Desorption of solute

Alternatively, peak asymmetry could arise from thermal effects. During the passage of a solute along the column the heats of adsorption and desorption that are evolved and adsorbed as the solute distributes itself between the phases. At the front of the peak, where the solute is being continually adsorbed, the heat of adsorption will be evolved and thus the front of the peak will be at a temperature above its surroundings. Conversely, at the rear of the peak, where there will be a net desorption of solute, heat will be adsorbed and the temperature or the rear of the peak will fall below its surroundings. 

The washing of filter cake is carried out to remove liquid impurities from valuable solid product or to increase recovery of valuable filtrates from the cake. Wakeman (1990) has shown that the axial dispersion flow model, as developed in Sec. 4.3.6, provides a fundamental description of cake washing. It takes into account such situations as non-uniformities in the liquid flow pattern, non-uniform porosity distributions, the initial spread of washing liquid onto the topmost surface of the filter cake and the desorption of solute from the solid surfaces. 

As shown by Wakeman, the solute material balance for the flowing liquid phase, allowing for axial dispersion and desorption of solute is given by the following defining partial differential equation... 

Program FILTWASH models the dimensionless filtration wash curves for the above case of a filter cake with constant porosity, axial dispersion in the liquid flow and desorption of solute from the solid particles of the filter bed (Boyd, 1993). 

Deactivating catalyst 319 Dead zones 159, 162, 163 Degree of segregation 471 Density influences 492 Desorption of solute 578, 579 Difference differential equation 579 Difference formulae for partial differential equations 268 Differential column 167... 

Many potential applications have been proposed which involve the desorption of solutes from matrix using SCF solvents at elevated pressure these include activated carbon regeneration [1,2,3,4,5] and soil remediation [6,7,8] using supercritical carbon dioxide. 

A particularly interesting example on the interplay between the proton exchange for the surface hydroxyls and the adsorption or desorption of solution species is provided by silica. As mentioned above, it is known that the hydrolysis of silica species is minimal at 6 < pH < 7. It is therefore expected that the number of >SiOH groups (Equation 8.102a) increases when the pH scale descends and the number of >SiO groups (Equation 8.102b) increases when the pH scale ascends. Moreover, it is expected that the stability of silica sols would be the least at 6 < pH < 7. As shown in Figure 8.31 this is exactly what is observed. - ... 

When the same process was repeated by first equilibrating a monolayer of TCE with a kaolinite siloxane surface and then adding water, after 350 ps of MD simulation only 20% of the TCE had desorbed. As this example shows, MD simulations can be employed in this way to explore the relative kinetics of adsorp-tion/desorption of solutes at the clay mineral/aqueous solution interface. 

Temperature, which is an important influence in vapor systems, is only a minor factor in the desorption of solutes. With liquid systems, a change in temperature within the range that can be employed seldom alters the adsorption sufficiently to provide effective desorption. Temperature can be an adjunct to other means, but when so used it must be kept in mind that solutes vary in their response to changes in temperature. Some show increased adsorption as the temperature rises others decrease, and still others show no significant change. 

To quantify this effect, two types of material porosity must be considered total porosity and accessible porosity. Total porosity, , is the volume fraction of pores in the material accessible porosity, is the volume fraction of pores which are members of clusters that extend to the surface of the material. In desorption of solute from a porous material, only solute which is initially present in accessible pores can be released. The fractional volume of accessible pores, e /e, is denoted . As the intuitive model of porous materials—developed in the previous paragraph—suggests, both accessible porosity and fractional accessible porosity increase as the total porosity increases. 

Incomplete desorption of target compounds from the SPE material may also be the reason for low recovery. Off-line configuration offen many ways for avoiding this shortcoming. However, when working with an online SPE—HPLC system, the choice of the mobile phase is mostly dictated by the separation conditions in the analytical column, and this choice is not always beneficial for the compact and complete desorption of solutes from the pre-column. 

Typical examples include studies of the underpotential deposition of various metals on metallic substrates. The structure of the upd-layer [33, 34], the position of adsorbed anions and water molecules on top of the upd-layer and the respective bond angles and lengths could be elucidated [35, 36]. Surface reconstruction caused by weakly adsorbed hydrogen [37], surface expansion effects of low-index platinum and gold surfaces correlated with adsorption/desorption of solution species [38] and... 

A variety of other SCF extraction processes have been explored fGupta and Johnston. 2008 Hover. 1985 McHugh and Krukonis. 1994 Paulaitis etal.. 19831. These include extraction of oils from seeds such as soybeans, removal of excess oil from potato chips, fruit juice extraction, extraction of oxygenated organics such as ethanol from water, dry cleaning, removal of lignite from wood, desorption of solutes from activated carbon, and treatment of hazardous wastes. Not all of these applications were successful, and many that were technically successful are not economical. 

Desorption Models. Various isotherm types, in most cases Langmuir isotherms, have been used by different authors to describe the desorption of solutes from the pore walls in tlie DDD type models. Recasens et.al. [47] developed a model accounting for the local adsorption/desorption kinetics. Since, in most cases the extraction, is controlled by both solute-solid interactions and solubility in the fluid phase, the solubility must be incorporated to make models applicable to a wide variety of materials. Goto, et. al. [12] have recently suggested a model, i.e. the BET model, for this purpose. 

Consider, in addition, the nature of the force beli used in separation. Since there is adsorption and/or desorption of solutes between the mobile fluid and the stationary solid phases, the potential profile under consideration for each solute is discontinuous, a simple step function (see Figure 3.2.2) there are no external forces. According to Section 3.2, such a system in a closed vessel without any flow does not have any multicomponent separation capability. Multicomponent separation capability is, however, achieved in elution chromatography by having bulk flow perpendicular to the direction of the discontinuous chemical potential profile. The velocity here functions exactly like the quantity (- ) in equation (3.2.37). 

---