Yield and selectivity

It is important to distinguish between conversion and yield. Conversion is to do with reactants yield with products. 

Conversion is a measure of the fraction of the reagent that reacts. 

To optimize reactor design and minimize byproduct formation, the conversion of a particular reagent is often less than 100%. If more than one reactant is used, the reagent on which the conversion is based must be specified. 

This definition gives the total conversion of the particular reagent to all products. 

In the manufacture of vinyl chloride (VC) by the pyrolysis of dichloroethane (DCE), the reactor conversion is limited to 55% to reduce carbon formation, which fouls the reactor tubes. 

Frequently, several reactions proceed simultaneously, and consequently selectivity and yield in networks of parallel and series reactions with respect to a certain desired target component D are essential quantities. 

The integral selectivity of a desired component D, So, is related to the corresponding consumption of a reactant A. Considering molar fluxes of components at the inlet and outlet of a continuously operated reactor, Sd is defined as follows  

Of more practical relevance is often the related yield of component D, Yd, which is 

If we consider a desired reaction A + B — D and an undesired consecutive reaction D + B — U the following rate laws could be postulated  

The selectivity and the yield with respect to D depend strongly on the ratio kn/ku. Typical results obtained solving numerically the mass balance equations of the PFTR model (Eq. (8)) are shown in Fig. 12.3. 

The conversion of reactants to products and by-products dining a chemical process is easily determined from the mass balance. The selectivity of a reaction is defined as the proportion of useful product obtained from the amount feedstock converted. Thus it is possible to obtain almost 100% selectivity and still have an uneconomic process if the conversion is veiy low. Many processes operate at less than 100% conversion to hmit heat evolution, to achieve higher selectivity or because of thermodynamic hmitations. In these instances, the unconverted feed must be recycled and conversion per pass can still be relatively low, but economic. The key parameter in these instances is the yield of the reaction, which is the conversion multiphed by selectivity. 

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Nevertheless, chemists have been planning their reactions for more than a century now, and each day they run hundreds of thousands of reactions with high degrees of selectivity and yield. The secret to success lies in the fact that chemists can build on a vast body of experience accumulated over more than a hundred years of performing millions of chemical reactions under carefully controlled conditions. Series of experiments were analyzed for the essential features determining the course of a reaction, and models were built to order the observations into a conceptual framework that could be used to make predictions by analogy. Furthermore, careful experiments were planned to analyze the individual steps of a reaction so as to elucidate its mechanism. 

Adjustable solvent strength to tailor selectivities and yields. 

With electron-deficient aromatic substrates (Entries 4 and 5), high yields and selectivities were observed, but enantioselectivities were variable and solvent-de-pendent (compare Entry 6 with 7 and see Section 1.2.1.3 for further discussion). With a,P-unsaturated tosylhydrazone salts, selectivities and yields were lower. The scope of this process has been extensively mapped out, enabling the optimum disconnection for epoxidation to be chosen [10]. 

The only other esterification method which rivals the present procedure in convenience, mildness of conditions, selectivity, and yield js the preparation of methyl esters wdth diazomethane [Methane, diazo-] 10 Esterification with trialkyloxonium salts, however, allows... 

Figure 8.39. Transient effect of electrochemical O2 pumping to (a) and from (b) a Ag catalyst film on selectivity and yield to ethylene oxide.42 Current applied at t=0 Pc2H4=1-5 kPa, p02= 10 kPa T=400°C. Reprinted with permission from Academic Press.

Similar studies utilizing Au electrodes on YSZ showed again that the selectivity and yield of C2 hydrocarbons can be significantly affected by applying currents or potentials to the cell.40,41,53 The behaviour with Au appears to be qualitatively similar with that obtained with Ag electrodes although electrophilic behaviour is also reported.40,41... 

The potential energy curves (Fig. 1), the non-adiabatic coupling, transition dipole moments and other system parameters are same as those used in our previous work (18,19,23,27). The excited states 1 B(0 ) and 2 B( rio) are non-adiabatically coupled and their potential energy curves cross at R = 6.08 a.u. The ground 0 X( Eo) state is optically coupled to both the and the 2 R( nJ) states with the transition dipole moment /ioi = 0.25/xo2-The results to be presented are for the cw field e(t) = A Yll=o cos (w - u pfi)t described earlier. However, for IBr, we have shown (18) that similar selectivity and yield may be obtained using Gaussian pulses too. 

A packed-bed nonpermselective membrane reactor (PBNMR) is presented by Diakov et al. [31], who increased the operational stability in the partial oxidation of methanol by feeding oxygen directly and methanol through a macroporous stainless steel membrane to the PB. Al-Juaied et al. [32] used an inert membrane to distribute either oxygen or ethylene in the selective ethylene oxidation. By accounting for the proper kinetics of the reaction, the selectivity and yield of ethylene oxide could be enhanced over the fixed-bed reactor operation. 

Elow intensification is made with the use of apparatuses in which flow follows a perfect plug flow the internal parts of the reactor have to be designed accordingly. Indeed, dead zones, that is, reactant accumulation, must be avoided not only in order to have better selectivity and yield but also to avoid formation of hot spots, which would generate safety problems. 

As expected, heat exchanged per unit of volume in the Shimtec reactor is better than the one in batch reactors (15-200 times higher) and operation periods are much smaller than in a semibatch reactor. These characteristics allow the implementation of exo- or endothermic reactions at extreme operating temperatures or concentrations while reducing needs in purifying and separating processes and thus in raw materials. Indeed, since supply or removal of heat is enhanced, semibatch mode or dilutions become useless and therefore, there is an increase in selectivity and yield. 

Previously, we performed single- and multi-response optimization works in order to address optimal catalyst composition (%CaO and %MnO) and optimal operating conditions (temperature and CO2/CH4 feed ratio [3]. The maximum C2 selectivity and yield of 76.6% and 3.7%, respectively were achieved in multi-responses optimization over the 12.8% CaO-6.4% Mn0/Ce02 catalyst corresponding to the optimum reactor temperature being 1127 K and CO2/CH4 ratio being 2 [3]. The recent contribution on the catalyst technology of CO2 OCM was... 

The catalytic pyrolysis of R22 over metal fluoride catalysts was studied at 923K. The catalytic activities over the prepared catalysts were compared with those of a non-catalytic reaction and the changes of product distribution with time-on-stream (TOS) were investigated. The physical mixture catalysts showed the highest selectivity and yield for TFE. It was found that the specific patterns of selectivity with TOS are probably due to the modification of catalyst surface. Product profiles suggest that the secondary reaction of intermediate CF2 with HF leads to the formation of R23. 

It was eoncluded that the enhanced selectivity and yield for TFE over Cu-Mixed catalyst may be attributed to the surface modifications by the attack of HF produced during the pyrolysis of R22. The results suggest that R23 is formed by the secondary reaction between intermediate CF2 and HF. 

Catalysts were prepared with 0.5, 1.0, 2.0 and 5.0 wt% of iron loaded on activated carbon. Benzene hydroxylation with hydrogen peroxide as oxidant was carried out. The conversion of benzene, selectivity and yield of phenol for these catalysts are shown in Fig. 4. As the weight of loaded metal increased the benzene conversion increased by about 33% but the selectivity to phenol decreased. The yield of phenol that was obtained with S.OFe/AC was about 16%. 

Obtain meaningful data on the catalyst Usually for kinetic purposes is it the turnover frequency per active site (TOP) that of interest. But other parameters such as selectivity and yield are also of great importance for judging the potential of the catalyst. Instead of expressing the activity as a turnover frequency, it can also be given in terms of ... 

Figure 4 shows the evolution of the initial conversion versus temperature at a space velocity of 0.03 h l. The equilibrium conversion of isobutane to isobutene is 100% in our conditions. An increase of the conversion with temperature up to 773-823 K is observed. When metals were added, we also noted a large increase in isobutane dehydrogenation. Table 2 gives initial isobutane conversions, isobutene selectivities and yields of the reaction at 823 K for the three tested samples. 

The effect of CH4 conversion on the total C2, C2H4, C2He hydrocarbon selectivity and yield is shown in detail on Figure 4 for the case of I=5mA. 

Figure 4. Effect of methane conversion for 1=5 mA on ethylene, ethane and total Cg hydrocarbon selectivity and yield. Lines from kinetic model discussed below. Solid lines CgH j and C2Hg Dashed lines C2...

The ethylene selectivity (Fig. 5) and thus the ethylene yield depend strongly on the adsorbent mass (Fig. 5). For fixed catalyst mass, oxygen supply I/2F and methane conversion there is an optimal amount of adsorbent for maximizing ethylene selectivity and yield (Fig. 5). Excessive amounts of adsorbent cause quantitative trapping of ethane and thus a decrease in ethylene yield according to the above reaction network. This shows the important synergy between the catalytic and adsorbent units which significantly affects the product distribution and yield. 

Figure 6. Continuous flow steady-state operation (a) Effect of oxygen stream flowrate on C2 selectivity and yield (b) corresponding effect of methane conversion on the selectivity and yield of C2H4 and C2He Catalyst Sr/LaaOa T=750°C recirculation flowrate 200 cm3/min.

Figure 6a shows the effect of F02 on the C2 selectivity and yield. The C2 yield is up to 53%. Figure 6b refers to the same experiments and shows the corresponding elBfect of CH4 conversion on the selectivity and yield of ethylene and ethane. The ethylene yield is up to 50% (65% ethylene selectivity at 76% methane conversion). To the best of our knowledge this is the maximum ethylene yield obtained for the OCM reaction under continuous-flow steady-state conditions. 

TS-1-catalyzed processes are advantageous from the environmental point of view as the oxidant is aqueous hydrogen peroxide, which turns into water, and the reactions are operated in liquid phase under mild conditions, showing very high selectivity and yields, thus reducing problems and the costs of by-product treatments. Confinement of the metal species in the well-defined MFl pore system endows TS-1 with shape selectivity properties analogous to enzymes. For these features the application of the terms mineral enzyme or zeozyme to TS-1 is appropriate [42]. 

The two reactants Rj and R2 form an intermediate Ij which reacts to a second intermediate I2 via two reaction pathways, one needing the presence of R2 [48, 108]. A number of additional reactions decrease the selectivity and yield. First, the reactants Rj and R2 are labile to moisture. Hydrolysis results in the formation of the side product reactants S j and 82- By more complex pathways, three other known... 

Figure 4.93 Comparison of selectivity and yield for the synthesis of 4-methoxybenzaldehyde depending on reaction temperature and amount of conducting salt [69].

Figure 5.16 Influence of the fluorine-to-toluene equivalents (0.1 M toluene in acetonitrile) on conversion, selectivity and yield [13].

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