Volatile compounds diffusion

Carbonization Cycle. Carbonization is basically a heating cycle. The precursor is heated slowly in a reducing or inert environment, over a range of temperature that varies with the nature of the particular precursor and may extend to 1300°C. The organic material is decomposed into a carbon residue and volatile compounds diffuse outtothe atmosphere. The process is complex and several reactions may take place at the same time such as dehydrogenation, condensation and isomerization. 

The mechanism of poisoning automobile exhaust catalysts has been identified (71). Upon combustion in the cylinder tetraethyllead (TEL) produces lead oxide which would accumulate in the combustion chamber except that ethylene dibromide [106-93-4] or other similar haUde compounds were added to the gasoline along with TEL to form volatile lead haUde compounds. Thus lead deposits in the cylinder and on the spark plugs are minimized. Volatile lead hahdes (bromides or chlorides) would then exit the combustion chamber, and such volatile compounds would diffuse to catalyst surfaces by the same mechanisms as do carbon monoxide compounds. When adsorbed on the precious metal catalyst site, lead haUde renders the catalytic site inactive. 

Volatile Compounds. For volatile chemicals, Goring (1) showed that diffusion In the soli could be described mathematically as though It occurred only In the air phase. 

The last two assumptions are the most critical and are probably violated under field conditions. Smith et al. (3) found that at least a half-hour was required to achieve adsorption equilibrium between a chemical in the soil water and on the soil solids. Solution of the diffusion equation has shown that many volatile compounds have theoretical diffusion half-lives in the soil of several hours. Under actual field conditions, the time required to achieve adsorption equilibrium will retard diffusion, and diffusion half-lives in the soil will be longer than predicted. Numerous studies have reported material bound irreversibly to soils, which would cause apparent diffusion half-lives in the field to be longer than predicted. 

Boars and reproductive females differ in their secretions with respect to the amount of compounds they have in common rather than the presence of type-specific compounds. Eighty percent of compounds common in boars and reproductive females occur at a higher concentration in boars than in reproductive females. Seventy eight percent of all compounds common to reproductive and non-reproductive females are at a lower concentration in non-reproductive females than in reproductive females. Reproductive females and boars may compensate the high diffusion rate of their volatile compounds by excreting them at a higher concentration compared to non-reproductive females. This may increase their persistence and advertising the presence of a reproductive female or boar for a... 

The vapour pressures of the main volatile compounds involved in esterification and polycondensation are summarized in Figure 2.25. Besides EG and water, these are the etherification products DEG and dioxane, together with acetaldehyde as the main volatile product of thermal PET degradation. Acetaldehyde, water and dioxane all possess a high vapour pressure and diffuse rapidly, and so will evaporate quickly under reaction conditions. EG and DEG have lower vapour pressures but will still evaporate from the reaction mixture easily. 

Hoftyzer and van Krevelen [100] investigated the combination of mass transfer together with chemical reactions in polycondensation, and deduced the ratedetermining factors from the description of gas absorption processes. They proposed three possible cases for poly condensation reactions, i.e. (1) the polycondensation takes place in the bulk of the polymer melt and the volatile compound produced has to be removed by a physical desorption process, (2) the polycondensation takes place exclusively in the vicinity of the interface at a rate determined by both reaction and diffusion, and (3) the reaction zone is located close to the interface and mass transport of the reactants to this zone is the rate-determining step. 

Both the mass-transfer approach as well as the diffusion approach are required to describe the influence of mass transport on the overall polycondensation rate in industrial reactors. For the modelling of continuous stirred tank reactors, the mass-transfer concept can be applied successfully. For the modelling of finishers used for polycondensation at medium to high melt viscosities, the diffusion approach is necessary to describe the mass transport of EG and water in the polymer film on the surface area of the stirrer. Those tube-type reactors, which operate close to plug-flow conditions, allow the mass-transfer model to be applied successfully to describe the mass transport of volatile compounds from the polymer bulk at the bottom of the reactor to the high-vacuum gas phase. 

Volatilization (also referred to as vaporization or evaporation) is the conversion of a chemical from the sohd or hquid phase to a gas or vapor phase. The partitioning of a volatile compound in the subsurface environment comprises two distinct patterns volatilization of contaminant molecules (from the liquid, sohd, or adsorbed phase) and dispersion of the resulting vapors in the subsurface gas phase or the overlying atmosphere by diffusive and turbulent mixing. Even though the two processes are fundamentally different and controlled by different chemical and environmental factors, they are not wholly independent under natural conditions only by integrating their effects can volatilization be characterized. 

Pheromone propagation by wind depends on the release rate of the pheromone (or any other odor) and air movements (turbulent dispersion). In wind, the turbulent diffusivity overwhelms the diffusion properties of a volatile compound or mixture itself. Diffusion properties are now properties of wind structure and boundary surfaces, and preferably termed dispersion coefficients. Two models have dominated the discussion of insect pheromone propagation. These are the time-average model (Sutton, 1953) and the Gaussian plume model. 

Simple Bottleneck Boundaries A Simple Noninterface Bottleneck Boundary Illustrative Example 19.1 Vertical Exchange of Water in a Lake Two- and Multilayer Bottleneck Boundaries Bottleneck Boundary Between Different Media Illustrative Example 19.2 Diffusion of a Volatile Compound from the Groundwater Through the Unsaturated Zone into the Atmosphere... 

Illustrative Example 19.2 Diffusion of a Volatile Compound from the Groundwater... 

A. Typical Systems. A simple system for the transfer of samples to an infrared gas cell or to a NMR sample tube consists of a fore pump, diffusion pump, trap, and manifold (Fig. 5.1). At the other extreme is a general-purpose chemical vacuum line, which permits the separation of volatile compounds, transfer of noncondensable gases, and storage of reactive gases and solvents (Fig. 5.2). When attack of stopcock grease is a serious problem, grease-free de-... 

The tube type samplers have lower diffusive uptake rates and are therefore less prone to starvation effects. In consequence they require longer exposure periods to collect the same mass of analyte. The occurrence of reverse diffusion depends on the analyte-sorbent interaction, but for weaker adsorbents such as Tenax TA this does occur with more volatile compounds for example, compounds more volatile than toluene. 

Back diffusion can be prevented by use of stronger adsorbents such as graphi-tized carbon but recovery of less volatile compounds by thermal desorption is less efficient. The analysis can be automated and commercial equipment is available. 

Yoshida, T., Matsunage, I., Tomioka, K. and Kumagai, S. (2006a) Interior air pollution in automotive cabins by volatile organic compounds diffusing from interior materials I Survey of 101 types of Japanese domestically produced cars for private use. Indoor and Built Environment, 15, 425-44. 

In a MD process, a microporous hydrophobic membrane is in contact with an aqueous heated solution on the feed or retentate side. The hydrophobic nature of the membrane prevents the mass transfer in liquid phase and creates a vapor/liquid interface at the entrance of each pore. Here, volatile compounds (typically water) evaporate, diffuse and/or convert across the membrane, and are condensed and/or removed on the permeate or distillate side. 

The detection limits for SPME headspace sampling are equivalent to SPME liquid sampling for volatile compounds. However, semivolatile organic compounds diffuse slowly into the headspace so that SPME headspace sampling is not appropriate for semivolatile compounds [134],... 

Static headspace extraction is also known as equilibrium headspace extraction or simply as headspace. It is one of the most common techniques for the quantitative and qualitative analysis of volatile organic compounds from a variety of matrices. This technique has been available for over 30 years [9], so the instrumentation is both mature and reliable. With the current availability of computer-controlled instrumentation, automated analysis with accurate control of all instrument parameters has become routine. The method of extraction is straightforward A sample, either solid or liquid, is placed in a headspace autosampler (HSAS) vial, typically 10 or 20 mL, and the volatile analytes diffuse into the headspace of the vial as shown in Figure 4.1. Once the concentration of the analyte in the headspace of the vial reaches equilibrium with the concentration in the sample matrix, a portion of the headspace is swept into a gas chromatograph for analysis. This can be done by either manual injection as shown in Figure 4.1 or by use of an autosampler. 

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