Washout Effects in Pumped Tank Reactors

Studies of multiple steady states in continuous-flow stirred tank reactors (CSTR) typically depend on the systematic variation of reactor residence time or some other system constraint such as reactant concentration. The concentration of an intermediate species is usually monitored as a function of time at a particular residence time until a steady state is established; the flow rate is then changed to give a new residence time and the next steady state concentration is determined. At short reactor residence times, the reaction mixture composition approaches the composition of the combined reactant streams; a t long residence times, the composition approaches that corresponding to a batch reaction at thermodynamic equilibrium. For most chemical reactions, the steady-state concentration of a particular species is a continuous, smooth function of residence time between the extremes of zero and infinity. However, in a number of autocatalytic reactions,1-8 the branch of steady states asymptotic to the reactant stream composition and the branch asymptotic to the equilibrium composition do not connect in a continuous fashion, but instead overlap one another. Thus, when a system constraint such as residence time is varied, a hysteresis effect is observed as the system moves from one branch of steady states to the other and back again. The system may exist in either of two steady states in the overlap region, Le., the system is bistable. The familiar pattern of steady-state multiplicity is dramatically altered when a minor modification is made in pumping the CSTR. A simple model system recently proposed by Gray and Scott,9 which combines third-order autocatalysis with a first-order decay of the autocatalytic species, exhibits a second hysteresis region and may even give rise to an isolated branch of steady states. We recognized that a first-order decay of all species in a tank reactor can be provided by supplying a constant flux of solvent to the CSTR in addition to the flux of reagents.lo In addition, the bistable iodate-arsenous acid reaction exhibits third-order autocatalysisll and, therefore, is an ideal chemical system for testing the model predictions. In this communication we report the first experimental characterization of a mushroom (two connected