Equilibrium yield pressure

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). 

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. 

Figure 9-3 shows this schematically. If the partial pressure of the vapor is less than the equilibrium value (as in Figure 9-3A), the rate of evaporation exceeds the rate of condensation until the partial pressure of the vapor equals the equilibrium vapor pressure. If we inject an excess of vapor into the bottle (as in Figure 9-3Q, condensation will proceed faster than evaporation until the excess of vapor has condensed. The equilibrium vapor pressure corresponds to that concentration of water vapor at which condensation and evaporation occur at exactly the same rate (as in Figure 9-3B). At equilibrium, microscopic processes continue but in a balance that yields no macroscopic changes.

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). 

Equilibrium vapor pressures were measured in this study by means of a mass spectrometer/target collection apparatus. Analysis of the temperature dependence of the pressure of each intermetallic yielded heats and entropies of sublimation. Combination of these measured values with corresponding parameters for sublimation of elemental Pu enabled calculation of thermodynamic properties of formation of each condensed phase. Previ ly reported results on the subornation of the PuRu phase and the Pu-Pt and Pu-Ru systems are correlated with current research on the PuOs and Pulr compounds. Thermodynamic properties determined for these Pu-intermetallics are compared to analogous parameters of other actinide compounds in order to establish bonding trends and to test theoretical predictions. 

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 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. 

Enantiomeric recognition was clearly displayed in films spread from solution and films in equilibrium with their crystals, and was sharply dependent on the acidity of the subphase. Protonation of the amide group appeared to be necessary for spreading to stable monolayers. For example, the crystals of the racemate deposited on a 10n H2S04 solution at 25°C spread quickly to yield a film with an ESP of 7.7 dyn cm"1, while the single enantiomers spread only to a surface pressure of 3.9 dyn cm-1 (Table 1). Similar effects are observed at 15 and 35°C. The effect of stereochemistry on equilibrium spreading is even more pronounced at lower subphase acidities. On 6n sulfuric acid, the racemate spread to an equilibrium surface pressure of 4.9 dyn cm-1, while the enantiomeric systems spread to less than 1 dyn cm-1. 

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. 

It is implicit in reaction 9.4 that the equilibrium yield of ammonia is favored by high pressures and low temperatures (Table 9.1). However, compromises must be made, as the capital cost of high pressure equipment is high and the rate of reaction at low temperatures is slow, even when a catalyst is used. In practice, Haber plants are usually operated at 80 to 350 bars and at 400 to 540 °C, and several passes are made through the converter. The catalyst (Section 6.2) is typically finely divided iron (supplied as magnetite, Fe304 which is reduced by the H2) with a KOH promoter on a support of refractory metallic oxide. The upper temperature limit is set by the tendency of the catalyst to sinter above 540 °C. To increase the yield, the gases may be cooled as they approach equilibrium. 

The coordinates of thermodynamics do not include time, ie, thermodynamics does not predict rates at which processes take place. It is concerned with equilibrium states and with the effects of temperature, pressure, and composition changes on such states. For example, the equilibrium yield of a chemical reaction can be calculated for given T and P, but not the time required to approach the equilibrium state. It is however true that the rate at which a system approaches equilibrium depends direcdy on its displacement from equilibrium. One can therefore imagine a limiting kind of process that occurs at an infinitesimal rate by virtue of never being displaced more than differentially from its equilibrium state. Such a process may be reversed in direction at any time by an infinitesimal change in external conditions, and is therefore said to be reversible. A system undeigoing a reversible process traverses equilibrium states characterized by the thermodynamic coordinates. 

The mass-action equations have been written in the same form as those given by Marynowski et al. (6) so that the equilibrium constants can be used directly. (Should more accurate data become available, the equilibrium yields calculated here will require revision.) The fourth equation, which applies to the heterogeneous equilibrium between carbon and nitrogen, is included for completeness but is unnecessary for the general solution. It can be shown that when the total pressure of the system is F, the partial pressure of cyanogen radicals is given by the equation ... 

Many industrially important chemical processes are high pressure processes. Examples are the production of ammonia and the production of low density polyethylene. Basically, the pressure affects both the equilibrium yield of a chemical reaction and the reaction rate. Here, only the influence on the equilibrium yield is discussed. 

The results of the calculations show that with increasing pressure the equilibrium yield of ammonia is increasing and that the non-ideality of the gas mixtures has in this case a positive effect on the equilibrium conversion. 

Equilibrium Constant. At the pressures used in commercial production of ethanol (6.1-7.1 MPa or 60-70 atm), alcohol yield per pass is significandy limited by equilibrium considerations. This fact has focused attention on determination of equilibrium constants and equilibrium yields (122—124). The results of these determinations are as follows ... 

Thus diffusion separation is effective in increasing low yield concentrations of product but as the equilibrium yield rises the efficiency of enrichment is reduced. One further drawback arises if the hydrogen sulphide/sulphur stream is recycled. Eventually, if the product sulphur is not also removed, the partial pressure of the sulphur will be so great as to depress the forward reaction even under upset equilibrium conditions. 

Many industrial reactions are not carried to equilibrium. In this circumstance the reactor design is based primarily on reaction rate. However, the choice of operating conditions may still be determined by equilibrium considerations as already illustrated with respect to the oxidation of sulfur dioxide. In addition, the equilibrium conversion of a reaction provides a goal by which to measure improvements in the process. Similarly, it may determine whether or not an experimental investigation of a new process is worthwhile. For example, if the thermodynamic analysis indicates that a yield of only 20 percent is possible at equilibrium and a 50 percent yield is necessary for the process to be economically attractive, there is no purpose to an experimental study. On the other hand, if the equilibrium yield is 80 percent, an experimental program to determine the reaction rate for various conditions of operation (catalyst, temperature, pressure, etc.) may be warranted. 

Of the four possible reactant species, attack by the hydroxyl radical yielded the most nearly constant value of p, but the constancy is unimpressive. It is true that the use of equilibrium partial pressures will be in error, but the disequilibrium parameter (y) of Bulewicz, James, and Sugden (4) depends more on temperature than on composition and should not affect the picture from an isothermal set of flames. 

In cases in which the reaction quickly proceeds to equilibrium, the yields are easily estimated as the equilibrium yields. Under these circumstances, the only possibilities for process optimization are to change the temperature, pressure, or feed composition, so as to obtain a different equilibrium mixture. The calculation of reaction equilibrium is easily carried out using commercial process simulation programs. 

Isomerization of 1-hexene. Figure 6 shows simulated equilibrium yields and conversions of 1-hexene versus temperature at the specified pressure. Cracking products were not considered in our analysis. Increase in temperature is seen to cause a slight decrease in equilibrium conversion and to have little effect on the isomer selectivities. Simulated equilibrium conversion at 250 C and 7,250 psia is 97.2%. This value compares with the experimental value of 40% obtained by Tiltsher et al. (16) in a catalytic flow reactor. Clearly, there is room for improving the experimentally reported conversion. 

Fig. 8.8 The calculated effect of pressure and temperature upon the equilibrium yield of CCI4 from the dismutation of phosgene [ICI8,ICI9].

Table 17-1 shows the effects of increases in temperature and pressure on the equilibrium yield of NH3, starting with 1 3 mole ratios of N2 H2. decreases by more than ten orders of magnitude, from 3.6 X 10 at 25°C to only 1.4 X 10 at 758°C. This tells us that the reaction proceeds very far to the left at high temperatures. Casual examination of the data might suggest that the reaction should be run at lower temperatures, because a higher percentage of the N2 and H2 is converted into NH3. The reaction occurs so slowly, however, even in the presence of a catalyst, that it cannot be run economically at temperatures below about 450°C. 

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