Solutions supersatured

A pilot-scale continuous MSMPR crystallizer of 10 litre capacity is used to crystallize potash alum from aqueous solution, supersaturation. This is being achieved using a 15-min residence time, a 100-ml slurry sample was taken and the crystals contained in this sample subjected to a size analysis. The results of this analysis are given below... 

This meehanism is therefore more likely to oeeur at high levels of solution supersaturation. 

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... 

Each stage of particle formation is controlled variously by the type of reactor, i.e. gas-liquid contacting apparatus. Gas-liquid mass transfer phenomena determine the level of solute supersaturation and its spatial distribution in the liquid phase the counterpart role in liquid-liquid reaction systems may be played by micromixing phenomena. The agglomeration and subsequent ageing processes are likely to be affected by the flow dynamics such as motion of the suspension of solids and the fluid shear stress distribution. Thus, the choice of reactor is of substantial importance for the tailoring of product quality as well as for production efficiency. 

It is evident that most studies reported to date have used number density, average size or weight per eent as eontrol variables. Often these variables are inferred from other measurements, ineluding density, solution supersaturation, refraetive index ete. Inferential teehniques have been shown to be partieularly suitable for industrial seale applieations where laser seattering deviees for on-line size distribution measurement are not yet praetieal for industrial eontrol purposes, although substantial progress is being made to that end. Even when usable, however, these measurement deviees are often eharaeterized by noise and require operation at very low solids eoneentration. 

In an electrochemical system, gas supersaturation of the solution layer next to the electrode will produce a shift of equilibrium potential (as in diffusional concentration polarization). In the cathodic evolution of hydrogen, the shift is in the negative direction, in the anodic evolution of chlorine it is in the positive direction. When this step is rate determining and other causes of polarization do not exist, the value of electrode polarization will be related to solution supersaturation by... 

The formation of epidote, K-feldspar, prehnite, wairakite and calcite in the geothermal area is considered to be due to the loss of CO2 gas and rapid precipitation from the solution supersaturated with respect to quartz (Browne, 1978). The widespread occurrence of these minerals in the Seigoshi district seems to be consistent with the above-mentioned consideration, namely that these minerals usually occur as veinlets rather than the replacements of original minerals and filling amygdule. In particular, many veinlets of epidote, prehnite and wairakite are found near the Au-Ag-quartz veins. 

Also, hydrates are more soluble in water-miscible solvents than are the corresponding anhydrous forms. For example, the solubility of caffeine hydrate is lower than that of anhydrous caffeine in water but higher in ethanol. The maximum concentration seen may be due to the solubility of the anhydrous crystalline phase or due to a temporary steady state in which the rate of dissolution of the metastable anhydrous form and the rate of crystallization of the stable hydrate are equal. The decreasing concentration represents crystallization of the stable hydrate from a solution supersaturated with respect to it. If the maximum concentration of the solute in the dissolution experiment corresponds to the solubility, then the initial increase in concentration follows the Noyes-Whitney equation [15]. Van t Hoff plots of log solubility versus the reciprocal of temperature give linear relationships (Fig. 16). 

A solution containing the maximum amount of solute per given amount of solvent at a given temperature is a saturated solution. An unsaturated solution has less than that maximum amount of solute dissolved. Sometimes, there may be more that that maximum amount of solute, resulting in a supersaturated solution. Supersaturated solutions are unstable and eventually expel the excess solute, forming a saturated solution. 

A disadvantage of the conventional precipitation method in which the supersaturation was allowed to decrease during the reactions, was that different calcium phosphate phases could form and subsequently dissolve during the course of the reactions. In the present work, the constant composition method was used to investigate the influence of sodium chloride, potassium chloride, and potassium nitrate, as background electrolyte upon the rate of crystallization of HAP in solutions supersaturated only with respect to this phase. These experiments were made in solutions containing totaj... 

For a solution supersaturated with solute, crystallization cannot take place in the absence of a higher-order process known as nucleation. In one sense, nucleation refers to any process affording a route for a substance from one physical phase to another, usually in the direction... 

An important group within the ammonium nitrate explosives are emulsion explosives, which consist essentially of a water-in-oil emulsion of an aqueous solution, supersaturated at room temperature, in an oil matrix which is a fuel. The oil phase is the continuous phase and includes small droplets of the supersaturated solution of the oxidizing agent. 

The term supersaturate was coined in 1788 and was deLned as to add to a solution beyond saturation. The state of supersaturation may be obtained through cooling, evaporation, oras a result of a chemical reaction between two homogeneous phases. In addition, in the case of drug solutions, supersaturation may be reached through dissolution of... 

Figure 12.1. The process of spray atomization. (A) Conventional atomization break up of a liquid film from the shear with surrounding air (B) UNICARB vigorous decompressive atomization produced by the expansive forces of the compressed carbon dioxide (C) RESS solute precipitation due to solution supersaturation followed by nucleation and spinodal decomposition.

Add a crystal of solute to each solution to see if the solute dissolves (unsaturated) or remains (saturated), or more solute precipitates out of the solution (supersaturated). 

At the higher saturation state, the seawater solution is more than 5 times supersaturated with respect to aragonite so that aragonite would be expected to precipitate on the aragonite seed crystal. Results indicated that Mg2+ is adsorbed between 25 to 40 times less on aragonite than on calcite from solutions supersaturated with respect to both minerals. 

Solubility The equilibrium solute concentration. The dimensions in which solubility is expressed include, but are not limited to, mass or mole fraction, mass or mole ratio of solute to solvent, and mass or moles of solute per unit volume of solvent or solution. Supersaturation The difference between existing and equilibrium conditions the quantity represents the driving force for crystal nucleation and growth. 

The curve shown in Fig. 3 cannot proceed indefinitely in either direction. In the cathodic direction, the deposition of copper ions proceeds from solution until the rate at which the ions are supplied to the electrode becomes limited by mass-transfer processes. In the anodic direction, copper atoms are oxidized to form soluble copper ions. While the supply of copper atoms from the surface is essentially unlimited, the solubility of product salts is finite. Local mass-transport conditions control the supply rate so a current is reached at which the solution supersaturates, and an insulating salt-film barrier is created. At that point the current drops to a low level further increase in the potential does not significantly increase the current density. A plot of the current density as a function of the potential is shown in Fig. 5 for the zinc electrode in alkaline electrolyte. The sharp drop in potential is clearly observed at -0.9 V versus the standard hydrogen electrode (SHE). At more positive potentials the current density remains at a low level, and the electrode is said to be passivated. 

These problems are avoided if a continuous process is employed for the precipitation however, this makes higher demands on the process control. In a continuous process all parameters as temperature, concentrations, pH, and residence times of the precipitate can be kept constant or altered at will. Continuous operation is, for instance, used for the precipitation of aluminum hydroxide in the Bayer process. Bayer aluminum hydroxide is the main source for the production of cata-lytically active aluminas. The precipitation step of the Bayer process is carried out continuously. An aluminum solution supersaturated with respect to Al(OH)3, but not supersaturated enough for homogeneous nu-cleation, enters the precipitation vessel which already contains precipitate so that heterogeneous precipitation is possible. The nucleation rate has to be controlled very carefully to maintain constant conditions. This is usually done by controlling the temperature of the system to within 2-3 degrees [7]. 

The heating of the reaction mixture induces the growth of nuclei-I of both zeolites from the solution supersaturated with soluble aluminosilicate species. Since the growth rate of zeolite X is considerably greater than the growth rate of zeolite Na-Pc (see Figure 6), zeolite X appears as the first crystalline phase. The... 

Crystallization is the formation of solid particles from supersaturated liquid solution. Supersaturation is produced by the following ways. 

The over-all process of crystal growth in a seeded solution is analogous to other mass transfer situations encountered in chemical engineering and may be treated as a diffusional step in series with a surface reaction step. Solution supersaturation provides the driving force required for each step, as portrayed schematically in Fig. 12. First, solute molecules or ions diffuse through the solution to the growing crystal. Second, upon reaching the surface, the molecules or ions must be accepted and incorporated into the crystal lattice. 

Note. V is the molar volume, JVyi is Advogadro s number, is the equilibrium concentration, D is the diffusion coefficient, sub-s surface, hHfiB the heat of fusion, t) is the Damkohler number. Ah is the thermal conductivity, i die area shape factor for surface nuclei , y, is the distance between steps, n is the equilibrium surface concentration, p = 1 - o-JS is one minus the maximum surface supersaturation divided by the solution supersaturation, and p is the density. ihG - pl- fPMpAkBT In S)... 

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