Metastable Equilibrium Solubilities And Related Thermodynamic Properties Of CaSO₄ And SrSO₄ In Pure Water At Multi-temperature And In Salts Solutions At 298.2 K

In this paper, the isothermal evaporation method was used to determine the metastable equilibrium solubilities and metastable zone of Ca SO4 and Sr SO4 in pure water and salts(Na Cl、Ca Cl2 and Mg Cl2) solutions. And thermodynamic properties at metastable state were calculated by the simplified Pitzer equation at the relevant temperature.The change of concentration of Ca SO4 in pure water was measured at(276.2, 283.2, 293.2, 298.2, 303.2, 313.2, 318.2 and 323.2) K and the values were fitted to empirical equations of evaporating times. Then, the metastable zone of Ca SO4 in pure water from(276.2 to 323.2) K was determined and the maximum metastable zone width(MSZW) was 31 mmol/kg H2 O, the minimum one was 25 mmol/kg H2 O. The forming of Ca SO4 crystal can be controlled easily in the metastable zone and the crystal immediately occurs once solubility of Ca SO4 is lager than metastable solubility at a certain temperature.The metastable equilibrium concentration of Ca SO4 in Na Cl, Ca Cl2 and Mg Cl2 solution were measured at 298.2 K. The concentration of Ca SO4 in Na Cl solutions(m0Na Cl =(0.10 and 0.50) mol/kg H2O) during the evaporation progress were measured at 298.2 K. And the metastable zone of Ca SO4 in from(0 to 5.50) mol/kg H2 O Na Cl solution were also determined, MSZW was about(15-46) mmol/kg H2 O. The concentration of Ca SO4 in the Ca Cl2 solutions(m0Ca Cl2 =(0.50 and 5.00) mmol/kg H2O) were measured and the metastable zone(MSZW ≈(3-16) mmol/kg H2O) of Ca SO4 in from(0 to 50.00) mmol/kg H2 O Ca Cl2 solution were also determined at 298.2 K. Similarly, the concentration of Ca SO4 in the Mg Cl2 solutions(m0Mg Cl2 =(0.50 and 20.00) mmol/kg H2O) were measured and the metastable zone(△mmax ≈ 16 mmol/kg H2 O, △mmin ≈ 3 mmol/kg H2O) of Ca SO4 in from(0 to 50.00) mmol/kg H2 O Mg Cl2 solution were also determined at 298.2 K. According to the MSZW of in the salts above, it is the most difficult to form Ca SO4 crystal in Na Cl solution.The change of the concentration of Sr SO4 in pure water was measured at(278.2, 283.2, 303.2 and 323.2) K and the values were fitted to empirical equations of evaporating times. Then, the metastable zone of Sr SO4 in pure water from(278.2 to 323.2) K was determined(MSZW ≈(1-25) mmol/kg H2O). It is more difficult to form Sr SO4 crystal than Ca SO4 crystal in the pure water based on the WSZW achieved in this work.The metastable equilibrium concentration of Sr SO4 in Na Cl, Ca Cl2 and Mg Cl2 solution were measured at 298.2 K. The concentration of Sr SO4 in Na Cl solutions(m0Na Cl =(0.01, 0.25 and 0.50) mol/kg H2O) during the evaporation progress were measured at 298.2 K. And the metastable zone of Sr SO4 in from(0.01 to 5.50) and(0.25 to 1.75) mol/kg H2 O Na Cl solution were also determined, MSZW was about(0.8 to 1.2) and(1.6 to 3.8) mmol/kg H2 O. The concentration of Sr SO4 in the Ca Cl2 solutions(m0Ca Cl2 =(0.01 and 0.045) mol/kg H2O) were measured and the metastable zone(MSZW ≈(1.5-2.1) mmol/kg H2O) of Ca SO4 in from(0.01 to 0.25) mol/kg H2 O Ca Cl2 solution were also determined at 298.2 K. Similarly, the concentration of Sr SO4 in the Mg Cl2 solutions(m0Mg Cl2 =(0.05 and 0.125) mol/kg H2O) were measured and the metastable zone(△mmax ≈ 3.2 mmol/kg H2 O, △mmin ≈ 1.6 mmol/kg H2O) of Sr SO4 in from(0 to 0.55) mol/kg H2 O Mg Cl2 solution were also determined at 298.2 K. Based on the MSZW of Sr SO4 in salt solution obtained in this study, it is most difficult to form Sr SO4 scale in Na Cl solution.The crystals of Ca SO4 and Sr SO4 can be controlled effectively in metastable zone, so the conclusions about obtained from this work can provide scientific basis for the researches of inhibitors of the Ca SO4 and Sr SO4 scales in oilfield water.The related thermodynamic properties(γ±, Φ, GE, Qsp, Ksp) of Ca SO4 and Sr SO4 were calculated by the simplified Pitzer equations based on the experiment.The related thermodynamic properties(γ±, Φ, GE, Qsp, Ksp) of Ca SO4 in pure water and the activity of water(aw) were calculated at(276.2, 283.2, 293.2, 298.2, 303.2, 313.2, 318.2 and 323.2) K. All these thermodynamic properties were calculated by the binary simplified Pitzer equations. Besides, the simplified Pitzer equations were used to calculate the mean activity coefficients of Ca SO4 in Na Cl, Ca Cl2 and Mg Cl2 solutions at 298.2 K.The related thermodynamic properties(γ±, Φ, GE, Qsp, Ksp) of Sr SO4 in pure water and the activity of water(aw) were calculated at(278.2, 283.2, 303.2 and 323.2) K. All these thermodynamic properties were calculated by the binary simplified Pitzer equations. Moreover, the simplified Pitzer equations was used to calculate the mean activity coefficients of Sr SO4 in Na Cl, Ca Cl2 and Mg Cl2 solutions at 298.2 K. The calculated values obtained from this work can also provide scientific basis for the prediction of Ca SO4 and Sr SO4 scales in oilfield water.