Dispersion size methods

Catalytic treatment of supported metals can lead to a change in the metal surface area. Therefore it is necessary to obtain the metal dispersion (size) as a function of the reaction conditions. EM methods including HRTEM, ETEM,... 

Walstra, P., Oortwijn, H. and Degraaf, J. J. 1969. Studies on milk fat dispersion. I. Methods for determining globule-size distribution. Neth. Milk Dairy J. 23, 12-36. 

Dispersion and Dispersed Species Characterization Table 2.1 Classification of some size methods. 

The gas dispersion tube technique is much easier to assemble and use than the dialysis membrane/wire mesh basket assembly, but it gives similar release profiles for 5-FU from EHCF or copolymers of EHCF with either HA or HHA. The gas dispersion tube method only requires 0.1 gram of material for an accurate kinetic profile of these prodrugs. The small gas dispersion tubes are useful for studies involving powders, but pellets would not fit into these tubes. The large size gas dispersion tubes or the wire basket technique could be used for powders or pellets, and can accommodate larger sample sizes. In summary the gas dispersion tube technique is reproducible, easy to assemble, easy to use, can distinguish polymers with different release rates, and the results can be correlated with our earlier studies. 

Several physical methods may be used to provide indirect estimates of the degree of dispersion. Sizes of particles, or of single crystallites, or of magnetic domains determined, respectively, by electron microscopy, X-ray line broadening, and magnetization measurements, can be used for this purpose but, in all cases, assumptions must be introduced into the calculations. 

The paifoimance of coalescers cannot be predicted theoretically and sech systems require experimental work to determine sizing, uamg the same media, flow rate per area of media, and dispersion preparation method, Tests need to be run over sufficient time to assure thet the media is folly loaded with the dispersed phase and that steady-state operation has been obtained. Periods up to 2 h may be needed to assure complete loading. A material balance of disparsed phase in and out of the system is a good check. 

The present work deals with sintering of Ni/AbOa catalysts under reducing and steamreforming reaction conditions. The effects of preparation method (impregnation and coprecipitation), lanthanum oxide promoter, oxide phases developed after calcination, sintering temperature and atmosphere were studied in terms of the time evolution of metal dispersion, size distribution properties and kinetic parameters obtained from a GPLE model. 

The polymerization of styrene to produce uniform microspheres with diameters of 1 jxm or larger in a single step has been a challenge. The use of conventional suspension polymerization techniques are unsatisfactory since they invariably lead to a very broad size distribution. The use of dispersion polymerization methods, i.e., polymerizations in media with little or no water, is promising, albeit many interrelated factors have to be controlled for success. Reference [85] reviewed this field and cites at least 16 leading references. 

For the production of polymer particles in particular, a range of devices has been designed to produce uniform droplets that are then polymerized into solid particles. Such devices are dedicated to the formation of uniform droplets of a predetermined size and dispersity. The method employed involves two streams of immiscible liquids that are brought together at a point in the device where the shear forces induce droplet formation or emulsification. Some examples of droplet-generating systems are shown in Figure 14.2. 

MMA was polymerized heterogeneously to very high conversions (>90%) and high degrees of polymerization (>3000 g/mol) in SCCO2. These polymerizations were conducted at 65°C and 20.7 MPa (207 bar) CO2 with AIBN or fluo-rinated derivatives of AIBN as initiators and PFOA (M = 1.1 x 10 or 2.0 x 10 g/mol) as the surfactant. The reactions resulted in kinetically stable dispersions of micrometer-sized particles with a narrow size distribution. In contrast, reactions that were performed without PFOA as a stabilizer resulted in precipitated polymer with an unstructured, nonspherical morphology. Several other monomers were also polymerized by dispersion polymerization methods in CO2 (Table 1). 

The ISO working group ISO/TC 24 Sieves, Sieving, and Other Sizing Methods is engaged in elaborating methods for the surface characterization of dispersed materials and just issued an ISO standard on the BET method. 

Recently, we produced monosize polyalkylcyanoacrylate based polymeric particles in micron-size range by a dispersion polymerization method, as potential carriers for diagnosis and therapy. Details of polymerization and degradation kinetics were discussed in detail elsewhere (79). Here, the preparation procedure and degradation behaviour of the polyethylcyanoacrylate (PECA) particles are briefly presented. 

There are many particle-sizing methods as summarized in Table 21.4. Each method has advantages and disadvantages. The method of choice depends on the specific type of powder or dispersion sample to be characterized. The main criteria for evaluating the quality of the particle size results are accuracy, precision, reproducibility, and speed of analysis. 

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