The Plug-Flow Reactor PFR

The Plug Flow Reactor (PFR)—Basic Assumptions and Design Equations... 

Continuous reactor a reactor characterized by a continuous flow of reactants into and a continuous flow of products from the reaction system examples are the plug flow reactor (PFR) and the continuous stirred tank reactor (CSTR). 

The plug flow reactor (PFR) is conceptually the simplest example of a LCFR all fluid elements have the same axial velocity, and therefore they have the same residence time or age at the exit, which would correspond to the batch reactor time. But, unlike the batch reactor in the CPFR there is no mixing of the species except by diffusion. Figure 11.9(a) schematizes a CPFR. 

This reactor has continuous input and output of material through a tube. Assumptions made for the plug flow reactor (PFR) are (1) material passes through the reactor in incremental slices (each slice is perfectly mixed radially but has no forward or backward mixing between slices each slice can be envisioned as a miniature CSTR), (2) composition and conversion vary with residence time and can be correlated with reactor volume or reactor length, and (3) the reactor operates at steady state. 

The two extremes of the state of mixedness arc represented by the plug flow reactor (PFR, no mixing) and by the perfectly stirred reactor (PSR, perfectly mixed). The reactant flow in the PFR is neither macro nor micro mixed, whereas in the PSR mixing occurs down to the molecular level, thus both macro and micro mixing take place (see Figure 6). A variety of real flows can be characterised by series, parallel or loop connections of PFR and PSR. Additionally there exist other models such as the dispersion model (dispersed plug flow) which allows to model mixing conditions between the two extremes of PFR and PSR. 

The simulation results show the calculated influence of the reburn temperature and the rebum fuel rate on the reduction rate R. To simulate the influence of the mixing conditions, once a simple plug flow (PFR) is imposed and once a combination of a mixed flow (PSR) and a PFR with different mean residence times, i.e, the the mean residence time is splitted between the mixed reactor (PSR) and the plug flow reactor (PFR). The total mean residence time for the reburn zone and the burnout zone models is fixed for all cases to 2 s. 

The plug-flow reactor (PFR) is a mathematical model that depicts a certain type of continuous reactor operation. The model is based on three assumptions ... 

The plug flow reactor (PFR) presents the same concentration curve along the reactor length, which is shown for the tank reactor with reaction time. In the steady state, the concentrations of substrates and products at distinct positions of the reactor do not change with time. The reactor has plug flow characteristics,... 

Figure 7.1c and d show two types of the steady-state flow reactors with a continuous supply of reactants and continuous removal of product(s). Figure 7.1c shows the continuous stirred-tank reactor (CSTR) in which the reactor contents are perfectly mixed and uniform throughout the reactor. Thus, the composition of the outlet flow is constant, and the same as that in the reactor. Figure 7.Id shows the plug flow reactor (PFR). Plug flow is the idealized flow, with a uniform fluid velocity across the entire flow channel, and with no mixing in the axial and radial...

The plug flow reactor (PFR) can be envisaged as a conveyor carrying microscopic batch reactors from inlet to outlet. The erstwhile reaction time that we measured as clock time during the course of a reaction in a BR is now replaced by space time, which is a measure of how long it takes an increment of feed (the microscopic BR) to travel the length of the conveyor/reactor at a constant speed. 

We have introduced four main reactor types in this chapter the batch reactor, the continuous-stirred-tank reactor (C TR), the semi-batch reactor, and the plug-flow reactor (PFR). Table 4.3 summarizes the mole... 

To derive an energy balance for the plug-flow reactor (PFR), consider the volume element in Figure 6.32. If we write Equation 6.5 for this element and neglect kinetic and potential energies and shaft work, we obtain... 

As the main responsible for the changes in the material balance, the chemical reactor must be modelled accurately from this point of view. Basic flowsheeting reactors are the plug flow reactor (PFR) and continuous stirred tank reactor (CSTR), as shown in Fig. 3.17. The ideal models are not sufficient to describe the complexity of industrial reactors. A practical alternative is the combination of ideal flow models with stoichiometric reactors, or with some user programming. In this way the flow reactors can take into account the influence of recycles on conversion, while the stoichiometric types can serve to describe realistically selectivity effects, namely the formation of impurities, important for separations. Some standard models are described below. 

Nonlinear phenomena can be induced by material recycles. For examples, the plug-flow reactor (PFR) with recycle of a fraction of reactor s effluent can exhibit state multiplicity, sustained oscillations around a unique steady state and chaotic behaviour. If the material recycle has the temperature equal to the reactor effluent the true cause of the nonlinear phenomena is the material feedback. 

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