Adiabatic reactor operation equilibrium conversion

Heats of reaction also influence the choice between adiabatic and isothermal reactors. When a reactant mixture of the same composition and temperature is fed to both an adiabatic reactor and an isothermal reactor, the equilibrium conversion is almost always less in the adiabatic reactor this is true for both endothermic and exothermic reactions. Endothermic reactions performed in adiabatic reactors are accompanied by a fall in temperature, decreasing conversion. In such situations, we try to improve both the rate and the conversion by feeding reactants at high temperatures. But if a high temperature cannot be maintained in an adiabatic reactor, then we should consider adding heat and operating the reactor isothermaUy. 

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...

When we can predict the response of the reacting system to changes in operating conditions (how rates and equilibrium conversion change with temperature and pressure), when we are able to compare yields for alternative designs (adiabatic versus isothermal operations, single versus multiple reactor units, flow versus batch system), and when we can estimate the economics of these various alternatives, then and only then will we feel sure that we can arrive at the design well fitted for the purpose at hand. Unfortunately, real situations are rarely simple. 

It is often necessary to employ more than one adiabatic reactor to achieve a desired conversion. In the first place chemical equilibrium may have been established in the first reactor and it is then necessary to cool and/or remove the product before entering the second reactor. This, of course, is one good reason for choosing a catalyst which will function at the lowest possible temperature. Secondly, for an exothermic reaction, the temperature may rise to a point at which it is deleterious to the catalyst activity. At this point the products from the first reactor are cooled prior to entering a second adiabatic reactor. To design such a system it is only necessary to superimpose on the rate contours the adiabatic temperature paths for each of the reactors. The volume requirements for each reactor can then be computed from the rate contours in the same way as for a single reactor. It is necessary, however, to consider carefully how many reactors in series it is economic to operate. 

The activity of the catalyst is roughly proportional to the surface area of nickel used. In an adiabatic prereformer, a high activity is desired to maximize the space velocity. In a tubular reformer the activity may be of less significance because the reactor volume is settled by mechanical criteria. Most industrial tubular reformers operate at space velocities of 2000-4000 hr However, equilibrium conversion can be... 

We know that an increase in temperature will decrease the equilibrium extent of an exothermic reaction. Yet to perform the exothermic reaction adiabatically is to induce a temperature increase. Similarly, an endothermic reaction has poorer equilibrium conversion at a lower temperature, and the temperature falls if it is allowed to proceed adiabatically. Thus at first blush there is something rather self-defeating about adiabatic reaction. However, adiabatic operation, involving no heat transfer equipment within the reactor, is so attractive for its simplicity that it is worth more careful examination. 

If the reactor were a single adiabatically operated fixed bed, the heat release would raise the temperature to 600 °C, which corresponds to an equilibrium conversion of SO2 of only 70% (Figure 6.3.4), but even this far from sufficient conversion would only be reached for an infinite residence time and reactor length. For isothermal operation, a conversion of about 98% would be possible, but this would require an expensive reactor (e.g., a multi-tubular reactor intensively cooled by a molten salt. Figure 4.10.7). 

The single bed reactor is simply a vessel of relatively large diameter, as sketched in Fig. 11.1. This simple reactor design is best suited for adiabatic processes and not applicable for very exothermic or endothermic processes. If the reaction is very endothermic, the temperature change may be such as to extinguish the reaction before the desired conversion is attained. Strongly exothermic reactions, on the other hand, can lead to a temperature rise that is prohibitive due to its unfavorable influence on the equilibrium conversion, the product selectivity, the catalyst stability, and in extreme cases unsafe operation. 

A reactor for the oxidation of S02 is operated adiabatically with heat interchange between feed and product streams in countercurrent. Inlet concentrations are 10% each of S02 and 02 and the balance N2. Preheat temperature is to be 725 K, equilibrium is attained at the outlet and the conversion of S02 is 70%. Pressure is atmospheric. Find the temperatures of the feed and of the outlet. 

Most reactors used in industrial operations run isother-mally. For adiabatic operation, principles of thermodynamics are combined with reactor design equations to predict conversion with changing temperature. Rates of reaction normally increase with temperature, but chemical equilibrium must be checked to determine ultimate levels of conversion. The search for an optimum isothermal temperature is common for series or parallel reactions, since the rate constants change differently for each reaction. Special operating conditions must be considered for any highly endothermic or exothermic reaction. 

The RFR has been used so far in two major applications. One is to conduct an exothermic equilibrium-limited reaction so that its conversion increases as the reactants pass over the leading front of the moving hot zone. The second is to conduct reactions in which the adiabatic temperature is lower than that needed to carry out the reaction — for example, the catalytic destruction of a dilute mixture of volatile organic compounds (VOCs). It was recently proposed to conduct coke-forming reactions in a RFR [13]. In this operation, the desired reaction is conducted in the upstream section of the reactor while the downstream section is regenerated at the same time from the coke dqmsited in the previous half-cycle. 

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