Process Intensification: An Overview of Principles and Practice - Process Intensification for Green Chemistry - page 10

In many processes, the heat and mass transfer are interrelated. Generally, what enhances
one enhances the other. Indeed, the mechanisms for transfer are often the same or are
closely related. Experiments in heat transfer have often been used to draw conclusions
about mass transfer (and vice versa) through analogies. Various equations describing one or
the other are based upon analogy. Compare for instance the Dittus-Boelter equations for
heat and mass transfer:

Re 0 : 8 Pr 0 : 33

Heat Nu ¼ C 1 : : (1.13)

:

Re 0 : 8 Sc 0 : 33 Mass : Sh ¼ C 2 : : (1.14)

An example of an intensified mass-transfer device is the rotating liquid-liquid extractor.
The conventional design of liquid-liquid extractors was based on using the density
difference between the liquids to drive a countercurrent flow, by inputting the denser
fluid at the top of the column and the lighter at the bottom. One of the variables, although
it may not appear to be a variable initially, is g, the acceleration due to gravity. This can
of course be increased by applying a centrifugal field, in which case the lighter fluid is
introduced from the outside and travels inward countercurrent to the denser fluid. The
first example of this kind of device was the Podbielniak liquid-liquid contactor,
originally developed in the 1940s for penicillin extraction. There are currently hundreds
of Podbielniak contactors in use worldwide for a range of applications, including
antibiotic extraction, vitamin refining, uranium extraction, removal of aromatics, ion
exchange, soap manufacture and extraction of various organics [21]. This illustrates that
there are many successful examples of PI in industry today, although they are not viewed
as such, as they are not a new technology (and the term 'process intensification' did not
exist when they were invented). Indeed, any continuous process is an example of an
intensified process.

1.3.3.3 Momentum Transfer

Momentum transfer occurs due to velocity gradients within fluids. Many of the tech-
nologies listed above to enhance mass and heat transfer, also involve enhanced momentum
transfer. Again, as illustrated by the equations in section 1.3.3.2 (between heat and mass
transfer), there are analogies between this transfer process and others that lead to
meaningful quantitative relationships. Theories such as the Reynolds analogy (see Ref
[22] for a concise explanation), and its more sophisticated and accurate descendants, are
based on heat, mass and momentum transfer processes having the same mechanism: in this
particular analogy, the mechanism for all is the transport of turbulent eddies from a bulk
medium to a surface.
Essentially, any technology that enhances the flow increases momentum transfer. The
rotational fields applied to flows in section 'Centrifugal Fields' (see section 1.4.1.1) and
the turbulence promoters mentioned in 1.3.3.1 are just two examples of enhanced
momentum transfer (along with enhancement of other transfer properties). It should be
noted that enhancement of momentum transfer is often not performed for its own sake,
but rather to promote other transfer properties.