Plate height pressure drop

At high mobile phase velocities the equation for reduced plate height becomes approximately h/v = c. Under conditions required to obtain the maximum number of plates, the pressure drop tends to infinity. 

The bed height is determined by consideration of the work required to overcome the pressure drop across the bed and distributor plate. The pressure drop across the bed is simply the weight of the bed. 

FIG. 14-29 Pressure-drop contributions for cross-flow plates, hj = pressure drop through cap or sieve, equivalent height of plate liquid = height of weir ... 

Kotai = drop through all the plates, in height of the flowing solvent Nact = number of actual plates ht = pressure drop across a single plate... 

QiPe " conversion constant 32.2 [(ft.)(lb. force)]/[(sec. ) (lb. mass)] h liquid depth above surface of plate he pressure drop through riser and undercap, in. of liquid her height of liquid above top of weir, in. 

Fig. 27A, Height of a theoretical plate and pressure drop ofMellaiak 2X and 2Y versus flie factor...

Elution volume, exclusion chromatography Flow rate, column Gas/liquid volume ratio Inner column volume Interstitial (outer) volume Kovats retention indices Matrix volume Net retention volume Obstruction factor Packing uniformity factor Particle diameter Partition coefficient Partition ratio Peak asymmetry factor Peak resolution Plate height Plate number Porosity, column Pressure, column inlet Presure, column outlet Pressure drop... 

It is convenient and consistent to relate all of these pressure-drop terms to height of equivalent clear liquid (deaerated basis) on the plate, in either milhmeters or inches of liquid. 

Commercial preparations of these supports are available in narrow mesh-range fractions to obtain particles of uniform size the material should be sieved to the desired particle size range and repeatedly water floated to remove fine particles which contribute to excessive pressure drop in the final column. To a good approximation the height equivalent of a theoretical plate is proportional to the average particle diameter so that theoretically the smallest possible particles should be preferred in terms of column efficiency. Decreasing particle size will, however, rapidly increase the gas pressure necessary to achieve flow through the column and in practice the best choice is 80/100 mesh for a... 

The height of the weir determines the volume of liquid on the plate and is an important factor in determining the plate efficiency (see Section 11.10.4). A high weir will increase the plate efficiency but at the expense of a higher plate pressure drop. For columns operating above atmospheric pressure the weir heights will normally be between 40 mm to 90 mm (1.5 to 3.5 in.) 40 to 50 mm is recommended. For vacuum operation lower weir heights are used to reduce the pressure drop 6 to 12 mm ( to in.) is recommended. 

A simple additive model is normally used to predict the total pressure drop. The total is taken as the sum of the pressure drop calculated for the flow of vapour through the dry plate (the dry plate drop hj) the head of clear liquid on the plate (hw + how) and a term to account for other, minor, sources of pressure loss, the so-called residual loss hr. The residual loss is the difference between the observed experimental pressure drop and the simple sum of the dry-plate drop and the clear-liquid height. It accounts for the two effects the energy to form the vapour bubbles and the fact that on an operating plate the liquid head will not be clear liquid but a head of aerated liquid froth, and the froth density and height will be different from that of the clear liquid. 

The Poppe plot is a log-log plot of H/uq = t(JN versus the number of plates with different particle sizes and with lines drawn at constant void time, t(). H is the plate height, Vis the number of plates, and u() is the fluid velocity (assumed equal to the void velocity). The quantity H/u() is called the plate time, which is the time for a theoretical plate to develop and is indicative of the speed of the separation, with units of seconds. In the Poppe plot, a number of parameters including the maximum allowable pressure drop, particle diameter, viscosity, flow resistance, and diffusion coefficient are held constant. 

The properties of a fractionating column which are important for isotope separation are (1) the throughput or boil-up rate which determines production (2) HETP (height equivalent per theoretical plate) which determines column length (3) the hold-up per plate which determines plant inventory and time to production (4) the pressure drop per plate which should be as small as possible. The choice of a particular column is invariably a compromise between these factors. The separation in a production column is of course less than it would be at total reflux (no product withdrawal). The concentration at any point in the enriching section can be calculated from the transport equation (see, e.g., London 1961)... 

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