Adsorptive enhanced reactor

In this study, Pt/AliOj having high activity for CO oxidation and different affinities for fee adsorption of CO and Hi was selected as a catalyst/adsorbent In a conventional packed bed reactor (PBR), fee surface of fee catalyst is dominantly covered by COads with small amotmt of Oads fee CO conversion is therefore low. Several investigations on periodic operation have illustrated feat fee reaction front wife comparable amount of fee two adsorbed species leads to enhancement of fee CO conversion. Conceptually, this type of the reaction front should be generated by application of a CMBR, as well. Figure 1 illustrates an image of... 

As already discussed, the enhanced conversion is due to the separation of the products from the reaction zone. This can be realized via different distribution coefficients of the compounds (and consequently, a separation of the components) or via (selective) adsorption on a support. Since in the first case the compound travels through the reactor with different speeds, a continuous feed would cause repeated mixing of the separated compounds. Therefore, no improvement can be expected. In the second case, a regeneration of the adsorbent is needed after a certain operative period. This is an inherent drawback of the discontinuous operation of the fixed-bed chromatographic reactor. 

Many of the characterization techniques described in this chapter require ambient or vacuum conditions, which may or may not be translatable to operational conditions. In situ or in opemndo characterization avoids such issues and can provide insight and information under more realistic conditions. Such approaches are becoming more common in X-ray adsorption spectroscopy (XAS) methods ofXANES and EXAFS, in NMR and in transmission electron microscopy where environmental instruments and cells are becoming common. In situ MAS NMR has been used to characterize reaction intermediates, organic deposits, surface complexes and the nature of transition state and reaction pathways. The formation of alkoxy species on zeolites upon adsorption of olefins or alcohols have been observed by C in situ and ex situ NMR [253]. Sensitivity enhancement techniques play an important role in the progress of this area. In operando infrared and RAMAN is becoming more widely used. In situ RAMAN spectroscopy has been used to online monitor synthesis of zeolites in pressurized reactors [254]. Such techniques will become commonplace. 

The use of continuous immobilized cell biofilm reactors eliminates downtime and hence results in superior reactor productivity (2,3). Adsorbed cell continuous biofilm reactors have been shown to favorably affect process economics (4). Application of these reactors reduces capital and operational cost, thus making the process simpler. Within these reactors, cells are immobilized by adsorption, which is a simpler technique than other techniques such as entrapment and covalent bonding (5). Adsorption is a simple technique and can be performed inside the reactors without the use of chemicals, whereas entrapment and covalent bonding are complicated techniques and require chemicals for bond formation. In anaerobic systems, such as butanol production, adsorption can be performed anaerobically within the reactor. An additional advantage of adsorption is that cells form uniform biofilm layers around the support, which lessens diffusion resistance compared to entrapped and covalently bonded cells. Hence, these reactors are called biofilm reactors. Because of reduction in diffusion resistance, the reaction rate is enhanced. For this reason, adsorption was chosen as the technique to be employed for Clostridium beijerinckii BA101 cell immobilization to produce butanol. In addition to being simple, it has the potential to be used in large-scale reactors. In the present study, clay brick was chosen as the cell adsorption support. It is available at a low cost and is easy to dispose of after use. 

Fig. 7.1. Principle of an adsorptive reactor for enhancing conversion of an equilibrium-limited reaction.

Whilst the enhancement of unwanted side reactions through excessive distortion of the concentration profiles is an effect that has been reported elsewhere (e.g., in reactive distillation [40] or the formation of acetylenes in membrane reactors for the dehydrogenation of alkanes to olefins [41]), the possible negative feedback of adsorption on catalytic activity through the reaction medium composition has attracted less attention. As with the chromatographic distortions introduced by the Claus catalyst, the underlying problem arises because the catalyst is being operated under unsteady-state conditions. One could modify the catalyst to compensate for this, but the optimal activity over the course of the whole cycle would be comprised as a consequence. 

Macrostructuring measures are a well-known concept for the suppression of hotspots by local dilution of catalyst in multitubular reactors, for example [53]. It is also intuitively obvious that the need for adsorption to enhance an equilibrium reaction is much lower at the front end of the reactor, where kinetics rather than equilibrium are decisive, than downstream. The converse, however is not true in the outlet of such an adsorptive reactor both catalyst and adsorbent are required for maximal conversion. By a judicious selection of the ratio between the two, one can ameliorate the front broadening effects that arise in the reactor outlet. 

The preceding review of various aspects of adsorptive reactors has hopefully provided some insights as to why this apparently so promising technology has yet to fulfil its potential. The often conflicting (and sometimes incompatible) demands placed on catalyst and adsorbent, the enhancement of unwanted side reactions or catalyst deactivation, and the difficulties of an expedient adsorbent regeneration with realistic overall cycle times can quickly disillusion those trying to harness this particular type of multifunctional reactor. 

A simultaneous countercurrent movement of solid and gaseous phases makes it possible to enhance the efficiency of an equilibrium limited reaction with only one product (Fig. 4(a)) [34]. A positive effect can be obtained for the reaction A B if the catalyst has a higher adsorption capacity for B than for A. In this case, the product B will be collected mainly in the upper part of the reactor, while some fraction of the reactant A will move down with the catalyst. Better performance is achieved when the reactants are fed at some side port of the column inert carrier gas comes to the bottom and the component B is stripped off the catalyst leaving the column (Fig 4(a)). The technique was verified experimentally for the hydrogenation of 1,3,5-trimethylbenzene to 1,3,5-trimethylcyclohexane over a supported platinum catalyst [34]. High purity product can be extracted after the catalytic reactor, and overequilibrium conversion can be obtained at certain operating conditions. 

The presence of even small amounts of SO2 in the feedstock con.sidenibly enhances the rate of desorption of these adsorbed species, probably due to competitive adsorption, as shown by bT-IR experimenis. This explains, on the other hand, the increase tn the selectivity at high conversion in anhydride formation in alkane oxidation in the flow reactor tests in the presence of SO2. Favouring the anhydride desorption, in fact, decreases the possibility of its consecutive oxidation to CO 116. ... 

Recently, Comas et al.219 performed the thermodynamic analysis of the SRE reaction in the presence of CaO as a C02 sorbent. The equilibrium calculations indicate that the presence of CaO in the ethanol steam reforming reactor enhances the H2 yield while reducing the CO concentrations in the outlet of the reformer. Furthermore, the temperature range at which maximum H2 yield could be obtained also shifts from above 700 °C for the conventional steam reforming reaction without CaO to below 700 °C, typically around 500 °C in the presence of CaO. It appears that the presence of CaO along with ethanol reforming catalyst shift the WGS equilibrium in the forward direction and converts more CO into C02 that will be simultaneously removed by CaO by adsorption. 

As shown in Fig. 1, an induction period of 30-75 min was observed over all zeolite and MCM catalysts in the vapor phase runs. During this induction period, no toluene or products were observed in the effluent toluene and nitrotoluene appeared only when the catalyst seemed to become saturated. Examination of the reactor effluent indicated that this adsorption only occurred when the two reactants were fed to the system simultaneously. AVhen either toluene or NOj was fed separately, no adsorption was observed. For example, in one experiment, toluene was passed over the catalyst for 90 min. without noticeable adsorption. Then, as soon as the NOj flow was started, the toluene signal disappeared for about 50 min., after which both unreacted toluene and nitrotoluene products started to appear. It seems that the presence of NOj enhances the adsorption of the aromatic molecule which in turn is necessary for the catalytic process. However, this extensive adsorption may, simultaneously, cause pore plugging and catalyst deactivation. 

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