Ind. Eng. C h e m . Res. 1988, 27, 279-283
volumes as large as a few liters, to situations where several positions are to be monitored simultaneously, and to experiments with transient times from about two-tenths of a second to several hours. It is capable of measuring transient, volume-averaged composition and observing spatial composition variations. In the embodiment demonstrated, dual-wavelength and time-shared detection permit the concentrations of two absorbing species to be followed simultaneously 30 times/s. Eight light beams, each passing through different locations of a 3-L cell (15.8-cm light path), enable the transient volume-averaged concentrations as well as composition variations to be monitored. An IBM personal computer records 600 readings/s (2 wavelengths, 8 light beams, 30 samples/s, plus 30 reference light intensities, 60 thermistor temperatures, and 30 impeller speeds). The operating characteristics of the current embodiment have been confirmed by two types of measurements. Steady-state absorbance measurements using various concentrations of azo dyestuffs (products of the azo coupling reactions) confirm that the method is accurate to within 2%. Transient absorbance measurements following the progress of the azo coupling reactions while mixing occurs confirm that the method is robust, fast in response time, and accurate to within 2.5% in measuring light absorption. Modifications of the current embodiment appear feasible for monitoring four species simultaneously. This extension requires expansion of the dual-wavelength analysis section of four wavelengths. The current data acquisition system is capable of sampling and processing the two extra light signals. Also, trade-offs are possible between the number of positions monitored and speed. For example, if only two positions are monitored rather than eight, the speed of response can be 4 times faster, so transient phenomena on the order of five-hundredths of a second can be handled. This change is accomplished by shifting the assembly of beam splitters and mirrors to the interfacce between the reactor cell and the receiver/collimators a t the receiving end of the optical fibers.
279
Acknowledgment Much needed financial support was provided by the National Science Foundation (NSF CPE 83-17142) and the Petroleum Research Fund, administered by the American Chemical Society. Nomenclature 4 = a-naphthol; see eq 9a A = 2 X 1 absorbance array; see eq 6 AI, A , = absorbances at wavelengths 435 nm (1)and 577.5 nm (2), respectively, over light path b, dimensionless b = length of light path, cm @ = diazotized sulfanilic acid; see eq 9a C =,2 X 1 concentration array; see eq 6 C1, C, = volume-averaged concentrations of species 1and 2, respectively, mol/L C1., C, = concentration of species 1 and 2, respectively, for iight beam j , mol/L D,, D? = photodiode signals for Piand P:, respectively, V E = 2 X 2 extinction coefficient matrix; see eq 6 k,,kz = second-order rate constants of the azo coupling reactions, eq 9a and 9b, L/(mol-s) P,, P,O = optical beam power of the light beam at wavelength i passing through a solution and a solvent free of absorbing species, respectively, W = optical beam power of the reference light beam bypassing the reactor cell, W R, S = monoazo and disazo dyestuffs, respectively; see eq 9a and 9b Literature Cited Bourne, J. R.; Hilber, C.; Tovstiga, G. Chem. Eng. Commun. 1985 37,293. Bourne, J. R.; Kozicki, F.; Rys, P. Chem. Eng. Sei. 1981, 36, 1643. Dickey, D. S.;Fenic, J. G. Chem. Eng. 1976, 83(1), 139. Palepu, P. T. Ph.D. Thesis, Case Western Reserve University, Cleveland, OH, 1985. Zollinger, H. (Translatedby Nursten, H.) Azo and Diazo Chemistry; Interscience: New York, 1961; Chapter 1.
Received for review December 3, 1986 Revised manuscript received August 7, 1987 Accepted September 4, 1987
Iron/Manganese Oxide Catalysts for Fischer-Tropsch Synthesis. 4. Activity and Selectivity R e i n e r Malessa and M a n f r e d B a e r n s * Lehrstuhl fur Technische Chemie, Ruhr- Universitat Bochum, 0-4630 Bochum, West Germany
Activity and selectivity of iron/manganese oxide Fischer-Tropsch (FT) catalysts of various compositions were studied. The catalysts were reduced a t 300 or 400 “C. FT experiments were performed = 11 bar, pco = in a tubular catalytic fixed bed reactor for approximately 200 h of operation (Ptot 2.75 bar, pH2 = 5.5 bar, pN2= 2.75 bar; T = 225 “C 75 h 270 O C ) . Catalysts of pure iron or low manganese content reduced a t 400 “C exhibited a high initial activity which decreased, however, rather rapidly with time. Catalysts reduced a t 300 “C were less active; their activity was, however, only slightly affected by time. Small amounts of manganese (3-15 w t %) enhanced the formation of olefins and lowered simultaneously the isomerization activity. Selectivity for oxygenated hydrocarbons was small but highest for catalysts reduced a t low temperature.
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Iron-based Fischer-Tropsch catalysts containing “more than 50% manganese the remainder being iron” (Kolbel and Tillmetz, 1976), or “about equal parts of iron and manganese” (Bussemeier et al., 1976) have been proposed for high olefin selectivity. In contrast to these results, van Dijk et al. (1982) did not find any changes in product
formation when adding manganese oxide to an iron catalyst; their work was, however, done at atmospheric pressure, while the former authors operated the catalysts close to industrial conditions. To elucidate these controversial results, the catalytic performances of iron/manganese oxides having different manganese contents varying from
0888-5885/88/2627-0279$01.50/0 0 1988 American Chemical Society
280 Ind. Eng. Chem. Res., Vol. 27, No. 2, 1988 Table I. Chemical and Crystal-Phase Composition as Well as Specific Surface Area of the Catalyst Samples after Reduction and FT Synthesis SBET/m2g-l after FT crystal-phase compositionb after chemical reduction code composition temp: O C calcinat. reduct. synth. reduction FT synth. 12.8 12.6 4.0 Fe Fe304;carbides (traces) 300 Fe lOOa 100% Fe 4.5
