Application of micromixers - Micromixers Fundamentals Design and Fabrication - page 325

micromixers brings advantages for the safety of the highly exothermic fluorination process with
fluorine. Chambers et al. [22] introduce the liquid substrate and fluorine in nitrogen into a mixing
channel. The two phases form a pipe flow where the outer ring is formed by the liquid. For a higher
throughput, the number of mixing channels was scaled up. Fluorination of ethyl acetoacetate was
performed in a nine-channel microreactor and lasted for several months [23] .
Radicals for chlorination are induced thermally [24] or by irradiation [25] . Photochemical chlo-
rination of toluene-2,4-diisocyanate was carried out in a microreactor with 32 parallel microchannels.
Gas-phase chlorine was irradiated through a quartz window to form chlorine radicals. The space-time
yield of this process is two orders of magnitude higher than that of a conventional reactor. The
selectivity and yield of monochlorination of acetic acid are much better in microreactors than in the
conventional counterpart [24] .
Addition reaction such as hydrogenation involved gas-phase hydrogen. Cyclohexane hydroge-
nation was carried out in micromixer made of silicon [26] . The reaction occurred with platinum on
alumina as catalyst under pressure and temperature close to standard room condition. A higher
production of cyclohexane can be carried out with palladium as catalyst and under higher pressure and
temperature [27] . The small amounts of reactants involved allow the potential use of micromixers for
addition reactions with toxic reagents such as HCN for hydrocyanation and CO for carbonylation.
Dehydration reactions can also benefit from the advantages offered by micromixers. The dehy-
dration of 1-hexanol to 1-hexene was carried out in a micromixer [15] . Compared to conventional
dehydration processes, the conversion efficiency was significantly increased from 30% to over 85%.
The same concept was applied to dehydration of ethanol to form ethylene, ethane, and methane.

9.1.4 Polymerization reactions
Polymerization reactions link up small monomers into a long polymer. Common polymerization
reactions that can be realized in micromixers are [1] :

Free radical polymerization,

Living radical polymerization, and

Cationic polymerization.

Free radical polymerization (FRP) is a method to form a polymer by successive addition of free
radical building blocks. Free radical monomer units can be formed by a number of different mecha-
nisms. Subsequently, polymer chains grow with the successive addition of building blocks onto the
free radical sites. Free radical polymerization is the most versatile form of polymerization available.
FRP follows three basics steps: initiation, propagation, and termination. In the initiation step,
a molecule called initiator breaks the double bond of a monomer molecule, forming the first unit of the
polymer chain. The first unit subsequently reacts with the double bond of another monomer molecule.
This process repeats in a chain reaction to form the polymer. The process is called propagation. The
termination step occurs if the radical end groups of two growing chains meet. Most of the common
polymers are synthesized by FRP of acrylic, vinylic, and stryrenic monomers. FRP reactions are
exothermic. Under uncontrolled conditions, thermal runaway or temperature increase may lead to
explosion. The small amount of reactants and the improved and controlled mass and heat transfer in
micromixers allow FRP to be carried out in a safe manner. Furthermore, the controlled reaction
condition in micromixers would maintain the quality of the polymers formed. Good mixing prevents