Nonisothermal reactions reactors

Nonisothermal reaction in a batch reactor Acetylated Castor Oil Hydrolysis... 

The design equations for a nonisothermal batch reactor include A-fl DDEs, one for each component and one for energy. These DDEs are coupled by the temperature and compositional dependence of 91/. They may also be weakly coupled through the temperature and compositional dependence of physical properties such as density and heat capacity, but the strong coupling is through the reaction rate. 

The solution of Eq. (173) poses a rather formidable task in general. Thus the dispersed plug-flow model has not been as extensively studied as the axial-dispersed plug-flow model. Actually, if there are no initial radial gradients in C, the radial terms will be identically zero, and Eq. (173) will reduce to the simpler Eq. (167). Thus for a simple isothermal reactor, the dispersed plug flow model is not useful. Its greatest use is for either nonisothermal reactions with radial temperature gradients or tube wall catalysed reactions. Of course, if the reactants were not introduced uniformly across a plane the model could be used, but this would not be a common practice. Paneth and Herzfeld (P2) have used this model for a first order wall catalysed reaction. The boundary conditions used were the same as those discussed for tracer measurements for radial dispersion coefficients in Section II,C,3,b, except that at the wall. 

For a single reaction in a nonisothermal batch reactor we can write the species and energy-balance equations... 

Govindarao10 also postulated generalized nonisothermal (constant reactor wall temperature) models for batch as well as cocurrent- and countercurrent-flow three-phase gas-liquid-solid systems carrying out a first-order reaction. 

The normal butenes were pyrolyzed in the presence of steam in a nonisothermal flow reactor at 730°-980°C and contact times between 0.04 and 0.15 sec to obtain conversion covering the range between 3% and 99%. Isomerization reactions accompanied the decomposition of these olefins however, the decomposition was the dominant reaction under these conditions. Pyrolysis of 1-butene is faster than that of either cis- or trans-2-butene. Methane, propylene, and butadiene are initial as well as major products from the pyrolysis of the n-butenes. Hydrogen is an initial product only from the 2-butenes. Ethylene appears to be an initial product only from 1-butene it becomes the most prominent product at high conversions. Over the range of conditions of potential practical interest, the experimental rate expressions for the disappearance of the respective butene isomers, have been derived. 

Consider accomplishing the reaction A + B C in nonisothermal batch reactor. The reaction occurs in the liquid phase. Find the time necessary to reach 80 percent conversion if the coolant supply is sufficient to maintain the reactor wall at 300 K. 

In this series reaction pathway, the desired species is the aldehyde. Since both reactions are exothermic (second reaction is highly exothermic), the reactor is operated nonisothermally. The reactor is a shell-and-tube heat exchanger consisting of 2500 tubes of 1 inch diameter. Should the heat exchanger be operated in a cocurrent or countercurrent fashion in order to provide a greater stabilization against thermal runaway ... 

Preferred flow patterns in nonisothermal membrane reactors. The discussions so far focus on flow patterns in an isothermal membrane reactor. In many situations, however, the membrane reactor is not operated under a uniform temperature. The choice between a plug flow (PFMR) and a perfect mixing membrane reactor (PMMR) depends on a number of factors. First of all, it depends on whether the reaction is endothermic or exothermic. 

Industrially, selectivity is often as important as conversion in considering the efficiency of the reactor. In isothermal reactions, the dilute phase and transition zone may cause better selectivity due to better contact in that region. But in nonisothermal reactions, the effect will be different because of the temperature effect. Mixing of gas and solids in the dilute phase is not sufficient, and this may cause a temperature distribution for exo- or endothermic reactions. 

When the heat of reaction is large, sizable temperature variations may be present even though heat transfer between the reactor and surroundings is facilitated. In such cases it is necessary to consider the effect of temperature on the rate of reaction. Reactors operating in this fashion are termed nonisothermal or nonadiabatic. 

In summary, the operation of commercial reactors falls into three categories isothermal, adiabatic, and the broad division of nonadiabatic, where attempts are made to approach isothermal conditions, but the magnitude of the heat of reaction or the temperature level prevents attaining this objective. Quantitative calculations for isothermal and nonisothermal homogeneous reactors are given in Chaps. 4 and 5. 

Nonuniform temperatures, or a temperature level different from that of the surroundings, are common in operating reactors. The temperature may be varied deliberately to achieve optimum rates of reaction, or high heats of reaction and limited heat-transfer rates may cause unintended nonisothermal conditions. Reactor design is usually sensitive to small temperature changes because of the exponential effect of temperature on the rate (the Arrhenius equation). The temperature profile, or history, in a reactor is established by an energy balance such as those presented in Chap. 3 for ideal batch and flow reactors. 

Use of Rate Equations in Reactor Design. The method of using the rate equations for catalytic reactions to calculate the reactor size and amount of catalyst needed for a specified conversion and feed rate is very similar to the method used for noncatalytic reactions. The calculations may be divided into three types, namely, those for isothermal reactors, adiabatic reactors, and nonisothermal nonadiabatic reactors. In all three cases where the feed rate F and the desired conversion x are specified, the weight of catalyst needed can be calculated from the expression... 

Figure 4.24 Plug-flow reactor in a nonisothermal reaction.

Tubular reactors are normally used in the chemical industry for extremely large-scale processes. When filled with solid catalyst particles, such reactors are referred to as fixed-bed or packed-bed reactors. In this section we treat general design relationships for tubular reactors in which isothermal homogeneous reactions take place. Nonisothermal tubular reactors are treated in Section 10.4 and packed-bed reactors in Section 12.7. 

The profiles of temperature, conversion, and concentration of the reactants in nonisothermal reactions taking place in batch, tank, or tubular reactors are shown in Figure 14.13. 

The temperature varies with the reaction time in the nonisothermal batch reactor. To perform the energy balance, we use the same energy balance equation 14.67, annulling the molar flow terms, but considering the variation of sensible heat with temperature and time. Then,... 

E14.16 An irreversible reaction A R is carried out in a nonisothermal CSTR reactor. Reactant is introduced into reactor at 130 kg/h at 20° C and the final conversion is 90%. The final temperature is 160°C. Calculate the heat required to reach this temperature and also the volume of the reactor operating isothermally at 160°C ... 

A perfectly mixed nonisothermal adiabatic reactor carries out a simple first-order exothermic reaction in the liquid phase ... 

The Professional Reference Shelf on the DVD-ROM describes a typical nonisothermal industrial reactor and reaction, the SO, oxidation, and gives many practical details. 

Calculation of the volume of a nonisothermal chemical reactor usually needs the use of numerical integration. For example, consider the first order reaction A B in liquid phase, taking place in an adiabatic plug flow reactor. Pure <4 enters the reactor, and it is desired to have the conversion X, at the outlet. The volume of this reactor is given by... 

Omoleye, J. A., Adesina, A. A., and Udegbunam, E. O., Optimal design of nonisothermal reactors Derivation of equations for the rate-temperature conversion profile and the optimum temperature progression for a general class of reversible reactions, Chem. Eng. Comm., Vol. 79, pp. 95-107, 1989. 

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