
5869 5807
746 1078
5069 4568
1795 1550
2.40 1.44
a
u,"/u:
Superscripts o and F imply, respectidy, "initial" and 'j%nal."
LEGEND COATED 4,
RUN6
0
I
2
3 4 5 6 RUN TIME in HOURS
UNCOATED 0
7
8
9
10
Figure 1 . Coating effect in evaporating a continuous feed containing 0.12% Cas04
For the pilot scale evaporator, the first run was made by using distilled water to obtain the heat transfer coefficient of the entire tube section. Then, these tubes were coated with McLube 1700. As in runs in the small evaporator, the salt solution run was operated by boiling distilled water at the beginning, then switching to the salt solution. Temperatures, steam condensate rate, feed rate, overflow liquor rate, and salt concentration of feed were measured. Since the capacity ratio of the feed tanks to the evaporator is small, salt concentration in the feed tank could be maintained a t an approximately constant level only by frequently adjusting the idle one of the two tanks to 0.12% CaS04 level in the feed. Results and Discussions
Experimental results of the small evaporator are summarized in Table I. Figure 1 shows a typical set of comparable runs indicating the beneficial results of coating. Dislodged flakes of scales were small, while those in a boiling batch solution (Myers, 1967) were large. 308
l & E C PRODUCT RESEARCH A N D DEVELOPMENT
The effect of temperature drop across the tube wall on heat transfer rate is shown by runs in groups 1 and 2. For a comparable overflow rate of evaporator liquid, run 13 indicates a 20% improvement at AT = 43OF., while runs 6 and 7 indicate 75 to 120% improvement at AT = 59°F. The improvement refers to the U,"/U," ratios in Table I. This effect may be due to the more vigorous boiling action a t the higher AT, causing higher shear forces which dislodge scales. Run groups 2, 3, and 4 explore the effect of Cas04 concentration level in the feed. The average improvements for groups 2, 3, and 4 are, respectively, 80, 103, and 254% for CaS04 on feed a t levels of 0.12, 0.24, and 0.31 weight %. Probably a better comparison may be made for runs with the same levels of overflow rate and temperature drop. Runs 6 and 7 (0.12% CaSO,) showed an improvement range of 75 to 120%; run 17 (0.24% C&O,), 116%; and run 2 1 (0.31% CaSO,), 254%. As indicated by the UF/Uy ratios, the larger improvement at a higher salt concentration in the feed is caused not by better performance of coated tubes but by deteriorated performance of the uncoated tubes. The rate of scale formation on uncoated tubes was increased when the salt concentration in the feed was increased. The effect of overflow rates on heat transfer coefficients can be observed in groups 1, 2, 3, and 5. Variations of ratios of U,"/U," in groups 1, 2, and 3 did not demonstrate a consistent trend. The important finding is indicated by the low U,"/Uf ratio values of group 5 . This definitely shows that suspended solids must be continuously withdrawn from an evaporator to enable coatings to exhibit their effectiveness. As observed by Myers (1967), scaling and descaling on the coated tube occur reversibly and continuously. Therefore, accumulation of suspended solids is expected to increase the scaling rate and thus nullify the potential improvement. Runs in group 6 tested the durability of coatings for prolonged periods. Figure 2 shows variation of heat transfer rate related to run time. The major differences occurred in the first hour and coatings appeared to be durable at the end of the 24-hour run. The difference between two U,"/U," ratios here was probably caused at
01"""P
I
L L ]
I
I
I
1 LEGEND COATED UNCOATED
I
found to be 708 B.t.u./ (hr.) (sq. ft.) (" F.), which compares favorably with a rating of 426 for runs with no overflow. This result proved importance of the removal of suspended solids and also the durability of the coating a t the end of 33 hours of operation. Future work may involve seeding the feed to see if additional improvement may be achieved. In these runs, the dislodged particles appeared too large to provide sufficient surface areas for scaling solute to deposit. Also, further tests must be made to observe the durability of coatings in continuous operations for many months. Conclusions
RUN TIME in HOURS
Figure 2. Variation of heat transfer coefficients in prolonged runs least partially by not being able to control all operating variables during these prolonged runs, whereas careful control was exercised for runs of short durations. For the pilot scale evaporator, the relatively small feed tank capacity caused large fluctuation of feed composition. Also, because of the limited steam supply, the steam pressure could not be raised to a level to provide AT values comparable to that of the small evaporator. Therefore, only qualitative confirmation of the small evaporator results was expected. A11 runs were made with feed having a nominal 0.12% CaSO4 and AT was maintained between 52" and 59" F. The uncoated tubes showed heat transfer coefficients of 374 and 229 B.t.u./(hr.)(sq. ft.)(OF.) a t the end of 14 and 28 hours of operation without overflow of evaporator liquor. Coated tubes under similar conditions showed 426 and 396 B.t.u./(hr.)(sq. ft.)(" F.) a t the end of 14 and 18 hours of opleration. This confirms the result observed for the small evaporator, that continuous accumulation of solids lowers the effectiveness of coating. Following an 18-hour run of the coated tubes with no overflow, the tube section was removed and the deposited scale was removed by soaking in water and washed with distilled water. The section was then installed without recoating. I t was then run a t AT = 42°F. with an overflow rate of 75.7 gallons per hour for 15 hours. At the end of the run, the heat transfer coefficient was
Coating of polyfluorocarbon on evaporator tubes prevents scaling in continuous evaporation of Cas04 solutions. Effects of temperature drop across tube walls, salt concentration in feed, and overflow rate of boiling liquor were demonstrated and some were confirmed in a pilot scale evaporator. Coatings appeared durable at the end of the longest run of 33 hours of operation. literature Cited
Githens, R. G., Minor, W. R., Tomsic, V. J., Chem. Eng. Progr. 61, No. 7 , 55 (1965). Hall, R. E., I d . Eng. Chem. 17, 283 (1925). Herbert, L. S., Sterns, U. J., Aduan. Chem. Ser. No. 38, 52 (1963). Jara, V., Nyvit, J., Micek, F., Czech. Patent 103,810 (May 15, 1962). Kos, Dennis D., M.S. thesis, Department of Chemical Engineering, University of Nebraska, 1968. Lu, C. H., Fabuss, B. M., Ind. Eng. Chem. Process Design Develop. 7, 206 (1968). Myers, R. G., M.S. thesis, Department of Chemical Engineering, University of Nebraska, 1967. Palen, J. W., Westwater, J. W., Chem. Eng. Progr. Symp. Ser. 62, No. 64, 77 (1966). Surgi, J., Nippon Shio Gekkaishi 9, 29 (1955). Young, R. K., Hummel, R. L., Chem. Eng. Progr. Symp. Ser. 61, No. 59, 264 (1965). RECEIVED for review March 5, 1969 ACCEPTED June 19, 1969 Financial support made by the Office of Water Resources Research, U. S. Department of Interior. Coating materials donated by McGee Chemical Co., Upper Darby, Pa.
VOL. 8 NO. 3 S E P T E M B E R 1 9 6 9
309
