Method And Apparatus For Continuously Dispersing Materials

Abstract

The method and apparatus of continuous high-shear dispersion of materials such as paint pigments in conventional vehicles and the like comprising: establishing a rotating zone of high-shear action along a horizontal plane with a conventional high-shear agitator, continuous admitting below the plane a supply of materials to be dispersed, inducing the materials to enter the zone, forming a consistent dynamic essentially curved laminar flow path for said materials after leaving the influence of said zone, repeating the influence of the high-shear action of said zone upon the same materials for a predetermined time to cause the materials to pass continuously through said path, confining the materials with the shape of the vessel to remain within said path to force said materials to repeatedly contact said zone, an outlet for withdrawing a portion of said materials continuously from above said zone during the movement of said materials near said zone and prior to a reentry into said zone, whereby to provide highly dispersed materials having been subjected repeatedly to the action of said zone which controls the period of retention within the vessel by means of the location of the exit path and by controlling the input and withdrawing.

Description

This invention relates generally to a method and apparatus for the continuous mixing or dispersing of materials. More particularly, the present invention is concerned with a method and apparatus for producing dispersions, homogeneous mixes or emulsions of dissimilar materials continuously through the use of a dispersion vessel in conjunction with a high-speed, high-shear agitator.

In the past, various processes and apparatus were employed in the paint, food, biological research, and related industries, wherein homogeneous mixes and dispersions or emulsions were desired. Equipment and methods that were applied for such purposes included the well-known ball mills, roller mills, sand mills, or similar mills which used other dispersion media, vibratory mills, sonic agitators, homogenizers, slow-pace mixers, and high-speed agitators.

While many of these approaches have been somewhat successful in achieving a desired result, one of the most serious drawbacks for all of the prior art attempts at achieving proper mixtures or dispersions, and the like, is the time consumed in batch production of the mixture.

The use of a high-speed agitator which produces a high rate of shear has long been known in the art as an effective means for use in a high degree of mixing. Such a high-speed agitator with peripheral speeds of 750 to 15,000 feet per minute, however, is usually limited to batch operations which bring about delays in transferring batches from the high-shear agitator apparatus. It has been found through experience that when utilizing the high-speed, high-shear agitator in the tank that is generally open, the most efficient mixing is achieved during the time which the materials being mixed form a flow pattern characteristically described as being in the shape of a doughnut or toroidal having the vortex in the center. While this pattern of flow of materials being mixed is not considered by the art to be essential, it is known that this pattern takes shape during periods of maximum dispersion.

Although the flow patterns for most efficient mixing are known to the art, there has never been a commercially successful continuous flow-through mixing providing continuous, repeated, and controlled contact with the high-speed, high-shear agitator.

Most commercial embodiments utilize an open tank or other vessel which necessarily must be larger than the batch to be mixed in order to avoid splashing or to permit further let-back of the dispersion. The size of these vessels in commercial operation range up to many hundreds of gallons and occupy floor space that in many modern plants results in a high cost factor.

Further, since the ratios of tank diameter and depth of batch to impeller diameter must be held within established limits to achieve efficient dispersion, open dispersion tanks must be properly proportioned and the dispersed batch transferred to a larger tank for let-back or further processing. Horsepower requirements in batch dispersion operation are high; i.e., a properly formulated dispersion mix of alkyd resin and pigment of 150 gallons dispersed in an open tank at a peripheral blade speed of 5,400 feet per minute will require 50 to 75 horsepower.

The batch must be loaded in proper sequence and attended throughout the process--then transferred to a larger tank for let-back.

Ideally, the dispersion would be able to be in a continuous flow. For instance, in a continuous dispersion vessel, the materials to be dispersed are premixed in much larger, low horsepower, slow mixers; then pumped through the continuous dispersion vessel directly to the letdown tanks, or through a second dispersion vessel in tandem on the same shaft or on a second agitator where a reducing vehicle is added at a predetermined rate, eliminating the need for a letdown tank.

The open or closed tank also possesses other inherent drawbacks such as permitting the ready admixture of air in the exposed flow pattern which is undesirable and often requires subsequent steps to remove the air so entrapped.

One of the most important drawbacks of the prior art high-speed, high-shear mixers utilizing the conventional tank or vessel or continuous units not using the exit system of this invention, is that the uniformity of the product is sometimes suspect since not all of the material in the tank can be assured of having been contacted by the high-shear blade a sufficient number of times to effect the proper mixing. It is the usual practice in the art to attempt to avoid this problem by repeatedly scraping down the agitator and the sides of the tank, and by maintaining the batch under the high-shear agitation for a substantially longer time than might be thought necessary in order to produce a relatively reliable dispersion. Attempts to diminish the length of time in which the material is mixed by the high-speed agitator often destroys the desired uniformity of mixing.

The art therefore is familiar with the use of the open or closed tank high-speed agitator, but has found at times that the mixture must be made in one properly dimensioned tank, let-back in another, an even then may be incomplete or else require extended time in the mixing apparatus to bring about a reasonable assurance of consistent quality.

Accordingly, it is the principal object of the present invention to provide a continuous flow-through high-speed, high-shear agitator method and apparatus.

Another object of the present invention is to provide a method and apparatus for reducing the mixing time for achieving consistent quality of a dispersion or mixture.

Another object of the present invention is to utilize at least a partially closed vessel of small dimensions compared to the conventional vessel to conserve floor space.

A further object of this invention is to obtain volume dispersion production with low horsepower.

The method and apparatus of the present invention also have as a particular object the provision of a vessel which conforms in internal shape to an ideal flow path of the materials being subjected to a high-shear agitator in order to assure complete mixing by repeated contact of the material with the high-shear zone of the agitator blade.

Another object of the present invention is to minimize in an open vessel, and eliminate in a sealed vessel, the air entrapment during the mixing of the materials with a high-shear agitator.

A further object of the present invention is to provide an apparatus and method for producing a high degree of dispersion and mixing in the minimum amount of time and to control the flow rate to produce any desired degree of dispersion or mixing.

These and other objects of the present invention will become apparent after careful study of the following specification and the accompanying drawings in which:

FIG. 1 is a perspective view of the continuous high-shear dispersion apparatus of the present invention;

FIG. 2 is a cross-sectional view, partly broken away, taken along the lines 2--2 of FIG. 1 best illustrating the important details of the preferred embodiment of the present invention;

FIG. 3 is a cross-sectional view of the side elevation, partly broken away, illustrating an embodiment of the vessel having a sealed tubular outlet pipe;

FIGS. 4 and 5 are cross-sectional views of the side elevation of another modification of the present invention illustrating both a variation in the shape of the vessel as well as the extent of the outlet.

FIG. 6 is a perspective view of another embodiment of the present invention illustrating another shape for tubular outlet and its use in an open vessel.

FIG. 7 is a side-elevational view partly broken away and taken along lines 7--7 of FIG. 6.

FIG. 8 is a schematic view of a continuous process and apparatus beginning with the raw materials in a premix tank through a tandem arrangement of the dispersion apparatus of FIG. 1.

Directing attention to FIGS. 1 and 2 which are the principal embodiments of the present invention, it can be seen that the apparatus in accordance with the present invention shown generally by the numeral 10 includes an overflow bowl 12 having sidewall 14 and bottom wall 16 which receives a tubular outlet pipe 18 which may optionally be sealed at 19 for pressurized operation, as best shown in FIG. 3. A rotatable shaft 20 is concentrically positioned within the pipe 18 and rotatably connected to a motor (not shown) by extending through the optional overflow bowl 12 at one end and out the bottom of the outlet 22 of the pipe 18 at the other end. The outlet pipe 18 is connected to side flow pipe 24 which is in fluid communication with the outlet pipe 18. Optionally, a screen member 26 may be positioned at the junction between the outlet pipe and the withdrawal pipe if sand or other media were to be used for improved grinding. The outlet pipe 18 extends through the cover housing 28 of the vessel 30. Suitable sealing means, such as welding 32, provides a leaktight fit between the outlet pipe 18 and the cover housing 28. The vessel 30 is provided with radial lip flanges 34 which are secured to corresponding edge portions 36 by means of bolts 38 or suitable conventional clamps. A sealing gasket 40 is positioned between the lip flanges 34 and edge portions 36 to provide proper sealing. A needle air-vent 39 is also provided in the cover housing 28.

Surrounding the vessel 30 is an outer and inner double wall 42, 44, respectively, which together form a fluid heat exchange means. Suitable fluid inlets and outlets 46 and 48 are provided as shown in FIG. 2. The heat exchange fluid is generally water and is desirable in order to remove the heat that is built up within the inner wall 44 of the vessel during the mixing, or to provide heat under some circumstances well known in the art. Materials inlet 50 is positioned through the outer and inner walls 42 and 44 to communicate with the interior 52 of the vessel. Optionally, screen 54 may be positioned within the inlet opening for the same purpose as screen 26 is positioned within the site withdrawal pipe 24.

Secured to the shaft 20 is a high-speed, high-shear blade 56 which, as shown, is disc-shaped having a substantially planar portion extending from the shaft 20 radially outwardly. The particular configuration of the periphery of the blade is not critical the present invention and, as shown in FIG. 2, the blade has alternating upwardly and downwardly directed fingers 58. However, these are not essential to the present invention, it being adequate for the purposes of the present invention to utilize a blade such as that shown in FIG. 5 at 60. The high-shear blade 60 is completely flat and disc-shaped having no vertical fingers. In fact, any conventional high-shear mixing means including blades, impellers, colloid mixing heads, rotors, stators, and such, may be utilized in the present apparatus and method. Blade configuration and the advantages of certain high-shear means over others is an art well known, but the principles of this invention do not require any particular shape or design to effect the beneficial results.

One embodiment of the present invention and one that produces particularly efficient operation is that configuration of the vessel which conforms primarily to the doughnut or toroidal shaped flow path of the materials to be mixed. To confine the interior of the vessel 52 essentially to the shape of the most efficient flow path, it can be seen that the inner bottom wall of 62 of the vessel 30 is provided with a cone-shape partition 64 which, in cross section as shown in FIG. 2, slopes upwardly from the bottom wall 62 and outwardly to contact the inner sidewall 44 at 63. Similarly, the cover housing 28 is provided with a conical partition 66 which contacts the inwardly protruding tubular outlet pipe 18 at 68 and the cover housing at 69. With the conical partitions 64 and 66, the interior 52 of the vessel 30 is substantially confined to a somewhat toroidal doughnut-shaped space above the blade 56.

It has been found not to be essential but rather far more efficient to confine the shape of the interior of the vessel above the blade or the tubular outlet pipe to the natural flow pattern that will be achieved by reason of the high-speed, high-shear laminar action of the blade. FIGS. 4, 5, 6 and 7 demonstrate the various other shapes that the interior of the vessel and the tubular outlet pipe may assume which generally conform to the doughnut or toroidal-shaped flow path.

One of the unique aspects of the present invention is the discovery that the outlet 22 at the end of the tubular outlet pipe 18 must be above the blade 56. In the embodiment as shown in FIG. 2, the outlet 22 is concentric the shaft 20. Concentricity is not essential, but improves the fluid flow. The diameter of the outlet 22 may vary from being slightly larger than the diameter of the shaft 20 to enable the passage of the mix upwardly from the outlet 22 to a diameter which approaches the diameter of the blade.

In the preferred embodiment, the outlet pipe 18 is tapered at 70 to provide selected withdrawal and also to provide for a smaller outlet 22 with respect to the diameter of the blade 56. It is important that the outlet be so positioned adjacent the high-shear zone of the blade 56 that the tendency for the material downwardly directed toward the blade 56 is to be directed radially outwardly away from the outlet and that only the controlled input at 50 forces the materials after repeated passes through the high-shear zone to pass upwardly into the outlet 22.

It has been found that the distance of the outlet above the inner bottom wall 62 may vary from one-half to four times the diameter of the blade. The blade 56, however, may be raised and lowered toward or away from the outlet 22 to attain the maximum effectiveness of the mixing. The size of the interior 52 of the vessel may vary from a vertical dimension between the cover 28 to the inner bottom wall 62 from between one to four or more blade diameters. The diameter of the interior 52, that is, the diameter of the space bounded by the inner sidewall 44, should vary between 1.5 to four times the blade diameter. These dimensions are not critical, but are particularly desirable to achieve the outstanding results of the present invention.

Optionally, as shown in FIG. 3, the overflow bowl may be omitted, and a bearing seal 19 secured by rings 21 and 23 closes the top of the tubular outlet pipe 18. Curved side flow pipe 25 may also be provided.

In FIGS. 4 and 5, the cover housing is more rounded to more precisely conform to the flow path of the materials. In FIG. 4, the cover housing 2 is almost half circular in cross section and receives the tubular outlet pipe 18 just above the tapered portion 74 to provide a smooth path for the materials around the bulbous portion 76 of the cover housing. The bottom wall 78 is essentially ball-shaped to meet the curved portion of the bulbous portion 76 of the cover housing 72. As shown, the lower sidewalls slope upwardly and outwardly at 80, while the cover housing follows the smooth curved portion to the flow path of the material downwardly and inwardly toward inlet 22.

In FIG. 5, the cover housing 82 is in the form of an arc of a circle in cross section and meets the cooperating bottom wall 84 also in the form of an arc of a circle when viewed in cross section to form an essentially toroidal-shaped interior 86 similar to that illustrated in FIG. 4. It should be noted that the tubular outlet pipe 18 of FIG. 5 is not tapered at its end as in the modifications of FIGS. 2 and 4 to illustrate the fact that it is not essential to the effective performance of the mixing to have a tapered sidewall for the outlet pipe. The end of the tubular outlet pipe 18 is provided with a closed conical flange 88 having concave walls 90 surrounding the outlet pipe. As can be seen the bottom surface 92 of the flange extends from the outlet 22 to a point adjacent to or slightly beyond the extremities of the shear blade 56. The flow of fluid within this embodiment will be essentially the same as in the previous embodiment except that the particles to be dispersed will be more likely to have greater contact with the shear blade before being able to navigate the path between the shear blade and the bottom surface 92 of the flange before exiting through the outlet 22.

Irrespective of the configuration of the housing, whether that of FIGS. 2, 4 or 5, the path taken by the materials will be consistently a dynamic essentially curved path as shown by the arrows curving upwardly above the blade 56 and downwardly to the influence by the shear action of the blade. The action below the blade 56 is also in a curved path as shown by the flow path arrows.

In FIGS. 6 and 7, there is shown a further embodiment of the present invention and illustrates a broader aspect of this invention. The vessel 94 is a conventional open-top tank. The tubular outlet pipe 18 and the shaft 20 are the same as previously described. Side flow pipe 24 is connected to a source of suction (not shown). The blade 56 may be the same as described and illustrated for the previous embodiments.

The unique aspects of the embodiments of FIGS. 6 and 7 lie in the open vessel and the formation of the ideal characteristic flow path by means of the inverted flange 94 having flat upper surface 96 and a concave side 98 which surrounds the outlet pipe 18. The dispersed material must pass up through outlet 22 and, therefore, will repeatedly impinge upon the whirling shear blade 56 before passing up through the outlet pipe by means of the suction provided from side flow pipe 24.

In FIG. 8 is shown a schematic view of the complete, continuous dispersion apparatus. As shown, a tandem arrangement of the dispersion vessels of FIG. 1 or even FIG. 4 or FIG. 5 is utilized which receives a premix combination.

The premix, such as resin and pigment, is mixed in a closed conventional tank 100 with the usual stirrer 102 mounted for rotation by a motor, not shown. At or near the base 104 of the tank 100, the premixed raw materials are withdrawn in pipe 106 under the action of the pump and injected up through inlet 50 into the lower dispersion vessel 30. From this point, the action is completely similar to that previously described for the apparatus of FIG. 1, with the outlet of dispersed material from the lower vessel at 22 into the upper vessel being through connecting pipe 108 into the base of the upper dispersion vessel.

For the continuous process of paint mixing, reducing vehicle is injected at 110 through pipe 112 to mix with the initially dispersed material in connecting pipe 108 and enter the upper dispersion vessel from which a finished paint, for instance, may exit at side flow pipe 24 and be passed directly into the fillers.

The blades 56 and 56' as shown to be on a common shaft 20 may be identical in shape and size, but preferably the diameter of the upper blade is smaller since the shear action, and therefore the peripheral speeds in the upper vessel, need not be as great as in the lower vessel.

With the tandem arrangement of FIG. 8, a letdown tank is unnecessary resulting in additional economies.

In the method of the present invention, the preferred method necessarily requires the formation of a high-shear action along a horizontal plane. It has been previously described that the conventional high-shear agitator blades rotating at peripheral speeds of 750 to 15,000 feet per minute or more would establish a rotating zone of high-shear action in the immediate vicinity of the blade.

In the vessel having an inlet preferably below the plane of the zone of the high-shear action, materials to be dispersed pass through the inlet and then will be withdrawn from the outlet. It has been found that, since the material will inherently assume a curved flow path, as shown, including being directed inwardly and downwardly again towards the blade, this path will necessarily repeatedly bring the materials into the zone of the high-shear action. This action occurs continually so that the materials are time and again brought into the high-shear action zone. As soon as the materials enter the high-shear action zone, the materials are accelerated rapidly to the periphery and motivated at approximately the peripheral speed of the blade which can be up to or greater than 15,000 feet per minute. The materials will continue to move in essentially curved path forming overall the toroidal-shaped path for most effective mixing action.

For the most efficient mixing action, it is desirable to confine the vessel or enclosure to a space which roughly conforms to the configuration of the dynamic curving path of the particles to be mixed. In any event it is preferred, but not essential, that the vessel be essentially closed in order to permit the controlled flow-through path of the materials from the inlet through the outlet.

One of the unique features of the method of the present invention is that the retention of the materials within the vessel where it is subjected repeatedly to the action of the high-shear zone is controlled primarily by the flow-through rate. For instance, if the size of the interior 52 of the vessel is such as to contain approximately 1 gallon of material and the flow-through rate of the materials to be mixed is pumped at the rate of one-half gallon per minute, then the materials will be subjected to the high-shear action for a period of 2 minutes. Any size vessel from 1-pint laboratory size or smaller to 100 gallon size or larger would be adequate to meet almost any need and, by simply controlling the input and output, the degree of dispersion can be regulated due to the length of time that the materials to be dispersed are mixed are within the high-shear zone in the vessel.

In withdrawing the materials from the vessel, it is preferable although not essential that the materials be withdrawn into the outlet during or immediately following the downward movement of the materials near the high-shear action zone and prior to reentry into the zone of the high-shear action. The purpose it has been found of placing the outlet or withdrawing the materials near the high-shear action zone is to assure that the materials will have passed repeatedly into contact with the high-shear blade a sufficient number of times before it passes out under the force of the pumped input or the suction withdrawal. The withdrawal must be above the high-shear action zone and not radially extended beyond the radial limit of the high-shear action zone. Generally, the high-shear action zone extends within the dimensions of the high-shear blade approximately. The outlet 22 should be below the upper level of the materials being mixed and while in their dynamic curved path.

It should be manifest that, with the apparatus and method of the present invention, the entrapment of air is essentially avoided since the mixing action in the preferable embodiments is not in the presence of air, and that the flow rate permits complete control over the quality of the dispersion, permitting adequate dispersion of amounts of materials on a continuous basis with far less space requirement as compared to the conventional batch-mixing apparatus.

As an example of the commercial advantages of the present invention, it should be noted that a 5-horsepower unit utilizing a dispersion vessel of 1-gallon capacity, as in FIG. 1, can process 60 to 120 gallons per hour of dispersed paste which lets back to 180 to 360 gallons of finished enamel. Equivalent 8-hour production by batch process in an open tank would require loading, dispersion, transfer, the cleanup of four 150-gallon batch dispersions and a 1,500 to 3,000-gallon let-back tank, using a conventional 60-horsepower, high-shear mixer, a premix mixer and a let-back mixer.

The method and apparatus of the present invention may be used to disperse paint pigments in vehicles such as varnish and the like or any solid materials in a nonsolvent to produce the dispersion desired. The present invention is adaptable to the paint pigment, ink industry, food field, and any biological or pharmaceutical preparations which should be in the form of air-free dispersion or homogeneous mixture or emulsion may utilize the present invention.

Claims

I claim:

1. A method of continuous high-shear dispersion of materials in a vessel comprising: establishing a rotating zone of high-shear action, continuously admitting below said zone a supply of materials to be dispersed, including said materials to enter said zone, forming a consistent dynamic essentially curved path for said materials after leaving the influence of said zone, repeating the influence of the high-shear action of said zone upon the same materials for a predetermined time to cause the materials to pass continuously through said path, confining said materials to remain within said path to force said materials to repeatedly contact said zone, continuously withdrawing all of said materials within said path from a sole outlet (a) above said zone, (b) below the upper level of said material in said vessel, (c) exclusively adjacent the axis of rotation of said zone, and (d) during the movement of said materials near said zone and prior to reentry into said zone whereby to provide highly dispersed materials having been subjected repeatedly to the action of said zone.

2. The method of claim 1 including said path being essentially radially out from said zone, curving upwardly from said zone and toward the axis of rotation of said zone and curving downwardly toward and into said zone.

3. The method of claim 2 including controlling the number of contacts of said materials with said zone by limiting the flow rate of said materials through said path.

4. The method of claim 2 including controlling the number of contacts of said materials with said zone by preselection of the volume of said path.

5. The method of claim 2 including sealing the path of said materials and the zone of high-shear action in a closed vessel to avoid gaseous admixture with said materials.

6. The method of claim 2 including withdrawing said materials at a point opposed to a point wherein said materials are admitted and at a distance between one-half to four times the diameter of said zone.

7. The method of claim 2 including withdrawing heat built up during said dispersion by partially surrounding said path with a fluid heat exchange.

8. The method of claim 2 including adding heat during said dispersion by partially surrounding said path with a fluid heat exchange.

9. The method of claim 2 including controlling the number of contacts of said materials with said zone by limiting the flow rate of said materials through said path, sealing the path of said materials and the zone of high-shear action in a closed vessel to avoid gaseous admixture with said materials.

10. The method of claim 2 including controlling the number of contacts of said materials with said zone by preselection of the volume of said path, sealing the path of said materials and the zone of high-shear action in a closed vessel to avoid gaseous admixture with said materials, and withdrawing said materials adjacent the radial extent of said zone.

11. The method of claim 2 including controlling the number of contacts of said materials with said zone by limiting the flow rate of said materials through said path, sealing the path of said materials and the zone of high-shear action in a closed vessel to avoid gaseous admixture with said materials, withdrawing said materials adjacent the axis of rotation of said zone, said withdrawing of said materials being at a point opposed to a point wherein said materials are admitted and at a distance between one-half to four times the diameter of said zone, and withdrawing heat built up during said dispersion by partially surrounding said path with a fluid heat exchange.

12. A continuous high-shear dispersion apparatus for fluid materials including a shaft rotatable about a vertical axis, means for rotating said shaft, a shear means secured to the end of said shaft, and shear means producing a zone of high-shear action, a vessel containing said shear means, an inlet positioned within said vessel to supply materials to be dispersed and for contact with said shear means to form during high-speed rotation a dynamic curved path of said materials, an outlet positioned around said shaft and located adjacent said shear means and below the top level of said materials in said dynamic curved path providing the sole outlet whereby to withdraw all said materials while above said shear means, and means connected to said outlet for withdrawing the dispersed materials through the said outlet only, said material confining means being continuous and positioned within said vessel surrounding and secured to said outlet above said shear means.

13. The apparatus of claim 12 including said path being radially outwardly from said shear means, curving upwardly and inwardly toward said axis and downwardly toward said shear means, and said vessel having walls surrounding said shear means, said walls sloping upwardly and outwardly from the bottom of said vessel and downwardly and inwardly from the top of said vessel to conform substantially to said curved path.

14. The apparatus of claim 12 including said outlet being positioned concentric to said shaft.

15. The apparatus of claim 12 including said inlet being positioned below said shear means wherein said material-confining means substantially conforms to the shape of said dynamic curved path.

16. The apparatus of claim 12 including said outlet extending to the edge of said shear means.

17. The apparatus of claim 16 wherein said outlet extends outside of the periphery of said shear means.

18. The apparatus of claim 12 wherein the outlet is positioned a distance one-half to four times the diameter of said shear means away from said shear means.

19. The apparatus of claim 12 wherein the vessel is circular in horizontal cross section.

20. The apparatus of claim 12 wherein the vessel above said shear means is essentially toroidally shaped.

21. The apparatus of claim 12 including an inverted flange surrounding said outlet, said flange having an upper radially outwardly extending portion and tapering down inwardly toward said outlet.

22. The apparatus of claim 12 wherein said vessel includes an interior having walls shaped to substantially the configuration of said dynamic curved path.