The IAPWS industrial formulation for the thermodynamic properties of seawater

Abstract

In 2008, the International Association for the Properties of Water and Steam (IAPWS) adopted a standard formulation for the thermodynamic properties of seawater as a sum of contributions to the Gibbs free energy from pure water and from dissolved sea salt. For pure water, the IAPWS formulation for general and scientific use (IAPWS-95) was used. However, for industrial uses such as desalination and seawater power-plant cooling, it is likely to be more convenient to use the computationally simpler IAPWS formulation for industrial use (IAPWS-IF97), which is standard in the steam power industry. This paper documents this approach and gives formulas for calculating thermodynamic properties of seawater and steam (volume, enthalpy, isobaric heat capacity, etc.). The calculation of colligative properties (such as boiling and freezing points and osmotic pressure) is also described, as is the calculation of properties of two-phase states such as brine-vapor and brine-ice (sea ice). The computing speeds for these calculations are faster than those using IAPWS-95 by factors on the order of 100–200. The use of IAPWS-IF97 instead of IAPWS-95 for industrial seawater calculations is endorsed in IAPWS Advisory Note No. 5: Industrial Calculation of the Thermodynamic Properties of Seawater. This use is valid for IAPSO Standard Seawater with sea salt of the Reference Composition in specified regions inside the pressure, temperature, and salinity ranges of 0.3 kPa ≤ p ≤ 100 MPa, 261 K ≤ T ≤ 353 K, and 0 ≤ S ≤ 0.12 kg kg⁻¹.

Keywords:

• Seawater
• Thermodynamics
• Industrial use
• Gibbs function
• Brine ice
• Brine vapor

Acknowledgment

The authors express their gratitude to the reviewers whose comprehensive comments improved the quality of this paper.

Notes

¹ Certain commercial products are identified in this paper, but only to adequately specify the procedure. Such identification neither constitutes nor implies recommendation or endorsement by either the US government or the National Institute of Standards and Technology.