New Segmented Flow Reactor Promises Baseline Nanotechnology for Coatings

By Mark Drukenbrod

Many coatings make use of precipitated minerals such as calcium carbonate and copper oxalate in their formulations. Their functions in formulations can vary widely, from moisture removers to antifoulants. Particle size and morphology weigh heavily in their effectiveness as additives.

The current process for manufacturing these ingredients involves loading the reactant suspensions of the mineral formulation into a sealed reactor vessel equipped with a compliment of axial and radial turbines to provide intense mixing. A properly mixed batch will approach bulk homogeneity, but due to localized eddy currents and dead spots, actual homogeneity is extremely difficult to obtain, especially for forced reactions in larger reactor vessels.

Poor mixing yields precipitates of wide particle size distribution, varying morphology, and generally smaller mean particle size. Add to this the inconvenience and inconsistency inherent in a batch process, and there is little wonder that much research has been expended in the search for a practical continuous flow reactor.

Up until now, coatings companies involved in the manufacture of aerospace and high technology coatings have had little choice but to reduce and normalize the particle size in their coatings using a media mill. Because the particle sizes being sought by these manufacturers are in the nanometer range, the process is time, labor and capital intensive.

Enter the segmented flow tubular reactor developed by the Laboratory of Powder Technology at the University of Lausanne with the financial support of Kemgas Ltd., a French technology company. This device promises better reaction control within the paradigm of a continuous process.

The reactor itself is made up of three distinct parts: a mixer, a segmenter and a tubular reactor. In the case of a forced precipitation reaction, such as that used to manufacture copper oxalate, the co-reactants are introduced through the mixer, where initial supersaturation occurs. A plug of air or an immiscible fluid such as kerosene segments the reacting mixture. Then the discrete small suspension volumes move through the tubular reactor, the length of which defines the residence (and thereby reaction) time.

Sometimes reactants can be mixed without precipitation before introduction into the reactor. This is the case of precipitation from a homogenous solution, where the precipitating reactant may be generated by thermal hydrolysis of an organic precursor. Nucleation and growth take place in the tubular reactor, whose length defines the reaction time. In this case, the reaction mixture is injected directly into the segmenter, and the mixer is not used.

In some reactions, particles are formed along two parallel growth processes—crystal growth and crystallite aggregation. Both crystallites and aggregates vary in size depending on the physical reaction conditions. In the case of copper oxalate, for example, larger aggregates are formed in the segmented flow tubular reactor and larger crystallites are formed in most batch reactions. This phenomenon is caused by inconsistencies in the batch process, as nucleation and aggregation kinetics are allowed to vary. Varying kinetics definitely influence particle characteristics. For this reason, the precipitate from the continuous tubular reactor exhibits a more regular morphology, even though the product of the batch and continuous process appears to be identical using laser particle size analysis.

The role of flow segmentation in the tubular reactor is demonstrated most easily with homogeneous precipitation reactions. Without segmentation, a laminar velocity gradient develops in a tubular reactor, just like with any fluid in a tube. Particles moving with various speeds according to their axial position in the tube will experience different residence times, leading to wider size distribution. The segmentation feature of the reactor ensures the same residence time for each individual microvolume of reactant.

Kemgas has been awarded several US and international patents on this process.

Kemgas sales and marketing director, Marc Janssens says "We are presently fully committed to the construction of a PCC (precipitated calcium carbonate) pilot plant to validate and demonstrate out unique new PCC technology. We are looking for industrial partners interested in the tubular plug flow reactor to help finance the step from pilot to production scale."

For more information: Marc Janssens, Kemgas Ltd. B.P. 31, 13 Chemin du Levant 01211 Ferney Voltaire, France. Tel: (0) 450 42 80 95, fax: (0) 450 40 59 61.

Graphics supplied by Kemgas Ltd.