Английский для Тх / adsorptive Bubble Separation and Dispersed Air Flotation

3.1.2.5. MACROFLOTATION AND ORE FLOTATION

Contrary to the microflotation (and colloid flotation), the terms of macroflotation and ore flotation represent the removal of macroscopic particles by foaming. Both macroflotation and ore flotation are practically the same, but it was felt necessary to take special note of the mineral dressing process, and thus the foam separation of minerals is termed ore flotation (59), which is a well-established process in the mining industry (67–70). In the field of environmental engineering, separation of biological sludge or other macroscopic pollutants from wastewater can be accomplished efficiently by macroflotation if a surface-active agent is added to the system as a collector of particulate species, drawing and attaching the species to the generated gas bubbles, and as a frother to produce a stable foam in which the macroscopic pollutants are concentrated and removed. In other cases, a surface-active agent already present in the wastewater can be utilized as a flotation agent (i.e., collector or frother) for the foam separation of macroscopic particles.

3.1.2.6. ADSORPTION FLOTATION

Adsorption flotation involves the removal of dissolved pollutants by activated carbon in a bubble reactor, and subsequent removal of activated carbon as well as other suspended particles by flotation technique (71 ). This process was found efficient for removing both dissolved organics and suspended solids from an industrial effluent (72), and for removing the emulsified oil from water (73). The mechanism of removal has been proposed by Wang (72,73 ).

All the aforementioned process terms can simply be called foam separation. It is important to note that although any type of technique can be used for bubble generation in a foam separation system, the most common bubble generation technique for a foam separation system is dispersed air flotation, which is also known as induced air flotation.

3.2. Nonfoaming Adsorptive Bubble Separation

The following are the classifications of nonfoaming adsorptive bubble separation:

(a) Bubble fractionation

(b) Solvent sublation

(c) Nonfoaming flotation

(c-1) Nonfoaming precipitate flotation (c-2) Nonfoaming adsorption flotation (c-3) Nonfoaming flotation thickening

Again, any type of technique can be used for generating gas bubbles in a nonfoaming adsorptive bubble separation system. The most effective bubble generation techniques for a nonfoaming system are dissolved air flotation and electrolytic flotation. The following are the process descriptions of selected nonfoaming processes.

3.2.1. Bubble Fractionation

Bubble fractionation is similar to foam fractionation except that there is no foam produced in the system; thus, it is applied to dilute surface-active solutions that do not foam; while foam fractionation is applied to surface-active solutes at high concentration (76,77). Technically speaking, bubble fractionation represents an operation in which gas is bubbled up through a vertical bubble reactor containing the surface-active solute(s)

soluble/colloidal impurities are precipitated by flocculants and the flocs as well as other suspended solids are floated.

3.2.3.2. NONFOAMING ADSORPTION FLOTATION

If powdered activated carbons (or other adsorbents) are added to water or wastewater for removing soluble pollutants, the spent carbons are then flocculated and floated by a dissolved air flotation cell (or by an electroflotation cell), in which no foam is produced, the process system is termed nonfoaming adsorption flotation (83).

3.2.3.3. NONFOAMING FLOTATION THICKENING

Sludge thickening and fiber separation by dissolved air flotation cells (2,3) are typical examples of nonfoaming flotation thickening in which the target sludges or fibers are originally in insoluble forms and ready to be floated.

There are too many terminologies used for classification of “adsorptive bubble separation processes.” In order to avoid the confusion which exists regarding the interrelationship of various adsorptive bubble separation methods, four important terms should always be remembered: (a) flotation (either a foaming or a nonfoaming system): the liquid to be processed is heterogeneous such as industrial effluents which contain both insoluble particles (or complexes) and dissolved organics; (b) fractionation (either a foaming or a nonfoaming system): the liquid to be processed is homogeneous, such as a detergent solution; (c) foam separation (either homogeneous or heterogeneous): the system involves the production of foam; and (d) nonfoaming bubble separation (either homogeneous or heterogeneous): the system involves no production of foam.

4.BUBBLE SEPARATION PROCESS DESCRIPTIONS AND DEFINITIONS ACCORDING TO THE OPERATIONAL MODES

4.1. Continuous Adsorptive Bubble Separation

An adsorptive bubble separation process is assumed to be a continuous process in which the influent is continuously fed into the process system, while the effluent is continuously withdrawn from the process system.

4.2. Sequencing Batch Reactor Adsorptive Bubble Separation

One of many sequencing batch reactor (SBR) processes developed by Wang, Kurylko, and Wang in 1994 ( 125) is a physicochemical sequencing batch reactor adsorptive bubble separation (SBR-ABS) process, which can be used for potable water purification, industrial water treatment, wastewater effluent treatment, and groundwater decontamination ( 126). There are various types of SBR-ABS systems: (a) physicochemical SBR flotation, (b) physicochemical SBR fractionation, (c) biological SBR flotation (2,3,4). The physicochemical SBR flotation has been used successfully in full-scale operation in Europe ( 123).

5. SURFACE ADSORPTION

Solids, liquids, and solutions exhibit many properties that can only be explained in terms of the action of their surfaces. A surface is actually a boundary, where the mass of one body ends and the mass of another begins. Consider a rising air bubble immersed in a liquid pool. The surface of the air faces a corresponding surface of the liquid; the region enclosed by these two surfaces is known as an interface, and it is within this interfacial region that adsorption occurs. There are five types of possible interfaces (43 ):

Gas–Liquid

•Liquid–Liquid

•Liquid–Solid

•Solid–Gas

•Solid–Solid

The action at these interfaces includes: interfacial tensions, adsorption, the spreading of liquids on surfaces, insoluble surface films, and the catalytic activity of various solid surfaces for many types of chemical reactions.

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.

A phenomenon of concentration of a substance on the interface is called adsorption. Surface activity is due to the unequal distribution of a solute between the surface and the bulk solution. Quantitative description of the adsorption of a solute at gas–liquid interfaces, under an equilibrium condition, is expressed by the Gibbs adsorption equation as

where r represents the surface tension of the solution, dyne/cm, R is the universal gas constant, dyne-cm/(g-mole K), T is the absolute temperature, K, Exi is the surface excess, g-mole/cm2, which is essentially the concentration the component i at the interface; and ai is the activity of the its component.

For an ideal two-component solution consisting of a solvent and a single surfaceactive solute at equilibrium condition, and with concentration assumed to be equivalent to activity (i.e., dilute solution), Eq. (1) may be written as

where dr/dL is the rate of variation of the surface tension of the solution with bulk concentration. L is the bulk solute concentration, g-mole/cm3. When the surface tension of

a solution decreases with concentration, dr/dL is negative, E is positive, and the sur-

x

face contains a higher concentration of solute than the bulk solution. Equation (2) is the form in which the Gibbs adsorption equation is usually quoted. In the adsorption of ionic surfactants into the surface phase, with or without the presence of inorganic electrolyte, equal numbers of cations and anions must enter the surface and different modifications must be made before the equation is formally applied. Theoretically, for the adsorption of a dilute univalent ionic surfactant, the following modified adsorption equation should be applied:

A detailed derivation of Eq. (3) may be found elsewhere. In the presence of excess inorganic electrolyte in the univalent ionic surfactant system, the factor 2 in Eq. (3) can be reduced to 1 by thermodynamic modification.

Adsorption from the liquid solution and adsorption of gases generally follow the same principles and are subject to the same factors. When adsorptive bubble separation processes are applied to the treatment of industrial effluents containing high suspended solids or applied to the purification of raw water containing high turbidity, some other mechanisms are responsible for liquid–solid as well as liquid–gas adsorption phenomena. The process of liquid–solid adsorption can be of three types—physical adsorption, chemical adsorption, and electrostatic adsorption. Physical adsorption results from molecular condensation (of adsorbate) in the micropores of the adsorbent by so-called inner van der Walls or dispersion forces. Chemical adsorption (or chemisorption) results in the formation of a chemically bound monomolecular layer of the adsorbate on the adsorbent surface through forces of residual valence of the solid surface molecules. Electrostatic adsorption is an attractive force responsible for adsorbing ionic solute on an oppositely charged adsorbent. Electrostatic adsorption forces can also result from ion exchange phenomena between adsorbate and adsorbent. It can be altered because of the affinity of hydrogen and hydroxide ions for the adsorbent surface.

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 solids content), hydrogen ion concentrations of the system, and the temperature of the system. In a multicomponent 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 line of contact between a smooth, rigid, solid phase, a liquid phase and a gas phase. The equilibrium contact angle Ae can be expressed in terms of the average surface tensions (i.e., interfacial tensions, dyne/cm) of the liquid-gas (rLG), solid-liquid (rSL), and solid-gas (rSG) interfaces, by the well-known Young’s equation:

The definition of contact angle is illustrated in Fig. 2. For a system containing no surface-active agents, the contact angle between a particle and an air bubble is said to be zero because of complete wetting. If some appropriate surface-active agents were added to this system, they would be adsorbed on the surface of the particle and the particle surface would become hydrophobic, thus it would attach itself to an air bubble which came in contact with it. In general, high contact angles are produced by interaction between the adsorbed layers of collector molecules at the solid–liquid interfaces

Fig. 2. Attachment of air bubble to sold particle at flotation.

in solution and frother molecules at the gas–liquid interfaces. The association produced

amixed monolayer at the solid–gas interfaces. Collectors are surface-active agents that normally attach themselves to nonfloating particles endowing them with hydrocarbon-like surfaces, thereby making them capable of adhering to air bubbles. Frothers are organic compounds such as pine oil and cresols slightly soluble in water. When frothers are added in small amounts to a solution that has air bubbling through it, the bubbles become smaller and a frothy layer several inches thick forms. This layer containing the solid pollutants can be skimmed off. Figure 2 shows how an air bubble is attached to

asolid particle at flotation. The solid surface, which is wetted by water, is surrounded by more or less rigid layers of water. The properties of these layers differ from those of ordinary fluid water—the vapor pressure is higher, for example.

6. BUBBLE PHENOMENA

Figure 1 shows the structure of a bubble ascending in a surfactant solution under the influence of drag force and buoyant force. In the Stokes’ law range, the fluid will flow smoothly over the bubble surface, leaving no wake. Only skin friction (viscosity shear stresses) contributes to the total drag on the bubble. Beyond the Stokes’ law range, increasing Reynolds number (NR) gradually separates the boundary layer from the bubble surface, produces wakes in the rear, and contributes more and more form drag to the total drag force. Form drag results from pressure differences caused by the acceleration of the fluid flowing around the bubble and from the high velocities of the turbulent eddies in the wake (77).

The interference of surface-active agents upon the bubble movement can also be understood by the same figure. During the bubble rise the surfactant film adsorbed on the bubble wall will be driven to the tail end of the gas bubble where it condenses, forming a solid cap. As a result a surface force will be set in the direction of the front end of the air bubble, opposing the liquid drag forces, which may cause a retardation in the bubble rising velocity and opposing the hydrodynamic stress responsible for interfacial renewal. Other interferences caused by the adsorption of a surfactant monomolecular film at the air–water interface (such as bubble wall) are a reduction in the gas transfer rate and gas adsorption rate.

Fig. 3. Various flow regimes encountered in two-phase flow.

Increasing surfactant concentrations in the aeration cell has been found to decrease bubble diameter, bubble velocity, axial diffusion coefficient, but increase bubble’s surface- to-volume ratio, and total bubble surface area in the system. The effect of a surface-active agent on the total surface area of the bubbles is also a function of its operating conditions. The surfactant’s effect is pronounced in the case of a coarse gas diffuser where the chances of coalescence are great; and the effectiveness of a surface-active solute in preventing coalescence increases with the length of its carbon chain.

7. MULTIPHASE FLOW

The mechanics and applications of multiphase flow has been an area of continuing interest to chemical, environmental, and civil engineers (23,77). The multiphase flow patterns may be classified as bubble flow, plug flow, stratified flow, wave flow, slug flow, annular flow, spray flow, and froth flow. Typical sketches of these various flow patterns are shown in Fig. 3. They are self-explanatory. In the field of absorptive bubble separation processes, only multiphase bubble flow and froth flow are of interest to the process engineer.

Type of flow pattern(s) involved in an adsorptive bubble separation system depends on the type of process used. For example, bubble fractionation involves two-phase (gasphase and liquid-phase) bubble flow, while solvent sublation involves multiphase bubble flow in their vertical bubble cells. Foam fractionation involves a two-phase bubble flow in the bottom bubble cell, and a two-phase froth flow in the top foam cell. However, all froth flotation processes (i.e., precipitate flotation, ion flotation, molecular flotation, ore flotation, microflotation, adsorption flotation, macroflotation, and adsorbing colloid flotation) involve multiphase bubble flow and multiphase froth flow.

All batch adsorptive bubble separation processes involve no net movement of liquid, but steady bubbling of gas through the stagnant liquid. The relative bubble velocity in the bubble cell is the function of buoyancy component and the superficial gas velocity.

In the bubble cell of any continuous adsorptive bubble separation process, the relative bubble velocity is the function of buoyancy component, the superficial gas velocity, and the superficial liquid velocity. The superficial velocity of gas and liquid are caused by the continuous entry of the gas and liquid phases into the bubble cell. Figures 4 (A) and 4 (B) show the relative bubble velocity in various two-phase bubble flow systems (43).

Fig. 4. (A) Relative bubble velocity in a two-phase bubble flow—systems (a), (b), and (c).

The bubble size distribution in a bubble flow is primarily dependent on the rate of air supply to the gas diffuser. The size of the largest bubbles may change somewhat with the rate of air discharge and orifice size. When the liquid moves relative to the orifices, the maximum bubble diameter is equal to 2.4 times the square root of the gas-flow rate per jet divided by the liquid velocity. However, the size distribution of bubbles below the largest can be obtained only from experiment.

8. MATERIAL BALANCES

In the batch adsorptive bubble separation processes, a feed solution was introduced to a bubble separation column (or chamber) containing an aqueous solution of surfaceactive 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:

in which Vi is the initial volume, Vt is the collapsed foam (as liquid) volume from the batch process, and Vb is the residual liquid volume in separation column. For the substance other than collectors (i.e., target solute or solid), which may be separated by the foaming process, the following material balance may be used:

Fig. 4. (B) Relative bubble velocity in a two-phase bubble flow—systems (d) and (e).

in which Li is the surface-active solute or solid concentration in the feed, Lt is the surfaceactive solute or solid concentration in the collapsed foam phase, and Lb is the residual surface-active solute or solid concentration in the drain. Considering the additional collector necessary for foaming, the material balance may be written as:

where Si is the collector concentration in the feed, St is the collector concentration in the foam phase, and Sb is the residual collector concentration in the drain.

In case of continuous adsorptive bubble separation processes, the following set of

in which Qi Qt and Qb are volumetric liquid flow rates of feed, overhead, and drain, respectively. For a single-solute fractionation system, Eqs. (7) and (10) are dropped because no collector is used. Another basic solute balance for the continuous bubble separation system is

where E is the surface excess, L is the bulk solute concentration, Q is the volumetric

x a

gas rate, and f is the surface to volume ratio of the bubble. Generally, the size of the bubbles are not uniform and an effective average radius is used:

re Σ(Ni ri3 )/Σ(Ni ri2) (12)

where Ni is the number of bubbles of radius ri and re is the effective average bubble radius. re is also defined as the bubble radius averaged by the ratio of the third moment

(volume) to the second moment (surface area). For spherical bubbles,

where d is the effective average bubble diameter. Equation (11) can then be written as:

9. FOAM SEPARATION BY DISPERSED AIR FLOTATION CELL

Many contaminants in wastewater today, such as dissolved dyestuffs, lignins, detergents, proteins, fatty acids, tannins, and so on, possess surface-active properties that decrease surface tension and oxygen transfer rate, but increase the demand for dissolved oxygen. Particularly, the sharp reduction in surface tension of water by these pollutants seems to be a basic cause of increasing the susceptibility of aquatic life to the surfactant poison.

Foam separation process involves the selective adsorption of the surface-active pollutants at the gas–liquid interfaces of fine air bubbles in a foam separation column. The surface-active pollutants, which are adsorbed on the surfaces of the rising bubbles, can be carried upward to the top of the foam separation column and thus removed from the aqueous system as condensed foam. Foam separation can be used for both waste treatment and water purification. This section presents the data on the feasibility of removing various organics and inorganics by the foam separation processes. A general survey of foam separation process and its fundamental principles are also presented.

The basic principle for solid/liquid and solute/liquid separation by the adsorptive bubble separation processes has been introduced previously. This section further presents fundamental principles on foam phenomena and foam separation cell’s operation.

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).