Pressure drop, packings average

Permeability is another method for obtaining information about pcirticle diameters. If one packs a tube with a weight of powder exactly equal to its density, and applies a calibrated gas pressure through the tube, the pressure drop can be equated to an average particle size. The instrument based on this principle is called the "Fisher Sub-Sieve Sizer ". Only one value can be obtained but the method is fast and reproducible. The instrument itself is not expensive and the method can be applied to quality control problems of powders. Permeametry is usefiil in the particle range of 0.5 to 50 n. 

Equation (I.IS) Is valid for open tubular columns under all normal conditions and for packed columns at low mobile phase velocities. The average carrier gas velocity is calculated from the outlet velocity by correcting the latter for the pressure drop across the column, and is simply given by u - ju, where j is the gas compressibility correction factor, defined In equation (1.2). 

The physical properties of silica are determined by its specific surface area, pore volume, average pore diameter, porosity, and the particle diameter and shape [8]. The latter two are responsible for the efficiency, the physical stability and the pressure drop of the packed columns and do not contribute to retention and selectivity. 

By backward calculation from the GPDC correlation, based on average performance of the packing at 0.5, 1.0, and 1.5 in of water pressure drop per foot of packed depth (1). This method proved un-... 

For computer calculations, a packed bed can be divided into several intervals. The bed pressure drop is the sum of all the interval pressure drops. Alternatively, the specific pressure drop can be calculated at the top of the bed and at the bottom of the bed. The average specific pressure drop is then calculated from (15)... 

Packed height = 3 ft Gas rate = 100 lb/h Liquid rate = 90 lb/h Average molecular weight of gas = 100 Average temperature = 110°F Total pressure drop = 12 mm Hg Liquid viscosity = 1.2 centipoises Liquid density = 55 lb/ft3... 

Majority of packing materials used in HPLC are porous particles with average diameters between 3 and 10 pm. For most pharmaceutical applications, 3-pm particle sizes are recommended. Porosity provides the surface area necessary for the analyte retention (usually between 100 and 400m /g). Interparticle space is large enough to allow up to 1-3 mL/min flow within acceptable pressure range (however, the pressure drop across the column depends on the particle size, length of column, temperature of separation, and type of mobile-phase composition). 

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