(580d) Combined Thermodynamic Model Development and Cape-Open Implementation – an Industry Case Study

The great benefit of having a thermodynamic model available as a CAPE-OPEN plug-in, is that it accelerates the adoption of a new model by practising engineers by eliminating the complex and lengthy step of computational implementation.

The steady increase in the number of CAPE-OPEN compliant thermodynamic modules tells us that the problem of making codes CAPE-OPEN compliant (while difficult) has been solved. Therefore the focus for thermodynamicists must be on good thermodynamic models. Producing a CAPE-OPEN compliant version of a readily available model (such as the Soave-Redlich-Kwong equation of state) does not meet a need.

Ten years of experience of using the Cubic-Plus-Association (CPA) equation of state [1-4] have demonstrated that it is a very successful model for handling multicomponent, multiphase equilibrium in systems containing hydrogen-bonding substances. Our long experience (which in turn means a sizeable parameter set) made CPA an obvious target for CAPE-OPEN development when we began work in this area in 2003. This has been done and the module is currently being used by our industrial collaborators.

Because of the very strong association that occurs in organic acids, systems containing these can be difficult to model. This is especially the case for vapour-liquid-equilibrium in the system acetic acid-water at conditions of industrial interest, as was brought to our attention by an industrial partner. While CPA performs as well (or better) than competing models (such as SAFT) for this system, the level of accuracy is unacceptable for real process modelling and design. Our proposed solution was to implement the classical Huron-Vidal (HV) mixing rules [5] for this binary. Optimization of the relevant parameters gave excellent results at the conditions of interest, resulting in the model CPA-HV.

A great advantage of using the classical HV mixing rules is that the overall model reduces to the reference model (in this case CPA) when binary pairs other than those for which HV parameters are required are considered. This is a tremendous advantage for computational implementation of the model, such as in CAPE-OPEN. Thus, HV parameters are only required for the acetic acid-water pair and no reparameterization is required for other components - the large database of CPA parameters is still valid and can be used without modification. This is traditionally a huge hindrance in the transfer of models from literature to engineering applications ? typically academic researchers may publish a new model with parameters for only a handful of components. Although a model may perform well, it will not necessarily be adopted in practice unless i) implementation is easy and ii) a sufficiently large component parameter set exists.

Once the industrial problem had been identified, work on model development and CAPE-OPEN implementation occurred in close collaboration. The resulting CAPE-OPEN module of the CPA-HV model was ready within 8 months of problem identification. This talk will contain a discussion of the CPA-HV model, as well as issues related to CAPE-OPEN implementation. A live demonstration of the model will be done in ASPEN Plus for complex systems containing (at least) acetic acid and water.

References

[1] G.M. Kontogeorgis, E.C. Voutsas, I.V. Yakoumis, and D.P Tassios, ?An Equation of State for Associating Fluids. Ind. Eng. Chem. Res. 1996, 35, 4310. [2] G.M. Kontogeorgis, I.V. Yakoumis, H. Meijer, E.M. Hendriks, T. Moorwood, ? Multicomponent phase equilibrium calculations for water ? methanol ? alkane mixtures,? Fluid Phase Equilib. 1999, 158 ? 160, 201. [3] G.M. Kontogeorgis, M.L. Michelsen, G.K. Folas, S.O. Derawi, N. von Solms and E.H. Stenby, ?Ten years with the CPA (Cubic-Plus-Association) equation of state. Part I. Pure compounds and Self-associating systems,? accepted Ind. Eng. Chem. Res. (2006). [4] G.M. Kontogeorgis, M.L. Michelsen, G.K. Folas, S.O. Derawi, N. von Solms and E.H. Stenby, ?Ten years with the CPA (Cubic-Plus-Association) equation of state. Part II. Cross-associating and multicomponent systems,? accepted Ind. Eng. Chem. Res. (2006). [5] M.-J. Huron, and J. Vidal, ?New Mixing Rules in Simple Equations of State for Representing Vapour-Liquid Equilibria of Strongly Non-ideal Mixtures,? Fluid Phase Equilib. 1979, 3, 255.