Adsorbents characteristics

The variety of the different framework structures result in different adsorbent characteristics acid strength, size of molecule adsorbed, adsorption/desorption rate of different molecules, capacity and stability. As a result, these differences characterize the adsorbent s selectivity to a specific molecule and adsorbent-adsorbate interactions. Take for example, the difference in selectivity of BaY and Ba-Mordenite [24] to p-xylene (PX), m-xylene (MX) and o-xylene (OX) ... 

The second adsorbent characteristic is capacity. Capacity is defined as the quantity of desired component for example normal paraffins adsorbed from the feed while the adsorbent is exposed to the feed. Capacity is reported either as a weight or volume of the desired component retained by the adsorbent per volume or weight of adsorbent. It is desirable to have capacity values as great as physically practical. Capacity, just like selectivity, is measured using the pulse test apparatus. 

The fifth and final adsorbent characteristic is zeolite type. The adsorbent used in the Molex process is a proprietary and is a particularly effective adsorbent for normal paraffin separation [4, 5] and has achieved purity and recovery targets for the Molex processes. A sampUng of various molecules (and their corresponding dimensions) that Molex can easily separate is listed in Table 8.1. As discussed in Chapter 6, a zeoUtes s pore structure is dependent on its silica aluminum ratio and the proprietary Molex adsorbent possess a uniform repeating three-dimensional porous structure with pores running perpendicular to each other in the x. 

The unique properties of carbon nanotubes include adsorbent characteristics that offer the prospect of yet more efficient hydrogen storage. Dillon et al. [64] used temperature programmed desorption to determine the hydrogen storage capacity of single-wall nanotubes. The authors predict a storage capacity of —50 kg Hi/m at ambient temperature and pressures for nanotubes with 20 A diameters. 

Table 3.12 Interrelationship between adsorbent characteristics and chromatographic properties.

The above adsorbent performance requirements can simply transfer to adsorbent characteristic requirements as follows ... 

Adsorbent Characteristics Commercial uses Strengths Weaknesses... 

All of the various applications require special adsorbent characteristics. The broadest and generally the most significant are the inherent adsorption capacity and selectivity. In many cases, the adsorption and desorption rates or kinetics and pressure drop are also important hence, particle size is important. In addition, nearly every different application has a different set of additional priorities. For example, the main prerequisite for municipal water purification and many other large-scale applications is low cost. Other adverse conditions can complicate adsorbent specifications. For example, properties such as density, color, fluid compatibility, and durability (e.g., attrition resistance, crush strength, and hardness) all may be important. Adsorbents that have the ideal combination of essential characteristics for a specific application may or may not exist. That implies that compromises are frequently necessary. 

Table II. Adsorbent characteristics and parameters used in the constant capacitance model...

Adsorbate characteristics. The adsorbates are hydrate metals (Zn, Cd, Pb) salt with determinate concentration of metal ion in solution. The adsorbates are supplied from Topilnica-Veles - metallurgical industry in Macedonia. The initial concentration for Zn and Pb are from 0-250 mg/dm3, and for the Cd are from 900-1400 mg/dm3, because the real wastewater from the metallurgical industry is polluted in this range of concentration with these heavy metals. 

The selection of a suitable zeolite adsorbent for CO2 removal from flue gas (mixture of CO2 and N2) has been carried out. The limiting heats of adsorption, Henry s Law constants for CO2 and N2, CO2 pure component adsorption isotherms and expected working capacity curves for Pressure Swing Adsorption (PSA) separation application were determined. The results show that the most promising adsorbent characteristics are a near linear CO2 isotherm and a low Si02/Al203 ratio with a cation in the zeolite structure that has strong electrostatic interaction. 

A second adsorbent characteristic that should be controlled is the water content of the silica. The water content can vary from 0 to 10 wt.%. As silica is a well-known desiccant (drying agent) care has to be taken with adsorbent types that are adjusted with low amounts of water. Here, signiflcant uptake of water from the air might... 

Sample Preparation Slurries, Solvent Evaporated Samples and Equilibrium Samples. The adsorption of probes on microcrystalline cellulose or native cellulose, on the surface of different pore size silicas or alumina, and on silicalite surfaces (or other zeolites) has to be made in a differentiated manner, according to each s adsorbent characteristics. Adsorption of probes onto these powdered solids can be performed from a solution containing the probe or from a gas phase. 

The cellulose phosphate was widely applied in the removal process of divalent and trivalent (Cu ", Zn ", Co, Ni, Pb ", Mn ", Fe, Fe , Cr ) ions because of the rapid adsorption of the metals ions in comparison with the synthetic polymers. Also it was observed that the cellulose phosphate functions as an ion exchanger for lanthanide ion removal from aqueous solution. In order to evaluate the adsorbent characteristics of each support in the removal process of metal ions from aqueous solution, the most widely used method by researchers has been the batchwise one. In any adsorption process the most important parameter is the pH of the medium, which depends on the nature of the support used and also on which metal ions are to be removed. In most cases, researchers worked with solutions that have an acid pH value in order to avoid the possible precipitation of the studied metal ions. A complex study of the pH influence upon the efficiency of Fe ", Cu ", Mn, Zn, Co ", and lanthanide(III) ions removal from aqueous solutions with PBC was made by Oshima et al. Their conclusion was that the lanthanide ions are adsorbed when the pH value of the aqueous solution is less than 3, and in the case of transition metal ions the adsorption percentage increases with increasing aqueous pH and reaches over 90% at a pH value of around 4.5. 

For most adsorption experiments the temperature at which the measurements are made is less than the triple point of the gas being used but above its freezing point. This being the case, one would normally expect that the adsorbate characteristics resemble the liquid phase rather than the solid phase of the adsorptive. This is the normal assumption used for most adsorption theories. The principle measurement performed as an adsorption experiment is the measurement of the adsorption isotherm. The adsorption isotherm is the measurement of amount adsorbed versus adsorptive pressure at constant temperature. This is the easiest measurement to make. Another type of measurement is calorimetry. One form of calorimetry measures the amount of heat evolved as the adsorptive is adsorbed. Another form measures the heat capacity of the adsorbate. There are various forms of calorimetry but the most accurate methods are very difficult to perform and only a few examples are available in the literature. Another form of calorimetry, which is easier to perform, is scanning calorimetry. This calorimetry form is a good tool to determine qualitative features of the adsorption and to yield a fair indication of the physical quantities. 

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