Equilibrium conversions feed temperature

Table VIII demonstrates the inverse relationship of conversion to S02 concentration in the feed that is a consequence of applying flow reversal to S02 oxidation using a single reactor. As the S02 concentration in the table moves from 0.8 to over 8 vol%, the conversion drops from 96-97% down to 85%. At the same time, the maximum bed temperature changes from 450 to 610°C. For an equilibrium-limited, exothermic reaction, this behavior is explained by variation of the equilibrium conversion with temperature.

The equilibrium conversion versus temperature and pressure for stoichiometric feed is shown in Figure 3-17. 

Single reactions. For single reactions, a good initial setting is 95 percent conversion for irreversible reactions and 95 percent of the equilibrium conversion for reversible reactions. Figure 2.9 summarizes the influence of feed mole ratio, inert concentration, temperature, and pressure on equilibrium conversion. ... 

The equilibrium constant can be determined at any temperature from standard state information on reactants and product. Considering the synthesis of NH3, the equilibrium conversion can be determined for a stoichiometric feed of Hj and Nj, at the total pressure. These conversions are determined by the number of moles of each species against conversion X by taking as a basis, 1 mole of N2. 

For many years, butadiene has been manufactured by dehydrogenating butene or butane over a catalyst at appropriate combinations of temperature and pressure. It is customary to dilute the butene feed with steam (10-20 moles H20/mole butene) to stabilize the temperature during the endothermic reaction and to help shift the equilibrium conversion in the desired direction by reducing the partial pressures of hydrogen and butadiene. The current processes suffer from two major disadvantages. 

Figure 3-17 Plot of eqdlibrimn convereion Xg vasus terr5)er-ature for ammonia synthesis starting with stoichiomeh ic feed. While the equilibrium is favorable at anbient tar5)erature (where bactaia fix N2), the convasion r dly falls off at elevated temperature, and commercial ammonia synthesis reactors operate with a Fe catalyst at pressiues as high as 300 atm to att 2 high equilibrium conversion.

Figiu 3-18 Plot of equilibrium conversion Xq versus temperatiwe for methanol synthesis starting witii stoichiomeh ic feed. While die equilibriimi is favorable at ambient temperature, die conversion rapidly decreases at higher temperature, and industrial reactors operate with a Cu/ZnO catalyst at pressures as high as 100 atm. 

This mode is used industrially for exothermic reactions such as NH3 oxidation and in CH3OH synthesis, where exothermic and reversible reactions need to operate at temperatures where the rate is high but not so high that the equilibrium conversion is low. Interstage cooling is frequently accomplished along with separation of reactants from products in units such as water quenchers or distillation columns, where the cooled reactant can be recycled back into the reactor. In these operations the heat of water vaporization and the heat removed from the top of the distillation column provides the energy to cool the reactant back to the proper feed temperature. 

In ammonia synthesis, high temperatures correspond to small reactor volumes. For exothermic reactions, the equilibrium conversion Xe decreases as the temperature increases. Therefore, these reactions are often carried out in a series of adiabatic beds with either intermediate heat exchangers to cool the gases or bypass the cold feed to decrease the temperatures between the beds. Some compromise can be achieved between high temperatures involving small reactor volumes and high equilibrium conversions. 

Table 6-5 shows the equilibrium constant Ket with the equilibrium partial pressure of NH3 starting with a stoichiometric mixture of H2 and N2 at pressures of 1, 10, and 100 atm. Figure 6-12 shows the relationship between equilibrium conversion Xe versus temperature and pressure for stoichiometric feed. 

Typical results are shown in Fig. 12.8. They demonstrate the expected tendency that increasing the temperature and residence time leads to higher conversions, with equilibrium conversion being reached at sufficient residence time. Figure 12.8 also illustrates the fact that a higher conversion of cyclohexane can be achieved if the feed is more diluted - a finding which is in agreement with the predictions of Eq. (16) (see Fig. 12.1). 

The equilibrium conversion is a function of temperature, pressure, and the steam-to-ethylene ratio in the feed. The efiects of all three variables are shown in Fig. 15.5. The curves in this figure come from calculations just like those illustrated in this example, except that a less precise equation for K as a function of T was used. 

The HDS reaction for a feed with the composition 15% Bz + 85% n-C5 + 30 ppmw sulfur is already completed at about 170°C, as shown in Figure 7.5. This shows that HYSOPAR is an efficient hydrodesulfiirization (HDS) catalyst under these conditions despite the effect of sulfur on benzene hydrogenation activity. To complete the benzene conversion the temperature needs to be increased from 225-235°C to 250-260°C with 30 ppmw S in the feed. At these high temperatures, thermodynamic equilibrium between nC5 and iC5 was practically reached, indicating that the impact of S on isomerization is not pronounced. Some ring cleavage of cyclic compounds was also observed, accompanied by an increase in gas production. The results are summarized in table 7.7. 

In a separate parametric study, Mohan and Govind(l)(9) analyzed the effect of design parameters, operating variables, physical properties and flow patterns on membrane reactor. They showed that for a membrane which is permeable to both products and reactants, the maximum equilibrium shift possible is limited by the loss of reactants from the reaction zone. For the case of dehydrogenation reaction with a membrane that only permeates hydrogen, conversions comparable to those achieved with lesser permselective membranes can be attained at a substantially lower feed temperature. 

Other equations, which contain CO and/or hydrogen as well, may be formulated, too. That a methanation reaction occurs is already obvious from the fact that the reaction product contains more methane than the naphtha feed and that the equilibrium conversion of the hydrocarbon in a direct reforming reaction according to equation (40) is lower than in reaction (56) at low temperature. The reaction of aromatic is principally similar but with higher risk of carbon formation. 

We now consider an adiabatic reactor of fixed sire or catalyst weight and investigate what happens as the feed temperature is varied. ITie reaction is reversible and exothermic. At one temperature extreme, using a very high feed temperature, the specific reaction rate will be large and the reaction will proceed rapidly, but the equilibrium conversion will be close to zero. Consequently, very little product will be formed. A plot of the equilibrium conversion and the conversion calculated from the adiabatic energy balance,... 

For a feed temp oire of 300 K, the adiabatic equilibrium temperature is 465 K and the conesponding adiabatic equilibrium conversion is 0.41,... 

What conversion could Im achieved in Example 8-8 if two interstage coolers were available that had the capacity to cool the exit stream to 350°K. Also determine the heat duty of each exchanger for a molar feed rate of A of 40 mol/s. Assume that 95% of equilibrium conversion is achieved in each reactor. The feed temperature to the first reactor is 300 K ... 

Figure S-11 Equilibrium conversion for different feed temperatures.

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