The Side-Reactor Concept

One approach to solving the incompatibility in the requirements of reaction (high liquid or catalyst hold-up) and separation (high vapor-Uquid interfacial area) is to employ the side-reactor or external reactor concept [51], Fig. 7.21. The liquid is withdrawn from stage j, passes through the side reactor, and is fed back to the column at stage k. The amount of liquid pumped around, Igp = Rp Lj, where RpA is the pump-around ratio. By providing adequate residence time for reaction, equilibrium conversion is achieved in the side reactor. 

For the production of MeOAc, let us compare the RD column with a distillation column with either three or five side reactors. Fig. 7.22. The side-reactor could be a tubular reactor, packed with, say, Amberlyst-15 for acid catalysis. It could also be a homogeneous liquid-phase tubular reactor, catalyzed by H2SO4. Clearly the location 

The (non-reactive) distillation columns linked to the side reactors can be much smaller in diameter than the RD column and no specially designed trays (e. g., with higher weirs or additional sumps) or proprietary devices such as Katapak-S are necessary. The side-reactor concept is particularly attractive when the conversion requirements are not as stringent as assumed in the MeOAc case study above. 

Many of the conflicting hardware issues can be resolved by de-coupling the separation and reaction function by employing the side-reactor concept. 

would like to express his appreciation to Dr. R. Baur for carrying out most of the simulation work presented in this chapter and assistance in the preparation of this paper. R. K. also acknowledges financial support from the Netherlands Organization for Scientific Research (NWO) in the form of a programmasubsidie for development of novel concepts in reactive separations. 

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The building block of the superstructure representation is the generic reactor unit, which follows the shadow reactor concept (32). This generic unit is illustrated in Figure 4. Each generic unit consists of reactor compartments in each phase of the system, and each processes the reaction. The shadow reactor compartment assumes a state from the set of homogeneous reactors. The default units in the set include CSTRs and PFRs with side streams. The interface between a given pair of... 

X 10 mol/s. Considering the thermodynamic limit of I8,7% the conversion achieved by the membrane reactor is extremely high, testifying to the validity of the membrane reactor concept. It should be noted that benzene leaving the membrane reactor is almost pure, and therefore a cyclohcxanc/benzene separator is not necessary. Furthermore, if a vacuum is applied on the permeate side, instead of an argon sweep gas stream, pure hydrogen gas is obtained as a reaction product. 

Figure 12.2. Schematic drawing of the original reactor concept of Temkin and Kul kova, a) side view of reactor, b) top view along reactor axis.

The conditions are substantially more favorable for the microporous catalytic membrane reactor concept. In this case the membrane wall consists of catalyti-cally active, microporous material. If a simple reaction A -> B takes place and no permeate is withdrawn, the concentration profiles are identical to those in a catalyst slab (Fig. 29a). By purging the permeate side with an inert gas or by applying a small total pressure difference, a permeate with a composition similar to that in the center of the catalyst pellet can be obtained (Fig. 29b). In this case almost 100% conversion over a reaction length of only a few millimeters is possible. The advantages are even more pronounced, if a selectivity-limited reaction is considered. This is shown with the simple consecutive reaction A- B- C where B is the desired product. Pore diffusion reduces the yield of B since in a catalyst slab B has to diffuse backwards from the place where it was formed, thereby being partly converted to C (Fig. 29c). This is the reason why in practice rapid consecutive reactions like partial oxidations are often run in pellets composed of a thin shell of active catalyst on an inert support [30],... 

It has been considered traditional applications of zeolite-membrane reactors those based on reactor concepts already demonstrated using mesoporous or dense membranes. These include conversion enhancement by equilibrium displacement or by the removal of inhibitors, and selectivity enhancement by reactant distribution. For such cases, the zeolite membrane is usually catalytically inert and is coupled with a conventional fixed bed of catalyst placed on one of the membrane sides. 

Other reactor configurations and concepts have also been discussed in the technical literature. Most commonly dted are hybrid concepts, where the membrane reactor is used as an add-on stage to an already existing conventional reactor. This particular configuration has a number of attractive features, especially for applications involving conventional type porous membranes, which are characterized by moderate (Knudsen-type) permselective properties. Staged membrane reactors have received mention and so have reactors with multiple feed-ports and recycle. To facilitate the transport across the membrane in laboratory studies one often applies a sweep gas or a vacuum in the permeate side or a pressure gradient across the membrane. It is unlikely that the first two approaches, effective as they may be in laboratory applications, will find widespread commercial application. 

Systems Analysis Figure 1 shows a concept identified by NETL for a integrated fuel processor/ fuel cell system targeted for diesel APUs. There are several favorable attributes of this system. For example, startup occurs by firing an internal combustor in the dual reactor reformer. This provides heat to the ATR reformer (via conduction) as well as supplying heat to the fuel cell cathode via direct exhaust from the combustor or preheated air from the heat exchanger (optional). If necessary, the ATR is fired in a partial oxidation mode to aid in heatup and to provide heat to the anode side of the... 

Most new concepts in fixed-bed reactor design are for very large scale production, and hence we only mention them briefly here. The radial reactor, a relatively well-known design, allows radial flow of gas between the tube and shell sides of the conventional multitubular reactor (through perforations in the tube walls). On the other hand, in the newer spherical fixed-bed reactor (see Hartig and Keil, 1993), flow across the catalyst is accomplished by placing it between two perforated spherical shells inside a solid spherical enclosure. The reactant is introduced between the outer shell and the enclosure and flows through the catalyst between the shells into the inner shell from where it exits the system. 

Two reactor concepts may be distinguished [209], which are schematized in Figure 9.30. One consists of the gas-phase and the liquid-phase flowing, respectively, on each side of the membrane (Figure 9.30a). In this case, one reactant is dissolved in the liquid phase, which is sucked by capillary forces into the catalytic membrane layer, getting the reactant in contact with the catalytic sites. The gaseous reactant is fed on the other side through the porous support of the membrane. As a result, a gas-liquid-phase boundary was established, which is determined by the pressure difference between the gas and the liquid side. The pressure must be controlled in order to have the phase boundary in the membrane layer where catalytic active sites are located so that the contact between the liquid reactant, the gas reactant, and... 

Over the years, several processes for the catalytic dehydrogenation of propane to propylene have been developed, which can be divided into processes based on an adiabatic or an isothermal reactor concept, respectively. The processes currently apphed on an industrial scale are based on adiabatic systems, such as the Catofin (Lummus/Air Products) and the Oleflex (UOP) process. As the dehydrogenation of propane to propylene comprises an equihbrium reaction (11), selective removal of hydrogen from the reaction mixture can shift the reaction towards the product side. At high temperatures, thermal cracking may occur. 

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