Partial pressures of reactants

Reactor diluents and solvents. As pointed out in Sec. 2.5, an inert diluent such as steam is sometimes needed in the reactor to lower the partial pressure of reactants in the vapor phase. Diluents are normally recycled. An example is shown in Fig. 4.5. The actual configuration used depends on the order of volatilities. 

For gas reactions where the gases are assumed to follow ideal behaviour this equation becomes AG° = RT]n Kp, where Kp is defined in terms of the partial pressures of reactants and products. Thus for the general reaction above,... 

In almost all modem plants, the ammonia is recovered by condensation and at modern synthesis pressures, ammonia is usually the source of refrigeration required. In order to maintain a high partial pressure of reactants, inerts entering with the make-up gas are normally removed using a purge stream. 

Example 12.4 illustrates a principle that you will find very useful in solving equilibrium problems throughout this (and later) chapters. As a system approaches equilibrium, changes in partial pressures of reactants and products—like changes in molar amounts—are related to one another through the coefficients of the balanced equation. 

Express the equilibrium partial pressures of all species in terms of a single unknown, x. To do this, apply the principle mentioned earlier The changes in partial pressures of reactants and products are related through tite coefficients of the balanced equation. To keep track of these values, make an equilibrium table, like the one illustrated in Example 12.4. 

Thus denoting by 0P the coverage of the promoter on the catalyst surface and by pj the partial pressures of reactants, j, of the catalytic reaction we can formulate mathematically the above definition as ... 

Notice that, as anticipated, the partial pressure of reactant has increased and the partial pressures of the products have decreased from their initial values (Fig. 9.12). 

The following plot shows how the partial pressures of reactant and products vary with time for the decomposition of compound A into compounds B and C. All three compounds are gases. Use this plot to do the following (a) Write a balanced chemical equation for the reaction, (h) Calculate the equilibrium constant for the reaction, (c) Calculate the value of Kc for the reaction at 25°C. 

We will list the elementary steps and decide which is rate-limiting and which are in quasi-equilibrium. For ammonia synthesis a consensus exists that the dissociation of N2 is the rate-limiting step, and we shall make this assumption here. With quasi-equilibrium steps the differential equation, together with equilibrium condition, leads to an expression for the coverage of species involved in terms of the partial pressures of reactants, equilibrium constants and the coverage of other intermediates. 

If all partial pressures except that of one reactant, say species A, are held constant and the partial pressure of species A is varied, tfye rate will go through a maximum. The rate increases initially with increasing partial pressure of reactant A as the fraction of sites occupied by species A increases. However, as this fraction increases, the fraction occupied by species B declines as B molecules are displaced by A molecules. Eventually one reaches a point where the decline ip the value of 0B more than offsets the increase in 0A and the product 6A6B goes through a maximum. 

A useful tool for dealing with reaction stoichiometry in chemical kinetics is a stoichiometric table. This is a spreadsheet device to account for changes in the amounts of species reacted for a basis amount of a closed system. It is also a systematic method of expressing the moles, or molar concentrations, or (in some cases) partial pressures of reactants and products, for a given reaction (or set of reactions) at any time or position, in terms of initial concentrations and fractional conversion. Its use is illustrated for a simple system in the following example. 

Increasing reactant gas utilization or decreasing inlet concentration results in decreased cell performance due to increased concentration polarization and Nernst losses. These effects are related to the partial pressures of reactant gases and are considered below. 

Equation 5 gives the concentration or partial pressure of reactant A as a function of the total pressure tt at time t, initial partial pressure of A, Pao, and initial total pressure of the system, ttq. 

If we choose a high value for the partial pressure of reactant P, p > 0.037 975, then the system does not cross the region of multiplicity as r is varied. The reaction rate varies monotonically with r. A similar situation occurs if the fixed partial pressure of P is too small, such that p < 0.020 133. However, with any value of p in the range 0.020 133 < p < 0.037975, there are multiple stationary-state reaction rates over some range or ranges of the partial pressure of R. 

Fig. 16. Effect of degree of crosslinking (% DVB) of standard (non-porous) ion exchanger on initial transesterification rate, r° (mol kg-1 h-1), of ethyl acetate with 1-propanol [436]. (1) Liquid phase at 52°C initial composition (mole%), 0.4 ethyl acetate, 0.4 1-propanol, 0.2 dioxan (solvent). (2) Vapour phase at 120°C partial pressure of reactants, 0.5 bar (ester—alcohol ratio 1 1).

If the adsorption of A is the rate determining step in the sequence of adsorption, surface reaction and desorption processes, then equation 3.71 will be the appropriate equation to use for expressing the overall chemical rate. To be of use, however, it is first necessary to express CA, Cv and Cs in terms of the partial pressures of reactants and products. To do this an approximation is made it is assumed that all processes except the adsorption of A are at equilibrium. Thus the processes involving B and P are in a state of pseudo-equilibrium. The surface concentration of B can therefore be expressed in terms of an equilibrium constant KB for the adsorption-desorption equilibrium of B ... 

Write the equilibrium equation by setting Kp equal to the equilibrium constant expression using partial pressures. Put the partial pressures of products in the numerator and the partial pressures of reactants in the denominator, with the pressure of each substance raised to the power of its coefficient in the balanced chemical equation. Then substitute the partial pressures into the equilibrium equation and solve for Kp. 

In actual galvanic cells, the concentrations and partial pressures of reactants and products seldom have standard-state values, and the values change as the... 

Total pressure/time data given use the integrated rate expression method. But first have to convert to partial pressure of reactant remaining. 

The total pressure has to be converted into partial pressure of reactant remaining at the various times. 

On the basis of the general formula (46), we can classify the dependences of the reaction rate on the three parameters partial pressure of reactants, temperature, and the total pressure. For such investigations, see Chap. 3, Sect. 3 of ref. 7. 

The reactor can operate with either a liquid-phase reaction or a gas-phase reaction. In both types, temperature is very important. With a gas-phase reaction, the operating pressure is also a critical design variable because the kinetic reaction rates in most gas-phase reactions depend on partial pressures of reactants and products. For example, in ammonia synthesis (N2 + 3H2 O 2NH3), the gas-phase reactor is operated at high pressure because of LeChatelier s principle, namely that reactions with a net decrease in moles should be mn at high pressure. The same principle leads to the conclusion that the steam-methane reforming reaction to form synthesis gas (CH4 + H20 O CO + 3 H2) should be conducted at low pressure. 

Thiele (17), Wheeler (12), and Weisz and Prater (1) have given ij vs. curves for some integral-order reaction kinetics. However, the kinetics of cumene cracking does not exhibit a simple constant order. Instead, the order is a function of partial pressure of reactants and products. To determine the jj vs. curve for this kinetics, the diffusion equation... 

A problem that must be carefully considered is the occurrence of side- and consecutive reactions. This is especially important for alkane activation, because severe reaction conditions are necessary to activate the C-H bonds. When reactions are fast, as in the HCN and acetylene syntheses, rapid quenching of the reaction products is possible. Another way of affecting selectivity is to limit the partial pressure of reactants, thus also reducing the partial pressure of the desired product. In this way in the maleic anhydride synthesis conversion is limited by diluting the gas and limiting the amount of oxygen available for the reaction. 

When AG = 0 the reaction is then at equilibrium and the concentrations (or partial pressures) of reactants and products are then those that appear in the equilibrium constant expression. 

It is very likely that the different adsorption modes as shown in Figure 1 are influenced differently by changes in temperature and partial pressure of reactants. The same holds for the influence of support induced changes in the electronic properties of the metal on the adsorption of the various intermediates. Instead of using the multiplicity, one should carefully analyze the observed selectivities and extract the contribution of the different adsorption modes in the exchange reaction. D1 represents the contribution of the G-T)1 intermediate. By using the developed Monte-Carlo model, the contribution to the formation of D2 of the di-G-46... 

The most important parameter for the optimization of this isomerization is reaction temperature. Better yields of 1,4 cyclooctadiene are obtained at higher temperatures, however selectivity to 1,3 cyclooctadiene is also enhanced. The metal loading and flow rate are also important parameters with low metal loading and high flow rate being optimal. The partial pressure of reactant 1,5 cyclooctadiene does not appear to be important. Time of reaction was found to be somewhat important because some catalyst deactivation occurs. 

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