US2584225A - Mixing apparatus - Google Patents

Description

Feb. 5, 1952 LM. PLUNGUIAN Erm. 2,584,225

MIXING APPARATUS Filed Ooi. l, 1949 NVENTORS- y MARK PLU NGU IAN FBG4. JESSE l.. RILEY Patented Feb. 5, 1952 UNITED STATES PATENT OFFICE Orange, N. J.,

assignors to Celanese Corporation of America, New York, N. Y., a corporation of Delaware Application October 1, 1949, Serial No. 119,121

This invention relates to mixing apparatus and relates more particularly to mixing apparatus for blending a fibrous solid and a liquid.

As is well known, in the preparation of organic acid esters of cellulose, a cellulosic material, preferably after a suitable pretreatment to increase its reactivity. is esteried by treatment with an organic acid anhydride in the presence of an esterilcation catalyst and a solvent for the ester being formed. During the esterification, it is necessary to achieve an intimate contact between all of the celluloslc material and the esterication medium, and also to control the temperature of the esterication mixture closely so as to obtain a product having the desired properties.

Heretofore, it has been the practice to carry out the esterication of the cellulosic material on a batch basis in jacketed vessels equipped with simple'low speed stirrers of the paddle bladeor helical ribbon type. AThe time required for the complete esterication of the cellulosic material has been at least several hours and it was found that during this time the low speed stirrers produced an adequate contact between the cellulosic material and the esterication medium. It was also found that the temperature of the esterilication mixture could be closely controlled by passing a heat-transfer medium at a suitable temperature through the jacket surrounding the reaction vessel.

However, when due to improvements in the process. the time required for the esterication of the cellulosic material was reduced to a fraction of its previous value, say to less than one hour, it was found that the low speed stirrers failed to produce an adequate contact between the cellulosic material and the esterication medlum. 'I'he low speed stirrers also failed to break up any lumps or wads of cellulosic material that were present in the esteriflcation mixture, with the result that these wads and lumps were not fully esteried and the cellulose ester that was formed contained an' appreciable quantity of unesteriiled or incompletely esteriiied cellulosic material. With a short reaction time. it was also found that the heat liberated during the esterilicatlon caused an excessive rise in the temperature of the esterication mixture resulting in a degradation of the cellulose molecule and the production of a cellulose ester that was not suited for the preparation of filaments and films. In addition, the esterication of the cellulosic material on a batch basis rather than in a continu- 'ous manner was found to be highly inefficientV 2 when the time required for the esterication was reduced to less than one hour.

It is an important object of this invention to provide a mixing apparatus which will be free from the foregoing and other disadvantages and which will be especially simple in construction and emcient in operation.

A further object of this invention is the provision of a mixing apparatus which is particularly adapted to blending a fibrous solid and a liquid.

Other objects of this invention will be apparent from the following detailed description and claims.

A preferred embodiment of our invention will now be described with particular reference to the accompanying drawing wherein Fig. 1 is an isometric view, partly in section and partly broken away, showing the general arrangement of the mixing apparatus;

Fig. 2 is a longitudinal cross-sectional view of the mixing apparatus shown in Fig. l,

Fig. 3 is a longitudinal cross-sectional view, on a reduced scale. showing the ducts for passing a heat-transfer medium through the stirring blades of the mixing apparatus shown in Fig. 1,

Fig. 4 is a cross-sectional detail view of a modii'led form of mixing apparatus, and

Fig. 5 is a cross-sectional view taken on the line 5--5 in Fig. 4 in the direction of the arrows.

Referring now to the drawing. the reference numeral il designates a hopper into which the cellulosic material to be esteried is entered, which hopper II opens into a passageway I2 extending through a casing I3. The passageway I2 has a cross-sectional contour defined by the peripheries of two intersecting curves of gener-y ally circular shape, whose centers are spaced apart a distance greater than the radius of either of the curves and less than the sum of the radii of the curves. With this spacing between the centers of the curves, the distance between the projections I4 and l5 on the contour of the passageway I2 at the point where the peripheries of the two generally circular curves intersect is less than the diameter of either of said curves. If the spacing between the centers of the curves is less than the value specied, the passageway I2 will not be large enough to accommodate the elements positioned therein as will be more fully described hereinafter. On the other hand, if the spacing between the centers of the curves is larger than the value specied, the curves will not intersect.

Shafts I8 and I1. which are rotated in synshafts I3 and I1 at the base of the hopper II and for a short distance into the passageway I2 are feed screws I3 and I3, whichserve to feed the cellulosic material from said hopper into and through said passageway. As the cellulosic material enters the passageway I2', it is contacted with an esteriiication medium that is fed into said passageway from a conduit 23 extending through the casing I3. The cellulosic material and esterificatiox'i` medium are blended and brought into intimate contact with each other in the passageway I2 by intermeshing, multi-lobed l stirring blades 2| and 22 mounted on the shafts I3 and I1, respectively, and extending longitudinally of the passageway I2. The cross-sectional contours of the stirring blades 2| and 22 are trochoidal, being either generally epicycloidal or epitrochoidal so that the clearance between the stirring blades 2| and 22 themselves, or between said stirring blades and the walls of the passageway I2, will not vary widely as the shafts I3 and I1 are rotated. The term trochoidal is employed in the appended claims to define an epicyclod or epitrochoid. After traversing the passageway I2, the esterifled cellulosic material enters a conduit 23 positioned midway between the shafts I3 and I1, through which conduit the esterified material is discharged from the mixing apparatus.

To control the temperature of the esteriiication mixture during the esteriicaion, the casing I3 is provided with a jacket 24 through which a heat-transfer medium at a suitable temperature may be circulated from conduits 23. In addition, the stirring blades 2| and 22 are each provided with ducts 23 into which a heat-transfer medium is entered from communicating ducts 21 in the shafts I3 vand I1 and from which it is discharged through communicating ducts 23 in said shafts.

The dimensions of the stirring blades 2| and 22, which effect the blending of the cellulosic material and esteriflcation medium in the passageway I2, are selected to provide a suicient clearance between the stirring blades 2| and 22 themselves, and/or between said stirring blades and the walls of the passageway I2 so that the material in said passageway will continually ow through said clearance when the shafts I3 and I1 are rotated. Any stagnation or hold-up of the material is thereby avoided. While the exact amount of such clearance will depend upon the nature and viscosity of the materials being blended, it is desirable to employ'a minimum clearance of about 0.03 inch at at least one of these places. If a smaller clearance is employed, the stirring blades 2| and 22 will tend to act as pumps rather than mixers. forcing the material in the passageway I2 transversely of the shafts I3 and I1, developing excessivepressures in the casing I3 and greatly increasing the power required to drive said shafts. Instead of only two lobes, each of the stirring blades 2| and 22 may have a greater number of lobes so-long as the lobes intermesh with each other. To accommodate the stirring blades 2| and 22, the centers of the curves whose peripheries define the contour of the passageway I2 must, as pointed out above, be spaced apart a distance greater than the radii of either of said curves.

Asthe shafts I3 and I1 rotate. the materials in the passageway I2 will be carried by the stirring blades 2| and 22 into the clearance between said stirring blades themselves. at which point they will be thoroughly blended and any wads or lumps therein will be crushed and dispersed. In addition, the rotation of the shafts I3 and I1 will force the materials in the passageway I2 through the clearance between the stirring blades 2| and 22 and the walls of the passageway I2, at which point a further blending of said materials will take place. The clearance between the stirring blades 2| and 22 themselves may be equal to the clearance between said stirring blades and the walls of the passageway I2. However. it is preferred to make the clearance between the stirring blades 2| and 22 themselves, at which point the most effective blending takes place. somewhat larger, say at least about 2 to 3 times as large as the clearance between said stirring blades and the walls of the passageway I2 so that the major portion of the materials being blended will be forced through the yclearance between the stirring blades 2| and 22 themselves during each revolution of the shafts I3 and I1.

' The clearance between the stirring blades 2| and 22 themselves, and between said stirring blades and the walls of the passageway I2 may be constant for the entire length of said stirring blades. Alternatively, one or both of said clearances may be varied longitudinally of the stirring blades, particularly when the viscosity of the materials being blended changes during their passage through the mixing apparatus. This variation in clearance may be obtained by altering the cross-sectional area of the passagelway I2, by altering the cross-sectional area of the stirring blades 2| and 22, by inclining the shafts I3 and I1 on which the stirring blades 2| and 22 are mounted, with respect to each other, or by a combination of these expedients.

The clearance between the stirring blades 2| and 22 themselves, and between said stirring blades and the walls of the passageway I2 may remain constant as the shafts I3 and I1 rotate. However, an adequate blending of the materials in the passageway I2 will also be obtained if these clearances vary during the rotation of the shafts I3 and I1. This variation in clearance will take place, if the curves whose peripheries define the contours of the passageway I2 are not truly equal and circular, if the shafts I3 and I1 are not positioned axially of said curves, and if the contours of the stirring blades 2| and 22 are not truly epitrochoidal or hypotrochoidal. It is also possible to eliminate the projections I3 and I5 from the contour of the passageway I2 and employ a passageway free from reentrant angles. However, this will produce a dead space in the passageway I2 through which the stirring blades 2| and 22 will not pass and in which the materials being blended will accumulate.

Not only are the materials traversing the passageway I2 effectively and rapidly blended, but the temperaturevof said materials is maintained within close limits, since. with each revolution of the shafts I3 and I1, all of said materials pass in the form of a relatively thin lm through the clearance between the stirring blades 2| and 22. themselves, and between said stirring blades and the walls of the passageway I2. In this manner, all of the materials being blended are brought reDOtedly into intimate contact with a surface whose temperature is controlled by a heat-transfer medium.

A modified form of mixing apparatus especially suited for use at high rates of material ow is shown in Figs. 4 and 5 of the drawing. In this modification grooves 29 and 3l are provided circumferentially of the stirring blades 2l and 22, respectively, and parallel baille disks 32 and 33 mounted transversely of the shafts I6 and I1, respectively, extend into said grooves. The baille disks 32 and 33 overlap each other forming a barrier to the flow of the materials being blended along the center of the passageway l2, forcing said materials to flow through the clearance between said balev disks and the walls of said passageway and thereby avoiding the channeling which might' otherwise take place at high rates of material flow. Alternatively, the

materials being blended may be led around thev baille disks 32 and 33 by means of one or a p lurality of conduits opening through` the casing i3 into the passageway I2 and extending from 'one side of said baille disks to the other. In this case, no clearance is necessary between the baffle disks 32 and 33 and the walls of the passageway l2. Treating agents may advantageously be added to the materials in the passageway I2 as said material llow around the baille disks 32 and 33 and will be thoroughly blended with said materials by the stirring blades 2l and 22.

While the mixing apparatus of this invention has been described with particular reference to the `preparation of organic acid esters of cellulose, it may also be employed for other purposes r ,blending materials on a batch basis as well as in a continuous manner by entering a charge into the passageway l2, holding the charge in said passageway until the desired blending has taken place and then removing the charge from said passageway.

It is to be understood that the foregoing detailed description is given merely by way of illustration and that many variations may be made therein without departing from the spirit of our invention.

Having described our invention, what we desire to secure by Letters Patent is:

l. In a mixing apparatus. the combination with a. casing having a passageway extending therethrough, said passage having a cross-sectional contour defined by the peripheries of two intersecting curves of generally circular shape, of means for feeding material to and longitudinally through said passageway, a pair of multi-lobed stirring blades insaid passageway for operating on said material, at least one of the clearances between'said stirring blades themselves and between said stirring blades and the walls of said passageway being suflicient to permit material in said passageway to pass through said clearance when the stirring blades are rotated, said stirring blades having a contour of trochoidal shape whereby the clearance between the stirring blades themselves and the stirring blades and the walls of the passageway will remain substantially constant as the stirring blades are rotated, and means for discharging the materialv from the passageway.

2. In a mixing apparatus, the combination with a casing having a passageway extending therethrough, said passageway having a cross-sectional contour defined by the peripheries of two v intersecting curves of generally circular shape, of means for feeding material to and longitudinally through said passageway, apair of multilobed stirring blades in said passageway for operating on said material, at least one of the clearare rotated, said stirring blades .having a con-4 tour of trochoidal shape thereby thev clearance between thestirring blades themselves and the stirring blades and the walls of the passageway will remain substantially constant as the stirring blades are rotated, and means for discharging the material from the passageway.

3. In a mixing apparatus, the combination with a casing having a passageway extending therethrough, said passageway having a crosssectional contour defined by the peripheries of two intersecting curves vof generally circular shape, of means for feeding material to and longitudinally through said passageway, a pair of multi-lobed stirring blades in said passageway for operating on said material, at least one of `the clearances between said stirring blades themselves and between said stirring blades and the walls of said passageway being a minimum of 0.03 inch to permit material in said passageway to pass through said clearance when the stirring blades are rotated, said stirring blades having a contour of trochoidal shape whereby the clearance between the stirring blades themselves and the stirring blades and the walls of the passageway will remain substantially constant as the stirring blades are rotated, means operatively connected to said stirring blades forvpassing a vheat transfer medium therethrough, and means for discharging the material from the passageway.

4. In a mixing apparatus, the combination with a casing having a passageway extending therethrough, of a plurality of shafts in said passageway, multi-lobed stirring blades, having grooves therein, mounted on said shafts in intermeshing relationship, at least one of the clearances between said intermeshed stirring blades and between said stirring blades and the walls of said passageway being suilcient to permit materials in saidpassageway to pass through said clearance when the stirring blades vare rotated, and bale disks, mounted on said shafts in said grooves in said stirring blades, the baille disks on one of said shafts being parallel to and in overlapping relationship with the baille disks on another of said shafts for preventing the material in said passageway from flowing along the center of said passageway.

5. In a mixing' .-apparatus'ythe combinationwith a casing having a passageway extending clearance -wh`e'n' v the stirring blades are rotated,

and baille disks/mounted o n said shafts in said groovesin saidstirring blades, the baille disks on one 'of'said*, shafts being. parallel to and in' 7 overlapping relationship with the baille disks on another of said shafts for preventing the msterial in said passageway from owing along the center of said passageway, thev clearance between said baille disks and the walls of said passageway being sufficient to permit materials in said passageway to pass through said clearance.

8. In a mixing apparatus, the combination with a casing having a passageway extending therethrough. said passageway having a cross-sectional contour defined by the peripheries of two intersecting curves of generally circular shape. of a plurality of shafts in said passageway. multilobed stirring blades, having grooves therein, mounted on said shafts in intermeshing relationship, said stirring blades having a contour of trochoidal shape, at least one of the clearances between said intermeshed stirring blades and between said stirring blades and the walls of said passageway being sufficient to permit materials in said passageway to pass through said clearance when the stirring blades are rotated, and

circular baille disks. mounted on said shafts inv said grooves in said stirring blades, the baille disks on one of said shafts being parallel to and in overlapping relationship with the baille disks on another of said shafts for preventing the material in said passageway from ilowing along the center of said passageway.

MARK PLUNGUIAN.

JESSE L. RILEY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Name Date Mills Aug. 27, 1867 Chesnut et al Mar. 31, 1874 Kempter Mar. 26, 1918 Reider et al. June 20, 1933 Pratt et al. Apr. 15, 1941 Number

Images