Overall selectivity membrane reactors

The removal of products to increase conversion in equilibrium-limited reactions is indeed one of the hrst applications that comes to mind when considering membranes in a reaction environment. To make beneficial the use of the membrane reactor, the membrane employed must be able to separate preferentially at least one of the products from the reaction mixture, at a reasonable rate and selectivity. If the selectivity of the membrane is insufficient, reactant loss will become significant and the overall conversion in the membrane reactor will be lower than in a conventional packed bed reactor. If the permeation rate is insufficient, the ratio between the membrane surface area required and the volume of the catalyst bed will become unrealistic. 

In recent years, new concepts to produce hydrogen by methane SR have been proposed to improve the performance in terms of capital costs reducing with respect to the conventional process. In particular, different forms of in situ hydrogen separation, coupled to reaction system, have been studied to improve reactant conversion and/or product selectivity by shifting of thermodynamic positions of reversible reactions towards a more favourable equilibrium of the overall reaction under conventional conditions, even at lower temperatures. Several membrane reactors have been investigated for methane SR in particular based on thin palladium membranes [14]. More recently, the sorption-enhanced steam methane reforming (Se-SMR) has been proposed as innovative method able to separate CO2 in situ by addition of selective sorbents and simultaneously enhance the reforming reaction [15]. 

Using membrane reactors, it has been found that incorporation of 10% ethane into a methane feed on the membrane oxidation surface can lead to a significant increase in ethylene yield [116]. In this work it was found that overall selectivity to ethylene was 94% and methane conversion 36%, indicating that the heuristic conversion -I- selectivity =100 % was significantly exceeded. 

Synthesis of membranes with high permeability and selectivity, that is, oriented and thin zeolite membranes. Optimal MR operation requires the membrane flux to be in balance with the reaction rate. A large number of factors - such as the support, organic additives, temperature, and profile - have a significant influence on the microstructure and overall quality of the membrane. However, the precise correlation between the synthesis procedure and conditions and the properties of the resultant zeolite membranes is not clear. In contrast, the majority of membranes synthesized so far are MFI-type zeolite membranes that have pore diameters 5 A, which are still too big to separate selectively small gaseous molecules. Zeolite membranes with pores in the 3 A range should be developed for membrane reactors, to separate small gas molecules on the basis of size exclusion. In addition, a method to produce zeolite membranes without non-zeolite pores or defects has to be found. 

Zeolites may show an intrinsic catalytic activity. This on the one hand suggests their application to reactions which are catalysed by zeolites. On the other hand, when the zeolite layer or the membrane reactor hosts a different catalyst, the possible negative effect of the zeolite on the overall selectivity or conversion needs to be evaluated in order to reduce it or to select the most proper zeolite. 

A second option is to apply the membrane on the particle level (millimeter scale) by coating catalyst particles with a selective layer. As a third option, application at the microlevel (submicrometer scale) is distinguished. This option encompasses, for example, zeolite-coated crystals or active clusters (e.g., metal nanoparticles). Advantages of the latter two ways of application are that there are no sealing issues, it is easy to scale-up, the membrane area is large per unit volume, and, if there is a defect in the membrane, this will have a very limited effect on the overall reactor performance. Because of these advantages, it is believed that using a zeolite... 

Multifunctional reactors. Apart from the applications exclusively in the reaction engineering field, fixed beds can also be found integrated in apparatus with separation purposes. Two major classes can be identified membrane [16, 17] and chromatographic (adsorptive) reactors [18]. In both cases, a familiar configuration involves a packed bed with a permeable, selective wall and/or packed with a catalyst/ adsorbent. The latter is found in simulated moving bed reactors [19, 20], which can be seen as an arrangement of several of these beds. The overall effect from the coexistence of reaction and separation functionalities is the increase of conversion in equilibrium-limited systems, which are shifted due to the simultaneous separation of the products. 

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