Precipitation from the Vapor Phase

A cast polymer solution that consists of polymer and solvent is brought into a nonsolvent vapor environment saturated with solvent vapor. The saturated solvent vapor suppresses the evaporation of solvent from the film the nonsolvent molecules diffuse into the film causing polymer coagulation [33]. 

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In the crystallization of melts, where relatively large degrees of supersaturation are attainable, nucleation and growth phenomena are more easily separated and studied experimentally than in crystallization from solution, which is characterized by rather narrow metastable regions. However, the basic concepts of nucleation are the same in both types of processes. In fact, much of the experimental verification of nucleation theory has come from studies of condensation and precipitation from the vapor phase. The highly publicized rainmaking experiments of several years ago made significant contributions (S6). 

There are a number of different techniques belonging to the category of phase inversion solvent evaporation, precipitation by controlled evaporation, precipitation from the vapor phase, thermal precipitation, and immersion precipitation (13,34—36). The most commercially available membranes are prepared by the last method. 

A number of authors have studied the interactions of electron-deficient group III acids with bases for the deliberate purpose of comparing the results with protonation measurements, and most of these data are available in Stone s review (329). It is usually possible to study such reactions entirely in the gas phase by vapor pressure measurements. In such cases the heat of formation of the adduct is a good criterion of basicity. If, however, one of the compounds precipitates from the vapor phase, the results contain lattice energies... 

Chemical Phase Inversion Svmrnetrical phase-inversion membranes (Fig, 22-71) remain the most important commercial MF membranes produced. The process produces tortiioiis-Bow membranes. It involves preparing a concentrated solution of a polvrner in a solvent. The solution is spread into a thin film, then precipitated through the slow addition of a nonsolvent, iisiiallv w ater, sometimes from the vapor phase. The technique is irnpressivelv v ersatile, capable of producing fairlv uniform membranes wFose pore size rnav be varied within broad limits. 

A two-step methanolysis-hydrolysis process37 has been developed which involves reaction of PET with superheated methanol vapors at 240-260°C and atmospheric pressure to produce dimethyl terephthalate, monomethyl terephthalate, ethylene glycol, and oligomeric products in the first step. The methanolysis products are fractionally distilled and the remaining residue (oligomers) is subjected to hydrolysis after being fed into the hydrolysis reactor operating at a temperature of ca. 270°C. The TPA precipitates from the aqueous phase while impurities are left behind in the mother liquor. Methanolysis-hydrolysis leads to decreases in the time required for the depolymerization process compared to neutral hydrolysis for example, a neutral hydrolysis process that requires 45 min to produce the monomers is reduced... 

Phase separation membranes. This category includes membranes made by the Loeb-Sourirajan technique involving precipitation of a casting solution by immersion in a nonsolvent (water) bath. Also covered are a variety of related techniques such as precipitation by solvent evaporation, precipitation by absorption of water from the vapor phase, and precipitation by cooling. 

It is important to note that while SCWO is formally defined in terms of the critical point of pure water, addition of any other constituents to the water will alter the critical point, and the system may or may not be supercritical with respect to this mixture critical point. Rather than a single critical point, for a binary system a critical curve exists that in the simplest cases joins the critical point of pure water to the critical point of the second substance across the composition space. For ternary mixtures the critical curve becomes a critical surface, and so on. In general, mixtures of water with higher volatility substances such as noncondensable gases or liquid organics will remain supercritical, while mixtures of water with lower-volatility substances such as salts will become subcritical and liquid or solid phases will precipitate from the vapor/ gas phase. 

Two different techniques have been employed for the precipitation of membranes from a polymer casting solution. In the first method, the precipitant is introduced from the vapor phase. In this case the precipitation is slow, and a more or less homogeneous structure is obtained without a dense skin on the top or bottom side of the polymer film. This structure can be understood when the concentration profiles of the polymer, the precipitant and the solvent during the precipitation process are considered. The significant feature in the vapor-phase precipitation process is the fact that the rate-limiting step for precipitant transport into the cast polymer solution is the slow diffusion in the vapor phase adjacent to the film surface. This leads to uniform and flat concentration profiles in the film. The concentration profiles of the precipitant at various times in the polymer film are shown schematically in Figure 13. 

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. 

Clays are actually secondary minerals—meaning that they are formed chiefly by the weathering of primary minerals. Primary minerals are those that form directly by precipitation from solution or magma, or by deposition from the vapor phase. In the case of clays their primary or parent minerals are feldspars, the mineral group with the greatest abundance in Earth s crust. Feldspars and clays are actually aluminosilicates. The formation of an aluminosilicate involves the replacement of a significant portion of the silicon in the tetrahedral backbone by aluminum. 

Nucleation plays a fundamental role whenever condensation, precipitation, crystallization, sublimation, boiling, or freezing occur. A transformation of a phase a, say, a vapor, to a phase p, say, a liquid, does not occur the instant the free energy of p is lower than that of a. Rather, small nuclei of p must form initially in the a phase. This first step in the phase transformation, the nucleation of clusters of the new phase, can actually be very slow. For example, at a relative humidity of 200% at 20°C (293 K), far above any relative humidity achieved in the ambient atmosphere, the rate at which water droplets nucleate homogeneously is about 10 54 droplets per cm3 per second. Stated differently, it would take about 1054 s (1 year is 3 x 107 s) for one droplet to appear in 1 cm3 of air. Yet, we know that droplets are readily formed in air at relative humidities only slightly over 100%. This is a result of the fact that water nucleates on foreign particles much more readily than it does on its own. Once the initial nucleation step has occurred, the nuclei of the new phase tend to grow rather rapidly. Nucleation theory attempts to describe the rate at which the first step in the phase transformation process occurs—the rate at which the initial very small nuclei appear. Whereas nucleation can occur from a liquid phase to a solid phase (crystallization) or from a liquid phase to a vapor phase (bubble formation), our interest will be in nucleation of trace substances and water from the vapor phase (air) to the liquid (droplet) or solid phase. 

Several dozen measurements of atmospheric H2O2 levels in air, precipitation, and cloudwater have been summarized by Gunz and Hoffmann (1990). A strong tendency for H2O2 to partition from the vapor phase to the liquid phase is reflected by its high Henry s law coefficient of about 10 M/atm (Yoshizumi et al., 1984 Martin and Damschen, 1981) there are also several mechanisms for direct in situ formation in the aqueous phase. 

The polymerization of VCM is highly exothermic (i.e., 100 kj mol ). Thus, the efficient removal of the reaction heat is very critical for the operation of large-scale reactors [44]. When all of the free liquid monomer has been consumed, the pressure in the reactor starts to fall as a result of the monomer mass transfer from the vapor phase to the polymer phase due to subsaturation conditions. In industrial PVC production, the reaction is usually stopped when a certain pressure drop has been recorded. Because the polymer is effectively insoluble in its own monomer, once the polymer chains are first generated, they precipitate immediately to form a separate phase in the polymerizing mixture. Thus, from a kinetic point of view, the polymerization of VCM is considered to take place in three stages [45]. 

Black Dull black metallic powder obtained by reduction and precipitation from solutions or by condensation from the vapor phase. 

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