Adsorptive bubble separation surface adsorption

Principle The adsorptive-bubble separation methods, or adsub-ble methods for short [Lemlich, Chem. Eng. 73(21), 7 (1966)], are based on the selective adsorption or attachment of material on the surfaces of gas bubbles passing through a solution or suspension. In most of the methods, the bubbles rise to form a foam or froth which carries the material off overhead. Thus the material (desirable or undesirable) is removed from the liquid, and not vice versa as in, say, filtration. Accordingly, the foaming methods appear to be particularly (although not exclusively) suited to the remov of small amounts of material from large volumes of hquid. 

Excess collector can also reduce the separation by forming micelles in the bulk which adsorb some of the colhgend, thus keeping it from the surface. This effect of the micelles on Ki for the colhgend is given theoretically [Lemhch, Principles of Foam Fractionation, in Periy (ed.). Progress in Separation and Purification, vol. 1, Interscience, New York, 1968, chap. 1] by Eq. (22-44) [Lemlich (ed.). Adsorptive Bubble Separation Techniques, Academic, New York, 1972] if F, is constant when C, > C-... 

Other plant-scale applications to pollution control include the flotation of suspended sewage particles by depressurizing so as to release dissolved air [Jenkins, Scherfig, and Eckhoff, Applications of Adsorptive Bubble Separation Techniques to Wastewater Treatment, in Lemlich (ed.). Adsorptive Bubble Separation Techniques, Academic, New York, 1972, chap. 14 and Richter, Internat. Chem. Eng, 16,614 (1976)]. Dissolved-air flotation is also employed in treating waste-water from pulp and paper mills [Coertze, Prog. Water TechnoL, 10, 449(1978) and Severeid, TAPPl 62(2), 61, 1979]. In addition, there is the flotation, with electrolytically released bubbles [Chambers and Cottrell, Chem. Eng, 83(16), 95 (1976)], of oily iron dust [Ellwood, Chem. Eng, 75(16), 82 (1968)] and of a variety of wastes from surface-treatment processes at the maintenance and overhaul base of an airline [Roth and Ferguson, Desalination, 23, 49 (1977)]. 

This natural process by which dissolved and/or particulate surface-active materials end up in the atmosphere has been modeled and studied in the laboratory. As summarized by Detwiler and Blanchard (ref. 46), tests in suspensions of bacteria (ref. 76,96,97), latex spheres (ref. 98), dyes (ref. 99), and in sea water and river water (ref. 96,100,101) have demonstrated successful transfer of all manner of surface-active material from the bulk fluid, or the bulk interface, to the droplets ejected when bubbles burst. (This situation can be pictured as an extension of the common industrial adsorptive-bubble-separation process (ref. 102) into a third dimension or phase — the atmosphere.) Further laboratory tests with various tap waters, distilled waters, and salt solutions have shown that no water sample was ever encountered that did not contain at least traces of surface-active material (ref. 46). 

A few simple differences in the properties of immiscible phases make possible their relative displacement. Most simply, if the phases have different densities they will automatically acquire a relative motion in a gravitational field. Thus in adsorptive bubble separation methods, bubbles injected into a column of liquid rise toward the upper surface. Separation occurs by combining the relative enrichment of components at the bubble interface with the continuous displacement of bubbles through the liquid [33-35]. 

Bubble Separation Process Descriptions and Definitions Based on the Techniques Used for Bubble Generation Bubble Separation Process Descriptions and Definitions According to the Techniques Used for Solids Separation Bubble Separation Process Descriptions and Definitions According to the Operational Modes Surface Adsorption Bubble Phenomena Multiphase Flow Material Balances... 

Adsorptive bubble separation process may be defined as the mass transfer process of a solid from the body of a liquid to the liquid surface by means of bubble attachment (42,75,84). The solids can be in dissolved, suspended, and/or colloidal form. The three basic mechanisms involved are bubble formation, bubble attachment, and solids separation (43,75). 

Table 1 indicates the solids or substances that can be effectively separated by the adsorptive bubble separation process. In general, the light-weight suspended solids, such as fibers, activated sludge, free oil, chemical floes, and fats, can be readily separated by the process in accordance with the physical-chemical bubble attachment mechanism shown in Fig. 1. The colloidal solids, soluble organics, soluble inorganics, and surface-active substances can be separated from the bulk liquid by the bubble separation process after they are converted from colloidal or soluble form into insoluble form (i.e., suspended solids), which can then be floated by gas bubbles. 

Alternatively, an adsorptive bubble separation process in accordance with its surface-adsorption phenomena, shown in Fig. 1, can separate the soluble surface-active substances easily. Non-surface-active suspended solids, colloidal solids, soluble organics, and soluble inorganics can all be converted into surface-active substances. All surface-active substances (in either soluble form or insoluble form) can be effectively floated by gas bubbles (75). 

In summation, the adsorptive bubble separation process, in theory, can remove or separate almost any kind of light-weight and/or surface-active substances from water. Because there are various types of adsorptive bubble separation processes, selection of an appropriate type for a specific application is an important skill (43,84). 

Of the five types of interfaces mentioned above, adsorption at gas-liquid (e.g., air-water) interfaces is of interest in all adsorptive bubble separation methods. In the liquid pool, a molecule is acted upon by molecular attractions, which are distributed more or less symmetrically about the molecule. However, at the air-water interface, a water molecule is only partially surrounded by other like molecules as a consequence, an attraction tends to draw the surface molecules inward, and in doing so makes the water behave as if it were surrounded by an invisible membrane. This behavior of the surface is called surface tension. Surface-active substances possess the ability to lower the surface tension of water even at low concentrations. 

There are several other physical and chemical variables that affect the adsorption rate and the adsorption equilibrium of an adsorption system involving the separation of a solute from aqueous onto an adsorbent. These include the total surface area of an adsorbent, concentration of adsorbent, concentration of adsorbate, nature of adsorbent, nature of adsorbate, nature of the mixture of solutes (such as dissolved soUds content), hydrogen ion concentrations of the system, and the temperature of the system. In a multi-component bubble separation system, several adsorption mechanisms are involved. True adsorption phenomena cannot be clear until laboratory experiments are conducted. 

In water solution containing small particles (i.e., suspended solids or turbidity) and non-surface-active solutes, when air is bubbled through it, little or no particles will be removed by any adsorptive bubble separation process. This is because the particles have virtually no natural affinity for air bubbles and hence there is no adhesion when contact is made. This particular phenomena may be explained by the contact angle between a particle and an air bubble. Consider the case of the three-phase fine of contact between a smooth, rigid, solid phase, a liquid phase and a gas phase. The equilibrium contact angle can be expressed in terms of the average surface tensions (i.e., interfacial tensions, dyne/cm) of the liquid-gas solid-liquid (r j ), and solid-gas (r ) interfaces, by the well-known Young s equation ... 

In the batch adsorptive bubble separation processes, a feed solution was introduced to a bubble separation column (or chamber) containing an aqueous solution of surface-active materials. Surface-active solutes or complexes that are hydrophobic and readily attachable to the air bubbles are carried up to the surface of the water by the bubbles. The enriched material at the top (whether collapsed foam from a foam separation column or overflow liquid from a nonfoaming bubble separation column) and the clarified drain solution at the bottom are withdrawn from the system. The overall material balance for the process is as follows ... 

For foam separation processes, adsorption takes place in solution, the essential basis exists for solute separation by foaming. Foam consists of gas bubbles separated by thin liquid films. The liquid films are often formed by the mutual approach of two already existing liquid surfaces (e.g., two bubbles below the surface). Foam structures may vary between two extreme situations. The first is wet foam, which consists of nearly spherical bubbles separated by rather thick liquid films. The second is dry foam, which may develop from the first type as a result of drainage (i.e., foam drainage). 

A number of processes that utilise the surface active nature of hydrophobic compounds in separating them from the aqueous phase are described in the literature. Of these the so-called adsorptive bubble separations form a class of techniques that utilise air bubbles to concentrate and separate surface active compounds (i). There are two main classifications of these, viz., foaming and non-foaming separations (2). The focus of attention has mostly b n on foam separations. However, in those cases that involve low concentrations of... 

In adsorptive bubble separation methods, surface active material collects at solution interfaces and, thus, a concentration gradient between a solute in the bulk and in the surface layer is established. If the (very thin) surface layer can be collected, partial solute removal from the solution will have been achieved. The major application of this phenomenon is in ore flotation processes where solid particles migrate to and attach themselves to rising gas bubbles and literally float out of the solution. This is essentially a three-phase system. 

Foam fractionation (6), a two-phase adsorptive bubble separation method, is a process where natural or chelate-induced surface activity causes a solute to migrate to rising bubbles and thus be removed as a foam. Two government-... 

As mentioned before, the basis of bioseparation and unit operation is based on the differences of physicochemical properties of the materials (Figure 2.2). Chromatography methods are not always the best option due to variable yield losses and high costs. ° Therefore, various attempts have been made to find new separation processes focusing on cost reduction. Among these options, a versatile and promising technique is that of the foam fractionation (Figure 2.2), an adsorptive bubble separation technique in which the principle of separation is based on the differences in the surface activity of molecules. It has been used to separate proteins, but it can also be used for other purposes [e.g. the concentration of plant secondary metabolites). ... 

The basis for the separation by bubbles and foam (adsorptive bubble separation) is the difference in the surface activities of the various materials present in the solution or the suspension of interest. The material may be cellular or colloidal substances, crystals, minerals, ionic or molecular compounds, precipitates, proteins, or bacteria, but in any case it must be surface active at the air-liquid interface (Fig. 17.1-1). These surface-active materials tend to attach preferentially to the air-liquid interfaces of the bubbles or foams. As the bubbles or foams rise through the column or pool of liquid, the attached material is removed. When this combination reaches the surface, the material can be removed in the relatively small volume of collapsed foam or surface scum. ... 

Foam fractionation (or separation) is an adsorptive bubble separation technique in which soluble, surface-active substances can be removed from solution by preferential adsorption at the gas-Uquid interface (Wang and Liu, 2003). Proteins contain both hydrophilic and hydrophobic amino acid residues that are surface active. During foam formation, bubbles... 

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