Ind. Eng. Chem. Res. 1991,30, 2514-2518
2514
Copper Chromite Catalyst Activity Correlation for the Hydrogenation of 2-Ethyl-3-propylacrolein L a r r y Novak* and Eugene Nebesh Engelhard Research and Development, 23800 Mercantile Road, Beachwood, Ohio 44122
A Thiele-type pore diffusion model was developed and successfully used to correlate CuCr catalyst activity with key measurable catalyst properties. The model is a potentially useful tool for catalyst research and development because it provides a fundamental basis for interpreting catalyst activity, checking the consistency of catalyst activity and physicochemical properties data, and optimizing catalyst performance. CuCr catalyst activity was defined in terms of a n 2-ethyl-3-propylacrolein (EPA) hydrogenation rate constant, The kinetics of EPA hydrogenation over CuCr catalysts were described by a rate expression which was zero order in EPA concentration and first order in hydrogen partial pressure over the range of conditions studied. Over the range of conditions studied, CuCr EPA hydrogenation activity was not found to be influenced by the type of catalyst supports and additives, except as they influenced a catalyst properties parameter. Introduction The'purpose of this paper is to describe and evaluate a model for correlating the catalyst activity of various formed copper chromite catalysts with key physicochemical catalyst properties. Catalyst activity was determined during the batch hydrogenation of 2-ethyl-3-propylacrolein (EPA) to 2-ethylhexanol (2-EH) and expressed in terms of an EPA rate constant without regard to product selectivities. An activity correlation model was developed to support the development of improved catalysts by providing a fundamental basis for interpreting catalyst activity, checking the consistency of catalyst activity and physicochemical properties data, and optimizing catalyst performance. A check on the consistency of catalyst activity and properties data is useful for identifying anomalous behavior which could be the result of erroneous data or anomalous catalyst behavior. 2-EH is primarily used as a plasticizer, particularly as the phthalate ester, for flexible poly(viny1 chloride). Practically all of the 2-EH produced in the United States is made by the oxo process. The oxo process produces 2-EH by the following series of steps: (1) hydroformylation of propylene to butyraldehyde, (2) aldolization of nbutyraldehyde to EPA, and (3) hydrogenation of EPA. The hydrogenation of EPA to 2-EH is accomplished with a base metal catalyst such as copper chromite (Kirshenbaum and Inchalik, 1981). Hydrogenations of oxo aldehydes are usually conducted at 100-250 "C and pressures up to 350 atm (Kirshenbaum and Inchalik, 1981). Both vapor-phase and two-phase hydrogenations are practiced. Kinetic Model. The experimental catalyst activity will be defined in terms of a lumped pseudo-zero-order EPA kinetic rate constant ( K )defined below. The validity of the assumed kinetics will be established later in this paper. EPA contains an aldehyde and an olefinic functional group which results in the following reaction pathway for the hydrogenation of EPA to 2-EH. ~
2-ethyl-2-hexenol
% 2-ethylhexanol
2-ethyl-2-hexenal
(EPN 2-ethylhexanal
d
(P-EH)
* To whom correspondence should be addressed at Lubrizol Corp., 29400 Lakeland Blvd., Wickliffe, OH 44092-2298. 0888-5885/91/2630-2514$02.50/0
The experimental catalyst activity for EPA hydrogenation at a fixed hydrogen pressure can be determined experimentally from the following equation. K = kl kz = (COEpA - CEpA)/t (1) where K = k, + k2 = experimental rate constant or experimental catalyst activity (min-l); CowA= starting weight fraction of EPA; CEpA = EPA weight fraction at time t; and t = minutes since the start of hydrogenation. Equation 1 can be used with batch EPA concentration data to determine the experimental catalyst activity for the liquid-phase hydrogenation of EPA. The use of eq 1to determine the experimental catalyst activity will only be useful for differentiating the activity of various catalysts when the catalyst loading level in the reactor is sufficiently low. In this paper catalyst loading is defined as the grams of catalyst loaded into the reactor. At high catalyst loading levels, d active catalysts will have the same experimental EPA hydrogenation activity due to the hydrogenation rate being controlled by gas-liquid hydrogen mass transfer. Interphase Mass Transfer. Levenspiel (1979) described a mass-transfer-reactionmodel for reaction systems analogous to the one described in this paper. The model incorporated interphase gas-liquid-solid mass transfer occurring with reaction in porous catalysts. This model illustrates that, at a sufficiently low reactor catalyst loading level, the experimental rate constant will describe the combined effects of liquid-solid mass transfer and the intrinsic reaction occurring within the porous catalyst. This experimental rate constant will be the rate constant calculated from eq 1 when an appropriate reactor catalyst loading level is used. The appropriate catalyst loading level for comparing catalyst activities can be determined by measuring the experimental hydrogenation rate constant as a function of the amount of catalyst loaded into the reactor. In the region where the experimentalhydrogenation rate constant increases linearly with catalyst loading, the hydrogenation rate is not limited by gas-liquid hydrogen mass transfer and the experimental activity of catalyst particles can be determined. Catalyst Activity Model. Previous work at Engelhard and data presented in this paper demonstrate that the experimental catalyst activity for EPA hydrogenation, under the reaction conditions listed in the following section, is pore diffusion controlled for the catalysts used in this work. Therefore, a simple Thiele-type pore diffusion model was used as a basis for the development of a model
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0 1991 American Chemical Society
Ind. Eng. Chem. Res., Vol. 30,No. 12, 1991 2515
to correlate CuCr EPA hydrogenation catalyst activity with catalyst physicochemical properties. For a single ideal pore, the theoretical rate constant or theoretical catalyst activity for a reaction which is zero order in EPA and first order in hydrogen can be defined as KT = (2kD/rL2)1/2
(2)
where KT = theoretical rate constant or catalyst activity (pore volume basis) (min-l); k = intrinsic rate constant based on total pore surface area (cm/min); D = ordinary diffusion coefficient for Hzin EPA (cm2/min);r = pore radius (cm); and L = Thiele pore length (cm). The derivation of eq 2 can be found in Levenspiel (1972). For a real catalyst, the theoretical catalyst activity due to reaction within the porous catalyst can be derived from eq 2 and is described below in eqs 3-8.
L = T/U,
(3)
where a, = specific geometric surface area of a catalyst particle (geometric surface area/particle volume) (cm-') and T = tortuosity factor (dimensionless). Levenspiel (1972) suggested the use of a;' to characterize the Thiele pore length. For a real catalyst, a tortuosity factor should be used to convert the Thiele pore length to a more realistic hypothetical pore length. Wakao and Smith (1962) found the following relationship to hold over porosity ranges typically encountered with catalysts. T
= 1/E
(4)
where E = catalyst porosity, cm3/cm3. The catalyst porosity used should represent the porosity in the pores which are large enough to accommodate the reactants. In this work the total catalyst porosity was used. Total catalyst porosity can be calculated by multiplying the particle density (g/cm3) times the total pore volume (cm3/g). Catalyst particle density and total pore volume are commonly determined by mercury and helium pycnometry. An average pore radius can be calculated from the catalyst total pore volume and surface area and substituted for "rn in eq 2.
r = 2 X 104(PV)/(SA)
(5)
where PV = catalyst total pore volume (cm3/g); SA = one point Nz BET pore surface area (m2/g);and r = an average pore radius, (cm). The derivation of eq 2 assumes that all pore surface areas are catalytically active surface areas. When supported catalysts are used, this is not an accurate assumption and the intrinsic rate constant based on the total pore surface area must be adjusted. In this work, the total pore surface area was adjusted by the weight fraction of CuCr (oxide basis), M, in the catalyst. With the above concepts, eq 2 can now be written as KT
= 70.71Ea,(2koDM(SA)/(PV))'/2
(6)
Since eqs 2 and 6 apply to a single pore (pore volume basis), eq 6 must be further modified to account for the ratio of total catalyst pore volume to EPA volume charged to the reactor. (total pore volume)/(EPA volume in autoclave) = (LL)(PV)/VL (7) where LL = g of catalyst charged to the reactor (catalyst loading level) and VL = cm3of EPA charged to the reactor. Multiplying eq 6 by 7 gives the final equation which relates the theoretical rate constant or catalyst activity for the batch reactor to the amount of EPA and catalyst charged
to the autoclave and the catalyst physicochemical properties.
'"
KoT = 70.7 1((LL)Ea,/ VL)(2koD(PV)(SA)M)
(8)
When liquid-solid mass transfer is negligible, KoT should be equal to the experimental rate constant or experimental catalyst activity calculated from eq 1. In a crude and simple manner, eq 8 expresses the concepts of pore diffusion and active surface area in terms of measurable catalyst physicochemical properties. More sophisticated refinements of eq 8 can be made for expressing pore diffusion and active surface area. However, in this paper we will evaluate the simple model. Equation 8 suggests that the experimental catalyst activity could potentially be predicted from standard catalyst physicochemical property measurements if the intrinsic rate constant (12") and ordinary diffusion coefficient (D) were known. It also suggests that the experimentalcatalyst activity for various catalysts should correlate with eq 8. Since the EPA reactor charge and temperature were held constant, the following equation was evaluated for correlating the EPA hydrogenation activity with catalyst properties: experimental rate constant or experimental CuCr activity = C(LL)EU,((PV)(SA)M)'/~(9) where C = proportionality constant and (LL)Ea,((PV)(SA)M)'12 = catalyst properties parameter. The catalyst properties incorporated are porosity, specific geometric surface area, pore volume, surface area, and CuCr content. Specific geometric surface area is inversely proportional to the effective catalyst particle size, which influences fixed-bed pressure drop and interparticle mass and heat transfer. Although catalyst activity can potentially be increased by decreasing effective particle size, catalyst manufacturing cost and reactor pressure drop will place lower limits on this variable., The left-hand side of eq 9 was determined from the experimental rate constant or experimental catalyst activity (eq 1). All catalyst activities were normalized relative to a standard catalyst to account for some variations in EPA feedstocks used during the course of this work.
Experimental Section Experimental catalyst activity was determined for EPA hydrogenation using the thimble-autoclave system illustrated in Figure 1. The thimble-autoclave equipment is commercially available through Autoclave Engineers (AutoclaveEngineers, 1990). This system is ideal for the laboratory-scale evaluation of formed catalysts in twophase hydrogenations since it allows complete wetting of formed catalyst particles which are held in a "thimble basket". Catalyst change-out is also relatively quick and easy. The following standard reaction conditions and procedure were employed. Conditions: temperature = 150 "C; pressure = lo00 psig; reactor = l-L 316 ss autoclave; agitation = 1500 rpm, Autoclave Engineers Dispersimax turbine; feed = 447.2 g of 2-ethyl-2-hexenal (EPA); catalyst loading = 6.27 g; reaction time = up to 2 h. The feedstock used in this work was obtained from an industrial source. Procedure. Following the loading of catalyst and feed, the autoclave was purged with nitrogen to remove air and then heated to 150 "C with agitation to destabilize the catalyst. The reaction was started by quickly pressurizing the autoclave with hydrogen after the desired operating temperature was reached.
2516 Ind. Eng. Chem. Res., Vol. 30,No. 12,1991
NITROaEN
SUPPLY
E
I T THERMOCOUPLE
RUPTURE ( 7 0 0 0 PILI>
CATALYST
4 -
I
u
HYDROGEN
REACTOR
BALLAST
Figure 1. Thimble basket autoclave reaction system. 1.00
,
I I 1 I
o'0040
/
I
i
P
i
I
p/ /
/
EPA Conconbation a. a F d i of
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--
-
-
-
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20
40
80
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BATCH REACTON TIM. n h
--
~ e a c t a 1 nn316 u ~ u i ~ c b v e Reactor EPA Charg. 4472 gnu 98 wt % EPA Reactor Tarpaatwe 150 deg C A&dii I s 0 0 rpm Reactor Catalyrt Loadhg 827 gnu Catalyd Cu-1107 T1/8
Reactor EPA Charg. 4472 m9B wt % EPA Reactor T m p r a t v e 150 deg C
060
020
0.40
0.80 100 IThOUund.1 REACTOR PRESSURE. pie
0.60
120
-
l.40
l.80
L80
Figure 2. Effect of reactor pressure on batch kinetics.
Figure 3. Effect of reactor pressure on rate constant.
Results and Discussion Verification of the Kinetics Model. Figure 2 illustrates the effect of reactor pressure on EPA hydrogenation kinetics using CuCr catalysts. The delay in the start of the reaction, demonstrated in Figure 2, suggests that the catalyst undergoes further destabilization or activation before hydrogenation starts. It is evident that following the initial delay in the start of the reaction, the kinetics can be described by a rate expression which is zero order in EPA concentration. The magnitude of the slopes (rate constants) increase with reactor pressure. The slopes from Figure 2 were plotted as a function of reactor pressure in Figure 3. Figure 3 demonstrates that the pressure dependence is essentially first order over the pressure range studied. Therefore, under the conditions studied, EPA hvdroeenation kinetics can be described - - -~~ - - __ bv
a rate expression which is first order in hydrogen and zero order in EPA. These findings support the use of eq 1for experimentally determining catalyst activity and eq 9 for correlating catalyst activity with catalyst properties. Figure 4 demonstrates that the kinetics can also be described as zero order in EPA for two different CuCr catalysts. Cu-1107 T is an unsupported tabletted catalyst, and Cu-1230 E is a supported extruded catalyst. These CuCr catalysts are commercially available from Engelhard Corp. Establishment of the Catalyst Loading Level. The establishment of an autoclave catalyst loading level, which will allow one to experimentally determine catalyst activity without confounding gas-liquid mass-transfer effects, should be accomplished by experiments with one of the more active catalysts being studied. The results of such
I
"
-..
-.I
Ind. Eng. Chem. Res., Vol. 30, No. 12, 1991 2517 Table I. Summary of Catalyst Activity and Properties for CuCr Catalysts relative catalyst activity 39.0 183.1 64.2 109.4 CuCr catalyst properties catalyst property parameter 366 1814 613 935 total porosity/ (cms/cm3) 0.52 0.63 0.62 0.65 reactor catalyst loading/g 6.21 6.21 6.21 6.21 total pore volume/(cma/g) 0.25 0.42 0.41 0.45 BET surface area/(m*/g) 52 164 126 132 CuCr composition/(wt % ) 77.1 63.0 45.5 10.0 supporta/additives clay alumina ZnO zeolite sample number 90-3 16-5 91-2 92-3 E1/8 E1/8 E1/16 catalyst nominal size* E1/8 36.0 35.1 12.0 specific geometric surface area/in-’ 35.3
84.6
59.1
57.7
47.1
850 672 623 0.62 0.68 0.68 6.21 4.50 4.50 0.54 0.40 0.49 135 107 84 70.0 70.0 70.0 zeolite diatomaceous earth 92-4 95-2 96-2 E1/8 E1/8 E1/8 36.1 35.8 36.4
96.2 142 0.64 6.27 0.25 90 93.5 SiO, 130-4 T1/8
605 0.63 4.50 0.40 130 69.8 Zda 97-2 E1/8 35.5
48.0
in. diameter; Tl/8” designata a cylin-
ORelative to reference catalyst Cu-1230 E1/8”. bE1/8” designates a cylindrical extrudate of drical tablet with diameter and length of ‘/ein.
00030 -
/
d /’ 8k
a
J
00015 /
,
/
own.
RaI. Caulnl”
1 0 om5 /
-
- -- -
___
40
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00
FvrLa
I yfn316 *I Auloc(lw Rirlu EPA C h r r U 7 2 W wt X EPAI R e r l u T q a h r r 160 d q C R.~lu Do0 pw A p t a l a 1500 rpn C.I+I cu-iinr TVB Rarstor
- -~
00~00
I
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Figure 4. Effect of catalyst.
Figure 6. Effect of catalyst loading on rate constant.
: . ACTMTY TEST 85 X
-1
/F&NT
00
02
04
-
C O N m M c E L M l 20 %
-
-
OF CORRELATKWI 97 X
06
OB
LO
n+”n&l
12
14
16
I8
20
CATALYST PROPERTESPARAMETER
Figure 5. Effect of catalyst loading on batch kinetics.
Figure 7. Catalyst activity correlation.
experiments are plotted in Figure 5. Figure 5 once again demonstrates kinetics which can be described as pseudo zero order in EPA concentration. The slopes from Figure 5 were plotted in Figure 6 to demonstrate that, under the conditions studied, the Cu1230 E1/8”reference catalyst can safely be loaded up to 8 g without gas-liquid mass transfer confounding the determination of catalyst activity. The experimental catalyst activity (eq 1)is essentially proportional to the weight of catalyst loaded in the 4-8-g range. At much higher catalyst loadings, the measured catalyst activity would eventually asymptote to a a constant value, reflecting the gas-liquid hydrogen mass-transfer rate. At catalyst loading levels below 4 g, the extrapolated line through the origin has some curvature. This curvature suggests the presence of catalyst poisoning. Evaluation of CuCr Catalyst Activity Correlation. Figure 7 illustrates the correlation of relative CuCr catalyst activity with the catalyst properties parameter (eq 9) for
nine experimental catalysts prepared by a variety of techniques. The properties used to calculate the catalyst properties parameter for the various catalysts are listed in Table I, along with the respective CuCr catalyst activities relative to the Cu-1230 E1/8” reference catalyst. Absolute experimental catalyst activities were calculated from eq 1prior to normalizing. It is evident that a range of catalyst properties and supports/additives were tested with the activity correlation model (eq 9). As predicted by theory, a straight-line correlation exists which passes through the origin. This suggests that eq 9 is an appropriate correlating model and liquid-solid mass transfer is apparently negligible under the conditions of this study for catalysts having a wide range of physicochemical properties. The coefficient of correlation is 9770, and deviations of data from the correlating line are essentially within the 95% confidence limits illustrated by the bracketed vertical line at 100% relative activity. The catalyst activity-
I n d . Eng. Chem. Res . .1991,30, 2518-2522
2518
properties correlation may not be strongly sensitive to the assumed kinetics. Data analysis by first-order EPA kinetics resulted in a coefficient of correlation of 96%. Attempts to correlate relative catalyst activity with any single catalyst property were unsuccessful in comparison to the more fundamental approach used in Figure 7. The supports/additives consisted of alumina and other oxides. It is apparent that under the conditions studied, the supports/additives did not have any apparent effect on catalyst activity for EPA hydrogenation, except as they influenced the catalyst properties parameter. The supports/additives evaluated did not result in any anomalous deviation from the correlation based on eq 9. Conclusions A simple extension of the Thiele pore diffusion model was successfully used to correlate EPA hydrogenation activity with measurable physicochemical catalyst properties for a variety of CuCr catalysts. The model presented here is a potentially useful tool for catalyst research and development because it provides a fundamental basis for interpreting catalyst activity, checking the consistency of catalyst activity and physicochemical properties data, and optimizing catalyst performance. The EPA hydrogenation activity for a variety of CuCr catalysts was found to be independent of catalyst support/additives under the conditions studied, except as they influenced the catalyst properties parameter. Acknowledgment We express our appreciation to Engelhard Corp. for granting permission to publish this work. Nomenciat ure a , = specific geometric surface area of a catalyst particle (geometric surface area/particle volume), cm-' C = proportionality constant CEpA = EPA weight fraction at time t COEpA = starting weight fraction of EPA D = ordinary diffusion coefficient for H2in EPA, cm2/min
E = catalyst porosity, cm3/cm3 k = intrinsic rate constant based on total pore surface area, cm/min k o = intrinsic rate constant based on active pore surfacearea, cm/min K = experimental rate constant or experimental catalyst activity, min-' k , = experimental rate constant for the hydrogenation of EPA aldehyde group, min-' k , = experimental rate constant for the hydrogenation of EPA olefin group, min-' KT = theoretical rate constant or theoretical catalyst activity (pore volume basis), min-' KoT = theoretical rate constant or theoretical catalyst activity (EPA volume basis), min-' L = Thiele pore length, cm LL = weight of catalyst charged to reactor (catalyst loading level), g M = CuCr content of catalyst (weight fraction of CuCr as oxide in the catalyst), dimensionless PV = catalyst total pore volume, cm3/g r = average radius of catalyst pores, cm SA = one point NzBET pore surface area, m2/g t = minutes since the start of hydrogenation T = tortuosity factor, dimensionless VL = volume of EPA charged to reactor, cm3 Registry No. EPA, 645-62-5; 2-EH, 104-76-7;cu-1107 T, 136503-78-1;Cu-1230 E, 136503-77-0;copper chromite, 11104-65-7. Literature Cited Autoclave Engineers. Autoclaue Catalytic Reactor Selection Guide. Bulletin 1200; Autoclave Engineers: Erie, PA, 1990. Kirshenbaum, I.; Inchalik, E. J. Oxo Process. In Kirk-Othmer Encyclopedia of Technology, 3rd ed.; John Wiley & Sons: New York, 1981; Vol. 16. Levenspiel, 0. Chemical Reaction Engineering; John Wiley & Sons: New York, 1972; Chapter 14, pp 470-475. Levenspiel, 0. Chemical Reactor Omnibook; OSU Bookstores, Inc.: Corvallis, OR, 1979; Section 24, pp 34.5-34.9. Wakao, N.; Smith, J. M. Diffusion in Catalyst Pellets. Chem. Eng. S C ~1962, . 17, 825-834.
Received for review January 25, 1991 Revised manuscript received June 27, 1991 Accepted July 17, 1991
Ozonation of Aqueous Solutions of Resorcinol and Phloroglucinol. 3. Instantaneous Kinetic Regime Fernando J. B e l t r i n * and Manuel Gonziilez Departamento de Ingenieria Quimica y Energetica, Universidad de Ertremadura, 06071 Badajoz, Spain
The ozonation of resorcinol and phloroglucinol in water is carried out at ozone partial pressures higher than 1700 Pa, 1-20 "C,and pH 7 and 8.5. At these conditions the kinetic regime of absorption is found to be instantaneous. This fact allows the determination of the volumetric mass-transfer coefficient kLa of the system. Values of kLa obtained in this way are in good agreement with others deduced from other chemical methods. Introduction As is known, the formation of organohalogen compounds such as trihalomethanes (THM) in water treatment plants is due to the chlorination of natural humic substances present in raw waters (Rook, 1977; Croui5 et al., 1989). The THM precursor character of these substances is often related to the presence of phenol-like structures in the humic macromolecules (Rook, 1980). Hence, the study of the elimination of these type of compounds is of interest 0888-5885/91/2630-2518$02.50/0
in reducing the THM formation potential of the waters in which they are solved. Reduction of THM concentration from the ozonation-chlorination of resorcinol and phloroglucinol in batch and semicontinuous mode (bubble column) has been presented in other papers (Sotelo et al., 1990; Beltrh et al., 1990). In these works, resorcinol and phloroglucinol are shown to be strong THM precursors, and ozonation of these phenols was an effective process for their elimination from the water. Therefore, it was 0 1991 American Chemical Society
