- 8:33 AM

A Novel Method for Reporting Adsorption Equilibria: Net ADSORPTION

Sasidhar Gumma, Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati, 781039, India and Orhan Talu, Cleveland State University, Department of Chemical Engineering, Cleveland, OH 44115.

Usually the experimental data reported in literature for gas adsorption equilibrium is for so-called (Gibbs') excess amount adsorbed. It is obtained by fixing the Gibbs' dividing surface. In practice this is done correcting experimental measurements for buoyancy in case of gravimetric experiments or for void volume in case of volumetric experiments. Both the corrections need a non-adsorbing gas to measure the impenetrable solid volume; helium is a chosen de facto standard for these measurements due to its relatively inert nature compared to any other gas.

In recent years several articles have appeared in literature, which question the validity of non-adsorbing helium assumption. Now, it is established beyond doubt that helium adsorbs at all conditions; at best adsorbed amount may be quite small. In any case, some error and differences result because of such an arbitrary assumption. The magnitude of the error increases when dealing with adsorption of gases whose adsorption is close to that of helium adsorption (for example hydrogen on several adsorbents at room temperature). This error also increases when correction term for experimental measurements becomes significant, usually at very high pressures. These conditions are encountered in most applications of adsorption for energy storage (hydrogen or methane storage).

In this work, we present a new method for reporting adsorption equilibrium data that does not necessitate any arbitrary assumptions or measurement of impenetrable solid volume. We call this as net adsorption. We believe net adsorption is better suited for reporting of adsorption data since correction terms are not needed in its calculation. Moreover, data across different labs at all conditions can be readily compared, since arbitrary assumptions are completely eliminated.

To further validate our claim, we present and compare both excess and net adsorption data for adsorption of methane and argon on a proprietary adsorbent. Amounts adsorbed and heats of immersion are calculated and presented using both excess and net adsorption data. A procedure to calculate mass and energy balances energy balances on an adsorption column is also presented, using this data. It is shown that there will be no difference, when one uses net adsorption data for performing balances on a column.

This work proposes a new method for reporting and using adsorption equilibrium data for designing adsorption processes, without necessity to fix Gibbs' dividing surface through arbitrary assumptions.