education for chemical engineers 6 (2011) e62–e70 Contents lists available at ScienceDirect Education for Chemical Engineers journal homepage: www.elsevier.com/locate/ece XSEOS: An evolving tool for teaching chemical engineering thermodynamics  M. Castier a,*,1 , Mohamed M. Amer a,b a Department of Chemical and Petroleum Engineering, United Arab Emirates University, P.O. Box 17555, United Arab Emirates b Chemical Engineering Department, Alfateh University, Tripoli, Libya abstract XSEOS – excess Gibbs energy models and equations of state – is an Excel ® add-in for computing properties with thermodynamic models often used in chemical process design. The program is free, has open source, and runs on a platform, Excel, commonly available in personal computers. The main targets are undergraduate and graduate courses in chemical engineering thermodynamics whose syllabi include phase and chemical equilibrium calcula- tions, but XSEOS may also be useful for research. The thermodynamic models available in XSEOS allow computing excess or residual properties, and activity or fugacity coefficients of pure components and mixtures (with any num- ber of components). The recent addition of several methods to characterize petroleum fluids and to estimate surface tensions extends the potential application of the package to oil refining and petroleum engineering courses. We also report assessments of XSEOS’s use in assignments and course projects, based on feedback provided by students. © 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Equations of state; Excess properties; Phase equilibrium; Chemical equilibrium; Education 1. Introduction Chemical engineering thermodynamics is a subject that many students find difficult to learn and some instructors find diffi- cult to teach. The first law of thermodynamics, i.e., the energy conservation principle, is easy to grasp. The second law and its far-reaching results are more difficult to understand. Solution thermodynamics and its applications to phase and chemical equilibria also pose great challenges. Within weeks, students need to learn about residual, mixing, infinite dilution, partial molar, and excess properties, as well as fugacities, activities, fugacity coefficients, and activity coefficients. They must mas- ter the conceptual aspects of these properties and how to estimate them with thermodynamic models. Simple meth- ods, such as ideal gas, ideal solution, and chart-based models, often allow quick hand calculations of great instructional value. However, modern applications of solution thermody- namics to chemical process design use equations of state (EOS) and excess Gibbs energy (G E ) models, which are often  A preliminary version of this paper was presented at the VIII Iberoamerican Conference on Phase Equilibria and Fluid Properties for Process Design – EQUIFASE 2009, Praia da Rocha, Portugal. ∗ Corresponding author. Current address: Texas A & M University at Qatar, Doha, Qatar. Tel.: +974 44230534. E-mail address: marcelo.castier@qatar.tamu.edu (M. Castier). Received 31 August 2010; Received in revised form 29 November 2010; Accepted 19 December 2010 1 On leave from the Federal University of Rio de Janeiro, Brazil. too complicated for hand calculations. Many of these models are available in programs routinely used for chemical process design, such as ASPEN and HYSYS, which perform fully auto- mated calculations of bubble and dew points, flashes, among several others. XSEOS (Castier, 2008) – excess Gibbs energy models and equations of state – is an Excel add-in that provides func- tions to calculate thermodynamic properties, such as residual and excess properties and fugacity and activity coefficients. XSEOS occupies a niche between hand and fully automated calculations. The idea is that students use the add-in func- tions to get the values of thermodynamic properties, allowing them to focus on developing procedures for flash calculations, bubble and dew points, and for finding out heat and power needs in process units. The authors’ view is that this is an effective approach in chemical engineering thermodynamics (CET), which exposes students to modern methods and gives them more confidence to solve problems that need numeri- cal methods. Many modern CET textbooks (e.g., Smith et al., 1749-7728/$ – see front matter © 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.ece.2010.12.002