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  • A BETTER “PICKER-UPPER”

    dis12cminGubbins and his colleagues are interested in understanding the more diverse and complex phenomena that occur in materials whose pores are only a few times larger than those of the fluid being taken up. They are using the advanced computing resources of the Cornell Theory Center to conduct their research. The results of investigations into such phenomena could point to new ways to remove poisonous gases from the atmosphere and to purify oil and water.

    For many science problems of practical interest, researchers must rely on “approximations”—guesses whose accuracy is questionable—to determine or predict the properties of assorted fluids and solids given different conditions. An alternative approach to approximation is to use molecular simulation, in which equations that predict properties of interest are solved by numerical methods, using a supercomputer. Many academic and industrial chemical engineers are finding applications for molecular simulation in a wide range of scientific and technological problems.

    At Cornell University, these techniques have been applied to problems of practical interest in chemical engineering, such as the storage of methane at high density in a porous material. This problem is important for the storage of natural gas, which is 95 percent methane. If the gas can be stored at low pressure but at high density, it may become desirable and practical to use natural gas as a fuel for cars and other vehicles.

    Keith Gubbins, professor of chemical engineering at Cornell, has studied adsorption of methane on microporous materials at a variety of temperatures and pressures. Adsorption is the take-up of a fluid by a usually porous solid material; a familiar example is the take-up of water by a sponge. In this case the pores of the sponge are very large compared to the water molecules, and the process by which the sponge takes up the water can be understood without having to look at the molecular structures of the sponge or the water.

    However, in studying changes in the properties of methane, Gubbins and his colleagues are attempting to determine both an optimum gas pressure and an optimum pore size for storing methane under different conditions. They have found that the optimum pore size is about three molecular diameters, a little more than one nanometer (about 40 billionths of an inch). Determining that involved understanding the more complex phenomena occurring in materials whose pores are only a few times larger than the fluid molecules themselves. Examples include the adsorption of unwanted or poisonous gases by activated carbon in gas masks and the use of silica gel packets to adsorb moisture in shoe boxes.

    A full understanding of those phenomena, coupled with the ability to control the nature of the pores—their size, shape, and interconnectedness— in experiments, would open the way to a new era in the design of many industrial processes that are based on the use of microporous materials. Such an understanding would be difficult to achieve with experimental techniques alone because of the difficulty of observing these phenomena at the molecular level, inside pores. Modeling these processes using molecular simulation and applying statistical mechanical theories can now complement experiments and can suggest the most profitable areas for study.

    One example of an industrial process that uses microporous materials is the catalytic “cracking” of petroleum (the breakup of large petroleum molecules, called hydrocarbons, into smaller hydro-carbon molecules, which are more suitable for use as fuels). In this process the vaporized petroleum mixture is passed through a bed of microporous alumina particles treated with a solution of a catalyst that speeds up the reaction. Microporous materials, such as activated carbons and silicas, are also frequently used industrially to remove pollutants and impurities from mixtures in which the component to be removed is more readily adsorbed than the other components.

    The most important molecular-simulation techniques Gubbins and his colleagues use are those of molecular dynamics and Monte Carlo. These techniques allow the researchers to simulate the movement of molecules, their speed, and their location within a porous material. The researchers can also incorporate into the simulation different conditions—such as changes in temperature, pressure, and density—and “observe” the interaction between the molecules and the porous material. Super-computing resources, such as those found at the Cornell Theory Center, are necessary to complete these simulations, particularly now that the researchers have begun to parallelize the algorithms involved in their simulation. In parallel computations, many parts of the calculation are performed simultaneously on different processors, in contrast to the more conventional procedure of making these calculations in sequence. Only supercomputers supply a tool for such sophisticated investigations of constant and changing properties of complicated systems, such as water confined in narrow graphite pores. An appropriate combination of a parallel algorithm and a supercomputer can speed up the investigations a hundredfold.

    The potential of molecular simulation as a tool for practical scientific research has expanded rapidly with improvements in the speed and availability of supercomputers. In the investigation of adsorption on porous materials, work has been extended to studies of the more complex fluids, such as water, aqueous solutions, and liquid crystals, and to other porous ceramic materials, including silicas and other oxides.

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