*3.3. Heat Transfer Fluid Selection*

The heat transfer fluid used on the shell side should be able to rapidly remove energy from the process without the requirement of impractically high flowrates. Molten salt heat transfer fluids were chosen due to their practicality for high temperature applications. Apart from being cost effective and nontoxic, molten salt has superior heat transport properties such as high heat capacities and high heat transfer coefficients such that the equipment size and surface areas for heat transfer can be held to a minimum. In addition, maintenance and power costs for circulating the molten salt are correspondingly low. Commercial HTS1 (40% KNO3 and 60% NaNO3) and HTS2 (53% KNO3, 40% NaNO2 and 7% NaNO3) molten heat transfer salts were selected for their suitable operating temperature ranges, i.e., 422–853 K and 563–823 K, respectively [12]. Molten salt physical properties, if modeled as a concentration of nitrate and nitrite salts in water, are grossly underestimated on Aspen Plus® (AspenTech, Bedford, MA, USA) and therefore do not accurately represent the heat transfer capability of molten salt. Therefore, a molten salt component was built into Aspen Plus® (AspenTech, Bedford, MA, USA) with the respective physical properties manually input. The physical properties were estimated as per correlations in literature [13].
