Plate efficiency factors influencing

Additional factors influencing column performance are the type and quality of the packing process, which mainly determines the theoretical plate count (N) of the column. In contrast to HPLC columns the efficiency of the separation itself is determined predominantly by the quality of the sorbent alone (pore... 

Column diameter for a particular service is a function of the physical properties of the vapor and liquid at the tray conditions, efficiency and capacity characteristics of the contacting mechanism (bubble trays, sieve trays, etc.) as represented by velocity effects including entrainment, and the pressure of the operation. Unfortunately the interrelationship of these is not clearly understood. Therefore, diameters are determined by relations correlated by empirical factors. The factors influencing bubble cap and similar devices, sieve tray and perforated plate columns are somewhat different. 

Electrolytic recovery systems work best on concentrated solutions. For optimal plating efficiency, recovery tanks should be agitated ensuring that good mass transfer occurs at the electrodes. Another important factor to consider is the anode/cathode ratio. The cathode area (plating surface area) and mass transfer rate to the cathode greatly influence the efficiency of metal deposition. 

LIQUID RESISTANCE TO INTERPHASE MASS TRANSFER. Liquid viscosity, gas solubility in absorbers, and relative volatility in rectification columns are important factors in determining the liquid resistance to interphase mass transfer. Increase in liquid viscosity, decrease in gas solubility for absorbers, and increase in relative volatility for rectification columns cause an increase in the liquid resistance to interphase mass transfer and a resultant reduction in plate efficiency. The ratio of the liquid rate to the gas rate influences the relative importance of the liquid resistance to interphase mass transfer. An increase in the ratio of liquid rate to gas rate reduces the importance of the liquid resistance and can cause an increase in the plate efficiency. 

Mass-transfer theory indicates that for trays of a given design the factors most likely to influence E in absorption and stripping towers are the physical properties of the fluids and the dimensionless ratio mGM/LM Systems in which the mass transfer is gas-film-controlled may be expected to have plate efficiencies as high as 50 to 100 percent, whereas plate efficiencies as low as 1 percent have been reported for the absorption of gases of low solubility (large m) into solvents of relatively high viscosity. 

When the sample is introduced into the column, usually in the form of a zone of vapor, it takes the form of a narrow band. During transit through the column, various factors influence the width of this band, which is continuously increased due to various dispersion processes. These include diffusion of the solute, resistance to mass transfer between and within phases, and the influence of flow irregularities and pertur-bations.f A simple concept, the theoretical plate, carried over from distillation processes, has been used to compare columns and account for the degree of dispersion that influences bandwidth. A chromatographic column may be considered to consist of numerous theoretical plates where the distribution of sample components between the stationary and mobile phase occurs. Hence, a measure of the efficiency of a GC column may be obtained by calculating the number of theoretical plates, N, in the column from ... 

FACTORS INFLUENCING PLATE EFFICIENCY. Although thorough studies of plate efficiency have been made, the estimation of efficiency is largely empirical. Sufficient data are at hand, however, to show the major factors involved and to provide a basis for estimating the efficiencies for conventional types of columns operating on mixtures of common substances. 

Equations 12.21 and 12.22 contain terms corresponding to column efficiency, column selectivity, and capacity factor. These terms can be varied, more or less independently, to obtain the desired resolution and analysis time for a pair of solutes. The first term, which is a function of the number of theoretical plates or the height of a theoretical plate, accounts for the effect of column efficiency. The second term is a function of a and accounts for the influence of column selectivity. Finally, the third term in both equations is a function of b, and accounts for the effect of solute B s capacity factor. Manipulating these parameters to improve resolution is the subject of the remainder of this section. 

Various authors [90] and researchers have published factors that influence the magnitude of HETP values. The lower the HETP value (i.e., number of feet (or inches) per theoretical plate/tray/stage) the more efficient is the packing (random or structured) for any particular separation system. Other influences include ... 

Other factors that can influence the separability of components of complex natural mixtures, such as adsorbent particle size and layer thickness, are similar to those used in analytical TLC. Mostly, adsorbents of wide dispersion of particle size — 5 to 40 pm and layers of 0.5 to 1 mm thickness — are used. Although the capacities of layers increase with their thickness, the separation efficiency decreases for thickness above 1.5 mm. Commercially available precoated preparative plates (e.g., silica, alumina, and RP2 plates) with fluorescence indicators and plates with preadsorbent zones are more convenient and commonly used. 

A wide range of operating conditions and design philosophies affect mercury cell efficiency. For example, the fundamental distinction between a resaturation and a waste brine process influences the temperature and brine strength profile along the length of the cell and hence the overall efficiency. Another important factor is the quality of the brine. Impurities in the brine can cause base-plate deposits, which tend to reduce the anode/cathode gap. This gradual reduction in gap requires either manual or automatic adjustment and, eventually, the cell must be taken off-line and the thick mercury removed. 

Factors Which Influence Resolution. In order to effectively apply SMGPC to separation problems, the influence of three factors on the resolution of sample components must be considered. Solvent effects play a minor role, but choice of eluent can alter selectivity in some cases. Column efficiency, as noted, has a major impact on the quality of separation. The number of peaks which can be resolved within the pore volume of a given column (i.e. peak capacity) is related to the square root of the number of theoretical plates (13). Finally, the nature of the calibration curve will influence resolution. Each... 

OTHER FACTORS. Design details of the column, such as vapor-opening dimensions, plate iayout, or the total number of trays can have an effect on the efficiencies. The exact influence of these factors is best determined by experimental tests. 

The procedures of partial wetting, evaporation and condensation depend on several factors and basically influence both the separation of the analyte spots and the speed of the mobile phase movement 39,40. A beneficial effect of the evaporation of the running mobile phase from the thin-layer plate and wetting the stationary phase with the mobile phase is that spots with Rf values over 0.5 are concentrated. Thereby, the efficiency of the TLC system can be highly increased in comparison with the column technique. 

Resolution can be increased if one of the three terms in Eq. 4.14 is increased. As mentioned in Chapter 2.4.3 the first term describes the influence of selectivity. This term should be maximized by maximizing a in a selectivity screening with different adsorbents and mobile phases. The second term should be kept in a certain range and not be maximized, because the maximum value of 1 is reached for an infinite retention factor. At infinite retention the productivity would decrease due to the high cycle time. The last term of Eq. 4.14 describes the efficiency of the column in terms of the number of plates. Resolution can be increased by selecting efficient adsorbents with small particle size and appropriate narrow particle size distribution. For these adsorbents fluid dynamic and mass transfer resistances are minimized. Con-... 

In isocratic elution, resolution depends on the column efficiency or plate number N, the selectivity a, and the retention factor k, all of which can be experimentally influenced through systematic changes in individual chromatographic parameters. In the isocratic mode of separation, resolution is determined from... 

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