Membrane separation modules

Chan, C. Y., Aatmeeyata, Gupta, S. K. and Ray, A. K. (2000). Multi-objective optimization of membrane separation modules using genetic algorithm, J.Memb. ScL, 176, pp. 177-196. 

Membrane separating modules can be (a) flat sheets (such as continuous column, supported liquid, or polymer film), (b) tabular, (c) capillary, (d) hollow fibers (either coated fibers or supported liquid) and (e) spiral wound. The hollow fiber is similar to the tubular mounting except that hollow fibers typically have a much smaller diameter. 

S.D. Kolev, W.E. van der Linden, Mathematical-modeling of a flow-injection system with a membrane separation module, Anal. Chim. Acta 268 (1992) 7. 

There is a multitude of different configurations that have been proposed in the literature in order to combine the membrane separation module and the reactor into a single unit (Figure 1.4b). Sanchez and Tsotsis [1.24] have classified these configurations for catalytic membrane reactors into six basic types, as indicated in Table 1.1 and Figure 1.5. This classification and acronyms are also applicable to other types of membrane reactors, and will be used throughout this book. 

FIGURE 27.8 Laboratory-scale pilot system consisting of three membrane separation modules in series used for the transport of thiocyanate, (a) Photographic image of the three modules in series, each containing a PIM sandwiched between two meander-shaped flow channels, (b) Photographic image of the membrane after exposure to the channels filled with blue ink. 

The feed is sent to a convective steam reformer where it is partially converted into hydrogen then hydrogen is recovered through a Pd alloy membrane separation module, while the retentate is sent to the next step or recycled to the first module. By means of a heat recovery system, the operating temperature can be reduced to about 450°C before the membrane unit and again increased before the second reactor. It is possible to replicate the RMM until the desired natural gas conversion is achieved. 

The coupling of membrane separation modules and the conventional WGSR reactors through this kind of architecture results in a better overall efficiencies (97.5% as compared to 91% for the reference case). By this configuration, the fuel stream is enriched in H2 by the membrane reactor and requires only polishing by PSA. 

In an alternative configuration, the selective membrane is placed outside the reactor in units located downstream (open architecture. Fig. 11.3). In this case, after the membrane separation module another reaction unit is required, in which the enhancement in hydrocarbon conversion may be observed. 

However, according to the literature (Sanchez Tsotsis, 1996), there are different configurations of MR to combine the membrane separation module and reactor into a single unit. The six basic types of configurations are indicated in Table 7.3. 

Han et al. developed a membrane separation module for a power equivalent of lOkWei [402], shown in Figure 7.42. A palladium membrane containing 40wt.% copper and of 2 5-pm thickness was diffusion bonded onto a metal frame. The separation module for a capacity of 10 Nm h hydrogen had a diameter of 10.8 cm and a length of 5 6 cm. The reformate fed to the modules contained 65 vol.% hydrogen... 

Figure 7.42 Membrane separation modules for a 10-kWei methanol fuel processor as developed by Han et al. [402].

A methanol fuel processor based on steam reforming in a fixed catalyst bed and membrane separation was described by LedjefF-Hey et al. [401]. The system consisted of an evaporator, a steam reformer, which was supplied with heat by a catalytic burner, and a membrane separation module, which carried membranes of a very high thickness of 7.5 mm. At 5-bar system pressure and S/C ratio of 2.0, a hydrogen flow equivalent to 1.1-kW thermal power was generated by the system, which had an overall efficiency of 54%. Between 40 and 62% of the hydrogen produced by the reformer could be separated by the membrane module. Leakages in the sealing of the membrane module led to carbon monoxide spill-over to the permeate, but this was limited to carbon monoxide concentrations well below 100 ppm. 

This development work was performed in several stages 2- and 10-kW systems [402] were built before the final size of 25 kW was achieved with the third generation prototype. The size of the 10-kW second generation methanol fuel processor was still fairly bulky at more than 860 L [402]. However, the efficiency of about 82% was relatively high already for membrane separation. The membrane separation modules have been described in Section 7.4. Figure 9.14 shows the gas flows and gas compositions for the 10-kWei system. About 95% methanol conversion was achieved and the reformate contained 3 vol.% carbon monoxide. The system had a high startup time demand of between 30 and 60 min. The response to the load changes required between 2 and 3 min [402]. 

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