Micro-scale steam reforming reactors

Micro-scale plate-t5q)e reformers and multi-channel reformers are being developed for compact units for small-scale operation [188] [282] [319] [488] — for instance for the use in cars. A better catalyst utilisation (larger catalyst effectiveness factor) can be achieved by catalysing the heat transfer surfaces (catalysed hardware) and by leaving the tubular constraint [166] (refer to Section 1.2.4). Some designs involve a 

This implies that the heat needed for the reforming reaction is provided by conduction, but still the heat transfer over the gas film determines the heating up of the process gas. The multi-channel reformer designs are compact with estimated hourly productivities in the order of 1000 Nm Hz/m reactor/h [319] [488]. The multi-channel reformers may result in fast heating up of the feed stream within milliseconds [313], hence with a possibility to avoid coke from thermal cracking in preheaters and over deactivated catalyst. 

High heat fluxes (100 kW/m ) can be obtained in spite of the mainly laminar flow in the multi-channel systems, where the Nusselt number is constant. This is mainly due to the short distance for heat transfer and temperature driving forces around 100 C as described by Haynes [223]. 

The micro-channel reformers used show high heat transfer rates and the design achieves a high productivity per volume [295]. However, there is little economy of scale and the feasibility is limited to small- to medium-size capacities. 

Rigorous modelling must take the selected geometry into consideration and this usually requires CFD. It must, in addition to heat transfer across the wall and film mass and heat transfer between the gases and catalysts, also include axial heat conduction in the metals due to steep temperature profiles. In addition transient behaviour and interaction between the steep temperature profiles must be understood for a proper design, especially when a reasonable catalyst deactivation is 

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Figure 1.9 Hybrid, multi-scale micro-reactor plant for catalyst testing for propane steam reforming [15],...

One of the benefits of applying micro structured heat exchanger/reactors is the potential of combining an endothermic reaction such as steam reforming with an exothermic reaction such as catalytic combustion. This idea was proposed for the macro scale as a so-called catalytic plate reactor (CPR) by Reay [100],... 

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. 

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... 

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