Precipitation and crystallization

Eor products having relatively low specific activity, such as some compounds labeled with and which are synthesized on the scale of several millimoles, classical organic chemical separation methods may be utilized, including extraction, precipitation, and crystallization. Eor separation of complex mixtures and for products having high specific activity, such as those labeled with tritium, etc, chromatographic methods utilizing paper, thin... 

Sludge-blanket clarifiers are difficult to start up because the first blanket must be estabUshed, and large-scale units require extensive excavation. Sizes range from 600 x 600 mm to 50 x 50 m. Precipitation and crystallization can be carried out in similar hopper-designed units, having overflow rates of 80 m/h or higher. 

Scale. Scale deposits are formed by precipitation and crystal growth at a surface in contact with water. Precipitation occurs when solubiUties are exceeded either in the bulk water or at the surface. The most common scale-forming salts that deposit on heat transfer surfaces are those that exhibit retrograde solubiUty with temperature. 

Dyes are synthesized in a reactor, then filtered, dried, and blended with other additives to produce the final product. The synthesis step involves reactions such as sulfonation, halogenation, amination, diazotization, and coupling, followed by separation processes that may include distillation, precipitation, and crystallization. 

Various theories have been postulated to explain the phenomenom that magnetic fields can change the way in which calcium carbonate precipitates and crystallizes, the most probable one being that the applied field distorts the electrical charge carried by small particles of calcium carbonate that have already formed in the water, thus affecting the way in which they adhere to form large particles. 

When salts in groundwater precipitate and crystallize within the cavities of buried materials such as pottery, cement, and wood, they may generate internal pressures sufficient to disrupt these materials and turn them into gravel. Salts are also active in blistering and scaling painted surfaces on a variety of materials. 

Erudc acid is a constituent of various natural oils but is most conveniently obtained from rape seed oil. The process described above is essentially that of Reimer and Will.1 Methods have been developed for obtaining pure erudc add free from saturated acids,2 but these involve time-consuming procedures of fractional precipitation and crystallization, and necessarily give poor yields. The product obtained above is satisfactory for most purposes. 

Figure 4.22 Slurry reactor synthesis of malic acid from fumaric acid applying a batch process followed by precipitation and crystallization...

The counterparts of dissolving particles are the processes of precipitation and crystallization the description and simulation of which involve several additional aspects however. First of all, the interest in commercial operations often relates to the average particle size and the particle size distribution at the completion of the (batch) operation. In precipitation reactors, particle sizes strongly depend on the (variations in the) local concentrations of the reactants, this dependence being quite complicated because of the nonlinear interactions of fluctuations in velocities, reactant concentrations, and temperature. 

Certain isolated aspects of precipitation and crystallization which have successfully been studied by means of LES, are discussed below to illustrate the progress being made in the field of precipitation and crystallization. The LB... 

Alkali ions (salts) influence the formation of the precursor gel for most of the synthetic zeolites (3,34,39,40). Na+ ions were shown to enhance in various ways the nucleation process (structure-directing role) (40-42), the subsequent precipitation and crystallization of the zeolite (salting-out effect) (JO and the final size and morphology of the crystallites (34,43). Informations on the various roles played by the inorganic (alkali) cations in synthesis of ZSM-5, such as reported in some recent publications (7,8,10,14,17,29,30,44,45) remain fragmentary, sometines contradictory and essentially qualitative. 

A better insight into the mechanisms of the individual steps in the formation of crystals would be of great help in explaining the creation and transformation of sedimentary deposits and biological precipitates. Valuable reviews are available on the principles of nucleation of crystals and the kinetics of precipitation and crystal growth (Zhang and Nancollas, 1990 Steefel and Van Cappellen, 1990 Van Cappellen, 1991). Only a few important considerations are summarized here to illustrate the wide scope of questions to be answered in order to predict rates and mechanisms of precipitation in natural systems. 

Preparation of a-Nitroguanidine. Five hundred cc. of concentrated sulfuric acid in a 1-liter beaker is cooled by immersing the beaker in cracked ice, and 400 grams of well-dried guanidine nitrate is added in small portions at a time, while the mixture is stirred with a thermometer and the temperature is not allowed to rise above 10°. The guanidine nitrate dissolves rapidly, with very little production of heat, to form a milky solution. As soon as all crystals have disappeared, the milky liquid is poured into 3 liters of cracked ice and water, and the mixture is allowed to stand with chilling until precipitation and crystallization are complete. The product is collected on a filter, rinsed with water for the removal of sulfuric acid, dissolved in boiling water... 

Most of the silicon component feed has been already involved in this precipitate, and crystallizations occurs internally inside the solid phase of the precipitate. The precipitate itself is the precursor of the crystal. In fact, seed materials have no enhancement effect on the crystal growth as described above. Therefore, the following three hypotheses were considered and trials were made to substantiate them. 

Due to the insolubility of the polymer in the monomer, the formed polymer precipitates and crystallizes during the polymerization within the droplets ca. 10-nm large polymer nanocrystals are formed. Pure PAN latexes have a crumpled appearance where the single polymer nanocrystals remain in the final structure and are easily identified by their sharp edges and flat surfaces. 

The structure of aluminate in aqueous solution is gradually decomposed and the moiety of the network structure of aluminate, aluminum tetrahydroxide, and the hydrates occur as the predominant chemical species in the aqueous solution. The structure of chemical species in aqueous solution changes from fourfold coordination structure to sixfold coordination structure with H20 molecules. At this time, polymerization begins to proceed. Hexaaquaaluminum cation serves as a core to form the network structure of Al(OH)3 by the progress of polymerization. Finally, Al(OH)3 precipitates and crystallizes. 

The particle size of precipitated potassium heptafluorotantalate is one of the more important parameters. In order to achieve a certain particle size, potassium salts are added to the hot tantalum strip solution as a hot solution. The mixture is cooled down at a specific rate in order to enable the precipitation and crystallization of K-salt in the form of small, individual crystals. 

Catalyst. One catalyst, prepared to contain equi-atomic amounts of lead, magnesium, and aluminum, was prepared for this study. Synthesis was accomplished by the hydrotalcite preparation procedure reported by Reichle 12). It is a standard aqueous precipitation and crystallization procedure that avoids filtering and washing problems associated with gel precipitates. A solution of Pb, Mg, and Al nitrates was added under agitation to a solution of NaOH and Na2C03 and maintained at 70 C for thirty hours. This caused precipitation of crystalline material. 

Many solids-handling operations have an effect on the particle size distribution (PSD) of the solid phase. The particle size distribution can also be an important product property. Aspen Plus allows the user to enter a particle size distribution as an attribute of a solid substream. In UniSim Design, the particle size distribution is entered on the PSD Property tab, which appears under worksheet on the stream editor window for any stream that contains a pure or hypothetical solid component. Unit operations such as yield-shift reactor, crusher, screen, cyclone, electrostatic precipitator, and crystallizer can then be set up to modify the particle size distribution, typically by using a conversion function or a particle capture efficiency in each size range. 

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