Revisiting the concept of extents for chemical reaction systems using an enthalpy balance

For the investigation of complex reaction systems, it has been proposed to decouple the various rate processes using a linear time-invariant transformation that is constructed from knowledge of stoichiometry, reaction enthalpies, inlet compositions and temperatures, and initial conditions, that is, without any kinetic information. The resulting transformed system is expressed in terms of vessel extents. The transformation uses a heat balance that is written in terms of reaction enthalpies evaluated at some reference temperature. This paper revisits the transformation to vessel extents using an enthalpy balance instead of the heat balance. In this transformation, the reaction enthalpies are not used explicitly. However, it is shown that, provided a not very restrictive assumption holds, the enthalpy balance equation is equivalent to the heat balance equation written with constant reaction enthalpies at some reference temperature. The two transformations to vessel extents are compared, first formally and then numerically via a simulated CSTR example. (C) 2019 Elsevier Ltd. All rights reserved.