278832 Microwave Assisted Flow Processing: Coupling of Electromagnetic and Hydrodynamic Phenomena

Tuesday, October 30, 2012: 1:36 PM
Oakmont (Omni )
Narendra Patil1, Erik Esveld2, Faysal Benaskar1, Evgeny V. Rebrov3, Lumbertus Hulshof1, Jan Meuldijk1, Volker Hessel1 and Jaap C. Schouten1, (1)Chemical Engineering and Chemistry, Eindhoven University of Technology, Eindhoven, Netherlands, (2)Food and Biobased Research, Wageningen UR , Wageningen, Netherlands, (3)School of Chemistry & Chemical Engineering, Queen's University Belfast, Belfast, United Kingdom

Microwave assisted flow processing: coupling of electromagnetic and hydrodynamic phenomena

 

Narendra G. Patil,1 Erik Esveld,2 Faysal Benaskar,1 Evgeny V. Rebrov,3 Lumbertus A. Hulshof,1 Jan Meuldijk,1 Volker Hessel1 and Jaap C. Schouten1

 

1Laboratory of Chemical Reactor Engineering, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, the Netherlands

2Wageningen UR Food and Biobased Research, P.O. Box 17, 6700 AA, Wageningen, The Netherlands

3School of Chemistry and Chemical Engineering, Queen's University Belfast, Stranmillis Road, Belfast, BT9 5AG, United Kingdom

Introduction

Fast and volumetric heating behavior of microwaves is attracting attention for their application in continuous flow synthesis of specialty chemicals. However, the use of tuned microwave field even in small defined reactors does not always provide the expected temperature patterns.1,2 This is majorly due to a lack of position specific information with respect to velocity profile of liquid. Additionally, the accuracy of these predictions in microwave heating is limited when no space distribution of volumetric heating source is considered.1 Understanding the space distribution of microwave heating process with detailed modeling is a prerequisite for process development and control. In this work, we take a closer look at the influence of liquid velocity profiles on the axial and radial temperature profiles in a tubular microwave integrated millireactor combined with a heat exchanger (Figure 1a).

Results and discussion

Horizontal co-current flow of a reactant (ethanol) and a microwave transparent coolant (toluene) was established in a teflon supported quartz tube (i.d.: 3 mm, o.d.: 4 mm) and shell (i.d.: 7 mm, o.d.: 9 mm), respectively (Figure 1a). The co-current flow of coolant was assumed to avoid overheating of the reactant by inherently fast microwave heating. The reactor was inserted in a microwave cavity with a length of 47 mm. The physical model included electromagnetic interactions, fluid dynamics and heat transport (Figure 1b). The temperature and flow profiles were obtained in a 3-D domain using the COMSOL software. As the dielectric properties of the reactant were changing with temperature, it was expected that they should influence the heating rate. However, this effect was of minor importance (Figure 1c) and the remaining simulations were made in a 2-D domain. Effect of gravitation was observed due to horizontal arrangement of the reactor-heat exchanger. The effect of gravity was clearly pronounced in the coolant flow where a recirculation pattern was observed due to difference in density (Figure 2). The coolant flow rate had a minor effect on temperature profiles and energy dissipation at a high heating rate  of 180 °C/s (Figure 3, a and b). Higher temperatures were observed at the wall (stagnant liquid films), leading to drastic drop in the energy dissipation (Figure 3, c and d). The laminar flow of ethanol was disturbed by a temperature probe (Figure 4). The probe was inserted either from the outlet (Figure 4a) or inlet (Figure 4b) side of the assembly. In the former case, measured temperature was lower as compared to the latter case (Figure 5). This effect could be explained by local hydrodynamics near the tip of the probe. When the probe was inserted from the outlet, the highest velocities were found at the tip. Conversely, when the probe was inserted from the inlet, a stagnant film assured lowest velocity at the probe tip. These observations were then validated by experiments (data points, Figure 5). The temperature profiles obtained by the modeling study agreed with the experimental observations.

Conclusions

The stagnancy of the flow of the microwave absorbing fluid influenced the temperature distribution in a microwave integrated flow reactor. The buoyancy influence (as a result of gravitational forces) was already visible for horizontal arrangement of reactor-heat exchanger assembly at millimeter sizes. The stagnant layer formation caused by any insertion or at reactor walls yielded higher temperatures and lower energy dissipation regions. The coolant flow, unless used as a heated jacket to minimize heat losses, was found to be ineffective. Lastly, complex 3-D geometry modeling efforts can be eliminated at very early stages unless there are specific disturbances in electric field pattern.

                                                                (a)

           

                                (b)                                                                                           (c)

Figure 1: a) Schematic view and process details of the microwave integrated reactor-heat exchanger assembly used for experimental validations, b) 3 D computational domain of the microwave cavity showing individual components of the assembly, c) Electric field intensity in and around the microwave integrated reactor-heat exchanger. Ethanol flow rate:  40 ml/min, toluene flow rate: 100 ml/min. Input microwave power: 55 W.

Figure 2: Effect of buoyancy in a horizontal arrangement of the microwave integrated reactor-heat exchanger assembly. Gray lines (with thickness indicating the strength) show velocity profiles of coolant. Axially sectioned colored pattern shows temperature in the reactant as well as the coolant section.

 

                                                (a)                                                                           (b)

                                                (c)                                                                           (d)

Figure 3: Temperature profiles (a,c) as a function of axial co-ordinate, and (b,d) energy dissipation of microwaves (QMW, W/ml), temperature rise (T-Tin, °C) and velocity profile (U, cm/s) as a function of radial co-ordinate. (a) near stagnant coolant flow, FEtOH = 100 ml/min, FTOL = 0.5 ml/min. (b) high coolant flow, conditions are the same as those in Fig 1c.

Probe influence.bmp

Figure 4: Influence of the direction of probe insertion on the temperature profiles obtained by modeling, a) probe inserted from the outlet, b) probe inserted from the inlet. Conditions are the same as those in Fig 1c.

Figure 5: Influence of the direction of probe insertion on the temperature profiles obtained by modeling (lines) and experiments (data points). Solid line and squares: probe inserted from inlet, dotted line and triangles: probe inserted from outlet. Conditions are the same as those in Fig 1c.

 

Reference:

1. Patil, N. G.; Hermans, A. I. G.; Benaskar, F.; Rebrov, E. V.; Meuldijk, J.; Hulshof, L. A.; Hessel, V.; Schouten, J. C. Energy efficient and controlled flow processing under microwave heating by using a milli reactor-heat exchanger. AIChE J. 2011, Accepted. DOI 10.1002/aic.13713.

2. Patil, N. G.; Benaskar, F.; Rebrov, E. V.; Meuldijk, J.; Hulshof, L. A.; Hessel, V.; Schouten, J. C. Continuous multi-tubular milli-reactor with a Cu thin film for microwave assisted fine-chemical synthesis. Ind. Eng. Chem. Res. 2012, submitted.


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