Process mass overload

A chromatographic separation can be developed in three ways, by displacement development, by frontal analysis, and by elution development, the last being almost universally used in all analytical chromatography. Nevertheless, for the sake of completeness, and because in preparative chromatography (under certain conditions of mass overload) displacement effects occur to varying extents, all three development processes will be described. 

The problem is made more difficult because these different dispersion processes are interactive and the extent to which one process affects the peak shape is modified by the presence of another. It follows if the processes that causes dispersion in mass overload are not random, but interactive, the normal procedures for mathematically analyzing peak dispersion can not be applied. These complex interacting effects can, however, be demonstrated experimentally, if not by rigorous theoretical treatment, and examples of mass overload were included in the work of Scott and Kucera [1]. The authors employed the same chromatographic system that they used to examine volume overload, but they employed two mobile phases of different polarity. In the first experiments, the mobile phase n-heptane was used and the sample volume was kept constant at 200 pi. The masses of naphthalene and anthracene were kept... 

Band dispersion from sample mass overload is a direct result of the chromatographic process proceeding under conditions, where the adsorption isotherm of the solute on the stationary phase, is no longer linear. The development of an equation that describes the extent, of band spreadinn as a function of mass of sample placed on the column, is complex. This problem has been elegantly approached by 6uiochon and his co-workers (15-18) from the basis of the adsorption isotherm of the solute on the stationary phase. 

The underlying equilibrium-dispersion model, for which the mass balance for solute / in a A component mixture and a volume element is given in equation (21-2), has been very often successfully applied to quantify chromatographic processes under overloaded conditions. 

The effect of excessive sample mass on the chromatographic process is extremely complex. The theory of mass overload is also intricate [5-7] and requires a considerable amount of basic physical chemical data, such as the adsorption isotherms of each solute measured over a wide range of concentration, before it can be applied to a practical problem. Unless the production size, and the production economy, will support the necessary basic data gathering, the problem of mass overload is more conveniently and thriftily approached from a simple experimental stance. 

Overloading effects seem even more complex at intermediate pH because silan-ols are now partially ionized and involved in the retention of bases. As mentioned previously, the overload of now-ionized silanols could at least be part of the cause of tailing peaks, even when very small amounts of ionized base are used [18,24]. However, it has been observed that as solute mass increases in experiments at pH 7, column efficiency may improve from an initially low value to a maximum, afterward declining in the usual way [33]. This observation could be due to the blocking or saffiration of ionized silanols by a portion of the sample, such that the rest interacts mainly by hydrophobic processes, resulting in better efficiency. At higher pH still, the solute should not be ionized if appreciably above its p a and therefore should... 

The ideal model and the equilibrium-dispersive model are the two important subclasses of the equilibrium model. The ideal model completely ignores the contribution of kinetics and mobile phase processes to the band broadening. It assumes that thermodynamics is the only factor that influences the evolution of the peak shape. We obtain the mass balance equation of the ideal model if we write > =0 in Equation 10.8, i.e., we assume that the number of theoretical plates is infinity. The ideal model has the advantage of supplying the thermodynamical limit of minimum band broadening under overloaded conditions. 

Thus, it will be extremely difficult at best to separate the influences of the various phenomena that may be responsible for the effects of a slow kinetics of mass transfer and a slow kinetics of the retention mechanism. The fitting of experimental data obtained in overloaded elution chromatography to various models of chromatography will not permit the choice of a best model, nor the identification of the slowest step in the chromatographic process. Independent measurements of the kinetic parameters are necessary. 

To describe the peak shapes of a separation under overload conditions a clear understanding of how the competitive phase equilibria, the finite rate of mass transfer, and dispersion phenomena combine to affect band profiles is required [ 11,66,42,75,76]. The general solution to this problem requires a set of mass conservation equations appropriate initial and boundary conditions that describe the exact process implemented the multicomponent isotherms and a suitable model for mass transfer kinetics. As an example, the most widely used mass conservation equation is the equilibrium-dispersive model... 

The TWTF will receive waste contaminated with transuranic nuclides (TRU waste) presently stored at the INEL Radioactive Waste Management Complex. Waste packages are opened and their contents are sorted, placed into charging containers, and charged into an incinerator. Combustibles will be burned and noncombustibles converted to a chemically inot, basalt-bke substance. This substance is cast into ingots and shipped to a federal waste repository. Because of the wide variety of shapes, sizes, and compositions of the waste, heavy reliance is placed on flssile material mass limits in the various process areas. The individual areas interact in the sense that an overload or accumulation of fissile material in one area may be propagated to the next area in the process chain. 

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