Time-averaged conversion measurements

Although time-averaged conversion measurements provide valuable information, a detailed analysis of the dynamic behavior of three-way catalysts requires direct observation of catalyst responses to rapid changes in exhaust composition. 

CO concentration at the outlet of each zone was continuously measured using a CO analyzer (Shimadzu CGT-7000). To evaluate the performance of the reactors, the conversion of CO for the PBR (Xco) with 4g of catalyst and the time-average conversion of CO for the SCMBR (Tea) with 2g of catalyst in each zone were calculated and compared. It should be noted that the CO concentration wave used for Eq. (1) was obtained whrai the system is at cyclic steady state (after 30 min of operation). 

Catalytic reactions were carried out in an isothermal plug flow reactor at 673K. Products were collected during the run and the average conversion measured. Reaction times varied between 1 and 30 minutes. 99.45% pure 2M obtained froia Aldrich was used without further purification. The principal impurity was 3-Methylpentane (0.55%). Experimental procedures and analytical techniques were outlined elsewhere (7 8). 

This procedure can also be applied to "cycled-A/F" experiments in which only time-averaged concentration measurements are recorded. Note that one must first average the inlet concentrations over the A/F cycle and then average the appropriate steady-state outlet concentrations over the A/F cycle before calculating an average "instantaneous response" conversion. One can not average the steady-state conversion levels themselves in order to get an average conversion. 

Figure 5. Time-averaged CO and NO conversions measured using the laboratory reactor system shown in Figure 4. A fresh, pelleted Pt/Rh/AltOs catalyst was operated at a middle-bed temperature of 820 K and a space velocity of 52,000 h 1 (STP). The feedstreams simulated exhaust that would be obtained with various engine air-fuel ratios (A/F) but did not contain SOs. The feedstream compositions were cycled at 0.25 and 1 Hz at an amplitude of 0.25 A/F about the mean A/F. For the curves labeled Steady-State, conversions were measured with feedstreams at the mean A/F values (8).

As already indicated Eq. (39) (Eq. (40)) gives the rate of ideal time and frequency resolved emission. If compared with experimental data gained by single photon counting, F(iv t) has to undergo a time averaging with the respective apparatus function which determines the possible time resolution of the measurement (for up conversion techniques see [44]). 

The results Illustrated by Figures 3 and 4 resemble those obtained in the Berty recycle reactor under similar conditions. The space-mean, time average rates for the fixed-bed reactor were only about 50% of those measured in the Berty reactor, because, of course the former reactor achieved conversions high enough for the back reaction to become important. The significance of these observations is that 1) CSTR and differential reactors, widely used for laboratory studies, seem to reflect performance improvements obtainable with fixed-bed, integral reactor which resemble commercial units, and 2) improvement from periodic operation are still observed even tfien reverse reactions become important. 

Consider the choice of reactor size r when the reactor is subject to catalyst deactivation. A measure of the performance of the reactor is the average conversion per unit volume of reactor in a given time period or some reference time period tr. If tf is defined as the duration of the reaction phase of the cycle, and b as the time required for catalyst regeneration, the number of cycles is ... 

Since we did not measure the conversion during the experiment, we computed the equilibrium vapor pressure at the average solution temperature. We believe that, for safety design, the equilibrium vapor pressure is an adequate estimate of the styrene vapor pressure. For example, even at a 50% conversion, the difference is only 10 at the experimental temperatures. Figures 6, 7 and 8 compared the observed pressures with the computed total pressures. The latter were based on the equilibrium vapor pressure. As expected, there were increasing variations in Tests 1, 2 and 3 respectively because of their higher initial conversions. From these figures we can verify that our pressure and temperature measurements were in phase with respect to time. 

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