GB2061746A

GB2061746A - Static mixer

Info

Publication number: GB2061746A
Application number: GB8034470A
Authority: GB
Original assignee: Sulzer AG; Gebrueder Sulzer AG
Current assignee: Sulzer AG
Priority date: 1979-10-26
Filing date: 1980-10-27
Publication date: 1981-05-20
Also published as: AU534423B2; MX151266A; DE8019476U1; FR2468401B1; JPS5662531A; IT1133888B; SE441061B; IT8025329A0; NL188561C; AU6353880A; CA1177061A; BE884546A; ES263049U; CH642564A5; NL8004240A; ES263049Y; DE2943688A1; NL188561B; SE8007497L; GB2061746B

Abstract

The invention provides a static mixing device comprising a tubular casing (1) and, disposed therein, at least one mixer element (2-5) in the form of crossed webs (6, 7) disposed at an angle with the tube axis, the webs being disposed in at least two groups, the webs of anyone group of elements extending substantially parallel to one another and the webs of one group crossing the webs of the other group, in which the maximum web width (b) is from 0.1 to 0.167 times the tube diameter (d), the normal between-webs distance (m) in each group is from 0.2 to 0.4 times the tube diameter (d) and the length (l) of the mixer element is from 0.75 to 1.5 times the tube diameter (d). <IMAGE>

Description

SPECIFICATION Static mixer The invention relates to a static mixer.

Static mixers of the general type to which the invention relates, are known e.g. from German Auslegeschrifts 2,328,795 and 2,522,106.

Static mixers are required to be as short as possible for economic and technical reasons. Material costs and pressure drop in use are the economic considerations, while overall length should be short for technical reasons to ensure that the mixture is of compact construction and that the dwell time of the media in the mixer is short.

It has previously been assumed in practice that if a required level of homogeneity is required, e.g. in relation to concentration or temperature, mixer elements must have a large number of webs and the webs must be arranged in a narrow "pack" to give a small "mesh size". Mixer length then becomes relatively short. Unfortunately, it has been found in practice that this arrangement results in a considerable pressure drop, resulting in high pumping powers and therefore high energy costs; also, the mixer elements have to be very strong. Another difficulty is that mixer elements of this kind are different to clean and become clogged fairly readily because of deposits forming on the webs.

It therefore began to be thought that the pressure drop could be decreased by some "loosening-up" of the mixture element structure, i.e., by using fewer webs and by increasing mesh size. However, the layering laws for determining homogeneity show tiat such a construction would reduce the layers p;oduced over a particular mixer length, so that the tangth would have to be increased, on the assumpton that mixer length would have to be increased approximately in the same proportion as the pressure drop would be reduced. This construction was therefore not used in practice.

It is the object of the invention to provide a geometry for the known structure which provides the required mixing quality in a mixer of relatively reduced length and having a low pressure drop.

Accordingly the present invention provides a static mixing device comprising a tubular casing and, disposed therein, at least one mixer element in the form of crossed webs disposed at an angle with the tube axis, the webs being disposed in at least two groups, the webs of any one group of elements extending substantially parallel to one another, and the webs of one group crossing the webs of the other group, in which the maximum web width (b) is from 0.1 to 0.167 times the tube diameter (d), the normal between-webs distance (m) in each group is from 0.2 to 0.4 times the tube diameter (d) and the length (I) of the mixer element is from 0.75 to 1.5 times the tube diameter (d).

The surprising knowledge underlying the invention is that if the above dimension relationships are observed, the resulting mixer is only slightly longer than the conventional mixer and has an unexpectedly low pressure drop, as will be described hereinafter with reference to embodiments.

The invention is of use more particularly for mixing processes of Newtonian and non-Newtonian liquids.

The tubular casing can be a circular section tube or a square section tube. In the former case the contour of the webs is adapted to the circular cross-section of the cylindrical tube.

The geometry of the mixer elements is determined by the dimensional specifications for the relationship of web width b to tube diameter d and the relationship of the normal between-webs spacing m between adjacent group pairs to the tube diameterd and the relationship of mixer element length / to tube diameterd. For instance, the statement b/d = 0.167 means that six webs are distributed over the same tube-cross-section, whereas the statement b/d = 0.1 indicates that ten webs are distributed over the same tube cross-section.

The relationship between the normal spacing m and the tube diameter d denotes the web density in the tube - i.e., mesh size along the tube axis and therefore the total web surface area.

The relationship between mixer element length / and tube diameterd gives the length of a mixer element.

In orderto promote a fuller understanding of the above and other aspects of the present invention, an embodiment will now be described, by way of example only, with reference to the accompanying drawings in which: Fig. I is a diagrammatic view in longitudinal section showing part of a mixer embodying the invention and Fig. 2 is a diagram in which mixing quality, as indicated by the variation coefficient X is plotted against x relative mixer length l/d.

Referring to Fig. 1, four mixer elements 2-5 are disposed one after another in a tube 1, each of the consecutive elements being rotated by 90 relatively to one another referred to the tube axis.

The elements of the embodiments each comprise two web groups 6,7 and each group comprises webs 6'a, 6"a, 6"'a - 6!d, 6"d, 6"'d and 7'a, 7"a, 7"'a - 7'd, 7"d, 7"'d, the inclination angle a of the webs of group 6 being opposite to that of the webs of group 7. In the embodiment angle a is 45". Each mixer element comprises three interleaved plate pairs 6'a 6'd, 7'a - 7'd; 6"a -6"d, 7"a - 7"d; and 6"'a - 6"'d, 7"'a - 7"'d, the webs of group 6 extending through the gaps between the webs of group 7 to cross the same while the webs of group 7 extend through the gaps between the webs of group 6 to cross the same.

In the embodiment each plate pair consists of eight webs, the webs of each plate being coplanar (see 6"a - 6"d of element 3 and 7"'a - 7"'d of element 5 in Fig. 1). However, the webs 6'a - 6'd, 7'a - 7'd and so on, instead of being coplanar, can be offset from one another stairwise. As described in German Offen leg ungsschrift 2,748,128, the webs of a single mixer element can be joined together at their contact places as a whole in a single working step by electric resistance welding.

The web widths have the references b, the tube diameter has the referenced, the normal distances of the group pairs between the webs have the referencem, the angle of inclination of the group 6 and group 7 to the tube axis has the reference a and the length of the mixer elements has the reference!.

Five types of mixer elements will be compared hereinafter for measured pressure loss and relative mixer length with reference to the diagram shown in Fig. 2.

In the diagram the variation coefficient aik is plotted along the ordinate, and the relative mixer length lid of the complete mixer comprising a number of mixer elements, is plotted along the abscissa. (r denotes the measured standard deviation from the calculated mean value X of a mixture produced in a static mixer.

The standard deviation from from the calculated mean value X of the homogeneity of ingredients for mixing which a mixer provides can be found by means of electrical conductivity measurements (see Chem.-lng. Techn. 51(1979), Nr. 5, pp. 353-354).

The formal equation:

is used for the pressure loss Ap found by measurements in static mixers, in the case of laminar flow.

"z" is the pressure drop multiple and represents the relationship of the pressure conditions in a static mixer to the empty tube atthe same viscosity 7, flow velocityw, length Land tube diameterd.

The following table gives the geometric data for mixer types I-V.

Type bid mid lid a 0,08 0,15 1,63 45" If 0,1 0,2 0,75 45o Ill 0,125 0,3 1 45" IV 0,167 0,4 1,5 45o V 0,25 0,5 1,6 45o The characteristic curves a/X = f(Lid) fortypes I-V are plotted in the diagram of Fig. 2 aiX = 1O-2 means that the standard deviation from the mean value is 1% and the mixture can be considered to be homogenous.

The table below gives measured values of relative mixer length for a/X = 10-2 and the associated pressure drop multipleszfortypes I - V.

Type Lid z 8 90 9 50 Ill 10 35 IV 14 20 V 30 16

It can be gathered from the foregoing data that the relative mixer lengths 11, III and IV are not much greaterthan fortype I, but the pressure drop multiple of types ll, Ill and IV can be reduced considerably below the pressure drop for type I.

It will also be apparentthat pressure drop reduction is not in approximatelythe same relationship to increase in relative mixer length as has previously been assumed but is much stronger and more pronounced. Type I is of a construction similar two constructions disclosed in the publications cited in the introduction hereof.

A comparison of type V with types II - IV shows that the substantial reduction of the pressure drop multiple is linked with a substantial increase in relative mixer length; the increase of Ud and the decrease ofz as compared with type 1 are in approximatelythe same relationship.

The interesting feature in a comparison of mixing devices with one another is the pressure dropithroughput for the same quality of mixing. The pressure drop and throughput are of course interconnected by way of the specific effect W which is a dimensionless characteristic (cf. e.g. E. Dolling: "Zur Darstellung von Mischvorgangen in hochviskosen Flussigkeiten", Dissertation, Techn. Hochschule Aachen/Germany/l971 and H. Brunemann and G.

John: "Statische Mischer", Aufbereitungstechnik, 1972, 1, pp. 16-23).

in which ApV denotes the flow work, 77 denotes viscosity and V denotes volume flow.

Fora given qualityofmixingW is lowestforthe technically optimal mixing device.

The following table gives the observed values of specific effectwformixing devices for which mixer elements of types I - V are used.

Type w 184 320 129 600 Ill 112 000 IV 125 440 V 460 800 As the table shows, a device having mixer elements III can be considered to be the technically optimal mixing device, although the difference from devices having mixer element types II and IV are so slight that the three types 11, III and IV can be regarded as virtually equivalent. However, the specific effect W differs considerably for types land V and can therefore be considered unsuitable for the purposes of the invention.

The surprising knowledge underlying the invention is based on the fact that the indirect proportionality previously assumed between pressure drop and mixer length is not continuous but that an optimisation range for the geometry of the known structures of static mixing devices exists where the mixers have a relatively short mixer length and an economicallytolerable pressure drop.

Claims

1. Astatic mixing device comprising atubular casing and, disposed therein, at least one mixer element in the form of crossed webs disposed at an angle with the tube axis, the webs being disposed in at least two groups, the webs of any one group of elements extending substantially parallel to one another and the webs of one group crossing the webs of the other group, in which the maximum web width (b) is from 0.1 to O.t67 times the tube diameter (d), the normal between-webs distance (m) in each group is from 0.2 to 0.4 times the tube diameter (d) and the length (!)of the mixer element is from 0.75 to 1.5 times the tube diameter (d).

2. A device as ctaimed in Claim 1, in which the maximum web width (b) is 0.1 in which the tube diameter td), the vertical between-webs distance (m) in each group is 0.2 of the web diameter and the length (!)of a mixer element is 0.75 of the tube diameter(d).

3. A device according to Claim 1, characterised in that the maximum web width tb) is 0.125 of the tube diameter (dt the vertical between-webs distance (m) in each group is 0.3 of the tube diameter (d) and the length (/) of a mixer element is equal to the tube diameter (d).

4. A device accordingto Claim t, characterised in that the maximum width (b) is 0.167 times the tube diameter (d), thevertical.between-webs distance (m)in each group is 0.4 times the tube diameter (d) and the length (/) of a mixer element is 1.5 times the tube diameter (d).

5. A device according to Claim 1, characterised in that at leasttwo mixer elements are arranged consecutively in the tube and the adjacent elements are pivoted preferably at right-anglesto one another referred to the tube axis

6. Astatic mixer substantially as herein described with referencetothe accompanying draw ings