I n d . Eng. Chem. Res. 1987,26, 18-22
18
Curtis, C. W.; Guin, J. A.; Tarrer, A. R.; Huang, W. J. Fuel Proc. Technol. 1983, 7, 277-291. Curtis, C. W.; Tsai, K. J.; Guin, J. A. Ind. Eng. Chem. Process Des. Deu. 1985, 24, 1259. Curtis, C. W.; Guin, J. A.; Kamajian, B. L.; Moody, T. Fuel Proc. Technol. 1986a, 12, 111. Curtis, C. W.; Guin, J. A.; Pass, M. C.; Tsai, K. J. Fuel Sci. Technol. Int. 1986b, in press. Curtis, C. W.; Tsai, K. J.; Guin, J. A. Fuel Proc. Technol. 1986c, in press. Garg, D.; Tarrer, A. R.; Guin, J. A.; Curtis, C. W. Fuel Proc. Technol. 1979, 2, 189. Garg, D.; Tarrer, A. R.; Guin, J. A.; Curtis, C. W.; Clinton, J. H. Fuel Proc. Technol. 1980a, 3, 245. Garg, D.; Tarrer, A. R.; Guin, J. A.; Clinton, J. H.; Curtis, C. W.; Paranjape, S. M. Fuel Proc. Technol. 1980b, 3, 263. Gatsis, J. G. US Patent 4 338 183, 1982. Gollakota, S. V.; Guin, J. A.; Curtis, C. W. Ind. Eng. Chem. Process Des. Dev. 1985, 24, 1148. Kottenstette, R. J. Sandia Report SAND82-2495, March 1983. Lebowitz, H.; Kulik, C.; Weber, W.; Johnson, T. W. Presented at the 74th AIChE Annual Meeting, New Orleans, LA, Nov 1981. Monnier, J. CANMET Report 84-53, March 1984. Moschopedis, S. E.; Hawkins, R. W.; Fryer, J. F.; Speight, J. G. Fuel 1980, 59, 647. Neuworth, M. B.; Moroni, E. C. Fuel Proc. Technol. 1984, 8 , 231. Rosenthal, J. W.; Dahlberg, A. J. US Patent 4 330 390, 1982. Rosenthal, J. W.; Dahlberg, A. J.; Kuehler, C. W.; Cash, D. R.; Freedman, W. Fuel 1982, 61, 1045. Schindler, H. D.; Chen, J. M.; Peluso, M.; Moroni, E. C.; Potts, J. D.; Presented a t the 74th AIChE Annual Meeting, New Orleans, LA, Nov 1981. Shinn, J. H.; Dahlberg, A. J.; Kuehler, C. W.; Rosenthal, J. W. Presented a t the EPRI Coal Liquefaction Contractor’s Meeting, May 1984. Stohl, F. V. Fuel 1983, 62, 122. Tarrer, A. R.; Curtis, C. W.; Guin, J. A.; Huang, W. J.; Lee, J. M. EPRI Report AP-1827, 1981.
Summary Highly effective and accessible catalysts are required to achieve high levels of oil production from the coprocessing of coal and heavy residua. Powdered hydrotreating catalyst at the higher loading and oil-soluble metal salts of organic acids catalyst precursors achieved the highest levels of activity for coal conversion and oil production. On a weight of active metal basis, the catalysts from the oilsoluble salts were the most effective in achieving both high levels of coal conversion and oil production. Pyrite was effective in achieving upgrading of asphaltenes from residuum and in achieving coal conversion in both singleand two-stage processing. Two-stage catalytic coprocessing using the first- and second-stage catalyst sequences of pyrite-NiMo/Alp03 and NiMo/A1203-NiMo/A1203 achieved the dual goals of coal conversion and oil production; however, the NiMo/A1,03-NiMo/A1203 sequence was much more effective in oil production. The products from the two-stage reactions were slightly more upgraded than those from the single-stage reaction.
Acknowledgment We gratefully acknowledge the support of this work by the US Department of Energy and Cities Service Research and Development Company under Contract DEFG2282PC50793. The provision of petroleum crudes and residua from Cities Service Research and Development Company and coal from the Wilsonville Advanced Coal Liquefaction Research and Development Facility is also gratefully acknowledged.
Literature Cited
Received f o r review February 10, 1986 Revised manuscript received July 16, 1986 Accepted August 26, 1986
Aldridge, C. L.; Bearden, R. US Patent 4 111787, 1978. Aldridge, C. L.; Bearden, R. US Patent 4 298454, 1981.
The Cup-and-Cap Reactor: A Device To Eliminate Induction Times in Mechanically Agitated Slurry Reactors Operated with Fine Catalyst Particles Ricardo J. Grau,t Albert0 E. Cassano,§and Miguel A. BaltanBs*S INTEC,’ Guemes 3450, 3000 S a n t a Fe, Argentina
A new three-phase mechanically agitated batch laboratory reactor is presented, featuring a cupand-cap holder for powdered catalyst. The apparatus allows precise determination of minute catalyst loadings, accurate control, and stability of process operating conditions, in situ preactivation of the catalyst a t any pressure and temperature without external devices for injection or introduction of solids, and zero induction time determinations of reaction rates. The catalytic hydrogenation of vegetable oil derivatives and the evaluation of mass-transfer coefficients are exemplified. Introduction The accurate determination of initial reaction times is a basic requirement for interpretation and data treatment of experimental results in time-dependent heterogeneous catalytic processes. Unfortunately, an “induction time” Instituto de Desarrollo Tecnoltgico para la Industria Quimica. Universidad Nacional de! Litoral (UNL) and Consejo Nacional d e Investigaciones Cientificas y TBcnicas (CONICET). Research Assistant from CONICET. $Member of CONICET’s Scientific and Technological Research Staff and Professor at UNL.
*
0888-5885/87/2626-0018$01.50/0
is all too often observed for hydrotreating processes and, as a general rule, for hydrogenation of vegetable oils and fats (List et al., 1974; Cordova and Harriott, 1975; Coenen, 1976; Drozdowski and Zajac, 1980). Hydrogenation processes for margarine and shortening production operate at 393-473 K and 140-1300 kPa under hydrogen pressure, in batch three-phase reactors. The hydrogenation catalysts consist of very fine particles of prereduced supported metal or metal oxides imbedded onto a protective, saturated-fat coating, which dissolves under process conditions into the oil to yield a catalyst slurry with 0.04-0.40% of solids. 0 1987 American Chemical Society
Ind. Eng. Chem. Res. Vol. 26, No. 1, 1987 19 Several causes for “induction time” appearances are identified in this process: (a) in situ reduction of the catalyst in parallel with the reaction under study, (b) noninstantaneous melting and diffusion of the protective coating, (c) reversible adsorption of impurities on the catalytic surface, and (d) time-dependent diffusive control of the initial reaction rate until stable concentration gradients have been reached. In situ reduction of the catalyst is perhaps the most important reason for the presence of induction times, since imbedded prereduced catalysts have often been subjected to haphazard histories prior to laboratory use. It is the purpose of this paper to present a novel laboratory technique for the elimination of induction times in kinetic and reaction engineering studies for vegetable oil hydrogenation processes in batch slurry reactors. The technique may, of course, be extended to other processes with similar characteristics.
Current Techniques in Use: Advantages and Disadvantages Two different groups of techniques have been used in the past to initiate reactions in well-stirred batch slurry reactors: (I) injection of one of the reactants, generally the gaseous one, into the suspension containing the remaining reactants and the solid catalyst and (11) insertion of the catalyst into the mixture of gaseous and liquid reactants. In the first group of techniques, the reactor containing the reactants and catalyst is taken to reaction conditions either under vacuum or under an inert atmosphere. The remaining reagent is then added to start the reaction. Both alternatives have disadvantages that are related to an actual lack of in situ activation of the catalyst prior to the start of the hydrogenation reactions. In vacuum techniques, minute leaks may cause a partial or total oxidation of the catalyst upon heating. This problem is avoided with the second alternative by using positive pressures of inert gases. In this case, however, a continuous purging of the inert gas is required whenever hydrogen has to be injected, adding to the uncertainties in defining its partial pressure a t zero reaction time. Two main methods are in use to reduce such difficulties: (1)preactivation of the catalyst in a separate closed vessel, followed by insertion under pressure into the reactor (Sin-hong Lo, 1981), and (2) breakage of fragile glass ampules containing the preactivated catalyst inside the reactor at zero reaction time (Kuboi et al., 1974). The first method offers severe uncertainties because of the lack of knowledge of the true amount of catalyst introduced into the reactor. The latter solves these uncertainties, but it is cumbersome and the catalyst and its coating are not exposed to an actual preactivation and saturation with the high-pressure hydrogen gas. Description and Operation of the Device A two-piece device was designed to overcome these deficiencies. It is applicable to autoclave semibatch slurry reactors that are provided with mechanical agitation. Its working principle is based on the “falling basket reactor” (Alcorn et al., 1978), an ingenious extension of the wellknown “Notre Dame continuous stirred tank catalytic reactor” (Carberry, 1964). It has been clearly indicated that Alcorn reactor cannot be used with catalytic particles of very small diameter. This is the case in many laboratory and bench-scale studies that involve the use of mechanically agitated slurry reactors. The proposed system consists of a fixed cover (cap) and a loose vase (cup) mounted on the reactor shaft, as indi-
Figure 1. Details of reactor with the CAC device: A, shaft; B, screw; C, cap; D, cup; E, grooved sleeve; F, cooling element; G, agitator; H, thermoresistance well; I, hydrogen inlet sampling tube.
cated in Figure 1. A helicoidal groove on the shaft allows the descent of the cup by means of a follower pin. Two horizontal grooves allow the positioning of the cup either a t the upper part of the helix in the gas phase or at the lowest position on top of the impeller blades submerged into the liquid phase. The cup contains the solid catalyst and avoids its contact with the liquid reactants while exposing the catalyst to pretreatment at real process temperature and pressure conditions. Thus, activation of the catalyst with simultaneous hydrogen saturation of both the protective coating and the oil phase can be achieved. An excellent and quick stabilization of the temperature and pressure can be obtained. The cap prevents the liquid reactants from splashing into the cup whenever the oil is mechanically agitated. Its internal diameter is slightly bigger than the one belonging to the vase. A conical upper external face avoids accumulation of liquid or slurry portions. The cup interior has to be carefully designed to prevent catalyst spills due to centrifugal forces and to guarantee a complete, fast mixing of the solid with the liquid reactants. A detailed description of recommended operational procedures follows: (a) A precise weight of catalyst is placed in the cup. A powdered catalyst should be added as a mixture with a chemically inert liquid to facilitate its later dispersion into the reacting mixture. A liquid reactant can be used whenever two reacting liquids are involved, whereas a final nonadsorbing reaction product is a good choice for a single liquid reactant. We have found that a liquid-to-solid ratio of 1.5-2.3 v/v is most suitable. Given the small catalyst-to-reagents ratio usually required, this extra liquid volume is generally negligible. Commercial vegetable oil catalysts are usually available in flakes of protective hydrogenated-fat coatings. These inert coatings melt below reaction temperatures and provide a convenient liquid carrier. The cup can be easily removed from the shaft and used for direct weighing of the catalyst load. (b) The cup is mounted by sliding it upward along the helix and rotating it horizontally into the upper suspension groove. The reactor, filled with the liquid reagents, is assembled. (c) The system is then purged of air at room temperature
20 Ind. Eng. Chem. Res. Vol. 26, No. 1, 1987 Y
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Figure 3. Typical temperature vs. time and H2 consumption vs. time curves with the CAC device.
by a flow of the gaseous reactant under mechanical agitation. Mild vacuum or an inert gas may also be used if needed. (d) An in situ preactivation of the catalyst and gas saturation of the liquid phases follow by heating the reacting system and pressurizing with the reactant gas under mechanical agitation. Oxidation of the catalyst is thus precluded. (e) Once the preactivation is finished, the reacting system may be taken to controlled and stabilized operating conditions (pressure, temperature, speed of agitation) prior to the kinetic measurements. A sudden stop of the impeller for about 3 s causes the cup to fall along the helix while full agitation is reinstated. The reaction starts a t zero time because the intense turbulence causes the dispersion of the catalyst to be almost instantaneous. This was optically verified by our group with a glass reactor of exactly the same geometry and hydrodynamic characteristics as the one used for regular runs. With this glass reactor it could be observed that the impeller blades produce an easy and fast washing of the cup content. Several unique advantages of this device have been identified: (I) precise determination of the amount of catalyst to be used and the initial time of reaction; (2) precise control and stability of process operating conditions (see the example below), with start-up being accomplished with a simple interruption of agitation; (3) in situ preactivation of the catalyst at any pressure and temperature, without external devices for injection or introduction of solids; (4) in situ saturation of the activated catalyst and the liquid reactants with the reacting gas; and (5) avoidance of vacuum or inert atmospheres.
Some disadvantages are also present: (a) the volumetric capacity of the cup places a limit on the attainable catalyst-to-liquid ratios (b) any nondesirable interruption of mechanical agitation will cause the cup to fall, spoiling the run; and (c) the device is only suitable for mechanically agitated reactors.
Application of the New Technique The device was used for hydrogenating 40 cm3 of soybean methyl esters, with a 2 mg of cat./g of liq loading throughout the experiments. Temperature and pressure ranges were 398-443 K and 272-542 kPa, respectively. Both induction time suppression and accurate initial rate determinations were achieved. The reaction kinetics were also suitable for mass-transfer coefficient estimation, as shown below. Induction Time Suppression. Two different experimental procedures were selected to show the induction time related improvements. Low operation pressure and temperature were chosen, because the induction time effect is more pronounced under those conditions. Figure 2 shows the hydrogen consumption vs. time profile observed when the gaseous reactant injection technique was used. The existence of an important induction time, mainly due to an in situ activation of the catalysts, is apparent. A substantial qualitative and quantitative change is depicted in Figure 3, which portrays the hydrogen consumption vs. time relation under the same experimental conditions when the cup-and-cap (CAC) device was used. A 2-h preactivation period at 398 K and 272 kPa was used for this experiment. Note the absence of temperature changes as well as a total suppression of the induction time.
Ind. Eng. Chem. Res. Vol. 26, No. 1, 1987 21 "I
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Preactivation procedures depend, of course, on the catalyst used and/or the process nature and have to be determined experimentally for each situation. We have chosen the above-mentioned conditions since it was found that higher temperatures and pressures or longer preactivation times yield the same catalytic activity. Be that as it may, this new technique allows preactivation procedures to be carried under process conditions that do not depend upon the subsequent reaction conditions. Reproducibility. Experimental reproducibility is a seldom mentioned but important subject. Coenen (1960) has paid special attention to this problem, describing reproducibility in terms of cumulative total hydrogen consumption during hydrogenation reactions. This description is unfortunately not rigorous enough and does not guarantee identical time evolution of concentration values for individual species in complex reacting systems, given equal values of the total hydrogen consumption vs. time chart. We have followed instead the concentration vs. time pattern of each of the liquid-phase reactants and products. Fiwre 4 shows the close agreement of these concentration patterns by using the CAC device in two experimental runs at identical process conditions. Estimation of Mass-Transfer Coefficients. The discussion that follows illustrates an effective application of the procedure for mass-transfer coefficient estimations from accurate initial reaction rate values. It is not meant to present a complete or detailed analysis of the subject. Intraparticle diffusional limitations were assessed by using the well-known criterion of Weisz and Prater (1954). The Weisz and Prater moduli, @, for hydrogen and methyl esters are 0.019 and 0.017, respectively, at 542 kPa and 443 K, the process conditions for maximum reaction rates. Since
