Precipitation liquid phase

In a second paper from Kirillov et al., synthetic aspects of liquid phase precipitation reactions are discussed in relation to the factors responsible for metal-oxide formation. In particular the Pechini reaction, a sol-gel process, was examined. A good reference section is provided to introduce the reader to previous literature. The authors stress that more work needs to be done to establish what factors are important for producing the highest quality metal-oxide powders from such reactions. The conclusion specifically lists the criteria needed in order to accomplish this task. 

The carboxyl terminated butadiene-acrylonitrile rubber, which is soluble initially in the liquid phase, precipitates out as a second phase during the crosslinking reaction of the epoxy. The cured specimens are... 

In the geological and soil science literature, ion exchange and precipitation are frequently considered as adsorption and thermodynamically described by adsorption equations, or isotherms. This is not correct because, as shown previously, the processes are principally different adsorption is directed by the decrease of surface energy, and it takes place on the free surface sites ion exchange is just a competitive process on an already covered surface, determined by the ionic composition of the liquid phase. Precipitation, including colloid formation, is governed by the composition of the liquid phase, the crystal structure (coprecipitation), or primary chemical forces. 

Molten solutions can also be used for crystallization. Fluxes are selected that have a high solubility of the to-be crystallized material. Borate fluxes are used for some oxide systems, sodium sulfide fluxes are used for sulfide systems, and molten metal fluxes are used for carbide and nitride systems. In both the melt and flux systems, the solubility is highly temperature dependent. The solubility in flux systems is not particularly well known except for a few systems. The general solubility behavior is like that in liquid phase precipitation, which was discussed in Chapter 6. In melt solidification, the supersaturation, S, is given by... 

Inorganic non-oxide materials, such as III-V and II-VI group semiconductors, carbides, nitrides, borides, phosphides and silicides, are traditionally prepared by solid state reactions or gas-phase reaction at high temperatures. Some non-oxides have been prepared via liquid-phase precipitation or pyrolysis of organometallic precursors. However, amorphous phases are sometimes formed by these methods. Post-treatment at a high temperature is needed for crystallization. The products obtained by these processes are commonly beyond the manometer scale. Exploration of low temperature technique for preparing non-oxide nanomaterials with controlled shapes and sizes is very important in materials science. 

Step number 3 in the above sequence is responsible for the formation of an anisotropic membrane. In Chapter 1, Strathmann describes the difference between a vapor-phase precipitation process (Figure 1.22) as used in MF membrane formation and the liquid-phase precipitation process (Figure 1.23) used in UF membrane formation. In the former case, the rate limiting step is the slow diffusion of precipitant (e.g., water vapor) from the vapor phase to the polymer solution. Since this is a relatively slow process, precipitation of polymer is also slow resulting in fairly large pores in the membrane. In the latter case, described here, bringing liquid water in contact with the polymer solution results in catastrophic precipitation under supersaturated conditions. 

Recently, a modification ofWakai s model has been created [49], based on the fact that the detailed examination of a nucleation process requires the addition of two new terms to the Wakai s free energy [81]. One term is related to the free energy increases due to the molecular volume change when the solute (which initially was dissolved in the liquid phase) precipitates into the crystal at a step. The second term arises from the fact that an oversaturated solution of crystalline material in the glassy phase, giving rise to precipitation phenomena, is not in equilibrium with the crystal, at least locally in the surroundings of the step [85, 86] (also see Ref. [49] for further details). 

These assumptions are violated at high pressures when the vapor phase becomes nonideal and in the cases where the multicomponent nature of the mixture cannot be neglected. The extreme case of non-Kelvin behavior is the behavior of hydrocarbon mixtures in od-gas-condensate reservoirs. Although the Kelvin equation is apphed to tests of the porous media of the reservoirs [74,75], it cannot be used for modeling of equilibria in such reservoirs, not only because of the high pressure and the multicomponent composition of the mixture but also because this mixture exhibits retrograde behavior when the liquid phase precipitates as the pressure decreases. Such a behavior cannot, in principle, be described in terms of a single-component model. 

Previous work (13,14) has shown that the glycothermal synthesis process, a liquid phase precipitation at elevated temperatures under autogeneous pressure using a glycol as solvent, for a-Al203 is mediated by the formation of a precursor pseudo-boehmite phase (15). The pseudo-boehmite is thermodynamically unstable under the reaction conditions, and transforms to the stable a-Al203 (similar to the formation of alumina in the hydrothermal system (16)). Thus, the overall reaction is as follows. 

Initially fermentation broth has to be characterised on the viscosity of the fluid. If the presence of the biomass or cells causes trouble, they have to be removed. Tire product is stored inside the cells, the cells must be ruptured and the product must be freed. Intracellular protein can easily be precipitated, settled or filtered. In fact the product in diluted broth may not be economical enough for efficient recovery. Enrichment of the product from the bioreactor effluents for increasing product concentration may reduce the cost of product recovery. There are several economical methods for pure product recovery, such as crystallisation of the product from the concentrated broth or liquid phase. Even small amounts of cellular proteins can be lyophilised or dried from crude solution of biological products such as hormone or enzymes.2,3... 

However, some interconversion reactions take place simultaneously and therefore the composition of the sulfane mixtime is not a mirror image of the composition of the polysulfide solution [103]. The sulfane mixture forms a yellow oily hydrophobic liquid which precipitates from the aqueous phase. At 20 °C it decomposes more or less rapidly to H2S and Sg. 

Solid solutions generally form in one of two ways, both of which involve forming the solid flxim the liquid phase. One way is to heat the solid solvent until it melts, add the solutes into the molten material, and then cool the melt until it solidifies. Solid solutions of one metal in another, such as brass and steel, are prepared in this way. A second method is to dissolve the solid solvent and solutes in an appropriate liquid, then cool or evaporate the liquid until a solid precipitates. Solid solutions of organic substances can form in this manner. 

Aluminum sulfate, AI2 (804)3 > widely used in water purification to remove finely divided particulate matter. When added to water, aluminum sulfate forms a precipitate of aluminum hydroxide that has a very open structure and large surface area. This precipitate, called a gel, traps dispersed particulate matter as it settles out of the liquid phase. 

Wahl and Deck were able to obtain an estimate of an assumed second-order rate coefficient ( 10 l.mole" .sec at 4°C) using a separation procedure based on the extraction of Fe(CN)e by a chloroform solution of Ph AsCl, in the presence of the ions Co(CN)g and Ru(CN)6, to reduce the exchange between the iron species in the two liquid phases. A similar estimate was obtained using a precipitation method in the presence of the carrier Ru(CN)6. A direct injection technique was used as short reaction times were necessary. Wahl has reviewed the large induced exchanges occurring in the chemical separation methods. The extraction procedure when the carriers Co(CN)6 and Ru(CN) are present provides the most satisfactory method of separation. ... 

Recalling the previous assertion that efficient fractionation requires liquid-liquid phase separation, we conclude that nitrobenzene and amyl acetate should be satisfactory solvents from which to fractionate polyethylene by successively lowering the temperature and that the better solvent xylene should be avoided for this purpose. The character of the phase diagram may, in fact, be used as a criterion of the efficacy of a given solvent for fractionation (see Chap. VIII, p. 344). If the curve representing the precipitation temperature plotted against concentration rises monotonically, crystalline separation is clearly indicated if it passes through a maximum at a low concentration, liquid-liquid separation is virtually assured, and the solvent may be assumed to be a satisfactory one to use for fractionation. 

Gas-Phase Grafting Liquid-Phase Grafting Deposition-Precipitation... 

The approach comprises deposition-precipitation (DP) of Au(OH)3 onto the hydroxide surfaces of metal oxide supports from an alkaline solution of HAUCI4 [26] and grafting of organo gold complexes such as dimethyl gold (Ill)acetylacetonate (hereafter denoted as Au acac complex) [27] and Au(PPh3)(N03) [28] either in gas and liquid phase are advantageous in that a variety of metal oxides commercially available in the forms of powder, sphere, honeycomb can be used as supports. 

Intelligent engineering can drastically improve process selectivity (see Sharma, 1988, 1990) as illustrated in Chapter 4 of this book. A combination of reaction with an appropriate separation operation is the first option if the reaction is limited by chemical equilibrium. In such combinations one product is removed from the reaction zone continuously, allowing for a higher conversion of raw materials. Extractive reactions involve the addition of a second liquid phase, in which the product is better soluble than the reactants, to the reaction zone. Thus, the product is withdrawn from the reactive phase shifting the reaction mixture to product(s). The same principle can be realized if an additive is introduced into the reaction zone that causes precipitation of the desired product. A combination of reaction with distillation in a single column allows the removal of volatile products from the reaction zone that is then realized in the (fractional) distillation zone. Finally, reaction can be combined with filtration. A typical example of the latter system is the application of catalytic membranes. In all these cases, withdrawal of the product shifts the equilibrium mixture to the product. 

Precipitation/Crystallization to Produce Nano- and Microparticles Because fluids such as C02are weak solvents for many solutes, they are often effective antisolvents in fractionation and precipitation. In general, a fluid antisolvent may be a compressed gas, a gas-expanded liquid, or a SCF. Typically a liquid solution is sprayed through a nozzle into CO2 to precipitate a solute. As CO2 mixes with the liquid phase, it... 

Table 7.89 lists the main characteristics of MDHPLC (see also Table 7.86). In MDHPLC the mobile-phase polarity can be adjusted in order to obtain adequate resolution, and a wide range of selectivity differences can be employed when using the various available separation modes [906]. Some LC modes have incompatible mobile phases, e.g. normal-phase and ion-exchange separations. Potential problems arise with liquid-phase immiscibility precipitation of buffer salts and incompatibilities between the mobile phase from one column and the stationary phase of another (e.g. swelling of some polymeric stationary-phase supports by changes in solvents or deactivation of silica by small amounts of water).

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