Equilibrium conversions endothermic reactions

The local reactor temperature affects the rates of reaction, equilibrium conversion, and catalyst deactivation. As such, the local temperature has to be controlled to maximize reaction rate and to minimize deactivation. In the case of an exothermic (endothermic) reaction, higher (lower) local temperatures can cause suboptimal local concentrations. Heat will have to be removed (added) to maintain more uniform temperature conditions. The mode of heat removal (addition) will depend on the application and on the required heat-transfer rate. 

Because the reaction is endothermic, the equilibrium conversion increases with increasing temperature. A typical equilibrium curve and temperature conversion trajectory for the reactor sequence are shown in Figure 11-8. 

In the Monsanto/Lummus Crest process (Figure 10-3), fresh ethylbenzene with recycled unconverted ethylbenzene are mixed with superheated steam. The steam acts as a heating medium and as a diluent. The endothermic reaction is carried out in multiple radial bed reactors filled with proprietary catalysts. Radial beds minimize pressure drops across the reactor. A simulation and optimization of styrene plant based on the Lummus Monsanto process has been done by Sundaram et al. Yields could be predicted, and with the help of an optimizer, the best operating conditions can be found. Figure 10-4 shows the effect of steam-to-EB ratio, temperature, and pressure on the equilibrium conversion of ethylbenzene. Alternative routes for producing styrene have been sought. One approach is to dimerize butadiene to 4-vinyl-1-cyclohexene, followed by catalytic dehydrogenation to styrene ... 

We can see from Table 9.2 that the equilibrium constant depends on the temperature. For an exothermic reaction, the formation of products is found experimentally to be favored by lowering the temperature. Conversely, for an endothermic reaction, the products are favored by an increase in temperature. 

As the temperature increases, the equilibrium conversion increases for endothermic reactions and decreases for exothermic reactions. 

Figure 6.4a shows the behavior of an endothermic reaction as a plot of equilibrium conversion against temperature. The plot can be obtained from values of AG° over a range of temperatures and the equilibrium conversion calculated as illustrated in Examples 6.1 and 6.2. If it is assumed that the reactor is operated adiabatically, a heat balance can be carried out to show the change in temperature with reaction conversion. If the mean molar heat capacity of the reactants and products are assumed constant, then for a given starting temperature for the reaction Ttn, the temperature of the reaction mixture will be proportional to the reactor conversion X for adiabatic operation, Figure 6.4a. As the conversion increases, the temperature decreases because of the reaction endotherm. If the reaction could proceed as far as equilibrium, then it would reach the equilibrium temperature TE. Figure 6.4b shows how equilibrium conversion can be increased by dividing the reaction into stages and reheating the reactants...

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. 

This behavior can be shown graphically by constructing the rD-7 -/A relation from equation 5.3-16, in which kp kr, and Keq depend on T. This is a surface in three-dimensional space, but Figure 5.2 shows the relation in two-dimensional contour form, both for an exothermic reaction and an endothermic reaction, with /A as a function of T and ( rA) (as a parameter). The full line in each case represents equilibrium conversion. Two constant-rate ( -rA) contours are shown in each case (note the direction of increase in (- rA) in each case). As expected, each rate contour exhibits a maximum for the exothermic case, but not for the endothermic case. 

In an endothermic reaction, the reactant temperature will fall as reaction proceeds unless heat is supplied from an external source. With a highly endothermic reaction, it may be necessary to supply a considerable amount of heat to maintain a temperature high enough to provide a rate of reaction and equilibrium conversion which are large enough for the process to be operated economically. Under these circumstances, the rate of heat transfer may effectively determine the rate of reaction and so dominate the problems involved in the reactor design. 

For an increase in temperature, equilibrium conversion rises for endothermic reactions and drops for exothermic reactions. 

For endothermic reactions a rise in temperature increases both the equilibrium conversion and the rate of reaction. Thus, as with irreversible reactions, the highest allowable temperature should be used. 

This reaction is strongly endothermic and proceeds to near-equilibrium conversion at -850°C at pressures up to -20 atm. The products also contain H2O and CO2. If syngas is desired, the CO2 and H2O are removed from this mixture (how ). 

The shapes of these curves is plotted in Figure 6-13 for endothermic and exothermic reactions. If AH > 0, then the shape of the X(T) curve is nearly unchanged because the equilibrium conversion is lower at low temperatures, but if AH < 0, then X(T) increases with T initially but then decreases at high T as the reversibihty of the reaction causes X to decrease. However, the multiplicity behavior is essentially unchanged with reversible reactions. 

The equation Kc = kf/kr also helps explain why equilibrium constants depend on temperature. Recall from Section 12.10 that rate constants increase as the temperature increases, in accord with the Arrhenius equation k = Ae E RT. In general, the forward and reverse reactions have different values of the activation energy, so kf and kT increase by different amounts as the temperature increases. The ratio kf/kT = Kc is therefore temperature-dependent. For an exothermic reaction, which has AE = Ea(forward) — Ea(reverse) < 0, Ea(reverse) is greater than Ea(forward). Consequently, kT increases by more than kf increases as the temperature increases, and so Kc = kt/kr for an exothermic reaction decreases as the temperature increases. Conversely, Kc for an endothermic reaction increases as the temperature increases. 

The second example of process intensification at DSM is the urea process (5). The history of the urea process at DSM is rather long, as shown in Table 5. Urea is produced in a two-step process. The first step is the formation of carbamate from NH3 and C02. This reaction is exothermic. The second step is the decomposition of carbamate into urea and water. This second reaction is slightly endothermic. Both reactions are equilibrium reactions. The conversion to urea in equilibrium is about 60%. This means that substantial recycle flow is necessary to obtain sufficient overall conversion. In the reaction section the main unit operations are ... 

The effect of temperature on equilibrium composition can be calculated using the van t Hoff equation. Since the standard heat of reaction is negative (-AHR) for an exothermic reaction, an increase in temperature results in a decrease in K and a subsequent decrease in conversion. Therefore, an exothermic reaction must be performed at as low a temperature as possible. An endothermic reaction (+AHR) is positive and K increases with an increase in temperature, as does the equilibrium conversion. Therefore, an endothermic reaction must be performed at an elevated temperature. 

For endothermic reactions, e.g. dehydrogenation, cracking reactions, etc., the equilibrium constant and the equilibrium conversion increase with temperature. 

Related Calculations. (1) Since the reaction is irreversible, equilibrium considerations do not enter into the calculations. For reversible reactions, the ultimate extent of the reaction should always be checked first, using the procedures outlined in Section 4. If equilibrium calculations show that the required conversion cannot be attained, then either the conditions of the reaction (e.g., temperature) must be changed or the design is not feasible. Higher temperatures should be investigated to increase ultimate conversions for endothermic reactions, while lower temperatures will favor higher conversions for exothermic reactions. 

Equation (15.17) gives the effect of temperature on the equilibrium constant hence on the equilibrium yield. If AH° is negative, i.e., if the reaction is exot the equilibrium constant decreases as the temperature increases. Converse increases with T for an endothermic reaction. 

The highest conversion that can be achieved in reversible reactions is the equilibrium conversion. For endothermic reactions, the equilibrium conversion increases with increasing temperature up to a maximum of 1.0. For exothermic reactions the equilibrium conversion decreases with increasing tetr erature. 

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