LIQUID-VAPOR PARTITIONING OF NaCl(aq) FROM CONCENTRATED BRINES AT TEMPERATURES TO 3 50" C

Compositions of coexisting liquid and vapor phases have been determined at temperatures from 250 to 350°C for brines containing NaCl and either HCI or NaOH by direct sampling of both phases from a static phase-equilibration apparatus. In these experiments, NaCl concentrations in the liquid phase ranged to 6.5 mol-kg", with corresponding vapor-phase NaCl concentrations varying strongly with temperature and brine composition. Acid or base was added to the brines to suppress unknown contributions of NaCl(aq) hydrolysis products to the observed volatilities. Thermodynamic partitioning constants for NaCl have been determined from the observed compositions of the coexisting phases combined with the known activity coefficients of NaCl(aq) in the liquid phase. An apparent dependence of the values of these partitioning constants on brine concentration is explained by considering the effect of decreasing pressure on the density of the vapor phase. Concentrations of HCI and NaCl in steam produced from various natural brines may be calculated as hnctions of temperature and brine composition based on these new results coupled with our previous determinations of the partitioning constants for HCl(aq). Application of these results to The Geysers will be discussed in terms of the composition of postulated brines which could be in equilibrium with observed steam compositions at various temperatures. INTRODUCTION The production of acidic, chloride-containing steam from wells at The Geysers has resulted in corrosion of well casings and steam piping through attack by corrosive condensed fluids. In extreme cases corrosion has forced wells to be taken out of production., While the incidence of acid-chloride steam was originally considered a problem in the hotter producing regions of the northwest portion of The Geysers, recently the possible production of acid-chloride steam from the lower-temperature regions of the central portion of the field has received renewed attention. While the subject of acid-chloride steam chemistry has been modeled and discussed (TRUESDELL et ul., 1988; HIRTZ et al., 1991), little new experimental information has been obtained on the partitioning of electrolytes between liquid and vapor phases over the temperature range commonly found in steam at The Geysers. The relative lack of reliable experimental values in this temperature range limits our ability to predict steam compositions over brines of varying pH and salinity. Conversely, given the observed composition and temperature of produced steam, the ability to predict a brine composition which would be in equilibrium with that steam phase is limited for many systems by a lack of data. A number of aspects of the basic physical chemistry of geothermal systems are under investigation at ORNL, including solubility and speciation of aluminum in aqueous solutions, thermodynamics and phase relations in granitic melts, liquid-vapor distribution of stable isotopes in brines, and liquid-vapor distribution of HCI including the production of acid-chloride-containing steam. We have previously reported (SIMONSON and PALMER, 1993) measured values obtained in this program for the partitioning ofHCl over HCl(aq) from 50 to 350°C. In a similar experimental study sponsored by the Electric Power Research Institute, we have measured liquid-vapor partitioning of NHJI over acidic and buffered aqueous solutions from 120 to 350°C (PALMER and SIMONSON, 1993) Ammonia and ammonium salts have been found in steam from The Geysers @RZ et ul., 1991). Our recent experimental studies have addressed possible sources of acidity in steam from hydrolysis of alkaline-earth chlorides (Mg and Ca) at high temperatures through extensive series of measurements on brines containing NaCI. These latter experiments were performed both to determine quantitatively the effects of NaCl concentration on HCI partitioning, and to reinvestigate the partitioning of NaCl to steam from multicomponent brines. These new measurements of NaCl partitioning, coupled with experimental values for HCI partitioning from pure aqueous acid and fiom acidic NaCl brines, may be used to provide new estimates of steam composition above {NaCl + HCl}(aq) at temperatures ranging to the solvent critical temperature. EXPERIMENTAL The apparatus and techniques used to equilibrate and analyze the samples taken in these equilibrium studies have