Membrane reactors example

In general the membrane reactor examples show that knowledge of the demands and requirements of each foreseen membrane application is very important in the choice and development of the membrane materials. 

Example 6-1 Gas-Phase Reaction in Microreactor—Molar Flow Rate Example 6-2 Membrane Reactor Example 6-3 Isothermal Semibatch Reactor Proressional Reference Shelf R6,1 UnsH udy CSTRs and Semihaich Reactors R6.1A Start-up of a CSTR... 

As an example the use of ceramic membranes for ethane dehydrogenation has been discussed (91). The constmction of a commercial reactor, however, is difficult, and a sweep gas is requited to shift the product composition away from equiUbrium values. The achievable conversion also depends on the permeabihty of the membrane. Figure 7 shows the equiUbrium conversion and the conversion that can be obtained from a membrane reactor by selectively removing 80% of the hydrogen produced. Another way to use membranes is only for separation and not for reaction. In this method, a conventional, multiple, fixed-bed catalytic reactor is used for the dehydrogenation. After each bed, the hydrogen is partially separated using membranes to shift the equihbrium. Since separation is independent of reaction, reaction temperature can be optimized for superior performance. Both concepts have been proven in bench-scale units, but are yet to be demonstrated in commercial reactors. 

Several L-amino acids are produced on a large scale by enzymatic resolution of N-acetyl-D,L-amino adds (Figure A8.4). Acylase immobilised on DEAE-Sephadex is for example employed in a continuous process while Degussa uses the free acylase retained in a membrane reactor. In the latter process the advantage of reuse of the enzyme and homogeneous catalysis are combined. 

Degussa AG uses immobilised acylase to produce a variety of L-amino adds, for example L-methionine (80,000 tonnes per annum). The prindples of the process are the same as those of the Tanabe-process, described above. Degussa uses a new type of reactor, an enzyme membrane reactor, on a pilot plant scale to produce L-methionine, L-phenylalanine and L-valine in an amount of 200 tonnes per annum. 

Membrane reactors are defined here based on their membrane function and catalytic activity in a structured way, predominantly following Sanchez and Tsotsis [2]. The acronym used to define the type of membrane reactor applied at the reactor level can be set up as shown in Figure 10.4. The membrane reactor is abbreviated as MR and is placed at the end of the acronym. Because the word membrane suggests that it is permselective, an N is included in the acronym in case it is nonpermselective. When the membrane is inherently catalytically active, or a thin catalytic film is deposited on top of the membrane, a C (catalytic) is included. When catalytic activity is present besides the membrane, additional letters can be included to indicate the appearance of the catalyst, for example, packed bed (PB) or fluidized bed (FB). In the case of an inert and nonpermselective... 

Concerning function integration, for example, micro-flow membrane reactors can exhibit similar process intensification, as shown already for their large-scale counterparts [75]. Separation columns for proteomics, immobilizing enzymes, utilize the large surface-to-volume ratios. Surface tension differences can guide and transport liquids selectively. 

An important problem in emulsified organic-aqueous systems is that of scale-up, which is concerned with the realization of stable emulsions and the separation of phases after the reaction. The use of biphasic membrane systems that contain the enzyme and keep the two phases separated is likely to solve this problem. In the case of 5-naproxen an ee of 92% has been demonstrated without any decay in activity over a period of two weeks of continuous operation. A number of examples of biocatalytic membrane reactors have been provided by Giorno and Drioli (2000) and include the conversion of fumaric acid to L-aspartic acid, L-aspartic acid to L-alanine, and cortexolone to hydrocortisone and prednisolone. 

Chemically functional membranes afford yet another intriguing platform upon which process-intensified chemistry can be performed. For example, an enzyme membrane reactor process is used to produce a... 

It is well known that dense ceramic membranes made of the mixture of ionic and electron conductors are permeable to oxygen at elevated temperatures. For example, perovskite-type oxides (e.g., La-Sr-Fe-Co, Sr-Fe-Co, and Ba-Sr-Co-Fe-based mixed oxide systems) are good oxygen-permeable ceramics. Figure 2.11 depicts a conceptual design of an oxygen membrane reactor equipped with an OPM. A detail of the ceramic membrane wall... 

Another example is the palladium catalyzed allylic substitution of 3-phenyl-2-propenyl-carbonic acid methyl ester to yield iV-(3-phenyl-2-propenyl)morpho-line reported by Reetz, Kragl and co-workers. This reaction was performed in the presence of phosphino-terminated amine dendrimers [17, 18] loaded with Pd11 cations as shown in Scheme 10. For this particular dendrimer with a molecular weight of 10 212 g/mol, a retention of 0.999 per residence time [35] was estimated in a membrane reactor with a SELRO MPF-50 membrane. It must be noted that a very high retention is a prerequisite for a continuous operating system, since a small leaching of the dendrimer leads to an exponential decrease in the amount... 

There is huge potential in the combination of biocatalysis and electrochemistry through reaction engineering as the linker. An example is a continuous electrochemical enzyme membrane reactor that showed a total turnover number of 260 000 for the enantioselective peroxidase catalyzed oxidation of a thioether into its sulfone by in situ cathodic generated hydrogen peroxide - much higher than achieved by conventional methods [52],... 

Grafting a modified cinchona alkaloid to hexagonal mesoporous molecular sieve SBA-15 afforded catalyst (27) with excellent activity. 1-Phenyl-1-propene was converted to the corresponding diol in 98% yield (98% ee), while trans-stilbene yielded the desired product in 97% yield (99% ee) [92]. Other examples in this field are the utilization of microencapsulated osmium tetroxide by Kobayashi [93] and the application of continuous dihydroxylation mns in chemzyme membrane reactors described by Woltinger [94]. 

The same hyperbranched polyglycerol modified with hydrophobic palmitoyl groups was used for a noncovalent encapsulation of hydrophilic platinum Pincer [77]. In a double Michael addition of ethyl cyanoacetate with methyl vinyl ketone, these polymer supports indicated high conversion (81 to 59%) at room temperature in dichloromethane as a solvent. The activity was stiU lower compared with the noncomplexed Pt catalyst. Product catalyst separation was performed by dialysis allowing the recovery of 97% of catalytic material. This is therefore an illustrative example for the possible apphcation of such a polymer/catalyst system in continuous membrane reactors. 

There are reports of numerous examples of dendritic transition metal catalysts incorporating various dendritic backbones functionalized at various locations. Dendritic effects in catalysis include increased or decreased activity, selectivity, and stability. It is clear from the contributions of many research groups that dendrimers are suitable supports for recyclable transition metal catalysts. Separation and/or recycle of the catalysts are possible with these functionalized dendrimers for example, separation results from precipitation of the dendrimer from the product liquid two-phase catalysis allows separation and recycle of the catalyst when the products and catalyst are concentrated in two immiscible liquid phases and immobilization of the dendrimer in an insoluble support (such as crosslinked polystyrene or silica) allows use of a fixed-bed reactor holding the catalyst and excluding it from the product stream. Furthermore, the large size and the globular structure of the dendrimers enable efficient separation by nanofiltration techniques. Nanofiltration can be performed either batch wise or in a continuous-flow membrane reactor (CFMR). 

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,... 

Another example of a gas-phase membrane reactor is a palladium tube through which only hydrogen can permeate. This can be used to run the reaction... 

Figure 12-4 Membrane reactor in wJiicIi a cataJyst promotes reaction in the membrane and maintains reactants and products separate. Examples sliown are CH4 oxidation to syngas and C2H6 reduction to C2H4. Tlie membrane eliminates N2 from the syngas and produces C2H4 beyond equilibrium by removing H2 in these apphcations.

Fuel cells and batteries are examples of membrane reactors in which a conducting membrane separates the anode and cathode compartments, which supply fuel and oxidant, respectively. With fuel cells we have the added complexity that we need an ion-conducting electrode, which is also a catalyst at each electrode so that we can extract electrical power from the energy of the reaction. A battery is similar to a fuel ceU except that now the fuel and oxidant are stored and supplied within the ceU rather than being supphed externally. A fuel cell is usually operated as a continuous-flow reactor, while a battery is a rechargeable batch reactor. 

As discussed in this volume, the use of membrane reactors (Bernstein, et oL), monoliths (Hickman and Schmidt), optimized catalyst distribution in pellets (Gavriilidis and Varma), and supercritical conditions (Azzam and Lee) are examples of engineering developments that may provide improvements over existing processes. 

The present study investigates a different approach. The membrane is used to allow the desired intermediate product to escape from the reaction zone before it is consumed by further reaction. This use of a membrane reactor was first suggested by Michaels [15]. The partial oxidation of methane, which is a challenging reaction of the type propos for this application of membrane reactors, has been analyzed herein. There is no thermodynamic limitation for the production of carbon dioxide and water, actually these products are favored. It is desired to remove any partial oxidation product, for example formaldehyde, before it has a chance to be further oxidized. 

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