Batch adsorption experiments are carried out by adding a known amount of adsorbent to a liquid solution at a known initial concentration and following the evolution in time of the concentration of ...the adsorbate. This is a very common method to obtain equilibrium and kinetic information in liquid systems, but in most cases kinetic results are analysed on the basis of empirical models. Two phenomenological models based on macropore diffusion in beads and shrinking core kinetics are used to generate data that are then interpreted with the widely used
unconstrained
linear regression methods. The results show that for both cases R
2
values close to unity are obtained leading to the incorrect interpretation of the mechanism of mass transport. It is recommended that batch adsorption experiments should be analysed using phenomenological models to obtain physical parameters that are applicable to other systems and to reduce the experiments required to characterise fully the kinetics of adsorption.
To match the dynamics of a linear driving force model and the diffusion equation is of great practical importance in the design and optimization of adsorption separation processes. A frequency ...response analysis is applied to show that it is not possible to arrive at an equivalence based on a single parameter. Using this as the basis, a universal equivalence for the linear problem is constructed and closed form analytical expressions for the two parameters are derived for the sphere and slab geometries. The two parameters represent the increased effective mass transfer coefficient and a reduction in the active volume of the particle, both corresponding to the internal concentration profiles of the diffusion equation at cyclic steady state.
The Rigid Adsorbent Lattice Fluid model has been shown to comply with all the requirements for thermodynamic consistency in the case of an adsorbent that does not undergo structural changes. This is ...achieved by introducing a correction to the reduced density function that multiplies the combinatorial term. A procedure to calculate the predicted adsorbed mixture activity coefficients has been presented that allows the production of excess Gibbs energy plots at a constant reduced grand potential. The predicted nonideality is structurally consistent with the Non-Ideal Adsorbed Solution Theory of Myers in terms of both its dependence on concentration and reduced grand potential. The ability to generate excess Gibbs energy values allows linking the new Rigid Adsorbent Lattice Fluid model to the traditional Real Adsorbed Solution Theory providing an alternative approach to predicting multicomponent adsorption based solely on pure component data.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
The zero length column technique has been developed over the past 30 years as a versatile experimental method to measure adsorption equilibrium and kinetics. In this review we discuss in detail the ...theory that forms the basis for the technique in order to understand how to design and operate efficiently a system. Experimental checks that should be performed to ensure the correct interpretation of the dynamic response are presented and examples are used to identify how to avoid major errors in determining diffusion time constants. The review concludes with an overview of all experimental studies available in the literature to date and a set of recommendations that should help improve the standard in the reported equilibrium and kinetic properties.
Chemical and petrochemical companies are increasingly realizing that their sustainable development critically depends upon development of new innovative processes that use more efficiently materials ...and energy. As overall separation/purification processes account for 40–60% of capital and operating costs, their amelioration can significantly reduce costs, energy use and waste generation by increasing profits. Gas separation by adsorption technology is a well-established unit operation in chemical and petrochemical industries due to its efficiency for dealing with a large range of gas separations including impurity removal, gas purification, and separation in recycle streams. The technology is far from being mature and opportunities to expand its domain of applicability and improve its efficiency are high in a context where better understanding of physical phenomena and technological progress in materials and engineering research are integrated. Major contribution for innovations in gas separation by adsorption technology relates to the discovery of new adsorbents with better separation characteristics coupled to process development and its optimization using multi-objective and multi-domain numerical approaches. This short review identifies technological gaps and drivers for accelerating the development of industrially important gas separations by adsorption.
Despite decades of research, quantitatively accurate molecular models of water sorption on activated carbons remain elusive, while many phenomena are not properly understood even qualitatively. Here ...we focus specifically on scanning phenomena. Scanning isotherms, obtained by reversing either adsorption or desorption process before the closure of the hysteresis loop, have been recently explored as a sensitive experimental probe for structural characterization of activated carbons and at the same time have been subject of several competing theories of adsorption. We employ adsorption experiments and molecular simulations to understand the nature of the states along the adsorption and scanning curves for water at 308 K in a high surface area activated carbon, Maxsorb. The molecular model considered here is based on a random packing of fullerene-like fragments, functionalized with carboxylate groups. The model is able to reproduce reasonably well the shape of the adsorption and scanning desorption isotherms. We investigate spatial organization, structure, size and interaction between the clusters on the adsorption branch. Furthermore, we show that the scanning desorption isotherm consists of a series of states of the system where a single cluster of water shrinks in size as water evaporates from its surface, before it finally disintegrates into separate smaller clusters.
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Adsorbents exhibiting non type I adsorption behaviour are becoming increasingly more important in industrial applications, such as drying and gas separation. The ability to model these processes is ...essential in process optimisation and intensification, but requires an accurate description of the adsorption isotherms under a range of conditions. Here we describe how the Rigid Adsorbent Lattice Fluid is capable of a priori predictions both type I and type V adsorption behaviour in silicalite-1. The predictions are consistent with experimental observations for aliphatic (type I) and polar (type V) molecules in this hydrophobic material. Type V behaviour is related to molecular clustering and the paper discusses the model parameters governing the presence/absence of this behaviour in the predicted isotherms. It is found that both the solid porosity and the adsorbate interaction energy/energy density are deciding factors for the isotherm shape. Importantly, the model, whilst thermodynamically consistent, is macroscopic and thus computationally light and requires only a small number of physically meaningful parameters.
Multiscale material screening strategies combine molecular simulations and process modeling to identify the best performing adsorbents for a particular application, such as carbon capture. The idea ...to go from the properties of a single crystal to the prediction of material performance in a real process is both powerful and appealing; however, it is yet to be established how to implement it consistently. In this article, we focus on the challenges associated with the interface between molecular and process levels of description. We explore how predictions of the material performance in the actual process depend on the accuracy of molecular simulations, on the procedures to feed the equilibrium adsorption data into the process simulator, and on the structural characteristics of the pellets, which are not available from molecular simulations and should be treated as optimization parameters. The presented analysis paves the way for more consistent and robust multiscale material screening strategies.
The availability of commercial gravimetric and volumetric systems for the measurement of adsorption equilibrium has seen also a growth of the use of these instruments to measure adsorption kinetics. ...A review of publications from the past 20 years has been used to assess common practice in 180 cases. There are worrying trends observed, such as lack of information on the actual conditions used in the experiment and the fact that the analysis of the data is often based on models that do not apply to the experimental systems used. To provide guidance to users of these techniques this contribution is divided into two parts: a discussion of the appropriate models to describe diffusion in porous materials is presented for different gravimetric and volumetric systems, followed by a structured discussion of the main trends in common practice uncovered reviewing a large number of recent publications. We conclude with recommendations for best practice to avoid incorrect interpretation of these experiments.
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