Forming emulsions

Details Forming emulsions Forming emulsions

2002-08-20

2004-07-20

Soohoo, Tony G. (Department: 1723)

Agitating

Having specified feed means

Liquid injector within mixing chamber

C366S176100, C366S181500

Reexamination Certificate

active

06764213

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to forming emulsions.
We use the term “emulsion” for a system comprising two immiscible liquid phases, with one phase dispersed as small droplets in the other phase. For simplicity we will call the dispersed phase “oil” and the continuous phase “water”, although the actual components may vary widely. As additional components, emulsifying agents, known as emulsifiers or surfactants, serve to stabilize emulsions and facilitate their formation, by surrounding the oil phase droplets and separating them from the water phase.
The uses of emulsions have been increasing for many years. Most processed food and beverage products, medicine and personal care products, paints, inks, toners, and photographic media are either emulsions or employ emulsions. In recent years, demand for emulsions with smaller and more uniform droplets has increased. Artificial blood applications, for example, require nearly uniform droplets averaging 0.2 micrometers. Jet-ink printing has similar requirements of size and distribution.
High pressure homogenizers are often used to produce small and uniform droplets or particles, employing a device which is commonly referred to as an homogenizing valve. The valve is kept closed by a plug forced against a seat by means of a spring or hydraulic or pneumatic pressure. The pre-mixed raw emulsion is fed at a high pressure, generally between 1,000 and 15,000 psi, to the center of the valve seat. When the fluid pressure overcomes the force closing the valve, a narrow annular gap (10-200 um) is opened between the valve seat and the valve plug. The raw emulsion flows through, undergoing rapid acceleration as well as sudden drop in pressure which breaks down the oil phase into small droplets. More recently, a new type of high pressure homogenizer was introduced, employing two or more fixed orifices, and capable of reaching 40,000 psi. When forced through these orifices, the pre-mixed raw emulsion forms liquid jets which are caused to impinge at each other. A description is found in U.S. Pat. Nos. 4,533,254 and 4,908,154.
The typical mechanism for emulsification in this type of device is the controlled use of shear, impact, and cavitation forces in a small zone. The relative effects of these forces generally depend on the fluid's characteristics, but in the vast majority of emulsion preparation schemes, cavitation is the dominant force.
Fluid shear is created by differential velocity within the fluid stream, generated by the sudden fluid acceleration upon entering the orifice or small gap, by the difference between the extremely high velocity at the center of the orifice and zero velocity at the surfaces defining the orifice, and by the intense turbulence which occurs after exiting the orifice.
Cavitation takes place when pressure drops momentarily below the vapor pressure of the water phase. Small vapor bubbles form and then collapse (within 10-3 to 10-9 sec.), generating shock waves which break down surrounding oil droplets. Cavitation occurs in homogenizing valves when the sudden acceleration in the orifice, with a simultaneous pressure drop, causes the local pressure to drop momentarily below the vapor pressure.
More generally, it has become known that cavitation occurs when two surfaces are separated faster than some critical velocity, and that cavitation bubbles affect their surrounding only during the formation of the cavities, and not during the collapse of the cavities, as had been long assumed. Another discovery of interest is that cavitation can occur either totally within the liquid, or at the solid-liquid interfaces, depending on the relative strength of solid-liquid adhesion and the liquid—liquid cohesion.
Typical emulsification schemes have several characteristics worth noting. Cavitation takes place only once, for a very short time (10-3 to 10-9 seconds), and equipment which employs high power density imparts emulsification energy only to a very small portion of the product at any given time. The emulsification process is thus highly sensitive to the uniformity of the feed stock, and several passes through the equipment are usually required before the desired average droplet size and uniformity are achieved. The final droplet size depends on the surfactant's rate of interaction with the oil phase. Because surfactants cannot generally surround the oil droplets at the same rate they are being formed by the emulsifying process, agglomeration takes place and average droplets size increases. There is a typical sharp increase in product temperature during the process, which limits the choice of emulsion ingredients and processing pressure, as well as accelerating the agglomeration rate of the droplets after the emulsification process. Some processes require very small solid polymer or resin particles; and this is often accomplished by dissolving solid polymers or resins in VOC's (volatile organic compounds), then employing mixing equipment to reduce the droplets size, and finally removing the VOC.
SUMMARY OF THE INVENTION
In general, in one aspect, the invention features a method for use in causing emulsification in a fluid. In the method, a jet of fluid is directed along a first path, and a structure is interposed in the first path to cause the fluid to be redirected in a controlled flow along a new path, the first path and the new path being oriented to cause shear and cavitation in the fluid.
Implementations of the invention may include the following features.
The first path and the new path may be oriented in essentially opposite directions. The coherent flow may be a cylinder surrounding the jet. The interposed structure may have a reflecting surface that is generally semi-spherical, or is generally tapered, and lies at the end of a well. Adjustments may be made to the pressure in the well, in the distance from the opening of the well to the reflecting surface, and in the size of the opening to the well. The controlled flow, as it exits the well, may be directed in an annular sheet away from the opening of the well. An annular flow of a coolant may be directed in a direction opposite to the direction of the annular sheet.
In general, in another aspect, the invention features a method for use in stabilizing a hot emulsion immediately after formation. The emulsion is caused to flow away from the outlet end of an emulsion forming structure, and a cooling fluid is caused to flow in a direction generally opposite to the flow of the emulsion and in close enough proximity to exchange heat with the emulsion flow.
Implementations of the invention may include the following features. The emulsion may be formed as a thin annular sheet as it flows out of the emulsion forming structure. The cooling fluid may be a thin annular sheet as it flows opposite to the emulsion. The cooling fluid may be a liquid or gas compatible with the emulsion. The flows of the emulsion and the cooling fluid may occur in an annular valve opening.
In general, in another aspect, the invention features a method for use in causing emulsification of a first fluid component within a second fluid component. In the method, an essentially stagnant supply of the first fluid component is provided in a cavity. A jet of the second fluid component is directed into the second fluid component. The temperatures and the jet velocities of the fluids are chosen to cause cavitation due to hydraulic separation at the interface between the two fluids.
Implementations of the invention may include the following features. The second fluid component may include a continuous phase of an emulsion or dispersion. The first fluid component may be a discontinuous phase in the emulsion, e.g., a solid discontinuous phase. The second fluid may be provided in an annular chamber, and the jet may be delivered from an outlet of an orifice which opens into the annular chamber. After emulsification by hydraulic separation, the product may be passed through an orifice to cause additional emulsification, or may be delivered to a subsequent processing chamber, where an additi

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