Impurity adsorption isotherms

The adsorption behavior of homologous sodium alcohol sulfates at the interface can be characterized by the adsorption isotherms. However, the adsorption parameters of these isotherms are very sensitive to impurities present in the surfactant. Wiinstneck et al. [145] determined the equilibrium values of... 

Figure 21-19. Region of complete separation (solid line) in the (mj/mm) plane for a feed concentration of 25 g racemate/L calculated with the parameter of the MDM adsorption isotherms and for the given lab-scale SMB unit (12 columns each 100-mm X 16-mm i.d., 3 in each zone). The four points mentioned illustrate the following scenarios 1, robust conditions to obtain pure enantiomers 2, less robust system with highest productivity 3, area of pure extract and impure raffinate 4, area of pure raffinate and impure extract. 1-A 2-0 3-D 4-0. (From reference 124, with permission.)...

The evaluation of the separation factor enables characterization of the initial slopes of the adsorption isotherm for the product and neighboring impurities under various conditions. The term linear conditions means, under analytical conditions or under conditions where the injection size is small and the injection concentration is in the linear region of the ad.sorption i.sotherm. Retention experiments enable evaluation of the thermodynamics under infinite dilution. 

The research of adsorption properties has shown, that adsorption isotherms of benzene for dealuminated (NH4)2SiF6 zeolite lay below adsorption isotherms of benzene for NaY obtained even at more high adsorption temperatures. In a fig. 1 are shown adsorption isotherms for a sample 2. It is characteristic that at different temperatures (180, 200, 230 C) isotherms are not divided. Apparently, it is connected that so a little benzene is kept on a surface, that the distinction in adsorption at given temperature interval is not fixed. Probably, the samples contain impurities of complex compounds of aluminium with fluorine, that is agreed XRD data. Also relative concentration of defect sites of structure is increased during dealumination, that also can affect on adsorption property. 

The key impurities present in a typical ROG for the recovery of H2 by a PSA process are bulk Ci and C2 and dilute C3 and C4 hydrocarbons. Figures 10.11 and 10.12 describe the pure gas adsorption isotherms of the components of ROG at 30 °C on the BPL activated carbon and a silica gel sample (Sorbead H produced by Engelhard Corp.), respectively.31 These data were also measured in Air Products and Chemicals, Inc. laboratories. It may be seen that the carbon adsorbs C3+ hydrocarbons very strongly. Consequently, desorption of these hydrocarbons from the carbon by H2 purge becomes rather impractical requiring a large volume of purge gas. 

There are, so far, only two reports on the determination of isotherm data from mixtures containing more than two components. In both cases, the FA method was used for ternary mixtures [135, 136], There are no reports on the experimental determination of adsorption isotherms for quaternary mixtures using any chromatographic method. The competitive quaternary adsorption isotherm parameters could be very valuable for the separations of the four isomers of a compound with two chiral centers, or for the preparative separation of two compounds in the presence of one or two impurities. 

For adsorption of impurities, the suiTounding phase of interest is typically the liquid phase. The adsorption isotherm can be built by determining the impurity levels in the solution before and after adding the solid (as adsorbent). Based upon the change of impurity levels in the solution, the adsorption isotherm can then be determined. Figiue 2-24 shows a Langmuir-type adsorption isotherm of R-ibuprofen S-lysinate on S-ibuprofen S-lysinate crystals in an ethanol/water solvent mixture. As shown in the figure, a trace amount of impurities can be adsorbed onto the crystal surface even when the impurity concentration in the solution is well below its solubility limit. 

This option was tested and did not work as a stand-alone process because the DMSO was too dilute in these impurities. The measured adsorption isotherms indicated that unrealistically large amounts of adsorbent would be required, even for marginal results. 

Figure ll-12a presents room temperature adsorption isotherms for some representative activated carbon samples on Technical Grade DMSO, showing the difficulty of reducing UV275 to the desired level of <0.10 without additional purification. Figure ll-12a is a schematic of the melt crystallization-carbon column recycle system which was employed to get around this problem. The higher concentration of impurities in the unfrozen melt altered the equilibrium concentration on the activated carbon. In the steady state (Fig. 1 l-12b) a reasonably sized carbon column could produce effluent suitable for further freeze CrystalUzation, and the yield of the total process was close to 100%. 

Du et al. [75] investigated the surface area of purified and pristine HiPco nanotubes by performing N2 and Ar adsorption isotherms. Interestingly, this study found that there were significant differences in the specific surface areas of the pristine HiPco samples, even when their reported impurity levels were similar. These authors analyzed their data using the Horvath—Kawazoe equation... 

Adsorption isotherms of poorly purified solutes on heterogeneous or impure adsorbents often pass through a maximum in adsorption. Although such phenomena are possible in adsorption from concentrated solutions or from the gas phase, it is difficult to justify on theoretical grounds the existence of these phenomena in adsorption from dilute solutions of surfactants. They often disappear upon purification of the adsorbent and the solute and are believed to be due to the presence of impurities (Kitchener, 1965). 

This phenomenon can be explained allowing for the usual presence of dodecanol in SDS solutions. At first the salt leads to an increase in surface activity (shift of about one order of magnitude of the adsorption isotherm to lower concentration), and secondly the potential impurity dodecanol, which strongly adsorbs at the interface water/air, will more or less transfer to the dodecane phase after it has been adsorbed at the water/dodecane interface. Thus, no different mechanism is needed to describe the relaxation behaviour, as done by Bonfillon Langevin (1993). 

Macroscopic models of adsorption are probably more useful for elucidating information about the nature of adsorption sites and valuable in developing models that can relate the concentration of adsorbed impurity to its concentration in the solution phase. There are several ways to mathematically model adsorption, but the most common method is with the use of an adsorption isotherm that relates the amount of impurity adsorbed per unit mass of crystals (or per unit area of crystal) at a fixed temperature to the concentration of impurity in solution. A type of adsorption isotherm commonly observed experimentally is the Langmuir isotherm (Langmuir 1918) of the general form... 

Black and Davey used Eq. (3.22) to study the effect of the tailor-made additive L-glutamic acid on L-asparagine monohydrate crystals. With the use of a linear adsorption isotherm, Eq. (3.22) fit the crystal growth rate data. Consistent with a structural model in which impurities are embedded in the growing crystal surface, the growth rate of the crystals tended to zero at a high L-glutamic level. 

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