Poisoning of Porous Catalyst Particles An Experimental Study Francesco Gioia Istituto d i Principi d i Ingegneria Chimica, Uniuersity of S a p l e s , Y a p l e s , Italy
The influence on the deactivation rate of a porous catalyst of step introduction of a reversible poison was studied theoretically and experimentally. Investigated in detail were the reversible poisoning of a powdered catalyst, and of porous catalyst particles whose Thiele modulus i s much smaller than unity but poison penetration i s slow. In the absence of data-fitting parameters, the experimental results can b e considered in good agreement with theory. The discrepancies are well within the limits of the uncertainty affecting the physical parameters independently evaluated, such as diffusivities, adsorption constants, etc. Part of the data were used in predicting heat transfer coefficients between the solid particles and the reactant gas stream.
In many iiidustrial processes the selectivit,?. and yield of cxtalytic r e a c t o i ~are controlled by introducing reversible p i h o ~ i sinto thc feed stream. Interesting examples of such techiiiqiies are the production of ethylene oxide on silversulq)ortcd c:it:ilysts and the hydrocarbon reforming process carried out oii a hifuiic4tioii:il catal>.st of 1)latiiiuni supported on a i l diiniiiiii h s c . I n most cases such processes are coiltrolled 011 a purely enipirical basis. In fact, while in the literaturc it, is pos"i\ilr to find niany indications (Imies, 1954; lInstec1, 1951) of tlie clieniirtry of the poisoning action of s1)cc'ific suhstnnccs on several catalysts, few works (I A, a i d a.wines t h a t c t >> et.*, the :itlsorhetl poi+oii peiirti,ation can be 1~egm.det1a i :I step of :I completely inactive zone l)eiiet,r:itilig iiisidr the slab a i i t l
where t h e is now a parameter. Equation Ii was solved hy Gioia (1967) for the semiinfinite slab for thc two liniitiiig cases: K1cc0 X
1 X I 0
X I
20-
10 -
A
0
0
T=7ODC
10
20 P H ~ O . mmHg
Figure 5. 208
Figure 4 also reports for one run the variation of the degree of conversion when a t time t the poison is switched off. The curve indicates t h a t t h e poison is reversibly adsorbed b u t t h a t its desorption is not a s fast as adsorption. Several runs with powdered catalyst were made a t several poison concentrations and a bath temperature of 60°C. According to Equation 21, in terms of conversion degree,
Evaluation of KF
Ind. Eng. Chem. Fundam., Vol. 10, No. 2, 1971
I n Figure 5 (z, - x , ) / x , is plotted vs. P H ~ O . The slope of the straight line through the data furnishes a value of K , = 1.52 mm of Hg. A few more data were taken a t a temperature different from 60°C in order t o see the dependence of K , on temperature. Figure 5 shows t h a t the temperature dependence of K , is small enough to be covered by the d a t a scattering. Kinetic Runs on Catalyst Particles. POISOSING. The catalyst was sintered by pressure at two different densities. T h e internal geometrical characteristics were obtained b y standard methods such as B E T a n d mercury porosimetry. Complete macro-micropore distribution curves were drawn according to the porosimeter results and from the nitrogen adsorption isotherm (Cranston and Inkley, 1957). Table I reports, besides other parameters of interest, the diffusivity values calculated according to the relationships proposed b y Wakao and Smith (1962). The values of cm* were calculated according to the relation
where a value of lO-'E for AazO as reported in t h e literature (Eipeltaver et al., 1964) is assumed. All experimental poisoning runs were done according t o the procedure described previously. The reactor was filled, taking care to distribute the catalyst particles uniformly along the bed. There was a good filling when each particle was completely surrounded by inert material (glass beads). For all runs the temperature along the reactor axis varied by no more than 5" or 6°C from t h a t of t h e thermostatic bath. Samplings through the chromatograph also equipped with the Porapak Q column showed a constant wat,er concentration in the product during the run. This concentration was equal to t h e one in the feed stream. The feed conditions are reported in Table 11. Each poisoning run was plotted as z vs. t . A typical plot is reported in Figure 6. A11 plots showed the same peculiar behavior-Le., faster initial decay followed by a slower one. The faster initial decay can be easily explained if one coiisiders t h a t a t zero time the temperature of the particle is higher than t h a t of the gas stream. 111 such case, as the poisoning proceeds the decrease in the reaction rate due t.o t h e covering of the active area by the poison molecules is amplified exponeiit.ially by the simultaneous lowering of the particle temperature according to Equation 28. When, as t,ime passes, the reaction rate has reached a value lorn enough t h a t t'he
Table II. Q H ~=
Run
1 2 3 4 5 6 7 8 9 10
P C
1.1 1.1 1.1 1.1 2.0 2.0 2.0 2.0 2.0 2.0
xo
7
0.516 0.690 0.338 0,292 0.257 0.248 0.295 0.232 0.289 0.264
0.20 0.17 0.24 0.26 0.08 0.08 0.07 0.08 0.07 0.08
Feed Conditions 1 = 0.25 cm
16 c c j s e c i Q c ~ H = ~ 1 cc/sec; XO*
0.062 0.069 0.103 0.037 0.011 0.016 0.010 0.015 0,009 0.020
Tb
r,
Re
JH from Data
JH from
rl*
1
296 303 295 293 297 296 296 296 296 296
354 370 333 345 395 384 40 1 383 402 381
4.3 4.3 4.5 5.3 4.3 4.3 4.3 4.3 4.3 4.3
0.098 0.12 0.11 0.10 0.06 0.06 0.06 0.06 0.06 0.07
0.14 0.14 0.14 0.13 0.14 0.14 0.14 0.14 0.14 0.14
1 1 1 1 1 1 1 1 1
1:
Eq. 37
Run N o 1
= 1 Ncclsec QH2 = 16 N c c / s e c PHZO= 2.1 m m H g
QCZH4
''8 - 0
- 8
. s
0.1:
e e e
Tb
= 2 3 "C
e
0.021 0
100
t, min
Figure 6.
Poisoning of particles
teiiiperatui~differciicc 1)etn.ceii t,he particles mid the renc+mt gas st'reain is n c g l i g h k ~ t,lie , poisoiiiiig proceeds only because t h e active :ireti is covered by the poison moleciiles. Let u s assiinie foi, t,he iiionieiit t h a t if isothermal conditions (110 t'eriipcr:itui,e differelice betweeii paiticles and reactaiit stream) held duriiig all the run the conversion would have beeii low enough to yield an effectiveness factor of uliit,y. , Equatioii 25 or 27 (if K"ct.0 > I, resl)ect'ively) should al)i)lx. In t e r m of conversion degree ' arc these equa t.ioiis
J
=
XO*
(1
&
- 1 2Dvcv0t )
(33) (34)
cetlure strictly a1q)lic:~l)lc.111 Figu1.c 8 tlic slol)cs of the correlating straight h i e s ni'c compared with t'lre values 1)1'cdictable froni Eqiiatmioiis33 aiid 34. 1iisi)ectioii of this figui,cl s h o w t,hat t'hr theory preseiitrd ovwcstiiiiatcs soniewhat t l i v rat,e of decay of the catalyst. 'I'his ~ i i \IC i due citliei, to tmlic' uiicertaiiit.ies t h a t affect sonic of the p:irarncters appc:iriiig in Equations 33 aiid 34 or to the restrictive :is.;unn1)tioii that' only a. moiiolayer of poisoii builds u p oii the c:it:ilyst surface. I n fact, t h e formitioil of Inore than :I layer of poison would furnish expeririieiit'al i,ates of decay snidlcr than t'hat prcdictable from the t,heory. Moreover, for sonic rims it was not definitely Kccvo> 1. Furthermore, it. seemed worthwhile to iisc h t 8 h espcri-
0.08
4 1
t h a t is, for each r u ~ ia plot, of z vs. should be linear. Actually, for each run, the data after t h e first initial decay showed a conversion low enough t h a t both isothermicity factor unity between particle and fluid and effc~t~iveiiess could have been safely assumed. Therefore, t h e runs were worked out by correlating all data points for each run (aft,er t h e first rapid decay) by a straight line as shown in Figure 7 . The int,ercept of the extrapolated lines with the ordinat,e axis a t = 0 was called zo*. It represent's t h e conversion t h a t the reacting system would have shown if a t t = 0 the particle temperature had been equal to t,hat of the reacting gas stream. Calculatioii of the effectiveness factor based on t h e zO*value showed 110 ineasurable reactant concentratmioil gradients in the part,icles. This rnatle the above outlined pro-
4;
400
300
200
1
p!
R u n No.1
0.06 X
0.04-
0.02
0
1 0
10
Figure 7.
fi , min"'
20
Example of data correlation
Ind. Eng. Chem. Fundam., Vol. 10, No. 2, 1971
209
u
al
which the hydrogeiiation of ethylene was taking place w:ii poisoned with water vapor. The dat'n show good agreement wit,h bheory, compat,ible with the iincertainties that affect the values of some physical p a r m e t e r s contailled in the derived equat,ionr. The poisoiiing experiments have been valuable in measuring heat transfer coefficients in packed beds; in fact,, poisoning experiments make it' possible to derive the heat transfer coefficient direct'ly froni a packed bed react'or. In contrast, most of t,he semiempirical correlat'ioiis reported in the literat'ure and used t'o calculate heat' transfer coefficiefits are derived not from experiments on cat'alytic react,ors, but from systems which only simulate the lat'ter.
Key PtiZ0,mmHg
L
-rg 3 m
0
V
2.1; 2.5 0.6
I pc = 2.0 gr/cc
Appendix
10.3 ; 11.7
Experimental Figure 8. Comparison perimental data
between
theoretical
and
ex-
nient'al values, x 0 , and intercept values, zo*, t.o calculate the heat transfer coefficient' betweeii particles and reactant stream. This \vould have been, in fact, a direct way to measure heat transfer coefficient:: in a catalytic packed bed. Aicomparison with the dimeiisionless correlations usually used t'o calculate such corfficients nould have been valuable. Such correlations are generally derived by siniulat'ing the catalytic reactor in a more or 1e.w good way. The calculations were made in t8he following way: From .Irrhenius' law and the definit'ion of effectiveness factor one writes XOV*
-~ =
exp
XO*V
[--R (- - k)] E
1 Ts
(35)
From t'his equation it was possible to calculate T,for pach run. The values of q* and q appearing in Equation 35, ethylene being always the liniit'ing reactant, mere calculated according to the procedure presented by Gioia et al. (1970b). The calculat~ionshowed t8hatm in all cases t.he efficiency, v*, was unity, given the low value of x0*, while 7 for the various run3 ranged between 0.07 and 0.25 (values report,ed in Table 11). But in no case was there an appreciable temperat,ure profile inside the pellets. Knowing the values of T,, it was possible t.0 calculat,e,by a heat, balance equation, the values of the J x factors defined as
The results of these calculations are reported 111 Table 11. The calculated J H values and those reported in the literature should be compared with the correlation reported by Eichhorn and V h i t e (1952) :
Jx
=
0.23(Re)-o.*6
(37)
derived for a Reynolds number range and for an experimental system t h a t better simulates the system used in the present work. In fact, the above authors heated plastic particles filling a bed b y an electrostatic field, thus generating the heat directly on the particles and simulating a catalyst particle on ivhich an exothermic reaction is taking place.
Deactivation by poison adsorption is discussed by h i e s (1954), Tvho describes two mechanism, bot'h advocated by a number of workers. X1t.houg.h the distinction is somewhat vague, the first. mechanism stresses reaction between poison and act'ive centers, whereas t'he second considers a less direct interference, the number of free electrons, unpaired elect,rons, or reaction sites being in some way adversely affected by t'he presence of poison molecules adsorbed on the catalyst surface. The choice of t'he first, or second mechanism leads t o a different' functional relationship bet'iveen t,he rate equation and t,he gas phase poison concent~ration. Let us consider the very simple case of a catalytic reactmion whose rate is proport,ional to the concentration of reactant A adsorbed on active sites--namely,
r
=
kFA
and let u s suppose t h a t t.he reaction takes place in the presence of a poison V t h a t , according to the first proposed mechanism, competes with -1in adsorbing on active sites. I n this case a balance on the available active sites, c1 =
S
-
(FA
+ Et>)
(38)
and t,he assuinption that' equilibrium holds in the processes of adsorpt.ion-Le.,
c,
=
Ea =
Kv'cocl
(39)
KA'CACi
(40)
lead to the ratmeequation
The K " s represent adsorption constants on t,he active sites and not on t,he catalytic surface as a TThole. O n the contrary, according t,o the second mechanism, "independent poisoning," the amount of poison adsorbed, and consequent.ly the number of active sites made ineffective, does not' depend on the aniouiit, of reactant, adsorbed. Therefore, if we further assume that a proportionality exists between active sites made ineffective and poison adsorbed, Equat'ion 38 becomes
from which we get the rate equ a t'ion (43)
Conclusions
A physical niodel for the reversible poisoning of porous catalysts haq been preqented. A porous Cu-MgO catalyst on 210
Ind. Eng.
Chem. Fundam., Vol. 10, No. 2, 1971
In the present paper we have assumed this second mechanism to be valid.
Nomenclature = = = = C
=
?A
=
catalyst surface area blocked by :in 1120 molecule, cm2/'molecule catalyst surface area available for reaction, em*/ e1113 catalyst' surface area poisoned a t :lily time, cin2 particle esternal area per unit volume of cat bed, cm2/ciii3 reactant concent~ration,grani 1nolesjcni3 adsorbed phase reactant conceiitrat'ioii, gram moles,
molal coiicentratioii of active sites, gram moles/cm3 reactant heat capacity, gram caljgram mole "C masiriiuin adsorbed phase poison coiicentratioii, cm * gram moles/cm3 = adsorbed phase poison concentration, grani moles, c, em3 = gas phase poison concentration, gram moles/cm3 C" C = gas phase poison concentration at pellet surface, gram moles, em3 CV* = gas phase poison conceiitratioii in equilibrium with e,* (for linearized Langmuir equation), gr:rni moles/cni3 D = effective diffusivity, cni2/sec E = activation energy, gram tal, grani mole F = C2H4molar flow rate, gram mole G Mt = molal velocity of gas based upon total cross-sect'ioiial area of bed, gram moles/cm2 see = nims velocity of gi\s based upon total cross-sectional area of bed, grnm9,'cm2 see I1 = heat transfer coefficient, grani cal, a n 2 sec "C = adsorption equilibrium constaiit (see Equation 9) K K , . = Laiigniuir adsorptioii constant, cm3/gram moles = Langmuir adsorptioii constant (mni of Hg) - l = mass transfer coefficient, cni;'sec = frequency factor, sec-l = reaction rate constant, sec-' = particle half dimension, c m = total molal active sites, gram moles cm3 n = Avogadro's iiumber P = tot'al pressure, mm of ITg = Praiidtl iiuniber Pr = partial pressure, nini of Hg P Q = reactant flox rate, cni3/sec = reaction rate based 011 catalyst volume, gram moles/ c1n3 sec R e = Reynolds number = ZLGt;b Th = thermostatic bath temperature, "I< T, = particle esteriial surface temperat'ure, OK = time measured from poison introduction, sec t = volume of catalyst bed, cm3 = volume per mole, cni3/gram mole = weight of catalyst bed, grams = conversion based on ethylene, gram moles C2H4 X conv./gram mole C2Ha fed
so
=
.xo*
=
s,
=
y
=
GREEKSYhfBOLS
C1
=
A
CP
= =
A
= =
p,. u
= =
p IJ
= =
1)
=
q*
=
V
C
steady-state conversion a t time zero, grain moles C1H4conv.!gram mole C?H4fed steady-state coiiversioii a t time zero, if T , = T b , grain moles C2H4coiiv./gram iiiole C,H4 fed steady-state conversion for poihoii coiiceiitration uniform tlirougli particle, grain moles C2H4 coiiv.,/grani mole C2H1fed. distance into pellet, cin
finite cliange of :i property poisoii penetratioii, e111 catalyst density, grilnih,'eni3 poison pseudo-diffusivity (see Equation 8), cni*,!sec poison flus, grain moles~cm2sec reactant viscosity, g r a n i c i n see effectiveness factor based on .zo effectiveness factor based on ro*
S~JBSCRIPTY c
= catalyst
v
= =
t
poison tot'al flow rate
literature Cited
Butt, J. B., Fourth Ihropeaii Symposiuni on Chemical Reaction Engineering, Bruxelles, September 1968. Cranston, R. R., Inkley, F. .4.,Adtian. Catal. 9 , 143 (1957). Cunningham, R . A , , Carberry, J. J., Smith, J. H., ii.I.Ch.E. J . 11, 636 (196,j). $:ichhorn, J., IVhite, li,Xoina VIII-42, 51.5 (1967). Gioia, F., Chint. Ind. (.lll'lnn) 48,237 (1966). Gioia, F., Gibilaro, L. G., (ireco, G., Jr., Cheni. Eng. J . 1, 9 (1970a). Gioia, F., Greco, G., Jr., Quad. Ing. Chint. Ita/. (Jlilan)6, 11 (1970). Gioia, F., Greco, G., Jr., Gibilaro, L. ( i . , Cheiii. Eng. Sci. 25, 969 (1970b). Hollis, 0. L., Hayes, W. V.,J . Gas Chromalog. 236 (1966). Innes, W.B., Catalysis, 1, 245 (19,54). lIaxted, E. B., Advan. Catd. 3 , 129 (1951l.Sada, E., Wen, C. Y., Chon. Eng. Sci. 22, .>*)9(1967). Wakao, S . , Smith, J . ll.,Chcni. Eng. Sei. 17,82a (1962). Wynkoop, R., Wilhelm, I{, H., Cheiit. Eng. Pmgr. 46, 300 il9a0). Work financed by a research grant from the Ente Nazionale Idrocarburi of Italy.
RECEIVED for review Xovember 14, 1969 ACCEPTEDDecember 21, 1970
Ind. Eng. Chem. Fundam., Vol. 10, No. 2, 1971
21 1
