High Shear Mixers

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GBH Enterprises, Ltd.

Process Engineering Guide: GBHE-PEG-MIX-709

'High Shear' Mixers Information contained in this publication or as otherwise supplied to Users is believed to be accurate and correct at time of going to press, and is given in good faith, but it is for the User to satisfy itself of the suitability of the information for its own particular purpose. GBHE gives no warranty as to the fitness of this information for any particular purpose and any implied warranty or condition (statutory or otherwise) is excluded except to the extent that exclusion is prevented by law. GBHE accepts no liability resulting from reliance on this information. Freedom under Patent, Copyright and Designs cannot be assumed.



Process Engineering Guide: 'High Shear' Mixers CONTENTS SECTION 0 INTRODUCTION/PURPOSE 3 1 SCOPE 3 2 FIELD OF APPLICATION 3 3 DEFINITIONS 3 4 SELECTION OF MIXER TYPE 3 4.1 The Shrouded Turbine 3 4.2 Turbine Mixers 4 4.3 Unshrouded Agitators 4 4.4 In-Line Mixers 5 4.5 Ultrasonic Homogenizers 5 5 SHROUDED TURBINE DATA 6 5.1 Recommended Duties 6 5.2 Equipment Data 6 6 HIGH SPEED INTERNAL (TURBINE & VESSEL

COMBINED) MIXERS 10

6.1 Recommended Duties 10 6.2 Equipment Data 11



7 HIGH SPEED UNSHROUDED (SAWTOOTH) AGITATORS 12 7.1 Duties 12 7.2 Equipment Data 13 TABLES

1 POWER INSTALLED FOR TORRANCE SAWTOOTH IMPELLERS 15

FIGURES 1 SHROUDED TURBINE 4 2 TURBINE AND VESSEL COMBINED 4 3 SAWTOOTH MIXER 5 4 IN-LINE MIXER (OAKES) 5 5 ULTRASONIC EMULSIFIER IN-LINE 6 6 ROTOR STATOR GEOMETRIES 7 7 SAWTOOTH IMPELLER RANGE OF GEOMETRIES

(AFTER TORRANCE) 13 8 MIXER POWER CURVES 14 DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE 16



0 INTRODUCTION/PURPOSE "High shear" mixers are used widely in the paint, food, pharmaceuticals, adhesives and coating industries, but their application in the chemical plant operations has been limited. The break-up of solid agglomerates, especially the dispersion into paints, is a major application. Many wet filter cakes may be redispersed by high shear mixers prior to drying. Emulsification and foaming feature in pharmaceutical, food and cosmetic applications. "High shear" mixers are particularly effective for dissolving high polymer additives. It should, however, be remembered that polymer chains in excess of about 20,000 monomer units are vulnerable to degradation in commercial high shear mixers. This degradation of polymers is used for viscosity control in the manufacture of shampoo and in other processes to obtain optimum rheological characteristics. The process engineer may be faced with the task of selecting a satisfactory device from one of the many proprietary designs available. The technical guidance available from the suppliers often falls short of the ideal; manufacturers may not always be able to supply such basic information as the power consumption of a mixer for a liquid of density greater than that of water. 1 SCOPE This Process Engineering Guide helps the user to select the appropriate mixer type for the duty and then recommends criteria to be met for satisfactory operation. 2 FIELD OF APPLICATION This Guide applies to Process Engineers in GBH Enterprises worldwide. 3 DEFINITIONS No specific definitions apply to this Guide. With the exception of terms used as proper nouns or titles, those terms with initial capital letters which appear in this document and are not defined above are defined in the Glossary of Engineering Terms.



4 SELECTION OF MIXER TYPE The following types of mixer are available commercially: 4.1 The Shrouded Turbine Shrouded turbines are manufactured by Greaves Ltd and Silverson Machines Ltd. Kinematica supply a machine for producing sub-micron sized dispersions. They are illustrated in Figure 1 and are recommended for emulsification, polymer or gel dissolution or 'soft' solid dispersion where the solids content is less than 15% w/w. Detailed recommendations and performance data are given in Clause 5. FIGURE 1 SHROUDED TURBINE



4.2 Turbine Mixers Turbine mixers, consisting of a vessel and the agitator which are sold as one unit, are exemplified by the Baker Perkins 'Hydisperser', see Figure 2. These units are particularly suited to slurrying filter cakes and are suitable for solid dispersion duties where the solids content exceeds 15% w/w. The standard system gives poor bulk circulation; Detailed design recommendations and performance data are given in Clause 6. FIGURE 2 TURBINE AND VESSEL COMBINED

4.3 Unshrouded Agitators Unshrouded agitators are often of the sawtooth disc type (see Figure 3 for an example of the Torrance device). They are recommended for solids dispersion duties requiring more than 15% w/w of solids. They can give problems with filter cakes and will be slow to emulsify liquid-liquid systems with a viscosity ratio greater than 3. Detailed recommendations and performance data are given in clause 7.



FIGURE 3 SAWTOOTH MIXER

4.4 In-Line Mixers Some manufacturers make in-line versions of their shrouded mixers. Oakes Ltd and Mondo Mix BV supply shrouded mixers with no batch equivalent. The Oakes mixer with its concentric rows of rotor and stator teeth should ensure that fluid does not by-pass the active zones. Oakes and Mondo mixers are particularly effective in the production of foams. Figure 4 illustrates an Oakes mixer. FIGURE 4 IN-LINE MIXER (OAKES)



Other mixer types may be subject to by-passing of the most intensive agitation zone and a single pass may therefore not be able to meet the requirements. Such designs should therefore be treated with caution. 4.5 Ultrasonic Homogenizers These devices, illustrated in Figure 5, perform well in some niche applications, e.g. the exfoliation of vermiculite, where the caviation process is effective. They should be effective inline emulsifiers with no restriction on the liquid viscosity ratio. FIGURE 5 ULTRASONIC EMULSIFIER IN-LINE

5 SHROUDED TURBINE DATA 5.1 Recommended Duties Shrouded turbines give the most intense extensional flow fields and are recommended for the following duties: (a) Emulsification

Shrouded turbines are particularly suited when the dispersed phase is the more viscous. When the ratio of the viscosity of the dispersed to that of the continuous phase is greater than 3, a shrouded turbine should be used.



(b) Polymer or Gel Dissolution

Polymers swell in solvents and in the early stages of dissolution the process is the breakdown of the viscous gel particles, somewhat analogous to emulsification. Quite large lumps of elastomeric solids can be digested by shrouded mixers.

(c) Polymer Degradation

High molecular weight polymers may be degraded by the action of a shrouded turbine. Significant heat can be generated in this process.

(d) Homogenization of Soft Solids

Soft solids, e.g. elastomers, may be dispersed by shrouded turbines to give a homogeneous paste. They give an intense disruptive action and should be used whenever possible. Their use is however restricted by their limited pumping action and by abrasion. Other devices are recommended for solids concentrations in excess of 15%!w/w.

5.2 Equipment Data (a) Rotor Stator Geometry

Typical geometries of the shrouded turbine mixers are shown in Figure 6. The rotor discharges the fluid through the radial ports in the stator. Manufacturers supply 'high shear' and 'high pumping' stators; the latter has fewer, larger ports to allow the impeller to digest coarse lumps and gives higher circulation rates.

The Greaves mixer includes an axial flow turbine and the circulation rate may be controlled by a deflector plate which, when raised, allows fluid to bypass the stator



FIGURE 6 ROTOR STATOR GEOMETRIES



The 'high pumping' head is preferred for most duties and has performed better than the 'high shear' head for polymer dissolution and degradation. In concept the 'high pumping' head allows the impeller to digest coarse lumps and circulate fluid at a higher rate, but should take longer to reach, for example, the same degree of emulsification because the fluid can bypass the zone of intense agitation. The most intense disruptive forces occur in stretching flow where the fluid is forced between the 'nip' of an advancing rotor blade and a stator port. Because of this the annular gap between the rotor and stator is not too critical for emulsification, and polymer dissolution and degradation duties where a gap of 0.25 to 0.5 mm is satisfactory. (b) Mixer Size for Standard Duties For routine applications manufacturers should be approached. They should then quote the size and power of a mixer for a particular duty. They are, however, often reluctant to commit themselves to a mixing time and thus to throughput. Greaves have provided data for sizing impellers for batch emulsification and polymer dissolution duties which are summarized by the dimensional equation:

The equation may be used to select a proprietary Greaves or Silverson mixer 'off the shelf' provided the liquid density is 1000 kg/m3 or less. If the liquid density exceeds 1000 kg/m3, increase the manufacturer's nominal motor rating pro rata with the density. Equation 1 implies a rotor to vessel ratio ranging from 0.12 in low viscosity (0.01 N s/m2) to 0.2 in high viscosity (30 N s/m2) batches and represents competitive sizing for standard duties.



(c) Mixer Size for Critical Duties Polymer degradation represents a critical duty because the zone of intense agitation is only a small fraction of the total volume of the vessel. To achieve economical batch times rotor to vessel diameter ratios as high as 0.33 have been required. Mixers for these duties should be scaled up from trials on small-scale mixers. The small scale impeller should have the same rotor to stator gap, blade angle and tip speed as that proposed for the larger scale. The D/T ratio need not be matched on both scales. Scale-up then involves the following steps: (1) Using a small, variable speed laboratory mixer, establish the tip speed

needed to give the required process effect.

Note that these mixers may not reach a full-scale tip speed. (2) From the laboratory results, specify conditions for a trial with a 1 to 5 hp

mixer in a batch up to 50 liters volume. This mixer will approximate reasonably closely to the full-scale geometry. A cooling coil would be required for isothermal operation.

(3) Calculate the full-scale batch mixing time to achieve the required end

result from:



(d) Mixer Power The manufacturer should normally be asked to specify the power required for a new agitator. Note however that manufacturers have not always been able to predict the power dissipated in unusual liquids. The power absorbed depends partly on the mixer's behavior as a turbine and partly on the torque generated in the rotor stator gap. The latter effect becomes more important as the liquid density increases. For most liquids, including solutions of low or degraded polymers (e.g. linear polymer Mw less than 400,000, Mw/Mn < 4), the power of Greaves and Silverson mixers has been predicted to within 20% by:

If the batch contains a higher polymer (e.g. a linear polymer of Mw of 400,000 or more and Mw/Mn > 4) additional power is dissipated in extensional flow as the viscoelastic fluid is forced into the 'nip' between the rotor and stator. At present it is not possible to predict the additional power dissipation which has been 2 - 3 times that predicted by the first term of equation 3. It is therefore recommended that the additional torque be measured on a small-scale with the rotor stator gap and tip speed as proposed for the full-scale operation. The full-scale torque is then calculated from:



D' = small-scale rotor diameter n = number of blades on full-scale rotor n' = number of blades on small-scale rotor. (e) Pumping Capacity There are no published data on pumping capacities of shrouded turbine mixers. The following tentative correlation is suggested for a 5 hp Greaves mixer:

(f) Bulk Mixing Times and Heat Transfer No data are available for bulk mixing times and heat transfer, both of which are flow sensitive. The high shear impellers typically generate about 1/3 of the flow of a turbine (W/D = 0.2) at a given speed. It is recommended that the turbine correlations for mixing time and heat transfer, given in GBHE-PEG-MIX-701, should be used with the speed term corrected to N/3.



(g) Shear Rate The nominal shear rate in the rotor stator gap is given by:

The shear rate is not normally a particularly important parameter in emulsification and polymer dissolution or degradation. See also GBHE Mixing and Agitation Manual, Sections D4.4 and D4.5. 6 HIGH SPEED INTERNAL (TURBINE & VESSEL COMBINED) MIXERS These machines tend to be sold in two performance ranges. The first, typified by the 'Hydisperser', has a power dissipation of 10-16 kW/m3, and is used by some operators to liquidize dispersible filter cakes, often before spray drying. The standard 'Hydisperser' is fitted with a back-swept vaned disc agitator (D/T = 0.45) in the base. The standard maximum tip speed is 11 m/s with W/D of 0.06-0.07 to economize on power. A cruciform baffle cage may be fitted for lower viscosity applications and a 'butterfly' impeller of D/T = 0.65 is also available. The second performance range includes the Baker Perkins 'Hynetic Mill' and the Bearsley & Piper 'Speed mixer'. These devices may incorporate mulling wheels and are better regarded as colloid mills. 6.1 Recommended Duties (a) Solids Dispersion

High speed internal mixers can handle 15 - 65% w/w of solids. The upper limit will depend on the apparent viscosity of the sheared batch and should be less than 50N s/m2.

(Consult GBHE-PEG-FLO-302 for the interpretation of viscometric data.) The sheared viscosity has to be measured experimentally as there is no simple relationship which covers particle shapes, sizes, packing fractions and interaction effects. A measurement at a shear rate of 300!l/s should cover the shearing effect expected in the 'Hydisperser'.



To estimate the sheared viscosity of an ideal system of non-inter-acting spherical particles, the Mooney equation may be used:

k can be as high as 1.9 and will be lower than 1.6 for particles with a wide size distribution. The limiting viscosity would be reached at 50% v/v solids (i.e. c = 0.5) with k = 1.6. (b) Filter Cake Dispersion Over 100 dispersible filter cakes have been examined and all are liquidized at shear rates of 300 l/s or below. All these filter cakes may be dispersed by a high speed internal mixer in the lower performance range (10-16 kW/m3). 6.2 Equipment Data (a) Scale-Up Trials, involving the manufacturer of the internal mixer, on small-scale machines are advised. Manufacturers offer a product range of machines operating at a constant maximum tip speed and scale up is by keeping the tip speed constant and increasing the batch time in proportion of the linear scale-up factor. This method works in most cases. Theoretically, if cake disintegration requires relatively low shear rate (ca. 300 l/s), then power per unit volume should be the relevant criterion. Following the manufacturers' scale-up method, the power input per unit volume will decrease with increasing size. This will not usually be critical, but it is advisable to check the dispensability of the cake at a lower power input by reducing the tip speed of the small scale device according to:



Only if the cake fails to disperse satisfactorily at this lower speed should the manufacturer be requested to supply a non-standard device based on constant power per unit volume. (b) Butterfly vs Vaned Disc Agitators Butterfly type agitators are used to provide a folding action in some paste mixers. They do not offer any particular advantage in high speed dispersion duties although the open structure of the butterfly agitator may assist in starting the movement of a cake with a high yield stress. (c) Power Consumption The estimation of the agitator power should be the responsibility of the manufacturer. Starting loads and transients are likely to be more critical than the power dissipated in the dispersed batch and, in any event, a variable speed motor may be selected. A power number (Po) of approximately 0.3 would be expected for the abbreviated vaned disc in an unbaffled vessel at high Reynolds numbers. (d) Pumping Capacity, Heat Transfer and Bulk Mixing Times The pumping capacity may be estimated for vaned discs of W/D = 0.065 as:

Mixing times will then be of the order of 3 times those expected for standard turbines working in baffled vessels at the same speed. Tangential velocities are similar to those generated by a 2-bladed paddle (W/D = 0.33) and this 'pseudo geometry' should be used for heat transfer calculation, using the method given in GBHE-PEG-MIX-701.



(e) Shear Rates For information on shear rates, consult the GBHE Mixing and Agitation Manual, Sections D4.4 and D4.5. 7 HIGH SPEED UNSHROUDED (SAWTOOTH) AGITATORS 7.1 Duties The major application of saw toothed impellers is in paints manufacture. (a) Solids Dispersion into Liquid A major application of the sawtooth mixer is in the dispersion of 15-65% w/w solids in a liquid resulting in a final batch viscosity of up to 25 N s/m2. The mixers will break flocculated pigments down to the basic particle size in the range 1-3 µm and can thus disperse modern, more loosely aggregated pigments into paint mill base without the need for further milling. They will not, however, break up strongly aggregated or fused particles. No formal methods exist for defining aggregate strengths. It is recommended that the agitator manufacturer be consulted and that small-scale trials are performed in doubtful situations. The mixers can break up large lumps of solid so that even in the more doubtful situations they could be used to prepare a feed for a colloid mill. The mixers are effective at tip speeds greater than 20 m/s; GBHE prefer the range 25- 36 m/s. The recommended D/T ratios range from 0.25 in low viscosity (1 N s/m2) batches to 0.5 in high viscosity (25 N s/m2) applications. Figure 7 shows the preferred range of geometries for the sawtooth disc impellers in a tank. (b) Emulsification The sawtooth mixer is not recommended for liquid mixing duties. It should, however, be effective in blending or emulsification of liquids into concentrated solid dispersions and is recommended for this duty.



(c) Polymer Dissolution The sawtooth mixer is not recommended for polymer dissolution as it can give a stratified batch in polymer systems where viscoelastic forces oppose vortex formation. (d) Filter Cake Dispersion Sawtooth mixers are not recommended for filter cake dispersion. They can suffer from stratification problems and these would prevent the lifting of the undispersed solid from the base of the vessel. FIGURE 7 SAWTOOTH IMPELLER RANGE OF GEOMETRIES (AFTER TORRANCE)



7.2 Equipment Data (a) Scale-Up Using values of D/T and tip speed recommended in 7.1 (a), scale-up from small-scale experiments at constant D/T and tip speed is recommended. The batch mixing time will increase in proportion to the linear scale. (b) Power Figure 8 shows the relationship between Power number and Reynolds number for sawtooth and other high speed impellers in baffled vessels and for a Torrance mixer and a plain disc working in unbaffled vessels. Hydraulic drives are usually fitted and are recommended for application with these limited power data as they cannot be overloaded and allow mixing speeds to be changed in a batch. A Paints manufacturer produced the power data shown in Table 1. The apparent power numbers near 0.1 are because the two of the three mixers are operating in vortex aerated conditions at the highest tip speeds. Using this data they have been able to replace the hydraulic drives with two speed electric motors which are easier to maintain. If overloaded (for example by an increase in velocity which causes loss of vortex) the motor trips to the lower speed. The trip power may then be calculated from Figure 8. This procedure should only be attempted with systems of less than 3 N s/m viscosity and then only in consultation with the manufacturer.



FIGURE 8 MIXER POWER CURVES

(c) Pumping Capacity, Heat Transfer and Bulk Mixing Times An estimate of the radial pumping rate may be made from:

Q = 0.15 N D3 . . . . . . (10) this is about one sixth of that of a turbine with W/D = 0.2. Heat transfer is influenced by circumferential velocities and these in turn are related to the effective vortex depth. The vortex factor, g x/D2N2, is about one third of that of a 2-bladed flat paddle with W/D = 0.33. Approximate heat transfer coefficients could be estimated assuming a standard paddle geometry operating at 1/3 times the speed of the saw toothed impeller.



Scale-up should be on the basis of constant tip speed and bulk mixing times will increase in proportion to the linear scale, i.e. be proportional to volume to the one-third power:

t'' = t' (V'' / V')1/3 . . . . . . (11) (d) Blade Geometry Experience indicates that there is little to choose between the different sawtooth blade configurations offered by the various manufacturers. Sawtooth impellers are more effective than plain or perforated discs in the creation of fine pigment dispersions. We have found that a circular saw blade is as effective as a proprietary device in the dispersion of pigment base. The proprietary devices incorporate blades with a vertical surface oriented circumferentially (along the direction of rotation) and these blades will be more effective in the initial break-up of coarse lumps by impaction. (e) Shear Rates Peak shear rates for sawtooth impellers are caused by shear and elongational flows. To determine the range over which peak shear rates apply, reference should be made to the GBHE Mixing and Agitation Manual, Sections D4.4 and D4.5.



TABLE 1 POWER INSTALLED FOR TORRANCE SAWTOOTH IMPELLERS

Batch Density 1300 - 1440 kg/m3 Viscosity 1 N s/m2



DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE This Process Engineering Guide makes reference to the following documents: GBHE ENGINEERING GUIDES GBH Enterprises Glossary of Engineering Terms

(referred to in Clause 3) GBHE-PEG-MIX-701 Mixing of Miscible Liquids

(referred to in 5.2 (f) and 6.1 (d)) GBHE-PEG-FLO-302 Interpretation and Correlation of Viscometric Data

(referred to in 6.1 (a)) OTHER GBHE DOCUMENTS GBH Enterprises Mixing and Agitation Manual

(referred to in 5.2 (g), 6.2 (e) and 7.2!(e)).

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