Microreactor Technology and Process Intensification - American

Chapter 10

Miniaturization of a Hydrogen Plant

Downloaded by UNIV OF CINCINNATI on May 26, 2016 | http://pubs.acs.org Publication Date: August 9, 2005 | doi: 10.1021/bk-2005-0914.ch010

J. D. Holladay, E. O. Jones, R. A. Dagle, G. G. Xia, C. Cao, and Y. Wang Battelle, Pacific Northwest Division, P. O. Box 999, MS K6-24, Richland, WA 99352

The development of a miniaturized hydrogen plant is discussed. The micro-scale system is capable of producing 1-5 sccm hydrogen that could be used as a fuel supply in a small fuel cell to produce 99.999% pure, thus no further purification is required. The high pressure requirements make this technology difficult to integrate in some microreactor systems. Additionally, in some cases, the membranes are not stable for multiple (>100) thermal cycles (10,28,29). The PrOx and methanation reactors are catalytic reactors. Note that although PrOx reactors are sometimes called selective oxidation reactors, selective oxidation actually refers to carbon monoxide reduction reactions occurring inside the fuel cell (75). The PrOx reactor increases the system complexity, because carefully measured concentrations of air must be added (12,13,30,31). However these reactors are compact and, i f excessive air is introduced, some hydrogen is burned. Selective methanation reactors are simpler in that no air is required; however, for every molecule of carbon monoxide reacted, three hydrogen molecules are consumed. Also, as the temperature increases, the carbon monoxide selectivity decreases, and carbon dioxide may react with hydrogen. Careful control of the reactor conditions is important to maintain the selectivity. The catalysts for both these systems are typically noble metals such as platinum, ruthenium, or rhodium supported on A 1 0 (12,13,30,31). Researchers at the Stevens Institute for Technology have had some success at fabricating a microscale PrOx reactor for a 0.5-W fuel cell (30-31). However, to simplify the reactor, a selective methanation step was used in the work discussed here. 2

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

168 Material Selection Materials selection takes into account chemically compatible materials, thermal properties, and manufacturing complexities (e.g., feature fabrication and sealing). Table 2 lists the benefits and drawbacks of metals, silicon (which includes materials containing silicon or that are processed with semiconductor fabrication techniques), low-temperature co-fired ceramics (LTCC) (33,34), and plastics (polymers). O f particular importance to microreactors is the requirement


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Table 2. Common Materials for Microreactors: Benefits and Challenges Substrate Benefits Metal Standard fabrication techniques Durable Low to modest costs No clean room required Silicon Well characterized silicon fabrication techniques High precision Low cost LTCC Flexible fabrication Refractory and durable Low cost No clean room required Polymers Low cost Flexible fabrication Low thermal conductivity

Challenges High thermal conductivity Poor compatibility with ceramics and glass Modest thermal conductivity Fragile Requires a clean room Non-standard fabrication Low thermal conductivity Sealing Chemical compatibility Thermal compatibility Sealing

Source: Adapted with permission from references 33 and 34. Copyright 2003 Elsevier.

that the materials have a low thermal conductivity (5,10) and that joining and sealing of dissimilar materials can easily be achieved (5,33,34). Table 3 lists the thermal conductivity and coefficient of thermal expansion for materials that can be considered for microreactors. Polyimide is an interesting material that is thermally stable at high temperatures (>400°C) and has a low thermal conductivity (36). However, it may not be compatible with methanol, steam, and the other components at elevated temperatures. Furthermore, sealing it is difficult. The other materials, which were chemically compatible, were considered in the micro processor design development.


169 Table 3. Thermal Conductivity and Coefficient of Thermal Expansion for Some Common Materials

Substrate Silicon (35) SiO (35) Silicon Nitride (35) Alumina (35) Zirconia (35) 316 Stainless Steel (35) Polyimide (36)


x

Thermal Conductivity (W/m*Kat600K) 62 5 11 16 ~3 18 0.12(at296K)

CTE (HQ 2.6*10" 5.0*10" 3.6* ΙΟ" 8.8*10* 1.0* ΙΟ* 16.0*10-* 20.0*10"* 6

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Reactor Design Considerations The strategy in this work was to identify feasible reforming reactor designs and then engineer a system that would thermally integrate the other unit operations. The steam reforming reactor incorporated a structured catalyst. In a structured catalyst, a catalyst powder is supported on a foam or felt. B y supporting the catalyst on structured substrates, plugging can be avoided and pressure drop can be reduced due to the large opening pores in the foams or felt (typically 100 to 300 μπι in diameter). Additionally, conventional catalysts typically used in packed-bed reactors can be used. Plugging and pressure drop can be rninimized by the design. The reactor is fabricated with an open side or port for loading the supported catalysts. This port requires sealing for practical use and can cause problems in the design and material selection. The reactor design must also minimize reactant bypass or "channeling" (/). A radial flow system design (Figure 2) was used to incorporate all the unit operations into the fuel processing system. It was desired to have all the unit operations located close together to rninimize the volume (especially the surface area) of the system in order to increase the efficiency. With the dimensions so small and the flow rates so low, the processing system would be designed to localize the heat to where it was needed the most (the reformer) and allow it to conduct to the other unit operations (primarily the vaporizers and preheaters).

Results and Discussion The first microscale fuel processor design involved using an engineered catalyst, i f possible, and low temperature co-fired zirconia, with its low thermal conductivity. A l l of the fuel processing systems were fabricated in house. The


170


system was designed with two vaporizers/preheaters, a catalytic combustor, catalytic methanol steam reformer, and a heat exchanger. The reactor walls varied in thickness, depending on where additional strength was needed, but ranged from 0.1 to 0.25 mm thick. The reactor channels were slightly larger, ranging from 0.1 to 0.5 mm thick, depending on the unit operation. The catalyst was engineered onto a felt that was inserted into the reformer section.

Radial-Flow

Figure 2. System Design. 3

The reforming reactor was sized to be approximately 5 mm for about 200 mW, hydrogen production with the gas flow rates at 1-2 seem. Also, at such low flow rates, less than 0.01 W was being carried in the gases. The combustor volume, also less than 5 mm , had a capacity of up to 3 W . The oversized combustor capacity allowed a wide range of operating conditions to be examined. B y keeping the design simple and using thin walls to minimize thermal conductivity, a small device could be designed. The premixed liquid methanol and water was fed and vaporized in the center of the processor. The reactant gaseous mixture was then distributed by a porous disk and flows through the catalyst bed counter-currently against the feed, whereby conversion takes place. The heat required for the reaction is supplied by the combustion chamber below the disk. The fabrication of the device was extremely difficult due to sealing issues and to the fragile nature of the zirconia. B y increasing the wall thicknesses (from 0.25 mm to less than 0.5 mm), a more structurally sound device could be built, but this would significantly increase the size o f the device and also eliminate the low thermal conductivity advantage of the zirconia. In addition, connecting the input and outlet lines was difficult, and no acceptable seals were achieved. Ultimately, 316 stainless steel was chosen for the fuel processor system (Figure 3), which provided a stronger reactor, while maintaining compactness. The combustor fuel consisted of hydrogen or methanol. The hydrogen feed during startup was less than 5 seem, and the methanol feed to the combustor was from 0.1 to 0.4 cm -hf to maintain the reactor temperatures at 300-450°C. In addition, 8-20 seem air was fed to the combustor. A thermocouple was inserted t

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171 into the combustor to monitor the device temperature. The processor could be started without the use of electric heating, by initially feeding hydrogen gas to the combustor and then, once above 70°C, decreasing the flow of the hydrogen while slowly increasing the flow of methanol. The reformer fuel consisted of distilled de-ionized water/methanol mixture (1.8:1 steam-to-carbon molar ratio) for the fiiist-generation reactor. The maximum flow rate was 0.05 cm -hr . The methanol I was Aldrich reagent grade (>99% purity). Gaseous product samples were analyzed with a microgas chromatograph (Agilent Quad G2891A). A combustion temperature of over 400°C was required to achieve >99% conversion of the methanol. This temperature was higher than anticipated, and the resultant product stream (reformate) contained higher amounts of carbon monoxide than desired. The reformate composition on a dry gas basis was 7274% hydrogen, 24-26% carbon dioxide, and 1-2% carbon monoxide (10).


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Figure 3. Microscale Fuel Processing System. (Reproduced with permission from reference 39. Copyright 2004 Elsevier.)

The thermal efficiency (η ) was calculated by dividing the lower heating value of the hydrogen in the reformate stream by the heating value of the methanol fed to the reformer plus the heating value of the fiiel fed to the combustor: 4

AH hydrogen

=

'

^

c

AH methanol(reformed) c

+ AHcombustion

· fuel

Where AHc is the lower heating value of hydrogen or methanol as indicated. Using equation 10, the maximum efficiency was found to be 13% (10).


172 From these initial tests, it was apparent that the vaporizers and preheaters were over designed. Therefore, a new design was developed that decreased the size o f the vaporizer and preheater units and increased the reactor volume by approximately a factor of three. The package size did not significantly change (

Microreactor Technology and Process Intensification - American

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