Abstract
This paper gives the first step of the development of a rigorous multicomponent reactive separation model. Such a model is highly essential to further the optimization of acid gases removal plants (CO2 capture, gas treating, etc.) in terms of size and energy consumption, since chemical solvents are conventionally used.Firstly, two main modelling approaches are presented: the equilibrium-based and the rate-based approaches.Secondly, an extended rate-based model with rigorous modelling methodology for diffusion-reaction phenomena is proposed. The film theory and the generalized Maxwell-Stefan equations are used in order to characterize multicomponent interactions. The complete chain of chemical reactions is taken into account. The reactions can be kinetically controlled or at chemical equilibrium, and they are considered for both liquid film and liquid bulk.Thirdly, the method of numerical resolution is described. Coupling the generalized Maxwell-Stefan equations with chemical equilibrium equations leads to a highly non-linear Differential-Algebraic Equations system known as DAE index 3. The set of equations is discretized with finite-differences as its integration by Gear method is complex. The resulting algebraic system is resolved by the Newton- Raphson method.Finally, the present model and the associated methods of numerical resolution are validated for the example of esterification of methanol. This archetype non-electrolytic system permits an interesting analysis of reaction impact on mass transfer, especially near the phase interface. The numerical resolution of the model by Newton-Raphson method gives good results in terms of calculation time and convergence. The simulations show that the impact of reactions at chemical equilibrium and that of kinetically controlled reactions with high kinetics on mass transfer is relatively similar. Moreover, the Fick’s law is less adapted for multicomponent mixtures where some abnormalities such as counter-diffusion take place.
Highlights
The wide interest in multifunctional unit operations involving reactive separation arises from the world’s industrial tendency towards process intensification
The film theory and the generalized Maxwell-Stefan equations are used in order to characterize multicomponent interactions
The rate-based models for reactive separation follow the principle of non-equilibrium stage, implying that actual rates of chemical reactions and multicomponent mass and mass transfer between phases are taken into account directly
Summary
The wide interest in multifunctional unit operations involving reactive separation arises from the world’s industrial tendency towards process intensification. Using more complex solvents in reactive separation units in order to obtain higher efficiencies, deal eventually with a huge chain of chemical reactions of different natures and velocities. In this case, any simplification in terms of reaction mechanism such as considering just one apparent chemical reaction would cause a loss in accuracy. The first part of this article describes the two major modelling approaches to simulate homogeneous reactive separation: the equilibrium-based and the rate-based approach. Both models are applied to mass and energy transfer, as well as to chemical reactions.
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