Mechanical contact vapor-phase

For effective volatilization using an enclosed mechanical aeration system, contaminated soil is mixed in a pug mill or rotary drum. The gasoline components are released from the soil matrix by the churning action of the air/soil contact. The induced airflow within the chamber captures the gasoline emissions and passes them through an air pollution control device (e.g., a water scrubber or vapor-phase carbon adsorption system) before they are discharged through a properly sized stack. 

Figure 4.3 Schematic chart of a CSTR reactor for the 0-methylation of phenols with DMC. Liquid reagents are vaporized by contact with the hot slurry (mechanically stirred) and bubbled through it. Reaction takes place instantaneously and anisoles are picked up from the vapor phase.

General purpose PS and HIPS are used in food packaging applications. In some packaging configurations, with no direct contact of a surface with the polymer the migration of residual styrene monomer via the vapor-phase with subsequent absorption into the food is believed to be a significant mechanism of accumulation. 

Absorption of a solute liquid or vapor into a polymer film can profoundly affect the viscoelastic behavior of the polymer. The magnitude of this effect depends on the nature of the solute/polymer interactions and on the amount of solute absorbed. The solute/polymer interactions can range fttun simple dispersion to hydrogen-bonding and other specific interactions. The extent of absorption can be described by the partition coefficient, AT, which quantifies the thermcxlynamic distribution of the solute between two phases (K = coiKentration in polymer divided by die concentration in the liquid or vapor phase in contact with the polymer). It has long been known that acoustic wave devices can be used to probe solubility and partition coefficients (53,67). Due to the relevance of these topics to chemical sensors, more comprehensive discussions of these interaction mechanisms and the significance of the partition coefficient are included in Chapter 5. 

In the chamber, steam jets are directed parallel to or slightly impinging the flow of feed. This serves two purposes to wet the solids and to cause particle movement resulting in collisions and coalescense. Contact of the steam with cold particle surfaces results in condensation and thermal energy transfer also, droplets can form in the vapor phase. Therefore, two different mechanisms contribute to the wetting processes in the agglomeration zone ... 

Over 150 years ago Thomas Young [104] proposed treating the contact angle of a liquid as the result of the mechanical equilibrium of a drop resting on a plane solid surface under the action of three surface tensions (Figure 1)—Vlv the interface of the liquid and vapor phases, VsL the interface of the solid and the liquid, and Vgv the interface of the solid and vapor. Hence,... 

C. Either mechanical or pneumatic foam whipping of the oligomer or polymer solution or emulsion contact of the inhibiting vapor phase in the presence of a structural surfactant. The foam is then fixed by lacing the polymer walls. 

Vapor permeation differs from pervaporation, as stated above, insofar as the feed mixture to be separated is supplied as a vapor. At least the more-permeable component is kept as close to saturation conditions as possible. Thermodynamically there is no difference between a liquid and ifs equilibrium vapor, the partial vapor pressure and thus the driving force for the transport through the membrane are identical and the same solution-diffusion mechanism is valid. However, the density of the vaporous feed and thus the concentration of molecules per volume is lower by two to three orders of magnitude than that of the liquid. As a consequence the membrane is usually less swollen than when in contact with a liquid feed. As the feed mixture getting in contact with the membrane is already in the vapor phase no phase change occurs across the membrane and thus no temperature polarization will be observed. Concentration polarization, however, is still an issue. Although the diffusion coefficient is much higher in a vapor than in a liquid, this is at least partially outbalanced by the lower density of the vapor, and therefore concentration polarization effects may be observed at all concentrations of the component to be removed. Minimum... 

The invention of the so-called vapor-phase transport (VPT) method [164] is another argument for the existence of a solid-phase transformation mechanism (Figure 12.11). A dried aluminosilicate gel (423-473 K) was contacted only with vapors of an aqueous SDA phase. The solution is placed at the bottom of an autoclave, while the dry gel was on top of a supporting device half-way up the autoclave [165]. In a variation of the method denoted as steam-assisted crystallization (SAC), the dry SDAactivated with steam [166]. The method gives high yields of crystals and is useful for several zeolite phases such as Al-and Ti-Beta (BEA), TS-1 (MFI), TS-2 (MEL), and EU-1... 

Currently, there is much uncertainty about the mechanisms of ice nucleation in the atmosphere, but it is thought that ice nuclei operate by three basic modes. In one mode, water is absorbed from the vapor phase onto the surface of the ice nucleus, and at sufficiently low temperatures, the adsorbed vapor is converted to ice. In another mode, the ice nucleus, which is inside a supercooled droplet either by collection or as a result of its participation in the condensation process, initiates the ice phase from inside the droplet. In the third mode, the ice nucleus initiates the ice phase at the moment of contact with a supercooled droplet (such nuclei are known as contact nuclei). The relative importance of these different modes of operation is not known with certainty, but the latter two modes are thought to be much more common than the first. 

GDEs typically consist, at least, of two layers a gas diffusion layer (GDL) and an active layer (AL). The GDL should provide mechanical support and electrical contact (current collector), optimal distribution of reactant gases and a pore structure suitable for the remova of liquid or vapor phase water (water management). The AL contains the catalyst, where the electrochemical reaction takes place, but only in those sites where reactant, electrolyte and catalyst meet, that is, in the three-phase zone or boundary (TPB). 

The problem of stress corrosion cracking becomes especially severe for those stainless steel parts that are intermittently exposed to boiler water. This exposure represents a much more severe condition for inhibition than in the case of parts that are submerged in water continually. Cracking in the parts that are in the vapor phase does not occur if water containing chloride does not come into contact with them by splashing or by some other mechanism. 

These two examples illustrate that 7) is, in general, an excess quantity for a given D-face. As a consequence of this result, there is no physical difference between interfacial and surface tension in thermodynamic equilibrium. In the literature, the term surface tension is attributed to the D-face, or interface, between a condensed phase and the corresponding vapor phase. Some authors define the surface tension as the interfacial tension of a condensed phase in contact with vacuum. This is incorrect in the framework of equilibrium thermodynamics. A condensed phase will fill a vacuum with its vapor phase to be at thermodynamic equilibrium. Hence, any interfacial tension results from a defined pair of phases, never fi om a single phase, and they need to be marked by the subscript AB or similar. Otherwise, the interfacial tension would be ill-defined. 7 5 accounts for the real distribution of pressure or stress inside the phase boundary. It is called a tension because it possesses the dimension of a force per unit length [N m ] in the 2D space of the D-face, and indeed acts somewhat like a tension in a membrane (except there is no elastic proportionality between the tension and extension). Moreover, (O 4.15b) reveals that the value for 7 5 usually depends on the position, here rjy, chosen for that D-face. is always positive. This is a condition of stability for the interphase and hence for the phase itself, and it is proven by experiment and by statistical mechanics calculations as well. The value remains almost constant when p decreases. Therefore, py-must he negative, at least inside the dominant part of the interphase, and as a consequence, the interphase and the corresponding D-face will adopt the minimum volume and area, respectively. 

For a static system, solid surface interfacial tensions can be calculated from the measured contact angles by using a mechanical equilibrium relation derived by Young in 1800 [38]. The liquid droplet contact angle on any solid surface can be defined by the mechanical equilibrium of the droplet under the action of three interfacial tensions. The interfacial tension a ij is defined as the amount of work that must be performed in order to separate a unit area of fluid j from k. The term aj is the surface tension between substance j and its own vapor phase (Figure 5.28). The work to separate two phases is expressed as... 

When determining the solubility and dissolution rate of amorphous or partially crystalline solids, the metastability of these phases with respect to the highly crystalline solid must be considered. While the low diffusivity of the molecules in the solid state can kinetically stabilize these metastable forms, contact with the solution, for example during measurements of solubility and dissolution rate, or with the vapor, if the solid has an appreciable vapor pressure, may provide a mechanism for mass transfer and crystallization. Less crystalline material dissolves or sublimes whereas more crystalline material crystallizes out. The equilibrium solubility measured will therefore approach that of the highly crystalline solid. The initial dissolution rate of the metastable form tends to reflect its higher... 

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