Reactors, membrane

The membrane reactors enable one to separate the reaction products increasing the production capacity and also to shift the reaction beyond the equilibrium under the same conditions of pressure and temperature. This type of reactor consists of a permeable membrane with selective porous walls. The membrane should be able to withstand high temperatures and sintering without blocking fluid flow. 

An example is the use of membrane reactors to generate and separate hydrogen to fuel cells. 

Hydrogen can be obtained by steam reforming and autothermal reforming of 

Partial oxidation of methane Water gas shift reaction  

Steam reforming and dry reforming are endothermic, bnt partial oxidation of methane and water gas shift reactions are exothermic. The autothermal process becomes exothermic and is thermodynamically favorable. 

The use of membrane reactors offers several advantages, including (1) a high specific surface area, (2) instantaneous reaction and separation of substrate and product, (3) reusing the lipase, and (4) continuous operation. Immobilization of lipase onto a membrane offers many advantages, such as a low drop in pressure when continuously operated, and high operational stability with low external and internal diffusional resistance (Giorno and Drioli, 2000). In these bioreactors, the lipase is immobilized on a membrane, which may be either a flat sheet (FSMR) or hollow fiber (HFMR). 

A variety of membrane materials (hydrophobic and hydrophilic) and reactor configurations have been studied. Hydrophobic membrane reactors are made of polypropylene (Atia et al., 2003 Malcata, 1992a Malcata et al., 1991), polytetrafluoroethylene 

Supercritical Fluids Technology in Lipase Catalyzed Processes 

FIGURE 3.5 Schematic diagram of flat sheet bioreactor apparatus used for butteroil hydrolysis 1, stirrer 2, products 3, membrane reactor 4, water bath 5, temperature controller, 6, oil 7, buffer 8, pump. (From Malcata, F. X., C. G. Hill, and C. H. Amundson, 1991, 

Gas-liquid-biosolid Bubble-free membrane gassing gas diffuses into the media without bubbles. Used for shear-sensitive animal cell cultures (ex insect cells) and for systems containing serum that are prone to foaming. Use 10 to 25 m /rrf for volume 150 L. Enzyme membrane reactor Power 10 kW/m maximum volume 0.5 m . Membrane allows diffusion of gas into the liquid without having to use bubbles. 

By having one of the products pass through (he membrane, we drive the reaction toward completion. 

Membrane reactors can be used to increase conversion when the reaction is thermodynamically limited, as well as to increase the selectivity when multiple reactions are occurring. Thermodynamically limited reactions are reactions where the equilibrium lies far to the left (i.e reactant side) and there is little conversion. If the reaction is exothermic, increasing the temperature will only drive the reaction further to the left, and decreasing the temperature will result in a reaction rate so slow that there is very little conversion. If the reaction is endothermic, increasing the temperature will move the reaction to the right to favor a higher conversion however, for many reactions these higher temperatures cause the catalyst to become deactivated. 

Isothermal Reactor Design Molar Flow Rates Chapter 6 

Hydrogen, species B, flows out through the sides of the reactor as it flows down the reactor with the other products that cannot leave until they exit the reactor. 

The mole balance on C is carried out in an identical manner to A. and the resulting equation is 

The principle of both membrane reactors and membrane bioreactors are the same but the origin is completely different. In the case of a bioreaction enzymes or microorganisms (bacteria, fungi, mammalian ceils, yeasts) are applied under very specific reaction conditions. Both concepts wil be discussed briefiy. 

Schematic drawing of various membrane reactor concepts for a tubular configuration (a) bore of the tube tilled with catalyst, (b) uaplayer with catalyst, and (c) membrane wall with catalyst. 

In membrane reactors, the combination of a chemical reactor with a membrane separation 

Due to fluctuations in the sulfur dioxide concentrations this reaction is very difficult to control in conventional reactor systems. The stoichiometry may be maintained by carry out the reaction within the wall of a porous ceramic membrane as shown schematically in figure VI - 75. 

In this case two different reactor configurations are usually distinguished the packed-bed membrane reactor (PBMR), and the fluidized-bed membrane reactor FBMR). 

Membrane reactors can also be classified according to the transport function of the membrane. A possible classification may be that depicted in Fig. 1.3 with three different reactor types for PCMRs and only two for PIMRs  

Polymeric catalytic membranes, when compared with inert membranes, offer the possibility of tuning the sorption of reactants and products in the close vicinity of the catalytic active sites, by a careful selection of the polymeric environment. As was mentioned above, in porous catalytic membranes the choice of the polymer is of less importance, since permeation does not take place through the polymer matrix. However, in the case of dense membranes, sorption and transport properties are crucial for the catalytic performance and are strongly affected by the polymeric matrix. 

Pervaporation-assisted catalysis is a typical example of an operation eflide-ntly carried out in extractor-type catalytic membrane reactors. Esterification is by far the most studied reaction combined with pervaporation. Esters are a class of compounds with wide industrial appUcation, from polymers to fragrance and flavour industries. Esterification, a reaction between a carboxylic acid and an alcohol with water as a by-product, is an equilibrium-limited reaction. So, this is a typical reaction that can be carried out advantageously in a extractor-type membrane reactor. By selectively removing the reaction product water, it is possible to achieve a conversion enhancement over the thermodynamic equilibrium value based on the feed conditions. 

The traditional process to carry out esterification reactions is reactive distillation. However, this process can only be applied if the difference of volatilities between the components is high enough and if azeotropes are not present. Pervaporation is an interesting alternative to reactive distillation, since it is not limited by the relative volatility of the components of the reaction mixture.  

Another type of reactor that may have considerable future potential for use in homogeneous catalytic reactions is called the membrane reactor. These reactors have been successfully used for the commercialization of manufacturing processes based on enzyme catalysis. In fact, 75% of the global production of l-methionine is performed in an enzyme reactor. A membrane is basically an insoluble organic polymeric film that can have variable thickness. The catalyst 

When it comes to combination with a reaction or conversion, membranes have mainly found application in a sequential mode, i.e. reaction followed by separation. In this chapter, we wiU focus on the integration of conversion and separation in so-caUed membrane reactors. As the separation function of the membrane can be used in various modes of operation, this leads to a broad variety of process options. In the past few years, several review papers have emerged, usually covering parts of this huge research field [1-5]. The general advantages of membrane reactors as compared to sequential reaction-separation systems are  

These advantages potentially lead to compact process equipment that can be operated with a high degree of flexibility. Because of the reduced byproduct forma- 

First applications of membrane reactors can be foimd in the field of bioprocess engineering using whole cells in fermentations or enzymatic bioconversions [6, 7]. Most of these processes use polymeric membranes, as temperatures seldomly exceed 60 °C. The development of inorganic membrane materials (zeolites, ceramics and metals) has broadened the application potential of membrane reactors towards the (petro) chemical industry [8]. Many of these materials can be applied at elevated temperatures (up to 1000°C), allowing their application in catalytic processes. 

The basic functions of the membrane in membrane reactors can be divided into (Fig. 5.1)  

As the membrane acts as a separating medium between two flow compartments, these basic functions can be applied to liquid/liquid, gas/liquid and gas/ gas systems, respectively. The physical shape of the membrane strongly depends on the membrane material used. For polymeric systems, these can be flat sheets in a plate-and-frame configuration, spiral-wound modules, and tubular mem- 

the majority of research investigations into CMRs are being conducted by many institutions, in addition to oil and chemical and utilities companies [5]. The use of mixed ionic-electronic membrane reactors for the partial oxidation of natural gas is undergoing active development by a number of consortia based around Air Products and Chemicals (USA), Praxair (USA), and/or Air Liquide (France). At present, the development of CMRs involving a pure ion-conducting electrolyte is restricted to a few reports of conceptual systems [12, 95]. 

Dr Samuel Georges (LEPMI) is warmly acknowledged for his critical reading of the manuscript. 

3 Sundmaichei.K..Rihko-Stnickmann,LK.and Galvita, V. (2005) Catalysis Today, 104,185-99. 

10 Fabry, P. and Kleitz, M. (1976) in Electrode Processes in Solid State Ionics (eds M. Kleitz and J. Dupuy), D. Reidel Publ. Comp., Dordrecht, pp. 331-65. 

12 Munder, B., Ye, Y, Rihko-Struckmann, L. and Sundmacher, K. (2005) Catal. Today, 

How ever. the mole balance on B (Hi) must be modified because hydrogen leaves through both the sides of the reactor and at the end of the reactor. 

Where is the overall mass transfer coefficient in m/s and Crs is the concentration of B in the sweep gas channel (mol/dm ). The overall mass transfer coefficient accounts for all resistances to transport the tube side resistance of the membrane, the membrane itself, and on the shell (sweep gas) side resistance. Further elaboration of the mass transfer coefficient and its correlations can be found in the literature and in Chapter 11. In general, this coefficient can be a function of the membrane and fluid properties, the fluid velocity, and the tube diameters. 

To obtain the rale of removal of B. we need to multiply the flux through the membrane by the surface area of membrane in the reactor. The rate at w hich B is removed per unit volume of reactor. / b- is just the flux IVb time.s the surface area membrane per volume of reactor,-o (mVm ) that is. 

Present research is being directed towards the development of catalysts that can maintain sufficient activity at high partial pressures of hydrogen and carbon dioxide and can operate at temperatures above the present limit of 500 °C. Process-intensification steps such as these should, if successful, improve efficiency and reduce both capital and operating costs. 

The catalyst technology for reforming can be roughly separated into nonnoble metal-based catalysts, the most prominent being nickel and noble metal-based catalyst, where rhodium shows the best performance, but also the average highest price. Hickman and Schmidt [25] were one of the first authors 

Qi et al. [32] tested autothermal reforming of n-octane over a ruthenium catalyst, which was composed of 0.5 wt.% ruthenium stabilized by ceria and potassium on y-alumina. It showed full conversion of n-octane for 800 h. However, the selectivity moved from carbon dioxide and methane toward carbon monoxide and light hydrocarbons, which has to be regarded as an indication of catalyst degradation during long-term tests despite the fact that full conversion was achieved. After 800 h the catalyst consequently showed incomplete conversion. Tests performed on the spent catalyst revealed losses of specific surface area and of 33 wt.% of the noble metal. 

Dreyer et al. [33] investigated the oxidative steam reforming of diesel surrogates over wash-coated ceramic monoliths carrying 5 wt.% rhodium supported by y-alumina. The conversion of M-decane dropped with increasing S/C ratio because the reaction temperature decreased significantly. The beneficial effect of 

Karatzas et al. [34] investigated the performance of rhodium catalyst containing 3 wt.% rhodium, 10 wt.% ceria, 10 wt.% lanthana on alumina carrier for the autothermal reforming of n-tetradecane, low sulfur diesel containing 6 ppm sulfur, and Fischer-Tropsch diesel over cordierite monolithic reactors. At temperatures exceeding 740°C, full conversion of the feed was achieved. 

The fuel injection turns out to be a critical issue for autothermal reforming of heavy hydrocarbon fuels such as diesel and kerosene. In particular the contact of air with the fuel at elevated 

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Membrane filtration Membrane module Membrane permeability Membrane process Membrane processes Membrane reactor Membrane roofing Membranes... 

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. 

Membrane Reactor. Another area of current activity uses membranes in ethane dehydrogenation to shift the ethane to ethylene equiUbrium. The use of membranes is not new, and has been used in many separation processes. However, these membranes, which are mostly biomembranes, are not suitable for dehydrogenation reactions that require high temperatures. Technology has improved to produce ceramic and other inorganic (90) membranes that can be used at high temperatures (600°C and above). In addition, the suitable catalysts can be coated without blocking the pores of the membrane. Therefore, catalyst-coated membranes can be used for reaction and 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. 

Fig. 7. Equihbrium conversion of ethane versus temperature at 210 kPa in a membrane reactor. The effect of hydrogen removal on ethane conversion is...

Catalytic A catalytic-membrane reactor is a combination heterogeneous catalyst and permselective membrane that promotes a reaction, allowing one component to permeate. Many of the reactions studied involve H9. Membranes are metal (Pd, Ag), nonporous metal oxides, and porous structures of ceran iic and glass. Falconer, Noble, and Speriy [in Noble and Stern (eds.), op. cit., pp. 669-709] review status and potential developments. 

Catalytic Membrane Reactors Membrane reactors combine reaction and separation in a single vessel. By removing one of the... 

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). 

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. 

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. 

CASE STUDY ENZYME KINETIC MODELS FOR RESOLUTION OF RACEMIC IBUPROFEN ESTERS IN A MEMBRANE REACTOR... 

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. 

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. 

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. 

The inhibition analyses were examined differently for free lipase in a batch and immobilised lipase in membrane reactor system. Figure 5.14 shows the kinetics plot for substrate inhibition of the free lipase in the batch system, where [5] is the concentration of (S)-ibuprofen ester in isooctane, and v0 is the initial reaction rate for (S)-ester conversion. The data for immobilised lipase are shown in Figure 5.15 that is, the kinetics plot for substrate inhibition for immobilised lipase in the EMR system. The Hanes-Woolf plots in both systems show similar trends for substrate inhibition. The graphical presentation of rate curves for immobilised lipase shows higher values compared with free enzymes. The value for the... 

M. Stoukides, Solid-Electrolyte Membrane reactors Current experience and future outlook, Catalysis Reviews - Science and Engineering 42(1 2), 1 -70 (2000). 

Membrane reactors, whether batch or continuous, offer the possibility of selective transpiration. They can be operated in the reverse mode so that some... 

Membrane Reactors. Consider the two-phase stirred tank shown in Figure 11.1 but suppose there is a membrane separating the phases. The equilibrium relationship of Equation (11.4) no longer holds. Instead, the mass transfer rate across the interface is given by... 

These component balances are conceptually identical to a component balance written for a homogeneous system. Equation (1.6), but there is now a source term due to mass transfer across the interface. There are two equations (ODEs) and two primary unknowns, Og and a . The concentrations at the interface, a and a, are also unknown but can be found using the equilibrium relationship, Equation (11.4), and the equality of transfer rates. Equation (11.5). For membrane reactors. Equation (11.9) replaces Equation (11.4). Solution is possible whether or not Kjj is constant, but the case where it is constant allows a and a to be eliminated directly... 

The most effective of these include immobilization [80], lipid coating [81], surfactant coating [82], use of cross-linked enzyme crystals [83], cross-linked enzyme aggregates [84], and membrane reactors [85]. 

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... 

Figure 10.4 Meaning of acronyms used to define types of membrane reactors at the reactor level. (After [2]).

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