Mixing Device

Abstract

The use of a screen structure between groups of static mixing elements located in a conduit through which tow or more liquids flow increases mixing efficiency.

Description

BACKGROUND OF THE INVENTION

This invention relates to a device for mixing two or more liquids of highly differing viscosities. In particular, the invention is concerned with an improvement on a mixer having no moving parts.

Armeniades et al., in U.S. Pat. No. 3,286,992 disclose a mixer that consists of a hollow tube or pipe containing a series of curved dividing elements that provide repeated division and recombination of the materials to be mixed as they flow through the tube. The chief mechanisms for accomplishing mixing in such a device are flow division and radial mixing. Flow inversion and backmixing contribute to a lesser extent. Where miscible liquid systems are being combined, diffusion across the flow strata in the mixer enhances mixing somewhat. However, for an all-liquid system in which the constituents possess widely differing viscosities, mixing performance is determined almost exclusively by the amount of flow division attained. In such systems in which the viscosity ratio of the liquids is 1,000 or higher, globules or "islands" of the high viscosity component tend to form after passing the first few curved dividing elements of the mixer. These discrete particles then pass the remaining elements essentially unchanged. This problem is aggravated in those systems in which the low viscosity component is in volumetric excess.

Attempts to overcome this problem by increasing the number of dividing elements often result in a prohibitively large pressure drop through the mixer. The problem of excessive pressure drop through such mixers is recognized by Grout et al., U.S. Pat. No. 3,664,638.

SUMMARY OF THE INVENTION

It has been found that the problem of undissolved high viscosity inhomogeneities in such systems may be overcome by the use of one or more fine mesh screens or other foraminous structures placed between sets of Armeniades et al. mixer elements. The use of a screen between the mixer elements greatly increases the mixing efficiency. Thus, fewer mixer elements are needed and a lower pressure drop results.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a partially sectioned view of one embodiment of the invention, showing a screen section between mixing sections;

FIG. 2 is a partial perspective view partly in section of the screen section of FIG. 1.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT

In the drawing, a hollow cylindrical tube 10 of uniform cross section consists of a premixing section 12, an initial mixing section 13, a screen section 14 and a final mixing section 15. Low viscosity liquid A is fed directly into premixing section 12 and high viscosity liquid B is introduced through flow distributor 16 having several orifices 17. From delivery end 18 of the tube issues a homogeneous mixture of A and B. Within the initial mixing section 13 and final mixing section 15 of tube 10 are sets of curved elements 20, 21, 22, etc., and 30, 29, etc., respectively. As more fully described in the aforementioned Armeniades et al. patent, the downstream edge of each of the elements 20, 21 . . . 29, 30, are positioned at a substantial angle to the upstream edge of the next adjacent one. The elements are preferably physically connected, e.g., by welding or brazing at their contact point 33. Two cylindrical screens 34 having solid end plates 36 are mounted in the end walls 35 of screen section 14.

The screens placed between sets of the curved mixer elements are positioned in such a way that the liquid stream is required to pass through the meshes of the screens. The screens may be planar, cylindrical or of any desired shape. If planar, they are conveniently positioned transversely in the hollow tube between sets of the mixing elements. If cylindrical, they may be positioned, for example, as shown in the drawing. In general, a screen having a mesh opening approximately equal to the maximum particle size of the undissolved high viscosity globules gives satisfactory results. In such a case, the screen does not serve as a filter since the average particle size is smaller than the mesh opening. However, the invention will operate satisfactorily even if the average particle size is not smaller than the mesh opening. Usually screens of 100 mesh or finer will be effective and are preferred.

In an example, a mixer consisting of a set of 35 Armeniades et al. elements followed by two 200-mesh cylindrical screens followed in turn by a set of 21 elements produced complete mixing in the following system: 4 parts of a high viscosity liquid (440 poises) and 6 parts of a low viscosity solvent (1 centipoise) at 30.degree.C. The viscosity ratio was 44,000/1. Good mixing was indicated by no discernible globules of the high viscosity component in the effluent and no buildup on the screens. Similar results were obtained in a two component liquid system having a viscosity ratio of approximately 150,000/1.

In the operation of the embodiment illustrated in the drawing, it is preferred that the coaxially oriented flow distributor 16 and the initial mixing section 13 be disposed within 10.degree. of a vertical position, most preferably, absolutely vertical. Flow direction, either up or down, has not been found to make a significant difference in efficiency of mixing.

The concept of using fine mesh screens between sets of Armeniades et al. mixer elements for the purpose of dividing or dispersing the higher viscosity component is applicable to any liquid system. It is especially beneficial in those cases where precise drop size is a critical objective or in those instances where an excessive number of mixer elements would otherwise be required to achieve the desired degree of mixing and/or in obtaining homogeneous mixing with minimum pressure drop.

Although the invention has been described using two spaced groups of elements or mixing sections 13, 15 of the same diameter, more than two mixing sections as well as mixing sections of differing diameters may be used with a screen section incorporated between at least two of the mixing sections.

Claims

What is claimed is:

1. In a device for mixing a plurality of flowing liquids including a hollow cylindrical tube and a plurality of curved mixing elements positioned in said tube, the improvement comprising: said elements being formed in spaced groups and a cylindrical shaped screen positioned between at least two spaced groups of elements, said screen having a diameter less than the diameter of the conduit, said screen being connected to the walls of the conduit at one end and being closed at the other end.

2. The device as defined in claim 1, including a second cylindrical shaped screen positioned between said groups of elements, said second screen being the same as the first screen and having its closed end adjacent the closed end of the first screen.