Residence-time Distribution and Models for Macromixing in the Reactors

RESIDENCE-TIME DISTRIBUTION AND MODELS FOR MACROMIXING IN THE REACTORS 

The performance of a chemical reactor depends not only on the relevant intrinsic kinetics of the reaction processes, but also on the physical processes occurring in the reactor. The physical processes such as interphase, interparticle, and intraparticle mass and heat transfer occurring within a multiphase reactor depend very significantly upon the mixing characteristics of the various phases involved. 

For the proper modeling of the performance of a multiphase chemical reactor, the RTD s of various fluid phases are of vital importance. The RTD curves allow 

In constructing a flow model for a given reactor, we must know the flow pattern through the reactor. This can be conveniently achieved by determining the age distribution of the elements of the fluid in the exit stream or the RTD within the reactor. 

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Chapters 2, 3, and 4 review the tools for modeling the performance of three-phase reactors. Chapter 2 evaluates the use of film and penetration theory for the calculation of absorption rate in three-phase reactors. Chapter 3 describes various techniques for characterizing residence time distribution and the models which take into account the macromixing in a variety of three-phase reactors. The concepts described in these two chapters are vital to the simulation of an entire reactor. Chapter 4 illustrates the development of the mathematical models for some important pilot scale and commercial reactors. In Chapter 5 some advantages and disadvantages of three-phase laboratory reactors are outlined. 

In a continuous reaction process, the true residence time of the reaction partners in the reactor plays a major role. It is governed by the residence time distribution characteristic of the reactor, which gives information on backmixing (macromixing) of the throughput. The principal objectives of studies into the macrokinetics of a process are to estimate the coefficients of a mathematical model of the process and to validate the model for adequacy. For this purpose, a pilot plant should provide the following ... 

The TIS and DPF models, introduced in Chapter 19 to describe the residence time distribution (RTD) for nonideal flow, can be adapted as reactor models, once the single parameters of the models, N and Pe, (or DL), respectively, are known. As such, these are macromixing models and are unable to account for nonideal mixing behavior at the microscopic level. For example, the TIS model is based on the assumption that complete backmixing occurs within each tank. If this is not the case, as, perhaps, in a polymerization reaction that produces a viscous product, the model is incomplete. 

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