Ind. Eng. Chem. Res. 1992,31,1583-1589
1583
KINETICS AND CATALYSIS
A Bench Scale Study of Reversed Flow Methanol Synthesis S. G. Neophytides and G. F. Froment* Laboratorium uoor Petrochemische Techniek, Rijksuniuersiteit, Krijgslaan 281, B 9000 Gent, Belgium
The reversed flow operation of a bench scale catalytic reactor was applied to the synthesis of methanol from CO-C02-H2mixtures a t 50 bar, over a IC1 copper-based commercial catalyst. Temperature and concentration profiles develop and wander back and forth through the reactor, depending upon the direction of the flow. The relaxation characteristics of the different reaction pathways on the catalytic surface are also responsible for the significant transients generated in the gas-phase composition. The experimental results lead to considerable insight into the mechanism of methanol synthesis. The influence of the cycle length, flow rate, and inlet temperature on the reactor performance are also reported.
Introduction The transient operation of catalytic reactors is receiving increased attention. Forced cycling of feed composition or temperature may lead to improved conversions and selectivities (Silveston, 1987; Froment, 1990) when compared with those obtained by steady-state operation, with a feed corresponding to the cycle average. Additional transients may also be generated on the reactor scale: temperature and concentration profiles may wander back and forth through the bed. Their shape may further improve conversions and selectivities. From the mid-1970s onward Boreskov and Matros (Boreakov and Matroe, 1983; Matroe, 1985,1989)described the behavior of a catalytic tubular reactor under transient conditions forced by periodic reversal of the gas flow. This technique leads to the formation and propagation of a comparatively narrow high-temperature reaction zone along the catalyst bed (Padberg and Wicke, 1967). The most important advantage of this mode of operation lies in the combination within the catalytic bed of a chemical reactor and a heat exchanger. The gas mixture is introduced in the reactor at a low temperature, so that the reaction rate is negligible. Temperature and concentration fronts are formed and move pardel to one another without distortion. After each flow reversal, the heat wave appears at the inlet of the reactor and then moves slowly in the direction of flow. T w o phenomena are competing with each other and are responsible for the main characteristics of the heat front: first, chemical transformation accompanied by the release of heat leads to a temperature increase in the catalyst bed. Second, the flow of the cool inlet reaction mixture through the previously heated catalyst bed, together with the interface heat exchange, extracts heat from the bed in the direction of flow. The transient temperature and concentration fields which are thus created possess a number of characteristics which are of great practical importance. The characteristic time of the various processes occurring in the reactor may differ from one another. Lee and Bailey (Leeand Bailey, 1974) showed that there exist three ranges of forced oscillations, according to the relative reoass-5805/92/2631-1583$03.00/0
lations between the cycling period t, and the time constant of the different processes t,. For t, > t, the process operates in a quasisteady state. When t, E t, the process is maintained under permanent transient conditions, while when t,
