In this section problems relating to the conveyed material are considered. In the previous section some of the problems were as a direct result of the materials being conveyed, but the problems were recognized in terms of the effects that the materials had on the system. This section includes problems that result from the effect that the system can have on the material being conveyed, such as absorption of moisture, the formation of angel hairs and particle degradation. The more obvious material proper- ties such as particle size, temperature and moisture content are also considered here.
The formation of angel hairs is a problem that can occur with plastic materials such as nylons, polyethylene and polyesters, particularly in pelletized form. The presence of angel hairs is undesirable since they can cause blockages at line diverters and in filters. Angel hairs are generally caused by sliding contact between the particle and the pipeline. The frictional heat generated is sufficient to cause melting of that part of the particle in contact with the pipeline surface. This problem is considered in Chapter 24 along with other particle degradation problems.
Coating of pipelines
Certain moist materials, pigments and similar ultra-fine materials, hygroscopic mater- ials, and food products with a high fat content, may have a tendency to stick to or coat
the walls of a pipeline. If the coating builds up it will gradually reduce the section area of the pipeline and generally results in blockage. Conveying with a very much higher air velocity is often successful with some materials. One method that often works is to convey the material through a rubber hose capable of withstanding the conveying air pressure. The natural flexing of the hose with the conveying of the material and pres- surizing and de-pressurizing is often sufficient to dislodge any build-up of material.
With cohesive materials the problems often relate to the difficulty of feeding the material into the pipeline. If difficulties are encountered in achieving flow rates with a system, and the conveying line pressure drop is below the expected value, the problem could well relate to the discharge of the material from the supply hopper, rather than the capability of the feeding device or the pipeline. In this case, the use of a suitable bin discharge aid should be considered. In the case of rotary valves, a blow through type should be used if there is any difficulty in discharging a cohesive material into a conveying line.
Consolidation of materials
Many bulk materials increase in strength with time. This is a particular problem with the storage of bulk solids in hoppers and silos. A material stored for 1 day may well flow freely from a hopper but refuse to flow at all after being stored for two days. Bulk density can also increase with time, and significantly so with some materials. If a material has consolidated in a hopper and is fed into a positive displacement feeder, the pipeline could block due to being overfed.
This can occur with rotary valve fed systems on start-up. The bulk density of materials such as barytes, cement and fly ash can increase by 30 to 40 per cent with compaction. The discharge rate of a rotary valve was given in Equation (3.1) and it will be seen that it is directly proportional to the bulk density of the material. Once the material in the hopper has been disturbed by flowing down into the rotary valve, and with aeration from a proportion of the air leaking across the rotary valve, operation could be back to normal once the blockage has been cleared and the system re-started.
Aeration of the material before being conveyed would always be recommended in road and rail vehicle transport systems. By the time such a vehicle arrives at a depot for off-loading a considerable degree of compaction will have resulted. This is one of the advantages of pressurizing bottom discharge blow tanks from the bottom, as shown in Figure 4.16. The air required to pressurize the vessel must pass through the material and this will aerate the material very effectively.
Degradation of materials
The fracture and breakage of pneumatically conveyed materials is a problem with all friable materials. Even if the presence of fines in the material is not a problem with respect to product quality, the fines produced will add unnecessarily to the duty on the filtration unit. The problem is influenced to a large extent by conveying air velocity. Since this is a major problem in the industry Chapter 24 is devoted to the subject.
If a granular material has to be conveyed, difficulties may be experienced in discharging the material into the conveying line. Rotary valves and blow tanks may cause problems here, and so reference should be made to the appropriate items in Chapters 3 and 4. Once the granular material is fed into the pipeline there should be no problem with its conveying, although it is almost certain that it will have to be conveyed in dilute phase suspension flow, unless the material has a very narrow particle size distribution and good permeability.
If a hygroscopic material is pneumatically conveyed it may absorb moisture from the air used to convey the material and become very cohesive, and have poor flowability as a result. Although the specific humidity of the air will reduce if it is compressed isothermally beyond the saturation point, its relative humidity will increase and is likely to be 100 per cent after compression. The added moisture will not only affect material quality but could cause subsequent handling problems.
Problems of moisture in conveying air are not so serious in negative pressure systems. Although the specific humidity will remain constant, the relative humidity of the air will constantly reduce along the pipeline as the conveying air pressure reduces. The problem can be overcome altogether by drying the air that is used for conveying the material, either by refrigeration or desiccant devices. The subject of moisture in air with respect to pneumatic conveying systems is considered in detail in Chapter 25.
Large particles can be conveyed quite successfully in pneumatic conveying systems but a general recommendation is that the diameter of the pipeline should be about three times that of the larger particles. This is simply an expedient measure to ensure that the pipeline will not block by the wedging action of two ‘rigid’ particles. There are exceptions to this rule, of course, and with very ‘pliable’ materials such as fish, it is possible to convey ‘particles’ that are slightly larger than that of the pipeline bore. With ‘rigid’ particles, shape may present a problem if a mean particle value is used in sizing, and the particles have an irregular shape.
Care must be exercised with the feeding of these materials in all cases. With materials such as coal, clinker and iron ore, gate valves are often used because they are very rugged, and heavy duty closing devices are employed. The trapping of these particles must be avoided for they may damage the seals. Trapped particles and damaged seals will both allow air to leak through the feeder and so affect the performance of the conveying system.
If a system is dedicated to a single duty with a single material, and the system has been optimized to the lowest specific energy, operating difficulties may be experienced if there is a change in material grade or quality. If a material is produced with a slightly
different shape or size it could be sufficient to cause the pipeline to be blocked. It must be appreciated that different grades of the same material can have very different conveying characteristics, and even the pneumatic conveying of a material can change its conveying characteristics. This was illustrated in Chapter 13 with light soda ash and a number of other materials.
High temperature materials can be conveyed quite successfully and conveying gas at any temperature can be used. Compatibility with system components is the determining factor. Conveying air velocities also have to be guaranteed if there are significant temperature changes. It is the evaluation of gas and conveyed material temperature that presents the difficulty, as discussed in Section 9.6.1.
At the feeding point, for example, cold air may be used to convey a high temperature material. Along the conveying line there will be a move towards thermal equilibrium between the air and the material, and there will be heat transfer from the pipeline to the surroundings. Since conveying times are very short it is unlikely that equilibrium will be established. It is quite possible, therefore, for the surface of the particles to be ‘cold’ and the inner core to be ‘hot’. Due to this it is often possible to use filter cloths in these high temperature situations. By the same reasoning the material in the reception hopper could be very hot once equilibrium has been established there.
The maintenance of conveying air velocities is particularly important in these situations but their evaluation can be difficult. Particle temperature transients represent a complex convection, radiation and three-dimensional conduction heat transfer problem. Since air density increases with decrease in temperature, however, the maintenance of air velocities is only likely to be a problem in situations where a very high temperature gas is used to convey a cold material. In this case the temperature gradient effect could over-ride the pressure gradient influence on air density.
Fine materials that are wet will tend to coat the pipeline and gradually block the pipeline. The problem can be relieved by heating the conveying air if the material is not too wet. Greater difficulty may be experienced in discharging a material from a hopper if it is wet. When wet granular materials are fed into a pipeline bends present a particular difficulty since these can become wet with moisture centrifuged off particles on impact, and fine material will adhere and gradually block the line at the bend. Single plug blow tanks, as illustrated in Figure 2.9, often work well with wet materials since the retarding force is wall friction, and the higher the moisture content the lower the wall friction.