Solve Probs Retrofit Patrones de Flujo

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Solve Solids Handling Problems by Retrofitting1

by

Herman Purutyan,

Brian H. Pittenger

and

Dr. John W. Carson

1Reprinted with permission from Chemical Engineering Progress, June, 2001. Copyright 1998 American Institute ofChemical Engineers. All rights reserved. Not to be uploaded to any other site without written permission from the AIChE.Individual downloads are permitted so long as a fee of $15 per article is paid directly to the Copyright Clearance Center, 222

Rosewood Drive, Danvers, MA 01923.

Most flow problems are caused by a funnelflow pattern and can be cured by altering the

pattern to mass flow, which requires

changing the hopper or feeder design. Other

methods to fix poor flow include mechanical

and chemical flow aids.

In plants and processes involving solids,ineffective and unreliable handling systems areoften a primary cause of startup delays, process

inefficiencies, and equipment downtime. This isa major problem, and its extent was confirmedby a six-year study by the Rand Corp. of 40solids processing plants in the U.S. and Canada[1]. The findings revealed that 80% of all plantsstudied experienced solids handling problems.Also, the affected facilities were slow in comingup to speed, with an average startup time for

some types (raw, unprocessed solids feedstock)

approaching 18months. Once startup began,poor performance continued to plague theseoperations with capacity ranging between 40%and 50% of design values.

Although considerable engineering resourcesare usually allocated to processing concerns,(e.g. reaction chemistry), design of materialhandling systems to prevent problems rarelygets much attention. It is not uncommon tofind, for example, a critical centrifugation step

beset with problems because of a frequentlyplugging transfer chute, or costly downtime (ofthe centrifuge and the whole process) due to aplugged silo.

Many, if not all, of these problems can beavoided by basing the design of solids handling


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systems on the flow properties of the bulkmaterials being handled and processed throughthem. However, in many cases, such systemsare designed using prior experience as a basis,which unfortunately usually only points to what

does not work instead of what works best.Other times, the design of the solids handlingsystem is left to the end of the process design,and fitted into the space left over from the restof the processing equipment.

What if the equipment is already in place andthe process is limping along? Although it ismuch easier to prevent solids handling problemsby using sound design methods at the outset,there are a number of effective retrofits that can

significantly improve equipment performance.Retrofitting is often more economical thancompletely replacing the faulty equipment, orliving with the problem. The exact choice of aretrofit will obviously depend on the nature ofthe problem, process, product, and constraints.A broad overview of retrofit options isdiscussed in this article, following a discussionof common problems encountered whenhandling bulk solids.

Common flow problems

While handling problems can be encountered ina variety of equipment, the most common occur

in silos.

(The terms silo, bin, tank, vessel, and bunker areinterchangeable. Silo will be used throughout

this article to represent such equipment. Hopperis the portion of a silo in which the cross-section

changes, such as a cone, wedge, pyramid, or

other shape.)

The problems in silos consist of:

No-flow from a silo is the most common andoften the most serious solids handling problem.It can be due to either arching (bridging) orratholing.

Arching occurs when an obstruction in theshape of an arch or a bridge forms above theoutlet and prevents any further materialdischarge. It can be an interlocking arch, wherethe particles mechanically lock to form theobstruction, or a cohesive arch. An interlockingarch occurs when the particles are largecompared to the outlet size of the hopper. Acohesive arch happens when particles pack

together to form an obstruction (Figure 1).

Ratholing can occur in those silos where flowtakes place through a channel that forms withinthe material. If the material has sufficientcohesive strength, once the channel is emptied,no more discharges (Figure2).

Erratic flow is the result of obstructionsalternating between an arch and a rathole. Arathole may collapse due to an external force,

such as vibrations created by a train or a fork-lift operating nearby, or a flow-aid device suchas an air cannon or a vibrator. While somematerial is likely to discharge, falling materialoften impacts over the outlet and forms an arch.An arch may break due to a similar externalforce, and material flow may resume until the

Figure 1. Interlocking and cohesive arches are commonproblems in silos.

Interlocking arch Cohesive arch


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flow channel is emptied and a rathole is formedagain.

Flooding/flushingcan occur when handling finepowders, such as pigments, additives, orprecipitates. When a rathole collapses, thefalling particles entrain air and becomefluidized. Since most solids-handlingequipment cannot handle fluids, material floods

through the feeder uncontrollably. The solidsbulk density can often undergo dramaticvariations, negatively impacting on downstreampackaging or processing equipment.

Flooding can also occur when handling finepowders (100 mm or smaller) in small vessels

with high fill and discharge rates. In suchsituations, the powder does not have sufficientresidence time to deaerate, resulting in floodingthrough the feeder.

Flow rate limitationis another problem uniqueto fine powders. Most fine powders have verylow permeabilities and are affected by anymovement of interstitial air (or other gases).The pressure gradients caused as a result of thisgas movement can retard discharge from a

hopper, significantly limiting the maximumachievable rates.

Particle segregation can occur both whenhandling blends consisting of a number of

ingredients, and when handling a homogenoussubstance with a wide variation in particle sizeor density. Segregation or de-mixing ofingredients may prevent a desired chemicalreaction, may cause an undesired reaction, ormay cause out-of-spec product, resulting incostly product waste or rework. Segregation offines from the coarse may also have similareffects [2] (Figure 3).

Associated flow problems

The following are some results of the flowproblems described above:

Limited live capacity Although the liquidvolume of the silo may be sufficient, its live(usable) capacity can be severely limited due tostagnant material inside. If a rathole forms, thenthe live capacity of the silo effectively becomeslimited to the size of the rathole (see Figure 4);

Product degradation When allowed toremain stagnant, some materials may cake,oxidize, spoil, or otherwise change physically orchemically;

Figure 2. Ratholing can occur in those silos where flowtakes place through a channel that forms within thematerial.

Rathole

Stagnant

Figure 3. Sifting segregation can result in costly productwaste or rework.


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Incomplete or non-uniform processing In

processing vessels where the product may bedried, cooled, reacted or purged of volatiles,flow problems give rise to incompleteprocessing which may severely reduce processefficiency, result in quality problems, or evenlead to dangerous situations, such as whencombustible volatiles are only partially removedfrom the product;

Vibration of structure Material flow throughsilos can result in significant vibrations, which

can be either high-frequency/low-amplitude orlow-frequency/high-amplitude [3]. If notaddressed, these vibrations can result in noisethat violates U.S. Occupational Safety andHealth Administration (OSHA) regulations, orworse, failure of the silo structure, or nearbyequipment;

Structural failure Structural failures canoccur when large masses of material fall andimpact on the outlet of the silo and the feeder,

for example as a result of collapsing arches andratholes. In addition, silo walls can dent orwrinkle as a result of uneven loads imposed byasymmetric flow channels. These dents anddistortions can significantly weaken a silo andcan eventually lead to catastrophic failure [4,5];and

Excessive power requirement Large amountsof stagnant material over a feeder cansignificantly increase the power required tooperate the feeder, in addition to contributing to

some of the other problems already discussed.

Flow patterns

Most of the problems discussed above occur insilos where flow takes place through a channelformed within the bulk solid. This describes afunnel flow pattern, in which some materialmoves while the rest remains stationary duringdischarge from the silo (Figure 5). Funnel flowoccurs when the sloping hopper walls of a silo

are not steep enough and smooth enough forparticles to flow along them. Particles slide onthemselves rather than the hopper walls, and aninternal flow channel develops.

Funnel flow silos often have flat bottoms orvery shallow hopper angles. They may be used

Figure 4. A rathole limits the live capacity of a silo.

Figure 5. Funnel flow occurs when the sloping hopperwalls of a silo are not steep enough and smooth enough forparticles to flow along them.

oving

tagnant


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reliably, provided that all of the following fourconditions are met:

1. Material handled consists entirely of coarseparticles usually 1/4in. or larger;

2. Material is free flowing particles do notstick to each other;

3. Particles are non-degrading spontaneouscombustion, spoilage, aging, or caking does notoccur when particles are stagnant for extendeddurations; and

4. Particle segregation is not a concern.

Unless all of these four conditions are met, flowrelated problems will occur in funnel flow silos.To prevent such problems, a mass flow siloshould be used.

Mass flow is defined as the flow pattern whereupon withdrawal of any material, all of thecontents of a silo move (Figure 6). Mass flowoccurs when sloping hopper walls are smoothenough and steep enough for particles to slide

along them. This type of flow eliminatesratholing and associated problems of floodingand stagnant material, as well as maximizes theusable capacity of the silo.

A third flow pattern, expanded flow, is acombination of mass flow and funnel flow. Anexpanded flow silo consists of a lower sectionthat is in mass flow and an upper section that isfunnel flow (Figure 7). It is typically used toovercome ratholing in large silos by using the

mass-flow section to enlarge the flow channeldiameter such that a rathole cannot form. Touse expanded flow, the material must be non-degrading, segregation must be unimportant,and the particles must be coarse enough not to

Figure 6. In mass flow, all of the contents of a silo moveupon withdrawal of any material.

Figure 7. An expanded flow silo consists of a lowersection that is in mass flow and an upper section that isfunnel flow.

Funnel

flow

Mass

flow

Exceeds critical

rathole diameter


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become fluidized. Unless these three conditionsare met, expanded flow should not be used.

RETROFIT OPTIONS

The most cost-effective retrofit option willdepend on the flow properties of the materialand the types of constraints that are imposed onthe design. In general, retrofit options consist ofthe following:

Hopper modifications;Use of inserts;Feeder modifications;Flow aids (mechanical and chemical);Air permeation systems; andFluidization.

Hopper modifications

If the problem is caused by funnel flow,modifications to the hopper to convert it to massflow have high probability of success. Theproblems that can be solved by this approach areflow stoppages or erratic discharge due toarching and ratholing, flooding or flushing offine powders, certain types of segregation,

incomplete or non-uniform processing, silovibrations, and structural problems.

To achieve mass flow, two conditions must bemet as a minimum:

1. The sloping walls of the hopper must be steepenough and must have sufficiently low frictionto allow particles to flow along them; and

2. The hopper opening must be large enough to

overcome arching.

Flow along walls

One or more of the following modifications tothe hopper may be considered to achieve flowalong hopper walls: change inner hoppersurface, the hopper slope, or the hoppergeometry.

Before modifications are made, the flow

properties of a material must be determined.With respect to flow along hopper walls, thecritical property is friction that developsbetween the bulk solid and the hopper wallsurface (wall friction). Wall friction of amaterial can be easily measured in a laboratoryusing a Jenike Shear Tester (Figure8), a smallsample of the bulk solid, and various wallsurfaces.

The test is conducted by first placing the bulk

solid in a retaining ring on a flat coupon of wallmaterial. Then, various normal forces areapplied by placing weights on the cover.Material in the ring is pushed along thestationary wall surface, and the shear force ismeasured as a function of applied normal force.

Figure 8. A Jenike Shear Tester determines friction between a solid and a wall surface (uncoated or coated).

Shear Stress =Force/Area

Ring

Cover

Sample of Wall MaterialBulk Solid

Bracket

Normal Pressure = Weight/Area


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A typical wall friction test result is shown inFigure 9. The applied normal pressures areplotted on the horizontal axis, and the measuredshear values on the vertical axis. Wall frictionangle, f',is then defined as the angle formed by

drawing a straight line from the origin to a pointon the curve. f' is another way of expressing

the coefficient of friction ( = tan f').

Once the wall friction angles have beenmeasured, hopper angles for mass flow can be

determined using a series of design chartsoriginally developed by Jenike [6]. A typicaldesign chart for a conical hopper geometry isshown in Figure 10. This is an example chartand should not be used for design. Completesets of design charts are given in Ref. 6.

In Figure 10, the hopper angles from vertical,qc, are plotted along the horizontal axis. The

wall friction angles, f', are on the vertical axis.

The chart contains three regions: mass flow,funnel flow, and an uncertain region (which is,in fact, a margin of safety).

When considering retrofitting an existinghopper, the wall friction tests should beconducted on a number of candidate wallsurfaces, which may include liners such as ultra-

high molecular-weight polyethylene, smooth orpolished stainless steel sheets, or coatings suchas various epoxies, polytetrafluoroethylene(PTFE), and others.

Changing the hopper wall surface can be anoption in converting a hopper to mass flow ifone of the surfaces tested has a wall frictionangle that results in mass flow at the existinghopper angle.

Example 1 A silo with a 20-deg. from verticalconical hopper exhibits funnel flow. Thehopper is constructed from mill finish (No. 1)stainless steel plate. A wall friction test on thiswall surface using the bulk solid handled, shows

that wall friction angle f ' is 30 deg. Thecombination of these two numbers plots a pointwithin the funnel flow region of the designchart, which confirms the observation. Anothertest conducted on a No. 2B-finish stainless steelsheet shows that the wall friction angle is20deg. The combination of this wall frictionand the hopper angle plots a point within themass-flow area of the design chart. Thus, liningthe existing hopper with a No. 2B finishstainless steel sheet will convert the silo to mass

flow.

Figure 9. A typical wall friction test result from the JenikeShear Tester.

Shear

stress,

psf

Normal pressure, psf

f'1

f'2

f' = Wall friction angle, deg

0

0

Figure 10. Data from Figure 9 are used to determinedesign charts for setting hopper angles for mass flow.

Mass

flow

Funnel

flow

Uncertain

40

30

20

10

0

50403020100

qc

f'

qc


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It is not always possible to find a hopper surfacethat results in mass flow at the given hopperangle. In this case, another option is to increasethe slope of the hopper. This may need to bedone in combination with changing the hopper

surface.

Example 2 A silo with a 30-deg. from verticalconical hopper exhibits funnel flow. Thehopper is constructed from carbon steel plate.Tests on various wall materials show the lowestwall friction angle to be 20 deg. (on a particularepoxy coating). The design chart shows that a20-deg. wall friction angle and a 30-deg. hopperangle still will result in funnel flow. However,if the hopper could be steepened to 22 deg. from

vertical, then a wall surface having a 20-deg.wall friction angle would result in mass flow.Thus, cutting off the existing hopper andinstalling a new, 22-deg.-from-vertical hoppercoated with the epoxy tested will convert thesilo to mass flow.

Making a hopper steeper is not always an optiondue to, for example, headroom constraints. Ifmass flow must be achieved in the existingheadroom, another option is to replace a conical

hopper with a plane flow hopper in whichconvergence is on two opposing sides only, andthe outlet is elongated such as a wedge ortransition hopper. The sloping sidewalls of aplane flow hopper can be made 10- to 12-deg.less steep than a cone with the same innersurface and still achieve flow along its walls. Inaddition, theory shows that in plane flow thereis no sharp boundary between the mass flow andthe funnel flow regions, so that mass flow is stillpossible to the right of the boundary in the

design charts. This makes the plane flowgeometry a more robust design, capable ofhandling wider variations in materialcharacteristics. Typical plane flow hoppers areshown in Figure11.

Example 3 A silo with a 35-deg. from verticalconical hopper exhibits funnel flow. The

hopper is constructed from carbon steel plate.Tests on several wall materials show the lowestwall friction angle to be 20 deg. The designchart shows that a 20-deg. wall friction angleand a 35-deg. conical hopper still will result infunnel flow. A 22-deg. or steeper cone isrequired for mass flow. However, a transitionhopper with 35-deg. sidewall angles will resultin mass flow, within the existing hopper height.Thus, replacing the existing hopper with a newtransition hopper with 35-deg. side walls will

convert the silo to mass flow.

In general, when using a plane flow hopper, ifthe end walls are converging, the length of theoutlet must be at least three times its width. Forvertical end walls, for example when a chiselhopper is used, a minimum length-to-width ratioof 2:1 is sufficient.

Figure 11. Plane flow geometry is a more robust design,capable of handling wider variations in materialcharacteristics.

Transition Hopper

Chisel Hopper

p

L W

q

c p

WL

qq

L/W 2

L/W 3


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Going from a circular outlet to an elongatedoutlet will likely require a change in the feederas well. Changes in feeder design are addressedlater in this article.

Overcoming arching

In addition to modifications to ensure flowalong hopper walls, mass flow requires that thehopper has an outlet large enough to overcomearching. As shown in Figure 1, two types ofarches are possible: interlocking and cohesive.To overcome interlocking arches requires thatthe diameter of a circular opening be at least 6to 8 times that of the largest particles handled.

Additional considerations may be necessary forflaked or stringy materials.

The second type of arching, namely cohesivearching, can be analyzed by measuring thecohesiveness of the material. If, at any givenpoint, the cohesive strength of a materialexceeds the stresses imposed on it, then a stablearch will form and flow will stop. To preventarching, it is necessary to ensure that thestresses acting on the material are always

greater than its cohesive strength.

Cohesive strengths of most materials are afunction of consolidating pressures. Toillustrate this, lets hypothetically assume that abulk solid is confined in a cylinder withfrictionless walls. This bulk solid is

consolidated by placing a uniform pressure onits top surface, P1. After some period of time,the consolidating pressure is taken off the solid,and the walls of the cylinder are removedwithout disturbing the column of material.

Once the material is unconfined (i.e.the wallsare removed), pressure is once again applied tothe top surface, increasing in magnitude untilthe column gives out at a failure pressure, F1.This failure pressure is, in effect, the materialscohesive strength at a consolidating pressure ofP1.

Given the obvious limitations of this test (suchas frictionless walls and a self-sustainingcolumn of material), it cannot be used to obtain

cohesive strength values for design. Instead, aJenike Shear Tester, for example, can be used[7] (Figure 12). Normal forces are applied tothe cover to consolidate the material, similar tothe wall friction tests, and the force required toshear the material is measured. The result ofthis test is a relationship of cohesive strength toconsolidating pressure, called a "FlowFunction," as shown in Figure 13.

Once a materials Flow Function has been

determined, minimum outlet sizes to overcomearching can be calculated through a series ofdesign charts created by Jenike [6].

Typically, the minimum outlet diameterrequired to prevent arching across a circularoutlet is roughly twice the minimum width

Figure 12. A Jenike Shear Tester is used to find thecohesive strength of a solid.

RingCover

Normal force

Shearingforce

Base

Bulk solid

Figure 13. Typical Flow Function of a solid as determinedby Jenike Shear Tester.

CohesiveStrength,

F

Major Consolidating Pressure, P1


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required for a slotted outlet. For example, if theminimum outlet diameter to prevent arching iscalculated to be 12in., a 6-in. wide slotted outlet(and at least 18in. long) is sufficient to preventarching.

It should be noted that given the same sizeoutlet, a bulk solid is more likely to arch in afunnel-flow hopper than in a mass-flow one.Therefore, modifying the hopper to achieveflow along hopper walls may be in itselfsufficient to overcome arching. Whether this isthe case or not can be confirmed by checkingthe minimum outlet requirements in mass flowcalculated using the cohesive strength test data.

Example 4 Arching and ratholing frequentlyoccur in a silo with a 25-deg. from verticalconical hopper having a 12-in. dia. outlet. Thehopper is constructed from mill finish (No. 1)stainless steel plate. Wall friction and cohesivestrength tests are performed on the material.Cohesive strength tests show that if handled in amass-flow hopper, the minimum outlet diameterto overcome arching is 12 in. If handled in afunnel-flow hopper, the outlet must be 36 in. orgreater to overcome arching and ratholing. In

this case, modifying the hopper to achieve flowalong hopper walls will solve the arching andratholing problems.

Example 5 Assume that, in the above example,the cohesive strength tests reveal that theminimum outlet required to prevent arching in amass-flow cone is 24 in. One option is toincrease the outlet size to 24 in. and modify thehopper to get mass flow; however, this will limitthe feeder choices. Another option is to replace

the conical hopper with a transition hopper witha 12 in. wide by 36 in. long outlet.

Combatting ratholing

Whether a rathole will form in a silo is also afunction of cohesive strength. A rathole will

develop if the cohesive strength of the materialis greater than the stresses acting on theboundary of a flow channel. Cohesive strengthof most materials increases with increasingconsolidation pressure. Since consolidation

pressure experienced by a bulk solid is afunction of the size of a silo, the tendency for amaterial to rathole increases with increasing silosize. On the other hand, it is common to findmaterials cohesive enough to form ratholes evenin small silos (5-ft. dia. or less). (Note thatratholes form only in funnel-flow silos.)

Once the Flow Function of a material isobtained, critical flow channel diameters toovercome ratholing can be calculated in a

similar manner to outlet sizes. This criticaldiameter, DF, is calculated as a function of silosize. To avoid ratholing in a funnel-flow silo,the flow channel must be greater than DF,which will ensure that the stresses at theboundary of the flow channel exceed thestrength of the bulk solid. The size of a flowchannel is set by the hopper outlet, and isapproximately equal to its diameter (if it iscircular) or the diagonal of a slotted outlet.

Example 6 Ratholes form in a 25-ft. diametersilo, causing flow stoppages and, at times,uncontrollable fluidized discharge of material,which occurs due to collapsing ratholes. Thesilo cylinder is 35-ft. tall, followed by a conicalhopper extending to a 12-in. dia. outlet with ascrew below.

Tests confirm that the 25-deg. from verticalconical hopper fabricated using No. 1 mill finishstainless steel plate results in funnel flow. The

least frictional of all the surfaces tested, apolyethylene liner, requires 19 deg. fromvertical to achieve mass flow. Given the size ofthe silo, it is not practical to replace the hopperwith a steeper one. In this case, another optionto consider is expanded flow.


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The 12-in. dia. outlet results in a flow channelthat starts at a 12-in. diameter and then expandssomewhat upwards. Cohesive strength testsshow that the critical rathole diameter, DF, forthis material in a silo of this size is 6 ft.

Therefore, ratholing can be overcome by cuttingthe existing hopper at a 6 ft. diameter andplacing below this a 19 deg. from verticalconical hopper lined with the polyethylene sheettested. This modification will result in massflow in the silo up to the top of the new hoppersection, and funnel flow above. Note that if thematerial is susceptible to segregation ordegradation with time, then other problems maypersist even though this modification will resultin reliable discharge.

Use of inserts

Inserts placed inside hoppers can be used tomodify or alter flow in a silo. Early insertsconsisted of inverted cones placed near the topsof conical hoppers (Figure 14). In theory, underthe right conditions. these inserts can force flow

along a portion of hopper wall. However, inpractice they rarely achieve flow along hopperwalls. Nevertheless, if placed correctly, theseinserts can increase the size of the flow channel,therefore activating more of the contents of a

silo. Thus if the only problem is one of limitedlive capacity of a silo, there may be some meritin considering such an insert. However, ifmisplaced, these inserts can make problemsworse by causing arching.

Another type of insert consists of a hopper-within-a-hopper, such as cone-within-a-cone, orwedge-within-a-wedge. (This design ispatented.) These inserts can force material flowalong hopper walls otherwise too shallow or too

frictional for flow (see Figure 15).

Example 7 Particle segregation has been aproblem in a silo. Although well-blendedmaterial is transferred to the silo, the dischargecontains large amounts of fines at the beginningand large amounts of coarse particles at the endof a run. The problem is caused by a funnel-

Figure 14. Inverted cone placed near the top of a conicalhopper can modify or alter flow in a silo.

Figure 15. Cone-within-a-cone insert forces material flowalong hopper walls otherwise too shallow or too frictional

for flow.


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flow pattern that results in the center of the silobeing discharged first, followed by the sides.Note that if the bulk solid is susceptible tosifting segregation, the process of filling the silowill result in a higher concentration of fines in

the center. If the silo is then emptied in afunnel-flow pattern, the center is withdrawnfirst, followed by the periphery. To remedy thesituation, the flow must be converted to massflow, which results in the sides and the centerdischarging roughly at the same time, therebyreducing the impact of this side-to-sidesegregation.

The silo consists of a conical hopper sloping at35 deg. from vertical. Test results show that the

existing wall surface requires a 20-deg. conicalhopper to achieve mass flow. Perhaps becauseof headroom limitations, the cost of replacingthe hopper, or the downtime required, a new 20-deg. hopper is not feasible. An alternative tothis would be to place an 18-deg. cone inside thehopper, forming a "cone-within-a-cone" system.This will force flow along the 35-deg. hopperwalls and convert the silo to mass flow. Inaddition, the use of this type of hopper insertsystem results in more-uniform velocities in the

silo, which further reduce the impact ofsegregation. A uniform velocity may also bedesirable in silos where residence time of thebulk solid is important, such as in purge vessels,or when cross-contamination as a result ofproducts mixing in the silo during discharge isa concern [8].

Example 8Funnel flow in a purge silo used toremove volatiles from granular polymers iscausing incomplete processing, resulting in the

volatiles being discharged with the material.This not only creates an explosion hazard, butalso causes quality problems. In addition,because of funnel flow, stagnant material in thesilo is causing cross-contamination betweenbatches.

Converting the purge silo to mass flow ensures auniform residence time, hence, uniform purging.Mass flow also prevents stagnant material,therefore, significantly reducing cross-contamination across batches. In continuous

processes where a different batch is placed ontop of another, a certain amount of mixingoccurs as the material flows through the silo.Ensuring a uniform velocity profile through thesilo can minimize this mixing and minimizechangeover waste.

Wall friction tests show that the existing hopperslope is too shallow for mass flow regardless ofthe surface. Since the silo is a pressure vessel,any external change to the hopper would require

extensive fabrication and retesting of thestructure. The most practical solution is to use a"hopper-within-a-hopper" insert to convert theflow pattern to mass flow, which also provides auniform velocity (Figure 16).

Pup tent

Another type of insert, often referred to as a puptent, is useful in combining flow channels toovercome ratholing (Figure 17). The diameter

of a flow channel created by combining twoflow channels is roughly equal to the sum of thediameters of the individual flow channels. Forexample, a pup tent could be used to combinethe flow channels that form above two adjacent

Figure 16. Retrofitted purge silo yields mass flow.


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outlets of a silo to dramatically increase the flowchannel size, to the point that a stable ratholecannot form.

Pup tents can also be used to reduce loading onfeeders with multiple screws. By carefullyplacing small pup tents between the screws, the

solids pressure acting on each screw, and,hence, power requirements can be significantlylowered. Care must be taken, however, to avoidformation of arches over the pup tents.

Feeder modifications

In addition to the two requirements stated earlier(flow along walls, and an outlet large enough toovercome arching), mass flow also requires thatthe entire hopper opening be active. Regardless

of the slope and the surface finish of the hopperabove, if the outlet is partially blocked, funnelflow will occur. The blockage of the outletcould be caused by a lip or a ledge formed dueto flange mismatches, partially closed gates, or afeeder that withdraws material from only a partof the outlet [9]. (A feeder is a piece of

equipment that is used to meter the discharge ofmaterial from a silo.)

A common problem, especially when elongatedoutlets are used, is the inability of a feeder to

activate the entire outlet. For flow to occurfrom the entire length of the outlet, the feedermust have a capacity that increases in thedirection of flow.

Two feeders commonly used with elongatedoutlets are belts and screws. When using a beltfeeder, the increase in capacity is achieved byusing a tapered interface as shown in Figure 18.The increasing capacity along the length isachieved by increasing size of the opening as a

result of the taper.

With a screw feeder, the increase in capacitymay be achieved through the use of a taperedshaft as shown in Figure19a, or by increasingthe pitch in the direction of flow, starting from ahalf pitch and ending with a full pitch, as shownin Figure 19b [10]. However, both of thesemethods are limited to a length-to-screwdiameter of about 3:1 or less. For longerlengths, the increase from one flight to the next

becomes so small that, because of fabricationtolerances, an increase in the direction of feedcannot be guaranteed.

Figure 17. A pup tent is useful in combining flow channelsto overcome ratholing.

Figure 18. A belt feeder increases in capacity in the

direction of flow by using a tapered interface.

hopper


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The combination of a tapered shaft andincreasing pitch sections is used to achievelength-to-diameter ratios of up to about 6:1. Forthe first half, a section with a tapered shaft andhalf pitch is used, which is followed by anincreasing pitch section (Figure19c).

Example 9 Arches and ratholes occur in a silowith a wedge hopper. The hopper has a 14-in.

wide by 6-ft. long outlet. A 6-in. dia. screw in aV-trough (tapered trough) is used as a feeder.The screw is constant diameter, constant pitch.

The ratholes form over the back of the screw.Since the screw is constant pitch and constantdiameter, its capacity is equal to the capacity ofthe first flight, and is constant along its length.

The first flight withdraws material from directlyabove it, and becomes full. The successiverotations of the screw convey the contents of thefirst flight forward to the second flight, which,having the same capacity, is filled with material

from the first flight. This prevents any morematerial entering the flight from above, so thematerial forward of the first flight remainsstagnant. This creates a flow channel over thefirst flight, and if the material is cohesive, arathole forms when the flow channel is emptied.(Structural failures have also occurred in anumber of cases as a result of the flow channelwhich develops along the walls of the silo.)

The solution is to replace the feeder with a 14-

in. dia. mass flow screw feeder, as shown inFigure 19c, placed in a U-trough. A mass-flowscrew feeder with a uniform increase in capacityalong the length of the outlet will activate theentire outlet. A screw with a diameter equal tothe width of the slot ensures that the entirewidth of the slot is active, which is imperative toachieve mass flow. Note that the shallowsidewalls of a V-trough prevent mass flow evenif a mass flow screw and mass flow hopper areused.

Rotary Valves

Rotary valves are frequently used as feeders. Insome applications, these feeders also serve as apressure seal, for example, feeding a pneumaticconveying line or a reactor at a higher pressurethan the silo.

Rotary valves used as feeders below hoppersoften tend to withdraw material only from a part

of the hopper outlet. As the rotor turns, itspockets begin to fill as they become exposed. Ifthe pocket fills with material before it travels theentire length of the outlet, then mass flow ishindered as a result of a partially active outlet.

Figure 19. Various methods for increasing capacity in theflow direction for screw feeders.

A C

a. Tapered Shaft

B C

b. Increasing Pitch

A B C

c. Combination Mass-Flow Screw Feeder

A = Conical Shaft and Constant Pitch (Feed Section)B = Constant Shaft and Increasing Pitch (Feed Section)

C = Constant Shaft and Constant Pitch (Conveying

Section)


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To overcome this problem, a vertical sectionshould be placed between the rotary valve andthe hopper outlet. Making this vertical sectionapproximately one dia. tall is typically sufficientto allow the material flow to expand so as not to

interfere with discharge from the hopper.

When feeding into higher-pressureenvironments, especially when handling finepowders, the effect of gas movement in thesystem becomes significant. If a rotary valve isnot vented properly, gas leakage through thevalve into the silo can significantly reduce themaximum rate at which material can exit thesilo. The upward moving gas acts as a bodyforce on the powder, opposite to gravity,

retarding flow. In some cases, this gas backflowcan actually cause material to arch in thehopper, completely stopping discharge. Thisproblem can be avoided by providing properventing to allow the gas to travel an alternativepath, and reducing the amount of gas leakage byproperly maintaining the rotary valve [11].

Use of flow aids

Flow aids are devices or substances often used

to assist gravity in promoting material flow.Flow aids can be grouped in two classes:mechanical and chemical. Common mechanicalflow aid devices include air cannons, vibrators,vibrating dischargers, and agitators. Chemicalflow aids include powdered additives such asfumed silica and magnesium stearate, or liquidadditives, such as freeze-conditioning agents.

Mechanical flow aids: Air cannons

Flow properties of many materials change withincreasing time under pressure. For example, abulk solid may flow out of a certain hopperreliably as long as discharge from the silo iscontinuous. However, after stopping dischargefor some time, say overnight or over a weekend,

the same material may not flow as a result of anarch that has formed.

Air cannons may be an excellent choice for asituation such as this. These devices work by

quickly releasing a volume of high-pressure airor other gas into the bin (Figure 20). Thisaction creates a pressure front, which appliesadditional stresses on an arch in an effort tobreak it.

The size, number, and location of the aircannons required depend on the cohesivestrength of the material and the dimensions ofthe silo. Once the materials strength ismeasured, mathematical analyses can be used to

determine the specifics of the air cannonsrequired.

While these devices are effective in breakingarches that form after material has been stored atrest, if the problem is due to arch formationduring continuous flow, air cannons are at best aband-aid solution, and an alternative means ofalleviating the problem should be consideredsuch as previously mentioned.

Air cannons are also ineffective in overcomingratholing. Once an air cannon is fired and apath is cleared from the air cannon to therathole, any subsequent operation of that aircannon becomes useless, as air just travels

Figure 20. Air cannons quickly release a volume of high-pressure air or other gas into the bin, creating a pressurefront which applies additional stresses on an arch to breakit.


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through the path without disturbing anyadditional material. To disturb sufficientmaterial to actually collapse a rathole wouldrequire an unreasonably large number of aircannons. Even so, a collapsing rathole with

large masses of material falling could causestructural damage to the silo and to theequipment below.

Example 10 Arches form in a silo everyMonday morning after a system has been shutdown for the weekend. The silo consists of aconical hopper with a 14-in. diameter outlet.Operators poke the silo with rods until flow isestablished, after which the silo operates wellfor the entire week.

Wall friction tests show that the silos hoppersection is sufficiently steep and has low enoughfriction to provide mass flow. Cohesivestrength tests indicate that the minimum outletsize required to overcome arching duringcontinuous flow in a mass flow silo is 10 in.However, after three days at rest under pressurein a silo, the cohesive strength increases suchthat the minimum outlet required to overcomearching becomes 3 ft. 6 in. An air cannon

analysis shows that two air cannons placed atthe 4 ft. diameter level are sufficient toovercome arching after a weekend at rest.

Vibrators

Vibrators impart continuous body forces to thematerial through the walls of the handlingequipment. These devices are often mounted onthe hopper sections of silos. Some modelsdeliver low-frequency/high-amplitude forces,

much like a hammer blow. Others producehigh-frequency/low-amplitude forces through,for example, rotating a set of unbalancedmasses.

The effect of such devices on most arching andratholing problems is minimal. The additional

force required is usually not delivered where itis needed, namely at an arch or a rathole.Furthermore, when dealing with pressure-sensitive materials (those materials that exhibitan increase in cohesive strength if subjected to

forces beyond gravity), such devices mayexacerbate the problem.

Vibrators are better used to reduce buildup inchutes. By definition, chutes are surfaces onwhich a bed of bulk solid slides, in contrast tohoppers, which are geometries 100% full ofmaterial. Such devices include cyclones anddust collection hoppers, which act as chutes.

Vibrating dischargers

Mounted at the outlet of a hopper, a vibratingdischarger activates a large cross-section.Activation is achieved by an insert, commonlyan inverted cone or a dish, that vibrates alongwith the outer shell in a gyratory motion causedby a set of unbalanced rotating weights or avertical motion by hydraulic or pneumaticactuators. Product flows around the insert into aconical section below, which typically operatesas a chute (Figure 21).

To the extent that it activates a large cross-section, vibrating this device can be used toovercome a number of flow problems, provideda few basics are adhered to.

Figure 21. A vibrating discharger activates a large cross-section via an insert, commonly an inverted cone or a dish.


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If a vibrating discharger is used at the outlet of afunnel-flow silo, then the flow channel createdwill approximate the size of the outlet (i.e., thetop diameter of the discharger). If this flowchannel is larger than the critical rathole

diameter discussed above, then stable ratholesshould not form, even though the pattern isfunnel flow. In this case, if segregation anddegradation of the solid is not a concern, adischarger could provide reliable flow. If, on theother hand, the flow channel is not large enoughto overcome ratholing, flow stoppages and otherflow problems related to funnel flow andratholing will occur.

It is possible to overcome this problem by

ensuring that the hopper above the discharger isdesigned for mass flow. Since the discharger isisolated from the hopper above, the vibrationsdo not affect flow in the hopper, and proceduresfor obtaining mass flow, as described above,apply.

Vibrating dischargers may cause more problemsthan they solve if used with pressure-sensitivesolids. The energy input can actually packmaterial and cause flow stoppages. With certain

materials (this depends on the material and thecircumstances), distinct flow channels form infour quadrants of the outlet, caused by the insertsupports, and this can asymmetrically load thesilo above. Such loads can result in structuraldamage to the silo [5].

A distinction must be made between adischarger and a feeder. A discharger is adevice that promotes flow, but does not controlits rate. A feeder, on the other hand, is used to

meter bulk solids. Therefore, if the rate ofdischarge is to be controlled, a feeder must beused below the discharger. In this case, caremust be taken that material does not fill the areabetween the discharger and the feeder and getcompacted by vibrations.

Vibrating dischargers should be operated on anon/off cycle as suggested by the manufacturer,even if the bulk solid seems to flow without anyvibrations. Not operating the vibratingdischarger can easily result in distinct flow

channels, which can load the siloasymmetrically. Structural failures haveoccurred in a number of silos where thedischarger was not being vibrated.

Agitators/flexible walls

Paddles, moving arms, and flexible walls aresometimes effective in overcoming arching andratholing problems; however, these solutions aretypically limited to relatively small hoppers

(several hundred pounds capacity), where solidspressures and the power required to operatethem are small. A number of commerciallyavailable feeders employ agitation effectively,and are especially useful when low dischargerates are required with cohesive bulk solids.

Chemical flow aids

A number of chemicals have been successfullyused as flow aids to alter the flow properties of

the bulk solid handled. These additives, ingeneral, work by preventing the host particlesfrom adhering to each other, which isaccomplished by coating the surface of eachparticle, removing free moisture, or preventingthe freezing of that moisture. Generally, theseadditives are dry powders, such as silicates,stearates, or phosphates. Their effectivenessand the amount required can easily bedetermined by conducting cohesive strengthtests on samples prepared in a lab.

Some of the considerations of this approachinclude the compatibility of the additive withthe material handled and the process, the cost ofthe additive, and the cost of an additionalhandling system to deliver and mix the additive.


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Generally, this approach is considered to be oneof the last resorts in solving a flow problem.

Liquid additives have been used as freeze-conditioning agents to prevent freezing of moist

materials that are subjected to low temperatures,such as coal transported by rail or ores storedoutside.

Air permeation systems

When handling fine powders (e.g.all particles100mm and smaller), the effect of interstitial air

or gas can be significant. As briefly discussedabove, one of these effects is limiting thedischarge rate. As the material moves from the

top of the silo toward its outlet, the pressuresexerted on it at first increase, resulting in a risein bulk density. This causes the particles tobecome packed closer together, which forcesout a certain amount of gas between them. Asthe material continues to move through thehopper toward the outlet, the consolidatingpressures are reduced and the bulk densitydecreases. This can result in a slight amount ofvacuum between the particles in the hopper,which often creates a gas flow inward through

the outlet, counter to the solids flow. As the rateof discharge increases, so does the amount ofvacuum; hence, the body forces exerted on thematerial by the counter-flowing gas go up.Eventually, this body force becomes equal andopposite to the force of gravity, setting theupper limit to material discharge.

The flow rate that is limited by this mechanismcan be increased by supplying gas into thematerial using an air-permeation system, which

reduces the amount of gas flow in through theoutlet.

Typical placements of air permeation systemsare shown in Figure 22. Care must be taken todesign to ensure the gas permeation system doesnot prevent mass flow. The air requirements for

these systems are a function of the permeabilityof the bulk solid and are typically very low.Injecting excessive amounts of air can result influidization of the hoppers contents, and maylead to flooding.

Fluidization

This may also be an option if the materialhandled is fine and easily fluidizable.Fluidization is especially useful where highdischarge rates (e.g., 1,000 ton/h) are required.Gas injected through permeable membranesnear or at the bottom of the hopper is used tofluidize the contents of a silo. This changes thecharacteristics of the bulk solid almost

completely, making it behave more like a fluid.

The gas must be uniformly distributed throughthe material to prevent localized fluidization orflow channels. If a fast-flowing flow channel or

Figure 22. Typical placements of air permeation systems.Care must be taken to ensure that the system does notprevent mass flow.


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a rathole develops, then most of the fluidizationgas moves through that path, losing its ability tofluidize any more material. Additionalconsiderations when evaluating this as an optioninclude the need for dry or conditioned gas, the

requirement of an additional system to handleand clean the gas after use, and discharging withlow density and fluctuating rate.

To sum up

While it is preferable to avoid handlingproblems by designing equipment based on flowcharacteristics of a bulk solid, problems inexisting equipment can be alleviated. Themajority of flow problems are caused by the

existence of a funnel-flow pattern. Theseproblems can be solved by altering the flowpattern to mass flow, which requires changing

the hopper or feeder design as outlined above.In addition, a number of other methods, such asthe use of mechanical and chemical flow aids,fluidization, and air permeation, may be usefulmeans of addressing these problems.

A quick guide to the potential effectiveness ofmost of the various options presented is given inTable I. The listing is intended as a startingpoint in considering alternatives, and is in noway definitive or all-inclusive. However,understanding the problem and its causes, aswell as the flow properties of the bulk solidhandled, should form the basis for any retrofit toalleviate the problem.

Table I. A quick guide to the effectiveness of the various retrofit options

Problem

Solution Arching Ratholing Flooding Rate Limiting Segregating

Hopper Modifications

Liner Good Good Good Poor GoodTransition Hopper Good Good Good Fair Good

Expanded Flow Good Good Good Poor PoorLarger Outlet Good Fair Poor Good Poor

Inserts

Inverted Cone Poor Fair Fair Poor FairHopper-In-Hopper Good Good Good Poor GoodPup Tent Poor Fair Poor Poor Poor

Feeder Modifications

Mass-Flow Screw Good Good Good Good FairMass-Flow Belt Interface Good Good Good Good FairVented Rotary Valve Good Poor Poor Good Poor

Flow-Aid DevicesAir Cannon Good Fair Poor Poor Poor

Vibration Fair Fair Poor Poor PoorAgitation Good Good Fair Poor Fair

Aeration

Air Permeation Poor Poor Poor Good PoorFluidization Good Fair Good Good Poor


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Literature cited

[1] Merrow, E. W.,"Estimating Startup Timesfor Solids-Processing Plants," Chem. Eng., p.89 (Oct. 24, 1988).

[2] Carson, J. W., et al.,"Understanding andEliminating Particle Segregation Problems,"Bulk Solids Handling, 6, pp. 139-144 (Feb.1986).

[3] Purutyan, H. , et al., "Identifying andControlling Silo Vibration Mechanisms,"Powder & Bulk Eng., Part I, 8(11), pp. 58-65(Nov. 1994); Part II, 8 (12), pp. 19-28 (Dec.1994).

[4] Jenkyn, R. T. and D. J. Goodwill,"SiloFailures: Lessons to be Learned," Eng. Digest(Sept. 1987).

[5] Carson, J. W. and R. T. Jenkyn, "LoadDevelopment and Structural Considerations inSilo Design," paper presented at Reliable Flowof Particulate Solids II, Oslo, Norway (Aug.1993).

[6] Jenike, A.W., "Storage and Flow ofSolids," Bulletin No. 123, University of UtahEngineering Experiment Station, Salt Lake City(Nov. 1964).

[7] American Society for Testing andMaterials,Standard Shear Testing Method forBulk Solids Using the Jenike Shear Cell,ASTM Standard D6128, ASTM, Philadelphia.

[8] Carson, J. W., et al., Bulk Solid Purge

and Conditioning Vessels, Chem. Proc., 58(8),pp. 77-80 (Aug. 1995).

[9] Carson, J. W., and G. Petro, FeederSelection Guidelines, Chem. Proc. 1997Powder & Solids Annual, pp. 40-43 (1977).

[10] Marinelli, J., and J. W. Carson,"UseScrew Feeders Effectively,"Chem. Eng.Progress, 88(12), pp. 47-51 (Dec. 1992).

[11] Carson, J. W.,"Interfacing Bulk Solids

Conveyors with Upstream Equipment," AIChESymposium on Solids Conveying andSeparation, AIChE, New York (Nov. 18, 1987).

[Biographies updated, 2003]

H. PURUTYANis a vice-president at Jenike &

Johanson, Inc. Westford, MA (Phone: 978/392-

0300; Fax: 978/392-9980; e-mai l :

[email protected]). Since joining the firmin 1991 he has been involved in designing

reliable handling systems for a variety of

applications, ranging from degassing/purging

vessels for olefins to storage and feeding

systems for cement plants. He has extensive

experience with a wide range of difficult-to-

handle materials such as synthetic gypsum,

kaolin clay, fertilizers and titanium dioxide.

Purutyan regularly lectures on the subject of

bulk solids handling through AIChE and to

individual companies. He has authored a

number of articles on the subject, and is the

holder of two patents. He received both his B.S.

and M.S. from Worcester Polytechnic Institute

and an MBA degree at Babson College.

B. H. PITTENGERis a senior consultant at

Jenike & Johanson, Inc. Westford, MA (Phone:

978/392-0300; Fax: 978/392-9980; e-mail:[email protected]). Since joining J&J,

he has designed bulk-solids storage and

handling systems ranging from conditioning

silos for food and chemicals, to simple and

accurate batch dosing systems in processing

plants, to developing anti-segregation systems


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for powdered metals and pharmaceuticals. He

previously worked at General Electric (GE) as a

production, systems, and quality engineer, and

as a shift and plant supervisor. With GE's

Corporate Audit Staff, he provided systemsdesigns to many GE businesses, including

Quartz Products, Plastics Europe, Aerospace,

and Power Generation. He has published many

papers and lectures on solids flow for several

professional organizations. Pittenger has a B.S.

from Renssalaer Polytechnic Institute, and an

MS from Worcester Polytechnic Institute.

J. W. CARSON is president of Jenike &

Johanson, Inc. Westford, MA (Phone: 978/392-0300; Fax: 978/392-9980; e-mai l :[email protected]). He joined the firm in1970 and has been active in research,consulting, and management. Carson is theauthor of over 50 technical papers and articles inthe areas of bin loads, flow of fine powders,vibration, and using computers to analyze solidsflow. He lectures extensively on the topic offine powder storage and flow of solids, anddevotes much of his time to consulting with

clients. He received a BS in mechanicalengineering from Northwestern University, aPhD from MIT, and is a member of AIChE.

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