Dense membrane reactors

In the case of dense membranes, where only hydrogen can permeate (permselectivity for H2 is infinite), the permeation rate is generally much lower than the reaction rate (especially when a fixed bed is added to the membrane). Experimental conditions and/or a reactor design which diminishes this gap will have positive effects on the yield. An increase of the sweep gas flow rate (increase of the driving force for H2 permeation) leads to an increase in conversion and, if low reactant flow rates are used (to limit the H2 production), conversions of up to 100% can be predicted [55]. These models of dense membrane reactors explain why large membrane surfaces are needed and why research is directed towards decreasing the thickness of Pd membranes (subsection 9.3.2.2.A.a). 

Gas/vapor phase hydfogen-consuming reactions using dense membrane reactors... 

Palladium and selective alloys with other metals can be fabricated into dense membrane reactors in foil or tubular form, mostly in thin layers to reduce permeation resistance. In... 

Similar to the case of dehydrogenation or other hydrogen-generating reactions, the use of a dense membrane reactor to remove oxygen from an oxygen-generating reaction such as decomposition of carbon dioxide displaces the reaction equilibrium and increases the conversion from 1.2% (limited by the equilibrium) to 22% [Nigara and Cales, 1986]. This has been confumed by Itoh et al. [1993]. 

Shown in Table 8.6 arc some literature data on the use of dense membrane reactors for liquid- or multi-phase catalytic reactions. Compared to gas/vapor phase application studies, these investigations are relatively few in number. Most of them involve hydrogenation reactions of various chemicals such as acetylenic or ethylenic alcohols, acetone, butynediol, cyclohexane, dehydrolinalool, phenylacetylene and quinone. As expected, the majority of the materials adopted as membrane reactors are palladium alloy membranes. High selectivities or yields are observed in many cases. A higher conversion than that in a conventional reactor is found in a few cases. 

Similar to corresponding cases of dense membrane reactors, the assumption of a packed bed only on the tube side calls for kf = 0 (/ = 1 to I). Likewise, ii = 0 if the catalytic reaction occurs on the shell side instead. 

As will become evident later, the counter-current flow conflguration appears to provide a clear-cut advantage over the co-current flow configuration with respect to the reaction conversion in a dense membrane reactor. 

The above discussion pertains to dense membrane reactors. For the case of a semipermeable membrane reactor which has finite permselectivities for the various reaction components, isothermal operations favor the plug flow membrane reactor over the perfect mixing membrane reactor for both endothermic and exothermic reactions. In the case of exothermic reactions, the difference between the two flow patterns is rather small for low feed temperatures [Mohan and Govind, 1988b]. 

As mentioned in Chapter 8, the understanding of how the material composition of a dense membrane interplays with the operating conditions and the reaction components is far from being sufficient for reliable design of a dense membrane reactor, particularly... 

Based on the above considerations, the types of reactions that are amenable to inorganic membrane reactors in the first wave of industrial implementation will probably be as follows (1) The reactions are heterogeneous catalytic reactions, particularly dehydrogenation processes (2) The reaction temperature exceeds approximately 200°C (3) When the reactions call for high-purity reactant(s) or produces) and the volume demand is relatively small, dense membrane reactors (e.g., Pd-based) can be used. On the other hand, if high productivity is critical for the process involved, porous membrane reactors are necessary to make the process economically viable. 

Silver membranes are permeable to oxygen. Metal membranes have been extensively studied in the countries of the former Soviet Union (Gryaznov and co-workers are world pioneers in the field of dense-membrane reactors), the United States, and Japan, but, except in the former Soviet countries, they have not been widely used in industry (although fine chemistry processes were reported). This is due to their low permeability, as compared to microporous metal or ceramic membranes, and their easy clogging. Bend Research, Inc. reported the use of Pd-composite membranes for the water-gas shift reaction. Those membranes are resistant to H2S poisoning. The properties and performance characteristics of metal membranes are presented in Chapter 16 of this book. 

W. Wang and Y. S. Lin, A theoretical analysis of oxidative coupling of CH4 in a tubular dense membrane reactor. Paper presented at the 3rd International Congress on Inorganic Membranes, July 10-14,1994, Worcester, MA, USA. 

The use of a dense membrane reactor has been studied experimentally by the group at Argonne/Amoco" and by the group at WPI both theoretically and experimentally. ... 

Dense membrane reactors are discussed, emphasizing the most frequently investigated configurations and applications. Factors influencing economics are analyzed and, as a case study, results on the water-gas shift reaction in Pd-based membrane reactors are presented. 

Dense Membrane Reactors for the Water-Cas Shift Reaction 251 Tab. 9.2 M ain factors influencing the economics of membrane reactors. 

Dense Membrane Reactors for the Water-Cas Shift Reaction... 

Jin, W., Gu, X., Li, S. et al. (2000) Experimental and simulation study on a catalyst packed tubular dense membrane reactor for partial oxidation of methane to syngas. Chemical Engineering and Science, 55 (14), 2617-2625. 

Lu Y P, Dixon A G, Moser W R, Ma Y H and Balachandran U (2000a), Oxidative couphng of methane using oxygen-permeable dense membrane reactors , Catal Today, 56,297-305. 

Tong J, Yang W, Cai R, Zhu B and Lin L (2002), Novel and ideal zirconium-based dense membrane reactors for partial oxidation of methane to syngas , Catal LeH, 78,129-137. 

Temperature (°C) Traditional reactor (g) Porous membrane reactor (g) Dense membrane reactor (g)... 

---