Dense phase suitability

Based on the previous classifications and discussions, as well as the author s own experiences, it is suggested that the following design procedure and considerations be adopted to provide an initial indication of dense-phase suitability, which as described below also has been found useful in assessing conveying performance over long distances. 

When a liquid-liquid interface is to be investigated using an electrode in the more dense phase, or for studies at the water-air interface, a submarine electrode can be deployed [18,19,34], depicted schematically in Fig. 3(b). In this case, the electrode is inverted in the cell, such that the tip points upwards, and an insulated connection is made through the solution. Metal electrodes down to the nanometer scale can also be fabricated by sealing an etched Pt or Pt-Ir wire in a suitable insulating material, leaving just the etched end exposed [35-37]. 

When evaluating a material for the purpose of establishing dense-phase and long-distance suitability, it is important to undertake all the necessary tests (e.g., particle sizing, particle and bulk densities, fluidization and deaeration). Also, if possible, it is useful to compare such results with those obtained on previously conveyed similar materials (e.g., fly ash). However, it should be noted that such an evaluation only is a qualitative one and it is not possible to predict say, minimum air flows or pipeline pressure drop based on such data (i.e., pilot-scale tests normally are required to confirm minimum velocities, friction factors, etc., especially over long distances and for large-diameter pipes). 

In supercritical extraction, the components are transferred from a liquid phase to a supercritical dense phase at equilibrium with the liquid. A system would be suitable for supercritical extraction if the extracted component, or solute, is adequately soluble in the supercritical solvent, and its solubility is a strong function of the solvent density. This phenomenon facilitates the recovery of the solute from the solvent since lowering its pressure or raising its temperature will lower its density. The solubility of the solute drops at the lower solvent density allowing the solute to be recovered, possibly with no need for a distillation column following the extractor. 

Matching the lubrication equation to thermodynamic theory requires some caution, since thermodynamic theory yielding an expression for pL should be applied to the entire system including dense (liquid) and dilute (vapor) phases in equilibrium, whereas only the dense phase may have a suitable aspect ratio. To make the approximation applicable, one has to assume that the interface dividing the dense and the dilute phase is only weakly inclined relative to the substrate and weakly curved, so that its position can be expressed by a function h x, t) with derivatives obeying the above lubrication scahng. Thermodynamic theory, either local or nonlocal, can be used to compute an equilibrium density profile across the interface (in the vertical direction), po z — h x, t)), which is weakly dependent on the horizontal 2D position and time only through its dependence on h, e.g. 

Both sessile and pendant drops are suitable for use in liquid-gas and liquid-liquid systems, provided, of course, that the liquid surrounding the drop is transparent. Changes in drop shape can be followed to determine time-dependent effects on interfadal tension. Both methods have been used to measure low interfacial tensions in liquid-liquid systems, but only the sessile drop works well for tensions below about 0.01 mN/m. Finally, it should be noted that sessile and pendant bubbles of the less dense phase can be employed where the whole apparatus is, in effect, turned upside down (Figure 1.8). 

Settler interface levels. The interface level in the end stage where the dense phase leaves the extractor is simply controlled by means of a dense phase overflow weir. This is set at a suitable height to maintain the interface below the level of the mixed phase port. 

We have used our pore models to discuss the effect of surface roughness and structural defects on the adsorption mechanism and on the nature of the dense phases. In Fig. 4 we present plots of the local density profile p r, z), as well as representative simulation snapshots for the three pore models at different values of P/Pq and r= 100 K. For pore model A, p also depends on the angular coordinate 0 nevertheless, a plot of pUp, z) can provide a suitable measure of the local state of the confined phase. As P/Po increases, the pore walls are covered by an adsorbate film whose thickness increases gradually with P/Po, until it reaches a point when there is formation of a condensate bridge between low density regions of adsorbate in... 

As two air movers are provided, the most suitable exhauster can be dedicated to the vacuum system and the most appropriate positive pressure system can be used for the onward transfer of material. If the vacuum off-loading section is only a short distance, it is possible that the material could be conveyed in dense phase over the entire conveying distance. 

Porous membranes are widely used in the filtration of liquid mixtures in pressure driven processes. Their rejection is mainly determined by the pore size and pore size distribution rather than by the membrane material properties. However, the direct contact of a porous membrane with a liquid phase can cause swelling of the polymer, which, in turn, causes complete squeezing of the nanopores, resulting in a dense membrane suitable for pervaporation [54]. Knudsen diffusion is the dominating mechanism in micropores (2-50 nm). It is characterized by a Hnear relation between the gas permeability and the inverse square root of the gas molecular weight. 

FIG. 3 (a) Block schematic of the typical instrumentation for SECM with an amperometric UME tip. The tip position may be controlled with various micropositioners, as outlined in the text. The tip potential is applied, with respect to a reference electrode, using a potential programmer, and the current is measured with a simple amplifier device. The tip position may be viewed using a video microscope, (b) Schematic of the submarine UME configuration, which facilitates interfacial electrochemical measurements when the phase containing the UME is more dense than the second phase. In this case, the glass capillary is attached to suitable micropositioners and electrical contact is made via the insulated copper wire shown. 

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