Microreactor Technology and Process Intensification - American

Chapter 20

Enhancement of Micromixing Tees Using Ultrasound Energy Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 9, 2016 | http://pubs.acs.org Publication Date: August 9, 2005 | doi: 10.1021/bk-2005-0914.ch020

L. Zeng and J. Palmer* Chemical Engineering Program, Louisiana Tech University, Ruston, LA 71272

Microfluidic opposed mixing tees were tested with hydraulic diameters of 100 μm, 177 μm, and 254 μm. Mixing performance was characterized by feeding one stream with hydrochloric acid and another stream with sodium hydroxide and dimethoxypropane. Under ideal mixing conditions, the instantaneous neutralization of sodium hydroxide with hydrochloric acid would prevent the catalytic hydrolysis of dimethoxypropane. Therefore, higher conversions of dimethoxypropane in this fast, competitive reaction is an indication of lower mixing performance. For a constant Reynolds Number, the mixing performance was increased by decreasing the channel dimensions. Ultrasound energy was externally applied to all three reactors and was found to increase mixing performance. Challenges in sealing the 100 μm mixer precluded operation at comparable pressures achieved with the 177 and 254 μm mixers. The highest mixing performance was observed with the 177 μm diameter mixer using a combination of pump work and ultrasound energy. A combination of ultrasound energy and pump work was demonstrated to be the most efficient mode of providing mixing for the 254 μm system. Therefore, externally applied ultrasound energy can be an effective and efficient means of improving the mixing performance of microfluidic systems.

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© 2005 American Chemical Society

Wang and Holladay; Microreactor Technology and Process Intensification ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

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Introduction Chemical reactors operating with homogeneous, liquid phases must begin with the mixing of two fluids. The product distribution of fast reactions with competitive pathways can be influenced by the intensity of this initial mixing (1). Reactions with half-lives of 10" to 10* seconds are considered sufficiently rapid to warrant special consideration. Researchers (2-6) have studied the effects of mixing on fast, competitive reactions in conventional scale reactors. In recent years, researchers have explored the use of microfluidic devices for synthesis. Microfluidic devices present unique challenges to mixing due to the predominance of laminar flow regimes within these structures. Ehrfeld et al. (7) authored a text describing a number of mixing strategies that have been tried on the microscale. Hessel, et al. (8) recent text provides many new designs along with applications and design equations of many of the previous approaches. One common approach is to minimize the thickness of the two contacting streams. These and other designs rely on the geometry of the microchannel to facilitate a passive mixing process. The research of utilizing externally applied energy to create an active micromixer has been limited to date. Active mixing devices have been demonstrated in microfluidics operating at very low Reynolds Numbers (9, 10). The microfluidics operating at low Reynolds Numbers are applicable to analytical sample preparation, process development, and catalyst/drug discovery; applications that seek to extract the maximum information from the minimum amount of sample. However, for synthesis, much higher flow-rates and therefore Reynolds Number can be achieved. The application of external energy to enhance the performance of a fast, competitive reaction in a device operating at a relatively high Reynolds Number has not been explored.

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 9, 2016 | http://pubs.acs.org Publication Date: August 9, 2005 | doi: 10.1021/bk-2005-0914.ch020

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6

Ultrasound energy is a method that has been utilized to increase the performance of both continuous (11) and semi-batch (12) conventional sized reactors. Monnier et al. (13) investigated ultrasound enhanced micromixing in a 50 m l flow cell. Yang et al. (14) presented the integration of a piezoelectric material in a microfluidic mixer. The mixing chamber was 6000 μιη long, 6000 μιη wide, and 60 μιη deep. The mixing time as measured by imaging the mixing of water with a fluorescent dye was estimated to be 2 s. In this study, ultrasound energy will be applied to a microfluidic mixing tee operating at relatively high Reynolds Numbers to enhance the mixing performance for fast, competitive reactions.



324 A number o f model reactions have been discussed i n the literature for characterization of mixing performance (15-18). The fast, competitive reaction utilizing dimethoxypropane and presented previously by Walker (19) was the reaction employed to characterize mixing performance in this study. In this system, a solution of dimethoxypropane, sodium hydroxide, ethanol, and water is mixed with a solution o f hydrochloric acid, ethanol, and water. The two competing reactions are the very fast hydrolysis of dimethoxypropane catalyzed by hydrochloric acid, and the near instantaneous neutralization of hydrochloric acid with the sodium hydroxide. They hydrolysis products of dimethoxypropane is acetone and methanol. Therefore, the conversion o f dimethoxypropane to methanol and acetone is inversely proportional to the mixing performance o f a given device.

NaOH +HCI

> NaCl +H 0 2

Reaction A - Fast k = 1.3x10" m /kmol s @25°C 3

CH C(OCH ) CH 3

3

2

3

+H 0—!^CH COCH +2CH OH 2

z

3

3

Reaction Β - Slow k = 700m /kmols@25°C 3

Experimental The following chemicals were obtained from Aldrich chemical and used without further purification: 2,2-dimethoxypropane, 98%; ethyl alcohol, absolute, 200 proof reagent grade; sodium hydroxide pellets, 97+% reagent grade; hydrochloric acid, 37% reagent grade; sodium chloride, 99+% reagent grade. Type 1 Reagent Grade Water (RGW) was obtained using a Barnstead Series 1090 Ε-Pure reverse osmosis purifier and was utilized for all experiments. A Hewlett Packard 5890 Series II gas chromatograph with a 30 m H P - F F A P column and a flame ionization detector was employed to determine the concentration o f the resulting methanol and therefore determine the conversion of the reaction. A lower conversion of dimethoxypropane was an indication of


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higher mixing performance. Ethanol was present in both solutions at a constant concentration of 25 weight percent for solvation of the dimethoxypropane and for use as an internal standard to measure the extent of reaction of dimethoxypropane. The two reactants were fed into an opposed mixing tee at equal mass flow-rates. Table I depicts the composition of the two feed streams. The 5% molar excess of sodium hydroxide was necessary to ensure complete neutralization of the hydrochloric acid after the mixing of the streams. The catalytic hydrolysis of dimethoxypropane requires that no residual quantities of hydrochloric acid are present after the mixing of the two streams, otherwise the performance of the mixing environment would not be measured.

Table I - Concentrations of reactants in aqueous solutions, balance of solution is Type 1, Reagent Grade Water Component

Mass

Fraction

Molar Concentration (gmol/m ) 3

Feedl Dimethoxypropane Ethanol Sodium hydroxide Sodium chloride Feed 2 Hydrochloric acid Ethanol

0.07 0.25 0.028 0.0197

648

0.0237 0.25

648

680 324

Figure 1 depicts the complete experimental apparatus. A Lab Alliance Series III pump with a P E E K head was used to feed in each reactant into the mixing tee. Three Ohaus Explorer balances with a resolution of 10 mg were used to measure the mass of each reactant feed solutions and the product to ensure that the proper mass flow of each stream had been achieved. Ultrasound energy was provided by a Sonics VC-750 processor operating at 20 kHz. Three sizes of mixing tees were tested in this study. The 127 μιη and 254 μπι mixers were commercially available tees with circular dimensions and had the same dimensions for both inlets and outlet streams. A third mixing tee was produced at Louisiana Tech in silicon to achieve smaller internal dimensions. The anisotropic etching in silicon resulted in rectangular dimensions.


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The energy required by the Sonics VC-750 processor to maintain a 20 K H z ultrasound is displayed by the instrument. Ultrasound energy measurements were performed on reactors with and without liquid flow. The ultrasound energy reported in this article is the difference of the energy observed for the flow and no flow conditions to account for inductive losses by the probe to the device. Figure 2 and figure 3 depict the position of the ultrasound probe in relation to the 254 [ifflL and 100 μιη mixers respectively. The silicon micomixers were fabricated on 4 inch diameter, 550 μιη thick double side polished wafers coated with 2.5 μιη S i 0 . The pattern was transferred from a chrome mask using a E V 420 mask aligner. A Buffered Oxide Etch was used to remove the S i 0 layer, and an Alctel A601 Ε Inductive Coupling Plasma etched the silicon using the Bosch process. The silicon microchannels were sealed by anodically bonding borosilicate glass to the top surface. The silicon mixer was placed into a Teflon holder to allow an interface with the fluidic connections and provide mechanical support to the borosilicate glass cap. The depth of the entire mixing tee was 100 μπι. The width of both inlet portions of the mixer was 33 μιη wide while the product outlet was 100 μιη wide. The channel widths were chosen to maintain a constant Reynolds Number between each inlet channel and the combined outlet channel. This was achieved because as the volumetric flow-rate doubled in the outlet channel, the hydraulic diameter of the outlet channel was also doubled. Neglecting the volume change of mixing of the two fluids and any differences in density and viscosity, maintaining a constant ratio of volumetric flow-rate to hydraulic diameter will result in a constant Reynolds Number. The outlet channel was 3 cm long while each inlet was 0.5 cm long. The length of the outlet was chosen to ensure complete conversion of the slow reaction at the flow rates of interest. 2

2

Discussion of Results As stated above, a decrease in conversion in dimethoxypropane is an indicator of improved mixing performance. Figure 4 depicts the trend that as the Reynolds Number for each of the 127 μ ι η and 254 μιη mixing tee's was increased, the mixing performance increased. The 100 μπι silicon mixer encountered challenges in sealing at the higher pressures, precluding the large variation in Reynolds Number tested for the previous two mixers. For a given Reynolds Number, the mixing performance increased as the diameter of channel inside the mixing tee was reduced. The pressure of the two feed streams was measured using a gauge integral to each pump. Pulsations from the positive displacement pump were minimized



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Figure 1. Experimental setup. (See page 3 of color insert.)

Figure 2. Ultrasound generator touching 254 μιη mixing tee. (See page 4 of color insert)



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Figure 3. Ultrasound generator touching 100 μπι mixing tee. (See page 4 of color insert.)

Note: open and filled symbols denote experiments conducted with and without ultrasound repectiveiy

60 λ

Figure 4. Mixing Performance of 100, 177, and 254 μm Micromixers as a Function ofReynolds Number.


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due to a diaphragm pulse dampener that was part of the system. The pulse dampener required pressure above approximately 100 psia before uniform pressures could be obtained. Unfortunately, the 100 μπα silicon mixer could not be operated above these pressures due to leaks. Therefore, the pressure data from the 100 μιη silicon mixer are suspect and not presented in the following analysis of system energy. Improvements are being implemented to allow operation of much higher pressures in the silicon microchannel. The total power input into the system was calculated for all experiments considering both ultrasound and pump work. The pump work delivered to the fluid was calculated from the inlet pressure and mass flow-rate of each microchannel feed streams. The experimental section describes how the ultrasound energy delivered to the fluid was measured. Figure 5 depicts the total system power of all three micomixers tested. A s in the previous figure, the open and filled symbols denote experiments with and without ultrasound energy respectively. The 177 μπι without ultrasound energy showed a dramatic increase in performance with the initial increases in system power. The increase in power is due to higher flow-rates and therefore higher pressure drops. The lowest conversion for this system was approximately 8%. The addition of ultrasound energy provided a step change in energy fed to the system. 3 Watts of power was fed to the system without substantial gains in performance. However, the additional energy input by the ultrasound was able to improve the performance of the 177 μπι reactor at the highest flow-rate tested to produce the highest level of mixing observed in this study. 4% conversion was observed at this highest flow-rate combined with ultrasound energy. Figure 6 depicts the specific work for the 177 μπι mixer. With the non-ultrasound experiments, the specific work increased as the pressure and therefore flow-rate was increased. The reverse slope of the ultrasound experiments is indicative of the large amount of energy provided into the mixer from the ultrasound generator. As the flow-rate was increased, the specific work decreased because the homogenizer made up the large majority of the energy into the system and that energy was now being distributed over a larger amount of mass. From the specific work, it is clear that much more energy was being delivered to the mixer than was effectively utilized by the fluid. Increasing the proportion of fluid in the channels by operating several mixers in parallel in a single mixer would likely have increased the efficient utilization of ultrasonic energy. The 254 μιη mixer exhibited the worst performance of the three mixers tested. This low performance is evident from Figure 5 depicting total power input into the system. As stated above, low power in Figure 5 was due to low flow-rates


330 70 legend: Square -254//m Triangle-177//m Circle -100 μ m

60

50 Note: open and filled symbols denote experiments conducted with and without ultrasound repectively


* g S

40

g

30

û.

D

ο

•

α

20

• 10

0

2

4

6

8

10

12

Total System Power (W)

Figure 5. Mixing Performance of100, 177, and254 μm Micromixers as a Function of Total System Power (Ultrasound Energy and Pressure Dissipation)

70

60 S?

S

40

â 30 177 #m micromixer

A 177 μπ\ micromixer with ultrasound enhancement

Δ Δ Δ 10000

20000

Δ

30000

40000

50000

Specific Work (J/kg)

Figure 6. Mixing Performance ofl77μm Micromixers as a Function of Specific Work



331 and correspondingly low pressures generated by the system. The mixing performance of the 254 μιη system increased nearly linearly with a corresponding increase in system power. The specific work was most interesting for the 254 μπι mixer and is depicted in Figure 7. A t a combined flow-rate of 8 g/min, the ultrasound enhanced mixer resulted in a conversion of only 35%, but resulted in a specific work of 22,000 J/kg. Experiments conducted without ultrasound at a combined flow-rate of 40 g/min resulted in a lower conversion of 27%. The pressure required for these two feeds was approximately 1050 psig, resulting in a specific work of 6,700 J/kg. Pump work, at least initially, provided a more efficient method of mixing versus the externally applied ultrasound energy. A t total flow-rates above 10 g/min, the ultrasound experiments achieve system powers that are comparable to extrapolated performances of the experiments without ultrasound. The ultrasound energy allows mixing performance unachievable due to pressure drop constraints of the pumping system. Therefore, a combination of high-pressure and ultrasound results in the most efficient and effective combination for fast mixing. Estimates o f the mixing times can be calculated based on the literature kinetic rate constant and the concentrations of the dimethoxypropane and hydrochloric acid. Prior literature utilized the resultant concentration of dimethoxypropane and hydrochloric acid that would theoretically exist i f the two feed streams combined with no reaction (20). Therefore, mixing times of 480 μδ were achieved with the 100 μπι system without ultrasound energy. A s described above, pressure limitations of this system prevented operation at higher flowrates that would have reduced this mixing time. The higher pressure allowed by the 177 μπι system allowed the mixing time without ultrasound energy to be reduced to 240 μβ. The addition of ultrasound energy further reduced the mixing time to 70 μβ. The 254 μιη system benefited the greatest from the ultrasound energy, with a reduction in mixing time from 1400 to 240 μβ as the ultrasound energy was added at the highest flow-rate tested.

Conclusions The mixing performance of the opposed mixing tees improved for a constant Reynolds Number as the channel dimension was decreased. A n increased amount of pump work was required to maintain a constant Reynolds Number as the channel dimension was decreased. Limitations on the maximum pressure achievable by both the pump and the mixer preclude further increases in mixing performance through pump work. Ultrasound energy, even externally applied as in the manner of this study, is an effective means of adding additional work to the system and therefore promoting faster mixing. The application of ultrasonic


332 70

I

•

50

• ^ 254 μπ\ micromixer

• • Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 9, 2016 | http://pubs.acs.org Publication Date: August 9, 2005 | doi: 10.1021/bk-2005-0914.ch020

30 π

• •

20

254 μνη micromixer with ultrasound enhancement

• •

10

• 0

10000

20000

30000

40000

50000

Specific Work (J/kg)

Figure 7. Mixing Efficiency of254 μιη Micromixers as a Function of Specific Work.

energy to very small diameter channels may not be efficient on a per unit mass basis, but does provide the highest level of mixing performance. A combination of pump work and ultrasound energy was demonstrated to give the lowest specific work while maximizing mixing performance. Further studies on smaller diameters channels with suitable sealing to withstand high pressures and the ultrasound energy are ongoing.

References 1. 2. 3. 4.

nd

Levenspiel, O; Chemical Reaction Engineering, 2 ed.; N Y , New York, 1972. Bourne, J.; Togvstiga, G.; Chem. Eng. Res. Des. 1988 vol 66, 26-32. Baldyga, J.; Bourne, J.; Chem. Eng. Sci. 1990, vol 45, 907. Bourne, J.; Y u ,S.;Ind. Eng. Chem. 1994, vol 33, 41-55.


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st

1

5. Baldyga , J.; Bourne, J.; Zimmerman, B.; Chem. Eng. Sci. 1994, vol 49, 1937-1946. 6. Knight, C.; Penny, W.; Fasano, J.; Presented at the 1995 AIChE Winter Annual Meeting, Miami Beach, F L , 1995. 7. Hessel, V.; Hardt, S.; Löwe, H. Chemical Micro Process Engineering, ed.; Weinheim, Germany, 2004. 8. Ehrfeld, W.; Hessel, V.; Löwe, H. Microreactors - New Technology for Modern Chemistry, 1 ed.; Weinheim, Germany; 2000. 9. L u , L.; Ryu, K.; L i u , C.; J. Microelectromechanical Sys. 2002, vol 11, 462-470. 10. Deshmukh, Α., Liepmann, D., Pisano, Α.; Proceedings of the 11 Internaional Conference on Solid State Sensors and Actuators, Munich, Germany, 2001, 950-953. 11. Okada, K ; Fuseya, S.; Nishimura, Y.; Matsubara, M. Chem. Eng. Sci. 1972, vol 27, 529-535. 12. Monnier, H.; Wilhelm, A.M.; Delmas, H. Chem. Eng. Sci. 1999, vol 54, 2953-2961. 13. Monnier, H.; Wilhelm, A.M.; Delmas, H.; Chem. Eng. Sci. 2000, vol 55, 4009-4020. 14. Yang, Z . ; Matsumoto, S.; Goto, H . ; Matsumoto, M.; Maeda, R. Sen. and Act. A 2001, vol 93, 266-272. 15. Bourne, J.; Kozicki, F.; Rys, P.; Chem. Eng. Sci. 1981 vol 36, 16431648. 16. Bourne, J.; Kut, O.; Lenzner, J.; Ind. Eng. Chem. Res. 1992 vol 31, 949-958. 17. Fournier, M.; Falk, L.; Villermaux, J.; Chem. Eng. Sci. 1996 vol 51 5187-5192. 18. Guichardon, P.; Falk, L . ; Chem. Eng. Sci. 2000 vol 55, 4233-4243. 19. Walker, B . ; Doctoral Dissertation, Swiss Federal Institute of Technology, Zurich, Switzerland, 1996. 20. Johnson, B . ; Prud'homme, R.; AIChE Journal 2003 vol 49, 2264-2282.


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Microreactor Technology and Process Intensification - American

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