Foam column

In practice, the performance of a well-operated foam column several feet tall may actually approximate the limiting equations, provided there is little channeling in the foam and provided that reflux is either absent or is present at a low ratio. 

Stable Foam. When a well is drilled with stable foam as the drilling fluid, there is a back pressure valve at the blooey line. The back pressure valve allows for a continuous column of foam in the annulus while drilling operations are under way. Thus, while drilling, this foam column can have significant bottom-hole pressure. This bottomhole pressure can be sufficient to counter formation pore pressure and thus control potential production fluid flow into the well annulus. 

The superficial gas velocity Og is G/A, where A is the horizontal cross-sectional area of the empty vertical foam column. Also, g is the acceleration of gravity, p is the liquid density, p is the ordinary liquid viscosity and p, is the effective surface viscosity. 

Transfer of gas from a rising bubble into a liquid is becoming increasingly important bubble and foam columns are often more efficient than... 

In contrast, most of the conventional foam separation techniques use large bubbles, requiring relatively high gas flow rates to generate sufficient interfacial area for adhesion of solid particles to bubbles. This causes turbulence at the foam/liquid boundary and, in order to prevent redispersion of floated particles, a rather tall foam column is required (ref. 36). 

S. L. C. Ferreira, H. C. dos Santos, R. C. Campos, The determination of molybdenum in water and biological samples by graphite furnace atomic spectrometry after polyurethane foam column extraction, Talanta, 61 (2003), 789-795. 

The excess pressure at a certain point z in the foam column which can be measured with a micromanometer [36] equals... 

By definition osmotic pressure equals the pressure that has to be applied onto a mobile semi-permeable membrane (filter) separating foam and liquid in order to prevent liquid phase from entering the foam. This pressure can be calculated as the difference between pressure of liquid column and pressure of foam column, both having the same height [84]... 

This means that the osmotic pressure differs from p L by a value equal to the difference between the pressures of foam columns with heights H and z. For example when a foam with expansion ratio n = 100 with constant cross-section area A and total height H = 10 cm, the difference between excess and osmotic pressures at level z = 2 cm is... 

When the pressure difference is large (Ap pgH + %oeG), the pressure in Plateau borders and the capillary pressure will be equal along the height of foam column... 

In a foam column that is not very high (of order of centimetres) this condition is realised even at Ap 103 Pa. 

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. 

Applying the relation of parameters n, a, r (Eqs. (4.9) and (4.10)) and the dependence of hydrostatic pressure on foam column height (Eq. (1.39)), it is possible to derive the distribution of local foam expansion ratio along the height H. Assuming a border foam (i.e. neglecting the amount of liquid in films) one obtains from Eq. (4.10) and Eqs. (1.40) and (1.42) which account for the p dependence on r, the following relation... 

Princen and Kiss [13] have divided the profile of the dependence of gas volume fraction in a foam (or volume fraction of the disperse phase in emulsion) on foam column height into three regions, giving the respective analytical expressions for each one of them. 

Calculating the largest liquid volume of a foam column with a cross-sectional unit area and using Eqs (4.15) - (4.17) gives... 

The analysis in [2] indicates that for a foam at hydrostatic equilibrium that is in contact with the foaming solution, Eqs. (4.15) and (4.16) cannot be employed to calculate the average expansion ratio or the critical foam height, which gives the boundary between the ability of a foam either to drain or to suck in liquid. That is so because the maximum volume of the liquid in a foam is a function of the 5-layer and actually does not depend on the whole foam column height. 

An important condition which has to be fulfilled when using this method for foam dispersity determination is the absence of an excess hydrostatic pressure in the foam liquid phase. This pressure is equalized to a considerable extent when an equilibrium distribution the foam expansion ratio and the border pressure along the height of the foam column is established. This can be controlled by measuring the pressure in the Plateau borders at a certain level of the foam column by means of a micromanometer. However, if this condition is overlooked, the hydrostatic pressure can introduce a considerable error in the results of bubble size measurements, especially in low expansion ratio foams. Probably, it is the influence of the unrecorded hydrostatic pressure that can explain the lack of correspondence between the bubble size in the foam and the excess pressure in them, observed by Aleynikov[49]. The... 

Very useful for the kinetic studies of dispersity changes in a foam is the method based on simultaneous measurement of local expansion ratio and border pressure at one and the same level in the foam column [5]. This method can be applied to polyhedral foams in which the liquid is mainly in the borders. A formula for the length of a dodecahedron edge is obtained when Eqs. (1.43), (1.45) and (4.10) are solved together... 

Another variant of this technique involves measuring the electrical resistance of a cylindrical column of a homogenous foam closed between two porous plates. The external sides of both plates are in contact with the foaming solution. Equal reduction of pressure Ap is created in the spaces above the upper and below the lower porous plates. If pressure is not strongly reduced and foam column is not very high, the capillary pressure in the whole foam volume is constant. 

It is experimentally proved that the electrical resistance of the porous plates is negligible compared to that of the foam column. In order to estimate the resistance of the porous plates, instead of glass cup 2, a similar cup made of aluminum is used. During the experiment the porous plate soldered to the lid is in contact with both the foam and the aluminum cup. The electrical conductivity of the latter cup is 106 times higher than that of the... 

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. 

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