Adsorbents used

Many of the adsorbents used have rough surfaces they may consist of clusters of very small particles, for example. It appears that the concept of self-similarity or fractal geometry (see Section VII-4C) may be applicable [210,211]. In the case of quenching of emission by a coadsorbed species, Q, some fraction of Q may be hidden from the emitter if Q is a small molecule that can fit into surface regions not accessible to the emitter [211]. 

The Aromax process was developed in the early 1970s by Toray Industries, Inc. in Japan (95—98). The adsorption column consists of a horizontal series of independent chambers containing fixed beds of adsorbent. Instead of a rotary valve, a sequence of specially designed on—off valves under computer control is used to move inlet and withdrawal ports around the bed. Adsorption is carried out in the Hquid phase at 140°C, 785—980 kPA, and 5—13 L/h. PX yields per pass is reported to exceed 90% with a typical purity of 99.5%. The first Aromax unit was installed at Toray s Kawasaki plant in March 1973. In 1994, IFP introduced the Eluxyl adsorption process (59,99). The proprietary adsorbent used is designated SPX 3000. Individual on-off valves controlled by a microprocessor are used. Raman spectroscopy to used to measure concentration profiles in the column. A 10,000 t/yr demonstration plant was started and successfully operated at Chevron s Pascagoula plant from 1995—96. IFP has Hcensed two hybrid units. 

The practical adsorbents used in most gas phase appHcations are limited to the following types, classified by their amorphous or crystalline nature. 

Each olefin is more soluble than the paraffin of the same chain length, but the solubiHty of both species declines as chain length increases. Thus, in a broa d-boiling mixture, solubiHties of paraffins and olefins overlap and separation becomes impossible. In contrast, the relative adsorption of olefins and paraffins from the Hquid phase on the adsorbent used commercially for this operation is shown in Figure 2. Not only is there selectivity between an olefin and paraffin of the same chain length, but also chain length has Httie effect on selectivity. Consequentiy, the complete separation of olefins from paraffins becomes possible. 

This is a process that takes place via specific chemical forces, and the process is unique to the adsorbent or adsorbate used. In general, it is studied at temperatures much higher than those of the boiling point of the adsorbate consequently, if supported metals are studied, little or no physical adsorption of the chemisorbing gas takes place on the high surface area support. 

The separation capacity of a TLC method can be easily improved by use of a two-dimensional high performance TLC technique (2D HPTLC). Various plant essential oils (menthae, thymi, anisi, lavandulae, etc.) have been analysed by 2D TLC with florisil (magnesium silicate) as the adsorbent, using dichloromethane/ -heptane (4 6) in the first direction and ethyl acetate/n-heptane (1 9) in the second direction (69). 

Liquid-solid chromatography (LSC). This process, often termed adsorption chromatography, is based on interactions between the solute and fixed active sites on a finely divided solid adsorbent used as the stationary phase. The adsorbent, which may be packed in a column or spread on a plate, is generally a high surface area, active solid such as alumina, charcoal or silica gel, the last... 

A large range of stationary phases is available, and according to their polarity they can be divided into normal phase and reversed phase types. Silica gel, aluminium oxide, and a nitrile-bonded-phase are normal adsorbents used to separate carotenoids... 

In Chapter 2 we touch on theoretical models of adsorption response of electrophysical characteristics of real semiconductor adsorbents used as sensitive elements of chemical sensors. 

Starting with this definition the semiconductor diemical sensors can be arbitrary classified with respect to following features the type of electrophysical characteristics diosen for monitoring, such as electric conductivity, thermal-electromotive force, work function of electron, etc. type and nature of semiconductor adsorbent used as an operational element of the sensor and, finally, the detection method used for monitoring the adsorption response of electrophysical characteristics of die sensor. 

General principles of selection of semiconductor adsorbents used as a operational sensor elements... 

Semiconductor adsorbents used as operational elements in sensors can be monocrystals or monocrystal films as well as polycrystals. The latter can be arbitrary divided into two groups differing in properties of contacts between monocrystals. The first group contains those polycrystal line adsorbents in which contacts between crystals can be represented as a double Shottky barrier or, in more general case, isotype hetero-... 

To resolve the problem applying methods of collimated atom beams, equilibrium vapour as well as radioactive isotopes, the Hall effect and measurement of conductivity in thin layers of semiconductor-adsorbents using adsorption of atoms of silver and sodium as an example the relationship between the number of Ag-atoms adsorbed on a film of zinc oxide and the increase in concentration of current carriers in the film caused by a partial ionization of atoms in adsorbed layer were examined. 

The principal adsorbents used in GSC are silica, alumina, graphltlzed carbon blacks, porous polymer beads, zeolites and cyclodextrlns [8,430,431,445]. The bonded phase sorbents discussed in section 2.2.3 could also be considered as modified adsorbents in many respects. 

The most common inorganic adsorbents used in TLC are silica gel, alumina and kieselguhr silica gel being by far the most important. Kieselguhrs are made from natural diatomites and are an impure form of silica, being about 90% or so silica with the... 

TYPICAL PROPERTIES OF INORGANIC OXIDE ADSORBENTS USED FOR TLC... 

Interest in the adsorption of sulfur-containing molecules at metal surfaces been stimulated by a desire to elucidate the decomposition mechanisms of thiols during the catalytic removal of sulfur from feedstocks and the position of thiols as the favoured head groups for adsorbates used to construct self-assembled monolayers. We shall not survey the extensive self-assembled film literature but restrict our discussion to the simpler thiols. 

A great deal of effort has been put into methods for removing only the caffeine from the extracting solvent, and somehow returning all of the other components to the coffee beans for reabsorption. The principle of the method most generally seen involves exposure of the extract-laden solvent to a caffeine-specific adsorbent. Once the solvent has been treated in this way, it is returned to remove more caffeine. Flowever, the solvent is already saturated with the other solvent-soluble components and does not extract them from the second and subsequent batches of steamed green coffee beans. Adsorbants used for this purpose include activated char-... 

Rimmer, P. G. and Bowen, J. H. Trans. Inst. Chem. Eng. 50 (1972) 168. The design of fixed bed adsorbers using the quadratic driving force equation. 

As documented in Chapter 5, zeolites are very powerful adsorbents used to separate many products from industrial process steams. In many cases, adsorption is the only separation tool when other conventional separation techniques such as distillation, extraction, membranes, crystallization and absorption are not applicable. For example, adsorption is the only process that can separate a mixture of C10-C14 olefins from a mixture of C10-C14 hydrocarbons. It has also been found that in certain processes, adsorption has many technological and economical advantages over conventional processes. This was seen, for example, when the separation of m-xylene from other Cg-aromatics by the HF-BF3 extraction process was replaced by adsorption using the UOP MX Sorbex process. Although zeolite separations have many advantages, there are some disadvantages such as complexity in the separation chemistry and the need to recover and recycle desorbents. 

In this particular case, the adsorption process can be used to overcome the distillation limitation. This is demonstrated in Figure 6.2, which represents the relative adsorption of C5 and C(, Hnear, branched and cycHc paraffins from the liquid phase of the 5A adsorbent used in the HOP GasoHne Molex process, licensed by HOP. In this process, only Hnear paraffins can enter the pores of 5A zeolite, while branched and cyclic paraffins are completely excluded due to their large kinetic diameters. Also, the selectivity for Hnear paraffins with respect to other types of paraffins is infinite. Consequently, the separation of Hnear paraffins from branched and cyclic paraffins becomes possible. 

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