(217ai) Validation of Micro-Reaction Technology for Polymerization Processes

The
structure of macromolecules produced in polymerization processes is strongly
dependent on the predefined boundary conditions in the system. In these
processes any disturbances in operating conditions of the reactor either due to
insufficient heat removal or poor mixing, even locally, will not only affect
the reaction kinetics but also the final product quality. These challenges arise
especially during the scale-up step from lab scale apparatuses to industrial
production units due to the changes imposed on the reactor geometries. It is
therefore of great interest to gather sufficient information on the state of
ongoing polymer reactions in order to conceptualize scalable systems, optimal
for precise regulation of final product characteristics.

Though
suffering from thermal and mass transfer limitations, (semi) batch reactors
have been the conventional choice for viscous medium of polymers for decades. The
inhomogeneities in these reactors increase with monomer
conversion and can never be fully overcome even by implementing energy
intensive mixing methods. Consequently occurrence of local hotspots and gel
effect is often observed in this reactor type leading to low level of control
over molecular weight distributions. As a common approach, the polymers are usually
synthesized at lower reagent concentrations and hence lower yields or alternatively
the advantages offered by tubular reactors in general and milli-reactor
as an example of it, are considered to design a continuous-working apparatus for
carrying out highly exothermic often diffusion limited polymer reactions.

Higher surface
to volume ratios as well as shorter diffusion paths in milli-reactors
minimize the mass and thermal gradients while increasing the heat removal
capacity as the limiting factor especially in case of free radical
polymerization. Greater throughputs needed for industrial production is
achieved by numbering-up strategies keeping the size of each reactor intact. These
features combined with a suitable mixing element can ensure a minimum dead zone
generation in reactor and make continuous milli-scaled
apparatuses a considerable alternative for the conventional batch ones.   

In the scope
of this work solution radical polymerization of acrylate monomers with AIBN as
initiator has been chosen as the test reaction to investigate the main design parameters
for an optimal milli-reactor layout suitable for
production of acrylate polymers.

The
thermal investigation of the system has been made possible through the
measurement of the axial temperature profile along the reactor length. For this
purpose an integrated Fiber Bragg Grating Sensor (FBGS) inside the reactor has
been utilized (Fig 1). The sensor, having the diameter of less than 3 mm
induces minimal disturbances on the flow regime but still enables the detection
of possible hotspots inside the reactor. The gathered data has been used to
optimize the employed cooling strategies needed to achieve certain product
quality (Fig 2). 

Fig 1: Fiber Bragg Grating Temperature Sensor

Fig 2: Measured Temperature Profiles

The
fundamental impact of mixing on free radical polymerization to avoid local dead
zones as well as intensifying heat and mass transfer has been demonstrated in publications.
In this work experiments have been conducted implementing different micro-mixers
with respectively different mixing efficiencies (Fig 3). As a result the influence
of mixing mechanism on the reactor operating condition as well as molecular
weight distribution of the final product has been determined. 

Fig 3: An Example for implemented Micro Mixers,

LH2, Ehrfeld Mikrotechnik BTS GmbH