Reactive adsorption reactor operation

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... 

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. 

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. 

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 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. 

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]. 

The pilot unit consists of six reactors, each six meters tall. The reactors contain beds of pellets that simultaneously catalyze the water gas shift reaction and adsorb CO2. The unit is operated as a reactive pressure swing adsorption installation. By switching valves, the reactors are operated in cycles of C02-adsorption and desorption. Since at any time, at least one reactor is in adsorption mode and at least one reactor in desorption mode, a continuous production of purified hydrogen (and CO2) is obtained. Design, engineering, construction, and commissioning of the unit took less than one year. 

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