Efficiency plate columns

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. 

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. 

The pulsed-plate column is typically fitted with hori2ontal perforated plates or sieve plates which occupy the entire cross section of the column. The total free area of the plate is about 20—25%. The columns ate generally operated at frequencies of 1.5 to 4 H2 with ampHtudes 0.63 to 2.5 cm. The energy dissipated by the pulsations increases both the turbulence and the interfacial areas and greatly improves the mass-transfer efficiency compared to that of an unpulsed column. Pulsed-plate columns in diameters of up to 1.0 m or mote ate widely used in the nuclear industry (139,140). 

These two types of flooding are usuaUy considered separately when a plate column is being rated for capacity. For identification purposes they are caUed entrainment flooding (or priming ) and downflow flooding. When counterflow action is destroyed by either type, transfer efficiency is lost and reasonable design hmits have been exceeded. 

Entrainment Entrainment in a plate column is that liquid which is carried with the vapor from a plate to the plate above. It is detrimental in that the effective plate efficiency is lowered because hquid from a plate of lower volatility is carried to a plate of higher volatility, thereby diluting distillation or absorption effects. Entrainment is also detrimental when nonvolatile impurities are carried upward to contaminate the overhead product from the column. 

Direct Scale-Up of Laboratory Distillation Ljficiency Measurements It has been found by Fair, Null, and Bolles [Ind. Eng. Chem. Process Des. Dev., 22, 53 (1983)] that efficiency measurements in 25- and 50-mm (1- and 2-in-) diameter laboratory Oldersbaw columns closely approach tbe point efficiencies [Eq. (14-129)] measured in large sieve-plate columns. A representative comparison of scales of operation is shown in Fig. 14-37. Note that in order to achieve agreement between efficiencies it is necessaiy to ensure that (1) tbe systems being distilled are tbe same, (2) comparison is made at tbe same relative approach to tbe flood point, (3) operation is at total reflux, and (4) a standard Oldersbaw device (a small perforated-plate column with downcomers) is used in tbe laboratoiy experimentation. Fair et al. made careful comparisons for several systems, utibzing as large-scale information tbe published efficiency studies of Fractionation Research, Inc. 

FIG. 14-37 Overall column efficiency of 25-mm Oldersbaw column compared with point efficiency of 1,22-m-diameter-sieve sieve-plate column of Fractionation Research, Inc, System = cyclohexane-n-heptane, [(Fair, Null, and Bolles, Ind, Eng, Chem, Process Des, Dev, 22, 53 (i.982),]... 

TABLE 15-10 Summary of Minimum HETS Values and Volumetric Efficiencies for a Reciprocating-Plate Column ... 

Vital, Grossel, Olsen, Estimating Separation Efficiency, Part 2—Plate Columns, Hydro. Proc., November 1984, p. 147. 

Column type Guaranteed minimum efficiency (plates/meter)... 

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. 

It follows that as the variance of the peak is inversely proportional to the number of theoretical plates in the column then the larger the number of theoretical plates, the more narrow the peak and the more efficiently the column has constrained the band dispersion. As a consequence the number of theoretical plates in a column has been given the term Column Efficiency and is used to describe its quality. 

Figure 2.12 Plot of the logaritlm of the specific retention volune and colunn efficiency (plates per aeter) as a function of column tM erature for benzaldehyde (BZA) and n-tridecane (C ) on the stationary phase tetra-n-butylanmonium picrate. (Reproduced with permission from ref. 58. Copyright Blsevier Scientific Publishing Co.)...

Having estimated the number of trays, the column diameter can then be estimated. This is usually estimated with reference to the flood point for the column. The flood point occurs when the relative flowrates of the vapor and liquid are such that the liquid can no longer flow down the column in such a way as to allow efficient operation of the column14. For plate columns, there are a number of different mechanisms that can create flooding, but in one way or another, either14 ... 

The more efficient the column, the smaller will be at a given value of Vr. To measure efficiency, we use quantities called the plate number (N) or the plate height (H) of the column, which are defined as follows ... 

Much of this work was carried out using a special distilling column called a bubble-plate column (Fig. 141). Each plate really does act like a distilling flask with a very efficient column, and one distillation is really carried out on one physical plate. To calculate the number of plates (separation steps, or distillations) for a bubble-plate column, you just count them ... 

Now you have a column with one-point-six theoretical plates. Is that good you ask. Relative to what, I say. If that column is six feet high, that s terrible. The Height Equivalent to a Theoretical Plate (HETP) is 3.7feet/ plate. Suppose another column also had 1.6 theoretical plates, but was only 6 inches (0.5 ft) high. The HETP for this column is 3.7in/plate, and if it were 6 feet high, it would have 19 plates. The smaller the HETP, the more efficient the column is. There are more plates for the same length. 

The height equivalent to a theoretical plate, H, is that length of column that represents one theoretical plate, or one equilibration step. Obviously, the smaller the value of this parameter, the more efficient the column. The more theoretical plates packed into a length of column, the better the resolution. It is calculated by dividing the column length by N ... 

For the optimal application of GPC to the separation of discrete small molecules, three factors should be considered. Solvent effects are minimal, but may contribute selectivity when solvent-solute interactions occur. The resolving power in SMGPC increases as the square root of the column efficiency (plate count). New, efficient GPC columns exist which make the separation of small molecules affordable and practical, as indicated by applications to polymer, pesticide, pharmaceutical, and food samples. Finally, the slope and range of the calibration curve are indicative of the distribution of pores available within a column. Transformation of the calibration curve data for individual columns yields pore size distributions from which useful predictions can be made regarding the characteristics of column sets. 

Effect of Injection Volume. Table II shows the effect of injection volume on peak broadening and measured column efficiency. The bandwidths listed in Table II are due to injection volume alone, and were measured using an injector connected directly into the flowcell of a low-bandwidth detector. The plate reductions were then calculated for a 24,000 plate column, such as that represented by the bottom line of Table I, assuming 5 and 10 ml, respectively, for exclusion and total permeation volumes. Efficiencies of 23,000 plates at exclusion and 25,000 plates at permeation were actually measured for the column indicated in Table II. The effect of large injection volumes is thus to lose 25 to 50% of the potential column efficiency. 

The bandwidth values in the table are those calculated for the total system the instrument plus the column. The values for number of plates are for the number of plates realized in the total system. It can be seen that the optimized system does not greatly impact column efficiency, the total loss in plates being only about ten percent at total exclusion for a 24,000 plate column. This is consistent with an instrumental bandwidth equal to a third of the bandwidth of the column. The conventional system, with a bandwidth equal to or greater than that of the column, exhibited a severe loss in realized efficiency, particularly at or near exclusion. 

The data of Table III represent calculated bandwidths and efficiencies. Actual realized efficiencies were measured for the four chromatograms of Figure 4. For the 10-ym gel column, the conventional system produced an effective efficiency of 11,000 plates, compared with an effective efficiency of 16,000 plates for the optimized systems. These values are in excellent agreement with the calculated values shown on the top line of Table III. Similar measurements on chromatograms obtained from the 5-vim gel columns yielded values of 16,000 and 20,000 plates, respectively, for the conventional and optimized systems. This also represents good agreement with calculated effective efficiencies at total exclusion for a 24,000 plate column. 

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