Process Intensification: An Overview of Principles and Practice - Process Intensification for Green Chemistry - page 20

Reactive Extraction: This is a separation process in which reaction is used to cause a
product to move into a different phase. This is usually between two liquid phases,
normally organic and aqueous, although it can be between solids and liquids. Again, this
is often a way of overcoming equilibrium limitations.
Reactive Distillation (RD): In this method, the distillation column has a second
function as a reactor, which removes the reactor from the flow-sheet. The greatest
advantage usually lies not in reducing the number of unit operations (although this can
be a significant benefit) in the plant, but in overcoming the equilibrium limitations of a
reaction, by removing the product through distillation.
Supercritical Operation: A key advantage of performing reactions in supercritical
carbon dioxide, for example, is that separation of the solvent (the CO 2 ) from the reaction
mixture becomes facile, and does not require a separate unit operation. Instead, all that is
required is that the pressure be released to the necessary degree for the reaction mixture
to come out of solution.

There are many other examples of combined reaction and separation: these are simply a
selection to illustrate the range and the possibilities for process improvement.
To focus on one example, RD, in the 200 kte 1 /yr Eastman process for methyl acetate
production, the following reaction takes place:

acetic acid þ methanol $ methyl acetate þ water (1.16)

Eastman successfully used RD to replace 11 distillation vessels (with associated con-
densers and reboilers) with just 3 RD vessels [70], resulting in significant running and
capital cost savings (as distillation columns are among the most energy-intensive unit
operations in any process). This intensification was shown to be successful because the
reaction fitted the requirements of RD, the most important of which was that one of the
products (the methyl acetate itself) had a significantly lower boiling point (57 C) than any
other species present.
When integrating process steps, as with any other intensification, the effects on the
wholeprocessshouldbeassessed.Itmaybethat the effects on the process as a whole
are minimal or even undesirable, or that the economics are not significantly affected by
the new design. However, there is a wide range of process steps that can be combined,
and with any process it is worth taking the time to assess the scope for intensification by
integrating steps.

1.4.3 Moving from Batch to Continuous Processing

Generally, continuous processing is more efficient than batch processing. Continuous
reactors are usually smaller than equivalent batch reactors for two reasons:

1. Greater Occupancy: Batch vessels are tied to the 'batch cycle', meaning that much
of their time is spent on processes such as filling, emptying, cleaning, heating and
cooling, all of which reduce their space-time yield, as during these processes the
reactor is not generating product.

1 Kilotonnes