Pressure 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. 

Many catalytic reactions require high pressures of reactant gases. Thus, an in-depth understanding of such catalytic systems requires truly in situ NMR and IR measurements and it has been necessary to develop appropriate High Pressure-spectroscopic cells the development and use of HP-NMR and HP-IR cells are reviewed in chapters 2 and 3 respectively. The use of both of these complementary methods/HP-techniques is probably best illustrated in chapters 5 - carbonyla-tion reactions, chapter 6 - hydroformylation and chapter 7 - alkene/CO copolymerisation, which deal with the recent advances in each of these important areas. 

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. 

In catalytic distillation the temperature also varies with position in the column, and this will change the reaction rates and selectivities as well as the equilibrium compositions. Temperature variations between stages and vapor pressures of reactants and products can be exploited in designing for multiple-reaction processes to achieve a high selectivity to a desired product with essentially no unwanted products. 

Production of CDt and CHDi with Equal Pressures of Reactants ... 

By plotting tlie initial overall rate with the initial pressure of reactant A, when B has not been yet produced, one can find which case is consistent with the experimental data and choose the appropriate rate form. Generally, the overall rate is expressed as the combination of three terms ... 

If the pressure of reactant P above the surface is maintained constant, the rate equation for the concentration of adsorbed reactant is... 

Fig. 12.5. (a) The region of multiple stationary-state behaviour for the Takoudis-Schmidt-Aris model of surface reaction, with = 10 3 and k2 = 2 x 10-3 (b), (c), and (d) show how the stationary-state reaction rate varies with the gas-phase pressure of reactant R for different values of p, giving isola, mushroom, and single hysteresis loop respectively. (Adapted and reprinted with permission from McKarnin, M. A. et al. (1988). Proc. R. Soc., A415, 363-87.)... 

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).

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