Phase Equilibrium Engineering

329 Pages·2013·8.879 MB·2-331\329

by  Esteban Brignole and Selva Pereda (Eds.)

SupercriticalFluidScienceandTechnology
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Foreword
It is with pleasure that I introduce the third volume in the Elsevier Book
Series on Supercritical Fluid Science and Technology, Phase Equilibrium
Engineering, which has been authored by Drs. Esteban Brignole and Selva
Pereda from Universidad National del Sur, Argentina, with one chapter also
contributed by Drs. Martin Cismondi and Marcelo S. Zabaloy from Universi-
dad National de Co´rdoba and Universidad National del Sur, Argentina,
respectively. They are all well-recognized names in the supercritical fluids
and phase equilibria community. The book reflects and benefits from their
many years of accumulated knowledge and practical expertise.
Phaseequilibriumisattheheartofchemicalprocesses,andphaseequilib-
rium at high pressures is a central theme in any application involving super-
critical fluids. The topic becomes even more relevant when systems under
consideration involve chemical transformations along a reaction coordinate
whichcontinuallyalterthecompositionalmakeupandtherebyalterthephase
equilibrium conditions.
This book starts out with a clear statement of the significance of phase
equilibrium in process development where there is a critical need to fill the
gap between reaction and separation stages by designing and controlling
thephaseconditionsthatareessentialforthesuccessoftheprocess.Thebook
emphasizes the importance and the need for effective information flow along
thepathwaysconnectingthechemicalplantorprocesstothelaboratory,tothe
thermodynamics and phase equilibria, and to modeling and simulations. This
four-node grid and their interplay form the essence of Phase Equilibrium
Engineering.
Toprovideapedagogicaldevelopmentoftherelevantengineeringconcepts,
theauthorsstartinChapter2withabriefreviewofintermolecularforces(attrac-
tiveandrepulsive)andmolecularinteractions(dispersive,polar,electrostatic,
induceddipole)thatareimportantinphaseequilibriaandseparationprocesses.
Chapter 3 provides the background on thermodynamics of phase equilibrium
andreviewsthephasediagramsforpuresubstancesandbinaryfluidmixtures
within the framework of the van Konynenburg and Scott classification of the
differenttypesofphasebehavior.Theauthorsprovideaclearandelegantgraph-
ical description of the changes in the binary mixture phase diagrams and the
behaviorofthecriticallines fromTypeItoTypeVIwith changesinthesize
of the molecules and the nature of the molecular interactions and the energy
asymmetriesencountered.Thischapterfurtherprovidesaclassificationforter-
narymixturephasediagramsthatarebasedonthepartialmiscibilityinone,two,
xi
xii Foreword
orthreeofthebinarypairs,whicharegraphicallydescribedinGibb’striangles.
Multicomponentsystemsarealsodiscussedintermsofpseudocomponentsthat
areusedtorepresentsimilarmolecules.
Chapter 4 is devoted to thermodynamic models and provides guidelines
for selecting the appropriate model from among the various options, ranging
from cubic equations of state to SAFT (Statistical Associating Fluid Theory)
for different scenarios which are accomplished by using real case studies for
separations of different levels of complexity. A comprehensive treatment of
a methodology for general phase equilibrium calculations and generation of
phase diagrams is provided in Chapter 5.
Chapter6shiftsthefocustoengineeringandprovidesapracticalperspective
on how the fundamental thermodynamics and phase equilibrium calculations
andpredictionsareusedinaddressingcomplexseparationprocessesusingsev-
eral case studies such as the supercritical biodiesel production process. These
arecontinuedinChapter7bydemonstratinghowphaseequilibriumengineering
comesintoplayindistillationprocessesbyanelegantdescriptionthatmakesthe
connections to the Type I to Type VI phase descriptions. The ethylene plant
recoverysectionisusedasacasestudy.InChapter8,discussionsareextended
to azeotropic mixtures and to the synthesis of solvents by computer-aided
molecular design (MOLDES) to break up the azeotropes. As case studies,
solvent design for recovery of aromatic fractions of reforming naphtha and
high-pressure azeotropic separation of ethaneþCO mixtures by extractive
2
distillationusingn-butaneassolventarepresented.
Chapter9isdevotedtogreenprocessesandhigh-pressuresupercriticalfluid
solvents. Solvent tuning for systems displaying Type V (propaneþvegetable
oil) and Type III (carbon dioxideþnatural oil) phase behavior are discussed
indetail.Chapter10continuesthediscussionsontheuseofsupercriticalfluids
in high-pressure fractionation and extraction of natural oils using orange oil
deterpenation as a case study. Chapter 11 is devoted to reactive systems and
supercriticalreactors,andthephasebehaviorofreactivemixturesandsolvents.
Solvent selection strategies are discussed according to the reaction pathway
using case studies such as selective hydrogenation of fatty acid methyl esters
or hydrogenation of vegetable oils. Feasible or unfeasible operational regions
arediscussedintermsoftheprevailingphasediagrams.Finally,Chapter12dis-
cusses how phase equilibrium engineering is used in the conceptual process
designusingproductionofbiodieselviatransesterificationofvegetableoilwith
methanolandalcoholextractionanddehydrationasexamples.
I trust you will find this volume with its application-oriented engineering
approach to be of great value and interest.
Erdogan Kiran
Series Editor
Blacksburg, Virginia
December 2012
Preface
Phase design engineering has valuable applications in a variety of chemical
processes as well as in many other fields such as metallurgy, geology, clima-
tology,materialsdesign,andoilreservoirengineering,tonameonlyafew.In
this book, we explore the phase design of fluid systems and we apply it to a
number of separation processes and chemical reactors.
The design of the phase scenario that meets the process needs is what we
have called a Phase Equilibrium Engineering (PEE) problem. We first used
this terminology in a paper titled “Phase Equilibrium Engineering of Super-
critical Reactors” published in 2002.1 This discipline combines methods of
phase equilibrium thermodynamics with process requirements in order to
obtain desired phase scenarios. The challenges posed by the phase design
of a reactor that works under homogeneous conditions have all the elements
of a chemical engineering design problem. Indeed, the solvent properties of
supercritical fluids, highly dependent on pressure, temperature, and composi-
tion, call for a systematic methodology for the design of the phase condition.
ThedevelopmentofastrategyforPEEstartswiththeanalysisofthemix-
ture components and their intermolecular forces. This volume thus begins
with an introduction to the classification of industrial separation problems
and explores the relationship between molecular interactions and separation
technologies. The first part of the book also assesses the effect of molecular
interactions on the general phase behavior and on the classification of binary
and multicomponent mixtures.
Thethermodynamicmodelingofphaseequilibriaisreviewedstartingwith
idealmixturesandfollowingwithmodelsformixturesofincreasingdegreeof
nonideality. The most popular models are presented and a strategy of model
selection is discussed, taking into account the class of mixtures and their
molecular interactions. The classification of molecules, separation processes,
andthermodynamicmodelsconfirmsthatthemolecularnatureofthemixture
is the key to determine an adequate thermodynamic model and a suitable
technology.
The above discussion is followed by the presentation of the GPEC pro-
gram, a tool that proves useful to explore the effect of model parameters on
thetopologicalphasebehaviorofbinarymixtures.TheGPECprogramisable
to automatically compute the general phase diagrams of univariant lines of
binary mixtures for the different types of phase behavior.
1. S.Pereda,S.B.Bottini,E.Brignole,Am.Inst.Chem.Eng.J.48(2002)2635–2645.
xiii
xiv Preface
In the second part of the book, the principles of PEE are developed
through the analysis of a series of case studies that look into the separation
of heavy aromatic mixtures, the cryogenic processing of natural gas, and the
separation of biodiesel mixtures obtained by supercritical alcoholysis. The
useofmaterialandenergybalances,combinedwiththemethodologiesdevel-
opedinthe firstpartofthe book, leadtothe formulationofan efficient strat-
egy for phase design. The principles of PEE are applied to the tuning of
thermodynamic models and to the analysis of thermodynamic sensitivity in
thecontextoffractionaldistillationofsinglecolumnsandfractionationtrains.
Thereafter,weexaminethequestionofsolvent selection forthe separation of
highly nonideal mixtures as well as the computer-aided molecular design of
solvents for liquid extraction and extractive distillation.
Finally,westudythephasedesignofbothsupercriticalprocessingofnatural
products (extraction and fractionation) and of supercritical reactors. In these
two cases, the selection of solvents and the identification of optimal operating
conditions are carried out while taking into account green chemistryprinciples
for sustainable development. These principles call for the use of new reactants
andcatalystsinordertoachievechemicalsynthesisthatresultsinimprovedatom
efficiency,higheryieldsandselectivities,andsaferandenvironmentallyfriendly
solvents.Thisdiscussionincludestwomorecasestudieswherethephasedesign
leadstothedevelopmentofnewtechnologiesforbiofuelsproduction.
PEE is thus the road that takes us in a systematic way from the molecular
level of the mixture to the art of process synthesis, through (i) the process
goals, (ii) thermodynamic modeling, (iii) algorithms for phase equilibrium
calculations,(iv)phasescenarioandtechnologyselection,(v)sensitivityanal-
ysis, and (vi) conceptual process design. Finally, this book highlights the fact
that the answer to a successful phase design problem lies in the nature of the
mixture we are working with.
Thecasestudiespresentedinthisbookarebasedonthepersonalexperiences
oftheauthorsandofmanyothercolleagues.Wewouldliketoparticularlythank
andacknowledgethecontributionsofSusanaBottini,SoledadD´ıaz,Guillermo
Mabe,GloriaFoco,PabloHegel,SusanaEspinosa,AlbertoArcodaci,Hernan-
Gros, Ticiana Fornari, Marta Lacunza, Alberto Bandoni, Rube´n Debeistegui,
Marcelo Zabaloy, Martin Cismondi, Olga Ferreira, Laura Rovetto, Nicola´s
Gan˜an,Francisco Sa´nchez, Pedro Valle,andAlexisVe´lez.Our recognitionis
alsoextendedtoCorPeters,PeterRasmussen,andthelateLilianaUrlic,Noemi
Petracci,andAageFredenslund.Mostimportantly,thisbookwouldneverhave
been possible without the encouragement received from the Editor of this
Series,Prof.ErdoganKiran.Wethankhimdeeplynotonlyforsupportingthis
bookprojectbutalsoforhisinsightfulcommentsandinvaluableeditinghelp.
Esteban Brignole
Selva Pereda
Bah´ıa Blanca, Argentina
December, 2012
Chapter 1
Phase Equilibrium and Process
Development
With this volume, we aim to fill the gap between the books on reactors and
separations process design, and the chemical engineering thermodynamics.
Ourgoalistochangethefocusfromtheuseofthermodynamicsrelationships
to compute phase equilibria to the design and control of the phase conditions
that a process needs. In this way, we can say that we put phase equilibrium
thermodynamics to work. Phase equilibrium from a theoretical and experi-
mental point of view belongs to the realm of thermodynamics. The ties to
the fundamental laws of thermodynamics are so strong that the teaching of
phase equilibria emphasizes more the relationships between the thermody-
namicvariablesofeachphaseinequilibriumthanitsengineeringapplications.
The main goal of phase equilibrium engineering is the design of the sys-
temconditionstoachievethedesiredphaseequilibriumscenariothatthepro-
cess at hand requires. The mixture under consideration and the process to be
accomplished illuminate the search for the thermodynamic and process vari-
ables that give the required phase conditions. The synthesis of new processes
basedonthephasebehaviorisalsodiscussedinthisbook.Dealingwiththese
types of problems, the engineer could make use of several design variables:
pressureandtemperature,feedcomposition,entrainers,solvents,supercritical
condition, etc. to modify the phase scenario.
1.1 THE WORLD OF PHASE EQUILIBRIA IN CHEMICAL
PROCESSES
The world of applied phase equilibrium thermodynamics has four cardinal
points:(1)thechemicalplantorprocess,(2)thelaboratory,(3)thethermody-
namicmodelingofphaseequilibria,and(4)thesimulator.Howallthesecom-
ponents can be focused and tuned in harmony to get the better design or
operation is the ultimate goal of phase equilibrium engineering.
Figure1.1illustratestheworldofphaseequilibriumandtheflowofinfor-
mation between the main players and how each pair defines different realms.
Theexchangeofinformationbetweenthechemicalplantandthelaboratoryis
commonplace, as well as the interaction between the thermodynamic
SupercriticalFluidScienceandTechnology,Vol.3.http://dx.doi.org/10.1016/B978-0-444-56364-4.00001-7
©2013ElsevierB.V.Allrightsreserved. 1
2 PhaseEquilibriumEngineering
Real world
Chemical plant Laboratory
P, T, z Data
Industry Test plant data Phase equilibrium P, T, y, x Data Academy
world
Model and parameters
Simulator Thermo modeling
Modeling
FIGURE1.1 Phaseequilibriumworlds:industryandacademyintherealandengineeringworld.
modelingandthelab;however,theinformationonthechemicalplantprocess
is seldom used in the modeling of phase equilibria. The parameterization of
the thermodynamic models for phase equilibria is usually based on experi-
mental binary information. These models and their parameters are available
in the modern process simulators. On the other hand, the process simulator
isnormallyusedintheindustrialplantwithoutpropertuningofthethermody-
namic models to the process needs. In the industrial world, the virtual plant
(the simulator) and the real process or chemical plant coexists, and often
rather crude assumptions are made to match the simulator results with the
plant data which could lead to serious errors in parameter estimation of plant
units,revampingprojects,processoptimization,andnewdesigns.Goingback
to Fig. 1.1, if we keep the game only on the sides, it will be a boring match
andourmain “goal” willnotbeachieved. Onthe contrary,if the information
flows from the plant or the actual process to the other players, that also
exchangeinformationamongthem,veryfruitfulresultsinthedesignandopti-
mizationoftheprocesswillbeobtained.Thedevelopmentofmodernprocess
simulators with a large collection of thermodynamic models and parameters
makes more critical the wise use of information from the real system to get
the most from the process simulators, and to avoid critical errors in design,
operation, and plant parameters estimation.
1.2 THERMODYNAMIC MODELING IN PROCESS
DEVELOPMENT
Theneedofinformationonphysicochemicalpropertiesofpurecompoundsand
their mixtures isalways present inthe synthesis,design,simulation, debottle-
necking,control,andoptimizationofchemicalplants.Fromthepointofview
ofthemodernchemicalindustryandtheirsustainabledevelopment,theproper-
tiesassociatedwiththeequilibriumbetweenphases,thatis,vapor–liquid(VL),
Chapter 1 PhaseEquilibriumandProcessDevelopment 3
liquid–liquid (LL), gas–liquid (GL), and solid–liquid, play a dominant role.
Otherpropertiesofinterestarerelatedtothethermodynamicenergyfunctions,
transport,andvolumetricproperties.
In the design or conception of a new process, a wealth of experimental
information is required. This information comes from laboratory and bench
scale studies, pilot plant, and industrial units. Even when a large amount of
phase equilibrium and physical properties data is available, the need for ther-
modynamicmodelpredictionsisalwayspresent.Thescenarioofphasecondi-
tions in a plant is infinitely varied, and the experimental data will cover only
some regions, hopefully the most critical, of the information required. The
experimental data availableare like ascaffoldingthat supports the thermody-
namicpredictionsofthevolumetricproperties,phaseequilibrium,andenergy
functionsrequiredforthedesignandsimulationofalltheprocessunits.When
moreabundantandreliablearetheexperimentaldata,betterpredictionsofthe
physical and thermodynamic properties will be obtained from the selected
thermodynamic model.
Therefore, a well-tuned thermodynamic model for the mixture under the
process conditions is a key engineering tool for the design and optimization
oftheplantunits.Theprocessthermodynamicsensitivitywillprovideastrat-
egy for parameter estimation of thermodynamic models, tailored to the pro-
cess needs. On the other hand, models with qualitative predictive capacity
are needed in development studies. A well-tuned thermodynamic model
allows studying different pressure and temperature conditions of operation,
exploring new phase scenarios to carry out the reactions, as well as, testing
new separation techniques.
The likelihood of finding the required information generally is limited to
pure compounds or binary mixtures. Modern equations of state, on the basis
of pure component physical constants and a few empirical parameters, are
used to obtain PVT relations for pure compounds and their mixtures. In gen-
eral, the mixture phase equilibria can be estimated on the basis of limited
experimental information using the tools of classical thermodynamics, com-
bined with the predictions of PVT properties from equations of state. Also,
group contribution models can be applied to extend the scarce information
available, to systems for which no experimental information has been
measured.
1.3 DEFINITION OF PHASE EQUILIBRIUM ENGINEERING
Thechemicalandpharmaceuticalindustryisblamedforitsnegativeenviron-
mentalimpact.TheGreenChemistryprinciplesforasustainabledevelopment
of the chemical industry claim for new reactants and catalyst for chemical
synthesis with improved atom efficiency, higher yields and selectivities, pro-
cess intensification and simplification, less waste, and safe and environmen-
tally friendly solvents. Among the benign solvents, the supercritical fluids
4 PhaseEquilibriumEngineering
(SCFs) have an increasing role. SCFs are classified as low or high critical
temperature(T )solvents.LowT SCFs,likeCO andethane,offeringeneral
c c 2
low solvent power, unless very high pressures are applied. In addition, these
solvents are selective for compounds of low to moderate polarity and low
molecular weight.High T SCFs have highsolventpowerbutlow selectivity.
c
Typical examples of high T solvents are supercritical water or toluene.
c
The SCFs have very attractive physical properties and exceptionally tune-
able solvent power. SCFs add a new dimensionto conventional solvents, that
is, the density-dependent solvent power, as the density of SCFs is very sensi-
tive to changes in pressure, temperature, and composition.
The design of the phase scenario that meets the process needs is what we
call a phase equilibrium engineering problem. This discipline combines the
methods of phase equilibrium thermodynamics with the process requirements
to obtain the desired phase scenario and thermodynamic properties.
We may look for homogeneous conditions in a supercritical reactor, for
phase split and selectivity in a separation process, or a tuneable phase condi-
tiontoachievebothreactionandseparationprocess inasinglestep.Foreach
problem, there will be a set of specifications which represent the process
restrictions. For instance, if we need to operate a reactor under homogeneous
conditions, we specify the reactants, degree of conversion, and the operating
temperature. Phase equilibrium engineering, by systematic analysis, develops
criteria for selecting the right solvent, the solvent-to-feed ratio, and the pres-
sure of operation, to keep the whole reaction trajectory in a single phase
region [1].
Amulticomponentfluidmixturecanbeinahomogenousphaseatsupercrit-
icalconditionsorasasubcooledliquidorsuperheatedvapor,orinaheteroge-
neousVLorLLphasecondition.Thephaseenvelopeorpressure–temperature
diagramisausefulplotforagivenmixturecomposition;itpresentsthebubble
anddewpointphasetransitioncurvesaswellasthemixturecriticalpoint.The
regionunderthecurveisheterogeneous.Abovethemaximumpressureatany
temperature,wewillhaveasinglephase.
Figure 1.2 presents a phase envelope sketch for a given mixture where
three typical phase scenario requirements are indicated. Of course, during
the course of a reaction or a separation process, with the change in composi-
tions, different phase envelopes will be obtained, and the process trajectory
should be always in the specified phase scenario: homogeneous or
heterogeneous.
1.4 PHASE SCENARIOS IN SEPARATION, MATERIALS,
AND CHEMICAL PROCESSES
Allsortsofphasescenariosarepresentinnature,inlivingorinertbodies:the
exchange of oxygen and carbon dioxide in our lungs (GL), the formation of
clouds and ice in the atmosphere, the evaporation of water from oceans and