Green Chemistry in Oscillatory Baffled Reactors - Process Intensification for Green Chemistry - page 165

this case, the safety was further improved by the fact the new operating temperature of
85 C was below the ambient-pressure boiling point of the solvent. The lower temperature
operation would also result in some energy savings, although the reduction would
be primarily due to the more efficient heat transfer and reduced residence time.
The size reduction in Table 5.1 was due to the decrease in required residence time and
the increase in occupancy time (100% for a continuous reactor, less than 10% for this
particular batch reactor).
A subsequent, similar study showed that similar benefits could be achieved for the
'biodiesel reaction' - that is, transesterification of vegetable oil using methanol [7] - as
the reaction time could be reduced to 15 minutes. More recent studies in mesoscale
OBRs have demonstrated that even this can be outdone, as it has become clear that the
residence time can be made as low as 2 minutes [8], resulting in a further 7.5-fold
reduction in reactor size. In so doing, the mesoreactor was used to determine the kinetics
of the biodiesel reaction and competing saponification reactions to show exactly why
this was possible: under the correct conditions the saponification is relatively slow
compared to the transesterification. This was only realizable in a reactor with extremely
well-defined mixing.

5.2.1.1 Green Chemistry Elements

This case study illustrates four aspects of green chemistry:

(1) Atom Economy: Greater selectivity for the desired product was demonstrated,
leading to reduced overall process waste and a reduced load on downstream
separation steps.
(2) Design for Energy Efficiency: The energy costs of constructing and running the
reactor were substantially reduced, as it was less than one-hundredth the volume of
the reactor that it could replace.
(3) Real-time Analysis for Pollution Prevention: As this was a continuous reactor, it
made sense to develop real-time analysis. An online infrared monitoring technique
was thus implemented, which was able to monitor the concentration of the main
reactant and product.

(4) Inherently Safer Chemistry for Accident Prevention: The OBR-based process no

longer required large inventories of solvent, nor that the solvent be above its boiling
point in operation.

5.2.2 A Three-phase Reaction with Photoactivation for Oxidation of Waste Water
Contaminants

At the University of Cambridge in the mid to late 1990s, an OBR was developed for an
application involving gas, liquid and solid phases, with in situultraviolet (UV)
irradiation (Figure 5.9). The liquid was water containing hydrocarbons, mimicking
typical waste water contaminants. The solid was a titanium dioxide catalyst that required
UV activation to catalyse oxidation of the hydrocarbons in the water. The oxidizing
agent was air, bubbled through the column. This illustrates a number of potential
advantages of OBRs, and that an OBR can be designed to exploit many of them
simultaneously: