I n d . Eng. Chem. Res. 1987,26, 1059-1066 Buckley, G. D.; Ray, N. H. U.S. Patent 2550767, 1951. Clark, K. G.; Gaddy, V. L.; Rist, C. E. Ind Eng. Chem. 1933,25,1092. Ebetino, F. F. U.S. Patent 3 254075, 1966. Etemad-Moghadam, G.; Gasc, M. B.; Klaebe, A.; Perie, J. Nouu. J . Chem. 1984,8, 285. Fischer, E.; Koch, H. Ann. Chem. 1886,232, 227. Herman, F. L.; Dixon, D. D. U S . Patent 4314067, 1982. Hermanns, K.; Meyer, B.; Andrews, B. A. Kottes Ind. Eng. Chem. Prod. Res. Deu. 1986, 25, 469. Hofmann, K. Imidazoles and Its Deriuatiues; Interscience: New York, 1953; Part I, Chapter VII. Ito, K.; Takano, S.; Yasuda, M.; Ishii, M. Jpn Kokai JP 82/98268, 120 570 and 175 170, 1982. Kohn, H.; Cravey, M. J. Arceneaux, J. H.; Cravey, R. L.; Willcott, M. R., 111. J. Org. Chem. 1977,42, 941. Krimm, H.; Buysch, H. J.; Lange, P. M.; Klipper, R. Ger. Offen. DE 3 207 031, 1983. Leiber, M. A.; Berk, H. C. Anal. Chem. 1985,57, 2792. Lesher, G. Y.; Brundage, R. P.; Opulka, C. J., Jr.; Page, D. F. Fr. Demande, FR 2478637, 1981. Matsuda, H.; Baba, A.; Nomura, R.; Kori, M.; Ogawa, S. Ind. Eng. Chem. Prod. Res. Deu. 1985,24, 239. Matsuda, H.; Urabe, A,; Nomura, R. Znd. Eng. Chem. Prod. Res. Deu. 1984, 23, 422. Moeller, H.; Osberghaus, R. Ger. Offen. DE 2 746 650, 1977. Morino, S.; Sakai, M.; Kashiki, I.; Suzuki, A.; Miki, M. Bull. Fac. Fish. Hokkaido Univ. 1978,29, 75.
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Mulvaney, J. F.; Evans, R. L. Ind. Eng. Chem. 1948,40, 393. Nomura, R.; Kimura, M.; Teshima, S.; Ninagawa, A.; Matsuda, H. Bull. Chem. SOC.Jpn. 1982,55, 3200. Nomura, R.; Kori, M.; Matsuda, H. Chem. Lett. 1985, 579. Nomura, R.; Ninagawa, A.; Matsuda, H.’J. Org. Chem. 1980, 45, 3735. Nomura, R.; Takabe, A.; Matsuda, H. Chem. Express 1986, I 375. Puschin, T.; Mitic, R. V. Ann. 1937, 532, 300. Rayland, R. L., Jr. US.Patent 2825732, 1958. Reich, M.; Maeren, H. Ger. Patent 1121 617, 1962. Richiter, R. H.; Tucker, B. W.; Ulrich, H. U.S.Patent, 4154931, 1979. Sandler, S. R.; Karo, W. Organic Functional Group Preparations; Academic: New York, 1971; Vol 11, Chapter 6. Say, G. R.; Hays, J. R.; Iyenger, J. N.; Hacker, B. A. US.Patent 4 282 194, 1981. Schweitzer, H. J . Org. Chem. 1950, 15, 471, 475. Sherlock, M. H. U S . Patent 4376760, 1983. Snyder, H. R., Jr. U S . Patent 3822255, 1974. Walles, W. E. U S . Patent 3365426, 1968. Walles, W. E. U.S. Patent 4462865, 1984. Walles, W. E.; Leff, S. S. U.S. Patent 3248399, 1966. Wilson, A. L. US.Patent 2517750, 1950.
Received for review July 7, 1986 Revised manuscript received December 16, 1986 Accepted March 11, 1987
Residence Time Distribution Studies in a Multiphase Reactor under High Temperature and Pressure Conditions Ramakrishna V. Nalitham* and Oliver L. Davies Catalytic, Inc., Advanced Coal Liquefaction R & D Facility, Wilsonville, Alabama 35186 The residence time distribution of the slurry phase in a coal liquefaction reactor is determined experimentally under high temperature and pressure conditions using “native” radioactive tracers. T h e experimental data are fitted to several exit age distribution models, and a model is selected based on the “best” fit. The effect of process conditions such as recycle gas rate, coal feed rate, reactor temperature, and solvent-to-coal ratio on the degree of backmixing and mean residence time is studied. Gas holdup is estimated from the experimental mean residence time, the nominal residence time, and the total reactor holdup. The effect of gas superficial velocity on gas holdup is studied. T h e base-line experiment is repeated by using a different tracer to verify that the experimental residence time distribution data are characteristic of the reactor slurry, and not of the tracer type. An integrated two-stage liquefaction (ITSL) process is currently being studied in a 6 ton/day pilot plant at Wilsonville, AL (Rao et al., 1984). The process consists of a thermal liquefaction (TL) unit, a proprietary critical solvent deashing (CSD) unit, and a hydrotreater unit. System integration is achieved by selectively recycling various streams between the three units. A large ITSL data base (process yields) has been developed at Wilsonville for a bituminous rank Illinois No. 6 coal from the burning Star mine over the last 2-3 years. Such data may not be directly applicable to a large-scale plant because of the different hydrodynamic behavior in the reactor systems. An understanding of the mixing phenomena in the reactors, in general, is necessary for the scale-up and improved design of the reactors. The Wilsonville process consists of essentially two reactors, namely, the thermal dissolver and the hydrotreater. The catalyst in the hydrotreater is ebullated by recycling the slurry around the reactor which provides good backmixing in the reactor. There is no such liquid recycle in the thermal dissolver. This raises doubts about the hy0888-5885/87/2626-l059$01.50/0
drodynamic behavior of the dissolver in comparison to that of a well-mixed reactor. Moreover, since the three-phase dissolver is operated at high temperatures, in the range of 800-830 O F , a significant portion of the liquid stream will be in the vapor phase. This results in a higher actual slurry mean residence time compared to the nominal residence time. The mean residence time is an important variable in the kinetic analysis and reactor scale-up. Radioactive tracer studies are the only workable means for characterizing mixing and determining the mean residence time. Residence time distribution (RTD) studies using radioactive tracers were conducted on preheaters and reactors of the H-Coal, Exxon Donor Solvent (EDS), and Solvent Refined Coal (SRC) processes. For the SRC dissolver at Ft. Lewis, WA, three types of tracers were used: colloidal gold for the slurry phase, 82Br-tagged bromophenanthrene for the liquid phase, and 41Arfor the gas phase (Pittsburgh and Midway Coal Mining Company, 1982). A slug of tracer was injected in the middle of the reactor, and the tracer concentration was measured at the 0 1987 American Chemical Society
1060 Ind. Eng. Chem. Res., Vol. 26, No. 6, 1987
top and bottom. The results showed that in the reactor gas flow was essentially plug flow, while the slurry was backmixed a t all test conditions. In the preheater, both the gas and the slurry phases were predominantly in plug flow. Panvelker et al. (1982) reported that the experimental dispersion coefficient in the SRC dissolver was lower than that calculated from the correlation proposed by Decker et al. (1974) by a factor of 11.4. In their paper, they emphasized the need to study backmixing at elevated temperatures and pressures using radioactive tracers with physical properties closer to those of the actual process material. Exxon Research and Engineering Company reported RTD studies using 19*Au-taggedcolloidal gold for the slurry-phase and 41Arfor the gas phase (Exxon Research and Engineering Company, 1982). Gas moved essentially in plug flow, while the slurry Peclet numbers were in the range 4-13, indicating the slurry was not well mixed. The gas holdup was shown to be a strong function of gas superficial velocity. Bickel and Thomas (1982) studied catalyst mixing in the H-Coal reactor using 6oCo-tagged American Cyanamid HDS-1442A CoMo catalyst. The Peclet number was 0.45, which indicates the catalyst is well mixed. Vasalos et al. (1979) reported bed expansion and gas holdup data from the H-Coal reactor. In the present work, the RTD of the slurry phase in the Wilsonville dissolver was determined experimentally by using radioactive tracers. The experimental RTD data were fitted to several mathematical models, and a model was selected based on the “best” fit. The effect of process conditions such as recycle gas rate, coal feed rate, reactor temperature, and solvent-to-coal ratio on degree of backmixing and mean residence time was studied. Gas holdup was estimated from the mean residence time, the nominal residence time, and the total reactor holdup. The effect of gas superficial velocity on gas holdup was studied. A t the end of the study, a different tracer was used to verify that the experimental mean residence time was characteristic of the reactor slurry, and not of the tracer type.
Experimental Section Thermal Liquefaction Unit. The tracer studies were conducted during Wilsonville run 247. A complete description of the process configuration and the yield data is given elsewhere (Catalytic, Inc., 1985). A brief description of the TL unit is presented here. In this unit, pulverized coal is mixed with a process-derived solvent (450+ OF distillate plus hydrotreated resid). The slurry along with the hydrogen-rich recycle gas stream (8590 hydrogen purity) is heated in a preheater. The preheated slurry-gas mixture reacts in the dissolver, operated at temperatures of 800-825 O F and a pressure of 2400 psi. Gases and distillates are separated in a series of separators and distillation columns. A schematic diagram of the dissolver is shown in Figure 1. The 23-ft-long dissolver, inside diameter of 12 in., has four product draw-off nozzles placed a t equal intervals along its length. Thus, 25%, 50%, and 75% draw-off nozzles correspond to dissolver heights of 5.75, 11.5, and 17.25 ft, respectively, from the bottom of the reactor. Gas-slurry mixture from the preheater enters the bottom of the dissolver through a bubble cap plate distributor. Accumulated solids are withdrawn from the bottom of the dissolver intermittently a t a net rate of approximately 6 w t % of the feed slurry. All the RTD experiments were performed with the 25% draw-off nozzle on the reactor outlet. Therefore, the reactor volume above the 2590 draw-off nozzle was essentially “stagnant” gas.
Figure 1. Schematic diagram of dissolver (R-101).
Tracer Selection. The dissolver may be treated as a bubble column containing an upward flowing three-phase mixture of unreacted coal, solvent, and hydrogen gas. Ideally, the RTD of all three phases should be determined for a complete characterization of the mixing behavior. However, a single tracer was used in the present study for the reasons mentioned below. The presence of empty volume in the dissolver above the 25 % take-off level introduces some uncertainties in the analysis and interpretation of the gas-phase tracer data. For this reason, gas-phase tracer experiments were not performed. The solid phase is a mixture of unconverted coal (UC) and mineral matter. Selecting an appropriate solid-phase tracer is difficult since the solid phase is depleted continuously along the reactor length. Another complicating factor is that the solids are not uniform in size. There is some evidence in the literature that the solid particles smaller than 200 pm will have the same RTD as the liquid phase (Kato et al., 1972; Kang et al., 1984). For these reasons, a single tracer was used in the Wilsonville studies. The tracers can be classified into two broad categories, namely, native tracers and foreign tracers. Native tracer, as the name implies, is a component in the process material itself. The single most important consideration in any tracer study, perhaps, is that the tracer should follow the actual material, and hence, it is highly desirable to use a native tracer whenever possible. Some of the native tracers are coal, ash, and cresol insolubles of the reactor feed (preheater outlet sample). Foreign tracers were used in other liquefaction reactors. These were colloidal gold and brominated organic compounds (Exxon, 1982; Pittsburg and Midway Coal Mining Co., 1982). Oak Ridge National Laboratory proposed gold on resin for the Wilsonville dissolver (Jolley et al., 1983). In the present study, the solid material (cresol insolubles) in the dissolver feed was selected as a tracer for three reasons. First, this material represents the actual solid feed to the reactor. Second, ash in the sample can be activated by using neutrons to produce y rays which can be detected through the pipe walls. Third, on the basis of literature data, the solid particles are considered t o behave like a liquid with respect to mixing. Near the end of the study, a second tracer was used to verify whether the results were affected by the type of tracer being used. The second tracer was the ash from the CSD ash concentrate.
Ind. Eng. Chem. Res., Vol. 26, No. 6, 1987 1061 Table I. Neutron Activation Analysis of P r e h e a t e r Extract" element concn, element irradiated half-life ppm irradiated half-life K 12.4 h 8297 Mg 9.5 min Na 15 h 1824 Mn 2.6 h Ca 8.7 min 14824 Ba 83 min A1 2 min 36532 Dy 2.3 h v 5min 73 ~
concn, ppm 9657 267 496 2.6
a Irradiation was done a t Texas A & M University. Irradiation conditions: 9 g of material irradiated at a neutron flux of 1 X lo3 n/cm2/s.
I , y
Tracer Preparation. The preheater outlet sample from run 247 was extracted with cresol. The extracted solids were dried and ground. The mean particle size of the solids is about 5 pm with a standard deviation of 3 pm. The largest particle is smaller than 50 pm. On the basis of literature data (Kato et al., 1972; Kang et al., 1984), these particles are considered small enough to behave like a liquid phase. Table I shows neutron activation analysis of the tracer, performed at Texas A & M University. Primary considerations in judging the suitability of a tracer are half-life, concentration, and y-ray abundance. The desired characteristics are adequate half-life, high concentration, and high-y-ray abundance. Short half-life can cause shipping problems, and long half-life can cause safety problems. As noted in Table I, calcium, aluminum, vanadium, magnesium, and barium have short half-lives. Dysprosium and manganese have moderate half-lives, but their concentrations were low in the sample. Teledyne Isotopes estimated the abundance of y-ray emissions from the irradiated sample and the relative activity of the isotopes at any time after the sample leaves the nuclear reactor. The results are given in Table 11. At 6 h from the time the sample leaves the reactor, manganese will have substantial activity. However, since the half-life for manganese is short (
