artículo thermodynamics of fluid-phase equilibria for standard chemical engineering operations.pdf

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  • ANNIVERSARY ARTICLE

    Thermodynamics of Fluid-Phase Equilibria forStandard Chemical Engineering Operations

    John M. PrausnitzDept of Chemical Engineering, University of California, Berkeley, CA 94720

    andChemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720

    Frederico W. TavaresDept of Chemical Engineering, University of California, Berkeley, CA 94720

    andEscola de Qumica, Universidade Federal do Rio de Janeiro, Caixa Postal 68542, CEP, 21949900, Rio de Janeiro, RJ, Brazil

    DOI 10.1002/aic.10069Published online in Wiley InterScience (www.interscience.wiley.com).

    Thermodynamics provides one of the scientic cornerstones of chemical engineering.This review considers how thermodynamics is and has been used to provide phaseequilibria as required for design of standard chemical engineering processes withemphasis on distillation and other conventional separation operations. While this reviewdoes not consider modern thermodynamics for high-tech applications, attention isgiven to 50 years of progress in developing excess-Gibbs-energy models and engineering-oriented equations of state; these developments indicate rising use of molecular physicsand statistical mechanics whose application for chemical process design is made possibleby increasingly powerful computers. As yet, results from molecular simulations have nothad a major inuence on thermodynamics for conventional chemical engineering; how-ever, it is likely that molecular simulation methods will become increasingly useful,especially when supported by quantum-mechanical calculations for describing intermo-lecular forces in complex systems. 2004 American Institute of Chemical Engineers AIChE J,50: 739761, 2004Keywords: uid-phase equilibria, chemical thermodynamics, equations of state

    Introduction

    Separation of uid mixtures is one of the cornerstones ofchemical engineering. For rational design of a typical separa-tion process (for example, distillation), we require thermody-namic properties of mixtures; in particular, for a system thathas two or more phases at some temperature and pressure, werequire the equilibrium concentrations of all components in allphases. Thermodynamics provides a tool for meeting that re-quirement.

    For many chemical products (especially commodity chemi-cals), the cost of separation makes a signicant contribution tothe total cost of production. Therefore, there is a strong eco-nomic incentive to perform separations with optimum ef-ciency. Thermodynamics can contribute toward that optimiza-tion. In this article, we indicate some of the highlights ofprogress in the thermodynamics of phase equilibria since theAIChE Journal began about 50 years ago.

    Although it is clear that, in addition to phase equilibria,thermodynamics also provides useful caloric information (en-thalpy balances) that affects separation operations, we do notconsider that here. Furthermore, we recognize that in recentyears, thermodynamic research has given rising attention toproduct (as opposed to process) design. There is good reason to

    Correspondence concerning this article should be addressed to J. M. Prausnitz atprausnit@cchem.berkeley.edu

    2004 American Institute of Chemical Engineers

    THERMODYNAMICS

    AIChE Journal 739April 2004 Vol. 50, No. 4

  • believe that modern chemical engineering will be increas-ingly concerned not with large-scale production of classical(commodity) chemicals, but with new (specialty) materials forapplication in biotechnology, medicine, pharmaceuticals, elec-tronics, and optics, as well as in the food and personal-careindustries. Thermodynamics can contribute to the developmentof these new areas and, indeed, much progress toward thatend has been reported in the ever-growing literature. In ourshort review here, it is not possible to give adequate attentionto new thermodynamics. This review is essentially connedto progress in thermodynamics for what is often called tradi-tional chemical engineering.

    Furthermore, during the last 50 years, the literature on ap-plied thermodynamics has grown to vast proportions, evenwhen we limit our attention to phase equilibria. Therefore, thisreview cannot be comprehensive. Indeed, it is unavoidablyselective, reecting the authors (right or wrong) opinions andpreferences. Space and time are nite, and some subconsciousprejudice is always with us. The authors, therefore, apologizeto their colleagues whose work is not sufciently mentionedhere or worse, not mentioned at all. Because the editor hasnecessarily limited the total number of pages available to us,we must, with regret, omit much that may merit inclusion.

    Fifty years ago, most chemical engineering thermodynamicswas based on representation of experimental data in charts,tables, and correlating equations that had little, if any, theoret-ical basis. Fifty years ago, most chemical engineering thermo-dynamics was in what we may call an empirical stage. How-ever, the word empirical has several interpretations. Whenwe represent experimental data by a table or diagram (forexample, the steam tables or a Mollier diagram), we call suchrepresentation empirical. When we t experimental ideal-gasheat capacities to, say, a quadratic function of temperature,wechoose that algebraic function only because it is convenient todo so. However, when, for example, we t vaporpressure dataas a function of temperature, we inevitably do so by expressingthe logarithm of the vapor pressure as a function of the recip-rocal absolute temperature. This expression is also empiricalbut in this case our choice of dependent and independentvariables follows from a theoretical basis, viz. the ClausiusClapeyron equation. Similarly, when for a binary mixture, werepresent vapor-liquid equilibrium data with, say, the Margulesequation for activity coefcients, we also call that representa-tion empirical, although it has a theoretical foundation, viz. theGibbsDuhem equation. We should distinguish betweenblind empiricism, where we t experimental data to a totallyarbitrary mathematical function, and thermodynamically-grounded empiricism where data are expressed in terms of amathematical function suggested by classical thermodynamics.

    If now, in addition to thermodynamics, we introduce into ourmethod of representation some more-or-less crude picture ofmolecular properties, for example, the van der Waals equationof state, we are still in some sense empirical but now we are inanother realm of representation that we may call phenomeno-logical thermodynamics. The advantage of proceeding fromblind to thermodynamically-grounded to phenomenolog-ical is not only economy in the number of adjustable param-eters but also in rising ability to interpolate and (cautiously)extrapolate limited experimental data to new conditions, whereexperimental data are unavailable. Because phenomenologi-cal thermodynamics uses molecular concepts, an alternate

    designation is to say molecular thermodynamics. The moststriking engineering-oriented examples of molecular thermo-dynamics are provided by numerous useful correlations, basedon the theorem of corresponding states or on the concept ofgroup contributions.

    In contrast to what we (somewhat carelessly) call empiri-cal thermodynamics, we also have modern theoretical ther-modynamics that uses statistical mechanics and molecular sim-ulations. In this theoretical area, we relate macroscopicthermodynamic properties to microscopic characteristics in amore-or-less rigorous manner.

    Although thermodynamically-grounded thermodynamicswas well known fty years ago, and whereas some correlationsbased on corresponding states or group contributions have along history, the last 50 years have provided signicantprogress in phenomenological or molecular thermodynam-ics with numerous applications in chemical process design.During the last 25 years, there has also been much progress instatistical thermodynamics and molecular simulation. How-ever, regrettably, with few exceptions, that progress has not yetseen signicant application in engineering design of uid-phase chemical processes.

    For engineering application, applied thermodynamics is pri-marily a tool for stretching experimental data: given somedata for limited conditions, thermodynamics provides proce-dures for generating data at other conditions. However, ther-modynamics is not magic. Without some experimental infor-mation, it cannot do anything useful. Therefore, for progress inapplied thermodynamics, the role of experiment is essential:there is a pervasive need for ever more experimental results.Anyone who does thermodynamics is much indebted to thosewho work in laboratories to obtain thermodynamic properties.It is impossible here to mention even a small fraction of thevast body of new experimental results obtained over a period offty years. However, it is necessary here to thank the hundredsof experimentalists who have provided essential contributionsto progress in chemical thermodynamics. Particular recognitionmust go to the laboratories of Grant Wilson (Provo, Utah) andD. Richon (Fontainebleau, France) who have pioneered inobtaining experimental results for difcult industrial sys-tems.

    A useful compilation of experimental phase-equilibrium datais provided by the multivolume DECHEMA series (Behrensand Eckermann, 19801997).

    Thermodynamic Properties of Pure Fluids

    For common uids (for example, water, ammonia, lighthydrocarbons, carbon dioxide, sulfur dioxide, and some freons)we have detailed thermodynamic data conveniently compiledin tables and charts; the outstanding example of such compi-lations is the steam table with periodic improvements andextensions (Harvey et al., 1998; Harvey and Parry, 1999).

    Although thermodynamic data for less common uids areoften sketchy, a substantial variety of thermodynamic proper-ties for normal uids can be estimated from

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