(74b) Microstructured Mixing Devices: an Efficient Tool for the Determination of Chemical Kinetic Data?

Entwurf eines Abstracts zum AICHE spring meeting, Houston , Texas, USA 22.-26. 4.2007

Microstructured Mixing Devices:

An Efficient Tool for the Determination of Chemical Kinetic Data?

K. Hecht¹⁾, M. Kraut²⁾, A. Kölbl²⁾

1.) Department of Chemical Engineering, University of Utah, 50 South Central Campus Drive, Room 3290 MEB, Salt Lake City, UT 84112, http://www.che.utah.edu

2.) Institute for Micro Process Engineering (IMVT), Forschungszentrum Karlsruhe GmbH, Hermann-von-Helmholtz Platz 1, 76344 Eggenstein-Leopoldshafen, Germany, http://www.fzk.de/imvt-en

The product distribution of fast, parallel or consecutive chemical reactions heavily depends on mixing.

Bourne and Kozicki have shown in 1977, utilizing stirred laboratory flasks, that the consecutive, competitive bromination of 1,3,5-trimethoxybenzene (TMB) to 1-bromo-2,4,6-trimethoxybenzene (1BR) and 1,3-dibromo-2,4,6-trimethoxybenzene (2BR) (Figure 1), yield mixing sensitive product distributions [Bourne 1977].

Figure 1: The Mixing Sensitive Bromination of 1,3,5-Trimethoxybenzene

Due to the deactivating effect of the Br substituent, the first bromination is much faster than the second (k₁ >> k₂). With better mixing more 1BR and less 2BR are formed in the resulting mixture. Hence mixing processes can be judged by the ratio 1BR/2BR.

Bourne and Kozicki observed 1BR/2BR ratios between 2.9 and 5.6 depending on the stirring speed used [Bourne1977].

Kinetic data like rate constants (k₁or k₂) or ratios of rate constants (k₁/k₂) can be derived from the experimentally obtained product distributions. Prerequisite to the determination of kinetic data is a sufficiently short mixing time compared to the reaction time. Otherwise mixing masked kinetic values are obtained.

Commonly used devices for the examination of fast chemical reactions are ?stopped flow apparatuses?. Bourne reports a 1BR/2BR ratio of about 10 for mixing in a stopped flow apparatus [Bourne2003].

We have applied the bromination reaction system for static mixers using Bourne's initial concentrations (0.05 mol/L TMB and bromine in methanol respectively). Using a cyclone type mixer (Figure 2) manufactured of PTFE, we observed 1BR/2BR ratios up to 100 indicating better mixing in the cyclone type mixer compared to the stopped flow mixing results reported by Bourne [Bourne2003].

Details of the cyclone type mixer can be seen in Figure 2 a-c. The height of each feed loop is 103 μm; the height of the exit loop is 145 μm. A detail of the feed loop feeding into the swirl chamber is shown in Figure 2c. The swirl chamber is 0.5 mm in diameter and 5 mm long. The feed loops fed into the swirl chamber in such a way that a three-dimensional rotational flow is created in the swirl chamber.

Figure 2a-c:

a: Cyclone mixer schematic

b: Cyclone mixer and housing; top plate is the exit, bottom plate contains the feeds

c: Optical micrograph of the edge of feed loop to the swirl chamber

[Bourne1977]

J.R. Bourne and F. Kozicki;

?Mixing Effects during the Bromination of 1,3,5-Trimethoxybenzene?

Chemical Engineering Science 32(1977)1538-1539.

[Bourne2003]

?Mixing and the Selectivity of Chemical Reactions?

Organic Process Research & Development 7(2003)471-508.