Selectivity Yield, fractional

Reactor Performance Measures. There are four common measures of reactor performance fraction unreacted, conversion, yield, and selectivity. The fraction unreacted is the simplest and is usually found directly when solving the component balance equations. It is a t)/oo for a batch reaction and aout/ciin for a flow reactor. The conversion is just 1 minus the fraction unreacted. The terms conversion and fraction unreacted refer to a specific reactant. It is usually the stoichiometrically limiting reactant. See Equation (1.26) for the first-order case. 

Triblock copolymers of the ABA type may be analyzed similar to the analysis of diblock copolymers. The two possible cases for this type of investigation are (a) the analysis with respect to the inner block B using the critical conditions of the outer blocks A and A, and (b) the analysis of the outer blocks A and A using the critical conditions of the inner block B. It is particularly useful to carry out experiments at the critical point of A and A. The separation then occurs with respect to the chain length of B, yielding fractions that are monodisperse with respect to B and polydisperse with respect to A and A. These fractions can be analyzed selectively with respect to the outer blocks A and A in separate experiments (Entelis et al., 1986 Adrian et al., 1998 Pasch and Augenstein, 1993, 1994, 2002 Pasch, 1997, 2000, 2004 Kilz and Pasch, 2000). 

If both reactions are first order (a = j8 = 1), then micromixing is irrelevant yield, selectivity, and fractional conversion depend solely on the RTD. If, however, either a or /3 is not equal to 1, then the degree of micromixing can have a significant impact upon performance, as illustrated in the following example. 

A possible separation protocol for a complex polymer mixture is presented in Fig. 4. The sample under investigation comprises molecules of different chemical compositions (different colors) and different sizes. In a first separation step this mixture is separated according to composition yielding fractions which are chemically homogeneous. These fractions are transferred to a size-selective separation method and analyzed with respect to molar mass. As a result of this two-... 

Methanol appears to be the most suitable relatively inert solvent for the extraction of dried plant material either in the cold or at higher tent eratu-re. After evaporation of the solvent, the residue is extracted with dilute add (e.g. 1 to 5% HCl or H2SO4) and filtered. The insoluble material is the best source of non-basic alkaloids (dihydro-, 0x0-, and norderivatives of QBA) [61,65]. The acidic filtrate is made alkaline with NaaCOs or NH3 and extracted repeatedly with a non-polar solvent. The most advantageous choice seems to be diethyl ether because its high selectivity yields a crude alkaloid fraction of relatively good purity (up to 95%). Other solvents, especially chloroform, dissolve much more of the tar material so that the crude fi action may contain up to 90% of non-alkaloidal matter. 

To bypass these potential problems, we manually browsed a small subset of the literature for papers that actually use computational tools. Rather than read through all the papers in the selected journals, a random subset of papers was examined to see which used computational chemistry techniques. To test the accuracy of this approach, the results obtained from one-quarter of the total number of issues in a given year was compared with the results found by browsing all issues in that volume. The test case was the 1994 volume of Journal of Organic Chemistry. The random selection yielded 14%, whereas inspection of all 1376 papers also yielded 14%. Thus browsing a fraction of the issues of a journal should suffice for our purposes. At least one-fourth of the total number of issues of the journals listed in Table 1 were read, but one-third to one-half were evaluated for the smaller journals. 

The yield of each of these fractions will depend on their retention volume which in turn wiil depend on the adsorbent selected and the eluting force of the solvents. 

The reaction of chlorine gas with a mixture of ore and carbon at 500—1000°C yields volatile chlorides of niobium and other metals. These can be separated by fractional condensation (21—23). This method, used on columbites, is less suited to the chlorination of pyrochlore because of the formation of nonvolatile alkaU and alkaline-earth chlorides which remain in the reaction 2one as a residue. The chlorination of ferroniobium, however, is used commercially. The product mixture of niobium pentachloride, iron chlorides, and chlorides of other impurities is passed through a heated column of sodium chloride pellets at 400°C to remove iron and aluminum by formation of a low melting eutectic compound which drains from the bottom of the column. The niobium pentachloride passes through the column and is selectively condensed the more volatile chlorides pass through the condenser in the off-gas. The niobium pentachloride then can be processed further. 

Oxidation of cumene to cumene hydroperoxide is usually achieved in three to four oxidizers in series, where the fractional conversion is about the same for each reactor. Fresh cumene and recycled cumene are fed to the first reactor. Air is bubbled in at the bottom of the reactor and leaves at the top of each reactor. The oxidizers are operated at low to moderate pressure. Due to the exothermic nature of the oxidation reaction, heat is generated and must be removed by external cooling. A portion of cumene reacts to form dimethylbenzyl alcohol and acetophenone. Methanol is formed in the acetophenone reaction and is further oxidized to formaldehyde and formic acid. A small amount of water is also formed by the various reactions. The selectivity of the oxidation reaction is a function of oxidation conditions temperature, conversion level, residence time, and oxygen partial pressure. Typical commercial yield of cumene hydroperoxide is about 95 mol % in the oxidizers. The reaction effluent is stripped off unreacted cumene which is then recycled as feedstock. Spent air from the oxidizers is treated to recover 99.99% of the cumene and other volatile organic compounds. 

The ease of oxidation varies considerably with the nature and number of ring substituents thus, although simple alkyl derivatives of pyrazine, quinoxaline and phenazine are easily oxidized by peracetic acid generated in situ from hydrogen peroxide and acetic acid, some difficulties are encountered. With unsymmetrical substrates there is inevitably the selectivity problem. Thus, methylpyrazine on oxidation with peracetic acid yields mixtures of the 1-and 4-oxides (42) and (43) (59YZ1275). In favourable circumstances, such product mixtures may be separated by fractional crystallization. Simple alkyl derivatives of quinoxalines are... 

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