Chemistry: electrical and wave energy – Processes and products – Electrical
Patent
1994-10-18
1996-12-03
Hruskoci, Peter A.
Chemistry: electrical and wave energy
Processes and products
Electrical
210748, 210708, 204568, B01D 1704, B01D 17035, B01D 1706
Patent
active
055804640
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to a method and apparatus for separating emulsions, in particular water-in-oil emulsions, into their component phases.
BACKGROUND OF THE INVENTION
Existing techniques whereby a water-in-oil emulsion may be resolved into its constituent phases make use of chemical and physical means for achieving the resolution.
The need for an emulsion breaking operation may arise in connection with a wide variety of processes. For example, in the production of crude oil at the well-head, where saline formation water is present as fine stable droplets in the oil coming to the surface, it is necessary to break the emulsion in order to dehydrate the oil.
In other cases, it may be essential to recover the components of an emulsion after it has been deliberately created for a specific purpose. This occurs in processes using emulsion liquid membrane technology where, for instance, a very stable water-in-oil emulsion is used to treat an aqueous effluent feed stream to remove an impurity. The species to be removed from the aqueous feed stream transfers through the outer oil phase of the emulsion into the water droplets or internal phase of the emulsion. To encourage this mass transfer to occur it is necessary to have some appropriate chemical or physical driving force. A simple example of such a process is the extraction of ammonia from an effluent water into an emulsion of sulphuric acid in kerosene.
A crucial feature of the economics of processes that employ emulsion liquid membranes concerns the ability to re-use the oil phase repeatedly. Ecohomic viability depends on being able to break the used emulsion speedily to remove the spent internal phase in bulk and recycle the oil phase to make fresh emulsion for re-use.
In the oil industry, crude oil emulsions that are slow to resolve using a purely physical method such as gravity separation, can often be treated with chemical additives which serve to destabilise the emulsion. However, the latter course of action is not feasible for liquid membrane emulsions. Here the desire to recreate a stable emulsion using the separated oil phase mitigates against the use of chemical demulsifiers. In the case of liquid membrane emulsions it is therefore necessary to consider the use of physical methods alone to speed up the separation of an emulsion.
In general, the resolution of emulsions requires that the small droplets of the dispersed (internal) phase coalesce together, until they become large enough to be removed easily from the continuous phase. Where the densities of the two phases are different, the denser phase simply gravitates from the emulsion and, given enough time, the two phases can be separated sufficiently for each to be drawn off. The time required for this separation is reflected by the size of the settling tanks which are typically required. These may be very large and may contain a large inventory of expensive liquids. In addition, the phase separation step may be the slowest stage of a more extensive process and therefore limit the throughput of the overall process.
During the resolution of a water-in-oil emulsion, those droplets that have grown by coalescence must gravitate through the emulsion to reach the bulk interface between the separated water layer and the unseparated emulsion. The viscosity of the emulsion tends to be high, especially for liquid membrane emulsions. This hinders the passage of the water droplets and they can be very slow to reach the bulk interface.
One method that has proved useful specifically for enhancing the rate of separation of a water-in-oil emulsion is to subject the emulsion to an applied high voltage gradient. The electric field assists the process of phase separation by promoting coalescence between the water droplets in the emulsion. Several possible mechanisms for electrically aided coalescence have been identified [Waterman, L. C. (1965) Chem. Eng. Progress, vol 61, (10), 51], all of which rely upon the attraction of opposite charges on adjacent droplets to cause an increased inciden
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Bradford University
Green Theodore M.
Hruskoci Peter A.
Schindler Edwin D.
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