Selectivity constants residence times

Figure 2.86 Isooctane steam reformer performance. At constant residence time the hydrogen selectivity is not affecteded by decreasing the S/C ratio while the isooctane conversion is lowered [135] (by courtesy of S. P. Fitzgerald).

The combination of low residence time and low partial pressure produces high selectivity to olefins at a constant feed conversion. In the 1960s, the residence time was 0.5 to 0.8 seconds, whereas in the late 1980s, residence time was typically 0.1 to 0.15 seconds. Typical pyrolysis heater characteristics are given in Table 4. Temperature, pressure, conversion, and residence time profiles across the reactor for naphtha cracking are illustrated in Figure 2. 

GP 2] [R 2] A residence time variation was performed imder constant gas composition, temperature and pressure, but with varying flow rate. On increasing the residence time from 0.5 to 8 s, reaction rates on OAOR-modified silver decreased notably from 9.5 10 mol s to about 1 10 mol s mT (5 vol.-% ethylene, 50 vol.-% oxygen, balance nitrogen 20 bar 0.5-8 s) [4]. The selectivity decreases from 43 to 21%. 

Possible solutions to overcome this problem are (1) decrease the residence time the decrease of conversion is more than compensated by an increase of selectivity (due to the lower extent of methacrylic acid combustion), and in overall the productivity increases (2) increase the total pressure, while simultaneously increasing both the oxygen and the isobutane partial pressure, as well as the total gas flow (so as to keep a constant contact time in the reactor). A higher pressure also implies smaller reactor volume, and hence lower investment costs. Under these circumstances, productivity as high as 6.4 mmol/h/gcat was reached, which is acceptable for industrial production. The additional heat required for the recirculation of unconverted isobutane and for increased pressure would be equalized by the higher heat generated by the reaction. 

Next, the residence time inside the catalyst bed was varied by altering the total reactant flow at a constant propene partial pressure of 2.1 bar. The selectivity for the desired hnear aldehyde was only influenced by temperature and not by residence time. Shorter residence times generally resulted in lower conversions, as expected. However, under non-differential conditions at higher conversions the observed TOFs decreased shghtly with longer residence times, due to lower mean levels of propene present in the reactor. 

In a variable-density reactor the residence time depends on the conversion (and on the selectivity in a multiple-reaction system). Also, in ary reactor involving gases, the density is also a function of reactor pressure and temperature, even if there is no change in number of moles in the reaction. Therefore, we frequently base reactor performance on the number of moles or mass of reactants processed per unit time, based on the molar or mass flow rates of the feed into the reactor. These feed variables can be kept constant as reactor parameters such as conversion, T, and P are varied. 

The best per pass yield to C2 + C3 products (aldehydes plus acids with two and three C atoms) with the said catalyst was obtained at a propene conversion of 61.3% (selectivity to acrolein 83.7%), at the reaction temperature of 355 °C, with the following feed composition C3H6/H20/N2 11.6 10.0 78.4 (mol.%), with a gas contact time of 2.4 s. A decrease in solids circulation rate, while keeping gas residence time constant, led to a considerable decrease in propene conversion, while selectivity to C2 + C3 oxygenated products was not much affected by circulation rate. With a less concentrated feed, the amount of solid to be circulated for a defined olefin conversion is lower, but productivity also becomes lower. Other catalysts based on Bi/Mo/O or on V/Mo/W/Cu/O [72c] afforded conversions >70% and selectivity >90% industrial... 

P 2] [R 18, modified] [C 2] To-date, the reaction has been carried out up until the residence-time module. The final hydration step [Figure 4.44, reaction (4)] has not taken place. Even so, the first results are very encouraging as shown in Figure 4.46. In order to evaluate the reaction conditions, the mole ratio of the two reactants, sulfur trioxide and toluene, was varied and the selectivity of the desired product (sulfonic acid) and of the by-products (sulfone and the anhydride mixture) was determined. Evidently, with increasing S03/toluene mole ratio, the selectivity of the undesired by-products decreases whereas the selectivity of sulfonic acid stays nearly constant. At a mole ratio of 13/100, the selectivity of sulfonic acid is approximately 80% whereas that of sulfone decreases to approximately 3% and that of the sulfonic acid anhydride to approximately 1.3%. 

A fixed bed reactor described by ASTM Method No. D3907 was employed for catalytic testing. A sour, imported heavy gas oil with properties described in Table II was used as the feedstock. Experiments were carried out at a reactor temperature of 800°K and catalyst residence time (9) of 30 seconds. Liquid and gaseous products were analyzed with gas chromatographs. Carbonaceous deposit on the catalyst was analyzed by Carbon Determinator WR-12 (Leco Corp., St. Joseph, MI). The Weight Hourly Space Velocity (WHSV) was varied at constant catalyst contact time to generate selectivity data of various products as a function of conversion. For certain experiments, conversion was also varied by varying the catalyst pretreatment conditions. 

An important advantage of the use of EOF to pump liquids in a micro-channel network is that the velocity over the microchannel cross section is constant, in contrast to pressure-driven (Poisseuille) flow, which exhibits a parabolic velocity profile. EOF-based microreactors therefore are nearly ideal plug-flow reactors, with corresponding narrow residence time distribution, which improves reaction selectivity. 

Fig. 12.3. Selectivity Sd and Yield Yd as a function of conversion Xa for the two consecutive reactions A + B —> D and D + B —> U calculated with the PFTR model (Eq. (8)) fixing the residence time and varying the feed composition in a wide range. The reaction rates were described with Eqs. (21) and (22)). Three different ratios of the rate constants of the desired and the undesired reaction were considered a) ko/ku = 10 b) ko/ku = 1 c) ko/ku = 0.1.

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