Micro-Scale Methanol Reactors

The drive to make fuel processors more compact is now being fired by the prospects of using compact methanol fuel cells for consumer electronics devices. In Chapter 6 we 

Mixer/vaporizer Catalytic reformer Palladium membrane microreactor 

Several research groups as well as consumer electronics companies are working on micro-scale reactor technology and it remains to be seen how successful these become. Many of the details of fabrication methods are proprietary and the example of the membrane shift reactor is just one example of a whole new area of development. 

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Studies at Mobil Research have shown that light olefins instead of gasoline can be made from methanol by modifying both the ZSM-5-type MTG (Methanol-to-Gasoline) catalyst and the operating conditions. Work carried out in micro-scale fluidized-bed reactors show that methanol can be completely converted to a mixture of hydrocarbons containing about 76 wt% C2-C5 olefins. The remaining hydrocarbons are 9% C1-C5 paraffins, of which the major component is isobutane, and 15% C6+, half of which is aromatic. 

Figure 8.16 shows a methanol/water vaporiser, followed by a catalytic steam reformer operating at about 250°C, in which the catalyst is a thin film of Cu/ZnO coated onto the silica reactor, and finally a membrane shift reactor consisting of a palladium diffusion layer mounted on top of a perforated copper-based shift catalyst. Built onto the chip are integrated resistive heaters for getting the reformer and vaporiser up to temperature, together with micro-scale sensors and control electronics. Whilst such systems are a long... 

Selectivity may also come from reducing the contribution of a side reaction, e.g. the reaction of a labile moiety on a molecule which itself undergoes a reaction. Here, control over the temperature, i.e. the avoidance of hot spots, is the key to increasing selectivity. In this respect, the oxidative dehydrogenation of an undisclosed methanol derivative to the corresponding aldehyde was investigated in the framework of the development of a large-scale chemical production process. A selectivity of 96% at 55% conversion was found for the micro reactor (390 °C), which exceeds the performance of laboratory pan-like (40% 50% 550 °C) and short shell-and-tube (85% 50% 450 °C) reactors [73,110,112,153,154]. 

Shah and Besser presented results from their development work targeted at a 20 Wei methanol fuel processor-fuel cell system [66]. The layout of the system consisted of a methanol steam reformer, preferential oxidation, a catalytic afterburner and an evaporator. Vacuum packaging was the insulation strategy for the device, which is in line with other small-scale systems described above. A micro fixed-bed steam reformer coupled to a preferential oxidation reactor was then developed by the same group with a theoretical power output of 0.65 W. 

The work at University College London by Cao and Gavriilidis was based on the use of ion etched micro-reactors made in silicon (see Figure 8.7). One of the principal reasons for the experiments was the lack of data on the kinetics of the catalytic selective oxidation of methanol to formaldehyde over a silver catalyst at relevant industrial reactor temperatures, which of course inhibits the design of reactors at the industrial scale able to match the kinetics. The reactor used had a channel of 600 microns in width and the depth is determined by the etching time. 

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