(65d) Fouling and Long Term Stability in Micro Heat Exchangers

For a couple of years, micro heat exchangers are available not only as tools for laboratory work but also for industrial applications [1]. They are well known for their superior heat transfer properties due to the large surface-to-volume ratio (values up to 30.000 m² ∙ m^-3 have been reported, [1]). Applications in chemical and pharmaceutical industry, automotive industry and many more have been presented [2]. Furthermore, the well-known excellent temperature control of fluids achievable with microstructure heat exchangers made them interesting for applications in bio-technology. But there are still little data available on the long term stability of these devices in conventional chemical processes as well as in bio-technological processing.

In this publication, we will present several examples for micro heat exchangers made of polymer and stainless steel. The polymer devices have been manufactured by micro stereo lithography, a rapid prototyping process. Details are given in [3]. The metal devices consists out of stainless steel foils providing numerous micro channels generated by mechanical micromachining or wet chemical etching. A number of the foils are arranged in a specific way and bonded together, i.e. by diffusion bonding. Details on the manufacturing process can be found in [2].

Single microstructure layer polymer devices have been tested in a special experimental arrangement. The polymer micro channel shell was covered with a non-structured stainless steel foil, sealed by a polymer o-ring. This arrangement was screwed on top of an electrically heated aluminium block. A solution of calcium nitrate and sodium hydrogen carbonate was used as test fluid. The fluid was pumped at a flow rate of 1,5 kg/h through the channels of the microstructures, using hose fittings. The Reynolds number Re was calculated to about Re ≈ 110, thus the experiments were performed at laminar flow. The solution was heated electrically by the hot stainless steel surface. At the beginning of the experiment the temperature rises from 25°C at the inlet to 86°C at the outlet. The high temperature causes the precipitation of solid calcium carbonate. Because of the fouling layer, the heat transfer capabilities decreases.

It was expected that a calcium carbonate layer would grow more or less homogeneously on the relatively rough stainless steel surface, but not on the smooth polymer micro channel walls. But from the experiments it is obvious that fouling mainly took place at the polymer micro channels and at the outlet manifold of the test configuration.

It is assumed that the flow velocity is too low to efficiently draw away calcium carbonate particles. Some particles are ripped off and re-deposited in the outlet manifold due to a reduction of the flow velocity. Furthermore, the temperature is equalised across to the flow direction due to the low flow velocity values of about 0.3 m/s and the relatively high heat conduction of water. Therefore, precipitation of calcium carbonate crystals occurred in the bulk of the fluid. Experiments at higher Reynolds numbers ? especially in the turbulent flow regime ? might lead to other results and will be performed in future.

To obtain relevant data on multilayer devices two stainless steel crossflow heat exchangers have been investigated for their long term stability. We used an existing test rig for characterisation of microstructure heat exchangers which was described in details before [4]. Both devices have been tested for more than 8000 hours each, using deionised water as test fluid. Experimental data on the heat transfer properties and the pressure drop will be given and compared. It was found that the heat transfer capabilities were decreased by almost 50% within the first few hundred hours of testing and then ran into a saturation state, still providing good heat transfer capabilities.

As a lab-scale application example, results of tests for the quench of a mammalian cell solution will be given. This method was developed to provide a possibility to stop the metabolism of mammalian cells as rapid as possible without damaging the cells, for conventional methods like cooling with liquid nitrogen lead to a large number of disrupted cells and therefore to an increased expenditure for the analysis. No fouling was observed during the experiments with the cell suspension [5]. It will be tried to engage the method to other bio-tech processes like, e.g., temperature control of fermentation.

References

[1] Brandner, J.J., Bohn, L., Henning, T., Schygulla, U., Schubert, K., Microstructure heat exchanger applications in laboratory and industry, Heat Transfer Engineering 28 (8-9), 761-771, 2007

[2] Kraut, M., Wenka, A., Bohn, L., Schubert, K., Successful upscale of laboratory micro reactor into industrial scale, Proc. of the 6th ANQUE International Congress of Chemistry, Dec 5-7, 2006, Puerto de la Cruz, Tenerife, Spain, paper T2-L-9

[3] Brandner, J.J., Microfabrication in Metals and Polymers, Advanced Micro- and Nanosystems Vol.5, pp.267-320, Wiley-VCH, Weinheim, Germany, 2006

[4] Brandner, J.J., Schubert, K., Fabrication and Testing of Microstructure Heat Exchangers for Thermal Applications, Proc. of ICMM2005, paper 75071, 5th Int. Conf. on Micro and Minichannels, June 13-15, 2005, Toronto, Canada

[5] Wiendahl, C., Brandner, J.J., Küppers, C., Luo, B., Schygulla, U., Noll, T., Oldiges, M., A Microstructure Heat Exchanger for Quenching the Metabolism of Mammalian Cells, Chem. Eng. Techn., 2007, 30, No.3, 1-8