Membrane reforming

One of the possible problems in a steam reforming membrane reactor is the formation of carbon, either by cracking of methane (Reaction 4) or the Bou-douard reaction (Reaction 5). 

Mixet/vaporizet Catalytic reformer membrane microreactor... 

La2Cu04, Sr2Cu04. As we show in chapter 6, when a perovskite forms a composite or intergrowth with other structures, new compounds of interest in catalysis can be formed (such as in high-temperature superconducting copper oxides) and EM is used to determine the structures and properties of these complex compounds. The merits of using perovskites in steam reforming, membrane catalysis and fuel cells are discussed in chapter 6. 

For operation in a steam reformer, membranes must be found with a proper balance between permeance and selectivity. Ideally, a membrane with both high selectivity and high permeance is required, but one may expect on forehand that, typically, attempts to maximise one are compromised by a reduction in the other. State-of-the-art hydrogen-selective membranes were already discussed in chapter 1 and the reader is referred to that for more information on suitable membrane types. 

Basile A, Tosti S, Capannelli G, VituUi G, lulianelli A, Gallucci F, Drioli E (2006) Co-current and counter-current modes for methanol steam reforming membrane reactor experimental study. Catal Today 118 237-245... 

Table 5.2 Operating conditions to be imposed for the steam reforming membrane reactor...

On the other hand, the main eritieism in steam reforming membrane reactors is the necessity to impose the same operating conditions for membrane and catalyst, whereas the catalyst should be at high temperature due to reactions endothermicity while membrane temperature must not exceed a threshold for assuring its stability. Considering that for the present technology the thermal limit for Pd-based selective dense membrane has to be imposed at values lower than 800 K, the MR performance is limited as well. Calculations reported in this chapter show that, at industrial operating conditions, methane conversion in MRs is limited at 35% about. 

A. Matzakos, S. L. Wellington, T. Mikus and J. M. Ward, Integrated flameless distributed combustion/steam reforming membrane reactor for hydrogen production and use thereof in zero emissions hybrid power system, U.S. Patent 6 821 501, 2004. 

Figure 1. Fuel heated pilot natural gas (feed) steam reforming membrane reactor (by ECN)...

Table 10.2 Main characteristics of the plants proposed in the literature using reforming membrane reactors for H2 separation... 

Another measure to increase the hydrogen partial pressure difference between permeate and retenate is to use sweep gas on the retenate side. Because the hydrogen requires humidification for low temperature PEM fuel cells to prevent membrane dry-out, steam is the preferred sweep gas [405]. OHany et al. highlighted the effect of steam as the sweep gas for the permeate in a methane steam reforming membrane reactor [406]. Higher methane conversion was observed, which originated from back-diffusion of steam from the permeate to the reaction side of the membrane, which increased the S/C ratio and consequently the conversion. 

Barbieri et al. also performed experimental work on a methane steam reforming membrane reactor, which will be discussed in Section 7.1.4. 

Marigliano et al. compared the performance of two different types of tubular methane steam reforming membrane reactors by numerical simulations [413]. In the first, the catalyst was packed into the palladium/silver membrane tube, and in the second it was positioned in the annular region surrounding the membrane tube. Both configurations were heated from the outside. Owing to the indirect heat transfer to the catalyst bed inside the membrane tube, methane conversion was lower in this instance, and under certain conditions it was even inferior to a conventional fixed bed... 

Caravella A, Maio F P D, Renzo A D (2008) Optimization of membrane area and catalyst distribution in a permeative-stage membrane reactor for methane steam reforming. / Membrane Sci, 321,209-221... 

Among the hydrogen-selective membranes that can be assembled in a steam reforming membrane reactor, the Pd-based dense membranes are surely the most interesting. 

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