Porous membranes reactions

The situation is somewhat different with porous membranes, where the permselectivities for all components do not equal zero but exhibit certain values determined in most cases by the Knudsen law of molecular masses. In general, when porous membranes are used as separators in a membrane reactor next to the catalyst or the reaction zone (Figure 7.2a), it has been shown experimentally (Yamada et al. 1988) and theoretically (Mohan and Govind 1986, 1988a, b, Itoh et al. 1984, 1985) that there is a maximum equilibrium shift that can be achieved. On the basis of simple mass balances one can calculate that this maximum depends on, besides the reaction mechanism, the membrane permselectivities (the difference in molecular weights of the components to be separated) and it corresponds to an optimum permeation to reaction-rate ratio for the faster permeating component (which is a reaction product). 

It is obvious from the above discussion that porous and dense membranes form two different cases, each with its own advantages and disadvantages. Dense membranes, (permeable only to one component) operating at optimum conditions, can be used to obtain complete conversions. However, because the permeation rate is low, the reaction rate has also to be kept low. Porous membranes (permeable to all components but at different permselectivities) are limited under optimum conditions to a maximum conversion (which is not 100%) due to the permeation of all the components. The permeation rates through porous membranes are, however, much higher than those through dense membranes and consequently higher reaction rates or smaller reactor volumes are possible. 

Microporous membranes (pore radius less than 10 A) are ideal materials to be used as separators in membrane reactor processes. Microporous membranes also combine the high selectivities to certain components with high permeation rates. The high selectivities mean that maximum conversions (and thus equilibria shifts) higher than those achieved by porous membranes can be attained, while the high permeation rates allow for high reaction rates... 

It has been mentioned earlier that using porous membranes for product separation during the course of an equilibrium reaction, maximum attainable conversions are limited because of reactant permeation. This is the case where the membrane forms the wall of the reactor in which a catalyst is packed. It has also been mentioned that in this mode equilibrium conversions for some slow reactions could be increased by factors ranging between 1.3 and 2.3. Another important operation mode arises when the membrane is inherently catalytic or when the catalytically active species are placed within the membrane pores (catalytically active membrane as shown in Figure 7.2b and 7.2c). In this case, reaction and separation take place simultaneously and are combined in parallel rather than in series as was the case in the previous mode. 

Abe, F. 1987. Porous membranes for use in reaction processes. European Patent. Appl. 0,228,885A2. 

It has recently been demonstrated that solutes can be extracted from ionic liquids by perevaporation. This technique is based on the preferential partitioning of the solute from a liquid feed into a dense, non-porous membrane. The ionic liquids do not permeate the membrane. This technique can be applied to the recovery of volatile solutes from temperature-sensitive reactions such as bioconversions carried out in ionic liquids (34). 

In high temperature separation, hydrogen is separated from the hot product gas at the reaction temperature using porous membranes made of materials such as zirconia. The porous membranes separate the gases through, in general, either mass diffusion or molecular effusion [3,74,79]. In mass diffusion, the... 

Over the past 20 years, membrane contactors, a technology based on the combination of membrane separation and chemical absorption, have been evaluated for C02 capture applications [106]. The nonwetting porous membrane is generally not selective, but solely acts as a barrier between the flue gas and the liquid adsorbent, see Figure 9.9 [106]. Separation is determined by the reaction of one component (typically C02 or H2S) in the gas mixture with the absorbent in the liquid. 

In another study, Tsum et al. [80] reported the use of porous Ti02 membranes having pores of several nanometers for a gas-phase photocatalytic reaction of methanol as a model of volatile organic component (VOC). In this system, the titanium dioxide is immobilized in the form of a porous membrane that is capable of selective permeation and also a photocatalytic oxidation that occurs both on the surface and inside the porous Ti02 membranes. In this way, it is possible to obtain a permeate stream oxidized with OH radicals after one-pass permeation through the Ti02 membranes. 

Lui et al. [109] have described an automated system for determination of total and labile cyanide in water samples. The stable metal-cyanide complexes such as Fe(CN)63 are photo-dissociated in an acidic medium with an on-line Pyrex glass reaction coil irradiated by an intense mercury lamp. The released cyanide is separated from most interferences in the sample matrix and is collected in a dilute sodium hydroxide solution by gas diffusion using a hydrophobic porous membrane separator. The cyanide ion is then separated from remaining interferences such as sulphide by ion exchange chromatography and is detected by an amperometric detector. The characteristics of the automated system were studied with solutions of free cyanide and metal-cyanide complexes. The results of cyanide determination for a number of wastewater samples obtained with this method were compared with those obtained with the standard method. The sample throughput of the system is eight samples per hour and the detection limit for total cyanide is 0.1 pg L 1. 

Figure 6. Tubular membrane reactor (fixed-bed catalyst + inert porous membrane) for dehydrogenation reactions [51].

The dehydrogenation of other hydrocarbons has also been studied in CMRs, generally with porous membranes. Conversions of ethane [47], propane [48], butane [49], and ethylbenzene [50] have been reported to be higher when membrane reactors were used. In the case of ethylbenzene dehydrogenation, the undesirable hydrodealkylation side reaction is slowed down due to the removal of H2, i.e. the membrane enables an increase in selectivity as well [50]. 

Modeling studies have also considered other aspects of CMRs. Sun and Khang [46] compared two types of CMRs, one with an inert (only separative) porous membrane associated with a fixed-bed catalyst, the other with the catalyst deposited within the porous framework of the membrane (thus leading to a catalytic membrane). For long contact times, the performance of the catalytic membrane is higher, due to the simultaneity of reaction and separation. 

It has also been proposed that porous catalytic membranes may improve the efficiency of gas-liquid solid reactions when the gas-liquid interface is placed within the porous framework of the porous membrane [77], This postulated increase in efficiency has been experimentally supported by Cini and Harold in a comparative study of a CMR and a single-pellet model reactor [78] in the hydrogenation of a-mcthylstyrene into cumene. The authors ascribe this observation to a decrease in the mass transfer resistance. 

Consider the electrochemical system shown in Fig. 19 consisting of three Pt-solution interfaces in room-temperature solutions. The middle chamber is separated from the outer chambers by a porous membrane that limits mass transport, allowing the solutions to remain at different pH. Chamber A represents a NHE reference electrode the pH is zero and hydrogen gas is present at 1 atm. At this Pt-solution interface, the HER reaction is in thermodynamic equilibrium. A... 

The membrane reactor shown in Fig. 6.5 consists of a tubular shell containing a tubular porous membrane. It defines two compartments, the inner and the outer (shell) compartments. The reactants are fed into the inner compartment where the reaction takes place. We can observe that when the reactants flow along the reactor, one or more of the reaction participants can diffuse through the porous membrane to the outer side. In this case, we assume that only one participant presents a radial diffusion. This process affects the local concentration state and the reaction rate that determine the state of the main reactant conversion. The rate of reaction of the wall diffusing species is influenced by the transfer resistance of the boundary layer (1/lq.) and by the wall thickness resistance (S/Dp). 

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