Micro-membrane reactor

Micro-structured (membrane) reactors are quite interesting due to their (i) improved mass and heat transfer owing to the reduction of the scale length in the micro-channels (ii) removal of mass transfer limitations (concentration polarization) (iii) high degree of process intensification by integrating different process steps in a small-scale device. 

In particular, by comparing the performance of the same membrane in different configurations, the authors showed that in tubular configuration the extent of concentration polarization is the limiting step for hydrogen permeation (as also indicated above), while with the same membrane used in micro-channel configuration the concentration polarization effect can be completely neglected [27, 57]. 

The reactor consists of 6 micro-channels 13 mm long and a section 1 mmx 1 mm, while the membranes used are self-supported Pd-based membrane thinner than 3 pm. It has been shown that in this configuration a 1.4 pm thick membrane can withstand a differential pressure 470 kPa. 

In another work [58], the authors studied the influence of CO and CO2 (components always present in reforming reactions) on the permeation of hydrogen. Although concentration polarization is not a problem, at low temperatures CO preferentially absorbs on the Pd surface (as found for all kind of configurations) depleting the hydrogen permeation rate. 

Other studies on membrane micro-reactors were carried out for integrating different process steps in one micro-device for portable hydrogen production for fuel cell applications (see a.o. Ref. [60-62]). 

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Figure 3.16 Schematic of Si-chip catalyst membrane micro reactor. Top view (A), end-on cross section of reaction channel (B) side-view cross section of reaction channel (C) [60].

Figure 3.17 Microfabrication sequence for the silicon component of the catalyst membrane micro reactor [57],...

Figure 3.29 Ammonia oxidation over a Pt catalyst in different membrane micro reactors. Experimental results show good temperature uniformity across the catalyst regions [19].

Franz et al. [93] developed a palladium membrane micro reactor for hydrogen separation based on MEMS technology, which incorporated integrated devices for heating and temperature measurement. The reactor consisted of two channels separated by the membrane, which was composed of three layers. Two of them, which were made of silicon nitride introduced by low-pressure chemical vapor deposition (0.3 pm thick) and silicon oxide by temperature treatment (0.2 pm thick), served as perforated supports for the palladium membrane. Both layers were deposited on a silicon wafer and subsequently removed from one side completely... 

O. Wolfrath, L. Kiwi-Minsker, A. Renken, Filamenteous catalytic beds for the design of membrane micro-reactor propane dehydrogenation as a case study, in M. Matlosz, W. Ehrfeld, J.P. Baselt (Eds.), Proceedings of the 5th International Conference on Microreaction Engineering (IMRET 5), Springer, Berlin, 2001, p. 191. 

Jani M, Aran HC, Wessling M, and Lammertink RGH. Modeling of gas liquid reactions in porous membrane micro-reactors. J. Membr. Sci. 2012 419-420 57-64. 

Different types of membrane reactors for hydrogen production have been proposed in the literature. Most of the previous work has been performed in packed bed membrane reactors (PBMRs) however, there is an increasing interest in novel configurations such as fluidized bed membrane reactors (FBMRs) and membrane micro-reactors (MMRs), especially because better heat management and decreased mass transfer limitations can be obtained in these novel reactor configurations. 

Finally, Alfadhel and Kothare [63] modelled a membrane micro-reactor for WGS reaction with a simplified (ID, isothermal) model, suggesting the limitation of the simple model in simulating a membrane micro-reactor. 

As mentioned, membrane micro-reactors are very interesting systems to be studied in case external mass transfer limitations could not be ignored (such as for high-flux Pd-based membranes) and when different steps need to be coupled (coupling exothermic and endothermic reaction is an example). However, more research is required before being... 

Rahman M A, Garcfa-Garcfa F R, Irfan Hatim M D, Kingsbury B F K and Li K (2011), Development of a catalytic hollow fibre membrane micro-reactor for high purity H2 production,/ Membrane Sci, 368,116-123. 

Development of a catalytic hollow fibre membrane micro-reactor for high purity production. Journal of Membrane Science, 368, 116-123. 

Garcia-Garcia, F.R. et al.. Catalytic hollow fibre membrane micro-reactor High purity H-2 production by WGS reaction. Catalysis Today, 2011.171(1) 281-289. 

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