Membrane reactor, selective removal product

When a sweep gas is used, a typical family of conversion curves for a membrane reactor where a product is selectively removed to effect equilibrium displacement to increase reaction conversion arc shown in Figure 9.2 for dehydrogenation of cyclohexane with Ar as the sweep gas [Itoh et al., 1985]. When no membrane is employed in the reactor, the maximum achievable conversion is that of the equilibrium value. When a permselective... 

In addition to using membrane reactors lo remove a reaction product in order to shift the equilibrium toward completion, we can use membrane reactors to increase selectivity in multiple reactions. This increase can be achieved by injecting one of the reactants along the length of the reactor. It is particularly effective in panial oxidation of hydrocarbons, chlorination, ethoxylation. hydrogenation, nitration, and sulfunation reactions to name a few. ... 

Other methods of combining reaction with separations, such as extraction, crystallization, and adsorption, are being explored, but none have been used on a large scale. Using reactors with membranes that selectively remove a reaction product is a very promising development, but improvements in membrane permeability, selectivity, and high-temperature stability are needed for practical processes. 

As an example the use of ceramic membranes for ethane dehydrogenation has been discussed (91). The constmction of a commercial reactor, however, is difficult, and a sweep gas is requited to shift the product composition away from equiUbrium values. The achievable conversion also depends on the permeabihty of the membrane. Figure 7 shows the equiUbrium conversion and the conversion that can be obtained from a membrane reactor by selectively removing 80% of the hydrogen produced. Another way to use membranes is only for separation and not for reaction. In this method, a conventional, multiple, fixed-bed catalytic reactor is used for the dehydrogenation. After each bed, the hydrogen is partially separated using membranes to shift the equihbrium. Since separation is independent of reaction, reaction temperature can be optimized for superior performance. Both concepts have been proven in bench-scale units, but are yet to be demonstrated in commercial reactors. 

Novel unit operations currently being developed are membrane reactors where both reaction and separation occur simultaneously. Through selective product removal a shift of the conversion beyond thermodynamic equilibrium is possible. The membrane itself can serve in different capacities including (i) a permselective diffusion barrier, (ii) a non-reactive reactant distributor and (iii) as both a catalyst and permselective membrane [44]. 

Improved selectivity in the liquid-phase oligomerization of i-butene by extraction of a primary product (i-octene C8) in a zeolite membrane reactor (acid resin catalyst bed located on the membrane tube side) with respect to a conventional fixed-bed reactor has been reported [35]. The MFI (silicalite) membrane selectively removes the C8 product from the reaction environment, thus reducing the formation of other unwanted byproducts. Another interesting example is the isobutane (iC4) dehydrogenation carried out in an extractor-type zeolite CMR (including a Pt-based fixed-bed catalyst) in which the removal of the hydrogen allows the equilibrium limitations to be overcome [36],... 

Fundamental aspects of chemical membrane reactors (MRs) were introduced and discussed focusing on the peculiarity of MRs. Removal by membrane permeation is the novel term in the mass balance of these reactors. The permeation through the membrane is responsible for the improved performance of an MR in fact, higher (net) reaction rates, residence times, and hence improved conversions and selectivity versus the desired product are realized in these advanced systems. The permeation depends on the membranes and the related separation mechanism thus, some transport mechanisms were recalled in their principal aspects and no deep analysis of these mechanisms was proposed. 

The field of chemical process miniaturization is growing at a rapid pace with promising improvements in process control, product quality, and safety, (1,2). Microreactors typically have fluidic conduits or channels on the order of tens to hundreds of micrometers. With large surface area-to-volume ratios, rapid heat and mass transfer can be accomplished with accompanying improvements in yield and selectivity in reactive systems. Microscale devices are also being examined as a platform for traditional unit operations such as membrane reactors in which a rapid removal of reaction-inhibiting products can significantly boost product yields (3-6). 

A novel type of membrane reactor, emerging presently, is the pervaporation reactor. Conventional pervaporation processes only involve separation and most pervaporation set-ups are used in combination with distillation to break azeotropes or to remove trace impurities from product streams, but using membranes also products can be removed selectively from the reaction zone. Next to the polymer membranes, microporous silica membranes are currently under investigation, because they are more resistant to chemicals like Methyl Tertair Butyl Ether (MTBE) [23-24], Another application is the use of pervaporation with microporous silica membranes to remove water from polycondensation reactions [25], A general representation of such a reaction is ... 

As pointed out in Section 9.3.2.1, the most common application of CMRs concerns equilibrium-restricted reactions [1-12]. The selective removal of a reaction product from the reaction zone through a membrane will shift the equilibrium, leading to higher conversions when compared to conventional (nonmembrane) reactors. 

In membrane reactors, the reaction and separation processes take place simultaneously. This coupling of processes can result in the conversion enhancement of the thermodynamically-limited reactions because one or more of the product species is/are continuously removed. The performance of such reactors depends strongly on the membrane selectivity as well as on the general operahng conditions which influence the membrane permeability. 

The core to the concept of an inorganic membrane reactor is to steer a membrane-selective species of a reaction mixture in or out of the membrane, which also serves as part of the reactor boundaries, to improve either the reaction conversion or product distribution. This is accomplished primarily by equilibrium displacement due to the permselective removal of a product or by avoiding or reducing undesirable side reactions (between certain chemical species including intermediate products) via controlled addition of some key reactant to the reaction zone through the membrane. 

Corrosive reaction streams. In some application environments, the reactive or corrosive nature of one or more of the reaction components in a membrane reactor can pose a great technical challenge to the selection as well as the design of the membrane element Feed streams often contain some Impurities that may significantly affect the performance of the membrane. Therefore, attention should also be paid to the response of the selected membrane material to certain impurities in the reactant or product streams. Care should be taken to pretreat the feed streams to remove the key contaminants as far as the membrane is concerned in these cases. For example, palladium alloy membranes can not withstand sulfur- or carbon-containing compounds at a temperature higher than, say, 500 C [Kamcyama et al., 1981]. Even at lOO C, the rate of hydrogen absorption (and, therefore, permeation) in a pure palladium disk is... 

For the first type of reactions (A n > 0) the PFR and the CSTR operated at the permeate-side pressure perform better than the CMR or the PBIMR. The performance of the CMR is slightly belter than that of PBIMR with catalysts on the feed side when the pressure drop across the membrane is low. For the second type of reactions (A n s 0), both CMR and PBIMR perform better than the conventional PFR and the CSTR due to the equilibrium displacement induced by selective removal of a product The PBIMR is preferred over the CMR at a longer space time. For the third type (A n < 0), the PBIMR outperforms any other reactors at a longer diffusional space time. The CMR in this case does not provide advantages due to the undesirable equilibrium effect induced by the pressure variation in the membrane. 

The membrane reactors and their models discussed so far utilize the permselective properties of the membranes. The membranes which can be catalytic or inert with respect to the reactions of interest benefit the reactor performances mostly by selectively removing a product or products to effect the equilibrium displacement. 

---