(478g) HETP Analysis of Structured Adsorbents for Gas Separation Processes

Structured adsorbents (SAs) can increase the performance of separation units when compared to traditional packed columns [1]. SAs applications span from VOC removal, hydrogen production, desiccant cooling, rapid cycle PSA and other processes where pressure drop can have a significant economic impact [2].

The aim of this work is to assess the performance of different types of SAs for gas separation processes. More specifically, the behaviour of monoliths with different channel shape, and corrugated metal foil with adsorbent coating have been investigated determining the Height Equivalent to a Theoretical Plate (HETP) and pressure drop. HETP equations for these structures have been derived with the use of the moment analysis described by Dutta and Leighton [3], and 3D numerical simulations have been produced to validate the proposed equations.

Both the moment analysis and the 3D simulations consider account an isothermal and linear system, with low concentration of the adsorbable component. In these conditions, the HETP can be written as sum of the contributions to the dispersion of the solute in the channel. Therefore, the relative weights of the single contributions can be studied.

The analysis of these structures shows that, for a gas separation process, the velocity profile inside the channel has little impact on the HETP. Furthermore, it also highlights how the main contribution to the dispersion is given by the diffusion in the solid.

In order to obtain the correct equation of the HETP Ahn and Brandani showed that for a rectangular channel of arbitrary aspect ratio a corrected wall thickness is needed [4]. The corrected wall thickness addresses the additional mass transfer resistance in the “corners” of the structures. This concept has been extended to all the structures analysed in this work and is shown to lead to the correct match between HETP and 3D simulations.

Finally, a 1-D equivalent model has been derived from the HETP correlation. The model consists of an axially dispersed plug flow with linear driving force (LDF) model. The axial dispersion is obtained from the Taylor-Aris dispersion coefficient [5], and the LDF constant is includes the corrected wall thickness. The 1-D model provides an accurate match of the 3-D breakthrough curves, leading to a quick and simple tool for the design of structures that include distributions of channel widths and wall thicknesses [6].

References

[1] Ruthven, D.M., Thaeron, C. Performance of a parallel passage adsorbent contactor. Gas Sep. Purif. 10, 63–73 (1996).

[2] Ruthven, D. M. Past Progress and Future Challenges in Adsorption Research. Ind. Eng. Chem. Res. 2127–2131 (2000).

[3] Dutta D, Leighton DT. Dispersion in large aspect ratio microchannels for open-channel liquid chromatography. Anal Chem. 75, 57-70 (2003).

[4] Ahn, H. & Brandani, S. Analysis of breakthrough dynamics in rectangular channels of arbitrary aspect ratio. AIChE J. 51, 1980-1990 (2005).

[5] Aris, R. On the dispersion of a solute in a fluid flowing through a tube. Proc. R. Soc. Lond. A. 235, 67-77 (1956).

[6] Ahn, H. & Brandani, S. Dynamics of Carbon Dioxide Breakthrough in a Carbon Monolith over a Wide Concentration Range, Adsorption, 11, 473–477 (2005).