Design of periodic adsorptive reactors for the optimal integration of reaction, separation and heat-exchange

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Conversion-temperature trajectories for well-mixed adsorptive reactors

Reactant conversion parameters which account for the solid- and fluid-phase distribution of adsorbate are used to yield information on the conversion-temperature characteristics of well-mixed and adiabatic adsorptive reactors. When applied to endothermic reactions in which there is the preferential adsorption of product species, favourable operating trajectories in the conversion-temperature plane are generated for both batch and steady flow type operation. The effect is attributed to a reduction in the net heat of consumption under the conditions of simultaneous adsorption and reaction; modified (effective) heats of reaction are derived to characterise this effect. Conditions for mean-isothermal reactor operation under adiabatic conditions are also derived, and shown to be functions of the heats of adsorption and reaction, and the capacity of the adsorbent for the various reaction species. For the flow reactor, the analysis is extended to mass-transfer-limited adsorption as described by a linear driving force model. Conversion-temperature trajectories are thus attained which account for the adsorption and reaction parameters, and the residence time of the adsorbent in the reactor. As an example of the potential reduction in the net heat of reaction, the dehydrogenation of methylcyclohexane to toluene in an adiabatic flow reactor, and in the presence of various commercial adsorbents, is considered

Total connectivity models for adsorptive reactor design

The synergetic combination of separation, reaction and heat exchange using multiple fixed beds of adsorbent and catalyst is theoretically explored through the optimization of a general configurational superstructure. The superstructure enables all possible connections between the beds (stages) and the feed and product reservoirs, and is thus referred to as a total connectivity model. Two-step cycle operations are considered involving a reactant feed step and adsorbent regeneration step. Gas flow in each step can be in the same overall direction, or in a reverse-flow arrangement. As a case study, the endothermic dehydrogenation of methylcyclohexane to toluene is considered over an admixture of Pt-alumina catalyst and zeolite 5A adsorbent. The method demonstrates an effective means for generating cyclically operated reactor and adsorber networks which substantially improve upon the production efficiency of an equivalent adiabatic steady-flow reactor, whilst adhering to user-specified bulk separation constraints. For example, optimization of a reverse-flow total connectivity model has led to a process which yields 65% conversion of methylcyclohexane (cf. 23% for an equivalent and optimally operated PFR), with 97% recovery of a toluene product which is 64% pure on an inert-free basis (cf. 11% purity for the PFR). These benefits are achieved for energy inputs which do not exceed that of the steady-flow reactor. When compared to sub-set structures in which, for example, gas recycle is not permitted, the calculations indicate that simple series-parallel connectivity of the stages can provide a comparable performance in terms of conversion and separation. Total connectivity simulation thus establishes the maximum system performance, against which simpler configurations can be evaluated

Total connectivity models for adsorptive reactor design

The synergetic combination of separation, reaction and heat exchange using multiple fixed beds of adsorbent and catalyst is theoretically explored through the optimization of a general configurational superstructure. The superstructure enables all possible connections between the beds (stages) and the feed and product reservoirs, and is thus referred to as a total connectivity model. Two-step cycle operations are considered involving a reactant feed step and adsorbent regeneration step. Gas flow in each step can be in the same overall direction, or in a reverse-flow arrangement. As a case study, the endothermic dehydrogenation of methylcyclohexane to toluene is considered over an admixture of Pt-alumina catalyst and zeolite 5A adsorbent. The method demonstrates an effective means for generating cyclically operated reactor and adsorber networks which substantially improve upon the production efficiency of an equivalent adiabatic steady-flow reactor, whilst adhering to user-specified bulk separation constraints. For example, optimization of a reverse-flow total connectivity model has led to a process which yields 65% conversion of methylcyclohexane (cf. 23% for an equivalent and optimally operated PFR), with 97% recovery of a toluene product which is 64% pure on an inert-free basis (cf. 11% purity for the PFR). These benefits are achieved for energy inputs which do not exceed that of the steady-flow reactor. When compared to sub-set structures in which, for example, gas recycle is not permitted, the calculations indicate that simple series-parallel connectivity of the stages can provide a comparable performance in terms of conversion and separation. Total connectivity simulation thus establishes the maximum system performance, against which simpler configurations can be evaluated

Conversion-temperature trajectories for well-mixed adsorptive reactors

Reactant conversion parameters which account for the solid- and fluid-phase distribution of adsorbate are used to yield information on the conversion-temperature characteristics of well-mixed and adiabatic adsorptive reactors. When applied to endothermic reactions in which there is the preferential adsorption of product species, favourable operating trajectories in the conversion-temperature plane are generated for both batch and steady flow type operation. The effect is attributed to a reduction in the net heat of consumption under the conditions of simultaneous adsorption and reaction; modified (effective) heats of reaction are derived to characterise this effect. Conditions for mean-isothermal reactor operation under adiabatic conditions are also derived, and shown to be functions of the heats of adsorption and reaction, and the capacity of the adsorbent for the various reaction species. For the flow reactor, the analysis is extended to mass-transfer-limited adsorption as described by a linear driving force model. Conversion-temperature trajectories are thus attained which account for the adsorption and reaction parameters, and the residence time of the adsorbent in the reactor. As an example of the potential reduction in the net heat of reaction, the dehydrogenation of methylcyclohexane to toluene in an adiabatic flow reactor, and in the presence of various commercial adsorbents, is considered

Design of periodic adsorptive reactors for the optimal integration of reaction, separation and heat exchange

Optimisation techniques are used for the design of novel forced-periodic reactors in which there is the integration of catalytic reaction, adsorptive separation and direct fluid to solid heat exchange within a single unit operation. For such configurations, dynamic operation is utilised to generate favourable temperature (catalyst activity) profiles, and, through in situ separation, to provide reaction enhancement and the enriched recovery of the primary product(s). The work thus involves aspects of thermal and concentration swing adsorption, reverse (or bidirectional) flow reactor operation, and thermal regeneration. The design methodology is based on fully discretised mathematical models, upon which periodic constraints are imposed to yield direct cyclic steady state solutions. The models are then formulated as non-linear programming problems to yield optimal operating schedules and conditions. Models are developed for general endothermic and equilibrium limited reaction schemes; as a case study, specific consideration is given to the dehydrogenation of methylcyclohexane to toluene over and admixture of Pt-Al2O3 catalyst and zeolite 5A adsorbent. When compared to an equivalent and optimally operated adiabatic plug flow reactor, design and operating conditions are calculated for both single and multistage configurations in which there are significant improvements in reactant conversion, with the additional benefit of the bulk separation of the product species. These qualities of the hybrid reactors are attained for energy inputs (i.e. feed pre-heat requirements) which do not exceed that of the equivalent steady flow reactor. The results also predict larger improvements in reactor performance when there is greater flexibility in the way in which heat and material are introduced into the reactor systems. For example, for a five-stage configuration involving mixed series-parallel connections of the stages, and the disproportional splitting of feed streams to each stage, a conversion of 59% is calculated for the production of 0.02 mol/m2 s toluene (cf. 23% for an equivalent reactor), with 72% recovery of the toluene in a near pure form (inert-free basis)

Design and optimisation of temperature cycled adsorptive reactors

This work describes a novel process in which reaction, separation and heat-exchange are combined within a single unit operation. Thermal and concentration swing steps are considered in the theoretical design and optimisation of the process configurations, and include co- and counter-current configurations. The operation is carried out over a fixed bed of an admixture of catalyst and adsorbent which selectively removes the primary product from the reaction zone. The adsorbent is periodically regenerated using hot inert gas, which also acts as an energy supply step for the endothermic reaction. An optimisation strategy is used to seek the optimal operating policy that enables the process to separate the primary product, and outperform an optimal PFR in terms of production yield and energy consumption. © 1998 Published by Elsevier Science Ltd. All rights reserved