Performances in membrane reactors

Research has, thereafter, focused on membrane development and progressively has addressed problems related to their scale-up and related to specific applications. 

12 Feed pressure dependence of CH4 conversion in a bench-scale tubular membrane reformer. Ex stands for experimental and Eq for equilibrium. (After Li eta ., 2011.) 

The experiment was carried out with a feed gas simulating both a classical reforming mixture (steam/CH, ratio = 3) and apre-reformed mixture in a natural gas combined cycle (NGCC) power plan with a steam/CH, ratio of 3.2 (0.2% CO, 3.5% CO2,14.6% Hj, 19.4% CH and 62.2% HO). As shown in Fig. 3.12, despite unfavourable thermodynamics, CH conversion increases with increasing feed pressure and remains well above the corresponding thermodynamic equilibrium values. The maximum CH conversion and H production rate were 73.4% and 1.3 Nm /h, respectively, at a feed/permeate pressure of 35/5 bar. Stable performance was found both for the membranes and the membrane reactor for a period of about 30 days. For comparison, performances obtained by other researchers with Pd membranes deposited by electroless plating, on various supports, are shown in Table 3.6. 

The WGS reaction has been extensively studied, as reported by Basile et al (2010) in an exhaustive literature review experiment has been initially focused on WGS at low temperatures, for example, 250-350°C et al, 1996), 

Membrane type Thickness (pm) Operating conditions T( C) P(bar) S/C Catalyst CH4 conversion (%) Reference 

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A very similar combined process scheme integrating OCM and steam reforming of methane (SRM) was suggested at the same time by Tiemersma et al [44]. OCM is performed in membrane reactor for distributed O2 feeding. The membrane reactor tubes are immersed into a fluidized-bed reforming reactor for optimal heat transfer. Simulation of such process based on kinetic data obtained... 

This chapter will focus on composite membranes for hydrogen separation and, specifically, on Pd and Pd-alloy composite membranes. Preparation methods will be described, and chemical and physical stability in relation to specific applications will be discussed. Performances in membrane reactors and significant developments up to and including the prototype scale will be also presented. 

Figure 3.30 Ignition/extinction loops for ammonia oxidation over platinum performed in micro reactors with different membranes [19],...

In both situations the interaction of the medium inside the pore with the pore wall (1) is increased (2) or changed which affect the transport and separation properties (surface diffusion, multilayer adsorption) and/or help overcome equilibrium constraints in membrane reactors. Membrane modifications can be performed by depositing material in the internal pore structure from liquids (impregnation, adsorption) or gases. Several modification possibilities are schematically shown in Figure 2.3. Some results obtained by Burggraaf, Keizer and coworkers are summarized in Table 2.7. Composite structures on a scale of 1-5 nm were obtained. 

Other evidence for the importance of catalysis in anode performance came from an examination of the products formed by Cu—ceria—YSZ and Cu—molyb-dena—YSZ anodes in membrane—reactor measurements. The anodes in these experiments both had... 

Membrane reactors can offer an improvement in performance over conventional reactor configurations for many types of reactions. Heterogeneous catalytic reactions in membrane reactors [1] and the membranes used in them [2,3] have been reviewed recently. One well studied application in this area is to remove a product from the reaction zone of an equilibrium limited reaction to obtain an increase in conversion [4-10]. The present study involves heterogeneous... 

All peptide-catalyzed enone epoxidations described so far were performed using insoluble, statistically polymerized materials (neat or on solid supports). One can, on the other hand, envisage (i) generation of solubilized poly-amino acids by attachment to polyethylene glycols (PEG) and (ii) selective construction of amino acid oligomers by standard peptide synthesis-linked to a solid support, to a soluble PEG, or neat as a well-defined oligopeptide. Both approaches have been used. The former affords synthetically useful and soluble catalysts with the interesting feature that the materials can be kept in membrane reactors for continuously oper-... 

The choice of reactor configuration depends on the properties of the reaction system. For example, bioconversions for which the homogeneous catalyst distribution is particularly important are optimally performed in a reactor with the biocatalyst compartmentalized by the membrane in the reaction vessel. The membrane is used to retain large components, such as the enzyme and the substrate while allowing small molecules (e.g., the reaction product) to pass through. For more labile molecules, immobilization may increase the thermal, pH and storage stability of biocatalysts. 

Reforming reactions have been studied in membrane reactors as well. Most well-known is the steam-reforming of various hydrocarbons [10-13], especially methane steam-reforming which is the major source of hydrogen in the world [14], Some research has been performed on CO2 reforming of methane [15] and also a considerable amount of effort has been put in performing the water-gas shift reaction in a membrane reactor [16,18],... 

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. 

Several investigators have faced the problem of modeling of membrane reactors either to achieve a proper interpretation of their experimental data or to assess the role of the various operating parameters (temperature, membrane permeability and permselectivity, feed flow rates, and concentrations, etc.) on the performance of membrane reactors. In some other cases [61,138] modeling studies helped to point the way toward future experimental work concerning, e.g., the need for thinner or more permeable or more stable membranes to outperform conventional technologies for given applications. 

The need to govern heat balances properly in membrane reactors will certainly become a major task if large-scale industrial units are ever to be put into operation. Whether the performed reaction is endothermic (dehydrogenation) or exothermic (oxidation), innovative means to supply or remove heat from large-scale membrane reactor modules will have to be designed. The isothermicity assumption valid for several lab-scale membrane reactors will not hold anymore, and much more complex modeling will certainly have to be developed. 

Several other recent modelling membrane reactor studies are also worth discussing, Varma and coworkers [127] have analyzed the effect that nonuniform catalyst distribution on the membrane itself (for CMR and PBCMR applications) and in the catalyst bed (for PBMR applications) has on membrane reactor performance. Conventional membrane reactor models were utilized by a number of groups to model their experimental data. Shu and co-workers [33]... 

On the other hand, the oxidative coupling reaction of CH4 in the presence of O2, even when performed in membrane type reactors,188 is mainly catalysed by metal oxides catalysts.185 Also, oligomerisation, aromatisa-tion, and the partial oxidation apply non-metallic heterogeneous catalysts (such as zeolites). The reader is therefore directed to some excellent reviews on these subjects.189,190 At this point, it is perhaps relevant to introduce the formation of carbon nanofibres or nanotubes from methane, these being catalysed by metal nanoparticles, but at this moment this is not considered as a Cl chemistry reaction. Again we direct the attention of the reader to some reviews on this type of process.191 192... 

Both theoretical and experimental studies have been performed on palladium-based membrane reactors for the water-gas shift reaction. Ma and Lund simulated the performance achievable in a high temperature water-gas shift membrane reactor using both ideal membranes and catalysts [18]. By comparing the results obtained with those related to the existing palladium membrane reactors, they concluded that better membrane materials are not needed, and that higher performances mainly depend on the development of a water-gas shift catalyst not inhibited by CO2. Marigliano et al. pointed out how the equilibrium shift conversion in membrane reactors is an increasing function of the sweep factor (defined as the ratio between the flow rate of the sweep at the permeate side and the flow rate of CO at the reaction side) [19]. The ratio is an index of the extractive capacity of the system. 

Due to the availability of the mentioned overviews it is not the goal of this chapter to consider the whole field of membrane reactors. Rather, the discussion below will be focused on presenting simplified and more detailed mathematical models capable of describing the performance of membrane reactors. Although there are several studies available for analyzing the combination of reaction and membrane separation (e.g. Salomon et al., 2000 Struis and Stucki, 2001 Wielandet al., 2002 Patil etal., 2005 Rohde etal., 2005) there is a need to analyze in more detail specific features of membrane reactors. The focus of this chapter will be the development and application of simplified and also more detailed mathematical models for packed-bed membrane reactors in which certain reactants are dosed over the reactor wall using nonselective membranes. This type of membrane reactor is sometimes also-called a distributor (Dalmon, 1997 Julbe et al, 2001). Despite this restricted focus of the work, most of the concepts considered should be applicable also in the analysis of other types of membrane reactors. 

While performance of catalytic reactors is usually plotted vs. catalyst weight (W to molar feed rare (Fo) ratio, the performance of membrane reactor depends on that parameter as well as on the membrane surface area. In fact in the limit of very fast reaction the performance depends only on the equilibrium coeflScient and the ratio of the feed rate to the overall hydrogen separation rate. This ratio is sometimes referred to as a membrane P6clet number ... 

The packed bed membrane reactor configuration is the first and most studied configuration for hydrogen production in membrane reactors. In fact, the first studies on membrane reactors mainly focussed on the effect of the hydrogen permeation through the membranes on the performance (in terms of conversion) of the reaction system. Thus, it was relatively straightforward to compare (both experimentally and theoretically) the performance of two packed bed reactors in one of which the tubular wall was replaced by a membrane. 

The kinetics of reactions is specific for different reaction systems and processes and valid for isothermal and nonisothermal reactors. The effects of the kinetics on the conversion, selectivity, or yield depend on the reaction and may be quite pronounced. Liquid or gas phase reactions with high heat capacity can be performed in specific reactors, which operate isothermally or not. We will study the most common cases such as semibatch reactors, recycle reactors, fixed-bed reactors, and reactors with membranes. 

Although reactor performance in the laboratory and assessments by mathematical model simulations have demonstrated the excellent potential of membrane integration in chemical processes, the performance of membrane reactors remains heavily dependent on the behavior of the selective membrane in terms of permeability, selectivity and stability. Some technological challenges remain, such as the optimization of the fabrication method, the durability of the membrane in a contaminating environment and the... 

Nishu T (2009), Reforming performance of hydrogen production module based on membrane on catalyst . Proceedings of 9th International Conference on Catalysis in Membrane Reactors, Lyon, France. 

Andres, M. B., Chen, Z., Grace, J. R., Elnashaie, S. S. E. H., Jim Lim, C., Rakib, M., et al. (2009). Comparison of fluidized bed flow regimes for steam methane reforming in membrane reactors a simulation study. Chemical Engineering Science, 64, 3598—3613. Ayturk, M. E., Kazantzis, N. K., Ma, Y. H. (2009). Modeling and performance assessment of Pd- and Pd/Au-based catalytic membrane reactors for hydrogen production. Energy Environmental Science, 2, 430—438. 

As a result, milder conditions are required in membrane reactors compared with a conventional reactor system to achieve the same performances. 

Thus, the current density, flowing across the cell in the direction perpendicular to the electrode surface, is one of the most important factors that influences the performance of the membrane reactor. Second, the activation energy is a function of temperature, and, in a sense, the equflibrium voltage deceases with increasing temperature. Therefore, a proper temperature can promote the electrode reactions. In addition, because the electrolyte concentration has an influence on the equilibrium potential of the electrochemical reaction and resistance in the electrolyte, the concentration of the electrolyte is also a key factor in membrane reactors. Combined with the previous contents, the operating parameters of the membrane reactor are mainly as follows pH, current density, temperature, and electrolyte concentration. 

Improvements in membrane reactor performance may also be obtained by further developments on new processes and methods for scaling up. Electrochemical desulfurization of gases by a membrane reactor is a promising and proven technology. 

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