Foam Pressure Drop Technique

The capillary pressure pa in a foam can be significantly increased by the Foam Pressure Drop Technique employed to produce dry foams [32-35]. The principle of this method is that the foam is brought into contact with a porous plate (usually sintered glass filter) under which a reduced pressure p 0 is created, and this pressure difference Ap = p0 - p 0 should not exceed the capillary pressure in filter pores 2acosd / rn (where 6 is the contact... 

The quantity xp is a much better defined characteristic of foam stability (since the pressure in the borders along the height of the foam column remains constant during its destruction). This parameter is also much more sensitive to the kind of surfactant, electrolyte concentration and other additives, compared to the lifetime of the foam in gravitational field, with an averaged pressure value from 0 to pgH. Estimation of the stability of foams from different surfactants by xp and by the Ross-Miles test has been reported in [16]. The results are discusses in Section 7.6.1. The advantages of the Foam Pressure Drop Technique and, respectively, xp as a characteristic of the foam stability, are clearly shown. 

As already mentioned (see Chapter 3), at the instant of foam formation the films and borders are in non-equilibrium state. The films thin mainly due to the capillary pressure, while the borders thin due to gravity or a pressure drop (when the foam is dried by the Foam Pressure Drop Technique [21-23]). The surfactant adsorption layers decrease the flow rate through the borders and films and the process of thinning becomes similar to the flow in thin gaps with solid surfaces. As indicated in Sections 3.2.1 and 5.3 the degree of retardation of the flow depends on the surfactant type and concentration as well as on the film type. A complete immobility at the film and border surfaces usually is not reached. 

These two examples with the homologous series of alkylsulphonates and alkylsulphates indicate the undoubted advantages of Foam Pressure Drop Technique for determining the foam stabilising properties of surfactants. This technique allows to distinguish small differences in the foam stabilising ability of surfactants. 

Though not quantitative, the comparison between the two techniques provides information about the effect of the pressure in the foam liquid phase as well as the effect of the foam film type. The advantages of the Foam Pressure Drop Technique for estimating the foam stabilising ability of the surfactants is indisputable. 

Experimental methods for the design parameters outlined here contain both physical and chemical techniques. The gas holdup can be accurately measured by the techniques described here except when pronounced foaming occurs. In the presence of foam, the gas holdup is best measured by photographic or pressure-drop methods. The bubble size can be measured by photographic methods however, this method contains inherent inaccuracies, as outlined earlier. 

Recently a new technique has been introduced for the study of foam drainage under pressure drop. The especially constructed apparatus allows automated calculation of foam expansion ratio at any instant of time (see Section 5.3.4). 

A special technique has been developed by Kuznetsova and Kruglyakov [28] for the study of liquid flow through a foam with either constant or variable border cross-sections. It allows to measure small liquid volumes and the number of independent foam borders. The foam studied is closed between two porous plates the external sides of which are in contact with the foaming solution. The measurements are carried out at identical (gravitational flow) as well as variable reduced pressure (flow at applied pressure drop). 

The second technique is based on a filter to capture the soot particulates. Common filters are wall flow monoliths or ceramic foams. Cordierite wall flow monoliths are probably currently the most used particulate traps. They can capture diesel particulates with an efficiency of 99%. At normal diesel engine exhaust gas temperatures, the captured soot is not reactive enough to prevent build up on the filter, with an intolerable high pressure drop over the exhaust system as a result. The oxidation rate of the soot should, therefore, be increased which can be achieved by increasing the temperature of the filter, resulting in higher fuel consumption and thus making this solution unfavourable. The other possibility is catalytic oxidation of the collected soot. Several catalytic systems will be discussed. 

In order to study reactions in liquid phase, it is necessary to develop new experimental techniques that will allow operando spectroscopy and transient studies of liquid phase heterogeneous catalytic reactions. Essential for such technique is a reactor module. Chromolith HPLC column (Merck) [1] with sihca foam in a polymer cartridge is suitable as a reactor for transient experiments because the high surface area silica foam can act as support with relatively low pressure drop. However, thermal stability of this HPLC column is limited to low temperatures because of the polymer housing (<150°C). It is... 

A diagnostic to distinguish between the foaming and nonfoaming regime was proposed by Charpentier et al [5] in which the propensity for foam to collapse was correlated to pressure drop. This foam collapse data is obtained in a special apparatus in which the propensity for two gas bubbles to collapse is measured in the liquid in question. This technique requires that the foam collapse data be obtained at reactor conditions. 

Many methods are available for the measurement of surface and interfacial tensions. Details of these experimental techniques and their limitations are available in several good reviews (75—27). Some methods that are used frequently in foam work are the du Nouy ring, Wilhelmy plate, drop weight or volume, pendant drop, and the maximum bubble-pressure method. In all cases, when solutions, rather than pure liquids, are involved, appreciable changes can take place with time at the surfaces and interfaces. 

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