Degradative transfer

In terms of the environmental fate of contaminants, three general process types can be distinguished phase transfer, degradation, and bioaccumulation. AU of these process types depend upon knowledge of the physical-chemical properties of the markers, transformation rates and pathways, measurements of the compositions of mixtures of compounds, and an understanding of the physics of the system under investigation. 

Thus, by the presence of inert gases, not only does the average temperature difference ATjjj become smaller but also the heat transfer degrades, since the heat transfer resistance on the condensation side increases. The cold walls are a sink for vapours. If vapour condenses, the inert gas remains and forms a layer at the heat exchange surface. In order to get to the cold wall, the vapour must diffuse through this inert gas layer, thus heat transfer is made more difficult. This effect is more pronounced the higher the inert gas fraction is or becomes during the condensation. Therefore, the heat transfer is worst at the condenser outlet. 

FIGURE 13.4 In zinc/air button cell, water vapor transfer is the dominant form of gas transfer degradation. (Courtesy of Duracell, Inc.)... 

The combination of the effects of self-discharge and gas transfer degradation determines the service life performance of a zinc/air battery. For most applications water transfer is the dominant factor. However, under some conditions electrolyte carbonation and direct oxidation mechanisms can adversely affect performance. 

When the incident beam of fast-moving atoms or ions impinges onto the liquid target surface, major events occur within the first few nanometers, viz., momentum transfer, general degradation, and ionization. 

The FAB source operates near room temperature, and ions of the substance of interest are lifted out from the matrix by a momentum-transfer process that deposits little excess of vibrational and rotational energy in the resulting quasi-molecular ion. Thus, a further advantage of FAB/LSIMS over many other methods of ionization lies in its gentle or mild treatment of thermally labile substances such as peptides, proteins, nucleosides, sugars, and so on, which can be ionized without degrading their. structures. 

Protective Coatings. Some flame retardants function by forming a protective Hquid or char barrier. These minimize transpiration of polymer degradation products to the flame front and/or act as an insulating layer to reduce the heat transfer from the flame to the polymer. Phosphoms compounds that decompose to give phosphoric acid and intumescent systems are examples of this category (see Flame retardants, phosphorus flame retardants). 

Fypass Flow Effects. There are several bypass flows, particularly on the sheUside of a heat exchanger, and these include a bypass flow between the tube bundle and the shell, bypass flow between the baffle plate and the shell, and bypass flow between the shell and the bundle outer shroud. Some high temperature nuclear heat exchangers have shrouds inside the shell to protect the shell from thermal transient effects. The effect of bypass flow is the degradation of the exchanger thermal performance. Therefore additional heat-transfer surface area must be provided to compensate for this performance degradation. 

Synthetic fluids are safe, noncorrosive, essentially nontoxic, and thermally stable when operated under conditions recommended by the manufacturers. Generally, these fluids are more expensive than petroleum oils, but the synthetics can usually be reprocessed to remove degradation products. There are several classes of chemicals offered permitting a wide temperature range of appHcation. Any heat-transfer fluid in use should be examined periodically to monitor degradation or contamination. 

Ucon HTF-500. Union Carbide Corp. manufactures Ucon HTE-500, a polyalkylene glycol suitable for Hquid-phase heat transfer. The fluid exhibits good thermal stabHity in the recommended temperature range and is inhibited against oxidation. The products of decomposition are soluble and viscosity increases as decomposition proceeds. The vapor pressure of the fluid is negligible and it is not feasible to recover the used fluid by distiHation. Also, because the degradation products are soluble in the fluid, it is not possible to remove them by filtration any spent fluid usuaHy must be burned as fuel or discarded. The fluid is soluble in water. 

Phosphonium salts are typically stable crystalline soHds that have high water solubiUty. Uses include biocides, flame retardants, the phase-transfer catalysts (98). Although their thermal stabiUty is quite high, tertiary phosphines can be obtained from pyrolysis of quaternary phosphonium haUdes. The hydroxides undergo thermal degradation to phosphine oxides as follows ... 

Storage tanks, lines, and pumps should be heat traced and insulated to enable product handling. Temperature control is required to prevent product degradation because of color alkan olamines have poor heat transfer properties. Exposure to air will also cause product discoloration. Storage tanks should be nitrogen-padded if low color product is required. 

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