Collaborative Research: Adsorption Equilibration of Binary Mixtures on Planar and Porous Sorbents

Southern Illinois University At Carbondale, Carbondale IL

Investigators

Abstract

Non-Technical Abstract After a mixture of two gases is introduced into a container where an absorbing material has been placed, the pressure and the ratio of the two gases of the mixture inside the container change with time, as particular molecules bind to the absorbing surface. After enough time elapses, the pressure and the composition of the gas reach constant values, and a condensed phase forms on the surface, in equilibrium with the gas phase. This project studies the kinetics of adsorption, that is, the evolution of this process from the introduction of the gas mixture until equilibrium is reached, and how it depends on parameters such as the composition and temperature of the starting gas mixture, how strongly each of the gas molecules attaches to the sorbent, and how fast the different molecules reach their final state. While this is a very complex process, with numerous aspects that are not well understood, most practical applications of adsorption (such as gas separation or gas storage) either depend on, or are limited by, these dynamics. A more complete understanding of the mixture adsorption dynamics will allow for better control of the process and more efficient practical applications. The kinetics of adsorption is studied by performing computer simulations for model systems, and comparing the results with those obtained through experiments in which pressure and composition are determined, as a function of time. The computer simulations provide a microscopic understanding of the process, and the experiments are a test for the validity of the simulations. Graduate and undergraduate students participating in the project can gain a broader perspective of the investigated phenomena provided by the experimental-theoretical character of this partnership. Special efforts are made towards recruitment and retention of students from underrepresented groups, through summer programs for local high school students and one-on-one mentoring programs that provide critical student support. Technical Abstract This project addresses the question of how a mixture of gases approaches adsorption equilibrium as it interacts with various kinds of solid surfaces and/or is confined within small pores. This is achieved by combining results from a series of studies that involve theory, experiments and simulations. Information on the equilibration of a single component (which is itself often difficult to find for many systems) is not sufficient to understand the sorption kinetics of mixtures, where the interplay between the species has a distinct and defining role. Understanding and taking full advantage of the adsorption characteristics of a sorbent when two distinct species compete to become adsorbed onto its surfaces requires further and explicit investigation of the kinetic processes present in the mixture that eventually lead to the final equilibrium state of the adsorbed phase. Three main aspects of these phenomena are investigated: a) How does the adsorption rate of a binary mixture of gases (and of its individual components) depend on the partial pressures and temperature? (experiments and simulations); b) What are the elementary processes involved in the competition during equilibration, and how do they affect the adsorption rates? (theory and simulations); c) How does the spatial distribution of each component on the surface change with time? (theory and simulations). These questions are addressed for two kinds of sorbents (planar and porous) and two kinds of adsorbates (spherical and linear), with a focus on the role of molecular interactions (especially between the distinct species), the effects of the orientational degrees of freedom of the adsorbates and, those of the confinement imposed by the porous sorbents. The experiments are conducted in a specially built setup that allows for volumetric adsorption measurements as well as for the determination of the composition of the gas mixture through mass spectrometry as the system approaches equilibrium, and which allows control of the temperature over extended periods of time. The modeling and simulation studies explore the same systems and behavior studied in the experiments by using a Kinetic Monte Carlo approach to simulate adsorption dynamics. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.