US3675903A

US3675903A - Mixing device

Info

Publication number: US3675903A
Application number: US98277A
Authority: US
Inventors: Robert C Harper
Original assignee: Milacron Inc
Current assignee: Milacron Inc
Priority date: 1970-12-15
Filing date: 1970-12-15
Publication date: 1972-07-11
Anticipated expiration: 1989-07-11

Abstract

A mixing device for intermixing liquids to a homogenous state. The device comprises a cylindrical housing and a generally cylindrical impeller rotatably positioned within the housing and includes a mixing section between the housing and impeller and means for recycling a portion of the mixture to the mixing section. They recycling of a portion of the flow provides improved homogenity of the final mixture.

Description

United States Patent Harper [451 July 11, 1972 [54] MIXING DEVICE [72] Inventor: Robert C. Harper, Loveland, Ohio [73] Assignee: Cincinnati Milacron Inc., Cincinnati, Ohio [22] Filed: Dec. 15, 1970 [21 Appl. No.: 98,277

[52] US. Cl. ..259/8, 259/D1G. 3O

[51] ..B0ll'7/24 [58] Field of Search ..259/7, 8, 23, 24, 43, 44, DIG. 30, 259/9, 10

[56] References Cited UNITED STATES PATENTS 3,427,003 2/1969 Schneider et al. ..259/9 l/l965 Frenkel ..259/9X l/l969 Gurley ..259/7 Primary Examiner-Walter A. Scheel Assistant Examiner-Philip R. Coe Attorney-Howard T. Keiser and Alfred J. Mangels [57] ABSTRACT A mixing device for interrnixing liquids to a homogenous state. The device comprises a cylindrical housing and a generally cylindrical impeller rotatably positioned within the housing and includes a mixing section between the housing and impeller and means for recycling a portion of the mixture to the mixing section. They recycling of a portion of the flow provides improved homogenity of the final mixture.

12 Claims, 5 Drawing Figures INVENTOR. ROB T HARPER SHEET 1 BF 3 FPYENTEUJUL 1 1 1912 SHEET 3 OF 3 MIXING DEVICE BACKGROUND OF THE INVENTION This invention relates to mixing devices and more particularly to mixing devices of the substantially cylindrical type incorporating an impeller rotatably positioned in a cylindrical housing.

It is well known to mix several materials in the form of viscous liquids in a shear-type mixer. Such a mixer frequently comprises a cylindrical chamber or housing within which a rapidly rotating impeller is positioned. The impeller can be of a cylindrical type and frequently includes spiral grooves on the outer periphery thereof to pump the materials in the intended flow direction. The mixing takes place within the spiral grooves in the impeller and in the clearance space between the housing and the impeller.

Oftentimes, and particularly when relatively viscous materials are to be intermixed, the output of such mixing devices is of a non-uniform character in that the resulting mixture is non-homogenous. More specifically, it has been found that when mixing devices of the type hereinabove described are utilized for intermixing the components of a two-part mixture for preparing foam plastics, the output of the mixer frequently is a mixture which results in undesirable properties of the parts which are molded therefrom. For example, when polyisocyanates and polyols are intermixed together with a foaming agent to provide a foamed polyurethane article on curing of the mixed material in a mold, the resulting molded parts frequently exhibit severe streaking, soft spots, blisters, and non-uniform density. In order to provide the degree of mixing necessary to overcome those conditions, it has been found that the through-put rate of material through the mixer has to be severely reduced.

Increasing the speed of the impeller in such a mixer to permit an increase in the flow rate was found to be impractical since it required excessive power inputs to the system to achieve the degree of mixing desired and caused considerable input of heat to the material. The latter condition is undesirable in that excessive heat will cause the foaming agent in the mixture to vaporize prematurely, thereby causing foaming to take place in the mixing head, where it is not desired, rather than in the forming mold, where it is desired.

It is therefore an object of the present invention to obviate the above-described difficulties by means of an improved mixing device permitting a greater material through-put rate and which also provides a mixture which when molded results in uniform, homogeneous articles.

SUMMARY OF THE INVENTION Briefly stated, in accordance with one aspect of the present invention, a mixing device is provided for mixing a plurality of liquid ingredients into a substantially homogeneous mixture. The device includes an inlet and a mixing section through which the material to be mixed passes and is mixed by shear forces and by turbulence. The mixing section is defined by the space between a pair of relatively rotatable, closely spaced cylindrical elements between which the materials to be mixed pass. A portion of the mixture which results after passage of the components through the mixing section is returned to the mixing section as recycle to improve the homogenity of the resulting mixture.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view of the mixing device of the present invention and illustrates a structure which provides the recycling contemplated by the present invention.

FIG. 2 is an exploded view of the mixing device of FIG. 1, partially broken away, to show the constructional details of the relatively rotatable elements between which the mixing takes place.

FIG. 3 is a cross-sectional view of another embodiment of the present invention which incorporates a single mixing zone.

FIG. 4 is a cross-sectional view showing an additional embodiment of the present invention within which the recycling contemplated by the present invention can take place.

FIG. 5 is a cross-sectional view of a further embodiment of the present invention involving a modification of the structure shown in FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings and particularly to FIGS. 1 and 2 thereof, there is shown the mixing device of the present invention which comprises a cylindrical housing 11 having an axial inlet opening 12 closed by a cover 13. Housing 11 includes an outlet aperture 14 which is joined to the housing body itself by means of a transition section 15 of generally frusto-conical configuration. Rotatably positioned within housing 11 is an impeller 16 which comprises a central disc portion 17 to which are attached an upstanding annular wall 18 and a depending annular skirt portion 19. Impeller 16 is rotated by means of a drive shaft 20, which is connected by known means to a source of rotational energy such as, for example, an electric or hydraulic motor (not shown). Shaft 20 extends axially through cover 13 and is keyed to disc portion 17 of impeller 16 by means of key 21. Upstanding wall portion 18 of impeller 16 and drive shaft 20 together define a first annular mixing section 22 into which the material to be mixed is introduced by means of

metering tubes

23, 24, which extend through cover 13 and into first annular mixing section 22. The outer diameter of impeller 16 and the inner diameter of housing 11 are so sized to provide a radial clearance space therebetween which ranges from about 0.010 inches to about 0.030 inches. The clearance space and the spiral grooves in the housing and impeller define the second mixing section.

In the embodiment shown in FIGS. 1 and 2, a pair of

metering tubes

23, 24 is shown by means of which the materials to be intermixed can be introduced into the mixer. However, it is not necessary that two such tubes be provided. In fact, the number can range from one to up to six or more, depending upon the number of streams of material to be mixed in the device. If desired, a preliminary mixing operation can be performed external to the device shown in FIGS. 1 and 2 and the latter utilized for the final mixing step in a multi-step mixing operation. In that event, only a single metering tube would be required. Additionally, although the

metering tubes

23, 24 are shown positioned for axial introduction of the material to be mixed, it is not absolutely necessary that that particular orientation be employed and the material can be introduced in any direction as, for example, radially through the housing 11 opposite the first annular mixing section 22. (See FIG. 3).

Impeller 16 includes a plurality of helical grooves 25 on its outermost surface. Grooves 25 are preferably of a generally semi-circular cross section and include generally triangularly shaped, rounded edges 26 on the intervening ridges 27. The general arrangement of grooves 25 on the exterior surface of impeller 16 is shown most clearly in FIG. 2.

Referring once again to FIG. 1, upstanding annular wall 18 is of such a radial thickness that grooves 25 extend at least partially therethrough in order to permit communication between first annular mixing chamber 22 and the radial clearance space between impeller 16 and housing 11. However, the radial thickness of depending annular skirt 19 is sufficiently large to preclude grooves 25 from extending therethrough, thereby defining a plurality of helical flow channels of generally semi-circular cross section.

Although shown as being of substantially semicircular cross section, grooves 25 in impeller 16 can be of any desired cross section such as, for example, rectangular, triangular, trapezoidal, or the like. Additionally, the helix angle which grooves 25 make with the axis of impeller 16 is preferably about to although helix angles within the range of from about 30 to about are also suitable. Furthermore, although in the embodiments shown in FIGS. 1 and 2 impeller 16 includes a total of 8 equally spaced helical grooves, the number of such grooves is not critical and any desired number can be utilized. However, although there is some latitude with respect to the size and spacing of the several grooves, in order to provide the desired pumping action on the material to be mixed it is normal practice that the grooves be so oriented that they define a left hand thread when impeller 16 is caused to rotate in a clockwise direction as viewed from above in FIG. 2. Similarly, if impeller 16 is rotated counter-clockwise as viewed from above in FIG. 2, the grooves normally are so oriented that they define a right hand thread.

As shown in FIGS. 1 and 2, housing 11 includes a continuous helical groove 28 on its innermost surface to define a continuous flow channel. This flow channel and the intervening portions of inner side wall of housing 1 l are of generally trapezoidal cross section since the groove 28 in the configuration shown in essentially an internal acme thread for convenience of manufacture. Groove 28 can be provided along the entire inner surface of housing 11, but preferably it extends from frusto-conical transition section 15 upwardly to a point below first annular mixing chamber 22. The helix angle of groove 28 relative to a transverse plane through the axis of housing 11 can range from 0.5 to 60, and preferably ranges from about 1 to about Although shown as being of substantially trapezoidal cross section, groove 28 can be of any other desired cross section such as, for example, semicircular, triangular, rectangular, or the like. Preferably, the radial depth of groove 28 ranges from 0.02 to 0.2 times the inside diameter of the housing. However, in order to provide the benefits of the present invention, the hand of the grooves in the housing side wall must be such that the rotational direction of motion imparted by the impeller to the material being mixed will induce an upward flow direction to the material in the grooves in the housing side wall. Thus, if when viewed from above as in FIG. 2 impeller 16 rotates in a clockwise direction, grooves 28 in the housing side wall should define a left hand progression for optimum operation of the device.

If desired, impeller 16 can be formed without upstanding skirt 18 to thereby eliminate first mixing section 22 shown in FIGS. 1 and 2. The resulting embodiment would have but a single mixing section defined by the space between impeller 16 and housing 11 as shown in FIG. 3. That structure could be employed, for example, when the process in which the mixing device of the present invention is utilized includes an external, preliminary mixing step.

In operation, the components to be intermixed are introduced through the several feed tubes 23, 24 (FIGS. 1 & 2) into first annular mixing section 22. The rotation of impeller 16 will cause some degree of mixing to take place in mixing section 22 and as the material accumulates therein, the centrifugal force imparted to the material by the rotation of impeller 16 forces the material radially outwardly and through those portions of grooves 25 in impeller 16 which extend through upstanding annular wall 18. Some mixing of the materials takes place in the course of their passing through the radial apertures defined by grooves 25 in upstanding annular wall 18 and subsequently recombining in the intervening space between impeller 16 and housing 11. Other means of pre-mixing the individual components prior to entering the primary mixing zone can also be used, or the individual components may be fed directly to the primary mixing zone between impeller 16 and housing 11 without pre-mixing.

Since the grooves in impeller 16 are normally so oriented with respect to the direction of rotation of impeller 16 that a downward component of motion is imparted to the material to cause the same to flow generally axially through the device toward the outlet aperture 14, the result is a pumping action which causes the material to flow downwardly. This downward flow can also be accomplished by the supply pressure of the individual components forced into the mixing device in the event that the impeller does not impart a pumping action. As the material flows downwardly through grooves 25, shear forces are imparted to the material by virtue of the relative rotation between the housing and the impeller. Those shear forces serve to further inter-mix the material both by laminar shear in the close clearance zone between housing 11 and impeller 16 and by turbulence set up within grooves 25. The mixed material then flows through transition section 15 and outlet aperture 14 and can then be deposited in a suitable receptacle for further processing. For example, the mixer hereinabove described is particularly suitable for use as a mixing device in connection with the preparation of polyurethane foams. In that case, one of the components to be intermixed as a polyisocyanate compound and the other component is a polyol. One of the components includes a foaming agent which can be activated by heat or chemical reaction in a known fashion and which causes the liberation of gas, thus forming a multiplicity of cells in the mixture, which when reacted becomes a rigid structure. Those materials are of a relatively viscous nature being on the order of l,000 20,000 centipoises. Although the use of the device in conjunction with the mixing of components to form a polyurethane foam has been disclosed, it would be apparent to those skilled in the art that other liquid materials including, for example, epoxy or polyester resins can also be intermixed by using the present invention. Furthermore, quasi-homogeneous liquids such as emulsions and suspensions can also be intermixed by using the present invention.

Groove 28 provided in the inner wall of housing 11 materially aids in mixing the components to a substantially homogeneous state in that it permits recycling or recirculation of material by upward or counter-current flow through the mixing device. The flow of the material in grooves 28 of housing 11 is upward because of the orientation of the groove with respect to the direction of rotation of impeller 16. Thus, that particular orientation provides a reverse pumping action, or feedback, as compared with the downward pumping action of the grooves in the impeller side wall. In providing the feedback through the mixing device, the described embodiment provides a more homogeneous mixture than would be the case if the grooves in the housing side wall were not provided. As a consequence, the throughput rate of materials to provide a given degree of homogeneity can be considerably increased by virtue of the improved mixing which takes place when the mixer of the present invention is employed. In essence, the grooves in the housing provide an upward path for a portion of the mixed or partially mixed material to be returned to the beginning of the primary mixing zone between impeller 16 and housing 11. In this manner, material in the mixing device is mixed with material that has entered the device at a later time. In effect, the residence time of the material is increased and short term errors in the ratios of ingredients are averaged to a homogeneous mixture by virtue of the configuration of the device. As a result, it is not necessary to reduce the throughput rate to achieve a homogeneous mixture, which would be the case if the grooves in the housing were not provided.

Another embodiment of the present invention is shown in FIG. 4. In that embodiment the mixing device incorporates substantially the same elements as shown in the embodiment of FIG. 1. However, in the embodiment of FIG. 4, housing 11 does not include groove 28 of FIGS. 1 and 2 and thus the inside wall of housing 11 is a smooth, cylindrical surface. The embodiment of FIG. 4 includes a baffle tube 29 positioned coaxially with outlet aperture 14 and which extends upwardly into the area enclosed by depending annular shirt 19 of impeller 16. Skirt 19 includes a plurality of radial apertures 30, which can be provided in a single transverse plane. Apertures 30 provide communication between the space enclosed by skirt l9 and the intervening space between the outer surface of impeller 16 and the inner surface of housing 1 1.

In operation, the entering material is intermixed in first annular mixing chamber 22 in a manner similar to that described in conjunction with the embodiment of FIGS. 1 and 2, and also travels axially between impeller 16 and housing 11 in a similar fashion but without the reverse flow provided by groove 28 in housing 11 shown in FIGS. 1 and 2. After passing around the lower portion of depending skirt 19 of impeller 16, the material flows upwardly between depending skirt l9 and baffle tube 29. In flowing upwardly the material passes radial apertures 30 and by virtue of the centrifugal force imparted to the rotating material, causing it to be urged against the inner surface of depending skirt 19, a portion of the material passes radially outwardly through apertures 30 and joins the material which is progressing downwardly between impeller 16 and housing 11 in the mixing zone therebetween. Thus, radial apertures 30 provide a feedback means whereby some recirculation is provided to further improve the mixing capabilities of the disclosed device.

Preferably, in order to insure that some material passes through radial feedback apertures 30, baffle tube 29 is positioned so that its topmost surface lies in a plane which passes through radial apertures 30, or in a transverse plane which is axially upwardly spaced from apertures 30. This orientation of the several elements will insure that the material passes the apertures, and thus a portion thereof will flow through apertures 30 to be mixed with material which is passing between impeller l6 and housing 11.

' A further embodiment of the present invention is shown in FIG. 5. That embodiment is similar in structure to that of FIG. 4 except that the inner surface of depending skirt 19 slopes upwardly and outwardly to facilitate flow of the mixture to radial apertures 30 by centrifugal action. The centrifugal action forces the material against the inner surface of skirt l9 and the slope thereof causes the material to flow upwardly toward apertures 30. Some of the material passes through apertures 30 while the remainder passes through baffle tube 29.

it can thus be seen that the present invention provides an improved mixing device for liquid materials by permitting recirculation of a portion of the already mixed material to join some that has not been completely mixed, thereby improving the homogeneity of the final product.

Further mixing can be provided by including a screen in first annular mixing section 22 of the embodiments of FIGS. 1, 2, 4 and 5. Such a screen is shown and described in the copending application of Donald Dunn, entitled Process for Mixing Coreactive Liquids Which Form Polyurethanes, Ser. No. 878,081, filed Nov. 19, 1969, and assigned to the assignee of the present invention.

While particular embodiments have been illustrated and described, it will be apparent to those skilled in the art that various changes and modifications can be made and it is intended to cover in the appended claims all such changes and modifications that are within the scope of this invention.

What is claimed is:

l. A mixing device for providing a homogeneous mixture of a plurality of liquid materials, said device comprising:

a. a mixing section having an inlet and an outlet and defined by the space between a pair of relatively rotatable, closely spaced cylindrical elements between which said materials pass and are mixed by shear and turbulent forces, said mixing section including a cylindrical housing and a closely fitting cylindrical impeller having at least one helical groove on its exterior surface;

b. recycle means comprising at least one helical groove formed on the inner surface of said housing for returning to said mixing section a portion of the output therefrom to improve the homogenity of said mixture, said helical groove being of the same hand as the helical groove on said impeller;

c. means for introducing the liquids to be mixed into said mixing section; and

d. means for rotating said impeller in a direction of rotation to cause said helical grooves in said impeller to impart to said material a component of motion in a direction toward said mixing section outlet.

2. The device of claim 1 wherein said helical groove on the inner surface of said housing is formed in a major portion thereof.

3. The device of claim 1 wherein said helical groove on the inner surface of said housing is of a smaller radial depth than that of said impeller groove.

4. The device of claim 1 wherein said helical groove on the inner surface of said housing is equal in radial depth so that of said impeller groove.

5. The device of claim 1 wherein said helical groove on the inner surface of said housing is greater in radial depth than that of said impeller groove.

6. The device of claim 1 wherein the helix angle of the helical groove on the inner surface of said housing is greater than the helix angle of the helical groove on said impeller.

7. The device of claim 1 wherein said helical groove on the inner surface of said housing is of a substantially trapezoidal cross-section and the intervening ridges as of substantially trapezoidal cross-section.

8. The device of claim 1 wherein said helical groove on said impeller is of substantially semi-circular cross-section and the intervening ridges between adjacent grooves are substantially V-shaped.

9. The device of claim 1 wherein said helical groove on said impeller is of substantially trapezoidal cross-section and the intervening ridges are also of substantially trapezoidal crosssection.

10. The mixing device of claim 1 wherein said device includes a premixing section positioned within the confines of said cylindrical housing into which said liquids are introduced and intermixed prior to entering a second mixing section defined by the space between said cylindrical elements.

11. The device of claim 10 wherein said first mixing section comprises an upstanding annular wall on said impeller defining an annular basket mixing section into which said materials are deposited and from which said materials are expelled by means of centrifugal force, said upstanding annular wall including a plurality of radial apertures therein to provide communication between the annular basket and the space between the housing and the impeller.

12. The device of claim 11 wherein said helical groove on the exterior surface of said cylindrical impeller extends into the region of said annular basket and through said upstanding annular wall to define said apertures.

Claims

1. A mixing device for providing a homogeneous mixture of a plurality of liquid materials, said device comprising: a. a mixing section having an inlet and an outlet and defined by the space between a pair of relatively rotatable, closely spaced cylindrical elements between which said materials pass and are mixed by shear and turbulent forces, said mixing section including a cylindrical housing and a closely fitting cylindrical impeller having at least one helical groove on its exterior surface; b. recycle means comprising at least one helical groove formed on the inner surface of said housing for returning to said mixing section a portion of the output therefrom to improve the homogenity of said mixture, said helical groove being of the same hand as the helical groove on said impeller; c. means for introducing the liquids to be mixed into said mixing section; and d. means for rotating said impeller in a direction of rotation to cause said helical grooves in said impeller to impart to said material a component of motion in a direction toward said mixing section outlet.

9. The device of claim 1 wherein said helical groove on said impeller is of substantially trapezoidal cross-section and the intervening ridges are also of substantially trapezoidal cross-section.