Scaleup of Batch Reactions

Section 1.5 described one basic problem of scaling batch reactors namely, it is impossible to maintain a constant mixing time if the scaleup ratio is large. However, this is a problem for fed-batch reactors and does not pose a limitation if the reactants are premixed. A single-phase, isothermal (or adiabatic) reaction in batch can be scaled indefinitely if the reactants are premixed and preheated before being charged. The restriction to single-phase systems avoids mass 

Section 5.3 discusses a variety of techniques for avoiding scaleup problems. The above paragraphs describe the simplest of these techniques. Mixing, mass transfer, and heat transfer aU become more difficult as size increases. To avoid limitations, avoid these steps. Use premixed feed with enough inerts so that the reaction stays single phase and the reactor can be operated adiabatically. This simplistic approach is occasionally possible and even economical. 

The numerical methods in this book can be applied to all components in the system, even inerts. When the reaction rates are formulated using Equation (2.8), the solutions automatically account for the stoichiometry of the reaction. We have not always followed this approach. For example, several of the examples have ignored product concentrations when they do not affect reaction rates and when they are easily found from the amount of reactants consumed. Also, some of the analytical solutions have used stoichiometry directly to ease the algebra. This section formalizes the use of stoichiometric constraints. 

Scaleup relationships for stirred tanks, both batch and continuous flow, are given in Section 4.4. 

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The curves in Figure 5.2 are typical of exothermic reactions in batch or tubular reactors. The temperature overshoots the wall temperature. This phenomenon is called an exotherm. The exotherm is moderate in Example 5.2 but becomes larger and perhaps uncontrollable upon scaleup. Ways of managing an exotherm during scaleup are discussed in Section 5.3. 

Small steady-state reactors are fiequently the next stage of scaleup of a process from batch scale to full commercial scale. Consequently, it is common to follow batch experiments in the laboratory with a laboratory-scale continuous-reactor process. This permits one both to improve on batch kinetic data and simultaneously to examine more properties of the reaction system that are involved in scaling it up to commercial size. Continuous processes almost by definition use much more reactants because they run continuously. One quickly goes from small bottles of reactants to barrels in switching to... 

Chapter 2 developed a methodology for treating multiple and complex reactions in batch reactors. The methodology is now applied to piston flow reactors. Chapter 3 also generalizes the design equations for piston flow beyond the simple case of constant density and constant velocity. The key assumption of piston flow remains intact there must be complete mixing in the direction perpendicular to flow and no mixing in the direction of flow. The fluid density and reactor cross section are allowed to vary. The pressure drop in the reactor is calculated. Transpiration is briefly considered. Scaleup and scaledown techniques for tubular reactors are developed in some detail. 

Reaction kinetics, catalyst handling, mass and heat transfer, corrosion and many other practical industrial chemistry and engineering considerations impact the success of scaleup from lab to commercial for batch processing. Since the starting point for scaleup studies is the ultimate intended commercial unit, the professional should scaledown from the design parameters and constraints of the proposed commercial unit. 

However, each set of factors entering in to the rate expression is also a potential source of scaleup error. For this, and other reasons, a fundamental requirement when scaling a process is that the model and prototype be similar to each other with respect to reactor type and design. For example, a cleaning process model of a continuous-stirred tank reactor (CSTR) cannot be scaled to a prototype with a tubular reactor design. Process conditions such as fluid flow and heat and mass transfer are totally different for the two types of reactors. However, results from rate-of-reaction experiments using a batch reactor can be used to design either a CSTR or a tubular reactor based solely on a function of conversion, -r ... 

The main problem with a living polymer is maintaining the strict cleanliness that is demanded by the chemistry. This is a particularly severe problem for large-scale batch polymerizations, but it is a problem more of economics than technology. Living polymerizations are usually run to near completion, so that end-point control is not a problem. Most living polymerizations operate at low temperatures, —40 to - -40°C, to avoid chain transfer reactions. Thus temperature control is a significant scaleup problem. The usual approach is to use 85-95 w % solvent and to rely on sensible heat transfer to the vessel walls. 

Go from batch to continuous operation. Scaleup is easier, especially for postreactor processing techniques that are typically continuous. For a stirred-tank reactor, the reaction rate will decline because the entire reaction is conducted at the highest conversion and thus, typically, at the lowest reaction rate. However, the decline in rate is more than compensated for by the full time utilization of the reactor compared to occasional utilization in a batch process. 

As a result of the increased illumination as well as the increased gas-liquid contact area per unit volume, the triple-channel microreactor exhibited better performance compared to the batch reactor, or even compared to a typical dual-channel microreactor [138,139]. Moreover, the scaleup process using the microreactor revealed higher productivity than the batch reactor, which would be valuable for the practical applications in a broad range of gas-liquid chemical reactions. 

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