Froths, physical properties

To predict the height of aerated liquid on the plate, and the height of froth in the downcomer, some means of estimating the froth density is required. The density of the aerated liquid will normally be between 0.4 to 0.7 times that of the clear liquid. A number of correlations have been proposed for estimating froth density as a function of the vapour flow-rate and the liquid physical properties see Chase (1967) however, none is particularly reliable, and for design purposes it is usually satisfactory to assume an average value of 0.5 of the liquid density. 

Separation of milled solid materials is usually based on differences in their physical properties. Of the various techniques to obtain ore concentrates, those of froth flotation and agglomeration exploit differences in surface activities, which in many cases appear to involve the formation of complexes at the surface of the mineral particles. Separation by froth flotation (Figure 4) depends upon conversion of water-wetted (hydrophilic) solids to nonwetted (hydrophobic) ones which are transported in an oil-based froth leaving the undesired materials (gangue) in an aqueous slurry which is drawn off from the bottom of the separator. The selective conversion of the ore particles to hydrophobic materials involves the adsorption of compounds which are usually referred to as collectors. 4... 

Materials mined from a mineral deposit usually consist of a heterogeneous mixture of solid phases that are generally crystalline and contain various minerals. Crushing and grinding operations are used to liberate the mineral species from one another and to reduce the size of the solids to a range suitable for subsequent processing. Of the various separation techniques, those of froth flotation and agglomeration exploit the chemical and physical properties of the surfaces of minerals, which can be controlled by various chemical interactions with species in an aqueous phase. 

When Fair s correlation was developed, little was known about the difference between spray and froth entrainment flooding, and the data base used was small and included both types. The author compared predictions from Fair s correlation to a much wider data bank available at present. The correlation predicted most of these data well, perhaps somewhat on the conservative side. However, the correlation has been less successful ii. reliably predicting some of the effects (described above) of physical properties, operating variables, and tray geometry on entrainment flooding. 

The clear liquid height at the froth-to-spray transition, hct, is calculated using the Jeronimo and Sawistowski (35) correlation, as modified for physical properties by Kister and Haas (86). The relevant equations are Eqs. (6.68) to (6.70). The recommended range of application of the correlation is shown in Table 6.8. 

The frothing process was developed by the Du Pont Co. (25). The process has the following advantages isotropic physical properties and lower foaming pressure. The method is preferably used for large-panel production at in-plant production or pour-in-place foaming in field applications, e.g., building panels, chemical tanks, etc. 

Frothing was conducted by the co-use of dichlorodifluoromethane, in which the mixing ratio was 2.2 parts of crude MDl per 1.0 part of the solution. The equivalent ratio of NCO/OH was 5.6 and the weight ratio of crude MDI/polyol was 0.14. The frothing mixture was poured into a 100 mm thick panel. The physical properties of the resulting foam were as follows overall density, 0.047 g/cm core density, 0.042 g/cm compressive strength in the direction of foam rise, 2.2 kg/cm the same in the perpendicular direction, 1.6 kg/cm closed-cell percent, 93.8 thermal conductivity, 0.015 kcal/mh C (71). 

The amount of liquid on the plate increases with the weir height and with the flow rate of liquid, but it decreases slightly with increasing vapor flow rate, because this decreases the density of the froth. The liquid holdup also depends on the physical properties of liquid and vapor, and only approximate methods of predicting the holdup are available. A simple method of estimating h, uses the weir height h, the calculated height of clear liquid over the weir and an empirical correlation factor /3 ... 

Plate efficiencies and HETP values are complex functions of measurable physical properties temperature, pressure, composition, density, viscosity, diflusivity, and surface tension measurable hydrodynamic factors pressure drop and liquid and vapor flow rates plus factors that cannot be predicted or measured accurately foaming tendency, liquid and gas turbulence, bubble and droplet sizes, flow oscillations, emulsification, contact time, froth formation, and others. Values for plate efficiency, HETP, or HTU, particularly those that purport to compare various devices, are usually taken over a limited range of concentration and liquid-to-vapor ratios. The crossovers in Fig. 2.5 and the rather strange behavior of the ethyl alcohol-water system, Fig. 2.6, demonstrate the critical need for test data under expected operating conditions. ... 

Camphor oils refer to the by-products during the extracting process of camphor from camphorwood [2]. The physical properties of various camphor oils can be seen from Table 3.1. Compared with pine oils, the selectivities of those camphor oils with low-boiling fractions (such as white oil) are relatively better. The frothing performances of those camphor oils with high-boiling fractions (such as red oil) are relatively stronger. 

Froth flotation (qv) is a significant use of foam for physical separations. It is used to separate the more precious minerals from the waste rock extracted from mines. This method reHes on the different wetting properties typical for the different extracts. Usually, the waste rock is preferentially wet by water, whereas the more valuable minerals are typically hydrophobic. Thus the mixture of the two powders are immersed in water containing foam promoters. Also added are modifiers which help ensure that the surface of the waste rock is hydrophilic. Upon formation of a foam by bubbling air and by agitation, the waste rock remains in the water while the minerals go to the surface of the bubbles, and are entrapped in the foam. The foam rises, bringing... 

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