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Creaming or sedimentation

Creaming or sedimentation occurs as a result of the density difference between the phases. This process results from external forces usually gravitational or centrifugal. When such forces exceed the thermal motion of the droplets (Brownain motion), a concentration gradient builds up in the system with the larger droplets moving faster to the top (if their density is lower than that of the medium) or to the bottom (if their density is larger than that of the medium) of the container. In the limiting cases, the droplets may form a close-packed (random or ordered) array at the top or bottom of the system with the remainder of the volume occupied by the continuous liquid phase. [28]

Flocculation

It is the process in which emulsion drops aggregate, without rupture of the stabilizing layer at the interface. Flocculation of emulsions may occur under conditions when the van der Waals attractive energy exceeds the repulsive energy and can be weak or strong, depending on the strength of inter-drop forces. The driving forces for flocculation can be:

1. Body forces, such as gravity and centrifugation causing creaming and sedimentation, depending on whether the mass density of the drops is smaller or greater than that of the continuous phase.

2. Brownian forces

3. Thermo-capillary migration (temperature gradients) may dominate the gravitational body force for very small droplets, less than 1 μm. [29]

Ostwald Ripening (Disproportionation)

This results from the finite solubility of the liquid phases. Liquids that are referred to as being immiscible often have mutual solubilities that are not negligible. With emulsions, which are usually polydisperse, the smaller droplets will have larger solubility when compared with the larger ones (due to curvature effects). With time, the smaller droplets disappear and their molecules diffuse to the bulk and become deposited on the larger droplets. With time, the droplet size distributionshifts to larger values.[27]

Coalescence

The efficiency of the emulsification is closely related to the adsorption kinetics of surface-active molecules. For surfactant concentrations of the order of one weight %, the adsorption times are much shorter than the characteristic times for emulsification, and surfactant adsorption kinetics do not therefore influence emulsification. In the presence of asphaltenes, one would expect that the emulsions will be much more stable when the skins are present. However, this type of film was observed after equilibration times much longer than the times of emulsion formation. Indeed aged crude oil emulsions were found more difficult to demulsify. In our opinion, spontaneous coalescence can only occur before the formation of skins, and in any case, when small amounts of asphaltenes are adsorbed.[20] Emulsions in which drops are covered by dense adsorbed layers do not coalesce spontaneously. Coalescence can be however produced by applying either compression or shear. In the first case, osmotic compression or centrifugation can be used. Theory predicts that coalescence rate increases with



increasing drop size, as observed experimentally. Surfactant emulsions undergoing coalescence then become very polydisperse [30]. Pickering emulsions are again unusual in this respect: Leal-Calderon and coworkers showed that coalescence stops when the drop coverage corresponds to full coverage, and that the drop size distribution is quite narrow. This is because, adsorption being irreversible, drop coverage increases during the growth process, and the coalescence rate decreases with drop size (even millimeter-sized drops are very stable against coalescence) [31].

Phase Inversion

This refers to the process whereby there will be an exchange between the disperse phase and the medium. For example, an O/W emulsion may with time or change of conditions invert to a W/O emulsion. In many cases, phase inversion passes through a transition state whereby multiple emulsions are produced.[27]

There are many factors that usually favour emulsion stability such as low interfacial tension, high viscosity of the bulk phase and relatively small volumes of dispersed phase. A narrow droplet distribution of droplets with small sizes is also advantageous, since polydisperse dispersions will result in a growth of large droplets on the expense of smaller ones.The potent stabilization of the emulsion is achieved by stabilization of the interface. [32, 33]

The presence of solids at interfaces may give rise to repulsive surface forces which

thermodynamically stabilize the emulsion. As concluded [34-36], many of the properties of solids in stabilizing emulsion interfaces can be attributed to the very large free energy of adsorption for particles of intermediate wettability (partially wetted by both oil and water phases). This irreversible adsorption leads to extreme stability for certain emulsions and is in contrast to the behavior of surfactant molecules which are usually in rapid dynamic equilibrium between the oil: water interface and the bulk phases. According to the asphaltene stabilization mechanism, coalescence requires the solid particles to be removed from the drop-drop contact region. Free energy considerations suggest that lateral displacement of the particles is most likely, since forcing droplets into either phase from the

interface require extreme energies [37-42]. The asphaltenes stabilization effect for water droplets has already been pictured in Figure 11, where droplet contact is prevented by a physical barrier around the particles

Figure 11. Steric stabilization of the interfaces

As the stability of w/o emulsions appeared clearly related to the presence of an interfacial network surrounding the water droplets [43], many rheological studies of water/asphalted oil interfaces have been made to clarify the mechanisms involved [44]. The rheological properties of these interfaces have been found to be strongly dependent on the nature of solvent used for dilution, the oil concentration, the asphaltenes and resin concentrations, the resin to asphaltenes ratio, and so on [45].

The breaking of emulsions (demulsification) is necessary in many applications such as environmental technology, painting, petroleum industry and waste water treatments. Methods currently available for demulsificationcan be broadly classified as chemical, electrical and mechanical [46].

Electrical double layer repulsion or charge stabilization by polymers and surfactants with protruding molecular chains may prevent the droplets to come into contact with each other [47,48]. Also, polymers, surfactants or adsorbed particles can create a mechanically strong and elastic interfacial film that act as a barrier against aggregation and coalescence. A film of closed packed particles has considerable mechanical strength, and the most stable emulsions occur when the contact angle is close to 900, so that the particles will collect at the interface.

Particles, which are oil-wet, tend to stabilize w/o emulsions while those that are water-wet tend to stabilize o/w emulsions. In order to stabilize the emulsions the particles should be least one order of magnitude smaller in size than the emulsion droplets and in sufficiently high concentration. Nevertheless, stable w/o emulsions have been generally found to exhibit high interfacial viscosity and/or elasticity modulus. It has been attributed to physical cross-links between the naturally occurring surfactants in crude oil (i.e. asphaltenes particles) adsorbed at the water–oil interface [49,50]

Chemical demulsification is the most widely applied method of treating water in oil and oil in water emulsions and involves the use of chemical additives to accelerate the emulsion breaking process. The formulation of an emulsion demulsifier for a specific petroleum emulsion is a complicated undertaking. In petroleum system, asphaltenes and resins substances comprise a major portion of the interfacially active components of oil [51].

Asphaltenes stabilize the crude oil emulsion by different modes of action. When asphaltenes disperse on the interface, the film formed at a water/ crude oil interface behaves as a skin whose rigidity can be shown by the formation of crinkles at interface when contracting the droplet to a smaller drop size [52].

They can also aggregate with resin molecules on the interfaces and prevent droplet coalescence by steric interaction. Some authors suggest that asphaltenes stabilize the emulsion by formation of hydrogen bonding between asphaltenes and water molecules [53].

The postulated that asphaltenes stabilize w/o emulsions in two steps. First, dislikeasphaltenes molecules aggregate into particles or micelles, which are interfacially active. Then, these entities upon adsorbing at the w/o interface aggregate through physical interactions and form an interfacial network. Different modes of action of asphaltenesare represented in Figure 12 [54].

 

Figure 12. Different modes of action of asphaltenes in stabilizing crude oil emulsions

Resins are thought to be molecular precursors of the asphaltenes. The polar heads of the resins surround the asphaltenes, while the aliphatic tails extend into the oil. Resins may act to stabilize the dispersion of asphaltene particles and can be converted to asphaltenes by oxidation. Unlike asphaltenes, however, resins are assumed soluble in the petroleum fluid. Pure resins are heavy liquids or sticky (amorphous) solids and are as volatile as the hydrocarbons of the same size. Petroleum fluids with high-resin content are relatively stable. Resins, although quite surface-active, have not been found to stabilize significantly water-in-oil emulsions by themselves in model systems. However, the presence of resins in solution can destabilize emulsions via asphaltenes solvation and/or replacement at the oil/water interface, Figure 13 [55].

Figure 13. Schematic illustration of asphaltene aggregates in absence and in presence of resins

1.4 Demulsifiers

The most common method of emulsion treatment is adding demulsifiers. These chemicals are designed to neutralize the stabilizing effect of emulsifying agents. Demulsifiers are surface-active compounds that, when added to the emulsion, migrate to the oil/water interface, rupture or weaken the rigid film, and enhance water droplet coalescence. Optimum emulsion breaking with a demulsifier requires a properly selected chemical for the given emulsion; adequate quantity of this chemical; adequate mixing of the chemical in the emulsion; and sufficient retention time in separators to settle water droplets. It may also require the addition of heat, electric grids, and coalescers to facilitate or completely resolve the emulsion.

Dehydration chemicals, or demulsifiers, are chemical compounds that are widely used to destabilize, and assist in coalescence of, crude-oil emulsions. This treatment method is popular because the chemicals are easily applied, usually are reasonable in cost, and usually minimize the amount of heat and settling time required.[56]

The chemical counteracts the emulsifying agent, allowing the dispersed droplets of the emulsion to coalesce into larger drops and settle out of the matrix[57]. To work, demulsifiers:

· must be injected into the emulsion

· must mix intimately with the emulsion and migrate to all the protective films surrounding all the dispersed droplets

· must displace or nullify the effect of the emulsifying agent at the interface[58]

For the oil and water to separate, there must also be a period of continual, moderate agitation of the treated emulsion to produce contact between and coalescence of the dispersed droplets, as well as a quiet settling period.[59]

Chemical selection. Selection of the right demulsifier is crucial to emulsion breaking. The selection process for chemicals is still viewed as an art rather than a science. However, with the increasing understanding of emulsion mechanisms, the availability of new and improved chemicals, and new technology, research, and development efforts, selection of the right chemical is becoming more scientific. Many of the failures of the past have been eliminated.

Demulsifier chemicals contain the following components:

· Solvents

· Surface-active ingredients

· Flocculants

Solvents, such as benzene, toluene, xylene, short-chain alcohols, and heavy aromatic naptha, are generally carriers for the active ingredients of the demulsifier. Some solvents change the solubility conditions of the natural emulsifiers (e.g., asphaltenes) that are accumulated at the oil/brine interface. These solvents dissolve the indigenous surface-active agents back into the bulk phase, affecting the properties of the interfacial film that can facilitate coalescence and water separation.

Surface-active ingredients are chemicals that have surface-active properties characterized by hydrophilic-lipophilic balance (HLB) values. The HLB scale varies from 0 to 20. A low HLB value refers to a hydrophilic or water-soluble surfactant. In general, natural emulsifiers that stabilize a water-in-oil emulsion exhibit an HLB value in the range of 3 to 8. Thus, demulsifiers with a high HLB value will destabilize these emulsions. The demulsifiers act by total or partial displacement of the indigenous stabilizing interfacial film components (polar materials) around the water droplets. This displacement also brings about a change in properties such as interfacial viscosity or elasticity of the protecting film, thus enhancing destabilization. In some cases, demulsifiers act as a wetting agent and change the wettability of the stabilizing particles, leading to a breakup of the emulsion film.[60]

Flocculants are chemicals that flocculate the water droplets and facilitate coalescence. A detailed process for selecting the appropriate demulsifier chemicals, described in the literature, includes the following steps.

Characterization of the crude oil and contaminants includes the API gravity of the crude oil, type and composition of oil and brine, inorganic solids, amount and type of salts, contaminant type and amounts.

Evaluation of operational data includes production rates, treating-vessel capabilities (residence time, electrostatic grids, temperature limitations, etc.), operating pressures and temperatures, chemical dosage equipment and injection points, sampling locations, maintenance frequency, and wash-water rates.

Evaluation of emulsion-breaking performance: past experience and operating data including oil, water, and solids content during different tests; composition and quality of interface fluids; operating costs; and amounts of water generated and its disposal.[61]

For the oil and water to separate, there must also be a period of continual, moderate agitation of the treated emulsion to produce contact between and coalescence of the dispersed droplets, as well as a quiet settling period [59].

Factors affecting demulsifier efficiency.

Several factors affect demulsifier performance including:

· Temperature

· pH

· Type of crude oil

· Brine composition

· Droplet size and distribution

As described previously, an increase in temperature results in a decrease in emulsion stability, and, hence, a lower dosage of demulsifier is required. pH also affects demulsifier performance. Generally, basic pH promotes oil-in-water emulsions and acidic pH produces water-in-oil emulsions. High pH, therefore, helps in destabilizing water-in-oil emulsions. It has also been reported that basic pH reduces demulsifier dosagerequirements [62].

Demulsifiers that work for a given emulsion may be completely ineffective for another. Demulsifiers are typically formulated with polymeric chains of:

· Ethylene oxides and polypropylene oxides of alcohol

· Ethoxylated phenols

· Ethoxylated alcohols and amines

· Ethoxylated resins

· Ethoxylated nonylphenols

· Polyhydric alcohols

· Sulphonic acid salts

Figire 14 shows typical demulsifier molecular formulas. Commercial demulsifiers may contain one or more types of active ingredient. There is a wide variation within the active ingredient type as well. For example, the molecular weight and structure of the ethylene or propylene oxides can be changed to effect a complete range of:[63]

· Solubilities

· HLBs

· Charge neutralization tendencies

· Solids-wetting characteristics

· Costs

 

Figure 14. Typical demulsifier molecular formulas.

Actions of a chemical demulsifier.The mechanisms to consider for a chemical demulsifier:

· Strong attraction to the oil/water interface. The demulsifier must be able to migrate rapidly through the oil phase to reach the droplet interface where it must counteract the emulsifying agent.

· Flocculation. The demulsifier must have an attraction for water droplets with a similar charge and bring them together. In this way, large clusters of water droplets gather, which under a microscope look like bunches of fish eggs.

· Coalescence. After flocculation, the emulsifier film remains continuous. If the emulsifier is weak, the flocculation force might be enough to cause coalescence; however, this usually is not true, and the demulsifier must enable coalescence by neutralizing the emulsifier and promoting rupture of the droplet interface film. In the flocculated emulsion, the film rupture causes increasing water-drop size.

· Solids wetting. Iron sulfides, clays, and drilling muds can be made water-wet, which causes them to leave the interface and be diffused into the water droplets. paraffins and asphaltenes can be dissolved or altered by the demulsifier to make their films less viscous, or they can be made oil-wet so that they will be dispersed in the oil [64].

 


Date: 2016-04-22; view: 1265


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