Mixing and dispersion

    mixing-and-dispersion-vapourtec

    Without detailed knowledge of reaction engineering it is difficult to judge the importance of mixing and dispersion to the outcome of the reaction. If the reaction has a long residence time at high temperature and both the starting materials and product are stable then we can probably conclude that our reaction will not be sensitive to mixing or dispersion. However if the reaction is complete in < one minute and / or the reaction is run at a temperature well below ambient we can start to assume that mixing may be a critical factor.
    Kevin D. Nagy et al documented their excellent work in this area and anyone involved in scaling flow reactions could do much worse than study the paper referenced below.



    Kevin D. Nagy†, Bo Shen‡, Timothy F. Jamison*‡, and Klavs F. Jensen*†
    †Departments of Chemical Engineering and ‡ChemistryNovartis−MIT Center for Continuous Manufacturing, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
    Org. Process Res. Dev., 2012, 16 (5), pp 976–981
    DOI: 10.1021/op200349f
    Publication Date (Web): January 13, 2012
    Copyright © 2012 American Chemical Society

    For mixing critical reactions Vapourtec recommends the use of micromixer chip reactors. These devices have limitations in scale up but give access to high rates of mixing even when the reagents are at low temperature (when Brownian motion is slowed).
    For less mixing critical reactions Vapourtec systems generally use tubular reactors under laminar flow conditions relying on diffusion for radial mixing. To quantify dispersion effects and the mixing characteristics of these reactors it is necessary to measure the residence time distribution (RTD).

     

    mixing-and-dispersion-vapourtec-reactor

    The picture on the left shows a typical Vapourtec reactor.  The reactor uses a fluoropolymer tube extruded from high purity Perfluoroalkoxy (PFA).

    Most reactors used by Vapourtec have a bore of 1.0 mm as this has been found to offer an acceptable compromise between reactor volume, mixing performance and resistance to blockages which can result from small quantities of precipitates.

    A 10 ml reactor has a total length of 12.73 m.


    mixing-and-dispersion-vapourtec-graph

    When scaling up tubular reactors it would be tempting to stay with small diameter (~1.0 mm) tubing a use very long reactors to achieve the high through puts required. Unfortunately much above 40 ml, the back pressure generated by 1 mm bore reactors becomes highly significant. The obvious alternative is to increase tube diameter, unfortunately the mixing performance drops rapidly with increasing tube diameter.

    For scale-up reactors Vapourtec favour the static mixer as shown at the top of the page.

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