ADMIX - Mixing and Blending

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  March 2001 ã PROCESSING’s 2001 Mixing, Blending & Size Reduction Handbook  1   The 2001Mixing,Blending and SizeReduction Handbook High shear mixing advances for foods, pharmaceuticals, cosmetics  p. 2  Mixing scale-up speeds new products to market  p. 5  A Putman PublicationA supplement to  Processing             I           N           S           I           D           E        : The 2001Mixing,Blending and SizeReduction Handbook  2 PROCESSING’s 2000 Mixing, Blending & Size Reduction Handbook  H igh shear mixing equipment has been used inhygienic processing industries over the past twodecades. The equipment that was first developed tohandle these difficult-to-dissolve formulas has con-sistently been improved upon to allow for less wear-ing parts, quicker processing, and easier cleaning.These changes have allowed for even more applica-tions that once required expensive homogenizers, lowflow colloid mills, high upkeep liqui-processors, andother high-powered equipment. Beyond its applica-bility in high viscosity creams and lotions and meattenderizers, high shear mixing is now used withoutmaintenance issues in such areas as the reconstitutionof milk powder, high temperature processes, and thedissolving of abrasive materials, such as TiO 2 . Background High shear mixers are an important tool for dispersing solidsinto liquids,emulsifying immiscible liquids,breaking downsolids and agglomerates,and other applications in which lowspeed agitation is often not sufficient.One of the character-istics that make high shear mixers different from traditionallow-to-moderate speed agitators is the use of high speedmotors,operating from 3600 rpm to as high as 10,000 rpm.High speed mixers typically provide a very high degree of shear and a lowlevel of pumpingand circulation.Shear is producedthrough a combi-nation of hydraulicforces,as the mix-ing head turns atvery high speeds,and by mechanicalforces,as particlessplit when contact-ing the edges of themixing blades,or when forcedthrough slots,portsor orifices. The Evolution of the Mixing Heads of Today The first few high shear mixing heads developed couldbe classified as either an open-disk impeller type or aclosed rotor-stator type.These worked great with respectto shear,but have limitations in other areas.Special impellers such as an open-disk type rely mostlyon hydraulic shear rather than mechanical and cannot beused for large batches without very high energy require-ments.Open-disk mixers utilize a flat disk ranging in diam-eter from 10”to 30”and “saw teeth”on the perimeter.They are designed topromote high shear and particle reduc-tion at moderate cir-culation rates,butthey require veryhigh tip speeds (upto 80 to 90 fps) for total effectiveness.Open-disk highshear mixers havefound wide applica-tion in the paint andcoating industry andrelated markets.Unfortunately,the“saw teeth”are worn down with products containing abra-sives and require replacement frequently.The closed rotating rotor-stator type of mixers relies onsomewhat different technology.The high shear rotor develops a pressure differential immediately below therotor and the process material,causing the product to becontinuously drawn from the bottom of the vessel andinto the mixing head.Once the product enters the mix-ing head,it is forced through the restricted openings of the stator,where mechanical shear causes further particlereduction.Most rotor-stator devices utilize a deflectionplate somewhere above the mixing head to create theproper circulation pattern dictated by the process.Theseunits can operate effectively at lower tip speeds (50 to 60fps) and therefore require less power than open-disk types.They can be difficult to clean and do not provide enoughpumping on their own to mix very viscous products. High shear mixing advancesfor foods,pharmaceuticals,cosmetics High shear disperser/dissolver High shear emulsifier   PROCESSING’s 2000 Mixing, Blending & Size Reduction Handbook  3 The closed rotating rotor-stator mixer has been modifiedto increase the positive aspects of both flow and shear.Thisgeneration of high shear mixers utilized the rotor-stator principle to create high shear,with the additional feature of a revolving stator rather than conventional stationary types.The revolving stator is driven by the fluid movement andis restrained by friction and drag so that it rotates at approx-imately 1/10th to 1/15th the speed of the rotor or motor speed.The revolving stator has extended arms,which dueto the reduced speed,provide high pumping capacity withminimal power requirements.These devices are normallyused in combination with special high flow propellers or impellers mounted below the high shear head to increasepumping while maintaining high shear.This type of head isextremely efficient and very useful in such industries asmeat processing in preparing meat brines and tenderizers.Virtually all of the problems associated with maintenance,cleaning,and hygiene were solved with the development of a new type of mixing head that uses a fixed rotor and stator combined into one part,which does not require bushings.Flow is drawn into the mixing head from above and below,where all materials are immediately mechanically sheared byteeth on the rotors at the top and bottom of the stator.Thetwo high velocity counter current streams converge withinthe stator causing high turbulence and hydraulic shear,with-out momentum loss from obstructions within the stator.Hydraulic pressure forces material to the periphery of thestator where it is subjected to further mechanical shear asmaterial passes through the sharpened slots.The high shear velocity radial discharge combines the slower moving tankflow for additional hydraulic shear and circulation.This newdesign allows for good top to bottom motion within a tank,high shear rates,great particle size reduction,mixing of highviscosity fluids,and large batches with the benefit of CIPcapability and containing no wearing parts. Advancements in Continuous Mixing Not only have advancements been made with high shear batch mixing,but also with high shear in-line mixing.Somein-line mixers consist of an axial flow rotor and stator or aradial flow rotor and stator.For even finer particle sizereduction,there are those featuring a system of interlockingchannels.While all work well with respect to emulsifyingand dissolving,there was room for improvement.Thosewith axial rotors and stators cannot achieve high flow ratesin small sizes and also require shims to maintain the gap dis-tance.Shims present cleaning and assembly issues.Radialflow in-line mixers have higher pumping rates,which mayalso correspond to larger motor requirements.To avoid these problems,a design that combines thedifferent types of mixing has been developed.This designconsists of both an axial stage and a radial stage,allowingfor the benefits of both.The high flow from the radialstage is obtained,but power is kept in check due to theefficiency of the two stages working together.Designadvancements have been made to allow for the elimina-tion of bushings,bearings,wear sleeves,and shims so thatcleaning and assembly are as easy as possible.When com-bined with stainless steel motors,the in-line mixer can beconsidered ultra sanitary. Scale-Up The more high shear batch mixers have been studied andimproved,the better understood the processing problemshave become.Scale-up is one such issue that has oftenbeen ineffectual or difficult to provide.The methods of using geometric similarity and torque per unit volumehave become obsolete with their lack of success.Nowhigh shear mixers are scaled-up by trying to match avariety of properties from the small scale to the large scaleto maximize similarities that define the mixing.High shear mixers are typically sized according to thefollowing:ã Tip speed - the peripheral speed or velocity of themixing head or impeller ã Bulk Fluid Velocity (Intensity) - the pumping capac-ity of the specific mixing head;a measure of how vigor-ous the mixing is within a volume of fluidã Blend Time - time required to turn over the vessel Summary of Typical Operating Conditions Tip SpeedTip Speed ã Open-Disk Impellers80-90 fps ã Closed Rotating Rotor-Stator50-60 fps ã Rotor-Stator w/Revolving Stator40-60 fps ã Fixed Rotor and Stator40-60 fps ã High Shear In-Line60-110 fpsBulk Fluid Bulk Fluid VVelocityelocity ã Low to moderate mixing forrelatively mixable components24-35 ft/min ã Fairly vigorous mixing , sufficient for most applications35-47 ft/min ã Vigorous mixing fordifficult applications47-59 ft/min ã  Extremely vigorous mixingfor very difficult jobs59-88 ft/minBlend Blend TimeTime ã Low concentrations5-10 min ã High concentrations10-15 min  4 PROCESSING’s 2000 Mixing, Blending & Size Reduction Handbook  contents enough times to cause complete,homogenousblendingScale-up can generally be achieved by maintainingequal ratios on all three parameters and typical valuesused are listed below.Scale-up with continuous mixers is done within theenlarging of the mechanical design,and different sizes arechosen based on required flow rates. High Shear Technology in Sanitary IndustriesCosmetics High shear mixers have been present in the cosmeticsindustry for years dealing with the rapid growth of viscous,exotic liquid makeup,face creams,and lipsticks.They wereand still are useful tools in dispersing talc to produce anultrasmooth,lump-free product.The new design improvements allow for the dispersing of metal oxides and abrasives without a worry about having tofrequently replace wearingparts.Titanium Dioxide andCalcium Carbonate can bedispersed for use in lotions,creams,and sunscreens.Any high temperatureprocesses can be done with-out a concern,such as thepreparation of hair gel.Hair conditioner can be madesmoother and more consis-tent than ever,reports onecosmetics manufacturer using a new axial and radial stage in-line mixer.Difficult viscosity building ingredients such asCarbopol™,Methocel™,and many other gums arequickly dispersed and 100% hydrated in room temperaturewater for use in shampoos and other products.These solu-tions can be prepared easily in high concentrations with-out any clumps.A major cosmetic/personal care manufac-turer was able to reduce water phase mix times with poly-mer gums from two hours to 15 minutes with the upgradefrom a low speed agitator to a fixed rotor and stator mixer.The emulsion droplet size reduction that Rotor-Stator mixers are capable of makes them a necessary tool whenpreparing shelf-stable lotions.Other common applicationsinclude hair dyes,liquid mascara,and perm solutionsbecause of the difficult-to-mix ingredients. Food and Beverage One of the most important advancements in high shear technology is the ability to quickly dissolve stabilizing andthickening agents,such as pectin and carrageenin,withconsiderably less power and faster process times.A major tomato company was able to reduce power requirementsfrom 150 hp to 40 hp for paste thin-down applications in750-gallon batches.A confection manufacturer was ableto increase the solid content of a slurry from 78% to 85%and higher with the use of a fixed rotor and stator mixer.Further modifications have allowed high shear mixers toalso be used in the dairy industry.Fixed rotor-stator mixersand sanitary in-line mixers can be used for the reconstitu-tion of milk powder or making sweetened condensed milkwithout bacteria and cleaning concerns.Developments havealso been made to reduce the air incorporation with thoseproducts to produce a uniform product with minimal foam. Pharmaceutical The new developments within high shear have led tomore sanitary equipment,which now meet the require-ments of pharmaceutical facilities.They are a much-wel-comed addition with the ability to process ingredi-ents for ointments,antibiotics,stomach medicine,coatings,etc.Pharmaceutical processing applicationsusing Aluminum Hydroxide and Zinc Oxide can bemixed smoothly.Not only can colloid mills and low speed agitatorsbe replaced for faster mixing times and less power,butalso homogenizers and other,less effective high shear mixers.A major pharmaceutical manufacturer was ableto go from having a high shear and low speed agitator in a vessel,to just one fixed rotor and stator high speedmixer.This head has such excellent pumping capabili-ties that it can mix even the thickest fluids by itself. Other Applications High shear equipment can also be used for non-sanitaryapplications.Rotor-stator units have been successfullyapplied in the addition of high viscosity,difficult-to-dis-perse polymer additives,used to enhance the elasticity of asphalt and rubber compounds.Rotor-stator units are also being used within thepetroleum industry when difficult-to-wet-out polymersare added into invert mud for improved drilling of oilwells.In addition,dramatic results have been reported intertiary oil recovery operations where high shear tech-nology is employed to disperse additives into oil fields.The new high shear devices can be applied in any pro-cessing area desiring lower power requirements,faster mix times,less maintenance,and product consistency.  Article written by Gretchen Ames, Applications Engineer, Admix,Inc. How it works 
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