Plates per unit time

This is an example for a case where we have to increase the inlet pressure to reduce analysis time, but we also have to explore ways to improve the column packing technique. A comparison of the results shows that the very well-packed column generates ten times more plates per unit time than the poorly packed column. In Eq. (2 ) r is multiplied by 4 and li is di vided by 2.5. Thus, analysis time and column length are divided by 10 and... 

Most analyses are run at carrier gas velocities that are 1.5 to 2 times as great as the optimum velocity at the minimum of the van Deemter curve. The higher velocity is chosen to give the maximum efficiency (most theoretical plates) per unit time. A small decrease in resolution is tolerated in return for faster analyses. 

Similarly, the number of theoretical plates per unit time can be calculated ... 

In pressure-driven operation, considerable band broadening was observed at high linear velocity, although the separation impedance was much lower than that of a particle-packed column owing to the much lower flow resistance. The separation impedance (E = AP to / r N2 = (AP / N) (to / N) (l/r )) expresses the total column performance in terms of the reciprocal number of theoretical plates per unit time and pressure drop. Because the contributions of the B- and C-terms are expected to be similar for a pressure-driven mode and an electro-driven mode, the difference in performance can be attributed to the greater contribution of the A-terms in Eqn. 5.2 in the pressure-driven mode. The contribution of the A-term is known to be less in CEC than in HPLC [6],... 

In some cases the speed of separation becomes crucial, so we change our focus, as in Section 8.9, to that of maximizing the number of plates per unit time Nit. From Eq. 9.17, N = XIH is... 

A typical HPLC separation using a 15-cm column of 15,000 theoretical plates produces peak capacity (Giddings, 1991) of about 80-100 under isocratic conditions and up to 150 under gradient conditions in 1 h(Eq. 7.3, n peak capacity, A number of theoretical plates of a column, and fR and t retention time of the last and the first peak of the chromatogram, respectively). An increase in the number of separated peaks per unit time can be achieved by increased separation speed made possible by monolithic silica columns (Deng et al., 2002 Volmer et al., 2002). This has also been shown for peptides and proteins (Minakuchi et al., 1998 Leinweber et al., 2003). 

Equations (29) and (30) allow us the calculation of column performance as a function of the particle diameter when the columns >are operated at minimum inlet pressure. We find that the pressure gradient is inversely proportional to the cube of the particle diameter and the number of plates generated per unit time is inversely proportional to the sc uare of the particle diameter. j... 

The HETP curve clearly shows, that for a packed column, the particle size has a profound effect on the minimum value of the HETP of a column and thus the maximum efficiency attainable. It would also appear that the highest efficiency column would be obtained from columns packed with the smallest particles. This will in due course be shown to be a fallacy, but what is true, is that the smaller the particle diameter the smaller will be the minimum HETP and thus, the larger the number of plates per unit length obtainable from the column. At this time it will suffice to point out that the total number of theoretical plates that can be obtained will depend on the length of the column which, in turn, must take into account the available inlet pressure... 

The energy transfer per unit time and area by radiation between two infinite parallel black plates is... 

This is known as the plate time and has units of seconds. It is equivalent to the amount of time it takes to generate one theoretical plate. Its inverse would be plates per second, N/to. Plates per second may also be expressed more generally as N/t for elution times other than the void time [2,3]. These terms more effectively describe the criteria of resolution per unit time that are desired to be maximized (actually, N/t is proportional to resolution squared per time) unfortunately, they are not widely used in the literature, and for the sake of continuity will not be used in this discussion. The following sections will look at what influences plate height and velocity and how best to minimize H/u. 

Now that the factors affecting plate height have been examined, it is time to turn to the effect of linear velocity and the limitation of pressure on the resolution per unit time. 

When compared to the batchwise preparative chromatography, Simulated moving bed (SMB) units exhibit a number of advantages. These advantages are primarily because of the continuous nature of the operation and the efficient use of the stationary and mobile phases, which allows a decrease in desorbent requirement and an improvement of the productivity per unit time and per unit mass of stationary phase. In addition, high performances can be achieved even at rather low values of selectivity and with a relatively small number of theoretical plates. Due to these positive features, SMB is particularly attractive in the case of enantiomer separations, since it is difTicult to separate enantiomers by conventional techniques. More recent applications related to chiral technology were reported [1-3]. 

When the partition ratio is large, the plate height at the minimum is 1.9r and the velocity 2. DJr. The important conclusion to be reached here is that, the smaller the diameter of the capillary, the smaller the optimum plate height and the higher the optimum flow velocity. This situation means more theoretical plates per unit length and the possibility of shorter analysis time for a given level of separation. 

After the column has come to equilibrium, the reflux ratio is adjusted. As was mentioned previously, it is usually advisable to maintain the reflux ratio equal to the number of theoretical plates the column possesses at total reflux. The ratio is usually measured either by noting the number of drops returned to the column by the head and the number withdrawn per unit time, or by the lengths of the off and on periods if a magnetically operated valve is used. 

This result indicates that the typical disk of 10 pm is approximately 5 times more efficient or has 5 times more effective plates per unit length than does the best syringe cartridge. 

Derive an expression for the rate at which the particles of a polydisperse aerosol settle on a horizontal plate from a stagnant gas. Dimensions arc number per unit time per unit area. Assume that Stokes law holds for the terminal settling velocity, and express your answer in terms of the appropriate moments. 

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