Wax formation from paraffinic mixtures: A simplified thermodynamic model based on sensitivity analysis together with a new modified predictive UNIQUAC Ehsan Ghanaei, Feridun Esmaeilzadeh ⇑ , Jamshid Fathikalajahi School of Chemical and Petroleum Engineering, Shiraz University, Shiraz, Iran article info Article history: Received 4 January 2012 Received in revised form 3 March 2012 Accepted 28 March 2012 Available online 9 April 2012 Keywords: Wax Thermodynamic model Heat capacity UNIQUAC Sensitivity analysis abstract In petroleum reservoir fluids, wax is mainly formed from n-alkanes (normal alkanes) at specific thermo- dynamic conditions. Through the wax formation phenomenon, in addition to the solid–liquid phase tran- sition, some n-alkanes undergo an order–disorder solid–solid phase transition. These phase transitions appear in the SL (Solid–Liquid) fugacity ratio of pure component in the forms of phase change enthalpy and heat capacity. In this work, a new predictive approach with the use of new generalized heat capacity correlations for liquid, disorder and order solid phases has been presented to calculate the SL fugacity ratio. These correlations have been obtained and evaluated by using 1912 data points of pure n-alkanes according to the group contribution method. Additionally, the effective parameters in the SL fugacity ratio calculation have been obtained by an exhaustive sensitivity analysis. The results show the impor- tant contributing parameters in the SL fugacity ratio are related to the phase changes’ enthalpies. Finally, the simplified model with the new modified P-UNIQUAC (Predictive UNIQUAC) has been evaluated by the experimental data of wax formation with an AAD% (Average Absolute Deviation percent) of 2.81%. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Wax formation is a difficulty in petroleum industries. Signifi- cant efforts have been devoted by the researchers to investigate wax formation from theoretical and experimental aspects [1–23]. This research is intended to investigate the thermodynamic model- ing of wax precipitation by the sensitivity analysis in order to sim- plify the model. Then, the simplified model is combined with a new modified version of P-UNIQUAC. For the thermodynamic modeling of wax formation, it is neces- sary to relate the fugacity of pure component in solid phase to its fugacity in liquid phase as denoted SL fugacity ratio in this work. In petroleum reservoir fluids, the most components encountered are n-alkanes which can form solid wax. Some n-alkanes undergo an order–disorder solid–solid phase transition. This phase transi- tion affects the SL fugacity ratio in the forms of phase change en- thalpy and heat capacity terms. It seems that, in the existing thermodynamic models, owing to the lack of a correlation for the prediction of heat capacity in the disorder solid phase, the effect of heat capacity terms related to the disorder solid phase has not been investigated. Also, many of thermodynamic models of wax formation [1–16] are based on a model of SL fugacity ratio in which a correlation of heat capacity difference between solid and liquid phases is used without consideration of the difference between disorder and order solid phases. In addition, some of these models [1–14] use the correlation of heat capacity difference obtained by the wax formation data instead of thermophysical property of pure component [1]. Therefore, in this work, three generalized correla- tions based on the group contribution method have been presented to estimate the heat capacity of n-alkanes in liquid, disorder and order solid phases. These correlations have been used to present a new predictive approach to obtain the SL fugacity ratio. Consid- ering all the terms of SL fugacity ratio related to phase transitions, in condition that the predictive characteristic is preserved, gives the ability to investigate its important contributing parameters with the use of exhaustive sensitivity analysis. The sensitivity anal- ysis shows the effective parameters are related to enthalpies of phase changes. In addition, to describe the non-ideality of solid wax phase, a new modified version of P-UNIQUAC has been presented. This modification is related to the new correlation of interaction energy. According to the sensitivity analysis and new P-UNIQUAC, a new thermodynamic model is created. The proposed model has been evaluated by 117 experimental data points of WDT (Wax Disap- pearance Temperature) or WAT (Wax Appearance Temperature) and the amount of precipitated wax as a function of temperature. 0016-2361/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2012.03.053 ⇑ Corresponding author. Address: School of Chemical and Petroleum Engineering, Shiraz University, P.O. Box 71936-16511, Shiraz, Iran. Tel.: +98 711 2303071; fax: +98 711 6474619. E-mail addresses: ghanaei@shirazu.ac.ir (E. Ghanaei), esmaeil@shirazu.ac.ir (F. Esmaeilzadeh), fathi@shirazu.ac.ir (J. Fathikalajahi). Fuel 99 (2012) 235–244 Contents lists available at SciVerse ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel