Mixing of fluid materials

[57] ABSTRACT

Method and apparatus are disclosed for the mixing of fluid materials comprising providing a mixing zone having an inlet and an outlet; introducing said material through said inlet into said mixing zone; agitating said material in said mixing zone with a rotating, interrupted spiral ribbon agitator; disrupting the material flow in a region of interruption of said agitator to effect more intimate mixing; and withdrawing intimately mixed material through said outlet.

Claims, 3 Drawing Figures

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Outlet

PATENTED JUL 11197E

3,675,902

SHEET 1 OF 3

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Outlet

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INVENTOR

WALTER R.MARSHALL

BY /bw

ATTORNEY

PATENTED JUL 111972

3,875,902

SHEET 2 OF 3

<img file="US3675902A_D0004.tif" />

INVENTOR

WALTER R. MARSHALL

BY /) . \ x. x

ATTORNEY

PAtENTEOMII 1972

3,675,902

SHEET 3 OF 3

<img file="US3675902A_D0005.tif" />

INVENTOR

WALTER R. MARSHALL

BY /ΊζΜ .Ά

ATTORNEY

3,675,902

MIXING OF FLUID MATERIALS

The present invention relates to method and apparatus for the mixing of fluid materials.

Heretofore, a wide variety of methods and apparatus have been proposed for the mixing of fluid materials. Most of such methods and apparatus present inherent difficulties in the ability to handle in a rapid manner the intimate mixing of such fluids over a broad range of fluid viscosities. This is particularly troublesome in situations where it is desirable to obtain the rapid admixture of a relatively high viscosity fluid with a relatively low viscosity fluid.

It is, accordingly, the prime object of the present invention to provide method and apparatus for the rapid admixture of fluids, regardless of the relative viscosities of such fluids.

Other aims and objectives of the present invention will be apparent from the following description and appended claims.

In accordance with the present invention, a method is provided for the mixing of fluid materials comprising: providing a mixing zone having an inlet and outlet; introducing said materials through said inlet into said mixing zone; agitating said materials in said mixing zone with a rotating, interrupted spiral ribbon agitator; disrupting the material flow in a region of interruption of said agitator to effect more intimate mixing; and withdrawing intimately mixed material through said outlet.

Apparatus suitable for practice of the method aspect of the invention comprises a vessel of substantially circular crosssection having inlet means and outlet means, spiral ribbon agitator means rotatably mounted in said vessel near the walls thereof, said agitator means having at least one interruption in continuity; and baffle means positioned in a region of said interruption of said agitator means.

For a more complete understanding of the apparatus of the invention, reference is made to the accompanying drawings, wherein:

FIG. 1 is an elevational cross-sectional view of apparatus for the admixture of fluid materials embodying the invention;

FIG. 2 is a sectional view of such apparatus taken along the line 2—2 of FIG. l;and

FIG. 3 is a schematic view of two-stage apparatus embodying the invention.

Referring specifically to the embodiment of FIGS. 1 and 2 of the drawings, a mixing apparatus is provided comprising an outer vessel 10 which is vertically positioned and cylindrically shaped. Volatile venting means 12 is positioned in the upper end of vessel 10, material outlet means 14 is positioned near the lower end thereof, and material inlet means 16 is positioned between volatile venting means 12 and material outlet means 14.

Mounted in a vertical position within vessel 10 are helical ribbon agitator means 18 which are supported by horizontal struts 20 mounted on vertically-axially aligned shaft 22 which is rotatably-mounted in vessel 10 and secured at the upper end in bearing-seal means 24.

It is to be understood that, while the illustrated preferred embodiment employs a generally vertical, cylindrical vessel having a helical agitator means, vertical orientation, cylindrical shape and helically-shaped agitator means are not required. The vessel may have any orientation and be of any shape having a generally circular cross-section; i.e., cylindrical, spherical, conical, etc. The agitator means may be of any spiral configuration, i.e., helical, tapered spiral, etc.

The rotation of shaft 22 by external drive means (not shown) in either a clockwise or counterclockwise direction will produce rotation of the helical ribbon agitator means 18 in the same direction.

At least one region of interruption 26 is provided in the flight continuity of helical ribbon agitator means 18. In this region of interruption 26 are positioned baffle means 28 which are positioned horizontally and radially (as shown) across the interior of vessel 10. The baffle means are preferably supported by the side walls of vessel 10 and may (as shown) be provided with a sleeve 30 for supporting the lower end of shaft and, consequently, the helical ribbon agitator means 18. Baffle means 28 may be provided with protrusions 32 for additionally assisting the baffling action. These protrusions may be oriented at varying angles from the main plane of the baffle means 28.

In operation, as an initial amount of fluid (liquids or solidcontaining liquids) is charged into vessel 10 through inlet means 16, the liquid level is brought up to that shown in the vicinity of lines 34 and 36 of FIG. 1. Rotation of the shaft and consequently of the helical ribbon agitator means 18 produces, in conjunction with the baffle means, an interrupted envelope region 38 generated by the revolution of the helical spiral. The direction of flow of fluid in the vicinity of the envelope is dependent upon the direction of the rotation of helical ribbon agitator means 18, a clockwise direction of rotation (viewed from top in FIG. 1) yielding a downward fluid flow 40 through envelope region 38 and an upward central flow 42 in that area of vessel 10 between shaft 22 and helical ribbon agitator means 18.

The interruption of flow and the break up of the fluid in the region of baffle means 28 provides the intimate admixture of the fluid components.

The clockwise rotation, shown in the embodiment of FIGS. 1 and 2, produces a curved fluid surface 34 within the vessel 10, while counterclockwise rotation would produce an oppositely-curved (vortex) surface 36 and an oppositely directed flow as against that shown in the envelope region 38 and in the region between shaft 22 and helical ribbon agitator means 18.

The clockwise rotation provides the preferred condition for rapid removal of the fluid from the vessel, but best over-all agitation mixing is produced with a counterclockwise rotation of the agitator means. Accordingly, it has been found advantageous to carry out mixing with a counterclockwise rotation and then to reverse the direction of rotation (clockwise) when evacuation of the vessel is desired.

It has been found that, in the absence of the provision of the interruption of helical ribbon agitator means 18 and the provision of baffle means 28 positioned therein, the helical ribbon agitator merely produces two vortices (one at the top surface of the fluid 34 and the other at an interface between the two immiscible fluid phases of different densities which separate readily) with inefficient agitation and consequent inefficient mixing of the two fluid phases.

Accordingly, the basic action within the apparatus of the invention comprises two compound actions: (1) the helical ribbon agitator means produces a pumping or conveying action in the envelope region generated by its revolution where it is submerged in the fluid; and (2) as a second component of action, a shear mixing of the fluid phases (constituting turbulence in less viscous fluids), which vary from maximum at the periphery of the large diameter to decreasing shear action toward the center or shaft of the helical ribbon assembly, is produced. It is to be understood that any volatile components of the fluid phase will pass to the space 44 within the vessel 10 above the fluid level and from there through volatile venting means 12.

It is to be further understood that the present invention may be employed in any application where mixing or agitation of two or more fluid components is desired. It is particularly desirable to employ the present invention in chemical reaction autoclaves for either batch or continuous operation. For example, the present invention may be employed in reaction vessels for polymerization or other fluid phase reactions. In such instances, the fluid may be thin or viscous, ranging to extremely high viscosities. A two or more phase fluid may be mixed, wherein the two or more immiscible fluids are of widely varying types, with either liquid containing solids, rubber or any dispersible medium.

It has been found that tolerable clearances between spiral or helical ribbon agitator means 18 and the inner side walls of vessel 10 are dictated by specific processing conditions and may be generally of the order of 4 to 15 percent of vessel diameter. It has also been found that two or more flights of

3,675,902 such ribbon agitator means may be employed. The baffle means 28 may comprise or include additional fluid inlet or outlet means, which may function to permit the entry of additional phases of the fluid mixture or an additional reaction fluid, as desired, while also physically functioning as the baffle means which produces the breaking up of the fluid and the consequent intimate mixing of the fluid phases. Similarly, baffle means 28 may also serve as sensing or instrument means or as mechanical supporting means for the agitator shaft.

Typical agitator ribbons having widths of 3 to 18 percent of vessel diameter, into a lead pitch angle of 5° to 45° may be employed in a mixing vessel of a preferably cylindrical or conical shape and having a height varying between 0.8 to 5.0 diameters.

Apparatus, as shown in the embodiment of FIGS. 1 and 2 of the drawings, may be employed to produce a wide variety of batch or continuous simple mixing operations or mixing operations carried out during the process of batch or continuous chemical reactions. Examples of a few such operations during chemical reactions are as follows:

1. Phenolic resins have been produced in batch autoclaves during mixing carried out in accordance with the invention;

2. Polystyrene plastics are similarly produced in continuous autoclave reactors during mixing carried out in accordance with the invention, wherein solids are dispersed in one of the phases of the mixture;

3. A wide variety of other polymers, such as epoxy resins, acrylates, and modified phenolic resins may be produced in a batch autoclave vessel during mixing carried out in accordance with the invention.

It has been found that a desirable decrease in cavitation (accumulation of vapor or gas pockets along trailing edges or flats of blades) can be obtained in the apparatus of the invention by streamlining the helical ribbon agitator means 18, rather than by employing a flat ribbon agitator means (i.e. having rectangular section and non-streamlined square corners). It is to be noted, however, that such decrease in cavitation is obtained at the expense of some mixing efficiency, thereby necessitating a compromise in ribbon design.

In a specific example of the use of the invention, a 2,000 gallon internal capacity autoclave, having an 84 inch internal diameter and a 132 inch height, was employed for the production of phenolic resin in a batch process operation. Requisite quantities of phenol, aqueous formaldehyde and catalyst were charged to the vessel through the inlet means until the vessel was substantially filled. The rotation of the agitator means (two 8 inch wide ribbons having a 1½ inch clearance to the side walls) was then begun and in a counterclockwise direction at a speed of 30 rpm. As the reaction progressed at a temperature of 60° to 100° C. (controlled by means of conventional jacket fluids and vapor heat removal), the agitator means, which consisted of two interrupted ribbon flights, produced continuous, intimate mixing. At the end of the 3 hour reaction, dehydration was carried out by steam jacket heating, using the same agitation and venting the vapors to an external condensing system, not shown. The reaction product was finally vacuum dehydrated and rapidly discharged. Ap proximately 10,000 lbs. of product was evacuated from the vessel in 5 minutes by reversing the direction of the agitator, to clockwise rotation, for this operation. Rapid discharge to cooling means not shown, is essential for heat reactive products.

It is to be noted that operation in accordance with the present invention may be carried out in two or more stages of mixing, rather than in a single vessel. As shown in FIG. 3 of the drawings two mixing vessels, each embodying the invention, may be employed in series, whereby in the first stage 50 the interrupted helical ribbon agitator may be rotated in that direction which gives best over-all agitation and mixing (counterclockwise as shown), while the second state 52 rotation is in the opposite direction (clockwise as shown) to effect more rapid evacuation of the fluid from the vessel.

A series of three such mixing vessel autoclaves was employed in the preparation of acrylonitrile-butadiene-styrene terpolymer in which different concentrations of reactants and different percent solids concentrations were maintained in each of the autoclave mixture-reactor vessels of the three stages. A clockwise direction of agitator rotation was employed in each stage to provide a more rapid evacuation of the vessel by producing a positive pumping pressure on each vessel.