Packed columns limiting flow rates

The amount of resin to pack in a column, column geometry, flow rates, pressure, column hardware, and wetted materials of construction should all be evaluated in development. Chromatography columns must be properly packed prior to validating the purification process. From a business perspective there should be some criteria other than purification of the product by which the quality of the packed column can be assessed prior to applying the feedstream, which by this time in the process is quite expensive. Height equivalent to a theoretical plate (HETP) and asymmetry determinations can be used to evaluate the quality of column packing, but may have limited value for some types of packed columns... 

According to eq.(6), the specific resolution in SEC is governed by both the slope D2 of the log - linear calibration curve and the peak dispersion of the solutes. The product of both quantities should approach a minimum. As already pointed out, the slope D2 is inversely proportional to gp of the column, i.e. the larger the intraparticle porosity of a column, the smaller D2. The upper limit of Ep is obviously dictated by the mechanical stability of the support when it Is subjected to pressure of 30 MPa during packing at high flow rates. 

The main disadvantage of smaller particles is the increased back-pressure during the operation of HPLC systems. The pressure can be calculated according to Darcy s law and is inversely proportional to the square of the particle size. For example, a 33 X 4.5mm column packed with 1.5pm nonporous silica particles needed a pressure of approximately 500 bar (the Umit for most commercial pumps in HPLC units) at a flow rate of 2ml min with acetonitrile and water. A general relation between particle size, pressure drop, plate number, and analysis time is provided in Fig. 7. The assumed specific conditions for viscosity, analyte diffusivity, retention factor, and other parameters are given in the legend. Fast analysis times combined with a limited flow rate also necessitates the need for fast detector systems, small volume detection cells (small volume injection loops, yet, all these challenges have been successfully resolved. 

The orifice-riser distributor is designed to lay the hquid carefully onto the bed, with a minimum of contact with gas during the process. It can be designed to provide a large number of liquid streams, with the limit of sufficient liquid head to provide uniform liquid flow through the orifices. The gas risers must oe designed to accommodate the expected variations in flow rate, often with a minimum of pressure drop. For veiy distribution-sensitive packings, it is necessaiy to include pour points in the vicinity of the column wall (to within 25 mm). 

Satisfactory operation must be between the upper and lower limits for both liquid and vapor flow rates. At liquid rates below 0.5 GPM per square foot of packing cross-section, liquid distribution is not uniform enough to ensure thorough wetting. At liquid rates between 25 GPM and 70 GPM per square foot of packing, the column is considered liquid-loaded and becomes very sensitive to additional liquid or vapor flow. 

Implementation of SFC has initially been hampered by instrumental problems, such as back-pressure regulation, need for syringe pumps, consistent flow-rates, pressure and density gradient control, modifier gradient elution, small volume injection (nL), poor reproducibility of injection, and miniaturised detection. These difficulties, which limited sensitivity, precision or reproducibility in industrial applications, were eventually overcome. Because instrumentation for SFC is quite complex and expensive, the technique is still not widely accepted. At the present time few SFC instrument manufacturers are active. Berger and Wilson [239] have described packed SFC instrumentation equipped with FID, UV/VIS and NPD, which can also be employed for open-tubular SFC in a pressure-control mode. Column technology has been largely borrowed from GC (for the open-tubular format) or from HPLC (for the packed format). Open-tubular coated capillaries (50-100 irn i.d.), packed capillaries (100-500 p,m i.d.), and packed columns (1 -4.6 mm i.d.) have been used for SFC (Table 4.27). 

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