Precipitation kinetics, determination

Zauner and Jones (2000a) describe an experimental set-up for determination of precipitation kinetics, as shown in Figure 6.19. Briefly, the jacket glass reactor (1) (300 ml, d = 65 mm) is equipped with a polyethylene draft tube and four baffles. The contents are stirred using a three-blade marine-type propeller (5) with motor (Haake), which pumps the suspension upwards in the annulus and downwards inside the draft tube. Measured power inputs ranged from 3.3 X 10- to 1.686 W/kg. 

The model is able to predict the influence of mixing on particle properties and kinetic rates on different scales for a continuously operated reactor and a semibatch reactor with different types of impellers and under a wide range of operational conditions. From laboratory-scale experiments, the precipitation kinetics for nucleation, growth, agglomeration and disruption have to be determined (Zauner and Jones, 2000a). The fluid dynamic parameters, i.e. the local specific energy dissipation around the feed point, can be obtained either from CFD or from FDA measurements. In the compartmental SFM, the population balance is solved and the particle properties of the final product are predicted. As the model contains only physical and no phenomenological parameters, it can be used for scale-up. 

TABLE 1 Theophylline Release Rates from Asymmetric-Membrane Tablet Coatings, Results of Fitting Release Profiles to Zero-Order Kinetics, and Precipitation Times Determined from Model Predictions... 

The permanganate ion was labeled with " Mn. The separation of Mn04 for kinetic determinations was achieved by co-precipitation with tetraphenylarsonium perrhenate, and the activity was determined with a scintillation counter. The rate constant, Arexch = 710 M s at 273 K, was obtained by fitting of the data to Eq. 65... 

Laboratory studies are in progress to determine the calcium sulfite precipitation kinetics and the oxidation kinetics of sulfite to sulfate. Until these reaction rate expressions are developed, the experimental data obtained from the pilot plant, prototype, and field units will be used to design the reaction tanks and scrubbers to eliminate calcium sulfite scaling. 

Mullis, A. M. 1991. The role of silica precipitation kinetics in determining the rate of quartz pressure solution, J. Geophys. Res., 96 pp. 10,007-10,013. 

In the model considered below, the role of both grain boundary and bulk diffusion in the transformation front and close to it, respectively, is analyzed within the problem of unambiguous determination of the discontinuous precipitation parameters in the binary Pb-Sn system at room temperature [9]. In order to complete this, we use the principle of maximum rate of free energy release and balance of entropy fluxes for the description of discontinuous precipitation kinetics for binary polycrystaUine alloys and independent determination of three basic parameters interlamellar distance, rate of phase transformation front, and concentration profile close to the transformation front. While solving the problem, we also find the optimal concentration distribution of components both along the precipitation lamella behind the transformation front and close to it, as well as the degree of the components separation. 

Similarly, several authors have presented MSMPR methods for kinetics determination from continuous crystallizer operation (Chapter 3), which have become widely adopted. In an early study, Bransom etal. (1949) anticipated Randolph and Larson (1962) and derived a crystal population balance to analyse the CSD from the steady state continuous MSMPR crystallizer for growth and nucleation kinetics. Han (1968) proposed a method of kinetics determination from the moments of the CSD from a cascade of continuous crystallizers and assessed the effect of sample position. Timm and Larson (1968) suggested the use of the extra information present in transient response data to determine kinetics, followed by Sowul and Epstein (1981), Daudey and de Jong (1984) and Jager etal. (1991). Tavare (1986) applied the j-plane analysis to the precipitation of calcium oxalate, again assuming nucleation and growth only. 

The kinetic parameters, which were determined from laboratory-scale continuous experiments as a function of the energy input and/or supersaturation, were applied to the semibatch mode of operation without any adjustments or parameter fitting. The SFM slightly underestimates the mean particle size in the range between 0.01 and 1 W/kg, but correctly predicts the smaller particle size obtained experimentally for the 25 1 reactor. On the same scale, the model also predicts a lesser degree of dependence of the particle size on the specific power input due to the interactions of mixing and the precipitation kinetics. This behaviour has also been observed experimentally in this research. 

Initially, we need to know the thermodynamic properties or particularly the constant of solubility, besides the energy involved and verify if the process is thermodynamically possible. It is also very important to know about the kinetics of precipitation for determining the precipitation rates. 

These relationships are generally determined empirically, because of the complex kinetics of the precipitation polymerization process and the large variations from one reaction system to another. Nevertheless, a review of the literature presents useful guidelines for process design experiments. 

An important result of the concepts discussed in this section and the preceding one is that precipitation and complexation reactions exert joint control over metal ion solubility and transport. Whereas precipitation can limit the dissolved concentration of a specific species (Me ), complexation reactions can allow the total dissolved concentration of that metal to be much higher. The balance between these two competing processes, taking into account kinetic and equilibrium effects, often determines how much metal is transported in solution between two sites. 

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