Lab Reactor Performance Problem Analysis by Jon Worstell (oct 2015).pdf

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    Analysis of SCG Olefin Metathesis Process Issues

    Jonathan H. Worstell 25 October 2015

    Abstract

    This report contains an analysis of trickle-bed olefin metathesis with regard to the process operated by SCG. It concludes with a list of recommended actions to prove or disprove various possibilities.

    Introduction SCG currently metathesize 2-butene with ethylene to produce propylene. The feed to their reactor is a mixture of ethylene and 1-butene; however, the 1-butene stream contains various C4 components. This feed mixture enters an MgO catalyst bed that isomerizes 1-butene to 2-butene; at full isomerization, the mole ratio is 25:75, respectively.

    This isomerized feed then enters an MgO/WO3 mixed catalyst bed that metathesizes 2-butene and ethylene to produce propylene. The WO3 catalyzes the metathesis reaction while the MgO isomerizes 1-butene to 2-butene. This latter reaction is necessary because, as metathesis occurs, the concentration of 2-butene declines; thus, propylene formation declines. However, the MgO catalyst maintains 2-butene concentration at full isomerization; i.e., at 75 mole percent, by isomerizing 1-butene. Note that the 2-butene can also metathesize with 1-butene and 2-butene. The former reaction produces a propylene molecule and 2-pentene while the latter reaction simply occupies catalytic sites.

    SCG have developed catalysts to replace one or both of the above catalysts. SCG are now preparing to test their new catalysts in their laboratory and pilot plant facilities. Unfortunately, testing the current catalysts in their laboratory facility has identified a problem. The problem is: at equivalent weight hourly space velocities (WHSV in per hours), their laboratory process demonstrates lower conversion than either their pilot plant or plant processes; and, their laboratory process demonstrates poorer catalyst stability; i.e., shorter catalyst life, than either their pilot plant or plant processes. The following illustrative information encapsulates their problem.1

    WHSV = 10 hr-1 (commercial scale WHSV which is used to test lab & pilot scale too)

    Conversion - Lab (3 g MgO, 9 g WO3 catalyst): conversion = < 10% - Pilot (35 g MgO, 105 g WO3 catalyst): conversion = 40% - Commercial (4.7 ton MgO, 14.2 ton WO3 catalyst): conversion =

    60%

    Catalyst stability days - Lab (3 g MgO, 9 g WO3 catalyst): stability = 1 day (TGA weight loss

    did not imply more cokes formed)

    1 Wiroon Tanthapanichakoon, e-mail communication, 5 October 2015.

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    - Pilot (35 g MgO, 105 g WO3 catalyst): stability = 3 days - Commercial (4.7 ton MgO, 14.2 ton WO3 catalyst): stability = 30

    days

    The question is: what is causing this disconnect between the laboratory process and the pilot plant and plant processes?

    Assumptions for Process Analysis

    1. The catalyzed process comprises one reactor containing two catalyst beds in series. 2. The first bed encountered by the olefin feed comprises MgO pellets. 3. The second bed comprises a mixture of MgO pellets and granulated WO3. 4. The MgO catalyst isomerizes the butenes present in the olefin feed. 5. The WO3 catalyst metathesizes the ethylene and butenes in the olefin feed. 6. The butene feed is a mixture and enters the reactor as a liquid. 7. The ethylene is gaseous (critical temperature is 49.1 oF or 9.5 0C and critical

    pressure is 742.7 barg, which is 49.5 atmospheres or 50.2 barg; operating conditions are 350 oC and 20 barg).

    Derivation of Pertinent Equations

    1-butene Isomerization SCG first isomerize their 1-butene to 2-butene through a reactor containing MgO pellets.2 MgO is a solid base, which, if properly activated, isomerizes carbon-carbon double bonds. For this analysis, we assume the SCG MgO catalyst to be porous. The butene mass balance, in circular cylindrical coordinates, is

    B2

    2

    2

    2

    2Bz

    r Rz

    B

    B

    r

    1

    r

    Br

    rr

    1D

    z

    Bv

    B

    r

    v

    r

    Bv

    t

    B

    where B represents butene molar concentration (mols/m3); t is time is seconds (s); r is the radial distance from the container centerline to the inner wall of the container; i.e., reactor; is the azimuthal angle about the container centerline (dimensionless); z is the physical height of the catalyst bed in the container (m); vr; v, and vz are the linear fluid velocities in the radial, azimuthal, and axial directions, respectively (m/s); DB is the

    diffusivity constant for butene in a butene mixture (m2/s); 2

    2

    2

    2

    2 z

    B

    B

    r

    1

    r

    Br

    rr

    1

    represents the diffusion of butene in each direction; and, BR is the rate of butene

    isomerization. The information provided for this analysis appears to be for the mixed MgO/WO3 catalyst bed and not for the separate MgO catalyst bed. Therefore, nothing can be said about dispersion effects in the MgO catalyst bed. Thus, we make these assumptions with regard to the MgO catalyst bed.

    2 We define a reactor as a discrete, distinct bed or charge of catalyst. Such a reactor can be in a

    container with a separate, distinct charge of a second catalyst, which comprises a second reactor in the same container.

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    0t

    B

    ; the process is at steady state;

    vr = v = 0; axial flow only;

    0z

    B

    B

    r

    1

    r

    Br

    rr

    12

    2

    2

    2

    2

    ; no dispersion of any kind.

    We make these assumptions to arrive at a simple model. If we need a more complex model, we can relax one of our assumptions to discuss a particular issue at that time. Implementing these assumptions, we can reduce the above mass balance to

    BBF

    z Rz d

    B dv

    Multiplying this equation by the ratio ACS/ ACS, where ACS is the cross sectional area of the empty container or reactor, we obtain

    BBF

    CS

    CSz R

    z d

    B d

    A

    Av

    which reduces to

    B

    CS

    BF Rz d A

    B d Q

    where Q is ACSvz. Rearranging and integrating the left side of the above equation gives

    B

    BF

    R

    B d

    Q

    V

    V/Q is space time; its inverse, Q/V is space velocity. Since the publication of Hougen and Watsons classic volume Chemical Process Principles: Kinetics and Catalysis in 1947, the parameter used for scaling fixed-bed processes, catalytic and non-catalytic, has been space velocity.3 Space velocity is defined as process volumetric flow rate divided by the volume of solid material through which the process fluid flows. Thus, its units are

    s

    1

    m

    sm

    3

    3

    3 O. Hougen and K. Watson, Chemical Process Principles: Kinetics and Catalysis, John Wiley and Sons, Inc, New York, NY, 1947.

  • 4

    Space velocity has the same dimensionality as a first-order reaction rate constant. This equivalence between a physical inverse time and a chemical inverse time is generally made when scaling a fixed-bed process. However, these two inverse times are only equivalent when the process is reaction rate limited and the reaction mechanism is first-order. These two inverse times are not equivalent if the process is diffusion rate limited.

    To evaluate BR , we must develop a simplified mechanism for butene

    isomerization. Figure 1 presents such a mechanism. It shows a schematic of olefin isomerization by a porous solid catalyst. When a fluid flows over a solid surface, a boundary layer forms along that surface. This boundary layer forms a stagnant film. Little or no convective mass transport occurs in this stagnant film. Thus, mass transport from the bulk fluid to the solid surface occurs via molecular diffusion.

    The reactant concentration at the bulk fluid/stagnant film boundary is the concentration of reactant in the feed. Movement of reactant from the bulk fluid/stagnant film boundary to the catalyst surface occurs by molecular diffusion, which is generally modeled as a linear concentration difference. That difference is BBF BSF, where BSF is the concentration of reactant at the surface of the catalyst and CBF is the concentration of reactant in the bulk fluid, both in mol/m3. Reactant must then diffuse from the surface of the catalyst along pores to the catalytic sites inside the porous solid. Movement of reactant in the pore is also by molecular diffusion, which is modeled as a linear concentration difference. Catalytic sites occur along the length of the pore, thus reactant concentration in a pore is a function of pore length. In other words, reactant concentration decreases along the length of the pore.

    Reactant concentration at a given catalyst site is BAS (mol/m3). Thus, the

    concentration difference to that point in the pore is BSF BAS. Equilibrium may be established at the catalytic site; reactant concentration at equilibrium is BEQ (mol/m

    3). For 1-butene isomerization, the mole ratio of 2-butene to 1-butene is 75:25, respectively.

    Olefin isomerization follows a first-order reaction mechanism with respect to olefin. Thus, for our case, reactant conversion rate is given as

    EQASRxnRxn BBkr Equation 1

    where rRxn is the reaction rate for olefin isomerization at the catalytic site (mol/m

    3*s); kRxn

    is the reaction rate constant (1/s). The rate of reactant movement along the pore is

    ASSFP

    PPDPD BBv

    akr

    Equation 2 where rPD is the molar rate at which olefin moves along the pores (mol/m

    3*s); kPD is the

    velocity at which olefin moves along the pores (m/s); aP is the cross sectional area of

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    the pore (m2); and, vP is the average catalyst pore volume (m3). The units of

    P

    PPD v

    ak

    are m3/m3*s. The rate of reactant movement through the stagnant film surrounding the catalyst

    pellet is

    SFBFSFSF BBVSkr Equation 3

    where rSF is the molar rate at which olefin moves along the pores (mol/m

    3*s); kSF is the

    velocity at which olefin moves through the stagnant film (m/s); S is the exterior surface area of the catalyst pellet (m2), and V is the volume of the catalyst pellet (m3). The units

    of V

    SkSF are m3/m3*s.

    The only reactant concentrations known with any accuracy are BBF and BEQ. Thus, the mathematical expression for the rate of reactant conversion must be in terms of BBF and BEQ. Solving Equation 1 for BAS yields

    ASEQ

    Rxn

    Rxn BBk

    r

    Solving Equation 2 for BSF gives

    SFAS

    P

    PPD

    PD BB

    va

    k

    r

    Substituting for BAS gives

    SFEQ

    Rxn

    Rxn

    P

    PPD

    PD CCk

    r

    va

    k

    r

    Solving Equation 3 for BBF yields

    BFSFSFSF BB

    VSk

    r

    then substituting for BSF provides us with

    BFEQSFSF

    P

    PPD

    PDs

    Rxn

    Rxn BB

    VSk

    r

    va

    k

    r

    k

    r

  • 6

    Rearranging the above equation gives reactant conversion rate in terms of BBF and BEQ. Thus

    EQBFSFSF

    P

    PPD

    PDs

    Rxn

    Rxn BB

    VSk

    r

    va

    k

    r

    k

    r

    Assuming rRxn = rPD = rSF = r, then rearranging the above equation, yields conversion in terms of BBF and BEQ

    EQBFSFP

    PPD

    Rxn

    BB

    VSk

    1

    va

    k

    1

    k

    1r

    or

    EQBF

    SF

    P

    PPD

    Rxn

    BB

    VSk

    1

    va

    k

    1

    k

    1

    1r

    The overall process rate constant kOverall (1/s) is, by definition

    VSk

    1

    va

    k

    1

    k

    1

    1k

    SF

    P

    PPD

    Rxn

    overalll

    Inverting kOverall yields

    V

    Sk

    1

    va

    k

    1

    k

    1

    k

    1

    SF

    P

    PPD

    RxnOverall

    Equation 4

    Equation 4 has the same mathematical form as a series of resistors in an electrical circuit. Thus, the overall resistance to olefin isomerization; i.e., 1-butene to 2-butene formation, is represented by 1/kOverall. The portion of the overall resistance

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    attributed to the chemical reaction rate is 1/kRxn. Pore diffusion resistance and film

    diffusion resistance are represented by

    P

    PPD v

    ak

    1 and V

    Sk1

    SF

    , respectively.

    Equation 4 can be simplified to yield

    V

    Sk

    1

    kv

    ak

    kv

    ak

    k

    1

    SFRxn

    P

    PPD

    RxnP

    PPD

    Overall

    which reduces to

    V

    Sk

    1

    k

    1

    k

    1

    SFRxnOverall

    Equation 5

    where is defined as

    RxnP

    PPD

    P

    PPD

    kv

    ak

    va

    k

    Thus, the overall resistance to 1-butene isomerization is the sum of two resistances: one related to film diffusion, the other to pore diffusion and reaction rate combined. If

    RxnP

    PPD kv

    ak

    , then = 1, which describes a reaction rate limited chemical

    process. If RxnP

    PPD kv

    ak

    , then < 1, which describes a pore diffusion rate limited

    chemical process. Olefin isomerization also depends upon the number of catalyst sites on the

    porous solid. Let IsomtN represent the number of isomerization catalyst sites functioning at

    time t. Note that IsomtN is on a solids basis. However, r is on a fluid basis. Multiplying Isom

    tN by

    1

    where is the void fraction for the catalyst bed, converts IsomtN from a solids basis to a

    fluid basis. Therefore, R for olefin isomerization is

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    EQBFOverallIsom

    t

    Isom

    t BB k N

    1r N

    1R

    Substituting this equation into the integral equation above yields

    ExitBF

    FeedBF

    ExitBF

    FeedBF

    BB

    BB EQBF

    BF

    Overall

    Isom

    t

    BB

    BBEQBFOverall

    Isom

    t

    BF

    BB

    B d

    k N

    1

    1

    BB k N

    1

    B d

    Q

    V

    where BFeed is the concentration of 2-butene entering and BExit is the 2-butene concentration exiting the MgO catalyst bed. Integrating gives

    EQFeed

    EQExit

    Isom

    tOverall

    BB

    BB ln

    Q

    V N

    1 k

    Note that N(t) changes with time as active catalyst sites deactivate or become poisoned. Levenspiel has proposed a method for evaluating the change in N(t) as a function of time.4

    Assume the rate at which the number of active catalyst sites disappears with respect to time is a power law equation; namely

    Isom

    tDecay

    Isom

    t Nkdt

    dN

    where kDecay is the catalyst decay rate constant. Integrating the above equation yields

    tkIsom

    0

    Isom

    tDecayeNN

    where IsomtN is the number of active catalyst sites at any time greater than t = 0 and is

    defined as

    0t at Rate ionIsomerizat

    t time at Rate ionIsomerizatNIsomt

    Isom

    0N is the number of active catalyst sites at t = 0. Isom

    tN and Isom

    0N are indications of

    catalyst efficiency. Isom0N is assumed to be unity.

    Substituting IsomtN into the isomerization equation above yields

    4 O. Levenspiel, Journal of Catalysis, 25, 265 (1972).

  • 9

    EQFeed

    EQExittkIsom

    0

    Overall

    BB

    BB lneN

    Q

    V

    1 k

    Decay

    Taking the logarithm again gives us

    EQFeed

    EQExitDecay

    Isom

    0Overall

    BB

    BB lnlntk-

    Q

    V N

    1 k

    Therefore, plotting

    EQFeed

    EQExit

    BB

    BB lnln as a function of time produces a straight line with a

    declining slope of kDecay. This analysis allows us to quantify kDecay. Butene isomerization in the MgO/WO3 catalyst bed can be evaluated using the above equations. Ethylene Diffusion The liquid phase ethylene mass balance, in circular cylindrical coordinates, is

    LE2

    L

    2

    2

    L

    2

    2

    LEB

    Lz

    LLr

    L Rz

    E

    E

    r

    1

    r

    Er

    rr

    1D

    z

    Ev

    E

    r

    v

    r

    Ev

    t

    E

    where EL represents ethylene molar concentration of ethylene in the butene mixture (mols/m3); t is time is seconds (s); r is the radial distance from the container centerline to the inner wall of the container; i.e., reactor; is the azimuthal angle about the container centerline (dimensionless); z is the physical height of the catalyst bed in the container (m); vr; v, and vz are the linear fluid velocities in the radial, azimuthal, and axial directions, respectively (m/s); DEB is the diffusivity constant for ethylene in the

    butene mixture (m2/s); 2

    L

    2

    2

    L

    2

    2

    L

    z

    E

    E

    r

    1

    r

    Er

    rr

    1

    represents the diffusion of ethylene

    in each direction; and, LE

    R is the rate of ethylene appearance or disappearance as the

    case may be. To simplify the above mass balance, we make these assumptions

    0t

    EL

    ; the process is at steady state;

    vr = v = 0; axial flow only;

    0z

    E

    E

    r

    1

    r

    Er

    rr

    12

    L

    2

    2

    L

    2

    2

    L

    ; no dispersion of any kind.

  • 10

    We make these assumptions to arrive at a simple model. If we need a more complex model, we can relax one of our assumptions to discuss a particular issue at that time. Implementing these assumptions, we can reduce the above mass balance to

    LEL

    z Rz d

    E dv

    To evaluate LE

    R , we must develop a simplified mechanism for the consumption

    of ethylene. Figure 2 presents such a mechanism. Gaseous ethylene moves from the left of Figure 2 to a gas-liquid interface where it is absorbed into the liquid butene mixture. Henrys Law controls the amount of ethylene absorbed by the butene mixture. Absorbed ethylene attains some concentration in the butene mixture. That ethylene concentration remains constant. Dissolved ethylene then diffuses across the stagnant liquid film surrounding each porous, solid catalyst pellet. Ethylene eventually enters one of the pores of the catalyst, diffuses along the length of the pore, and finally encounters a catalyst site where metathesis occurs. Each of these steps has a rate. The rate at which the mixed butene liquid absorbs ethylene is

    IEAbsAbs PPkr

    where rAbs is the molar absorption rate of ethylene (atm/s); kAbs is the absorption mass transfer coefficient (1/s); PE is ethylene pressure (atm); and, PI is ethylene pressure at the gas-liquid interface (atm). We discount the presence of a stagnant gas film on the gas side of this physical interface because

    the ethylene is essentially pure;

    the process operating pressure is high enough to preclude significant butene vaporization.

    Henrys Law controls the dissolved ethylene concentration at the gas-liquid

    interface. That ethylene interface concentration is

    PI = (He) EI

    where He is Henrys Law constant (atm*m3/mol) and EI is ethylene concentration at the

    gas liquid interface (mol/m3). Substituting for PI in rAbs gives

    IEAbsAbs EHePkr

    Rearranging this last equation yields

    IE

    Abs

    Abs E He

    P

    kHe

    r

  • 11

    There is a stagnant liquid film on the liquid side of the gas-liquid interface. This stagnant film develops due to the mixing of ethylene and butene molecules near the physical interface. The rate of ethylene movement through this liquid stagnant film is

    BIL

    LLGLSF EE v

    akr

    where rGLSF is the molar rate of ethylene movement through the gas-liquid stagnant film (mol/m3*s); kL is the velocity at which ethylene moves through the gas-liquid stagnant

    film (m/s); L

    L

    va

    is the ratio of the cross sectional area through which ethylene diffuses

    to the volume of that stagnant film (m2/m3); and, EB is the ethylene concentration in the bulk mixed butene liquid (mol/m3). Rearranging rGLSF gives

    IB

    L

    LL

    GLSF E E

    va

    k

    r

    There is a liquid-solid stagnant film surrounding each catalyst pellet in the container. The rate of ethylene movement through this liquid-solid stagnant film is

    PSBLSFLSF EE VSkr

    where rLSF is the molar rate of ethylene movement through the liquid-solid stagnant film (mol/m3*s); kLSF is the velocity at which ethylene moves through the liquid-solid stagnant

    film (m/s); V

    S is the ratio of the surface area of the catalyst pellet to the volume of the

    catalyst pellet (m2/m3); and, EPS is the ethylene concentration at the surface of the catalyst pellet (mol/m3). Rearranging rLSF gives

    BPSLSFLSF EE

    VSk

    r

    At the catalyst pellet surface, ethylene molecules enter pores leading to the interior of each catalyst pellet. The rate at which ethylene diffuses along these catalyst pores is

    ASPSP

    PPDPD EE v

    akr

    where rPD is the molar rate of ethylene movement along the catalyst pores (mol/m

    3*s);

    kPD is the velocity at which ethylene moves along the catalyst pores (m/s); P

    P

    va

    is the

    ratio of the cross sectional area of the catalyst pores to the volume of catalyst pores

  • 12

    (m2/m3); and, EEQ is the equilibrium ethylene concentration of ethylene at an active catalytic site (mol/m3). There will be an equilibrium ethylene concentration, EEQ, for this reaction due to the various metathesis reactions occurring at each catalytic site, such as

    C1=C2 + C3=C4 C1=C3 + C2=C4

    C1=C2 + C3=C4-C5-C6 C1=C3 + C2=C4-C5-C6

    C1=C2-C3-C4 + C5=C6-C7-C8 C1=C5 + C4-C3-C2=C6- C7-C8

    Rearranging rPD gives

    PSEQ

    P

    PPD

    PD E E

    va

    k

    r

    Back substituting each of the concentrations yields

    EQE

    P

    PPD

    PD

    LSF

    LSF

    L

    LL

    GLSF

    Abs

    Abs E He

    P

    va

    k

    r

    VSk

    r

    va

    k

    r

    kHe

    r

    But, ethylene cannot diffuse through the process faster than the slowest molar rate; therefore

    r = rAbs = rGLSF = rLSF = rPD

    Making the appropriate substitution gives

    EQE

    P

    PPD

    LSF

    L

    LL

    Abs

    E He

    P

    va

    k

    1

    VSk

    1

    va

    k

    1

    kHe

    1 r

    koverall for ethylene diffusion, in resistance notation, is

    P

    PPD

    LSF

    L

    LL

    AbsOverall

    va

    k

    1

    VSk

    1

    va

    k

    1

    kHe

    1

    k

    1

    The form of kOveral for ethylene diffusion is thus similar to that found for 1-butene isomerization.

    Ethylene/2-butene Metathesis The desired metathesis reaction is

  • 13

    C=C + C-C=C-C 2 C=C-C

    However, other metathesis reactions can occur at an active catalyst site. Those reactions are

    C=C-C + C=C-C-C C=C + C-C=C-C-C

    C1=C2-C3 + C4-C5=C6-C7 C1=C5-C6 + C3-C2=C6-C7

    C1=C2 + C3=C4-C5-C6 C1=C3 + C2=C4-C5-C6

    C1=C2-C3 + C4=C5 C2=C3-C4 + C1=C4

    The above are the main metathesis reactions that we will consider in this analysis. The first metathesis reaction above produces the product, propylene. The second metathesis reaction above consumes propylene and produces an unwanted by-product, which can enter into various metathesis reactions as well; however, we will not consider those reactions in this analysis. The last three metathesis reactions above produce no change in the overall product distribution exiting the reactor. They do, however, occupy the active catalytic site. Therefore, they act as a diluent in the process.

    From the above two sections, using the same assumptions, the mass balance for propylene formation is

    PR

    P d

    Q

    V

    where P is propylene molar concentration (mol/m3) and RP is the molar rate of propylene formation (mol/m3*s). RP is

    PB-1N

    1kB2E N

    1k Metat

    Meta

    OverallEQ

    Meta

    t

    Meta

    Overall

    where MetaOverallk is the overall metathesis rate constant, which we assume to be the same

    for the metathesis reaction forming propylene and for the metathesis reaction

    consuming propylene; N Metat represents the number of active metathesis catalytic sites

    at time t of the catalyst bed life; and (EEQ), (2-B), (P), and (1-B) are the molar concentrations of ethylene, 2-butene, propylene, and 1-butene (mol/m3). The above mass balance then becomes

    PB-1N

    1kB2E N

    1k

    P d

    Q

    V

    Meta

    t

    Meta

    OverallEQ

    Meta

    t

    Meta

    Overall

  • 14

    If we let

    B2E N

    1k a EQ

    Meta

    t

    Meta

    Overall

    and

    B-1N

    1kb Metat

    Meta

    Overall

    we can then write the above integral equation as

    Pba

    P d

    Q

    V

    The boundary conditions are: at z = 0, P = PFeed; at z = L, P = PProduct. L is the length of the catalyst bed in the reactor. Integrating the above equation gives

    Product

    Feed

    Feed

    Product

    Pba

    Pbaln

    b

    1

    Pba

    Pbaln

    b

    1

    Q

    V

    If PFeed = 0, the above becomes

    Product

    Product

    Pba

    aln

    b

    1

    a

    Pbaln

    b

    1

    Q

    V

    Substituting for a and b gives us

    B2E N

    1k

    PB-1N

    1kB2E N

    1k

    ln

    B-1N

    1k

    1

    Q

    V

    EQ

    Meta

    t

    Meta

    Overall

    Product

    Meta

    t

    Meta

    OverallEQ

    Meta

    t

    Meta

    Overall

    Meta

    t

    Meta

    Overall

    Note that MetaOverallk is

    V

    Sk

    1

    va

    k

    1

    k

    1

    k

    1

    SF

    P

    PPD

    RxnMeta

    Overall

    Flow Patterns through Trickle-bed Reactors

    Inlet distributors and local catalyst packing characteristics have an inordinate impact on the process performance of trickle-bed reactors. This situation arises because the liquid flow through such reactors is generally slow. If liquid flow is gravity-driven, then there

  • 15

    are few degrees of freedom with which to adjust the liquid distribution through the catalyst mass. Depending upon catalyst pellet properties and liquid flow rate, void-filling liquid flow may be as films, rivulets, pendular structures, or filaments. Stagnant fluid pockets may also exist in trickle-bed catalyst masses.5 Figure 3 illustrates these flow patterns.

    The flow patterns in Figure 3 control the residence time distribution of the process. In other words, it is the wetted area of the catalyst mass that defines residence time for a catalyst mass. Normalizing conversion with catalyst wetted area can lead to correlations between process units, but the issue is: how to determine catalyst wetted area? Since so many flow types may occur in a trickle-bed catalyst mass and since there are so few variables for controlling flow type, it is not surprising that these processes are so difficult to upscale and downscale. We should not necessarily expect a laboratory tickle-bed reactor to correlate well with a pilot plant trickle-bed reactor, which, in turn, may not correlate well with a plant trickle-bed reactor.6

    Analysis of SCG Olefin Metathesis Processes

    Mass Transport Each reaction, 1-butene isomerization and 2-butene metathesis, have a variety of resistances. While butene isomerization and metathesis occur in the liquid phase, it is doubtful that metathesis is pore diffusion rate limited. Porous, solid catalyzed olefin metathesis is generally not pore diffusion rate limited at molecular weights less than octene or decene. It is unlikely that the current SCG olefin metathesis process is pore diffusion rate limited. The same cannot be said for the 1-butene isomerization process. The number of catalytically active isomerization sites depends upon the MgO activation procedure. If few sites are activated during a particular activation procedure, then the SCG isomerization process could become pore diffusion rate limited. If fewer catalytically active isomerization sites are generated in the SCG laboratory process than in the SCG plant process, then the laboratory process will demonstrate lower conversion and shorter catalyst life. Of course, the same is true for the SCG pilot plant. It is doubtful that either the SCG isomerization process or the metathesis process are liquid-solid stagnant film diffusion rate limited.7 Most likely, the SCG olefin metathesis process is reaction rate limited, which means it becomes highly dependent upon the number of active sites generated during each activation procedure. It also becomes highly dependent upon the pattern of liquid flow through the catalyst mass.

    Process Poisons

    All the above process rate equations contain the term IsomtN or Meta

    tN , which measure in

    some fashion the number of catalytically active sites for each process. If the catalyst

    5 V. Ranade, R. Chaudhari, and P. Gunjal, Trickle-bed Reactors: Reactor Engineering and Applications,

    Elsevier, Amsterdam, The Neatherlands, 2011. 6 L. Ross, Chemical Engineering Progress, 61(10), 77 (1965).

    7 D. Mears, Chemical Engineering Science, 26, 1361 (1971).

  • 16

    activation procedure varies, then IsomtN and Meta

    tN will vary. For low Isom

    tN and Meta

    tN ,

    process conversion will be low and catalyst life will be short compared to a catalyst

    activation procedure that produces larger IsomtN and Meta

    tN numbers.

    Also, if the olefin metathesis process contains a poison, then IsomtN and Meta

    tN will

    decline faster than normal; thus, the olefin metathesis process will demonstrate lower process conversion and shorter catalyst life. The MgO solid base catalyst is poisoned by Lewis acids; it is particularly sensitive to water. The WO3 metathesis catalyst is most likely susceptible to water poisoning. If isomerization catalyst sites are decaying preferentially to metathesis catalyst sites, then 3-hexene will accrue in the process. If metathesis catalyst sites are decaying preferentially to isomerization catalyst sites, then 2-pentene will accrue in the process. Axial Aspect Ratio --- Axial Dispersion

    Carberry suggests that axial dispersion,

    2

    L

    2

    EBz

    ED , can be neglected if the axial

    aspect ratio, Pd

    Z , where Z is the physical height of the catalyst mass or bed and dP is

    the diameter of the catalyst pellet, is greater than 150.8 Carberrys criterion is considerably more conservative than the common rule of thumb for axial dispersion;

    namely, 30d

    ZP

    to ensure against axial dispersion.9 Mears has published a more

    general rule for deciding whether a concurrent, gas-liquid, trickle-bed reactor suffers axial dispersion. His general rule is

    Out

    In

    a

    zPP C

    Cln

    Dvd

    20n

    d

    Z

    where n is reaction order; vz is superficial liquid velocity; Da is the axial diffusivity coefficient; [C]In is the concentration of reactant entering the trickle-bed reactor, and [C]Out is the concentration of reactant exiting the trickle-bed reactor. If axial dispersion occurs in a process, then its conversion will be less than expected. Radial Aspect Ratio --- Radial Dispersion

    The opposite is true for radial dispersion,

    r

    Er

    rr

    1D LEB . If the radial aspect ratio

    PdR where R is the radius of the container and dP is the diameter of the catalyst pellet,

    is less than 3 or 4, then we can neglect radial dispersion.10 However, radial dispersion

    8 J. J. Carberry, Canadian Journal of Chemical Engineering, 36, 207 (1958).

    9 D. Mears, Chemical Engineering Science, 26, 1361 (1971).

    10 H. Lee, Heterogeneous Reactor Design, Butterworth Publishers, Boston, MA, 1985, page 291.

  • 17

    arises only when a fixed-bed reactor is externally cooled. In other words, the reaction has to be exothermic, thereby requiring external cooling. It is the temperature difference between the centerline portion of the catalyst mass and the container wall that induces radial flow; i.e., radial dispersion. Since olefin isomerization is nearly thermally neutral and olefin metathesis is thermally neutral, it is doubtful that radial dispersion occurs in the SCG olefin metathesis processes. Pellet Aspect Ratio

    There is also a pellet aspect ratio. It is Pd

    D , where D is the diameter of the container

    and dP is the diameter of the catalyst pellet. If 10dD

    P

    , then we generally assume

    process fluid is channeling along the container wall.11 If wall channeling occurs, then process conversion will be lower than expected. Mears writes that at ninety-two percent conversion, a wall flow of 1.1 percent is sufficient to increase the required reactor length by five percent.12 Summary of Aspect Ratios The below table gives the aspect ratios for the various SCG olefin metathesis processes.13

    Process Scale Axial Aspect Ratio Radial Aspect Ratio Pellet Aspect Ratio

    Laboratory 3.5 1.6 3

    Pilot Plant 13 3.8 8

    Plant 758 300 600

    The above table indicates that the SCG laboratory and pilot plant olefin metathesis process conversions might be lower than expected due to axial dispersion. Conversion at the plant does not suffer from axial dispersion. On the other hand, the plant may experience radial dispersion, but it is doubtful that such dispersion adversely impacts process conversion. The laboratory process does not experience radial dispersion. The pilot plant may or may not experience radial dispersion. The above table does indicate that conversion in the laboratory process is probably adversely impacted by wall channeling. Conversion in the pilot plant process probably suffers from some wall channeling. Conversion at the plant is not impacted by wall channeling.

    Recommendations 1. Determine koverall for isomerization as a function of MgO activation temperature

    and activation procedure. 2. Monitor the process for 2-pentene and 3-hexene and determine whether they can

    be correlated to specific catalyst poisoning.

    11

    H. Rase, Chemical Reactor Design for Process Plants, Volume One, Principles and Techniques, John Wiley and Sons, Inc, New York, NY, 1977, page 537. 12

    D. Mears, Chemical Engineering Science, 26, 1361 (1971). 13

    Wiroon Tanthapanichakoon, e-mail communication, 5 October 2015.

  • 18

    3. Determine whether pre-wetting the catalyst bed improves process performance. If pre-wetting improves process performance, then channeling through the catalyst bed is most likely happening.14

    4. Determine whether increasing ethylene velocity through the catalyst bed improves process performance. Increasing gas velocity increases the local liquid flow through the central portion of the catalyst bed, which will improve process conversion.15

    5. Determine whether spherical catalyst pellets improve process performance. Cylindrical catalyst pellets are more likely to create wall channeling than are spherical pellets.16

    6. Determine the impact of catalyst bed length on process performance. If wall channeling is occurring, then process performance will improve as a function of catalyst bed length. Process performance will increase as catalyst bed length increases and eventually it will plateau.17

    7. Confirm the presence of wall channeling by building and operating a laboratory

    reactor with a minimum diameter of 50 mm. Such a reactor will give 10d

    DP

    .

    Or, reduce the catalyst pellet diameter so that the present process meets

    10d

    DP

    .

    8. Start measuring kDecay for all process catalysts. Use these kDecay to compare catalyst stability.

    9. Start developing process performance correlations between each SCG trickle-bed process. Because of the various liquid flow patterns through a trickle-bed catalyst mass, it is unlikely that a fundamental scaling rule will work. SCG will have to develop empirical correlations between their various sized trickle-bed processes.

    14

    A. Sederman and L. Gladden, Chemical Engineering Science,56, 2615 (2001). 15

    N. Sylvester and P. Pitayagulsarn, Canadian Journal of Chemical Engineering, 53, 599 (1975). 16

    M. Borda et al, American Institute of Chemical Engineering Journal, 24, 439 (1978). 17

    D. Mears, Chemical Engineering Science, 26, 1361 (1971).

  • 19

    Figure 1. Schematic of Butene Isomerization

    Bulk Fluid BBF Pore Catalytic Site BAS BEQ (BSF BAS) (BAS BEQ) (BBF BSF)

    Stagnant Film

    BSF

  • 20

    Figure 2. Schematic of Ethylene Absorption into Mixed Butenes with Reaction.

    Gaseous Ethylene PE PE = HeEPI Bulk Fluid EB Pore Catalytic Site EPD ERxn (ESF EPD) Stagnant Film (EPD ERxn) ESF Stagnant (EB ESF) Film

    (EI EB)

  • 21

    Figure 3.

    Liquid Flow Patterns through a Trickle-bed Catalyst Mass (From: V. Ranade, R. Chaudhari, and P. Gunjal, Trickle-bed Reactors: Reactor Engineering and Applications, Elsevier, Amsterdam, The Neatherlands, 2011,

    page186.)