Solid−Liquid Equilibrium in the System of CaCl2−H2O with Special Regard to the Transition Points

Abstract

Experimental data on the T−x and p−T solid−liquid equilibrium relations of the CaCl2−H2O system have been used simultaneously to determine temperatures, pressures, and solution compositions at the transition points between calcium chloride hydrates. The (T, p, x) coordinates of the transition points have been determined from intersections of the curves corresponding to the adjacent hydrate branches of the SLE phase diagram in the T−x and p−x planes. For this purpose, a set of empirical temperature−molar fraction and pressure−molar fraction expressions has been developed describing the solid−liquid equilibrium relations of the CaCl2−H2O system. The equations for the T−x relation cover the full composition range while the equations describing the p−x relation cover compositions up to the salt mole fraction of x = 0.35 (i.e., mass fraction of w = 0.77), corresponding to the transition from one-third hydrate to anhydrate. Equations of a special structure were used, involving the coordinates of the transition points as parameters, which makes possible their direct nonlinear optimization via least-squares fits to experimental data. The estimates of uncertainties in the obtained values of the coordinates of the transition point range from 0.1 K to 5 K for temperatures and from 1·10−4 to 1.6·10−3 for solution composition expressed in salt mole fraction.