Precipitation time constants

In the SFM the reactor is divided into three zones two feed zones fj and (2 and the bulk b (Figure 8.1). The feed zones exchange mass with each other and with the bulk as depicted with the flow rates mi 2, i,3 and 2,3 respectively, according to the time constants characteristic for micromixing and mesomix-ing. As imperfect mixing leads to gradients of the concentrations in the reactor, different supersaturation levels in different compartments govern the precipitation rates, especially the rapid nucleation process. 

Co-administration of ofloxacin and chitosan in eyedrops increased the bioavailabUity of the antibiotic [290]. Trimethyl chitosan was more effective because of its solubility (plain chitosan precipitates at the pH of the tear fluid). On the other hand, N-carboxymethyl chitosan did not enhance the corneal permeability nevertheless it mediated zero-order ofloxacin absorption, leading to a time-constant effective antibiotic concentration [291]. Also W,0-carboxymethyl chitosan is suitable as an excipient in ophthalmic formulations to improve the retention and the bioavailability of drugs such as pilocarpine, timolol maleate, neomycin sulfate, and ephedrine. Most of the drugs are sensitive to pH, and the composition should have an acidic pH, to enhance stability of the drug. The delivery should be made through an anion exchange resin that adjusts the pH at around 7 [292]. Chitosan solutions do not lend themselves to thermal sterilization. A chitosan suspension, however. 

Nucleation Fischer found that, in PFHS, nucleation takes place early and only growth occurs thereafter. In the homogeneous precipitation of barium sulfate by sulfate generation from sulfamic acid in a solution containing 0.01 Af barium ion, he found (Section 8-2) that the number of particles was sensibly constant from the beginning to the end of precipitation. Evidently nucleation was complete within the first small fraction of the total precipitation time. From the rate of sulfate generation it... 

The rate constants for dissolution, kd,x. and precipitation, Jcpx (mol m s ) refer only to the species X. If the mineral dissolves congruently, that is, if the ratio of the constituents released to solution is the same as in the solid, then the solid stoichiometric coefficient times the total mineral dissolution and precipitation rate constants can be used. 

IMPORTANCE OF CHARACTERISTIC MATERIAL AND TIME CONSTANTS FOR PRECIPITATION... 

This chapter mainly concerns mixing and precipitation in a jet. Therefore we illustrate some general scaleup principles by considering a nozzle scale-up based on the combined time constants for mixing (tm), nucleation (tn), and growth (tq), as defined in Section 5. One can classify the precipitation subprocesses from the point of view of their competition with mixing. 

Precipitation at constant pH is also a prerequisite in order to coprecipitate two catalyst components at the same time. This is illustrated for example by the preparation of Pd/La catalysts for methanol decomposition.13 The preparation procedure and the La203/Pd weight ratio of these Pd/La203/Si02 catalysts (5wt% Pd) was found to be important for high catalytic activities in methanol decomposition. If the... 

Anoxic precipitation rate constants of —0.5-0.7/day we estimated from the cylindrical coordinate model. A traditional one-dimensional model predicts rates —1.5-2 times higher. Agreement is relatively close between the two models in this case because Mn production rates attenuate rapidly with depth. Fe " precipitation rates of about 10-120/day are estimated from a three-layer one-dimensional model. 

The left term represents the oxygen concentration normalized to the solubility limit C (T) for a given temperature, Co denotes the starting concentration of the oxygen in solution. The right hand side gives the precipitation rate with the time constant V. 

The values of the precipitation rate constant and the equilibrium constant at 25°, 50°, and 75°C (Rimstidt and Barnes, 1980) given in Table 3.2 were used to calculate the concentration versus time graphs shown in Figure 3.5. 

The precipitation of O was monitored by means of infra-red measurements, and the time constant for an exponential approach to equilibrium was obtained at 650 to 1050C. The number density of precipitates was then used to estimate the diffusivity of O in the bulk. It was found that the data could be described by ... 

The effect of macromixing on the performance of chemical reactors is dealt with in standard text books [2] [4] [6] and will not be discussed here. Practical examples of micromixing effects are less known. From the discussion presented in Sec. 4 and influence of micromixing may be expected when a controlling step of the chemical process (time constant t ) competes with the micromixing process (time constant. This may happen especially in three domains fast and complex reaction systems, polymerization reactions, and precipitation (crystallization) reactions. Three examples of such micromixing effects are presented below. 

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