21 Material Degradation Problems
1
INTRODUCTION
Many materials that have to be conveyed are friable, and particles are liable to be broken when they impact against retaining surfaces, such as bends in the pipeline. As a consequence there is often a reluctance to use pneumatic conveying systems for this category of materials, particularly if the material has to be conveyed in dilute phase and hence at high velocity. There are, however, numerous means by which the problem can be reduced to an acceptable level. 1.1
Breakage Mechanisms
In some bulk solids handling processes intentional breakdown of the material is required, as in crushing, grinding and comminution. In many handling and storage situations, however, unintentional breakage occurs. This is usually termed degradation or attrition, depending on the mechanism of particle breakage. Bulk materials, when pneumatically conveyed, will impact against bends in the pipeline, and there may be a significant amount of particle to particle interaction in addition. There may also be frequent impacts against the pipeline walls, and in dense phase flows particles will slide along the pipeline walls. These collisions and interactions will produce forces on the particles that may lead to their breakage.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
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If particle breakdown occurs readily the bulk solid is said to be friable. Tendency to particle breakdown covers three main situations. The first is a tendency to shatter or degrade when the bulk solid is subject to impaction or compressive loading. The second is the tendency for fines and small pieces to be worn away by attrition when bulk solids either rub against each other or against some surface, such as a pipeline wall or bend. The third is the tendency for materials such as nylons and polymers to form angel hairs when conveyed, as a result of micromelting occurring due to the particles sliding against pipeline walls. 1.2
Magnitude of Problem
Of all conveying systems, dilute phase probably results in more material degradation and attrition than any other. This is because particle velocity is a major variable in the problem and, in dilute phase conveying, high velocities have to be maintained. The potential influence of a pneumatic conveying system on a material is demonstrated in Figures 21.1 and 21.2 [1], This is a consequence of conveying a friable material at an excessively high velocity in dilute phase suspension flow in a conveying system with a large number of small radius bends. Figure 21.1 shows the influence on the cumulative particle size distribution for the material before and after conveying. The mean particle size, based on the 50% value, has changed from about 177 to 152 urn. The really significant effect, however, is shown in the fractional size distribution plot in Figure 21.2. In this magnified plot the effect of degradation on the material can be clearly seen. A considerable number of fines are produced and even on a percentage mass basis these cause a significant secondary peak in the particle size distribution. 100 Material before conveying
80 60 3
Material after conveying
40
20
40
80
120
160
240
Particle Size - urn Figure 21.1 Possible influence of pneumatic conveying on cumulative particle size distribution of a friable material.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
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Particle Degradation
3 40 on
Material before conveying
c C3
os
Particle size
§
c/5 u 30
Material after conveying
•S 20
a 10
40
80
120
160
200
240
Particle Size - u.m Figure 21.2 Possible influence of pneumatic conveying on fractional size distribution of a friable material.
1.3
Operating Problems
Particle degradation can cause problems in a number of areas on account of the changes in particle shape and particle size distribution that can result. It is a particular problem with chemical materials that are coated, for it is the coating that is generally the friable element of the resulting material. Plant operating difficulties are often experienced because of the fines produced, and problems in handling operations can also result after the material has been conveyed. Apart from the obvious problems of quality control with friable materials, changes in particle shape can also lead to subsequent process difficulties with certain materials. The appearance of the material may also change so that it is not so readily sold. Changes in particle size distribution can affect flow characteristics, which in the extreme, can change a free-flowing material into one which will only handle with great difficulty and, with materials for subsequent sale, this can lead to customer problems. 1.3.1 Filtration Problems In pneumatic conveying systems plant, operating difficulties can result if degradation causes a large percentage of fines to be produced, particularly if the filtration equipment is not capable of handling the fines satisfactorily. Filter cloths and screens will rapidly block if they have to cope with unexpectedly high flow rates of fine powder. The net result is that there is usually an increase in pressure drop
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
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across the filter, and this could be a significant proportion of the total pressure available in a low pressure system. This means that the pressure drop available for conveying the material will be reduced, which in turn means that the mass flow rate of the material will probably have to be reduced in order to compensate. If this is not done there will be the risk of blocking the pipeline. Alternatively, if the filtration plant is correctly specified, with material degradation taken into account, it is likely to cost very much more as a result. This, therefore, provides a direct financial incentive to ensure that particle degradation is minimized., even if it does not represent a problem with respect to the material itself [2], 1.3.2 Flow Problems In many systems there is a need to store the conveyed material in a hopper or silo. Flow functions can be determined for bulk particulate materials, from which hopper wall angles and opening sizes can be evaluated, to ensure that the material flows reliably at the rate required. A change in particle size distribution of a material, as a result of conveying operations, however, can result in a significant change in flow properties. Thus a hopper designed for a material in the "as received" condition may be totally unsuitable for the material after it has been conveyed. As a result it may be necessary to fit an expensive flow aid to the hopper to recover the situation. 1.3.3 Potential Explosion Problems Many materials, in a dust cloud, can ignite and cause an explosion. Dust clouds are clearly quite impossible to avoid somewhere in a pneumatic conveying system, and so this poses a problem with regard to the safe operation of such systems. Of those materials that are explosive, research has shown that it is only the fraction with a particle size less than about 200 u.m that poses the problem. Degradation and attrition caused by pneumatic conveying, however, can result in the generation of a considerable number of fines, particularly if the material is friable. Even if the material did not represent a problem with respect to explosions in the "as received" condition, the situation could be very different after the material has been conveyed. 1.4
Test Rigs and Data Sources
Little data is available on the degradation of materials in pneumatic conveying systems. This is partly due to the complexity of obtaining and analyzing the data, but mainly to the fact that so many variables are involved, together with the problem of relating the data from one material and situation to another. A particular problem with data obtained from a pneumatic conveying system pipeline is that it is very difficult to separate the individual contributions made by the bends and the straight pipeline.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
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Particle Degradation
A further problem is that in a pipeline there is a gradual expansion of the conveying air, which means that the particle velocity is constantly changing. Velocity is a major variable in particle degradation and so this makes attempts at devising experimental plans and analysis of results very difficult. The major source of information is probably from the basic research that has been undertaken with small bench scale test rigs in which particles have been impacted against test materials under controlled conditions. This work has often been carried out to assist in an understanding of erosive wear problems. Although much of this work cannot be related directly to pneumatic conveying situations, it can provide valuable information of a comparative nature on a number of variables in the process. 1.4.1 Acceleration Tube Device Test facilities employed are very similar to those used in erosive wear research, such as whirling arm and acceleration tube devices. A device used by Salman et al [3] is shown in Figure 21.3 and consists of a linear air gun. One particle was tested at a time. Compressed air was used to accelerate the particles, and particle velocity could be varied by adjusting the air pressure. A cage was used to collect the particles and fragments after impact. The particle impact velocity was determined by measuring the time required for a particle to travel from the end of the barrel to the target. A photodiode was located at the end of the barrel and a loudspeaker was mounted behind the target.
Pressure Regulating Valve
Cage
Particle Impact .Angle Speaker
\
Compressed Air Supply
Computer
High-Speed Electronic Timer
Figure 21.3 Schematic arrangement of acceleration tube test apparatus and measuring system for particle impact studies.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
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In order to study the particle degradation process, brittle materials were used to ensure that no plastic deformation should take place. Three types of particle were used and tested. These were aluminum oxide, polystyrene and glass, and all the particles were spherical. The majority of the work was carried out with 0-2 in diameter aluminum oxide particles, with particle velocities up to about 6000 ft/min. For every test, 100 particles were impacted, and the number of unbroken particles was counted to provide an assessment of the degradation. 2
INFLUENCE OF VARIABLES
The variables in particle degradation are similar to those associated with erosive wear. Velocity, once again, is probably the most important, but particle size and concentration also play a part. Particle impact angle is equally important, and has a major influence with respect to the selection of pipeline bend geometry. The influence of both particle materials and surface materials must also be given due consideration. As with erosive wear, much of the research work into the subject has been carried out for various other purposes, and so the range of parameters investigated is often beyond those associated with pneumatic conveying, but it does provide useful information on the general trends of the variables. 2.1
Velocity
The relative velocity between particles and surfaces has a major influence on the nature and extent of the degradation and is probably the most important variable in the problem. In any collision the kinetic energy of the particles has to be absorbed and may provide sufficient energy for fracture. If the collision is elastic, with a high coefficient of restitution, much of the kinetic energy will reappear as particle velocity. In plastic collisions much of the kinetic energy will be converted to heat. Low velocity impacts tend to knock small chips from the edges of particles, whereas high velocity collisions are more likely to shatter particles. In general the rate of damage has been found to be a power law function of velocity, in much the same way as the erosive wear process. The range in value of the power coefficient is also large, and can vary between one and five, depending upon the conveyed material and the system being considered. The possibility of there being a threshold value of velocity, below which no degradation occurs, is also a possibility. 2. /. /
Peas
Agricultural products have been widely used in test work. Segler [4] investigated the effects of air velocity, moisture content, pipeline diameter and material concentration on the damage of peas, as a result of pneumatic conveying. His test loop was 240 ft long, 4'/2 in bore and contained 4 bends. The results of his tests on the effect of aii- velocity are presented in Figure 21.4. These showed that the damage increased approximately with the cube of air velocity.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
Particle Degradation
609
12
IX
o
^H
S 4
4000
2000
6000
Conveying Air Velocity - ft/min Figure 21.4 2.7.2
The influence of air velocity on the breakage of peas.
Quartz
Tilly et al [5] carried out impact studies with quartz particles against an alloy steel target in a rotating arm test rig. They found that the particles incurred a substantial degree of fragmentation which was dependent upon the velocity of impact. Their results are presented in Figure 21.5.
100
Particle size Taraet Material
80
- 125-150 urn - 11% Cr steel
60
I i-M
40
I g?
20
o
0
0
20,000
40,000
60,000
Particle Velocity - ft/min Figure 21.5
The influence of particle velocity on the degradation of quartz particles.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
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The velocity range comes as a result of their work being applied to aircraft engines. From this it would appear that for fragmentation to occur it is necessary to exceed a threshold value of velocity. Below this velocity the particles may be considered to behave elastically. From Figure 21.5 this would appear to occur at a velocity of about 3000 ft/min for this material. In work by Tilly and Sage [6] they impacted quartz particles in the size range of 100 to 225 urn at velocities of 12,000, 26,000 and 60,000 ft/min. Their results, in terms of particle size distribution, are presented in Figure 21.6. Although this data is for conveying velocities very much higher than those that would be encountered in a pneumatic conveying system, they relate to a single impact and so help to illustrate the nature of the problem, for many materials that are conveyed are significantly more friable than quartz. 2.7.3 A lum inum Oxide The results of a program of tests carried out with 0-2 inch aluminum oxide particles impacted at 90° against a steel target are presented in Figure 21.7 [3"|. In this plot the experimental data has been included to show how the relationship was derived and to show the limits of scatter in the results. The relationship is typical of the results obtained and so where families of curves are presented in subsequent figures from this program of work, experimental data has been omitted for clarity. It will be seen from Figure 21.7 that there is a very rapid transition in particle velocity from zero breakage to total degradation. Below a particle velocity of about 1800 ft/min only elastic deformation occurs and there is no particle degradation. Above a particle velocity of about 5000 ft/min, however, the stress induced by the impact is always sufficient to damage every particle.
100
Particle Velocity - ft/min
80 60
Before Impact
40
Particles - Quartz Target - 11% Cr Steel
20
50
100
150
200
Mean Particle size Figure 21.6
Influence of particle velocity on size distribution generated with quartz.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
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Particle Degradation
100
Particle size = 0-2 in Impact angle = 90° Target material = Steel
80 60 C
D
40
X
20
2000
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Particle Velocity - ft'min Figure 21.7 particles.
The influence of particle velocity on the degradation of aluminum oxide
It is interesting to note that within the transition region the number of unbroken particles at any given velocity is very consistent and that a smooth transition is obtained from one extreme to the other over this range of velocity. This was probably one of the first research programs to focus on particle degradation in the velocity range appropriate to pneumatic conveying. 2.2
Particle Size
Tilly et al [5] carried out impact studies with quartz particles against an alloy steel target in a rotating arm test rig. They found that the particles incurred a substantial degree of fragmentation which was dependent upon the initial particle size. Their results are presented in Figure 21.8. From this it would appear that for fragmentation to occur it is necessary for the particles to exceed a threshold size of about 10 urn. Below this size the particles probably behave elastically, for in their test rig the particles would have impacted the target since the tests were carried out in a vacuum. The results of tests carried out with three different sizes of spherical aluminum oxide particles are shown in Figure 21.9 [3], The data for the 0-2 in particles, which was the reference material in the work, was presented above in Figure 21.7. Results from similar tests with 0-12 and 0-28 in aluminum oxide particles, also impacted at 90° against the same steel target are additionally presented in Figure 21.9. A very significant particle size effect is shown. As the particle size increases, the maximum velocity at which no degradation occurs decreases. The transition from no degradation to total degradation also changes, with the transition occurring over a narrower velocity range with increase in particle size.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
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100
80 60
Particle Velocity - 60,000 ft/min Target Material - 11% Cr Steel
S 01)
S
40
ob 20
50
100
150
200
Initial Mean Particle size - u.m Figure 21.8
The influence of initial particle size on the degradation of quartz particles.
2.2.7 Particle Velocity Influence In more recent work on the influence of particle size, fertilizer particles, also having particle diameters of 0-12, 0-20 and 0-28 in, were pneumatically conveyed in a test facility to assess their degradation [7]. In this case the velocity used was that of the conveying air and not that of the particles. In terms of air velocity the 0-12 in particles degraded the most and the 0-28 in particles the least. 100
0-
g 60
^
XJ
c
40
o !-~
Impact Angle -90° Target Material - Steel
O
X)
20 0-12 1000
2000
3000
4000
5000
Particle Velocity - ft / min Figure 21.9 The influence of particle velocity and particle size on the degradation of aluminum oxide particles.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
613
Particle Degradation
The reason for this is that when it is the air velocity that is held constant, the smaller particles are accelerated to a higher velocity than the larger particles, as illustrated earlier with Figure 15.10. It is because particle velocity has a greater influence on degradation than particle size that a reversal in the influence of particle size has occurred. 2.3
Surface Material
With erosive wear of surface materials it has been found that the resilience of the surface material can have a significant influence on erosive wear, and that rubbers and polymers can offer better wear resistance than metals having a very high hardness value in certain cases. Since the mechanisms of erosion and degradation have many similarities, it is quite possible that resilient materials could offer very good resistance to particle degradation. 2.3.1 Material Type Further work by Tilly and Sage [6] showed that fragmentation is also dependent upon the type of target material. Figure 21.10 shows a comparison of their results for quartz impacted against nylon and fiberglass, which with their earlier results for alloy steel demonstrates the complex nature of the problem. Degradation in terms of the influence of initial particle size is used for the comparison in this case. The results of tests carried out on four different target materials are presented in Figure 21.11 [3]. In each case the targets were 0-2 in thick and they were impacted by 0-2 in diameter aluminum oxide particles at an angle of 90°. 100
c
80
I
60
'•o
40
Particle,Velocity /
Target Material
xo o-
Impact Angle - 90°
Steel - 60,000 ft/min
5h cd
a Q 20
Fiberglass - 50,000 ft/min
0 50
100
150
200
250
Initial Mean Particle size Figure 21.10 The influence of initial particle size and target material on the degradation of quartz particles.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
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Chapter 21
100 r
•*—- •"•—
—"•= ^
80L
—
^
Plexiglass . . .
Aluminum
Particle Size - 0-2 in Inroad Angle - 90°
60 5 40
