Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D17/00—Separation of liquids, not provided for elsewhere, e.g. by thermal diffusion
  • B01D17/06—Separation of liquids from each other by electricity
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B03—SEPARATION OF SOLID MATERIALS USING LIQUIDS OR USING PNEUMATIC TABLES OR JIGS; MAGNETIC OR ELECTROSTATIC SEPARATION OF SOLID MATERIALS FROM SOLID MATERIALS OR FLUIDS; SEPARATION BY HIGH-VOLTAGE ELECTRIC FIELDS
  • B03C—MAGNETIC OR ELECTROSTATIC SEPARATION OF SOLID MATERIALS FROM SOLID MATERIALS OR FLUIDS; SEPARATION BY HIGH-VOLTAGE ELECTRIC FIELDS
  • B03C11/00—Separation by high-voltage electrical fields, not provided for in other groups of this subclass
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B03—SEPARATION OF SOLID MATERIALS USING LIQUIDS OR USING PNEUMATIC TABLES OR JIGS; MAGNETIC OR ELECTROSTATIC SEPARATION OF SOLID MATERIALS FROM SOLID MATERIALS OR FLUIDS; SEPARATION BY HIGH-VOLTAGE ELECTRIC FIELDS
  • B03C—MAGNETIC OR ELECTROSTATIC SEPARATION OF SOLID MATERIALS FROM SOLID MATERIALS OR FLUIDS; SEPARATION BY HIGH-VOLTAGE ELECTRIC FIELDS
  • B03C2201/00—Details of magnetic or electrostatic separation
  • B03C2201/02—Electro-statically separating liquids from liquids

Definitions

• the present invention concerns a method for inducing the merging of drops of water in an emulsion (coalescence) by applying an electric voltage across the emulsion to simplify subsequent separation of water from the emulsion.
• the invention concerns an electrocoalescer for performing the method.
• Oil/ water separators are used to separate water from oil continuous emulsions.
• oil continuous emulsions water-in-oil emulsions
• an electric field applied to the emulsion will cause water drops to coalesce.
• the water drops become larger and therefore settle at a higher rate allowing use of smaller settling tanks.
• US patent 4,804,453 describes an electrical system to achieve coalescence by which a decreasing field from the inlet side towards the outlet side of the coalescer is desired, since such arrangement is believed to enhance the separation of water from the emulsion.
• WO 03/049834 correspondingly describes equipment for separation of water from emulsions by coalescence.
• the particular feature of this design is the method by which the electrode plates in a vessel are connected to a low voltage power supply outside the vessel so that all parts having a high voltage are isolated from the environment in an as compact way as possible.
• Equipment for coalescence is usually designed as large tanks holding substantially stagnant emulsions.
• the companies Kvaerner and ABB have created a new generation of electrocoalescers utilizing alternating current and turbulent flow, cf. US patent 6,136,174.
• alternating current When alternating current is applied the drops become small dipoles between which strong forces occur when the shear in the emulsions brings one drop close to another.
• the objective of the present invention has been to provide a method that allows electro- coalescence for separation of water from water-in-oil emulsions to take place with a higher efficiency so that larger volumes of emulsions may be treated within a certain period of time or so that the equipment may be downsized for a certain required rate of separation.
• a further objective of the invention has been to provide a method that is well suited for AC coalescers and particularly of the kind with covered electrodes, thereby avoiding the problems particularly related to resistive voltage distribution.
• the invention concerns a method as defined by claim 1. According to another aspect the invention concerns an electrocoalescer as defined by claim 7. Preferred embodiments of the invention are disclosed by the dependent claims.
• the essential feature of the present invention is to utilize an ac voltage shaped as a substantially bipolar pure square wave.
• the frequency of the square wave can be adapted to obtain a capacitive voltage distribution and thus full voltage over the emulsion, ensuring a high electric field around the drops in the emulsion.
• the force or thrust effect on the drops in a neutral emulsion is not dependent on polarity which means that the forces have the same direction and magnitude during positive and negative half cycles.
• the forces between drops are the same as with a DC voltage of the same amplitude, supplied to emulsion where the oil phase has good insulating properties but without the negative effects of DC voltage with respect to resistive voltage distribution and possibly of charge relaxation.
• Figure 1 is a schematic illustration of two drops in an electric field.
• Figure 2a and /2b show how a water drop changes its shape in an electric field.
• Figure 3 is a diagram showing the relation between critical voltage and drops radius for a single drop.
• Figure 4 is a schematic comparison of conditions between coalescence with sinusoidal AC and AC in the form of square waves.
• Figure 1 shows two drops of water in a vertical electric field. The deformation of the drop with larger radius of curvature and less internal pressure is easily observed. If the voltage exceeds a critical limit the deformation of the larger drop will lead to an unstable condition and the larger drop will eventually be "injected” into the smaller one.
• Figure 2a shows how a round water drop (uppermost) in a horizontal electrical field will stretch from the induced dipole moment in the water (middle). Beyond a critical limit the drop becomes unstable which is what is desired in a coalescence process.
• Figure 2b illustrates how a critical voltage for a drop is reduced with increasing drop radius.
• the lowermost graph is for DC and square waves and the middle one for 50 Hz AC. It is easily recognized that a square wave shaped voltage supply allows lower field strength for instability than a sinusoidal voltage supply. The same can be observed with respect to critical voltage level for coalescence between two drops.
• Figure 3 shows a scheme diagram of the voltage distribution in a coalescer with covered electrodes.
• the emulsion and the electrode insulation may be described by a capacitance and a resistance in parallel.
• the voltage distribution will follow the solid-drawn line and there is thus no voltage drop between the two drops.
• the voltage distribution will follow the dotted line and the drops will be positioned at different voltage levels.
• Figure 4 depicts the conditions for a pair of drops at a critical voltage level for coalescence.
• a sinusoidal voltage supply will have a field strength above the critical field strength only a part of the time, while a square wave with sufficient amplitude will have a field strength above the critical field strength all the time.
• the sinusoidal voltage will in addition provide an (unnecessary) over- voltage part of the time.
• the voltage in the system may be held lower than with a coalescer with a sinusoidal voltage that has to be supplied with a high voltage to obtain a voltage above the critical level most of the time.
• a coalescer with a sinusoidal voltage that has to be supplied with a high voltage to obtain a voltage above the critical level most of the time.
• the RMS value is approximately 0.7 while for a square wave shaped alternating voltage it is approximately 1. This means that the potential for improvement is about 40 %.
• the voltages may conveniently be in the range 1 to 40 kV — of course dependent on dimensions (electrode separation). Frequencies in the range 10 to 10 000 Hz may be convenient.
• Technically square waves may be realized by means of transformers, rectifiers, and capacitors where the voltage is turned on/ off mechanically or possibly with a solid state switch. Alternatively power electronics may be used.
• Rise-time for the square wave voltages depends on impedances and capacitances in the circuit. Since the process depends on the voltage level it is not sensitive to changes in rise-time or ripple (deviations from ideal pulse shape). What is important is that the voltage quickly rises to the desired level and mainly stays at that level for the duration of a half-cycle.
• the term "mainly” in this context is understood to mean that fluctuations in voltage over a longer period - compared to the half-cycle - do not at any time bring the voltage below a value corresponding to a critical electric field level in the emulsion.
• the necessity of using electrode insulation is related to the risk of a water bridge that forms a short circuit between the electrodes. If no such risk exist the electrode insulation may be omitted.
• the frequency of the voltage to be applied should be adapted to the conditions.
• the frequency of the supplied voltage should be so high that a half-cycle is short compared to the emulsion time constant. If the efficiency of a coalescer increases with increasing frequency there is no problem in going higher. Presently, though, it seems that the optimal design is to go as high as required to overcome the emulsion time constant but no higher than that. Recent attempts indicate that increasing the frequency further may prevent a strong agitational movement imparted by drops with a net charge, thereby contributing to a more rapid coalescence in emulsions. This effect may imply that a higher frequency may be advantageous.

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• Physics & Mathematics (AREA)
• Thermal Sciences (AREA)
• Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Water Treatment By Electricity Or Magnetism (AREA)

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Cited By (2)

Citations (5)

• 2004
  • 2004-10-08 NO NO20044265A patent/NO20044265L/no not_active Application Discontinuation
• 2005
  • 2005-10-03 WO PCT/NO2005/000369 patent/WO2006043819A1/en active Application Filing

Patent Citations (5)

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