Equilibrium yield

Although the left to right reaction is exothermic, hence giving a better equilibrium yield of sulphur trioxide at low temperatures, the reaction is carried out industrially at about 670-720 K. Furthermore, a better yield would be obtained at high pressure, but extra cost of plant does not apparently justify this. Thus the conditions are based on economic rather than theoretical grounds (cf Haber process). 

These multicomponent calculations are now computerized, and complicated systems, such as tire Si-C-H-Cl quaternaty, may be solved by the use of commercially available software, e. g. the IVTAN database. The solution to this multicomponent system which is obtained by this means is somewhat subjective, since, at the time of writing for example, data are available for 72 gaseous species in the quaternary system Si-C-H-Cl. Choosing 19 of the most probable of tlrese, and using tire IVTAN software to solve this multicomponent equilibrium, yields the following results for tire most probable species (see Table 3.2). 

The reaction is reversible and strongly exothermic. The equilibrium yield of CH3OH decreases as the temperature increases. Hence, a low temperature and increased pressure will be kept. 

In fad the aconitase enzyme in A. niger is active even when dtric add is accumulating. This aconitase, if allowed to come to equilibrium, yields 90% dtrate, 3% ris-aconitate ferrous ions 7% isocitrate. To lower the activity of the enzyme, ferrous ions (essential for... 

The enzyme rate equation with two dissociation relations at equilibrium yields ... 

Vapor-phase fugacity coefficients are needed not only in high-pressure phase equilibria, but are also of interest in high-pressure chemical equilibria (D6, K7, S4). The equilibrium yield of a chemical reaction can sometimes be strongly influenced by vapor-phase nonideality, especially if reactants and products have small concentrations due to the presence in excess of a suitably chosen nonreactive gaseous solvent (S4). 

ILLUSTRATION 2.1 CALCULATION OF EQUILIBRIUM YIELD FOR A CHEMICAL REACTION... 

The temperature affects the equilibrium yield primarily through its influence on the equilib-... 

Equation 2.6.9 is an extremely useful relation for determining the effects of changes in process parameters on the equilibrium yield of a given product in a system in which only a single gas phase reaction is important. It may be rewritten as... 

The equilibrium constant Ka is independent of pressure for those cases where the standard states are taken as the pure components at 1 atm. This case is the one used as the basis for deriving equation 2.6.9. Tjie effect of pressure changes then appears in the terms KfjP and ps + t+ b c . The influence of pressure on KfjP is quite small. However, for cases where there is no change in the total number of gaseous moles during the reaction, this is the only term by which pressure changes affect the equilibrium yield. For these... 

The only term in equation 2.7.1 that is influenced by the addition of inert gases is nr Thus, for reactions in which there is no change in the total number of gaseous moles, addition of inerts has no effect on the equilibrium yield. For cases where there is a change, the effect produced by addition of inert gases is in the same direction as that which would be produced by a pressure decrease. 

The equilibrium constant and equilibrium yield are independent of whether or not a catalyst is... 

Other reactions will have somewhat different forms for the curve of Qq versus T. For example, in the case of a reversible exothermic reaction, the equilibrium yield decreases with increasing temperature. Since one cannot expect to exceed the equilibrium yield within a reactor, the fraction conversion obtained at high temperatures may be less than a subequilibrium value obtained at lower temperatures. Since the rate of energy release by reaction depends only on the fraction conversion attained and not on the position of equilibrium, the value of Qg will thus be lower at the higher temperature than it was at a lower temperature. Figure 10.2 indicates the general shape of a Qg versus T plot for a reversible exothermic reaction. For other reaction networks, different shaped plots of Qg versus T will exist. 

In order to minimize the required reactor volume for a given type of reactor and level of conversion, one must always operate with the reactor at a temperature where the rate is a maximum. For irreversible reactions the reaction rate always increases with increasing temperature, so the highest rate occurs at the highest permissible tepiperature. This temperature may be selected on the basis of constraints established by the materials of construction, phase changes, or side reactions that become important at high temperatures. For reversible reactions that are endothermic the same considerations apply, since both the reaction rate and the equilibrium yield increase with increasing temperature. 

The constraint of thermodynamic equilibrium for the butene dehydrogenation reaction is effectively removed since hydrogen is converted to water by oxidation. Equilibrium yields then approach 100% over the complete temperature and partial pressure range of interest. 

Nitrosobenzenes react with the carbonyl group of aldehydes to yield hydroxamic acids 73, according to reaction 20. Recently, the reactions between some X-substituted nitrosobenzenes (X = H, p-Me, p-C 1, m-Cl, p-Br) and formaldehyde were reported194 in order to investigate the mechanism of the hydroxamic acid formation. The mechanism reported in Scheme 9 involves a first equilibrium yielding the zwitterionic intermediate 74 which rearranges (by acid catalysis) into hydroxamic acid 75. The presence of a general acid catalysis, the substituent effect (p values of the Hammett equation equal —1.74),... 

For example, classic thermodynamic methods predict that the maximum equUi-brium yield of ammonia from nitrogen and hydrogen is obtained at low temperatures. Yet, under these optimum thermodynamic conditions, the rate of reaction is so slow that the process is not practical for industrial use. Thus, a smaller equilibrium yield at high temperature must be accepted to obtain a suitable reaction rate. However, although the thermodynamic calculations provide no assurance that an equUibrium yield will be obtained in a finite time, it was as a result of such calculations for the synthesis of ammonia that an intensive search was made for a catalyst that would allow equilibrium to be reached. 

If the value of for a reaction is calculated from the value of AG, we must have values of the 7, to substimte into Equation (16.27) or Equation (16.30) to obtain equilibrium yields in terms of m, or A,. The determination of these quantities from experimental data will be discussed in Chapters 17 and 19. 

Table 6.1 lists the stoichiometric yields of hydrogen and percentage yields by weight from steam reforming of some representative model compounds present in biomass pyrolysis oils, and also several biomass and related materials. The table also shows the equilibrium yield of H2, as a percentage of the stoichiometric yield, predicted by thermodynamic calculations at 750 °C and vdth a steam-to-carbon (S/C) ratio of 5 [32]. 

The reaction is exothermic, hence the highest equilibrium yield is obtained at low temperatures and high pressures. The catalyst functions by inducing the formation of a nitrogen complex with the catalyst surface this complex is far more readily hydrogenated to NH3 than is nitrogen with its triple bond (Somorjai and Salmeron, 1986). 

Chemistry The first topic to examine is the chemical reactions one wants to run and all the reactions that can occur. One immediately looks up the A and AGr,- to determine the heat release or absorption and the equilibrium composition. Equilibrium considerations also govern the temperature and pressure necessary for an acceptable equilibrium yield. This was the subject of Chapter 2. 

These reactors operate below room temperature to attain a high equilibrium yield, and refrigeration equipment is a major component of an alkylation process. 

We have shown that Cli can be oxidized directly to synthesis gas over a Pt or Rh monolith catalyst with surprisingly high selectivities for contact times of 10 to lO" s. Although these experiments do not give the equilibrium yields of H2 and CO reported in o Aer work at much longer residence times, they do show that both H2 and CO are primary products of the direct oxidation of CH4 over a noble metal catalyst. 

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