Reactive adsorption reactors

Another interesting example of reactive adsorption is the so-called gas-solid-solid trickle flow reactor, in which adsorbent trickles through the fixed bed of catalyst, removing selectively in situ one or more of the products from the reaction zone. In the case of methanol synthesis this led to conversions significantly exceeding the equilibrium conversions under the given conditions (67). 

The suitability of regenerating the adsorbent by reactive means can only be judged on a case-by-case basis. With some adsorptive reactors - for example, for the total denitrification of flue gases or the unsteady-state Deacon process - the reactive regeneration is the object of the exercise. The prerequisite that the adsorbate does not undergo further reaction and is adsorbed in reasonable amounts at moderate temperatures means that the molecules being adsorbed tend to be small and stable, and thus do not lend themselves to reactive regeneration. 

Whilst the development of new adsorbents on monolithic [62] or fibrous supports [63] to cut pressure drops, of high-capacity metal organic frameworks (MOFs) [64], or of highly selective molecularly imprinted polymers (MIPs) [65], is certainly beneficial for the realization of novel adsorptive reactive concepts, the serendipity of catalytic chemistry and the accompanying adsorption process remains the crucial factor for the success or otherwise of an adsorptive reactor. Thus, although a healthy degree of skepticism is appropriate when assessing the suitability of an adsorptive... 

Of course, this reactive adsorption is favoured by removal of hydrogen from the reaction zone. When 80% of the hydrogen is removed in the membrane reactor, the H2S tolerance of the catalyst is about halve the tolerance when no hydrogen is removed from the reaction zone. A higher degree of sulphur removal from the feed stream should be accomplished when operating a membrane steam reformer. 

In this section reactive distillation, reactive extraction, reactive adsorption and membrane reactors are discussed. The references cited in this section are recommended for those wishing to seek out more detail on these or other reactive extractions. 

Adsorption is a basic separation technique that can be used in parallel with a reaction to, for example, increase conversion by removing a product. Reactive adsorption combines the separation role of, for example, a solid adsorbent with the reaction taking place on a different surface. The abihty to remove the adsorbent or to desorb one or more of the products of the reaction can be convenient and intensive , although adsorbents do not necessarily respond rapidly. There could well be, for instance, synergy between the fluidized-bed reactors and such adsorption processes. 

Multi-functional operations are discussed under two sections of reactive separations and hybrid separation platforms. Reactive separations of reactive distillation, reactive adsorption, and membrane reactors are presented in more detail including their principles, advantages and applicability to different systems. Hybrid separations incorporating different unit operations are discussed briefly along with their application and scope. 

Maas R, Chaudhari S (2005) Adsorption and biological decolourization of azo dye Reactive Red 2 in semicontinuous anaerobic reactors. Process Biochem 40 699-705... 

The reactor system may consist of a number of reactors which can be continuous stirred tank reactors, plug flow reactors, or any representation between the two above extremes, and they may operate isothermally, adiabatically or nonisothermally. The separation system depending on the reactor system effluent may involve only liquid separation, only vapor separation or both liquid and vapor separation schemes. The liquid separation scheme may include flash units, distillation columns or trains of distillation columns, extraction units, or crystallization units. If distillation is employed, then we may have simple sharp columns, nonsharp columns, or even single complex distillation columns and complex column sequences. Also, depending on the reactor effluent characteristics, extractive distillation, azeotropic distillation, or reactive distillation may be employed. The vapor separation scheme may involve absorption columns, adsorption units,... 

The most active formulation (ZSNbPt) was tested in a conventional reactor using as feedstream a mixture of light n-alkanes [n-pentane (20 wt%), n-hexane (60 wt.%) and n-heptane (20 wt%)] to simulate an industrial stream. Experiments were carried out in a conventional reaction system using a fixed-bed continuous -flow reactor. Reaction was carried out under the same conditions as the poisoning resistance experiments. The activity and selectivity of this catalyst (Fig. 5.13) have been compared with those obtained with sulfated zirconia impregnated with platinum (ZS). Fig. 5.13 represents the evolution of the conversion with reaction temperature. Clearly, the reactivity of the n-paraffm follows the order n-heptane > n-hexane > n-pentane for both catalysts, as expected when taking into account the adsorption heats of the different hydrocarbons [34]. 

In the second chapter, Anil Agiral and Han J.G.E. Gardeniers take us to a fascinating world wherein "chemistry and electricity meet in narrow alleys." They claim that microreactor systems with integrated electrodes provide excellent platforms to investigate and exploit electrical principles as a means to control, activate, or modify chemical reactions, or even preparative separations. Their example of microplasmas shows that the chemistry can take place at moderate temperatures where the reacting species still have a high reactivity. Several electrical concepts are presented and novel principles to control adsorption and desorption, as well as the activity and orientation of adsorbed molecules are described. The relevance of these principles for the development of new reactor concepts and new chemistry is discussed. 

The feasibility of combining chemical reaction and adsorption separation in a single unit has been discussed in this chapter. In particular, two units allowing continuous operation have been considered, namely annular reactive chromatography and simulated moving-bed reactors. 

At first sight, adsorption and reaction are well-matched functionalities for integrated chemical processes. Their compatibility extends over a wide temperature range, and their respective kinetics are usually rapid enough so as not to constrain either process, whereas the permeation rate in membrane reactors commonly lags behind that of the catalytic reaction [9]. The phase slippage observed in extractive processes [10], for example, is absent and the choice of the adsorbent offers a powerful degree of freedom in the selective manipulation of concentration profiles that lies at the heart of all multifunctional reactor operation [11]. Furthermore, in contrast to reactive distillation, the effective independence of concentration and temperature profiles... 

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. 

The combination of a second unit in peroxidase reactors may be helpful in different situations, depending on the nature of the substrate to be treated. For instance, a two-stage reactor system for the continuous decolorization of direct dyes was used [44]. The first unit consisted on a fixed bed reactor connected to a second column of activated silica, which helped in the adsorption of toxic reactive species, therefore reducing the biotoxicity of the effluent. The same system was applied for the decolorization of textile effluents and was capable of decolorizing 40% effluent even after 2 months of continuous operation [45]. 

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