solids handling problems

8/9/2019 Solids Handling Problems

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SOLVING SOLIDS HANDLINGPROBLEMS BY RETROFITTING

KILARU HARSHA VARDHAN


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Overview

Most flow problems are caused by

funnel flow patterns and can be curedby altering the pattern to mass flow,which requires changing the hopper

and feeder designs.

Mechanical and chemical flow aids


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Introduction

In effective and unreliable handling systems -

start delays, process inefficiencies and equipmentdowntime.

Engineering resources are usua y a ocate toprocessing concerns( reaction chemistry ),

Design of materials handling systems to prevent

problems rarely gets much attention.


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Introduction

Solids handling problems can be avoided by

basing the design on the flow properties of thebulk materials being handled

Although it is much easier to prevent solids

methods, there are a number of effective retrofitsthat can significantly improve equipmentperformance.


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Common flow problems No flow

Erratic flow

Flooding / flushing

Flow rate limitations

Particle segregation


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No flow


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Erratic flow

This is the result of obstructionsalternating between an arch and arathole.


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Flooding/flushing

Occurs when handling fine powders

such as pigments and additives

When a rathole collapses, the falling

fluidized. material floods through thefeeder uncontrollably.

The solids bulk density can oftenundergo dramatic variations


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Flow rate limitations

Flow rate limitation is another problemunique to fine powders.

Most fine powders have very low

any movement of interstitial air.

The pressure gradients caused as aresult of this gas movement canretard discharge from a hopper.


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Particle segregation canoccur

when handling blends

of ingredients,

when handling ahomogenous substancewith a wide variation inparticle size or density.


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Segregation or de-mixing ofingredients may prevent

A desired chemical,

May cause an undesiredreaction, or

May results in costlyproduct waste or rework.


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Associated flow problems

m e ve capac y Product degradation Incomplete or non-uniform processing Vibration of structures Structural failure Excessive power requirement


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Flow patterns

Funnel flow

Mass flow

xpan e ow


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Funnel flow

Some material moves while rest remainsstationary during the discharge from the

silo.


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Funnel flow

1. Material handled consists entirely of coarseparticles usually in. 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


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Mass flow

Mass flow occurs whensloping hopper walls

are smooth enoughand steep enough forthe particle to slide

.

This type of floweliminates ratholing

and flooding andmaximizes the usablecapacity of the silo.


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Mass flow

To achieve mass flow

1) The sloping walls of the hoopermust be steep enough and musthave the sufficiently low frictionto allow particles to flow along

em

2) The hopper opening must belarge enough to overcomearching

3) Entire hopper opening must beactive


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Expanded flow

Used to overcome

ratholing in large silos

using the mass flow

sec on o en arge eflow channel diametersuch that a ratholecannot form.


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Expanded flow

To use expanded

flow the materialmust be

The particle mustbe coarse enough

not to becomefluidized.


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Retrofitting options

Hopper modifications

Use of inserts Feeder modifications

Air permeation systems

Fluidization.


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Hooper modification

Flow stoppages or erratic discharges due to arching and ratholing,flooding or flushing of the fine powders, silo vibrations andstructural problems.

Inner hopper surface

Ho er slo e

Hopper geometry

Flow along walls

Shear force is measured as a

function of applied normal force

Wall friction is the friction between

bulk solids and hopper wall surface


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Hooper modification

Ultra high molecular weight Polyethylene

Smooth or polishedstainless steel sheets

,

polytetrafluroethylene(PTFE)

= tan()


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Hooper modification

The test is conducted by first placing the bulk solidin a retaining ring on a flat coupon of wall material.

Various normal forces are applied by placingweights on the cover. Material in the ring is

pushed along the stationary wall surface, and theshear force is measured as a function of appliednormal force

Wall friction angle is the angle formed by drawinga straight line from the origin to a point on thecurve


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Hooper modification cont


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Hopper geometry

Making a hopper steeper isnot always an option due to

head constraints.

For mass flow in existingho er re lace a conical

hopper with a plane flowhopper in which convergenceis on two opposing sides only,and the outlet is elongated

such as a wedge or transitionhopper.


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Hopper geometry

plane flow geometry ismore robust design,

capable of handlingwider variations inmaterial characteristics.

Circular outlet to anelongated outlet will likely

require a change in thefeeder


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Use of inserts


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Use of inserts


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Use of inserts

If placed correctly, these inserts canincrease the size of the flow channel

Activating more contents of a silo

If the only problem is one of limited livecapacity of a silo, there may be some meritin considering an insert

If misplaced, these inserts can makeproblems worse by causing arching.


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Feeder Modifications

A feeder is a piece of equipment thatis used to meter the discharge of

material from silo

When elongated outlets are used,

the feeder inabilit to activate theentire outlet.

For flow to occur from the entirelength of the outlet, the feeder musthave a capacity that increases in

the direction of flow


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Feeder Modifications

To make entire hopper opening active

Belt feeder increase in capacity isachieved by tapered interface andincreasing size of opening

Screw feeder increase in capacity is

achieved by tapered shaft or byincreasing the pitch in the direction offlow

Length to screw ratio

3:1

6:1


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Rotary valves

Rotary valves are frequently used as feeders.

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

of the hopper outlet

If a rotary valve is not vented properly, gasleakage through the valve into the silo cansignificantly reduce the maximum rate at which

material can exit the silo.


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Flow aids

Flow aids are devices or substances often used toassist gravity in promoting material flow.

Flow aids can be grouped in two classes:mechanical and chemical.

Mechanical flow aid devices include air cannons,vibrators, vibrating dischargers, and agitators.

Chemical flow aids include powdered additivessuch as fumed silica and magnesium stearate, orliquid additives, such as freeze-conditioning agents


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Mechanical flow aids

Effective for breaking arches

Ineffective for ratholing


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Mechanical flow aids

Vibrators impart continuous body forces to the material through the walls of the handling

Low-frequency / high-amplitude forces, much like a hammer blow.Others produce high-frequency/low-amplitude forces

Vibrators are better used to reduce buildup in chutes


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Discharger and Feeder

Discharger is a device that promotes flow, butdoes not control its rate.

A feeder is used to meter bulk solids.

If the rate of discharge is to be controlled, afeeder must be used below the discharger.

Care must be taken that material does not fill the

area between the discharger and the feeder andget compacted by vibrations


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Agitators / Flexible walls

Paddles, moving arms, and flexible walls aresometimes effective in overcoming arching and

ratholing problems

These are typically limited to relatively smalloppers w ere so s pressures an e power

required to operate them are small.

Especially useful when low discharge rates are

required with cohesive bulk solids.


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Chemical flow aids

Work by preventing the host particles fromadhering to each other - by coating the surface

of each particle, removing free moisture

Generally, these additives are dry powders,

Liquid additives have been used as freeze-conditioning agents to prevent freezing ofmoist materials that are subjected to low

temperatures, such as coal transported by railor ores stored outside.


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Air permeation systems

When handling fine powders, the effect of interstitialair or gas limits the discharge rate.

As the material moves from the top of the silo towardits outlet, the pressures exerted on it at first increase,resulting in a rise in bulk density.

This causes the particles to become packed closertogether, which forces out a certain amount of gasbetween them.

Flow rate can be increased by supplying gas into the

material using this systems, which reduces theamount of gas flow in through the hopper


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Air permeation systems cont


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Fluidization

Material should be fine and easily fluidizable

Useful when high discharge rates are needed

The as must be uniforml distributed throu h thematerial to prevent localized fluidization or flowchannels.

If a rathole develops, then most of the Fluidizationgas moves through that path, losing its ability tofluidize any more material.

Requires dry or conditioned gas

Eff ti f th i t fit ti


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Effectiveness of the various retrofit options


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References

Purutyan, H.,Pittinger, B.H., Carson, J.W., Solve

solids handling problems by retrofitting,Chem.Eng.Prog.,pp 37-49, (April 1998).


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Thank you


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solids handling problems

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