Flow dissolution experiments

Figure 10 Rotational (tangential) flow (UA) as a function of stirring rate (co) for paddle (filled circles) and basket (open circles) Mean SD position S2 approximately 1 cm above the paddle and midway between the paddle shaft and the wall of the dissolution vessel. (Please note that, in contrast to simulation techniques such as, for instance, computational fluid dynamics, these data are based on dissolution experiments.) Source Data from Ref. 10, UPE method.

In the paddle method, bulk Reynolds numbers range from Re = 2292 (25 rpm, 900 mL) up to Re = 31025 (200 rpm, 500 mL). In contrast, Reynolds numbers employing the basket apparatus range from Re = 231 to Re = 4541. These Reynolds numbers are derived from dissolution experiments in which oxygen was the solute [(10), Chapter 13.4.8] and illustrate that turbulent flow patterns may occur within the bulk medium, namely for flow close to the liquid surface of the dissolution medium. The numbers are valid provided that the whole liquid surface rotates. According to Levich (9), the onset of turbulent bulk flow under these conditions can then be assumed at Re 1500. 

ICENHOWER, J. P., McGRAIL, B. P., SCHAEF, H. T. Rodriquez, E. A. 2000. Dissolution kinetics of titanium pyrochlore ceramics at 90 °C by singlepass flow-through experiments. In Smith, R. W. Shoesmith, D. W. (eds) Scientific Basis for Nuclear Waste Management XXIII. Materials Research Society Symposium Proceedings, 608, 373-378. 

The first single-mineral dissolution experiments to utilize radiogenic isotopes investigated the dissolution of the feldspars bytownite, microcline, and albite in flow-through cells at a pH 3 and at 25 °C (Brantley et al., 1998). Solutions from major element experiments (Stillings and Brantley, 1995) were reanalyzed for strontium and rubidium concentrations and Sr/ Sr ratios with the goal of... 

Figure 13.5. Transport vs surface controlled dissolution. Schematic representation of concentration in solution, C, as a function of distance from the surface of the dissolving mineral. In the lower part of the figure, the change in concentration (e.g., in a batch dissolution experiment) is given as a function of time, (a) Transport controlled dissolution. The concentration immediately adjacent to the mineral reflects the solubility equilibrium. Dissolution is then limited by the rate at which dissolved dissolution products are transported (diffusion, advection) to the bulk of the solution. Faster dissolution results from increased flow velocities or increased stirring. The supply of a reactant to the surface may also control the dissolution rate, (b) Pure surface controlled dissolution results when detachment from the mineral surface via surface reactions is so slow that concentrations adjacent to the surface build up to values essentially the same as in the surrounding bulk solution. Dissolution is not affected by increased flow velocities or stirring. A situation, intermediate between (a) and (b)—a mixed transport-surface reaction controlled kinetics—may develop.

Medication can be delivered via the eye in the form of eyedrops or in an ocular device. Sodium or zinc insulin was incorporated into a Gelfoam sponge-based device. An in vitro dissolution test indicated that the release of insulin from the device was proportional to the flow rate of the dissolution medium. An in vivo dissolution experiment provided support for the hypothesis that there was a direct relationship between the prolonged pharmacological response to insulin and its release from the device. The ocular device with or without the aid of an enhancer was placed in the eye of rabbits as an ocular insert and produced a uniform blood glucose-lowering effect of 60% over 8h. The blood glucose... 

In most penetration scans performed in surfactant dissolution experiments the phases are homogeneous and the interface between them is sharp. However, in some cases the interface becomes unstable and dramatic instobilities can be observed. There are many examples of instabilities that are well understood that maybe rationalized in terms of kinetic maps or dissolution paths, or dynamic instabilities involving fluid flow (e.g. Marangoni effects) or other Laplacian growth instabilities , such as Mullins-Sekerka instabilities (3J). However, myelins (Figure 1) are an example of an instability that remains poorly understood. 

If the rate of change of the solution concentration is measured at the onset of the experiment and again at several later times, the slope of a graph of the left-hand side of this equation versus time is kp = lti = 2rVJD ), which can be used to find k, and the dissolution rate constant (k+). This approach would be especially suitable for mixed flow reactor experiments where the rates are determined directly from the solution composition and flow rate. 

To evaluate this hypothesis, experimentalists have been trying to provide data on mineral solubility and dissolution kinetics as a function of temperature, pH, and fluid composition. Experimental techniques and results of published studies are summarized below along with new data from our laboratory, including the results of pH-buffered, flow-through experiments that track variations in pore-fluid chemistry through time (Reed 1990 Reed and Hajash 1990, 1992 Franklin 1991 Franklin et al. 1990, 1991). 

Reed CL, Hajash A (1990) Effect of flow rate on dissolution of granitic sand in oxalic acid Flow-through experiments at 100 °C, 345 bars. Geol Soc Am Abstr Programs 22 A291... 

During the dissolution reactions, the RHSE merely reduces in size. Thus, the change in the flow characteristics will be insignificant within a reasonably short duration of an experiment. 

Dissolution rate of carbonates as a function of pH. These experiments were carried out with a continuous flow reactor in open systems with controlled pco2-... 

Figures 6 and 7 are derived from laboratory experiments and illustrate that flow can become turbulent close to particle walls even when the bulk flow remains laminar. The turbulent vortices bore into the particle surface, magnifying cavitations and abrading protrusions, and hence accelerating the dissolution process [(10), Chapter 4.3.5]. However, irregulari-...

Work on the rate of dissolution of regular shaped solids in liquids has been carried out by Linton and Sherwood(1), to which reference is made in Volume 1. Benzoic acid, cinnamic acid, and /3-naphthol were used as solutes, and water as the solvent. For streamline flow, the results were satisfactorily correlated on the assumption that transfer took place as a result of molecular diffusion alone. For turbulent flow through small tubes cast from each of the materials, the rate of mass transfer could be predicted from the pressure drop by using the 1 j-factor for mass transfer. In experiments with benzoic acid, unduly high rates of transfer were obtained because the area of the solids was increased as a result of pitting. 

However, the differences between an experiment whereby 1.4 1/s of the total fines flow of 1.7 1/s is dissolved and an experiment at the same residence time of 1.25 hrs without fines dissolution, are negligible. This is probably due to the small cut size which is attained with the present configuration. Increasing the cut size by using an insert in the annular zone, may be one method of increasing the effect of fines removal. 

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