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Ind. Eng. Chem. Prod. Res. Dev. 1983,22,127-131
Elastomers for Use in Dichlorotetrafluoroethane, Brine, and Oil at Elevated Temperatures Rlchard J. Thibeau Sperry Research Center, Sudbury, Massachusetts 0 1776
Elastomeric materials which do not degrade under severe service conditions are required in many novel applications. Twenty elastomer compounds were tested for compatibility with dichlorotetrafluoroethane, geothermal brine, and lubricating oil at 350 O F (176.7 “ C )to determine which elastomers were best suited for use in a binary geothermal energy conversion system. Sealed tube tests were conducted to duplicate downwell conditions in the laboratory. Although elastomer swelling was a problem in dichlorotetrafluoroethane, fluoroelastomer and EPDM compounds were found to be acceptable in that fluid and in brine. Fluoroelastomersand silicones were found to be compatible with lubricating oil.
Introduction For several years, the Sperry Corporation has been engaged in the development of an improved electrical generation system using low-temperature hot water geothermal resources for the energy source. The system uses an organic working fluid at temperatures above its critical temperature to transfer energy from the underground water to an electrical generator. Because of the design of the equipment, reliable high-temperature elastomeric seals are extremely important to the operation of the geothermal system. It was not known which elastomers are best suited for use in the severe environment encountered; therefore, a laboratory study of elastomer stability in simulated geothermal well conditions was undertaken. The generating system is a binary conversion plant using a novel heat engine approach to extract energy from hot geothermal brine in a much more efficient manner than in conventional binary systems. The key elements of the Sperry “gravity head” system (Matthews and McBee, 1979), are a 2OOO-ft vertical, underground heat exchanger to transfer heat to the working fluid and, below that, an underground brine pump driven by an organic-powered turbine. The optimum organic working fluid is dependent on the available brine temperature. In the Sperry well at East Mesa, CA, the brine temperature is 350 O F (177 “C), so dichlorotetrafluoroethane (R-114) was selected as the working fluid to give the most efficient energy transfer. The use of downwell equipment necessitates the use of very reliable elastomers in seals for rotating machinery and sliding joints under difficult service conditions. In the well at East Mesa, seals would be exposed to temperatures of 350 O F and pressures as high as 1100 psi. Seals were required between pairs of the fluids dichlorotetrafluoroethane, brine, and lubricating oil for the downwell turbine-pump unit. The program described here determined which elastomeric compounds could be used for seals based on laboratory immersion tests. The elastomers selected for testing were the materials recommended by manufacturers of elastomeric seals. While the choice of elastomers for temperatures of 400 OF or higher is essentially limited to silicones and fluoroelastomers, at 350 OF EPDM, acrylic, and phosphonitrile compounds should be compatible with one or more of the fluids. Samples of each of these types were tested, together with a few compounds known to give good performance in R-114 at low temperatures (nitrile and ethylene-acrylic). While little is known about the resistance of elastomers to degradation by hot dichlorotetrafluoroethane, prelim0196-4321/83/1222-0127$01.50/0
Table I. Analysis of Geothermal Brine from Well 87-6 flashed (used for elastomer unflashed testing)
PH total dissolved solids calcium bicarbonate chloride sulfate silica sodium potassium specific conductance carbon dioxide nitrogen methane ammonia hydrogen sulfide
2450 ppm 24 ppm 330 ppm 928 ppm 179 ppm 232 ppm 768 ppm 56 ppm
9.2 2700 ppm
