Static Mixer

Abstract

The static mixer has a static rigid mixing unit contained in a coaxial barrel which is mounted at its ends, respectively, in an inlet header through which materials to be mixed are fed under pressure to and through the unit, by which they are mixed, and in an outlet header through which the mixture is discharged. The unit comprises a row of vanes arranged to divide the material into separate streams, reassemble them in different combinations, and redivide and combine the combinations, repeatedly, as the material passes through the unit. The characteristic feature is that the barrel and unit are composed of different materials so related that the barrel can be destroyed readily, or rendered readily destructible, by means which, when and as applied to the combined barrel and unit so that both are subjected thereto, leaves the unit substantially intact.

Description

This invention relates to a static mixer and particularly to a static mixer of the general type disclosed in U.S. Pat. No. 3,286,992 of Nov. 22, 1966.

Such static mixers have been found very effective for mixing moldable plastic compositions, usually by the introduction thereinto of soluble plastic material, filler, solvent, and catalytic setting agent. Generally such a mixer can be used for a few hours after which it must be flushed out with a suitable solvent, but after a period of about a week or two it must be replaced with an entirely new and clean mixer.

When used, however, for mixing quick-setting plastic compositions, an additional problem is involved. Due to the quick-setting characteristics, the flow through the mixer must be at a rate above a predetermined minimum, or else the plastic will set up within the mixer instead of being discharged into the usual succession of molds which generally are moved on suitable belts past the discharge of the mixer in timed relation to the discharge therefrom. If for any reason a slight delay or decrease in rate of the passage of the plastic mixture through the mixer occurs, the mixture sets up in portions of the mixer and restricts the flow, thus further slowing the discharge and resulting in the entire mixer being clogged with the hard-setting plastic. Generally such static mixers are in the form of a tubular barrel in which is arranged a rigid static mixing unit. The mixing unit generally comprises fixed mixing vanes arranged in a row which extends endwise of the barrel. The vanes are arranged so that as the material is discharged from one vane it discharges with a swirling action about the axis of the barrel and strikes the next vane which subdivides the stream before passing it on to the next succeeding vane which subdivides these subdivisions. At each subdivision, each subdivided stream is recombined with a subdivided stream different from the one from which it was subdivided the instant before. Thus as the materials pass through the length of the barrel, the stream is subdivided and recombined in many different subcombinations so many times that a completely homogeneous mixture is discharged from the discharge end of the mixer. By the very nature of these vanes, however, and their arrangement in the barrel, they cannot be cleaned readily once any appreciable quantity of plastic has set up in the various interstices and along the serpentine passages defined by the unit and the barrel.

Attempts have been made to make the barrel, headers, and unit readily separable for cleaning, but due to the cementing and interlocking effects of the plastic, this approach has proven impractical.

Also attempts have been made to clean by introducing plastic solvents, but they cannot be circulated effectively through the tortuous passages in the units, especially when the entire flow has been blocked at some location in the mixer.

Heretofore the vanes of these units have been made of very durable material, such as a stainless steel, carbon steel, glass, titanium, or Teflon, and the barrel made of ordinary steel. While this type of structure can be cleaned, provided only a very small amount of settable plastic accumulates therein and is of a type that can be dissolved readily, it cannot be cleaned once a substantial amount of plastic has formed in any one place or the mixer has become completely clogged. This is due primarily to the inaccessibility of the plastic, by solvents, in portions of the passages through the barrel and vane unit, or inaccessibility inherent if the vanes are arranged for efficient mixing. For like reasons, an effective air supply cannot be applied to the clogging plastic in the mixer for use in burning out the plastic effectively.

In accordance with the present invention, however, the outer case or barrel is made of a material which is so related to the material of which the vanes are composed that the barrel can readily be destroyed or rendered readily destructible by means which when applied in the intended manner to the combined barrel and unit for the destruction of the barrel leave the vane units and headers substantially intact.

The specific material of the barrel per se and of vane unit per se is secondary. It is the relation of the material of the one to the material of the other that is controlling. If the barrel is removed by destroying it by means which do not affect the headers and unit when applied to the entire assembly, then all parts of the vane unit are rendered readily accessible for cleaning by the application of chemicals, heat, mechanical forces, or other means.

For example, the barrel may be formed of plastic, ceramic, or other material which can be readily fragmented, burnt or melted off, slit and pulled off, shattered at room temperature or in frozen condition, or dissolved chemically by means which, when and as applied to the mixer, does not deleteriously affect the material of the vane unit. The vanes, therefore, must be of a material such that they will not be deleteriously affected or changed substantially in any manner as the result of the means used for destroying the barrel. Assuming that the vanes are of a hard metal, such as steel or the like, the barrel may be made of a plastic which can be burnt off or rendered very frangible readily at temperatures around 400.degree. F. which, of course, would not affect the steel appreciably. Generally the quick setting plastic is of a composition which can be burnt off the vanes at this same temperature, once adequate air can gain access to it. In the method of cleaning by heat, the entire mixer is placed in an oven and heated to the temperature requisite to burn up and consume the barrel or to render it very frangible so that it can be fragmented and broken off readily, thereby exposing the vane unit and all the intricate passages within the vane unit for cleaning. Generally all that is necessary is to heat the entire combination of casing and unit and then with a heavy brush, brush off the weakened plastic barrel and the plastic adhering to the vanes themselves.

Polyvinylchloride, polyethelene, and other types of commercially available synthetic plastic pipes have been found to be effective for the barrels, so long as the unit is of a material which would not be destroyed by the destruction of the barrel.

On the other hand, the outer barrel may be of a metal which can be dissolved chemically by a solvent which would not adversely affect the vane unit, in which case the vane units are readily exposed for cleaning by any conventional means.

Again, the barrel might by of ceramic or glass or a composition which can readily be fragmented by a series of sharp blows without damage to the vane unit or headers.

Thus by using for the barrel a material properly related to the material of the vane unit, the cleaning problem is simplified so that cleaning can be effected at very little expense in man-hours, materials, and down time. Such cleaning can be effected by an average workman in the plant where the unit is in use.

A preferred embodiment of the invention is illustrated in the drawings, in which:

FIG. 1 is a side elevation of a mixer embodying the principles of the present invention;

FIG. 2 is a cross-sectional view of the mixer taken on line 2--2 of FIG. 1; and

FIG. 3 is a cross-sectional view of the mixer taken on line 3--3 of FIG. 1.

In the illustrative example, the mixer, indicated generally at 1, comprises a tubular barrel 2 having at its inlet end a feed header 3 and at its outlet end a discharge header 4. The feed header is provided with one or more feedpipes or ducts 5 which supply the separate ingredients, or a preliminary mixture thereof, as preselected, into the header under pressure sufficient to cause a relatively rapid flow of the material through the mixer.

Within and coaxial with the barrel 2 is a static vane mixing unit 6 which is in the form of a plurality of blades 7 which may be directly fastened together or fastened to a center rod or support. The blades are arranged in a row extending from one end of the barrel to the other and, if desired, the row may be connected at its ends to the headers. The outer periphery of the vanes of this mixing unit may fit rather snugly against the interior wall of the barrel 2, but may terminate close thereto and not necessarily be in sealing relation thereto.

The vanes 7 are arranged so that the material entering through the header 3 is divided initially at that end of the barrel and unit. The two streams resulting from this division pass to the next vanes which subdivide each of these streams and recombines each subdivided stream with another subdivided stream different from the one from which it was previously subdivided. As the material passes to successive vanes, further subdividing and combining are caused so that by the time the material reaches the discharge header 4, a multitudinous number of streams have been divided, subdivided, and intermixed with each other to an extent such that the mixture discharged from the mixer is substantially homogeneous and uniform throughout.

The discharge header 4 is provided with a discharge pipe 8 which is arranged to discharge into molds, not shown, which are moved successively therebeneath. The timing for as short a cycle as possible is important from the economic standpoint and hence it is desirable to pass the material through the mixer at the maximum rate permissible, this rate, of course, depending on the nature of the plastic material which is passed through the mixer and the viscosities and chemical characteristics. The materials are supplied to the mixer under pressure so as to maintain the proper flow, but in seeking as short a cycle as possible, the danger of "setting up" of the material because of a slight increase in resistance to flow slowing down the stream is increased.

The specific type of vane unit, of course, is dictated to some extent by the type of materials which are to be mixed. This dictates, in turn, the type of material to be used for the barrel and the type of destruction of the barrel casing to be employed to afford access to the vane unit for cleaning.

It is apparent from the foregoing description that regardless of how badly clogged the unit may become, cleaning is made possible by the relative destructibility and indestructibility of the barrel and vane unit, respectively.

The invention is also applicable to mixers with other types of mixing units operating in a barrel or container where clogging by "setting up" or by sludge due to increase in viscosity, and the like, occur and the interior is inaccessible for cleaning in the assembled relation of the parts.

Claims

Having thus described my invention, I claim:

1. A continuous static type continuous mixer for flowable ingredients of a quick-setting plastic fed continuously under pressure and for mixing the ingredients and discharging the resultant plastic in flowable moldable condition, comprising:

a tubular barrel;

a header on one end of the barrel adapted for connection to a source for receiving the ingredients under pressure and admitting the received ingredients into the barrel;

discharge means at the other end of the barrel;

a static rigid mixing unit within the barrel and comprising a row of mixing vanes extending endwise of the barrel and arranged so that the stream of material is caused to flow through tortuous paths and to be subdivided and the subdivided streams mixed repeatedly as the material passes through the barrel to the discharge means;

said barrel being composed of a material different from that of the vanes, said material being such, relative to the material of the vanes, that said material can be destroyed or rendered readily destructible by means which, when and as applied so that the combined barrel and unit are subjected thereto, leave the vanes substantially intact.

2. The structure according to claim 1 wherein the barrel is composed of material which is rendered readily destructible by the application of heat to the combined barrel and unit at predetermined low temperatures below temperatures which have deleterious effects on the unit.

3. The structure according to claim 1 wherein the barrel is composed of material which is soluble readily in a solvent in which the unit is insoluble.

4. The structure according to claim 1 wherein the barrel is composed of material which can be broken into fragments readily by mechanical forces which, as applied to the combined barrel and unit, leave the vanes substantially intact.

5. The structure according to claim 1 wherein the barrel is set plastic material.

6. The structure according to claim 5 wherein the barrel is ceramic material.

7. The structure according to claim 5 wherein the unit is of hard metal.

8. The structure according to claim 1 wherein the barrel is composed of synthetic organic plastic and filler and the vanes are ferrous metal.