REACTORS, KINETICS, AND CATALYSIS Diagnostic NonlinearAnalysisof Fischer-Tropsch Synthesis in Stirred-Tank Slurry Reactors

Multiplicity of steady states was experimentally obser®ed, somewhat sporadically, for ( ) Fischer Tropsch FT synthesis in stirred tank slurry reactors in the reactor originally ( ) at the wax-producing normal steady state suddenly jumping to a high-temperature ( ) methane-producing steady state for some time and e®entually returning to the normal steady state. A diagnostic nonlinear analysis showed a plausible interpretation of this nonlinear beha®ior under two settings: 1. assuming that the controlled system is stable and moti®ated by simplicity, as it ob®iates the inclusion of an energy balance for the cooler, thus reducing the dimension of the problem; 2. examining the more realistic problem by discarding the foregoing assumption, thus ha®ing to increase the analysis ( ) le®el. The first approach showed that the Stanton number for heat transfer St and H ( ) the Damkohler number Da could be two key process parameters accounting for the ̈ obser®ed multiplicity characteristics of FT synthesis. The decrease in St , attributable to H deteriorating performance of the cooler, is the likely cause for the sudden jump of the reactor from the normal to high-temperature steady state. The e®entual reco®ery of the original steady state is, spurred by a decrease in Da, possibly attributable to catalyst deacti®ation. Depending on the initial conditions andror startup history from the initial to the target conditions, the multiplicity beha®iors of FT synthesis may or may not be obser®ed, which is why the obser®ation of multiplicity is sporadic. The second approach incorporating the cooler’s energy balance increases the region of multiplicity o®er that with the first. Simple proportional control law is applied to the system to examine the effect of the control parameter settings on reactor beha®ior. Except for this feature, the findings from the two analysis modes are in harmony.