Reactor mixing models

TABLE I. REACTOR MIXING MODELS The derivations of the equations are given by Treybig (32)... 

In this chapter the simulation examples are described. As seen from the Table of Contents, the examples are organised according to twelve application areas Batch Reactors, Continuous Tank Reactors, Tubular Reactors, Semi-Continuous Reactors, Mixing Models, Tank Flow Examples, Process Control, Mass Transfer Processes, Distillation Processes, Heat Transfer, and Dynamic Numerical Examples. There are aspects of some examples which relate them to more than one application area, which is usually apparent from the titles of the examples. Within each section, the examples are listed in order of their degree of difficulty. 

SPBEDRTD - Spouted Bed Reactor Mixing Model System... 

Bioremediation of Soil Particles 591 Spouted Bed Reactor Mixing Model 390 Steady-State, Two-Pass Heat Exchanger 515 Multicomponent, Semi-Batch Steam Distillation 508 Space-Time-Yield and Safety in a Semi-Continuous Reactor 365... 

Given the parameters and rate constants in Table 8.3, calculate xa and yc versus Qr for the two-reactor mixing model shown in Figure 8.32, and compare the result to the single, well-mixed reactor. Then calculate the residence-time distribution P(0) for tracer injected with the A feed stream for the two models. Discuss whether or not the residence-time distribution is a reliable indicator for problems with yield in the imperfectly mixed. reactor. 

Segregrated flow model The fluid in a flow reactor is assumed to behave as a macrofluid. Each clump functions as a miniature batch reactor. Mixing of molecules of different ages occurs as late as possible. 

In what follows, both macromixing and micromixing models will be introduced and a compartmental mixing model, the segregated feed model (SFM), will be discussed in detail. It will be used in Chapter 8 to model the influence of the hydrodynamics on a meso- and microscale on continuous and semibatch precipitation where using CFD, diffusive and convective mixing parameters in the reactor are determined. 

In order to account for both micromixing and mesomixing effects, a mixing model for precipitation based on the SFM has been developed and applied to continuous and semibatch precipitation. Establishing a network of ideally macromixed reactors if macromixing plays a dominant role can extend the model. The methodology of how to scale up a precipitation process is depicted in Figure 8.8. 

Mixing Models. The assumption of perfect or micro-mixing is frequently made for continuous stirred tank reactors and the ensuing reactor model used for design and optimization studies. For well-agitated reactors with moderate reaction rates and for reaction media which are not too viscous, this model is often justified. Micro-mixed reactors are characterized by uniform concentrations throughout the reactor and an exponential residence time distribution function. 

Evaluation of Mixing Models. The micro-mixed reactor will produce polymer disttibutions with increasing amounts of high molecular weight tail as the degree of polymerization of the polymer product increases over that of the original seed polymer. 

David, R., Muhr, H. and Villermaux, J., The Yield of a Consecutive-Competitive Reaction in a Double Jet Semi-Batch Reactor Comparison between Experiments and a Multizone Mixing Model, Chem. Eng. Sci. 1992, 47 (9-11), 2841-2846. 

Tanks-in-series reactor configurations provide a means of approaching the conversion of a tubular reactor. In modelling, they are employed for describing axial mixing in non-ideal tubular reactors. Residence time distributions, as measured by tracers, can be used to characterise reactors, to establish models and to calculate conversions for first-order reactions. 

In this model of non-ideal reactor mixing, a fraction, fi, of the volumetric feed rate, F, completely by-passes the mixing in the reactor. In addition, a fraction, f2, of the reactor volume, V, exists as dead space. F3 is the volumetric rate of exchange between the perfectly mixed volume Vi and the dead zone volume V2 of the reactor. 

Timm, Gilbert, Ko, and Simmons O) presented a dynamic model for an isothermal, continuous, well-mixed polystyrene reactor. This model was in turn based upon the kinetic model developed by Timm and co-workers (2-4) based on steady state data. The process was simulated using the model and a simple steady state optimization and decoupling algorithm was tested. The results showed that steady state decoupling was adequate for molecular weight control, but not for the control of production rate. In the latter case the transient fluctuations were excessive. 

The classical CRE model for a perfectly macromixed reactor is the continuous stirred tank reactor (CSTR). Thus, to fix our ideas, let us consider a stirred tank with two inlet streams and one outlet stream. The CFD model for this system would compute the flow field inside of the stirred tank given the inlet flow velocities and concentrations, the geometry of the reactor (including baffles and impellers), and the angular velocity of the stirrer. For liquid-phase flow with uniform density, the CFD model for the flow field can be developed independently from the mixing model. For simplicity, we will consider this case. Nevertheless, the SGS models are easily extendable to flows with variable density. 

The consequences of the differences in these and other mixing models for reactor performance are explored in Chapter 20. 

Figure 5.24. A four-environment mixing model can be developed for reactors with three feed streams. In environment 1, the two components of the mixture-fraction vector are null = 0. In environment 2, fi = 1 and 2 = 0, while, in environment 3, = 0 and 2 = 1- Chemical...

---