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Project

Implementation of flow technology for continuous microencapsulation through emulsion based techniques

Microcapsules are spherical particles containing a liquid core of active ingredients, protected by a shell material. The release of core material is controlled by the shell material. These principle of microencapsulation, sparked the advent of an extensive range of new controlled release products. As these products have flooded the market, the demand of large scale production methods has increased. In both product development and commercial production of microcapsules, traditional batch reactors dominate. However transferring batch processes to a continuous process presents several benefits. Continuous operation reduces the production cost per unit and avoids batch to batch variations as it operates in steady state conditions. Furthermore, flow reactors have been proven to facilitate scale-up for the production of fine chemicals, mainly due to maintaining process conditions in larger scale equipment. This makes the process of scale-up less costly in continuous reactors, compared to batch reactors which require a time consuming multistep scale-up approach. However, continuous processes for microencapsulation are scarce. Therefore, this research project studied the implementation of flow technology for the production of microcapsules. To generate microcapsules two chemical techniques are used, the interfacial polymerisation and the in-situ polymerisation. These processes are based on a polymerisation reaction at the droplet interphase of an emulsion. The two consecutive steps required for the microencapsulation process are, emulsification and encapsulation. During the emulsification step, intensive mixing produces a certain droplet size distribution, while the second step, the shell formation, is performed in gentle mixing conditions at elevated temperatures.

Emulsification and encapsulation are first studied separately for both emulsion based microencapsulation processes. For the interfacial polymerisation process the emulsification is studied in a recycle loop driven by different pump types and static mixers. It is shown that the recycle pump dominates the droplet break-up. The shell formation of the interfacial polymerisation process takes only 6 min to be completed. The encapsulation step is therefore studied in a tubular reactor which obtains the reaction temperature in a few minutes. Curing within this tubular reactor had no major influence on the capsule size distribution. This setup successfully produces polyurea microcapsules with a hexyl acetate core, with a mean diameter of 13 µm at a flow rate of 200 g/h. For the in-situ polymerisation, the recycle loop is driven by an inline rotor-stator mixer and is compared to the batch rotor-stator mixer. The geometric differences between batch and flow devices result in large discrepancies in droplet generation. However the inline setup with recycle loop produces similar emulsions at lab scale (200 ml/h) and production scale (20 L/h), eliminating scale-up problems as rotor-stator geometry remains unchanged, while the processed flow rate is increased. The encapsulation process for the in-situ microencapsulation is performed in a multistage continuously stirred tank reactor (MCSTR). A temperature gradient is needed to avoid phase separation because the emulsion is thermally unstable in the first stage of the 150 min curing process. This gradient is realised through the jacketed construction of consecutive reactor vessels using it as a counter current heat exchanger. The thermal stability of the emulsion is improved by aging the prepolymer, which reduced the total time needed for encapsulation by 50%. Finally the two process steps of microencapsulation are combined resulting in a flow device, that produces melamine formaldehyde capsules of 10 µm mean diameter at 8.4 kg/h. Overall, emulsion based microencapsulation processes can successfully be performed an scaled in a continuous reactor, whereby the emulsion is generated by an active mixer within a recycle loop and the encapsulation is performed in a reactor tailored to the chemical system requirements.

Date:1 Oct 2013 →  4 Nov 2020
Keywords:Microencapsulation, Reactor characterisation
Disciplines:Process engineering, Polymeric materials, Catalysis and reacting systems engineering, Chemical product design and formulation, General chemical and biochemical engineering, Transport phenomena, Other (bio)chemical engineering
Project type:PhD project