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Catalytically active catalyst poisons: the activity replacement problem
Shorter Communications significant error. The values of k. and D., have been found to be 0.136 kcal/m hr “C and 0.01584 m*/hr respectively. AH for the reaction Ca(OH)2+CaO+H20 has been taken as 23.75 kcal/mol on averaging the reported values. The dotted line in Fig. 4 thus shows the reaction curve for the decomposition of calcium hydroxide. The reaction curve almost coincides with the equilibrium curve plotted on the basis of the data published by Tamaru and Soimi[7]. The thermal decomposition is thus found to be a diffusion-controlled process and to proceed virtually at equilibrium. The drying process has been assumed to occur also at equilibrium and the reaction points are therefore plotted on the saturation line, as shown in the Fig. 4. The tie lines, i.e. the lines joining the operating points and the corresponding reaction points, are found to be parallel, as observed more clearly by the enlarged diagram of a section of it. The assumption is thereby proved correct. For any sample of Ca(OH), (the values of k. and D_, being known), such diagrams can be directly applied to find out the reaction-surface temperature and concentration (X and x,,) at any operating temperature, The equations available for nonisothermal analysis (see, for example, Ishida and Shirai[2]) can then readily yield the exact kinetics of these processes. S. DUTTAt T. SHIRAI Research Laboratory of Resources Utilization Tokyo Znstitute of Technology Tokyo, Japan NOTATION
a stoichiometric coefficient of solid-gas reaction C total gas concentration, M/L’ C, concn of gas component A, MIL’ C,, initial concentration of solid, M/L’ tPresent address: Department ing, West Virginia University, 26506, U.S.A.
Chemicol
Engineering
Science,
Pergamon
D
molecular diffusivity of HzO in air, L’/e effective diffusivity of gas A in the ash layer, L’/O heat of reaction, HIM permeability coefficient of the ash layer, L2 effective thermal conductivity, HILOT chemical reaction rate constant, L*/MO total gas pressure, M/L8’ partial pressure of the reactantlproduct gas A, M/L# R initial radius of the reacting solid sphere, L radius, L 4 temperature, T t time, 8 U gas velocity, LI0 minimum fluidization velocity, L/8 l(rnf mol fraction of reactant gas; x.,., at the reaction XA surface; xAO,in the bulk.
De.4 AHA k k. k, P PA
symbols 6h equivalent thickness for gas film in heat transfer, L 6m equivalent thickness for gas film in mass transfer, L RERERENCES
111 Ishida M. and Shirai T.,‘J. Chem. Engng. Japan, 1%9 2 184. [21 Ishida M. and Shirai T., J. Chem. Engng. Japan, 1970 3 1%. [31 IshidaM.andWenC.Y.,A.Z.Ch.E.JLl%814311. 141 Levenspiel O., Chemical Reaction Engineering, John Wiley, New York 1972. [51 Perry J. H., Chemical Engineers’ Handbook, p. 462. McGraw-Hill 1950. [61 Shen J. and Smith J. M., I.E.C. Fund. 1%5 4 293. [71 Tamaru S. and Soimi K., 2. phys. Chem. 1932 Al61 421.
of Chemical EngineerMorgantown, W. Va.
1974, Vol. 29, pp. 2003-2005.
2003
Press.
[8] Wen [91 Wen [lOl Yagi tion,
Printcd
C. Y., I.E.C. 1%8 60 34. C. Y. and Wang S. C., Z.E.C. 1970 62 30. S. and Kunii D., Proc. Int. Symp. on Combus1955 23 1.
in Graat Britain
Catalytically active catalyst poisons: the activity replacement problem (Accepted 8 April 1974) It sometimes occurs in industrial practice that a porous catalyst is poisoned by a poison precursor which, upon deposition over the catalytic surface, has its own catalytic activity. Such processes can take place, as an example, when an automotive catalyst is being poisoned by a lead containing exhaust gas in an oxidative environment. Experiments[l] have shown that the catalyst cannot be poisoned fully; much more, it appears that lead oxide has some catalytic activity which, upon deposition over the original catalytic surface (e.g. Pt), gradually replaces the first kind of activity by its own activity. In this quoted example, of course, the poison is much less active than the original catalyst but the problem can be easily generalized
and, at least hypothetically, any combination of the two activities can be envisioned. The deposited active poison might alter both the rates and selectivities of the reactions taking place over the catalytic surface. Let US consider the simplest possible problem here where the original activity is gradually replaced by a second kind of activity in a porous, isothermal, single catalyst pellet of arbitrary geometry. The main reaction is characterized by the simple mechanism A + B, while P + W wil1 describe the activity replacement (i.e. poisoning) process. Both extemal and internal mass transfer resistances wil1 be considered. The formulation of poisoning problems for isothermal single pellets is exp-
Shorter
2004
Communications
lained in more detail in the works of Masamune and Smith [2] and Hegedus 131. If the activity’replacement is much slower than the rate of the main reaction, quasi-steady state prevails: *_ W
h 1-7
a** - h12Bm$An - hz*(l - %)“&t” = 0 a7
*_ a+
- A 1-r)
a+ -ar,
h32%o$p8 = 0
(1)
(2)
- %y+” = g
(3)
tiA(O>s)=f(rl)
(4)
bP(O,7))=0
(9
%(O, 71) =
1
(6)
c$T> 0) = PA[$A(7,O)- 11
ternal and internal mass transfer control. The salient feature of these plots is the residual activity after the activity replacement process has been completed. Although the above analysis might be an oversimplified representation of an actual catalytic process, it provides a qualitative picture which helps to identify the parameters and variables which affect the system and their relalionship to each other. In the author’s experience hypothetical computer ‘experiments’ such as these often provide valuable hints toward fruitful experimentation. Acknowledgements-J. computations.
Melbardis
assisted
L. LOUIS
during
the
HEGEDUS
Research Laboratories Genera1 Motors Corporation Warren, Michigan 48090, U.S.A.
(7) NOTATION
~(~,0)=Lw.w-11
(8)
3,
í)=0
(9)
$$(T,
1) = 0
Dimensioned
uariables a,, A, c,=
(10) c,i,
The addition of the last term to Eq. (1) accounts for the activity replacement process which, together with the main reaction, can have an arbitrary kinetic order. The dimensionless parameters and variables are defined in the Notation. Equations (1-10) were solved by a quasilinearization technique[4], for h = 2 (spherical pellets), m = n = p = q = a = 6 = 1 (first order reactions throughout), and various values of p and h, representing varying degrees of external and internal mass transfer control for the main reaction and the poisoning reaction. Figure 1 shows how the overall reaction rate changes with dimensionless time for a variety of examples. Run 5 replaces the original catalyst with another one of equal activity. Runs 1-4 employ poisons of increasing catalytic activity toward the main reaction at various degrees of ex-
10
Fig. 1. Activity
20
30
transition
c, 0. k,, k,, kx
ki
R - R IR,, t
40
during activity
50
replacement.
60
specific active surface area, cm2/cm’ pellet area covered by surface 1 mol of W, cm2/mol W concentrations far away from the pellet, mol/cm’ concentration of species i, molicm concentration, mol/ poison cm’ pellet effective diffusivity of species i, cm’/sec heterogeneous rate constant, units depend on reaction orders mass transfer coefficient of species i, cmlsec distance coordinate, cm characteristic pellet radius, cm fractional reaction rate time, sec
T
Shorter
Communications
2005
T = t(k2AwaO-‘CP,) distance
parameter
fraction of original surface remaining geometrie factor: h = 0 infinite flat slabs; h = 1 infinite cylinders; A= 2 spheres. reaction orders (see text)
Rkp PP = DP
of the main the original
Thiele parameter reaction over face
of the main the new sur-
Thiele parameter poisoning reaction
Chemical
Engineering
Science,
1974, Vol. 29, pp. 2005-2008.
Du rôle des agents tensio-act&
of
Pergamon
the
Press.
Biot number precursor
time of
of
the
the
main
poison
REFERENCES
[11 Unpublished
Thiele parameter reaction over catalyst
dimensionless
RkA BA = Biot number DA reacant
Genera1 Motors Research Laboratories, Michigan. [2] Masamune J. M., A.I.Ch.E..Jl. 1966 12 384. L. L., On the Poisoning of Porous Cata[31 Hegedus lysts by an Impurity in the Feed. Ind. and Engng chem. Fund. in press; ACS, Los Angeles, April 1974. J., Ind. and Engng chem. Fund. 1968 7 141 Newman 514.
Printed
in Great
results, Warren, S., Smith
Britain
dans la séparation de mélanges par un procédé à trois phases liquides
(Received 7 February
1974; accepted
La mise en oeuvre d’un procédé d’extraction liquide-liquide repose sur la disponibilité d’un solvant sélectif. De plus, la solubilité du soluté doit être assez grande sinon la quantité de solvant requise rend le procédé peu économique. Grâce à I’utilisation d’agents tensio-actifs, la méthode de séparation que nous présentons ici fait intervenir deux solvants: I’un est sélectif mais sans que la solubilité du soluté soit nécessairement importante, l’autre parfaitement miscible avec tous les constituants d’alimentation est le véritable solvant. Divers travaux effectués en extraction[l-31 ont déjà montré que dans certaines conditions la présence d’agents tensio-actifs, en augmentant les aires d’échanges [2] permet d’accroître l’échange global[2], méme si elle entraîne une diminution du coefficient de transfert[l-31. N.N. Li a constaté qu’une gouttelette d’hydrocarbures traversant une solution aqueuse de saponine, tensio-actif naturel non ionique, s’enrobe d’un film visible apparemment si stable qu’il lui permet de pénétrer dans un solvant organique sans se détruire. 11 a en outre remarqué que les hydrocarbures diffusent avec des vitesses différentes à travers cette membrane, si bien que ce solvant organique, quoique non sélectif, s’enrichit préférentiellement en un des constituants [4]. Cependant, sous cette forme la séparation n’est par quantitative à cause de la durée de vie très courte et de la fragilité des gouttelettes[5]. N.N. Li a alors proposé[5] d’émulsifier préalablement le mélange à séparer dans la solution tensio-active. Notons que ses travaux ont porté sur des expériences du premier type plutôt que sur l’étude du mécanisme de I’opération et son développement industriel. Sous sa deuxième forme, cependant, le procédé présente des
12 April 1974)
caractéristiques particulières et nous nous sommes attachés à déterminer ici les difficultés techniques, les étapes fondamentales du transfert et le rôle des tensioactifs et de la phase aqueuse. 1. ETUDES EXPERIMENTALES Dans cette optique, nous avons repris les études antérieures pour le couple à séparer, N-heptane-toluène, avec la saponine aqueuse comme tensio-actif et le kérosène comme solvant. Pour déterminer I’influence de la solubilité des constituants dans I’eau[69], nous avons ensuite effectué la séparation de trois autres couples (N-heptane-tétrachlorure de carbone, cyclohexène-toluène chlorobenzènetoluène) et nous avons comparé la solubilité relative, S, (Tableau 1) définie comme le rapport de la solubilité dans l’eau du constituant préférentiellement transféré à la somme des solubilités des deux composants du couple à la sélectivité, 4, rapport de l’enrichissement du kérosène en constituant préférentiellement transféré à son enrichissement global en mélange. Les tensio-actifs pouvant par leur nature et leur concentration (formation de micelles [lol) modifier les solubilités, nous avons utilisé différents tensio-actifs commerciaux tels que la sandozine N.J (non ionique) et le lissapol (oleyl para anisidine sulfonate de sodium, anionique) à des concentrations allant jusqu’à 5 pour cent en poids. Formation des émulsions-contact mélange à séparer solution aqueuse Tous les tensio-actifs employés dans la gamme de concentrations couverte (0,5 à 5% en poids) ont le même comportement en tant qu’émulsificateurs: tous les pro-
a stoichiometric coefficient of solid-gas reaction C total gas concentration, M/L’ C, concn of gas component A, MIL’ C,, initial concentration of solid, M/L’ tPresent address: Department ing, West Virginia University, 26506, U.S.A.
Chemicol
Engineering
Science,
Pergamon
D
molecular diffusivity of HzO in air, L’/e effective diffusivity of gas A in the ash layer, L’/O heat of reaction, HIM permeability coefficient of the ash layer, L2 effective thermal conductivity, HILOT chemical reaction rate constant, L*/MO total gas pressure, M/L8’ partial pressure of the reactantlproduct gas A, M/L# R initial radius of the reacting solid sphere, L radius, L 4 temperature, T t time, 8 U gas velocity, LI0 minimum fluidization velocity, L/8 l(rnf mol fraction of reactant gas; x.,., at the reaction XA surface; xAO,in the bulk.
De.4 AHA k k. k, P PA
symbols 6h equivalent thickness for gas film in heat transfer, L 6m equivalent thickness for gas film in mass transfer, L RERERENCES
111 Ishida M. and Shirai T.,‘J. Chem. Engng. Japan, 1%9 2 184. [21 Ishida M. and Shirai T., J. Chem. Engng. Japan, 1970 3 1%. [31 IshidaM.andWenC.Y.,A.Z.Ch.E.JLl%814311. 141 Levenspiel O., Chemical Reaction Engineering, John Wiley, New York 1972. [51 Perry J. H., Chemical Engineers’ Handbook, p. 462. McGraw-Hill 1950. [61 Shen J. and Smith J. M., I.E.C. Fund. 1%5 4 293. [71 Tamaru S. and Soimi K., 2. phys. Chem. 1932 Al61 421.
of Chemical EngineerMorgantown, W. Va.
1974, Vol. 29, pp. 2003-2005.
2003
Press.
[8] Wen [91 Wen [lOl Yagi tion,
Printcd
C. Y., I.E.C. 1%8 60 34. C. Y. and Wang S. C., Z.E.C. 1970 62 30. S. and Kunii D., Proc. Int. Symp. on Combus1955 23 1.
in Graat Britain
Catalytically active catalyst poisons: the activity replacement problem (Accepted 8 April 1974) It sometimes occurs in industrial practice that a porous catalyst is poisoned by a poison precursor which, upon deposition over the catalytic surface, has its own catalytic activity. Such processes can take place, as an example, when an automotive catalyst is being poisoned by a lead containing exhaust gas in an oxidative environment. Experiments[l] have shown that the catalyst cannot be poisoned fully; much more, it appears that lead oxide has some catalytic activity which, upon deposition over the original catalytic surface (e.g. Pt), gradually replaces the first kind of activity by its own activity. In this quoted example, of course, the poison is much less active than the original catalyst but the problem can be easily generalized
and, at least hypothetically, any combination of the two activities can be envisioned. The deposited active poison might alter both the rates and selectivities of the reactions taking place over the catalytic surface. Let US consider the simplest possible problem here where the original activity is gradually replaced by a second kind of activity in a porous, isothermal, single catalyst pellet of arbitrary geometry. The main reaction is characterized by the simple mechanism A + B, while P + W wil1 describe the activity replacement (i.e. poisoning) process. Both extemal and internal mass transfer resistances wil1 be considered. The formulation of poisoning problems for isothermal single pellets is exp-
Shorter
2004
Communications
lained in more detail in the works of Masamune and Smith [2] and Hegedus 131. If the activity’replacement is much slower than the rate of the main reaction, quasi-steady state prevails: *_ W
h 1-7
a** - h12Bm$An - hz*(l - %)“&t” = 0 a7
*_ a+
- A 1-r)
a+ -ar,
h32%o$p8 = 0
(1)
(2)
- %y+” = g
(3)
tiA(O>s)=f(rl)
(4)
bP(O,7))=0
(9
%(O, 71) =
1
(6)
c$T> 0) = PA[$A(7,O)- 11
ternal and internal mass transfer control. The salient feature of these plots is the residual activity after the activity replacement process has been completed. Although the above analysis might be an oversimplified representation of an actual catalytic process, it provides a qualitative picture which helps to identify the parameters and variables which affect the system and their relalionship to each other. In the author’s experience hypothetical computer ‘experiments’ such as these often provide valuable hints toward fruitful experimentation. Acknowledgements-J. computations.
Melbardis
assisted
L. LOUIS
during
the
HEGEDUS
Research Laboratories Genera1 Motors Corporation Warren, Michigan 48090, U.S.A.
(7) NOTATION
~(~,0)=Lw.w-11
(8)
3,
í)=0
(9)
$$(T,
1) = 0
Dimensioned
uariables a,, A, c,=
(10) c,i,
The addition of the last term to Eq. (1) accounts for the activity replacement process which, together with the main reaction, can have an arbitrary kinetic order. The dimensionless parameters and variables are defined in the Notation. Equations (1-10) were solved by a quasilinearization technique[4], for h = 2 (spherical pellets), m = n = p = q = a = 6 = 1 (first order reactions throughout), and various values of p and h, representing varying degrees of external and internal mass transfer control for the main reaction and the poisoning reaction. Figure 1 shows how the overall reaction rate changes with dimensionless time for a variety of examples. Run 5 replaces the original catalyst with another one of equal activity. Runs 1-4 employ poisons of increasing catalytic activity toward the main reaction at various degrees of ex-
10
Fig. 1. Activity
20
30
transition
c, 0. k,, k,, kx
ki
R - R IR,, t
40
during activity
50
replacement.
60
specific active surface area, cm2/cm’ pellet area covered by surface 1 mol of W, cm2/mol W concentrations far away from the pellet, mol/cm’ concentration of species i, molicm concentration, mol/ poison cm’ pellet effective diffusivity of species i, cm’/sec heterogeneous rate constant, units depend on reaction orders mass transfer coefficient of species i, cmlsec distance coordinate, cm characteristic pellet radius, cm fractional reaction rate time, sec
T
Shorter
Communications
2005
T = t(k2AwaO-‘CP,) distance
parameter
fraction of original surface remaining geometrie factor: h = 0 infinite flat slabs; h = 1 infinite cylinders; A= 2 spheres. reaction orders (see text)
Rkp PP = DP
of the main the original
Thiele parameter reaction over face
of the main the new sur-
Thiele parameter poisoning reaction
Chemical
Engineering
Science,
1974, Vol. 29, pp. 2005-2008.
Du rôle des agents tensio-act&
of
Pergamon
the
Press.
Biot number precursor
time of
of
the
the
main
poison
REFERENCES
[11 Unpublished
Thiele parameter reaction over catalyst
dimensionless
RkA BA = Biot number DA reacant
Genera1 Motors Research Laboratories, Michigan. [2] Masamune J. M., A.I.Ch.E..Jl. 1966 12 384. L. L., On the Poisoning of Porous Cata[31 Hegedus lysts by an Impurity in the Feed. Ind. and Engng chem. Fund. in press; ACS, Los Angeles, April 1974. J., Ind. and Engng chem. Fund. 1968 7 141 Newman 514.
Printed
in Great
results, Warren, S., Smith
Britain
dans la séparation de mélanges par un procédé à trois phases liquides
(Received 7 February
1974; accepted
La mise en oeuvre d’un procédé d’extraction liquide-liquide repose sur la disponibilité d’un solvant sélectif. De plus, la solubilité du soluté doit être assez grande sinon la quantité de solvant requise rend le procédé peu économique. Grâce à I’utilisation d’agents tensio-actifs, la méthode de séparation que nous présentons ici fait intervenir deux solvants: I’un est sélectif mais sans que la solubilité du soluté soit nécessairement importante, l’autre parfaitement miscible avec tous les constituants d’alimentation est le véritable solvant. Divers travaux effectués en extraction[l-31 ont déjà montré que dans certaines conditions la présence d’agents tensio-actifs, en augmentant les aires d’échanges [2] permet d’accroître l’échange global[2], méme si elle entraîne une diminution du coefficient de transfert[l-31. N.N. Li a constaté qu’une gouttelette d’hydrocarbures traversant une solution aqueuse de saponine, tensio-actif naturel non ionique, s’enrobe d’un film visible apparemment si stable qu’il lui permet de pénétrer dans un solvant organique sans se détruire. 11 a en outre remarqué que les hydrocarbures diffusent avec des vitesses différentes à travers cette membrane, si bien que ce solvant organique, quoique non sélectif, s’enrichit préférentiellement en un des constituants [4]. Cependant, sous cette forme la séparation n’est par quantitative à cause de la durée de vie très courte et de la fragilité des gouttelettes[5]. N.N. Li a alors proposé[5] d’émulsifier préalablement le mélange à séparer dans la solution tensio-active. Notons que ses travaux ont porté sur des expériences du premier type plutôt que sur l’étude du mécanisme de I’opération et son développement industriel. Sous sa deuxième forme, cependant, le procédé présente des
12 April 1974)
caractéristiques particulières et nous nous sommes attachés à déterminer ici les difficultés techniques, les étapes fondamentales du transfert et le rôle des tensioactifs et de la phase aqueuse. 1. ETUDES EXPERIMENTALES Dans cette optique, nous avons repris les études antérieures pour le couple à séparer, N-heptane-toluène, avec la saponine aqueuse comme tensio-actif et le kérosène comme solvant. Pour déterminer I’influence de la solubilité des constituants dans I’eau[69], nous avons ensuite effectué la séparation de trois autres couples (N-heptane-tétrachlorure de carbone, cyclohexène-toluène chlorobenzènetoluène) et nous avons comparé la solubilité relative, S, (Tableau 1) définie comme le rapport de la solubilité dans l’eau du constituant préférentiellement transféré à la somme des solubilités des deux composants du couple à la sélectivité, 4, rapport de l’enrichissement du kérosène en constituant préférentiellement transféré à son enrichissement global en mélange. Les tensio-actifs pouvant par leur nature et leur concentration (formation de micelles [lol) modifier les solubilités, nous avons utilisé différents tensio-actifs commerciaux tels que la sandozine N.J (non ionique) et le lissapol (oleyl para anisidine sulfonate de sodium, anionique) à des concentrations allant jusqu’à 5 pour cent en poids. Formation des émulsions-contact mélange à séparer solution aqueuse Tous les tensio-actifs employés dans la gamme de concentrations couverte (0,5 à 5% en poids) ont le même comportement en tant qu’émulsificateurs: tous les pro-