Agitating – Having specified feed means – Pump forces material through restriction
Patent
1996-04-16
1998-07-21
Cooley, Charles E.
Agitating
Having specified feed means
Pump forces material through restriction
366340, B01F 506
Patent
active
057825572
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to homogenization heads and is more particularly, although not exclusively, concerned with homogenization heads for producing photographic dispersions.
BACKGROUND OF THE INVENTION
Homogenization is the process by which very small particles are produced from an initial coarse emulsion, suspension or dispersion to give a more stable emulsion, suspension or dispersion of smaller particle size. The process takes place in a valve or head wherein the coarse emulsion, suspension or dispersion is pumped through a small gap at high pressure. Due to the high pressure, the material being homogenized has a very high velocity and as a result a large pressure drop is experienced in the valve or head. Cavitation, turbulence and shear forces act to divide the coarse material into finer particles.
For example, if oil and water are to be homogenized to produce the smallest possible droplet size, a coarsely mixed liquid or `premix` is passed through a high pressure homogenizer. In this process, the `premix` may be pumped at pressures up to 1000 bar or even higher through a head wherein the pressure is reduced suddenly by a series of steps and narrow gaps back to atmospheric pressure. The high shear induced on the `premix` causes the oil droplets to break up in a sequence of eddies, vortices, cavitations or impacts.
It has been found empirically that when homogenizing discrete phases of high viscosity, such as are found in photographic dispersions for example, then it is necessary to use an orifice type homogenizing system to produce the smallest droplet size. This type of homogenizing system comprises a plurality of orifice plates, each plate having at least one small hole formed therein and each plate is separated from an adjacent plate by spacers. As the `premix` passes through the stack of plates, the dispersion is sequentially sheared. This type of homogenizing system provides a finer product than systems in which the liquid is forced between plates held together by an adjustable spring.
GB-A-1 577 567 describes such a system. A high pressure `premix` is directed through a multi-stage device in which, at each stage, the material is directed through a nozzle at high speed and then impinges on a wall of an orifice block perpendicular to the direction of flow. The first break up and dispersion of the `premix` occurs due to the shearing force exerted on the `premix` as it passes through the nozzle. On impingement on the wall, the direction of flow of the `premix` at high pressure is rapidly changed and a second break up and dispersion occurs. The material then flows through axially bored holes offset from the nozzle formed in the orifice block on to the next stage. A high degree of dispersion is obtained by the multi-stage effect.
However, with a fixed orifice homogenizing system, the pressure and flow are inseparably linked and one cannot be adjusted independently of the other. This is normally acceptable if the material being homogenized is made in a batch-wise fashion where the batch can be made under any reasonable pressure/flow conditions and stored prior to use.
DE-A-3 728 946 discloses a homogenizing head in which a piston-valve arrangement is used. Liquid to be homogenized is passed through an inlet formed in the valve seat at high pressure through a homogenizing channel defined by a piston and a valve seat each having a diamond-sintered layer. The valve has a surface which is angle at 5.degree. to the end surface of the piston. Homogenizing material is then collected from a collection chamber through an outlet.
US-A-4 348 116 describes a homogenizing head arrangement in which a casing is provided with an internal cylindrical surface defining a bore, a portion of internal surface being threaded. The casing has an open end into which a rotatable closing plug is inserted, a portion of the outer surface of the plug being threaded for engagement with the threaded portion of the internal surface of the casing.
The casing also includes an inlet port through which the
REFERENCES:
patent: 1898894 (1933-02-01), Richards et al.
patent: 4348116 (1982-09-01), Bordas
patent: 5421650 (1995-06-01), Meyer
Bocchetti Mark G.
Cooley Charles E.
Eastman Kodak Company
Rosenstein Arthur H.
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