Liquid-phase adsorption studies

We wUl now touch upon some of these factors. First, let s look at what we mean by system isotherm. Freundlich liquid phase isotherm studies can be used to establish the adsorptive capacity of activated carbon over a range of different concentrations. Under standard conditions, the adsorptive capacity of activated carbon increases as the concentration increases, until we reach a point of maximum saturation capacity. An example of an isotherm for phenol is shown in Figure 8. 

When developing a liquid phase adsorptive separation process, a laboratory pulse test is typically used as a tool to search for a suitable adsorbent and desorbent combination for a particular separation. The properties of the suitable adsorbent, such as type of zeolite, exchange cation and adsorbent water content, are a critical part of the study. The desorbent, temperature and liquid flow circulation are also critical parameters that can be obtained from the pulse test. The pulse test is not only a critical tool for developing the equilibrium-selective adsorption process it is also an essential tool for other separation process developments such as rate-selective adsorption, shape-selective adsorption, ion exchange and reactive adsorption. 

A novel and simple method for determination of micropore network connectivity of activated carbon using liquid phase adsorption is presented in this paper. The method is applied to three different commercial carbons with eight different liquid phase adsorptives as probes. The effect of the pore network connectivity on the prediction of multicomponent adsorption equilibria was also studied. For this purpose, the Ideal Adsorbed Solution Theory (lAST) was used in conjuction with the modified DR single component isotherm. The results of comparison with experimental data show that incorporation of the connectivity, and consideration of percolation processes associated with the different molecular sizes of the adsorptives in the mixture, can improve the performance of the lAST in predicting multicomponent adsorption equilibria. 

The pore network connectivity is usually determined by gas sorption analysis [2-4] or mercury intrusion [5] based on percolation theory. Recently, Ismadji and Bhatia [6] have successfully employed the liquid phase adsorption isotherms to determine the pore network connectivity and the pore size distribution of three commercial activated carbons. In our recent study [7], the pore network connectivity of three commercial activated carbons was characterized using liquid phase adsorption isotherms of eight different compounds. In that study we used ester molecules with complex structure, as probe molecules. 

Modeling in multicomponent adsorption systems is an extension to that of single component adsorption. Many models have been reported in the literature for the prediction of concentration versus time decay curves in single component batch adsorption stem. However, there are very few research papers on the topic of multicCHnponent mass transport studies for liquid phase adsorption, therefore, it is a valuable contribution and novel development to adsorption research. 

In the present work, we have investigated the liquid phase adsorption behavior of n-alkanes on a ZSM-S zeolite. The saturation capacity of linear CS-C22 alkanes was determined to study alkane packing effects. Adsorption isotherms of binary mixtures were... 

Kinetic studies of ion exchange on partially ion-exchanged type A zeolites of Mg Ca and Mn " revealed that mini-mums and maximums characterize the differential coefficients of internal diffusion for every exchange of 2 Na " ions for one divalent cation per unit cell of the zeolite. On the basis of these observations, assuming definite interactions between the cations and the zeolite lattice, predictions can be made concerning the distribution and arrangement of cations in the unit cells of a type A zeolite. Research on liquid phase adsorption of n-alkanes on partially ion-exchanged type A zeolites indicated that the differential diffusion coefficients for alkane adsorption are influenced likewise by cation distribution in the unit cells of the zeolite. 

This case study involves the recovery of highly valued and high demand ethylbenzene (EB) and mixed-xylenes (comprising of p-xylene (PX), m-xylene (MX) and o-xylene (OX)) from a C8-aromatics mixture (C8A). As point out above, C8A is isomers mixture, so their separation (recovery) is not simple, that why there is only one commercial process of liquid-phase adsorptive separation available for EB recovery from C8A. [8] However, this process requires high investment cost and generates huge volume of waste adsorbent that may become an environmental problem. Therefore, another green process should be considered for the EB purification. The ratio of various properties of the key components (EB and PX) were tested to examine the possibly alternatives. The result showed, by vapor pressure ratio, the solvent-based extractive distillation can be employed for their purification. [7]... 

Despite its industrial importance, adsorption from the liquid phase has been studied much less extensively than adsorption from the vapor phase. There is no difference in principle between adsorption from liquid and vapor phases since, thermodynamically, the adsorbed phase concentration in equilibrium with a liquid must be precisely the same as that which is in equilibrium with the saturated vapor. The differences arise in practice because in adsorption from the liquid phase one is almost invariably concerned with high adsorbed phase concentrations close to the saturation limit. The simple model isotherms, developed primarily to describe adsorption from the vapor phase, are at their best at low sorbate concentrations and become highly unreliable as saturation is approached. Such models are therefore of only very limited applicability for the correlation of liquid phase adsorption data. 

Abuzaid and Nakhla, and Vidic et al. found that the adsorption of phenol by activated carbons from aqueous solutions in the presence of molecular oxygen in the test environment resulted in a threefold increase in the adsorption capacity of the carbon. This has been attributed to the oxygen induced polymerization reactions on the surface of the carbon. Juang et al. studied liquid-phase adsorption of eight phenohc compounds on a PAN-based activated carbon Fiber in the concentration range of 40 to 500 mg/L and observed that the chlorinated phenols showed better adsorption than methyl substituted phenols. Moreno-CastiUa et al. studied the adsorption of several phenols from aqueous solutions on activated carbons prepared from original and deminerahzed bituminous coal and found that the adsorption capacity depended upon the surface area and the porosity of the carbon, the solubility of the phenolic compound, and the hydrophobicity of the substituent. The adsorption was attributed to the electron donor-acceptor complexes formed between the basic sites on the surface of the carbon and the aromatic ring of the phenol. 

Let us also mention a combined micro calorimetry and adsorption study aimed at characterizing the surface acidity of a series of MCM-41 aluminosilicates (referred to as SiAlxC , where x is the molar Si Al ratio and n the chain length of the surfactant template). With the exception of H - SiAl32Ci4 and SiAlsCu, all samples were found to present low surface acidity [281]. The behavior of these materials in liquid phase (adsorption of 1-butanol and immersion in various solvents) has also been studied [282],... 

Normally cation type and amount are used to tune the selectivity for aromatic compounds (e.g. xylenes). Additionally, unexpected packing induced selectivity effects were observed for the liquid phase adsorption of aromatics. The adsorption of benzene, toluene, m-xylene and mesitylene from their binary mixtures with octene or octane was studied on Na-FAU having different Si Al-ratios. It was found that NaY (Si Al 2.79 low cation content) is a more selective adsorbent compared to NaX (Si Al 1.23 high cation content). As an example, the data for benzene are given in Figure 5. Furthermore, no differences were observed between the adsorption of aromatics on NaX and LSNaX (Si Al 1.02 very high cation content). 

Recently, Zajacetal. [50] suggested that two-cycle ammonia adsorption studies coupled with microcalorimetric data can give additional information on surface acidity. In addition, liquid phase adsorption calorimetric studies can give, with some limitations, interesting data [51]. 

Copper oxides CuO and CU2O are semiconductors and effective for redox-type reactions such as oxidation or dehydrogenation. However, in spite of numerous studies on these types of reactions, except for a work by Shibata el a/., who determined the acidity of CuO calcined at 773 K to be 0.170 mmol g" or 0.113 mmol m by liquid-phase adsorption of butylamine, no direct studies on the acidic or basic nature of the oxides have been conducted. 

The kinetics and equilibrium of autocatalyzed and ion exchange resin (Amberlyst-15) catalyzed esterification of acetic acid with methanol and hydrolysis of methyl acetate were studied by Popken et. al. (2000) in a temperature range of 303 - 343 K. The homogeneous reaction has been described with a simple power-law model. To compare pseudo-homogeneous and adsorption-based kinetic models for the heterogeneously catalyzed reaction, independent binary liquid phase adsorption experiments were used to estimate the adsorption equilibrium constants to keep the number of adjustable parameters the same for each model. 

In a separate study using the JKR technique, Chaudhury and Owen [48,49] attempted to understand the correlation between the contact adhesion hysteresis and the phase state of the monolayers films. In these studies, Chaudhury and Owen prepared self-assembled layers of hydrolyzed hexadecyltrichlorosilane (HTS) on oxidized PDMS surfaces at varying degrees of coverage by vapor phase adsorption. The phase state of the monolayers changes from crystalline (solidlike) to amoiphous (liquid-like) as the surface coverage (0s) decreases. It was found that contact adhesion hysteresis was the highest for the most closely packed... 

Although MIL-47, and especially MIL-53(A1), had been found on many occasions to dynamically respond to adsorption of particular compounds, referred to as breathing [35] in the literature, in these liquid phase conditions, only minor changes of the lattice parameters have been observed. A study of xylene separations in vapor phase on MIL-5 3(A1) shows that breathing profoundly influences the shape of the obtained breakthrough profiles as a function of adsorbate concentration [97]. 

We have developed a compact photocatalytic reactor [1], which enables efficient decomposition of organic carbons in a gas or a liquid phase, incorporating a flexible and light-dispersive wire-net coated with titanium dioxide. Ethylene was selected as a model compound which would rot plants in sealed space when emitted. Effects of the titanium dioxide loading, the ethylene concentration, and the humidity were examined in batches. Kinetic analysis elucidated that the surface reaction of adsorbed ethylene could be regarded as a controlling step under the experimental conditions studied, assuming the competitive adsorption of ethylene and water molecules on the same active site. 

---