Controlling Residence Time

Researchers will be freed from the limitations posed by the flask chemistry and will develop, with great anticipation, synthetic reactions that have been impossible with flasks, and will reveal the true values of flow reactions in the near future. Considerations needed in developing such flow reactions by taking advantage of characteristic features of flow microreactors will be discussed in the following chapters. 

Chapter 1 describes the need to depart from flask reactions that researchers have long been accustomed to and move our focus on flow reactions to understand flow microreactor synthesis. This chapter describes one of the specific characteristics of flow reactions, i.e, residence time, and explores how such a characteristic can be used in controlling chemical synthesis. 

A flow reaction is usually carried out at a steady state after some period of time passes from the initiation of the operation. At a specific location in the reactor, the concentrations of chemical species including starting materials, products, and reaction intermediates are temporally constant. However, the concentrations of the chemical species differ at different locations in the reactor. In contrast, the 

2 Monitoring the Progress of a Reaction in a Flow Reactor Inline Analysis 

A question now is how we can monitor the progress of a reaction occurring in a flow reactor. The reactor using a microchip may be entirely imaged to determine the concentrations of specific chemical species at various locations. However, imaging the microtube reactor is not so easy. Also, types of chemical species that can be imaged are rather limited, and the accuracy of the method depends on the nature of the chemical species that we want to quantify. The concentrations of chemical species at different locations in a flow reactor may also be measured by sensors or measuring devices installed at the different locations in the reactor (inline analysis). A simpler method is to measure the concentrations by inline analysis at the exit of the reactor. 

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The above-mentioned targets refer to general advantages of micro reactors [42, 80, 100, 114, 119]. Enhanced transfer and better controlled residence time improve conversion and selectivity. The tools have small internal volumes, allowing one to generate flexibly a multitude of samples in serial or parallel fashion. Synthesis can be combined with a multi-step procedure. The economy of micro-reactor processes has not really been analyzed so far however, it is clear that as laboratory tools they allow in a number of cases technical expenditure, personnel and costs to be reduced. 

An important feature is the ability of the roller grooves to aid the transport of material through the mill, thus providing a means to control residence time and mill capacity. This transporting action is particularly important with materials which would not readily flow by gravity. Pastes and sticky materials fall into this category. 

For flat die extrusion of sheet, critical variables are temperature control, residence time and flow channel streamlining. Recent developments have been presented (175, 176). Sheet and film extrusion lines include cooling and polishing rolls. 

Twin-screw extruders (with the exception of most counterrotating profile extruders) are designed to be starve fed. Therefore, throughput is independent of screw speed. This permits the processor to control residence time, degree of fill, and specific energy input (kw/kg). 

Plastic microcapillary flow disk (MFD) reactors have been constructed from a flexible, plastic microcapillary film (MCF), comprising parallel capillary channels with diameters in the range of 80-250 jxm. MCFs are wound into spirals and heat treated to form solid disks. These reactors are capable of carrying out continuous flow reactions at elevated temperatures and pressures with a controlled residence time. ... 

Selectivity, which is one of the most important characteristics of an industrial process, depends on several parameters temperature control, residence time distribution, gas and liquid hold-ups, catalyst loading, catalyst type, mass transfer rates etc... If homogeneous side reactions are awkward, fixed beds give better results.But if the desired product can react further on the catalyst, small catalyst particles have to be preferred to avoid concentration gradients in the pores and slurry reactors are the best. In this last case, the poor residence time distri-... 

An example of a microflow continuous palladium-catalyzed Mizoroki-Heck coupling between iodobenzene with butyl acrylate, in combination with continuous microextraction/ catalyst recyclation was reported by Liu and coworkers (Liu et al., 2004). Their reaction was catalyzed by a [Pd(PPh3)Cl2(BMIM)] carbene complex, which was immobilized in the low-viscosity ionic liquid l-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIM][(CF3S02)2N]). Using automated microflow apparatus (Figure 7), iodobenzene, butyl acrylate, and tripropylamine were introduced from one inlet of the micromixer (channel width 0.1 mm, inner volume 2 mL), and the ionic liquid containing the Pd catalyst was introduced from the other inlet. Two solutions were mixed in the microreactor and were pnimped into the temperature controlled residence time unit. 

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