Parameters membrane reactors

The values of the Michaelis-Menten kinetic parameters, Vj3 and C,PP characterise the kinetic expression for the micro-environment within the porous structure. Kinetic analyses of the immobilised lipase in the membrane reactor were performed because the kinetic parameters cannot be assumed to be the same values as for die native enzymes. 

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... 

The design of the Pd-membrane reactor was based on the chip design of reactor [R 10]. The membrane is a composite of three layers, silicon nitride, silicon oxide and palladium. The first two layers are perforated and function as structural support for the latter. They serve also for electrical insulation of the Pd film from the integrated temperature-sensing and heater element. The latter is needed to set the temperature as one parameter that determines the hydrogen flow. 

It is evident, that in any membrane reactor operation mode there are important parameters which determine the performance of the process (Shah, Remmen and Chiang 1970). These are (1) the total and partial pressures on both sides of the membrane, (2) the total and partial pressure differences across the membrane, (3) the diffusion mechanism through the support and the membrane layer (membrane structure), (4) the thickness of the membrane, (5) the reactant configuration (i.e. whether the reactants are supplied from the same or from opposite sides of the membrane, in counter or co-current flow) and (6) the catalyst distribution. 

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. 

Table III. Critical dimensionless parameters for series reactions in a membrane reactor...

Reaction engineering helps in characterization and application of chemical and biological catalysts. Both types of catalyst can be retained in membrane reactors, resulting in a significant reduction of the product-specific catalyst consumption. The application of membrane reactors allows the use of non-immobilized biocatalysts with high volumetric productivities. Biocatalysts can also be immobilized in the aqueous phase of an aqueous-organic two-phase system. Here the choice of the enzyme-solvent combination and the process parameters are crucial for a successful application. 

In the preceding section, we analyzed an immobilized enzyme process and calculated some important parameters such as productivity. In this section, we investigate another process configuration for retaining biocatalysts, the membrane reactor. The advantages and disadvantages of immobilization and membrane retention have already been discussed in Chapter 5. As in the case of immobilization, retention of catalyst by a membrane vastly improves biocatalyst productivity, a feature important on a processing scale but usually not on a laboratory scale. 

The overall mass-transfer rates on both sides of the membrane can only be calculated when we know the convective velocity through the membrane layer. For this, Equation 14.2 should be solved. Its solution for constant parameters and for first-order and zero-order reaction have been given by Nagy [68]. The differential equation 14.26 with the boundary conditions (14.28a) to (14.28c) can only be solved numerically. The boundary condition (14.28c) can cause strong nonlinearity because of the space coordinate and/or concentration-dependent diffusion coefficient [40, 57, 58] and transverse convective velocity [11]. In the case of an enzyme membrane reactor, the radial convective velocity can often be neglected. Qin and Cabral [58] and Nagy and Hadik [57] discussed the concentration distribution in the lumen at different mass-transport parameters and at different Dm(c) functions in the case of nL = 0, that is, without transverse convective velocity (not discussed here in detail). 

Parameters Influencing the Photocatalytic Membrane Reactors (PMRs) Performance... 

In the development of a photocatalytic membrane reactor it is important to take into account some parameters that influence the performance of the system and its applicability to the industrial level. 

The selection ofthe membrane to be used in enzymatic membrane reactors should take into account the size of the (bio)catalyst, substrates, and products as well as the chemical species ofthe species in solution and ofthe membrane itself. An important parameter to be used in this selection is the solute-rejection coefficient, which should... 

Fig. 12.12. Calculated dependence of cyclohexane conversion (Eq. (37)) as a function of the Damkohler number (Eq. (41)) for a) the conventional fixed-bed reactor b) the diluted fixed-bed reactor and c) the membrane reactor with an optimized thickness ofVycor glass membrane (the dashed line corresponds to a hypothetically higher membrane selectivity, Sm). The range in which the membrane reactor experiments were performed is also indicated. Parameters T = 473 K, x r H =...

Cost calculations for a membrane reactor are very cumbersome. Numerous uncertainties and assumptions have to be made for a large number of parameters. Tube length and sealing costs are very important and up to now it is not even sure whether a sealing material can be developed that is able to withstand the severe operation conditions. This makes a proper... 

It may be necessary to improve membrane selectivities, so that further purification of the produced hydrogen before re-use in the desulphurisation units can be limited as far as possible. Moreover the membrane reactor can be optimised for various variables, such as H2S conversion, hydrogen recovery, membrane area and temperature. In a techno-economic evaluation combined with advanced process design the impact of different operating parameters on the investment and operating costs should be studied. 

Gumi T, Femandez-Delgado Albacete J, Paolucci-Jeanjean D et al (2008) Study of the influence of the hydrodynamic parameters on the performance of an enzymatic membrane reactor. J Membr Sci 311 147-152... 

Zaspalis and Burggraaf [47] have summarized typical membrane reactor configurations, different membrane/ catalyst combinations, and a large number of membrane reactor studies. Their article clearly shows that inorganic membranes prepared by the sol-gel method, with their dual ability in catalysis and separation, have many unique advantages over other product forms. At the same time, it is important to realize that the parameters which affect a membrane s characteristics and the advantages which the sol-gel process offers are similar to what has been presented thus far. 

Important parameters in a catalytic membrane reactor for dehydrogenation are the reaction rate, the permeability (i.e. permeation rate) and the permselectivity for hydrogen. It appears at first sight that good conditions are those where the permeation rate (removal of H2) and the reaction rate (formation of H2) are close to each other, but the role of the permselectivity is also important. 

In the set of relations (3.182)-(3.188), P represents the coefficient for the velocity increase due to the species transport through the wall, Bi is the heat transfer Biot number (Bi = (arj)/ ), Bip is the mass transfer Biot number for the gaseous phase (Bi[) = (kri)/DA) and Bip is the Biot number for the porous wall (Bip = (k5xx,)/DAw)- Two new parameters and D w, respectively, represent the wall thickness and the wall effective diffusion coefficient of species. The model described by the set of relations (3.182)-(3.188) can easily be modified to respond to the situation of a membrane reactor when a chemical reaction occurs inside the cylindrical space and when one of the reaction products can permeate through the wall. The example particularized here concerns the heat and mass transfer of a... 

In a separate parametric study, Mohan and Govind(l)(9) analyzed the effect of design parameters, operating variables, physical properties and flow patterns on membrane reactor. They showed that for a membrane which is permeable to both products and reactants, the maximum equilibrium shift possible is limited by the loss of reactants from the reaction zone. For the case of dehydrogenation reaction with a membrane that only permeates hydrogen, conversions comparable to those achieved with lesser permselective membranes can be attained at a substantially lower feed temperature. 

Figure 9.2 Calculated total conversion profile of cyclohexane to benzene in a porous shell-and-tube Vycor glass membrane reactor with membrane thickness as a parameter [ltohetal.,1985]...

The choice of the above three modes of catalyst placement relative to the membrane can significantly affect the reactor performance. From the analysis of catalytically active and passive (inert) membrane reactors [Sun and Khang, 1988], it appears that the critical parameter determining the choice is the reaction residence time. At low residence times, the difference between a catalytically active and a catalytically passive membrane is not significant. However, as the reaction residence time becomes high, the catalytically active membrane shows a higher reaction conversion. 

Besides the critical issue of containment and sealing, the choice of the materials for the membrane and other membrane reactor components affects the permeability and permselectivity, operable temperature, pressure and chemical environments and reaction performance. Important material parameters include the particular chemical phase, thickness, thermal properties and surface contamination of the membrane, membrane/support microstructure, and sealing of the end surfaces of the membrane elements and of the joining areas between elements and module components. The conventional permeability versus permselectivity dilema associated with membranes needs to be addressed before inorganic membrane reactors are used in bulk processing. 

As mentioned earlier, most membrane reactor models are based on isothermal macroscopic balances in the axial direction and do not solve the transport equations for the membrane/support matrix. They all account for the effects of membrane permeation through the use of some common relevant parameters (as a permeation term) in the transport equations for both the feed and permeate sides. Those parameters are to be determined experimentally. The above approach, of course, is feasible only when the membrane (or membrane/support) is not catalytic. 

Some other unceitainties associated with packed-bed catalytic membrane reactors arc the following. The reaction rate in the membrane layer is not easy to assess and it is likely to be different from that in the catalyst bed. The extremely small thickness of the membrane layer compared to the dimensions in the tubular and the annular regions makes the direct determination of the membrane-related parameters difficult, if not impossible in some cases. Furthermore, obtaining accurate data of the effective diffusivity for the membrane, particularly in the presence of a support layer, is not straightforward and often involves a high degree of uncertainty. 

Figure 10.5 Temperature and conversion profiles in an adiabaiic packed-bed membrane reactor with a dimensionless effective thermal conductivity of the membrane as a parameter [Itoh and Govind, 1989b]...

The beneficial effects of a fluidized-bed membrane reactor on methane conversion and hydrogen yield can be optimized by systematically varying the operating parameters such as catalyst amount, membrane area for permeation, rate of heat supply, and reactor bed height and bed volume. A hydrogen yield of 3.70 and an accompanying methane conversion of 92% can be attained at a relatively moderate temperature of 733 C which... 

Shown in Figure 10.19 is a comparison of the overall reaction rate as a function of the liquid flow rate for the three models Just mentioned [Harold et al., 1989]. Three different values of the catalytic activity (k ) arc used as a parameter. The solid line, dashed line (-) and dotted line (- -) represent the results predicted by the CNMMR model with a well-mixed liquid stream, the CNMMR model with a laminar-flow liquid stream and the string-of-pellcts reactor model, respectively. The membrane is assumed to be 0.1 cm thick for the two membrane reactors. The liquid feed is saturated with A and contains a... 

Furthermore, various membrane reactor parameters and configurations result in different performance levels. All the above factors and other engineering aspects will be reviewed in this chapter with both modeling predictions and experimental data. 

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