Y

Main fan Venturi

Filter

Main fan Venturi

Figure 9. Multipass (a) and air recycle (b) arrangements in flash dryers.

porting band or alternatively upward. Sometimes a combination of the two may be dictated by the nature of the wet feed. It occasionally happens that extruded materials have a tendency to coalesce when deposited on the band, in which case one or more sections at the wet end of the dryer may be arranged for upward airflow to reduce the effect. Wherever possible, through-air circulation is used as opposed to transverse airflow. This results in greatly increased evaporative rates as may be seen from Table 1.

An illustration of the relatively high performance of a band dryer operating on this system as compared with a unit having transverse airflow can be cited in a case involving the processing of a 70% moisture content filter cake. When this material is dried in a conventional unit, the cycle time is in the region of 28 h. This is reduced to 55 min in the through-circulation band dryer largely as a result of using an extruder-performer designed to produce a dimensionally stable bed of sufficient porosity to permit air circulation through the feed.

In view of this, transverse airflow usually is used only where the type of conveyor necessary to support the product does not allow through-flow or where the product form is not suitable for this method of airflow. The most usual method of heating is by steam through heat exchangers mounted in the side plenums or above the band, although direct oil and gas firing sometimes is used. In such cases, the products of combustion normally are introduced to a hot well or duct at an elevated temperature from where they are drawn off and mixed with circulating air in each zone or section of the dryer.

Another alternative with direct firing is to use a series of small individual burners positioned so that each serves one or more zones of the dryer. Typical single-pass dryers of modular construction are illustrated in Figures 10, 11, and 12.

With this type and size of dryer, the average product throughput is about 5600 lb/h and involves an evaporation of 1600 lb/h moisture. It is not unusual, however, to find equipment with evaporative capacities of 3000 lb/h. Such outputs involve quite a large band area with correspondingly large floor area requirements. Various types of feeding arrangements are available to spread or distribute the wet product over the width of the band. Here again, the nature of the feed is an important prerequisite for efficient drying. Steam-heated, finned drums have been used as a means of producing a partially dried, preformed feed. While the amount of predrying achieved is reflected in increased output for a given dryer size or, alternatively, enables a smaller dryer to be used, these items are usually much more costly than many of the mechanical extruders that are available.

Generally, these extruders operate with rubber covered rollers moving over a perforated die plate with feed in the form of pressed cakes or more usually, as the discharge from a rotary vacuum filter. Others of the pressure type employ a gear pump arrangement, with extrusion taking place through a series of individual nozzles, while some use screw feeds that usually are set up to oscillate in order to obtain effective coverage of the band. Alternative designs include rotating cam blades or conventional bar-type granulators, although the latter often produce a high proportion of fines because of the pronounced shearing effect. This makes the product rather unsuitable because of the entrainment problems that can occur.

Each of the types available is designed to produce continuous-discontinuous extrudates or granules, with

Figure 10. Continuous conveyor band dryer arrangement for direct gas firing.

Figure 10. Continuous conveyor band dryer arrangement for direct gas firing.

Figure 11. Multistage band dryer.

the grid perforations spaced to meet product characteristics. In selecting the proper type of extruder, it is essential to carry out tests on semiscale equipment as no other valid assessment of suitability can be made.

As a further illustration of the desirability of using a preforming technique, tests on a designated material exhibited a mean evaporative rate of 1.9 lb ft2 • h when processed in filter-cake form without preforming. When extruded, however, the same material being dried under identical conditions gave a mean evaporative rate of 3.8 lb ft2 • h. This indicates, of course, that the effective band area required when working on extruded material would be only 50% of that required in the initial test. Unfortunately, the capital cost is not halved as might be expected since the feed and delivery ends of the machine housing the drive and terminals remain the same and form an increased proportion of the cost of the smaller dryer. While the cost of the extruder also must be taken into account in the comparison, cost reduction still would be about 15%. Of course, other advantages result from the installation of the smaller dryer. These include reduced radiation and convection losses and a savings of approximately 40% in the floor area occupied.

This type of plant does not involve high installation costs, and both maintenance and operating labor requirements are minimal. Since they generally are built on a zonal principle, with each zone having an integral heater and fan, a good measure of process control can be achieved.

Figure 12. Band dryer and extruder with dye stuffs.

Figure 12. Band dryer and extruder with dye stuffs.

Furthermore, they provide a high degree of flexibility because of the provision of variable speed control on the conveyor.

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