ELECTRICAL CONDUCTIVITY OF LOW DIELECTRIC CONSTANT LIQUIDS by

This experimental investigation was carried out in order to determine the electrical characteristics of several hydrocarbons of low dielectric constant. All measurements of the conduc ivity were below 1 picomho/m (u.10A/V-m 10-14 ohm'lcml). This category ncludes most petroleum products, petroleum distillates made conductive by adding anti-static additives, and liquified gases. Of particular interest was the determination of the electrical conductivity of liquified natural gas (LNG), which is given special consideration due both to its increasing usage and to the electrostatic hazards associated with its transfer and storage. Electrostatic potentials can be generated during the flow of these insulating liquids and, if the voltage is sufficient, it will produce a spark. Liquified natural gas and many other petroleum products are highly combustible and the electrical conductivities of these liquids are among the properties related to the hazards of their combustion, since the conductivity controls the rate at which charges accumulate and are dissipated, At the ver low conductivities found for several of these liquids (< 10 ohmlcm ), high velocities and long residence times are necessary for static charge buildup to occur. However, any charge accumulation which does take place would require a long relaxation time to dissipate. Two methods were used to measure the electrical conductivities: (1) direct current measurements, and (2) liquid capacitance measurements. In preliminary tests the conductivities of several petroleum solvents were measured. A comparison of this data with published values for these liquids indicated that the apparatus was working properly. Any variation between these two sets of values could be accounted for by the presence of small uantities of dissolved ionic impllrities such as dissociated aqueous salts, chlorides, sulfur compounds, and water. Support for this explanation partially stems from the different readings obtained for reagent rade toluene and purified tolXuene. Another type of ion has been postulnted to exist in these insulating linuids. Work done by other investigators indicates that "chemically pure liquids," although showing no electrical conductivity of their own, are slightly ionized by natural background radiation. These ions are continually being formed as others are lost by recombination. Because of the very small extent of ionization in these pure liquids, their electrical conductivities are much lower than for the less pure liquids. From the present study, it has been found that the maximum electrical conductivity for ING is below 10-17 ohm'lcm 'l. This extremely low value indicates that any electrostatic charges which do accumulate during LNG transfer operations would persist over a long time interval. The author is in agreement with conclusions reached from other studies that cryogenic fluids are nearly perfect insulators and that the observed conduction is due to the presence of charge carriers produced from atmospheric rad iation. Thesis Supervisor: Kenneth A. Smith Professor of Chemical Engineering Department of Chemical Engineering Massachusetts Institute of Technology Cambridge, Massachusetts 02139 August 11, 1975 Professor Frederick J. McGarry Secretary of the Faculty Massachusetts Institute of Technology Cambridge, Massachusetts 02139 Dear Professor McGarrys In accordance with the regulations of the Faculty, I herewith submit a thesis, entitled "Electrical Conductivity of Low Dielectric Constant Liquids" in partial fulfillment of the requirements for the degree of Master of Science in Chemical Engineering at the Massachusetts Institute of Technology. Respectfully submitted, Evelyn Mauren White . . Evelyn Maureen White