Stoichiometric limit

Tcoi and Trii are both important in the understanding of relative merits of candidate cooling systems, and we shall later emphasise the difference between Tcoi and Tn,. Without improvements in materials and/or heat transfer, it is doubtful whether much higher values can be achieved in practice as a result, a practical limit on plant efficiency may be near, before the stoichiometric limit is reached. Below we refer to Tcoi as T, the maximum... 

Fig. 5.6 shows the results of a set of computer calculations for the [CBTJics plant in the form of (arbitrary) overall efficiency (tjq) against pressure ratio (r) with the combustion temperature T ox as a parameter. Fig. 5.7 shows tjq plotted against T x with r as a parameter and Fig. 5.8 shows a contour plot of tjq against T ox and r. There is a flat efficiency plateau around T ox 1750°C, less than the maximum value used in these calculations, which approaches the stoichiometric limit. 

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. 

The variable / depends on the particular species chosen as a reference substance. In general, the initial mole numbers of the reactants do not constitute simple stoichiometric ratios, and the number of moles of product that may be formed is limited by the amount of one of the reactants present in the system. If the extent of reaction is not limited by thermodynamic equilibrium constraints, this limiting reagent is the one that determines the maximum possible value of the extent of reaction ( max). We should refer our fractional conversions to this stoichiometrically limiting reactant if / is to lie between zero and unity. Consequently, the treatment used in subsequent chapters will define fractional conversions in terms of the limiting reactant. 

It makes no difference in this example which endgroup is stoichiometrically limiting. The polymer is sometimes called a terpolymer because three monomers are involved, but the term copolymer is used inclusively for any polymer formed from more than one monomer. In the absence of BZB, AMA and BNB will polymerize to form a strictly alternating copolymer. When BZB is added, the polymer still alternates with respect to the M mers but is random with respect to the N and Z mers. Of course, the enthusiastic chemist might add some AYA or even some AWB to the mix. They will all happily copolymerize, albeit at different rates. 

Substance that stoichiometrically limits the amount of product(s) that can be formed. 

If the concentrations of the stoichiometrically-limiting reactant in the two phases are in equilibrium and if the chemical potential is the driving force, then, from thermodynamics, it is clear that the reaction rate is unaffected by the nature of the phase with which the solid is in contact, provided that no mass- and heat-transfer gradients exist and no blockage of the catalyst sites by the impurities occurs. However, the competitive adsorption of impurities in the liquid, even if these are inert to reaction, can markedly affect catalytic behavior. 

Note that we solved this problem by first performing a stoichiometric (limiting reactant) calculation and then an equilibrium calculation. A similar strategy works if a strong base such as OH is added instead of a strong acid. The base reacts with formic acid to produce formate ions. Adding 0.10 mol of OH to the HCOOH/HCOO buffer of Example 15.7 increases the pH only to 3.58. In the absence of the buffer system, the same base would raise the pH to 13.00. 

Thirdly thereis the Freundlich equilibrium applicable to the adsorption of direct and vat dyes by cellulosic fibres. In this case the attachment is not at specific sites so that there is no stoichiometric limiting factor. If attachment is brought about by hydrogen bonds and physical forces the limitation is the available surface within the pores. In this case, therefore, adsorption is rapid at first because the sites are easily accessible but becomes slower as the dye molecules have to seek out the more remote points of attachment. The [0]//[7)]s curve, therefore, is not a straight line nor does it reach a point at which it becomes parallel with the horizontal axis, as shown in Fig. 12.16. 

Apply Equation 1.53 to calculate the mean residence time needed to achieve 90% conversion in a CSTR for (a) a first-order reaction and (h) a second-order reaction of the type A -I- B - products. The rate constant for a first-order reaction has units of reciprocal time. For the current example, assume k = 0.1 s. The rate constant for a second-order reaction has units of reciprocal time and reciprocal concentration. It is common practice to multiply a second-order rate constant by the initial or inlet concentration of the stoichiometrically limiting coefficient. This gives a rate constant with units of reciprocal time. For the second-order reaction suppose ajnk = 0.1 s . ... 

Table 1.2 summarizes the design equations for elementary reactions in ideal reactors. Note that component A is the only component or else is the stoichiometrically limiting component. Thus a = alao for batch reactions and a = afor flow reactors and Ya = a in both cases. For the case of a second-order reaction with two reactants, the stoichiometric ratio is also needed ... 

Determine the initial half-life for an elementary, second-order reaction for which ao < bo. Note that the half-life should be based on the stoichiometrically limiting component, A. 

In tempcrature-swing regeneration, there ate two steps necessary to accomplish desorption ( ) heat must be added to the ndsorbate and ndsorbent to raise them to desorption conditions and to provide the endothermic heat of desorption (2) the desorbed species must be transferred sway from ihe adsorbent. When the former is the limiting step, ii is termed heating- (or stoichiometric-) limited regeneration,3 The latter is referred to as stripping- (or equilibrium-) limited regeneration. 

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