Adiabatic operations conversion

Adiabatic plug flow reactors operate under the condition that there is no heat input to the reactor (i.e., Q = 0). The heat released in the reaction is retained in the reaction mixture so that the temperature rise along the reactor parallels the extent of the conversion. Adiabatic operation is important in heterogeneous tubular reactors. 

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...

In the case of adiabatic operation the temperature must be related to the fraction conversion so that equation A can be integrated. 

For an exothermic reaction, adiabatic operation gives an increase in temperature with increasing conversion. However, the optimum... 

If the flow rate of each feed stream is 0.139 m3/ksec and if 50% conversion is to be achieved, determine the reactor volumes required for isothermal operation and for adiabatic operation. 

For exothermic, reversible reactions, the existence of a locus of maximum rates, as shown in Section 5.3.4, and illustrated in Figures 5.2(a) and 18.3, introduces the opportunity to optimize (minimize) the reactor volume or mean residence time for a specified throughput and fractional conversion of reactant. This is done by choice of an appropriate T (for a CSTR) or T profile (for a PFR) so that the rate is a maximum at each point. The mode of operation (e.g., adiabatic operation for a PFR) may not allow a faithful interpretation of this requirement. For illustration, we consider the optimization of both a CSTR and a PFR for the model reaction... 

Operation is adiabatic and conversion is to be 95%. Find the volumes of (a) a tubular flow reactor (b) a CSTR (c) a batch reactor when the down time is 1 hr per batch and the daily charge is 3(1440) cuft/day. 

Adiabatic operations of an exothermic reaction give a rising temperature with conversion. However, the desired progression is one of falling temperature. Thus,... 

Figure 10b shows that CO conversion is much higher under adiabatic operation due to the higher bed temperatures. Note that the conversion of the C02 becomes important as soon as the CO is nearly depleted. The rippling in the C02 curve is a result of the axial orthogonal collocation.14 Numerical solution problems such as this will be discussed in Section VII. 

As we see, for a specific reaction, the higher the inlet concentration, the higher the conversion and the exit temperature. This is a result of the positive effect of the temperature rise, due to the exothermic nature of the reaction, on the rate coefficient and thus on the reaction rate and conversion. Note that for higher inlet CO concentration, the conversion for the isothermal operation is the same, while for the adiabatic operation the conversion is higher for higher inlet concentrations. Furthermore, the conversion in the adiabatic fixed bed is always higher in comparison to the isothermal fixed bed. Of course, these results are such because the reaction is of first order in respect to CO. 

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. 

A theoretical and experimental study of multiplicity and transient axial profiles in adiabatic and non-adiabatic fixed bed tubular reactors has been performed. A classification of possible adiabatic operation is presented and is extended to the nonadiabatic case. The catalytic oxidation of CO occurring on a Pt/alumina catalyst has been used as a model reaction. Unlike the adiabatic operation the speed of the propagating temperature wave in a nonadiabatic bed depends on its axial position. For certain inlet CO concentration multiplicity of temperature fronts have been observed. For a downstream moving wave large fluctuation of the wave velocity, hot spot temperature and exit conversion have been measured. For certain operating conditions erratic behavior of temperature profiles in the reactor has been observed. 

For the following reactor feed, use the AREAC block of the CACHE FLOWTRAN simulation to determine the exit temperature for adiabatic operation at 90 percent conversion of toluene. 

The primary reason for choosing a particular reactor type is the influence of mixing on the reaction rates. Since the rates affect conversion, yield, and selectivity we can select a reactor that optimizes the steady-state economics of the process. For example, the plug-flow reactor has a smaller volume than the CSTR for the same production rate under isothermal conditions and kinetics dominated by the reactant concentrations. The opposite may be true for adiabatic operation or autocata-lytic reactions. For those situations, the CSTR would have the smaller volume since it could operate at the exit conditions of a plug-flow reactor and thus achieve a higher overall rate of reaction. 

These equations correspond to isothermal and adiabatic operation respectively. Using the total conversion from Equation 5.54, Equations 5.60 and 5.61 can be rewritten in terms of measurable quantities ... 

Higher conversions than those shown in Figure E8-8.1 can be achieved for adiabatic operations by connecting reactors in series with interstage cooling ... 

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. 

Table 5-6 Conversion vs reactor length for chlorination of propylene NON ADIABATIC OPERATION...

Even adiabatic operation results in the formation of considerable amounts of the undesirable dichloropropane. This occurs in the first part of the reactor, where the temperature of the flowing mixture is low. This is an illustration of the discussion at the beginning of the chapter with respect to Fig. 5-la and b. The conditions correspond to the low-conversion range of Fig. 5-lb before the maximum rate is reached. A tubular-flow reactor is less desirable for these conditions than a stirred-tank unit. The same reaction system is illustrated in Example 5-3 for a stirred-tank unit. 

Solution The rate of each reaction (allyl chloride and dichloropropane formation) will be a constant and should be evaluated at the temperature and cornposition of the stream leaving the reactor. The temperature is determined by Eq. (3-6). For adiabatic operation and zero conversion in the feed this becomes, using molal units for F,... 

The energy balance [Eq. (3-6)] for adiabatic operation and zero conversion in the feed is... 

A reactor for the production of drying oils by the decomposition of acetylated castor oil is to be designed for a conversion of 70%. The initial charge will be 500 lb and the initial temperature 340°C, as in Example 5-1. In fact, all the conditions of Example 5-1 apply, except instead of adiabatic operation, heat will be supplied electrically with a cal-rod unit in the form of a l-in.-OD coil inraiersed in the reaction mixture. The power input and the stirring in the reactor will be such that the surface temperature of the heater is maintained constant at 700°K. The heat-transfer coefficient may be taken equal to 60 Btu/ (hr)(ft )(°F). What length of heater will be required if the conversion of 70% is to be obtained in 20 min ... 

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