Scale-Up in Batch and Continuous-Flow

Most examples of microwave-assisted chemistry published to date and presented in this book (see Chapters 6 and 7) were performed on a scale of less than 1 g (typically 1-5 mL reaction volume). This is in part a consequence of the recent availability of single-mode microwave reactors that allow the safe processing of small reaction volumes under sealed-vessel conditions by microwave irradiation (see Chapter 3). While these instruments have been very successful for small-scale organic synthesis, it is clear that for microwave-assisted synthesis to become a fully accepted technology in the future there is a need to develop larger scale MAOS techniques that can ultimately routinely provide products on a multi kg (or even higher) scale. 

In this chapter, microwave scale-up to volumes 100 mL in sealed vessels is discussed. An important issue for the process chemist is the potential for direct seal-ability of microwave reactions, allowing rapid translation of previously optimized small-scale conditions to a larger scale. Several authors have reported independently on the feasibility of directly scaling reaction conditions from small-scale singlemode (typically 0.5-5 mL) to larger scale multimode batch microwave reactors (20-500 mL) without reoptimization of the reaction conditions [24, 87, 92-94], 

Recently published examples of continuous-flow organic microwave synthesis include, for example, 1,3-dipolar cydoaddition chemistry in the CEM CF Voyager system (see Figs. 3.23 and 3.24). The cycloaddition of dimethyl acetylenedicarboxy-late with benzyl azide in toluene was first carefully optimized with respect to solvent, temperature, and time under batch conditions. The best protocol was then translated to a continuous-flow procedure in which a solution 0.33 m in both build- 

2035 C.-J. Li, T.-H. Chan, Organic Reactions in Aqueous Media, Wiley, New York, NY, 

Bhimalapuram, K. Koga, Phys. Chem. Chem. Phys. 2003, 5, 3085-3093. 

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Conventional batch and continuous-flow peptide synthesizers typically produce one to three peptides at a time in quantities from 0.025 to 2 mmol. To satisfy the demand for a greater number of sequences, robotic instruments have been adapted for peptide assembly which are able to construct 8 to 144 peptides simultaneously. Production scale synthesizers have also been designed that are able to prepare up to 5 moles of a peptide. 

Ultrasound can be scaled up to large volumes in batch or continuous flow systems. It can also be combined with an extruder, where it speeds up extrusion and allows the processing of polymers of higher molecular weight that may have improved tensile strength. It is a technique that deserves to be used more often, for it can often save money by reducing reaction times. 

This chapter treats the effects of temperature on the three types of ideal reactors batch, piston flow, and continuous-flow stirred tank. Three major questions in reactor design are addressed. What is the optimal temperature for a reaction How can this temperature be achieved or at least approximated in practice How can results from the laboratory or pilot plant be scaled up ... 

Methane can be oxidatively coupled to ethylene with very high yield using the novel gas recycle electrocatalytic or catalytic reactor separator. The ethylene yield is up to 85% for batch operation and up to 50% for continuous flow operation. These promising results, which stem from the novel reactor design and from the adsorptive properties of the molecular sieve material, can be rationalized in terms of a simple macroscopic kinetic model. Such simplified models may be useful for scale up purposes. For practical applications it would be desirable to reduce the recycle ratio p to lower values (e.g. 5-8). This requires a single-pass C2 yield of the order of 15-20%. The Sr-doped La203... 

In building up multiple units (Sections 5.1.2 and 5.1.3), alternative systems of electrical and hydraulic connections are possible, each of which have advantages. The choice often depends on the scale of the process and whether batch or continuous operation is to be used. In batch processes with flow electrolysis, the conversions per cell pass are usually small and the type of hydraulic connection will depend on mechanical aspects of reactor design and operation. With parallel plate cells, parallel electrolyte flow is certainly the most common option. In continuous processes the type of hydraulic connections—series or parallel—may affect the production capacity of a unit. Further information can be found in Picket s book" on reactor design. 

We will consider only the batch reactor in this chapter. This is a type of reactor that does not scale up well at all, and continuous reactors dominate the chemical industry. However, students are usually introduced to reactions and kinetics in physical chemistry courses through the batch reactor (one might conclude fi om chemistry courses that the batch reactor is the only one possible) so we wiU quickly summarize it here. As we vrill see in the next chapter, the equations and their solutions for the batch reactor are in fact identical to the plug flow tubular reactor, which is one of our favorite continuous reactors so we will not need to repeat all these definitions and derivations in the section on the plug flow tubular reactor. 

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