Dynamic axial compression columns

Colin, H., Hilaireau, P., and de Tournemire, J., Dynamic axial compression columns for preparative high performance liquid chromatography, LC-GC, 8, 302, 1990. 

Correct column packing is obtained by using preparative columns equipped with dynamic axial compression (DAC) technology [10, 11]. The column includes a movable piston attached to a hydraulic jack. The piston is used to pack and unpack the column and to maintain the stationary phase under dynamic compression, ensuring perfect particle stacking (Figure 12.5) and bed stability through time. 

Analytical columns are packed by the slurry technique, where a dilute suspension of the packing is pumped at a high flow rate and a high pressure through the column. Particles are retained by a porous frit at the end of the column. Preparative columns with dp between 10 and 30 xm are, preferably, packed by the dynamically axial compression technique (Unger, 1994). The column contains a movable piston that keeps the packing under an external pressure during operation. The operation pressure should be always less than the piston pressure. 

Decisive parameters in the dynamic axial compression technique with respect to optimum column performance and column stability are ... 

Table 3.15 Column performance of several types of silicas packed into dc = 50 mm columns by the dynamic axial compression technique (Marme, 1991).

Column packing - dynamic axial compression technology... 

Figure 11.5. Schematic diagram of continuous compression columns for high-pressure Preparative-scale liquid chromatography. The arrows represent the force directions. A = radial compression, B = dynamic axial compression, and C = annular expansion.

Electrically or hydraulically moved piston In terms of column technology, there is a consensus today that the best (and probably only) feasible approach at any size is dynamic axial compression (DAC). The piston can be driven by a hydraulic jacket, or pushed by a liquid, by a spring, or by an electrical motor. Radial compression is an alternative but it does not seem to be available at (very) large column diameter sizes. It also requires the use of prepacked cartridges, thus reducing the operator s freedom compared to DAC. Under appropriate conditions, the DAC technology ensures bed stability and reproducible performance. DAC can be scaled-up to very large size with remarkable reproducibility in performance. This has been verifled with columns up to 1600 mm internal diameter. 

A series of pharmaceutical active ingredients and intermediates are produced industrially by means of SMB separations (see Table 7). The quantities of enantiomericaUy pure compounds isolated by this approach currently amount to several hundred tonnes per annum. On a production scale, SMB systems consist of five or six dynamic axially compressed H PLC columns with internal diameters up to 1 m. 

The use of the so-called flip-flop technique [13] in which the direction of flow in the column is reversed and the sample is injected alternately at either end of the column has been demonstrated to provide higher throughputs with reduced solvent consumption when used with elution chromatography. This approach requires the use of a column such as the Dynamic Axial Compression system manufactured by Prochrom, Champigneulles, France, where the stability of the packed bed is unaffected by the direction of flow. 

It is now well accepted that, for liquid chromatography, the obtention of large diameter, very efficient and stable columns requires the dynamic axial compression technology. The similarity between the HPLC and SFC columns and stationary phases suggest that the dynamic axial compression is the good solution and indeed efficiencies of 63 000 plates per meter were obtained with a good stationary phase. 

---