Column particle shape

Spherical microparticles have been preferred in modem column technology since they form more hMiogeneous, stable and permeable column beds. Irregular microparticles are less expensive and still widely used, largely because in practice, it has not been shorn that their properties are significantly inferior. Particle shape may become more important as the particle size is reduced, and spherical microparticles are considered superior for particle dieuaeters less than 5 micrometers [33]. 

Practical Aspects There are a number of process-specific concerns that are accounted for in good design. In regenerate systems, sorbents age, losing capacity because of fouling by heavy contaminants, loss of surface area or crystallinity, oxidation, and the like. Mass-transfer resistances may increase over time. Because of particle shape, size distribution, or column packing method,... 

For example for the preparation of a 15 cm long, 4.6 mm i.d. stainless tube column, 2.5 g of octadecyl-bonded silica gel was suspended in 25 ml of hexanol-methanol mixture, and kept in an ultrasonic bath for a few minutes to remove air. After the reservoir was filled with the slurry, methanol was pumped in at 10 ml min -1 under constant pressure, 45 MPa (450 bar). After the replacement of slurry solvent by methanol, the flow was stopped and the pressure allowed to drop. When 0 MPa was reached the reservoir was removed. Then, 20 ml of water was added and methanol was again pumped in under the same conditions as before. Again, the flow was stopped and the pressure allowed to drop until it reached 0 MPa. The pre-column was removed and the analytical column closed. The maximum pressure that can be applied in the filling stage is based on the pore size, particle shape, and purity of the silica gel. This reproducible packing procedure is performed at constant temperature by using a water bath (60-BO °C). 

X depends on the irregularity of the particles (particle shape) and on the column material (steel gives more disturbance than glass) the relative effect will always increase as the column diameter decreases. The above-mentioned effects have led to an optimum internal diameter of 2-5 mm for LC columns. In a column with this internal diameter, which is uniformly packed with small spherical particles, the eddy diffusion will be limited. The X value is about 1 for spherical particles. 

Variation in column packing material from different manufacturers can be significant yet it is not always feasible to specify a given manufacturer in a method, due to problems such as availability. Therefore, a variety of column manufacturers were selected as a factor in the ruggedness test. These columns and their specifications are shown in Table 5.22, all columns had a specified particle size of 5pm and a spherical particle shape. 

The effective viscosity depends on the solid hold-up, on particle size and distribution, on the surface properties, on the particle shape and density, on the properties of the liquid (p, p, d), on temperature, and the shear stress in the column. Depending on the solid concentration encountered in BSCR, we can classify the suspensions into "dilute" and "concentrated" groups. 

Column packing Brand name Chemical composition Particle shape... 

Several studies have been conducted on particle capture by collectors of different shapes. Among the first studies were those of Albrecht (6), and Kaufman (7), who investigated the capture of airborne particles flowing past simple collector geometries. In packed columns, particle capture can be quantified in terms of the filter coefficient, X, defined by ... 

Most geometry-related properties of packing materials are related to the column efficiency and fiow resistance particle size, particle shape, particle size distribution, packing density, and packing uniformity. Surface-chemistry-related properties are mainly responsible for the analyte retention and separation selectivity. 

It is most often prepared by acid hydrolysis of sodium silicate followed by emulsification in an alcohol water mixture and subsequent condensation to give solid silica gel. This is then washed and dried for use as HPLC column packing. The exact conditions under which these procedures are carried out (e.g. pH, catalysts, temperature) will affect the properties of the resulting material. The most important qualities with regard to the chromatographic performance of the gel are the average particle size, the particle shape, the specific surface area and the pore size. Other factors which are also important are the pH of the gel surface, the number of active silanol groups and the presence of metal ions. 

We conducted cold flow model experiments in a air-water/glycerin system to investigate a cause of maldistribution in a catalyst bed. The apparatus used was a 30 cm I.D. acrylic column equipped with a liquid distributor at the top and a liquid collector with 33 compartments at the bottom. Bed depth can be varied by combining the pipes. Liquid distribution at a given depth of the bed was estimated by measuring the liquid flow from each compartment of the collector. We examined effects of gas and liquid velocity, liquid viscosity, particle shapes, and ways of catalyst loading on liquid distribution in the bed. An increase in liquid velocity or viscosity slightly improved liquid distribution. However, gas flow rate did not affect liquid distribution. 

The catalyst particle sizes used in RD are usually of 1-3 mm range to avoid intraparticle diffusion limitations. To overcome the flooding limitations, the catalyst particles are contained within wire gauze envelopes. Most commonly, the catalyst envelopes are packed inside the column. Various shapes of catalyst envelopes have been patented. Some of these structures include ... 

Even within the best supports available there wiU be some irregularities in the particle shapes which will lead to non-uniform channels through which the molecules will permeate. Optimisation of the physical parameters of the column, the type of stationary phase and the method of packing are normally directed at overcoming these effects. 

Particle shape and size is there a need for greater column efficiencies and faster chromatography ... 

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