Process Intensification: An Overview of Principles and Practice - Process Intensification for Green Chemistry - page 16

the falling film reactor, but other reactor configurations such as the thin-film SDR or
microreactor might potentially be applied for more efficient processing of photochemical
processes. This will be dealt with in more detail in Chapter 3, where the SDR is considered.

1.4.1.2 Enhanced Surface Configurations

The influence of well-defined structures or enhanced surface configurations on the
increased efficiency of chemical processes, in terms of heat/mass transfer and mixing
rates, is well established. Such structures can be classified according to the time and length
scales at which they function. For example, molecular-scale structures such as zeolite
supports, which are termed 'molecular reactors' [6], have been used to improve chemical
transformations at molecular scales. A range of examples of such molecular reactors
applied in chemical reactions is provided in a recent paper by Van Gerven and Stankiewicz
[6]. At the meso- and macroscales, structures such as channel reactors (including milli- and
microchannels), monoliths, foams and static mixers have all been used to improve process
performances such as yield and selectivity. A brief overview of these areas is given in this
subsection; a selection of them will form the subject of later chapters in the topic.

Micro/millichannel Reactors. The use of micrometre- or millimetre-scale (typical dimen-
sions of 10-100

mand 0.5-2.0mm, respectively) reaction spaces in the formof channels of
various shapes (Figure 1.7) allows formuchmore precise control of diffusion, heat exchange,
retention/residence times andflowpatterns inchemical reactions [51].Althoughflowin these
types of reactor systems tends to be mainly laminar (Re

m

100, typically), effective mass
transfer can be achieved via thevery short diffusion path lengths, determined by the diameter
of the channels. Thus, given that the characteristic diffusion time, t D , is expressed as:

<

l 2
D l

t D ¼ (1.15)

Figure 1.7 A glass microreactor. Reproduced from [ref 52] by permission of John Wiley &

Sons. 2005.

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