Coalescence demulsification

DEMULSIFICATION TESTS. Demulsification tests were conducted using standard bottle test procedures to evaluate the relative performance of Thin Film Spreading Agents in coalescing emulsions of formation brine in crude oil under reservoir conditions. 

Demulsification with electrostatic fields appears to be the most effective and economic way for breaking of W/0 emulsion in ELM processes 190, 91]. Electrostatic coalescence is a technique widely used to separate dispersed aqueous droplets from nonconducting oils. Since this type of technique is strictly a physical process, it is most suitable for breaking emulsion liquid membranes to recover the oil membrane phase for reuse. 

Freshly prepared macroemulsions change their properties with time. The time scale can vary from seconds (then it might not even be appropriate to talk about an emulsion) to many years. To understand the evolution of emulsions we have to take different effects into account. First, any reduction of the surface tension reduces the driving force of coalescence and stabilizes emulsions. Second, repulsive interfacial film and interdroplet forces can prevent droplet coalescence and delay demulsification. Here, all those forces discussed in Section 6.5.3 are relevant. Third, dynamic effects such as the diffusion of surfactants into and out of the interface can have a drastic effect. 

Some emulsions are undesirable when they occur. In process industries chemical demulsification is commonly used to separate water from oil in order to produce a fluid suitable for further processing. The specific kind of emulsion treatment required can be highly variable, even within the same industry. The first step in systematic emulsion breaking is to characterize the emulsion in terms of its nature (O/W, W/O, or multiple emulsion), the number and nature of immiscible phases, the presence of a protective interfacial film around the droplets, and the sensitivity of the emulsifiers [295,408,451], Demulsification then involves two steps. First, there must be agglomeration or coagulation of droplets. Then, the agglomerated droplets must coalesce. Only after these two steps can complete phase separation occur. It should be realized that either step can be rate determining for the demulsification process. 

It should be noted that the droplet sizes in Hallworth s emulsions are considerably greater than those investigated by Davis and Smith. The importance of the two possible routes of degradation of the emulsions, coalescence or molecular diffusion, may be dependent upon the droplet size and size distribution. Also an interfacial coherent film may reduce the demulsification by either mechanism, i.e. by reducing the rate of coalescence or by presenting an interfacial barrier to... 

Stevens et al. [86] proposed the replacement of the surfactant with fluids to modify the rheological properties and stabilize the emulsion. The aim was to slow the drainage of the film between the coalescing drops, thereby increasing the stability of the membrane. Their study on the removal of chromium with Alamine 336 showed that the emulsion stabihty could be controlled with the addition of smaU amounts of polymer to the organic phase and that demulsification could be achieved by heating the system. 

Coalescence of the large water and oil droplets with their respective continuous phases in the coalescer To date, chemical or physical treatment is the method used for demulsification. 

Equipment Several types of ac and dc electrostatic coalescers have been developed. Documented equipment for electrostatic demulsification includes the continuous insulated electrode coalescer [130,136], two-phase insulated electrodes in parallel [126], circular coalescer [137], continuous coalescer [138], horizontal insulated electrode [130], box electrostatic demulsifier [139], tubular coalescer [140], and batch cylinder demulsifier [141]. Design criteria for electrostatic demulsifiers have been reported by Draxler and Marr [142] and Draxler et al. [143]. 

Feng et al. [138] found that coalescence efficiency could be improved with increasing frequency. Draxler et al. [103] demonstrated that for a given degree of demulsification the voltage could be reduced if the frequency was enhanced. It has... 

Goto M, Irie J, Kondo K, and Nakashio F. Electrical demulsification of W/O emulsion by continuous tubular coalescer. J Chem Eng Jpn 1989 22 401 06. 

The techniques used for demulsification of a transport emulsion may include raising the temperature of the emulsion, addition of emulsionbreaking additives, addition of diluents to reduce the viscosity of the heavy crude oil, and the use of equipment designed to promote coalescence of the crude-oil droplets. Raising the temperature of the emulsion increases the... 

Coalescence being the secondary process, the number of distinct droplets decreases leading to a stage of irreversibility and finally complete demulsification takes place. Coalescence rate very likely depends primarily on the film-film repulsion, film drainage and on the degree of kinetics of desorption. Kinetically, coalescence is a unimolecular process and the probability of merging of two droplets in an aggregate is assumed not to affect the stability at other point of contact (32). 

It was observed that the formulations consisting of ethoxylated sulfonates and petroleum sulfonates are relatively insensitive to divalent cations. The results show that a minimum in coalescence rate, interfacial tension, surfactant loss, apparent viscosity and a maximum in oil recovery are observed at the optimal salinity of the system. The flattening rate of an oil drop in a surfactant formulation increases strikingly in the presence of alcohol. It appears that the addition of alcohol promotes the mass transfer of surfactant from the aqueous phase to the interface. The addition of alcohol also promotes the coalescence of oil drops, presumably due to a decrease in the interfacial viscosity. Some novel concepts such as surfactant-polymer incompatibility, injection of an oil bank and demulsification to promote oil recovery have been discussed for surfactant flooding processes. 

Demulsification can be usefully broken down into a series of stages. In the first instance, droplets must approach one another to form a loose aggregate. At this point, it is important to sweep up small droplets before the next stage, coalescence, progresses too far. During coalescence, there is a dramatic reduction in interfacial area, resulting in the concentration of solids and other debris at the interface. These materials, which may have contributed to the stabilisation of the original emulsion, must be dealt with by dispersal into the bulk of one or other of the liquid phases. The formation of distinct phases can then be completed and the water removed. 

In contrast to the large achievements in investigations of kinetic stability, modest attention has been paid to the fundamentals of thermodynamic stability in emulsions, especially regarding the surfactant adsorption layer s influence on the coalescence time. There are several investigations devoted to the surface chemistry of adsorption related to emulsification and demulsification. However, the link between the chemical nature of an adsorption layer, its structure, and the coalescence time is not yet quantified. 

Emulsification and demulsification are both eomplex processes. However, as noted earlier, demulsifieation is by no means the opposite of emulsification (200,201). This is especially the case in the petroleum industry. In order to demulsify a crude W/O emulsion efficiently, it has been emphasized that it is advantageous to understand first the characteristics of the emulsions, the nature of interfaeial films, and hence the causes of stability. Accordingly, in choosing a demulsification protocol, one would first identify key factors responsible for the stabihty, find the target properties to modify toward destabilization, introduce sufficient energy to promote coalescence, and find the best conditions to allow phase separation. 

In industry, if crude oil emulsions do not coalesce and/or phase separate in a given time frame, and persist throughout the process, the emulsion is deemed stable or tight. Demul-sification can be monitored by bulk phase separation over time and/or by a more fundamental approach of examining the interfacial dynamics which provides some understanding of the demulsification mechanisms. The first is used in... 

They showed that the coalescence rate constant, K, increases while the flocculation rate constant decreases with increased demulsifier concentration. Flocculation is high at low demulsifier concentration. At increased concentration it breaks the interfacial film and promotes coalescence. A plot of initial coalescence rate constant versus dosage indicates that the demulsification of this system was in a flocculation-rate controlling state, within its environment. Aggregation is reversible and the drop identity is not lost. 

Most chemical agents used for demulsification are preferentially oil-soluble blends consisting of HMW polymers. These blends commonly consist of (1) floc-culants (large, slow acting polymers) (2) coalescers (LMW polyethers) (3) wetting agents and (4) sol-vents/cosolvents. Some chemical structures of demulsifiers used for breaking crude oil emulsions have been listed by Jones et al. (42). Much work has been carried out in order to identify and understand the mechanisms behind chemical demulsification. Fiocco (43) concluded that the inter facial viscosity was kept at a low level when demulsifiers were present. Later on it was realized that the interfacial shear viscosity of crude oil emulsions does not have to be very low in order to ensure accelerated water separation (44). 

The overall efficacy of microemulsion-based extraction of heavy metals (particularly mercury) from contaminated water involving oleic acid was reported and successfully modeled by Wiencek and coworkers [153,154], who used experimentally determined equilibrium extraction, stripping, and the initial reaction kinetics. This model accurately predicts both the initial extraction kinetics and final mercury extraction equilibrium, Good agreement between theory and experiment on the mechanism of extraction using a microemulsion to that of coarse emulsions has been found. Electrostatic coalescence and butanol addition were evaluated as potential demulsification techniques for recovery of the components from mercury-rich microemulsions [155]. 

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