Vapor-phase precipitation

Some reactants in atmospheric-pressure reactors must be highly diluted with inert gases to prevent vapor-phase precipitation, while generally no dilution is necessary at low pressure. However, atmospheric pressure reactors are simpler and cheaper. They can operate faster, on a continuous basis and, with recent design improvements, the quality of the deposits has been upgraded considerably and satisfactory deposits of many materials, such as oxides, are obtained. 

At a deposition temperature of 1300°C., a low-density material is obtained (1.5 g/cm ). Density increases with increasing temperature and reaches 2.0 g/cm at 1600°C. Vapor phase precipitation can be a problem in the high-temperature range. A more convenient reaction uses boron fluoride ... 

The first major application of microfiltration membranes was for biological testing of water. This remains an important laboratory application in microbiology and biotechnology. For these applications the early cellulose acetate/cellulose nitrate phase separation membranes made by vapor-phase precipitation with water are still widely used. In the early 1960s and 1970s, a number of other membrane materials with improved mechanical properties and chemical stability were developed. These include polyacrylonitrile-poly(vinyl chloride) copolymers, poly(vinylidene fluoride), polysulfone, cellulose triacetate, and various nylons. Most cartridge filters use these membranes. More recently poly(tetrafluo-roethylene) membranes have come into use. 

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. 

Because of the flat concentration profiles, the solution precipitates at virtually the same time over the entire film cross section, and no macroscopic gradients of activity or concentration of the polymer are obtained over the film cross-section. On a microscopic scale, however, because of thermal molecular motions, there are areas of higher and lower polymer concentration, which act as nucleation centers for polymer precipitation. These microscopic areas of higher polymer concentration are randomly distributed throughout the cast polymer film. Therefore, a randomly distributed polymer structure is obtained.during precipitation. This structure is also shown in Figure 13 in the form of a scanning electron microscope picture of the cross section of a symmetric membrane obtained with a vapor phase precipitant. 

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. 

Commercial sols Commercial gel (large pore) Condensed from vapor phase Precipitated from hot solution... 

Experimentation shows that the best, fully dense, and homogeneous carbon deposits are produced at an optimum negative value of AG. For smaller negative values, the reaction rate is very low and, for higher negative values, vapor-phase precipitation and the formation of soot can occur. Such factors are not revealed in the simple free-energy change calculation. A more complete analysis is often necessary. 

Inhibitors act and are classified in a variety of ways (1,3,37,38). The classifications used herein closely foUow the discussion in Reference 37. Types of inhibitors include (/) anodic, (2) cathodic, (3) organic, (4) precipitation, and (5) vapor-phase inhibitors. 

Precipitation and Vapor-Phase Inhibitors. Precipitation inhibitors are film-forming compounds that produce barrier films over the entire surface. Phosphates and siUcates, which are the most common, do not provide the degree of protection afforded by chromate inhibitors, but are useful in situations where nontoxic additives are required. Two main drawbacks to the use of phosphates and siUcates are the dependence on the water composition and the control required to achieve maximum inhibition (37,38). 

Other Esters. The esterification of acetic acid with various alcohols in the vapor phase has been studied using several catalysts precipitated on pumice (67). 

Reaction Conditions. Alcoholysis commonly takes place in one Hquid phase, sometimes with one of the reactants being only partially soluble and going into solution gradually as the reaction proceeds. Unless an excess of one of the reactants is used, or unless one of the products is withdrawn from the reaction phase by vaporization or precipitation, the reaction does not proceed to completion but comes to a standstill with substantial proportions of both alcohols and both esters in equilibrium. The concentrations present at equilibrium depend on the characteristics of the alcohols and esters involved, but in most practical uses of the reaction, one or both of the devices mentioned are used to force the reaction toward completion. 

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. 

The iron, now in a reduced ferrous form, is not consumed instead, it is continuously regenerated by bubbling air through the solution. The sulfur precipitates out of the solution and is removed from the reactor with a portion of the reagent. The sulfur slurry is pumped to a melter requiring a small amount of heat and then to a sulfur separator where the reagent in the vapor phase is recovered, condensed, and recycled back to the reactor. 

Based upon a vapor pressure of 159 mm Hg at 20 °C, chloroform is expected to exist almost entirely in the vapor phase in the atmosphere (Boublik et al. 1984 Eisenreich et al. 1981). Large amounts of chloroform in the atmosphere may be removed by wet deposition since chloroform has significant solubility in water. This is confirmed by its detection in rainwater (Kawamura and Kaplan 1983). Most of the chloroform removed in precipitation, however, is likely to reenter the atmosphere by volatilization. Trace amounts of... 

Cu-Mn mixed-oxide binary spinel catalysts (CuxMn3 x04, where x = 0, 0.25, 0.5, 0.75 and 1) prepared through co-precipitation method exhibit phenol methylation activity imder vapor phase conditions [75]. All of the catalysts, irrespective of the compositions, produced only C-methylated phenols. However, a total ortho selectivity of 100% with 2,6-xylenol selectivity of 74% was observed over x = 0.25 compositions at 400°C. This composition was found to be relatively stable under reaction conditions compared with the other compositions studied. The catalysts with high copper content suffered severe reduction under methylation conditions whereas, catalysts with low copper content had a hausmannite phase (Mu304) that sustained... 

In situ precipitation by vapor phase application Chemical immobilization ... 

The electrostatic precipitator was very efficient (— 98% ) for most trace elements based on analyses of the fly ash particulate specimens collected from the precipitator inlet and outlet. An exception was selenium. Although a reasonable mass balance was obtained for this element (see NAA results, Table III), it was not removed efficiently by the precipitator. This may indicate that a significant fraction of the material is in the vapor phase in the flue gas and that it is being adsorbed in... 

Evaporators employ heat to concentrate solutions or to recover dissolved solids by precipitating them from saturated solutions. They are reboilers with special provisions for separating liquid and vapor phases and for removal of solids when they are precipitated or crystallized out. Simple kettle-type reboilers [Fig. 8.4(d)] may be adequate in some applications, especially if enough freeboard is provided. Some of the many specialized types of evaporators that are in use are represented on Figure 8.16. The tubes may be horizontal or vertical, long or short the liquid may be outside or inside the tubes, circulation may be natural or forced with pumps or propellers. 

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