Construction of Enthalpy-Composition Diagram for Manufacture of Common Salt from Sea Water

Abstract

The enthalpy-composition diagram, Fig. 1, has been constructed, with a view to facilitating the computation of heat balance in the manufacture of common salt. Although the method of construction adopted is substantially the same as that for ordinary binary system, special attention has been paid to the following points, because the sea water system is composed of many components as de- noted in Table 1.
1. Expression of composition: From many experiments, it has been found that the relative proportion of various salts in the sea water is constant anywhere on earth. At any concentration stage, the salts in the sea water may be conveniently expressed in a mass as "total salts" S, which includes salts both in the solution phase and the solid phase. Therefore, the sea water system may be denoted as a binary system of S and H2O as shown in Fig. 1. The weight per cent of S has been calculated from the following equation:
Per cent of S=(weight of salts in soln.)+(weight of deposited salts)/(weight of solution)+(weight of deposited salts)×100
2. State of system: For the calculation of enthalpy, the state of system or the composition of the solution and deposit must be known. This state was estimated by M. Ishibashi⁵⁾ Fig. 2) and Y. Onodera¹¹⁾ from the experiments on isothermal concentration at 1 atm, and by J.D'Ans¹⁾, from the researches in the solution equilibrium of ocean salts.
3. Enthalpy: The enthalpy of the system is given as follows:
H=H_l+H_s
H_s was obtained from the enthalpy of each deposited component.
H_l was approximately evaluated by the following equation:
H_l=n₁H₁+n₂H₂+……
where ni is a number of moles of component i in the brine and H_i, the enthalpy of i th single salt solution at the same molality as the total molality of the brine.
4. Lines of isobaric boiling point: The lines have been derived from the experiments by S. Uchida^{14), 15)}.
From the above calculations and data, Fig. 1 has been obtained. Although not free from faults, this diagram, aided by the pressure table for saturated steam, would meet the purpose of making easy the calculation of heat balance in the concentration process.