Conversion extent

Figure 3. Extent of conversion and composition of liquid products in cumene cracking over NiCaY catalyst (2.4 wt % nickel), depending on temperature (volume rate 1.10 h l, ratio of hydrogen. cumene 10) 1, total conversion extent 2, benzene 3,...

Physical properties (porosity, permeability, thermal conductivity, specific heat and fluid viscosity) depend on temperature and conversion extent. 

The nitrogen fixation reaction to form ammonia, considered in Illustration 13.1-4, is run at higher temperatures in commercial reactors to take advantage of the faster reaction rates. However, since the reaction is exothermic, at a fixed pressure the equilibrium conversion (extent of reaction) decreases with increasing temperature. To overcome this, commercial reactors are operated at high pressures. The operating range of commercial reactors is pressures of about 350 bar and temperatures from 350°C to 600°C. 

In Sect. 1.7, the extent of conversion (extent of reaction) was introduced as a measure of the progress of a reaction. The amounts rit and therefore the concentrations c, = riilV oi the substances involved change with increasing Starting with the initial values n, o and c, o, respectively, we obtain ... 

In an enantioselective reaction, such as an asymmetric hydrogenation of a ketone, the enantiomeric excess of the chiral product (eCp oj) is generally constant with conversion. This is not true in a KR, where enantiomeric excess of the recovered starting material (ee jj ) and enantiomeric excess of the product (eCp j) change with conversion extent. These points are discussed on a quantitative basis in the next sections. 

The reaction time t was classically used as the parameter to discuss the course of a KR [6-8, 19]. Conversion extent C gives equations that are easier to handle, especially if taken with values lying between 0 (initial state) and 1 (full transformation) [20, 21]. Conversion C is denoted by C= 1-([R]- -[S])/Xq, where Xq is the initial concentration of the racemic mixture. Enantiomeric excess of the remaining starting material (ee jj ) is defined as follows. 

There is the possibility to combine several KR reactions and to approach the theoretical maximum values of 50% recovery and 100% ee for one of the enantiomers. This can be achieved after isolation of the components of a first KR [30]. Consecutive KRs will also enhance the enantiomeric excess [31]. The amount of recovered material will decrease. KR has been used to increase the enantiomeric excess of an aheady enantioenriched material (obtained by KR or asymmetric synthesis). The conversion extent needed for going from 90% to 95% ee, for example, for a given s value, has been calculated for pseudo-first-order reactions [23]. Enantioconvergent reactions (see below. Section 2.5) is a rare but convenient way to optimize a KR from a racemic mixture where the enantiomers are susceptible to interconversion. 

Thermoset Cure Chracterization by DMA. On the basis of the information on crosslinking and Tg noted above, DMA can be nsed for the cure characterization of Thermosets (qv). There are two aspects of this. First, as the chemical conversion (extent of cure) and the degree of crosshnking advance, Tg increases. Thus the Tg measured by DMA correlates with the cure state of the material. In cases where Tg is not overly broad and a well-defined maximum in the loss modulus is evident, the Tg value obtained is related to crosslink density. Second,... 

Modem calorimeters permit relatively rapid and truly precise measurement of heat exchanges in a wide variety of reactions. Since heat evolution is proportional to the conversion (extent of reaction) in a chemical, physical, or biological reaction, calorimetric measurement constitutes one method for quantitative evaluation of the reaction itself. Measurement is possible not only of the total heat (and therefore the total conversion) of a reaction but also of the course of the reaction... 

To begin, let s ask the question What is the minimum number of variables that are required to completely describe the composition of a system in which R independent reactions take place In Chapter 4, we saw that a single variable, e.g., fractional conversion, extent of reaction, or the concentration of one compound, is sufficient to describe the composition of a system, when only one reaction takes place. How many variables are required if R reactions occur ... 

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