Viscosity modelling of alcohols using the CPA EoS and the friction theory


W. Yan1, C. Langlais2 and E. Stenby1

1Technical University of Denmark, Denmark
2University of Pau, France

Keywords: EoS, modelling, friction theory
property: viscosity
material: alcohol

Alcohols, especially methanol, are widely used in various industry processes, such as chemical industry, oil and gas production, and medical and pharmaceutical related applications. Accurate modelling of their phase equilibrium and thermophysical properties is important to the design of the relevant processes. Those components are difficult to model due to their associating nature. For phase equilibrium modelling, the traditional cubic equations of state (EoS) can be modified empirically by introducing special temperature dependency functions and advanced mixing rules. A more rigorous way to describe those associating compounds is to use EoS that explicitly accounts for association, such as the Cubic-Plus-Association EoS [1] which incorporates the Wertheim association term. For viscosity modelling, in contrast to the abundant studies on hydrocarbon systems over wide temperature and pressure ranges, the viscosity models for alcohols are relatively rare. One of the EoS based semi-empirical methods for viscosity modelling is the friction viscosity theory (f-theory) [2], which utilizes the repulsive pressure and the attractive pressure in a specific EoS to calculate fluid viscosity. The method has been applied to a variety of systems. Coupled with the Soave-Redlich-Kwong (SRK) EoS and the Peng-Robinson (PR) EoS, the f-theory has also been applied to alcohols [3]. In this work, the f-theory viscosity model is coupled with the CPA EoS to model the viscosities of pure alcohols, with emphasis on methanol and ethanol. The results are compared with those from the SRK f-theory model and the PR f-theory model, both refitted with an extended viscosity database for pure alcohols. For methanol, comparison is also made with a reference correlation developed by Xiang et al. [4]. This study shows that as a methodology for developing EoS based viscosity models, the f-theory can be successfully applied to advanced EoS such as CPA. The developed viscosity model is therefore consistent with the model used for phase equilibrium calculation. The CPA f-theory model is a little more accurate than the SRK f-theory model and the PR f-theory model. The study also reveals that there is still a need for more viscosity data if the developed viscosity models are to be used for a wide pressure range.

References
  1. G.M. Kontogeorgis, I.V. Yakoumis, H. Meijer, E. Hendriks, T. Moorwood, Fluid Phase Equilibria 158-160, 201 (1999)

  2. S.E. Quiñones-Cisneros, C.K. Zéberg-Mikkelsen, E.H. Stenby, Fluid Phase Equilibria 169, 249 (2000)

  3. C.K. Zéberg-Mikkelsen, S.E. Quiñones-Cisneros, E.H. Stenby, Fluid Phase Equilibria 194-197, 1191 (2002)

  4. H.W. Xiang, A. Laesecke, M.L. Huberb, J. Phys. Chem. Ref. Data 35, 1597 (2006)

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