Membrane reactors production

Membrane reactors, where the enzyme is adsorbed or kept in solution on one side of an ultrafHtration membrane, provides a form of immobilized enzyme and the possibiHty of product separation. 

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. 

Two reactions for the production of L-phenylalanine that can be performed particularly well in an enzyme membrane reactor (EMR) are shown in reaction 5 and 6. The recently discovered enzyme phenylalanine dehydrogenase plays an important role. As can be seen, the reactions are coenzyme dependent and the production of L-phenylalanine is by reductive animation of phenylpyruvic add. Electrons can be transported from formic add to phenylpyruvic add so that two substrates have to be used formic add and an a-keto add phenylpyruvic add (reaction 5). Also electrons can be transported from an a-hydroxy add to form phenylpyruvic add which can be aminated so that only one substrate has to be used a-hydroxy acid phenyllactic acid (reaction 6). 

Many procedures have been suggested to achieve efficient cofactor recycling, including enzymatic and non-enzymatic methods. However, the practical problems associated with the commercial application of coenzyme dependent biocatalysts have not yet been generally solved. Figure A8.18 illustrates the continuous production of L-amino adds in a multi-enzyme-membrane-reactor, where the enzymes together with NAD covalently bound to water soluble polyethylene glycol 20,000 (PEG-20,000-NAD) are retained by means of an ultrafiltration membrane. 

In this case study, an enzymatic hydrolysis reaction, the racemic ibuprofen ester, i.e. (R)-and (S)-ibuprofen esters in equimolar mixture, undergoes a kinetic resolution in a biphasic enzymatic membrane reactor (EMR). In kinetic resolution, the two enantiomers react at different rates lipase originated from Candida rugosa shows a greater stereopreference towards the (S)-enantiomer. The membrane module consisted of multiple bundles of polymeric hydrophilic hollow fibre. The membrane separated the two immiscible phases, i.e. organic in the shell side and aqueous in the lumen. Racemic substrate in the organic phase reacted with immobilised enzyme on the membrane where the hydrolysis reaction took place, and the product (S)-ibuprofen acid was extracted into the aqueous phase. 

Substrate and product inhibitions analyses involved considerations of competitive, uncompetitive, non-competitive and mixed inhibition models. The kinetic studies of the enantiomeric hydrolysis reaction in the membrane reactor included inhibition effects by substrate (ibuprofen ester) and product (2-ethoxyethanol) while varying substrate concentration (5-50 mmol-I ). The initial reaction rate obtained from experimental data was used in the primary (Hanes-Woolf plot) and secondary plots (1/Vmax versus inhibitor concentration), which gave estimates of substrate inhibition (K[s) and product inhibition constants (A jp). The inhibitor constant (K[s or K[v) is a measure of enzyme-inhibitor affinity. It is the dissociation constant of the enzyme-inhibitor complex. 

Furthermore, the application of the SOD membrane in a FT reaction has been investigated. The advantages of water removal in a FT reaction are threefold (i) reduction of H20-promoted catalyst deactivation, (ii) increased reactor productivity, and (iii) displaced water gas shift (WGS) equilibrium to enhance the conversion of CO2 to hydrocarbons [53]. Khajavi etal. report a mixture of H2O/H2 separation factors 10000 and water fluxes of 2.3 kg m h under... 

Whilst the basic process for generation and conversion of syngas is well established, production from biomass poses several challenges. These centre on the co-production of tars and hydrocarbons during the biomass gasification process, which is typically carried out at 800 °C. Recent advances in the production of more robust catalysts and catalytic membrane reactors should overcome many of these challenges. 

Figure 4 Membrane reactor for 4-hydroxybenzoate production using phenolphosphate carboxylase. A membrane (A) separates the reaction space containing the enzyme (B) from water phase where the product is collected (C).

Since in continuous degradation processes it is expected to reach a molecular weight distribution of the products, which is optimal for their further use, the investigation was devoted to test the effect of a key parameter such as the enzyme to substrate ratio (E/S). For a fixed mean retention time in the UF-membrane reactor, the following behaviour can be... 

Enzymatic degradation of pectin can be satisfactory performed in UF-membrane reactors which have been proved to be helpful tool for laboratory scale investigations. Reaction products can be continuously recovered in a sequence of filtration stages. The obtained product distribution depends on the enzyme to substrate ratio, which affects particularly the... 

Oligouronides production in a membrane reactor by enzymatic degradation of pectins from Citrus peel. A preliminary study... 

Membrane reactors are known on the macro scale for combining reaction and separation, with additional profits for the whole process as compared with the same separate functions. Microstructured reactors with permeable membranes are used in the same way, e.g. to increase conversion above the equilibrium limit of sole reaction [8, 10, 11, 83]. One way to achieve this is by preparing thin membranes over the pores of a mesh, e.g. by thin-fihn deposition techniques, separating reactant and product streams [11]. 

The production process for (S)-phenylalanine as an intermediate in aspartame perpetuates the principle of reracemization of the nondesired enantiomer (Figure 4.5) in a hollow fiber/ liquid membrane reactor. Asymmetric hydrolysis of the racemic phenylalanine isopropylester at pH 7.5 leads to enantiopure phenylalanine applying subtilisin Carlsberg. The unconverted enantiomer is continuously extracted via a supported liquid membrane [31] that is immobilized in a microporous membrane into an aqueous solution of pH 3.5. The desired hydrolysis product is charged at high pH and cannot, therefore, be extracted into the acidic solution [32]. 

Enzyme membrane reactor for production of diltiazem intermediate. A solution of the racemic ester in organic solvent enters the port at the bottom of the reactor and flows past the strands of microporous, hollow-fiber membrane that contain an enzyme. The enzyme catalyzes hydrolysis of one enantiomer of the ester that undergoes decarboxylation to 4-methoxyphenylacetaldehyde (which in turn forms a water-soluble bisulfite complex that remains in the aqueous phase). The other enantiomer of the ester remains in the aqueous stream that leaves the reactor via the port at the top. Courtesy of Sepracor, Inc. 

Decomposition of methane to H2 and carbon over Ni/Si02 was carried out in a membrane reactor (membrane 90Pd-10Ag) [106]. The use of the membrane reactor allowed increasing the H2 yield by shifting the reaction equilibrium toward the products. An excellent review of the literature data on nonoxidative methane activation over the surface of transition metals was recently published by Choudhary et al. [107]. 

Barbiery, G. et al., Hydrogen production using membrane reactor, Korean Membrane., 5,68,2003. 

Ishihara, T. et al., Decomposition of methane over Ni/Si02 catalysts with membrane reactor for the production of hydrogen, Chem. Lett., 93, 1995. 

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