Fluid Phase Equilibria 235 (2005) 50–57 Monte Carlo adiabatic simulation of equilibrium reacting systems: The ammonia synthesis reaction Martin L´ ısal a,b, ∗ , Magdalena Bendov´ a a , William R. Smith c a E. H ´ ala Laboratory of Thermodynamics, Institute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic, Rozvojov´ a 135, 165 02 Prague 6-Suchdol, Czech Republic b Department of Physics, Institute of Science, J. E. Purkynˇ e University, 400 96 ´ Ust ´ i n. Lab., Czech Republic c Faculty of Science, University of Ontario Institute of Technology, 2000 Simcoe St. N., Oshawa, ON, Canada L1H7K4 Received 4 February 2005; received in revised form 20 June 2005; accepted 23 June 2005 Abstract We present an application of the recently developed Monte Carlo method for simulations at fixed total enthalpy [W. R. Smith, M. L´ ısal, Phys. Rev. E 66 (2002) 01114-1–01114-3], combined with the reaction ensemble Monte Carlo method, for the direct prediction of equilibrium reactive adiabatic processes. For the industrially important ammonia synthesis reaction in an adiabatic plug-flow reactor, we perform direct simulations of the equilibrium reaction temperature and the composition of the exit stream as a function of the temperature and pressure of the inlet stream. The chemical species of the system are represented by all-atom potentials with interaction parameters taken from the literature. The accuracy of the molecular model is validated by comparing simulation results with experimental data. We also compare the simulation results with a macroscopic thermodynamic model based on the Soave–Redlich–Kwong equation of state. The simulation results for the reaction conversion show very good agreement with available experimental data over a wide range of temperatures and pressures, whereas the corresponding results from the macroscopic thermodynamic model slightly deteriorate with increasing pressure. Based on these comparisons, the predicted values of the reaction temperature and composition of the exit stream from the simulations are more accurate than the corresponding predicted values from the macroscopic thermodynamic model. © 2005 Elsevier B.V. All rights reserved. Keywords: Adiabatic plug-flow reactor; Ammonia synthesis reaction; Constant enthalpy Monte Carlo simulation; Reaction ensemble Monte Carlo simulation 1. Introduction Calculations of the properties of fluid systems at fixed total internal energy U or at fixed total enthalpy H are important problems of both theoretical and practical interest. The effi- cient computation of such properties by means of molecular- based models is an important area of computational science in physics, chemistry, and chemical engineering. Two examples are (1) adiabatic flash (Joule–Thomson expansion) calcula- tions for non-reacting pure fluids and for mixtures at fixed (H, P ), where P is the pressure, and (2) adiabatic reaction temperature and composition calculations for reacting mix- tures at either fixed (U, V ), where V is the system volume, or at fixed (H, P ). For such problems a main objective is to ∗ Corresponding author. Tel.: +420 220390301; fax: +420 220920661. E-mail address: lisal@icpf.cas.cz (M. L´ ısal). calculate the system (absolute) temperature T and the system composition. Despite its practical importance, molecular-level com- puter simulation studies relating to either the Joule–Thomson expansion or adiabatic reaction temperature are rather scarce overall, and have primarily focused on calculations of the Joule–Thomson coefficient, the Joule–Thomson integral effect and the Joule–Thomson inversion curve [1–11]. Furthermore, the results have all been obtained indirectly by means of calculations of other properties. Recently, we proposed a new direct Monte Carlo (MC) method [12] for property predictions of fluid systems at fixed (H, P ) or (U, V ), given a molecular-level system model. The method is also applicable to chemically reacting systems by combining it with the reaction ensemble MC (REMC) approach [13,14]. This (H, P ) MC method was the first approach capable of directly calculating Joule–Thomson expansion properties, 0378-3812/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.fluid.2005.06.013