Patterson Kelle;Process Equipmei

Solids Processing

Fundamentals ofBulk Solids Mixingand Blending

Leam about mixing teciinoiogjn types of biondingequipment and i(ey sampiing practices to meet

today's requirements for robust processes

Eric MaynardJenike and Johanson

Mixing and blending of bulksolids is a common process-ing step in many indus-tries. For example, in phar-maceutical manufacturing of soliddosage formulations (tablets or cap-sules), small amounts of a powderedactive drug are carefully blended withexcipients, such as sugar, starch, cel-lulose, lactose and lubricants. Withfoods, many powder-form consumerproducts result from custom mixedbatches; consider cake mix, ice teaand dry seasonings. Thousands of pro-cesses in the chemical process indus-tries (CPI) involve mixing or blendingof specialty chemicals, explosives, fer-tilizers, glass or ceramics, detergentsand resin compounds.

Today's production operations re-quire robust mixing processes thatprovide fast blend times, recipe flex-ibility, ease of equipment cleaning forminimizing grade change-over time,and assurances that de-mixing (seg-regation, for example) does not resultwith a blended material [7].

Over the past two decades, mixingand blending technology has greatlyimproved to address needs for largerbatch sizes, faster blend times andsegregation minimization. Thoughmany blenders are capable of mix-ing all kinds of powders, the processof selecting a blender remains an "artform" because of the many variablesinvolved. There are many types of sol-ids blenders available, and while oneblender may have a lot of flexibility.

others may be highly specialized for adifficult blending application. Knowl-edge gains in the area of samplingand segregation have allowed a moreholistic approach to the typical blend-ing unit operation, thereby often pre-venting problems with the uniformlyblended material once it has been dis-charged from the mixer.

This article provides an overview ofbasic powder-blending technology andsampling considerations.

Mixing versus blendingThe terms "mixing" and "blending" canbe synonjonous to some, however, theytechnically can be considered slightlydifferent. Mixing is defined as the pro-cess of thoroughly combining differentmaterials to achieve a homogenousmass. In most cases, the mixture isa combination of dissimilar materi-als (such as polyethylene pellets andblack pigment to make trash bags)using significant agitation. A mix canalso be made with a chemically homog-enous material that requires uniformdistribution of its particles.

Blending, like mixing, is an actof combining materials. This opera-tion, however, usually occurs in agentle fashion with multiple compo-nents (such as blending fertilizer in-gredients without generating fines).For the scope of this article, we willuse the terms mixing and blendinginterchangeably.

The goals of producing an accept-able blend, maintaining it through ad-

FIGURE 1 . The tumbler blender comesin a V-shaped configuration

ditional handling steps, and verifyingthat both the blend and the finishedproduct are sufficiently homogeneouscan be difficult to achieve on the firstattempt. The costs for troubleshootinga poorly performing blending systemcan far outweigh the initial invest-ment costs. For example, an inad-equate blend or segregation of a phar-maceutical "blockbuster drug" cancause the batch to fail, which couldlead to costs in the millions of dollars,even though the equipment used toblend and transfer the powder can bea small percentage of this cost.

Batch versus continuousBlenders come in all shapes, sizes, ar-rangements and modes of operation,but they fit into one of two categories:batch or continuous.Batch blending. A batch blendingprocess typically consists of three se-quential steps: weighing and loadingblend components; mixing; and dis-charge of the blended product.

In a batch blender, solids motion isconfined only by the vessel, and di-rectional changes are frequent. Theretention time in a batch blender iscarefully controlled, while for a con-tinuous blender, this is generallynot the case. Blending cycles cantake from a few seconds with high-intensity units to 30 min or morewhere additional processing, such asheating or cooling, may be involved.Blender discharge may be rapid ortake substantial time, particularly ifthe blender is used as a surge vesselto feed a downstream process. Ideally,a blender should not be used for stor-age capacity, because this can createa process bottleneck, given that theblender cannot perform operationsof storage and blending concurrently.Batch blenders [2\ are often used inthe following situations: When quality control requires strict

batch control6 6 CHEMICAL ENGINEERING WWW.CHE.COM SEPTEMBER 2013

TABLE 1 . TYPICAL BLENDER FEATURESBlender

Ribbon,plowTumble

In-bintumblerPlanetary

FluidjzedHighshear

Typicalcapocity

30-28,000 L(1-1,000 ft3)15-5,000 L(0.5-175 ft3)750-3,000 L(25-100ft3)30-28,000 L(1-1,000 tt3)2,800-85,000 L(100-3,000 ft3)30-10,000 L(1-350 ft3)

Typicalspeed

15-100 rpm

5-30 rpm

5-30 rpm

15-100 rpm

0.03-0.33 m/s(0.1-1 ft/s)Tip > 3 m/s(600 tt/min)

Powerrequired

High

Moderate

Moderate

Moderate

Low

High

Lumpbreaking

Good

Poor

Poor

Good

Poor

Excellent

Jacketvessel

Yes

Difficult

Difficult

Yes

Yes

Yes

Ability toadd liquid

Yes

Difficult

Difficult

Yes

Yes

Yes

TABLE 2. BLENDER COMPARISONSBlender

Ribbon, plowTumbleIn-bin tumblerPlanetaryFluidizedHigh shear

Range ofmaterialsWideModerateModerateModerateNarrowModerate

Can handle co-hesive materialsYesWith intensifierWith intensifierYesNoYes

BlendingtimeFastLongLongModerateFastFast

Easy tocleanModerateYesYesModerateYesModerate

GentleblendingModerateYesYesYesModerateNo

If ingredient properties change overtime

When the blender cannot be dedi-cated to a specific product line

When production quantities aresmall

When many formulations are pro-duced on the same production line

Major advantages of batch over contin-uous blending include the following: Lower installed and operating costs

for small to medium capacities Lower cleaning costs when product

changes are frequent Production fiexibility Pre-blending of minor ingredients is

easily accomplished Control of blending timeContinuous blending. In a continu-ous blending process, the weighing,loading, blending and discharge stepsoccur continuously and simultane-ously. Blending occurs during trans-port of the material fi-om the in-feedpoint toward the mixer outlet. Unlikebatch blenders where product reten-tion time is carefully controlled, ma-terial retention time with continuousblenders is not uniform and can bedirectly affected by blender speed, fee-drate, blender geometry and designof internals. Continuous blending [2]is typically used under the followingconditions: A continuous, high production rate

process is required Strict batch integrity is not

essential Combining several process streams Smoothing out product variationsSome of the advantages of a continu-ous blending system are the following:

Ease of equipment integration intocontinuous processes

Less opportunity for batch-to-batchvariation caused by loading errors

Automation can improve qualityand reduce labor costs

Higher throughputs are oftenpossible

Blending mechanismsThere are three primary mechanismsof blending, namely: convection, diffu-sion and shear. Convectiue blendinginvolves gross movement of particlesthrough the mixer either by a forceaction from a paddle or by gentle cas-cading or tumbling under rotationalmotion. Diffusion is a slow blendingmechanism and will pace a blendingprocess in certain tumbling mixersif proper equipment fill order andmethod are not utilized. Lastly, theshear mechanism of blending involvesthorough incorporation of materialpassing along high-intensity forcedslip planes in a mixer. Often thesemixers will involve dispersion of a liq-uid or powdered binder into the blendcomponents to achieve granulation.

Achieving a uniform blend is thegoal of any industrial process involv-ing mixing, and defining uniformitystrongly depends upon the scale ofuniformity. For instance, loading twocomponents into a tumble blenderdoes not guarantee blend uniformityacross the range of sample sizes. Ifthe entire quantity in the blenderwere analyzed, then uniformity maybe present. However, taking smallersamples fi-om either side of theblender will result in substantial dif-

FIGURE 2. With tumbling in-bin blend-ers, the storage container itself be-comes a blender

ferences, which clearly does not meetuniformity requirements.

Think of a tumble blender contain-ing a side-by-side loading of salt andpepper. Perhaps after 20 revolutionsof the blender, the salt remains pre-dominantly on the left side while thepepper resides on the right side of theblender. Though diffusion has allowedsome intermixing, in general, thereis a large-scale non-uniformity inthe blender that indicates additionalblend time is required. As sample sizeis reduced, even with a good blend ofsalt and pepper, there is a chance thatrandom selection will yield some sam-ples mostly composed of salt and oth-ers of pepper. This example illustrateswhy it is important to collect samplesizes representative of the final prod-uct size when evaluating uniformity.

There are two types of blend struc-tures: random and ordered. A randomblend occurs when the blend compo-nents do not adhere or bind with eachother during motion through the blendvessel. In this case, dissimilar par-ticles can readily separate from eachother and collect in zones of similarparticles when forces such as gravity,airflow or vibration act on the blend.An example of a random blend is saltand pepper.

More commonly, ordered or struc-tured blends, result in most industrialprocesses. This occurs when the blendcomponents interact with one anotherby physical, chemical or molecularmeans and some form of agglomera-tion or coating takes place. The pro-cess of granulation involves this ap-proach, whereby larger particles arecreated from smaller building-blockingredient particles, and each "super"particle has ideally the correct blenduniformity. A blend of perfect super

CHEMICAL ENGINEERING WWW.CHE.COM SEPTEMBER 2013 6 7

Solids Processing

particles of identical size will notsegregate after discharge from theblender, which is clearly an advantageover a random blend. However, if theseparticles are not mono-sized, then seg-regation by size may occur and induceproblems with bulk density, reactivityor solubility in post-blend processing.

A word of caution regarding blendstructure: There are cases where someingredients have a tendency to adhereonly to themselves, without adheringto dissimilar ingredients. This oftenhappens with fine materials, such asfumed silica, titanium dioxide andcarbon black. At times, a blend canreach "saturation," where minor finecomponents will no longer coat largerparticles, and concentrations of thefine component will build (and seg-regate from the blend). Fortunately,some blender manufacturers have rec-ognized this problem and have devel-oped technology, like chopper bladesplaced in dead-zone locations, to miti-gate its harmful effects.

Types of blendersThere are four main types of blend-ers: tumbler; convective; hopper; andfluidization. A general description ofeach blender type, including its typi-cal operation and possible concernsfollows. Tables 1 and 2 also providean at-a-glance feature comparison foreach blender.Tumhler. The tumble blender is amainstay in the pharmaceutical andfood industries because of its posi-tive attributes of close quality control(batch operation only), effective con-vective and diffusive mechanisms ofblending, and gentle mixing for par-ticles prone to breakage. This type ofrotating blender comes in double-coneor V-shaped (Figure 1) configurations,and in some cases, these geometriesare given asymmetric features to re-duce blend times and improve blenduniformity. Typical tumble blenderfeatures, speeds and capacities aregiven in Table 1.

Rotational speed is generally notas much of a factor on achieving uni-formity as loading method and blendtime (number of rotations). Thoughthere is no proven method of calculat-ing required blender run time, there isa preferred loading method for tumble

blenders, especiallywith symmetric ge-ometries. A top-to-bottom componentloading is betterthan a side-to-sideloading. In this case,ingredients are al-lowed to cascadeinto one another with diffusive effectsoccurring perpendicular to the mainflow. This approach 3delds far fasterblend times than side-to-side loading.

It is also important to prevent in-gredient adherence to the walls of theblender. This is common with fine ad-ditives, such as pigments and fumedsilica. Component loss can occur withthe blend if the material does not leavethe wall surface. In some cases, thesticky ingredient can be pre-blendedinto another component (called mas-ter-batching) to help pre-dispersethe material and avoid wall adher-ence. It is also important to considerblend cohesiveness, which directlycorrelates to a material's tendency toform a bridge over the blender's out-let. Highly cohesive blends shouldnot be handled in tumble blenders ifbridging or ratholing flow obstruc-tions have been experienced in pastprocessing equipment. Additionally,cohesive material mixing in a tumbleblender takes significant time, usuallyrequires an internal agitator (calledan intensifier), may not achieve inti-mate mixing, and thus may not be themost suitable equipment.In-bin tumbler. To reduce blendingprocess bottlenecks and segregationpotential, tumbling in-bin blenders(Figure 2) have been developed wherethe storage container itself becomesa blender. Blend components can beloaded into the container, blended andtransferred in the container to point-of-use or to a storage area. This pro-cess leads to highly flexible productionand has been popular in the pharma-ceutical, food and powdered metalindustries. Typical in-bin blender fea-tures, speeds and capacities are givenin Table 1.

In-bin tumble blending is likelythe foremost solids-mixing technol-ogy improvement that has occurredin the past 25 years [3]. The great-est benefit of this technology is its

FIGURE 3. Paddle (left) and ribbon (right) blenders areconvection-type units

elimination of a transfer step from ablender into a container, by which seg-regation by various mechanisms canresult. Additional benefits include:no cleaning between batches; and theblend is stored in a sealed containeruntil use. Optimum in-bin tumbleblenders incorporate mass-fiow tech-nology (all of the material is in mo-tion whenever any is discharged) toensure the blend does not segregateduring container discharge.Ribbon, paddle, plow. Convectionblenders use a fixed U-shaped or cy-lindrical shell with an internal rotat-ing element (impeller) like a ribbon,paddle, or plow (Figure 3). Due to theaction of the impeller, the particlesmove rapidly from one location to an-other within the bulk of the mixture.The blending action can be relativelygentle to aggressive, depending on theagitator design and speed and the useof intensifiers (choppers).

Ribbon and paddle blenders tend tocreate cross-wise, recirculating cut-ting planes within the vessel to allowrapid mixing at an intimate unifor-mity level. With fine powder mixtures,the action of the ribbons induces anear fiuidized state with minimal in-terparticle friction, thereby allowingfast blend times.

Plow blenders operate slightly dif-ferently. The main plows divide thepowder bed and have back-side plowsthat fold in the remaining powderbehind the main plow segments.This effectively blends highly cohe-sive materials without inducing par-ticle breakage. Additionally, the plowblender is renowned for having mini-mal dead zones since the clearance be-tween the plows and the blender shellis very small. Ribbons and paddles, onthe other hand, tend to have largerdead zones due to the requirement forthe clearances to be bigger.

The convective blenders work wellwith cohesive materials, which nor-

6 8 CHEMICAL ENGINEERING WWW.CHE.COM SEPTEMBER 2013

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FIGURE 4. Tube-typebienders are weli-suited forfree-fiowing, granuiar solidsmixing

FIGURE 5. A conicaiscrew, or Nauta-type mixeris commonly used for co-hesive powder biending

mally take substantially longer blendtimes in tumbling-type mixers. Theyalso have the advantages of takingup less headroom, allowing liquid ad-dition, heating and/or cooling, andpotential for continuous operationinstead of only batch mixing as withtumble blenders. Also, these blend-ers are less likely to experience blendsegregation during discharge becausethe impellers typically operate dur-

ing this process. Typi-cal convective blenderfeatures, speeds andcapacities are given inTable 1.Hopper. Hopper blend-ers are usually cone-in-cone to tube-type units,where particles flowunder the influence ofgravity in a contact-bedwithout moving parts(attractive for highlyabrasive bulk materials

given their wear potential). With theformer unit, the inner cone producesa pronounced faster flow through theinner hopper as compared to the outerannulus section, thereby allowing mod-erate blending of material. These hop-pers typically require two to four passeswith a recirculation system to achieveproper uniformity. Tube blenders (Fig-ure 4) utilize open pipes within a bin;the pipes have notches in them to allow

pellet or granular material to partiallynow in and out of the tubes over theheight of the bin, or for reintroductioninto a lower portion of the bin (such asin a mixing chamber).

These blenders can handle muchlarger volumes of material than tum-bling or convective blenders, since nofree-board space is required, and theirtechnology can be applied to storagebins or silos. Typical gravity-flow hop-per blender features and capacitiesare given in Table 1.Planetary. Another type of hopperblender, called a planetary or conical-screw mixer (Figure 5), is commonlyused for cohesive powder blending.The planetary screw is composed of anear-vertical screw conveyor inside aconical hopper. The screw is located sothat one end is near the apex of thecone and the other end is near thetop of the hopper, with the tip of theflights near the wall of the hopper. Thescrew rotates while revolving around

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Dynamic Air

Solids Processing

the walls of the hopper, pullingmaterial up from the bottom.Advantages include the abilityto handle a wide range of mate-rials, from free-fiowing to highlycohesive.

Potential concerns with thisblender include possible segrega-tion during blend discharge and adead region at the bottom of the coneduring blending. These blenders arecommonly jacketed for heating and/orcooling of a material during the blendcycle. Typical convective blender fea-tures, speeds and capacities are givenin Table 1. The Nauta-type blendercan also be fitted with a verticallyoriented ribbon blender, though thereare limitations on its capacity giventhe high level of operating torque andhorsepower.Fluidization. Fluidization mixers(Figure 6) use high fiowrates of airor inert gas to fully fluidize powdersin order to rapidly blend components.The gas can also be used to process(heat or cool) the blend. Not all pow-der blends are well-suited for fiuidiza-tion mixing. Ideal candidates are fine,free-fiowing powders that have a nar-row size distribution and are close inparticle density. Highly cohesive pow-der blends may experience channelingand non-uniform blend quality.High shear. These mixers (Figure 7)combine fluidization and convectivefeatures, yielding rapid blend timeswith a high degree of blend uniformity.This type of blender consists of twincounter-rotating paddled agitatorsthat mechanically fluidize the ingredi-ents. Rotation is such that the blendis lifted in the center, between the ro-tors. Mixing is intensive, producing in-timate blends in a short period of time.Blend cycles are often less than a min-ute, and "bomb-bay" doors allow rapiddischarge of the entire blend. Thesefeatures combine to give this blendera high throughput capacity relative toits batch size, and highly cohesive ma-terials can be readily blended.

In another type of high-shearblender, a rapidly rotating impel-ler with integral choppers createshigh-intensity blending. The impel-ler clearance is very small to avoidblender dead zones. This type ofblender is routinely used for blend-

FIGURE 6. Fluidizationmixers rapidly blend com-ponents using high gasfiowrates

ing highly cohesive powders and foragglomeration processes, such as themanufacture of dry laundry deter-gent. Rapid blend times are commonwith this type of mixer.

Sampling of blendsEffective sampling is essential in de-termining the state of the blend in amixer and in downstream equipment,such as a bin, hopper or packagingsystem. To achieve a high level of con-fidence in the quality of the samplesextracted from a process, considerthese five points regarding sampling(see Refs. 4 and 5 for good technicalarticles on sampling).1. A perfect blend does not guaran-tee uniform product. Consider thatevery time a transfer step occurs in apowder handling process, the mix orblend has the potential to segregate.Common segregation mechanisms [6]occurring during industrial powderhandling applications include sifting(Figure 8), fluidization and dusting.Depending upon which mechanism ofsegregation occurs, the fine and coarseparticles will concentrate in differentlocations in the bin or hopper, thusrendering location-specific samplingresults. Sample at each piece of equip-ment that the powder has transferredinto to evaluate if segregation has re-sulted due to powder transfer.2. Beware of thief. A sample thief iscommonly used to collect powder sam-ples from a stationary bed of materialin a blender, drum or bin. A thief is ametal rod with recessed cavities ca-pable of receiving powder after beinginserted into a powder bed. Care mustbe made with thief-collected samples,because this method will disturb thepowder sample in-situ and some com-ponents may or may not fiow into orstick to the thief cavity. Numerousstudies have shown that thief sam-pling results can be dependent onoperator technique (such as thief in-

FIGURE 7. These high-shear mixers combinefluidization and convectivefeatures Dynamic Air

sertion angle, penetrationrate, angle and twisting). Iam not proposing that thief samplersbe abandoned. Rather, I suggest thatthe resulting data be carefully scruti-nized and observations (for example,static cling, agglomeration and smear-ing) of the thief cavity and extractedpowder sample be recorded.3. Use stratified sampling. Improvethe quality of thief sampling with astratified (nested) approach and sta-tistical analysis to differentiate blendvariability from sampling error (fromthe thief, laboratory analysis or col-lection method). Instead of samplingonly once from a given location in ablender, multiple (minimum of three)thief samples should be extracted fromthe same location. This should then berepeated throughout several distinctlocations, especially in known "dead-zones" like at the blender walls. Afteranalysis of these samples, assess-ments can be made to within-locationversus between-location variabihty.If the three samples collected at thesame point have large variability, thenquestions should be raised regardingthe thief or analytical testing method.If large variability exists between thesamples collected around the blender,then it is likely that the blend is notyet complete and additional time oragitation will be required; it is alsopossible that segregation may haveoccurred within the blender due toover-blending. Nested sampling is alsoeffective for thief sampling of bins,hoppers, drums or other vessels con-taining the bulk-solid mixture.4. Collect full-stream samples.Consider an alternative sampling ap-proach, such as full-stream samplingduring blender discharge. This tech-nique provides a true "snapshot" ofblend uniformity exiting the blenderand overcomes many of the pitfallscommon to the sample thief. If a full-size sample is extracted, it may re-

7 0 CHEMICAL ENGINEERING WWW.CHE.COM SEPTEMBER 2013

FIGURE 8. Sifting Is a common segre-gation methodquire reduction in size for analysis.In this case, a rotary sample splitter also called a rotary or spinning rif-fler should be used to properly dis-tribute fine and coarse particles to thereduced sample quantity.5. Handle collected sample care-fully. Ideally, use the entire collectedsample for analysis. However, inmany cases, the gross sample will berequired to be split down to a smaller

size for the analysis (such as chemi-cal assay, pH and particle size). Forexample, imagine that a 500-g sampleis collected from a hopper, and it seg-regates in the sample container. If thelaboratory technician then collects asmall 5-g grab sample for analysis,this smaller sample may not repre-sent the true particle size distribu-tion of the entire sample, and errorresults. In this case, a sample splitter,such as a rotary riffler, can be usedto accurately reduce the sub-samplesize. Avoid using error-prone split-ting methods like cone and quarter or

References1. Carson, J. and Purutyan, H., Predicting, Di-

agnosing, and Solving Mixture SegregationProblems, Powder and Bulk EngineeringMagazine, Voi. 21, No. 1, January 2007.

2. Clement, S. and Prescott, J., "Blending, Seg-regation, and Sampling", Encapsulated andPowdered Foods, C. Onwulata, Ed. (Taylor &Francis Group, NY), Food Sciences and Tech-nology Series Vol. 146,2005.

3. Maynard, E., A Retrospective of Mixing &

Blending Over the Past 25 Years, PowderBulk Solids Magazine, June 2007.

4. Trottier, R., Dhodapkar, S., Sampling Partic-ulate Materials the Right Way, Chem. Eng.,pp.42-49, April 2012.

5. Brittain, H., The Problem of Sampling Pow-dered Solids, Pharmaceutical Technology, pp.67-73, July 2002.

6. Williams, J., The Segregation of ParticulateMaterials: A Review, Powder Technology, Vol.15, 1976.

chute riffling. Additionally, samplescollected over time and combined intoa composite sample can only tell youat-best what is the quality of materialover that period. Furthermore, if thecomposite sample is not well-mixed,sampling bias can result.

Edited by Dorothy Lozowski

AuthorEric Maynard is the directorof education and a senior con-sultant at Jenike & Johanson,Inc. (J&J; 400 Business ParkDr., Tyngsboro, MA 01879;Phone: 978-649-3300; Fax978-649-3399; Email: epmay-nard@jenike.com; Website:www.jenike.com). The firmspecializes in the storage,flow, conveying, and process-ing of powders and bulk sol-

ids. During his 18 years at J&J, Maynard hasworked on nearly 500 projects and has designedhandling systems for bulk solids includingchemicals, plastics, foods, Pharmaceuticals, coal,cement, and other materials. He is the principalinstructor for the AIChE courses "Flow of solidsin bins, hoppers, chutes, and feeders" and "Pneu-matic conveying of bulk solids." He received hisB.S. in mechanical engineering from VillanovaUniversity and an M.S. in mechanical engineer-ing from Worcester Polytechnic Institute.

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