Efficiency, packing plate

The performance of real stages is related to an equilibrium stage by the concept of plate efficiencies for plate contactors, and height of an equivalent theoretical plate for packed columns. 

Fig. 1.18. Examples of chromatographic separation of a ihree-componcnt sample mixture and possible ways lo improve the separation during HPLC melhtxl development. tA) Satisfactory separation. (B) Unsatisfactory separation — ttw low retention. The elution strength of the mobile phase should be decreased. (C) Good resolution, but too long time of separation. The elution strength of the mobile phase should be increased. (D) Unsatisfactory separation — too low column efficiency. The plate number should be increased by using finer packing panicles or a longer column. (E) Unsatisfactory separation — gixxl retention and column efficiency, but too low separation selectivity. The components of the mobile phase can be changed, a ternary or a quaternary mobile phase, selective mobile phase additives, or another type of the stationary phase can be used.

Economic lot size, 350 Effective interest, 218-222, 224, 241 Efficiency, packing, 702-706 plate, 661-667 pump, 517-518, 520 Ejectors, cost of, 528 Electrical installation, cost of, 174, 807 Electricity, cost of, 815 Electromotive series of metals, 433 Emissivity of surfaces, 582-585 Energy balance, mechanical, 479-480 for reactor design, 715-716 total, 479-480... 

Gas absorption can be carried out in a column equipped with sieve trays or other types of plates normally used for distillation. A column with trays is sometimes chosen instead of a packed column to avoid the problem of liquid distribution in a large diameter tower and to decrease the uncertainty in scaleup. The number of theoretical stages is determined by stepping off plates on a y-x diagram, and the number of actual stages is then calculated using an average plate efficiency. The plate and local efficiencies are defined in the same way as for distillation [Eqs. 

An efficient packed gas chromatography column will have several thousand theoretical plates, and capillary columns will have in excess of 10,000 theoretical plates. The H value for a 1-m column with 10,000 theoretical plates would be 100 cm/ 10,000 plates = 0.01 cm/plate. In a high-performance liquid chromatography (below), efficiency on the order of 400 theoretical plates per centimeter is typically achieved, and colunms are 10 to 50 cm in length. 

Thus, a minimum of more than 400 theoretical plates is required to achieve the desired separation this is an enormous number. Fortunately, high-efficiency packings are now available with equivalent heights HETP of the order of 1 cm. This limits the size of the column to heights that are not excessive and can be implemented in practice. 

In the case of a plate column the performance of a real plate is related to the performance of a theoretical one by the plate efficiency. In the case of a packed column the height equivalent to a theoretical plate HETP) gives a measure of the contacting efficiency of the packing. 

To minimize the multiple path and mass transfer contributions to plate height (equations 12.23 and 12.26), the packing material should be of as small a diameter as is practical and loaded with a thin film of stationary phase (equation 12.25). Compared with capillary columns, which are discussed in the next section, packed columns can handle larger amounts of sample. Samples of 0.1-10 )J,L are routinely analyzed with a packed column. Column efficiencies are typically several hundred to 2000 plates/m, providing columns with 3000-10,000 theoretical plates. Assuming Wiax/Wiin is approximately 50, a packed column with 10,000 theoretical plates has a peak capacity (equation 12.18) of... 

Microcolumns use less solvent and, because the sample is diluted to a lesser extent, produce larger signals at the detector. These columns are made from fused silica capillaries with internal diameters of 44—200 pm and lengths of up to several meters. Microcolumns packed with 3-5-pm particles have been prepared with column efficiencies of up to 250,000 theoretical plates. 

Open tubular microcolumns also have been developed, with internal diameters of 1-50 pm and lengths of approximately 1 m. These columns, which contain no packing material, may be capable of obtaining column efficiencies of up to 1 million theoretical plates.The development of open tubular columns, however, has been limited by the difficulty of preparing columns with internal diameters less than 10 pm. 

Nonisothermal Gas Absorption. The computation of nonisothermal gas absorption processes is difficult because of all the interactions involved as described for packed columns. A computer is normally required for the enormous number of plate calculations necessary to estabUsh the correct concentration and temperature profiles through the tower. Suitable algorithms have been developed (46,105) and nonisothermal gas absorption in plate columns has been studied experimentally and the measured profiles compared to the calculated results (47,106). Figure 27 shows a typical Hquid temperature profile observed in an adiabatic bubble plate absorber (107). The close agreement between the calculated and observed profiles was obtained without adjusting parameters. The plate efficiencies required for the calculations were measured independendy on a single exact copy of the bubble cap plates installed in the five-tray absorber. 

Rate of Mass Transfer in Bubble Plates. The Murphree vapor efficiency, much like the height of a transfer unit in packed absorbers, characterizes the rate of mass transfer in the equipment. The value of the efficiency depends on a large number of parameters not normally known, and its prediction is therefore difficult and involved. Correlations have led to widely used empirical relationships, which can be used for rough estimates (109,110). The most fundamental approach for tray efficiency estimation, however, summarizing intensive research on this topic, may be found in reference 111. 

Pulsed Columns. The efficiency of sieve-plate or packed columns is increased by the appHcation of sinusoidal pulsation to the contents of the column. The weU-distributed turbulence promotes dispersion and mass transfer while tending to reduce axial dispersion in comparison with the unpulsed column. This leads to a substantial reduction in HETS or HTU values. 

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