Preparative Chromatography - Packing Preparative Columns > LC Columns > Page 35
Figure 16 Reduction in Bed Stabilization by Wall Effects in Preparative Columns
The lack of wall support makes the whole bed 'active' in that it is not only free to rearrange but also particles (usually the very small 'fines' in the packing) can move through the bed. The rearrangement of the more poorly packed areas results in the formation of voids or channels in the packed bed, while the migration of fines results in sudden increases in operating pressure with time which are sometimes a feature of large columns. The problem of bed stability in large diameter columns has been addressed by use of technologies which support the packed bed and allow changes in bed volume to be accommodated without effecting column efficiency. For production columns, the compensation for the variation of bed volume must be automatic, since it is clearly poor practice to wait for a problem to arise before taking avoiding action. Two techniques are in wide use, both relying upon some form of compression system to preserve the bed lifetime the radial compression column and the longitudinal compression column. The principles of both techniques are schematically shown in figures 17 and 18.