Description
Due to their intrinsic dynamic nature, adsorption processes (TSA, PSA, SMB) are typically designed with the aid of process simulators. Remarkably, the extended Langmuir and variants thereof (e.g. the dual process Langmuir model) are widely used equilibrium models in many process simulations when dealing with type I isotherms (Langmuirian). The extended Langmuir model captures essential competition and displacement effects. However, it is well-known that this model is thermodynamically inconsistent when the molar saturation capacities of the adsorbing species differ[1]. Furthermore, it fails to account for adsorbate size effects predicted by the ideal adsorbed solution theory (IAST), where smaller adsorbates can be preferred at high pressures. Both the extended Langmuir as IAST offer predictions based on pure component isotherms alone. While IAST is a well-established model, it comes in an implicit form, offering some disadvantages for process simulations.Here, we first compared various equilibria model based on the Langmuir framework with non-interacting species, to the (1) extended Langmuir model. Examples are the (2) multisite Langmuir (MSL) or Nitta model, (3) IAST applied on pure component Langmuir isotherms and (4) the explicit (analytic) LeVan-Vermeulen approximations of the latter. Furthermore, we have included (5) several variants of an adapted extended Langmuir model[2], maintaining a simple and explicit form. Beyond the extended Langmuir model, all other models can meet the thermodynamic consistency criteria, even when the saturation capacities differ. A key difference with the extended Langmuir model is the high pressure preference towards the component with the largest saturation capacity. Differences between models become important with increased qsat,max/qsat,min ratio of the mixture, and the fractional loading. For cyclic processes, the fractional loading range must thus be considered.
Using a TSA process simulator, a case study with 10 hydrocarbon components was performed. Here, the qsat,max/qsat,min ratio reaches a very high 6.3, and the fractional loadings go beyond 0.99. Under such conditions the differences between the extended Langmuir model and the other models become large, evident by comparing the cyclic steady state recoveries. Furthermore, the execution times of the models were studied, confirming that explicit models can be executed faster than IAST (FASTIAS), although the manner of implementation in the process simulator is important.
References:
1. S. Sircar, Adsorption 23 (2017) 121-130.
2. T. R. C. Van Assche, G. V. Baron, J. F. M. Denayer, Adsorption 24 (2018) 517-530.
| Periode | 22 mei 2022 → 27 mei 2022 |
|---|---|
| Evenementstitel | 14th International Conference on Fundamentals of Adsorption (FOA14) |
| Evenementstype | Conference |
| Locatie | Broomfield, United States, ColoradoToon op kaart |