Diameter packing particle

Re = R nolds number, dpS UolV Sc = Schmidt number, V/D D = axial dispersion coefficient dp = Diameter of particle or empty tube = Fraction voids in packed bed Uq = Superficial velocity in the vessel. 

The type of distribution to select depends on the sensitivity of the tower performance to the liquid distribution as discussed earlier. Norton s [83] data indicate that the sensitivity of tower performance to liquid distribution quality depends only on the number of theoretical stages in each bed of packing achierable at its System Base HETP [83]. Tower beds of high efficiency packing are more sensitive to liquid distribution quality than shorter beds of medium efficiency packing [83]. It is important to extend the uniformity of the distributor all the way to within one packing particle diameter of the tower wall [85]. 

Glaser and Lichtenstein (G3) measured the liquid residence-time distribution for cocurrent downward flow of gas and liquid in columns of -in., 2-in., and 1-ft diameter packed with porous or nonporous -pg-in. or -in. cylindrical packings. The fluid media were an aqueous calcium chloride solution and air in one series of experiments and kerosene and hydrogen in another. Pulses of radioactive tracer (carbon-12, phosphorous-32, or rubi-dium-86) were injected outside the column, and the effluent concentration measured by Geiger counter. Axial dispersion was characterized by variability (defined as the standard deviation of residence time divided by the average residence time), and corrections for end effects were included in the analysis. The experiments indicate no effect of bed diameter upon variability. For a packed bed of porous particles, variability was found to consist of three components (1) Variability due to bulk flow through the bed... 

Glaser and Litt (G4) have proposed, in an extension of the above study, a model for gas-liquid flow through a b d of porous particles. The bed is assumed to consist of two basic structures which influence the fluid flow patterns (1) Void channels external to the packing, with which are associated dead-ended pockets that can hold stagnant pools of liquid and (2) pore channels and pockets, i.e., continuous and dead-ended pockets in the interior of the particles. On this basis, a theoretical model of liquid-phase dispersion in mixed-phase flow is developed. The model uses three bed parameters for the description of axial dispersion (1) Dispersion due to the mixing of streams from various channels of different residence times (2) dispersion from axial diffusion in the void channels and (3) dispersion from diffusion into the pores. The model is not applicable to turbulent flow nor to such low flow rates that molecular diffusion is comparable to Taylor diffusion. The latter region is unlikely to be of practical interest. The model predicts that the reciprocal Peclet number should be directly proportional to nominal liquid velocity, a prediction that has been confirmed by a few determinations of residence-time distribution for a wax desulfurization pilot reactor of 1-in. diameter packed with 10-14 mesh particles. 

A column 3 cm long, 4.6 mm in diameter packed with particles 3 Jim in diameter will give about 6,000 theoretical plates at the optimum velocity. This efficiency is typical for a commercially available column. 

The second valve controls a sample loop, 5 cm long and 1 mm in diameter, packed with dimethyloctadecyl reverse phase comprising of fairly coarse particles 100-120 im in diameter to reduce flow impedance. The sample pump is supplied via a two-way tap from either of two reservoirs, one containing pure water and the other, normal saline. The output of the pump can be used to either force the contents of the open loop sample tube through the packed loop, or to permit washing with an appropriate solvent. The separate pump is necessary to overcome the impedance of the packed loop. 

The column used in the separation depicted in figure 1 was 25 cm long and 6.2 mm in diameter packed with silica gel having a mean pore diameter of 100 A and a particle diameter of 5 jum. Thus, the column would have an HETP of approximately 0.001 cm (twice the particle diameter). Consequently, a column 25 cm long would have an 25... 

Another example of the use of a C8 column for the separation of some benzodiazepines is shown in figure 8. The column used was 25 cm long, 4.6 mm in diameter packed with silica based, C8 reverse phase packing particle size 5 p. The mobile phase consisted of 26.5% v/v of methanol, 16.5%v/v acetonitrile and 57.05v/v of 0.1M ammonium acetate adjusted to a pH of 6.0 with glacial acetic acid and the flow-rate was 2 ml/min. The approximate column efficiency available at the optimum velocity would be about 15,000 theoretical plates. The retention time of the last peak is about 12 minutes giving a retention volume of 24 ml. 

The two different concepts are depicted schematically in Figure 1.15. The fixed bed is assumed to have a cross-section Sp and a height Hp and is fiUed with non-deformable spherical particles with diameter dp, where the density of the packing (particles per m ) is denoted by n-p. The micro-channel reactor has a cross-section and a height and comprises channels of diameter d with a specific density (number of channels per m ) n-j. ... 

Five types of columns are routinely used in gas chromatography classical packed columns with internal diameters greater than 2 mm containing particles in the range 100 to 250 micrometers micropacked columns having diameters less than 1 mm with a packing density similar to classical packed columns (dp/d less than 0.3, where dp is the particle diameter and d the column diameter) packed capillary lumns have a column diameter less than 0.5 mm and a packing density less than classical packed columns (dp/d 0.2-0.3) SCOT columns (support-coated open... 

Particle diameter (packed columns) (dp) Pressure drop increased with smaller particles retention time longer at same inlet pressure Smaller particles sharper peaks... 

The Carman-Kozeny equation relates the drop in pressure through a bed to the specific surface of the material and can therefore be used as a means of calculating S from measurements of the drop in pressure. This method is strictly only suitable for beds of uniformly packed particles and it is not a suitable method for measuring the size distribution of particles in the subsieve range. A convenient form of apparatus developed by Lea and Nurse 22 1 is shown diagrammatically in Figure 4.4. In this apparatus, air or another suitable gas flows through the bed contained in a cell (25 mm diameter, 87 mm deep), and the pressure drop is obtained from hi and the gas flowrate from h2. 

Very large LC columns up to 1.2 m in diameter have been used on occasion since the 1940s. These were low pressure units with very large packing particles and were therefore relatively slow and inefficient. The use of smaller particles and higher velocities, both requiring large column pressure differences (40-400 bar), has been a key factor in the development of modern HPLC since 1967 (Section 19.3.2.) The other two factors were improved stationary phases (Section 19.4.3) and better detectors. 

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