Membrane reactors deactivation

One of the most studied applications of Catalytic Membrane Reactors (CMRs) is the dehydrogenation of alkanes. For this reaction, in conventional reactors and under classical conditions, the conversion is controlled by thermodynamics and high temperatures are required leading to a rapid catalyst deactivation and expensive operative costs In a CMR, the selective removal of hydrogen from the reaction zone through a permselective membrane will favour the conversion and then allow higher olefin yields when compared to conventional (nonmembrane) reactors [1-3]... 

After reaching its maximum productivity (after ca. 8 hours.) the [Gl]-Nii2 showed a fast deactivation when applied in continuous catalysis performed in a membrane reactor (Figure 4.12). The fast loss of activity cannot be due to a lack of retention of the catalyst. Due to the high retention measured, this process should be much slower. A model study revealed that this deactivation process probably takes place by the formation of insoluble Ni(III) species (see Section 4.5 for further details). 

The use of such an oxazaborolidine system in a continuously operated membrane reactor was demonstrated by Kragl et /. 58] Various oxazaborolidine catalysts were prepared with polystyrene-based soluble supports. The catalysts were tested in a deadend setup (paragraph 4.2.1) for the reduction of ketones. These experiments showed higher ee s than batch experiments in which the ketone was added in one portion. The ee s vary from 84% for the reduction of propiophenone to up to >99% for the reduction of L-tetralone. The catalyst showed only a slight deactivation under the reaction conditions. The TTON could be increased from 10 for the monomeric system to 560 for the polymer-bound catalyst. 

The other major issue in reactor design concerns catalyst deactivation and membrane fouling. Both contribute to loss of reactor productivity. Development of commercially viable processes using inorganic membrane reactors will only be possible if such barriers are overcome. These subjects will receive greater attention as current R D efforts expand beyond laboratory scale evaluations into field demonstrations. 

Lipases (E.C. 3.1.1.3.) catalyze the hydrolysis of lipids at an oil/water interface. In a membrane reactor, the enzymes were immobilized both on the side of the water phase of a hydrophobic membrane as well as on the side of the organic phase of a hydrophilic membrane. In both cases, no other means for stabilization of the emulsion at the membrane were required. The synthesis reaction to n-butyl oleate was achieved with lipase from Mucor miehei, which had been immobilized at the wall of a hollow fiber module. The degree of conversion reached 88%, but the substrate butanol decomposed the membrane before the enzyme was deactivated. 

We will calculate the reactor performance itself as well as the productivity over time we will see that productivity is influenced by retention of the enzyme catalyst as much as by its deactivation behavior. In the schematic of an enzyme membrane reactor... 

An enzyme membrane reactor allows continuous transketolase-catalyzed production of L-erythrulose from hydroxypyruvate and glycolaldehyde with high conversion, stable operational points, and good productivity (space-time yield) of 45 g (L d) 1, thus best overcoming transketolase deactivation by substrates (Bongs, 1997). 

Removing the product as fast as possible from the reaction mixture could possibly prevent the enzyme from deactivating. This was experimentally tried on pilot scale using an ultrafiltration unit, parallel to the reactor. In this filtration unit the product is separated and the unreacted components and the enzyme are returned to the reactor. An even better option, using a true membrane reactor in which the catalyst (enzyme) would remain on one side and the product would remain on the other side, was not tested. Both options are given schematically in Figure 7. 

K. Hou, M. Fowles and R. Hughes, Potential Catalyst Deactivation Due to Hydrogen Removal in a Membrane Reactor Used for Methane Steam Reforming , Chem. Eng. Sci., 54 3783-91 (1999). 

The first step for the design of an EMR is to select the type of reactor. Extractive membrane reactors are desirable when one of the substrates or products is poorly soluble in aqueous solution or when an undesirable by-product has to be separated, as the membrane acts as a solvent extraction step [99]. Immobilized enzyme reactors are usually applied with materials that enable enhancement of enzymatic stability or preserve enzyme from deactivation by a direct contact with an organic solvent [99]. Finally, direct-contact membrane reactors are the most versatile alternative in processes with soluble compounds. 

In addition to this increase in conversion, other benefits can be expected when using a membrane reactor. The same yields can be achieved at lower temperatures, leading to energy savings and reduced catalyst deactivation (one of the major problems of alkane dehydrogenation), increased selectivities when temperature-promoted side reactions exist or when the permeating species arc involved in these side reactions. Moreover, the formation and separation of products in the same unit leads to a reduction in capital costs. 

As will be pointed out later, catalysts and membranes may become gradually deactivated and, when this occurs, they need to be regenerated often by some combustion process using dilute oxygen. In cases where coke deposit becomes significant on the cat yst or the membrane and needs to be removed by combustion, the membrane reactors are potentially subject to thermochemical cycling as part of the normal operation. 

The nature of many high-temperature hydrocarbon reactions which potentially can benefit from inorganic membrane reactors (particularly catalytic membrane reactors) is such that the catalysts or the catalytic membranes are subject to poisoning over time. Deactivation and regeneration of many catalysts in the form of pellets are well known, but the same issues related to either catalyst-impregnated membranes or inherently catalytic membranes are new to industrial practitioners. They are addressed in this section. 

Poisoning. Both the membrane and the catalyst in a membrane reactor may become deactivated over time in the application environment. This poisoning arises from some species present in the feed stream or from some product(s) of the reaction. When the poison is present in the feed stream at a relatively high concentration and is weakly adsorbed onto the catalyst or membrane surface or when the poison is formed by reaction, it is uniformly distributed throughout the catalyst. On the other hand, if the poison is present in the feed stream in a relatively low concentration and is strongly adsorbed, the outer pore surfaces can completely lose catalytic activity before the inside pore surfaces do. When significant deactivation of either or both occurs, effective... 

Enhancement of reaction conversion by employing a permselective membrane often has the implication that, for a given conversion, it is possible to run the reaction at a lower temperature in the membrane reactor than in a conventional reactor. Catalyst deactivation due to coke formation generally becomes more severe as the reaction temperature increases. Therefore, the use of a membrane reactor to replace a conventional one should, in principle, reduce the propensity for coke formation because for the same conversion the membrane reactor configuration may be operated at a lower temperature than a conventional reactor. This is particularly true for such reactions as dehydrogenation. 

However, the addition of an oxidant such as oxygen is not without some trade-off. To help solve the problem of catalyst deactivation due to carbon deposit in an alumina membrane reactor for dehydrogenation of butane, oxygen is introduced to the sweep gas, helium, on the permeate side at a concentration of 8% by volume. The catalyst service life increa.scs from one to four or five hours, but the selectivity to butene decreases from 60 to 40% at 480 C [Zaspalis et al., 1991b]. If oxygen is added to the feed stream entering the membrane reactor in order to inhibit coke formation, the butene selectivity decreases even more down to 5%. 

Finally, possible causes for deactivation of catalytic membranes are described and severad aspects of regenerating catalytic membrane reactors are discussed. A variety of membrane reactor configurations are mentioned and some unique membrane reactor designs such as double spiral-plate or spiral-tube reactor, fuel cell unit, crossflow dualcompartment reactor, hollow-fiber reactor and fluidized-bed membrane reactor are reviewed. 

Deactivation of the catalyst is always an industrially important problem. For fixed-bed reactors, to which class the cross-flow reactors also belong, catalyst poisoning is a particularly delicate matter, since the reactivation is often complicated and expensive. Some poisoning effects may be difficult to explain and understand, and this of course causes extra uncertainty. One example of such poisoning was the observation by Amor and Farris [33] that a special deactivation effect appeared in liquid-phase hydrogenation of toluene using a spiral tubular membrane reactor. Toluene was not hydrogenated at all over the palladium foil used. This phenomenon and reactivation of the catalyst have recently been studied by Ali et al. [56]. 

The most suitable driving force in BI is the reduction of the diffusion path that already operates in transport processes across biological bilayers. Consequently, biocatalyst membranes and specially designed bioreactors, such as jet loop and membrane reactors, are available to intensify biochemical reactions. " " Supported biocatalysts are often employed to enhance catalytic activity and stability and to protect enzymes/ microorganisms from mechanical degradation and deactivation.f Immobilization of the cells is one of the techniques employed to improve the productivity of bioreactors. 

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