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Column efficiency plots

To increase the number of theoretical plates without increasing the length of the column, it is necessary to decrease one or more of the terms in equation 12.27 or equation 12.28. The easiest way to accomplish this is by adjusting the velocity of the mobile phase. At a low mobile-phase velocity, column efficiency is limited by longitudinal diffusion, whereas at higher velocities efficiency is limited by the two mass transfer terms. As shown in Figure 12.15 (which is interpreted in terms of equation 12.28), the optimum mobile-phase velocity corresponds to a minimum in a plot of H as a function of u. [Pg.562]

The curves represent a plot of log (h ) (reduced plate height) against log (v) (reduced velocity) for two very different columns. The lower the curve, the better the column is packed (the lower the minimum reduced plate height). At low velocities, the (B) term (longitudinal diffusion) dominates, and at high velocities the (C) term (resistance to mass transfer in the stationary phase) dominates, as in the Van Deemter equation. The best column efficiency is achieved when the minimum is about 2 particle diameters and thus, log (h ) is about 0.35. The optimum reduced velocity is in the range of 3 to 5 cm/sec., that is log (v) takes values between 0.3 and 0.5. The Knox... [Pg.265]

The effect of pore size on CEC separation was also studied in detail [70-75]. Figure 9 shows the van Deemter plots for a series of 7-pm ODS particles with pore size ranging from 10 to 400 nm. The best efficiency achieved with the large pore packing led to a conclusion that intraparticle flow contributes to the mass transfer in a way similar to that of perfusion chromatography and considerably improves column efficiency. The effect of pore size is also involved in the CEC separations of synthetic polymers in size-exclusion mode [76]. [Pg.18]

FIGURE 6.3 Plot of column efficiency against sample mass for three neutral compounds (3-phenylpropanol, caffeine, phenol), and three charged compounds (propranolol, nortriptyline, 2-NSA [2-naphthalenesulfonic acid]) on XTerra MS (15 x 0.46 cm, 3.5 pm particles). Mobile phase acetonitrile-formic acid (overall concentration 0.02 M) pH 2.7 (28 72, v/v) except for caffeine (12.5 77.5, v/v). Flow rate 1 mL min . Column temperature 30°C. Injection volume 5 pL. [Pg.311]

Efficiency (N)—A measure of the narrowness of elution bands, the sharpness of peaks, and the performance of a column. Results are in theoretical plates. The Huber equation calculates efficiency versus flow rate, which is plotted on as a Van Deampter plot, which compares column efficiency with flow rate. [Pg.215]

Figure 7.5 The O Connell correlation for overall column efficiency, (a) Plot for distillation (b) plot for absorbers. (Prom H. E, O Connell, Trane. AlChE, 42. p. 741, 1946, Reprinted courtesy of the American Institute of Chemical Engineers.)... Figure 7.5 The O Connell correlation for overall column efficiency, (a) Plot for distillation (b) plot for absorbers. (Prom H. E, O Connell, Trane. AlChE, 42. p. 741, 1946, Reprinted courtesy of the American Institute of Chemical Engineers.)...
Van Deemter plot. A graph of column efficiency, expressed as HETP versus linear velocity of the mobile phase. This plot indicates the optimum linear velocity (and, thus, flow rate) for a particular column. [Pg.25]

The basis and various parameters for the economic analysis are given in Table II. The overall column efficiency used was obtained from a plot of efficiency vs. the product of relative volatility and liquid viscosity (9), corrected to match predicted (10) data for the propane-propylene system. The value from the plot (9) was increased by a factor required to make the efficiency of the propane-propylene binary distillation equal to 100%. Costs were calculated by the Venture Analysis method (II), because this method yields the appropriate weighting factors for the fixed and operating costs in order to calculate the total costs. Results are expressed as annual costs, before taxes. The important process variables are discussed below. [Pg.33]

Figure 3.25 Experimental isotherms of Troger s base enantiomers on microcrystalline cellulose triacetate. Experimental data by frontal analysis (symbols) and best quadratic isotherm (solid line). Experimental conditions column length, 25 cm column efficiency, N = 106 plates phase ratio, F = 0.515 flow velocity 0.076 cm/s, pure ethanol. Column (250 x4.6 mm) packed with cellulose microcrystalUne triacetate (CTA, 15-25ftm), previously boiled in ethanol for 30 min. (a) Isotherm data. Top line, (+)-TB, bottom line, (-)-TB. (b) Plot of q/C versus C. Reproduced with permission from A. Seidel-Morgenstem and G. Guiochon, Chem. Eng. Scl, 48 (1993) 2787 (Figs. 4 and 5). Figure 3.25 Experimental isotherms of Troger s base enantiomers on microcrystalline cellulose triacetate. Experimental data by frontal analysis (symbols) and best quadratic isotherm (solid line). Experimental conditions column length, 25 cm column efficiency, N = 106 plates phase ratio, F = 0.515 flow velocity 0.076 cm/s, pure ethanol. Column (250 x4.6 mm) packed with cellulose microcrystalUne triacetate (CTA, 15-25ftm), previously boiled in ethanol for 30 min. (a) Isotherm data. Top line, (+)-TB, bottom line, (-)-TB. (b) Plot of q/C versus C. Reproduced with permission from A. Seidel-Morgenstem and G. Guiochon, Chem. Eng. Scl, 48 (1993) 2787 (Figs. 4 and 5).
The hodograph transform is valid only within the framework of the ideal model. It has been shown, however, that the hodograph plots derived from actual chromatograms are very similar to those predicted by the ideal model [18]. If the column efficiency exceeds 100 to 200 theoretical plates, there is no significant difference between the hodograph plot obtained with the ideal model and the plot derived from the profiles calculated with the equilibrium-dispersive model, except very near the axes of coordinates (Figure 8.13). Figure 8.14a compares the... [Pg.423]

Figure 8.13 Influence of column efficiency on the hodograph plot for a wide rectangular band. H - 0.02 cm column length (L, cm) and number of theoretical plates (N) 1, 1.0 (50) 2, 2.0 (100) 3,4.0 (200) 4, 8.0 (400). Reproduced with permission from Z. Ma and G. Guiochon,. Chromatogr., 603 (1992) 13 (Fig. 2). Figure 8.13 Influence of column efficiency on the hodograph plot for a wide rectangular band. H - 0.02 cm column length (L, cm) and number of theoretical plates (N) 1, 1.0 (50) 2, 2.0 (100) 3,4.0 (200) 4, 8.0 (400). Reproduced with permission from Z. Ma and G. Guiochon,. Chromatogr., 603 (1992) 13 (Fig. 2).
Figure 10.3 Correlation between the column efficiency and the amount of sample injected [26]. Plot of N/Nq versus Wx,xiiVs, where Nq is the column efficiency under linear conditions (very small sample size), x,N = Noi o/(1 + with Wx,... Figure 10.3 Correlation between the column efficiency and the amount of sample injected [26]. Plot of N/Nq versus Wx,xiiVs, where Nq is the column efficiency under linear conditions (very small sample size), x,N = Noi o/(1 + with Wx,...
Figure 10.9 Comparison between the band profiles predicted by the ideal model and the numerical solution of the equilibrium-dispersive model for a Langmuir isotherm. Constant column efficiency, 2000 theoretical plates, (a) Classical C vs. f profile. Sample size given as loading factor, (b) Reduced profiles, plots of bC vs. (t — fo)/(fR,o — to)- Sample size given as apparent loading factor, m = [Icq/(1 + J q)] NLj. Similar chromatograms, corresponding to intermediate loading factors, are given in Figure 10.8. Figure 10.9 Comparison between the band profiles predicted by the ideal model and the numerical solution of the equilibrium-dispersive model for a Langmuir isotherm. Constant column efficiency, 2000 theoretical plates, (a) Classical C vs. f profile. Sample size given as loading factor, (b) Reduced profiles, plots of bC vs. (t — fo)/(fR,o — to)- Sample size given as apparent loading factor, m = [Icq/(1 + J q)] NLj. Similar chromatograms, corresponding to intermediate loading factors, are given in Figure 10.8.
Figure 11.1 Plot of the apparent column efficiency versus the retention factor. Curves 1-3, forward-backward calculation scheme, Eq. 11.17 (ai = 2) curves 4r-6, backward-forward calculation scheme, Eq. 11.18 (fli = 0.5). fcg = 4. Reprinted from Czokand G. Guiochon, Comput. Chem. Eng., 14 (1990) 1435 (Fig. 1). Figure 11.1 Plot of the apparent column efficiency versus the retention factor. Curves 1-3, forward-backward calculation scheme, Eq. 11.17 (ai = 2) curves 4r-6, backward-forward calculation scheme, Eq. 11.18 (fli = 0.5). fcg = 4. Reprinted from Czokand G. Guiochon, Comput. Chem. Eng., 14 (1990) 1435 (Fig. 1).
Figure 18.20 Plot of the specific production in elution versus the column efficiency, N. a = 1.2 and fcj = 6. Less ( ) and more (o) retained components of a 3 1 mixture less (A) and more (+) retained components of a 1 3 mixture. Reproduced from A. Fdinger and G. Guiochon, AlChE 40 (1994) 594. Fig. 3a). Reproduced by permission of the American Institute of Chemical Engineers. ( )1994 AIChE. All rights reserved. Figure 18.20 Plot of the specific production in elution versus the column efficiency, N. a = 1.2 and fcj = 6. Less ( ) and more (o) retained components of a 3 1 mixture less (A) and more (+) retained components of a 1 3 mixture. Reproduced from A. Fdinger and G. Guiochon, AlChE 40 (1994) 594. Fig. 3a). Reproduced by permission of the American Institute of Chemical Engineers. ( )1994 AIChE. All rights reserved.
Some of the variables that affect column efficiency can be controlled to improve separations. The extent of band broadening, and thus colunm efficiency, depends on the amount of contact time between the mobile and stationary phases. Therefore, colunm efficiency depends on the flow velocity of the mobile phase. Plots of plate height vs. velocity are called van Deemter curves. The plots are fit to the van Deemter equation... [Pg.492]

Calculate the IBW (4 a) using the tangent method as shown in Figure 4.19b. An IBW of -60 pL was found for this HPLC. Figure 4.20 shows a chart plotting column efficiencies of 150-mm long 3-pm columns of various inner diameters versus retention factors (k) on an HPLC system with a 60-pL IBW. Note that efficiency loss from instrumental dispersion can be severe for small inner diameter columns where peak volumes are smaller. Note that peaks of low k are affected more by system dispersion since peak volumes are proportional to k. [Pg.106]

In open tubular colvunns, because the thin hquid film is deposited directly on the wall of the column rather than the sohd supports, the A term is zero, therefore ehminating one of the major contributor to zone broadening. Comparing to packed columns, the resistance to mass transfer is also reduced in both the hquid phase due to the apphcation of very thin film of the stationary phase, and in the mobile phase due to the apphcation of very narrow internal diameter columns. The typical open tubular colmnns have an internal diameter of 0.25 mm and a film thickness of 0.25 pm. A combination of ah these factors makes for the fact that capillary GC columns have much lower plate height value and substantiaUy more theoretical plates. The effect of carrier gas and hnear velocity on capihary column efficiency is illustrated in Figure 4, which shows a family of van Deemter plots for common carrier gases. [Pg.75]

The flat slope of plate height vs mobile phase velocity plot (Figure 7.2 a) in LC permits the use of high mobile phase velocity without much loss in column efficiency. In this respect, LC differs considerably from GC in which plate height varies more steeply with mobile phase velocity at large values of the latter (Figure 7.2 b). [Pg.121]

Figure 11.8 The relationship between capacity factor and resolution from a plot of Equation (11.35). A column efficiently of 10,000 theoretical plates and a selectivity factor of 2 was used. There is great increase in resolution for modest increases in capacity factor but this advantage is soon lost. Figure 11.8 The relationship between capacity factor and resolution from a plot of Equation (11.35). A column efficiently of 10,000 theoretical plates and a selectivity factor of 2 was used. There is great increase in resolution for modest increases in capacity factor but this advantage is soon lost.
The zirconia particles described above are stable at high temperatures. Polystyrene-coated porous zirconia particles have been used at 200° C. Peter Carr at the University of Minnesota, in studies on high-temperature fast LC, demonstrated that column efficiency at high velocity improves at higher temperatures, especially for solutes that are strongly retained. Also, the van Deemter plot flattens out significantly as the temperature is increased. Carr performed separations at 150°C at flow rates of 15 mL/min in a 4.6-mm X 5-cm column with 3-ju,m particles. [Pg.618]

For a given column length, optimum column efficiency is obtained when the equilibrium step or plate height is at a minimum, that is, the column band broadening processes described by the van Deemter equation (see section 2.5.1) are minimised by selecting the optimum velocity, t>f the mobile phase. Figure 5.3 shows the van Deemter plot of H against n for... [Pg.169]


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