Agitating – Having specified feed means – Pump forces material through restriction
Reexamination Certificate
2000-06-13
2003-04-22
Soohoo, Tony G. (Department: 1723)
Agitating
Having specified feed means
Pump forces material through restriction
C138S042000, C138S043000
Reexamination Certificate
active
06550956
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the mixing of liquids, particularly viscous liquids, for example plastic materials such as polymers, and especially the mixing of such materials having widely different viscosities, and when a minor phase is highly viscous. However, the invention can also be used for mixing other liquids, for example milk homogenization and preparation of mayonnaise in the food industry, preparation of explosive emulsions in the explosive industry, and homogenisation of molten soaps in the chemical industry.
2. Prior Art
One form of the present invention is an improvement of the motionless extensional flow mixer described in our U.S. Pat. No. 5,451,106, issued Sep. 19, 1995, which gives a detailed review of the prior art in this field.
Briefly, it is known to mix polymers by distributive mixing effected by so-called “motionless mixers” placed between a screw feeder and a die. In most cases these mixers have a number of alternating right and left-handed helical elements placed in a tubular housing equipped with temperature control. The energy for mixing is provided by the pressure loss across the mixer. The splitting and recombination of streams results in a predictable number of striations. The advantage of such mixers is that they are accessories to standard type of compounding or processing equipment, not their integral part, and their main disadvantages are lack of easy adjustment, limited effectiveness in mixing, and inability to provide dispersive mixing. The basic principle behind their design is division and recombination of the flow streams. Since the flow division is of the shear type, the dispersive forces are usually weak, limited to the cases where the two liquids show similar viscosity.
Theoretical calculations and experiments have shown that dispersive mixing of two Newtonian liquids is more efficient in extensional than in shear flow. Extensional flow occurs for example when fluid converges from a reservoir to a capillary. In shear flow fields it is impossible to disperse liquids that have viscosity higher than that of the matrix fluid by more than a factor of 3.8. By contrast, the dispersing capability of the extensional flow field is only slightly affected by the viscosity ratio. From the kinematics point of view, the extensional flow field engenders deformation much more rapidly (note the absence of vorticity in the elongational flow field). At a given stress level, the generated interphase (that is the accepted measure of adequacy of mixing or “mixedness”) is orders of magnitude greater than that generated in shear. Similarly, the amount of energy required to generate a given degree of mixedness in elongation is orders of magnitude smaller than that in shear. Furthermore, the mechano-chemical degradation of the macromolecules is much less extensive in the elongational than in the shear field.
In spite of all these advantages present mixing equipment (including the twin-screw extruders) operates mainly in shear. This is due to the ease of designing equipment that operates on the shear flow principle. By contrast, it is difficult to envisage geometry that will engender very large deformations in the extensional flow field. However, one may by-pass this problem by designing a mixer in which the elongational flow field is engendered in a series of convergent-divergent geometries, preferably with semi-quiescent zones in between.
One prior patent describing an extensional flow mixer was U.S. Pat. No. 4,334,783 of Suzaka, which issued Jun. 15, 1982. The drawbacks of the Suzaka mixer are described in our aforesaid '106 patent. The mixer described in our '106 patent was intended to overcome these drawbacks, and to provide a mixer having the following characteristics:
1. The mixture of two fluids is exposed to strong extensional flow fields, each followed by a semi-quiescent zone;
2. The flow fields are generated by a series of convergences and divergences of progressively increasing intensity;
3. To reduce the pressure drop, as well as to prevent blockage of the restrictive openings, a series of holes (e.g. of the Suzaka design) are replaced by slits;
4. The slit gaps are made adjustable.
The mixer of our '106 patent has a series of chambers separated by several convergent/divergent surfaces providing narrow openings between the chambers. The openings are in the form of slits defined by the inner edges of protrusions formed on die members which provide the convergent/divergent surfaces. Also, the die members subject the liquids to gradually increasing stress, since the protrusions of the die members are concentric and are arranged so that during mixing the liquids pass radially inwards between the die members in passing from the inlet to the outlet of the mixer. At least one of the die members is made movable to adjust the slit gap, thereby adjusting the stress level.
In the design shown in our '106 patent, the movable die member is held at the lower end of a cylindrical block or mandrel which is slidable in a cylindrical chamber of a housing. Movement of the block, for adjustment of the gap width, is effected by rotating a wedge-shaped disc between an end of the housing and a sloping top end of the block. Passageways for the supply of the mixed liquids to the edges of the die members are formed around the sides of the block, and communicate with a side inlet into the housing. This construction has been found to have two drawbacks.
Firstly, when using high pressures in the mixer, for example 3,000 psi or 20 MPa, the liquid pressure at the side of the block adjacent the side inlet tends to tilt the block causing asymmetrical flow to the edges of the die members. Secondly, the wedge-shaped disc used to vary the slit gaps was difficult to adjust. The present invention overcomes these problems.
Another form of the present invention combines features of the '106 motionless mixer patent with some features of known dispersive mixers that are used in association with screw extruders, particularly single screw extruders, to improve the mixing capability of such extruders. Such mixers generally have a housing defining a cylindrical cavity with inlet and outlet ends, and a mandrel of generally cylindrical form which is rotatable in the cavity. The mandrel has protrusions which may resemble screw threads, but which are interrupted by gaps, or separated by other, discrete protrusions or indentations, so that the material being mixed is not merely progressed along the cavity, as in a screw extruder, but is also caused to move through slits between the outer edges of the protrusions and the inside surface of the cavity. The side surfaces of the protuberances provide convergent entrances into, and divergent exits from, the slits.
SUMMARY OF THE INVENTION
The extensional flow mixer of this invention is similar to that of our '106 patent in having:
a housing providing a cavity having an internal surface, and having an inlet into the cavity which inlet is connectable to a pressurized source of the liquids, the end of the housing remote from the inlet having an outlet for the mixed liquids;
a mandrel located in the cavity;
the mandrel carrying protrusions having side surfaces which converge towards their outer edges, the outer edges cooperating with the internal surface of the cavity to divide the space between the protrusions and the internal surface into a series of chambers separated by slits such that liquid passes successively through all the chambers and slits in moving from the inlet to the outlet, the side surfaces providing convergent entrances to, and divergent exits from, the slits, and the slits having cross-sectional areas which decrease in the liquid flow direction, from an upstream chamber adjacent the inlet, to the outlet; and means for adjusting the slit gaps.
To overcome problems with asymmetrical flow of liquids into the outermost cavity, in accordance with this invention the inlet into the housing is at an end of the housing, rather than at the side, and the mandrel h
Bourry Daniel Jude Joseph
Luciani André
Utracki Lechoslaw Adam
Anissimoff & Assoc.
Koenig Hans
National Research Council of Canada
Soohoo Tony G.
LandOfFree
Extensional flow mixer does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Extensional flow mixer, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Extensional flow mixer will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3069713