Reactive mixing stirred tanks

In order to calculate the rate of polymerisation, the monomer concentration in the particles, [M,]p, the average number of radicals per particle, n, the number of particles, Np, as well as the reactivity ratios should be available. The calculation of [M,]p has been described in Section 4.2. The calculation of n and Np can be found in Chapters 2 and 3 of this book. Note that to calculate the time evolution of the instantaneous copolymer composition, the time history of the variables [M,]p, n and Np must also be known. The unreacted or free monomer present in the reactor can be computed from the general macroscopic material balance for a perfectly mixed stirred tank reactor by omitting inlet and outlet streams ... 

For typical reactive mixing systems in a stirred tank, a dual impeller system, Figure 9.23, with a radial-flow impeller at the bottom and an axial flow on the top is recommended [31-33], We recommend the following ratios as initial starting points for a typical batch with liquid height equal to tank diameter ... 

Stirred tank case, it has been difflcnlt to find experimental evidence of segregation for single-phase reactions. Real CSTRs approximate perfect mixing when observed on the time and distance scales appropriate to indnstrial reactions provided that the feed is premixed. Even with unmixed feed, the experimental observation of segregation requires very fast reactions. The standard assumption of perfect mixing in a CSTR is usually justified. Worry when a highly reactive component is separately fed and when the reaction is sensitive to mixing time. See Section 4.6. 

Despite all these complexities, the stirred tank is successfully used in many sectors of the chemical and process industries. It is flexible, of moderate cost and ultimately controllable. Many processes are batch, and poor scale up can always be countered by longer batch times or turning up the impeller speed. This can however never completely disguise losses in selectivity and reactivity arising from imperfections in the mixing and the resulting transport limitations. 

Analogously to batch distillation and the RCM, the simplest means of reactive distillation occurs in a still where reaction and phase separation simultaneously take place in the same unit. Additionally, we can choose to add a mixing stream to this still, and the overall process thus consists of three different phenomena chemical reaction, vapor liquid equilibrium, and mixing. Such a system is referred to as a simple reactive distillation setup. This setup is shown in Figure 8.1 where a stream of flowrate F and composition Xp enters a continuously stirred tank reactor (CSTR) in which one or more chemical reaction(s) take place in the liquid phase with a certain reaction rate r =f(kf, x, v) where v represents the stoichiometric coefficients of the reaction. Reactants generally have negative stoichiometric coefficients, while products have positive coefficients. For example, the reaction 2A + B 3C can... 

The experimental setup, as shown in Fig. 8.6 can also be used for reactive extrusion with prepolymerization (3). In a stirred tank reaetor that can be heated and cooled the mixture is allowed to prepolymerize thermally (without the addition of initiator). This prepolymerization oeeurred batchwise. The contents of the reactor were heated to 135°C, and onee the thermal polymerization started, it was eooled to maintain this temperature. At a conversion of 25% the reactor was cooled to 60°C to stop the reaction. The tubing from this reactor vessel to the extruder was heated to avoid viscosity problems. In another vessel the initiators were dissolved in a small amount of monomer mixture at room temperature. The prepolymer and the monomer-initiator mixture were pumped to the extruder in a ratio of 10 to 1 and mixed in a small statie mixer at the feed port. The volume of the static mixer was chosen such that the residence time in this mixer was in the order of one second to prevent polymerization inside the mixer. 

In addition to processes involving gas-liquid reactions, stirred-tank reactors can also be used for single (liquid)-phase reactions. Moreover, their operation is not limited to the continuous mode, and they can be easily adapted for use in semibatch and batch modes. The absence of a gas phase does not pose important structural and operational differences from those stated earlier for multiphase systems. However, in the case of single-phase operation, the aspect ratio is usually kept lower ( 1) to ensure well mixing of the reactive liquid. Regardless of the number of phases involved, stirred-tank reactors can approach their ideal states if perfect mixing is established. Under such conditions, it is assumed that reaction takes place immediately just... 

Understanding the interplay of the three mechanisms—reaction, diffusion, and convection—is essential. In light of what happens in 2D reactive chaotic flows, we can attempt to extend the analysis to 3D reactive mixing applications. Let us take a fresh look at a common 3D example, a mixing tank stirred with three... 

Figure 3-29 The evolution of a fast reaction in a mixing tank stirred with three Rushton impellers at Re = 20. The reactive zones in a stirred tank are identieal to the location of the intermaterial contact area in a mixing after 10, 20, 40, and 60 impeller revolutions. Each figure shows half of the vertical cross-section, where the shaft and three impeller blades are seen on the left. The upper half is a photograph of the reactive flow and the bohom is the computed chaotic mixing structure.

---