Partial oxidation reactions membrane reactors

One example of membrane reactors is oxidation, in which oxygen from one phase diffuses from one side of an oxygen-permeable membrane to react with a fuel on the other side of the membrane. This avoids a high concentration of O2 on the fuel side, which would be flammable. A catalyst on the fuel side of the membrane oxidizes the fuel to partial oxidation products. One important process using a membrane reactor is the reaction to oxidize methane to form syngas,... 

The viability of one particular use of a membrane reactor for partial oxidation reactions has been studied through mathematical modeling. The partial oxidation of methane has been used as a model selective oxidation reaction, where the intermediate product is much more reactive than the reactant. Kinetic data for V205/Si02 catalysts for methane partial oxidation are available in the literature and have been used in the modeling. Values have been selected for the other key parameters which appear in the dimensionless form of the reactor design equations based upon the physical properties of commercially available membrane materials. This parametric study has identified which parameters are most important, and what the values of these parameters must be to realize a performance enhancement over a plug-flow reactor. 

Figure 1. A schematic representation of a membrane reactor for improving yields of intermediate products in a partial oxidation reaction.

The heterogenization of catalysts in membrane is particularly suitable for catalyst design at the atomic and molecular level. One of the main advantages of the membrane reactors, compared to traditional reactors, is the possibility to recycle easily the catalyst. Moreover, the selective transport properties of the membranes can be used to shift the equilibrium conversion (e.g., esterihcation reaction), to remove selectively products and by-products from the reaction mixture, to supply selectively the reagents (e.g., oxygen for partial oxidation reactions). 

Whether such devices (which is this chapter are referred to by the acronym CNMR, namely catalytic nonpermselective membrane reactors) should be called membrane reactors in the first place will stay a matter of debate among the purists nevertheless these membrane reactors are attracting a growing share of attention with a number of hydrogenation, oxidation and partial oxidation reactions studied. 

A number of research groups [83-86] have used a rather related concept for carrying out various selectivity limited reactions in a membrane reactor. The concept is illustrated schematically in the top part of Fig. 11.8 which is from a study by Harold and coworkers [84]. It applies to partial oxidation reactions where the desired reaction product can further react with oxygen to produce an imdesirable total oxidation product. In some instances it makes better sense (in terms of maximizing the reactor yield) to feed the two reactants separately on either side of the membrane rather that to co-feed them on either side. This is shown in the bottom part of Fig. 11.8 which shows the yield to the desired product as a fimction of the Thiele modulus as the degree of feed segregation... 

Fig. 11.8. The use of membrane reactors in partial oxidation reactions. Upper figure represents a schematic of the concept. Lower figure gives the calculated )deld as a function of the Thiele modulus.

Realizing hybrid approaches by coupling either a reactor or separation unit with a membrane is another very interesting possibility [50, 51]. There are two options in which membranes could be used coupled to reactors. The first is to distribute the feed of one of the reactants to a packed bed of catalyst, and thus realize a better profile of concentration along the reactor to minimize hot spots and consecutive reactions. This approach has resulted in increased selectivity in partial oxidation reactions. The second approach is to either remove a product that inhibits a reaction (e.g., in enzymatic reactions) or to remove a product to shift the equilibrium. Examples are in the continuous removal of water in dehydration reactions, or of H2 in dehydrogenation reactions. 

The objective of this study was to determine the conversion and selectivity of the methane partial oxidation reaction when using high feed rates and low methane/oxygen feed ratios in the membrane reactor configuration. 

Further increases in methane conversion were attained by using an additional bed downstream from the membrane bed. In addition, the reactor temperature was increased so that the second bed operates at temperatures higher than the autothermal operation. This allows for the dry reforming reaction to occur in the second bed thus increasing the conversion of the methane not consumed in the first bed. In this case the highest methane conversion was about 90% with CO and H2 selectivities of about 90% when the external temperature is 700°C. Similar results can be attained without heating if a third feed of O2 is added between the membrane bed and just before the second fixed bed. In this case, the temperature increase is realized by the partial oxidation reaction with no major loss of selectivity. 

The vanadium pentoxide cataKtic membrane reactor was prepared by coating its sol inside the Vycor tube membrane. After heat treatment of the prepared membrane, the [010] planes of vanadium pentoxide layer were grown largely which contributes to partial oxidation reaction of 1-butene to maleic anhydride. The partial oxidation of 1-butene to maleic anhydride was carried out in the catalytic membrane reactor. The maximum selectivity of 95% was obtained at 350 °C when the surface velocity was 500cm/h. And at this condition, oxygen permeability was almost four times higher than the reaction had not occured. 

One of the most industrially important reactions using vanadium pentoxide(V205) catalyst is the partial oxidation of 1-butene to maleic anhydride [1]. Partial oxidation reactions are inherently unselective and often make by-products of little or no value. Oxygen-rich feeds result in low product selectivities and high hydrocarbon conversions [2]. Because partial oxidation and total oxidation always proceed competitively, the selectivity of maleic anhydride from 1-butene is low. Though fixed bed reactors or fludized bed reactors have been used for partial oxidation for the past 30 years, the selectivity of maleic anhydride has not been obtained higher than 69% [3]. Some attempts have been reported on a new type of reactor to overcome the above limit. This is a membrane reactor which offers some advantages. A membrane reactor plays a... 

Another partial oxidation reaction that is attracting industrial attention for the application of reactive separations is the production of synthesis gas from methane [Stoukides, 2.127]. The earlier efforts made use of solid oxide solutions as electrolytes. Stoukides and coworkers (Eng and Stoukides [2.200, 2.126], Alqahtany et al. [2.201, 2.202]), for example, using a YSZ membrane in an electrochemical membrane reactor obtained a selectivity to CO and H2 of up to 86 %. They found that a Fe anodic electrode was as active as Ni in producing synthesis gas from methane (Alqahtany et al. [2.201, 2.202]), and that electro-chemically produced O was more effective in producing CO than gaseous oxygen (no ef-... 

Dixon recently [5.44] provided a comparative study of the yield to intermediate products for partial oxidation for a distributed-feed membrane reactor, and the conventional cooled-tube fixed-bed reactor (FBR). They studied a generic parallel/consecutive partial oxidation reaction scheme (reactions 5.25 and 5.26 in Section 5.1.4 with all the stoichiometric coefficients being equal to 1). The reactor resembled that shown in Figure... 

A model for a similar membrane reactor, shown schematically on the top of Figure 5.15, has been developed by Harold et al [5.54] to simulate reactant and products concentrations in parallel-consecutive reaction networks. The membrane reactor compartments on either side of the membrane were assumed to be completely stirred with no pressure gradient across the membrane. The model reaction studied is of relevance to hydrocarbon partial oxidation reactions, where the intermediate oxidation product can further react with oxygen to produce the undesirable total combustion products. Here the goal is to maximize the yield of the intermediate desired product. The calculation results... 

The study presented was based on reaUstic data originating from the important class of partial oxidation reactions that might be favorably performed in such membrane reactors. The oxidative dehydrogenation of ethane to ethylene using a vanadium oxide catalyst was considered. Concerning the properties of the membranes different permeabilities were studied, being in the range of currently available porous materials. 

M., Roberts, G.L. and Cox, J.L., 1996b. Experimental Investigations of Inorganic Membrane Reactors A Distributed Feed Approach for Partial Oxidation Reactions. Chemical Engineering Science, 51(5) 789-806. 

The three-phase membrane reactors have been mainly investigated for applications in both hydrogenation and partial oxidation reactions. 

Tonkovich ALY,Zilka J L, JimenezD M,Roberts G L,Cox J L (1996), Experimental investigations of inorganic membrane reactors a distribnted feed approach for partial oxidation reactions , Chem. Eng. 5ci.,51,789-806. 

Indeed, Roy et aL first demonstrated that autothermal operation could be achieved by directly adding oxygen to a fluidized bed reformer to provide all of the heat required via the partial oxidation reaction [24], The extension of this work to a membrane fluidized bed reactor has been performed by the same... 

Table 2.9 Partial oxidation reactions performed in porous membrane reactors... 

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