(209h) Thermal Mass Flow Sensors for Monitoring Esterification Reactions in Residence Time Micro Reactors

Motivation

Recently, in chemical micro process engineering [1] the performance of homogeneously as well as heterogeneously catalyzed esterification reactions in micro plants became an interesting field [2,3]. Esters are used as high quality organic solvents with significant industrial relevance, e.g. used for lacquer production. Strong mineral or organic acids are used as liquid catalyst and acidic ion exchange resins immobilized in the micro channel as solid catalyst. Micro residence time reactors or micro fixed bed reactors are usually applied for processing this class of reactive systems in micro plants. Compared to classical batch processing, a micro plant enables fast mixing, precise temperature control, a well defined residence time and interfacing area to the solid catalyst. The micro reactor can be operated in steady state mode with continuous educt flow or non-steady state condition, where one educt is fed in pulsed mode. When educts and products have different adsorptivities to the solid catalyst, the device behaves like a chromatographic reactor. Inline analysis of the dynamic reactor behavior is fundamental for the development, optimization and control of novel integrated chemical processes.

With regard to state of the art micro residence time reactors, there is a lack in sensitive, robust, scalable and low priced inline analytics for monitoring chemical reactions based on miniaturized sensors. General process parameters of interest are conversion rate, reaction kinetics, phase changes, residence time, dispersion effects and separation efficiency. On micro scale quite expensive flow through chambers with fiber optics attached to spectrometers (UV-VIS-NIR) and light microscopes with attached digital camera are applied. Scaling up for multiple micro reactors or channels in parallel is a problem.

Objectives and approach

In this contribution, we present a novel residence time micro reactor with integrated thermal mass flow sensors based on the thermo-transfer (calorimeter) principle [4] for inline analysis of esterification reactions. The setup was optimized for monitoring relative changes in isobaric heat capacity along the micro channel. This class of micro sensors is state of the art in measuring mass flow rates in micro fluidic systems. Furthermore, it is known that the measurement signal is also sensitive to changes in isobaric heat capacity. The transducer consists of a stainless steel capillary with heater/ temperature coils around for temperature field generation and deformation measurement. The reactive system is only in contact with a monolithic capillary, which enables high chemical and pressure resistance. Insofar, it emerges as an attractive sensor principle for inline monitoring of chemical reactions in micro reactors by means of changes of the isobaric heat capacity. For the discrimination between sensor signal changes due to chemical conversion and flow pulsation multiple sensors are connected in series. With a reasonable capillary length all sensors are affected simultaneously by changes in mass flow rate. When using non-reactive liquids differences in the sensor signal can be attributed to different sensor transduction characteristics. For the optimization of the measurement sensitivity as a function of heat dissipation and thermal properties of the reactive system a static 2D CFD simulation based on the Finite Element Method (FEM) was conducted.

As a reactive model system we analyzed the homogeneously catalyzed synthesis of butyl acetate with sulfuric acid. The reactor was integrated in a micro plant that consists basically of two pumps for educt feed connected to an interdigital micro mixer, a climate chamber for temperature control and PTFE-capillaries for interconnection and as flow restrictor at the outlet. Residence time was adjusted using different micro channel length. First, differences in sensor transduction behavior were characterized using a high precision syringe pump and different pure solvents with known thermal properties. Focus was on the measurement sensitivity against relative changes in isobaric heat capacity in the presence of flow pulsation and noise induced by sensor electronics. Single sensor signal and differential sensor signal evaluation are compared. Second, the homogeneously catalyzed esterification reaction was processed with equimolar educt feed and 1 % sulfuric acid premixed with acetic acid. Before starting an experiment the setup was flushed several times with 1-butanol.

Summary and conclusion

In the non-reactive case our first results imply that the measurement sensitivity in terms of relative changes in heat capacity is predominantly limited by flow pulsation and not by the sensor characteristics itself. In an isobaric heat capacity range (cp = 1500 ? 2500 J/(kg*K)) the sensor response can be treated as linear (Pearson's correlation coefficient: 0.996). When comparing single sensor signals with differential sensor signals flow induced noise can be reduced up to one order of magnitude dependent on the type of the pump (HPLC pump, syringe pump?). It is known that during synthesis of butyl acetate a phase transition from one liquid phase to two liquid phases occurs at a certain mole fraction. This mole fraction is related to a certain residence time or micro channel length. The phase transition could be detected as oscillations in sensor signal (Fig. 1).

Fig. 1: oscillations in sensor signal during esterification reaction at the end of the reactor capillary due to phase transition at a certain residence time/ micro channel length

Positive signal shifts in reference to the base line result from the water-rich phase passing the sensing elements. Based on our first results, thermal mass flow sensors using the thermo-transfer principle can be applied for the inline analysis of reaction kinetics and phase transition in residence time micro reactors.

[1]

H. Pennemann, V. Hessel and H. Löwe, ?Chemical microprocess technology - from laboratory-scale to production?, Chem. Eng. Sci., vol. 59, nr. 22-23, pp. 4789-4794, 2004.

[2]

A. A. Kulkarni, K.-P. Zeyer, T. Jacobs, A. Kienle, ?Miniaturized Systems for Homogeneously and Heterogeneously Catalyzed Liquid-Phase Esterification Reaction?, Ind. Eng. Chem. Res., vol. 46, nr. 16, pp. 5271-5277, 2007.

[3]

A.A. Kulkarni, K.-P. Zeyer, T. Jacobs, M. Kaspereit and A. Kienle, ?Feasibility studies and dynamics of catalytic liquid phase esterification reactions in a micro plant?, Chem. Eng. J., in press.

[4]

J. Lötters, ?Liquid flow sensor for nano- and micro-flow ranges?, Sensor Review, vol. 25, nr. 1, pp. 20-23, 2005.