Introduction and Objectives
1.1 Introduction
1.1.4 Membrane reactors
1.1.4.1 Classification of MRs based on their configuration
Catalytic reactors based on ceramic membrane are classified into three categories and these are (a) Inert membrane reactors (IMR), used to add or remove certain species from the
reactor without direct participation of membrane in the reaction, (b) Catalytic membrane reactors (CMR), itself a catalyst or become catalytically active during preparation by addition of catalyst precursors, and (c) Combination of CMR and IMR, catalytically active both by inside and outside of the membrane. These categories are explained by different configurations and these are labelled as Configuration A, B, C, D, E, F, G and H as shown in Figure 1.3. Inert membrane reactors (IMR) are explained by configuration A, used to add or remove particular species from reactor without direct participation of the membrane in reaction. The removal of at least one reaction product involves equilibrium displacement and offers higher reaction yield, e.g., Removal of hydrogen in dehydrogenation reactions is the most widely used application for this type of configuration. This configuration is also been fit to other processes such as decomposition of H2S and H2O and production of synthesis gas [11,12].
Configuration A provides both selective (e.g. Pd or Ag-based alloys on ceramic
substrates) and preferential (removal of product silica, alumina, titania, glass, zeolite membranes etc.) separation as it is confirmed by several scientists. In this case, the membrane does not participate in the reaction directly, but it is used to add or remove certain species from the reactor. The most widely used application involves equilibrium displacement by removal of at least one reaction product [13,14].
Configuration B offers higher yield through reaction coupling. In this case, on both sides of the membrane, complementary processes are run that use either the permeated species (chemical coupling, e.g., dehydrogenation/hydrogenation, or dehydrogenation/combustion reactions), or the heat generated in the reaction (thermal coupling, exothermic/endothermic processes). The reactions often use different catalysts, which would be packed on opposite sides of the membrane tube [15,16].
Configuration C describes the distribution of a reactant to a fixed bed of catalyst packed in the opposite side of the membrane. IMRs of this configuration (may be meso- or microporous membranes) have already been used successfully as oxygen distributors in methane oxidative coupling [17,18] and the production of olefins and oxygenates [19,20] from the oxidation of alkanes. The membrane used for the distribution of oxygen in oxidation processes is safer as it reduces formation of hot spots, runaway reactions and gives greater selectivity through control of the concentration of the selected species along the reactor with respect to the conventional feed arrangements. Porous membranes with a dense layer can also be applied for reactant distribution but have some difficulties in attaining high permeation fluxes related to oxidation reactions.
Reactant distribution can also be achieved using porous membranes with a thin but dense permselective layer. In the case of oxidation reactions, this would have the important advantage of using air instead of oxygen in the oxygen-supply side. However, there are few results reported to date, which is mainly due to the difficulties in attaining sufficiently high permeation fluxes (which is usually achieved by reducing the thickness of the dense layer), while at the same time maintaining the membrane properties during prolonged exposure at operating conditions. The only clear advantage with respect to reactant distribution using porous membranes would be available from Configuration D where at least in principle, the oxygen species transported through the membrane could react before recombination and desorption takes place. This would completely avoid the presence of gas phase oxygen, and could certainly represent a valuable alternative provided that a membrane with sufficiently high reaction selectivity and permeability to oxygen can be developed. In this case the reaction on one side of the membrane acts as an efficient oxygen sink resulting in an enhanced oxygen transport across the membrane.
Configuration E defines the separation of reactants on both sides of a porous catalytic
membrane in which the reaction takes place in a small zone or a plane inside the porous structure as the reaction rate is higher than the mass transport, avoiding the slip of reactants to the opposite side and also helps to reduce the unwanted side reactions. By changing the reactant concentrations outside the membrane, the position of the reaction plane can be shifted to a new location where transport rates to the reaction zone are again matched by the reaction stoichiometry, which gives a lower residence time in the reaction zone, and also reduces further reaction of partial oxidation products [21]. The same principle has been demonstrated with experiments and model calculation by CO [22] and H2S oxidation [23]. The performance of a membrane reactor with separate feeding of reactant for the catalytic combustion of methane based on this concept has been reported recently [24].
Configuration F is a modified version of configuration C. The objective of this
configuration is to minimize the concentration of oxygen in the reacting environment and to deliver a sharp distribution of the active component across the entire membrane [25-27]. In this case, the goal is the same (reduce the concentration of oxygen in the reacting environment), although the oxygen partial pressure is now lowered by feeding it through a diffusion layer of sufficient resistance. Oxygen diffusion can take place by itself or in the presence of a stagnant fluid filling the pores of the membrane. This can be achieved by feeding an inert species at approximately the same partial pressure to both sides of the membrane. The diffusion zone is followed by a catalytic layer, where the reaction of oxygen and the reactant permeated from the opposite side takes place.
The purpose of using Configuration G is to improve the contact in gas-liquid-solid systems by providing a distinct contact zone, as shown in Figure 1.3.
Figure 1.3 Membrane reactors based on the principle of configurations A to H H2
A B+H2
Configuration A
H2
AB+H2
Configuration B O2
O2
Configuration C Hydrocarbon
Product
Porous inert layer Dense selective catalytic layer
Air
O2 O2 O2
Hydrocarbon Product Configuration D H2 O2
H2O
C A B
Reaction Zone
B C
A C
A + B C Configuration E
O2
Configuration F
Diffusion
Diffusion/Reaction
A C
A C
D O2
O2 O2 (Desired) (Undesired)
Gas flow
Liquid flow Configuration G
Configuration H CO2, H2O
CO2, H2O Air +
VOCs
This configuration has the potential to overcome the difficulty of catalyst recovery that appears in slurry reactors. The concept has already been popular in the case of hydrogenation reactions over Pt/Al2O3 catalysts [28,29].
Configuration H delivers the same idea like configuration G. The last two configurations aim to improve contact efficiency as well as conversion by decreasing mass transfer resistance.
This approach has been employed to prepare a catalytically modified fly ash filters for alcohol dehydration and for the reduction of the nitrogen oxides with NH3 [30,31].