Fractionation foaming

Fractionation. See also Plasma fractionation foam, 12 22 physical, 10 813-814 Fractionation methods, for particle size measurement, 18 139, 140-146 Fractionation process, in paper recycling, 21 441... 

Perhaps the most important and striking features of high internal phase emulsions are their rheological properties. Their viscosities are high, relative to the bulk liquid phases, and they are characterised by a yield stress, which is the shear stress required to induce flow. At stress values below the yield stress, HIPEs behave as viscoelastic solids above the yield stress, they are shear-thinning liquids, i.e. the viscosity varies inversely with shear rate. In other words, HIPEs (and high gas-fraction foams) behave as non-Newtonian fluids. 

The viscous properties of HIPEs and high gas fraction foams have also been studied extensively, using a two dimensional, monodisperse, hexagonal cell model. Khan and Armstrong [52] showed that, under steady shear flow (i.e. beyond the yield point of the system), the foam viscosity was inversely proportional to shear rate. At high rates of shear, a constant viscosity value was approached. Gas fraction, <)>, was assumed to be very close to unity. 

It has been suggested the methods of separation involving adherence of particles of different dispersity on bubble surface as a result of adsorption or adhesion, to be named adsorptive-bubble (adsubble) methods [27,28]. The methods of surface separation are termed differently in the different publications in the case of surfactant extraction they are referred to as adsorption flotation, foam flotation, foam fractionation, foam separation or adsorptive accumulation in the case of ion extraction, they are called ion flotation, foam flotation of hydrophobic precipitates, etc. 

Khan, S. A., Foam rheology Relation between extensional and shear deformations in high gas fraction foams, Rheological Acta, Vol. 26, No. 1, pp. 78-84, 1987. 

If a C02 flood is already underway, then the initiation of C02-foam mobility control will cause changes in the injection schedule. Because in many cases C02 is the major adjustable operating expense, this alone may call for an increase in rate of outlay. On the other hand, industry experience has shown that other things being equal, the rate of oil production is proportional to the rate of injection of C02, which would be greater by a factor of 2—3 in a high-C02-fraction foam flood over a 2 1 WAG flood. Consequently, this cost increase can be expected to be matched quickly by increasing production. 

VES-A Mass fraction/% Foam volume/mL Half time/s Foam integraed value/mL-s... 

Although it is hard to draw a sharp distinction, emulsions and foams are somewhat different from systems normally referred to as colloidal. Thus, whereas ordinary cream is an oil-in-water emulsion, the very fine aqueous suspension of oil droplets that results from the condensation of oily steam is essentially colloidal and is called an oil hydrosol. In this case the oil occupies only a small fraction of the volume of the system, and the particles of oil are small enough that their natural sedimentation rate is so slow that even small thermal convection currents suffice to keep them suspended for a cream, on the other hand, as also is the case for foams, the inner phase constitutes a sizable fraction of the total volume, and the system consists of a network of interfaces that are prevented from collapsing or coalescing by virtue of adsorbed films or electrical repulsions. 

About 2 X 10 Ib/year of 1 2 epoxypropane is produced in the United States as an intermediate in the preparation of various polymeric materials including polyurethane plastics and foams and polyester resins A large fraction of the 1 2 epoxypropane is made from propene by way of its chlorohydrm... 

Foamed polymers Foamed sheet Foamed silicone rubber Foam fractionation... 

The amount and physical character of the char from rigid urethane foams is found to be affected by the retardant (20—23) (see Foams Urethane polymers). The presence of a phosphoms-containing flame retardant causes a rigid urethane foam to form a more coherent char, possibly serving as a physical barrier to the combustion process. There is evidence that a substantial fraction of the phosphoms may be retained in the char. Chars from phenohc resins (qv) were shown to be much better barriers to pyrolysate vapors and air when ammonium phosphate was present in the original resin (24). This barrier action may at least partly explain the inhibition of glowing combustion of char by phosphoms compounds. 

The fraction of open cells expresses the extent to which the gas phase of one cell is in communication with other cells. When a large portion of cells are intercoimected by gas phase, the foam has a large fraction of open cells, or is an open-ceUed foam. Conversely, a large proportion of noninterconnecting cells results in a large fraction of closed cells, or a closed-ceUed foam. 

As a good first approximation (187), the heat conduction of low density foams through the soHd and gas phases can be expressed as the product of the thermal conductivity of each phase times its volume fraction. Most rigid polymers have thermal conductivities of 0.07-0.28 W/(m-K) and the corresponding conduction through the soHd phase of a 32 kg/m (2 lbs/fT) foam (3 vol %) ranges 0.003-0.009 W/(m-K). In most cellular polymers this value is deterrnined primarily by the density of the foam and the polymer-phase composition. Smaller variations can result from changes in cell stmcture. 

The permeabUity of ceUular polymers to gases and vapors depends on the fraction of open ceUs as weU as the polymer-phase composition and state. The presence of open ceUs in a foam allows gases and vapors to permeate the ceU stmcture by diffusion and convection dow, yielding very large permeation rates. In closed-ceUed foams the permeation of gases or vapors is governed by composition of the polymer phase, gas composition, density, and ceUular stmcture of the foam (194,199,215,218,219). 

Rheology. The rheology of foam is striking it simultaneously shares the hallmark rheological properties of soHds, Hquids, and gases. Like an ordinary soHd, foams have a finite shear modulus and respond elastically to a small shear stress. However, if the appHed stress is increased beyond the yield stress, the foam flows like a viscous Hquid. In addition, because they contain a large volume fraction of gas, foams are quite compressible, like gases. Thus foams defy classification as soHd, Hquid, or vapor, and their mechanical response to external forces can be very complex. 

Although aH these models provide a description of the rheological behavior of very dry foams, they do not adequately describe the behavior of foams that have more fluid in them. The shear modulus of wet foams must ultimately go to zero as the volume fraction of the bubbles decreases. The foam only attains a solid-like behavior when the bubbles are packed at a sufficiently large volume fraction that they begin to deform. In fact, it is the additional energy of the bubbles caused by their deformation that must lead to the development of a shear modulus. However, exactly how this modulus develops, and its dependence on the volume fraction of gas, is not fuHy understood. 

Foams have a wide variety of appHcations that exploit their different physical properties. The low density, or high volume fraction of gas, enable foams to float on top of other fluids and to fiU large volumes with relatively Httle fluid material. These features are of particular importance in their use for fire fighting. The very high internal surface area of foams makes them useful in many separation processes. The unique rheology of foams also results in a wide variety of uses, as a foam can behave as a soHd, while stiH being able to flow once its yield stress is exceeded. 

Foams for firefighting appHcations are typically made from a concentrated foaming agent diluted with water and then mixed with air. Rather than consider the volume fraction of air in the foam, firefighting foams are characterized by their expansion ratio, which is the increase in volume of the Hquid after the foam is formed. Expansion ratios range from 5 1 to over 1000 1 ratios of 5 1 to 20 1 are called low expansion ratios of 21 1 to 200 1, medium expansion and ratios greater than 200 1, high expansion. 

The pipe stiU furnace heats the feed to a predeterrnined temperature, usuaUy a temperature at which a predeterrnined portion of the feed changes into vapor. The vapor is held under pressure in the pipe in the furnace until it discharges as a foaming stream into the fractional distUlation tower. Here the nonvolatile or Hquid portion of the feed descends to the bottom of the tower to be pumped away as a bottom nonvolatile product, while the vapors pass up the tower to be fractionated into gas oU, kerosene, and naphtha. 

Chemicals responsible for odor in some PUR foams were synthesised by polymerisation of PO in CH2CI2 with Bp2(C2H )20 catalyst (114). The yield was 25% volatile material and 75% polymeric material. The 25% fraction consisted of dimethyldioxane isomers, dioxolane isomers, DPG, TPG, crown ethers, tetramers, pentamers, etc, and 2-ethy1-4,7-dimethyl-1,3,6-trioxacane (acetal of DPG and propionaldehyde). The latter compound is mainly responsible for the musty odor found in some PUR foams. This material is not formed under basic conditions but probably arises during the workup when acidic clays are used for catalyst removal. 

Accurate information oa the size of the defoamer market is impossible to obtaia. There are too many types of materials and suppHers iavolved. Particularly for the more common oils and surfactants, defoaming is a very small part of their total usage, and no pubHc information is available on what fraction of manufacturers sales is ia the area of foam coatrol. Evea for more expeasive materials such as the poly(alkyleae oxide) block copolymers, there is ao way of distinguishing betweea their use as defoamers and other significant surfactant uses such as de-emulsifiers. 

Foam Fractionation. An interesting experimental method that has been performed for wastewater treatment of disperse dyes is foam fractionation (88). This method is based on the phenomenon that surface-active solutes collect at gas—Hquid iaterfaces. The results were 86—96% color removal from a brown disperse dye solution and 75% color removal from a textile mill wastewater. Unfortunately, the necessary chemical costs make this method relatively expensive (see Foams). 

Among the methods of foam separation, foam fractionation usually implies the removal of dissolved (or sometimes colloidal) material. The overflowing foam, after collapse, is called thefoamate. The solid lines of Fig. 22-42 illustrate simple continuous foam fractionation. (Batch operation would be represented by omitting the feed and bottoms streams.)... 

FIG. 22-42 Four alternative modes of contimioiis-flow operation with a foam-fractionation column (1) The simple mode is illustrated hy the solid lines. (2) Enriching operation employs the dashed reflux line. (3) In stripping operation, the elevated dashed feed line to the foam replaces the solid feed line to the pool. (4) For combined operation, reflux and elevated feed to the foam are both employed. 

A separation can sometimes be obtained even in the absence of any foam (or any floated floe or other surrogate). In bubble fractionation this is achieved simply by lengthening the bubbled pool to form a vertical column [Dorman and Lemlich, Nature, 207, 145 (1965)]. The ascending bubbles then deposit their adsorbed or attached material at the top of the pool as they exit. This results in a concentration gradient which can serve as a basis for separation. Bubble fractionation can operate either alone or as a booster section below a foam fractionator, perhaps to raise the concentration up to the foaming threshold. 

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