Supersaturation mixing

In order for an inclusion to form, the crystal-liquid interface must break up and grow in an unstable manner (Slaminko and Myerson 1981). Instability of the interface can be related to a number of factors including growth rate, mixing, supersaturation, and presence of certain impurities. In the latter case, impurities have been found, in different instances, to both promote and suppress inclusion formation (Senol and Myerson 1982), hence impurities in the crystallizer are often, but not always, responsible for this undesirable growth behavior. 

Another type of crystallizer is the Oslo-type unit shown in Figure 24. In units of this type, the object is to form a supersaturated solution in the upper chamber and then reHeve the supersaturation through growth in the lower chamber. The use of the downflow pipe in the crystallizer provides good mixing in the growth chamber. 

Mixing of two saturated streams at different temperatures. This is commonly seen in the plume from a stack. Since vapor pressure is an exponential function of temperature, the resultant mixture of two saturated streams will be supersaturated at the mixed temperature. Uneven flow patterns and cooling in heat exchangers make this route to supersaturation difficult to prevent. 

These mechanisms can be observed in many common situations. For example, fog via mixing can be seen in the discharge of breath on a cold day. Fog via adiabatic expansion can be seen in the low-pressure area over the wing of an airplane landing on a humid summer day and fog via condensation can be seen in the exhaust from an automobile air conditioner (if you follow closely enough behind another car to pick up the ions or NO molecules needed for nucleation). All of these occur at a veiy low supersaturation and appear to be keyed to an abundance of foreign nuclei. All of these fogs also quickly dissipate as heat or unsaturated gas is added. 

When a process is continuous, nucleation frequently occurs in the presence of a seeded solution by the combined effec ts of mechanical stimulus and nucleation caused by supersaturation (heterogeneous nucleation). If such a system is completely and uniformly mixed (i.e., the product stream represents the typical magma circulated within the system) and if the system is operating at steady state, the particle-size distribution has definite hmits which can be predic ted mathematically with a high degree of accuracy, as will be shown later in this section. 

In accordance with these data, ionic associates (lA) can be precipitated at phosphate concentrations more than 10 M. Below this concentration stabile supersaturated solutions of lA are formed. Colour of lA appeal s immediately after mixing of the solutions and remains constant during several hours. There is a new band in spectmm at 570-590 nm. Appearance of color is caused by formation of stable solid phase in the solution. 

At pressures and temperatures above the eritieal point, where liquid and vapour phases beeome indistinguishable, supereritieal fluids (SCFs) exhibit very different properties to those of the liquids or gases at ambient. Partiele formation in SCFs oeeurs as a result of a rapid inerease in supersaturation, either by means of expansion or by antisolvent mixing proeesses. Thus Chang and Randolph (1989) demonstrated that small (<1 pm) uniform partieles of... 

Solution enters the vessel (Figure 3.4) and is well-mixed throughout i.e. all eonditions - temperature, eoneentration, veloeity, turbulenee ete. are uniform (homogeneous). Supersaturation is generated (by evaporation, eooling, ete.) and nuelei form and grow into erystals. Sinee erystals have varying probabilities of residenee time in the vessel, however, the slurry exhibits a erystal size distribution (CSD). Produet slurry is eontinuously withdrawn and has exaetly the same eomposition as the vessel. 

A secondary particle formation process, which can increase crystal size dramatically, is crystal agglomeration. This process is particularly prevalent in systems exhibiting high levels of supersaturation, such as from precipitation reactions, and is considered along with its opposite viz. particle disruption in Chapter 6. Such high levels of supersaturation can markedly accentuate the effects of spatial variations due to imperfect mixing within a crystallizer. This aspect is considered further in Chapter 8. 

Thus in a mixed system, as e.g. in a stirred tank, the rate of agglomeration additionally depends on the shear field and therefore on the energy dissipation e in the vessel. Furthermore, in precipitation systems solution supersaturation plays an important role, as the higher the supersaturation, the stickier the particles and the easier they agglomerate (Mullin, 2001). This leads to a general formulation of the agglomeration rate... 

In the SFM the reactor is divided into three zones two feed zones fj and (2 and the bulk b (Figure 8.1). The feed zones exchange mass with each other and with the bulk as depicted with the flow rates mi 2, i,3 and 2,3 respectively, according to the time constants characteristic for micromixing and mesomix-ing. As imperfect mixing leads to gradients of the concentrations in the reactor, different supersaturation levels in different compartments govern the precipitation rates, especially the rapid nucleation process. 

The reactor has been successfully used in the case of forced precipitation of copper and calcium oxalates (Jongen etal., 1996 Vacassy etal., 1998 Donnet etal., 1999), calcium carbonate (Vacassy etal., 1998) and mixed yttrium-barium oxalates (Jongen etal., 1999). This process is also well adapted for studying the effects of the mixing conditions on the chemical selectivity in precipitation (Donnet etal., 2000). When using forced precipitation, the mixing step is of key importance (Schenk etal., 2001), since it affects the initial supersaturation level and hence the nucleation kinetics. A typical micromixer is shown in Figure 8.35. 

The triethylamine salts of peptide acids are often relatively insoluble in acetonitrile or nitromethane therefore, the supersaturated solution formed on mixing the amine and the acid should be added to this reaction mixture immediately, before crystallization occurs. If crystallization does occur, the mixture should be heated to dissolve the salt, cooled rapidly, and added to the reaction mixture immediately. If it is impossible to obtain a solution of the salt, the peptide acid and then the triethylamine solution may be added separately to the reaction mixture with only a small sacrifice in yield. 

Deposition commonly reflects a combination of physicochemical processes and localized effects. It may occur through fouling as a result of contamination by process materials, perhaps plus scaling from the supersaturation of dissolved salts, and coupled with some active under-deposit corrosion. As a consequence, deposits forming within a boiler are almost never single mineral scales but typically consist of a variable mix of scale and corrosion debris, chemical treatment residuals, process contaminants, and the like. 

The gas formed by mixing two unsaturated gases may be either unsaturated, saturated, or supersaturated. The possibility of producing supersaturated gas arises because the 100 per cent humidity line on the humidity-enthalpy diagram is concave towards the humidity... 

Supersaturation can also be achieved by adding a liquid that is miscible with the solvent and decreases the solubility of the solute in the mixed solvent. This is called precipitation. In fine chemicals manufacture, the solid is usually dissolved in an organic solvent and water is used as the desalting agent. Precipitation also occurs when a solid product, which is insoluble in the reaction mixture, is formed by chemical reaction. For instance, a phenolic product can be purified by three possible routes ... 

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