Chemical Reaction Modeling with ThermoFluid/MF and MultiFlash

The free Modelica library THERMOFLUID (see [2] and [11]) was developed for simulation of thermohydraulic applications, both for single-species applications like the water-steam cycle in a thermal power plant and for multi-species applications with gas mixtures. It has demonstrated its flexibility for modeling thermodynamic and process applications in a variety of industrial and academic projects, see [10], [3] and [7]. This article describes how support for chemical reactions and membrane diffusion has been added to THERMOFLUID, thus expanding the area of possible applications to include reacting flows, chemical batch reactors, catalytic converters, etc. Another crucial part of the modeling work has to be spent on getting physical property data of sufficient accuracy and with acceptable computational complexity for engineering purposes into the model. This has been adressed in the development of a commercial interface to the industry-standard physical property package MultiFlash. The new Modelica library THERMOFLUID/MF provides the modeler with two toolboxes. Firstly, a low-level Modelica function interface to MultiFlash. MultiFlash consists of a core of physical property calculation routines and a basic database of the most comman chemical components and a number of add-on property databases. The interface gives access to multi-component, multi-phase property calculations including gas, several liquid and condensed phases, wax formations and hydrates. Secondly, a high-level Modelica model library which is fully integrated with the THERMOFLUID library and implements robust and efficient dynamical models for the most common process engineering equipment. In addition, reliable crossing functions for detecting phase boundaries in multi-phase, multi-component mixtures have been implemented for the first time in a highlevel modeling language. The crossing functions make it possible to simulate processes correctly even at offdesign operating points and under start-up conditions. A flash volume may in such cases be filled with only liquid or only gas. Crossing functions for phase transitions ensure high performance simulation even in these cases. 1 Flexible handling of chemical reactions In standard chemical textbooks, reactions are treated as source terms in component concentration balances: dci dt c i c out i ri (1) where ri are the component reaction rates, given by a kinetic expression. In a more general way, we can include the reaction terms in the component mass balance and total energy balance dMi dt ṁ i ṁ out i rZi MWi (2)