Monolith Reactors for Intensified Processing in Green Chemistry - Process Intensification for Green Chemistry - page 182

lead to higher atom efficiency and decrease the production of waste byproducts for
disposal.
Alternative Process Routes: Monolith reactors may enable a hazardous process route to
be converted to a less hazardous one. This might occur, for example, if monolith reactors
make feasible the oxidation of organic chemicals using molecular oxygen under mild
conditions [17], compared with the alternative route of aggressive peroxides [41] or
extreme reaction conditions of high temperature or pressure.
Lower Running Costs, Saving Process Energy: The pressure drop in monolith channels
is lower than that for alternative catalytic reactors, such as trickle beds, which leads to
lower requirement for gas compression and pumping. This can lead to savings in the
electricity used for pumping. However, monolith catalysts do have a higher capital cost
compared with pelleted catalysts.
Improved Heat Transfer: For applications such as hydrogenation reactions, it may be
necessary to cool the reaction mixture in order to keep the temperature under control. For
strongly exothermic or fast reactions, an external heat exchanger can be used, located in
the liquid circulation loop [24]. Alternatively, compact heat-exchanger reactors have
been developed, which are a variant of monolith reactors in which alternating channels
of a labyrinth design are used for the flow of the reaction mixture and the heating or
cooling of fluid [42]. If the heat can be used elsewhere in the plant through process
integration principles, such as the coupling together of endo- and exothermic reactions,
the overall energy requirements of the plant may be decreased [43].
Safer Processing: Monolith reactors present an inherently safer design than stirred tank
reactors. Cybulski et al. [24] has summarized their advantages as: (1) lower reaction
volume of hazardous material; (2) fail-safe operation, since in the event of pump failure
the reaction stops automatically because the liquid drains down from the reaction zone;
(3) avoidance of risky filtration operations (involving pyrophoric materials in the fine
chemicals industry); and (4) improved control of temperature, preventing thermal runaway.
Reduction of Emissions: Even though according to the Principles of Green Chemistry
one should try to avoid the production of waste products, monoliths present a good
catalyst design for end-of-pipe pollutant removal due to their low pressure drop. This
makes them suitable for the removal of pollutants from a range of media, including car
exhaust emissions, NO x in stationary combustion sources [9] and oxidized pharmaceu-
tical intermediates in wastewater [44].
Process Miniaturization and Local Manufacture: The intensification of processes
using monoliths may enable the development of small chemical reactors or distributed
power plants [45], rather than a large centralized site such as an oil refinery or coal-
fired power station. These portable plants could be operated at dispersed locations or
even transported by trucks to the locations of different feedstocks. This type of
localized operation could lead to lower transport distances of raw materials and
products. Additionally, monoliths could be used in mobile applications, such as the
cleaning of air in aircraft cabins [46] or the reforming of ethanol and dimethyl ether
for onboard hydrogen production in a hydrogen vehicle [47]. The latter type of
technology will help to enable the transformation of the carbon-based economy to the
hydrogeneconomyandcouldhelptoreducereliance on fossil fuels, depending on
the method of hydrogen generation.