Material degradation effects
Pneumatic conveying is potentially one of the most aggressive means of transporting materials. Only in low velocity conveying systems can the conveying of the material be described as being ‘gentle’ but even then the material is in constant contact with the pipeline walls and there is considerable particle to particle interaction. In dilute phase suspension flow there may be very little contact between the material and the pipeline walls, but in this case most of the damage occurs with the high velocity impact of the material against the pipeline bends. The subject of particle degradation in pneumatic conveying systems is dealt with specifically in Chapter 24.
If friable materials are conveyed, therefore, there is the potential for damage to the material. Degradation will cause a change in particle size and there is a tendency for ‘fines’ to be generated. This effect was illustrated earlier with Figure 13.15. Particle size distribution has the effect of reducing the permeability of a material and of increasing the air retention. This effect was mentioned in relation to many of the materials considered in the previous chapter. The minimum conveying air velocity of the granulated sugar, for example, having a very narrow size distribution, was about 16 m/s, and yet the minimum velocity for granular coal with a much larger mean particle size was only 13 m/s.
In the work reported above, to find a correlation between material properties and conveying performance, two materials were represented twice. These were coal and granulated sugar, in the ‘as received’ and ‘degraded’ conditions. Although each material was identical chemically, re-circulation in the conveying facility changed the mater- ial so much that in terms of their conveying characteristics, each was a completely different material. This made it possible to include the degraded materials as additional materials in the analysis.
The conveying characteristics for the granulated sugar in the ‘as supplied’ condition were shown in Figure 12.11b. The material was conveyed through pipeline no. 3 (Figure 12.12). The sugar had a mean particle size of about 460 f.Lm and it had neither good permeability nor good air retention properties.
It was clearly a material that would not convey in dense phase in a conventional conveying system. This was confirmed during conveying trials, for as soon as conveying was attempted with a conveying line inlet air velocity below 16 m/s, the pipeline would block very rapidly. It will be seen from Figure 12.11b that the maximum solids
loading ratio that could be achieved was only 16, despite the fact that high pressure air was used for conveying.
The conveying characteristics for the degraded sugar conveyed through the same pipeline (no. 3) are shown in Figure 13.27a. With this material the minimum conveying air velocity was now down to 7 m/s and the maximum solids loading ratio that could be achieved was over 50.
If the material had been degraded further, it is possible that conveying with much lower velocities, and at much higher solids loading, would have been possible. In the ‘as supplied’ condition the sugar had a relatively narrow particle size distribution. Dilute phase pneumatic conveying of this friable material rapidly caused the generation of a considerable amount of fines in the material and so it very quickly obtained a degree of air retention.
With this material there was no significant change in the conveying capability with respect to material flow rate for a given pressure drop and air flow rate. As a conse- quence the conveying characteristics for the degraded material are simply an extension of the conveying characteristics for the fresh material. The two together are shown in Figure 13.27b and the influence of degradation in extending the range of conveying capability can be clearly seen. This situation is not common, however, for with the other materials included in this section on degradation, significant changes in both minimum velocity and material flow rates are reported.
Coal is a particularly friable material but it does convey very well. Degradation is gen- erally not a problem since the conveyed material is often pulverized in the end for
combustion purposes. The changes that can occur with respect to conveying characteristics for the material, however, are worth reporting for they are very common effects that can occur with many materials. The coal, as supplied, had a mean particle size of about 778 f.Lm. It was conveyed through pipeline no. 3 (Figure 12.12) and the conveying characteristics are shown in Figure 13.28a. The minimum conveying air velocity for the material was about 12 m/s.
When the coal had been degraded to the extent that the mean particle size had reduced to 146 f.Lm it was tested again in the same pipeline, and the conveying charac- teristics are presented in Figure 13.28b. In this case they are presented alongside the data for the fresh material and with exactly the same set of axes for direct visual com- parison. Whereas with the sugar there was no change with respect to the location of the pressure drop lines but there was a major shift in minimum conveying air velocity, the situation is completely reversed with the coal. There is very little change in the minimum conveying air velocity, but there is a significant increase in the material flow rate for a given pressure drop and air flow rate with the degraded coal.
The conveying characteristics for a ground grade of sodium sulphate conveyed through pipeline no. 3 were presented in Figure 12.23c. A limited programme of tests was undertaken with an unground grade of the sodium sulphate in the same pipeline in order to check on the conveying capability. This data is presented in Figure 13.29a. If the two sets of conveying characteristics are compared it will be seen that there is very little difference between them. The minimum value of conveying air velocity in each case was about 12 m/s.
The sodium sulphate is very friable and after deliberately degrading the mate- rial, another set of conveying characteristic were obtained. These are presented in Figure 13.29b. The change in conveying characteristics for the degraded material are different yet again. In this case there was a change in both minimum conveying air velocity and material flow rates achieved. With a two bar pressure drop, for example, the material flow rate for the degraded material was 100 per cent greater, based on minimum conveying air conditions, and the conveying air velocity reduced from 12 to 8 m/s.
Light sodium carbonate (light soda ash) has a mean particle size of about 115 f.Lm and has something of a reputation for being a difficult material to convey, as mentioned earlier with regard to heavy soda ash in Section 184.108.40.206. It is a friable material and is slightly hygroscopic. In order to learn something of its conveying capability a controlled programme of conveying trials was undertaken .
The programme of work, therefore, started with the knowledge that significant changes were likely to occur, and to occur quickly. As a consequence the conveying characteristics for the fresh, as supplied, material were undertaken with a fresh batch of material for every test point. A sketch of the pipeline used is presented in Figure 13.30.
The conveying characteristics obtained for the fresh material are presented in Figure 13.31a and those for the degraded material in Figure 13.31b. The two sets of conveying characteristics are presented together in Figure 13.31 and the same set of axes have been employed to allow direct visual comparison. Although there is no significant
or apparent change in solids loading ratio values and conveying air velocities, mater- ial flow rates achieved with the fresh material, for a given conveying line pressure drop, are considerably different. This difference increases with decrease in air flow rate, for the slope of the constant pressure drop lines is different for the two cases.
Particle size changes
For reference purposes a batch of material was re-circulated and samples were taken after every pass to show how the re-circulation influenced the mean particle size.
A typical set of results is shown in Figure 13.32. The pipeline was only 37 m long and in the case shown the material degraded from a mean particle size was about 97–117f.Lm in the first pass. After 10 passes, the mean particle size had reduced to about 73 f.Lm. The maximum conveying air velocity was only 17.8 m/s in this programme.
Pressure drop changes
With such a dramatic change in conveying performance another controlled pro- gramme of tests was undertaken in order to monitor the gradual changes more closely. For this purpose the material was re-circulated with exactly the same air flow rate in each test, and the material flow rate was held constant each time. The influence on the conveying line pressure drop is shown in Figure 13.33. From this it will be seen that
there is a gradual and significant reduction in pressure drop as the material is conveyed, particularly for the first few passes.
There are serious implications here for system design. If a material such as this is conveyed a couple of times to get a ‘feel’ for the material before undertaking a test to record conveying data, so that scaling can be carried out from the test pipeline to the plant pipeline, a significant change could occur, as shown with Figure 13.33. The scal- ing process would magnify the differences caused by re-circulation and the ultimate design could be in significant error. In nearly all cases re-circulation of the material results in an increase in conveying capability and so it follows that the material flow rate actually achieved with the plant would be well below that expected as a consequence. It must be emphasized, however, that this is an unusual situation.
From complete sets of conveying characteristics for the fresh and degraded materials the 1.0 bar pressure drop lines have been compared in Figure 13.34. The characteristics of the two soda ash materials are very different. For the conveying line pressure drop of 1.0 bar selected, the fresh material shows a pressure minimum point in its characteristics and a limit on solids loading ratio of about 20. The degraded material shows no inter- mediate pressure minimum point and the two lines diverge widely at low air flow rates, with the degraded material being conveyed at a solids loading ratio in excess of 80.
Most reputable pneumatic conveying systems manufacturing companies have test facilities for carrying out conveying trials with materials in order to generate system design data for the given material. It is clearly important to establish whether the nature of the material is likely to change with conveying, and whether the conveying characteristics of the material will change as a result. This is particularly important if the batch size of material available for testing is limited, such that only a single batch is available.