Limitations of Breakthrough Curve Analysis in Fixed-Bed Adsorption

Abstract

This work examined in detail the a priori prediction of the axial dispersion coefficient from available correlations versus obtaining both it and mass transfer information from experimental breakthrough data and the consequences that may arise when doing so based on using a 1-D axially dispersed plug flow model and its associated Danckwerts outlet boundary condition. These consequences mainly included determining the potential for erroneous extraction of the axial dispersion coefficient and/or the LDF mass transfer coefficient from experimental data, especially when nonplug flow conditions prevailed in the bed. Two adsorbent/adsorbate cases were considered, i.e., CO 2 and H 2 O vapor in zeolite 5A, because they both experimentally exhibited significant nonplug flow behavior, and the H 2 O-zeolite 5A system exhibited unusual concentration front sharpening that destroyed the expected constant pattern behavior (CPB) when modeled with the 1-D axially dispersed plug flow model. Overall, this work showed that it was possible to extract accurate mass transfer and dispersion information from experimental breakthrough curves using a 1-D axial dispersed plug flow model when they were measured both inside and outside the bed. To ensure the extracted information was accurate, the inside the bed breakthrough curves and their derivatives from the model were plotted to confirm whether or not the adsorbate/adsorbent system was exhibiting CPB or any concentration front sharpening near the bed exit. Even when concentration front sharpening was occurring with the H 2 O-zeolite 5A system, it was still possible to use the experimental inside and outside the bed breakthrough curves to extract fundamental mass transfer and dispersion information from the 1-D axial dispersed plug flow model based on the systematic methodology developed in this work.

Calibration and uncertainty analysis of a fixed-bed adsorption model for CO2 separation

“…While there are many correlations (both empirical and semi-empirical) that allow researchers to directly predict the axial dispersion of their system without comparing their model directly to experimental data or higherfidelity simulations, none of these correlations account for every possible mechanism of axial dispersion. Furthermore, the available empirical correlations do not account for wall channeling, causing them to drastically underpredict the apparent axial dispersion in small-diameter beds, such as those used in laboratory experiments to obtain fitted parameters 25,33 . Knox et al 33 investigated the effect of using such empirical correlations to predict axial dispersion when fitting a onedimensional, fixed-bed adsorption model to typical lab-scale, experimental-breakthrough data.…”

“…Furthermore, the available empirical correlations do not account for wall channeling, causing them to drastically underpredict the apparent axial dispersion in small-diameter beds, such as those used in laboratory experiments to obtain fitted parameters 25,33 . Knox et al 33 investigated the effect of using such empirical correlations to predict axial dispersion when fitting a onedimensional, fixed-bed adsorption model to typical lab-scale, experimental-breakthrough data.…”

“…Knox et al 33 focused on the effect of using such predictions of axial dispersion on subsequently fit model parameters (e.g., LDF mass transfer coefficient). They found that using empirical prediction of axial dispersion fitted to the effluent caused an error in LDF coefficient and proposed an experimental method for reducing this error using in-bed centerline concentration measurements where the near-wall apparent dispersion effects are minimized.…”

“…The second approach maintains the plug-flow assumption and uses a larger, apparent axial dispersion to account for the wall-channeling (e.g., Knox et al 33 ).…”

“…Conclusions in the literature regarding sensitivity to axial dispersion are less consistent; while many researchers concluded that adsorption models are insensitive to axial dispersion 3,4,10,12,14,21,26,30 , others found the opposite to be true 16,24,25,33 . Yet others are equivocal in their conclusions.…”