190
Ind. Eng. Chem. Prod. Res. Dev. 1981, 20, 190-193
leaning effect of alcohols on the engine’s air/fuel mixture may be quite noticeable to the customer, particularly in colder weather or in the many cars which were designed for and are already operating with lean mixtures on gasoline. Increased corrosion with alcohol is typical. Blockage of in-line fuel filters by rust and sediment, picked up as a result of the surface active characteristics of alcohols, is not uncommon. A particularly difficult problem to overcome with alcohol blends is their ready extraction by water contact, whether in the distribution system or in the car fuel tank. In the distribution system, not only an economic loss can occur but provisions must be made for collection and disposal of the biologically toxic water/alcohol tank bottoms. If separation occurs in the car’s fuel tank, the customer will have no difficulty recognizing the degradation of car performance. In comparison, gasoline made by methanol conversion has none of these problems, and we believe the advantages of such a gasoline, already at a high octane level, will prove to be very attractive to consumers. Literature Cited
Table IX. Storage Stability Tests on Methanol-Derived Gasoline (Test Condition: 16 Weeks at 110 O F ) base + addibase tive Dackarre start of test residue on evap., mg/100 mL existent gum, mg/100 mL increase after 16 weeks at 110 “Fa residue on evaporation, mg/100 inL existent gum, mg/100 mL
3.8 1.3
6.4 2.6
0
0.7 0
0
a An increase of 2 mg or less/100 mL in this test would indicate satisfactory field storage stability for 1 year at typical ambient temperatures.
changes in gum contents during the 16-week period, satisfactory stability also would be expected in the field. Direct Blending vs. Conversions of Alcohols Alcohols have long been recognized as potential octane improvers in gasoline, and more recently they have received new attention as volume extenders. However, there are well-documented problems associated with their use, particularly for methanol and, to a lesser extent, for ethanol. From the consumer’s standpoint, for example, 10% additions of methanol or ethanol, typically advocated usage levels, increase the gasoline’s vapor pressure about 3 and 1 psi, respectively. These increases, plus substantial changes in distillation characteristics of the blends, contribute to greater vapor locking tendencies in hot weather. Adjustments (removal) of light hydrocarbons are necessary for equivalent vapor lock control, thus decreasing the original gasoline’s energy content. Degraded warm-up driveability performance occurs in cold weather, because the alcohol blends have higher latent heats of vaporization and also give leaner air/fuel mixtures. Adjustments for equivalent cold weather performance require a more difficult rebalancing of components, and satisfactory performance may not be possible in very cold temperatures with methanol because of insolubility problems. The
Chang, C. D.; Silvestri, A. J. J. Catal. 1977, 47, 249. Kam. A. J.; Lee W. Final Report, DOE Contract No. EX-764-01-2490, Apr 1978. Lee, W.; Chen, N. Y.; Perry, R. H., paper presented at Brazlllan Petroleum Congress, R b de Janeiro, Nov 1978. Meisel, S. L.; McCullough, J. P.; Lechthaler, C. H.;Weisz, P. B. CHEMECH 1976, 8 , 86. Meisel, S.L.; McCullough, J. P.; Lechthaler, C. H.; Welsz, P. B., paper presented at the 174th National Meeting of the American Chemlcal Society, Chlcago, Ill.,Aug 1977. V o k . S. E.; Wise, J. J., Final Report, ERDA Contract No. E (49-18).1773, Nov 1976.
Received for review February 29, 1980 Accepted July 21, 1980 This paper was presented at the 179th National Meeting of American Chemical Society, Houston, Texas, Mar 1980, Division of Petroleum Chemistry. This work was conducted under the Department of Energy (DOE) Contract No. EX-76-C-01-2490,which was jointly funded by DOE and Mobil Research and Development Coporation.
Dichlorotetrafluoroethane Sensitivity of Various Elastomers in Geothermal Energy Conversion Applications Barna Toekes Sperry Research Center, Sudbuty, Massachusetts 0 1776
Speny has developed a novel power generating system utitizii relatively low temperature hydrothermal resources. In this binary plant, an organic working fMd is heated by geothermal brine in a primary heat exchanger installed in a production well to a depth of 2000 ft. An organic propelled turbine, which drives a brine pump directly, is installed below the downwell heat exchanger. The heated organic flukl drives a turbinegenerator on the surface: the exhaust is condensed and returned to the well. Highly reliable elastomers as self-energized seals are required in the unaccessible locatbn of the downwell turbinepump and organic conduits. Nine elastomer candidates were tested for compatiMlky with C,CI#,, the organic working fluld in this applicatkm. Test results of dimensional stablllty and hardness of the elastomer and chemical stability of the organic working fluid after exposure at varlous temperatures are presented and discussed. The project was sponsored by the U.S. Department of Energy and tests were performed in part by Allied Chemical, Specialty Chemicals Division, Buffalo Research Laboratory.
Introduction In the quest to supplement the nation’s energy supply, Sperry has been engaged for several years in the devel0196-4321/81/1220-0190$01.25/0
opment of an improved energy conversion system utilizing low-temperature hydrothermal resources. Efforts to date resulted in the ongoing U.S.Department of Energy spon0 1981 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 1, 1981 1 B l Table I. Formulation of DuPont Vamac 91680 and 0803 com Pound
91680-187
ON 80 3-47
Vamac B-124b Armeen 18Da Stearic Acida Zelec UNasb FEF Black SRF Black Santicizer 409 Diak # lb MDA DPG
124 0.5 2.0 2.0 66
124 0.5 2.0 2.0
__
25 1.25
-_
2.5
Table 11. Formulation of Fluorel Cpd Ca Fluorel 2178 Fluorel 2179 Thermax MT N990 N774 SRF Black Maglite D Ca(OH)* Lithrage
__
35
__
__ 1.25 4
Synergistic intemal release system. b Vamac, Zelec, and Diak are registered DuPont trademarks. Laboratory cure: 20’1177 “C.
a
Press 10 ‘ at 350 F; post cure 16 h at 500 OF.
Table 111. Formulation of Epcar EPDM”
PHR 100 40 10 5 1 1 2 2
a
sored demonstration of the so-called Sperry “gravity-head” system (Matthews and McBee, 1979), a binary conversion plant utilizing a novel heat engine cycle, which permits a significant (37%) efficiency increase over a conventional binary conversion system. Thii improvement is due to two novel design approaches: (a) The primary heat exchanger is located in the production well extending from ground elevation to some 2000 ft depth which eliminates the heat transfer “pinch effect” between brine (“Brine” in this presentation is used for “geothermal fluid,which is a more correct description of the material involved, and presently under advisement of the appropriate ASTM Committee) and organic, and eliminates the organic circulating pump as it can circulate by thermosyphon, and (b) the brine pump, which is installed beneath the downwell heat exchanger, is directly driven by an organic-powered turbine. Both of these design approaches necessitate the use of highly reliable elastomers as seals under unusual service conditions. Compatibility of all prevailing and imposed materials with each other in the system were to be messed and properties of the organic working fluid were to be determined also to permit proper system design. The organic working fluid selection is governed by the brine temperature. In the Sperry geothermal well in East Mesa, Calif., 350 OF brine temperature is expected and a commercial refrigerant, R-114, was selected as the suitable working fluid for maximum conversion efficiency. R-114 is a mixture of two isomers of dichlorotetrafluoroethane. The resulting downwell pressures are expected to approach 1000 psia during operation. Allied Chemical, Specialty Chemicals Division, Buffalo Research Laboratory, was contracted to perform a battery of compatibility evaluations. These included: testing property changes of elastomers after exposure to R-114 at two temperature levels and testing decomposition of R-114 during the same exposures; testing compatibility of ten metal and two ceramic candidate components of the downwell turbine-pump unit with the R-114; testing the long-range compatibility of metal couples with dry and wet R-114 in the presence of lubricating oil at elevated temperatures; and determining properties of R-114 in previously uncharted (supercritical) domain. The following presentation is restricted to the discussion of elastomerR-114 compatibility assessment. Experimental Section Elastomers Selection. In order to identify and evaluate suitable elastomers, major manufacturers were solicited to recommend and supply candidate materials. The following selection resulted. (1) Vamac 91680 and (2) Vamac 0803. DuPont submitted two Vamac compounds described as “essentially a copolymer of ethylene and methyl acrylate with a third
20 80 40 15 3 6 10
Epcar 346 N-330 Sunpar 2280 zinc oxide AgeRite resin D Vanfre AP-2 Saret 500 methyl niclate DiCup 40C
2 168
Air oven aged, 168 h at 300 O F .
monomer which allows diamine curing”. This third monomer is described as containing carboxylic curing sites. (-R-)
I
F=O
I
OH
The formulation is given in Table I. (3) Fluorel Cpd C. Submitted by 3M Chemical Division, Fluorel is described as a fluoroelastomer, a highly fluorinated synthetic rubber. The formulation is given in Table 11. (4) FA Polysulfide Rubber. Thiokol submitted the FA polysulfide sample which is described as the reaction product of bis(2-chloroethyl) formal and ethylene dichloride with sodium polysulfide. (5) Krynac 50.75. Polysar Ltd. submitted the Krynac sample, described as “a 75 Mooney viscosity, 50% acrylonitrite content N B R . (6)Rulon A. This material is not an elastomer; it is a DuPont Teflon compounded with undisclosed fillers by Dixon Industries. It was included in the test program as a fall-back material, since in several seal applications a stainless steel spring in a Teflon envelope can be substituted for elastomers (Bal-seal). (7) Epcar EPDM. This material is an ethylene-propylene-diene terpolymer by B. F. Goodrich with the formulation presented in Table 111. (8) Silastic LS 2249 and (9) Silastic LS 2332. These Dow Corning products were described as two forms of methyl trifluoropropyl silasene. No formulation was given. The sample designated as LS 2249 was cured for 4 h at 200 OC, and sample LS 2332 for 8 h at 200 OC. (10) PNF. PNF is phosphonitrilic fluoroelastomer, a highly fluorinated polyphosphasene, manufactured by Firestone. It was primarily selected for brine application but tested in all runs with the other elastomers. Test Procedures This work entails the dimensional stability, hardness, and/or extraction testing of 10 elastomers in contact with dry R-114 at 120 and 350 OF. Additional test criteria are by visual inspection for discolorations and deposits and chloride analysis. The test procedures used are similar to those of Parmelee (1965), Walker (1965), and Armstrong
192
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20,No. 1, 1981
Table IV. Results of Dimensional Stability Tests
material Vamac 91680 Vamac 0803 Fluorel Cpd C FA polysulfide rubber Krynac 50.75 Rulon A Epcar EPDM Silastic LS2332U Silastic LS2249U 10. PNF 1. 2. 3. 4. 5. 6. 7. 8. 9.
% linear change, (Lf - L i ) / ( L i )x 100 120 " F 350 "F
+ 3.3
visual observations (60 days at 350 O F )
+ 3.2 +4.2 + 5.5 -5.7 + 0.8 -0.5 + 5.3 + 10.0 + 11.0 -4.6
2.8 + 6.1 + 0.8 + 0.2 + 3.2 f
+4.8
+11.0 + 10.0 + 15.5
solution yellow dissolving and distorting
distorted distorted distortion, solution yellow
Table V. Results of Hardness Tests" (120 OF) before heating 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
material Vamac 91680 Vamac0803 Fluorel Cpd C FA polysulfide Krynac 50.75 Rulon A Epcar EPDM Silastic LS2332U Silastic LS2449U PNF
Hi 73 68 90 74 77 86 65 57 48 77
Hf 65 59 88 70 67 86 61 52 43 73
after heating Hi
Hf
embrittleinent
76 65 87 77 86 94 65 51 41 71
66 55 85 72 73 92 59 44 36 66
N N N N N N N N N N
comments
softening softening
Shore A units.
(1965). All test samples were elastomer sheets having thicknesses between 0.050 and 0.125 in. Dimensional Stability. The testing procedure is similar to ASTM D-1460-60. The sheets were wiped with a clean cloth and samples were cut approximately 2 in. (120 O F ) or 11/2in. (350 O F ) X in. sheet of linear thickness. Tests were run on the same elastomer with transverse and longitudinal cuts, and the dimensional changes were averaged. Assuming isotropic expansion of the averaged linear change, the volumetric expansion was estimated. The samples were placed in 9-mm glass tubes sealed at one end. The initial dry length (Li)was measured at room temperature by vertically suspending the tube and measuring the distance from each end with an optical cathetometer accurate to *0.05 mm. Sufficient R-114 was distilled into the tube to ensure that the elastomer sample was totally immersed to an excel1 of in. at the test temperature, for temperatures below the critical. The solution was degassed and the tube was sealed. Tubes were placed in 1/2-in.copper tubing sheathes and run at 120 f 0.5 O F or at 350 f 1 O F . The samples were pulled out of the oven once per week, cooled to room temperature, and the lengths (Lf) were determined by cathetometer on the immersed sample. This procedure was repeated until no further length change was observed greater than 0.1 mm. Exceptions were the two elastomer samples, which were disintegrating at 350 OF. Maximum swelling was generally reached in 2-3 weeks. Total observation period was 4 weeks. (See Table IV.) Hardness. The testing procedure is similar to ASTM D-2240-68. Elastomer samples were cleaned by wiping with a clean cloth dampened with alcohol and were cut into pieces approximately 1 X 3/a in. X sheet of linear thickness. The samples were stored in jars for at least 24 h to come to ambient temperature and humidity. The samples were piled where necessary to a total thickness of about 'I4 in. Initial hardness measurements were performed on duplicate samples utilizing a calibrated Shore A constant load (822 g) durometer. Although this durometer is not a
maximum reading durometer, an estimate of cold flow, or "creep" was made by taking the difference between the instantaneous reading Hi (as soon as the foot made firm contact with the sample) and a final reading Hf after a time interval of 15 s. The samples were then placed in brass sample tubes (120 OF) or stainless tubes (400 OF). The sample tubes were sealed and pressurized with nitrogen to test for leaks. When no leaks were apparent the sample tubes were evacuated and R-114 was pressure titrated into the tubes. Selected samples were analyzed after 3 weeks and the remainder of the samples were analyzed after 5 weeks at the test temperature. Weights were taken weekly on all the sample tubes to detect leaks. A final hardness reading after thermal exposure was taken by venting the R-114 content and pumping the tube to vacuum 1 h before measurement, or until the sample reached thermal equilibrium with surroundings. The samples were removed from the tubes and immediately remeasured with the durometer. After hardnew measurement, the samples were flexed for embrittlement and compared with flexure of fresh samples. Test temperatures of the experiments were 120 and 400 O F . (The results are given in Tables V and VI.) Thermal Stability Testing. Elastomer sheets were cut into strips 4 X 3/16 in. for testing. The strips were washed by placing them in a Fischer & Porter glass aerosol tube and R-114 was distilled into the tube until the samples were completely immersed. The tube was shaken, the R-114 was drawn off, and the tube was then evacuated. The 3/4-in.diameter heavy wall Pyrex tubes were sealed at one end, joined to 9-mm tubing, and triple annealed. The sample tube was cleaned by washing with 4 N nitric acid, followed by copious rinsing with deionized water. The tubes were rinsed with reagent acetone and dried overnight at 120-150 "C. The tubes while warm were flushed with dry nitrogen and capped with plastic plugs. The elastomer strip was inserted into the tube, evacuated to less than 50 pm, and sufficient dried and degassed R-114 was distilled into the tube to produce 400 psia at
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 1, 1981 193 Table VI. Results of Hardness Testsa (400 O F ) material 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
Vamac91680 Vamac0803 Fluorel Cpd C FA polysulfide Krynac 50.75 Rulon A EpcarEPDM Silastic LS2332U Silastic LS2449U PNF
before heating Hi Hf
after heating Hi Hf
70 67 92 75 74 87 65 58 48 77
92 91 85 95 89 88 61 40 31 84
62 58 90 70 67 87 61 52 43 74
91 88 83 95 89 88 58 21 22 78
embrittlement
Y Y N Y Y N N N N Y
comments breaks on flex breaks on flex strong odor, shatters strong odor, shatters gummy, disintegrates gummy, disintegrates gummy, breaks on flex
Shore A units. Table VII. Results of Thermal Stability Test
1. 2. 3. 4.
elastomer Vamac 91680 Vamac 0803 Fluorel Cpd C FA polysulfide
5. Krynac 50.75 6. Rulon A 7. Epcar EPDM
8. Silastic LS2332U 9. Silastic LS2449U 10. PNF
visual observation (after 30 days) pale yellow liquid pale yellow liquid N.C. Brown liquid, sample distortion, possible clear liquid, some fine white particles (filler?) N.C. small amount of dark fluid at bottom liquid clear, sample curled liquid clear, sample curled pale yellow liquid, I / * a gray solid on bottom
chlorine content, PPm 11 19 11 11000 760 27 304 27 20 66
the test temperature. The sample tube was double degassed by freezethaw techniques with liquid nitrogen and the tip was sealed and flame-annealed with a torch while frozen. Duplicate samples of material were prepared; the tubes were placed in aluminum safety shields. The tubes were placed vertically in an oven regulated at 350 f 1 OF. After 30 days visual appearance was recorded and R-114 stability was checked by chloride analysis (titration method). These test results are presented in Table VII. Discussion of Test Results Dimensional Stability Test. This test is expected to indicate excessive solvent absorption and/ or chemical degradation at service temperature. Some swelling (positive linear change) may be acceptable for O-ring seals, but excessive swelling especially if it is followed by collapse (negative linear change) indicates failure. On these bases, samples 4, 8, 9, and 10 have failed as also indicated by visual observation. Of the survivors, numerical test resulta indicate the following gradation: (a) sample 5, Krynac 50.75; (b) sample 6, Rulon A; (c) samples 1and 2, Vamac; (d) sample 7, Epcar EPDM; (e) sample 3, Fluorel Cpd C. The results of these testa are presented in Table IV. Hardness Tests. Elastomers used for O-ring seals typically have Shore hardness values between 50 and 90. Excessive hardening or loss of elasticity will render the elastomer useless as self-energized sealant. Excessive softening increases undesirable extrudability and can be
an indication of chemical decomposition. The hardness test performed at 120 OF (Table V) indicates that samples 8, 9, and 10 failed. Based on the “instantaneous” hardness change (Hi)before and after exposure, the surviving elastomers rated as follows: (a) sample 7, Epcar EPDM (b) sample 3, Fluorel Cpd C; (c) sample 4, FA polysulfide; (d) samples 1and 2, Vamac; (e) sample 6, Rulon A; (f) sample 5, Krynac 50.75. The “final hardness” (Hf) change gave similar results, although with somewhat different grading sequence. Hardness tests were also performed at 400 O F (Table VI). This was an error, since 350 O F was requested. While these results are not applicable to the 350 O F service condition, they do indicate that only three candidates survived at 400 OF: (a) sample 6, Rulon A; (b) sample 7, Epcar EPDM; (c) sample 3, Fluorel Cpd C. Conclusions In conclusion, for the specific application of elastomers in R-114 refrigerant, aside from the Rulon A material, two elastomers (Epcar EPDM and Fluorel Cpd C) were found acceptable and two additional ones (Krynac and the two Vamac samples) were found marginal (at 400 OF). Since the service condition calls for 350 O F , tests will be repeated at this lower temperature before final selection is made. Acknowledgment This work was supported in part by the United States Department of Energy. Acknowledgment is also made to Mr. S. R. Orfeo and Dr. D. P. Wilson, under whose direction a portion of the work was performed at Allied Chemical Company, Specialty Chemicals Division, Buffalo Laboratories. Literature Cited Armstrong, T. D., Jr., “Chkrkle Analysis as a Measure for Evaluation of Sealed-Tube Tests”, ASHRAE Semiannual Meeting, Chlcago, IL, Jan 1965. M a t h w s , H. B., McBee, W. D., ‘QavltyHead C%othermal Energy Converslon System”, Qeothermal Resources Council Trans., Vol. 3, Calif., 1979. Parmelee, H. M., “Sealed Tube StaMllty Tests on Refrigeration Materials”, ASHRAE Semiannual Meetlng, Chicago, IL, Jan 1965. Walker, W. O., “Sealed Tube Tests; A Comparison of Methods”, ASHRAE Semiannual Meetlng, Chicago. IL, Jan 1965.
Received for reuiezu April 14, 1980 Accepted June 23, 1980 This paper was presented at the 179th National Meeting of the American Chemical Society, Division of Polymer Chemistry, Houston, TX, Mar 24,1980.
