lecture 4- stoichiometry

Upload: imran-azman

Post on 20-Feb-2018

223 views

Category:

Documents


0 download

TRANSCRIPT

  • 7/24/2019 Lecture 4- Stoichiometry

    1/50

    Stoichiometry

    andRate Laws

    Suchithra Thangalazhy Gopakumar

    H83RED Reactor Design

    Autumn Semester 2015

  • 7/24/2019 Lecture 4- Stoichiometry

    2/50

    Outline

    Rate Law and Reaction Rate Constant

    Arrhenius Law

    Elementary reaction

    Order of Reaction

    Rate Law for reversible reactions Stoichiometric Table

    Batch systems:

    Constant volume and

    Constant pressure

    Flow systems:

    Liquid phase

    Gas phase reactions that have a change in the total number of moles

    Reactors with variable volumetric flowrates

  • 7/24/2019 Lecture 4- Stoichiometry

    3/50

    Chemical kinetics deals with reaction rates

    Types of Reactions

    Homogeneous reaction

    involves substances of only one phase

    Heterogeneous reaction

    involves more than one phase, and thereaction usually occurs at or very near the

    interface between the phases

    Phase

    Irreversible reaction

    proceeds in only one direction and continues

    until the reactants are exhausted

    Reversible reaction

    proceeds in both forward and reverse

    directions; continues until the equilibrium state

    is reached

    Equilibrium

    A B

    A B

    Types of Reactions

  • 7/24/2019 Lecture 4- Stoichiometry

    4/50

    The Rate Law and the Reaction Rate Constant

    General form of the rate law, which is an algebraic equation representing the

    reaction rate as a function of the concentrations (activities) of various species:

    (1)

    k = reaction rate constant

    ~ independent of the species concentrations

    ,...)C,C(f)T(kr BAA

    The specific reaction rate

    constant, kA, is always referred to

    a particular species

    Exception

    OHNaClHClNaOH kkkkk 2

    NaOH + HCl NaCl + H2O

    Liquid Phase:

    Reaction rate constantis an intensive parameter concentration-independent

    Gas Phase:

    k=f(temperature, total pressure, ionic strength, choice of solvent )

    k=f(temperature, catalysts, total pressure )

    Rate Law

  • 7/24/2019 Lecture 4- Stoichiometry

    5/50

    A = pre-exponential factor or frequency factorE= activation energy (J/mol or cal/mol),

    R = gas constant (8.314 J/mol.K=1.987 cal/mol.K)

    T = absolute temperature (K)

    (2)

    Activation energy, E characterise energy state of molecules which could

    result in their chemical interaction

    Minimum energy that molecules must have to react

    (3)

    Temperature Dependency from Arrhenius Law

    RT/EAe)T(k

    1/T, K-1

    ln

    k,sec-1

    slope= -E/R

    1/T, K-1

    ln

    k,sec-1

    Low E

    High E

    TR

    EAkA

    1lnlnActivation energy from experimental results

    E , k~ sensitive to the temperature

    E , k~ less sensitive to thetemperature

    Rate Law

  • 7/24/2019 Lecture 4- Stoichiometry

    6/50

    Temperature rise needed to double the rate of reaction

    Average

    TemperatureActivation Energy, E

    40 kJ/mol 160 kJ/mol 280 kJ/mol 400 kJ/mol

    0o

    C 11o

    C 2.7o

    C 1.5o

    C 1.1o

    C400 oC 65 oC 16 oC 9.3 oC 6.5 oC

    1000 oC 233 oC 58 oC 33 oC 23 oC

    Reactions with high activation energies are very temperature-sensitive

    Any given reaction is more temperature-sensitive at lower temperatures thanat higher temperatures

    Rules of thumb:

    Rate of reaction doubles for every 10o

    Cincrease in temperature

    ~ Only true for a specific combination

    of activation energy and temperature1/T, K-1

    ln k, sec-1

    Low E

    High E

    Arrhenius Law

  • 7/24/2019 Lecture 4- Stoichiometry

    7/50

    Task

    Consider the following elementary reactions :

    Solut ion

    1) Which reaction has the higher activation energy?

    2) Which reactions have the same activation energy?

    3) Which reaction is virtually temperature insensitive?

    Arrhenius Law

  • 7/24/2019 Lecture 4- Stoichiometry

    8/50

    Solut ion

    4) Which reaction will dominate (i.e. take place the fastest) at high temperatures?

    5) Which reaction will take place the fastest at low temperatures?

    Arrhenius Law

  • 7/24/2019 Lecture 4- Stoichiometry

    9/50

    Finding k(T) given k(T0)

    0/0)( RTEAeTk

    RTEAeTk /)(

    k(T)k(T0)exp E

    R

    1

    T0

    1

    T

    If we know:

    The specific reaction rate k0(T

    0) at a temperature T

    0 The activation energy, E,

    We can find the specific reaction rate constant k(T) at any other

    temperature,

    T, for that reaction

    0/

    0RTEeTkA substitute

    RT

    E

    RT

    E

    eeTkTk

    0)()( 0

    Arrhenius Law

  • 7/24/2019 Lecture 4- Stoichiometry

    10/50

    Task

    Determine the activation energy, E and frequency factor, A from the following data:

    Solut ion

    Arrhenius equation in logarithm form:

    k (min-1) 0.001 0.050

    T (oC) 0.0 100.0

    TRE

    AkA1

    lnln

    RT/EAe)T(k

    Arrhenius Law

  • 7/24/2019 Lecture 4- Stoichiometry

    11/50

    Elementary and Non- elementary Reactions

    Assumption from Collision theory:

    Reactant particles must collide with each other.

    Reaction is a result of the collision of a single molecule A with a singlemolecule B

    E: Energy required to activate the molecules into a state from whichreactant bonds can change into product bonds.

    Only those collisions with enough energy to exceed E can lead toreaction.

    Rate law can be derived from collision theory

    (1),...)C,C(f)T(kr BAA

    Rate of reaction is proportional to the number of such collisions.

    The number of collisions is proportional to the concentration ofreactant.

    Elementary Reaction

  • 7/24/2019 Lecture 4- Stoichiometry

    12/50

    The dependence of possible collisions on the reactant concentrations.

    A

    A

    B

    B

    A

    A

    B

    B

    A

    4 collisions

    Add anothermolecule of A

    6 collisions

    Add anothermolecule of B

    A

    A

    B

    B

    A B

    9 collisions

    Elementary Reaction

  • 7/24/2019 Lecture 4- Stoichiometry

    13/50

    Elementary Reactions

    corresponds to the stoichiometric

    equationBAA CkCr

    If kinetic experiments results in a rate law, which corresponds tothe stoichiometric equation, we call such reactions elementary.

    Molecularity Elementary step Rate law

    1. Unimolecular A -> products rate = k CA

    2. Bimolecular2A -> productsA + B -> products

    rate = k C2Arate = k C

    AC

    B

    3. Termolecular3 A -> productsA + 2 B -> productsA + B + C -> products

    rate = k C3Arate = k CAC

    2B

    rate = k CACBCC

    A + B R

    The molecularity of an elementary reaction is the number of molecules

    (atoms) involved (colliding) in the reaction.

    Elementary Reaction

  • 7/24/2019 Lecture 4- Stoichiometry

    14/50

    Rate law reflects a limiting step or combination of limiting steps

    The rate limiting step is the slowest step in the sequence of stepsleading to product formation

    The overall rate of reaction depends on this slowest step

    Non-elementary reactions involve more complex, multi-step reactions

    H2 + I2 2HIElementary Reaction

    222 IHH CkCr

    H2 + Br2 2HBr

    2

    22

    BrHBr2

    21BrH1

    HBrC/Ck

    CCkr

    /

    Non-ElementaryReaction

    Elementary and Non-Elementary Reactions

    Elementary Reaction

  • 7/24/2019 Lecture 4- Stoichiometry

    15/50

    The Reaction Order and Rate Law

    General form of the rate law: (4)

    2 NO + O2 2 NO2

    ...CCkr BAAA

    Temperature-dependentterm

    Concentration-dependentterms

    The order of a reaction refers to the power to which the concentrations are rais

    The overall order of the reaction: n = + +

    2O

    2

    NONONO CCkr

    CO + Cl2 COCl2

    23CO

    /ClCO CkCr

    H2 + Br2 2HBr

    2

    22

    BrHBr2

    21

    BrH1

    HBr

    C/Ck

    CCkr

    /

    2N2O 2N2 + O2

    2

    22

    2

    O

    ONON

    ON1 Ck

    Ckr

    2OCk1>> Pseudo-first order with

    respect to nitrous oxide

    2OCk1

  • 7/24/2019 Lecture 4- Stoichiometry

    16/50

    Examples of Forms of Rate Law and Units of Reaction Rate Constant

    Zero-order:AA kr

    secdm

    mol

    secdm

    mol33 AA

    kr secdm

    mol3

    k

    First-order:AAA Ckr

    [k]=sec-1

    secdm

    mol

    dm

    mol

    sec

    1

    secdm

    mol333 AAA Ckr

    Second-order: 2

    AAA Ckr

    secmol

    dm3k

    secdm

    mol

    dm

    mol

    secmol

    dm

    secdm

    mol36

    22

    3

    3 AAA Ckr

    Third-order: 3

    AAA Ckr

    secmol

    dm2

    6

    k

    secdm

    mol

    dm

    mol

    secmol

    dm

    secdm

    mol39

    33

    2

    6

    3 AAA Ckr

    Rate Law

  • 7/24/2019 Lecture 4- Stoichiometry

    17/50

    TaskWhat is the reaction rate law for the reaction:if the reaction is elementary?Calculate the rates of A, B, and C in a CSTR where the concentrationsareCA = 1.5 mol/dm

    3, CB = 9 mol/dm3 and kA = 2 (dm

    3/mol)(1/2)(1/s).

    C B2

    1A

    Rate Law

  • 7/24/2019 Lecture 4- Stoichiometry

    18/50

    (6)

    One more limit condition for reversible reactions: Equilibrium State

    The rate of reaction is identical for all species and equal zero

    (5)

    Rate Law for Reversible Reactions

    aA + bB k

    cC + dDk-1

    At thermodynamic equilibrium:

    b

    Be

    a

    Ae

    d

    De

    c

    Ce

    CCC

    CCK

    KC is an equilibrium constant Units

    abcd

    3

    dm

    mol

    The equilibrium constant characterise: equilibrium concentration of all species

    ratio between rates of forward andreverse reactions

    1-reverse

    forward

    k

    k

    k

    k

    KC

    -rA 0

    k= Forward reaction rate constantk-1 = Reverse reaction rate constant

    The state in which the concentrations of the reactants and productshave no net change over time

    Rate Law (Reversible Reaction)

  • 7/24/2019 Lecture 4- Stoichiometry

    19/50

    Kinetic rate law for reversiblereaction must be consistentwith

    thermodynamicequilibrium relationship

    additional limit conditionwith specified rate law offorward and reversereactions

    AkA

    Bk-A

    An elementary reversible reaction

    (7)

    formation of species A (8)

    Reaction rate with respect to species A

    AAA Ckr disappearance of species A

    AAA Ckr

    Reaction rate with respect to species A

    BAA Ckr (9)formation of species A

    b

    Be

    a

    Ae

    d

    De

    c

    CeC

    CCCCK

    Forward reaction

    Reverse reaction

    A BkA

    ABk-A

    Rate Law (Reversible Reaction)

  • 7/24/2019 Lecture 4- Stoichiometry

    20/50

    The overall rate of formation ofspecies A

    (10)BAAA

    reverseAforwardAA

    CkCk

    rrr

    ,,

    (11)

    The overall rate of disappearance of species A:

    )( BA

    AAABAAA

    reverseAforwardAA

    C

    k

    kCkCkCk

    rrr

    ,,

    (12)

    Rate law for elementary monomolecular reversible reaction:

    )(C

    BAAA

    K

    CCkr

    Check of the consistency with thermodynamic equilibrium:

    )(0C

    BeAeAA

    KCCkr

    After rearranging:C

    BeAe

    K

    CC

    Ae

    Be

    C C

    CK

    Ae

    Be

    C C

    CK

    Thermodynamics tells that:

    At equilibrium:

    AkA

    Bk-A

    1-reverse

    forward

    k

    k

    k

    kKC

    Rate Law (Reversible Reaction)

  • 7/24/2019 Lecture 4- Stoichiometry

    21/50

    TaskWrite the rate law for the elementary reaction

    Here kfA and krA are the forward and reverse specific reaction rates bothdefined with respect to A.

    Rate Law (Reversible Reaction)

  • 7/24/2019 Lecture 4- Stoichiometry

    22/50

    Stoichiometry provides information on relative quantities of reactants

    and products in chemical reactions. We have already addressed related topics such as limiting reactant

    and excess in our previous lecture.

    Stoichiometry

    Stoichiometry

    Rate of reaction ri is a variable in all the design equations for

    reactors. Rate of reaction is a function of thermodynamic quantities

    such as temperature and also concentration of the species.

    Conversion of the limiting reactant is a function of the concentrationand therefore rate of reaction is a function of the conversion: ri=f(Xi).

    To evaluate the integrals associated with the design equations ofthe form

    the functional form of ri(X) has to be found.

    Stoichiometry provides a solution for this challenge.

    )( jA Cfr )(XhC jj )(XgrA

    rate law From a stoichiometric table

  • 7/24/2019 Lecture 4- Stoichiometry

    23/50

    Mole Balance for Different Reactors

    Reactor Type Differential Algebraic Integral

    Batch

    CSTR

    PFR

    PBR

    Vrdt

    dXN AA 0

    )(

    0

    0

    tX

    A

    A

    A

    Vr

    dXNt

    exit

    0

    )( A

    A

    r

    XFV

    A

    A r

    dV

    dXF0

    ou t

    in

    X

    X A

    Ar

    dXFV 0

    A

    A rdW

    dXF0

    ou t

    in

    X

    X A

    Ar

    dXFW 0

    exit)r(

    XC

    A

    A

    0

    ou t

    in

    X

    X A

    Ar

    dXC 0

    Conversion: Revision of concepts

  • 7/24/2019 Lecture 4- Stoichiometry

    24/50

    Relative quantities (extent)

    Stoichiometry: Extent

    + + 0 0 0 0

    0 0 0 0

    Reaction

    Initial quantities

    Quantities remaining at

    t=t

    Quantities reacted

    Amount of B react with A = 0

    0 = 0 0

    = 0

    0 =

    0 =

    0 =

    0 =

    Therefore the relative quantities are given by the following equation

    [13]

    The extent E of the reaction is the relative number of moles reacted/ generat

  • 7/24/2019 Lecture 4- Stoichiometry

    25/50

    Relative reaction rates

    Stoichiometry: Relative rates

    + + Reaction Rates

    Rate of disappearance of A =

    Rate of disappearance of B = Rate of appearance of C =

    =

    =

    =

    Therefore the relative reaction rates are given by

    [14]

  • 7/24/2019 Lecture 4- Stoichiometry

    26/50

    Constructing stoichiometric table

    Stoichiometry: Stoichiometric table

    Consider the reaction

    t=0

    NA0NB0NC0ND0NI0

    t=t

    NANBNCNDNI

    + +

    I stands for inert substances, e.g.solventsReaction was terminated at time t when

    the conversion of the limiting reactantA

    reachedX. Composition of the product:

    NA, NB, NC, ND and NI moles of substances

    A, B, C, D and I, respectivelyFrom the definition of conversion

    = 0 0

    Xreachedconversion

    whenreacted

    Aofmoles

    volumesystem

    theintointroduced

    Aofmoles

    Xconversionat

    volumesystema

    inAofmoles

    For every mole of A reacted, b/a moles of B must react

    )(reactedmolesreactedBmoles 0XNa

    bA

    a

    bA

  • 7/24/2019 Lecture 4- Stoichiometry

    27/50

    Stoichiometry: Stoichiometric table

    Species Initially

    (mol)

    Change

    (mol)

    Remaining

    (mol)

    A NA0

    B NB0

    C NC0

    D ND0

    I NI 0

    Total NT0

    )( 0XNA

    )( 0XNab A

    )( 0XNa

    cA

    )( 0XN

    a

    dA

    XNNN AAA 00

    XNabNN ABB 00

    XNa

    cNN ACC 00

    XN

    a

    dNN ADD 00

    XNa

    b

    a

    c

    a

    dNN ATT 00 1

    0II NN

    A + b/a B c/a C + d/a D

    XNa

    d

    a

    c

    a

    bA01

    )(

    0

    0

    tX

    A

    A

    A

    Vr

    dXNt

    i i i i i

  • 7/24/2019 Lecture 4- Stoichiometry

    28/50

    Total number of moles changed

    Stoichiometry: Stoichiometric table

    The total number of moles of all active species (reactants and products) at

    conversionX of speciesA:

    = 0+ +

    1 0

    The change in the total number of moles per mole of A

    reacted

    =+

    1 =0 0

    = 0+ 0 where

    = + = /

    0 = 0+ 0

    We will introduce concentrations in thestoichiometric table using:

    =

    [15]

    [16]

    St i hi t St i hi t i t bl

  • 7/24/2019 Lecture 4- Stoichiometry

    29/50

    Stoichiometry: Stoichiometric table

    In order to define all concentrations on a basis per mole of A we

    introduce the parameter :

    The final equation for concentration of species B:

    0

    0

    0

    0

    0

    0

    A

    i

    A

    i

    A

    ii

    y

    y

    C

    C

    N

    N

    V

    XN)a/b(N

    V

    XN)a/b(NC AABABB

    0000

    V

    Xa/bNC BAB

    )(0 [18]

    Expressing all in terms of NA0

    We now need Vas a function of conversionX in order to obtain )(XhC jj

    [17]

    St i hi t St i hi t i t bl

  • 7/24/2019 Lecture 4- Stoichiometry

    30/50

    Stoichiometry: Stoichiometric table

    = 00

    ()

    Initially

    (mol)

    Change

    (mol)

    Remaining

    (mol) (mol/dm3)

    A NA0

    B

    C

    D

    I

    Total NT0

    )( 0XNA

    )( 0XNa

    bA

    )( 0XNa

    cA

    )( 0XNad

    A

    XNNN AAA 00

    Xa

    bNN BAB 0

    Xa

    cNN CAC 0

    X

    adNN DAD 0

    0II NN

    XNNN ATT 00

    V

    X1NC 0AA

    )(

    V

    NC II

    0

    V

    Xa/bNC BAB

    )(0

    V

    Xa/cNC CAC

    )(0

    V

    Xa/dNC DAD)(0

    A + b/a B c/a C + d/a D

    00 ABB NN

    00 ACC NN

    00 ADD NN

    00 AII NN

    St i hi t St i hi t i t bl

  • 7/24/2019 Lecture 4- Stoichiometry

    31/50

    Stoichiometry: Stoichiometric table

    Constant-Volume (or Density) Reaction Systems

    Sealed vessel (gas phase reaction)

    ~Example:

    Laboratory bomb calorimeter

    (to measure the heat of combustion

    of a particular reaction)

    Isothermal gas-phase reaction with no change in the total number of moles

    (ideal gas law applies)

    ~ Example: coal gasification

    Liquid phase reaction in an incompressible solvent

    (except polymerizations)

    ~ Solvent dominates

    ~ Changes in the density of the solute do not affect

    the overall density of the solution significantly

    CO + H2O CO2 + H2P

    T

    nRV

    nRTPV

    St i hi t St i hi t i t bl

    http://upload.wikimedia.org/wikipedia/commons/3/34/Bombenkalorimeter_mit_bombe.jpghttp://upload.wikimedia.org/wikipedia/commons/3/34/Bombenkalorimeter_mit_bombe.jpg
  • 7/24/2019 Lecture 4- Stoichiometry

    32/50

    Stoichiometry: Stoichiometric table

    = 0

    0

    ()

    Initially

    (mol)

    Change

    (mol)

    Remaining

    (mol) (mol/dm3)

    A NA0

    B NB0

    C NC0

    D ND0

    I NI 0

    Total NT0

    )( 0XNA

    )( 0XNa

    bA

    )( 0XNa

    cA

    )( 0XNad

    A

    XNNN AAA 00

    Xa

    bNN BAB 0

    Xa

    cNN CAC 0

    X

    adNN DAD 0

    0II NN

    XNNN ATT 00

    )1(0 XCC AA

    XabCC BAB )( /0

    Xa/cCC CAC )(0

    XadCC DAD )/(0

    0II CC

    Stoichiometric Table for a Batch, Constant-Volume System

    A + b/a B c/a C + d/a D

    St i hi t St i hi t i t bl

  • 7/24/2019 Lecture 4- Stoichiometry

    33/50

    Stoichiometry: Stoichiometric table

    )( XCC AA 10

    A + b/a B c/a C + d/a D

    BAA CkCr

    XabCXkCr BAAA )( /).1( 00

    XabCC BAB )( /0

    XabXkCr BAA )( /)1(02 )(XfrA

    exit)(

    0

    A

    A

    r

    XFV

    Rate law

    Design Equation ou t

    in

    X

    X A

    Ar

    dXFV 0

    Sizing of the batch, CSTR, PFR system

    St i hi t i T bl E l

  • 7/24/2019 Lecture 4- Stoichiometry

    34/50

    Task

    A component of soap (sodium stearate) is produced by saponification of glyceryl

    stearate with aqueous caustic soda according the following equation:

    3 NaOH + (C17H35COO)3C3H5 3 C17H35COONa + C3H5(OH)3

    Production is carried out in a batch reactor. The initial mixture consists solely of sodium

    hydroxide at a concentration of 10 mol/dm3 and glyceryl stearate at a concentration

    of 2 mol/dm3. What is the concentration of glycerine when the conversion of sodium

    hydroxide is (a) 20% and (b) 90%?

    Stoichiometric Table: Example

    St i hi t i T bl Fl t

  • 7/24/2019 Lecture 4- Stoichiometry

    35/50

    Stoichiometric Table for Flow Systems

    EnteringFA0FB0FC0FD0FI0

    LeavingFAFBFCFDFI

    A + b/a B c/a C + d/a D

    Batch System

    )1(000 XNXNNN AAAA

    [19]

    Concentration of each species in a stream:

    volume

    moles

    timevolume

    timemoles

    q

    FC

    j

    j

    /

    /

    Flow systems

    )X(FXFFF AAAA 1000

    Nj0

    Nj

    Fj0

    Fj

    In flow systems we deal with molar

    fluxes

    Replaced by

    Stoichiometric Table: Flow system

    St i hi t i T bl Fl t

  • 7/24/2019 Lecture 4- Stoichiometry

    36/50

    Concentrations of species A, B, C, and D in terms of entering molarflow rate (FA, FB, FC, FD), the conversion X, and the volumetric flowrate

    [20]

    )1(0 Xq

    F

    q

    FC AAA

    q

    XFabF

    q

    FC ABBB

    00 )/(

    q

    XFacF

    q

    FC ACCC

    00 )/(q

    XFadF

    q

    FC ADDD

    00 )/(

    0

    0

    0

    0

    0

    0

    0

    0

    A

    i

    A

    i

    A

    i

    A

    ii

    y

    y

    C

    C

    vC

    vC

    F

    F

    The parameter :

    Expressing all in terms of FA0 + +

    Stoichiometric Table: Flow system

    Stoichiometric Table: Flow system

  • 7/24/2019 Lecture 4- Stoichiometry

    37/50

    Species Initially(mol / time) Change(mol / time) Remaining(mol / time)

    A FA0

    B

    C

    D

    I

    Total FT0

    00 ABB FF

    00 ACC FF

    00 ADD FF

    00 AII FF

    )( 0XFA

    )( 0XF

    a

    bA

    )( 0XFa

    cA

    )( 0XFa

    dA

    )1(0 XFF AA

    Xa

    bFF BAB 0

    Xa

    cFF CAC 0

    Xa

    dFF DAD 0

    IAI FF 0

    XFa

    b

    a

    c

    a

    dFF ATT 00 1

    XFFF ATT 00

    Stoichiometric Table for Flow Systems

    Stoichiometric Table: Flow system

    Stoichiometric Table: Flow system

  • 7/24/2019 Lecture 4- Stoichiometry

    38/50

    For liquid-phase reactions, the volumetric flow rate does not practically

    change in the course of reaction: v =v0 = Const

    )1(0 XFF AA

    X

    a

    bFF BAB 0

    Xa

    cFF CAC 0

    Xa

    dFF DAD 0

    Stoichiometric Table for Flow Systems (Liquid-Phase Reactions)

    )1()1( 00

    0 XCXq

    FC A

    AA

    X

    a

    bCC BAB 0

    Xa

    cCC CAC 0

    Xa

    dCC DAD 0

    Therefore for a given rate law we have:)(Xgr

    A

    Stoichiometric Table: Flow system

    Stoichiometric Table

  • 7/24/2019 Lecture 4- Stoichiometry

    39/50

    For most batch and liquid-phase reactions and some gas-phase reactions

    the volumetric flow rate or reaction volume does not change, however:

    Volume Change with Reaction

    volume (or volumetric flow rate) will change in the following situations:

    Flow system with gas-phase reaction with a change in the total

    number of moles

    Flow system with non-isothermal gas-phase reaction

    Batch system with variable volume

    01a

    b

    a

    c

    a

    d

    Stoichiometric Table

    + +

    N2 + 3H2 2NH3

    Stoichiometric Table

  • 7/24/2019 Lecture 4- Stoichiometry

    40/50

    Equation of State is a fundamental equation for gas systems:

    RTZNPV T [21]Z = a compressibility factorNT= an overall number of moles in a gas mixture

    [22]

    By dividing equation (21) by equation (22):

    00000 T

    T

    N

    N

    Z

    Z

    VP

    PV

    T

    T

    Volume Change with Reaction

    The first four columns of the stoichiometric tables are always valid

    Assumptions concerning a volume change were made when expressing

    concentration in terms of conversion

    Expressing volume as a function of conversion V = f (X) (Batch Systems)

    000

    00

    T

    T

    N

    N

    Z

    Z

    P

    PVV

    T

    T [23]

    00000

    RTNZVPT

    time = 0

    Stoichiometric Table

    Stoichiometric Table

  • 7/24/2019 Lecture 4- Stoichiometry

    41/50

    Total number of moles in system as a function of

    conversion:

    yA0 is the molar fraction of

    species A in an initial

    mixture

    [24]

    [25]

    000

    0

    0T

    T

    N

    N

    Z

    Z

    P

    PVV

    T

    T

    XNNN ATT 00

    XyXN

    N

    N

    NA

    T

    A

    T

    T

    0

    0

    0

    0

    11

    Let the parameter :

    [26]

    reactorthetofedmolesofnumbertotal

    conversioncompleteformolesofnumberin totalchange

    00

    0

    0

    0

    1 AT

    A

    T

    A

    yN

    N

    N

    N

    a

    b

    a

    c

    a

    d

    Equation [25] now becomes:

    XXy

    N

    NA

    T

    T 11 00

    Equation [23]:

    )1(

    00

    00 X

    T

    T

    Z

    Z

    P

    PVV [28]

    Volume Change with Reaction in a Batch System

    Stoichiometric Table

    [27]

    Stoichiometric Table

  • 7/24/2019 Lecture 4- Stoichiometry

    42/50

    In gas-phase systems, at the certain ranges of temperatures and pressuresthe compressibility factor does not change significantly during the course of

    the reaction: Z0Z

    [29]

    The volume of gas mixture for a variable volume batch reactorat anytime/conversion:

    )1(00

    0

    0 XTT

    ZZ

    PPVV

    0

    00 )1(

    T

    TX

    P

    PVV

    Volume Change with Reaction in a Batch System

    If the reactor is a rigid steel containerof constant volume, then:

    0VV

    and equation [29] can be used to calculate the pressure change as afunction of TandX

    Stoichiometric Table

    Stoichiometric Table: Flow system

  • 7/24/2019 Lecture 4- Stoichiometry

    43/50

    Consider the total concentration at

    any point within the reactor:

    [30]ZRT

    P

    qZRT

    Pq

    q

    FC TT

    [31]

    At the entrance to the

    reactor:

    00

    0

    0RTZ

    PCT

    Taking the ratio of the equations(31) and (30) and assuming that

    Z0Z:

    [32]0

    0

    0

    0T

    T

    P

    P

    F

    Fqq

    T

    T

    XXyX

    F

    F

    F

    FA

    T

    A

    T

    T 111 00

    0

    0

    Total molar flux rate as a

    function of conversion in acontinuous reactor:

    [33]XFFF ATT 00

    [34]

    Volumetric flow rate as a functionof conversion can be obtained bycombining the equations (32) and

    (34) :

    0

    00 )1(

    T

    T

    P

    PXqq [35]

    Volume Change with Reaction in a Flow System

    Stoichiometric Table: Flow system

    Stoichiometric Table: Flow system

  • 7/24/2019 Lecture 4- Stoichiometry

    44/50

    If the volumetric flow rate is known as a function of conversion, then theconcentration of any species in a flow system can be defined as a functionof conversion:

    Thus:

    [36]

    0

    00 )1(

    T

    T

    P

    PXqq

    q

    FC

    j

    j

    Molar flow rate of each species as a function of conversion is:

    XSFF jjAj 0

    T

    T

    P

    P

    Xq

    XSFC

    jjA

    j 0

    00

    0

    )1(

    )(

    T

    T

    P

    P

    X

    XSCC

    jjA

    j0

    0

    0

    )1(

    )(

    Concentrations as a Function of Conversion

    Stoichiometric Table: Flow system

    Stoichiometric Table: Flow system

  • 7/24/2019 Lecture 4- Stoichiometry

    45/50

    T

    T

    P

    P

    X

    XsCC

    jjA

    j0

    0

    0

    )1(

    )(

    Initially

    (mol/time)

    Change

    (mol/time)

    Remaining

    (mol/time) (mol/volume. time)

    A FA0

    B

    C

    D

    I

    Stoichiometric Table for a Variable-Volume Gas Flow System

    T

    T

    P

    P

    X

    XCC AA

    0

    0

    0)1(

    1

    T

    T

    P

    P

    X

    Xa/bCC BAB

    0

    0

    0)1(

    )(

    T

    T

    P

    P

    X

    Xa/c

    CC

    C

    AC

    0

    00 )1(

    )(

    T

    T

    P

    P

    X

    Xa/dCC DAD

    0

    0

    0)1(

    )(

    T

    T

    P

    P

    X

    CC IAI

    0

    0

    0

    )1(

    IAI FF 0

    Xa

    dFF DAD 0

    Xa

    cFF

    CAC 0

    Xa

    bFF BAB 0

    )1(0 XFF AA

    00 ABB FF

    00 ACC FF

    00 ADD FF

    00 AII FF

    )( 0XFA

    )( 0XFa

    bA

    )( 0XFa

    cA

    )( 0XFa

    dA

    Stoichiometric Table: Flow system

    + +

    Stoichiometric Table: Flow system-Example

  • 7/24/2019 Lecture 4- Stoichiometry

    46/50

    Task

    A mixture of 28% SO2 and 72% air is charged to a flow reactor in which SO2 is

    oxidized.

    2 SO2 + O2 2 SO3

    Calculate concentrations of all species at the following conversion of SO2: 0; 0.25;

    0.5; 0.75; 1, if the total pressure is 1485 kPa and the temperature is constant at

    227oC. Note that air composition is 21% O2 and 79% N2.

    Solution

    Taking SO2 as the basis of calculations:

    +12

    Stoichiometric Table: Flow system Example

    2 SO2 + O2 2 SO3

    Stoichiometric Table: Flow system-Example

  • 7/24/2019 Lecture 4- Stoichiometry

    47/50

    Task

    The elementary gas phase reaction

    3A + 2B

    3 C +5 D

    is carried out in a flow reactor operated isothermally at 427C and 28.7 atmospheres.

    Pressure drop can be neglected. Express the rate law and the concentration of each

    species as a function of conversion and as a function of the total molar flow rates. The

    entering volumetric flow rate is 10 dm3/s and the specific reaction rate,k is 200

    dm12/mol4s. The feed is equal molar in A and B.

    Solut ion

    k1

    A is the limiting reactant

    Stoichiometric Table: Flow system Example

    Volume change (Flow system) -Example

  • 7/24/2019 Lecture 4- Stoichiometry

    48/50

    Task

    The elementary gas phase reaction

    3A + 2B

    3 C +5 D

    is carried out in a flow reactor operated isothermally at 427C and 28.7 atmospheres.

    Pressure drop can be neglected. Express the rate law and the concentration of each

    species as a function of conversion and as a function of the total molar flow rates. The

    entering volumetric flow rate is 10 dm3/s and the specific reaction rate is 200 dm12/mol4s.

    The feed is equal molar in A and B.

    k1

    Volume change (Flow system) Example

    Volume change (Flow system) -Example

  • 7/24/2019 Lecture 4- Stoichiometry

    49/50

    Task

    The elementary gas phase reaction

    3A + 2B

    3 C +5 D

    is carried out in a flow reactor operated isothermally at 427C and 28.7 atmospheres.

    Pressure drop can be neglected. Express the rate law and the concentration of each

    species as a function of conversion and as a function of the total molar flow rates. The

    entering volumetric flow rate is 10 dm3/s and the specific reaction rate is 200 dm12/mol4s.

    The feed is equal molar in A and B.

    k1

    Volume change (Flow system) Example

    Summary

  • 7/24/2019 Lecture 4- Stoichiometry

    50/50

    Summary