Foam destruction

Esters, for example, dialkyl polypropyleneglycol adipate and dibutyl adipate, also find use as defoamers in the removal of H2S and CO2 from natural gas by bubbling it through an amine solution [659]. Use of the aforementioned components increases the efficiency of foam destruction. 

The measurement of the decrease in foam column height induced by a-particle irradiation can be considered to belong to the macroscopic techniques. As shown in [325,330,331 ] the rate of foam destruction depends on the activity of the oc-source. 

In order to compare the structural parameters of the foam model studied by Kruglyakov et al. [18] with the respective parameters of a real polydisperse foam (individual bubbles of different degree of polyherdisity) Kachalova et. al. [19] performed measurements of the average border radius of curvature of foams with variable expansion ratio. The foam studied, generated by the set-up shown in Fig. 1.4, was obtained from a nonionic surfactant solution of Triton-X-100 (a commercial product) to which NaCl (0.4 mol dm 3) was added. The expansion ratio was determined conductometrically with correction of the change in electrolyte concentration due to the internal foam destruction. The electrolyte concentration... 

Thus, the precise value of foam expansion ratio, calculated from Eq. (4.32) can be evaluated only if the condition h r a is fulfilled (see Section 4.1). However, the change in solution composition in the foam during foaming, caused both by the different adsorption of solution components and the increase in surfactant concentration in the foam liquid phase because of foam destruction, restricts the use of Eq. (4.32). The effect of surface conductivity is another restriction. 

The main features of this technique are the absence of contact between the foam and the ambient space (i.e. no foam/gas interface) and constant capillary pressure along the whole foam height. This technique allows to study the kinetics of internal foam destruction at various capillary pressures, i.e. decrease in the specific foam surface area without destruction of the foam column. Thus, the influence of surface foam films on foam lifetime and the character of foam destruction can be estimated. 

The effect of gas solubility on the rate of foam destruction is of major practical importance. For example, in the production of firefighting foams for underground use in coal mines, it is advisable to use exhaust gases as a disperse phase. However, they contain a considerable amount of carbon dioxide and water vapour that sharply decrease the expansion ratio and stability of the foam produced. 

A significant effect of pressure on the rate of foam destruction was not established for coarsely dispersed foams with NBF, formed by blowing air through a single capillary in a NaDoS solution (10 3 mol dm 3 containing 0.4 mol dm 3 NaCl). In these experiments the pressure drop in the Plateau borders changed from 103 to 4.5-104 Pa and the maximum... 

Systematic studies of the influence of border pressure on the kinetics of foam column destruction and foam lifetime have been performed in [18,24,41,64-71], Foams were produced from solution of various surfactants, including proteins, to which electrolytes were added (NaCI and KC1). The latter provide the formation of foams with different types of foam films (thin, common black and Newton black). The apparatus and measuring cells used are given in Fig. 1.4. The rates of foam column destruction as a function of pressure drop are plotted in Fig. 6.11 [68]. Increased pressure drop accelerates the rate of foam destruction and considerably shortens its lifetime. Furthermore, the increase in Ap boosts the tendency to avalanche-like destruction of the foam column as a whole and the process itself begins at higher values of foam dispersity. This means that at high pressure drops the foam lifetime is determined mainly by its induction period of existence, i.e. the time interval before the onset of its avalanche-like destruction. This time proves to be an appropriate and precise characteristic of foam column destruction. 

All the results presented so far give reason to conclude that the avalanche-like destruction of a foam column at definite temperature, pressure drop and foam dispersity, depends mainly on the equilibrium pressure reached. However, in order to establish the mechanism of action of the critical pressure drop, further studies of single foam films and foams are required. They should be performed under conditions that reveal the role of all elements of the foam (films, borders and vertexes) in the process of foam destruction. 

The increase in angular rotation velocity to 104.9 s 1 and 315 s 1 (1000 and 3000 rpm, respectively) leads to a reduction in the lifetime of a NaDoS foam layer (A/ = 1.5 cm) and to a decrease in the time of establishing the capillary pressure in it, i.e. to 6 and 3 min, respectively. However, raising the centrifuge rotation speed does not affect the value of the capillary pressure at which foam destruction occurs. For example, a NaDoS foam layer of 1,5 cm thickness decays at the same average capillary pressure of 7 kPa at a rotation speed of 1000 and 3000 rpm. 

In Section 6.5 it was emphasised that critical pressure in a foam produced in a porous plate cell corresponds to the equilibrium capillary pressure at which an avalanchelike foam destruction occurs at given dispersity and temperature (the latter is of particular importance for nonionic surfactants foams). 

Fig. 7.2. Photographs illustrating foam destruction by an a-particle beam (a) and the traces of the ex-...

The described above principle has served as a basis for the development of a method for foam destruction which is particularly useful for highly stable foams [20]. This method has also been employed for establishing the CBF/NBF transition in a foam. The critical electrolyte concentrations thus obtained proved to be close to those determined with free foam films (see Chapter 3). 

The kinetics of establishing equilibrium pressure in a foam shows that the time needed is much shorter (about 6-7 times) than the time for foam destruction. Thus, with respect to pressure, foam destruction can be considered to occur under equilibrium conditions. 

It is necessary to mention that an avalanche-like destruction is also observed in a NaDoS foam but it occurs at significantly higher pressure drops with respect to the equilibrium pressure pa (see Fig. 6.12,a). That is why it is important to distinguish between the destruction at equilibrium critical Apcr,e and at non-equilibrium critical Apcrne pressures since probably the causes are different. Foam destruction at Apcre is perhaps due to foam film rupture while at Apcr ne the destruction results from other phenomena occurring in the disperse... 

Foamed emulsions are disperse systems with two disperse phases (gas and liquid) in the disperse medium (surfactant solution). Water foamed emulsions are formed when foams or aqueous surfactant solutions are used to clean up oil deteriorated surfaces, in the process of oil flotation of waste waters, in firefighting when the foam contacts various organic liquids and in the processes of chemical defoaming (foam destruction by antifoams). Individual foamed emulsions can have practical importance e.g. a foamed emulsion of bitumen is used in road coating foamed emulsions from liquid fuels are used as explosives. 

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