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Gas-Solid Flows

1 INTRODUCTION

There is essentially no limit to the capability or a pneumatic conveying system forthe conveying of dry bulk particulate materials. Almost any material can be con-veyed and high material flow rates can be achieved over long distances. There are,however, practical limitations and these are mainly imposed by the fact that theconveying medium, being a gas, is compressible. The limiting parameters are thenmainly the economic ones of scale and power requirements.

Conveying capability depends mainly upon five parameters. These are pipebore, conveying distance, pressure available, conveying air velocity and materialproperties. The influence of many of these variables is reasonably predictable butthat of the conveyed material is not fully understood at present.

1.1 Pipeline Bore

The major influence on material flow rate is that of pipeline bore. If a greater ma-terial flow rate is required it can always be achieved by increasing the pipelinebore, generally regardless of the other parameters. In a larger bore pipeline a largercross sectional area is available and this usually equates to the capability of con-veying more material.

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

108 Chapter 4

1.2 Conveying Distance

In common with the single phase flow of liquids and gases, conveying line pres-sure drop is approximately directly proportional to distance. Long distance con-veying, therefore, tends to equate to high pressure, particularly if a high materialflow rate is required. For the majority of conveying applications, however, it is notconvenient to use high pressures. As a consequence, long distance, with respect topneumatic conveying, means about one mile. This limitation, and means of ex-tending distance capability, are discussed at various points in this handbook. Inthis chapter the basic fundamentals are considered.

1.3 Pressure Available

Although air, and other gases, can be compressed to very high pressures, it is notgenerally convenient to use air at very high pressure. The reason for this is that airis compressible and so its volumetric flow rate constantly increases as the pressuredecreases. In hydraulic conveying, pressures in excess of 2000 lbf/in2 can be usedso that materials can be conveyed over distances of 70 miles and more with a sin-gle stage. With water being essentially incompressible, changes in the velocity ofthe water over this distance are not very significant.

In pneumatic conveying, air at pressures above about 15 lbf/in2 gauge isgenerally considered to be 'high pressure', as mentioned in Chapter 1. With air at15 lbf/in2 expanding to atmospheric pressure, for example, the conveying air ve-locity will double over the length of the pipeline. Although the air expansion canbe accommodated to a certain extent by stepping the pipeline to a larger bore partway along its length, this is a complex design procedure. As a consequence, airpressures above 100 lbf/in2 gauge are rarely used for pneumatic conveying sys-tems that deliver materials to reception points at atmospheric pressure.

Where pneumatic conveying systems are required to deliver materials intoreactors and vessels that are maintained at pressure, however, high air supply pres-sures can be used, and 300 lbf/in2 is not unusual. With a high back pressure theexpansion of the air is significantly limited and relatively few, if any, steps wouldbe required in the pipeline. It is on this basis that staged pneumatic conveying sys-tems would be designed for very long distance conveying.

1.4 Conveying Air Velocity

The parameter here is volumetric flow rate, for this has to be quoted, along withsupply pressure, when specifying a blower, compressor or exhauster for a pneu-matic conveying system. The critical design parameter with respect to pneumaticconveying, however, is conveying air velocity, and more particularly, conveyingline inlet air velocity or pick-up velocity. Since the air expands along the length ofthe pipeline it will always be a minimum at the material feed point at the start ofthe pipeline, in a single bore pipeline, regardless of whether it is a positive pres-sure or a vacuum conveying system.


Gas-Solid Flows 109

In a single bore pipeline the velocity will be a maximum at the end of thepipeline. It is the value of the minimum velocity of the air that is critical to thesuccessful operation of a pneumatic conveying system. Volumetric flow rate, ofcourse, is given simply by multiplying conveying air velocity by pipe section area.In this process, however, the correct velocity has to be used and this is consideredin detail in the next chapter on 'Air Requirements'.

The minimum value of conveying air velocity depends to a large extent onthe properties of the bulk particulate material to be conveyed and the mode ofconveying. For dilute phase conveying this velocity is typically about 3000 ft/min,although this does depend upon particle size, shape and density, as will be dis-cussed.

For dense phase conveying the minimum velocity is about 600 ft/min. Forfine powders that are capable of being conveyed in dense phase the minimumvalue of conveying air velocity also depends upon the concentration of the mate-rial in the air, or the solids loading ratio, and this will be considered in detail in thischapter.

In dilute phase conveying the particles are conveyed in suspension in the airand this relatively high value of velocity is due, in part, to the large difference indensity between the particles and the air. In hydraulic conveying typical velocitiesfor suspension flow are only about 300 ft/min, but the difference in density be-tween water and particles is very little in comparison. The difference in densitybetween water and air is about 800:1. Since the difference in conveying mediumvelocity is only of the order of about 10:1 it will be seen that the pressure of theair, and hence its density, will not have a major effect on the value of minimumconveying air velocity for general pneumatic conveying.

1.5 Material Properties

The properties of the conveyed material have a major influence on the conveyingcapability of a pneumatic conveying system. It is the properties of the material thatdictate whether the material can be conveyed in dense phase in a conventionalconveying system, and the minimum value of conveying air velocity required. Forthis reason the conveying characteristics of many different materials are presentedand featured in order to illustrate the importance and significance of materialproperties.

Although it is the properties of the bulk material, such as particle size andsize distribution, particle shape and shape distribution, and particle density that areimportant in this respect, at this point in time it is the measurable properties ofmaterials in bulk that are more fully understood, These include air-material inter-actions, such as air retention and permeability, and are more convenient to use. Ingeneral, materials that have either good air retention or good permeability will becapable of being conveyed in dense phase and at low velocity in a conventionalconveying system. Materials that have neither good air retention nor good perme-ability will be limited to dilute phase suspension flow.


110 Chapter 4

7.5.7 Dense Phase Conveying

There are two main mechanisms of low velocity, dense phase flow. For materialsthat have good air retention, the material tends to be conveyed as a fluidized mass.In a horizontal pipeline the vast majority of the material wi l l flow along the bot-tom of the pipeline, rather like water, with air above, but carrying very little mate-rial. At a solids loading ratio of about 150 the pipeline is approximately half full.For dense phase flows there is a distinct pulsing of the flow, with the materialflowing smoothly and then suddenly stopping for a second or two and then flow-ing smoothly again. In vertically upward flow, the flow of material also pulses,and for the second that the flow halts the material falls momentarily back down thevertical pipe.

For materials that have good permeability the material tends to be conveyedin plugs through the pipeline. The plugs fill the full bore of the pipeline and areseparated by short air gaps. As the conveying air velocity is reduced, the air gapbetween the plugs gradually fills with material along the bottom of the pipelineand the plug ultimately moves as a ripple along the top of an almost static bed ofmaterial. As the air flow rate reduces, to give very low conveying air velocities,the material flow rate also reduces.

Materials composed almost entirely of large mono-sized particles, such aspolyethylene and nylon pellets, peanuts, and certain grains and seeds, convey verywell in plug flow. In dilute phase conveying, nylons and polymers can suffer dam-age in the formation of angel hairs, and grains and seeds may not germinate as aconsequence of damage caused at the high velocities necessary for conveying.Because of the very high permeability necessary, air will readily permeate throughthe material while it is being conveyed and so maximum values of solids loadingratios will typically be about 30.

2 MATERIAL CONVEYING CHARACTERISTICS

If a pneumatic conveying system is to be designed to ensure satisfactory operation,and to achieve maximum efficiency, it is necessary to know the conveying charac-teristics of the material to be handled. The conveying characteristics will tell adesigner what the minimum conveying velocity is for the material, whether thereis an optimum velocity at which the material can be conveyed, and what pipelinediameter and air mover rating will be required for a given material flow rate andconveying distance.

Alternatively, for an existing pneumatic conveying plant, the appropriateconveying characteristics will tell a designer what flow rate to expect if it is neces-sary to convey a different material, and whether the air flow rate is satisfactory.Conveying characteristics can also be used to check and optimize an existing plantif it is not operating satisfactorily.

In order to be able to specify a pipe size and compressor rating for a re-quired duty it is necessary to have information on the conveying characteristics of


Gas-Solid Flows 111

the material. If sufficient previous experience with a material is available, suchthat the conveying characteristics for the material are already established, it shouldbe possible to base a design on the known information.

If previous experience with a material is not available, or is not sufficient fora full investigation, it will be necessary to carry out pneumatic conveying trialswith the material. These should be planned such that they will provide data on therelationships between material flow rate, air flow rate and conveying line pressuredrop, over as wide a range of conveying conditions as can be achieved with thematerial.

The trials should also provide information on the minimum conveying airvelocity for the material and how this is influenced by conveying conditions. Thisis particularly important in the case of dense phase conveying, for the differencesin conveying characteristics between materials can be very much greater thanthose for dilute phase conveying.

If the investigation is to cover the entire range of conveying modes with thematerial, then the previous experience must be available over a similar range ofconveying conditions. Scale up in terms of air supply pressure, pipe bore, convey-ing distance and pipeline geometry from existing data is reasonably predictable,provided that the extrapolation is not extended too far. Scale up in terms of modeof conveying, into regions of much higher solids loading ratios and lower convey-ing air velocities, however, should not be attempted unless evidence of the poten-tial of the material for such conveying is available.

2.1 Conveying Mode

With high pressure air, conveying is possible in the dense phase mode, providedthat the material is capable of being conveyed in this mode. It is the influence ofmaterial properties on the possible mode of conveying, as well as differences inmaterial flow rates achieved for identical conveying conditions, that makes it es-sential for conveying trials to be carried out with an untried material before de-signing a pneumatic conveying system. In conveying tests with high pressure airthere is an additional need, therefore, to establish the limits of conveying and thismay be over a very wide range of conveying conditions.

In addition to material properties, conveying distance can have a significantinfluence on the solids loading ratio at which a material can be conveyed, andhence mode of conveying that is possible. The influencing factor here is simplypressure gradient, and this will limit conveying potential regardless of the capabili-ties of the material. This aspect of conveying pipeline performance is consideredin more detail in Chapter 8.

2.1,1 The A ir Only Datum

In order to illustrate how conveying characteristics can be used it is necessary toshow first how they are built up and to examine the influence of the main vari-ables.


112 Chapter 4

30

40 80 120 160 200

Free Air Flow Rate - ftVmin

Figure 4.1 Air only pressure drop data for pipeline shown in figure 4.2.

The simplest starting point is to consider the air only flowing through thepipeline. If a graph is drawn of pressure drop against air flow rate for a conveyingline the result will be similar to that shown in Figure 4.1.

The data in Figure 4.1 relates to a 165 ft long pipeline of 2 inch nominalbore which includes nine ninety degree bends. Details of the pipeline are presentedin Figure 4.2. This pipeline was used for conveying many of the materials forwhich conveying characteristics are presented in the first part of this chapter, andseveral subsequent chapters. As a consequence, both the pipeline in Figure 4.2,and the air only pressure drop datum in Figure 4.1, will serve as a reference formuch of the data that follows.

The line representing the air only pressure drop on Figure 4.1 is effectivelythe lower limit for conveying and will appear on subsequent graphs with a zero toindicate that this is the datum for conveying and represents a material flow rate ofOlb/h.

It will be seen from Figure 4.1 that the air only pressure drop increasesmarkedly with increase in air flow rate. When material is added to the air in thepipeline, at any given value of air flow rate, there will be an increase in pressure.This is as a consequence of the drag force of the air on the particles to enable themto be conveyed through the pipeline.

The air, however, has to be at a velocity that is sufficiently high to conveythe material, otherwise the particles will not convey, and a build up of such mate-rial could cause blockage of the pipeline.


Gas-Solid Flows 113

Pipeline:165 ft long2 inch nominal bore9 * 90° bends

D/d = 24

Figure 4.2 Details of pipeline used for conveying trials.

In some situations, when fine dust is fed into a pipeline, there will be a slightreduction in pressure drop, and this relates to modification of the boundary layer.The flow rates of material involved are very small and have no relevance to pneu-matic conveying. It will be seen from Figure 4.1 that if an air mover having a lowpressure capability is to be employed, the pressure drop available for conveyingmaterial will be very limited, particularly if a high air flow rate is required for di-lute phase conveying. Pipeline bore, of course, can be increased in order to com-pensate if the pressure available for conveying is limited.

2.1.1.1 Pressure Drop EvaluationFigure 4.1 relates to single phase flow and the analysis of such flows is well estab-lished and quite straightforward. The pressure drop, Ap, for a fluid of density p,flowing through a pipeline of a given diameter, d, and length, L, can be determinedfrom Darcy's Equation:

fLpC2

Ap a lbf/in2 - - (1)d

where / is the friction factor, which is a function of the Reynoldsnumber for the flow and the pipe wall roughness,

and C is the mean velocity of the flow - ft/min

It can be seen from this mathematical model, which is presented in more de-tail in Chapter 6 on 'The Air Only Datum', that pressure drop follows a square lawrelationship with respect to velocity. This means that if the velocity is doubled the


114 Chapter 4

pressure drop will increase by a factor of approximately four. Velocity, therefore,is a very important parameter in this work and so in graphical representations ofexperimental results and data, velocity needs to be represented on one of the axes.

2.7.2 Conveying Air Velocity

A major problem with using velocity, however, is that it is not an independentvariable. Gases are compressible and their densities vary with both pressure andtemperature. Since density decreases with decrease in pressure, the velocity of theconveying gas will gradually increase along the length of a constant bore pipeline.In Figure 4.1 it will be noticed that free air flow rate has been used instead of ve-locity. Velocity, however, can be determined quite easily from the volumetric flowrate by use of the two following equations:

D V D V D Vr\ \ _ f T . 2 _ ^0 0 .-T*.

T T T•M *2 -'0

where p = absolute pressure of air - lbf/in2

V = volumetric flow rate of air - ftVmin

and T = absolute temperature of air - R(°F + 460)and the subscripts relate to:

1 = conveying line inlet2 = conveying line exit0 = free air conditions

and for a circular pipeline:

576 VC = — ft/min - - - - - . . . . - (3)

where C = conveying air velocity - ft/minand d = pipeline bore - inch

This shows quite clearly how velocity is influenced by both gas pressure andtemperature, for a given volumetric flow rate of free air, and that for any given setof conditions the gas velocity can be evaluated quite easily. These equations aredeveloped further in the next chapter.

In Figure 4.3 a graph is presented that will allow the conveying air velocityto be evaluated for any given free air flow rate and conveying air pressure for con-veying data relating to Figures 4.1 and 2. Conveying air velocity values up toabout 6000 ft/min have been considered as this is ideally the maximum value thatshould normally be employed in dilute phase conveying.


Gas-Solid Flows 115

6000 L

c

IL 4000

_o>

•= 2000

coU

Conveying Air Pressure- Ibf/in2 gauge

Atmospheric Pressure= 14-7 Ibf7in2 absolute

Pipeline Bore = 2 in nominalAir Temperature = 60 F

40 80 120 160 200

Free Air Flow Rate - ft/min

Figure 4.3 The influence of air flow rate and pressure on conveying air velocity for testpipeline and data.

2.2 Pneumatic Conveying

If a small quantity of a granular or powdered material is fed into a gas stream at asteady rate there will be an increase in the conveying line pressure drop, above theair only value, if the gas flow rate remains constant. For a given material the mag-nitude of this increase depends upon the concentration of the material in the gas.As the material flow rate into the conveying line increases, therefore, the convey-ing line pressure drop will also increase.

In a two phase flow system consisting of a gas and solid particles conveyedin suspension, part of the pressure drop is due to the gas alone and part is due tothe conveying of the particles in the gas stream. In such a two phase flow the par-ticles are conveyed at a velocity below that of the conveying gas. There is, there-fore, a drag force exerted on the particles by the gas.

For dilute phase, suspension flow, this drag force is the main contributor tothe conveying line pressure drop, whether it is accelerating the particles from thefeed point or conveying them through straight pipeline or around bends, and so itis not surprising that different materials will behave very differently. These differ-ences will be highlighted in this chapter, and they will be a major theme throughthe handbook.

2.2.7 Slip Velocity

The difference in velocity between the conveying gas and the particles is called theslip velocity. The magnitude of the slip velocity will depend upon the size, shape


116 Chapter 4

and density of the particles. For horizontal conveying, low density 20 micron sizedparticles are likely to be conveyed at about 90% of the velocity of the conveyinggas, and for high density 1000 micron sized particles the value will be about 50%.A typical representative value for the velocity of powdered materials is about 85%of the gas velocity for horizontal conveying and 75% of the gas velocity for con-veying vertically up.

2.2.2 Cases Considered

The influence of particle concentration on conveying line pressure drop over awide range of conveying air flow rates, and hence velocities, is illustrated withthree very different materials. These are ordinary portland cement, a sandy gradeof alumina and polyethylene pellets. They are representative of materials capableof the range of conveying modes discussed above and so are used to illustrate theconveying characteristics typical of these three groups of material.

Identical sets of axes have been used for presenting the conveying data foreach of the three materials so that direct visual comparisons can be made betweenthe conveying capabilities of the three materials. Each of the three materials con-sidered was conveyed through the pipeline shown in Figure 4.2. 200 ftVmin offree air was available at a pressure of 100 Ibf/in2 gauge, although the maximumvalue of pressure employed for conveying any of the materials was limited toabout 40 Ibf/in" gauge. A top discharge blow tank was used to feed each of thematerials into the pipeline.

It should be emphasized that the data presented here for the various materi-als relates only to the materials tested and to this particular pipeline. This aspect ofthe problem is considered in more detail in Chapters 7 and 8 where scaling pa-rameters are presented, which will allow the conveying data presented here to bescaled to any other pipeline required.

2.3 The Conveying of Cement

Pressure drop data for the cement is presented in Figure 4.4. This is a graph ofconveying line pressure drop plotted against free air flow rate, and lines of con-stant cement flow rate have been drawn as the family of curves. Within the limit ofthe 30 Ibf/in2 pressure drop the cement was conveyed at flow rates up to about35,000 Ib/h through this two inch nominal bore pipeline.

2.3.1 Conveying Limits

The zero line at the bottom of the graph is the curve representing the variation ofconveying line pressure drop with air flow rate for air only, which comes fromFigure 4.1 for the pipeline used. This, therefore, represents the lower limit withrespect to the material conveying capacity for the given system. Apart from thelower limit of zero for material conveying capacity, there are three other limita-tions on the plot in Figure 4.4.


Gas-Solid Flows 117

30

Q 20

0.o

10

coU

0

Material Flow Rate- I b / h * 1000 30

0 40 160 20080 120

Free Air Flow Rate - ItVmin

Figure 4.4 Pressure drop data for cement.

The first is the limit on the right hand side of the graph, but this is set onlyby the volumetric capacity of the compressor or blower used. This was 200ftVmin, and by reference to Figure 4.3 it will be seen that conveying air velocitiesare up to about 8000 ft/min at the end of the pipeline. For the majority of pneu-matic conveying systems this is considered to be the upper limit.

This upper limit is partly influenced by problems of material degradationand bend erosion in the conveying line, but it is mainly due to the adverse effecton the conveying line pressure drop and hence material flow rate. This aspect ofthe problem is considered in more detail in the next section. In terms of the overallconveying characteristics, the shape of the curves is quite clearly establishedwithin this maximum limit.

The second limit is that at the top of the graph and this is set by the pressurerating of the compressor or blower used. Once again this is not a physical limit, forif air is available at a higher pressure, it can be used for conveying, but it wouldnormally be recommended that the pipeline be stepped to a larger bore in order tolimit the very high values of conveying air velocity. This aspect of system designis considered in Chapter 9.

The third is the limit on the left hand side of the graph and this representsthe approximate safe minimum conditions for successful conveying with the mate-rial. The lines actually terminate and conveying is not possible in the area to theleft at lower air flow rates. This limit is governed by a complex combination ofmaterial properties, material concentration and conveying distance, and is consid-ered in more detail later in this section.


118 Chapter 4

Any attempt to convey with a lower air flow rate would result in blockage ofthe pipeline, in a conventional conveying system. This is because the air flow ratewould be below the minimum required to convey the material. The terminologyemployed for these situations is choking, when conveying vertically up, and salta-tion when conveying horizontally.

2.3.2 Conveying Air Velocity Effects

An alternative way of presenting the conveying data on Figure 4.4 is to plot thematerial flow rate against the air flow rate and to have a series of curves at a con-stant value of the conveying line pressure drop. Such a plot is presented in Figure4.5a. Although the air only datum is lost, this alternative plot shows the influenceof excessively high conveying air velocities very well.

The lines of constant pressure drop can be seen to slope quite steeply to theair flow rate axis, and hence to zero material flow rate at very high air flow rates,and hence velocities. This is because of the square law relationship of pressuredrop with respect to velocity, presented in Equation 1 for air only, but which ap-proximately applies to suspension flow for high velocity dilute phase conveying.

60

,50

40

_0

20

a

10

Conveying LinePressure Drop

- Ibt7in2

\35

30

(a)

0 50 100 150 200

Free Air Flow Rate - fVVmin

60

Solids LoadingRatio

Conveyingo 50oo

> 40

<uCS

30

_o

I 20O"o3

S10

0

-AREA

ConveyingPressure Dri

- Ibf/iiv

(b)

0 50 100 150 200

Free Air Flow Rate - ft3/min

Figure 4.5 Performance data for cement conveyed through the pipeline shown in Fig-ure 4.2. (a) Material flow rate data and (b) conveying characteristics.


Gas-Solid Flows 119

If the conveying system has a compressor or blower with a maximum ratingin terms of delivery pressure, a considerable amount of this available pressure willbe taken up by moving the air through the line if the air flow rate, and hence ve-locity, is too high.

Part of the pressure drop is due to the material being conveyed and thegreater the concentration of the material in the air, the greater the pressure drop. Ifthe conveying air velocity is too high, therefore, the concentration of the materialin the air will have to be reduced in order to match the available pressure drop, andso the resulting material flow rate will be much lower.

2.3.3 Solids Loading Ratio

Solids loading ratio is the term generally used by pneumatic conveying engineersto describe the conveyed gas-solids suspension flow. Solids loading ratio is theratio of the mass flow rate of the solids conveyed to the mass flow rate of the airused. The particular advantages over particle concentration are that it is a dimen-sionless quantity and its value does not vary with the conveying gas pressure. Withthe graph in Figure 4.5a being a plot of material flow rate against air flow rate,lines of constant solids loading ratio can be superimposed quite easily as they willbe straight lines through the origin. Such a plot is shown in Figure 4.5b.

The plot presented in Figure 4.5b is referred to as the conveying characteris-tics for the material and is, in effect, a performance map for the material in thegiven pipeline. A conveying limit for the material is also identified on this plot.From Figure 4.5b it will be seen that solids loading ratios up to about 140 havebeen achieved and this is quite clearly dense phase conveying. With a low airpressure and a high air flow rate, however, the cement is conveyed at solids load-ing ratios below ten and this is quite clearly dilute phase, suspension flow. It willbe seen that there is no transition between dilute and dense phase flow and so thedividing line between the two modes of flow is not clearly defined.

2.3.4 Minimum Conveying Air Velocity

The conveying limit represented on Figure 4.5b appears a little strange at firstsight. If reference is made to Figure 4.3, or if conveying air velocities are other-wise calculated, it will be seen that at the upper part of the conveying limit curvethe conveying air velocity is about 600 ft/min. This is where the solids loadingratio is about 140 and so a minimum conveying air velocity of 600 ft/min is con-sistent with that appropriate for dense phase conveying. At very low values ofconveying air pressure, and hence low values of solids loading ratio, the minimumconveying air velocity is about 2000 ft/min and this is consistent with that neces-sary for the dilute phase conveying of this type of material.

The slope of the conveying limit curve is positive in both of these extremeareas of dilute and dense phase conveying. This is due to the compressibility effectof the air. In these two regions the conveying air velocity is reasonably uniform,being about 2000 ft/min for the dilute phase conveying of the cement, and 600


120 Chapter 4

ft/min for the dense phase conveying. As the pressure of the conveying air in-creases, a greater volumetric flow rate of air is required to maintain the same valueof conveying air velocity, and hence the positive slope to the conveying limitcurve in these areas.

Between these two regions two opposing effects come into play. One is theproblem of compressibility, which means that a greater air flow rate is required asthe air supply pressure increases. The other relates to the considerable increase insolids loading ratio that is possible with an increase in conveying line pressuredrop. This means that the cement can be conveyed at a lower velocity, which inturn means that a lower air flow rate is required. The combination of these twoeffects dictates the shape of the transition between the dilute phase and the verydense phase portions of the conveying limit curve.

2.3.4.1 Solids Loading Ratio InfluenceThe relationship between the minimum conveying air velocity and the solids load-ing ratio at which a material is conveyed can be determined experimentally withthe material in a pipeline. This is typically derived during the conveying trials car-ried out with a material in order to determine the conveying characteristics for thematerial, since the determination of conveying limits is generally an integral partof the test work.

Pneumatic conveying trials with bulk particulate materials are considered inChapter 23. The approximate influence of solids loading ratio on the minimumconveying air velocity for the cement is presented in Figure 4.6.

3000

• 2000

enc

coU

1000

20 40 60 80 100

Solids Loading Ratio

Figure 4.6 Approximate influence of solids loading ratio on the minimum value ofconveying air velocity for the pneumatic conveying of ordinary portland cement.


Gas-Solid Flows 121

This curve is typical of the relationship between minimum conveying air ve-locity and solids loading ratio for air retentive materials that are capable of beingconveyed in the sliding bed mode of dense phase flow. This relationship has amajor influence on the operation and pneumatic conveying capability of this typeof material and will feature at many points throughout this Handbook. Possibly thegreatest effect is the change that occurs with increase in conveying distance, whichis considered in Chapter 7.

Since high solids loading ratios can only be achieved with a high value ofpressure gradient, an increase in conveying distance will mean that the value ofsolids loading ratio must be reduced if there is no increase in the air supply pres-sure. A reduction in solids loading ratio, as will be seen from Figure 4.6, will re-quire an increase in conveying air velocity and this will consequently require anincrease in air flow rate.

In the extreme the solids loading ratio will reduce to a value at which thematerial can only be conveyed in dilute phase. This relationship is introduced laterin this Chapter.

2.4 The Conveying of Alumina

The grade of alumina used and reported here is one that is generally referred to asbeing sandy or coarse. The alumina was conveyed through the pipeline shown inFigure 4.2 and the pressure drop data for the material is presented in Figure 4.7.This is a graph of conveying line pressure drop plotted against free air flow rate,and lines of constant alumina flow rate have been drawn as the family of curves.

D.eQu

IOJ

OH

DOC

CoU

30

20

10

Material Flow Rate- l b / h x 1000

40 80 120 160 200

Free Air Flow Rate - fr/min

Figure 4.7 Pressure drop data for sandy alumina.


122 Chapter 4

Within the limit of the 30 Ibf7in2 pressure drop the alumina was conveyed atflow rates up to about 25,000 Ib/h through this two inch nominal bore pipeline. Ifthis is compared with the corresponding data for the cement in Figure 4.4 it will beseen that the maximum value of flow rate for the alumina is very much lower andthat the air flow rate required to achieve 25,000 Ib/h is significantly greater thanthat required to convey the cement at 35,000 Ib/h.

The same conveying limits, as discussed in relation to the conveying of ce-ment, apply to the alumina. It is the same pipeline and so the air only pressuredrop relationship is the same. It is the same air supply and so the air flow rate andpressure considered are also the same. It is the conveying limit for the material thatdiffers. Conveying capability and conveying limits, however, do differ widelyfrom one material to another, and this is why conveying data is so essential.

2.4.1 Conveying A ir Velocity Effects

An alternative presentation of the data, in terms of material flow rate plottedagainst air flow rate, with lines of constant conveying line pressure drop superim-posed, is presented in Figure 4.8a. Once again this graph is drawn with the sameaxes as that for the cement in Figure 4.5a so that a direct visual comparison of thetwo materials can be made.

60

50

40

g30

o

20od

10

(a)


- lbf/in2

25

50 100 150 200


60

50

o

* 40.o

I30•5

320

10

(b)

NO

GO

AREA

Solids LoadingRatio

ConveyingLimit

Conveying Line 20Pressure Drop

- lbf/in2

50 100 150 200


Figure 4.8 Performance data for sandy alumina conveyed through the pipeline shownin figure 4.2. (a) Material flow rate data and (b) conveying characteristics.


Gas-Solid Flows 123

The comparison is striking in terms of the small area of the graph in whichthe data for the sandy alumina appears. It will be noted in Figure 4.8a that the linesof constant conveying line pressure drop terminate at progressively higher airmass flow rates as the material flow rate increases. This does not mean that theminimum conveying air velocity increases. This is entirely due to the influence ofair pressure and the compressibility of the air. By reference to Figure 4.3, it will beseen that the minimum conveying air velocity for this material is about 2600ft/min and that it changes little over this range of material concentration.

This slope of the minimum conveying limit on Figure 4.8b is a characteristicfeature of all materials conveyed in dilute phase and will be seen on the conveyingcharacteristics for most of the materials presented here. It applies equally to mate-rials capable of being conveyed in dense phase, if the pressure gradient is low, aswill be seen in the very low pressure area on Figure 4.5b for the cement.

With the cement it was possible to convey the material with higher air sup-ply pressures. From Figure 4.5a it will be seen that within the limit of 60,000 Ib/hof material, conveying line pressure drop values up to 40 Ibf/in2 were employed.From Figure 4.8a for the alumina it will be seen that 25 Ibf/in2 is close to the maxi-mum pressure that could be employed. Although the air pressure with the test fa-cility was available at 100 Ibf/in2 gauge, a pressure higher than 25 Ibf/in2 could notbe used because the volumetric flow rate of the air was limited to 200 ftVmin.

The locus of the conveying limit line is included on Figure 4.8b and it willbe seen that this passes through the 200 ftVmin air flow rate limit with an air sup-ply pressure of about 30 Ibf/in2. At these air supply pressures the minimum con-veying air velocity for the alumina is about 2600 ft/min compared with only 600ft/min for the cement, and so for a given air supply pressure the air flow rate ismore than four times greater.


The conveying capability for the alumina is clearly illustrated with the conveyingcharacteristics presented in Figure 4.8b. The maximum value of solids loadingratio achieved is only just over 25 and this, together with the minimum conveyingair velocity of 2600 ft/min, equates to dilute phase, suspension flow for the mate-rial. Despite the fact that a high air supply pressure was available, the material isonly conveyed in dilute phase. It must be stressed, therefore, that high pressure isnot synonymous with dense phase conveying.

Solids loading ratios for dilute phase conveying are generally much lowerthan 25. The fact that a solids loading ratio as high as 25 was achieved in this caseis due to the fact that the pipeline was relatively short at 165 ft and the air supplypressure was relatively high at 25 Ibf/in2. To complete the picture for the alumina aplot of the minimum conveying air velocity versus solids loading ratio is presentedin Figure 4.9. This is simply a horizontal line at a value of 2600 ft/min over a lim-ited range of solids loading ratios. This is typical of materials that can not be con-veyed in dense phase, and there is generally little change in the value of the mini-mum conveying air velocity value over the range of solids loading ratios.


124 Chapter 4

3000^

I

<5

2000

.S 1000

ooc

oO

20 40 60

Solids loading ratio

80 100

Figure 4.9 Approximate influence of solids loading ratio on the minimum value ofconveying air velocity for the pneumatic conveying of sandy alumina.

2.5 Comparison of Materials

To illustrate the influence of the material on conveying capability further, the con-veying characteristics for two more materials conveyed through the Figure 4.2pipeline are presented in Figures 4.10a and 4.1 Ob. The first of these is a fine gradeof pulverized fuel ash, obtained from the electrostatic precipitators of a boilerplant, and had a mean particle size of about 25 micron.

The second is a silica sand, obtained from a quarry, and air classified to givea mean particle size of about 70 micron. It will be seen that this pair of materialsare very similar to the cement and alumina in terms of overall characteristics, par-ticularly with regard to minimum conveying limits.

The conveying line pressure drop curves for the pulverized fuel ash, how-ever, are very much steeper than those of the cement and so very much highermaterial flow rates were achieved at low values of air flow rate. As a consequencevery much higher values of solids loading ratio were achieved.

Apart from differences in density, the shape of the particles are also verydifferent, with the cement coming from a grinding process and fly ash being de-rived from a combustion process. It is not surprising, therefore, that the conveyingcharacteristics are very different. Values of minimum conveying air velocity,however, are very similar.

The differences between the alumina and the sand are not so pronounced, al-though the sand has a slightly lower value of minimum conveying air velocity, buta much lower material flow rate was achieved for a given pressure drop.


Gas-Solid Flows 125

60

50

40

ai

o20

S 10

(a)

300200 120 100 8060

50 100 150 200

Free Air Flow Rate - frYmin

40

.320

10

0

(b)

NO

GO

AREA

Solids LoadingRatio

ConveyingLimit

Conveying LinePressure Drop 25

\ 35 V.20

15

10

0 50 100 150 200


Figure 4.10 Conveying characteristics for materials conveyed through the pipelineshown in figure 4.2. (a) A fine grade of pulverized fuel ash and (b) silica sand.

2.6 The Conveying of Polyethylene Pellets

Polyethylene pellets were also conveyed through the pipeline shown in Figure 4.2and the pressure drop data for the material is presented in Figure 4.11. This is agraph of conveying line pressure drop plotted against free air flow rate, and linesof constant material flow rate have been drawn as the family of curves. The axesused are the same as those employed for the cement in Figure 4.4 and the aluminain Figure 4.8 and so a direct visual comparison can be made.

Within the limit of the 30 lbf/in2 pressure drop the pellets were conveyed atflow rates up to about 30,000 Ib/h through this two inch nominal bore pipeline.This compares with 35,000 Ib/h for the cement and 25,000 Ib/h for the alumina butthe main point is that the overall conveying data is very different once again. Con-veying is possible at very low values of air flow rate, like the cement, but themaximum value of material flow rate was achieved at the highest value of air flowrate, like the alumina.

The same conveying limits, as discussed in relation to the conveying of boththe cement and alumina, apply to the polyethylene pellets. It is the same pipelineand so the air only pressure drop relationship is the same. It is the same air supplyand so the air flow rate and pressure considered are also the same. In this case it isthe conveying limit for the material that differs and the behavior of the material atlow values of conveying air velocity.


126 Chapter 4

30MaterialFlow RateIb/h x 1000

80 120 160 200Free Air Flow Rate - ftVmin

Figure 4.11 Pressure drop data for polyethylene pellets.

2.6.1 Conveying Air Velocity Effects

An alternative presentation of the data, in terms of material flow rate plottedagainst air flow rate, with lines of constant conveying line pressure drop superim-posed, is presented in Figure 4.12a. Once again this graph is drawn with the sameaxes as those for the cement in Figure 4.5a and the alumina in Figure 4.8a so that adirect visual comparison of the three materials can be made. Once again the pat-tern of the data is totally different from that of the previous two materials.

With this material a distinct pressure minimum point is observed. The linesof constant conveying line pressure drop on Figure 4.12a change slope at the pointwhere the material flow rate is a maximum. The term 'pressure minimum' is actu-ally derived from Figure 4.11 where a minimum value of conveying line pressuredrop can be seen for each of the lines of constant material flow rate.

With the pressure drop lines changing slope below the pressure minimumpoint, material flow rates reduce considerably with further decrease in air flowrate. Although conveying is possible at lower air flow rates, unlike the alumina,material flow rates are significantly lower than those achieved with the cement. Itwill be seen that the pressure drop lines all merge below the pressure minimumpoints and this is why the curves on Figure 4.11 rise vertically below the pressureminimum point.

The material flow rate, however, was reasonably uniform over the entirerange of conveying conditions, although this is not always the case with this typeof material. With the pipeline being of relatively small bore it was not possible toseparate the lines of constant pressure in this area.


Gas-Solid Flows 127

60

50

ooo

40

« 30cd

as

FT 20

10

60

50


- lbf/in2

NO

GO

AREA

\ Conveying LinePressure Drop

- lbf/in%

Solids LoadingRatio

(a)

0 50 100 150 200Free Air Flow Rate - ftVmin (b)

50 100 150 200Free Air Flow Rate - ftVmin

Figure 4.12 Performance data for polyethylene pellets conveyed through the pipelineshown in figure 4.2. (a) Material flow rate data and (b) conveying characteristics.


The conveying characteristics for the polyethylene pellets are presented in Figure4.12b and from this it will be seen that the maximum value of solids loading ratioat which the material was conveyed is no different from that of the sandy alumina,at about 25. The polyethylene pellets, however, were successfully conveyed withconveying air velocities down to about 600 ft/min and so this is clearly densephase flow at low values of air flow rate.

The material, having a relatively large particle size, and being mono-sized,means that it is very permeable. As a consequence the air will pass through apacked bed of the material relatively easily. It is as a result of the material beingvery permeable that it will naturally convey in dense phase in a plug flow mode.With the material being so permeable it is possible that the material could be con-veyed with very much lower air velocities than 600 ft/min without blocking thepipeline. As the material flow rate decreases with decrease in air flow rate, belowthe optimum point, however, the benefits of ultra low velocity conveying need tobe carefully considered.

As with the cement, a natural transition from dilute phase conveying todense phase conveying occurs, but the value of the solids loading ratio provides noguidance in this case. It can, however, be determined by the value of the convey-ing line inlet air velocity and this is considered further in Chapter 7.


128 Chapter 4

3 THE INFLUENCE OF MATERIALS

The conveying characteristics for different materials can vary significantly, asillustrated with the representative group of materials considered above. This isparticularly so for materials that are capable of being conveyed in dense phase. Atlow values of air flow rate the lines of constant conveying line pressure drop canhave a wide variety of slopes. There is also the added complexity of different ma-terials having different minimum conveying limits. Thus for a given air flow rateand conveying line pressure drop, material flow rates for different materials canvary considerably, and the air flow rate necessary to convey different materials canalso vary considerably.

Some of these differences were illustrated earlier with the materials used toshow how conveying characteristics are determined. These differences, however,are not just a feature of conveying with high pressure air but will be found in lowpressure systems also.

3.1 Low Pressure Conveying

If only low pressure air is available for conveying a material through a pipeline,such as that from a positive displacement blower or any vacuum conveying sys-tem, and with a pressure drop below about 15 lbf/in2, a material will only be con-veyed in dilute phase through a pipeline, unless the conveying distance is veryshort. Conveying at high values of solids loading ratio typically requires high val-ues of conveying line pressure drop.

The influence of solids loading ratio on pressure drop is illustrated in Figure4.13. This is an extension of the data presented in Figures 4.4 to 4.6 for the cementand is a plot of conveying line pressure drop against free air flow rate, with linesof constant solids loading ratio superimposed.

For the pipeline shown in Figure 4.2, for which the data relates, a conveyingline pressure drop in excess of 10 lbf/in" is required before the material can beconveyed in true dense phase and at low velocity. It will be seen that the volumet-ric flow rate of the air has a significant effect in this respect and helps to illustratewhy high values of solids loading ratio are not possible for materials that can notbe conveyed in dense phase.

If the pipeline is very much shorter, and with fewer bends, however, the aironly curve and the pressure drop axis on Figure 4.13 will be significantly reducedand low velocity dense phase conveying will be possible at much lower values ofpressure drop.

By positioning the reception vessel on the quayside close to bulk containerships, conveying distances can be kept very short and materials such as cementcan be off-loaded in dense phase, and at very high flow rates, with a vacuum con-veying system. Fly ash can similarly be transferred from electrostatic precipitatorson boiler plants to intermediate reception vessels in dense phase by vacuum con-veying systems.


Gas-Solid Flows 129

30

o,eD

20

g ' l O

coU

Solids LoadingRatio

NO

GO

AREA

Conveyin:Limit

120 100 80 60 50 4030

20

40 80 120

Free Air Flow Rate - frVmin

160 200

Figure 4.13 Solids loading ratio data for cement.

Conveying data for four different materials is presented in Figure 4.14. Eachmaterial was conveyed in a positive pressure conveying system up to a limit of 8lbf/in2 in terms of conveying line pressure drop. All four materials were conveyedthrough the same pipeline, a sketch of which is given in Figure 4.15. Althougheach material could only be conveyed in dilute phase, because of the limit on pres-sure available, it will be seen that there are significant differences in their convey-ing capabilities.

The differences between materials are mainly in terms of the material flowrates achieved, varying from 8500 Ib/h for the pearlite to 3500 Ib/h for the ironpowder, for a pressure drop of 8 lbf/in2. Since all the materials were conveyed indilute phase, and they were all either powders or fine granular materials, suchmarked differences would not be expected in terms of minimum conveying airvelocities. With a 3 lbf/in2 pressure drop, these varied between 2400 ft/min for thepearlite and 3200 ft/min for the iron powder.

Although the iron powder achieved the lowest flow rate of the four materialspresented, it should be noted that the iron powder conveyed very well, regardlessof the fact that particle density was 355 lb/ft3 and the bulk density about 150 lb/ft3.Metal powders can be conveyed pneumatically; the main problem is that many ofthem are potentially explosive and so require to be conveyed with nitrogen.

Uranium with even higher density values is regularly conveyed in pneu-matic systems because of the safety aspects of the conveying system. At the otherextreme the pearlite had a bulk density of only 6 !b/ft' and a particle density of 50lb/ft'. With a higher pressure gradient available both the iron powder and pearlitehave dense phase conveying potential.


130 Chapter 4

o 7

x 6X3

~̂T 5<L>"8

3ox,

2u

!2

i0

(a)

Conveying LinePressure Drop 8

- lbf/in2

Solids LoadingRatio

40 60 80 100 120Free Air Flow Rate - ft'/min

Solids LoadingConveying Line Ra(io

Pressure Drop- lbf/in2

(b)40 60 80 100 120

Free Air Flow Rate - frVmin

o2 6X

_c£ 5

8. 4

3

B 2

.Conveying Line- Pressure Drop

- lbf/in2

Solids LoadingRatio

oo

(c)

40 60 80 100 120


1

0

(d)


- lbf/in2

Solids LoadingRatio

40 60 80 100 120


Figure 4.14 Conveying characteristics for low pressure conveying of materials, (a)Pearlite, (b) sodium chloride (salt), (c) iron powder, and (d) sodium carbonate (soda ash).


Gas-Solid Flows 131

Pipeline:length 115f tbore 2 inbends 8 x 90°D/d = 5

Figure 4.15 Details of pipeline used for low pressure conveying trials.

Many different materials have been tested in the pipeline presented in Figure4.15. To illustrate how the conveying characteristics of different materials canvary in such a low pressure system, the 8 lbf/in2 constant conveying line pressuredrop curves from a number of such materials are compared on Figure 4.16.

With additional materials it will be seen that the conveying performance, interms of material flow rate achieved, does not correlate with material density.Soda ash is little better than iron powder and pulverized fuel ash is better thanpearlite in terms of material flow rate achieved. Lump coal is better than finegranular salt, although a slightly higher value of conveying line inlet air velocity isrequired, and so performance does not correlate with particle size either.

(U^ .oi 4

• g 2

Pulverized Fuel Ash(fine grade)

Pearlite

Soda Ash

Silica Sand

Iron Powder

20 40 60 80 100 120 140

Free Air Flow Rate - fr/min

Figure 4.16 Comparison of performance of different materials conveyed through thepipeline shown in figure 4.15 with a conveying line pressure drop of 8 lbf/in2.


132 Chapter 4

Different conveying capabilities and air requirements mean that particularcare must be taken if an existing system is to be used to convey another material,or if one system is required to convey a number of different materials. If the capa-bility of a system is dictated by the pressure rating of the air mover, then differentmaterial flow rates must be expected, and the feeding device must be capable ofmeeting the needs of any other material. A different air flow rate may also be re-quired, as shown by the different minimum values for conveying line inlet air ve-locity.

3.2 High Pressure Conveying

If high pressure air is available for conveying a material, and the pipeline is not toolong, then the material could be conveyed in dense phase if the material is capableof being conveyed in dense phase. Conveying data for a further four materials ispresented in Figure 4.17. All four materials were conveyed through the samepipeline once again, so that direct comparisons of performance can be made. Thepipeline is the same as that used for the earlier high pressure conveying trials, asketch of which was given in Figure 4.2. The compressor used for this high pres-sure work was capable of delivering 200 frVmin of free air at 100 lbf/in2 gauge.

The four materials presented include two food products and two metal prod-ucts, and from each group, one material could not be conveyed in dense phase andone could. The materials that could be conveyed in dense phase were conveyed atsolids loading ratios of well over 100 and were conveyed in the sliding bed modeof dense phase flow. These were wheat flour and iron powder and the conveyingcharacteristics for these materials are very similar in form to those for the cementin Figure 4.5b and the fly ash in Figure 4.10a presented earlier. With high pressureconveying air, and at high values of solids loading ratios, all four of these materi-als could be conveyed with conveying line inlet air velocities as low as 600 ft/min.

That high pressure is not synonymous with dense phase conveying is clearlyshown with the granulated sugar. A minimum conveying air velocity of 3200ft/min had to be maintained and, as a result, the maximum pressure that could beused was only 25 lbf/in2, because of the limit of 200 ftVmin on air flow rate avail-able. As a consequence the maximum solids loading ratio achieved was well be-low 20. Granulated sugar has both poor air retention properties and poor perme-ability.

Flour and sugar are often materials that are required to be conveyed with acommon system and often through the same pipeline. It will be seen that there aresignificant differences between the conveying capabilities of these two materials.The specification of air requirements represents a particular problem, apart fromchoice of feeder and controls.

Many different materials have been conveyed in the pipeline shown in Fig-ure 4.2. To show how the conveying characteristics of different materials can varyin such a high pressure system, the 20 lbf/in2 constant conveying line pressure dropcurves from a number of such materials are compared on Figure 4.18.


Gas-Solid Flows 133

50

I40

I30

° 20fcu

is'C0)

'S10

0

ConveyingLimit

Conveying Line

Solids LoadingRatio

50

>40

I730

Bi

.220~is<BJS10

(a)

0 40 80 120 160 200Free Air Flow Rate - ft3/min

NO

GO

AREA

Conveying LinePressure Drop -

- lbf/in2

Solids LoadingRatio

ConveyingLimit

(b)

40 80 120 160 200


60

50ooo

^ 4 0£

£ 30

_oi 2°.5

'8S 10

ConveyingConveying Line Limit" Pressure Drop

Solids LoadingRatio

AREA

50 100 150 200

(C)Free Air Flow Rate - ft/min

60

050

X

=5 4Q

u

(S30s_oE|20

110

-

: NO• GO"

: AREA; c

• Conveying LinePressure Drop

; - lbf/in2 \

; 15

: 5.̂

SolidsRe

Convey!Limit

^^r=-j .

(d)

0 50 100 150 200Free Air Flow Rate - ftVmin

Figure 4.17 Conveying characteristics for high pressure conveying of materials, (a)Wheat flour, (b) granulated sugar, (c) iron powder, and (d) zircon sand.


134 Chapter 4

50

ooo 40

30

FT 20

10

Pulverized Fuel Ash - fine

SugarGranulated

Barite

Iron Powder

Cement

Wheat Flour

PVC Powder

Magnesium/Sulfate

50 100 ^ 150Free Air Flow Rate - ft'/min

200

Figure 4.18 Comparison of performance of different materials conveyed through thepipeline shown in figure 4.2 with a conveying line pressure drop of 20 !bf/in2.

It will be noted that at the extreme right of Figure 4.18, at high air flowrates, all the materials are conveyed in dilute phase and the degree of scatter inmaterial flow rates is similar to that shown in Figure 4.16. All the pressure dropcurves have a negative slope in this area and each one will probably reach the airflow rate axis at a value of about 600 ft3/min.

As a result of the different slopes of the pressure drop curves, at low valuesof air flow rate, for the different materials, quite remarkable differences in materialflow rate can be obtained. This is for materials conveyed through exactly the samepipeline and under exactly the same conveying conditions. Differences in mini-mum conveying air velocities, for materials that will not convey in dense phase,significantly add to the problems of reliable system design, particularly for a newor unknown material.

3.2.7 Conveying L im its

Conveying limits in terms of minimum conveying air velocities and maxi-mum solids loading ratios vary widely for different materials. This point is clearlyillustrated in Figure 4.19 with the limits for three materials presented. Each mate-rial was conveyed through the 165 ft long pipeline shown in Figure 4.2. Althoughthe fine grade of fly ash could be conveyed at solids loading ratios in excess of200 and with minimum conveying air velocities close to 600 ft/min the copperconcentrate could not be conveyed above a solids loading ratio of about 55.


Gas-Solid Flows 135

50

§40

o20

.3j§

10

Limit for Copper Concentrate

'100 /80

20

10

Solids Loading Ratio

50 100

Free Air Flow Rate - fr'/min

150 200

Figure 4.19 Comparison of material conveying limits for conveying under identicalconveying conditions.

The minimum conveying air velocity for the copper concentrate was about1600 ft/min. With the granulated sugar, however, conveying at a solids loadingratio of 20 could not be achieved and the minimum value of conveying air velocitywas about 3200 ft/min.

4 MATERIAL CHARACTERIZATION

Certain material characteristics can be used to predict the potential behavior of amaterial when pneumatically conveyed. These are mostly based on bulk propertiesof the material that relate to material-air interactions, such as fluidization, air re-tention and permeability.

The air retention capabilities of a bulk material are a good indicator ofwhether a material will convey in dense phase or not. Powdered materials such asfly ash, cement, and flour have very good air retention properties and are generallycapable of being conveyed at low velocities in a sliding bed mode of dense phaseflow. Large mono-sized particles having very good permeability, such as polyeth-ylene pellets are generally capable of being conveyed at low velocities in a plugmode of dense phase flow.

Coarse granular materials such as sand and alumina, that have very poor airretention and permeability are generally only capable of being conveyed in dilute


136 Chapter 4

phase suspension flow in conventional pneumatic conveying systems, particularlyif they have a wide particle size distribution.

4.1 The Geldart Classification

The Geldart classification of materials is essentially in terms of two material prop-erties [1J. One is the difference in densities between the material particles and thefluidizing medium. For air this can simply be taken as the particle density, sincethe density of air is negligible in comparison.

The other property is the mean particle size of the material. This classifica-tion is shown here in Figure 4.20. It includes four broad areas that identify thebehavior of bulk materials when fluidized. It has often been considered that thisform of classification could be used to assess the suitability of materials for densephase conveying.

Group A materials retain air and the fluid bed collapses very slowly whenthe gas is turned off. These materials are generally capable of being conveyed indense phase. Group B materials do not retain air and the fluid bed collapses almostinstantaneously when the gas supply is turned off. These materials are not gener-ally capable of being conveyed in dense phase in a conventional conveying systemand so are restricted to dilute phase, suspension flow.

Group C materials are essentially cohesive and will behave in a similarmanner to Group A materials but are more difficult to handle. They will generallyconvey in dense phase but the main problem is often one of feeding them into thepipeline. Group D materials are likewise an extension of Group B in terms ofpneumatic conveying.

"o

I

500

100

50

10

10 50 100 500 1000

Mean Particle Size - micron

5000

Figure 4.20 Geldart's classification of fluidization behavior for fluidization with ambi-ent air.


Gas-Solid Flows 137

This Group D classification, however, being in terms of mean particle size,is not capable of identifying materials that are capable of being conveyed in densephase in plug flow mode, for this is only appropriate to essentially mono-sizedparticles.

By the same reasoning the line separating Groups A and B is not particularlyreliable in identifying the division between dilute and dense phase conveying ca-pability.

4.2 Dixon's Slugging Diagram

Dixon [2], realized the importance of material type on the mode of conveying anddevised a classification known as the Slugging Diagram, which is shown in Figure4.21. The axes are the same as those for the Geldart classification and the samedivisions are identified. This classification, however, clearly identifies the capabil-ity of large mono-sized particles for conveying in the plug mode of dense phaseflow.

An understanding of the role of particle properties such as size, and size dis-tribution, shape or fractal properties and density will probably provide the ultimatesolution to the problem. It is, however, very difficult to quantify properties such asparticle shape and size distribution, and so measurable bulk properties relating togas-particle interactions offer the best short-term means of using property valuesto predict pneumatic conveying performance. Air retention, permeability and spe-cific surface are probably the best properties to consider for this purpose, althoughthe first two are probably the easiest to measure and determine.

500

100

t 50gQ

10

Group D

StrongAxisymmetric

Slugs

Group C

10 50 100 500 1000

Mean Particle Size - micron

5000

Figure 4.21 Dixon's slugging diagram.


138 Chapter 4

4.3 Aeration Property Classification

Jones and Mills [3, 4] used a vibrated de-aeration constant and permeability factorto produce an empirical material classification for conventional pneumatic con-veying systems. The correlation that they produced is presented in Figure 4.22.For convenience the de-aeration rate was determined by vibrating the materialfrom the 'as poured' condition rather than measuring it from the fluidized state.

This clearly identifies the three main modes of conveying. Dense phase,moving bed flow, will naturally occur with materials that have very poor perme-ability and very low values of de-aeration. Dense phase, plug type flow, will natu-rally occur with materials that have very good permeability and a very rapid rateof de-aeration. The center grouping represents materials that are generally re-stricted to dilute phase flow in a conventional conveying system.

Materials that have very good air retention, and hence a low vibrated de-aeration rate value, such as cement, flour and fly ash, fall into the Group 1 cate-gory, and will convey very well in a conventional conveying system. A simple testto apply is to half fill a glass jar, preferably having a screw top lid, with a sampleof the material to be conveyed. Invert the jar a few times to aerate the material,place it on a surface, remove the lid, and drop a ball bearing or similar object intothe jar. If the ball bearing falls through the material and hits the bottom of the jar,the material is likely to have good air retention properties and be a potential candi-date for dense phase conveying.

With a material such as cement, the ball bearing will hit the bottom of thejar, even if it is dropped in the jar several minutes after the material has been aer-ated and left standing, as it has such good air retention properties.

T3& 0-5

S10

GROUP 2

DILUTE PHASE

GROUP 1 \ (SusPension Flow)Q

GROUP 3

PLUG TYPEFLOW

MOVING BEDTYPE FLOW

0-1 1 10 100Permeability Factor - ft3 in/lb x l(r6

Figure 4.22 Bulk material property classification for pneumatic conveying.


Gas-Solid Flows 139

If the material is granular, the ball bearing is unlikely to penetrate the mate-rial and will simply come to rest on the top of the surface. In this case the materialis unlikely to have sufficient air retention to allow it to be conveyed in dense phasein a conventional conveying system.

If the material has good permeability, however, such that it falls into Group3, it is possible that the material will convey at low velocity in the plug type densephase mode of flow. Pelletized materials, such as polyethylene and nylon, areideal candidates and will convey very well in a conventional conveying system.Coarse granular materials having a wide particle size distribution, however, do notgenerally have sufficient permeability to be capable of dense phase conveying inthe plug phase mode.

5 CONVEYING SYSTEM CAPABILITY

For a given material a particular problem with pneumatic conveying systems is theevaluation of their conveying potential. The capability of a pneumatic conveyingsystem in terms of achieving a given material mass flow rate, depends essentiallyon the following three parameters:

D the diameter of the pipeline,D the distance to be conveyed, andD the conveying line pressure drop available.

Within normal limits, and for a given material, air flow rate is a secondaryfunction, being primarily dependent upon the pipeline bore and air pressure. It is,however, important with respect to achieving optimum conveying conditions in agiven pipeline. The properties of the material to be conveyed are also of para-mount importance. Their main influence, however, in terms of material mass flowrate, is in placing an upper limit on the solids loading ratio at which the materialcan be conveyed under particular conditions, as shown in Figure 4.19.

5.1 Solids Loading Ratio - </>

The solids loading ratio of a conveyed material is the dimensionless ratio of themass flow rate of the material being conveyed to the mass flow rate of the air usedfor conveying.

m<t> = — (4)

ma

where mp = mass flow rate of material - Ib/h

and ma = mass flow rate of air - Ib/h


140 Chapter 4

Since air is a compressible fluid its density changes with pressure and so thevolumetric flow rate, and hence velocity, of the conveying air can increase quitesignificantly along the length of a pipeline. Solids loading ratio, therefore, is aparticularly useful parameter for describing the concentration of the material in theair in pneumatic conveying system pipelines, for it is a dimensionless quantity andits value remains essentially constant. This applies to stepped bore pipelines aswell as single bore lines.

5.2 The Influence of Pipe Bore

The mass flow rate of a material can be expressed in terms of the solids loadingratio at which the material is conveyed, by:

m = m Ib/h (5)

Note:To convert free air flow rate, in ftVmin, to a mass flow rate, in Ib/h,multiply by the density of the air, in lb/ff, and by 60 min/h:

x 0-0765 x 60 Ib/h (6)

Thus

Therefore

m,.

where F0 = volumetric flow rate of free air - ftVmin

ma cc

V =

and for a circular pipeline

n d2

576x C ft7min (7)

where d = pipeline bore - inand C = conveying air velocity - ft/min

oc Ccf (8)

As a first order approximation, for simplicity, conveying air velocity, C, canbe considered as being constant, so that:

m cf (9)

For a given system, therefore, throughput capability can be increased quiteconsiderably by increasing the pipe bore and so enable high material flow rates tobe achieved. The air requirements, of course, also have to be increased in the sameproportion in order to maintain an equivalent air velocity.


Gas-Solid Flows 141

In terms of achieving a given material flow rate over a specified distance,pipeline bore is probably the main variable. Pressure drop is also important, but anincrease in air supply pressure is not always possible. Pipeline bore also has a sig-nificant effect on the air only pressure drop value, and this is particularly importantif a low pressure air supply is to be used. A significant proportion of the availablepressure could be used in getting the air through the pipeline. This aspect of sys-tem design is considered in Chapter 6.

5.3 The Influence of Pressure and Distance

The inter-relating effects of conveying line pressure drop and conveying distanceare illustrated for low pressure systems in Figure 4.23, and for high pressure sys-tems in Figure 4.24. The data is in terms of an approximate value of solids loadingratio that might be achieved for typical combinations of air supply pressure andconveying distance. It must be stressed that these figures are only approximationsfor the purpose of illustration and should not be used for design purposes. Pipebore, conveying air velocity and, more particularly, material type, all have an in-fluence on the overall relationship.

For very short distances it is quite possible to convey a material at high val-ues of solids loading ratio, even with the limited pressure drop available withnegative pressure systems, as will be seen in Figure 4.23, provided that the mate-rial is capable of being conveyed in this mode. Pressure gradient, therefore, is theparameter that will dictate the potential mode of conveying for a material that iscapable of being conveyed in dense phase.

150 100 80 60 40

o003

15

10

£ 5

.D.3

t/3

< -10

200 „ 300 400Conveying Distance"^

500

5

-10

100 80 60 40 30

Figure 4.23 Influence of air supply pressure and conveying distance on solids loadingratio for low pressure systems.


142 Chapter 4

60

50

40

3 30cn

a0-

20

., ,0

30

500 1000 1500 2000

Conveying Distance - ft

2500

Figure 4.24 Influence of air supply pressure and conveying distance on solids loadingratio for high pressure systems.

Figure 4.24 shows that if very long conveying distances are required, thesolids loading ratio will be relatively low, even with a high pressure system. Witha low pressure system the maximum value of solids loading ratio that can beachieved will be very low, and then only with a large bore pipeline. It must bestressed once again that the high values of solids loading ratio are only applicableif the material being considered is capable of being conveyed in dense phase.

REFERENCES

1. D. Geldart. Types of gas fluidization. Powder Technology. Vol 7. pp 285-292. 1973.2. G. Dixon. The impact of powder properties on dense phase flow. Proc Int Conf on

Pneumatic Conveying. London. Jan. 1979.3. M.G. Jones and D. Mills. Product classification for pneumatic conveying. Powder

Handling and Processing. Vol 2. No 2. June 1990.4. D. Mills. Pneumatic Conveying Design Guide. Butterworth-Heinemann. 1990.


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