Multicomponent Mass Transfer

Book Reviews/The Chemical Engmeenng Journal 60 (1995) 173480

This 1s followed by a quahtatlve dlscusslon of when to use chemical solvents, on the thermal effects found m absorption and desorptlon, and on the differences between simulations for design and for operating purposes Packed columns have their own chapter with bnef dlscusslons of the different types of packmg, of liquid maldlstrlbutlon, liquid hold-up, pressure drop relations and relations for the different mass transfer parameters This part 1s very empmcal, but the authors have made choices out of the many available correlations It may be a useful quick reference This chapter contains adlscusslon on the shooting method for solvmg countercurrent multlcomponent problems This 1s a method which, unfortunately, seldom converges because it 1s so sensitive to mltlal estimates The chapter on plate columns 1s slmdar, with discussions on flooding, weeping and operating regimes, mlxmg m the hqmd and gas phase and a small collection of empirical equations for all mass transfer parameters It also contains a fairly extensive derivation of the Kremser equation and a discussion of the matrix method for solving eqmhbrmm stage problems Unfortunately, I found the dlscusslon of the most Important problem rather superficial That 1s on how to include multlcomponent mass transfer and chemical reactions m computer calculations The book ends with a short (again mamly emplrIcal) chapter on bubble columns I found this book unbalanced The first part 1s a good starting point for an advanced course on multlcomponent mass transfer, but the rest 1s introductory and very much m the conventional fragmented style of treating separation processes One strong point 1s the large number of worked examples Teachers may find the the book a treasury for assignments

Multtcomponent Mass Transfer by Ross Taylor and R Knshna, John Wdey and Sons, 1993, ISBN O471-57417-1, pnce f58 00, xxxlv+579 pp

This book also consists of three, roughly equal, parts The first covers the theory of multicomponent, molecular dlffuslon The second deals with the description of multlcomponent mass transfer m the thm layers or ‘films’ next to phase boundaries The third part deals with multlcomponent noneqmhbnum design methods for dlstdlatlon, absorption and condensation equipment Part I of the book covers ‘Molecular Diffusion’ The weakest chapter of the book 1s the first ‘Prehmmary Concepts’ It contams a dour and dry summary of the different forms of the balance equations for multlcomponent mass transport Run through It quickly, the book really begins with ‘The Maxwell-Stefan Relations’ This starts with a qualitative, but mstructlve derlvatlon of the multicomponent diffusion relations from an ideal gas mixture, then extends these by analogy to hqmd mixtures It ends with extensions of the MaxwellStefan (MS) equations to include dnvmg forces such as

177

centrifugal and electrical gradients ‘Flck’s Law’ introduces an alternative descrlptlon of multicomponent mass transfer It also describes a few aspects of the behavlour of dlffuslvltles m non-ideal solutions This subject is pursued further m ‘Estlmatlon of Diffusion Coefficients’ This shows how MS-dlffuslvltles behave much more simply than Flck dlffuslvltles, especially m non-ideal and m multicomponent mixtures It brlefly describes how dlffuavltles can be estimated m dilute solutions These values are then used wtth mterpolatlon to estimate MS-dlffuslvltles m concentrated mixtures, as from these the Flck dlffuslvltles We now know the transport equations and have values of the dlffuslvltles So we progress to ‘The Linearized Theory’, which deals with various methods for solving the multlcomponent Flck equations Here we also see some ‘strange’ effects that may be expected m multlcomponent diffusion problems One of these 1s ‘reverse dlffuslon’, where a component diffuses against its concentration gradient Surpnsmgly, the elegant explanation that the MSequations provide for these phenomena 1s hardly discussed The last section of this part, ‘Effective Dlffuslvlty Models’, shows that these methods are not good These methods, which have been popular, consider the mixture to behave as a pseudo binary They often give acceptable results, but unfortunately it 1s difficult to see when and where they will fall Part II discusses ‘Interphase Mass Transfer’ In most parts of engineering equipment, convection dominates transport However, m thm layers near phase boundaries, much of the flow normal to the boundary dies out There, diffusion becomes the dominant transport mechanism This part of the book analyses multlcomponent mass transfer m such boundary layers or ‘films’ Chapter 7 defines Flck mass transfer coefficients m multicomponent systems, showing how their behavlour parallels that of the Flck dlffuslvltles It then dlscusses the problem that diffusion equations only describe relative motion wlthm a mixture, and that an extra ‘bootstrap’ relation 1s required to determine the problem Such relations can be obtained from the special behavlour of one component (for example if the component cannot traverse the phase boundary because it IS msoluble) They can also be obtamed from mass or energy balances, or from the stolchlometry of chemical reactions ‘Film Theory’ 1s one of the longest chapters of the book It begins with the binary form of the theory including flux corrections This 1s worked out for several different cases which are important m dlstdlatlon, absorption, condensation and m chemical reactors The multlcomponent equations are ‘Just’ matnx generahsations of the binary forms Different schemes for solving these equations are dlscussed, mcludmg lmearlsatlon and effective dlffuslvlty approxlmatlons Then the effects of thermodynamic non-ldeahties are taken into account The chapter finishes on making estimations of multicomponent mass transfer coefficients from dilute binary data (We usually do not have any better data available > This chapter parallels the developments on molecular diffusion m Part I ‘Unsteady State Mass Transfer Models’ summarlses the various binary surface renewal models and show which extensions have been made so far to

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Book Renews/The

Chemcal Engrneenng Journal 60 (1995) 173-180

multlcomponent systems The chapter ends with multlcomponent versions of the penetration model for spheres and cylinders, these are rough models for mass transfer to bubbles and drops and to Jets ‘Mass Transfer m Turbulent Flow’, contams a descnptlon of turbulence models of boundary layers It shows how molecular and turbulent dlffuslvltles are combined m binary models and extended to multicomponent mixtures with analogous matnx equations The chapter also contains a dlscusslon of the estlmatlon of mass transfer coefficients flmd/sohd boundaries ‘Simultaneous Mass and Energy Transfer’ covers a partial form of the transport equations for energy, which includes conduction and convection The diffusion equation for non-isothermal diffusion 1s presented, but otherwise not used m the text For different mass transfer models, the influence of heat and mass transfer on each other, are then worked out These effects are pnmarily convective conmbutlons of mass transfer to heat transfer and thermal effects on phase equlhbrla Part III, ‘Design’, takes up almost 40 percent of the book It deals with the non-eqmhbrmm or ‘rate based’ multlcomponent models of the classical chemical engmeermg operations of dlstlllatlon, absorption and condensation This part of the book brings the reader to the state of the art m this subject Chapter 12 deals with multicomponent dlstlllatlon in tray and packed columns It starts with binary models, dlscussing conventional concepts such as overall mass transfer coefficients and numbers of mass transfer stages, which lead to efficiency models This part ends with a model of a tray which describes both the bubble and the spray regimes, with then transition (I find the mathematics a little detailed, compared with the rough way that two phase flow 1s modelled Also, the model parameters cannot be easily predicted ) The extension of the model equations to multlcomponent systems 1s then worked out This mvolves a lot of matrix algebra The chapter ends with a parallel development of design equations for packed columns ‘Efficiency Models’ deals with the common method of designing dlstlllatlon columns using eqmhbrmm stages, corrected with an ‘efficiency’ In binary mixtures such a Murphree efficiency 1s well behaved It often has a value of around 0 7 for both components and varies little over the length of the column No such simple behavlour 1s observed m multicomponent dlstdlatlon The chapter contams many examples, showing how efficiencies vary wildly, even m dlstlllatlon of ideal mixtures They can be anywhere between plus and minus infinity 1 The ‘efficiency’ m multlcomponent systems 1s shown to be a confusing concept, which 1s probably best avoided altogether ‘A Non-eqmhbrmm Stage Model’ develops a complete multlcomponent model of a dlstdlatlon column This includes heat and mass transfer resistances m both phases, but only simple models of two phase flow m the column Solving this model on a computer is discussed and the results of many design wmulatlons are presented These include dlstdlatlon of simple ideal mixtures, extractlve dlstlllatlon with strong nomdeahties, vacuum dlstlllatlon with pronounced pressure variations and an absorber with large heat effects A comparison with

expenmental studies show that the non-equlhbrmm multicomponent model 1s supenor to the combmatlon of the eqmhbrmm model with efficiencies, although the differences are usually not dramatic The last chapter deals with ‘Condensation of Vapor Mixtures’ In multlcomponent condensers heat and mass transfer interact strongly Here the difference between ‘simple conventional models and the more fundamental multlcomponent noneqmhbrmm models can be substantial Occasionally the conventional models even predict a wrong direction of the transport of certain components in the condenser The book contams a large number of worked examples throughout the text Most are given as Mathcad files on a separate disk, so you can play with them (The column simulations are not on the disk) The book has been wntten by two men with long experience both m mdtlcomponent mass transfer and m computation This shows Even so, the book does have a few weaknesses 1 It 1s not as general as the title suggests The book 1s primarily on mass transfer m dlstlllatlon and condensation It does set up the multlcomponent mass transfer relations clearly and generally, but the general form of the equations 1s only used m a few examples 2 The book treats multlcomponent mass transfer with a mix of two methods the Maxwell-Stefan and the multlcomponent Flck descrlptlons The MS-equations usually give the clearest description of what 1s going on The Flck equations are more convenient numerically, but difficult to understand I would have preferred to have more of the explanations using the MS equations 3 I find the authors defensive m then position against conventional descrlptlons of mass transfer The ideas m the equations of multlcomponent diffusion are simple and straightforward (which does not mean that SubJect is easy!) They form a much better base, than the mess of (pseudo) binary models with empirical modlficatlons, efficiencies, numbers of mass transfer units and HETPs that we engineers are still using 4 In the comparison between the different models, only a little 1s said about the effects of transport parameters m the models These parameters (such as mass transfer coefficients and mterfaclal areas) are very unpredictable, and the values used will certainly have an influence on the outcome of calculations Having said this, I must say that I found this a very good book Anyone seriously involved m research or development of multicomponent separation processes should read it Even designers (who may be using the techniques of the book m their programs without knowing), can learn a lot They can probably best start m the last part of the book to see what the results of the ‘new’ way of modellmg are A final word of warning The authors say that they think the content of the book can be covered m a single semester of a graduate course I think this 1s optlmlstlc (although it does depend on the length of your semester ) Mastering

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Book Revrews / The Chemrcal Engmeenng Journal 60 (1995) 173-180

multlcomponent mass transfer, may take a few years For a young chemical engineer it 1s worth the trouble J A Wessebngh Unwersrry of Gronmgen, The Netherlands

Experiments m Heat Transfer and Thermadynamtcs edlted by R A Granger, 521-44925-I Hardback, f14 95

Cambndge Unwerslty Press, 1994, ISBN Of35 00, ISBN O-521-45115-9 Paperback,

The preface states that “we learn by doing” and suggests that engineering students will gam a better understanding of the prmclples of thermodynamics and heat transfer by performing well-designed experiments The editor has collected together 32 expenments from well-known figures m the heat transfer literature and produced a very readable volume which should be useful to anyone interested m teaching heat transfer at the graduate or undergraduate level The chapters are organized mto sections on heat transfer (conduction (six experiments), convection (ten), bollmg (four), diffusion (two), radiation (one), heat exchange (two) and thermodynamics (seven) ) The book 1s aimed at heat transfer so the thermodynamics section covers topics related to heat transfer (water superheat, vapour pressure, latent heat and multlphase flow volume fractions) rather than heat engines or other areas of engineering thermodynamics The book 1s intended as a resource to supplement taught courses and this 1s reflected m the appendlces, which include comprehensive lists of other experiments and demonstrations m heat transfer and thermodynamics as well as films dlustratmg heat transfer phenomena Each chapter describes an experiment and follows a general format The prmclple, obJective and theoretlcal background are followed by descrlptlons of the experimental apparatus and procedure The experimental section 1s supplemented by questions for the student, samples of results and references for further reading The description of expenmental apparatus and procedure 1s generally very good but the references are likely to be required reading for some of the more complicated experiments Each chapter fimshes with a brief pen portrait of the contributor The experiments cover a range of difficulty and complexity Several are ideally suited for lecture demonstrations while others require space, apparatus or condltlons m a suitably equipped laboratory Few of the experiments involve expensive pieces of equipment and most feature water or au- as the transfer medium The most evident trend m the book 1s that the experiments from Japanese contributors tend to involve more complex apparatus and analysis There 1s a notable lack of reference to safety m a book of this nature, there 1s more dlscusslon of burnout of heaters rather than potential sources Of lnJury to personnel One expenment involves evaporation

of drops of benzene or tetrachloromethane, which 1s unlikely to comply with the reviewer’s departmental safety gmdelmes The mixture of US, Japanese and European authors is also evident from the mixing of SI and US units The editor refers to other texts for a description of error analysis This book does not provide a template for a heat transfer teaching laboratory It does, however, represent a readable and thought-provokmg resource for anyone involved m teaching heat transfer to students m chemical or mechanical engineering Dr D I Wdson Unwerstty of Cambridge,, UK

Thermodynamics of Irreversrble Processes D@-ustonand Rheology

Applications to

by G D C Kulken, Wdey, 1994, ISBN O-471-94844-6, f29 95

xxxu + 425pp,

This book 1s a thorough and clear exposition of the underlying prmclples of the linear tbermodynamlcs of irreversible processes (TIP) The monograph admirably satisfies the goals of the Wiley tutorial senes m theoretical chemistry and although it 1s primarily almed at undergraduate students it should also serve as an excellent reference textbook for research as well as teaching Introductory concepts are provided by the author m Chapters 1 and 2 The first chapter clearly defines the scope to which macroscopic (contmuum) theory 1s restrlcted and those topics which are of pnmary concern m the monograph The concept of local equlhbrmm 1s discussed at length and the author ratlonahzes how truly equlllbnum intensive measures can be considered to be useful defimtlons for non-equlhbrmm systems In Chapter 2 the fundamental concepts and laws of classical equlhbnum thermodynamics are introduced and an interesting method of heat engine representation 1s provided via vector diagrams I am also pleased to see the inclusion of Caratheodory’s statement with the Clausms and Kelvin-Planck statements of the second law Chapters 3,4, and 5 are, m sum, an important prelude to the maJor emphasis of the book which 1s to appear later m Chapters 6 and 7 In Chapter 3 the axioms of TIP are carefully enumerated and described, with prehmmary comments on entropy dlsslpatlon and the phenomenologlcal equations, the Onsager-Caslmlr reciprocal relations, and axioms which specify invariance of the TIP equations to V~IYOUS transformations A clear mathematical development of the symmetry prmclples for both lsotroplc and amsotroplc materials 1s also provided In Chapter 4 the author extends the topics of classical equlhbrmm and non-eqmllbrmm thermodynamics to multlcomponent fluids which do not possess mternal “hidden” variables The material balance, diffusion equations, and elec-