14. Equation of States and Ammonia Decomposition in Ammonothermal Systems

Thermodynamic modeling of the ammonia decomposition under ammonothermal conditions (T > 600 K, P > 100 MPa) is presented given recent advances in developing a new, simple equation of state (EOS) describing ammonia, hydrogen, nitrogen and their mixtures under these conditions. The simplified EOS is based on the traditional form of the Beattie-Bridgeman (BB) EOS and expands relevant parameters to second order in density. As a consequence, a seven-parameter EOS is presented and compared to experimental data for ammonia up to 250 MPa and 810 K. Experimental data for ammonia, hydrogen and nitrogen are simulated using the reference multiparameter EOS for each individual gas. Gas mixtures are formed by applying mixing rules with separated contributions for polar and non-polar interactions. The accuracy of the expanded BB EOS is suggested to be 1–2% in pressure for temperatures greater than 700 K for ammonia, hydrogen, nitrogen and NH₃–N₂–H₂ mixtures. Additionally, a general thermodynamic expression for the equilibrium constant is presented and applied to the ammonia decomposition reaction by using non-ideal mixing contributions from the second virial coefficient using the expanded BB EOS. Comparison with experimental data (P <210 MPa, T <810 K) suggests an accuracy of ~2% in pressure.