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Ind. Eng. Chem. Process Des. Dev. 1983, 22, 662-665
size, the encapsulated catalyst ends as particles with surface area roughly 100 times greater than that of the powder. Microencapsulation of coal liquefaction catalysts is not limited to AM. Other catalytic metal salts, such as ferrous sulfate, nickelous chloride, or stannous chloride, could be encapsulated and used in the same way. It is also likely that air-sensitive catalysts (some organometallics) can be similarly prepared and used, since they will be protected by the polymeric shell of the microcapsules against attack by air. Acknowledgment The authors wish to thank Moore Business Forms for
its continued encouragement of this research, and the Department of Energy for its financial support of much of the work. Registry No. Ammonium heptamolybdate, 12027-67-7.
Literature Cited Moll. N. G.; Quarderer, G. J. U.S. Patents 4 136013 and 4172814, 1979. Ruus, H. U.S. Patent 3429 827, 1969, Scinta, J.; Weller, S.W. Fuel Process Tecbnol. 197711978, 1 279. Weller, S.; Pelipetz. M. G. Ind. End. Chem. 1951, 43, 1243. Weller, S. W. 4th International Molybdenum Chemistry Conference, Golden, CO, Aug 1982.
Received for review January 3, 1983 Accepted March 21, 1983
Oxygen Chemisorption and Olefin Disproportionation Activity of W03/Si02 Suk J. Choung and Sol W. Weller" Department of Chemical Enginwring, State University of New York at Buffalo, Buffalo, New York 14260
Tungsten oxide-silica catalysts have been characterized by measurement of oxygen chemisorption and activity for the disproportionation of propylene, over the loading range 5.3 to 15.2 wt % W03. Oxygen chemisorption at -78 "C,on samples reduced in situ in a vacuum microbalance, increases smoothly with W 0 3 loading; the shape of the curve suggests that the higher the loading, the lower the dispersion. Catalytic activities after N, activation vary almost linearly with the O2chemisorption values, when correction is made for the actual extent of reduction during the prereduction (preceding the chemisorption measurement). This correlation suggests, but does not prove, that the relative tungsten oxide area, as measured by O2chemisorption on reduced catalysts, may be causally related to the activity for propylene conversion.
Introduction Molybdenum oxide and tungsten oxide on silica have been the subject of various studies since the report by Banks and Bailey (1964) of their activity in the disproportionation of olefins (Banks, 1980; Heckelsberg et al., 1969; Luckner et al., 1973; Thomas et al., 1979). Until recently, the research has been concerned largely with reaction kinetics and mechanism. In the past several years a series of papers has attempted to elucidate the nature of the active sites in these catalysts by a variety of techniques (Kerkhof et al., 1977; Stork and Pott, 1977; Thomas et al., 1979; Van Roosmalen et al., 1980). There appear to be no published reports on characterization of reduced W03/Si02 by gas chemisorption. Since selective chemisorption of O2 at low temperature has been profitably employed for the study of supported molybdena catalysts (Parekh and Weller, 1977; Liu and Weller, 1980; Garcia Fierro et al., 1980), an attempt has been made in the present work to apply this method to the W03/Si02system. Catalysts with a loading range of 5.3 to 15.2 wt % WO, have been studied, for O2 chemisorption (after prereduction) at -78 "C, and for the disproportionation of propylene to ethylene and butenes at 500 "C. Experimental Section Catalyst Preparation. Catalysts were prepared by impregnation of 60 to 80 mesh SiOzgel (Davison Chemical grade 57; BET surface area 253 m2/g) with aqueous am0196-4305/83/1122-0662$01.50/0
monium metatungstate, (NH4)6H2W12040.5H20, by the no-excess solution technique. After impregnation, the catalysts were dried at 100 "C and calcined at 550 "C for 3 h in flowing dry air. The range of loading was 5.3 to 15.2 wt % W03 in the finished catalyst. The calcined samples have a bright yellow color, characteristic of WO,. Apparatus and Procedures. Oxygen Chemisorption. A static, gravimetric adsorption system, composed of a Cahn 2000 Electrobalance and a high-vacuum system, has been used for prereduction and adsorption measurements. Details of the apparatus and corrections have been give elsewhere (Choung, 1982). The O2 chemisorption values were determined as the difference between two O2 adsorption isotherms a t -78 "C. Prior to the first adsorption isotherm measurement, (typically) 120 mg of catalyst was prereduced in situ with static H2 (400 torr) at 500 "C for 10 h and then outgassed at 500 "C for another 10 h. Between the first and second isotherm measurements, the sample was pumped for 1h at -78 "C in order to desorb the physically adsorbed oxygen. Activity for Propylene Disproportionation. A continuous flow microcatalytic reactor was used for the activity measurements. The reactor was 4.6 mm i.d. stainless steel tube, in which 300 mg of catalyst was placed and held by plugs of glass wool. The propylene used was Matheson C.P. grade (99.2% purity), and its flow rate through the reactor was 40 cm3/min at 14 psig. Product analysis was by G.C. with an analytical column (4.8 m X 0.635 cm) of 20% BMEA on 60-80 mesh Chromosorb P. Column and 0 1983 American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 22, No. 4, 1983 883
Table I. 0, Chemisorption o n Reduced WO, /SiOZo
0, chemiwt%
WO, O(Si 0, ) 5.3
10.0 15.2
sorption, extent run mgof O,/ of no. g of cat. redn
1 2 1 2
0 0.42 0.32 0.60 0.56 0.80
0.66 0.46 0.81 0.84 0.89
app disp (D)b
?-
0.114 0.087 0.088 0.081 0.076
a 0, chemisorption a t -78 "C after reduction in situ, H,, 500 "C, 10 h . Apparent dispersion (D) = (no. of chemisorbed 0 atoms)/(total no. of W atoms in sample).
detector were maintained at room temperature. Data processing of the G.C. output was by a Varian CDS-111. Three sets of activity measurements a t 500 "C were conducted: (1) no pretreatment, (2) prior nitrogen purge at 600 "C for 1 h, and (3) prereduction with a steam-hydrogen mixture for 1 h.
Results and Discussion Prereduction. Since prereduction is a necessary step for interpretation of the chemisorption results, the reduction behavior was followed in the Cahn microbalance. While the reduction of W03 in dry H2 proceeds directly to the metal, it is known to be possible to form the intermediate W02 by the use of controlled temperature and H2/H20mixtures (Glemser and Sauer, 1943; Hougen et al., 1956). The details of extensive reduction studies of supported and unsupported W03 catalysts have been give elsewhere (Choung, 1982). In the present work, instead of adding H 2 0 to the H2, the water content of the silica support was used to control the reduction process. For this reason, the sample in the microbalance was outgassed at room temperature (2 h) rather than high temperature prior to reduction. Blank determinations of the amount of support-derived water were made in order to permit evaluation of the true reduction values. For the prereduction step, 400 torr of H2 was introduced at room temperature, and the sample was heated in the microbalance to 500 "C, where it was held for 10 h. The mass change became insignificant, for most samples, after 10 h. The computed extent of reduction (defined as 1.0 when W03 is converted to W02) ranged from about 0.6 to 0.9, depending on the W03 loading; the higher the loading, the greater the extent of reduction. This suggests the presence of difficultly reducible material in the catalysts with lower W content. Kerkhof et al. (1977) have also concluded that W03/Si02is composed of easily reducible, crystalline, bulk W03 and a difficultly reducible surface compound, the ratio of these species being a function of W03 loading. Oxygen Chemisorption. After the prereduction, the sample was outgassed at 500 "C for 10 h and cooled under vacuum to -78 "C for chemisorption studies. A first isotherm (corresponding to physisorption as well as chemisorption) was measured at -78 "C; then the sample was pumped at the same temperature for 1 h and a second isotherm was measured. Both isotherms were linear in the range of O2 pressure of 120 to 360 torr, and the second isotherm (representing physisorption) passed through the origin as well. (This provides a useful apparatus check). Oxygen chemisorption values were calculated from the difference between the two isotherms. Results of these studies and of the prereduction are summarized in Table I.
0
~
io
5
45
Wt 96 W03 (in oxidized stote)
Figure 1. Oxygen chemisorption (at -78 "C)on prereduced WOs/ SiOp as a function of gross compositon (wt % WOs in catalyst). I IZ
2
8 P
io
t t I
V
0
I
5
I
40 Wt % W03 x Extent of Reduction ("Corrected Loading")
I
15
Figure 2. Oxygen chemisorption on prereduced W03/Si02 as a function of corrected loading ( w t % W03 X extent of reduction).
The apparent dispersion, D, is defined as the ratio of number of chemisorbed 0 atoms to the total number of W atoms in the sample. As in the case or reduction, precise determination of chemisorption becomes increasingly difficult as the W03 loading is decreased. Figure 1 is a plot of O2 chemisorption on the reduced catalyst vs. the gross composition (wt % ' W03 in the catalyst prior to reduction). Oxygen chemisorption increases as the loading increases, as one would expect. The curve is convex upward, suggesting that the apparent dispersion decreases as the loading increases. The values of D in Table I reflect this behavior, which is common in supported catalysts. Since catalysts with a higher extent of reduction may be expected to have a greater fraction of sites that are able to chemisorb 02,the gross composition (wt % W03) was corrected for the measured extent of reduction of each sample. For example, the "corrected loading" for the 15.2 wt % sample was calculated as 15.2 X 0.89 = 13.5 w t % W03. Figure 2 is a plot of O2 chemisorption vs. this "corrected loading". The points fall on a smooth curve which, like that in Figure 1, is again convex upward. The implication remains that the apparent dispersion decreases as W03 loading increases. Propylene Disproportionation. Activity measurements at 500 "C were carried out after each of three different pretreatments: (a) normal air calcination (550 "C) only; (b) flowing dry N2 at 600 "C for 1 h; or (c) prere-
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Ind. Eng. Chem. Process Des. Dev., Vol. 22, No. 4, 1983
withouY/ Correction
0
5
40 wt % wo,
45
20
Figure 3. Conversion of propylene by N2-pretreated catalyst as a function of gross composition ( w t % W03 in sample).
duction with flowing H20/H2(72 torr of water vapor in H2 stream) at 630 "C for 1 h. With no additional pretreatment ((a) above), the activities quickly approached a maximum conversion level of about 19 to 21% and remained level. The "% conversion" is defined here as (moles of propylene consumed by reaction/moles propylene fed) X 100. There was no sign of deactivation during a 6-h run. Surprisingly, the activity was almost independent of W03 loading over the range studied. The ratio of ethy1ene:butenes in the product remained close to 1 at all times; this is the stoichiometric ratio for the disproportionation reaction. Totally different behavior was shown by catalysts prereduced in H20/H2((c) above). In this case, even the initial conversion was relatively low, and it decreased to a steady value of only about 1%. This severe deactivation may have resulted from the combined effects of water adsorption during the prereduction process and of side reactions occurring ower bulk W03. The depression of the catalytic activity of W03/Si02 by moisture was previously reported by Luckner et al. (1973),who postulated that the loss of activity was related to retention of moisture by the catalyst surface. In the present study, an attempt was made to reactivate the prereduced catalyst by heating it at 630 "C for 100 min in flowing N2,following which the activity was measured again at 500 "C. Almost the same activity vs. time profile was reproduced: initial activity was low, and severe deactivation occurred within 1 h on stream. This experiment argues against the hypothesis of poisoning by water vapor. In remarkable contrast, the conversion with N2 pretreatment ((b) above) showed both an effect of W03 loading and a significant increase over the "nopretreatment" case for each sample. A characteristic "break-in" period was observed. It took 4 to 8 h, depending upon W03 loading, to obtain steady-state conversion; the higher the loading, the longer the "break-in" period. No sign of deactivation was observed up to 8 h on stream. Biloen and Pott (1973) suggested that @-tungstenoxide (WO,,) may be formed in the surface of the W03 crystallites during N2 pretreatment at elevated temperature. Westhoff and Moulijn (1977) measured the conversion of propylene on variously reduced W03/Si02catalysts and found that the samples intermediate between W03 and W02.95show maximum activity. Samples more reduced than this exhibit lower conversion. Our results are con-
'V Y
0
I
I
02
04
Corrected
I
I
I
06 08 40 0, Chemisorption (mg O,/g cat 1
I
I
42
44
Figure 4. Conversion of propylene by N2-pretreated catalyst vs. oxygen chemisorption on prereduced catalyst. "Without correction" in conversion vs. measured chemisorption. "Corrected" is conversion vs. chemisorption corrected for extent of reduction.
sistent with these earlier reports in the sense that some loss of oxygen is necessary (e.g., by N2pretreatment at 630 "C) for high activity, but extensive reduction (even with a controlled H20/H2stream) is harmful. Figure 3 shows the plot of steady-state conversion, after N2 pretreatment of 630 "C, against W03 loading. A smooth, convex-upward curve was obtained, resembling the oxygen chemisorption plots in Figures 1 and 2. This similarity suggested cross-plotting of propylene conversion vs. O2 chemisorption. Figure 4 is such a plot, made on two bases. The curve labeled "without correction" shows conversion vs. the measured O2 chemisorption value. Since, as mentioned above, the O2chemisorption may be better associated with the amount of reduced W03than with total WO, the curve labeled "corrected" in Figure 4 represents conversion vs. 90W03 corrected for the actual extent of reduction before O2 chemisorption. Although not much reliance can be placed on the accidental occurrence of linearity in nature, the fact that the points fall almost on a straight line thrugh the origin does provide encouragement for the belief that the apparent tungsten oxide areas, as measured by oxygen chemisorption, are causally related to the activity for propylene disproportionation. Registry No. Propylene, 115-07-1;tungsten oxide, 1314-35-8. Literature Cited Banks, R. L.; Bailey, G. C. Ind. Eng. Chem. Prod. Res. Dev. 1064, 3 , 170. Banks, R. L. "Catalysis in OrQnic Synthesis"; Academic Press: New York, 1980; p 233. Biloen, P.; Pott, G. T. J . &fa/. 1073. 30,169. Chouna. S.J. Ph.D. Dissertation. State University of New York at Buffalo, Bufalo, NY, 1982. Garcia Fierro, J. L.; Mendioroz, S.; PaJares, J. A.; Weller, S. W. J . Catal. 1880. 65. 263. Gh3iiser; O.;'Sauer, H. Z.Anorg. Chem. 1843, 252, 144. Heckelsberg, L. F.; Banks, R. L.; Bailey, G. C. Ind. Eng. Chem. Prod. Res. D e v . 1969, 8 , 259. Hougen. J. 0.;Rooves, R. R.; Mannella. G. I d . Eng. Chem. 1056, 4 8 , 318. Kerkhof, F. P. J. M.; Thomas, R.; Mouiijn, J. A. R e d . Trav. Chim. 1977, 9 6 ,
M121.
Liu, H.G.; Weller, S. W. J . Cera/. 1080, 66, 65
Ind. Eng. Chem. Process Des. Dev. 1983, 22, 665-672
885
A. J.; Koster, D.; Mol, J. C. J. phvs. Chem. 1980. 84, 2090. Westhoff, R.; MoullJn, J. A. J . Catel. 1977, 46. 414. Van Roosmalen.
Ludtner, R. C.; McConchle. 0. E.; Wills, 0. B. J . Catel. 1973. 28, 63.
Parekh, B. S.; WeHer, S. W. J. Catel. 1977, 47, 100. Stork, W. H. J.; Poit, G. T. R e / . Trav. Chlm. 1977, 96, M105. Thomas, R.; MlttelmelJer-Hazeieger,M. C.; Kerkhoff, F. P. J. M.; Moulijn, J. A.; Medema, J.; de Beer, V. H. J. 3rd Intemetionel Conference on Chembtry and Uses of Molybdenum, 1979; Clhnax Molybdenum Co.: Ann Arbor, p 05.
Received for review January I, 1983 Accepted March 28, 1983
Limiting Analytical Relationships for Prediction of Spray Dryer Performance Rodger A. Ooffredlt and E. Johansen Crosby" Department of Chemical Engineering, The University of Wisconsin; Madison, Wisconsin 53 706
Tractable analytical relatlonshlps that allow prediction of performance trends for existing spray dryers with well-insulated chambers in which sprays are monodispersed and fine have been developed for certain limiting conditions. Drying mechanisms based on the evaporation of water drops and the diffusion of water in solid particles in combination with cocurrent and completely mixed gas flow were considered. The sensitivity of the drying process to changes In operating variables for directly fired dryers with outlet temperature control is illustrated.
conditions when exposed to the outlet drying gas conditions. The results were formulated as expected overall heat-transfer rates, and reasonable agreement was reported between prediction and experimental results for small laboratory dryers. The object of this study was to develop a set of limiting but relatively simple and tractable analytical models which would bracket the performance of large, existing spray dryers equipped with single or multiple atomizers which produce sprays consisting of droplet diameters less than ca. 100 pm. Analysis Regardless of the prevailing conditions within a spray dryer, the total transfer of moisture between the spray and the drying medium and of heat between the drying medium, the spray and the surroundings can be determined from the overall mass and energy balances, respectively. However, the drying chamber geometry and size in addition to the operating conditions necessary to meet any given set of product specifications determine the rates at which transfer occurs. The appropriate rate expressions can be combined with the differential forms of the mass and energy balances which upon integration yield a design or performance equation that relates the most important operating parameters to the dryer size. A complete analytical model is, then, a set of three equations composed of a mass balance, an energy balance, and a design or performance relationship. In the present formulation of the performance relationship, the complex mixing effects in the vicinity of the atomizer are neglected and the spray is assumed to be dispersed ideally and instantaneously into the drying medium because of the smallness of the drops. Further, the drops are assumed to be uniform in size and to have negligible velocity relative to the drying medium. Two limiting mechanisms of drying are considered, viz., the evaporation of pure water drops by adiabatic humidification and diffusion-controlleddrying of solid particles. The manner in which each of these mechanisms influences the performance of a well-insulated spray dryer is examined for both cocurrent and mixed-flow operation.
Introduction A major obstacle in the development of predictive procedures for use in the design, scale-up, and performance description of spray dryers is the complexity of the flow patterns and interactions between the drying medium and droplets. As a rault, most of the more detailed procedures require lengthy numerical computations. The work of Edeling (1950)is illustrative of first attempts to consider such flow patterns and interactions. He observed experimentally that high initial drop velocities caused substantial penetration into the dryer before sufficient deceleration occurred to allow the swirling motion of the gas to manifest itself. Relationships were developed to describe the spiral motion of drying drops. However, drop motion and drying were not coupled. Many of the more recently proposed procedures to describe the simultaneous exchange of momentum, mass, and thermal energy between droplets and the drying medium have been reviewed by Crowe (1980).A number of these procedures take the zone of droplet penetration into consideration and allow for coupling of droplet drying and motion. Such techniques yield exact results within the limitations of the models as illustrated by the computations of Keey and Pham (1976).However, lengthy numerical analyses do not give rapid insight relative to expected dryer performance under variable operating conditions. Consequently, it would be advantageous to have a simple system of equations which gives a clear understanding of the drying principles involved, is relatively easy to use, and is sophisticated enough to predict reasonably accurately the interrelationships among the most important design and operating parameters. Gluckert (1962)proposed such relationships for the design of spray dryers equipped with a single atomizer of either rotary, pneumatic, or pressure nozzle variety. These models were based on particle trajectory from the point of drop formation with drying described by likening the largest drop in the spray to a water drop of constant diameter evaporating under stationary 'Power Division, Stone & Webster Engineering Corp., Denver,
co.
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0
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American Chemical Society
