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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
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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
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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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Young Industries Hosokawa Bepex
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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CHEMICAL ENGINEERING WWW.CHE.COM SEPTEMBER 2013 6 9
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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
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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: [email protected]; 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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