Ingeniero Quimico

Fluid Phase Equilibria 233 (2005) 28–33

Paraffin crystallization in synthetic mixtures: Predictive local composition models revisited
Jo˜ o A.P. Coutinho a,∗ , Carla Goncalves a , Isabel M. Marrucho a , a ¸ Jerome Pauly b , Jean-Luc Daridon b
b a CICECO, Departamento de Qu´mica, Universidade de Aveiro, 3810-193 Aveiro, Portugal ı Laboratoire Haute Pression, Centre Universitaire de RechercheScientifique, Universit´ de Pau, Avenue de l’Universit´ , BP 1155, 64013 Pau, France e e

Received 4 January 2005; received in revised form 12 April 2005; accepted 12 April 2005 Available online 17 May 2005

Abstract Aiming at identify the shortcomings of the available models for the description of the solid phase non-ideality, new solid liquid equilibrium data for multicomponent hydrocarbonsystems was measured. The Predictive UNIQUAC and Wilson models for the description of the solid phase non-ideality, previously proposed, are here compared against this new data. The weaknesses displayed by these models on the predictive description of the measured phase behavior helps to identify their limitations and this is expected to lead to a new improved model for the solid phase non-ideality.Following this approach a new UNIQUAC based model is proposed that seems to overcome the limitations of the models identified on this work. © 2005 Elsevier B.V. All rights reserved.
Keywords: Paraffin crystallization; Wax formation; Modelling; Solid–liquid equilibria; Solid phase nonideality

1. Introduction The development of predictive local composition models for the description of waxformation in hydrocarbon fluids [1–19] lead to a highly successful tool for the description of wax formation in diesels and fuels [7–10,15,16] and dead and live oils [11,13,14,17,19]. Yet the time and experience of various users brought to light some limitations and some behaviors not always well understood. With the acquired experience it is now possible to revisit these models and try to identify andbetter understand the models’ limitations and develop strategies to overcome their problems. For this purpose new data was collected, and modeled to shed new light over the solid phase models, since only on well-defined systems with good quality data can the model deficiencies be pinpointed for future correction. This approach was started in a previous paper recently published for systems with threeparaffins in a solvent [20] and is here extended to somewhat
∗

Corresponding author. Tel.: +351 234401507; fax: +351 234370084. E-mail address: jcoutinho@dq.ua.pt (J.A.P. Coutinho).

more complex systems of five paraffins in a solvent. The new data were expected to bridge the gap between the very simple systems with one or two paraffins in a solvent [2,3] and complex systems with 10 or moreparaffins and complex distributions previously reported by us [3,6,21]. One of the anomalous situations previously identified was that although Predictive modified UNIQUAC was much better than Wilson for complex mixtures [4–6], for simple mixtures Wilson provided better predictions. This odd behavior still holds for the mixtures previously studied [20] and, as will be shown, for the mixtures reported onthis paper. Another important limitation relates to the overestimation of the crystallization of light compounds in mixtures. This is particularly clear in this paper for mixture B where octadecane precipitation just below the cloud point is predicted by both models. Although this was expected from Wilson that cannot predict phase split, and thus all the compounds must be present in the solid phaseformed, it was unexpected from the modified UNIQUAC. A new version of the UNIQUAC model for the description of the solid phase non-ideality, where the Z, r and q

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J.A.P. Coutinho et al. / Fluid Phase Equilibria 233 (2005) 28–33

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parameters are those of original UNIQUAC is here...