Ideal isothermal reactors performance

Comparison ofthe Performance of Ideal Isothermal Reactors... 

Another view is given in Figure 3.1.2 (Berty 1979), to understand the inner workings of recycle reactors. Here the recycle reactor is represented as an ideal, isothermal, plug-flow, tubular reactor with external recycle. This view justifies the frequently used name loop reactor. As is customary for the calculation of performance for tubular reactors, the rate equations are integrated from initial to final conditions within the inner balance limit. This calculation represents an implicit problem since the initial conditions depend on the result because of the recycle stream. Therefore, repeated trial and error calculations are needed for recycle... 

In the previous section we indicated how various mathematical models may be used to simulate the performance of a reactor in which the flow patterns do not fit the ideal CSTR or PFR conditions. The models treated represent only a small fraction of the large number that have been proposed by various authors. However, they are among the simplest and most widely used models, and they permit one to bracket the expected performance of an isothermal reactor. However, small variations in temperature can lead to much more significant changes in the reactor performance than do reasonably large deviations inflow patterns from idealized conditions. Because the rate constant depends exponentially on temperature, uncertainties in this parameter can lead to design uncertainties that will make any quantitative analysis of performance in terms of the residence time distribution function little more than an academic exercise. Nonetheless, there are many situations where such analyses are useful. 

A very important form of such disturbances is caused by the presence of the wall of the tube containing the packed bed. Vortmeyer and Schuster (1983) have used a variational approach to evaluate the steady two-dimensional velocity profiles for isothermal incompressible flow in rectangular and circular packed beds. They used the continuity equation, Brinkman s equation (1947), and a semiempirical expression for the radial porosity profile in the packed bed to compute these profiles. They were able to show that significant preferential wall flow occurs when the ratio of the channel diameter to the particle diameter becomes sufficiently small. Although their study was done for an idealized situation it has laid the foundation for more detailed studies. Here CFD has definitely contributed to the improvements of theoretical prediction of reactor performance. 

The most important feature of a CSTR is its mixing characteristics. The idealized model of reactor performance presumes that the reactor contents are perfectly mixed so that the properties of the reacting fluid are uniform throughout. The composition and temperature of the effluent are thus identical with those of the reactor contents. This feature greatly simplifies the analysis of stirred-tank reactors vis-h-vis tubular reactors for both isothermal and nonisothermal... 

For a basic understanding of chemical reactor design, start with Sections 4.10.1 and 4.10.2, where different ideal and isothermal reactor types are introduced and the respective performance equations are derived. You should then study the behavior of real reactors (non-ideal flow and residence time distribution, Section 4.10.4) and the simplest model to account for deviations of real systems from ideal reactors, the tanks-in-series model (Section 4.10.5). 

In order to deduce fundamental information on intrinsic catalyst performance it is important to reduce the influence of the chosen reactor set-up on catalyst performance to a minimum. The first reactor requirement is ideal isothermal operation conditions. The second requirement is continuously operated ideal plug flow without axial hackmixing, this being identical to a series of infinitesimally small, continuously stirred tank reactors each fulfilling the stationary concentration requirement The realization of such an optimum reactor concept is not trivial, and in 1969 Temkin and Kul kova developed a concept in which actual-size catalyst bodies could be tested under ideal conditions. Catalyst spheres and inert cylinders are alternately placed in a tube with a diameter slightly bigger than the catalyst spheres. Inert cylinders and catalyst spheres are fixed by three wires. Excellent heat transport... 

In this chapter we develop the performance equations for a single fluid reacting in the three ideal reactors shown in Fig. 5.1. We call these homogeneous reactions. Applications and extensions of these equations to various isothermal and noniso-thermal operations are considered in the following four chapters. 

The axial dispersion plug flow model is used to determine the performance of a reactor with non-ideal flow. Consider a steady state reacting species A, under isothermal operation for a system at constant density Equation 8-121 reduces to a second order differential equation ... 

Perform energy balance analyses around isothermal and adiabatic ideal reactors... 

Topic 4.10.1 Influence of changing volume on the performance of isothermal ideal reactors (single first-order reaction)... 

In true single phase batch reactors the reactants are mixed before the reaction starts. It would seem that transport of matter could never influence the course of chemical reactions then, since a mixture remains mixed. The performance of the reactor could then not be scale dependent. The ideal models presented in section 3.2.1 would then always be applicable, as long as the reactor can be considered isothermal. That is mostly true, but deviations from the ideal situation may occur when the reaction approaches complete conversion. The concentrations of the reactants may become so low, that the average diffusion path becomes large in relation to molecular dimensions, so that diffusion times can no longer be neglected. This is only of practical importance in exceptional cases. Interesting examples are certain polymerizations and polycondensations, see sections 13.3.1 and 13.7. 

Problem 6-6 (Level 1) The catalytic, gas-phase reaction A B is taking place in an isothermal, ideal plug-flow reactor that is filled with catalyst particles. Some data on the performance of the reactor are shown in the following table. 

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