(612b) Group Contribution Methods In Undergraduate Chemical Engineering Thermodynamics

Thermodynamics at the undergraduate level in a chemical engineering curriculum traditionally covers topics ranging from the first law of thermodynamics, the second law of thermodynamics, properties of pure components and mixtures as well as phase equilibria and chemical equilibria.  The challenge is often to provide the necessary depth in these important areas while still preparing the student for the activities/endeavors that require thermodynamics in subsequent design courses or in the workplace.  Thus, design of out-of-class assignments that allow the student to explore some of the concepts not covered in depth in the classroom is a mechanism that allows newer topics relevant to chemical engineering practice/design to be included in the course, yet not at the expense of essential topics in the curriculum.

One concept/topic that often is not covered in depth in the undergraduate chemical engineering thermodynamics sequence is the use of group contribution methods for estimation of physical properties and for estimation of vapor-liquid equilibria.  For many compounds, necessary physical property data (critical properties, acentric factor, vapor pressure, heat capacity, phase change enthalpies) may be readily available in textbooks [1-4] or in data compilations [5-7].  However, upon graduation, chemical engineers may develop and design new chemical processes and/or products involving compounds for which these data are not available.  Experimental measurement of these types of data is expensive, requiring precise and specialized equipment, and often is undertaken only after a process is sufficiently developed to indicate a favorable economic return on investment.  At MSU, Chemcad (Chemstations, Inc.) is used as the process simulation software package.  However, for compounds not included in its database, the user must provide a minimum set of data, including critical temperature, critical pressure and acentric factor among others.  An assignment employing the student version of Cranium [8] was developed to allow students to explore group contributions methods with respect to estimation of these properties for organic chemicals, including isomers.  Exposure to group contribution methods for physical property estimation at the undergraduate level is beneficial.  Not only does the student become conversant in the use of these methods, they also are made aware of the limitations inherent in certain methods through use of properly designed assignments.

The expanded use of process simulation software in the senior design courses also warrants inclusion of group contribution methods at an earlier point in the undergraduate chemical engineering curriculum.  Chemcad is also equipped with a ‘Thermodynamics Wizard’; the program will identify an appropriate model for estimation of K-values (phase equilibria) based on the specified components and upper/lower bounds on temperature and pressure provided by the user.  When experimental data for a given chemical system are lacking, ‘UNIFAC’ may be identified as the appropriate model for phase equilibria estimation.  A project focused on use of UNIFAC for estimation of activity coefficients for a binary system taken from the literature allowed students to gain an appreciation for this method and for its implementation in Chemcad.

Incorporation of group contribution models into undergraduate thermodynamics instruction for both pure component properties as well as phase equilibria is beneficial.  Such inclusion results in students having the necessary background in estimation techniques, allowing them to readily employ these tools in future endeavors.

References

[1] Sandler, Stanley I. Chemical, Biochemical and Engineering Thermodynamics, (4th ed., Wiley & Sons: 2006).

[2] Smith, J.M.; Van Ness, .C.; Abbott, M.M. Introduction to Chemical Engineering Thermodynamics. (7th edition, McGraw-Hill:  2005).

[3] Elliott, J.R.; Lira, C.T. Introductory Chemical Engineering Thermodynamics, (Prentice Hall:  1999).

[4] Koretsky,M. D. Engineering and Chemical Thermodynamics (Wiley  & Sons: 2003).

[5] Poling, B.E.; Prausnitz, J.M.; O'Connell, J.P. The Properties of Gases and Liquids. (5th ed., McGraw-Hill Professional:  2000).

[6] Green, D.; Perry, R. Perry's Chemical Engineers Handbook. (8th ed., McGraw-Hill:  2008).

[7] Yaws, C. Chemical Properties Handbook:  Physical, Thermodynamics, Engironmental Transport, Safety & Health Related Properties fo Organic & Inorganic Chemicals. (McGraw Hill:  1999).

[8] Molecular Knowledge Systems, Inc., CRANIUM - Property Estimation Software, <http://www.molknow.com/Cranium/cranium.htm>.