Miniaturized thermal flow and impedimetric sensors for the inline chemical process analysis in micro-plants

In parallel to the advances in micro-fabrication techniques chemical processes in micro-reactors and micro-plants became more and more of interest. Understanding of the static and dynamic behavior is fundamental for the development, optimization and control of novel integrated chemical processes. Up to now there is a lack in sensitive, robust and low priced sensor chips for the inline monitoring of chemical processes in micro-reactors and micro-plants. In conventional batch reactors chemical conversion is monitored as a function of time, here it is a function of position in the micro-fluidic reaction channel.

In this work three novel sensor systems were developed based on capillary-type thermal flow sensors and thermoelectric flow sensors combined with impedimetric sensors. It involves two residence time micro-reactors and a micro-fluidic measurement chamber. One reactor contains a network of 6 capillary-type thermal flow sensors for low mass flow rates. The second one contains a multi-parameter micro-sensor network comprising of 8 sensing positions with pairwise organized thermoelectric flow and impedimetric sensors. The micro-fluidic measurement chamber as third system represents a single sensing position of the network. Selected physics of fluid flow and heat transfer in micro-fluidic channels as weil as of dielectrics and electrolytes in electric fields were reviewed. Principles of established miniaturized thermal flow and impedimetric sensors were concerned.

Capillary-type thermal flow sensors were derived from commercially available mass flow sensors. Based on a finite element model the convective and conductive heat transfer/transport in the sensor were analyzed comparing the single and dual heater mode of the device. Parameters of interest were e.g. the gap in between both heater, flow velocity and thermal properties of the liquid. Based on an existing thermoelectric flow sensor, a new chip with a perforated membrane was developed. The perforation enables medium flow on both sides and hence overcomes the limited pressure stability of the existing chip. A finite element and equivalent circuit model were derived and the heat transfer and fluid f10w on both sides of the perforated membrane were simulated. Parameters of interest were e.g. the perforation size, flow velocity and thermal properties of the liquid. Robust impedimetric sensors based on silicon and ceramics technology were developed. The electric field and capacitance/conductance in the presence of a conductive micro-reaction channel close to the interdigital electrode were simulated. Special focus was on the comparison of both sensor fabrication technologies and the impact of the channel height, passivation thickness, electrode gap and the electric properties of the liquid. Equivalent circuit models for chips integrated in a complex voltage divider were derived for the prediction and theoretical analysis of sensor signals.

A micro-plant was developed for the characterization of all sensor systems based on selected reactive and non-reactive model systems. Fabricated chips were experimentally characterized predominantly in terms of measurement sensitivity to relative changes in the thermal and electric properties of liquids. Both thermal sensors were operated as constant current anemometer. The pulsed and constant current feed of the thermoelectric flow sensor were compared in terms of single and differential thermopile voltages. Here the impact of Johnson-Nyquist noise and boiling/degassing nearby the heater on the sensor signal were of special interest. Furthermore the single and dual heater mode of the capillary-type thermal flow sensor were experimentally compared. The cross-sensitivity to propagating flow alterations and short-term pulsation in ambient and overpressure regime was investigated. While a method for pulsation compensation within the sensor network was demonstrated.

Finally both micro-reactors and the measurement chamber were applied for analyzing different relevant processes and unit operations. Mixing and dispersion of miscible and immiscible liquids as weil as chemical conversion during the synthesis of n-butyl acetate were addressed. Furthermore multi-phase flows, here a biphasic system, and transient flow patterns, e.g. bubbly and/or slug flow, were characterized.