Design, Fabrication, and Performance of an Optimized Flow Reactor

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Design, fabrication, and performance of an optimized flow reactor with parallel micro packed beds Hirotada Hirama, Hokichi Yoshioka, Yoshihiro Matsumoto, Takuya Amada, Yousuke Hori, Kenichiro Ohtaki, Ming Lu, and Tomoya Inoue Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03589 • Publication Date (Web): 02 Nov 2017 Downloaded from http://pubs.acs.org on November 6, 2017

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Industrial & Engineering Chemistry Research

Design, fabrication, and performance of an optimized flow reactor with parallel micro packed beds Hirotada Hirama1, Hokichi Yoshioka2, Yoshihiro Matsumoto2, Takuya Amada2, Yousuke Hori2, Kenichiro Ohtaki1, Ming Lu1, and Tomoya Inoue1,* 1

Research Center for Ubiquitous MEMS and Micro Engineering, National Institute of Advanced

Industrial Science and Technology, 1-2-1, Namiki, Tsukuba, Ibaraki 305-8564, Japan 2

TECNISCO Ltd., 2-2-15, Minami-Shinagawa, Shinagawa-ku, Tokyo 140-0004, Japan

ABSTRACT. Herein, we build on the results of our previous studies and describe the design and fabrication of a hybrid glass/silicon flow reactor with 32 parallel packed beds using an eight-inch wafer process. Increasing throughput of exothermic reaction requires sufficient heat removal for maintaining reasonable productivity as well as for safe operation. In this work, the glass/silicon combination was chosen as reactor materials. We predicted the heat dissipation effect of bonding of a silicon substrate to the glass microreactor, which was validated by the reaction experiment monitored by IR thermography. The reactor withstood pressures of up to 13 MPa, which ensures safe reactor operation in spite of brittleness of materials (silicon and glass) used. It was designed

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to serve for the direct synthesis of hydrogen peroxide (10 wt%) with 0.5 kg/h productivity per single reactor, and its parallel operation was demonstrated. Here we can show that the reactor is well suited for the safe and efficient operation of hazardous processes and their upscaling.

INTRODUCTION Flow chemical processing (consisting of continuous chemical synthetic procedures) is frequently used for mass production, requiring a smaller reactor volume than batch and/or continuous stirred-tank reactor-based processing of identical throughput. Recently, flow chemical processing has been successfully applied to small-scale processes such as active pharmaceutical ingredient manufacturing to maximize their profitability and reduce waste production.1-3 Flow reactors exhibit certain advantages over batch reactors under harsh operating environment, including high pressure operation4 and precise temperature control in a wide temperature range. The above advantages are even more pronounced in microfabricated channel-based microreactors, wherein channels of sub-millimeter width and depth act as reaction chambers and thus result in a high surface area/volume ratio, enhancing mass and heat transfer. Among microreactors made from diverse materials such as glass,5-8 silicon,9 stainless steel,10,

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ceramics,12 and polymers,13-16 glass ones exhibit good resistance to acids and bases and allow flow regime monitoring during operation. Hence, combining the enhanced heat dissipation ability of microreactors with the chemical resistance of glass allows one to perform exothermic


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reactions in aggressive media, for example, direct oxidation and even fluorination (after surface modification).4, 8, 17-20 Herein, we describe the direct synthesis of hydrogen peroxide in glass microreactors with catalyst-packed beds. The sub-millimeter width and depth of the packed beds prevented the explosive reaction between hydrogen and oxygen, and a pressure drop–based flow control design based on a combination of microchannels was used to achieve proper gas-liquid flow and proper paralleled packed bed operation (i.e., gas-liquid flow was stably distributed from a single-packed bed to 16 parallel ones), resulting in well-controlled paralleled flow reactor operation. Using a proper catalyst, we could obtain 10 wt% hydrogen peroxide, preventing the direct production of water under ambient pressure (1 MPa) and temperature (~300 K) in the absence of methanol, despite the high reaction exothermicity.4, 8, 17-19 Despite the scalability of glass flow reactor processes, the available reactor capacities remain insufficient to support large-scale chemical production. Hence, we attempted to scale up the reactor by increasing the number (32) and length of the packed beds, accounting for the potential overheating arising from the small heat conductivity of the reactor material and the large expected heat generation. Based on the above considerations, we designed and fabricated a hybrid glass/silicon 32-channel (32ch) flow reactor capable of facile heat dissipation, relying on calculated microreactor heat transfer coefficients and experimental verification tests. To address the potential problem posed by the brittleness of glass-Si, the fabricated reactor was subjected to destructive tests to evaluate its durability at high pressure. Finally, we utilized the fabricated flow reactor for the direct synthesis of hydrogen peroxide and evaluated its productivity.


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EXPERIMENTAL Reactor design The designed flow reactor comprised three layers and 32 parallel micro packed beds (Figure 1), featuring microchannels with dimensions and functions similar to those in previously reported 1–16ch microreactors.19 Layer 1 contained liquid distribution and catalyst loading channels, catalyst-packed beds, and gas-liquid outlet channels. Layer 2 contained channels for gas (oxygen and hydrogen) and liquid (water) delivery to the catalyst-packed beds in layer 1, whereas layer 3 contained 32 gas (oxygen and hydrogen) distribution channels. The holes in layer 2 connected distribution channels to each delivery channel in order to direct gases and liquids into each packed bed channel.


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Figure 1. Configuration of the 32ch flow reactor. (a) Structure of layers. Layer 1: mainly catalyst-packed bed channels with width = 0.6–2.4 mm and depth = 0.005–0.9 mm (See Figure S1 for detailed dimensions). Layer 2: channels for gas delivery and liquid distribution/delivery with width = 0.05 mm and depth = 0.02 mm. Layer 3: gas distribution channels with width = 0.6 mm (in 32 branched channels) or 1 mm (upstream of 32 branched channels) and depth = 0.3 mm. Steady gas/liquid flow was achieved by using narrow channels to enhance the pressure drop. (b) Schematic representation of layers and description of their functions. (c) Fabricated 32ch flow reactor with dimensions of 128 mm × 182 mm. C and C’: catalyst introduction ports; L: liquid introduction port; G1 and G2: gas introduction ports; O: gas and liquid outlet.

Reactor fabrication The abovementioned flow reactor was fabricated by TECNISCO Ltd. using a procedure developed at the National Institute of Advanced Industrial Science and Technology (AIST).4, 8, 1719

Briefly, layer 1 was fabricated by end milling using a sintered diamond tool manufactured in-

house by TECNISCO Ltd. Because milling could easily result in the formation of small cracks, which could weaken the bond strength between the layers under high pressure reaction, any such cracks were removed by mild chemical etching with dilute hydrofluoric acid to afford flat and smooth surfaces. The channels of layer 2, which is used for gas and liquid distribution, were fabricated using photolithography based on chemical etching with hydrofluoric acid. Layer 3 was utilized for gas distribution, with the corresponding channels fabricated by sandblasting and the surface processed by chemical etching as in layer 1. Finally, the fabricated layers were bonded by thermal fusion.


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The use of glass as a reactor material faces the challenge of proper heat management, with one advantage of a microreactor being its suitability for hazardous chemical processes such as highly exothermic reactions. Hence, we estimated the influence of reaction heat (vide infra) and modeled the effect of using highly thermally conductive silicon on heat dissipation, while also demonstrating the effect of silicon hybridization for 8ch microreactors. The 32ch flow reactor was fabricated by TECNISCO Ltd. and the 1ch, 8ch, and 16ch microreactors were fabricated by AIST using a previously reported procedure.8 The silicon substrate (thermal conductivity = 168 W m–1 K–1) was attached to the bottom (layer 3) of the glass (thermal conductivity = 1.1 W m–1 K–1) microreactor by anodic bonding.21 Destructive test The 8ch microreactor used for evaluating high-pressure stability was fabricated and tested by TECNISCO Ltd. The reactor channels were loaded with oil and increasing pressure was applied at room temperature using a hydraulic hand pump until reactor failure. The produced fractures were inspected to determine the mechanism of their generation. Direct synthesis of hydrogen peroxide The direct synthesis of hydrogen peroxide was performed in a catalyst-loaded 32ch flow reactor using a previously described procedure. Briefly, Pd/TiO2 (2wt%) catalysts was prepared using palladium nanocolloid methanol solution stabilized by PVP (Pd: particles 5 nm in diameter, Nano Cube Japan Co., Ltd., Japan), and TiO2 porous spherical particles, mainly rutiletype, 0.06 ± 0.01 mm in diameter, Covalent Materials Co., Japan) by impregnation procedure, followed calcination under air flow, and hydrogen flow afterwards at 600 K. Thus prepared catalyst was suspended in deionized water as a slurry, and loaded into channels as described for


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1ch, 8ch, and 16ch microreactors.19 For the 32ch flow reactor, catalyst loading was performed using port C (or C’), as shown in Figure 1c. The loaded catalyst particles formed packed beds due to a dam structure at the end of the channels (Figure S1), whereas water flowed out of the channels. Once loading was completed, ports C and C’ were tightly closed to prevent reactor drainage during the reaction. As described in our previous paper19, the degraded catalysts after the reaction can be replaced, by opening port C, flushing the catalysts with the water introduced from port O, and re-loading the catalysts into the channels. The instrumental setup used for the reaction was similar to that reported previously.8, 17-19 The catalyst-packed flow reactor was fixed on a set of aluminum jigs to connect inlet and outlet tubing for the reaction fluids (Figure S2). This design enabled parallel operation of the flow reactor, as demonstrated previously for 16ch microreactors.19 The reaction was performed at an outlet pressure of 0.95 MPaG and room temperature (296 K), with no further temperature control. During start-up, hydrogen, nitrogen, and the reaction solution were introduced into the reactor, with pressure (up to 0.95 MPaG at the outlet) subsequently applied using a back pressure regulator. Finally, nitrogen was replaced by oxygen. The reaction solution was prepared by mixing dilute sulfuric acid (0.025 M), phosphoric acid (0.005 M), and sodium bromide (5.1 × 10-4 M) solutions to stabilize the produced hydrogen peroxide.19 The flows of hydrogen, nitrogen, and oxygen were adjusted using mass flow controllers (Model 5850E, Brooks Instruments, USA), and the reaction solution was fed by a high-performance liquid chromatography pump (NP-KX-2001UP, Nippon Seimitsu Kagaku, Japan). The pressure was controlled using a backpressure valve (Model 5866, Brooks Instruments, USA), and the surface temperature was occasionally monitored using an infrared (IR) thermometer.


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The products of this reaction are hydrogen peroxide and water. The amount of hydrogen peroxide was measured by permanganate titration, while that of unreacted hydrogen was measured by MS-5A in a micro GC (Agilent3000A, Inficon, Switzerland) to estimate the hydrogen conversion.8, 18, 19 The amount of unreacted hydrogen, hydrogen conversion that is, for the 32ch reactor could not be measured due to an experimental set-up issue. Prediction of the heat removal effect Prior to designing real reactor with 32ch paralleled packed beds, we tried to obtain images of the reactor to fabricate, by estimating the reactor temperature during operation using a COMSOL Multiphysics 5.0 heat transfer module. In doing so, we used a simplified reactor model focusing on catalyst packed beds and ignoring fluid flow distribution channels, as shown in Figure 2. The obtained results were used to evaluate the microreactor performance and determine its design. In the verification test, heat dissipation was visualized using two types of 8ch microreactors (with and without a silicon substrate) fabricated by TECNISCO Ltd., as mentioned above. The reactor temperature was monitored using an IR thermography camera (Nippon Avionics Co., Ltd., Japan).


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Figure 2. Schematic heat dissipation model: (a) Real reactor, (b) Simplified view of reactor, consisting of catalyst packed bed (colored as blue) and inlet/outlet connected, (c) Cross-section of the above model showing heat dissipation, with catalyst-packed beds acting as a heat source. T: reactor temperature; Q1: heat removal from the reactor wall; Q2: heat removal with the product flow, details are in the text (vide infra).

RESULTS AND DISCUSSION Pressure test We already demonstrated that the microreaction system we have been using is available for the reaction up to 20 atm (the leakage generally occurred at the interconnect between the reactor and the tubings).18 Still, the pressure tolerance of the reactor itself had been unclear hence we tried to study in order to know the limitation of the reaction condition we could apply. The microreactor was fixed in a jig, and its channels were filled with oil and subjected to pressure applied using a hydraulic hand pump (Figure 3a). Although the microreactor was destroyed at pressures above 13 MPa, the glass-bonding interface was preserved without detachment (Figure 3b). The above test was performed in triplicate, with destruction observed at 13 MPa in all cases. Hence, we concluded that the reactor pressure resistance equaled 13 MPa, being remarkable compared to values observed in previous studies of glass- or glass-Si systems, even taking into account the fact that these systems featured narrower microchannels.22,

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Although the

breakdown mechanism remained unclear, analysis of the generated fractures revealed that failure was highly unlikely to occur at the glass-glass fusion site.


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Figure 3. Pressure test of the glass microreactor. (a) Experimental setup, featuring an 8ch microreactor fixed by jigs and subjected to pressure applied using a hydraulic hand pump. (b) Destroyed microreactor.

Simulation of heat dissipation As mentioned in the experimental section, we attempted to estimate the effect of heat dissipation by simulating the reaction temperature for different reactor material assemblies. In doing so, we relied on the steady-state approximation, assuming that the heat generated by the reaction was balanced by dissipation processes. Mathematically, the heat balance can be described as follows: T = Tin,

(1)

Q1 = hr Asurf (Text − T),

(2)

Q2 = CW fl (Text − T) = hw Ach (Text − T),

(3)

QR = Q1 + Q2,

(4)


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where Tin [K] denotes the reactant temperature at the inlet, hr [W m–2 K–1] denotes the overall heat transfer coefficient, T [K] denotes the temperature inside the microreactor (i.e., in channels for catalyst-packed beds), Text [K] denotes the temperature outside the microreactor, Asurf [m2] denotes the microreactor surface area, Ach [m2] denotes the area of the channels within the microreactor, CW [J g–1 K–1] denotes the specific heat of water, fl [g s–1] denotes the mass flow rate of liquid inside the microreactor, Q1 [W] denotes heat removal from the reactor wall, Q2 [W] denotes heat removal due to product flow, and QR [W] denotes the heat generated by the reaction. In this set, Eq. (1) describes the boundary condition for the inlet, while Eqs. (2) and (3) describe those for the reactor surface and the outlet, respectively. Eq. (4) describes the dissipation of reaction heat through the reactor wall (Q1, Eq. (2)) and product outflow (Q2, Eq. (3)). Introduction of a parameter hw (used in Eq. (3)) would require further explanation; hw [W m–2 K–1] denotes the imaginary coefficient of heat removal by water, introduced in order to analyze within “heat transfer in solid” module in COMSOL 5.0. As indicated in Eq. (3), this imaginary parameter hw has a relationship with real physical parameters, shown in Eq. (5): hw = CW fl / Ach.

(3)’

We also calculated the heat transfer module in the solid components of the microreactor. Hence, the steady-state approximation was applied; in Eq. (3), heat removal was viewed as heat dissipation from the wall, which had an overall heat transfer coefficient equivalent to the heat capacity of the outflowing liquid. The considered reactions corresponded to hydrogen peroxide and water formation:


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H2 + O2 → H2O2 (ΔH = −187.78 kJ mol–1)

(5)

H2 + 0.5O2 → H2O (ΔH = −285.83 kJ mol–1)

(6)

The heat generated by the reaction was estimated based on previous observations, as summarized in Table 1. Moreover, analysis was performed by assuming that the reaction heat was generated by heaters occupying the positions of the catalyst-packed beds.

Table 1. Simulated reactor performances, with selectivity defined as (amount of H2O2 in the effluent [mol])/(amount of H2 fed [mol] – amount of H2 unreacted [mol]) × 100%. Reaction rate (mol s–1)

Generated H2O2 Number of channels

Concentration

Generation rate

Selectivity (%)

H2O2

H2O

Heat of reaction

inside the

(wt%)

(mL min–1)

1

3

0.01

60

1.47 × 10–7

9.80 × 10–8

0.056

8

3

0.08

60

1.18 × 10–6

7.84 × 10–7

0.45

16

3

0.16

60

2.35 × 10–6

1.57 × 10–6

0.89

32

10

0.32

60

1.57 × 10–5

1.05 × 10–5

6.0

(W)

microreactor

Initially, we estimated hr to consistently reproduce Text, demonstrating that reasonable reactor temperatures could be obtained using hr = 5-10 W m–2 K–1, which was in good agreement with conventionally obtained values. Figure 4a shows the maximum surface temperature simulated with 8ch packed bed reactor, by changing hr value from 0 (totally adiabatic wall) to 10.


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We simulated the reactor temperatures for 1ch, 16ch, and 32ch reactors and compared the maximum temperature with fixing hr = 5. The comparison is shown in Figure 4b. The order of maximum temperature achieved was 32ch > 16ch ～ 8ch > 1ch, indicating that not only the degree of reaction (the 32ch reactor holds longer residence time than others, see Table 2), but also the density of the catalyst packed bed (the 16ch reactor has double size of the 8ch one) determine the reactor temperature.

Simulated surface temperature distribution is shown in

Figure 4 c), accompanied with the real reactor image and the reactor structure used for the simulation.


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Figure 4. (a) Simulated maximum surface temperatures of the 8ch reactor with varying hr from 0 to 10, (b) comparison of maximum surface temperatures simulated for the 1ch, 8ch, 16ch, and 32ch reactors, with fixing hr = 5, and (c) schematic of temperature simulation procedure for the 8ch reactor In the next step, we determined a suitable material combination for the 32ch reactor by calculating heat transfer coefficients for three types of devices: (1) a reactor without a silicon substrate (i.e., “all_glass” in Figure 5a), (2) a top-silicon-bonded glass reactor (i.e., “top_Si_attached” in Figure 5a), and (3) a bottom-silicon-bonded glass reactor (i.e., “bottom_Si_attached” in Figure 5a). Using the model shown in Figure 2b, we estimated that (i) the maximum surface temperature of the 32ch reactor with a silicon substrate dropped by 5–10 K regardless of whether the top or bottom side of the reactor was bonded to the above substrate


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(Figure 5a) and (ii) the silicon-bonded reactor showed a more uniform heat distribution than the non-bonded one (Figure 5b). Thus, bonding to a silicon substrate was demonstrated to be a powerful way of dissipating the reaction heat in a reactor to avoid hot spot generation and allow safe operation. Furthermore, we compared a grooved-surface (Figure S4) and flat-surface silicon-bonded reactors, revealing that the increased surface area of the grooved surface structure did not affect heat transfer.

Figure 5. Effects of silicon substrate on reaction heat dissipation. (a) Estimated heat dissipation, represented by the maximum surface temperature of reactors with/without a bonded silicon


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substrate, and visualization of the simulated surface temperature distribution in the 32ch reactors (b) without and (c) with a silicon substrate, with fixing hr = 5

Verification of heat dissipation efficiency To ensure that silicon substrate bonding suppressed microreactor heating during the exothermal reaction, we performed a verification test using microreactors with and without a silicon substrate. As a preliminary experiment, we evaluated the temperature reached during the exothermic reaction in 8ch microreactors with/without a silicon substrate, revealing that the use of silicon resulted in a uniform temperature distribution on the microreactor surface, whereas in the absence of bonded silicon, the catalyst-packed bed channels showed a significantly elevated temperature (Figure 6). Such heat dissipation control was not observed in conventional microreactors with packed beds, thus being a characteristic feature of our silicon-bonded microreactor.

Figure 6. Visualization of increased heat dissipation efficiency achieved by bonding a silicon substrate to an 8ch microreactor. (a) Photograph of a microreactor fixed in a jig, and infrared thermographs of microreactors (b) without and (c) with a silicon substrate. Hydrogen, nitrogen,


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an inert gas, and water were introduced into the reactor channels prior to the reaction, with nitrogen subsequently replaced by oxygen (after reaction initiation). Notably, no hot spots were observed in the microreactor with a silicon substrate.

The currently preferred optimization strategy involves using reactor materials with high heat conductivities to compensate for the higher heat flux caused by upscaling, with a good example being Corning’s Advanced Flow Reactor (AFR) series, in which the reactor material was changed from glass (low throughput) to silicon carbide (SiC, thermal conductivity = 114 W m–1 K–1).24 Herein, we showed that heat transfer can also be promoted by attaching a heatconductive material to a glass reactor with poor heat conductivity, simultaneously preserving favorable glass properties, such as transparency and enhancing heat conductivity, to compensate for the increased heat flux due to upscaling.

Hydrogen peroxide productivity We summarized the 8ch and the 32ch reactor performances in Table 2. Considering longer residence (by three times, compared with the 8ch one) in this condition, the 32ch reactor could have 10 wt% of hydrogen peroxide solution, although merely 6 wt% solution was obtained. We recognize that such reduction of the reactor performance would be due to the catalyst deactivation during the start-up of the reaction system. For both 32ch reactors (with and without silicon substrate) the reactor surface temperature increased rapidly after the start-up, while that of steady state was lower by 8 K (with silicon substrate bonded) and 24 K (glass only). We suspect that hydrogen depletion occurred during the start-up in both cases (oxygen-rich operation is


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preferable for the direct synthesis of hydrogen peroxide), which lead to palladium elution for it is likely to occur under oxidative and acidic condition. Still, it should be noted that the temperature difference between the initial stage and steady state stage was plunged by the heat removal effect by the silicon bonding, and obtained hydrogen peroxide concentration was higher with the reactor with intensified heat removal. More specifically, the temperature of the microreactor with a silicon substrate increased to ≤ 307 K and eventually stabilized at 300 K, while that of the microreactor without a silicon substrate reached almost 323 K. These results verified that (i) the silicon-bonded microreactor proposed in this study featured more efficient reaction heat removal than a conventional packed bed microreactor and (ii) the anodic bonding of silicon (which has a heat conductivity equivalent to that of metals) to a glass microreactor enabled operation below 308 K, whereas temperatures of 323 K were reached in the absence of Si. The results of four parallel reactor operation are also included in Table 2, and the reactor set-up is shown in Figure 7. We have to note that this is rather preliminary results, and the effort to overcome catalyst deactivation is in progress by improving the start-up procedures.

Table 2. Microreactor performance for hydrogen peroxide synthesis at H2/O2 = 1/9 sccm ch–1, fl = 0.01 mL min–1 ch–1, pressure = 1.0 MPa, and temperature = 298 K. Reactor

Reactor temperature

Max (K)

8ch glass

321

H2O2

Steady state

Concentration

(K)

(wt%)

306

3.3

Selectivity (%)

Conversion (%)

66

64


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8ch glass + Si

310

301

3.2

70

61

32ch glass*

332

308

6.0

-

-

32ch glass + Si*

312

304

6.9

-

-

32ch glass + Si*,**

-

305

4.2

(4 parallel operation) 32ch glass + Si*,**,***

(4 parallel operation)

(forefront) -

306

6.4

(forefront)

* The selectivity and conversion of the 32ch microreactors was measured due to the reaction system set-up problem. ** Initial temperature could not be measured due to a technical problem. *** fl was reduced to 0.005 mL min–1 ch–1 (i. e. 0.64 mL min-1 as total)

Figure 7. 32ch microreactors used for parallel operation.

CONCLUSIONS


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We have successfully designed and fabricated a hybrid 32ch glass/silicon microreactor with high heat dissipation ability and increased pressure resistance (up to 10 MPa) for safe and efficient operation of exothermal reactions. Bonding of a silicon substrate to the glass microreactor enhanced reaction heat dispersion and allowed the reactor temperature to stabilize ~300 K. The optimized 32ch microreactor was used for the direct synthesis of hydrogen peroxide, with the operation of twenty parallelized such microreactors allowing 10 kg of 10 wt% hydrogen peroxide to be produced per day. The described reactor design can be widely used to improve the productivity of hazardous exothermal reactions, including hydrogenation in the pharmaceutical industry, and we expect upscaled glass microreactors with catalyst-packed beds to find numerous industrial applications.

ASSOCIATED CONTENT Supporting Information. Channel dimensions, connection of tubing to the microreactor, microreactors used for preliminary experiments, and grooved-surface silicon substrate. AUTHOR INFORMATION Corresponding Author *Tel: +81-29-861-2000. E-mail: [email protected] ACKNOWLEDGMENTS This work has been partially funded by NEDO, JST, and JSPS. We would also like to acknowledge collaboration with the Mitsubishi Gas Chemical Company, Inc.


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REFERENCES

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TOC graphic


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Design, Fabrication, and Performance of an Optimized Flow Reactor

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