Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

30 EXPERIMENTAL METHODS FOR SLOW REACTIONS3.2.3 Comparison of flow and static systemsIn a static system, a finite time is required for the reactant or reactants to fillthe RV and acquire the RV temperature. This renders initial pressure determination(by graphical extrapolation) and initial rate measurements somewhat suspect.(Heating may be assisted by having a heated dosing or mixing vessel.) In practice,filling and heating take 5-15 sec, depending on the conditions, and have to be verymuch faster than the rate of the reaction studied. This limits the study of thedecomposition of dtBP, for example, to a maximum temperature of 160-170" C.Leads to and from the RV and pressure-measuring devices contribute to the productionof a "dead space". Ideally, this dead space should amount to no morethan 4 % of the RV volume. Corrections for dead space have to be made withpressure measurements and subsequent kinetic expressions' 03. However, for lowextents of reaction no (or very simple) corrections are necessary. For adiabaticreactions in large diameter RV's, errors in rate coefficient values may be significant',due to the production of thermal gradients. Very often, for a partly heterogeneousreaction, it is difficult to isolate or to determine the rate of the homogeneous process2.Flow systems, with low contact times, have the advantage that higher temperaturesmay be used, thus minimising or obviating heterogeneous processes. Also,percentage conversion may be kept low, while still retaining precision in determiningthe extent of reaction, since a run can last until sufficient product has accumulated,thus minimising any further reactions of the initial products. The inherenterrors associated with rate coefficients determined using conventional flow system~''~have led to relatively little use of this technique in spite of its advantages.However, the use of a stirred flow RV obviates most of these difficultie~~~ andtogether with a static system allows a reaction to be studied over a wide pressureand large temperature range. Two difficulties remain. In spite of heat exchangethe heat capacity effect of the gases entering the RV lowers the temperature of theRV centre relative to the walls9', and instantaneous cooling still does not occurupon leaving the reaction zone. Thermal gradients will also still be present dueto the endothermicity or exothermicity of a reaction. Low pressures should beused to minimise the heat capacity effect. However, the use of low pressures maylead to pressure-dependent rate coefficients"5 and therefore make bond dissociationenergies, determined for example by the toluene carrier gas technique'06,suspect. Also, side reactions may not be insignificant'". Conversely, the flowsystem is ideal for the study of pressure-dependent decompositions or isomerisationsof free radicals, whose rate coefficients are close to their high pressure limitsat normal pressures with static systems, At the higher temperatures used in theflow system, the transition pressure from second to first order kinetics for a unimolecularreaction will also occur at a higher pressure'.t The transition pressure varies as the (s-3)th power of the absolute temperature, where sis the number of oscillators contributing to the decomposition (see ref. 108).