Precipitation-dispersion model

Very fine mineral particles precipitate by a rapid mixing of hydrothermal solution with seawater and transport laterally, affected by ocean current. Vertical settling of mineral particle from the plume is controlled by Stokes equation 

The calculations on the settling velocity using (4.12) indicate that the particles with more than 100 pm in diameter settle from the plume onto seafloor, but the particles with less than 100 pm do not. Electron microscopic observation of the chimneys from Kuroko and mid-oceanic ridge deposits supports this calculation results. 

Above consideration concerns only with the vertical settling of mineral particles. Feely et al. (1987) considered the effect of dispersion of black smoker mineral particles from active vents on the Juan de Fuca ridge in addition to the settling and obtained the distribution of the size of mineral particles settled onto the seafloor from the equation 

The particles are assumed to have sufficiently large settling velocity that diffusion in the vertical direction has little effect on overall particle movement. The results of calculations clarified that the pyrite particles with small grain size less than 100 pm do not settle near the vent of hydrothermal solution but the majority of large-grained black smoker particles should be deposited within a few hundred meters of the vent (Fig. 4.9). 

---

Tritium measurements are frequently used to calculate recharge rates, rates or directions of subsurface flow, and residence times. For these purposes, the seasonal, yearly, and spatial variations in the tritium content of precipitation must be accurately assessed. This is difficult to do because of the limited data available, especially before the 1960s. For a careful discussion of how to calculate the input concentration at a specific location, see Michel (1989) and Plummer et al. (1993). Several different approaches (e.g., piston-flow, reservoir, compartment, and advective-dispersive models) to modeling tritium concentrations in groundwater are discussed by Plummer et al. (1993). The narrower topic of using environmental isotopes to determine residence time is discussed briefly below. 

Parameter Estiiiiation results. The dispersion model including the growth and nucleation models was applied to the experimental precipitation data in the dispersion range. Typical fit of the calculated cumulative particle size distribution and the experimental values for these data are presented in Figure 10. Calculated values and experimental data are in agreement within 15%. Details can be found in (13). 

The semi-empirical model used for an MSMPR and the dispersion model for the tubular precipitator proved to be capable of deterniining the kinetic parameters (nucleation rate, aggregation rate, and growth rate) from the experimental data. 

These block copolymers can act as effective steric stabilizers for the dispersion polymerization in solvents with ultralow cohesion energy density. This was shown with some polymerization experiments in Freon 113 as a model solvent. The dispersion particles are effectively stabilized by our amphi-philes. However, these experiments can only model the technically relevant case of polymerization or precipitation processes in supercritical C02 and further experiments related to stabilization behavior in this sytem are certainly required. 

Aggregates are microprecipitates of tens to thousands of polymer molecules which can be invisible to the naked eye or appear as a faint cloudiness in the solution. They form when a poor solvent such as methanol is added to a molecularly dispersed solution of the polymer in a good solvent, such as toluene or THF, or vice versa. Since aggregates are the first stage of a solid precipitate, they are also useful as model materials of the solid bulk state polymers and are amenable to study by many solution techniques such as solution-state UV and CD spectroscopy. Chiral molecules... 

Conventional routes to ceramics involve precipitation from solution, drying, size reduction by milling, and fusion. The availability of well-defined mono-dispersed particles in desired sizes is an essential requirement for the formation of advanced ceramics. The relationship between the density of ceramic materials and the sizes and packing of their parent particles has been examined theoretically and modeled experimentally [810]. Colloid and surface chemical methodologies have been developed for the reproducible formation of ceramic particles [809-812]. These methodologies have included (i) controlled precipitation from homogeneous solutions (ii) phase transformation (iii) evaporative deposition and decomposition and (iv) plasma- and laser-induced reactions. 

Mechanistic Multiphase Model for Reactions and Transport of Phosphorus Applied to Soils. Mansell et al. (1977a) presented a mechanistic model for describing transformations and transport of applied phosphorus during water flow through soils. Phosphorus transformations were governed by reaction kinetics, whereas the convective-dispersive theory for mass transport was used to describe P transport in soil. Six of the kinetic reactions—adsorption, desorption, mobilization, immobilization, precipitation, and dissolution—were considered to control phosphorus transformations between solution, adsorbed, immobilized (chemisorbed), and precipitated phases. This mechanistic multistep model is shown in Fig. 9.2. 

The classical thermodynamic and kinetic model is that of a rigid sphere impenetrable by water. A spherical geometry has been observed in many polysaccharide systems, notably hyaluronic acid-protein complexes (Ogston and Stainer, 1951), dispersed gum arabic (Whistler, 1993), and spray-dried ungelatinized starch granules (Zhao and Whistler, 1994). Spherulites of short-chain amylose were obtained by precipitation with 30% water-ethanol (Ring et al., 1987), and spherulites of synthetic polymers were obtained... 

Particularly sophisticated models deal with reactive mass transport, including both the accurate description of the convective and dispersive transport of species, as well as the modeling of interactions of species in water, with solid and gaseous phases (precipitation, dissolution, ion exchange, sorption). 

Model the carbonate precipitation in this carbonate channel by means of a Id transport with 40 cells of 10 m length each. Dispersivity is assumed with lm. Use the key words KINETICS and RATES and the BASIC program for calcite from the data set PHREEQC.dat describing the kinetics for both the calcite dissolution and the calcite precipitation. How much calcite precipitates each year within the channel s first 400 meter after the discharge How much C02 degasses at the same time ... 

Like crystallization, US also successfully assists the formation of extremely finely divided and uniform particles, which can be termed sonoprecipitation. This effect, which has not yet been used in analytical chemistry and might facilitate sample preparation in nephelometric or turbidimetric methods, has been widely exploited by the pharmaceutical industry to prepare liquid dispersions of drugs for oral or subcutaneous administration where extremely small particle sizes ensure stable suspensions of the drug and faster assimilation into the body. On a laboratory scale, US-assisted precipitation of magnesium carbonate in a model system has been studied [65]. 

---