Collecting Plate Height

Figure 6.39 Parallel extraction of 96 samples with an Empore 96 individual disk SPE plate (A) and vacuum manifold system, including an acrylic vacuum manifold top (.B), a polypropylene deep-well collection plate (C), two shims (D) for height adjustment of the collection plate, and a Delrin vacuum manifold (E) bottom. (Reprinted with permission from Simpson et al., 1998. Copyright 1998 John Wiley Sons.)...

When the terms contributing to band broadening are collected and equilibrium spreading is assumed to be negligible, the following general expression for overall plate height is obtained ... 

Recall that one measure of the efficiency of a chromatographic column is the number of theoretical plates. This analogy is drawn from distillation The more collection plates in the column, the more effective is the separation. The maximum number of plates (N) is achieved when the distance between them (i.e., the smallest possible height of a plate, HETP) is minimized. 

Tilted-Plate Clarifiers Lamella or tilted-plate separators have achieved increased use for clarification. They contain a multiplicity of plates inchned at 45 to 60° from the horizontal. Various feed methods are employed so that the influent passes into each inchned channel at about one-third of the vertical height from the bottom. This results in the solids having to settle only a short distance in each channel before sliding down the base to the collection zone beneath the plates. The clarified liquid passes in the opposite direction beneath the ceiling of each channel to the overflow connection. 

The filter structure consists of a stack of plates attached to a hollow shaft which are mounted inside a pressure vessel with each plate covered with a suitable filter medium. The slurry is fed under pressure into the vessel and the cake, which is retained by the filter medium, forms on the top of each plate whilst the filtrate passes through the hollow shaft further to the process. Filter sizes may vary but generally the maximum is 60 m area and designed for a 6 bar operating pressure. Each circular plate in the stack is constructed with radial ribs that are welded to the bottom and support a horizontal coarse mesh screen which is covered with a finer woven metal screen or filter cloth to retain the cake. The bottom of the plate slopes towards the hollow central shaft which lets the filtrate flow freely through circumferential holes and further down the shaft to the filtrate outlet. The clearance between the plates is maintained by special spacers with "o" rings to positively seal between the slurry that surrounds the plates and the shaft that collects the filtrate. The height of the spacers determine the clearance for cake build-up and may be replaced to meet various process conditions. 

Concentration in the sample (c). This is normally calculated using both peak areas and peak heights as it is a good idea to postpone the selection of a calibration technique until after the ruggedness study. Mean number of theoretical plates, N, there are several methods to calculate N. The following calculation is often employed due to its convenience as it uses values which are previously collected as part of the data handling. 

The liquid begins to flow into the top plate and collects on the plate because of the retention made by the vapour flow. When the liquid level passes the weir height (thus filling the holdup), the liquid begins to fall to the plate below and the same phenomenon is repeated until the reboiler is reached. In practice some liquid also trickles down from the plate holes when the flows are initially established. 

---