D.D. Do

The preparation of active carbons from coal by chemical and physical activation

A series of activated carbons was prepared from bituminous coal by chemical activation with potassium hydroxide and zinc chloride and also by physical activation with carbon dioxide. The effect of process variables such as carbonization time, temperature, particle size, chemical agents, method of mixing and impregnation ratio in the chemical activation process was studied in order to optimize those preparation parameters. Partial gasification of the high surface area carbon obtained by zinc chloride activation in CO2 with different exposure times shows some improvement in adsorption. The physical properties of the chemically activated carbon was also compared with those obtained purely by physical activation. The most important parameter in chemical activation of coal with both of the chemical agents was found to be impregnation ratio. Carbonization temperature is another variable which had a high effect on pore volume evolution. While increasing the carbonization temperature enhances surface area and pore volumes of KOH-activated carbon, it destroys carbon structure in the ZnCl2 carbon series. Under the experimental conditions investigated, the optimum conditions for high surface area carbons with KOH and ZnCl2 activation are identified.

A Model for water adsorption in activated carbon

In this paper we present a new model to describe the adsorption equilibrium of water in activated carbon. The model is based on the growth of the water cluster at the functional groups and the penetration of water clusters into the micropore in the form of pentamer, which has sufficiently high dispersion energy to remain in the micropore. This model is able to describe all possible behaviours of the water adsorption isotherm observed in the literature, ranging from type V for hydrophobic carbon to type IV for highly oxidised carbon. Testing of the model against a number of experimental data shows that the model is able to describe data well

Review of time lag permeation technique as a method for characterisation of porous media and membranes

The time lag permeation technique has proven to bean effective method for characterisation. Because of the simple nature of the permeation experiment, transport parameters can be directly obtained from experimental data hence avoiding the intensive mathematical treatment required by other techniques. The method has historically been applied to diffusion and adsorption in porous membranes and diffusion in polymer membranes. Since its origins in 1920, interest in the time lag method has expanded because of its value in characterising simple permeation processes and also complex systems of diffusion with simultaneous adsorption and surface diffusion. This review focuses on presenting the asymptotic solution of the mass balance diffusion equations and includes applications of time lag analysis, in order to give a critical and broad perspective of this method as a tool for characterisation. It includes much of the previously published literature in order to show that for most cases the asymptotic solution of the transport equations is simple, and for more complex cases that an analytical solution is possible hence avoiding cumbersome numerical techniques.

Capillary condensation of adsorbates in porous materials.

Hysteresis in capillary condensation is important for the fundamental study and application of porous materials, and yet experiments on porous materials are sometimes difficult to interpret because of the many interactions and complex solid structures involved in the condensation and evaporation processes. Here we make an overview of the significant progress in understanding capillary condensation and hysteresis phenomena in mesopores that have followed from experiment and simulation applied to highly ordered mesoporous materials such as MCM-41 and SBA-15 over the last few decades.

The Dubinin–Radushkevich equation and the underlying microscopic adsorption description

Abstract The Dubinin–Radushkevich (DR) equation is widely used for description of adsorption in microporous materials, especially those of a carbonaceous origin. The equation has a semi-empirical origin and is based on the assumptions of a change in the potential energy between the gas and adsorbed phases and a characteristic energy of a given solid. This equation yields a macroscopic behaviour of adsorption loading for a given pressure. In this paper, we apply a theory developed in our group to investigate the underlying mechanism of adsorption as an alternative to the macroscopic description using the DR equation. Using this approach, we are able to establish a detailed picture of the adsorption in the whole range of the micropore system. This is different from the DR equation, which provides an overall description of the process.

Adsorption of supercritical fluids in non-porous and porous carbons : Analysis of adsorbed phase volume and density

In this paper, we revisit the surface mass excess in adsorption studies and investigate the role of the volume of the adsorbed phase and its density in the analysis of supercritical gas adsorption in non-porous as well as microporous solids. For many supercritical fluids tested (krypton, argon, nitrogen, methane) on many different carbonaceous solids, it is found that the volume of the adsorbed phase is confined mostly to a geometrical volume having a thickness of up to a few molecular diameters. At high pressure the adsorbed phase density is also found to be very close to but never equal or greater than the liquid phase density

A new model for the description of adsorption kinetics in heterogeneous activated carbon

A new model for the description of adsorption kinetics in heterogeneous activated carbon is presented in this paper. The activated carbon particle is composed of the fluid phase and the adsorbed phase, the latter of which is heterogeneous. This heterogeneity is assumed to be described by a distribution in the energy of interaction between the two phases. This distribution is obtained from the information of the adsorption equilibria of all species. The kinetics model assumes three processes occurring within the porous particle: (1) the pore volume diffusion, (2) the adsorbed phase diffusion and (3) the finite mass interchange between the molecules in the fluid phase and those in the energy distributed adsorbed phase. The distribution of energy of interaction is accounted for in the last two processes. These three processes are found to have rates that are comparable in magnitude, and depending on the adsorbate, the operating conditions and the mode of operation (adsorption or desorption) one or two of these processes dominate the overall uptake. The model is tested with the experimental data collected in our laboratories using the volumetric isotherm apparatus for equilibria and the differential adsorption bed for kinetics. Seven adsorbates were used, and a wide range of parameters as well as operating conditions were also used to validate the mathematical model. It is found that the model proposed describes well the adsorption as well as desorption kinetics.

The Role of Deposited Poisons and Crystallite Surface Structure in the Activity and Selectivity of Reforming Catalysts

Abstract Over the past three decades catalytic reforming has evolved very rapidly to the point where it is now one of the most important industrial applications of catalysis [1]. The process was originally developed to produce gasoline components of high antiknock quality, in response to the fuel requirements of high compression ratio automobile engines. The objective of the process is to convert saturated hydrocarbons (alkanes and cyclo-alkanes) in petroleum naphtha fractions to aromatic hydrocarbons as selectively as possible, since the latter have excellent antiknock ratings. Naphtha fractions are composed of hydrocarbons having boiling points within the approximate range of 320–470 K. Reaction temperatures of 700–800 K and pressures of 10–35 atm are employed in the process. The catalysts employed commonly contain platinum or a combination of platinum and a second metallic element such as rhenium or iridium [2].

A new adsorption isotherm for heterogeneous adsorbent based on the isosteric heat as a function of loading

A new adsorption isotherm model is proposed in this paper for a heterogeneous solid. The basis of this isotherm equation is that the degree of heterogeneity is reflected through the variation of the isosteric heat of adsorption with respect to loading. This degree as well as the pattern of heterogeneity is assumed to be independent of adsorbate used. The influence of the adsorbate on the isotherm will be through the interaction energy at zero loading as well as parameters which reflect the way in which the adsorbate fits into the adsorption sites. The resulting isotherm equation is very general, and under certain conditions it reduces to many isotherms commonly used in the literature, such as the Langmuir, Langmuir-Freundlich, Toth, Fowler-Guggenheim and Nitta et al. equations. The new isotherm is tested with adsorption isotherm data of many adsorbates on various samples of activated carbon and zeolite, and the parameters extracted for these adsorbates shed some light on the system heterogeneity. Implications of this new isotherm are discussed in this paper.

Pore characterization of carbonaceous materials by DFT and GCMC simulations: A review

A review is given of the pore characterization of carbonaceous materials, including activated carbon, carbon fibres, carbon nanotubes, etc., using adsorption techniques. Since the pores of carbon media are mostly of molecular dimensions, the appropriate modern tools for the analysis of adsorption isotherms are grand canonical Monte Carlo (GCMC) simulations and density functional theory (DFT). These techniques are presented and applications of such tools in the derivation of pore-size distribution highlighted.